Cu(I/II) redox control of molecular conformation and shape in chiral tripodal ligands: Binary exciton-coupled circular dichroic states

This manuscript describes the design, synthesis, and characterization of a coordination complex that demonstrates reversible interconversion between two redox states that show very different chiroptical properties. Reported here are three chiral N₄-tetradentate ligands, prepared in enantiopure form by asymmetric synthesis, and their circular dichroic and other properties, as well as those of numerous Cu(I) and Cu(II) complexes. While the chiroptical spectra of the free ligands are unremarkable, the CD spectra of the complexes are very intense, and spectral features indicate that they derive from exciton coupling. Comparison of complexes with one, two, or three chromophores provides strong supportive evidence for this assignment, and semiquantitative correlation with intensity as rationalized by additivity. A Cu^I(L)/Cu^II-(L) redox couple was identified that showed dramatically different circular dichroic properties that was further shown to be easily oxidized and reduced chemically between two states with remarkable retention of signal. A structural hypothesis for the interconversion is given and supported by a variety of spectroscopic evidence.