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

The inclusion complexes of 10-methyl-12H-benzo[a]phenothiazine (10-MeBPHT) with beta -cyclodextrin (beta -CD) and with 2-hydroxypropyl-beta -cyclodextrin (HP-beta -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-beta -CD (Phi (F) = 0.097 +/-0.02) and beta -CD (Phi (F) = 0.025 +/-0.004) than in water (Phi (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 degreesC), depending on the nature of the CD. In the case of the 10-MeBPHT: HP-beta -CD complex, the complexation reaction was endothermic (DeltaH(0) = 12.4 kJ mol(-1)) and presented a positive entropy value (DeltaS(0) = 75 J mol(-1) K-1). AM1 semiempirical calculations of the 10-MeBPHT geometry indicate a partial inclusion of this molecule in HP-beta -CD, in agreement with our experimental results.