Electronic tuning of metal hydrides enables precise control over potentials, mechanisms, selectivity, and rates of electrocatalytic reactions by regulating bond dissociation free energies such as the hydricity (.G degrees H-) and pKa of the catalyst. Here, we investigate a series of electronically tuned ruthenium hydrido complexes that are isostructural at the metal center: [Ru(4,4'-R-2-bpy)2(CO)H]+ (R = CF3, Cl, H, CH3, and CH3O; bpy = 2,2'bipyridine) (denoted as (R)Ru-H+). A substantial 22 kcal mol-1 hydricity range is available across five complexes in three stable oxidation states: (R)Ru-H+, (R)Ru-H0, and (R)Ru-H-. Thermodynamic and mechanistic predictions of electrocatalytic proton reduction were tested experimentally by reducing protons from weak acids to H2. Two mechanisms are observed, depending on the acid strength and the catalyst hydricity. The rate constants for hydride transfer and protonation of the catalyst were, in some cases, extracted from the analysis of cyclic voltammetry data. A key finding is a 400 mV decrease in the catalytic overpotential for H-2 production by using a doubly reduced electron-poor metal hydride instead of a singly reduced electron-rich metal hydride. The former also exhibits a higher rate constant for hydride transfer, representing a strategy to disconnect rate and free energy relationships.

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.

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.

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.

Stimuli-responsive particles have gained considerable interest in many fields of materials science. Among the various possible triggers, light is particularly advantageous due to its easy and efficient spatiotemporal control. In this work, we report the synthesis of a novel light-cleavable bis-alkoxysilane linker and its use for the preparation of mesoporous organosilica particles. We demonstrate that the resulting porous particles can be completely degraded upon exposure to UV light. These light-breakable nanocontainers can encapsulate a large variety of molecules, and they are able to quantitatively release their cargo upon light exposure. We proved the loading and release of a biologically important molecule, provitamin D3, upon UV-light irradiation.

This chapter addresses the main principles, challenges and achievements in ECL using metal complexes. Selected applications in diagnostics and biosensing are described.

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.

Two ruthenium compounds were anchored to mesoporous nanocrystalline TiO2 thin films to probe local electric fields generated by TiO2 reduction or cation adsorption in acetonitrile electrolytes. The metal-to-ligand charge-transfer (MLCT) excited states were well formulated as [Ru-III(dtb)(2)(dcb(-))](2+)* and [Ru-III(btfmb)(dcb)(btfmb(-))](2+)*, where dtb is 4,4'-(tert-butyl)(2)-2,2-bipyridine, dcb is 4,4'-(CO2H)(2)-2,2-bipyridine, and btfmb is 4,4'-(CF3)(2)-2,2-bipyridine. The MLCT excited state was orientated toward the TiO2 surface linker (antiparallel to the field) for Ru(dtb)(2)(dcb(-))*/TiO2 and away from the surface for Ru(btfmb)(dcb)(btfmb(-))*/TiO2. Reduction of the TiO2 in an electrochemical cell resulted in a blue shift of the Ru(dtb)(2)(dcb(-))*/TiO2 photoluminescence spectrum, while the Ru(btfmb)(dcb)(btfmb(-))*/TiO2 spectrum was either unchanged or was red-shifted. Accompanying these spectral shifts were bimodal changes in the PL intensity and excited-state lifetime that first increased and then decreased as the quasi-Fermi level of the TiO2 thin films was raised toward the vacuum level with a potentiostat. The initial increase was coincident with a lower excited state injection yield, while the origin(s) of the decrease observed at more negative applied potentials remains speculative. The potential dependence and magnitude of the spectral shifts and intensity/lifetime changes were acutely sensitive to the identity of the cations present in the acetonitrile electrolytes.

Correction for ‘Dye-sensitized electron transfer from TiO2 to oxidized triphenylamines that follows first-order kinetics’ by Brian N. DiMarco et al., Chem. Sci., 2018, DOI: 10.1039/c7sc03839a.

Mesoporous thin films comprised of similar to 15 nm diameter SnO2 nanocrystallites were synthesized and characterized in acetonitrile electrolytes by electrochemical and spectroscopic techniques. Spectroelectrochemical reduction of the thin films resulted in broad, non-superimposable UV/vis absorption changes. Simultaneous analysis of potential-dependent spectra, by a process termed "potential associated spectra", resulted in the identification of three unique absorption spectra for reduced SnO2, while only one spectrum was identified for TiO2. Reduction of SnO2 resulted in the appearance of (1) a broad absorption that spans across the visible and near-IR regions, (2) a blue-shifted fundamental absorption, and (3) an absorption band in the blue region. The absorption onsets were dependent on the electrolyte cation, present as the perchlorate salt of Li+, Na+, Mg2+, Ca2+, and TBA(+), where TBA(+) is tetrabutylammonium. Correlations between the charge within the thin film and the absorbance intensity revealed that significant charge was transferred to SnO2 films before significant visible color changes were observed. This suggested the presence of electrons within the SnO2 thin films that did not absorb visible light and were termed "phantom electrons".

Dye-sensitized solar cells combine the robustness of nanocrystalline TiO2 with molecular dyes or “sensitizers” that absorb broadly across the visible. The surface anchored sensitizer undergoes a photo-induced electron transfer to the TiO2 acceptor states and is subsequently regenerated by a redox mediator present in the external electrolyte. Efficient light-to-energy conversion requires that the injected electron be transported to the external circuit prior to recombination with the oxidized redox mediator. Despite the importance of this unwanted recombination reaction the mechanism remains elusive. This is due in part to complex kinetics that usually cannot be described with first- or second order kinetic models, but satisfactory fitting instead require sums of exponentials or stretched exponential functions. The non-exponential kinetics are likely due to transport of the injected electron and/or the electron acceptor prior to recombination. Here we report charge recombination studies of charge recombination to a family of oxidized amines that provide compelling evidence for first-order kinetics with abstracted rate constants that show a driving force dependence. The recombination reaction between nanocrystalline anatase TiO2 and a series of symmetrically substituted triphenylamine (TPA) redox mediators that possessed electron donating or withdrawing groups at the para-position of the phenyl rings was quantified on nanosecond and longer time scales. The groups utilized afforded an ~0.5 V change in TPA+/0 reduction potential. The nanocrystalline TiO2 thin films were sensitized by a ruthenium polypyridyl complex bearing either a carboxylic or a phosphonic acid functional group for surface anchoring. Pulsed light excitation of the film resulted in excited-state injection to yield TiO2(e-)|S+. Sensitizer regeneration and the subsequent charge recombination reaction TiO2(e-) + TPA+ → TiO2 + TPA were then monitored spectroscopically. Concentration dependent data revealed that recombination could be described by a distribution of first-order rate constants that became strictly first-order when the driving force was larger. The reaction rate increased with more positive TPA+/0 reduction potentials, consistent with electron transfer in the Marcus normal region. The recombination reaction was monitored as a function of temperature, and an Arrhenius analysis provided activation energies. Interestingly, the same activation barrier was abstracted for all the TPA derivatives, suggesting that a common rate limiting step exists for all these reactions. A model for recombination consistent with both the thermodynamics and activational parameters will be presented.

Three ruthenium(II) sensitizers, [Ru(L)(2)(dcb)](2+), were anchored to mesoporous TiO2 thin films where the ligand L = 4,4'-(CH3)(2)-bpy (dmb), 4,4'-(C(CH3)(3))(2)-bpy (dtb), and 4,4'-(CF3)(2)-bpy (bpyCF(3)) controls the thermodynamics and electronic coupling for self-exchange intermolecular Ru-III/II "hole hopping". Apparent electron difussion coefficients, D-app, were reported to increase in the order bpyCF(3) << dtb < dmb. Nanosecond transient absorption measurements made over an 80 degrees temperature range were conducted to abstract average charge recombination rate constants, k(cr), under conditions of sub-percolation and saturated sensitizer surface coverages. For sensitizers [Ru(dmb)(2)(dcb)](2+) and [Ru(dtb)(2)(dcb)](2+), the kcr values at saturation coverages were significantly larger than those at low coverages, by a degree that followed the trend in D-app. The inability of [Ru(bpyCF(3))(2)(dcb)](2+) to introduce hole transport was afirmed by recombination kinetic data that were insensitive to the sensitizer surface converage. An Arrhenius analysis indicated that lateral Ru-III/II hole hopping decreased the barrier for electron transfer that ultimately led to faster recombination rates.

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.

Self-exchange intermolecular Ru-III/II electron transfer, a process commonly referred to as "hole-hopping", is of great interest as it provides a means of charge transport across the surface of nanocrystalline (anatase) TiO2 mesoporous thin films without the loss of free energy. This process was characterized by cyclic voltammetry and chronoabsorptometry for three homologous Ru diimine compounds of the general form [Ru(LL)(2)(dcbH(2))](PF6)(2), where LL is 2,2'-bipyridine (bpy), 4,4'-dimethyl-2,2'-bipyridine (dmb), or 4,4'-di-tert-butyl-2,2'-bipyridine (dtb) and dcbH(2) is 2,2'-bipyridyl-4,4'-dicarboxylic acid. Apparent electron diffusion coefficients, D, abstracted from this data increased with dtb < bpy < dmb. Both techniques were consistent with this trend, despite differences in the magnitude of D between the two methods. Temperature dependent measurements revealed an activation barrier for electron self-exchange of 250 +/- 50 meV that was within this error the same for all three diimine compounds, suggesting the total reorganization energy, lambda, was also the same. Application of Marcus theory, with the assumption that the 900 +/- 100 meV total reorganization energy for self-exchange electron transfer was independent of the Ru compound, revealed that the electronic coupling matrix element, H-AB, followed the trend dtb (0.02 meV) < bpy (0.07 meV) < dmb (0.10 meV). The results indicate that insulating side groups placed on redox active molecules can be utilized to tune the electronic coupling and hence self-exchange rate constants without significantly altering the reorganization energy for electron transfer on TiO2 surfaces.

Mesoporous TiO2 thin films sensitized to visible light with [Ru(dtb)(2)(dcb)](2+), where dtb is 4,4'-((CH3)(3)C)(2)-2,2'-bipyridine and dcb is 4,4'-(CO2H)(2)-2,2'-bipyridine, abbreviated TiO2 vertical bar Ru, were investigated in acetonitrile electrolytes that contained 0.1 M Na+, Li+, Mg2+, or Ca2+ perchlorate solutions with the electron donor trip-tolylamine (TPTA) or phenothiazine (PTZ). Pulsed 532 nm light excitation of TiO2 vertical bar Ru with >25 mM donor concentrations quantitatively yielded TiO2(e(-))vertical bar Ru + D+, and the corresponding recombination reaction between the injected electrons and the oxidized donors was monitored on nanosecond and longer time scales. The recombination kinetics were nonexponential, but were well described by the Kohlrausch-Williams-Watts stretched exponential function. For the TPTA donor, the abstracted rate constants followed the trend Na+ > Li+ > Mg2+ > Ca2+. A similar trend was seen for PTZ, although no significant difference in the recombination kinetics for Na and Li was observed, Na+ similar to Li+ > Mg2+ > Ca2+. The origins of this cation dependence are discussed in terms of the thermodynamic driving force, the electron diffusion lengths of the injected electrons, and the electric field strength present.

The sensitizer [Ru(dtb)(2)(dcb)](2+), where dtb is 4,4'-di-tert-butyl-2,2'-bipyridine and dcb is 4,4'-dicarboxylic acid-2,2'-bipyridine, was anchored to mesoporous TiO2 thin films and characterized by visible spectroscopy in 0.1 M Na+, Li+, Mg2+, and Ca2+ perchlorate acetonitrile solutions on nanosecond and longer time scales. Relative to neat acetonitrile, the presence of these electrolyte cations induced a red shift in the metal-to-ligand charge transfer (MLCT) absorption of Ru(dtb)(2)(dcb)/TiO2. The magnitude of the shift increased with increasing valence of the metal cation. Pulsed 532 nm light excitation of Ru(dtb)(2)(dcb)/TiO2 resulted in the appearance of a long-lived bleach that returned to pre-excitation values on an approximately millisecond time scale under all conditions studied. Global analysis, spectral modeling, and single wavelength kinetic analysis revealed that two dynamic processes were operative: (1) charge recombination, Ru-III(dtb)(2)(dcb)/TiO2(e(-)) -> Ru-II(dtb)(2)(dcb)/TiO2, and (2) an electric field created by the injected electron. These two distinct nonexponential processes were observed in the same spectral region and on similar time scales. The ability of global analysis, specifically the decay-associated spectra, to kinetically and spectrally resolve these two processes was assessed. Single wavelength kinetic measurements and spectral modeling provided quantitatively different rate constants, but both led to the surprising conclusion that there was no evidence for charge screening of the electric field by cations present in the electrolyte. The decay of the electric field was cation independent, behavior very different from that previously reported in the presence of redox mediators. The charge recombination kinetics revealed a small yet measurable dependence on the nature of the cation present in the electrolyte with the divalent cations inducing the fastest recombination.

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.