Structural, electronic, and optical properties of representative Cu-flavonoid complexes.
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, O 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 p(z) 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.
Department of Materials Science and Engineering, University of Ioannina, Ioannina 45110, Greece. chlekka@cc.uoi.gr
Ch E
ChE
Lekka
Jun
J
Ren
Sheng
S
Meng
Efthimios
E
Kaxiras
D003300:Copper / Q000737:chemistry*; D004583:Electrons; D005419:Flavonoids / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D015394:Molecular Structure; D055095:Optics and Photonics; D013056:Spectrophotometry, Ultraviolet
D005419:Flavonoids; D003300:Copper