The Parallel Coplanar Alignment of Planar Non‐π‐Conjugated [Pb 3 Br 3 ] Rings for a Stable Ultraviolet Nonlinear Optical Crystal

Ultraviolet (UV) nonlinear optical (NLO) crystals play an indispensable and critical role in UV solid‐state lasers. However, the targeted synthesis of high‐performance UV NLO crystals remains an ongoing challenge due to their mutual constraint and conflicting properties. Herein, we have proposed the strategy of replacing the π‐conjugated group with the planar non‐π‐conjugated unit to widen bandgap and finely reduce the birefringence while maintaining strong second‐harmonic generation (SHG) response, resulting in a new UV NLO crystal with comprehensive performance, K 2 Pb 3 [H 2 N(CH 2 COO) 2 ] 3 Br 5 . It is intriguing that all planar non‐π‐conjugated [Pb 3 Br 3 ] rings are aligned into a parallel coplanar arrangement, giving rise to the superposition of the microscopic second‐order susceptibility of the large [Pb 3 Br 5 O 6 ] clusters. Remarkably, K 2 Pb 3 [H 2 N(CH 2 COO) 2 ] 3 Br 5 simultaneously possesses the widest bandgap among lead oxybromide NLO crystals, a broad high transparency window extending to 340 nm, a phase‐matching SHG response reaching 2.0 × KH 2 PO 4 , suitable birefringence of 0.089@1064 nm, high thermal stability up to 270°C in air and good crystal growth habit. On the basis of structural analysis and theoretical calculations, the [Pb 3 Br 5 O 6 ] cluster makes the main contribution to SHG response. This study puts forward a new method for the exploration of high‐performance UV NLO crystals.