The photophysics of thioxanthone (TX) dissolved in methanol (MeOH) and 2,2,2,-trifluoroethanol (TFE) was studied by time-resolved fluorescence and absorption spectroscopy. The spectrally integrated stimulated emission is seen to lose amplitude within ∼5-10 ps. This is much shorter than the fluorescence lifetimes of the compound (2.7 ns for MeOH and 7.6 ns for TFE). The initial reduction in amplitude is attributed to reversible intersystem crossing between the primarily excited (1)ππ* and a triplet (3)nπ* state. The latter one is energetically slightly (∼0.02 eV) above the former one. Addition of the quencher 1-methylnaphthalene (1-MN) reduces the fluorescence lifetime and yields triplet excited 1-MN, giving further evidence for the equilibrium of singlet and triplet excitations. The depopulation of these two states on the nanosecond time scale results in the rise of the lowest triplet state, a (3)ππ* state. Temperature dependencies attribute this to an activated internal conversion process between the two triplet states. Kinetic and energetic parameters derived from the experimental data will be compared with quantum chemical results in the accompanying paper [Rai-Constapel , V. , Villnow , T. , Ryseck , G. , Gilch , P. , and Marian , C. M. J. Phys. Chem. A 2014 , DOI: 10.1021/jp5099415].

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).

In this paper we investigate the photochromic ring-opening reaction of 2,2-diphenyl-5,6-benzo(2H)-chromene. In particular, we study the uncertainties and contradictions in various published reaction models using a combination of transient absorption and fluorescence spectroscopy with femtosecond time resolution. We propose a simplified reaction scheme which is in good agreement with theoretical studies. Here, photoexcitation populates a Franck-Condon state, whose fast vibrational wave packet motion, vibrational relaxation, bond-alternation and/or solvent rearrangement processes occur on the sub-picosecond timescale. Our data suggest that the resulting excited state minimum with picosecond lifetime still features structural characteristics of the closed form. Subsequently, the ring-opened photoproducts are formed in a concerted step from the excited state. The velocity of the photoreaction hence only depends on the time that the molecule needs to reach the transition region between the ground and excited states where the crucial bond breakage occurs.