Periodic Trends in Intra-ionic Excited State Quenching by Halide.

The preassociation of reactants in a photoinitiated redox reaction through the use of noncovalent interactions can have a significant impact on excited state reactivity. As these noncovalent interactions render some stabilization to the associated species, they impact the kinetics and thermodynamics of photoinitiated electron transfer. Reported herein is a novel iridium(III) photocatalyst, equipped with an anion-sensitive, amide-substituted bipyridine ligand, and its reactivity with the halides (X = I-, Br-, Cl-) in acetonitrile and dichloromethane. A noteworthy periodic trend was observed, where the size and electron affinity dramatically altered the observed photoredox behavior. The binding affinity for the halides increased with decreasing ionic radius (Keq ∼103 to >106) in a polar medium but association was stoichiometric for each halide in a nonpolar medium. Evidence for the static quenching of iodide and bromide is presented while dynamic quenching was observed with all halides. These results highlight how the photophysics of halide adducts and the thermodynamics of intra-ionic photo-oxidation are impacted as a consequence of preassociation of a quencher through hydrogen bonding.

Factors that Impact Photochemical Cage Escape Yields.

The utilization of visible light to mediate chemical reactions in fluid solutions has applications that range from solar fuel production to medicine and organic synthesis. These reactions are typically initiated by electron transfer between a photoexcited dye molecule (a photosensitizer) and a redox-active quencher to yield radical pairs that are intimately associated within a solvent cage. Many of these radicals undergo rapid thermodynamically favored "geminate" recombination and do not diffuse out of the solvent cage that surrounds them. Those that do escape the cage are useful reagents that may undergo subsequent reactions important to the above-mentioned applications. The cage escape process and the factors that determine the yields remain poorly understood despite decades of research motivated by their practical and fundamental importance. Herein, state-of-the-art research on light-induced electron transfer and cage escape that has appeared since the seminal 1972 review by J. P. Lorand entitled "The Cage Effect" is reviewed. This review also provides some background for those new to the field and discusses the cage escape process of both homolytic bond photodissociation and bimolecular light induced electron transfer reactions. The review concludes with some key goals and directions for future research that promise to elevate this very vibrant field to even greater heights.

Reductive Dynamic and Static Excited State Quenching of a Homoleptic Ruthenium Complex Bearing Aldehyde Groups.

A new homoleptic Ru polypyridyl complex bearing two aldehyde groups on each bipyridine ligand, [Ru(dab)3](PF6)2, where dab is 4,4'-dicarbaldehyde-2,2'-bipyridine, was synthesized, characterized, and utilized for iodide photo-oxidation studies. In acetonitrile (CH3CN) solution, the complex displayed an intense metal-to-ligand charge transfer (MLCT) absorbance maximum at 475 nm (ε = 22,000 M-1 cm-1) and an infrared (IR) band at 1712 cm-1 assigned to the pendent aldehyde groups. Visible light excitation in air-saturated solution resulted in room temperature photoluminescence (PL) with a maximum at 675 nm, a quantum yield, ϕPL = 0.048, and an excited state lifetime, το = 440 ns, from which radiative and nonradiative relaxation rate constants were extracted, kr = 9.1 × 104 s-1 and knr = 1.8 × 106 s-1. Pulsed visible light excitation yielded transient UV-vis and IR absorption spectra consistent with an MLCT excited state; relaxation occurred with the maintenance of two isosbestic points in the visible region, and a lifetime that agreed with that measured by time-resolved PL. Cyclic voltammetry studies in a CH3CN solution with 0.1 M TBAPF6 electrolyte revealed a quasi-reversible oxidation, E°(RuIII/II) = +1.25 V vs. Fc+/0, and three sequential one-electron reductions at -1.10, -1.25, and -1.54 V vs. Fc+/0. An excited state reduction potential of E°(Ru*2+/+) = +0.89 V vs. Fc+/0 was estimated with the Rehm-Weller expression. Titration of tetrabutylammonium iodide, TBAI, into a CD3CN solution of [Ru(dab)3](PF6)2 resulted in significant shifts in the aldehyde H atom and 3,3'-biypridyl resonances that were analyzed with a 1:1 equilibrium model, from which Keq = 460 M-1 was extracted, increasing to 5800 M-1 when the solvent was changed to acetone-d6. Iodide titrations resulted in a significant quenching of the [Ru(dab)3]*2+ lifetime and quantum yield in both CH3CN and acetone solvents. In CH3CN, the quenching was mainly dynamic and well described by the Stern-Volmer model, from which a quenching rate constant, kq, of 4.5 × 1010 M-1 s-1 and an equilibrium constant, Keq, of 8.3 × 103 M-1 were obtained. In acetone, the static quenching pathway by iodide was greatly enhanced, with a Keq of 1.2 × 104 M-1 and a higher kq of 9.2 × 1010 M-1 s-1.

Impact of Diimine Ligand on Singlet-to-Triplet Charge Transfer Electroabsorption in Iron and Ruthenium Complexes.

The electroabsorption and absorption spectra of eight homoleptic complexes of the general form [M(LL)3]2+ where M = Ru, Fe, and LL = 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and 4,4',-(R)2-bpy where R = -OCH3, -CF3, were quantified at 77 K in a butyronitrile glass. Intense metal-to-ligand charge transfer (MLCT) absorption bands were evident in the visible region. Electroabsorption spectra measured with applied electric fields >0.2 MV/cm were analyzed by the two-state Liptay model. Significant light-induced dipole moment changes of Δµâ‡€ = 4-13 D were found consistent with a metal-to-ligand charge transfer (MLCT) excited state comprised an electron localized on a single diimine ligand, [MIII(LL-)(LL)2]*2+, in the initially formed Franck-Condon excited state. A low energy feature evident in the electroabsorption spectra was assigned to a direct singlet-to-triplet MLCT excited state. The identity of the diimine ligand had an unexpected and large impact on these transitions. Analysis relative to the higher energy absorption provides a comparison of spin-allowed and disallowed transitions for first- and second-row transition metal complexes. With the notable exception of [Fe(CF3bpy)3]2+, the change in dipole moment for the 3MLCT excited states was less than or equal to that of the 1MLCT excited states. The charge transfer distances for the iron complexes were generally larger than those for the Ru complexes, a behavior attributed to a smaller degree of iron-diimine coupling in the ground state. A striking result was the sensitivity of the extinction coefficient and spectral profile of the low energy electroabsorption assigned to the identity of the diimine ligand; data that suggests electronic coupling with ligand localized triplet states and high spin metal centered states must be considered when modeling the Franck-Condon excited state.

Reduction of CO to Methanol with Recyclable Organic Hydrides.

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP+.

Direct Evidence for a Sequential Electron Transfer-Proton Transfer Mechanism in the PCET Reduction of a Metal Hydroxide Catalyst.

The proton-coupled electron transfer (PCET) mechanism for the reaction Mox-OH + e- + H+ → Mred-OH2 was determined through the kinetic resolution of the independent electron transfer (ET) and proton transfer (PT) steps. The reaction of interest was triggered by visible light excitation of [RuII(tpy)(bpy')H2O]2+, RuII-OH2, where tpy is 2,2':6',2″-terpyridine and bpy' is 4,4'-diaminopropylsilatrane-2,2'-bipyridine, anchored to In2O3:Sn (ITO) thin films in aqueous solutions. Interfacial kinetics for the PCET reduction reaction were quantified by nanosecond transient absorption spectroscopy as a function of solution pH and applied potential. Data acquired at pH = 5-10 revealed a stepwise electron transfer-proton transfer (ET-PT) mechanism, while kinetic measurements made below pKa(RuIII-OH/OH2) = 1.3 were used to study the analogous interfacial reaction, where electron transfer was the only mechanistic step. Analysis of this data with a recently reported multichannel kinetic model was used to construct a PCET zone diagram and supported the assignment of an ET-PT mechanism at pH = 5-10. Ultimately, this study represents a unique example among Mox-OH/Mred-OH2 reactivity where the protonation and oxidation states of the intermediate were kinetically and spectrally resolved to firmly establish the PCET mechanism.

Tailoring Interfaces for Enhanced Methanol Production from Photoelectrochemical CO2 Reduction.

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor-catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving force for CO2 reduction. These structures together create a unique microenvironment for effective reduction of CO2 to methanol, leading to a remarkable Faradaic efficiency reaching 20% together with a partial current density of 3.4 mA cm-2, surpassing the previous record based on planar silicon photoelectrodes by a notable factor of 17. This work demonstrates a new pathway for enhancing photoelectrocatalytic CO2 reduction through meticulous interface and microenvironment tailoring and sets a benchmark for both Faradaic efficiency and current density in solar liquid fuel production.

General Kinetic Model for pH Dependence of Proton-Coupled Electron Transfer: Application to an Electrochemical Water Oxidation System.

The pH dependence of proton-coupled electron transfer (PCET) reactions, which are critical to many chemical and biological processes, is a powerful probe for elucidating their fundamental mechanisms. Herein, a general, multichannel kinetic model is introduced to describe the pH dependence of both homogeneous and electrochemical PCET reactions. According to this model, a weak pH dependence can arise from the competition among multiple sequential and concerted PCET channels involving different forms of the redox species, such as protonated and deprotonated forms, as well as different proton donors and acceptors. The contribution of each channel is influenced by the relative populations of the reactant species, which often depend strongly on pH, leading to complex pH dependence of PCET apparent rate constants. This model is used to explain the origins of the experimentally observed weak pH dependence of the electrochemical PCET apparent rate constant for a ruthenium-based water oxidation catalyst attached to a tin-doped In2O3 (ITO) surface. The weak pH dependence is found to arise from the intrinsic differences in the rate constants of participating channels and the dependence of their relative contributions on pH. This model predicts that the apparent maximum rate constant will become pH-independent at higher pH, which is confirmed by experimental measurements. Our analysis also suggests that the dominant channels are electron transfer at lower pH and sequential PCET via electron transfer followed by fast proton transfer at higher pH. This work highlights the importance of considering multiple competing channels simultaneously for PCET processes.

Nanosecond Intra-Ionic Chloride Photo-Oxidation.

Transition-metal photocatalysts capable of oxidizing chloride are rare yet serve as an attractive means to controllably generate chlorine atoms, which have continued to garner the interest of researchers for notable applications in photoredox catalysis and solar energy storage. Herein, a new series of four Ir-photocatalysts with different dicationic chloride-sequestering ligands were synthesized and characterized to probe the relationship between chloride binding affinities, ion pair solution structures, and rate constants for chloride photo-oxidation in acetonitrile at room temperature. The substituents on the quaternary amines of dicationic bipyridine ligands had negligible effects on the photocatalyst excited-state reduction potential, yet dramatically influenced the affinity for chloride binding, indicating that synthetic design can be utilized to independently tune these important properties. An inverse correlation was observed between the equilibrium constant for chloride ion pairing and the rate constant for intra-ionic chloride oxidation. Exceptions to this trend suggest structural differences in the ion-paired solution structures, which were probed by 1H NMR binding experiments. This study provides new insights into light-induced oxidation of ion-paired substrates, a burgeoning approach that offers to circumvent diffusional constraints of photocatalysts with short excited-state lifetimes. Ground-state association of chloride with these photocatalysts enables intra-ionic chloride oxidation on a rapid nanosecond timescale.

Impact of Molecular Orientation on Lateral and Interfacial Electron Transfer at Oxide Interfaces.

Molecular dyes, called sensitizers, with a cis-[Ru(LL)(dcb)(NCS)2] structure, where dcb is 4,4'-(CO2H)2-2,2'-bipyridine and LL is dcb or a different diimine ligand, are among the most optimal for application in dye-sensitized solar cells (DSSCs). Herein, a series of five sensitizers, three bearing two dcb ligands and two bearing one dcb ligand, were anchored to mesoporous thin films of conducting tin-doped indium oxide (ITO) or semiconducting TiO2 nanocrystallites. The number of dcb ligands impacts the surface orientation of the sensitizer; density functional theory (DFT) calculations revealed an ∼1.6 Å smaller distance between the oxide surface and the Ru metal center for sensitizers with two dcb ligands. Interfacial electron transfer kinetics from the oxide material to the oxidized sensitizer were measured as a function of the thermodynamic driving force. Analysis of the kinetic data with Marcus-Gerischer theory indicated that the electron coupling matrix element, Hab, was sensitive to distance and ranged from Hab = 0.23 to 0.70 cm-1, indicative of nonadiabatic electron transfer. The reorganization energies, λ, were also sensitive to the sensitizer location within the electric double layer and were smaller, with one exception, for sensitizers bearing two dcb ligands λ = 0.40-0.55 eV relative to those with one λ = 0.63-0.66 eV, in agreement with dielectric continuum theory. Electron transfer from the oxide to the photoexcited sensitizer was observed when the diimine ligand was more easily reduced than the dcb ligand. Lateral self-exchange "hole hopping" electron transfer between surface-anchored sensitizers was found to be absent for sensitizers with two dcb ligands, while those with only one were found to hop with rates similar to those previously reported in the literature, khh = 47-89 µs-1. Collectively, the kinetic data and analysis reveal that interfacial kinetics are highly sensitive to the surface orientation and sensitizers bearing two dcb ligands are most optimal for practical applications of DSSCs.

Multi-Electron Transfer at H-Terminated p-Si Electrolyte Interfaces: Large Photovoltages under Inversion Conditions.

Photovoltages for hydrogen-terminated p-Si(111) in an acetonitrile electrolyte were quantified with methyl viologen [1,1'-(CH3)2-4,4'-bipyridinium](PF6)2, abbreviated MV2+, and [Ru(bpy)3](PF6)2, where bpy is 2,2'-bipyridine, that respectively undergo two and three one-electron transfer reductions. The reduction potentials, E°, of the two MV2+ reductions occurred at energies within the forbidden bandgap, while the three [Ru(bpy)3]2+ reductions occurred within the continuum of conduction band states. Bandgap illumination resulted in reduction that was more positive than that measured with a degenerately doped n+-Si demonstrative of a photovoltage, Vph, that increased in the order MV2+/+ (260 mV) < MV+/0 (400 mV) < Ru2+/+ (530 mV) ∼ Ru+/0 (540 mV) ∼ Ru0/- (550 mV). Pulsed 532 nm excitation generated electron-hole pairs whose dynamics were nearly constant under depletion conditions and increased markedly as the potential was raised or lowered. A long wavelength absorption feature assigned to conduction band electrons provided additional evidence for the presence of an inversion layer. Collectively, the data reveal that the most optimal photovoltage, as well as the longest electron-hole pair lifetime and the highest surface electron concentration, occurs when E° lies energetically within the unfilled conduction band states where an inversion layer is present. The bell-shaped dependence for electron-hole pair recombination with the surface potential was predicted by the time-honored SRH model, providing a clear indication that this interface provides access to all four bias conditions, i.e., accumulation, flat band, depletion, and inversion. The implications of these findings for photocatalysis applications and solar energy conversion are discussed.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands.

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)5Cl to form complexes of the type Re(bpy)(CO)3Cl, which are related to known catalysts for CO2 reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO2 on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO2 reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

A quantitative model of charge injection by ruthenium chromophores connecting femtosecond to continuous irradiance conditions.

A kinetic framework for the ultrafast photophysics of tris(2,2-bipyridine)ruthenium(II) phosphonated and methyl-phosphonated derivatives is used as a basis for modeling charge injection by ruthenium dyes into a semiconductor substrate. By including the effects of light scattering, dye diffusion, and adsorption kinetics during sample preparation and the optical response of oxidized dyes, quantitative agreement with multiple transient absorption datasets is achieved on timescales spanning femtoseconds to nanoseconds. In particular, quantitative agreement with important spectroscopic handles-the decay of an excited state absorption signal component associated with charge injection in the UV region of the spectrum and the dynamical redshift of a ∼500 nm isosbestic point-validates our kinetic model. Pseudo-first-order rate coefficients for charge injection are estimated in this work, with an order of magnitude ranging from 1011 to 1012 s-1. The model makes the minimalist assumption that all excited states of a particular dye have the same charge injection coefficient, an assumption that would benefit from additional theoretical and experimental exploration. We have adapted this kinetic model to predict charge injection under continuous solar irradiation and find that as many as 68 electron transfer events per dye per second take place, significantly more than prior estimates in the literature.

Reorganization Energies for Interfacial Proton-Coupled Electron Transfer to a Water Oxidation Catalyst.

The reorganization energy (λ) for interfacial electron transfer (ET) and proton-coupled ET (PCET) from a conductive metal oxide (In2O3:Sn, ITO) to a surface-bound water oxidation catalyst was extracted from kinetic data measured as a function of the thermodynamic driving force. Visible light excitation resulted in rapid excited-state injection (kinj > 108 s-1) to the ITO, which photo-initiated the two interfacial reactions of interest. The rate constants for both reactions increased with the driving force, -ΔG°, to a saturating limit, kmax, with rate constants consistently larger for ET than for PCET. Marcus-Gerischer analysis of the kinetic data provided the reorganization energy for interfacial PCET (0.90 ± 0.02 eV) and ET (0.40 ± 0.02 eV), respectively. The magnitude of kmax for PCET was found to decrease with pH, behavior that was absent for ET. Both the decrease in kmax and the larger reorganization energy for an unwanted competing PCET reaction from the ITO to the oxidized catalyst showcases a significant kinetic advantage for driving solar water oxidation at high pH. Computational analysis revealed a larger inner-sphere reorganization energy contribution for PCET than for ET arising from a more significant change in the Ru-O bond length for the PCET reaction. Extending the Marcus-Gerischer theory to PCET by including the excited electron-proton vibronic states and the proton donor-acceptor motion provided an apparent reorganization energy of 1.01 eV. This study demonstrates that the Marcus-Gerischer theory initially developed for ET can be reliably extended to PCET for quantifying and interpreting reorganization energies observed experimentally.

Chloride Oxidation by One- or Two-Photon Excitation of N-Phenylphenothiazine.

Chloride oxidation has tremendous utility in the burgeoning field of chlorine-mediated C-H activation, yet it remains a challenging process to initiate with light because of the exceedingly positive one-electron reduction potential, E° (Cl•/-), beyond most common transition-metal photooxidants. Herein, two photocatalytic chloride oxidation pathways that involve either one- or consecutive two-photon excitation of N-phenylphenothiazine (PTH) are presented. The one-photon pathway generates PTH•+ by oxidative quenching that subsequently disproportionates to yield PTH2+ that oxidizes chloride; this pathway is also accessed by the electrochemical oxidation of PTH. The two-photon pathway, which proceeded through the radical cation excited state, 2PTH•+*, was of particular interest as this super-photooxidant was capable of directly oxidizing chloride to chlorine atoms. Laser flash photolysis revealed that the photooxidation by the doublet excited state proceeded on a subnanosecond timescale through a static quenching mechanism with an ion-pairing equilibrium constant of 0.36 M-1. The PTH photoredox chemistry was quantified spectroscopically on nanosecond and longer time scales, and chloride oxidation chemistry was revealed by reactivity studies with model organic substrates. One- and two-photon excitation of PTH enabled chlorination of unactivated C(sp3)-H bonds of organic compounds such as cyclohexane with substantial yield enhancement observed from inclusion of the second excitation wavelength. This study provides new mechanistic insights into chloride oxidation catalyzed by an inexpensive and commercially available organic photooxidant.

Reorganization Energies for Interfacial Electron Transfer across Phenylene Ethynylene Rigid-Rod Bridges.

A family of three ruthenium bipyridyl rigid-rod compounds of the general form [Ru(bpy)2(LL)](PF6)2 were anchored to mesoporous thin films of tin-doped indium oxide (ITO) nanocrystals. Here, LL is a 4-substituted 2,2-bipyridine (bpy) ligand with varying numbers of conjugated phenylenethynylene bridge units between the bipyridine ring and anchoring group consisting of a bis-carboxylated isophthalic group. The visible absorption spectra and the formal potentials, Eo(RuIII/II), of the surface anchored rigid-rods were insensitive to the presence of the phenylene ethynylene bridge units in 0.1 M tetrabutyl ammonium perchlorate acetonitrile solutions (TBAClO4/CH3CN). The conductive nature of the ITO enabled potentiostatic control of the Fermi level and hence a means to tune the Gibbs free energy change, -ΔG°, for electron transfer from the ITO to the rigid-rods. Pseudo-rate constants for this electron transfer reaction increased as the number of bridge units decreased at a fixed -ΔG°. With the assumption that the reorganization energy, λ, and the electronic coupling matrix element, Hab, were independent of the applied potential, rate constants measured as a function of -ΔG° and analyzed through Marcus-Gerischer theory provided estimates of Hab and λ. In rough accordance with the dielectric continuum theory, λ was found to increase from 0.61 to 0.80 eV as the number of bridge units was increased. In contrast, Hab decreased markedly with distance from 0.54 to 0.11 cm-1, consistent with non-adiabatic electron transfer. Comparative analysis with previously published studies of bridges with an sp3-hybridized carbon indicated that the phenylene ethynylene bridge does not enhance electronic coupling between the oxide and the rigid-rod acceptor. The implications of these findings for practical applications in solar energy conversion are specifically discussed.

Resolving Halide Ion Stabilization through Kinetically Competitive Electron Transfers.

Stabilization of ions and radicals often determines reaction kinetics and thermodynamics, but experimental determination of the stabilization magnitude remains difficult, especially when the species is short-lived. Herein, a competitive kinetic approach to quantify the stabilization of a halide ion toward oxidation imparted by specific stabilizing groups relative to a solvated halide ion is reported. This approach provides the increase in the formal reduction potential, ΔE°'(Χ•/-), where X = Br and I, that results from the noncovalent interaction with stabilizing groups. The [Ir(dF-(CF3)-ppy)2(tmam)]3+ photocatalyst features a dicationic ligand tmam [4,4'-bis[(trimethylamino)methyl]-2,2'-bipyridine]2+ that is shown by 1H NMR spectroscopy to associate a single halide ion, K eq = 7 × 104 M-1 (Br-) and K eq = 1 × 104 M-1 (I-). Light excitation of the photocatalyst in halide-containing acetonitrile solutions results in competitive quenching by the stabilized halide and the more easily oxidized diffusing halide ion. Marcus theory is used to relate the rate constants to the electron-transfer driving forces for oxidation of the stabilized and unstabilized halide, the difference of which provides the increase in reduction potentials of ΔE°'(Br•/-) = 150 ± 24 meV and ΔE°'(I•/-) = 67 ± 13 meV. The data reveal that K eq is a poor indicator of these reduction potential shifts. Furthermore, the historic and widely used assumption that Coulombic interactions alone are responsible for stabilization must be reconsidered, at least for polarizable halogens.

Visible Light Generation of a Microsecond Long-Lived Potent Reducing Agent.

Photoexcitation of molecular radicals can produce strong reducing agents; however, the limited lifetimes of the doublet excited states preclude many applications. Herein, we propose and demonstrate a general strategy to translate a highly energetic electron from a doublet excited state to a ZrO2 insulator, thereby increasing the lifetime by about 6 orders of magnitude while maintaining a reducing potential less than -2.4 V vs SCE. Specifically, red light excitation of a salicylic acid modified perylene diimide radical anion PDI•- anchored to a ZrO2 insulator yields a ZrO2(e-)|PDI charge separated state with an ∼10 µs lifetime in 23% yield. The ZrO2(e-)s were shown to drive CO2 → CO reduction with a Re catalyst present in micromolar concentrations. More broadly, this strategy provides new opportunities to reduce important reagents and catalysts at low concentrations through diffusional electron transfer.

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•/-)

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light.

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)2]+ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)3]2+. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.