The inclusion behavior of 4-methoxybenzoic acid (4MBA) with α-cyclodextrin and β-cyclodextrin in buffer solutions of pH ~3, ~7, and ~11 was examined using UV-visible, steady-state and time-resolved fluorescence spectroscopy, along with PM3 computational analysis. Ag: 4MBA: CD nanomaterials were synthesized and characterized by SEM, DSC, FTIR, XRD, and ¹H NMR techniques. Because 4MBA predominantly exists as a carboxylate anion in pH ~7 medium, the spectra of its neutral and monoanion forms were also recorded at pH ~3 and pH ~11, respectively. In both cyclodextrin solutions, smooth emission profiles were obtained at pH ~3, while structured emission bands appeared at pH ~7 and pH ~11, with greater band structure observed in alkaline medium. The CD-induced spectral changes at different pH values indicate that the geometries of the resulting inclusion complexes vary across media. Since the carboxylate group is ionized, ground-state dimer is not formed, however, excimer formed in the excited state. Lifetime measurements reveal that the β-CD: 4MBA complex is more stable than the α-CD counterpart. The calculated HOMO-LUMO energy gap, total energy, free energy, enthalpy, entropy, dipole moment, and zero-point vibrational energy of the CD: 2AP complex differed significantly from those of the isolated 4MBA, α-CD and β-CD molecules, and both the vertical and horizontal bond lengths between the amino and hydroxy groups are smaller than the β-CD cavity size confirming the formation of an inclusion complex. SEM-EDX analysis confirms the presence of silver in the composite. FTIR, XRD, and NMR results collectively suggest strong interactions between 4MBA and the silver nanoparticles.
Rajendiran et al. (Fri,) studied this question.