Sulfur‐Free Ultrahigh‐Refractive‐Index Polymers Enabled by Densely Packed Oxygen‐Bridged Dibenzofuran‐Cardo Architectures

ABSTRACT

Sulfur‐free polymeric materials simultaneously achieving ultrahigh refractive index ( n > 1.8), low optical dispersion, high visible transparency, and thermal robustness have remained elusive due to the fundamental polarizability–dispersion trade‐off. Here, we introduce an oxygen‐bridged dibenzofuran‐cardo (DBFX) motif as a sulfur‐ and heavy‐atom‐free platform combining enhanced intrinsic polarizability through benzofuran ring annulation with efficient intermolecular packing through a rigid 3D cardo geometry, thereby mitigating this trade‐off. DBFX was incorporated into polycarbonate (DBFX‐PC), polyurethane (DBFX‐PU), and polyacrylate (mDBFX‐PMA) backbones, yielding transparent amorphous polymers. The resulting materials achieve refractive indices of n _D = 1.753–1.848 at 589 nm, Abbe numbers of ν _D = 19.7–33.3, optically isotropic behavior, glass‐transition temperatures of 140°C–240°C, and visible internal transmittance of 91.7%–97.6%. Lorentz–Lorenz analysis supported by polymer density measurements, single‐crystal x‐ray diffraction, Hirshfeld surface analysis, and WAXS reveals that exceptional performance originates from simultaneous optimization of molar refraction and packing efficiency. Crucially, the orthogonally aligned dibenzofuran cardo unit inhibits charge transfer complex formation and maintains structural isotropy, providing a mechanistic basis for the unusually high Abbe numbers despite the ultrahigh refractive index. These results establish the DBFX platform as a powerful strategy for advanced photonic, optical coating, and optoelectronic applications.