LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF JET-COOLED 2-METHYLANTHRACENE S1 (PI, PI-ASTERISK) - EVIDENCE FOR METHYL CONFORMATION CHANGE UPON ELECTRONIC EXCITATION

The laser-induced fluorescence excitation spectrum of jet-cooled 2-met hylanthracene (2-MA) near the S1-S0 origin reveals a prominent torsion al progression due to the internal rotation of the methyl group or its torsional vibration against the aromatic ring frame. From the torsion al level spacings a low, threefold hindered internal rotation barrier of 70 cm-1 has been determined for S1 (pi, pi). A Franck-Condon inten sity analysis of the progression in the excited state torsion suggests that the most stable rotamer of 2-MA changes from an eclipsed-stagger ed (e-s) conformation in the electronic ground state to a (s-e) confor mation in the first singlet excited state. This 60-degrees change in t he dihedral conformation can be rationalized by the change of the pi-b ond order of the adjacent ring bonds upon electronic excitation, such that one of the C-H methyl bonds prefer to eclipse the adjacent C-C bo nd with the stronger pi-bond character.