Photosalience and Thermal Phase Transitions of Azobenzene- and Crown Ether-Based Complexes in Polymorphic Crystals

Abstract

<p>Stimuli-responsive molecular crystals have attracted considerableattention as promising smart materials with applications in various fields such assensing, actuation, and optoelectronics. Understanding the structure−mechanicalproperty relationships, however, remains largely unexplored when it comes tofunctionalizing these organic crystals. Here, we report three polymorphic crystals(Forms A, B, and C) formed by the non-threaded complexation of adibenzo[18]crown-6 (DB18C6) ether ring and an azobenzene-based ammoniumcation, each exhibiting distinct thermal phase transitions, photoinduceddeformations, and mechanical behavior. Structural changes on going from FormA to Form B and from Form C to Form B during heating and cooling, respectively,are observed by single-crystal X-ray crystallography. Form A shows photoinduced reversible bending, whereas Form B exhibitsisotropic expansion. Form C displays uniaxial negative expansion with a remarkable increase of 44% in thickness underphotoirradiation. Force measurements and nanoindentation reveal that the soft crystals of Form A with a low elastic modulusdemonstrate a significant photoresponse, attributed to the non-threaded molecular structure, which permits flexibility of theazobenzene unit. This work represents a significant advance in the understanding of the correlation between structure−thermomechanical and structure−photomechanical properties necessary for the development of multi-stimulus-responsive materialswith tailored properties<br></p>