Spectroscopic determination of vibrational potential energy surfaces in ground and excited electronic states

Vibrational potential energy surfaces (PESs) for selected vibrations can provide detailed information on the conformational changes of non-rigid molecules in both ground and excited electronic states. These have been investigated using several spectroscopic techniques including laser-induced fluorescence (LIF), both fluorescence excitation spectroscopy (FES) and single vibrational level fluorescence (SVLF) or dispersed fluorescence (DF), infrared (IR) and ultraviolet (UV) absorption, and laser Raman spectroscopy. These methods allow detailed quantum energy maps for the ground electronic state (S0) and excited electronic states [S1(π,π*) and S1(n,π*)] to be established. Knowledge of these states allows the PESs to be determined in terms of the relevant molecular vibrations. The results for coumaran and 1,4-benzodioxan (14BZD) will be presented. Coumaran is puckered in both S0 and S1(π,π*) states with inversion barriers of 154 and 34 cm−1, respectively, and the energy minima lie at dihedral angles of 25° and 14°. 14BZD is twisted in both S0 and S1(π,π*) states with high barriers to planarity. In the electronic excited state the inversion barrier is reduced considerably.