Photoinduced electron transfer in supramolecular complexes of a π-extended viologen with porphyrin monomer and dimer

Abstract

A π-extended viologen has been synthesized, forming supramolecular complexes with a freebase tetraphenylporphyrin (H₂TPP) and the cofacial porphyrin dimer with an anthracene spacer [H₄(DPA)] through π–π interaction in benzonitrile (PhCN). Formation of the H₂TPP-BHV²⁺ supramolecular complex was probed by UV-vis and fluorescence spectra. The fluorescence of H₂TPP was strongly quenched by electron transfer from the singlet excited state (¹H₂TPP^*) to BHV²⁺ in the supramolecular complex. The transient absorption spectrum of the charge-separated (CS) state (H₂TPP^•+ and BHV^•+) was successfully detected by the laser flash photolysis measurements of the H₂TPP-BHV²⁺ supramolecular complex in PhCN. The lifetime of the CS state of the supramolecular complex was determined as 0.37 ms in PhCN at 298 K. The H₄(DPA)-BHV²⁺ supramolecular complex was formed with a larger formation constant as compared to the H₂TPP-BHV²⁺ supramolecular complex because of the stronger π–π interaction of BHV²⁺ with the cofacial porphyrin dimer and the lifetime of the CS state was determined to be 0.59 ms in PhCN at 298 K.

---

Introduction

The essential importance of the function of photosynthesis has been well recognized as the energy and environmental problems associated with enormous consumption of fossil fuels evolving unacceptable amounts of CO₂ has prompted design and preparation of a large number of donor–acceptor covalently linked ensembles including dyads, triads, tetrads, and pentads that can mimic the electron transfer processes in the photosynthetic reaction center.^1–7 However, achieving the long lifetime and the high efficiency of charge separation like natural photosynthesis has often been hampered by the synthetic difficulties of making a number of covalent bonds to connect each component. In contrast to the covalently linked donor–acceptor ensembles, the use of non-covalent binding such as metal–ligand coordination, electrostatic interaction, hydrogen bonds and rotaxane formation has recently merited increasing attention as a simpler but more elegant way to construct electron donor–acceptor ensembles mimicking the biological photosynthetic reaction center model systems.^8–20 The development of such supramolecular systems requires molecular modification such as introduction of recognition sites for hydrogen bonding, metal ions for coordination bond formation, and opposite charges for electrostatic interaction. In contrast to these cases, no special molecular modification is required to initiate π–π interactions once appropriate compounds containing π-systems are chosen as components to construct electron donor–acceptor supramolecular ensembles.^7–20 However, there has been no report on the introduction of π-conjugation to electron acceptors, which makes possible formation of supramolecular π-complexes with electron donors.

We report herein synthesis of a π-extended viologen, which can form supramolecular complexes with porphyrins through π–π interactions in benzonitrile (PhCN). We have chosen a viologen derivative as an electron acceptor because viologens have frequently been used as an electron mediator for hydrogen production.^21–24 Photoexcitation of supramolecular complexes with porphyrin monomers and dimers resulted in efficient photoinduced electron transfer to achieve the long-lived electron-transfer state. Porphyrins and viologen derivatives used in this study are shown in Fig. 1.

Fig. 1 Porphyrins and viologens used in this study.

Results and discussion

Formation of a supramolecular complex of a π-extended viologen with a porphyrin monomer and dimer

The synthesis and characterization of 1,4-bis(N-hexyl-4-pyridinium)butadiene diperchlorate [BHV²⁺(ClO₄⁻)₂] are described in the experimental section and the electronic supplementary information (ESI) S1 and S2.† First, we examined the supramolecular complex formation of monomer porphyrin (H₂TPP) with hexyl viologen (HV²⁺) and BHV²⁺. The UV-visible spectrum of H₂TPP in PhCN at 298 K was not changed by addition of HV²⁺ (Fig. 2a). On the contrary, the spectrum of H₂TPP was changed upon addition of BHV²⁺ with appearance of isosbestic points (Fig. 2b). The absorption change at 650 nm is shown in Fig. 3a. Such a saturated dependence on the BHV²⁺ concentrations indicates that H₂TPP forms a 1 : 1 complex with BHV²⁺ (Scheme 1). According to Scheme 1, the absorbance change is given by eqn 1,²⁵ which predicts a linear correlation

[H₂TPP]₀/(A₀ − A) = 1/(ε_c − ε_p) + (K [BHV²⁺](ε_c − ε_p))⁻¹ (1)

between [H₂TPP]₀/(A₀ − A) and [BHV²⁺]⁻¹, where A₀ and A are the absorbance of H₂TPP at 650 nm in the absence and presence of BHV²⁺, and ε_p and ε_c are the molar absorption coefficients of H₂TPP at 650 nm in the absence and presence of BHV²⁺, respectively. From a linear plot of [H₂TPP]₀/(A₀ − A) vs. [BHV²⁺]⁻¹ shown in Fig. 3b, the formation constant (K) of the H₂TPP–BHV²⁺ complex was determined to be 3.1 (±0.5) × 10² M⁻¹ in PhCN at 298 K. In this way, porphyrin formed supramolecular complex with viologen through π–π interactions by using π-extended viologen (BHV²⁺).

Fig. 2 (a) UV-visible spectra of H₂TPP (6.0 × 10⁻⁵ M) in the presence of various concentration of HV²⁺ (0 to 5.5 × 10⁻³ M) in PhCN. (b) UV-visible spectra of H₂TPP (6.0 × 10⁻⁵ M) in the presence of various concentration of BHV²⁺ (0 to 5.4 × 10⁻³ M) in PhCN.

Fig. 3 (a) Plots of absorbance at 650 nm vs. [BHV²⁺]. (b) Plots of [H₂TPP]₀/(A − A₀) vs. [BHV²⁺]⁻¹ at 650 nm.

Scheme 1 Supramolecular formation of BHV²⁺ with H₂TPP.

The supramolecular complex formation between BHV²⁺ and cofacial porphyrin dimer (H₄(DPA) in Fig. 1)²⁶ was also examined by UV-visible spectroscopy (Fig. S3 in ESI†). An evolution of the spectra was observed in analogy with H₂TPP by addition of BHV²⁺ with isosbestic points (Fig. S4a in ESI†). This indicates that H₄(DPA) forms a 1 : 1 complex with BHV²⁺ (Scheme 2). The binding constant (K) of this supramolecular complex was determined to be 4.0 (± 0.2) × 10² M⁻¹ from the plot of the absorption change at 500 nm (Fig. S4b in ESI†). This binding constant is larger than for monomer porphyrin, H₂TPP, suggesting that the cofacial porphyrin dimer has stronger π–π interactions than the monomer porphyrin, because BHV²⁺ can be inserted between the two porphyrin rings of H₄(DPA).²⁷

Scheme 2 Supramolecular formation of BHV²⁺ with H₄(DPA).

Photoexcitation of the Q band of H₂TPP at 532 nm in PhCN results in fluorescence at λ_max = 653 and 714 nm. Addition of BHV²⁺ to a PhCN solution of H₂TPP resulted in a significant change in the fluorescence spectrum of H₂TPP (Fig. 4a). The fluorescence intensity of H₂TPP decreased to reach a constant value with increasing BHV²⁺ concentration as the H₂TPP–BHV²⁺ supramolecular complex was formed (inset of Fig. 4a). Virtually the same K value (3.3 (± 0.5) × 10² M⁻¹) was obtained from the change in the fluorescence spectrum of H₂TPP in the presence of BHV²⁺ (Fig. 4b) as the value obtained from the absorption spectral change (vide supra). Such agreement strongly indicates that fluorescence quenching occurs exclusively in the complex rather than intermolecular reaction between the singlet excited state (¹H₂TPP*) and BHV²⁺.

Fig. 4 (a) Fluorescence spectra of H₂TPP (6.0 × 10⁻⁵ M) in the presence of various concentrations of BHV²⁺ (0 to 4.6 × 10⁻³ M) in deaerated PhCN. The arrows indicate the direction of change. Inset: Plots of the fluorescence intensity at 720 nm vs. [BHV²⁺]. (b) Plots of [H₂TPP]₀/(I − I₀) vs. [BHV²⁺]⁻¹ at 720 nm.

The fluorescence quenching of H₄(DPA) also occurs by BHV²⁺ in the supramolecular complex between H₄(DPA) and BHV²⁺ (Fig. 4Sa in ESI†). The formation constant (K = 4.0 (± 0.2) × 10² M⁻¹) determined by the fluorescence quenching (Fig. 4b in ESI) agrees with the K value obtained from the absorption spectral change.

One-electron redox potentials of supramolecular complexes

In order to determine the free energy change of photoinduced electron transfer, the redox potentials of H₂TPP and BHV²⁺ were determined by cyclic voltammetry measurements in PhCN (Fig. 5). The one-electron oxidation potential (E⁰_ox) of H₂TPP and the reduction potential (E⁰_red) of BHV²⁺ were determined to be 1.06 V and −0.50 V, respectively (Fig. 5a and 5b). The change in the redox potentials of H₂TPP and BHV²⁺ by the supramolecular complex formation was also examined by CV. The ratio of complexation of the supramolecular complex is calculated as 66% under the experimental conditions, [H₂TPP] = 6.0 mM, [BHV²⁺] = 1.0 mM. The E⁰_ox value of H₂TPP was shifted from 1.06 V to 0.84 V upon addition of excess BHV²⁺ (Fig. 5c). Similarly, the E⁰_red value of BHV²⁺ was also shifted from −0.50 V to −0.52 V (Fig. 5b and 5d). The energy of the electron-transfer state determined by the E⁰_ox value of H₂TPP and the E⁰_red value of BHV²⁺ is higher than the triplet state of H₂TPP (1.43 eV).²⁸ However, the energy of the electron transfer (ET) state in the supramolecular complex determined by the change in the redox potentials by formation of the supramolecular complex is lower than the triplet state of H₂TPP. The free energy change of photoinduced electron transfer (ΔG⁰_ET) from ¹H₂TPP^* to BHV²⁺ in the H₂TPP-BHV²⁺ supramolecular complex in PhCN was determined to be −0.55 eV from the E⁰_ox and E⁰_red values of the H₂TPP-BHV²⁺ supramolecular complex, and the excitation energy (S₁ = 1.91 eV)²⁹ of H₂TPP in PhCN (Table 1). A similar potential shift was observed in the case of H₄(DPA)–BHV²⁺ (Fig. S5 in ESI†), which is summarized in Table 1. Photoinduced electron transfer in H₄(DPA)–BHV²⁺ is also favorable because of the negative ΔG⁰_ET value (−0.49 eV) as shown in Table 1. Thus, the observed fluorescence quenching of H₂TPP and H₄(DPA) by BHV²⁺ results from electron transfer from the singlet excited states (¹H₂TPP^* and ¹H₄(DPA)^*) to BHV²⁺ in the supramolecular complexes.

Fig. 5 Cyclic voltammograms of (a) H₂TPP (1.0 × 10⁻⁴ M), (b) BHV²⁺ (1.0 × 10⁻⁴ M), (c) H₂TPP (1.0 × 10⁻⁴ M) with BHV²⁺ (6.0 × 10⁻³ M), and (d) H₂TPP (6.0 × 10⁻³ M) with BHV²⁺ (1.0 × 10⁻³ M) in deaerated PhCN containing 0.10 M TBAPF₆. Scan rate: 100 mV s⁻¹.

Table 1 Redox potentials (E⁰_ox and E⁰_red) and driving Force of photoinduced electron transfer (−ΔG_ET) of supramolecules in PhCN

Compound	E ^0 ox vs. SCE (V)^a	E ^0 red vs. SCE (V)^a	ΔGET (eV)^a
a Values in the parenthesis are given for the uncomplexed form.
H2TPP-BHV^2+	0.84(1.06)	−0.52(−0.50)	−0.55(−0.35)
H4(DPA)-BHV^2+	0.92(1.03)	−0.56(−0.50)	−0.49(−0.44)

---

Photoinduced electron transfer in supramolecular complexes

The occurrence of the photoinduced electron transfer in the supramolecular π-complex was confirmed by the transient absorption spectra of the π-complex measured in PhCN using nanosecond laser flash photolysis. The transient absorption bands observed in Fig. 6 agree with those of superposition of the absorption bands due to H₂TPP^•+ and those of BHV^•+ in PhCN (Fig. 7).²⁶ Thus, the transient absorption spectrum in Fig. 6 clearly indicates formation of the ET state of H₂TPP–BHV²⁺ by photoinduced electron transfer from the singlet excited state of H₂TPP (¹H₂TPP^*) to BHV²⁺ in the supramolecular complex.

Fig. 6 Transient absorption spectrum of H₂TPP (6.0 × 10⁻⁵ M) in the presence of BHV²⁺ (5.4 × 10⁻³ M) in deaerated PhCN at 298 K taken at 10 μs after laser excitation at 570 nm.

Fig. 7 Differential spectrum of BHV^•+ in the electron-transfer reduction of BHV²⁺ with tetramethylsemiquinone radical anion in PhCN.

In the case of the H₄(DPA)–BHV²⁺ supramolecular complex, similar transient spectra were observed in PhCN (Fig. 8), indicating formation of the ET state of H₄(DPA)–BHV²⁺. Fig. 9a and 9c show time profiles of the decay of the ET states, [H₂TPP^•+–BHV^•+] and [H₄(DPA)^•+–BHV^•+] in PhCN using different laser intensities, respectively. The first-order plots (Fig. 9b and 9d) for the decay time profiles of the ET state at different concentrations afforded linear correlations with the same slope for the long-lived component, irrespective of the difference in concentration of the ET state, whereas the corresponding second-order plots are clearly non-linear and the initial slope varies depending on concentration of the ET state (Fig. S6 in ESI†). The non-linear minor component in the first-order plot (Fig. 9b and 9d) indicates bimolecular charge recombination between charge-separated states or an unbound radical ion pair. Thus, the long-lived decay process is ascribed to back electron transfer from BHV^•+ to H₂TPP^•+ and H₄(DPA)^•+ in the supramolecular complexes rather than intermolecular back electron transfer.

Fig. 8 Transient absorption spectrum of H₄(DPA) (6.0 × 10⁻⁵ M) in the presence of BHV²⁺ (5.5 × 10⁻³ M) in deaerated PhCN at 298 K taken at 10 μs after laser excitation at 570 nm.

Fig. 9 (a) Time profiles of the absorption at 640 nm of H₂TPP^•+-BHV^•+ obtained upon nanosecond flash photolysis (λ_ex = 570 nm) with different laser power in deaerated PhCN at 298 K.; [H₂TPP] = 6.0 × 10⁻⁵ M; [BHV²⁺] = 5.4 × 10⁻³ M. (c) Time profiles of the absorption at 640 nm of H₄(DPA)^•+-BHV^•+ obtained upon nanosecond flash photolysis (λ_ex = 570 nm) with different laser power in deaerated PhCN at 298 K; [H₄(DPA)] = 6.0 × 10⁻⁵ M; [BHV²⁺] = 5.5 × 10⁻³ M. (b, d) First-order plots.

The lifetimes of the ET state of H₂TPP–BHV²⁺ and H₄(DPA)–BHV²⁺ were determined as 0.37 ms and 0.59 ms, respectively at 298 K. The quantum yields of the ET state of H₂TPP-BHV²⁺ and H₄(DPA)–BHV²⁺ were determined from the comparison of absorbance due to the triplet excited state of H₂TPP (ε₄₃₀ = 83 000 M⁻¹ cm⁻¹)³⁰ and the absorption of the CS state (ε₆₄₀ = 54 000 M⁻¹ cm⁻¹)³¹ to be 0.84 and 0.83, respectively. The ε₆₄₀ value was determined from the absorption spectra of BHV^•+, which was produced by the electron transfer reduction of BHV²⁺ with tetramethylsemiquinone radical anion in PhCN. (Fig. 7). Tetramethylsemiquinone radical anion was produced by the proportionation reaction of tetramethyl-p-benzoquinone and tetramethylhydroquinone in the presence of a base.²⁸

Formation of the ET state was further confirmed by the EPR measurements after photoirradiation of H₄(DPA)–BHV²⁺ in frozen PhCN at 173 K. The resulting EPR spectrum is shown in Fig. 10a.³² The observed isotropic EPR signal at g = 2.0028 consists of the superposition of that of H₄(DPA)^•+ (isotropic signal at g = 2.0029 in Fig. 10b) produced by the one-electron oxidation of H₄(DPA) with [Fe(bpy)₃]³⁺ (bpy = 2,2′-bipyridine) and that of BHV^•+ (isotropic signal at g = 2.0027) produced by the one-electron reduction of BHV²⁺ with a tetramethylsemiquinone radical anion (Fig. 10c).³³

Fig. 10 (a) EPR spectrum of (a) charge-separated state obtained from photoirradiation of a deaerated PhCN solution containing H₄(DPA) (6.0 × 10⁻⁵ M) and BHV²⁺ (5.5 × 10⁻³ M) with a high pressure Hg lamp at 173 K. (b) EPR spectrum of a MeCN solution of H₄(DPA)^•+ at 173 K generated by the electron transfer oxidation of H₄(DPA) (1.5 × 10⁻⁴ M) with [Fe(bpy)₃](PF₆)₃ (8.6 × 10⁻⁵ M). (c) EPR spectrum of a MeCN solution of BHV^•+ at 173 K generated by the electron-transfer reduction of BHV²⁺ (2.9 × 10⁻³ M) with tetramethylsemiquinone radical anion (9.8 × 10⁻⁵ M). The asterisk * denotes Mn²⁺ marker.

Conclusion

In conclusion, a π-extended viologen (BHV²⁺) forms supramolecular π-complexes with a porphyrin monomer and dimer, which affords long-lived ET states upon the photoexcitation. Such π-extension of electron acceptors provides a new strategy for formation of supramolecular π-complexes to achieve efficient photoinduced electron transfer and slow back electron transfer.

Experimental

General

¹H NMR spectra were measured on a JEOL JMN-AL300 (300 MHz). Matrix-assisted laser desorption/ionization (MALDI) time-of-flight mass spectra (TOF) were measured on a Kratos Compact MALDI I (Shimadzu).

Materials

A cofacial bisporphyrin, H₄(DPA) [DPA = 1,8-bis[5-(2,8,13,17-tetraethyl-3,7,12,18-tetramethylporphyrinyl)]anthracene] was synthesized according to literature.²⁶ 5,10,15,20-Tetraphenyl-21H,23H-porphyrin (H₂TPP) and 4-(bromomethyl)pyridine hydrobromide was obtained from Aldrich Chemical Co., Inc. Tetra-n-butylammonium perchlorate (TBAP) used as a supporting electrolyte for electrochemical measurements was obtained from Tokyo Chemical Industry (TCI). Triphenylphosphine, β-(4-pyridyl)acrolein oxalate, 1-bromohexane, sodium carbonate, sodium hydroxide, dichloromethane, methanol, ethanol, acetone, benzonitrile (PhCN) and acetonitrile (MeCN) were purchased from Wako Pure Chemical Ind., Ltd. Benzene was obtained from Nacalai Tesque, Inc. PhCN and MeCN were purified by successive distillation over calcium hydride.³⁴

Synthesis of 1,4-bis(N-hexyl-4-pyridinium)butadiene diperchlorate [BHV²⁺(ClO₄⁻)₂]

A solution of 4-(bromomethyl)pyridine hydrobromide (1.8 g, 7 mmol) in H₂O was basified with Na₂CO₃ and extracted with benzene. To this organic layer was added PPh₃ (7.9 g, 30 mmol) and stirred at room temperature for 60 h under air. The reaction mixture was then precipitated out of benzene, filtered to give the cream colored solid (2.1 g, 4.8 mmol, 69% yield). To the solution of that solid (0.87 g, 2 mmol) and β-(4-pyridyl)acrolein oxalate (0.45 g, 2 mmol) in CH₂Cl₂ were added a 50% NaOH solution under nitrogen atmosphere. The reaction mixture was stirred for 2 h at room temperature, and then diluted with CH₂Cl₂ and H₂O. The organic layer was concentrated in vacuo, and the residue was dissolved in anhydrous EtOH. Passing HCl gas through the EtOH solution gave a brown precipitate, which was collected and then dissolved in H₂O. The solution was basified with Na₂CO₃ and CH₂Cl₂ extraction followed by solvent evaporation gave 1,4-di(4-pyridyl)butadiene. The product was recrystallized from MeOH–H₂O (1 : 1) to give a brown solid (75 mg, 0.36 mmol, 18% yield). 1,4-Di(4-pyridyl)butadiene (75 mg, 0.36 mmol) and 1-bromohexane (206 mg, 1.26 mmol) were dissolved in MeCN. The mixture was heated to reflux for 72 h. After cooling, the brown precipitate was filtered and rinsed with Et₂O. The obtained dibromide (186 mg, 0.35 mmol) and sodium perchlorate (2.3 g, 18.7 mmol) were dissolved H₂O and stirred for 20 h. The yellow precipitate was rinsed with EtOAc and recrystallized from acetone and yellow solid (172 mg, 0.3 mmol) was obtained.³⁵¹H NMR (300 MHz, CD₃CN) δ 8.55 (d, J = 6.6 Hz, 4H), 8.03 (d, J = 6.6 Hz, 4H), 7.64 (dd, J = 3.0 Hz and 9.3 Hz, 2H), 7.14 (dd, J = 3.0 Hz, 9.3 Hz, 2H), 4.44 (t, J = 7.2 Hz, 4H), 1.31–1.50 (m, 4H), 1.34 (m, 12H), 0.90 (t, J = 6.6 Hz, 6H); MALDI-TOF MS m/z 378.31 (BHV^•+ requires 378.30). Anal. Calcd for C₂₆H₃₈Cl₂N₂O₈: C, 54.07; H, 6.63; N, 4.85. Found: C, 53.78; H, 6.42; N, 4.80.

Photophysical measurements

UV-visible absorption spectra were obtained on a Shimadzu UV-3100PC spectrometer or a Hewlett Packard 8453 diode array spectrophotometer at 298 K. Corrected fluorescence spectra were taken using a SHIMADZU spectrofluorophotometer (RF-5300PC). The solutions were deaerated by argon purging for 15 min prior to the measurements. For nanosecond laser flash photolysis experiments, deaerated PhCN solutions were excited by a Panther OPO pumped by Nd:YAG laser (Continuum, SLII-10, 4–6 ns fwhm) at λ = 570 nm. The photochemical reactions were monitored by continuous exposure to a Xe-lamp (150 W) as a probe light and a photomultiplier tube (Hamamatsu 2949) as a detector. Transient absorption spectra were recorded using fresh solutions in each laser excitation.

Electrochemical measurements

Cyclic voltammetry measurements were performed at 298 K on an ALS-630B voltammetric analyzer in deaerated solvent containing 0.10 M TBAPF₆ as a supporting electrolyte at 298 K. A conventional three-electrode cell was used with a platinum working electrode and a platinum wire as a counter electrode. The measured potentials were recorded with respect to the Ag/AgNO₃ (1.0 × 10⁻² M). The E⁰_ox and E⁰_red values (vs. Ag/Ag⁺) are converted to those vs. SCE by adding 0.29 V, respectively.³⁶

ESR measurements

An argon-saturated PhCN solution of H₄(DPA) (6.0 × 10⁻⁵ M) and BHV²⁺ (5.5 × 10⁻³ M) was irradiated at 173 K with a high pressure mercury lamp (USH-1005D) through a water filter focused at the sample cell in the ESR cavity. The ESR spectra were taken on a JEOL JES-RE1XE and were recorded under non-saturating microwave power conditions. The magnitude of the modulation was chosen to optimize the resolution and the signal to noise ratio (S/N) of the observed spectra. The g values were calibrated using an Mn²⁺ marker.

Acknowledgements

This work was financially supported by a Grant-in-Aid (No. 20108010 and 23750014) and the Global COE (center of excellence) program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University from MEXT, Japan, NRF/MEST of Korea through WCU (R31-2008-000-10010-0) and GRL (2010-00353) programs, and The Centre National de la Recherche Scientifique (ICMUB, UMR 6302), France.

References

1. (a) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2001, 34, 40; (b) D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2009, 42, 1890.
2. (a) M. R. Wasielewski, Chem. Rev., 1992, 92, 435; (b) M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910.
3. (a) G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 676; (b) N. Martin, L. Sanchez, M. A. Herranz, B. Illescas and D. M. Guldi, Acc. Chem. Res., 2007, 40, 1015.
4. (a) S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609; (b) S. Fukuzumi, Bull. Chem. Soc. Jpn., 2006, 79, 177; (c) S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283; (d) K. Ohkubo and S. Fukuzumi, J. Porphyrins Phthalocyanines, 2008, 12, 993; (e) S. Fukuzumi and T. Kojima, J. Mater. Chem., 2008, 18, 1427.
5. (a) S. Fukuzumi, Eur. J. Inorg. Chem., 2008, 1351; (b) S. Fukuzumi, Y. Yamada, T. Suenobu, K. Ohkubo and H. Kotani, Energy Environ. Sci., 2011, 4, 2754.
6. (a) H. Imahori, K. Tamaki, D. M. Guldi, C. Luo, M. Fujitsuka, O. Ito, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 2607; (b) H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 6617; (c) D. M. Guldi, H. Imahori, K. Tamaki, Y. Kashiwagi, H. Yamada, Y. Sakata and S. Fukuzumi, J. Phys. Chem. A, 2004, 108, 541.
7. (a) D. Curiel, K. Ohkubo, J. R. Reimers, S. Fukuzumi and M. J. Crossley, Phys. Chem. Chem. Phys., 2007, 9, 5260; (b) H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito, H. Yamada and S. Fukuzumi, Chem.–Eur. J., 2004, 10, 3184; (c) S. H. Lee, A. G. Larsen, K. Ohkubo, J. R. Reimers, S. Fukuzumi and M. J. Crossley, Chem. Sci., 2012, 3, 257; (d) C. P. Gros, F. Brisach, A. Meristoudi, E. Espinosa, R. Guilard and P. D. Harvey, Inorg. Chem., 2007, 46, 125; (e) M. A. Filatov, R. Guilard and P. D. Harvey, Org. Lett., 2010, 12, 196.
8. (a) D. Gust, T. A. Moore and A. L. Moore, In From Non-Covalent Assemblies to Molecular Machines, ed. J.-P. Sauvage and P. Gaspard (ed.), Wiley-VCH, Weinheim, 2011, pp. 321–354; (b) V. Balzani, A. Credi and M. Venturi, in Organic Nanostructures, ed. J. L. Atwood and J. W. Steed, Wiley-VCH, Weinheim, 2008, pp. 1–31.
9. (a) L. Martin-Gomis, K. Ohkubo, F. Fernandez-Lazaro, S. Fukuzumi and Á. Sastre-Santos, Chem. Commun., 2010, 46, 3944; (b) M. E. El-Khouly, K.-J. Han, K.-Y. Kay and S. Fukuzumi, ChemPhysChem, 2010, 11, 1726–1734.
10. (a) D. I. Schuster, K. Li, D. M. Guldi, A. Palkar, L. Echegoyen, C. Stanisky, R. J. Cross, M. Niemi, N. V. Tkachenko and H. Lemmetyinen, J. Am. Chem. Soc., 2007, 129, 15973; (b) T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima and S. Fukuzumi, J. Phys. Chem. C, 2010, 114, 14290; (c) F. J. Cespedes-Guirao, K. Ohkubo, S. Fukuzumi, Á. Sastre-Santos and F. Fernandez-Lazaro, J. Org. Chem., 2009, 74, 5871.
11. (a) F. D'Souza and O. Ito, Chem. Commun., 2009, 4913; (b) R. Chitta and F. D'Souza, J. Mater. Chem., 2008, 18, 1440; (c) F. D'Souza and O. Ito, Coord. Chem. Rev., 2005, 249, 1410.
12. (a) J. L. Sessler, E. Karnas, S. K. Kim, Z. Ou, M. Zhang, K. M. Kadish, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2008, 130, 15256; (b) S. Fukuzumi, K. Ohkubo, Y. Kawashima, D. Kim, J. S. Park, A. Jana, V. M. Lynch, D. Kim and J. L. Sessler, J. Am. Chem. Soc., 2011, 133, 15938.
13. (a) T. Kojima, T. Honda, K. Ohkubo, M. Shiro, T. Kusukawa, T. Fukuda, N. Kobayashi and S. Fukuzumi, Angew. Chem., Int. Ed., 2008, 47, 6712; (b) T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima and S. Fukuzumi, J. Am. Chem. Soc., 2010, 132, 10155; (c) J. Rosenthal, J. M. Hodgkiss, E. R. Young and D. G. Nocera, J. Am. Chem. Soc., 2006, 128, 10474.
14. S. Fukuzumi, T. Honda, K. Ohkubo and T. Kojima, Dalton Trans., 2009, 3880.
15. M. Tanaka, K. Ohkubo, C. P. Gros, R. Guilard and S. Fukuzumi, J. Am. Chem. Soc., 2006, 128, 14625.
16. (a) A. Takai, M. Chkounda, A. Eggenspiller, C. P. Gros, M. Lachkar, J.-M. Barbe and S. Fukuzumi, J. Am. Chem. Soc., 2010, 132, 4477; (b) A. Takai, C. P. Gros, J.-M. Barbe and S. Fukuzumi, Phys. Chem. Chem. Phys., 2010, 12, 12160.
17. (a) M. E. El-Khouly, D. K. Ju, K.-Y. Kay, F. D'Souza and S. Fukuzumi, Chem.–Eur. J., 2010, 16, 6193; (b) F. D'Souza, C. A. Wijesinghe, M. E. El-Khouly, J. Hudson, M. Niemi, H. Lemmetyinen, N. V. Tkachenko, M. E. Zandler and S. Fukuzumi, Phys. Chem. Chem. Phys., 2011, 13, 18168.
18. (a) S. Fukuzumi, K. Saito, K. Ohkubo, V. Troiani, H. Qiu, S. Gadde, F. D'Souza and N. Solladie, Phys. Chem. Chem. Phys., 2011, 13, 17019; (b) S. Fukuzumi, K. Saito, K. Ohkubo, T. Khoury, Y. Kashiwagi, M. A. Absalom, S. Gadde, F. D'Souza, Y. Araki, O. Ito and M. J. Crossley, Chem. Commun., 2011, 47, 7980; (c) S. Fukuzumi and Y. Kashiwagi, J. Porphyrins Phthalocyanines, 2007, 11, 368.
19. (a) H. Nobukuni, Y. Shimazaki, H. Uno, Y. Naruta, K. Ohkubo, T. Kojima, S. Fukuzumi, S. Seki, H. Sakai, T. Hasobe and F. Tani, Chem.–Eur. J., 2010, 16, 11611; (b) M. Murakami, K. Ohkubo, T. Hasobe, V. Sgobba, D. M. Guldi, F. Wessendorf, A. Hirsch and S. Fukuzumi, J. Mater. Chem., 2010, 20, 1457.
20. (a) F. D'Souza, N. K. Subbaiyan, Y. Xie, J. P. Hill, K. Ariga, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2009, 131, 16138; (b) F. D'Souza, E. Maligaspe, K. Ohkubo, M. E. Zandler, N. K. Subbaiyan and S. Fukuzumi, J. Am. Chem. Soc., 2009, 131, 8787; (c) T. Kojima, T. Nakanishi, H. Honda and S. Fukuzumi, J. Porphyrins Phthalocyanines, 2009, 13, 14; (d) T. Kojima, T. Nakanishi, R. Harada, K. Ohkubo, S. Yamauchi and S. Fukuzumi, Chem.–Eur. J., 2007, 13, 8714.
21. T. S. Teets and D. G. Nocera, Chem. Commun., 2011, 47, 9268.
22. (a) H. Kotani, T. Ono, K. Ohkubo and S. Fukuzumi, Phys. Chem. Chem. Phys., 2007, 9, 1487; (b) H. Kotani, K. Ohkubo, Y. Takai and S. Fukuzumi, J. Phys. Chem. B, 2006, 110, 24047.
23. M. Ogawa, G. Ajayakumar, S. Masaoka, H.-B. Kraatz and K. Sakai, Chem.–Eur. J., 2011, 17, 1148.
24. P. Du, J. Schneider, P. Jarosz and R. Eisenberg, J. Am. Chem. Soc., 2006, 128, 7726.
25. S. Fukuzumi, Y. Kondo, S. Mochizuki and T. Tanaka, J. Chem. Soc., Perkin Trans. 2, 1989, 1753.
26. S. Faure, C. Stern, R. Guilard and P. D. Harvey, J. Am. Chem. Soc., 2004, 126, 1253.
27. A similar sandwiched π-complex between H₄(DPA) and acridinium cation was reported in ref.15.
28. (a) A. Harriman, J. Chem. Soc., Faraday Trans. 2, 1979, 75, 1532; (b) E. G. Ermolina, R. T. Kuznetsova, R. M. Gadirov, G. V. Maier, N. N. Semenishin, N. V. Rusakova and Y. V. Korovin, High Energy Chem., 2010, 44, 387.
29. The energies of the 0-0 transition between the S₁ and the S₀ state were determined by averaging the energies of the corresponding (0,0) peaks in the fluorescence and the absorption bands..
30. (a) L. Pekkarinen and H. Linschitz, J. Am. Chem. Soc., 1960, 82, 2407; (b) H. Gratz and A. Penzkofer, Chem. Phys., 2000, 254, 363.
31. (a) C. Inisan, J.-Y. Saillard, R. Guilard, A. Tabard and Y. Le Mest, New J. Chem., 1998, 22, 823; (b) K. Okamoto, K. Ohkubo, K. M. Kadish and S. Fukuzumi, J. Phys. Chem. A, 2004, 108, 10405.
32. We could not observe the signal with zero-field splitting pattern around g = 2 (3230 G ± 300 G) and the triplet marker signal at g = 4 due to the triplet charge-separated state in this study. The spin–spin interaction of radical ion pair may not be strong..
33. K. Ohkubo, K. Suga, K. Morikawa and S. Fukuzumi, J. Am. Chem. Soc., 2003, 125, 12850.
34. W. L. F. Armarego and C. L. L. Chai, Purification of Laboratory Chemicals, 5th ed., Butterworth Heinemann. Amsterdam, 2003.
35. S. Fukuzumi, Jpn. Kokai Tokkyo Koho, JP, 2005, 2005255603.
36. C. K. Mann and K. K. Barnes, Electrochemical Reactions in Nonaqueous Systems, Marcel Dekker, New York, 1990.

---

Footnote

† Electronic supplementary information (ESI) available: Absorption spectra in PhCN, femtosecond transient absorption spectra in toluene, and nanosecond transient absorption spectra in benzonitrile. See DOI: 10.1039/c2ra20130e

---