Laser-Induced Fluorescence Spectra and Torsional Potential Energy Functions of Jet-Cooled 4,4‘-Dimethyl-trans-Stilbene

Abstract

The laser-induced fluorescence excitation spectra of jet-cooled 4,4‘-dimethyl-trans-stilbene have been recorded, and the phenyl and methyl internal rotation levels have been assigned for the S0 and the S1(π,π*) electronic states. Dispersed fluorescence spectra and vapor-phase Raman spectra were utilized to help assign the low-frequency vibrations in the electronic ground state. The methyl torsions have energy levels and potential functions very similar to those of m-xylene. The S0 state has V6 = 15 ± 15 cm-1 (negligible barrier), while S1 has V3 = 85 ± 10 cm-1 and V6 = −30 ± 20 cm-1. The barrier to simultaneous internal rotation of the phenyl groups in S0 was found to be 3100 cm-1. This is identical to that of trans-stilbene within experimental error. For the S1 state, the barrier is 3140 cm-1 as compared to 3000 cm-1 for trans-stilbene. The barrier to rotation about the CC bond in the S0 state was estimated to be 41 ± 5 kcal/mol based on one assigned frequency. In the S1 state the trans-twist barrier was...