On the photochemistry of two hydroxy-substituted styrylnaphthalenes

The photoisomerization of 1,2-diarylethenes is still the object of lively investigations though the studies on the mechanism of this photoreaction were started more than 50 years ago.1 The reaction depends on various external factors, such as temperature, solvent polarity and viscosity, presence of sensitizers or quenchers and on molecular factor, such as nature of the aryl groups linked to the ethenic bridge,2 presence of heavy atoms3 or push-pull groups4 and existence of conformers.5,6 In this communication we refer about the photoisomerization of 1-(1-naphthyl)-2-(3-OH- phenyl)ethene [1,3-NOH] and its (2-naphthyl) positional isomer [2,3-NOH]. The prominent feature of the excited state behavior of trans-2,3-NOH in cyclohexane is the dependence on the excitation wavelength of the fluorescence and photoisomerization quantum yields, due to the presence of two conformers arising from the rotation of the naphthyl group around the aryl-ethenic bridge (a quasi-single bond). The most important peculiarity of the 1,3-NOH photochemistry in apolar solvent is photoisomerization with a significative contribution of the adiabatic process 1trans*?1cis*. The efficiency of the adiabatic reaction is lower in polar and H-bonding solvents. The role of the acid-base equilibrium in the lowest singlet excited state on the photochemistry of the trans isomers of 1,3-NOH and 2,3-NOH was also investigated.