Article pubs.acs.org/JPCA
Enhanced Photoinduced Electron-Transfer Reduction of Li+@C60 in Comparison with C60 Yuki Kawashima,† Kei Ohkubo,† and Shunichi Fukuzumi*,†,‡ †
Department of Material and Life Science, Graduate School of Engineering, Osaka University and ALCA, Japan Science and Technology Agency (JST), 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan ‡ Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *
ABSTRACT: Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ionencapsulated C60 (Li+@C60) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C60. Femtosecond laser flash photolysis measurements of Li+@C60 revealed that singlet excited state of Li+@C60 (λmax = 960 nm) underwent intersystem crossing to the triplet excited state [3(Li+@C60)*: λmax = 750 nm] with a rate constant of 8.9 × 108 s−1 in deaerated benzonitrile (PhCN). The lifetime of 3(Li+@C60)* was determined by nanosecond laser flash photolysis measurements to be 48 μs, which is comparable to that of C60. Efficient photoinduced electron transfer from a series of electron donors to 3(Li+@C60)* occurred to produce the radical cations and Li+@C60•−. The rate constants of photoinduced electron transfer of Li+@C60•− are significantly larger than those of C60 when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of 3 (Li+@C60)* as compared with 3C60* results from the much higher one-electron reduction potential of Li+@C60 (0.14 V vs SCE) than that of C60 (−0.43 V vs SCE). The rate constants of photoinduced electron transfer reactions of Li+@C60 and C60 were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The reorganization energy of electron transfer of Li+@C60 was determined from the driving force dependence of electron transfer rate to be 1.01 eV, which is by 0.28 eV larger than that of C60 (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li+ upon electron transfer in PhCN.
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We report herein photodynamics of Li+@C60 and photoinduced electron-transfer reactions of Li+@C60, which were examined using femtosecond and nanosecond laser flash photolysis. Rate constants of photoinduced electron transfer from a series of electron donors to the triplet excited state of Li+@C60 were determined in comparison with those of C60. The results were analyzed in light of the Marcus theory of electron transfer,31 to determine the reorganization energy of electron transfer of Li+@C60. Li+@C60 is one of the abundantly and commercially available endohedral metallofullerenes. The knowledge of electron-transfer properties of Li+@C60 provides valuable information to expand the metallofullerene chemistry.
INTRODUCTION
Fullerenes have attracted considerable interest for a wide range of practical applications because of the unique electrical and chemical properties, in particular the excellent electron-transfer properties at the ground and photoexcited states.1−20 Because fullerenes have spacious inner cavities, some metals can be encapsulated inside the fullerene cages to form endohedral metallofullerenes.21−28 The metal(s) are normally encapsulated in fullerenes having higher cages (so-called higher fullerenes such as C72, C80 and C82), in which the fullerene cage is reduced by encapsulated metals.21−27 As a typical metallofullerene, La@ C82 has a La3+ in a three-electron reduced C82 trianion radical cage.21−23 On the other hand, lithium ion-encapsulated C60 (Li+@C60) was recently isolated, and the X-ray crystal structure was determined.28,29 In the case of Li+@C60, the C60 cage remains neutral.28 The electron acceptor ability of Li+@C60 was significantly enhanced as compared to pristine C60.28−30 The enhanced electron acceptor ability of Li+@C60 has made it possible to form a supramolecular electron-transfer complex between a tetrathiafulvalene calix[4]pyrrole (TTF-C4P) and Li+@C60 with chloride anion.29 Although photoinduced electron-transfer reactions of C60 have extensively been studied, the photoexcited state and the electron transfer reactivity of Li+@C60 have yet to be examined. © 2012 American Chemical Society
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EXPERIMENTAL SECTION Materials. Chemicals were purchased from commercial sources and used without further purification, unless otherwise noted. Lithium ion-encapsulated C60 (Li+@C60: 96%) was obtained from Daiichi Jitsugyo Co. Ltd., Japan. C60, 1,2,4,5tetrametylbenzene, pentamethylbenzene, hexamethylbenzene, 1,4-dimethoxybenzene, ferrocene (Fc), and Fe(bpy)3(PF6)2 (bpy =2,2′-bipyridine) were also purchased from commercial Received: June 15, 2012 Revised: August 7, 2012 Published: August 22, 2012 8942
dx.doi.org/10.1021/jp3059036 | J. Phys. Chem. A 2012, 116, 8942−8948
The Journal of Physical Chemistry A
Article
electron donors. The observed rate constants were determined by a least-squares curve fit. All experiments were performed at 298 K. Electrochemical Measurements. Cyclic voltammetry (CV) measurements of Li+@C60 were performed with an ALS630B electrochemical analyzer in deaerated PhCN containing 0.1 M Bu4N+ClO4− (TBAP) as a supporting electrolyte at 298 K. The platinum working electrode (BAS, surface i.d. 1.6 mm) was polished with BAS polishing alumina suspension and rinsed with acetone before use. The counter electrode was a platinum wire (0.5 mm dia.). The measured potentials were recorded with respect to an Ag/AgNO3 (0.01 M) reference electrode. The values of redox potentials (vs Ag/ AgNO3) are converted into those vs SCE by addition of 0.29 V.34 Theoretical Calculations. Density functional theory (DFT) calculations were performed on an 8CPU workstation (PQS, Quantum Cube QS8-2400C-064). Geometry optimizations were carried out using the RB3LYP/6-31G(d) basis set for Li+@C60 and C60 and UB3LYP/6-31G(d) basis set for Li+@ C60•− and C60•− as implemented in the Gaussian 09 program Revision A.02.35,36
sources and used without further purification. 1-Benzyl-1,4dihydronicotinamide (BNAH) was obtained from Tokyo Chemical Industry, Japan, and purified by recrystallization from methanol. 10-Methyl-9,10-dihydroacridine (AcrH2) was synthesized by a literature method and purified by recrystallization from ethanol.32 Benzonitrile (PhCN) used as a solvent was distilled over phosphorus pentoxide.33 Emission Spectral Measurements. Phosphorescence was measured on a Horiba FluoroMax-4 spectrofluorophotometer. A 2-methyltetrahydrofuran solution of Li+@C60 in a quartz tube (3 mm in diameter) was degassed by nitrogen bubbling for 10 min prior to the measurements. The sample tube was put in a quartz liquid nitrogen dewar. The measurements were carried out by excitation at 300 nm. Laser Flash Photolysis Measurements. Femtosecond transient absorption spectroscopy experiments were conducted using an ultrafast source, Integra-C (Quantronix Corp.), an optical parametric amplifier, TOPAS (Light Conversion Ltd.), and a commercially available optical detection system, Helios provided by Ultrafast Systems LLC. The source for the pump and probe pulses were derived from the fundamental output of Integra-C (λ = 786 nm, 2 mJ/pulse and fwhm =130 fs) at a repetition rate of 1 kHz. 75% of the fundamental output of the laser was introduced into a second harmonic generation (SHG) unit: Apollo (Ultrafast Systems) for excitation light generation at λ = 393 nm, while the rest of the output was used for white light generation. The laser pulse was focused on a sapphire plate of 3 mm thickness and then white light continuum covering the visible region from λ = 410 to 800 nm was generated via self-phase modulation. A variable neutral density filter, an optical aperture, and a pair of polarizers were inserted in the path to generate a stable white light continuum. Prior to generating the probe continuum, the laser pulse was fed to a delay line that provides an experimental time window of 3.2 ns with a maximum step resolution of 7 fs. In our experiments, a wavelength at λ = 393 nm of SHG output was irradiated at the sample cell with a spot size of 1 mm diameter where it was merged with the white probe pulse in a close angle (
