Approaching the Spin-Statistical Limit in Visible-to-Ultraviolet 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 in biological applications and for improving the spectral match in solar energy conversion. While high TTA-UC quantum yields have been reported for, for example, red-to-green TTA-UC systems, there are only a few examples of visible-to-ultraviolet (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. We report a record-setting internal TTA-UC quantum yield (Φ_UC,g) of 16.8% (out of a 50% maximum) for 1,4-bis((triisopropylsilyl)ethynyl)naphthalene, 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 has never been used for TTA-UC previously, also showed impressive performances with Φ_UC,g above 12%. In addition, a new method to determine the rate constant of TTA 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 toward highly efficient visible-to-UV TTA-UC systems that hold great potential for driving high-energy photochemical reactions.