Hong-Feng Li

A new strategy for achieving white-light emission of lanthanide complexes: effective control of energy transfer from blue-emissive fluorophore to Eu(III) centres

Two novel bifunctional 1,8-naphthalimide-based ligands incorporating coordinated carboxyl groups and blue-emitting 1,8-naphthalimide, 2-(1,8-naphthalimido)ethanoic acid (HL1) and 3-(1,8-naphthalimido)propanoic acid (HL2) were designed for synthesis of white-light-emitting lanthanide complexes. Self-assembly of ligands with lanthanide ions was used to form two series of 1D coordination polymers: {Ln(L1)3(CH3OH)(H2O)}n (Ln = Eu3+ (1), Gd3+ (2)) and {[Ln(L2)3(H2O)]·H2O}n (Ln = Eu3+ (3), Gd3+ (4)). Single-crystal X-ray crystallography analyses reveal the complexes feature 1D chain structure. In 1 and 3, partial energy transfer from the 1,8-naphthalimide moieties to the metal centres results in sensitized Eu(III)-based emission in addition to the ligand-based blue/green luminescence. Through changing the length of the linkers between the chromophore and the metal centre, energy transfer efficiency of ligands to Eu is effectively controlled. In 1, the emission spectrum is mainly dominated by the red light of the Eu(III) ions. However, the red light emission is suppressed in 3 due to the decrease of chromophore-to-Eu energy transfer efficiency. This effective adjustment balances the three primary colours, thus resulting in white-light emission in 3. This result opens up a new synthetic strategy for white-light-emitting materials.

Complementation and joint contribution of appropriate intramolecular coupling and local ion symmetry to improve magnetic relaxation in a series of dinuclear Dy2 single-molecule magnets

A series of lanthanide binuclear complexes with formula [Dy2(DMOP)2(NTFA)4] (1), [Dy2(DMOP)2(NTFA)4(CH3OH)2·2H2O] (1a), [Dy2(DMOP)2(BTFA)4] (2), [Dy2(DMOP)2(TTA)4] (3), [Y2(DMOP)2(TTA)4] (3Y), [Tb2(DMOP)2(NTFA)4] (4), [Tb2(DMOP)2(NTFA)4(CH3OH)2·2H2O] (4a), [Er2(DMOP)2(NTFA)4] (5), [Gd2(DMOP)2(NTFA)4(CH3OH)2·2H2O] (6), and [Gd2(DMOP)2(TTA)4] (7) (DMOP = 2,6-dimethoxyphen, NTFA = 2-naphtoyltrifluoroacetone, BTFA = benzoyltrifluoroacetone, TTA = thenoyltrifluoroacetone) have been structurally and magnetically characterized. They possess nearly the same dinuclear Ln2 cores bridged by double μ2-phenoxyl oxygen atoms and differ in the peripheral coordinated β-diketonate ligands or coordinated neutral MeOH molecule. The Dy(III)-based derivatives show significant zero-field single-molecule magnet behavior. It is noteworthy that the single-molecule magnet (SMM) performance of 1 was dramatically improved after transforming to 1a with a nearly double energy barrier, which is mostly due to the enhancement of local symmetry around Dy(III) ions in 1a by the introduction of a CH3OH molecule. In addition, the nature of intramolecular Dy–Dy interaction changed from antiferromagnetic to ferromagnetic coupling. The detailed ab initio calculation reveals the alternate domination of the intramolecular exchange coupling and dipole–dipole interactions in 1 and 1a, respectively. Furthermore, improved SMM performance with a hysteresis temperature of 3 K was observed in diluted samples of 3. In the present work, the joint contribution of intramolecular coupling and local symmetry to improve their dynamic magnetic relaxation was unambiguously affirmed.

Near-infrared luminescent hybrid materials – PMMA doped with a neodymium complex: synthesis, structure and photophysical properties

A new β-diketone, 5,6-dimethoxy-2-(2,2,2-trifluoroethyl)-1-indone (5,6-DTFI) and two series of 5,6-DTFI lanthanide complexes, namely, Nd(5,6-DTFI)3(H2O)(CH3OH) (1), Nd(5,6-DTFI)3(phen) (2), Yb(5,6-DTFI)3(H2O)2 (3) and Yb(5,6-DTFI)3(phen) (4) (phen = 1,10-phenanthroline), have been isolated. X-ray diffraction analysis reveals that all complexes 1–3 have mononuclear structures. NIR luminescent spectra of complexes 1–4 show strong characteristic emissions of the corresponding Nd3+ and Yb3+ ions. Upon doping poly(methylmethacrylate) (PMMA) with complex 1 to form a film, the PMMA polymer matrix acts as a co-sensitizer for Nd3+ ion and increases the NIR luminescent lifetimes, overall quantum yields and the thermal stability in comparison with the precursor complex 1.

Anion-dependent assembly of Dy complexes: structures and magnetic behaviors

The self-assembly of 2-aldehyde-8-hydroxyquinoline, histamine dihydrochloride and LnX3·6H2O (X− = OAc−, NO3− and ClO4−) affords a series of lanthanide complexes, namely Nd3(nma)2(OAc)7·3CH3OH·0.5H2O (1a), Ln3(nma)2(OAc)7·2CH3OH [Ln = Tb (1b) and Dy (1c)], [Ln(nma)(NO3)2(DMSO)]·CH3OH [Ln = Nd (2a), Tb (2b), Dy (2c) and Er (2d)] and [Dy(nma)2]·ClO4·0.5CH3OH (3) (Hnma = N-(2-(8-hydroxylquinolinyl)methane(2-(4-imidazolyl)ethanamine))). It is noted that the formation and structures of 1–3 are anion-dependent, where diverse coordination modes are detected for acetates in trinuclear 1 as compared to the single coordination mode for nitrates in mononuclear 2. In the case of 3, the Dy3+ ion is completely encapsulated by two ligands with an uncoordinated perchlorate anion balancing the charge. Magnetic measurement shows that the Dy complexes of 1c, 2c and 3 exhibit slow relaxations under zero dc field. It is noted that single-molecule magnet behavior is obtained for 2c and 3 under an applied dc field of 2000 Oe.

Synthesis, structure, and tunable white light emission of heteronuclear Zn2Ln2 arrays using a zinc complex as ligand

New heteronuclear Zn2Ln2 clusters [Zn2L2Ln2(hfac)6] (Ln = Eu (1), Tb (2) and Dy (3)) have been synthesized based on a flexible Schiff base (H2L = N,N′-bis(salicylidene)-3,6-dioxa-1,8-diaminooctane) and β-diketonate ligand (hfac = hexafluoroacetylacetonate). The structures of the isomorphous complexes 1–3 were determined by single-crystal X-ray crystallography. Photophysical studies indicate that the short intramolecular distance of Zn⋯Ln allows energy transfer from the Zn2L2-based sensitizers to the Eu(III) and Tb(III) centres; the lanthanide luminescence is indeed “lighted up” by the excitation of the zinc complex. More interestingly, white light emission was realized by codoping Eu(III) ion into the Dy(III) complex for the first time.

Crystallization of triple- and quadruple-stranded dinuclear bis-β-diketonate-Dy(III) helicates: single molecule magnetic behavior

Helical structures are vital in chemistry and biochemistry and their importance has been reconsidered since the structure of DNA was revealed. Over the past decades, the advantages of the helical structures of bis-β-diketonate-based multiple-stranded Ln3+ complexes have been speculated with respect to their unique structures. Based on our previous study, we have designed a V-shaped bis-β-diketone ligand, H2MBDA, which has been utilized to crystallographically synthesize triple-stranded and quadruple-stranded dinuclear Dy3+ complexes. In contrast to the absence of crystallographical results in previous studies, the successful crystallization in this study has contributed to the functionalization of the –CF3 groups into the ligand as the termini, which have played a key role in the crystallization through intermolecular weak interactions. Both complexes display slow magnetic relaxation. The auxiliary ligand, phenanthroline, contributes to both tuning the structure and strengthening the anisotropy barrier.

A series of lanthanide(III) complexes constructed from Schiff base and β-diketonate ligands

A series of lanthanide(III) complexes constructed from Schiff base and β-diketonate ligands have been synthesized by the same method. They are mononuclear complex [Ln(hfac)3(L)] (Ln = Nd (1), Sm (2), Eu (3)), tetranuclear complex [Gd4(OAc)2(hfac)10(L)3] (4), dinuclear–mononuclear cocrystal {[Dy(hfac)2(L)]2(OAc)}·[Dy(hfac)4] (5), and mononuclear–mononuclear cocrystal [Yb(hfac)2(L)]·[Yb(hfac)4] (6) (L = N,N′-bis(pyridine)-1,2-ethanediamine, hfac = hexafluoroacetylacetonate). Complexes 1–3 and 6 exhibit characteristic metal-centred emission not only in the solid state but also in solution. Lifetimes and quantum yields of luminescence are also determined.

A series of dinuclear lanthanide(III) complexes constructed from Schiff base and β-diketonate ligands: synthesis, structure, luminescence and SMM behavior

A series of new isostructural lanthanide(III) complexes are reported which are constructed from Schiff base and β-diketonate ligands, namely, [Ln2(DBM)6(L)] (Ln = Sm (1), Eu (2), Gd (3), Dy (4), Yb (5); L = N,N′-bis(pyridin-2-ylmethylene)ethane-1,2-diamine, DBM = dibenzoylmethanide). The structures of the isomorphous complexes 1–5 were determined by single-crystal X-ray crystallography. The two Ln(III) ions in the molecules of 1–5 are bridged by a Schiff base ligand that displays a curved configuration. Complexes 1, 2 and 5 exhibit lanthanide ion-based luminescence not only in the solid state but also in methanol solution. The lifetimes and quantum yields of luminescence were also determined. The magnetic analysis reveals that complexes 4 and 5 exhibit field-induced single-molecule magnet behavior with energy barriers (Ueff/kB) of 46.8 and 23.0 K at direct current fields of 2000 Oe, respectively.

A salen-type trinuclear Zn2Gd complex

In the trinuclear title complex, di-μ-acetato-1:2κ2 O:O′;2:3κ2 O:O′-bis{μ-6,6′-dimethoxy-2,2′-[cyclohexane-1,2-diylbis(methanylylidene)]diphenolato}-1:2κ6 O 1,N,N′,O 1′:O 6,O 6′;2:3κ6 O 6,O 6′:O 1,N,N′,O 1′-2-gadolinium(III)-1,3-dizinc hexafluoridophosphate methanol monosolvate monohydrate, [GdZn2(C22H24N2O4)2(CH3COO)2]PF6·CH3OH·H2O, the two ZnII ions are located in the inner N2O2 cavities of two 6,6′-dimethoxy-2,2′-[cyclohexane-1,2-diylbis(methanylylidene)]diphenolate (L) ligands. Both ZnII ions are five-coordinated by two O atoms and two N atoms from the L ligand and one O atom of an acetic acid molecule, giving rise to a square-pyramidal geometry around the ZnII ions. The GdIII ion is nine-coordinated by four O atoms from the outer O2O2 sites of one ligand, and three O atoms from another ligand, in which there is one non-coordinating methoxy O atom. Two further O atom from different acetate ligands complete the nine-coordinate environment.

Dramatic impact of the lattice solvent on the dynamic magnetic relaxation of dinuclear dysprosium single-molecule magnets

Two dinuclear dysprosium single-molecule magnets (SMMs) with a nearly identical core structure, namely [Dy2(DMOP)2(DBM)4(CHCl3)4] (1) and [Dy2(DMOP)2(DBM)4(C2H4Cl2)2] (2) (DMOP = 2,6-dimethoxyphenol, DBM = dibenzoylmethane), and their Y(III) (3) and Gd(III) (4) analogues were structurally and magnetically characterized. Despite the fact the phenoxyl of DMOP can only transfer weak exchange coupling and the local coordination environment around the individual Dy(III) ion is in a low geometrical symmetry, their joint contribution ultimately leads to significant zero-field slow magnetic relaxation behaviours. It is interesting to note that the effective energy barrier (Ueff) of the magnetization reversal was nearly doubly improved by only deliberately replacing or losing the lattice solvents while maintaining the Dy2 core structure. Ab initio calculations confirmed that changing the lattice solvents induced the intramolecular coupling change and impacted the dynamic magnetic relaxations. The significant enhancement of an SMMs performance depending on the lattice solvents indicated that it is possible to improve an SMMs properties by tuning the guest molecules.