Dual Fluorescence and Excited-State Structural Relaxations in Donor−Acceptor Stilbenes

The time-resolved fluorescence behavior of two derivatives of 4-(dimethylamino)-4‘-cyanostilbene (DCS) bearing a more voluminous (JCS) and less voluminous anilino group (ACS) was investigated in ethanol by reconstructing the emission spectra using picosecond time-resolved single-photon-counting technique. For JCS, these spectra exhibit a temporary isosbestic point, a clear indication of level dynamics between two emitting excited singlet states (LE and CT). Kinetic evaluation yielded a precursor−successor relationship between LE and CT and CT formation time constants of 4 ps for ACS and 8 ps for JCS. This slowing of the reaction for the compound with the larger volume of the donor moiety supports the assumption of a twisting mechanism being a major component of the reaction coordinate. An additional transient red shift of the CT band is observed for both compounds and follows the relatively slow solvation dynamics (ethanol).