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

Abstract

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.

Investigation of the Excited-State Electron Transfer and Cage Escape Yields Between Halides and a Fe(III) Photosensitizer

“…41 These Φ CE are small compared with values recorded in organic solvents for photosensitizers where X 2 •− was observed with values of 0.25− 0.96. 34,60,68 In some instances, low Φ CE (∼0.04) with observation of I 2…”

“…This could imply that the in-cage geminate charge recombination is much faster than the cage escape process . These Φ CE are small compared with values recorded in organic solvents for photosensitizers where X 2 •– was observed with values of 0.25–0.96. ,, In some instances, low Φ CE (∼0.04) with observation of I 2 •– in acetonitrile have also been reported using Ru­(II) photosensitizers …”

“…Whereas evidence for iodide or bromide oxidation in organic solvent with Ru(II) and Ir(III) photosensitizers was unambiguously confirmed by transient absorption spectroscopy, 32−34 it is only very recently that evidence for chloride oxidation in waterlike conditions was provided using Ir(III) dinuclear photosensitizers. 34 Notably bromide and chloride photo-oxidation have been monitored in acetone using Ru(II) photosensitizers, 35−37 as well as by oneor two-photon excitation of N-phenylphenothiazine in acetonitrile. 38 The use of Ir(III) and Ru(II) photosensitizers helps perform these challenging oxidations using single-photon excitation but represents a drawback in terms of sustainability.…”

“…This wide range of potentials was shown to be narrower in organic solvents where E °(X •/– ) of 1.23, 1.35, and 1.46 V vs NHE have been roughly estimated in acetonitrile for iodide, bromide, and chloride, respectively. , Hence, such oxidations are thermodynamically very demanding in water and often require the use of photosensitizers based on rare ruthenium and iridium metals. Whereas evidence for iodide or bromide oxidation in organic solvent with Ru­(II) and Ir­(III) photosensitizers was unambiguously confirmed by transient absorption spectroscopy, − it is only very recently that evidence for chloride oxidation in waterlike conditions was provided using Ir­(III) dinuclear photosensitizers . Notably bromide and chloride photo-oxidation have been monitored in acetone using Ru­(II) photosensitizers, − as well as by one- or two-photon excitation of N -phenylphenothiazine in acetonitrile …”

Photoinduced One-Electron Chloride Oxidation in Water Using a Pentacationic Ir(III) Photosensitizer

“…There are only a handful of reported photosensitizers that can perform chloride oxidation. Noteworthy, these examples mostly report chloride oxidation in organic solvent ,− or acetonitrile/water mixtures with water content up to 50%, but none of these prior examples were performed in neat water. In this context, we present a unique instance of light-induced chloride oxidation in the aqueous phase mediated by a novel pentacationic iridium photosensitizer.…”