Towards the isomer-specific synthesis of higher fullerenes and buckybowls by the surface-catalyzed cyclodehydrogenation of aromatic precursors.

Carbon-based materials, such as fullerenes, carbon nanotubes, and graphene, are attracting increasing interest because of their remarkable properties and potential applications. Fullerenes are a unique family of cage molecules with a variety of sizes and shapes. Most effort thus far has focused on the study of C60 fullerene, for which superconducting properties have been demonstrated, along with high mechanical and heat resistance. The investigation of properties of higher fullerenes requires their production in macroscopic quantities. Whereas the synthesis of the most-familiar members of the fullerene family—C60 and C70—in such amounts by graphite vaporization in an inert atmosphere is well-established, the synthesis of higher fullerenes remains a challenge because of the low yield of the evaporation technique and accompanying purification issues. For example, the two major isomers of C84 obtained with this method (out of 24 possible structures satisfying the isolated-pentagon rule), cannot be fully separated even after 20 rounds of recycling highperformance liquid chromatography. Therefore, synthetic methods are needed for the production of a single isomer of a desired fullerene, free from impurities of other isomers or fullerenes of different sizes. A promising route for the selective synthesis of fullerenes is based on planar polycyclic aromatic hydrocarbon precursor molecules that already contain the carbon framework required for the formation of the target fullerene cage. Such an unfolded fullerene can be stitched up through an intramolecular cyclodehydrogenation to form the desired fullerene isomer. Flash vacuum pyrolysis has been found to be an effective technique for such intramolecular cyclization. Many small curved fullerene fragments have been obtained by this method. However, the yield of the target fullerenes is still too low for preparative synthesis. C60, for example, has been obtained in 0.1–1% yield, whereas higher fullerenes were only detected in trace amounts by mass spectrometry. The first step towards the controlled synthesis of nonplanar carbon nanostructures on a surface was made by Rim et al., who demonstrated the formation of carbon halfspheres (so called buckybowls) from planar precursor molecules on a Ru(0001) surface. The method used—surfacecatalyzed cyclodehydrogenation (SCCDH)—exhibits high dehydrogenation selectivity as a result of the catalytic activity of the surface and a high conversion ratio of the deposited precursors into nonplanar cap structures. Otero et al. recently demonstrated the efficiency of this SCCDH method for the synthesis of C60 fullerene cages. In their study, the precursor molecules C60H30 and C57N3H30 were deposited onto a Pt(111) surface under ultrahigh-vacuum conditions and annealed at 750 K. The surface acts as a support for the precursors and final products and thus enables the use of scanning tunneling microscopy (STM) investigations at the single-molecule level. Importantly, the surface also serves as a catalyst for the SCCDH reaction. A simple annealing step led to the formation of the corresponding C60 and C57N3 cages with an unprecedented high relative yield. Despite the variety of experimental and theoretical methods employed to characterize the process, the particular stability of the C60 target molecule prevented demonstration of the selectivity of the C C bond-formation process. In this case, it cannot be excluded that C C bonds rearrange to form the most stable compound, the C60 Ih isomer. Herein we address this important question by studying the SCCDH process of specifically designed precursor molecules. We provide strong evidence that the reaction indeed occurs in a selective manner and that only correctly programmed precursors yield the desired molecule. That is, we show that the SCCDH process does not involve C C bond rearrangement, and that cage formation proceeds through dehydrogenation and the zipping of newly formed bonds at preselected positions only. On the basis of this result, we proceeded to the synthesis of the higher fullerene C84. The production of fullerenes by means of surface-assisted cyclodehydrogenation is an efficient and controlled method, as the isomeric identity of the fullerene can already be built in at the stage of precursor synthesis. The cyclodehydrogenation process used in this study consists of the following steps: first, the designated precursor molecules are deposited by organic molecular beam epitaxy onto the surface; second, the sample is annealed to induce the surface-assisted reaction, which results in the aforementioned [*] Dr. K. Amsharov, N. Abdurakhmanova, Dr. S. Stepanow, Dr. S. Rauschenbach, Prof. M. Jansen, Prof. K. Kern Max-Planck-Institut f r Festk rperforschung Heisenbergstrasse 1, 70569 Stuttgart (Germany) Fax: (+49)711-689-1662 E-mail: [email protected]