TY - JOUR
T1 - Superexchange electron tunneling mediated by solvent molecules
T2 - Pulsed electron paramagnetic resonance study on electronic coupling in solvent-separated radical ion pairs
AU - Kobori, Yasuhiro
AU - Yago, Tomoaki
AU - Akiyama, Kimio
AU - Tero-Kubota, Shozo
AU - Sato, Hirofumi
AU - Hirata, Fumio
AU - Norris, James R.
PY - 2004/7/22
Y1 - 2004/7/22
N2 - Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay onstants (β) of the squared electronic coupling matrix element (V DA 2) in transient, solvent-separated radical ion pairs (RIP) composed of quinone anions and several cation radicals in aprotic liquid solutions of N,N-dimethylformamide, DMSO, and benzonitrile. The distance dependence of singlet-triplet energy splitting (2J) is shown to be described by β in V DA for charge-recombination processes. We show that the radical pair mechanism (RPM) electron spin polarization (PRPM) is quite sensitive to β. The β value is characterized by using the stochastic Liouville equation to fit the experimental PRPM values. The β values (from 0.8 to 1.0 Å -1) manifest that V DA is governed by the superexchange mechanism mediated by the intervening solvent molecules from a result that the β increases with increasing the tunneling energy gap (ΔG eff) for solvent oxidation or reduction in several intermolecular electron-transfer systems. We propose a simple three-dimensional model of V DA, in which the through-solvent tunneling pathways are exponentially increased with the increase in the intermolecular distance in bulk, condensed media. This model explains the ΔG eff dependence of β, including the data previously reported on the charge-transfer reactions both in liquid and frozen (77 K) solutions. Effective solvent-solvent coupling is estimated to be V B ≈ 850 cm -1 at a mean nearest-neighbor distance of 5.7 Å. This relatively large magnitude of v B may agree with dynamical amplifications of the effective coupling by low-frequency motions of the mediators as reported in charge-transfer reactions in biological systems. (Balabin, I. A.; Onuchic, J. Science 2000, 290, 114 and Troisi, A.; Orlandi, G. J. Phys. Chem. B 2002, 106, 2093.).
AB - Nanosecond pulsed electron paramagnetic resonance spectroscopy is applied to characterize exponential decay onstants (β) of the squared electronic coupling matrix element (V DA 2) in transient, solvent-separated radical ion pairs (RIP) composed of quinone anions and several cation radicals in aprotic liquid solutions of N,N-dimethylformamide, DMSO, and benzonitrile. The distance dependence of singlet-triplet energy splitting (2J) is shown to be described by β in V DA for charge-recombination processes. We show that the radical pair mechanism (RPM) electron spin polarization (PRPM) is quite sensitive to β. The β value is characterized by using the stochastic Liouville equation to fit the experimental PRPM values. The β values (from 0.8 to 1.0 Å -1) manifest that V DA is governed by the superexchange mechanism mediated by the intervening solvent molecules from a result that the β increases with increasing the tunneling energy gap (ΔG eff) for solvent oxidation or reduction in several intermolecular electron-transfer systems. We propose a simple three-dimensional model of V DA, in which the through-solvent tunneling pathways are exponentially increased with the increase in the intermolecular distance in bulk, condensed media. This model explains the ΔG eff dependence of β, including the data previously reported on the charge-transfer reactions both in liquid and frozen (77 K) solutions. Effective solvent-solvent coupling is estimated to be V B ≈ 850 cm -1 at a mean nearest-neighbor distance of 5.7 Å. This relatively large magnitude of v B may agree with dynamical amplifications of the effective coupling by low-frequency motions of the mediators as reported in charge-transfer reactions in biological systems. (Balabin, I. A.; Onuchic, J. Science 2000, 290, 114 and Troisi, A.; Orlandi, G. J. Phys. Chem. B 2002, 106, 2093.).
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U2 - 10.1021/jp036925t
DO - 10.1021/jp036925t
M3 - Article
AN - SCOPUS:3442901077
VL - 108
SP - 10226
EP - 10240
JO - Journal of Physical Chemistry B
JF - Journal of Physical Chemistry B
SN - 1520-6106
IS - 29
ER -