Highly Effective Near-Infrared to Blue Triplet–Triplet Annihilation Upconversion Nanoparticles for Reversible Photobiocatalysis

Near-infrared (NIR) to blue triplet-triplet annihilation upconversion (TTA-UC) shows unique applications in optogenetics, photocaging, and stereoscopic three-dimensional printing, etc. Here, we disclose a unique strategy that narrowed the energy gap between the triplet states of the NIR photosensitizer and annihilator, with the aim of maximally suppressing the photoexcitation energy loss during TET. Hence, we produced a NIR-to-blue TTA-UC pair that exhibited an exceptionally large anti-Stokes shift (0.76 eV) and achieved a record upconversion quantum yield (15.5%, out of 50%). We further prepared for the first time small, water-dispersed, oxygen-resistant upconversion nanoparticles with an upconversion quantum yield of up to 1.8%. Such upconverted nanoparticles were successfully utilized as NIR-responsive photocatalysts for the reversible transformation of enzyme cofactor NAD⁺/NADH in a photobiocatalytic system in air-saturated aqueous solutions.