Photophysics of Caffeic, Ferulic and Sinapic Acids with α- and β-Cyclodextrins: Spectral and Molecular Modeling Studies

Encapsulation of caffeic acid (CAA), ferulic acid (FEA) and sinapic acid (SIA) molecules with α-CD and β-CD was studied by UV-visible, fluorescence, time-resolved fluorescence and molecular modelling techniques. This analysis reports the probing of cyclodextrin (CD) cavities with the help of dual emission properties of the above hydroxycinnamic acids (HCAs) in aqueous solution. The normal Stokes shifted bands originated from the locally excited state and the large Stokes shifted bands due to the emission from an intramolecular charge transfer (ICT) state were observed. The ratio of the TICT emission to the normal emission increased with α-CD and β-CD concentration. CD studies indicates (i) HCAs forms 1:1 inclusion complex and (ii) acrylic group present in the interior part of the CD cavity and OH/OCH3 groups present in the upper part of the CD cavity. pH studies suggest proton transfer reactions follow the same trend in these molecules. A mechanism is proposed to explain the inclusion process. PM3 optimizations were also carried out to assign the encapsulation of the HCA molecules.