On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations

Understanding the reaction mechanisms of dehydrogenative C-aryl-C-aryl coupling is the key to directed formation of pi-extended polycyclic aromatic hydrocarbons. Here we utilize isotopic labeling to identify the exact pathway of cyclodehydrogenation reaction in the on-surface synthesis of model atomically precise graphene nanoribbons (GNRs). Using selectively deuterated molecular precursors, we grow seven-atom-wide armchair GNRs on a Au(111) surface that display a specific hydrogen/deuterium (H/D) pattern with characteristic Raman modes. A distinct hydrogen shift across the fjord of C-aryl-C-aryl coupling is revealed by monitoring the ratios of gas-phase by-products of H-2, HD, and D-2 with in situ mass spectrometry. The identified reaction pathway consists of a conrotatory electrocyclization and a distinct [1,9]-sigmatropic D shift followed by H/D eliminations, which is further substantiated by nudged elastic band simulations. Our results not only clarify the cyclodehydrogenation process in GNR synthesis but also present a rational strategy for designing on-surface reactions towards nanographene structures with precise hydrogen/deuterium isotope labeling patterns.

Automated summary

This study investigates the cyclodehydrogenation reaction mechanism in the on-surface synthesis of graphene nanoribbons using isotopic labeling. A reaction pathway involving conrotatory electrocyclization and a [1,9]-sigmatropic D shift, followed by H/D eliminations, is identified and substantiated by experiments and simulations. The research not only helps clarify the reaction process in GNR synthesis, but also presents a rational strategy for designing precise hydrogen/deuterium isotope labeling patterns in on-surface reactions.