Through-Bond Electronic Coupling and Strong Ligand Binding Facilitate Blue-to-UV Photon Upconversion by CsPbBr₃ Quantum Dots

CsPbBr3 quantum dots (QDs) have garnered interest as triplet sensitizers for blue-to-UV triplet–triplet annihilation upconversion (TTA-UC), which can be leveraged to drive processes, such as photocatalysis and high-resolution 3D printing. To facilitate upconversion, molecular transmitters bound to a QD accept energy in the form of a triplet exciton and pass it to molecules in solution that engage in TTA-UC. Here, we investigated how different binding groups affixing naphthalene-based ligands to CsPbBr3 QDs impact their performance as transmitters. We find that naphthylamine ligands bind weakly to CsPbBr3 QDs, preventing them from functioning as effective transmitters. In contrast, 1- and 2-naphthoic acid ligands each enable blue-to-UV TTA-UC, with 1-naphthoic acid showing stronger performance with a 50% lower threshold intensity. This arises from improved through-bond coupling due to a slight difference in geometry that facilitates triplet energy transfer. Our results highlight the importance of binding and orbital interactions when designing QD triplet transmitters.