A bromine-substituted thermally activated delayed fluorescent (TADF) molecule AQCzBr₂ is designed with both small singlet–triplet splitting (ΔE_ST) and increased spin–orbit coupling (SOC) to boost intersystem crossing (ISC) for singlet oxygen generation. AQCzBr₂ nanoparticles (NPs) demonstrate high productivity of singlet oxygen generation (Φ_Δ = 0.91) which allows highly efficient photodynamic therapy toward cancer cells.