Photophysical properties and excited-state dynamics of donor-acceptor-heavy-atom molecules and their application in triplet-triplet annihilation upconversion

In the field of triplet-triplet annihilation upconversion (TTA-UC), the design of an optimized UC system with a high quantum yield at a low excitation power density is a challenging issue. To achieve this goal, it is essential to improve the triplet quantum yields (Φ_T) and the UC quantum yields (Φ_UC) of the triplet photosensitizers. In this study, we synthesized a novel series of donor-acceptor-heavy-atom (D-A-H) molecules where we have focused on pyrene, BODIPY, and halogen atoms (Cl, Br, and I) as D, A, and H moieties, respectively. The TTA-UC properties of the D-A-H molecules were examined, and their electrochemical properties and excited-state dynamics were investigated. The highest singlet oxygen and UC quantum yields were estimated for PY-BDP-2I (PY = pyrene and BDP = BODIPY) with the values of 0.93 and 4.84%, respectively. In addition, the threshold intensities (I_th) of PY-BDP-2Cl, PY-BDP-2Br, and PY-BDP-2I (112, 27, and 42 mW cm⁻², respectively) were lower than that of PY-BDP (406 mW cm⁻²). In UC quantum yields, the iodine-containing samples (PY-BDP-2I and BEN-BDP-2I) show higher quantum yields than other compounds, which demonstrates that the introduction of iodine into the D-A-H photosensitizer system is an effective strategy for improving the TTA-UC performance.