Optimizing Intersystem Crossing Dynamics for Triplet–Triplet Annihilation Upconversion: Aromatic Donor Modulation in Donor–Acceptor–Heavy Atom Molecular Architectures

Triplet–triplet annihilation upconversion (TTA-UC) is a photophysical process that converts low-energy photons to higher-energy light. It has gained significant attention owing to its unique anti-Stokes photoluminescence properties and potential applications. However, developing an optimal triplet photosensitizer capable of achieving a high UC efficiency at a low power density remains a major challenge. In this study, we synthesize a series of donor–acceptor–heavy atom (D–A–H) molecules (BEN-BDP-2I, NAP-BDP-2I, ANT-BDP-2I, PY-BDP-2I, and PERY-BDP-2I) by integrating aromatic hydrocarbon moieties (benzene, naphthalene, anthracene, pyrene, and perylene) as donors (D), BODIPY as the acceptor (A), and iodine as the heavy atom (H). All of the synthesized compounds exhibit exceptionally low threshold intensities (Ith), ranging from 16 to 26 mW/cm2. Notably, PY-BDP-2I demonstrates the highest singlet oxygen quantum yield (ΦΔ = 0.948) and UC quantum yield (ΦUC = 5.21%). Additionally, computational chemistry and electrochemical analyses explore the correlation between the donor type and intersystem crossing (ISC) mechanisms. Femtosecond and nanosecond transient absorption spectroscopies provide an in-depth investigation of excited-state dynamics. The D–A–H-type triplet photosensitizer proposed in this study is expected to play a crucial role in the future design of high-performance UC systems.