Flash-Quench Technique Employed To Study the One-Electron Reduction of Triiodide in Acetonitrile: Evidence for a Diiodide Reaction Product

Halogen-Bonding-Mediated Radical Reactions: The Unexpected Behavior of Piperazine-Based Dithiooxamide Ligands in the Presence of Diiodine

N,N'-Dialkylpiperazine-2,3-dithiones (R₂pipdt) were recognized as a class of hexa-atomic cyclic dithiooxamide ligands with peculiar charge-transfer donor properties toward soft electron-acceptors such as noble metal cations and diiodine. The latter interaction is nowadays better described as halogen bonding. In the reaction with diiodine, R₂pipdt unexpectedly provides the corresponding triiodide salts, differently from the other dithiooxamides, which instead typically achieve ligand·nI₂ halogen-bonded adducts. In this paper, we report a combined experimental and theoretical study that allows elucidation of the nature of the cited products and the reasons behind the unpredictable behavior of these ligands. Specifically, low-temperature single-crystal X-ray diffraction measurements on a series of synthetically obtained R₂pipdt (R = Me, ⁱPr, Bz)/I₃ salts, complemented by neutron diffraction experiments, were able to experimentally highlight the formation of [R₂pipdtH]⁺ cations with a -S-H bond on the dithionic moiety. Differently, with R = Ph, a benzothiazolylium cation, resulting from an intramolecular condensation reaction of the ligand, is obtained. Based on density functional theory (DFT) calculations, a reasonable reaction mechanism where diiodine plays the fundamental role of promoting a halogen-bonding-mediated radical reaction has been proposed. In addition, the comparison of combined experimental and computational results with the corresponding reactions of N,N'-dialkylperhydrodiazepine-2,3-dithione (R₂dazdt, a hepta-atomic cyclic dithiooxamide), which provide neutral halogen-bonded adducts, pointed out that the difference in the torsion angle of the free ligands represents the structural key factor in determining the different reactivities of the two systems.

On the Determination of Halogen Atom Reduction Potentials with Photoredox Catalysts

The standard one-electron reduction potentials of halogen atoms, E°'(X^•/-), and many other radical or unstable species, are not accessible through standard electrochemical methods. Here, we report the use of two Ir(III) photoredox catalysts to initiate chloride, bromide, and iodide oxidation in organic solvents. The kinetic rate constants were critically analyzed through a derived diffusional model with Marcus theory to estimate E°'(X^•/-) in propylene carbonate, acetonitrile, butyronitrile, and dichloromethane. The approximations commonly used to determine diffusional rate constants in water gave rise to serious disagreements with the experiment, particularly in high-ionic-strength dichloromethane solutions, indicating the need to utilize the exact Debye expression. The Fuoss equation was adequate for determining photocatalyst-halide association constants with photocatalysts that possessed +2, +1, and 0 ionic charges. Similarly, the work term contribution in the classical Rehm-Weller expression, necessary for E°'(X^•/-) determination, accounted remarkably well for the stabilization of the charged reactants as the solution ionic strength was increased. While a sensitivity analysis indicated that the extracted reduction potentials were all within experimental error the same, use of fixed parameters established for aqueous solution provided the periodic trend expected, E°'(I^•/-) <E°'(Br^•/-) <E°'(Cl^•/-), in all of the organic solvents investigated; however, the potentials were more closely spaced than what would have been predicted based on gas-phase electron affinities or aqueous reduction potentials. The origin(s) of such behavior are discussed that provide new directions for future research.

Understanding the Role of Surface States on Mesoporous NiO Films

Surface states of mesoporous NiO semiconductor films have particular properties differing from the bulk and are able to dramatically influence the interfacial electron transfer and adsorption of chemical species. To achieve a better performance of NiO-based p-type dye-sensitized solar cells (p-DSCs), the function of the surface states has to be understood. In this paper, we applied a modified atomic layer deposition procedure that is able to passivate 72% of the surface states on NiO by depositing a monolayer of Al₂O₃. This provides us with representative control samples to study the functions of the surface states on NiO films. A main conclusion is that surface states, rather than the bulk, are mainly responsible for the conductivity in mesoporous NiO films. Furthermore, surface states significantly affect dye regeneration (with I^–/I₃^– as redox couple) and hole transport in NiO-based p-DSCs. A new dye regeneration mechanism is proposed in which electrons are transferred from reduced dye molecules to intra-bandgap states, and then to I₃^– species. The intra-bandgap states here act as catalysts to assist I₃^– reduction. A more complete mechanism is suggested to understand the particular hole transport behavior in p-DSCs, in which the hole transport time is independent of light intensity. This is ascribed to the percolation hole hopping on the surface states. When the concentration of surface states was significantly reduced, the light-independent charge transport behavior in pristine NiO-based p-DSCs transformed into having an exponential dependence on light intensity, similar to that observed in TiO₂-based n-type DSCs. These conclusions on the function of surface states provide new insight into the electronic properties of mesoporous NiO films.

Halide Photoredox Chemistry

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-to-ligand 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.

Excited-State Dynamics of [Ru(bpy)3 ]2+ Thin Films on Sensitized TiO2 and ZrO2

The excited state dynamics of Tris(2,2'-bipyridine)ruthenium(II) hexafluorophosphate, [Ru(bpy)3 (PF6 )2 ], was investigated on the surface of bare and sensitized TiO2 and ZrO2 films. The organic dyes LEG4 and MKA253 were selected as sensitizers. A Stern-Volmer plot of LEG4-sensitized TiO2 substrates with a spin-coated [Ru(bpy)3 (PF6 )2 ] layer on top shows considerable quenching of the emission of the latter. Interestingly, time-resolved emission spectroscopy reveals the presence of a fast-decay time component (25±5 ns), which is absent when the anatase TiO2 semiconductor is replaced by ZrO2 . It should be specified that the positive redox potential of the ruthenium complex prevents electron transfer from the [Ru(bpy)3 (PF6 )2 ] ground state into the oxidized sensitizer. Therefore, we speculate that the fast-decay time component observed stems from excited-state electron transfer from [Ru(bpy)3 (PF6 )2 ] to the oxidized sensitizer. Solid-state dye sensitized solar cells (ssDSSCs) employing MKA253 and LEG4 dyes, with [Ru(bpy)3 (PF6 )2 ] as a hole-transporting material (HTM), exhibit 1.2 % and 1.1 % power conversion efficiency, respectively. This result illustrates the possibility of the hypothesized excited-state electron transfer.

Visible Light Driven Bromide Oxidation and Ligand Substitution Photochemistry of a Ru Diimine Complex

The complex [Ru(deeb)(bpz)₂]²⁺ (RuBPZ²⁺, deeb = 4,4'-diethylester-2,2'-bipyridine, bpz = 2,2'-bipyrazine) forms a single ion pair with bromide, [RuBPZ²⁺, Br^-]⁺, with K_eq = 8400 ± 200 M^-1 in acetone. The RuBPZ²⁺ displayed photoluminescence (PL) at room temperature with a lifetime of 1.75 μs. The addition of bromide to a RuBPZ²⁺ 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 [RuBPZ²⁺, Br^-]⁺ and free RuBPZ²⁺, respectively. The single ion-pair [RuBPZ²⁺, Br^-]⁺* had a lifetime of 45 ± 5 ns, consistent with an electron transfer rate constant, k_et = (2.2 ± 0.3) × 10⁷ s^-1. In contrast, RuBPZ²⁺* was dynamically quenched by bromide with a quenching rate constant, k_q = (8.1 ± 0.1) × 10¹⁰ M^-1 s^-1. Nanosecond transient absorption revealed that both the static and dynamic pathways yielded RuBPZ⁺ and Br₂^•- products that underwent recombination to regenerate the ground state with a second-order rate constant, k_cr = (2.3 ± 0.5) × 10¹⁰ M^-1 s^-1. Kinetic analysis revealed that RuBPZ⁺ was a primary photoproduct, while Br₂^•- was secondary product formed by the reaction of a Br^• with Br^-, k = (1.1 ± 0.2) × 10¹⁰ M^-1 s^-1. Marcus theory afforded an estimate of the formal reduction potential for E⁰(Br^•/-) in acetone, 1.42 V vs NHE. A ¹H NMR analysis indicated that the ion-paired bromide was preferentially situated close to the Ru^II center. Prolonged steady state photolysis of RuBPZ²⁺ and bromide yielded two ligand-substituted photoproducts, cis- and trans-Ru(deeb)(bpz)Br₂. A photochemical intermediate, proposed to be [Ru(deeb)(bpz)(κ¹-bpz)(Br)]⁺, was found to absorb a second photon to yield cis- and trans-Ru(deeb)(bpz)Br₂ photoproducts.

Chloride Oxidation by Ruthenium Excited-States in Solution

Photodriven HCl splitting to produce solar fuels is an important goal that requires strong photo-oxidants capable of chloride oxidation. In a molecular approach toward this goal, three ruthenium compounds with 2,2'-bipyrazine backbones were found to oxidize chloride ions in acetone solution. Nanosecond transient absorption measurements provide compelling evidence for excited-state electron transfer from chloride to the Ru metal center with rate constants in excess of 10¹⁰ M^-1 s^-1. The Cl atom product was trapped with an olefin. This reactivity was promoted through pre-organization of ground-state precursors in ion pairs. Chloride oxidation with a tetra-cationic ruthenium complex was most favorable, as the dicationic complexes were susceptible to photochemical ligand loss. Marcus analysis afforded an estimate of the chlorine formal reduction potential E°(Cl^•/-) = 1.87 V vs NHE that is at least 300 meV more favorable than the accepted values in water.

Peripheral ligands as electron storage reservoirs and their role in enhancement of photocatalytic hydrogen generation

The contrasting early-time photodynamics of two related Ru/Pt photocatalysts are reported.

Role of the triiodide/iodide redox couple in dye regeneration in p-type dye-sensitized solar cells

A series of perylene dyes with different optical and electronic properties have been used as photosensitizers in NiO-based p-type dye-sensitized solar cells. A key target is to develop dyes that absorb light in the red to near-infrared region of the solar spectrum in order to match photoanodes optically in tandem devices; however, the photocurrent produced was found to decrease dramatically as the absorption maxima of the dye used was varied from 517 to 565 nm and varied strongly with the electrolyte solvent (acetonitrile, propionitrile, or propylene carbonate). To determine the limitations of the energy properties of the dye molecules and to provide guidelines for future sensitizer design, we have determined the redox potentials of the diiodide radical intermediate involved in the charge-transfer reactions in different solvents using photomodulated voltammetry. E°(I(3)(-)/I(2)(•-)) (V vs Fe(Cp)(2)(+/0)) = -0.64 for propylene carbonate, -0.82 for acetonitrile, and -0.87 for propionitrile. Inefficient regeneration of the sensitizer appears to be the efficiency-limiting step in the device, and the values presented here will be used to design more efficient dyes, with more cathodic reduction potentials, for photocathodes in tandem dye-sensitized solar cells.