Inclusion complexation of diquat and paraquat by the hosts cucurbit[7]uril and cucurbit[8]uril

The binding interactions in aqueous solution between the dicationic guest diquat (DQ(2+)) and the cucurbit[7]uril (CB7) and cucurbit[S]uril (CB8) hosts were investigated by H-1 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=350M(-1)) inside CB7 and more effectively (K=4.8x10(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 1H NMR spectroscopic data revealed that DQ(2+) is not fully included by CB7, the crystal structure of the CB8.DQ(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.