Internal Rotation of Methyl Group in Electronically Excited State

The fluorescence excitation and dispersed fluorescence spectra of jet-cooled o-, m- and p-fluorotoluene were observed. It was found that the spectra exhibit a feature characteristic of the internal rotation of methyl group in the ground and excited states. The detailed analyses of the spectra provided us with the potentials for the internal rotation in both the ground and excited states. In o-fluorotoluene, a great potential barrier for the internal rotation in the ground states was found to be practically removed in the excited state (S 1 ). It was also found that the most stable conformation of methyl group with respect to the benzene plane in the ground state is the most unstable conformation in the excited state. The situation is entirely different in m-fluorotoluene, where almost free rotation of methyl group in the ground state is greatly hindered in the excited state.