Controlling the edges: from nanographenes towards to graphene nanoribbons

Up to now, “bottom-up” synthesis has mainly focused on armchair-edged PAHs, whereas PAHs with cove-type or zigzag edges have remained elusive. However, depending on the size and periphery structure, dramatically different optical and chemical properties are observed. For this reason, we have developed a series of different edge structure of nanographenes in this thesis to investigate the electronic and optoelectronic properties. The results can be summarized as follows: 1. A series of new cove-type PAHs, even towards cove-edged graphene nanoribbons, are presented in the second chapters to investigate the electronic and optoelectronic properties. As model subunits of the infinite cove-edged GNRs, fused chrysene-based oligomers or PAHs, namely the dimer (two units of chrysene) and tetramer (four units of chrysene), can be synthesized via the Ullmann coupling reaction of 11,11'-dibromo-5,5'-bischrysene followed by intramolecularly oxidative cyclodehydrogenation in solution, and much higher GNR homologues via on-surface synthesis. The unambiguous crystallographic characterization of the tetramer reveals that the cove-edge structure causes it to twist away from planarity due to the steric repulsion, and shows alternative “up-down” geometry. DFT calculation further reveals that the “up-down” geometry of the tetramer has the lowest energy compared to the “mix” and “helix” conformers, which is in agreement with the experiment crystal result. One would safely conclude that the corresponding infinite GNRs also adopt the alternating "up-down” rolling hill. Moreover, the calculation predicts that these type GNRs turn to low band gap (Eg=1.70 eV) and high mobility with the cove edge at the periphery. From our surface synthesis, the length of the ribbons is around 20 nm. The unique optical electronics properties of such oligomers and GNRs suggest that they are promising candidates for the OFET, OLED and photovoltaic applications. 2. PAHs with zigzag periphery display fascinating electronic and optoelectronic properties resulting from the spin-polarized state at the zigzag edges. We demonstrated a PAHs with zigzag periphery in chapter 3. A novel tetrabenzo[a,f,j,o]perylene, namely “bistetracene” in which two tetracenes are connected side by side with two bonds, was synthesized and characterized. An optical energy gap of about 1.56 eV is derived from the UV-vis absorption spectrum, showing the low optical gap feature of such zigzag-edged type polycyclic aromatic hydrocarbons (PAHs). Theoretical calculations and physical property investigations manifest that such a PAH possesses a prominent biradical character in the ground state, which is the first example among bistetracene derivatives. However, the bistetracene can easily undergo oxidation into tetrabenzo[a,f,j,o]perylene-9,19-dione (diketone) under ambient conditions. Such zigzag-edged PAH can be considered as a short segment of infinite graphene nanoribbons with exceptional zigzag edges. Therefore, our studies contribute to an understanding of the edge states of expanded PAH homologues (such as peri-tetracenes or peri-pentacenes) and graphene nanoribbons which may possess a localized nonbonding π-electron around the zigzag edges. It is envisioned that these zigzag edged PAHs are promising candidates for the development of spintronics given that the edge localized spins can be polarized by applying external magnetic/electrical fields. 3. In chapter 3, we have demonstrated the synthesis of a novel tetrabenzo[a,f,j,o]perylene, namely “bistetracene” in which two tetracenes are connected side by side with two bonds. However, theoretical calculations and physical property investigations manifest that such bistetracene possesses a prominent biradical character in the ground state. Accordingly, this bistetracene can easily undergo oxidation into tetrabenzo[a,f,j,o]perylene-9,19-dione (diketone) under ambient conditions. Thereby, the stabilization is the critical issue for such PAHs. In chapter 4, a novel dibenzo[a,m]rubicene, pentagon-embendded bistetracene, was synthesized and characterized. This pentagon-embendded molecular is more stable compared to the traditional six-membered bistetracene. In chapter 4, we report the cyclodehydrogenation reaction of compound 6,7,13,14-tetraphenylbenzo[k]tetraphene, which gives two unexpected products, namely 5,14-diphenyldibenzo[a,m]rubicene and the (R)-9b,14-diphenyldibenzo[a,m]rubicen-5(9bH)-one, instead of 5,14-diphenyltetrabenzo[a,cd,j,lm]perylene, during the Scholl reaction. Nevertheless, to the best of our knowledge, this is the first example that a position exchange of substituents has been observed and formation the five-membered rings during the cyclodehydrogenation process. More importantly, this pentagon-embedded molecular is more stable than the traditional six-membered bistetracene. 4. Indenofluorene is a prominent 20-π-electrons fully conjugated antiaromatic analogues of acenes with a 6-5-6-5-6 skeleton. In chapter 6, we have developed a new efficient synthetic method toward an unknown antiaromatic dibenzene indeno[1,2-b]fluorene and its derivatives. The physical and chemical properties were systematically studied by various experimental methods and DFT calculations. The results unambiguously demonstrated that compounds were new type air stable and fully conjugated antiaromatic system. The design concept and the new synthetic strategy starting from the corresponding methyl group to construct the fivemember ring can be applied to synthesis of other stable antiaromatic polycyclic hydrocarbons or even hybrid structures