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.