Computational determination of the dominant triplet population mechanism in photoexcited benzophenone

Abstract

In benzophenone, intersystem crossing occurs efficiently between the S₁(nπ*) state and the T₁ state of dominant nπ* character, leading to excited triplet states after photoexcitation. The transition mechanism between S₁(nπ*) and T₁ is still a matter of debate, despite several experimental studies. Quantum mechanical calculations have been performed in order to assess the relative efficiencies of previously proposed mechanisms, in particular, the direct S₁ → T₁ and indirect S₁ → T₂(ππ*) → T₁ ones. Multiconfigurational wave function based methods are used to discuss the nature of the relevant states and also to determine minimum energy paths and conical intersections. It is found that the T₁ state has a mixed nπ*/ππ* character and that the T₂(ππ*) state acts as an intermediate state between the S₁ and T₁ states. This result is in line with recent experiments, which suggested a two-step kinetic model to populate the phosphorescent state after photoexcitation [Aloïse et al., J. Phys. Chem. A, 2008, 112, 224–231].