Crystal chemistry of dithienylethene photochromic switches

Abstract

Diarylethene (DAE)-based photochromic molecules, e.g., dithienylethenes, have emerged as versatile candidates for molecular switching over a very broad application range. Their popularity can be rationalized by the robust reversible photo-induced transformations, high thermal stability, excellent fatigue resistance, and reactivity in the solid state. This highlight emphasizes DAE performance in the solid-state. It explores, among other things, crystal structures, co-crystals, and Metal-Organic Frameworks (MOFs). Although the DAE photochromic switches have been indispensable for research into light-responsive materials since their discovery in 1988, recent structural surveys and database studies reveal critical design challenges. Analysis of data in the Cambridge Structural Database, CSD show that dithienylethenes (DTEs) such as 1,2-Bis[2-methylbenzo[b]thiophen-3-yl]-3,3,4,4,5,5-hexafluoro-1-cyclopentene, account for around 80 % of crystal structures. Antiparallel geometry and short C···C distances govern photoactivity, but crystal packing frequently enforces non-reactive conformations, complicating rational design. Notable advances include Metal-Organic Frameworks combining two distinct photoswitches (DAEs and spiropyrans), photoswitching between porous and non-porous phases, and rare single-crystal-to-single-crystal transformations in interpenetrated frameworks. Photomechanical crystals capable of face-dependent twisting and lifting weights 900 × heavier than the crystal illustrate the potential of crystal engineering for actuators. These findings underscore both the opportunities and limitations in translating molecular photochromism into functional solid-state devices.