Conical Pentaaryl[60]fullerene Thiols: Self-Assembled Monolayers on

Sep 17, 2010 - ... 113-0033, Japan, and Nakamura Functional Carbon Cluster Project, ERATO, .... Yoshimitsu Itoh , Bumjung Kim , Raluca I. Gearba , Noa...
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J. Phys. Chem. C 2010, 114, 17741–17752

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Conical Pentaaryl[60]fullerene Thiols: Self-Assembled Monolayers on Gold and Photocurrent Generating Property Yutaka Matsuo,*,†,‡ Sebastian Lacher,† Aiko Sakamoto,† Keiko Matsuo,‡ and Eiichi Nakamura*,†,‡ Department of Chemistry, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan, and Nakamura Functional Carbon Cluster Project, ERATO, Japan Science and Technology Agency, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan ReceiVed: June 28, 2010; ReVised Manuscript ReceiVed: August 20, 2010

Conical-shaped penta(aryl)[60]fullerene thiol derivatives bearing one alkyl thiol linker surrounded by five phenyl or biphenyl groups were synthesized in good yield, and characterized by X-ray crystallographic analysis. These conical fullerene thiols were used for formation of self-assembled monolayers (SAMs) on gold surface in anaerobic and aerobic condition. Under nitrogen atmosphere, SAMs on gold were successfully prepared for all thiols (C60Ph5(CH2)3SH, C60Ph5(CH2)4SH, C60Ph5(CH2)6SH, C60(biphenyl)5(CH2)6SH) used in this work. The SAMs of penta(aryl)[60]fullerene thiols exhibited anodic and cathodic photocurrent generation upon light irradiation (λ ) 400 nm) in the presence of sacrificial electron-donating and -accepting reagents, respectively. The SAM formation under air gave either SAMs or aggregates on gold depending on the length of alkyl chains of the linker. When the linker is long enough (i.e., C60Ph5(CH2)6SH) so that the disulfide group is not sterically protected (i.e., C60Ph5(CH2)6S-S(CH2)6C60Ph5), the molecules formed SAMs and generated anodic and cathodic photocurrent. Further aggregation of the molecules occurred when the linker is short relative to the arene cone consisting of five aryl groups (i.e., C60Ph5(CH2)3SH). These aggregates showed unusual anodic photocurrent behavior, likely because of diffusion issue for electrolyte and sacrificial reagents. The aggregates were further investigated by cyclic voltammetry, atomic force microscopy, and photoelectron yield spectroscopy. Introduction Self-assembled monolayers (SAMs) are a simple and convenient system used to tailor the interfacial properties of metals, metal oxides, and semiconductors.1 SAMs are currently studied intensively for various applications as, for example, work function tuning and molecular sensors. Another very important application of SAMs is in optoelectronic devices for photocurrent generation in photoelectric conversion cells.2 For such devices, it was shown before that the structural and chemical properties of functional cores and linkers have a large influence on electron transfer characteristics.3 Besides that, the linker length has great impact on electron transfer.4 For the gold substrate, electron transfer competes with energy-transfer quenching of the excited singlet state by surface plasmon in the vicinity of the gold surface.5 Nevertheless, high quantum yields for cathodic photocurrent generation of fullerene-terminated SAMs on gold of 50% and more were claimed in earlier reports.6 Penta(organo)[60]fullerenes are especially suitable for building SAMs due to the compact and rigid Apollo lunar landing module-like structure (Figure 1) that makes it possible to form dense monolayers on surfaces with the fullerene cores separated from each other to prevent direct contact, which would cause excited-state annihilation.7,8 Furthermore, these molecules show light absorption in the visible region of the * Corresponding author. E-mail: [email protected] (Y.M.), [email protected] (E.N.). † The University of Tokyo. ‡ ERATO, Japan Science and Technology Agency.

solar spectrum, making them suitable for photocurrent generation devices. This was demonstrated by SAMs of penta(organo)[60]fullerenes linked to indium-tin oxide (ITO) by five carboxylic acid groups where a dependency of the photocurrent on the molecular orientation was found (compound 1 in Figure 1).7 In a similar fashion, pentapod deca(organo)[60]fullerenes were linked to ITO showing the ability of forming photoelectrochemical bilayers.9 Alternatively, a separate alkyl linker was introduced in the system with a phosphonic acid headgroup that can be linked to ITO surface (compound 2 in Figure 1).8 This made a fast and clean SAM formation possible because phosphonic acids are known to form very stable and reproducible SAMs on metal oxides as, for example, ITO.10,11 We here introduce a thiol group connected to the penta(organo)[60]fullerenes core by an alkyl linker to obtain SAMs on gold. In fact, the formation of clean monolayers of fullerenes on gold surface is still a challenging task. Tour et al.12 found that instead of clean monolayers, multilayer formation of fullerenes often occurs due to the tendency of fullerenes to form clusters13 as well as head-to tail assemblies due to the high gold-fullerene interaction (30-60 kcal/mol, vide infra),14 which is of strength similar to the sulfur-gold interaction. In this Article, we describe the synthesis of conical-shaped penta(organo)[60]fullerene thiols 3-6 with various linker length relative to the size of the cones consisting of five aryl groups and report about SAM formation on gold for the construction of photocurrent generation devices (Figure 2). We will discuss the effect of the linker length on reactivity of oxidatively

10.1021/jp1059402  2010 American Chemical Society Published on Web 09/17/2010

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Figure 1. Penta(organo)[60]fullerene-carboxylic acid, -phosphonic acid, and -thiols for preparation of SAMs on ITO (1 and 2) and gold (3-6).

Figure 2. Models of pentaaryl[60]fullerene on gold surfaces. These models are based on X-ray crystallographic structures of corresponding thiol precursors. (a) Side view of 3. (b) Side view of 6. (c) Top view of 6.

generated disulfides in protective cone-shaped cavities,15 their aggregation behavior,16 and surface coverage to show how we

can obtain the desired penta(organo)[60]fullerene SAMs. This knowledge will be applicable to recent interests in covering gold

Conical Pentaaryl[60]fullerene Thiols SCHEME 1: Synthesis of Penta(organo)[60]fullerenes Thiols 3 Starting from C60Ph5Ha

a Intermediates, halides and thioacetates, are denoted by a and b, respectively. The letters a and b indicate the halide and thioacetate reaction products of the corresponding compound, respectively.

surface with rigid aromatic systems, which was reported earlier for porphyrins on gold nanoparticles.17 Results and Discussion Synthesis and Characterization of Pentaaryl[60]fullerene Thiol Derivatives. The pentaphenyl[60]fullerene thiol derivatives were prepared as shown for compound 3 in Scheme 1. The synthesis for compounds 4 and 5 was analogous; compound 6 started with a penta(biphenyl)[60]fullerene.18 The five aromatic groups were used as addends to construct rigid conical-shaped structures on the [60]fullerene core. The synthesis started from the penta-adducts, C60Ar5H,19 which can be prepared in almost quantitative yield starting from C60, the corresponding Grignard reagent, and copper salt.20 Deprotonation of the pentaaryl[60]fullerenes with KOtBu in ortho-dichlorobenzene (ODCB) generated a cyclopentadienyl anion,21 which was subsequently treated with R,ω-alkane diiodide and afforded the iodoalkyl adduct 3a in 93% yield.22 Replacement of iodine by thioacetate in a THF/ DMF (1:1) solvent mixture gave the protected thiol compound 3b in 75% yield. Subsequently deprotection was done in ODCB with sodium methoxide as base and gave thiol 3 in 70% yield.

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17743 TABLE 1: Disulfide Formation for Compound 3 Stirred in ODCB under Air as a Function of Stirring Time time/h

dimer percentage

0 18 28 43 138 174

1.4 2.3 3.0 3.2 8.0 8.9

The disulfide dimer was identified as a byproduct, which was separated subsequently by HPLC. It is worth mentioning that during the deprotection reaction in the basic medium the thiolate was very sensitive toward oxidation to disulfide. However, the neutral thiol after reaction was reasonable stable under air, with slow oxidation and disulfide fullerene dimer formation occurring when stirring in ODCB solution in air as shown in Figure 3 and Table 1. Crystal structure analyses were performed for C60Ph5(CH2)4Br (4a′), an analogue of C60Ph5(CH2)4I (4a) (Figure 4), and a penta(biphenyl)[60]fullerene thioacetate derivative, C60(biphenyl)5(CH2)6SAc (6b) (Figure 5). Protrusion of the arene groups is clearly seen even in the case of the pentaphenyl derivative. Diameter of the space filling surface for the pentaphenyl moiety is ca. 1.5 nm, as compared to that (ca. 1.0 nm) for the fullerene core. This protrusion becomes bigger for the penta(biphenyl) derivative. The diameter spanned by the penta(biphenyl) moiety is roughly double (2.0 nm) as compared to that of the fullerene core. The heights of the molecules are approximately 1.5 and 1.8 nm for the phenyl and the biphenyl derivatives, respectively. Photophysical and Electrochemical Properties of Penta(organo)[60]fullerene Thiols in Solution. We studied the photophysical and electrochemical properties of the synthesized penta(organo)[60]fullerenes in solution. The UV/vis spectrum of compound 3 shown in Figure 6a displays a characteristic absorption maximum at 397 nm (ε ) 5.1 × 103 M-1 cm-1), which is due to the π-π* excitation of the fullerene core. Thus, for photocurrent measurements, the SAMs were irradiated with monochromatic light of the wavelength of 400 nm. Cyclic voltammogram (CV) was measured for compound 3 in ODCB solution containing supporting electrolyte (nBu4NPF6) under argon atmosphere using glassy carbon as working electrode, platinum wire as counter electrode, and an Ag/Ag+ electrode as reference to investigate the electrochemical properties of the compound. Two reversible one-electron reduction processes could be observed at -1.26 and -1.69 V vs Ag/Ag+ (Figure 6b). All other penta(organo)[60]fullerene thiols showed similar UV/vis and CV spectra.

Figure 3. Formation of fullerene disulfide. (a) HPLC chart of a freshly prepared solution of compound 3 in ODCB. (b) HPLC chart after stirring for 174 h in air. Peak at ca. 8.7 min is due to the dimer (8.9%). Column: Cosmosil Buckyprep 4.6 × 250 mm. Eluent: toluene:2-propanol (7:3). Flow rate: 1.5 mL/min.

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Figure 4. Crystal structure of C60Ph5(CH2)4Br (4a′). The unit cell contains two toluene molecules for one fullerene bromide molecule. Solvent molecules are omitted for clarity.

Figure 5. Crystal structure of C60(biphenyl)5(CH2)6SAc (6b). The unit cell contains two toluene molecules for one fullerene thioacetate molecule. Solvent molecules are omitted for clarity.

Figure 6. Photophysical and electrochemical properties of the penta(organo)[60]fullerene thiol. (a) UV/vis absorption spectrum of 3 in ODCB (1 × 10-5 M). (b) CV of 3 against Ag/Ag+ reference electrode in ODCB. Two reversible reduction peaks were observed.

Photocurrent Generation Properties of SAMs of Penta(biphenyl) and Pentaphenyl Compounds 3 and 6 on the Gold Electrodes. SAMs were built according to the following procedures: Gold electrodes were cleaned by a 60 s sulfochromic acid pretreatment23 and subsequently immersed in an ODCB solution of fullerene thiol derivatives (0.1 mM) for 3 h in the glovebox if not mentioned differently. After SAM formation, the substrates were rinsed with fresh ODCB and dichloromethane to remove nonbounded fullerene derivatives and were dried in an argon stream. The process yielded a completely covered surface with the penta(organo)[60]fullerenes. Comple-

tion of SAM formation was also confirmed by measurements on quartz crystal microbalance (Affinix-Q, Initium). Photoelectrochemical measurements for 3 and 6 were performed upon irradiation with λex ) 400 nm. In the presence of an electron sacrificial donor such as ascorbic acid (AsA), SAMs of the penta(organo)[60]fullerenes exhibited anodic photocurrent generation upon illumination (Figure 7). In the presence of an electron acceptor such as oxygen or methyl viologen (MV2+), the SAMs generated cathodic current (Figure 8). An explanation of this ambivalent current generation behavior has been given in our previous report on a phosphonic acid/indium tin oxide

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Figure 7. Anodic photocurrent generation of the pentaaryl[60]fullerene thiols on the gold surface in the presence of AsA as a sacrificing donor. (a) An on-off profile for compound 3 and (b) for compound 6.

TABLE 2: Quantum Yields of Compounds 3 and 6 for Anodic (Sacrificer, AsA) and Cathodic (Sacrificer, MV2+) Photocurrent at 0 and 0.1 V Applied Bias Voltage anodic current, φa %

Figure 8. Cathodic photocurrent with MV2+ as sacrificer for compound 3 self-assembled on gold surface.

system.8 The irradiation of the SAM at 400 nm causes photoexcitation of the fullerene part, which accepts one electron from the AsA in the case of the AsA sacrificer or the gold substrate in the case of the MV2+ sacrificer. The electron then is transferred to the gold electrode for the anodic current or to MV2+ for the cathodic current generation. The action spectra (Figure 9) are in good agreement with the absorption spectrum of compound 3 shown in Figure 6, indicating that excitation of the penta(organo)[60]fullerenes is the origin of the photocurrent. Quantum yields for compounds 3 and 6 at 0 and 0.1 V applied bias voltage are displayed in Table 2. Quantum yields for both compounds are approximately comparable to the corresponding phosphonic acids on ITO reported earlier.8 Anodic quantum yields for compound 6 are much higher and double as compared to compound 3 that is closer to the gold surface. This distance dependency of the quantum yield was expected, because it is well-known that the excited states of dyes on metal surfaces are quenched efficiently by the metal via energy transfer,24-26 and also surface plasmon quenching might play a role.5 The quantum yields for cathodic photocurrent for both compounds are in a similar range and relatively low.

cathodic current, φc %

compound

no bias

0.1 V bias

no bias

0.1 V bias

3 6

5.2 11.8

5.4 12.9

2.8 2.3

2.8 3.0

Investigation of the Influence of Dimer Formation on Photocurrent Generation. For the cathodic photocurrent shown in Figure 8, a curved photocurrent shape was observed, which is a slow rising and falling of the current upon illumination and in the dark, respectively. This kinetic behavior was observed before for the phosphonic acid of penta(organo)[60]fullerenes on ITO.8 Interestingly, the anodic photocurrent in Figure 7 also showed an unusual shape; the photocurrent rises to a peak and declines to a constant value. After the light was stopped, the same behavior is observed in reverse. Because this current behavior is quite unique and must be due to irregularities of the fullerene SAM, we analyzed the SAM in greater detail. We investigated first the influence of disulfide formation. Fullerene dimers are less soluble and will precipitate after formation as we showed in Figure 3. For this purpose, we built the same SAMs in air for 3 h instead of building them in the glovebox. The anodic photocurrents for air-built SAMs of the compounds 3 and 6 in the presence of AsA sacrificer are shown in Figure 10a,d. Most important, the small peak that can be seen in Figure 7 is greatly enhanced this time. Especially for compound 3 this additional current increased significantly. It is plausible that oxygen causes fullerene dimer formation through the generation of disulfide bonds (Figures 3 and 11). Although disulfides usually can form SAMs on gold as well, the disulfide bond in case of the compound 3 and 6 is protected by the aryl groups attached to the fullerene core (Figure 11a).15 We thus

Figure 9. Photocurrent action spectra for compound 3 on gold. (a) Anodic photocurrent with AsA as sacrificer and (b) cathodic photocurrent with MV2+.

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Figure 10. Anodic photocurrent with AsA as sacrificer for the SAMs built under air on gold electrodes for (a) compound 3, (b) compound 4, (c) compound 5, and (d) compound 6.

examined compounds with longer alkyl linker relatively to the arene groups to investigate the behavior of an unprotected thiol group. Compound 4 with four and compound 5 with six carbon atoms as alkyl linker were used for the SAM formation under ambient conditions. Anodic photocurrent with AsA as sacrificer is shown in Figure 10b,c. The peak current decreases with increasing alkyl linker (compound 4) and disappears completely for compound 5. In that case, the disulfide bond is not protected by the phenyl groups of the pentaaryl[60]fullerenes (Figure 11b), making it possible for disulfides to form SAMs on the gold surface (Figure 11c). The effect of other electron sacrificer on the photocurrent behavior was investigated using triethanol amine (TEA) as electron donor. The photocurrent for air-built SAMs of compound 3 is shown in Figure 12. As observed earlier for the phosphonic acid counterparts on ITO, TEA is less effective for photocurrent generation leading to lower quantum yields due to an energy level mismatch that makes charge transfer less effective. As already observed for AsA as sacrificer, a current overshoot was observed, which we assigned to the presence of aggregates on the SAM. However, this time the lifetime of the additional current is significantly reduced as compared to Figure 7b, proving that the sacrificer plays a crucial role in its decay. Because the time scale of the decay is in the order of seconds, diffusion effects of the sacrificer in the aggregates play probably a major role in generation of this unusual additional anodic photocurrent behavior. Characterization of SAMs of Compounds 3 and 5 on Gold Substrate. We employed several characterization techniques to investigate the SAMs of the different compounds built under ambient conditions. Photoelectron yield spectroscopy (PYS) is a very useful technique for work function investigation of metals and semiconductors.27 In addition, it is possible to investigate the ionization potentials of molecules. PYS measurements were performed on SAMs of compounds 3 and 5 that were built under

air. The cubic root of the photoelectron yield after background subtraction of gold electrons is shown in Figure 13. For SAMs of compound 3 on gold (Figure 13a), two well-resolved ionization potentials were observed at 5.80 and 5.99 eV, respectively. It is well-known that the ionization potentials for compounds in the bulk differ from those of single molecules.28 Bulk state ionization potentials of fullerenes are lower than that of single fullerene molecules due to relaxation effects. The result thus indicates that cluster formation took place, because ionization potentials of molecules in the bulk as well as in the isolated state are observed. The bulk cluster shown in 3/gold can be considered to be aggregates of insoluble fullerene disulfide. The PYS measurement of compound 5 self-assembled on gold substrate showed a different behavior (Figure 13b). The lower ionization potential for the bulk state was almost not possible to observe, indicating the cleaner SAM formation. We then performed atomic force microscopy (AFM) measurements on cleaned gold substrate as well as SAMs of compounds 3 and 5 on gold that were self-assembled under air. A large overview of clean gold substrate is shown in Figure 14a. A surface with grain-like structure was observed. These grains have diameters of several hundred nanometers. The SAM of the compound 5 on gold (Figure 14b) showed a surface similar to that of the cleaned gold substrate with the island-like structure and a small number of bright spots that were several nanometers in height. For compound 3, however, a larger number of structures being 10-12 nm in height were found in theimage(Figure14c).Thesestructuresareprobablypenta(organo)[60]fullerene disulfide aggregates, which are in agreement with the ionization potential data in the PYS spectra. Because these structures for compound 3 appear only for the SAMs built in the presence of oxygen, we believe that aggregates formed by disulfides are the reason for the unusual photocurrent behavior of penta(organo)[60]fullerenes.

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Figure 11. CPK models of penta(phenyl)[60]fullerene disulfides. (a) A protected disulfide bond in 3. (b) An unprotected disulfide in 5. (c) A possible model for 5 for reaction with the gold surface.

Figure 12. Photocurrent of air-built SAMs of compound 3 on gold with TEA as sacrificial electron donor. Peak current decay time is shorter as compared to AsA.

The quality of the SAMs of compounds 3 and 5 was further investigated by cathodic scanning in a CV cell (Figure 15). The reduction wave for compound 5 is higher in intensity, which corresponds to a higher surface coverage. However, with subsequent scans, these waves decrease in intensity and disap-

pear after three or four scans. It is well-known that thiol SAMs on gold undergo electrochemically reductive desorption at higher negative voltages.29 Note that we can rule out such a fluxionality of fullerene thiols on gold in photocurrent generation processes described in the previous sections, because electron transfer in photocurrent generation processes is much faster (femtosecond to picosecond) than on the CV time scale (∼second). The observation of clear reduction peaks for this system itself is an interesting fact. It was shown earlier that the electrochemical behavior of fullerene SAMs is coverage dependent.30 In specific, a complex electrochemical behavior was observed for tightly packed SAMs of fullerenes due to the inhibition of charge-compensation ion transport within the SAM film.31 In case of direct fullerene-fullerene contact within the SAMs, it was found before that CV waves look broad and featureless.12 Because the CV signals of the penta(organo)[60]fullerenes on gold here are resolved, it provides additional evidence that the five phenyl groups act as a spacer, separating each single fullerene core.

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Figure 13. Photoemission yield spectroscopy of SAMs of 3 and 5. (a) SAM of compound 3 on gold. (b) SAM of compound 5 on gold. The ionization potential of penta(organo)[60]fullerenes in the bulk is displayed in red (dashed line); the one in the SAM is in blue (blue line). One can judge SAM of compound 3 is more aggregated than that of compound 5. We ascribe aggregates in 3 to reprecipitation of insoluble disulfide dimer.

Figure 14. AFM images. (a) Cleaned gold substrate. (b) SAM and small number of fullerene aggregates of compound 5 on gold. (c) SAM and large number of fullerene aggregates of compound 3 on gold. Bright spots in (b) and (c) are fullerene aggregates, which is probably due to penta(organo)[60]fullerene disulfides.

Integration of the first reduction wave allowed us to roughly estimate the surface coverage. Note that due to the abovementioned reductive desorption it was not possible to calculate the exact surface coverage by using CV. We found that SAMs of compound 5 on gold show a charge flow of 2.87 × 10-6 C, which corresponds to a surface coverage of 1.41 × 10-10 mol/ cm2. Compound 3 shows a charge flow of 8.76 × 10-7 C, corresponding to 4.54 × 10-11 mol/cm2. These values are comparable to the theoretical value of 9.39 × 10-11 mol/cm2 derived from the diameter of the X-ray structure of penta(phenyl)[60]fullerenes (Figure 4) and 5.29 × 10-11 mol/cm2 for

penta(biphenyl)[60]fullerenes. This last value fits well with the experimental value of 6.64 × 10-11 mol/cm2 (40 molecules on 100 nm2) obtained from a STM image for a penta(biphenyl carboxylic acid) compound, C60(C6H4C6H4COOH)5Me, on gold (see also compound 1 in Figure 1).16 The higher value for compound 5 as compared to 3 is probably due to the long alkyl linker. The flexibility of the alkyl chain should make it easier for pentaaryl[60]fullerenes to reach denser surface packing. The higher surface coverage of compound 5 was also shown by work function measurement of the gold substrates done with the PYS method. In this measurement, gold functionalized by compound

Conical Pentaaryl[60]fullerene Thiols

Figure 15. CV scan of SAM of compounds 3 on gold self-assembled in glovebox. Scanning was performed in acetonitrile containing n Bu4NClO4 as an electrolyte under argon. The intensity of the first reduction wave declines as indicated by the arrow. First scan is shown in blue, then red, yellow, green, and violet.

5 showed a higher work function shift and a lower background electron yield, which is due to a denser fullerene SAM. Conclusion Penta(organo)[60]fullerene thiols were synthesized and selfassembled on gold electrodes via thiol linkers. The properties of these compounds were characterized with UV/vis and CV in solution as well as with photoelectrochemical studies, PYS, AFM, and CV measurements of the formed SAMs on gold electrode surfaces. We found that SAM formation in aerobic condition depends on the relative length of alkyl linkers and aromatic spacers of the penta(aryl)[60]fullerene-thiol molecules. The compounds bearing long alkyl chains and short aryl groups (i.e., compound 5) afforded clean SAMs with high surface coverage. On the other hand, the compounds with short alkyl chains (i.e., compound 3) gave fullerene aggregates besides SAM formation because of precipitation of insoluble disulfide dimers, which formed upon oxidation and are unreactive due to the steric protection effect of the five aryl groups. These functionalized gold electrodes show an unusual behavior in the anodic photocurrent generation and are characterized with PYS and AFM measurements. SAMs built under anaerobic condition generated anodic and cathodic photocurrent in the presence of ascorbic acid and methyl viologen, respectively. Furthermore, longer linkers were also found to be beneficial with respect to higher surface coverage. We believe that the present studies on penta(organo)[60]fullerene thiols give us valuable information of how to obtain clean fullerene-coated gold substrates, which are expected to have various applications, for instance, construction of photoelectric conversion systems and sensors as well as surfaces for separation technology. In addition, we think penta(organo)[60]fullerene thiols will be useful to obtain gold nanoparticles covered with semiconducting fullerene derivatives, which should have different interesting properties for organic electronic applications as compared to their counterparts covered by insulating alkane thiols and citric acids. Experimental Section Materials. All manipulations were carried out under a nitrogen or argon atmosphere with standard Schlenk techniques. C60 was purchased from Frontier Carbon Co. A THF solution of KOtBu and MV2+ were purchased from Sigma-Aldrich Co. and used as received. 1,3-Diiodopropane, 1,6-diiodohexane, Na2SO4, and AsA were purchased from Kanto Chemicals. These were used as received without further purification. nBu4NClO4 for electrochemical measurement was purchased from Kanto Chemicals and used after recrystallization from ethanol. All

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17749 anhydrous organic solvents were purchased from Kanto Chemicals and purified by a solvent purification system (GlassContour) equipped with columns of activated alumina and supported copper catalyst (G-5) as well as degassed with argon for 15 min before use. C60Ph5H and C60(biphenyl)5H were synthesized as described earlier in the literature.18 C60Ph5C4H8OH was synthesized according to our previous report.32 Flash column chromatography was performed on Kanto Silica gel 60 (spherical, neutral, 140-325 mesh). All reactions were monitored by HPLC (Cosmosil Buckyprep column, 4.6 × 250 mm, Nacalai Tesque; flow rate, 1.5 mL/ min; eluent, toluene/2-propanol; detected at 350 nm with a UV spectrophotometric detector, Shimadzu SPD-6A). Isolated yields were calculated on the basis of the starting fullerene compounds. The gold electrodes (1000 Å) were prepared by a sputtering technique with gold onto fresh mica and cleaned with the procedure explained above previous to use. Characterization. NMR spectra were measured on JEOL ECA-500 (500 MHz) and JEOL AL-400 (400 MHz). Spectra are reported in parts per million from internal tetramethylsilane (δ 0.00 ppm) or residual protons of the deuterated solvent for 1 H NMR, and from solvent carbon (e.g., δ 77.00 ppm for chloroform) for 13C NMR. High-resolution mass spectra were recorded by APCI using a time-of-flight mass analyzer on a JEOL JMS-T100LC (AccuTOF) spectrometer with a calibration standard of C60Ph5- (MW 1105.1956). PYS and AFM measurements were performed with PYS-201 (Sumitomo Heavy Industries, Ltd.) and JSPM-4200 (JEOL), respectively. Cyclic voltammetry was carried out in a standard onecompartment cell under an argon atmosphere using a Pt-wire counter electrode and an Ag/Ag+ reference electrode [10 mM AgClO4 in 0.1 M NBu4ClO4-ODCB, E0′ (Fc/Fc+) ) 0.27 V vs Ag/Ag+ (Fc: ferrocene)] with a HZ-5000 voltammetric analyzer (HOKUTO DENKO). A glassy carbon rod was embedded in Pyrex glass, and the cross section was used as a working electrode. In case of CV measurements of SAMs, no ferrocene was added, and acetonitrile was used for solvent instead of ODCB. Photoelectrochemical measurements were carried out in a onecompartment cell, being irradiated with monochromatic excitation light with an Ag/AgCl reference electrode and a Pt counter electrode. Light intensity at 400 nm was 173 µW for anodic photocurrent and 81 µW for cathodic photocurrent measurements, respectively. The photocurrent was detected with an HZ5000 voltammetric analyzer. Anodic current was investigated with 50 mM ascorbic acid as electron donor in an argonsaturated 0.1 M Na2SO4 aqueous solution. For cathodic current, a 50 mM methyl viologen aqueous solution with 0.1 M Na2SO4 as electrolyte was prepared. Quantum efficiencies of anodic and cathodic current were obtained by eq 1.

Φ ) (i/e)/[I(1 - 10-A)]

(1)

where I ) (Wλ/hc). I is the number of photons per unit area and unit time, A is the absorbance of the adsorbed dyes at λ nm, i is the photocurrent density, e is the elementary charge, W is the light power irradiated at λ nm, λ is the wavelength of light irradiation, h is the Planck constant, and c is the speed of light. A was estimated from eq 2.

A ) εc′l

(2)

ε was determined from the UV/vis spectrum of the corresponding compound in ODCB solution. c′l is surface coverage. Here,

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we use the theoretical surface coverage determined from the diameter measured by X-ray single crystal analysis (9.39 × 10-11 mol/cm2 for compound 3 and 5.29 × 10-11 mol/cm2 for compound 6) due to the above-mentioned good agreement with earlier experimental results as well as with our values obtained by CV. Synthesis of C60Ph5C3H6I (3a). A solution of KOtBu in THF (1.0 M, 0.19 mmol, 0.19 mL) was added to a suspension of C60Ph5H (200 mg, 0.18 mmol) in ODCB (8.3 mL). The color of the reaction mixture changed from red to black. 1,3Diiodopropane (0.44 mmol, 0.05 mL) was added to the mixture, which was heated subsequently to 140 °C. After being stirred for 24 h, the reaction was stopped by addition of aqueous saturated NH4Cl, and then the solvent was removed under reduced pressure. The residual orange solid was dissolved in CS2 and filtered through a pad of silica gel. The orange filtrate was concentrated under reduced pressure and was diluted with methanol. The resulting precipitate was collected by filtration to obtain the title compound (213 mg, 93% yield). 1H NMR (500 MHz, CDCl3): δ 1.85 (m, 2H, CH2), 2.04 (m, 2H, CH2), 2.55 (m, 2H, CH2), 7.05-7.12 (m, 3H, Ar), 7.20-7.33 (m, 14H, Ar), 7.57-7.63 (m, 4H, Ar), 7.71-7.75 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 30.21, 41.83, 43.99, 58.54, 61.10, 63.24, 64.52, 127.2-156.7 (aromatic carbons of C60 and attached phenyl groups). Synthesis of C60Ph5C3H6SAc (3b). KSAc (914 mg, 8.0 mmol) was added to the solution of C60Ph5C3H6I (200 mg, 0.16 mmol) in THF/DMF (1:1, 28 mL). After the mixture was stirred for 24 h, the solvent was removed under reduced pressure. The compound was precipitated with methanol, and the resulting solid was washed with methanol. The product was purified with silica gel column chromatography (CS2/EtOAc ) 1/0 to 1/1 as an eluent) to afford the title compound (147 mg, 75% yield) as a red solid. 1H NMR (500 MHz, CDCl3): δ 1.36-1.42 (m, 2H, CH2), 1.62-1.72 (m, 2H, CH2), 2.18-2.25 (t, J ) 8.00 Hz, 2H, CH2), 2.23 (s, 3H, CH3), 7.03-7.09 (m, 3H, Ar), 7.21-7.35 (m, 14H, Ar), 7.58-7.65 (m, 4H, Ar), 7.67-7.72 (m, 4H, Ar). 13 C NMR (125 MHz, CDCl3): δ 26.61, 28.09, 30.40, 39.37, 58.55, 61.16, 63.30, 64.84, 127.01, 127.58, 127.84, 127.93, 127.98, 128.54, 128.80, 129.02, 130.56, 138.57, 140.10, 142.45, 142.67, 143.54, 143.82, 143.91, 144.03, 144.13, 144.28, 144.39, 144.55, 144.60, 145.23, 145.40, 147.25, 147.30, 147.41, 147.81, 148.04, 148.17, 148.26, 148.43, 148.56, 148.69, 148.77, 151.29, 153.01, 156.13, 156.76, 194.06. Synthesis of C60Ph5C3H6SH (3): 1-(Propylthiol)-6,9,12,15,18pentaphenyl-1,6,9,12,15,18-hexahydro[60]fullerene. To an ODCB (6.1 mL) solution of C60Ph5C3H6SAc (30 mg, 0.020 mmol) was added a methanol solution of NaOMe (1.0 M, 0.5 mL, 0.5 mmol) at 25 °C. This mixture was warmed to 50 °C. After stirring for 2 h, the reaction was stopped by addition of 1 N aqueous HCl (1 mL). The product was precipitated by the addition of excess methanol and was washed with methanol. The crude product was subject to silica gel column chromatography (CS2/EtOAc ) 1/0 to 1/1 as an eluent) to afford the title compound (9 mg, 0.008 mmol, 27% yield) as an orange solid. 1H NMR (400 MHz, CDCl3): δ 1.38-1.48 (m, 2H, CH2), 1.62-1.72 (m, 2H, CH2), 1.82-1.91 (m, 2H, CH2), 7.05-7.11 (m, 3H, Ar), 7.22-7.34 (m, 14H, Ar), 7.59-7.65 (m, 4H, Ar), 7.70-7.76 (m, 4H, Ar). 13C NMR (100 MHz, CDCl3): δ 24.49, 30.08, 30.55, 39.91, 58.86, 61.44, 63.61, 65.15, 127.35, 128.05, 128.17, 128.22, 128.88, 129.06, 129.16, 130.89, 139.13, 140.48, 142.85, 142.96, 143.79, 144.07, 144.11, 144.33, 144.40, 144.60, 144.68, 144.77, 144.86, 145.55, 145.68, 147.34, 147.54, 147.61, 147.75, 148.11, 148.34, 148.47, 148.55, 148.72, 148.86, 149.00,

Matsuo et al. 149.08, 151.56, 153.42, 156.76, 156.93. APCI-HRMS (-): calcd for C126H58S [M]- 1603.4259, found 1603.4260. Synthesis of C60Ph5C4H8Br (4a′). To an ODCB (21.0 mL) solution of C60Ph5C4H8OH (396 mg, 0.336 mmol) were added pyridine (82.8 mL, 1.01 mmol), PPh3 (82.8 mL, 0.470 mmol), and CBr4 (166 mg, 0.500 mmol) at 25 °C. After being stirred for 1 h, the solvent was removed under reduced pressure. The mixture was subjected to silica gel column chromatography (toluene/EtOAc ) 1/0 to 1/1 as an eluent) and reprecipitated with methanol to afford the C60Ph5C4H8Br (399 mg, 0.321 mmol, 96% yield) as a red solid. 1H NMR (500 MHz, CDCl3): δ 1.24-1.30 (m, 2H, CH2), 1.34-1.37 (m, 2H, CH2), 1.62-1.69 (m, 2H, CH2), 3.12 (t, J ) 5.70 Hz, 2H, CH2), 7.09-7.10 (m, 3H, Ar), 7.29-7.34 (m, 14H, Ar), 7.66-7.68 (m, 4H, Ar), 7.80-7.81 (m, 4H, Ar). 13C NMR (100 MHz, CDCl3): δ 24.06, 31.89, 34.04. 39.45, 58.57, 61.03, 63.30, 64.90, 126.89, 127.65, 127.78, 127.85, 127.89, 128.54, 128.80, 128.98, 130.57, 138.63, 139.87, 142.57, 142.60, 143.51, 143.69, 143.77, 143.99, 144.09, 144.27, 144.34, 144.46, 144.53, 145.17, 145.43, 147.20, 147.25, 147.47, 147.77, 148.00, 148.14, 148.21, 148.36, 148.51, 148.65, 148.73, 151.32, 153.08, 156.01, 156.68. APCI-MS (-) m/z 1242 [M]-. Synthesis of C60Ph5C4H8SAc (4b). KSAc (3.57 g, 31.3 mmol) was added to the solution of C60Ph5C4H8Br (389 mg, 0.313 mmol) in ODCB (74.0 mL) and DMF (31.0 mL) at 25 °C. After stirring for 24 h, the solvent was removed under reduced pressure. The mixture was subjected to silica gel column chromatography (CS2/EtOAc ) 1/0 to 2/1 as an eluent) and reprecipitated with methanol to afford the C60Ph5C4H8SAc (339 mg, 0.274 mmol, 89% yield) as an orange solid. 1H NMR (500 MHz, CDCl3): δ 0.97-1.03 (m, 2H, CH2), 1.34-1.37 (m, 2H, CH2), 1.43-1.50 (m, 2H, CH2), 2.31 (s, 3H, CH3), 2.52 (t, J ) 6.85 Hz, 2H, CH2), 7.07-7.09 (m, 3H, Ar), 7.26-7.30 (m, 14H, Ar), 7.61-7.63 (m, 4H, Ar), 7.74-7.76 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 24.74, 28.68, 29.55, 30.45, 39.96, 58.40, 60.95, 63.16, 64.89, 126.83, 127.56, 127.72, 127.77, 128.38, 128.72, 128.79, 130.46, 138.55, 139.98, 143.36, 143.61, 143.69, 143.92, 144.03, 144.17, 144.23, 144.27, 144.45, 145.06, 145.06, 145.28, 147.11, 147.16, 147.29, 147.68, 148.03, 148.12, 148.28, 148.42, 148.57, 148.62, 151.12, 152.98, 156.12, 156.56, 192.32. Synthesis of C60Ph5C4H8SH (4): 1-(Butylthiol)-6,9,12,15,18pentaphenyl-1,6,9,12,15,18-hexahydro[60]fullerene. Synthesis of the title compound was performed by similar condition employing the procedure of 3 described above using C60Ph5C4H8SAc as a starting material. The title compound was isolated in 97% yield as orange crystals by purification with reprecipitation with methanol. 1H NMR (500 MHz, CDCl3): δ 1.12-1.15 (m, 2H, CH2), 1.43-1.46 (m, 2H, CH2), 1.55-1.56 (m, 2H, CH2), 2.24 (t, J ) 6.35 Hz, 2H, CH2), 7.10-7.11 (m, 3H, Ar), 7.27-7.30 (m, 14H, Ar), 7.67-7.69 (m, 4H, Ar), 7.79-7.80 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 25.00, 29.81, 32.93, 39.82, 58.43, 60.97, 63.21, 64.98, 126.82, 127.61, 127.71, 127.76, 128.83, 128.72, 128.79, 130.51, 138.58, 139.91, 143.37, 143.62, 143.70, 143.93, 144.03, 144.18, 144.23, 144.25, 144.28, 144.46, 145.08, 145.29, 146.92, 147.11, 147.17, 147.31, 147.71, 148.06, 148.14, 148.30, 148.44, 148.58, 148.64, 151.11, 153.00, 156.10, 156.30. Anal. Calcd for C94H29S: C, 94.45; H, 2.87. Found: C, 94.19; H, 3.11. Synthesis of C60Ph5C6H12I (5a). Synthesis of the title compound was performed by similar condition employing the procedure of 3a described above using 1,6-diiodohexane. The title compound was isolated in 87% yield. 1H NMR (500 MHz, CDCl3): δ 0.73-0.82 (m, 2H, CH2), 1.06-1.13 (m, 2H, CH2), 1.35-1.44 (m, 4H, CH2), 1.48-1.55 (m, 2H, CH2), 3.03-3.07

Conical Pentaaryl[60]fullerene Thiols (t, J ) 6.9 Hz, 2H, CH2), 7.09-7.13 (m, 3H, Ar), 7.26-7.37 (m, 14H, Ar), 7.62-7.67 (m, 4H, Ar), 7.74-7.78 (m, 4H, Ar). 13 C NMR (125 MHz, CDCl3): δ 6.95, 24.80, 27.58, 30.31, 32.41, 40.52, 58.49, 61.07, 63.28, 65.17, 126.87, 127.69, 127.75, 127.82, 127.88, 128.27, 128.43, 128.74, 128.75, 130.59, 138.78, 140.17, 142.57, 142.60, 143.44, 143.72, 143.84, 143.97, 144.06, 144.23, 144.26, 144.32, 144.55, 145.19, 145.36, 146.97, 147.18, 147.24, 147.41, 147.76, 148.09, 148.18, 148.36, 148.49, 148.63, 148.70, 151.18, 153.17, 156.36, 156.70, 192.40. Synthesis of C60Ph5C6H12SAc (5b). Synthesis of the title compound was performed by similar condition employing the procedure of 3b described above using C60Ph5C6H12I as a starting material. The title compound was isolated in 84% yield as red crystals by purification with silica gel column chromatography (CS2/EtOAc ) 1/0 to 1/1 as an eluent). 1H NMR (500 MHz, CDCl3): δ 0.67-0.76 (m, 2H, CH2), 0.96-1.03 (m, 2H, CH2), 1.19-1.28 (m, 2H, CH2), 1.29-1.38 (m, 4H, CH2), 2.32 (s, 3H, CH3), 2.70 (t, J ) 7.40 Hz, 2H, CH2), 7.03-7.08 (m, 3H, Ar), 7.21-7.32 (m, 14H, Ar), 7.58-7.62 (m, 4H, Ar), 7.68-7.72 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 24.80, 28.22, 28.65, 28.74, 28.95, 30.41, 40.48, 58.46, 61.04, 63.27, 65.17, 126.84, 127.63, 127.72, 127.79, 127.86, 128.37, 128.70, 128.74, 130.57, 138.73, 140.13, 142.53, 142.59, 143.43, 143.71, 143.84, 143.96, 144.05, 144.21, 144.30, 144.54, 145.17, 145.34, 146.95, 147.16, 147.22, 147.39, 147.74, 147.97, 148.07, 148.17, 148.34, 148.47, 148.62, 148.68, 151.17, 153.16, 156.31, 156.39, 194.97. Synthesis of C60Ph5C6H12SH (5): 1-(Hexylthiol)-6,9,12,15,18pentaphenyl-1,6,9,12,15,18-hexahydro[60]fullerene. Synthesis of the title compound was performed by similar condition employing the procedure of 3 described above using C60Ph5C6H12SAc as a starting material. The title compound was isolated in 40% yield as orange crystals by purification with precipitation with methanol. 1H NMR (400 MHz, CDCl3): δ 0.68-0.77 (m, 2H, CH2), 1.01-1.09 (m, 2H, CH2), 1.15-1.22 (m, 2H, CH2), 1.25-1.33 (m, 2H, CH2), 1.32-1.38 (m, 2H, CH2), 2.32-2.38 (m, 2H, CH2), 7.03-7.08 (m, 3H, Ar), 7.21-7.32 (m, 14H, Ar), 7.58-7.63 (m, 4H, Ar), 7.69-7.73 (m, 4H, Ar). 13C NMR (100 MHz, CDCl3): δ 24.93, 25.04, 28.38, 40.81, 58.81, 61.39, 63.63, 65.51, 127.16, 127.96, 128.04, 128.11, 128.20, 128.71, 129.03, 129.09, 130.91, 139.09, 140.51, 142.91, 143.77, 144.05, 144.19, 144.29, 144.38, 144.54, 144.59, 144.63, 144.88, 145.50, 145.69, 147.29, 147.50, 147.56, 147.73, 148.08, 148.30, 148.41, 148.50, 148.68, 148.81, 148.95, 149.02, 151.52, 153.50, 156.64, 157.04. APCI-HRMS (-): calcd for C126H58S [M]- 1603.4259, found 1603.4260. Synthesis of C60(C6H4-Ph-4)5C6H12I (6a). Synthesis of the title compound was performed by similar condition employing the procedure of 3a described above using C60(C6H4-Ph-4)5H and 1,6- diiodohexane. The title compound was isolated in 89% yield as red crystals by purification with silica gel column chromatography (CS2/toluene ) 1/0 to 1/1 as an eluent) and reprecipitation with methanol. 1H NMR (500 MHz, CDCl3): δ 0.86-0.89 (m, 2H, CH2), 1.13-1.16 (m, 2H, CH2), 1.49-1.61 (m, 6H, CH2), 2.91-2.94 (m, 2H, CH2), 7.32-7.50 (m, 20H, Ar), 7.58-7.65 (m, 15H, Ar), 7.80-7.81 (m, 4H, Ar), 7.93-7.94 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 7.33, 25.25, 27.84, 30.44, 32.36, 40.79, 58.38, 60.96, 63.21, 65.50, 126.61, 126.90, 127.02, 127.10, 127.32, 127.46, 127.58, 128.45, 128.68, 128.68, 128.77, 128.84, 128.88, 129.30, 131.07, 137.98, 139.66, 139.86, 140.01, 140.15, 140.58, 140.67, 1401.79, 142.74, 143.52, 143.83, 143.86, 144.20, 144.36, 144.45, 144.66, 145.32, 145.47, 147.05, 147.26, 147.33, 147.53, 147.85, 148.07, 148.20, 148.29, 148.47, 148.58, 148.72, 148.80, 151.32, 153.31, 156.77.

J. Phys. Chem. C, Vol. 114, No. 41, 2010 17751 Synthesis of C60(C6H4-Ph-4)5C6H12SAc (6b). Synthesis of the title compound was performed by similar condition employing the procedure of 3b described above using C60(C6H4-Ph4)5C6H12I as a starting material. The title compound was isolated in 87% yield as orange crystals by purification with silica gel column chromatography (CS2/toluene ) 1/0 to 1/1 as an eluent) and reprecipitation with methanol. 1H NMR (500 MHz, CDCl3): δ 0.86-0.89 (m, 2H, CH2), 1.07-1.13 (m, 2H, CH2), 1.32-1.38 (m, 2H, CH2), 1.57-1.61 (m, 4H, CH2), 2.09 (s, 3H, CH3), 2.65-2.68 (m, 2H, CH2), 7.32-7.37 (m, 6H, Ar), 7.39-7.51 (m, 15H, Ar), 7.57-7.66 (m, 16H, Ar), 7.80-7.81 (m, 4H, Ar), 7.92-7.94 (m, 4H, Ar). 13C NMR (125 MHz, CDCl3): δ 25.18, 28.37, 28.73, 28.93, 30.33, 40.75, 58.38, 60.97, 63.23, 65.52, 126.62, 126.91, 127.04, 127.10, 127.16, 127.40, 127.47, 127.54, 128.17, 128.42, 128.66, 128.82, 128.86, 128.98, 129.25, 131.05, 137.99, 139.49, 139.66, 139.91, 140.08, 140.23, 140.56, 140.67, 141.81, 142.72, 143.52, 143.84, 143.89, 144.07, 144.18, 144.31, 144.38, 144.43, 144.66, 145.32, 145.46, 147.03, 147.24, 147.31, 147.53, 147.83, 148.04, 148.18, 148.26, 148.45, 148.55, 148.69, 148.77, 151.35, 153.31, 156.76, 156.80, 195.52. Synthesis of C60(C6H4-Ph-4)5C6H12SH (6): 1-(Hexylthiol)6,9,12,15,18-pentabiphenyl-1,6,9,12,15,18-hexahydro[60]fullerene. Synthesis of the title compound was performed by similar condition employing the procedure of 3 described above using C60(C6H4-Ph-4)5C6H12SAc as a starting material. The title compound was isolated in 97% yield as orange crystals by purification with reprecipitation with methanol. 1H NMR (500 MHz, CDCl3): δ 0.87-0.91 (m, 2H, CH2), 1.10-1.16 (m, 2H, CH2), 1.32-1.38 (m, 2H, CH2), 1.57-1.61 (m, 4H, CH2), 2.25-2.29 (m, 2H, CH2), 7.33-7.50 (m, 20H, Ar), 7.58-7.65 (m, 17H, Ar), 7.81-7.83 (m, 4H, Ar), 7.94-7.95 (m, 4H, Ar). 13 C NMR (125 MHz, CDCl3): δ 24.49, 25.14, 28.11, 28.35, 33.00, 40.81, 58.41, 60.99, 63.26, 65.55, 125.26, 126.60, 126.90, 127.02, 127.06, 127.15, 127.45, 127.59, 128.18, 128.46, 128.70, 128.85, 128.90, 128.99, 129.33, 131.09, 138.01, 139.47, 139.69, 139.90, 140.07, 140.19, 140.57, 140.69, 141.84, 142.76, 143.56, 143.86, 143.92, 144.11, 144.22, 144.38, 144.42, 144.46, 144.69, 145.35, 145.50, 147.09, 147.30, 147.36, 147.56, 147.89, 148.11, 148.23, 148.50, 148.61, 148.76, 148.82, 151.37, 153.34, 156.76, 156.81. APCI-HRMS (-): calcd for C126H58S [M]- 1603.4259, found 1603.4260. Acknowledgment. This study was partially supported by KAKENHI (#18105004) and the Global COE Program for Chemistry Innovation of the MEXT, Japan. Supporting Information Available: CIF files for two crystal structures. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (2) (a) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Chem. Commun. 2000, 1921–1922. (b) Nomoto, A.; Mitsuoka, H.; Ozeki, H.; Kobuke, Y. Chem. Commun. 2003, 1074–1075. (c) Nomoto, A.; Kobuke, Y. Chem. Commun. 2002, 1104–1105. (d) Kawasaki, M.; Aoyama, S. Chem. Commun. 2004, 988–989. (e) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367–8368. (f) Imahori, H.; Hasobe, T.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Fukuzumi, S. Langmuir 2001, 17, 4925–4931. (g) Terasaki, N.; Iwai, M.; Yamamoto, N.; Hiraga, T.; Yamada, S.; Inoue, Y. Thin Solid Films 2008, 516, 2553–2557. (h) Shiro, Y.; Morita, T.; Imanishi, Y.; Kimura, S. Science 2004, 304, 1944–1946. (3) Yasseri, A. A.; Syomin, D.; Loewe, R. S.; Lindsey, J. S.; Zaera, F.; Bocian, D. F. J. Am. Chem. Soc. 2004, 126, 15603–15612. (4) (a) Creager, S.; Yu, C. J.; Bamdad, C.; O’Connor, S.; MacLean, T.; Lam, E.; Chong, Y.; Olsen, G. T.; Luo, J.; Gozin, M.; Kayyem, J. F.

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