Abstract
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 (ΦUC,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 ΦUC,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.
Highlights
Unconventional strategies for expanding the use of solar energy have attracted significant attention in recent years.[1,2]Using photon upconversion (UC), in which low-energy photons are combined to form high-energy light, it is expected that the conventional limits in photovoltaics can be shifted upward.[3]
We show that the internal UC quantum yield of visible-to-UV triplet annihilation photon UC (TTA-UC) systems may approach the oftenencountered spin-statistical limit of 20%
The results show that the TTA-UC pair 4CzBN/TIPS-Naph achieve a record-setting 16.8% internal UC quantum yield, and high internal quantum yields are reached when using PPO (14.0%), PPF (13.0%), or TP (12.6%) as annihilators as well
Summary
Unconventional strategies for expanding the use of solar energy have attracted significant attention in recent years.[1,2]Using photon upconversion (UC), in which low-energy photons are combined to form high-energy light, it is expected that the conventional limits in photovoltaics can be shifted upward.[3]. The mechanism called triplet−triplet annihilation photon UC (TTA-UC) is of specific interest as this process functions under low-intensity, noncoherent light.[10,11] By using a donor, or sensitizer, species in conjunction with a fluorescent annihilator, triplets generated by the sensitizer from incident long-wavelength light may be converted into a highly energetic singlet state within the annihilator species in a spin-allowed TTA process This scheme has been demonstrated for many different spectral ranges and with a variety of compounds,[12] spanning purely organic systems,[13,14] nanocrystals,[15−19] metallic complexes,[20−24] and metal−organic frameworks,[25] to name a few
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