The birefringence optimization by hydroxyl in organic nonlinear optical crystals

Owing to the urgent requirement of short-wave ultraviolet (UV) nonlinear optical (NLO) crystals, organics with planar groups attract much attention, which usually exhibit a high optical anisotropy and a large second harmonic generation (SHG) response. Herein, we found glycolamide crystals based on the structure modulation of acetamide by hydroxyl. With a staggered arrangement of planar groups, acetamide crystal exhibits such an insufficient optical anisotropy that could not satisfy the phase-matching condition. To address this limitation, a targeted molecular engineering approach was implemented by introducing hydroxyl into methyl, which induces a configuration reconstruction by dual hydrogen bonds, resulting in a uniformly arranged glycolamide structure. As a bifunctional unit (acting as both a hydrogen donor and an acceptor), the hydroxyl improves not only the uniformity of molecular arrangement, but also their spatial density. Therefore, glycolamide crystals exhibit a substantially modified birefringence (0.07 @ 546 nm) and an enhanced SHG response (1.6 × KH 2 PO 4 ), while they maintain a short UV absorption cutoff edge (about 210 nm). This work establishes a novel paradigm for performance optimization in organic NLO crystals by rational hydrogen bond engineering.