Fluorescence spectra and torsional potential functions for trans ‐stilbene in its S 0 and S 1 (π,π*) electronic states
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The fluorescence excitation spectra and dispersed fluorescence spectra of trans-stilbene have been recorded and analyzed. Vibrational assignments for the eight low-frequency modes have been made for both the S0and S1(π, π*) electronic states, and these differ substantially from those of previous workers. Two-dimensional kinetic and potential energy calculations were carried out in order to determine the potential energy surfaces for the two phenyl internal rotations ν37and ν48. The function V(φ1, φ2) = 1/2V2(2 + cos 2φ1+ cos 2φ2)+V12cos 2φ1cos 2φ2+V′12sin 2φ1sin 2φ2, with V2= 1550 cm-1, V12= 337.5 cm-1, and V′12= 402.5 cm-1for the S0state and with V2= 1500 cm-1, V12= -85 cm-1, and V′12= -55 cm-1for the S1(π,π*) state fits the observed data (nine frequencies for S0and six for S1) extremely well. The barriers to simultaneous internal rotation of both phenyl groups are given by twice the V2values. The fundamental frequencies for these torsions are ν37=9 cm-1and ν48=118 cm-1for the S0state and ν37=35 cm-1and ν48=110 cm-1for the S1excited state. The third torsion ν35, which is the internal rotation about the C=C bond, was assigned at 101 cm-1for the S0state based on a series of overtone frequencies (202, 404 cm-1, etc.). For S1, ν35=99 cm-1based on observed frequencies at 198, 396 cm-1, etc. Kinetic energy calculations were also carried out for this mode, and a one-dimensional potential energy function of the form V(θ)=1/2V1(1-cos θ)+1/2V2(1-cos 2θ)+1/2V4(1-cos 4θ) was utilized to reproduce the frequencies for the ground state. For the excited state, an additional V8term was added in order to fit the data for the trans potential energy well. The data indicate that the trans→twist barrier for the S1state is higher than 1400 cm-1. However, a somewhat revised frequency assignment would be compatible with a barrier of 1250 cm-1, which is close to the value of 1200 cm-1determined from dynamics studies. © 1994 American Institute of Physics.
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