A state-switchable TADF macrocycle for multi-analyte sensing and hydrogen gas-driven emission enhancement

Photochemical sensing demands systems that adjust their optical properties without altering chromophore structure. Here, we report CPCQ, a thermally activated delayed fluorescence (TADF)-active macrocycle, as a responsive platform for optical sensing. CPCQ shows a photoluminescence quantum yield (PLQY) of 78%, a delayed lifetime of 243 ns, and an efficient reverse intersystem crossing rate of 2.68 × 10⁷ s⁻¹. Unlike conventional fluorescent and TADF emitters, CPCQ with cyclic array of four TADF active donor-acceptors enables reversible state switching of its emission mechanism. Electron-deficient guests form exciplexes with reducing fluorescence and shifting emission. In contrast, electron-rich guests enhance delayed fluorescence (248 ns) via the heavy atom effect. Oxygen quenches charge-transfer emission and restores blue-shifted locally excited (LE) emission. Strikingly, hydrogen flips the effect, triples LE emission (kr = 2.07 × 108 s-1, PLQY = 93%), and hints at superradiance from cooperative behavior of donor–acceptor units. These adaptive responses establish CPCQ as a highly sensitive and selective molecular tool for next-generation sensing and optoelectronic applications. Host–guest frameworks can exhibit thermally-activated delayed fluorescence (TADF), but the role of the cavity environment on TADF modulation remains underexplored. Here, the authors study a TADF-active macrocycle, CPCQ, in the presence of small molecular guests and under various gaseous environments, showing that electron-deficient guests and oxygen gas reduce fluorescence signals while electron-rich guests and hydrogen enhance delayed fluorescence and fluorescence, respectively.