Synthesis of molecular graphenes with continuous non-hexagonal topologies

Abstract

Nanographenes, can be regarded as segments of graphene sheet, have received great attention due to their controllable molecular sizes, edge structures and topological geometries, granting them potential applications in electronic, optical, and magnetic devices. Non-hexagonal defects, normally, can be observed in the growth of graphene and present remarkable effects on their electronic properties and performance accordingly. However, the number and orientation of defects in graphene occur randomly and uncontrollably, making them difficult to be quantified and be predicted for their impacts on the electronic perturbation. Fortunately, bottom-up approach has been successfully employed to obtain graphene nanostructures with fascinating architectures and physicochemical properties, by which the precise control of the sizes and shapes of graphene nanostructures are regarded as an indispensable tool to tailor their intriguing electronic and optoelectronic properties. Recently, embedding of pentagon-heptagon pair (an azulene unit) into nanographenes has been developed, which indicates that incorporation of non-alternant topology into hexagonal lattice could lead to significant changes in electronic states, optical properties, molecular geometries, and aromaticity. Nevertheless, integrating the continuous non-hexagonal defects into nanographenes is a very challenging task due to the lack of efficient synthetic methodologies. Accordingly, my research interests mainly focus on the precise synthesis of non-alternant nanographenes with continuous non-hexagonal topologies through bottom-up synthetic strategy, as well as careful characterization of their physicochemical properties, then attempts to gain further insights into their structure-related properties. In Chapter 2, we demonstrated a bottom-up synthesis of three novel nanographenes (2-1, 2-2, and 2-3) with well-defined defects, in which seven-five-seven (7-5-7) membered rings were introduced into their sp2 carbon frameworks and confirmed by X-ray crystallographic analysis. The resultant defective nanographenes 2-1, 2-2, and 2-3 were well investigated by absorption spectra, cyclic voltammetry, time-resolved absorption spectra and further corroborated by density functional theory (DFT) calculations. Detailed experimental and theoretical investigations elucidated that these three nanographenes 2-1, 2-2, and 2-3 exhibited anti-aromatic character in their ground states and displayed high stability under ambient conditions. In Chapter 3, N-doped defective nanographenes were synthesized (3-2 and 3-3) by one-pot Pd-mediated cascade coupling reaction involving Suzuki-Miyaura reaction and N-H arylation. The formation of N-heterocyclic rings led to clear change in their (opto)electronic properties, supported by ultraviolet–visible (UV-vis), transient absorption (TA) and cyclic voltammetry (CV) measurements associated with theoretical calculations. Additionally, the N-doped defective nanographenes exhibited lower oxidation potential values and longer decay process compared to their phenyl-substituted analogue. In Chapter 4, we successfully embedded the inverse Stone-Wales (ISW) defect into π-conjugated system (4-1) by combining the in-solution and on-surface synthesis, in which non-alternant topology was confirmed by scanning tunneling microscopy (STM) images. Meanwhile, we carefully investigated the (opto)electronic properties of non-alternant nanographenes with gradual fusion of polygons including hexagons and heptagons through experimental and theoretical studies. Both results indicated that the incorporation of heptagons into π-conjugated framework could lead to abnormal changes in aromatic and electronic properties when compared to the introduction of fused hexagons.