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 torsional 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 different compounds in all solvents studied. In fact, the relaxation seems to be almost independent of solvent at room temperature, indicating that most of the local friction felt by the twisting carbonyl is internal friction rather than solvent friction. Apparently the small carbonyl is effectively shielded from the solvent by the anthracene ring so the solvent friction is not strong. In contrast to the weak solvent coupling, previously reported gas‐phase spectra indicate that the torsion may be strongly coupled to the other vibrational modes of the molecule. Energy flow out of the torsion into these other vibrational modes appears to occur on the subpicosecond time scale.