Chain-Length- and Solvent-Dependent Intramolecular Proton Transfer in Styrene-Amine Exciplexes

The photochemical and photophysical behavior of several ((N,.N-dimethylamino)alkyl)styrenes in which the amino group is attached to the styrene α- or β-carbon by a methyl, ethyl, propyl, or butyl polymethylene chain has been investigated. Efficient intramolecular addition of an aminomethyl C-H to styrene is observed in nonpolar solvents for the (aminoethyl)styrenes, and addition of an aminomethylene C-H is observed for the (aminobutyl)styrenes. However, the (aminomethyl)- and (aminopropyl)styrenes do not undergo intramolecular addition reactions. Both the reactive and unreactive (aminoalkyl)styrenes form fluorescent singlet exciplexes in nonpolar and polar solvents. The results of exciplex and product quenching by an added primary amine indicate that the fluorescent exciplex is an intermediate in the addition reactions of the (aminoalkyl)styrenes. Activation parameters for both exciplex formation and exciplex proton transfer have been determined. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy folded conformations of the singlet exciplex intermediates in nonpolar solvents. The solvent dependence of exciplex proton transfer, fluorescence, intersystem crossing, and nonradiative decay is attributed to a change in exciplex conformation from folded in nonpolar solvents to extended in solvents more polar than diethyl ether.