Long-lived photoinduced charge separation for solar cell applications in supramolecular complexes of multi-metalloporphyrins and fullerenes

Abstract

Monomers, dimers, trimers, dendrimers and oligomers of metalloporphyrins form supramolecular complexes with fullerene derivatives via electrostatic interactions, π–π interactions and coordination bonds. Photoexcitation of the supramolecular complexes resulted in photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the charge-separated states as revealed by laser flash photolysis measurements. The rate constants of photoinduced charge separation and charge recombination in supramolecular complexes of multi-metalloporphyrins and fullerenes were also determined by laser flash photolysis measurements and the results depending on the number of porphyrins in the supramolecular complexes are discussed in terms of efficiency of photoinduced energy transfer and charge separation as well as the lifetimes of charge-separated states. The photoelectrochemical performances of solar cells composed of supramolecular complexes of monomers, dimers, dendrimers and oligomers of metalloporphyrins with fullerenes are compared in relation to the rate constants of photoinduced charge separation and charge recombination.

---

Shunichi Fukuzumi

Shunichi Fukuzumi earned a Ph.D. degree in applied chemistry at the Tokyo Institute of Technology in 1978. After working as a postdoctoral fellow (1978–1981) at Indiana University in USA, he joined the Department of Applied Chemistry, Osaka University, as an Assistant Professor in 1981 and was promoted to a Full Professor in 1994. He is now a special distinguished professor at Osaka University and the director of an ALCA (Advanced Carbon Technology Research and Development) project that started in 2011.

Kei Ohkubo

Kei Ohkubo earned his Ph.D. degree from the Graduate School of Engineering, Osaka University in 2001. He was working as a JSPS fellow and a JST research fellow at Osaka University from 2001 to 2005. He has been a designated associate professor at Osaka University since 2005.

---

1. Introduction

Photosynthesis is one of the most fundamental and indispensable processes in nature, because it converts light energy into chemical energy required to maintain life.^1,2 Photosynthesis is initiated by the multistep electron-transfer reactions in the photosynthetic reaction centres following light energy harvesting by antenna chlorophylls, funnelled to a bacteriochlorophyll dimer, the so-called special pair, to attain the long-lived charge-separated (CS) state.^1,2 The redox-active components such as chlorophyll, pheophytin and quinones are appropriately located in the protein matrix by non-covalent interactions.^1,2 Extensive efforts have so far been devoted to the design of electron donor–acceptor composites using covalently and non-covalently linked systems to form the long-lived CS state upon photoexcitation for artificial photosynthesis.^3–29 Porphyrins, which are involved in a number of important biological electron-transfer systems including the primary photochemical reactions of chlorophylls (porphyrin derivatives) in the photosynthetic reaction centres, are particularly attractive building blocks as electron acceptors as well as light-harvesting compounds for the construction of supramolecular electron donor–acceptor composites due to their excellent photophysical and electron-transfer properties.^8–29 With regard to electron acceptors, fullerenes, which are extensively conjugated three-dimensional π systems, are ideal electron acceptors because of the minimal changes of structure and solvation associated with the electron-transfer reduction.^30–38 Thus, combination of porphyrins and fullerenes is regarded as ideal donor–acceptor ensembles, because the combination results in a small reorganization energy, which allows to accelerate photoinduced electron transfer and to slow down charge recombination, leading to the generation of long-lived CS states with high quantum yields.^39–63 However, non-covalent binding between monomer porphyrins and fullerenes is usually not strong enough in polar solvents which are generally used for studies on photoinduced electron-transfer reactions.^64–68 Among non-covalent interactions, an electrostatic interaction is relatively strong in polar solvents.^69–73 Muti-point binding sites can be introduced by using multi-metalloporphyrins (dimers, trimers, dendrimers and oligomers), allowing strong binding between multi-metalloporphyrins and fullerenes in polar solvents.^29,30

In this perspective, we review our recent development on photoinduced charge separation in supramolecular complexes of porphyrin anions and fullerene cations with electrostatic interactions and those composed of multi-metalloporphyrins and fullerenes, which are strongly bound in polar solvents, towards construction of supramolecular solar cells based on the long-lived photoinduced charge separation.

2. Supramolecular complexes of monomer porphyrin sulfonates and Li⁺@C₆₀

Zinc tetraphenylporphyrin tetrasulfonate anion [(Bu₄N⁺)₄ZnTPPS⁴⁻] forms a strong supramolecular binding with a cationic lithium ion encapsulated fullerene (Li⁺@C₆₀)^74–77 in benzonitrile (PhCN) by electrostatic and π–π interactions (Scheme 1).⁷⁸ The Job's plots of the absorbance change confirmed the 1 : 1 stoichiometry between ZnTPPS⁴⁻ and Li⁺@C₆₀.⁷⁸ Free base tetraphenylporphyrin tetrasulfonate anion [(Bu₄N⁺)₄H₂TPPS⁴⁻] also forms a 1 : 1 complex with Li⁺@C₆₀. The formation constants (K) of the ZnTPPS⁴⁻/Li⁺@C₆₀ and H₂TPPS⁴⁻/Li⁺@C₆₀ complexes were determined from the absorption change to be 1.6 × 10⁵ and 3.0 × 10⁵ M⁻¹, respectively.⁷⁸ The same formation constants were obtained from the fluorescence quenching of ZnTPPS⁴⁻ and H₂TPPS⁴⁻ and by Li⁺@C₆₀ in PhCN.⁷⁸

Scheme 1 Supramolecular complex formation and photoinduced charge separation of MTPPS⁴⁻ (M = Zn and H₂) with Li⁺@C₆₀ in PhCN.

The occurrence of the photoinduced energy transfer in the supramolecular complex was confirmed by the transient absorption spectra of the ZnTPPS⁴⁻–Li⁺@C₆₀ complex measured in PhCN using femtosecond laser flash photolysis (Fig. 1a).⁷⁸ The transient absorption bands taken at 2 ps observed at 620 and 737 nm are assigned to the singlet excited state of ZnTPPS⁴⁻. This band decays with the rate constant (k_EN) of 9.7 × 10¹⁰ s⁻¹ (Fig. 1b) to form the singlet excited state of Li⁺@C₆₀ at 100 ps (Fig. 1a). The decay rate constant of ¹[Li⁺@C₆₀]* was determined to be 8.9 × 10⁸ s⁻¹, which agrees with the rate constant of the intersystem crossing of Li⁺@C₆₀.⁷⁸ Thus, efficient energy transfer occurred from ¹[ZnTPPS⁴⁻]* to Li⁺@C₆₀ rather than electron transfer.

Fig. 1 (a) Transient absorption spectra of ZnTPPS⁴⁻ (2.5 × 10⁻⁵ M) in the presence of Li⁺@C₆₀ (5.0 × 10⁻⁵ M) in deaerated PhCN at 298 K taken at 2, 10 and 100 ps after femtosecond laser excitation at 388 nm. (b) Time profile at 737 nm.

The transient absorption spectra taken by nanosecond laser flash photolysis shown in Fig. 2a demonstrate the formation of [ZnTPPS⁴⁻]˙⁺ (λ_max = 670 nm) and that of Li⁺@C₆₀ radical anion (λ_max = 1035 nm).⁷⁸ Thus, the electron transfer from ZnTPPS⁴⁻ to ³[Li⁺@C₆₀]* or from ³[ZnTPPS⁴⁻]* to Li⁺@C₆₀ occurs in the supramolecular complex to produce the triplet charge-separated (CS) state. The lifetime of the triplet CS state of the supramolecular complex was determined to be 300 μs for ZnTPPS⁴⁻ from the first-order decay of the CS state (Fig. 2b).⁷⁸ It was confirmed that back electron transfer occurred in the supramolecular complex, because the first-order decay rate constant remains the same irrespective of the difference in the laser intensity (inset of Fig. 2b).⁷⁸ Similarly the CS lifetime of 310 μs was determined for [(H₂TPPS⁴⁻)˙⁺–Li⁺@C₆₀˙⁻].⁷⁸ This is the longest lifetime of the CS state ever reported for monomer porphyrin/fullerene systems linked non-covalently in solution. The quantum yield of the CS state is determined to be 0.39 using the absorption of the CS state (Li⁺@C₆₀˙⁻: ε₁₀₃₅ = 7300 M⁻¹ cm⁻¹).⁷⁸

Fig. 2 (a) Transient absorption spectra of ZnTPPS⁴⁻ (2.5 × 10⁻⁵ M) in the presence of Li⁺@C₆₀ (5.0 × 10⁻⁵ M) in deaerated PhCN at 298 K taken at 20 and 200 μs after nanosecond laser excitation at 550 nm; (b) decay time profiles at 1035 nm with different laser intensities (1, 3, 6 mJ per pulse). Inset: first-order plots.

The activation enthalpies of the charge-recombination (CR) processes were determined to be 3.0 kcal mol⁻¹ for ZnTPPS⁴⁻–Li⁺@C₆₀ and 5.4 kcal mol⁻¹ for H₂TPPS⁴⁻–Li⁺@C₆₀.⁷⁸ This indicates that there is a significant energy difference between the singlet and triplet CS states and that the CR processes may occur through the thermally activated singlet CS state. The lifetime of the CS state at 77 K is estimated as long as 60 h for H₂TPPS⁴⁻–Li⁺@C₆₀.⁷⁸ Such a long-lived triplet CS state was detected by the EPR measurements by photoirradiation of the H₂TPPS⁴⁻–Li⁺@C₆₀ complex in frozen PhCN as shown in Fig. 3. The spin–spin interaction in the triplet radical ion pair of the supramolecular complex is clearly shown at 77 K, where the fine structure due to the triplet CS state is clearly observed at g = 2. From the zero-field splitting values (D = 52 G for ZnTPPS⁴⁻ and 56 G for H₂TPPS⁴⁻) the distances (r) between two electron spins were estimated using the relation, D = 27 800/r³,^79,80 to be 8.1 and 7.9 Å, respectively.⁷⁸ These r values agree with the centre-to-centre distance of a reported crystal structure of porphyrin/C₆₀.

Fig. 3 EPR spectra of (a) (ZnTPPS⁴⁻)˙⁺–Li⁺@C₆₀˙⁻ and (b) (H₂TPPS⁴⁻)˙⁺–Li⁺@C₆₀˙⁻ in PhCN generated by photoirradiation with a high-pressure Hg lamp (1000 W) at 77 K.

By mixing PhCN solutions of the supramolecular complexes of MTPPS⁴⁻ and Li⁺@C₆₀ with acetonitrile (MeCN), nanoclusters were produced and they were deposited on an optically transparent electrode (OTE) of nanostructured SnO₂ (OTE/SnO₂) by application of a dc electric field (∼100 V cm⁻¹) to construct photovoltaic cells.⁸¹ The (MTPPS⁴⁻/Li⁺@C₆₀)_n films are composed of closely packed Li⁺@C₆₀ clusters of about 80 nm size, which render a nanoporous morphology to the film as shown in the TEM images in Fig. 4.⁸¹

Fig. 4 TEM images of (a) Li⁺@C₆₀/ZnTPPS⁴⁻ and (b) Li⁺@C₆₀/H₂TPPS⁴⁻ nanoclusters.

The photoelectrochemical measurements of a robust thin film of OTE/SnO₂/(MTPPS⁴⁻/Li⁺@C₆₀)_n were performed using a standard two-electrode system consisting of a working electrode and a Pt wire gauze electrode in air-saturated MeCN containing 0.5 M LiI and 0.01 M I₂ (Fig. 5).⁷⁷ The IPCE (incident photon-to-photocurrent efficiency) values were determined by normalizing the photocurrent values for incident light energy and intensity and using eqn (1).^82–85

IPCE (%) = 100 × 1240 × i_sc/(I_inc × λ) (1)

where i_sc is the short circuit photocurrent (A cm⁻²), I_inc is the incident light intensity (W cm⁻²) and λ is the wavelength (nm). The IPCE value of OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n is much higher than the sum of the two individual IPCE values of the individual systems OTE/SnO₂/(ZnTPPS⁴⁻)_n and OTE/SnO₂/(Li⁺@C₆₀)_n in the visible region (Fig. 6). The maximum IPCE value of OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n was 77% at 450 nm. Such a high IPCE value indicates that photocurrent generation is initiated via photoinduced electron transfer from ZnTPPS⁴⁻ to Li⁺@C₆₀, followed by the charge transport to the collective surface of an OTE/SnO₂ electrode (Fig. 5). When ZnTPPS⁴⁻ was replaced by H₂TPPS⁴⁻, a significantly low IPCE value was observed as 7% at 440 nm probably because of the self-aggregation of H₂TPPS⁴⁻ without binding with Li⁺@C₆₀.⁸¹

Fig. 5 Schematic image of photoelectrochemical cell of OTE/SnO₂/MTPPS⁴⁻/Li⁺@C₆₀ and electron-transfer pathways to generate photocurrent.

Fig. 6 Photocurrent action spectra of OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n, (red) OTE/SnO₂/(ZnTPPS⁴⁻)_n (blue) and OTE/SnO₂/(Li⁺@C₆₀)_n (black). Electrolyte: 0.5 M LiI and 0.01 M I₂ in MeCN–PhCN (3 : 1 v/v).

The power conversion efficiency (η) of the OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n electrode was calculated by using eqn (2):^82–85

η = FF × I_sc × V_oc/W_in (2)

in which the fill factor (FF) is defined as FF = [IV]_max/I_scV_oc and V_oc is the open-circuit photovoltage and I_sc is the short-circuit photocurrent. The OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n electrode has an overall power conversion efficiency (η) of 2.1% at an input power (W_in) of 28 mW cm⁻², whereas FF = 0.37, V_oc = 460 mV and I_sc = 3.4 mA cm⁻² in the OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n. The η value is two orders of magnitude greater than that of the previously reported simple porphyrin and C₆₀ composite system (∼0.03%).⁸³ Such a significant enhancement of the η value indicates that the strong ordering in the clusters and the efficient charge separation in (ZnTPPS⁴⁻/Li⁺@C₆₀)_n improved the light energy conversion properties.

3. Supramolecular complexes of cyclic porphyrin dimers with C₆₀ and Li⁺@C₆₀

As compared to porphyrin monomers, porphyrin dimers with appropriate linkage can accommodate electron acceptor guest molecules by π–π interactions to form sandwich complexes.^86–96 For example, a cyclic Ni porphyrin dimer (Ni-CPD_Py) linked by butadiyne moieties bearing 4-pyridyl groups (Fig. 7) forms a sandwich complex with C₆₀ (C₆₀ ⊂ Ni₂-CPD_Py) as shown in the X-ray crystal structure (Fig. 8), where the dimer bites a C₆₀ molecule by tilting the porphyrin rings with respect to each other and there are strong π–π interactions between the porphyrin rings and C₆₀.⁹⁷ The adjacent dimers are linked by hydrogen bonds and π–π interactions.⁹⁷ The C₆₀ molecules are linearly arranged in the inner channel to give a supramolecular peapod.^97–102

Fig. 7 Chemical structure of Ni₂-CPD_Py.

Fig. 8 Crystal structures of tubular assemblies of C₆₀ ⊂ Ni₂-CPD_Py. Hydrogen atoms are omitted for clarity.

The linear arrangement of C₆₀ in C₆₀ ⊂ Ni₂-CPD_Py high electron mobilities of ∑μ = 0.72 and 0.20 cm² V⁻¹ s⁻¹ along the b and c axes, respectively, which were determined by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements.⁹⁷ The TRMC technique can evaluate the intrinsic mobility without being affected by the chemical or physical defects in the material and/or the organic/metal–electrode interfaces.^103–105 The observed value along the b axis of the single crystal of C₆₀ ⊂ Ni₂-CPD_Py is comparable to that of the single crystal of C₆₀ (∑μ = 0.50 cm² V⁻¹ s⁻¹ measured by TOF).¹⁰⁶ The observed high electron mobility along the b axis is due to the well-ordered linear arrangement of C₆₀ in the porphyrin nanotube. However, the expected charge-separated state could not be observed in the time-resolved transient absorption spectra of C₆₀ ⊂ Ni₂-CPD_Py because the singlet excited state of the nickel porphyrin immediately changes to the triplet excited state by intersystem crossing and the low energy triplet excited state of C₆₀ (³C₆₀*) is formed by energy transfer.⁹⁷ The estimated energy level of the charge-separated state (1.98 eV) is higher than that of ³C₆₀* (1.60 eV).⁹⁷ When Ni₂-CPD_Py was replaced by a free base porphyrin dimer (H4-CPDPy), a complete charge-separated state {H₄-CPDPy˙⁺ + C₆₀˙⁻)} was observed by femtosecond laser flash photolysis of C₆₀ ⊂ H₄-CPD_Py in the solid state with a lifetime of 470 ps.¹⁰⁷ The photovoltaic activity of C₆₀ ⊂ Ni₂-CPD_Py and C₆₀ ⊂ H₄-CPD_Py was evaluated by using solar cells composed of modified electrodes and I⁻/I₃⁻ solution.¹⁰⁷ The C₆₀ ⊂ H₄-CPD_Py-modified electrode exhibited IPCE of 17% and a power conversion efficiency (η) of 0.33%, which was more than 16 times larger than that of OTE/SnO₂/(C₆₀ ⊂ Ni₂-CPD_Py)_n (0.02%).¹⁰⁷ Such a significant enhancement of the η value demonstrates that the formation of highly ordered clusters and the efficient charge separation of (C₆₀ ⊂ H₄-CPD_Py)_n contributes to the improvement of the light energy conversion properties.¹⁰⁷

When C₆₀ is replaced by Li⁺@C₆₀, porphyrin dimers with four long alkoxy substituents on the meso-phenyl groups (MCPD_Py(OC₆) in Fig. 9) form strong supramolecular complexes in even a polar solvent such as PhCN.¹⁰⁸ The association constants (K_assoc) of Li⁺@C₆₀ ⊂ MCPD_Py(OC₆) in PhCN at 298 K were determined to be 2.6 × 10⁵ M⁻¹ for Li⁺@C₆₀ ⊂ H₄-CPD_Py(OC₆) and 3.5 × 10⁵ M⁻¹ for Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆).¹⁰⁸

Fig. 9 Supramolecular formation and photoinduced charge separation between MCPD_Py(OC₆) and Li⁺@C₆₀.

Upon laser excitation of Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆), transient absorption bands due to Ni₂-CPD_Py(OC₆)˙⁺ and Li⁺@C₆₀˙⁻ were observed as shown in Fig. 10a.¹⁰⁸ In this case, electron transfer occurs from Ni₂-CPD_Py(OC₆) to the triplet excited state of Li⁺@C₆₀ (³Li⁺@C₆₀*) rather than from ³[Ni₂-CPD_Py(OC₆)]* to Li⁺@C₆₀ as indicated by the disappearance of the absorption band at 750 nm due to ³Li⁺@C₆₀*, accompanied by the appearance of the absorption band at 1035 nm due to Li⁺@C₆₀˙⁻ (Fig. 10b).¹⁰⁸ The rate constant of electron transfer from Ni₂-CPD_Py(OC₆) to ³Li⁺@C₆₀* to produce the CS state was determined from the rise in the absorbance at 1035 nm due to Li⁺@C₆₀˙⁻ to be 5.7 × 10⁷ s⁻¹.¹⁰⁸ The absorbance at 1035 nm due to Li⁺@C₆₀˙⁻ in the CS state decayed obeying first-order kinetics with the same slope irrespective of the difference in the laser intensity (Fig. 10c).¹⁰⁸ This clearly indicates that the decay of the CS state occurs via intrasupramolecular back electron transfer rather than a bimolecular back electron-transfer reaction between the CS states. The CS lifetime was determined from the slope of the first-order plots in Fig. 10c to be 0.67 ms, which is the longest value ever reported for non-covalent monomer dimer porphyrin-fullerene supramolecules in solution.¹⁰⁸ The CS state was also observed for Li⁺@C₆₀ ⊂ H₄-CPD_Py(OC₆). The quantum yields of the CS states were estimated to be 0.13 for Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆) and 0.32 for Li⁺@C₆₀ ⊂ H₄-CPD_Py(OC₆) and by means of the comparative method with the absorption intensities of the CS states (Li⁺@C₆₀˙⁻: ε(1035 nm) = 7300 M⁻¹ cm⁻¹).¹⁰⁸ When Li⁺@C₆₀ was replaced by pristine C₆₀, no CS states were produced as predicted by their higher energy levels than those of the triplet excited states of CPD_Py(OC₆) and C₆₀.¹⁰⁸

Fig. 10 (a) Transient absorption spectra of Ni₂-CPD_Py(OC₆) with Li⁺@C₆₀ in deaerated PhCN at room temperature taken at 4 and 28 μs after nanosecond laser excitation at 520 nm. [Ni₂-CPD_Py(OC₆)] = 2.5 × 10⁻⁵ M, [Li⁺@C₆₀] = 5.0 × 10⁻⁵ M. (b) Rise and (c) decay time profiles at 1035 nm with different laser intensities (1, 3, 5 mJ per pluse). Inset: first-order plots.

The mechanisms of intrasupramolecular photoinduced charge separation in Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆) are shown in Scheme 2.¹⁰⁸ The singlet excited state of Ni₂-CPD_Py(OC₆) (¹[Ni₂-CPD_Py(OC₆)]*) is generated upon photoexcitation of Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆) at 420 nm, where the porphyrin moiety is exclusively excited. Even if the Li⁺@C₆₀ moiety is excited, energy transfer from ¹[Li⁺@C₆₀]* (E_s = 1.94 eV)⁷⁷ to Ni₂-CPD_Py(OC₆) (E_s = 1.97 eV) occurs to produce ¹[Ni₂-CPD_Py(OC₆)]*. Although electron transfer from ¹[Ni₂-CPD_Py(OC₆)]* to Li⁺@C₆₀ is energetically possible (Scheme 2), the fast intersystem crossing occurs to generate ³[Ni₂-CPD_Py(OC₆)]* (k_ISC > 10¹² s⁻¹).¹⁰⁸ Then, electron transfer occurs from ³[Ni₂-CPD_Py(OC₆)]* to Li⁺@C₆₀ with the driving force of 0.30 eV to produce the CS state. The CS state decays slowly via intrasupramolecular BET with the lifetime of 0.67 ms (Scheme 2).¹⁰⁸

Scheme 2 Energy diagram for Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆); broken arrow: minor pathway.

4. Supramolecular complex of a porphyrin tripod with C₆₀

The tripod conformation of a porphyrin trimer (TPZn₃) in Fig. 11 makes it possible to capture a fullerene derivative containing a pyridine moiety (PyC₆₀)¹⁰⁹ inside the cavity strongly by π–π interactions together with the coordination bond between Zn²⁺ and the pyridine moiety (Scheme 3).^110–113 The association constant of TPZn₃ with PyC₆₀ (1.1 × 10⁵ M⁻¹ in toluene) determined from the UV-vis absorption spectral titration (Fig. 12a) is much larger as compared with those of the corresponding monomer (MPZn) and dimer porphyrin (DPZn₂).¹⁰⁹ The ¹H NMR signals of TPZn₃ exhibit downfield shifts upon complexation with PyC₆₀, whereas the pyridyl protons of PyC₆₀ exhibit large upfield shifts by the complexation, which is ascribed to the influence of the large porphyrin aromatic ring current.¹¹³ This result clearly shows that the pyridyl group of PyC₆₀ coordinates to the central zinc ions of TPZn₃. The encapsulation of PyC₆₀ into the cavity of TPZn₃ was supported by the DFT-optimized structure (B3LYP/3-21G(*) basis set) in Fig. 12b.¹¹³

Fig. 11 A porphyrin tripod and the reference dimer and monomer.

Scheme 3 Formation of a supramolecular complex between TPZn₃ and PyC₆₀.

Fig. 12 (a) UV-Vis spectral changes upon addition of PyC₆₀ (0 to 48 μM) to an o-DCB solution of TPZn₃ (3 μM) at 298 K. (b) Schematic view of photoinduced electron transfer in the TPZn₃–PyC₆₀ complex. The structure of the TPZn₃–PyC₆₀ complex was optimized by DFT at the B3LYP/3-21G(*) level.

The occurrence of photoinduced electron transfer from ¹TPZn₃* to PyC₆₀ was confirmed by femtosecond laser flash photolysis measurements in Fig. 13a, where the transient absorption spectrum due to ¹TPZn₃* changes as time elapses to afford the absorption bands at λ_max = 1000 nm due to the monofunctionalized fullerene radical anion^114,115 and at 670 nm due to the one-electron oxidized species of TPZn₃ (TPZn₃˙⁺).^113,116,117

Fig. 13 Transient absorption spectra of (a) TPZn₃ (7.0 × 10⁻⁶ M) and (b) MPZn (1.1 × 10⁻⁵ M) in the presence of PyC₆₀ (2.3 × 10⁻⁵ M) obtained at 2 ps (black), 62 ps (red), and 2800 ps (green) after femtosecond laser pulse irradiation at 410 nm in deaerated toluene at 298 K.

In sharp contrast to the TPZn₃–PyC₆₀ complex, the transient absorption spectrum of the monomer porphyrin (MPZn) in the presence of PyC₆₀ (Fig. 13b) exhibits the absorbance change due to the energy transfer from ¹MPZn* to PyC₆₀ to give the singlet excited state ¹PyC₆₀* (1.76 eV), followed by the conversion to the triplet excited states ³MPZn* and ³PyC₆₀* at 2800 ps (green line in Fig. 13b), accompanied by the recovery of the ground state.¹¹³

The energy diagrams of photodynamics for TPZn₃ and MPZn in the presence of PyC₆₀ in toluene are shown in Scheme 4a and 4b, respectively.¹¹³ The energy level (1.49 eV) of the CS state (TPZn₃˙⁺−PyC₆₀˙⁻) is lower than the energy level of the triplet excited state of PyC₆₀ moieties (1.56 eV). The rate constant (k_ET) of photoinduce electron transfer from ¹TPZn₃* to PyC₆₀ is larger than the rate constant of intersystem crossing. From the rate constant of back electron transfer (k_BET = 1.9 × 10⁹ s⁻¹), the lifetime of the CS state is determined to be τ_CS = 0.53 ns. In contrast, only energy transfer from ¹MPZn* to PyC₆₀ occurs to produce ¹PyC₆₀*, in competition with intersystem crossing to ³MPZn*.¹¹³

Scheme 4 Energy diagrams for photodynamics of (a) TPZn₃ and (b) MPZn in the presence of PyC₆₀ in toluene.

TPZn₃ also forms a stable 1 : 1 complex with gold(III) tetra(4-pyridyl)porphyrin (AuTPyP⁺) in nonpolar solvents.¹¹⁸ The strong binding of TPZn₃ with AuTPyP⁺ results from the encapsulation of AuTPyP⁺ inside the cavity of TPZn₃ through multiple coordination bonds. The efficient quenching of the singlet excited state of TPZn₃ occurs via a photoinduced electron-transfer pathway in the TPZn₃–AuTPyP⁺ complex as the case of TPZn₃–PyC₆₀ complex.¹¹⁸

5. Supramolecular complexes of porphyrin oligopeptides and C₆₀

Multiple photosynthetic reaction centres composed of light-harvesting multiporphyrin units and charge-separation units were obtained by using both the coordination bond and π–π interaction. Zinc porphyrinic oligopeptides with various numbers of porphyrin units [P(ZnP)_n; n = 2, 4, 8]^119,120 were used as light-harvesting multiporphyrin units (Fig. 14), which are bound to electron acceptors of fulleropyrrolidine bearing a pyridine (PyC₆₀)¹¹³ or imidazole coordinating ligand (ImC₆₀)⁸² as shown in Fig. 15.¹²¹

Fig. 14 Chemical structures of P(ZnP)₂, P(ZnP)₄ and P(ZnP)₁₆.

Fig. 15 Illustration of supramolecular complex composed of porphyrin-peptide octamer [P(ZnP)₈, Ar = 3,5-(t-Bu)₂C₆H₃] and PyC₆₀ or ImC₆₀.

The binding constant (K) of PyC₆₀ to P(ZnP)_n increased with increasing number of zinc porphyrins in an oligopeptide unit.¹²¹ No supramolecular complex formation was observed in the case of zinc tetraphenylporphyrin in PhCN.¹²¹ The strong binding between P(ZnP)₈ and PyC₆₀ results from the strong π–π interactions between two zinc porphyrins and PyC₆₀ in addition to the axial coordination of PyC₆₀ to zinc ions of the porphyrins. In the case of ImC₆₀, however, the highest K value was obtained in the P(ZnP)₄–ImC₆₀ complex. This indicates that ImC₆₀ is much more strongly bound by the oligopeptide, P(ZnP)₄, than PyC₆₀.¹²¹ The apparent binding constants (K) determined from the fluorescence quenching of P(ZnP)_n were significantly larger than those determined from the UV-vis spectral change, and the difference in the values increased with increasing the generation of porphyrinic oligopeptides (with increasing the number of the porphyrin units).¹²¹ This indicates that the excited energy migration between the porphyrin units occurs efficiently prior to the electron transfer to the bound C₆₀ moiety. An extremely efficient energy transfer also occurs in P(ZnP)₈–ImC₆₀ judging from the large difference in the K values determined by the absorption change and by the fluorescence quenching (1.5 × 10⁴vs. 3.3 × 10⁵ M⁻¹).

The occurrence of photoinduced electron transfer in the supramolecular complex in PhCN was confirmed by the transient absorption spectra of the supramolecular complex of P(ZnP)₈ with PyC₆₀ using nanosecond laser flash photolysis.¹²¹ The laser photoexcitation at 561 nm of the supramolecular complex of P(ZnP)₈ with PyC₆₀ results in formation of the CS state as indicated by the transient absorption spectra in Fig. 16a, where the absorption band due to PyC₆₀˙⁻ is clearly observed at 1000 nm together with that due to ZnP˙⁺ at 630 nm.¹²¹ The CS state detected decays obeying first-order kinetics (Fig. 16b) and the first-order plots at different initial CS concentrations afford linear correlations with the same slope (inset of Fig. 16b).¹²¹ If there is any contribution of intermolecular back electron transfer from unbound PyC₆₀˙⁻ to ZnP˙⁺, the second-order kinetics would be involved for the decay time profile. In fact, the corresponding second-order plots (Fig. 16c) are clearly non-linear and the initial slope varies depending on the CS concentration. Thus, the decay process is ascribed to back electron transfer in the supramolecular complex rather than intermolecular back electron transfer between ZnP˙⁺ and PyC₆₀˙⁻.¹²¹ The CS lifetimes of the supramolecular complexes of other porphyrins [P(ZnP)_n] and the fullerene derivative (ImC₆₀) become longer with increasing generation of porphyrinic oligopeptides (with increasing the number of the porphyrin units).¹²¹ Such elongation of the CS lifetimes results from efficient hole migration between the porphyrin units following the photoinduced electron transfer in the supramolecular complexes.

Fig. 16 (a) Transient absorption spectra of P(ZnP)₈ (2.9 × 10⁻⁶ M) in the presence of PyC₆₀ (4.9 × 10⁻³ M) in deaerated PhCN at 298 K taken at 70 (solid line with black circles) and 350 μs (solid line with white circles) after laser excitation at 561 nm (4 mJ per pulse), respectively. (b) Time profiles of the absorption at 1000 nm due to PyC₆₀˙⁻ with different laser powers (4 and 1 mJ per pulse) at 298 K. Inset: first-order plots. (c) Second-order plots.

Multiple photosynthetic reaction centres have also been constructed using supramolecular complexes of zinc porphyrin dendrimers [D(ZnP)_n: n = 4, 8, 16] with PyC₆₀.¹²² Efficient energy migration occurs more efficiently between the ZnP units of dendrimers prior to the photoinduced electron transfer with increasing the generation of dendrimers to attain an extremely long CS lifetime e.g., 0.25 ms for the D(ZnP)₁₆–PyC₆₀ complex in PhCN at 298 K.¹²²

Multiple photosynthetic reaction centres composed of supramolecular complexes of harvesting multiporphyrin units and charge-separation units have enabled us to construct supramolecular organic solar cells by the electrodeposition of mixed porphyrin-peptide oligomers [P(H₂P)_n or P(ZnP)_n] and C₆₀ clusters [(P(H₂P)_n + C₆₀)_m or (P(ZnP)_n + C₆₀)_m] onto an optically transparent electrode (OTE) of a nanostructured SnO₂ electrode (OTE/SnO₂), to obtain modified electrodes [denoted as (P(H₂P)_n + C₆₀)_m or (P(ZnP)_n + C₆₀)_m (n = 1, 2, 4, 8, 16)].¹²³ The IPCE value increased with increasing the number of porphyrins in a polypeptide unit in both (P(H₂P)_n + C₆₀)_m and (P(ZnP)_n + C₆₀)_m (n = 1, 2, 4, 8, 16) systems as shown in Fig. 17. Such a good photoelectrochemical performance results from efficient photoinduced electron-transfer from the excited state of the porphyrin unit to C₆₀ in the supramolecular complex with longer CS lifetimes as the number of porphyrins in a polypeptide unit increases (vide supra). The maximum IPCE value of (P(ZnP)₁₆ + C₆₀)_m (56%) is larger than that of (P(H₂P)₁₆ + C₆₀)_m (48%) probably because of the larger driving force of the photoinduced electron transfer.

Fig. 17 (A) The photocurrent action spectra (IPCE vs. wavelength) of (a) (P(H₂P)₁₆ + C₆₀)_m, (b) (P(H₂P)₈ + C₆₀)_m, (c) (P(H₂P)₄ + C₆₀)_m, (d) (P(H₂P)₂ + C₆₀)_m and (e) (P(H₂P)₁ + C₆₀)_m modified OTE/SnO₂ electrodes. (B) The photocurrent action spectra of (a) (P(ZnP)₁₆ + C₆₀)_m, (b) (P(ZnP)₈ + C₆₀)_m, (c) (P(ZnP)₄ + C₆₀)_m, (d) (P(ZnP)₂ + C₆₀)_m and (e) (P(ZnP)₁ + C₆₀)_m modified electrodes. (C) The photocurrent action spectra of (a) (P(ZnP)₁₆ + ImC₆₀)_m, (b) (P(ZnP)₁₆ + PyC₆₀)_m and (c) (P(ZnP)₁₆ + C₆₀)_m modified OTE/SnO₂ electrodes. See text for the employed concentration of the individual species.

The maximum IPCE values of (P(ZnP)₁₆ + PyC₆₀)_m (20%) and (P(ZnP)₁₆ + ImC₆₀)_m (15%) are much smaller than that of (P(ZnP)₁₆ + C₆₀)_m (56%), whereas the binding constant of P(ZnP)₁₆–C₆₀ is smaller than those of P(ZnP)₁₆–ImC₆₀ and P(ZnP)₁₆–PyC₆₀.¹²³ The lower IPCE values of P(ZnP)₁₆–ImC₆₀ and P(ZnP)₁₆–PyC₆₀ systems as compared with that of P(ZnP)₁₆–C₆₀ system may result from the poor electron-transport properties of C₆₀ derivatives due to the steric hindrance of the ligand moiety.¹²³ Thus, a key element for efficient photocurrent generation is mainly the hole and electron transport in the thin film rather than the charge separation between porphyrins and C₆₀.¹²³

I/V characteristics of (a) (P(H₂P)₁₆ + C₆₀)_m, (b) (P(H₂P)₈ + C₆₀)_m and (c) (P(H₂P)₁ + C₆₀)_m modified electrodes under visible light irradiation (λ > 400 nm) are shown in Fig. 18. The (P(H₂P)₁₆ + C₆₀)_m system has a larger fill factor (FF) of 0.47, an open circuit voltage (V_oc) of 320 mV, a short circuit current density (I_sc) of 0.36 mA cm⁻², and the overall power conversion efficiency (η) of 1.6% at input power (W_in) of 3.4 mW cm⁻².¹²³

Fig. 18 (A) Current–voltage characteristics of (a) (P(H₂P)₁₆ + C₆₀)_m, (b) (P(H₂P)₈ + C₆₀)_m, and (c) (P(H₂P)₁ + C₆₀)_m modified electrodes. (B) Current–voltage characteristics of (a) (P(ZnP)₁₆ + C₆₀)_m and (b) (P(ZnP)₁ + C₆₀)_m. Electrolyte: 0.5 M NaI and 0.01 M I₂ in acetonitrile. Input power: 3.4 mW cm⁻², λ > 400 nm.

The η values of the (P(H₂P)₁₆ + C₆₀)_m system was remarkably enhanced (around 40 times) in comparison with the (P(H₂P)₁ + C₆₀)_m modified electrode (η = 0.043%) under the same experimental conditions. The η value of (P(ZnP)₁₆ + C₆₀)_m is also determined as 1.4% and this value is much larger than that of (P(ZnP)₁ + C₆₀)_m (0.047%) as shown in Fig. 18B.¹²³

6. Conclusions

As described above, porphyrin monomers, dimers, trimers and oligomers form supramolecular complexes with fullerene derivatives via electrostatic interactions, π–π interactions and coordination bonds. Photoexcitation of the supramolecular complexes resulted in efficient photoinduced electron transfer from the porphyrin moiety to the fullerene moiety to produce the long-lived CS states as revealed by laser flash photolysis measurements. In particular, a supramolecular complex of a cyclic Ni porphyrin dimer with Li⁺@C₆₀ [Li⁺@C₆₀ ⊂ Ni₂-CPD_Py(OC₆)] affords a long-lived triplet CS state with 0.63 ms lifetime. A high IPCE value (77% at 450 nm) was achieved for a supramolecular solar cell using the OTE/SnO₂/(ZnTPPS⁴⁻/Li⁺@C₆₀)_n electrode. The use of porphyrin oligomer peptides has also enabled to construct multiple photosynthetic reaction centres composed of light-harvesting multiporphyrin units and charge-separation units linked by both the coordination bond and π–π interactions, which afforded long-lived CS states. Supramolecular organic solar cells composed of porphyrinic oligopeptides and C₆₀ exhibited higher overall power conversion efficiency with increasing the number of porphyrin units. Supramolecular complexes formed between porphyrins and fullerenes in particular Li⁺@C₆₀ provide promising materials for more efficient solar energy conversion.

Acknowledgements

The authors gratefully acknowledge the contributions of their collaborators and co-workers mentioned in the cited references, in particular Prof. Fumito Tani (Kyushu University) and Prof. Taku Hasobe (Keio University). Financial supports from the Grants-in-Aid (no. 20108010 to S.F. and 23750014 to K.O.) from MEXT of Japan and KOSEF/MEST of Korea through WCU project (R31-2008-000-10010-0) are gratefully acknowledged.

References

1. Anoxygenic Photosynthetic Bacteria, ed. R. E. Blankenship, M. T. Madigan and C. E. Bauer, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1995.
2. W. Leibl and P. Mathis, in Electron Transfer in Photosynthesis. Series on Photoconversion of Solar Energy, 2004, vol. 2, p. 117.
3. T. A. Faunce, W. Lubitz, A. W. Rutherford, D. MacFarlane, G. F. Moore, P. Yang, D. G. Nocera, T. A. Moore, D. H. Gregory, S. Fukuzumi, K. B. Yoon, F. A. Armstrong, M. R. Wasielewski and S. Styring, Energy Environ. Sci., 2013, 6, 695.
4. S. Fukuzumi, Y. Yamada, T. Suenobu, K. Ohkubo and H. Kotani, Energy Environ. Sci., 2011, 4, 2754.
5. A. Harriman and J.-P. Sauvage, Chem. Soc. Rev., 1996, 25, 41.
6. M. N. Paddon-Row, Adv. Phys. Org. Chem., 2003, 38, 1.
7. M. N. Paddon-Row, Acc. Chem. Res., 1994, 27, 18.
8. D. Gust, T. A. Moore and A. L. Moore, Acc. Chem. Res., 2009, 42, 1890.
9. D. Gust, T. A. Moore and A. L. Moore, Faraday Discuss., 2012, 155, 9.
10. M. R. Wasielewski, Acc. Chem. Res., 2009, 42, 1910.
11. C. Thilgen and F. Diederich, Chem. Rev., 2006, 106, 5049.
12. F. Giacalone and N. Martín, Chem. Rev., 2006, 106, 5136.
13. W.-S. Li and T. Aida, Chem. Rev., 2009, 109, 6047.
14. C. M. Drain, A. Varotto and I. Radivojevic, Chem. Rev., 2009, 109, 1630.
15. (a) F. D'Souza and O. Ito, Coord. Chem. Rev., 2005, 249, 1410; (b) F. D'Souza and O. Ito, Chem. Commun., 2009, 4913; (c) F. D'Souza, A. S. D. Sandanayaka and O. Ito, J. Phys. Chem. Lett., 2010, 1, 2586; (d) F. D'Souza and O. Ito, Chem. Soc. Rev., 2012, 41, 86.
16. (a) S. Fukuzumi, Bull. Chem. Soc. Jpn., 2006, 79, 177; (b) S. Fukuzumi, Phys. Chem. Chem. Phys., 2008, 10, 2283; (c) S. Fukuzumi, Pure Appl. Chem., 2007, 79, 981; (d) S. Fukuzumi, Org. Biomol. Chem., 2003, 1, 609.
17. H. Imahori and S. Fukuzumi, Adv. Funct. Mater., 2004, 14, 525.
18. D. M. Guldi, G. M. A. Rahman, F. Zerbetto and M. Prato, Acc. Chem. Res., 2005, 38, 871.
19. G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 676.
20. S. Fukuzumi, T. Honda, K. Ohkubo and T. Kojima, Dalton Trans., 2009, 3880.
21. S. Fukuzumi and T. Kojima, J. Mater. Chem., 2008, 18, 1427.
22. D. M. Guldi and V. Sgobba, Chem. Commun., 2011, 47, 606.
23. D. M. Guldi, Phys. Chem. Chem. Phys., 2007, 9, 1400.
24. S. Fukuzumi and K. Ohkubo, J. Mater. Chem., 2012, 22, 4575.
25. (a) K. Ohkubo and S. Fukuzumi, J. Porphyrins Phthalocyanines, 2008, 12, 993; (b) K. Ohkubo and S. Fukuzumi, Bull. Chem. Soc. Jpn., 2009, 82, 303.
26. G. Bottari, G. de la Torre, D. M. Guldi and T. Torres, Chem. Rev., 2010, 110, 6768.
27. M. V. Martinez-Diaz, N. S. Fender, M. S. Rodriguez-Morgade, M. Gomez-Lopez, F. Diederich, L. Echegoyen, J. F. Stoddart and T. Torres, J. Mater. Chem., 2002, 12, 2095.
28. (a) C. G. Claessens and T. Torres, Chem. Commun., 2004, 1298; (b) C. G. Claessens and T. Torres, J. Am. Chem. Soc., 2002, 124, 14522.
29. S. Fukuzumi, K. Ohkubo, F. D'Souza and J. L. Sessler, Chem. Commun., 2012, 48, 9801.
30. J.-F. Nierengarten, U. Hahn, T. M. F. Duarte, F. Cardinali, N. Solladié, M. E. Walther, A. Van Dorsselaer, H. Herschbach, E. Leize, A.-M. Albrecht-Gary, A. Trabolsi and M. Elhabiri, C. R. Chim., 2006, 9, 1022.
31. L. Echegoyen, F. Diederich and L. E. Echegoyen, in Fullerenes: Chemistry, Physics and Technology, ed. K. M. Kadish and R. S. Ruoff, Wiley-Interscience, New York, 2000, pp. 1–51.
32. T. M. Figueira-Duarte, A. Gegout and J.-F. Nierengarten, Chem. Commun., 2007, 109.
33. S. Fukuzumi and D. M. Guldi, in Electron Transfer in Chemistry, ed. V. Balzani, Wiley-VCH, Weinheim, 2001, vol. 2, pp. 270–337.
34. S. Fukuzumi, in Functional Organic Materials, ed. T. J. J. Müller and U. H. F. Bunz, Wiley-VCH, 2007, pp. 465–510.
35. J.-F. Nierengarten, New J. Chem., 2004, 28, 1177.
36. S. Fukuzumi, I. Nakanishi, T. Suenobu and K. M. Kadish, J. Am. Chem. Soc., 1999, 121, 3468.
37. S. Fukuzumi, K. Ohkubo, H. Imahori and D. M. Guldi, Chem.–Eur. J., 2003, 9, 1585.
38. Y. Kawashima, K. Ohkubo and S. Fukuzumi, J. Phys. Chem. A, 2013, 117, 6737.
39. S. Fukuzumi, K. Ohkubo, H. Imahori, J. Shao, Z. Ou, G. Zheng, Y. Chen, R. K. Pandey, M. Fujitsuka, O. Ito and K. M. Kadish, J. Am. Chem. Soc., 2001, 123, 10676.
40. D. Wróbel and A. Graja, Coord. Chem. Rev., 2011, 255, 2555.
41. 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.
42. H. Imahori, D. M. Guldi, K. Tamaki, Y. Yoshida, C. Luo, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2001, 123, 6617.
43. K. Ohkubo, H. Imahori, J. Shao, Z. Ou, K. M. Kadish, Y. Chen, G. Zheng, R. K. Pandey, M. Fujitsuka, O. Ito and S. Fukuzumi, J. Phys. Chem. A, 2002, 106, 10991.
44. H. Imahori, K. Tamaki, Y. Araki, Y. Sekiguchi, O. Ito, Y. Sakata and S. Fukuzumi, J. Am. Chem. Soc., 2002, 124, 5165.
45. K. Ohkubo, H. Kotani, J. Shao, Z. Ou, K. M. Kadish, G. Li, R. K. Pandey, M. Fujitsuka, O. Ito, H. Imahori and S. Fukuzumi, Angew. Chem., Int. Ed., 2004, 43, 853.
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. H. Imahori, Y. Sekiguchi, Y. Kashiwagi, T. Sato, Y. Araki, O. Ito, H. Yamada and S. Fukuzumi, Chem.–Eur. J., 2004, 10, 3184.
48. F. D'Souza, R. Chitta, S. Gadde, S. D.-M. Islam, A. L. Schumacher, M. E. Zandler, Y. Araki and O. Ito, J. Phys. Chem. B, 2006, 110, 25240.
49. F. D'Souza, R. Chitta, S. Gadde, L. M. Rogers, P. A. Karr, M. E. Zandler, A. S. D. Sandanayaka, Y. Araki and O. Ito, Chem.–Eur. J., 2007, 13, 916.
50. R. Chitta, K. Ohkubo, M. Tasior, N. K. Subbaiyan, M. E. Zandler, D. T. Gryko, S. Fukuzumi and F. D'Souza, J. Am. Chem. Soc., 2008, 130, 14263.
51. F. D'Souza, E. Maligaspe, P. A. Karr, A. L. Schumacher, M. El Ojaimi, C. P. Gros, J.-M. Barbe, K. Ohkubo and S. Fukuzumi, Chem.–Eur. J., 2008, 14, 674.
52. F. D'Souza, E. Maligaspe, K. Ohkubo, M. E. Zandler, N. K. Subbaiyan and S. Fukuzumi, J. Am. Chem. Soc., 2009, 131, 8787.
53. F. D'Souza, G. M. Venukadasula, K. Yamanaka, N. K. Subbaiyan, M. E. Zandler and O. Ito, Org. Biomol. Chem., 2009, 7, 1076.
54. 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.
55. E. Maligaspe, T. Kumpulainen, N. K. Subbaiyan, M. E. Zandler, H. Lemmetyinen, N. V. Tkachenko and F. D'Souza, Phys. Chem. Chem. Phys., 2010, 12, 7434.
56. M. E. El-Khouly, D. K. Ju, K.-Y. Kay, F. D'Souza and S. Fukuzumi, Chem.–Eur. J., 2010, 16, 6193.
57. 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.
58. F. D'Souza, A. N. Amin, M. E. El-Khouly, N. K. Subbaiyan, M. E. Zandler and S. Fukuzumi, J. Am. Chem. Soc., 2012, 134, 654.
59. S.-H. Lee, A. G. Larsen, K. Ohkubo, J. R. Reimers, S. Fukuzumi and M. J. Crossley, Chem. Sci., 2012, 3, 257.
60. V. Garg, G. Kodis, M. Chachisvilis, M. Hambourger, A. L. Moore, T. A. Moore and D. Gust, J. Am. Chem. Soc., 2011, 133, 2944.
61. J. Frey, G. Kodis, S. D. Straight, T. A. Moore, A. L. Moore and D. Gust, J. Phys. Chem. A, 2013, 117, 607.
62. Y. Kashiwagi, K. Ohkubo, J. A. McDonald, I. M. Blake, M. J. Crossley, Y. Araki, O. Ito, H. Imahori and S. Fukuzumi, Org. Lett., 2003, 5, 2719.
63. D. Curiel, K. Ohkubo, J. R. Reimers, S. Fukuzumi and M. J. Crossley, Phys. Chem. Chem. Phys., 2007, 9, 5260.
64. J. Iehl, M. Vartanian, M. Holler, J.-F. Nierengarten, B. Delavaux-Nicot, J.-M. Strub, A. Van Dorsselaer, Y. Wu, J. Mohanraj, K. Yoosaf and N. Armaroli, J. Mater. Chem., 2011, 21, 1562.
65. F. D'Souza, G. R. Deviprasad, M. S. Rahman and J.-P. Choi, Inorg. Chem., 1999, 38, 2157.
66. 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.
67. T. Da Ros, M. Prato, D. M. Guldi, M. Ruzzi and L. Pasimeni, Chem.–Eur. J., 2001, 7, 816.
68. F. D'Souza, G. R. Deviprasad, M. E. El-Khouly, M. Fujitsuka and O. Ito, J. Am. Chem. Soc., 2001, 123, 5277.
69. T. Honda, T. Nakanishi, K. Ohkubo, T. Kojima and S. Fukuzumi, J. Am. Chem. Soc., 2010, 132, 10155.
70. T. Honda, T. Kojima, N. Kobayashi and S. Fukuzumi, Angew. Chem., Int. Ed., 2011, 50, 2725.
71. S. Fukuzumi, T. Honda and T. Kojima, Coord. Chem. Rev., 2012, 256, 2488.
72. 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.
73. E. Karnas, S. K. Kima, K. A. Johnson, J. L. Sessler, K. Ohkubo and S. Fukuzumi, J. Am. Chem. Soc., 2010, 132, 16617.
74. S. Aoyagi, E. Nishibori, H. Sawa, K. Sugimoto, M. Takata, Y. Miyata, R. Kitaura, H. Shinohara, H. Okada, T. Sakai, Y. Ono, K. Kawachi, K. Yokoo, S. Ono, K. Omote, Y. Kasama, S. Ishikawa, T. Komuro and H. Tobita, Nat. Chem., 2010, 4, 678.
75. S. Fukuzumi, K. Ohkubo, Y. Kawashima, D. S. Kim, J. S. Park, A. Jana, V. Lynch, D. Kim and J. L. Sessler, J. Am. Chem. Soc., 2011, 133, 15938.
76. N. L. Bill, M. Ishida, S. Bähring, J. M. Lim, S. Lee, C. M. Davis, V. M. Lynch, K. A. Nielsen, J. O. Jeppesen, K. Ohkubo, S. Fukuzumi, D. Kim and J. L. Sessler, J. Am. Chem. Soc., 2013, 135, 10852.
77. Y. Kawashima, K. Ohkubo and S. Fukuzumi, J. Phys. Chem. A, 2012, 116, 8942.
78. K. Ohkubo, Y. Kawashima and S. Fukuzumi, Chem. Commun., 2012, 48, 4314.
79. J. S. Park, E. Karnas, K. Ohkubo, P. Chen, K. M. Kadish, S. Fukuzumi, C. W. Bielawski, T. W. Hudnall, V. M. Lynch and J. L. Sessler, Science, 2010, 329, 1324.
80. M. Murakami, K. Ohkubo, T. Nanjo, K. Souma, N. Suzuki and S. Fukuzumi, ChemPhysChem, 2010, 11, 2594.
81. K. Ohkubo, Y. Kawashima, H. Sakai, T. Hasobe and S. Fukuzumi, Chem. Commun., 2013, 49, 4474.
82. T. Hasobe, Y. Kashiwagi, M. A. Absalom, J. Sly, K. Hosomizu, M. J. Crossley, H. Imahori, P. V. Kamat and S. Fukuzumi, Adv. Mater., 2004, 16, 975.
83. T. Hasobe, H. Imahori, S. Fukuzumi and P. V. Kamat, J. Phys. Chem. B, 2003, 107, 12105.
84. T. Hasobe, H. Imahori, P. V. Kamat, T. K. Ahn, D. Kim, T. Hanada, T. Hirakawa and S. Fukuzumi, J. Am. Chem. Soc., 2005, 127, 1216.
85. T. Hasobe, P. V. Kamat, V. Troiani, N. Solladié, T. K. Ahn, S. K. Kim, D. Kim, A. Kongkanand, S. Kuwabata and S. Fukuzumi, J. Phys. Chem. B, 2005, 109, 19.
86. D. Sun, F. S. Tham, C. A. Reed, L. Chaker, M. Burgess and P. D. W. Boyd, J. Am. Chem. Soc., 2000, 122, 10704.
87. D. Sun, F. S. Tham, C. A. Reed, L. Chaker and P. D. W. Boyd, J. Am. Chem. Soc., 2002, 124, 6604.
88. M. Tanaka, K. Ohkubo, C. P. Gros, R. Guilard and S. Fukuzumi, J. Am. Chem. Soc., 2006, 128, 14625.
89. L. P. Hernández-Eguía, E. C. Escudero-Adán, I. C. Pintre, B. Ventura, L. Flamigni and P. Ballester, Chem.–Eur. J., 2011, 17, 14564.
90. A. L. Kieran, S. I. Pascu, T. Jarrosson and J. K. M. Sanders, Chem. Commun., 2005, 1276.
91. B. Grimm, J. Schornbaum, C. M. Cardona, J. D. van Paauwe, P. D. W. Boyd and D. M. Guldi, Chem. Sci., 2011, 2, 1530.
92. L. P. Hernández-Eguía, E. C. Escudero-Adán, J. R. Pinzón, L. Echegoyen and P. Ballester, J. Org. Chem., 2011, 76, 3258.
93. S. K. Samanta and M. Schmittel, Org. Biomol. Chem., 2013, 11, 3108.
94. K. Börjesson, J. G. Woller, E. Parsa, J. Mårtensson and B. Albinsson, Chem. Commun., 2012, 48, 1793.
95. B. Kang, R. K. Totten, M. H. Weston, J. T. Hupp and S. T. Nguyen, Dalton Trans., 2012, 41, 12156.
96. S. Fukuzumi, I. Amasaki, K. Ohkubo, C. P. Gros, R. Guilard and J.-M. Barbe, RSC Adv., 2012, 2, 3741.
97. H. Nobukuni, F. Tani, Y. Shimazaki, Y. Naruta, K. Ohkubo, T. Nakanishi, T. Kojima, S. Fukuzumi and S. Seki, J. Phys. Chem. C, 2009, 113, 19694.
98. T. Yamaguchi, N. Ishii, K. Tashiro and T. Aida, J. Am. Chem. Soc., 2003, 125, 13934.
99. R. Harada, Y. Matsuda, H. Okawa and T. Kojima, Angew. Chem., Int. Ed., 2004, 43, 1825.
100. T. Kojima, T. Nakanishi, R. Harada, K. Ohkubo, S. Yamauchi and S. Fukuzumi, Chem.–Eur. J., 2007, 13, 8714.
101. T. Nakanishi, T. Kojima, K. Ohkubo, T. Hasobe, K. Nakayama and S. Fukuzumi, Chem. Mater., 2008, 20, 7492.
102. T. Kojima, R. Harada, T. Nakanishi, K. Kaneko and S. Fukuzumi, Chem. Mater., 2007, 19, 51.
103. A. Saeki, Y. Koizumi, T. Aida and S. Seki, Acc. Chem. Res., 2012, 45, 1193.
104. R. J. O. M. Hoofman, M. P. de Hass, L. D. A. Siebbeles and J. M. Warman, Nature, 1998, 392, 54.
105. J. H. Choi, T. Honda, S. Seki and S. Fukuzumi, Chem. Commun., 2011, 47, 11213.
106. E. Frankevich, Y. Maruyama and H. Ogata, Chem. Phys. Lett., 1993, 214, 39.
107. 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.
108. T. Kamimura, K. Ohkubo, Y. Kawashima, H. Nobukuni, Y. Naruta, F. Tani and S. Fukuzumi, Chem. Sci., 2013, 4, 1451.
109. F. D'Souza, P. M. Smith, S. Gadde, A. L. McCarty, M. J. Kullman, M. E. Zandler, M. Itou, Y. Araki and O. Ito, J. Phys. Chem. B, 2004, 108, 11333.
110. N. Armaroli, F. Diederich, L. Echegoyen, T. Habicher, L. Flamigni, G. Marconi and J.-F. Nierengarten, New J. Chem., 1999, 77.
111. T. Da Ros, M. Prato, D. M. Guldi, E. Alessio, M. Ruzzi and L. Pasimeni, Chem. Commun., 1999, 635.
112. F. Hauke, A. Swartz, D. M. Guldi and A. Hirsch, J. Mater. Chem., 2002, 12, 2088.
113. A. Takai, M. Chkounda, A. Eggenspiller, C. P. Gros, M. Lachkar, J.-M. Barbe and S. Fukuzumi, J. Am. Chem. Soc., 2010, 132, 4477.
114. 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.
115. D. I. Schuster, K. Loa, D. M. Guldi, A. Palkar, L. Echegoyen, C. Stanisky, R. J. Cross, M. Niemi, M. V. Tkachenko and H. Lemmetyinen, J. Am. Chem. Soc., 2007, 129, 15973.
116. A. Takai, C. P. Gros, R. Guilard and S. Fukuzumi, Chem.–Eur. J., 2009, 15, 3110.
117. A. Takai, C. P. Gros, J.-M. Barbe and S. Fukuzumi, Chem.–Eur. J., 2011, 17, 3420.
118. A. Takai, C. P. Gros, J.-M. Barbe and S. Fukuzumi, Phys. Chem. Chem. Phys., 2010, 12, 12160.
119. N. Solladié, A. Hamel and M. Gross, Tetrahedron Lett., 2000, 41, 6075.
120. F. Aziat, R. Rein, J. Peon, E. Rivera and N. Solladié, J. Porphyrins Phthalocyanines, 2008, 12, 1232.
121. S. Fukuzumi, K. Saito, K. Ohkubo, V. Troiani, H. Qiu, S. Gadde, F. D'Souza and N. Solladié, Phys. Chem. Chem. Phys., 2011, 13, 17019.
122. 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.
123. T. Hasobe, K. Saito, P. V. Kamat, V. Troiani, H. Qiu, N. Solladié, T. K. Ahn, K. S. Kim, S. K. Kim, D. Kim, F. D'Souza and S. Fukuzumi, J. Mater. Chem., 2007, 17, 4160.

---