Thioxanthone: on the shape of the first absorption band

The equilibrium ground state geometry of thioxanthone (TX) has been investigated and its effect on the vertical excitation energies and photophysical behaviour has been explained. In line with this purpose, the first absorption band of TX has been simulated and analysed in detail. The calculations show that TX is planar, C(2v) symmetric in its ground state. The energy of the low-lying excited states seems to be rather insensitive along the butterfly motion coordinate. The shoulder in the first absorption band (at around 3.43 eV) is shown to be caused by vibrational progression of various in-plane modes and does not justify the hypothesis that two photophysically distinct conformers of TX exist. The calculated vertical excitation spectrum (in vacuum) has been compared with the experimental absorption bands in non-polar solvents.