Sterically protected π-electron systems for efficient solid-state photon upconversion

A solid-state visible-to-ultraviolet triplet-triplet annihilation-based photon upconversion system driven by low-intensity light at sunlight levels is developed. Realizing such a solid-state photon upconversion system has been challenging because it is difficult to achieve high fluorescence quantum yield and fast triplet exciton diffusion simultaneously, which requires methodologies to precisely control interactions among chromophores and suppress quenching of both singlet and triplet excited states. Here, we report that a group of dihydroindeno[2,1-a]indene derivatives functionalized with alkyl chains above and below the π-plane satisfies all these requirements. Among three derivatives investigated, we identify the optimal emitter structure that exhibits the highest photon upconversion quantum yield in both solution and crystalline states. The solid-state system is less affected by crystalline defects, exhibiting high photoluminescence quantum yield, long triplet lifetime, and fast triplet diffusion, with an absolute photon upconversion quantum yield of 1.9% and a low threshold excitation intensity of 1.2 mW cm^-2.