Rotational dynamics in crystalline molecular machines: A novel paradigm for tuning solid-state photophysical properties

Solid-state molecular materials with tunable photophysical properties are at the forefront of developing advanced photoelectronic devices and security technologies. Typically, modulation of the photophysical properties generally depends on molecular arrangement and conformational changes. However, controlling these structural factors in rigid crystalline matrices remains a significant challenge. Recent advances in crystalline molecular rotors, a class of solid-state molecular machines, have enabled precise modulation of molecular dynamics, providing a novel strategy for controlling photophysical behaviors. This review highlights recent studies that explore the interplay between solid-state molecular rotational motion and photophysical properties, encompassing fluorescence, phosphorescence, chiroptical responses, and related phenomena. We also discuss how designed molecular rotational motion influences intra or intermolecular electronic environments, alters crystal symmetry, and modifies the relaxation pathways, thereby governing the mechanisms underlying the modulation of photophysical properties in molecular crystalline solids.