Anion-modulated self-assembly and on-surface polymerization of 2-Bromothiophene on Au(111)

We report the anion-modulated self-assembly and electrochemical polymerization of 2-bromothiophene (BrT) on Au(111), probed by cyclic voltammetry and in situ scanning tunneling microscopy (STM). BrT forms ordered adlayers with distinct unit cells—(3 × 3), (3 × 2√3), (4 × √31), and (√7 × √13)—whose packing density depends on the applied potential and electrolyte anion. In H₂SO₄, bisulfate coadsorption stabilizes open, dynamic BrT structures, whereas in HClO₄, weak perchlorate adsorption enables compact packing at higher potentials. At anodic bias (≥ 0.9 V verse Ag/AgCl), BrT undergoes irreversible oxidative coupling, yielding surface-bound oligothiophenes aligned with Au(111) crystallography. The dual S/Br anchoring of BrT ensures stable adsorption, preferential alignment, and robust film growth. These results reveal a critical role of anion–molecule interactions in directing surface assembly and demonstrate a tunable pathway for potential-controlled nanostructure fabrication.