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Efficient, selective and recyclable heterogeneous catalysts for photocatalytic CO2 reduction to CO under visible light irradiation are readily prepared by immobilization of cobalt molecular catalysts into Zr(iv)-based MOFs.

In the quest for designing efficient and stable photocatalytic materials for CO2 reduction, hybridizing a selective noble-metal-free molecular catalyst and carbon-based light-absorbing materials has recently emerged as a fruitful approach. In this work, we report about Co quaterpyridine complexes covalently linked to graphene surfaces functionalized by carboxylic acid groups. The nanostructured materials were characterized by X-ray photoemission spectroscopy, X-ray absorption spectroscopy, IR and Raman spectroscopies, high-resolution transmission electron microscopy and proved to be highly active in the visible-light-driven CO2 catalytic conversion in acetonitrile solutions. Exceptional stabilities (over 200 h of irradiation) were obtained without compromising the selective conversion of CO2 to products (>97%). Most importantly, complete selectivity control could be obtained upon adjusting the experimental conditions: production of CO as the only product was achieved when using a weak acid (phenol or trifluoroethanol) as a co-substrate, while formate was exclusively obtained in solutions of mixed acetonitrile and triethanolamine.

International audience; Conversion of CO2 into valuable compounds, including fuels, with renewable energy sources and sustainable compounds is a challenge addressed by artificial photosynthesis research. In particular, the application of solar assisted electrochemical (EC) processes, in which electrons are furnished by a photovoltaic (PV) cell, is a promising approach. A PV-EC system is described, consisting of a CIGS (copper indium gallium selenide) PV unit linked to a carbon electrode loaded with cobalt phthalocyanine as molecular catalyst, able to achieve the CO2 reduction to CO and then to methanol in aqueous media with limited bias voltage. Using CO as starting material, a partial current density of ca. 0.6 mA cm−2 for methanol is obtained at a bias voltage corresponding to a low 240 mV overpotential. Remarkably, the liquid fuel production can be sustained for at least 7 h. Under ideal conditions, the CO2-to-CH3OH reaction shows a global Faradaic efficiency of 28 %.

Carbon dioxide (CO2) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H+/6e− reduction of CO2 affords CH3OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO2 reduction to CH3OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO2 reduction to CH3OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH3OH synthesis. This Review identifies competitive advantages and drawbacks of heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis, for CO2 reduction to methanol.

In the quest for designing efficient and stable photocatalytic materials for CO

International audience; Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO 2 electrochemical and visible-light-driven reduction. Since the first reports in the 1970s, an enormous body of work has been accumulated regarding the two-electron two-proton reduction of the gas, along with mechanistic and spectroscopic efforts to rationalize the reactivity and establish guidelines for structure-reactivity relationships. The ability to fine tune the ligand structure and the almost unlimited possibilities of designing new complexes have led to highly selective and efficient catalysts. Recent efforts toward developing hybrid systems upon combining molecular catalysts with conductive or semi-conductive materials have converged to high catalytic performances in water solutions, to the inclusion of these catalysts into CO 2 electrolyzers and photo-electrochemical devices, and to the discovery of catalytic pathways beyond two electrons. Combined with the continuous mechanistic efforts and new developments for in situ and in operando spectroscopic studies, molecular catalysis of CO 2 reduction remains a highly creative approach.

Achieving visible-light-driven carbon dioxide reduction with high selectivity control and durability while using only earth abundant elements requires new strategies. Hybrid catalytic material was prepared upon covalent grafting a Co-quaterpyridine molecular complex to semiconductive mesoporous graphitic carbon nitride (mpg-C

International audience; Achieving visible-light-driven carbon dioxide reduction with high selectivity control and durability while using only earth abundant elements requires new strategies. Hybrid catalytic material was prepared upon covalent grafting a Co–quaterpyridine molecular complex to semiconductive mesoporous graphitic carbon nitride (mpg-C3N4) through an amide linkage. The molecular material was characterized by various spectroscopic techniques, including XPS, IR, and impedance spectroscopy. It proved to be a selective catalyst for CO production in acetonitrile using a solar simulator with a high 98% selectivity, while being remarkably robust since no degradation was observed after 4 days of irradiation (ca. 500 catalytic cycles). This unique combination of a selective molecular catalyst with a simple and robust semiconductive material opens new pathways for CO2 catalytic light-driven reduction.

Solar fuels may be generated upon visible light induced catalytic reduction of carbon dioxide. This appealing approach remains highly challenging, especially when earth abundant catalysts, mild conditions, and water as a solvent were used. Employing an iron tetraphenyl porphyrin complex substituted by positively charged trimethylammonio groups at the para position of each phenyl ring and reduction with three electrons by the excited state of an iridium sensitizer (λ > 420 nm) reduce CO2 to CO and to CH4 in both acetonitrile and aqueous solutions (acetonitrile/water 3:7 v:v) with good selectivity. Stability of the catalytic system remains a weakness and the reasons were analyzed.

This review examines recent advances in photocatalytic CO2 reduction using heterogenized molecular catalysts. The main part of the discussion is focused on the chemistry used to attach catalysts to different supports to produce hybrid materials, and how this effects photocatalytic performance. Examples of hybrid materials used for colloidal dispersions and solid suspensions are presented, including those based on carbon nitride, chalcogenide and perovskite quantum dots, and metal oxides. Some key examples in which this chemistry has been employed to make electrodes and photoelectrodes for photoelectrochemical CO2 reduction are also presented. In addition, the incorporation of molecular catalysts into ordered, porous frameworks (MOFs and COFs) is discussed because it offers many new and unique chemical pathways for heterogenization. Lastly, an outlook for this field and the potential future impact of these systems on solar fuels research is given.

An iron-substituted tetraphenyl porphyrin bearing positively charged trimethylammonio groups at the para position of each phenyl ring catalyzes the photoinduced conversion of CO2 . This complex is water soluble and acts as a molecular catalyst to selectively reduce CO2 into CO under visible-light irradiation in aqueous solutions (acetonitrile/water=1:9 v/v) with the assistance of purpurin, a simple organic photosensitizer. CO is produced with a catalytic selectivity of 95 % and turnover number up to 120, illustrating the possibility of photocatalyzing the reduction of CO2 in aqueous solution by using visible light, a simple organic sensitizer coupled to an amine as a sacrificial electron donor, and an earth-abundant metal-based molecular catalyst.

An iron tetraphenylporphyrin complex is shown to catalyse the reduction of carbon dioxide to methane upon visible light irradiation at ambient temperature and pressure. Converting CO2 into fuel or feedstock chemicals could reduce fossil fuel consumption and climate changing CO2 emissions, but a major stumbling block is the lack of efficient and selective catalysts for such conversions. Heng Rao et al. report that an iron tetraphenylporphyrin complex that is known as the most efficient and selective molecular catalyst for converting CO2 to CO can also catalyse the reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. The overall performance makes the system unsuitable for practical applications, but the basic principles by which it operates might guide the development of more efficient catalysts for the production of solar fuels from CO2 under mild conditions. Converting CO2 into fuel or chemical feedstock compounds could in principle reduce fossil fuel consumption and climate-changing CO2 emissions1,2. One strategy aims for electrochemical conversions powered by electricity from renewable sources3,4,5, but photochemical approaches driven by sunlight are also conceivable6. A considerable challenge in both approaches is the development of efficient and selective catalysts, ideally based on cheap and Earth-abundant elements rather than expensive precious metals7. Of the molecular photo- and electrocatalysts reported, only a few catalysts are stable and selective for CO2 reduction; moreover, these catalysts produce primarily CO or HCOOH, and catalysts capable of generating even low to moderate yields of highly reduced hydrocarbons remain rare8,9,10,11,12,13,14,15,16,17. Here we show that an iron tetraphenylporphyrin complex functionalized with trimethylammonio groups, which is the most efficient and selective molecular electro- catalyst for converting CO2 to CO known18,19,20, can also catalyse the eight-electron reduction of CO2 to methane upon visible light irradiation at ambient temperature and pressure. We find that the catalytic system, operated in an acetonitrile solution containing a photosensitizer and sacrificial electron donor, operates stably over several days. CO is the main product of the direct CO2 photoreduction reaction, but a two-pot procedure that first reduces CO2 and then reduces CO generates methane with a selectivity of up to 82 per cent and a quantum yield (light-to-product efficiency) of 0.18 per cent. However, we anticipate that the operating principles of our system may aid the development of other molecular catalysts for the production of solar fuels from CO2 under mild conditions.

A substituted tetraphenyl iron porphyrin, bearing positively charged trimethylammonio groups at the para position of each phenyl ring, demonstrates its ability as a homogeneous molecular catalyst to selectively reduce CO2 to CO under visible light irradiation in organic media without the assistance of a sensitizer and no competitive hydrogen evolution for several days.

International audience; Substituted tetraphenyl Fe porphyrins are versatile molecular catalysts for the activation of small molecules (such as O2, H+ or CO2), which could lead to renewable energy storage, the direct production of fuels or new catalytic relevant processes. Herein, we review the recent studies of these earth-abundant metal catalysts for the electrochemical activation of dioxygen on the one hand and for the photostimulated reduction of carbon dioxide on the other hand. These two prototype reactions illustrate how mechanistic studies are the only rational approach to gain fundamental insights into the elementary steps that drive the catalysis and for identification of the key intrinsic parameters controlling the reactivity, offering in turn the possibility to rationally tune the structure of the catalysts as well as the catalytic conditions.

Using a phenoxazine-based organic photosensitizer and an iron porphyrin molecular catalyst, we demonstrated photochemical reduction of CO(2) to CO and CH(4) with turnover numbers (TONs) of 149 and 29, respectively, under visible-light irradiation (λ > 435 nm) with a tertiary amine as sacrificial electron donor. This work is the first example of a molecular system using an earth-abundant metal catalyst and an organic dye to effect complete 8e(−)/8H(+) reduction of CO(2) to CH(4), as opposed to typical 2e(−)/2H(+) products of CO or formic acid. The catalytic system continuously produced methane even after prolonged irradiation up to 4 days. Using CO as the feedstock, the same reactive system was able to produce CH(4) with 85% selectivity, 80 TON and a quantum yield of 0.47%. The redox properties of the organic photosensitizer and acidity of the proton source were shown to play a key role in driving the 8e(−)/8H(+) processes.

Earth-abundant metal complexes have emerged as promising surrogates of platinum for catalyzing the hydrogen evolution reaction (HER). In this study, we report the design and synthesis of two novel nickel quinolinethiolate complexes, namely [Ni(Hqt) 2 (4, 4′-Z-2, 2′-bpy)] (Hqt = 8-quinolinethiol, Z = H [ 1 ] or CH 3 [ 2 ], bpy = bipyridine). An efficient three-component photocatalytic homogeneous system for hydrogen generation working under visible light irradiation was constructed by using the target complexes as catalysts, triethylamine (TEA) as sacrificial electron donor and xanthene dyes as photosensitizer. We obtain turnover numbers (TON, vs. catalyst) for H 2 evolution of 5923/7634 under the optimal conditions with 5.0 × 10 −6 M complex 1/2 respectively, 1.0 × 10 −3 M fluorescein and 5% (v/v) TEA at pH 12.3 in EtOH/H 2 O (1:1, v/v) mixture after 8 h irradiation (λ > 420 nm). We discuss the mechanism of H 2 evolution in the homogeneous photocatalytic system based on fluorescence spectrum and cyclic voltammetry data.

Molecular catalysis of carbon dioxide reduction using earth-abundant metal complexes as catalysts is a key challenge related to the production of useful products--the "solar fuels"--in which solar energy would be stored. A direct approach using sunlight energy as well as an indirect approach where sunlight is first converted into electricity could be used. A Co(II) complex and a Fe(III) complex, both bearing the same pentadentate N5 ligand (2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene), were synthesized, and their catalytic activity toward CO2 reduction was investigated. Carbon monoxide was formed with the cobalt complex, while formic acid was obtained with the iron-based catalyst, thus showing that the catalysis product can be switched by changing the metal center. Selective CO2 reduction occurs under electrochemical conditions as well as photochemical conditions when using a photosensitizer under visible light excitation (λ460 nm, solvent acetonitrile) with the Co catalyst. In the case of the Fe catalyst, selective HCOOH production occurs at low overpotential. Sustained catalytic activity over long periods of time and high turnover numbers were observed in both cases. A catalytic mechanism is suggested on the basis of experimental results and preliminary quantum chemistry calculations.

Photodeprotection of 1,3-dithianes in the presence of thiapyrylium was performed to return to the parent carbonyl compound, and the mechanism was studied by steady state photolysis, laser flash photolysis, and theoretical calculations. Electron transfer from dithianes to triplet sensitizers is extremely fast, and the decay of dithiane radical cations was not affected by the presence of water or oxygen as the consequence of a favorable unimolecular fragmentation pathway. Similar behaviors were observed for dithianes bearing electron-releasing or electron-withdrawing substituents on the aryl moiety, evidenced by C-S bond cleavage to form a distonic radical cation species. The lack of reaction under nitrogen atmosphere, requirement of oxygen for good conversion yields, inhibition of the photodeprotection process by the presence of p-benzoquinone, and absence of a labeled carbonyl final product when the reaction is performed in the presence of H218O all suggest that the superoxide anion drives the deprotection reaction. Density functional theory computational studies on the reactions with water, molecular oxygen, and the superoxide radical anion support the experimental findings. Fil: Oksdath Mansilla, Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Hajj, Viviane. Université Paris Diderot - Paris 7; Francia Fil: Andrada, Diego Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Argüello, Juan E.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina Fil: Bonin, Julien. Université Paris Diderot - Paris 7; Francia Fil: Robert, Marc. Université Paris Diderot - Paris 7; Francia Fil: Peñeñory, Alicia Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Córdoba. Instituto de Investigaciones en Físico-química de Córdoba. Universidad Nacional de Córdoba. Facultad de Ciencias Químicas. Instituto de Investigaciones en Físico-química de Córdoba; Argentina

A substituted tetraphenyl iron porphyrin, bearing positively charged trimethylammonio groups at the para position of each phenyl ring, demonstrates its ability as a homogeneous molecular catalyst to selectively reduce CO

Converting CO

Small Molecule Activation; International audience; Reduction of CO2 is a fruitful approach for storage of renewable electric energy as well as for the production of value added chemicals. Using variously substituted iron tetraphenylporphyrins and iron and cobalt macrocyclic pentadendate N5 complexes electrochemically or photochemically reduced to their active states, selective and efficient molecular catalysis of the CO2-to-CO and of the CO2-to-HCOOH conversion was achieved at low overpotential. From fruitful joint experimental and mechanistic studies rooted in the analysis of cyclic voltammograms, the mechanisms for catalysis were fully deciphered, allowing not only to give a complete picture of the catalytic processes but also to identify the main parameters for optimizing the catalyst structure. Efficient CO2 catalysis was performed not only in aprotic but also in aqueous conditions, opening the way to the design of electrochemical or photo-electrochemical cells, a key step for future applied devices.

A photochemical catalytic reduction of CO2 was performed in an organic solvent with iron(0) porphyrins as homogeneous molecular catalysts under visible light irradiation. With modified tetraphenylporphyrins consisting of internal phenolic groups, the photochemical process led to the production of CO, with H2 as a minor product. High catalytic selectivity for CO formation and turnover numbers up to 30 were obtained. Degradation of the catalyst occurred at longer irradiation times, along with decreased selectivity. Furthermore, addition of a weak acid, which increased the reduction efficiency under electrochemical conditions, led to rapid deactivation of the catalyst. With the unmodified tetraphenylporphyrin as catalyst, we observed lower performance and higher proportion of H2, which highlighted differences in the reduction pathways followed. A combination of a spectroscopic study and product analysis performed under various conditions led to detailed reduction mechanisms and helped pave the way for designing durable photocatalytic systems.

Proton-coupled electron transfer (PCET) reactions have received much attention over the past 10 years, from an experimental as well as from a theoretical point of view. At the heart of many chemical and biological processes, such reactions are of particular interest in energy conversion and enzymatic processes. Among the numerous examples of PCET reactions, photosynthesis and particularly reactions inside the Photosystem II (PSII) subunit, involving a global four electrons and four protons process to perform water oxidation and respiration, is the most emblematic one. This review focuses on the photochemical approaches of PCET reactions involving phenolic molecules. Indeed, a significant part of photochemical PCET studies were conducted on tyrosine or phenol relevant to PSII and charge transport in enzymes. The mechanisms of these reactions, sequential or concerted, with particular emphasis on the influence of pH, temperature, solvent nature and H-bonding pattern are presented based on photochemical techniques and related theoretical analysis.

Three experimental techniques, laser flash photolysis, redox catalysis, and stopped-flow, were used to investigate the variation of the oxidation rate constant of phenol in neat water with the driving force offered by a series of electron acceptors. Taking into account a result previously obtained with a low–driving force electron acceptor thus allowed scanning more than half an electron-volt driving force range. Variation of the rate constant with pH showed the transition between a direct phenol oxidation reaction at low pH, where the rate constant does not vary with pH, and a stepwise reaction involving the prior deprotonation of phenol by OH - , characterized by a unity-slope variation. Analyses of the direct oxidation kinetics, based on its variation with the driving force and on the determination of H/D isotope effects, ruled out a stepwise mechanism in which electron transfer is followed by the deprotonation of the initial cation radical at the benefit of a pathway in which proton and electron are transferred concertedly. Derivation of the characteristics of counterdiffusion in termolecular reactions allowed showing that the concerted process is under activation control. It is characterized by a remarkably small reorganization energy, in line with the electrochemical counterpart of the reaction, underpinning the very peculiar behavior of water as proton acceptor when it is used as the solvent.

Using pump–probe transient absorption spectroscopy, we studied the solvation dynamics of the electron in liquid polyalcohols: ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol and propane-1,2,3-triol. Time-resolved absorption spectra ranging from 440 to 720 nm were measured. Our study shows that the excess electron in the diols presents an intense and wide absorption band in the visible and near-IR spectral domain at early time after two-photon ionization of the neat solvent. Then, for the first tens of picoseconds, the electron spectrum shifts toward the blue domain and its bandwidth decreases as the red part of the initial spectrum rapidly drops, while the blue part hardly evolves. In contrast, in the triol, the absorption spectrum of the electron is early situated in the visible range after the pump pulse and then solely evolves in the red part. The Bayesian data analysis of the observed picosecond solvation dynamics with different models is in favor of a heterogeneous continuous relaxation. That is corroborated by the analogy between the change in the absorption band with increasing time or decreasing temperature. That tends to indicate a similar organization disorder of the solvent. Moreover, the electron solvation dynamics is very fast in propane-1,2,3-triol despite its high viscosity and highlight the role of the OH-group in that process.

Using pump-probe transient absorption spectroscopy we studied the solvation dynamics of the electron in liquid polyalcohols: ethane-1,2-diol, propane-1,2-diol, propane-1,3-diol and propane-1,2,3-triol. First, transmission measurements allowed us to assess that electrons were produced via two-photon ionization of the solvent with 263 nm femtosecond laser pulses, and to determine the two-photon absorption coefficient of the polyols. Second, time-resolved absorption spectra ranging from 440 to 710 nm were measured. Our study shows that the excess electron in the diols presents an intense and wide absorption band in the visible and near-IR spectral domain at early time after photoionization. Then, for the first tens of picoseconds the electron spectrum shifts toward the blue domain and its bandwidth decreases as the red part of the initial spectrum drops rapidly while the blue part hardly evolves. Using Bayesian data analysis method, the observed picosecond solvation dynamics were reconstructed with three models: a two-step mechanism and two continuous relaxation models. Comparison between the ability of models to reproduce the experimental kinetics is in favor of a heterogeneous continuous relaxation. Recent results obtained in propane-1,2,3-triol show that the electron solvation dynamics is very fast in this solvent despite its high viscosity and highlight the role of the OH group in that process.

The solvation dynamics of excess electrons in glycerol have been measured by the pump-probe femtosecond laser technique at 333 K. The electrons are produced by two-photon absorption at 263 nm. The change in the induced absorbance is followed up to 450 ps in the spectral range from 440 to 720 nm. The transient signals of electron solvation have been analyzed by two kinetic models: a stepwise mechanism and a continuous relaxation model, using a Bayesian data analysis method. The results are compared with those previously published for ethylene glycol (J. Phys. Chem. A 2006, 110, 175) and for propanediols (J. Phys. Chem. A 2007, 111, 4902). From the comparison, it is pointed out that solvation dynamics in glycerol is very fast despite its high viscosity. This is interpreted as the existence of efficient traps for the electrons in glycerol with low potential energy. The small shift of the absorption band of the excess electron indicates that the potential of these traps is very close to that corresponding to the fully solvated electron.

Temporal evolution of transient absorption spectra of electrons produced by two-photon ionization of two isomers, propane-1,2-diol (12PD) and propane-1,3-diol (13PD), with 263 nm femtosecond laser pulses has been studied on picosecond time scale. The two-photon absorption coefficients of 12PD and 13PD at 263 nm were determined to be beta = (2.0 +/- 0.3) x 10(-11) and (2.4 +/- 0.3) x 10(-11) m W(-1), respectively. Time-resolved absorption spectra ranging from 440 to 720 nm have been measured, showing a blue shift for the first tens of picoseconds for both solvents. However, the observed solvation dynamics of electron appears faster in 13PD than in 12PD. The transient signals of electron solvation have then been reconstructed with different models (stepwise mechanism or continuous relaxation model) using a Bayesian data analysis method. Results are discussed, compared with those previously obtained in ethylene glycol (J. Phys. Chem. A 2006, 110, 1705) and corroborate the interpretation, according to which the solvation of electrons is mainly governed by continuous solvent molecular motions.

The rate constants for the reactions of phenol with the hydroxyl radical (OH*) in water have been measured from room temperature to 380 degrees C using electron pulse radiolysis and transient absorption spectroscopy. The reaction scheme designed to fit the data shows the importance of an equilibrium, giving back reactants (OH* radical and phenol) from the dihydroxycyclohexadienyl radical formed by their reaction, and the non-negligible contribution of the hydroxycyclohexadienyl radical absorption from H* atom addition. The accuracy of the reaction scheme and the reaction rate constants determined from it have been determined by the analysis of two different experiments, one under pure N2O atmosphere and the second under a mixture a N2O and O2. We report reaction rates for the H* and OH* radical addition to phenol, the formation of phenoxyl, the second-order recombination, the reaction of dihydroxycyclohexadienyl with O2, and the decay of the peroxyl adduct. Nearly all of the reaction rates deviate strongly from Arrhenius behavior.

Converting CO2 into valuable compounds using sunlight as the energy input and an earth-abundant metal complex as the catalyst is an exciting challenge related to contemporary energy issues as well as to climate change. By using an inexpensive organic photosensitizer under visible-light excitation (λ > 400 nm) and a substituted iron(0) tetraphenylporphyrin as a homogeneous catalyst, we have been able to generate carbon monoxide from CO2 selectively with high turnover numbers. Sustained catalytic activity over a long time period (t > 50 h) did not lead to catalyst or sensitizer deactivation. A catalytic mechanism is proposed.

The reactivity of the pair formed by magnesium perchlorate with solvated electron in THF with aromatic molecules and halogenated hydrocarbons was investigated by pulse radiolysis. The kinetics of electron transfer from the pair to aromatic molecules with reduction potentials covering a 1 V range were recorded. The corresponding rate constants were determined and show that the pair presents a strong reducing character because it can reduce biphenyl which has a low redox potential. Nevertheless, the pair is still a weaker reducing agent than the solvated electron in THF. Moreover, we estimated the rate constants of the reaction between the pair and different types of alkyl halogenides.

Besides its own interest, tryptophan oxidation by photogenerated Ru complexes is one of the several examples where concerted proton-electron transfer (CPET) to water as proton acceptor endowed with a pH-dependent driving force has been invoked to explain the data. Since this notion is contrary to the very basic principles of chemical physics, it was interesting to attempt uncovering the source of this contradiction with an easily accessible substrate. Careful examination of the oxidation of the tryptophan (ethyl ester derivative) bearing a NH3(+)/NH2 group showed that there is no trace of such an unconventional H2O-CPET with a pH-dependent driving force. The reaction mechanism simply consists, with both the NH3(+) acid and NH2 basic forms of the tryptophan derivative, in a rate-determining electron-transfer step followed by deprotonation steps. The same is true with the ethyl ester-methyl amide derivative of tryptophan, whose behavior is even simpler since the molecule does not bear an acid-base group. No such unconventional H2O-CPET was found with phenol, another easily accessible substrate. It may thus be inferred that the same applies to less easily available systems in which electron transfer occurs intramolecularly. These observations help to rid the road of such artificial obstacles and improve present models of H2O-CPET reactions, a landmark towards the understanding of the role of water chains in natural systems.

On the course to sustainable energy, artificial photosynthesis is one of the greatest scientific challenges of our time. From a chemist point of view, the understanding of proton-coupled electron transfers which take place during the process is a fundamental issue. To decipher them, transient absorption spectroscopy, applied to model systems, is a method of choice. General methods and most recent results obtained by this technique are the object of the present review.

Reaction mechanisms in which electron and proton transfers are coupled are central to a huge number of processes, both natural and synthetic. Moreover, most of the new approaches to address modern energy challenges involve proton-coupled electron transfer (PCET). Recent research has focused on the possibility that the two steps are concerted, that is, concerted proton-electron transfer (CPET) reactions, rather than stepwise pathways in which proton transfer precedes (PET) or follows (EPT) electron transfer. CPET pathways have the advantage of bypassing the high-energy intermediates of stepwise pathways, although this thermodynamic benefit may have a kinetic cost. Concerted processes require short distances between the group being oxidized and the proton acceptor (and vice versa for a reduction process), which usually involves the formation of a hydrogen bond. Unlike the electron in outer-sphere electron-transfer reactions, the distance a proton may travel in a CPET is therefore rather limited. The idea has recently emerged, however, that this distance may be substantially increased via a H-bond relay located between the electron-transfer-triggered proton source and the proton acceptor. Generally speaking, the relay is a group bearing a H atom able to accept a H-bond from the moiety being oxidized and, at the same time, to form a H-bond with the proton-accepting group without going through a protonated intermediate. Although these molecules do not retain all the properties of chains of water molecules engaged in Grotthuss-type transport of a proton, the OH group in these molecules does possess a fundamental property of water molecules: namely, it is both a hydrogen-bond acceptor and a hydrogen-bond donor. Despite centuries of study, the mechanisms of proton movement in water remain active experimental and theoretical research areas, but so far with no connection to CPET reactions. In this Account, we bring together recent results concerning (i) the oxidative response of molecules containing a H-bond relay and (ii) the oxidation of phenol with water (in water) as the proton acceptor. In the first case, a nondestructive electrochemical method (cyclic voltammetry) was used to investigate the oxidation of phenol molecules containing one H-bond relay and an amine proton acceptor compared with a similar amino phenol deprived of relay. In the second, the kinetics of phenol oxidation with water (in water) as proton acceptor is contrasted with that of conventional proton acceptors (such as hydrogen phosphate and pyridine) to afford evidence of the concerted nature of Grotthuss-type proton displacement with electron transfer. First indications were provided by the same electrochemical method, whereas a more complete kinetic characterization was obtained from laser flash photolysis. Older electrochemical results concerning the reduction of superoxide ion in the presence of water are also examined. The result is a timely picture of current insight into concerted mechanisms involving electron transfer coupled with proton transport over simple H-bond relays and over H-bond networks.

The oxidation of PhOH in water by photochemically generated Ru(III)(bpy)(3) is taken as prototypal example disclosing the special character of water, in the solvent water, as proton acceptor in concerted proton-electron transfer reactions. The variation of the rate constant with temperature and driving force, as well as the variation of the H/D kinetic isotope effect with temperature, allowed the determination of the reaction mechanism characterized by three intrinsic parameters, the reorganization energy, a pre-exponential factor measuring the vibronic coupling of electronic states at equilibrium distance, and a distance-sensitivity parameter. Analysis of these characteristics and comparison with a standard base, hydrogen phosphate, revealed that electron transfer is concerted with a Grotthus-type proton translocation, leading to a charge delocalized over a cluster involving several water molecules. A mechanism is thus uncovered that may help in understanding how protons could be transported along water chains over large distances in concert with electron transfer in biological systems.

The photoinduced reductive cleavage of the carbon–chlorine bond in some chlorobenzylic nitro- and cyano-substituted compounds has been studied by transient absorption spectroscopy. The influence of the nature of the electroattractive group as well as its relative position and of the mixture composition of the solvent were investigated to give new clues into the mechanisms and into the factors that control the concerted or stepwise character of the process. Experimental results have been compared with previous results obtained by electrochemical techniques on the same molecules. Analysis leads to the conclusion that, beside the molecular structure, the solvation conditions and the driving force offered to the reaction, the formation of an excited state may control the reactivity.

Solvated electrons have been produced in ethylene glycol by two-photon ionization of the solvent with 263 nm femtosecond laser pulses. The two-photon absorption coefficient of ethylene glycol at 263 nm is determined to be beta = (2.1 +/- 0.2) x 10(-11) m W(-1). The dynamics of electron solvation in ethylene glycol has been studied by pump-probe transient absorption spectroscopy. So, time-resolved absorption spectra ranging from 430 to 710 nm have been measured. A blue shift of the spectra is observed for the first tens of picoseconds. Using the Bayesian data analysis method, the observed solvation dynamics are reconstructed with different models: stepwise mechanisms, continuous relaxation models, or combinations of stepwise and continuous relaxation. Comparison between models is in favor of continuous relaxation, which is mainly governed by solvent molecular motions.

Taking pyridine as a prototypal example of biologically important nitrogen bases involved in proton-coupled electron transfers, it is shown with the example of the photochemically triggered oxidation of phenol by Ru(III)(bpy)(3) that this proton acceptor partakes in a concerted pathway whose kinetic characteristics can be extracted from the overall kinetic response. The treatment of these data, implemented by the results of a parallel study carried out in heavy water, allowed the determination of the intrinsic kinetic characteristics of this proton acceptor. Comparison of the reorganization energies and of the pre-exponential factors previously derived for hydrogen phosphate and water (in water) as proton acceptors suggests that, in the case of pyridine, the proton charge is delocalized over a primary shell of water molecules firmly bound to the pyridinium cation.

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Photocatalytic solar fuel production is an effective means of chemical storage for solar energy and it provides a potential fruitful option for achieving a zero-emissions energy system. The cornerstone of a practical solar fuel production process is to design and optimize stable, efficient, and scalable photocatalysts, including a semiconductor material that accommodates photon absorption, efficient charge carriers generation, transport, and then catalytic reactions. Molecular catalysis plays an essential role in both natural and artificial photosynthesis. In this doctoral work, I have (i) investigated a bimetallic complex for selective CO2 reduction, illustrating that molecular catalysis is one of the promising way to activate metal cooperativity with outstanding intrinsic activities; (ii) synthesized inorganic material (semiconductive carbon nitride and graphitic acid) able to be functionalized with a molecular molecule (cobalt quaterpyridine) through a covalent amide bond. Effective heterogenization of the two components was thoroughly accessed. These hybrid systems showed high catalytic performance towards CO2 photoreduction. They illustrate that the development of molecular catalysts is worth to be explored for the invention of scalable catalytic systems that are needed for large scale, practical solar fuel production. In the mechanism of material-supported molecule catalysts, challenges still remain regarding both to molecular stability, heterogenization techniques, and control of the electronic interaction between these components.; La production photocatalytique de carburants solaires est un moyen efficace de stockage chimique de l'énergie solaire et offre une option potentiellement fructueuse pour parvenir à un système énergétique à zéro émission de carbone. La pierre angulaire d'un processus pratique de production de carburant solaire est de concevoir et d'optimiser des photocatalyseurs stables, efficaces et déployables à grande échelle, et comprenant un matériau semi-conducteur permettant l'absorption de photons, la génération efficace de porteurs de charge, leur transport jusqu’à la réalisation de réactions catalytiques.La catalyse moléculaire joue un rôle essentiel dans la photosynthèse naturelle et artificielle. Dans ce travail de doctorat, j’ai étudié (i) un complexe bimétallique pour la réduction sélective du CO2, illustrant le fait que la catalyse moléculaire est l'un des moyens prometteurs pour mettre en œuvre la coopérativité des métaux avec des activités intrinsèques exceptionnelles ; (ii) des matériaux inorganiques (nitrure de carbone semi-conducteur et acide graphitique) pouvant être fonctionnalisés par une molécule (complexe quaterpyridine de cobalt) par le biais d’une liaison amide covalente. Une hétérogénéisation efficace des deux composants a été réalisée. Ces systèmes hybrides ont montré des performances catalytiques élevées vis-à-vis de la photoréduction au CO2. Ils illustrent que le développement de catalyseurs moléculaires vaut la peine d'être exploré afin de parvenir à des systèmes catalytiques évolutifs qui sont nécessaires pour la production pratique de carburant solaire à grande échelle. Dans le mécanisme de fonctionnement des catalyseurs moléculaires supportés par des matériaux, des défis subsistent en ce qui concerne à la fois la stabilité de l’unité moléculaire, les techniques d'hétérogénéisation et le contrôle de l'interaction électronique entre les composants.

Photocatalytic solar fuel production is an effective means of chemical storage for solar energy and it provides a potential fruitful option for achieving a zero-emissions energy system. The cornerstone of a practical solar fuel production process is to design and optimize stable, efficient, and scalable photocatalysts, including a semiconductor material that accommodates photon absorption, efficient charge carriers generation, transport, and then catalytic reactions. Molecular catalysis plays an essential role in both natural and artificial photosynthesis. In this doctoral work, I have (i) investigated a bimetallic complex for selective CO2 reduction, illustrating that molecular catalysis is one of the promising way to activate metal cooperativity with outstanding intrinsic activities; (ii) synthesized inorganic material (semiconductive carbon nitride and graphitic acid) able to be functionalized with a molecular molecule (cobalt quaterpyridine) through a covalent amide bond. Effective heterogenization of the two components was thoroughly accessed. These hybrid systems showed high catalytic performance towards CO2 photoreduction. They illustrate that the development of molecular catalysts is worth to be explored for the invention of scalable catalytic systems that are needed for large scale, practical solar fuel production. In the mechanism of material-supported molecule catalysts, challenges still remain regarding both to molecular stability, heterogenization techniques, and control of the electronic interaction between these components.; La production photocatalytique de carburants solaires est un moyen efficace de stockage chimique de l'énergie solaire et offre une option potentiellement fructueuse pour parvenir à un système énergétique à zéro émission de carbone. La pierre angulaire d'un processus pratique de production de carburant solaire est de concevoir et d'optimiser des photocatalyseurs stables, efficaces et déployables à grande échelle, et comprenant un matériau semi-conducteur permettant l'absorption de photons, la génération efficace de porteurs de charge, leur transport jusqu’à la réalisation de réactions catalytiques.La catalyse moléculaire joue un rôle essentiel dans la photosynthèse naturelle et artificielle. Dans ce travail de doctorat, j’ai étudié (i) un complexe bimétallique pour la réduction sélective du CO2, illustrant le fait que la catalyse moléculaire est l'un des moyens prometteurs pour mettre en œuvre la coopérativité des métaux avec des activités intrinsèques exceptionnelles ; (ii) des matériaux inorganiques (nitrure de carbone semi-conducteur et acide graphitique) pouvant être fonctionnalisés par une molécule (complexe quaterpyridine de cobalt) par le biais d’une liaison amide covalente. Une hétérogénéisation efficace des deux composants a été réalisée. Ces systèmes hybrides ont montré des performances catalytiques élevées vis-à-vis de la photoréduction au CO2. Ils illustrent que le développement de catalyseurs moléculaires vaut la peine d'être exploré afin de parvenir à des systèmes catalytiques évolutifs qui sont nécessaires pour la production pratique de carburant solaire à grande échelle. Dans le mécanisme de fonctionnement des catalyseurs moléculaires supportés par des matériaux, des défis subsistent en ce qui concerne à la fois la stabilité de l’unité moléculaire, les techniques d'hétérogénéisation et le contrôle de l'interaction électronique entre les composants.

The transition from fossil fuels to renewable energy sources is a major challenge in today’s society. The first step to address this challenge is to find a way to store and carry this energy, which can be done under the form of chemical bonds through the activation of small molecules like CO2, O2 and H2O. In this context, our work was focused on the catalytic photoreduction of carbon dioxide by three iron porphyrins showing remarkable electrocatalytic properties. We have first studied a homogeneous molecular photochemical approach. A careful analysis of the results, comparing the three porphyrins and the effect of the addition of a weak Brønsted acid, allowed us to propose a reaction mechanism correlating the different behaviour of the porphyrins with their molecular structures. Furthermore, a detailed analysis of the experimental conditions required for the catalysis revealed that they induced the degradation of the porphyrins. To overcome these limits, we studied the use of a photosensitizer, resulting in improved catalytic performances and higher system stability. We also studied a homogeneous molecular photoelectrochemical approach, using p-type Si photoelectrodes. We observed the reduction of CO2 with a photovoltage of 350 mV, thus validating this strategy, but the process is limited by the sensitivity of the photoelectrodes towards O2. We are currently developing new electrodes made of copper oxide to obtain better catalytic performances and a higher stability.; La transition vers l’utilisation des énergies d’origine renouvelable est un enjeu majeur pour notre société. Elle implique cependant de développer des moyens pour stocker et transporter l’énergie produite, ce que permet, sous forme de liaisons chimiques, l’activation de petites molécules telles que CO2, O2 et H2O. Dans ce cadre, notre travail a porté sur la photoréduction catalytique de CO2 par trois porphyrines de fer qui possèdent des propriétés électrocatalytiques remarquables. Dans un premier temps, nous avons mis en œuvre une approche photochimique moléculaire homogène. Une analyse détaillée des résultats obtenus avec ces trois porphyrines, ainsi que de l’effet de l’ajout d’un acide de Brønsted faible, nous a permis de proposer un mécanisme réactionnel corrélant les comportements des porphyrines avec leur structure moléculaire. De plus, par une étude attentive des conditions expérimentales, nous avons mis en évidence que les conditions nécessaires à la catalyse induisaient une dégradation des porphyrines, limitant d’autant leurs performances catalytiques. Constatant les limites de cette approche, l’utilisation d’un photosensibilisateur a été abordée, et les résultats préliminaires ont montré une amélioration nette de la catalyse et une longévité accrue du système. Dans un deuxième temps, nous avons étudié une approche photoélectrocatalytique moléculaire homogène utilisant des photoélectrodes en p-Si et ces mêmes porphyrines en solution. Malgré des performances contraintes par l’instabilité des photoélectrodes vis-à-vis de O2, une catalyse de la réduction de CO2 avec un photovoltage de 350 mV a été obtenu, démontrant la validité de cette approche. De nouvelles photoélectrodes à base d’oxyde de cuivre sont en cours d’élaboration, afin de parer au manque de stabilité du silicium et ainsi obtenir une catalyse efficace et durable.

The transition from fossil fuels to renewable energy sources is a major challenge in today’s society. The first step to address this challenge is to find a way to store and carry this energy, which can be done under the form of chemical bonds through the activation of small molecules like CO2, O2 and H2O. In this context, our work was focused on the catalytic photoreduction of carbon dioxide by three iron porphyrins showing remarkable electrocatalytic properties. We have first studied a homogeneous molecular photochemical approach. A careful analysis of the results, comparing the three porphyrins and the effect of the addition of a weak Brønsted acid, allowed us to propose a reaction mechanism correlating the different behaviour of the porphyrins with their molecular structures. Furthermore, a detailed analysis of the experimental conditions required for the catalysis revealed that they induced the degradation of the porphyrins. To overcome these limits, we studied the use of a photosensitizer, resulting in improved catalytic performances and higher system stability. We also studied a homogeneous molecular photoelectrochemical approach, using p-type Si photoelectrodes. We observed the reduction of CO2 with a photovoltage of 350 mV, thus validating this strategy, but the process is limited by the sensitivity of the photoelectrodes towards O2. We are currently developing new electrodes made of copper oxide to obtain better catalytic performances and a higher stability., La transition vers l’utilisation des énergies d’origine renouvelable est un enjeu majeur pour notre société. Elle implique cependant de développer des moyens pour stocker et transporter l’énergie produite, ce que permet, sous forme de liaisons chimiques, l’activation de petites molécules telles que CO2, O2 et H2O. Dans ce cadre, notre travail a porté sur la photoréduction catalytique de CO2 par trois porphyrines de fer qui possèdent des propriétés électrocatalytiques remarquables. Dans un premier temps, nous avons mis en œuvre une approche photochimique moléculaire homogène. Une analyse détaillée des résultats obtenus avec ces trois porphyrines, ainsi que de l’effet de l’ajout d’un acide de Brønsted faible, nous a permis de proposer un mécanisme réactionnel corrélant les comportements des porphyrines avec leur structure moléculaire. De plus, par une étude attentive des conditions expérimentales, nous avons mis en évidence que les conditions nécessaires à la catalyse induisaient une dégradation des porphyrines, limitant d’autant leurs performances catalytiques. Constatant les limites de cette approche, l’utilisation d’un photosensibilisateur a été abordée, et les résultats préliminaires ont montré une amélioration nette de la catalyse et une longévité accrue du système. Dans un deuxième temps, nous avons étudié une approche photoélectrocatalytique moléculaire homogène utilisant des photoélectrodes en p-Si et ces mêmes porphyrines en solution. Malgré des performances contraintes par l’instabilité des photoélectrodes vis-à-vis de O2, une catalyse de la réduction de CO2 avec un photovoltage de 350 mV a été obtenu, démontrant la validité de cette approche. De nouvelles photoélectrodes à base d’oxyde de cuivre sont en cours d’élaboration, afin de parer au manque de stabilité du silicium et ainsi obtenir une catalyse efficace et durable.