Photophysical properties of a highly fluorescent push-pull stilbene

A new push-pull stilbene was synthesized, and its photophysical properties investigated by spectroscopic and theoretical methods. Solvatochromic shifts in the fluorescence were in accordance with Onsagerʹs continuum model, whereas shifts in the absorption spectra only correlated with the polarizability of the solvents. Excited state dipole moments were determined and compared to semi-empirically calculated values. With increasing solvent polarity the fluorescence quantum yield increases steeply as the solvent dielectric permittivity exceeds 8. It is concluded that a strongly luminescent and highly dipolar π, π∗ state becomes the lowest excited state in polar solvents, and radiative deactivation therefore competes effectively with non-radiative pathways. Preventing internal rotation of molecular subunits is thus no prerequisite for obtaining highly fluorescent push-pull stilbenes.