Approaching the Spin-Statistical Limit in Visible-to-UV Photon Upconversion

Triplet-triplet annihilation photon upconversion (TTA-UC) is a process in which triplet excitons combine to form emissive singlets, and holds great promise to improve the spectral match in solar energy and biological applications. While high TTA-UC quantum yields has been reported for e.g., red-to-green TTA-UC systems there are only a few examples of visible-to-UV transformations in which the quantum yield reaches 10%. In this study we investigate the performance of six annihilators when paired with the sensitizer 2,3,5,6-tetra(9H-carbazol-9-yl)benzonitrile (4CzBN), a purely organic compound that exhibits thermally activated delayed fluorescence (TADF). We report a record-setting TTA-UC quantum yield of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene (TIPS-Naph), demonstrating the first example of a visible-to-UV TTA-UC system approaching the classical spin-statistical limit of 20%. Three other annihilators, of which 2,5-diphenylfuran (PPF) has never been used for TTA-UC previously, also showed impressive performances with TTA-UC quantum yields above 12%. In addition, a new method to determine the rate constant of triplet-triplet annihilation is proposed in which only time-resolved emission measurements are needed, circumventing the need for more challenging transient absorption measurements. The results reported herein represent an important step towards highly efficient visible-to-UV TTA-UC systems which hold great potential for driving high-energy photochemical reactions.