Volume 15 / Number 1 / Pages 1-98 - World Scientific · Tomé, Augusto C. 1 Y Yedukondalu, Meesala 83 Z Zhang, Ying 66 Zhu, Weihua 66 Ziegler, Thomas 39 Journal of Porphyrins and

Volum

e 15 Num

ber 1 Pages 1-98 Journal of P

orphyrins and Phthalocyanines 2011 S

PP

MICA (P) 207/01/2009

2011 - ISSN 1088-4246

Volu

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15 /

Num

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1 /

Page

s 1-

98January 2011

JPP Vol 15 N1.indd 1 3/9/11 9:56 AM

Editor-in-ChiefProfessor Karl M. KadishDepartment of ChemistryUniversity of HoustonHouston, Texas 77204-5003USAemail: [email protected]

Associate EditorsProfessor Francis D’SouzaDepartment of ChemistryWichita State University1845 FairmountWichita, KS 67260-0051USAemail: [email protected]

Professor Atsuhiro OsukaDepartment of ChemistryGraduate School of ScienceKyoto UniversityKitashirakawa-oiwake-cho, Sakyo-kuKyoto 606-8502Japanemail: [email protected]

Professor Roberto PaolesseDept. of Chemical Science and TechnologiesUniversity of Rome "Tor Vergata"Via della Ricerca Scientifica00133 RomeItalyemail: [email protected]

Professor Kevin M. SmithDepartment of ChemistryLouisiana State University130 David Boyd HallBaton Rouge, Louisiana 70803USAemail: [email protected]

Professor Tomas TorresDepartamento de Quimica OrganicaUniversidad Autonoma de Madrid28049 MadridSpainemail: [email protected]

The official journal of the Society of Porphyrins & Phthalocyanines.

An international journal devoted to research in the chemistry, physics, biology and technology of porphyrins, phthalocyanines and related macrocycles.

Editorial Board

Vefa Ahsen, Gebze Institute of Technology, TurkeyJosé A. S. Cavaleiro, University of Aveiro, PortugalMichael J. Cook, University of East Anglia, United KingdomMaxwell J. Crossley, University of Sydney, AustraliaShunichi Fukuzumi, Osaka University, JapanAbhik Ghosh, University of Tromso, NorwaySergiu M. Gorun, New Jersey Institute of Technology, USAZeev Gross, Technion - Israel Institute of Technology, IsraelRoger Guilard, Université de Bourgogne, FranceDirk M. Guldi, Friedrich-Alexander-Universität, GermanyDevens Gust, Arizona State University, USAMichael Hanack, Universität Tübingen, GermanyTakashi Hayashi, Osaka University, JapanDongho Kim, Yonsei University, KoreaNagao Kobayashi, Tohoku University, JapanLechoslaw Latos-Grazynski, University of Wroclaw, PolandChang-Hee Lee, Kangwon National University, KoreaFranz-Peter Montforts, Universität Bremen, GermanyRavindra K. Pandey, Roswell Park Cancer Institute, USAJonathan Sessler, University of Texas, USAMartin J. Stillman, University of Western Ontario, CanadaKenneth S. Suslick, University of Illinois at Urbana-Champaign, USADieter Wöhrle, University of Bremen, Germany

Editorial StaffWSPC (Singapore)

T. Yugarani (Desk Editor) Eliana Sidharta (Marketing)email: [email protected] email: [email protected]

SPP Office (Dijon)

Virginie Mollinier Alain Tabardemail: [email protected] email: [email protected]

Aims and ScopeThe Journal of Porphyrins and Phthalocyanines covers research in the chemistry, physics, biology and technology of porphyrins, phthalocya-nines and related macrocycles. Research papers, review articles and short communications deal with the synthesis, spectroscopy, processing and applications of these compounds.

Editorial_notes.indd 1 3/9/2011 1:35:03 PM

CONTENTS

Journal of Porphyrins and PhthalocyaninesJ. Porphyrins Phthalocyanines 2011; 15: 1–98

See Jing Du, Masanori Sono and John H. Dawson* pp 29–38

His93Gly myoglobin has been extensively used as a versatile scaffold to build heme iron complexes of defined structure with imidazole, alkylamine and thiolate ligation as natural heme protein models, and for generating mixed ligand adducts as well as oxyferrous and ferryl moieties. Herein, the authors describe the analysis of the pH-de-pendent heme iron coordination structures of exogenous ligand-free ferric His93Gly myoglobin and the preparation and spectral characterization of novel thioether- and selenothiolate-ligated heme iron states to mimic methionine- and selenocysteine-bound heme proteins.

About the Cover

Review

Articles

pp. 29–38Ferric His93Gly myoglobin cavity mutant and its complexes with thioether and selenolate as heme protein modelsJing Du, Masanori Sono and John H. Dawson*

Starting from the exogenous ligand-free ferric H93G Mb which is a pH-dependent mixture of two high-spin species with pKa

app = 6.6 as shown, ferric mono- and bis-thioether (tetrahydrothiophene, THT)- and ben zene selenolate (Ph-Se-)- bound proteins have been successfully prepared and characterized with UV-visible absorption and magnetic circular dichroism spectroscopy as models for mono-Met-, bis-Met- and Se-Cys-ligated heme proteins, respectively.

N N

N N

FeIII

S

S

+ THT

OH

N N

N N

FeIII

S

+ Ph-SeH

Se

N N

N N

FeIII

Exogenous ligand-free His93Gly myoglobin

OH

N N

N N

FeIII

distal side

proximal side

N N

N N

OH2

OH2

FeIII

+ H2O, + H+

- H2O, - H+

pKa(app) = 6.6

NN

NN

-O O-O O

FeN N

N N

Fe

+ THT

pp. 1–28Atropisomerism and conformational aspects of meso-tetraarylporphyrins and related compoundsAugusto C. Tomé, Artur M. S. Silva,* Ibon Alkorta and José Elguero

The atropisomerism of meso-di- and tetraarylporphyrins with substi-tuents in ortho-positions of the aryl ring, as well as in corroles and in conveniently substituted phthalocyanines, is comprehensively reviewed. The study of the atropisomers, the tautomerism of the inner protons, the restricted rotation of the meso-aryl groups and the influence of the metal on the conformation of the macrocycle involved the use of a number of experimental and theroretical methods.

15-01 Contents.indd 1 3/3/2011 12:57:48 PM

CONTENTS

J. Porphyrins Phthalocyanines 2011; 15: 1–98

pp. 54–65Synthesis and spectral study of tetra(2,3-thianaphthe no) porphyrazine, its tetra-tert-butyl derivative and their Mg(II), Al(III), Ga(III) and In(III) complexesEkaterina S. Taraymovich*, Andrey B. Korzhenevskii, Yulia V. Mitasova, Roman S. Kumeev, Oscar I. Koifman and Pavel A. Stuzhin*

Synthesis of novel porphyrazines bearing four 2,3-annulated thianaphthene moieties has been elaborated starting from easily available thiophenols and oxalylchloride. The free base tetra(2,3-thianaphtheno)porphyrazine, its tetra-tert-butylsubstituted derivative and their complexes with Mg(II), Al(III), Ga(III) and In(III) have been prepared and the effect of extension of the porphyrazine π-chromophore by fusion of four 2,3-thianaphthene fragments has been studied.

pp. 47–53Singlet molecular oxygen generation by water-soluble phthalocyanine dendrimers with different aggregation behaviorMasakazu Nishida, Hiroaki Horiuchi, Atsuya Momotake, Yoshinobu Nishimura, Hiroshi Hiratsuka and Tatsuo Arai*

Phthalocyanines having hydrophilic or lipophilic dendrons were synthesized to investigate the efficiencies of singlet molecular oxygen (1Δg) formation. The introduction of higher generation of dendrons to the central metal (Si) of phthalocyanine in vertical direction to their ring plane has resulted in the suc-cessful improvement in avoiding aggregate formation that resulted in efficient generation of 1Δg even in water.

pp. 39–46Aggregation behavior and UV-vis spectra of tetra- and octaglycosylated zinc phthalocyaninesAlexey Lyubimtsev, Zafar Iqbal, Göran Crucius, Sergey Syrbu, Ekaterina S. Taraymovich, Thomas Ziegler and Michael Hanack*

Several tetra- and octaglycosylated PcZn’s were investigated for their aggre-gation behavior using different concentrations of the PcZn-species in pure DMSO, water and in various DMSO/water mixtures by discussing their UV-vis spectra.

pp. 66–74Reductive dechlorination of DDT electrocatalyzed by synthetic cobalt porphyrins in N,N ′-dimethyl-formamideWeihua Zhu, Yuanyuan Fang, Wei Shen, Guifen Lu, Ying Zhang, Zhongping Ou* and Karl M. Kadish*

Two cobalt porphyrins, (OEP)CoII and (TPP)CoII, were examined as electroca talysts for the reductive dechlorination of DDT. The effect of porphyrin struc ture and reaction time on the dechlorination products was examined by GC-MS, cyclic voltammetry, controlled-potential electrolysis and UV-visible spectro electrochemistry.

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CONTENTS

J. Porphyrins Phthalocyanines 2011; 15: 1–98

pp. 75–82Tin(IV) porphyrin functionalization of electro-chemically active fluoride-doped tin-oxide (FTO) via Huisgen [3+2] click chemistryShiva Prasad, Mohan Bhadbhade and Pall Thordarson*

The tin(IV) porphyrin 3 was attached to transparent fluoride tin oxide (FTO) surfaces using copper(I) catalyzed Huisgen [3+2] click chemistry and the resulting tin(IV) porphyrin modified FTO electrodes 7 were then characterized by UV-vis, XPS and CV. These tin(IV) porphyrine electro-active surfaces could be used in various photo-driven devices, including for hydrogen production.

pp. 83–98Meso-meso phenyl bridged unsymmetrical por-phyrin dyads: synthesis, spectral, electro che mi-cal and pho tophysical propertiesMeesala Yedukondalu, Dilip K. Maity* and Mangalampalli Ravikanth*

The meso-meso phenyl bridged unsymmetrical porphyrin dyads contain-ing two different types of porphyrin sub-units were synthesized and the electronic interactions between the two porphyrin sub-units were inves-tigated by spectral, electrochemical and computational studies.

M = Zn; X = NH;M = 2H; X = NH;M = Zn; X = O;M = 2H; X = O;M = Zn; X = S;M = 2H; X = S;

N N

NN

N X

NY

CH3 CH3

CH3

CH3CH3

H3C M

Y = S : 1Y = S : 2Y = S : 3Y = S : 4Y = S : 5Y = S : 6

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AUTHOR INDEX (cumulative)

AAlkorta, Ibon 1Arai, Tatsuo 47

BBhadbhade, Mohan 75

CCrucius, Göran 39

DDawson, John H. 29Du, Jing 29

EElguero, José 1

FFang, Yuanyuan 66

HHanack, Michael 39Hiratsuka, Hiroshi 47Horiuchi, Hiroaki 47

IIqbal, Zafar 39

KKadish, Karl M. 66Koifman, Oscar I. 54Korzhenevskii, Andrey B. 54Kumeev, Roman S. 54

LLu, Guifen 66Lyubimtsev, Alexey 39

MMaity, Dilip K. 83Mitasova, Yulia V. 54Momotake, Atsuya 47

NNishida, Masakazu 47Nishimura, Yoshinobu 47

OOu, Zhongping 66

PPrasad, Shiva 75

RRavikanth, Mangalampalli 83

SShen, Wei 66Silva, Artur M.S. 1Sono, Masanori 29Stuzhin, Pavel A. 54Syrbu, Sergey 39

TTaraymovich, Ekaterina S. 39, 54Thordarson, Pall 75Tomé, Augusto C. 1

YYedukondalu, Meesala 83

ZZhang, Ying 66Zhu, Weihua 66Ziegler, Thomas 39


JPP Volume 15 - Number 1 - Pages 1–98

15-01 AI.indd 1 3/3/2011 1:01:18 PM

Aaggregation 39, 47aluminium(III) 54atropisomerism 1atropisomers 1

Bbacterioferritin model system 29

Ccobalt porphyrins 66corroles 1

DDDT 66dendrimer 47

Eelectrocatalysis 66electrochemistry 66, 75electronic absorbtion spectra 54energy transfer 83

Ffluorescence lifetime 47FTO 75

Ggallium(III) 54

Hheteroporphyrin 83His93Gly myoglobin 29

Iindium(III) 54

Lligand binding 29

Mmagnesium(II) 54magnetic circular dichroism

spectroscopy 29

Pphenyl bridge 83phthalocyanine 1, 39, 47picket fence 1porphyrazines 54porphyrin boronate 83porphyrins 1

Rreductive dechlorination 66restricted rotation 1

Sseleno-Cys-ligated heme protein 29

singlet oxygen yield 47spectroelectrochemistry 66sugar-substituents 39surface functionalisation 75

Ttetra(2,3-thianaphtheno)

porphyrazines 54tetrahydrothiophene-bound H93G

myoglobin 292,3-thianaphthene derivatives 54tin(IV) porphyrin 75transient absorption 47

Uunsymmetrical porphyrin dyad 83UV-vis spectra 39

Wwater-soluble 47

XXPS 75

KEYWORD INDEX (cumulative)


JPP Volume 15 - Number 1 - Pages 1–98

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DOI: 10.1142/S1088424611002994

Published at http://www.worldscinet.com/jpp/

Copyright © 2011 World Scientific Publishing Company

INTRODUCTION

This review deals with the atropisomerism of meso-tetraarylporphyrins 1 and related compounds, like phthalo cyanines 2 (Fig. 1). As most authors, we prefer atropisomer to atropoisomer although both terms appear in the literature. It refers to the restricted rotation about the C–C bond marked with an arrow in the figures below. There are some much less frequent examples of C–N [1, 2, 3] or C–O bonds [2] (meso-N- or -O-substituted porphyrins).

Related to the atropisomerism of porphyrins and phtha-locyanines is the deformation of the central 16-mem-bered ring (tetrapyrroles or tetraisoindoles). For instance, the presence of a metal in the middle distorts the central ring, the distortion being a direct result of shrinkage of the macrocycle cavity, caused by the complexation with a central metal atom with a small ionic radius. The result-ing conformations have received several names: saddled, ruffled, domed, etc [4–8]. In phthalocyanines one of such conformations is called shuttlecock [9–11] (Fig. 2).

There are a series of IUPAC’s definitions worth to remember here.

Atropisomers

A subclass of conformers which can be isolated as separate chemical species and which arise from restricted rotation about a single bond (see rotational barrier) e.g. ortho-substituted biphenyl and 1,1,2,2-tetra- tert-butylethane.

Rotational barrier

In a rotation of groups about a bond, the potential energy barrier between two adjacent minima of the molecular entity as a function of the torsion angle.

Axial chirality

Term used to refer to stereoisomerism resulting from the non-planar arrangement of four groups in pairs about a chirality axis. It is exemplified by allenes abC=C=Ccd (or abC=C=Cab) and by the atropisomerism of ortho-substituted biphenyls.

The configuration in molecular entities possessing axial chirality is specified by the stereodescriptors Ra and Sa (or by P or M ).

Atropisomerism and conformational aspects of meso-tetra arylporphyrins and related compounds

Augusto C. Toméa, Artur M.S. Silva*a, Ibon Alkortab and José Elguerob

a Department of Chemistry and QOPNA, University of Aveiro, 3810-193 Aveiro, Portugal b Instituto de Química Médica (CSIC), Juan de la Cierva 3, 28006 Madrid, Spain

Received 15 November 2010Accepted 30 December 2010

ABSTRACT: This review provides a comprehensive description of the atropisomerism of meso-di- and tetraarylporphyrins with substituents in ortho-positions of the aryl ring, as well as in corroles and in conveniently substituted phthalocyanines. Different methods of study were examined: X-ray crystallography, NMR spectroscopy (both static and dynamic aspects), classical kinetics, HPLC and theoretical calculations. Then the four atropisomers, the tautomerism of the inner protons, the ‘picket fence’ concept, conformationally restricted meso-tetraarylporphyrins and the influence of the metal on the conformation were discussed based on 250 references.

KEYWORDS: atropisomers, atropisomerism, picket fence, restricted rotation, porphyrins, corroles, phthalocyanines.

SPP full member in good standing

*Correspondence to: Artur M.S. Silva, email: [email protected], fax: +351 234-370-084

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http://dx.doi.org/10.1142/S1088424611002994

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Copyright © 2011 World Scientific Publishing Company J. Porphyrins Phthalocyanines 2011; 15: 2–28

2 A.C. TOmé et al.

Fig. 1. Porphyrins and phthalocyanines atropisomerism (in bold the atropisomeric bond)

1 2

N

HN

N

N

N

NH

N

N

φ

12

3

4

5

6

78

9

1011

1213

1415

16

17

18

19

20

2122

23

24 25

2627

28

29

30

32

31N

NH N

HN

φ

1

2

34

56 7

8

9

10

11

12

1314

1516

17

18

20

21 22

232419

N

N N

N

4, Zn(To-CNPP)

Zn

CN

NC

NC

CN

N

N N

N

Zn

NH

HN

5

O

O

N

N

N

NH N

HN

NH

HN

3, H2(TpivPP)

t-Bu

O

t-BuO

HN

NH

O

t-Bu

Ot-Bu

Chirality axis (also axis of chirality)

An axis about which a set of ligands is held so that it results in a spatial arrangement which is not superpos-able on its mirror image. For example with an allene abC=C=Ccd the chiral axis is defined by the C=C=C bonds, and with an ortho-substituted biphenyl the atoms C-1, C-l′, C-4 and C-4′ lie on the chiral axis.

AIMS AND SCOPE

The aim of this review is to report the static and dynamic aspects of the conformation of meso-tet-raarylporphyrins and related compounds including

phthalocyanines. The conformation was mainly studied by crystallography (static aspects) and by NMR (dynamic aspects) while theoretical publica-tions have provided a bridge between these aspects. Chiral HPLC and racemization experiments have shed light in the barriers to the rotation of the aryl groups. Synthetic aspects will be mentioned only when necessary. Other important properties of the compounds under survey are outside the scope of

this review; they include aspects such as absorption spec-troscopy, photophysics, biochemistry, biological applica-tions or molecular electronics.

METHODS OF STUDY

Information concerning the rotation about the C-C bond can be obtained by different methods. X-ray crys-tallography provides information about the dihedral angle φ and on the preferred atropisomer. A search in the CSD [12] shows that there are about 3025 and 660 structures of porphyrins and phthalocyanines of the 1 and 2 types susceptible to present atropisomerism.

X-ray crystallography

Since the information on X-ray structures is too large to be commented, we have selected a few examples to illustrate the information that this technique provided.

Fig. 2. Conformations of porphyrins and phthalocyanines

ruffled saddled shuttlecock

b

a

a

b

C C Cab

a

b

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ATrOPISOmerISm AnD COnfOrmATIOnAl ASPeCTS Of meso-TeTrA ArylPOrPhyrInS 3

Besides the paradigmatic ‘picket fence’ porphyrin 3 determined by Rheingold [13], Hatano isolated the four atropisomers of meso-tetrakis(o-cyanophenyl)porphyrinatozinc(II) (4), Zn(To-CNPP), and determined the X-ray structure of the α,β,α,β-atropisomer [14]. He also studied the atropisomerism of the free ligand, To-CNPP [15]. The same author reported the structure of 5α-10β-bis(o-nicotinamidophenyl)-15,20-diphenylporphyrinato]zinc(II) (5) [16]. The nicotinamide side arms coordinate the zinc ions of adjacent molecules to form a linear chain containing six-coordinate zinc ions.

In the structure of the antimony(V) complex of meso-tetra(p-tolyl)porphyrin (TTP), {Sb(TTP)[OCH-(CH3)2]2}

+Cl- (6), the antimony atom exhibit approximate octahedral geometry [17]. Atropisomerism in porphyrins bearing quaternary heterocycles at the meso-positions is less frequent. An example is the determination of the structure of the α,α,α,β-atropisomer of [Cu(H2O)T (2-NMePy)P]4+ (7) [18].

The structures of two porphyrins bearing 3-pyrazolyl (8) or 5-pyrazolyl (9) groups at the meso-positions were determined: in both cases they correspond to the α,α,β,β-atropisomer [19, 20]. Other meso-tetrapyrazolyl-porphyrins were characterized by 1H NMR [21]. The fluo-rescence of meso-tetrakis(1-arylpyrazol-4-yl) porphyrins was also reported [22].

The much distorted structure of the class I porphy-rin (see Fig. 8) with a Ni atom was described as “chiral ruffled basked-handle porphyrin” [23]. The same author reported the X-ray structures of several strapped porphy-rins [24]. Rose et al. reported the structure of a ‘double picket fence’ [5,10,15,20-tetrakis(2,6-diacrylamido-4- t-butylphenyl) porphyrinato]zinc(II) (10) [25]. The struc-tures of other ‘picket fence’ porphyrins were discussed in another publication [26].

When the porphyrin has only two asymmetric meso-aryl rings bearing ortho-substituents the nomenclature is simplified, like in the α,α-atropisomer 11 [27].

Bringmann et al. determined the structure of the race-mic β,β'-bis(5,10,15,20-tetraphenylporphyrin) (12) [28]. Another spectacular work was the determination of the X-ray structure of a hexameric wheel by Osuka et al. [29] obtained through the use of a meso-3-pyridyl substituent and Zn cations (Fig. 3).

NMR spectroscopy

In NMR spectroscopy, several nuclei have been used for the study of atropisomerism of porphyrins in two ways: static and dynamic. Static studies have been used to determine the nature of the atropisomers present in solu-tion and, much less frequently, in the solid state. Dynamic

N

N N

N

Sb

N

N N

N

N

N

N

N

Cu

7, [Cu(H2O)T(2-NMePy)P]4+

6, [Sb(TTP)(OCH(CH3)2)2]+

OPr-i

i-PrO

Me

Me

Me

Me

H2O

Me

Me

Me

Me

N

NH N

HN

α,α,β,β

NN

N

NH N

HN

NN

N N

NN

NN

R1

R1

R1

R1

Me

Me

Me

Me

8, R1 = CH2OCH2CH2SiMe3

α,α,β,β

Bn

Me

NN

N N

NN

Bn

Me

Bn

MeBn

Me

9

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4 A.C. TOmé et al.

NMR (DNMR) studies have been used to determine the barriers of rotation of meso-substituted porphyrins.

Static aspects. Static studies in solution, i.e. the deter-mination of the equilibrium constants and structure of

the four atropisomers of porphyrins are very numerous. Ann Walker [30] reported the first study of atropiso-merism of porphyrins using NMR. Using the nickel complex of meso-tetrakis(o-methylphenyl)porphyrin, Ni(To-CH3PP), she noted that the ortho-phenyl pro-tons lie outside the porphyrin ring π system, and are thus expected to be deshielded by the magnetic field of the ring current, while the protons of the methyl group in the ortho-position of the phenyl ring lie inside the perime-ter of the π system of the porphyrin ring, and are thus expected to be shielded by the magnetic field of the ring current. The second obvious feature of the NMR spec-trum of Ni(To-CH3PP) is the multiple methyl resonances. This phenomenon is observed for all metal complexes of meso-tetrakis(o-methylphenyl)porphyrin, whether they are diamagnetic or paramagnetic, although the peaks of the latter type are quite broad and overlap extensively.

If the porphyrin is formed by completely random linking of pyrrole and o-methylbenzaldehyde units, the statistical ratio of the atropisomers is 1:4:2:1. The α,α,α,β-isomer contains three types of methyl groups, abundance 1:2:1, while the other three isomers contain sets of 4 equivalent methyl groups. This analysis predicts a maximum of six types of methyl groups of idealized intensity ratios 1:1:2:2:1:1 [31].

More scarce are the solid state (CPMAS) NMR stud-ies of porphyrins and phthalocyanines and this surely will change in time since this technique is both powerful and common. Two main groups of authors have studied the tautomerism of porphyrins and phthalocyanines in the solid state, those of Limbach [32–34] and Frydman [35–37]. The crystal structure of meso-tetrakis(p-meth-ylphenyl)porphyrin was determined to verify the NMR conclusions concerning the 21,23-position of the inner protons [38]. Other papers on the proton exchange on phthalocyanines were published [39, 40].

Sanders et al. [41] used solid state 13C CPMAS NMR to study the properties of monomers (Fig. 4), dimers, trim-ers and various host-guest adducts of metalloporphyrins.

The black meso-tetraferrocenylporphyrin solvate, H2TFcP·DMSO (13), was studied by 1H and 13C NMR in

N

N N

N N

NH N

HN

R R

t-Bu

R

R

t-Bu

R

R

t-Bu

R R

t-Bu10, R = NH-CO-CH=CH2

Double picket fence

Zn

OH2

H2O

NO2

Me

Me

Me

Me

Me

Me

α,α -11

NO2

N

N N

N

HH

N

N N

N

HH

12

Fig. 3. Hexamer of formula C258H258N30O6Zn6

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CDCl3 solution [42]. The 1H NMR spectra (at 60, 100, and 220 MHz) indicate the presence of atropisomers which could not be separated by chromatographic techniques. The four atropisomers were observed in the 1:4:2:1 statistical ratio. picket-fence porphyrins (see the ‘picket-fence’ concept sec-tion) bearing four ferrocenes were reported by Beer [43].

The paper by Abraham et al. [44] is the most impor-tant one in the NMR study of meso-tetraarylporphyrins. Using porphyrin 14 as model, they separated the four atropisomers and fully characterized them by 1H and 13C NMR based on symmetry arguments not only of the meso signals but also of the α,β-pyrrole positions. There is a detailed discussion of ring current shifts and their effect on the φ angle.

Porphyrins 15 and 16 bear (arylsulfonyl)oxy substit-uents: on account of a 1H NMR upfield shift in CDCl3 solution of 2–5 ppm for the aryl protons, a folded confor-mation is assumed in which the substituted aryl groups lie right above and below the porphyrin plane [45]. The same authors extended their work to zinc derivatives and

to the influence of adding pyridine [46]. A series of zinc meso-tetraarylporphyrins (aryl = phenyl, 1-naphthyl, 2-naphthyl, 9-phenanthryl and 1-pyrenyl) were synthe-sized and characterized by 1H NMR spectroscopy [47].

Besides these most common nuclei, several authors have used 19F NMR. Thus, Sternhell et al. reported the 376.5 MHz 19F NMR spectrum of an equilibrium mixture of the four atropisomers of 5,10,15,20-tetrakis(2-fluoro-5-methoxyphenyl)porphyrin in CDCl3 at 298 K [48]. Simonneaux et al. used the signal of the CF3 substituent of chiral ‘picket fence’ porphyrins [o-NH-CO-C*(OMe)(CF3)Ph] to characterize the four atropisomers [49]. The atropisomers of porphyrins 17 were separated and fully characterized by 19F NMR [50]. The use of 31P NMR spec-troscopy was also reported for the study of phosphorus(V) meso-tetra-p-tolylporphyrin complexes [17].

Gerothanassis et al. have discussed the 17O chemical shifts and the 16O/18O isotope effects (on the 13C chemical shifts) of carbon monoxide of the Fe–CO unit of several carbonmonoxy hemoprotein strapped models [51].

N

N N

N

Zn

R1

R1

N

N N

N

Zn

R2

R2

R1R1

Fig. 4. Monomers used to build up supramolecular structures

N

NH N

HN N

NH N

HN

13, H2TFcP 14

Fe

Fe

Fe

Fe

MeO

OMe

OMe

MeO

N HN

NNH

O

O

15 16

OSO2

MeOO2S

MeN HN

NNH

O

O

O2S

SO2

O

O

NMe

Me

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6 A.C. TOmé et al.

Dynamic aspects. Dynamic studies were summarized in Oki’s book published in 1985 [52]. Internal rotation of phenyl rings in 5,10,15,20-tetraarylporphyrins (1) has been extensively studied. If the tetraarylporphyrin ring contains a metal cation in its middle and the cation has two different axial ligands, the phenyl protons and car-bons of the ortho- and meta-positions are diastereotopic due to the fact that the phenyl rings are not coplanar with the porphyrin ring. Therefore, topomerization by rotation about the bond connecting the phenyl ring to the porphy-rin skeleton can be observed.

Eaton and Eaton [53] using 1H NMR reported some values that range between 60 and 80 kJ.mol-1 the metal being Ru, In and Ti, with different ligands and different substituents in para-position. They demonstrated that the identity of the ligand on the central metal ion has little effect on the rotational barrier of the aryl group, but this barrier is strongly dependent on the nature of the metal ion due to steric effects, i.e. distortion of the porphyrin ring by the metal ion. Gallium complexes are known to exhibit much lower barriers to rotation [54] than those observed for complexes involving the previous metal ions. The effect of para substituents on the rotational barrier is similar to that observed in the biphenyl series, i.e. electron-donating substituents lower the magnitude of the barrier [52].

In a free-base 5,10,15,20-tetraarylporphyrin or in a metal complex with two identical axial ligands, it is not possible to observe topomerization by examination of the o,o′-protons of the aryl rings. However, if a substitu-ent is introduced into the aryl ring in ortho- or meta-positions, it is possible to do so. In this respect, the dynamic behavior of 5,10,15,20-tetrakis(o-methoxyphenyl)porphyrin has been studied [55]. This compound can exist in four characteristic conformations of o-substituted phenyl derivatives. The α,α,α,β-isomer (see Scheme 1) can be converted into any other conformer by rotation of a single aryl group, and the

other isomers are converted to it by a rotation of a suitable ring. There are six rate processes, there-fore, that can be observed by the NMR method in the isomerization. The observed lineshapes were analyzed and, assuming identical rate constants for any of the processes, a ∆G‡

432 = 108.4 kJ.mol-1 was calculated. This high barrier to rotation rela-tive to those previously quoted must be attributed to the steric effect of the 2-substituent.

The results presented above suggest that atropi-somers of meso-tetraarylporphyrins should be isol-able at room temperature if the aryl groups contain an ortho substituent. Indeed, meso-tetrakis(o-hy-droxyphenyl)porphyrin [56] and meso-tetrakis-(o-aminophenyl)porphyrin [57] can be separated into stable rotamers. By classical kinetics, the

barrier to rotation in the former is 100.4 kJ.mol-1 at 23 °C, and the latter isomerizes only partially in solution after 24 h at 25 °C. meso-tetrakis(o-tolyl)porphyrin is reported to exhibit a barrier to rotation in excess of 109 kJ.mol-1 [30].

The nitrate ligand in Bi(Tp-CH3PP)NO3 (18) makes the ortho and meta protons diastereotopic in the absence of free rotation about the meso bond [17]. DNMR experiments lead to a barrier ∆G‡ in the range 60–64 kJ.mol-1, similar to those reported by Eaton and Eaton [53].

N

NH N

HN

F

FF

F

F

F F

R1

FF

R2

F

F

F

F

R3

F F

FFa, R1 = R2 = pz, R3 = Fb, R1 = R3 = pz, R2 = F

NN

pz =17

N

N N

N

18

Bi

NO3

Me

Me

Me

Me

N

NHN

HN

19

N

NH N

HN

OMe

20'

N

NHN

HN20''

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The unlike-configured [58] β,meso-linked porphy-rin trimer 19 was studied by Senge and Bringmann in what concerns the rotation about the meso bonds 20′ and 20′′ [59].

Other spectroscopies

Electronic spectroscopy (UV and visible, both absorp-tion and emission) is of fundamental importance in the science of porphyrins and phthalocyanines but only of minor interest in what concerns to atropisomerism. The four possible isomers of meso-tetrakis(o-pivalamido-phe-nyl)porphyrin, the α,α,α,α-one being known as the picket fence porphyrin (see The ‘picket fence’ concept section) were prepared separately [60]. The visible absorption spectra of the isomers are all different, especially in DMF. The wavelength of the absorption maxima is most blue-shifted in the α,α,α,α-isomer. Phyllo-type [61] ten-dency was in the order α,α,α,α > α,α,β,β ≈ α,α,α,β > α,β,α,β. The photophysical properties (absorption and fluorescence spectra) of the atropisomers of 5,10,15,20-tetrakis(2,6-dichloro-3-N-alkyl sulfamoylphenyl)por-phyrins were also reported [62]. In vibrational (IR) spectroscopy, the carbonmonoxy hemoprotein strapped models show ν(CO) frequencies correlated to 13C chemi-cal shifts for a great number of compounds; this dem-onstrated that both heme models and heme proteins are homogeneous from the structural and electronic view point [51].

The absorption and resonance Raman (RR) spec-tra of the bis-N-methylimidazole and bis-1,5-dicyclo-hexylimidazole complexes of the meso-α,α,β,β- and meso-α,β,α,β-atropisomers of Fe(II)-tetrakis(o-pivalami-dophenyl)porphyrins, Fe(II)TpivPP, were recorded. The different spatial arrangements of the o-pivalamide pickets in these two Fe(II)TpivPP compounds control the abso-lute and relative positions of the axial ligand rings with respect to the Fe-N(pyrrole) bonds. These spectral effects were associated with a change in relative position of the axial imidazole rings from nearly parallel in the bis-N-methylimidazole complex to nearly perpendicular in the bis-dicyclohexylimidazole complex. On the basis of stereochemical considerations, the data were interpreted

in terms of change in porphyrin structure from planar to saddled [63].

Classical kinetics

Since in some cases (generally bulky ortho-substitu-ents) the four atropisomers of porphyrins can be isolated and are sufficiently stable to carry out isomerization exper-iments, classical kinetics experiments were reported from 1969 onwards. Thus, Gottwald and Ullman [56] found ∆G‡ = 100 kJ.mole-1 for the rotation of a o-hydroxyphenyl group in H2(To-OHPP), and a ∆G‡ = 106 kJ.mole-1 for the same process in its copper(II) complex.

The four diastereomeric atropisomers of meso-tetrakis(2-cyanophenyl)porphyrin (To-CNPP) were iso-lated and their mutual isomerization processes studied by thin layer chromatography: the free energy of activation is in average ∆G‡ = 110 kJ.mole-1 at 50 °C [15]. Photo-induced (photo-atropisomerization) and thermal isomer-ization of two ‘picket fence’ porphyrins and their metal complexes were studied by Whitten et al. [64]. Using HPLC to monitor the reaction they measured activation parameters in the 113–131 kJ.mol-1 range for the ther-mal process suggesting that it involves a simple one-bond isomerization mechanism. Irradiation with visible or ultraviolet light results in the interconversion of the atropisomers, however, this process involves a different path, probably two trans porphyrin-phenyl bonds rotate simultaneously in a very distorted (warped or ruffled) structure [65]. Hatano et al. have determined the rates of thermal interconversion of atropisomers of 5,10,15,20-tetrakis(o-aminophenyl)porphyrin and 5,10,15,20-tetrakis (o-pivalamidophenyl)porphyrin [H2(TpivPP)] [66]. The following ∆G‡, ∆H‡ (54 to 93 kJ.mol-1) and ∆S‡ (-60 to -177 J.K-1.mol-1) were determined.

Sternhell et al. [48] determined the rates of aryl ring rotation in a series of 5,10,15,20-tetrakis(2-X-5-meth-oxyphenyl)porphyrins, where X = H, F, Cl, Br and I, by NMR and chromatography. The rotational barriers increased monotonically as the van der Waals radii of the substituent X.

Hayashi, Ogoshi et al. described that meso-tetrakis(o-hydroxyphenyl)porphyrins, like 20, that exists in solution

N

N N

N

OH

C9H19

OH

C9H19

C9H19

C9H19

MeO

MeO

O

OMe

OMe

O

NNH

NHN

O

O

C9H19

OO

C9H19

C9H19

C9H19

OMeMeOO

OMeMeO

OH

H

HH

20

HO

OH

21

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8 A.C. TOmé et al.

as a statistical mixture of the four atropisomers, when complexed with ubiquinone analogues such as tetrame-thoxy-p-benzoquinone (see 21) are transformed into the α,α,α,α-atropisomers due to the formation of four hydro-gen bonds, two of them bifurcated [67, 68]. The authors determined the rotation rate of one carbon-carbon bond of porphyrin-phenyl ring (k = 1.7 × 10-5 s-1 at 25 °C in CHCl3 by HPLC analysis) [68]. Compound 21 was used as an intermolecular quinone shuttle [69].

Kuroda, Ogoshi et al. [70, 71] described an unusual phenomenon of atropisomerization of meso-tetrakis(2-carboxyphenyl)porphyrins that they studied by classical kinetics using reverse phase HPLC chromatography and computer analyses. They explain the observed behavior by the formation of a porphyrin dimer linked by four CO2H···CO2H hydrogen bonds.

HPLC

We have already cited HPLC experiments to study por-phyrins atropisomerism [28, 29, 59, 64]; this very pow-erful chromatographic technique has replaced GC, TLC and older methods. Elliot used a continuous partition-reequilibrium apparatus to obtain the α,α,α,α-isomer of meso-tetrakis(o-aminophenyl)porphyrin [72]. Quaternary N-alkyl pyridiniumylporphyrins (related to 7 [18]) and their metal complexes were separated by HPLC [73]. Pre-parative HPLC was used by Bayer et al. to obtain signifi-cant amounts of “tailed picket fence” porphyrins [74].

The zinc complex of 5,10-bis(2-hydroxynaphthyl)octaethylporphyrin was prepared and its chiral trans isomer 22a and the di(benzyl ether) derivative 22b were resolved into enantiomers by means of chiral HPLC using MK-165 and FC-41 columns, respectively [75].

The first optical resolution by chiral HPLC (OA-3100 column) of a meso-meso-linked diporphyrin was described by Yoshida and Osuka [76]. Bringmann et al. reported that some intrinsically chiral directly β,β-linked bisporphyrins (see X-ray crystallography section) can be resolved by chiral HPLC (Chirex 3010 column) [28, 77]. Both separations were monitored by CD.

Theoretical calculations

Although there is a number of interesting contri-butions concerning theoretical calculations of meso-tetra arylporphyrins, it is expected that this field would

increase considerably in the next years. We will report them by the topic covered.

Concerning geometries, Maseras reported that the hybrid quantum mechanics/molecular mechanics method IMOMM was applied to the calculation of the structure of the oxygenated picket-fence porphyrin complex of iron Fe(TpivPP)(1-MeIm)(O2) with results in good agreement with available X-ray data [78]. Using non-local density functional theory, the B3LYP exchange correlation functional, the 6-311G(d,p) basis set and full geometry optimizations the factors control-ling ruffling deformations of porphyrins with small cen-tral ions such as Si(IV), P(V), Ge(IV), and As(V) were explored [5]. A DFT study on the influence of meso-phenyl substitution on the geometric, electronic struc-ture and vibrational spectra of free-base porphyrins was carried out [79]. A theoretical model of inhibition by indazoles of the nitric oxide synthase (NOS) that sup-poses the interaction with a porphyrin (both by the lone pair and by π-π stacking) was proposed [80, 81]. The shuttlecock inversion of phthalocyanines was studied theoretically [11].

In what concerns the relative stabilities of atropiso-mers, Marchon, Daku et al. have calculated some bridled chiroporphyrins like 23 [M = Fe(III) or Mn(III)] [82] and related ones [83].

Some rotational barriers characteristic of meso-arylporphyrins were calculated. Okuno et al. calculated the barriers of compounds 24a and 24b [84]. They found that along the reaction path, the structure changes from a planar porphyrin-ring at the stable state to a considerably distorted one at the transition state, and that the small potential-energy barrier, which is consistent with experi-ments, largely stems from the deformation of the por-phyrin ring. An increase in the potential-energy barrier resulting from the substitution at the ortho-position of the aryl group was also found to originate mainly from an additional deformation of the porphyrin ring at the transi-tion state. Another study concerns the molecular dynamic simulations of iron(II) “basket-handle porphyrins” (BHP, 25, class I of Fig. 8) [85].

Optical properties calculations include circular dichr-oism (CD) [28, 77], electronic spectra [86] and optical properties of H2TPP [87]. Finally, the acidity of porphy-rin and related heterocycles was the subject of B3LYP/ 6-31+G(d,p) calculations [88].

A theoretical study of the conformation of meso-tetraphenylporphyrin (H2TPP), its anions, cations and metal complexes [Mg(II), Ca(II) and Zn(II)] was carried out recently [89]. Two properties were analyzed: the first one considers the conformation of the meso-phenyl rings and the deformation of the porphyrin macrocycle and the second one relates to the barriers of rotation of the meso-phenyl rings (atropisomerism). In the case of the Zn complex (ZnTPP), the

N

N N

NOR

OR

Zn

N

N N

N

OR

O

R

Zn

a, R = Hb, R = benzyl22

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coordination effects with the N3 of 1H-imidazole (Im) were calculated.

Using theoretical calculations, a comprehensive pic-ture of the geometry and atropisomerism of TPP has emerged. The picture is consistent with the experimental data on these compounds: (i) tautomerism results in an increase of the energy and a deformation of TPP; (ii) the charges, both positive and negative, produce profound alterations in the geometry and the energy; (iii) divalent cations always stabilize the TPP structures; (iv) coordina-tion with imidazole transforms the square-planar Zn(II) into square-pyramidal and it is accompanied by a small stabilization.

Theoretical studies on atropisomeric phthalocyanines are much less frequent; AM1 calculations were carried out on poly(pyrrol-1-yl)phthalocyanines as model of the synthesized hexadeca (pyrazol-1-yl)phthalocyanine [90].

PORPHYRINS

General information concerning porphyrins can be found in some books: The Porphyrin Handbook [91], Expanded, Contracted & Isomeric Porphyrins [92], Phthalocyanines: Properties and Applications [93] and Handbook of Porphyrin Science [94].

THE FOUR ATROPISOMERS

Generalities

The first reported example of atropisomerism in por-phyrins involved the separation of the four atropisomers of meso-tetrakis(2-hydroxyphenyl)porphyrin [H2(To-OHPP)] [56]. The authors were able to separate ana-lytically pure H2(To-OHPP) into four components, in a 1:4:2:1 ratio, by TLC on silica. These four compo-nents correspond to atropisomers where the 2-hydroxy groups are oriented “up” or “down” relatively to the porphyrin plane, as indicated in Scheme 1. However, after one hour at 23 °C, a methanol solution of the most abundant atropisomer (α,α,α,β) reaches again the 1:4:2:1 equilibrium ratio, indicating that the energy bar-rier for the interconversion of this isomer into any of the other isomers by rotation of the o-hydroxyphenyl rings about the porphyrin-phenyl bond is quite low. A few years later, atropisomers with higher rotational barrier were obtained by SnCl2 reduction of meso-tetrakis(2-nitrophenyl)porphyrin [95]. Interconversion of the atropisomers of the resulting meso-tetrakis(2-aminophenyl)porphyrin [H2(To-NH2PP)] is sufficiently slow at room temperature to allow their separation by silica gel chromatography affording successively the four atropisomers α,β,α,β (higher Rf), α,α,β,β, α,α,α,β and α,α,α,α (lower Rf) in the ratio 1:2:4:1, respectively [57]. In the following years it was found that the most polar atropisomer (α,α,α,α) is the most convenient for the synthesis of ‘picket fence’ porphyrins (see below) and a few methods were developed to convert the origi-nal atropisomeric mixture into the α,α,α,α-atropisomer [57, 72, 96]. A simple but tedious procedure involves the separation of the most polar atropisomer by silica gel column chromatography, then the mixture containing the other three atropisomers is reequilibrated in boiling tolu-ene and a new separation by chromatography allows to isolate more of the α,α,α,α-atropisomer. Repetition of these steps leads to ultimate conversion of nearly all H2(To-NH2PP) into the desired α,α,α,α-atropisomer [57].

N

N N

NM

O

O

OO

OO

O

O23

NNH

N HN

R

a, R = Hb, R = OMe

N

N N

NFe

NHOHN

O

NH

HN

O

O25

24

Fig. 5. Optimized structure of ZnTPPIm

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10 A.C. TOmé et al.

Alternatively, the original mixture of atropisomers can be converted, in a single step, into the α,α,α,α-atropisomer by isomerization in benzene (20 h at 75–80 °C) in the presence of silica gel [96]. The silica gel-porphyrin slurry is then poured into a chromatography column and eluted to obtain the α,α,α,α-atropisomer in 66% yield. Methods for efficient preparation of the α,β,α,β- [97] and α,α,β,β-atropisomers [98] of H2(To-NH2PP) were reported.

Kinetic studies of the thermal rotational isomerization of H2(To-CNPP) [15] and H2(To-NH2PP) [66] allowed to calculate the activation parameters for the rotation of the o-substituted phenyl ring about the porphyrin-phenyl bond. Czuchajowski et al. [99, 100] and Quici et al. [100] reported the study of the stereoisomerism of meso-tetrakis[2.2]paracyclophanylporphyrins.

Tautomerism

Tautomerism (Fig. 6) [99] is a phenomenon concomi-tant with atropisomerism although the activation barriers and the dependence on external factors are rather differ-ent. Tautomerism involves the proton transfer between the nitrogen atoms and results in the existence of two isomers of class III, even considering only the 21,23-diH and the 22,24-diH tautomers. Remember that most solid state CPMAS NMR studies concern tautomerism [32–40].

The mechanism of inner-hydrogen migration in free-base porphyrins was studied at the MP2 level [102] and using quantum dynamics simulations [103]. There are some examples of the study of the tautomerism of the meso substituents: 1H-pyrazoles [104] and 1H-1,2,3-triazoles [105].

N

N

N

NH

H

α,α,α,α (C4v)

N

N

N

NH

H

α,α,β,β (C2h)

α,α,α,β (Cs)

α,β,α,β (D2d)

N

N

N

NH

H

N

N

N

NH

H

Scheme 1. Equilibria of the four atropisomers of 5,10,15,20-tetrakis(2-substituted-phenyl)porphyrins

I II III IV

α,α,α,α α,α,α,β α,α,β,β α,β,β,α α,β,α,β

Fig. 6. Tautomerism and atropisomerism

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THE ‘PICKET FENCE’ CONCEPT

In 1975, Collman et al. [57] introduced the ‘picket fence’ concept (Fig. 7).

The conformation of the α,α,α,α-atropisomer 26 can be frozen by formation of the tetra-pivalamide 27 (Scheme 2). In fact, the barrier to phenyl ring rotation in tetra-pivalamide 27 is so high that only after 16 h of heating under reflux in THF could any interconversion be detected. Complete equilibration to the statistical mix-ture of tetra-pivalamide requires several hours in boiling xylene [95]. Iron(II) complex of the tetra-pivalamide 27, also known as “picket fence porphyrin”, has been exten-sively studied as a synthetic model for the active site of oxygen-binding hemoproteins (myoglobin and oxymyo-globin) [57, 106, 107].

Reaction of α,α,α,α-26 with p-toluenesulfonyl chlo-ride gave a 75% yield of the corresponding α,α,α,α-tetrasulfonamide while reaction with isophthaloyl dichloride afforded porphyrin 28 (with isophthalamide groups between adjacent amino substituents) in 57% yield [57]. The search of nitric oxide reductase active site models bearing imidazole groups in the arms led Coll-man et al. to prepare a α,α,α,α-trisimidazole and glutaric acid derivative [108] and an α,α,α,β-derivative possess-ing a distal and three proximal imidazoles [109].

Kinetic studies of the thermal rotational isomeriza-tion of H2(To-CNPP) [15], H2(To-NH2PP) [66] and

H2(TpivPP) [66] allowed to calculate the activation parameters for the rotation of the o-substituted phenyl ring about the porphyrin-phenyl bond. Another kinetic study suggests that the thermal isomerization and the photoatropisomerization of the free-base meso-tetrakis(2-hexanamidophenyl)porphyrin and its zinc and palladium complexes occur by different paths [64].

Many other papers reported ‘picket fence’ porphyrins [110–132] as useful scaffolds for many applications. For instance, the X-ray structure of a series of Fe(III) complexes of the potentially binucleating ligand α,α,-α,α-meso-tetrakis(o-nicotinamidophenyl)porphyrin was described; the structure consists of polymeric chains, with the Fe atom of one molecule coordinated to a pyridine N of the nicotinamide unit of a second molecule

N

N

N

N

NH HN

HN

HNH H

O

O OO

α,α,α,α-27

N

N

N

N

NH2 H2N

H2N

NH2

H H

Cl

O

α,α,α,α-26

Scheme 2. Synthesis of picket fence porphyrin [95]

O

O

Fe

N

N

R

oxygen binding pocket

picket fence

porphyrin ring

base prevents oxygencoordination on unhinderedside

bulky R group disfavorscoordination of base onpicket fence side

Fig. 7. Picket fence concept

N

N

N

N

NH HN

HN

HNH H

O

O

O

O

28

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and the chloride ion occupying the sixth coordination site, inside the “pocket” of the four nicotinamide groups [133]. The reactivity of picket fence porphyrins, i.e. meso-tetrakis[o-(alkylcarbonyl)aminophenyl]porphyrins, toward metalation with Cu(II) or Zn(II) shows an extremely wide rate range in aqueous anionic surfactant solutions [134].

The unusual conformational stability of a sterically crowded α-α,α,α,α-atropisomer of methyl[5,10,15, 20-tetrakis(o-phenylphenyl)porphyrinato]aluminum is related to the possibility of intramolecular CH-π bond-ing interactions between the methyl group bonded to the central metal atom and the phenyl rings located at the ortho-position of the meso-phenyl substituents [135]. Zn(II), Ni(II), Cu(II), and Fe(III) complexes of bimeta-lating tris(pyridine- and imidazole-appended) picket fence naphthylporphyrins with benzyl ether spacers have implications for cytochrome c oxidase active-site model-ing [136]. The influence of pendant arms bearing ligating groups on the structure of bismuth porphyrins and their

implications for labeling immunoglobulins used in medi-cal applications was studied [137]. A study of the tetra-urea picket porphyrin-chloride anion complex has shown the anion to be situated between two adjacent ureas and hydrogen bonded via four NH protons [138]. This por-phyrin receptor also binds a DMSO molecule and utilizes it as a participant in its anion recognition unit, in a man-ner similar to enzymes that bind water for use as part of their substrate recognition unit [139]. Picket fence meta-loporphyrins were used as anion sensors and the effect of the metal center on the anion binding properties of amide-functionalized and tetraphenyl metalloporphyrins determined [140].

CONFORMATIONALLY RESTRICTED MESO-TETRAARYLPORPHYRINS

Many possibilities of restricting the conformation of meso-tetraarylporphyrins by creating bridges between

A B C D

E F G H I

J M NK

ROP

Q S

T V X Y

up down free to rotate

B'

U

L

up & down

PP

Fig. 8. Some possible isomers of meso-linked porphyrins (the porphyrin ring in blue). In red, reported situations

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the aryl rings exist. Some of these possibilities were materialized and in some cases received amusing names: “pocket”, “basket handle”, “picnic basket”, etc. We have represented in Fig. 8 the most common possibilities.

The curved links can be a covalent chain or a hydrogen bond and the small black point in the middle corresponds to a molecule covalently bonded to the aryl groups. We have summarized in Table 1 the different situations.

Starting from the α,α,α,α- and α,α,α,β-atropisomers of H2(To-NH2PP), Collman et al. [141] prepared a number of capped and strapped tris-imidazole porphy-rins, being one of them the closest structural analog yet reported of the metal free cytochrome c oxidase (CcO) active site.

OTHER CONFORMATIONALLY REST-RICTED PORPHYRINS

The phenomenon of restricted rotation (atropisomer-ism) in porphyrins is usually discussed for meso-di- and tetraarylporphyrins with substituents in ortho-positions

of the aryl ring. However, restricted rotation can also be observed in meso-monoaryl porphyrins, such as 29 [142]. In this compound, the presence of the bulky spiro-bifluorenyl group, which hinders the rotation around the Cmeso–Caryl bond, leads to the formation of two different topological faces after ruthenium complexation. Porphy-rins bearing bulky chiral aryl groups are another interest-ing class of compounds. Curiously, in the D2-symmetric porphyrin 30, due to the presence of the two C2-sym-metric chiral groups, no atropisomers are possible [143]. The manganese complex of porphyrin 30 is a catalyst for the enantioselective epoxidation of cis-β-methylstyrene. Other chiral porphyrins bearing 1,1′-binaphth-2-yl or 1,1′-binaphthyl-2-carboxamidophenyl groups at meso-positions were also synthesized and their iron(III) and manganese(III) complexes used as catalysts in asymmet-ric epoxidations [144, 145].

The heptabromo-2-(2-methoxyphenyl)-5,10,15,20-tetra - phenylporphyrinato]zinc(II) 31 is a chiral nonplanar porphyrin. This compound was synthesized and two fractions could be separated by HPLC using a Chiral-cel OD column. These fractions had identical optical

Table 1. References corresponding to the known situations depicted in Fig. 8

Structure Common name References

A “adjacent-linked” [200–203]

B “cross-linked” [51, 200–213]

B “strapped”a [214]

B′ [215–217]

C [57]

F “tren-capped” (α,α,α) [218–221]

G “adjacent trans-linked” [24, 200, 222, 223]

H “adjacent trans-linked” [24, 200, 201]

I “basket handle” or “cross trans-linked” (αβαβ) [23, 24, 85, 200, 201, 224–233]

J “picnic basket” (α,α,α,α) [234, 235]

K [236, 237]

L [238, 239]

R “capped” [240–247]

U related to cytochrome P450 [248–250]

a The bridge (i.e. a 3,5-substituted pyridine) coordinates to the metal (i.e. Zn) of the porphyrin.

N

NH N

HN

29

N

NH N

HN

30

t-Bu t-Bu

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spectra but their CD spectra were mirror images of each other [146]. Porphyrin 31 was calculated to have a very high barrier for rotation of the 2-methoxyphenyl group (> 146 kJ.mol-1), a low barrier for inversion of the porphy-rin macrocycle (ca. 40 kJ.mol-1), and a strong preference (∆E = 8 kJ.mol-1) for structures A and B vs. structures C and D. Unfortunately, the structures of the two porphy-rins separated by HPLC could not be confirmed by X-ray crystallography.

Quadruply azulene porphyrins 32 showing atropisom-erism were prepared together with their conformationally restricted analogs 33 [147]. The selective synthesis of a number of single atropisomers of meso-naphthylporphy-rins, namely compounds 34 and 35, was reported [148].

In 1981 Dolphin, Hiom and Paine published a review on covalently linked dimeric porphyrins [149] where

many beautiful examples involving meso-tetraaryl deriv-atives are discussed: position 1,4-linked (36), positions 1,3-linked (37), positions 1,2-linked (38) and positions 1,2,3,4-linked (39). Booker and Bruice [150] reported quadruply two- and three-atom, aza-bridged, cofacial bis(5,10,15,20-tetraphenylporphyrins) related to 39.

Other interesting compounds with high conformational rigidity are the “gyroscope-like” porphyrins. These com-pounds are obtained from the reaction of 5,15-bis(2,6-diaminophenyl)porphyrin derivatives with dicarboxylic acid chlorides [151–153, 227].

Doubly hydrogen-bonded structures, typical of ben-zoic acids, results in a self-assembled dimer correspond-ing to the Zn(II) complex of the α,α,α,α-atropisomer of meso-tetrakis(2-carboxy-4-nonylphenyl)porphyrin (40) [154].

N

N

N

NZn

MeO

N

N

N

NZn

N

N

N

NZn

OMe

N

N

N

NZn

OMe MeOPh

Ph

Ph

Ph Ph

PhPh

Ph

Br

Br

Br

Br BrBr

Br

Br

Br

Br

Br

Br

Br Br

Ph

Ph

Ph

Ph

Ph

Ph

Ph

Ph

Br

Br

BrBr

BrBr Br

Br

Br

Br

Br Br

Br

Br

Mirror plane

31A B

C D

N

N N

N

Ni

RO2C

CO2R

RO2C

RO2C

N

N N

N

Ni

CO2R

CO2R

RO2C

RO2C

32 33

R = Me, (CH2)7CH3, R =

NNH N

HNPh

Ph

O

O

NNH N

HN

C6F5

C6F5O O

34 35

00299.indd 14 3/2/2011 6:59:25 Pm

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Mono(imidazolyl)-substituted Co(II) porphyrin 41, with a ‘picket fence’ structure, forms the dimeric struc-ture 42 which behaves as an artificial hemoglobin model, binding two dioxygen molecules reversibly. However,

the oxygen affinity of dimer 42 is significantly lower than that of monomer 41 [155].

Echegoyen et al. described rotaxanes like 43 where the conformation of one of the meso aryl rings is restricted

O

O O

O

O

O

O

O

HN

HNO

HN

HNO

HN

HNO

HN

HN

O

NH

HN O

HN

HN

O

36 37

38 39

Zn

Zn= CO2H

N

N

N

N

CO2H CO2H

HO2C

CO2HH H

α,α,α,α-40

RR

R

R

R = (CH2)8CH3

NN

NN

NH

HN

HN

O

OO

N

HN

Co

NN

NN

NH

HN

HN

t-Bu

t-Bu

O

OO

t-Bu

N

HN

Co

NN

NN

HNNH

NH

t-Bu

t-Bu

O

OO

t-Bu

N

NH

Co

41

42

00299.indd 15 3/2/2011 6:59:26 Pm

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N

N N

N

Ar

Ar

Ar

OZn

N

N N

N

Ar

Ar

Ar

O

OZn

N

N

N

NCu

O

O

PF6–

43

O

O

O

O

O

O O

O

O

O

O

NN N

N

Ar

Ar

NN

NNAr

Ar NN

NN Ar

ArZnZn

NN N

N

Ar

Ar

NN

NNAr

Ar NN

NN Ar

ArZnZn

Zn

N

NN

44

Zn

N N

NN

N N

NN

Ar

Ar

N N

NN

Ni

Ni

NN

NN

Ni

N N

NN

Ar

ArN

NN Ni

ArArAr

Ar

46

N

NHN

NNH

Ar

Ar

N

HNN

NH

Ar

Ar

N

HN

N

NH

Ar

Ar

NH

HN

NH

45

00299.indd 16 3/2/2011 6:59:26 Pm

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[156]. Similar rotaxanes were reviewed by Durot, Revir-iego and Sauvage [157].

Lindsay et al. reported the synthesis of covalent arrays of six porphyrins in a wheel-like architecture [158]. Those shape persistent macrocycles were used as hosts to bind a tripyridyl guest molecule, generating complexes such as 44 [159]. Pyrrole-bridged porphyrin nanorings of type 45 [160] and the porphyrin nanobarrel 46 [161] were recently described. For excellent recent papers and reviews on multiporphyrin systems see [162–166].

Moreover less frequently discussed, atropisomers are also observed in meso-tetraheteroarylporphyrins. Especially relevant examples are the meso-N-alkylpyri-dinium-2-yl porphyrins (such as compound 7) and the meso-pyrazolylporphyrins (such as compounds 8 and 9). The alkylation of meso-tetrakis(2-pyridyl)porphyrin affords a mixture of four possible rotational isomers as a result of hindered rotation of the N-alkylpyridinium-2-yl groups with respect to the porphyrin plane. The statisti-cal distribution of the α,α,α,α-, α,α,α,β-, α,α,β,β- and α,β,α,β-atropisomers is 12.5, 50, 25, and 12.5%, respec-tively [167]. Groves et al. report that the alkylation of meso-tetrakis(2-pyridyl)porphyrin with bromoac-etamides bearing bulky groups leads to the formation of α,α,β,β-atropisomer as the major product (up to 79% yield) [167].

An iron(III) complex of a tetracationic porphyrin of type 7, with N-tetradecyl groups, was synthesized and the corresponding atropisomer were separated in two fractions: the less polar (the α,α,β,β- and α,β,α,β-atropisomers) and the most polar one (the α,α,α,α- and α,α,α,β-atropisomers). The two fractions were used as catalysts for cyclohexane hydroxylation and it was found that the most polar fraction is a better catalyst than the less polar one [168].

Takeoka et al. have shown that the 5,15-substituted methyluracyl porphyrin 47 exists at room temperature as the α,α- and α,β-atropisomers [169]. The atropisomer-ization is regulated by the steric repulsion between the methyl group at the 6-position of the uracyl units and the

methyl substituents at the β pyrrolic positions. Each atropisomer was mixed with an alkylated melamine as a com-plementary hydrogen-bonding unit and the resulting hydrogen-bonded assemblies were shown by diffusion-ordered spectroscopy (DOSY) in solu-tion to be completely different: while the α,α-isomer formed a face-to-face dimer, the α,β-isomer acquired a zig-zag structure.

Recently it was reported that por-phyrins bearing uracyl motifs at the four meso-positions, such as 48, self-organize via homo-complementary hydrogen bonds and π-stacking into

nanofibers, nanorods and thin films on mica and glass surfaces depending on deposition conditions [170]. The atropisomers do not significantly influence the H-bond energetics and kinetics of self-assembly in solution. In the supramolecular structures the uracyl moieties remain orthogonal to the porphyrin.

THE INFLUENCE OF THE METAL ON THE CONFORMATION

In the metalloporphyrins, coordination of the arms of the meso-aryl groups to the central metal modifies their conformation. Structure 49 illustrates the fixed α,α,α,α-conformation of Fe porphyrin [171].

To this section belong the large field of multiple-decker porphyrins and phthalocyanines [91]. We will only cite some examples. The synthesis and spectroscopic charac-terization of neutral, oxidized, and reduced homo- and heteroleptic complexes of double-decker actinide porphy-rins and phthalocyanines was described [172]. Related to it, mixed-metal triple-decker sandwich complexes with the porphyrin/phthalocyanine/porphyrin ligand system were prepared [173]. Rotational libration of a double-decker porphyrin was visualized using scanning tunnel-ing microscopy (STM) [174]. A porphyrinic molecular box was self-assembled using double decker strategy [175]. Finally, a very interesting paper by Shinkai et al.

N

N N

N

M

48

NHN

HN

N

NH

N

HNN

O

O

O O

O

O

O O

M = 2H, Zn

N

NH N

HN

47

NHN

HN N

O

O

O

O

O

O

HexylHexyl

Hexyl Hexyl

49

NN N

N

NH

NH

HN

HNFe

CuN

N

N

N

CO

COOC

CO

N

C

2+

00299.indd 17 3/2/2011 6:59:27 Pm

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describes the behavior of a bevel-gear-shaped rotor bear-ing a double-decker porphyrin complex [176].

OTHER RELATED COMPOUNDS

Corroles

The atropisomerism in corroles is much less studied than in the corresponding porphyrins. However there are several spectroscopic characteristics that are similar in both types of compounds.

An estimate of the α,β,α:α,α,β:α,α,α ratio of the 5,10,15-tris(o-aminophenyl)corrole [H3(To-NH2PC), 50] atropisomers was ca. 0.25:0.60:0.15, but it changes with the temperature and the polarity of the solvent [177]. Unlike the corresponding porphyrin H2(To-NH2PP), chromatography separation was not entirely satisfactory for the isolation of pure atropisomers of 50, because they equilibrate in solution. For example, the original ratio is restored in solution (at low concentration) within 15 min at ca. 40 °C, within 1 h at room temperature, sev-eral hours at 0 °C, or several days at -20 °C. These data suggest a low rotational barrier of the o-aminophenyl groups adjacent to the bipyrrole moiety. Calculations with CAChe (PM3 procedure) for corrole 50 gives as the barrier to rotation of the 5- and 15-o-aminophenyl groups (adjacent to the bipyrrole unit) 46–59 kJ.mol-1, whereas for the 10-o-aminophenyl group (opposite to the bipyr-role) it gives 100 kJ.mol-1, which is similar to the bar-rier to rotation of o-aminophenyl groups in the porphyrin analog (93 kJ.mol-1). Spontaneous rotation at room tem-perature is thus attributed to pickets 5 and 15, whereas

the 10-o-aminophenyl group is more porphyrin-like and would rotate more slowly [177].

The calculation of the heat of formation of the atro-pisomers of corrole 50 shows that the most stable atro-pisomer is α,α,β (1038 kJ.mol-1), followed by α,β,α and α,α,α (1035 kJ.mol-1), which agrees with their TLC profile.

To prepare the picket fence corrole 51, the triazacy-clononane-capped corrole 52, or the tris(imidazole) picket corrole 53, it was necessary a preliminary enrichment of H3(To-NH2PC) in the α,α,α-atropisomer. This was carried out by adsorbing a statistical mixture of H3(To-NH2PC) atropisomers (α,β,α:α,α,β:α,α,α) on silica and heating in toluene/hexane under N2, as previously devel-oped for porphyrins [96]. This resulted in nearly quanti-tative conversion into the α,α,α-atropisomer with little decomposition after 10 min. To overcome the low rota-tional barrier of aminopickets, α,α,α-50 was handled at -20 °C and reacted immediately at this temperature with the appropriate reagents. Compounds 51 and 52 are suf-ficiently stable at room temperature to allow chromato-graphic separation without rotation [177].

Collman and Decréau also described the synthesis of α,α,β-corroles at room temperature using a statistical mixture of atropisomers of 50, because the most abundant atropisomer is α,α,β [177]. These authors synthesized, for the first time, corroles having a cis-A2B geometry (54 as example); the separation of the enantiomers was achieved by HPLC using a Chiralcel OD column. PM3 calculation shows that the distal phenyl strap and the cor-role are nearly coplanar. This might explain why the 1H NMR signals of the strap are shifted upfield (ca. 7 ppm

N

NH HN

HNNH2 H2N

H2N

50, H3(To-NH2PC)

N

NH HN

HNNH HN

HN

t-BuO

t-Bu

O

t-BuO

51, α,α,α -TpivPCpicket fence corrole

N

NH HN

HNNH HN

HN

O

O

α,α,α -52

ONN

N

N

NH HN

HNNH HN

HN

O

O

α,α,α -53

O

N N MeN NMe

NN

Me

00299.indd 18 3/2/2011 6:59:27 Pm

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for the aromatic proton meta to the CH2). This is one of the first reports of the strong ring current induced by a corrole.

Bröring et al. prepared sterically demanding bis- picket-fence A2B corroles bearing four pivalamido groups in the ortho-positions of the meso-aryl substituents and in some cases the disubstituted corroles were obtained as a single α,α-atropisomer [178, 179]. They have proved that, despite the steric hindrance imposed by two bulky pivalamido substituents on each side of the corrole plane of 55, the insertion of iron ion into the N4 blocked cavity of the new corrole ligand proves successful when directed towards the nitrosyl derivative [178]. The X-ray crystal-lographic investigation of such an iron complex allows a detailed view of the steric restraints imposed by the bulky amido substituents. The tert-butyl groups on either side of the macrocyclic plane are situated about 2.5 Å apart from each other. The attempts of Bröring et al. to prepare an A3 corrole, with six pivalamido pickets in the ortho-positions of the meso-aryl substituents, failed [179].

Recently, Dehaen et al. synthesized a variety of meso- pyrimidinyl-substituted A2B- and A3-corroles (A = 4,6-dichloropyrimidin-5-yl) by careful optimization of the macrocyclization conditions [180]. A2B-corroles appear as a mixture of atropisomers of different ratios depending on the reaction conditions. They also demon-strate that the anti-isomer of picket-fence A2B corroles bearing bulky groups in the ortho-positions of the meso-aryl substituents is the more abundant one due to steric reasons.

The broadness of the resonances of the ortho-pyridyl groups of ortho-pyridyl-substituted corroles was justified by Gross et al. due the presence of various atropisomers

(and equilibrium between them) regarding to the posi-tioning of the nitrogen atoms above and below the mac-rocyclic ring [181]. Therefore, the β-pyrrole protons appear as the characteristic four doublets with cou-pling constants of 4.1–4.9 Hz. However, in the case of 5,10,15-tris(N-methylpyridinium-2-yl)corrole and 10- (pentafluorophenyl)-5,15-bis(N-methylpyridinium-2-yl)corrole all the possible atropisomers were formed in the statistically predicted ratio: α,α,α, α,α,β, and α,β,α for the former, and α,α and α,β for the latter where α and β represent opposite positioning of the N-methyl groups relative to the macrocycle plane. This information was deduced from the number and relative integration of the resonances attributed to the methyl groups, such as the 1:1 ratio of singlets at 4.13 and 4.15 ppm for bis-pyridinium-substituted corrole. For tris-pyridinium-substituted cor-role, each isomer has two identical (on C5 and C15) and one different (on C10) methyl groups; and three such 2:1 pairs were evident in a 1:2:1 ratio for the α,α,α-, α,α,β-, and α,β,α-atropisomers, respectively. The separation of atropisomers was successfully carried out by HPLC, but despite several methods being applied for evapora-tion of the isolated fractions, those of tris-pyridinium-substituted isomerized back to the original distribution of atropisomers.

In a study on the electrochemical behavior of (5,10,15-tris-X-phenyl-2,3,7,8,12,13,17,18-octamethyl corrolato)cobalt(III) triphenylphosphine complexes 56 Kadish et al. showed that the ortho-Cl derivative exists as dif-ferent atropisomers in solution, and a thermal intercon-version between them was achieved at 338 K in toluene [182]. Activation parameters (∆Sǂ, ∆Hǂ) for intercon-version between the atropisomers were obtained from 1H NMR measurements and were similar in magnitude to values reported for ortho-substituted meso-tetraphe-nylporphyrin derivatives. In the 1H NMR spectra of the corrolato complexes 56 an increase in the line widths going from the para-C1 derivative to the meta-C1 one is observed. Therefore the resonances of the ortho-Cl complex are split into three major components, due to the presence of stable atropisomers at room temperature as a result of the chlorine atoms located on either side of the macrocyclic ring. In fact, the presence of the bound PPh3 axial ligand makes the two faces of the complex diastereotopic.

N

NH HN

HNNH HN

HN

O

O

α,α,β -54

O

N

N

N

NH HN

HNNH HN

HN

O

O

α,α,α-54

O

N

N

N

N N

NNH HN

O O

Fe

NO

NHO

MeOOMe

OMe

HNO

55

00299.indd 19 3/2/2011 6:59:28 Pm

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Koszarna and Gryko [183] have shown by spectro-scopic methods (absorption and emission spectra) that the components of the meso(10)-meso(10′) linked corroles 57 and 58 are weakly electronically linked. Fluorescence data demonstrate that these corroles induce a different response from that of meso-meso linked porphyrins. In the meso-meso linked porphyrin the fluorescence quan-tum yield slightly increases vs. that of the monomer, while in the case of the meso(10)-meso(10′) linked corroles 57 and 58 the fluorescence quantum yield is considerable lower than the corrole monomer. Therefore they found a puzzling difference between the 1H NMR spectrum of each one of these dimers. In the 1H NMR spectrum of 58 there are a significant line broadening, especially in the β-pyrrole protons and NH-protons regardless the solvent used. In a variable-temperature study of the 1H NMR spectrum of corrole 58 in DMSO-d6 it was shown that at 303 K the signals are sharper than in other solvents like CDCl3, CD2Cl2 or THF-d8, but at 363 K the spectrum shows significantly broadened aromatic signals in com-parison to the spectrum recorded at 303 K. For NH, the spectrum at lower temperature has two distinct signals (at -1.95 and 0.6 ppm) which coalesce at 363 K to give a very broad signal (0.0 ppm). Generally this type of spec-trum suggests the presence of hindered rotation and/or NH tautomeric equilibrium of two conformers with slow exchange relative to the NMR time scale at 303 K. How-ever, the referred broadening effect was not observed in the standard 1H NMR spectrum of the meso-meso dimer 57. Since one can assume that the free rotation of both corrole moieties is restricted to a similar extent in dim-ers 57 and 58. The authors attributed this phenomenon to some complex dynamic processes that are taking place and that can only be clarified to a certain extent by X-ray

crystallography (however, it was not possible to obtain suitable crystals for an X-ray crystal structure analysis).

Ziegler et al. revisited the single X-ray crystal struc-tures of the meso-substituted free-base 5,10,15-triphenyl-corrole and 5,10,15-tris(pentafluorophenyl)corrole [184]. In both cases the quality of the data of the original studies limited the internal proton assignment [185, 186]. In the high resolution structures of these compounds the inter-nal NH protons are clearly assignable to different nitro-gens, which represent the two tautomer limits of corrole (Fig. 9) [184]. Crystals of 5,10,15-triphenylcorrole were growth by slow evaporation of dichloromethane and in this structure the NH protons are located as depicted in 59. In the case of 5,10,15-tris(pentafluorophenyl)corrole the crystals resulted from ethyl acetate solution and the structure was assigned as 60. The mean plane of the corrole ring (based on the 19 external atoms of the skeleton) is deformed from planarity due to the steric hindrance of the internal protons, the two aryl rings of the 5- and 10-positions are tilted by 39–66° relatively to the mean plane while the third one is tilted by 68–73°. The unambiguous structural elucidation of the two tau-tomers of free-base corrole led to the authors to begin to evaluate the contribution of tautomerization to electronic structure [184].

Phthalocyanines

Atropisomerism in phthalocyanines has been much less studied than in corroles and porphyrins. However, there are some interesting studies on the conformational aspects of deformed phthalocyanines, mainly those bear-ing four (61) [187–189] or eight (62) [10, 190, 191] aryl substituents at the α-positions. Phthalocyanines bear-ing eight bulky alky, alkyloxy or aryloxy substituents at

the α-positions also show conformation-ally stressed structures [187, 192–196]. An extensive review of the single-crystal structures of phthalocyanines and their metal complexes is available [197].

The X-ray data of the nickel complex of C4h phthalocyanine 61a shows that the nickel(II) ion is placed in the plane formed by the four nitrogen atoms. The dihe-dral angles between the o-methylphenyl groups and the Pc plane are 72°, 63°, 64° and 84° with a relative α,α,β,β-orientation

N

N N

N

Me

Me

Me

Me

Me

Me Me

MeR

R

R

Co

PPh3

R = H, p-OMe, p-Me, p-Cl,m-Cl, o-Cl, o-F, m-F

56

N

NH HN

HN

N

NH HN

HN

Me

Me

MeMe

Me

Me

Me

MeMe

Me

MeMe

N

NH HN

HN

N

NH HN

HN

Cl

Cl

Cl

Cl

Cl

ClCl

Cl

57 58

N

NH HN

HN

60

A B

CDNH

NH N

HN

A B

CD

59

C6F5

C6F5C6F5Ph Ph

Ph

Fig. 9. The two tautomers of corroles

00299.indd 20 3/2/2011 6:59:28 Pm

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for the o-methyl groups [188]. The 1H NMR spectrum of compound 61b shows only two signals correspond-ing to the resonances of the aromatic protons of the di(t-butyl)phenyl ring (a triplet of one proton and a doublet of two protons) and a singlet corresponding to the t-butyl protons [189]. This clearly indicates that 61b is a single positional isomer and a single atropisomer. Similar con-clusions were deduced from the 1H NMR spectrum of Pc 61c [189].

1H NMR and steady-state fluorescence data show that the isomeric phthalocyanine-C60 dyads 63 and 64, resulted from ABAB- and AABB-type phthalocyanines, respectively, have very different structures: while in dyad 63 phthalocyanine and fullerene are in a face-to-face ori-entation, in dyad 64 they are in a face-to-tail orientation [189]. It was confirmed that the orientation of the two moieties strongly affects the electron transfer from the phthalocyanine to the fullerene moiety [198].

The presence of eight phenyl groups in Pc 62 leads to a highly deformed saddle-shaped skeletal structure, where the maximum deviation from the planarity reaches more than 1 Å at the β-carbon of the pyrroles. This reduces the

symmetry of the Pc skeleton from the D4h of a normal metalloPc to D2d [190, 191].

ZnPor-ZnPc dyads of type 65, in which the phthalo-cyanine moiety is directly bonded to a β-pyrrolic position of a meso-tetraphenylporphyrin, have hindered rotation through the direct linking bond. PM3 calculations sug-gest a pseudo-orthogonal arrangement of the Pc unit with respect to the main porphyrin plane [199]. The 1H NMR spectrum of dyad 65 and the correlations found in the COSY and HSQC spectra allowed to conclude that there is no rotation of the meso-phenyl rings, since the ortho-protons of each phenyl ring are not equivalent. The proton resonances of the 20-phenyl ring of the porphyrin moiety appear at d = 5.61 (meta), 6.00 (ortho), 6.07–6.13 (meta, para), and 8.69 (ortho) ppm. The low frequency values of these resonances are due to the shielding effect of the phthalocyanine unit.

Typically, metal phthalocyanines are planar with D4h symmetry. However, if the metal ion is too large to fit between the four complexing nitrogen atoms it adopts a position out of the plane of the Pc macrocycle and signif-icant doming occurs, giving these molecules shuttlecock

N HNN

NN

NHN

N

O OO

OO

O

OO

N

HN

N

N N

NH

N

N

O

O

O

O

O O

O

O

tBu

tButBu

tBu

63 64

tBu

tBu

tBu

tBu

N HN

N

N

N

NH

N

N

R

RR

R

N HN

N

N

N

NH

N

N

Ph

Ph

Ph

Ph

Ph

Ph

Ph

Ph

6162a, R = 2-CH3

b, R = 3,5-di-t-Buc, R = 3-CH2CH2CH2OH

00299.indd 21 3/2/2011 6:59:29 Pm

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shape. When these shuttlecock shaped metallophthalo-cyanines are adsorbed on a substrate surface the metal atom can be either down or up (Fig. 10). The mechanism of the internal conformational inversion of germanium, tin and lead phthalocyanines in terms of the geometry, energy barrier of inversion and redox properties of the central metal atom was investigated using DFT calcula-tions [11]. It was found that the mechanism of inversion is the same for GePc and SnPc but PbPc has a different one. The energy barrier of inversion calculated for GePc, SnPc and PbPc is, respectively, 2.12, 3.16 and 4.27 eV.

CONCLUSION

As discussed throughout this review, the atropisomerism in tetrapyrrolic compounds has been extensively studied since the pioneer work of Gottwald and Ullman in 1969 [56]. Most part of those studies is related with static and dynamic aspects of conformationally restricted porphyrins, including their characterization by NMR and X-ray crystal-lography. Typically, the NMR characterization of atropiso-mers was carried out by 1H and 13C NMR spectra but other nuclei, such as 19F and 31P, were also used. The solid-state NMR studies of porphyrins and phthalocyanines are scarce but this surely will change in the near future since this pow-erful technique is becoming common in the structural char-acterization of organic compounds.

The chromatographic separation of the four possible atropisomers of meso-tetrakis(o-substituted-phenyl)por-phyrins and the kinetics of their interconversion is also a

recurrent subject. It is shown that, in general, even when it is not possible to separate the atropisomers, the slow rate of isomerization allows their detection and quantification. In other cases, after being separated, each pure atropiso-mer reaches again the starting equilibrium composition, even at room temperature. A number of experimental pro-cedures were reported for the large scale synthesis of a single atropisomer. In this context, the atropisomers of the meso-tetrakis(o-aminophenyl)porphyrin, and in particular

the α,α,α,α-isomer, which is the most convenient for the synthesis of picket-fence porphyrins, are the most studied.

The atropisomers of meso-bis(o-sub-stituted-phenyl)porphyrins, meso-di-and tetraheteroarylporphyrins, meso-tris(o-substituted-phenyl)corroles and of phtha-locyanines bearing aryl substituents at the α-positions are also reviewed.

The influence of the metal on the con-formation of metalloporphyrins was also considered. Frequently the metal leads to highly distorted macrocycles or to self-assembled multiporphyrinic systems. In addition, the presence of axial ligands in metalloporphyrins with restricted rotation

may turn diastereotopic the ortho and meta protons of the meso-aryl rings.

A number of interesting contributions concerning the-oretical calculations of the geometries, stabilities, energy barriers to rotation, structure changes and conformations of the atropisomers of the tetrapyrrolic macrocycles are discussed. The results of those theoretical studies are usually in good agreement with NMR and X-ray data. Due to the diversity and relevance of the data obtained from the theoretical calculations, it is expected that this type of studies applied to atropisomerism will increase considerably in the next years.

Acknowledgements

Thanks are due to Fundação para a Ciência e a Tec-nologia (Portugal) and FEDER for funding the Organic Chemistry Research Unit (nº 62) and the project PTDC/QUI/74150/2006. Financial support from the FCT-CSIC bilateral convention (26/CSIC/08) is also acknowledged. This work was carried out with financial support from the Ministerio de Ciencia e Innovación (Project no. CTQ2009-13129-C02-02) and Comunidad Autónoma de Madrid (Project MADRISOLAR2, reference S-2009/PPQ/1533).

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N

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N

N

N

N

N

N

N

N

M

M = Ge, Sn, Pb

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DOI: 10.1142/S1088424610002872



INTRODUCTION

It is well-known that the axial ligands coordinated to the iron center play an essential role in modulating the properties and reactivity of heme proteins [1–3]. For example, oxygen binding and transporting proteins such as myoglobin (Mb) or hemoglobin (Hb) and the dioxygen activating enzyme cytochrome P450 have histidine (His) and cysteine (Cys) as their proximal ligands, respectively. Methionine (Met), in combination with His, is the axial ligand in numerous electron transport c-type cytochromes [4]. Two heme proteins, the heme storage protein bacte-rioferritin [5] and the heme transporting protein known as streptococcal heme-associated protein (Shp) [6, 7], have bis-Met-ligated heme iron active sites. Although

Met ligation in heme proteins is much less common than His or Cys ligation, thioether ligation has been addressed by heme iron coordination chemists in the preparation of a variety of model complexes [8–11]. Nonetheless, there are limitations to the types of complexes that can be prepared by that approach. Additional examples of thioether-ligated heme complexes of biological relevance would be useful, for example, to address the coordina-tion found in ferric H102M cytochrome b562, where the His102 axial ligand was replaced by Met. Using magnetic circular dichroism (MCD) and electron paramagnetic resonance (EPR) spectroscopy, Barker et al. reported that this mutant was high-spin at neutral pH and concluded that only one of the two potential Met ligands was coordi-nated, but could not identify the sixth ligand [11].

His93Gly sperm whale Mb (H93G Mb) has the native proximal His ligand replaced with a much smaller non-coordinating glycine (Gly) residue. This creates a cavity on the proximal side of the heme prosthetic group and it

Ferric His93Gly myoglobin cavity mutant and its complexes with thioether and selenolate as heme protein models

Jing Dua, Masanori Sonoa and John H. Dawson*a,b

a Department of Chemistry and Biochemistry and b School of Medicine, University of South Carolina, 631 Sumter St., Columbia, SC 29208, USA

Received 19 August 2010Accepted 10 December 2010

ABSTRACT: The composition of ferric exogenous ligand-free His93Gly sperm whale myoglobin (H93G Mb) at neutral pH has been determined by examination of the spectral properties of the protein over the pH range from 3.0 to 10.5. An apparent pKa value of ~6.6 has been observed for the conversion of a postulated six-coordinate bis-water-bound coordination structure at pH 5.0 to a five-coordinate hydroxide-bound form at pH 10.5. Starting from the exogenous ligand-free ferric H93G protein, ferric mono- and bis-thioether (tetrahydrothiophene, THT)-ligated adducts have been prepared and characterized by UV-visible (UV-vis) absorption and magnetic circular dichroism (MCD) spectroscopy. The mono-THT ferric H93G Mb species has hydroxide as the sixth ligand. The bis-THT derivative is a model for the low-spin ferric heme binding site of native bis-Met-ligated bacterioferritin or streptococcal heme-associated protein (Shp). A novel THT-bound ferryl H93G Mb moiety has been partially formed. The high-spin five-coordinate ferric H93G(selenolate) Mb complex has been prepared using benzene selenol and characterized by UV-vis and MCD spectroscopy as a model for Se-Cys-ligated ferric cytochrome P450. The results described herein further demonstrate the versatility of the H93G cavity mutant for modeling the coordination structures of novel heme iron protein active sites.

KEYWORDS: His93Gly myoglobin, ligand binding, magnetic circular dichroism spectroscopy, seleno-Cys-ligated heme protein, bacterioferritin model system, tetrahydrothiophene-bound H93G myoglobin.


*Correspondence to: John H. Dawson, email: [email protected], tel: +1 (803)-777-7234, fax: +1 (803)-777-9521

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30 J. Du et al.

has been shown that various exogenous ligands are able to enter the cavity and coordinate to the heme iron [12, 13]. This model system has provided a very simple way to mimic the coordination structure of native heme pro-teins. In previous studies, the H93G Mb cavity system has been shown to be a versatile template for model-ing heme complexes of defined structures [13–20]. For example, ferric H93G Mb (imidazole) and (alkylthiolate) adducts provided active site mimics of native Mb and cytochrome P450 with six-coordinate His/water [13] and five-coordinate Cys [15] ligation, respectively.

In the present study, we have first examined the compo-sition of ferric H93G Mb sample in the absence of exoge-nously added ligands, referred to as exogenous ligand-free ferric H93G Mb, by analysis of the spectral properties of the protein from pH = 5.0 to 10.5. An apparent pKa (pKa

app) value of ~6.6 has been determined for the conversion of a six-coordinate bis-water-bound coordination structure on the acidic side to a five-coordinate hydroxide-bound form on the alkaline side likely via a water/hydroxide-bound six-coordinate complex. Next, we have explored how the H93G Mb cavity mutant model approach can contribute to an understanding of the structural features of thioether ligation in the ferric and ferryl heme states. Mono- and bis-thioether adducts of the ferric H93G Mb cavity mutant have been prepared using tetrahydrothiophene (THT) as an exogenous ligand and a ferryl moiety has been partially generated. The resulting complexes have been character-ized by UV-visible (UV-vis) absorption and MCD spec-troscopy. MCD spectroscopy has been repeatedly shown to provide invaluable information on the axial ligands coordinated to the heme iron [21].

Cysteine thiolate is the proximal iron ligand in all cytochromes P450 [22]. Selenocysteine (Se-Cys), the 21st amino acid, occurs naturally in all kingdoms of life [23–26]. Using the H93G cavity mutant system, we have generated and spectroscopically characterized a five- coordinate high-spin benzeneselenolate-ligated Mb deriva-tive as a model for Se-Cys-ligated ferric P450 [27].

EXPERIMENTAL

Materials

Sperm whale H93G Mb was expressed and purified in the presence of 10 mM imidazole (Im) as previously described [13, 16]. THT, benzenethiol, benzeneselenol, hydrogen peroxide and sodium dithionite were obtained from Sigma/Aldrich.

Sample preparation

H93G Mb was purified as Im complex as previously reported [13] and its concentrations were determined by the pyridine hemochromogen method [28]. Complete oxi-dation of the protein heme iron is accomplished by addi-tion of a few crystals of potassium ferricyanide (Fluka)

followed by gel-filtration column chromatography. Im can be completely removed from the proximal cavity by means of heme extraction followed by reconstitution with hemin [29] as previously reported [15]. THT stock solu-tions were prepared by dissolving liquid THT (99%) in ethanol. All spectral measurements were obtained in 100 mM potassium phosphate buffer (pH = 7.0) at 4 °C unless otherwise specified at protein concentration of about 25 or 50 µM for 0.5 or 0.2 cm cuvettes, respectively.

For the pKa determination experiment, 25 µL of 0.5 mM exogenous ligand-free ferric H93G Mb was added to 325 µL of 0.1 M potassium phosphate at vari-ous pH values from 3.0 to 10.5. For THT titration experi-ments, stock solutions of 5 M THT in ethanol were added to 0.4 mL (l = 0.2 cm) of 35 µM ferric exogenous ligand-free H93G Mb. Comparable amounts of ethanol did not produce observable spectral changes. Kd values were determined from hyperbolic saturation plots. The ferryl [Fe(IV)=O] derivative was generated from ferric H93G (THT) Mb by adding 2.0 equivalents of H2O2 (relative to the H93G Mb concentration) in 100 mM potassium phosphate buffer, pH = 7.0 at 4 °C.

The anaerobic experiments were done either in a glove box (Coy) or in anaerobic cuvettes (NSG Precision Cells, Inc) sealed with septa at 4 °C. The oxygen level in the glove box was monitored by an oxygen sensor. Anaero-bic 0.1 M potassium phosphate buffer, pH = 7.0 was pre-pared by degassing for 20 min and bubbling nitrogen gas through the solution on ice. A stock solution of benzene-thiol (0.1 M) or benzeneselenol (0.5 M) was prepared in ethanol under nitrogen. The benzenethiol (thiophenol) or benzeneselenol ligand solution was added into the protein sample in the cuvette through a gas tight syringe (Hamilton, Reno, NV) and mixed by slight shaking.

Spectroscopic techniques

UV-vis absorption spectra were recorded with a Cary 400 spectrophotometer interfaced to a Dell PC. MCD spectra were measured at a magnetic field strength of 1.41 T using a JASCO J815 spectropolarimeter equipped with a JASCO MCD-1B electromagnet and interfaced with a Silicon Solutions PC through a JASCO IF-815-2 interface unit. Data acquisition and manipulation using Cary or JASCO software has been described previously [30]. UV-vis absorption spectra were recorded before and after the MCD measurements to verify sample integrity. The spectra of H93G Mb (Im) complexes and horse heart Mb are taken from previous published work [16, 18].

RESULTS AND DISCUSSION

Determination of the apparent pKa of ferric exogenous ligand-free H93G Mb

We have previously demonstrated that the heme iron coordination structure of ferric exogenous ligand-free

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FerrIC His93Gly myOGlObIn CavIty mutant anD ItS COmPlexeS 31

H93G Mb changes depending on the solution pH [16, 31]. It was reported that exogenous ligand-free ferric H93G Mb has a single water bound at pH = 5.0 and below (high-spin), a single hydroxide ligand at pH = 10.0 and above (high-spin), and consists of a mixture of species at pH = 7.0 including five-coordinate hydroxide-bound and six-coordinate (low-spin) structures [16]. Identifica-tion of the structure of H93G at alkaline and neutral pH values was based on resonance Raman and MCD spec-troscopy, namely H2

18O and D2O isotope effects on the resonance Raman spectra [32] as well as detailed MCD spectral examinations in comparison with other oxygen donor-ligated heme systems [31]. However, in the previ-ous study, clear isosbestic points were not observed in the UV-vis absorption spectra over the full pH range inves-tigated [31]. Using freshly prepared reconstituted exog-enous ligand-free ferric H93G Mb samples that had never been frozen and thawed, we have been able to measure its pKa value over the pH range from 5.0 to 10.5 in this study. We have also found that, with samples generated in this manner, the formation of the five-coordinate water-ligated ferric heme state is not apparent until the pH is below 5 (as opposed to nearly complete conversion at pH = 5.0 as previously reported (vide supra)).

Figure 1a shows visible region (500–700 nm) absorp-tion spectra of exogenous ligand-free ferric H93G Mb recorded over a pH range of pH = 3.0–10.5. For this pur-pose, it was necessary to prepare separate samples at dif-ferent pH values because changing the pH value in a single sample by adding small aliquots of either 1-2 M NaOH or HCl solution did not provide reproducible results due to an unstable nature of the H93G Mb protein for such treatments. The spectrum of ferric H93G Mb at pH = 5.0 has a peak at ~624 nm in the visible region. As the pH is increased, the trough at 605 nm grows into a maximum. When samples were also examined at lower pH values (pH = 3.0 and 4.0), the absorption spectrum at pH = 5 changed further and the peak at ~624 nm shifted to 629 nm (pH = 4) and to ~640 nm (pH = 3.0). Over a pH range of 5.0–10.5, a single set of isosbestic points is observed at ~628 nm and ~555 nm except for a detectable deviation at pH = 3.0. In the Soret region (Fig. 1b), no large absorption spectral change of ferric H93G Mb was observed between pH = 10.5 and 5 and the peak posi-tion remained at ~406 nm. However, further lowering the pH to 4.0 to 3.0 caused a drastic spectral change with a Soret peak shift from 406 nm to ~370 nm (pH = 3.0). The Soret CD intensity at pH = 3.0 is considerably dimin-ished (data not shown) suggesting that a significant pro-tein conformational change at the heme-binding site may have occurred due to protonation of amino acid group(s). Nonetheless, a single set of isosbestic point is seen at ~388 nm during the second step spectral transition. This suggests that when the pH was lowered from 10.5 to 3.0, only two heme species exist during each of the two-step spectral changes (thus a total of three species in the overall process), with the first and the second phases

occurring between pH = 10.5 and ~5.0 and between pH = ~5 and 3, respectively.

As mentioned above, the high (pH = 10.5) and low pH (pH = 3.0) species have previously been assigned to mono-hydroxide and mono-water-bound five-coordinate high-spin complexes, respectively [16]. This pH-depen-dent spectral (i.e. heme iron coordination structural) change of ferric H93G Mb appears to differ somewhat from analogous spectral changes seen in water-soluble heme systems (the iron complex of 5,10,15,20-tetrakis(l-methylpyridinium-2-yl)porphine (Fe(III)T2MP) and 5,10,15,20-tetrakis(l-methylpyridinium-3-yl)porphine (Fe(III)T3MP)) [33, 34]. In these free heme systems, direct inter-conversion between the mono-hydroxide and mono-water-ligated five-coordinate high-spin species occurs with pKa values ranging from 4.8–6.4 [33]. If the inter-conversion of these two five-coordinate ferric H93G Mb derivatives involves another species as an interme-diate, two possible candidates are bis-water- (H2O-Fe(III)-OH2) and water/hydroxide (H2O-Fe(III)-OH) six-coordinate complexes. To identify the intermediate H93G Mb species that is observable at pH ≈ 5.0, fur-ther spectral analysis has been performed as described below. The intensities of the peak at 605 nm (A605) are

Fig. 1. Visible (a) and Soret (b) region electronic absorption spectra of exogenous ligand-free ferric H93G Mb in 0.1 M potassium phosphate buffer at different pH values from 3.0 to 10.5 at 4 °C. Vertical arrows indicate the directions of absor-bance change with increasing pH. Only selected spectra are shown in the Soret region above pH = 5.0. Inset in A: absor-bance at 605 nm (charge transfer band) as a function of pH from 5.0 to10.5. Sigmoid plot fit yields pKa = 6.6

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32 J. Du et al.

plotted as a function of pH value between 10.5 and 5.0 in Fig. 1a (inset). The apparent isosbestic points at ~555 nm and ~628 nm over the pH range studied support the use of a sigmoid plot to calculate the pKa value. The solid line that has been drawn for a sigmoid fit nicely match the data. The pKa value of the heme iron active site of exogenous ligand-free ferric H93G Mb thus deter-mined is ~6.6. This pKa value is nearly within the range of 4.8–6.4 reported for direct inter-conversion between the mono-hydroxide and mono-water-ligated five-coordinate high-spin species of Fe(III)T2MP and Fe(III)T3MP [33]. However, we rule out such a direct interconversion (see above). Judging from the visible region spectral charac-teristic of the intermediate species (A) observed at pH = 5 (dot-dot-dash line in Fig. 1a), namely the presence of charge-transfer bands at ~500 and 624 nm and the absence of dominant peaks between these two bands, the interme-diate species (A) is clearly high-spin. Unfortunately, this does not distinguish between bis-water- (H2O-Fe(III)-OH2) or water/hydroxide-bound (H2O-Fe(III)-OH) six-coordinate complexes. However, the apparent pKa value for deprotonation of species (A) to form the hydroxide-bound derivative (C) is ~6.6, which is relatively low and more consistent with direct deprotonation of a heme iron-ligated water with a trans neutral ligand (water) than with a trans anionic ligand (hydroxide) [‡a].

Based on the above observations and considerations, we propose a pH-dependent equilibrium (pKa ≈ 6.6) for the heme iron coordination states of exogenous ligand-free ferric H93G Mb between a six-coordinate bis-water-ligated mid pH form (A) and mono-hydroxide-ligated high pH form (C) as illustrated in Scheme 1. Since complex A will not convert to C in one step, we pro-pose another species, a six-coordinate water/hydroxide-ligated complex (B) formed upon deprotonation of A, as an intermediate. Species B must be in a pH-independent equilibrium with C that is considerably shifted towards C (with a constant, Keq = [C]/[B] > 10) [‡b]. The apparent pKa value of 6.6 indicates that A and C exist in a ratio of 1:2.45 (i.e. 29% A and 71% C) at pH = 7.0.

Previous resonance Raman studies [31, 32] have not established definitively which position of the heme iron (proximal or distal site) the hydroxide ligand (in C) coordinates, but speculated that the proximal site was the likely location [32]. We consider the more polar dis-tal site rather than more hydrophobic proximal site to be the likely location for hydroxide to bind (in both B and C) because the distal His in wild-type ferric Mb is known to stabilize binding of those ligands via hydro-gen bonding [35]. Furthermore, a vacant proximal site (in C in Scheme 1) would facilitate binding of relatively large size ligands to that side of the heme. Such ligands include imidazole [13, 18] and its derivatives [12, 16], pyridine [12, 18], alkylamines [19], alkylthiols [12, 15], phenol [12, 31, 36], and carboxylate [17, 37]. The crystal structures of the imidazole-, β-mercaptoethanol- and acetate-bound ferric H93G Mb complexes that support such coordination modes have been determined [13, 17]. The small amount of unidentified six-coordinate low-spin species that was seen with resonance Raman spectroscopy upon lowering pH from 10 to 7 [31] was likely a minor contaminant(s) not present in the current freshly prepared (vide supra) exogenous ligand-free samples.

Cavity mutants of other heme proteins in which the proximal His ligand is replaced with a smaller amino acid such as Ala or Gly have also been prepared and their spectroscopic properties examined. Such mutants include His170Ala horseradish peroxidase (HRP) [37], His175-Gly yeast cytochrome c peroxidase (CCP) [38] and His25Ala human liver heme oxygenase (HO) [31, 39]. H175A HRP forms a five-coordinate high-spin species at pH = 4 that likely contains mono-water-ligated heme, which changes to a six-coordinate low-spin state when pH is raised to 6, with two axial ligands likely provided by distal histidine and an unknown ligand [37]. Unlike His93Gly Mb cavity mutant, neither of the H175G CCP and H25A HO mutants has been shown to exhibit pH-dependent spectral change between pH = 5–7 (H175G CCP, in non-phosphate buffer) or pH = 6–10 (H25A HO).

UV-vis absorption (H175G CCP) and MCD (H25A HO) spectra of these species are characteristic of mono-water-ligated five-coordinate and/or bis-water-ligated six-coordi-nate-(H175G CCP) [38] and mono-carboxylate-ligated five-coordinate (H25A HO) high-spin heme, lat-ter of which likely has a nearby glutamyl or aspartic residue as an axial ligand [31]. The presence of a six-coordinate bis-aqua (water)-ligated structure of ferric H175G CCP was found in its crystalline state [40]. However, such a six- coordinate high-spin species has not been identified in solution or

N N

N N

OH

OH2

FeIII

B

OH

N N

N N

FeIIIN N

N N

OH2

OH2

FeIII

+ H+

- H+pKa

+ H2O

- H2O

Keq

CA

pH = 5 pH = 106.6 ≈ pKa

(app) = pKa - logKeq(Keq = [C]/[B] > 10)

proximal side

(bottom)

distal side(top)

A CpKa

(app)

Scheme 1. pH-dependent interconversions between two species (A and C) involving intermediate B for exogenous ligand-free ferric H93G Mb

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in the crystal with resonance Raman spectroscopy [41]. Thus, the absence of resonance Raman spectroscopic support for the bis-water-ligated six-coordinate high-spin H93G Mb complex (A in Scheme 1) at pH = 7.0 [31, 33] is not necessarily inconsistent with our proposal shown in Scheme 1.

Figure 2 shows the MCD absorption spectra of exog-enous ligand-free ferric H93G Mb at pH = 3.0, 5.0, 7.0 and 10.5. These spectra are similar to those previously reported from our lab at pH = 5.0, 7.0 and 10.0 [31], respectively; i.e. the new spectrum at pH = 3.0 (Fig. 2) resemble that at pH = 5.0 in [31]. Alkaline ferric H93G Mb has the most intense derivative-shaped MCD band in the visible region centered at 605 nm, a feature that is typical for five-coordinate high-spin ferric heme centers [31].

Thioether adducts of H93G Mb

As just discussed, exogenous ligand-free ferric H93G Mb at pH = 7 consists of a mixture of a six-coordinate bis-water-bound species and a five-coordinate hydroxide adduct and exhibits an UV-vis absorption spectrum with a dominant Soret peak at 405–406 nm and a charge trans-fer band at ~600 nm. Stepwise addition of low concentra-tions of THT produces spectral changes in the Soret and visible regions with a clear set of isosbestic points dur-ing the titration (Fig. 3). This indicates that there are two optically distinct heme ligation states, exogenous ligand-free and THT-bound, in the binding reaction. Addition of THT to ferric H93G Mb causes a decrease in intensity and a red-shift of the Soret absorption maximum, while the absorption intensity of the charge transfer band at ~600 nm increases. The isosbestic points observed in the

titration support the use of a simple bimolecular associa-tion scheme to describe the binding reaction. The titration data have been analyzed by using a hyperbolic saturation plot (Fig. 4). The solid line that has been drawn for the non-linear fit nicely matches the data. The dissociation constants (Kd) for THT binding to exogenous ligand-free ferric H93G Mb (at pH = 7.0, 4 °C) is 2.1 mM. The Kd values vary only slightly between pH = 5.0 and 8.0 (Kd = 2.8, 2.2, 2.1 and 1.6 mM at pH = 5.0, 6.0, 7.0, and 8.0, respectively).

Figure 5 compares the UV-vis absorption and MCD spectra of exogenous ligand-free ferric H93G Mb, ferric H93G(THT) Mb and ferric H93G(Im) Mb. The band pat-terns of the UV-vis absorption and MCD spectra of fer-ric H93G(THT) Mb are relatively similar to those of the exogenous ligand-free ferric H93G Mb, but quite differ-ent from those of ferric H93G(Im) Mb. The overall MCD spectrum intensity of ferric H93G(THT) Mb is slightly stronger compared to that of the ferric H93G(Im) Mb complex. However, the derivative-shaped MCD spec-tral features of ferric H93G(THT) Mb are red-shifted

Fig. 2. MCD spectra of exogenous ligand-free ferric H93G Mb at pH = 3.0 (dot-dashed line), pH = 5.0 (dashed line), pH = 7.0 (solid line) and pH = 10.5 (dotted line) recorded using freshly prepared and never frozen protein samples. The spectrum at pH = 3.0, which is plotted with the same scale as the others, is offset downward for clarity. All spectra were measured in 0.1 M potassium phosphate buffer at 4 °C. The spectra are very sim-ilar to those reported in [31] at pH values 5.0, 7.0 and 10.5, respectively

Fig. 3. (a) Soret and (b) visible region UV-vis absorption spec-tral changes upon titration of exogenous ligand-free ferric H93G Mb (36 μM in a 0.2-cm cuvette) with THT in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. Vertical arrows indicate the directions of absorbance change on addition of 0, 1.3, 2.6, 5.2, 10.5 and 21.0 mM THT. The short diagonal arrows show isosbestic points

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34 J. Du et al.

about 2 nm in the Soret band and blue-shifted about 5 nm in the visible region compared with those of the ferric H93G(Im) Mb derivative. As mentioned above, exogenous ligand-free ferric H93G Mb exists in a ~30/70 mixture of six-coordinate bis-water- and five-coordinate

hydroxide-bound structures at neutral pH. The similar-ity between the UV-vis absorption and MCD spectra of the THT adduct and the exogenous ligand-free alkaline ferric form of H93G Mb at pH 10.5 indicates that the proximal site is occupied by THT, a neutral thioether ligand. We therefore conclude that, the heme iron-ligand coordination mode of ferric H93G(THT) adduct is a six-coordinate THT/hydroxide structure. This is the first report of a THT/hydroxide ferric heme iron adduct. By comparing the UV-vis absorption and MCD spectra of ferric H93G(THT) Mb (Fig. 6) with those of the ferric high-spin H102M cytochrome b562 at neutral pH [11], we propose that Met and hydroxide are the heme iron ligands in that mutant.

Upon further increasing the concentration of THT, the resulting spectra deviate from the first set of isos-bestic points in Fig. 4 (data not shown). The Soret absorption band is further red-shifted to 412 nm. Fea-tures in the visible region of the UV-vis absorption spectrum that are typical of low-spin ferric heme com-plexes around 530 nm and 560 nm increase. A second set of isosbestic points appears. These indicate that the reaction enters the second phase process, i.e. a sec-ond THT ligand coordinates to the heme iron center. Formation of a bis-THT adduct requires substantially higher concentrations of THT at neutral pH value and it is not possible to fully form the ferric H93G(bis-THT)

Fig. 4. Hyperbolic saturation plots for THT binding to exog-enous ligand-free ferric H93G Mb. Maximum absorbance changes in difference spectra (not shown) (ΔA423 - ΔA404, where 423 and 404 are wavelengths at which maximum positive (peak) and negative (trough) absorbance changes are observed, respectively) in the Soret region are plotted as a function of total ligand concentration. The line drawn is a non-linear fit for a bimolecular association model to the data

Fig. 5. (Top) MCD and (bottom) UV-vis absorption spectra of ferric H93G(THT) Mb (solid line, 21 mM THT), ferric H93G (Im) Mb (dashed line, 1 mM Im) and ferric exogenous ligand-free H93G Mb (dotted line) in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. The spectra for ferric H93G(Im) Mb are replotted from [16]

Fig. 6. (Top) Magnetic circular dichroism and (bottom) UV-vis absorption spectra of ferric H93G(bis-THT) Mb (solid line, 90 mM THT) in 0.1 M potassium phosphate buffer, pH = 5.0, at 4 °C and ferric H93G (bis-Im) Mb (dashed line, 4 M Im) in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. The spectra for ferric H93G(bis-Im) Mb are replotted from [18]

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Mb adduct at pH = 7 due to the limited THT solubility. However, a nearly homogenous ferric H93G(bis-THT) Mb complex has been successfully prepared with 90 mM THT at pH = 5.0 at 4 °C. Figure 6 reveals that the band patterns of the UV-vis absorption and MCD spec-tra of ferric H93G(bis-THT) Mb at pH = 5.0 are quite similar to those of the ferric H93G(bis-Im)Mb at pH = 7. The ferric H93G(bis-THT) Mb derivative has spec-tral parameters indicative of asix-coordinate low-spin structure with peaks at 415 nm, 535 nm and 562 nm, a band pattern that is quite similar to those of the ferric bis-Met-ligated heme iron site in E. coli bacterioferritin (peaks at 418 nm, 531 nm and 570) [11] and the ferric bis-Met coordinated heme-binding protein Shp (peaks at 417, 530, and 560 nm) [7]. The MCD spectra of the former two heme species are also similar (Fig. 7, solid line and [11]). The bis-THT-ligated ferric H93G Mb is a heme model for bacterioferritin and Shp with the comparable electronic structures and heme iron-ligand coordination modes.

Incubation of the ferric H93G(mono-THT) Mb at pH = 7, 4 °C with 2 equiv. hydrogen peroxide causes the Soret peak to red-shift from 408 nm to 410 nm. The intensity of the charge transfer band at 603 nm for fer-ric H93G(THT) Mb gradually decreases, while two new peaks at 542 nm and 575 nm appear together in

the UV-vis absorption spectrum (Fig. 7). The spectral changes suggest that the ferric state is slowly converting to a ferryl complex (FeIV=O). However, the 603 nm peak in UV-vis absorption spectra and the 612 nm trough in MCD spectra did not completely disappear even after longer incubation time and/or treatment with higher con-centrations of hydrogen peroxide. The observed MCD spectrum of the ferryl H93G(THT) Mb species (Fig. 7) suggests that the ferryl moiety is not fully formed. Given the expectation that the ferryl derivative would have little MCD signal above 600 nm, the fact that the MCD trough at 623 nm (Fig. 7, labeled b) is about half as intense as the same trough prior to hydrogen peroxide addition (Fig. 7, labeled a), leads to the calculated UV-vis absorption and MCD spectra of ferryl H93G(THT) Mb generated by subtraction of 47% of the spectrum of the ferric starting form from the observed spectrum (and normalization of the resulting spectra). The UV-vis absorption spectrum thus obtained has a Soret peak at 416 nm and two peaks at 542 nm and 575 nm in the visible band (Fig. 7). The MCD spectra of ferryl horse heart Mb (HH Mb) has a derivative-shaped feature (peak at 415 nm, trough at 433 nm and crossover point at 424 nm) in the Soret region and an intense trough around 600 nm [42, 43]. The extrapolated MCD spectrum of ferryl H93G(THT) shows a very similar spectral band pattern. However, its overall spectrum is blue-shifted compared to that of ferryl HH Mb, presumably due to differences in the donor properties of THT and His. For the purpose of comparison, the MCD spectrum of ferryl HH Mb has been blue-shifted by 10 nm and over-plotted vs. the extrapolated MCD spectrum of ferryl H93G(THT) (Fig. 8). The band patterns of the two MCD spectra (Fig. 8) are quite similar to each other, consis-tent with the conclusion that the extrapolated spectrum of ferryl H93G (THT) is accurate.

Fig. 7. (Top) Magnetic circular dichroism and (bottom) UV-vis absorption spectra of ferric H93G(THT) Mb (a) before (dashed line) and (b) after (solid line) addition of 50 µM H2O2 as recorded and (c) extrapolated ferryl H93G(THT) spectrum (dotted line) in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. The H93G Mb and THT concentration were 25 µM and 21 mM, respectively

Fig. 8. MCD spectra of extrapolated ferryl H93G(THT, 21 mM) spectrum (solid line) and the ferryl horse heart Mb blue-shifted by 10 nm (dashed line) in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. The spectrum of ferryl horse heart Mb was taken from [42]

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36 J. Du et al.

Selenolate adducts of H93G Mb

Selenocysteine (Se-Cys), the 21st amino acid, is a Cys residue analogue with a selenium-containing sele-nol group in place of the sulfur-containing thiol group. Selenoproteins naturally exist in all kingdoms of life [23–26]. Owing to the replacement of the sulfur atom with selenium, Se-Cys has somewhat dissimilar prop-erties compared to Cys. The most obvious difference is the larger molecular weight and lower pKa of Se-Cys (Mr = 79.0, pKa = 5.2) (Mr = relative molecular mass) compared to that of Cys (Mr = 32.0, pKa = 8.3). Se-Cys is also a stronger nucleophile than Cys and contributes to the efficient catalytic activity of the enzymes in which it is found. Recently, Hilvert and coworkers described a Se-Cys-ligated mutant of cytochrome P450-CAM [27]. In addition, Jian et al. reported that the binding of a seleno-late ligand to the heme iron of the H25A proximal cavity mutant of heme-iron-bound human heme oxygenase-1, under anaerobic conditions, led to reduction of the heme iron to the ferrous state [44].

We have previously reported that the H93G cav-ity mutant reacts with benzenethiol (Ph-SH) to form a thiolate-ligated adduct that accurately mimics the active site of Cys-ligated five-coordinate high-spin ferric

P450-CAM [15]. The Se form of benzenethiol, ben-zeneselenol (Ph-SeH), has the S replaced by Se in the molecular structure. Ph-SeH is easily oxidized by air to give diphenyl diselenide. Under anaerobic condition, ferric H93G(Ph-Seˉ) Mb is only stable for about 5 min at 4 °C; the selenolate ligand is such a strong reductant that the ferric heme iron of H93G Mb is then reduced to the ferrous state. The UV-vis absorption and MCD spectra of ferric H93G(Ph-Seˉ) Mb have been recorded and are compared to those of the ferric H93G(thiolate) Mb complex and ferric camphor-bound P450-CAM in Fig. 9. The most characteristic UV-vis spectral features of the ferric H93G (Ph-Seˉ) Mb are the dominant Soret peak at 394 nm, with the corresponding trough at 398 nm in the MCD spectrum, as well as the charge trans-fer band at 646 nm (Fig. 9). All these indicate that fer-ric H93G (Ph-Seˉ) Mb has a five-coordinate high-spin structure in the heme active site. The comparison of MCD and UV-vis absorption spectra of the selenolate-ligated H93G Mb with those of the H93G(Ph-Seˉ) Mb and camphor-bound P450-CAM (Fig. 9) indicates that the electronic structures and thus the heme iron-ligand coordination modes of the three ferric protein derivatives are comparable (Fig. 9).

CONCLUSION

An apparent pKa value of exogenous ligand-free ferric H93G Mb for its pH-dependent spectral change (between pH = 5.0 and 10.0) has been successfully determined in the present study. The data indicate that at pH ≈ 6.6, ferric exogenous ligand-free H93G Mb exists as a ~70/30 mix-ture of five-coordinate hydroxide- (alkaline form) and a six-coordinate bis-water-bound structure (pH = 5 spe-cies). Ferric mono- and bis-thioether-ligated H93G Mb adducts have been prepared starting from the exogenous ligand-free ferric protein. In the ferric H93G(THT) Mb complex, hydroxide is the ligand trans to THT. This is the first report of a THT/hydroxide ferric heme iron adduct. The bis-thioether adduct is a model for low-spin ferric heme binding site of native bis-Met-ligated bacteriofer-ritin [11] or streptococcal heme-associated protein (Shp) [7]. A thioether-ligated ferryl H93G Mb complex has been partially generated. The extrapolated MCD and UV-vis absorption spectra of homogenous ferryl H93G(THT) feature similar band patterns to those of ferryl HH Mb, except for approximately a 10 nm blue-shift likely due to differences in the donor properties of the respective prox-imal ligands. A ferric H93G(selenolate) Mb adduct has been generated and characterized by MCD and UV-vis absorption spectroscopy as a model for Se-Cys- ligated cytochrome P450-CAM [27]. The results described herein provide another example of the versatility of the H93G cavity mutant for as a scaffold for modeling the coordination structures of novel heme iron protein active sites.

Fig. 9. (Top) MCD and (bottom) UV-vis absorption spectra of ferric H93G(benzeneselenolate) Mb (solid line, 67.6 µM ben-zeneselenol), H93G(benzenethiolate) Mb (blue dashed line, 110 mM benzenethiol) and P450-CAM (dotted line, 1 mM camphor) in 0.1 M potassium phosphate buffer, pH = 7.0, at 4 °C. The spectra for benzenethiolate complex are very simi-lar to those reported in [15] and those for ferric P450-CAM are replotted from [15]

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Acknowledgements

We thank Professor Steven G. Boxer for the H93G expression system. This work was supported by the National Institutes of Health (GM 26730) and Research Corp. (to J.H.D.).

REFERENCES

‡ a) Deprotonation of the water in six-coordinate water/hydroxide form of H93G Mb would yield a complex with bis-hydroxide ligands. However, despite con-siderable effort, we have been unable to coordinate two anions to ferric H93G Mb [15]. Bis-hydroxide-ligated low-spin ferric hemes have been generated with water-soluble free heme systems at pH ≈ 14 [33, 34]. b) The three-species equilibrium will yield the following equations: Ka = [B][H+]/[A] and Keq = [C]/[B]. By combining these two equations, Ka·Keq = [C][H+]/[A] can be obtained. Taking the logarithm of this equation and considering pKa

(app) = pH when [A] = [C], one obtains pKa

(app) = pKa - logKeq, where pKa

(app), pKa represent apparent (between A and C) and intrinsic pKa values (between A and B), respectively.

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DOI: 10.1142/S1088424611002891



IntroductIon

We investigate Pc and Nc carbohydrate conju-gates because of the potential of other Pc’s and Nc’s in photodynamic therapy (PDT) [1]. This is due to their ability to photosensitize the formation of highly reactive singlet oxygen via transfer of energy from the triplet exited state of the Pc or Nc macrocycle to the triplet ground state of oxygen. Singlet oxygen is a potent oxidant that reacts with several functional groups of biomolecules [2]. Diverse research groups have synthesized a variety of porphyrinoid-carbohy-drate conjugates [3] assuming that the presence of the carbohydrate moiety could improve the membrane

interaction, therefore increasing their tumor selectiv-ity. Moreover, various types of glucose transporters are specific for different monosaccharides in cancer cells [4]. Recently we published a series of papers dealing with the syntheses of anomerically tetra- and octagly-cosylated zinc phthalocyanines (PcZn) and zinc naph-thalocyanines (NcZn) [5–10].

First we synthesized tetra-glycosylated PcZn’s 1a–e with D-glucopyranose, 1-thio-b-D-glucopyranose, 1-thio- b-D-galactopyranose, 1-thio-b-D-cellobiose and 1-thio-b-D-lactobiose in b-positions [5, 6] and the PcZn’s 2a–e with D-glucopyranose, 1-thio-b-D-glucopyranose, D-ga-lactopyranose, 1-thio-b-D-galactopyranose, and D-malt-ose in the a-positions [9].

Aggregation behavior and UV-vis spectra of tetra- and octa glycosylated zinc phthalocyanines

Alexey Lyubimtseva,b, Zafar Iqbala,c, Göran Cruciusa, Sergey Syrbub, Ekaterina S. Taraymovicha,b, Thomas Zieglera and Michael Hanack*a

a Institut für Organische Chemie der Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany b Ivanovo State University of Chemistry and Technology, F. Engels str. 7, 15300 Ivanovo, Russia c Karakoram International University, Department of Chemistry, Gilgit, Pakistan


ABStrAct: Several tetra- and octaglycosylated PcZn’s 1a, 2a–2d, 3a-b and 4 were investigated for their aggregation behavior using different concentrations of the PcZn-species in pure DMSO, water and in various DMSO/water mixtures by comparing their UV-vis spectra. The PcZn’s 1–4 are independent of the concentration in pure DMSO and in up to 25 vol.% water/DMSO mixtures are non aggregated. Increasing amounts of water leads to higher aggregation ratios. The aggregation behavior is influenced by the nature and the position of the sugar substituents on the Pc-ring. PcZn 4 was found the least aggregated compound even in pure water.

KEYWordS: phthalocyanine, sugar-substituents, aggregation, UV-vis spectra.


*Correspondence to: Michael Hanack, email: [email protected], tel: +49 7071-2972432, fax: +49 7071-295268

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40 A. LyubImtSev et al.

The synthesized tetraglycosylated PcZn’s were isolated as mixtures of structural isomers. No attempts were made to separate the isomers. Mixtures of Pc- isomers are reported to exhibit even better PDT-properties than the single isomers due to their enhanced solubility [11].

In addition, we were successful to obtain the anomerically octaglycosylated zinc(II) phthalocya-nines 3a–i. Also in these cases the sugars glucopy-ranose, galactose, lactose, cellobiose and maltose were anomerically attached either via oxygen or via sulphur to the Pc ring in the b-positions [10] ZnPc 4 with eight glucopyranose moieties in the b-positions connected via carbon-6 of the sugar ring was also synthesized [7].

Beside porphyrins and Pc’s naphthalocyanines (Nc’s) also have been used as photosensitizers in PDT. Nc’s are of particular interest due to their absorption maxima in the range of 750–800 nm, where light penetration through skin and tissues is approximately twice as that of Pc’s with their absorption maxima ~630 nm [12]. This would

allow treatment of larger and more deeply lying tumors [13]. Several peripherally substituted metal naphthalocyanines have been synthesized and stud-ied for their use in PDT [14]. We have extended our work on glycoconjugated Pc’s also to Nc’s with the synthesis of the anomerically tetraglucose substi-tuted Zn naphthalocyanine 5 [8].

The tetraglycosylated (1a–e, 2a–e) and octaglyco-sylated ZnPc’s (3a–i and 4) show very good solubility in water and relatively lower solubility in DMSO and e.g. DMF.

Pc’s have a high tendency to aggregate especially in aqueous solutions, whereby mostly H-type aggre-gates are formed which is indicated by their broad Q-bands in the UV-vis-spectra. By aggregation, the photosensitizing ability of the Pc’s is decreased by self-quenching.

For the generation of singlet oxygen from the glycosylated PcZn’s and NcZn’s their aggregation behavior in different solvents therefore is an impor-tant issue. The aggregation performance of the syn-thesized glycosylated PcZn’s and NcZn’s has been investigated here and will be discussed in some detail in the following.

Not many studies on Pc or Nc aggregation in solution are available in recent literature, the results on Pc aggregation up to 2001 have been reviewed [15]. The aggregation behavior of Pc’s and Nc’s is difficult to predict because it is influenced by many factors, e.g. by the strong π-π electron interaction of the macrocycles, coloumbic forces, the central metal, axial substituents on the central metal, the temperature and most importantly by the solvent used and the concentration of the dissolved Pc’s or Nc’s. In addition, the aggregation performance is quite dependent on the position and the size of

N

NN

N

N

NN N

OHO

OHO

OOH

Zn

Glycoses

glucopyranoseglucopyranosegalactopyranosegalactopyranoselactoselactosecellobiosecellobiosemaltose

Y

OSOSOSOSO

3a3b3c3d3e3f3g3h3i

Y

OOH

Y

Y

Y

OOH

OOH

YY

Y

Y

OHO

OHO

N

NN

N

N

NN N

O

O

YHO

YHO

OHO

Y

OOH

YZn

Glycoses

glucopyranoseglucopyranosecellobiosegalactopyranoselactobiose

Y

OSSSS

1a1b1c1d1e

N

NN

N

N

NN N

O

YHO

OHO

Y

OOH

YZn

Glycoses

glucopyranoseglucopyranosegalactopyranosegalactopyranosemaltose

Y

OSOSO

2a2b2c2d2e

Y

OOH

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AggregAtIOn behAvIOr AnD uv-vIS SPeCtrA Of tetrA- AnD OCtA gLyCOSyLAteD zInC PhthALOCyAnIneS 41

the substituents on the Pc macrocycle. Substituents in a- positions of the Pc have pronounced different influ-ences on the aggregation properties in comparison with substituents in the b-positions. As an example: Pc’s with eight linear alkylgroups substituted in the a-positions exi-bit almost no aggregation in cyclohexane up to a concen-tration of 10-4 M [16]. This effect is due to steric crowding of the substituents in the a-positions causing a deviation of the Pc-Ring out of planarity which leads to a reduc-tion of the π-π interaction of the macrocycles. Also elec-tronic effects caused by the substituents in the a- and b-positions contribute to the different aggregation prop-erties (see below).

Bulky, e.g. tert-butyl groups and water solubiliz-ing groups such as carboxylates, sulfonates, PEGs etc. have been introduced either in peripheral or axial

positions of metal Pc’s to minimize their aggregation tendency [17].

In the UV-vis-spectra a sharp single narrow Q-band points to non-aggregated PcM-species. Substitution in a-positions is known to lead to larger bathochromic shifts in PcM-complexes than b-substitution [17a,h].

The aggregation of water soluble Pc’s were mostly studied with peripherally sulfonated metal free Pc’s and Pc’s containing Zn, Co, Fe and others as central metals [11, 15, 18, 19]. With the sulfonated Pc’s, the influence of the position and number of the peripheral sulfonate groups as well as the effects of temperature and mixtures with organic solvents on the aggregation behavior was investigated [19].

In the following section, we describe the aggre-gation behavior of the glycosylated PcZn’s 1–4 with different concentrations in water, DMSO and DMSO/water mixtures by analyzing their UV-vis spectra in these solvents.

rESuLtS And dIScuSSIon

First the electronic absorption spectra of the tetra- (1a and 2a–e) and octaglyco-sylated PcZn’s 3a–i and 4 were recorded in DMSO. The results are summarized in Table 1.

The PcZn’s presented in Table 1 gave typical UV-vis spectra for non-aggregated phthalocyanines showing an intense and sharp Q-band in the red vis-ible region between 678 and 710 nm [20]. The B-bands at 350–370 nm do not exhibit special properties, the same is true for the vibronic bands at 350–370 nm. Therefore no further discussion will be done concerning these absorptions.

DMSO being a coordinating solvent, binds axially to the PcZn(II) macrocycles reducing their aggregation tendency. In DMSO as the solvent, the non-aggregated status is independent of the concentration

Table 1. Electronic absorption data for glycosylated PcZn’s 1–4 in DMSO

Compound λmax, nm Compound λmax, nm

1a 681, 613, 354 3d 708, 635, 371, 3142a 702, 633, 352 3e 680, 3502b 710, 639, 336, 260 3f 708, 3702c 704, 633, 333 3g 679, 612, 361, 2892d 712, 639, 336, 261 3h 709, 636, 3723a 678, 612, 360, 288 3i 679, 611, 3613b 710, 636, 371 4 679, 613, 360, 2913c 680, 613, 360, 290

N

NN

N

N

NN N

Zn

OO

O

O

OO

O

O

O

OH

HO

OH

OH

O

OH

OH

HO

HO

O

HO

HO

OH

OH

O

OH

HO

OH

OH

O

OH

OH

HO

HO

O

HO

OH

HOHO

O

HO

OH

HO

HO

O

HO

HO

OH

OH

4

N

NN

N

N

NN N

OHO

OHO

OOH

Zn O

O

O

O

OHO

5

00289.indd 3 3/3/2011 4:10:03 Pm

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in the range shown in Fig. 1 with octaglucopyranosyl substituted zinc phthalocyanine 3a as an example.

As the concentration was increased (see Fig. 1, from F to A) the Q-band absorption intensity also increased and there were no new bands due to aggregation species being formed. Lambert-Beer’s law was obeyed for the complexes 3a in the concentration ranging from 2 × 10-5 to 10-6 M (see inset of Fig. 1). The other PcZn’s 1–4 displayed a similar behavior in DMSO solution.

As mentioned in the introduction the Q-band positions depend on the number, position and nature of the sub-stituents (see e.g. [21]).

Figure 2 shows the UV-vis spectra of the glycosylated PcZn’s 1a, 2a, 2b and 3a respectively in DMSO.

For PcZn 2a with O-glucopyranosyl units in the a- positions the Q-band absorption appears at longer wave-length compared with PcZn 1a containing the same substit-uents in the b-positions (λmax = 702 nm for 2a and 683 nm for 1a respectively). The observed red shift of the Q-band

is typical for phthalocyanines with substituents in the a-positions and has been explained beside the above men-tioned steric effects to be due to linear combinations of the atomic orbitals (LCAO) coefficients at the a-positions of the HOMO being greater than those at the b-positions. As a result, the HOMO level is destabilized more in the a-position than in the b-position. Thereby the energy gap (DE) between the HOMO and LUMO becomes smaller, resulting in a bathochromic shift [22].

An increase in the number of glycopyranosyl substitu-ents in the b-positions from four to eight results in an even more blue shifted Q-band from 683 for 1a to 678 for 3a respectively. These results are in agreement with other a- and b-substituted PcZn’s [23].

As can be seen in Fig. 2 and also in Table 1 the thiog-lucosylated PcZn 2b show red shifted Q-bands compared with O-glucosylated PcZn 2a of 10 and 8 nm respec-tively. The Q-band of the octa-S-glucopyranosyl PcZn 3b also was significantly red shifted (by 32 nm) compared with the Q-band of the octa-O-glucopyranosyl PcZn 3a.

Fig. 1. Electronic absorption spectra of PcZn 3a at different concentrations in DMSO. Concentration range: (A) 18 μM, (B) 12 μM, (C) 9 μM, (D) 6 μM, (E) 2 μM, (F) 1 μM. Inset: Lambert-Beer law plot

Fig. 2. Electronic absorption spectra of PcZn’s 1a, 2a, 2b and 3a in DMSO

Fig. 3. Electronic absorption spectra of 3a at different concen-trations in water. Concentration range: (A) 36 μM, (B) 24 μM, (C) 18 μM, (D) 12 μM, (E) 9 μM, (F) 6 μM, (G) 3 μM. Inset: Lambert-Beer law plot

Table 2. Selected UV-vis data of PcZn’s 2a–2d, 3a and 3b in DMSO, water and in various DMSO/water mixtures

Ratio εQ1/εQ2

% H2O

0 25 50 75 100

1a * * 0.98 ** **2a * * 0.89 0.68 **2b * * * 1.10 0.622c * * 3.32 1.64 0.892d * * * 0.72 **3a * * 1.77 0.70 **3b * * 2.03 0.69 **

*very large; **very small.

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Within the O- and S-glycosylated series no effects of the different sugar moieties on the UV-vis spectra were observed.

In water as solvent, all investigated PcZn’s 1–4 were highly aggregated, as shown as an example for the octa-glycosylated PcZn 3a in Fig. 3.

In general, aggregation results in a decrease of the Q-band intensity (Q1) at ~680–710 nm indicative for a monomeric species. Correspondingly a new broader and blue shifted band at ~680 nm is observed. The shift to lower wavelengths is caused by H-type aggregates.

As shown in Fig. 3, if the concentration of PcZn 3a in water was increased, the intensity of the Q-band absorp-tion corresponding to an aggregation species (Q2) also increased. For 3a the Lambert-Beer law was obeyed in water in concentrations ranging from 4 × 10-5 to 10-6 M (see inset of Fig. 3). The linear curve with increasing concentrations shows that under the described conditions only one aggregated Pc-species, probably a dimer, can be present. The other glycosylated PcZn-complexes 1–4 also displayed a similar behavior in water with minor differences in solubility.

Fig. 4. Electronic absorption spectra of PcZn’s 2a–2d, 3a and 3b in DMSO, water and in DMSO/water mixtures

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The octaglycosylated PcZn’s 3a–i and 4 are more sol-uble in water compared with the tetrasubstituted PcZn’s 1a–e and 2a–e. This is due to the higher number of sugar substituents containing OH-groups thereby increasing the water solubility even more than the tetrasubstituted Pc’s in contrast with tetraalkylsubstituted Pc’s which are more soluble in organic solvents than the corresponding octasubstituted Pc’s [24].

Table 2 and Fig. 4 show the UV-vis data and the corre-sponding spectra respectively of the tetra-a-glycosylated PcZn’s 2a–2d, as well as the data of the octaglycosylated PcZn’s 3a and 3b in DMSO, water and DMSO/water mixtures.

The tetra-substituted Pc’s 2a–2d are almost non-aggre-gated in DMSO as well as in DMSO/water mixtures up to 25 vol.% water (see UV-vis spectra in Fig. 4). Increasing the amounts of water in the DMSO/water mixtures leads to a decrease of the Q1-band of 2a–2d and an increase of the broad Q2-band demonstrating the formation of aggre-gated species [19].

Within the series of the tetraglycosylated PcZn’s 2a–2d specific differences in the aggregation behavior depend-ing on the DMSO/water ratio are observed. As can be clearly seen in Fig. 4 and Table 2 the S-glycosides 2b and 2d are less aggregated up to 50 vol.% water than the O-glycosides 2a and 2c for which already with 50 vol.% water the ε1/ε2 ratio are 0.89 and 3.32 respectively.

Comparing the O-glycosylated PcZn’s 2a and 2c with increasing amounts of water in the DMSO/water mix-tures also leads to pronounced differences in the UV-vis spectra. Tetra-a-galactose substituted PcZn 2c in more than 50 vol.% water shows less aggregation than the tetra-a-glucose substituted PcZn 2a, which is seen for 2c by the still comparatively large Q1-band in pure water (cf. also Table 2).

The octa-O- and octa-S-glucopyranosyl substituted PcZn’s 3a and 3b do not differ very much in their aggre-gation behavior in the DMSO/water mixtures (Fig. 4).

The aggregation behavior of the octasubstituted glucopyranose PcZn 4 in which the glucopyranose is attached via its 6-position to the macrocycle is quite dif-ferent from the other Pc’s (Fig. 5). PcZn 4 shows high solubility in water and exhibits less aggregation in pure water compared with the anomerically glycosylated PcZn’s 1–3. This points to a larger steric effect of the glucopyranose moieties in the b-positions connected via carbon-6 of the sugar than the anomerically attached sugar substituents.

As mentioned above, the low aggregation tendency of the glycosylated PcZn’s 1–4 in DMSO is due to the special solvation characteristics of DMSO forming axial PcZn-DMSO complexes and thereby avoiding the forma-tion of H-type PcZn aggregates. In the protonic and polar water as solvent, H-briding between the sugar substitu-ents with the many OH group will be supported under these conditions leading to formation of least dimeric and probably also higher aggregates.

To avoid the high aggregation tendency of the PcZn’s 1–4 in water other sugar substituted metal Pc’s contain-ing axial substituents, e.g. RnPcMR’m in which R = sug-ars substituents as in 1–4, n = 4, 8; m = 1, 2; M = Al, In, Si; R′ = akyl, phenyl are synthesized in our group and investigated for their aggregation properties [25].

Another method to minimize aggregation in the sugar substituted PcZn’s is to introduce, beside the sugar moi-eties, larger substituents, e.g. tert-butyl groups into the Pc-macrocycle. Work in this direction is underway.

Contrary to the completely water soluble tetraglucose substituted PcZn’s 1a and 2a the tetraglucose substituted NcZn 5 is less soluble in water at room temperature but soluble in hot water and in DMSO and DMF [8]. The lower solubility of 5 in water in comparison with the PcZn’s 1a and 2a is due to the larger hydrophobic Nc-macrocycle for which the four hydrophylic glucose substituents are not sufficient enough to induce water solubility.

The tetraglucosylated NcZn 5 dissolved in DMSO exhibits a strong Q-band at 773 nm, indicating that it is practically non-aggregated and thereby showing a similar behavior as the PcZn’s 1–4 described here.

concLuSIon

By discussing their UV-vis spectra the anomerically tetra- and octaglycosylated PcZn’s 1a, 2a–2d, 3a and 3b, as well as the 2,3,9,10,16,17,24,25-octakis(a,b-D-galacto-pyranos-6-yl)phthalocyaninato zinc (4) were studied for their aggregation behavior in pure DMSO, water and in various DMSO/water mixtures using different concentra-tions of the PcZn-species in the respective solvents. All investigated PcZn’s are non-aggregated in pure DMSO and in up to 25 vol.% water/DMSO mixtures. Increasing amounts of water in all cases lead to higher aggregation ratios. PcZn 4 was found to be the least aggregated com-pound even in pure water.

As described for the PcZn’s the tetraglucosylated NcZn 5 dissolved in pure DMSO is also non-aggregated.

Fig. 5. Electronic absorption spectra of PcZn’s 4 in DMSO, water and in DMSO/water mixtures

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Acknowledgements

Financial support of the Deutsche Forschungsgemein-schaft (DFG) (ZI: 338/8-1) is highly acknowledged.

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DOI: 10.1142/S1088424611003008



IntroductIon

Phthalocyanines (Pcs) have extended π-conjugation system and various central metals [1], which determine the profile of absorption spectrum. Characteristic dura-bility enables us to apply as paint under bright sunshine, rewritable optical disc by diode laser, and color filer in liquid crystal vision [2]. Pcs have also been used as pho-tosensitizers due to large extinction coefficient and effi-cient triplet state formation that can provide possibilities of a singlet-oxygen sensitizer.

Pcs are known to form aggregates because of their extended π-conjugation. This aggregation leads to self-quenching of their singlet-excited state through very fast excitation migration followed by excitation trap. Sup-pressing these processes are necessary to achieve effi-cient photochemistry of Pcs. In this sense, long alkyl chains have been introduced to Pcs to increase homoge-neous distribution in condensed phase due to improve-ment of solubility, although it causes to form columnar discotic mesophases [3]. Pioneering work has been done by Kimura et al. bearing polyether-amide group as a water-soluble dendron, reporting that second-generation

dendrimer gave monomer-like absorption spectra and showed photoactive property in photoinduced electron transfer [4].

The aim of this article is to accomplish avoiding aggregation of Pcs in aqueous solution as well as pro-viding preferable reaction space in a molecule. We suc-cessfully synthesized two kinds of Pcs having dendrons located at vertical (V3, WV3, and V0 as a model com-pound) and horizontal (G2 and WG2) [5] arrangement toward Pc plane. Chemical structures of dendrimers are shown in Fig. 1. The aggregation behaviors were inves-tigated by steady state spectroscopy and fluorescence lifetime measurement. Obtained results indicate that the introduction of various types of dendrons effectively suppress unfavorable aggregation to perform useful photoreaction.

results And dIscussIon

Materials

To achieve vertical arrangement of dendron, we chose six coordinated Si as a central metal. The third generation lipopholic dendrimer V3 and its potassium salt WV3 were synthesized to explore their ground and excited behavior

Singlet molecular oxygen generation by water-soluble phthalocyanine dendrimers with different aggregation behavior

Masakazu Nishidaa, Hiroaki Horiuchib, Atsuya Momotakea, Yoshinobu Nishimuraa, Hiroshi Hiratsukab and Tatsuo Arai*a

a Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan b Department of Chemistry and Chemical Biology, Gunma University, Kiryu 376-8515, Japan

Received 8 July 2010Accepted 17 December 2010

ABstrAct: Phthalocyanines having hydrophilic or lipophilic dendrons were synthesized to investigate the efficiencies of singlet molecular oxygen (1∆g) formation. The introduction of higher generation of dendrons to the central metal (Si) of phthalocyanine in vertical direction to their ring plane has resulted in the successful improvement in avoiding aggregate formation that resulted in efficient generation of 1∆g even in water.

KeYWords: phthalocyanine, dendrimer, water-soluble, aggregation, transient absorption, fluorescence lifetime, singlet oxygen yield.

*Correspondence to: Tatsuo Arai, email: [email protected]

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48 M. NIShIDa et al.

in organic and aqueous media, respectively. Triethyle-neglycol-substituted phthalocyanine (V0), which can be dissolved both in water and in organic solvent, was used as a reference compound. In addition, photochemistry and aggregation behavior of lipophilic dendrimer G2 and water-soluble dendrimer WG2 having dendrons in differ-ent direction were also compared as references.

steady-state absorption and fluorescence spectra

V0, V3, and G2 dendrimers in organic solvent. The effect of dendrimerization on both steady-state absorp-tion and fluorescence spectra was not significant in THF, where all compounds were monomerically dispersed. Figure 2 shows the absorption spectra of V0, V3, and G2 dendrimers in THF. The spectral profile and the extinction coefficient (ε) of V0 were very similar to that of V3. The Q-band of G2 was located at much longer wavelength (699 nm) due to difference of central metal [6]. Since the bandwidth of Pc in aggregate state should be broadened compared to that in monomeric state [5], the observed sharp absorption spectra indicate that all compounds are considered to be singly-dispersed.

The fluorescence (Fig. 3) and the corresponding absorption spectra (Fig. 2) of V0, V3, and G2 in THF were almost mirror image relationship. The fluorescence maximum of V3 was located at 682 nm, appeared at lon-ger wavelength than that of V0 (675 nm). The emission peak was further red-shifted in G2 (705 nm). Similarity of the shape of fluorescence spectra for all compounds supports monomeric dispersion in THF. According to the fluorescence quantum yields and the lifetime for V0

(Φf = 0.12, τs = 5.3 ns) and V3 (Φf = 0.13, τs = 5.4 ns) (Table 1), the electronic properties of emissive state of V0 and V3 are similar to each other, indicating den-drimer effect is weak when compounds are monomeri-cally dispersed.

V0, WV3, and WG2 dendrimers in water. The remarkable “dendrimer effect” was observed in aqueous solution. First, the spectrum of V0 (Fig. 4) was totally different from that in THF, indicating that V0 aggregates in water due to its hydrophobic interacition. The decrease of the molar extinction coefficient caused from aggregate formation was previously reported [7]. The large spectral change, compared to that of G2 in THF, was also found in WG2 despite of its dendron units. In this case, stack-ing should take place mainly in the vertical direction as

O

NN

NN

NN

NN

Si

O

O

OO

O

O

R

RO

O

O

O

O

R

R

O

O

O

O

O

R

RO

OO

O

O

R

R

O

O

O

O

O

O

O

NN

NN

NN

NN

Si

O

N

N

N

N

N N

N N

Zn

O

O

O

O

O O

OR O

R

G2 (R = )WG2 (R = O-K+)

O

O

O

OR

O

ROO

ORO

R

O

O

OR

O

R

V3 (R = OMe)WV3 (R = O-K+)

V0

Fig. 1. Chemical structures of phthalocyanine dendrimers

Fig. 2. Absorption spectra of V0 (dotted line), V3 (solid line) and G2 (broken line) in THF

00300.indd 2 3/2/2011 6:50:39 PM

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SINglet MOleCular OxygeN geNeratION by Water-SOluble PhthalOCyaNINe DeNDrIMerS 49

expecting from extention of bulky dendrons in horizon-tal direction [5]. Unlike the case of V0 and WG2, the absorption spectrum of WV3 was very similar to that of V3 in THF, indicating that WV3 is mononerically dis-persed even in water. The result is consistent with those of reported Pc dendrimers in which much larger dendritic substituents were required to be monodispersed in water when dendrons come out in horizontal direction [4].

Figure 5 depicts fluorescence spectra of V0, WV3, and WG2 in water and the relevant data were listed in Table 2. According to the absorption spectrum in Fig. 4, V0 mainly exist as an aggregate in the ground state in

water, whereas the fluorescence spectra of V0 in Fig. 5 is probably due to the excited monomer that partially exist, because (1) the bandwidth is sharpen and the decay curve was fitted to single exponential indicating monodes-persity (2) the fluorescence from V0 aggregates should not be observed due to their immediate self-quenching. Fluorescence spectra of WG2 and WV3 are also attrib-uted to the excited monomer for the same reasons. For all compounds, characteristic red-shift relative to those in THF were observed. It may come from the high polar environment by surrounding water molecules. The red-shifted fluorescence of WV3 in water suggests that hori-zontal sphere remains still vacant even in WV3, leading to inclusion of water molecules. Thus, WV3 is probable candidate as an effective sensitizer for small moleclues in water.

transient spectra

V0, V3, and G2 dendrimers in organic solvent. Relatively high concentration of 10-4 M was chosen to investigate generation effect on an excited triplet state behaviors. In this concentration the excited triplet spe-cies can be self-quenched even when triplet state species exist as a monomer. Therefore the decay curves are suited to biexponential modeling. Figure 6 shows time-resolved spectra and transient decay curves (inset) of V0 and V3 in THF on excitation of 308 nm by a XeCl excimer laser. Obtained transient spectra are attributed to triplet-triplet (Tn-T1) absorption of Pc moiety judging from previously

Fig. 3. Fluorescence spectra of V0 (dotted line, 1.9 × 10-6 M), V3 (solid line, 2.1 × 10-6 M) and G2 (broken line, 1.1 × 10-6 M) in THF

Fig. 4. Absorption spectra of V0 (dotted line), WV3 (solid line) and WG2 (broken line) in water

table 1. Fluorescence maximum wavelengths (λmax), quantum yields (Φf), lifetimes (τs) quantum yields of intersystem crossing (ΦISC), and their rate constants (kISC) of V0, V3, and G2 in THF

Compound λmax, nm Φf τs, ns ΦISC kISC, s-1

V0 675 0.12 5.3 0.32 6.0 × 107

V3 682 0.13 5.4 0.43 8.0 × 107

G2 705 0.28 2.8 0.73 2.6 × 107

table 2. Fluorescence maximum wavelengths (λmax), quantum yields (Φf) and lifetimes (τs) of V0, WV3, and WG2 in water

Compound λmax, nm Φf τs, ns

V0 686 0.009 1.1

WV3 719 0.006 2.3

WG2 695 0.038 3.8

Fig. 5. Fluorescence spectra of V0 (dotted line, 1.2 × 10-6 M), WV3 (solid line, 0.9 × 10-6 M) and WG2 (broken line, 0.5 × 10-6 M) in water

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reported spectra [8, 9]. Time development of the tran-sient spectra of V0 and V3 gave similar dissipation of the triplet state. In contrast, triplet lifetimes τt of V0 and V3 showed quite different values: 6.0 µs and 22 µs for V0, and 10 µs and 45 µs for V3, respectively as listed in Table 3. Although the origin of biexponential decay of Tn-T1 absorption of V0 and V3 is still unknown, one can assume that the T-T anihilation takes place under high concentration (10-4 M). Remarkable decrease of long component of V0 relative to V3 indicates that the trip-let state in V0 is perturbed by surrounding environment more dominantly than in V3. Since V0 has relatively small substituents at the vertical position, the intermolec-ular association in the ground state is still considerable.

V0, WV3, and WG2 dendrimers in water. Figure 7 illustrates time-resolved spectra of V0 and WV3 with excitation at 308 nm in water. Transient absorption spec-tra were observed with maximum at 520 nm and 495 nm

for V0 and WV3, respectively. The peak wavelength of WV3 at 1 µs after excitation was similar to those of V0 and V3 in THF, whereas V0 in water exhibited red-shifted spectra with respect to WV3. Lifetimes of all compounds that were monitored at peak wavelength and were fitted to biexponential decay as listed in Table 4. This suggests the possibility of occuring triplet-triplet annihilation in all compounds. Interestingly, built-up components for both V0 and WV3 were observed at 600 nm only in water. The time constants of rise at 600 nm are in agreement with that of decay at peak wave-length, indicating that the build-up species must be gen-erated from the triplet state of Pcs. Since the build-up species at 600 nm has a long lifetime, it can be assigned to ground state product such as phthalocyanine anion radical that has been reported to form in protic solvents via the triplet excited state [10–12].

Lifetime of WV3 was longer than those of V0 and WG2 as listed in Table 4. One possibility to explain above results is occuring self-quenching in V0 and WG2 in water. Pc unit in WG2 may stack in a face-to-face manner because of lack of substituent in vertical direc-tion. Pc unit in V0 may also aggregate in various stack-ing styles, since the absorption spectrum of V0 gave a remarkably different profile from that of WG2. Quench-ing rate constant of the triplet state of V0 by oxygen was larger (1.7 × 108 M-1.s-1) than those of other dendrim-ers. This may reflect the oxgen molecules can approach easier to Pc unit in V0 than to that in higher generation dendrimers.

Fig. 6. Time resolved spectra with superimposed smooth lines of (a) V0 and (b) V3 in THF under Ar. Inset: transient decay curves monitored at 500 nm

table 3. Lifetimes of excited triplet state (tT, tTq) in the absence

and presence of oxygen (10-4 M), quenching rate constants by oxygen (kq), quenching efficiency of triplet state by oxygen (Fq), and quantum yields of singlet-oxygen formation (FD) of V0, V3 and G2 in THF

Compound τT, µs τTq, µs kq, M

-1.s-1 Φq Φ∆

V0 6.0, 22 0.19 5.3 × 108 0.99 NA

V3 10, 45 0.12 8.3 × 108 ~1 0.42

G2 31, 94 0.14 7.3 × 108 ~1 0.73

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efficiency of singlet-oxygen generation

V3 and G2 dendrimers in THF. Phosphorescence spectra of singlet molecular oxygen (1∆g) sensitized by dendrimers were shown in Fig. 8. In THF, phosphores-cence intensity and quantum yield of singlet oxygen pro-duction (Φ∆) G2 was larger than those of V3 (Fig. 8a and Table 3). The lower ΦT value for V3 is a main reason for lower Φ∆. This difference in photophysical values are probably attributed to the different central metal of V3 (Si) and G2 (Zn).

WV3 and WG2 dendrimers in water. On the contrary, the phosphorescence in water was more intense and τT was much longer for SiPc WV3 (Fig. 8b and Table 4) compared to those for ZnPc WG2, and as a consequence, Φ∆ values of WV3 become about 3 times larger than that of WG2. The result indicate that the substitution of bulky dendrons in a vertical direction of the Pc ring is quite effective to generate singlet molecular oxygen in water, because the key point for higher Φ∆ is the avoidance of

aggregation that promote self-quenching of the triplet state of Pc. Interestingly, phosphorescence of singlet-oxygen could not be detected by V0 excitation in water, despite the fact that T-T absorption spectra of V0 was observed (Fig. 7a). One possible reason is that the pro-duced singlet oxygen from triplet state V0 immediately react to form non radiative species. For example the rapid electron transfer from ground state V0 to the produced singlet oxygen took place to give super oxide anion radi-cal. Another posibility is that the observed T-T absorp-tion is not due to the monomer but due to the aggregate that does not sensitize ground state oxygen. It may be related to the result that the transient spectra of V0 in water (Fig. 7a) is slightly different from others. However, there is still no evidence for the above assumptions.

To discuss Φ∆ via the excited triplet state more preci-cely, quenching efficiencies (Φq) of excited triplet states by the ground state oxygen molecule were calculated by the following equation:

Φq = 1 – (τqT / τT) (1)

where τT and τqT is the triplet lifetime in the absence and

presence of oxygen. Φq in THF gave almost unity in all Pcs (Table 3), while that of WG2 in water is relatively lower than those of others (Table 4), which may reflects the aggregated manner.

The deactivation process of 1Pc* and 3Pc* in the pres-ence of oxygen is shown below;

3 32

1 12Pc O Pc O* *+ → +k∆ (2)

3 32

1 32Pc O Pc OTO* + → +k (3)

table 4. Lifetimes of excited triplet state (τT, τTq) in the absence

and presence of oxygen (10-4 M), and quenching rate constants by oxygen (kq), quenching efficiency of triplet state by oxygen (Φq), and quantum yields of singlet-oxygen formation (Φ∆) of V0, WV3 and WG2 in water

Compound τT, µs τTq, µs kq, M

-1.s-1 Φq Φ∆

V0 27, 103 0.58 1.7 × 108 0.99 < 0.01

WV3 32, 299 3.59 2.8 × 107 0.99 0.13

WG2 12, 91 7.01 6.9 × 107 0.92 0.04

Fig. 7. Time resolved spectra with superimposed smooth lines of (a) V0 and (b) WV3 in water under Ar. Inset: transient decay curves monitored at (a) 520 nm (solid line) and 600 nm (broken line), (b) 495 nm (solid line) and 600 nm (broken line)

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Usage of above rate constants can give the quantum yield of singlet oxygen generation, Φ∆, and fraction of energy transfer from 3Pc* to oxygen, f∆

T, can be given by following equations;

Φ Φ Φ∆ ∆= ⋅ ⋅ISC q f T (4)

f∆ ∆ ∆T = +k k kTO( ) (5)

From Equation 4, f∆T was determined to be almost

unity in V3 and G2, which were found to have an advan-tage as an energy donor to oxygen [13]. Although the T-T absorption spectra of WV3 and WG2 were observed, we could not determine the quantum yield of intersystem crossing of WV3 and WG2 and therefore it is not possi-ble to calculate and compare the f∆

T values of these phtha-locyanines in water. However, the observed results show that only WV3 gave sufficient yield of singlet oxygen, but V0 and WG2 gave no detectable emission of singlet oxy-gen and the difference in existing form in water, clearly indicate that the monomeric form should be essential to produce the singlet oxygen during the quenching process of excited state phthalocyanine by oxygen.

In order to use sensitizers having absorption spec-tra at near IR region with high quantum efficiency of

production of singlet oxygen, the solubility of sensitiz-ers in solvent water is indispensable and also, the high quantum yield of singlet oxygen production is essential. The present results clearly revealed that the phthalocya-nines with vertical as well as horizontal dendron groups can produce singlet oxygen in organic solvent, probably because of the monomeric nature of the phthalocya-nines. On the other hand, only the phthalocyanine with vertical dendron substituent can produce singlet oxygen rather efficiently, which should be related to the mono-meric solubility in water. With these results in mind, we can propose that the introduction of vertical substituent with hydrophobic dendron surrounded by water-soluble substituent group should be a good candidate of singlet oxygen sensitizer in biological environment.

exPerIMentAl

Methods

Absorption and fluorescence spectra were measured on Shimadzu UV-1600 spectrophotometer and Hitachi F-4500 fluorescence spectrometer, respectively. Fluores-cence lifetimes were determined by time-correlated sin-gle-photon counting method described elsewhere [14]. Fluorescence quantum yield was determined by using Rhodamine 101 (ΦF = 1.0) as a standard. A correction in the difference in refractive index among the solvents was made for each sample. The absorbance of the sample solution at the excitation wavelength was adjusted to less than 0.1, and the integration of the fluorescent spectra over wavenumber was plotted against absorbance at the excitation wavelength. The slope of these plots gave the relative value of the fluorescence quantum yield, and the quantum yield of fluorescence emission was then determined. The quantum yield of singlet oxygen emis-sion was determined by photon counting method reported previously [15]. The transient decay measurement was performed by using an excimer laser (Lambda Physik LPX-100, 308 nm 20 ns, fwhm) as an excitation light source and a xenon lamp (Ushio UXL-159) was used as a monitor light source. A photomultiplier (Hamamatsu R-928) and a storage oscilloscope (LeCroy LT264) were used for detection of absorption decay. The quantum yield of intersystem crossing from singlet to triplet state was determined according to an article [16].

synthesis

V0. Triethyleneglycol (67.2 mg, 0.409 mmol) in toluene (5 mL) was added slowly to sodium hydride (23.6 mg, 0.982 mmol) in an ice bath under N2 atmo-sphere. The mixture was stirred for 30 min. Silicon-phthalocyanine dichloride (50.0 mg, 0.0818 mmol) in toluene (20 mL) was added slowly to the mixture and then refluxed for 46 hours. The solvent was removed under reduced pressure. The residue was subjected to

Fig. 8. Phosphorescence spectra of singlet oxygen sensitized by (a) V3 (solid line) and G2 (dotted line) in THF; (b) WV3 (solid line) and WG2 (dotted line) in water

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column chromatography using CHCl3/methanol (50/1) as eluent to give the product. Yield 33.2 mg (47%), 0.0383 mmol. 1H NMR (270 MHz, CDCl3): δH, ppm 9.68–9.55 (m, 8H, 1,4-Pc H), 8.41–8.35 (m, 8H, 2,3-Pc H), 3.13 (s, 6H, OCH3), 3.10 (t, 4H), 2.96–2.92 (t, 4H), 2.47–2.43 (t, 4H), 1.70–1.66 (t, 4H), 0.42–0.37 (t, 4H), 1.89–1.93 (t, 4H).

V3. The dendron G3-OH was synthesized according to the previously reported method [5]. A mixture of anhy-drous potassium carbonate (33.9 mg, 0.245 mmol) and the dendron G3-OH (207 mg, 0.204 mmol) dissolved in distilled toluene (4 mL) was stirred for 30 min. Silicon-phthalocyanine dichloride (50.0 mg, 0.0818 mmol) and 18-crown-6-ether (10.8 mg, 0.0409 mmol) were added to the mixture. The obtained mixture was stirred for 15 hours at 150 °C. The crude products were concen-trated by evaporation. Then the crude product was chro-matographed with CHCl3/MeOH as eluent and purified by HPLC (rt = 7.77 min) with GPC to give the product as a green solid. Yield 30.0 mg (15%), 0.0120 mmol. 1H NMR (270 MHz, CDCl3): δH, ppm 9.56-9.53 (m, 8H, 1,4-Pc H), 8.22–8.20 (m, 8H, 2,3-Pc H), 8.02–7.99 (d, J = 8.3 Hz, 16H, dendron terminal 2,6-Ar H), 7.45–7.40 (d, J = 8.3 Hz, 16H, dendron terminal 3,5-Ar H), 6.62–6.50 (m, 6H, dendron Ar H), 6.45-6.31 (m, 12H, dendron Ar H), 5.01 (s, 16H, ArCH2O), 4.03 (s, 8H, ArCH2O), 3.90 (s, 24H, OCH3), 3.51 (s, 4H, ArCH2OSi). MS (MALDI-TOF): m/z 2461.4 calcd. for [M]+ 2490.7.

WV3. KOH dissolved in water/MeOH (2/9) was added to V3 including minimum quantity of THF. The mixture was stirred for 6 hours at room temperature. The reaction mixture was concentrated by evaporation to give an aque-ous solution by the addition of water. The aqueous solu-tion was stirred for 18 hours at room temperature. The solution was purified by dialysis for 48 hours. The resul-tant solution was concentrated by evaporation to give a green solid. 1H NMR (270 MHz, DMSO-d6): δH, ppm 9.66–9.52 (m, 8H, 1,4-Pc H), 8.23–8.19 (m, 8H, 2,3-Pc H) 8.11–8.09 (d, J = 8.4 Hz, 16H, dendron terminal 2,6-Ar H), 7.51–7.47 (d, J = 8.4 Hz, 16H, dendron terminal 3,5-Ar H), 6.70–6.61 (bs, 6H, dendron Ar H), 6.47–6.31 (m, 12H, dendron Ar H), 5.11 (s, 16H, ArCH2O), 4.12 (s, 8H, ArCH2O), 3.39 (s, 4H, ArCH2OSi). MS (MALDI-TOF): m/z 2313.5 calcd. for [M]+ 2380.4.

conclusIon

Consequently, we can develop the efficient sensitizer for singlet oxygen formation in water. It is important to introduce vertical dendrons to prevent aggregation for-mation followed by self-quenching of the triplet state. Higher generation of dendrimer is valuable to avoid

aggregation of Pcs even in water, which is similar envi-ronment of in vivo rather than organic solvent like THF. This approach is expected to develop more efficient sen-sitizer of singlet oxygen formation for photodynamic therapy.

Acknowledgements

This work was supported by a Grant-in-Aid for Sci-entific Research in a Priority Area “New Frontiers in Photochromism” (No. 471) and (No. 19550176) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan.

reFerences

1. Leznoff CC and Lever ABP. Phthalocyanines: Proper-ties and Applications, Wiley-VCH: New York, 1989.

2. McKeown NB. Phthalocyanine Materials: Synthe-sis, Structure and Function, Cambridge University Press: Cambridge, 1998.

3. Piechocki C, Simon J, Skoulios A, Guillon D and Weber P. J. Am. Chem. Soc. 1982; 104: 5245.

4. Kimura M, Nakada K, Yamaguchi Y, Hanabusa K, Shirai H and Kobayashi N. Chem. Commun. 1997; 1215.

5. Nishida M, Momotake A, Shinohara Y, Nishimura Y and Arai T. J. Porphyrins Phthalocyanines 2007; 11: 448.

6. Dirk CW, Inabe T, Schoch KF and Marks TJ. J. Am. Chem. Soc. 1983; 105: 1539.

7. Yang YC, Ward JR and Seiders RP. Inorg. Chem. 1985; 24: 1765.

8. Ohno T, Kato S and Lichtin NN. Bull. Chem. Soc. Jpn. 1982; 55: 2753.

9. Nishimura Y, Kaneko Y, Arai T, Sakuragi H, Toku-maru K, Kiten M, Yamamura S and Matsunaga D. Chem. Lett. 1990; 19: 1935.

10. Frink ME, Geiger DK and Ferraudi GJ. J. Phys. Chem. 1986; 90: 1924.

11. Mack J and Stillman MJ. J. Am. Chem. Soc. 1994; 116: 1292.

12. Mack J and Stillman MJ. J. Porphyrins Phthalocya-nines 2001; 5: 67–76.

13. Tanielian C and Wolff C. J. Phys. Chem. 1995; 99: 9831.

14. Nishimura Y, Kamada M, Ikegami M, Nagahata R and Arai T. J. Photochem. Photobiol. A 2006; 178: 150.

15. Schmidt R, Tanielian C, Dunsbach R and Wolff C. J. Photochem. Photobiol. A 1994; 79: 11.

16. Kumar CV, Qin L and Das PK. J. Chem. Soc. Faraday Trans. 2 1984; 80: 783.

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DOI: 10.1142/S1088424611002969



INTRODUCTION

Initially observed as unexpected byproducts [1, 2], now phthalocyanines, [MPc], are produced in million tons per year [3]. They are one of the major types of tetra-pyrrole derivatives showing a wide range of practical applications in various high technology fields such as nonlinear optics, photosensitizers, gas sensors, catalysis, liquid crystals,

optical data storage, electrodes in fuel cell, photoelectric conversion materials and others [4–8]. A great number of promising applications of phthalocyanines arise from the presence of the unique 18π-electron conjugated aromatic porphyrazine (PA) core which determines their remark-able optic and coordination properties and endows them with high thermal and chemical stability. Useful proper-ties of phthalocyanine materials can be tuned by struc-tural modification of the periphery of the macrocyclic ligand. Along with introduction of different appendages in benzene rings of phthalocyanine, the growing attention is now given to heterocyclic phthalocyanine analogues containing aromatic heterocycles instead of the benzene

Synthesis and spectral study of tetra(2,3-thianaphtheno)-porphyrazine, its tetra-tert-butyl derivative and their Mg(II), Al(III), Ga(III) and In(III) complexes

Ekaterina S. Taraymovich*a, Andrey B. Korzhenevskiia,†, Yulia V. Mitasovaa, Roman S. Kumeevb, Oscar I. Koifmana and Pavel A. Stuzhin*a

a Department of Chemistry and Technology of Macromolecular Compounds, Ivanovo State University of Chemistry and Technology, Ivanovo 153000, Russia b Institute of Solution Chemistry, Russian Academy of Science, Ivanovo 153000, Russia

Received 19 October 2010Accepted 17 December 2010

ABSTRACT: Starting from easily available thiophenols (PhSH (1a), 4-tert-butyl-PhSH (1b)) and oxalylchloride we have prepared 2,3-thianaphtenequinones 2a,b which were then successively converted to thianaphthene-2,3-dicarboxylic acids 4a,b their imides 10a,b, diamides 9a,b and finally to thianaphthene-2,3-dicarbonitriles 11a,b — the key precursors for the series of novel porphyrazines bearing four 2,3-annulated thianaphthene moieties. The free-bases 12a,b were obtained by cyclotetramerization of the dinitrile 11a,b in the presence of lithium in n-pentanol, while the reaction with magnesium(II) butoxide in n-butanol leads to the Mg(II) complex 13a. Complexes with Al(III) (14a,b), Ga(III) (14a,b) and In(III) (14a,b) were obtained by the template cyclotetramerization of the dinitriles 11a,b in a melt with the corresponding (hydroxy)acetates. Tetra(2,3-thianaphtheno)porphyrazine 12a and its metal complexes 13a–15a are only sparingly soluble in common organic solvents, the solubility is enhanced for their tert-butyl substituted derivatives 12b, 14b–16b. The study of the electronic absorbtion spectra has revealed that the extension of the porphyrazine π-chromophore by fusion of four thianaphthene fragments due to the angular type of their annulation (similar to that found in 1,2-naphthalocyanines) and negative inductive effect of the sulfur atoms has an effect on its spectral properties which is less than in the case of the isoelectronic naphthalene rings fusion and is comparable with the influence of four benzene rings in phthalocyanines.

KEYWORDS: 2,3-thianaphthene derivatives, porphyrazines, tetra(2,3-thianaphtheno)porphyrazines, magnesium(II), aluminium(III), gallium(III), indium(III), electronic absorbtion spectra.

*Correspondence to: Ekaterina S. Taraymovich, email: [email protected] and Pavel A. Stuzhin, email: [email protected], tel/fax: +7 4932-416693

†Deceased.

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SyntheSIS anD SPeCtral StuDy Of tetra(2,3-thIanaPhthenO) POrPhyrazIne 55

rings [9, 10]. Azaanalogues of phthalocyanine containing 6-membered N-heterocycle (pyridine or pyrazine) fused to the porphyrazine core instead of benzene rings (i.e. tetrapyrido- and tetrapyrazinoporphyrazines, [MPyPA] and [MPyzPA]) and their benzohomologues containing qui-noline and quinoxaline fragments are mostly well studied [9–13]. In the recent decade considerable advances have been also achieved in the synthesis and study of the chalco-gen analogues of pyrazinoporphyrazines containing S or Se atom instead of the pyrazine C=C bond, i.e. porphyrazines with annulated 1,2,5-thiadiazole or 1,2,5-selenadiazole rings [14]. Thiaanalogues of phthalocyanine — porphyra-zines with fused thiophene rings, [MThPA], which were first reported by Linstead in 1937 [15], since then have been only scarcerely studied [16–18]. Linstead and cowork-ers [15] have also observed formation of thiaanalogues of naphthalocyanine — upon melting of 2,3-thianaphthenedi-carbonitrile with metallic reagents (CuCl, Cu, AlCl3, Mg) green products were obtained. However, these species were not characterized and, to the best of our knowledge, since then no reports appeared on tetra(2,3-thianaphtheno)porphyrazine complexes [MSNc]. In our opinion, such porphyrazine macrocycles containing “mild” peripheral S-atoms with accessable d-sublevel might exhibit inter-esting and unusual properties and deserve more detailed investigation.

In this work, we report on the improved preparation of the precursors for tetra(2,3-thionaphtheno)porphyrazines

and on the synthesis and characterization of Mg(II), Al(III), Ga(III) and In(III) complexes. The latter might be especially interesting for investigation of their non-linear optical properties, since In(III) complexes of phthalocyanine and its analogues (porphyrazines, naph-thalocyanines) often exhibit enhanced performance as optical limiting materials [19–22].

EXPERIMENTAL

General

All chemicals were commercially available reagent grade species (Aldrich, Merck, Reakhim, Vecton, Tech-nolog) and used without further purification otherwise specified. Solvents such as n-butanol, DMF and pyridine were distilled before use. Infrared spectra were measured on IR-spectrometer AVATAR 360 FT-IR in KBr pellets, UV-vis spectra were recorded with Perkin Elmer spec-trometer Lambda 20, elemental analyses were performed on a CHN analyser Flash EA 1112. Mass-spectrometric measurents were carried out by mass-spectrometer Saturn 2000R (Varian Chrompack) for porphyrazine precursors and MALDI-TOF Bruker Ultraflex spectrometer for por-phyrazines. The melting points for porphyrazine precur-sors were determined using Buchi Melting Point B-540 apparatus.

Chart 1. Molecular structures of metal phthalocyanines and its heterocyclic and π-extended analogues

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56 e.S. taraymOvICh et al.

Preparation of precursors

4-tert-butylbenzenesulfonyl chloride. It was syn-thesized as described elsewhere [23]. Yield ca. 80%, mp 78–80 °C (lit. data 79–80 °C [24]). Anal. calcd. for C10H13SO2Cl (%): C, 51.61; H, 5.63; S, 13.78; O, 13.76. Found (%): C, 49.94; H, 6.10; S, 13.58; O, 13.95. IR (KBr): ν, cm-1 3070w and 3030w (ν(CH)Ar), 2970s and 2872m (ν(CH)tBu), 1926w, 1793w, 1664w, 1589s, 1477m, 1462m, 1402s, 1373vs (ν(SO2Cl)), 1298m, 1269m, 1236m, 1201s, 1176vs (ν(SO2Cl)), 1111s, 1080s, 1012m, 835m, 752m, 615s, 575s, 534s. MS: m/z 217 (100%, [M]+). 1H NMR (CDCl3): δ, ppm 8.00 (2H, m, ArH), 7.60 (2H, m, ArH), 1.36 (9H, s, tBu).

4-tert-butylbenzenethiol (1b). Zinc dust (22 g, 0.336 mol) was added to a mixture of chopped ice (125 g) and concentrated sulfuric acid (35 mL) under intensive stir-ring in a three-neck flask. Then 4-tert-butylbenzenesul-fonyl chloride (15 g, 0.06 mol) was immersed in small portions. Reaction mixture was stirred for 3 h and then refluxed 2 h on the water bath. The resulting suspension was cooled and the product was extracted in chloroform or ether. Extract was purged with water to neutrality, dried with CaCl2 and after evaporation of the solvent 4-tert-butylbenzenethiol was obtained as yellow oil. Yield 9.1 g (85%), bp 120–122 °C at 20 mmHg (lit. data 112–114 °C at 15 mmHg [24]). Anal. calcd. for C10H14S (%): C, 72.23; H, 8.49; S, 19.28. Found (%): C, 71.54; H, 7.99; S, 19.18. IR (KBr): ν, cm-1 3080w and 3030w (ν(CH)Ar), 2962vs, 2904m and 2868m (ν(CH)tBu), 2550w (ν(SH)), 1492s, 1398m, 1362m, 1267m, 1117s, 990s, 823s, 551m. MS: m/z 166 (45%, [M]+). 1H NMR (CDCl3): δ, ppm 7.54 (2H, m, ArH), 7.37 (2H, m, ArH), 3.37 (1H, s, -SH), 1.29 (9H, s, tBu).

2,3-thianaphthenequinone [2,3-dihydrobenzo[b]thiophene-2,3-dione] (2a). Oxalyl chloride (14.6 mL, 0.144 mol) was added to thiophenol (1a) (15.0 mL, 0.144 mol) under vigorous stirring which was continued till solid-ification of the reaction mixture. Then carbon disulfide (45 mL) was added, solution was cooled to 0–5 °C and anhy-drous AlCl3 (38.5 g, 0.288 mol) was immersed in portions to regulate acylation reaction. Stirring was continued for further 10 min and then the solvent was evaporated by heating on a water-bath at 42–45 °C. Dark residue was mixed with ice and kept overnight. The orange precipitate was filtered and treated with saturated aqueous NaHCO3 solution. Filtrate was collected and carefully acidified by concentrated HCl solution until complete precipitation of the bright orange product (yield 53%), mp 119–121 °C. Anal. calcd. for C8H4SO2 (%): C, 58.54; H, 2.41; S, 19.51; O, 19.51. Found (%): C, 58.03; H, 2.27; S, 19.95; O, 19.81. IR (KBr): ν, cm-1 1726m, 1714s (ν(C=O)), 1589m, 1468s, 1452m, 1360s, 1342s, 1282s, 1242m, 1147s, 1108s, 1060m, 962m, 843s, 748m, 449m.

5-tert-butyl-2,3-thianaphthenquinone [5-tert-butyl- 2,3-dihydrobenzo[b]thiophene-2,3-dione] (2b). It was prepared similarly to unsubstituted quinone 2a. Yield

7.6 g (63%), mp 103–105 °C. Anal. calcd. for C12H12SO2 (%): C, 65.43; H, 5.49; S, 14.56; O, 14.53. Found (%): C, 64.78; H, 4.88; S, 13.69; O, 13.98. IR (KBr): ν, cm-1 3409s (ν(COOH)), 3072m (ν(CH)Ar), 2964s, 2871m (ν(CH)tBu), 1708vs (ν(C=O)), 1596s, 1562m, 1477s, 1365m, 1288m, 1257m, 1201m, 1110m, 1004m, 919m, 850m. 1H NMR (CDCl3): δ, ppm 7.84 (1H, m, ArH), 7.69 (1H, m, ArH), 7.39 (1H, m, ArH), 1.33 (9H, s, tBu).

2-oxalylphenylthioglycolic acid [2-(2-carboxy-methylsulfanylphenyl)-2-oxoacetic acid] (3a). It was prepared by modification of the earlier reported proce-dure [25]. Solutions of 2,3-thionaphthenequinone 2a (5 g, 30.5 mmol) and chloroacetic acid (2.88 g, 30.5 mmol) each in 20 mL of 10% aqueous NaHCO3 were mixed together, heated for 10–15 min and cooled to 5 °C. Addi-tion of conc. aqueous HCl leads to precipitation of 3a as yellow product, which was filtered and dried. Yield 6.95 g (95%), mp 164–168 °C. Anal. calcd. for C10H8SO5 (%): C, 50.00; H, 3.33; S, 13.33; O, 33.3. Found (%): C, 48.31; H, 3.15; S, 13.65; O, 30.94. IR (KBr): ν, cm-1 3414m (ν(OH)), 1711s (ν(COOH)), 1676s (ν(C=O)), 1612s (ν(C=O)), 1591m, 1554vs, 1522vs, 1493vs, 1458s, 1412s, 1373m, 1333m, 1290s, 1271vs, 1244s, 1142m, 1107s, 1014w, 974m, 891w, 802m, 789s, 762vs, 723s, 689s, 638m, 575m, 480m, 446vs, 426s. 1H NMR (DMSO-d6): δ, ppm 3.89 (2H, s, -S-CH2-COOH), 7.36 (1H, t, ArH ), 7.49 (1H, d, ArH), 7.67 (1H, t, ArH ), 7.79 (1H, dd, ArH ).

5-tert-butyl-2-oxalylphenylthioglycolic acid {5-tert-butyl-2-[(carboxymethyl)thio]phenyl}(oxo)acetic acid (3b). It was prepared from quinone 2b similarly to unsubstituted acid 3a. After acidification of the reaction mixture with aqueous HCl, the product was extracted with chloroform to give 3b as a ductile yellow liquid. Yield 8.5 g (83%). Anal. calcd. for C14H16SO5 (%): C, 56.74; H, 5.44; S, 10.81. Found (%): C, 50.64; H, 5.71; S, 8.85. IR (KBr): ν, cm-1 2964s, 2906m, 2868w (ν(CH)tBu), 1708vs (ν(C=O)), 1596s, 1477m, 1468m, 1396m, 1363m, 1288m, 1263m, 1207m, 1169m, 1004m, 921m, 851m, 585m. 1H NMR (CDCl3): δ, ppm 8.56 (1H, m, Ar-H), 8.10 (1H, m, ArH), 7.72–7.54 (1H, m, ArH), 3.73 (2H, s, -S-CH2-COOH), 1.33 (9H, s, tBu).

Thianaphthene-2,3-dicarboxylic acid [benzo[b]thiophene-2,3-dicarboxylic acid] (4a). It was prepared by modification of the earlier reported procedure [25]. Acid 3a (6.95 g, 0.029 mol) was dissolved in 30% aque-ous NaOH solution (30 mL) with heating. After transfor-mation of the red solution to dense gruel of beige color, distillated water (40 mL) was added to dissolve the pre-cipitate. Then the solution was cooled and acidified with conc. aqueous HCl to precipite the acid 4a which was filtered and dried. Yield 5.21 g (81%), mp 248–252 °C. Anal. calcd. for C10H6O4S (222.21) (%): C, 54.05; H, 2.7; S, 14.41; O, 28.83. Found (%): C, 53.21; H, 2.41; S, 13.92; O, 27.45. IR (KBr): ν, cm-1 1675m (ν(COOH)), 1554m, 1492s, 1263s, 1243m, 1106m, 802s, 761m.

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1H NMR (DMSO-d6): δ, ppm 7.45–7.60 (2H, m, ArH), 8.00–8.15 (2H, m, ArH).

5-tert-butylthianaphthene-2,3-dicarboxylic acid [5- tert-butylbenzo[b]thiophene-2,3-dicarboxylic acid] (4b). It was prepared similarly to the unsubstituted acid 4a, but the reaction time was increased to 1 h. Yield 5.2 g (65%), mp 132–137 °C with decomposition. Anal. calcd. for C14H14SO4 (%): C, 60.43; H, 5.07; S, 11.51; O, 22.99. Found (%): C, 58.50; H, 4.87; S, 11.25; O, 20.11. IR (KBr): ν, cm-1 2965s, 2927m, 2871m (ν(CH)tBu), 2366w, 1841w, 1708vs (ν(COOH)), 1596s, 1562m, 1477s, 1467m, 1413m, 1398m, 1365m, 1299m, 1288m, 1259m, 1207m, 1110m, 1089m, 1047m, 1004m, 921m, 906w, 879w, 850w, 836w, 775w, 755w, 730w, 709w, 690w, 667w, 588w, 543w, 497w, 449w, 418vw. MS: m/z 277 (40%, [M]+). 1H NMR (CDCl3): δ, ppm 7.89 (2H, m, COOH), 7.70 (1H, m, Ar-H), 7.46 (1H, m, Ar-H), 7.39 (1H, m, Ar-H), 1.33 (9H, s, tBu).

Thianaphthene-2,3-dicarboxylic acid anhydride [1,3-dihydrobenzo[4,5]thieno[2,3-c]furan-1,3-dione] (6a). It was obtained following the procedure of Linstead [15] by refluxing of acid 3a in acetic anhydride for 1 h with yield ca. 95%, mp 171–173 °C. IR (KBr): ν, cm-1 3088w (ν(CH)arom), 1832vs and 1762vs (ν(-CO-O-CO-)), 1597m, 1563m, 1522s, 1470s, 1419m, 1394m, 1324m, 1267s, 1170m, 1151s, 1119s, 1050m, 1006m, 961w, 890s, 830s, 785s, 757s, 719s, 700m, 661m, 632m, 563m, 492m, 425m. Anal. calcd. for C10H5O3S (204.19) (%): C, 58.82; H, 1.96; S, 15.69; O, 23.53. Found (%): C, 57.91; H, 1.68; S, 14.18; O, 21.82.

Thianaphthene-2,3-dicarboximide [2,3-dihydro-1H- benzo[4,5]thieno[2,3-c]pyrrole-1,3-dione] (10a). Method A. Dry NH3 was bubbled through a stirred solu-tion of the anhydride 6a (2 g, 0.01 mol) in CHCl3 for 2–3 h at room temperature. This leads to precipitation of amidoacids 8a, which were filtered, dried, dissolved in DMF (10 mL) and treated with POCl3 (2.6 mL, 0.03 mol) in an ice bath overnight. The precipitate formed after pouring of the reaction mixture into water was filtered, dried and sublimed to give 0.98 g of the imide 10a (yield ca. 50%). Method B. Dicarboxylic acid 4b (5.79 g, 0.0261 mol) was refluxed with thionyl chloride (40 mL, 0.56 mol) for 6 h. Excess SOCl2 was removed under reduced pressure, the residue (see remark below) was dissolved in chloroform (40 mL) and dry ammonia gas was bubbled through the solution for 2–3 h. The obtained suspension was kept in a closed flask overnight. The precipitate containing a mixture of amidoacids 8a and diamide 9a (mp 178–192 °C, yield 3.1 g, 54%) was fil-tered, dried and dissolved in DMF (20 mL). The solu-tion was cooled in an ice bath to 0–5 °C and POCl3 (5.14 mL, 0.056 mol) was added in portions to keep the temperature at 5–7 °C. The stirring of the reaction mix-ture was continued for another 3 h and then it was left overnight. Precipitate formed upon pouring of the reac-tion mixture on ice was filtered, dried and washed with CHCl3. Evaporation of the extract gave 0.48 g (10%) of

dinitrile 11a (see characterization details below). Sub-limation of the insoluble residue gave the imide 10a as yellow crystals. Yield 2.1 g (39.5%), mp 235–240 °C. Anal. calcd. for C10H5NO2S (203.21) (%): (C 59.10% H 2.48% N 6.89% O 15.75% S 15.78%). Found (%): (C, 59.73; H, 2.56; N, 7.42, O, 13.11; S, 16.34). IR (KBr): ν, cm-1 3242s (ν(NH)), 3066m (ν(CH)Ar); 1834m, 1768s 1730s and 1709s (ν(imide), 1597w, 1520m, 1473m, 1429m, 1392m, 1329s, 1306s, 1259m, 1194m, 1173w, 1063m, 1047s, 995s, 947w, 899w, 856w, 841m, 787m, 752s, 721, 665m, 644m, 565m, 519m, 496m, 428s. MS: m/z 203 (100%, [M]+). 1H NMR (CDCl3): δ, ppm 7.5–7.6 (2H, m, ArH), 7.95–8.00 (1H, m, ArH), 8.2–8.3 (1H, m, ArH). Remark. The residue obtained after the reaction of dicarboxylic acid 4a with thionyl chloride is a mix-ture consisting from anhydride 6a (m/z = 204) and easily hydrolizable dichloroanhydride 7a (m/z = 259). mp 64 (partly) and 156–159 °C (completely). IR (KBr): ν, cm-1 3579m (ν(OH)), 3082m (ν(CH)Ar), 2093m, 1837vs and 1756vs (ν(-CO-O-CO-)), 1662s, 1633w, 1597s, 1562m, 1522vs, 1470vs, 1419s, 1394s, 1325s, 1267vs, 1218w, 1203w, 1171s, 1149vs, 1118vs, 1049m, 1005s, 960m, 891vs, 831vs, 785vs, 756vs, 731s, 719s, 700s, 661s, 633m, 563m, 513m, 487m, 484m, 426s. Anal. found (%): C, 53.24; H, 1.77; S, 15.00; O, 17.49. Calcd. (%) for C10H4SO3 (204.19): C, 58.82; H, 1.96; S, 15.69; O, 23.53; for C10H4ClSO2 (259.10) C, 46.36; H, 1.56; S, 12.37; O, 12.35. Separation of this mixture is wasteful and it was used as obtained in further ammonolysis.

5-tert-butylthianaphthene-2,3-dicarboximide [2,3-dihydro-7-tert-butyl-1H-benzothieno[2,3-c]pyrrole- 1,3(2H)-dione (10b). The synthetic method of substituted imide is the same as described above for 10a (method B). Yield 1.5 g (31%), mp 179–181 °C. Anal. calcd. for C14H13SNO2 (%): C, 64.84; H, 5.05; S, 12.36; N, 5.40; O, 12.34. Found (%): C, 64.30; H, 5.09; S, 12.13; N, 5.46. IR (KBr): ν, cm-1 3444m, 3211m (ν(NH)), 3058m (ν(CH)At), 2964m (ν(CH)tBu), 2904m, 2865m, 1765s, 1734vs (ν(C=O)imide), 1637m, 1545w, 1519m, 1459m, 1433m, 1394m, 1357m, 1331m, 1294s, 1255m, 1184m, 1103m, 1068m, 1039m, 997m, 883m, 819m, 740m, 725m, 704m, 681m, 640m, 519m, 451w, 428w. 1H NMR (CDCl3): δ, ppm 8.20 (1H, s, Ar-H ), 8.01 (1H, m, -CO-NH-CO-), 7.89 (1H, d, ArH ), 7.65 (1H, m, ArH ), 1.43 (9H, s, tBu).

Thianaphthene-2,3-dicarboxylic acid diamides (9). [Benzo[b]thiophene-2,3-dicarboxamides] (9a). Imide 10a or 10b (3.69 mmol) was suspended in the saturated ammonia solution (0.88 g/cm3, 30–40 mL). On the next day, the white precipitate of diamides 9a or 9b was fil-tered and dried. Diamide 9a. Yield 0.68 g (84%), mp 221–225 °C (lit. mp 204–205 °C [15]). Anal. calcd. for C10H8N2O2S (220.24) (%): C, 54.54; H, 3.64; S, 14.55; N, 12.73. Found (%): C, 54.84; H, 3.35; S, 14.61; N, 11.65. IR (KBr): ν, cm-1 3381s and 3269m (ν(NH2)), 3059w (ν(CH)Ar), 1668vs and 1616s (ν(C=O)amide), 1567s, 1519m, 1456w, 1404s, 1381w, 1313m, 1252w, 1147m,

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1117m, 1103m, 1051m, 937m, 881m, 800m, 733s, 704s, 667s, 648s, 636s, 471m, 434m, 407m. 1H NMR (DMSO-d6): δ, ppm 8.37 (2H, s, CONH2), 8.18 (2H, s, CONH2), 8.0–8.1 (1H, m, ArH), 7.9–8.0 (1H, m, ArH), 7.45–7.55 (2H, m, ArH). Diamide 9b. Yield 1.3 g (81%), mp 221–225 °C with decomposition. Anal. calcd. for C14H16SN2O2 (%): C, 60.85; H, 5.84; S, 11.60; N, 10.14. Found (%): C, 60.93; H, 5.63; S, 11.78; N, 9.35. IR (KBr): ν, cm-1 3332s and 3197s (ν(NH2)), 2963s and 2869m (ν(CH)), 1764m, 1722m, 1658vs and 1612s (ν(C=O)amide), 1525m, 1442s, 1398m, 1363m, 1325m, 1292m, 1259m, 1201m, 1160m, 1105m, 979w, 914w, 879w, 810w, 646m, 586m, 526w, 441w. 1H NMR (DMSO-d6): δ, ppm 8.42 (1H, s, Ar-H), 8.15–8.12 (2H, s, CONH2), 7.98 (1H, d, ArH), 7.87 (2H, s, CONH2), 7.63 (1H, m, ArH), 1.35 (9 H, s, tBu).

2,3-dicyanothianaphthenes [benzo[b]thiophene-2,3-dicarbonitriles] (11). POCl3 (2.5 mL, 0.028 mol) was added in portions to a solution of diamide 9a or 9b (3.09 mmol) in DMF (10–15 mL) at 0–5 °C. The reac-tion mixture was stirred further for 2 h, kept overnight and then poured on ice. The precipitate was filtered, washed with water, dried and sublimed. Dinitrile 11a. Yield 0.5 g (88%), mp 151–153 °C (lit. 148 °C [15]). Anal. calcd. for C10H4N2S (184.21) (%): C, 65.22; H, 2.17; S, 17.39; N, 15.22. Found (%): C, 64.43; H, 2.16; S, 17.03; N, 14.00. IR (KBr): ν, cm-1 2229vs (ν(C≡N)), 1982w, 1940w, 1817w, 1726m, 1589m, 1498s, 1460m, 1423m, 1348m, 1321w, 1267m, 1182m, 1157s, 1134m, 1020w, 993w, 955m, 864m, 764vs, 729s, 661w, 642m, 518m, 488m, 438s, 413m. MS: m/z 184 (100%, [M]+). 1H NMR (CDCl3): δ, ppm 7.69 (2H, m, ArH), 7.96 (1H, d, ArH), 8.08 (1H, d, ArH). Dinitrile 11b. Yield 0.95 g (84%), mp 96–98 °C. Anal. calcd. for C14H12SN2 (%): C, 69.97; H, 5.03; S, 13.34; N, 11.66. Found (%): C, 68.71; H, 5.01; S, 12.98; N, 10.98. IR (KBr): ν, cm-1 3086m and 3064m (ν(CH)Ar), 2964vs, 2837s and 2871s (ν(CH)tBu), 2229s (ν(C≡N)), 1934m, 1795vs, 1754m, 1653m, 1544m, 1510s, 1467s, 1440s, 1417s, 1367vs, 1317m, 1297m, 1253s, 1203m, 1159vs, 1105m, 1054m, 1016s, 925m, 916m, 879m, 831s, 777w, 755w, 734m, 690m, 669m, 592m, 517w, 499m, 441m. 1H NMR (CDCl3): δ, ppm 8.02 (1H, m, ArH), 7.91–7.88 (1H, m, ArH), 7.81 (1H, m, ArH), 1.45 (9H, s, tBu).

Preparation of tetra(2,3-thianaphtheno)porphyrazines

Free-bases [H2SNc] (12a) and [H2SNctBu4] (12b). Dinitrile 11a (0.5 g, 2.7 mmol) or 11b (0.5 g, 2.1 mmol) were added to a solution of lithium (0.05 g, 7 mmol) in freshly distilled n-amyl alcohol (15 mL), and the reaction mixture was refluxed until dark green color was achieved (4–6 h). The solvent was evaporated, the residue was washed with ethanol, 50% aqueous acetic acid, water, and dried. [H2SNc] (12a). Low solubil-ity prevents further purification. Yield 0.15 g (30%). UV-vis (CHCl3): λmax, nm (A/Amax) 325 (2.11), 365sh (1.64), 440sh, 629 (1.01), 642 (0.97), 694 (1). UV-vis

(pyridine): λmax, nm (A/Amax) 319 (2.87), 370sh (1.73), 631 (1.05), 649 (1.02), 696 (1). [H2SNctBu4] (12b). The product was purified by column chromatogra-phy on alumina (eluent: CHCl3). Yield 0.26 g (52%). Anal. calcd. for C56H50SN8 (963.30) (%): C, 69.82; H, 5.23; N, 11.63; S, 13.31. Found (%): C, 68.01; H, 5.86; N, 10.98; S, 12.98. UV-vis (CHCl3): λmax, nm (log ε) 329 (4.56), 636 (4.22), 653 (4.24), 702 (4.42). UV-vis (pyridine): λmax, nm (log ε) 328 (4.44), 640sh (4.12), 659 (4.28), 705 (4.42). IR (KBr): ν, cm-1 3288w (ν(NH)); 2956s, 2927s and 2858m (ν(CH3)); 1602m, 1571m, 1525m, 1456m, 1417s, 1373s, 1317m, 1297m, 1257m, 1155vs, 1072m, 1016s, 925m, 833m, 809w, 798w, 730w, 690m, 580w, 516w, 487w.

MgII complex [MgSNc] (13a). To a solution of mag-nesium chips (15 mg, 0.62 mmol) in dry butanol (15 mL) (dissolution was initiated by iodine crystal) dinitrile 11a (80 mg, 0.435 mmol) was added and the mixture was refluxed for 20 h. After evaporation of the solvent the residue was stirred with aqueous acetic acid (50% v/v, 30 mL) at room temperature for 30 min. The precipitate was centrifugated and washed thoroughly with water and methanol. After purification by column chromatography (silica, CHCl3) the MgII complex 13a was obtained . Yield 34 mg (42%). UV-vis (CHCl3): λmax, nm (A/Amax) 337 (1.96), 611 (0.43), 659 (1), 669 (1). UV-vis (pyridine): λmax, nm (A/Amax) 326 (1.34), 381 (1.19), 601 (0.34), 658 (0.98), 670 (1). IR (KBr): ν, cm-1 1654s, 1596s, 1523m, 1457s, 1407s, 1317m, 1267m, 1122s, 1095m, 1079m, 1043m, 917w, 858m, 750m, 725m.

Complexes with AlIII (14), GaIII (15) and InIII (16), [(HO)MSNc] and [(HO)SNctBu4]. General method of preparation. Mixture of the dinitrile 11a (0.5 g, 2.7 mmol) or 11b (0.5 g, 2.1 mmol) with the corresponding metal acetate ([Al(OAc)3] [26]), or hydroxodiacetate ([M(OH)(OAc)2] M = GaIII or InIII produced by “Reakhim”) in a 2:1 molar ratio was placed in a glass test tube and rapidly heated to 250 ºC in a metallic bath till complete solidi-fication (1–3 min). Unreacted dinitriles were extracted with diethyl ether and the residue purified by column chromatography on Al2O3 (eluent: chloroform).

[(HO)AlSNc] (14a). Yield 26%. UV-vis (CHCl3): λmax, nm (A/Amax) 331 (1.12), 383 (0.98), 468 (0.37), 611 (0.38) 665 (0.97), 673 (1). UV-vis (pyridine): λmax, nm (A/Amax) 387 (0.84), 470 (0.28) 609 (0.33), 670 (1.01), 678 (1). IR (KBr): ν, cm-1 1498m, 1462m, 1417s, 1402m, 1373m, 1319m, 1261m, 1155s, 1107m, 1076m, 1016m, 923m, 762m, 728m, 690w.

[(HO)AlSNctBu4] (14b). Yield 19%. Anal. calcd. for C56H49S4N8OAl + 2H2O (1041.31) (%): C, 64.59; H, 5.13; N, 10.76; S, 12.32. Found (%): C, 65.21; H, 5.04; N, 10.55; S, 12.08. UV-vis (CHCl3): λmax, nm (A/Amax) 335 (3.24), 628 (0.56), 684 (1). IR (KBr): ν, cm-1 2958vs and 2863m (ν(CH)tBu); 1636m, 1509m, 1463s, 1411s, 1384m, 1360s, 1296s, 1253s, 1222s, 1185s, 1099s, 1015m, 921m, 915s, 883m, 809s, 740s, 701m, 572m, 447m.

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[(HO)GaSNc] (15a). Yield 65%. UV-vis (CHCl3): λmax, nm (log ε) 334 (3.99), 378sh (3.88), 470sh, 616sh (3.66), 677 (4.08), 684 (4.08). UV-vis (pyridine): λmax, nm (A/Amax) 385 (0.73), 469sh, 611 (0.32), 678 (1). IR (KBr): ν, cm-1 3644m ν(OH), 3058m ν(CH)Ar, 1598s, 1498s, 1465s, 1421s, 1400s, 1309s, 1257s, 1228s, 1201s, 1189s, 1157s, 1128s, 1093s, 1060s, 1014s, 921m, 885s, 856m, 819m, 765s, 734s, 686s, 557s. MS (MALDI-TOF): m/z 805 [M - OH]+, 822 [M]+, 840 [M + H2O]+.

[(HO)GaSNctBu4] (15b). Yield 58%. Anal. calcd. for C56H49S4N8OGa + 2H2O (1084.04) (%): C, 62.05; H, 4.93; N, 10.34; S, 11.83. Found (%): C, 62.51; H, 4.84; N, 9.68; S, 11.68. UV-vis (CHCl3): λmax, nm (log ε) 341 (4.82), 369sh, 433 (4.49), 483 (4.49), 621 (4.27), 688 (4.92). IR (KBr): ν, cm-1 2960vs and 2865m (ν(CH)tBu), 1639s, 1506s, 1463s, 1407s, 1394s, 1361s, 1296s, 1255s, 1222s, 1197vs, 1180vs, 1105s, 1016m, 937m, 916s, 890m, 809s, 757m, 740s, 701m, 632m, 570m, 559m, 457m. 1H NMR (DMSO-d6): δ, ppm 8.22–8.06 (8H, m, ArH ), 7.89–7.75 (4H, m, ArH ), 1.39 (36H, s, tBu).

[(HO)InSNc] (16a). Yield 73%. UV-vis (CHCl3): λmax, nm (log ε) 333 (4.23), 382 (4.01), 483sh, 618 (3.74), 684 (4.35). UV-vis (pyridine): λmax, nm (A/Amax) 330sh, 391(0.68), 616sh (0.22), 683 (1). IR (KBr): ν, cm-1 3616m ν(OH), 3060w and 3023w ν(CH)Ar , 1558m, 1519m, 1492m, 1454m, 1398m, 1309m, 1255m, 1224m, 1195m, 1182m, 1157m, 1124s, 1106s, 1095s, 1064s, 1037m, 914w, 875w, 811vw, 763w, 730w, 698w. MS (MALDI-TOF): m/z 851 [M - OH]+, 868 [M]+.

[(HO)InSNctBu4] (16b). Yield 61%. Anal. calcd. for C56H49S4N8OIn + 3H2O (1147.16) (%): C, 58.63; H, 4.83; N, 9.77; S, 11.18. Found (%): C, 58.71; H, 5.04; N, 9.38; S, 10.98. UV-vis (CHCl3): λmax, nm (log ε) 347 (4.63), 386 (4.51), 420sh, 489 (4.24), 625 (4.26), 693 (4.94). IR (KBr): ν, cm-1 2962s, 2925s and 2867s (ν(CH)tBu); 1633s, 1514s, 1463s, 1415s, 1363s, 1296s, 1257s, 1222s, 1155s, 1088s, 1049s, 1016s, 917s, 885m, 809m, 740m, 701m, 597m, 574m, 435m. 1H NMR (CDCl3): δ, ppm 7.98–7.87 (8H, m, ArH), 7.68 (4H, m, ArH), 1.44 (36H, s, tBu).


Synthesis of the precursors

Template cyclotetramerization of dinitriles of ortho-dicarboxylic acids in the presence of metals or metal salts is the most convenient methodology for the preparation of porphyrazines with various annulated heterocycles [9, 27, 28]. In the original Linstead�s work [15] 2,3-thia-, 28]. In the original Linstead�s work [15] 2,3-thia-28]. In the original Linstead�s work [15] 2,3-thia-]. In the original Linstead�s work [15] 2,3-thia-naphthenedicarboxylic acid 4a, the key intermediate in the synthesis of the corresponding dinitrile 11a, was

prepared according to Friedlaender [25] from 2,3-thia-naphtenequinone 2a which in turn was synthesised star-ting from thioindoxyl (benzo[b]thiophen-3-ol, 5) using the 3-stage Mayer�s procedure [29] (Scheme 1). In our work we have prepared 2,3-thianaphthenquinone 2a in one stage from more easily available thiophenol (1a) and oxalyl chloride following the approach suggested by Dutta [30] for thiophene-fused phenanthrene. Taking into account the peculiarities of intramolecular acylation pro-cesses [31] we have optimized the conditions and carried out the reaction at 0–5 °C under vigorous stirring and short reaction time. As a result, the yield of the quinone 2a was increased from 13% to 53%.

While the dinitrile 11a could be easily obtained by dehydration of the diamide 9a (e.g. upon refluxing with acetic anhydride, 80–90% yield [15]), the conversion of the diacid 4a to its diamide 9a implicates some diffi-culties (Scheme 2). According to the procedure used in [15] the diacid 4a upon treatment with acetic anhydride was dehydrated to the anhydride 6a, which on heating with PCl5 was converted to the dichloroanhydride 7a with high overall yield (ca. 90%). However, ammonoly-sis of the dichloroanhydride 7a gave after recrystalliza-tion from water only 13% of the diamide 9a along with ca. 13% of amidoacids 8a from the mother liquid. The disadvantage of this synthetic approach and low overall yield of the dinitrile 11a (ca. 9% on the used diacid 4a), is evidently connected with high sensitivity of the dichlo-roanhydride 7a to hydrolysis. This leads to formation of considerable amounts of amidoacids 8a which were mainly lost during recrystallization. We have elaborated the improved synthetic procedure taken into account that the diamide 9a can be prepared by ammonolysis of the imide 10a with almost quantitative yield. According to Linstead�s report [15] the imide 10a is formed upon dis-tillation of amidoacids 8a with P2O5, but the attempts of

Scheme 1. Synthesis of 2,3-thianaphthendicarboxylic acid 4

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its direct preparation from the anhydride 6a by its melting with urea or ammonium carbonate gave unsatisfactory results. We have synthesized the imide 10a in two dif-ferent ways. (A) Ammonolysis of the anhydride 6a with dry NH3 in chloroform solution leads to precipitation of the mixture of aminoacids 8a, which upon dehydration with POCl3 in DMF gave the imide 10a (50% yield after sublimation) along with some amount of mononitrile. (B) In another procedure, the imide 10a was obtained from the diacid 4a in three stages by consecutive treatment with SOCl2, dry NH3, POCl3 in DMF and sublimation with 39–40% overall yield. In line with observations made in [15] we have found that treatment of the diacid 4a with SOCl2 leads to a mixture of the dichloroanhy-dride 7a with the anhydride 6a. It was partly melted at 58–65 °C and completely at 155–165 °C. Melting points for the dichloroanhydride 7a and the anhydride 6a repor-ted in [15] are 72 °C and 173 °C, respectively. We could not find conditions for complete conversion of the diacid 4a to the dichloroanhydride 7a under action of SOCl2 and separation of the anhydride 6a and the dichloroanhydride 7a was a wasteful procedure. Instead we have directly treated their mixture of with dry NH3 to obtain mixture of amidoacids 8a and the diamide 9a which were then dehydrated with POCl3 to the imide 10a and the dinitrile 11a. The latter is soluble in CHCl3 and could be isolated with ca. 10% yield (on the used diacid 4a). The reaction of the insoluble imide 10a with conc. aqueous NH3 solu-tion yielded the diamide 9a (84%) which upon treatment with POCl3 in DMF gave the target dinitrile 11a (88%).

So the overall combined yield of the dinitrile 11a from the diacid 4a in the “one-pot” procedure (B) (ca. 45%) was larger than that in the procedure (A) (30–35%), and much higher than in the original Linstead�s procedure (9–10%).

A similar strategy has being used for the synthesis of the tert-butyl substituted dinitrile 11b from tert- butylthophenol 1b which was prepared by reduction of tert- butylbenzenesulfochloride with zinc dust in concen-trated sulfuric acid.

Synthesis of porphyrazines

The dinitriles 11 afford tetra(2,3-thianaphtheno)por-phyrazines upon cyclotetramerization. Refluxing of the dinitriles 11 in n-amyl alcohol in the presence of lithium amylate leads to the dilithium complexes which upon isolation are demetalated to give the corresponding free-base macrocycles [H2SNc] (12a) or [H2SNctBu4] (12b). Cyclotetramerization of 11a in n-butanol in the presence of magnesium butylate gives the MgII complex [MgSNc] (13a). Complexes with GaIII (15a, 15b) and InIII (16a, 16b) were very easily produced with 65–75% yields by tem-plate cyclotetramerization of the dinitriles 11 when melt-ing with the corresponding hydroxydiacetate [M(OH)(OAc)2] in a metallic bath for 3–5 min at 170–250 °C. The AlIII complexes (14a, 14b) could be obtained from AlIII acetate only with 20–25% yield. Purification were troublesome for the free-base 12a and AlIII complex 14a due to their extremely low solubility in organic solvents, but complexes of GaIII (15a, 15b) and InIII (15a, 15b) as well as tert-butyl substituted species 12b, 14b–16b could be purified by column chromatography. Thianaphthene rings can be 2,3-annulated to four pyrrole rings of the porphyrazine macrocycle in a different manner and we obtained tetra(2,3-thionaphtheno)porphyrazines as a mixture of four randomers having different symmetry (2,3:2,3:2,3:2,3 – C4h, shown in Chart 1; 2,3:2,3:2,3:3,2 – Cs; 2,3:2,3:3,2:3,2 – C2v and 2,3:3,2:2,3:3,2 – D2h, see Chart 2). Unfortunately, our column chromatographic procedure was not effective enough for isolation of the individual randomers. The presence of the mixture of randomers leads to complex multiplets in the aromatic region of the 1H NMR spectra and to broadening or split-ting of the Q-band in the UV-vis spectra (see below). Ana-lytical data indicate that metal complexes 14b–16b were obtained as hydrated materials. All new porphyrazines have been characterised by UV-vis and IR spectroscopy, and some of them by MALDI-TOF mass-spectrometry and 1H NMR spectra.

As is usual for MgII complexes of phthalocyanine [32] and its heterocyclic analogues (see e.g. [33]) for the MgII complex 13a one water molecule is assumed to be coor-dinated [(H2O)MgSNc]. Complexes of AlIII, GaIII and InIII were obtained as hydroxo complexes [(HO)MSNc] (M = AlIII, GaIII, InIII). In the MALDI-TOF spectra recorded for 15a and 16a the intense peaks of [MSNc+]

Scheme 2. Conversion of 2,3-thianaphthenedicarboxylic acid 4 to dinitrile 11

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(100%) are accompanied by lower intensity peaks of hydroxocomplexes [(HO)MSNc+] (30% for 15a and 10% for 16a), and in the case of the GaIII complex 15a an additional peak of the aquahydroxo complex [(HO)(H2O)GaSNc+] is also seen in Fig. 1. In the IR spectra, the bands of the stretching OH vibrations ν(OH) can be seen at 3644 cm-1 for 15a and at 3616 cm-1 for 16a. Elemental analysis data obtained for 14b–16b provide evidence that these species were isolated as hydrated materials.

UV-vis spectra

Due to essentially planar structure of the macrocycle [H2SNc] (12a) and its metal complexes [MSNc] 13a–16a have low solubility and exhibit some tendency to aggre-gate. They are relatively well soluble in pyridine, DMF, α-chloronaphthalene and partly aggregated in CHCl3. The corresponding tert-butyl substituted derivatives [H2SNcBu4] (12b) and [MSNcBu4] (14b–16b) have better solubility and are less aggregated. The electronic absorp-tion spectra (UV-vis spectra) recorded for the free-bases 12a,b and for their metal complexes 13–16 in CHCl3 and/or in pyridine are shown in Fig. 2. Table 1 compares some

spectral characteristics of tetra(thianaphtheno)porphyra-zines 12-16 with the data for the related porphyrazines, phthalocyanines and naphthalocyanines taken from the literature.

UV-vis spectra obtained for the metal complexes of the present tetra(thianaphtheno)porphyrazines, [MSNc] and [MSNcBu4] (13–16) (see Fig. 2), are typical for porphyrazine and phthalocyanine metal complexes (see [9, 34, 35]) and contain the intense absorption bands in the far-red visible and in the UV-region. For the tert-butyl substituted species the absorption maxima are shifted slightly to the longer wavelength as compared to the unsubstituted tetra(thianaphtheno)porphyrazines. The low-lying HOMO→LUMO π-π* transitions of the porphyrazine π-chomophore are responsible for the appearance of the intense Q-band absorption at 665–695 nm, which is accompanied by vibronic sat-ellites on the blue side. It should be noted that unlike metal complexes of symmetrically substituted por-phyrazines and phthalocyanines which have a strict D4h symmetry of the π-chromophore and hence the degenerated eg* type LUMO, the present metal com-plexes of tetra(thionaphtheno)porphyrazines 13–16 have lower symmetry due to “angular” 2,3-annulation of

Chart 2. Structural formulae of four possible randomers of tetra(2,3-thianaphtheno)porphyrazines [MSNc] (12a–16a) and [MSNcBu4] (12b,14b–16a). M = 2H (12a,b), (H2O)Mg (13a), (HO)Al (14a,b), (HO)Ga (15a,b), (HO)In (16a,b). R = H (a), R = tert-Bu (b)

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thionaphthene moieties. The symmetry of four possible randomers is C4h, C2v, Cs or D2h (see Chart 2) and there-fore two LUMO is not degenerated. Indeed, the Q-band for the metal complexes [MSNcBu4] and [MSNcBu4] (13–16) is broader than for the symmetrical metal por-phyrazines [MPA] and phthalocyanines [MPc], and the splitting of the Q-band is definitely seen for the MgII complex 13a (658 and 670 nm in pyridine), for the AlIII complex 14a [(HO)AlSNc] (670 and 678 nm in pyrid-ine) and for the GaIII complex 15a [(HO)GaSNc] (677 and 684 nm in chloroform). A similar situation was observed [36] for the MgII complex of 1,2-naphthalo-cyanine [Mg1,2Nc] containing “angularly” fused naph-thalene units. Four randomers of this latter species were separated chromatographically and distinguished by slight differences in the Q-band maximum position and its splitting (up to 4 nm) taking into account the theo-retical prediction of the Q-band splitting values [37]. In our case, although we have observed some spectral dif-ferences in the successively eluted fractions of the broad band during column chromatography, the effective sep-aration of the individual randomers was not achieved. The observed splitting of the Q-band for [MSNc] does not exceed 12 nm, indicating that effective symmetry of the π-chromophore in metal complexes is only slightly lower than D4h. This is unlike the free-bases [H2SNc]

(12a) and [H2SNctBu4] (12b) which, due to the presence of the inequivalent pyrrole and pyrrolenine units, have effective sym-metry of the π-chromophore close to D2h and, as can be seen from Fig. 2, exhibit considerable splitting of the Q-band into two components (629 and 694 nm for 12a and 653 and 702 nm for 12b in CHCl3).

As can be seen from the data presented in Table 1, the Q-band maxima for the free-bases of the thianaphthene annulated por-phyrazines 12a,b and their metal complexes 13–16 is shifted bathochromically by ca. 80 nm as compared to the corresponding β-unsubstituted or tert-butyl-substituted of porphyrazines ([MPA] and [MPAtBu4], M = 2H, MgII) and by 30–40 nm as com-pared to the octaphenyl substituted species ([MPAPh8] M = 2H, Mg, ClAlIII, ClGaIII and ClInIII [38, 39]). Such bathochromic shift is a typical consequence of the exten-sion of π-chromophore by annulation of peripheral aromatic fragments [34]. The bathochromic shift of the Q-band (∆λQ) caused by 2,3-fusion of four 10π-electron thianaphthene moieties (∆λQ = 79 nm for [H2SNc]) is larger than the effect of four 6π-electron thiophene rings (∆λQ = 59 nm for [H2

2,3ThPA] [17]), but comparable with annulation of benzene rings in phthalocy-

anines (∆λQ ca. 80 nm for [H2Pc] [35, 40]. The spectral data available for the MgII complexes indicate that the bathochromic shift upon 2,3-fusion of four thianaph-thene fragments (∆λQ = 86 nm for [MgSNc]) is slightly less that the effect of isoelectronic 1,2-fusion of the naphthalene rings (∆λQ = 96-107 nm for [Mg1,2Nc] [36]), but much less that the effect of 2,3-nahthalene annulation (∆λQ = 190 nm for [Mg2,3Nc] [41]). These facts show that benzene rings of the thionaphthene units are not effectively involved in conjugation with the central porphyrazine π-chromophore. This is due to the “angular” type of annulation also observed in 1,2-naphthalocyanines. Smaller bathochromic shift for [MSNc] as compared to [M1,2Nc] species is consist-ent with the electron-withdrawing effect of the more electronegative S-atom as compared to the ethylene CH=CH unit.

The maximum of the Q-band for the InIII complexes is shifted slightly bathochromically as compared with the MgII, AlIII and GaIII complexes. This is typical for complexes of porphyrazines [MPAPh8] [38, 39] and phthalocyanines [MPc] [35, 42] (see Table 1) and can be explained by larger ionic radius of InIII which causes doming of the macrocyclic ligand and destabilization of its HOMO.

The broad absorption band in the UV-region has maximum at 320–340 nm and maximum or shoulder

Fig. 1. MALDI-TOF mass-spectrum of the GaIII complex 15a and comparison of the experimental and calculated isotopic distribution for the molecular ion peaks

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at 380–390 nm. The latter can be assigned to the por-phyrazine Soret band due to strongly mixed transitions from the lower lying occupied π-MOs to LUMO. Indeed for unsubstituted porphyrazines the Soret band

appear near 330 nm (326 nm in [MgPA] [43]), but shifted bathochromically upon introduction of substit-uents in pyrrole rings (376 nm for [MgPAPh8] [44]) and/or extention of the conjugated π-system by annu-lation (377–381 nm for [Mg1,2Nc] [36]). In the spec-tra of metal porphyrazines and phthalocyanines only single maximum is present in the 300–400 nm region. The second UV maximum observed for [MSNc] at 320–340 nm can be therefore assigned to the π-π* transitions localized on the thionaphthene fragments. The broad lower intensity absorption at 450–500 nm can be tentatively assigned to the charge transfer from thionaphthene units to the central porphyrazine macrocycle.

CONCLUSION

We have modified and improved the procedure for the preparation of thianaphthene-2,3-dicarbonitriles — the key intermediates for template synthesis of thianaph-thene annulated porphyrazines. This allows us to prepare the symmetrical tetra(2,3-thianaptheno)porphyrazine, its tetra-tert-butyl substituted derivative and their complexes with MgII, AlIII, GaIII and InIII, which were spectroscopi-cally chasracterized. It is shown that the extension of the porphyrazine π-chromophore by annulation of four thianaphthene fragments have only a moderate effect on its spectral properties which is comparable with fusion of four benzene rings in phthalocyanines. This is due to the angular type of their annulation (similar to that found in 1,2-naphthalocyanines) and negative inductive effect of the sulfur atoms. The effect of thianaphthene annula-tion on the acid-base properties of porphyrazine macro-cycle and on the non-linear spectral properties (optical limiting) is currently under investigation. Our interest on new classes of annulated porphyrazines is also related to possibilities of further structural modification of thianaphthene fragments.

Fig. 2. UV-vis spectra of tetra(2,3-thianaphtheno)porphyrazines 12a–16a (solid and dotted lines) and 12b, 15b, 16b (dashed lines) in pyridine (solid lines) and in chloroform (dotted and dashed lines). Absorption maxima for tert-butyl substituted porphyrazines 12b, 15b, 16b are given in italics

Table 1. Position of the Q-band (λmax, nm) in the UV-vis spectra of tetra(2,3-thianaphtheno)porphyrazines [MSNc] and related porphyrazine, phthalocyanine and naphthalocyanine analogues

Porphyrazine 2H MgII AlIII GaIII InIII Reference

[MPA] 545, 617a 584b [43]

[MPAtBu4] 547, 620c 591c [35]

[MPAPh8] 599, 664d 637d 636e 636e 642e [38, 40, 44]

[MPc] 665, 698a 675f 691g 700g 690d [35, 40, 42]

[MPctBu4] 668, 703g 678h 683i 689j 697d [35, 42]

[M2,3Nc] 780g 776g 815g 815g 823d [19, 42]

[M1,2Nc] 678, 691i,k 694l 703l [35, 36]

[MSNc] 629, 642, 694d 658, 670f 670, 678f 678f 683f this work

[MSNctBu4] 636, 653, 702d 684d 688d 693d this work

[M2,3ThPA] 632, 676m [17]

aChlorobenzene. bMeOH. cHexane. dCHCl3. eCH2Cl2.

fPyridine. g1-Chloronaphthalene. giso-AmOH. iDMSO. jMethyl methacrylate. kD2h randomer. lo-Dichlorobenzene. mTetrahydrofurane.

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Acknowledgements

This work was supported by the state grants of the President of Russian Federation for leading scientific schools (NSh-2642.2008.3) and young candidates of sciences (MK-4752.2009.3), as well by the Federal Pro-gram “Scientific and Educational Personnel of Innovative Russia” (state contracts 02.740.11.0116, P-1105, P-2077). The authors are thankful to Dr. Y. Enakieva from the Institute of Physical Chemistry Russian Academy of Sciences (Moscow) for mass-spectrometric measurements.

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DOI: 10.1142/S1088424611002957



INTRODUCTION

DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane) was widely used for three decades as a potent pesticide for controlling malaria, typhus and other insect borne diseases but its use in agriculture has been banned in many countries since the mid-1970s because its exposure to human and wildlife can result in severe health prob-lems [1–4]. Unfortunately, DDT is resistant to degrada-tion in the environment and is still present in significant quantities in soil and water at numerous locations around the world. For this reason, research and development into new techniques for the degradation of DDT by various

methods of dechlorination have attracted a great deal of interest in recent years [5–15].

It has long been known that macrocycles containing transition-metal ions can be used as catalysts for the reductive dehalogenation of organohalides [16–31]. One example is vitamin B12 [15] which has been examined as a catalyst for the biomimetic reductive dechlorina-tion of chlorinated pollutants such as DDT. The reduced Co(I) form of this porphyrin-like macrocycle can act as a supernucleophile which reacts with organohalides to form alkylated Co(III) complexes while also accom-plishing dehalogenation [15, 25–28]. Other examples of cobalt macrocycles which have been studied as dehalo-genation catalysts include phthalocyanines [19, 32] and porphyrins [16, 33].

Most cobalt(III) and cobalt(II) porphyrins can be reversibly reduced to their Co(I) forms under the applica-tion of an applied potential [34–37] and the compound in

Reductive dechlorination of DDT electrocatalyzed by synthetic cobalt porphyrins in N,N′-dimethylformamide

Weihua Zhua, Yuanyuan Fanga, Wei Shena, Guifen Lua, Ying Zhanga, Zhongping Ou*a◊ and Karl M. Kadish*b◊

a School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, P. R. China b Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA

Received 24 September 2010Accepted 21 December 2010

ABSTRACT: Two cobalt porphyrins, (OEP)CoII and (TPP)CoII, where OEP and TPP are the dianions of octaethylporphyrin and tetraphenylporphyrin, respectively, were examined as electrocatalysts for the reductive dechlorination of DDT (1,1-bis(4-chlorophenyl)-2,2,2-trichloroethane) in N,N′-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). No reaction is observed between DDT and the porphyrin in its Co(II) oxidation state but this is not the case for the reduced Co(I) forms of the porphyrins which electrocatalyze the dechlorination of DDT, giving initially DDD (1,1-bis(4-chlorophenyl)-2,2-dichloroethane), DDE (1,1-bis(4-chlorophenyl)-2, 2-dichloroethylene) and DDMU (1,1-bis(4-chloro phenyl)-2-chloroethylene) as determined by GC-MS analysis of the reaction products. A further dechlorination product, DDOH (2,2-bis(4-chlorophenyl)ethanol), is also formed on longer timescales when using (TPP)Co as the electroreduction catalyst. The effect of porphyrin structure and reaction time on the dechlorination products was examined by GC-MS, cyclic voltammetry, controlled potential electrolysis and UV-visible spectroelectrochemistry and a mechanism for the reductive dechlorination is proposed.

KEYWORDS: cobalt porphyrins, electrochemistry, spectroelectrochemistry, DDT, electrocatalysis, reductive dechlorination.


*Correspondence to: Zhongping Ou, email: [email protected], tel: +86 510-88791928, fax: +86 510-88791800 and Karl M. Kadish, email: [email protected], tel: +1 (713)-743-2740

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http://dx.doi.org/10.1142/S1088424611002957

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ReDuCtIve DeChlORInatIOn Of DDt eleCtROCatalyzeD by SynthetIC CObalt PORPhyRInS 67

this oxidation state will react with alkyl halides (RX) to give the σ-bonded Co(III) porphyrin [31, 38] which can be further reduced to its Co(II) form as shown by Equa-tions 1–4 where Por represents the dianion of a given porphyrin. The dechlorination step occurs in reaction 3.

[(Por)Coiii]+ + e →← (Por)CoII (1)

(Por)Coii + e →← [(Por)CoI]- (2)

[(Por)Coi]- + RX →← (Por)Coiii(R) + X- (3)

(Por)Coiii(R) + e →← [(Por)CoII (R)]- (4)

If the resulting Co(III) or Co(II) σ-bonded porphyrins formed in Equations 3 and 4 are unstable, a cleavage of the cobalt-carbon bond will occur to generate the mono-dechlorinated axial ligand in its anionic or radical forms (R- or R•) and [(Por)CoIII]+ or (Por)CoII, the latter of which is immediately reduced to its Co(I) form (Equation 2) under the application of an applied potential sufficient to reduce the homogeneously generated (Por)Co(R) (Equa-tion 4). The newly formed [(Por)CoI]- can then further react with the original organohalide compound in solution or it can react with its dechlorinated product if it also con-tains a halogen group. This suggests that simple synthetic cobalt(II) porphyrins might be used as efficient catalysts for the electroreductive multiple dechlorination of DDT.

This is examined in the present paper using (OEP)CoII and (TPP)CoII as electrocatalysts, where OEP and TPP

are the dianions of octaethylporphyrin and tetraphenyl-porphyrin, respectively (Chart 1a). Each porphyrin was reduced to its cobalt(I) form and reacted with DDT in solutions of N,N′-dimethylformamide (DMF) containing 0.1 M tetra-n-butylammonium perchlorate (TBAP). The resulting dechlorination products are shown in Chart 1b and were identified by GC-MS analysis. The effect of porphyrin structure and reaction time on the dechlorina-tion products were also examined by cyclic voltammetry, controlled potential electrolysis and UV-visible spec-troelectrochemistry and a mechanism for the reductive dechlorination is proposed.

ExpERImENTAl

Chemicals

The cobalt porphyrins (OEP)Co and (TPP)Co and the chlorinated hydrocarbons, 1,1-bis(4-chlorophenyl)- 2,2,2-trichloroethane (DDT), 1,1-bis(4-chlorophenyl)-2, 2-dichloroethane (DDD), 1,1-bis(4-chloro-phenyl)-2,2- dichloroethylene (DDE) and 1,1-bis(4-chlorophenyl)-2- chloroethylene (DDMU) were purchased from Sigma-Aldrich and used as received. N,N-dimethylformamide (DMF) was purchased from Sigma-Aldrich and used as received or from Shanghai Guoyao Co. and freshly dis-tilled before use. Tetra-n-butylammonium perchlorate (TBAP) was purchased from Fluka Chemical Company and used without further purification. Other solvents were of analytical grade and obtained from Shanghai Guoyao Co.

Instrumentation

Cyclic voltammetry was performed in a three- electrode cell using a BiStat or Chi-730C electrochemistry sta-tion. A glassy carbon disk electrode was utilized as the working electrode while a platinum wire and a saturated calomel electrode (SCE) were employed as the counter and reference electrodes, respectively. An “H” type cell with a fritted glass layer to separate the cathodic and anodic sections of the cell was used for bulk electroly-sis. The working and counter electrodes were made from platinum mesh and the reference electrode was an SCE. Both the working and reference electrodes were placed in one compartment while the counter electrode was in other compartment of the cell.

Before GC-MS measurements, a pre-treatment proce-dure for removing TBAP from the electrolysis solution was utilized. The solution was transferred into a 150 mL flask and evaporated to remove the DMF solvent under vacuum at 45 °C in a roto-evaporator. Cyclohexane was then added in the flask containing the residue to extract DDT and its possible dechlorinated products. After stir-ring and centrifuging, the TBAP supporting electro-lyte was removed and the solution was subjected to the GC-MS analysis.

Chart 1. Structures of (a) cobalt porphyrins and (b) DDT along with its dechlorinated products

N N

N

N

N

N

N

NCoCo

(OEP)Co

(a) Cobalt porphyrins

DDE

(b) DDT and its dechlorinated products

C

H

CCl3

Cl Cl

C

H

HCCl2

Cl Cl

C

HCCl

Cl Cl

C

CCl2

Cl Cl

(TPP)Co

DDMU

DDT

DDD

C

CH2OH

Cl Cl

DDOH

H

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68 W. zhu et al.

GC-MS system Model HP6890-GC and HP5975-MSD was utilized to separate and identify the products of catalytic reductive dechlorination. The GC was equipped with an HP-5 5% phenyl methyl siloxane column (length 30 m, ID 250 μm, film 0.25 μm). Temperature pro-gramming was applied to the GC analysis. The initial temperature was 100 °C and increased at a rate of 15 °C/min to 300 °C. An area-normalized method was applied for analysis of the peaks. The molar response factor of DDT was defined as 1.00 and the relative values for DDE, DDD and DDMU were measured as 1.076 ± 0.039, 0.934 ± 0.053, 1.003 ± 0.031, respectively.

UV-visible spectroelectrochemical measurements were performed with a home-made optically transparent thin-layer cell with Pt mesh as the working electrode. The potential was applied using a BiStat or Chi-730C poten-tiostat. Time-resolved UV-visible spectra were recorded with a HP 8453A diode array spectrophotometer. All electrochemical and spectroelectrochemical measure-ments were carried out under a nitrogen atmosphere.

RESUlTS AND DISCUSSION

Cyclic voltammetry of (por)CoII in the absence and presence of DDT

The electrochemical properties of (OEP)CoII and (TPP)CoII were previously characterized in DMF con-taining 0.1 M TBAP [39, 40]. Both porphyrins undergo a reversible one-electron reduction under these experi-mental conditions to give [(Por)CoI]- at E1/2 = -0.78 V (Por = TPP) or -0.99 V (Por = OEP) (see Equation 2). A one-electron reduction is also seen in other non-aque-ous solvents and this process may be followed by elec-trogeneration of a cobalt(I) porphyrin π-anion radical at potentials close to the negative limit of the non-aqueous solvent [36, 41–49].

The CoII/CoI reaction of (OEP)Co and (TPP)Co becomes irreversible in DMF when DDT is added to the solution as shown by the cyclic voltammograms in Figs 1 and 2. At the same time, the reduction peak cur-rent for the CoII/CoI process increases with increasing concentration of DDT while the reverse anodic peak for re-oxidation of [(Por)CoI]- disappears. This is consis-tent with the occurrence of a catalytic process involving DDT and the electrogenerated Co(I) porphyrin to give a σ-bonded Co(III) derivatives as shown in Equation 3. Similar oxidative addition reactions have been shown to occur between simple alkyl or aryl halides and the elec-trogenerated Co(I) form of different cobalt macrocycles [15, 25–30, 38, 46, 50].

Additional irreversible reduction peaks are observed following the CoII/CoI reduction of (Por)CoII in the DMF solutions containing DDT. These are indicated by aster-isks in the cyclic voltammograms of Figs 1 and 2 and are assigned to irreversible reductions of one or more

σ-bonded Co(III) porphyrins which are generated from the reaction of [(Por)CoI]- and DDT. For example, (TPP)CoIII(R) where R = CH3, C2H5 or CH2Cl is reduced at E1/2 ≈ -1.40 V in CH2Cl2 [46] and this potential is about 600 mV more negative than for reduction of (TPP)CoII (E1/2 = -0.78 V) under the same solution conditions.

The increase in reduction peak current for the CoII/CoI process of (Por)Co as a function of increasing DDT concentration is shown in Fig. 3, where ip and ip0 are the measured cathodic peak currents obtained from cyclic voltammograms of the porphyrin in the presence and absence of DDT, respectively. The measured ip/ip0 ratio is larger for (OEP)Co than for (TPP)Co in DMF solutions containing a given DDT concentration and suggests that the rate to regenerate the corresponding catalyst is larger for [(OEP)CoI]- than [(TPP)CoI]-. This observation is not consistent with the thermodynamic ease for generating

Fig. 1. Cyclic voltammograms of (OEP)Co (1.15 × 10-3 M) with added 0.0–1.0 eq DDT and DDT (0.02 M) in the absence of catalyst in DMF containing 0.1 M TBAP. Scan rate = 0.10 V/s. The peak with as asterisk is associated with a product of the initial reduction

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the Co(I) species since the first reduction of (OEP)Co is located at E1/2 = -0.99 V while (TPP)Co is reduced to [(TPP)CoI]- at -0.78 V, implying that factors other than thermodynamic potentials dominate the catalytic process.

Controlled-potential electrolysis and GC-mS ana lysis

In order to determine the products of DDT dechlori-nation, controlled-potential bulk electrolysis was carried out in DMF solutions containing 2.0 × 10-4 M (Por)CoII, 0.1 M TBAP and 15 eq DDT (3.0 × 10-3 M). The bulk electrolysis potential was set at -1.70 V which is suffi-cient to generate the singly reduced species [(Por)CoI]- and also to reduce the expected homogeneously generated σ-bonded Co(III) porphyrin products.

Because the TBAP supporting electrolyte and its decomposition components formed during the high tem-perature GC process will block the column and prevent determination of the DDT dechlorination products, it was necessary to first remove all TBAP from solution before it could be examined by GC analysis. The procedure adopted for solution “pre-treatment” in this present work is shown in Scheme 1. After each hour of electrolysis at a given reduction potential, 1.0 mL solution was removed from the H-cell and pretreated for GC-MS analysis. Most of the TBAP in solution was successfully removed after which the product distribution of DDT and its dechlorinated products was determined by GC-MS. Well-defined GC data from these solutions were obtained as shown by the examples in Figs 4 (OEP) and 5 (TPP). The distribution of DDT and its dechlorinated products, DDD, DDE, DDMU and DDOH were calculated from the chromatographic data and are summarized in Table 1. The distribution as a function of time is also graphically illustrated in Fig. 6.

As seen from Table 1 and Fig. 6, 15 eq DDT are com-pletely removed from the solution within 6–8 h after start-ing bulk electrolysis at an applied potential of -1.70 V. Three main DDT dechlorinated products (DDD, DDE and DDMU) are detected during the first 6 h of electroly-sis, when using (OEP)Co as the catalyst and an additional dechlorinated product, DDOH, is also seen in the case of (TPP)Co. These four dechlorinated products were the only ones observed after 4 h or longer of electrolysis time (see Fig. 6b).

The DDT distribution decreased as expected with increased reaction time and no DDT remained in solu-tion after 6–8 h of bulk-electrolysis. The measured dis-tribution of DDD was 11.1% after one hour in solutions of (OEP)Co and 8.7% in solutions of (TPP)Co. Both solutions showed a maximum amount of DDD formation (~30%) after 4 h and then this value leveled off as seen in Table 1 and Fig. 6.

The DDE distribution was 6.1% after one hour of electrolysis with the (OEP)Co catalyst, 13% with the (TPP)Co catalyst and in both cases reached a maximum of about 30% before decreasing at longer times. The

Fig. 2. Cyclic voltammograms for the first reduction of (TPP)Co (7.49 × 10-4 M) with added 0.0–2.0 eq DDT in DMF con-taining 0.1 M TBAP. Scan rate = 0.10 V/s. The peak with an asterisk is associated with a product of the initial reduction

4.5

3.5

2.5

1.5

0.50.0 0.2 0.4 0.6 0.8 1.0

(OEP)Co

(TPP)Co

i p/i p

0

[DDT]/[(Por)Co]

Fig. 3. Plot of ip/ip0 vs the ratio of [DDT]/[(Por)Co], where ip and ip0 are the measured cathodic peak currents obtained from cyclic voltammograms for the first reduction of (Por)Co in the pres-ence and absence of DDT in DMF containing 0.1 M TBAP

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70 W. zhu et al.

same time, the DDMU distribution increased continu-ously over the 8 h of electrolysis and ranged from 4.1% after one hour in the case of (OEP)Co to 44.7% after 6 h. This distribution data implies that DDE is initially formed after which DDMU is generated at longer times of electrolysis.

Similar changes in the DDD and DDE dis-tribution were obtained for the two porphyrins as a function of time but different distribution are seen for DDMU and DDOH, the latter of which is only seen for the (TPP)Co catalyst and is accompanied by a decrease in the dis-tribution of DDMU after 8 h of electrolysis.

It summary, three main dechlorinated products are detected when DDT reacts with singly reduced (OEP)Co but four products are seen when using (TPP)Co as the catalyst. Furthermore, the type, number and the distri-bution of the dechlorinated products depends not only on the catalyst used but also varies with the reaction time.

Spectroelectrochemical monitoring of the reduction products and proposed mecha-nism for dechlorination of DDT

The fate of [(Por)CoI]- electrogenerated from (Por)Co in the presence of DDT was spectroelectrochemically monitored to pro-vide a better understanding of the DDT reduc-

tive dechlorination mechanism. The UV-visible spectra of [(Por)Co]n+ where n = 3, 2 or 1 are known for Por = OEP and TPP as are the spectra for (TPP)Co(R) where R is a simple alkyl or aryl group. Thus the progress of the catalytic reaction could be monitored by measuring

Solution of(Por)Co, DDT, TBAP, DMF

Electrolysis(Controlled-potential)

Solution(after electrolysis)

Solvent(DMF)

Residue

Residue(Porphyrins,TBAP)

Solution for GC-MS analysis(DDT and its dechlorinated products)

Rotating evaporation(45oC under vacuum)

(i) Cyclohexane extraction (ii) Centrifuging

Rec

ycle

Scheme 1. The procedure for reductive dechlorination of DDT catalyzed by cobalt(II) porphyrins

Fig. 5. GC analysis of the DDT and its dechlorinated products during the controlled-potential electrolysis at -1.70 V using (TPP)Co as the catalyst

Fig. 4. GC analysis of the DDT and its dechlorinated products during the controlled-potential electrolysis at -1.70 V using (OEP)Co as the catalyst

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the UV-visible spectra as a function of time during the electroreduction.

Neutral (OEP)Co in DMF has a Soret band at 393 nm and two visible bands at 525 and 547 nm while (TPP)Co is characterized by a Soret band at 414 nm and a broad visible band at 520 nm. The two cobalt(I) porphyrins both display a split Soret band and these spectra are observed during the CoII/CoI process in the absence of DDT as shown in Fig. 7 where λmax = 354 and 414 nm for [(OEP)CoI]- and at 365 and 432 nm for [(TPP)CoI]-. Similar spectral transformations have been reported in

the literature for reduction of related cobalt(II) porphy-rins in other non-aqueous solvents [44, 47].

No spectral changes are seen when DDT is added to solutions containing unreduced (OEP)CoII or (TPP)CoII, indicating the lack of a reaction with the neutral cobalt porphyrins. However, the spectra of the electroreduced compounds change significantly in solutions containing DDT. As seen from Fig. 7, the Soret band of (OEP)Co (393 nm) and (TPP)Co (414 nm) both decrease in intensity during controlled potential reduction, but the split Soret band characteristic of Co(I) formation is not observed in

Table 1. Analysis data of DDT and its dechlorinated products obtained at different reaction time during the controlled-potential electrolysis at -1.70 V

Compound Rxn time, h Distribution, %a

DDT DDD DDE DDMU DDOH

(OEP)Co 0.0 100.0 0.0 0.0 0.0 0.0

1.0 78.8 11.1 6.1 4.1 0.0

2.0 58.1 19.7 12.3 9.9 0.0

3.0 30.1 27.5 25.6 16.8 0.0

4.0 15.5 30.2 28.7 25.6 0.0

5.0 3.9 29.9 30.6 35.6 0.0

6.0 0.0 30.2 25.1 44.7 0.0

(TPP)Co 0.0 100.0 0.0 0.0 0.0 0.0

1.0 72.0 8.7 13.0 6.2 0.0

2.0 55.4 16.8 16.9 10.9 0.0

3.0 32.2 24.4 23.2 20.2 0.0

4.0 15.2 27.7 27.8 26.3 3.0

5.0 4.1 29.0 29.6 33.5 3.7

8.0 0.0 30.6 13.4 28.9 27.2

a Distribution (%) = [(moles of a specific compound)/(moles of all compounds)] × 100%.

100

80

60

40

20

0

0 1 2 3 4 5 6

(a) (OEP)CoII

Dis

trib

utio

n of

pro

duct

s (%

)

(b) (TPP)CoII

time (h)

100

80

60

40

20

0

Dis

trib

utio

n of

pro

duct

s (%

)

time (h)10 2 3 4 5 6 7 8

DDT

DDD

DDE

DDMU

DDOH

DDT

DDD

DDE

DDMU

Fig. 6. Distribution of DDT and its dechlorinated products obtained in DMF at different electrolysis time during controlled-potential reduction of (a) (OEP)Co and (b) (TPP)Co at -1.70 V

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72 W. zhu et al.

Fig. 7. Thin-layer UV-visible spectral changes obtained during the first reduction of (a) (OEP)CoII and (b) (TPP)CoII without or with DDT in DMF containing 0.1 M TBAP

CoIII CoICoII

CoI C HC

Cl

Cl

Cl

Cl

ClCoIII C HC

Cl

Cl

Cl

ClCl-

CoIII C HC

Cl

Cl

Cl

Cl

CoII C HC

Cl

Cl

Cl

Cl

C HC

Cl

Cl

Cl

Cl

C HC

Cl

Cl

Cl

Cl

H

H+

DDD

DDT

C HC

Cl

Cl

Cl

Cl

CC

Cl

Cl

Cl

ClH+

+

e e

+

[(Por)CoIII]+

(Por)CoII

DDE

(1)

(2)

(3)

Co = (Por)Co

+ e

(Por)CoII

Scheme 2. Proposed mechanism of DDT dechlorination catalyzed by cobalt porphyrins

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the solutions containing DDT. Instead the final spectrum obtained from the reduction of (OEP)Co or (TPP)Co displays a Soret band at 390 or 410 nm, respectively, and this spectrum is assigned to the transient formation of a (Por)CoIII(R) species. A similar spectrum has been reported for (TPP)CoIII(C2H5) which has a Soret band at 407 nm [46].

In summary, the electrochemistry, spectroelectro-chemistry and GC analysis of the dechlorinated products suggest the proposed catalytic cycle for reductive dechlo-rination of DDT which is illustrated in Scheme 2. The neutral (OEP)CoII and (TPP)CoII initially undergo a one-electron reduction to yield the corresponding cobalt(I) complex, [(Por)CoI]- which then reacts with DDT to generate the organometallic intermediate with a Co-C σ bond, (Por)CoIII(R). This intermediate can undergo a photoinduced cleavage of the Co-C bond leading to DDD or DDE and the unreduced CoII or CoIII porphyrins which would then be immediately reduced under the application of an applied reducing potential. The σ-bonded Co(III) porphyrin can also be reduced by one-electron to form a [(Por)CoII(R)]- species at the applied potential of -1.70 V after which the electrogenerated σ-bonded Co(II) com-pound will undergo a cleavage of the Co-C bond leading to formation of DDD as shown in the last reaction step of Scheme 2.

CONClUSION

Cyclic voltammetry, controlled potential bulk elec-trolysis and thin-layer UV-visible spectroelectrochemis-try of (OEP)CoII and (TPP)CoII in the presence of DDT show that the two singly reduced porphyrins, represented as [(Por)CoI]-, will catalyze the dechlorination of DDT to generate DDD, DDE, DDMU as well as some less chlorinated products on longer electrolysis timescales. The Co(I) forms of the reduced porphyrins will also react with DDT leading to the formation of intermediates hav-ing a Co-C σ bond, but a cleavage of the bond will occur with formation of the DDT dechlorination products.

Acknowledgements

Support from the Robert A. Welch Foundation (KMK, Grant E-680) and the Natural Science Foundation (BK2008226) of Jiangsu Province, P. R. China is grate-fully acknowledged.

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48. Kadish KM, Guo N, Van Caemelbecke E, Paolesse R, Monti D and Tagliatesta P. J. Porphyrins Phtha-locyanines 1998; 2: 439–450.

49. Kadish KM, Ou ZP, Tan XY, Boschi T, Monti D, Fares V and Tagliatesta P. J. Chem. Soc., Dalton Trans. 1999; 1595–1601.

50. Guilard R and Kadish KM. Chem. Rev. 1988; 88: 1121–1146.

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DOI: 10.1142/S1088424611002970



INTRODUCTION

Currently, there is an extensive amount of work focused on the study of photo-driven processes and devices, par-ticularly in the field of renewable energy research in the form of solar cells [1–6]. Most of this effort is focused on searching for chromophores capable of efficiently generating photocurrent on appropriately designed elec-trodes. The ideal chromophores for this purpose requires a combination of unique electronic [7], photonic [8] and catalytic properties [9, 10]. One particular class of chro-mophores that has drawn a lot of interest are metallopor-phyrins. Metalloporphyrins are long been known to act as good photosensitizers due to their long-lived excited states, good quantum yields of excitation and excellent visible light absorption and redox properties [11, 12]. Representive examples of frequently used metallopor-phyrins as photosensitizers include zinc [13], tin [14],

nickel [15], rhodium [16] and iron [17] porphyrins. In particular, zinc(II) and tin(IV) metalloporphyrins are most commonly used as photosensitizers as they have most of the above properties for a good photocatalyst. This includes a number of recent demonstration of the use of zinc(II) porphyrins in dye-sensitized solar cells [18–20] and the application of tin(IV) porphyrins as pho-tocatalyst to generate metallic nanostructures [21, 22].

To be useful in photo-driven devices such as light activated biosensors, biofuel cells and solar cells prefer-ably requires the metalloporphyrin chromophores to be attached to transparent conductive electrodes. Recent studies show that electrodes functionalized with free-base porphyrin chromophores can generate photocurrent in the presence of light and a sacrificial electron donor. The free-base tetraphenyl porphyrin-fullerene dyad mod-ified indium tin oxide (ITO) surface reported by Fuku-zumi and coworkers showed up to 100 nA photocurrent in the presence of triethanolamine as an electron donor and an applied potential bias [23]. Similarly, Hirano and coworkers have been able to generate photocurrent with a mesoporous silica-porphyrin hybrid coated on a

Tin(IV) porphyrin functionalization of electrochemically active fluoride-doped tin-oxide (FTO) via Huisgen [3+2] click chemistry

Shiva Prasada, Mohan Bhadbhadeb and Pall Thordarson*a

a School of Chemistry, The University of New South Wales, Sydney, NSW 2052, Australia b Analytical Centre, The University of New South Wales, Sydney, NSW 2052, Australia

Received 5 November 2010Accepted 15 January 2011

ABSTRACT: A novel tetra-alkyne terminated tin(IV) porphyrin 3 was synthesized in good yields and characterized using NMR spectroscopy, high resolution mass spectrometry and X-ray crystallography, the latter revealing interactions with hexane molecules that stabilize the crystal structure of the tin(IV) porphyrin 3. It was then linked to a conductive fluorine-doped tin oxide (FTO) surfaces using Huisgen [3+2] click chemistry. The attachment of the tin(IV) porphyrin to the FTO surface 6 was characterized by X-ray photoelectron spectroscopy (XPS), indicating the presence of the 1,2,3-triazole unit. Electro-chemical measurements of the tin(IV) porphyrin modified FTO surface 7 show that it is still electro-chemically active with oxidation (Epa) and reduction peaks (Epc) for the ferricyanide redox couple observed at Epc and Epa of -0.144 and +0.568 V vs. Ag|AgCl respectively, representing a modest shift of ca. +/- 0.1–0.15 V, compared to unmodified FTO.

KEYWORDS: surface functionalization, tin(IV) porphyrin, XPS, FTO, electrochemistry.


*Correspondence to: Pall Thordarson, email: [email protected], tel: +61 2-9385-4478

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76 S. PraSaD et al.

fluorine-doped tin oxide (FTO) electrode [24]. When the system is exposed to a switching light source under a bias of -0.4 to 0.3 V, a photocurrent is detected.

Here we describe the synthesis of novel tin(IV) por-phyrin that has been linked to a FTO surface using the Huisgen [3+2] click reaction. The resulting tin(IV) por-phyrin modified FTO surface were characterized by X-ray photoelectron spectroscopy (XPS) and shown to be electrochemically active. This represents an effective and generic way to generate metalloporphyrin modified electrodes for use in photo-driven devices.

RESUlTS AND DISCUSSION

Synthesis and characterization of tin(IV) porphyrins

The tin(IV) porphyrin 3 was obtained in three steps as shown in Scheme 1. The precursor trimethylsilane (TMS) protected free-base porphyrin 1 was synthesized in 37% yield following the method reported by McDon-ald and coworkers [25]. The subsequent metalation to afford dichlorotin(IV) porphyrin 2 was achieved using the Rothemund method [26] where 1 was refluxed with an excess of tin(II) chloride to afford 2 in 97% yield. The TMS-groups were then removed to afford the target tetra-alkyne tin(IV) porphyrin 3 in 30% yield by reflux-ing potassium carbonate with tin(IV) porphyrin 2 in a dichloromethane/methanol mixture (Scheme 1) [27].

In each step of this synthesis, the porphyrins were isolated in good purity by precipitation or by washing the organo-soluble porphyrins (1–3) with water. The structure of these compounds was determined by a combi-nation of spectroscopic techniques, including 1H and high resolution mass spectrometry. The UV-vis (Fig. 1a) and 1H NMR spectra for 2 (Fig. 1b) and 3 (Fig. 1c) are similar to previously reported tin(IV) porphyrins [28]. Following the metalation, 1H NMR the -2.83 ppm resonance for the inner pyrrolic N-H in 1 disappeared and the inser-tion of the tin metal was confirmed by MS (1000.53 m/z) and UV-vis with a Soret band at 430 nm and two Q-bands at 602 and 561 nm, respectively. After removal of the TMS-group to form 3, the coupling constant of the 119Sn NMR (-590.08) suggested that the axial ligands were chlorides [28]. This was somewhat unexpected as the conditions used for converting 2 and 3 have been reported to result in ligand exchange from a chloride to a hydroxide. It is possible that chloroform, which was used as a solvent for recrystallatization, could have degraded to form phosgene and hydrochloric acid. The resulting chlorides from hydrochloric acid could then exchange with the labile hydroxyl ligands, accounting for the observed chlorides in 3. Signs of 3 converted to the dihydroxy ligated species did show in MALDI-TOF mass spectrometry analysis but this ligand exchange may have happened during sample preparation or in situ in the mass spectrometer.

Single crystals of tin(IV) porphyrin 3 were grown for X-ray crystallography by slow diffusion of hexane into a solution of 3 in chloroform. Due to the small crys-tal size, acquisition of data for X-ray crystallographic analysis was carried out at the synchrotron X-ray source at the Australian Synchrotron Centre. The data was collected with 10223 reflections with an R factor of 0.1143. The quality of the data obtained was sufficient to confirm the main structural features of the tin(IV) porphyrin 3, including the nature of the axial ligands as chlorides.

Scheme 1. Reagents and conditions: (i) tin(II) dichlo-ride, pyridine, reflux, overnight; (ii) potassium carbonate, methanol:dichloromethane (1:4), reflux, 16 h

N

NH

NHN

TMS TMS

TMSTMS

i)

N

N

N

N

TMS TMS

TMSTMS

SnCl

Cl

297%

N

N

N

N

SnCl

Cl

ii)

1

330%

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TIn(IV) POrPhyrIn funCTIOnalIzaTIOn Of eleCTrOChemICally aCTIVe fluOrIDe-DOPeD TIn-OxIDe (fTO) 77

As illustrated in Fig. 2a, the tin metal ion sits expect-edly within the porphyrin macrocycle with a N-Sn-N bond angle of 180° and a N-Sn bond length of 2.1 Å. The Cl-Sn-N bond angle, which is expected to be approx-imately 90°, was observed to be 87°. This is because the axial chloride ligand sits next to the hexane molecule. Moreover, the meso-porphyrin phenyl rings of the tin(IV) porphyrin 3 are rotated approximately 79° to the plane of the porphyrin macrocycle. The tin(IV) porpyhrin mac-rocycles packs in a unique way, where one macrocycle is approximately 40° to the following tin(IV) porphyrin macrocycle (Fig. 2b). The three- dimensional packing of tin(IV) porphyrin 3 is shown in Fig. 3.

The porphyrin packs into a crystal lattice in the pres-ence of hexane that was used during the growth of these

crystals. These hexane molecules are important for the stability of the crystal lattice of tin(IV) porphyrin 3. Along the crystallographic a-axis, these porphyrin mol-ecules pack as close to each other as possible, with the hexane molecules sitting in between each porphyrin. The axial chlorides of the porphyrin interacts with the hydro-gen of the C3-hexane molecules, while the phenyl ring of an adjacent porphyrin molecule interacts with the same hexane, however at the C1-position, causing the alkyne group (CaC−C) to be slightly kinked (178°). Along the crystallographic b-axis, each tin(IV) porphyrin mac-rocycle sits approximately 40° to the following tin(IV) porphyrin, creating a unique form of packing.

Synthesis and characterization of tin(IV) porphyrin-modified FTO surfaces

The tin(IV) porphyrin modified surface 7 was pre-pared using the copper(I) catalyzed Huisgen [3+2] click

Fig. 1. Spectra of 2 and 3; (a) UV-vis spectra (CHCl3) of 2 and 3. 1H NMR (300 MHz, CDCl3) of (b) 2 and (c) 3 (solvent impu-rities are labeled with *)

Fig. 2. X-ray crystallographic analysis of tin(IV) porphyrin 3. (a) Front view and (b) the unique packing of tin(IV) porphyrin 3 along the crystallographic b-axis. The hydrogen atoms are excluded for clarity

Fig. 3. A perspective view of the tin(IV) porphyrin 3 along the crystallographic a-axis. The hydrogen atoms are excluded for clarity. The packing of tin(IV) porphyrin 3 (wireframe) is shown in the presence of hexane (spacefill)

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78 S. PraSaD et al.

chemistry approach [29] outlined in Scheme 2. The (3-azidopropyl)triethoxysilane 5 precursor was initially synthesized in 95% yields using methods adopted from Khoukhi and coworkers [30]. The azide linker 5 was added to the activated FTO surface 4 in a solution of toluene (Scheme 2). The resulting azide-modified FTO surface 6 was washed and sonicated in dichloromethane, chloroform, methanol and acetone to clean the surface of any absorbed compounds. The tetra-alkyne terminated tin(IV) porphyrin 3 was then attached to surface 6 using the copper(I) catalyzed Huisgen [3+2] click chemistry method adopted from Ortega-Munoz [31]. The azide-modified FTO surface 6 was reacted with a catalytic

amount of copper(II) sulfate, ascorbic acid and tin(IV) porphyrin 3 in N,N-dimethylformamide and water to yield the tin(IV) porphyrin triazole-modified FTO sur-face 7. Scheme 2 outlines a step-wise modification of tin(IV) porphyrin to FTO surfaces.

The tin(IV) porphyrin-modified FTO surface 7 was analyzed using XPS (Fig. 4a,b) and surface reflectance UV-vis spectroscopy (Fig. 4c). The N1s XPS spectra of the azide-modified FTO surface 6 (Fig. 4a) show two dis-tinct peaks at 404.7 eV and 400.6 eV in a 1:2 ratio which correspond to the center electron deficient azide nitro-gen and the two nearly equivalent terminal azide nitro-gen atoms as previously reported in the literature [32]. Following the Huisgen [3+2] click chemistry reaction to form the tin(IV) porphyrin triazole-modified FTO surface 7, the 404.7 eV peak disappears (Fig. 4b), leaving one peak at 400.4 eV for the near-equivalent 1,2,3-triazole nitrogens, indicating that the reaction of the azide with the terminal alkyne of tin(IV) porphyrin 3 to a 1,2,3- triazole was successful and therefore effectively linking the tin(IV) porphyrin 3 to the surface [32]. In addition, surface reflectance UV-vis spectral data (Fig. 4c) shows the characteristic Soret peak of the tin(IV) porphyrin chro-mophore at 432 nm following the attachment of tin(IV)

OHOH

OHOHOH

Si

OO O

NN

NNSn

Cl

Cl

i)

ii)

NN

N

N3

Si

OO O

Si OEtOEt

OEtN3

4

5

6

7

Scheme 2. (Left) the attachment of tin(IV) porphyrin 3 to the azide-modified FTO surface 6; (i) (3-azidopropyl)triethoxysi-lane 5 (40 mM), toluene, 2 h, room temperature; (ii) tin(IV) porphyrin 3, copper(II) sulfate, ascorbic acid, N,N-dimethyl-formamide, water, overnight (the presence of chloride axial ligands are assumed but cannot be verified by XPS or UV-vis spectroscopy)

Fig. 4. Characterization spectra for FTO modified surfaces; the N1s XPS spectra of FTO modified surfaces (a) 6 and (b) 7. (c) The UV-vis reflectance of FTO modified surface 7 showing the porphyrin peak at 432 nm (absorption below ≈ 380 nm is due to the glass in FTO)

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porphyrin 3 to the azide-modified FTO surface 6. It should be noted that surface reflectance UV-vis spectroscopy does not distinguish between covalently attached ligands or surface absorbed molecules. However, the absorbance spectrum observed for the tin(IV) porphyrin triazole-modified FTO surface 7 did not change even after con-tinuous sonication to remove adhered contaminants in solvents such as dichloromethane, chloroform, toluene, acetone, methanol and N,N-dimethylformamide.

Electrochemical characteristics of surface 7

Cyclic voltammetry (CV) measurements of the tin(IV) porphyrin-modified FTO electrodes were performed in an effort to gain information about the electroactivity of the modified FTO surfaces, using a ferricyanide probe.

Previous electrochemical studies of tin(IV) porphy-rins suggest that the first reduction of tin(IV) porphyrins occurs at about -0.7 V vs. SCE [33, 34]. However, this falls outside the linear current region of unmodified FTO surfaces, as reduction of tin in FTO surfaces occurs at approximately -0.5 to -0.6 V vs. Ag|AgCl (Fig. 5, thin black line). For this reason, the reduction peak of tin(IV) porphyrin-modified on FTO surfaces was not observed directly. In order to obtain information about the porphy-rin layers, ferricyanide (Fe(CN)6

3+) was used as a probe to measure the changes in response during each modifi-cation step. The cyclic voltammogram (-0.4 to 1.0 V vs. Ag|AgCl) for each modification step leading up to the synthesis of tin(IV) porphyrin triazole-modified FTO surface 7, using ferricyanide as a redox probe, is shown in Fig. 4. The activated FTO surface 4 (Fig. 5, thin gray line) shows large reversible ferricyanide with oxidation (Epa) and reduction peaks (Epc) at +0.441 V vs. Ag|AgCl

and +0.006 V vs. Ag|AgCl respectively, consistent with an accessible electrode surface. After modification, this response decreases, consistent with a surface blocking layer. The azide-modified FTO surface 6 (Fig. 5, thick gray line) has an Epc at +0.084 V vs. Ag|AgCl and an Epa at +0.384 V vs Ag|AgCl. Following the attachment of the tin(IV) porphyrin 3 chromophore, peaks with Epc and Epa of -0.144 and +0.568 V vs. Ag|AgCl were detected on the tin(IV) porphyrin triazole-modified FTO surface 7 respec-tively (Fig. 5, thick black line). The change in Epc and Epa are attributed to the increased distance from the FTO surface to the terminal tin(IV) porphyrin chromophores. As the length of the monolayer increases from surface 6 to 7, the rate of electron transfer from the redox probe to the surface decreases, resulting in an increase in ∆E. The ratio of the area under the reduction and oxidation peaks for surfaces 6 and 7 (ipa/ipc) was calculated to be approxi-mately 1, indicating the redox processes are reversible and stable under the working conditions (0.05–0.60 V/s). The tin(IV) porphyrin triazole-modified FTO surface 7 also shows a reduction in current when compared to the azide-modified precursor FTO surface 6. The azide- modified FTO surface 6 has a cathodic peak current (ipc) and an anodic peak current (ipa) of -77 and +51 μA respectively, whilst the tin(IV) porphyrin triazole-modified FTO sur-face 7 has an ipc and ipa of -51 and +25 μA.

ExpERImENTAl

Chemicals and instruments

All chemicals were purchased from Sigma Aldrich with the exception of propionic acid, potassium carbon-ate, pyridine, anhydrous sodium sulfate (Ajax Finchem Pty. Ltd.) and tin(II) chloride dihydrate (Merck). Solvents were used as is from the manufacturers. Dichloromethane and methanol were distilled before use or obtained from Pure Solv dry solvent system (Innovative Technology Inc. #PS-MD-7). Pyrrole was purchased from Merck and freshly distilled or purified over aluminum oxide before use. The 1H Nuclear Magnetic Resonance (1H NMR) spectra were recorded on a Bruker Avance DPX 200 Bruker Avance DPX 300 or on an Avance III 400 MHz spectrometers at 300 K. Signals were reported in ppm relative to tetramethylsilane (SiMe4, (

1H) = 0). The 119Sn NMR spectra were obtained on a Bruker Avance III 400 MHz spectrometer as stated, quoted in ppm relative to tetramethyltin (Sn(CH3)4,

119Sn = 0 ppm) as an external standard. Infra-red (IR) spectroscopy was recorded on a ThermoNicolet Avatar model 370 FT-IR spectrometer or on a Shimadzu FTIR-8400S by solid state. UV-vis spec-tra were recorded either on a Varian Cary 5E UV-vis-NIR or a Varian Cary 50Bio UV-visible spectrophotometer. Electrospray ionisation (ESI) mass spectra were recorded on a ThermoQuest Finnigan LCQ-DECA electrospray instrument equipped with a Xcaliber processing software

Fig. 5. Cyclic voltammograms for the bare activated FTO sur-face 4, FTO with ferricyanide, the azide-modified FTO surface 6 and the tin(IV) porphyrin triazole-modified FTO surface 7. These results were obtained using ferricyanide (1 mM) and potassium chloride (200 mM) as supporting electrolyte in phosphate buffer (100 mM, pH 7.0) with platinum counter and Ag|AgCl reference electrodes at a scan rate of 0.1 mV/s

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or on a Waters Micromass ZQ 2000 ESCi equipped with MassLynx version 4.1. Matrix assisted laser desorption ionisation (MALDI) was performed on a Applied Bio-systems Voyager DE STR MALDI (reflectron mode) with the matrix trans-2-[3-(4-tert-butylphenyl)-2-methyl- 2-propenylidene]-malononitrile. High resolution ESI (HR-MS) mass spectrometry was performed on a Thermo Linear Quadropole Ion Trap Fourier Transform Ion Cyclotron Resonance (LQT FT Ultra) mass spectrometer in electrospray mode with a 7 T superconducting magnet at the BMSF, Analytical Centre at the University of New South Wales. Melting points were recorded using a Mel-Temp II melting point apparatus and are uncorrected. standard. Cyclic Voltammetry was performed on either a Electrochemical Analyzer BAS100B (BASi) utilizing BAS100W software or on a Autolab Potentiostat PGSTAT 12 equipped with GPES version 4.9. Surface modi-fied electrodes were analyzed in a solution of 100 mM phosphate buffer (pH 7.0) with 200 mM potassium chlo-ride and 1 mM ferrocyanide with Ag|AgCl reference and platinum counter electrodes. Each sample was deoxygen-ated by bubbling dry nitrogen for 5 min prior to elec-trochemical analysis. X-ray Crystallography work in this research was undertaken on the macromolecular crystal-lography beamline at the Australian Synchrotron, Victo-ria, Australia. Surface analysis by X-ray photoelectron spectroscopy (XPS) was performed on a VG ESCALAB 220-IXL imaging XPS microscope with an Al Ka X-ray (1486.6 eV) anode source. Surface UV-vis absorption measurements were made using a Cary 5 UV-vis-NIR fit-ted with a diffuse reflectance apparatus. The 5,10,15,20-tetrakis[(4-trimethylsilyl)ethynyl-phenyl]porphyrin 1 was synthesized according to literature procedure [25]. UV-vis (CH2Cl2): λmax, nm (log ε) 422 (5.65), 516 (4.38), 551 (4.18), 589 (3.88), 645 (3.86). 1H NMR (200 MHz, CDCl3, Me4Si): δH, ppm -2.83 (2H, s, pyrrole-NH), 0.38 (36H, s, Si(CH3)3), 7.79 and 8.16 (16H, ABq, J = 9.0 Hz, Ph), 8.81 (8H, s, pyrrole-H). FTIR (KBr): ν, cm-1 3313 (w), 2956 (m), 2156 (s), 1497 (s), 1474 (s), 1249 (s), 966 (s), 869 (s). MS (ESI): m/z 1000.53 (calcd. for [M + H]+ 999.55).

Dichloro[5,10,15,20-tetrakis((4-trimethylsilyl)ethynyl phenyl)porphyrinato] tin(IV) (2). The 5,10, 15,20-tetrakis [(4-trimethylsilyl)ethynylphenyl]porphy-rin 1 (300 mg, 300 μmol) was dissolved in pyridine (100 mL) and heated to reflux. Tin(II) chloride dihy-drate (270 mg, 1.20 mmol) was added and the solution was allowed to reflux in air for 12 h in the absence of light. After cooling, dichloromethane was evaporated and water (150 mL) was added. The aqueous layer was then extracted with dichloromethane (3 × 100 mL). The com-bined organic extracts were then washed with water (3 × 100 mL) and aqueous hydrochloric acid (1 M, 2 × 100 mL). The organic layers were combined, dried over anhydrous sodium sulfate and filtered. The dichloro[5,10,15,20-tetrakis((4-trimethylsilyl)ethynylphenyl)porphyrinato]-tin(IV) porphyrin 2 was collected under reduced pressure

as fine purple crystals (347 mg, 97%), mp > 300 °C. UV-vis (CH2Cl2): λmax, nm (log ε) 431 (5.74), 562 (4.29), 603 (4.27). 1H NMR (300 MHz, CDCl3, Me4Si): δH, ppm 0.39 (36H, s, Si(CH3)3),

7.93 and 8.25 (16H, ABq, J = 8.2 Hz, Ph), 9.19 (8H, s, satellites, 4J1H-Sn = 14.8 Hz, pyrrole-H). 119Sn NMR (149 MHz, CDCl3, Me4Sn): δSn, ppm -590.08 (Sn). FTIR (KBr): ν, cm-1 2956 (m), 2898 (w), 2157 (s), 1499 (m), 1249 (s), 1030 (s), 864 (s). MS (MALDI): m/z 1116.26 (calcd. for [M - 2Cl]+ 1116.29). HR-MS (FT-ESI): m/z 1147.3091 (calcd. for C65H63N4OSi4Sn: [M - 2Cl + OCH3]

+ 1147.2836), 1133.2935 (calcd. for C64H61N4OSi4Sn: [M - 2Cl + OH]+ 1133.2958.

Dichloro[5,10,15,20-tetrakis(4-ethynylphenyl)porphyrinato]tin(IV) (3). The dichloro[5,10,15,20-tetrakis((4-trimethylsilyl)ethynylphenyl)porphyrinato]tin(IV) porphyrin 2 (84.3 mg, 71.0 μmol) was dissolved in dichloromethane (40 mL). To this was added a solution of potassium carbonate (1.10 g, 7.97 mmol) in metha-nol (10 mL) and the mixture was refluxed for 16 h in the absence of light. After cooling, the solvents were removed under reduced pressure and the crude solid was redissolved in dichloromethane (200 mL). The solution was washed with water (3 × 100 mL) and the organic layer dried over anhydrous sodium sulfate. The final product was recovered by filtration and solvent removed under reduced pressure to give the dichloro[5,10,15,20- tetrakis(4-ethynylphenyl)porphyrinato]tin(IV) porphyrin 3 as a purple crystalline solid (22.1 mg, 29%), mp > 300 °C. UV-vis (CH2Cl2): λmax, nm (log ε) 430 (5.84), 561 (4.37), 602 (4.29). 1H NMR (300 MHz, CDCl3, Me4Si): δH, ppm 3.37 (4H, s, C≡CH), 7.97 and 8.29 (ABq, 16H, J = 8.2 Hz, Ph), 9.21 (8H, s, satellites, 4J1H-Sn = 14.8 Hz, pyrrole-H). 119Sn NMR (149 MHz, CDCl3, Me4Sn): δSn, ppm -588.77 (Sn). FTIR (KBr): ν, cm-1 3289 (m), 2107 (w), 1498 (m), 1234 (s), 1029 (s), 856 (s), 813 (s). MS (MALDI): m/z 861.72 (calcd. for [M - 2Cl + 2OH]+ 861.53), 828.15 (calcd. for [M - 2Cl]+ 828.13). HR-MS (FT-ESI): m/z 859.1528 (calcd. for C53H31N4OSn [M - 2Cl + OCH3]

+ 859.1532).Activating FTO surface electrodes [38]. The FTO

slides were washed by sonicating in acetone for 5 min followed by isopropanol for 5 min. The resulting slides were treated with a solution of H2O2:H2O (30%) and NH4OH:H2O (25%) at 80 °C for 20 min. The activated transparent slides were again rinsed with water and dried under nitrogen prior to use.

(3-azidopropyl)triethoxysilane (5) [31, 39]. To a pre-heated solution (60 °C) of sodium azide (1.01 g, 15.5 mmol) in dimethyl sulfoxide was added (3-chlo-ropropyl)triethoxysilane (2.00 g, 8.33 mmol) and stirred overnight. After cooling, water (100 mL) was added and the aqueous phase was extracted with diethyl ether (3 × 100 mL), dried over anhydrous sodium sulfate and fil-tered. The (3-azidopropyl)triethoxysilane 5 was retrieved under reduced pressure as a clear yellow oil (1.96 g, 95%). 1H NMR (CDCl3, 200 MHz): δ, ppm 3.86–3.79 (m 2H, CH3CH2Si), 3.26 (t, 2H, N3CH2CH2, J = 7.1 Hz),

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1.74–1.68 (m, 2H, -CH2CH2CH2), 1.23 (t, 3H, CH3CH2Si, J = 7.1 Hz), 0.71–0.65 (m, 2H, SiCH2CH2). These results were in good agreement with those found in literature [31].

(3-azidopropyl)triethoxysilane-modified FTO (6). To a pre-washed activated fluorine-doped tin oxide sur-face (ethanol then water, sonicated twice, each for 5 min) was added a solution of (3-azidopropyl)triethoxysilane 3 (98 mg, 396 μmol) in toluene (10 mL). The mixture was left to stand for 1.5 h and washed with toluene, then sonicated twice (5 min each) with toluene, ethanol and water. The resulting (3-azidopropyl)triethoxysilane-mod-ified FTO surface 6 was then dried under nitrogen. XPS (orbital) eV: Si2p (102.29, OSiC), C1s (285.01 C-C) C1s (286.61, C-N), C1s (288.01, C-Hx), N1s (400.73, N=N-N), N1s (404.74, N=N-N), O1s (531.01, O-C), O1s (532.14, O-Si), O1s (533.24, O-H). The XPS spectral data were in good agreement with those reported in literature [32].

Dichloro[5,10,15,20-tetrakis(4-(ethynylphenyl))porphyrinato]tin(IV) porphyrin-modified FTO (7). The (3-azidopropyl)triethoxysilane-modified FTO sur-face 6 was pre-washed in toluene, water, ethanol and acetone, then nitrogen dried prior to use. The FTO sur-face 6 was then sonicated in dry N,N-dimethylforma-mide. Separately, ascorbic acid (105 mg, 0.60 mmol) and copper(II) sulfate (100 mg, 626 μmol) were pre-dissolved in dry N,N-dimethyl formamide (20 mL) and allowed to stir with 3 (35.9 mg, 41.6 μmol) for 15 min. The FTO surface 6 was then added and the deep purple mixture was allowed to stand overnight at room temperature in the absence of light. The surface was then washed with N,N-dimethylformamide, water, ethanol and acetone (sonicated, each for 5 min) and nitrogen dried to afford the tin(IV) porphyrin-modified FTO surface 7. UV-vis surface reflectance spectroscopy: 432 nm; XPS (orbital) eV: C1s (285.06 C-C) C1s (286.55, C-N), C1s (288.23, C-Hx), N1s (400.38, N-C), Sn3d5 (487.20, Sn-O), O1s (531.22, O-C), O1s (532.34, O-Si), O1s (533.47, O-H).

x-ray structure determination

The X-ray diffraction measurement for 3 were car-ried out at the Australian Synchrotron Facility using graphite-monochromated synchrotron X-ray radiation (λ = 0.65253 Å) at 120(2) K. The crystal, mounted on the goniometer using cryo loops for intensity measurements, was coated with paraffin oil and then quickly transferred to the cold stream using Oxford Cryo stream attachment. Symmetry related absorption corrections using the pro-gram XDS were applied and the data were corrected for Lorentz and polarization effects using the XDS software [36]. All structures were solved by Direct methods and the full-matrix least-squares refinements were carried out using SHELXL [37].

Compound 3. C52H28Cl2N4Sn. The data crystal had the form of a thin needle, dark red and had approximate dimension of 0.11 × 0.09 × 0.02 mm; triclinic, space group

P-1, a = 12.185(2) Å, b = 12.512(3) Å, c = 18.173(4) Å, a = 71.95(3)º, β = 70.69(3)º, γ = 80.68(3)º. Z = 2, ρcalcd = 1.260 Mg/m3. μ = 0.661 mm-1. F(000) = 954.0. A total of 30,826 reflections were measured, 10,223 unique. The structure was refined on F2 to 0.1143, with R(F) equal to 0.2723 and a goodness of fit, S = 1.902.

CONClUSION

We were successful in the preparation and character-ization of a novel alkyne-terminated tin(IV) porphyrin 3, including obtain a crystal structure for 3. We also described the successful synthesis of tin(IV) porphyrin modified FTO transparent electrode 7. The tetra-alkyne terminated tin(IV) porphyrin 3 was attached to a transparent FTO electrode using Huisgen [3+2] click chemistry to link the porphy-rin to the electrode via a 1,2,3-triazole structure. Electro-chemical measurements of surface 7 using a ferricyanide probe show oxidation and reduction peaks at -0.144 and +0.568 V vs. Ag|AgCl, respectively. Following the suc-cessful attachment of tin(IV) porphyrin to the FTO sur-faces described here, future studies will focus on the use of these electrodes in the photocatalytic generation of hydro-gen which has been demonstrated by Shelnutt and cowork-ers using tin(IV) porphyrin–platinum nanoaggregates [22, 35]. We are also currently exploring the use of these FTO surfaces for the regeneration of the oxidatively active bio-logical markers such as the coenzyme NAD+ and FAD but the reductive power of photoexcited tin(IV) porphyrins is well-placed to reduce these coenzymes [34].

Acknowledgements

We thank the Australian Research Council for a dis-covery grant and an Australian Research Fellowship (DP0666325) to Pall Thordarson and the University of Sydney for a PhD scholarship to Shiva Prasad. We thank Dr. Tom Caradoc-Davies (Australian Synchrotron) for assistance. We would also like to thank Dr. Bill Bin Gong for XPS analysis and Prof. J.J. Gooding for access and assistance with electrochemical equipment.

Supporting information

Crystallographic data for compound 3 have been deposited at the Cambridge Crystallographic Data Center (CCDC) under deposition number CCDC 759436. Cop-ies can be obtained on request, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cam-bridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223-336-033 or email: [email protected]).

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DOI: 10.1142/S1088424611002982



INTRODUCTION

Photosynthesis starts by the absorption of a photon by light-harvesting complexes followed by a rapid and an efficient energy transfer over many pigments within the antenna system until a reaction center is encountered, where photoinduced charge separation takes place for

fixation of the solar energies harvested [1]. Numerous artificial molecular architectures based on porphyrins have been explored with the aim of achieving efficient and directed energy transfer. Most covalent synthetic models linked by bridging groups such as phenyls, biphenyls, aromatic heterocycles, alkenes or alkynes [2] were synthesized and their energy transfer properties at singlet state was studied. However, the fact is that the porphyrins in covalently linked porphyrin assemblies possessing two identical porphyrins do not allow for a selective excitation for an elucidation of the energy

Meso-meso phenyl bridged unsymmetrical porphyrin dyads: synthesis, spectral, electrochemical and photophysical properties

Meesala Yedukondalua, Dilip K. Maity*b and Mangalampalli Ravikanth*a

a Department of Chemistry, Indian Institute of Technology, Mumbai 400076, India b Theoretical Chemistry Section, Chemistry Group, Bhabha Atomic Research Centre, Mumbai 400085, India


ABSTRACT: Seven phenyl bridged meso-meso unsymmetrical porphyrin dyads containing two different porphyrins either two types of heteroporphyrins or one porphyrin and one heteroporphyrin were synthesized by coupling of readily accessible appropriate mono-meso porphyrin boronate/heteroporphyrin boronate with meso-bromoheteroporphyrin in the presence of Cs2CO3/Pd(PPh3)4 in toluene/DMF at 80 °C for 4 h. The dyads are freely soluble in common organic solvents and were characterized by mass, NMR, absorption, electrochemical and fluorescence techniques. The NMR, absorption and electrochemical studies indicated that the two porphyrin sub-units in dyads interact weakly and they retain their characteristic features. In six out of seven dyads, the free-base porphyrin with N4 core or its Zn(II) derivative possess singlet state energy level at higher energy hence acts as energy donor and the hetero porphyrin sub-unit with N3S, N2SO and N2S2 cores having singlet state energy level at lower energy acts as energy acceptor. Our preliminary photophysical studies on six unsymmetrical porphyrin dyads indicated a possibility of energy transfer at singlet state from donor porphyrin sub-unit to acceptor porphyrin sub-unit on selective excitation of donor porphyrin sub-unit. To gain the structural information and to demonstrate possible electronic interaction between the donor and acceptor porphyrin sub-units, DFT calculations were carried out on dyads 1, 3 and 5 by adopting B3LYP hybrid functional with Gaussian atomic basis functions. The theoretical studies predicted a considerable modification of electronic energy levels in these dyads with the change of porphyrin core in acceptor sub-unit. Calculations also indicate most efficient donor → acceptor energy transfer in case of dyad 5 supporting the experimental results.

KEYWORDS: phenyl bridge, heteroporphyrin, porphyrin boronate, unsymmetrical porphyrin dyad, energy transfer.

*Correspondence to: Dilip K. Maity, email: [email protected] and Mangalampalli Ravikanth, email: [email protected], tel: +91 22 25767176, fax: +91 22-25767152

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84 M. YeDukOnDalu et al.

transfer pathway upon irradiation. The energy gradient between two porphyrin sub-units in dimer is necessary for photo-induced energy transfer at singlet state from one porphyrin sub-unit to another. The energy gradient between two porphyrin sub-units in covalently linked dimer or higher oligomers can be induced by any of the following ways: (1) selective metallation of one of the porphyrin sub-unit and leaving the other porphyrin sub-unit in metal free state [3]; (2) using two different types of meso-substituents such as six membered tolyl groups vs. five membered furyl groups [4] and (3) using two different types of macrocycles such as porphyrin-pyropheophorbide [5], porphyrin-corrole [6], porphyrin-phthalocyanine [7] and porphyrin-heteroporphyrin [8] sub-units. Unsymmetrical porphyrin arrays containing two different types of macrocycles are potential mod-els for photosynthetic energy transfer and should pro-vide information about the role played by the electronic properties of the porphyrins in the yield and direction of energy transfer [9]. However, the number of reports on unsymmetrical porphyrin arrays containing two or more different types of porphyrin sub-units are very few to understand their potential for various studies. Our group is involved in the design and synthesis of unsymmetrical porphyrin arrays containing core-modified porphyrins or heteroporphyrins as one of the porphyrin sub-unit [8b–i].

Heteroporphyrins resulting from the replacement of one or two pyrrole rings with other heterocycles such as thio-phene, furan, selenophene, tellurophene possess differ-ent properties compared to normal porphyrins in terms of their electronic properties and metal binding proper-ties [10]. Heteroporphyrins are known to stabilize met-als in unusual oxidation states unlike normal porphyrins. The heteroporphyrins also possess low lying singlet state energy levels compared to porphyrins. Hence in a por-phyrin assembly containing porphyrin and heteropor-phyrin sub-units, there is a possibility of energy transfer from porphyrin unit to heteroporphyrin unit [11]. We demonstrated an efficient energy transfer from porphy-rin to heteroporphyrin sub-unit in a number of diphenyl ethyne bridged unsymmetrical porphyrin dyads, triads, tetrads and pentads [8d–h]. However, except our own preliminary communication [12], there is no report on phenyl bridged meso-meso unsymmetrical porphyrin dyads containing porphyrin and heteroporphyrin sub-units. In this paper, we report the synthesis of seven phe-nyl bridged meso-meso porphyrin dyads 1–7 (Chart 1) containing two different porphyrin sub-units by coupling of appropriate borylated porphyrin building blocks with bromoporphyrin building blocks under Pd(0) coupling conditions. The preliminary fluorescence studies indi-cated a possibility of energy transfer at singlet state from

Chart 1. Unsymmetrical meso-meso phenyl bridged porphyrin dyads 1-7

N

N N

N

CH3

H3C

CH3

N

S N

HN

CH3

CH3

CH3MN

N N

N

CH3

H3C

CH3

N

S N

O

CH3

CH3

CH3M

N

N N

N

CH3

H3C

CH3

N

S N

S

CH3

CH3

CH3MN

NH N

S

CH3

H3C

CH3

N

S N

S

CH3

CH3

CH3

M = Zn :1M = H, H :2

M = Zn :3M = H, H :4

M = Zn :5M = H, H :6

7

00298.indd 2 3/2/2011 7:35:50 PM

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Meso-Meso PhenYl brIDgeD unSYMMetrICal POrPhYrIn DYaDS 85

one porphyrin sub-unit to another. It is known that the electronic interactions between two porphyrin sub-units in dyad systems can be tuned by connecting two porphy-rins at different positions such as meso-meso, β-meso and β-β which helps electron delocalization within the dyads [13]. Tuning energy gaps between the highest doubly occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) of such extended π-conjugated systems should play crucial role in optimizing the performance of electronic devices of these porphyrin based active organic components. Hence the structure and electronic properties of three dyads 1, 3 and 5 along with their porphyrin monomers are also calculated by applying density functional theory with all electron Gaussian atomic basis functions. Selected frontier orbital contour plots are visualized to identify orbitals of the donor and acceptor moieties of the dyads interacting through phenyl bridge.


The required borylated porphyrin building blocks 8–11 (Chart 2) were prepared by following well-established procedures [12, 14] and meso-bromo heteroporphyrin building blocks 12–14 (Chart 2) were synthesized from

their corresponding meso-unsubstituted heteroporphyrins 15–17 (Chart 2) reported recently in literature [15]. All compounds were characterized by using various spectro-scopic techniques and the data obtained is in agreement with the reported data. The phenyl bridged unsym-metrical porphyrin dyads 1, 3 and 5 containing Zn(II) porphyrin and heteroporphyrin sub-units were synthe-sized as shown in Scheme 1. Coupling of mono meso-(4-borylphenyl)zinc(II)porphyrin [14] with appropriate meso-bromoheteroporphyrin [15] building blocks with N3S, N2SO and N2S2 cores respectively in toluene/DMF in the presence of Pd(PPh3)4/Cs2CO3 at 80 °C for 4 h fol-lowed by column chromatographic purification yielded phenyl bridged meso-meso porphyrin dyads 1, 3 and 5 in 80–88% yields [16]. The phenyl bridged meso-meso porphyrin dyads containing free-base porphyrin and heteroporphyrin sub-units 2, 4 and 6 were synthesized by demetallation of the corresponding dyads 1, 3 and 5 respectively. The dyads 1, 3 and 5 were treated with trif-luoroacetic acid in CH2Cl2 at room temperature followed by flash silica gel column chromatographic purification afforded dyads 2, 4 and 6 in decent yields. The phenyl bridged meso-meso porphyrin dyad 7 containing N3S and N2S2 porphyrin sub-units was synthesized by following two methods as shown in Scheme 2. In the first method,

N

N N

N

CH3

H3C

CH3

MN

Y N

X

CH3

H3C

CH3

BO

O

X = S; Y = NH :12X = S; Y = O :13X = S; Y = S :14

N

XN

YBr

CH3

CH3

H3C

M = Zn :8M = 2H :9

X = S; Y = NH :10X = S; Y = S :11

BO

O

X = S; Y = NH :15X = S; Y = O :16X = S; Y = S :17

N

XN

YH

CH3

CH3

H3C

Chart 2. Functionalized porphyrin monomers 8–14

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+

X = NH; Y = S :1 (85%)X = O; Y = S :3 (80%)X = S; Y = S :5 (88%)

TFA/CH2Cl2rt

X = NH; Y = S :2 (94%)X = O; Y = S :4 (90%)X = S; Y = S :6 (96%)

N X

NYBr

CH3

CH3

CH3

N N

NN

CH3

B

CH3

H3C Zn

N

N N

N

CH3

H3C

CH3

Y

N X

N

CH3

CH3

CH3

Pd(PPh3)4

N

NH N

HN

CH3

H3C

CH3

Y

N X

N

CH3

CH3

CH3

ZnO

O

8 X = NH; Y = S :12 X = O; Y = S :13 X = S; Y = S :14

Cs2CO380 ˚C, 4 h

Scheme 1. Synthesis of meso-meso phenyl bridged unsymmetrical porphyrin dyads 1–6

N

SN

S

CH3

H3C

CH3

BO

O

N

S N

HNBr

CH3

CH3

CH3

N

NH N

S

CH3

H3C

CH3

N

S N

S

CH3

CH3

CH3

Pd(PPh3)4Cs2CO3

80 ˚C, 4 h

+

N

SN

NH

CH3

H3C

CH3

BO

O

N

S N

SBr

CH3

CH3

CH3

+

11

10

12

14

7(86%)

Scheme 2. Synthesis of meso-meso phenyl bridged unsymmetrical porphyrin dyad 7

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the mono meso N2S2 porphyrin boronate 11 was coupled with meso-bromo N3S porphyrin 12 in the presence of Pd(PPh3)4/Cs2CO3 in THF at 80 °C for 4 h followed by column chromatographic purification yielded dyad 7 in 86% yield. Alternately, dyad 7 was also prepared by cou-pling meso-bromo N2S2 porphyrin 14 with mono meso N3S porphyrin boronate 10 under identical reaction con-ditions yielded dyad 7 in 84% yield. Thus, both methods work efficiently and dyad 7 was obtained in decent yield. The dyads 1–7 are freely soluble in common organic sol-vents and characterized by mass, NMR, absorption, elec-trochemical and fluorescence techniques. The dyads 1–7 showed molecular ion peak in ES-MS mass spectra con-firming the identity of the dyads. The dyads 1–7 along

with their corresponding monomers were characterized in detail by NMR spectroscopy. The 1H NMR resonances of dyads 1–7 were assigned on the basis of 1H NMR spec-tra of their corresponding monomers and 1H-1H COSY spectra recorded for selective dyads. A comparison of 1H NMR spectra of dyad 5 along with its correspond-ing monomers 8 and 14 in selected region is shown in Fig. 1a and 1H-1H COSY spectrum of dyad 5 is shown in Fig. 1b. It is noted that the number of signals observed for β-pyrrole and β-thiophene protons in dyads 1–7 are more than the number of signals observed for their correspond-ing monomers indicating that the dyads are asymmetric in nature. For example, the meso-borylated Zn(II)N4 porphyrin monomer 8 showed set of one signal for

(b)

(a)

β-th -th-Py

b-Ph b-Py

Ar Ar

β-th

-th

-Py

-Py

-Py

ββ

β

β

β

β

b-Ph

Ar

b-Ph

Ar

Fig. 1. (a) Comparison of partial 1H NMR spectra of dyad 5 with monomers 9 and 14 recorded in CDCl3. (b) 1H-1H COSY spectrum of dyad 5

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eight β-pyrrole protons and the meso-bromo N2S2 por-phyrin monomer 14 showed set of three signals for four β-pyrrole protons. However, the ZnN4-N2S2 porphyrin dyad 5 showed set of six signals for 12 β-pyrrole protons corresponding to both constituted porphyrin sub-units of dyad 5. Similarly, the β-thiophene protons also showed more number of signals in ZnN4-N2S2 porphyrin dyad 5 compared to the corresponding meso-bromo N2S2 por-phyrin monomer 14 supporting the asymmetric nature of dyads. Furthermore, the bridging phenyl protons showed set of two doublets at 7.60 and 8.20 ppm which were clearly identified by their cross peaks in 1H-1H COSY spectrum. All other dyads showed similar features in their 1H NMR spectra and the peaks were assigned easily with the help of the 1H NMR spectra of their corresponding porphyrin monomers. Furthermore, the chemical shifts of various protons of dyads experienced negligible shifts compared to their corresponding porphyrin monomers indicating that the two porphyrin units in dyads interact very weakly and retain most of their 1H NMR spectral features. Thus, 1H NMR spectral studies were helpful in characterizing the dyads and the studies supported weak interaction between the porphyrin sub-units in phenyl bridged meso-meso linked porphyrin dyads 1–7.

The absorption properties of phenyl bridged meso-meso unsymmetrical porphyrin dyads 1–7 and their cor-responding porphyrin monomers were studied in CH2Cl2 and the data is presented in Table 1. A comparison of

normalized absorption spectra of ZnN4-N2S2 porphyrin dyad 5 along with its corresponding monomers 8 and 14 is shown in Fig. 2. As it is clear from Fig. 2 and data in Table 1 that the porphyrin dyads 1–7 exhibit the features of their corresponding two porphyrin sub-units with neg-ligible changes in their peak maxima. For example the ZnN4-N2S2 porphyrin dyad 5 showed two Soret bands

Table 1. Absorption data of porphyrin dyads 1–7 along with their corresponding monomers recorded in dichloromethane

Compound Soret band λ, nm (log ε) Q-bands λ, nm (log ε)

8 421 (5.39) — 550 (4.18) 590 (3.67) — —

9 418 (5.38) 516 (4.28) 552 (4.03) 592 (3.76) 648 (3.70) —

10 431 (5.89) 515 (4.59) 551 (4.16) 619 (3.66) — 680 (3.95)

11 437 (5.83) 515 (4.67) 550 (4.27) 634 (3.55) — 698 (4.01)

12 430 (5.18) 516 (4.05) 551 (3.45) 620 (3.11) — 681 (3.23)

13 434 (5.73) 517 (4.02) 547 (3.47) — 649 (3.14) 717 (3.32)

14 438 (5.48) 516 (4.29) 549 (3.79) 638 (3.12) — 702 (3.61)

1 420 (6.14) 515 (4.62) 549 (4.66) 587 (4.12) 622 (3.83) 682 (4.03)

432 (5.99)

2 419 (6.10) 516 (4.80) 552 (4.45) 592 (3.99) 648 (3.96) 682 (3.97)

432 (5.98)

3 420 (6.05) 514 (4.65) 548 (4.64) 588 (4.06) 648 (3.56) 717 (4.11)

435 (5.77)

4 419 (5.92) 516 (4.89) 549 (4.69) 590 (3.49) 647 (3.28) 713 (3.24)

433 (5.64)

5 420 (6.05) 515 (4.64) 549 (4.64) 588 (3.99) 636 (3.43) 701 (4.00)

437 (5.89)

6 419 (5.99) 516 (4.74) 551 (4.40) 591 (3.89) 646 (3.92) 701 (3.96)

432 (5.89)

7 430 (5.74) 516 (4.78) 551 (4.39) 625 (3.74) 684 (4.04) 695 (4.04)

440 (5.75)

Fig. 2. The normalized Q-bands and Soret band (inset) absorp-tion spectra of dyad 5 along with their porphyrin monomers 8 and 14 recorded in dichloromethane. The concentrations used were 5 × 10-5 M for Q-band spectra and 5 × 10-6 M for Soret band spectra

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at 420 and 437 nm and five Q-bands at 515, 549, 588, 636 and 701 nm. In this dyad, the Soret band at 420 nm and Q-bands at 549 and 588 nm are mainly due to ZnN4 porphyrin sub-unit and the Soret band at 437 nm and Q-bands at 515, 636 and 701 nm are because of N2S2 porphyrin sub-unit. The other dyads also exhibited similar absorption features with peak maxima matching closely with their constituted porphyrin sub-units with slight alterations in absorption coefficients supporting that the two porphyrin sub-units in dyads almost behave independently and the sub-units in dyads interact very weakly.

The redox chemistry of dyads 1–7 and their corre-sponding porphyrin monomers was followed by cyclic voltammetry. All the cyclic voltammograms were recorded in CH2Cl2 containing 0.1 M TBAP as the sup-porting electrolyte at a scan rate of 50 mV/s. Differen-tial pulse voltammograms have also been recorded for a precise measurement of redox potentials. Cyclic voltam-mograms of dyads 1 and 5 in 0.1 M TBAP in CH2Cl2 obtained at a saturated calomel electrode are shown in Fig. 3 and the redox data of porphyrin dyads 1–7 along with their corresponding porphyrin monomers are pre-sented in Table 2. In general, the porphyrin monomers

Table 2. Electrochemical redox data (V) of porphyrin dyads 1–7 and their corresponding monomers recorded in dichloromethane containing 0.1 M TBAP as supporting electrolyte using scan rate of 50 mV/s. The redox potentials were reported

Compound E° (oxdn, soln), (∆EP, mV) E° (redn, soln), (∆EP, mV) E0-0, eV

I II III IV I II III IV

8 — 1.20 (115) 1.34a — -1.19 (101) — -1.40 (129) — 1.90

9 0.75 (63) 1.05 (71) — — — — - 1.33a -1.61a 2.03

10 — 1.16a — 1.53a -0.97 (95) — -1.39 (120) — 1.80

11 — 1.18a — 1.56a -1.15 (69) -1.27 (69) — — 1.75

12 — 1.15a — 1.53a -0.96 (69) — -1.39 (84) — 1.80

13 — — — — -1.18a — — -1.50a 1.72

14 — 1.19a — 1.58a -1.14a -1.25 (91) — — 1.75

15 — — 1.30a 1.60a -0.83 (98) -1.13 (95) — — 1.84

16 — — — — -0.78a — -1.31a — 1.76

17 — — 1.36a -1.50a -0.73 (84) -1.05 (94) — — 1.78

1 0.77 (66) 1.08 (72) — 1.47a -0.98 (64) -1.34 (104) — -1.73a —

2 — 0.99a 1.38a — -0.88 (62) — -1.53 (91) — —

1.16a -1.18 (75)

3 0.78 (55) 1.08 (62) — — -1.14 (60) — -1.55 (81) -1.73a —

4 — — 1.38a — -1.16a -1.37 (71) -1.56a — —

5 0.78 (55) 1.08 (120) — 1.55a -0.92 (55) — -1.56 (105) -1.75a —

-1.19 (60)

6 — 0.99a 1.38a — -0.88 (57) — -1.54 (72) — —

1.16a -1.18 (58)

7 — 1.26a — 1.48a -1.00a -1.30a — — —

a peak potential Epc is quoted.

Fig. 3. The cyclic voltammogram of porphyrin dyads 1 and 5 in dichloromethane containing 0.1 M TBAP as supporting electrolyte recorded at 50 mV.s-1 scan speed. All measurements were carried out using glassy carbon electrode as working electrode, platinum wire as counter electrode and saturated calomel electrode as refer-ence electrode. The concentrations used were ~10-3 M

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show one electron two quasi-reversible/irreversible oxi-dations and two one electron reversible reductions. For example, the Zn(II) derivative of porphyrin monomer 8 showed two oxidations at 0.75 and 1.05 V and two reduc-tions at -1.33 and -1.61 V whereas the free-base N3S porphyrin monomer 12 showed two oxidations at 1.15 and 1.53 V and two reductions at -0.96 and -1.39 V. The dyads 1–7 exhibited the electrochemical features of both their constituted porphyrin units with negligible potential shifts compared to their respective porphyrin monomers which helped in the assignment of redox potentials to the corresponding porphyrin sub-unit in dyads. How-ever, the potential at which both components exhibited redox wave was attributed to both components in dyads. The cyclic voltammogram study of dyads 1–7 indicated that the dyads generally showed two or three ill-defined oxidations and two or three well defined reductions. For example, the ZnN4-N3S porphyrin dyad 1 exhibited three oxidations at 0.77, 1.08 and 1.47 V and three reductions at -0.98, -1.34 and -1.73 V. In this dyad, the oxidation at 0.77 V was assigned exclusively to ZnN4 porphyrin sub-unit on the basis of the fact that this value is close to that observed for the first oxidation of 8; the irreversible oxi-dation at 1.47 V was due to N3S porphyrin sub-unit on the basis that the potential is closer to N3S porphyrin mono-mer 12 and the oxidation at 1.08 V was due to oxidation of both ZnN4 and N3S porphyrin sub-units. Similarly, the reduction at -0.98 V was exclusively due to N3S por-phyrin sub-unit, since N3S porphyrin is easier to reduce than ZnN4 porphyrin sub-unit; the reduction at -1.73 V was mainly due to reduction of ZnN4 porphyrin sub-unit and the reduction at -1.34 V was due to reduction of both the porphyrin sub-units. The redox potentials of all other dyads were assigned based on their corresponding porphyrin monomers and the potentials of dyads were closely in line with the potentials of their correspond-ing porphyrin monomers indicating that the interaction between the porphyrin sub-units in dyads are negligible and does not influence each other.

The steady state fluorescence properties of dyads 1–6 were studied in CH2Cl2 at two different excitation wave-lengths at which the donor and acceptor porphyrin sub-units independently absorbs strongly. The comparison of fluorescence spectra of ZnN4-N2S2 porphyrin dyad 5 and its corresponding 1:1 mixture of porphyrin monomers 8 and 14 recorded at 550 nm is shown in Fig. 4 and the relevant data of dyads 1–6 are presented in Table 3. In dyads 1–6, the ZnN4 porphyrin and N4 porphyrin sub-units act as energy donor and heteroporphyrin sub-units act as energy acceptors because of favorable arrangement of their singlet state energy levels. Thus, when dyads 1–6 were excited at wavelength at which the heteroporphy-rin sub-units absorb relatively strongly, the emission was observed mainly from heteroporphyrin sub-unit with quantum yield almost matching with the correspond-ing monomeric porphyrin. However, when dyads 1–6 were excited at wavelength where ZnN4 or N4 porphyrin

sub-units absorb strongly, the emission from ZnN4 or N4 porphyrin sub-units quenched significantly and the major emission was noted from the heteroporphyrin sub-unit. For e.g., the ZnN4-N2S2 dyad, on excitation at 550 nm where ZnN4 porphyrin sub-unit absorbs relatively strongly, the emission from ZnN4 porphyrin sub-unit was quenched by 99% and the major emission was observed from N2S2 porphyrin sub-unit. Under same excitation conditions, its corresponding 1:1 mixture of porphyrin monomers 8 and 14 exhibited emission exclusively from ZnN4 porphyrin sub-unit. Similarly, the ZnN4-N3S and

Fig. 4. Comparison of emission spectra of dyad 5 with 1:1 mix-ture of porphyrin monomers 8 and 14 recorded at λex = 550 nm in dichloromethane. The concentrations used were 2 × 10-6 M

Table 3. Emission data of porphyrin dyads 1–6 recorded in dichloromethane

Compound λex, nm Sub-unit Φf % of quenching

H2TPPa 420 — 0.110 —

ZnTPPa 550 — 0.033 —

STPPHa 430 — 0.016 —

SOTPPa 430 — 0.005 —

S2TPPa 435 — 0.007 —

1 550 ZnN4em 0.0008 97.5

530 N3Sem 0.014 —

3 550 ZnN4em 0.0003 99.1

530 N2SOem 0.0048 —

5 550 ZnN4em 0.0002 99.4

530 N2S2em 0.0068 —

2 420 N4em 0.004 96.4

450 N3Sem 0.015 —

4 420 N4em 0.0003 98.7

450 N2SOem 0.005 —

6 420 N4em 0.0011 99.0

450 N2S2em 0.0057 —

a taken from reference 10.

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ZnN4-N2SO dyads on excitation at 550 nm, the emis-sion was noted from N3S and N2SO porphyrin sub-units respectively. Furthermore, the dyads 2, 4 and 6 contain-ing N4 porphyrin sub-unit and appropriate heteroporphy-rin sub-unit, on excitation at 420 nm where N4 porphyrin absorbs relatively strongly, the emission was observed from the corresponding heteroporphyrin sub-unit. These results indicated that there is an efficient energy transfer from ZnN4 or N4 porphyrin sub-unit to heteroporphyrin sub-unit in dyads 1–6. The N3S and N2S2 porphyrin sub-units in dyad 7 absorb and emit closely to each other hence it is not possible to study the selective excitation of donor porphyrin sub-unit energy transfer properties at singlet state. Thus, our steady state photophysical stud-ies indicated a possibility of energy transfer from donor ZnN4 porphyrin or N4 porphyrin to heteroporphyrin sub-unit in meso-meso phenyl bridged unsymmetrical porphyrin dyads.

Since the dyads are unsymmetrical containing two dif-ferent macrocycles, there is also a possibility of photoinduced electron transfer between the two porphyrin sub-units which also quench the fluorescence of donor porphyrin unit in dyads. Hence, we evaluated the rela-tionships of the energy levels for dyads 5 and 6 with Equations 1 and 2, respectively, using fluorescence and redox potential data [17].

∆G(1ZnN4P → N2S2) = ECT(ZnN4P+N2S2

-) -E0-0(ZnN4P) (1) ∆G(ZnN4P → 1N2S2) = ECT(ZnN4P

+N2S2-)

-E0-0(N2S2) (2)

The singlet, excited and charge-transfer states for dyads 5 and 6 are shown in Fig. 5. In dyad 5, the charge-transfer state is lower than the singlet state of the donor ZnN4, whereas in dyad 6, the charge-transfer state is higher than the singlet state of the donor N4 porphyrin. Thus, the free energy change ∆GPET is negative for dyad 5, indicat-ing that electron transfer is also possible in addition to

energy transfer. However, the electron transfer possibility (∆G = -0.1 eV) in dyad 5 is lower than the energy transfer (∆G = -0.34 eV) indicating that the quenching of donor porphyrin fluorescence is mainly due to energy transfer to acceptor porphyrin unit in dyad 5. In the case of dyad 6, the ∆GPET is positive, indicating that the PET is not possible, and energy transfer is the predominant process. The time-resolved studies are required to estimate energy transfer parameters in these novel unsymmetrical meso-meso phenyl bridged porphyrin dyads containing two different porphyrin sub-units.

Since we do not have crystal structural data of these dyads, we have applied first principle based electronic structure theory to calculate the structure of these dyads under gas phase isolated condition. We have also stud-ied frontier orbitals of these systems to gather knowl-edge on orbital interactions especially between the donor and acceptor sub-units. Most stable minimum energy structures calculated applying B3LYP/6-31G(d) level of theory for the three dyads 1, 3 and 5 are displayed with selected geometrical parameters in supplementary data (Fig. S22, see Supporting information section). Note that the two porphyrin sub-units are in the same plane in all these three dyad systems. The bridging phenyl group is at ~65° with either of the porphyrin units and the dis-tance between the donor and acceptor sub-units is 5.83 Å in all three dyads. Furthermore, the heteroporphyrin sub-unit in all four dyads is deviated from planarity as evident from the calculated fully optimized structures. Calculated distance between two opposite N-N atoms in donor sub-unit is 4.09 Å in three dyads whereas the same in acceptor sub-unit is quite different (Fig. S22) showing a significant modification of the acceptor core. Frontier orbitals (FO) are also calculated for dyads 1, 3 and 5 and displayed in Fig. 6 as frontier orbital correlation diagram showing corresponding FOs and the energies of their respective porphyrin sub-units. The HOMO orbital in FO correlation diagram of dyad 1 (Fig. 6a) indicates that only the ring atoms of acceptor porphyrin sub-unit par-ticipate in this orbital and shows no participation of meso substituents or bridging phenyl group. A similar feature is also manifested in LUMO of this dyad indicating poor possibility of energy transfer from donor unit to accep-tor sub-unit. This observation is in support of fluores-cence studies on dyad 1 which also indicated relatively poor energy transfer between the porphyrin sub-units in dyad 1. Higher energy states like LUMO+1, LUMO+2 or LUMO+3 do not exhibit any π electron delocaliza-tion between the donor and acceptor porphyrin sub-units. Stable state like HOMO-2 and HOMO-3 shows com-prehensive π electron delocalization within dyad from donor to acceptor sub-units through phenyl bridge. The HOMO of dyad 1 is destabilized by 0.04 and 0.08 eV whereas LUMO is stabilized by 0.25 and 0.10 eV with respect to that of donor ZnN4 porphyrin and acceptor N3S porphyrin sub-units respectively (Fig. 7a). This opposite energetic trends of the MOs reduces the HOMO-LUMO

Fig. 5. Energies of singlet and charge-transfer states of the dyads 5 and 6

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gap substantially to 2.56 eV for 1, down from 2.85 and 2.75 eV for 8 and 15, respectively (Table 4).

In case of dyad 3 HOMO orbital, an extended π elec-tron delocalization is observed from the donor to acceptor through the phenyl bridge. Similar π electron delocaliza-tion is manifested in case of HOMO-1 orbital. However, only ring atoms of acceptor porphyrin unit participate in the LUMO orbital showing switching behavior. In case of more stable states like HOMO-2 or HOMO-3 either the donor or acceptor sub-unit only participate as depicted in Fig. 6b. Similar features are observed in case of higher orbitals like LUMO+1, LUMO+2 and LUMO+3. The calculated HOMO-LUMO energy gap is 2.42 eV, which is lower than that in dyad 1. The HOMO of dyad 3 is destabilized by 0.02 and 0.07 eV whereas LUMO is sta-bilized by 0.31 and 0.09 eV with respect to that of donor ZnN4 porphyrin and acceptor N2SO porphyrin sub-units respectively. Figure 6c displays the FO correlation dia-gram of dyad 5 showing selected MOs of the dyad as well as the constituent monomers. Visualization of dis-played HOMO and LUMO orbitals suggests the possi-bility of significant energy transfer from the donor ZnN4 porphyrin sub-unit to the acceptor N2S2 porphyrin unit. The HOMO of dyad 5 is destabilized by 0.09 and 0.22 eV

whereas LUMO is stabilized by 0.38 and 0.11 eV with respect to that of donor ZnN4 porphyrin and acceptor N2S2 porphyrin sub-units respectively. As a result, the calculated HOMO-LUMO energy gap is 2.38 eV. Fig-ure 7 depicts a comparative chart of HOMO-LUMO energy gaps in these three dyads and the correspond-ing porphyrin monomer donor and acceptor sub-units. This demonstrates that the modified core tunes HOMO-LUMO energy gap significantly showing electronic inter-action in these newly synthesized meso-meso porphyrin dyads. Dyad 5 has the maximum gap between HOMO energy levels of the donor and the acceptor (Fig. 7) prom-ising to have the maximum energy transfer out of these three dyads. HOMO orbitals of these dyads show that in case of dyad 3, atoms from the donor, linker as well as acceptor participate in this MO but in the case of dyads 1 and 5, either the donor or acceptor unit only partici-pate. However, in LUMO orbitals only atoms from the acceptor sub-unit participate showing these systems to be potential candidates for molecular switch. Dyad 5 has the lowest HOMO-LUMO energy gap and screened to be the best candidate for molecular level device applica-tions among the present meso-meso substituted porphy-rin dyads reported at present.

N

N N

N

CH3

CH3

CH3

N

S N

HN

CH3

CH3

CH3

1

Zn

(a)

Fig. 6. (a) Frontier orbital correlation diagram for meso-meso ZnN4P-N3SP unsymmetrical dyad 1. (b) Frontier orbital correlation diagram for meso-meso ZnN4P-N2SOP unsymmetrical dyad 3. (c) Frontier orbital correlation diagram for meso-meso ZnN4-N2S2 unsymmetrical dyad 5

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N

N N

N

CH3

CH3

CH3

N

S N

O

CH3

CH3

CH3

3

Zn

N

N N

N

CH3

CH3

CH3

N

S N

S

CH3

CH3

CH3

5

Zn

(b)

(c)

Fig. 6. (Continued )

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EXPERIMENTAL

General1H NMR spectra were recorded with Varian 400 MHz

instrument using tetramethylsilane as an internal standard. 13C NMR spectra were recorded on Varian spectrometer operating at 100.6 MHz. All NMR measurements were carried out at room temperature in dueterochloroform

with TMS as an internal standard. Absorption spectra were obtained with Perkin-Elmer Lambda-35 instrument and steady state fluorescence spectra were obtained with PC1 photon counting spectrofluorometer manufactured by ISS, USA. The IR spectra were recorded with a Nicolet Imapact-400 FTIR spectrometer and ES-MS spectra were recorded with a Q-Tof micromass spectrometer. The fluo-rescence quantum yields (Φf) of all the compounds were estimated from the emission and absorption spectra by comparative method using H2TPP (Φf = 0.11) or ZnTPP (Φf = 0.033) as reference compounds. MALDI-TOF spec-tra were obtained from Axima-CFR manufactured by Kratos Analyticals. Cyclic voltammetric (CV) and differ-ential pulse voltammetric (DPV) studies were carried out with BAS electrochemical system utilizing the three elec-trode configuration consisting of a glassy carbon (work-ing electrode), platinum wire (auxiliary electrode) and saturated calomel (reference electrode) electrodes in dry dichloromethane using 0.1 M tetrabutylammonium per-chlorate as supporting electrolyte. Under these conditions, ferrocene shows a reversible one electron oxidation wave (E1/2 = 0.42 V). The solution was deaerated by bubbling argon gas, and during the acquisition argon was slowly

Fig. 7. Pictorial representation of Kohn Sham HOMO and LUMO energy levels of the four dyads (a) 1, (b) 3 and (c) 5 and the corresponding donor and acceptor macrocycles

Table 4. HOMO and LUMO energy levels of porphyrin dyads 1, 3 and 5 and their corresponding monomers 8, 15–17 (in eV)

Compound εHOMO εLUMO εHOMO-εLUMO

8 -4.90 -2.05 2.85

15 -4.94 -2.19 2.75

16 -4.95 -2.27 2.68

17 -5.03 -2.31 2.72

1 -4.86 -2.30 2.56

3 -4.88 -2.36 2.42

5 -4.81 -2.43 2.38

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flowed above the solution. All general chemicals and sol-vents were procured from S.D. Fine Chemicals, India. Column chromatography was performed using silica obtained from Sisco Research Laboratories, India. Tet-rabutylammonium perchlorate was purchased from Fluka and used without further purifications. All other chemicals used for the synthesis were reagent grade unless otherwise specified.

DFT calculations are carried out on three dyads (1, 3, 5) shown in Chart 1 and their subunits to find out the mini-mum energy structures and their electronic properties. Search for minimum energy structures are performed in the ground electronic state (S0) applying Becke’s three parameters correlated hybrid density functional, namely, B3LYP adopting 6-31G(d) atomic basis functions. Equi-librium structures of these systems are calculated based on full geometry optimization without any symmetry restriction following Newton Raphson procedure. All these calculations were carried out applying GAMESS suite of program for ab initio electronic structure calcula-tions on a LINUX cluster platform [18]. Visualization of molecular structures and orbital contour plots were car-ried out using molden visualization software [19].

General synthesis of meso-meso phenyl bridged unsymmetrical porphyrin dyads

Samples of meso-bromoporphyrin and mono meso-porphyrin boronate were dissolved in dry toluene/DMF (3 mL, 2:1) in a 25 mL two-necked round-bottomed flask. The flask was fitted with a reflux condenser, gas inlet and gas outlet tubes for nitrogen purging. After purging with nitrogen for 15 min, Cs2CO3 (1.5 equiv.) followed by Pd(PPh3)4 (0.1 equiv.) were added, and the resulting mixture was heated at 80 °C for 4 h. TLC anal-ysis of the reaction mixture indicated the appearance of a dark new spot apart and almost disappearance of spots corresponding to the starting porphyrin building blocks. After standard work up, the crude compound was puri-fied by silica gel chromatography with petroleum ether/dichloromethane to remove the small amounts of unre-acted porphyrin building blocks and the pure dyad was collected using dichloromethane in 80–88%.

Meso-meso phenyl bridged ZnN4-N3S porphyrin dyad (1). Yield 85%, mp > 300 °C. IR (KBr film): ν, cm-1 3046, 2915, 2840, 2211, 1641, 1458, 967, 820, 654. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm -2.47 (s, 1H, NH), 2.70 (s, 18H, CH3), 7.53–7.59 (m, 12H, aryl), 7.66 (d, J = 7.7 Hz, 2H, aryl), 8.04–8.15 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.59 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.68 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.71 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.94 (s, 4H, β-pyrrole), 8.96 (d, J = 4.8 Hz, 2H, β-pyrrole), 8.97 (s, 2H, β-pyrrole), 9.02 (d, J = 4.6 Hz, 2H, β-pyrrole), 9.27 (d, J = 4. 8 Hz, 1H, β-pyrrole), 9.86 (d, J = 5.1 Hz, 1H, β-thiophene), 10.32 (d, J = 5.1 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.5, 22.7, 23.0, 23.7,

68.1, 112.1, 121.1, 121.4, 124.4, 124.9, 125.9, 127.3, 127.6, 128.4, 128.9, 130.9, 131.0, 131.8, 132.4, 132.9, 133.6, 134.1, 134.3, 134.7, 135.2, 135.5, 137.1, 137.5, 137.9, 138.4, 138.7, 138.9, 139.3, 139.5, 139.8, 143.3, 146.5, 148.1, 148.4, 148.9, 150.3, 150.9, 151.2, 157.6, 158.9. MS (MALDI-TOF): m/z 1313.2 (calcd. for [M]+ 1313.7).

Meso-meso phenyl bridged ZnN4-N2SO porphyrin dyad (3). Yield 80%, mp > 300 °C. IR (KBr film): ν, cm-1 3035, 2914, 2835, 2220, 1645, 1454, 967, 820, 654. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm 2.71 (s, 18H, CH3), 7.47–7.61 (m, 12H, aryl), 7.65 (d, J = 7.7 Hz, 2H, aryl), 8.07–8.12 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.45 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.56 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.73 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.93 (s, 4H, β-pyrrole), 8.95 (s, 1H, β−pyrrole), 8.96 (d, J = 5.1 Hz, 2H, β-pyrrole), 9.01 (d, J = 4.6 Hz, 1H, β-pyrrole), 9.32 (d, J = 4.8 Hz, 1H, β-pyrrole), 9.36 (d, J = 5.5 Hz, 1H, β-furan), 9.66 (d, J = 5.5 Hz, 1H, β-furan), 9.72 (d, J = 5.1 Hz, 1H, β-thiophene), 10.13 (d, J = 5.1 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.3, 22.8, 23.2, 23.9, 68.5, 112.1, 121.2, 121.4, 124.4, 124.6, 125.9, 127.3, 127.5, 128.4, 128.9, 130.6, 131.0, 131.6, 131.9, 132.6, 133.4, 134.1, 134.3, 134.7, 135.5, 135.9, 137.1, 137.5, 137.9, 138.1, 138.7, 138.9, 139.3, 139.5, 141.2, 143.6, 146.5, 148.1, 148.4, 148.9, 150.1, 150.5, 151.2, 157.6, 158.9. MS (MALDI-TOF): m/z 1314.4 (calcd. for [M]+ 1314.5).

Meso-meso phenyl bridged ZnN4-N2S2 porphyrin dyad (5). Yield 88%, mp > 300 °C. IR (KBr film): ν, cm-1 3040, 2912, 2842, 2220, 1644, 1458, 967, 820, 651. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm 2.72 (s, 18H, CH3), 7.52–7.55 (m, 8H, aryl), 7.61 (d, J = 7.7 Hz, 2H, aryl), 7.64 (dd, J = 7.6 Hz, J = 2.1 Hz, 4H, aryl), 8.07–8.15 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.66 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.69 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.77 (d, J = 4.3 Hz, 1H, β-pyrrole), 8.94 (s, 4H, β-pyrrole), 8.98 (d, J = 4.9 Hz, 2H, β-pyrrole), 9.04 (d, J = 4.6 Hz, 2H, β-pyrrole), 9.29 (d, J = 4.6 Hz, 1H, β-pyrrole), 9.64 (d, J = 5.2 Hz, 1H, β-thiophene), 9.69 (d, J = 5.2 Hz, 1H, β-thiophene), 9.82 (d, J = 5.2 Hz, 1H, β-thiophene), 10.32 (d, J = 5.2 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.4, 22.9, 23.1, 23.7, 68.9, 110.1, 120.1, 121.4, 124.5, 124.9, 125.6, 127.1, 127.9, 128.4, 128.9, 130.9, 131.5, 131.8, 132.4, 132.6, 133.2, 134.1, 134.4, 134.7, 135.2, 135.5, 137.3, 137.5, 137.8, 138.5, 138.7, 138.9, 139.1, 139.5, 139.8, 143.3, 146.5, 148.3, 148.4, 148.9, 150.2, 150.9, 152.4, 157.6, 158.1. MS (ES-MS): m/z 1330.6 (calcd. for [M]+ 1330.4).

General procedure for the synthesis of unsymmetrical meso-meso phenyl bridged N4 porphyrin-heteropor-phyrin dyads 2, 4 and 6

Samples of the corresponding ZnN4 porphyrin-het-eroporphyrin dyads 1, 3 and 5 in CH2Cl2 were treated

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with excess of TFA for 15 min. The progress of the reac-tion was followed by absorption spectroscopy. After standard work-up, the crude compounds were purified by silica gel column chromatography using CH2Cl2 and afforded pure N4 porphyrin-heteroporphyrin dyads 2, 4 and 6 respectively in ~90–96% yields.

Meso-meso phenyl bridged N4-N3S porphyrin dyad (2). Yield 94%, mp > 300 °C. IR (KBr film): ν, cm-1 3044, 2918, 2840, 2195, 1641, 1456, 967, 820, 655. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm -2.80 (s, 2H, NH), -2.47 (s, 1H, NH), 2.70 (s, 18H, CH3), 7.53–7.59 (m, 12H, aryl), 7.65 (d, J = 7.7 Hz, 2H, aryl), 8.04–8.15 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.59 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.68 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.71 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.83 (s, 4H, β-pyrrole), 8.87 (d, J = 5.1 Hz, 2H, β-pyrrole), 8.92–8.94 (m, 4H, β-pyrrole), 9.27 (d, J = 4.8 Hz, 1H, β-pyrrole), 9.86 (d, J = 5.1 Hz, 1H, β-thiophene), 10.32 (d, J = 5.1 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.4, 22.9, 23.1, 23.7, 68.4, 111.9, 121.0, 121.4, 124.4, 124.6, 125.9, 127.3, 127.6, 128.4, 128.9, 130.9, 131.4, 131.8, 132.4, 132.9, 133.6, 134.2, 134.3, 134.7, 135.2, 135.5, 137.3, 137.5, 137.9, 138.5, 138.7, 138.9, 139.3, 139.6, 139.8, 143.3, 146.5, 148.2, 148.4, 148.9, 150.3, 150.9, 151.2, 157.6, 158.8. MS (MALDI-TOF): m/z 1251.7 (calcd. for [M]+ 1251.5).

Meso-meso phenyl bridged N4-N2SO porphyrin dyad (4). Yield 90%, mp > 300 °C. IR (KBr film): ν, cm-1 3040, 2912, 2841, 2194, 1641, 1460, 967, 820, 650. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm -2.80 (s, 2H, NH), 2.71 (s, 18H, CH3), 7.47–7.61 (m, 12H, aryl), 7.65 (d, J = 7.7 Hz, 2H, aryl), 8.07–8.12 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.45 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.56 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.72 (d, J = 4.8 Hz, 1H, β-pyrrole), 8.92 (s, 4H, β-pyrrole), 8.95 (s, 1H, β-pyrrole), 8.96 (d, J = 5.1 Hz, 2H, β-pyrrole), 9.01 (d, J = 4.6 Hz, 1H, β-pyrrole), 9.32 (d, J = 4.8 Hz, 1H, β-pyrrole), 9.36 (d, J = 5.5 Hz, 1H, β-furan), 9.65 (d, J = 5.5 Hz, 1H, β-furan), 9.72 (d, J = 5.1 Hz, 1H, β-thiophene), 10.12 (d, J = 5.1 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.4, 22.6, 23.2, 23.9, 68.5, 112.1, 121.2, 121.4, 124.0, 124.6, 125.9, 127.3, 127.5, 128.4, 128.9, 130.6, 131.0, 131.6, 131.9, 132.6, 133.4, 134.1, 134.3, 134.7, 135.5, 135.9, 137.1, 137.6, 137.9, 138.4, 138.7, 138.9, 139.3, 139.5, 141.4, 143.6, 146.5, 148.1, 148.5, 148.9, 150.1, 150.5, 151.2, 157.6, 158.6. MS (MALDI-TOF): m/z 1275.4 (calcd. for [M + Na]+ 1252.5).

Meso-meso phenyl bridged N4-N2S2 porphyrin dyad (6). Yield 96%, mp > 300 °C. IR (KBr film): ν, cm-1 3041, 2912, 2842, 2214, 1646, 1458, 967, 820, 653. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm -2.8 (s, 2H, NH), 2.69 (s, 6H, CH3), 2.72 (s, 6H, CH3), 2.73 (s, 6H, CH3), 7.52–7.55 (m, 8H, aryl), 7.61 (d, J = 7.7 Hz, 2H, aryl), 7.64 (dd, J = 7.6 Hz, J = 2.1 Hz, 4H, aryl), 8.06–8.10 (m, 12H, aryl), 8.18 (d, J = 7.7 Hz, 2H, aryl), 8.68 (d, J = 4.6 Hz, 2H, β-pyrrole), 8.77 (d, J = 4.6 Hz, 1H,

β-pyrrole), 8.83 (s, 4H, β-pyrrole), 8.87 (d, J = 4.9 Hz, 2H, β-pyrrole), 8.94 (d, J = 4.3 Hz, 2H, β-pyrrole), 9.29 (d, J = 4.3 Hz, 1H, β-pyrrole), 9.64 (d, J = 5.2 Hz, 1H, β-thiophene), 9.69 (d, J = 5.2 Hz, 1H, β-thiophene), 9.82 (d, J = 5.2 Hz, 1H, β-thiophene), 10.32 (d, J = 5.2 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.3, 22.9, 23.1, 23.7, 68.9, 110.4, 120.1, 121.4, 124.5, 124.9, 125.6, 127.1, 127.9, 128.1, 128.9, 130.9, 131.5, 131.8, 132.4, 132.6, 133.2, 134.3, 134.5, 134.7, 135.2, 135.5, 137.3, 137.5, 137.8, 138.5, 138.7, 138.9, 139.1, 139.3, 139.8, 143.3, 146.5, 148.1, 148.4, 148.9, 150.2, 150.9, 152.1, 157.6, 158.6. MS (ES-MS): m/z 1268.3 (calcd. for [M]+ 1268.5).

Meso-meso phenyl bridged N3S-N2S2 porphyrin dyad (7). Meso-bromo porphyrin 12/14 and N2S2 por-phyrin boronic ester 11/10 were dissolved in anhydrous toluene-triethylamine (6 mL, 5:1) in a 25-mL two-necked, round-bottomed flask under a nitrogen atmo-sphere. Cs2CO3 (1.5 equiv.) followed by Pd(PPh3)4 (0.1 equiv.) were added and the resulting mixture was heated at 80 °C for 4 h. After work-up, the crude compound was subjected to silica gel chromatography to afford dyad 7 as purple solid. Yield 86%, mp > 300 °C. IR (KBr film): ν, cm-1 3042, 2913, 2845, 2217, 1644, 1458, 967, 820, 652. 1H NMR (400 MHz; CDCl3; Me4Si): δH, ppm -2.55 (s, 1H, NH), 2.63 (s, 3H, CH3), 2.64 (s, 9H, CH3), 2.67 (s, 6H, CH3), 7.47 (d, J = 7.6 Hz, 2H, aryl), 7.50-7.54 (m, 10H, aryl), 7.58 (d, J = 7.6 Hz, 2H, aryl), 7.99 (d, J = 7.9 Hz, 2H, aryl), 8.03–8.12 (m, 12H, aryl), 8.52 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.59 (s, 2H, β-pyrrole), 8.61–8.62 (m, 2H, β-pyrrole), 8.64 (d, J = 4.6 Hz, 1H, β-pyrrole), 8.70 (d, J = 4.3 Hz, 1H, β-pyrrole), 8.87 (dd, J = 10.7, J = 2.1 Hz, 2H, β-pyrrole), 9.19 (d, J = 4.6 Hz, 1H, β-pyrrole), 9.59 (s, 2H, β-thiophene), 9.63 (d, J = 4.9 Hz, 1H, β-thiophene), 9.69 (d, J = 5.2 Hz, 1H, β-thiophene), 9.79 (d, J = 5.2 Hz, 1H, β-thiophene), 10.22 (d, J = 5.2 Hz, 1H, β-thiophene). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 21.3, 22.7, 23.0, 23.4, 68.9, 112.5, 121.2, 121.4, 122.4, 124.0, 125.1, 127.1, 127.6, 128.1, 128.6, 130.9, 131.0, 131.8, 132.4, 132.6, 133.0, 134.3, 134.6, 135.2, 135.9, 137.2, 137.5, 137.9, 138.4, 138.7, 138.9, 139.3, 139.5, 139.8, 143.3, 146.5, 148.1, 148.4, 148.9, 150.3, 150.9, 151.2, 157.6, 159.4. MS (MALDI-TOF): m/z 1285.6 (calcd. for [M]+ 1285.4).

CONCLUSION

We synthesized phenyl bridged meso-meso linked unsymmetrical porphyrin dyads containing two different porphyrin sub-units such as either two heteroporphyrin sub-units or one porphyrin and one heteroporphyrin sub-unit by coupling of appropriate mono meso-porphyrin boronate and meso-bromoporphyrin under Pd(0) coupling conditions. The ground state properties indicated that the two porphyrin sub-units in dyads interact weakly and they retain their individual characteristic features. The preliminary fluorescence studies supported a possibility

00298.indd 14 3/2/2011 7:36:28 PM

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of intramolecular singlet-singlet energy transfer from donor porphyrin sub-unit to acceptor porphyrin sub-unit on selective excitation of donor porphyrin sub-unit. The computational studies carried out on unsymmetrical por-phyrin dyads containing metallo porphyrin and heteropor-phyrin sub-units do support an interaction between the porphyrin sub-units in dyads. Our computational studies indicated that meso-meso phenyl bridged unsymmetrical ZnN4P-N2S2P porphyrin dyad 5 is the potential candidate for molecular level device applications among the dyads presented here. The design and synthesis of more such unsymmetrical covalent dyads containing two porphy-rins or related macrocycles with their singlet state energy levels arranged in favorable manner would be useful for molecular electronics applications. We will return with time-resolved results on these systems in the near future.

Acknowledgements

Mangalampalli Ravikanth thanks the Council of Sci-entific and Industrial Research (CSIR) and Department of Science and Technology (DST) for financial support and YK thanks CSIR for fellowship. Computer Centre, Bhabha Atomic Research Centre (BARC) is gratefully acknowledged for generous CPU time in ANUPAM par-allel computer systems.

Supporting information

1H NMR, mass, uv-visible, Emission and Reduction data are given in the supplementary material (Figs S1-S23). This material is available free of charge via the Internet at http://www.worldscinet.com/jpp/jpp.shtml.

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CONTENTS


Atropisomerism and conformational aspects of meso-tetraarylporphyrins and related compounds 1Augusto C. Tomé, Artur M. S. Silva,* Ibon Alkorta and José Elguero

Ferric His93Gly myoglobin cavity mutant and its complexes with thioether and selenolate as 29 heme protein models

Jing Du, Masanori Sono and John H. Dawson*

Aggregation behavior and UV-vis spectra of tetra- and octaglycosylated zinc phthalocyanines 39Alexey Lyubimtsev, Zafar Iqbal, Göran Crucius, Sergey Syrbu, Ekaterina S. Taraymovich,

Thomas Ziegler and Michael Hanack*

Singlet molecular oxygen generation by water-soluble phthalocyanine dendrimers with different 47 aggregation behavior

Masakazu Nishida, Hiroaki Horiuchi, Atsuya Momotake, Yoshinobu Nishimura, Hiroshi Hiratsuka and Tatsuo Arai*

Synthesis and spectral study of tetra(2,3-thianaphthe no)porphyrazine, its tetra-tert-butyl derivative 54 and their Mg(II), Al(III), Ga(III) and In(III) complexes

Ekaterina S. Taraymovich*, Andrey B. Korzhenevskii, Yulia V. Mitasova, Roman S. Kumeev, Oscar I. Koifman and Pavel A. Stuzhin*

Reductive dechlorination of DDT electrocatalyzed by synthetic cobalt porphyrins in 66 N,N ′-dimethyl formamide

Weihua Zhu, Yuanyuan Fang, Wei Shen, Guifen Lu, Ying Zhang, Zhongping Ou* and Karl M. Kadish*

Tin(IV) porphyrin functionalization of electro chemically active fluoride-doped tin-oxide (FTO) via 75 Huisgen [3+2] click chemistry

Shiva Prasad, Mohan Bhadbhade and Pall Thordarson*

Meso-meso phenyl bridged unsymmetrical por phyrin dyads: synthesis, spectral, electro che mi cal and 83 pho tophysical properties

Meesala Yedukondalu, Dilip K. Maity* and Mangalampalli Ravikanth*

15-01 Contents-back.indd 1 3/3/2011 12:59:02 PM

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Volum

e 15 Num

ber 1 Pages 1-98 Journal of P

orphyrins and Phthalocyanines 2011 S

PP

MICA (P) 207/01/2009

2011 - ISSN 1088-4246

Volu

me

15 /

Num

ber

1 /

Page

s 1-

98

January 2011

JPP Vol 15 N1.indd 1 3/9/11 9:56 AM

Documents

Volume 15 / Number 1 / Pages 1-98 - World Scientific · Tomé, Augusto C. 1 Y Yedukondalu, Meesala 83 Z Zhang, Ying 66 Zhu, Weihua 66 Ziegler, Thomas 39 Journal of Porphyrins and