Single molecule magnet features in luminescent lanthanide coordination polymers with heptacoordinate Dy/Yb(III) ions as nodes

Two sets of 1D/2D lanthanide coordination polymers with formulas of Ln(oqa)3·2H2O [Hoqa = 2-(4-oxoquinolin-1(4H)-yl) acetic acid, Ln = Dy (1), Yb (2)] and Ln(oaa)2(HCOO)(H2O) [Hoaa = 2-(9-oxoacridin-10(9H)-yl) acetic acid, Ln = Dy (3), Yb (4)] have been synthesized and their physical properties were investigated. All four complexes are constructed from seven-coordinate lanthanide ions and corresponding organic linkers. The lanthanide ions in 1 and 2 adopt a pentagonal bipyramid coordination geometry, whereas the coordination geometry of lanthanide ions in 3 and 4 can be described as a capped octahedron. Slow magnetic relaxation behaviors were observed in these four products at a zero/non-zero static magnetic field. Complexes 1, 2 and 4 exhibit the characteristic emission of Ln(III) ions, whereas complex 3 shows ligand-based emission. Bright yellow light emission was also observed when a voltage was applied, demonstrating the potential of 1 for application in light-emitting diodes (LEDs). Compounds 3 and 4 are the first examples of lanthanide complexes based on Hoaa ligands.

Triangulo-{ErIII 3} complex showing field supported slow magnetic relaxation

The triangulo-{Er₃} complex [Er₃Cl(o-van)₃(OH)₂(H₂O)₅]Cl₃·nH₂O (n = 9.4; H(o-van) = o-vanillin) (1) was generated by an in situ method. The isolated Er(iii) complex 1 was characterized by elemental analysis and molecular spectroscopy. The results of single crystal X-ray diffraction studies have shown that 1 is built up of trinuclear [Er₃Cl(o-van)₃(OH)₂(H₂O)₅]³⁺ complex cations, chloride anions and water solvate molecules. Within the complex cation the three Er(iii) central atoms are placed at the apexes of a triangle which are bridged by three (o-van)^- ligands with additional chelating functions and two μ₃-OH^- ligands. Additionally five aqua and one chlorido ligands complete the octa-coordination of the three Er(iii) atoms. AC susceptibility measurements reveal that the compound exhibits slow magnetic relaxation with two relaxation modes.

Seven-Coordinate Tb3+ Complexes with 90% Quantum Yields: High-Performance Examples of Combined Singlet- and Triplet-to-Tb3+ Energy-Transfer Pathways

Seven-coordinate, pentagonal-bipyramidal (PBP) complexes [Ln(bbpen)Cl] and [Ln(bbppn)Cl], in which Ln = Tb³⁺ (products I and II), Eu³⁺ (III and IV), and Gd³⁺ (V and VI), with bbpen^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)ethylenediamine and bbppn^2- = N,N'-bis(2-oxidobenzyl)-N,N'-bis(pyridin-2-ylmethyl)-1,2-propanediamine, were synthesized and characterized by single-crystal X-ray diffraction analysis, alternating-current magnetic susceptibility measurements, and photoluminescence (steady-state and time-resolved) spectroscopy. Under a static magnetic field of 0.1 T, the Tb³⁺ complexes I and II revealed single-ion-magnet behavior. Also, upon excitation at 320 nm at 300 K, I and II presented very high absolute emission quantum yields (0.90 ± 0.09 and 0.92 ± 0.09, respectively), while the corresponding Eu³⁺ complexes III and IV showed no photoluminescence. Detailed theoretical calculations on the intramolecular energy-transfer rates for the Tb³⁺ products indicated that both singlet and triplet ligand excited states contribute efficiently to the overall emission performance. The expressive quantum yields, Q_Ln^L, measured for I and II in the solid state and a dichloromethane solution depend on the excitation wavelength, being higher at 320 nm. Such a dependence was rationalized by computing the intersystem crossing rates (W_ISC) and singlet fluorescence lifetimes (τ_S) related to the population dynamics of the S₁ and T₁ levels. Thin films of product II showed high air stability and photostability upon continuous UV illumination, which allowed their use as downshifting layers in a green light-emitting device (LED). The prototypes presented a luminous efficacy comparable with those found in commercial LED coatings, without requiring encapsulation or dispersion of II in host matrixes. The results indicate that the PBP environment determined by the ethylenediamine (en)-based ligands investigated in this work favors the outstanding optical properties in Tb³⁺ complexes. This work presents a comprehensive structural, chemical, and spectroscopic characterization of two Tb³⁺ complexes of mixed-donor, en-based ligands, focusing on their outstanding optical properties. They constitute good molecular examples in which both triplet and singlet excited states provide energy to the Tb³⁺ ion and lead to high values of Q_Ln^L.

One-dimensional lanthanide coordination polymers supported by pentadentate Schiff-base and diphenyl phosphate ligands: single molecule magnet behavior and photoluminescence

Four one-dimensional lanthanide coordination polymers constructed from Ln₂ dimers showing single molecule magnet behavior and photoluminescence are reported.

Near-Infrared Photoluminescence and Reversible Trans-to-Cis Photoisomerization of Mononuclear and Binuclear Ytterbium(III) Complexes Functionalized by Azobenzene Groups

Two mononuclear and one binuclear ytterbium complexes with dual near-infrared (NIR) photoluminescence and reversible trans-to-cis photoisomerization functions were synthesized and characterized. The central ytterbium(III) ion coordinates with two β-diketonate (4,4,4-trifluoro-1-phenylbutane-1,3-dionate (tfd)) ligands and one deprotonated azobenzene-containing tetradentate ligand [(E)-4-(phenyldiazenyl)-N,N-bis(pyridin-2-ylmethyl) benzohydrazide (HL), (E)-4-((4-(dimethylamino)phenyl)diazenyl)-N,N-bis(pyridin-2-ylmethyl)benzohydrazide (HNL), or (E)-4,4'-N',N'-bis(pyridin-2-ylmethyl)benzohydrazide azobenzene (H2DL)] to form a neutral ternary complex ([Yb(tfd)2L], [Yb(tfd)2(NL)], or [Yb2(tfd)4(DL)], respectively), where the ytterbium(III) ion is eight-coordinated to N3O5 donor sets. X-ray crystallographic analysis shows that all three complexes form a trigonal dodecahedron geometry with similar -N=N- distances that are slightly longer than those of the pure azobenzene-containing ligands. The NIR luminescence properties of the Yb(III) complexes were determined at a wavelength of about 980 nm with quantum yields in the range of 0.4-0.6% in ethanol and acetonitrile solutions at room temperature, and trans-to-cis photoisomerization was determined with the quantum yields (Φt→c = 10-2) at the same level as their pure ligands. The trans-to-cis photoisomerization rates of the complexes (10-4 s-1) are slightly higher than those of the pure ligands and similar to azobenzene (10-5 to 10-4 s-1). From time-dependent density functional theory calculations of the energy levels of the first excited triplet states of the ligands, the energies of the lowest excited triplet states of all of the ligands are higher than the resonance level of Yb3+ (2F5/2, 1.2722 eV). We suggest that these azo-containing ligands may participate in energy transfer to the ytterbium ion, in addition to the main "antenna effect" ligand tfd. This is the first report of azobenzene group-functionalized ytterbium complexes with dual NIR luminescence and photoisomerization properties, indicating that azobenzene-containing lanthanide(III) complexes have potential applications as dual function materials in biological systems.

A salen-type Dy4 single-molecule magnet with an enhanced energy barrier and its analogues

A discrete hexadentate salen-type tetranuclear Dy complex with a unique {Ln₄O₈} core exhibits two slow magnetic relaxations with the highest energy barrier of 48.14 K among the reported tetranuclear salen-type dysprosium SMMs.

Near-IR luminescence and field-induced single molecule magnet of four salen-type ytterbium complexes

A series of rigid hexadentate salen-type (H2L) ytterbium complexes, namely, [Yb2L3(CH3OH)]·3CH3CN (1), [Yb2LL'L″(CH3OH)(H2O)2](ClO4)2·CH3OH·H2O (2), [Yb2L(OAc)4(CH3OH)2]·2CH3OH (3), and {[Yb2L(OAc)4]·3H2O}n (4) (H2L = N,N'-bis(2-oxy-3-methoxybenzylidene)-1,2-phenylenediamine, HL' = 2-(2'-hydroxy-3'-methyloxy-phenyl)benzimidazole and HL" = 3-methoxysalicylaldehyde) have been synthesized by reactions of H2L with multifarious Yb(3+) salts. X-ray crystallographic analyses demonstrate that complex 1 is of a triple-decker sandwich-type Yb2L3 structure with a ratio of H2L/Yb = 3:2, 2 and 3 possess the unique Yb2 core with a ratio of H2L/Yb = 2:2 and 1:2, respectively, 4 exhibits one dimensional coordination polymers in which the polymeric structures are formed by acetate (OAc(-)) groups. All complexes 1-4 exhibit near-IR luminescence, which can be rationalized on the basis of the disparate structural effects. The magnetic analysis unveils that all complexes 1-4 are of field-induced single-molecule magnet behavior with the energy barriers (Ueff/kB) of 14.5, 2.0, 9.5, and 2.4 K at 3 kOe direct current fields, respectively.