Study of the Binding Interactions of a Hemicyanine Dye with Nanotubes of β-Cyclodextrin and Effect of a Hofmeister Series of Potassium Salts

The photophysical properties of a hemicyanine dye, 4-[4-(dimethylamino)styryl]-1-docosylpyridinium bromide (DASPC22), have been studied in homogeneous media of pure solvents and mixed solvents using UV-vis absorption and fluorescence spectroscopies. These properties were explored to study the binding interactions between DASPC22 and nanotubes of beta-cyclodextrin (beta-CD) using UV-vis absorption, steady-state fluorescence and fluorescence anisotropy, and time-correlated single-photon-counting (TCSPC) fluorescence measurements of the dye. DASPC22 molecules form H-aggregates in pure water. beta-CD forms a simple inclusion complex (1:2 stoichiometry) below its critical aggregation concentration (cac) by encapsulating the chromophoric part of the dye. The H-aggregate dissociates significantly to the monomeric form of the dye only when the nanotubes of beta-CD molecules start to form above its cac. The dye molecule exists in its monomeric form upon inclusion of its chromophoric part along with the aliphatic tail inside the hydrophobic nanotubular cavity of beta-CD. A 350-fold increase in fluorescence intensity of DASPC22 inside the nanotubular cavities formed by an 8 mM concentration of beta-CD compared to the fluorescence intensity in the form of a simple inclusion complex was observed. The effects of a Hofmeister series of potassium salts, namely, KClO4, KI, KCl, and KF, in both low and high concentration ranges on the binding strength between host and guest molecules were studied. Salts in their very low concentration ranges enhanced the stability of the host-guest complexes, resulting in a further increase in fluorescence intensity. The fluorescence properties can be tuned by the selective addition of potassium salts with various anions. The tuning of the optical properties of the dye in beta-CD nanochannels could help materials scientists to develop novel supramolecular materials.