Development of Innovative Luminescent Functional Molecules Based on Fine Organic Synthesis

The synthesis of fluorescent organic molecules with superior luminescent properties is a highly active research area due to their extensive applications in life sciences for fluorescence imaging and in materials science for organic electroluminescence. Even though molecules exhibit efficient luminescence in dilute solution, they often experience concentration quenching in aggregated or solid states. However, recent advancements have led to an increase in solid-state-emissive organic molecules by suppressing intermolecular interactions or utilizing aggregation-induced emission. Some of them exhibit mechanochromic luminescence (MCL), where emission color changes reversibly in response to mechanical stimuli, such as grinding with a spatula. In typical MCL of organic crystals, mechanical stimuli induce amorphization and a red shift in emission wavelength. Subsequent stimuli, like heating or solvent exposure, restore the original emission color. Initially, designing organic crystals with MCL properties was challenging and relied on serendipity. This paper summarizes research on the syntheses of stimuli-responsive luminescent organic molecules, particularly those exhibiting MCL in the crystalline state. By synthesizing various donor-acceptor-type luminescent molecules, the steric effects of substituents were found to significantly influence luminescent properties and mechanoresponsive behavior in the crystalline state. Additionally, creating segregated crystals by mixing pyrene derivatives with other luminescent molecules enhanced the mechanoresponsive shift in emission wavelength. Furthermore, utilizing asymmetric synthesis of secondary alcohols, chiral bispyrene derivatives were developed that enabled the switching of circularly polarized luminescence. These findings represent significant achievements in the rational control of luminescence in organic molecules and have substantial implications for advancing synthetic organic chemistry, photochemistry, and materials science.