Birefringent crystals are indispensable for polarizing optics in applications such as laser modulation, UV detection, and quantum state control. Current commercial inorganic crystals (e.g., α?BBO, Δn=0.122 at 546 nm) exhibit limited birefringence, hindering the development of miniaturized optical devices, especially those operating in the solar-blind UV region (200?280 nm). Organic π?conjugated functional building units (FBUs) offer superior optical anisotropy due to strong electron delocalization and tunable weak interactions. This work aims to design a high-birefringence molecular crystal for solar-blind UV applications by strategically combining two anisotropic FBUs—melamine (C₃N₆H₆) and cyanuric acid (H₃C₃N₃O₃)—through hydrogen-bond-directed assembly.

The melamine-cyanuric acid cocrystal (CNCO) was synthesized hydrothermally by reacting melamine and cyanuric acid (1∶1 molar ratio) at 180 ℃ for 10 h, followed by slow cooling. Phase purity was confirmed via powder X-ray diffraction (XRD), with experimental data matching theoretical simulations. Hydrogen bonding was characterized using Fourier-transform infrared spectroscopy (FTIR). The UV cutoff edge was determined from diffuse reflectance spectra, with the band gap derived via the Kubelka?Munk method. First-principles calculations were performed using the CASTEP package within the framework of density functional theory (DFT). The generalized gradient approximation (GGA-PBE) was used for geometry optimization, while the HSE06 hybrid function was employed to calculate electronic properties. Refractive indices and birefringence were derived from frequency-dependent dielectric functions. Hirshfeld surface analysis (Fig. 3) and differential charge density maps elucidated intermolecular interactions.

CNCO crystallizes in the I 2/m space group, forming a 2D layered structure via N—H…O (2.063—2.103 ?) and N—H…N (2.011 ?) hydrogen bonds between melamine and cyanuric acid molecules. These layers stack vertically with a spacing of 3.108 ?, stabilized by π-π interactions. Hirshfeld surface analysis confirmed strong in-plane N—H…O (proportion of contact area of 43.01%) and N—H…N (16.39%) hydrogen bonds, alongside out-of-plane π-π stacking (24.97%). FTIR revealed a significant redshift in N—H stretching vibrations (3362?3396 cm?1 vs. 3419?3470 cm?1 in pure melamine), validating strong H-bond-induced proton delocalization. UV spectroscopy showed a wide band gap of 5.06 eV (cutoff edge of ~250 nm) in CNCO, confirming its solar-blind UV transparency. Crucially, CNCO exhibited a giant birefringence of 0.508 at 400 nm—approximately four times larger than α-BBO (0.130 at 400 nm)—and 0.377 at 1064 nm. This surpasses most reported cyanurate or melamine-based crystals (e.g., K₂Pb(H₂C₃N₃O₄)₄(H₂O)₄, Δn=0.325 at 532 nm; (C₃N₆H₇)BF₄·H₂O, Δn=0.37 at 546 nm). The exceptional Δn arises from synergistic effects: hydrogen bonds enforce parallel alignment of FBUs within layers, while π?π stacking enhances in-plane polarizability anisotropy.Electronic structure analysis (Fig. 6) revealed that the valence band maximum (VBM) is dominated by N 2p orbitals of melamine, while the conduction band minimum (CBM) comprises C 2p orbitals of the [C₃N₃] rings (from both melamine and cyanuric acid) and O 2p orbitals of cyanuric acid. The delocalized pπ electrons and anisotropic O 2p contributions underpin the huge optical anisotropy.

This work demonstrates CNCO as a record-high birefringence material (Δn=0.508 at 400 nm) for the solar-blind UV region. Its performance stems from the synergy of hydrogen bonds and π-π interactions, which enforce planar, parallel alignment of π?conjugated FBUs, maximizing in-plane polarizability. Experimental characterization (XRD, FTIR, UV-Vis) and DFT calculations consistently validate its wide band gap (5.06 eV), structural integrity, and giant birefringence. This study establishes hydrogen-bond-regulated molecular ordering as a powerful strategy for designing birefringent crystals, addressing the critical need for high-performance materials in solar-blind UV optics.