Deciphering Substrate-Driven Hierarchical Self-Assembly of 1,3,6,8-Tetrabromopyrene (Br4Py): First-Principles Insights into Interfacial Halogen Bonding and Strain-Mediated Epitaxy

The hierarchical self-assembly of 1,3,6,8-tetrabromopyrene (Br4Py) into two-dimensional monolayers was systematically elucidated through first-principles calculations, revealing complementary Br–H hydrogen bonding and Br–Br halogen interactions as synergistic driving forces. During the assembly process, three distinct molecular chain intermediates were identified, which further organize into two thermodynamically stable monolayer configurations with nearly identical binding energies. Structural parameters derived from CASTEP simulations exhibit excellent agreement with experimental scanning tunneling microscopy (STM) data, with deviations below 6% in lattice constants (b/c = 2.170 nm) and Br–H bond lengths (0.323 nm). Detailed electron density analysis quantifies the competitive nature of intermolecular interactions, showing charge accumulation at Br–H bonding regions and depletion zones in Br–Br repulsive domains. Notably, substrate-mediated strain effects from Au(111) induce lattice distortions of up to 5.8%, underscoring the critical influence of surface-molecule coupling in dictating final configurations. This computational-experimental correlation establishes a mechanistic framework for designing substrate-selective functional materials via halogen-mediated self-assembly, with potential applications in surface-confined molecular electronics or photonic materials.