Strategy to Mechanical Activation of Centrosymmetrically Packed Organic Luminogens

Abstract

A new strategy has been developed to sensitize centrosymmetrically packed organic luminogens, which generally do not respond to external stimuli owing to their zero gross dipole moment and degenerate electronic energy states in solid-state packing. To achieve this goal, a series of fused biheterocyclic luminogens exhibiting centrosymmetric packing (P-1 and P21/c space group) organic luminogens have been synthesized via a cost-effective, atom-economical, metal-catalyzed “one-pot” C–H bond activation route. The designed luminogens do not respond to external mechanical stimulus. However, the application of heat as an effective stimulus leads to “centrosymmetric to noncentrosymmetric crystal to crystal” phase transitions at their corresponding crystallization temperature. This thermo-induced phase transition has been probed by polarized light microscopic (PLM) techniques, corroborated by powder X-ray diffraction. After “centrosymmetric to noncentrosymmetric” phase transitions, we have successfully activated our luminogens to external mechanical stimuli. The higher dipole moments of luminogens in noncentrosymmetric crystal packing are probably responsible for exhibiting such remarkable mechanochromic properties. We believe that the above-mentioned strategy may be useful to activate large numbers of centrosymmetrically packed organic luminogens and, subsequently, for promising applications in optical storage, mechanical sensors, security systems, and optoelectronic devices.