No description available.

No description available.

No description available.

No description available.

No description available.

This thesis is concerned with an experimental study into the effects of weak static magnetic fields on radical recombination reactions, weak magnetic fields being defined as those which are similar to the average hyperfine coupling of the radical pair. It is the aim of this thesis to examine the effect of these weak magnetic fields on the recombination reactions of neutral radical pairs, as influenced by the microenvironments in which they are created, using the technique of time resolved optical absorption spectroscopy. In the introductory chapters the theoretical concepts underlying the phenomenon of magnetic field effects (MFEs) are presented, and experimental methods applied to study these effects are discussed. A variety of structures used to confine the radical pairs after creation are introduced, and their relevant properties are listed. A discussion of the experimental apparatus used to study the MFEs in neutral radical pairs is presented, alongside the methods used for data analysis. The opening two results chapters are concerned with neutral radical pairs contained in synthetic micelles. A variety of minor alterations to both the radical pair and the micelle structure are introduced, such as addition of hydrogen donor molecules to the micelle, and deuteration of one or both members of the radical pair. An experimental investigation is also presented which tests the theoretical prediction that changing the magnetic field strength from absolute zero magnetic field to a very weak field will lead to a large alteration in radical recombination rate. For the first time at weak magnetic field strengths MFEs are demonstrated in synthetic vesicles, and optimised as a function of concentration and temperature, both above and below the phase transition temperature of the vesicle. Lysophospholipid micelles, formed by phospholipid derivatives are investigated, and form a stepping stone between the synthetic micelles and vesicles investigated in earlier chapters and the biologically relevant vesicles formed by phospholipid molecules. A new probe molecule is introduced, and the MFEs are investigated as a function of chemical concentration and solution temperature. Finally, MFEs are investigated in phospholipid vesicles. Experiments are performed in systems of differing chemical composition, and the effect of solution preparation is evaluated.

Structural and magnetic studies of citrate coordination clusters are presented for reaction systems involving cobalt(II), copper(II) and nickel(II). The cobalt(II) compounds incorporate cubane structural motifs that are of particular interest for the generation of single molecule magnet (SMM) behaviour. The product family includes a Co4 SMM cluster, a Co6 SMM cluster and a 3D network of linked SMM units. A new route to the erinaceous Co6 cluster crystals is reported, which allows further investigation of the hydration dependent magnetic behaviour of the samples. The structures of a range of copper(II) cluster sizes are described, including a monomer, dimers, trimers and a tetramer. Correlations between structure and magnetic properties are discussed, including the important structural role of the citrate ligand. Two types of citrate-based copper(II) dimers are shown to undergo unusual structural transformations at high pressure. Initial results of the development of 3d-4f systems based on cobalt(II) and copper(II) are described, including a discussion of reaction methodology. Finally, a brief examination of the nickel(II) citrate system includes an overview of the complicated synthetic conditions, then considers the magnetic properties of the nickel(II) analogue of the Co4 cluster.

Les complexes à transition de spin sont des molécules qui peuvent être commutées entre deux états, l’un diamagnétique et l’autre paramagnétique. Cette commutation peut s’effectuer, entre autre, à l’aide d’une excitation lumineuse, ouvrant la voie vers un possible stockage de l’information au niveau d’une molécule unique. Toutefois, l’information photo-inscrite n’est stable qu’au-dessous d’une certaine température appelée T(LIESST). L’objectif principal de cette thèse a constitué en des modifications chimiques (modification du ligand organique, désolvatation, modifications d’anion,…) d’un complexe macrocyclique de Fe(II) qui présente à l’heure actuelle l’un des T(LIESST) les plus élevés (134 K), et ce afin d’augmenter encore cette valeur. Parmi les divers résultats expérimentaux, un T(LIESST) de 160 K a été obtenu. / Spin transition complexes are molecules that can be commuted between two different states, one being diamagnetic and the other one paramagnetic. This commutation can be triggered by different ways, including light irradiation, opening a way to the storage of information at molecular level. However, the photo-inscribed information is stable only below a given temperature called T(LIESST). The main goal of this thesis has been to modify chemically (modification of organic ligand, desolvation, modification of anions, …) a Fe(II) macrocyclic ligand presenting one of the highest T(LIESST) known today (134 K) with the aim to further increase this temperature. Among the different results obtained, a T(LIESST) of 160 K has been reached.

L’étude des matériaux à transition de spin constitue un important challenge du fait de leurspotentialités dans le traitement de l’information, notamment par voie optique. En effet, inscrire uneinformation par la lumière permet d’accéder à des vitesses de traitement extrêmement rapides. Leszones de stabilité de cette information photo-inscrite varient beaucoup avec la température. Cetravail de thèse a consisté à faire converger ces zones vers un même point via la rencontre entre lestransitions de spin thermique et photo-induite. Pour cela, nous avons utilisé deux approcheschimiques : la réalisation de solutions solides sur des systèmes très coopératifs et l’obtention denanoparticules photomagnétiques. Ce manuscrit détaille les résultats obtenus durant ce travail. / The study of spin crossover materials is an important challenge regarding their potentiality ininformation process, especially by optical way. Indeed, writing information with light can allow tospeedup the treatment. The stability areas of this photo-induced information is sensitive to thetemperature. The aim of this work consisted of realise the convergence of this areas toward thesame point through the meeting of the thermal and photo-induced spin transitions. For this purpose,we used two chemical approaches: achievement of metal dilution on very cooperative systems andsynthesis of photo-magnetic nanoparticles. This manuscript describes the results obtained during thiswork.

Les polymères de Fe(II) à transition de spin à base de ligand triazole [Fe(Rtrz)3]X2 possèdent des propriétés de commutation proches de la température ambiante. Cette bistabilité détectable à la fois par voie optique et magnétique leur confère un intérêt applicatif. Il est maintenant acquis que leurs propriétés sont très dépendantes de leur composition : la nature du substituant du ligand 1,2,4-triazole et/ou de l’anion et/ou du taux d’hydratation. Ce travail a permis d’aborder diverses problématiques. L’influence du phénomène de déshydratation/réhydratation sur les propriétés de transition de spin est d’abord étudiée pour quelques complexes. Ensuite, la synthèse de nouveaux matériaux est envisagée grâce à la méthode des alliages moléculaires. Puis, l’influence des paramètres de synthèse sur les propriétés est examinée pour quatre composés choisis. Enfin, l’effet du traitement en milieu supercritique sur les caractéristiques de ces composés moléculaires est étudié. / Spin crossover polymers based on Fe(II) and 1,2,4-triazole ligands [Fe(Rtrz)3]X2 have switching capabilities near room temperature. Optical and magnetic bistabilities observed for these systems confer potential applications. It is well established that their properties are highly dependent on their composition : the nature of the 1,2,4-triazole substituent, the anion and the rate of hydration. Our work has addressed several different topics. The influence of the dehydration/rehydration phenomenon on the spin transition is first studied for some complexes. Next, molecular alloy method is considered to obtain new materials. Then, the influence of synthesis parameters on the properties is examined for four selected compounds. Finally, the effect of treatment in a supercritical medium on the molecular characteristics of these compounds is studied.