Abstract
The binding interactions in aqueous solution between the dicationic guest diquat (DQ2+) and the cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) hosts were investigated by 1H 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 (PQ2+). DQ2+ was found to bind poorly (K=350 m−1) inside CB7 and more effectively (K=4.8×104 m−1) inside CB8. One‐electron reduction led to increased binding affinity with both hosts (Kr=1×104 m−1 with CB7 and Kr=6×105 m−1 for CB8). While 1H NMR spectroscopic data revealed that DQ2+ is not fully included by CB7, the crystal structure of the CB8⋅DQ2+ 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.
Be included! The hosts cucurbit[7]uril (CB7) and cucurbit[8]uril (CB8) exhibit a wide range of binding affinities with the guests diquat and paraquat and their one‐electron reduced forms. The host CB7 forms a less stable inclusion complex with diquat than with paraquat, but one‐electron reduction of the guest decreases the binding affinity with paraquat and increases it with diquat. The larger host CB8 is less selective, forming reasonably stable complexes (an example is shown here) with diquat and its one‐electron‐reduced radical cation.
