High-resolution Laser Spectroscopy Of Trans-stilbene : Nonplanar Structure In The Ground State

Trans-stilbene os og great interest in the excited-state dynamics such as cis-trans isomerization in the electronic excited state. Zewail at al. reported the results of time-resolved spectroscopy and suggested its nonplanar structure in the ground S$_0$ state \footnote{J. A. Syage, P. M. Felker, and A. H. Zewail, J. Chem. Phys. {\bf 81}, 4685 (1984).}. In contrast, Pratt et al. concluded that the molecule is essentially planar both in the S$_0$ and S$_1$ states by analyzing the rotationally resolved high-resolution speoctrum of the S$_1$ $\leftarrow$ S$_0$ 0-0 band \footnote{D. W. Pratt, W. L. Meerts et al., J. Phys. Chem. {\bf 94}, 6 (1990).}. We observed the spectrum with much higher accuracy and quality, and re-determined the rotational constants. Although it is impossible to accurate determine the abosolute value of $A$ for the $a$-type transition, We could conclude that trans-stilbene is nonplanar in the S$_0$ state. Theoretical calculation using WB97XD functional provided the nonplanar structure in which the phenyl rings are rotated around the C-C bond axis and take the $C_2$ symmetry. It suggests that steric repulsion between H atoms of {\it ortho}-position in a phenyl ring and in an ethylene part surpasses stabilization by $\pi$ conjugation