PHOTOPHYSICS OF STILBENOID DYE SYSTEMS - A COMPARISON OF EXPERIMENT AND THEORY

Stilbenes undergo multidimensional adiabatic photochemistry in the excited state involving the twist of the central double and of the adjacent single bonds. By bridging the single bonds chemically, dimensionality is reduced, and only one product state, the ''phantom singlet state'' P*, remains. Froin the solvent polarity dependence of fluorescence quantum yields and lifetimes, the solvatokinetic response of the photo-reaction towards P* can be extracted. It is shown that P* is highly polar in the case of unsubstituted stilbene but weakly polar for dimethylamino-cyano-stilbenes, consistent with a change of the electronic nature of P* in agreement with recent theoretical models describing double-bond twisting. Quantum chemical calculations within the CNDO/S framework support these results.