PHOTOPHYSICS AND PHOTOCHEMISTRY OF INTRAMOLECULAR STILBENE-AMINE EXCIPLEXES

The photophysical and photochemical behavior of a series of trans-(ami noalkyl)stilbenes in which a primary, secondary, or tertiary amine is appended to the stilbene ortho position with a methyl, ethyl, or propy l linker has been investigated. The tertiary (aminoalkyl)stilbenes for m fluorescent exciplexes and undergo trans-cis isomerization, but fail to undergo intramolecular reactions analogous to intermolecular addit ion reactions observed for stilbene with tertiary amines. The photophy sical behavior of the tertiary (aminoalkyl)stilbenes is dependent upon the choice of linker, solvent, and temperature. The secondary (aminoa lkyl)stilbenes do not form fluorescent exciplexes but undergo intramol ecular N-H addition to the stilbene double bond. Unlike the intermolec ular reactions of substituted stilbenes with secondary amines, which y ield mixtures of regioisomeric adducts and reduction products, the int ramolecular reactions are highly selective, providing an efficient met hod for the synthesis of tetrahydrobenzazepines. Direct irradiation of the primary (aminoalkyl)stilbenes results only in trans-cis isomeriza tion. However, irradiation in the presence of the electron acceptor p- dicyanobenzene results in regioselective intramolecular N-H addition t o the stilbene double bond. These results are discussed in terms of th e mechanisms of direct and electron-transfer-sensitized irradiation.