Dehalogenation and Coupling of a Polycyclic Hydrocarbon on an Atomically Thin Insulator

Abstract

Catalytic activity is of pivotal relevance in enabling efficient and selective synthesis processes.Recently, covalent coupling reactions catalyzed by solid metal surfaces opened the rapidly evolving field of on-surface chemical synthesis. Tailored molecular precursors in conjunction with the catalytic activity of the metal substrate allow the synthesis of novel, technologically highly relevant materials such as atomically precise graphene nanoribbons. However, the reaction path on the metal substrate remains unclear in most cases and the intriguing question is how a specific atomic configuration between reactant and catalyst controls the reaction processes. In this study, we cover the metal substrate with a monolayer of hexagonal boron nitride (h-BN), reducing the reactivity of the metal, and gain unique access to atomistic details during the activation of a polyphenylene precursor by sequential dehalogenation and the subsequent coupling to extended oligomers. We use scanning tunneling microscopy (STM) and density functional theory (DFT) to reveal a reaction site anisotropy, induced by the registry mismatch between the precursor and the nano-structured h-BN monolayer. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 An atomically thin layer of insulating h-BN is a structurally analogous counterpart for graphene -a single layer of sp 2 -hybridised carbon atoms 1 -matching the graphene lattice almost perfectly with a small mismatch of approx. 2%. Currently, the fabrication of two-dimensional materials follows two main approaches: i) the bottom-up synthesis by substrate supported chemical vapor deposition (CVD) with suitable precursors and ii) the top-down approach by exfoliation. The assembly of graphene/h-BN heterostructures, leading to novel devices or devices with enhanced performance, usually relies on elaborate, sequential transfer processes of the produced layers. 2 The main obstacles are possible misalignment, introduction of defects and contaminations resulting from transferring layers that are just one atom thick. Only recently, the direct CVD growth of graphene on h-BN was demonstrated, offering superior properties. [3][4][5] However, the growth conditions are harsh (long exposure time, high temperatures, several cycles, etc.). Metal substrates are favored, because of their high catalytic activity, 6 but they have the disadvantage that they strongly alter the properties of the grown layers, which therefore cannot be directly used for electronic device fabrication.A common motif in on-surface chemical reactions is the activation of a precursor, the intermittent complex formed between precursor and substrate, and finally the coupling reaction. 7A long-standing question is the impact of the specific atomic configuration between substrate and reactant on the catalytic efficiency and how does the specific atomic arrangement chan...

Side-functionalized two-dimensional polymers synthesized via on-surface Schiff-base coupling

“…To confirm the formation of oxazoline network we have 10 performed total reflection infrared spectroscopy (ATR-FT IR) on the sample prepared at 75°C and 205°C, respectively, as illustrated in Figure S6. In the spectrum obtained on the sample prepared at 75°C the peak at 1650 cm -1 is attributed to the vibration of imine C=N, while for the sample annealed at 205°C, 15 the peak red shift to 1676 cm -1 , which is in good agreement with previously reported N=CO vibration of oxazoline group. [40][41] The peak at 1170 cm -1 is attributed to the vibration of CO bond of the phenol group, appears for both samples, also agrees with the conclusion that not all imine groups were transformed to 20 oxazoline even after annealing at 205°C.…”

“…[1][2][3][4][5][6] Although on-surface synthesis, using a solid surface as template, has been proved to be an efficient approach toward such intriguing materials, the realization of 15 long-range ordered two-dimensional polymer with low defect density still remains a foremost challenge for surface scientists. [7][8] Up to now, different types of two-dimensional polymers, or put to another term, surface covalent organic frameworks (surface COFs), based on diverse chemical reactions have been 20 accomplished through on surface synthesis via different reactions.…”

Chemical Reactions on Metal-supported Hexagonal Boron Nitride Investigated with Density Functional Theory

“…2) molecules on the nanomesh, we have conducted DFT calculations in order to better understand the experimental observations. [37] Here we discuss further aspects of the DFT results. respectively, and E nm and E dnm are the total energies of pristine and defective nanomesh.…”

Molecular assembly on two-dimensional materials

“…A number of molecules such as C 60 [57], naphthalocyanine [83], CuPc [62,78], H 2 Pc [78], CoPc [79], porphine [80], 5,5',5",5"',5"",5""'-hexaiodocyclohexa-m-phenylene (I 6 CHP) [84], TCNQ [80] selectively adsorb on the pores of the hBN surface. Similar to the case of graphene, the adsorption mechanism is related to the presence of in-plane electric field due to the strong work function modulation.…”

“…The difficulties in using the radical coupling on surfaces other than bulk metals are highlighted by Dienel et al [84]. They describe the reactivity of 5,5',5",5"',5"",5""'-hexaiodocyclohexa-m-phenylene (I 6 CHP) on epitaxial hexagonal boron nitride on Rh(111) ( Fig.…”