Molecular interactions between pillar[5]arene and bis(pyridinium) derivatives

Electronic structure of pillar[5]arene (P5) host and its interactions with a series of bis(pyridinium) guests, namely, 1,2-ethylenedipyridine (‘g1’), 1,2-propylenedipyridine (‘g2’), 1,2-butylenedipyridine (‘g3’), 1,2-pentamethylenedipyridine (‘g4’) and 1,2-hexamethylenedipyridine (‘g5’) are analyzed using density functional theory. The lowest energy conformer of the P5 host was obtained by alternate rotation of two phenolic units which leads to extended intra-molecular hydrogen bonding network. Molecular electrostatic potential reveals electron-rich regions in P5 are largely localized near flipped hydroquinone monomers. Theoretical calculations predicted encapsulation of one or two pyridinium rings depends on the alkyl chain length in bis(pyridinium) guests. The presence of solvent (DMSO or acetone) brings about reversal of conformational stability in ‘g4’- and ‘g5’ ⊂ P5 complexes compared to gas phase data. Host–guest binding pattern of bis(pyridinium) guests reflects in 1H NMR chemical shifts. The methylene- and α-protons encapsulated in the host cavity emerge with up-field signals in 1H NMR spectra of the complex while protons interacting with portals engender down-field signals.