ULTRAFAST TORSIONAL RELAXATION FROM THE BARRIER REGION FOR AN EXCITED-STATE ISOMERIZATION IN SOLUTION - 9-CARBONYLANTHRACENES

The class of compounds containing a carbonyl group substituted at the 9 position of anthracene have been studied as new model systems for to rsional isomerization in solution. Franck-Condon excitation yields an excited-state distribution near the barrier for isomerization in S-1. Using time-resolved fluorescence spectroscopy, we have determined that the relaxation from the barrier region is subpicosecond for three dif ferent compounds in ah solvents studied. In fact, the relaxation seems to be almost independent of solvent at room temperature, indicating t hat most of the local friction felt by the twisting carbonyl is intern al friction rather than solvent friction. Apparently the small carbony l is effectively shielded from the solvent by the anthracene ring so t he solvent friction is not strong. In contrast to the weak solvent cou pling, previously reported gas-phase spectra indicate that the torsion may be strongly coupled to the other vibrational modes of the molecul e. Energy flow out of the torsion into these other vibrational modes a ppears to occur on the subpicosecond time scale.