Complex interactions of pillar[5]arene with paraquats and bis(pyridinium) derivatives

The complexation behavior of a series of paraquats (G1 center dot 2PF(6)-G5 center dot 2PF(6)) and bis(pyridinium) derivatives (G6 center dot 2PF(6)-G14 center dot 2PF(6)) with pillar[5] arene (P5A) host has been comprehensively investigated by H-1 NMR, ESI mass and UV-vis absorption spectroscopy. It is found that P5A forms 2 : 1 external complexes with N,N'-dialkyl-4,4'-bipyridiniums (G1-G4 center dot 2PF(6)); while it forms 1 : 1 pseudorotaxane-type inclusion complexes with methylene [-(CH2)(n)-] linked bis(pyridinium) derivatives possessing appropriate chain lengths (n = 3-6, G7-G10 center dot 2PF(6)). Host-guest association constants in dimethyl sulfoxide (DMSO) were determined, indicating G7-G10 center dot 2PF(6) axles form stable [2]pseudorotaxanes with P5A wheel in this very high polarity solvent and 1,4-bis(pyridinium) butane (G8 center dot 2PF(6)) was the most suitable axle unit. Meanwhile, the nature of the substituents attached to 1,4-bis(pyridinium) butane dramatically affects the molecular recognition behavior. The introduction of pyridyls (G13 center dot 2PF(6)) increases not only the K-a value (4.5 x 10(2) -> 7.4 x 10(2) M-1), but also the charge transfer (CT) absorption (colorless -> yellow). Furthermore, the solvent effects have also been investigated, showing they significantly influence the association strength during the course of host-guest complexation. Particularly, the K-a value of P5A-G13 center dot 2PF(6) in 1 : 1 (v: v) acetone-d(6)/DMSO-d(6) is enhanced by a factor of 7.3 compared with pure DMSO-d(6) (7.4 x 10(2) -> 5.4 x 10(3) M-1).