Ab initio investigation of the cyclodehydrogenation process for polyanthrylene transformation to graphene nanoribbons

Abstract

Graphene nanoribbons (GNRs) can be synthesized from molecular precursors with atomic precision. A prominent case is the 7atom-wide armchair GNR made from 10,10′-dibromo-9,9′-bianthryl (DBBA) precursors on metal substrates through dehalogenation/ polymerization followed by cyclodehydrogenation. We investigate the key aspects of the cyclodehydrogenation process by evaluating the energy profiles of various reaction pathways using density functional theory and the nudged elastic band method. The metal substrate plays a critical catalytic role by providing stronger adsorption for products and facilitating H desorption. For polyanthrylene on an extra layer of GNR on Au, the underlying GNR insulates it from the Au substrate and increases the reaction barriers, rendering the polyanthrylene "quasi-freestanding". However, positive charge injection can induce localized cyclodehydrogenation. We find that this is due to the stabilization of an intermediate state through an arenium ion mechanism and favorable orbital symmetries. These results provide mechanistic insight into the effects of the metal substrate and charge injection on cyclodehydrogenation during GNR synthesis and offer guidance for the design and growth of new graphitic structures.

On-Surface Synthesis with Atomic Hydrogen

“…The on-surface path toward extended molecular nanoarchitectures most often relies on aryl–aryl coupling within the Ullmann-like reaction scheme with the metallic substrate providing catalytic activity. ,,− The usual design approach is based on precursors equipped with halogen atoms being protectors of the prepared active sites. As a result, when the polymerization step is activated, halogens remain as the major byproduct contaminating the surface and hence influencing the overall reaction process. − While bromine adsorbed on a surface may influence the reaction processes, e.g.…”

“…On-surface synthesis of extended molecular nanostructures is commonly achieved by aryl–aryl coupling following an Ullmann-like reaction scheme on appropriately chosen metallic surfaces playing a catalytic role in polymerization. ,,, Most often, custom-designed molecular precursors are equipped with halogen atoms as leaving groups on the desired active sites. As a result, when the polymerization step is activated, halogens remain as the major byproduct contaminating the surface and hence influencing the overall reaction process. − While halogen atoms exposed at the surface might be removed by higher temperature annealing, which is usually associated with cyclodehydrogenation leading to planarization of previously generated polymers, , removal of surface halogens is still a challenging problem if higher temperature annealing is not needed or especially impossible to carry out. Simultaneously, there is an intensive search for alternative surface-assisted reaction pathways that would improve the abilities to control, specify, and tune the functionalities of molecular systems and extend the versatility of the approach …”

On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations

“… 35 Without the stabilization effect of Au(111) surface, the energy costs for the entire cyclodehydrogenation process will be generally higher. 51 On the other hand, the substrate can render steric hindrance and confinement effects on the intermediates. As a result, the conrotatory electrocyclization will be hampered, and the sigmatropic hydrogen shift will only take place for the top-side hydrogen in a suprafacial instead of antarafacial way.…”

Step edge-mediated assembly of periodic arrays of long graphene nanoribbons on Au(111)

“…Perdew-Burke-Ernzerhof exchange-correlation functional, 32 which has a good performance in modeling metal substrate. 33 A non-local vdw-df functional 34 was used to achieve higher accuracy in the simulation of van der Waals interaction between polymer and Au, 35 which is shown to be a good choice to produce accurate adsorption energies for graphene on Au, 36 especially on substrate steps. 28 The energy cutoffs were 30 Ry and 300 Ry for the wavefunctions and charge density, respectively.…”