Time-dependent density functional theory study on intramolecular charge transfer and solvent effect of dimethylaminobenzophenone

The lower singlet excited states for dimethylaminobenzophenone have been investigated as a function of the twisting motion with inclusion of solvent effects. Theoretical calculations have been performed using time-dependent density functional theory. The B3LYP and MPW1PW91 functionals with a 6-311+G(2d,p) basis set have been used to compute transition energies. The solvent effects have been described within the polarizable continuum model. Ground-state geometries are optimized using density functional theory with both B3LYP and MPW1PW91 functionals combined with 6-31G(d) basis sets. Vertical absorption energy calculations characterize the lower singlet excited states both in vacuum and in different kinds of solvents. A large redshift of the absorption maximum in the polar solvent suggests an intramolecular charge transfer character of the excited state. We have constructed the potential energy curves of two possible twisting motions of the excited states both in vacuum and in the polar solvent of acetonitrile: the twisting of only the dimethylamino group and the twisting of the dimethylaminophenyl group with respect to the benzoyl group. Both twisting processes predict the formation of the twisted intramolecular charge transfer state associated with the crossing of a low barrier. The presence of the polar solvent significantly changes the shape of the energy curves. Calculated emission energies for both the isolated and the solvated systems show a large Stokes shift between the absorption and fluorescence maxima. Two possible twisting motions produce similar fluorescence spectroscopic consequences. Our results including solvent effects explain the weak "dual-fluorescence" feature of dimethylaminobenzophenone, and imply that the two possible twisting motions may occur in the excited-state relaxation dynamics, but the twisting of the dimethylamino group seems to take place easier.