Excited-state dynamics in polar solvents of push-pull polyenes designed for nonlinear optics

Subpicosecond spectroscopy of push-pull polyenes, previously designed to achieve large optical nonlinearities, reveals an isosbestic point in the time-resolved gain band in polar solvents. The compounds possess a diethylthiobarbituric acid electron-withdrawing group and a dibutylaniline electron-releasing group coupled by a pi-conjugated chain. The final gain peak exhibits a regular red shift with increasing chain length, whereas the ground-state absorption shift levels off. A fast photoinduced process toward a rigid, fully conjugated cyanine-like structure is proposed. This observation supports a recent proposal that a proper description of the excited-state dynamics of these push-pull polyenes requires more than two valence bond states.