A Hydrogen-Bonded Molecular Crystal with Strong Optical Anisotropy and Large Second-Harmonic Generation Response

Functional crystals that exhibit a wide ultraviolet transparency window, a large second-harmonic generation (SHG) response, and high optical anisotropy are currently of great interest because of their promising applications in laser systems. Herein, the reported 4-hydroxyprine-3-sulfonic acid (C5H5NSO4, compound 1), was identified as a promising candidate due to its combination of non-π-conjugated [SO3C] tetrahedra and π-conjugated [4-C5H5O] groups connected via S-C bonds. Notably, compound 1 exhibits strong optical anisotropy with a birefringence value of 0.295 at 550 nm (calculated value 0.369), surpassing those of most recently reported sulfate- and sulfonate-based functional crystals. It also combines a large, phase-matchable SHG response with efficiency of 1.5 times that of KH2PO4 (KDP), a wide optical bandgap of 4.32 eV, and high thermal stability (up to 340 °C). These findings highlight that molecular sulfonate crystals involving π-conjugation are promising candidates for the exploration of functional crystals with targeted optical properties.