N-Salicylideneaniline (SA) and its derivs. are known to exhibit photochromism, which is a reversible color change induced by photo irradn. 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 mol. conformations. A monohydrated co-crystal (1H) of SA derivs. did not exhibit photochromism upon UV light irradn. However, upon heating, 1H transformed to the anhyd. 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 mols. 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 mol. conformation change. Addnl., proton transfer between SA and the co-former mol. was obsd. 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". [on SciFinder(R)]
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