Investigating Cooperative Reactivity in Photomechanical Crystals Using First-Principles Density Functional Theory

Organic photomechanical crystals convert light into mechanical work via molecular photochemical reactions. Density functional theory (DFT) calculations have proved useful for rationalizing the observed photomechanical response properties and establishing design principles in these materials. However, recent DFT studies have focused on idealized crystals that react completely and instantaneously, while real-world crystals exhibit a variety of reaction kinetics, cooperativity, and other more complex behaviors. To obtain insights into how the photomechanical response differs in partially reacted crystals and the role of thermodynamic cooperativity, the present study models stepwise reactions for five different photomechanical crystals, ranging from photodimerizations of anthracene and cinnamaldehyde malonitrile derivatives to photochemical ring-closing reactions of diarylethenes. Although the details differ across the five systems, the DFT models find that all five crystals exhibit some degree of thermodynamic reaction cooperativity and that the work densities obtained from partially reacted crystals can be substantially smaller compared to those from fully reacted crystals. These behaviors are rationalized based on the atomistic structures of the materials, and the experimental implications of the findings are discussed.