STUDIES ON CURCUMIN AND CURCUMINOIDS .24. CHARACTERIZATION OF THE SPECTROSCOPIC PROPERTIES OF THE NATURALLY-OCCURRING CURCUMINOIDS AND SELECTED DERIVATIVES

The sensitivity of the absorption and emission properties of the natur ally occurring curcuminoids and selected derivatives to the solvent's polarity and hydrogen bond donor-acceptor capability was investigated in order to obtain information about the intra- and intermolecular hyd orgen bonding capacity of these substances. Intermolecular hydrogen bo nd formation was observed both in the ground state and in the excited state of the curcuminoids possessing a phenolic group. The keto-enol u nit of these molecules did not, or only weakly, participate in intermo lecular hydrogen bond formation. Fluorescence was favoured by strong i ntramolecular hydrogen bonding in the keto-enol unit. The phenolic gro ups in curcumin (1) showed H-bond acceptor properties while the phenol ic groups in bisdemethoxycurcumin (3) acted as H-bond donors. This res ults in a difference in affinity of 1 and 3 to the various alcohols. T he geometric arrangement of nuclei in the excited slate was similar to the ground state for all the compounds except 5 where an increase of polarity in the excited stale was observed.