Structural, Electronic, and Optical Properties of Representative Cu−Flavonoid Complexes

Abstract

We present density functional theory (DFT) results on the structural, electronic, and optical properties of Cu-flavonoid complexes for molar ratios 1:1, 1:2, and 1:3. We find that the preferred chelating site is close to the 4-oxo group and in particular the 3-4 site followed by the 3'-4' dihydroxy group in ring B. For the Cu-quercetin complexes, the large bathochromic shift of the first absorbance band upon complexation, which is in good agreement with experimental UV-vis spectra, results from the reduction of the electronic. energy gap. The HOMO states for these complexes are characterized by pi-bonding between the Cu d orbitals and the C, 0 p orbitals except for the case of 1:1. complex (spin minority), which corresponds to sigma-type bonds. The LUMO states are attributed to the contribution of Cu pz orbitals. Consequently, the main features of the first optical absorption maxima are essentially due to pi -> pi* transitions, while the 1:1 complex exhibits also sigma -> pi* transitions. Our optical absorption calculations based on time-dependent DFT demonstrate that the 1: 1 complex is responsible for the spectroscopic features at pH 5.5, whereas the 1:2 complex is mainly the one responsible for the characteristic spectra at pH 7.4. These theoretical predictions explain in detail the behavior of the optical absorption for the Cu-flavonoid complexes observed in experiments and are thus useful in elucidating the complexation mechanism and antioxidant activity of flavonoids.