[2]Pseudorotaxanes based on the cryptand/monopyridinium recognition motif

The first cryptand/monopyridinium [2]pseudorotaxanes were prepared from five bis(m-phenylene)-32-crown-10- and one bis(m-phenylene)-26-crown-8-based cryptand hosts and three monopyridinium guests. These pseudorotaxanes were studied by proton NMR spectroscopy, mass spectrometry, and X-ray crystallography. Association constants ranged from 141 M⁻¹ to 1.86×10⁴ M⁻¹ in 1:1 acetone: chloroform at 22 °C.     

[3]Pseudorotaxanes based on the cryptand/monopyridinium salt recognition motif

The first cryptand/monopyridinium salt [3]pseudorotaxanes were prepared from two cryptand hosts and two bispyridinium guests as confirmed by proton NMR characterization, electrospray ionization mass spectrometry, and X-ray analysis. It was found that the two monopyridinium binding sites are independent of each other for the formation of one [3]pseudorotaxane.     

A bis(m-phenylene)-32-crown-10-based fluorescence chemosensor for paraquat and diquat

A bis(m-phenylene)-32-crown-10-based host to which are covalently attached two pyrene groups as fluorescence chromophores was designed and synthesized. Its complexations with paraquat (PQ) and diquat (DQ) were studied by proton NMR, ESI mass spectrometry, and UV-vis spectroscopy. Its chemosensor behavior to PQ and DQ was revealed by fluorescence emission spectroscopy. This new host can function as a fluorescence chemosensor for PQ and DQ due to the inhibition of photoinduced electron transfer between the bis(m-phenylene)-32-crown-10 moiety and the pyrene groups by the addition of PQ (or DQ).     

An acid-base adjustable pseudocryptand-type [2]pseudorotaxane based on a bis(meta-phenylene)-32-crown-10 derivative and paraquat

A pseudocryptand-type [2]pseudorotaxane was formed via the self-assembly of a dipyridyl bis(meta-phenylene)-32-crown-10 (BMP32C10) derivative and a paraquat derivative. Due to the basicity of the pyridyl group, which forms the third pseudo-bridge of the pseudocryptand, this pseudorotaxane possesses two finite acid-base adjustable association constants.     

Efficient syntheses of bis(m-phenylene)-26-crown-8-based cryptand/paraquat derivative [2]rotaxanes by immediate solvent evaporation method

A novel bis(m-phenylene)-26-crown-8-based cryptand has been synthesized. It has been used to prepare two 1:1 complexes with two paraquat derivatives with high association constants (6.5×10⁵ and 4.0×10⁵ M⁻¹) in acetone. In the solid state the cryptand forms a 2:1 threaded structure with paraquat and an interesting supramolecular poly[2]pseudorotaxane threaded structure with a dihydroxyethyl-substituted paraquat derivative, respectively. It has been further used to prepare cryptand/paraquat derivative [2]rotaxanes efficiently by the immediate solvent evaporation method using easily available 3,5-dimethylphenyl groups as the stoppers.     

Host size effect in the complexation of two bis(m-phenylene)-32-crown-10-based cryptands with a diazapyrenium salt

N,N′-Dimethyl-2,7-diazapyrenium bis(hexafluorophosphate) binds a C₃-symmetric bis(m-phenylene)-32-crown-10-based cryptand more strongly than N,N′-dimethyl-4,4′-bipyridinium bis(hexafluorophosphate) partly because of its better fit in size with the cryptand host cavity, while it binds a relatively smaller pyridyl cryptand less strongly due to its worse fit in size with the cryptand host cavity and the lack of hydrogen bonding to the pyridyl nitrogen atom of the host.     

Bis(benzimidazolium)methane salts: a potential guest for dibenzo-24-crown-8 towards [2]pseudorotaxanes

We investigated the threading of bis(benzimidazolium) methane moieties with DB24C8 at 35 °C (room temperature) and found a high degree of association. The presence of threaded complexes was determined by ¹H NMR and also supported by high resolution mass spectrometry and B3LYP/6-31G**++calculations. A 2D NMR study was done to elucidate the host-guest geometry and the molecular interactions present. The variable temperature NMR experiment was done over a temperature range of 243-323 K. The threading was stable and visible even at 323 K. Acid-base equilibrium established the reversibility of the process. DFT optimized structures establish that two H-bonded links are present between O₁?O_4′ and O₃?O_2′ across the dibenzo axis of the crown.     

Bis(meta-phenylene)-32-crown-10-based cryptand/diquat inclusion [2]complexes

Bis(meta-phenylene)-32-crown-10-based cryptands have been proved to complex diquat much more strongly than bis(meta-phenylene)-32-crown-10 itself; in fact, one containing a pyridyl moiety has one of the highest Ka values yet reported.     

An efficient synthesis of 4,4′,5,5′-tetraiododibenzo-24-crown-8 and its highly conjugated derivatives

4,4′,5,5′-Tetraiododibenzo-24-crown-8 (9), a practical building block, was prepared under efficient and mild reaction conditions starting from the simple starting material, catechol (1). Highly conjugated 4,4′,5,5′-tetraethynyldibenzo-24-crown-8 (10a,b) were prepared via a Sonogashira coupling reaction from tetraiodocrown ether 9. These highly conjugated crown ethers form complexes in CD₂Cl₂ with dibenzylammonium hexafluorophosphate in a 1:1 ratio. Emission spectrum of pseudorotaxane 11 shows a dramatic shift from the non-complexed precursor.     

Taco grande: a dumbbell bis(crown ether)/paraquat [3](taco complex)

The first paraquat-based [3](taco complex) was successfully prepared from a linear bis(crown ether) host and paraquat as shown by proton NMR characterization and X-ray analysis. It has a dumbbell shape in the solid state. The two crown ether binding sites are independent of each other during their complexation in solution.     

Pseudocryptand-type [2]pseudorotaxanes based on bis(meta-phenylene)-32-crown-10 derivatives and paraquats with remarkably improved association constants

The first dual component pseudocryptand-type [2]pseudorotaxanes were designed and prepared via the self-assembly of synthetically easily accessible bis(meta-phenylene)-32-crown-10 pyridyl, quinolyl, and naphthyridyl derivatives with paraquat. The formation of the pseudocryptand structures in the complexes remarkably improved the association constant by forming the third pseudobridge via H-bonding with the guest and π-stacking of the heterocyclic units.     

Inclusion [2]complexes based on the cryptand/diquat recognition motif

Seven diquat-based inclusion [2]complexes were studied by proton NMR spectroscopy, electrospray ionization mass spectrometry, and X-ray analysis. The hosts used in these inclusion [2]complexes are bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10, a bis(m-phenylene)-26-crown-8-based cryptand, and five bis(m-phenylene)-32-crown-10-based cryptands. Bis(m-phenylene)-32-crown-10-based cryptands have been proved to be able to complex diquat much more strongly than bis(m-phenylene)-32-crown-10 itself and one containing a pyridyl moiety has one of the highest K_a values reported to date. These hosts form 1:1 complexes with diquat in solution and in the solid state. It was found that the improved binding from bis(m-phenylene)-32-crown-10 to bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 was due to a supramolecular cryptand structure formed by chelation of the two terminal OH moieties of bis(5-hydroxymethyl-1,3-phenylene)-32-crown-10 with a water molecule as a hydrogen-bonding bridge.     

Bis-p-xylyl[26]crown-6/pyridinium ion recognition: one-pot synthesis of molecular shuttles

Simple one-pot syntheses allow the preparation of [2]rotaxane-based degenerate molecular shuttles featuring the recognition of pyridinium ions by BPX26C6 macrocycles. Because of the weak interactions between the BPX26C6 and pyridinium units in the [2]rotaxanes in CD₃COCD₃, the rates of shuttling of the BPX26C6 moieties between the pyridinium stations are rapid, relative to those of DB24C8-based shuttles, on the NMR spectroscopic time scale at ambient temperature.     

Slow-exchange C3-symmetric cryptand/trispyridinium inclusion complexes containing non-linear guests: a new type of threaded structure

Slow-exchange C₃-symmetric inclusion complexes were successfully prepared based on a new cryptand/trispyridinium recognition motif as confirmed by proton NMR spectroscopy, electrospray ionization mass spectrometry, and molecular modeling. These inclusion complexes are the first examples of a new type of threaded structure with non-linear guests.     

Paraquat guest induced hydrogen-bonding modes of a hydrazide-derived bis(meta-phenylene)-32-crown-10 host in the solid state

A hydrazide-derived bis(meta-phenylene)-32-crown-10 host showed a dimeric structure via quadruple N-H?O hydrogen bonds, but a polymeric structure via two N-H?O hydrogen bonds and two C-H?O hydrogen bonds at each knot in the presence of paraquat in the solid state, which led to a novel poly(taco complex) and ordering arrangement of the guest molecules indirectly.     

A chemical-responsive bis(m-phenylene)-32-crown-10/2,7-diazapyrenium salt [2]pseudorotaxane

A chemical-responsive bis(m-phenylene)-32-crown-10/2,7-diazapyrenium salt [2]pseudorotaxane was prepared. It was found to form a supramolecular poly[2]pseudorotaxane in the solid state driven by π-π stacking interactions.     

Novel [2]rotaxanes based on the recognition of pillar[5]arenes to an alkane functionalized with triazole moieties

Host-guest complexation between an alkane functionalized with triazole moieties and three pillar[5]arenes was studied. Three pillar[5]arene-based [2]rotaxanes were constructed based on this new recognition motif. The sequence of the yields of these [2]rotaxanes was consistent with the order of association constants between the three corresponding pillar[5]arenes and the alkane. ¹H NMR, electrospray ionization mass spectrometry, and NOESY NMR were employed to characterize these [2]rotaxanes.     

Preparation of two new [2]rotaxanes based on the pillar[5]arene/alkane recognition motif

Based on the pillar[5]arene/alkane recognition motif, two [2]rotaxanes were successfully prepared. Their formation was confirmed by NMR spectroscopy and ESI mass spectrometry. We also demonstrated that 3,5-dinitrophenyl group and 3,5-bis(trifluoromethyl)phenyl group are big enough to work as stoppers for DPPillar[5]arene, which lays a foundation for the preparation of more complex and functional supramolecular structures.     

Detection of cationic guest molecules by quenching of luminescence of a self-assembled host molecule consisting of terbium(III) and calix[4]arene-p-tetrasulfonates

By self-assembly in aqueous solution, calix- (CAS) and thiacalix[4]arene-p-tetrasulfonate (TCAS) formed luminescent complexes Tb^III·(CAS)₂ and Tb^III·TCAS, respectively, which were utilized as a host for cationic guests. Addition of 1-ethylpyridinium guest quenched luminescence of Tb^III·(CAS)₂ in accordance with the Stern-Volmer (SV) relation with a low detection limit (D.L.) of 5.94 × 10⁻⁸ M (S/N = 3, M ≡ mol dm⁻³). On the other hand, 1-ethylquinolinium quenched luminescence of Tb^III·TCAS most efficiently, affording a very low D.L. (6.71 × 10⁻¹⁰ M). The agreement of the SV coefficients obtained with luminescent intensity (K_SV,all = 6.74 × 10⁶ M⁻¹) and lifetime (K_SV,Tb = 6.50 × 10⁶ M⁻¹) implied that dynamic quenching of ⁵D₄ excited state of Tb^III was predominant in the quenching processes. The quenching rate was estimated to be k_q,Tb = 9.94 × 10⁹ M⁻¹ s⁻¹, which was as fast as diffusion-limited rate. Quenching of Tb^III·(CAS)₂ was also applied to detection of NAD⁺, with a D.L. of 2.78 × 10⁻⁷ M.