Naphthalimide-Tagged Iron(II) Spin Crossover Complex with Synergy of Ratiometric Fluorescence for Thermosensing.

Spin crossover (SCO) materials that possess switchable and cooperative fluorescence have long focused interest in photonic sensor devices to monitor the variations in the physicochemical parameters of the external environment. However, the lack of quantified cooperativity for the SCO transition operating in isolated molecules is detrimental to short-term technological applications. In this study, a pretwisted energy D-A system combining the deep-blue naphthalimide fluorophore (donor) and the FeN6 SCO chromophore (switchable acceptor) has been developed with the formula of Fe(naph-abpt)2(NCS)2·2DMF (1), where naph-abpt is N-[3,5-di(pyridin-2-yl)-4H-1,2,4-triazol-4-yl]-1,8-naphthalimide. Dual emission from the naphthalimide function based on its vibronic structure exhibits a different synergy effect with SCO, providing a new platform for ratiometric fluorescence thermosensing. Theoretical calculations and optical experimental results demonstrate an excellent correlation between luminescence intensity ratio signals and magnetic data of spin transition, promising a high sensitivity of the optical activity of the ligand to the spin state of the active iron(II) ions, with the maximum relative sensitivity as 0.7% K-1 around T1/2.

Ferroelectric Single-Molecule Magnet with Toroidal Magnetic Moments.

Materials that coexist magnetic and electric properties on the molecular scale in single-molecule magnets (SMMs) with peculiar quantum behaviors have promise in molecular electronics and spintronics. Nevertheless, such molecular materials are limited in potentials because their magnetic signal cannot be transformed into an electrical signal through magnetoresistance or Hall effects for their high insulativity. The discovery of an entirely new material, ferroelectric SMMs (FE SMMs) is reported. This FE SMM also shows single-molecule magnetic behaviors, toroidal magnetic moments, and room-temperature ferroelectricity. The toroidal moment is formed by a vortex distribution of magnetic dipoles in triangular Dy3 clusters. The analysis of ac magnetic susceptibility reveals the coexistence of three distinct magnetic relaxation processes at low temperatures. The ferroelectricity is introduced by incorporating polar alcohol molecules in the structure, which is confirmed by the X-ray diffraction and optical second harmonic generation (SHG) measurements. Moreover, the dielectric measurements reveal a ferroelectric-to-ferroelectric phase transition around 150 K due to the symmetry change from Pc to Pna21 . The coexistence of toroidal moment and ferroelectricity along with quantum magnetism in the rare-earth single-molecule magnets yields a unique class of multiferroics.

Vapor-triggered reversible crystal transformation of a nickel-based magnetic molecular switch.

Vapor-triggered crystal-to-crystal transformation between a discrete trinuclear complex [Ni3(sih)2(py)8] and a two-dimensional (2D) coordination polymer [Ni3(sih)2(py)2]n·2DMF·2H2O was demonstrated. It provides an example of a solid-state coordination-induced spin state switch behavior attributed to the structural phase transition triggered by solvent signal. The reversible nature can be detected by both optical (spectral) and magnetic responses in cycles.

Auxiliary alkyl chain modulated spin crossover behaviour of [Fe(H2Bpz2)2(Cn-bipy)] complexes.

Three new alkyl chain substituted complexes [Fe(H2Bpz2)2(Cn-bipy)] (pz = pyrazolyl, Cn-bipy = bipyridine alkyl chain diester, n = 3 (3), 4 (4) and 5 (5)) show versatile spin state switching behaviour with different "tail" lengths as revealed by structural and magnetic analyses. The most striking phenomenon is observed for 5 which undergoes an abrupt spin transition accompanied by thermal hysteresis of ca. 10 K, which is attributed to crystal packing effects derived from the competition between π⋯π and C-H⋯O interactions. Interestingly, each of the complexes exhibits similar gradual and complete spin crossover in methanol solution with a transition temperature around 249 K, as deduced from temperature-dependent UV-vis spectroscopy. This highlights the differences between the solid state (ligand field; crystal packing) and solution (ligand field; solvation) effects on spin crossover. This work demonstrates that the length of the complex's alkyl chain substituents on the complex can have a large impact on the transition temperature and profile of solid state spin crossover, offering a potential path to the fabrication of soft matter spin crossover materials.

Spin Crossover Behavior in a Homologous Series of Iron(II) Complexes Based on Functionalized Bipyridyl Ligands.

A series of bulky substituted bipyridine-related iron(II) complexes [Fe(H2Bpz2)2(L)] (pz = pyrazolyl) were prepared, where L = 5,5'-dimethyl-2,2'-bipyridine (bipy-CH3, 1), L = dimethyl-2,2'-bipyridyl-5,5'-dicarboxylate (MeObpydc, 2), L = diethyl-2,2'-bipyridyl-5,5'-dicarboxylate (EtObpydc, 3), or L = diisopropyl-2,2'-bipyridine-5,5'-dicarboxylate ( i-PrObpydc, 4). The crystal structures of five new iron(II) complexes were determined by X-ray diffraction: those of 1, 3, and 4 and two modifications of 3 (3B) and 4 (4B). Complexes 1 and 3B display incomplete spin crossover (SCO) behavior because of a freezing-in effect, whereas 3 and 4B undergo gradual and incomplete SCO behaviors. Complexes 2 and 4 show a completely gradual and steep SCO, respectively. Such different SCO behaviors can be attributed to an electronic substituent effect in the bipyridyl ligand conformation and a crystal packing effect. Importantly, the electronic substituent effect of the isopropyl acetate group and C-H···O supramolecular interactions in 4 contribute to a highly cooperative behavior, which leads to an abrupt thermally induced spin transition.

Tuning the Magnetic Interactions and Relaxation Dynamics of Dy2 Single-Molecule Magnets.

Efficient modulation of single-molecule magnet (SMM) behavior was realized by deliberate structural modification of the Dy2 cores of [Dy2(a'povh)2(OAc)2(DMF)2] (1) and [Zn2Dy2(a'povh)2(OAc)6]⋅4 H2O (2; H2a'povh = N'-[amino(pyrimidin-2-yl)methylene]-o-vanilloyl hydrazine). Compound 1 having fourfold linkage between the two dysprosium ions shows high-performance SMM behavior with a thermal energy barrier of 322.1 K, whereas only slow relaxation is observed for compound 2 with only twofold connection between the dysprosium ions. This remarkable discrepancy is mainly because of strong axiality in 1 due to one pronounced covalent bond, as revealed by experimental and theoretical investigations. The significant antiferromagnetic interaction derived from bis(µ2-O) and two acetate bridging groups was found to be crucial in leading to a nonmagnetic ground state in 1, by suppressing zero-field quantum tunneling of magnetization.

Field-induced multiple relaxation mechanism of Co(III)2Dy(III) compound with the dysprosium ion in a low-symmetrical environment.

A defective cubane-shaped heterometallic trinuclear Co(III)2Dy(III) compound with only one magnetically interesting ion (Dy(III)) has been assembled by virtue of a multifunctional acylhydrazone ligand. Because of the nonaxial ground state of Dy(III) ion derived from a low-symmetrical crystal field, the title compound displays field-induced multiple relaxation processes which are of molecular and a dipolar-dipolar coupling origin, as revealed by combined experimental and theoretical investigations. The results demonstrate that such a mononuclear dysprosium(III) compound with a low-symmetrical environment of magnetic center appears to be a model system for further investigations to shed light on the complex relaxation mechanism of lanthanide-based single ion magnets.

Molecular magnetic investigation of a family of octanuclear [Cu6Ln2] nanoclusters.

Reaction of in situ prepared acylhydrazone ligand with Ln(NO3)3·6H2O and Cu(OAc)2·H2O resulted in the formation of novel isostructural octanuclear Cu6Ln2 compounds (Ln = Dy (1), Tb (2), Gd (3), Y (4)) with an unprecedented octametallic structure, which can be described as an oblate wheel built up from two structurally similar Cu3 fragments linked together by two nodelike mononuclear lanthanide units. A detailed magnetic analysis reveals that the strong antiferromagnetic Cu···Cu interactions via the Cu-N-N-Cu-N-N-Cu linkage and the anticipated ferromagnetic Cu···Gd coupling makes an overall high-spin ground state in favor of the observation of significant magnetic caloric and SMM-like properties in the isotropic and anisotropic derivatives.

An NCN-pincer ligand dysprosium single-ion magnet showing magnetic relaxation via the second excited state.

Single-molecule magnets are compounds that exhibit magnetic bistability purely of molecular origin. The control of anisotropy and suppression of quantum tunneling to obtain a comprehensive picture of the relaxation pathway manifold, is of utmost importance with the ultimate goal of slowing the relaxation dynamics within single-molecule magnets to facilitate their potential applications. Combined ab initio calculations and detailed magnetization dynamics studies reveal the unprecedented relaxation mediated via the second excited state within a new DyNCN system comprising a valence-localized carbon coordinated to a single dysprosium(III) ion. The essentially C2v symmetry of the Dy(III) ion results in a new relaxation mechanism, hitherto unknown for mononuclear Dy(III) complexes, opening new perspectives for means of enhancing the anisotropy contribution to the spin-relaxation barrier.

Unique Y-shaped lanthanide aggregates and single-molecule magnet behaviour for the Dy4 analogue.

The assembly of N'-(amino-(pyrimidin-2-yl)methylene)-o-vanilloyl hydrazine ligands (H2L, Scheme 1) with different lanthanide perchlorates produces novel Y-shaped tetranuclear complexes, [Ln4(µ3-OH)(L(2-))4(H2O)6]·(ClO4)3·6H2O, where Ln = Tb (1), Dy (2) and Ho (3). The formation of this unprecedented Y-shaped topology is largely ascribed to the versatility of the mixed-donor hydrazone ligands in terms of their potential denticity. Analysis of the susceptibility data shows that only the Dy-based molecule features SMM-like behaviour. The synthetic methodology of employing H2L and related ligands has provided a very promising route towards new families of magnetic coordination clusters with novel metal topologies and properties.

Two bulky-decorated triangular dysprosium aggregates conserving vortex-spin structure.

The self-assembly of dysprosium(III) with the tailored chemical modification of the vanillin group affords two decorated Dy(3) compounds, namely, [Dy(3)(µ(3)-OH)(2)(Hpovh(-))(3)(NO(3))(3)(CH(3)OH)(2)H(2)O]·NO(3)·3CH(3)OH·2H(2)O (2) and [Dy(3)(µ(3)-OH)(2)(H(2)vovh(-))(3)Cl(2)(CH(3)OH)(H(2)O)(3)][Dy(3)(µ(3)-OH)(2)(H(2)vovh(-))(3)Cl(2)(H(2)O)(4)]·Cl(4)·2CH(3)OH·2CH(3)CN·7H(2)O (3), where H(2)povh = N-(pyridylmethylene)-o-vanilloylhydrazone and H(3)vovh = N-vanillidene-o-vanilloylhydrazone. Of particular interest is that those two title Dy(3) compounds maintain the peculiar vortex-spin structure of the ground nonmagnetic doublet. Complex 2 displays frequency-dependent slow magnetic relaxation, while 3 still inherits the single-molecule-magnet behavior as the parent Dy(3) prototype. The dissimilar dynamic magnetic behavior originates from the structural differences in light of the coordination environment of Dy(III) ions, which influence the local tensor of anisotropy and crystal-field splitting on each Dy site.

The use of a versatile o-vanilloyl hydrazone ligand to prepare SMM-like Dy3 molecular cluster pair.

A novel lanthanide molecular cluster pair (MCP), displaying single molecule magnet behaviour, was assembled using the novel o-vanilloyl hydrazone ligand, versatile in terms of denticity, tautomerism and the rotatable C-C bond.

Macrocyclic ligand encapsulating dysprosium triangles: axial ligands perturbed magnetic dynamics.

Two Dy(3) triangles encapsulated inside the cavity of a macrocycle ligand have been successfully synthesized, providing a unique opportunity to probe the relaxation dynamics of the complexes by altering the axial ligands.

Field enhanced thermally activated mechanism in a square Dy4 aggregate.

A rare µ(4)-OH centred square Dy(4) aggregate displays a field enhanced thermally activated mechanism with 3-fold increase in the single-molecule magnetic relaxation barrier upon application of an optimal field.

M(III)Dy(III)3 (M = Fe(III), Co(III)) complexes: three-blade propellers exhibiting slow relaxation of magnetization.

[Dy(III)(HBpz(3))(2)](2+) moieties (HBpz(3)(-) = hydrotris(pyrazolyl)borate) and a 3d transition-metal ion (Fe(III) or Co(III)) have been rationally assembled using an dithiooxalato dianion ligand into 3d-4f [MDy(3)(HBpz(3))(6)(dto)(3)]·4CH(3)CN·2CH(2)Cl(2) (M = Fe (1), Co (2) complexes. Single-crystal X-ray studies reveal that three eight-coordinated Dy(III) centers in a square antiprismatic coordination environment are connecting to a central octahedral trivalent Fe or Co ion forming a propeller-type complex. The dynamics of the magnetization in the two isostructural compounds, modulated by the nature of the central M(III) metal ion, are remarkably different despite their analogous direct current (dc) magnetic properties. The slow relaxation of the magnetization observed for 2 mainly originates from isolated Dy ions, since a diamagnetic Co(III) metal ion links the magnetic Dy(III) ions. In the case of 1, the magnetic interaction between S = 1/2 Fe(III) ion and the three Dy(III) magnetic centers, although weak, generates a complex energy spectrum of magnetic states with low-lying excited states that induce a smaller energy gap than for 2 and thus a faster relaxation of the magnetization.

Two new Dy3 triangles with trinuclear circular helicates and their single-molecule magnet behavior.

Self-assembly of polydentate Schiff base 2,6-diformyl-4-methylphenol di(benzoy1hydrazone) (H(3)L), with dysprosium thiocyanate and sodium azide, affords two novel trinuclear triangular circular helicate dysprosium(III) complexes, [Dy(3)(µ(3)-OCH(3))(2)(HL)(3)(SCN)]·4CH(3)OH·2CH(3)CN·2H(2)O (1) or [Dy(3)(µ(3)-N(3))(µ(3)-OH)(H(2)L)(3)(SCN)(3)](SCN)·3CH(3)OH·H(2)O (2), depending on the presence or absence of base. Single-crystal X-ray analyses show that two µ(3)-methoxy oxygens cap the Dy(3) triangle in complex 1 and that one µ(3)-OH and one µ(3)-N(3)(-) cap the Dy(3) triangle of complex 2, representing the first example of a µ(3)-N(3)(-)-capped lanthanide complex reported to date. Ac susceptibility measurements reveal that multiple relaxation processes and the onset of slow magnetization relaxation occur for complex 1 and 2, respectively. Theoretical calculations are required to elucidate the underlying mechanism; however, the different magnetic anisotropy of the respective structures, which is dictated by the coordination environment of Dy(III) ions and structural parameters of the triangles, is mostly responsible for the distinctive relaxation dynamics observed.

Molecular assembly and magnetic dynamics of two novel Dy6 and Dy8 aggregates.

Complexation of dysprosium(III) with the heterodonor chelating ligand o-vanillin picolinoylhydrazone (H(2)ovph) in the presence of a carbonato ligand affords two novel Dy(6) and Dy(8) clusters, namely, [Dy(6)(ovph)(4)(Hpvph)(2)Cl(4)(H(2)O)(2)(CO(3))(2)]·CH(3)OH·H(2)O·CH(3)CN (2) and [Dy(8)(ovph)(8)(CO(3))(4)(H(2)O)(8)]·12CH(3)CN·6H(2)O (3). Compound 2 is composed of three petals of the Dy(2) units linked by two carbonato ligands, forming a triangular prism arrangement, while compound 3 possesses an octanuclear core with an unprecedented tub conformation, in which Dy(ovph) fragments are attached to the sides of the carbonato core. The static and dynamic magnetic properties are reported and discussed. In the Dy(6) aggregate, three Dy(2) "skeletons", having been well preserved (see the scheme), contribute to the single-molecule-magnet behavior with a relatively slow tunneling rate, while the Dy(8) cluster only exhibits a rather small relaxation barrier.

Steric hindrances create a discrete linear Dy4 complex exhibiting SMM behaviour.

Two linear tetranuclear lanthanide complexes of general formula [Ln(4)(L)(2)(C(6)H(5)COO)(12)(MeOH)(4)], where HL = 2,6-bis((furan-2-ylmethylimino)methyl)-4-methylphenol, () and Ln(III) = Dy(III) (1) and Gd(III) (2), have been synthesized and characterized. The crystal structural analysis demonstrates that two Schiff-base ligands inhibit the growth of benzoate bridged 1D chains, leading to the isolation of discrete tetranuclear complexes due to their steric hindrances. Every Ln(III) ion is coordinated by eight donor atoms in a distorted bicapped trigonal-prismatic arrangement. Alternating current (ac) susceptibility measurements of complex 1 reveal a frequency- and temperature-dependent out-of-phase signal under zero dc field, typical of single-molecule magnet (SMM) behaviour with an anisotropic barrier Δ(eff) = 17.2 K.