Triplet-Triplet Annihilation Upconversion with Reversible Emission-Tunability Induced by Chemical-Stimuli: a Remote Modulator for Photocontrol Isomerization

Triplet-triplet annihilation upconversion (TTA-UC) has been widely studied, but color-tunable TTA-UC system triggered by chemical stimuli has not yet been proposed. Herein, reversible acid/base switching of TTA-UC emission wavelength is achieved for the first time by a simple platform, composed of a direct singlet-triplet (S₀-T₁) absorpting photosensitizer, and pH-responsive 9,10-di(pyridin-4-yl)anthracene (DPyA) as acceptor. The photosensitizer-acceptor pair exhibits efficient UC emission (quantum yield up to 3.3%, anti-Stokes shift up to 0.92 eV) with remarkable contrast upon base/acid treatment (Δλ_em,max = 82 nm, 0.46 eV). In a proof-of-concept study, the color-adjustable TTA-UC emission was applied as a remote modulator to photo-control reversible chemical reactions for the first time. This platform enriches the portfolio of color-switchable TTA-UC, and the mechanism would inspire further development of smart UC systems and extend the application field of upconversion.