The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Control of magnetic properties through external stimuli.

Digital information processing in molecular systems.

The literature has shown numerous contributions on the synthesis and physicochemical properties of persistent organic radicals but there are a lesser number of reports about their use as building blocks for obtaining molecular magnetic materials exhibiting an additional and useful physical property or function. These materials show promise for applications in spintronics as well as bistable memory devices and sensing materials. This critical review provides an up-to-date survey to this new generation of multifunctional magnetic materials. For this, a detailed revision of the most common families of persistent organic radicals—nitroxide, triphenylmethyl, verdazyl, phenalenyl, and dithiadiazolyl—so far reported will be presented, classified into three different sections: materials with magnetic, conducting and optical properties. An additional section reporting switchable materials based on these radicals is presented (257 references).

Playing with organic radicals as building blocks for functional molecular 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.

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Catechol-Based Biomimetic Functional Materials

Emerging trends in spin crossover (SCO) based functional materials and devices

Valence tautomeric complexes combine redox-active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candidates for future use in molecular electronic devices and switches Most of the valence tautomeric complexes reported thus far are based on quinone or quinone-type ligands with a series of transition metal ions such as Co, Cu, Ni and Mn Nevertheless, in the last few years, the number of electroactive ligands identified to be active in valence tautomeric complexes is being expanded by including new radical ligands such as polychlorotriphenylmethyl, phenoxyl or tetraphenylporphyrin radicals (© Wiley-VCH Verlag GmbH & Co KGaA, 69451 Weinheim, Germany, 2005)

Valence Tautomerism: New Challenges for Electroactive Ligands

We acknowledge the financial support of the Ministerio de Educacion y Ciencia (MEC) through projects MAT2006–13765-C02–01, CTQ2004–02539, and CTQ2006–01040. F.B. and J.H. thank MCyT-ERDF for their “Ramon y Cajal” contracts, E.E. thanks the MEC for a predoctoralgrant, and G.G.B. thanks DURSI and FSE for a predoctoral grant.

Catechol derivatives as fluorescent chemosensors for wide-range pH detection.

The idea of developing magnetic molecular materials into real functional electronic devices with low-cost and scalable techniques appeared with the emergence of the field several years ago. Today, even though great advances have been done with this aim, the promise of a functional device working at the micro-/nanoscale and at room temperature has unfortunately not completely materialized yet, as their use still strongly depends on the fabrication methodology of a robust device that can be handled and integrated without compromising their functionality. Here we propose the use of polymeric matrices as a platform for the development of such robust switchable structures exhibiting reproducible results independently of the dimension -from macro to micro-/nanoscale- and morphology -from thin-films to nanoparticles and nanoimprinted motives- while allowing to induce an irreversible hysteresis, reminiscent of a non-volatile memory, by synchronization with the polymer phase transition.

/pdf/robust-spin-crossover-platforms-with-synchronized-spin-21b3orxhuz.pdf

Robust spin crossover platforms with synchronized spin switch and polymer phase transition

In this letter, we report on the development of a surface molecular sensor for the detection of acidity. Lithographically controlled wetting deposition has been applied to form the nanostructure of...

Surface-structured molecular sensor for the optical detection of acidity.

Valence tautomeric complexes combine redox-active ligands and transition metal ions with two or more accessible oxidation states, exhibiting two nearly degenerated electronic states with localized electronic structures. Charge distribution in such electronic isomers has an appreciable sensitivity to the environment so an external perturbation, like photons, temperature and/or pressure, may lead to an intramolecular electron transfer between both redox active units and therefore to a reversible interconversion between the two degenerated electronic states. Moreover, since each electronic isomer exhibits different optical, electronic and/or magnetic properties, these complexes are being proposed as candidates for future use in molecular electronic devices and switches. Most of the valence tautomeric complexes reported thus far are based on quinone or quinone-type ligands with a series of transition metal ions such as Co, Cu, Ni and Mn. Nevertheless, in the last few years, the number of electroactive ligands identified to be active in valence tautomeric complexes is being expanded by including new radical ligands such as polychlorotriphenylmethyl, phenoxyl or tetraphenylporphyrin radicals. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Emilia Evangelio

Papers

Valence Tautomerism: New Challenges for Electroactive Ligands

Catechol derivatives as fluorescent chemosensors for wide-range pH detection.

Robust spin crossover platforms with synchronized spin switch and polymer phase transition

Surface-structured molecular sensor for the optical detection of acidity.

Valence Tautomerism: New Challenges for Electroactive Ligands