The photochemical behavior of the three positional isomers of trans- aminostilbene is reported and compared to that for unsubstituted trans- stilbene. The absorption spectrum of the para isomer displays a single intense long-wavelength band; however the meta and ortho isomers display two less intense bands as a consequence of configuration interaction. All three isomers are fluorescent and display similar solvent-induced shifts of their fluorescence maxima. The fluorescence rate constant of the para isomer is larger than that of the ortho or meta isomer. The para isomer has a short singlet lifetime and high photoisomerization quantum yield, similar to those for trans-stilbene. In contrast, the ortho and meta isomers have long singlet lifetimes and low photoisomerization quantum yields. Isomerization of the para isomer is a singlet-state process, whereas isomerization of both the ortho and meta isomers is a triplet-state process. The lower bound for the barrier for singlet-state torsion of the ortho and meta isomers is 7 kcal/mol. The barrier height is found to be determined by the effects of the amino substituent upon the relative energies of the fluorescent singlet and twisted singlet states.
ASJC Scopus subject areas
- Colloid and Surface Chemistry