Mechanism of Complexation of Cucurbiturils with Styryl Dyes in the Presence of Sodium Cations

The complexation of styryl dyes 4-[(E)-2-(3,4-dimethoxyphenyl)]-1-ethylpyridinium perchlorate and trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide with cucurbit[n]urils (CBn) (n = 6, 7) in aqueous solutions in the presence of sodium sulfate with a concentration of 0 to 1 mol/L has been studied by stationary and time-resolved optical spectroscopy, electrochemical, and quantum chemical methods. Adding the electrolyte to the solution decomposes 1 : 1 inclusion complexes with the dyes (D+ · CBn) due to competitive formation of complexes of the cavitand with sodium cations: Na+ · CBn and Na+ · CBn · Na+. Their total formation constants have been determined to be β = 2760 and 168 600 M−2 for CB7 and CB6, respectively, which agrees with the results of quantum chemical calculations. At the same time, no evidence of the presence of Na+ · CBn · D+ type complexes has been found in solutions.