Dendritic Effects on Structure and Photophysical and

J. Phys. Chem. C 2007, 111, 2777-2786

2777

Dendritic Effects on Structure and Photophysical and Photoelectrochemical Properties of Fullerene Dendrimers and Their Nanoclusters Kohei Hosomizu,† Hiroshi Imahori,*,† Uwe Hahn,‡ Jean-Franç ois Nierengarten,*,‡ Andrea Listorti,§ Nicola Armaroli,*,§ Takashi Nemoto,⊥ and Seiji Isoda⊥ Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8510, Japan, Laboratoire de Chimie de Coordination du CNRS, 205 route de Narbonne, 31077 Toulouse Cedex 4, France, Istituto per la Sintesi Organica e la FotoreattiVita` , Molecular Photoscience Group, Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy, and Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan ReceiVed: October 13, 2006

Multifullerene-terminated dendrimers Gn (n ) 1-5) were synthesized and structural, photophysical, and photoelectrochemical properties were studied for the fullerene dendrimers and their nanoclusters. The fullerene dendrimers formed clusters when toluene solutions of the fullerene dendrimers were injected into acetonitrile. Dynamic light scattering and atomic force and scanning electron microscopic measurements on these clusters revealed that the cluster size decreased with increasing the generation number of the dendrimers. The negatively charged clusters were deposited electrophoretically onto a nanostructured SnO2-coated ITO electrode by applying DC voltage to the electrode. Photoelectrochemical measurements were carried out in acetonitrile dissolved 0.5 M LiI and 0.01 M I2 with the standard three electrodes containing the fullerene dendrimermodified SnO2 working electrode, a platinum wire as a counter electrode, and I-/I3- as a reference electrode. An incident photon-to-photocurrent efficiency of the dendrimer photoelectrochemical devices increased with increasing the generation number. Such a close relationship between the structure and photophysical and photoelectrochemical properties of the fullerene dendrimers and their nanoclusters will provide knowledge of photophysics regarding photoactive molecular assemblies with dendritic architectures.

Introduction Recently, molecular assemblies onto electrode surfaces have attracted much attention toward the development of molecular devices and machines1 including organic solar cells.2 Various methods such as chemical adsorption,3 spin coating,4,5 vacuum evaporation,6 Langmuir-Blodgett films,7 layer-by-layer deposition,8 self-assembled monolayers (SAM),9-11 electrophoretic deposition,12-15 and others16 have been applied to construct photoelectrochemical devices. In particular, an electrophoretic deposition method of the clusters of chromophores in mixed solvents on a nanostructured SnO2 electrode seems to be highly promising for controlling three-dimensional architecture of the molecules on the electrode surface, resulting in an efficient lightcollecting property in UV-visible regions and high light energy conversion efficiency.12-15 The clusters of fullerenes,12 porphyrins and phthalocyanines,13 donor-linked fullerenes,12c,14 and C60-porphyrin composites15 in toluene/acetonitrile mixed solutions have been deposited electrophoretically onto the nanostructured SnO2 electrode to exhibit efficient photocurrent generation. However, it is still difficult to control the initial process of cluster formation from a molecular level to a few hundred nanometers by modulating various parameters including concentrations, solvents, and mixing methods. One rational way * Address correspondence to this author. E-mail: [email protected] (H.I.); [email protected] (J.-F.N.); [email protected] (N.A.). † Graduate School of Engineering, Kyoto University. ‡ Laboratoire de Chimie de Coordination du CNRS. § Consiglio Nazionale delle Ricerche. ⊥ Institute for Chemical Research, Kyoto University.

to surmount such a problem is to employ three-dimensionally preorganized chromophores in which three-dimensional assembly of the chromophores can be tailored by attaching them to the nanoscaffold such as branched dendritic architectures,17 nanoparticles,18 and peptide oligomers.19 In such a case the preorganized chromophores are expected to be further assembled to large clusters with a well-defined architecture that would be favorable for the improvement of photoelectrochemical properties. In this context, multiporphyrin dendrimers have been successfully associated with fullerenes to be deposited electrophoretically onto the nanostructured SnO2 electrode to yield efficient photocurrent generation.17 Nevertheless, the dendritic effects on the structure and photophysical properties of the clusters of chromophore-functionalized dendrimers and dendrons in solutions have not been fully understood. As such, no dentritic effects have been reported in terms of the structure and photophysical and photoelectrochemical properties of the clusters of chromophore-functionalized dendrimers and dendrons deposited electrophoretically on semiconductor electrodes.20 We report herein the first preparation of fullerene dendrimer clusters from fullerene dendrimers in mixed solvents and their assemblies as nanostructured films on a nanostructured SnO2 electrode under application of a DC electric field. Although a variety of fullerene dendrimers and dendrons have been reported,21 they have yet to be applied to molecular photoelectrochemical devices by using the electrophoretic deposition. A series of dendrimers containing a different number of C60 spheres at the periphery of the dendritic wedges (G1, G2, G3, G4, and G5) were designed and synthesized22 to examine the dendritic effects on the structure and photophysical properties

10.1021/jp066730w CCC: $37.00 © 2007 American Chemical Society Published on Web 01/23/2007

2778 J. Phys. Chem. C, Vol. 111, No. 6, 2007

Hosomizu et al.

Figure 1. Fullerene dendrimers used in this study.

of the fullerene dendrimer clusters in solutions as well as the structure and photophysical and photoelectrochemical properties of the fullerene dendrimer nanoclusters electrophoretically deposited on the nanostructured SnO2 electrode (Figure 1). Different molecular assemblies from the fullerene dendrimers of different generation make it possible to control the resulting three-dimensional structure and photodynamics of the molecular assemblies, which is essential for efficient photocurrent generation.

Results and Discussion UV-Visible Absorption Spectra of Fullerene Dendrimers in Toluene. Figures 2 and 3 show absorption spectra of G1G5 in toluene (dashed lines in Figure 2). Herein the concentration of the C60 chromophore is taken as identical for all of the dendrimer solutions: [C60] ) 0.033 mM. The monofunctionalized C60 dendrimers in toluene have a strong UV band (λmax ) 330 nm) as well as a broad and weak visible bands similar

Multifullerene-Terminated Dendrimers

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2779

Figure 2. Absorption spectra of (a) G1, (b) G2, (c) G3, (d) G4, and (e) G5 in toluene ([C60] ) 0.033 mM, 1 cm cuvette, dashed line) and of (a) (G1)m, (b) (G2)m, (c) (G3)m, (d) (G4)m, and (e) (G5)m in 1/6 (v/v) toluene/acetonitrile mixture ([C60] ) 0.23 mM, 1 mm cuvette, solid line).

to that of C60 in toluene.12 Characteristic absorption bands due to monofunctionalized C60 appear near 430 and 690 nm for G1G5 (Figure 3).12,23-26 The dendrimer absorption spectra evidence a gradual increase of absorption with the dendrimer generation in the 400-500 nm window. This is typical for fullerene systems in which the carbon sphere is in close contact with other molecular units and has been documented in many cases such as fullerodendrimers,21b compact hybrid systems,27 and hostguest supramolecular adducts.28 Steady-State and Time-Resolved Emission Spectra of Fullerene Dendrimers. The fluorescence spectra of G1-G5 in toluene exhibit the typical shape of methanofullerenes with an intense band at ∼700 nm with a shoulder at ∼770 nm (see the Supporting Information, S1).23-26 The corresponding emission quantum yields (Φem) and singlet lifetimes (τ) are summarized in Table 1. Little red shift and intensity decrease are found upon an increase of the dendrimer size starting from G3. Also the lifetime is shorter for larger dendrimers, going from 1.6 to 1.1 ns along the series. These effects are probably related

to some degree of intramolecular interaction also observable from the absorption spectra in the ground state; however, it can be concluded that dendritic effects on the short-lived singlet excited state of G1-G5 are not very significant. Triplet Transient Absorption Spectra and Lifetimes of Fullerene Dendrimers. Light excitation of methanofullerenes populates the short-lived lowest singlet state, which undergoes intersystem crossing to the triplet level with high yield (g95%).25,29,30 The latter has been characterized by transient absorption spectroscopy both in simple systems25,29,30 and in more complex architectures.26,31,32 Monofunctionalized fullerenes are also known to be potent singlet oxygen sensitizers25,26,29,30,33 and their long-lived lowest triplet level has been conveniently utilized to monitor dendritic effects in fullerodendrimers.21a-21c In Table 1 are collected the triplet lifetimes of G1-G5 in toluene and benzonitrile under air-equilibrated and oxygen-free conditions. Toluene as a nonpolar solvent and benzonitrile as a polar solvent were employed for the photodynamical measurements. We tried to dissolve the fullerene dendrimers also in


Hosomizu et al.

Figure 3. Comparison of absorption spectra of G1 (black), G2 (red), G3 (green), G4 (blue), and G5 (magenta) in toluene solutions ([C60] ) 0.033 mM, 1 cm cuvette). The inset depicts the expanded spectra at 400-500 nm.

TABLE 1: Fluorescence and Triplet Lifetime Data of Dendrimers G1-G5 in Toluene and Benzonitrile triplet lifetime fluorescence toluene

toluene

benzonitrile

λmax/nm Φem/×10-4 τ/ns AERa/ns DEAb/µs AERa/ns DEAb/µs G1 G2 G3 G4 G5 a

700 702 702 704 704

3.0 3.0 2.8 2.7 2.7

1.6 1.4 1.2 1.1 1.1

360 390 430 460 490

29 12 7 6 6

530 540 600 590 610

34 10 6 5 5

Air equilibrated solution. b Oxygen-free solution.

Figure 4. Transient absorption spectra of G1 (black), G2 (red), G3 (green), G4 (blue), and G5 (magenta) in deaerated toluene solutions, having the same absorbance at λex) 355 nm; delay ) 2 µs, laser energy ) 0.5 mJ/pulse. Inset: Transient absorption decay profiles at 710 nm. Excitation of more than one fullerene moiety within dendrimers is excluded (see the Experimental Section).

other solvents, but they turned out to be too scarely soluble to make optical spectroscopy. Furthermore, reliable data could not be obtained in toluene/acetonitrile mixture because of intense light scattering from the clusters (vide infra). In Figure 4 are depicted the nanosecond transient absorption spectra of a set of solutions of G1-G5 in oxygen-free toluene, having the same absorbance at 355 nm, i.e., the same concentration of C60 units; in the inset of Figure 4 are given the corresponding absorbance decays at 710 nm, which allows us to monitor the triplet excited state of the fullerene dendrimers.25,29,30 The transient absorption signal intensity is dramatically decreased by increasing the generation number, and the spectrum of G5 is hardly detectable over the signal noise. In the case of G1, to avoid triplet-triplet annihilation processes and get monoexponential triplet decays,21c,24,25 the laser excitation energy has to be kept lower than 0.3 mJ/pulse. In contrast, the

Figure 5. Transient absorption spectra of G1 (black), G2 (red), G3 (green), G4 (blue), and G5 (magenta) in air-equilibrated toluene solutions, having the same absorbance at λex ) 355 nm; delay ) 150 ns, laser energy 0.5 mJ/pulse. Inset: Transient absorption decay profiles at 710 nm.

absorbance decay of G2-G4 is monoexponential also at laser energy as high as 10 mJ/pulse. To the best of our knowledge, the trend observed for spectral intensity and lifetimes in G1G5 (Figure 4 and Table 1) has never been reported before with fullerodendrimers or even fullerene clusters.35,36 At present a satisfactory rationalization is difficult; however, it is reasonable to assume that the observed trend is related to the dendrimer size and structure in solution (possibly leading to the formation of small molecular aggregates) characterized by strongly reduced intersystem crossing yields, and shorter triplet lifetimes (a factor of 5 between G1 and G5 in both investigated solvents, Table 1). Reduced intersystem crossing yields have been reported earlier for C60 aggregates,37 but due to enhanced relaxation of the singlet excited state, which is observed here only to a negligible extent (vide supra). We have also considered the possibility that the reduced signal intensity could be an artifact due to a lower “actual” absorbance of the fullerodendrimers in solution, thanks to light scattering induced by the large molecular size.38 However, the nearly identical fluorescence quantum yield of G1-G5, whatever the excitation wavelength, tends to discard this possibility. Passing to air equilibrated solutions, triplet absorption decays become faster due to oxygen quenching and are monoexponential for the whole series of dendrimers G1-G5, also at relatively high laser energy (1-3 mJ/pulse), as shown in Figure 5. The signal intensity is decreased with increasing the generation number, but to a lesser extent than observed in oxygenfree solution (-70% at 720 nm from G1 to G5). The fitted lifetimes are prolonged with an increase of the generation number both in toluene and in benzonitrile (+36% and +15% from G1 to G5, respectively, Table 1). The trend in lifetimes follows what was previously observed with fullerodendrimers having the carbon sphere as the central core, where the prolongation of the C60 triplet lifetime was attributed to a protective effect of the external dendritic skeleton toward interactions with O2.21a,b In the present fullerodendrimers, where the carbon spheres are the external units, we might conceive a sort of C60 self-protection from other C60 moieties, particularly effective for the dendrimers with the higher generation. Such molecular architectures are likely to assume very complex and hardly predictable conformations in solution, where the excited carbon sphere could be partially shielded by other C60 moieties. Nevertheless, we propose a plausible quenching mechanism of the C60 excited triplet state in the dendrimers with the higher generation by oxygen in Scheme 1: in such a case, oxygen may quench the triplet excited state of the outside C60, which


J. Phys. Chem. C, Vol. 111, No. 6, 2007 2781

SCHEME 1

TABLE 2: Reduction Potentials of Fullerene Dendrimers G1-G5 in CH2Cl2 potential/V vs SCEa 0/-1

compd

E1/2

G1 G2 G3 G4 G5

-0.64 -0.66 -0.67 -0.68 -0.69

E1/2-1/-2

E1/2-2/-3

-1.01 -1.02 -1.03 -1.04 -1.05

-1.47 -1.47 -1.47 -1.48 -1.47

a

Measured in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 0.01 V s-1.

is generated by direct excitation or energy migration from the inside C60, and/or penetrate into the C60 aggregates to quench the triplet excited state of the inside C60, thereby leading to the longer lifetime of the triplet excited state in the dendrimers with the higher generation. The sensitized singlet oxygen luminescence spectra of G1-G5 obtained upon excitation at 355 nm support this interpretation (see the Supporting Information, S1), for a decreased sensitization yield is found along the series (-40% from G1 to G5 in toluene). The bimolecular oxygen quenching process is fast enough to partially mask the pronounced trend of faster intrinsic triplet deactivation with dendrimer size, observed in deaerated solution and commented on above (Table 1). Electrochemical Properties of Fullerene Dendrimers. Differential pulse voltammetric measurements were performed for fullerene dendrimers G1-G5 in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 0.01 V s-1 to compare the redox properties of the C60 moieties. Three successive cathodic peaks due to the first, second, and third one-electron reductions of the C60 moieties are observed (see the Supporting Information, S2). The results on the reduction potentials of G1-G5 are summarized in Table 2. The first reduction potential shifts progressively to a more negative direction by up to 0.05 V as the generation number of the fullerene dendrimers increases. Similar negative shift is noted for the second reduction step of G1-G5 by up to 0.04 V. On the other hand, the third reduction potential of G1-G5 remains nearly constant. The first and second reduction processes of the C60 moieties become thermodynamically disfavored inside the dendrimer, because the C60 moieties tend to be embedded into the dendrimer with increasing the generation, leading to destabilization of the negative charges on the C60 moieties in the less polar environment.39 Thus, the less polar environment results in the negative shift of the first and second reduction potentials. After twoelectron reduction of the C60 moieties, electrostatic repulsion among the negatively charged C60 spheres becomes large. Accordingly, in the three-electron reduction of the C60 moieties, exposure of the C60 moieties to the solvent molecules may lead to the C60 moieties in similar electrochemical environment. This explains the occurrence of the three-electron reduction of the C60 moieties for G1-G5 at a similar potential, despite the difference in the generation number. Only the first and second

Figure 6. Particle size distribution of clusters of G1 (dotted line with solid circles), G2 (solid line), G3 (dashed line), G4 (dashed line with open circles), and G5 (dotted line) in 1/6 (v/v) toluene/acetonitrile mixture ([C60] ) 0.23 mM) at the incubation period of 15 min after rapid injection of the toluene solutions into acetonitrile.

reduction potentials become broad with an increase in the generation number, probably due to the heterogeneous environment of the C60 moieties in the higher generation.40 A large solvent effect on the first and second reduction potentials of C60 relative to the third reduction potential is also consistent with the electrochemical trend in the present system.41 Spectroscopic and Microscopic Studies on Fullerene Dendrimer. Figure 2 also displays absorption spectra of G1G5 in toluene/acetonitrile (1/6 ) v/v) mixed solvent (solid line). Herein the concentration of the C60 chromophore is taken identical for all of the dendrimer solutions: [C60] ) 0.23 mM. Fullerene dendrimers G1-G5 in the toluene/acetonitrile mixture exhibit structureless broad absorption in 300-800 nm relative to the monomeric form in toluene. These results confirm that these fullerene dendrimers aggregate and form large clusters in the mixed solvents (denoted as (Gn)m (n ) 1-5)), as reported previously.12 The solid lines in Figure 2 for the absorption around 500 nm at equal C60 concentration in the toluene/ acetonitrile mixture follow the trend G1 > G2 > G5 ≈ G3 > G4. The trend may be resulted from the light scattering from the clusters of which the sizes (100-1000 nm) are similar to the wavelength for the absorption measurement. It should be noted here that G1 in the mixed solvent reveals further broad absorption with a maximum at about 800 nm. This suggests that the cluster structure of G1 in the mixed solvent is quite different from those of G2-G5 (vide infra). The particle size of these clusters in the mixed solvent was measured by dynamic light scattering (DLS). There is no significant change in the size of the clusters from fullernene dendrimers G2-G5 as a function of time elapsed after injection of the toluene solution into acetonitrile. On the other hand, with increasing the incubation time, the size of clusters from fullerene dendrimer G1 increases gradually and reaches a constant value of 1000 nm in 50 min (see the Supporting Information, S3). Thus, although the dimension of the clusters mainly depends on the dendrimer generation, for G1 the incubation time has a large impact on the size of the clusters. In a mixture (1/6) of toluene/ acetonitrile at an incubation period of 15 min after the injection of the toluene solution into acetonitrile, the size distribution of G1-G5 is found to be relatively narrow with different mean diameters (DM) of 790 nm for (G1)m, 210 nm for (G2)m, 170 nm for (G3)m, 100 nm for (G4)m, and 90 nm for (G5)m (Figure 6 and Table 3). The order of the mean diameters [i.e., (G5)m < (G4)m < (G3)m < (G2)m < (G1)m] is not consistent with that of


Hosomizu et al.

TABLE 3: Cluster Size Determined from Various Spectroscopic and Microscopic Measurements AFM

compd G1 G2 G3 G4 G5

DLS mean diameter/nm

SEM mean size/nm

mean horizontal size/nm

mean vertical size/nm

790 210 170 100 90

900 230 180 110 100

900 200 200 100 100

170 190 100 70 70

their molecular sizes. Dendrimers and dendrons are known to become a compact, rigid structure with an increase in the generation number.42 This suggests that in the process of the cluster formation with the higher dendrimer generation, each dendritic branch is subject to interactions with branches belonging to the same dendrimer molecule (intramolecular) rather than to other molecules (intermolecular), resulting in the formation of densely packed dendrimer clusters with a small, compact size (i.e., 90-100 nm). In other words, in the lower dendrimer generations, intermolecular interactions among branches prevail, leading to the formation of poorly packed dendrimer clusters with a large size (vide infra). The size of (G1)m (790 nm) in the mixed solvent is much larger than those of (Gn)m (n ) 2-5) and of the clusters of C60 derivatives (∼300 nm).12 We have already reported unusual association behavior of the C60 derivative in which a phenylpyrrolidine group with two long alkoxy groups at the meta positions of the phenyl ring is attached to the C60 surface by the Prato method.12d In toluene and acetonitrile mixture (1/3 ) v/v), the C60 derivative was proposed to give a bilayer vesicle structure, irrespective of the hydrophobic nature of both the C60 and alkoxy chain moieties. From these results, G1 molecules may be associated in a specific manner to make large aggregates in the mixed solvent compared with G2-G5 (vide supra). To assess the shape and morphology of (Gn)m clusters (n ) 2-5), we performed scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. The samples for the SEM measurements were prepared by spin-coating the cluster solution onto mica. The SEM images of (Gn)m clusters disclose spherical structure with an average diameter of 900 nm for (G1)m, 230 nm for (G2)m, 180 nm for (G3)m, 110 nm for (G4)m, and 100 nm for (G5)m (see the Supporting Information, S4, and Table 3). The size of the clusters is in good agreement with the value obtained from the DLS measurements. Figure 7 reveals the AFM images of the clusters prepared by spin-coating of the (Gn)m cluster solutions on mica surface. To remove solvent, the resulting substrates were heated under reduced pressure. The (Gn)m clusters are spherical with an average horizontal diameter of 900 nm for (G1)m, 200 nm for (G2)m, 200 nm for (G3)m, 100 nm for (G4)m, and 100 nm for (G5)m (Figure 7 and Table 3). The size of the clusters agrees well with the values obtained from the DLS and SEM measurements (vide supra). The vertical size of the clusters on mica (Table 3) correlates largely with the horizontal size of the clusters except for the case of (G1)m. The AFM image of the (G1)m cluster reveals a rather disc-like structure with an average diameter of 900 nm and an average maximum thickness of 170 nm (see the Supporting Information, S5). Namely, the vertical size of the (G1)m cluster is much smaller than the horizontal size. Although detailed structure at the molecular level is not yet clear, self-organization of G1 in the mixed solvent would lead to the formation of multilayer vesicle. In such a case solvent evaporation from the inner space of the multilayer vesicle may

Figure 7. AFM images of (a) (G1)m, (b) (G2)m, (c) (G3)m, (d) (G4)m, and (e) (G5)m on mica. The samples were prepared by spin-coating the cluster solutions onto mica.

yield the disk-like structure. Similar proposed structures were reported for amphiphilic fullerene derivatives in water, mixed organic solvents, and cast films.43,44 Electrophoretic Deposition of Fullerene Dendrimer Clusters. Clusters of fullerene derivatives can be deposited electrophoretically onto a nanostructured SnO2 electrode (denoted as ITO/SnO2) by applying dc voltage to the electrode.12-15 In a similar manner, the clusters of (Gn)m clusters (n ) 1-5) were attached onto the ITO/SnO2 electrode by the electrophoretic deposition method [denoted as ITO/SnO2/(Gn)m (n ) 1-5)]. Under application of a high dc electric field (200 V for 1 min), the clusters of G1-G5 in toluene/acetonitrile became negatively charged as they were driven toward the positively charged electrode surface. After the electrophoretic deposition, the ITO/ SnO2 electrode turned brown, whereas discoloration of the cluster solution took place. As a result, cluster films with a thickness of 4-5 µm could be obtained. Absorption spectra of ITO/SnO2/(Gn)m (n ) 1-5) are shown in Figure S6 (see the Supporting Information, S6). The broad absorption of these films as well as high molar absorptivity in the visible region enables these films to be suitable for harvesting photon energy. AFM was used to evaluate the surface morphology of ITO/ SnO2/(Gn)m (n ) 1-5) films (see the Supporting Information, S7). The ITO/SnO2/(G1)m, ITO/SnO2/(G2)m, ITO/SnO2/(G3)m, ITO/SnO2/(G4)m, and ITO/SnO2/(G5)m films are composed of closely packed clusters with a size of 800, 200, 200, 100, and 100 nm, respectively. The size of the clusters largely agrees with the diameters of the clusters on the mica obtained from the cluster solutions. These results also confirm that fullerene dendrimer clusters are successfully transferred onto the nanostructured SnO2 electrodes. Photoelectrochemical Properties of Fullerene Dendrimer Clusters. Photoelectrochemical measurements were performed in deaerated acetonitrile containing 0.5 M LiI and 0.01 M I2 with ITO/SnO2/(Gn)m as a working electrode, a platinum wire as a counter electrode, and an I-/I3- reference electrode. An inset in Figure 8 displays anodic photocurrent response of the


J. Phys. Chem. C, Vol. 111, No. 6, 2007 2783 SCHEME 2

Figure 8. Current-potential characteristic of ITO/SnO2/(G5)m (input power: 1.7 mW cm-2 (λ > 380 nm)). The current-potential characteristic was measured with controlled-potential scan (1 mV s-1) under 0.5 Hz chopped light. Photocurrent response was shown as an inset. Applied potential: 0.11 V vs SCE; 0.5 M LiI and 0.01 M I2 in acetonitrile; input power: 25 µW cm-2 (λex ) 420 nm).

Figure 9. Photocurrent action spectra of ITO/SnO2/(G1)m (black), (b) ITO/SnO2/(G2)m (red), (c) ITO/SnO2/(G3)m (green), (d) ITO/SnO2/(G4)m (blue), and (e) ITO/SnO2/(G5)m (magenta) devices. Applied potential: +0.11 V vs SCE; 0.5 M LiI and 0.01 M I2 in acetonitrile.

ITO/SnO2/(G5)m device illuminated at λex ) 420 nm (input power: 25 µW cm-2) at an applied potential of 0.11 V vs SCE. The photocurrent responses are prompt, steady, and reproducible during repeated on/off cycles of visible light illumination. Blank experiments conducted with a bare ITO/SnO2 electrode yield no detectable photocurrent under similar experimental conditions. Similar photoelectrochemical responses are noted for ITO/ SnO2/(Gn)m (n ) 1-4) devices. The current vs potential characteristics of ITO/SnO2/(G5)m under white light illumination (>380 nm) are shown in Figure 8. With increasing the positive bias, the photocurrent increases relative to the dark current. This demonstrates that an electron flows from the electrolyte to the ITO/SnO2 electrode via the excited states of C60. Similar current vs potential characteristics are observed for ITO/SnO2/(Gn)m (n ) 1-4) devices. A series of photocurrent action spectra were recorded to evaluate the response of fullerene dendrimer clusters toward photocurrent generation. The photocurrent action spectra of ITO/ SnO2/(Gn)m (n ) 1-5) devices are shown in Figure 9. Incident photon-to-photocurrent efficiency (IPCE) was calculated by normalizing the photocurrent density for incident light energy and intensity by using the expression

IPCE (%) ) 100 × 1240 × i/(Win × λ) where i is the photocurrent density (A cm -2), Win is the incident light intensity (W cm-2), and λ is the excitation wavelength

(nm). The action spectra of ITO/SnO2/(Gn)m (n ) 1-5) devices largely agree with the absorption spectra on ITO/SnO2, supporting the involvement of the C60 moieties for photocurrent generation. These results are consistent with photoelectrochemical properties of the clusters of fullerene derivatives on SnO2 electrodes.12 The IPCE values of ITO/SnO2/(Gn)m (n ) 1-5) devices are compared under the same conditions. The IPCE value at an excitation wavelength of 400 nm increases with increasing the generation number: 1.7% for ITO/SnO2/(G1)m, 1.9% for ITO/SnO2/(G2)m, 4.1% for ITO/SnO2/(G3)m, 4.1% for ITO/SnO2/(G4)m, and 6.0% for ITO/SnO2/(G5)m devices.45 The maximum IPCE value (6.0%) for the ITO/SnO2/(G5)m device is also larger than the values (2.3-4%) for pristine C60 devices,12a,d showing the dendritic effect on photocurrent generation efficiency. On the basis of previous studies on a similar photoelectrochemical system of C60 and C60 derivatives,12 the photocurrent generation diagram is illustrated in Scheme 2. The primary step in the photocurrent generation is initiated by photoinduced electron transfer from I- (I3-/I-, 0.5 V vs NHE) in the electrolyte solution to the excited states of fullerene dendrimer clusters (C60•-/1C60* ) 1.45 V vs NHE, C60•-/3C60* ) 1.2 V vs NHE).12 The electron-transfer rate is controlled by diffusion of I- (∼109 s-1) in the electrolyte solution.12 The resulting reduced C60 (C60•-/C60 ) -0.3 V vs NHE) injects an electron directly into the SnO2 nanocrystallites (ECB ) 0 V vs NHE)12 or the electron is injected into the SnO2 nanocrystallites through an electron hopping process between the C60 molecules.18 The electron transferred to the semiconductor nanocrystallines is driven to the counter electrode via external circuit to regenerate the redox couple. It is noteworthy that the IPCE values are dependent on the dendritic generation. With increasing the dendritic generation the IPCE value increases. It does not match the fact that the lifetimes of excited singlet and triplet states of the dendrimers in solutions decrease with increasing the generation number under the deaerated conditions. The structural investigation on the fullerene dendrimers reveals that the higher dendrimer generation leads to the formation of densely packed dendrimer clusters with a smaller, compact size (vide supra). Such structures of fullerene dendrimer clusters on ITO/SnO2 in the higher generation would make it possible to accelerate the electron injection process from the reduced C60 to the conduction band of SnO2 via the more efficient electron hopping through the C60 moieties where the average distance between the C60 moieties is smaller.

2784 J. Phys. Chem. C, Vol. 111, No. 6, 2007 In conclusion, we have successfully examined the structural and photophysical and photoelectrochemical properties of the fullenene dendrimers and their nanoclusters for the first time. Significant change in the structural and photophysical and photoelectrochemical properties of the fullerene dendrimers and their nanoclusters was observed with increasing the generation number of the dendrimers. These unique properties are the result of the specific dendritic structure which, by increasing the generation, tends to promote intramolecular interactions among branches, leading to smaller cluster size with substantially different physicochemical properties including, remarkably, higher photocurrent generation efficiency in photoelectrochemical devices. Experimental Section Materials and Methods. All solvents and chemicals were of reagent grade quality, purchased commercially, and used without further purification unless otherwise noted. Dendrimers G1-G5 were synthesized as described elsewhere.22 ITO electrodes (190-200 nm ITO on transparent glass slides) were commercially available from Sanyo Shinku, Inc. (Japan). The ITO electrodes were washed by sonication in 2-propanol and cleaning in O3 atmosphere in advance. Nanostructured SnO2 film was prepared by spin-coating a dilute (10%) SnO2 colloidal solution (particle size: 15 nm; Chemat Technology, Inc.) onto the ITO electrode, followed by annealing at 673 K for 1 h. Preparation of Cluster Solutions and Films. The cluster suspensions of fullerene dendrimers were prepared in a 1-cm cuvette by injecting 0.25 mL of a toluene solution of G1-G5 (1.64 mM) into 1.5 mL of acetonitrile (toluene:acetonitrile ) 1:6 (v/v), final concentration of fullerenes ) 0.23 mM). Two electrodes (ITO and ITO/SnO2) were kept at a distance of 6 mm by use of a Teflon spacer and set in the cuvette, and then a dc voltage (200 V) was applied for 1 min between these two electrodes by using an ATTO AE-8750 power supply. The thickness of the cluster film on the nanostructured SnO2 electrode was determined by using a surface roughness/profile measuring instrument (Surfcom 130A, Accretech). Spectroscopic and Microscopic Methods. UV-visible absorption spectra of solutions and films were recorded on a Lambda 900 spectrophotometer (Perkin-Elmer, USA). Distribution of cluster size was evaluated by dynamic light scattering (DLS) on a LB-550 particle size analyzer (Horiba, Japan). The surface morphology of the clusters on mica and the films on ITO/SnO2 were observed by atomic force microscopy (AFM) measurements with Nanoscope IIIa (Veeco, USA) in the tapping mode. Scanning electron microscopy (SEM) measurements were carried out with ABT-150 (Topcon, Japan). Electrochemical Measurements. All electrochemical measurements were carried out in a standard three-electrode system with a 660A electrochemical analyzer (ALS/CH Instruments). Differential pulse voltammetric measurements were performed for fullerene dendrimers in CH2Cl2 containing 0.1 M n-Bu4NPF6 with a sweep rate of 0.01 V s-1. Photophysical Measurements. Absorption spectra were recorded with a Perkin-Elmer λ40 spectrophotometer. Emission spectra were obtained with an Edinburgh FLS920 spectrometer (continuous 450 W Xe lamp), equipped with a Peltier-cooled Hamamatsu R928 photomultiplier tube (185-850 nm) or a Hamamatsu R5509-72 supercooled photomultiplier tube (193 K, 800-1700 nm range). Emission quantum yields were determined according to the approach described by Demas and Crosby,46 using [Ru(bipyridine)3Cl2] (λem ) 0.028 in airequilibrated water solution)47 as standard. Emission lifetimes

Hosomizu et al. were determined with the time correlated single photon counting technique by using an Edinburgh FLS920 spectrometer equipped with a laser diode head as excitation source (1 MHz repetition rate, λexc ) 407 nm, 200 ps time resolution upon deconvolution) and an Hamamatsu R928 PMT as detector. Transient absorption spectra in the nanosecond-microsecond time domain were obtained by using the nanosecond flash photolysis apparatus Proteus by Ultrafast Systems LLC. The excitation source was the second or third harmonic (532 or 355 nm) of a Continuum Surelite II Nd:YAG laser with 5 ns pulse duration at 0.1-10 mJ/pulse. Light signals were passed through a Chromex/Bruker 500IS monochromator (equipped with two gratings blazed at 600 or 1200 nm), and collected on a high-speed Silicon (DET210) or InGaAs (DET410) Thorlabs detector in the VIS (400-800 nm) and NIR (800-1700 nm) regions, respectively. The signal was then amplified by means of a variable gain wideband voltage amplifier (Femto DHPVA-200) interfaced with a Tektronix TDS 3032B digital oscilloscope connected to a PC having the acquisition software Proteus. The probe light source was a 150 W CW Xe Arc lamp Spectra Physics 69907. Triplet lifetimes were obtained by averaging 64 or 132 different decays recorded around the maximum of the absorption peak (720 nm). The samples were placed in fluorimetric 1 cm path cuvettes and, when necessary, purged from oxygen by at least 4 freeze-thaw-pump cycles. Typical laser power has been taken as 0.5 mJ/pulse which, taking into account the photon energy at 355 nm, the concentration of the sample, and the volume of solution effectively excited, leads to an excitation of about 10% of the overall fullerene molecules, thus making negligible the chance of multiple excitation events within dendrimers. All measurements were carried out in spectroscopy grade solvents, used without further purification. Experimental uncertainties are estimated to be 8% for lifetime determinations, 20% for emission quantum yields, 5% for relative emission intensities in the NIR, and 1 and 5 nm for absorption and emission peaks respectively. Photoelectrochemical Measurements. All photoelectrochemical measurements were carried out in a standard threeelectrode system with an ALS 630a electrochemical analyzer. The fullerene modified ITO/SnO2 electrode as a working electrode was contacted with the deaerated electrolyte solution containing 0.5 M LiI and 0.01 M I2 in acetonitrile. Pt wire, covered with glass luggin capillary whose tip was located near the working electrode, and Pt coil were used as quasi-reference and counter electrodes, respectively. The potential measured was converted to the saturated calomel electrode (SCE) scale by adding +0.05 V. The stability of the reference electrode potential was confirmed under the experimental conditions. A 500-W xenon lamp (XB-50101AA-A; Ushio, Japan) was used as a light source. The monochromatic light through a monochromator (MC-10N; Ritsu, Japan) was illuminated on the modified area of the working electrode (0.20 cm2) from the backside. The light intensity was monitored by an optical power meter (ML9002A; Anritsu, Japan) and corrected. Acknowledgment. This work was supported by Grant-inAid for 21st Century COE on Kyoto University Alliance for Chemistry from the Ministry of Education, Culture, Sports, Science, the Technology Agency, Japan, and the Italian CNR (commessa PM-P04-ISTM-C1-ISOF-M5, Componenti Molecolari e Supramolecolari o Macromolecolari con Proprieta` Fotoniche ed Optoelettroniche). H.I. also thanks Sekisui and Kurata foundations for financial support. K.H. thanks a JSPS fellowship for young scientists. U.H. thanks the German Aca-

Multifullerene-Terminated Dendrimers demic Exchange Service (DAAD) for a postdoctoral fellowship. J.-F.N. and N.A. thank the EU for the RTN contract “FAMOUS” no. HPRN-CT-2002-00171. Supporting Information Available: Steady-state fluorescence spectra in toluene (S1), differential pulse voltammogram (S2), Time profile of cluster size distribution (S3), SEM images (S4), Section profile of AFM (S5), absorption spectra on electrode (S6), and AFM images of dendrimer-modified SnO2 electrodes (S7). These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Balzani, V.; Venturi, M.; Credi, A. Molecular DeVices and Machines; Wiley-VCH: Weinheim, Germany, 2003. (2) Organic PhotoVoltaics: Mechanisms, Materials and DeVices; Sun, S.-S., Sariciftci, N. S., Eds.; CRC: London, UK, 2005. (3) (a) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (b) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (c) Clifford, J. N.; Palomares, E.; Nazeeruddin, Md. K.; Gra¨tzel, M.; Nelson, J.; Li, X.; Long, N. J.; Durrant, J. R. J. Am. Chem. Soc. 2004, 126, 5225. (4) (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (b) Padinger, F.; Rittberger, R. S.; Sariciftci, N. S. AdV. Funct. Mater. 2003, 13, 85. (c) Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Knol, J.; Hummelen, J. C.; van Hal, P. A.; Janssen, R. A. J. Angew. Chem., Ind. Ed. 2003, 42, 3371. (d) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467. (5) (a) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498. (b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119. (c) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (6) (a) Hiramoto, M.; Fujiwara, H.; Yokoyama, M. Appl. Phys. Lett. 1991, 58, 1062. (b) Tsuzuki, T.; Shirota, Y.; Rostalski, J.; Meissner, D. Solar Energy Mater. Solar Cells 2000, 61, 1. (c) Xue, J.; Rand, B. P.; Uchida, S.; Forrest, S. R. AdV. Mater. 2005, 17, 66. (7) (a) Luo, C.; Huang, C.; Gan, L.; Zhou, D.; Xia, W.; Zhuang, Q.; Zhao, Y.; Huang, Y. J. Phys. Chem. 1996, 100, 16685. (b) Zhang, W.; Shi, Y.; Gan, L.; Huang, C.; Luo, H.; Wu, D.; Li, N. J. Phys. Chem. B 1999, 103, 675. (c) Fukuda, N.; Mitsuishi, M.; Aoki, A.; Miyashita, T. J. Phys. Chem. B 2002, 106, 7048. (d) Matsui, J.; Mitsuishi, M.; Aoki, A.; Miyashita, T. J. Am. Chem. Soc. 2004, 126, 3708. (8) (a) Abdelrazzaq, F. B.; Kwong, R. C.; Thompson, M. E. J. Am. Chem. Soc. 2002, 124, 4796. (b) Luo, C.; Guldi, D. M.; Maggini, M.; Menna, E.; Mondini, S.; Kotov, N. A.; Prato, M. Angew. Chem., Int. Ed. 2000, 39, 3905. (c) Lahav, M.; Heleg-Shabtai, V.; Wasserman, J.; Katz, E.; Willner, I.; Du¨rr, H.; Hu, Y.-Z.; Bossmann, S. H. J. Am. Chem. Soc. 2000, 122, 11480. (d) Fushimi, T.; Oda, A.; Ohkita, H.; Ito, S. Langmuir 2005, 21, 1584. (e) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828. (9) (a) Imahori, H.; Azuma, T.; Ajavakom, A.; Norieda, H.; Yamada, H.; Sakata, Y. J. Phys. Chem. B 1999, 103, 7233. (b) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (c) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (d) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T.; Hasobe, T.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124, 532. (10) (a) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. AdV. Mater. 2002, 14, 892. (b) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129. (c) Imahori, H.; Kimura, M.; Hosomizu, K.; Sato, T.; Ahn, T. K.; Kim, S. K.; Kim, D.; Nishimura, Y.; Yamazaki, I.; Araki, Y.; Ito, O.; Fukuzumi, S. Chem. Eur. J. 2004, 10, 5111. (d) Chukharev, V.; Vuorinen, T.; Efimov, A.; Tkachenko, N. V.; Kimura, M.; Fukuzumi, S.; Imahori, H.; Lemmetyinen, H. Langmuir 2005, 21, 6385. (11) (a) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (b) Ikeda, A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc. 2001, 123, 4855. (c) Enger, O.; Nuesch, F.; Fibbioli, M.; Echegoyen, L.; Pretsch, E.; Diederich, F. J. Mater. Chem. 2000, 10, 2231. (d) Cho, Y.-J.; Ahn, T. K.; Song, H.; Kim, K. S.; Lee, C. Y.; Seo, W. S.; Lee, K.; Kim, S. K.; Kim, D.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 2380. (e) Ma, H.; Kang, M.-S.; Xu, Q.-M.; Kim, K.-S.; Jen, A. K.-Y. Chem. Mater. 2005, 17, 2896. (12) (a) Kamat, P. V.; Barazzouk, S.; Thomas, K. G.; Hotchandani, S. J. Phys. Chem. B 2000, 104, 4014. (b) Kamat, P. V.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004, 108, 5166. (c) Kamat, P. V.; Barazzouk, S.;

J. Phys. Chem. C, Vol. 111, No. 6, 2007 2785 Hotchandani, S.; Thomas, K. G. Chem. Eur. J. 2000, 6, 3914. (d) Hotta, H.; Kang, S.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Imahori, H. J. Phys. Chem. B 2005, 109, 5700. (13) (a) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Mater. Chem. 2003, 13, 2515. (b) Kira, A.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Isosomppi, M.; Tkachenko, N. V.; Lemmetyinen, H.; Imahori, H. Langmuir 2006, 22, 5497. (14) Imahori, H.; Hasobe, T.; Yamada, H.; Kamat, P. V.; Barazzouk, S.; Fujitsuka, M.; Ito, O.; Fukuzumi, S. Chem. Lett. 2001, 784. (15) (a) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 12105. (b) Kang, S.; Umeyama, T.; Ueda, M.; Matano, Y.; Hotta, H.; Yoshida, K.; Isoda, S.; Shiro, M.; Imahori, H. AdV. Mater. 2006, 18, 2549. (16) (a) Liu, Y.; Xiao, S.; Li, H.; Li, Y.; Liu, H.; Lu, F.; Zhuang, J.; Zhu, D. J. Phys. Chem. B 2004, 108, 6256. (b) Sudeep, P. K.; Ipe, B. I.; Thomas, K. G.; George, M. V.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Nano Lett. 2002, 2, 29. (c) Imahori, H.; Liu, J.-C.; Hosomizu, K.; Sato, T.; Mori, Y.; Hotta, H.; Matano, Y.; Araki, Y.; Ito, O.; Maruyama, N.; Fujita, S. Chem. Commun. 2004, 2066. (d) Imahori, H.; Liu, J.-C.; Hotta, H.; Kira, A.; Umeyama, T.; Matano, Y.; Li, G.; Ye, S.; Isosomppi, M.; Tkachenko, N. V.; Lemmetyinen, H. J Phys. Chem. B 2005, 109, 18465. (e) Huang, C.-H.; McClenaghan, N. D.; Kuhn, A.; Hofstraat, J. W.; Bassani, D. M. Org. Lett. 2005, 7, 3409. (17) (a) Hasobe, T.; Kashiwagi, Y.; Absalom, M. A.; Sly, J.; Hosomizu, K.; Crossley, M. J.; Imahori, H.; Kamat, P. V.; Fukuzumi, S. AdV. Mater. 2004, 16, 975. (b) Hasobe, T.; Kamat, P. V.; Absalom, M. A.; Kashiwagi, Y.; Sly, J.; Crossley, M. J.; Hosomizu, K.; Imahori, H.; Fukuzumi, S. J. Phys. Chem. B 2004, 108, 12865. (18) (a) Hasobe, T.; Imahori, H.; Kamat, P. V.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 14962. (b) Hasobe, T.; Imahori, H.; Kamat, P. V.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fujimoto, A.; Hirakawa, T.; Fukuzumi, S. J. Am. Chem. Soc. 2005, 127, 1216. (c) Imahori, H.; Fujimoto, A.; Kang, S.; Hotta, H.; Yoshida, K.; Umeyama, T.; Matano, Y.; Isoda, S. AdV. Mater. 2005, 17, 1727. (d) Imahori, H.; Fujimoto, A.; Kang, S.; Hotta, H.; Yoshida, K.; Umeyama, T.; Matano, Y.; Isoda, S.; Isosomppi, M.; Tkachenko, N. V.; Lemmetyinen, H. Chem. Eur. J. 2005, 11, 7265. (e) Imahori, H.; Mitamura, K.; Shibano, Y.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Araki, Y.; Ito, O. J. Phys. Chem. B 2006, 110, 11399. (19) Hasobe, T.; Kamat, P. V.; Troiani, V.; Solladie´, N.; Ahn, T. K.; Kim, S. K.; Kim, D.; Kongkanand, A.; Kuwabata, S.; Fukuzumi, S. J. Phys. Chem. B 2005, 109, 19. (20) Dendrimers have been assembled on electrodes by using electrostatic interaction or chemical modification for photocurrent generation. See: (a) Boubbou, K. H.; Ghaddar, T. H. Langmuir 2005, 21, 8844. (b) Bustos, E.; Manrı´quez, J.; Echegoyen, L.; Godıńez, L. A. Chem. Commun. 2005, 1613. (c) Li, C.; Mitamura, K.; Imae, T. Macromoelcules 2003, 36, 9957. (21) (a) Rio, Y.; Accorsi, G.; Nierengarten, H.; Rehspringer, J.-L.; Ho¨nerlage, B.; Kopitkovas, G.; Chugreev, A.; Van Dorsselaer, A.; Armaroli, N.; Nierengarten, J.-F. New J. Chem. 2002, 26, 1146. (b) Rio, Y.; Accorsi, G.; Nierengarten, H.; Bourgogne, C.; Strub, J.-M.; Van Dorsselaer, A.; Armaroli, N.; Nierengarten, J.-F. Tetrahedron 2003, 59, 3833. (c) GutierrezNava, M.; Accorsi, G.; Masson, P.; Armaroli, N.; Nierengarten, J.-F. Chem. Eur. J. 2004, 10, 5076. (d) Wooley, K. L.; Hawker, C. J.; Frećhet, J. M. J.; Wudl, F.; Srdanov, G.; Shi, S.; Li, C.; Kao, M. J. Am. Chem. Soc. 1993, 115, 9836. (e) Cardullo, F.; Diederich, F.; Echegoyen, L.; Habicher, T.; Jayaraman, N.; Leblanc, R. M.; Stoddart, J. F.; Wang, S. Langmuir 1998, 14, 1955. (f) Camps, X.; Scho¨nberger, H.; Hirsch, A. Chem. Eur. J. 1997, 3, 561. (g) Dardel, B.; Deschenaux, R.; Even, M.; Serrano, E. Macromolecules 1999, 32, 5193. (22) Synthesis and characterization of G1-G5 were reported. See: (a) Hahn, U.; Hosomizu, K.; Imahori, H.; Nierengarten, J.-F. Eur. J. Org. Chem. 2006, 85. (b) Herschbach, H.; Hosomizu, K.; Hahn, U.; Leize, E.; Van Dorsselaer, A.; Imahori, H.; Nierengarten, J.-F. Anal. Bioanal. Chem. 2006, 386, 46. (23) Ma, B.; Bunker, C. E.; Guduru, R.; Zhang, X.-F.; Sun, Y. P. J. Phys. Chem. A 1997, 101, 5626. (24) Armaroli, N.; Diederich, F.; Dietrich-Buchecker, C. O.; Flamigni, L.; Marconi, G.; Nierengarten, J.-F.; Sauvage, J.-P. Chem. Eur. J. 1998, 4, 406. (25) Bensasson, R. V.; Bienvenuë, E.; Fabre, C.; Janot, J.-M.; Land, E. J.; Leach, S.; Leboulaire, V.; Rassat, A.; Roux, S.; Seta, P. Chem. Eur. J. 1998, 4, 270. (26) Armaroli, N.; Accorsi, G.; Felder, D.; Nierengarten, J.-F. Chem. Eur. J. 2002, 8, 2314. (27) Rio, Y.; Enderlin, G.; Bourgogne, C.; Nierengarten, J.-F.; Gisselbrecht, J.-P.; Gross, M.; Accorsi, G.; Armaroli, N. Inorg. Chem. 2003, 42, 8783. (28) Eckert, J.-F.; Byrne, D.; Nicoud, J.-F.; Oswald, L.; Nierengarten, J.-F.; Numata, M.; Ikeda, A.; Shinkai, S.; Armaroli, N. New J. Chem. 2000, 24, 749. (29) Prat, F.; Stackow, R.; Bernstein, R.; Qian, W.; Rubin, Y.; Foote, C. S. J. Phys. Chem. A 1999, 103, 7230.

2786 J. Phys. Chem. C, Vol. 111, No. 6, 2007 (30) Bensasson, R. V.; Berberan-Santos, M. N.; Brettreich, M.; Frederiksen, J.; Go¨ttinger, H.; Hirsch, A.; Land, E. J.; Leach, S.; McGarvey, D. J.; Scho¨nberger, H.; Schro¨der, C. Phys. Chem. Chem. Phys. 2001, 3, 4679. (31) Holler, M.; Cardinali, F.; Mamlouk, H.; Nierengarten, J.-F.; Gisselbrecht, J.-P.; Gross, M.; Rio, Y.; Barigelletti, F.; Armaroli, N. Tetrahedron 2006, 62, 2060. (32) Armaroli, N.; Accorsi, G.; Song, F.; Palkar, A.; Echegoyen, L.; Bonifazi, D.; Diederich, F. ChemPhysChem 2005, 6, 732. (33) Hamano, T.; Okuda, K.; Mashino, T.; Hirobe, M.; Arakane, K.; Ryu, A.; Mashiko, S.; Nagano, T. Chem. Commun. 1997, 21. (34) Kunieda, R.; Fujitsuka, M.; Ito, O.; Ito, M.; Murata, Y.; Komatsu, K. J. Phys. Chem. B 2002, 106, 7193. (35) Biju, V.; Sudeep, P. K.; George Thomas, K.; George, M. V.; Barazzouk, S.; Kamat, P. V. Langmuir 2002, 18, 1831. (36) George Thomas, K.; George, M. V.; Kamat, P. V. HelV. Chim. Acta 2005, 88, 1291. (37) Quaranta, A.; McGarvey, D. J.; Land, E. J.; Brettreich, M.; Burghardt, S.; Scho¨nberger, H.; Hirsch, A.; Gharbi, N.; Moussa, F.; Leach, S.; Go¨ttinger, H.; Bensasson, R. V. Phys. Chem. Chem. Phys. 2003, 5, 843. (38) Garcia-Rubio, L. H. Macromolecules 1987, 20, 3070. (39) (a) Toba, R.; Quintela, J. M.; Peinador, C.; Romań, E.; Kaifer, A. E. Chem. Commun. 2001, 857. (b) Ceroni, P.; Vicinelli, V.; Maestri, M.; Balzani, V.; Mu¨ller, W. M.; Mu¨ller, U.; Hahn, U.; Osswald, F.; Vo¨gtle, F. New J. Chem. 2001, 989. (40) Aoki, K. Electroanalysis 2005, 17, 1379.

Hosomizu et al. (41) Dubois, D.; Moninot, G.; Kutner, W.; Jones, M. T.; Kadish, K. M. J. Phys. Chem. 1992, 96, 7137. (42) (a) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. ReV. 1999, 99, 1665. (b) Grayson, S. M.; Frećhet, J. M. J. Chem. ReV. 2001, 101, 3819. (43) (a) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (b) Sano, M.; Oishi, K.; Ishi-i, T.; Shinkai, S. Langmuir 2000, 16, 3773. (c) Cassell, A. M.; Asplund, C. L.; Tour, J. M. Angew. Chem., Int. Ed. 1999, 38, 2403. (44) (a) Brettreich, M.; Burghardt, S.; Bo¨ttcher, C.; Bayerl, T.; Bayerl, S.; Hirsch, A. Angew. Chem., Int. Ed. 2000, 39, 1845. (b) Nakashima, N.; Ishii, T.; Shirakusa, M.; Nakanishi, T.; Murakami, H.; Sagara, T. Chem. Eur. J. 2001, 7, 1766. (c) Nakanishi, T.; Morita, M.; Murakami, H.; Sagara, T.; Nakashima, N. Chem. Eur. J. 2002, 8, 1641. (d) Nakanishi, T.; Ohwaki, H.; Tanaka, H.; Murakami, H.; Sagara, T.; Nakashima, N. J. Phys. Chem. B 2004, 108, 7754. (e) Nakanishi, T.; Schmitt, W.; Michinobu, T.; Kurth, D. G.; Ariga, K. Chem. Commun. 2005, 5982. (45) For the improvement of IPCE value, we added various porphyrin monomers and dimers with a different spacer into the fullerene dendrimers to make the composite clusters for the photoelectrochemical devices. However, the IPCE value was not improved relative to that of the present device. (46) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (47) Nakamaru, K. Bull. Chem. Soc. Jpn. 1982, 55, 2697.

Dendritic Effects on Structure and Photophysical and

Recommend Documents