The large amplitude out-of-plane vibrations of 1,3-Benzodioxole in the S0 and S1 states: an analysis of fluorescence and excitation spectra by ab-initio calculations.

We report the analytical expressions of the two-dimensional potential energy surfaces (PES) spanned by the puckering and flapping vibrations in the S0 and S1 states of 1,3-Benzodioxole (BDO). Both PES are obtained from S0 and S1 energies computed on a grid of 2500 molecular geometries at the CASPT2 level. Both S0 and in S1 PES are anharmonic and the planar geometry corresponds to a barrier that separates two minima at non-planar geometries along the puckering/flapping deformations. Eigenvalues and eigenvectors of the mixed puckering/flapping modes are calculated by the Meyer flexible model. Improved vibronic levels, in better agreement with the observed spectra, are obtained by suitably optimized CASPT2 surfaces. To assign the lower energy (0-500 cm-1) portion of emission and absorption spectra we evaluate the band intensities by estimating the Franck-Condon factors between the puckering-flapping eigenvectors of the S0 and S1 states. From these calculations we obtain a satisfactory assignment of the ground state IR spectra and of the fluorescence excitation spectrum. Both assignments are supported by the analysis of the vibrational structures of several SVL fluorescence spectra. The successful interpretation of these spectra shows that the S0 and S1 PES that we derive for BDO are substantially correct. The barrier heights in the two states are similar: 125.7 and 190.4 cm-1 in S0 and in S1, respectively. In S0 the barrier is associated essentially with the puckering motion. In S1 it involves to a considerable extent also the flapping coordinate, whose vibrational frequency is much lower in S1 than in S0. This fact introduces a substantial Duschinsky effect in the S0 - S1 transitions of BDO.