Synthesis of Single-Handed Helical Spiro-Conjugated Ladder Polymers through Quantitative and Chemoselective Cyclizations*

Abstract

We report the unprecedented synthesis of one-handed helical spiro-conjugated ladder polymers with well-defined primary and secondary structures, in which the spiro-linked dibenzo[a,h]anthracene fluorophores are arranged in a one-handed twisting direction, through quantitative and chemoselective acid-promoted intramolecular cyclizations of random-coil precursor polymers composed of chiral 1,1'-spirobiindane and achiral bis[2-(4-alkoxyphenyl)ethynyl]phenylene units. Intense circular dichroism (CD) and circularly polarized luminescence (CPL) were observed, whereas the precursor polymers exhibited negligible CD and CPL activities. The introduction of 2,6-dimethyl substituents on the 4-alkoxyphenylethynyl pendants is of key importance for this simple, quantitative, and chemoselective cyclization. This strategy is applicable to the defect-free precise synthesis of other varieties of fully π-conjugated molecules and coplanar ladder polymers that have not been achieved before.

Keywords: alkyne benzannulations; circularly polarized luminescence; helical structures; ladder polymers; spiro-conjugated structures.