On-surface cyclodehydrogenation reaction pathway determined by selective molecular deuterations.

Area of Science:

• Materials Science
• Organic Chemistry
• Surface Science

Background:

• Directed synthesis of polycyclic aromatic hydrocarbons relies on understanding dehydrogenative C_aryl-C_aryl coupling.
• On-surface synthesis offers precise control over nanostructure formation.

Purpose of the Study:

• To elucidate the reaction mechanism of cyclodehydrogenation in on-surface synthesis of graphene nanoribbons (GNRs).
• To utilize isotopic labeling for precise pathway identification.

Main Methods:

• On-surface synthesis of GNRs using selectively deuterated precursors on Au(111).
• Isotopic labeling (deuterium) to track hydrogen movement.
• In situ mass spectrometry to analyze gas-phase by-products (H₂, HD, D₂).
• Nudged elastic band simulations to support proposed mechanisms.

Main Results:

• Identified a specific hydrogen/deuterium (H/D) pattern in seven-atom-wide armchair GNRs.
• Observed distinct H/D ratios in by-products, indicating a hydrogen shift during C_aryl-C_aryl coupling.
• Elucidated the pathway involving conrotatory electrocyclization and a [1,9]-sigmatropic deuterium shift.

Conclusions:

• Clarified the cyclodehydrogenation mechanism in GNR synthesis.
• Demonstrated a strategy for designing on-surface reactions to create nanographenes with specific isotope patterns.
• Opened avenues for precise control over nanostructure synthesis.