trans-Stilbene-Amine Exciplexes. Stereoelectronic Control of Amine Dimer Cation Radical Formation

The interaction of singlet trans-stilbene with several trialkylamines, a,ω)-diaminoalkanes, and the bicyclic diamine Dabco in benzene solution has been investigated. The rate constants for fluorescence quenching equal or exceed the rate of diffusion and are not subject to steric hindrance. Exciplex energies and monoamine oxidation potentials show little variation with structure, suggesting that steric destabilization and inductive stabilization counterbalance each other. Intersystem crossing to yield triplet trans-stilbene is the predominant exciplex decay pathway at low amine concentrations. At higher amine concentrations, exciplex quenching by ground-state amines occurs and is subjected to a pronounced steric effect in the case of monoamines and a chain-length effect in the case of diiaminoalkanes. On the basis of these effects, exciplex quenching is proposed to occur by the interaction of cation radical and neutral amines to yield a triplex of trans-stilbene anion radical and a three-electron, ϭ-bonded amine dimer cation radical. The energy of this triplex is estimated to be lower than that of triplet trans-stilbene.