Development of Diketopyrrolopyrrole-Based Dichroic Dyes for Improved Transmittance Control in Liquid Crystal Smart Windows

Two diketopyrrolopyrrole (DPP)-based dichroic dyes were synthesized and investigated for application in dye-doped liquid crystal (LC) smart windows. These dyes exhibited strong absorption in the long-wavelength region, particularly DPP-2, addressing the limited absorption of benzothiadiazole (BT)-based dyes beyond 650 nm. Both DPP dyes demonstrated high dichroic ratios and excellent alignment within the LC host, enabling effective transmittance modulation under an electric field. To enhance full visible spectral coverage, a multidye-doping strategy was implemented by combining DPP and BT dyes with complementary absorption properties, yielding an optimized mixture mix-10 with broad visible-light coverage and strong dimming capability. Finally, a smart window prototype incorporating mix-10 was assembled, which showed continuous and reversible transmittance modulation under varying voltages. The addition of a polarizer film further reduced off-state transmittance, making this approach particularly suitable for automotive applications requiring enhanced privacy. Moreover, the device could effectively modulate polarized light with high contrast, expanding its potential utility. This study presents an effective strategy for achieving full-spectrum tunable smart windows through complementary dichroic dye mixtures, providing insights for future research and practical applications.