Encapsulation of 3,5-dihydroxybenzoic acid and 3,4,5-trihydroxybenzoic acid by α- and β -cyclodextrins : Spectral and theoretical studies

Department of Chemistry, Annamalai University, Annamalainagar-608 002, Tamilnadu, India

E-mail : drrajendiran@rediffmail.com

Manuscript received online 22 February 2014, accepted 06 April 2014

Inclusion complex formation of α-CD, β-CD, hydroxy propyl-α-CD (Hp-α-CD) and hydroxy propyl-β-CD (Hp-β-CD) of 3,5-dihydroxybenzoic acid (DHBA) and 3,4,5-trihydroxybenzoic acid (THBA) were investigated. The inclusion complexes of both guest molecules with the four CDs were analysed by UV-Visible, steady state and time resolved fluorescence, FTIR, ¹H NMR and PM3 methods. The results shows (i) in pH~1, pH~7 and pH~10 solutions both HBAs form 1 : 1 inclusion complex, (ii) the entrapment of both HBAs in the four CDs cause ICT emission at pH~1 and pH~7 and (iii) at pH~10, the hydroxyl anion and IPT prevents ICT emission. PM3 results indicate that dianion : CD complex is significantly more favorable than the others complexes. The geometry of the most stable complex shows that the hydroxyl group is entrapped in the interior part of the CD and carboxyl group is present in the hydrophilic part of the CD cavity in DHBA. In THBA, the carboxyl group is entrapped in the interior part of the CD and hydroxyl group is present in the hydrophilic part of the CD cavity.