Identifying efficiency-loss pathways in triplet-triplet annihilation upconversion systems

Triplet-triplet annihilation upconversion (TTA-UC) systems have been studied extensively recently, and have been proposed for use in a wide range of applications. Identification of the dominant mechanisms of upconversion-efficiency loss (UEL) will assist in the development of efficient TTA-UC systems. In this work, we combine experiments and kinetic analysis to study UEL. We identify exciplex formation and reverse triplet energy transfer (TET) as the two most important UEL mechanisms in the model TTA-UC system of platinum octaethylporphyrin (PtOEP) and 9,10-diphenylanthracene (DPA). Based on spectral analysis and time-resolved photoluminescence experiments, we show that exciplex formation is a potent UEL pathway in the PtOEP-DPA system. We demonstrate that prolonged sensitizer phosphorescence arises from reverse TET from annihilator triplet states, and that the reverse TET is likely facilitated by thermal population of low-frequency vibrational states in the sensitizer and the annihilator. Additionally, we demonstrate how the rate constants for reverse TET and exciplex formation can be estimated based on knowledge of a few key parameters and the experimental value of the optimum sensitizer concentration.