Proton-Coupled Group Transfer Enables Concerted Protonation Pathways Relevant to Small-Molecule Activation.

The mechanistic identification of Nature's use of concerted reactions, in which all bond breaking and bond making occurs in a single step, has inspired rational designs for artificial synthetic transformations via pathways that bypass high-energy intermediates that would otherwise be thermodynamically and kinetically inaccessible. In this contribution we electrochemically activate an organometallic Ruthenium(II) complex to show that, in acetonitrile solutions, the movement of protons from weak Brønsted acids, such as water and methanol, is coupled with the transfer of its negatively charged counterpart to carbon dioxide (CO2)─a process termed proton-coupled group transfer─to stoichiometrically produce a metal-hydride complex and a carbonate species. These previously unidentified pathways have played key roles in CO2 and proton reduction catalysis by enabling the generation of key intermediates such as hydrides and metallocarboxylic acids, while their applicability to carbon acids may provide alternative approaches in the electrosynthesis of chemical commodities via alkylation and carboxylation reactions.

Structural and Electronic Influences on Rates of Tertpyridine-Amine CoIII -H Formation During Catalytic H2 Evolution in an Aqueous Environment.

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII -Cl]+ species results in the formation of [CoII (κ4 -L)(OH2 )]2+ . Further reduction produces [CoI (κ4 -L)(OH2 )]+ , which undergoes a rate-limiting structural rearrangement to [CoI (κ5 -L)]+ before being protonated to form [CoIII -H]2+ . The rate of [CoIII -H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII -H]2+ estimated from PR experiments, we found that while the protonation of [CoIII -H]2+ is unfavorable, [CoII -H]+ reacts with protons to produce H2 . The catalytic activity for H2 evolution tracks the hydricity of the [CoII -H]+ intermediate.

Efficiency Considerations for SnO2-Based Dye-Sensitized Solar Cells.

A comparative study of mesoporous thin films based on SnO2 (rutile) and TiO2 (anatase) nanocrystallites sensitized to visible light with [Ru(dtb)2(dcb)](PF6)2, where dtb = 4,4'-(tert-butyl)2-2,2'-bipyridine and dcb = 4,4'-(CO2H)2-2,2'-bipyridine, in CH3CN electrolyte solutions is reported to identify the reason(s) for the low efficiency of SnO2-based dye-sensitized solar cells (DSSCs). Pulsed laser excitation resulted in rapid excited state injection (kinj > 108 s-1) followed by sensitizer regeneration through iodide oxidation to yield an interfacial charge separated state abbreviated as MO2(e-)|Ru + I3-. Spectral features associated with I3- and the injected electron MO2(e-) were observed as well as a hypsochromic shift of the metal-to-ligand charge-transfer absorption of the sensitizer attributed to an electric field. The field magnitude ranged from 0.008 to 0.39 MV/cm and was dependent on the electrolyte cation (Mg2+ or Li+) as well as the oxide material. Average MO2(e-) + I3- → recombination rate constants quantified spectroscopically were about 25 times smaller for SnO2 (6.0 ± 0.14 s-1) than for TiO2 (160 ± 10 s-1). Transient photovoltage measurements of operational DSSCs indicated a 78 ms lifetime for electrons injected into SnO2 compared to 27 ms for TiO2; behavior that is at odds with the view that recombination with I3- underlies the low efficiencies of nanocrystalline SnO2-based DSSCs. In contrast, the average rate constant for charge recombination with the oxidized sensitizer, MO2(e-)|-S+ → MO2|-S, was about 2 orders of magnitude larger for SnO2 (k = 9.8 × 104 s-1) than for TiO2 (k = 1.6 × 103 s-1). Sensitizer regeneration through iodide oxidation were similar for both oxide materials (kreg = 6 ± 1 × 1010 M-1 s-1). The data indicate that enhanced efficiency from SnO2-based DSSCs can be achieved by identifying alternative redox mediators that enable rapid sensitizer regeneration and by inhibiting recombination of the injected electron with the oxidized sensitizer.

Correction: Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics.

[This corrects the article DOI: 10.1039/C7SC03839A.].

Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics.

Two sensitizers, [Ru(bpy)2(dcb)]2+ (RuC) and [Ru(bpy)2(dpb)]2+ (RuP), where bpy is 2,2'-bipyridine, dcb is 4,4'-dicarboxylic acid-2,2'-bipyridine and dpb is 4,4'-diphosphonic acid-2,2'-bipyridine, were anchored to mesoporous TiO2 thin films and utilized to sensitize the reaction of TiO2 electrons with oxidized triphenylamines, TiO2(e-) + TPA+ → TiO2 + TPA, to visible light in CH3CN electrolytes. A family of four symmetrically substituted triphenylamines (TPAs) with formal Eo(TPA+/0) reduction potentials that spanned a 0.5 eV range was investigated. Surprisingly, the reaction followed first-order kinetics for two TPAs that provided the largest thermodynamic driving force. Such first-order reactivity indicates a strong Coulombic interaction between TPA+ and TiO2 that enables the injected electron to tunnel back in one concerted step. The kinetics for the other TPA derivatives were non-exponential and were modelled with the Kohlrausch-William-Watts (KWW) function. A Perrin-like reaction sphere model is proposed to rationalize the kinetic data. The activation energies were the same for all of the TPAs, within experimental error. The average rate constants were found to increase with the thermodynamic driving force, consistent with electron transfer in the Marcus normal region.

Evidence that ΔS‡ Controls Interfacial Electron Transfer Dynamics from Anatase TiO2 to Molecular Acceptors.

Recombination of electrons injected into TiO2 with molecular acceptors present at the interface represents an important loss mechanism in dye-sensitized water oxidation and electrical power generation. Herein, the kinetics for this interfacial electron transfer reaction to oxidized triphenylamine (TPA) acceptors was quantified over a 70° temperature range for para-methyl-TPA (Me-TPA) dissolved in acetonitrile solution, 4-[N,N-di(p-tolyl)amino]benzylphosphonic acid (a-TPA) anchored to the TiO2, and a TPA covalently bound to a ruthenium sensitizer, [Ru(tpy-C6H4-PO3H2)(tpy-TPA)]2+ "RuTPA", where tpy is 2,2':6',2''-terpyridine. Activation energies extracted from an Arrhenius analysis were found to be 11 ± 1 kJ mol-1 for Me-TPA and 22 ± 1 kJ mol-1 for a-TPA, values that were insensitive to the identity of different sensitizers. Recombination to RuTPA+ proceeded with Ea = 27 ± 1 kJ mol-1 that decreased to 19 ± 1 kJ mol-1 when recombination occurred to an oxidized para-methoxy TPA (MeO-TPA) dissolved in CH3CN. Eyring analysis revealed a smaller entropy of activation |ΔS‡| when the a-TPA was anchored to the surface or covalently linked to the sensitizer, compared to that when Me-TPA was dissolved in CH3CN. In all cases, Eyring analysis provided large and negative ΔS‡ values that point toward unfavorable entropic factors as the key contributor to the barrier that underlies the slow recombination kinetics that are generally observed at dye-sensitized TiO2 interfaces.

Azadipyrromethene cyclometalation in neutral Ru(II) complexes: photosensitizers with extended near-infrared absorption for solar energy conversion applications.

In the on-going quest to harvest near-infrared (NIR) photons for energy conversion applications, a novel family of neutral ruthenium(ii) sensitizers has been developed by cyclometalation of an azadipyrromethene chromophore. These rare examples of neutral ruthenium complexes based on polypyridine ligands exhibit an impressive panchromaticity achieved by the cyclometalation strategy, with strong light absorption in the 600-800 nm range that tails beyond 1100 nm in the terpyridine-based adducts. Evaluation of the potential for Dye-Sensitized Solar Cells (DSSC) and Organic Photovoltaic (OPV) applications is made through rationalization of the structure-property relationship by spectroscopic, electrochemical, X-ray structural and computational modelization investigations. Spectroscopic evidence for photo-induced charge injection into the conduction band of TiO2 is also provided.

Excited state electron transfer after visible light absorption by the Co(I) state of vitamin B12.

The first example of excited state electron transfer from cob(I)alamin is reported herein. Vitamin B12 was anchored to a mesoporous TiO2 thin film and electrochemically reduced to the cob(I)alamin form. Pulsed laser excitation resulted in rapid excited state electron transfer, ket > 10(8) s(-1), followed by microsecond interfacial charge recombination to re-form cob(I)alamin. The supernucleophilic cob(I)alamin was found to be a potent photoreductant. The yield of excited state electron transfer was found to be excitation wavelength dependent. The implications of this dependence are discussed.