(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

Materials with π-conjugated units usually show strong second-order harmonic generation but excessively large birefringence (Δn), a trade-off intensified by planar restrictions from π-π stacking. Here, we report a monoclinic P2₁ NLO crystal, (C₅H₇N₂)(C₈H₄O₄)PbBr (APPB), which resolves this conflict by tilting π-conjugated planes at a large angle relative to the Y optical axis to reduce Δn. The second-order Jahn-Teller distortion associated with the lone pair on Pb²⁺ yields to a low-symmetry distorted [PbO₄Br₂] 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 (Δn = n_z-n_x) to a favorable Δ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:msubsup><mml:mi>n</mml:mi> <mml:mrow><mml:mi>obv</mml:mi> <mml:mo>.</mml:mo></mml:mrow> <mml:mrow><mml:mo>(</mml:mo> <mml:mn>010</mml:mn> <mml:mo>)</mml:mo></mml:mrow> </mml:msubsup> <mml:annotation>$n_{{\mathrm{obv}}{\mathrm{.}}}^{{\mathrm{(010)}}}$</mml:annotation></mml:semantics> </mml:math> = 0.11@546 nm. Interestingly, APPB exhibits a strong SHG response (5.74 × K H₂PO₄). This study provides a novel structural tuning approach for balancing strong SHG and suitable Δn, offering design principles for high-performance NLO crystals.