Molecular design strategies for application-oriented triplet-triplet annihilation upconversion

Triplet-triplet annihilation upconversion (TTA-UC) efficiently converts low-energy photons into high-energy photons under low power density excitation via a series of photophysical processes of sensitizers and annihilators. A few classical TTA-UC pairs have been successfully applied to diverse fields such as photocatalysis, photovoltaics, organic photoredox chemistry. However, molecular optimization is often overlooked in applied research, leading to suboptimal performance enhancement. The persistent disconnection between TTA-UC optimization and practical implementation motivates this review to bridge the critical gap. This review first analyzes application-specific requirements for TTA-UC pairs and provides design guidelines for molecular selection. Subsequently, we comprehensively review the design and optimization of various components within the TTA-UC process, including molecular classification of sensitizers and strategies to achieve efficient intersystem crossing, strong absorption, and extended triplet lifetimes; summarize methods for developing annihilators with tunable energy levels and suitable for specific applications; describe design principles of the third component while evaluating their impacts when introducted. A molecular library encompassing diverse sensitizers and annihilators for selection is provided. Furthermore, this review systematically examines emerging TTA-UC applications in biosensing, 3D printing, and anti-counterfeiting, highlighting how molecular optimization dictates application outcomes. This review will offer a comprehensive perspective for researchers, thereby advancing broader implementation of TTA-UC across multidisciplinary fields.