Chain‐Length Dependence of the Optical Activity of Helical Triptycene‐Based π‐Conjugated Ladder Polymers

Abstract

AbstractHelical ladder polymers are designed by connecting planar indenofluorene chromophores by chiral triptycene linkers. The resulting hexagonal helices exhibit a substantial degree of homoconjugation across the triptycene knots, with saturation above ≈20 repeat units. The chiroptical anisotropy values gabs = Δε/ε of the lower‐energy lobes of the red‐most couplets in the circular dichroism (CD) spectra arise from the π−π* transition of the planar indenofluorene chromophores and increase almost sevenfold when going from a model dimer to the corresponding polymeric helical ladder structures. This increase represents a signature of “chiral amplification” that is observed at the molecular, single‐chain level. Here, the incorporated chiral linker units determine the chiroptical properties of the helical polymer chain, with chiral interactions arising across many achiral indenofluorene building blocks (>12). This behavior is observed for a fully covalently bound single polymer chain, in contrast to the more commonly studied supramolecular assemblies of appropriate chiral molecules.