Characterization of just one atom using synchrotron X-rays.

Since the discovery of X-rays by Roentgen in 1895, its use has been ubiquitous, from medical and environmental applications to materials sciences1-5. X-ray characterization requires a large number of atoms and reducing the material quantity is a long-standing goal. Here we show that X-rays can be used to characterize the elemental and chemical state of just one atom. Using a specialized tip as a detector, X-ray-excited currents generated from an iron and a terbium atom coordinated to organic ligands are detected. The fingerprints of a single atom, the L2,3 and M4,5 absorption edge signals for iron and terbium, respectively, are clearly observed in the X-ray absorption spectra. The chemical states of these atoms are characterized by means of near-edge X-ray absorption signals, in which X-ray-excited resonance tunnelling (X-ERT) is dominant for the iron atom. The X-ray signal can be sensed only when the tip is located directly above the atom in extreme proximity, which confirms atomically localized detection in the tunnelling regime. Our work connects synchrotron X-rays with a quantum tunnelling process and opens future X-rays experiments for simultaneous characterizations of elemental and chemical properties of materials at the ultimate single-atom limit.

Circular Heterochiral Titanium-Based Self-Assembled Architectures.

Circular trinuclear helicates have been synthesized from a bis-biphenol strand (LH4), titanium isopropoxide, and various diimine ligands. These self-assembled architectures constructed around three TiO4N2 nodes have a heterochiral structure (C1 symmetry) when 2,2'-bipyridine (A), 4,4'-dimethyl-2,2'-bipyridine (B), 4,4'-bromo-2,2'-bipyridine (C), or 4,4'-dimethyl-2,2'-bipyrimidine (D) is employed. Within these complexes, one nitrogen ligand is endo-positioned inside the metallo-macrocycle, whereas the other two diimine ligands point outside the helicate framework. This investigation highlights that the nitrogen ligand which does not participate in the helicate framework of the complex controls the overall symmetry of the helicate since the 2,2'-bipyrimidine chelate (F) ends in the formation of a homochiral aggregate (C3 symmetry). The lack of symmetry found in the solid state for the trinuclear species ([Ti3L3(B)3], [Ti3L3(C)3], and [Ti3L3(D)3]) is observed for these complexes in solution (dichloromethane or chloroform). Remarkably, the 2,2'-bipyrazine ligand (ligand E) ends in the formation of a hexameric aggregate formulated as [Ti6L6(E)6], whereas the use of 4,4'-dimethyl-2,2'-bipyrimidine (ligand D) permits to generate the dinuclear complexes ([Ti2L(D)2(OiPr)4] and [Ti2L2(D)2]) in addition to the trimeric structure [Ti3L3(D)3]. The behavior of [Ti3L3(A)3] in solution, on the other hand, is unique since an equilibrium between the homochiral and the heterochiral form is reached within 17 days after the complex has been dissolved in dichloromethane (C3-[Ti3L3(A)3]/C1-[Ti3L3(A)3] ratio = 0.3). In chloroform, the heterochiral form of [Ti3L3(A)3] is stable for the same period of time, evidencing the dependence of this stereochemical transformation toward the solvent medium. The thermodynamic and kinetic parameters linked to this stereochemical equilibrium have been obtained and point to the fact that the transformation is intramolecular and not induced by the presence of external ligands. The thermodynamic constant of the C1-[Ti3L3(A)3]/C3-[Ti3L3(A)3] equilibrium is found to be K = 0.34 ± 10%. Further evidence to rationalize this solvent-induced symmetry switch is obtained via a DFT calculation and classical molecular dynamics. In particular, this computational investigation elucidates the reason why the stereochemical transformation of a heterochiral architecture into a homochiral structure is possible only for a trinuclear assembly containing ligand A.

Binuclear Copper(I) Complexes for Near-Infrared Light-Emitting Electrochemical Cells.

Two binuclear heteroleptic CuI complexes, namely Cu-NIR1 and Cu-NIR2, bearing rigid chelating diphosphines and π-conjugated 2,5-di(pyridin-2-yl)thiazolo[5,4-d]thiazole as the bis-bidentate ligand are presented. The proposed dinuclearization strategy yields a large bathochromic shift of the emission when compared to the mononuclear counterparts (M1-M2) and enables shifting luminescence into the near-infrared (NIR) region in both solution and solid state, showing emission maximum at ca. 750 and 712 nm, respectively. The radiative process is assigned to an excited state with triplet metal-to-ligand charge transfer (3 MLCT) character as demonstrated by in-depth photophysical and computational investigation. Noteworthy, X-ray analysis of the binuclear complexes unravels two interligand π-π-stacking interactions yielding a doubly locked structure that disfavours flattening of the tetrahedral coordination around the CuI centre in the excited state and maintain enhanced NIR luminescence. No such interaction is present in M1-M2. These findings prompt the successful use of Cu-NIR1 and Cu-NIR2 in NIR light-emitting electrochemical cells (LECs), which display electroluminescence maximum up to 756 nm and peak external quantum efficiency (EQE) of 0.43 %. Their suitability for the fabrication of white-emitting LECs is also demonstrated. To the best of our knowledge, these are the first examples of NIR electroluminescent devices based on earth-abundant CuI emitters.

Chemistry on the Complex: Derivatization of TiO4 N2 -Based Complexes and Application to Multi-Step Synthesis.

The chemistry on octahedral TiO4 N2 -complexes is described. The Ti(IV)-based precursors are composed of two 3,3'-diphenyl-2,2'-biphenolato ligands (1) and one substituted 1,10-phenanthroline ligand (2-5). The application of imine condensation, palladium-catalyzed C-C bond formation or copper-catalysed azide-alkyne cycloaddition allowed the grafting of various new groups onto these species. In particular Sonogashira reactions permitted to observe an excellent conversion of the starting complexes. This systematic study enabled to compile the factors required to preserve the framework of the complexes in the course of a chemical transformation. Thanks to this chemistry realized on the complex, the Ti(1)2 fragment was used as a protecting group to develop a multi-step synthesis of a bis-phenanthroline compound (12), for which the synthesis without this protection failed. Thus, a dinuclear complex [Ti2 (1)4 (12)] was first prepared starting from complex precursor bearing an acetylenic function via a Hay coupling reaction. This was followed by a deprotection step affording 12. Overall, this work illustrates how the Ti(1)2 fragment could be an useful tool for the preparation of unprecedented diimine compounds.

Bent 1,10-Phenanthroline Ligands within Octahedral Complexes Constructed around a TiO4N2 Core.

The synthesis of monomeric octahedral complexes constructed around a TiO4N2 core bearing neocuproine derivatives is detailed. These architectures follow the [Ti(1)2(N-N)] general formulas, where 1 is the 6,6'-diphenyl-2,2'-biphenolato ligand and N-N is a 1,10-phenanthroline derivative. Single-crystal analysis revealed that the neocuproine-based ligands within these architectures adopt a nonplanar geometry. The distortion of these aromatic diimine systems has been quantified through measurement of a torsion angle (α) and a dihedral angle (ß) defined by two planes within the aromatic diimine molecule (π1 and π2), permitting one to evaluate the twisting and bending of a coordinated nitrogen ligand, respectively. Next, the scope of this investigation was extended to the synthesis of a dimeric architecture, [Ti2(1)4(3)], where 3 is the 5,5'-bis(neocuproine) ligand. Again, a strong distortion of the neocuproine fragments was characterized in the crystalline state for such a complex. The UV-visible properties of these complexes were interpreted with the help of time-dependent density functional theory calculations. The solution behavior as well as good hydrolytic stability of these species has been established.

Dipyrrolyldiketonato Titanium(IV) Complexes from Monomeric to Multinuclear Architectures: Synthesis, Stability, and Liquid-Crystal Properties.

Dipyrrolyldiketone ligands (dpkH) are used with Ti(OiPr)4 to afford monomeric titanium(IV) complexes displaying the general formula C2-[Ti(dpk)2(OiPr)2]. The dpkH ligands employed consist of two dipyrrolyldiketone compounds (2H and 3H) and three diphenyl-substituted analogues (4H-6H). The behavior of these octahedral [Ti(dpk)2(OiPr)2] species in solution was investigated by 1H NMR at variable temperatures. Dynamic phenomena were evidenced, and the activation parameters associated with these processes (ΔH⧧, ΔS⧧, and ΔG⧧) were retrieved. [Ti(dpk)2(OiPr)2] complexes are precursors for the formation of high-nuclearity aggregates whose structures depend on the substituents on the diketone backbone. The crystal structures of monomeric ([Ti(1)2(OiPr)2]; 1 is the 1,3-diphenyl-1,3-propanedionato ligand) and [Ti(2)2(OEt)2]), dimeric ([Ti2(1)4(µ2-O)2]), and tetrameric ([Ti4(4)8(µ2-O)4]) species have been established, and the origin of this structural diversity is discussed. The solid-state optical properties of several complexes were determined and interpreted with the help of DFT calculations. Finally, the dinuclear complex [Ti(6)2(µ2-O)2] was synthesized, where ligand 6 incorporates six long alkyl chains (C16H33). This complex shows rich mesomorphic properties, with an original room-temperature plastic crystal phase followed by a hexagonal columnar liquid-crystalline phase.

Counterintuitive torsional barriers controlled by hydrogen bonding.

The torsional barriers along the Caryl-Caryl axis of a pair of isosteric disubstituted biphenyls were determined by variable temperature 1H NMR spectroscopy in three solvents with contrasted hydrogen bond accepting abilities (1,1,2,2-tetrachloroethane-d2, nitrobenzene-d5 and dimethyl sulfoxide-d6). One of the biphenyl scaffolds was substituted at its ortho and ortho' positions with N'-acylcarbohydrazide groups that could engage in a pair of intramolecular N-HO=C hydrogen bonding interactions at the ground state, but not at the transition state of the torsional isomerization pathway. The torsional barrier of this biphenyl was exceedingly low despite the presence of the hydrogen bonds (16.1, 15.6 and 13.4 kcal mol-1 in the three aforementioned solvents), compared to the barrier of the reference biphenyl (15.3 ± 0.1 kcal mol-1 on average). Density functional theory and the solvation model developed by Hunter were used to decipher the various forces at play. They highlighted the strong stabilization of hydrogen bond donating solutes not only by hydrogen bond accepting solvents, but also by weakly polar, yet polarizable solvents. As fast exchanges on the NMR time scale were observed above the melting point of dimethyl sulfoxide-d6, a simple but accurate model was also proposed to extrapolate low free activation energies in a pure solvent (dimethyl sulfoxide-d6) from higher ones determined in mixtures of solvents (dimethyl sulfoxide-d6 in nitrobenzene-d5).

Phosphorus(V) Porphyrin-Based Molecular Turnstiles.

A new cationic molecular turnstile based on a P(V) porphyrin backbone bearing two pyridyl interaction sites, one at the meso position of the porphyrin and the other on the handle connected to the porphyrin through P-O bonds, was designed and synthesized. The dynamic behavior of the turnstile 2, investigated by 1D and 2D 1H NMR techniques, showed that in the absence of an effector, the turnstile is in its open state and undergoes a free rotation of the rotor (the handle) around the stator (the porphyrin backbone). In the presence of an external effector such as Ag+ cation or H+, the turnstile is switched to its closed states 2-Ag+ and 2-H+, respectively. The locking/unlocking process is reversible and may be achieved by precipitation of AgBr upon addition of Et4NBr in the case of the silver-locked turnstile or by addition of Et3N in the case of the proton-locked turnstile.

Welding Molecular Crystals.

Both for fundamental and applied sciences, the design of complex molecular systems in the crystalline phase with strict control of order and periodicity at both microscopic and macroscopic levels is of prime importance for development of new solid-state materials and devices. The design and fabrication of complex crystalline systems as networks of crystals displaying task-specific properties is a step toward smart materials. Here we report on isostructural and almost isometric molecular crystals of different colors, their use for fabrication of core-shell crystals, and their welding by 3D epitaxial growth into networks of crystals as single-crystalline entities. Welding of crystals by self-assembly processes into macroscopic networks of crystals is a powerful strategy for the design of hierarchically organized periodic complex architectures composed of different subdomains displaying targeted characteristics. Crystal welding may be regarded as a first step toward the design of new hierarchically organized complex crystalline systems.

A bowl-shaped circular trinuclear helicate generated from a TiO4 N2 motif by a multicomponent self-assembly approach.

The synthesis of a bowl-shaped trinuclear circular titanium-based helicate is reported. The strategy allowing access to this neutral architecture is based on a multicomponent self-assembly approach in which the ligands involved in the process are a bis-biphenol strand and 2,2'- bipyrimidine. By reacting the bis-biphenol ligand and 2,2'-bipyrimidine with an equimolar amount [Ti(OiPr)4 ], a bowl-shaped architecture is obtained through the formation of 18 new coordination bonds. This aggregate built from three octahedral TiO4 N2 nodes displays an unusually high stability in solution compared to related species. In addition, by modifying the stoichiometry of the initial components, two assemblies incorporating two titanium centers bridged by a 2,2'-bipyrimidine ligand are obtained. The crystal structures of these species are reported.

Molecular Tectonics: Design of Enantiopure Luminescent Heterometallic Ir(III)-Cd(II) Coordination Network.

With the aim of combining luminescence and chirality in heterometallic Ir(III)-Cd(II) coordination networks, synthetic strategies for the formation of new Ir(III)-based chiral metallatectons ([Ir(dFppy)2(1)][PF6]), both as a racemic mixture of Δ and Λ enantiomers (rac-[Ir(dFppy)2(1)][PF6]) and as enantiopure complexes (Δ-[Ir(dFppy)2(1)][PF6] and Λ-[Ir(dFppy)2(1)][PF6]), were developed. The final compounds were characterized both in solution and in the crystalline phase. Notably, their crystal structures were determined by single crystal X-ray diffraction, and their photophysical properties in solution and in the solid state were investigated. Combination of the cationic linear metallatecton with Cd(2+) iodide salt ([CdI3](-)), behaving as an anionic two-connecting node, leads to the formation of 1D chiral and neutral heterometallic Ir(III)-Cd(II) luminescent coordination networks both as a racemic mixture and as enantiomerically pure infinite architectures. The latter have been structurally studied in the solid state by X-ray diffraction both on single crystals and on microcrystalline powders. The infinite coordination networks display phosphorescence in the solid state at ca. 600 nm upon excitation at 400 nm.

Transient self-assembly of metal-organic complexes.

Implementing transient processes in networks of dynamic molecules holds great promise for developing new functional behaviours. Here we report that trichloroacetic acid can be used to temporarily rearrange networks of dynamic imine-based metal complexes towards new equilibrium states, forcing them to express complexes otherwise unfavourable in their initial equilibrium states. Basic design principles were determined for the creation of such networks. Where a complex distribution of products was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily yielded a simplified output, forcing a more structured distribution of products. Where a single complex was obtained in the initial equilibrium state of the system, the transient rearrangement temporarily modified the properties of this complex. By doing so, the mechanical properties of an helical macrocyclic complex could be temporarily altered by rearranging it into a [2]catenane.

Strapped-porphyrin-based molecular turnstiles.

The synthesis of a series of molecular turnstiles that contained both H-bond-donor and -acceptor sites was achieved. Their structures were based on tetra-aryl X(2)Sn(IV) porphyrins (X=Cl or OH) as H-bond-acceptor sites that were equipped with a rotor that contained a pyridyldiamide moiety as a H-bond donor. In the solution phase, 1D and 2D NMR spectroscopic analysis showed that switching between the closed state, which resulted from the formation of intramolecular H-bonds, and the open state of the turnstile was achieved by using external H-bond-acceptor molecules, such as DMSO. The solid-state structure of the closed state of the turnstile was established by single-crystal X-ray diffraction.

Giant core-shell nanospherical clusters composed of 32 Co or 32 Ni atoms held by 6 p-tert-butylthiacalix[4]arene units.

Using combinations of p-tert-butylthiacalix[4]arene (TCA) and [M(DMSO)(6)(BF(4))(2)] salts (M = Co(II) or Ni(II)), two almost isostructural core-shell-type thermally stable giant nanoclusters, composed of 32 metal centers, 6 deprotonated calix units binding the metal centers by both their O and S atoms, 24 µ-oxo or µ-hydroxo bridging groups, and 6 MeOH molecules, have been prepared under mild and reproducible conditions. For both giant clusters, the oxidation state II [M(II)(32)O(16)(OH)(8)(CH(3)OH)(6)TCA(6) (M = Co or Ni)] for the metal center was demonstrated by X-ray photoelectron and electronic absorption spectroscopies.

New 2-D silver(I) coordination network constructed from thiomethyl group-substituted p-tert-butylthiacalix[]arene.

New ligand based on p-tertbutyltetrathiacalix[4]arene blocked in 1,3-alternate conformation, was achieved via a multistep synthesis with the introduction of thiomethylpropoxy groups on the lower rim which leads to a neutral tecton. The combination of this p-tertbutyltetrathiacalix[4]arene (p-TCA-2), in 1, 3-alternate imposed conformation, with tetrahedral Ag(I)SbF6 salt, leads to the formation of neutral, new 2-D coordination network, which was structurally investigated in the solid state by X-ray diffraction methods on a single crystal.

1-D and 2-D coordination networks based on tetrathiacalix[]arene derivative generated with mercury and cobalt salts.

The combination of four iso-nicotinate appended tetrathiacalix[4]arene (TCA-1) in 1,2-alternate imposed conformation behaving as neutral tectons with Co(II)Cl2 and Hg(II)Cl2 metallic salts, leads to the formation of neutral, new coordination networks. Indeed, the tetrasubstituted TCA-1 derivative leads to a 1D coordination polymer with Co(II)Cl2 and a 2D grid-like network with, Hg(II)Cl2. The effect of the nature of the metal used on the dimensionality was demonstrated by X-ray diffraction studies on a single crystal.

Bright V-Shaped bis-Imidazo[1,2-a]pyridine Fluorophores with Near-UV to Deep-Blue Emission.

Ten novel small-molecule fluorophores containing two electron-accepting imidazo[1,2-a]pyridine (ImPy) units are presented. Each ImPy core is functionalized at its C6 position with groups featuring either electron accepting (A) or donating (D) properties, thus providing emitters with general structure X-ImPy-Y-ImPy-X (X=either A or D; Y=phenyl or pyridine). The molecules bear either a phenyl (series 4) or a pyridine (series 5) π bridge that connects the two ImPys via meta (phenyl) or 2,6- (pyridine) positions, yielding an overall V-shaped architecture. The final compounds are synthetized straightforwardly by condensation between substituted 2-aminopyridines and α-halocarbonyl derivatives. All the compounds display intense photoluminescence with quantum yield (PLQY) in the range of 0.17-0.51. Remarkably, substituent effect enables tuning the emission from near-UV to (deep-)blue region while keeping Commission Internationale de l'Éclairage (CIE) y coordinate ≤0.07. The emitting excited state is characterized by a few nanoseconds lifetime and high radiative rate constant, and its nature is modulated from pure π-π* to intramolecular charge transfer (ICT) by the electronic properties of the peripheral X substituent. This is further corroborated by the nature of the frontier orbitals and vertical electronic excitations computed at (time-dependent) density functional level of theory (TD-)DFT. Finally, this study enlarges the palette of bright deep-blue emitters based on the interesting ImPy scaffolds in view of their potential application as photo-functional materials in optoelectronics.

Porphyrin-based switchable molecular turnstiles.

The design, synthesis and structural characterisation, in solution, of two new molecular turnstiles based on Sn-porphyrin derivatives are described. The system is composed of a stator (5-(4-pyridyl)-10,15,20-triphenylporphyrin), a hinge (Sn(IV)) and a rotor (handle equipped with 2,6-pyridinedicarboxamide as a tridentate coordinating site or its Pd(II) complex). The presence of interaction sites, both on the stator and the rotor, offers the possibility of switching between an open state (free rotation of the handle around the porphyrin) and a closed state (blockage of the rotation) by either establishment of hydrogen bonds between the stator and the rotor or by the simultaneous binding of Pd by both coordinating groups.

Luminescent 1D heterometallic (Ir,Cd) coordination polymers based on bis-cyclometalated Ir(III) metallatectons and trinuclear Cd(II) dianionic nodes.

Seven isostructural heterometallic luminescent (Ir,Cd) coordination polymers were prepared upon the combination of tris-chelate cationic Ir(III) complexes behaving as metalloligands with Cd(II) salts. Three octahedral Ir(III) complexes have been considered in the present report. They consist of a bipyridine unit functionalised with 3-pyridyl moieties as peripheral coordinating sites and two 2-phenylpyridyl cyclometalating derivatives. Three cadmium halide salts CdX2 (X = Cl, Br, I) were used and rearranged themselves during the self-assembly process with the metallatectons to afford a dianonic trinuclear Cd node [Cd3X8]2-. Seven out of the nine possible metallotecton-metal salt combinations could be characterised in the crystalline phase by X-ray diffraction on single crystals proving the isostructurality of the seven extended architectures studied. All of the CPs are luminescent with small shifts observed in the emission wavelength compared to the discrete complexes. Depending on the degree of fluorination of the two cyclometalating units, tuning of the emission wavelength of the discrete complexes as well as of the resulting coordination polymers is achieved.

Enhanced photoreduction of water catalyzed by a cucurbit[8]uril-secured platinum dimer.

A cucurbit[8]uril (CB[8])-secured platinum terpyridyl chloride dimer was used as a photosensitizer and hydrogen-evolving catalyst for the photoreduction of water. Volumes of produced hydrogen were up to 25 and 6 times larger than those obtained with the corresponding free and cucurbit[7]uril-bound platinum monomer, respectively, at equal Pt concentration. The thermodynamics of the proton-coupled electron transfer from the Pt(ii)-Pt(ii) dimer to the corresponding Pt(ii)-Pt(iii)-H hydride key intermediate, as quantified by density functional theory, suggest that CB[8] secures the Pt(ii)-Pt(ii) dimer in a particularly reactive conformation that promotes hydrogen formation.