Thermally stable and highly soluble UV absorbers for colorless polyimide film: Star-shaped hydroxyphenyl benzotriazole with a tailored trade-off between molecular planarity and distortion

Colorless polyimide (CPI) is a key material of flexible displays characterized by high thermal stability and flexibility. However, it is susceptible to photodegradation induced by ultraviolet (UV) light exposure in its operating environments, thus necessitating the development of a new UV absorber to address this issue. Meanwhile, the high-temperature processing required for CPI synthesis makes the development of novel UV absorbers challenging. Here, we developed a novel thermally stable and highly soluble star-shaped trimeric UV absorber compatible with CPI. Various hydroxyphenyl benzotriazoles (HBTs) were designed to explore molecular structure dependent changes in thermal stability and solubility that are changed following molecular trade-offs between planarity and distortion. HBTs were synthesized by substituting them on a central phenyl linker to obtain 3 dimers (p-PBT, m-PBT, and o-PBT) and 1 trimer (TPBT). UV/VIS spectroscopy, computational calculation, solubility test, thermalgravimetric analysis, and powder X-ray diffraction result supported the molecular structure-dependent changes in optical absorption, solubility, thermal stability, and crystallinity. Interestingly, among the synthesized UV absorbers, a star-shaped trimer TPBT with optimized distortion and planarity exhibited great solubility (>8 wt% in NMP solvent) and outstanding thermal stability (1 wt% loss at 360 °C). Furthermore, due to its strong UV absorption characteristics, the TPBT efficiently prevented the photodegradation of CPI film after xenon lamp irradiation. This study highlights the key roles of tailoring trade-offs in molecular planarity and distortion to develop thermally stable and highly soluble UV absorbers for CPI films.