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

Three iridium photosensitizers, [Ir(dCF(3)ppy)(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 dCF(3)ppy 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, epsilon = 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) x 10 10 M(-1)s(-1). A Marcus analysis provided a total reorganization energy of 0.7 +/- 0.1 eV for excited-state electron transfer.