Trimesic Acid Molecule in a Hexagonal Pore: Central versus Noncentral Position

Self-assembly of trimesic acid (TMA) molecules into the honeycomb structure with filled pores and the resulting host-guest chemistry are studied by the density functional theory (DFT) and Monte Carlo (MC) simulations. The DFT calculations demonstrate that a guest TMA molecule prefers a noncentral position in a relaxed hexagonal pore formed of six TMA molecules, and it is binded by two intermolecular interactions. The symmetric central position of the guest molecule is energetically favorable only in the honeycomb structure, which is compressed by more than 3%. Based on the estimated host-guest dimeric interactions, a model is proposed to identify the conditions for central and noncentral positioning of TMA molecules within the pore during their ordering into the honeycomb structure with partly filled pores. The MC simulations reveal that increase of the molecule-substrate interaction in the center of the pore or interactions of the central molecule with the cage molecules have a significant effect in preserving the central position of the guest molecule. However, if these interactions are not too strong, the noncentral position is favored due to multiplicity of noncentral arrangements in a hexagonal cage.