Inorganic-organic hybrid compounds based on face-sharing octahedral [PbI₃]_∞ chains: Self-assemblies, crystal structures, and ferroelectric, photoluminescence properties

Eight inorganic-organic hybrid compounds with a formula of [R-Bz-1-APy][PbI₃] (R-Bz-1-APy⁺ = mono-substituted benzylidene-1-aminopyridinium Schiff base derivative; R = m-CN (1), m-CH ₃ (2), H (3), p-F (4), p-Cl (5), p-Br (6), o-Cl (7), o-Br (8)) have been synthesized and characterized structurally. The common characteristic of the crystal structures of 1-8 is that the inorganic components form straight and face-sharing octahedral [PbI₃]_∞ chains and the Schiff base cations surround the [PbI₃]_∞ chains to form molecular stacks. The substituent (R) on the phenyl ring of the Schiff base cation clearly influences the packing structures of 1-8, and the hybrid compound crystallizes in the space group P6₃ when R = CN (1) in the meta-position of the phenyl ring, and in a central symmetric space group when R is in the ortho- or para-position of the phenyl ring. The conformation of the Schiff base cation is related to the R position, and the dihedral angle between the phenyl and pyridyl rings increases in the order of para- < meta- < ortho-position substitution of the phenyl ring. The long molecular axis of the Schiff base cation adopts a manner approximately parallel to the straight inorganic [PbI₃]_∞ chain in the para-substituted hybrid compounds, and perpendicular to the straight inorganic [PbI ₃]_∞ chain in the ortho-substituted hybrid compounds. 1 is second harmonic generation (SHG) active with a comparable response as that of urea and also exhibits ferroelectricity with larger P_s and P _r values; 1-8 emit multi-band luminescence in the 300-650 nm regions under the excitation of ultraviolet light.