Structural characterisation of molecular conformation and the incorporation of adatoms in an on-surface Ullmann-type reaction

Abstract

The on-surface synthesis of covalently bonded materials differs from solution-phase synthesis in several respects. The transition from a three-dimensional reaction volume to quasi-two-dimensional confinement, as is the case for on-surface synthesis, has the potential to facilitate alternative reaction pathways to those available in solution. Ullmann-type reactions, where the surface plays a role in the coupling of aryl-halide functionalised species, has been shown to facilitate extended one- and two-dimensional structures. Here we employ a combination of scanning tunnelling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and X-ray standing wave (XSW) analysis to perform a chemical and structural characterisation of the Ullmann-type coupling of two iodine functionalised species on a Ag(111) surface held under ultra-high vacuum (UHV) conditions. Our results allow characterisation of molecular conformations and adsorption geometries within an on-surface reaction and provide insight into the incorporation of metal adatoms within the intermediate structures of the reaction.

On‐Surface Synthesis of Nitrogen‐Doped Kagome Graphene

“…Their coexistence indicates an incomplete polymerization reaction at this stage and implies that the formation of the Kagome lattice is governed by the formation of organometallic intermediates 2 . This observation is typical of hierarchical Ullman polymerization which has been corroborated in previous works using X‐ray photoelectron spectroscopy [24, 27, 28] . A final annealing to T 3 =220 °C promotes only the N‐KG 3 as observed by AFM (Figure 2 d), indicative of a complete polymerization reaction.…”

“…Compared to the surrounding carbon rings, [4,26] the PZQ moiety is identified by a darker and more elongated contrast as confirmed by AFM simulations ( Figure S4b This observation is typical of hierarchical Ullman polymerization which has been corroborated in previous works using X-ray photoelectron spectroscopy. [24,27,28] A final annealing to T 3 = 220 8C promotes only the N-KG 3 as observed by AFM (Figure 2 d), indicative of a complete polymerization reaction. Phenazine-based molecules and nanographene have shown thermal stability up to 500 8C.…”

“…[31] This suggests a substantial mechanical stress in the Kagome structure as a result of polymerization. [28,32] To assess the internal strain of the Kagome structure, we first relax it using DFT in the gas phase starting from two geometries-flat and twisted ones, characterized by the different torsion angles along the biphenyl-like CÀC bonds (Figure 2 a). In isolated biphenyl, the two benzene rings are known to rotate by about 448 in order to minimize the steric interaction between H-atoms.…”

On‐Surface Synthesis of Nitrogen‐Doped Kagome Graphene

“…In comparison to the debrominated TBQP molecules that always lie flat, twisted PZQ bridges in the N‐KG (Figure 1 d and black arrow in Figure 2 d) are systematically observed using the high vertical precision of AFM measurements [31] . This suggests a substantial mechanical stress in the Kagome structure as a result of polymerization [28, 32] …”

Identification of Topotactic Surface‐Confined Ullmann‐Polymerization

“…We considered coupling of a dimer in which the OM Cu is assumed to be an adatom, but found that ejection of the adatom during coupling involves an excessively high barrier of approximately 2.4 eV. The origin of the source of bridging metal atoms is still unsettled [25] and we thus followed the approach of lifting surface atoms. [12a,26] We found that dimerization along [1 −1 ±1] occurs via two elementary steps (Figure 4a).…”