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

The titration of iodide into acetonitrile solutions of BiI resulted in the formation of [BiI]. Ligand-to-metal charge transfer (LMCT) excitation of [BiI] yielded a transient species assigned as the diiodide anion I directly ligated to Bi, [Bi(I)I]. With 20 ns time resolution, transient absorption measurements revealed the appearance of two species assigned on the analysis of the iodine molecular orbitals as an η ligated I, [(η-I)BiI] (λ = 640 nm), and an η species [(η-I)BiI] (λ = 750 nm). The rapid appearance of this intermediate was attributed to intramolecular I-I bond formation. The [(η-I)BiI] subsequently reacted with 1 equiv of iodide to yield [(η-I)BiI]. Interestingly, [(η-I)BiI] decayed to ground state products with a first-order rate constant of k = 2 × 10 s. Under the same experimental conditions, I in CHCN rapidly disproportionates with a tremendous loss of free energy, ΔG = -2.6 eV. The finding that metal ligation inhibits this energy wasting reaction is of direct relevance to solar energy conversion. The photochemistry itself provides a rare example of one electron oxidized halide species coordinated to a metal ion of possible relevance to reductive elimination/oxidation addition reaction chemistry of transition metal catalysts.

The relatively unexplored luminophore architecture [Ir(N^N^N)(C^N)L](+) (N^N^N = tridentate polypyridyl ligand, C^N = 2-phenylpyridine derivative, and L = monodentate anionic ligand) offers the stability of tridentate polypyridyl coordination along with the tunability of three independently variable ligands. Here, a new family of these luminophores has been prepared based on the previously reported compound [Ir(tpy)(ppy)Cl](+) (tpy = 2,2':6',2″-terpyridine and ppy = 2-phenylpyridine). Complexes are obtained as single stereoisomers, and ligand geometry is unambiguously assigned via X-ray crystallography. Electrochemical analysis of the materials reveals facile HOMO modulation through ppy functionalization and alteration of the monodentate ligand's field strength. Emission reflects similar modulation shifting from orange to greenish-blue upon replacement of chloride with cyanide. Many of the new compounds exhibit impressive room temperature phosphorescence with lifetimes near 3 μs and quantum yields reaching 28.6%. Application of the new luminophores as photosensitizers for photocatalytic hydrogen generation reveals that their photostability in coordinating solvent is enhanced as compared to popular [Ir(ppy)2(bpy)](+) (bpy = 2,2'-bipyridine) photosensitizers. Yet, the binding of their monodentate ligand emerges as a source of instability during the redox processes of cyclic voltammetry and mass spectrometry. DFT modeling of electronic structure is provided for all compounds to elucidate experimental properties.

Electron-transfer theories predict that an increase in the quantum-mechanical mixing (H) of electron donor and acceptor wavefunctions at the instant of electron transfer drives equilibrium constants toward unity. Kinetic and equilibrium studies of four acceptor-bridge-donor (A-B-D) compounds reported herein provide experimental validation of this prediction. The compounds have two redox-active groups that differ only by the orientation of the aromatic bridge: a phenyl-thiophene bridge (p) that supports strong electronic coupling of H > 1,000 cm; and a xylyl-thiophene bridge (x) that prevents planarization and decreases H < 100 cm without a significant change in distance. Pulsed-light excitation allowed kinetic determination of the equilibrium constant, K In agreement with theory, K(p) were closer to unity compared to K(x). A van't Hoff analysis provided clear evidence of an adiabatic electron-transfer pathway for p-series and a nonadiabatic pathway for x-series. Collectively, the data show that the absolute magnitude of the thermodynamic driving force for electron transfers are decreased when adiabatic pathways are operative, a finding that should be taken into account in the design of hybrid materials for solar energy conversion.

The "Franck−Condon" (FC) excited state is the first state created when a molecule absorbs a visible photon. Here we report Stark and visible absorption spectroscopies that interrogate the FC state of rigorously diamagnetic [M(bpy) 3 ] 2+ complexes, where bpy is 2,2′-bipyridine and M = Fe, Ru, and Os. Direct singlet-to-triplet metal-to-ligand charge transfer (MLCT) transitions are evident in the 550−750 nm region of the absorbance spectrum of [Os(bpy) 3 ] 2+ , yet are poorly resolved or absent for [Ru(bpy) 3 ] 2+ and [Fe(bpy) 3 ] 2+ . In the presence of a strong 0.4−0.8 MV/cm electric field, well-resolved transitions are observed for all the complexes in this same spectral region. In particular, an electroabsorption feature at 633 nm (15 800 cm −1 ) provides compelling evidence for the direct population of a high spin [Fe(bpy) 3 ] 2+ * MLCT excited state. Group theoretical considerations and Liptay analysis of the Stark spectra revealed dramatic light-induced dipole moment changes in the range μ Δ ⎯⇀ ⎯ = 3−9 D with the triplet transitions consistently showing shorter charge transfer distances. The finding that the spin of the initially populated FC excited state differs from that of the ground state, even with a relatively light first row transition metal, is relevant to emerging applications in energy up-conversion, dye sensitization, spintronics, photoredox catalysis, and organic light emitting diodes (OLEDs).T he electron spin in the metal-to-ligand charge-transfer (MLCT) excited states utilized for applications in displays, sensing, solar energy conversion, and organic synthesis differ from that of the ground state. Stark and visible absorption spectroscopies directly probe the "Franck−Condon" (FC) excited state of [M(bpy) 3 ] 2+ complexes, where bpy is 2,2′-bipyridine and M is Fe, Ru, and Os. It is shown herein that Stark spectroscopy is a more sensitive technique than absorption spectroscopy for ΔS ≠ 0 electronic transitions and provides experimental values of the excited state dipole moment. In particular, a previously unresolved low energy transition for solvated [Fe(bpy) 3 ] 2+ * indicates a paramagnetic FC excited state. Group theoretical and Liptay analysis provides a large dipole moment change, μ |Δ | ⎯⇀ ⎯