Interaction of Carotenoids and Cu²⁺in Cu-MCM-41:  Distance-Dependent Reversible Electron Transfer

Carotenoids (Car), β-Carotene (I), 8‘-apo-β-caroten-8‘-al (II), and canthaxanthin (III), incorporated into activated Cu−MCM-41 were examined by UV/vis and EPR spectroscopies. A Cu 2+ −Car complex was formed for I and III but not for II. Formation of a complex results in distortion of the all-trans carotenoid geometry and a tetragonal geometry for Cu 2+ . The binding energies of Car and Cu 2+, the changes in the maximum absorption, and the bond lengths of Car after the formation of the Cu 2+ −Car complex were examined by semiempirical ZINDO/1 and ZINDO/S calculations. Formation of a complex between Car and Cu 2+ favors both forward and back electron transfer (ET) reactions due to the short distance (∼2 Å) between Car and Cu 2+, and reversible ET appears upon temperature cycling.