Quantum chemical study of the torsional motions for 9-vinylanthracene

The torsional motions for 9-vinylanthracene both in the ground and excited states were investigated by using quantum chemical calculations. In the calculations, the restricted Hartree-Fock (RHF) method and the configuration interaction with single excitation (CIS) were used for the ground (S0) and excited (S1) states, respectively. First, the stable structures in both states were obtained and compared with experimental results. Next, the harmonic vibrational frequencies were calculated; it was clarified that there is a low frequency mode in the S0 state of which the normal coordinate is approximated by the displacement of the torsional dihedral angle while there are two torsional modes in the S1 state which are coupled to the out-of-plane bending of the anthracene ring. Finally, the multi-dimensional Franck-Condon factors were calculated based on the assumption that all the normal modes of vibrations are small-amplitude motions in each state. The anharmonic effect of the torsional motions on the Franck-Condon factors is also examined in brief. These results are in agreement with the laser-induced fluorescence (LIF) study by Barbara et al.