Abstract
Controlling the regioselectivity of C–H activation in unimolecular reactions is of great significance for the rational synthesis of functional graphene nanostructures, which are called nanographenes. Here, we demonstrate that the adsorption of tetranaphthyl-p-terphenyl precursors on metal surfaces can completely change the cyclodehydrogenation route and lead to obtaining planar benzo-fused perihexacenes rather than double [7]helicenes during solution synthesis. The course of the on-surface planarization reactions is monitored using scanning probe microscopy, which unambiguously reveals the formation of dibenzoperihexacenes and the structures of reaction intermediates. The regioselective planarization can be attributed to the flattened adsorption geometries and the reduced flexibility of the precursors on the surfaces, in addition to the different mechanism of the on-surface cyclodehydrogenation from that of the solution counterpart. We have further achieved the on-surface synthesis of dibenzoperioctacene by employing a tetra-anthryl-p-terphenyl precursor. The energy gaps of the new nanographenes are measured to be approximately 2.1 eV (dibenzoperihexacene) and 1.3 eV (dibenzoperioctacene) on a Au(111) surface. Our findings shed new light on the regioselectivity in cyclodehydrogenation reactions, which will be important for exploring the synthesis of unprecedented nanographenes.
Highlights
Nanographenes (NGs) are regarded as segments of graphene featuring nanometer sizes in one or two dimensions.[1]
The synthesis of NG molecules has typically been carried out through the oxidative cyclodehydrogenation of tailor-made oligoarylene precursors in solution in the presence of Lewis acids and oxidants.[9−19] the cyclodehydrogenation in solution is sometimes hampered by side reactions furnishing undesired and often inseparable mixtures of products.[20−24] On the other hand, cyclodehydrogenation on metal surfaces has emerged during the past decade as an alternative and complementary method enabling the synthesis of atomically precise NG molecules under ultrahigh vacuum (UHV) conditions.[25−27] The on-surface cyclodehydrogenation proceeds at elevated temperatures, typically at >400 °C, with the help of the catalytic effect of the metal surfaces such as Received: February 1, 2019
We investigated the on-surface cyclodehydrogenation of tetranaphthyl-p-terphenyl precursors 1a and 1b on Cu(110) and Au(111) surfaces (Cu(111) was used for comparison), which led to the formation of dibenzoperihexacenes with distinct regioselectivity of the reaction compared with the solution synthesis
Summary
Nanographenes (NGs) are regarded as segments of graphene featuring nanometer sizes in one or two dimensions.[1]. To this end, bottom-up molecular synthesis is the most suitable approach for providing NG molecules, i.e., large polycyclic aromatic hydrocarbons (PAHs) with uniform and defined chemical structures. The synthesis of NG molecules has typically been carried out through the oxidative cyclodehydrogenation of tailor-made oligoarylene precursors in solution in the presence of Lewis acids and oxidants.[9−19] the cyclodehydrogenation in solution is sometimes hampered by side reactions furnishing undesired and often inseparable mixtures of products.[20−24] On the other hand, cyclodehydrogenation on metal surfaces has emerged during the past decade as an alternative and complementary method enabling the synthesis of atomically precise NG molecules under ultrahigh vacuum (UHV) conditions.[25−27] The on-surface cyclodehydrogenation proceeds at elevated temperatures, typically at >400 °C, with the help of the catalytic effect of the metal surfaces such as Received: February 1, 2019 Published: April 16, 2019
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