Heating and mechanical force-induced luminescence on–off switching of arylamine derivatives with highly distorted structures

A triphenylamine-based organic luminophor (TPA-CO) with a highly distorted structure has been designed and effortlessly obtained by an Ullmann reaction. The luminophor exhibits a stimuli-induced emission enhancement effect and intramolecular charge transfer properties. The fluorescence efficiency of its crystals is dramatically increased from 0.4% to 12.3% upon grinding. The emission enhancement is also realized by a heating process. The "bright" state can recover its original state and turn "dark". The luminescence "on–off" behaviour is repeatedly transformed by a grinding–vapour process or by a heating process. The XRD patterns of the "bright" and "dark" states show that the change of emission intensity is related to the reversible transition between the crystalline state and the metastable amorphous state. At the molecular level, the emission enhancement upon external stimuli may be attributed to conformational planarization and weak intermolecular interactions.