Luminescent polymorphic crystals: mechanoresponsive and multicolor-emissive properties

Abstract

Mechanoresponsive luminescent organic crystals have recently attracted increasing interest owing to their potential applications in advanced optoelectronic devices, mechanosensors, security technologies, etc. In the past few years, an increasing number of polymorphic organic crystals have emerged to elucidate the relationship between the crystal structure and mechanoresponsive properties of luminescent organic crystals. This highlight article will introduce the recent progress in luminescent organic polymorphs with mechanoresponsive properties. Multicolor emission and multistimuli-responsive switching of fluorescence have been achieved for various polymorphic organic crystals that exhibit mechanochromic luminescence (MCL). Remarkable studies have also been carried out for the development of polymorphic MCL crystals showing different thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) properties depending on their crystal structures. In addition to the formation of polymorphs, the crystallization of pseudopolymorphs and cocrystals is also effective in controlling the luminescence and MCL properties of organic luminescent compounds. Moreover, polymorphic crystals that exhibit other mechanoresponsive behaviors, such as mechanoluminescence (ML), have been highlighted. Based on the deep insights into the luminescence properties and mechanoresponsive behaviors of polymorphic crystals, further research is expected to establish rational guidelines for the prediction and creation of organic crystals with desired photophysical and mechanical properties.