Kinetic Study on Achiral-to-Chiral Transformation of Achiral Poly(diphenylacetylene)s via Thermal Annealing in Chiral Solvent: Molecular Design Guideline for Conformational Change toward Optically Dissymmetric Structures

Achiral poly(diphenylacetylene)s (PDPAs: pMe3, pEt3, piPr3, pMe2O1, pMe2OD1, and mMe3) with different alkyl side chains at the para or meta position of the side phenyl ring were prepared to examine achiral-to-chiral transformations upon thermal annealing in a chiral solvent. All the para-substituted PDPAs showed significant circular dichroism (CD) enhancement upon annealing, whereas the meta-substituted polymer, mMe3, was inert to the same treatment. To investigate the kinetics, the asymmetric conformational change was monitored by observing the changes in the magnitude of circular polarization (g_CD) or optical rotation. PDPAs with bulkier, round-shaped side groups (pEt3 and piPr3) had greater g_CD values at equilibrium than pMe3 with a smaller side group. Moreover, the activation energy for the forward reaction (Ea_f) was lower in the bulkier polymers than in pMe3 owing to enhanced miscibility with the chiral solvent. Similarly, the long alkyl chains of pMe2O1 and pMe2OD1 acted as internal plasticizers to lower their Ea_f values relative to that of pMe3, whereas their g_CD values at equilibrium were smaller than that of pMe3. The kinetics of the achiral-to-chiral transformation is discussed in detail based on the spectroscopic changes observed during the annealing process.