Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy

Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl

Chemical bonding induces one-dimensional physics in bulk crystal BiIr4Se8

One-dimensional (1D) systems persist as some of the most interesting because of the rich physics that emerges from constrained degrees of freedom. A desirable route to harness the properties therein is to grow bulk single crystals of a physically three-dimensional (3D) but electronically 1D compound. Most bulk compounds which approach the electronic 1D limit still field interactions across the other two crystallographic directions and, consequently, deviate from the 1D models. In this paper, we lay out chemical concepts to realize the physics of 1D models in 3D crystals. These are based on both structural and electronic arguments. We present BiIr4Se8, a bulk crystal consisting of linear Bi2+ chains within a scaffolding of IrSe6 octahedra, as a prime example. Through crystal structure analysis, density functional theory calculations, X-ray diffraction, and physical property measurements, we demonstrate the unique 1D electronic configuration in BiIr4Se8. This configuration at ambient temperature is a gapped Su-Schriefer-Heeger system, generated by way of a canonical Peierls distortion involving Bi dimerization that relieves instabilities in a 1D metallic state. At 190 K, an additional 1D charge density wave distortion emerges, which affects the Peierls distortion. The experimental evidence validates our design principles and distinguishes BiIr4Se8 among other quasi-1D bulk compounds. We thus show that it is possible to realize unique electronically 1D materials applying chemical concepts.This research was primarily supported by the Princeton Center for Complex Materials, a National Science Foundation (NSF)-MRSEC program (DMR-2011750), the Gordon and Betty Moore Foundation’s EPIQS initiative (grant numbers GBMF9064 and GBMF9466), and the David and Lucille Packard foundation. C.J.P. is supported by the NSF Graduate Research Fellowship Program under grant number DGE-2039656. G.S. is supported by the Arnold and Mabel Beckman foundation through an AOB postdoctoral fellowship. NSF’s ChemMatCARS, Sector 15 at the Advanced Photon Source, Argonne National Laboratory, is supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation, under grant number NSF/CHE-1834750. This research used resources of the Advanced Photon Source, a U.S. Department of Energy Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract no. DE-AC02-06CH11357.M.G.V., I.E., and M.G.A acknowledge the Spanish Ministerio de Ciencia e Innovacion (grants PID2019-109905GB-C21, PID2022-142008NB-I00, and PID2022-142861NA-I00). I.E. acknowledges the Department of Education, Universities and Research of the Eusko Jaurlaritza and the University of the Basque Country UPV/EHU (Grant no. IT1527-22). M.G.V. thanks support to the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) GA 3314/1-1─FOR 5249 (QUAST) and partial support from European Research Council grant agreement no. 101020833. M.G.A. thanks the Department of Education of the Basque Government for a predoctoral fellowship (grant no. PRE_2019_1_0304). This work has been financially supported by the Ministry for Digital Transformation and of Civil Service of the Spanish Government through the QUANTUM ENIA project call - Quantum Spain project, and by the European Union through the Recovery, Transformation and Resilience Plan - NextGenerationEU within the framework of the Digital Spain 2026 Agenda.Peer reviewe

Electrically conductive porous catecholate metal-organic frameworks

Metal-organic frameworks (MOFs) are porous crystalline solids made up from countless possible combinations of organic ligands and metal nodes. MOF research has seen rapid expansion in the past two decades owing to the high surface areas of MOFs – and since the first studies in the mid-1990s, these materials have already found their first real-life applications in gas storage and separation. Of particular recent interest are electrically conductive MOFs, seen not only as porous conductors, but also as designer conductors thanks to the high degree of control over their structures. In this thesis, we attempt to improve our understanding of charge transport in MOFs by focusing on the compounds of 2,3,6,7,10-11- hexahydroxytriphenylene (H₆HOTP), which is essentially a trimerized 1,2-dihydroxybenzene, or catechol. Chapter 1 introduces basic concepts of electrical conductivity in MOFs and explores in detail the current state of experimental investigations into record-setting two-dimensional (2D) conductive MOFs with extended π-d conjugation throughout the layers. Chapter 2 challenges the common assumption that this extended conjugation is the primary pathway for conduction in these materials: we explored MOFs based on the lanthanides (Ln) and H₆HOTP, Ln₁+ₓHOTP (x ~ 0.2) with no in-plane conjugation, and found that high conductivities could be achieved only through π-π stacking interactions of the organic linkers. Chapter 3 supports the findings of Chapter 2 with another new 2D conductive MOF, Ga₉HOTP₄. We found that despite little electron delocalization within the 2D layers, Ga₉HOTP₄ possesses conductivities matching those of its heavily delocalized transition-metal analogs, and that the conductivity similarly originates in ππ stacking. In Chapter 4, we find that high quality crystals of La₁.₅HOTP and Nd₁.₅HOTP are one-dimensional metals with record-high conductivities surpassing 1000 S/cm at room temperature. Importantly, the crystals also transition to a charge density wave phase below 370 K – such transitions are characteristic to one-dimensional metals and have not been reported previously for MOFs or any other porous solids. Lastly, in Chapter 5, we present a novel family of conductive MOFs based on rare-earth metals and H₆HOTP with isotropic cubic structures – a feature that is surprisingly rare in conductive MOFs.Ph.D

Electrical Conductivity in a Porous, Cubic Rare-Earth Catecholate

Electrically conductive metal-organic frameworks (MOFs) provide a rare example of porous materials that can efficiently transport electrical current, a combination that is favorable for a variety of technological applications. The vast majority of such MOFs are highly anisotropic in both their structures and properties: Only two electrically conductive MOFs reported to date exhibit cubic structures that enable isotropic charge transport. Here we report a new family of intrinsically porous frameworks made from rare-earth nitrates and hexahydroxytriphenylene. The materials feature a novel hexanuclear secondary building unit and form cubic, porous, and intrinsically conductive structures, with electrical conductivities reaching 10-5 S/cm and surface areas of up to 780 m2/g. By expanding the list of MOFs with isotropic charge transport, these results will help us to improve our understanding of design strategies for porous electronic materials. ©2020Army Research Office (grant no. W911NF-17-1-0174)National Science Foundation (grant no. CHE-0946721

Electrical Conductivity in a Porous, Cubic Rare-Earth Catecholate

Electrically conductive metal-organic frameworks (MOFs) provide a rare example of porous materials that can efficiently transport electrical current, a combination favorable for a variety of technological applications. The vast majority of such MOFs are highly anisotropic in both their structures and properties: only two electrically conductive MOFs reported to date exhibit cubic structures that enable isotropic charge transport. Here, we report a new family of intrinsically porous frameworks made from rare earths and hexahydroxytriphenylene that are cubic, porous, and intrinsically conductive with conductivities reaching 10−5 S/cm and surface areas of up to 780 m2/g. By expanding the list of MOFs with isotropic charge transport, these results will help improve our understanding of design strategies for porous electronic materials.<br /

Electrically Conductive Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are intrinsically porous extended solids formed by coordination bonding between organic ligands and metal ions or clusters. High electrical conductivity is rare in MOFs, yet it allows for diverse applications in electrocatalysis, charge storage, and chemiresistive sensing, among others. In this Review, we discuss the efforts undertaken so far to achieve efficient charge transport in MOFs. We focus on four common strategies that have been harnessed toward high conductivities. In the “through-bond” approach, continuous chains of coordination bonds between the metal centers and ligands’ functional groups create charge transport pathways. In the “extended conjugation” approach, the metals and entire ligands form large delocalized systems. The “through-space” approach harnesses the π–π stacking interactions between organic moieties. The “guest-promoted” approach utilizes the inherent porosity of MOFs and host–guest interactions. Studies utilizing less defined transport pathways are also evaluated. For each approach, we give a systematic overview of the structures and transport properties of relevant materials. We consider the benefits and limitations of strategies developed thus far and provide an overview of outstanding challenges in conductive MOFs.National Science Foundation (Grant 1122374

Aperiodic metal–organic frameworks

© The Royal Society of Chemistry. Metal-organic frameworks (MOFs) represent one of the most diverse structural classes among solid state materials, yet few of them exhibit aperiodicity, or the existence of long-range order in the absence of translational symmetry. From this apparent conflict, a paradox has emerged: even though aperiodicity frequently arises in materials that contain the same bonding motifs as MOFs, aperiodic structures and MOFs appear to be nearly disjoint classes. In this perspective, we highlight a subset of the known aperiodic coordination polymers, including both incommensurate and quasicrystalline structures. We further comment upon possible reasons for the absence of such structures and propose routes to potentially access aperiodic MOFs. This journal i

Reversible Topochemical Polymerization and Depolymerization of a Crystalline Three-Dimensional Porous Organic Polymer with C–C Bond Linkages

Three-dimensionally connected porous organic polymers are of interest because of their potential in adsorption, separation, and sensing, among others. When crystalline, they also afford accurate structure description, which in turn can enable particular functions. However, crystallization of three-dimensional (3D) polymers is challenging. This is especially true when targeting polymerization via stable C–C bonds, whose formation is usually irreversible and does not allow for error correction typically required for crystallization. Here, we report polyMTBA, the first 3D-connected crystalline organic polymer with permanent porosity, here formed via C–C linkages. High crystallinity is achieved by solid-state topochemical reaction within monomer MTBA crystals. polyMTBA is recyclable via thermal depolymerization and is solution-processable via its soluble monomers. These results reveal topochemical polymerization as a compelling methodology for generating stable, crystalline, and porous 3D organic frameworks

Electrical conductivity through π–π stacking in a two‐dimensional porous gallium catecholate metal–organic framework

Metal-organic frameworks (MOFs) are hybrid materials known for their nanoscale pores, which give them high surface areas but generally lead to poor electrical conductivity. Recently, MOFs with high electrical conductivity were established as promising materials for a variety of applications in energy storage and catalysis. Many recent reports investigating the fundamentals of charge transport in these materials focus on the role of the organic ligands. Less consideration, however, is given to the metal ion forming the MOF, which is almost exclusively a late first-row transition metal. Here, we report a moderately conductive porous MOF based on trivalent gallium and 2,3,6,7,10,11-hexahydroxytriphenylene. Gallium, a metal that has not been featured in electrically conductive MOFs so far, has a closed-shell electronic configuration and is present in its trivalent state-in contrast to most conductive MOFs, which are formed by open-shell, divalent transition metals. Our material, made without using any harmful solvents, displays conductivities on the level of 3 mS/cm and a surface area of 196 m2 /g, comparable to transition metal analogs