Emissions of sterically hindered stilbene derivatives and related compounds. Part IV. Large conformational differences between ground and excited states of sterically hindered stilbenes: implications regarding stokes shifts and viscosity or temperature dependence of fluorescence yields

Large effects of molecular overcrowding on fluorescence spectra (Stokes shifts) and quantum yields were observed in a series of stibene derivatives. The measured Stokes shifts depend strongly on the extent of molecular over-crowding. The fluorescence quantum yields increase sharply with increasing viscosity and decreasing temperature. As the extent of overcrowding increases, the temperature range over which fluorescence is observed is shifted downwards, and the importance of viscosity increases. This reflects a corresponding complementary variation of the radiationless processes competing with fluorescence. CFF–SCF–PI calculations of the ground and first excited singlet state geometries of the trans- and cis-isomers of stilbene and of αα′-dimethylstilbene chosen as model molecules indicate that excitation results in large changes in the ring–α and α–α′ torsion angles of the relaxed sterically hindered molecules, and therfore in substantial relaxation energies. Good agreement is obtained between observed and calculated Stokes shifts. The viscosity dependence of fluorescence quantum yields is correlated with the calculated Franck–Condon factors for the radiationless transitions competing with fluorescence.