Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Metal-organic frameworks (MOFs) are a class of porous inorganic materials with promising properties in gas storage and separation, catalysis and sensing. ...

Universitat de València

The dinuclear Cu2+ and Zn2+ as well as the mixed Cu2+-Zn2+ complexes of a 5,5''-pentaazaterpyridinophane ligand (L) are able to incorporate imidazolate (Im-) as a bridging ligand. The crystal structure of [Cu(2)L(Im)(Br)(H2O)](CF(3)SO(3))(2).3H2O (1) shows one copper coordinated by the three pyridine nitrogens of the terpyridine unit, one nitrogen of the imidazolate bridge (Im-) and one bromide anion occupying the axial position of a distorted square pyramid. The second copper atom is coordinated by the remaining imidazolate nitrogen, the three secondary nitrogens at the centre of the polyamine bridge and one water molecule that occupies the axial position. Magnetic measurements have been performed in the 2.0-300.0 K temperature range. Experimental data could be satisfactorily reproduced by using an isotropic exchange model H = -JS(1)S(2) with J = -52.3 cm(-1) and g = 2.09. Potentiometric studies have provided details of the speciation and stability constants for the mixed Cu2+-L-HIm, Zn2+-L-HIm (HIm = imidazole) and Cu2+-Zn2+-L-HIm systems. The apparent stability constant obtained at pH = 9 for the addition of imidazole to the dinuclear Cu2+ complexes is one of the highest so far reported (log K = 7.5). UV-Vis spectroscopy and paramagnetic NMR data show that imidazole coordinates to the Cu2+ ions as a bridging imidazolate ligand from pH 5 to 10. Electrochemical reduction of the Cu2+-Zn2+-L complex occurs in two successive one-electron per copper ion quasi-reversible steps. The formal potential of the Cu2+-Zn2+-L/Cu+-Zn2+-L couple is close to that of SOD. The IC50 values measured at pH 7.8 by means of the nitro blue tetrazolium method show significant SOD activity for the dinuclear Cu2+ complexes (IC50 = 2.5 microM) and moderate activity for the Cu2+-Zn2+ mixed systems (IC50 = 30 microM).

Imidazolate bridged Cu(II)-Cu(II) and Cu(II)-Zn(II) complexes of a terpyridinophane azamacrocycle: a solution and solid state study.

Fast shuttling motions in solution have been observed by cyclic voltammetry in a Cu(I/II)-based [2] rotaxane. In the reported system, the different coordination preferences of both copper oxidation states are exploited to promote the electrochemically-triggered gliding of the ring from a tetra to a pentacoordinated site and vice versa. The thread of this rotaxane consists of a tridentate 2,2':6',2''-terpyridine chelating unit directly bonded through its 5-position to the 3-position of the bidentate 1,10-phenanthroline unit. This distribution reduces to a minimum the distance between the two coordination sites and lessens the congestion around the tetrahedral environment. These two factors have been demonstrated to highly increase the kinetics of the switching process. In addition, the electrochemical experiments carried out in different solvent mixtures evidenced the influence of the solvent on the shuttling mechanism. 

Fast redox-triggered shuttling motions in a copper rotaxane based on a phenanthroline-terpyridine conjugate.

Self-assembled monolayers (SAMs) are nowadays broadly used as surface protectors or modifiers and play a key role in many technological applications. This has motivated the study of their formation in all kind of materials; however, and despite the current interest in molecular spintronics, the study of SAMs on ferromagnetic surfaces remains almost unexplored. In this paper, we report for the first time a methodology for the formation of SAMs of n-alkylphosphonic acids on permalloy in ambient conditions. The formed monolayers have been fully characterized by means of contact angle measurements, atomic force microscopy, X-ray photoelectron spectroscopy, matrix assisted laser desorption ionization time-of-flight mass spectrometry, infrared reflection absorption spectroscopy, and X-ray reflectometry. Additionally, the magnetic stability of the modified permalloy after the solution process required for the SAM formation has been confirmed by magneto-optical Kerr effect magnetometry. Moreover, by means of microcontact printing lithography, very accurate SAM patterns have been transferred onto permalloy surfaces and used as resist mask in a chemical etching process giving rise to submicrometric permalloy surface patterns with potential interest in nanomagnetism, spintronics, and storage technologies. 

Self-assembled monolayers on a ferromagnetic permalloy surface.

Currently available methodologies arguably lack the exquisite control required for producing metal-organic framework (MOF) thin films of sufficient quality for electronic applications. By directing MOF transfer with self-assembled monolayers (SAMs), we achieve very smooth, homogeneous, highly oriented, ultrathin films across millimeter-scale areas that display moderate conductivity likely due to electron hopping. Here, the SAM is key for directing the transfer thereby enlarging the number and nature of the substrates of choice. We have exploited this versatility to evolve from deposition onto standard Si and Au to nonconventional substrates such as ferromagnetic Permalloy. We believe that this strategy might be useful for the integration of MOFs as active interfaces in electronic devices. 

High-Quality Metal-Organic Framework Ultrathin Films for Electronically Active Interfaces.

Chemically functionalized surfaces are studied for a wide range of applications going from medicine to electronics. Whereas non-magnetic surfaces have been widely studied, functionalization of magnetic surfaces is much less common and has almost never been used for spintronics applications. In this article we present the functionalization of La2/3Sr1/3MnO3, a ferromagnetic oxide, with self-assembled monolayers for spintronics. La2/3Sr1/3MnO3 is the prototypical half-metallic manganite used in spintronics studies. First, we show that La2/3Sr1/3MnO3 can be functionalized by alkylphosphonic acid molecules. We then emphasize the use of these functionalized surfaces in spintronics devices such as magnetic tunnel junctions fabricated using a nano-indentation based lithography technique. The observed exponential increase of tunnel resistance as a function of alkyl chain length is a direct proof of the successful connection of molecules to ferromagnetic electrodes. For all alkyl chains studied we obtain stable and robust tunnel magnetoresistance, with effects ranging from a few tens to 10 000%. These results show that functionalized electrodes can be integrated in spintronics devices and open the door to a molecular engineering of spintronics. 

Self-assembled monolayers based spintronics: from ferromagnetic surface functionalization to spin-dependent transport.

Robust catechol coatings for enhanced moisture tolerance were produced in one step by direct reaction of Hong Kong University of Science and Technology (HKUST) with synthetic catechols. We ascribe the rapid formation of homogeneous coatings around the metal-organic framework particles to the biomimetic catalytic activity of Cu(II) dimers in the external surface of the crystals. Use of fluorinated catechols results in hydrophobic, permeable coatings that protect HKUST from water degradation while retaining close to 100% of its original sorption capacity.

Surface Functionalization of Metal-Organic Framework Crystals with Catechol Coatings for Enhanced Moisture Tolerance.

Though generally considered insulating, recent progress on the discovery of conductive porous metal-organic frameworks (MOFs) offers new opportunities for their integration as electroactive components in electronic devices. Compared to classical semiconductors, these metal-organic hybrids combine the crystallinity of inorganic materials with easier chemical functionalization and processability. Still, future development depends on the ability to produce high-quality films with fine control over their orientation, crystallinity, homogeneity, and thickness. Here self-assembled monolayer substrate modification and bottom-up techniques are used to produce preferentially oriented, ultrathin, conductive films of Cu-CAT-1. The approach permits to fabricate and study the electrical response of MOF-based devices incorporating the thinnest MOF film reported thus far (10 nm thick).

Sergio Tatay

Instituto de Ciencia Molecular (ICMol)

Universitat de València

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Sergio Tatay

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Instituto de Ciencia Molecular (ICMol)

Universitat de València

Surface Functionalization of Metal-Organic Frameworks for Improved Moisture Resistance

Instituto de Ciencia Molecular (ICMol)