Knowledge Bank

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 $S0$ and $S1(\pi \pi )$ 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, $v37$ and $v48$. The function $V(\varphi 1,\varphi 2)=1/2V2(2+\cos 2\varphi 1+\cos 2\varphi 2)+V12\cos 2\varphi 1\cos 2\varphi 2+V12\sin 2\varphi 1\sin 2\varphi 2$, with $V2=1550cm-1,V12=337.5cm-1$ and $V12\prime =402.5cm-1$ for the $S0$ state and with $V2=1500cm-1,V12=-85cm-1$ and $V12\prime =5.5cm-1$ for the $S1(\pi \pi )$ state fits the observed data (nine frequencies for $S0$, six for $S1$) extremely well. The fundamental frequencies for these torsions are $v37=9cm-1$ and $v48=118cm-1$ for the $S0$ state and $v37=35cm-1$ and $v48=110cm-1$ for the $S1$ excited state. The third torsion, the infernal rotation about the C=C bond, was assigned at 101 $cm-1$ for the $S0$ state based on a series of overtone frequencies. For $S1v35=99cm-1$ based on observed frequencies at 198, 396 $cm-1$, etc. A one-dimensional potential energy function of the form $V(\theta )=1/2V1(1-\cos \theta )+1/2V2(1-\cos 2\theta )+1/2V4(1-\cos 4\theta )$ was utilized to reproduce the frequencies for the ground state, For the excited slate an additional $VB$ term was added in order to lit The data for the trans potential energy well. The data indicate that the {trans} $\to$ twist barrier for the $S1$ state is higher than $1400cm-1$.