Photophysical properties and photoisomerization behaviour of 1-(9-anthryl)-2-(n-pyridyl)ethenes (n=2, 3 or 4), aza analogues 1-(9-anthryl)-2-phenylethene

The quantum yields of fluorescence and photoisomerization of trans-1-(9-anthryl)-2-(n-pyridyl) ethenes (n=2, 3 or 4) (t-n-APyE) and the corresponding cis isomers (n=2 or 4) (c-n-APyE) were measured in various solvents. t-3-APyE is strongly fluorescent, but does not undergo photoisomerization, similar to trans-1-(9-anthryl)-2-phenylethene (t-9-APE). The fluorescence and photoisomerization of t-2-APyE and t-4-APyE are strongly influenced by the solvent polarity. In non-polar solvents, they show relatively strong fluorescence, but no photoisomerization to the corresponding cis isomers, As the solvent polarity is increased, the fluorescence intensity decreases and the photoisomerization to the cis isomer becomes efficient, An intramolecular charge transfer excited state is suggested to contribute to the photoisomerization. A singlet mechanism is proposed from the inverse relationship between the fluorescence and photoisomerization quantum yields over the range of solvent polarity and temperature used. A much weaker solvent effect was observed on the fluorescence and photoisomerization of c-2-APyE, and the fluorescence and photoisomerization of c-4-APyE were found to be independent of the solvent polarity. Efficient isomerization to the trans isomer was observed in both non-polar and polar solvents. (C) 1997 Elsevier Science S.A.