Ferrocenyl BODIPYs: synthesis, structure and properties{

Prabhat Gautam, Bhausaheb Dhokale, Shaikh M. Mobin and Rajneesh Misra*

Received 28th August 2012, Accepted 3rd October 2012

DOI: 10.1039/c2ra21964f

A set of donor–p-acceptor–p-donor type ferrocenyl substituted

BODIPYs were designed and synthesized by the Sonogashira

coupling reaction. These compounds exhibit red shifted absorp-

tion and poor fluorescence. The electrochemical properties of

these compounds exhibit strong donor–acceptor interactions. The

crystal structure of 3b exhibits an extensive hydrogen bonded 2-D

network.

Boron-dipyrromethane (BODIPY) dyes have attracted considerable

attention because of their distinguished properties, such as high

absorption coefficients, high quantum yields and high photostabil-

ity.1 These dyes are used for a variety of applications in light

harvesting molecular arrays, photodynamic therapy, sensors, energy

transfer cassettes and photovoltaic devices.2 The electronic properties

of the BODIPYs can be tuned by functionalization at the meso-

position, as well as at the pyrrolic position. Functionalization of the

BODIPY dye at the pyrrolic position perturbs the electronic

properties more significantly compared to the meso-position because

the pyrrolic site favors a coplanar geometry for the attached group,

which maximizes the electronic coupling, whereas the meso-site

favors an orthogonal geometry which minimizes the electronic

communication.3 The BODIPY unit acts as an electron acceptor.4 It

has been established that the structural motifs of D–A–D type

compounds show promising nonlinear optical (NLO) behavior.5

Therefore, we were interested in incorporating the donor groups at

both ends of the BODIPY dye and to explore its properties. There

are many reports where donor groups are attached at the pyrrolic

position of the BODIPY.6 Ferrocene is a strong donor and is highly

stable.7 In this work, we have incorporated a ferrocenyl group on

both the pyrroles of the BODIPY and generated a D–p-A–p-D type

molecular system. Here, our aim was to explore the effect of the

ferrocene unit on the photophysical and electrochemical behavior of

BODIPY on enhancement of the p-conjugation length.

The synthetic route for ferrocenyl substituted BODIPYs 3a–3d is

shown in Scheme 1. BODIPY 1 was synthesized following earlier

reports.8 The 2,6-diiodo BODIPY 2 was synthesized by the

iodination reaction of BODIPY 1.9 The ferrocenyl substituted

BODIPYs 3a–3d were synthesised by Sonogashira coupling reactions

of diiodo BODIPY 2 with the corresponding ferrocenyl derivatives.

Diiodo substituted BODIPY 2 was reacted with ethynylferrocene,

4-ferrocenylphenylacetylene, 3-ferrocenylphenylacetylene and 4-ethy-

nyl-phenylethynylferrocene, which resulted in compounds 3a, 3b, 3c

and 3d in 60%, 40%, 50% and 55% yields respectively.

The Sonogashira coupling reaction of 2,6-dibromo BODIPY with

the corresponding ferrocenyl alkyne resulted in multiple products

which were difficult to purify. On the other hand, the Sonogashira

coupling reactions of 2,6-diiodo BODIPY 2 with the ferrocenyl

derivatives were straight forward and the purification was achieved

by column chromatography followed by repeated recrystallization of

the product in a mixture of chloroform and ethanol. The isolation of

ferrocenyl substituted BODIPY 3b was difficult because of its poor

solubility. It is sparingly soluble in chloroform, dichloromethane and

toluene and due to this we were able to record the 1H NMR

spectrum only. The ferrocenyl BODIPYs 3a–3d were well character-

ized by 1H, 13C NMR and HRMS techniques. Compounds 3a–3b

were also characterized by single crystal X-ray diffraction.

UV-visible absorption spectra of all the compounds in toluene

were recorded on a Carry-100 Bio UV-visible Spectrophotometer

with a concentration of 1.5 6 1024 M at room temperature. Cyclic

voltammetric (CV) studies for BODIPYs 3a–3b were carried out

with an electrochemical system utilizing a three-electrode configura-

tion consisting of a glassy carbon (working) electrode, platinum wire

(auxiliary) electrode and a saturated calomel (reference) electrode.

The experiments were performed in dry dichloromethane with 0.1 M

tetrabutylammonium hexafluorophosphate (Bu4NPF6) as the sup-

porting electrolyte at a concentration of 1.5 6 1024 M . Crystals of

Department of Chemistry, Indian Institute of Technology Indore, MP,452017, India. E-mail: rajneeshmisra@iiti.ac.in; Fax: +91 7312361482;Tel: +91 7312438710{ Electronic supplementary information (ESI) available: Synthetic proce-dures and spectroscopic characterizations for new compounds 3a–3d andcrystallographic data for 3a and 3b. CCDC 891093 and 891094. For ESI andcrystallographic data in CIF or other electronic format details of anysupplementary information available. See DOI: 10.1039/c2ra21964f Scheme 1 General procedure for the synthesis of 3a–3d.

RSC Advances Dynamic Article Links

Cite this: RSC Advances, 2012, 2, 12105–12107

www.rsc.org/advances COMMUNICATION

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 12105–12107 | 12105

Dow

nloa

ded

on 2

9/04

/201

3 09

:55:

16.

Publ

ishe

d on

29

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2RA

2196

4FView Article Online / Journal Homepage / Table of Contents for this issue

BODIPYs 3a and 3b suitable for X-ray analysis were obtained by the

slow diffusion of ethanol into their chloroform solutions. The vial

containing this solution was loosely capped to promote crystal-

lization upon ethanol diffusion. The structures were solved by direct

methods using SHELXS-97 and were refined by full matrix least-

squares with SHELXL-97, refining on F2.

The BODIPYs exhibit a strong absorption band at around 500

nm along with a broad transition in the higher energy region.10 The

BODIPYs 3a–3d exhibit a strong absorption band between 575–590

nm, corresponding to the S0 A S1 (p A p*) transition, and a weak

absorption band between 400–410 nm due to the S0 A S2 (p A p*)

transition (Fig. 1). The substitution of the ferrocenyl group on the

BODIPY results in a substantial red shift in the absorption maxima.

The absorption maxima of 3a–3d is considerably red shifted

compared to the unsubstituted BODIPY 1, which reflects the strong

electronic interaction between the ferrocene and the BODIPY.11 The

meta branching in compound 3c disrupts the extended p-conjugation

compared to the other phenylacetylene spacers.12 Thus, a lower red

shift is observed for compound 3c compared with the other

ferrocenyl BODIPYs. The absorption maxima are broad in 3a–3d

when compared to the sharp absorption of unsubstituted BODIPY 1

and this may be due to the overlap of the charge-transfer

absorption.13 Compounds 3a–3d are poorly emissive in nature

compared to BODIPY 1 due to the fast non-radiative deactivation of

the exited state via intramolecular charge transfer.14,16

The electrochemical data of BODIPYs 3a–3b is listed in Table 1

and a representative cyclic voltammogram is presented in Fig. 2. The

BODIPYs exhibit one oxidation and one reduction wave corre-

sponding to the formation of a mono p-radical cation and mono

p-radical anion respectively.15 The ferrocenyl substituted BODIPYs

3a–3d show one reversible reduction wave in the region of 21.08 to

21.05 V. The reduction potential of 3a–3d is shifted to lower values

compared to unsubstituted BODIPY 1, indicating that the boron–

dipyrromethane unit is easier to reduce.16 Oxidation peaks

corresponding to the oxidation of ferrocene to ferrocenium and the

formation of a BODIPY p-radical cation are observed for

compounds 3a–3d. The ferrocenyl BODIPYs 3a–3d show one

irreversible oxidation wave in the 1.24–1.20 V region. The trend in

the oxidation potential of the ferrocenyl moiety in the ferrocenyl

substituted BODIPYs 3a–3d follows the order 3a . 3b . 3d . 3c.

Compared to the oxidation potential of free ferrocene, the oxidation

of the ferrocenyl moiety became harder, which reflects the strong

electronic communication between the ferrocenyl unit and the

BODIPY core in 3a–3d.17

BODIPY 3a crystallizes in the monoclinic space group P21/c and

BODIPY 3b crystallizes in the monoclinic space group C2/c, with a

crystallographic two-fold axis which passes through atoms B1, C3

and C6–H6. Fig. 3 shows the single crystal X-ray structure of 3a and

3b.

The BODIPY core, B(N2F2), exhibits an F–B–F angle of 109.0(5)ufor 3a and 110.2(4)u for 3b. The average bond length of the B–N

bond for BODIPYs 3a and 3b lies in the range of 1.537(4) A–1.557(7)

A and the average bond length of the B–F bond lies in the range of

1.372(7) A–1.407(7) A. The dihedral angles between the meso-

substituted phenyl group and the BODIPY core are 89.15u and

79.79u for BODIPYs 3a and 3b respectively. This may be attributed

to the steric hindrance from the 1,7-dimethyl-substituents of the

BODIPY core. The meso-phenyl mean plane is tilted by 20.11u with

respect to the BF2 mean plane in 3b, whereas they are almost parallel

in 3a (2.61u). The two cyclopentadienyl rings of the ferrocene moiety

Fig. 1 UV-visible absorption spectra of ferrocenyl substituted BODIPYs 1

and 3a–3d in toluene. The concentration used was 1025 M.

Table 1 Photophysical and electrochemical data of BODIPYs 1 and3a–3d

Compound

Photophysical dataa Electrochemical datad

labs (nm)b e (M21cm21)c E1ox (V) E2

oxid (V)e E1red (V)

Ferrocene — — 0.38 — —1 503 93 989 — 1.16 21.253a 583 15 500 0.53 1.24 21.053b 590 42 800 0.52 1.21 21.063c 576 69 600 0.44 1.20 21.083d 586 69 200 0.51 1.21 21.06a Measured in toluene. b labs (nm): absorption wavelength of the firstabsorption maximum. c e: extinction coefficient. d Recorded by cyclicvoltammetry in a 0.1 M solution of Bu4NPF6 in DCM (1.5 6 1024 M)at a scan rate of 100 mV s21, vs. a saturated calomel electrode. e Forthe irreversible redox process, the peak potential is quoted.

Fig. 2 Cyclic voltammogram of BODIPY 3a in DCM containing 0.1 M

Bu4NPF6 as the supporting electrolyte, recorded at a scan speed of 100

mVs21.

Fig. 3 Single crystal X-ray structures of BODIPY 3a and 3b. (i) Front view

and (ii) side view.

12106 | RSC Adv., 2012, 2, 12105–12107 This journal is � The Royal Society of Chemistry 2012

Dow

nloa

ded

on 2

9/04

/201

3 09

:55:

16.

Publ

ishe

d on

29

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2RA

2196

4F

View Article Online

are parallel with the syn-periplanar eclipsed conformation in both 3a

and 3b. The tilt between the BODIPY and the ferrocene units is

more prominent in 3b, with a dihedral angle of 35u, while for 3a it is

3.70u. The ferrocene unit in both 3a and 3b lies on the opposite sides

of the BODIPY mean plane. The important bond lengths and bond

angles are listed in Table S1 (see the ESI for details{). Moreover, in

the case of 3a, the chloroform solvation is linked to F1 via C–H…F

hydrogen bonding of 2.46(3) A.

The packing diagram of 3b (Fig. 4) reveals intermolecular C–

H…F and C–H…p interactions. The C–H…F interaction involves

the meso-phenyl hydrogen (H6) of one molecule with both the

F-atoms of a neighboring molecule along the b-axis, forming a 1D-

linear chain. The other phenyl hydrogen atoms (two H4 atoms) are

involved in C–H…p interactions with adjacent molecules along the

a-axis, leading to the formation of a 2D-network.

In summary, we have synthesized a series of ferrocenyl substituted

BODIPYs via a Pd catalyzed Sonogashira coupling reaction. The

UV-visible absorption and electrochemical properties of these

molecules show strong donor–acceptor interactions. The crystal

structure of 3b shows a 2D-network.

References

1 A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891.2 (a) T. Cheng, T. Wang, W. Zhu, X. Chen, Y. Yang, Y. Xu and X. Qian,

Org. Lett., 2011, 13, 3656; (b) O. A. Bozdemir, R. Guliyev, O. Buyukcakir,S. Selcuk, S. Kolemen, G. Gulseren, T. Nalbantoglu, H. Boyaci and E. U.Akkaya, J. Am. Chem. Soc., 2010, 132, 8029; (c) H. S. Kim, T. C. T. Pham

and K. B. Yoon, Chem. Commun., 2012, 48, 4659; (d) M. Benstead, G. H.Mehl and R. W. Boyle, Tetrahedron, 2011, 67, 3573; (e) Y. Zhou, Y. Xiao,S. Chi and X. Qian, Org. Lett., 2008, 10, 633; (f) S. J. Lord, N. R. Conley,H. I. D. Lee, R. Samuel, N. Liu, R. J. Twieg and W. E. Moerner, J. Am.Chem. Soc., 2008, 130, 9204; (g) R. West, C. Panagabko and J. Atkinson,J. Org. Chem., 2010, 75, 2883; (h) A. B. Descalzo, H. J. Xu, Z. L. Xue, K.Hoffmann, Z. Shen, M. G. Weller, X. Z. You and K. Rurack, Org. Lett.,2008, 10, 1581; (i) S. R. Marder, B. Kippelen, A. K. Y. Jen and N.Peyghammbarian, Nature, 1997, 388, 845–851; (j) S. M. LeCours, H. W.Guan, S. G. DiMagno, C. H. Wang and M. J. Therien, J. Am. Chem. Soc.,1996, 118, 1497.

3 (a) G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed., 2008,47, 1184; (b) V. Lakshmi and M. Ravikanth, J. Org. Chem., 2011, 76,8466.

4 F. Algi and A. Cihaner, Org. Electron., 2009, 10, 453.5 (a) Y. Wang, D. Zhang, H. Zhou, J. Ding, Q. Chen, Y. Xiao and S. Qian,

J. Appl. Phys., 2010, 108, 033520; (b) K. D. Belfield, A. R. Morales, B. S.Kang, J. M. Hales, D. J. Hagan, E. W. V. Stryland, V. M. Chapela and J.Percino, Chem. Mater., 2004, 16, 4634; (c) V. Hrobarikova, P. Hrobarik,P. Gajdos, I. Fitilis, M. Fakis, P. Persephonis and P. Zahradnik, J. Org.Chem., 2010, 75, 3053; (d) P. Gautam, B. Dhokale, V. Shukla, C. P. Singh,K. S. Bindra and R. Misra, J. Photochem. Photobiol., A, 2012, 239, 24.

6 D. K. Zhang, Y. C. Wang, Y. S. Xiao, X. Qian and X. H. Qian,Tetrahedron, 2009, 65, 8099.

7 M. L. H. Green, S. R. Marder, M. E. Thompson, J. A. Bandy, D. Bloor,P. V. Kolinsky and R. J. Jones, Nature, 1987, 330, 360.

8 (a) W. Yang, Y. Li, H. Liu, L. Chi and Y. Li, Small, 2012, 8, 504; (b) X.Yin, Y. Li, Y. Li, Y. Zhu, X. Tang, H. Zheng and D. Zhu, Tetrahedron,2009, 65, 8373; (c) M. Yuan, Y. Li, J. Li, C. Li, X. Liu, J. Lv, J. Xu, H.Liu, S. Wang and D. Zhu, Org. Lett., 2007, 9, 2313; (d) M. Yuan, X. Yin,H. Zheng, C. Ouyang, Z. Zuo, H. Liu and Y. Li, Chem.–Asian J., 2009, 4,707; (e) M. Zhu, L. Jiang, M. Yuan, X. Liu, C. Ouyang, H. Zheng, X. Yin,Z. Zuo, H. Liu and Y. Li, J. Polym. Sci., Part A: Polym. Chem., 2008, 46,7401.

9 J. Godoy, G. Vives and J. M. Tour, Org. Lett., 2010, 12, 1464.10 Y. Chen, J. Zhao, H. Guo and L. Xie, J. Org. Chem., 2012, 77, 2192.11 T. K. Khan, R. R. S. Pissurlenkar, M. S. Shaikh and M. Ravikanth, J.

Organomet. Chem., 2012, 697, 65.12 (a) J. S. Melinger, Y. Pan, V. D. Kleiman, Z. Peng, B. L. Davis, D.

McMorrow and M. Lu, J. Am. Chem. Soc., 2002, 124, 12002; (b) R. Misra,R. Kumar, T. K. Chandrashekar, C. H. Suresh, A. Nag and D. Goswami,J. Am. Chem. Soc., 2006, 128, 16083.

13 R. Ziessel, P. Retailleau, K. J. Elliott and A. Harriman, Chem.–Eur. J.,2009, 15, 10369.

14 (a) S. Fery-Forgues and B. Delavaux-Nicot, J. Photochem. Photobiol., A,2000, 132, 137; (b) B. Dhokale, R. Misra and P. Gautam, TetrahedronLett., 2012, 53, 2352; (c) V. A. Nadtochenko, N. N. Denisov, V. Y. Gak,N. V. Abramova and N. M. Loim, Russ. Chem. Bull., 1999, 148, 1900; (d)S. Barlow and S. R. Marder, Chem. Commun., 2000, 1555.

15 X. Yin, Y. Li, Y. Zhu, X. Jing, Y. Li and D. Zhu, Dalton Trans., 2010, 39,9929.

16 M. R. Rao, K. V. Pavan Kumar and M. Ravikanth, J. Organomet. Chem.,2010, 695, 863.

17 R. Maragani, T. Jadhav, S. M. Mobin and R. Misra, Tetrahedron, 2012,68, 7302–7308.

Fig. 4 Packing diagram of 3b forming a 2D-network along the a-axis.

This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 12105–12107 | 12107

Dow

nloa

ded

on 2

9/04

/201

3 09

:55:

16.

Publ

ishe

d on

29

Oct

ober

201

2 on

http

://pu

bs.r

sc.o

rg |

doi:1

0.10

39/C

2RA

2196

4F

View Article Online