Designed Polynuclear Lanthanide Complexes for Quantum Information Processing

The design of dissymmetric organic ligands featuring combinations of 1,3-diketone and 2,6-diacetylpyridine coordination pockets has been exploited to produce dinuclear and trinuclear lanthanide-based coordination compounds. These molecules exhibit two or more non-equivalent Ln ions, most remarkably enabling the access to well-defined heterolanthanide compositions. The site-selective disposition of each metal ion within the molecular entities allows the study of each centre individually as a spin-based quantum bit, affording unparalleled versatility for quantum gate design. The inherent weak interaction between the Ln ions permits the performance of multi-qubit quantum logical operations realized through their derived magnetic states, or implementing quantum-error correction protocols. The different studies performed to date on these systems are revised, showing their vast potential within spin-based quantum information processing

Accessing Lanthanide-to-Lanthanide Energy Transfer in a Family of Site-Resolved [LnIIILnIII'] Heterometallic Complexes

The ligand H3 L (6-[3-oxo-3-(2-hydroxyphenyl)propionyl]pyridine-2-carboxylic acid), which exhibits two different coordination pockets, has been exploited to engender and study energy transfer (ET) in two dinuclear [LnIII LnIII '] analogues of interest, [EuYb] and [NdYb]. Their structural and physical properties have been compared with newly synthesised analogues featuring no possible ET ([EuLu], [NdLu], and [GdYb]) and with the corresponding homometallic [EuEu] and [NdNd] analogues, which have been previously reported. Photophysical data suggest that ET between EuIII and YbIII does not occur to a significant extent, whereas emission from YbIII originates from sensitisation of the ligand. In contrast, energy migration seems to be occurring between the two NdIII centres in [NdNd], as well as in [NdYb], in which YbIII luminescence is thus, in part, sensitised by ET from Nd. This study shows the versatility of this molecular platform to further the investigation of lanthanide-to-lanthanide ET phenomena in defined molecular systems

Selective Lanthanide Distribution within a Comprehensive Series of Heterometallic [LnPr] Complexes

The preparation of heterometallic, lanthanide-only complexes is an extremely difficult synthetic challenge. By a ligandbased strategy, a complete isostructural series of dinuclear heterometallic [LnPr] complexes has been synthesized and structurally characterized. The two different coordination sites featured in this molecular entity allow study of the preferences of the praseodymium ion for a specific position depending on the ionic radii of the accompanying lanthanide partner. The purity of each heterometallic moiety has been evaluated in the solid state and in solution by means of crystallographic and spectrometric methods, respectively, revealing the limits of this strategy for ions with similar sizes. DFT calculations have been carried out to support the experimental results, confirming the nature of the siteselective lanthanide distribution. The predictable selectivity of this system has been exploited to assess the magnetic properties of the [DyPr] and [LuPr] derivatives, showing that the origin of the slow dynamics observed in the former arises from the dysprosium ion

A Dissymmetric [Gd2] Coordination Molecular Dimer Hosting six Addressable Spin Qubits

Artificial magnetic molecules can host several spin qubits, which could then implement small-scale algorithms. In order to become of practical use, such molecular spin processors need to increase the available computational space and warrant universal operations. Here, we design, synthesize and fully characterize dissymetric molecular dimers hosting either one or two Gadolinium(III) ions. The strong sensitivity of Gadolinium magnetic anisotropy to its local coordination gives rise to different zero-field splittings at each metal site. As a result, the [LaGd] and [GdLu] complexes provide realizations of distinct spin qudits with eight unequally spaced levels. In the [Gd2] dimer, these properties are combined with a Gd-Gd magnetic interaction, sufficiently strong to lift all level degeneracies, yet sufficiently weak to keep all levels within an experimentally accessible energy window. The spin Hamiltonian of this dimer allows a complete set of operations to act as a 64-dimensional all-electron spin qudit, or, equivalently, as six addressable qubits. Electron paramagnetic resonance experiments show that resonant transitions between different spin states can be coherently controlled, with coherence times TM of the order of 1 µs limited by hyperfine interactions. Coordination complexes with embedded quantum functionalities are promising building blocks for quantum computation and simulation hybrid platforms

Controlled heterometallic composition in linear trinuclear [LnCeLn] lanthanide molecular assemblies

The combination of two different β-diketone ligands facilitates the size-controlled assembly of pure heterometallic [LnLn′Ln] linear compounds thanks to two different coordination sites present in the molecular scaffold. [HoCeHo], [ErCeEr], and [YbCeYb] analogues are presented here and are characterized both in the solid state and in solution, demonstrating the selectivity of this unique method to produce heterometallic 4f molecular entitie

Zn2+ ion surface enrichment in doped iron oxide nanoparticles leads to charge carrier density enhancement

Here, we report the development of monodisperse Zn-doped iron oxide nanoparticles (NPs) with different amounts of Zn (ZnxFe3-xO4, 0 < x < 0.43) by thermal decomposition of a mixture of zinc and iron oleates. The as-synthesized NPs show a considerable fraction of wüstite (FeO) which is transformed to spinel upon 2 h oxidation of the NP reaction solutions. At any Zn doping amounts, we observed the enrichment of the NP surface with Zn2+ ions, which is enhanced at higher Zn loadings. Such a distribution of Zn2+ ions is attributed to the different thermal decomposition profiles of Zn and Fe oleates, with Fe oleate decomposing at much lower temperature than that of Zn oleate. The decomposition of Zn oleate is, in turn, catalyzed by a forming iron oxide phase. The magnetic properties were found to be strongly dependent on the Zn doping amounts, showing the saturation magnetization to decrease by 9 and 20% for x = 0.05 and 0.1, respectively. On the other hand, X-ray photoelectron spectroscopy near the Fermi level demonstrates that the Zn0.05Fe2.95O4 sample displays a more metallic character (a higher charge carrier density) than undoped iron oxide NPs, supporting its use as a spintronic material

Thermodynamic stability of heterodimetallic [LnLn] complexes: synthesis and DFT studies

The solid-state and solution configurations of the heterodimetallic complexes (Hpy)[LaEr(HL)(3)(NO3)(py)(H2O)] (1), (Hpy)[CeEr(HL)(3)(NO3)(py)(H2O)] (2), (Hpy)[CeGd(HL)(3)(NO3)(py)(H2O)] (3), (Hpy)[PrSm(HL)(3)(NO3)(py)(H2O)] (4), and (Hpy)(2)[LaYb(HL)(3)(NO3)(H2O)](NO3) (5), in which H3L is 6-(3-oxo-3-(2-hydroxyphenyl)propionyl)pyridine-2-carboxylic acid and py is pyridine, were analyzed experimentally and by using DFT calculations. Complexes 3, 4, and 5 are described here for the first time, and were analyzed by using single-crystal X-ray diffraction and mass spectrometry. The theoretical study was also extended to the [LaCe] and [LaLu] analogues. The results are consistent with a remarkable selectivity of the metal distribution within the molecule in the solid state, enhanced by the size difference between the different ions. This selectivity was reduced in solution, particularly for ions with the most similar radii. This unique entry into 4f-4f heterometallic chemistry establishes for the first time the difference between the selectivity in solution and that in the solid state, as a result of changes to the coordination that follow the dissociation of terminal ligands upon dissolution of the complexes

A heterometallic [LnLn ' Ln] lanthanide complex as a qubit with embedded quantum error correction

We show that a [Er-Ce-Er] molecular trinuclear coordination compound is a promising platform to implement the three-qubit quantum error correction code protecting against pure dephasing, the most important error in magnetic molecules. We characterize it by preparing the [Lu-Ce-Lu] and [Er-La-Er] analogues, which contain only one of the two types of qubit, and by combining magnetometry, low-temperature specific heat and electron paramagnetic resonance measurements on both the elementary constituents and the trimer. Using the resulting parameters, we demonstrate by numerical simulations that the proposed molecular device can efficiently suppress pure dephasing of the spin qubits

Pathway selection as a tool for crystal defect engineering: A case study with a functional coordination polymer

New synthetic routes capable of achieving defect engineering of functional crystals through well- controlled pathway selection will spark new breakthroughs and advances towards unprecedented and unique functional materials and devices. In nature, the interplay of chemical reactions with the diffusion of reagents in space and time is already used to favor such pathway selection and trigger the formation of materials with bespoke properties and functions, even when the material composition is preserved. Following this approach, herein we show that a controlled interplay of a coordination reaction with mass transport (i.e. the diffusion of reagents) is essential to favor the generation of charge imbalance defects (i.e. protonation defects) in a final crystal structure (thermodynamic product). We show that this syn- thetic pathway is achieved with the isolation of a kinetic product (i.e. a metastable state), which can be only accomplished when a controlled interplay of the reaction with mass transport is satisfied. Account- ing for the relevance of controlling, tuning and understanding structure-properties correlations, we have studied the spin transition evolution of a well-defined spin-crossover complex as a model system

Design, synthesis and study of coordination complexes for quantum computing

[spa]El trabajo realizado en esta tesis doctoral se basa en el diseño, la síntesis y el estudio de complejos de coordinación, centrándose en la comprensión de sus propiedades magnéticas y la posibilidad de su aplicación en la computación cuántica. Para el diseño de estos materiales moleculares, tres diferentes propuestas han sido llevadas a cabo. En primer lugar, se han desarrollado ligandos capaces de agregar metales paramagnéticos en dos grupos diferentes, definiendo de esta manera los dos posibles bits cuánticos de una puerta lógica. Complejos de coordinación homo- y heterometálicos con NiII, CoII y CuII han sido sintetizados y caracterizados para tal efecto. La segunda estrategia seguida ha estado centrada en el diseño de complejos de coordinación lineales para su posterior ensamblaje en parejas de compuestos. Se han desarrollado ligandos que favorezcan la complejación de este tipo de topología, obteniéndose un compuesto de CoII con las propiedades estructurales idóneas para su ensamblaje. Utilizando el ligando bifuncional 4.4’-bipiridina, se ha podido unir dos entidades [Co4] obteniendo así otro prototipo de “parejas moleculares”. La tercera estrategia se ha centrado en el diseño de moléculas asimétricas para facilitar la definición de cada bit cuántico dentro de la entidad molecular. Para ello, se ha sintetizado un ligando no simétrico, que ha sido utilizado para obtener complejos dinucleares homo- y heterometálicos de iones lantánido. Se ha obtenido compuestos con todos los elementos de la serie de los lantánidos. Su estudio magnético y estructural ha mostrado que los dos centros metálicos de estas entidades moleculares son distintos, lo que ha permitido definir el espín de cada ion lantánido como un bit cuántico. El estudio magnético a muy bajas temperaturas de un compuesto de dos átomos de terbio(III), por ejemplo, ha permitido definir dos puertas lógicas: la CNOT y la √SWAP. Utilizando el espectro de energías de los estados magnéticos de la molécula, se han observado las transiciones entre dichos estados en relación a las dos operaciones lógicas.[eng]This thesis presents different strategies for the design of molecular complexes with the requirements to be used as two-qubit quantum gates. The approaches followed towards the preparation of potential qubit systems have been carried out focusing on the synthesis of ligands with β-diketone coordination units, which are very versatile for the design of metallocluster assemblies. One of the main advantages of using this kind of ligands is that they can be easily prepared through simple Claisen condensation, providing different combinations and possibilities for the addition of big variety of donor atoms and pockets. Each ligand has been designed for the preparation of predefined magnetic coordination complexes that can fulfill the conditions of a two-qubit molecular quantum gate. The different complexes synthesized within this thesis can be defined in two different categories: Molecular pairs of well-defined and weakly coupled metal clusters, and complexes of two dissimilar and weakly coupled anisotropic metal ions. In addition, the use of the designed ligands for the preparation of metallo-helicates has been also carried out. The description and study of their helicoidal structure is also shown, using the ligand H4L1 with trivalent and tetravalent metal ions like FeIII, GaIII or UIV. - Molecules featuring two weakly coupled clusters: The approach is based on the design of molecules featuring two well defined coordination clusters that could represent ideal systems for realizing two-qubit quantum gates, as long as each cluster exhibits the appropriate spin properties. Two different ligand-based strategies have been followed for the preparation of such molecular pairs of well-defined metal clusters. The first one is based on the design of poly-β-diketone ligands exhibiting two groups of coordination pockets, which serve to aggregate metals in close proximity. Following this approach, the ligand is responsible for having metals grouped into two clusters, as well as for keeping each metal group together within each subsystem. The structural characteristics of H4L1 provides the requirements for the construction of this kind of clusters, since it might align and separate metals into two dimetallic entities. The goal has been achieved by using the deprotonated ligand that organizes the metals in two groups within molecular linear arrays, saturating the equatorial positions. Compounds with NiII, CuII and CoII metal ions are described and studied. The second strategy is based on the preparation of poly-β-diketone ligands with an additional X donor atom in the middle for acting as a “template” for the aggregation of metals into linear clusters, further linked as molecular pairs by auxiliary ligands Organic ligands like H4L2 or H2L3 fulfill the requirements to aggregate closely spaced metals, which can be then used as building blocks to be linked into molecular pairs by other bifunctional external ligands. An example using CoII is described and studied. - Dinuclear complexes of anisotropic metal ions: The synthesis of complexes of two dissimilar and weakly coupled lanthanides has been used as approach for the construction of molecular prototypes of CNOT quantum gates. The ligand-based strategy considers the design of non-symmetric ligands as a possible way of having two inequivalent lanthanide qubits within a molecule. The ligand H3L4 exhibits a collection of donor groups disposed to favor the aggregation of two metals in different coordination environments. The use of lanthanide ions are good candidates for encoding quantum information following such approach, since they can exhibit strong anisotropy and very well isolated ground state doublets ±mJ (effective S = ½). In addition, lanthanide ions have been proved to have spin states with long decoherence, with T2 timescales that can reach values up to 7 μs.[41] A detailed study of a vast number of dinuclear homo- and heterometallic lanthanide coordination complexes is exposed, including an exhaustive study for some of them to prove their possibilities as CNOT and √SWAP quantum gates