Daniel Ruiz-Molina

Catechol-based biomimetic functional materials

Catechols are found in nature taking part in a remarkably broad scope of biochemical processes and functions. Though not exclusively, such versatility may be traced back to several properties uniquely found together in the o-dihydroxyaryl chemical function; namely, its ability to establish reversible equilibria at moderate redox potentials and pHs and to irreversibly cross-link through complex oxidation mechanisms; its excellent chelating properties, greatly exemplified by, but by no means exclusive, to the binding of Fe(3+); and the diverse modes of interaction of the vicinal hydroxyl groups with all kinds of surfaces of remarkably different chemical and physical nature. Thanks to this diversity, catechols can be found either as simple molecular systems, forming part of supramolacular structures, coordinated to different metal ions or as macromolecules mostly arising from polymerization mechanisms through covalent bonds. Such versatility has allowed catechols to participate in several natural processes and functions that range from the adhesive properties of marine organisms to the storage of some transition metal ions. As a result of such an astonishing range of functionalities, catechol-based systems have in recent years been subject to intense research, aimed at mimicking these natural systems in order to develop new functional materials and coatings. A comprehensive review of these studies is discussed in this paper.

Magnetic nanoporous coordination polymers

The use of organic molecules as an alternative strategy for achieving new nanoporous metal–organic materials has become an attractive prospect. So far, the use of chemical coordination or crystal engineering techniques allows the systematic design of open-framework structures with a considerable range of modulatable pore sizes and functionalities using different organic ligands such as phosphane, cyano groups, N,N′-type ligands and polycarboxylic acids. Furthermore, the use of transition metal ions opens the possibility to obtain nanoporous materials with additional electrical, optical or magnetic properties. Among them, the search for magnetic open-framework structures has become a major challenge due to their potential applications in the development of low-density magnetic materials, magnetic sensors and intelligent or multifunctional materials.

Metal–Organic Spheres as Functional Systems for Guest Encapsulation†

Music of the spheres: Infinite coordination polymerization of Zn(2+) ions and a multitopic ligand produces metal-organic micro- and nanospheres that can be used as functional matrices. The spheres can encapsulate combinations of active substances, such as organic dyes, magnetic nanoparticles, or luminescent quantum dots (see image), which results in spheres that are luminescent in the blue, green, and red regions of the spectrum.

Valence-Tautomeric Metal–Organic Nanoparticles†

This work was supported by projects MAT2006-13765. D.M. thanks the Ministerio de Ciencia y Tecnologia for a RyC contract. The authors thank the Servei de Microscopia of the UAB, Serveis Cientifico-Tecnics of the UB, and the Laboratori de Nanotecnologia de MATGAS. I.I. thanks the ICN for financial support.

Versatile Nanostructured Materials via Direct Reaction of Functionalized Catechols

A facile one-step polymerization strategy is explored to achieve novel catechol-based materials. Depending on the functionality of the catechol, the as-prepared product can be used to modify at will the surface tension of nano and bulk structures, from oleo-/hydrophobic to highly hydrophilic. A hydrophobic catechol prepared thus polymerized shows the ability to self-assemble as solid nanoparticles with sticky properties in polar solvent media. Such a versatile concept is ideal for the development of catechol-based multifunctional materials.

Radical para-Benzoic Acid Derivatives: Transmission of Ferromagnetic Interactions through Hydrogen Bonds at Long Distances

Investigation of the transmission of magnetic interactions through hydrogen bonds has been carried out for two different benzoic acid derivatives which bear either a tert-butyl nitroxide (NOA) or a poly(chloro)triphenylmethyl (PTMA) radical moiety. In the solid state, both radical acids formed dimer aggregates by the complementary association of two carboxylic groups though hydrogen bonding. This association ensured that atoms with most spin density are separated from one another by more than 15 A. Thus, no competing through-space magnetic exchange interactions are expected in these dimers and, hence, they provide good models to investigate whether noncovalent hydrogen bonds play a role in the long-range transmission of magnetic interactions. The nature of the magnetic exchange interaction and their strengths within similar dimer aggregates in solution was assessed by electron spin resonance (ESR) spectroscopy. In the case of radical NOA, low-temperature ESR experiments showed a weak ferromagnetic interaction between the two radicals in the dimer aggregates (which have the same geometry as in the solid state). In contrast, the corresponding solution ESR study performed with radical PTMA did not lead to any conclusive results, as aggregates were formed by noncovalent interactions other than hydrogen bonds. However, the bulkiness of the poly(chloro)triphenylmethyl radical prevented interdimer contacts in the solid state between regions of high spin density. Hence, solid-state measurements of the alpha phase of PTMA radical provided evidence of the intradimer interaction to confirm the transmission of a weak ferromagnetic interaction through the carboxylic acid bridges, as found for the NOA radical. Moreover, crystallization of the PTMA radical in presence of ethanol to form the beta phase of PTMA radical prevented the dimer formation; this resulted in the suppression of this interaction and provides further evidence of the magnetic exchange mechanism through noncovalent hydrogen bonds at long distances.

Chiral, single-molecule nanomagnets: synthesis, magnetic characterization and natural and magnetic circular dichroism

The first three chiral dodecamanganese clusters that behave as single-molecule magnets are reported. All reveal natural optical activity, which is stronger for the 2-chloropropionate derivative than for either the (S)-6-methoxy-α-methyl-2-naphthaleneacetate or the (S)-2-phenylbutyric acetate compounds. For the cluster with 2-chloropropionate moieties at its periphery, the magnetic circular dichroism was investigated and found to display large optical hysteresis, which depends on the direction in which the magnetic field direction is swept.

Controlling the Number of Proteins with Dip‐Pen Nanolithography

This work was supported by projects MAT2009-13977-C03, Nanomateria-DGA, and PI091/08. D.M. thanks the Ministerio de Ciencia y Tecnologia for a RyC contract. A. L. thanks ARAID for financial support. R. de M. is indebted to MICINN for receiving a predoctoral FPU fellowship. Funding from the European Network of Excellence MAGMANet is also acknowledged.

A New Photomagnetic Molecular System Based on Photoinduced Self‐Assembly of Radicals

An irreversible trans→cis isomerization of the imino group occurs during the irradiation of the new ferrocenyl Schiff-base polychlorotriphenylmethyl radical 1 by light. Low-temperature ESR investigations of frozen solutions revealed that the cis isomer exists as the (cis-1)2 dimer with strong antiferromagnetic interactions. The radical 1 constitutes an example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground-state species is promoted by a photoinduced self-assembly process driven by the formation of hydrogen bonds.

Coexistence of Two Thermally Induced Intramolecular Electron Transfer Processes in a Series of Metal Complexes [M(Cat‐N‐BQ)(Cat‐N‐SQ)]/[M(Cat‐N‐BQ)2] (M=Co, Fe, and Ni) bearing Non‐Innocent Catechol‐Based Ligands: A Combined Experimental and Theoretical Study

The different thermally induced intermolecular electron transfer (IET) processes that can take place in the series of complexes [M(Cat-N-BQ)(Cat-N-SQ)]/[M(Cat-N-BQ)(2)], for which M = Co (2), Fe (3) and Ni(4), and Cat-N-BQ and Cat-N-SQ denote the mononegative (Cat-N-BQ(-)) or dinegative (Cat-N-SQ(2-)) radical forms of the tridentate Schiff-base ligand 3,5-di-tert-butyl-1,2-quinone-1-(2-hydroxy-3,5-di-tert-butylphenyl)imine, have been studied by variable-temperature UV/Vis and NMR spectroscopies. Depending on the metal ion, rather different behaviors are observed. Complex 2 has been found to be one of the few examples so far reported to exhibit the coexistence of two thermally induced electron transfer processes, ligand-to-metal (IET(LM)) and ligand-to-ligand (IET(LL)). IET(LL) was only found to take place in complex 3, and no IET was observed for complex 4. Such experimental studies have been combined with ab initio wavefunction-based CASSCF/CASPT2 calculations. Such a strategy allows one to solicit selectively the speculated orbitals and to access the ground states and excited-spin states, as well as charge-transfer states giving additional information on the different IET processes.