Chimeric behavior of excited thioxanthone in protic solvents: II. Theory.

The chimeric behavior of thioxanthone in protic solvents has been investigated employing computational chemistry methods. In particular, methanol and 2,2,2-trifluoroethanol have been chosen in this study. The solvent environment has been modeled using microsolvation in combination with a conductor-like screening model. The vertical excitation spectrum within the same solvent is seen to depend on the number of specific bonds formed between the chromophore and the solvent molecules. Two different models have been discussed in this work, namely, one and two H-bond models. In particular, the formation of the second H-bond causes the energy gap between the πHπL* and nOπL* states to increase further. Excited-state absorption spectra for the photophysically relevant electronic states have been theoretically determined for comparison with the time-resolved spectra recorded experimentally [Villnow, T.; Ryseck, G.; Rai-Constapel, V.; Marian, C. M.; Gilch, P. J. Phys. Chem. A 2014]. The equilibration of the 1(πHπL*) and 3(nOπL*) states holds responsible for the chimeric behavior. This equilibrium sets in with a calculated time constant of 23 ps in methanol and 14 ps in TFE (5 and 10 ps in experiment, respectively). The radiative decay from the optically bright 1(πHπL*) state is computed to occur with a time constant of 25 ns in both solvents (14–25 ns in experiment).