Field-induced single-ion magnets exhibiting tri-axial anisotropy in a 1D Co(II) coordination polymer with a rigid ligand 4,4'-(buta-1,3-diyne-1,4-diyl)dibenzoate

Topochemical Polymerization at Diacetylene Metal-Organic Framework Thin Films for Tuning Nonlinear Optics

Developing the topochemical polymerization of metal-organic frameworks (MOFs) is of pronounced significance for expanding their functionalities but is still a challenge on third-order nonlinear optics (NLO). Here, we report diacetylene MOF (CAS-1-3) films prepared using a stepwise deposition method and film structural transformation approach, featuring dynamic structural diversity. The MOF structures were determined by the three-dimensional electron diffraction (3D ED) method from nanocrystals collected from the films, which provides a reliable strategy for determining the precise structure of unknown MOF films. We demonstrate the well-aligned diacetylene groups in the MOFs can promote topological polymerization to produce a highly conjugated system under thermal stimulation. As a result, the three MOF films have distinct NLO properties: the CAS-1 film exhibits saturable absorption (SA) while CAS-2 and CAS-3 films exhibit reverse saturable absorption (RSA). Interestingly, due to the topochemical polymerization of the MOF films, a transition from SA to RSA response was observed with increasing temperatures, and the optical limiting effect was significantly enhanced (∼46 times). This study provides a new strategy for preparing NLO materials and thermally regulation of nonlinear optics.

Enhancement of intermolecular aggregation on solvent-influenced strong hydrogen-bonded single crystal

The intermolecular aggregation between the solvent and organic molecules is covered in the current article. 4,4'-(Buta-1,3-diyne-1,4-diyl)dibenzoic acid (DADBA) was used as an organic molecule and dimethyl sulfoxide (DMSO) as a solvent to create the target compound DADBA-DMSO. The material's hydrogen bonding and intermolecular aggregation were determined by appropriate characterization methods, including single-crystal X-ray diffraction (XRD), Fourier-transform infrared (FTIR), photoluminescence (PL), and cyclic voltammetry (CV) analysis. Each hydrogen of the carboxylic group is coordinated by oxygen from the DMSO molecule in the stiff planar layer packing that makes up the DADBA-DMSO crystal structure.

Change in the Electronic Structure of the Cobalt(II) Ion in a One-Dimensional Polymer with Flexible Linkers Induced by a Structural Phase Transition

A new 1D-coordination polymer [Co(Piv)₂(NH₂(CH₂)₆NH₂)]_n (1, Piv is Me₃CCO₂^- anion) was obtained, the mononuclear fragments {Co(O₂CR)₂} within which are linked by μ-bridged molecules of hexamethylenediamine (NH₂(CH₂)₆NH₂). For this compound, two different monoclinic C2/c (α-1) and P2/n (β-1) phases were found at room temperature by single-crystal X-ray diffraction analysis, with a similar structure of chains and their packages in unit cells. The low-temperature phase (γ-1) of crystal 1 at 150 K corresponds to the triclinic space group P-1. As the temperature decreases, the structural phase transition (SPT) in the α-1 and β-1 crystals is accompanied by an increase in the crystal packing density caused by the rearrangements of both H-bonds and the nearest ligand environment of the cobalt atom ("octahedral CoN₂O₄ around the metal center at room temperature" → "pseudo-tetrahedral CoN₂O₂ at 150 K"). The SPT was confirmed by DSC in the temperature range 210-150 K; when heated above 220 K, anomalies in the behavior of the heat flow are observed, which may be associated with the reversibility of SPT; endo effects are observed up to 300 K. The SPT starts below 200 K. At 100 K, a mixture of phases was found in sample 1: 27% α-1 phase, 61% γ-1 phase. In addition, at 100 K, 12% of the new δ-1 phase was detected, which was identified from the diffraction pattern at 260 K upon subsequent heating: the a,b,c-parameters and unit cell volume are close to the structure parameters of γ-1, and the values of the α,β,γ-angles are significantly different. Further heating leads to a phase transition from δ-1 to α-1, which both coexist at room temperature. According to the DC magnetometry data, during cooling and heating, the χ_MT(T) curves for 1 form a hysteresis loop with ~110 K, in which the difference in the χ_MT values reaches 9%. Ab initio calculations of the electronic structure of cobalt(II) in α-1 and γ-1 have been performed. Based on the EPR data at 10 K and the ab initio calculations, the behavior of the χ_MT(T) curve for 1 was simulated in the temperature range of 2-150 K. It was found that 1 exhibits slow magnetic relaxation in a field of 1000 Oe.

Self-assembly of fish-bone and grid-like CoII-based single-molecule magnets using dihydrazone ligands with NNN and NNO pockets

Three Co^II complexes, [Co₂(H₂L¹)₂](ClO₄)₄·4CH₃OH (1), [Co₂(H₄L²)₂](ClO₄)₄ (2) and [Co₄(H₄L²)₄](ClO₄)₈ (3), were constructed by the self-assembly of the symmetrical dihydrazone ligands H2L1 and H4L2 with Co^II ions under different synthetic conditions. The fish-bone-like complex 1 was obtained using the ligand H2L1 in methanol via the solvothermal method, while the self-assembly of H4L2 with Co^II ions is solvent-dependent, producing the fish-bone-like complex 2 and [2 × 2] grid-like complex 3. Magnetic susceptibility measurements and theoretical calculations reveal that the large negative D values for the three complexes stem from their easy-axis magnetic anisotropy. Ac magnetic susceptibility measurements disclosed field-induced slow magnetic relaxation behaviors and the presence of Raman and/or direct processes of the three complexes at various applied dc fields.

Trigonally Distorted Hexacoordinate Co(II) Single-Ion Magnets

By simple reactions involving various cobalt(II) carboxylates (acetate and in situ prepared pivalate and 4-hydroxybenzoate salts) and neocuproine (neo), we were able to prepare three different carboxylate complexes with the general formula [Co(neo)(RCOO)₂] (R = –CH₃ for 1, (CH₃)₃C– for 2, and 4OH-C₄H₆– for 3). The [Co(neo)(RCOO)₂] molecules in the crystal structures of 1–3 adopt a rather distorted coordination environment, with the largest trigonal distortion observed for 1, whereas 2 and 3 are similarly distorted from ideal octahedral geometry. The combined theoretical and experimental investigations of magnetic properties revealed that the spin Hamiltonian formalism was not a valid approach and the L-S Hamiltonian had to be used to reveal very large magnetic anisotropies for 1–3. The measurements of AC susceptibility showed that all three compounds exhibited slow-relaxation of magnetization in a weak external static magnetic field, and thus can be classified as field-induced single-ion magnets. It is noteworthy that 1 also exhibits a weak AC signal in a zero-external magnetic field.