Photochromism compounds generally exhibit reversible color changes when exposed to photoirradiation. This phenomenon has been observed in the crystals of N-Salicylideneaniline derivatives, therefore, explaining their wide interest in various applications. The photochromism color change has been found to particularly depend on their crystal structure comprising of molecular conformations and arrangements. Consequently, there exists an interesting relationship between the photochromism property and molecular conformations of the N-Salicylideneaniline crystals. As such, researchers have developed several methods to assess the various factors affecting the photochromism properties.
However, in-situ changes in the photochromic properties which are permanently fixed upon the formation of the targeted material or crystal have not been fully explored. Currently proposed analysis strategies should overcome the difficulty experienced in preparing necessary single crystals. Recently, researchers have realized the possibility to alter the crystal structures and photochromatic properties based on the solid-state phase transitions such as deammoniation. This approach also allows clarification of the photochromic property change mechanism concerning the crystal structures.
To this effect, Tokyo Institute of Technology scientists: Dr. Kohei Johmoto, Dr. Akiko Sekine and Dr. Hidehiro Uekusa together with Dr. Haruki Sugiyama from Keio University in collaboration demonstrated a photochromic property switching mechanism of N-Salicylideneaniline ammonium salt crystals based on the deammoniation reaction. Two polymorphic ammonium salt crystals of N-Salicylidene-4-carboxyaniline were analyzed. Both of the polymorphs were heated to deammoniation phase transition. Additionally, the authors determined the photochromic properties of the deammoniated N-Salicylideneaniline crystals. Eventually, the crystal structures of all the forms were examined to elucidate their relationship with the physicochemical properties. The work is currently published in the research journal, Crystal Growth & Design.
The authors confirmed that the photochromic properties of N-Salicylideneaniline derivatives are closely related to their molecular conformations and crystal structures. For instance, the in-situ conformation molecular change from photochromic nonplanar conformation to non-photochromic planar conformation within the crystal successfully switched the photochromic properties of the N-Salicylidene-4-carboxyaniline ammonium salt crystal through deammoniation. On the other hand, N-Salicylidene-4-carboxyaniline ammonium crystal adsorbed the ammonium gas during the heating process thus transforming into nonionic N-Salicylideneaniline crystals. This phase transition change resulted in jumping crystal phenomenon which was further examined.
It was worth noting that the deammoniation phase transition with photochromic switching was reversible. Thus, nonionic N-Salicylideneaniline crystal reverted to N-Salicylidene-4-carboxyaniline ammonium salt crystals when exposed to ammonia-water vapor. Furthermore, the two polymorphs of ammonium salt crystal exhibited different colors attributed to the molecular conformation. They also showed the jumping crystal effects when heated due to the partial mechanical strains induced by deammoniation process.
According to the authors, ammonium salt crystals containing N-Salicylideneaniline derivates can undergo both dynamic mechanical and chemical property changes upon deammoniation phase transition. Therefore, crystals that undergo structural and property changes due to external stimuli are potential and promising materials for various applications such as switching and chemical sensors.
Sugiyama, H., Johmoto, K., Sekine, A., & Uekusa, H. (2019). In-Situ Photochromism Switching with Crystal Jumping through the Deammoniation of N-Salicylideneaniline Ammonium Salt. Crystal Growth & Design, 19(8), 4324-4331.