Adiabatic Photoinduced Electron Transfer and Back Electron Transfer

17694

J. Phys. Chem. C 2008, 112, 17694–17701

Adiabatic Photoinduced Electron Transfer and Back Electron Transfer in a Series of Axially Substituted Silicon Phthalocyanine Triads Luis Martıń-Gomis,† Kei Ohkubo,‡ Fernando Fernańdez-La´zaro,† Shunichi Fukuzumi,*,‡ and ´ ngela Sastre-Santos*,† A DiVisioń de Quı´mica Orgańica, Instituto de Bioingenierıá, UniVersidad Miguel Hernańdez, Elche 03202, Alicante, Spain, and Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ReceiVed: June 5, 2008; ReVised Manuscript ReceiVed: July 25, 2008

A series of new electron acceptor bearing silicon phthalocyanine (SiPc) triads have been synthesized, using the six-coordinated nature of the central silicon atom, by attachment of two electron-acceptor units, fullerene SiPc-(C60)2, trinitrofluorenone SiPc-(TNF)2, and trinitrodicyanomethylenefluorene SiPc-(TNDCF)2. The redox and photophysical properties of SiPc triads in benzonitrile are determined to evaluate the energy of the charge-separated (CS) states and driving force of photoinduced electron transfer in SiPc triads. Photoexcitation of SiPc triads in benzonitrile results in efficient formation of the CS states, which were detected by femtosecond laser flash photolysis measurements. The fate of the CS state whether it decays to the triplet excited-state of the SiPc unit or to the ground-state was examined by nanosecond laser flash photolysis measurements for the longer time scale. The driving force dependence of the rate constants of photoinduced electron transfer and back electron transfer in SiPc triads is analyzed in light of the Marcus theory of adiabatic intramolecular electron transfer. CHART 1: Chemical Structure of C60-SiPc-C60 Triad 1

Introduction (Pcs),1,2

structural analogues of porphyrins Phthalocyanines with a strong absorption in the visible region, are robust chromophores with special photophysical and photochemical properties that make them, alone or in combination with many other electro- and photoactive moieties, ideal building blocks for the construction of molecular materials with unique electronic and optical properties. Generally, Pcs present intermolecular π-π stacking interactions between the planar faces of the macrocycles, even at low concentration, giving rise to dimerization or aggregation which results in fluorescence quenching and low solubility. In contrast, axially substituted silicon Pcs have gained great attention because they are not able to aggregate due to their special structural features,3 thus avoiding fluorescence quenching. Therefore, they are very attractive targets to study photophysical processes.4 In the past few years, different axially substituted silicon Pcs have been synthesized bearing a wide variety of active moieties such as carotenoid,5 azo,6 tetrathiafulvalene,7 ferrocene,8 porphyrin,9 and [C60] fullerene,10 which have been studied to determine energy or electron transfer processes. Recently, we have described the synthesis of a novel C60-SiPc-C60 triad 1 (Chart 1),11 with electron-demanding groups placed axially on a Pc skeleton due to the six-coordinated nature of the central silicon atom, leading to a long-lived chargeseparated (CS) state. With these results in hand, a extension of the methodology was sought. In this vein, we have prepared new electron acceptor bearing silicon phthalocyanine hybrids based on trinitrofluorenone (TNF) and trinitrodicyanomethyl* Corresponding author. Phone: +34 9666658408. E-mail: [email protected]; [email protected]. † Divisio ´ n de Quı´mica Orgańica, Instituto de Bioingenierıá, Universidad Miguel Hernańdez. ‡ Graduate School of Engineering, Osaka University.

enefluorene (TNDCF). TNF and TNDCF derivatives, due to their high electron acceptor capability, have been extensively investigated in many research areas, for example, as photosensitizers in organic photorefractive materials.12 Moreover, a TNFappended gemini-shaped amphiphilic hexabenzocoronene has been shown to selectively form nanotubes or microfibers with different photochemical properties,13 and a low band gap system tetrathiafulvalene-R-TNDCF has been used as molecular rectifier.14 We report herein the synthesis and characterization of the triads TNF-SiPc-TNF 2 and TNDCF-SiPc-TNDCF 3 (see Scheme 1). Furthermore, we describe the photoinduced processes occurring upon excitation of one of the moieties, and in particular the electron-transfer processes from SiPc to TNF and TNDCF. The rates of the forward and back electron transfer processes have been obtained by time-resolved emission and transient absorption spectroscopy and analyzed in light of the Marcus theory of electron transfer for intramolecular electron transfer.15 Experimental Section Materials. Bis[p-(N-methyl-3′,4′-fulleropyrrolidin-2′-yl)benzoate](2,9,16,23-tetra- tert-butylphthalocyaninato)silicon (1),11

10.1021/jp804983k CCC: $40.75  2008 American Chemical Society Published on Web 10/14/2008

Axially Substituted Silicon Phthalocyanine Triads

J. Phys. Chem. C, Vol. 112, No. 45, 2008 17695

SCHEME 1: Synthesis of TNF-SiPc-TNF Triad 2 and TNDCF-SiPc-TNDCF Triad 3

CHART 2: Chemical Structures of SiPc (4), TNF (5), and TNDCF (6)

bis(p-methylbenzoate)(2,9,16,23-tetra-tert-butylphthalocyaninato)silicon (4),11 methyl-2,5,7-trinitrofluorenone-4-carboxylate (MTNFC, 5),16 methyl-2,5,7-trinitro-9-dicyanomethylenfluorene4-carboxylate (MTNDCM, 6),16 (see Chart 2), dichloro-(2,9,16,23tetra-tert-butylphthalocyaninato)silicon,17 and 2,5,7-trinitro-9fluorenone-4-carboxylic acid18 were prepared following previously reported synthetic procedures. All chemicals were reagent grade, purchased from commercial sources, and used as received, unless otherwise specified. Column chromatography was performed on silica gel 60 ACC 40-63 µm. Thin layer chromatography (TLC) was carried out on TLC plates coated with SiO2 60F254. Solvents and reagents were dried by standard methods under an inert gas atmosphere prior to use. Characterization. NMR spectra were measured with a Bruker AC 300 and with a Bruker AVANCE DRX-500. UV-vis spectra were recorded with a Helios Gamma spectrophotometer and IR spectra with a Nicolet Impact 400D spectrophotometer. Mass spectra were obtained from a Bruker Reflex III matrix-assisted laser desorption/ionization time-offlight (MALDI-TOF) spectrometer. Elemental analyses were performed on a Thermo Finnigan Flash 1112 CHN elemental analyzator. Bis(2′,5′,7′-trinitrofluorenone-4′-carboxylate)(2,9,16,23tetra-tert-butylphthalo-cyaninato)silicon (2). A mixture of 2,5,7-trinitrofluorenone-4-carboxylic acid18 (230 mg, 0.640 mmol), dichloro-(tetra-tert-butylphthalocyaninato)silicon17 (58 mg, 0.160 mmol), and diethylenglycol dimethyl ether (3 mL) was stirred for 12 h at 150 °C. Quenching of the reaction mixture in water and filtration of the resulting precipitate gave the crude product as a blue-green solid. After flash chromatography (silica gel, CHCl3:AcOEt 95:5) the desired product was isolated as a bright green solid (16 mg, 16%). 1H NMR (500 MHz, CDCl3): δ ) 9.52-9.48 (4H, m, Pc-Ar-H), 9.45-9.40 (4H, m, Pc-Ar-H), 8.55-8.49 (4H, m, Pc-Ar-H), 8.06 (2H, d, J ) 2.1 Hz, 2×H-TNF), 7.99 (2H, d, J ) 1.9 Hz, 2×H-TNF), 7.72 (2H, d, J ) 1.9 Hz, 2×H-TNF), 5.94 (2H, br s, 2×H-TNF),

1.90-1.88 (36H, m, 4×tBu). FT-IR (KBr): ν ) 3089, 2960, 2868, 1737, 1714, 1614, 1536, 1343, 1260, 1147, 1082, 940 cm-1. UV/vis (CHCl3): λmax/nm (log ) ) 282 (4.99), 360 (4.98), 639 (4.54), 705 (5.20). MS (MALDI-TOF-dithranol): m/z ) 1480 [M+]. Anal. Calcd. for C76H56N14O18Si (1480.37): C, 61.62; H, 3.81; N, 13.24 (%). Found: C, 61.75; H, 4.05; N, 12.85. Bis(2′,5′,7′-trinitro-9′-dicyanomethylenfluorene-4′-carboxylate)(2,9,16,23-tetra- tert-butylphthalocyaninato)silicon (3). A mixture of (tBu)4PcSi(TNF)2 2 (10 mg, 0.007 mmol), malononitrile (500 mg, 7.575 mmol) and dry DMF (1 mL), was stirred 36 h under argon at 70 °C. The reaction mixture was diluted with dichloromethane and washed with brine and water. The organic layer was dried, and the residue was washed with hexane, to give compound 3 as a bright green solid (6 mg, 56%). 1H NMR (500 MHz, CDCl ): δ ) 9.63-9.36 (8H, m, 3 Pc-Ar-H), 9.00 (2H, d, J ) 1.7 Hz, 2×H-TNDMF), 8.84 (2H, d, J ) 1.7 Hz, 2×H-TNDMF), 8.63-8.49 (4H, m, Pc-Ar-H), 7.73 (2H, d, J ) 1.7 Hz, 2×H-TNDMF), 5.86 (2H, br s, 2×HTNDMF), 1.95-1.84 (36H, m, 4×tBu). FT-IR (KBr): ν ) 3091, 2961, 2869, 2191, 1714, 1613, 1539, 1343, 1259, 1155, 1077, 942 cm-1. UV/vis (CHCl3): λmax/nm (log ) ) 300 (4.80), 359 (4.84), 632 (4.44), 705 (4.99). MS (MALDI-TOF-dithranol): m/z ) 1577 [M+]. Anal. Calcd. for C82H56N18O16Si (1576.39): C, 62.43; H, 3.58; N, 15.98 (%). Found: C, 63.02; H, 4.04; N, 15.24. Cyclic Voltammetry. Electrochemical measurements were performed on an ALS630B electrochemical analyzer in deaerated MeCN containing 0.1 M Bu4NPF6 (TBAPF6) as supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode (surface area of 0.3 mm2), and a platinum wire as the counter electrode. The Pt working electrode (BAS) was routinely polished with BAS polishing alumina suspension and rinsed with acetone before use. The measured potentials were recorded with respect to the Ag/AgNO3 (0.01 M) reference electrode. All potentials (vs Ag/

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Ag+) were converted to values vs SCE by adding 0.29 V.19 All electrochemical measurements were carried out under an atmospheric pressure of Ar. Laser Flash Photolysis. Nanosecond time-resolved transient absorption measurements were carried out using the laser system provided by UNISOKU Co., Ltd. An examined benzonitrile (PhCN) solution was excited by a Panther OPO pumped by Nd: YAG laser (Continuum, SLII-10, 4-6 ns fwhm) at λ ) 531 nm with the powers of 1.5 and 3.0 mJ per pulse. The nanosecond transient absorption measurements were performed using a continuous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu 2949) as a probe light and a detector, respectively. The output from the photodiodes and a photomultiplier tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz). Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source: Integra-C (Quantronix Corp.), an optical parametric amplifier: TOPAS (Light Conversion Ltd.) and a commercially available optical detection system: Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses were derived from the fundamental output of Integra-C (780 nm, 2 mJ/pulse and fwhm ) 130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into TOPAS which has optical frequency mixers resulting in tunable range from 285 to 1660 nm, while the rest of the output was used for white light generation. Typically, 2500 excitation pulses were averaged for 5 s to obtain the transient spectrum at a set delay time. Theoretical Calculations. Density-functional theory (DFT) calculations were performed on a COMPAQ DS20E computer. Geometry optimizations were carried out using the Becke3LYP functional and 3-21G basis set,20-22 with the restricted Hartree-Fock (RHF) formalism and as implemented in the Gaussian 03 program. Graphical outputs of the computational results were generated with the Gauss View software program (ver. 3.09) developed by Semichem, Inc.

Figure 1. (a) 1H NMR of TNF-ref 5. (b) 1H NMR of TNF-SiPc-TNF triad 2.

Results and Discussion

Figure 2. (a). 1H NMR of the TNF ref 6. (b) 1H NMR of TNDCF-SiPc-TNDCF triad 3.

Synthesis and Characterization of SiPc Triads. The synthesis of the TNF-SiPc-TNF 2 involved the nucleophilic displacement of two chloride substituents from (tBu)4PcSi(Cl)2, by reaction with the 2,5,7-trinitro-9-fluorenone-4-carboxylic acid in diglyme at high temperature. The TNDCF-SiPc-TNDCF 3 was afforded by a double Knoevenagel reaction between the triad 2 and malononitrile (Scheme 1). The presence of both TNF or TNDCF groups in the axial position and the tert-butyl groups in the equatorial positions of the phthalocyanine precludes aggregation phenomena, thus allowing the full characterization by FT-IR, 1H NMR, UV-vis and MALDI-TOF-MS. For example, the FTIR spectra of 2 and 3 show, respectively, the characteristic absorption patterns of TNF and TNDCF, with bands at around 1537 and 1342 cm-1 corresponding to the NO2 groups, and bands at 1714-cm-1 corresponding to the ester functionality. The band at 1737 cm-1, indicating the presence of the ketone group in the triad 2, disappeared in the triad 3 appearing a band at 2191 cm-1 corresponding to the nitrile substituents. The good resolution of the 1H NMR spectra of SiPc 2 (Figure 1b) and SiPc 3 (Figure 2b) in chloroform clearly indicates the lack of aggregation phenomena in solution. The signals corresponding to the three different aromatic protons of the phthalocyanine are located around 9.5, 9.4, and 8.5 ppm. Because of the effect of the strong ring current of the Pc, the signals corresponding to the aromatic trinitroflurorenone protons in the

TABLE 1: 1H-NMR Chemical shifts of TNF Ref 5, TNF-SiPc-TNF triad 2, TNDCF Ref 6, and TNDCF-SiPc-TNDCF Triad 3 compound

δ H1 (ppm)

δ H3 (ppm)

δ H6 (ppm)

δ H8 (ppm)

TNF ref 5 TNF-SiPc-TNF 2 TNDCF ref 6 TNDCF-SiPc-TNDCF 3 ∆δ 5-2 ∆δ 6-3

8.77 7.99 9.62 8.84 0.78 0.78

8.89 5.94 8.91 5.86 2.95 3.05

8.97 7.72 9.00 7.73 1.25 1.27

8.83 8.06 9.70 9.00 0.77 0.70

triad 2 are upfield shifted when compared with those of MTNFC 5. The most dramatic shift of almost 3 ppm corresponds to H3, which moves from 8.89 ppm in 5 to 5.94 ppm in 2. This result is to be expected, because H3 lies the closest to the Pc core. H6 experiences also a remarkable shielding effect, thus indicating its close location to the Pc moiety. Finally, the ring current effect on H1 and H8 is quite modest. The same trend was observed comparing the 1H NMR spectrum of the triad TNDCF-SiPcTNDCF 3 with that of MTNDCM 6 (see Figure 1, Figure 2, and Table 1). Figure 3shows a bathochromic shift of the Q-band of the phthalocyanine in triads 1-3 in comparison with the SiPc reference 4, indicating in all cases a ground-state interaction. between the SiPc and the electron acceptor moieties. Larger



Figure 3. UV- vis spectra of SiPc-ref 4 (black), SiPc-(C60)2 triad 1 (blue), SiPc-(TNF)2 triad 2 (green), and SiPc-(TNDCF)2 triad 3 (red) in CHCl3.

bathochromic shifts are observed in the trinitrofluorene derivatives than in the C60 one, probably due to the closer distance of the subunits to the SiPc core. Moreover, a decrease in the extinction coefficient of the phthalocyanine Q-band is observed for triads TNF-SiPc-TNF 2 and TNDCF-SiPc-TNDCF 3 in comparison with the reference compound 1 and with C60-SiPc-C60 triad 1. This last effect could be explained by a possible π-π interaction between the trinitrofluorene ring with the phthalocyanine core as a consequence of the close distance of both moieties. Finally, evidence concerning the purity of all the new compounds is given by elemental analysis, which is in accordance with the expected values. Redox Properties of SiPc Triads. The cyclic voltammograms of SiPc-(C60)2 1, C60-ref, SiPc-ref, SiPc-(TNF)2 2, and SiPc-(TNDCF)2 3 in deaerated PhCN containing 0.10 M Bu4NPF6 at 298 K are shown in parts a-e of Figure 4, respectively. The one-electron oxidation potential (Eox) of SiPc-(C60)2 1 (1.02 V, Figure 4a) is the same as the Eox value of SiPc-ref (1.02 V vs SCE, Figure 4c). The first reduction potential (Ered) of C60-ref (-0.57 V vs SCE, Figure 4b) is close to the Ered value of SiPc-ref (-0.61 V), and both waves are overlapped in 1 (Figure 4a). In the case of SiPc-(TNF)2 2, the Eox and Ered values are shifted slightly to a positive and negative direction, as compared with those of SiPc-ref and TNFref,16 respectively. Such potential shifts result from the electronwithdrawing effect of the TNF moiety, which causes lowering the highest-occupied molecular orbital (HOMO) level (-5.347 eV) together with raising the lowest-unoccupied molecular orbital (LUMO) level (-4.165 eV) of 2 as compared with the HOMO level (-4.954 eV) and the LUMO level (-3.412 eV) of 1. These energy levels were calculated using the DFT method (see Experimental Section). It should be noted, however, that the change in the HOMO and LUMO levels are attenuated by solvation of polar solvent molecules such as PhCN. The HOMO and LUMO orbitals of 1-3 with the optimized structures are shown in Figure 5. The electron acceptor ability is strongest in SiPc-(TNDCF)2 judging from the most positive Ered value (0.01 V vs SCE, Figure 4e). In this case, the energy of the CS state, which is obtained as the difference between the Eox and Ered values, is the lowest (1.04 eV) among investigated SiPc triad compounds. Photoinduced Charge Separation in SiPc Triads. The photoexcitation of a deaerated PhCN solution containing SiPc-(C60)2 1 with 360 nm monochromatized light results in

Figure 4. (a) Cyclic voltammograms of (a) SiPc-(C60)2 1, (b) C60ref, (c) SiPc-ref, (d) SiPc-(TNF)2 2 and (e) SiPc-(TNDCF)2 3 in PhCN containing 0.10 M Bu4NPF6.

fluorescence with the emission maximum at 708 nm, which is significantly quenched as compared with the reference compound SiPc-ref. The quenching rate constant is determined to be 3.3 × 109 s-1 from the fluorescence quenching and the fluorescence lifetime of SiPc-ref (9.0 ns). The free energy change of photoinduced electron transfer (∆GET) from the 1SiPc* unit to the C60 unit in PhCN is determined to be -0.15 eV from the one-electron oxidation potential (Eox ) 1.02 V vs SCE) and the excitation energy (S1 ) 1.77 eV) of the SiPc unit and the one-electron reduction potential of the C60 unit (Ered ) -0.60 V vs SCE) in SiPc-(C60)2 1. In such a case, the singlet excited state of SiPc may be quenched efficiently by the intramolecular electron transfer to C60. The occurrence of photoinduced electron transfer was examined by femtosecond laser flash photolysis measurements. The photoexcitation of a deaerated PhCN solution containing SiPc-(C60)2 1 with a 400 nm femtosecond laser pulse affords transient absorption bands at 880 and 1000 nm (Figure 6a). The NIR bands at 880 and 1000 nm are diagnostic of the radical cation of phthalocyanines10,23 and the radical anion of C60 derivatives.24 The SiPc•+ has also a distinctive absorption band at 580 nm.10 Thus, the transient absorption spectra in Figure 6a clearly indicates the formation of the CS state [SiPc•+-(C60)2•-], in which one of the two C60 unit becomes the radical anion by electron transfer from 1SiPc* to C60 in the triad. The fate of the CS state was examined by nanosecond laser flash photolysis measurements. The nanosecond laser excitation

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Figure 5. HOMO and LUMO orbitals of (a) SiPc-(C60)2 1, (b) SiPc-(TNF)2 2, and (c) SiPc-(TNDCF)2 3 calculated by the DFT method at the B3LYP/3-21G level.

of a deaerated PhCN solution containing 1 affords the transient absorption band at 520 nm due to the triplet excited state [3SiPc*-(C60)2] as shown in Figure 6b. The same spectrum was obtained for the reference compound without C60 (SiPc-ref). This indicates that back electron transfer from C60•- to SiPc•+ gives the triplet excited state rather than the ground state. The triplet state energy of SiPc was determined to be 1.26 eV from the phosphorescence spectrum of SiPc-ref in 2-MeTHF/ethyliodide (1:1 v/v). The CS state energy (1.62 eV) determined from the Eox value (1.02 V vs SCE) of the SiPc unit and the Ered value (-0.60 V vs SCE) of the C60 unit is higher than the energy of the triplet excited state of SiPc unit [3SiPc*-(C60)2]. The triplet excited-state energy of C60 (SiPc-3C60*, 1.56 eV)24 is also lower than the energy of the CS state. The reason why the back electron transfer affords 3SiPc*-(C60)2 rather than SiPc-3C60* and the ground state will be discussed in the next section. The CS state is also formed in photoinduced electron transfer from 1SiPc* to TNF and TNDCF in SiPc-(TNF)2 2 and SiPc-(TNDCF)2 3 as shown in parts a and b of Figure 7, respectively. In each case, the transient absorption band at 580 nm due to SiPc•+ is seen together with the absorption at 800 nm. In the case of SiPc-(TNDCF)2 3, the transient absorption band at 720 nm is assigned to TNDCF•-. Time profiles of formation and decay of the CS states of SiPc triads 1, 2, and 3 are shown in parts a-c of Figure 8, respectively. From the rise in absorbance at 880 nm due to SiPc•+ in Figure 8a the rate constant of formation of the CS state of the triad 1 is determined to be 3.2 × 109 s-1, which agrees well with the value determined from the fluorescence quenching (3.3 × 109 s-1). At 1000 nm, the decay of 1SiPc* is

Figure 6. (a) Transient absorption spectra of SiPc-(C60)2 1 (1.0 × 10-5 M) in deaerated PhCN at 298 K after femtosecond laser excitation at 400 nm. (b) Transient absorption spectra of SiPc-(C60)2 1 (1.0 × 10-5 M) in deaerated PhCN at 298 K after nanosecond laser excitation at 430 nm.

overlapped with the rise of C60•-. The lifetime of the CS state in PhCN is determined to be 5.0 ns from the decay of absorbance at 880 nm as well as 1000 nm.26 Similarly, the rate constants of formation and decay of the CS state of SiPc-(TNF)2 2 are determined form the rise and decay of absorbance at 580 nm to be 2.0 × 1011 s-1 and 4.5 × 1010 s-1, respectively. The rate constants of formation and decay of the CS state of SiPc-(TNDCF)2 3 are also determined to be 2.9 × 1012 and 4.6 × 1011 s-1, respectively. The CS lifetimes of 2 and 3 are 22 and 2.2 ps, respectively. The energy diagrams of photoinduced electron transfer in SiPc-(C60)2 1, SiPc-(TNF)2 2, and SiPc-(TNDCF)2 3 are summarized in parts a-c of Scheme 2, respectively, where one acceptor unit is omitted for clarity. In each case, photoinduced electron transfer occurs from the 1SiPc* unit to the acceptor unit to produce the CS state. In the case of SiPc-(C60)2 1, there are three pathways for the charge recombination (CR): the first is CR to 3C60*, the second CR to 3SiPc*, and the third CR to the ground state. The third pathway is unlikely to occur because the decay of the CS state affords 3SiPc* (Figure 6) and the intensity of the T-T absorption was virtually the same as that observed for the reference compound without C60 (3SiPc*-ref).



Figure 7. (a) Transient absorption spectra of (a) SiPc-(TNF)2 2 and (b) SiPc-(TNDCF)2 3 (1.0 × 10-5 M) in deaerated PhCN at 298 K after laser excitation at 400 nm.

Thus, the CS state decays to 3C60* followed by rapid energy transfer to produce 3SiPc* or direct BET to 3SiPc*. In the case of SiPc-(TNF)2 2, the CS state decays via CR to produce 3SiPc* rather than the ground state. This is not the case of SiPc-(TNDCF)2 3, because the energy of the CS state (1.04 eV) is lower than the energy of the triplet excited state (3SiPc* ) 1.26 eV). According to the Marcus theory of intramolecular electron transfer,15 the driving force dependence of the rate constant of nonadiabatic intramolecular electron transfer (kET) is given by eq 1

( )

[

(∆GET + λ)2 4π3 2 V exp kET ) 2 4λkBT h λkBT

]

(1)

where V is the electronic coupling matrix element, h is the Planck constant, T is the absolute temperature, ∆GET is the free energy change of electron transfer (-∆GET is the driving force), and λ is the reorganization energy of electron transfer. The driving force dependence of the rate constants of photoinduced electron transfer from the 1SiPc* unit to the acceptor unit in triads 1-3 (kET) and back electron transfer to the triplet excited state (kBET), which is shown in Figure 9, is well fitted by the Marcus equation (eq 1) with a small reorganization energy of 0.80 eV and a large electronic coupling matrix V ) 70 cm-1. This indicates that the CS and CR processes of a series of axially substituted silicon phthalocyanine triads proceed by adiabatic electron transfer. In the case of adiabatic electron transfer, the rate of electron transfer is determined solely by the driving force and reorganization energy of electron transfer, and thereby it is insensitive to the degree of interaction depending on the distance and spatial distribution of the acceptor moiety as long as the interaction energy is large enough to make the electron-transfer processes adiabatic.15

Figure 8. Time profiles of (a) absorbance at 880 and 1000 nm of SiPc-(C60)2 1 and those of absorbance at 580 nm of (b) SiPc-(TNF)2 2 and (c) SiPc-(TNDCF)2 3 (1.0 × 10-5 M) in deaerated PhCN at 298 K after laser excitation at 400 nm.

The λ value (0.80 eV) in the present triad systems is similar to the value (0.66 eV) reported for zinc porphyrin-fullerene dyad with an amide linkage.24 In the case of CR, back electron transfer from the singlet CS state to the ground-state is deeply in the Marcus inverted region, and thereby CR must occur to the triplet excited state. In the case of CR to the triplet excited state, the spin flip in the CS state may occur from the singlet to the triplet state via electron-nuclear hyperfine coupling-induced intersystem crossing (ISC), followed by back electron transfer from the triplet excited state.28,29 However, the rapid CS decay rate in the present systems precludes the ISC in the CS state, which typically occurs on a time scale of a few nanoseconds.28,29 The large V value may also preclude the ISC in the CS state, because the singlet-triplet splitting (2J) in the CS state may also be too large to undergo the electron-nuclear hyperfine couplinginduced ISC. In the case of SiPc-(C60)2 1, the spin-orbit interaction between the CS state and SiPc-3(C60)2* may be much stronger than that between the CS state and 3SiPc*-(C60)2 because of the much smaller energy difference (Scheme 2a). This may be the reason why the kBET value for CR to 3C60* rather than to 3SiPc* fits well with eq 1 with large V value for

17700 J. Phys. Chem. C, Vol. 112, No. 45, 2008 SCHEME 2: Energy Diagrams of Photoinduced Electron Transfer in (a) SiPc-(C60)2 1, (b) SiPc-(TNF)2 2, and (c) SiPc-(TNDCF)2 3a

Martıń-Gomis et al. solider Ingenio 2010 project HOPE CSD2007-00007) and a Grant-in-Aid (Nos. 19205019 and 19750034) and a Global COE program, Global Education an Research Center for BioEnvironmental Chemistry from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (S.F.). References and Notes

a

Bold arrows denote main reaction pathways.

Figure 9. Driving force dependence of log (kET and kBET) for photoinduced electron transfer (CS) and back electron transfer (CR) in SiPc triads (1-3). The solid lines are drawn as the best fit line to eq 1 using the values of λ ) 0.80 eV and V ) 70 cm-1.

adiabatic intramolecular electron transfer [(1CR) in Figure 9]. The CR of 2 to 3SiPc* and CR of 3 to the ground state also fit well with eq 1. Thus, both the CS and CR processes in SiPc triads occur via adiabatic electron transfer with the same reorganization energy (λ ) 0.80 eV).30 Conclusions In conclusion, the newly synthesized SiPc triads linked with two electron-acceptor units [SiPc-(C60)2, SiPc-(TNF)2, and SiPc-(TNDCF)2] undergo efficient intramolecular photoinduced electron transfer from the singlet excited-state of the SiPc unit to the electron acceptor unit to afford the CS states in PhCN. The fate of the CS state is different depending on the CS state energy relative to the triplet energy of the SiPc unit. In the case of SiPc-(C60)2 and SiPc-(TNF)2, the CS states decay via the spin-orbit interaction rather than the hyperfine interaction to produce the triplet excited state [3SiPc*-(C60)2 and 3SiPc*(TNF)2]. On the other hand, the CS state of SiPc-(TNDCF)2 decays to the ground state, because the energy of 3SiPc* is higher than the energy of the CS state. Acknowledgment. We thank support from the Spanish Government CICYT (Grant MAT2005-07369-C03-02 and Con-

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JP804983K

Adiabatic Photoinduced Electron Transfer and Back Electron Transfer

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