Modulation of halogen-bonded 2D self-assemblies of benzothiadiazole derivative: Concentration and solvent effects

Halogen bonding (XB) with definite directivity and moderate strength like hydrogen bonding has been widely utilized in medical science, organic optoelectronics, and crystal engineering. Whereas, the mechanisms of hetero-halogen bonding (Br···S) induced 2D supramolecular self-assembly are still poorly understood. In this paper, an asymmetrical Br-substituted thiophene-based benzothiadiazole derivative (BTZ-Br) was selected as a building block to explore the self-assembly behaviors driven by XBs using scanning tunneling microscopy (STM). Besides, the short-chain solvents (heptanoic acid and nonanoic acid) and long-chain solvents (n-tridecane and n-tetradecane) were used to study the solvent effect on the BTZ-Br's assemblies. The BTZ-Br molecules could form frame-like patterns on graphite in all solvents at relatively high concentrations. Whereas, at lower concentration, bamboo-like and linear arrangements were observed in short-chain solvents and long-chain solvents, respectively. Such structural difference is partially caused by the co-absorption of long-chain solvents, which changes the intermolecular bonding mode and stabilizes the molecular arrays via van der Waals forces between BTZ-Br and n-alkanes. Density functional theory (DFT) calculation further confirms the important role of XBs during assembling. This work is helpful for the in-depth understanding of XBs and may provide feasible guidance for the fabrication of novel 2D textures via XBs.