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-hexa-azatriphenylene (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) x 10(10) and (0.0036-2.9) x 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.

Automated summary

Two iridium(III) binuclear photosensitizers were investigated for the photooxidation of iodide, bromide, and chloride. The excited states of the photosensitizers were efficiently quenched by the halides, and evidence of reductive excited-state electron transfer was found in solvent mixtures containing water.