Structures, Properties, and Host-Guest Chemistry of P(O)-Bridged Pillar[n]arenes: A DFT Study

The structural construction and application development of pillararenes have been hot topics in supramolecular chemistry. To enrich the pillararene chemistry, this work proposed a new type of heteroatom-bridged pillararene molecules: phosphorus- (P-) and phosphoryl- (PO-) bridged pillararenes. Density functional theory (DFT) calculations were used to systematically predict and analyze the structures, properties, and host-guest complexes of P(O)-bridged pillar[n]arenes (PnP and PnPO, n = 4-7). Medium-sized PnP and PnPO maintained a prismatic molecular skeleton containing an ordered network of H-bonds. Compared with the parent pillar[n]arene, the substitution of P(O) groups significantly increased their cavity size and reduced the energy levels of HOMO and LUMO orbitals due to elongated P-C bonds and σ*-π* conjugation effects. The change of bridging groups affected the position of the maximum absorption peak of pillar[n]arenes; the absorption spectrum had a significant redshift, and the solvation effect in polar solvents had also been improved. PnP and PnPO preferred to form host-guest complexes with electron-deficient guests. Due to the electron-withdrawing property of phosphoryl groups, the inclusion capacity of PnPO toward neutral molecules was obviously stronger than that of PnP. These properties were expected to provide new ideas for expanding the pillararene chemistry.