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Bn-doped Pi-conjugated materials: from monomeric to polymeric borazine arraysS

Dosso, Jacopo 2020. Bn-doped Pi-conjugated materials: from monomeric to polymeric borazine arraysS. PhD Thesis, Cardiff University.
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Since the first isolation of graphene by Geim and Novoselov in 2004, a new field involving the preparation of tailored derivatives of this material has been exponentially growing. One of the major targets of this scientific endeavour is the conversion of graphene from conductor to semiconductor, leading to a new generation of miniaturized high-performance transistors and spintronics. To prepare semiconductors based on graphene, many different strategies have been pursued, mostly based on physical methods, or doping of graphene. In the latter approach, the most interesting results have been achieved using surface assisted chemistry, resulting in the formation of graphene presenting various heteroatoms as dopants (B, N, O, S) and semiconducting properties. However, this approach has been often limited by a difficult control over structure and inclusion of the dopants. A possible solution to these limitations comes from the preparation of BN-doped graphene. This highly polar couple is isoelectronic and isosteric with CC double bonds, thus allowing for an inclusion of the dopant without important changes in graphene structure. This results in a material with very similar morphology but different opto-electronic properties, featuring a wider bandgap compared to pristine graphene. From this point of view, evidence of a dependence of the bandgap size of graphene from doping percentage and position was obtained from theoretical investigations, highlighting the importance of a precise synthesis of the materials, which is not achievable with top-down methods. Consequently, a bottom-up approach in which BN-doped PAHs are synthesised using organic synthesis and then used as building blocks to form extended systems, represents the most reliable strategy towards precisely doped materials. A vast library of BN-doped PAHs has been synthesised by many different groups using different precursors, doping patterns, and strategies. Among these, the use of functionalized borazines as precursors has been often neglected and limited to the synthesis of the archetypal borazine-doped nanographene, namely “Hexa-perihexabenzoborazinocoronene” (HBBNC). The high interest towards this molecule, which can be considered as the smallest unit of a borazine-doped graphene (Figure 1A), is related to theoretical studies predicting an efficient HOMO-LUMO gap widening in this derivative compared to the full carbon congener. Synthetic attempts resulted in HBBNC formation in low yield as very insoluble product, thus resulting in incomplete characterizations, which failed to produce comprehensive experimental evidence for the effect of the borazine doping on nanographene systems. Abstract VII Figure 1A: Example of a borazine-doped graphene and HBBNC as its smallest unit. In this dissertation we present a synthetic strategy towards soluble HBBNC derivatives, leading to an extensive characterization of the borazine doping effects, thus providing a solid experimental confirmation of the ability of this doping strategy to widen the HOMO-LUMO gap of PAHs. This work is presented in 5 chapters, starting with an introduction regarding the stateof- the-art results in the field in Chapter I. In this part the most important achievements in the preparation of doped graphene and PAHs are reported, thus giving an overview on the open questions in the area, which will be addressed in the following chapters. Chapter II reports the synthesis of ortho halogen functionalised borazines and their use in the preparation of soluble HBBNC derivatives (Figure 2A). Various approaches are investigated to obtain the full planarization of the borazine precursors, leading to the successful use of fluoro hexaaryl-borazines in presence of silylium ion reagents resulting in a six-fold Friedel-Crafts like reaction producing the desired soluble HBBNC. Along with the target molecule, different by-products were formed depending on the precursors used. These PAHs, presenting B2N3H2 and B3N2O doping patterns, were isolated and characterized as well. Moreover, the suitability of the reaction towards a variety of functional groups has been tested, leading to a clear understanding of the viable precursors for this reaction. Abstract VIII Figure 2A: Strategy towards the synthesis of soluble HBBNC. In Chapter III a complete structural and opto-electronic characterization of the BNdoped materials previously obtained is reported. This is enriched by the comparison with tailored reference compounds, leading to an unequivocal assessment of the effect of doping on the molecule properties. The results displayed in this chapter represent an exhaustive characterization of a borazine-doped hexabenzocoronene, thus filling an important gap in the literature on the topic by proving the ability of borazine doping to widen the HOMO-LUMO gap of PAHs. Chapter IV presents the preparation and study of extended borazine precursors (multiborazines) which can be used for the synthesis of borazine-doped nanoribbons. The synthesis is enriched by a complete study on the stability and properties of these unprecedented molecules. The results of these investigations allowed us to synthesise a fluorinated dimeric multi-borazine system which can be used in a future synthesis of borazine-doped extended graphene systems. Finally, in Chapter V, an approach towards borazine-doped polymeric materials is developed relying on the use of ethynyl functionalised borazines undergoing [4+2] cycloadditions with dimeric cyclopentadienone reagents. The result of this reaction is the formation of a borazine-doped polymeric polyphenylene, which was extensively studied using solid state NMR. The material proved to form gels efficiently in chlorinated solvents and due to the presence of the borazine rings and the high thermal and chemical stability, was used as a support material in solid state electrolytes.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Date of First Compliant Deposit: 22 September 2020
Last Modified: 22 Sep 2021 01:30

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