Inclusion Complexation of Diquat and Paraquat by the Hosts Cucurbit[7]uril and Cucurbit[8]uril

Abstract

The binding interactions in aqueous solution between the dicationic guest diquat (DQ(2+)) and the cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) hosts were investigated by (1)H NMR, UV/Vis, and fluorescence spectroscopy; mass spectrometry; single-crystal X-ray diffraction; and electrochemical techniques. The binding data were compared with previously reported results for the related paraquat guest (PQ(2+)). DQ(2+) was found to bind poorly (K=350 m(-1)) inside CB7 and more effectively (K=4.8 x 10(4) m(-1)) inside CB8. One-electron reduction led to increased binding affinity with both hosts (K(r)=1 x 10(4) m(-1) with CB7 and K(r)=6 x 10(5) m(-1) for CB8). While (1)H NMR spectroscopic data revealed that DQ(2+) is not fully included by CB7, the crystal structure of the CB8DQ(2+) complex-obtained from single-crystal X-ray diffraction-clearly establishes its inclusion nature. Overall, both diquat and its one-electron reduced radical cation are bound more effectively by CB8 than by CB7. In contrast to this, paraquat exhibits selectivity for CB7, but its radical cation forms a highly stable dimer inside CB8. These differences highlight the pronounced sensitivity of cucurbit[n]uril hosts to guest features such as charge, charge distribution and shape.

Complexation study and spectrofluorometric determination of the binding constant for diquat and p-sulfonatocalix[4]arene

“…The large value of k DQ for free DQ is consistent with literature values on related systems, and it is well established that many bipyridinium dication/radical cation couples show fast heterogeneous electron transfer. 27,38,39 The significant reduction in k DQ on addition of C4S is consistent with the DQ bound in an inclusion complex. A number of studies on the electrochemistry of encapsulated redox active guests indicate that the direct electron transfer from the inclusion complex does not occur.…”

“…As DQ is known to be weakly fluorescent at an excitation wavelength of 310 nm with an emission band between 320 and 500 nm, 27 it is possible to determine K c by monitoring the change of the DQ fluorescence intensity as a function of added C4S. Typical fluorescence spectra of DQ in the absence and presence of varying concentrations of C4S, to give an excess of C4S, are shown in Fig.…”

“…This is a somewhat unusual result, as generally, a fluorescence enhancement is recorded upon the addition of cationic aromatic molecules to uncharged macrocyclic hosts. 27 The inclusion results in the guest experiencing a less polar environment, which reduces the number of nonradiative pathways available to deactivate the excited state. However, hosteguest complexation by p-sulfonatocalix[n] arenes have been previously reported to have a quenching effect on the guest fluorescence.…”

Encapsulation of alkyl and aryl derivatives of quaternary ammonium cations within cucurbit[n]uril (n = 6,7) and their inverted diastereomers: density functional investigations

“…The number of glycouril units in cucucrbituril homologues renders varying size to their hydrophobic cavity while portal carbonyl groups on portals provide hydrophilic exterior to these macrocycles. The CB[n] macrocycles show efficient and selective binding toward simple gaseous molecules [3], aliphatic/aromatic cations [16][17][18][19][20][21], metal complexes [22][23][24][25][26] Semiempirical quantum chemical investigations on novel tubular nanostructures of modified CB[n] combined with transition metals have been reported [37]. Furthermore density functional theory have been successfully applied to characterize the electronic structure and spectral features of CB [n] homologues and modified CB[n] hosts [37][38][39][40][41][42][43][44].…”

A study of the interaction between inverted cucurbit[7]uril and symmetric viologens

“…[4][5][6][7] In particular, they examined the inclusion of 4,4 0 -pyridinium dicationic guests, measured the binding constants (typically 1 Â 10 9 L mol À1 ) and investigated the effect that one electron reduction of the viologen had on the binding constants. 4 Work was also extended to the cucurbit [8] uril system (Q [8]), 8 whilst viologens covalently bonded to Newkome-type dendritic structures have been encapsulated by Q [7]. Of more relevance to the work herein are the investigations into the effects of varying the aliphatic chain lengths in dialkyl-4,4 0 -bipyridinium dicationic guests on interaction with Q [7].…”