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J. Phys. Chem. C 2009, 113, 19694–19699
Anisotropic High Electron Mobility and Photodynamics of a Self-Assembled Porphyrin Nanotube Including C60 Molecules Hirofumi Nobukuni,† Fumito Tani,*,† Yuichi Shimazaki,‡ Yoshinori Naruta,† Kei Ohkubo,§ Tatsuaki Nakanishi,§ Takahiko Kojima,| Shunichi Fukuzumi,§ and Shu Seki⊥ Institute for Materials Chemistry and Engineering, Kyushu UniVersity, Higashi-ku, Fukuoka 812-8581, Japan, College of Science, Ibaraki UniVersity, Bunkyo, Mito 310-8512, Japan, Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity and SORST, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba 305-8571, Japan, and Department of Applied Chemistry, Graduate School of Engineering, Osaka UniVersity and PRESTO, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ReceiVed: August 9, 2009; ReVised Manuscript ReceiVed: October 5, 2009
A cyclic porphyrin dimer (Ni2-CPDPy) linked by butadiyne moieties bearing 4-pyridyl groups includes a C60 molecule inside its cavity in solution to give a 1:1 inclusion complex (C60⊂Ni2-CPDPy). The charge-transfer (CT) band is observed at 645 nm in the UV-vis absorption spectrum of the solution of C60⊂Ni2-CPDPy. In the cyclic voltammogram of C60⊂Ni2-CPDPy, a small anodic shift of the porphyrin oxidation potential and a small cathodic shift of the fullerene reduction potential compared with their original redox potentials are indicative of CT interaction from the porphyrin to C60. In the crystal structure of C60⊂Ni2-CPDPy, a porphyrin nanotube is formed by the self-assembly of Ni2-CPDPy. Ni2-CPDPy molecules link together through nonclassical C-H · · · N hydrogen bonds and π-π interactions of the pyridyl groups along the crystallographic b axis. The included C60 molecules are linearly arranged in the nanotube to afford a supramolecular peapod. The charge-carrier mobility of the single crystal of C60⊂Ni2-CPDPy was determined by flash-photolysis timeresolved microwave conductivity (FP-TRMC) measurements. It has an anisotropic high electron mobility (∑µ ) 0.72 cm2 V-1 s-1) along the linear array of C60 (crystallographic b axis). Femtosecond laser flash photolysis of C60⊂Ni2-CPDPy in the solid state with photoexcitation at the Soret band of the porphyrin shows the formation of a triplet exciplex 3{Ni2-CPDPy · · · C60}*, which decays with a lifetime of 34 ps to the ground state without observation of a complete charge-separated state. Introduction Porphyrin assemblies have been synthesized as functional models of light-harvesting antenna for energy migration and reaction center for charge separation in natural photosynthesis.1 Especially, much attention has been devoted to the design and synthesis of nanotubular assemblies composed of porphyrin derivatives with the intention of forming functionalized onedimensional spaces surrounded by large π-planes.2 Recently, porphyrin nanochannels of highly distorted porphyrin dications including electron donor molecules such as hydroquinone in the inner channels were synthesized, and the photoinduced electron transfer from the donors to the acceptor porphyrins was reported.2c,d However, the rational strategy for creating a porphyrin nanotube has still remained to be developed, and only a few examples of porphyrin nanotubes crystallographically characterized at the atomic level have been reported.2b,e,u On the other hand, fullerene (C60) and its derivatives have been used as ultimate electron acceptors owing to their favorable reduction potentials and small reorganization energies in electron-transfer reactions.3 Thus, supramolecular architectures consisting of porphyrin derivatives (donors) and fullerenes * To whom correspondence should be addressed. E-mail: tanif@ ms.ifoc.kyushu-u.ac.jp. † Kyushu University. ‡ Ibaraki University. § Osaka University and SORST (JST). | University of Tsukuba. ⊥ Osaka University and PRESTO (JST).
(acceptors) are currently attracting much interest for the development of artificial photosynthesis and photovoltaic devices.4,5 A linear arrangement of C60 inside a porphyrin nanotube is expected to perform photoinduced charge separation and vectorial smooth charge transport, both of which are the most fundamental prerequisites for efficient photovoltaics. Moreover, a synthetic supramolecular peapod in which fullerenes are aligned within an organic nanotube has been of interest as a chemical analogue to carbon nanotubes including fullerenes.2a,6-9 In order to construct organic nanotubes, one of the most straightforward ways is to introduce self-assembling substituents into rigid cyclic molecules to align them in one direction.10 We have reported a porphyrin nanotube synthesized by applying this method to a rigid cyclic porphyrin dimer (Ni2-CPDPy) bearing self-assembling pyridyl groups.11 The porphyrin nanotube can include C60 in its inner channel to produce a one-dimensional array of C60 in the crystalline state (Figure 1). We report herein the photoelectrochemical properties of this C60-including porphyrin nanotube. The charge-carrier mobility of the inclusion complex (C60⊂Ni2-CPDPy) in the single crystal was determined by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements.12 We also investigated the ultrafast excited state behaviors of the solid of C60⊂Ni2-CPDPy by femtosecond laser flash photolysis. Experimental Section Materials. Commercially available reagents and solvents were used without further purification unless otherwise noted.
10.1021/jp9076849 CCC: $40.75 2009 American Chemical Society Published on Web 10/20/2009
Anisotropic High Electron Mobility and Photodynamics
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19695 and sum of the mobilities of charge carriers as given in eqs 1 and 2.
〈∆σ〉 ) (1/A)(∆Pr /Pr) ∆σ ) e
Figure 1. Crystal structures of tubular assemblies of C60⊂Ni2-CPDPy. Hydrogen atoms are omitted for clarity. (a) side view; (b) top view.
1,1,2,2-Tetrachloroethane was purified by distillation under reduced pressure after being stirred with CaCl2 for several days. Instruments. Steady-state UV-vis absorption spectra were recorded on a Shimadzu UV-3100PC spectrophotometer. Steadystate fluorescence spectra were obtained on a Shimadzu RF5300PC spectrofluorophotometer. Cyclic voltammetry was performed on a BAS 100B potentiostat in a deaerated 1,1,2,2tetrachloroethane solution containing 0.10 M n-Bu4NPF6 as a supporting electrolyte. The typical scan rate was 100 mV s-1. A 6 mm diameter platinum electrode was used as the working electrode, while a platinum wire served as the counter electrode. A Ag/AgNO3 electrode in CH3CN, separated by a Vycor tip, was used as the reference. Redox potentials were determined with respect to that of the Fc+/Fc redox couple. All electrochemical measurements were carried out under an atmospheric pressure of argon. Flash-Photolysis Time-Resolved Microwave Conductivity (FP-TRMC) Measurements. Nanosecond laser pulses from a Nd: YAG laser (second harmonic generation, THG (532 nm) from Spectra Physics, INDY-HG, fwhm 5-8 ns) were used as excitation sources. The power density of the laser was set at 4.3-110 mJ/cm2 (1.1-30 × 1016 photons cm-2). For timeresolved microwave conductivity (TRMC) measurements, the microwave frequency and power were set at ≈9.1 GHz and 3 mW, respectively, so that the motion of the charge carriers was not disturbed by the low electric field of the microwaves. A Rohde & Schwarz SMA-100A signal generator was used as a microwave continuum source. A crystal was mounted on a quartz rod with poly(vinylalcohol) binder, and set in the microwave cavity resonator (TE012 mode). The mounted crystal was back-excited via a quartz rod, and rotated relatively to the vector of the electric field in the cavity. The TRMC signal picked up by a diode (rise time
