Elucidating the relationship between the phenolic substituents and the excited state dynamics of curcuminoids

Curcumin, which is essentially non-toxic in the ground state for both bacterial and mammalian cells, acquires notable cytotoxicity if photosensitized to its first excited singlet state. Nevertheless, a full exploitation of its photosensitizing properties is prevented by its very fast non-radiative decay from S1. In solution, deactivation is due to either the transfer of the enolic proton to the ketonic oxygen (excited-state intramolecular proton transfer, prevailing in non-H-bonding environment) or interactions with solvent molecules (intermolecular energy transfer, which is dominant in H-bonding environment). In this Chapter we report a systematic study on the dependence of the proneness of curcuminoids to decay by both excited-state intramolecular proton transfer and intermolecular energy transfer from the number and type of phenyl substituents. We show that excited-state intramolecular proton transfer is the more efficient the strongest is the phenyl rings electron withdrawing character, and that intermolecular energy transfer takes place only if both the hydroxyl and the methoxy substituents found in curcumin are present. An explanation for these observations is attempted. © 2012 Nova Science Publishers, Inc. All rights reserved.