Enhanced Coplanarity and Giant Birefringence in Hydroxypyridinium Nitrate via Hydrogen Bonding between Planar Donors and Planar Acceptors.

Birefringent crystals, which possess optical anisotropy, are important optical components. However, designing and synthesizing birefringent crystals faces the challenge of achieving anisotropic structures, especially coplanar geometries. Herein, we achieve a significant birefringence in an ionic compound (C₅H₆ON)⁺(NO₃)^-, (4-hydroxypyridinium nitrate, 4HPN) by hydrogen bonding between planar donors and planar acceptors. We demonstrate that the interactions between the planar hydrogen bond donor ((C₅H₆ON)⁺) and planar hydrogen bond acceptor ((NO₃)^-) ensure the coplanarity during the crystal packing, generating the desired giant optical anisotropy. On two manually cut crystal chips, we observe a $\Delta n^{obv.}$ =0.494 ( $\Delta n_{max}^{cal.}$ =0.593), which is the largest among nitrates, or hydroxypyridinium derivatives. This $\Delta n^{obv.}$ value already surpasses those of the benchmark crystals, e.g., YVO₄ and CaCO₃, commonly used in the UV to visible and near IR spectral range. 4HPN also exhibits a strong second harmonic generation response (9.55×KDP). This strategy offers a promising avenue for the design and development of birefringent crystals with potential applications in optical communication, sensing and signal processing devices.