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, Lambda,Lambda-[LnCr(1)(3)](6+) (Ln = Eu, Gd, Tb), which contain segmental di-imine ligands. For the mononuclear precursor of these helicates, Lambda-[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 Lambda,Lambda-[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 coupling) is also predicted to have a significant impact on the observed spectrum. We were able to "deconstruct" the observed CD spectra into a set of competing exciton coupling effects and show that the sign of these spectra does not correlate with the absolute configuration of the individual metal centers.