Molecular recognition and complexation thermodynamics of dye guest molecules by modified cyclodextrins and calixarenesuffonates

The inclusion complexation behavior of some dye guest molecules with two kinds of molecular receptors, i.e., native alpha-, beta-, and gamma-cyclodextrins, and chemically modified beta-cyclodextrins, and water-soluble calix[n]-arenesulfonates (n = 4, 6. 8) and their lower rim alkylated derivatives, has been investigated, It was found that the fluorescence intensity of guest molecules evident]), changes upon the addition Of molecular receptors. The calix[n]arenesulfonates (n = 4. 6, 8) cause gradual decreases in the fluorescence intensity, whereas native (alpha-. beta-, and gamma-cyclodextrins cause gradual increases in the fluorescence intensity, Interestingly, the fluorescence intensity of dye guest molecules gradually increases upon the addition of the lower rim alkylated calix[n]L arenesulfonate derivatives (n = 4. 6, 8), in contrast to their parent calix[n]arenesulfonates (n = 4, 6, 8). Meanwhile, chemically modified cyclodextrins still show the same effect on the fluorescence intensity of dye guest molecules; however, the hydrophobicity or hydrophilicity Of the substituent group influences the increment in the fluorescence intensity. The fluorescence phenomena are explained in terms of photophysical behavior on the basis of the changes in the fluorescence quantum yield and the molecular extinction coefficient of the guests due to the complexation with the two molecular receptors, i.e., cyclodextrins and calixarenesulfonates. The stability constants of the resulting complexes are determined via spectrofluorometric titration. The molecular recognition behavior is discussed from the viewpoint of size/shape-fit, electrostatic interaction, hydrogen-C bonding, hydrophobic interactions, etc. The thermodynamic parameters for the inclusion complexation of (guest molecules with the two kinds of molecular receptors were determined through van't Hoff analysis. The complexation mechanism is also discussed from the thermodynamic viewpoint.