Fluorescence and Absorption Spectra and Potential Energy Surfaces of 1,3-Benzodioxole and Coumaran in Their S$_{0}$ and S$_{1}(\pi $,$\pi $*) States

The fluorescence excitation spectra and dispersed fluorescence spectra of jet-cooled molecules along with ultraviolet and far-infrared absorption spectra of the bicyclic molecules 1,3-benzodioxole and coumaran have been recorded. These spectra have allowed us to determine the two-dimensional potential energy surfaces for the ring-puckering and ring-flapping vibrations for the S$_{0}$ and S$_{1}(\pi $,$\pi $*) electronic states. Both molecules are bent with barriers to planarity in both their ground and excited states. The bent structures for coumaran result from --CH$_{2}$-CH$_{2}$- torsional forces while those for 1,3-benzodioxole are caused by the anomeric effect.