Probing Triplet Photochemistry in Ru(II) Porphyrins Using Singlet Oxygen Generation and Triplet–Triplet Annihilation Upconversion

Abstract Porphyrins are known for their rich triplet-state photophysics, making them attractive platforms for light-driven applications. Compared with extensively studied Pd(II) and Pt(II) porphyrins, Ru(II) porphyrins remain less systematically explored, particularly for triplet-driven reactivity. Here, we demonstrate that a single porphyrin framework can perform complementary functions: generate singlet oxygen (1O2) and photosensitize triplet–triplet annihilation upconversion (TTA-UC). We examine six related systems: free-base porphyrins (1A/1B) and Ru(II) complexes 2A/2B with CO and 3A/3B with CO plus PyPX (10-(pyridin-4-yl)-10H-phenoxazine). Complexes 3A and 3B are newly synthesized Ru(II) porphyrins, enabling direct assessment of axial ligation effects within this series. Optical spectroscopy, DFT/TD-DFT calculations, 1O2 measurements, and TTA-UC experiments investigate how metalation, bromination, and axial ligation influence triplet-state properties. Ru(II) insertion promotes intersystem crossing, leading to 1O2 quantum yields up to 70%. Bromination further increases ΦΔ in the free-base and monoaxial complexes. In contrast, PyPX coordination significantly increases ΦΔ in the nonbrominated system (2A to 3A) but has negligible influence in the brominated analogue (2B to 3B). Complexes 3A/3B also sensitize TTA-UC, achieving efficiencies of ∼1.3–1.4% with perylene in THF. Collectively, our results establish design principles for Ru(II) porphyrins as triplet sensitizers, enabling control over complementary triplet-driven photochemical processes within a single molecular platform.