Exciplex formation and behavior in the 9,10-dicyanoanthracene-stilbene system

Quenching of singlet 9,10-dicyanoanthracene by several substituted trans- and cis-stilbenes in non-polar solvents results in the formation of singlet exciplexes, some of which are weakly fluorescent. Rate constants for fluorescence quenching and exciplex fluorescence maxima and intensities are dependent upon the stilbene oxidation potential, as expected for the formation of a charge transfer stabilized exciplex. Non-radiative decay results in moderately efficient isomerization of the cis-stilbenes, but not of the trans-stilbenes. Several mechanisms are considered for the selective isomerization of the cis isomers. Formation of a singlet biradical which cleaves selectively to yield trans- vs. cis-stilbenes is considered to be more likely than mechanisms involving the formation of stilbene singlet or triplet excited states.