Colorless polyimides derived from rigid trifluoromethyl-substituted triphenylenediamines

Colorless polyimides (CPIs) have wide applications in flexible photoelectric devices, thanks to their high transparency and heat-resistance. Herein, three rigid aromatic triphenylenediamine monomers containing varied number and position of trifluoromethyl (-CF3) substituents, i.e., c-3FTP, c-6FTP, and s-6FTP, were designed. The corresponding CPI films were fabricated by polycondensation with the commercial 4,4'-(hexa-fluoroisopropylidene)diphthalic anhydride (6FDA) or 1,2,4,5-cyclohexanetetracarboxylic dianhydride (CHDA). Due to the steric hindrance of -CF3 groups, three diamines all exhibit large torsion angles within 46.1 degrees -87.1 degrees between benzene rings determined via single crystal analysis. The structure-property analysis reveals that -CF3 substitution located on the central phenyl ring facilitates the high glass transition temperature (Tg), while more -CF3 substitutions are beneficial for high optical transparency. Among all the CPIs, the 6FDA/c-6FTP film ex-hibits the most remarkable balance in thermal, optical, and dielectric properties, including high thermal resis-tance (Tg = 385 degrees C, Td5% = 537 degrees C), excellent optical transparency (lambda cutoff = 350 nm, T450 = 84.5%, and b* = 1.44), low dielectric nature (Dk = 2.84, Df = 0.0053) at 10 GHz, and high surface hydrophobicity (theta = 100 degrees, WH2O = 0.25%). This systematic study would complement the basic structure-property relationship of aromatic CPIs and inspire rational development of high-performance aromatic CPI materials toward applications in flexible display or 5G technology.