The surface-mediated synthesis of epitaxially aligned and separated polyphenylene lines on Cu(110) by exploiting the Ullmann dehalogenation reaction is reported. Scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) show that the C-I bonds of 1,4-diiodobenzene and 1,3-diiodobenzene (C(6)H(4)I(2)) are catalytically cleaved when dosed onto the surface. Subsequent annealing transforms the copper-bound phenylene intermediates into covalent conjugated structures: linear chains of poly(p-phenylene) for 1,4-diiodobenzene and zigzag chains of poly(m-phenylene) as well as macrocyclic oligomers in the case of 1,3-diiodobenzene. The chains are strongly bound to the surface (likely through C--Cu bonds at the chain-ends) while the macrocycles are very mobile and can only be imaged by STM at low temperature. The detached halogens adsorb on the surface and separate the polymer chains from each other.

We provide insight into surface-catalyzed dehalogenative polymerization, analyzing the organometallic intermediate and its evolution into planar polymeric structures. A combined study using scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED), near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and first-principles calculations unveils the structural conformation of substrate-bound phenylene intermediates generated from 1,4-dibromobenzene precursors on Cu(110), showing the stabilizing role of the halogen. The appearance of covalently bonded conjugated structures is followed in real time by fast-XPS measurements (with an acquisition time of 2 s per spectrum and heating rate of 2 K/s), showing that the detaching of phenylene units from the copper substrate and subsequent polymerization occur upon annealing above 460 ± 10 K.

We report the synthesis of extended two-dimensional organic networks on Cu(111), Ag(111), Cu(110), and Ag(110) from thiophene-based molecules. A combination of scanning tunnelling microscopy and X-ray photoemission spectroscopy yields insight into the reaction pathways from single molecules towards the formation of two-dimensional organometallic and polymeric structures via Ullmann reaction dehalogenation and C-C coupling. The thermal stability of the molecular networks is probed by annealing at elevated temperatures of up to 500 °C. On Cu(111) only organometallic structures are formed, while on Ag(111) both organometallic and covalent polymeric networks were found to coexist. The ratio between organometallic and covalent bonds could be controlled by means of the annealing temperature. The thiophene moieties start degrading at 200 °C on the copper surface, whereas on silver the degradation process becomes significant only at 400 °C. Our work reveals how the interplay of a specific surface type and temperature steers the formation of organometallic and polymeric networks and describes how these factors influence the structural integrity of two-dimensional organic networks.

On-surface covalent self-assembly of organic molecules is a very promising bottom–up approach for producing atomically controlled nanostructures. Due to their highly tuneable properties, these structures may be used as building blocks in electronic carbon-based molecular devices. Following this idea, here we report on the electronic structure of an ordered array of poly(para-phenylene) nanowires produced by surface-catalysed dehalogenative reaction. By scanning tunnelling spectroscopy we follow the quantization of unoccupied molecular states as a function of oligomer length, with Fermi level crossing observed for long chains. Angle-resolved photoelectron spectroscopy reveals a quasi-1D valence band as well as a direct gap of 1.15 eV, as the conduction band is partially filled through adsorption on the surface. Tight-binding modelling and ab initio density functional theory calculations lead to a full description of the band structure, including the gap size and charge transfer mechanisms, highlighting a strong substrate–molecule interaction that drives the system into a metallic behaviour.