Photochromism, thermochromism, and solvatochromism of naphthalene-based analogues of salicylideneaniline in solution

A series of salicylideneaniline (SA) derivatives 1–3 were prepared to study the substituent effect on the chromic properties. Compounds 1 and 3, introduced naphthyl group on the A ring in the parent compound SA, exhibited the solvatochromism, thermochromism and photochromism, while compound 2, introduced the naphthyl group on the B ring in SA, did not show the solvatochromism or thermochromism. The discussion in this study is mainly focused on the behaviour of compounds 1 and 2. The UV absorption spectra of 1 dramatically changed with increasing the solvent polarity as well as cooling temperature, suggesting that 1 takes Z-NH form in polar solvents at room temperature or even in non-polar solvents at below 120 K. Compound 1 emits fluorescence (FL) only at low temperature with small Stokes shift, while 2 exhibits fluorescence emission with considerably large Stokes shift. On UV irradiation, 1 in benzene underwent intramolecular hydrogen atom transfer to give E-NH form. The large Stokes shift observed in 2 indicates that 2 takes E-OH form even at low temperature in any solvents, and underwent intramolecular hydrogen atom transfer in the excited state to give Z-NH form, which emits the fluorescence at low temperature, or undergoes isomerization around CC double bond to give E-NH form. The potential energy diagrams for 1 and 2 were also determined.