Photophysical and Photochemical Properties of a Curcumins Family: A Combined Computational and Experimental Investigation

In the present study, photophysical and photochemical properties of curcumin (1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione) and its seven derivatives, encompassing esterified curcumins and their bisdemethoxy conjugates have been studied computationally and experimentally to explore their suitability as photosensitizers for photodynamic therapy. We found out that enol forms of curcumins are more stable than keto ones. We observed that the main electronic levels of the curcumin derivatives well agree with the observed spectroscopic features (i.e. absorption, fluorescence and phosphorescence spectra). Based on the spin–orbit coupling matrix elements and the associated energy gaps, we suggested that the most plausible mechanism involves excitation from S0 to S1, followed by intersystem crossing from S1 to T3. Subsequent internal conversion occurs from T3 to T2 and T1, culminating in phosphorescence from T1 to S0. The computed vertical phosphorescence energy of the first triplet state (T1) for the studied curcumin derivatives exceed both the computed first excited-state energy of O2─1.06 eV in vacuum and 1.05 eV in water─and the measured value of 0.98 eV in vacuum. These findings indicate that the studied curcumin derivatives are theoretically capable of photosensitizing the production of 1O2. In this context, curcumin HL4a exhibits the best yield for singlet oxygen production (∼15%) and the esterification and the presence of methoxy groups poorly affect both photophysical and photochemical properties.