PHOTOPHYSICS OF 4-DIMETHYLAMINO 4'-CYANOSTILBENE AND MODEL COMPOUNDS - DUAL EXCITED-STATES REVEALED BY SUBPICOSECOND TRANSIENT ABSORPTION AND KERR ELLIPSOMETRY

4-Dimethylamino 4'-cyanostilbene (DCS) and two selectively bridged com pounds are investigated using sub-picosecond time-resolved absorption and Kerr ellipsometry experiments. The latter technique makes it possi ble to work at low excitation energy and low concentrations, thereby a voiding intensity and concentration effects which exist in DCS derivat ives. Using this technique; in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied co mpounds, In polar solvents, the bridged derivative where twisting of t he anilino moiety is prevented but double bond twisting is allowed als o reveals the presence of only one excited state, On the other hand, i n polar solvents, for the DCS compound and the related bridged derivat ive where the anilino moiety is still able to twist, a precursor-succe ssor relationship is clearly observed between two different excited st ates. These results can be understood within a four excited states mod el derived from a previously suggested diagram: the DE state (delocali zed excited state), the ICT state (internal charge transfer, highly po lar, nearly planar configuration, formed quasi-instantaneously from th e DE state by electronic reorganization), the CRICT state (conformatio nal relaxed ICT state, highly polar and fluorescent, involves conforma tional geometric changes such as twisting of the anilino group which e nhances charge transfer) and the ''phantom'' state P on the trans rev ersible arrow cis isomerization pathway (twisted double bond, low pola r, non-fluorescent). In this study, we observed the formation of the C RICT excited state on a time scale ranging from 2 to 20 ps depending o n the solvent characteristics (polarity, viscosity and hydrogen bondin g ability).