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. Turn on the light: Room‐temperature phosphorescence “turn on” is realized in a purely organic thioether system, where protons serve to activate a charge‐transfer phosphorescence state. The technology is promising for background free sensing and imaging.