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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 π-bonding between the Cu d orbitals and the C, O p orbitals except for the case of 1:1 complex (spin minority), which corresponds to σ-type bonds. The LUMO states are attributed to the contribution of Cu pzorbitals. Consequently, the main features of the first optical absorption maxima are essentially due to π → π* transitions, while the 1:1 complex exhibits also σ → π* 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. [ABSTRACT FROM AUTHOR]