Rational Synthesis of Functional Carbon Nanostructures

Language
en
Document Type
Doctoral Thesis
Issue Date
2020-09-28
Issue Year
2020
Authors
Uka, Arber
Editor
Abstract

The main goal was the synthesis of precursor molecules for initiation of the grown of atomically precise carbon nanostructures by chemical vapor deposition onto metal surfaces. The main attention was paid to investigate general possibility of the rational introduction of heteroatoms to carbon nanotubes and shell-shaped nanostructures. Initially, two different aldehydes 40 and 41 were prepared, which reacted with 4,7-dihydro-2H-4,7-ethanoisoindoles 39 to the corresponding nanotube and nanocone precursors (tetrabenzoporphyrin derivatives HH-72 and HH-57). Subsequently, both macromolecules were deposited by onto a purified Pt(111) platinum surface under ultra-high vacuum conditions. Controlled and continuous increase in temperature allowed a cyclodehydrogenation leading to the respective nanocone/nanotube seeds, which can be grown to the respective carbon nanostructures by CVD, as presented in chapter 3.2.3 (Figure 36). Subsequently, grown carbon nanotubes and nanocones should be visualized by scanning tunneling microscopy. As described in chapter 3.2.3 (Figure 37), the nanocone precursor HH-57 was successfully deposited onto Pt(111) platinum surface and transformed to the target ultra-short nanocone via thermally triggered cyclodehydrogenation.In another project, the idea was to produce graphene nanoribbons which could form nanotubes with preprogrammed chirality employing a “rolling-up” process directly on metal surfaces (Figure 52). The goal of this project was to synthesize the required precursor and to investigate the general possibility of carbon nanotubes formation via controlled “rolling-up” of the GNRs. The special here were the developed molecules, which had given a chirality by their basic structure (see Figure 53). In order to produce long nanoribbons and later also long carbon nanotubes, dibrominated bisanthracene 105 was added. For surface-assisted cyclodehydrogenation, an Au(111) gold surface was used. It should be mentioned that the primary focus was the synthesis of key molecules 71 and 94. The “roll-up” process is illustrated by the example of molecule 7-armchair GNR terminated by 94 block, which is preprogrammed for the formation of the (10,3) SWCNT. After the successful synthesis of 94, it was deposited together with dibrominated bisanthracene 105 on an Au (111) gold surface by chemical vapor deposition. After continuous temperature rise, a polymer first was formed which was made to roll up by the formation of pentagons at a temperature about 400 °C, as described in chapter 3.2.3 (Figure 39). Scanning tunneling microscopy was also used in these experiments to visualize the outcome of the reaction. First experiments already have shown promising results. At the beginning of annealing, the formation of required 7-armchair GNRs terminated by 94 block was observed. With temperature rise, polymer chain formation began, and graphene nanoribbons were formed at temperatures around 400 °C. The bright “lumps” or the bright end of each ribbon seems to represent a rolled-up cap. Determining the apparent barrier heights of head and tail resulted in higher values for the tail, which suggested a start of the rolling process. However, these were not ready-rolled carbon nanotubes. In order to solve this problem, further investigations are required and experiments with the second molecule 71 must be carried out. Since the first results looked promising, another molecule 97 was synthesized, which should be subjected to similar STM studies (Figure 54). The synthesis of molecule 97 was carried out on the basis of available in our laboratory 4-bromo-13,16-difluorobenzo-[s]-picene (95). After bromination and cyclodehydrofluorination on activated alumina, the target molecule 97 was prepared, as described in detail in chapter 3.3 (Scheme 36). The special feature of molecule 97 is the asymmetric arrangement of two bromine atoms. The target molecule 97 was placed on an Au(111) gold surface together with excess of 105 and annealed resulting in the formation of two GNRs, which join together via block 97. It is expected that further cyclodehydrogenation should lead to the formation of hybrid Bowl-CNR nanostructure (nanospoon), presented in chapter 2 (Figure 27 and Figure 28). Further experiments and STM measurements are intended to be performed in near future. At the end of this work, attempts to synthesize fluorinated cyclometaphenylenes, representing prospective building block for the synthesis of heteroatom containing nanostructures, were performed. Building blocks were obtained from available 2,6-difluorotoluene (98), as presented in chapter 3.4. Since attempts to synthesize [12]F-[6]Me-[6]CMP 104 from starting material 98 failed, syntheses were carried out to prepare the desired cyclic hexamer 104 from trimer 103 (Scheme 41). Although the target molecule 104 could be detected by mass spectrometry, it should be noted that the cyclic hexamer 104 and the linear hexamer 103 were in a mixture (Figure 55). Attempts to separate the two molecules by preparative LC or HPLC failed because of the similar retention times. The NMR analysis confirmed the fact that both molecules 103 and 104 existed as a mixture. The results obtained show the general possibility of the formation of fluorinated cyclometaphenylenes, so their synthesis plan is to be continued in the group.

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