Photoreactive Crystals Exhibiting [2 + 2] Photocycloaddition Reaction and Dynamic Effects.

ConspectusConducting a reaction in the solid state eliminates the usage of solvents. If such reactions are conducted in a single-crystal to single-crystal (SCSC) fashion, then structural characterization by single-crystal X-ray crystallography (SCXRD) techniques provides unequivocal structural details. Although topochemical principles govern, getting single crystals at the end of a SCSC reaction purely depends on the experimental skills of the researchers. SCSC reactions are common among solid-state [2 + 2] cycloaddition reactions (hereafter "photoreaction") after the classical work of Schmidt and co-workers in 1960s. Synthons and tectons in the crystal engineering box can be exploited to bring the functional groups into the required alignment and packing to achieve the desired chemical reactivities and physical properties, respectively. Bringing a pair of alkenes closer together in the organic molecules provides an effective starting point to achieve the goal of crystal engineering.Further, understanding and controlling photoreactivity in the solid state provide a gateway to designing new advanced materials, for example, making cycloreversible optical storage materials, photosalient and photomechanical materials, highly crystalline or even single-crystalline organic polymers, covalent organic framework structures, and organic polymers incorporated inside metal-organic frameworks (MOFs). Photoreactions often proceed in a SCSC manner due to the limited movements of the closely disposed reactive functional groups in the crystals. Thus, these photoreactions yield not only quantitative photoproducts but also regio- and stereospecificity, which are otherwise inaccessible by solution syntheses.The traditional definition of crystals being hard, rigid, and brittle is no longer valid ever since the mechanically responsive crystals were discovered. These dynamic crystals undergo various movements like curling, jumping, hopping, popping, splitting, and wiggling, when exposed to light (called "photosalient effect") or heat (called "thermosalient" effect). These crystals generate new methods of transforming light and heat energy into mechanical work. Recently, photosalient behavior during the [2 + 2] cycloaddition reaction under UV light has been frequently observed. With the emergence of the field of "crystal adaptronics", dynamic photoreactive crystals have emerged as smart actuating materials.This Account aims to provide an overview of the development in this area, since it has garnered much attention among solid state chemists. While presenting selected examples of important strategies, we try to illustrate the intentions and concepts behind the methods developed, which will help in a rational approach for the fabrication of advanced solid state materials. Apart from topochemical transformations, the important roles played by weak interactions, guest solvents, and mechanical grinding have been highlighted in several classes of compounds to show structural transformations that defy the expected outcomes. Overall, the progress of [2 + 2] cycloaddition reaction in solid state materials has been discussed from UV induced structural transformations to the development of smart actuating materials.