Ultrafast Electron Transfer from CuInS2 Quantum Dots to a Molecular Catalyst for Hydrogen Production: Challenging Diffusion Limitations.

Systems integrating quantum dots with molecular catalysts are attracting ever more attention, primarily owing to their tunability and notable photocatalytic activity in the context of the hydrogen evolution reaction (HER) and CO2 reduction reaction (CO2RR). CuInS2 (CIS) quantum dots (QDs) are effective photoreductants, having relatively high-energy conduction bands, but their electronic structure and defect states often lead to poor performance, prompting many researchers to employ them with a core-shell structure. Molecular cobalt HER catalysts, on the other hand, often suffer from poor stability. Here, we have combined CIS QDs, surface-passivated with l-cysteine and iodide from a water-based synthesis, with two tetraazamacrocyclic cobalt complexes to realize systems which demonstrate high turnover numbers for the HER (up to >8000 per catalyst), using ascorbate as the sacrificial electron donor at pH = 4.5. Photoluminescence intensity and lifetime quenching data indicated a large degree of binding of the catalysts to the QDs, even with only ca. 1 µM each of QDs and catalysts, linked to an entirely static quenching mechanism. The data was fitted with a Poissonian distribution of catalyst molecules over the QDs, from which the concentration of QDs could be evaluated. No important difference in either quenching or photocatalysis was observed between catalysts with and without the carboxylate as a potential anchoring group. Femtosecond transient absorption spectroscopy confirmed ultrafast interfacial electron transfer from the QDs and the formation of the singly reduced catalyst (CoII state) for both complexes, with an average electron transfer rate constant of ≈ (10 ps)-1. These favorable results confirm that the core tetraazamacrocyclic cobalt complex is remarkably stable under photocatalytic conditions and that CIS QDs without inorganic shell structures for passivation can act as effective photosensitizers, while their smaller size makes them suitable for application in the sensitization of, inter alia, mesoporous electrodes.

A Combined Spectroscopic and Theoretical Study on a Ruthenium Complex Featuring a π-Extended dppz Ligand for Light-Driven Accumulation of Multiple Reducing Equivalents.

The design of photoactive systems capable of storing and relaying multiple electrons is highly demanded in the field of artificial photosynthesis, where transformations of interest rely on multielectronic redox processes. The photophysical properties of the ruthenium photosensitizer [(bpy)2 Ru(oxim-dppqp)]2+ (Ru), storing two electrons coupled to two protons on the π-extended oxim-dppqp ligand under light-driven conditions, are investigated by means of excitation wavelength-dependent resonance Raman and transient absorption spectroscopies, in combination with time-dependent density functional theory; the results are discussed in comparison to the parent [(bpy)2 Ru(dppz)]2+ and [(bpy)2 Ru(oxo-dppqp)]2+ complexes. In addition, this study provides in-depth insights on the impact of protonation or of accumulation of multiple reducing equivalents on the reactive excited states.

Hydrogen Production at a NiO Photocathode Based on a Ruthenium Dye-Cobalt Diimine Dioxime Catalyst Assembly: Insights from Advanced Spectroscopy and Post-operando Characterization.

The production of hydrogen by efficient, low-cost, and integrated photoelectrochemical water splitting processes represents an important target for the ecological transition. This challenge can be addressed thanks to bioinspired chemistry and artificial photosynthesis approaches by designing dye-sensitized photocathodes for hydrogen production, incorporating bioinspired first-row transition metal-based catalysts. The present work describes the preparation and photoelectrochemical characterization of a NiO photocathode sensitized with a phosphonate-derivatized ruthenium tris-diimine photosensitizer covalently linked to a cobalt diimine dioxime hydrogen-evolving catalyst. Under simulated AM 1.5G irradiation, hydrogen is produced with photocurrent densities reaching 84 ± 7 µA·cm-2, which is among the highest values reported so far for dye-sensitized photocathodes with surface-immobilized catalysts. Thanks to the unique combination of advanced spectroscopy and surface characterization techniques, the fast desorption of the dyad from the NiO electrode and the low yield of electron transfer to the catalyst, resulting in the Co demetallation from the diimine dioxime framework, were identified as the main barriers limiting the performances and the stability of the system. This work therefore paves the way for a more rational design of molecular photocathodes for solar fuel production and represents a further step toward the development of sustainable processes for the production of hydrogen from sunlight and water.

Electrocatalytic reduction of protons to dihydrogen by the cobalt tetraazamacrocyclic complex [Co(N4H)Cl2]+: mechanism and benchmarking of performances.

The cobalt tetraazamacrocyclic [Co(N4H)Cl2]+ complex is becoming a popular and versatile catalyst for the electrocatalytic evolution of hydrogen, because of its stability and superior activity in aqueous conditions. We present here a benchmarking of its performances based on the thorough analysis of cyclic voltammograms recorded under various catalytic regimes in non-aqueous conditions allowing control of the proton concentration. This allowed a detailed mechanism to be proposed with quantitative determination of the rate-constants for the various protonation steps, as well as identification of the amine function of the tetraazamacrocyclic ligand to act as a proton relay during H2 evolution.

Electrocatalytic Hydrogen Evolution with a Cobalt Complex Bearing Pendant Proton Relays: Acid Strength and Applied Potential Govern Mechanism and Stability.

[Co(bapbpy)Cl]+ (bapbpy: 6,6'-bis(2-aminopyridyl)-2,2'-bipyridine) is a polypyridyl cobalt(II) complex bearing both a redox-active bipyridine ligand and pendant proton relays. This compound catalyzes electro-assisted H2 evolution in DMF with distinct mechanisms depending on the strength of the acid used as the proton source (pKa values ranging from 3.4 to 13.5 in DMF) and the applied potential. Electrochemical studies combining cyclic voltammetry and bulk electrolysis measurements enabled one to bring out four distinct catalytic processes. Where applicable, relevant kinetic information were obtained using either foot-of-the-wave analysis (FOWA) or analytical treatment of bulk electrolysis experiments. Among the different catalytic pathways identified in this study, a clear relationship between the catalyst performances and stability was evidenced. These results draw attention to a number of interesting considerations and may help in the development of future adequately designed catalysts.

Tuning the Electron Storage Potential of a Charge-Photoaccumulating RuII Complex by a DFT-Guided Approach.

Molecular photosensitizers that are able to store multiple reducing equivalents are of great interest in the field of solar fuel production, where most reactions involve multielectronic reduction processes. In order to increase the reducing power of a ruthenium tris-diimine charge-photoaccumulating complex, two structural modifications on its fused dipyridophenazine-pyridoquinolinone ligand were computationally investigated. Addition of an electron-donating oxime group was calculated to substantially decrease the reduction potentials of the complex, thus guiding the synthesis of a pyridoquinolinone-oxime derivative. Its spectroscopic and (spectro)electrochemical characterization experimentally confirmed the DFT predictions, with the first and second reduction processes cathodically shifted by -0.24 and -0.14 V, respectively, compared to the parent complex. Moreover, the ability of this novel artificial photosynthetic system to store two photogenerated electrons at a more reducing potential, via a proton-coupled electron-transfer mechanism, was demonstrated.

Earth-Abundant Molecular Z-Scheme Photoelectrochemical Cell for Overall Water-Splitting.

A push-pull organic dye and a cobaloxime catalyst were successfully cografted on NiO and CuGaO2 to form efficient molecular photocathodes for H2 production with >80% Faradaic efficiency. CuGaO2 is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with 4-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO2 photocathode was combined with a TaON|CoO x photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H2 and O2 from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4 × 10-3%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme.

Identification of Three-Way DNA Junction Ligands through Screening of Chemical Libraries and Validation by Complementary in Vitro Assays.

The human genome is replete with repetitive DNA sequences that can fold into thermodynamically stable secondary structures such as hairpins and quadruplexes. Cellular enzymes exist to cope with these structures whose stable accumulation would result in DNA damage through interference with DNA transactions such as transcription and replication. Therefore, the chemical stabilization of secondary DNA structures offers an attractive way to foster DNA transaction-associated damages to trigger cell death in proliferating cancer cells. While much emphasis has been recently given to DNA quadruplexes, we focused here on three-way DNA junctions (TWJ) and report on a strategy to identify TWJ-targeting agents through a combination of in vitro techniques (TWJ-screen, polyacrylamide gel electrophoresis, fluorescence resonance energy transfer-melting, electrospray ionization mass spectrometry, dialysis equilibrium, and sulforhodamine B assays). We designed a complete workflow and screened 1200 compounds to identify promising TWJ ligands selected on stringent criteria in terms of TWJ-folding ability, affinity, and selectivity.

Structure of Ni(OH)2 intermediates determines the efficiency of NiO-based photocathodes - a case study using novel mesoporous NiO nanostars.

We report the wet chemical synthesis of mesoporous NiO nanostars (NS) as photocathode material for dye-sensitized solar cells (DSSCs). The growth mechanism of NiO NS as a new morphology of NiO is assessed by TEM and spectroscopic investigations. The NiO NS are obtained upon annealing of preformed ß-Ni(OH)2 into pristine NiO with low defect concentrations and favorable electronic configuration for dye sensitization. The NiO NS consist of fibers self-assembled from nanoparticles yielding a specific surface area of 44.9 m2 g-1. They possess a band gap of 3.83 eV and can be sensitized by molecular photosensitizers bearing a range of anchoring groups, e.g. carboxylic acid, phosphonic acid, and pyridine. The performance of NiO NS-based photocathodes in photoelectrochemical application is compared to that of other NiO morphologies, i.e. nanoparticles and nanoflakes, under identical conditions. Sensitization of NiO NS with the benchmark organic dye P1 leads to p-DSSCs with a high photocurrent up to 3.91 mA cm-2 whilst the photoelectrochemical activity of the NiO NS photocathode in aqueous medium in the presence of an irreversible electron acceptor is reflected by generation of a photocurrent up to 23 µA cm-2.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode.

Dye-sensitized photo-electrochemical cells (DS-PECs) form an emerging technology for the large-scale storage of solar energy in the form of (solar) fuels because of the low cost and ease of processing of their constitutive photoelectrode materials. Preparing such molecular photocathodes requires a well-controlled co-immobilization of molecular dyes and catalysts onto transparent semiconducting materials. Here we used a series of surface analysis techniques to describe the molecular assembly of a push-pull organic dye and a cobalt diimine-dioxime catalyst co-grafted on a p-type NiO electrode substrate. (Photo)electrochemical measurements allowed characterization of electron transfer processes within such an assembly and to demonstrate for the first time that a CoI species is formed as the entry into the light-driven H2 evolution mechanism of a dye-sensitized photocathode. This co-grafted noble-metal free H2-evolving photocathode architecture displays similar performances to its covalent dye-catalyst counterpart based on the same catalytic moiety. Post-operando time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis of these photoelectrodes after extensive photoelectrochemical operation suggested decomposition pathways of the dye and triazole linkage used to graft the catalyst onto NiO, providing grounds for the design of optimized molecular DS-PEC components with increased robustness upon turnover.

Electron transfer in a covalent dye-cobalt catalyst assembly - a transient absorption spectroelectrochemistry perspective.

Various oxidation states of the catalytically active cobalt center in a covalent dyad were electrochemically prepared and the light-induced excited-state processes were studied. Virtually identical deactivation processes are observed, irrespective of the oxidation state of the cobalt center, varying from CoIII to CoI, indicating the absence of oxidative quenching within the dye-catalyst assembly.

Photophysics of a Ruthenium Complex with a π-Extended Dipyridophenazine Ligand for DNA Quadruplex Labeling.

The light-switch mechanism of the complex [Ru(bpy)2(Br-dpqp)](PF6)2 (1, bpy = 2,2'-bipyridine, Br-dpqp = 12-bromo-14-ethoxydipyrido[3,2- a:2',3'- c]quinolino[3,2- h]phenazine), i.e., a light-up probe for the selective labeling of G-quadruplexes, is investigated by time-resolved transient absorption and emission spectroscopy. We show that, in contrast to the prototypical light-switch complex [Ru(bpy)2(dppz)](PF6)2 (2, dppz = dipyrido[3,2- a:2',3'- c]phenazine), a 3ππ* state localized on the π-extended ligand is the state determining the excited-state properties in both protic and aprotic environments. In aprotic environments, emission originates from a bright 3MLCTphen state, which is thermally accessible from the 3ππ* state at ambient temperature. In the presence of water, i.e., in environments resembling in cellulo situations, the thermally accessible 3MLCT state is altered and becomes close in energy to the 3ππ* state, which induces a rapid excited-state deactivation of the 3ππ* state and a comparably weak emission.

An artificial photosynthetic system for photoaccumulation of two electrons on a fused dipyridophenazine (dppz)-pyridoquinolinone ligand.

Increasing the efficiency of molecular artificial photosynthetic systems is mandatory for the construction of functional devices for solar fuel production. Decoupling the light-induced charge separation steps from the catalytic process is a promising strategy, which can be achieved thanks to the introduction of suitable electron relay units performing charge accumulation. We report here on a novel ruthenium tris-diimine complex able to temporarily store two electrons on a fused dipyridophenazine-pyridoquinolinone π-extended ligand upon visible-light irradiation in the presence of a sacrificial electron donor. Full characterization of this compound and of its singly and doubly reduced derivatives thanks to resonance Raman, EPR and (TD)DFT studies allowed us to localize the two electron-storage sites and to relate charge photoaccumulation with proton-coupled electron transfer processes.

A protocol for quantifying hydrogen evolution by dye-sensitized molecular photocathodes and its implementation for evaluating a new covalent architecture based on an optimized dye-catalyst dyad.

A protocol that combines gas chromatography and a high-sensitivity micro Clark-type electrode is described to quantify hydrogen production across gas and solution phases for systems operating at very low currents such as dye-sensitized H2-evolving photocathodes. Data indicate that a significant fraction of H2 remains in aqueous solution even after several hours of experiments. Using this protocol, re-evaluation of a dye-sensitized H2-evolving photocathode based on a dye-catalyst dyad showed a reproducible 66% increase of the faradaic efficiency compared with previously reported headspace GC measurements [Kaeffer et al., J. Am. Chem. Soc., 2016, 138, 12308-12311]. This dyad was based on an organic push-pull dye where donor and acceptor are separated by one thiophene group. Insertion of a second thiophene group between the donor and acceptor led to a more efficient system with 30% improved faradaic efficiency for H2 evolution.

Aqueous Photocurrent Measurements Correlated to Ultrafast Electron Transfer Dynamics at Ruthenium Tris Diimine-Sensitized NiO Photocathodes.

Understanding the structural and electronic factors governing the efficiency of dye-sensitized NiO photocathodes is essential to optimize solar fuel production in photoelectrochemical cells (PECs). For these purpose, three different ruthenium dyes, bearing either two or four methylphosphonate anchoring groups and either a bipyridine or a dipyridophenazine ancillary ligand, were synthesized and grafted onto NiO films. These photoelectrodes were fully characterized by XPS, ToF-SIMS, UV-vis absorption, time-resolved emission and femtosecond transient absorption spectroscopies. Increasing the number of anchoring groups from two to four proved beneficial for the grafting efficiency. No significant modification of the electronic properties compared to the parent photosensitizer was observed, in accordance with the non-conjugated nature of the grafted linker. The photoelectrochemical activity of the dye-sensitized NiO electrodes was assessed in fully aqueous medium in the presence of an irreversible electron acceptor and photocurrents reaching 190 µA.cm-2 were recorded. The transient absorption study revealed the presence of two charge recombination pathways for each of the sensitizers and evidenced a stabilized charge separated state in the dppz derivative, supporting its superior photoelectrochemical activity.

CuAAC-based assembly and characterization of a ruthenium-copper dyad containing a diimine-dioxime ligand framework.

The design of molecular dyads combining a light-harvesting unit with an electroactive centre is highly demanded in the field of artificial photosynthesis. The versatile Copper-catalyzed Azide-Alkyne Cycloaddition (CuAAC) procedure was employed to assemble a ruthenium tris-diimine unit to an unprecedented azide-substituted copper diimine-dioxime moiety. The resulting RuIICuII dyad 4 was characterized by electrochemistry, 1H NMR, EPR, UV-visible absorption, steady-state fluorescence and transient absorption spectroscopies. Photoinduced electron transfer from the ruthenium to the copper centre upon light-activation in the presence of a sacrificial electron donor was established thanks to EPR-monitored photolysis experiments, opening interesting perspectives for photocatalytic applications.

Selective Luminescent Labeling of DNA and RNA Quadruplexes by π-Extended Ruthenium Light-Up Probes.

A series of RuII complexes exhibiting π-extended, acridine-based ancillary chelating heterocycles display high affinity and selectivity for DNA and RNA quadruplexes. The most promising candidates (3, 4) possess remarkable light-up luminophore properties (up to 330-fold luminescence enhancement upon interaction with quadruplexes), enabling them to discriminate quadruplexes from genomic DNA owing to a photochemical mechanism involving DNA protection against non-radiative decay (DAND), thus deviating from the other complexes of this series of ligands that exhibit an excited-state intramolecular proton transfer (ESIPT) that quenches their luminescence. The in vitro and preliminary in cellulo results shown here confirm the interest of this new family of fluorophores as invaluable molecular tools to detect G-quadruplexes in cells.

Synthesis of three series of ruthenium tris-diimine complexes containing acridine-based π-extended ligands using an efficient "chemistry on the complex" approach.

The preparation and characterization of three series of novel ruthenium(ii) complexes are reported, each series differing by the nature of the ancillary ligands (2,2'-bipyridine - bpy, 1,10-phenanthroline - phen or 1,4,5,8-tetraazaphenanthrene - TAP). The third ligand was either the heptacyclic heterocycle dipyrido[3,2-a:2',3'-c]quinolino[3,2-h]phenazine (dpqp) substituted at position 12 by an hydroxyl (oxo), 2,2-dimethoxyethylamine (DMEA) or halogeno (Cl or Br) substituent, or the octacyclic dipyrido[3,2-a:2',3'-c]pyrido[2,3,4-de]quinolino[3,2-h]phenazine (dppqp), prepared by a multi-step "chemistry on the complex" strategy from [RuL2(oxo-dpqp)](PF6)2. The three steps, halogenation, substitution by a dimethoxyethylamino group and cyclization in trifluoroacetic acid, were performed in reasonable to high yields depending on the nature of the ancillary ligands. Isolation and purification processes were facilitated by the ability to switch the solubility of the complex from aqueous to organic solvents, depending on the counter-ion. All new complexes were fully characterized; in particular their absorption properties were compared by UV-vis spectroscopy. Finally, π-stacking properties induced by these extended ligands were studied by 1H NMR studies and quantum chemical calculations.

Covalent Design for Dye-Sensitized H2-Evolving Photocathodes Based on a Cobalt Diimine-Dioxime Catalyst.

Dye-sensitized photoelectrochemical cells (DS-PECs) for water splitting hold promise for the large-scale storage of solar energy in the form of (solar) fuels, owing to the low cost and ease to process of their constitutive photoelectrode materials. The efficiency of such systems ultimately depends on our capacity to promote unidirectional light-driven electron transfer from the electrode substrate to a catalytic moiety. We report here on the first noble-metal free and covalent dye-catalyst assembly able to achieve photoelectrochemical visible light-driven H2 evolution in mildly acidic aqueous conditions when grafted onto p-type NiO electrode substrate.

Design and synthesis of novel organometallic dyes for NiO sensitization and photo-electrochemical applications.

Two metallo-organic dyes were synthesized and used for NiO sensitization in view of their photoelectrochemical applications. The new dyes present an original π-conjugated structure containing the [Ru(dppe)2] metal fragment with a highly delocalized allenylidene ligand on one side and a σ-alkynyl ligand bearing an electron-rich group, i.e. a thiophene or triphenylamine unit, and one or two anchoring functions on the other side. The optoelectronic, electrochemical and photoelectrochemical properties of the dyes were systematically investigated. A broad photoresponse was observed with the absorption maximum at 600 nm. The X-ray crystal structure of one precursor was obtained to elucidate the structural conformation of the organometallic complexes and theoretical calculations were performed in order to address the photophysical properties of the new dyes. These photosensitizers were further implemented in NiO-based photocathodes and tested as photocurrent generators under pertinent aqueous conditions in association with [Co(NH3)5Cl]Cl2 as an irreversible electron acceptor. The dye-sensitized photocathodes provided good photocurrent densities (40 to 60 µA cm(-2)) at neutral pH in phosphate buffer and a high stability was observed for the two dyes.