Spectroscopic, photophysical and thermodynamic studies of inclusion complexes of β-cyclodextrin and 2-hydroxypropyl-β-cyclodextrin with 10-methylbenzophenothiazine

Abstract

The inclusion complexes of 10-methyl-12H-benzo[a]phenothiazine (10-MeBPHT) with β-cyclodextrin (β-CD) and with 2-hydroxypropyl-β-cyclodextrin (HP-β-CD) were investigated by electronic absorption and fluorescence spectroscopies in aqueous solutions. 10-MeBPHT fluorescence quantum yields were significantly larger in the presence of HP-β-CD (Φ_F  = 0.097 ± 0.02) and β-CD (Φ_F  = 0.025 ± 0.004) than in water (Φ_F  = 0.00435 ± 0.00007). A 1 : 1 stoichiometry was found for these CD complexes and formation constants (K_f , evaluated by means of the Benesi–Hildebrand treatment) ranged from 14 ± 7 to 50 ± 11 M⁻¹ (at 20 °C), depending on the nature of the CD. In the case of the 10-MeBPHT : HP-β-CD complex, the complexation reaction was endothermic (ΔH⁰  = 12.4 kJ mol⁻¹) and presented a positive entropy value (ΔS⁰  = 75 J mol⁻¹ K⁻¹). AM1 semiempirical calculations of the 10-MeBPHT geometry indicate a partial inclusion of this molecule in HP-β-CD, in agreement with our experimental results.