Excited state properties of donor-acceptor substituted trans-stilbenes: The meta-amino effect

The photochemical behavior of 3-dimethylaminostilbene (3DS), 3-amino-3'-cya nostilbene (3A3'CS), and 3-dimethylamino-4'-cyanostilbene (3DCS) has been i nvestigated and compared to that of the analogous 1-amino derivatives. The absorption spectra of the 4-aminostilbenes display a single strong band whe reas the spectra of the 3-aminostilbenes display multiple bands or lesser i ntensity as a consequence of configuration interaction which results from l oss of symmetry. The 3-aminostilbenes have substantially higher fluorescenc e quantum yields and longer fluorescence Lifetimes when compared to the 4-a minostilbenes. Based on analysis of the solvatochromic shifts of the absorp tion and fluorescence spectra, the 3- and 4-anlinostilbenes have similar ex cited state dipole moments which are substantially larger than their ground state dipole moments. The long lifetimes of the 3-aminostilbene charge tra nsfer singlet states are a consequence of low fluorescence rate constants a nd a large barrier for singlet state torsion. Thus the "meta-amino effect" reported previously for 3-aminostilbene is also observed in the case of don or-acceptor substituted stilbenes. In nonpolar solvents the 3-aminostilbene s decay predominantly by fluorescence and intersystem crossing to the tripl et state which decays to yield a mixture of trans and cis isomers. In polar solvents a second nonradiative decay channel is operative and is tentative ly assigned to internal conversion on the basis of its energy Sap dependenc e. The primary 3-amino-3'-cyanostilbene displays specific solvation in alco hol solvents at room temperature and aggregation in a nonpolar solvent at l ow temperature.