Synthesis and electronic properties of alkyl- and alkyloxy-curcuminoids

Two series of p-alkyl- and p-alkoxy-substituted curcuminoids were synthesized. The validity of the proposed structures was checked on the basis of their high resolution mass spectra as well as NMR, IR and electronic spectra. The spectra were essentially similar for both series. NMR, IR and electronic spectra showed that the compounds exist mainly or exclusively as the enolic tautomer in their solutions. Theoretical study for the electronic spectra was conducted to account for the origins of the electronic transitions that are responsible for the UV-VIS bands. The theoretical calculations were done at the B3LYP/6-31G(d) level of theory and predicted that the similarity of the electronic spectra is may be due to the close resemblance of their molecular orbitals.