Benzothiadiazole-Based Dichroic Dyes: Novel Approaches for Electrically Tunable Liquid Crystal Smart Windows

Abstract

A series of benzothiadiazole (BT)-based dichroic dyes with donor–acceptor–donor (D–A–D) molecular frameworks is designed and synthesized, exhibiting absorption spectra that cover the visible light region and are tunable through molecular modifications. Most of the newly synthesized dyes show high dichroic ratios and order parameters, ensuring strong optical anisotropy and good alignment within the liquid crystal host mixture. The performance of individual dyes in smart window applications is evaluated, with transmittance changes observed under dynamic electric fields, demonstrating their potential for use in electrically tunable smart windows. To achieve full visible-light modulation, the combination of complementary absorption dyes is screened and an optimized mixture Mix-7 is obtained, which not only achieves full visible-light absorption coverage but also demonstrates smooth and dynamic transmittance modulation under varying voltages. Finally, a prototype smart window filled with Mix-7 is fabricated to validate the continuous tunability of dye-doped liquid crystal-based systems. Additionally, further introduction of a polarizer film significantly reduces the transmittance of the demo, especially in the OFF state which presents an almost completely dark state, highlighting the potential for automotive smart window applications.