Chirality induction of achiral polydialkylfluorenes by chiral solvation: odd–even and side chain length dependence

Abstract

In this contribution, we report the first odd–even and side chain length dependence on the chiral induction of achiral polydialkylfluorenes (PFs) by chiral solvation. Firstly, a series of PFs, substituted at the C9-position by linear alkyl chains of different lengths and branched side chains (PF5, PF6, PF7, PF8, PF9, PF10 and PF2/6), were designed and synthesized by the Kumada catalyst transfer polycondensation (KCTP) or typical Yamamoto-type coupling reaction. PFs with linear alkyl chains of different lengths (PF5, PF6, PF7, PF8, PF9 and PF10) are all good achiral candidates to form chiral supramolecular assemblies in neat limonene. UV-visible absorbance spectra validated the production of chiral β-phase conformers in PF5, PF7, PF8, PF9 and PF10/limonene aggregates, and another chiral conformer in PF6/limonene aggregates. Interestingly, an “odd–even” effect in circular dichroism (CD) signs for the chiral β-phase of PF/limonene aggregates was firstly observed, depending on whether the alkyl side chain was of odd or even length. Moreover, the disassembling temperature for optically active PF gels or aggregates also depends on both the length and odd–even number of side chains. Our results indicate that there is a complex interplay between the transfer of solvent chirality into supramolecular helicity and the length of the alkyl side chain. These results are critical for building optically active π-conjugated polymers from achiral ones, since subtle modifications in the length of the side alkyl chain can strongly influence the chirality sign.