Two-step crystal-crystal phase transformation of N-salicylidene-p-aminobenzoic acid by gas-solid reaction with aqua-ammonia vapour.

Crystal-crystal phase transformation by external stimuli has attracted significant attention for application in switchable materials, which can change their structures and properties. Herein, it is revealed that N-salicylidene-p-aminobenzoic acid crystals undergo a two-step crystal-crystal phase transformation through a gas-solid reaction with aqua-ammonia vapour. The photochromic behaviour of the crystals switched from nonphotochromic to photochromic and back to nonphotochromic via a phase transformation. The two-step phase transformation and photochromic behaviour change were characterized and correlated by X-ray crystal structure analysis, UV-Vis spectroscopy, differential scanning calorimetry and scanning electron microscopy. This article is the first report to capture the stepwise structural change in the gas-solid (acid-base) reaction of ammonia with benzoic acid derivatives.