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Synthesis, Characterisation and Conformational Studies of Novel Functionalised Polyarene Dendrimers Containing Pentaaryl and Hexaaryl Branching Units
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
Polyarene dendrimers are composed entirely of aromatic rings and are of interest as they possess rigid and shape-persistent structures, which have potential applications in materials chemistry and as scaffold for functional molecules. This thesis describes the synthesis and characterisation of a number of new polyarene dendrimers containing pentaphenylbenzene-like (pentaaryl dendrimers) and hexaphenylbenzene-like (hexaaryl dendrimers) branching units which are capable of focal and peripheral expansions. These structures are based on terminal and internal 1,3-dialkyne cores, onto which two polyaryl sections were introduced via Diels-Alder reaction. These compounds possess inherent axial chirality as a result of the dialkyne position on the core, and a range of differently substituted polyaryl units which result in further chiral axes. Several crystal structures of pentaryl dendrimers were obtained, with these crystallising either as meso conformers or conformers with C2 symmetry. Introduction of a chiral auxiliary onto the central aromatic ring of these dendrimers provided evidence of the atropisomers in solution. These were studied via VT 13C NMR, revealing fast rotation and an energy barrier of about 66 kJ mol-1. The presence of additional aromatic rings on a hexaaryl dendrimer enabled observation of atropisomerism in solution owing to the greater steric hindrance of rotation of its branching units about the chiral axes. Fluorine-containing analogues of these dendrimers were synthesised and subjected to conformational studies through 19F–19F Exchange Spectroscopy (EXSY). Qualitative and quantitative analysis of the spectra revealed the kinetic and thermodynamic parameters, showing that the energy barrier of rotation of these dendrimers was within the range of 75-80 kJ mol-1, which was sufficient for spectroscopic observation but not for physical separation.Polyaryls with symmetry or lacking symmetry have been synthesised. The lack of symmetry was achieved by stepwise introduction of the polyaryl fragments. This demonstrates a route to polyaryls with sections possessing different structural features and thus potentially engineered properties and future chemical differentiations.