A pillar[5]arene prepared from monomer 1 has four constitutional isomers. Although these constitutional isomers have the same molecular formula, their other properties that we have investigated are more or less different. They have different melting points, different NMR spectra and different crystal structures. Furthermore, the corresponding four anionic constitutional pillar[5]arene isomers derived from these four constitutional isomers showed different binding abilities with paraquat G in water. The association constants of 3a⊃G, 3b⊃G, 3c⊃G and 3d⊃G were determined to be (3.33±0.46)×10⁴ L·mol⁻¹, (5.00±0.63)×10⁴ L·mol⁻¹, (3.33±0.28)×10⁴ L·mol⁻¹, and (1.43±0.09)×10⁴ L·mol⁻¹, respectively.

A pillar[5]arene[1]quinone and a difunctionalized pillar[6]arene were synthesized. Their application in host–guest chemistry was investigated by using 1-adamantanylammonium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (3), which possesses a weakly coordinating counteranion, as a model guest. These hosts showed different binding affinities for 3 with different association constants.

A new pillar[5]arene-based heteroditopic receptor containing a urea anion recognition site was designed and made. It was demonstrated that the introduction of the urea group remarkably promoted the binding affinity to n-octyltriethylammonium salts with different counterions in chloroform, as the corresponding monotopic host, 1,4-dimethoxypillar[5]arene, showed weak complexation with these guests. The important role of the urea unit was investigated and it was shown that the urea group and the macrocyclic unit not only provided positioned binding sites for the guests but also acted cooperatively in binding these guests.

All the previously reported supramolecular polymers based on crown ether-based molecular recognition have been prepared in anhydrous organic solvents. This is mainly due to the weakness of crown ether-based molecular recognition in the presence of water. Here we report a linear supramolecular polymer constructed from a heteroditopic monomer in an aqueous medium driven by crown ether-based molecular recognition through the introduction of electrostatic attraction. In addition, the reversible transition between the linear supramolecular polymer and oligomers is achieved by adding acid and base. This study realizes the breakthrough of the solvent for supramolecular polymerization driven by crown ether-based molecular recognition from anhydrous organic solvents to aqueous media. It is helpful for achieving supramolecular polymerization driven by crown ether-based molecular recognition in a completely aqueous medium.

Efficient host–guest complexation and interesting self-assembled structures formed between two crown ether-based cryptands and a 1,2-bis(4-pyridinium)ethane derivative 3 are reported. By self-assembly of cis-dibenzo-24-crown-8-based cryptand 1 and guest 3, a [3]pseudorotaxane was formed in solution, which further formed a supramolecular poly[3]pseudorotaxane structure in the solid state driven by π-π stacking interactions. Meanwhile, a [2]pseudorotaxane, obtained from self-assembly of a bis(m-phenylene)-32-crown-10-based cryptand 2 and guest 3, can form a supramolecular poly[2]pseudorotaxane structure in the solid state. This difference in the binding model reflects the diversity of host–guest chemistry of crown ether-based cryptands. Furthermore, these host–guest recognition processes and self-assembled structures were fully characterized by ¹H NMR, UV/Vis spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray analysis. Interestingly, formation of the [3]pseudorotaxane between cryptand 1 and guest 3 can be reversibly controlled by adding and removing potassium cations in acetone. This reversible complexation process provides a simple on/off mechanism that can be used in the construction of controllable molecular switches.

Cavity-extended pillar[5]arenes containing electron-rich naphthyl groups have been demonstrated to have enhanced binding affinity to linear guests containing electron-deficient pyridinium units. The importance of size effect, charge density, cooperative effect, and C–H···π interactions were investigated, and these factors play significant roles in the complexation of these host–guest systems.

The facile and efficient preparation of pillar[n]arenes (n = 5 or 6) was achieved by cyclooligomerization of 2,5-dialkoxybenzyl alcohols or 2,5-dialkoxybenzyl bromides with an appropriate Lewis acid catalyst at room temperature. The mechanism for this cyclooligomerization is presumed to be a Friedel–Crafts alkylation.

2,2′-Dihydroxy-bis(m-phenylene)-32-crown-10 (2,2′-dihydroxy-BMP32C10, 1a) was synthesized and used to prepare the [2]catenane 4 in an unexpected yield of 68 %, three times the corresponding value for the case in which BMP32C10 (1b) was used and close to the corresponding value for the case in which bis(p-phenylene)-34-crown-10 was used. This indicated that 1a and paraquat derivatives formed pseudorotaxanes rather than the previously reported “taco complexes” between BMP32C10 and paraquat derivatives. Another unique feature of 1a in relation to other previously reported BMP32C10 derivatives was that its binding to paraquat derivatives in solution could be switched off and back on by addition of K⁺ and then dibenzo-18-crown-6. In the solid state, a 2:1 [3]pseudorotaxane of 1a with a paraquat derivative was formed.

A bis(1,2,3-phenylene) cryptand has been synthesized and used to prepare 1:1 complexes with paraquat and diquat, with association constants of 2.2 × 10³M^–1 and 3.7 × 10³M^–1, respectively, in CHCl₃/CH₃CN (1:1). In the solid state this cryptand forms a taco complex with paraquat, which has never been found before in cryptand/paraquat complexes. Furthermore, its binding to paraquat and diquat in solution can be switched off (and back on) by addition of acid or K⁺ (and then base or 18-crown-6).

A novel bis(m-phenylene)-26-crown-8-based lariat ether (i.e., 3b) was synthesized and characterized. It can bind paraquat derivatives more strongly than bis(m-phenylene)-26-crown-8 in solution. It forms pseudorotaxanes with two paraquat derivatives in the solid state. N-Methyl substitution was found to play an important role on the binding strength of lariat ether 3b. Furthermore, due to the introduction of two benzyloxy groups, its binding to paraquat derivatives can be switched off (and back on) by adding K⁺ (and then dibenzo-18-crown-6), and the disassociation percentage depends on the concentration of the added K⁺ ions.

The complexation of tightly ion-paired divalent salts such as paraquat dichloride by cryptands based on crown ethers can be improved by the introduction of ion-pair recognition as a means of also binding the counteranions. A series of diamide-based cryptands derived from bis(m-phenylene)-[32]crown-10 and designed to complex the bipyridinium dication with anion assistance was synthesized. The ion-pair recognition process was fully characterized by ¹H NMR spectroscopy, UV/Vis spectroscopy, electrospray ionization mass spectrometry and single crystal X-ray analysis. ¹H NMR spectroscopy demonstrated that these new heteroditopic cryptand hosts can complex both the positive and negative components of the paraquat dichloride salt. UV/Vis spectroscopy showed that the addition of chloride anion into equimolar solutions of cryptands 3 c or 3 g with paraquat bis(hexafluorophosphate) salt (2 a) improves the binding of the cryptands to the paraquat guest. Electrospray ionization mass spectrometry and single-crystal X-ray analysis confirmed the 1:1 stoichiometries and ion-pair recognition of these cryptand/paraquat complexes. It was found that the cryptand 3 g, with 13 atoms and an isophthalamide moiety in the third chain, exhibited the best binding affinity for tightly ion-paired paraquat dichloride (2 b), due to the combination of its spatial compatibility and additional anion-binding site.

Based on the dibenzo-24-crown-8/1,2-bis(pyridinium)ethane recognition motif, a hyperbranched mechanically interlocked polymer was prepared by polyesterification of an easily available dynamic trifunctional AB₂ pseudorotaxane monomer. It was characterized by various techniques including ¹H NMR, COSY, NOESY, GPC, viscosity, TGA, dynamic laser light scattering, AFM, and SEM. Its GPC M_n was determined to be 191 kDa with polydispersity 1.7 and its hydrodynamic diameter in a dilute solution in acetone was about 70 nm. This measured M_n value corresponds to about 93 repeating units. The study reported here presents not only a new polymer topology but also a novel and convenient way to prepare mechanically interlocked polymers. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4067–4073, 2010

The successful preparation of the first bis(m-phenylene)-32-crown-10/paraquat [2]rotaxane showed unambiguously that psuedorotaxane-type complexation, rather than the taco-complex-type complexation previously observed in the solid state and in solution, exists – in solution – for complexation between bis(m-phenylene)-32-crown-10 derivatives and paraquat derivatives.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The complexation between dibenzo-24-crown-8 (1) and diquat (2) was investigated in detail by NMR, MS and X-ray analysis. It was found that dibenzo-24-crown-8 and diquat formed a 1:1 complex 1·2 in acetone with K_a=2.0×10² L·mol⁻¹, but, as shown by X-ray analysis, a crystalline 2:1 host:guest inclusion complex 1₂·2 was isolated, in which a single molecule of diquat is enclosed in the concave cavity provided by two dibenzo-24-crown-8 host molecules. Both results are different from the previously assumed stoichiometry of the complexation between dibenzo-24-crown-8 and diquat. This result enriches the range of host-guest complexes based on dibenzo-24- crown-8 and provides new opportunities for developing more complicated structures and chemosensors for diquat.

Two [3]rotaxanes were synthesized from two bis(m-phenylene)-32-crown-10-based cryptands and a bisparaquat derivative by using a threading-followed-by-stoppering method. As a result of strong association and positive cooperative complexation between the cryptands and the bisparaquat derivative, high yields and high selectivities were achieved. No [2]rotaxanes were found during the preparation.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)