Synthesis of Boron-Nitrogen-Carbon (BNC) hybrid materials : from 2D doped polyphenylenes to 3D porous architectures

• Main Author: Fasano, Francesco
• Published: Cardiff University 2017
• Subjects: QD Chemistry
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.742824

In the growing field of polycyclic aromatic hydrocarbons (PAHs), the replacement of carbon with isostructural atoms is becoming a versatile functionalisation strategy to tailor the optoelectronic properties of the materials. Among the different dopants, the substitution of C-C pairs by isoelectronic B-N covalent couples leads to isostructural molecular material bearing strong local dipole moments. This imparts a series of physical-chemical properties to the molecule, such as wider HOMO-LUMO gap, the introduction of anchoring points for gas storage and peculiar self-assembly behaviour on metal surfaces. 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 isoelectronic and isostructural relationship withbenzene, borazine ring is commonly known as “inorganic benzene”. Hence, borazine and its derivatives are valuable molecular modules to be inserted as doping units in graphiticbased carbon materials to tailor their physical-chemical properties. This dissertation focuses on the design, synthesis and characterisation of novel organic materials doped with borazine modules, with the aim of exploring the photophysics, electronics and self-assembly properties of hybrid boron-nitrogen-carbon (BNC)frameworks. Before addressing the detailed investigations of this thesis work, in Chapter I, a brief introduction on the past and recent achievements in the organic synthesis of borazine and its derivatives are given to the reader. This chapter also includes the main synthetic methods used for their functionalisations, as well as their chemical properties and materials applications. Chapter II addresses the design and synthesis of three-branched borazine doped polyphenylenes, in which one or more aryl units are replaced by borazine rings. Owing the possibility of functionalising the borazine ring with different groups on the aryl substituents at the N and B atoms, BNC polyphenylenes were prepared through the decarbonylative [4 + 2] Diels-Alder cycloaddition reaction. To achieve this, two types of molecular modules were synthesised: core and branching units. Therefore, having the possibility of introducing the borazine in different ratios, orientations and positions, three-branched hybrid polyphenylenes featuring controlled orientation and dosages of the doping B3N3-rings were prepared (Figure A.1). At last, the photophysical properties of borazine-doped polyphenylenes have been investigated, describing the relation between the light emission properties and the orientation / dosage doping of these hybrid materials. The second project tackled in this doctoral work is described in Chapter III. Inspiring by the local polar character of the borazine ring, B3N3 scaffold can theoretically interact with polar or polarisable gas molecules, thus making BN-materials promising candidates for tailoring the gas absorptions. In this chapter, the preparation of 3D BN metal organic framework (BN-MOF), composed by three-carboxyl borazine 3-1 as organic linker and [Zn4O(CO)6] as metal cluster is described (Figure A.2). Additionally, to evaluate the effect of the BN doping units in gas storage, the isostructural 3D C-MOF reference, in which the borazine core is replaced by the benzene unit, has been prepared (Figure A.2). The last section of the chapter is dedicated to the description of the X-ray crystal structures of the MOFs as well as their morphology, surface area and thermal stability. Finally, Chapter IV is dedicated to the investigation of three-pyridyl borazine derivatives, amenable to self-assemble and form 2D metal-organic porous network upon deposition on metal surfaces. The first part of the chapter deals with the synthesis of borazine 4-6 through a Suzuki cross-coupling reaction between borazine bearing three-OTf groups and an organoboron pyridyl moiety. The second part describes, by scanning tunnelling microscopy (STM), the tailoring of the 2D self-assembly of borazine 4-6 on Ag(100) and Cu(111) via coordination and thermal treatments. Specifically, densely packed honeycomb network on Ag(100) was observed, whereas interconnected chains were found on Cu(111). Upon deposition of additional Cu atoms at 420 K, a structural transformation can be induced on Cu(111), which leads to a fully three-fold coordinated triangular network (Figure A.3).