Direct Observation of Modulated Radical Spin States in Metal–Organic Frameworks by Controlled Flexibility

Abstract

Owing to their switchable spin states and dynamic electronic character, organic-based radical species have been invoked in phenomena unique to a variety of fields. When incorporated in solid state materials, generation of organic radicals proves challenging due to aggregation. Metal−organic frameworks (MOFs) are promising candidates for immobilization and stabilization of organic radicals because of the tunable spatial arrangement of organic linkers and metal nodes, which sequesters the reactive species. Herein, a flexible, redox-active tetracarboxylic acid linker bearing two imidazole units was chosen to construct a new Zr 6 -MOF, NU-910, with scu topology. By exploiting the structural flexibility of NU-910, we successfully modulate the dynamics between an isolated organic radical species and an organic radical π-dimer species in the MOF system. Single-crystal X-ray diffraction analysis reveals that through solvent exchange from N,N-diethylformamide to acetone, NU-910 undergoes a structural contraction with interlinker distances decreasing from 8.32 Å to 3.20 Å at 100 K. Organic radical species on the bridging linkers are generated via UV light irradiation. Direct observation of temperature-induced spin switches from an isolated radical species to a magnetically silent radical π-dimer in NU-910 after irradiation in the solid state was achieved via variable-temperature single-crystal X-ray diffraction and variable-temperature electron paramagnetic resonance spectroscopy. Ultraviolet−visible−near infrared spectroscopy and density functional theory calculations further substantiated the formation of a radical cation π-dimer upon irradiation. This work demonstrates the potential of using flexible MOFs as a platform to modulate radical spin states in the solid phase.

Application of Copper(II)–Organic Frameworks Bearing Dilophine Derivatives in Photocatalysis and Guest Separation

“…In addition, from the structure of MOFs, the different number of connection points in the composition of the MOF structure will lead to the presentation of diverse topological network structures. − Therefore, creating accurate topologies, such as clear topology sites, stereo network chemistry, and molecular building blocks, is still an insurmountable problem for the construction of MOFs. − , For example, the PtS topology, usually using four-connected metal nodes and ligands to form the expected topology, sometimes gets the lvt topology. Therefore, more examples are needed to provide rich topological information for the establishment of a clear structure–activity relationship.…”

Nitroxyl radical-containing flexible porous coordination polymer for controllable size-aelective aerobic oxidation of alcohols

“…Unlike their rigid counterparts, flexible porous coordination polymers (PCPs) exhibit tunable conformations and structural softness due to the reversible nature of coordination bonds and the multitude of possible metal coordination geometries. 1,2 Depending on the guest molecule, pressure, and temperature, flexible PCPs can switch between configurations with different pore sizes and shapes, [3][4][5][6][7][8][9][10][11][12][13] which makes them attractive for applications such as gas separation, storage, and sensing; however, the corresponding catalytic applications remain underexplored. [14][15][16][17][18][19] The intriguing catalytic activity of PCPs makes them promising heterogeneous catalysts.…”

Synthesis and Proton Conductivity of Two Molybdate Polymers Based on [Mo8O26]4− Anions

“…Compound 2 is a 1D β ‐Mo 8 based framework consisting of [ β ‐Mo 8 O 26 ] 4− , [Ni II (H 2 O) 2 (bibp) 2 ] 2+ and lattice water (Figure 2a). Each β ‐Mo 8 anion has eight edge‐sharing {MoO 6 } octahedra with two {Mo 4 O 13 } subunits stacking [24] . All the Mo atoms exhibit +VI oxidation state according to BVS calculations (Table S6).…”