Défense de thèse de doctorat en sciences chimiques : "Aromatic engineering", par Dario Mosca

In spite of the tremendous technological progress occurred in the last century, there are still unresolved rebus in history, art, and chemistry (among other disciplines). If the meaning behind Monnalisa’s portrait and the location of “The battle of Anghiari“ at Palazzo Vecchio in Florence probably represent the main rebus in art, then the definition of aromaticity could be the most important one in organic chemistry. As well as for the two aforementioned Da Vinci’s paintings, we have a large set of information (theoretical and experimental) but still we miss a multidimensional definition to univocally define aromatic compounds on the basis of the electronic, geometric, chemical reactivity, magnetic and energetic characteristics. As an example, Gomes and Mallion have mentioned the long diatribe between Katritzky and von Rague Schleyer concerning the orthogonality of classical and magnetic criteria in aromatic systems. In the meanwhile, organic chemists have designed new aromatic molecules and planned creative synthetic strategies for the generation of aromatic building blocks that are currently used for a broad range of chemical applications. Before addressing the detailed investigations of this thesis work, Chapter 1 provides to the reader a brief insight into the concept of aromaticity and aromatic interactions, along with several examples of the scientific topics and terms discussed in this manuscript. Chapter 2 describes the first part of the present doctoral work, which address the gelation mechanism occurring under ultrasound irradiation of a novel and unusual organogelator. Organogels (R,R)-1 and (S,S)-1 have immediately attracted our interest due to the absence of the typical functional groups that usually favour the gelation process (such as urea, polyvinyl or long alkyl chains), the mandatory use of acoustic irradiation and the enantiomerically-pure conditions to trigger the gelation. These organogels show very fascinating properties such as the low amount of organogelator needed for the formation of the material (0.22%w). Upon gelation in MeOH it is possible to swap from an organogel to a hydrogel by a simple solvent exchange procedure (sample immersion in water). Variable temperature NMR measurements and theoretical calculation highlights the presence of two conformers, representing the “extended form” (E) and “folded form” (F) molecular arrangements, which seems to be the responsible for the generation of E-F supramolecular aggregates, as predicted computationally. Microscopy-based characterizations such as SEM, TEM and AFM shed light on the morphology of the supramolecular aggregates forming the organogel (spheres and fibres) and highlight the relation between enantiomeric purity and morphological organization of these materials. Such findings are sustained also by the results from polarimetry and relaxation time (T2) measurements, which allowed building a gel-sol phase diagram that describe the enantiomeric effect in the formation of these systems detected by the microscopic techniques. On the basis of these results we have proposed gel mechanism at the end of the Chapter 1, namely the nucleation-growth mechanism.