Photophysics of 4-dimethylamino 4'-cyanostilbene and model compounds: Dual excited states revealed by sub-picosecond transient absorption and Kerr ellipsometry
Résumé
4-Dimethylamino 4′-cyanostilbene (DCS) and two selectively bridged compounds 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 concentrations, thereby avoiding intensity and concentration effects which exist in DCS derivatives. Using this technique, in a non-polar solvent (cyclohexane), only the presence of a single excited state is observed for all studied compounds. In polar solvents, the bridged 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 compound and the related bridged derivative where the anilino moiety is still able to twist, 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 DE state (delocalized excited state), the ICT state (internal charge transfer, highly polar, nearly planar configuration, formed quasi-instantaneously from the DE state by electronic reorganization), the CRICT state (conformational relaxed ICT 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⇄cis isomerization pathway (twisted double bond, low polar, non-fluorescent). In this study, we observed the formation of the CRICT excited state on a time scale ranging from 2 to 20 ps depending on the solvent characteristics (polarity, viscosity and hydrogen bonding ability).