Tris(bipyridine)Metal(II)-Templated Assemblies of 3D Alkali-Ruthenium Oxalate Coordination Frameworks: Crystal Structures, Characterization and Photocatalytic Activity in Water Reduction

A Ru(II)-Strained Complex with 2,9-Diphenyl-1,10-phenanthroline Ligand Induces Selective Photoactivatable Chemotherapeutic Activity on Human Alveolar Carcinoma Cells via Apoptosis

[Ru(bipy)2(dpphen)]Cl2 (where bipy = 2,2'-bipyridine and dpphen = 2,9-diphenyl-1,10-phenanthroline) (complex 1) is a sterically strained compound that exhibits promising in vitro photocytotoxicity on an array of cell lines. Since lung adenocarcinoma cancer remains the most common lung cancer and the leading cause of cancer deaths, the current study aims to evaluate the plausible effect and uptake of complex 1 on human alveolar carcinoma cells (A549) and mesenchymal stem cells (MSC), and assess its cytotoxicity in vitro while considering its effect on cell morphology, membrane integrity and DNA damage. MSC and A549 cells showed similar rates of complex 1 uptake with a plateau at 12 h. Upon photoactivation, complex 1 exhibited selective, potent anticancer activity against A549 cells with phototoxicity index (PI) values of 16, 25 and 39 at 24, 48 and 72 h, respectively. This effect was accompanied by a significant increase in A549-cell rounding and detachment, loss of membrane integrity and DNA damage. Flow cytometry experiments confirmed that A549 cells undergo apoptosis when treated with complex 1 followed by photoactivation. In conclusion, this present study suggests that complex 1 might be a promising candidate for photochemotherapy with photoproducts that possess selective anticancer effects in vitro. These results are encouraging to probe the potential activity of this complex in vivo.

Enhanced circularly polarized luminescence dissymmetry of [Ru(bpy)3]2+ complexes in a 3D chiral framework: a study of transparent thin films

Circularly polarized luminescence (CPL) plays an important role in the development of advanced optical devices. However, the design of CPL-active materials with a decent dissymmetry factor is still challenging. Here, we report CPL-active transparent thin films made from optically active ruthenium complexes [Ru(bpy)3]2+ embedded in chiral inorganic frameworks. Due to the unique chiral-in-chiral combination, the obtained [Ru(bpy)3][Zn2(C2O4)3] displays CPL activity with a dissymmetry factor of 5 × 10-3. The CPL measurements show that the luminescence dissymmetry factor can be effectively enhanced by one order of magnitude when an optically active [Ru(bpy)3]2+ complex is incorporated into a chiral inorganic framework compared to its solution form. This study not only emphasizes the potential of constructing CPL-active thin films with coordination polymers but also points out the importance of introducing extra chiral environment to improve the CPL effect.

Steric hindrance, ligand ejection and associated photocytotoxic properties of ruthenium(II) polypyridyl complexes

Two ruthenium(II) polypyridyl complexes were prepared with the {Ru(phen)₂}²⁺ moiety and a third sterically non-hindering bidentate ligand, namely 2,2'-dipyridylamine (dpa) and N-benzyl-2,2'-dipyridylamine (Bndpa). Hence, complexes [Ru(phen)₂(dpa)](PF₆)₂ (1) and [Ru(phen)₂(Bndpa)](PF₆)₂ (2) were characterized and their photochemical behaviour in solution (acetonitrile and water) was subsequently investigated. Compounds 1 and 2, which do not exhibit notably distorted octahedral coordination environments, contrarily to the homoleptic "parent" compound [Ru(phen)₃](PF₆)₂, experience two-step photoejection of the dpa and Bndpa ligand upon irradiation (1050-430 nm) for several hours. DNA-binding studies revealed that compounds 1 and 2 affect the biomolecule differently upon irradiation; while 2 solely modifies its electrophoretic mobility, complex 1 is also capable of cleaving it. In vitro cytotoxicity studies with two cancer-cell lines, namely A549 (lung adenocarcinoma) and A375 (melanoma), showed that both 1 and 2 are not toxic in the dark, while only 1 is significantly cytotoxic if irradiated, 2 remaining non-toxic under these conditions. Light irradiation of the complex cation [Ru(phen)₂(dpa)]²⁺ leads to the generation of transient Ru species that is present in the solution medium for several hours, and that is significantly cytotoxic, ultimately producing non-toxic free dpa and [Ru(phen)(OH₂)₂]²⁺.

Elucidating the formation and active state of Cu co-catalysts for photocatalytic hydrogen evolution

The design of active and selective co-catalysts constitutes one of the major challenges in developing heterogeneous photocatalysts for energy conversion applications. This work provides a comprehensive insight into thermally induced bottom-up generation and transformation of a series of promising Cu-based co-catalysts. We demonstrate that the volcano-type HER profile as a function of calcination temperature is independent of the type of the Cu precursor but is affected by changes in oxidation state and location of the copper species. Supported by DFT modeling, our data suggest that low temperature (<200 °C) treatments facilitate electronic communication between the Cu species and TiO2, which allows for a more efficient charge utilization and maximum HER rates. In contrast, higher temperatures (>200 °C) do not affect the Cu oxidation state, but induce a gradual, temperature-dependent surface-to-bulk diffusion of Cu, which results in interstitial, tetra-coordinated Cu+ species. The disappearance of Cu from the surface and the introduction of new defect states is associated with a drop in HER performance. This work examines electronic and structural effects that are in control of the photocatalytic activity and can be transferred to other systems for further advancing photocatalysis.

General Design Concept for Single-Atom Catalysts toward Heterogeneous Catalysis

As a new and popular material, single-atom catalysts (SACs) exhibit excellent activity, selectivity, and stability for numerous important reactions, and show great potential in heterogeneous catalysis due to their high atom utilization efficiency and the controllable characteristics of the active sites. The composition and coordination would determine the geometric and electronic structures of SACs, and thus greatly influence the catalytic performance. Based on atom economy, rational design and controllable synthesis of SACs have become central tasks in the fields of low-cost and green catalysis. Herein, an introduction to the recent progress in the precise synthesis of SACs including the regulation of the coordination structure and the choice of different systems is presented. Thereafter, the potentials of SACs in different applications are comprehensively summarized and discussed. Furthermore, a detailed discussion of the recent developments regarding the large-scale preparation of SACs is provided, including the major issues and prospects for industrialization. Finally, the main challenges and opportunities of rapid large-scale industrialization of SACs are briefly discussed.

Modulation of Osmium(II) Tris(2,2'-bipyridine) Photophysics through Encapsulation within Zinc(II) Trimesic Acid Metal Organic Frameworks

The Os(II) tris(2,2'-bipyridine) (OsBpy) complex exhibits optical properties that are particularly attractive for light harvesting systems due to the broad absorption spectrum extending throughout the solar spectrum. However, the relatively short lifetime of the triplet metal to ligand charge transfer state (³MLCT) relative to the related Ru(II)tris(2,2'-bipyridine) (RuBpy) has limited applications. Here, the encapsulation of OsBpy within two distinct Zn(II)-trimesic acid MOFs, HKUST-1(Zn) and USF-2 is demonstrated in an effort to extend the ³MLCT lifetime. Encapsulation results in a hypsochromatic shift of the steady-state emission band in both frameworks resulting from a destabilization of the ³MLCT. The encapsulated OsBpy also exhibits extended emission lifetimes in both HKUST-1(Zn) (104 ns in MOF vs 50 ns in methanol) and USF-2 (81 ns in MOF vs 50 ns in methanol) arising from changes in the nonradiative decay constants in both systems. The data are also consistent with vibronic perturbations involved in mixing between higher energy ³MLCT* and ligand field states.

Giant Enhancement of Carrier Mobility in Bimetallic Coordination Polymers

Electrically conductive metal-organic coordination polymers (CPs) are promising candidates for a variety of technological applications. However, poor energetic and spatial overlap between the sp-electrons of organic ligands and the d-electrons of metal ion often blocks an effective charge transport (mobility) across CPs. Herein, we present a bimetallic design principle for enhancing carrier mobility in CPs. Bimetallic CPs of Fe(III) and Cr(III) ions coordinated to 1,3,5-benzenetricarboxylic acid (BTC) ligand (Fe-BTC-Cr) exhibited remarkably high carrier mobility at the matching mole ratio (1:1) with enhancement factors of 10² and 10⁴ in comparison to those of monometallic parents, Fe-BTC and Cr-BTC, respectively. The observation was substantiated by lowering of the band gap between the valence band and the conduction band upon the formation of a hybrid p-n-type structure in the bimetallic CPs. The direct current conductivity values of the CPs measured by four-probe technique were in good agreement with the alternating current conductivity values obtained from the electrochemical impedance spectroscopy. Our flexible approach of picking and choosing the appropriate combination of metal ions from the periodic table is expected to generate various CPs with desirable semiconducting properties.

Designing Kitaev Spin Liquids in Metal-Organic Frameworks

Kitaev's honeycomb lattice spin model is a remarkable exactly solvable model, which has a particular type of spin liquid (Kitaev spin liquid) as the ground state. Although its possible realization in iridates and α-RuCl_{3} has been vigorously discussed recently, these materials have substantial non-Kitaev direct exchange interactions and do not have a spin liquid ground state. We propose metal-organic frameworks (MOFs) with Ru^{3+} (or Os^{3+}), forming the honeycomb lattice as promising candidates for a more ideal realization of Kitaev-type spin models, where the direct exchange interaction is strongly suppressed. The great flexibility of MOFs allows generalization to other three-dimensional lattices for the potential realization of a variety of spin liquids, such as a Weyl spin liquid.

Influence of the Steric Bulk and Solvent on the Photoreactivity of Ruthenium Polypyridyl Complexes Coordinated to l-Proline

Ruthenium polypyridyl complexes are good candidates for photoactivated chemotherapy (PACT) provided that they are stable in the dark but efficiently photosubstitute one of their ligands. Here the use of the natural amino acid l-proline as a protecting ligand for ruthenium-based PACT compounds is investigated in the series of complexes Λ-[Ru(bpy)₂(l-prol)]PF₆ ([1a]PF₆; bpy = 2,2′-bipyridine and l-prol = l-proline), Λ-[Ru(bpy)(dmbpy)(l-prol)]PF₆ ([2a]PF₆ and [2b]PF₆; dmbpy = 6,6′-dimethyl-2,2′-bipyridine), and Λ-[Ru(dmbpy)₂(l-prol)]PF₆ ([3a]PF₆). The synthesis of the tris-heteroleptic complex bearing the dissymmetric proline ligand yielded only two of the four possible regioisomers, called [2a]PF₆ and [2b]PF₆. Both isomers were isolated and characterized by a combination of spectroscopy and density functional theory calculations. The photoreactivity of all four complexes [1a]PF₆, [2a]PF₆, [2b]PF₆, and [3a]PF₆ was studied in water (H₂O) and acetonitrile (MeCN) using UV–vis spectroscopy, circular dichroism spectroscopy, mass spectrometry, and ¹H NMR spectroscopy. In H₂O, upon visible-light irradiation in the presence of oxygen, no photosubstitution took place, but the amine of complex [1a]PF₆ was photooxidized to an imine. Contrary to expectations, enhancing the steric strain by the addition of two ([2b]PF₆) or four ([3a]PF₆) methyl substituents did not lead, in phosphate-buffered saline (PBS), to ligand photosubstitution. However, it prevented photoxidation, probably as a consequence of the electron-donating effect of the methyl substituents. In addition, whereas [2b]PF₆ was photostable in PBS, [2a]PF₆ quantitatively isomerized to [2b]PF₆ upon light irradiation. In pure MeCN, [2a]PF₆ and [3a]PF₆ showed non-selective photosubstitution of both the l-proline and dmbpy ligands, whereas the non-strained complex [1a]PF₆ was photostable. Finally, in H₂O–MeCN mixtures, [3a]PF₆ showed selective photosubstitution of l-proline, thus demonstrating the active role played by the solvent on the photoreactivity of this series of complexes. The role of the solvent polarity and coordination properties on the photochemical properties of polypyridyl complexes is discussed.

Three ruthenium polypyridyl l-proline complexes with increasing strain (R, R′ = H or Me) were synthesized and their photoreactivities studied in phosphate-buffered saline, pure acetonitrile (MeCN), and water−MeCN mixtures. Depending on the number of methyl groups, on the presence of air, and on the nature of the solvent, either photoisomerization, photooxidation of l-proline, selective photosubstitution, or nonselective photosubstitution was observed.