Luminescent Supramolecular Ionic Frameworks based on Organic Fluorescent Polycations and Polyanionic Phosphorescent Metal Clusters.

Emissive ionic supramolecular frameworks are designed by associating tetraphenylethylene-based tetra-cationic units and di-anionic molybdenum or tetra-anionic rhenium octahedral clusters. Obtained structures were characterized by single-crystal X-ray diffraction. The emission properties of the hybrids were investigated as dry powders or in various solvents by one photon and two photon absorption leading to a O2 concentration dependent luminescence color for the Mo based hybrid.

In-Situ Electrochemical Exfoliation and Methylation of Black Phosphorus into Functionalized Phosphorene Nanosheets.

Two-dimensional black phosphorus (BP) has attracted great attention as a perspective material for various applications. The chemical functionalization of BP is an important pathway for the preparation of materials with improved stability and enhanced intrinsic electronic properties. Currently, most of the methods for BP functionalization with organic substrates require either the use of low-stable precursors of highly reactive intermediates or the use of difficult-to-manufacture and flammable BP intercalates. Herein we report a facile route for simultaneous electrochemical exfoliation and methylation of BP. Conducting the cathodic exfoliation of BP in the presence of iodomethane makes it possible to generate highly active methyl radicals, which readily react with the electrode's surface yielding the functionalized material. The covalent functionalization of BP nanosheets with the P-C bond formation has been proven by various microscopic and spectroscopic methods. The functionalization degree estimated by solid-state 31P NMR spectroscopy analysis reached 9.7%.

The Cluster Polyazide Complexes: Synthesis, Crystal Structures, and 14N NMR Studies of [{Re3(µ-X)3}(N3)9]3- (X = Br or I).

Azide cluster complexes [{Re3(µ-Br)3}(N3)9]3- and [{Re3(µ-I)3}(N3)9]3- were obtained by reaction of Re3X9 (X = Br or I, respectively) with sodium azide in methanol. The complexes were crystallized as cesium salts of the compositions Cs3[{Re3(µ-Br)3}(N3)9]·H2O (1) and Cs3[{Re3(µ-I)3}(N3)9]·H2O (2) and characterized by X-ray single-crystal diffraction and elemental analyses, mass spectrometry, 14N NMR spectroscopy, and DFT calculations. In the anions, each rhenium atom is coordinated by three azide ligands. To the best of our knowledge, these compounds represent the first case of metal cluster polyazide complexes (i.e., where each metal atom bears more than one azide ligand). Both complexes are stable in air but are very shock sensitive, and spontaneous explosive decomposition is also possible.

Coordination Polymers Based on Rhenium Octahedral Chalcocyanide Cluster Anions and Ag+ Cations with Bipyridine Analogs.

A series of six coordination polymers based on octahedral cluster anions [Re6Q8(CN)6]4- (Q = S or Se) and Ag+ cations coordinated by bipyridine analogs were synthesized under solvothermal conditions. Their structures have been characterized by single crystal X-ray diffraction. Compounds 1 and 2 described by the general formula [{Ag(phen)}4Re6Q8(CN)6] (Q = Se (1), S (2); phen = 1,10-phenanthroline) exhibit layered structures assembled into a supramolecular network by CH…π contacts. At the same time, compounds [{Ag(bipym)}2Ag2Re6Se8(CN)6] (bipym = 2,2'-bipyrimidine) (3), [{Ag2(bipy)}Ag2Re6Se8(CN)6]·CH3CN (bipy = 4,4'-bipyridine) (4) and [{Ag(dpbp)}4Re6Q8(CN)6]·2H2O·2CH3CN (Q = Se (5), S (6); dpbp = 4,4'-Di(4-pyridyl)biphenyl)) evince framework structures. In 1, 2, 5 and 6 weak Ag⋯Ag interactions are observed. All the compounds show luminescence in the red region. The luminescence quantum yields and lifetimes were found to be notably higher than those for most of the coordination polymers based on the octahedral rhenium cluster complexes.

Octahedral Rhenium Cluster Complexes with 1,2-Bis(4-pyridyl)ethylene and 1,3-Bis(4-pyridyl)propane as Apical Ligands.

A series of eight new octahedral rhenium cluster complexes with the general formula trans-[{Re6Q8}L4X2] (Q = S or Se; L = 1,2-Bis(4-pyridyl)ethylene (bpe) or 1,3-Bis(4-pyridyl)propane (bpp); X = Cl or Br) was synthesized and investigated. While bpe is a ligand with a conjugated aromatic system, bpp represents a molecule of opposite type and has independent aromatic systems of the two pyridine rings. It was shown that this difference in the electronic structure of the ligands has a fundamental effect on the electronic structure, electrochemical and luminescent properties of the corresponding cluster complexes. Specifically, the [{Re6Q8}(bpe)4X2] complexes in solutions show multiple quasi-reversible electrochemical transitions associated with reduction of the organic ligands. At the same time, the trans-[{Re6Q8}(bpp)4X2] complexes show multielectron quasi-irreversible reduction processes, which correlate with the mixed character of the lowest unoccupied molecular orbitals of these complexes. All the obtained new compounds exhibit red photoluminescence. The photophysical parameters (emission lifetimes and quantum yields) measured for the bpp complexes exceed those revealed for bpe complexes by more than an order of magnitude.

Soluble Molecular Rhenium Cluster Complexes Exhibiting Multistage Terminal Ligands Reduction.

Substitution of terminal halide ligands of octahedral rhenium cluster complexes [Re6Q8X6]4- in a melt of 4,4'-bipyridine (bpy) led to us obtaining four new compounds with the general formula trans-[Re6Q8(bpy)4X2] (Q = S or Se; X = Cl or Br) in high yield. In contrast to most of the known molecular rhenium cluster complexes with heteroaromatic terminal ligands, compounds 1-4 are soluble in organic solvents. This made it possible to carry out a detailed characterization of the new compounds both in solids and in solutions. In particular, it was shown that compounds 1-4 in the DMSO solution exhibit four reversible reduction processes. A comparison of the obtained data with the results of DFT calculations of the electronic structure suggests that these processes correspond to two-electron reduction of all four bpy ligands. The reduction potentials are shifted to the positive region compared with the potential of free bipyridine, and the value of the shift depends on the composition of the cluster core. The presence of four transitions also suggests that electronic exchange between terminal ligands in the cis-position is possible. The study of the luminescence of the compounds showed that emission maxima of selenide clusters almost coincide with those of sulfide ones, while luminescence spectra of the complexes with chloride terminal ligands (1 and 3) are slightly blue-shifted relative to the spectra of the complexes with bromide ligands (2 and 4).

Cyanide Complexes Based on {Mo6I8}4+ and {W6I8}4+ Cluster Cores.

Compounds based on new cyanide cluster anions [{Mo6I8}(CN)6]2-, trans-[{Mo6I8}(CN)4(MeO)2]2- and trans-[{W6I8}(CN)2(MeO)4]2- were synthesized using mechanochemical or solvothermal synthesis. The crystal and electronic structures as well as spectroscopic properties of the anions were investigated. It was found that the new compounds exhibit red luminescence upon excitation by UV light in the solid state and solutions, as other cluster complexes based on {Mo6I8}4+ and {W6I8}4+ cores do. The compounds can be recrystallized from aqueous methanol solutions; besides this, it was shown using NMR and UV-Vis spectroscopy that anions did not undergo hydrolysis in the solutions for a long time. These facts indicate that hydrolytic stabilization of {Mo6I8} and {W6I8} cluster cores can be achieved by coordination of cyanide ligands.

[{Re6Q8}(SO3)6]10- (Q = S or Se): Facile Synthesis and Properties of the Most Highly Charged Octahedral Cluster Complexes and High Magnetic Relaxivity of Their Colloids with Gd3+ Ions.

New octahedral rhenium cluster complexes [{Re6Q8}(SO3)6]10- (Q = S or Se) were synthesized starting from [{Re6Q8}(H2O)4(OH)2]·12H2O. The complexes were crystallized as sodium salts and characterized by X-ray single-crystal diffraction and elemental analyses, IR, UV/vis and luminescence spectroscopies. Magnetic relaxation data demonstrate the complex formation of the cluster units with gadolinium ions. The analysis of the magnetic relaxation rates measured at various Gd:cluster ratios and different concentrations revealed the conversion of the aggregates (Gdx[{Re6Se8}(SO3)6]y)n- into a nanoparticulate form even at x = 1 and y ≥ 1. Thus, the self-assembly of the cluster units into the nanoparticles is greatly facilitated by counterion binding with sodium cations. The concentration conditions were optimized for the formation and hydrophilization of NaxGdy[{Re6Q8}(SO3)6]-based colloids with the magnetic relaxivity values of r1(2) = 21.0(24.1) and r1(2) = 25.9(29.8) mM-1 s-1 for the {Re6S8}2+ and {Re6Se8}2+ derivatives, respectively.

Host-Guest Binding Hierarchy within Redox- and Luminescence-Responsive Supramolecular Self-Assembly Based on Chalcogenide Clusters and γ-Cyclodextrin.

Water-soluble salts of anionic [Re6 Q8 (CN)6 ]4- (Q=S, Se, Te) chalcogenide octahedral rhenium clusters react with γ-cyclodextrin (γ-CD) producing a new type of inclusion compounds. Crystal structures determined through single-crystal X-ray diffraction analysis revealed supramolecular host-guest assemblies resulting from close encapsulations of the octahedral cluster within two γ-CDs. Interestingly, nature of the inner Q ligands influences strongly the host-guest conformation. The cluster [Re6 S8 (CN)6 ]4- interacts preferentially with the primary faces of the γ-CD while the bulkier clusters [Re6 Se8 (CN)6 ]4- and [Re6 Te8 (CN)6 ]4- exhibit specific interactions with the secondary faces of the cyclic host. Furthermore, analysis of the crystal packing reveals additional supramolecular interactions that lead to 2D infinite arrangements with [Re6 S8 (CN)6 ]4- or to 1D "bamboo-like" columns with [Re6 Se8 (CN)6 ]4- and [Re6 Te8 (CN)6 ]4- species. Solution studies, using multinuclear NMR methods, ESI-MS and Isothermal titration calorimetry (ITC) corroborates nicely the solid-state investigations showing that supramolecular pre-organization is retained in aqueous solution even in diluted conditions. Furthermore, ITC analysis showed that host-guest stability increases significantly ongoing from S to Te. At last, we report herein that deep inclusion alters significantly the intrinsic physical-chemical properties of the octahedral clusters, allowing redox tuning and near IR luminescence enhancement.

Sensing activity of cholinesterases through a luminescence response of the hexarhenium cluster complex [{Re6S8}(OH)6](4.).

The present work describes a new method to sense cholinesterase-catalyzed hydrolysis of acetylcholine (ACh) through a luminescence response of the hexarhenium cluster complex [{Re6S8}(OH)6](4-). A proton released from acetylcholinesterase (AChE)- or butyrylcholinesterase (BuChE)-catalyzed hydrolysis of ACh results in time-resolved sensitization of cluster-centered luminescence. The sensitization results from protonation of apical hydroxo-groups of the cluster complex. The protonation is affected by a counter ion effect. Thus, optimal conditions for adequate sensing of acetic acid produced by ACh hydrolysis are highlighted. Time-resolved luminescence and pH measurements under conditions of AChE-catalyzed hydrolysis of ACh show a good correlation between the cluster-centered luminescence and pH-induced inhibition of AChE. The inhibition is not significant within the first two minutes of ACh hydrolysis. Thus, the luminescence response measured within two minutes is dependent on both substrate and enzyme concentrations, which fits with AChE and BuChE kinetics. The usability of cluster-centered luminescence for monitoring the concentration-dependent inhibition of AChE with irreversible inhibitors is demonstrated, using a carbamylating agent, pyridostigmine bromide, as a model.

Synthetic Tuning of Redox, Spectroscopic, and Photophysical Properties of {Mo6I8}(4+) Core Cluster Complexes by Terminal Carboxylate Ligands.

The reactions between the tetra-n-butylammonium salt of [{Mo6I8}I6](2-) and silver carboxylates RCOOAg (R = CH3 (1), C(CH3)3 (2), α-C4H3O (3), C6H5 (4), α-C10H7 (5), or C2F5 (6)) in CH2Cl2 afforded new carboxylate complexes [{Mo6I8}(RCOO)6](2-). The complexes were characterized by X-ray single-crystal diffraction and elemental analysis, cyclic/differential pulse voltammetry, and IR, NMR, and UV-visible spectroscopies. The emission properties of the complexes 1-6, and those of the earlier reported complexes with R = CF3 (7) and n-C3F7 (8), were studied both in acetonitrile solution and in the solid state. In deaerated CH3CN at 298 K, all of the complexes 1-8 exhibit intense and long-lived emission with the quantum yield and lifetime being 0.48-0.73 and 283-359 µs, respectively. The oxidation (Eox)/reduction (Ered) potentials of the complexes correlate linearly with the pKa value of the terminal carboxylate ligands L = RCOO (pKa(L)). Reflecting the pKa(L) dependences of Eox/Ered, the emission energy (νem) of the complexes was also shown to correlate with pKa(L). The present study successfully demonstrates synthetic tuning of the redox, spectroscopic, and photophysical characteristics of a {Mo6I8}(4+)-based cluster complex with pKa(L).

The first water-soluble hexarhenium cluster complexes with a heterocyclic ligand environment: synthesis, luminescence, and biological properties.

The hexarhenium cluster complexes with benzotriazolate apical ligands [{Re6(µ3-Q)8}(BTA)6](4-) (Q = S, Se; BTA = benzotriazolate ion) were obtained by the reaction of [{Re6(µ3-Q)8}(OH)6](4-) with molten 1H-BTA (1H-benzotriazole). The clusters were crystallized as potassium salts and characterized by X-ray single-crystal diffraction, elemental analyses, and UV-vis and luminescence spectroscopy. In addition, their cellular uptake and toxicity were evaluated. It was found that both clusters exhibited luminescence with high lifetimes and quantum yield values; they were taken up by the cells illuminating them under UV irradiation and, at the same time, did not exhibit acute cytotoxic effects.

Emissive supramolecular ionic crystals combining a red-NIR phosphorescent [Re6Se8CN6]4- cluster anion and a blue fluorescent tetraphenylethene counter-cation.

A blue fluorescent tetraphenylethylene-based dication and a red-NIR phosphorescent rhenium octahedral cluster tetra-anion are associated electrostatically to generate a supramolecular ionic framework which crystallizes in the P1̄ centric space group. The emission properties of the hybrids are studied in the crystalline state and in solution revealing a resonant energy transfer and a high sensitivity toward oxygen.

Can Re cluster complexes be an efficient catalyst for hydrogen evolution reaction? Insights from experiments and computations.

To date, researchers in chase of economic cost-efficiency are faced with the problem of developing effective catalysts for water splitting without the use of platinoids. Herein, catalytic properties of hexanuclear rhenium cluster complexes are investigated in application to the hydrogen evolution reaction (HER). A paste composite electrode containing the cluster complexes was obtained, producing a current density of 10 mA cm-2 at an extraordinarily low overpotential of 90 mV (RHE). The {Re6Se8}-based complexes have shown very favorable reaction kinetics via 102 mV dec-1 value of the Tafel slope for HER reaction within the composition of the paste electrode. Model calculations of kinetic parameters using density functional theory also support the experimental findings. This work underscores the perspectivity of rhenium cluster compounds in HER and opens a promising avenue toward the practical implementation of hydrogen production through electrochemical water splitting.

Alkynyl complexes of high-valence clusters. synthesis and luminescence properties of [Mo6I8(C≡CC(O)OMe)6]2-, the first complex with exclusively organometallic outer ligands in the family of octahedral {M6X8} clusters.

The reaction of [Mo6I14](2-) with methyl propiolate HC≡CC(O)OMe in the presence of Ag(+) and Et3N yielded the new luminescent complex [Mo6I8(C≡CC(O)OMe)6](2-), the first fully organometallic complex in the family of octahedral {M6X8} clusters. The cluster was crystallized as tetraphenylphosphonium salt and characterized by X-ray single-crystal diffraction and elemental analyses, mass spectrometry, (13)C NMR, UV-vis, and luminescence spectroscopies.

Molecular and Nano-Structural Optimization of Nanoparticulate Mn2+-Hexarhenium Cluster Complexes for Optimal Balance of High T1- and T2-Weighted Contrast Ability with Low Hemoagglutination and Cytotoxicity.

The present work introduces rational design of nanoparticulate Mn(II)-based contrast agents through both variation of the µ3 (inner) ligands within a series of hexarhenium cluster complexes [{Re6(µ3-Q)8}(CN)6]4- (Re6Q8, Q = S2-, Se2- or Te2-) and interfacial decoration of the nanoparticles (NPs) K4-2xMnxRe6Q8 (x = 1.3 - 1.8) by a series of pluronics (F-68, P-123, F-127). The results highlight an impact of the ligand and pluronic for the optimal colloid behavior of the NPs allowing high colloid stability in ambient conditions and efficient phase separation under the centrifugation. It has been revealed that the K4-2xMnxRe6Se8 NPs and those decorated by F-127 are optimal from the viewpoint of magnetic relaxivities r1 and r2 (8.9 and 10.9 mM-1s-1, respectively, at 0.47 T) and low hemoagglutination activity. The insignificant leaching of Mn2+ ions from the NPs correlates with their insignificant effect on the cell viability of both M-HeLa and Chang Liver cell lines. The T1- and T2-weighted contrast ability of F-127-K4-2xMnxRe6Q8 NPs was demonstrated through the measurements of phantoms at whole body 1.5 T scanner.

New Approach toward Dual-Emissive Organic-Inorganic Hybrids by Integrating Mn(II) and Cu(I) Emission Centers in Ionic Crystals.

Inorganic-organic hybrid luminescent materials have received great attention for their potential applications in a wide range of clean/renewable energy-related areas, including photovoltaics and solid-state lighting. Herein, we present a unique and general "Mn + Cu" approach by blending two earth-abundant luminogenic metals, manganese and copper, within a single ionic structure to construct a remarkable family of low-cost and multifunctional hybrid materials featuring dual emission, as well as triboluminescence and second-harmonic generation response. The novel hybrid materials are made of diphosphine dioxide-chelated [Mn(O∧O)3]2+ cations and various anionic [CuxIy](y-x)- clusters, ensuring manifestation of dual phosphorescence streamed from octahedral Mn2+ ions (605-648 nm) and iodocuprate anions (480-728 nm). Noteworthily, the relative ratio of the emission bands, and hence a resulting emission chromaticity, can be tuned in a wide range through modification of cluster [CuxIy](y-x)- modules. The structural diversity, enhanced robustness, and up to 100% luminescence quantum yield make the designed materials promising phosphors for lighting and sensing applications.

pH-Driven Intracellular Nano-to-Molecular Disassembly of Heterometallic [Au2L2]{Re6Q8} Colloids (L = PNNP Ligand; Q = S2- or Se2-).

The present work introduces a simple, electrostatically driven approach to engineered nanomaterial built from the highly cytotoxic [Au2L2]2+ complex (Au2, L = 1,5-bis(p-tolyl)-3,7-bis(pyridine-2-yl)-1,5-diaza-3,7-diphosphacyclooctane (PNNP) ligand) and the pH-sensitive red-emitting [{Re6Q8}(OH)6]4- (Re6-Q, Q = S2- or Se2-) cluster units. The protonation/deprotonation of the Re6-Q unit is a prerequisite for the pH-triggered assembly of Au2 and Re6-Q into Au2Re6-Q colloids, exhibiting disassembly in acidic (pH = 4.5) conditions modeling a lysosomal environment. The counter-ion effect of polyethylenimine causes the release of Re6-Q units from the colloids, while the binding with lysozyme restricts their protonation in acidified conditions. The enhanced luminescence response of Re6-S on the disassembly of Au2Re6-S colloids in the lysosomal environment allows us to determine their high lysosomal localization extent through the colocalization assay, while the low luminescence of Re6-Se units in the same conditions allows us to reveal the rapture of the lysosomal membrane through the use of the Acridine Orange assay. The lysosomal pathway of the colloids, followed by their endo/lysosomal escape, correlates with their cytotoxicity being on the same level as that of Au2 complexes, but the contribution of the apoptotic pathway differentiates the cytotoxic effect of the colloids from that of the Au2 complex arisen from the necrotic processes.

Surface modification of silica nanoparticles by hexarhenium anionic cluster complexes for pH-sensing and staining of cell nuclei.

The surface deposition of luminescent anionic cluster complex [{Re6S8}(OH)6]4- advantages to the design and synthesis of composite luminescent silica nanoparticles (SNs) for intracellular imaging and sensing, while the encapsulation of the cluster units into SNs lacks for efficient luminescence. The deposition of the Re6 clusters resulted from their assembly at the silica surface functionalized by amino-groups provides the synthetic route for the composite SNs with bright cluster-centered luminescence invariable in pH range from 4.0 to 12.0. The pH-dependent supramolecular assembly of the cluster units with polyethyleneimine (PEI) at the silica surface is an alternative route for the synthesis of the composite SNs with high cluster-centered luminescence sensitive to pH-changes within 4.0-6.0. The sensitivity derives from the pH-driven conformational changes of PEI chains resulting in the release of the clusters from the PEI-based confinement under the acidification within pH 6.0-4.0. The potential of the composite SNs in cellular contrasting has been also revealed by the cell viability and flow cytometry measurements. It has been found that the PEI-supported embedding of the cluster units facilitates cell internalization of the composite SNs as well as results in specific intracellular distribution manifested by efficient staining of the cell nuclei in the confocal images.