Relaxation of the second excited singlet states of aromatic thiones: the role of specific solute-solvent interactions

Abstract

Second excited singlet state (S2) fluorescence quantum yields and lifetimes of six aromatic thiones have been measured in solution at room temperature. Intramolecular S2 S, internal conversion dominates S2 decay in inert perfluoroalkane solvents, but intermolecular photochemical and photophysical processes dominate in more strongly interacting solvents. Stern-Volmer quenching is observed when perfluoroalkane solutions of 2,2,3,3-tetramethylindanthione, a model thione, are spiked with addends at concentrations up to ca. 0.3 M. Measurements of the rate constants for the bimolecular quenching of S2 fluorescence by 26 different addends reveal that the thione S2 state is highly reactive and that the initial intermolecular interaction path divides into physical and chemical branches, the branching ratio being a function of the nature of the quencher.

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

“…As solvents we used: perfluoro1,3-dimethylcyclohexane, PFDMCH, 3-methylpentane, 3MΡ, and benzene, C6H6. As we have shown many times [2][3][4][5][6][7][8][9][10]29], only the use of perfluoroalkanes as solvents ensures the possibility of investigating intramolecular properties of thiones in the S2-and Τ1-states. The use of 3MΡ, a typical aliphatic hydrocarbone, ensures that the interactions with thioketones in the S2 and T1 -state are limited to the reaction of hydrogen abstraction only [5, 7-10, 15, 16] as the universal (physical) interactions are in it negligibly weak [5, 8-10, 29, 30].…”

“…One of them is very efficient quenching of the excited states, mainly the triplet ones but also the long-lived S2-state, by the thione molecules in the ground state, the socalled selfquenching [7][8][9][10], which has serious consequences [ΙΙ]. Another specific property of thioketones (thiones) is their unusually high reactivity in the excited states, mainly S2 but Τ1 as well, stemming from the properties of the thio group (C=S) [7,10] and long lifetimes of these states. In all solvents except perfluoroalkanes, thioketones excited to the S2-state undergo very efficient deactivation in intermolecular processes which is manifested by a very significant shortening of the decay time and a decrease in the quantum yield of fluorescence.…”

“…As indicated by measurements of the quantum yield of net photochemical consumption [4,5,10,12] and an analysis of quantum yield of phosphorescence determined upon selective excitation [13,14], these individua deactivate almost quantitatively to the thione and the solvent molecules in the ground state and the deactivation runs mostly with the involvement of Τ1-state. Without the assumption about formation of these individua, no correct interpretation of photophysical properties, results of photochemical studies [7,9,10] and studies on the quenching process [8,10] would be possible. It s assumed that in the solvents sufficiently strongly interacting (e.g.…”

“…It s assumed that in the solvents sufficiently strongly interacting (e.g. C6 H6 ) and simultaneously chemically inert [7,10] the processes taking place between the thione in the S2-state and the solvent molecules lead to formation of exciplexes (in this case exciplexes in the S2-state) while in the chemically reactive solvents [7,10,15,16] in which the interactions can be even very weak (e.g. alkanes) the transient individua are radicals.…”

“…alkanes) the transient individua are radicals. Despite many attempts, emission studies have so far failed to provide either direct evidence of existence of such individua or indirectly determine their properties [7,10,17]. Moreover, a few attempts have been made to use transient absorption spectroscopy with picosecond resolution to study the S2-thione decay and to search for the transient individua.…”