4CzBN possess several of the sought-after properties of a sensitizer, with weak prompt fluorescence, a long-lived delayed component, and slow rISC (Figure ). Its major drawback is the (for most systems) unnecessarily high singlet and triplet energies, which forbid excitation at wavelengths >430 nm, leading to significant energy loss during ISC and TET. Additionally, some photo instability of the UC samples was detected which was ascribed to the degradation of 4CzBN, an issue that can be alleviated by the addition of bulky substituents. Finding complementary compounds with similar characteristics to 4CzBN but with lower excited state energies will be needed to further improve green-to-UV UC, which is especially interesting for solar applications given the vast amounts of green light in the solar spectrum. Considerations on Annihilator Design. Not only could novel TADF sensitizers contribute to improved vis-to-UV TTA-UC systems, but perhaps even more crucial is the pursuit of new annihilators. Design principles that hold true for annihilators in general must obviously be upheld, such as high FF and a long tT, but for UV-emitting systems additional considerations should be taken into account. As touched upon previously many vis-to-UV TTA-UC systems suffer from low photostability, which follows from the relatively high energy of the states involved. This aspect has recently been investigated in greater detail by Murakami et al., gaining important insights as to how the energy levels of the sensitizer and annihilator affect the photostability of TTA-UC systems in solution. They observed a correlation between the main degradation pathway and the energy difference between the LUMO levels of annihilator and solvent. In our study we found no evidence of annihilator degradation during UC experiments, and we primarily ascribe the slight decrease in UC emission over time to sensitizer degradation.