Optimizing Triplet-TripletAnnihilation Upconversionvia Control of Annihilator Diradical Character

Abstract Triplet–triplet annihilation (TTA) upconversion (UC) is a process that converts two low-energy photons into one of higher energy. Due to efficient operation under incoherent low-fluence excitation, TTA-UC has an extensive and promising application scope from photovoltaics to night-vision optics, among others. TTA-UC requires two species, sensitizers which absorb low energy photons, and annihilators which emit high energy photons. However, the limited number of annihilators has hindered the practical application of TTA-UC. In this work, we introduce p-azaquinodimethanes (AQMs) as a novel family of TTA-UC annihilators. While commonly studied annihilators are limited to benzenoid compounds, we show that quinoidal chromophores exhibiting a high ground-state diradical character (y0) can undergo TTA-UC. By gradual optimization of AQMs via aryl functionalization, we demonstrate maximum achievable TTA-UC quantum yields (ΦUC,g∞) of up to 0.36%. Increasing y0 leads to small S1[An]–T2[An] energy gaps. This increases the likelihood of T2 to S1 reverse intersystem crossing after TTA, enhancing ΦUC,g∞ and the statistical probability factor f values. We demonstrate that diradical character is an important parameter when designing quinoidal annihilators, which can be used to control their excited-state energy landscape, optimizing TTA-UC performance.