Theoretical study of the physisorption of organic molecules on conjugated microporous polymers: the critical role of skeleton structures on binding strength

We present a computational study of the physisorption of benzene and its derivatives on a series of planar conjugated microporous polymers (CMPs) composed of alternative phenylene and ethynylene units, using a density-functional tight-binding method with a dispersion correction. We focus on the significant role of the skeleton structures on the binding strength, which is one of the key factors determining both the adsorption performance and adsorption selectivity in experiments. Our calculations show that the meta-oriented phenylene moiety in the node demonstrates a stronger binding energy to organic molecules than the para-oriented phenylene unit in the linker. Consistent with previous experimental findings, compared with CMP networks with a sole meta-oriented phenylene moiety, the existence of the para-oriented phenylene unit in the linker of the CMP frameworks will lead to smaller average binding energies to benzene derivatives. Compared to benzene, the benzene derivatives (phenol, aniline, and nitrobenzene) exhibit stronger physisorption. We further find that by enlarging the size (area) of the linker or adding substituent groups in the node, the binding energy between the CMPs and adsorbates will increase significantly, which contributes to a better adsorption performance demonstrated in experiments. Our calculations not only deepen the understanding of the physisorption mechanism between aromatic molecules and CMP networks, but also provide theoretical guidance for the rational design of novel CMP superhydrophobic materials for adsorption/separation of organic pollution from water.