Photoisomerization and photohydration of 3-hydroxystyrylnaphthalenes

Abstract

Fluorescence and trans → cis photoisomerization are the main deactivation paths following excitation of trans-1-(2′-naphthyl)-2-(3′-hydroxyphenyl)ethene (trans-2,3NOH) in cyclohexane, methanol and acetonitrile. The quantum yield of both processes is wavelength dependent: this is due to the presence of conformational isomers deriving from rotation of the naphthyl group around the single bond with ethene. Addition of water to acetonitrile quenches the fluorescence (λ_max = 380 nm). In CH₃CN/H₂O (4/6, v/v) the emission spectrum displays a broad band with maximum at ∼550 nm besides the original quenched fluorescence. This indicates that 2,3-NOH undergoes acid–base equilibration in the singlet excited state as supported by the enhancement of the fluorescence quantum yield with increasing acidity of the medium. Ground and excited state acidity constants have been determined. The main photochemical process is photohydration, i.e. water addition to the ethene bond. Fluorescence and photohydration have the same sigmoidal dependence on the acid concentration, which indicates that the undissociated form of singlet excited 2,3NOH is the photoreactive species. Laser flash photolysis experiments allowed identification of the reactive intermediates. The photophysics of trans-1-(1′-naphthyl)-2-(3′-hydroxyphenyl)ethene (trans-1,3NOH) is similar to that of 2,3NOH as regards the effect of water on fluorescence and the acid–base behaviour in the ground and first excited singlet state; the main photochemical process is trans → cis photoisomerization together with photocylization to hydroxychrysene in neutral water/acetonitrile, but with lower yield compared to cyclohexane, and photohydration in strongly acidic medium.