On-surface synthesis of carbon nanostructures with non-benzenoid and non-alternant topology

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The selective bottom-up synthesis of sp2 carbon nanostructures provides a versatile route to novel materials with unusual (opto)electronic properties. Prime examples include graphene nanoribbons and nanographenes, which have been successfully prepared by on-surface synthesis with atomic precision. So far, the main focus has been on systems with benzenoid and alternant topology. These systems are composed of sp2 carbon atoms forming hexagonal rings, and limited control over their electronic properties is possible through size, shape, and edge structure. An alternative and not yet well-developed method to modify the properties of these sp2 carbon nanostructures is to change their topology in a non-hexagonal manner, for example by introducing pentagonal and heptagonal rings. The resulting non-benzenoid and non-alternant pi-electron systems have unique (opto)electronic and other properties, according to theoretical considerations and our experimental results for selected systems. Here, we propose to employ a combined solution synthesis / on-surface synthesis approach to achieve structurally well-defined sp2 carbon nanostructures with non-benzenoid and non-alternant topology on metal- and metal-oxide surfaces in ultrahigh vacuum. Selectivity will be steered by a combination of precursor design, surface interaction, kinetic on-surface reaction control, and other means. This methodology combines the expertise in organic synthesis from the Hilt group with the surface science competence in the Gottfried group and has been refined during a long-standing collaboration. The products targeted in this proposal include long-range periodic azulene and azupyrene polymers, non-alternant carbon nanoribbons based on azulene and azupyrene structural elements, as well as non-alternant cycloarenes. The structural and electronic properties of the products will be characterized in detail by scanning probe microscopies/spectroscopies, X-ray and UV photoelectron spectroscopies, X-ray absorption spectroscopy, as well as optical spectroscopies. These complementary techniques will also be used for the monitoring of the on-surface reactions and for mechanistic studies, which aim at the further development of the principles of on-surface synthesis.