Improving photochromism and thermochromism in the solid state by co- crystallization: an integrated physico-chemical investigation

Dynamic materials are characterized by their potential to reversibly adjust their properties in response to external stimuli, such as light and temperature. Photo- and thermochromic materials can lead to a broad range of applications, i.e. in information storage or in photomechanical materials. Anil derivatives are among the most common photo- and thermo- switchable molecules. But these systems have limitations: reduced resistance to photodegradation and solvent effects. Recent efforts to overcome these problems have been proposed, such as encapsulation in a molecular host-capsule or dispersion in a polymeric matrix. In this context, the project aims at using solid-state co-crystallization to develop new photochromic and thermochromic materials by adding a second component (co-former) to the compound. The introduction of a co-former is expected to change the physical properties of the final assembly without altering the chemical nature of the starting component. To achieve this goal, an integrated physico-chemical approach will be developed, combining the preparation of new chromophores, their co-crystallization, and the determination of structural and optical properties. The selected chromophores (N-salicylideneanilines) will be synthesized by mechanochemical synthesis. After co-crystallization with appropriate co- formers, structures of the co-crystals will be approached with single-crystal X-ray diffraction (SCXRD) or powder X-ray data. The influence of external stimuli (temperature or irradiation) will also be established by SCXRD. For thermochromic co-crystals, data will be collected at low / room temperatures and similarly, for photochromic solids, crystallographic data will be collected after prolonged irradiation. Optical analysis of all obtained crystals and co- crystals will be mainly performed by solid UV/Vis Diffuse Reflectance spectroscopy to assess which co-crystals have effectively become photoresponsive, by the presence of typical absorption bands.