Thioxanthone in apolar solvents: ultrafast internal conversion precedes fast intersystem crossing.

The photophysics of thioxanthone dissolved in cyclohexane was studied by femtosecond fluorescence and transient absorption spectroscopy. From these experiments two time constants of ∼400 fs and ∼4 ps were retrieved. With the aid of quantum chemically computed spectral signatures and rate constants for intersystem crossing, the time constants were assigned to the underlying processes. Ultrafast internal conversion depletes the primarily excited (1)ππ* state within ∼400 fs. The (1)nπ* state populated thereby undergoes fast intersystem crossing (∼4 ps) yielding the lowest triplet state of (3)ππ* character.