Strong vibronic coupling in the first excited singlet state of diphenylhexatriene by an asymmetric low-frequency mode

Raman studies of diphenylhexatriene in the first excited singlet state evidence strong vibronic coupling between two close 1B_u/2A_g states. The geometrical change after photoexcitation contains a dominant contribution of a low-frequency mode with b_u symmetry near 40 cm^-1. This mode is identified as a relatively strong line in the Raman spectrum. Vibronic coupling mainly affects two C=C stretching Raman bands near 1700 cm^-1. A two-dimensional effective potential for the lowest excited singlet state is derived which models the coupling between the coordinate of the 40 cm^-1 mode and the C=C stretching coordinate, thereby explaining the occurrence of the two bands and their strong solvent shift. The model gives, in the zero gap limit, a double-well potential for the C=C stretching coordinate, due to the pseudo-Jahn-Teller effect. Modulation of this potential by the low-frequency cycle explains the spectral broadening observed in the Raman spectra.