The role of aromaticity in the cyclization and polymerization of alkyne-substituted porphyrins on Au(111)

Abstract Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, in on-surface chemistry – where the interaction with the substrate can significantly alter the electronic and geometric structure of the adsorbates – the role of aromaticity is not understood. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity of the reactions by scanning tunneling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, revealing a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights will help to design and understand reactions in on-surface chemistry and heterogeneous catalysis.