FORMATION, PHOTOPHYSICS AND PHOTOCHEMISTRYFORMATION, PHOTOPHYSICS AND PHOTOCHEMISTRY OFOF WATER-SOLUBLE CADMIUM(II) PORPHYRINS;WATER-SOLUBLE CADMIUM(II) PORPHYRINS;THE EFFECTS OF BROMINATIONTHE EFFECTS OF BROMINATION AND THE AXIAL HYDROXO LIGANDAND THE AXIAL HYDROXO LIGAND

Zsolt ValicsekZsolt Valicsekaa, Ottó Horváth, Ottó Horváthaa, György Lendvay, György Lendvaya,ba,b, Ilijana Kikaš, Ilijana Kikašcc, Irena Škorić, Irena Škorićcc

a Department of General and Inorganic Chemistry, Institute of Chemistry, University of Pannonia H-8201 Veszprém, P.O.B. 158., Hungary Phone/Fax: 36(88)624548 E-mail: valicsek@vegic.uni-pannon.hu

b Institute of Structural Chemistry, Chemical Research Center, Hungarian Academy of Sciences, H-1525 Budapest, P.O. Box 17, Hungaryc Department of Organic Chemistry, Faculty of Chemical Engineering and Technology, University of Zagreb, Marulićev trg 19, 10000 Zagreb, Croatia

ReferencesReferences1. O. Horváth, R. Huszánk, Z. Valicsek, G. Lendvay, Coord. Chem. Rev. 2006, 250, 1792-1803. 2. M. Tabata, J. Nishimoto, A. Ogata, T. Kusano, N. Nahar, Bull. Chem. Soc. Jpn. 1996, 69, 673-677.3. Z. Valicsek, O. Horváth, G. Lendvay, I. Kikaš, I. Škorić, J. Photochem. Photobiol. A. 2011, 218, 143-155.

ExperimentalExperimental

Formation and absorption spectraFormation and absorption spectraIn aqueous solution cadmium(II) reacted easily with H2P4-, forming a complex with 1:1 composition at pH 8-12 (HOCdP5-). Using the spectrophotometric titration method, the equilibrium constant was also determined in this system(lgK’ = 6.90) at adjusted pH of 8.

H2P4- + CdOH+ HOCdP5- + 2 H+

PhotophysicsPhotophysicsMetalation of the porphyrin results in a hypsochromic effect in the fluorescence (Table 1), in contrast to the red-shift in the absorption.

This blue-shiftred-shift anomaly is virtual, because the absorption shift is referred to the average of Qy(0,0)- and Qx(0,0)-bands of the free-base ligand, while the emission derives not from a hypothetical average level, but from the energetically lower S1x-state (populated in Qx(0,0) absorption). The Stokes-shift is a bit larger in CdP4- than in H2P4-, although the complex is slightly nonplanar already in the ground state, while the free base is quite planar. Out-of-plane monoporphyrin complexes of numerous different metal ions were found to have very similar vibronic overtones, thus in the S1-excited-state these metalloporphyrins can be assumed to have the same degree of ring deformation [1]. Coordination of the axial hydroxo ligand significantly increases the Stokes-shift, indicating a more nonplanar distortion in the S1-excited state compared to that of the ground state.

Primary photochemistryPrimary photochemistryWhile the normal (in-plane) metalloporphyrins do not undergo efficient and irreversible photoinduced ligand-to-metal

charge-transfer reactions, due to their kinetically inert, planar structure, OOP complexes display a characteristic photoredox chemistryfeatured by photodegradation caused by the effective separation of the reduced metal center and the oxidized macrocycle following the LMCT reaction, finally leading to irreversible ring cleavage of the ligand, e.g., the formation of the open-chain dioxo-tetrapyrrol derivatives. The irradiations were carried out at both the Soret- and the Q-bands, in both aerated and deoxygenated systems (Table 2).

IntroductionIntroductionMetalloporphyrins represent one of the most significant families of compounds in bio-, coordination-, and photochemistry. Within

this group the so-called out-of-plane (OOP) or sitting-atop (SAT) metalloporphyrins are characterized by special properties originated from the nonplanar structure, for which, first of all, the size of the metal center is responsible. In these complexes, the metal, due to its large ionic radius (> 80-90 pm), does not fit coplanar into the cavity of the ligand, hence it is located above the porphyrin plane, distorting it [1].The formation rate of metalloporphyrins is much slower for the in-plane or normal types than for the OOP complexes because of the inflexibility of porphyrins. In an OOP complex the distortion of the porphyrin caused by the out-of-plane location of the metal center makes two diagonal pyrrolic nitrogens more accessible on the other side of the ligand due to the increase of their sp3 hybridization. Thus, another metal ion, even with smaller ionic radius can easily coordinate to them. Hence, larger metal ions such as Pb2+, Hg2+, or Cd2+ can catalyse the formation of normal (in-plane) metalloporphyrins via generation of OOP complex intermediates. Since the deformation of the porphyrin ring proved to be the main factor governing the acceleration of the metalloporphyrin formation, this can also be achieved, e.g., by substituents at the peripheral ring [2].

ConclusionConclusionIn slightly alkaline solution (pH = 8) Cd(II) ion and 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2P4-) form a kinetically labile

complex (HOCdP5-), in which the metal center is located out of the ligand plane, due to the effects of the axial hydroxo ligand and the relatively large radius of Cd2+. Both acidification and irradiation at the Soret-band can result in the dissociation of the axial ligand, reducing the out-of-plane distance of the metal center and the distortion of the macrocycle (CdP4-). Besides, irradiation of both types of metalloporphyrins promotes an irreversible ligand-to-metal charge transfer leading to the oxidative degradation of the coordinated porphyrin. Under the same conditions, in the case of the octabromo derivative of this water-soluble porphyrin (H2BrP4-), the distorted structure accelerates the formation of the corresponding complex with cadmium(II) compared to its reaction with the parent, unbrominated ligand. The structure of this porphyrin (HOCdBrP5-), similarly to the free base and CdBrP4-, strongly distorted by the Br substituents, significantly affects the characteristic features of the absorption and emission spectra, red-shifting the position of the main bands of these porphyrins compared to those of the unbrominated species. Also the emission quantum yields and lifetimes are dramatically dimished by bromination. Deviating from the unbrominated species, photodegradation of the brominated derivatives proved to be very oxygen sensitive. DFT calculations of the geometrical structures and the absorption bands show good correlations with the observed photophysical and photochemical properties, due to the drastic distortions of the macrocyclic ligand [3].

Figure 1. Structures of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin, H2P4-, and its

octabromo derivative, H2BrP4-.

AcknowledgementAcknowledgement This work was supported by the National Development Agency (TÁMOP 4.2.2.-08/1/2008-0018, Livable environment and healthier people – Bioinnovation and Green Technology research at the University of Pannonia, the project is being co-financed by the European Social Fund with the support of the European Union) and also in the frame of the Hungarian–Croatian Intergovernmental S&T Cooperation Program for 2009-2010 jointly financed by the Hungarian National Office of Research and Technology (OMFB-01247/2009).

Electronic structure calculations involved molecular geometry optimization and the necessary vibrational frequency analysis as well as the determination of vertical electron excitation energies. For both purposes we used density functional theory, in particular, the B3LYP combination of functionals, and time-dependent density functional. In the geometry optimizations we used the Hay-Wadt valence double-zeta (LANL2DZ) basis set. In the calculations we modeled H2TSPP4- (H2P4-) and H2TSPPBr8

4- (H2BrP4-) by H2TPP and H2TPPBr8, respectively. In our test calculations we found that the sulfonato substituent has a negligible effect on the coordination site. All calculations were performed using the Gaussian 03 suite of programs.

Preparation of 2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (H6TSPPBr8, hereinafter H6BrP, Fig.1 ) was realized by a modified procedure of Tabata [2]. The analytical grade tetrasodium 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin Na4H2TSPP.12H2O (hereinafter Na4H2P) and CdCl2.2.5H2O (Sigma-Aldrich) were used for the experiments. The pH of each solution was adjusted to 8 by application of borate buffer, also keeping the ionic strength at constant value of 0.01 M.The absorption spectra were recorded and the photometric titrations were monitored using a Specord S-600 diode array spectrophotometer. For the measurement of fluorescence spectra a Perkin ELMER LS 50-B spectrofluorimeter was applied. The spectrum analyses were carried out by fitting Gaussian and Lorentzian curves in MS Excel. Rhodamine-B and Ru(bpy)3Cl2 were used as references for correction of the detector sensitivity and for determination of the fluorescence quantum yields. Luminescence lifetime measurements were performed using a laser flash photolysis system. A Quantel Brilliant Nd:YAG laser yielding 355- and 532-nm pulses of about 5 ns duration served as a light source. The measurement data were recorded by a Tektronix DPO 4034 digital oscilloscope. Since the fluorescence lifetimes of the compounds studied are comparable with the laser half-width, a deconvolution method was applied for their determination. For continuous irradiations an AMKO LTI photolysis equipment (containing a 200-W Xe-Hg-lamp and a monochromator) was applied. All measurements were carried out at room-temperature.

]][CdOHP[H][HOCdP

][HK

K' 42

5

2

According to our experiences, the complex with axial hydroxo ligand (HOCdP5-) was quantitatively formed at pH 8. In order to confirm the coordination of hydroxide, the solution of this complex was acidified to pH≈5. Under such a slightly acidic condition the axial HO- ligand was removed, which was manifested in significant blue shifts of the characteristic absorption bands (Figs. 2, 3 ). Irradiation can also promote the dissociation of the axial hydroxo ligand, leading to the CdP4- species. Notably, this complex does not form directly in the reaction of Cd2+ and H2P4- below pH 6, but its formation can be detected in the range of pH 7-8, suggesting that it is produced only through HOCdP5-. Above pH 8, where the partial molar fraction of the aqua complex compared to the monohydroxo complex of cadmium(II) ion becomes more negligible, CdP4- cannot form through HOCdP5-. It is also important to note that CdP4- and CdBrP4- formed by acidification at pH 5 or by photochemical reactions of HOCdP5- and HOCdBrP5- complexes, respectively, are stable at pH 8, moreover, an increase in pH above 8 cannot promote a fast re-coordination of a hydroxide ion into axial position, indicating that CdP4- and CdBrP4- are not so labile, otherwise the central cadmium(II) ion would easily dissociate and its position would be occupied with a [Cd(OH)]+ species, resulting in the formation of the original complexes, HOCdP5- and HOCdBrP5-. However, such a phenomenon has not been observed.Octabrominated porphyrin (H2BrP4-) forms a complex with cadmium(II) even more readily than the unbrominated ligand does. The equilibrium constant can only be estimated in our system (lgK’≈7.36). The formation rate constant (lgk+

(HOCdBrP5-) = 4.82) was ~19 times higher than that of the unbrominated complex (lgk+(HOCdP5-) = 3.55).As a consequence of the coordination of cadmium(II) ion, both the Soret-bands (Fig. 2) and the Q-bands (Fig. 3) are red-shifted in the case of the unbrominated compounds. The molar absorbances of both the main Soret- and the Q-bands of the metalloporphyrins are higher than the corresponding values for the free-base porphyrin. According to our earlier observations [1], this type of spectral properties is unambiguously characteristic for OOP or SAT complexes, confirming the expectations based on the size (95 pm ionic radius) of Cd(II). Also axial coordination of, e.g., a hydroxo ligand to CdP4- is accompanied by bathochromic shifts of the characteristic absorption bands because the axial ligand pulls out the metal center from the cavity of the ligand, resulting in the increase of dome distortion. In the case of the brominated porphyrins, both the Soret- and the Q-bands are significantly red-shifted compared to those of the corresponding unbrominated compounds. While metalation of the unbrominated free-base porphyrin leads to considerable bathochromic shifts of both the Soret- and the Q-bands, insertion of cadmium(II) into the cavity of the brominated free base results in significant hypsochromic shifts of these bands. This very unusual phenomenon in the case of the OOP complexes may be attributed to the decrease of the saddle distortion of the brominated macrocycle. Coordination of the axial hydroxo ligand may appreciably increase the distortion again, as the red-shifts of the characteristic bands indicate.

Figure 2. Molar absorption spectra of H2P4- and its cadmium(II) complexes, CdP4- and HOCdP5- (a), and those

of the corresponding octabromo derivatives, H2BrP4-, CdBrP4-, and HOCdBrP5- (b) at the B- or Soret-band.

a)

b)

Figure 3. Molar absorption spectra of H2P4- and its cadmium(II) complexes, CdP4- and HOCdP5- (a), and those

of the corresponding octabromo derivatives, H2BrP4-, CdBrP4-, and HOCdBrP5- (b) at the Q-bands (the dotted blue line represents the average of energy of Qy(0,0) and

Qx(0,0) of free-base porphyrin).

a)

b)

Coordination of cadmium(II) ion to the unbrominated porphyrin significantly decreases both the estimated quantum yield and lifetime of S1-fluorescence. The efficiency of nonradiative decay increases with the deformation, especially with the out-of-plane position of metal center. The observation that the fluorescence lifetimes are hardly affected by the nature of the metal center implies that probably the geometry is the determining factor in the balance of excited-state processes. Coordination of the axial hydroxo ligand dramatically decreases both the quantun yield and the lifetime of the fluorescence.The fluorescence spectra of the brominated porphyrins could not be measured in the range of wavelength beyond 900 nm (Fig. 3b) because of the detection limit of our equipment. Even so the emission bands belonging to the S1(0,0) and partly the S1(0,1) transitions were recorded and analysed (Table 1). In these cases the resolution by Gaussian curves can compensate, to some extent, the truncation of the spectra. Compared to the emission bands of the corresponding unbrominated species, those of the brominated porphyrins display very large red-shifts, predominantly due to the strongly distorted structure of the macrocycle. Moreover, the Stokes-shifts in these cases are much higher than those for the corresponding unbrominated porphyrins. This phenomenon indicates that, in spite of the considerably nonplanar ground state, much stronger structural distortions occur upon excitation of these species than in the case of the unbrominated compounds.

a)

b)

a)

b)

Figure 3. The analysed fluorescence spectra of H2P4- and its cadmium(II) complexes, CdP4- and HOCdP5- (a), and those of the corresponding octabromo derivatives, H2BrP4-, CdBrP4-,

and HOCdBrP5- (b).

species H2P4- CdP4- HOCdP5- H2BrP4- CdBrP4- HOCdBrP5-

S1-shift (metalation) /cm-1 - 980 450 - 1460 199 S1-shift (bromination) /cm-1 -3360 -2880 -3620

S1-Stokes /cm-1 360 390 450 1170 1350 1690 (S1) 0.075 (0.062b) 0.026 0.010 2.7×10-3 1.5×10-3 1.0×10-2

(S1-B) 0.056 0.022 0.0027 4.4×10-4 3.9×10-4 1.1×10-3 (IC) 0.75 (0.83b) 0.83 0.26 0.17 0.27 0.10

(S1) /ns 10.0 3.4 0.36 (0.26a) 0.15a 0.062a 0.35a

kr(S1) /106s-1 7.5 7.6 28.8 knr(S1) /107s-1 9.2 28.6 279

kr(Strickler-Berg) /106s-1 8.2 33.7 39.5 17 25 29

Table 1. The S1-fluorescence data of porphyrins .

Φ(S1B) = φ(IC S2S1)×φ(S1) and kr(S1) = φ(S1)/τ(S1);. a estimated by the Strickler-Berg-equation; b from Qy-state.

Photolysis of CdP4- at the B- or Soret-band results in the decrease of the absorption at the characteristic bands, indicating an irreversible degradation of the complex, the quantum yield of which is two orders of magnitude higher than that of the free base in aerated system. Besides the LMCT process, dissociation of CdP4- (to the free base and the metal center) can also beobserved. Upon irradiation of HOCdP5- at the Soret-band, both redox degradation of the complex and dissociation of the axial hydroxo ligand take place (Fig. 4a). The overall quantum yield at Soret-band excitation is one order of magnitude higher than that for the reaction of CdP4- because of the pulling effect of the axial ligand, i.e., the higher out-of-plane distance of the metal center.The primary photochemical reactions of brominated porphyrins are very oxygen sensitive probably because of the non-bonding electron-pairs on the peripheral bromine atoms, which may be suitable targets of electrophile attack by oxygen molecules. Deviating from the corresponding unbrominated species, irradiation of CdBrP4- results in a less efficient transformation than that observed for CdP4-. The decrease of the characteristic absorption of CdBrP4- is accompanied by the appearance of a new species, which still reserved the conjugated bond system, probably a chlorin-like product on the basis of its individual spectrum (Fig 4b). Similarly to the case of HOCdP5-, irradiation of the corresponding brominated species, HOCdBrP5-, leads also to both dissociation of the axial hydroxo ligand and irreversible ring cleavage. However, the overall quantum yield in this case is about one order of magnitude lower than that for the unbrominated species in aerated system.

In Table 3 are summarized the distances of the atoms from the plane that passes through only the diagonally situated N1 and N2 atoms and is perpendicular to the axis that is the remnant of the fourfold axis of the undistorted porphyrin ring. The degree of out-of-plane distortion of the porphyrin ring is characterized by these data as well as by the torsion angles of the phenyl rings relative to the mean plane of the macrocycle (defined by the C- Cm- C1- C2 dihedral angle, where C1 and C2 are the corresponding carbon atoms of the phenyl group).In the CdTPPX8 (X=H or Br, Fig. 5) complex, the diagonal N-N distances (i.e., the size of the coordination cavity) are noticeably longer than in the free bases; this significant expansion is neccessary for the approximately in-plane position of the metal center. In the CdTPPBr8 the saddle distorsion decreases, which is manifested in the slight blue-shift of the Q-band compared to that of Qx-band of the free-base porphyrin, deviating from the tendency observed in the case of the unbrominated species. The hydroxo ligand in the axial position (forming HOCdTPPX8

-) pulls the Cd2+ ion out of the ligand center so that it is located about 100 pm above the plane of the nitrogen atoms. Thus, the N-N distances decrease, the expansion disappears.

Figure 4. Spectral changes during the irradiationof HOCdP5- (a), CdBrP4- (b).

a: c(H2P4-)=2.16×10-6 M, c(Cd2+)=9.99×10-6 M, irr=429 nm, I0=7.98×10-6 M photon s-1, irradiation time 65 s.

b: c(H2BrP4-)=2.70×10-5 M, c(Cd2+)=3.31×10-5 M, irr=669 nm, I0=3.55×10-5 M photon s-1, irradiation time 230 min.

species H2P4- CdP4- HOCdP5- H2BrP4- CdBrP4- HOCdBrP5-

(B) /10-5 0.60 39 730 15.6 16.3 104 % structural - 3% 20% - 79% 23% (B-Ar) /10-5 0.33 44 610 1.65 8.8 12.3 %structural - 5% 15% - 86% 22% (Q) /10-5 - 70 114 5.4 23 77

% structural - - - - 100% 81% (Q-Ar) /10-5 - 76 108 1.5 4.4 2.3 % structural - - - - 100% 85%

Table 2. The overall photochemical quantum yields of porphyrins in air-saturated and deoxygenated solution.

Electronic Structure CalculationsElectronic Structure Calculations

Figure 5. The calculated structure of porphyrins.

distance/pm H2TPP CdTPP HOCdTPP- H2TPPBr8 CdTPPBr8 HOCdTPPBr8-

Cd,H 2 4 99 41 4 102 C 1 2 11 54 30 42 C 13 10 44 138 106 133 Cm 2 3 9 8 4 3

d(N1-N2) 409 431 422 422 437 431 d(N3-N4) 424 431 425 427 431 425

Phenyl twisting /deg 65.5 63.8 60.5 55.3 55.5 51.5 λmax(B) /nm 367 384 414 433 454 459 λmax(Q) /nm 565 566 652 661 (697) 688 734

Table 3. The calculated structural and spectral data of porphyrins.

CdTPPBr8

H2TPPBr8

CdTPP

H2TPP

HOCdTPP- HOCdTPPBr8-

There is a correlation between the magnitude of distortion of the porphyrin ring and the red-shift of the absorption maxima. The increasing twist of the phenyl rings towards the plane of the macrocycle promotes its orbitals to contribute to the conjugated bond system of the porphyrin to a larger extent, resulting in red-shifts of the absorption bands.