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Updated: Mar 29, 2026

Spatial Separation of Molecular Conformers and Clusters
Published on: January 9, 2014
(C5H7N2) (C8H4O4) PbBr: Jahn-Teller Distortion-Induced Tilting of π-Conjugated Planes Resolving the Second-Order
Ming-Yan Yu1, Can Yang1, Yun-Jie Wang1
1State Key Laboratory of New Textile Materials and Advanced Processing, Wuhan Textile University, Wuhan, People's Republic of China.
Abstract:
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 = nx), lowering the birefringence (Δn = nz-nx) to a favorable Δ = 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.
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