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Theocharis Stamatatos

Research Group

Research Interests

“Hybrid” Molecular Magnetic Materials

Molecular electronics is undoubtedly an exciting area of research which promises to deliver new technology and modern devices to society. It is based on the construction and fabrication of molecular species with intriguing magnetic properties, pronounced stability and robustness, and ability for deposition on electrical conducting surfaces. In order to gain access into some real applications for these species, such as molecular spintronics, transistors and spin valves, we need to combine their magnetic properties with one or more additional properties, such as conductivity, chirality and luminescence.

Owing to important perspectives in fundamental science and applications in nanotechnology and molecule-based electronics, these "hybrid" (or multifunctional) molecular magnetic materials are the subject of considerable efforts which involve coexistence, interplay or synergy between the multiple physical properties. One of our research projects deals with the synthesis of dual-acting species such as emissive molecular magnetic materials, which are coordination compounds exhibiting both photoluminescence and single-molecule magnetism behaviors.

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High-Spin Molecules and Single-Molecule Magnets

Magnetic materials find widespread utility in numerous areas of modern society, from sensors, refrigerants, switches, speakers, and computer hard-drives, to medical applications such as magnetic resonance imaging. Miniaturization of magnetism-based devices and machines is a major technological imperative, and thus a bottom-up molecular source of nanosized magnets would be invaluable. Paramagnetic metal ions, such as transition metals and lanthanides, can occasionally interact ferromagnetically with each other when they are bridged by organic or inorganic ligands that are known to propagate parallel spin alignments.

Hence, the resulting coordination compounds are high-spin molecules with abnormally large spin ground state (S) values that -upon combination with a large magnetoanisotropy of the Ising-type ("easy axis" type)- can act as single-molecule magnets (SMMs). SMMs exhibit hysteresis loops, the diagnostic property of any magnet, and they also show interesting quantum properties such as quantum tunneling of magnetization. SMMs are proposed as candidates for the new generation of quantum computing and spintronics. Our group has been systematically involved in the area of high-spin molecules and SMMs through the employment of 3d-, 4f-, and mixed 3d/4f-metal combinations in conjunction with flexible and versatile ligands such as azides, peroxides, cyanates, alkoxides, oximates and carboxylates.

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Polynuclear Nanoscale Coordination Compounds

Polynuclear metal complexes with moderate oxidation states, also known as coordination clusters, are high-nuclearity molecular species which do not involve metal-metal bonding, but instead, are assembled by multidentate N- and/or O-donor bridging/chelating ligands. The nuclearity of a metal cluster is associated with its size, with some of these clusters reaching the nanoscale regime. The smallest classical nanoparticles fabricated to date via the top-down approaches are of the same order of magnitude as the largest molecule-based metal clusters synthesized by bottom-up methods.

However, the synthesis and crystallization of these species has been always a challenging task for coordination chemists. One successful synthetic route to metal clusters with large dimensionalities has been the employment of polydentate chelating/bridging organic ligands that are able to coordinate to several metal centers and adopt a variety of different binding modes. Our group has had a longstanding interest in the synthesis of new 3d-, 4f-, and 3d/4f-metal cluster compounds with aesthetically pleasing structures, record nuclearities and impressive physical properties (i.e., magnetic and optical). The aggregation of such molecular species has been achieved through the employment of 'smart' ligands which belong to the families of Schiff bases, pyridyl- and pyrazine-alcohols, oximes and dioximes. We have recently been able to isolate and characterize a novel {Ni26} cluster with a beautiful 'rabbit-face' topology and a record nuclearity for Ni(II) clusters, as well as a {Mn25} barrel-like cluster which is extended into covalently-linked supramolecular chains.

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Molecular Magnetic Refrigerants

Molecular magnetic materials have been proposed as a promising candidate for magnetic refrigeration, an alternative to the increasingly rare and expensive helium-3 traditionally used for low-temperature refrigeration. Magnetic refrigeration uses the magnetocaloric effect (MCE) to affect changes in temperature by applying a magnetic field to magnetic materials. The larger the MCE of a magnetic material, the greater the potential temperature change and the more ideal it would be for magnetic refrigeration applications.

The molecular magnetic materials developed to date do not have sufficient thermal conductivity or high enough MCE values to be practical for use in magnetic refrigeration. With this research, we propose to develop novel ferromagnetic and isotropic molecular materials with record MCE values at extremely low temperatures. The synthesized materials will have potential applications for the development of magnetic refrigeration systems which are more energy-efficient and environmentally friendly as they do not require the use of hazardous chemicals or greenhouse gases.

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Biological Inorganic Chemistry

Our research direction into the field of bioinorganic chemistry includes two different projects; firstly, the synthesis and detailed study of synthetic analogues (molecular models) of the oxygen-evolving complex (OEC) that would greatly enhance our understanding of the spectroscopic, physical and catalytic properties of the water oxidation process, which is the source of essentially all the O2 on this planet. Secondly, the synthesis and biological study of platinum(II) and palladium(II) complexes bearing new chelate ligands with pronounced medicinal and pharmaceutical activities on the cardiovascular system, sedative, antidepressant and antispasmodic activity, as well as analgesic and anti-inflammatory activity.

Photosynthesis is the sunlight-driven process by which green plants, algae, and cyanobacteria convert CO2 and H2O into carbohydrates and molecular O2. Such a light-powered process utilizes a {Mn4CaO5} cluster which exists in the active site of Photosystem II (PSII), a multicomponent assembly of proteins and cofactors that absorbs four photons to sequentially oxidize the Mn ions through a four-electron process. Towards the search for new Mn-Ca molecular materials that could potentially mimic the structure and function of the oxygen-evolving complex in PSII, our group is currently trying to synthesize compounds possessing: (i) the preferred Mn4Ca metal stoichiometry, (ii) the extended, distorted cubane conformation, (iii) high oxidation states for Mn atoms, (iv) appropriate ancillary bridging ligands with biological and optical implications. In addition, the majority of synthetic metal complexes used as antitumor chemotherapeutic compounds are structural analogues of the neutral cisplatin, cis-[PtIICl2(NH3)2]. Recently, an increasing interest is shown in "rule breakers", in hope of finding new candidates that could interact differently with biological targets as compared to that of cisplatin. In addition to the prolonged biological interest in Pt(II) complexes, the Pd(II) analogues were recently included among the newly introduced structural types of metal complexes, designed for increased antitumor efficiency and decreased toxicity to normal cells. Our group is seeking for organic chelates which will form new Pt(II) and Pd(II) complexes with enhanced biological and medicinal properties.

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