CD spectra of d-f heterobimetallic helicates with segmental di-imine ligands.

We have investigated the CD spectra of a series of enantiomerically pure heterobimetallic helicates, Λ,Λ-[LnCr(1)3]6+ (Ln = Eu, Gd, Tb), which contain segmental di-imine ligands. For the mononuclear precursor of these helicates, Λ-[Cr(1)3]3+, a positive exciton couplet was observed around 330 nm, as expected for a tris(di-imine) complex with this absolute configuration. The titration of Ln(III) ions into a solution of this complex leads to the formation of Λ,Λ-[LnCr(1)3]6+. During this process, the CD signal was observed to invert to give a signal which was negative at lower energies. We investigated the observed changes in the CD spectra using a ZINDO-based computational method which we have previously developed. We were able to show that the exciton coupling of the chromophores coordinated to the Cr and Ln ions give rise to CD signals of opposite phase, despite having the same nominal absolute configuration. Exciton coupling between chromophores located on different metal centers (“internuclear” exciton...