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

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.