Complejos metálicos biestables para la electrónica molecular: síntesis y organización sobre superficies de rotaxanos y moléculas imán

RESUMEN En Capítulo 1 se aborda la síntesis de un nuevo sistema biestable basado en un rotaxano coordinado de cobre, incorporando en el fragmento lineal unidades de terpiridina y fenantrolina directamente enlazadas por sus posiciones T5 y F3 así como los intentos de utilizarlo en la construcción de un rotaxano biestable. En el Capítulo 2 se demuestra la posibilidad de utilizar un conjugado de 3,5-fenantrolina terpiridina (L) en la construcción de diferentes sistemas de interés. En una primera parte, se presenta la síntesis una serie de nuevos complejos luminiscentes mono-, di- y trinucleares basados en Ru/Os, estudiándose sus propiedades de absorción y emisión, así como los tiempos de vida del estado excitado. En la segunda parte, la naturaleza heteroditópica de L se explota en el diseño de sistemas metalosupramoleculares: cajas y polígonos moleculares, homo- y heterometálicos, con índices de coordinación variables. En el Capítulo 3 se presenta una estrategia que permite la deposición con precisión nanométrica de derivados catiónicos de la molécula imán Mn12 sobre superficies de silicio. El método combina la utilización de la técnica de oxidación local y las interacciones electrostáticas entre la molécula y una superficie funcionalizada. ____________________________________________________________________________________________________In Chapter 1 the synthesis of a rigid linear ligand based on directly connected phenanthroline and terpyridine units by means of their T5 and F3 positions is presented accompanied by our attempts to use the ligand in the construction of a bistable rotaxane. In Chapter 2, the feasibility of using a 3,5-phenanthroline terpyridine conjugate (L) for the construction of different systems of interest is demonstrated. In the first part, the synthesis of a series of new luminescent mono-, di- and trinuclear Ru/Os complexes is presented; their absortion, emission and excited state lifetimes have also been studied. In the second part, the heteroditopic nature of L is exploited in the design of metallosupramolecular systems: molecular cages and homo- and heterometallic polygons with variable coordination indexes and finally in Chapter 3 a strategy for the deposition over silicon surfaces of cationic Mn12 derivative single molecule magnets with nanometric precision is presented. This method combines the use of local oxidation nanolithography and the electrostatic between the molecule and a functionalized monolayer

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. However, the main issue limiting their applicability is their poor stability in humid conditions. The common methods to overcome this problem involve the formation of strong metal-linker bonds by using highly charged metals, which is limited to a number of structures, the introduction of alkylic groups to the framework by post-synthetic modification (PSM) or chemical vapour deposition (CVD) to enhance overall hydrophobicity of the framework. These last two usually provoke a drastic reduction of the porosity of the material. These strategies do not permit to exploit the properties of the MOF already available and it is imperative to find new methods to enhance the stability of MOFs in water while keeping their properties intact. Herein, we report a novel method to enhance the water stability of MOF crystals featuring Cu2(O2C)4 paddle-wheel units, such as HKUST (where HKUST stands for Hong Kong University of Science & Technology), with the catechols functionalized with alkyl and fluoro-alkyl chains. By taking advantage of the unsaturated metal sites and the catalytic catecholase-like activity of CuII ions, we are able to create robust hydrophobic coatings through the oxidation and subsequent polymerization of the catechol units on the surface of the crystals under anaerobic and water-free conditions without disrupting the underlying structure of the framework. This approach not only affords the material with improved water stability but also provides control over the function of the protective coating, which enables the development of functional coatings for the adsorption and separations of volatile organic compounds. We are confident that this approach could also be extended to other unstable MOFs featuring open metal sites

Origin of the Chemiresistive Response of Ultrathin Films of Conductive Metal-Organic Frameworks

Conductive metal-organic frameworks are opening new perspectives for the use of these porous materials for applications traditionally limited to more classical inorganic materials, such as their integration into electronic devices. This has enabled the development of chemiresistive sensors capable of transducing the presence of specific guests into an electrical response with good selectivity and sensitivity. By combining experimental data with computational modelling, a possible origin for the underlying mechanism of this phenomenon in ultrathin films (ca. 30 nm) of Cu‐CAT‐1 is described

Direct visualization of pyrrole reactivity by confined oxidation in a Cyclodextrin Metal‐Organic Framework

Metal-organic frameworks can be used as porous templates to exert control over polymerization reactions. Shown here are the possibilities offered by these crystalline, porous nanoreactors to capture highly‐reactive intermediates for a better understanding of the mechanism of polymerization reactions. By using a cyclodextrin framework the polymerization of pyrrole is restricted, capturing the formation of terpyrrole cationic intermediates. Single‐crystal X‐ray diffraction is used to provide definite information on the supramolecular interactions that induce the formation and stabilization of a conductive array of cationic complexes

Ultrathin films of 2D Hofmann-type coordination polymers: influence of pillaring linkers on structural flexibility and vertical charge transport

Searching for novel materials and controlling their nanostructuration into electronic devices is a challenging task ahead of chemists and chemical engineers. Even more so when this new application requires an exquisite control over the morphology, crystallinity, roughness and orientation of the films produced. In this context, it is of critical importance to analyze the influence of the chemical composition of perspective materials on their properties at the nanoscale. We report the fabrication of ultrathin films (thickness < 30 nm) of a family of FeII Hofmann-like coordination polymers by using an optimized liquid phase epitaxy (LPE) set-up. The series [Fe(L)2{Pt(CN)4}] (L = pyridine, pyrimidine and isoquinoline) conform an ideal platform for correlating the effect of the axial nitrogenated ligand with changes to their structural response to guests or electrical resistance. All film properties relevant to device integration have been thoroughly analyzed with complementary surface techniques for a meaningful comparison. Our results reveal that changes to this ligand can hinder the structural transformation triggered by the absorption of guest molecules previously reported for the pyridine phase. Also important, it can substantially hinder vertical charge transport across the layers, even at the ultrathin film limit

Epitaxial thin-film vs single crystal growth of 2D Hofmann-type iron(II) materials: a comparative assessment of their bi-stable spin crossover properties

Integration of the ON−OFF cooperative spin crossover (SCO) properties of FeII coordination polymers as components of electronic and/or spintronic devices is currently an area of great interest for potential applications. This requires the selection and growth of thin films of the appropriate material onto selected substrates. In this context, two new series of cooperative SCO two-dimensional FeII coordination polymers of the Hofmann-type formulated {FeII(Pym)2[MII(CN)4]·xH2O}n and {FeII(Isoq)2[MII(CN)4]}n (Pym = pyrimidine, Isoq = isoquinoline; MII = Ni, Pd, Pt) have been synthesized, characterized, and the corresponding Pt derivatives selected for fabrication of thin films by liquid-phase epitaxy (LPE). At ambient pressure, variable-temperature single-crystal X-ray diffraction, magnetic, and calorimetric studies of the Pt and Pd microcrystalline materials of both series display strong cooperative thermal induced SCO properties. In contrast, this property is only observed for higher pressures in the Ni derivatives. The SCO behavior of the {FeII(L)2[PtII(CN)4]}n thin films (L = Pym, Isoq) were monitored by magnetization measurements in a SQUID magnetometer and compared with the homologous samples of the previously reported isostructural {FeII(Py)2[PtII(CN)4]}n (Py = pyridine). Application of the theory of regular solutions to the SCO of the three derivatives allowed us to evaluate the effect on the characteristic SCO temperatures and the hysteresis, as well as the associated thermodynamic parameters when moving from microcrystalline bulk solids to nanometric thin films

WS2/MoS2 Heterostructures through Thermal Treatment of MoS2 Layers Electrostatically Functionalized with W3S4 Molecular Clusters

The preparation of 2D stacked layers that combine flakes of different nature, gives rise to countless number of heterostructures where new band alignments, defined at the interfaces, control the electronic properties of the system. Among the large family of 2D/2D heterostructures, the one formed by the combination of the most common semiconducting transition metal dichalcogenides WS2/MoS2, has awaken great interest due to its photovoltaic and photoelectrochemical properties. Solution as well as dry physical methods have been developed to optimize the synthesis of these heterostructures. Here a suspension of negatively charged MoS2 flakes is mixed with a methanolic solution of a cationic W3S4-core cluster, giving rise to a homogeneous distribution of the clusters over the layers. In a second step, a calcination under N2 of this molecular/2D heterostructure leads to the formation of clean WS2/MoS2 heterostructures where the photoluminescence of both counterparts is quenched, proving an efficient interlayer coupling. Thus, this chemical method combines the advantages of a solution approach (simple, scalable and low-cost) with the good quality interfaces reached by using more complicated traditional physical methods

Bottom‐Up Fabrication of Semiconductive Metal-Organic Framework Ultrathin Films

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)

Spin-crossover nanoparticles anchored on MoS2 layers for heterostructures with tunable strain driven by thermal or light-induced spin switching

In the last few years, the effect of strain on the optical and electronic properties of MoS2 layers has been deeply studied. Complex devices have been designed where strain is externally applied on the 2D material. However, so far, the preparation of a reversible self-strainable system based on MoS2 layers has remained elusive. In this work, spin-crossover nanoparticles are covalently grafted onto functionalized layers of semiconducting MoS2 to form a hybrid heterostructure. We use the ability of spin-crossover molecules to switch between two spin states upon the application of external stimuli to generate strain over the MoS2 layer. This spin crossover is accompanied by a volume change and induces strain and a substantial and reversible change of the electrical and optical properties of the heterostructure. This strategy opens the way towards a next generation of hybrid multifunctional materials and devices of direct application in highly topical fields like electronics, spintronics or molecular sensing

Path to overcome material and fundamental obstacles in spin valves based on Mo S2 and other transition-metal dichalcogenides

Experimental studies on spin valves with exfoliated 2D materials face the main technological issue of ferromagnetic electrode oxidation during the 2Ds integration process. As a twofold outcome, magnetoresistance (MR) signals are very difficult to obtain and, when they finally are, they are often far from expectations. We propose a fabrication method to circumvent this key issue for 2D-based spintronics devices. We report on the fabrication of NiFe/MoS2/Co spin valves with mechanically exfoliated multilayer MoS2 using an in situ fabrication protocol that allows high-quality nonoxidized interfaces to be maintained between the ferromagnetic electrodes and the 2D layer. Devices display a large MR of 5%. Beyond interfaces and material quality, we suggest that an overlooked more fundamental physics issue related to spin-current depolarization could explain the limited MR observed so far in MoS2-based magnetic tunnel junctions. This points to a path towards the observation of larger spin signals in line with theoretical predictions above 100%. We envision the impact of our work to be beyond MoS2 and its broader transition-metal dichalcogenides family by opening the way to an accelerated screening of other 2D materials that are yet to be explored for spintronics