Sterically Protected π-Electron Systems Enables Efficient, Sunlight-Driven Solid-State Photon Upconversion

A solid-state visible-to-UV triplet-triplet annihilation-based photon upconversion (TTA-UC) system driven by low-intensity light at sunlight levels has been developed. Such a TTA-UC system has never been available due to the difficulty to realize high fluorescence quantum yields and fast triplet exciton diffusion simultaneously, which requires the development of methodologies to optimize interchromophore interactions and suppress the quenching of the excited singlet and triplet states. Here, we report that a group of dihydro-indenoindene derivatives functionalized with alkyl chains above and below the π-conjugated electron system satisfies all these requirements. We found an optimal emitter structure exhibiting the highest UC efficiency in both solution and crystalline state. Notably, it is the first solid-state UC system unaffected by crystalline defects, exhibiting high photoluminescence quantum yield, long triplet lifetime, and fast triplet diffusion, showing high absolute UC efficiency of 3.8% with a remarkably low excitation threshold intensity of 1.2 mW cm−2.