Study on the energy level limitations of triplet-triplet annihilation upconversion with anthracene-isomerized dimers as annihilators

The enhancement in the efficiency of triplet-triplet annihilation upconversion (TTA-UC) is mainly determined by the triplet energy transfer (TET) and triplet-triplet annihilation (TTA) between the sensitizers and annihilators. The TET process works efficiently by adjusting the concentration ratio of the sensitizers and annihilators. The efficiency of TTA is determined by the properties of the annihilator. Because TTA is a Dexter-type energy transfer and is affected by the diffusion rate, the energy levels of the excited states and the molecular size are both crucial in TTA. In this study, four isomerized dimers of 9,10-diphenlanthracene (DPA) and anthracene (An) were designed and prepared as annihilators for TTA-UC. The singlet and triplet energy levels could be adjusted by altering the connection position while maintaining the molecular weight and size. When PtOEP was used as the sensitizer, the maximum upconversion efficiency of 9-[4-(9-anthracenyl)phenyl]-10-phenylanthracene (9DPA-9An) was ∼11.18%. This is four times higher than that of 9,10-diphenyl-2,9′-bianthracene (2DPA-9An, 2.63%). The calculation of the energies of T₁ and the higher triplet state (T₃, because E(T₂) is similar to the E(T₁) of these dimers) for these dimers has provided insights into the underlying reasons. These indicated that the energy gap value of 2 × E(T₁) − E(T₃) is the determining factor for TTA efficiency. This work may provide a better understanding of the excited-state energy levels, which is crucial for designing novel annihilators to enhance the TTA-UC efficiency.