Self-Assembly of Triple-Stranded Lanthanide Molecular Quasi-Lantern Containing 2,2'-Bipyridine Receptor: Luminescence Sensing and Magnetic Property

Ln₂L₃-type supramolecular architectures have received significant attention recently due to their unique magnetism and optical properties. Herein, we report the triple-stranded Ln₂L₃-type lanthanide molecular quasi-lanterns, which are fabricated by the deprotonation self-assembly of a linear ligand featuring a β-diketone chelating claw and 2,2'-bipyridine (bpy) moiety with lanthanide ions (Ln = Eu³⁺ and Dy³⁺). The crystal structure analysis indicates that Eu³⁺ and Dy³⁺ ions are all coordinated by eight oxygen donors but in different coordination geometries. The eight oxygen donors in Eu₂L₃ and Dy₂L₃ are arranged in a square antiprism and triangular dodecahedron geometry, respectively. Taking into account the fact that the bpy moiety has a strong coordination affinity for transition metal ions, luminescence sensing toward Cu²⁺ ions has been demonstrated with Eu₂L₃, bearing a detection of limit as low as 2.84 ppb. The luminescence sensing behavior of Eu₂L₃ is ascribed to the formation host-guest complex between Eu₂L₃ and Cu²⁺ ions with a 1:2 binding ratio. Dynamic AC susceptibility measurements for Dy₂L₃ reveal the relaxation of magnetization in it. This work provides a potential way for design and fabrication of lanthanide-based molecular materials with functions endowed by the ligands.

Heterometallic Molecular Architectures Based on Fluorinated β-Diketone Ligands

This review summarizes the data on the synthesis of coordination compounds containing two or more different metal ions based on fluorinated β-diketonates. Heterometallic systems are of high interest in terms of their potential use in catalysis, medicine and diagnostics, as well as in the development of effective sensor devices and functional materials. Having a rich history in coordination chemistry, fluorinated β-diketones are well-known ligands generating a wide variety of heterometallic complexes. In this context, we focused on both the synthetic approaches to β-dicarbonyl ligands with additional coordination centers and their possible transformations in complexation reactions. The review describes bi- and polynuclear structures in which β-diketones are the key building blocks in the formation of a heterometallic framework, including the examples of both homo- and heteroleptic complexes.

Helicate versus Mesocate in Quadruple-Stranded Lanthanide Cages: A Computational Insight

To drive the synthesis of metallo-supramolecular assemblies (MSAs) and to fully exploit their functional properties, robust computational tools are crucial. The capability to model and to rationalize different parameters that can influence the outcome is mandatory. Here, we report a computational insight on the factors that can determine the relative stability of the supramolecular isomers helicate and mesocate in lanthanide-based quadruple-stranded assemblies. The considered MSAs have the general formula [Ln₂L₄]²⁻ and possess a cavity suitable to allocate guests. The analysis was focused on three different factors: the ligand rigidity and the steric hindrance, the presence of a guest inside the cavity, and the guest dimension. Three different quantum mechanical calculation set-ups (in vacuum, with the solvent, and with the solvent and the dispersion correction) were considered. Comparison between theoretical and experimental outcomes suggests that all calculations correctly estimated the most stable isomer, while the inclusion of the dispersion correction is mandatory to reproduce the geometrical parameters. General guidelines can be drawn: less rigid and less bulky is the ligand and less stable is the helicate, and the presence of a guest can strongly affect the isomerism leading to an inversion of the stability by increasing the guest size when the ligand is flexible.

Coordination-Directed Self-Assembly of Functional Polynuclear Lanthanide Supramolecular Architectures

Lanthanide supramolecular chemistry is a fast growing and intriguing research field due to the unique photophysical, magnetic, and coordination properties of lanthanide ions (Ln^III). Compared with the intensively investigated mononuclear Ln-complexes, polymetallic lanthanide supramolecular assemblies offer more structural superiority and functional advantages. In recent decades, significant progress has been made in polynuclear lanthanide supramolecules, varying from structural evolution to luminescent and magnetic functional materials. This review summarizes the design principles in ligand-induced coordination-driven self-assembly of polynuclear Ln-structures and intends to offer guidance for the construction of more elegant Ln-based architectures and optimization of their functional performances. Design principles concerning the water solubility and chirality of the lanthanide-organic assemblies that are vital in extending their applications are emphasized. The strategies for improving the luminescent properties and the applications in up-conversion, host-guest chemistry, luminescent sensing, and catalysis have been summarized. Magnetic materials based on supramolecular assembled lanthanide architectures are given in an individual section and are classified based on their structural features. Challenges remaining and perspective directions in this field are also briefly discussed.

Bis-pyrazolone-based dysprosium(III) complexes: zero-field single-molecule magnet behavior in the [2 × 2] grid DyIII4 cluster

We assembled two new Dy(III) compounds, namely, [Dy4(L1)4(OH)4]·2CH3OH (1) and [Dy2(L2)2(HL2)2(CH3CN)2]N(C2H5)3 (2) (where H2L1 and H2L2 are methyl-substituted and p-tolyl substituted ligands respectively) using a bis-pyrazolone carbohydrazide based multi-dentate hydrazone...

Novel quadruple-stranded heterometallic Ln2Na complexes hosting sodium ions inside the cryptand-like cavity

The first examples of lanthanide tetrakis-complexes based on bis-chelating carbacylamidophosphate were synthesized and characterized.

Point Chirality Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence in Quadruple-Stranded Europium(III) Helicates

Aromatic β-diketones have been extensively employed as highly effective sensitizers in luminescent lanthanide complexes. However, the difficulties to make the chiral modified groups effectively participate in the frontier molecular orbital (FMO) distributions limit their applications on lanthanide circularly polarized luminescence (CPL) fields. Considering the inherent chirality of the helical structure, a pair of enantiopure dinuclear europium quadruple-stranded helicates, ΔΔ/ΛΛ-(HNEt₃)₂(Eu₂L₄) (ΔΔ/ΛΛ)-1; L = R/S-1,2-bis(4,4'-bis(4,4,4-trifluoro-1,3-dioxobutyl)phenoxyl)propane are assembled via a point chirality induced strategy. The comprehensive spectral characteristics combined with density functional theory (DFT) calculations demonstrate that the one point chirality at the spacer of the ligand successfully controls the Δ or Λ configuration around the Eu(III) ion center and the P or M helical patterns of the helicates. The mirror-image CPL and CD spectra further confirm the formation of the enantiomer pairs. As expected, the helicate presents a higher luminescence quantum yield (QY) of 68% and a large |g_lum| value (0.146). This study effectively combines the excellent sensitization capability of β-diketone and the helical chirality of helicates. This strategy provides an effective path for the synthesis of lanthanide material with excellent CPL performance.

Crystallization and single molecule magnetic behavior of quadruple-stranded helicates: tuning the anisotropic axes

It has been successfully proven that the bis-β-diketone ligands with proper lengths and flexibility are essential for the construction of multiple-stranded helicates, where two Dy3+ centers in subtly different environments allow the tuning of the anisotropic axes. Based on our previous work, we have designed a flexible bis-β-diketone ligand BTT (BTT = 3,3''-bis(4,4,4-trifluoro-1,3-dioxobutyl)-m-terphenyl), which is successfully utilized to construct quadruple-stranded helicates with the formula of [C6H16N]2[Ln2(BTT)4]·2CH2Cl2·4CH3OH [Ln = Ce (1), Dy (2)]. Structural analysis indicates that they crystallize in the tetragonal space group P4/n, and each Ln3+ center is chelated by four diketonate moieties from four ligands, giving rise to a dinuclear quadruple-stranded helicate. Magnetic measurements show that 2 displays single molecular magnet behavior under an applied DC field of 2000 Oe. Further investigations indicate that the anisotropic axes of the Dy3+ centers can be tuned depending on the bis-β-diketonate ligands used to assemble the Dy helicates.

Dinuclear Lanthanide(III) Complexes Showing Single-Molecule Magnet Behaviour and Optical Properties

Three dinuclear lanthanide(iii) complexes with an acetate bridge, which are synthesised by a Salen-type Schiff base and β-diketonate ligands, i.e. [Ln2L2(TTA)4(OAc)2]·CH2Cl2 (Ln=Eu (1), Gd (2), Dy (3); TTA=thenoyltrifluoroacetonate, H2L=N,N′-ethylenebis(salicylideneimine), are investigated. We obtained the structures of complexes 1–3 by X-ray crystallography. Notably, in terms of the structure of these complexes, what is intriguing is that the acetate groups link two Lniii ions, whereas the cadmium ions do not coordinate. Lanthanide-based luminescence is exhibited by complex 1, which exists in both the solid state and a methanol solution. Through magnetic analysis, it is found that a field-induced single-molecule magnet behaviour is exhibited by complex 3, and the energy barrier is shown to be Ueff=45.97K.

Synthesis of Bis-β-Diketonate Lanthanide Complexes with an Azobenzene Bridge and Studies of Their Reversible Photo/Thermal Isomerization Properties

The ligand, bis-β-diketone with an azobenzene bridge (4,4'-(4,4,4-trifluoro-1,3-butanedione)azobenzene, H 2 L), was prepared for the synthesis of a series of dinuclear lanthanide complexes with the formula [Ln 2 L 3 (DMSO) 4 ] (Ln = Eu3+, Gd3+, Tb3+, and DMSO = dimethyl sulfoxide). X-ray crystallographic analysis reveals that the three complexes are triple-stranded dinuclear structures formed by three bis-β-diketonate ligands with two lanthanide ions (Ln3+). The trans-to-cis photoisomerization rates of the azobenzene group of the three [Ln 2 L 3 (DMSO) 4 ] complexes in ethanol and acetonitrile solutions are similar to those of the pure H 2 L ligand and other azobenzene-containing mononuclear lanthanide complexes, but the trans-to-cis quantum yields (Φt→c = 10-3) are 1 order of magnitude smaller. The first-order rate constant for the cis-to-trans thermal isomerization at 50 °C of the H 2 L ligand is similar to those of azobenzene derivatives, while those for the [Ln 2 L 3 (DMSO) 4 ] complexes (k iso = 10-4 s-1) are higher than those of the mononuclear azobenzene-containing lanthanide complexes. Furthermore, as the lanthanide ionic radius becomes smaller from Eu3+ to Gd3+ to Tb3+, the thermal isomerization rate constant decreases and the half-life increases. All these results are proposed to arise from the rigidity at both ends of the azo group by coordination to the dinuclear lanthanide ions and the different isomerization mechanisms. These are the first examples of bis-β-diketonate dinuclear lanthanide complexes with an azobenzene bridge and help illustrate the mechanism of azobenzene isomerization.

Structures and magnetic properties of dysprosium complexes: the effect of crystallization temperature

Two new dysprosium complexes, [Dy(H₅L)(NO₃)₂(CH₃OH)₂]·4CH₃OH (1) and [Dy₂(H₅L)₂(NO₃)₄(H₂O)₂]·10CH₃OH (2), were isolated from the reaction of a novel 1,8-naphthalenediol-based ligand N,N'-((1,8-dihydroxynaphthalene-2,7-diyl)bis(methanylylidene))bis(2-hydroxybenzohydrazide), H₆L with dysprosium(iii) nitrate upon crystallization at different temperatures. Because of a low-symmetrical coordination environment, both complexes display only field-induced single molecular magnetic (SMM) behavior. Interestingly, complex 2 containing two dysprosium ions shows field-induced multiple relaxation processes, whereas only one relaxation process is observed for complex 1. The remarkably different behavior observed in 2 is mainly ascribed to the weak intra- or intermolecular interactions between the two Dy^III centres in this complex.

Enhancement of near-infrared luminescence of ytterbium in triple-stranded binuclear helicates

A bis-β-diketone, bis(4,4,4-trifluoro-1,3-dioxobutyl)(2,2'-bithienyl) (BTT), which can be looked upon as coupling of two mono-β-diketones (2-thenoyltrifluoroacetone, TTA) at the 5,5'-position of thiophene ring, has been designed for exploring the advantages of binuclear helical structure in sensitizing the lanthanide NIR luminescence. The Yb(iii) ion was selected as the luminescent center, and its corresponding mono-β-diketone complex Yb(TTA)3(DMSO) () and bis-β-diketone complex Yb2(BTT)3(DMSO)4 () were synthesized and isolated. X-ray crystallographical analysis reveals that the bis-β-diketone complex Yb2(BTT)3(DMSO)4 adopts a triple-stranded dinuclear structure, in which the two Yb(iii) ions are helically wrapped by three ligands, and each Yb(iii) ion is eight-coordinated by six oxygen atoms from three ligands and two oxygen atoms from the coordinated DMSO molecules. Whereas, the mono-β-diketone complex Yb(TTA)3(DMSO) is a mononuclear structure, the central Yb(iii) ion is coordinated by seven oxygen atoms from three ligands and a DMSO molecule. The photophysical properties related to the electronic transition are characterized by the absorbance spectra, the emission spectra, the emission quantum yields, the emission lifetimes, and the radiative (kr) and nonradiative rate constants (knr). The luminescence quantum yield experiment reveals that the dinuclear complex has about 10 times luminescence enhancement compared with the mononuclear complex. This enhancement mainly benefits from its helical structure, which effectively depresses the nonradiative transition caused by high-energy oscillators in ligands, and the part-encapsulated structure decreases the probability of solvents entering the metal centers.

Solvent triggered structural diversity of triple-stranded helicates: single molecular magnets

Solvents have been found to be the key for the construction of diverse triple-stranded helicates. Distinct solvent molecules are found to coordinate to the Dy³⁺ centers to replace the original hydrate molecules.