Chemically Induced Dynamic Electron Polarization Study on the Mechanism of Exchange Interaction in Radical Ion Pairs Generated by Photoinduced Electron Transfer Reactions

Abstract

Photoinduced electron transfer reactions were studied by using the continuous wave time-resolved electron paramagnetic resonance and Fourier-transformed electron paramagnetic resonance spectroscopy in polar solvents. The chemically induced dynamic electron polarization was investigated in both singlet and triplet precursor intermolecular electron transfer systems. The signs of the exchange interaction, which are defined by the energy differences between the singlet and triplet radical ion pairs, depended on the free energy changes for the charge recombination processes. The results are interpreted in terms of the mechanism that the spin selective stabilization and destabilization are caused by the perturbation through the electronic coupling from the ground state and the locally excited triplet state of the donor-acceptor pair at the equilibrium nuclear coordinate. In the singlet precursor electron transfer systems, the positive exchange interaction resulted from the selective triplet stabilization in the radical ion pair, when the ion pair state crossed with the locally excited triplet state at the normal region. In the triplet precursor electron transfer systems, the negative exchange interaction resulted from the selective singlet stabilization when the ion pair state crossed with the singlet ground state at the normal region. When the free energy change is larger than about 1.8 eV, the positive exchange interaction resulted from the spin-selective destabilization in the singlet ion pair, since the level crossing occurs at the inverted region.

Wire-like charge transport at near constant bridge energy through fluorene oligomers

“…No resonance was observed for compound 1, which most likely results from the magnitude of 2 J being larger than the magnetic field that our apparatus can apply (1.2 T). Measurements of 2 J have been shown to provide a measure of the electronic coupling for charge recombination by superexchange (11)(12)(13) and have been used as a probe of electron transfer mechanism in a number of studies (2,8,9,16,18,50,51). Most importantly, the energetic splitting between the singlet and triplet states within the radical ion pair arises from an indirect exchange between the two spins made possible by their interaction with the bridge.…”

“…The indirect electronic coupling between the electron donor and acceptor via the bridge orbitals, V DA , that is a result of superexchange (10) is the same interaction responsible for long-distance communication between the spins within a radical pair (RP), provided that the initial states are paramagnetic (11)(12)(13)(14)(15), which is true for the charge recombination process in the systems studied here. The magnetic field effect (MFE) on the yield of spin-selective RP recombination products directly reveals the magnitude of magnetic coupling between the spins of the RP and is proportional to V DA 2 (2,(16)(17)(18). The mechanistic details of the radical pair intersystem crossing mechanism (RP-ISC) and the theory behind the MFE have been researched extensively (19) and applied to many donor-acceptor systems (2,8,9,(20)(21)(22)(23)(24).…”

Effect of charge transfer state on the exchange interaction of radical-triplet encounter pairs

“…According to these results, we concluded that the net absorption CIDEP was produced in the triplet quenching with positive J value. Radical-triplet pairs combined with covalent bond or ligand [4] and several radical ion pairs [5][6][7][8][9] have been reported exceptionally to show positive J value (for radical ion pair, J is defined as energy splitting of triplet and singlet states and the negative sign means the triplet state lies higher than the singlet), but there are so far no reports on positive J value of radical-triplet encounter pairs.…”

“…Recently, Tero and his coworkers [8,9] studied the sign of J values of various radical ion pairs and demonstrated a beautiful correlation between the sign of J and the free energy changes for the charge recombination processes. They concluded that the sign of J is controlled by the spin-selective stabilization and destabilization of ion pair states through the electronic coupling with ground and locally excited triplet states of electron donor-acceptor pair at equilibrium nuclear coordinate.…”

Time-Resolved EPR Study on Photoinduced Charge-Transfer Trap State in Thiophene-Thiazolothiazole Copolymer Film

“…According to the configuration interaction model, the J-coupling is induced by the singlet perturbation from the ground state configuration: 2J = -|V| 2 /E CR where E CR = G CR +  is the vertical energy gap (Fig. 5) for the charge-recombination to the ground state [16]. In the P3HT:PCBM domain interfaces, the free energy change of G CR > 1 eV in the CS state of P3HT +• PCBM -• is evidently larger at the crystalline region than the reorganization energy of , [17] supporting the positive J because of the negative E CR .…”