Molecular Design Approach for Directed Alignment of Conjugated Polymers

Macroscopic alignment of conjugated polymers (CPs) is essential to fully realize the anisotropic optical and electronic properties of CPs, originated from the one-dimensional Ï-orbital overlap along the conjugated backbone, in the solid-state devices. Various CP-processing approaches to achieve CP alignment have been explored. However, molecular design to enable CP alignment has not been fully understood. Herein, we report a thorough investigation into molecular design parameters critically affecting CP alignment characteristics. First, we present a series of CPs with newly designed building blocks reflecting the previously identified preliminary design principles to validate the general applicability of the identified CP design parameters for alignment. Furthermore, newly defined design factors correlating with CP alignment characteristics are systematically studied, including the planarity of CP main chain, the effect of the side-chain design, intramolecular interaction moiety for induced chain planarity, and the surface energy of CPs. Utilizing aligned CP films, we also demonstrated the optical switching of organic thin-film transistor (OTFT) devices. Depending on the orientation of polarized light illumination, different amounts of photocurrent gain were observed. The on-to-off switching ratio (I_on/I_off) under illumination was ca. 7.2 × 10⁴, which is large enough for an OTFT to operate by an optical as well as an electrical trigger.