Improved Visible Light Absorption of Potent Iridium(III) Photooxidants 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, e = 9800 M–1cm–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 separa-tion and energy storage where each excited-state oxidized seven different electron donors with rate constants between 0.089 to 3.06 x1010 M–1s–1. A Marcus analysis provided a total reorganization energy of 0.7±0.1 eV for excited-state electron transfer.