Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X−X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-toligand charge-transfer excited states. Kosower's salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights. CONTENTS 1. Introduction, Background, and Motivation

Hydrobromic acid (HBr) has significant potential as an inexpensive feedstock for hydrogen gas (H) solar fuel production through HBr splitting. Mesoporous thin films of anatase TiO or SnO/TiO core-shell nanoparticles were sensitized to visible light with a new Ru polypyridyl complex that served as a photocatalyst for bromide oxidation. These thin films were tested as photoelectrodes in dye-sensitized photoelectrosynthesis cells. In 1 N HBr (aq), the photocatalyst undergoes excited-state electron injection and light-driven Br oxidation. The injected electrons induce proton reduction at a Pt electrode. Under 100 mW cm white-light illumination, sustained photocurrents of 1.5 mA cm were measured under an applied bias. Faradaic efficiencies of 71 ± 5% for Br oxidation and 94 ± 2% for H production were measured. A 12 μmol h sustained rate of H production was maintained during illumination. The results demonstrate a molecular approach to HBr splitting with a visible light absorbing complex capable of aqueous Br oxidation and excited-state electron injection.

Dye-sensitized bromide oxidation was investigated using a series of four ruthenium polypyridyl photocatalysts anchored to SnO/TiO core/shell mesoporous thin films through 2,2'-bipyridine-4,4'-diphosphonic acid anchoring groups. The ground- and excited-state reduction potentials were tuned over 500 mV by the introduction of electron withdrawing groups in the 4 and 4' positions of the ancillary bipyridine ligands. Upon light excitation of the surface-bound photocatalysts, excited-state electron injection yielded an oxidized photocatalyst that was regenerated through bromide oxidation. High injection quantum yields (Φ) and regeneration quantum yields (Φ) were essential to obtain efficient bromide oxidation yet required a photocatalyst that is both a potent photoreductant and a strong oxidant after excited-state injection. The four photocatalysts utilized in this manuscript ranged from unity Φ (1.0) and minimal Φ (0.037) to minimal Φ (0.09) and unity Φ (1.0). The photocatalyst that displayed the highest overall dye-sensitized photoelectrosynthesis cell performances exhibited near unity Φ (0.99), while a significant Φ was still preserved (0.59). Thus, these results highlighted the delicate interplay between the ground- and excited-state reduction potentials of photocatalysts for dye-sensitized hydrobromic acid splitting.

We describe here the preparation and characterization of a photocathode assembly for CO2 reduction to CO in 0.1 M LiClO4 acetonitrile.

The complex [Ru(deeb)(bpz)] (RuBPZ, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ, Br], with K = 8400 ± 200 M in acetone. The RuBPZ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ acetone solution led to significant PL quenching and Stern-Volmer plots showed upward curvature. Time-resolved PL measurements identified two excited state quenching pathways, static and dynamic, which were operative toward [RuBPZ, Br] and free RuBPZ, respectively. The single ion-pair [RuBPZ, Br]* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k = (2.2 ± 0.3) × 10 s. In contrast, RuBPZ* was dynamically quenched by bromide with a quenching rate constant, k = (8.1 ± 0.1) × 10 M s. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ and Br products that underwent recombination to regenerate the ground state with a second-order rate constant, k = (2.3 ± 0.5) × 10 M s. Kinetic analysis revealed that RuBPZ was a primary photoproduct, while Br was secondary product formed by the reaction of a Br with Br, k = (1.1 ± 0.2) × 10 M s. Marcus theory afforded an estimate of the formal reduction potential for E(Br) in acetone, 1.42 V vs NHE. A H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru center. Prolonged steady state photolysis of RuBPZ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ-bpz)(Br)], was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br photoproducts.