Pressure-Triggered Photochromism in Chiral Salicylaldehyde Schiff Bases

Abstract

Salicylaldehyde Schiff base is a kind of important photochromism system, whose photochromism process is widely acknowledged to be co-determined by the electronic structure and molecular conformation. Normally, salicylaldehyde Schiff base derivatives with α-type structures tend to exhibit photochromism while those with β-type structures tend to be photostable. However, more and more counterexamples are found, and the root cause of photochromism property of salicylaldehyde Schiff base is still unclear. In this work, a series of chiral salicylaldehyde Schiff bases and their racemates were prepared. The formers are photochromic while the latters are photostable. Influenced by Wallach's rule, the homochiral counterparts have looser packing modes than that of the racemic counterparts. Mechanism study revealed that the pressure should be the root cause of photochromism property. Close molecule packing can restrict the photochromism property effectively after being pressed, which also explains why the photochromism of salicylaldehyde Schiff base is usually observed in solid state. This work not only reports the unique photo-responsive difference between the chiral salicylaldehyde Schiff bases and their racemates, more importantly, provides a new perspective to understand the influence of molecule pressure on their photophysical properties.