Impact of Electron-Withdrawing Groups on the Performance of Triplet-Triplet Annihilation Upconversion Annihilators

Abstract

Triplet-triplet annihilation (TTA) upconversion (UC) is a process converting two low-energy photons into one of higher energy and has a wide range of promising technological applications. Typically, TTA-UC systems are comprised of a sensitizer that absorbs low-energy light and an annihilator responsible for emission of a higher-energy photon via TTA. The performance of most annihilators relies on both steric and electronic effects. Despite recent progress in steric design rules, there is a current lack of structure-property relationships related to the electronic properties of annihilators. Most common annihilator families, such as acenes, are difficult to functionalize, impeding the investigation of electronic structure-property relationships. In this work, we employ easily tunable dipyrrolonaphthyridinedione (DPND) annihilators to explore how electron-withdrawing groups (EWGs) affect their TTA-UC performance. By targeted functionalization with EWGs, we show that stabilization of radical character is essential for improved upconversion efficiency. Additionally, we demonstrate that methyl moieties can be used to control the distribution of radical character across aryl substituents, facilitating stabilization via EWGs. By integrating our design rules, we synthesized an optimized DPND derivative exhibiting a high NIR-to-visible TTA-UC quantum yield of up to 10.9% (out of 50%), a record for DPNDs. This demonstrates the optimization of annihilators via electronic design principles is a powerful strategy for optimizing upconversion efficiency, paving the way for the use of TTA-UC across a multitude of practical applications.