Synergistic Effect of Sulfur and Chalcogen Atoms on the Enhanced Refractive Indices of Polyimides in the Visible and Near-Infrared Regions

To develop thermally stable optical polymers for visible and near-infrared sensor applications, a series of sulfur-containing polyimides (PIs) with chalcogen atoms were successfully synthesized via conventional two-step polycondensation of 4,4′-[p-thiobis­(phenylene­sulfanyl)]­diphthalic anhydride (3SDEA) with four diamines: 4,4′-oxidianiline (ODA), 4,4′-thiodianiline (TDA), 4,4′-selenodianiline (SEDA), and 4,4′-tellurodianiline (TEDA). Because of the large atomic polarizabilities of the sulfur, selenium, and tellurium atoms, the resultant PIs exhibited significantly high refractive indices in the 1.738–1.778 range at 637 nm along with a transmittance >90% at 650–1500 nm. In addition, the PIs exhibited good thermal stability, with high thermal decomposition and glass transition temperatures (T d5% > 390 °C and T g = 183–217 °C, respectively). Owing to the good affinity of SEDA–3SDEA for TiO2 nanoparticles, a nanocomposite film with a 3.0 wt % loading of TiO2 nanoparticles exhibited a refractive index of 1.774 at 637 nm.