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 bbpen2- = N,N′-bis(2-oxidobenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine and bbppn2- = 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, QLnL, 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 (WISC) and singlet fluorescence lifetimes (τS) 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 QLnL

Temperature- and Light-Induced Spin Crossover Observed by X-ray Spectroscopy on Isolated Fe(II) Complexes on Gold

Using X-ray absorption techniques, we show that temperature- and light-induced spin crossover properties are conserved for a submonolayer of the [Fe(H2B(pz)2)2(2,2′-bipy)] complex evaporated onto a Au(111) surface. For a significant fraction of the molecules, we see changes in the absorption at the L2,3 edges that are consistent with those observed in bulk and thick film references. Assignment of these changes to spin crossover is further supported by multiplet calculations to simulate the X-ray absorption spectra. As others have observed in experiments on monolayer coverages, we find that many molecules in our submonolayer system remain pinned in one of the two spin states. Our results clearly demonstrate that temperature- and light-induced spin crossover is possible for isolated molecules on surfaces but that interactions with the surface may play a key role in determining when this can occur

Unbiased evaluation of zero-field splitting D parameter in high-spin molecules from DC magnetic data with incomplete powder averaging

A simple scheme is presented to account for preferential orientation effects in the DC magnetic response of polycrystalline samples of anisotropic high-spin molecules, like single-molecule magnets. A single additional least-squares parameter is introduced in the fitting of isothermal magnetization vs. field data to describe the leading part of a non-spherical distribution of anisotropy axes. The procedure is shown to afford an accurate D parameter and is potentially applicable whenever complete powder averaging cannot be achieved

A rational approach to the modulation of the dynamics of the magnetisation in a dysprosium–nitronyl-nitroxide radical complex

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Stoichiometrically Controlled Assembly of Lanthanide Molecular Complexes of the Heteroditopic Divergent Ligand 4′-(4-Pyridyl)-2,2′:6′,2″-terpyridine N-Oxide in Hypodentate or Bridging Coordination Modes. Structural, Magnetic, and Photoluminescence Studies

Mononuclear rare-earth tris-β-diketonato complexes RE(tta)3dme [RE = Y (1), La (2), Dy (3), or Eu (4); Htta = 2-thenoylacetone; dme = 1,2-dimethoxyethane] react cleanly at room temperature in a 1:1 molar ratio with the heteroditopic divergent ligand 4′-(4-pyridyl)-2,2′:6′,2″-terpyridine N-oxide (pyterpyNO) to yield RE2(tta)6(pyterpyNO)n, where n = 2 for RE = Y (5), Dy (6), or Eu (7) and n = 3 for RE = La (8). The crystal structure of 5 revealed a dinuclear compound with two pyterpyNO's bridging through the oxygen atom in a hypodentate mode leaving the terpyridine moieties uncoordinated. Using a metal:pyterpyNO molar ratio of 2 for RE = Y (9), Dy (10), or Eu (11), it was possible to isolate the molecular complexes RE4(tta)12(pyterpyNO)2, while using a 5:3 molar ratio, the product La5(tta)12(pyterpyNO)3 (12) can be obtained. 89Y nuclear magnetic resonance spectroscopy revealed two different yttrium centers at room temperature for 9. An X-ray diffraction study of 10 showed a symmetrical tetranuclear structure resulting from the coordination of two Dy(tta)3 fragments to the two hypodentate terpyridines of the dinuclear unit and presenting two different coordination sites for metals with coordination numbers of 8 and 9. Magnetic studies of 6 and 10 revealed the presence of an antiferromagnetic interaction between the two Dy(III) atoms bound by the NO bridges. These compounds displayed a slow relaxing magnetization through Orbach (6) and Raman (10) processes in the absence of an applied magnetic field; the rate increased upon application of a 1 kOe field. 7 and 11 showed a bright red emission typical of Eu3+. The two complexes have similar emission properties mainly determined by the employed β-diketonato ligands

Heterometallic [Ln(hfac)3Cu(acac)2] complexes with late 4f ions

Heterometallic [Ln(hfac)3Cu(acac)2] complexes (Ln = Eu 1, Dy 2, Er 3) can be prepared along the f-transition series. X-rays diffraction studies on well-shaped single crystals revealed for 1 - 3 a dinuclear structure. No ligand scrambling occurred, a Lewis acid-base interaction has been observed with the two metals bridged through two oxygen atoms of the acetylacetonato ligands and an oxygen atom of a hexafluoroacetylacetonato ligand. Compounds 2 and 3 can be obtained only in anhydrous conditions. (1) can be sublimed under reduced pressure, and crystals of the sublimed product, 1a, showed a polymorph structure with the same molecular identity. Magnetic studies have been carried out on 2, displaying an antiferromagnetic interaction and a field-induced slow relaxing magnetic moment with a significant phonon bottleneck effect. Compound 3, on the other hand, did not present any magnetization dynamics, with or without applied field. Quantum mechanical calculations rationalize the variation of the bond strengths from the reagents to the heterometallic complexes and quantify the Ln and Cu direct interaction, which is relevant, especially for 2

Magnetic Anisotropy of Tetrahedral CoII Single-Ion Magnets: Solid-State Effects

This study reports the static and dynamic magnetic characterization of two mononuclear tetrahedral CoII complexes, [CoiPr2P(E)NP(E)iPr22], where E = S (CoS4) and Se (CoSe4), which behave as single-ion magnets (SIMs). Low-temperature (15 K) single-crystal X-ray diffraction studies point out that the two complexes exhibit similar structural features in their first coordination sphere, but a disordered peripheral iPr group is observed only in CoS4. Although the latter complex crystallizes in an axial space group, the observed structural disorder leads to larger transverse magnetic anisotropy for the majority of the molecules compared to CoSe4, as confirmed by electron paramagnetic resonance spectroscopy. Static magnetic characterization indicates that both CoS4 and CoSe4 show easy-axis anisotropy, with comparable D values (â14 â30 cm-1). Moreover, alternating-current susceptibility measurements on these CoII complexes, magnetically diluted in their isostructural ZnII analogues, highlight the role of dipolar magnetic coupling in the mechanism of magnetization reversal. In addition, our findings suggest that, despite their similar anisotropic features, CoS4 and CoSe4 relax magnetically via different processes. This work provides experimental evidence that solid-state effects may affect the magnetic behavior of SIMs. © 2016 American Chemical Society