[Recognition of cucurbit [7 and 8] urils toward methylene blue with spectroscopy]

The molecular recognition of cucurbit[7, 8]urils toward a methylene blue dye was investigated by using electronic absorption spectroscopy and fluorescence spectroscopy. The experimental results revealed that the formation of host-guest complexes between Q[7] and the methylene blue dye were formed with a stoichiometry of 2 : 1, while the formation of host-guest complexes between Q[8] and the methylene blue dye were formed with a stoichiometry of 1 : 2. The calculated binding constants are K(Q)[7]-g = 1.34 x 10(12) L2 * mol(-2) and K(Q)[8]-g = 5.34 x 10(12) L2 x mol(-2) respectively based on the absorption spectrophotometric analysis, while the calculated binding constants are K(Q[7]-g = 1.53 x 10(12) L2 x mol(-2) and K[8]-g = 6.11 x 10(12) L2 x mol(-2) respectively based on the fluorescence spectroscopy analysis. The values of K are reasonably consistent. The fluorescence of the guest was found to rapidly increase with the increase in the concentration of Q[7], while the fluorescence of the guest was found to rapidly decrease with the increase in the concentration of Q[8]. Based on the variation of the absoption or fluorecence of the host-guest complexes of Q[7] or Q[8]-methylene blue dye with the variation of pH, the interaction of the Q[7] or Q[8] and the methylene blue dye with an unchangeable ratio can be observed in the range of pH between 0. 5 and 13.