An electrochiroptical molecular switch: mechanistic and kinetic studies

Previously, we reported the synthesis and preliminary characterization of Cu(I/II) complexes of N,N-bis(2-quinilylmethyl)-(l)-methionine (Zahn, S.; Canary, J. W. Science 2000, 288, 1404-7). The chemically oxidized and reduced forms of the complexes gave nearly mirror image circular dichroism (CD) spectra as a result of reorganization of the inner coordination sphere of the copper atom. The reorganization involved exchange of oxygen for sulfur in proceeding from the Cu(II) to Cu(I) oxidation state and corresponding ligand conformational changes required to accommodate this exchange. In this paper, we demonstrate that the complex can be triggered by electrochemical means. The electrochemical and stereochemical details of the redox-induced ligand reorganization were probed by independent synthesis of alternative chemical intermediates, CD spectroelectrochemistry, curve fitting of cyclic voltammograms, CD titration, and scanning electrochemical microscopy (SECM). A square-type mechanism was most consistent with the data: the Cu(II) complex is reduced followed by a ligand reorganization step to give the stable reduced form of the complex. The Cu(I) complex is then oxidized in a fast step followed by another ligand reorganization. A millisecond time scale rate was found by SECM for one of the key conformational conversion steps.