Characterization of guest molecule concentration dependent nanotubes of β-cyclodextrin and their secondary assembly: Study with trans-2-[4(dimethylamino)styryl]benzothiazole, a TICT-fluorescence probe

The formation of nanotubes of β-cyclodextrins (β-CD) and their secondary assembly induced by trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT) as a guest molecule has been explained using twisted intramolecular charge transfer (TICT) fluorescence, steady state fluorescence anisotropy, time-correlated single-photon counting fluorescence decay of DMASBT, atomic force microscopy and also transmission electron microscopy. It has been demonstrated that although at lower concentration of β-CD, DMASBT molecules form simple adduct of 1:1 stoichiometry but they form extended nanotubes at relatively high concentration of β-CD which further lead to the secondary assembly through intertubular hydrogen bondings forming rod-like structures. The changes in fluorescence properties of DMASBT yield critical aggregation concentration (CAC) of β-CD very close to 2.5 mM. The extent of formation of these supramolecular structures is dependent on the concentration of guest molecule. It has been observed that perchlorate ion (ClO4−) reduces the stability of supramolecular structures at higher concentration of it but at low concentration (<7 mM) it enhances the stability by providing the anchor sites for intermolecular hydrogen bonding between neighboring β-CD molecules. The polarity and viscosity dependent TICT fluorescence characteristics of DMASBT are exploited to determine the micropolarity and microviscosity of environment around it inside the nanotubes.