AbstractThe replacement of CC double bound in organic π-systems by a more polarized isoelectronic and iso-structure BN units leads to a strong local dipole moments. The polarity of such heteroatomic bonds widens the HOMO–LUMO gap of the molecule, imparting strong UV-emitting/absorption and electrical insulating properties while maintaining the existing structural features, resulting in new and desirable chemical and physical properties. The BN/CC isosterism concept goes back to the seminal discovery of the borazine (H3B3N3H3) by Stock and Pohland in 1926. Thus, in reference to its iso-electronic and iso-structural relationship with benzene, borazine core is commonly referred to as “inorganic benzene”. Successful incorporation of BN-heterocyclic building blocks into π-organic materials is intimately associated with the elaboration of versatile synthetic routes to access these building blocks. For this, borazine and its derivatives are valuable molecular scaffolds to be inserted as doping units in graphitic-based carbon materials to tailor their optoelectronic characteristics.
This research thesis is focusing on the synthesis and studies of novel organic materials based on the borazine core, taking advantage the synthetic versatility of borazine cores. With the aim of exploring the photophysics and electronic properties of this obtained boron-nitrogen doped compounds.
Before addressing the detailed investigations of this thesis work, in the first chapter of the manuscript, a brief insight into the past and recent developments in the organic synthesis and functionalization of borazine and its derivatives reported in the literature, including the main synthetic methods used for their preparation, as well as their chemical properties and materials applications is given to the reader. In addition, recent prototype electronic devices using boron-nitrogen based materials such as azaborines and BN-doped polycyclic aromatic hydrocarbons (PAHs), oligomers and polymers will be discussed, along with a succinct outline of the manuscript.
Encouraged by the particular importance of photoresponsive nanomaterials in the fundamental and applied fields, the first doctoral project, described in Chapter 2, was focused on the design and formation of different nanostructures formed by a novel borazine derivative coupled to three azobenzene moieties and for which reversible regulation using external photo stimuli is expected. In this chapter, the reader is first introduced (Section 2.1) to an overview of the azobenzene properties with a perspective of the current advances of formation of azobenzene based nanoparticles and their photoinduced reversible phase transitions, through the description of key examples reported in the literature. This leads to the objective of the project (Section 2.2): motivated by the wide range of the application of the macromolecules containing azobenzene molecule as a photoresponsive group in a controlled and reproducible manner, and the remarkable self-assemble and morphological transition obtained exploiting the trans-to-cis isomerization of the azobenzene moiety. The idea was to develop synthetic strategies to prepare azobenzene-functionalized borazines and to explore their photophysics properties in addition to the investigation of the properties of supramolecular nanostructures derived from. In the third section (2.3) the development of a protocol for the preparation of borazines decorated with azobenzene moieties (azo-borazine-1, azo-borazine-2 and azo-borazine-3) was undertaken.
Two synthetic strategies were proposed based on the functionalization of the “outer shell” of the borazine prepared by the so-called condensation-based synthesis. First one involved the azo-coupling reaction between borazine bearing amino-group and aniline to generate the azobenzene linked to the borazine core. In the second strategy we opted to work with Suzuki cross coupling reaction between a borazine bearing three OTf-groups and an organoboron azobenzene moiety. The detailed photo-physical including absorption and emission spectral studies were used to investigate the self-assemble behaviour and the switching mechanism. While the morphological change and the nanostructure formation will be investigated by SEM and TEM measurements.
Considering the importance of thiophene-based polymers for the production of low-cost, large-area, and flexible electronic devices and their optoelectronic properties can often be easily tuned simply by variation of their building blocks, for example, to construct donor-acceptor polymers. The incorporation of BN units into π-conjugated polymers appears as an evident route to broaden the scope of organic electronic materials. The second project tackled in this doctoral work is described in Chapter 3. It focuses on the extension of the “outer shell” functionalization of the borazine core from simple molecules described in the preview chapter to polymer architectures. The propose was to design, synthesize and characterize a BN-doping polythiophene with the borazine cores (linear and hyperbranched P(AB-3HT-borazine)). Thermal optical, physical and electrochemical properties were also investigated. In addition, density functional theory (DFT) analyses on model compounds constituting the corresponding repeat unit monomers (AB-3HT (A) and borazine (B)), dimer (AB), trimer (ABA), tetramer (BABA) and star (BA3) were performed.Finally, preliminary morphological studies by AFM were reported.
|Date of Award||22 Feb 2017|
|Supervisor||Davide BONIFAZI (Supervisor), Olivier Coulembier (Co-Supervisor), Roberto Lazzaroni (President), STEVE LANNERS (President), Christophe Detrembleur (Jury), Jean François Nierengarten (Jury) & Philippe H. Dubois (Jury)|
- boron nitrogen
- self- assembly
- Supramolecular chemistry
- hydrogen storage
- optoelectronic devices
- polycyclic aromatic hydrocarbons