Selective decarboxylative dimerization enabled by a supramolecular template on Ag(111).

On-surface synthesis with high selectivity remains a central challenge in surface-confined organic reactions. This study investigates the decarboxylative coupling reactions of heptaazaphenalene-tribenzoic acid (HTBA) molecules on the Ag(111) surface and demonstrates that a thermally persistent supramolecular ribbon assembly can strongly bias the reaction pathway. Although HTBA contains three chemically similar carboxyl groups, sequential annealing leads predominantly (>90%) to a single dumbbell-shaped dimer. Temperature-dependent STM measurements combined with in situ heating directly reveal that anisotropic steric confinement and cooperative inter-ribbon displacement within the pre-organized assembly play a key role in directing this selectivity. Control experiments using pseudo-high dilution conditions suppress dimer formation, highlighting the decisive influence of molecular templating. This study offers mechanistic insight into how temperature-stable supramolecular organization can regulate reaction selectivity in multi-functional molecules on surfaces.