Theoretical prediction of structures and inclusion properties of heteroatom-bridged pillar[n]arenes

In this paper, the heteroatom-bridged pillar[n]arene derivatives, namely bora-, aza-, and oxa-pillar[n]arenes (PnX, X = B, N, and O respectively, n = 4 - 6) are studied by quantum chemical calculations at the omega B97XD/6-311G(d,p) level of theory. The geometries, energetics, electronic structures, absorption spectra, solvent effects, and the inclusion complexation with paraquat dication are discussed in detail. The calculated results show the structures of pentamers (P5X) are more stable compared with tetramers (P4X) and hexamers (P6X) and the PnB have relatively loose and irregular geometry structures. The molecular cavities of PnX are electron-rich and capable of accommodating cationic guests. Absorption spectra of PnN and PnO are very similar to those of Pn with one intense peak (lambda(max)) at around 260 nm, while PnB indicates another characteristic peak between 300 and 400 nm. P5X can form inclusion complexes of 1:1 stoichiometry with paraquat dication, and P5N exhibits strongest combination ability with paraquat dication among them. These thorough understanding of the structure and properties of heteropillar[n]arenes will broaden the designs, syntheses, and applications of pillararenes.