Driving Multi-Step Regioselectivity in On-Surface Polymer Synthesis by Molecular Coverage

On-surface synthesis has revolutionized the design of low-dimensional organic nanomaterials, introducing unprecedented families of compounds while offering precise control over their structural and functional properties. A critical challenge in this field is achieving regioselective control over covalent bond formation, which is essential for tailoring polymerization processes. Here, we demonstrate a novel approach toward regioselective polymerization on Au(111) using a dicyclopentaanthracene precursor, an acene derivative that comprise two terminal indene moieties. Through a combination of scanning tunneling microscopy, non-contact atomic force microscopy, and density functional theory calculations, we reveal a molecular coverage-dependent regioselective homocoupling mechanism. At low coverage, one-dimensional polymers formed via indenyl couplings in anti-configuration dominate, involving both five-membered rings of the precursor. Increasing the coverage to half a monolayer shifts the regioselectivity, yielding a quasi-1D staircase polymer through indenyl couplings that engage only one five-membered ring of each precursor. At higher coverages, a two-dimensional porous network emerges, driven by the activation of the four benzylic positions of the molecular precursor. Our findings highlight the ability to steer regioselectivity and, consequently, polymer dimensionality by controlling molecular coverage, significantly advancing the fields of on-surface synthesis and polymer science.