Metal ion coordination enhancing quantum efficiency of ligand phosphorescence in a double-stranded helical chain coordination polymer of Pb2+ with nicotinic acid

A new 1D phosphorescence coordination polymer (CP) [Pb2O(C6H4NO2)2]n (1; C6H4NO2 = nicotinate) was synthesized by a solvothermal reaction and PbO was used as a Pb(II) source instead of traditional Pb(II) salts. This remarkably thermal-stable CP crystallizes in the space group I41/a. In the crystal structure of 1, two different Pb(II) ions show a five-coordinated and hemidirected coordination geometry, two nonequivalent nicotinate ligands link to Pb(II) ions in μ2–η1:η1 and μ4–η2:η2 modes, and the hemidirected coordination polyhedra of Pb(II) form a helical lead-oxide chain via an edge-sharing fashion along the c-axis. Under ambient conditions, 1 emits cyan ligand-based phosphorescence with an absolute quantum yield as high as 59.4% and a lifetime of 9.86 ms under UV-light irradiation. Under the same conditions, nicotinic acid emits simultaneously fluorescence and phosphorescence with a total absolute quantum yield of 4.8%. The great enhancement of phosphorescence quantum yield in 1, regarding nicotinic acid, is assigned to the heavy atom effect of Pb(II) and negligible π⋯π interaction between pyridyl rings. Noticeably, the vibronic fine structure is observed in the emission spectrum of 1 at room temperature. Additionally, 1 shows thermochromic behavior, and such functionality probably has realistic application in the field of temperature detection.