Activating Solid-State Triplet-Triplet Annihilation Upconversion via Bulky Annihilators

Triplet-triplet annihilation (TTA) upconversion (UC), a process converting two low-energy photons into one of a higher energy, has numerous promising applications. Commonly, TTA-UC systems consist of a sensitizer, which absorbs low-energy radiation, and an annihilator, responsible for TTA and emission of high-energy radiation. Despite solid-state operation being essential for the most attractive applications of TTA-UC, most studies on annihilator properties have been performed in solution. In this work, we develop a strategy to activate solid-state TTA- UC in singlet fission materials such as diketopyrrolopyrroles (DPPs) and dipyrrolonaphthyridinediones (DPNDs). Furthermore, we demonstrate a generalized approach to progressively enhance UC yields with bulky alkyl moieties. Our design motif, combined with optimization of TTA-UC thin films, enables solid-state TTA-UC for the first time in these highly stable DPP and DPND derivatives, reaching UC quantum yields up to 1.5%. We demonstrate that careful molecular design of TTA-UC materials is a powerful strategy towards efficient solid- state UC, and an important step towards the realization of the vast potential of TTA-UC across a multitude of applications.