Chemical engineering of quasicrystal approximants in lanthanide-based coordination solids

Tunable valence tautomerism in lanthanide-organic alloys

The inimitable electronic structures of the lanthanide (Ln) ions are key to advanced materials and technologies involving these elements. The trivalent ions are ubiquitous and are used much more widely than the divalent and tetravalent analogues, which possess vastly different optical and magnetic properties. Hence, alteration of the valence electron count by external stimuli can lead to dramatic changes in materials properties. Compounds exhibiting a temperature-induced complete Ln(III) ⇄ Ln(II) switch, referred to as a valence tautomeric (VT) transition, are rare. Here we present an abrupt and hysteretic VT transition in a lanthanide-based coordination polymer, SmI2(pyrazine)3, driven by the interconversion of Sm(II)-pyrazine(0) and Sm(III)-pyrazine(·-) redox pairs. Alloying SmI2(pyrazine)3 with Yb(II) yields isomorphous Sm1-xYbxI2(pyrazine)3 solid solutions with VT transition critical temperatures ranging widely from 200 K to ∼50 K at ambient pressure. These findings demonstrate a simple strategy to realize thermally switchable magnetic materials with chemically tunable transition temperatures.

Towards frustration in Eu(II) Archimedean tessellations

Self-assembly of trans-{EuI₂} nodes and ditopic ligands leads to isoreticular 2D frameworks featuring a rare, non-kagome Archimedean tessellation. The topology and intra-layer Eu(II)-Eu(II) antiferromagnetic interactions provide the prerequisites for geometrical spin frustration, which, due to the spin state degeneracy, is key for novel phenomena such as enhanced magnetic refrigeration.

2D Covalent Organic Frameworks with Topology

A large number of covalent organic frameworks (COFs) with two-dimensional (2D) layered structures have been reported, but their network structures are restricted to only seven topologies (namely, hcb, hxl, kgm, sql, tth, bex, and kgd) because of the limited choice of building blocks. In this work, we illustrate how linking pseudo-fivefold symmetric 1,2,3,4,5-penta(4-formylphenyl)pyrrole with linear aromatic diamines through dynamic imine bonds produces three 2D porous COFs with an unprecedented cem topology, which represent the first examples of five-vertex semiregular Archimedean tessellations in COFs. The three 2D COFs are isostructural, and each adopts an eclipsed stacking structure with unidirectional hierarchical pores, in which the pyrrole unit is utilized as the five-vertex of network to form both square and triangular pores in a 3³.4² sequence. With high thermal and chemical resistances, the COF-packed HPLC columns show excellent performance to provide separation of 10 different polycyclic aromatic hydrocarbons, a group of the most widespread organic environmental pollutants. The implementation of five-vertex Archimedean tessellations thus couriers a strategy to design COFs with new topologies and paves a new way to expand the inimitable properties of COF materials.

Structure-Dependent Accessibility of Phonon-Coupled Radiative Relaxation Pathways Probed by X-ray-Excited Optical Luminescence

Rare-earth scheelites represent a diverse family of compounds with multiple degrees of freedom, which enables the incorporation of a wide range of lanthanide color centers. Precise positioning of quantum objects is attainable by the choice of alkali cations and lattice connectivity of polyanion units. Herein, we report the structure-dependent energy transfer and lattice coupling of optical transitions in La³⁺- and Dy³⁺-containing scheelite-type double and quadruple molybdates NaLa_1-xDy_x(MoO₄)₂ and Na₅La_1-xDy_x(MoO₄)₄. X-ray excitation of La³⁺ core states generates excited-state electron-hole pairs, which, upon thermalizing across interconnected REO₈ polyhedra in double molybdates, activate a phonon-coupled excited state of Dy³⁺. A pronounced luminescence band is observed corresponding to optical cooling of the lattice upon preferential radiative relaxation from a "hot" state. In contrast, combined X-ray absorption near-edge structure and X-ray-excited optical luminescence studies reveal that such a lattice coupling mechanism is inaccessible in quadruple molybdates with a greater separation of La³⁺-Dy³⁺ centers.

Magnetic Archimedean Tessellations in Metal-Organic Frameworks

The self-assembly of lanthanide ions with ditopic organic spacers results in the formation of complex tiling patterns that mimic the structural motifs of quasi-periodic 2D materials. The linking of trans-{LnI₂}⁺ nodes (Ln = Gd, Dy) by both closed-shell and anion radicals of 4,4'-bipyridine affords rare examples of Archimedean tessellations in a metal-organic framework. We furthermore demonstrate the occurrence of sizable magnetic exchange interactions and slow relaxation of magnetization behavior in a complex tessellation pattern. The implementation of Archimedean tessellations in lanthanide(III) coordination solids couriers a strategy to design elusive quasi-periodic metal-organic frameworks with inimitable magnetic properties.

Two-dimensional oxide quasicrystal approximants with tunable electronic and magnetic properties

Recently, the discovery of the quasiperiodic order in ultra-thin perovskite films reinvigorated the field of 2-dimensional oxides on metals, and raised the question of the reasons behind the emergence of the quasiperiodic order in these systems. The effect of size-mismatch between the two separate systems has been widely reported as a key factor governing the formation of new oxide structures on metals. Herein, we show that electronic effects can play an important role as well. To this end, the structural, thermodynamic, electronic and magnetic properties of freestanding two-dimensional oxide quasicrystalline approximants and their characteristics when deposited over metallic substrates are systematically investigated to unveil the structure-property relationships within the series. Our thermodynamic approach suggests that the formation of these aperiodic systems is likely for a wide range of compositions. In addition, the magnetic properties and work functions of the thin films can be controlled by tuning their chemical composition. This work provides well-founded general insights into the driving forces behind the emergence of the quasiperiodic order in ternary oxides grown on elemental metals and offers guidelines for the discovery of new oxide quasicrystalline ultra-thin films with interesting physical properties.

Geometrical manipulation of complex supramolecular tessellations by hierarchical assembly of amphiphilic platinum(II) complexes

Here we report complex supramolecular tessellations achieved by the directed self-assembly of amphiphilic platinum(II) complexes. Despite the twofold symmetry, these geometrically simple molecules exhibit complicated structural hierarchy in a columnar manner. A possible key to such an order increase is the topological transition into circular trimers, which are noncovalently interlocked by metal···metal and π-π interactions, thereby allowing for cofacial stacking in a prismatic assembly. Another key to success is to use the immiscibility of the tailored hydrophobic and hydrophilic sidechains. Their phase separation leads to the formation of columnar crystalline nanostructures homogeneously oriented on the substrate, featuring an unusual geometry analogous to a rhombitrihexagonal Archimedean tiling. Furthermore, symmetry lowering of regular motifs by design results in an orthorhombic lattice obtained by the coassembly of two different platinum(II) amphiphiles. These findings illustrate the potentials of supramolecular engineering in creating complex self-assembled architectures of soft materials.