Chloride, Bromide, and Iodide Photooxidation in Acetonitrile/Water Mixtures Using Binuclear Iridium(III) Photosensitizers.

Two iridium(III) binuclear photosensitizers, [Ir(dFCF 3 ppy) 2 (N-N)Ir(dFCF 3 ppy) 2 ] 2+ , where N-N is tetrapyrido[3,2- a :2',3'- c :3″,2″- h :2‴,3‴- j ]phenazine (Ir-TPPHZ) and 1,4,5,8-tetraazaphenanthrene[9,10- b ]-1,4,5,8,9,12-hexaazatriphenylene (Ir-TAPHAT) are reported for iodide, bromide, and chloride photooxidation in acetonitrile and acetonitrile/water mixtures using blue-light irradiation. Excited-state reduction potentials E red * of +2.02 and +2.09 V vs NHE were determined for Ir-TPPHZ and Ir-TAPHAT, respectively. Both photosensitizers' excited states were efficiently quenched by iodide, bromide, and chloride with quenching rate constants in the (3.5-9.2) × 10 10 and (0.0036-2.9) × 10 10 M -1 s -1 ranges in neat acetonitrile and acetonitrile/water mixtures, respectively. Nanosecond transient absorption spectroscopy provided unambiguous evidence of reductive excited-state electron transfer, with all halides in the solvent mixtures containing up to 50% water. Cage-escape yields were large (55-96%) in acetonitrile and dropped below 32% in 50:50 acetonitrile/water mixtures.