Synthesis and Characterization of Indolenine‐Based Squaraine Dyes and their Photophysical Properties in Polymer Thin Films

AbstractFour squaraine dyes bearing various N substituents with different electron‐withdrawing abilities were synthesized in this study. The effects of these substituents on the photophysical properties and stabilities of the squaraine dyes were analyzed. Squaraine‐doped polymer films were fabricated using the synthesized squaraine dyes, and all films demonstrated excellent transmittances of ~100 % in the non‐absorptive region, along with extremely low transmittances in the 630–640 nm wavelength region. The thermal stabilities of the squaraine films were evaluated after thermal treatment at elevated temperatures. The film fabricated using polysulfone, which possesses a high glass‐transition temperature, maintained an absorption of >97 % after heat treatment, indicating its excellent thermal stability. Furthermore, all squaraine films maintained absorptions of >70 % after 24 h of light irradiation, surpassing the photostabilities of conventional polymethine dyes. It was concluded that squaraine dyes are sufficiently effective and applicable for producing optical films, owing to their excellent optical properties and high stabilities.