Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates.

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

Monitoring SEIRAS on a Graphitic Electrode for Surface-Sensitive Electrochemistry: Real-Time Electrografting.

The ubiquity of graphitic materials in electrochemistry makes it highly desirable to probe their interfacial behavior under electrochemical control. Probing the dynamics of molecules at the electrode/electrolyte interface is possible through spectroelectrochemical approaches involving surface-enhanced infrared absorption spectroscopy (SEIRAS). Usually, this technique can only be done on plasmonic metals such as gold or carbon nanoribbons, but a more convenient substrate for carbon electrochemical studies is needed. Here, we expanded the scope of SEIRAS by introducing a robust hybrid graphene-on-gold substrate, where we monitored electrografting processes occurring at the graphene/electrolyte interface. These electrodes consist of graphene deposited onto a roughened gold-sputtered internal reflection element (IRE) for attenuated total reflectance (ATR) SEIRAS. The capabilities of the graphene-gold IRE were demonstrated by successfully monitoring the electrografting of 4-amino-2,2,6,6-tetramethyl-1-piperidine N-oxyl (4-amino-TEMPO) and 4-nitrobenzene diazonium (4-NBD) in real time. These grafts were characterized using cyclic voltammetry and ATR-SEIRAS, clearly showing the 1520 and 1350 cm-1 NO2 stretches for 4-NBD and the 1240 cm-1 C-C, C-C-H, and N-È® stretch for 4-amino-TEMPO. Successful grafts on graphene did not show the SEIRAS effect, while grafting on gold was not stable for TEMPO and had poorer resolution than on graphene-gold for 4-NBD, highlighting the uniqueness of our approach. The graphene-gold IRE is proficient at resolving the spectral responses of redox transformations, unambiguously demonstrating the real-time detection of surface processes on a graphitic electrode. This work provides ample future directions for real-time spectroelectrochemical investigations of carbon electrodes used for sensing, energy storage, electrocatalysis, and environmental applications.

Calibrating the Oxidative Capacity of Microdroplets.

Recently, redox chemical transformations have been reported to occur spontaneously in microdroplets. The origins of such novel reactivity are still debated, and any systematic correlation of the oxidative/reductive yield with the reactivity of the reactant is yet to be established. Towards this end, we report the simple, outer-sphere, one-electron oxidation of a series of ferrocene derivatives spanning a range of oxidation potentials from -0.1 V to +0.8 V vs. Ag/AgCl in acetonitrile microdroplets generated via nebulization and measured by mass spectrometry of the corresponding ferrocenium ions. The reaction environments and dynamics in the droplets are complex, and it is still unclear whether such reactivity correlates with any bulk thermodynamic values. Our key finding is that the ion yields decrease monotonically with the oxidation potential of the ferrocenes, which is a thermodynamic quantity. The ion yields emphatically do not obey the Nernstian ratio, revealing the redox processes in the droplets do not follow the assumptions of bulk steady-state electrochemistry. Furthermore, oxidative competition in the mixture of several ferrocenes suggest a finite oxidative capacity or oxidant concentration. These results demonstrate that even though ion generation could be an out-of-equilibrium and kinetically limited process, the oxidative yield in microdroplets does correlate with thermodynamics, suggesting a possible free energy relationship between the kinetics and thermodynamics of the process.

Electro-inductive Effect Dominates Vibrational Frequency Shifts of Conjugated Probes on Gold Electrodes.

Interfacial electric fields play a critical role in electrocatalysis and are often characterized by using vibrational probes attached to an electrode surface. Understanding the physical principles dictating the impact of the applied electrode potential on the vibrational probe frequency is important. Herein, a comparative study is performed for two molecular probes attached to a gold electrode. Both probes contain a nitrile (CN) group, but 4-mercaptobenzonitrile (4-MBN) exhibits continuous conjugation from the electrode through the nitrile group, whereas this conjugation is interrupted for 2-(4-mercaptophenyl)acetonitrile (4-MPCN). Periodic density functional theory calculations predict that the CN vibrational frequency shift of the 4-MBN system is dominated by induction, which is a through-bond polarization effect, leading to a strong potential dependence that does not depend significantly on the orientation of the CN bond relative to the surface. In contrast, the CN vibrational frequency shift of the 4-MPCN system is influenced less by induction and more by through-space electric field effects, leading to a weaker potential dependence and a greater orientation dependence. These theoretical predictions were confirmed by surface-enhanced Raman spectroscopy experiments. Balancing through-bond and through-space electrostatic effects may assist in the fundamental understanding and design of electrocatalytic systems.

Controlling Water Delivery to an Electrochemical Interface with Surfactants.

Most electrochemical reactions require delivery of protons, often from water, to surface-adsorbed species. However, water also acts as a competitor to many such processes by directly reacting with the electrode, which necessitates using water in small amounts. Controlling the water content and structure near the surface is an important frontier in directing the reactivity and selectivity of electrochemical reactions. Surfactants accumulate near surfaces, and therefore, they can be used as agents to control interfacial water. Using mid-IR spectro-electrochemistry, we show that a modest concentration (1 mM) of the cationic surfactant CTAB in mixtures of 10 M water in an organic solvent (dDMSO) has a large effect on the interfacial water concentration, changing it by up to ∼35% in the presence of an applied potential. The major cause of water content change is displacement due to the accumulation or depletion of surfactants driven by potential. Two forces drive the surfactants to the electrode: the applied potential and the hydrophobic interactions with the water in the bulk. We have quantified their competition by varying the water content in the bulk. To our knowledge, for the first time, we have identified the electrochemical equivalent of the hydrophobic drive. For our system, a change in applied potential of 1 V has the same effect as adding a 0.55 mole fraction of water to the bulk. This work illustrates the significance of surfactants in the partitioning of water between the bulk and the surface and paves the way toward engineering interfacial water structures for controlling electrochemical reactions.

Inductive Effect Alone Cannot Explain Lewis Adduct Formation and Dissociation at Electrode Interfaces.

Understanding breaking and formation of Lewis bonds at an electrified interface is relevant to a large range of phenomena, including electrocatalysis and electroadsorption. The complexities of interfacial environments and associated reactions often impede a systematic understanding of this type of bond at interfaces. To address this challenge, we report the creation of a main group classic Lewis acid-base adduct on an electrode surface and its behavior under varying electrode potentials. The Lewis base is a self-assembled monolayer of mercaptopyridine and the Lewis acid is BF3, forming a Lewis bond between nitrogen and boron. The bond is stable at positive potentials but cleaves at potentials more negative of approximately -0.3 V vs Ag/AgCl without an associated current. We also show that if the Lewis acid BF3 is supplied from a reservoir of Li+BF4- electrolyte, the cleavage is completely reversible. We propose that the N-B Lewis bond is affected both by the field-induced intramolecular polarization (electroinduction) and by the ionic structures and ionic equilibria near the electrode. Our results indicate that the second effect is responsible for the Lewis bond cleavage at negative potentials. This work is relevant to understanding the fundamentals of electrocatalytic and electroadsorption processes.

Direct Determination of Plasmon Enhancement Factor and Penetration Depths in Surface Enhanced IR Absorption Spectroscopy.

Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) is a powerful tool for studying a wide range of surface and electrochemical phenomena. For most electrochemical experiments the evanescent field of an IR beam partially penetrates through a thin metal electrode deposited on top of an attenuated total reflection (ATR) crystal to interact with molecules of interest. Despite its success, a major problem that complicates quantitative interpretation of the spectra from this method is the ambiguity of the enhancement factor due to plasmon effects in metals. We developed a systematic method for measuring this, which relies upon independent determination of surface coverage by Coulometry of a surface-bound redox-active species. Following that, we measure the SEIRAS spectrum of the surface bound species, and from the knowledge of surface coverage, retrieve the effective molar absorptivity, εSEIRAS. Comparing this to the independently determined bulk molar absorptivity leads us to the enhancement factor f = εSEIRAS/εbulk. We report enhancement factors in excess of 1000 for the C-H stretches of surface bound ferrocene molecules. We additionally developed a methodical approach to measure the penetration depth of the evanescent field from the metal electrode into a thin film. Such systematic measure of the enhancement factor and penetration depth will help SEIRAS advance from a qualitative to a more quantitative method.

Inferring the Energetics of CO2-Aniline Adduct Formation from Vibrational Spectroscopy.

Control of atmospheric CO2 is an important contemporary scientific and engineering challenge. Toward this goal, the reaction of CO2 with amines to form carbamate bonds is an established method for CO2 capture. However, controllable reversal of this reaction remains difficult and requires tuning the energetics of the carbamate bond. Through IR spectroscopy, we show that a characteristic frequency observed upon carbamate formation varies as a function of the substituent's Hammett parameter for a family of para-substituted anilines. We present computational evidence that the vibrational frequency of the adducted CO2 serves as a predictor of the energy of formation of the carbamate. Electron donating groups typically enhance the driving force of carbamate formation by transferring more charge to the adducted CO2 and thus increasing the occupancy of the antibonding orbital in the carbon-oxygen bonds. Increased occupancy of the antibonding orbital within adducted CO2 indicates a weaker bond, leading to a red-shift in the characteristic carbamate frequency. Our work serves the large field of CO2 capture research where spectroscopic observables, such as IR frequencies, are more easily obtainable and can stand in as a descriptor of driving forces.

Electrode-Potential-Driven Dissociation of N-Heterocycle/BF3 Adducts: A Possible Manifestation of the Electro-Inductive Effect.

Recently, non-Faradaic effects were used to modify the electronic structure and reactivity of electrode-bound species. We hypothesize that these electrostatic perturbations could influence the chemical reactivity of electrolyte species near an electrode in the absence of Faradaic electron transfer. A prime example of non-Faradaic effects is acid-base dissociation near an interface. Here, we probed the near-electrode dissociation of N-heterocycle-BF3 Lewis adducts upon electrode polarization, well outside of the redox potential window of the adducts. Using scanning electrochemical microscopy and confocal fluorescence spectroscopy, we detected a potential-dependent depletion of the adduct near the electrode. We propose an electro-inductive effect where a more positive potential leads to electron withdrawal on the N-heterocycle. This study takes a step forward in the use of electrostatics at electrochemical interfaces for field-driven electrocatalytic and electro-synthetic processes.

Can Brønsted Photobases Act as Lewis Photobases?

Dative bonding or Lewis acid-base chemistry underpins a large number of chemical phenomena in a variety of fields, such as catalysis, metal-ligand interactions, and surface chemistry. Developing light-controlled Lewis acid-base interactions could offer a new way of controlling and understanding such phenomena. Photoinduced proton transfer, that is, excited-state Brønsted acidity and basicity, has been extensively studied and applied. Here, in direct analogy to excited-state Brønsted basicity, we show that exciting a photobasic molecule with light generates a thermodynamic drive for the transfer of a Lewis acid from a donor to a photobasic molecule. We have used the archetypal BF3 as our Lewis acid and our photoactive Lewis bases are a family of quinolines, which are known Brønsted photobases as well. We have constructed the experimental Förster cycle for this system and have verified it computationally to demonstrate that a significant drive (0.2-0.7 eV) exists for the transfer of BF3 to a photoexcited quinoline. The magnitude of this drive is similar to those reported for Brønsted photobasicity in quinolines. Computational results from TDDFT and energy decomposition analysis show that the origin of such an effect is similar to the Brønsted photoactivity of these molecules, in that they follow the Hammett parameter of substituent groups. These results suggest that photobases may be capable of controlling the chemical phenomena beyond proton transfer and may open opportunities for a new handle in photocatalysis.

In Situ Investigation of Ultrafast Dynamics of Hot Electron-Driven Photocatalysis in Plasmon-Resonant Grating Structures.

Understanding the relaxation and injection dynamics of hot electrons is crucial to utilizing them in photocatalytic applications. While most studies have focused on hot carrier dynamics at metal/semiconductor interfaces, we study the in situ dynamics of direct hot electron injection from metal to adsorbates. Here, we report a hot electron-driven hydrogen evolution reaction (HER) by exciting the localized surface plasmon resonance (LSPR) in Au grating photoelectrodes. In situ ultrafast transient absorption (TA) measurements show a depletion peak resulting from hot electrons. When the sample is immersed in solution under -1 V applied potential, the extracted electron-phonon interaction time decreases from 0.94 to 0.67 ps because of additional energy dissipation channels. The LSPR TA signal is redshifted with delay time because of charge transfer and subsequent change in the dielectric constant of nearby solution. Plateau-like photocurrent peaks appear when exciting a 266 nm linewidth grating with p-polarized (on resonance) light, accompanied by a similar profile in the measured absorptance. Double peaks in the photocurrent measurement are observed when irradiating a 300 nm linewidth grating. The enhancement factor (i.e., reaction rate) is 15.6× between p-polarized and s-polarized light for the 300 nm linewidth grating and 4.4× for the 266 nm linewidth grating. Finite-difference time domain (FDTD) simulations show two resonant modes for both grating structures, corresponding to dipolar LSPR modes at the metal/fused silica and metal/water interfaces. To our knowledge, this is the first work in which LSPR-induced hot electron-driven photochemistry and in situ photoexcited carrier dynamics are studied on the same plasmon resonance structure with and without adsorbates.

Modeling and Characterization of Exciplexes in Photoredox CO2 Reduction: Insights from Quantum Chemistry and Fluorescence Spectroscopy.

Interactions between excited-state arenes and amines can lead to the formation of structures with a distinct emission behavior. These excited-state complexes or exciplexes can reduce the ability of the arene to participate in other reactions, such as CO2 reduction, or increase the likelihood of degradation via Birch reduction. Exciplex geometries are necessary to understand photophysical behavior and probe degradation pathways but are challenging to calculate. We establish a detailed computational protocol for calculation, verification, and characterization of exciplexes. Using fluorescence spectroscopy, we first demonstrate the formation of exciplexes between excited-state oligo-(p-phenylene) (OPP), shown to successfully carry out CO2 reduction, and triethylamine. Time-dependent density functional theory is employed to optimize the geometries of these exciplexes, which are validated by comparing both emission energies and their solvatochromism with the experiment. Excited-state energy decomposition analysis confirms the predominant role played by charge transfer interactions in the red shift of emissions relative to the isolated excited-state OPP*. We find that although the exciplex emission frequency depends strongly on solvent dielectric, the extent of charge separation in an exciplex does not. Our results also suggest that the formation of solvent-separated ionic radical states upon complete electron transfer competes with exciplex formation in higher-dielectric solvents, thereby leading to reduced exciplex emission intensities in fluorescence experiments.

Adiabatic Frequency Conversion Using a Time-Varying Epsilon-Near-Zero Metasurface.

A time-dependent change in the refractive index of a material leads to a change in the frequency of an optical beam passing through that medium. Here, we experimentally demonstrate that this effect-known as adiabatic frequency conversion (AFC)-can be significantly enhanced by a nonlinear epsilon-near-zero-based (ENZ-based) plasmonic metasurface. Specifically, by using a 63-nm-thick metasurface, we demonstrate a large, tunable, and broadband frequency shift of up to ∼11.2 THz with a pump intensity of 4 GW/cm2. Our results represent a decrease of ∼10 times in device thickness and 120 times in pump peak intensity compared with the cases of bare, thicker ENZ materials for the similar amount of frequency shift. Our findings might potentially provide insights for designing efficient time-varying metasurfaces for the manipulation of ultrafast pulses.

Mechanistic Insights about Electrochemical Proton-Coupled Electron Transfer Derived from a Vibrational Probe.

Proton-coupled electron transfer (PCET) is a fundamental step in a wide range of electrochemical processes, including those of interest in energy conversion and storage. Despite its importance, several mechanistic details of such reactions remain unclear. Here, we have combined a proton donor (tertiary ammonium) with a vibrational Stark-shift probe (benzonitrile), to track the process from the entry of the reactants into the electrical double layer (EDL), to the PCET reaction associated with proton donation to the electrode, and the formation of products. We have used operando vibrational spectroscopy and periodic density functional theory under electrochemical bias to assign the reactant and product peaks and their Stark shifts. We have identified three main stages for the progress of the PCET reaction as a function of applied potential. First, we have determined the potential necessary for desolvation of the reactants and their entry into the polarizing environment of the EDL. Second, we have observed the appearance of product peaks prior to the onset of steady state electrochemical current, indicating formation of a stationary population of products that does not turn over. Finally, more negative of the onset potential, the electrode attracts additional reactants, displacing the stationary products and enabling steady state current. This work shows that the integration of a vibrational Stark-shift probe with a proton donor provides critical insight into the interplay between interfacial electrostatics and heterogeneous chemical reactions. Such insights cannot be obtained from electrochemical measurements alone.

Photochemistry of 2-Nitroarenes: 2-Nitrophenyl-α-trifluoromethyl Carbinols as Synthons for Fluoroorganics.

Facile synthesis of a new series of 2,2'-bis(trifluoroacetyl) azoxybenzene derivatives and trifluoromethylated benzo[c]isoxazoline systems, along with trifluoroacetyl nitrosobenzene derivatives was achieved by solvent controlled photolysis of appropriate 2-nitrobenzyl alcohols. Corresponding photoactive 2-nitrobenzyl chromophore plays a distinct role in this photosynthetic process, while, quite unprecedented, pertinent fluoromethyl substitution leads to high value fluoromethylated products, whose direct access is not feasible by common synthetic protocols. The significance of fluorine and fluoroalkyl substitution and its prominent biological effects makes this new photochemical approach an important discovery in synthetic methodology. Plausible mechanistic pathways involved in the formation of the products during steady-state photolysis are further established by picosecond laser flash photolysis experiments.

Donor-acceptor preassociation, excited state solvation threshold, and optical energy cost as challenges in chemical applications of photobases.

Photobases are molecules with increased pKa in the excited state that can serve to transduce light energy into proton removal capability. They can be used to control chemical reactions using light, such as removing protons from a catalytic site in reactions that are rate-limited by proton transfer. We identify and explore several major challenges toward their practical applications. Two important challenges are the need for pre-association (or ground state hydrogen bonding) between the proton donor and the photobase, and the need for excited state solvation of the photogenerated products. We investigate these two challenges with the photobase 5-methoxyquinoline as the proton acceptor and a low-pKa alcohol, 2,2,2-trifluoroethanol, as the proton donor. We vary the concentration of the donor in a background non-hydrogen-bonding solvent. Using absorption spectroscopy, we have identified that the donor-acceptor concentration ratio must exceed 100 : 1 to achieve appreciable ground state hydrogen bonding. Interestingly, emission spectroscopy reveals that the onset of ground state hydrogen bonding does not guarantee successful excited state proton transfer. It takes an additional order of magnitude increase in donor-acceptor ratio to achieve that goal, revealing that it is necessary to have excess donor molecules to reach the solvation threshold for the photogenerated products. The next challenge is reducing the large ground-excited state energy gap, which often requires UV photons to drive proton transfer. We show experimental and computational data comparing the photobasicity and optical energy gap for a few N-aromatic heterocyclic photobases. In general, we find that reducing the energy gap by increasing the conjugation size necessarily reduces photobasicity, while adding substituents of varying electron-withdrawing strength allows some fine-tuning of this effect. The combination of these two factors provide a preliminary design space for creating new photobasic molecules.

Hot electron-driven photocatalysis and transient absorption spectroscopy in plasmon resonant grating structures.

Plasmon resonant grating structures provide an effective platform for distinguishing between the effects of plasmon resonant excitation and bulk metal absorption via interband transitions. By simply rotating the polarization of the incident light, we can switch between resonant excitation and non-resonant excitation, while keeping all other parameters of the measurement constant. With light polarized perpendicular to the lines in the grating (i.e., TE-polarization), the photocatalytic reaction rate (i.e., photocurrent) is measured as the angle of the incident laser light is tuned through the resonance with the grating. Here, hot holes photoexcited in the metal are used to drive the oxygen evolution reaction (OER), producing a measurable photocurrent. Using TE-polarized light, we observe sharp peaks in the photocurrent and sharp dips in the photoreflectance at approximately 9° from normal incidence, which corresponds to the conditions under which there is good wavevector matching between the incident light and the lines in the grating. With light polarized parallel to the grating (i.e., TM), we excite the grating structure non-resonantly and there is no angular dependence in the photocurrent or photoreflectance. In order to quantify the lifetime of these hot carriers, we performed transient absorption spectroscopy of these plasmon resonant grating structures. Here, we observe one feature in the spectra corresponding to interband transitions and another feature associated with the plasmon resonant mode in the grating. Both features decay over a time scale of 1-2 ps. The spectral responses of grating structures fabricated with Ag, Al, and Cu are also presented.

Kinetic Evidence for the Necessity of Two Proton Donor Molecules for Successful Excited State Proton Transfer by a Photobase.

Photobases are molecules that convert light to proton transfer drive and therefore have potential applications in many areas of chemistry. Previously, we studied the photobasicity of quinolines and explored their applications. While it is possible to tether a photobase near a target proton donor, for the sake of versatility it is desirable to explore their capability to deprotonate molecules dispersed in a solution. Previous evidence suggested that in this scenario at least two proton donors were necessary for successful excited state proton transfer: one to donate a proton and the second to stabilize the photogenerated donor anion. Here we report kinetic evidence from transient absorption (TA) and time-correlated single photon counting (TCSPC) in support of this hypothesis. We used 5-methoxyquinoline as the photobase and 2,2,2-trifluoroethanol (TFE), a low pKa alcohol, as the proton donor. A constant concentration of the photobase was used for a range of proton-donor dilutions spanning several orders of magnitude in an aprotic background solvent. Absorption spectra confirm that over most of the studied range the majority of the photobase population is hydrogen bonded to at least one donor. Short-pulse TA was used to measure the faster (2-500 ps) dynamics, while TSCPC was used to measure the slower (>500 ps) dynamics. The measured proton transfer time constants varied as a function of donor concentration over a wide range. A log-log plot of the proton transfer rate constant as a function of proton-donor concentration shows two regimes: nondiffusive at high donor concentrations where multiple proton donors are near the photobase and diffusive at low donor concentrations where proton donors are more dilute. The nondiffusive regime has a slope of approximately one, suggesting that the proton transfer process is dependent on one donor molecule in addition to the donor molecule already hydrogen bonded with the photobase. The diffusive regime reasonably follows diffusion kinetics. We propose a model for how the second proton-donor molecule may interact with the photogenerated alkoxide to stabilize it. This work highlights the importance of inducing irreversible changes, in this case solvation of the alkoxide, after proton transfer. Understanding of such details is likely to be important in applications of photobases.

Controlled deposition of size-selected MnO nanoparticle thin films for water splitting applications: reduction of onset potential with particle size.

Emulating water oxidation catalyzed by the oxomanganese clusters in the photosynthetic apparatus of plants has been a long-standing scientific challenge. The use of manganese oxide films has been explored, but while they may be catalytically active on the surface, their poor conductivity hinders their overall performance. We have approached this problem by using manganese oxide nanoparticles with sizes of 4, 6 and 8 nm, produced in a sputter-gas-aggregation source and soft-landed onto conducting electrodes. The mass loading of these catalytic particles was kept constant and corresponded to 45%-80% of a monolayer coverage. Measurements of the water oxidation threshold revealed that the onset potential decreases significantly with decreasing particle size. The final stoichiometry of the catalytically active nanoparticles, after exposure to air, was identified as predominantly MnO. The ability of such a sub-monolayer film to lower the reaction threshold implies that the key role is played by intrinsic size effects, i.e., by changes in the electronic properties and surface fields of the nanoparticles with decreasing size. We anticipate that this work will serve to bridge the knowledge gap between bulk thick film electrocatalysts and natural photosynthetic molecular-cluster complexes.

Photodriven Deprotonation of Alcohols by a Quinoline Photobase.

Control of proton transfer is relevant to many areas in chemistry, particularly in catalysis where the kinetics of (de)protonation reactions are often rate limiting. Photobases, which are molecules with enhanced basicity in the excited state, allow for control of proton transfer with light and have the potential to be used as functional units in catalytic systems. Alcohols are the feedstock in many catalytic reactions, where their deprotonation or dehydrogenation is often important. We report that the photobase 5-methoxyquinoline can deprotonate a series of alcohols upon excitation by light. We measure both the thermodynamic limits and the relevant kinetics of this process. A series of alcohols and water spanning the p Ka range of 12.5-16.5 were used as the proton donors. First, we show evidence from absorption and emission spectroscopy that photoexcited 5-methoxyquinoline deprotonates all donors more acidic than methanol and fails to deprotonate donors that are more basic. Interestingly, in methanol a quasi-equilibrium between the protonated and unprotonated forms of the photobase is established in the excited state, suggesting that the excited state p Ka of the photobase is near the p Ka of methanol (15.5). Second, using ultrafast transient absorption spectroscopy, we find that the time constants for excited state proton transfer range from a few picoseconds to tens of picoseconds, with faster speeds for the more acidic donors. Such a correlation between the thermodynamic drive and kinetics suggests that the same mechanism is responsible for proton transfer throughout the series. These results are necessary fundamental steps for applying photobases in potential applications such as deprotonation of alcohols for catalytic and synthetic purposes, optical regulation of pH, and transfer of protons in redox reactions.