Surface Patterning of Benzenecarboxylic Acids: Influence of Structure, Solvent, and Concentration on Molecular Self-Assembly

We investigated the molecular self-assembly of pyromellitic acid (1,2,4,5-benzenetetracarboxylic acid), trimellitic acid (1,2,4-benzenetricarboxylic acid), and 1,3,5-benzenetriacetic acid at the liquid/graphite interface to assess how the number and orientation of carboxylic acid groups modulate network formation. In addition, we studied the roles solvent composition and solution concentration play in the self-assembly of pyromellitic acid and trimellitic acid by investigating each in various alkanoic acid/alkanol solvents and solvent mixtures under saturated and diluted conditions. For pyromellitic acid, three distinct, ordered monolayer structures were observed, depending on the solvent type and solution concentration. Trimellitic acid was observed to form highly disordered monolayers due to the molecule’s inherent asymmetry, whereas high-symmetry networks are observed from the symmetric building block pyromellitic acid. No solvent or concentration effects were observed for trimellitic acid monolayers. Self-assembly was not observed for 1,3,5-benzenetriacetic acid, likely due to an insufficient adsorption energy and difficulty in forming extended hydrogen-bonded networks because of its nonplanar molecular structure. Density functional theory geometry optimizations and relaxed potential energy surface scans were used to explore the conformational preferences of isolated analogue molecules, benzoic acid and phthalic acid (1,2-benzenedicarboxylic acid), providing a qualitative picture of surface adsorption and network formation. The observed network structures and solvent/concentration effects are rationalized using basic principles of thermodynamics and equilibrium.