Photophysics and Photochemistry of Intramolecular Stilbene—Amine Exciplexes

The photophysical and photochemical behavior of a series of trans-(aminoalkyl)stilbenes in which a primary, secondary, or tertiary amine is appended to the stilbene ortho position with a methyl, ethyl, or propyl linker has been investigated. The tertiary (aminoalkyl)stilbenes form fluorescent exciplexes and undergo trans—cis isomerization, but fail to undergo intramolecular reactions analogous to intermolecular addition reactions observed for stilbene with tertiary amines. The photophysical behavior of the tertiary (aminoalkyl)stilbenes is dependent upon the choice of linker, solvent, and temperature. The secondary (aminoalkyl)stilbenes do not form fluorescent exciplexes but undergo intramolecular N—H addition to the stilbene double bond. Unlike the intermolecular reactions of substituted stilbenes with secondary amines, which yield mixtures of regioisomeric adducts and reduction products, the intramolecular reactions are highly selective, providing an efficient method for the synthesis of tetrahydrobenzazepines. Direct irradiation of the primary (aminoalkyl)stilbenes results only in trans-cis isomerization. However, irradiation in the presence of the electron acceptor p-dicyanobenzene results in regioselective intramolecular N-H addition to the stilbene double bond. These results are discussed in terms of the mechanisms of direct and electrontransfer-sensitized irradiation.