Anomalous behaviour of emission anisotropy for short living derivatives of stilbene

Stilbene and its derivatives are often used as fluorescent probes to study photophysical properties of different media. These compounds are characterized by a high quantum yield and a very short mean excited state lifetimes what allows studying changes of emission anisotropy r as a function of both excitation and observation wavelength. In this Letter absorption, fluorescence as well as polarization excitation and emission spectra are presented for stilbenes in solvents of different viscosity and polarity. Higher emission anisotropy values in the absorption band are observed which stay at the same level upon excitations different than those corresponding to the 0–0 transition wavelength. It can be explained by the presence of two luminescent centers contributing to emitted light or by incomplete relaxation of the excited states before emission.