A quantum chemical study of intramolecular charge transfer in a closely-spaced, donor-acceptor molecule

This investigation explores the use of contemporary quantum chemistry to mimic the light-induced, intramolecular charge-transfer processes that occur in 2-methyl-2,3-dihydrobenz[d,e]isoquinoline (DHBIQ) in polar solvents. Thus, the computed excited-state manifold, comprising two locally excited pi,pi* singlets, a locally excited pi,pi* triplet, and a charge-transfer (CT) state, is in excellent agreement with the experimental findings. It is shown that, whereas the energies of the various locally excited states are insensitive to molecular geometry and environment, the energy of the CT state depends markedly on structure and solvent polarity. The most favorable charge-transfer interactions occur within a distorted geometry that is midway between the axial and equatorial conformers identified for the ground state. The calculated nuclear and solvent reorganization energies are in good agreement with prior experimental work. Molecular dynamics simulations were employed to estimate the change in Gibbs free energy accompanying charge transfer and this latter value, used in conjunction with the reorganization energy, allows reproduction of the experimental activation energy. Finally, the electronic coupling matrix element for charge transfer was computed by identifying the intersection point for potential energy surfaces associated with the CT state and the lowest-energy pi,pi* excited singlet state. The derived value (T-DA = 206 cm(-1)) is close to the experimental result (T-DA = 140 cm(-1)) obtained by application of classical Marcus theory. Overall, it is concluded that quantum chemical methods allow meaningful calculation of the parameters controlling the rate of charge transfer in this system.