Article pubs.acs.org/JPCC
Electron Transfer in a Supramolecular Complex of Zinc Chlorin Carboxylate Anion with Li+@C60 Affording the Long-Lived Charge-Separated State Yuki Kawashima,† Kei Ohkubo,† Mase Kentaro,† 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: A supramolecular complex was formed between zinc chlorin carboxylate (ZnCh−) and lithium ion-encapsulated [60]fullerene (Li+@C60) by an electrostatic interaction in benzonitrile (PhCN). The binding constant was determined to be 7.7 × 104 M−1. No fluorescence quenching of ZnCh− was observed upon addition of Li+@C60, indicating that no electron transfer (ET) from the singlet excited state of ZnCh− (1[ZnCh−]*) to Li+@C60 occurred. In contrast, the transient absorption band due to triplet excited state of ZnCh− (3[ZnCh−]*) was efficiently quenched by ET from 3 [ZnCh−]* to Li+@C60 to produce the charge-separated (CS) state, [ZnCh−]•+/Li+@ C60•−, with the rate constant of kET = 5.3 × 104 s−1. The charge-recombination dynamics was monitored by the decay of the transient absorption band at 1035 nm due to Li+@ C60•−. The lifetime of the CS state was determined to be 170 μs. The spin state of CS state was triplet determined by EPR measurements at low temperature. The reorganization energy (λ) and electronic coupling term (V) of ET and back electron transfer (BET) were determined from the temperature dependence of kET and kBET to be λ = 0.46 ± 0.02 eV and V = 0.095 ± 0.030 cm−1 for ET and λ = 1.26 ± 0.04 eV and V = 0.066 ± 0.010 cm−1 for BET based on the Marcus theory of nonadiabatic electron transfer. Such small V values result from the small orbital interaction between ZnCh− and Li+@C60 moieties to afford the long-lived CS state.
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where the kET is rate constant of intramolecular electron transfer, λ is the reorganization energy of electron transfer, V is the electronic coupling term, −ΔGET is the driving force for electron transfer, and kB is the Boltzmann constant.72,73 In the case of the tetrad, the V value is determined to be 1.7 × 10−4 cm−1, which is extremely small, because ferrocenium ion and C60 radical anion of CS state (Fc+−ZnP−H2P−C60•−) are spatially separated by a long edge-toedge distance (48.9 Å). In contrast to the tetrad, zinc porphyrin− fullerene dyad (ZnP−C60) afforded a much larger V value (3.9 cm−1) and a shorter CS lifetime (0.77 μs).70 Extensive efforts have so far been devoted to modulate and predict V values.74−78 In contrast to covalently linked CS systems, there has been poor discussion about V values of supramolecular CS systems. A nonpolar solvent is utilized to construct supramolecular CS systems with a strong binding.79−86 However, CS lifetimes are normally extremely short in a nonpolar solvent because the CS states are not stabilized by solvation and usually decay to produce the triplet excited states rather than the ground state because of the higher energy of CS state than the triplet excited states.87 Thus, it is preferred that to construct a supramolecular CS system by utilizing the electrostatic interaction in a polar solvent which
INTRODUCTION In the photosynthetic reaction centers, photoinduced electron transfer from the excited chlorophyll dimer (so-called “special pair”) occurs to attain the final charge-separated (CS) state, which is utilized to synthesize high-energy compounds such as NADPH (nicotinamide adenine dinucleotide phosphate) and ATP (adenosine triphosphate).1,2 In the field of artificial photosynthesis, there have been many reports on electron donor−acceptor linked systems to achieve long lifetimes of CS states (τCS).3−49 In particular, fullerenes have been widely used as three-dimensional π-electron acceptor due to their small reorganization energy, which results from the π-electron system being highly delocalized over the three-dimensional curved surface together with the rigid and confined structure of the aromatic π-sphere.50−53 The mimicry of the natural photosynthesis has prompted the design of synthetic donor−acceptor linked ensembles such as triad, tetrad, and pentads.54−71 For instance, a covalently linked ferrocene−zinc porphyrin−free base porphyrin−fullerene tetrad (Fc−ZnP−H2P− C60) afforded a long CS lifetime (0.38 s), which is comparable to the CS lifetime of the photosynthetic reaction center.69,70 Such an extremely long-lived CS state is well explained by the Marcus equation for nonadiabatic electron-transfer reaction (eq 1) ⎡ (ΔG + λ)2 ⎤ ⎛ 4π 3 ⎞1/2 1 ET ⎥ ⎟ V 2 exp⎢– = kET = ⎜ 2 4λkBT τCS ⎦ ⎣ ⎝ h λkBT ⎠ © 2013 American Chemical Society
Received: August 9, 2013 Revised: September 4, 2013 Published: September 5, 2013
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dx.doi.org/10.1021/jp407976b | J. Phys. Chem. C 2013, 117, 21166−21177
The Journal of Physical Chemistry C
Article
UV−vis Absorption Spectral Measurements. Absorption spectra were recorded on a Hewlett-Packard 8453 diode array spectrophotometer at room temperature. Emission Spectral Measurements. Phosphorescence was measured on a Horiba FluoroMax-4 spectrofluorophotometer. A 2-methyltetrahydrofuran (2-MeTHF)/ethyl iodide(EtI) (9:1) solution of TBA+ZnCh− 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 flask. The measurements were carried out by excitation at 550 nm for TBA+ZnCh−. Fluorescence spectra of TBA+ZnCh− were recorded on a Horiba FluoroMax-4 spectrofluorophotometer. The measurements were carried out by excitation at 445 nm for TBA+ZnCh− in deaerated PhCN. The PhCN solutions degassed by nitrogen bubbling for 10 min prior to the measurements. The fluorescence lifetimes (τfl) of the TBA+ZnCh− was determined in deaerated PhCN at 298 K by single photon counting using a Horiba FluoroMax-4 time-resolved spectrofluorophotometer. 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 was derived from the fundamental output of Integra-C (λ = 786 nm, 2 μJ/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 polarizer were inserted in the path in order to generate 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 (
