FLUORESCENCE-SPECTRA AND TORSIONAL POTENTIAL-ENERGY FUNCTIONS FOR 4-METHOXY-TRANS-STILBENE IN ITS S-0 AND S-1(PI,PI-ASTERISK) ELECTRONIC STATES

The fluorescence excitation spectra and dispersed fluorescence spectra of 4-methoxy-trans-stilbene have been recorded and analyzed. Assignments were made for all eight of the low-frequency vibrational modes, excluding the methoxy and methyl torsions, for the So ground state and for five of the low-frequency vibrations in the S-1(pi,pi*) electronic excited state. Numerous transitions involving the two-phenyl torsions and the C=C torsion were observed. These were utilized to determine the two-dimensional vibrational energy surface for the phenyl torsions and to evaluate the one-dimensional ethylenic torsion for both the S-0 and S-1 states. For the phenyl torsions the barriers to simultaneous internal rotation were calculated to be 2860 cm(-1) for both the S-0 and S-1 states, about 200 cm(-1) lower than the trans-stilbene values. The C=C torsional barrier for the S-0 state was estimated to be 52 +/- 3 kcal/mol based on its observed overtone frequency at 172 cm(-1) . For the excited state the C=C torsional overtone series can be observed beginning at 171 cm(-1). Analysis of the potential function for this mode indicates that the barrier to trans-twist interconversion is approximately 245 cm(-1) higher than that for trans-stilbene.