EFFECT OF SUBSTITUENTS ON THE VISCOSITY DEPENDENCE OF FLUORESCENCE AND ON THE S-1-T-1 ENERGY-GAP OF DONOR-ACCEPTOR-SUBSTITUTED TRANS-STILBENES

The dependence of fluorescence quantum yields and S-1-lifetimes of don or-acceptor substituted trans-stilbenes on temperature was measured in the temperature range from 298 to 100 K, using solutions of stilbenes in 3-methylpentane (3-MP). Measurements of fluorescence and phosphore scence spectra show that the triplet energy is almost independent of t he acceptor. The S-1-energy and the S-1-T-1 energy gap decrease with i ncreasing acceptor strength. For all compounds intersystem crossing (I SC) S-1 --> T-1 is negligible.