Improved Visible Light Absorption of Potent Iridium(III) Photo-oxidants for Excited-State Electron Transfer Chemistry

Three iridium photosensitizers, [Ir­(dCF₃ppy)₂(N–N)]⁺, where N–N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino­[2,3-a]­phenazine (pzph), or benzo­[a]­pyrazino­[2,3-h]­phenazine (bpph) and dCF₃ppy is 2-(3,5-bis­(trifluoromethyl-phenyl)­pyridine), were found to be remarkably strong photo-oxidants with enhanced light absorption in the visible region. In particular, judicious ligand design provided access to Ir-bpph, with a molar absorption coefficient, ε = 9800 M^–1 cm^–1, at 450 nm and an excited-state reduction potential, E(Ir^+*/0) = 1.76 V vs NHE. These complexes were successful in performing light-driven charge separation and energy storage, where all complexes photo-oxidized seven different electron donors with rate constants (0.089−3.06) × 10¹⁰ M^–1 s^–1. A Marcus analysis provided a total reorganization energy of 0.7 ± 0.1 eV for excited-state electron transfer.