View Online

PAPER

www.rsc.org/pccp | Physical Chemistry Chemical Physics

Electron transfer reaction of light harvesting zinc naphthalocyanine–subphthalocyanine self-assembled dyad: spectroscopic, electrochemical, computational, and photochemical studiesw Mohamed E. El-Khouly*ab

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

Received 19th May 2010, Accepted 8th July 2010 DOI: 10.1039/c0cp00612b Electron transfer reaction of a self-assembled donor–acceptor dyad formed by axial coordination of zinc naphthalocyanine, ZnNc, and subphthalocyanine appended with pyridine coordinating ligand, SubPc(py), was investigated in the present study. The SubPc(Py) : ZnNc self-assembled dyad absorbs the light in a wide section of the UV/Vis/NIR spectra. The formation constant of SubPc(py) : ZnNc in o-dichlorobenzene was found to be 1.2  105 M 1 from the steady-state absorption and emission measurements, suggesting stable complex formation. The geometric and electronic calculations by using ab initio B3LYP/6-311G methods showed the majority of the highest occupied frontier molecular orbital (HOMO) on the zinc naphthalocyanine entity, while the lowest unoccupied molecular orbital (LUMO) was on the subphthalocyanine entity, suggesting that the charge-separated state of the supramolecular complex is (SubPc(py)) : ZnNc +. The electrochemical results suggest the exothermic charge-separation process via the singlet states of both SubPc(py) and ZnNc entities. Upon coordination the pyridine appended subphthalocyanine to ZnNc; the main quenching pathway involved charge separation via the singlet excited states of ZnNc and SubPc(py). A clear evidence of the intramolecular electron transfer from the singlet state of ZnNc to SubPc(py) was monitored by femtosecond laser photolysis in o-dichlorobenzene by observing the characteristic absorption band of the ZnNc radical cation in the NIR region at 960 nm. The rate of charge-separation process was found to be 1.3  1010 s 1, indicating fast and efficient charge separation. The rate of charge recombination and the lifetime of the charge-separated state were found to be 1.0  109 s 1 and 1 ns, respectively. The absorption in a wide section of the solar spectrum and high charge-separation/charge-recombination ratio suggests the usefulness of self-assembled SubPc(Py) : ZnNc for being a photosynthetic model.

Introduction The X-ray structures of the bacterial photosynthetic reaction centers have revealed that the electron donor and acceptor entities are arranged via noncovalent incorporation into a well-defined protein matrix.1–4 Inspired by this natural phenomenon, researchers have been attempting to mimic such complex processes with the help of synthetic molecular architectures, often termed as artificial photosynthesis.5–43 In artificial photosynthesis, the light induced energy transfer and electron transfer events occur between the well-organized pigments with a high quantum efficiency to mimic the antenna and reaction center functionalities, respectively. The use of supramolecular assemblies to model the functionality of the reaction center is an attractive and fruitful strategy to develop photofunctional materials and devices. a

Department of Chemistry, Faculty of Science, Kafr El-Sheikh University, Kafr El-Sheikh University, Egypt. E-mail: [email protected] b Department of Material and Life Science, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan. E-mail: [email protected] w Electronic supplementary information (ESI) available: Absorption spectrum of ZnNc radical cation, and nanosecond transient spectra of the examined compounds. See DOI: 10.1039/c0cp00612b

12746

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

Among them, self-assembly via metal–ligand axial coordination is one of the successful approaches developed to study photoinduced electron transfer in donor–acceptor dyads.44–53 In the majority of these studies, porphyrins and phthalocyanines have been used due to their close resemblance to the photosynthetic pigment, chlorophyll, and the established synthetic methodologies. In the present study, a novel self-assembled dyad composed of zinc naphthalocyanine, as electron donor, and subphthalocyanine, as electron acceptor, is constructed via axial ligand coordination and its photochemical properties examined herein (Fig. 1). To achieve axial coordination, subphthalocyanine has been functionalized with a pyridine entity at the para position. The rationale for choosing the naphthalocyanine and subphthalocyanine are as follows: naphthalocyanine is a suitable primary electron donor due to its resemblance to the natural photosynthetic chlorophyll, porphyrin and phthalocyanine pigments, rich redox behavior and strong absorption in the visible and near-IR regions.54–58 The naphthalocyanine derivative chosen in the present study is zinc 2,11,20,29-tetra-tert-butyl2,3-naphthalocyanine, ZnNc, which has good solubility in many organic solvents and an intense absorption band of the radical cation in the near IR region. This would serve as clear evidence for charge-separation in light initiated This journal is

c

the Owner Societies 2010

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

View Online

Fig. 1

Structure of the investigated SubPc(py) : ZnNc self-assembled dyad, as well as the ZnNc and SubPc(py) references.

electron-transfer reactions in the presence of electron acceptors. Although the excellent electron-donating properties of ZnNc compared with porphyrins and phthalocyanines, the studies of ZnNc-based dyads and triads are rare.54–58 Most of these studies are related to the electron transfer process of ZnNc–C60 self-assembled dyads.55–57 Quite recently, D’Souza et al. reported the energy transfer process, but not the electron transfer, for ZnNc-porphyrin self-assembled dyads.58 On the other hand, the comparatively electron deficient subphthalocyanine, SubPc, was chosen because of its strong absorption in the visible region, relatively low reorganization energies, and higher energy of the singlet excited state (2.16 eV) and triplet excited state (1.40 eV) compared with those of phthalocyanines.59–74 For these reasons, it is not surprising that some examples of SubPc- and ZnNc-containing dyads and triads have been described in the literature.54–75 Nonetheless, the study of the intramolecular interactions of the covalently or self-assembled SubPc(py) : ZnNc has not been explored yet. The formed SubPc(py) : ZnNc dyad has an advantage in that it absorbs the light in a wide section (up to 800 nm) of the UV/Vis spectrum. Owing to the particular electronic properties of ZnNc and SubPc(py), such an ensemble seems to be perfectly suited for the study of intramolecular electron-transfer process via the singlet states of both entities. This behavior was confirmed in this study by the steady-state emission, electrochemical, computational, and femtosecond laser photolysis measurements.

Experimental section Materials and instruments Zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine was obtained from Aldrich Chemicals. Pyridine-appended subphthalocyanine, SubPc(py), was prepared according to the literature.75 Anhydrous o-dichlorobenzene (Aldrich; >99.9%) was purchased as reagent grade and used without further purification. Steadystate fluorescence spectra were measured on a Shimadzu RF-5300 PC spectrofluorophotometer equipped with a photomultiplier This journal is

c

the Owner Societies 2010

tube with high sensitivity in the 700–800 nm region. Cyclic voltammograms were measured on a BAS CV-50W Voltammetric Analyzer. A platinum disk electrode was used as working electrode, while a platinum wire served as a counter electrode. SCE electrode was used as a reference electrode. All measurements were carried out in CH2Cl2 containing 0.1 M (n-C4H9)4NClO4 as the supporting electrolyte. The scan rate = 100 mV s 1. Densityfunctional theory (DFT) calculations were performed on a COMPAQ DS20E computer. Geometry optimizations were carried out using the Becke3LYP functional and 3-21G basis set, 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. The studied compounds were excited by a Panther OPO pumped by Nd : YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at l = 430 nm with powers of 1.5 and 3.0 mJ per pulse. The 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 a tunable range from 285 nm 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. Kinetic traces at appropriate wavelengths were assembled from the time-resolved spectral data. All measurements were conducted at 298 K. The transient spectra were recorded using fresh solutions in each laser excitation. Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

12747

View Online

Results and discussion

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

Formation and characterization of SubPc(py) : ZnNc dyad, self-assembled by axial coordination The absorption spectrum of ZnNc revealed a main absorption band in the near-IR region at 779 nm, in addition to weak absorption bands at 738, 692, 424, 350 and 326 nm. On the other hand, the SubPc(py) exhibited main absorption band at 562 nm, in addition to weak absorption bands at 517 and 308 nm. From this observation, it is clear that the absorption bands of ZnNc and SubPc(py) entities are well separated, providing the possibility for selective excitation of either ZnNc or SubPc(py). Fig. 2 shows the UV/Vis optical absorption spectral changes observed during the complexation of SubPc(py) with ZnNc in o-dichlorobenzene. The binding of ZnNc to SubPc(py) was characterized by diminished intensity of 779 nm bands with 2–4 nm blue shifts. The Benesi–Hildebrand plot76 constructed from the absorbance data revealed a straight line with a K value of 1.2  105 M 1 suggesting moderately stable complex formation. It should be noted that the K value of SubPc(py) : ZnNc is considerably higher than the earlier reported porphyrin- and phthalocyanine self-assembled dyads.77,78 This could be attributed to the electron-rich ZnNc macrocycle compared to ZnPc and ZnP macrocycles.

Emission studies The photochemical behavior of the SubPc(py) : ZnNc dyad was investigated, first by using steady-state fluorescence measurements in o-dichlorobenzene. When the most intense visible band at 690 nm was excited, the emission spectra of the SubPc(py) : 1ZnNc* exhibited emission bands at 781 nm and 712 nm (sh). Addition of SubPc(py) to an argon saturated o-dichlorobenzene solution of ZnNc decreased significantly the fluorescence intensity of ZnNc, accompanied by a nearly 3 nm blue shift (Fig. 3a). It is most likely that the quenching pathway of SubPc(py) : ZnNc is due to the electron transfer from the singlet ZnNc to SubPc(py), taking into account that the energy transfers from the low-lying 1ZnNc* (1.70 eV) to the SubPc (2.10 eV) is unfavorable due to energetic considerations. The binding constant for the SubPc(py) : ZnNc self-assembled

Fig. 2 (a) UV-visible spectral changes observed during the complexation of SubPc(py) with ZnNc in o-dichlorobenzene. (b) Benesi–Hildebrand analysis of the absorbance data.

12748

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

Fig. 3 (a) Fluorescence spectra of ZnNc (3.7 mM) in the presence of various amounts of SubPc(py) (0–0.28 mM) in o-dichlorobenzene (lex = 630 nm). (b) Benesi–Hildebrand analysis of the fluorescence data. (c) Stern–Volmer plot for the fluorescence quenching of ZnNc by SubPc(py).

dyad was found to be 1.1  105 M 1 (Fig. 3b), which is in a good agreement with the obtained K from the steady-state absorption measurements. Fig. 3c shows the Stern–Volmer plot for quenching the singlet ZnNc by SubPc(py). The plot revealed a higher slope, indicating the occurrence of efficient quenching. The KSV value calculated from the linear segment of the plot was found to be 2.4  104 M 1, from which the quenching rate constant, kq, was calculated to be 1.0  1013 M 1s 1 by employing a fluorescence lifetime of 2.4 ns for the singlet state of ZnNc.55–57 This kq value is nearly 3 orders of magnitude higher than what was expected for intermolecular type diffusion controlled quenching (kdiff) of 3.6  109 M 1 s 1 in o-dichlorobenzene.79 These results clearly demonstrate the occurrence of an intramolecular quenching of the singlet ZnNc by SubPc(py). When the most intense visible band at 500 nm was excited, the emission spectrum of the singlet SubPc(py) exhibited emission band at 577 nm (Fig. 4). Adding amounts of ZnNc (0–18 mM) to a dichlorobenzene solution of SubPc(py) resulted in a decrease of the emission of SubPc(py). The quenching process may involve the electron transfer and/or energy transfer processes. Scanning the emission wavelength of longer wavelength regions (700–850 nm) revealed no emission of the ZnNc moiety. These results suggest that the energy transfer from the singlet excited SubPc(py) to ZnNc is not necessarily a cause of the fluorescence quenching. The inset of Fig. 4 shows the Stern–Volmer plot for the quenching of SubPc(py), by ZnNc, where the quenching rate constant, kq, This journal is

c

the Owner Societies 2010

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

View Online

Fig. 4 Fluorescence spectra of SubPc(py) (6.45 mM) in the presence of various amounts of ZnNc (0–18 mM) in o-dichlorobenzene (lex = 500 nm). Inset: Stern–Volmer plot for the fluorescence quenching of SubPc(py) by ZnNc.

was calculated to be 6.85  1013 M 1 s 1 by employing a fluorescence lifetime of 2.1 ns for the singlet state of subphthalocyanine.59–62 Since this kq value is much higher than what was expected for intermolecular type diffusion controlled quenching, this clearly demonstrates the occurrence of an intramolecular quenching process of 1(SubPc(py))* by the ZnNc.

Femtosecond and nanosecond transient absorption spectral studies In order to provide evidence for the charge separation in the picosecond time region as revealed by the fluorescence studies, the femtosecond transient absorption measurements of SubPc(py) : ZnNc dyad have been done in o-dichlorobenzene by utilizing 430 nm light, which excites the ZnNc entity. Fig. 5 shows the time-resolved absorption spectra of the SubPc(py) : ZnNc dyad in o-dichlorobenzene; the spectrum obtained at 1 ps after the 150 femtosecond-laser pulse has been attributed to the singlet (S1)–singlet (Sn) transitions of the ZnNc entity.80 Between 10–200 ps, the characteristic absorption band of the ZnNc radical cation (ZnNc +) is clearly seen at 956 nm.55–57,81 The characteristic band of the ZnNc radical cation in the near-IR region was employed as a reliable probe to determine the rate constant of the charge recombination, kCR, because the ZnNc + band does no overlap the bands of other species. The time profile (inset) was well fitted by a single-exponential function. From the rise profile of ZnNc +, the rate of the charge-separation (kCS) process was estimated to be 1.30  1010 s 1. After reaching a peak maximum after about 200 ps, the ZnNc + begins to decay slowly due to the charge-recombination process. An analysis of the absorption time profile of ZnNc + in the 0–3 ns region yielded a rate constant of 1.0  109 s 1 for the decay rate of the radical-ion pair, from which the lifetime of the singlet radical-ion pair was found to be 1 ns. This journal is

c

the Owner Societies 2010

Fig. 5 Femtosecond transient absorption spectra of SubPc(py) : ZnNc self-assembled dyad in deaerated dichlorobenzene. lex = 430 nm.

The charge-separation process via 1ZnNc* was supported from the viewpoint of thermodynamics of electron-transfer process from the ZnNc to the SubPc(py). By sweeping the voltage applied to the solution containing (n-C4H9)4NClO4 as supporting electrolyte, the redox potentials of the SubPc(py) : ZnNc were measured in dichloromethane. The differential pulse voltammogram (DPV) of SubPc(py) : ZnNc self-assembled dyad showed the first reduction potential (Ered) of the SubPc(py) entity at 1.03 V vs. SCE, and the corresponding first oxidation potential (Eox) of the ZnNc moiety was located at 0.60 V vs. SCE (Fig. 6). Based on the first oxidation potential of the ZnNc, the first reduction potential of the SubPc(py) and a solvent correction term (DGS), the thermodynamic driving forces for charge-recombination process ( DGCR) in o-dichlorobenzene was calculated as 1.55 eV.82 Based on DGCR and the energy of the singlet state of ZnNc (1.70 eV) and SubPc(py) (2.16 eV), the driving forces for chargeseparation processes ( DGCS) via 1ZnNc* and 1(SubPc(py))* were found to be 0.15 and 0.61 eV, respectively, indicating exothermic photoinduced electron transfer processes.

Fig. 6 The differential pulse voltammogram (DPV) of SubPc(py) : ZnNc self-assembled dyad in dichloromethane. Scan rate = 50 mV s 1.

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

12749

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

View Online

Moreover, the charge-separation from the ZnNc entity to the SubPc(py) forming the radical-ion pair was supported by studying the molecular geometries and electronic structures of the SubPc(py) : ZnNc dyad on the basis of molecular orbital calculation using density functional method (DFT) at the B3LYP/6-311G level. The geometric parameters of the conjugates were obtained after complete energy minimization. In the optimized structure, the radius of the ZnNc and SubPc(py) was estimated as 8.89 and 6.08 A˚, respectively. The Zn : N distance of the newly formed axial coordination bond was found to be 2.126 A˚. The center-to-center distance (RCC), that is, the distance between the central zinc and the boron atom of subphthalocyanine, was found to be 7.41 A˚. Fig. 7 shows that the electron distribution of the HOMO was found to be entirely located on the ZnNc entity, which suggests no charge transfer interaction between ZnNc and SubPc(py) entities in the ground state. Similarly, the electron distribution of the LUMO was found to be entirely located on the subphthalocyanine. The observations confirm the formation of (SubPc(py)) +–ZnNc as charge-separated state. The calculated HOMO–LUMO gap (gas phase) was found to be 1.42 eV. This value agree fairly well with the electrochemically measured (difference between the first oxidation and first reduction potential) values. The complementary nanosecond transient absorption spectra of SubPc(py) : ZnNc self-assembled dyad in deaerated o-dichlorobenzene was performed by utilizing 430 nm laser light, which excited the ZnNc entity. The transient spectra show no evidence of electron transfer from triplet ZnNc to SubPc(py), but instead the triplet–triplet (T–T) absorption of zinc naphthalocyanine (3ZnNc*) was observed in the visible region with a maximum at 600 nm that may arise from the charge-recombination process (Fig. S3 and S4w).55–57 This in a

Fig. 7 Optimized structure and calculated frontier HOMO and LUMO orbitals of the SubPc(py) : ZnNc dyad by using ab initio B3LYP/6-311G methods.

12750

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

Fig. 8 Nanosecond transient spectra of ZnNc (0.1 mM) : SubPc(py) (0.1 mM) in o-dichlorobenzene via triplet SubPc(py). lex = 500 nm.

good agreement with the lower energy of 3ZnNc* (0.97 eV) compared with the (SubPc(py)) : ZnNc +. Finally, the 3 ZnNc* decayed to the ground state with a rate constant of 4.4  104 s 1. By selective excitation of SubPc(py) with 490–600 nm laser light, the nanosecond transient absorption spectra of the SubPc(py) : ZnNc exhibited the characteristic absorption band of the triplet SubPc(py) at 450 nm (Fig. S5w),59–62,66,69 which may be populated from the intersystem crossing (ISC) of the singlet SubPc(py) and/or the charge-recombination of the radical-ion pair. With the decay of triplet SubPc(py), the concomitant formation of triplet ZnNc was observed at 600 nm (Fig. 8 and Fig. S6w). This observation points to the energy transfer from the triplet SubPc(py) (1.45 eV)70 to the low-lying triplet ZnNc (0.97 eV). From the rise profile of the 3ZnNc* at 600 nm, the rate of T–T energy transfer was found to be 2.2  105 s 1, while the decay of 3ZnNc* to the ground state was found to be 1.7  104 s 1. As depicted in Fig. 9 for the photoinduced intramolecular process of SubPc(py) : ZnNc in o-dichlorobenzene, the 430 nm laser excitation pumps up the ZnNc moiety to its singlet excited state, from which the charge-separation process takes place efficiently as confirmed by femtosecond laser

Fig. 9 Energy level diagram of SubPc(py) : ZnNc in o-dichlorobenzene via the singlet ZnNc.

This journal is

c

the Owner Societies 2010

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

View Online

measurements. The formed charge-separated state decayed to populate the ZnNc triplet state, which in turn decayed to the ground state. On the other hand, by selective excitation of the SubPc(py) entity, the electron transfers from the ZnNc to the singlet SubPc(py) is highly anticipated ( DGCS = 0.61 eV). However, it is difficult to follow the quenching pathway via the singlet SubPc(py) due the limitation in the excitation light of the employed femtosecond laser system. It is likely that the charge recombination results in formation of the triplet SubPc(py), which in turn decayed to populate the low lying triplet ZnNc through a T–T energy transfer process as confirmed from the nanosecond transient spectra.

Conclusions Supramolecular zinc naphthalocyanine-pyridine appended subphthalocyanine dyad via a metal ligand axial coordination approach have been formed in non-coordinating dichlorobenzene and characterized by spectroscopic, computational, and electrochemical methods. The measured binding constant was found to be on the order of 1.2  105 M 1, suggesting moderately stable complex formation. The structure of the ZnNc : SubPc(py) was deduced from computational studies using the B3LYP/6-31G method. The redox potential of the donor and acceptor entities were measured using differential pulse technique, and the performed free-energy calculations suggested electron transfer to be an efficient process when either ZnNc or SubPc(py) is selectively excited in this dyad. Efficient and fast electron transfer from the singlet state of ZnNc to SubPc(py) was observed by utilizing the femtosecond laser technique. From the time profile of the ZnNc radical cation in the NIR region, the rate of charge recombination was found to be 1.0  109 s 1, from which the lifetime of the charge-separated state was found to be 1 ns. The strong absorption in the visible and NIR region and high charge-separation/charge-recombination ratio of SubPc(py) : ZnNc suggests its potential to be a photosynthetic model.

Acknowledgements The author is grateful to Prof. Dr Shunichi Fukuzumi (Osaka University) for supporting this work. The author is also thankful to Atsuro Takai for providing the subphthalocyanine sample.

References 1 The Photosynthetic Reaction Center, ed. J. Deisenhofer and J. R. Norris, Academic Press, San Diego, CA, 1993. 2 J. Deisenhofer, O. Epp, K. Miki, R. Huber and H. Michel, J. Mol. Biol., 1984, 180, 385. 3 R. A. Wheeler, Introduction to the Molecular Bioenergetics of Electron, Proton, and Energy Transfer, ACS Symposium Series 883, Molecular Bioenergetics, American Chemical Society, Washington, DC, 2004, pp. 1–6. 4 Electron Transfer in Photosynthesis, in Molecular to Global Photosynthesis, ed. M. D. Archer and J. Barber, Photoconversion of Solar Energy 2, Imperial College Press, London, 2004, p. 117. 5 Photochemical Conversion and Storage of Solar Energy, ed. J. S. Connolly, Academic, New York, 1981.

This journal is

c

the Owner Societies 2010

6 Molecular Level Artificial Photosynthetic Materials, ed. G. J. Meyer, Wiley, New York, 1997. 7 M. R. Wasielewski, Chem. Rev., 1992, 92, 435. 8 J. W. Verhoeven, Adv. Chem. Phys., 1999, 106, 603. 9 A. Osuka, N. Mataga and T. Okada, Pure Appl. Chem., 1997, 69, 797. 10 M. J. Blanco, M. Consuelo Jimenez, J.-C. Chambron, V. Heitz, M. Linke and J.-P. Sauvage, Chem. Soc. Rev., 1999, 28, 293. 11 V. Balzani, P. Ceroni, A. Juris, M. Venturi, S. Campagna, F. Puntoriero and S. Serroni, Coord. Chem. Rev., 2001, 219–221, 545. 12 V. Balzani, A. Credi and M. Venturi, ChemSusChem, 2008, 1, 26. 13 L. Flamigni, F. Barigelletti, N. Armaroli, J.-P. Collin, I. M. Dixon, J.-P. Sauvage and J. A. G. Williams, Coord. Chem. Rev., 1999, 190–192, 671. 14 F. Diederich and M. Gomez-Lopez, Chem. Soc. Rev., 1999, 28, 263. 15 F. D. Lewis, R. L. Letsinger and M. R. Wasielewski, Acc. Chem. Res., 2001, 34, 159. 16 D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 1993, 26, 198. 17 D. Gust and T. A. Moore, The Porphyrin Handbook, ed. K. M. Kadish, K. Smith and R. Guilard, Academic Press, San Diego, 2000, vol. 8, pp. 153–190. 18 S. Fukuzumi and D. M. Guldi, Electron Transfer in Chemistry, ed. V. Balzani, Wiley-VCH, Weinheim, 2001, vol. 2, pp. 270–337. 19 S. Fukuzumi, The Porphyrin Handbook, ed. K. M. Kadish, K. Smith and R. Guilard, Academic Press, San Diego, 2000, vol. 8, pp. 115–151. 20 S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283. 21 Y. Sakata, H. Imahori, H. Tsue, S. Higashida, T. Akiyama, E. Yoshizawa, M. Aoki, K. Yamada, K. Hagiwara, S. Taniguchi and T. Okada, Pure Appl. Chem., 1997, 69, 1951. 22 H. Imahori and Y. Sakata, Eur. J. Org. Chem., 1999, 2445. 23 H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2002, 124, 5165. 24 T. Umeyama and H. Imahori, Energy Environ. Sci., 2008, 1, 120. 25 D. M. Guldi, Chem. Commun., 2000, 321. 26 D. M. Guldi, Chem. Soc. Rev., 2002, 31, 22. 27 V. Sgobba and D. M. Guldi, Chem. Soc. Rev., 2009, 38, 165. 28 A. Mateo-Alonso, D. M. Guldi, F. Paolucci and M. Prato, Angew. Chem., Int. Ed., 2007, 46, 8120. 29 D. M. Guldi, Phys. Chem. Chem. Phys., 2007, 9, 1400. 30 L. Sa´nchez, M. Nazario and D. M. Guldi, Angew. Chem., Int. Ed., 2005, 44, 5374. 31 J. S. Sessler, B. Wang, S. L. Springs and C. T. Brown, in Comprehensive Supramolecular Chemistry, ed. J. L. Atwood, J. E. D. Davies, D. D. MacNicol and F. Vogtle, Pergamon, New York, 1996, ch. 9. 32 Y. Chen, M. E. El-Khouly, J. J. Doyle, Y. Lin, Y. Liu, E. Notaras and S. M. O’Flaherty, in Handbook of Organic Electronics and Photonics, ed. H. R. Nalwa, American Scientific Publishers, Stevenson Ranch, California, USA, 2008, vol. 2, p. 151. 33 An Introduction to Molecular Electronics, ed. M. C. Petty, M. R. Bryce and D. Bloor, Oxford University Press, New York, 1995. 34 Molecular Switches, ed. B. L. Feringa, Wiley-VCH GmbH, Weinheim, 2001. 35 D. Gust, T. A. Moore and A. L. Moore, Chem. Commun., 2006, 1169. 36 V. Balzani, A. Credi and M. Venturi, Organic Nanostructures, ed. J. L. Atwood and J. W. Steed, Wiley-VCH Verlag GmbH & Co., Weinheim, Germany, 2008, pp. 1–31. 37 Energy Harvesting Materials, ed. D. L. Andrews, World Scientific, Singapore, 2005. 38 Molecular Electronics; Commercial Insights, Chemistry, Devices, Architectures and Programming, ed. J. M. Tour, World Scientific, River Edge, NJ, 2003. 39 S. Fukuzumi, Pure Appl. Chem., 2007, 79, 981. 40 S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609. 41 S. Fukuzumi, Bull. Chem. Soc. Jpn., 2006, 79, 177. 42 S. Fukuzumi and T. Kojima, J. Mater. Chem., 2008, 18, 1427. 43 S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283. 44 F. D’Souza, G. R. Deviprasad, M. E. Zandler, V. T. Hoang, A. Klykov, M. VanStipdonk, A. Perera, M. E. El-Khouly, M. Fujitsuka and O. Ito, J. Phys. Chem. A, 2002, 106, 3243.

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

12751

Downloaded by OSAKA UNIVERSITY on 30 September 2010 Published on 27 August 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00612B

View Online

45 M. E. El-Khouly, O. Ito, P. M. Smith and F. D’Souza, J. Photochem. Photobiol., C, 2004, 5, 79. 46 F. D’Souza, P. M. Smith, M. E. Zandler, A. L. McCarty, M. Itou, Y. Araki and O. Ito, J. Am. Chem. Soc., 2004, 126, 7898. 47 F. D’Souza and O. Ito, Coord. Chem. Rev., 2005, 249, 1410. 48 F. D’Souza, M. E. El-Khouly, S. Gadde, A. L. McCarty, P. A. Karr, M. E. Zandler, Y. Araki and O. Ito, J. Phys. Chem. B, 2005, 109, 10107. 49 F. D’Souza and O. Ito, Handbook of Organic Electronics and Photonics, ed. H. R. Nalwa, American Scientific Publishers, Stevenson Ranch, CA, 2008, vol. 1, ch. 13, pp. 485–521. 50 R. Chitta and F. D’Souza, J. Mater. Chem., 2008, 18, 1440. 51 T. Kojima, T. Honda, K. Ohkubo, M. Shiro, T. Kusukawa, T. Fukuda, N. Kobayashi and S. Fukuzumi, Angew. Chem., Int. Ed., 2008, 47, 6712. 52 F. D’Souza and O. Ito, Chem. Commun., 2009, 4913. 53 Y. Rio, W. Seitz, A. Gouloumis, P. Va´zquez, J. L. Sessler, D. M. Guldi and T. Torres, Chem.–Eur. J., 2010, 16, 1929. 54 N. Kobayashi, Coord. Chem. Rev., 2002, 227, 129. 55 M. E. El-Khouly, L. M. Rogers, M. E. Zandler, G. Suresh, M. Fujitsuka, O. Ito and F. D’Souza, ChemPhysChem, 2003, 4, 474. 56 F. D’Souza, G. Gadde, M. E. El-Khouly, M. E. Zandler, Y. Araki and O. Ito, J. Porphyrins Phthalocyanines, 2005, 9, 698. 57 M. E. El-Khouly, Y. Araki, O. Ito, S. Gadde, M. E. Zandler and F. D’Souza, J. Porphyrins Phthalocyanines, 2006, 10, 1156. 58 E. Maligaspe, T. Kumulainen, H. Lemmetyinen, N. V. Tkachenko, N. K. Subbaiyan, M. E. Zandler and F. D’Souza, J. Phys. Chem. A, 2010, 114, 268. 59 M. E. El-Khouly, S. H. Shim, Y. Araki, O. Ito and K.-Y. Kay, J. Phys. Chem. B, 2008, 112, 3910. 60 J.-H. Kim, M. E. El-Khouly, Y. Araki, O. Ito and K.-Y. Kay, Chem. Lett., 2008, 37, 544. 61 M. E. El-Khouly, J. B. Ryu, K.-Y. Kay, O. Ito and S. Fukuzumi, J. Phys. Chem. C, 2009, 113, 15444. 62 M. E. El-Khouly, D. K. Ju, K.-Y. Kay, F. D’Souza and S. Fukuzumi, Chem.–Eur. J., 2010, 16, 6193. 63 B. del Rey and T. Torres, Tetrahedron Lett., 1997, 38, 5351. 64 C. G. Claessens and T. Torres, J. Am. Chem. Soc., 2002, 124, 14522. 65 C. G. Claessens, D. Gonza´lez-Rodrı´ guez and T. Torres, Chem. Rev., 2002, 102, 835. 66 D. Gonza´lez-Rodrı´ guez, T. Torres, D. M. Guldi, J. Rivera and L. Echegoyen, Org. Lett., 2002, 4, 335. 67 C. G. Claessens and T. Torres, Chem. Commun., 2004, 1298.

12752

Phys. Chem. Chem. Phys., 2010, 12, 12746–12752

68 D. Gonza´lez-Rodrı´ guez, C. G. Claessens, T. Torres, S. Liu, L. Echegoyen, N. Vila and S. Nonell, Chem.–Eur. J., 2005, 11, 3881. 69 D. Gonza´lez-Rodrı´ guez, T. Torres, M. M. Olmstead, J. Rivera, M. A. Herranz, L. Echegoyen, C. Atienza Castellanos and D. M. Guldi, J. Am. Chem. Soc., 2006, 128, 10680. 70 A. Medina, C. G. Glassens, G. M. Aminur Rahman, A. Lamsabhi, O. Mo’, M. Yan˜ez, D. M. Guldi and T. Torres, Chem. Commun., 2008, 1759. 71 X.-Y. Li and D. K. P. Ng, Eur. J. Inorg. Chem., 2000, 1845. 72 G. Berber, A. N. Cammidge, I. Chambrier, M. J. Cook and P. W. Hough, Tetrahedron Lett., 2003, 44, 5527. 73 A. N. Cammidge, G. Berber, I. Chambrier, P. W. Hough and M. J. Cook, Tetrahedron, 2005, 61, 4067. 74 X. Leng and D. K. P. Ng, Eur. J. Inorg. Chem., 2007, 4615. 75 H. Xu and D. K. P. Ng, Inorg. Chem., 2008, 47, 7921. 76 H. A. Benesi and J. H. Hildebrand, J. Am. Chem. Soc., 1949, 71, 2703. 77 J. R. Miller and G. D. Dorough, J. Am. Chem. Soc., 1952, 74, 3977. 78 K. M. Kadish and L. R. Shiue, Inorg. Chem., 1982, 21, 1112. 79 Handbook of Photochemistry, ed. S. I. Murov, Marcel Dekker, New York, 1985. 80 J. D. Spikes, J. E. van Lier and J. C. Bommer, J. Photochem. Photobiol., A, 1995, 91, 193. 81 The absorption band in the NIR region with a maximum at 960 nm is assigned to the one-electron oxidized species of ZnNc (ZnNc +) by comparison with the spectrum obtained in the one-electron oxidation reaction of ZnNc with (Ru(bpy)3)3+ (Fig. S1w) and by the spectro-electrochemical studies (Fig. S2w). It should be noted that the absorption band of ZnNc + is red-shifted by about 300 and 120 nm compared with ZnP + and ZnPc +. 82 The driving forces for charge recombination ( DGCR) and charge separation ( DGCS) were calculated according to the equations Ered + DGS and DGCS = E0,0 ( DGCR), DGCR = Eox where Eox is the first oxidation potential of the ZnNc entity, Ered is the first reduction potential of the subphthalocyanine entity and DE0,0 is the energy of the 0–0 transition between the lowest excited state and the ground state of the ZnNc as evaluated from the fluorescence peak. DGS refers to the static energy, calculated by using the ‘dielectric continuum model’ DGS = e2/(4pe0eSRCt–Ct). The center-to-center distance between the ZnNc and SubP(py) entities, RCt–Ct, was estimated as 7.41 A˚. The symbols e0 and eS represent vacuum permittivity and dielectric constant of the solvent, respectively.

This journal is

c

the Owner Societies 2010