Benzo-FusedPeriacenes or Double Helicenes? DifferentCyclodehydrogenation Pathways on Surface and in Solution

摘要

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 approximately2.1 eV (dibenzoperihexacene) and 1.3 eV (dibenzoperioctacene) on aAu(111) surface. Our findings shed new light on the regioselectivityin cyclodehydrogenation reactions, which will be important for exploringthe synthesis of unprecedented nanographenes.