Time-Resolved Mechanistic Depiction of Photoinduced CO Reduction Catalysis on a Urea-Modified Iron Porphyrin.

The development of functional artificial photosynthetic devices relies on the understanding of mechanistic aspects involved in specialized photocatalysts. Modified iron porphyrins have long been explored as efficient catalysts for the light-induced reduction of carbon dioxide (CO) towards solar fuels. In spite of the advancements in homogeneous catalysis, the development of the next generation of catalysts requires a complete understanding of the fundamental photoinduced processes taking place prior to and after activation of the substrate by the catalyst.

Proton Domino Reactions at an Imidazole Relay Control the Oxidation of a Tyr-His Artificial Mimic of Photosystem II.

A close mimic of P680 and the Tyrosine-Histidine pair in photosystem II (PS II) has been synthesized using a ruthenium chromophore and imidazole-phenol ligands. The intramolecular oxidation of the ligands by the photoproduced Ru(III) species is characterized by a small driving force, very similar to PS II where the complexity of kinetics was attributed to the reversibility of electron transfer steps. Laser flash photolysis revealed biphasic kinetics for ligand oxidation.

Second Coordination Sphere Effect Shifts CO to CO Reduction by Iron Porphyrin from Fe to Fe.

Iron porphyrins are among the most studied molecular catalysts for carbon dioxide (CO ) reduction and their reactivity is constantly being enhanced through the implementation of chemical functionalities in the second coordination sphere inspired by the active sites of enzymes. In this study, we were intrigued to observe that a multipoint hydrogen bonding scheme provided by embarked urea groups could also shift the redox activation step of CO from the well-admitted Fe(0) to the Fe(I) state. Using EPR, resonance Raman, IR and UV-Visible spectroscopies, we underpinned a two-electron activation step of CO starting from the Fe(I) oxidation state to form, after protonation, an Fe(III)-COOH species.

Bimetallic Molecular Catalyst Design for Carbon Dioxide Reduction.

The core challenge in developing cost-efficient catalysts for carbon dioxide (CO ) conversion mainly lies in controlling its complex reaction pathways. One such strategy exploits bimetallic cooperativity, which relies on the synergistic interaction between two metal centers to activate and convert the CO substrate. While this approach has seen an important trend in heterogeneous catalysis as a handle to control stabilities of surface intermediates, it has not often been utilized in molecular and heterogenized molecular catalytic systems.

Identification of a Ubiquinone-Ubiquinol Quinhydrone Complex in Bacterial Photosynthetic Membranes and Isolated Reaction Centers by Time-Resolved Infrared Spectroscopy.

Ubiquinone redox chemistry is of fundamental importance in biochemistry, notably in bioenergetics. The bi-electronic reduction of ubiquinone to ubiquinol has been widely studied, including by Fourier transform infrared (FTIR) difference spectroscopy, in several systems. In this paper, we have recorded static and time-resolved FTIR difference spectra reflecting light-induced ubiquinone reduction to ubiquinol in bacterial photosynthetic membranes and in detergent-isolated photosynthetic bacterial reaction centers.

Bio-Inspired Bimetallic Cooperativity Through a Hydrogen Bonding Spacer in CO Reduction.

At the core of carbon monoxide dehydrogenase (CODH) active site two metal ions together with hydrogen bonding scheme from amino acids orchestrate the interconversion between CO and CO. We have designed a molecular catalyst implementing a bimetallic iron complex with an embarked second coordination sphere with multi-point hydrogen-bonding interactions. We found that, when immobilized on carbon paper electrode, the dinuclear catalyst enhances up to four fold the heterogeneous CO reduction to CO in water with an improved selectivity and stability compared to the mononuclear analogue.

Photocatalytic generation of a non-heme Fe(iii)-hydroperoxo species with O in water for the oxygen atom transfer reaction.

Coupling a photoredox module and a bio-inspired non-heme model to activate O for the oxygen atom transfer (OAT) reaction requires a vigorous investigation to shed light on the multiple competing electron transfer steps, charge accumulation and annihilation processes, and the activation of O at the catalytic unit. We found that the efficient oxidative quenching mechanism between a [Ru(bpy)] chromophore and a reversible electron mediator, methyl viologen (MV), to form the reducing species methyl viologen radical (MV˙) can convey an electron to O to form the superoxide radical and reset an Fe(iii) species in a catalytic cycle to the Fe(ii) state in an aqueous solution. The formation of the Fe(iii)-hydroperoxo (Fe-OOH) intermediate can evolve to a highly oxidized iron-oxo species to perform the OAT reaction to an alkene substrate.

Heterolytic O-O Bond Cleavage Upon Single Electron Transfer to a Nonheme Fe(III)-OOH Complex.

The one-electron reduction of the nonheme iron(III)-hydroperoxo complex, [Fe (OOH)(L )] (L =N-methyl-N,N',N'-tris(2-pyridylmethyl)ethane-1,2-diamine), carried out at -70 °C results in the release of dioxygen and in the formation of [Fe (OH)(L )] following a bimolecular process. This reaction can be performed either with cobaltocene as chemical reductant, or electrochemically. These experimental observations are consistent with the disproportionation of the hydroperoxo group in the putative Fe (OOH) intermediate generated upon reduction of the Fe (OOH) starting complex.

Proton-controlled Action of an Imidazole as Electron Relay in a Photoredox Triad.

Electron relays play a crucial role for efficient light-induced activation by a photo-redox moiety of catalysts for multi-electronic transformations. Their insertion between the two units reduces detrimental energy transfer quenching while establishing at the same time unidirectional electron flow. This rectifying function allows charge accumulation necessary for catalysis.

Intercepting a transient non-hemic pyridine -oxide Fe(III) species involved in OAT reactions.

In the context of bioinspired OAT catalysis, we developed a tetradentate dipyrrinpyridine ligand, a hybrid of hemic and non-hemic models. The catalytic activity of the iron(III) derivative was investigated in the presence of iodosylbenzene. Unexpectedly, MS, EPR, Mössbauer, UV-visible and FTIR spectroscopic signatures supported by DFT calculations provide convincing evidence for the involvement of a relevant Fe-O-N active intermediate.

Tracking light-induced electron transfer toward O in a hybrid photoredox-laccase system.

Photobiocatalysis uses light to perform specific chemical transformations in a selective and efficient way. The intention is to couple a photoredox cycle with an enzyme performing multielectronic catalytic activities. Laccase, a robust multicopper oxidase, can be envisioned to use dioxygen as a clean electron sink when coupled to an oxidation photocatalyst.

Phthalocyanine as a Bioinspired Model for Chlorophyll f-Containing Photosystem II Drives Photosynthesis into the Far-Red Region.

The textbook explanation that P pigments are the red limit to drive oxygenic photosynthesis must be reconsidered by the recent discovery that chlorophyll f (Chlf)-containing Photosystem II (PSII) absorbing at 727 nm can drive water oxidation. Two different families of unsymmetrically substituted Zn phthalocyanines (Pc) absorbing in the 700-800 nm spectral window and containing a fused imidazole-phenyl substituent or a fused imidazole-hydroxyphenyl group have been synthetized and characterized as a bioinspired model of the Chlf/Tyrosine /Histidine cofactors of PSII. Transient absorption studies in the presence of an electron acceptor and irradiating in the far-red region evidenced an intramolecular electron transfer process.

Through-Space Electrostatic Interactions Surpass Classical Through-Bond Electronic Effects in Enhancing CO Reduction Performance of Iron Porphyrins.

In his pioneering work to unravel the catalytic power of enzymes, Warshel has pertinently validated that electrostatic interactions play a major role in the activation of substrates. Implementing such chemical artifice in molecular catalysts may help improve their catalytic properties. In this study, a series of tetra-, di-, and mono-substituted iron porphyrins with cationic imidazolium groups were designed.

Visible light-driven simultaneous water oxidation and quinone reduction by a nano-structured conjugated polymer without co-catalysts.

In artificial photosynthesis, chemists are aiming to borrow principles from natural photosynthesis to develop photoelectrochemical cells (PEC) for water splitting. The water plastoquinone photo-oxidoreductase enzyme, also known as photosystem II, uses light to perform the four-electron, four-proton oxidation of water to dioxygen and stores reducing equivalents in reduced forms of quinones which are ultimately used in dark reactions for the synthesis of energy-rich molecules. We report a nano-structured semiconducting conjugated polymer based on poly(diphenylbutadiyne) (nano-PDPB) and its photocatalytic activities towards the water oxidation reaction under visible light irradiation when dispersed in water in the absence of any sacrificial agents or co-catalysts.

Atropisomeric Hydrogen Bonding Control for CO Binding and Enhancement of Electrocatalytic Reduction at Iron Porphyrins.

The manipulation of the second coordination sphere for improving the electrocatalytic CO reduction has led to breakthroughs with hydrogen bonding, local proton source, or electrostatic effects. We have developed two atropisomers of an iron porphyrin complex with two urea functions acting as multiple hydrogen-bonding tweezers to lock the metal-bound CO in a similar fashion found in the carbon monoxide dehydrogenase (CODH) enzyme. The αα topological isomer with the two urea groups on the same side of the porphyrin provides a stronger binding affinity to tether the incoming CO in comparison to the αβ disposition.

Spectroscopic Characterisation of a Bio-Inspired Ni-Based Proton Reduction Catalyst Bearing a Pentadentate N S Ligand with Improved Photocatalytic Activity.

Inspired by the sulfur-rich environment found in active hydrogenase enzymes, a Ni-based proton reduction catalyst with pentadentate N S ligand was synthesised. When coupled with [Ru(bpy) ] (bpy=2,2'-bipyridine) as photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared with a homologous counterpart bearing a tetradentate N S ligand. The mechanistic pathway of photoinduced hydrogen evolution was comprehensively analysed through optical transient absorption and time-resolved X-ray absorption spectroscopy, which revealed important electronic and structural changes in the catalytic system during photoirradiation.

A Reversible Electron Relay to Exclude Sacrificial Electron Donors in the Photocatalytic Oxygen Atom Transfer Reaction with O in Water.

Using light energy and O for the direct chemical oxidation of organic substrates is a major challenge. A limitation is the use of sacrificial electron donors to activate O by reductive quenching of the photosensitizer, generating undesirable side products. A reversible electron acceptor, methyl viologen, can act as electron shuttle to oxidatively quench the photosensitizer, [Ru(bpy) ] , generating the highly oxidized chromophore and the powerful reductant methyl-viologen radical MV .

Second-Sphere Biomimetic Multipoint Hydrogen-Bonding Patterns to Boost CO Reduction of Iron Porphyrins.

Inspired by nature's orchestra of chemical subtleties to activate and reduce CO , we have developed a family of iron porphyrin derivatives in to which we have introduced urea groups functioning as multipoint hydrogen-bonding pillars on the periphery of the porphyrinic ring. This structure closely resembles the hydrogen-bond stabilization scheme of the carbon dioxide (CO ) adduct in the carbon monoxide dehydrogenase (CODH). We found that such changes to the second coordination sphere significantly lowered the overpotential for CO reduction in this family of molecular catalysts and importantly increased the CO binding rate while maintaining high turnover frequency (TOF) and selectivity.

Photoinduced electron transfer in a molecular dyad by nanosecond pump-pump-probe spectroscopy.

The design of robust and inexpensive molecular photocatalysts for the conversion of abundant stable molecules like H2O and CO2 into an energetic carrier is one of the major fundamental questions for scientists nowadays. The outstanding challenge is to couple single photoinduced charge separation events with the sequential accumulation of redox equivalents at the catalytic unit for performing multielectronic catalytic reactions. Herein, double excitation by nanosecond pump-pump-probe experiments was used to interrogate the photoinduced charge transfer and charge accumulation on a molecular dyad composed of a porphyrin chromophore and a ruthenium-based catalyst in the presence of a reversible electron acceptor.

Water Molecules Gating a Photoinduced One-Electron Two-Protons Transfer in a Tyrosine/Histidine (Tyr/His) Model of Photosystem II.

We investigate a biomimetic model of a Tyr /His pair, a hydrogen-bonded phenol/imidazole covalently attached to a porphyrin sensitizer. Laser flash photolysis in the presence of an external electron acceptor reveals the need for water molecules to unlock the light-induced oxidation of the phenol through an intramolecular pathway. Kinetics monitoring encompasses two fast phases with distinct spectral properties.

Snapshots of Light Induced Accumulation of Two Charges on Methylviologen using a Sequential Nanosecond Pump-Pump Photoexcitation.

Methylviologen (MV) is perhaps the most used component as a reversible electron acceptor in photophysical studies. While MV is most commonly implicated as a reversible one-electron mediator, its electrochemical properties clearly evidence two successive one-electron reduction processes. In this report, we have investigated on the light driven two-charge accumulation on MV using a multicomponent system composed of the prototypical molecular photosensitizer [Ru(bpy)] and MV in the presence of ascorbate as reversible electron donor.