Chemical Physics Letters 467 (2008) 179–185

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Chemical Physics Letters

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Effects of meso-substituents and core-modification on photophysicaland electrochemical properties of porphyrin–ferrocene conjugates

Smita Rai a, G. Gayatri b, G. Narahari Sastry b,*, M. Ravikanth a,*

a Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400 076, Maharashtra, Indiab Molecular Modeling Group, Indian Institute of Chemical Technology, Hyderabad 500 007, Andhra Pradesh, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 July 2008In final form 28 October 2008Available online 5 November 2008

0009-2614/$ - see front matter � 2008 Published bydoi:10.1016/j.cplett.2008.10.076

* Corresponding authors. Fax: +91 22 25723480 (ME-mail addresses: gnsastry@gmail.com (G. Narah

iitb.ac.in (M. Ravikanth).

The effects of meso-substituents and porphyrin core-modification on electronic communication betweenferrocene and porphyrin in covalently linked porphyrin–ferrocene conjugates are described. The electro-chemical and photophysical studies indicated that the electronic communication between porphyrin andferrocene is strong when meso-substituents are five membered aryl groups than six membered arylgroups. This may be traced to the near orthogonal arrangement of porphyrin ring with six memberedmeso-aryl groups leading to weaker interaction between the porphyrin and ferrocenyl groups in conju-gates, while the five membered furyl and thienyl groups are closer to the porphyrin plane than beingorthogonal. Molecular orbital studies are performed at semiempirical PM3 and BLYP levels to rationalizethe results.

� 2008 Published by Elsevier B.V.

1. Introduction

In the recent past, the research has been focused towards devel-oping molecule based electronic devices [1]. Covalent linkage of apendant redox and photoactive group to a conjugated luminescentsystem will generate new macromolecular systems which are ex-pected to show strong electronic interaction between the molecu-lar components [2]. The photochemical and electrochemicalproperties of the molecular components of the macromoleculescan be modulated via changes in the oxidation state of the redoxactive group and photochemical activity of the luminescentgroups. Specifically, the ferrocenyl groups have been used as redoxactive groups with many macrocycles such as porphyrins. Sinceboth ferrocenyl and porphyrin groups have a rich and reversibleelectrochemistry, the linking of these two groups in one moleculecan lead to interesting electrochemical devices, provided thatground-state electronic communication between the componentsin substantial [3]. Linking ferrocene moieties to porphyrin is alsoof interest because the ferrocene, according to thermodynamicconsiderations, is able to reduce porphyrin hence porphyrin–ferro-cene hybrid molecules can be used in various photochemical de-vices, including donor–acceptor molecules that mimic the initialstages in the photosynthetic process at the molecular level. Thereare many reports available in literature, where one or more ferr-ocenyl moieties are linked to porphyrin directly or through variousspacer groups [4–7]. The direct linkage of ferrocene to the porphy-

Elsevier B.V.

. Ravikanth).ari Sastry), ravikanth@chem.

rin induces strong electronic coupling between the two systemsand the individual characteristic features of porphyrin and ferro-cenyl groups would be altered significantly. Alternately, usingspacer which is conjugated would help in retaining the indepen-dent characteristic features of the two components and help inunderstanding the communication between the components. In-spite of extensive literature on porphyrin–ferrocene conjugates,the reports on systematic study on electronic communication be-tween the porphyrin and ferrocenyl groups in porphyrin–ferroceneconjugates are scarce. In this Letter, we wished to report the sys-tematic investigation on electronic communication between por-phyrin and ferrocenyl groups in porphyrin–ferrocene conjugatesby changing the meso-substituents from six membered tolylgroups to five membered thienyl and furyl groups and also modi-fying the porphyrin core from N4 to N3S and N2S2 cores (Chart 1).It is well established that the electronic properties can be alteredsignificantly by introducing five membered groups such as thienyls[8] and furyls [9] in place of six membered tolyl groups at meso-positions and also modifying the porphyrin core by substituting in-ner nitrogen atom(s) with other hetero atoms such as sulfur, oxy-gen, selenium and tellurium [10]. It is shown in this Letter that theelectronic communication between the porphyrin and ferrocenylgroup in covalently linked porphyrin–ferrocene conjugates de-pends on type of meso-substituents and nature of porphyrin core.

2. Experimental

The reference porphyrin monomers such as meso-5,10,15,20-tetra(p-tolyl)porphyrin (H2TTP) [11], meso-5,10,15,20-tetra-(2-thienyl)porphyrin (H2TThP) [12], meso-5,10,15,20-tetra-

180 S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185

(2-furyl)porphyrin (H2TFP) [13], meso-5,10,15,20-tetra(p-tolyl)21-thiaporphyrin (STTPH) [14] and meso-5,10,15,20-tetra(p-tolyl)-21,23-dithiaporphyrin (S2TTP) [15] were synthesized by followingthe reported procedures. The porphyrin building blocks required tosynthesize porphyrin–ferrocene conjugates such as 5-(4-iodo-phenyl)-10,15,20-tri(p-tolyl)porphyrin [16], 5-(4-iodophenyl)-10,15,20-tri(p-tolyl)-21-thiaporphyrin [17] and 5-(4-iodophenyl)-10,15,20-tri(p-tolyl)-21,23-dithiaporphyrin [18] were prepared asreported previously. The other two unknown porphyrin buildingblocks such as 5-(50-bromo-20-thienyl)-10,15,20-tri(2-thienyl)por-phyrin (H2TThPBr) and 5-(50-bromo-20-furyl)-10,15,20-tri(2-furyl)porphyrin (H2TFPBr), respectively, were obtained in 5–7%yields by mixed condensation of one equivalent of 50-bromo-thio-phene-20-carboxaldehyde or 50-bromo-furan-20-carboxaldehydewith three equivalents of thiophene-20-carboxaldehyde or furan-20-carboxaldehyde and four equivalents of pyrrole under mild acidcatalyzed porphyrin forming conditions [11] followed by columnchromatographic purification on silica gel. The porphyrin–ferro-cene conjugates such as H2TTPFc, H2TThPFc, H2TFPFc, STTPHFcand S2TTPFc were synthesized in 70–75% yields under Lindsey’scopper free coupling conditions [19] by coupling of one equivalentof appropriate porphyrin building block with one equivalent of a-ethynyl ferrocene in toluene/triethylamine (3:1) in the presence ofcatalytic amount of AsPh3/Pd2(dba)3 at 35 �C for overnight andpurified by silica gel column chromatography using petroleumether/dichloromethane mixture as eluent. The compounds werecharacterized by all spectroscopic techniques and data for selectedporphyrin building blocks and porphyrin–ferrocene conjugates arepresented below:

2.1. H2TThPBr

1H NMR (400 MHz, CDCl3, d in ppm):�2.67 (s, 2H, NH), 7.24–7.48(m, 3H, thienyl), 7.64 (d, J = 7.68 Hz, 2H, thienyl), 7.84–7.86 (m, 2H,

Fig. 1. Q-band and Soret band (inset) absorption spectra of: H2TFP (—) and H2TFPFc (—)for Soret band was 5 � 10�6 M.

thienyl), 7.90–7.91 (m, 4H, thienyl), 9.06–9.08 (m, 8H, b-pyrrole).13C NMR (100 MHz, CDCl3, 25 �C): d = 112.2, 114.6, 127.4, 128.8,130.5, 135.4, 136.6, 139.8, 145.9, 152.2. ES MS C36H21N4S4Br calcd.av. mass, 717.8, obsd. m/z 717.1 (M � H+, 100%). Anal. calcd.: C,60.24; H, 2.95; N, 7.81. Found: C, 60.28; H, 2.91; N, 7.78.

2.2. H2TFPBr

1H NMR (400 MHz, CDCl3, d in ppm): �2.62 (s, 2H, NH), 6.95–6.99 (m, 2H, furyl), 7.02–7.04 (m, 3H, furyl), 7.32 (d, J = 1.2 Hz,3H, furyl), 8.12 (s, 3H, furyl), 9.15–9.18 (m, 8H, b-pyrrole). 13CNMR (100 MHz, CDCl3, 25 �C): d = 111.4, 112.7, 113.7, 114.6,122.8, 131.4, 132.2, 135.1, 145.6, 146.2, 146.9, 154.6. ES MS:C36H21N4O4Br, calcd. av. mass, 653.5, obsd. m/z 655.6 ([M+ + 2],100%). Anal. calcd.: C, 66.17; H, 3.24; N, 8.57. Found: C, 66.22; H,3.20; N, 8.52.

2.3. H2TTPFc

1H NMR (400 MHz, CDCl3, 25 �C): d = �2.77 (s, 2H, NH), 2.70 (s,9H, Ar–CH3), 4.30–4.38 (m, 7H, ferrocenyl), 4.62 (s, 2H, ferrocenyl),7.52–7.58 (m, 4H, Ar), 7.82–7.92 (m, 8H, Ar), 8.06–8.12 (m, 4H, Ar),8.82–8.87 (m, 8H, b-pyrrole) ppm. 13C NMR (100 MHz, CDCl3,25 �C): d = 21.7, 77.4, 83.8, 126.4, 128.9, 134.2, 134.8, 137.1,137.8, 138.4, 142.2, 146.6 ppm. ES-MS: C59H44FeN4, calcd. av. mass864.9, obsd. m/z 864.7 (M+, 100%). Anal. calcd.: C, 81.74; H, 5.97; N,6.15. Found: C, 81.79; H, 5.92; N, 6.20.

2.4. H2TThPFc

1H NMR (400 MHz, CDCl3, 25 �C): d = �2.64 (s, 2H, NH), 4.32–4.34 (m, 7H, ferrocenyl), 4.61 (s, 2H, ferrocenyl), 7.51–7.54 (m,3H, thienyl), 7.62 (d, J = 4.0 Hz, 1H, thienyl), 7.78 (d, J = 4.4 Hz,1H, thienyl), 7.84–7.86 (m, 3H, thienyl), 7.92 (s, 3H, thienyl),

recorded in toluene. The concentration used for Q-band spectra was 5 � 10�5 M and

Table 1Absorption data of porphyrin–ferrocene conjugates and their corresponding porphy-rin monomers recorded in toluene.

Compound Soret band k(nm) (log e)

Absorption Q-bands k (nm) (log e)

H2TTP 419 (5.74) 514 (4.36) 548 (4.06) 590 (3.90) 647 (3.86)H2TTPFc 420 (5.62) 516 (4.02) 550 (3.98) 592 (3.62) 648 (3.56)H2TThP 426 (5.59) 523 (4.26) 558 (3.70) 594 (3.48) 661 (3.40)H2TThPFc 427 (5.12) 530 (4.02) 560 (3.38) 596 (3.10) 662 (2.98)H2TFP 433 (4.92) 526 (3.88) 571 (3.84) 605 (sh) 670 (3.20)H2TFPFc 434 (4.34) 527 (3.26) 573 (3.01) 605 (sh) 672 (2.82)STTPH 428 (5.56) 514 (4.47) 550 (4.04) 618 (3.62) 680 (3.69)STTPHFc 432 (5.30) 516 (4.24) 552 (3.88) 620 (3.38) 680 (3.61)S2TTP 435 (5.40) 514 (4.41) 547 (3.85) 633 (3.34) 696 (3.65)S2TTPFc 437 (5.33) 516 (4.46) 551 (4.08) 635 (3.31) 699 (3.78)

Error limits: kmax ± 1 nm; log e ± 10%.

Table 2Electrochemical redox data (V) of porphyrin–ferrocene conjugates and their corre-sponding porphyrin monomers in dichloromethane containing 0.1 M TBAP assupporting electrolyte.

Compound Oxidation Reduction

I II III I II

EthynylFc 0.65 – – – –H2TTP – 1.03 1.30 �1.23 �1.55H2TTPFc 0.61 1.02 – �1.29 �1.67H2TThP – 0.89 1.13 �1.06 �1.41H2TThPFc 0.62 1.03 – �0.99 �1.37H2TFP – 0.80 – �0.98 �1.37H2TFPFc 0.69 0.99 – �0.90 �1.28STTPH – 1.11 1.51 �1.03 �1.35STTPHFc 0.61 1.07 – �1.07 �1.40S2TTP – 1.18 – �0.94 �1.23S2TTPFc 0.61 1.16 1.41 �0.95 �1.27

Error limit: T12, Tp ± 0.02 V.

S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185 181

9.10–9.13 (m, 8H, b-pyrrole) ppm. 13C NMR (100 MHz, CDCl3,25 �C): d = 72.2, 86.4, 127.1, 127.8, 128.6, 130.3, 131.2, 136.6,136.8, 137.2, 146.2, 148.1, 152.8 ppm. ES-MS: C48H30N4S4Fe, calcd.av. mass 846.9, obsd. m/z 847.1 (M+, 100%). Anal. calcd.: C, 68.71;H, 4.88; N, 6.16. Found: C, 68.69; H, 4.83; N, 6.12.

2.5. H2TFPFc

1H NMR (400 MHz, CDCl3, 25 �C): d = �2.58 (s, 2H, NH), 4.20–4.22 (m, 7H, ferrocenyl), 4.46 (s, 2H, ferrocenyl), 7.03–7.06 (m,4H, furyl), 7.33 (d, 3H, J = 4.4 Hz, furyl), 8.14 (s, 4H, furyl), 9.15–9.17 (m, 8H, b-pyrrole) ppm. 13C NMR (100 MHz, CDCl3, 25 �C):d = 74.8, 96.8, 127.1, 109.2, 109.8, 112.6, 114.6, 131.0, 136.4,137.5, 138.82, 143.7, 149.8, 152.8, 155.4 ppm. ES-MS:C48H30N4O4Fe, calcd. av. mass 782.6, obsd. m/z 783.1 (M+, 100%).

Fig. 2. Comparison of oxidation waves of cyclic voltammograms (scan rate,50 mV s�1) of (a) ferrocene (—) and ethynylferrocene (---), (b) H2TTPFc (—) andethynylferrocene (---), (c) H2TFPFc (—) and ethynylferrocene (---), (d) S2TTPFc (—)and ethynylferrocene (---), (e) H2TTPFc (—) and H2TFPFc (. . .) and (f) H2TTPFc (—)and S2TTPFc (. . .) in CH2Cl2 containing 0.1 M TBAP as supporting electrolyte.

Anal. calcd.: C, 73.93; H, 5.25; N, 6.63. Found: C, 73.97; H, 5.22;N, 6.69.

The various instruments used for characterization and studiesare as follows: 1H and 13C NMR spectra were recorded using a Var-ian 400 MHz NMR instrument and are reported in d (ppm), referredto 1H (of residual proton; d 7.26) and 13C (d 77.0) signals of CDCl3.Electro-spray mass spectra (ES-MS) were recorded with a Q-Tofmicro (YA-105) mass spectrometer. Elemental analyses were per-formed on Carlo-Erba MOD 1106 CHN analyzer. Absorption spectrawere recorded using Perkin–Elmer Lambda-35 UV–Vis spectropho-tometer. Cyclic voltammetric (CV) and differential pulse voltam-metric (DPV) studies were carried out with BAS electrochemicalsystem utilizing the three electrode configuration consisting of aGlassy carbon (working electrode), platinum wire (auxiliary elec-trode) and saturated calomel (reference electrode) electrodes.The experiments were done in dry dichloromethane using 0.1 M

Fig. 3. Comparison of reduction waves of cyclic voltammograms (scan rate,50 mVs�1) of (a) H2TTP (—) and H2TTPFc (---), (b) H2TFP (—) and H2TFPFc (---)in CH2Cl2 containing 0.1 M TBAP as supporting electrolyte.

Fig. 4. Comparison of emission spectra of (a) H2TTP (—) and H2TTPFc (---), (b) H2TThP (—) and H2TThPFc (---), (c) H2TFP (—) and H2TFPFc (-- -) and (d) H2TTPFc (—),H2TThPFc (---) and H2TFPFc (. . .) recorded in toluene. The concentration used was 5 � 10�6 M.

182 S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185

tetrabutylammonium perchlorate as supporting electrolyte. Halfwave potentials were measured using DPV and also calculatedmanually by taking the average of the cathodic and anodic peakpotentials.

Table 3Emission data of porphyrin–ferrocene conjugates along with their correspondingporphyrin monomers.

Compound Q (0, 0) Q (0, 1) /f sf (10�9 s) krad/106 s�1 knr/108 s�1

H2TTP 648 714 0.11 9.32 11.81 0.95H2TTPFc 648 715 0.046 6.55 6.99 1.45H2TThP 668 728 0.0046 9.27 5.00 1.07H2TThPFc 664 723 0.0008 1.85 0.43 5.40H2TFP 695 – 0.011 5.77 1.89 1.71H2TFPFc 694 – 0.0038 0.18 20.2 52.9STTPH 678 750 0.0168 1.77 9.49 5.55STTPHFc 686 756 0.0035 0.765 4.58 13.03S2TTP 706 781 0.0076 1.34 5.67 7.41S2TTPFc 707 778 0.0058 0.782 7.42 12.71

Error limits: /f ± 10%, sf ± 0.007.

Table 4PM3 optimized bond lengths (in Å) of the linker region between the porphyrin and ferroc

R Fc

R=O S, ,

r1 r2 r3

H2TTPFcSTTPHFcS2TTPFcH2TFPFcH2TThPFc

The steady state fluorescence spectra were recorded on Perkin–Elmer LS-55 luminescence spectrometer. The fluorescence quan-tum yields (/f) were estimated from the emission and absorptionspectra by comparative method using the following equation:

/f ¼ ½FðsampleÞ�½AðstandardÞ�=½FðstandardÞ�½AðsampleÞ�� /f ðstandardÞ ð1Þ

where F(sample) and F(standard) are the integrated fluorescenceintensities of the donor porphyrin (meso-tolylporphyrin) and thestandard, respectively, and A(sample) and A(standard) are theabsorbances of the meso-tolyl porphyrins and the standard, respec-tively, at the excitation wavelength and /f(standard) represents thequantum yield of the standard sample. Meso-tetratolylporphyrin(H2TTP, /f = 0.11) was used as the standard for all quantum yieldmeasurements [20].

The time-resolved fluorescence decay measurements were car-ried out at magic angle using a picosecond diode laser based timecorrelated single-photon counting (TCSPC) fluorescence spectrom-eter from IBH, UK. All the decays were fitted to single exponential.The good fit criteria were low chi-square (1.0) and random distri-butions of residuals.

enyl units.

r1 r2 r3

1.415 1.195 1.4001.415 1.195 1.3991.415 1.196 1.3991.403 1.195 1.3981.399 1.197 1.398

S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185 183

3. Results and discussion

The absorption spectra of 5,10,15,20-meso-tetrafurylporphyrin(H2TFP) and tetrafurylporphyrin–ferrocene conjugate (H2TFPFc)recorded in toluene at room temperature are shown in Fig. 1 anddata of porphyrin monomers and porphyrin–porphyrin–ferroceneconjugates are presented in Table 1. In general, it is known thaton replacement of six membered tolyl groups with five memberedthienyl [8] and furyl groups [9] at meso-positions, the absorptionbands shifts towards higher wavelengths and maximum shiftswere observed for meso-tetrafurylporphyrins [9]. This is becausethe thienyl and furyl groups are almost in plane with the porphyrinring unlike tolyl groups which are perpendicular with the porphy-rin ring resulting in greater extension of porphyrin p-delocaliza-tion in to meso-thienyl and meso-furyl groups compared to meso-tolyl groups. In porphyrin–ferrocene conjugates, the absorptionbands experienced negligible shifts but the extinction coefficientswere reduced compared to their corresponding porphyrin mono-mers supporting electronic interaction between ferrocenyl andporphyrin units. Similarly, on modification of porphyrin core from

Fig. 5. HOMO and LUMO orbital diagrams along with the D

N4 to N3S to N2S2, the absorption bands experiences red shifts dueto alteration of p-delocalization in porphyrin ring on sulfur substi-tution in place of nitrogen(s) [10]. In heteroporphyrin–ferroceneconjugates, the absorption bands showed minimal shifts butreduction of extinction coefficients supporting the electronic inter-action between ferrocene and heteroporphyrin units.

The electrochemical properties of porphyrins and porphyrin–ferrocene conjugates were followed by cyclic voltammetry at ascan rate of 50 mV/s and differential pulse voltammetry at a scanrate of 20 mV/s using tetrabutylammonium perchlorate as sup-porting electrolyte (0.1 M) in dichloromethane. In general, porphy-rins exhibit two oxidation and two reduction waves correspondingto the formation of mono, dications and mono, dianions of porphy-rin ring, respectively. The absolute E1/2 values depend on the nat-ure of the porphyrin. In porphyrin–ferrocene conjugates, anadditional redox couple corresponding to the oxidation of ferro-cene ring is also expected [5]. We carried out electrochemical stud-ies of porphyrin–ferrocene conjugates and their correspondingporphyrin monomers along with ferrocene and a-ethynylferro-cene. A comparison of ferrocene oxidation waves in various sets

EHOMO�LUMO values in eVs at BLYP/6-31G*//PM3 level.

184 S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185

of compounds such as ferrocene–ethynylferrocene, ethynylferro-cene–H2TTPFc, ethynylferrocene–H2TFPFc, ethynylferrocene–S2TPPFc, H2TTPFc–H2TFPFc and H2TTPFc–S2TTPFc is shown inFig. 2 and the data is presented in Table 2. The free ferrocene showsa reversible oxidation couple at 0.38 V which is shifted anodicallyin ethynylferrocene by 270 mV indicating that the oxidation be-comes difficult due to distribution of electron density of ferroceneonto ethynyl groups. In porphyrin–ferrocene conjugates, the ferr-ocenyl oxidation has experienced positive or negative shiftsdepending on the kind of porphyrin to which the ferrocene is at-tached. As clear from the Fig. 2 and data in Table 2, that in H2TTPFc(Fig. 2b), STTPHFc and S2TTPFc (Fig. 2d), the ferrocene is easier tooxidize whereas in H2TThPFc and H2TFPFc (Fig. 2c), it is difficult tooxidize compared to a-ethynylferrocene. This observation can berationalized as follows. When porphyrin is tetratolylporphyrin,the aryl groups at meso-position are out of plane with theporphyrin ring thus preventing the electron density distributionbetween ferrocene and porphyrin groups resulting easier oxidationof ferrocene group compared to a-ethynylferrocene. However,when porphyrins are tetrathienyl and tetrafurylporphyrins, thefive membered thienyl and furyl groups are relatively more inplane with the porphyrin and the electron density between ferro-cene and porphyrin ring is better distributed resulting in difficultoxidation of ferrocene group compared to ethynylferrocene, whichare corroborated by molecular orbital calculations (vide infra).

The porphyrin ring oxidation and reduction potentials are alsoexperienced shifts upon linking the ferrocenyl group to the por-phyrin ring. The porphyrin oxidations were found to be chemicallyirreversible hence only peak potentials are tabulated and reduc-tions were reversible or quasi-reversible (DEp = 60–120 mV). Thecomparison of reduction waves of H2TTP and H2TFPFc are shownin Fig. 3a and comparison of reduction waves of H2TFP andH2TFPFc are shown in Fig. 3b. Generally, the introduction of fivemembered thienyl and furyl groups in place of six membered

N

N Y

XC C

CH3

H3C

Fe

CH3

N

N HN

NH

O

O

O

X = NH; Y = NH; H2TTPFc

X = NH; Y = S; STTPHFc

X = S; Y = S; S2TTPFc

Chart 1. Structures of porphy

meso-tolyl groups results in easier oxidations and reductions[8,9]. The porphyrin–ferrocene conjugates, H2TTPFc and H2TFPFcshowed opposite trend. In H2TTPFc, the oxidation potentials wereslightly shifted towards less positive and reduction potentials wereshifted towards more negative compared to H2TTP (Fig. 3a) indi-cating that the porphyrin ring in H2TTPFc is easier to oxidize butdifficult to reduce compared to H2TTP. However, in H2TFPFc andH2TThPFc, the oxidations were shifted to more positive by 140–180 mV and reductions were shifted to less negative compared toH2TFP (Fig. 3b) and H2TThP, respectively, indicating that the por-phyrin in H2TFPFc and H2TThPFc is difficult to oxidize and easierto reduce compared to H2TFP and H2TThP. The ferrocene–hetero-porphyrin conjugates STTPHFc and S2TTPFc showed similar trendas observed for H2TTPFc. Thus the electrochemical data confirmsthe electronic communication between the ferrocene and porphy-rin groups in porphyrin–ferrocene conjugates. The magnitude ofthe electronic communication between porphyrin ring and ferroce-nyl group in porphyrin–ferrocene conjugates depends on the typeof meso-substituent present on the porphyrin ring rather than thenature of porphyrin core.

The fluorescence properties of porphyrin monomers and por-phyrin–ferrocene conjugates were studied by steady state andtime-resolved fluorescence techniques. The comparison of steadystate fluorescence spectra of reference porphyrins and porphy-rin–ferrocene conjugates are shown in Fig. 4, and the data is tabu-lated in Table 3. Inspection of Table 3, reveal the followingobservations: (1) the emission peak maxima of porphyrin in por-phyrin–ferrocene conjugates is almost identical with the emissionpeak maxima of corresponding monomeric porphyrin. (2) Thequantum yields of porphyrin in porphyrin–ferrocene conjugatesare decreased compared to the corresponding monomeric porphy-rins. (3) The magnitude of decrease of quantum yields depends onthe type of meso-substituents and nature of porphyrin core (Table3). The singlet excited state lifetime for porphyrin monomers and

N

N HN

NHC C

Fe

S

S

S S

C C

FeO

H2TThPFc

H2TFPFc

rin–ferrocene conjugates.

S. Rai et al. / Chemical Physics Letters 467 (2008) 179–185 185

porphyrin–ferrocene conjugates were measured by single-photoncounting technique. All compounds were excited at 406 nm andwere detected at the emission peak maxima of the compounds.The fluorescence decays of porphyrin–ferrocene conjugates andtheir corresponding reference compounds were fitted to singleexponential decay functions. The singlet state lifetime s, rate ofradiative decay kr and rate of nonradiative decay knr presented inTable 3 reveal the following as compared to their correspondingreference porphyrins: (1) the singlet state lifetime s of porphyrinin porphyrin–ferrocene conjugates is decreased. (2) The radiativelifetime, kr is decreased and nonradiative lifetime knr is increasedfor porphyrin in porphyrin–ferrocene conjugates. (3) The decreasein s and kr and increase in knr depends on type of meso-substituentand nature of porphyrin core. Thus, the significant quenching ofthe porphyrin fluorescence in porphyrin–ferrocene conjugatescould be due to the electron transfer from ferrocene to singlet ex-cited state of porphyrin [21]. However, the maximum effects wereseen for H2TThPFc and H2TFPFc compared to the meso-six mem-bered aryl analogues. These results indicate maximum electroniccommunication between ferrocene and porphyrin is present inH2TThPFc and H2TFPFc due to in plane orientation of ferrocenylgroup and porphyrin compared to H2TTPFc in which the ferrocenylgroup and porphyrin ring are out of plane with each other. Thus,the presence of five membered groups at meso-positions helps inbetter electronic communication between porphyrin and ferroce-nyl groups.

To rationalize the observations in the foregoing discussions weresort to carry out molecular orbital calculations. Quantum chem-ical calculations are carried out on all five porphyrin–ferroceneconjugates H2TTPFc, STTPHFc, S2TTPFc, H2TFPFc and H2TThPFc.Previous studies showed that the core-modified porphyrins under-go significant out of plane distortions and sensitive to the level oftheory, but the qualitative trends at semiempirical PM3 and AM1levels of theory are in agreement with higher level calculations[22]. In this study, optimizations are performed at semiempiricalPM3 level using SPARTAN’06 program package [23]. This is followedby energy evaluations at BLYP/6-31G* on the PM3 optimizedgeometries.

Since the extent of oxidation in conjugated systems depends onp-delocalization which in turn can be explained based on the bondlengths and frontier molecular orbitals, we examined the bondlengths between the linker atoms (Table 4) and the nature of fron-tier orbitals. The HOMO–LUMO energy gap and the nature ofHOMO have revealed that the delocalization is higher in H2TFPFcand H2TThPFc (Fig. 5). Thus the electronic communication be-tween the porphyrin and ferrocenyl moieties is very sensitive tothe meso-substituents, which provides a good handle to tune theproperties. Table 4 reveal the r1 distance between the linker atomsis shorter in the case of meso-furyl and meso-thienyl substitutedporphyrins compared to meso-tolyl substituted porphyrins. Thismay be traced to the higher levels of delocalization in the H2TFPFcand H2TThPFc. The energy difference between the HOMO and

LUMO (Fig. 5) can also indicative of the extent of electron delocal-ization and stability of the systems considered here (see Chart 1).

4. Conclusion

The electronic communication between porphyrin and ferro-cene in the covalently linked porphyrin–ferrocene conjugateswere studied by varying the type of meso-substituents and natureof porphyrin core. It is shown clearly in this Letter that theelectronic communication is relatively stronger when porphyrinand ferrocene units are in one plane and the interactions are weak-er when porphyrin and ferrocene units are out of plane. The studiessupported a strong interaction between porphyrin and ferrocenylgroups in porphyrin–ferrocene conjugates when five memberedgroups such as thienyls and furyls are present at meso-positionscompared to six membered tolyl groups. Molecular modeling stud-ies are carried to corroborate the experimental results.

Acknowledgements

This research was supported by Department of Atomic Energyand Department of Science and technology, Government of India,New Delhi to MR.

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