Helical Complex Ladder Polymer with Amplification of Asymmetry

We report a Cu–salen helical complex ladder polymer
(HLP-Cu) built from covalently fused Cu(Salen) units.
Point
chirality from enantiopure diamines directs the formation of a rigid,
π-conjugated helix. Compared with a planar analogue (LLP-Cu) and the Cu(Salen) monomer, HLP-Cu exhibits up to 5-fold
stronger circular dichroism and a maximum dissymmetry factor of 7.4
× 10–3. Titration experiments suggest a cooperative
amplification of asymmetry arising from extended conjugation and helical
geometry during helix formation. Time-dependent density functional
theory calculations are employed to support this hypothesis, showing
that increasing conjugation length and helical order enhance the rotational
strength of contributing excited states via elevated magnetic transition
moments and improved alignment of electric and magnetic transition
moments. Our findings provide a new polymer platform for designing
functional chiral materials with tunable chiroptical and spin-dependent
properties.