Disseny, preparació i estudi de compostos rics en bor amb propietats fotoluminiscents

Abstract

Following in the same line of research initiated by the Catalysis and Inorganic Synthesis Group within the ICMAB time ago, this work is mainly focused on the synthesis of carborane derivatives with styrenyl groups via nucleophilic substitution reactions, followed by the incorporation of these derivatives to different types of platforms and structures in order to study their photoluminescence properties. Starting from these compounds, in which the o-carborane cluster is of closo nature (therefore neutral from electronic point of view), their corresponding nido (anionic) derivatives have been obtained, by the cluster’s partial deboronation reaction in basic medium. These derivatives have been fully characterised using common spectroscopic techniques such as infrared spectroscopy, NMR spectroscopy, mass spectrometry, elemental analysis and in some cases, X-ray diffraction. Subsequently, photoluminescence studies have been performed, indicating that although the derivatives of methyl-o-carborane and monosubstituted o-carborane exhibit an intense fluorescence, their phenyl-o-carborane analogues show almost no light emission at all. In order to explain such phenomenon, the cyclic voltammetry has been studied as well as DFT theoretical calculations performed, leading to the conclusion that the lack of fluorescence for the phenyl-o-carborane derivative is most probably due to a photoinduced electronic transfer process (PET). Different types of carbosilane and poly(aryl-ether) dendrimers have been chosen as platforms to which the styrenyl group-containing compounds have been incorporated into. To achieve this objective, these carboranyl derivatives have been adequately functionalised to contain a terminal Si-H bond and, in this manner, to carry out the hydrosilylation reaction of them with different type and generations of dendrimers that contain a variable number of terminal alkenes. In this way, structures bearing from 3 to 12 carborane clusters at the periphery have been obtained. Polyanionic dendrimers have also been generated via partial deboronation reactions of closo clusters to the respective nido derivatives, and isolated as alkaline salts. All of these synthesised dendrimers have been fully characterised. For the poly(aryl-ether) dendrimers, their thermal properties have been studied by thermogravimetric analysis (TGA), as well as their photophysical properties. The influence of the different substituents of the clusters (phenyl or methyl), number of clusters and the electronic nature of the cage on the photoluminescence has been studied. Those compounds that contain methyl-o-carborane have shown the highest emission quantic yields. These dendrimers, due to their elevated boron content, are considered to be good candidates for future biomedical applications, among others Boron Neutron Capture Therapy (BNCT) and other applications such as antivirals or bactericides. The family formed by cubic silsesquioxanes or POSS constitutes another interesting platform to be taken into account, when considering the incorporation of our carborane derivatives bearing the styrenyl group. With the aim of retaining the structure of these compounds, they have been reacted with octavinylsilsesquioxane via olefin metathesis reactions, yielding macromolecules that contain 8 carborane clusters at the periphery. As was previously accomplished for the poly(aryl-ether) dendrimers, their corresponding polyanionic derivatives have been also obtained, using the same partial deboronation reaction of the closo cluster to the respective nido species. All resulting compounds have been fully characterised by different spectroscopic techniques and elemental analysis. The thermal properties of neutral POSS with closocarboranes have been studied by TGA, showing a remarkable thermal stability. This fact lead us to carry out a more complete study of POSS, previously treated at 600 ºC for 1 hour, using powder X-rays diffraction (XRD) technique and transmission electron microscopy (TEM). Photoluminescence studies of neutral POSS have revealed a different behaviour concerning emissions data, respect to their styrenyl-carborane precursors. In this case, the phenyl-o-carborane derivative presents the highest intensity of emission, whereas the monosubstituted o-carborane species shows a total fluorescence quenching. DFT calculations have been done, both in fundamental and excited states, corroborating the experimental results. By analysing the photoluminescent properties of the isolated starting materials and after anchoring them to different platforms (dendrimers and POSS), it has been observed that the carborane cluster and its substituents play an important and interesting role within these properties. Therefore, we decided to go even further and synthesize, using diverse synthetic strategies (addition, hydrosilylation, nucleophilic substitution or Heck reactions, etc), compounds bearing carborane clusters and different fluorophore groups, such as fluorene, anthracene or stilbene. All of these compounds have been adequately characterised and there photoluminescent properties studied. In all cases, it has been observed that when the fluorophore group is not directly bonded to the cluster and is situated away from it, the emission intensities are quite elevated, whereas if there is an aromatic group directly bonded to the cluster (phenyl or fluorene) and the fluorophore group is close to it in space, a quenching of the fluorescence is produced, which, depending on the type of fluorophore, is more or less efficient. This phenomenon is in close agreement with the results obtained for the phenyl-o-carborane derivatives, for the starting carboranestyrenyl- carborane derivatives as well as for the poly(aryl-ether) dendrimers. During the three months stage at the University of York, different compounds derived from the o-carborane and incorporating at least 1 mesogen derivative of cholesterol have been synthesised, with the aim of obtaining new materials with liquid crystal properties. Once these compounds have been characterised, they have also been analysed by differential scanning calorimetry (DSC) and polarised optical microscopy (POM), to confirm that in general all compounds synthesised by hydrosilylation reaction do not show mesogenic properties, with the exception of the compound bearing 4 mesogens within its structure. On the other hand, all of the compounds synthesised via olefinic metathesis, show interesting mesogenic properties, exhibiting, in all cases, a chiral nematic mesophase (N*). Moreover, in the case of the methyl-o-carborane derivative, a Blue Phase (BP*) and a chiral smectic mesophase A (SmA*) are exhibited during the cooling cycle. These results indicate that for the latter compounds, the incorporation of the carborane clusters does not destroy the mesomorphic behaviour of the mesogen, but in addition, confers it new and interesting mesogenic properties.