Second-Harmonic Generation and Photoluminescence Properties of Sn(II)- and Bi(III)-Based Lone Pair Cation-Pyridine Dicarboxylate Coordination Compounds

Lone pair cation-based novel coordination compounds Sn[(pdc)(H₂O)] (Sn-I) and (H₂bpy)[Bi(pdc)₂(Hpdc)]·5H₂O (Bi-I) (pdc = pyridine-2,6-dicarboxylate; bpy = 4,4'-bipyridine) were synthesized through mild hydrothermal reactions. While Sn-I crystallizing in the polar space group, Pca2₁, exhibits a helical chain structure consisting of SnO₃N distorted seesaws, 2,6-pdc linkers, and water molecules, Bi-I crystallizing in the centrosymmetric (CS) space group, P1̅, reveals a pseudo-3D network composed of BiO₅N₃ polyhedra, 2,6-pdc ligands, H₂bpy²⁺ cations, and isolated H₂O molecules. The lone cations Sn²⁺ and Bi³⁺ in the title compounds are in a highly deformed polyhedral environment. The single-crystal-to-single-crystal transformation from Sn-I to the anhydrous Sn[(pdc)] (Sn-II) with the polar noncentrosymmetric structure was successfully achieved upon heating crystals of Sn-I. UV-vis diffuse reflectance spectra indicate that the introduction of Sn²⁺ or Bi³⁺ red-shifts the adsorption edges upon coordination. Powder second-harmonic generation (SHG) measurements indicate that Sn-I and Sn-II are type-I phase-matchable and exhibit SHG intensity of ca. 15 and 35 times that of α-SiO₂, respectively. Solid state photoluminescence (PL) measurements indicate that Bi-I is an excellent green emitting phosphor with the quantum efficiency up to 26% and outstanding decay lifetime of 1.82 ms at room temperature.