On the scalability of supramolecular networks--high packing density vs optimized hydrogen bonds in tricarboxylic acid monolayers.

We present a scanning tunneling microscopy (STM) based study of 1,3,5-tris[4'-carboxy(1,1'-biphenyl-4-yl)]benzene (TCBPB) monolayers at the liquid-solid interface. In analogy to smaller aromatic 3-fold symmetric tricarboxylic acids, this compound was aimed to yield two-dimensional nanoporous networks with large cavities. Depending on the solute concentration, three crystallographically distinct phases with pores of different size and shape were observed on graphite (001) with heptanoic acid as solvent. All three phases have the same dimer motif as basic building block in common. Yet, as opposed to other carboxylic acid assemblies, these dimers are not interconnected by 2-fold O-H...O hydrogen bonds as anticipated, but by two energetically inferior C-H...O hydrogen bonds. Instead of the common head-to-head arrangement, this bonding pattern results in displaced dimers, which allow for higher packing density, and due to their lower symmetry give rise to chiral polymorphs. In accordance with studies of comparable systems, a positive correlation between solute concentration and average surface packing density is identified and rationalized by thermodynamic arguments.