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Title: Non-covalent nanostructuration of chromophoric organic materials
Authors: Marangoni, Tomas
Supervisore/Tutore: Prato, Maurizio
Cosupervisore: Bonifazi, Davide
Issue Date: 26-Mar-2012
Publisher: Università degli studi di Trieste
In the last few decades materials possessing well-defined structural properties on the nanoscale and microscale have shown to be extremely promising for applications in several fields, such as microelectronics, biology, and solar cells fabrication. This is due to the fact that the manufacture of organic-based devices, for any kind of application, requires the development of reproducible protocols to engineer materials featuring precise structural properties. To improve control on the nanoscale level, both bottom-up and top-down approaches have been intensively exploited to date. Although nowadays the second is still predominant at applicative level, Moore’s law foresees its final limit in a few years. In this context strong hope is coming from the possibility to control, in a defined way, the assembly of opportunely functionalized molecules, called building blocks, through the exploitation of particular type of non-covalent interactions. For this purpose the key concepts proper of the supramolecular chemistry has been revealed to be extremely promising for the preparation of nano-aggregates provided with well defined structural and functional properties. In this context one of the factors that crucially affects the process of nanostructuration through non-covalent interactions is the geometrical and structural property of the single building blocks used. Indeed, the geometric structure of molecules can considerably influence their ability to self-organize into more complex objects and therefore by an accurate development of the structural characteristic of the single molecular module it will be possible to tune the structure and the properties of the final material.
Unfortunately in this context, even if great efforts have been undertaken by the scientific community to prepare well defined nanostructures through a supramolecular approach, the possibility to perfectly control the transmission of the geometrical informations from the molecular level to the final nanostructure still remains a partially unresolved task due to the high number of physical and chemical variables correlated to the self-assembly/self-organization process.
The aim of this thesis consists into the design and synthesis of a novel library of molecules, equipped with desired molecular functionalities, which by means of hydrogen bonding interactions can self-assemble and generate different types of nanostructured materials that can be studied at the geometrical and morphological level by means of the combined use of different microscopic techniques such as Transmission Electron Microscopy (TEM) or Atomic Force Microscopy (AFM). Intrinsically, our goal is to shed further light on the structural features of the molecular recognition process, leading to the formation of the final nanostructured material, giving the maximum importance to the investigation of the transfer of geometrical informations from the single building block to the final nanostructure.
In the first part of Chapter 1, the reader is introduced to the basic principle regarding the engineering of nanostructured materials through the different types of non-covalent interactions (hydrogen bonds, electrostatic, aromatic-aromatic and coordinative interactions) with a particular emphasis on the operative procedure developed in the last ten years. In the second part of the chapter instead, the attention is focused on the detailed description of the design and preparation of the nanostructuration process of the material through hydrogen bonds systems.
In Chapter 2, the first part of the experimental work of this thesis is introduced. In this context the synthesis of a molecular library of building blocks able to self-assembly via heterocomplementary H-bonds interactions and self-organize into different types of nanostructure if thermally stimulated, is reported. As for our precedent studies on the subject, the molecular modules used feature complementary terminal H-bonding sites, namely 2,6-di(acetylamino)pyridyl) and uracil moieties, which are connected to different aromatic units through linear ethynyl spacers. The peculiarity of the building blocks adopted for this study is centred on the fact that they possess as H-bonds recognition units uracil moieties protected with the tert-butyloxycarbonyl (BOC) group at the level of their imidic nitrogen. Due to the thermal instability of the BOC groups, the heating of the modules results into the cleavage of this protective group, inducing in this way the self-assembly process between the complementary building blocks. The first part of the chapter guides the reader through the synthetic pathway adopted for the preparation and the spectroscopic characterization of the single building blocks, but also through the investigation of the different aspects of the thermal induced self-assembly process, such as the BOC deprotection phenomena and the molecular recognition process. In the second part of the chapter instead, great space will be given to the investigation of the microscopic characterization of the nanostructured morphologies by means of TEM and AFM. In order to have more detailed informations of the nanostructuration process not only the molecular geometry of the single building blocks was studied but also additional physical and chemical factors, such as the solvent composition or the temperature and concentration used, were taken in consideration to obtain the final nanostructure. A further development of the previous work is reported in Chapter 3, in which the self-assembly and self-organization behaviour of axially chiral building blocks based on binaphthol core is studied The principal task of this project regards the investigation of the transmission mechanism of chiral informations from the single building block to the resulting nano-object obtained by the self-assembly process. In the first part of this chapter the synthetic pathway toward the preparation of the single building blocks is discussed and their self-assembly mechanism in solution, is elucidated by means of different spectroscopic techniques, such as 1H-NMR, UV-Visible and Circular-Dichroism spectroscopy. The second part of the chapter is instead focused on the morphological aspects of the self-organized nanostructures deriving from the assembly of the chiral building blocks. In this context the morphology and the geometrical aspects of the resulting nanostructured materials were investigated by means of different microscopy techniques such as TEM and AFM. Moreover, a detailed evaluation of the morphological changes affecting the structure of the nanomaterial in relation with the solvent composition (i.e polarity) is performed, in order to determine at the same time the best conditions necessary for the preparation of nanostructures provided with a controlled shape and to shed some light on the organization mechanism. As last topic performed during this thesis, in Chapter 4 the supramolecular polymerization process was exploited in order to prepare nanostructured material provided with a certain degree of functionality. For this purpose a template approach was used in order to create hybrid material based on the self-assembly of organic supramolecular polymers onto an electroactive support. In this work we decided to use as template nanomaterial Multi-Wall Carbon Nanotubes (MWCNTs), due to their outstanding electronical properties, and high aspect ratio character that makes them excellent candidate for any eventual application in nanoelectronic devices. Unfortunately the main drawback of this kind of nanomaterial is their low solubility in almost any organic solvent that decreases drastically their applicability. To avoid this drawback, we decided therefore to functionalize the pristine MWCNT following a supramolecular approach. For this purpose a series of di-porphyrin derivatives, able to form a supramolecular polymer through axial coordination, are synthesized. The ability of these compounds to produce polymers by coordination with the bidentate ligand 1,2-(4-(bispyridyl))-ethane was evaluated by means of different spectroscopic techniques, such as UV-Visible and Fluorescence spectroscopy, whereas the morphological aspects of the nanostructure resulting from their self-organization was studied by AFM images. Finally the obtained supramolecular polymers were used to prepare highly soluble MWCNTs, provided at the same time of a large number of antenna systems that can be of high importance for the preparation of nanoelectronic devices. All the nanostructured systems described in this thesis provide a remarkable series of examples of the tremendous potential that the supramolecular approach possess for the fabrication of molecular devices of new generation, which are hardly achievable using the miniaturizing methods that are nowadays the most exploited.
Ciclo di dottorato: XXIV Ciclo
Keywords: Nanostructured Materials
Self- Assembly
Hydrogen Bond
Language: en
Type: Doctoral Thesis
Settore scientifico-disciplinare: CHIM/06 CHIMICA ORGANICA
NBN: urn:nbn:it:units-4454
Appears in Collections:Scienze chimiche

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