MULTIPLE RELAXATION PATHWAYS IN PHOTOEXCITED DIMETHYLAMINONITRO-STILBENES AND DIMETHYLAMINOCYANO-STILBENES

The photophysical behaviour of para-nitro-, para-cyano- and meta-nitro-4'-dimethylaminostilbenes is compared and explained. The increase of the fluorescence quantum yield with solvent polarity is the result of a negative solvatokinetic behaviour of the nonradiative channel leading to the ground state via the double-bond twisted species P* which is of low-polarity nature. The maximum in the fluorescence quantum yield at intermediate solvent polarities, observable for the nitro compounds, is due to a further nonradiative channel specifically linked with the nitro group and with positive solvatokinetic behaviour. This quenching channel becomes important for high-polarity solvents but is absent for compounds without the nitro group. TICT state formation (twisted nitro group) as a candidate for this channel is discussed.