On-Surface Polymerization of 1,6-Dibromo-3,8-diiodpyrene - A Comparative Study on Au(111) Versus Ag(111) by STM, XPS, and NEXAFS

The surface chemistry of 1,6-dibromo-3,8-diiodopyrene (Br₂I₂Py) is comparatively studied on Au(111) versus Ag(111) surfaces under ultrahigh vacuum conditions by a combination of high-resolution scanning tunneling microscopy (STM) and X-ray spectroscopy. The chemical state of the molecular networks, that is, the dehalogenation and the possible formation of organometallic intermediates, is assessed by X-ray photoelectron spectroscopy. In addition, pyrene tilt angles are quantified by carbon K-edge near edge X-ray absorption fine structure experiments. Upon room-temperature (RT) deposition of Br₂I₂Py onto Au(111), only partial deiodination was found, and STM revealed the coexistence of ordered arrangements of both intact Br₂I₂Py molecules and organometallic dimers as well as few larger aggregates. Further annealing to 100 °C triggered full deiodination followed by the formation of organometallic chains of otherwise still brominated molecules. By contrast, on Ag(111), iodine is fully and bromine is partly dissociated upon RT deposition of Br₂I₂Py. The initially disordered organometallic aggregates can be reorganized into more ordered structures by mild annealing at 125 °C. Yet, the conversion of the organometallic intermediates into well-defined cross-linked quasi 2D covalent networks was neither possible on Au(111) nor on Ag(111). This is attributed to the large steric hindrance in the covalently linked adsorbed state.