Benzonitrile and its van der waals complexes studied in a free jet. III. Enhancement of the intersystem crossing rate in the benzonitrile dimer and other complexes

By using the sensitized phosphorescence spectroscopy, the intensity of the phosphorescence has been recorded upon excitation of the benzonitrile dimer to the S₁ vibronic states in a free jet. The results indicate that the strong vibrational energy dependence of the fluorescence quantum yield, reported previously, is attributable to the increasing rate of intersystem crossing with increasing vibrational energy. Similar behavior is also observed in other van der Waals complexes of benzonitrile though the increase is less obvious. The enhancement of the intersystem crossing can be correlated with the state density of van der Waals modes in the S₁ electronic state. In case of the benzonitrile trimer and benzonitrile-Kr complex, intersystem crossing is found to be fully efficient even without vibrational excitation.