Dual excited states in 4-dimethylamino 4'-cyanostilbene (DCS) revealed by sub-picosecond 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 investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possible to work at low excitation energy and low concentration, thereby avoiding intensity 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 polar solvents, DCS-B24 derivative where twisting of the anilino moiety is prevented but double-bond twisting is allowed also reveals the presence of only one excited state. On the other hand, in polar solvents, for the DCS a precursor-successor relationship is clearly observed between two different excited states. These results can be understood within a four excited states model derived from a previously suggested diagram: the delocalized excited (DE) state, the internal charge transfer (ICT) state I: highly polar, nearly planar configuration, formed quasi instantaneously from the DE state by electronic reorganization), the conformational relaxed ICT (CRICT) state (highly polar and fluorescent, involves conformational geometric changes such as twisting of the anilino group which enhances charge transfer) and the ''phantom'' state P* on the trans reversible arrow cis isomerization pathway (twisted double bond, lowly polar, non-fluorescent). (C) 1997 Elsevier Science S,A.