Jet-cooled fluorescence spectra, Raman spectra, and potential energy surfaces of stilbenes

The jet-cooled fluorescence excitation spectra and dispersed fluorescence spectra of trans- stilbene (t-S) and 4-methoxy-trans-stilbene (MS) have been recorded and analyzed. Preliminary results for 4,4'-dimethyl-trans-stilbene have also been obtained. The high- temperature vapor-phase and liquid phase Raman spectra of these molecules along with cis- stilbene have also been recorded. The low-frequency vibrational modes have been assigned, and the vibrational progressions associated with the two phenyl torsions and the ethylinic internal rotation, which primarily governs the photoisomerization process, have been analyzed. For t-S and MS the two-dimensional phenyl torsion potential energy surfaces and the one-dimensional carbon-carbon double bond torsional potential energy functions for both the S0 and S1((pi) , (pi) *) states were determined. The barriers to simultaneous phenyl internal rotation are 3100 cm-1 for the ground state and 3000 cm-1 for the excited state. The data for the C equals C torsion are consistent with a trans yields cis barrier of 48.3 kcal/mole for the S0 state and a trans yields twist barrier of 2000 to 3500 cm-1 for the S1 state. For MS the barrier for simultaneous phenyl torsion for both the S0 and S1 states is 2860 cm-1. The C equals C torsional barrier for MS in both the S0 and S1 states is approximately 10% higher than that of t-S.