Tailored Formation of N-Doped Nanoarchitectures by Diffusion-Controlled on-Surface (Cyclo)Dehydrogenation of Heteroaromatics

Abstract

Surface-assisted cyclodehydrogenation and dehydrogenative polymerization of polycyclic (hetero)aromatic hydrocarbons (PAH) are among the most important strategies for bottom-up assembly of new nanostructures from their molecular building blocks. Although diverse compounds have been formed in recent years using this methodology, a limited knowledge on the molecular machinery operating at the nanoscale has prevented a rational control of the reaction outcome. We show that the strength of the PAH-substrate interaction rules the competitive reaction pathways (cyclodehydrogenation versus dehydrogenative polymerization). By controlling the diffusion of N-heteroaromatic precursors, the on-surface dehydrogenation can lead to monomolecular triazafullerenes and diazahexabenzocoronenes (N-doped nanographene), to N-doped oligomeric or polymeric networks, or to carbonaceous monolayers. Governing the on-surface dehydrogenation process is a step forward toward the tailored fabrication of molecular 2D nanoarchitectures distinct from graphene and exhibiting new properties of fundamental and technological interest.

Spectroscopic characterization of the on-surface induced (cyclo)dehydrogenation of a N-heteroaromatic compound on noble metal surfaces

“…In particular, in ref. [1] we demonstrated that the structures formed on different coinage metal surfaces from the same PAH (namely DiPy [5]DBH) are analogous, but their relative formation temperatures strongly depend on the substrate. Most of the works published until now in this field use Scanning Tunnelling Microscopy (STM) or non-contact Atomic Force Microscopy (nc-AFM) 8 images to identify both, precursors and reaction products, while spectroscopic characterization of the processes is less common.…”

“…1 The experiments have been carried out in three ultra-high vacuum (UHV) chambers, all with a base pressure of 1×10 -10 mbar. In all of them, the molecular powder was sublimated from a homemade Ta crucible annealed at 575 K controlled by a type-K thermocouple spot-welded to it.…”

“…NEXAFS and XPS measurements were performed for all the 3 systems: DiPy[5]DBH on Cu(110), Cu(111) and Au(111) with analogous results, some of them already reported. 1,42 We selected the Cu(111) case for representing the NEXAFS and the Cu(110) for representing the XPS results because higher annealing temperatures have been reached for these substrates. To shed more light into how the temperature-induced nanostructure growth process is produced a series of DFT-based CI-NEB calculations have been carried out.…”

“…In particular, thermally induced (cyclo)dehydrogenation (and/or dehalogenation) of PAHs followed by covalent coupling on transition metal surfaces may result in the synthesis of new zero-, one-and two-dimensional nanoarchitectures. [1][2][3][4][5][6] In the field of on-surface synthesis of new nano-objects from their functional molecular building blocks, the selection of a correctly designed precursor is vital to achieve a desired outcome. In particular, the electronic doping of the resultant structure can be controlled by introducing foreign atoms into the precursor.…”

“…Indeed, our group has shown that the diffusion of the PAH precursor on the surface defines the dimensionality of the reaction products. 1 On one hand, a highly reactive surface such as Pt(111) does not allow surface diffusion, impeaching the interaction between neighbouring molecules, so that thermal activation results in the intramolecular cyclodehydrogenation and the formation of new zerodimensional objects. On the other hand, when a PAH is deposited on a weakly reactive surface such as Au(111), the molecules diffuse at room temperature; upon annealing, they undergo cyclodehydrogenation and dehydrogenative polymerization, and as a consequence, twodimensional polymeric chains form since the diffusion allows the molecules to meet and react with each other.…”