Excited State Properties of Donor−Acceptor Substituted trans-Stilbenes: The meta-Amino Effect

The photochemical behavior of 3-dimethylaminostilbene (3DS), 3-amino-3‘-cyanostilbene (3A3‘CS), and 3-dimethylamino-4‘-cyanostilbene (3DCS) has been investigated and compared to that of the analogous 4-amino derivatives. The absorption spectra of the 4-aminostilbenes display a single strong band whereas the spectra of the 3-aminostilbenes display multiple bands or lesser intensity as a consequence of configuration interaction which results from loss of symmetry. The 3-aminostilbenes have substantially higher fluorescence quantum yields and longer fluorescence lifetimes when compared to the 4-aminostilbenes. Based on analysis of the solvatochromic shifts of the absorption and fluorescence spectra, the 3- and 4-aminostilbenes have similar excited state dipole moments which are substantially larger than their ground state dipole moments. The long lifetimes of the 3-aminostilbene charge transfer singlet states are a consequence of low fluorescence rate constants and a large barrier for singlet state torsion. Thus the “ meta -amino effect” reported previously for 3-aminostilbene is also observed in the case of donor−acceptor substituted stilbenes. In nonpolar solvents the 3-aminostilbenes decay predominantly by fluorescence and intersystem crossing to the triplet state which decays to yield a mixture of trans and cis isomers. In polar solvents a second nonradiative decay channel is operative and is tentatively assigned to internal conversion on the basis of its energy gap dependence. The primary 3-amino-3‘-cyanostilbene displays specific solvation in alcohol solvents at room temperature and aggregation in a nonpolar solvent at low temperature.