Effects of Solute-Solvent Interactions on Radiationless Decay of Thioketones Excited to Their S₂- and T₁-States

Abstract

Spectral and photophysical properties of a few aromatic thioketones in their S2-and T1-state, in particular those determined by their interactions with solvents, are discussed. The reasons for a drastically different behaviour of thioketones in interactions with benzene as well as saturated hydrocarbons and perfluoroalkanes are analysed in more detaił. Results of the time-resolved transient absorption measurements in the pico-and nanosecond time scale are given. An analysis of these results proves that a product of the decay of the S2-state of xanthione in benzene is a new transient (τile > 60 ps) individuum which, regarding the system properties and the conditions of the experiment, has been identified as an exciplex in S 2 -state formed as a result of an efficient interaction with benzene molecule. Also in xanthione/alkane systems the involvement of a transient individuum was proved. Most probably, this individuum was a thioketyl radical which could be formed by hydrogen abstraction from a hydrocarbon molecule by xanthione in the S2 -state. Such individua act as intermediates in passing excitation to the triplet states of thioketones and are effective channels of the S2-state decay. The changes observed in the transient absorption spectra of xanthione in C 6 Η6 in the time range of 10 -7 -10 -6 s can be attributed to the formation of an excimer in the T1-state as a result of the T1-state selfquenching.

Intra- and Intermolecular Electronic Relaxation of the Second Excited Singlet and the Lowest Excited Triplet States of 1,3-Dimethyl-4-thiouracil in Solution¶

“…For each of the DMTU decays, the respective instrument response function was obtained by measuring the decays of S 2 ‐fluorescence of xanthione (XT), whose fluorescence lifetimes, τ, were accurately measured previously under identical experimental conditions as τ= 14.0 ps in CH 3 CN (27) and τ= 38.0 ps in CCl 4 (26). Measurements of the triplet state lifetimes, τ, were made on a modified laser spectrometer of nanosecond time‐resolution (28). The samples were excited at 355 nm.…”

Spectral Dynamics of Nitro Derivatives of Xanthione in Solutions

“…The absorption spectra are characterized by three vibronic bands in the UV region (280–320, 320–360, and 360–410 nm), which according to the literature − correspond to electronic transitions S 4 ← S 0 , S 3 ← S 0 , and S 2 ← S 0 . The transitions S 4 ← S 0 and S 2 ← S 0 are allowed (π,π*) transitions and their molar extinction coefficients are ε ≥ 10 4 M –1 cm –1 .…”

“…The low-intensity transition S 0 ← S 3 is forbidden in symmetry and it becomes allowed, for example, in 1,3-dimethoxy-9 H -xanthene-9-thione, in which the symmetry is lower than that in the XT . The S 0 ← S 1 transition in the XT and its derivatives is strongly forbidden, and its molar extinction coefficient is estimated to be ε ≃ 20 M –1 cm –1 . − Therefore, it practically appears neither in the absorption spectra nor in the fluorescence spectra (Figure A). Such an unusual fact for the organic compound spectroscopy is due to the presence of a heteroatom of sulfur in the XT molecule.…”

“…Considerable interest of researchers in thioketones during past decades is associated with their remarkable spectral and photophysical properties. − Spectral properties are closely related to the electronic structure of their highest excited electronic states . For the thioketones in solutions, experimentalists can distinguish separate S 1 , S 2 , and S 3 absorption bands, as well as direct absorption T 1 ← S 0 , thermally activated delayed fluorescence from S 1 state, fluorescence and phosphorescence from S 2 and T 2 , correspondingly .…”

“…For the thioketones in solutions, experimentalists can distinguish separate S 1 , S 2 , and S 3 absorption bands, as well as direct absorption T 1 ← S 0 , thermally activated delayed fluorescence from S 1 state, fluorescence and phosphorescence from S 2 and T 2 , correspondingly . The essential feature of thioketones is strong fluorescence from the second excited state and negligible one from the first excited state that is a bright example of the Kasha’s rule violation. ,,, The latter is caused by the large energy gap between S 2 and S 1 states and appears in S 2 long lifetimes of thioketones τ ∼ 10 –9 –10 –11 s. ,, …”