AbstractIn 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 a sol-gel transition temperature (Tgs) at 104°C in iso-BuOH, and being much higher than the boiling point of cyclohexane (81°C) and MeOH (64°C), undergo the explosion of the materials. 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.
Chapter 3 describes the investigation of an enantiomerically pure propeller-like molcule, based on the combination of BINOL and hexakis(bromomethyl)benzene 19, which is soluble and amenable for chiral assessment. The resulting products (R,R,R)-14 and (S,S,S)-14, labelled molecular Trinacria due to their structural similarity with such mythological symbol, can be used as pre-organized and chiral system for chiral carbon nanostructures thanks to:
• the intrinsic chiral properties connected with the presence of a chiral axis in the BINOX moiety;
• The pre-organized propeller-like shape;
• The wide variety of chemical modification that can be performed onto naphthalene rings, along with the possibility to modify or extend such aromatic moieties;
• The ability of BINOX moieties to transfer the chirality to achiral substituents.
Before to address the synthesis of chiral carbon nanostructures, we have prepared new Trinacria derivatives decorated with iodine atoms in different positions of the BINOX moieties: (R,R,R)-15-18 and (S,S,S)-15-18. Iodine was the substituent of choice because it allowed the direct functionalization through Pd-catalysed C-C cross-coupling reactions, and for its higher bulkiness compared to hydrogen and other halogen atoms. Among all of them, (R,R,R)-15 and 17 have been isolated in enantiomeric and regioisomeric pure forms, while (R,R,R)-16 resulted as a regioisomeric mixture of compounds having iodine substituents in the BL and/or WE naphthalene rings.
Thanks to the possibility to obtain regioisomerically- and enantiomerically-pure halogenated Trinacria derivatives, we tackled the synthesis of chiral carbon nanostructures having pentaphenyl or coronene moieties. In particular, for the synthesis of coronene-Trinacria derivatives we have investigated three different synthetic pathways, namely through condensation (Scholl reaction), Sonogashira or Suzuki cross-coupling reactions. The lack of regioisomeric purity of (R,R,R)-16 obliged us to build up the aromatic substituents in the BINOL moiety and then react them with hexakis(bromomethyl)benzene 19. (R,R,R)-28 was thus obtained enantiomerically- and regioisomerically-pure. During the production of (R,R,R)-35 and (R,R,R)-37 we got synthetic issues that forced us to focus only on (R,R,R)-55. Unfortunately, our coronene-Trinacria derivatives shows strong π-π stacking interactions, and thus impossibility of performing NMR characterizations. Nonetheless, the LR-MS of (R,R,R)-55 shows the molecular peak of the desired compound, suggesting the feasibility of our synthetic approach.
Finally, Chapter 4 describes the design of a new ink used for as electrochromic layer in ECDs. No other details can be disclosed due to its filing on patent register.
|Date of Award||19 Dec 2016|
|Supervisor||Davide BONIFAZI (Supervisor), Daniel Vercauteren (President), Stephane VINCENT (Jury), Olivier Coulembier (Jury) & Pier Giorgio Cozzi (Jury)|
- Aromatic compounds
- Supramolecular chemistry