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Long-Lived Charge Separation in a Dyad of Closely-Linked Subphthalocyanine-Zinc Porphyrin Bearing Multiple Triphenylamines Mohamed E. El-Khouly,† Jung Bok Ryu,‡ Kwang-Yol Kay,*,‡ Osamu Ito,§ and Shunichi Fukuzumi*,† Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan, Department of Molecular Science and Technology, Ajou UniVersity, Suwon 443-749, South Korea, and Institute of Multidisciplinary Research for AdVanced Materials, Tohoku UniVersity, Katahira, Sendai, 980-8577, Japan ReceiVed: May 8, 2009; ReVised Manuscript ReceiVed: June 30, 2009

Photoinduced intramolecular events of the newly synthesized multimodular system composed of three triphenylamine (TPA) entities covalently substituted at the meso positions of the zinc porphyrin ring (ZnP), which is linked with the dodecafluorosubphthalocyanine SubPc(F)12 at its axial position with the B-O bond have been examined. Appreciable electronic interactions between the meso-substituted TPA entities and the ZnP π-system were observed, and as a consequence, ZnP(TPA)3 acts as an electron-donor, whereas the SubPc(F)12 moiety acts as an excellent electron-acceptor unit in the multimodular conjugate, ZnP(TPA)3SubPc(F)12. The computational studies performed by the DFT-B3LYP at the 6-31G level revealed delocalization of the highest occupied molecular orbital (HOMO) over the (TPA)3 entities and the porphyrin macrocycle, while the lowest unoccupied molecular orbital (LUMO) is localized on SubPc(F)12. Free-energy calculations suggested that the light-induced processes from the excited states of ZnP(TPA)3 are exothermic in both polar benzonitrile and nonpolar toluene and benzene. The occurrence of fast and efficient charge-separation processes (∼1012 s-1) via the singlet excited state of ZnP(TPA)3 was confirmed by the femtosecond transient absorption spectral measurements in polar and nonpolar solvents. The delocalization of the π-cation radical species over the donor ZnP(TPA)3, the lower energy of the radical-ion pair, the particular characteristics of the axial B-O bond, and the triplet radical-ion pair character rationalize the charge stabilization of (ZnP(TPA)3)•+(SubPc(F)12)•- with extremely long lifetime (370 µs) compared with the reported phthalocyanine-based compounds. Introduction Studies on light-induced electron transfer in covalently linked donor-acceptor systems have witnessed enormous growth in recent years, which mainly addresses the mechanistic details of electron transfer in chemistry and biology, resulting in the developments of artificial photosynthetic systems for light energy conversions and also the developments of molecular electronic devices.1-12 As is well-known, in order to achieve the high light-energy conversion, it is advantageous that the forward electron transfer between large π-electron systems in the natural photosynthetic systems is much faster than the corresponding reverse one. There are continuing efforts to construct donor-acceptor model systems that can mimic such behavior of natural photosynthetic systems. Compared with the electron-donating properties of flat phthalocyanines, the conically shape of the electron-accepting subphthalocynaines (SubPc) with three N-fused units arranged around a central boron atom and π-electron aromatic core affords unique photophysical and photochemical properties showing the strong electronaccepting ability.13-16 Because of the versatile chemistry of SubPc derivatives, their assemblies as multicomponent photoactive systems are produced via different routes involving * To whom correspondence should be addressed. E-mail: [email protected] chem.eng.osaka-u.ac.jp; [email protected] † Osaka University. ‡ Ajou University. § Tohoku University.

peripheral and/or axial approaches.17-19 Advantage of the axial approach is to preserve the electronic characteristics of the SubPc macrocycles. As a few photochemical studies for SubPcs have been already reported by Torres-Guldi group17 and our group,18 examinations of the electron-accepting properties of SubPcs are particularly attractive to employ as new building blocks for the artificial photosynthetic systems; the appealing points of SubPcs are (1) the strong absorption in the visible region (500-700 nm), (2) the higher energy of the singlet excited state (2.16 eV) and triplet excited state (1.40 eV) compared with those of phthalocyanines, and (3) the relatively low reorganization energies.19a Taking these properties into consideration, we present here the study of the photoinduced electron transfer of a dyad composed of a zinc porphyrin-subphthalocyanine (ZnP-SubPc) covalently bonded conjugate at the axial position with the B-O bond as an electron donor-acceptor pair. In order to increase the donor ability of ZnP, we have employed multimodular system composed of three triphenylamine (TPA) entities covalently linked at the meso position of the zinc porphyrin ring, producing ZnP(TPA)3. Because of the close proximity of the TPA entities to the porphyrin π-ring in the multimodular conjugate, delocalization of the porphyrin π-system to the TPA entities is expected. The ZnP(TPA)3 with its low oxidation potential and relatively high-lying triplet state appears to be superior photoinducible electron donor relative to other ZnP derivatives. The rich and extensive absorption features of

10.1021/jp904310f CCC: $40.75  2009 American Chemical Society Published on Web 08/05/2009

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Figure 1. Molecular formula and optimized structures of ZnP(TPA)3-SubPc(F)12 1.

ZnP(TPA)3 guarantee an increased absorption cross-section and an efficient use of the solar spectrum.20,21 On the other hand, to increase the electron-acceptor ability of SubPc, we employed dodecafluorosubphthalocyanine SubPc(F)12 entity by introducing the electron-withdrawing fluorine atoms. Thus, the newly synthesized conjugate, ZnP(TPA)3-SubPc(F)12 shown in Figure 1, is expected to promote electron-transfer rather than energy transfer by light excitation. From its design, the ZnP(TPA)3SubPc(F)12 multimodular system seems to be a promising material for enhancing the light-harvesting efficiency throughout the solar spectrum, as well as for converting the harvested light to the radical-ion pair, which can be applicable to molecular photovoltaic devices. This research plan is quite a contrast to the several porphyrin-phthalocyanine dyads linked through various spacers reported in the past decade, in which it has been established that an efficient photoinduced energy transfer from the porphyrin to the phthlaocyanine unit occurs.12 Several considerations led to the design of the ZnP(TPA)3-SubPc(F)12 multimodular system; (1) due to the relatively linear linkage of the phenol unit, the ZnP plane lays perpendicularly to the convex center of the cone-shaped SubPc(F)12, (2) to design the studied compound with welladjusted energies of the donor and acceptor molecules usually possesses large negative free energies for charge separation processes, (3) thus, the energy of the radical-ion pair should be lower than the lowest excited singlet and triplet states of ZnP(TPA)3 and SubPc(F)12, (4) as demonstrated here, the ZnP(TPA)3-SubPc(F)12 multimodular system will result in slowing down the undesired charge-recombination process after charge separation, thus, generating relatively long-lived chargeseparated species. Results and Discussions Synthesis and Characterization. The preparation of ZnP(TPA)3-SubPc(F)12 1 and its reference compounds ZnP(Ph)4 2, ZnP(TPA)3 3, and SubPc(F)12 4 was performed by following the steps as depicted in Scheme 1. Every step of the reaction sequence proceeded smoothly and efficiently to give a good or moderate yield of the product (see Experimental Section for the synthetic details). Commercially available triphenylamine was reacted with POCl3/DMF under Vilsmeier condition22 to give N-(4-formylphenyl)-N,N-diphenylamine 5 in 85.0%. Metalfree asymmetrical porphyrin 6 bearing three triphenylamine units at the meso position was prepared according to a typical method for the porphyrin synthesis23 in a yield of 10.1%. The methoxy group in 6 was demethylated by using BBr3 to afford 7 in 78.0% and then metalated with Zn(OAc)2 to provide the zinc porphyrin 8 in 83.0%. Subphthalocyanine 9 was synthesized in 52.0% by condensation reaction of tetrafluorophthalonitrile in the presence of boron trichloride according to the literature procedure24 and

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15445 the axial chlorine atom of the subphthalocyanine 9 was then replaced with the hydroxy group in 8 to give ZnP(TPA)3SubPc(F)12 1 in 25.4%. Analogously, porphyrin reference compounds, ZnP(Ph)4 (2) and ZnP(TPA)3 (3) were prepared via porphyrins 10 and 6, respectively. The reference compound SubPc(F)12 4 was also synthesized by the reaction of 9 with phenol under the same condition for the synthesis of ZnP(TPA)3-SubPc(F)12 1 to give a yield of 15.0%. ZnP(TPA)3-SubPc(F)12 1 and its reference compounds 2-4 and their precursor compounds are very soluble in aromatic solvents (i.e., toluene, o-dichlorobenzene, and benzonitrile) and other common organic solvents (i.e., acetone, CH2Cl2, CHCl3, and THF). The structure and purity of the new compounds were confirmed mainly by 1H NMR and elemental analysis. 1H and 19 F NMR spectra of 1 in (CD3)2CO are consistent with the proposed structure, showing the expected features with the correct integration ratios (Figures S1-S6 in the Supporting Information). The MALDI-TOF mass spectrum provided a direct evidence for the structure of 1,showing a singly charged molecular ion peak at m/z ) 1805.59 that matches the calculated value for the molecular weight (Figure S7). Further confirmation of the hybrid 1 was obtained from the steady-state UV/vis measurements as shown in the forthcoming sections. Ab Initio B3LYP/6-31G Studies. Computational studies were performed using the density functional methods (DFT) at the B3LYP/6-31G level to support the charge-separation process of ZnP(TPA)3-SubPc(F)12. As shown in Figure 2, the centerto-center distance (dCC) between the Zn center of ZnP(TPA)3 and the B center of SubPc(F)12 was 10 Å. The distance between the Zn and the N atom of the TPA entities was 9.2 Å, and this value was compared to the distances 13.6, 13.9, and 19.4 Å between the center of SubPc(F)12 and the N atoms of TPA entities. These results clearly show that the TPA entities dispose away from the SubPc(F)12 entity, but they are closer to the ZnP. The dCC value between the components is significantly shorter than that the reported dCC ()18 Å) for C60-ZnP(TPA)3.25 In the optimized structure, the electron cloud of the HOMO was delocalized over the porphyrin macrocycle and the peripheral TPA entities, but not on the SubPc(F)12 entity. The delocalization of the HOMO over the ZnP and TPA substituents is helping to stabilize the radical cation, ZnP(TPA)3)•+. The LUMO was fully localized over the SubPc(F)12 entity. These results suggest that the ZnP(TPA)3 acts as an electron-donor, while the SubPc(F)12 acts as an electron acceptor, to afford (ZnP(TPA)3)•+(SubPc(F)12)•-. One-Electron Redox Potentials and ET Driving Force. An accurate determination of the driving force for the electrontransfer processes requires the redox potentials of the studied chromophores. In the present study, the measurements were carried out by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques of deaerated benzonitrile (PhCN) solution containing tetra-n-butylannmonium hexafluorophosphate (TBAPF6; 0.1 M) as a supporting electrolyte (Figure 3). The ZnP(Ph)4 2 exhibits the first oxidation potential (Eox) at 630 mV versus Ag/AgNO3. By attaching ZnP with TPA, the oxidation potentials of ZnP-TPA were located at 256 and 624 mV versus Ag/AgNO3 reflecting the higher electron-donating properties of the ZnP(TPA) as compared to the ZnP(Ph)4 2. On the other hand, the first reduction potential (Ered) of SubPc(F)12 4 unit appears at -930 mV versus Ag/AgNO3 that is nearly 370 mV positive shift relative to the unsubstituted SubPc (-1300 mV). When turning to ZnP(TPA)3-SubPc(F)12 1, the first reduction potential (Ered) of SubPc(F)12 entity appears at -930 mV. While,

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SCHEME 1: Synthesis of compounds 1-4a

a (a) POCl3, DMF, 1,2-dichloroethane, reflux, 12 h, 85.0%. (b) p-Anisaldehyde, pyrrole, isopropyl alcohol, dichloromethane, BF3.OEt2, rt, 1.5 h and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), rt, 70 min and then Et3N, rt, 30 min, 10.1%. (c) BBr3, dichloromethane, rt, 16 h, 78.0%. (d) Zn(OAc)2, dichloromethane, methanol, rt, 1 h, 83.0% for 8, 97.8% for 2, 94.8% for 3. (e) Compound 9, toluene, reflux, 90 h, 25.4%. (f) Pyrrole, isopropyl alcohol, dichloromethane, BF3.OEt2, rt, 1.5 h and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, rt, 70 min and then Et3N, rt, 30 min, 6.20%. (g) Phenol, toluene, reflux, 95 h, 15.0%.

the oxidation potentials of ZnP(TPA)3 were located at 230, 440, 740, and 1030 mV. Based on the first Eox and Ered values, the driving forces for the charge recombination (-∆GCR) of (ZnP(TPA)3)•+-(SubPc(F)12)•- to the ground state were calculated as 1.16 eV in benzonitrile and 1.41 eV in toluene and benzene.26 However, as frequently pointed out, the -∆GCR values in the nonpolar solvents may contain considerable estimation errors. Based on the -∆GCR, the driving forces of the charge-separation process (-∆GSCS) via 1(ZnP(TPA)3)* (2.00 eV) and via 1(SubPc(F)12)* (2.16 eV) in benzonitrile were estimated as 0.84 and 1.0 eV, respectively. In addition, the

S values via the triplet states suggest exothermic negative ∆GCS charge separation (CS) processes via the 3(ZnP(TPA)3)* (-∆GTCS T ) 0.40 eV) and 3(SubPc(F)12)* (-∆GCS ) 0.30 eV) in polar benzonitrile, but no charge separation process via the triplet states in nonpolar solvents.27 Steady-State UV-Visible Spectral Studies. The steady-state absorption spectra of ZnP(TPA)3-SubPc(F)12 1 as well as the reference compounds in benzonitrile are shown in Figure 4a. The absorption spectra of the reference SubPc(F)12 4 consist of a high-energy B-band (312 nm) and lower energy Q-bands (516 and 573 nm) arisen from π-π* transitions, which are associated

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Figure 2. HOMO and LUMO of ZnP(TPA)3-SubPc(F)12 obtained with the B3LYP/6-31G on the basis of the optimized structure.

Figure 3. Cyclic voltammogram and differential pulse voltammetry of ZnP(TPA)3-SubPc(F)12 1 in deaerated benzonitrile.

with 14 π-electron systems, analogues to those of porphyrins and phthalocyanines. The absorption spectrum of SubPc(F)12 4 exhibited nearly 7 nm red-shifted as compared with that of unsubstituted SubPc. The absorption spectrum of the ZnP(Ph)4 2 exhibited a Soret band at 428 nm and Q-bands at 557 and 599 nm, while the absorption band of the TPA 5 was located at 303 nm. By attaching one TPA and three TPA entities at the meso position of the ZnP ring, the ZnP Soret and visible bands were red-shifted by nearly 3 and 8 nm as compared with the ZnP(Ph)4 2, respectively, suggesting interactions between the porphyrin π-system and peripheral TPA entities. For ZnP(TPA)3-SubPc(F)12 1, the electronic absorption spectra display common features of the electron-donor 3 and electronacceptor 4 subunits, suggesting that the donor and acceptor are not significantly electronically coupled in the ground state. With changing the solvent to toluene, the absorption spectrum of ZnP(TPA)3-SubPc(F)12 exhibits the similar features to that in benzonitrile; the only difference is observing 2-nm blue-shifted in toluene. Photoinduced Intramolecular Events in Polar and Nonpolar Solvents. The photophysical behavior was first investigated using steady-state fluorescence. When the most intense visible band at 430 nm was excited, the fluorescence spectra of the ZnP(Ph)4 2 revealed emission bands at 603 and 653 nm.

The emission bands of ZnP-TPA and ZnP(TPA)3 3 were redshifted by nearly 13 and 19 nm as compared with the pristine ZnP, respectively (Figure 4b). The red-shift and enhancement in the emission maxima (603-622 nm) of ZnP-TPA and ZnP(TPA)3 3 are also consequence of the interaction between the ZnP and TPA entities. When SubPc(F)12 is connected to the ZnP(TPA)3 entity, the emission intensity of the ZnP(TPA)3 moiety (λmax ) 622 nm) was decreased very much (∼97%) as compared to their control compounds in both polar and nonpolar solvents. These observations together with the calculated negative ∆GCS values via 1(ZnP(TPA)3)*, suggest that the charge-separation process occurs from the electron-donating 1 (ZnP(TPA)3)* to the attached electron-accepting SubPc(F)12, generating (ZnP(TPA)3)•+-(SubPc(F)12)•- in both polar and nonpolar solvents. Spectroscopic evidence for the radical-ion pair formation was obtained from the femtosecond transient absorption measurements by using 430-nm laser light, with which the ZnP(TPA)3 moiety is selectively excited. The time-resolved spectrum in the polar benzonitrile and nonpolar toluene and benzene at 1 ps showing an absorption band at 470 nm that is assigned to 1 (ZnP(TPA)3)* (Figure 5) and the spectra at >6 ps showing sharp absorption peaks around 480 and 740 nm together with a broadband in the whole near-IR region with a maximum at 1310 nm can be ascribed to the formation of the radical ion pair (ZnP(TPA)3)•+-(SubPc(F)12)•-. This assignment is confirmed as follows: The absorption bands in the visible region and in the NIR region with a maximum at 1310 nm are assigned to the one-electron oxidized species (ZnP(TPA)3)•+ by comparison with the spectrum obtained in the one-electron oxidation reaction of ZnP(TPA)3-SubPc(F)12 with nitronium hexafluoroantimonate and (Fe(bpy)3)3+ (Figure S8),25 which is quite different from that typically observed for ZnP radical cations.28 The absorptions of (ZnP(TPA)3)•+ in the NIR region are rationalized by the delocalization of the radical cation over the π-conjugated ZnP and three TPA units. Moreover, the radical-ion species were confirmed by recording the nanosecond transient spectra of a mixture system of ZnP(TPA)3 3 and SubPc(F)12 4 in dearerated benzonitrile (Figure 6), in which the 430 nm laser light excites ZnP(TPA)3. In the intermolecular electron-transfer experiments, the triplet state of the ZnP(TPA)3 at 470 and 820 nm was efficiently quenched by the external SubPc(F)12 (Figure 6). The pseudo-first-order triplet quenching was accompanied by the appearance of a strong transient absorption at the visible and NIR regions characteristic of the radical anion and radical cation with a rate of 1.7 × 1010 M-1 s-1, which then decayed, as

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Figure 4. (a) Steady-state absorption spectra of ZnP(TPA)3-SubPc(F)12 1 and reference compounds in benzonitrile. (b) Steady-state fluorescence spectra of ZnP(TPA)3-SubPc(F)12 1 and the reference compounds in benzonitrile; λex ) 420 nm.

Figure 6. Nanosecond transient spectra of mixture of ZnP(TPA)3 (0.05 mM) and SubPc(F)12 (0.05 mM) in dererated benzonitrile observed with 430 nm laser excitation at room temperature.

Figure 5. Differential transient absorption spectra obtained upon femtosecond photoexcitation (430 nm) of 1 in (a) benzene and (b) benzonitrile with several time delays at room temperature. Inset: Time profiles at 1310 nm.

expected, by second-order kinetic. The rate of back electron transfer (kbet) was calculated as 3.2 × 109 M-1 s-1 in benzonitrile. Returning to Figure 5, important finding is similarity of the transient spectra in nonpolar toluene (dielectric constant; εs ) 2.38) and even in benzene (εs ) 2.28) to that in benzonitrile (εs ) 25.70). An explanation for this analogy is based on the corresponding energy levels of (ZnP(TPA)3)•+-(SubPc(F)12)•relative to 1(ZnP(TPA)3)*, guaranteeing high driving forces for the associated charge-separation and charge-recombination processes in both polar and nonpolar solvents. The absorption bands in the NIR region due to (ZnP(TPA)3)•+ are clear marker bands that provide a powerful tool for the assignment of the observed transient absorptions and to examine kinetics of the charge-separation and charge-recombination processes. The

time-profile in the NIR region (Figure 5, inset) shows an extremely fast rise of (ZnP(TPA)3)•+-(SubPc(F)12)•-, from which the rate constants of the charge separation processes (kSCS) via 1(ZnP(TPA)3)* were calculated as 1.0 × 1012, 9.0 × 1011 and 9.0 × 1011 s-1 in benzonitrile, toluene and benzene, S values are almost independent from respectively. Since the kCS solvent polarities, it is suggested that the CS process is occurring near the top region of the Marcus parabola.29-31 Compared with S values of ZnP(TPA)3the reported related systems, the kCS SubPc(F)12 with dCC ) 10 Å are about 2000 times faster than that of the recently reported C60-ZnP(TPA)3 with dCC ) 18 Å.25 The decay-profile of (ZnP(TPA)3)•+-(SubPc(F)12)•- in benzene was observed as shown in inset of Figure 5a, from which the rate constant of the charge-recombination processes (kCR) and lifetimes of the radical-ion pair (τRIP ) 1/kCR) were evaluated to be 7.2 × 109 s-1 and 138 ps, respectively. Similar kCR and τRIP values were evaluated in toluene to be 5.9 × 109 s-1 and 170 ps, respectively. On the other hand, in benzonitrile, relatively fast decay of (ZnP(TPA)3)•+-(SubPc(F)12)•- was observed as shown in inset of Figure 5b, from which kCR and τRIP values were evaluated to be 5.0 × 1010 s-1 and 20 ps, respectively. These fast charge-recombination processes suggest that the radical-ion pair keeps the singlet-spin character, because of the charge separation via 1(ZnP(TPA)3)*. The observation that the kCR value increases with increasing the solvent polarity (i.e., the lifetime of the charge-separated state is longer in nonpolar solvents compared to that in polar solvents) can be understood, if charge recombination is occurring in the Marcus inverted region.29-31 Indeed, compared with the reorganization energy ranging 0.5-1.0 eV estimated above, the ∆GCR values (1.41 eV in toluene and benzene) are sufficiently large, whereas the ∆GCR value 1.16 eV in benzonitrile is closer to the reorganization energy. Similar features were observed in the

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Figure 7. Nanosecond transient spectra obtained by 430-nm laser excitation of ZnP(TPA)3-SubPc(F)12 (0.05 mM) in deaerated benzonitrile at room temperature. Inset: time profiles at 1310 nm with increasing the laser power.

case of subphthalocyanines-ferrocences17d and subphthalocyanine-triphenylamine dyads.17e The kCS/kCR ratios via 1(ZnP(TPA)3)*, which are a measure of the excellence in the photoinduced electron-transfer systems, were determined to be 20, 140, and 170 in benzonitrile, toluene, and benzene, respectively, suggesting the usefulness of the studied compound as light-harvesting systems especially in the nonpolar solvents. In addition, these ratios are higher than those of SubPc(F)12-TPA,17e indicating the significant role of the electrondonating ZnP(TPA)3 in stabilizing the radical-ion pair by delocalization of the hole over the ZnP and TPA entities. Photoinduced Intramolecular Event in Microsecond Time Regions. It is essential to follow dynamics in microsecond time regions by using nanosecond laser flash photolysis to follow the contribution of the triplet excited states on the electrontransfer process. The transient absorption spectra of ZnP(TPA)3-SubPc(F)12 in deaerated benzonitrile measured with 430 nm nanosecond laser light exciting ZnP(TPA)3 are shown in Figure 7. At the initial stages, we could see only the characteristic absorptions of 3(ZnP(TPA)3)* at 480 and 820 nm, suggesting that the intersystem crossing (ISC) process from 1 (ZnP(TPA)3)* occurs after the fast charge-separation and charge-recombination processes in the picoseconds time region. Such fast intersystem crossing (ISC) processes were reported in compact donor-acceptor molecules, especially when the donor is placed to be nearly in perpendicular orientation to that of the acceptor.32-35 With the decay of 3(ZnP(TPA)3)*, the absorption bands appeared at 480, 770, and 1310 nm as seen in the transient spectra in the microsecond region (inset of Figure 7). This observation demonstrates the generation of (ZnP(TPA)3)•+-(SubPc(F)12)•- via 3(ZnP(TPA)3)* in benzonitrile. From the energetic point of view, the charge-separated state is lower energy than 3(ZnP(TPA)3)* in benzonitrile. On the other hand, in toluene and benzene, formation of the radical-ion pairs was not observed in nanosecond transient absorption spectra, indicating the absence of a charge-separation process via 3 (ZnP(TPA)3)* in the nonpolar solvents. Such behavior is T value via 3(ZnPexpected because of the unfavorable ∆GCS (TPA)3)* in nonpolar solvents. An interesting observation is that the radical-ion pair (ZnP(TPA)3)•+-(SubPc(F)12)•- is alive in long time scale reaching 2 ms in deaerated benzonitrile, but not in oxygensaturated benzonitrile. The transient spectra shown in Figure 7 exhibit clearly the presence of (ZnP(TPA)3)•+-(SubPc(F)12)•-. The remarkable feature seems that the charge recombination of (ZnP(TPA)3)•+ at 1310 nm decayed by first-order kinetic in polar

Figure 8. Energy level diagram showing the intramolecular events of ZnP(TPA)3-SubPc(F)12 induced by the excitation of ZnP(TPA)3 in the studied solvents; time constants in parentheses are the values at room temperature. Energy level of 3RIP is depicted to be slightly lower than that of 1RIP.

benzonitrile at room temperature, ruling out the possibility of the intermolecular electron transfer. We believe that this slow charge recombination (2.7 × 103 s-1) is a consequence of the persistence of the triplet-correlated radical-ion pair character in the charge-separated state, which results in the corresponding forbidden character of the charge recombination back to the singlet ground state. In other words, the triplet multiplicity of the radical ion-pair, which is populated from the electron transfer from 3(ZnP(TPA)3)* to the attached SubPc(F)12, plays an important role in determining its very long lifetime of 370 µs with a quantum yield reaching 29%.36 The quick decay of the radical species in the oxygen-saturated benzonitrile confirms the triplet-spin character of the (ZnP(TPA)3)•+-(SubPc(F)12)•-; thus, the slow decay in deaered benzonitrile is closely related to the spin forbidden character of charge recombination from triplet radical-ion pair (3RIP) to the ground state.14d,17b,d The observed long 3RIP lifetime in fact implies that intersystem crossing (ISC) within the RIP states (1RIP T 3RIP) is slow rate determining step, because 3RIP only could decay via 1RIPrecombination channel.37,38 The small kCR values were found to be temperature dependent in benzonitrile (Figure S13). On the basis of the Marcus theory of electron transfer,29 the reorganization energy (λreorg) and the electronic coupling (VCR) were estimated as 0.88 eV and 0.3 cm-1, respectively. The small reorganization energy is in good agreement with that reported for the subphthalocyaninestriphenylamine mixture by Schehl and co-workers.19a The small value of VCR may result from the absence of the electron cloud in both HOMO and LUMO on the B-O bond. In the huge body of literature, only a few examples of the short-distance donor-acceptor dyads show such long-lived radical-ion pair with a reasonable quantum yield.39 The photoinduced intramolecular processes of ZnP(TPA)3SubPc(F)12 1 are summarized in the energy diagram as shown in Figure 8, in which the energy levels of ZnP(TPA)3SubPc(F)12 are depicted on the bases of electrochemical data in the studied solvents. For the short bridge used here, the singlet radical ion-pair (1RIP) state slightly destabilizes compared with the triplet radical ion-pair (3RIP). In accord with expectations

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based on the energy considerations, excitation of the ZnP(TPA)3 at 430 nm leads to formation of the charge-separated state with high efficiency via 1(ZnP(TPA)3)* as confirmed by femtosecond laser measurements. The CS state, (ZnP(TPA)3)•+-(SubPc(F)12)•-, with singlet spin character decays to the ground state with lifetimes of 20-169 ps in the studied solvents. On the other hand, the charge separation from 3(ZnP(TPA)3)* to SubPc(F)12 was also observed in benzonitrile generating (ZnP(TPA)3)•+-(SubPc(F)12)•- with triplet spin character with an extremely long lifetime of the compact dyad. From a point of view of mechanistic information, the studied compound, ZnP(TPA)3-SubPc(F)12, has the advantage that both the lifetimes of the 1RIP and 3RIP could be determined. As demonstrated by the values of the τRIP, the spin state of the RIP has a very substantial effect on its lifetime. Conclusions A novel model compound with well-adjusted energies of the triphenylamines substituted zinc porphyrin and dodecafluorosubphthalocyanine entities ZnP(TPA)3-SubPc(F)12 has been elegantly designed, and finely tuned electronic coupling between the donor and acceptor entities has been extensively studied herein. The occurrence of fast and efficient charge separation processes (≈012 s-1) via the singlet state of ZnP(TPA)3 was observed by detecting the radical ion species in both polar and nonpolar solvents by the femtosecond transient absorption spectral measurements. The lifetimes of the singlet radical-ion pair (20-170 ps at room temperature) decrease substantially in more polar media, whereas the rate of charge separation is much less solvent-dependent. This is qualitatively in line with the fact that charge separation occurs near to the Marcus top region, while charge recombination occurs in the Marcus inverted region. The nanosecond transient measurements of such a simple donor-acceptor bearing multimodular conjugate show a long-lived radical-ion pair (370 µs) in deaerated benzonitrile at room temperature, which is rationalized by the triplet spin character of the radical-ion pair. Such an unusually large ratio of charge separation rate to charge recombination rate is a rare and desirable property of molecular systems designed for photoinduced charge separation. Experimental Section Materials and Instruments. Reagents and solvents were purchased as reagent grade and used without further purification. All reactions were performed using dry glassware under nitrogen atmosphere. Analytical TLC was carried out on a Merck 60 F254 silica gel plate and column chromatography was performed on a Merck 60 silica gel (230-400 mesh). Melting points were determined on an Electrothermal IA 9000 series melting point apparatus and are uncorrected. NMR spectra were recorded on a Varian Mercury-400 (400 MHz) spectrometer with TMS peak as reference. IR spectra were recorded on a Nicolet 550 FT infrared spectrometer and measured as KBr pellets. UV/vis spectra were recorded on a Jasco V-550 spectrometer. MALDITOF MS spectra were recorded with an Applied Biosystems Voyager-DE-STR. Elemental analyses were performed with a Perkin-Elmer 2400 analyzer. Steady-state absorption and fluorescence spectra were measured on a JASCO V-550 spectrometer (UV-vis) and Shimadzu spectrofluorophotometer equipped with a photomultiplier tube having high sensitivity in the longer wavelength region, respectively. Electrochemical measurements were performed on an ALS630B electrochemical analyzer in deaerated PhCN containing TBAPF6 (0.1 M) as supporting electrolyte at 298 K.

El-Khouly et al. A conventional three-electrode cell was used with 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 (0.1 M) reference electrode. All electrochemical measurements were carried out under an atmospheric pressure of argon. The nanosecond transient absorption measurements were performed by excitation with a Panther OPO laser-light pumped by Nd:YAG laser (Continuum, SLII-10, 4-6 ns fwhm) nm with the powers of 1.5 and 3.0 mJ per pulse. A continuous xenon lamp (150 W) and an InGaAs-PIN photodiode (Hamamatsu 2949) were used as a probe light and a detector, respectively. The output from the photodiodes 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 was 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 a tunable range from 285 to 1660 nm, whereas 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. Kinetic traces at appropriate wavelengths were assembled from the timeresolved spectral data. All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation. Density-functional theory (DFT) calculations were performed on a COMPAQ DS20E computer. Geometry optimizations were carried out using the density functional methods (DFT) at the B3LYP/6-31G level basis set,40 with the unrestricted HartreeFock (UHF) formalism and as implemented in the Gaussian 03 program revision C.02. Graphical outputs of the computational results were generated with the Gauss View software program (ver. 3.09) developed by Semichem, Inc. Preparation of N-(4-Formylphenyl)-N,N-diphenylamine (5). To a solution of DMF (2.35 mL, 30.28 mmol) in 1,2-dichloroethane (20 mL) was added triphenylamine (1.80 g, 7.57 mmol) and carefully poured POCl3 (2.77 mL, 30.28 mmol). The mixture was refluxed for 12 h, cooled to room temperature, and then poured into a saturated aqueous sodium acetate solution (300 mL). The product was extracted with dichloromethane (3 × 50 mL), and the extract was dried over MgSO4 and evaporated. The residue was chromatographed on silica gel with dichloromethane to give compound 5 (1.85 g, 85.0%) as a yellow solid. Mp 147 °C; 1H NMR (400 MHz, CDCl3): δ ) 9.78 (s, 1H), 7.64 (d, J ) 4.8 Hz, 2H), 7.31 (t, J ) 7.6 Hz, 4H), 7.14 (m, 6H), 6.98 (d, J ) 8.0 Hz, 2H); Anal. Calcd for C19H15NO: C, 83.49%; H, 5.53%; N, 5.12%. Found: C, 83.42%; H, 5.57%; N, 5.09%. Preparation of 5-(4-Methoxyphenyl)-10,15,20-tris{(4diphenylamino)phenyl}-21H,23H-porphine (6). To a solution of compound 5 (3.42 g, 12.5 mmol) in dichloromethane (1.65 L) were added p-anisaldehyde (0.57 g, 4.15 mmol), pyrrole (2.3 mL, 16.7 mmol), isopropyl alcohol (11 mL), and BF3 · OEt2 (0.84 mL, 6.90 mmol), and the mixture was stirred at room temperature for 1.5 h. To the solution was added 2,3-dichloro-5,6dicyano-1,4-benzoquinone (DDQ, 3.79 g, 16.7 mmol), and the

Long-Lived Charge Separation mixture was stirred for 70 min. Finally, triethylamine (2.35 mL, 16.7 mmol) was added to the mixture, and it was stirred for 30 min. The solvent was evaporated, and the product was chromatographed on silica gel with chloroform/hexane (3:2) to give compound 6 (0.48 g, 10.1%) as a purple solid. Mp > 325 °C; 1 H NMR (400 MHz, CDCl3): δ ) 8.97 (s, 4H), 8.95 (d, J ) 4.4 Hz, 2H), 8.86 (d, J ) 4.4 Hz, 2H), 8.12 (d, J ) 8.4 Hz, 2H), 8.05 (q, J ) 12.4 Hz, 6H), 7.43 (q, J ) 17.6 Hz, 6H), 7.39 (m, 24H), 7.27 (d, J ) 8.4 Hz, 2H), 7.12 (m, 6H), 4.07 (s, 3H), -2.63 (s, 2H); Anal. Calcd for C81H59N7O: C, 84.87%; H, 5.19%; N, 8.55%. Found: C, 84.96%; H, 5.29%; N, 8.63%. Preparation of 5-(4-Hydroxyphenyl)-10,15,20-tris{(4diphenylamino)phenyl}-21H,23H-porphine (7). To a solution of compound 6 (0.24 g, 0.21 mmol) in dichloromethane (50 mL) was added boron tribromide (1.0 M solution in dichloromethane, 0.21 g, 0.83 mmol) at 0 °C, and the solution was stirred for 30 min. The reaction mixture was stirred at room temperature for 16 h, carefully poured into iced water (100 mL), and then stirred for 30 min. The organic layer was separated and dried over MgSO4. The solvent was evaporated and the product was chromatographed on silica gel with dichloromethane/methanol (20:1) to give compound 7 (0.18 g, 78.0%) as a brown solid. Mp > 350 °C; 1H NMR (400 MHz, CDCl3): δ ) 8.97 (s, 4H), 8.95 (d, J ) 4.4 Hz, 2H), 8.86 (d, J ) 4.4 Hz, 2H), 8.12 (d, J ) 8.4 Hz, 2H), 8.05 (q, J ) 12.4 Hz, 6H), 7.43 (q, J ) 17.6 Hz, 6H), 7.39 (m, 24H), 7.27 (d, J ) 8.4 Hz, 2H), 7.12 (m, 6H), -2.63 (s, 2H); Anal. Calcd for C80H57N7O: C, 84.85%; H, 5.07%; N, 8.66%. Found: C, 84.81%; H, 5.18%; N, 8.62%. Preparation of [5-(4-Hydroxyphenyl)-10,15,20-tris{(4diphenylamino)phenyl}-21H,23H-porphine]zinc (8). To a solution of compound 7 (0.16 g, 0.14 mmol) in dichloromethane/ methanol (3:1, 150 mL) was added zinc acetate (0.29 g, 1.59 mmol), and the mixture was stirred at room temperature for 1 h. The solvent was evaporated, and the product was chromatographed on silica gel with dichloromethane to give compound 8 (0.14 g, 83.0%) as a purple solid. Mp > 350 °C; 1 H NMR (400 MHz, CDCl3): δ ) 8.97 (s, 4H), 8.95 (d, J ) 4.4 Hz, 2H), 8.86 (d, J ) 4.4 Hz, 2H), 8.12 (d, J ) 8.4 Hz, 2H), 8.05 (q, J ) 2.4 Hz, 6H), 7.43 (q, J ) 17.6 Hz, 6H), 7.39 (m, 24H), 7.27 (d, J ) 8.4 Hz, 2H), 7.12 (m, 6H); Anal. Calcd for C80H55N7OZn: C, 80.36%; H, 4.64%; N, 8.20%. Found: C, 80.42%; H, 4.76%; N, 8.15%. Preparation of SubPc(F)12-Cl (9). 3,4,5,6-Tetrafluorophthalonitrile (0.87 g, 4.35 mmol) was added to boron trichloride (1.0 M solution in p-xylene, 4.30 mL, 4.30 mmol). The mixture was refluxed for 20 min and cooled to room temperature, and the solvent was evaporated. The product was chromatographed on silica gel with hexane/ethyl acetate (3:1) to give chlorododecafluorosubphthalocyanine (9) (0.50 g, 52.0%) as a dark purple solid. Mp > 250 °C (dec.); 19FNMR (CD3)2CO: δ ) -43.5, -53.8; UV/vis (toluene): λmax) 303, 533, 515, 576 nm; Anal. Calcd for C24BClF12N6: C, 44.58%; N, 13.00%. Found: C, 44.46%; N, 12.91%. Preparation of ZnP(TPA)3-SubPc(F)12 (1). To a solution of compound 8 (0.19 g, 0.16 mmol) in toluene (3.80 mL) was added compound 9 (0.021 g, 0.032 mmol), and the solution was refluxed for 90 h. The reaction mixture was cooled to room temperature, and the solvent was evaporated. The product was chromatographed on silica gel with dichloromethane/hexane (2: 1) to give compound 1 (0.015 g, 25.4%) as a blue solid; Mp > 370 °C (dec.); 1H NMR (400 MHz, (CD3)2CO: δ ) 9.11 (s, 4H), 9.05 (d, J ) 4.8 Hz, 2H), 8.68 (d, J ) 4.8 Hz, 2H), 8.08 (d, J ) 8.4 Hz, 6H), 7.68 (d, J ) 12.4 Hz, 2H), 7.43 (t, J )

J. Phys. Chem. C, Vol. 113, No. 34, 2009 15451 17.2 Hz, 30H), 7.27 (d, 6H), 5.75 (d, J ) 6.8 Hz, 2H); 19FNMR (CD3)2CO): δ ) -42.8, -53.6; UV/vis (toluene): λmax) 307.4, 438.3, 515, 554, 573.2 nm; MS (MALDI-TOF): m/z for C104H54BF12N13OZn Calcd 1805.81. Found 1805.59; Anal. Calcd for C104H54BF12N13OZn: C, 69.17%; H, 3.01%; N, 10.08%. Found: C, 69.02%; H, 3.12%; N, 10.13%. Preparation of 5,10,15,20-Tetrakisphenyl-21H,23H-porphine (10). To a solution of benzaldehyde (0.89 g, 8.39 mmol) in dichloromethane (0.84 L) were added pyrrole (0.56 g, 8.36 mmol), isopropyl alcohol (6 mL), and BF3OEt2 (0.43 mL, 3.38 mmol), and the mixture was stirred at room temperature for 1.5 h. To the solution was added 2,3-dichloro-5,6-dicyano-1,4benzoquinone (1.94 g, 8.39 mmol), and the mixture was stirred for 70 min. Finally, to the mixture was added Et3N (0.82 g, 8.40 mmol), and the solution was stirred for 30 min. The solvent was evaporated, and the product was chromatographed on silica gel with chloroform/hexane (3:2) to give compound 10 (0.32 g, 6.20%) as a dark purple solid. Mp > 350 °C; 1H NMR (400 MHz, CDCl3): δ ) 8.82 (s, 8H), 8.18 (d, 8H), 7.72 (t, 12H), -2.78 (s, 2H); Anal. Calcd for C44H30N4: C, 85.97%; H, 4.92%; N, 9.11%. Found: C, 85.87%; H, 4.99%; N, 9.14%. Preparation of (5,10,15,20-Tetrakisphenyl-porphine)zinc (2). To a solution of compound 10 (0.30 g, 0.48 mmol) in dichloromethane/methanol (3:1, 250 mL) was added zinc acetate (0.89 g, 4.88 mmol), and the solution was stirred at room temperature for 1 h. The solvent was evaporated, and the product was chromatographed on silica gel with dichloromethane to give compound 2 (0.32 g, 97.8%) as a purple solid; Mp > 350 °C (dec.); 1H NMR (400 MHz, CDCl3): δ ) 8.86 (s, 8H), 8.20 (d, 8H), 7.72 (t, 12H); UV/vis (toluene): λmax) 402, 423, 552 nm; Anal. Calcd for C44H28N4Zn: C, 77.93%; H, 4.16%; N, 8.26%. Found: C, 77.84%; H, 4.23%; N, 8.20%. Preparation of ZnP(TPA)3 (3). To a solution of compound 6 (0.20 g, 0.17 mmol) in dichloromethane/methanol (3:1, 170 mL) was added zinc acetate (0.32 g, 1.74 mmol), and the mixture was stirred at room temperature for 1 h. The solvent was evaporated, and the product was chromatographed on silica gel with dichloromethane/hexane (2:1) to give compound 3 (0.20 g, 94.8%) as a dark purple solid. Mp > 350 °C; 1H NMR (400 MHz, CDCl3): δ ) 8.97 (s, 4H), 8.95 (d, J ) 4.4 Hz, 2H), 8.86 (d, J ) 4.4 Hz, 2H), 8.12 (d, J ) 8.4 Hz, 2H), 8.05 (q, J ) 12.4 Hz, 6H), 7.43 (q, J ) 17.6 Hz, 6H), 7.39 (m, 24H), 7.27 (d, J ) 8.4 Hz, 2H), 7.12 (m, 6H), 4.07 (s, 3H). UV/vis (toluene): λmax ) 304, 430 nm. Anal. Calcd for C81H57N7OZn: C, 80.42%; H, 4.75%; N, 8.10%. Found: C, 80.34%; H, 4.87%; N, 8.18%. Preparation of SubPc(F)12-OPh (4). To a solution of compound 9 (0.10 g, 0.15 mmol) in toluene (5.00 mL) was added phenol (0.07 g, 0.74 mmol), and the solution was refluxed for 95 h. The reaction mixture was cooled to room temperature and the solvent was evaporated. The product was chromatographed on silica gel with dichloromethane/hexane (2:1) to give compound 4 (0.015 g, 15.0%) as a dark purple solid. Mp 330 °C; 1H NMR (400 MHz, (CD3) 2CO): δ ) 6.81 (t, J ) 16.0 Hz 2H), 6.69 (t, J ) 16.0 Hz, 1H), 5.58 (d, J ) 12.0 Hz, 2H); UV/vis (toluene): λmax ) 305, 533, 574 nm. Anal. Calcd for C30H5BF12N6O: C, 51.17%; H, 0.72%; N, 11.93%. Found: C, 51.09%; H, 0.78%; N, 11.89%. Acknowledgment. K.-Y. Kay acknowledges the financial support from Brain Korea 21 Program in 2007. This work was also supported by KOSEF/MEST through WCU project (R312008-000-10010-0) and a Global COE program, “the Global Education and Research Center for Bio-Environmental Chem-

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Long-Lived Charge Separation in a Dyad of Closely ...

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