(C ₅ H ₇ N ₂ ) (C ₈ H ₄ O ₄ ) PbBr: Jahn–Teller Distortion‐Induced Tilting of π‐Conjugated Planes Resolving the Second‐Order Polarizability–Structural Anisotropy Conflicts

ABSTRACT Materials with π‐conjugated units usually show strong second‐order harmonic generation but excessively large birefringence (Δ n ), a trade‐off intensified by planar restrictions from π–π stacking. Here, we report a monoclinic P 2 1 NLO crystal, (C 5 H 7 N 2 )(C 8 H 4 O 4 )PbBr (APPB), which resolves this conflict by tilting π‐conjugated planes at a large angle relative to the Y optical axis to reduce Δ n . The second‐order Jahn–Teller distortion associated with the lone pair on Pb 2+ yields to a low‐symmetry distorted [PbO 4 Br 2 ] pentagonal pyramid with two distinct Pb–Br bond lengths of 3.14 and 2.83 Å. The weaker Pb–Br bond strengthens the trans‐positional Pb–O coordination (2.82 Å vs. the normal range of 2.54–2.60 Å), tilting the m ‐phthalate ligand plane by 58° relative to the Y optical axis (i.e., the b axis). This tilt raises the minimum refractive index along the a ‐axis ( n min = n x ), lowering the birefringence (Δ n = n z— n x ) to a favorable Δ = 0.11@546 nm. Interestingly, APPB exhibits a strong SHG response (5.74 × K H 2 PO 4 ). This study provides a novel structural tuning approach for balancing strong SHG and suitable Δ n , offering design principles for high‐performance NLO crystals.