Mechanochromic Luminescence of Organic Crystals: Current Progress and Emerging Functions

Abstract

Recent advances in solid-state luminescence have stimulated the development of mechanochromic luminescence (MCL) in organic crystals. This review primarily summarizes recent progress in MCL-active organic crystals from the viewpoints of emission-state diversification and crystal-state control. First, multicolor luminescence switching is discussed, including two-step responses induced by mechanical stimuli of different intensities, multistimuli-responsive switching, and mechanoresponsive polymorphs and solvates. Multicomponent crystalline systems are then reviewed, with emphasis on cocrystals and salts, segregated crystals, and doped crystals, which provide additional strategies for tuning intermolecular interactions and emission properties. The scope of MCL has also expanded beyond conventional prompt fluorescence (PF) to triplet-state-related processes, including thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP). In addition, recent developments combining MCL with circularly polarized luminescence (CPL) are highlighted. These advances demonstrate the growing potential of luminescent organic crystals as platforms for multicolor switching, information encoding, sensing, and other stimuli-responsive optical functions.