Deformation-induced increase of spin—orbit coupling in the benzophenone molecule

Applying phosphorescence and phosphorescence excitation spectroscopy to a 6 K benzophenone glass, we found that the T₁ ← S_o transition can be enhanced by a factor of 300 if the absorbing molecule is distorted. Distortion also induces strong coupling to the antisymmetric torsional mode of the phenyl rings. It is concluded that twisting of the phenyl rings increases spin—orbit coupling in the 3nπ* state.