THE QUENCHING OF SHORT-LIVED S-2 STATES OF AROMATIC THIOKETONES BY HYDROCARBONS .1. IMPORTANT CONTRIBUTIONS OF THE TRANSIENT EFFECT FROM STEADY-STATE MEASUREMENTS

The very efficient fluorescence quenching of seven aromatic thioketone s in the second excited singlet state S-2 (lifetime tau(S2) = 53-857 p s in inert perfluoro-1,3-dimethylcyclohexane) has been examined by qua ntum yield and/or intensity measurements. Five n-alkanes (C6H14, C8H18 , C8D18, C10H22 and C16H34) as well as 2,2,4,4,6,8,8-heptamethylnonane , cyclohexane, benzene and their deuterated derivatives were used as q uenchers. The contribution of the transient effect to the quenching ra te constant k(q), which depends on the S-2-thioketone lifetime as well as on the size and diffusion coefficient of the quencher, has been in vestigated. The role of the transient effect in the quenching process was found to be very significant. Taking the transient effect into acc ount, the quenching rate constants calculated from the Stern-Volmer re lation ranged from 0.2 x 10(10) M(-1) s(-1) to the very large value of 12 x 10(10) M(-1) s(-1). These experimental results have been rationa lized by assuming the Smoluchowski-Collins-Kimball model. A comparison has been made between the measured and calculated values of k(q) for almost 40 donor-quencher systems in perfluoro-1,3-dimethylcyclohexane.