Photophysical properties and energy transfer mechanism of three novel lanthanide upconverting materials (UCMs)

Three novel lanthanide upconverting materials [LN(2,5-HPA)(2,5-PA)]n (LN ​= ​Sm, 1; Dy, 2; Ho, 3; 2,5-H2PA ​= ​2,5-pyridinedicarboxylic acid) have been synthesized through hydrothermal reactions and structurally characterized by single crystal X-ray diffraction technique. These materials are crystallographically isostructural and are characteristic of three-dimensional (3-D) frameworks. The photoluminescence measurements with solid-state samples reveal that compounds 1–3 exhibit upconversion photoluminescence emissions in red, yellow and green region, respectively. The photoluminescence emission bands could be assigned to the characteristic emission of the 4f electrons intrashell transition of the 4G5/2 ​→ ​6H7/2, 4G5/2 ​→ ​6H9/2 of the Sm3+ ions in 1, 4F9/2 ​→ ​6H17/2, 4F9/2 ​→ ​6H15/2, 4F9/2 ​→ ​6H13/2 of the Dy3+ ions in 2, and 5G6 → 5I8, 5S2 → 5I8 of the Ho3+ ions in 3. An energy transfer mechanism is explained by the energy level diagrams of the lanthanide ions and the 2,5-H2PA ligand. Solid-state UV/Vis diffuse reflectance spectra reveal that they are potential wide optical band gap semiconductors with the band gaps being of 3.73, 3.56 and 3.45 ​eV for 1–3, respectively. TG curves reveal that all of them are very thermal stable with the onset temperature being of 219 ​°C–322 ​°C. The photoluminescence quantum yields of compounds 1–3 were determined to be 14.1%, 18.3% and 19.6%, respectively.