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

Abstract

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 β-cyclodextrin (β-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. β-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 β-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 β-CD. A 350-fold increase in fluorescence intensity of DASPC22 inside the nanotubular cavities formed by an 8 mM concentration of β-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 β-CD nanochannels could help materials scientists to develop novel supramolecular materials.

Secondary assembly mechanism of cyclodextrin based on LiNTf2@β-CD system

“…The shapes roughly correspond to nanosheet structures after being fractured and replicated. This kind of nanosheet structure is apparently different from the nanotubes and their SA as reported before [23][24][25][26][27][28][29].…”

“…Later similar SA structures of CD nanotubes have also been reported by some other research groups [26][27][28][29][30][31][32][33]. Jaffer et al and Sowmiya et al found that trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) could induce the formation of CD nanotubes and their rodlike SA [26][27][28], although they suggested that the SA consisted of nanotubes filled with DMASBT only and no empty CD was involved [28].…”

“…However, the salts at low concentration enhanced the stability of binding between the host and guest molecules by providing anchor sites for intermolecular hydrogen-bonding between neighboring b-CD molecules. This is possibly because of the strong Coulombic interaction between these ions and the chromophore of the dye [29]. These results indicate that inorganic ions may influence the formation of the CD SA.…”

“…Jaffer et al and Sowmiya et al found that trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) could induce the formation of CD nanotubes and their rodlike SA [26][27][28], although they suggested that the SA consisted of nanotubes filled with DMASBT only and no empty CD was involved [28]. Recently, they discovered rodlike SA in a hemicyanine dye, 4-[4-(dimethylamino)-styryl]-1-docosylpyridinium bromide (DASPC22) and b-CD system [29]. It was observed that inorganic salts (e.g.…”

“…It was observed that inorganic salts (e.g. KClO 4 , KI, KCl, KF) reduced the stability of the supramolecular structures at high concentration, because they acted as salting-in agents and reduced the binding strength between the dye and b-CD, giving H-aggregates of the dye [29]. However, the salts at low concentration enhanced the stability of binding between the host and guest molecules by providing anchor sites for intermolecular hydrogen-bonding between neighboring b-CD molecules.…”