PhD Candidate, University of Chicago
My research investigates redox activity (i.e. the ability to remove or add electrons) of porous materials. These inorganic-organic hybrid materials have intrinsically large surface area due to the design of their framework structures that feature metal atoms or clusters linked by rigid organic molecules. Redox chemistry is route by which interesting magnetic and electronic properties may be instilled in these materials. For example, magnetic coupling and electrical conductivity have been reported in redox active metal organic frameworks. Porous materials with these properties may be useful for a variety of applications, including hybrid sensors, batteries and thermoelectric devices, among others. The work I will be presenting on focuses on sulfur-based organic molecules which link paramagnetic metal (i.e. metal atoms with unpaired electrons) sites to form a framework material. Redox activity may be possibly at both the metal and organic sites of the framework, allowing for tunability of the material's properties.
Abstract: We report the magnetism and conductivity for a redox isomeric pair of iron-quinoid metal-organic frameworks (MOFs). The oxidized isomer, (Me2NH2)2[Fe2L3]·2H2O·6DMF (LH2 = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) was previously shown to magnetically order below 80 K in its solvated form, with the ordering temperature decreasing to 26 K upon desolvation. Here, we demonstrate this compound to exhibit electrical conductivity values up to σ = 1.4(7) × 10(-2) S/cm (Ea = 0.26(1) cm(-1)) and 1.0(3) × 10(-3) S/cm (Ea = 0.19(1) cm(-1)) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of Cp2Co, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (Cp2Co)1.43(Me2NH2)1.57[Fe2L3]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [Fe(III)2(L(3-•))3](3-). Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal-organic radical coupling to give a magnetically ordered phase below Tc = 105 K, one of the highest reported ordering temperatures for a MOF. This high ordering temperature is significantly increased relative to the oxidized compound, and stems from the overall increase in coupling strength afforded by an additional organic radical. In line with the high critical temperature, the new MOF exhibits magnetic hysteresis up to 100 K, as revealed by variable-field measurements. Finally, this compound is electrically conductive, with values up to σ = 5.1(3) × 10(-4) S/cm with Ea = 0.34(1) eV. Taken together, these results demonstrate the unique ability of metal-quinoid MOFs to simultaneously exhibit both high magnetic ordering and high electrical conductivity.
Pub.: 24 Feb '17, Pinned: 28 Jun '17
Abstract: Reversible magnetic control by electrical means, which is highly desired from the viewpoint of fundamentals and technological applications such as data storage devices, has been a challenging topic. In this study, the authors demonstrate in situ magnetic phase switching between the ferrimagnetic and paramagnetic states of an electron-donor/-acceptor metal-organic framework (D/A-MOF) using band-filling control mediated by the Li+-ion migration that accompanies redox reactions, i.e., “magneto-ionic control”. By taking advantage of the rechargeability of lithium-ion battery systems, in which Li+-ions and electrons are simultaneously inserted into/extracted from a cathode material, the reversible control of nonvolatile magnetic phases in a D/A-MOF has been achieved. This result demonstrates that the combination of a redox-active MOF with porous flexibility and ion-migration capability enables the creation of new pathways toward magneto-electric coupling devices in the field of ionics.
Pub.: 29 Dec '16, Pinned: 28 Jun '17
Abstract: A series of isoreticular metal-organic frameworks (MOFs) of the formula M(BDC)(L) (M = Fe(II) or Co(II), BDC = 1,4-benzenedicarboxylate, L = pyrazine (pyz) or 4,4'-bipyridine (bipy)) has been synthesized and characterized by N2 gas uptake measurements, single crystal and powder X-ray diffraction, magnetometry, X-ray absorption spectroscopy, and Mössbauer spectroscopy. These studies indicate the formation of a permanently porous solid with high-spin Fe(II) and Co(II) centers that are weakly coupled, consistent with first-principles density functional theory calculations. This family of materials represents unusual examples of paramagnetic metal centers coordinated by linkers capable of mediating magnetic or electronic coupling in a porous framework. While only weak interactions are observed, the rigid 3D framework of the MOF dramatically impacts the properties of these materials when compared with close structural analogues.
Pub.: 04 Mar '17, Pinned: 28 Jun '17