Thermally Activated Delayed Fluorescence via Triplet Fusion.

Manipulating triplet states is technically intriguing for organic semiconductors and photochemical conversion. Sensitized photon upconversion that converts low-energy photons into higher-energy ones via triplet fusion holds great potential in augmentation of photovoltaic devices and photocatalysis. Conventional requirement of exothermic triplet-triplet energy transfer in sensitized upconversion constrains the development of upconversion donor-acceptor pairs. We demonstrate an upconversion system by utilizing a triplet energy-deficient donor within a triplet acceptor matrix, in which efficient thermally activated triplet-triplet energy transfer overcoming an energy barrier of up to 180 meV and an over 5% upconversion quantum yield in the solid state are achieved upon 635 nm irradiation at 195 mW/cm2. The effective solid-state upconversion through thermally activated triplet-triplet energy transfer establishes a new pathway to extract triplet energy from low-lying sensitizers, transcending traditional restriction in matching components energy level.