Electron Spin Polarization Transfer to the Charge-Separated State from Locally Excited Triplet Configuration: Theory and Its Application to Characterization of Geometry and Electronic Coupling in the Electron Donor-Acceptor System † Yasuhiro Kobori,* Masaaki Fuki, and Hisao Murai Department of Chemistry, Faculty of Science, Shizuoka UniVersity, 836 Ohya Surugaku, Shizuoka 422-8529 Japan ReceiVed: March 15, 2010; ReVised Manuscript ReceiVed: May 11, 2010 We present a theoretical model of analysis of the time-resolved electron paramagnetic resonance (TREPR) spectrum of the charge-separated (CS) state generated by the photoinduced electron transfer (ET) reaction via the locally excited triplet state in an electron donor-acceptor (D-A) system with a fixed molecular orientation. We show, by the stochastic-Liouville equation, that chemically induced dynamic electron polarization (CIDEP) of the triplet mechanism is explained by lack of transfer of quantum coherence terms in the primary triplet spin state, resulting in net emissive or absorptive electron spin polarization (ESP) which is dependent on anisotropy of the singlet-triplet intersystem crossing in the precursor excited state. This disappearance of the coherence is clearly shown to occur when the photoinduced ET rate is smaller than the angular frequency of the Zeeman splitting: the transferred coherence terms are averaged to be zero due to effective quantum oscillations during the time that the chemical reaction proceeds. The above theory has been applied to elucidate the molecular geometries and spin-spin exchange interactions (2J) of the CS states for both folded and extended conformers by computer simulations of TREPR spectra of the zinc porphyrin-fullerene dyad (ZnP-C 60 ) bridged by diphenyldisilane. On the extended conformation, the electronic coupling is estimated from the 2J value. It has been revealed that the coupling term is smaller than the reported electronic interactions of the porphyrin-C 60 systems bridged by diphenylamide spacers. The difference in the electronic couplings has been explained by the difference in the LUMO levels of the bridge moieties that mediate the superexchange coupling for the long-range ET reaction. Introduction For development of the molecular solar energy conversion systems, it is crucial to investigate how both the molecular geometry and the electronic structure of electron donor-acceptor (D-A) molecules contribute to the electronic coupling for the charge-separation (CS) and for the charge-recombination (CR) processes. 1–7 A wide variety of D-A molecules have been synthesized and extensively investigated to mimic the efficient light-energy conversion systems of the natural photosynthetic reaction centers (PRCs). 1,2,8–12 The PRCs undergo the efficient stepwise CS processes 13 initiating from the photoexcited singlet configurations of a pigment. The time-resolved EPR (TREPR) method has been a powerful tool to investigate several photochemical processes. By using the TREPR, electron spin polarization (ESP) has been observed at nanosecond and microsecond regions on radicals, radical pairs, excited triplet states, and so on. 4,14–34 Basically, the ESP is a signature of the primary photochemical processes, 35 and several ESP mechanisms have been established to account for non-Boltzmann spin distributions of the reaction intermediates observed by the TREPR technique. 36 The ESP mechanisms have been utilized to characterize the photochemical processes and the electronic properties in the photosynthetic proteins, 24,37–46 the electronic molecular wires, 30–32 and so on. Molecular geometries of the CS states have been examined in detail using the TREPR method by analyzing the ESP of the spin-correlated radical pair (SCRP) for several PRCs 24,37–46 and for the artificial D-A systems 3,7 in which the singlet precursor CS reactions are predominant. Contrarily, only a few studies have been performed to elucidate the molecular conformations and the exchange interactions in the CS states of the D-A systems in which the photoinduced CS reactions take place via the excited triplet configurations. 4,28,33 The photoexcited triplet states are well-known to possess characteristic anisotropic properties in the spin system. Rates of the singlet-triplet intersystem crossing (ISC) to the three spin sublevels of X, Y, and Z in the excited triplet state are anisotropic according to the spin-orbit coupling interaction. 47 The spin-spin dipolar interaction (or zero-field splitting interac- tion) in the triplet state is strongly dependent on the direction of the magnetic field (B 0 ) with respect to the principal axes (X, Y, and Z) of the triplet spin configuration. 47 Transfer of the ESP in the precursor triplet state takes place to the spin states of the radical pair during the CS reaction when the spin-lattice relaxation of the polarized triplet state is slower than the reaction. 4,26 Thus, in principle, the anisotropic triplet characters should be quite powerful to determine the molecular geometry and the electronic properties in the CS state of D +• A -• in detail by using the TREPR method. 4 Electron spin polarization transfer (ESPT) phenomena have been investigated in several photochemical systems by the TREPR. 4,21,22,26,34,48–50 To account for the TREPR spectra obtained by the triplet-triplet (TT) energy transfer and by the triplet-precursor photoinduced ET processes, the ESPT mech- anisms have been discussed in terms of the conservation of the spin angular momentum: since the operators responsible for the electronic couplings of the reactions have no effect on the † Part of the “Michael R. Wasielewski Festschrift”. * Corresponding author. E-mail: sykobor@ipc.shizuoka.ac.jp. J. Phys. Chem. B 2010, 114, 14621–14630 14621 10.1021/jp102330a  2010 American Chemical Society Published on Web 05/28/2010