Plasticizer-induced Enhancement of Mesoscale Dissymmetry in Thin Films of Chiral Polymers with Variable Chain Length.

Conjugated polymers with chiral side chains are of interest in areas including chiral photonics, optoelectronics, and chemical and biological sensing. However, the low dissymmetry factors of most neat polymer thin films have limited their practical application. Here, we demonstrate a robust method to increase the absorption dissymmetry factor in a poly-fluorene-thiophene (PF8TS series) system by varying molecular weight and introducing an achiral plasticizer, polyethylene monoalcohol (PEM-OH). Extending chain length within the optimal range and adding this long-chain alcohol significantly enhanced the chiroptical properties of spin-coated and annealed thin films. Mueller matrix ellipsometric analysis showed good agreement with the steady-state transmission measurements confirming a strong chiral response (circular dichroism and circular birefringence), ruling out linear dichroism, birefringence, and specific reflection effects. Solid-state NMR studies of annealed hybrid chiral polymer systems showed enhancement of signals associated with aromatic π-stacked backbone and the ordered side-chain conformations. Further studies using Raman spectrometry, X-ray diffraction, differential scanning calorimetry, atomic force microscopy, and polarized optical microscopy indicate that PEM-OH facilitates mesoscopic crystal domain ordering upon annealing. This provides new insights into routes for tuning optical activity in conjugated polymers. This article is protected by copyright. All rights reserved.