o-Amino conjugation effect on the photochemistry of trans-aminostilbenes

The excited-state behavior of a series of trans-2-(N-arylamino)stilbenes (aryl = phenyl (o1H), 4-methylphenyl (o1Me), 4-methoxyphenyl (o1OM), and 4-cyanophenyl (o1CN)) and trans-2-(N,N-diphenylamino)stilbene (o2) in both nonpolar and polar solvents is reported and compared to that of the parent trans-2-aminostilbene and the corresponding meta- and para-isomers (m1R and p1R, where R = H, Me, OM, and CN, and m2 and p2). Two types of torsional motions, the D-A torsion that results in a nonfluorescent twisted intramolecular charge transfer (TICT) state and the C═C torsion that leads to the cis isomers, account for the radiationless decays of o1R and o2. The relative efficiencies of these torsions can be readily evaluated from their quantum yields for fluorescence (Φ(f)) and trans → cis isomerization (Φ(tc)). The propensities of the D-A torsion are similar for the ortho and meta isomers, which is 1OM > 1Me and negligible for 1H, 1CN, and 2. The activation parameters determined from temperature-dependent fluorescence lifetimes suggest that the C═C torsion occurs mainly via the triplet state for the ortho systems, a behavior again similar to that of the meta isomers. Whereas the intersystem crossing in o1R, m1R, and m2 is essentially a nonactivated process, it encounters a barrier of 2.7-3.8 kcal mol(-1) in o2. As a result of the barriers that decelerate the radiationless decays and the slow fluorescence rate for o2 in acetonitrile, the observed long fluorescence lifetime 24.5 ns at room temperature reaches a new record for unconstrained trans-stilbenes.