2,2′‐Tethered Binaphthyl‐Embedded One‐Handed Helical Ladder Polymers: Impact of the Tether Length on Helical Geometry and Chiroptical Property

Abstract

Synthetic breakthroughs diversify the molecules and polymers available to chemists. We now report the first successful synthesis of a series of optically‐pure 2,2′‐tethered binaphthyl‐embedded helical ladder polymers based on quantitative and chemoselective ladderization by the modified alkyne benzannulations using the 4‐alkoxy‐2,6‐dimethylphenylethynyl group as the alkyne source, inaccessible by the conventional approach lacking the 2,6‐dimethyl substituents. Due to the defect‐free helix formation, the circular dichroism signal increased by more than 6 times the previously reported value. The resulting helical secondary structure can be fine‐tuned by controlling the binaphthyl dihedral angle in the repeating unit with variations in the 2,2′‐alkylenedioxy tethering groups by one carbon atom at a time. The optimization of the helical ladder structures led to a strong circularly polarized luminescence with a high fluorescence quantum yield (28 %) and luminescence dissymmetry factor (2.6×10⁻³).