Nature of the S 2-state quenching process of benzopyranthione by hydrocarbon solvents measured by pico- and femtosecond laser spectroscopy

Abstract

Femtosecond transient absorption has been used to study the mechanism and dynamics of the quenching of an aromatic thioketone, 4H-1-benzopyrane-4-thione (BPT), in the S 2 state, by saturated hydrocarbons ( n-hexane, dodecane, hexadecane, cyclohexane, cyclohexane-d 12). The results demonstrate the involvement of the hydrocarbon C–H bonds in the quenching process. Two quenching mechanisms are proposed: efficient H-atom abstraction followed by ultrafast back hydrogen transfer, or ‘aborted hydrogen abstraction’. In the latter case, the progress along the reaction path to a conical intersection is assumed to deactivate the S 2 state through transition to the S 1 state energy surface.