Reversible on/off switching of photochromic properties in: N-salicylideneaniline co-crystals by heating and humidification

N-Salicylideneaniline (SA) and its derivatives are known to exhibit photochromism, which is a reversible colour change induced by photo irradiation. Therefore, we performed on/off switching of the photochromic properties of an SA crystal by the hydration-dehydration phase transition. The photochromic properties of SA crystals are closely related to their crystal structures and molecular conformations. A monohydrated co-crystal (1H) of SA derivatives did not exhibit photochromism upon UV light irradiation. However, upon heating, 1H transformed to the anhydrous form 1A, which exhibited photochromism. Moreover, 1A re-hydrated to 1H under high humidity conditions, and the photochromic properties were switched off. Thus, the photochromic properties were reversibly switched on/off through the hydration-dehydration phase transition. X-ray crystal structural analyses revealed that the molecules adopted a planar (non-photochromic) conformation in 1H, but after dehydration, they changed to a non-planar (photochromic) conformation in 1A due to large crystal structure changes. Therefore, on/off switching of photochromism was clearly explained by the SA molecular conformation change. Additionally, proton transfer between SA and the co-former molecule was observed along with structural changes resulting from dehydration and hydration. This is the first report to switch the photochromic properties of SA crystals and explain the switching mechanism based on the crystal structure comparison. We intend to emphasize that the photochromism switching phenomena by external stimuli, namely heat and humidity, pave the way for utilizing these crystals in "smart materials".