Porphyrin-Based Supramolecular Nanoarchitectures for Solar Energy

Perspective pubs.acs.org/JPCL

Porphyrin-Based Supramolecular Nanoarchitectures for Solar Energy Conversion Taku Hasobe* Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama, 223-8522, Japan ABSTRACT: Photofunctional molecular architectures with well-defined shapes and sizes are of great interest because of various applications such as photovoltaics, photocatalysis, and electronics. Porphyrins are promising building blocks for organized nanoscale superstructures, which perform many of the essential light-harvesting and photoinduced electron/energy transfer reaction. In this Perspective, we present the recent advances in supramolecular architectures of porphyrins for solar energy conversion. First, we state preparation and light energy conversion properties of porphyrin (donor: D) and fullerene (acceptor: A)-based composite spherical nanoassemblies. The interfacial control of D/A molecules based on our supramolecular strategy successfully demonstrates the drastic enhancement of light energy conversion properties as compared to the corresponding nonorganized systems. Then, bar-shaped structures composed of two different D and A molecules with separated inside and outside layers are discussed. This unusual rod formation shows a possibility for a novel zeolite-like photoreaction cavity with efficient visible light absorption. Finally, photophysical and phoelectrochemical properties of supramolecular composites between porphyrins and carbon naotubes/ graphenes are briefly described.

A

The ability to control the morphology of nanoscale organic materials by means of molecular design and synthesis is increasingly gaining attention since these materials show atypical properties.18−20 By carefully adjusting these intermolecular interactions such as hydrogen bonds, metal coordination, and van der Waals, π−π interactions, diverse supramolecular structures with considerable complexity, such as size and shape-controlled aggregates, have been widely investigated so far.21,22 The successful control and organization of macroscopic structures through a proper choice of the molecular components opens a way to design and synthesize materials capable of exhibiting specific properties and functions. Therefore, such a supramolecular technique is also considered to be one of the useful strategies to control the functionalities of light energy conversion such as light-harvesting and chargeseparation properties in aggregate states.14,23,24 One of the promising candidates for photofunctional molecules is a porphyrin that is involved as a chromophore and an electron donor in the photosynthetic reaction centers. Additionally, rich and extensive absorption features of porphyrinoid systems guarantee increased absorption cross sections and an efficient use of the solar spectrum.25 On the other hand, porphyrins are also promising building blocks for superstructures.23,26,27 A variety of nano- and microscale assemblies composed of porphyrins from spherical particle to bar-shaped fiber formations are systematically reported.21,24,25

major scientific and technological challenge facing humanity is developing a secure and sustainable energy source to replace our reliance on fossil fuels.1−4 This source should be ideally clean, abundant, inexpensive, and widely distributed regionally in the world. Among renewable energy sources, sunlight is the most attractive and largest exploitable resource since it provides more energy in 1 h to the earth surface than the total energy consumed by humans throughout an entire year.2 Therefore, recent attention and interest have been drawn to the development of photovoltaic3,5−9 and photocalytic10−13 systems (e.g., hydrogen evolution) using solar energy. Photovoltaic cells convert sunlight directly into electricity by the photovoltaic effect. The energy conversion is generally performed by the following three processes: (i) light-harvesting and exciton diffusion, (ii) charge separation, and (iii) carrier transport.14 On the other hand, photocatalytic systems such as hydrogen evolution are also originally composed of three-component units such as a photosensitizer, an electron relay, and a catalyst.15−17 In both cases, light absorption and the subsequent electron transfer via the excited state are consequently essential processes for the final energy conversion.

In both cases, light absorption and the subsequent electron transfer via the excited state are consequently essential processes for the final energy conversion. © XXXX American Chemical Society

Received: March 7, 2013 Accepted: May 9, 2013

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dx.doi.org/10.1021/jz4005152 | J. Phys. Chem. Lett. 2013, 4, 1771−1780

The Journal of Physical Chemistry Letters

Perspective

Chart 1. A Schematic Illustration of Preparation of Porphyrin Nanoparticles Using Mixing Solvent Techniques in This Studya

a

TEM image shows (A) H2P(CO2H)4/TriEG and (B) H2P(CO2H)4/HeptaEG, respectively. Adapted from ref 30.

Moreover, a combination of porphyrins and other acceptor molecules/materials (e.g., fullerenes, carbon nanotubes and semiconductor nanoparticles etc) seems ideal for fulfilling an enhanced light-harvesting efficiency of chromophores throughout the solar spectrum and a highly efficient conversion of the absorbed light into the high energy state of the charge separation by photoinduced electron transfer.28,29 In this Perspective, based on the above concept, the focus is on the recent advances in supramolecular architectures of porphyrins for solar energy conversion. A porphyrin dye is soluble in nonpolar and good solvents such as toluene and dichloromethane, but less so in polar and poor solvents such as acetonitrile and water. By controlling the proper choice of polar to nonpolar solvent, we can achieve a controlled aggregation. The ratio of poor/good mixed solvent as well as the mode of mixing is an important factor in achieving the desired size and shape of porphyrin aggregates. In our study we employed a fast-injection method in which a concentrated solution of porphyrin in good solvent is syringed into a pool of poor solvent, whereas in some cases, addition of surfactants and sonicating were useful to prepare stable aggregate formations in suspended solution. Finally, such a procedure produced transparent suspension of porphyrin nanoassemblies. On the basis of this strategy, first, we report an ability to control the structures and photodynamics of mesotetra(4-carboxyphenyl) porphyrin [H2P(CO2H)4]-based nanoassemblies prepared in poor/good solvent mixture conditions (H2O/tetrahydrofuran (THF)) by using ethylene glycols (EG surfactants) with different spacer lengths, such as triethylene glycol (TriEG) and heptaethylene glycol (HeptaEG) (Chart1).30 With increasing the chain lengths, the diameters of H2P(CO2H)4/EG composite nanoparticles systematically increase in the range 90−350 nm. In contrast with H2P(CO2H)4/EG composite nanoparticles, pristine H2P(CO2H)4 assemblies show long rod-shaped assemblies with micrometer scales.30 The quenching processes of the excited singlet and triplet states of H2P(CO2H)4 nanoparticles were analyzed by annihilation theory, which is largely dependent on the sizes of nanoparticles.30 These self-assembled nanoparticles of can be deposited as thin films on nanostructured electrode using an electrophoretic technique for fabrication of photoelectrochemical cells (Figure 1). A known amount (∼2 mL) of 5,15-bis(3,5-di-tertbutylphenyl)porphyrin [H2P(tBu)2] nanoparticle in acetonitrile/toluene (9/1, v/v) is transferred to a cuvette in which optically transparent electrode (OTE) and TiO2-coated film electrode (OTE/TiO2). By applying electric field between two electrodes, the deposition of the film can be visibly seen as the solution becomes colorless with simultaneous brown coloration of the OTE/TiO2 electrode [denoted as OTE/TiO2/(H2P-

Figure 1. (A) Schematic illustrations of preparation processes of porphyrin nanoparticles (left) and porphyrin and fullerene composite nanoparticles (right). The atomic force microscopy (AFM) image shows composite nanoparticles of H2P(tBu)2 and C60 on an OTE film. (B) A photocurrent action spectrum (IPCE value) of OTE/SnO2/ (H2P(tBu)2 + C60)n. Electrolyte: 0.5 M NaI and 0.01 M I2 in acetonitrile. Adapted from ref 32.

(tBu)2)n]. The OTE/TiO2/(H2P(tBu)2)n film is highly photoactive and capable of undergoing charge separation under visible light excitation. Photoelectrochemical performance of OTE/TiO2/(H2P(tBu)2)n was measured in a standard twocompartment electrolyte cell. Photoexcitation of the porphyrin film electrode in a photoelectrochemical cell with visible light produces relatively high photocurrent generation. A maximum photocurrent of 0.15 mA cm−2 and a photovoltage of 250 mV were attained using I3−/I− redox couple under white light illuminatiom (∼100 mW/cm2). The incident photon-tophotocurrent generation efficiency (IPCE) was calculated by normalizing the photocurrent values for incident light energy and intensity using eq 114 IPCE (%) = 100 × 1240 × Isc/(Win × λ)

(1) 2

where Isc is the short circuit photocurrent (A/cm ), Win is the incident light intensity (W/cm2), and λ is the wavelength (nm). The maximum IPCE of 1.2% has been achieved in the photoelectrochemical cell.31 By using the above strategy, organic nanoparticles composed of two different moieties (e.g., donor (D) and acceptor (A) moieties) in suspended solution were easily deposited onto nanostructured SnO2 films [OTE/SnO2].32 These composite molecular nanoparticles of C60 (A) and H2P(tBu)2 (D) prepared in acetonitrile/toluene mixed solvent absorb light over entire spectrum of visible light. The highly colored 1772



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porphyrin units in H4−CPD was successfully confirmed by the single crystal structure.34 A transmission electron microscopy (TEM) image of AlPor−C60 dyad composites [denoted as (AlPor−C60)n], which were prepared in odichlorobenzene/acetonitrile solution, displays well-controlled size and shape (Figure 2C). The final concentrations are fixed as a constant (0.043 mM) in mixed solution. Photoelectrochemical measurements were also performed with a similar standard two-electrode system using a I−/I3− redox couple. The OTE/SnO2/(AlPor−C60)n system indicates the maximum IPCE value of 24% at 450 nm as well as a broad photoresponse in the visible and near-infrared regions (Figure 2D). On the other hand, the OTE/SnO2/(H4−CPD + C60)n system also gives rise to an especially broad photoresponse in the long wavelength region (700−800 nm) because of the extended charge-transfer (CT) absorption of highly ordered assemblies of H4−CPD and C60. The maximum IPCE values for (AlPor−C60)n (24%) and (H4−CPD + C60)n (17%) are much larger than that of (H2P(tBu)2 + C60)n (∼4%).32 To further develop the high-order organization system between porphyrins and fullerenes, novel quaternary selforganization of porphyrin and fullerene moieties units by clusterization with gold nanoparticles was performed (Figure 3A).35,36 First, porphyrin alkanethiolate monolayer-protected gold nanoparticles (H2PCnMPC: n is the number of methylene groups in the spacer) are prepared (secondary organization) starting from the primary component (porphyrin−alkanethiol). These porphyrin-modified gold nanoparticles form complexes with fullerenes (tertiary organization) and they are further clusterized in acetonitrile/toluene mixed solvent (quaternary organization). The composites are stable in solution for spectroscopic measurement. (H2PCnMPC+C60)m in the mixed solvent exhibit much broader and more intense absorption in the visible and near IR regions than those of parent H2PCnMPC and C60 in toluene. This demonstrates that the composite clusters of H2PCnMPC and C60 are superior as light absorbers to the single component systems of H2PC11MPC or C60, since the composite assemblies absorb throughout the visible part of the solar spectrum. The schematic structure in Figure 3B affords the center-tocenter distances between two porphyrins in H2PC5MPC, H2PC11MPC and H2PC15MPC as 11.5 Å, 15.2 Å and 17.3 Å, respectively.36 On the other hand, the closest distance between a carbon of C60 and the center of the porphyrin ring has been reported as 2.856 Å by the X-ray crystal structure of the C60 complex with a jaw-like bis-porphyrin.37 Thus, the smallest center-to-center distance of two porphyrin rings which can accommodate C60 between the rings is estimated as 12.8 Å by adding the diameter of C60 (7.1 Å) to twice of the closest distance between a carbon of C60 and the center of porphyrin ring (5.7 Å). The estimated distances between two porphyrins in H2PC11MPC (15.2 Å) and H2PC15MPC (17.3 Å) are long enough for the two porphyrins to accommodate C60 between the rings in contrast with H2PC5MPC (11.5 Å). The longer porphyrin−porphyrin distance in H2PC15MPC than in H2PC11MPC and H2PC5MPC may result in accommodation of more C60 molecules between the porphyrin rings (Figure 3B). The binding constants of H2PC15MPC, H2PC11MPC, and H2PC5MPC are also calculated to be 64 000 M−1, 47 000 M−1, and 16 000 M−1, respectively, by fluorescence quenching results. These trends are in good agreement with the IPCE results (vide inf ra).36

composite nanoparticles can be assembled as a three-dimensional array onto nanostructured SnO2 films using the same electrophoretic deposition approach [OTE/SnO2/(H2P(tBu)2 + C60)n]. The OTE/SnO2/(H2P(tBu)2 + C60)n film exhibits an IPCE as high as ∼4% as shown in Figure 1B.32 The IPCE observed with H2P(tBu)2 and C60 composites demonstrate the synergy of these systems toward yielding efficient photoinduced charge separation within these composite nanoassemblies. The main reason for the difference between IPCE and absorption spectra may be due to the measurement setup in the electrolyte system (SnO2 nanocrystallites and I−/I3− redox couple in acetonitrile) since the incident light is absorbed by the electrolyte solution and SnO2 nanocrystallites in the shortwavelength region. Thus, composite nanoparticles of porphyrins (D) and fullerenes (A) effectively work as photoactive materials in photovoltaic cells. As shown above, molecular nanoassembly composed of porphyrins and fullerenes by simple blend is a useful way for construction of photovoltaic systems. However, in such a system, the serious problem is limited IPCE value (∼4%) under no electrochemical bias. To get the high photovoltaic properties, organization of D and A moieties for efficient charge separation is required. Therefore, the next strategies for self-organization of porphyrin and fullerene moieties were performed. To control the D/A interface at a molecular level, we employed two different molecules: aluminum(III) porphyrin−fullerene dyad (AlPor−C60)33 and cyclic free-base porphyrin dimer (H4−CPD),34 respectively, as shown in Figure 2A,B. In the AlPor−C60 dyad, the C60 unit is bound axially to the aluminum(III) porphyrin (AlPor) via a benzoate spacer, whereas supramolecular insertion of C60 between two

Figure 2. Chemical structures of (A) aluminum(III) porphyrin− fullerene dyad (AlPor−C60) and (B) cyclic free-base porphyrin dimer (H4−CPD). (C) TEM image of molecular assemblies of AlPor−C60 dyad prepared in o-dichlorobenzene/acetonitrile (1/6, v/v). [AlPor− C60] = 0.043 mM. (D) Photocurrent action spectra of (a) OTE/ SnO2/(AlPor−C60)n, (b) OTE/SnO2/(AlPor)n, (c) OTE/SnO2/ (C60)n. Adapted from ref 33 and reprinted with permission from ref 34 (Copyright 2010 Wiley-VCH). 1773



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Figure 3. (A) An illustration of quaternary self-organization of porphyrin and C60 units with gold nanoparticles. (B) An illustration of photoelectrochemical cell and insertion of C60 between two porphyrin rings in H2PC15MPC. (C) OTE/SnO2/(H2PCnMPC + C60)m ([H2P] = 0.19 mM, [C60] = 0.31 mM) (a) n = 5, (b) n = 11, (c) n = 15, (d) [H2P] = 0.19 mM, [C60] = 0.38 mM; n = 15. Electrolyte; 0.5 M NaI and 0.01 M I2 in acetonitrile. (D) Current−voltage characteristics of (a) OTE/SnO2/(H2PC15MPC + C60)m and (b) OTE/SnO2/(H2P-ref + C60)m under visible light illumination (λ > 400 nm); electrolyte 0.5 M NaI and 0.01 M I2 in acetonitrile; input power: 11.2 mW/cm2. Adapted from ref 36.

ches as high as 1.5%, which is 45 times higher than that of the reference system composed of porphyrin monomer [H2Pref: 5,10,15,20-tetra(3,5-di-tert-butylphenyl)porphyrin] and fullerene (Figure 3D). The above strategy can generally be applied to other systems, such as porphyrin−peptide oligomers. Figure 4A shows chemical structures of porphyrin−peptide oligomers with different number of porphyrins in a polypeptide unit (porphyrin-functionalized α-polypeptides).38,39 Such porphyrin oligomers with a polypeptidic backbone are flexible enough to accommodate C60 between the porphyrin units. Figure 4A also shows the preparation procedure and TEM image of (P(H2P)16 + C60)m. The sizes and shapes of (P(H2P)16 + C60)m are much controlled as compared with those of (P(H2P)1 + C60)m.38 Figure 4B shows the IPCE of the (P(H2P)n + C60)m (n = 1, 2, 4, 8, 16) modified electrode at a constant concentration ratio of porphyrin to C60 units. The IPCE value of (P(H2P)n + C60)m system exhibits a remarkable increase with increasing the number of porphyrins in a polypeptide unit. In particular, the (P(H2P)16 + C60)m system has a maximum IPCE value of 48% at 600 nm as well as a broad photoresponse, extending into the infrared region up to 1000 nm.38 Such an effective light energy conversion property is ascribed to the polypeptide structure to control three-dimensional organization between porphyrins and C60. The (P(ZnP)n + C60)m system also exhibits a remarkable increase with increasing the number of porphyrins in a

Then, the highly colored composite assemblies can be assembled as three-dimensional arrays onto nanostructured SnO 2 films (OTE/SnO 2 ) to afford the OTE/SnO 2 / (H2PCnMPC+C60)m electrode using an electrophoretic deposition method. The evaluation of photovoltaic properties is performed using standard two-compartment cell in the electrolyte (Figure 3B). Figure 3C shows the effect of the alkanethiolate chain lengths on the IPCE values. The action spectra indicate that the higher IPCE and the broader photoresponse are attained with the longer chain length of H2PCnMPC. In particular, OTE/SnO2 /(H2PC15MPC+C60)m ([H2P] = 0.19 mM, [C60] = 0.38 mM) affords the maximum IPCE value (54%) and very broad photoresponse (up to ∼1000 nm), which extends to the near-IR region.36 In OTE/SnO2/ (H 2 PC15MPC+C 6 0 ) m , a long methylene spacer of H2PC15MPC allows one to leave enough space for fullerene molecules to effectively insert them between two neighboring porphyrin rings, as compared with the assemblies with a shorter methylene spacer, leading to more efficient photocurrent generation. The power conversion efficiency η of OTE/ SnO2/(H2PC15MPC + C60)m electrode is calculated by eq 2,14 η = FF × Isc × Voc/Win

(2)

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 power conversion efficiency of the OTE/SnO2/(H2PC15MPC+C60)m composite electrode rea1774



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Figure 4. (A) Chemical structures of porphyrin−peptide oligomers employed in this study (left). The right figure shows an organization process of P(H2P)16 with C60 [denoted as P(H2P)16 + C60]m]. The TEM image shows (P(H2P)16 + C60)m. (B) The photocurrent action spectra (IPCE vs wavelength) of (a) (P(H2P)16+ C60)m, (b) (P(H2P)8 + C60)m, (c) (P(H2P)4 + C60)m, (d) P(H2P)2 + C60)m and (e) (P(H2P)1 + C60)m on OTE/SnO2 electrodes. (C) Time-resolved absorption spectra recorded following 387 nm laser pulse excitation of (P(H2P)8 + C60)m. The sample was prepared in argon-saturated acetonitrile/toluene (3/1, v/v), and measurements were made at 298 K. Reprinted with permission from ref 38 (Copyright 2007 Royal Society of Chemistry).

between the porphyrin units occurs efficiently prior to the electron transfer. The association constant between porphyrins and C60 increases with increasing number of porphyrins in a polypeptide unit. This indicates that the interaction between porphyrin and C60 becomes stronger with increasing the number of porphyrins in the porphyrinic polypeptide. Such differences in the association constants are in good agreement with the IPCE results. The photodynamics of the composite molecular assemblies was further measured by femtosecond time-resolved transient absorption spectra to examine the charge-separation process. Upon 387 nm laser pulse excitation, a strong transient bleaching (around 610 nm) arises from the fluorescence in the case of (P(H2P)1 + C60)m. In contrast to the (P(H2P)1 + C60)m system, the strong absorption arising from singlet and

polypeptide unit. The maximum IPCE of (P(ZnP)16 + C60)m is determined to be 56% at 480 nm.38 The major reason for increased IPCE values with an increase of the number of porphyrins in a polypeptide unit is the chargeseparation process between porphyrins and fullerenes. The UV−vis absorption and fluorescence spectral changes give us useful information of supramolecular formation between porphyrins and fullerenes. The binding constants between porphyrins and fullerenes are thereby evaluated quantitatively. The spectral changes indicate that C60 forms a 1:1 complex with the porphyrin moiety. The apparent formation constant (K) between P(H2P) 16 and C60 determined from the fluorescence quenching (8.2 × 104 M−1) is significantly larger than that determined from the UV−vis spectral change (1.4 × 104 M−1). This indicates that the excited energy migration 1775



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triplet excited states of porphyrins (450−510 nm) is missing in the case of (P(H2P)16 + C60)m. Instead, a broad absorption at around 650 nm appears after the laser pulse excitation, as shown in Figure 4C. Such broad absorption spectra are clear indications of the formation of the porphyrin radical cation. It is also interesting to note that the bleaching originating from fluorescence (610 nm region) is absent in the transient spectra (Figure 4C). Thus, different supramolecular formations between porphyrins and fullerens have a qualitative effect on the photoinduced electron transfer process, which leads to the final light energy conversion properties. Additionally, the fluorescence lifetime and electron spin resonance (ESR) results also support the investigation of photoinduced electron transfer process.38 We have also observed similar drastic improvements in different systems such as porphyrin dendrimers,40,41 TiO2 nanoparticle/nanotubes,42 and other dye molecules.43−47

Different supramolecular formations between porphyrins and fullerens have a qualitative effect on the photoinduced electron transfer process, which leads to the final light energy conversion properties.

Figure 5. An illustration of fullerene nanoparticle-promoted organization of ZnP(Py)4 nanorods. CTAB surfactant 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. Adapted from ref 54.

Photocurrent generation in the present system may be initiated by photoinduced charge separation from the porphyrin excited singlet state (1H2P*/H2P•+ = −0.7 V vs normal hydrogen electrode (NHE))32 in the porphyrin−peptide oligomer to C60 (C60/C60•− = −0.2 V vs NHE)32 in the porphyrin−peptide oligomer−C60 complex rather than direct electron injection to the conduction band of SnO2 (0 V vs NHE).32 The reduced C60 injects electrons into the SnO2 nanocrystallites, whereas the oxidized porphyrin (H2P/H2P•+ = 1.2 V vs NHE)32 undergoes electron transfer reduction with the iodide (I3−/I− = 0.5 V vs NHE)32 in the electrolyte system. To date, bar-shaped porphyrin nanoassemblies are mainly composed of single molecular species.48−52 Considering the photovoltaic mechanism, a bar-shaped structure composed of two different molecules with separated inside and outside layers is a good candidate for the efficient carrier generation (hole and electron). We therefore have succeeded in construction of a new type of molecular composite: fullerenes-encapsulated porphyrin hexagonal nanorods composed of zinc meso-tetra(4-pyridyl)porphyrin [ZnP(Py)4] and C60 [denoted as C60ZnP(Py)4 rod], which are prepared by adding surfactant: cetyltrimethylammonium bromide (CTAB) in a dimethylformamide (DMF)/acetonitrile mixed solvent (Figure 5).53,54 With an increase in the bulkiness of substituent groups of fullerene derivatives, the average diameters increased from 15 to 80 nm. In ZnP(Py)4 assemblies, with the diffusion of DMF into acetonitrile, the Zn−N axial coordination of pyridyl Natoms to zinc atoms of ZnP(Py)4 promotes the growth of aggregates, which continue to grow into a flake structure. In this case, the organization process of ZnP(Py)4 moieties is initially derived from the coordination bond in contrast with C60 assemblies based on relatively weak π−π interactions. Then, fullerene-ZnP(Py)4 nanorods are finally formed (Figure 5B,C). In scanning electron microscopy (SEM) measurement, ZnP(Py)4 pristine hexagonal nanotubes with a large hollow structure [denoted as ZnP(Py)4 tubes] are observed, whereas

the hollow hole is completely closed in the case of nanorods composed of fullerenes (C 60 and C 70 ) and ZnP(Py) 4 [fullerene−ZnP(Py)4 rods].53,54 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. The reference ZnP(Py)4 hexagonal nanotube without C60 was also prepared in the same manner for comparison [denoted as ZnP(Py)4 tube]. Additionally, in the case of the preparation of ZnP(Py)4 and fullerene composite assemblies prepared without CTAB, nonuniform rectangular structures are observed.51,52 This indicates that CTAB surfactant plays an important role for organized ZnP(Py)4 hexagonal formations. The absorption spectrum of a fullerene−ZnP(Py)4 rod exhibits much broader and more intense absorption in the visible and near-infrared regions than those of the corresponding monomers: ZnP(Py)4 or C60 in DMF because of aggregate formations. Such a broad absorption property is useful for solar energy conversion. The photocurrent action spectrum of OTE/ SnO2/fullerene-ZnP(Py)4-rod shows a broad photoresponse in the visible region, and the maximum IPCE (∼35%) is much larger than the sum of two individual IPCE values (∼6.5%) of OTE/SnO2/ZnP(Py)4-tube and OTE/SnO2/C60-assembly under the same condition as the above-mentioned electrolyte system.53,54 These results clearly indicate that the organized structure between C60 and ZnP(Py)4 as well as the excellent electron acceptor property of C60 play a role for the improvement of light energy conversion properties. An efficient visible light-induced hydrogen evolution system has been developed by using an internal cavity of supramolecular porphyrin hexagonal nanotubes. The bar-shaped structure composed of Pt/TiO2 (inside layer) and zinc mesotetra(4-pyridyl)porphyrin [ZnP(Py)4] (outside layer) was formed with the aid of a surfactant, CTAB in a DMF/H2O mixture solution [denoted as Pt/TiO2−ZnP(Py)4 rod], by the above-mentioned method as shown in Figure 6A. The X-ray 1776



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ing process, photoinduced electron transfer from 1ZnP(Py)4* to TiO2 is likely to be the initial event for photocatalytic hydrogen evolution. Then, the injected electrons migrate toward Pt nanoparticles on TiO2 surface to reduce H+ to H2. The oxidized porphyrins [ZnP(Py)4•+] undergo the electrontransfer reduction with AsA or NADH. Carbon nanotubes and graphenes can also provide an ideal network to promote charge transfer with dye molecules such as porphyrins and transport electrons to the collecting surface because of the characteristic π-conjugated structure. So far, various elegant covalently linked and noncovalently assembled donor−acceptor (D−A) hybrids containing single-wall carbon nanotubes (SWCNTs), multiwall carbon nanotubes (MWCNTs), carbon nanohorns (CNHs), stacked-cup carbon nanotubes (SCCNTs), and graphenes have been reported by several groups.56−63 Especially, with regard to SWCNT composites, all of these studies used a mixture of SWCNTs, being both metallic as well as semiconducting with different (n,m) indices, making it impossible to derive meaningful and conclusive structure−reactivity relationships. In this study, we have overcome this barrier by constructing D−A hybrids made out of semiconducting SWCNTs of largely a single (n,m) index type, that is, two types of semiconducting nanotubes, viz., (6,5) and (7,6) having different band gaps (difference up to 0.1 eV) have been utilized (Figure 7A).64 The rates of charge separation were found to be slightly higher for (7,6)-SWCNT-possessing zinc porphyrin [ZnP(pyr)4] hybrids compared to the (6,5)SWCNT-possessing ZnP(pyr)4 hybrids. Charge recombination revealed an opposite effect, indicating that the (7,6)-SWNTs are slightly better for charge stabilization compared to the (6,5)-SWNTs. The IPCE of the OTE/SnO2/ZnP(pyr)4/ SWCNT(7,6) electrode reaches 1% as a maximal value at 420 nm with the second maximum near 460 nm, which is about 3 times larger than that of the OTE/SnO2/ZnP(pyr)4/ SWCNT(6,5) electrode (0.35%).64 Additionally, CNHs,65−67 SCCNTs,68 and graphene69,70-based molecular composites with porphyrins also demonstrate new potentials for light energy conversion. In particular, the composite electrode of SCCNT and porphyrins exhibits an IPCE of 32% under an applied potential.68 Summary and Future Outlook. Thus, utilization of supramolecular techniques successfully enables us to organize molecular aggregates from the molecular to mesoscopic level, which possess light energy conversion functionalities such as light-harvesting and charge separation. Especially in the case of photovoltaic applications, the nanoarchitectures are easily transferred onto an electrode using appropriate deposition techniques (e.g., electrophoretic deposition) maintaining the molecular-level information of D and A composites in solution. This is a clear advantage as compared to the conventional approaches such as vacuum deposition and spin-coat techniques. On the other hand, the important issue for future improvement of energy conversion property is how to effectively align these molecular assemblies at the macroscopic level (i.e., thin film). Recent surface patterning techniques may approximate a final solution.48,71−75 The other conventional method for light energy conversion is to synthesize simple D−A arrays such as molecular dyads and triads. In this case, efficient photoinduced charge separation occurs in D−A arrays by fine synthetic structural tunes following the well-established electron transfer theory.76 However, the main problem is the limited absorption region because of the monomeric form. By contrast, ideally, our

Figure 6. (A) A schematic illustration for photocatalytic hydrogen evolution in this study. (B) Time dependence of hydrogen evolution per unit weight of Pt (1 g) under photoirradiation (λ > 420 nm) to the aqueous solution at pH 4.5 containing AsA (3.8 × 10−2 M). (●) Pt/ TiO2−ZnP(Py)4 rods; (■) nonencapsulated Pt/TiO2 + ZnP(Py)4 composites. Adapted from ref 55.

diffraction (XRD) and SEM results clearly indicate that Pt colloid-deposited TiO2 nanoparticles (Pt/TiO2) were successfully encapsulated within a ZnP(Py)4 hexagonal nanotube. Pt/ TiO2−ZnP(Py)4 rods also show a broadened absorption in the visible region because of aggregation of ZnP(Py)4.

Considering the photovoltaic mechanism, a bar-shaped structure composed of two different molecules with separated inside and outside layers is a good candidate for the efficient carrier generation (hole and electron). Figure 6B shows the time course of hydrogen evolution in the reaction system composed of ascorbic acid (AsA) or dihydronicotinamide adenine dinucleotide (NADH) as a sacrificial electron donor and Pt/TiO2−ZnP(Py)4 rods as a photocatalyst, respectively, under visible light irradiation (λ > 420 nm). Pt/TiO2−ZnP(Py)4 rods exhibited efficient hydrogen evolution under visible light irradiation, whereas no hydrogen was evolved in the case of Pt/TiO2 without ZnP(Py)4. In addition, the hydrogen evolution efficiency of Pt/TiO2− ZnP(Py)4 rods per unit weight of Pt was 2 orders of magnitude greater than that of the nonencapsulated system: Pt/TiO2 and ZnP(Py) 4 nanotube composites [Pt/TiO 2 + ZnP(Py) 4 composites].55 The photodynamics of the excited state of Pt/ TiO2−ZnP(Py)4 rods was examined by femtosecond timeresolved transient absorption spectroscopy to clarify the photocatalytic mechanism. Judging from the ultrafast quench1777



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Figure 7. Schematic illustrations of porphyrin-based composites with (A) SWCNTs, (B) CNHs, (c) SCCNTs, and (D) graphenes. Adapted from refs 64, 66, and 68 and reprinted with permission from ref 69 (Copyright 2012 Royal Society of Chemistry).

Notes

Especially in the case of photovoltaic applications, the nanoarchitectures are easily transferred onto an electrode using appropriate deposition techniques (e.g., electrophoretic deposition) maintaining the molecularlevel information of D and A composites in solution. This is a clear advantage as compared to the conventional approaches such as vacuum deposition and spin-coat techniques.

The authors declare no competing financial interest. Biography Taku Hasobe obtained his Ph.D. degree at Osaka University in 2004. Prior to starting an associate professor position at Keio University in 2010, he was a postdoc at the University of Notre Dame and an assistant professor at JAIST. Additional information regarding our research can be found at http://www.chem.keio.ac.jp/∼hasobe/.

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ACKNOWLEDGMENTS The author thanks and expresses gratitude to my collaborators and co-workers whose names appear in the references. This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 19710119, 21710104, 23681025 & 23108721 to T.H.), the PRESTO project (JST), special coordination funds for promoting science and technology (MEXT), and the MEXT-Supported Program for the Strategic Research Foundation at Private Universities, 2009-2013. All figures and charts are reproduced from the related references with permission of the American Chemical Society, the Royal Society of Chemistry, and Wiley-VCH.

supramolecular strategies effectively improve the light-harvesting property based on aggregation while maintaining the charge separation property. Careful screening of D and A molecular composites is currently underway to further improve the light energy conversion properties. Such a supramolecular strategy combined with photophysical design and estimation will provide a new perspective for the construction of solar energy conversion systems.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. 1778



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