Low temperature magnetic properties and spin dynamics in single crystals of Cr8Zn antiferromagnetic molecular rings

Under the terms of the CC BY license.-- et al.A detailed experimental investigation of the effects giving rise to the magnetic energy level structure in the vicinity of the level crossing (LC) at low temperature is reported for the open antiferromagnetic molecular ring CrZn. The study is conducted by means of thermodynamic techniques (torque magnetometry, magnetization and specific heat measurements) and microscopic techniques (nuclear magnetic resonance line width, nuclear spin lattice, and spin-spin relaxation measurements). The experimental results are shown to be in excellent agreement with theoretical calculations based on a minimal spin model Hamiltonian, which includes a Dzyaloshinskii-Moriya interaction. The first ground state level crossing at μH = 2.15 T is found to be an almost true LC while the second LC at μH = 6.95 T has an anti-crossing gap of Δ = 0.19 K. In addition, both NMR and specific heat measurements show the presence of a level anti-crossing between excited states at μH = 4.5 T as predicted by the theory. In all cases, the fit of the experimental data is improved by introducing a distribution of the isotropic exchange couplings (J), i.e., using a J strain model. The peaks at the first and second LCs in the nuclear spin-lattice relaxation rate are dominated by inelastic scattering and a value of Γ ∼ 10 rad/s is inferred for the life time broadening of the excited state of the open ring, due to spin phonon interaction. A loss of NMR signal (wipe-out effect) is observed for the first time at LC and is explained by the enhancement of the spin-spin relaxation rate due to the inelastic scattering.This work was financially supported by the Italian FIRB Project No. RBFR12RPD1 of the Italian MIUR “New Challenges in Molecular Nanomagnetism: From Spin Dynamics to Quantum-Information Processing.”Peer Reviewe

g-engineering in hybrid rotaxanes to create AB and AB2 electron spin systems: EPR spectroscopic studies of weak interactions between dissimilar electron spin qubits

Hybrid [2]rotaxanes and pseudorotaxanes are reported where the magnetic interaction between dissimilar spins is controlled to create AB and AB2 electron spin systems,allowing independent control of weakly interacting S =1=2 centers

Self Assembly of Copper(I) and Silver(I) Butterfly Clusters with 2-Mercaptothiazoline

X-ray data obtained from poor crystals which formed from the reaction of copper(II) acetate with 2-mercaptothiazoline reveal the formation of a product that is a polymer formed of tetranuclear, butterfly shaped Cu4(MT)4, 1, clusters. Preparation, isolation and structural characterization of a series of isostructural butterfly complexes was accomplished by addition of a Lewis base (pyridine, PPh3, or ASPI13) to the precipitate obtained from the reaction of copper(II) and/or silver(I) acetate with the appropriate stoichiometric amount of 2-mercaptothiazoline. The general formula of these clusters is L2M4(MT)4; 2, L = PPI13 and M = Cu; 3, L = AsPh3 and M = Cu; 6, L = PPI13 and M = Ag; MT = C3H4NS2_, known as 2-mer- captothiazolinate. The polymer [pyCu4(MT)4]„, 4, formed by the addition of pyridine to 1, was also characterized crystallographically. A mixed metal butterfly complex, (PPh3)2Ag2Cu2(MT)4, 8, is formed by addition of PPI13 to a suspension of the precipitate formed upon reaction of the free HMT ligand with a 1:1 mixture of copper(II) and silver(I) acetates in CH2CI2. FD-MS results of each of the precipitates obtained from the metal acetates and the free ligand indicate that the monomeric unit is M4(MT)4. 1H-NMR and 31P{1H}-NMR, both in solution and in the solid state are presented and interpreted

Linking Wheels for use in quantum information processing

Heterometallic Cr7Ni-containing wheels have been identified as potential qubits for use in quantum information processing. The work described in this thesis details attempts to form a variety of multi-qubit systems for the purposes of better understanding the interactions occurring between the wheels and also forming potential 2-qubit quantum gates using redox/photo-active links. Mono-substituted wheels of type [Pr2NH2][Cr7MF8(O2CCMe3)15(L)], where L is acarboxylate with a pendant coordinating group, have been synthesised for use as bulky ligands in coordination chemistry with metal complexes. Various carboxylates have been substituted into the wheels and the products reacted with first row transition metal complexes in order to extend the series of linked-wheel systems. Many of these novel complexes have been characterised by X-ray crystallography, and in certain cases EPR studies have been undertaken to probe the strength of interactions occurring via different bridging units. The first well-established substituted wheel, [Pr2NH2][Cr7NiF8(O2CCMe3)15(O2CPy)] (Py= pyridine), has also been used in reactions with second and third row transition metal centres to show its ability to act as a ligand under more harsh conditions. In addition, the disubstituted product [Pr2NH2][Cr7NiF8(O2CCMe3)14(O2CPy)2] has been reacted with a copper complex in order to form a polymeric structure. Purple wheels of type [Cr7NiF3(O2CCMe3)15(EtGu)(H2O)] have been linked through a variety of extended organic molecules containing a minimum of two pendent pyridyl groups. Use of a ligand containing four pyridyl groups, 2,2´:4,4´´:4´,4´´´-quaterpyridyl(qpy) produced a three-wheel-containing system, creating an interesting modification of the wheel backbone not seen before. A series of transition metal complexes, containing qpy and its extended derivative bbpe, has been synthesised in order to form linked-wheel systems. While none of these systems has to date yielded a crystal structure, a significant amount of evidence has been collected to confirm successful formation of the desired products. UV-vis spectroscopic and electrochemical measurements show that these compounds are photo and/or redox-active,and preliminary luminescence studies indicate that the presence of the wheels quenches the emission from metal-to-ligand charge-transfer excited states. A series of mixed-wheel systems has been synthesised by reacting four different monosubstituted green wheels with four purple wheels (Cr7M, where M = Ni, Co, Mn or Zn). A total of fourteen novel systems has been formed, with two of the attempted reactions failing to occur. EPR studies on Cr7Ni-Cr7Ni linked systems show that the strongest interaction occurs when using isonicotinate as a link, with 4-pyridazinecarboxylate giving the weakest coupling. Studies have also been undertaken on Cr7Ni-Cr7Mn and Cr7Ni-Cr7Zn systems, with the former showing interactions and the latter showing none. Initial work to link wheels together indirectly via their templates was ultimately unsuccessful, but a variety of potential linker molecules were synthesised and are described.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Microstrip Resonators and Broadband Lines for X-band EPR Spectroscopy of Molecular Nanomagnets

We present a practical setup to perform continuous-wave X-band electron paramagnetic resonance spectroscopy by using planar microstrip lines and general purpose instrumentation. We fabricated Ag/alumina and Nb/sapphire microstrip resonators and transmission lines and compared their performance down to 2 K and under applied magnetic field. We used these devices to study single crystals of molecular Cr3 nanomagnets. By means of X-band planar resonators we measured angle-dependent spectra at fixed frequency, while broadband transmission lines were used to measure continuous wave spectra with varying frequency in the range 2–25 GHz. The spectra acquired at low temperatures allowed to extract the essential parameters of the low-lying energy levels of Cr3 and demonstrate that this method is particularly suitable to study small crystals of molecular nanomagnets

EPR spectroscopy of antiferromagnetically-coupled Cr3+ molecular wheels

Currently, there is interest in the development of molecular-scale devices for use in quantum information processing (QIP). With this application in mind, physical studies on antiferromagnetically coupled molecular wheels [Cr7MF3(Etglu)(O2CtBu)15(phpy)], where M is a divalent metal cation (M = Mn2+, Zn2+, Ni2+) have been pursued. The heterometallic wheels contain an octagon of metal centres, which are bridged by fluoride ions, pivalate groups and a chiral N-ethyl-D-glutamine molecule which is penta-deprotonated and bound to the metal sites through all available O-donors. They are deep purple in colour and they have been named purple-Cr7M. There is antiferromagnetic coupling between adjacent metal centres, J » -8 cm-1, resulting in a non-zero net spin ground state. The spin-Hamiltonian parameters of this family have been determined.At the heterometal site of purple-Cr7M wheels there is a terminal ligand which can be substituted for a variety of N-donor organic ligands. A series of bidentate N-donor linkers has been used to link Cr7Ni wheels (each wheel Seff = 1/2) to create prototype two-qubit systems. Multi-frequency EPR spectroscopy and SQUID magnetometry has been used to extract the spin-Hamiltonian parameters of this family. It has been shown that the single wheels can be linked together electronically as well as chemically. It has been found that for the unsaturated linkers, there is a weaker interaction between Cr7Ni wheels when longer linkers are used. The strength of interaction is smaller for the saturated linkers than for the unsaturated linkers.The formation of 'green'-Cr7M wheels is different, being templated around a cation. Two new types of wheels have been studied: [tBuCONHC6H12NH2C6H12NHCOtBu][Cr7M2+F8(O2CtBu)16] and [Cs?Cr7MF8(O2CtBu)16]·0.5MeCN (where, M = Mn2+, Zn2+, Ni2+), where the former is templated around a long dialkylammonium group and the latter around a caesium cation. The effect of the templating cation on spectroscopic properties has been determined.Physical studies on a family of antiferromagnetically-coupled homometallic clusters have been pursued. They consist of cyclic arrays of homometallic Cr3+ ions in either a octametallic wheel or hexametallic horseshoes. The horseshoes have the general formula: [CrxFx+5L2x-2]n3- (where L = carboxylate). Cr3+ centres are bridged by pivalate groups and fluorides, while Cr3+ centres at the ends of the chain have terminal fluorides completing their coordination sphere. These terminal fluoride groups are labile enough to be substituted, e.g. [EtNH2][Cr6F7(O2CtBu)10(acac)2] is the product of a substitution reaction with acetylacetone.EThOS - Electronic Theses Online ServiceEPSRCGBUnited Kingdo