Photofunctional Hybrid Nanocarbon Materials - The Journal of

Feature Article pubs.acs.org/JPCC

Photofunctional Hybrid Nanocarbon Materials Tomokazu Umeyama†,‡ and Hiroshi Imahori*,†,§ †

Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan PRESTO, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Institute for Integrated Cell-Material Sciences (WPI-iCeMS), Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan ‡

S Supporting Information *

ABSTRACT: Zero-, one-, and two-dimensional nanostructured carbon allotropes, i.e., fullerenes, single-walled carbon nanotubes (SWNTs), and graphenes, in combination with electron-donating conjugated molecules are promising building blocks for artificial photosynthesis and solar energy conversion. This feature article focuses on the fundamental aspects of covalently linked composites of porphyrins with fullerenes, SWNTs, and graphenes. The linkage structures between the porphyrin and nanocarbons have been found to exert a substantial impact on their interaction between the components in the ground and excited states. We also highlight recent developments of supramolecular nanocarbon hybrid materials of fullerenes and SWNTs, where SWNTs are utilized as scaffolds or wires of self-assembled fullerenes for photoelectrochemical devices and organic photovoltaics.

1. INTRODUCTION Harnessing solar energy is a potential choice to solve today’s energy issues. Much attention has been focused on photoinduced electron transfer (ET) processes in donor−acceptor assemblies in connection with natural and artificial photosynthesis.1−4 In this regard it has been well established that fullerenes, made up of alternating hexagons and pentagons of carbon atoms, are zero-dimensional (0D) excellent electron acceptors in artificial photosynthesis and solar energy conversion because of their small reorganization energies of ET, which result from the π-electron systems being delocalized over the spherical curved surface together with the rigid and confined structure of the aromatic π sphere.5−14 This has allowed fullerene-based donor−acceptor systems to exhibit fast photoinduced charge separation (CS) and charge shift as well as slow charge recombination (CR).5−14 Single-walled carbon nanotubes (SWNTs) and graphenes are even more fascinating considering their similarity as carbon allotropes and extended dimensionality as one- and twodimensional (1D and 2D) nanostructures. Namely, graphenes are emerging materials possessing a 2D planar atomic layer consisting of sp2-hybridized carbon,15,16 whereas SWNTs arise from rolling-up one graphene sheet to form the cylindrical shape.17 The structures and electronic properties of SWNTs and graphenes can also be modulated by covalent and noncovalent functionalization.18−21 Thus, one can envision that SWNTs and graphenes would be used instead of fullerenes in donor−acceptor systems. Specifically, a combination of electron-donating photoactive molecules with SWNTs or graphenes hold great promise as photofunctional nanomaterials for the construction of artificial photosynthetic and solar energy conversion systems. © 2012 American Chemical Society

On the other hand, intensive research has been conducted toward the development of low-cost solar cell technologies, among which organic photovoltaics (OPVs) are one of the representative examples.22−24 Nowadays, donor−acceptor bulk heterojunction (BHJ) solar cells have stimulated broad interest due to their potential advantages such as facile fabrication, low cost, light weight, and flexibility. To attain the high device performances, control of a film structure in the photoactive donor−acceptor mixed layer is essential because of the following reasons: (i) the domain size of the donor−acceptor network structures should be comparable to or even smaller than the exciton diffusion length (10−30 nm)25 so that the probability that an exciton reaches the donor−acceptor interface and dissociates into electrons and holes is high; (ii) for efficient electron and hole transport to respective electrodes, donor and acceptor molecules must form interpenetrating bicontinuous network nanostructures, inhibiting undesirable CR. Fullerenes and their derivatives have been ubiquitously employed as the electron-accepting component in BHJ solar cells, as rationalized by their small reorganization energies in ET.5−14 Highly controlled 1D and 2D nanostructures can be regarded as an ideal charge-transporting highway in the active layer of OPVs, and in particular, 1D SWNTs and 2D graphenes seem to fit the requirements. Several recent review papers have already highlighted the key findings of this intriguing area involving donor−fullerene or −SWNT hybrids5−14,26−33 and utilizations of SWNTs in OPVs.34−39 In this Feature Article, we focus on the Received: September 14, 2012 Revised: December 7, 2012 Published: December 10, 2012 3195

dx.doi.org/10.1021/jp309149s | J. Phys. Chem. C 2013, 117, 3195−3209

The Journal of Physical Chemistry C

Feature Article

fundamental aspects of covalently linked composites of porphyrins with fullerenes, SWNTs, and graphenes. The main reason for studying such covalently linked systems is to eliminate complex factors arising from diffusion in solutions as much as possible and to use the high energy of singlet excited states, of which the lifetime is usually too short, to undergo efficient intermolecular ET in solutions. In fact, the linkage structures between the porphyrin and nanocarbons have been found to exert a substantial impact on their interactions in the excited and ground states. In addition, we summarize a recent advancement in supramolecular nanocarbon hybrid materials of fullerenes and SWNTs, where the SWNT was utilized as scaffolds or wires of fullerene alignments, to seek for novel photoactive materials for photoelectrochemical devices and solar cells. The emphasis will be placed on the photophysical events that are essential for a better understanding of the photovoltaic properties of these nanocarbon composites.

2. PORPHYRIN−NANOCARBON LINKED SYSTEMS 2-1. Covalently Linked Porphyrin−Fullerene Systems. Porphyrins play essential roles in both light-harvesting and CS in natural and artificial photosynthesis.40−42 Intense Soret and moderate Q bands of porphyrins lead to efficient collection of visible light in the light-harvesting process. The large conjugated π system of porphyrins is also suitable for efficient CS because small reorganization energies of porphyrins in ET result in fast forward of ET and slowing of ET, as in the case of fullerenes. In this context, porphyrins have been frequently employed as donors in combination with fullerenes.5−14,26−33 A large number of porphyrin−fullerene linked systems have been designed and synthesized to mimic photosynthetic CS and CR. In porphyrin−fullerene linked dyads, efficient photoinduced ET occurs to yield the charge-separated state in competition with the decay of the excited states to the ground state. For instance, photoexcitation of ZnP-A-C60 with a rather longlength linear spacer (Figure 1) in polar solvents results in the occurrence of photoinduced ET, evolving all the excited states, that is, from the zinc porphyrin excited singlet state (1ZnP*) and the zinc porphyrin excited triplet state (3ZnP*) to C60 as well as from the zinc porphyrin to the C60 excited singlet state (1C60*) and the C60 excited triplet state (3C60*), creating the same charge-separated state, ZnP●+-A-C60●−.43−47 The energy diagram is shown in Figure 2 to illustrate the different relaxation pathways of photoexcited ZnP-A-C60. The total formation efficiency of the charge-separated state from the initial excited states in benzonitrile was estimated to be 99%. It is noteworthy that in polar solvents photoinduced CS from 1 ZnP* to C60 (∼1010 s−1) occurs in the Marcus top region, whereas CR from C60●− to ZnP●+ (∼106 s−1) takes place in the Marcus inverted region, which are typical characteristics of nonadiabatic ET where electronic coupling between the donor and acceptor is moderate or weak.1−4 In contrast, porphyrin−fullerene linked dyads with a shortlength spacer disclose different photophysical behavior. Let us exemplify the ultrafast photodynamics of a series of porphyrin− fullerene dyads, in which the separation distance between the porphyrin and C60 moieties is varied systematically at close proximity.48−50 More specifically, in ZnP-D-C60 the pyrrolidine ring of the C60 moiety is directly connected with the porphyrin ring at the meso position, whereas in ZnP-O-C60, ZnP-M-C60, and ZnP-P-C60 those of the C60 moieties are linked with the porphyrin moieties through the benzene ring at the ortho, meta, and para positions, respectively (Figure 1). Note that the

Figure 1. Structures of porphyrin−fullerene linked dyads.

Figure 2. Energy diagram for ZnP-A-C60.

calculated edge-to-edge distance (Ree) between the porphyrin and the C60 moieties for the optimized geometries increases in the order: ZnP-D-C60 (2.6 Å) < ZnP-O-C60 (3.4 Å) < ZnP-MC60 (3.7 Å) < ZnP-P-C60 (5.2 Å) ≪ ZnP-A-C60 (11.9 Å). The charge-transfer (CT) bands are observable for ZnP-D-C60 and ZnP-O-C60 in the visible-near-infrared absorption spectra, whereas no CT band is seen for ZnP-M-C60 and MP-P-C60. Time-resolved absorption spectral measurements revealed that the photoexcitation of ZnP-D-C60 in both benzonitrile and toluene results in formation of a new intermediate state, different from the locally excited chromophores and the complete charge-separated state, which can be assigned to an exciplex with partial CT. 1C60* also generates the same exciplex state. The exciplex of ZnP-D-C60 decays rapidly to the ground state (∼1010 s−1) without forming the charge-separated state. The strong interaction between the ZnP and the C60 moieties due to the small Ree value in ZnP-D-C60, as indicated by the observation of the strongest CT band in the ground state, leads to exclusive formation of the exciplex. This can be also 3196



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Groups. An approach to the covalent functionalization of SWNTs involves the reaction of the SWNTs with strong acids, usually mixtures of concentrated sulfuric and nitric acids.51,52 This oxidizing treatment yields shortened, uncapped SWNTs bearing oxygen-containing groups, such as carboxylates, at the open ends and defective sites of the sidewalls. Therefore, the carboxyl groups can be readily derivatized to acid chlorides by treatment with thionyl chloride and subsequently coupled to amines or alcohols. There are some pioneering works53,54 and following various studies26−33 on noncovalently linked porphyrin−SWNT composites. Meanwhile, the covalent linking of porphyrins to SWNTs was achieved by esterification/amidation of hydroxyl/ amino group appended porphyrins with the oxidized SWNT.55−58 Sun et al. presented the first porphyrin−SWNT linked composites where two different porphyrins bearing hydroxyl groups connected with long and short alkyl linkages are further covalently appended to SWNTs through esterification (H2P-L-SWNT and H2P-S-SWNT, respectively, Figure 5).55 The absorption spectra of H2P-L-SWNT and H2P-S-

rationalized by the fact that the energy of the exciplex is lower than that of the charge-separated state even in a polar solvent such as benzonitrile (Figure 3). Unlike ZnP-D-C60, the

Figure 3. Energy diagram for ZnP-D-C60.

photoexcitation of ZnP-O-C60 with slightly larger Ree value in benzonitrile results in the formation of the charge-separated state via the exciplex formation, which is higher in energy than the charge-separated state (Figure 4). Meanwhile, in toluene,

Figure 4. Energy diagram for ZnP-O-C60.

exclusive formation and decay of the exciplex occurs in ZnP-OC60, as seen in ZnP-D-C60. The photodynamical behavior of ZnP-M-C60 and ZnP-P-C60 is similar to that of ZnP-O-C60 as a result of further elongation of the separation distance between the donor and acceptor. An analogous trend was observed for the corresponding free-base porphyrin−C60 dyads.48−50 It should be noted here that the photodynamics of exciplex formation of ZnP-D-C60 and ZnP−O-C60 with the short separation distance is characterized by the extremely fast formation rate from the porphyrin second excited singlet state (S2ZnP*) and 1ZnP* due to the strong interaction between the porphyrin and C60 moieties. In the case of ZnP-D-C60, the exciplex formation from 1ZnP* occurs at an ultrafast time scale with a time constant of 160 fs, and that from S2ZnP* occurs even faster with a time constant less than 50 fs. The observed trends in the exciplex and CT dynamics indicate that both the separation distance between the porphyrin and C60 and the surrounding environment (i.e., solvent polarity) are important in determining the energy balance between the chargeseparated state and the exciplex (vide infra). 2-2. Covalently Linked Porphyrin−SWNT Systems. 22-1. Functionalization of Oxidized SWNTs Using Carboxyl

Figure 5. Structures of porphyrin−SWNT linked hybrids prepared using carboxylic acid at the ends and defect sites.

SWNT are similar to those of the porphyrin references, suggesting no significant interactions between the porphyrin and SWNT in the ground state. The fluorescence intensity of H2P-L-SWNT was decreased by ∼30% compared to the corresponding porphyrin reference, which was attributed to photoinduced energy transfer (EN) from the free-base porphyrin excited singlet state (1H2P*) to the SWNT framework. The possibility of photoinduced ET was ruled out owing to independent fluorescence quantum yield as a function of solvent polarity. Because of the long flexible linkage between the porphyrin and SWNT in H2P-L-SWNT, the porphyrin ring may take a close position to the nanotube surface or interact 3197



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Figure 6. Structures of ZnP-E-SWNT, ZnP-P-SWNT, and H2P-D-SWNT.

with the other porphyrin directly to quench 1H2P*. No fluorescence quenching in the spectrum of H2P-S-SWNT is not consistent with the plausible strong interaction between the porphyrin and SWNT through the short spacer, suggesting significant loss of the electronic structure in the covalently functionalized SWNT. Baskaran et al. reported an analogous porphyrin−SWNT conjugate with a slightly shorter spacer relative to H2P-SSWNT (H2P-SWNT, Figure 5).56 Upon excitation at 550 nm, efficient quenching (>95%) of the porphyrin emission at 650 and 725 nm was observed, as contrasted with Sun’s results. Although the authors attributed this photophysical behavior solely to ET from 1H2P* to SWNT via ester linkage, other processes such as photoinduced EN and exciplex formation should be considered. Recently, the ferrocene−porphyrin−SWNT (Fc-H2 PSWNT, Figure 5) triad was prepared by amidation reaction between oxidized SWNT and aminoporphyrin bearing an appended ferrocenyl substituent.58 The steady-state emission spectrum of Fc-H2P-SWNT showed efficient quenching (95%) in comparison with the corresponding porphyrin reference. In this case, the significant decrease of the fluorescence quantum yield of Fc-H2P-SWNT with increasing solvent polarity suggested that the quenching process is dominated by ET from 1H2P* to SWNT. From the results of time-resolved transient absorption spectra, the authors proposed the formation of the ferrocenium cation (Fc+) and SWNT radical anion (SWNT●−) pair in Fc-H2P-SWNT. The lifetime (τ) of the charge-separated state was estimated to be 62.9 μs in DMF, which was significantly increased compared to the estimated value of the corresponding porphyrin−SWNT reference (90%) relative to the corresponding porphyrin reference. An increase in the fluorescence quenching with increasing solvent polarity suggested photoinduced ET from 1H2P* to SWNT. From time-resolved nanosecond transient absorption measurements, the authors claimed formation of the charge-separated state, which decays with a lifetime of 57 ns in DMF.66 In contrast to the moderate fluorescence quenching of ZnP-E-SWNT62 and ZnP-PSWNT,65 the effective intramolecular quenching of H2P-DSWNT can be explained by the strong electronic coupling between the conjugated π-systems of the porphyrin and SWNT mediated by a through-bond interaction due to the short, rigid phenylene spacer (Ree = 5.8 Å). To shed light on the photoelectrochemical properties and excited state interactions of porphyrin−SWNT composites, we prepared SWNTs covalently functionalized with bulky porphyrin units utilizing both the direct aryl addition reaction to the sidewall and the amidation of carboxylic groups at terminal and defect sites (H2P-SWNT-H2P, Figure 7).68 The photoelectrochemical devices were fabricated using a standard three-electrode system consisting of the modified working electrode (FTO/SnO2/H2P-SWNT-H2P), a platinum wire counter electrode, and a Ag/AgNO3 reference electrode in 0.5 M LiI and 0.01 M I2 of acetonitrile solution as the electrolyte. The device revealed anodic electron flow with a maximum IPCE value of 5% at 400 nm, but the porphyrin absorption of the FTO/SnO2/H2P-SWNT-H2P electrode did not contribute to the photocurrent generation. In other words, only direct electron injection from the excited SWNT to the CB of the SnO2 electrode is responsible for the moderate photocurrent generation. This is in marked contrast with the 3199



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efficient quenching (93%) of 1H2P* by SWNT in H2P-SWNTH2P, which was observed in the steady-state fluorescence spectra.68 The photocurrent generation by the excitation of the porphyrin moieties would be expected if EN or ET from 1H2P* to SWNT occurs in H2P-SWNT-H2P. Accordingly, the EN or ET quenching mechanism can be excluded. As is the case of porphyrin−C60 linked dyads in close proximity,48−50 the evolution of an exciplex between the porphyrin excited singlet state and the SWNT and the subsequent rapid decay to the ground state without generating the charge-separated state are likely to occur due to the strong interaction between the porphyrin and SWNT through the short spacers (vide infra). This interpretation rationalizes the unusual photoelectrochemical behavior. The difference in the employed SWNT (HiPco SWNT with diameter of 0.8−1.2 nm for H2P-SWNT-H2P68 and arc-discharge SWNT with diameter of 1.4−1.7 nm for H2PD-SWNT66) may also have a profound effect on the electronic interaction between the porphyrin and SWNT in the excited and ground states. 2-2-3. Direct Addition of Porphyrin onto the Fullerene Peapod Sidewall. One of the fundamental approaches for controlling the electronic properties of SWNTs is the inner space doping of suitable-sized organic molecules like fullerenes.71 It is known that encapsulated fullerenes cause changes in the Fermi levels and band gap energies of SWNTs.72 When a suitable donor is combined with a fullerene-encapsulated SWNT, i.e., fullerene peapods or C60@SWNT, the enhanced electron-accepting character arising from CT interaction between C60 and SWNT in the ground state would be able to promote ET from the excited donor molecule to the fullerene peapod.73 To evaluate the effects of the fullerene encapsulation on the structure and photophysical properties, we designed and synthesized fullerene peapods covalently linked with zinc porphyrin by a short rigid phenylene spacer (Ree = 5.8 Å) (C60@SWNT-ZnP, Figure 8) as well as the

can be assigned to the ground state photobleaching due to the SWNT absorptions of M11 (the lowest transitions between van Hove singularities in the valence band (VB) and CB of the metallic SWNT) and S22 (the second lowest transitions in semiconducting SWNTs). The spectrum of p-SWNT-PhI also exhibited a similar negative signal. Additionally, a broad and featureless positive absorption band in the visible region emerged in the spectra of p-SWNT-ZnP with τ of 1073 ps. This broad and featureless absorption in the visible region can be assigned to the exciplex state comprised of the zinc porphyrin and SWNT, which is consistent with the photoelectrochemical properties of the FTO/SnO2/H2P-SWNT-H2P device exhibiting no photocurrent generation from the porphyrin excitation (vide supra).49,50,68 The absorption changes recorded upon the excitation of C60@SWNT-ZnP at 420 nm differ from those of p-SWNTZnP.70 In addition to the negative signal of the ground state photobleaching of the C60@SWNT moiety, the exciplex absorption appears, but the lifetime (τ = 24 ps) is much shorter than that of p-SWNT-ZnP (τ = 1073 ps). Furthermore, an additional long-lived component (τ > 2 ns) with two minima at 560 and 600 nm emerges. Importantly, the third component exhibits weak positive absorption in the 650−750 nm region. The third component was assigned to the charge-separated state considering the similarity between this band and that of ZnP●+. In contrast with p-SWNT-ZnP, where the resultant exciplex decays without forming the charge-separated state, C60@SWNT-ZnP generates the ZnP●+ and the fullerene peapod radical anion after the exciplex formation. The energy level of the charge-separated state would become lower than that of the exciplex state by the inclusion of the fullerene molecules. It should be noted here that no clear signals for C60●− at 1080 nm are observable. The lack of C60●− detection may be rationalized by no occurrence of the consecutive ET from the exterior frame of the SWNT to the encapsulated C60 or low molar absorption coefficient of C60●− at 1080 nm, making it difficult to detect the C60●− absorption. These photodynamical results are in good agreement with their photoelectrochemical properties. The FTO/SnO 2/C 60 @ SWNT-ZnP device exhibited significant photocurrent generation from the porphyrin absorption, whereas the FTO/SnO2/ SWNT-ZnP did not show such photoresponse. However, the direct decay of the exciplex state to the ground state may compete with the formation of the charge-separated state, leading to a considerable decrease in the formation efficiency of the charge-separated state. The differences in the photodynamics of C60@SWNT-ZnP and p-SWNT-ZnP highlight the effect of fullerene encapsulation on the electronic communications between SWNT and ZnP in the excited state. Our results exemplify that the encapsulation of C60 into the SWNT inner space have significant impacts on the excited-state interactions between porphyrins and SWNTs. The results obtained here demonstrate that the inner space doping of the SWNT will have considerable merit for tuning the electronic properties of SWNTs in the hybrid materials with photoactive molecules for the applications in artificial photosynthesis and solar energy conversion. 2-3. Covalently Linked Porphyrin−Graphene Systems. There have been several methods for the preparation of graphene, mechanical exfoliation of graphite,75 epitaxial growth,76 chemical vapor deposition (CVD), 77 solvent dispersion of graphite,78 and reduction of graphene oxide

Figure 8. Structure of C60@SWNT-ZnP.

corresponding porphyrin-linked SWNT reference without fullerenes (p-SWNT-ZnP).70 Note that SWNTs with diameters of 1.3−1.6 nm produced by the direct-injection-pyrolytic synthesis (DIPS)74 method are employed. The absorption spectrum of C60@SWNT-ZnP in DMF revealed that the Soret band was broadened and red-shifted by 3 nm compared to the porphyrin reference, suggesting significant interaction between the porphyrin and C60@SWNT in the ground state. The transient absorption component spectrum of p-SWNTZnP showed a negative signal in the full range of the measurement (500−1100 nm) with τ = 0.3 ps.70 This signal 3200



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Figure 9. Structures of porphyrin−graphene linked hybrids.

(GO),79,80 generated by oxidation of graphite using strong oxidants.81 Among them, chemical reduction of GO is a feasible technique for access to graphene-based materials in a large scale. Although GO is electrically insulating due to the disrupted sp2 bonding networks, the reduction treatments can restore the π-network significantly and recover electronic and optical properties to some extent including the electrical conductivity.82 There are some examples of noncovalently linked porphyrin−graphene composites.83−89 Meanwhile, covalently linked GO and porphyrin composites were prepared by amidation90−92 and esterification93 of carboxylates as in the case of oxidized SWNTs. Although porphyrin−CCG linked systems seem to be a better model system to understand the interactions between the linked porphyrins and graphene in the excited and ground states, the examples of covalently linked porphyrin−CCG composites have been limited. The aryl addition reactions of diazonium compounds and cycloadditions of azomethine ylides are also applicable to the πnetwork of the graphene surface94,95 and have been utilized to fabricate the covalently linked porphyrin−graphene hybrids.96−99 Zhang et al. prepared the first porphyrin−graphene covalent composite (ZnP-T-CCG, Figure 9) by the two-step functionalization procedure, i.e., the preparation of prefunctionalized CCG with p-ethynylphenyl groups by the direct aryl addition reaction, and subsequent click reactions with azideterminated porphyrin.96 This composite has a relatively long, tilted phenylene−triazole−phenylene spacer between the porphyrins and CCG. The absorption spectrum of ZnP-TCCG in DMF revealed the broadening and redshift (3 nm) of the Soret band relative to the porphyrin reference, suggesting the significant interaction between the porphyrin and CCG in the ground state. Upon excitation at 430 nm, the porphyrin fluorescence of ZnP-T-CCG at 610 and 660 nm in DMF was quenched by ca. 94% relative to the porphyrin reference. The effective emission quenching of the porphyrin in the CCG

composite is indicative of strong electronic interaction between ZnP* and CCG. The authors fabricated photoelectrochemical devices in a three-electrode configuration using ITO electrodes coated with ZnP-T-CCG and nonfunctionalized CCG films as the working electrodes.96 The device with the ITO/ZnP-TCCG electrode showed reversible moderate photocurrent response (∼310 nA cm−2) when the light was switched on and off under white light illumination (500 W xenon), whereas the device with the ITO/CCG electrode produced a negligibly small photocurrent. The photocurrent generation efficiency and photocurrent action spectrum as well as photophysical properties of ZnP-T-CCG were not noted.96 Feringa and co-workers synthesized the porphyrin−graphene covalent composite by the one-step cycloaddition reaction using azomethine ylide with free-base porphyrin units (H2P-Pgraphene, Figure 9).97 They prepared graphene by solvent (odichlorobenzene) dispersion of graphite because it gives the advantage of retaining the intrinsic properties of graphene as well as maintaining the dispersibility of graphene in certain solvents.78 The absorption spectrum of H2P-P-graphene in DMF displayed a broadening and redshift (2 nm) of the Soret band relative to the porphyrin reference, indicating significant interaction between the porphyrin and graphene in the ground state. Steady-state fluorescence of the porphyrin in H2P-Pgraphene was quenched by 93% compared to the porphyrin reference, suggesting considerable interaction between the porphyrin and graphene in the excited state. For H2P-Pgraphene the fluorescence decay was found to be biexponential with time constants of 99%). The fluorescence quenching of the covalently linked porphyrin by CCG in ZnP-D-CCG is more intensive than in ZnP-T-CCG and H2P-P-graphene. The direct attachment of the porphyrin moiety onto the π-conjugated planar surface of CCG with short rigid phenylene spacer may facilitate the 1ZnP* quenching by CCG in ZnP-D-CCG. To shed light on the ultrafast photodynamical process, the femtosecond pump−probe transient absorption measurements were carried out for IPh-D-CCG and ZnP-D-CCG with a laser excitation at 420 nm where both the porphyrin and CCG of ZnP-D-CCG were excited.98 The transient absorption decay component spectrum of IPh-D-CCG displayed broad negative absorption in the visible region with τ = 0.4 ps. This was attributed to the CCG excited state. According to a report on the photodynamics of graphene films made by the CVD method,100 this component originates from the carrier relaxation process governed by carrier-optical phonon scattering in the CCG moiety. This result demonstrates the similarity in photodynamics between the CCG and the high-quality graphene produced by CVD. In contrast, two components (τ = 0.3, 38 ps) are reasonably derived from ZnP-D-CCG. Given the analogy of the absorption and lifetime, the fast decay component could be assigned to the CCG excited state.98 On the other hand, the slower decaying component has distinct negative peaks at ∼560 and ∼600 nm with the broad and structureless positive absorption signals. The lifetime of this component (38 ps) was remarkably shorter than that of the excited singlet state of the porphyrin reference (2.0 ns). No characteristic absorption arising from ZnP●+ was observed in the region of 600−700 nm,101 ruling out the possibility of ET from the 1ZnP* to the CCG in ZnP-D-CCG. We can speculate that this component results from 1ZnP*, and EN occurs from 1ZnP* to CCG with a time constant of 38 ps, followed by the extremely fast relaxation of the CCG excited state (0.3 ps) to the ground state. Alternatively, the slower component may be assigned to the exciplex between the porphyrin and CCG considering the same phenylene spacer in ZnP-D-CCG, p-SWNT-ZnP, and H2P-SWNT-H2P (vide supra).68,70 To evaluate the photoelectrochemical properties of IPh-DCCG and ZnP-D-CCG, thin films were fabricated onto the nanostructured SnO2 electrodes by the electrophoretic method (denoted as FTO/SnO2/IPh-D-CCG or ZnP-D-CCG).98 Photoelectrochemical measurements were performed in acetonitrile containing 0.5 M LiI and 0.01 M I2 with the FTO/SnO2/IPh-D-CCG or FTO/SnO2/ZnP-D-CCG as a working electrode, a Pt wire counter electrode, and a reference electrode. The device with the FTO/SnO2/IPh-D-CCG

3. SWNTS AS SCAFFOLDS OR WIRES OF FULLERENE MOLECULES 3.1. SWNTs as Scaffolds of Fullerenes. On the basis of a series of our studies and others on the covalently linked composites of porphyrins with fullerenes, SWNTs, and graphenes, fullerenes have been found to be most suitable as an acceptor for photoinduced CS. Meanwhile, SWNTs have a highly controlled 1D nanostructure with excellent chargetransporting properties.17 Therefore, supramolecular 1D arrangement of fullerene molecules along the SWNT as a template is an attractive strategy to attain both efficient photoinduced CS and charge transportation in artificial photosynthetic systems including photoelectrochemical devices and OPVs.103 Although there have been some reports on the endohedral functionalization of SWNTs with fullerene molecules by the covalent linkages57,104,105 and the pyrenemediated attachments,106−108 it seems impractical to cover the surface of SWNTs densely with mono- or multilayers of fullerenes by these methods. Interactions between the inner surface of SWNTs and the outer surface of densely encapsulated fullerenes were extensively demonstrated by fullerene peapods,71,72 whereas the examples of supramolecular adsorption of fullerene molecules onto the outer surface of the SWNT by π−π interaction have been limited. Takaguchi et al. developed fullerodendrons (Figure 10) that can disperse SWNTs well in both water and organic solvents.109 C60 molecules at the focal point of the dendron interact with SWNTs and cover the surface tightly, rendering the soluble branched moieties outward. The coaxial fullerene−SWNT hybrid causes photoinduced CS from the SWNT to the C60 under visible light irradiation.110 The charge-separated state of the SWNT−fullerodendron composite is capable of transferring the electron to methyl viologen (MV2+), yielding MV●+. As a result, MV●+ is accumulated to yield an electron pool. In addition, a ternary nanohybrid photocatalyst consisting of SWNT−fullerodendron and SiO2 exhibited a high activity for hydrogen evolution from water under visible light irradiation.111 3202



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Figure 10. Structure of fullerodendron.

Figure 12. Formation of the fullerene−f-SWNT composite.

A different supramolecular approach has been used by Nakanishi and co-workers to construct a hybrid of fullerene with the trialkoxyphenyl group (C60(OC20)3, Figure 11) and

fullerenes and f-SWNT in the mixed solvent by lyophobic and π−π interactions leads to the formation of supramolecular 1D arrangement of fullerene molecules on the surface of fSWNT. In the case of the C60−f-SWNT composite, the desirable composite of f-SWNT covered with a few layers of C60 molecules was formed in addition to spherical-shaped C60 clusters with a size of 500−1000 nm containing f-SWNT and spherical-shaped C60 clusters with a size of 150−250 nm.113 However, the composite of f-SWNT and C70 yielded the selective 1D arrangement of C70 molecules on the surface of fSWNT.114,115 The ellipsoidal shape and large size of C70 relative to C60 may contribute to the enhanced π−π interaction with the sidewall of f-SWNT, leading to the exclusive formation of the composite cluster. To evaluate the charge-carrier mobility (μ) and excited state interactions of the C70−f-SWNT composite as well as single components C70 and f-SWNT, we measured the flashphotolysis time-resolved microwave conductivity (TRMC) of the electrophoretically deposited films on FTO/SnO2 electrodes (denoted as FTO/SnO2/C70−f-SWNT, FTO/SnO2/C70, and FTO/SnO2/f-SWNT).114,115 Upon exposure to a laser pulse with an excitation wavelength of 355 nm, all samples revealed a rise of the transient conductivity (ϕΣμ), in which ϕ is the quantum efficiency of CS and Σμ is the sum of the mobilities of all the transient-charge carriers. The major charge carriers were found to stem from electrons for all films. The Σμ value (3.2 cm2 V−1 s−1) of the FTO/SnO2/f-SWNT electrode is the highest, demonstrating the potentially superior electrontransporting property of f-SWNT. Notably, a 26% increase in the Σμ value (2.4 cm2 V−1 s−1) and 13% increase in the ϕ value (3.5%) of the FTO/SnO 2 /C 70 −f-SWNT electrode are discernible in comparison with those of the FTO/SnO2/C70 electrode. The rise profile of the transient conductivity for the FTO/SnO2/C70−f-SWNT electrode was close to that for the FTO/SnO2/C70 electrode, both reaching the conductivity maxima within 0.4 μs, while that for the FTO/SnO2/f-SWNT electrode revealed a slower rise component. Similarity in the photoresponse behavior of the TRMC signals for the FTO/ SnO2/C70-f-SWNT and FTO/SnO2/C70 electrodes implies that a large majority of the photocarriers in the C70−f-SWNT composites are generated by excitation of C70. On the other hand, decay kinetics of the conductivity transients for the FTO/

Figure 11. Structure of C60(OC20)3.

the SWNT.112 Driven by the interaction between the long alkyl chain of C60(OC20)3 and the SWNT probably due to good lattice matching between the SWNT surface and all-trans conformation of the −OC20H41 units, SWNTs were almost debundled and dispersed in organic solvents. Namely, the interaction between the C60 moiety and SWNT surface is not significant in this hybrid system, in contrast with the fullerodendron system.109−111 The photoelectrochemical device was fabricated in a three-electrode configuration using FTO electrodes coated with the hybrid C60(OC20)3−SWNT film (FTO/C60(OC20)3−SWNT) as the working electrodes.112 The device showed anodic photocurrent response, and a moderate maximum IPCE of ca. 5% at 400 nm was obtained. The value is much higher than those of the devices with the FTO/ C60(OC20)3 or FTO/SWNT electrodes under the same conditions. Photoinduced CS within the C60(OC20)3−SWNT hybrid was probed by a series of spectroscopic techniques. Steady-state and time-resolved fluorescence studies suggested efficient quenching of 1C60* in the assembly. More direct evidence for the photoinduced ET was obtained from the nanosecond transient absorption studies, where C60●− was observed upon laser excitation. The spectroscopic results provide clues for the future design of nanocarbon hybrid systems for more efficient photovoltaic applications. We prepared nanocarbon composites of fullerene (i.e., C60 and C70) and highly soluble, chemically functionalized singlewall carbon nanotubes (f-SWNT) by the rapid injection of a poor solvent (i.e., acetonitrile) into a mixed solution of fullerene and f-SWNT in a good solvent (i.e., o-dichlorobenzene) (Figure 12).113−115 Here, self-assembly of pristine 3203



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SnO2/C70−f-SWNT electrode exhibited a pseudo-first- to second-order profile, which is different from those of the FTO/SnO2/C70 and FTO/SnO2/f-SWNT electrodes.114,115 The second-order decay profile was observed only for the FTO/SnO2/C70−f-SWNT electrode. All these features can be explained by the occurrence of ET from C70●− to f-SWNT, followed by bulk recombination of charge carriers during electron transportation through f-SWNT, in addition to electron hopping on C70 arrays due to arrangement of C70 on the sidewalls of f-SWNTs in the FTO/SnO2/C70−f-SWNT electrode. This result is in marked contrast with the systems of Takaguchi and Nakanishi, where ET from the SWNT to the C60 molecules occurs.109−112 The differences in the used SWNTs (i.e., functionalized vs pristine), fullerenes (i.e., pristine vs functionalized), and the layer structures of fullerenes on SWNTs (i.e., multilayer vs monolayer) may influence the excited state interactions between fullerenes and SWNTs. The photocurrent action spectra of the FTO/SnO2/C70−fSWNT, FTO/SnO2/f-SWNT, and FTO/SnO2/C70 devices were measured.114,115 The maximum IPCE value (26%) of the FTO/SnO2/C70−f-SWNT device at 400 nm is 2.6 times that (10%) of the FTO/SnO2/C70 device and 10 times larger than that (2.6%) of the FTO/SnO2/f-SWNT device. The IPCE value is the highest one ever reported for analogous SWNTbased photoelectrochemical devices.116−120 Here, photocurrent generation (Figure 13) is initiated by photoinduced ET from

Figure 14. Structure of Pc-C60.

the basis of the atomic force microscopy (AFM) studies they proposed monolayer formation of Pc-C60 around the SWNT where Pc-C60 adopts a radial, upright position with the C60 moiety interacting with the SWNT. Although the optical and photophysical properties of this fascinating supramolecular assembly are not reported, its application in the field of nanoelectronics is of interest. Huang et al. discovered that GO can act as a surfactant sheet to disperse graphite and carbon nanotubes in water, whereas fullerene powders cannot be dispersed very well by GO.122−124 Recently, they extended the work to create ternary nanocarbon hybrids of fullerenes, SWNTs, and graphenes.125,126 Stable dispersions of C60-SWNT-GO can be successfully created by sonicating SWNTs and C60 in aqueous GO solution. While the hybrid dispersion exhibits the characteristic absorption bands of each component, the overall absorption spectrum was redshifted in regard to the individual constituents. This indicates intimate interaction in the ground state between these carbon nanomaterials. High-resolution TEM studies revealed that all SWNTs have C60 coatings on the surface with thicknesses of 1−2 nm, which is dispersed by GO in the aqueous media (Figure 15). The ternary complex dispersions can be readily

Figure 13. Photocurrent generation diagram for the FTO/SnO2/C70− f-SWNT device. We used the HiPco SWNT which is a mixture of semiconducting (∼70%) and metallic (∼30%) carbon nanotubes. Most of the semiconducting SWNTs including the most abundant SWNT (8,6) satisfy the requirement for exothermic ET from the fullerene radical anion to the CB of the semiconducting SWNTs, thereby contributing the photocurrent generation.

Figure 15. Schematic image of C60-SWNT-GO.

used for spin coating to make smooth thin films with peak-tovalley roughnesses of only a few nanometers, making it possible to use them as the active layers in OPV devices. To construct the OPV devices, C60-SWNT-GO thin film was s pi n - c a s t o n t o t h e I T O / P E D O T : P S S (p o l y ( 3 , 4 ethylenedioxythiophene):poly(styrenesulfonate)) electrode. Thermal annealing at 150 °C yielded the partially reduced GO (rGO). Next, an electron-transporting layer of C60 (e-C60) and Al electrodes were thermally evaporated and deposited to make the device (ITO/PEDOT:PSS/C60-SWNT-rGO/e-C60/ Al).125,126 The device displayed an unprecedented photovoltaic response for devices using nanocarbon-based materials as the active layer, with a short circuit current (JSC) of 1.23 mA cm−2, an open circuit voltage (VOC) of 0.59 V, and a fill factor (FF) of 0.29, giving rise to a power conversion efficiency (PCE) of 0.21% with a maximum IPCE of more than 35% at 400 nm. Furthermore, the PCE was significantly improved to 0.85% by replacing C60 with better absorber C70 (JSC = 4.95 mA cm−2, VOC = 0.59 V, FF = 0.29).126 The authors attributed the

the iodide ion in the electrolyte to the C70 excited states, and C70●− injects electrons into the CB of f-SWNT. The excellent electron mobility of f-SWNT, as revealed by the TRMC measurements, facilitates the electron flow toward the SnO2 electrode. Additionally, C70●− injects electrons into the CB of the SnO2 by electron hopping through the C70 arrays on the SWNT sidewalls. These results will provide a basic clue for the design of nanocarbon-composite-based molecular devices including OPVs. The noncovalent interaction between the SWNT and fullerene enables a phthalocyanine−C60 linked dyad (Pc-C60, Figure 14) to self-organize on the surface of the SWNT.121 When drop-cast on a silicon oxide surface on which the SWNT is grown by catalyst-assisted chemical vapor deposition (SiO2/ SWNT), Pc-C60 is able to self-organize on SiO2/SWNT. On 3204



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signals implies that a large majority of the photocarriers in the H2P-C60−f-SWNT composite are generated by the excitation of H2P-C60, that is, photoinduced CS between the porphyrin and C60. Therefore, the improved Σμ value of the FTO/SnO2/H2PC60−f-SWNT electrode, which is almost the same as that of the FTO/SnO2/f-SWNT electrode, can be interpreted by the occurrence of charge shift from the resulting C60●− to f-SWNT, followed by bulk recombination of charge carriers during efficient electron transportation through f-SWNT. In accordance with the f-SWNT wiring, the maximum IPCE value (22%) of the FTO/SnO2/H2P-C60−f-SWNT device is twice as large as that of the FTO/SnO2/H2P-C60 device. These results unambiguously corroborate that electric communication between the D−A nanoaggregates is enhanced remarkably by the SWNT wiring. The mechanism of a photocurrent generation for the FTO/ SnO2/H2P-C60−f-SWNT device can be illustrated as in Figure 17.102 Photocurrent generation is initiated by the photoinduced

improved photovoltaic output characteristics to the presence of numerous donor (SWNT)−acceptor (C60) interfaces distributed throughout the photoactive layer. Photogenerated excitons can thus undergo spontaneous dissociation and subsequent separation along the bicontinuous BHJ, thus improving the overall photovoltaic performance. Although the investigations on photodynamics are necessary to elucidate thoroughly the excited state interactions in the ternary composite, these results suggest the possibility of making a solution processed all-carbon solar cell. 3.2. SWNT as a Wire of Fullerene−Porphyrin Microcrystalline. Charge-transporting properties of organic thin films have been found to be crucial in the device performances. It is well-known that charge transport is limited by grain boundaries in the films as well as molecular arrangements within the grains. Therefore, a new method enhancing electrical communication between the grains as well as modulating the arrangements within the grains is essential to improve device performances. To address these issues we focused on the wiring effect of the SWNT on the charge-transporting properties. We attempted to link the microcrystalline of H2P-C60 prepared by the rapid injection method with f-SWNT to enhance the electric communication (Figure 16).102 Namely, initial self-

Figure 17. Photocurrent generation diagram for the FTO/SnO2/H2PC60−f-SWNT device.

ET from 1H2P* to the C60 moiety. Then, the C60 arrays mediate electrons to the CB of f-SWNT. ET from C60●− to fSWNT is energetically favorable and demonstrated by results of the TRMC measurements. In addition, intimate contact between f-SWNT and the microcrystalline of H 2 P-C 60 promotes the electron mediation from C60●− to f-SWNT. The superb electron mobility (3.2 cm2 V−1 s−1) of the f-SWNT facilitates the electron flow toward the SnO2 electrode by electrically wiring the microcrystalline of H2P-C60. On the other hand, the porphyrin arrays shift holes until the oxidized porphyrin accepts electrons from I−/I3− redox couple to regenerate the initial state. Finally, the electrons injected into the CB of the SnO2 nanocrystallines are driven to the counter electrode via an external circuit to regenerate the I3−/I− redox couple. We believe that supramolecular self-assembly of D−A linked molecules with molecular wires like SWNTs will be a highly promising method to achieve excellent device performances in OPVs and organic transistors.

Figure 16. Schematic representation of self-assembly processes of H2P-C60 with f-SWNT onto the FTO/SnO2 electrode. Step 1: rapid injection of poor solvent. Step 2: electrophoretic deposition.

assembly of H2P-C60 with f-SWNT in the mixed solvent (i.e., odichlorobenzene and acetonitrile) leads to the formation of H2P-C60−f-SWNT ternary composites and subsequent electrophoretic deposition of the H2P-C60−f-SWNT composite onto a FTO/SnO2 electrode to give the deposited electrode (denoted as FTO/SnO2/H2P-C60−f-SWNT). The TRMC measurement on the FTO/SnO2/H2P-C60-fSWNT electrode exhibited 1 order of magnitude higher transient conductivity than that of the FTO/SnO2/H2P-C60 electrode to yield a Σμ value of 3.1 cm2 V−1 s−1,102 which is comparable to the value (3.2 cm2 V−1 s−1)114 of the FTO/ SnO2/f-SWNT electrode without H2P-C60. Note that the rise profile of the transient conductivity for the FTO/SnO2/H2PC60−f-SWNT electrode is different from that for the FTO/ SnO2/f-SWNT electrode but close to that for the FTO/SnO2/ H2P-C60 electrode, reaching the conductivity maxima within 1 μs. Similarity in the photoresponse behavior of the TRMC

4. CONCLUSIONS This Feature Article has overviewed the recent advancement in the photophysical and photoelectrochemical properties of covalently linked composites of porphyrins with nanocarbon materials, i.e., fullerenes, SWNTs and graphenes, and supramolecular nanocarbon hybrid materials of fullerenes and 3205



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Biographies

SWNTs, where the SWNTs were utilized as scaffolds or wires for the self-assembled fullerene molecules. Our comprehensive studies revealed that the linker structures in the porphyrin−nanocarbon covalent composites have a large impact on their optical and photophysical properties. Specifically, moderate length-spacers between the donor and acceptor are essential to produce a long-lived charge-separated state efficiently. As such fast decay of photoinduced CT states (i.e., exciplex state) to the ground state has been often observed in not only simple donor−acceptor linked systems48−50 but also a more complex dye-semiconducting electrode interface for dye-sensitized solar cells101,127,128 and a donor−acceptor mixed interface for BHJ solar cells,129,130 integrated views and interpretation on the undesirable unique phenomena should be developed to give us a universal guideline on efficient photoinduced CS even without losing some fraction of the charge-separated state. In terms of photoinduced CS fullerenes have been found to be excellent acceptors relative to SWNTs and graphene, whereas SWNTs and graphene exhibit superior charge-transporting properties. For efficient photoinduced CS, optimization of the electronic coupling in the porphyrin− SWNT or −graphene composites is far behind that in the porphyrin−fullerene composites, but therefore there is plenty of room for improvement. Although fullerenes are “molecules” that can be structurally identified by the conventional methods for organic synthetic chemistry such as NMR and XRD, the accurate structures of covalently modified SWNTs and graphenes cannot be determined at the molecular level due to their huge molecular weights and heterogeneous entity. The characterization by the state-of-the-art transmission electron microscopy (TEM) with ultrahigh resolution will provide information on the structure such as the linking position (i.e., basal surface, defect sites, or edges) and the position relationship between the attached porphyrins. Utilization of model systems for SWNTs and graphenes with defined structures131,132 would be also beneficial. On the other hand, the supramolecular hybridization of SWNTs with pristine fullerenes or their derivatives showed a synergetic effect that presents new opportunities for the development of novel photoactive materials with fascinating nanosized structures. Interestingly, SWNTs can act as both an electron acceptor and donor in the SWNT−fullerene composites, as predicted from the positions of CB and VB in semiconducting SWNTs. More studies are necessary to fully understand and control the interactions between the two different carbon allotropes, e.g., the utilization of SWNTs sorted by chiralities.133 We expect to bring out the full potential of nanocarbon materials for the realization of a further boost in artificial photosynthetic systems and diverse devices including organic solar cells.

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Hiroshi Imahori was born in Kyoto, Japan in 1961. He completed his doctorate in organic chemistry at Kyoto University. From 1990 to 1992, he was a postdoctoral fellow at the Salk Institute for Biological Studies, USA. In 1992, he became an Assistant Professor at ISIR, Osaka University. In 1999, he moved to the Graduate School of Engineering, Osaka University, as an Associate Professor. Since 2002, he has been a Professor of Chemistry, Graduate School of Engineering, Kyoto University. He received the Japanese Photochemistry Association Prize (2004), JSPS Prize (2006), CSJ Award for Creative Work (2006), Tokyo Techno Forum 21 Gold Medal Prize (2007), Osaka Science Prize (2007), and NISTEP Researcher Award (2007). His current interests involve artificial photosynthesis, organic solar cells, organic functional materials, and drug delivery systems. To date, Hiroshi Imahori has written more than 250 original papers.

Tomokazu Umeyama was born in 1976 and studied chemistry at Kyoto University. He received his BS (1999), MS (2001), and PhD (2004) in polymer chemistry under the guidance of Prof. Y. Chujo. He was also a fellow of the Japan Society for the Promotion of Science (JSPS) in 2003−2004. Then he moved to the Department of Molecular Engineering in the same institute and has been an Assistant Professor in the group of Prof. Imahori. His current interests involve covalent and noncovalent functionalization of carbon nanotubes, preparation of nanocarbon composite materials, and their application to photoelectrochemical devices and cells.

ASSOCIATED CONTENT

* Supporting Information S

References with full list of authors. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS The authors are deeply indebted to the work of all collaborators and co-workers whose names are listed in the references (in particular, Prof. H. Lemmetyinen, Prof. N. V. Tkachenko, Prof. S. Seki). H.I. thanks Grants-in-Aid (MEXT, Japan, No. 21350100 to H.I.), Strategic Japanese-Finnish Cooperative Program (JST), Advanced Low Carbon Technology Research

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 3206



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(42) Imahori, H. J. Phys. Chem. C 2004, 108, 6130−6143. (43) Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem. Soc. 2000, 122, 6535−6551. (44) Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 325−332. (45) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Am. Chem. Soc. 2001, 123, 2571−2575. (46) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607− 2617. (47) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 6617−6628. (48) Tkachenko, N. V.; Rantala, L.; Tauber, A. Y.; Helaja, J.; Hynninen, P. H.; Lemmetyinen, H. J. Am. Chem. Soc. 1999, 121, 9378−9387. (49) Kesti, T. J.; Tkachenko, N. V.; Vehmanen, V.; Yamada, H.; Imahori, H.; Fukuzumi, S.; Lemmetyinen, H. J. Am. Chem. Soc. 2002, 124, 8067−8077. (50) Tkachenko, N. V.; Lemmetyinen, H.; Sonoda, J.; Ohkubo, K.; Sato, T.; Imahori, H.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 8834− 8844. (51) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Iverson, T.; Shelimov, K.; Huffman, C. B.; et al. Science 1998, 280, 1253−1256. (52) Chen, J.; Hamon, M. A.; Hu, H.; Chen, Y.; Rao, M. A.; Eklund, P. C.; Haddon, R. C. Science 1998, 282, 95−98. (53) Kataura, H.; Maniwa, Y.; Abe, M.; Fujiwara, A.; Kodama, T.; Kikuchi, K.; Imahori, H.; Misaki, Y.; Suzuki, S.; Achiba, Y. Appl. Phys. A: Mater. Sci. Process. 2002, 74, 349−354. (54) Murakami, H.; Nomura, T.; Nakashima, N. Chem. Phys. Lett. 2003, 378, 481−485. (55) Li, H. P.; Martin, R. B.; Harruff, B. A.; Carino, R. A.; Allard, L. F.; Sun, Y. P. Adv. Mater. 2004, 16, 896−900. (56) Baskaran, D.; Mays, J. W.; Zhang, X. P.; Bratcher, M. S. J. Am. Chem. Soc. 2005, 127, 6916−6917. (57) Giordani, S.; Colomer, J. F.; Cattaruzza, F.; Alfonsi, J.; Meneghetti, M.; Prato, M.; Bonifazi, D. Carbon 2009, 47, 578−88. (58) Zhao, H.; Zhu, Y.; Chen, C.; He, L.; Zheng, J. Carbon 2012, 50, 4894−4902. (59) Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. J. Am. Chem. Soc. 2002, 124, 760−761. (60) Bahr, J. L.; Yang, J.; Kosynkin, D. V.; Bronikowski, M. J.; Smalley, R. E.; Tour, J. M. J. Am. Chem. Soc. 2001, 123, 6536−6542. (61) Campidelli, S.; Sooambar, C.; Lozano Diz, E.; Ehli, C.; Guldi, D. M.; Prato, M. J. Am. Chem. Soc. 2006, 128, 12544−12552. (62) Arai, T.; Nobukuni, S.; Sandanayaka, A. S. D.; Ito, O. J. Phys. Chem. C 2009, 113, 14493−14499. (63) Palacin, T.; Khanh, H. L.; Jousselme, B.; Jegou, P.; Filoramo, A.; Ehli, C.; Guldi, D. M.; Campidelli, S. J. Am. Chem. Soc. 2009, 131, 15394−15402. (64) Ho, K. H. L.; Rivier, L.; Jousselme, B.; Jegou, P.; Filoramo, A.; Campidelli, S. Chem. Commun. 2010, 127, 727−733. (65) Das, S. K.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; Zandler, M. E.; Ito, O.; D’Souza, F. Chem.Eur. J. 2012, 18, 11388−11398. (66) Guo, Z.; Du, F.; Ren, D.; Chen, Y.; Zheng, J.; Liu, Z.; Tian, J. J. Mater. Chem. 2006, 16, 3021−3030. (67) Cheng, F.; Adronov, A. Chem. Mater. 2006, 18, 5389−5391. (68) Umeyama, T.; Fujita, M.; Tezuka, N.; Kadota, N.; Matano, Y.; Yoshida, K.; Isoda, S.; Imahori, H. J. Phys. Chem. C 2007, 111, 11484− 11493. (69) He, L.; Zhu, Y.-Z.; Zheng, J.-Y.; Ma, Y. F.; Chen, Y. S. J. Photochem. Photobiol. A 2010, 216, 15−23. (70) Umeyama, T.; Mihara, J.; Hayashi, H.; Kadota, N.; Chukharev, V.; Tkachenko, N. V.; Lemmtyinen, H.; Yoshida, K.; Isoda, S.; Takano, M.; et al. Chem. Commun. 2011, 47, 11781−11783. (71) Smith, B. W.; Monthioux, M.; Luzzi, D. E. Nature 1998, 396, 323.

and Development Program (ALCA, JST), and WPI Initiative (MEXT, Japan) for financial support.

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REFERENCES

(1) Wasielewski, M. R. Chem. Rev. 1992, 92, 435−461. (2) Osuka, A.; Mataga, N.; Okada, T. Pure Appl. Chem. 1997, 69, 797−802. (3) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2009, 42, 1890−1898. (4) Magnuson, A.; Anderlund, M.; Johansson, O.; Lindblad, P.; Lomoth, R.; Polivka, T.; Ott, S.; Stensjö, K.; Styring, S.; Sundström, V.; et al. Acc. Chem. Res. 2009, 42, 1899−1909. (5) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 9, 537−546. (6) Imahori, H.; Sakata, Y. Eur. J. Org. Chem. 1999, 2445−2457. (7) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22−36. (8) Imahori, H.; Mori, Y.; Matano, Y. J. Photochem. Photobiol. C 2003, 4, 51−83. (9) Imahori, H. Org. Biomol. Chem. 2004, 2, 1425−1433. (10) Imahori, H.; Fukuzmi, S. Adv. Funct. Mater. 2004, 14, 525−536. (11) D’Souza, F.; Ito, O. Coord. Chem. Rev. 2005, 249, 1410−1422. (12) Imahori, H. Bull. Chem. Soc. Jpn. 2007, 80, 621−636. (13) Imahori, H. J. Mater. Chem. 2007, 17, 31−41. (14) Imahori, H.; Umeyama, T. J. Phys. Chem. C 2009, 113, 9029− 9039. (15) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183−191. (16) Allen, M. J.; Tung, V. C.; Kaner, R. B. Chem. Rev. 2010, 110, 132−145. (17) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1996. (18) Karousis, N.; Tagmatarchis, N.; Tasis, D. Chem. Rev. 2010, 110, 5366−5397. (19) Yan, L.; Zheng, Y. B.; Zhao, F.; Li, S.; Gao, X.; Xu, B.; Weiss, P. S.; Zhao, Y. Chem. Soc. Rev. 2012, 41, 365−375. (20) Swager, T. M. ACS Macro Lett. 2012, 1, 3−5. (21) Sun, Z.; James, D. K.; Tour, J. M. J. Phys. Chem. Lett. 2011, 2, 2425−2432. (22) Risko, C.; McGehee, M. D.; Brédas, J.-L. Chem. Sci. 2011, 2, 1200−1218. (23) Beaujuge, P. M.; Fréchet, J. M. J. Am. Chem. Soc. 2011, 133, 20009−20029. (24) Li, G.; Zhu, R.; Yang, Y. Nat. Photonics 2012, 6, 153−161. (25) Brabec, C. J.; Heeney, M.; McCulloch, I.; Nelson, J. Chem. Soc. Rev. 2011, 40, 1185−1199. (26) Martín, N.; Sanchez, L.; Herranz, M. A.; Illescas, B.; Guldi, D. M. Acc. Chem. Res. 2007, 40, 1015−1024. (27) Fukuzumi, S.; Kojima, T. J. Mater. Chem. 2008, 18, 1427−1439. (28) Chitta, R.; D’Souza, F. J. Mater. Chem. 2008, 18, 1440−1471. (29) Sgobba, V.; Guldi, D. M. Chem. Soc. Rev. 2009, 38, 165−184. (30) Guldi, D. M.; Illescas, B. M.; Atienza, C. M.; Wielopolskia, M.; Martín, N. Chem. Soc. Rev. 2009, 38, 1587−1597. (31) D’Souza, F.; Sandanayaka, A. S. D.; Ito, O. J. Phys. Chem. Lett. 2010, 1, 2586−2593. (32) Bottari, G.; de la Torre, G.; Guldi, D. M.; Torres, T. Chem. Rev. 2010, 110, 6768−6816. (33) D’Souza, F.; Ito, O. Chem. Soc. Rev. 2012, 41, 86−96. (34) Sgobba, V.; Guldi, D. M. J. Mater. Chem. 2008, 18, 153−157. (35) Umeyama, T.; Imahori, H. Energy Environ. Sci. 2008, 1, 120− 133. (36) Sgobba, V.; Guldi, D. M. Chem. Commun. 2011, 47, 606−610. (37) Ratier, B.; Nunzi, J.-M.; Aldissi, M.; Kraft, T. M.; Buncel, E. Polym. Int. 2012, 61, 342−354. (38) Cataldo, S.; Salice, P.; Menna, E.; Pignataro, B. Energy Environ. Sci. 2012, 5, 5919−5940. (39) Song, T.; Lee, S. T.; Sun, B. J. Mater. Chem. 2012, 22, 4216− 4232. (40) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40−48. (41) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol. C 2004, 5, 79−104. 3207



Feature Article

(72) Lee, J.; Kim, H.; Kahng, S.-J.; Kim, G.; Son, Y.-W.; Ihm, J.; Kato, H.; Wang, Z. W.; Okazaki, T.; Shinohara, H.; et al. Nature 2002, 415, 1005−1008. (73) Tezuka, N.; Umeyama, T.; Matano, Y.; Shishido, T.; Yoshida, K.; Ogawa, T.; Isoda, S.; Stranius, K.; Chukharev, V.; Tkachenko, N. V.; et al. Energy Environ. Sci. 2011, 4, 741−750. (74) Ago, H.; Ohshima, S.; Uchida, K.; Yumura, M. J. Phys. Chem. B 2001, 105, 10453−10456. (75) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Science 2004, 306, 666−669. (76) Berger, C.; Song, Z. M.; Li, X. B.; Wu, X. S.; Brown, N.; Naud, C.; Mayo, D.; Li, T. B.; Hass, J.; Marchenkov, A. N.; et al. Science 2006, 312, 1191−1196. (77) Li, X. S.; Cai, W. W.; An, J. H.; Kim, S.; Nah, J.; Yang, D. X.; Piner, R.; Velamakanni, A.; Jung, I.; Tutuc, E.; et al. Science 2009, 324, 1312−1314. (78) Hernandez, Y.; Nicolosi, V.; Lotya, M.; Blighe, F. M.; Sun, Z. Y.; De, S.; McGovern, I. T.; Holland, B.; Byrne, M.; Gun’ko, Y. K.; et al. Nat. Nanotechnol. 2008, 3, 563−568. (79) Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Nat. Nanotechnol. 2008, 3, 101−105. (80) Park, S.; Ruoff, R. S. Nat. Nanotechnol. 2009, 4, 217−224. (81) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (82) Eda, G.; Fanchini, G.; Chhowalla, M. Nat. Nanotechnol. 2008, 3, 270−274. (83) Xu, X.; Zhao, L.; Bai, H.; Hong, W. J.; Li, C.; Shi, G. Q. J. Am. Chem. Soc. 2009, 131, 13490−13497. (84) Wojcik, A.; Kamat, P. V. ACS Nano 2010, 4, 6697−6706. (85) Geng, J.; Jung, H.-T. J. Phys. Chem. C 2010, 114, 8227−8234. (86) Geng, J.; Kong, B.-S.; Yang, S. B.; Jung, H.-T. Chem. Commun. 2010, 46, 5091−5093. (87) Tu, W.; Lei, J.; Zhang, S.; Ju, H. Chem.Eur. J. 2010, 16, 10771−10777. (88) Hayashi, H.; Lightcap, I. V.; Tsujimoto, M.; Takano, M.; Umeyama, T.; Kamat, P. V.; Imahori, H. J. Am. Chem. Soc. 2011, 133, 7684−7687. (89) Jahan, M.; Bao, Q.; Loh, K. P. J. Am. Chem. Soc. 2012, 134, 6707−6713. (90) Xu, Y. F.; Liu, Z. B.; Zhang, X. L.; Wang, Y.; Tian, J. G.; Huang, Y.; Ma, Y. F.; Zhang, X. Y.; Chen, Y. S. Adv. Mater. 2009, 21, 1275− 1279. (91) Liu, Z. B.; Xu, Y. F.; Zhang, X. Y.; Zhang, X. L.; Chen, Y. S.; Tian, J. G. J. Phys. Chem. B 2009, 113, 9681−9686. (92) Karousis, N.; Sandanayaka, A. S. D.; Hasobe, T.; Economopoulos, S. P.; Sarantopouloua, E.; Tagmatarchis, N. J. Mater. Chem. 2011, 21, 109−117. (93) Krishna, M. B. M.; Venkatramaiah, N.; Venkatesan, R.; Rao, D. N. J. Mater. Chem. 2012, 22, 3059−3068. (94) Lomeda, J. R.; Doyle, C. D.; Kosynkin, D. V.; Hwang, W.-F.; Tour, J. M. J. Am. Chem. Soc. 2008, 130, 16201−16206. (95) Quintana, M.; Spyrou, K.; Grzelczak, M.; Browne, W. R.; Rudolf, P.; Prato, M. ACS Nano 2010, 4, 3527−3533. (96) Wang, H.-X.; Zhou, K.-G.; Xie, Y.-L.; Zeng, J.; Chai, N.-N.; Li, J.; Zhang, H.-L. Chem. Commun. 2011, 47, 5747−5749. (97) Zhang, X.; Hou, L.; Cnossen, A.; Coleman, A. C.; Ivashenko, O.; Rudolf, P.; van Wees, B. J.; Browne, W. R.; Feringa, B. L. Chem.Eur. J. 2011, 17, 8957−8964. (98) Umeyama, T.; Mihara, J.; Tezuka, N.; Matano, Y.; Stranius, K.; Chukharev, V.; Tkachenko, N. V.; Lemmetyinen, H.; Noda, K.; Matsushige, K.; et al. Chem.Eur. J. 2012, 18, 4250−4257. (99) Kim, S. K.; Jeon, S. Electrochem. Commun. 2012, 22, 141−144. (100) Shang, J.; Luo, Z.; Cong, C.; Lin, J.; Yu, T.; Gurzadyan, G. G. Appl. Phys. Lett. 2010, 97, 163103. (101) Imahori, H.; Kang, S.; Hayashi, H.; Haruta, M.; Kurata, H.; Isoda, S.; Canton, S. E.; Infahsaeng, Y.; Kathiravan, A.; Pascher, T.; et al. J. Phys. Chem. A 2011, 115, 3679−3690.

(102) Umeyama, T.; Tezuka, N.; Kawashima, F.; Seki, S.; Matano, Y.; Nakao, Y.; Shishido, T.; Nishi, M.; Hirao, K.; Lehtivuori, H.; et al. Angew. Chem., Int. Ed. 2011, 50, 11706−11709. (103) Vizuete, M.; Barrejón, M.; Gómez-Escalonilla, M. J.; Langa, F. Nanoscale 2012, 4, 4370−4381. (104) Delgado, J. L.; de la Cruz, P.; Urbina, A.; Navarrete, J. T. L.; Casado, J.; Langa, F. Carbon 2007, 45, 2250−2252. (105) Wu, W.; Zhu, H.; Fan, L.; Yang, S. Chem.Eur. J. 2008, 14, 5981−5987. (106) Guldi, D. M.; Menna, E.; Maggini, M.; Marcaccio, M.; Paolucci, D.; Campidelli, S.; Prato, M.; Rahman, G. M. A.; Schergna, S. Chem. Eur. J. 2006, 12, 3975−3983. (107) Sandayanaka, A. S. D.; Maligaspe, E.; Hasobe, T.; Ito, O.; D’Souza, F. Chem. Commun. 2010, 46, 8749−8751. (108) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; D’Souza, L.; Araki, Y.; Ito, O. J. Am. Chem. Soc. 2007, 129, 15865− 15871. (109) Takaguchi, Y.; Tamura, M.; Sako, Y.; Yanagimoto, Y.; Tsuboi, S.; Uchida, T.; Shimamura, K.; Kimura, S.; Wakahara, T.; Maeda, Y.; et al. Chem. Lett. 2005, 34, 1608−1609. (110) Sandanayaka, A. S. D.; Takaguchi, Y.; Sako, Y.; Tamura, M.; Ito, O. Adv. Sci. Lett. 2010, 3, 353−357. (111) Tajima, T.; Sakata, W.; Wada, T.; Tsutsui, A.; Nishimoto, S.; Miyake, M.; Takaguchi, Y. Adv. Mater. 2011, 23, 5750−5754. (112) Shen, Y.; Reparaz, J. S.; Wagner, M. R.; Hoffmann, A.; Thomsen, C.; Lee, J.-O.; Heeg, S.; Hatting, B.; Reich, S.; Saeki, A.; et al. Chem. Sci. 2011, 2, 2243−2250. (113) Umeyama, T.; Tezuka, N.; Fujita, M.; Hayashi, S.; Kadota, N.; Matano, Y.; Imahori, H. Chem.Eur. J. 2008, 14, 4875−4885. (114) Umeyama, T.; Tezuka, N.; Seki, S.; Matano, Y.; Nishi, M.; Hirao, K.; Lehtivuori, H.; Tkachenko, N. V.; Lemmetyinen, H.; Nakao, Y.; et al. Adv. Mater. 2010, 22, 1767−1770. (115) Tezuka, N.; Umeyama, T.; Seki, S.; Matano, Y.; Nishi, M.; Hirao, K.; Imahori, H. J. Phys. Chem. C 2010, 114, 3235−3247. (116) Rahman, G. M. A.; Guldi, D. M.; Cagnoli, R.; Mucci, A.; Schenetti, L.; Vaccari, L.; Prato, M. J. Am. Chem. Soc. 2005, 127, 10051−10057. (117) Sheeney-Haj-Ichia, L.; Basnar, B.; Willner, I. Angew. Chem., Int. Ed. 2005, 44, 78−83. (118) Campidelli, S.; Ballesteros, B.; Filoramo, A.; Diaz, D. D.; Torre, G. D. L.; Torres, T.; Rahman, G. M. A.; Ehli, C.; Kiessling, D.; Werner, F.; et al. J. Am. Chem. Soc. 2008, 130, 11503−11509. (119) Das, S. K.; Subbaiyan, N. K.; D’Souza, F.; Sandanayaka, A. S. D.; Hasobe, T.; Ito, O. Energy Environ. Sci. 2011, 4, 707−716. (120) Sandanayaka, A. S. D.; Subbaiyan, N. K.; Das, S. K.; Chitta, R.; Maligaspe, E.; Hasobe, T.; Ito, O.; D’Souza, F. ChemPhysChem 2011, 12, 2266−2273. (121) Bottari, G.; Olea, D.; López, V.; Gomez-Navarro, C.; Zamora, F.; Gómez-Herrero, J.; Torres, T. Chem. Commun. 2010, 46, 4692− 4694. (122) Kim, F.; Cote, L. J.; Huang, J. X. Adv. Mater. 2010, 22, 1954− 1958. (123) Kim, J.; Cote, L. J.; Kim, F.; Yuan, W.; Shull, K. R.; Huang, J. X. J. Am. Chem. Soc. 2010, 132, 8180−8186. (124) Cote, L. J.; Kim, J.; Tung, V. C.; Luo, J.; Kim, F.; Huang, J. X. Pure Appl. Chem. 2011, 83, 95−110. (125) Tung, V. C.; Huang, J.-H.; Tevis, I.; Kim, F.; Kim, J.; Chu, C.W.; Stupp, S. I.; Huang, J. J. Am. Chem. Soc. 2011, 133, 4940−4947. (126) Tung, V. C.; Huang, J.-H.; Kim, J.; Smith, A. J.; Chu, C.-W.; Huang, J. Energy Environ. Sci. 2012, 5, 7810−7818. (127) Ardo, S.; Meyer, G. J. Chem. Soc. Rev. 2009, 38, 115−164. (128) Imahori, H.; Umeyama, T.; Kurotobi, K.; Takano, Y. Chem. Commun. 2012, 48, 4032−4045. (129) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736− 6767. (130) Deibel, C.; Strobel, T.; Dyakonov, V. Adv. Mater. 2010, 22, 4097−4111. (131) Omachi, H.; Segawa, Y.; Itami, K. Acc. Chem. Res. 2012, 45, 1378−1389. 3208



Feature Article

(132) Zhi, L.; Müllen, K. J. Mater. Chem. 2008, 18, 1403−1414. (133) Liu, H.; Nishide, D.; Tanaka, T.; Kataura, H. Nat. Commun. 2011, 2, 309−316.

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