Spectroscopic studies of inclusion complexation between ortho derivatives of p-methylaminobenzoate and α- and γ-cyclodextrins

Using the steady-state and time-resolved spectroscopic techniques and quantum-chemical calculations, photophysical and photochemical properties of the two ortho derivatives of p-methylaminobenzoate (methyl o-methoxy p-methylaminobenzoate (I) and methyl o-hydroxy p-methylaminobenzoate (II)) have been studied in binary mixture THF-H2O and aqueous solutions containing different concentrations of α- and γ-CD. Nonlinear solvatochromic shifts of the absorption and fluorescence bands were observed for both fluorophores in a mixture of polar aprotic (THF) and polar protic (H2O) solvents. This nonlinearity has been explained in terms of the non-specific (dielectric enrichment of the solvent around the polar solute) and specific (hydrogen bond) solute-solvent interactions. Spectroscopic measurements were used to investigate the role of H-bonding solute-solvent interactions, and the excited-state intramolecular proton transfer process in the formation of inclusion complexes between fluorophore and cyclodextrins. The obtained results were used to calculate, according to Benesi-Hildebrand's plot and nonlinear least-squares regression analysis, equilibrium constants of the fluorophore-cyclodextrin inclusion complexes. Performed analysis indicates also that both 1:1 and 1:2 inclusion complexes were formed between studied compounds and α- and γ-cyclodextrins.