Deactivation of the S1-State of ω-SubstituteD4-Dimethylamino-trans-Styrenes in Alkane Solutions

Deactivation of three ω-substituted acceptors in 4-dimethylamino-trans-styrenes (P(S)Ph02 (la), P(0)Ph2(2a) and SO3CH3(3a)), dissolved in n-paraffins (from n = 5 to n = 16) at 293 K. was investigated. Trans-cis isomerization and intersystem crossing are the main processes responsible for radiationless deactivation of the S1 excited state. High fluorescence anisotropy, r, observed in low-viscous n-paraffins, is due to strong fluorescence quenching resulting in the considerable shortening of the lifetime r of these molecules. The fluorescence depolarization which occurs when lowering the viscosity, n, of the solution is caused by Brownian rotations and conformational changes due to fast adiabatic twisting of the planar trans-S1form around the double bond, leading to the formation of the perp.-S1structure with perpendicularly oriented parts of the molecule, and to the concomitant change in the transition moment direction. Linear dependence of ζ2/(r0/r — 1) on n. ζin the whole viscosity range was only observed for molecule 3a.