Ultrafast quenching of the excited S 2 state of benzopyranthione by acetonitrile G. Burdzinski a, * , A. Maciejewski b,c , G. Buntinx d , O. Poizat d , C. Lefumeux d a Quantum Electronics Laboratory, Faculty of Physics, Adam Mickiewicz University, ul. Umultowska 85, 61-614 Poznan, Poland b Photochemistry Laboratory, Faculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland c Center for Ultrafast Laser Spectroscopy, Adam Mickiewicz University, Umultowska 85, 61-614 Poznan, Poland d LASIR, CNRS, Centre d’Etude et de Recherches Lasers et Applications, b^ at. C5, Universit e de Lille I, 59655 Villeneuve d’Ascq Cedex, France Received 24 November 2003; in final form 2 December 2003 Published online: 1 January 2004 Abstract Femtosecond and nanosecond transient absorption and picosecond time-correlated single photon counting techniques have been used to study the mechanism and dynamics of the efficient quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT) in the S 2 state, by acetonitrile. The results suggest the involvement of two aborted processes in the quenching mechanism: exciplex formation and hydrogen abstraction. The occurrence of the latter process is supported by the observation of a clear isotope effect on going from acetonitrile to deuterated acetonitrile. Ó 2003 Elsevier B.V. All rights reserved. 1. Introduction Aromatic thioketones show many interesting and unusual spectral and photophysical properties including direct S 0 ! T 1 absorption, well-resolved S 0 ! S 1 , S 0 ! S 2 and S 0 ! S 3 absorption bands, thermally acti- vated S 1 -fluorescence, and efficient fluorescence from the S 2 state and phosphorescence from the T 1 state in so- lution at room temperature [1–3]. The long S 2 state lifetime of thioketones (s S 2 ¼ 10 9 –10 11 s), due to a large DEðS 2  S 1 Þ energy gap, is responsible for the S 2 - state fluorescence, whereas emission from the S 1 state (radiative rate constant of about 10 5 s 1 ) is insignificant due to an ultrafast intersystem crossing process to the T 1 state (s S 1  10 12 s) [4]. The S 2 state is known to be extremely reactive in solution because of efficient inter- molecular quenching by most solvents including aceto- nitrile, but except perfluorohydrocarbons (PF) in which the S 2 state decay is exclusively intramolecular [1,3,5–8]. Recently, we have reported an analysis of the quenching mechanism of the S 2 state of 4H-1-benzopyrane-4-thi- one (BPT) by hydrocarbons using femtosecond transient absorption spectroscopy [9]. We have demonstrated the involvement of the hydrocarbon C–H bonds in the quenching process. Two possible quenching mechanisms have been proposed: efficient H-atom abstraction fol- lowed by ultrafast back hydrogen transfer, or Ôaborted hydrogen abstractionÕ. In the latter case, the progress along the reaction path was assumed to deactivate the S 2 state to the S 1 state through a conical intersection between the S 2 and S 1 energy surfaces. The quenching of the S 2 state of thioketones by acetonitrile has been investigated only for xanthione (XT) from picosecond emission (time-correlated single photon counting) and femtosecond transient absorption measurements [5,7]. The formation of a S 2 state solute– solvent exciplex has been suggested as the intermolecu- lar interaction responsible for the XT S 2 state quenching process. A very weak transient absorption signal, ob- served after subtraction of the S 2 and T 1 absorption bands, has been tentatively attributed to this exciplex [7]. The purpose of the present work is to extend this study to the case of BPT and determine whether the formation of such an exciplex can be confirmed or not. Moreover, the isotope effect induced by deuteration of * Corresponding author. Fax: +48-61-829-5155. E-mail address: gotardb@amu.edu.pl (G. Burdzinski). 0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.12.029 Chemical Physics Letters 384 (2004) 332–338 www.elsevier.com/locate/cplett