Highly Efficient, Noble-Metal-Free, Fully Aqueous CO2 Photoreduction Sensitized by a Robust Organic Dye

The development of efficient, selective, and durable CO2 photoreduction systems presents a long-standing challenge in full aqueous solutions owing to the presence of scarce CO2 and the fierce competition against H2 evolution, which is even more challenging when noble metals are not utilized. Herein, we present the facile decorations of four phosphonic acid groups on a donor-acceptor-type organic dye to obtain a water-soluble photosensitizer (4P-DPAIPN), which succeeds the excellent photophysical and photoredox properties of its prototype, exhibiting long-lived delayed fluorescence (>10 μs) in aqueous solutions. Combining 4P-DPAIPN with a cationic cobalt porphyrin catalyst has accomplished record-high apparent quantum yields of 9.4-17.4% at 450 nm for CO2-to-CO photoconversion among the precedented systems (maximum 13%) in fully aqueous solutions. Remarkable selectivity of 82-93% and turnover number of 2700 for CO production can also be achieved with this noble-metal-free system, outperforming a benchmarking ruthenium photosensitizer and a commercial organic dye under parallel conditions. Such high performances of 4P-DPAIPN can be well maintained under real sunlight. More impressively, no significant decomposition of 4P-DPAIPN was detected during the long-term photocatalysis. Eventually, the photoinduced electron transfer pathways were proposed.

Efficient Visible-Light-Driven Carbon Dioxide Reduction using a Bioinspired Nickel Molecular Catalyst

Inspired by natural enzymes, this study presents a nickel-based molecular catalyst, [Ni‖(N2S2)]Cl2 (NiN2S2, N2S2=2,11-dithia[3,3](2,6)pyridinophane), for the photochemical catalytic reduction of CO2 under visible light. The catalyst was synthesized and characterized using various techniques, including liquid chromatography-high resolution mass spectrometry (LC-HRMS), UV-Visible spectroscopy, and X-ray crystallography. The crystallographic analysis revealed a slightly distorted octahedral coordination geometry with a mononuclear Ni2+ cation, two nitrogen atoms and two sulfur atoms. Photocatalytic CO2 reduction experiments were performed in homogeneous conditions using the catalyst in combination with [Ru(bpy)3]Cl2 (bpy=2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor. The catalyst achieved a high selectivity of 89 % towards CO and a remarkable turnover number (TON) of 7991 during 8 h of visible light irradiation under CO2 in the presence of phenol as a co-substrate. The turnover frequency (TOF) in the initial 6 h was 1079 h-1, with an apparent quantum yield (AQY) of 1.08 %. Controlled experiments confirmed the dependency on the catalyst, light, and sacrificial electron donor for the CO2 reduction process. These findings demonstrate this bioinspired nickel molecular catalyst could be effective for fast and efficient photochemical catalytic reduction of CO2 to CO.

A Phosphine Oxide-Functionalized Cyclam as a Specific Copper(II) Chelator

Although cyclam-based ligands are among the strongest copper(II) chelators available, they also usually present good affinity for other divalent cations [Zn(II), Ni(II), and Co(II)], with no copper(II)-specific cyclam ligands having been described so far. As such a property is highly desirable in a wide range of applications, we present herein two novel phosphine oxide-appended cyclam ligands that could be efficiently synthesized through Kabachnik-Fields type reactions on protected cyclam precursors. Their copper(II) coordination properties were closely studied by different physicochemical techniques [electron paramagnetic resonance (EPR) and ultraviolet-visible (UV-vis) spectroscopies, X-ray diffraction, and potentiometry]. The mono(diphenylphosphine oxide)-functionalized ligand demonstrated a copper(II)-specific behavior, unprecedented within the cyclam family of ligands. This was evidenced by UV-vis complexation and competition studies with the parent divalent cations. Density functional theory calculations also confirmed that the particular ligand geometry in the complexes strongly favors copper(II) coordination over that of competing divalent cations, rationalizing the specificity observed experimentally.

Copper(II) and Zinc(II) Complexation with N Ethylene hydroxycyclams and Consequences on the Macrocyclic Backbone Configuration

We report a series of four cyclams and cross-bridged cyclams, N-functionalized by one hydroxyethyl arm, which may incorporate additional methyl(s) group(s). The Cu(II) and Zn(II) complexes of these ligands were...

Time-Resolved Spectroscopy and Electronic Structure of Mono-and Dinuclear Pyridyl-Triazole/DPEPhos-Based Cu(I) Complexes

Chemical and spectroscopic characterization of the mononuclear photosensitizers [(DPEPhos)Cu(I)(MPyrT)]0/+ (CuL, CuLH) and their dinuclear analogues (Cu2 L', Cu2 L'H2 ), backed by (TD)DFT and high-level GW-Bethe-Salpeter equation calculations, exemplifies the complex influence of charge, nuclearity and structural flexibility on UV-induced photophysical pathways. Ultrafast transient absorption and step-scan FTIR spectroscopy reveal flattening distortion in the triplet state of CuLH as controlled by charge, which also appears to have a large impact on the symmetry of the long-lived triplet states in Cu2 L' and Cu2 L'H2 . Time-resolved luminescence spectroscopy (solid state), supported by transient photodissociation spectroscopy (gas phase), confirm a lifetime of some tens of μs for the respective triplet states, as well as the energetics of thermally activated delayed luminescence, both being essential parameters for application of these materials based on earth-abundant copper in photocatalysis and luminescent devices.

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.

Cyclam-Based Chelators Bearing Phosphonated Pyridine Pendants for 64Cu-PET Imaging: Synthesis, Physicochemical Studies, Radiolabeling, and Bioimaging

Herein we present the preparation of two novel cyclam-based macrocycles (te1pyp and cb-te1pyp), bearing phosphonate-appended pyridine side arms for the coordination of copper(II) ions in the context of ⁶⁴Cu PET imaging. The two ligands have been prepared through conventional protection-alkylation sequences on cyclam, and their coordination properties have been thoroughly investigated. The corresponding copper complexes have been fully characterized in the solid state (X-ray diffraction analysis) and in solution (EPR and UV-vis spectroscopies). Potentiometric studies combined with spectrometry have also allowed us to determine their thermodynamic stability constants, confirming their high affinity for copper(II) cations. The kinetic inertness of the complexes has been verified by acid-assisted dissociation experiments, enabling their use in ⁶⁴Cu-PET imaging in mice for the first time. Indeed, the two ligands could be quantitatively radiolabeled under mild conditions, and the resulting ⁶⁴Cu complexes have demonstrated excellent stability in serum. PET imaging demonstrated a set of features emerging from the combination of picolinates and phosphonate units: high stability in vivo, fast clearance from the body via renal elimination, and most interestingly, very low fixation in the liver. This is in contrast with what was observed for monopicolinate cyclam (te1pa), which had a non-negligible accumulation in the liver, owing probably to its different charge and lipophilicity. These results thus pave the way for the use of such phosphonated pyridine chelators for in vivo ⁶⁴Cu-PET imaging.

Molecular catalysis of CO2 reduction: recent advances and perspectives in electrochemical and light-driven processes with selected Fe, Ni and Co aza macrocyclic and polypyridine complexes

Earth-abundant Fe, Ni, and Co aza macrocyclic and polypyridine complexes have been thoroughly investigated for CO2 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 CO2 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 CO2 reduction remains a highly creative approach.

The good, the neutral, and the positive: buffer identity impacts CO2 reduction activity by nickel(ii) cyclam

Development of new synthetic catalysts for CO2 reduction has been a central focus of chemical research efforts towards mitigating rising global carbon dioxide levels. In parallel with generating new molecular systems, characterization and benchmarking of these compounds across well-defined catalytic conditions are essential. Nickel(ii) cyclam is known to be an active catalyst for CO2 reduction to CO. The degree of selectivity and activity has been found to differ widely across electrodes used and upon modification of the ligand environment, though without a molecular-level understanding of this variation. Moreover, while proton transfer is key for catalytic activity, the effects of varying the nature of the proton donor remain unclear. In this work, a systematic investigation of the electrochemical and light-driven catalytic behaviour of nickel(ii) cyclam under different aqueous reaction conditions has been performed. The activity and selectivity are seen to vary widely depending on the nature of the buffering agent, even at a constant pH, highlighting the importance of proton transfer for catalysis. Buffer binding to the nickel center is negatively correlated with selectivity, and cationic buffers show high levels of selectivity and activity. These results are discussed in the context of molecular design principles for developing increasingly efficient and selective catalysts. Moreover, identifying these key contributors towards activity has implications for understanding the role of the conserved secondary coordination environments in naturally occurring CO2-reducing enzymes, including carbon monoxide dehydrogenase and formate dehydrogenase.

A Molecular CO2 Reduction Catalyst Based on Giant Polyoxometalate {Mo368}

Photocatalytic CO2 reduction in water is one of the most attractive research pursuits of our time. In this article we report a giant polyoxometalate {Mo368} based homogeneous catalytic system, which efficiently reduces CO2 to formic acid with a maximum turnover number (TON) of 27,666, turnover frequency (TOF) of 4,611 h-1 and external quantum efficiency of the reaction is 0.6%. The catalytic system oxidizes water and releases electrons, and these electrons are further utilized for the reduction of CO2 to formic acid. A maximum of 8.3 mmol of formic acid was observed with the loading of 0.3 μmol of the catalyst. Our catalyst material is also stable throughout the reaction. The starting materials for this experiment are CO2 and H2O and the end products are HCOOH and O2. The formic acid formed in this reaction is an important H2 gas carrier and thus significant in renewable energy research.

"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

A photoactive semisynthetic metalloenzyme exhibits complete selectivity for CO2 reduction in water

A series of artificial metalloenzymes containing a ruthenium chromophore and [NiII(cyclam)]2+, both incorporated site-selectively, have been constructed within an azurin protein scaffold. These light-driven, semisynthetic enzymes do not evolve hydrogen, thus displaying complete selectivity for CO2 reduction to CO. Electrostatic effects rather than direct excited-state electron transfer dominate the ruthenium photophysics, suggesting that intramolecular electron transfer from photogenerated RuI to [NiII(cyclam)]2+ represents the first step in catalysis. Stern-Volmer analyses rationalize the observation that ascorbate is the only sacrificial electron donor that supports turnover. Collectively, these results highlight the important interplay of elements that must be considered when developing and characterizing molecular catalysts.

ZnSe quantum dots modified with a Ni(cyclam) catalyst for efficient visible-light driven CO2 reduction in water

A precious metal and Cd-free photocatalyst system for efficient CO2 reduction in water is reported. The hybrid assembly consists of ligand-free ZnSe quantum dots (QDs) as a visible-light photosensitiser combined with a phosphonic acid-functionalised Ni(cyclam) catalyst, NiCycP. This precious metal-free photocatalyst system shows a high activity for aqueous CO2 reduction to CO (Ni-based TONCO > 120), whereas an anchor-free catalyst, Ni(cyclam)Cl2, produced three times less CO. Additional ZnSe surface modification with 2-(dimethylamino)ethanethiol (MEDA) partially suppresses H2 generation and enhances the CO production allowing for a Ni-based TONCO of > 280 and more than 33% selectivity for CO2 reduction over H2 evolution, after 20 h visible light irradiation (λ > 400 nm, AM 1.5G, 1 sun). The external quantum efficiency of 3.4 ± 0.3% at 400 nm is comparable to state-of-the-art precious metal photocatalysts. Transient absorption spectroscopy showed that band-gap excitation of ZnSe QDs is followed by rapid hole scavenging and very fast electron trapping in ZnSe. The trapped electrons transfer to NiCycP on the ps timescale, explaining the high performance for photocatalytic CO2 reduction. With this work we introduce ZnSe QDs as an inexpensive and efficient visible light-absorber for solar fuel generation.

Highly efficient and selective visible-light driven CO2-to-CO conversion by a Co-based cryptate in H2O/CH3CN solution

A Co-based cryptate exhibits highly efficient and selective photocatalytic CO₂-to-CO conversion under either pure CO₂ or 10% CO₂ in a water-containing system.

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 Ru^IICu^II 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.

Organometallic cobalamin anticancer derivatives for targeted prodrug delivery via transcobalamin-mediated uptake

We report the synthesis and study of new water-soluble vitamin B₁₂ prodrugs bearing metal complexes at the β-upper side of the cobalt center for targeted prodrug delivery.

Iron-catalyzed photoreduction of carbon dioxide to synthesis gas

Photocatalytic processes to convert CO₂ to useful products including CO and HCOOH are of particular interest as a means to harvest the power of the sun for sustainable energy applications. Herein, we report the photocatalytic reduction of CO₂ using iron-based catalysts and visible light generating varying ratios of synthesis gas.

Photochemical CO2 reduction in water using a co-immobilised nickel catalyst and a visible light sensitiser

Dye-sensitised photocatalytic CO₂ reduction in water using an immobilised Nickel catalyst operates with efficiencies greatly exceeding the equivalent solution based system.

CuAAC click reactions for the design of multifunctional luminescent ruthenium complexes

CuAAC (Cu(i) catalyzed azide–alkyne cycloaddition) click chemistry has emerged as a versatile tool in the development of photoactive ruthenium complexes with multilateral potential applicability. Three general concepts for their synthesis and selected applications are discussed.

A functionalised nickel cyclam catalyst for CO2 reduction: electrocatalysis, semiconductor surface immobilisation and light-driven electron transfer

Covalent immobilisation of a low cost electrocatalyst leads to an enhanced rate of photoelectron transfer from a light absorbing semiconductor.

Selective and efficient photocatalytic CO2 reduction to CO using visible light and an iron-based homogeneous catalyst

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.