Journal of Fluorescence, Vol. 10, No. 2, 2000 Deactivation of the S 2 and Two Lowest Triplet States of 4H-1-Benzopyran-4-thione and Its Alkyl Derivatives in Methanol Solution Marek Milewski, 1,3 Justyna Baksalary, 2 Pawel Antkowiak, 2 Wlodzimierz Augustyniak, 1 Michal Binkowski, 2 Jerzy Karolczak, 2 Dariusz Komar, 2 Andrzej Maciejewski, 1 Marian Szyman ´ ski, 2 and Wojciech Urjasz 1 Received September 28, 1999; accepted December 22, 1999 Spectral and photochemical properties of three aromatic thioketones, 4H-1-benzopyran-4-thione and its two alkyl derivatives, in their second excited singlet and two lowest triplet states have been a subject of stationary and time-resolved studies. Quantum yields of emissions from the above- mentioned states as well as lifetimes of these states have been measured. The results indicate an important role of intermolecular solvent-induced processes in deactivation of the excited states of the thioketones studied. The possible mechanism of these processes involves hydrogen atom abstrac- tion and/or formation of exciplex. For relatively high thione concentrations, the contribution of self-quenching in deactivation of the triplet states becomes significant. The effect of the alkyl chain in a thione molecule on deactivation parameters is discussed. KEY WORDS: S 2 fluorescence; room-temperature phosphorescence; thioketones; quenching; deactivation dynamics. INTRODUCTION from the two lowest thermally equilibrated triplet states [1,8–12]. Photochemical reactions can occur from the Aromatic thioketones have attracted considerable second excited singlet [1,13,14], the triplet states of thione interest due to their intriguing spectral, photophysical, molecules [1,15,16], and, more seldom, the lowest excited and photochemical properties. The energy gaps between singlet state [17]. Extensive stationary and time-resolved the second and the lowest excited singlet states of their investigations of the deactivation of the excited states of molecules are sufficiently large to inhibit radiationless thiocarbonyl compounds have shown the high reactivity relaxation of the S 2 state, so that a radiative process, S 2 of the S 2 state [6,7,18–22] and, to some extent, also the fluorescence, can compete with radiationless deactivation triplet states [12,23,24]. This property of thioketones is processes of the S 2 efficiently enough for the S 2 fluores- the reason that, in all common solvents except perfluoro- cence to be readily detected [1–7]. Another relatively alkanes and perfluorocycloalkanes (PF), intermolecular strong room-temperature emission is phosphorescence processes connected with interaction of an excited thio- ketone molecule with a solvent are the dominating path- 1 Faculty of Chemistry, Adam Mickiewicz University, 60-780 ways of deactivation of the S 2 state [25]. The contribution Poznan ´, Poland. of a solvent in deactivation of the triplet states depends 2 Institute of Physics, Adam Mickiewicz University, 61-614 Poznan ´, on their energy [26], the kind of solvent [12,23], and the Poland. concentration of the thioketone [12,23,24,27]. One of the 3 To whom correspondence should be addressed. e-mail: milewmar@ main.amu.edu.pl most interesting processes involving thione triplet is its 89 1053-0509/00/0600-0089$18.00/0  2000 Plenum Publishing Corporation