Encapsulation of thiazolyazoresorcinol and thiazolyazocresol dyes with α- and β-cyclodextrin cavities: Spectral and molecular modeling studies

• The presence of azo tautomer is higher than that of hydrazo tautomer.
• Intramolecular hydrogen bonding present in between the OH⋯NN groups.
• Thiazole ring of the dye is included in the CD cavities.
• Brick forms are observed in the TAC/α-CD complexes.
• TAR/β-CD shows micro rod structure.

Encapsulation behavior of 4-(2-thiazolyazo)resorcinol (TAR) and 2-(2-thiazolyazo)-p-cresol (TAC) with two cyclodextrins (α-CD and β-CD) were analyzed by UV–visible, fluorescence, time resolved fluorescence, scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), powder X-ray diffraction (PXRD), proton nuclear magnetic resonance (1H NMR) and molecular modeling methods. With an increase in the CD concentrations, no considerable absorbance difference was observed in TAR and TAC but the fluorescence intensity increased. Compared to water solution, both azo dyes show longer life time in CD solutions. In non-polar solvent, the absorption wavelength observed at 440 nm suggests intramolecular hydrogen bonding (IHB) present between the OH⋯NN groups. The chemical shift values suggest that thiazole ring of the dye is included in the CD cavities and the phenol ring present to edge of the CD cavities. In TAC, SEM image revealed the presence of irregular shape crystals agglomerated rod with CD. Binding energy, ΔH and ΔG values indicate that both azo dyes form stable inclusion complex.