Preparation and Photophysical and Photoelectrochemical Properties

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J. Phys. Chem. C 2009, 113, 18369–18378

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Preparation and Photophysical and Photoelectrochemical Properties of Supramolecular Porphyrin Nanorods Structurally Controlled by Encapsulated Fullerene Derivatives Atula S. D. Sandanayaka,*,† Tatsuya Murakami,‡ and Taku Hasobe*,†,§ School of Materials Science, Japan AdVanced Institute of Science and Technology (JAIST), Nomi, Ishikawa, 923-1292 Japan, Center for Nano Materials and Technology, JAIST, Nomi, 923-1292 Japan, and PRESTO, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 Japan ReceiVed: July 6, 2009; ReVised Manuscript ReceiVed: August 25, 2009

A new class of porphyrin nanorods structurally controlled by encapsulated fullerene derivatives is prepared via a solvent mixture technique. These nanorods, composed of fullerenes (C60, C60 derivatives and C70) and zinc meso-tetra(4-pyridyl)porphyrin [ZnP(Py)4], are formed with the aid of a surfactant, cetyltrimethylammonium bromide (CTAB), in a DMF/acetonitrile mixture. In scanning electron microscopy (SEM) measurement, ZnP(Py)4 pristine hexagonal nanotubes with a large hollow structure [denoted as ZnP(Py)4 tube] are observed, whereas the hollow hole is completely closed in the case of nanorods composed of fullerenes (C60 and C70) and ZnP(Py)4 [fullerene-ZnP(Py)4 rod]. In C60 derivative-ZnP(Py)4 rods, the distorted polygonal columnar structures with large diameter and length are formed, as compared to the hexagonal structures of C60-ZnP(Py)4 and C70-ZnP(Py)4 rods. X-ray diffraction (XRD) analyses also reveals that ZnP(Py)4 alignment in the nanorod is based on the stacked assemblies of ZnP(Py)4 coordinated hexagonal formations. Elemental analysis and titration experiment by absorption measurement were also performed to quantitatively check the relative molecular ratio between porphyrins and fullerenes. Steady-state and time-resolved fluorescence spectra show efficient fluorescence quenching, suggesting the forward electron-transfer process from the singlet excited state of ZnP(Py)4 to fullerenes. Moreover, the back electron-transfer processes are detected by nanosecond transient absorption measurements. The forward and back electron-transfer rate constants are largely dependent on the structures of the nanorods. To construct photoelectrochemical solar cells, fullerene-ZnP(Py)4 rods are deposited onto nanostructured SnO2 films (OTE/SnO2). Fullerene-ZnP(Py)4 rod-modified electrodes exhibited efficient light energy conversion properties, such as a power conversion efficiency (η) of 0.63% and an incident photon to current conversion efficiency (IPCE) of 35%, which are much larger than those of ZnP(Py)4 tube. Introduction Self-assembly as a method of organizing molecules and controlling size and shape is a simple and convenient way to design organized assemblies.1 Such organized assemblies make it possible to show the new phenomena and properties in comparison with those of the corresponding monomeric forms.2,3 Recent extensive results demonstrate that the driving force of self-assembly in molecular assembly systems is mainly dependent on a single interaction, such as the π-π interaction.4,5 However, by introducing new functional groups onto the peripheral positions, which induces additional intermolecular interactions including van der Waals,6 hydrogen,7 and coordination bonds,8 we can construct high-order organization systems. Thus, cooperative and competitive effects between two different interactions lead to organization of functional molecular materials with different morphologies. Such a strategy enables us to construct a variety of superstructures with different sizes and shapes. Promising applications are photonics and electronics, etc.9 The requirement to utilize renewable energy resources has stimulated new strategies and approaches for fabrication of efficient and low-cost organic photovoltaic systems.10-12 The fundamental photovoltaic mechanism is (i) light-harvest, (ii) * Corresponding author. E-mail: [email protected]. † School of Materials Science, JAIST. ‡ Center for Nano Materials and Technology, JAIST. § PRESTO, JST.

charge-separation for carrier generation, and (iii) carrier transport within the thin film.13,14 These processes are simultaneously and continuously performed within the molecular assembly (i.e., thin film). Therefore, structural and electrochemical tuning for molecular assemblies, which is based on the above three processes, is crucial for the final energy conversion properties. Although a variety of supramolecular assemblies with different sizes and shapes have been widely reported, little effort has been made to construct molecular assemblies for photovoltaic applications.15 Porphyrins contain an extensively conjugated two-dimensional π-system and are suitable not only for synthetic lightharvesting systems but also for efficient electron transfer, since the uptake or release of electrons results in minimal structural and solvation change upon electron transfer.16-18 Therefore, synthetic covalent or noncovalent systems, such as molecular dyad, triad, oligomers, and dendrimers, etc.16-20 have been extensively reported to investigate sequential energy and electron-transfer steps, as observed in natural photosynthetic systems. Moreover, the combination of porphyrins (dyes and electron donor) and fullerenes (electron acceptor) is a good candidate for efficient photoinduced charge separation because of the small reorganization energy in electron transfer reactions.17,18 Porphyrins are also promising major building blocks for superstructures. So far, a variety of nanostructures have been demonstrated to be tubes,21 rods,22,23 rings,24 particles,25,26 sheets,27 and wires.28 Particularly, bar-shaped assemblies of

10.1021/jp9063577 CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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SCHEME 1: Molecular Structures of ZnP(Py)4 and Fullerene Derivatives, Such As C60, C70, C60Ph and C60tBu Employed in This Study

porphyrins (i.e., porphyrin nanorods and nanotubes) have great potential for fabrication of optoelectronics because of the anisotropic structures. To date, these porphyrin nanoassemblies have been mainly composed of single molecular species.21-28 Considering the photovoltaic mechanism (vide supra), a barshaped structure composed of two different molecules with separated inside and outside layers is a good candidate for efficient carrier generation (hole and electron). Recently, we reported novel C60-encapsulated porphyrin hexagonal nanorods prepared in a mixed solvent;23c however, the controlled structural and detailed photochemical properties have yet to be explored. Herein, we report preparation and photophysical and photoelectrochemical properties of porphyrin nanorods composed of zinc meso-tetra(4-pyridyl) porphyrin, which are encapsulated by fullerene derivatives such as C60, C70, C60Ph, and C60tBu, as shown in Schemes 1 and 2. With the increasing bulkiness of fullerene derivatives, the sizes of nanorod structures systematically increase, and hexagonal shapes change into the distorted polygonal ones. We have also revealed the photoinduced electron transfer process from ZnP(Py)4 assembly to fullerene assembly to give the charge separation, which results in the formation of a radical ion pair in nanorods. Moreover, the photoelectrochemical behavior was measured to systematically compare with the structural and photophysical properties. Experimental Section General Information. Zinc meso-tetra(4-pyridyl)porphyrin [ZnP(Py)4] and cetyltrimethylammonium bromide (CTAB) were purchased from Aldrich. C60 and C70 were purchased from MTR Ltd. C60Ph and C60tBu were synthesized according to a previously reported method.29,30 All solvents and reagents of the best grade available were purchased from commercial suppliers and were used without further purification. All experiments were performed at room temperature. Fluorescence images were measured by a Nikon Eclipse LV100 optical microscope connected to a digital camera (Digital Sight DSRI1-U2) and filter cube (C-FL V-2A). Optically transparent electrodes (OTEs) were purchased from Ceramic Forum Co., Ltd.23c Preparation of ZnP(Py)4 Nanotube. ZnP(Py)4 nanotubes were prepared using the following procedure: 0.4 mL of 0.25 mM ZnP(Py)4 in DMF solution was injected into 3 mL of continuously stirred 0.20 mM CTAB/acetonitrile solution at

Sandanayaka et al. room temperature (final concentration [ZnP(Py)4] ) 0.03 mM). The resulting green solution was employed for analysis of the structures. Preparation of ZnP(Py)4 Nanorod. ZnP(Py)4 nanorods were prepared by the same method as ZnP(Py)4 nanotubes: 0.4 mL of 0.25 mM ZnP(Py)4 with fullerene or fullerene derivatives in DMF solution was injected into 3 mL of continuously stirred acetonitrile/0.20 mM CTAB solution at room temperature (final concentrations, [ZnP(Py)4] ) 0.03 mM and [fullerenes] ) 0.02 mM). For thermogravimetric and elementary analysis, the products were centrifuged at 14 000 rpm, and the solvent was decanted. The greenish residue was collected and dried under vacuum. Finally, the greenish solid powder was used for TGA and elementary analysis. Electron Micrograph Measurements. Transmission electron micrograph (TEM) measurements were recorded by applying a drop of the sample to a copper grid. Images were recorded on a Hitachi H 7100 transmission electron microscope at an accelerating voltage of 100 kV for imaging. SEM images of porphyrin assemblies were recorded using a Hitachi S-4100 scanning electron microscope. Dynamic Light Scattering (DLS) Measurements. The particle size and distribution were measured in DMF/acetonitrile, 2/15, v/v, using light-scattering equipment (Zetasizer nano ZS). Thermogravimetric Analysis. The thermogravimetric analysis was performed using a Perkin-Elmer Pyris 6 TGA instrument in an inert atmosphere of nitrogen and helium. In a typical experiment, 10 mg of the material was placed in the sample pan, and the temperature was equilibrated at 150 °C. Subsequently, the temperature was increased to 1000 °C at a rate of 10 °C/min, and the weight changes were recorded as a function of temperature. Steady-State Spectroscopic Measurements. Steady-state absorption spectra in the visible and near-IR regions were measured on a Perkin-Elmer (Lamda 750) UV-vis-NIR spectrophotometer. Steady-state fluorescence spectra were measured on a Perkin-Elmer (LS-55) spectrofluorophotometer equipped with a photomultiplier tube having high sensitivity in the 400-800 nm region. Time-Resolved Fluorescence Measurements. The timeresolved fluorescence spectra were measured by a single photon counting method using a streakscope (Hamamatsu Photonics, C5680) as a detector and a laser light (Hamamatsu Photonics M10306, laser diode head, 408 nm) as an excitation source. Lifetimes were evaluated with software attached to the equipment. Nanosecond Transient Absorption Measurements. Nanosecond transient absorption measurements were carried out using THG (532 nm) of a Nd:YAG laser (Spectra-Physics, QuantaRay GCR-130, 5 ns fwhm) as an excitation source. For transient absorption spectra in the near-IR region (600-1200 nm) and the time-profiles, monitoring light from a pulsed Xe lamp was detected with a Ge-APD (Hamamatsu Photonics, B2834). For the measurements in the visible region (400-1000 nm), a SiPIN photodiode (Hamamatsu Photonics, S1722-02) was used as a detector. X-ray Diffraction Measurement. X-ray diffraction (XRD) measurement was carried out with a Bruker-axs M18XHF-SRA using filtered Cu KR radiation. The sample for XRD analysis was prepared by the drying suspension liquid over a glass substrate in air. AFM Measurement. Sample surface morphologies were analyzed with atomic force microscopy (AFM) using a Nanoscope III (Veeco Instruments) fitted with a 20 µm scan. Scanning

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SCHEME 2: An Illustration of Fullerene Nanoparticle-Promoted Organization of ZnP(Py)4 Nanorods in This Studya

a CTAB is omitted for clarity. The electron micrographs show (A) immediate formations of C60tBu and ZnP(Py)4 composites (1 min) after injection, (B) hexagonal C60-ZnP(Py)4 rod, and (C) distorted polygonal C60tBu-ZnP(Py)4 rod. The crystal structure of ZnP(Py)4 ‘Hexagonal Formation’ was analyzed by data of ref..31

was performed in tapping mode. The AFM was operated at a scan rate of 1 Hz. Preparation of Nanorod-Deposited Films. ZnP(Py)4 and C60 composite films were simply prepared by an electrophoretic deposition method. A known amount of ZnP(Py)4 C60 or the mixture solution in DMF/acetonitrile (2/15, v/v, 2 mL) was transferred to a 1 cm cuvette in which two optically transparent electrodes were kept at a distance of 6 mm using a Teflon spacer. A DC electric field (∼200 V/cm) was applied for 1 min between these two electrodes using a Power Pac HV (Bio-Rad). The deposition of the film can be visibly seen as the solution becomes colorless with simultaneous green coloration of the OTE electrode. The OTE electrode coated with fullerene-ZnP(Py)4 rods is referred to OTE/SnO2/ fullerene-ZnP(Py)4. Measurement of Photoelectrochemical Solar Cells. Photoelectrochemical measurements were carried out in a standard two-compartment cell consisting of a working electrode and a Pt wire gauze counter electrode in the electrolyte. The electrolyte was 0.5 M LiI and 0.01 M I2 in acetonitrile. Keithley 2400 was used for recording photocurrent and photovoltage responses under an AM1.5 simulated light source (Otento-Sun II, BunkohKeiki Co., Ltd). In the case of IPCE measurement, a monochromator (SM-25, Bunkoh-Keiki Co.,Ltd) was introduced into the path of the excitation beam (300 W xenon lamp, BunkohKeiki Co., Ltd) for the selected wavelength. The lamp intensity at each wavelength was determined by a Si photodiode (Hamamatsu Photonics S1337-1010BQ) and corrected. Results and Discussion Preparation of Nanorods and Their Structural Characterization. C60Ph and C60tBu were synthesized according to the reported literature.29,30 The nanorods are formed with the aid of a surfactant, cetyltrimethylammonium bromide, in a DMF/

acetonitrile mixed solvent. The detailed organization procedure between ZnP(Py)4 and fullerene derivatives (C60, C70, C60Ph and C60tBu) is shown in Scheme 2. A mixture of the appropriate ratio in DMF solution was injected into 7.5 times the volume of acetonitrile while continuously stirring a 0.20 mM CTAB acetonitrile solution at room temperature. The final concentrations of ZnP(Py)4 and fullerene are 0.03 and 0.02 mM in DMF/ acetonitrile (2/15, v/v), respectively. On injection, they immediately form ZnP(Py)4 flake assemblies and fullerene-based nanoparticles (2. Initial Assembly in Scheme 2), separately. The resulting solution becomes a greenish colloidal suspension, since the aggregated assembly is formed. In Scheme 2A (TEM image), we can clearly see a lot of fullerene nanoparticles and ZnP(Py)4 flake assemblies, separately.23c In the diffusion process of DMF into acetonitrile, the Zn-N axial coordination induces the initial hexagonal and subsequent flake aggregates of ZnP(Py)4. The organization process of ZnP(Py)4 moieties is originated from a Zn-N axial coordination for hexagonal formation and π-π interaction for the subsequent flake organization.21a,23c,31 On the other hand, the driving force for nanoparticle formation of fullerenes is based on relatively weak π-π interaction. Here, it should be noted that the particle diameters are largely dependent on the structures of fullerenes. Figure 1 shows dynamic light scattering (DLS) results of the respective fullerene single nanoparticles prepared under the same conditions [denoted as (fullerene)n]. With an increase in the bulkiness of substituent groups in fullerene derivatives, the average diameters increased from 15 to 80 nm.32,33 This trend has a great effect on the final structures of nanorods (diameter and length) (vide infra). After several minutes, repeated centrifugations (14 000 rpm) and a filtration are performed to remove CTAB surfactant, unbound ZnP(Py)4, and fullerene assemblies, then fullerene-encapsulated ZnP(Py)4 nanorods are

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Figure 1. Size distributions of fullerene nanoparticles estimated by dynamic light scattering measurement: (A) (C60)n, (B) (C70)n, (C) (C60Ph)n, and (D) (C60tBu)n in DMF/acetonitrile ) 2/15, v/v.

finally formed [denoted as fullerene-ZnP(Py)4 rod]. The reference ZnP(Py)4 nanotubes without fullerenes were also prepared in the same manner for comparison [denoted as ZnP(Py)4 tube]. These self-assembled nanostructures can be maintained for several days without precipitation. Figure 2 shows SEM images of fullerene-encapsulated ZnP(Py)4 nanorods. ZnP(Py)4 tubes without fullerenes show a bar-shaped assembly with a large hollow hole (Figure 2A) whereas the hollow hole is completely closed in C60-ZnP(Py)4, C70-ZnP(Py)4, C60Ph-ZnP(Py)4, and C60tBu-ZnP(Py)4 rods (Figure 2B-E). This indicates that fullerene assemblies are effectively encapsulated within a ZnP(Py)4 tube. On the basis of these SEM results, the detailed distribution of length and diameter can also be analyzed. The length distribution figure of C60tBu-ZnP(Py)4 rod (Figure 2E) is shown in Figure 2F. All distributions are shown in Figure S1 of the Supporting Information (SI). The detail values of length and outside diameter are also summarized in Table 1. In the case of the reference ZnP(Py)4 tube, the structure is analyzed as 2.13 µm in length and 540 nm in outside diameter. On the other hand, the structure of C60-ZnP(Py)4 rod is 4.12 ( 0.94 µm in length and 490 ( 90 nm in outside diameter, which is very similar to those of C70-ZnP(Py)4 rod (4.50 ( 0.71 µm in length and 480 ( 40 nm in diameter). In these three systems, the diameters are very similar (∼500 nm) whereas a large increase in length from 2.13 to ∼4 µm is observed. This anisotropic growth in length is largely dependent on π-π interaction of encapsulated C60 or C70 moieties within the ZnP(Py)4 tube. In the case of C60 derivative composites, such as C60Ph-ZnP(Py)4 (6.45 ( 1.71 µm in length and 700 ( 160 nm in diameter) and C60tBu-ZnP(Py)4 (7.82 ( 2.07 µm in length and 700 ( 160 nm in diameter), the corresponding length as well as outside diameter increases as compared to those of C60-ZnP(Py)4 rod. As stated above, the average sizes of the C60, C70, C60Ph, and C60tBu nanoparticles are 15, 20, 60, and 80 nm, respectively as shown in Table 1. The increased sizes of bar-shaped structures are in good agreement with the trend of fullerene nanoparticles in part “2. Initial Assembly” process of Scheme 2 (vide supra). This indicates that an encapsulated

Figure 2. SEM images of (A) ZnP(Py)4 tube, (B) C60-ZnP(Py)4 rod, (C) C70-ZnP(Py)4 rod, (D) C60Ph-ZnP(Py)4 rod, (E) C60tBu-ZnP(Py)4 rod. (F) The length distribution of C60tBu-ZnP(Py)4 rod.

TABLE 1: Analyzed Structures of ZnP(Py)4 Tube, and C60-ZnP(Py)4 Rod, C70-ZnP(Py)4 Rod, C60Ph-ZnP(Py)4 Rod, C60tBu-ZnP(Py)4 Rod, and the Corresponding Fullerene Nanoparticles nanorod/nanotube

rod length (µm)a

ZnP(Py)4 tube C60-ZnP(Py)4 rod C70-ZnP(Py)4 rod C60Ph-ZnP(Py)4 rod C60tBu-ZnP(Py)4 rod

2.13 ( 0.27 4.12 ( 0.94 4.50 ( 0.71 6.45 ( 1.71 7.82 ( 2.07

a

rod diameter fullerene particle (nm)a diameter (nm)b 540 ( 30 490 ( 90 480 ( 40 700 ( 160 940 ( 240

15 20 60 80

Analyzed by SEM. b Average values analyzed by DLS.

process of fullerene assemblies by ZnP(Py)4 flake assemblies plays an important role in the final structural organization. Additionally, the macroscopic cross-sectional shape of C60-ZnP(Py)4 rod (Figure 2B) and C70-ZnP(Py)4 rod (Figure 2C) is a hexagonal structure, which is very similar to that of ZnP(Py)4 tube (Figure 2A). In contrast to these three systems, the shape of C60Ph-ZnP(Py)4 (Figure 2D) and C60tBu-ZnP(Py)4 (Figure 2E) is a distorted polygonal shape. As discussed above, the initial step of ZnP(Py)4 moieties is a molecular hexagonal formation after injection (crystal structure in Scheme 2). Therefore, the basic formation largely reflects the macroscopic ZnP(Py)4 tube assembly (Figure 2A). The difference between hexagonal formation [C60-ZnP(Py)4 rod and C70-ZnP(Py)4 rod] (Scheme 2B) and a distorted polygonal one [C60Ph-ZnP(Py)4 rod and C60tBu-ZnP(Py)4 rod] (Scheme 2C) is presumably due to the sizes of the fullerene nanoparticles. With increasing nanoparticle sizes of fullerenes, the relative size of the ZnP(Py)4 flake

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Figure 4. TGA analyses of (a) C70 starting material, (b) ZnP(Py)4 monomer (c) ZnP(Py)4 tubes, and (d) C70-ZnP(Py)4 rod.

Figure 3. XRD patterns of (a) a simulated pattern from the crystal structure of ZnP(Py)4 reported in ref 31, (b) ZnP(Py)4 starting material, (c) ZnP(Py)4 tube, (d) C60-ZnP(Py)4 rod, (e) C70-ZnP(Py)4 rod, (f) C60Ph-ZnP(Py)4 rod, and (g) C60tBu-ZnP(Py)4 rod.

assembly decreases toward the fullerene nanoparticle. Consequently, in the organization process [“3. organization of ZnP(Py)4 and fullerene assemblies” in Scheme 2], the macroscopic organization of ZnP(Py)4 assemblies in C60Ph-ZnP(Py)4 and C60tBu-ZnP(Py)4 rods may hamper formation of hexagonal structures, which leads to the final distorted structures.34-36 X-ray Diffraction (XRD) Measurement. To examine the internal structures, we also measured X-ray diffraction (XRD) patterns of these assemblies. In Figure 3, patterns a and b are obtained from the simulated pattern from the reported crystal structure of ZnP(Py)431 and ZnP(Py)4 bulk starting material, respectively. Patterns c, d, e, f, and g are derived from the ZnP(Py)4 tube, C60-ZnP(Py)4 rod, C70-ZnP(Py)4 rod, C60Ph-ZnP(Py)4 rod, and C60tBu-ZnP(Py)4 rod, respectively. The patterns of self-assembled ZnP(Py)4 assemblies (patterns c-g) are consistent with that of the bulk starting material, ZnP(Py)4 (pattern b). This means that ZnP(Py)4 assemblies consist of pristine ZnP(Py)4 moieties. To further examine the detailed crystal structures and selfassembled aggregate modes, we also compared the patterns of self-assembled ZnP(Py)4 assemblies (pattern c-g) with the simulated pattern from the crystal structure of ZnP(Py)4 (pattern a). The peaks in patterns c-g are assigned according to the simulated one. The pattern of fullerene-ZnP(Py)4 nanorod (patterns d-g) is approximately the same as that of the ZnP(Py)4 tube (pattern c). This suggests that ZnP(Py)4 internal alignment of the nanorods has structures quite similar to the ZnP(Py)4 tube, and fullerene moieties are encapsulated within the ZnP(Py)4 tube structure, as shown in Scheme 2. Moreover, we can see strong diffraction peaks of a and b axes such as (110) and (220). In contrast, the diffraction intensity based on the c axis is very weak. Considering the unit cell structure (monoclinic structure; dihedral angle: 90° in axes a-c and b-c), the growth direction of the rod assemblies is in the c axis direction. This indicates continuous stacked structure of ZnP(Py)4 hexagonal formation contributes to the final macroscopic bar-shaped structure (vide supra). Although the cross-sectional shape in C60Ph-ZnP(Py)4 rod and C60tBu-ZnP(Py)4 rod is a polygonal structure, the basic

crystal formation of ZnP(Py)4 moieties is likely the same as those of ZnP(Py)4 tube, C60-ZnP(Py)4 rod, and C70-ZnP(Py)4 rod.37 Finally, here it should be noted that we cannot exclude the possibility of composite molecular aggregation between ZnP(Py)4 and fullerene due to the well-known attractive interaction of porphyrins and fullerene.3b Definite discrimination is difficult because of a continuous organization process in solution. However, on the basis of SEM, XRD, fluorescence images (vide infra), elemental analysis,36 titration experiment by absorption measurement,35 and a controlled photoinduced electron transfer process via the singlet excited state of porphyrins (vide infra), we can conclude that such fullerene-encapsulated organization occurs mainly in these systems. TGA Measurement. Thermal gravimetric analyses of C70-ZnP(Py)4 rod and reference compounds were measured (Figure 4). The TGA curve shows a decrease in the initial sample weight (∼10 mg) in all systems with increasing temperature. C70 demonstrates excellent thermal stability up to 800 °C, whereas it exhibits a monotonic and continuous weight loss of about 60% up to 1000 °C (Figure 4a). Figure 4b and c shows ZnP(Py)4 monomer and ZnP(Py)4 tube, respectively. They basically show a large weight loss above 600 °C due to decomposition of ZnP(Py)4.21e The weight loss of ZnP(Py)4 tube is relatively decreased as compared to that of ZnP(Py)4 monomer. This may be due to the strong interaction of ZnP(Py)4 moieties within a nanotube structure. In C70-ZnP(Py)4 rod, we can observe cooperative trends of C70 and ZnP(Py)4 (especially in the range 600-700 °C). Thus, TGA results ensure the formation of nanorods composed of ZnP(Py)4 and fullerene moieties, suggesting that C70 and ZnP(Py)4 molecules in nanorods have an appropriate chance to contact each other. This observation further suggests that nanorod formation should be stable up to 600 °C.38 Steady-State Spectroscopic Measurement. Electronic absorption spectroscopy was used to probe the formation of ZnP(Py)4 nanorod structures as well as the ground state electronic interactions between the individual components in the nanorods. In Figure 5, absorption spectra of ZnP(Py)4 monomer, ZnP(Py)4 tube, and C60tBu-ZnP(Py)4 rod are shown. In the measurement of absorption spectra, we employed an integrating sphere to avoid a scattering effect on the apparent absorption. The characteristic Soret and Q bands of ZnP(Py)4 monomer are identified at 424, 558, and 597 nm, respectively (spectrum a in Figure 5). The Soret and Q bands of ZnP(Py)4 tube and C60tBu-ZnP(Py)4 rod (spectra b and c in Figure 5) become split, broadened, and red-shifted by approximately 40 nm as compared to the corresponding bands of monomer solution (spectrum a). These results suggest that electronic

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Figure 5. Steady-state absorption spectra of (a) 6 µM ZnP(Py)4 monomer, (b) ZnP(Py)4 tube, and (c) C60tBu-ZnP(Py)4 rod in DMF/ acetonitrile (15/2, v/v).

interaction occurs in the ground states of ZnP(Py)4 nanorods. Furthermore, this sharp splitting of Soret band may be attributed to exciton coupling.20a The broadened and red-shifted Q-bands mainly suggest the occurrence of the related J-type interaction in the nanorods.26a As discussed above, the length direction of the nanorod assembly is the c axis. In the c axis direction, head-to-tail structure of ZnP(Py)4 moieties are mainly observed (SI, Figure S3).23c,31 Additionally, the absorption spectrum of ZnP(Py)4 nanorods also becomes broader than that of ZnP(Py)4 nanotubes because of cooperative aggregated interactions of fullerenes and ZnP(Py)4 assemblies. A quite broad absorption in the visible region is useful for solar energy conversion. The extent of electronic communication in the excited states of the ZnP(Py)4 nanorods was evaluated by fluorescence spectroscopy. Spectra a, b, and c in Figure 6 show the fluorescence spectra of C60Ph-ZnP(Py)4 rod, ZnP(Py)4 tube, and ZnP(Py)4 monomer, respectively. Upon photoexcitation with the 408-nm light, the strong fluorescence emission peaks of ZnP(Py)4 tube appear at 616 and 665 nm (spectrum b). In spectrum a for C60Ph-ZnP(Py)4 rod, the quenching of the porphyrin emission peaks at 620 and 666 nm, which becomes slightly red-shifted, suggests that the singlet excited state of ZnP(Py)4 is effectively deactivated by encapsulated fullerenes via energy, electron transfer, or both. Moreover, a significant red shift with broadening was observed for the fluorescence band of C60Ph-ZnP(Py)4 rod as compared to that of ZnP(Py)4 monomer, following the trend observed during the absorption measurements (Figure 5). Thus, the ZnP(Py)4 and encapsulated fullerenes play a critical role in the efficiency of energy and electron transfer. Similar trends (fluorescence quenching, spectral red shift, and broadening) are also observed in C60-ZnP(Py)4 rod, C60tBu-ZnP(Py)4 rod, and C60-ZnP(Py)4 rod. Moreover, the fluorescence images of C60-ZnP(Py)4 rod and C60Ph-ZnP(Py)4 rod are also shown in Figure 6B and C. Since the outside layer is covered by ZnP(Py)4 assemblies, we can see uniform red, luminous structures. These structural sizes are similar to analyzed sizes in Table 1. Fluorescence Lifetime Measurement. Additional quantitative electronic interplay on the photoexcited ZnP(Py)4 nanorods could be evaluated by time-resolved fluorescence spectroscopy (Figure 7). In this context, the fluorescence emission decays of ZnP(Py)4 nanorods were found to proceed faster than the one observed on the ZnP(Py)4 monomer (τf ) 2.2 ns). By the biexponential fitting of the fluorescence decay of fullerene-ZnP(Py)4 rods, the fluorescence lifetimes (τf) were evaluated to be range of 180-320 ps (Table 2), which are shorter than the one for the ZnP(Py)4 tube (390 ps). For τf values,

Figure 6. (A) Steady-state fluorescence spectra of (a) C60Ph-ZnP(Py)4 rod, (b) ZnP(Py)4 tube in DMF/acetonitrile ) 2/15, v/v, and (c) 3 µM ZnP(Py)4 monomer in DMF. (Spectra b and c are normalized at 653 and 663 nm, respectively) λex ) 408 nm. Fluorescence images of (B) C60-ZnP(Py)4 rod and (C) C60Ph-ZnP(Py)4 rod.

Figure 7. Time-resolved fluorescence decay spectra of (a) C70-ZnP(Py)4 rod, (b) C60Ph-ZnP(Py)4 rod, and (c) C60tBu-ZnP(Py)4 rod in DMF/acetonitrile ) 2/15, v/v. λex ) 408 nm.

TABLE 2: Fluorescence Lifetimes (τf) and Quenching Rates (kSq) for C60-ZnP(Py)4, C70-ZnP(Py)4, C60Ph-ZnP(Py)4, and C60tBu-ZnP(Py)4 Nanorods in DMF/Acetonitrile 2/15, v/v C60-ZnP(Py)4 rod C70-ZnP(Py)4 rod C60Ph-ZnP(Py)4 rod C60tBu-ZnP(Py)4 rod

kSq/s-1b

τf/psa

nanorod/nanotube 180 200 250 320

(85%), (90%), (97%), (95%),

1100 1281 2720 2480

(15%) (10%) (3%) (5%)

3.0 × 109 2.5 × 109 1.5 × 109 0.6 × 109

a Fraction of two-component fitting. b kSq ) 1/τf - 1/τf0; τf0 ) 390 ps for ZnP(Py)4 tube [390 ps (65%), 1870 ps (35%)].

the following tendency was observed: C60-ZnP(Py)4 rod < C70-ZnP(Py)4 rod < C60Ph-ZnP(Py)4 rod < C60tBu-ZnP(Py)4 rod. From such efficient fluorescence quenching, it is presumed that the formation of fullerene assembly encapsulated the nanorods. This trend is in good agreement with the steady-state fluorescence intensity quenching. Considering the fluorescence lifetimes and transient spectroscopy (vide infra), the quenching

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Figure 8. Nanosecond transient absorption spectra of C70-ZnP(Py)4 rod in Ar-saturated DMF/acetonitrile (2/15, v/v) after 532 nm laser irradiation at 0.1 µs (b) and 1.0 µs (O). Inset: the time profiles of the anion radical of C70 monitored at 1380 nm.

process is photoinduced electron transfer (PET) from 1ZnP*(Py)4 to fullerene. Further, τf and kqs values of ZnP(Py)4 nanorods are largely dependent on the basic structure of the encapsulated molecules, as listed in Table 2. Transient Absorption Spectroscopy. Time-resolved transient absorption spectra of ZnP(Py)4 nanorods in DMF/acetonitrile (2/15, v/v) were measured by nanosecond laser photolysis with 532 nm laser excitation (6 ns laser pulse). Figure 8 shows the nanosecond transient absorption spectra of C70-ZnP(Py)4 rod in DMF/acetonitrile (2/15, v/v). The broad transient absorption band at 1380 nm observed immediately after the laser pulse (6 ns) was assigned to the anion radical of the C70 (C70•-) moiety.39 Upon nanosecond photoexcitation of C70-ZnP(Py)4 rod, the characteristic triplet-triplet transition of ZnP(Py)4 (460 and 840 nm regions) and C70 (980 nm region) is completely absent, suggesting that the singlet excited state of ZnP(Py)4 was quenched by C70, as observed in the fluorescence quenching experiments. In the transient absorption spectrum at 0.1 µs, the broad peak observed at 500-780 nm can be attributed to the radical cation of the ZnP(Py)4, [ZnP•+(Py)4] moieties.40 Similar results were also observed for other nanorods, such as C60-ZnP(Py)4 rod, C60Ph-ZnP(Py)4 rod, and C60tBu-ZnP(Py)4 rod (SI: Figure S4). The inset time profile in Figure 8 shows the rapid decay of the C70•- at 1380 nm. This indicates that the charge separation process takes place via the 1 ZnP*(Py)4 moiety. Thus, the formation of a charge-separation state, C70•--ZnP•+(Py)4, in the nanorods was suggested by the transient bands. The decay rates of the transient absorption bands can be attributed to charge recombination, after the formation of the charge separation state, C70•--ZnP•+(Py)4. In the case of C60-ZnP(Py)4 rod, C60Ph-ZnP(Py)4 rod, and C60tBu-ZnP(Py)4 rod, the decay time profiles of the radical anion were recovered at 1080 or 1020 nm (SI: Figure S4), respectively. Analysis of the decay time profiles of these transient bands aids in the calculation of the rate of the charge recombination (kCR) process, in the range of 0.44-1.1 × 107 s-1 (Table 3). The lifetime of the radical ion pairs (τCR) of C60-ZnP(Py)4 rod,23c C70-ZnP(Py)4 rod, C60Ph-ZnP(Py)4 rod, and C60tBu-ZnP(Py)4 rod are thus calculated to be 100, 90, 190, and 230 ns, respectively. Although the τCR of C60-ZnP(Py)4 rod (100 ns) is very similar to that of C70-ZnP(Py)4 rod (90 ns), the τCR values of C60Ph-ZnP(Py)4 rod (190 ns) and C60tBu-ZnP(Py)4 rod (230 ns) demonstrate quite longer τCR values as compared with C60-ZnP(Py)4 rod. This indicates that τCR values are dependent on the sizes of the nanorods. These photophysical properties play an important role in light energy conversion (vide infra). On the other hand, we have also

Figure 9. (A) Absorption spectra of (a) OTE/SnO2/C70-ZnP(Py)4, (b) OTE/SnO2/C60Ph-ZnP(Py)4, and (c) OTE/SnO2/ZnP(Py)4. (B) AFM image of OTE/SnO2/C70-ZnP(Py)4.

TABLE 3: Charge-Recombination Rates (kCR) and Lifetimes of the Radical Ion Pair (τCR); for C60-ZnP(Py)4 Rod, C70-ZnP(Py)4 Rod, C60Ph-ZnP(Py)4 Rod, and C60tBu-ZnP(Py)4 Rod in DMF/Acetonitrile 2/15, v/v nanorods

kCR/s-1a

C60-ZnP(Py)4 rod C70-ZnP(Py)4 rod C60Ph-ZnP(Py)4 rod C60tBu-ZnP(Py)4 rod

1.0 × 10 1.1 × 107 5.3 × 106 4.4 × 106

τCR/ns 7

100 90 190 230

a

From the decay at 1080 nm for C60, 1380 nm for C70, and 1020 nm for C60Ph, C60tBu.

prepared the reference system, ZnP(Py)4 and C60 composites prepared without CTAB. In this case, nonuniform bulky structures are observed. Moreover, the photoinduced electron transfer occurs via 3ZnP*(Py)4 (SI: Figure S5).41 Fabrication of Photoelectrochemical Cells and Their Properties. The highly colored molecular assemblies composed of fullerene and ZnP(Py)4 are assembled onto a nanostructured SnO2 electrode (denoted as OTE/SnO2). The electrophoretic deposition procedure was followed to deposit these molecular composites from a DMF/acetonitrile suspension. Upon application of a dc electric field of 200 V/cm for 1 min between the OTE/SnO2 and OTE electrodes, which were kept parallel in a DMF/acetonitrile suspension containing fullerene and ZnP(Py)4 composites, the composite assemblies are deposited on the SnO2 nanocrystallites. As the deposition continues, the suspension loses color with simultaneous coloration of the electrode.42 OTE/ SnO2 electrodes coated with C60-ZnP(Py)4 rod, C70-ZnP(Py)4 rod, C60Ph-ZnP(Py)4 rod, C60tBu-ZnP(Py)4 rod, and ZnP(Py)4 tube are referred to as OTE/SnO2/C60-ZnP(Py)4, OTE/SnO2/ C70-ZnP(Py)4, OTE/SnO2/C60Ph-ZnP(Py)4, OTE/SnO2/ C60tBu-ZnP(Py)4, and OTE/SnO2/ZnP(Py)4, respectively. Figure 9A shows absorption spectra of the representative electrodes. The absorptivity of fullerene and ZnP(Py)4 composites such as OTE/SnO2/C70-ZnP(Py)4 and OTE/SnO2/C60Ph-ZnP(Py)4 (spectra a and b) is enhanced as compared with that of OTE/SnO2/ ZnP(Py)4. This is likely due to the composite effect between fullerenes and ZnP(Py)4 moieties. Figure 9B shows an AFM image of an OTE/SnO2/C70-ZnP(Py)4 electrode to reveal the molecular aggregation on an OTE/SnO2 film. This result also

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Figure 11. Photocurrent action spectra of (a) OTE/SnO2/C60-ZnP(Py)4 electrode, (b) OTE/SnO2/C70-ZnP(Py)4 electrode, (c) OTE/SnO2/ C60Ph-ZnP(Py)4 electrode, (d) OTE/SnO2/C60tBu-ZnP(Py)4 electrode, and (e) OTE/SnO2/ZnP(Py)4 electrode. Electrolyte, 0.5 M LiI and 0.01 M I2 in acetonitrile.

Figure 10. (A) Photocurrent response and (B) photovoltage response of OTE/SnO2/C60Ph-ZnP(Py)4 electrode under white light illumination (AM 1.5). (C) I-V characteristics of (a) OTE/SnO2/C60Ph-ZnP(Py)4 electrode and (b) OTE/SnO2/ZnP(Py)4 electrode under white light illumination (AM 1.5); electrolyte 0.5 M LiI and 0.01 M I2 in acetonitrile; input power 20 mW/cm2.

suggests that the electrophoretic method of these nanorods leads to the deposition of nanorods on the nanostructured SnO2 electrode. The size of the OTE/SnO2/C70-ZnP(Py)4 is approximately same as that measured by the corresponding SEM image (Figure 2C). This indicates that organization interpenetrating of fullerene and ZnP(Py)4 molecules is achieved on OTE/SnO2. To evaluate the photoelectrochemical performance of fullerene and ZnP(Py)4 composite films, we used the OTE/SnO2 electrode as a photoanode in a photoelectrochemical cell. Photocurrent measurements were performed in acetonitrile containing LiI (0.5 M) and I2 (0.01 M) as redox electrolyte using a Pt gauge counter electrode.43 The photocurrent and photovoltage responses that were recorded following the excitation of the OTE/SnO2/ C60Ph-ZnP(Py)4 electrode in the visible light region (AM 1.5) are shown in Figure 10A and B, respectively. The photocurrent response is prompt, steady, and reproducible during repeated on/off cycles of visible light illumination. The short circuit photocurrent density (Isc) of 1.0 mA/cm2 and open circuit voltage (Voc) of 350 mV were reproducibly obtained during these measurements. Blank experiments conducted with OTE/SnO2 (i.e., by excluding composite C60Ph-ZnP(Py)4 rod) produced no detectable photocurrent under similar experimental conditions. These experiments confirmed the role of C60Ph-ZnP(Py)4 rod toward harvesting light energy and generating photocurrent during the operation of a photoelectrochemical cell. We also evaluated the power characteristics of the OTE/SnO2/ C60Ph-ZnP(Py)4 electrode (Figure 10C). The power conversion efficiency, η, is calculated by eq 1,43

η ) FF × Isc × Voc /Win

(1)

where the fill factor (FF) is defined as FF ) [IV]max/IscVoc, where Voc is the open circuit photovoltage, and Isc is the short circuit photocurrent. The OTE/SnO2/C60Ph-ZnP(Py)4 has a much larger fill factor of 0.36, open circuit voltage of 350 mV, short circuit current density of 1.0 mA cm-2, and the overall power conversion efficiency (η) of 0.63% at an input power (Win) of 20 mW cm-2 as compared to the reference ZnP(Py)4 tube system [OTE/SnO2/ZnP(Py)4]. The η value of OTE/SnO2/ C60Ph-ZnP(Py)4 (0.63%) is more than 20 times as large as that of OTE/SnO2/ZnP(Py)4 (0.03%). Such a significant enhancement of the η value demonstrates that the fullerene-encapsulated ZnP(Py)4 bar-shaped assembly contributes to the improvement of light energy conversion properties. To evaluate spectral photoresponses of these fullerene and ZnP(Py)4 composites on OTE/SnO2, photocurrent action spectra were also measured using a standard two-electrode system in photoelectrochemical cells (Figure 11).42 The IPCE values were calculated by normalizing the photocurrent values for incident light energy and intensity using eq 244

IPCE (%) ) 100 × 1240 × Isc /(Win × λ)

(2)

where Isc is the short circuit photocurrent (A/cm2), Win is the incident light intensity (W/cm2), and λ is the wavelength (nm). The overall response of fullerene-ZnP(Py)4 composites on OTE/SnO2 parallels the broad absorption spectral features (Figure 9A), indicating the involvement of fullerene and ZnP(Py)4 in the photocurrent generation. Especially, in OTE/ SnO2/C70-ZnP(Py)4, a broad photoresponse is observed in the long wavelength region (600-700 nm) because of extended absorption of C70 aggregates (spectrum b). The maximum IPCE values of fullerene-ZnP(Py)4 composites (16-35%, spectra a-d) are much larger than that of the ZnP(Py)4 alone assembly (4%, spectrum e). Considering the above photophysical results, the PET process from 1ZnP*(Py)4 to fullerenes in nanorod assemblies is a crucial pathway for efficient photocurrent generation. Moreover, the maximum IPCE values of OTE/SnO2/C60Ph-ZnP(Py)4 and OTE/SnO2/C60tBu-ZnP(Py)4 (35% and 25%) are much larger than those of OTE/SnO2/C60-ZnP(Py)4 and OTE/SnO2/ C70-ZnP(Py)4 (20% and 16%). As stated above, we need to consider the three processes, such as light-harvest, chargeseparation, and carrier transport as the photovoltaic mechanism.45-47 In this case, the charge-separation and carrier-transport processes are key roles for the final energy conversion, since the absorption

Supramolecular Porphyrin Nanorods properties are very similar in all systems (Figure 9A). In C60Ph-ZnP(Py)4 rod and C60tBu-ZnP(Py)4 rod, the lifetimes of charge recombination (τCR) are longer than those of C60-ZnP(Py)4 rod and C70-ZnP(Py)4 rod (vide supra), which indicates the radical species become more stabilized in C60Ph-ZnP(Py)4 rod and C60tBu-ZnP(Py)4 rod. The other possibility is a structural aspect for the resulting carrier transport. As discussed above, the particle sizes of (C60Ph)n (60 nm) and (C60tBu)n (80 nm) are much larger than those of (C60)n (15 nm) and (C70)n (20 nm). Accordingly, the macroscopic diameters of rod-shaped structures also increase (Table 1). Such an increased grain-size (i.e., particle size) may accelerate the carrier transport properties and decelerate the charge recombination.48 Thus, we demonstrate that a series of structural control of nanorod assemblies are largely dependent on the light energy conversion properties. Photocurrent generation in the present system is initiated by photoinduced electron transfer from the excited singlet state of ZnP(Py)4 (1ZnP*/ZnP•+ ) -1.0 V vs NHE)44 to the fullerene (e.g., C60/C60•- conduction band of SnO2 (0 V vs NHE)44 system. The reduced C60 injects electrons into the SnO2 nanocrystallites, whereas the oxidized porphyrin (ZnP/ZnP•+ ) 1.0 V vs NHE)44 undergoes electron-transfer reduction with the iodide (I3-/I- ) 0.5 V vs NHE)44 in the electrolyte system.44 Conclusion In this study, we demonstrate the utility of porphyrin nanorods toward new photoactive nanomaterials with uniform sizes and shapes. The organized sizes and shapes are efficiently controlled by changing the structures of the encapsulated molecules (i.e., introduction of substituents). The morphology of ZnP(Py)4-based nanorods was investigated with SEM, TGA, XRD, and fluorescence images, from which the average length and diameter of the nanorods were evaluated. Elemental analysis and titration experiment by absorption measurement were also performed to quantitatively check the relative molecular ratio between porphyrins and fullerenes. Steady-state electronic absorption and fluorescence emission spectroscopies give evidence for the formation of ZnP(Py)4 nanorods, whereas time-resolved fluorescence and transient absorption measurements demonstrated that photoexcited ZnP(Py)4 serves as an electron donor, resulting in a charge-separated state, such as the radical anion of fullerenes and radical cation of ZnP(Py)4. These nanorod-deposited electrodes onto the SnO2 nanocrystallites exhibited an excellent power conversion efficiency (η) as well as incident photon-tocurrent efficiency (IPCE), as compared to the reference pristine ZnP(Py)4 tube in a standard two-compartment electrochemical cell. Acknowledgment. This work was supported in part by a Grant-in-Aid for Scientific Research (no. 21710104 to T.H.) and special coordination funds for promoting science and technology from the Ministry of Education, Culture, Sports, Science and Technology, Japan. A.S.D.S. gratefully acknowledges the support of a Japan Society for the Promotion of Science (JSPS) Fellowship. We also thank Dr. Y. Araki (Tohoku University) for measurement of transient absorption spectroscopy. Supporting Information Available: The distributions of diameters and lengths of nanorods (S1), TEM images of fullerene nanoparticle (S2), crystal structure of ZnP(Py)4 assembly (S3), nanosecond transient absorption spectra of nanorods (S4), and Structural and photophysical data of composite nanoparticles composed of ZnP(Py)4 and C70 prepared without

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Sandanayaka et al. Int. Ed. 2003, 42, 1505. (c) Shirakawa, M.; Kawano, S. i.; Fujita, N.; Sada, K.; Shinkai, S. J. Org. Chem. 2003, 68, 5037. (d) Kishida, T.; Fujita, N.; Sada, K.; Shinkai, S. J. Am. Chem. Soc. 2005, 127, 7298. (e) Nobukuni, H.; Shimazaki, Y.; Tani, F.; Naruta, Y. Angew. Chem., Int. Ed. 2007, 46, 8975. (29) Wang, G.-W.; Zhang, T.-H.; Hao, E.-H.; Jiao, L.-J.; Murata, Y.; Komatsu, K. Tetrahedron 2003, 59, 55. (30) Imahori, H.; Ozawa, S.; Ushida, K.; Takahashi, M.; Azuma, T.; Ajavakom, A.; Akiyama, T.; Hasegawa, M.; Taniguchi, S.; Okada, T.; Sakata, Y. Bull. Chem. Soc. Jpn. 1999, 72, 485. (31) Krupitsky, H.; Stein, Z.; Goldberg, I.; Strouse, C. E. J. Inclusion Phenom. Macrocyclic Chem. 1994, 18, 177. (32) A similar trend was reported. See the reference paper. Hotta, H.; Kang, S.; Umeyama, T.; Matano, Y.; Yoshida, K.; Isoda, S.; Imahori, H. J. Phys. Chem. B 2005, 109, 5700. (33) In DLS measurement of ZnP(Py)4 pristine assembly, we can see a broad distribution with a maximum peak at around 300 nm under the same preparation method. In Scheme 2A (TEM), the large flake assemblies are likely attributable to ZnP(Py)4 moieties, whereas particle shapes are composed of fullerene moieties. (34) C60Ph pristine nanoparticles [(C60Ph)n] have large and nonuniform structures, as compared to C60 nanoparticles [(C60)n] according to the corresponding TEM images (SI: Figure S2). This result is similar to the DLS result. (35) The final molar ratios between ZnP(Py)4 and fullerene moieties were estimated by absorption spectra. ZnP(Py)4/C70 ) ∼4:1, ZnP(Py)4/C60 ) ∼4: 1, ZnP(Py)4/C60Ph ) ∼3.5:1. (36) The molecular ratios based on elemental analyses of ZnP(Py)4/C70 and ZnP(Py)4/C60 approximately agree with the ones calculated by absorption spectra (∼4:1). ZnP(Py)4 tube: H, 3.98%; C, 64.49%. C70-ZnP(Py)4rod: H, 1.63%; C, 86.02%. C60-ZnP(Py)4 rod: H, 2.02%; C, 84.01%. (37) The intense XRD peak at ∼30° may be attributable to the interactive structure between ZnP(Py)4 and fullerene in Figure 3c and d. (38) Because of thermal decomposition of molecules at high temperature and residual CTAB surfactants within the rod assembly, it may be difficult to perform the clear quantitative discussion. (39) Alam, M. M.; Watanabe, A.; Ito, O. Bull. Chem. Soc. Jpn. 1997, 70, 1833. (40) D’Souza, F.; Chitta, R.; Sandanayaka, A. S. D.; Subbaiyan, N. K.; D’Souza, L.; Araki, Y.; Ito, O. Chem.sEur. J. 2007, 29, 8277. (41) In the case of composite assemblies of ZnP(Py)4 and C60 prepared without CTAB, nonuniform rectangular structures are observed (SI: Figure S5). The reason for the photoinduced electron transfer via 3ZnP* may be attributable to the isolated diffusive molecules in solution, since the D/A layers are not effectively organized. On the other hand, in the case of nanorods, intramolecular interaction of fullerene and ZnP(Py)4 molecules within rods mainly influences for photoinduced electron transfer via 1ZnP* moiety (see Figures 7 and 8). (42) (a) Hasobe, T.; Fukuzumi, S.; Hattori, S.; Kamat, P. V. Chem. Asian J. 2007, 2, 265. (b) Hasobe, T.; Hattori, S.; Kamat, P. V.; Fukuzumi, S. Tetrahedron 2006, 62, 1937. (c) Hasobe, T.; Murata, H.; Kamat, P. V. J. Phys. Chem. C 2007, 111, 16626. (43) Hasobe, T.; Hattori, S.; Kamat, P. V.; Urano, Y.; Umezawa, N.; Nagano, T.; Fukuzumi, S. Chem. Phys. 2005, 319, 243. (44) Hasobe, T.; Imahori, H.; Fukuzumi, S.; Kamat, P. V. J. Phys. Chem. B 2003, 107, 12105. (45) As shown in Figure 9B (AFM image), the fullerene-ZnP(Py)4deposited film is composed of multilayers with interpenetrating networks. (46) The various light-absorption cross sections toward nanorods are possible considering the nanorod orientation on film (Figure 9B). On the other hand, the exciton diffusion length of organic dyes such as porphyrins is generally limited (ca. several nm). Considering these points and quenching properties of time-resolved measurements (fluorescence lifetimes and transient absorption), we can propose that light-absorbed D/A interfacial layers contribute mainly to the immediate charge-separation process. See the reference paper for exciton diffusion of porphyrins:Huijser, A.; Savenije, T. J.; Meskers, S. C. J.; Vermeulen, M. J. W.; Siebbeles, L. D. A. J. Am. Chem. Soc. 2008, 130, 12496. (47) On the basis of the discussion in ref 46, the broad photoresponse in the 600-700 nm region may contribute to charge separation via the excited state of fullerene assemblies. (48) (a) Horowitz, G.; Hajlaoui, M. E. Synth. Met. 2001, 122, 185. (b) Horowitz, G.; Hajlaoui, M. E. AdV. Mater. 2000, 12, 1046. (c) Di Carlo, A.; Piacenza, F.; Bolognesi, A.; Stadlober, B.; Maresch, H. Appl. Phys. Lett. 2005, 86, 263501.

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