Proton-Activated "Off-On" Room-Temperature Phosphorescence from Purely Organic Thioethers

Room-temperature phosphorescence (RTP)-based sensors have distinctive advantages over the fluorescence counterparts, such as larger Stokes shifts and longer lifetimes. Unfortunately, almost all RTP sensors are operated on quenching-based mechanisms given the sensitive nature of the emissive triplet state. Here we report a type of thioether RTP molecules that shows RTP "turn-on" when volatile acid vapors such as HCl are in contact. To elucidate the underlying mechanism, model thioethers containing different donor/acceptor combinations are investigated via fluorescence spectroscopy and theoretical calculations aided by molecular coordinates obtained from single-crystal X-ray diffraction. It is revealed that a charge-transfer character in the phosphorescence state is crucial. The "turn-on" design concept may significantly broaden the sensing application scope for organic RTP molecules.