On‐Surface Ullmann‐Type Coupling: Reaction Intermediates and Organometallic Polymer Growth

Abstract

Ullmann‐type coupling is the most widely used on‐surface reaction to form rationally designed bottom‐up molecular nanoarchitectures. A commonly observed reaction product in this reaction is an organometallic phase, however little is known about the formation of this phase. The on‐surface polymerization of the prochiral precursor 6,12‐dibromochrysene (DBCh) on Ag(111) is studied. Upon annealing of DBCh on Ag(111), a linear organometallic polymer forms. However, the delicate energy balance involved in the polymerization of DBCh is such that, at room temperature, several reaction intermediates, which eventually lead to the formation of the organometallic polymer, can be observed experimentally. Organometallic monomers, dimers, and trimers are finds, that self‐assemble into distinct networks. The experimental availability of these reaction intermediates provides key insights into the formation of the organometallic polymer. Comparing the chirality of the intermediates and the polymer sheds additional light on the reaction mechanism leading to the formation of the polymer. The main finding is that the organometallic polymer is not formed by a simple coupling of the reaction intermediates, but rather requires the breaking and re‐establishing of the C─Ag bonds. Additionally, a Br‐enhanced growth mode is observed, where the split‐off halogens align the polymers, which results in an increased polymer length.

Predicting Organometallic Intermediates in the Surface-Assisted Ullmann Coupling of Chrysene Isomers

“…The good agreement between the results simulated for ch6_12 and the corresponding STM data [28][29][30][31] confirms that simplified directional interaction Monte Carlo modeling is capable of accurately predicting the architecture of organometallic precursors comprising other tectons. This enables the experimental realization of such superstructures.…”

“…We start the discussion with the ch6_12 isomer which corresponds to 6_12-dibromochrysene (DBCh)-a molecule whose Ullmann coupling on metallic substrates has been recently studied experimentally [28][29][30][31]. This previously tested molecule [32] was included here primarily to validate our model using the available experimental data.…”

“…In this study, inspired by the recent experimental report on the Ullmann coupling of dihalogenated chrysene monomers [28][29][30][31], we investigate how our model can predict the structure of metal-organic precursors created by molecules of this type. Chrysene is an intrinsically prochiral molecule that provides the opportunity to study chirality aspects at the nanoscale and fabricate chirality-selective structures, such as graphene nanoribbons.…”

“…Chrysene is an intrinsically prochiral molecule that provides the opportunity to study chirality aspects at the nanoscale and fabricate chirality-selective structures, such as graphene nanoribbons. Due to the high potential for application, previous experimental studies have focused on analyzing the metal-organic and covalent structures that chrysene can form on the surface of copper, gold, and silver [28][29][30][31]. To that purpose, Scanning Tunneling Microscopy (STM) has been used, and the formation of both homochiral (copper) and heterochiral (silver and gold) metal-organic precursor chains has been observed.…”

Kinetics of On-Surface Dechlorinated Aryl–Aryl Coupling

“…In our TP-XPS maps, we observed no shift in the C 1s core level during the heating progression, suggesting the immediate coupling following dechlorination and the absence of the organometallic species on the Au(111) surface postdechlorination. 43 These observations imply that the reaction is not kinetically constrained.…”