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, p<i>i</i>Pr3, 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 (<i>g</i>_CD) or optical rotation. PDPAs with bulkier, round-shaped side groups (pEt3 and p<i>i</i>Pr3) had greater <i>g</i>_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 <i>g</i>_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

Annealing-Induced Circular Dichroism Enhancement in Luminescent Conjugated Polymers with an Intramolecular Stack Structure

Two poly­(diphenylacetylene) derivatives containing identical chiral pinanyl groups on the <i>para</i>- and <i>meta</i>-positions of the side phenyl ring were prepared, and their circular dichroism (CD) and photoluminescence (PL) spectra were compared. The magnitudes of circular polarization (<i>g</i>_CD) of the <i>para</i>- and <i>meta</i>-polymers were determined to be 3.1 × 10^–3 and 1.4 × 10^–3, respectively. The PL quantum yield (PLQY) of the <i>para</i>-polymer was much greater (27.8%) than that of the <i>meta</i>-polymer (2.61%). When the two polymers were annealed at 80 °C in toluene, their CD spectra were remarkably enhanced and reached equilibrium at <i>g</i>_CD values of 9.6 × 10^–3 and 6.0 × 10^–3, respectively. The <i>para</i>-polymer was kinetically more favored for the CD enhancement as known from the fact that the activation energies for the reactions of <i>para</i>- and <i>meta</i>-polymers were determined to be 88 and 187 kJ mol^–1, respectively. The PLQYs of both polymers were unaffected by annealing