Thermal-Induced Twisting and Photoinduced Planarization of Salicylideneaniline in Alcohols

Thermofluorochromism and photochromism of salicylideneaniline (SA) in alcohol were investigated using steady-state and time-resolved fluorescence and absorption spectroscopy. The planar trans-enol form of SA in alcohols is converted into the twisted trans-enol form on heating. This conversion results change in the emission maximum from the 530 to the 440 nm region with an increase in fluorescence intensity, which confirms the absence of intramolecular hydrogen bonding between imine nitrogen and phenolic hydrogen in the twisted trans-enol form. The activation barrier for thermal-induced formation of the twisted trans-enol form in methanol was determined experimentally and was found to be 20.15 ± 2.22 kcal/mol. The rotation of the phenolic C₇-C₈ and C₇-N₁ bond followed by breaking of the intramolecular hydrogen bond and formation of an intermolecular hydrogen bond with alcohol solvent molecules results in the thermally stable twisted trans-enol form in alcohol solvents. The biexponential nature of the fluorescence decay of the twisted trans-enol form of SA confirms that the emission originates from multiple (π-π* and n-π*) excited states. On photolysis under UV light, the twisted trans-enol form is converted back into the planar trans-enol form. The time-resolved absorption and excitation-resolved fluorescence spectrum of SA in methanol confirm the existence of the twisted cis-keto form as a transient photochromic intermediate in the light-induced planarization of SA in alcohols. In alcohols, an interplay between the intra- and intermolecular hydrogen-bonding controls excited-state reaction dynamics and conformational relaxation of SA, which are responsible for the photochromism of salicylideneaniline.