Stimuli-Responsive Molecular Photon Upconversion.

The addition of stimuli-responsiveness to anti-Stokes emission provides a unique platform for biosensing and chemosensing. Particularly, stimuli-responsive photon upconversion based on triplet-triplet annihilation (TTA-UC) is promising due to its occurrence at low excitation intensity with high efficiency. This Minireview summarizes the recent developments of TTA-UC switching by external stimuli such as temperature, oxygen, chemicals, light, electric field, and mechanical force. For the systematic understanding of the underlying general mechanisms, the switching mechanisms are categorized into four types: 1) aggregation-induced UC; 2) assembly-induced air-stable UC; 3) diffusion-controlled UC; and 4) energy-transfer-controlled UC. The development of stimuli-responsive smart TTA-UC systems would enable sensing with unprecedented sensitivity and selectivity, and expand the scope of TTA-UC photochemistry by combination with supramolecular chemistry, materials chemistry, mechanochemistry, and biochemistry.