DUAL EXCITED-STATES IN 4-DIMETHYLAMINO 4'-CYANOSTILBENE (DCS) REVEALED BY SUBPICOSECOND TRANSIENT ABSORPTION AND KERR ELLIPSOMETRY

4-dimethylamino 4'-cyanostilbene (DCS) and p-dimethylamino, p'-cyano, 1-1'-bi-indanylidene (substituted ''stiff stilbene'' DCS-B24) are inve stigated using sub-picosecond time-resolved absorption and Kerr ellips ometry experiments. The latter technique makes it possible to work at low excitation energy and low concentration, thereby avoiding intensit y and concentration effects which exist in DCS derivatives. Using this technique, in non-polar solvent (cyclohexane), only the presence of a single excited state is observed for the two studied compounds. In po lar solvents, DCS-B24 derivative where twisting of the anilino moiety is prevented but double-bond twisting is allowed also reveals the pres ence of only one excited state. On the other hand, in polar solvents, for the DCS a precursor-successor relationship is clearly observed bet ween two different excited states. These results can be understood wit hin a four excited states model derived from a previously suggested di agram: the delocalized excited (DE) state, the internal charge transfe r (ICT) state I: highly polar, nearly planar configuration, formed qua si instantaneously from the DE state by electronic reorganization), th e conformational relaxed ICT (CRICT) state (highly polar and fluoresce nt, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the ''phantom'' stat e P on the trans reversible arrow cis isomerization pathway (twisted double bond, lowly polar, non-fluorescent). (C) 1997 Elsevier Science S,A.