Photon upconversion based on triplet–triplet annihilation using thermally activated delayed fluorescence sensitizers

Photon upconversion based on triplet–triplet annihilation (TTA-UC) enables the conversion of low-energy photons into higher-energy ones, even under low-intensity, incoherent light. This process has recently gained renewed attention as a route toward efficient light-energy conversion for solar, photocatalytic, and biological technologies. Organic molecules exhibiting thermally activated delayed fluorescence (TADF) have recently attracted attention as efficient heavy-metal-free sensitizers owing to their strong visible-light absorption, efficient intersystem crossing (ISC), and tunable excited-state energy levels. The use of TADF compounds has expanded the accessible spectral window of TTA-UC, allowing large anti-Stokes shifts such as visible-to-UV and near-infrared (NIR)-to-visible TTA-UC. This review provides an overview of recent advances in TADF-sensitized TTA-UC systems and discusses the critical factors that determine their performance, including ISC dynamics, triplet energy transfer, and material stability. Challenges and prospects for designing efficient molecular architectures to achieve high TTA-UC efficiencies, lower threshold excitation intensities, and broader wavelength coverage are also highlighted. • Thermally-activated delayed fluorescence (TADF) sensitizers for triplet–triplet annihilation (TTA) photon upconversion. • Molecular design strategies for donor–acceptor and multiple-resonance TADF sensitizers. • Visible-to-UV and NIR-to-visible upconversion toward energy and biological applications. • Future challenges in achieving stable and efficient photon upconversion with large anti-Stokes shifts.