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

Abstract

Three iridium photosensitizers, [Ir(dCF₃ppy)₂(N-N)]⁺, where N-N is 1,4,5,8-tetraazaphenanthrene (TAP), pyrazino[2,3-<i>a</i>]phenazine (pzph), or benzo[<i>a</i>]pyrazino[2,3-<i>h</i>]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 <b>Ir-bpph</b>, with a molar absorption coefficient, ε = 9800 M^-1 cm^-1, at 450 nm and an excited-state reduction potential, <i>E</i>(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.