Laser-induced fluorescence spectroscopy of jet-cooled 2-methylanthracene S₁ (π,π^*). Evidence for methyl conformation change upon electronic excitation

The laser-induced fluorescence excitation spectrum of jet-cooled 2-methylanthracene (2-MA) near the S₁-S_o origin reveals a prominent torsional progression due to the internal rotation of the methyl group or its torsional vibration against the aromatic ring frame. From the torsional level spacings a low, threefold hindered internal rotation barrier of 70 cm^-1 has been determined for S₁(p,p^*). A Frank-Condon intensity analysis of the progression in the excited state torsion suggests that the most stable rotamer of 2-MA changes from an eclipsed-staggered (e-s) conformation in the electronic ground state to a (s-e) conformation in the first singlet excited state. This 60° change in the dihedral conformation can be rationalized by the change of the p-bond order of the adjacent ring bonds upon electronic excitation, such that one of the CH methyl bonds prefer to eclipse the adjacent CC bond with the stronger p-bond character. © 1993.