Potential-Dependent Balance between Mobility and Substrate Control in the Electrochemical Self-Assembly of π-Conjugated Molecules on Au(111)

Electrode potential provides a powerful external parameter for directing molecular organization at electrified interfaces, yet how molecular backbone chemistry converts this control into distinct assembly pathways remains poorly understood. Here we investigate the potential-dependent adsorption of two π-conjugated brominated molecules, PPr-4Br and DTPPr-4Br, on Au(111) using in situ electrochemical scanning tunneling microscopy together with voltammetry. Single-crystal X-ray diffraction confirms the planar geometry of the pyrrolopyrrole core in both molecules, enabling extended π-conjugation. Within a low-potential window (0.2–0.4 V vs Ag/AgCl), both molecules adsorb in predominantly flat-lying configurations aligned along the ⟨110⟩ direction of Au(111), forming sparse chain-like adlattices characteristic of π–Au interactions. Upon positive polarization, however, their structural evolution diverges markedly. PPr-4Br retains substantial lateral mobility and reorganizes reversibly into denser coincidence structures with increasing coverage. In contrast, incorporation of thiophene units in DTPPr-4Br strengthens molecule–substrate coupling, producing coverage-insensitive low-density arrangements and inducing premature lifting of the Au(111) reconstruction, ultimately limiting long-range order. These results demonstrate that electrode potential governs electrochemical self-assembly through a competition between admolecular mobility and substrate pinning, and that subtle variations in π-conjugated backbone structure strongly shift this balance. The findings provide direct atomic-scale insight into how molecular design and electrochemical control cooperate to determine interfacial organization of π-conjugated systems on metal surfaces.