1, 2-Diarylethylene-Diene Exciplexes. Solvent-Induced Changes in Chemical and Physical Processes1

The fluorescence of several 1, 2-diarylethylenes can be quenched by conjugated dienes. Quenching of trans-stilbene and diphenylvinylene carbonate results in the formation of cycloadducts and the appearance of weak red-shifted fluorescence. Both cycloaddition and fluorescence are shown to occur from a singlet exciplex intermediate. The temperature dependence of 1, 2-diarylethylene fluorescence quenching rate constants indicates that the exciplex intermediates are formed reversibly, more stable exciplexes being formed less reversibly. The effect of solvent on diphenylvinylene carbonate-2, 5-dimethyl-2, 4-hexadiene exciplex lifetime, radiative decay, and cycloaddition has been investigated. In moderately polar solvents cycloaddition reactivity decreases rapidly with increasing solvent polarity leading to an increase in exciplex lifetime and fluorescence quantum yield. This decrease in cycloaddition reactivity is attributed to greater solvent stabilization of the polar exciplex intermediate than the transition state leading to nonpolar cycloadducts. In highly polar solvents, both exciplex cycloaddition and fluorescence rate constants decrease. A solvated ion pair may be of lower energy than the exciplex in highly polar solvents. The chemical consequences of exciplex-solvent interactions are discussed.