Impact of steric effects on the statistical probability factor in triplet-triplet annihilation upconversion

The statistical probability factor (f) in triplet-triplet annihilation (TTA) upconversion (UC) is one of the key elements governing UC performance. This factor indicates the probability that an emissive singlet state is generated upon two triplet states undergoing TTA. Therefore, UC quantum yield has a maximum value of f/2 if all other processes within the system approach unity. As an intrinsic property of annihilator compounds, the f value is thought to mostly depend on the annihilator's excited state energy level distribution. However, emerging results in the field of TTA-UC indicate that steric effects have a substantial influence on the f factor and UC performance. In this work, we demonstrate that the f factor is composed of both energetic and steric components. We study a series of energetically similar diketopyrrolopyrrole (DPP) annihilators. By modulating the various alkyl groups appended to the DPPs, we demonstrate a strategy to enhance the f factor, and therefore UC performance, by limiting steric interactions. Herein, we develop structural design rules for upconversion annihilators. By removing sterically bulky groups, collisions are optimized, leading to annihilators with enhanced statistical probability factors and higher upconversion quantum yields.