Infoscience

Thesis

Functionalized Carbon Nanostructures from Oligoyne Carboxylates

Functional nanostructured carbon materials, including two-dimensionally extended nanosheets, offer intriguing perspectives for applications in emerging technologies such as hydrogen storage, lithium storage, transition metal free catalysis, or solar absorbers. The preparation of such materials with a control over the structure formation and chemical functionalization would render them better processable and suited for tailored applications. In this context, the present thesis investigated a novel approach for the low-temperature, wet-chemical preparation of two-dimensional carbon nanostructures. Novel amphiphilic molecules with reactive oligoyne segments were envisioned as precursors that would self-assemble into defined aggregates and subsequently be carbonized under mild conditions while preserving the morphology and the embedded chemical functionalization. The foundation of this novel approach was the development of a reliable synthetic procedure for the preparation of the desired precursor molecules. To this end, we employed a palladium-catalyzed coupling protocol based on the Negishi reaction that allowed for the direct bond formation between two sp-hybridized carbons. We then followed a versatile linear synthetic strategy that allowed for the successful preparation of hexayne carboxylates on the multi-gram scale. In the course of our synthetic work, we encountered an unprecedented single-crystal-to-single-crystal dimerization that we carefully investigated by means of spectroscopy, X-ray analysis of crystalline specimen, as well as DFT computations. The dimerization provided the first synthetic access to unsymmetric 1,2-dibromoeneyne products and, thus, extended the scope of single-crystal-to-single-crystal reactions, illustrating their potential as a tool for complex transformations. The obtained hexayne carboxylates were reactive, carbon-rich siblings of typical fatty acid ester amphiphiles, designed to self-assemble into monolayers at the air-water interface. The film formation of these amphiphiles was thoroughly investigated, and a detailed molecular model of their packing was established by means of different spectroscopy and diffraction techniques, in combination with computational modeling. Our results unambiguously confirmed the presence of a well-defined monolayer that comprised a densely packed array of the hexayne moieties. The complete carbonization of the films at the air-water interface was successfully accomplished by cross-linking of the hexayne layer through UV irradiation at room temperature. The furnished carbon microstructure was found to resemble that of reduced graphene oxide or amorphous carbon materials otherwise obtained at annealing temperatures beyond 800°C. In this way, we prepared mechanically stable and rigid, functionalized carbon films with a molecularly defined thickness below 2 nm and lateral dimensions on the order of centimeters.

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