Photoisomerization mechanism and conformational equilibria of styrylnaphthalenes. A study based on photophysical properties and molecular-orbital calculations

The photophysical properties of trans-1- and trans-2-styrylnaphthalene (1-StN and 2-StN) have been studied. The conformational isomers of 2-StN have been reinvestigated by measuring the solvent and temperature effects and by m.o. calculations (CNDO/S) on the nature of the lowest excited states. Fluorescence quantum yields and lifetimes and kinetic parameters of decay have been obtained for the two conformers in n-hexane and ethanol. The energy difference between the two conformers and the activation energy for their trans→cis photoisomerization have been obtained from the temperature effect in both solvents. Evidence has been obtained for two photoisomerization mechanisms, a triplet mechanism predominant below room temperature and a singlet mechanism favoured in ethanol and/or at higher temperatures.