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 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 the fact that solid-state operation is essential for many of the most attractive applications of TTA-UC, most studies on annihilator properties have been performed in solution. Here, we develop a strategy to activate solid-state TTA-UC in materials previously shown to undergo singlet fission such as diketopyrrolopyrroles (DPPs) and dipyrrolonaphthyridinediones (DPNDs). Furthermore, we demonstrate a general approach to progressively enhancing UC yields with bulky alkyl moieties. Our design motif, combined with optimization of TTA-UC thin films, enables solid-state TTA-UC 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 toward efficient solid-state UC and an important step toward the realization of the vast potential of TTA-UC across a multitude of applications.