The statistical probability factor in triplet mediated photon upconversion: a case study with perylene.

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process where two low-energy photons are converted into one higher-energy photon. A crucial component for an efficient upconversion process is the statistical probability factor (), defined as the probability of the formation of a high-energy singlet state upon coupling of two low-energy triplet states. Theoretically, depends on the energy level distribution, molecular orientation, inter-triplet exchange coupling of triplet dyads, and spin-mixing of resulting spin states (singlet, triplet, and quintet). However, experimental values of for acene-based annihilators have been subject to large variations due to many factors that have resulted in the reporting of different values for the same molecule. In this work, we discuss these factors by studying perylene as a case study annihilator, for which by far the largest variation in = 16 to 100% has been reported. We systematically investigated the TTA-UC of PdTPBP:perylene, as a sensitizer-annihilator pair and obtained the experimental = 17.9 ± 2.1% for perylene in THF solution. This limits the maximum TTA-UC quantum yield to 9.0% (out of 50%) for this annihilator. We found that such a low value for perylene is largely governed by the energy-gap law where higher non-radiative losses due to the small energy gap between 2 × T and T affect the probability of singlet formation. Interestingly, we found this observation true for other acene-based annihilators whose emission ranges from the UV to the yellow region, thus providing a blueprint for future design of efficient TTA-UC systems.