Preparation, structures, photophysical properties and energy transfer mechanism of two novel samarium and thulium compounds

Two novel samarium and thulium compounds [Sm(H2O)2(IA)3Hg2Cl5](HgCl2)1.5·H3O (1) and [Tm(H2O)2(IA)3Hg2Cl5](HgCl2)1.5·H3O (2) (IA ​= ​isonicotinate anion) have been prepared under hydrothermal conditions and structurally characterized by a single-crystal X-ray diffraction technique. They are isostructural and characterized by a two-dimensional (2-D) layer structure. The solid-state photoluminescence experiments unveiled that compound 1 shows a brownish emission band, while compound 2 exhibits three emission bands. These emission bands are resulted from the characteristic emissions of the 4f electrons intrashell transitions of 4G5/2 ​→ ​6H7/2 of the Sm3+ ion and 1G4 → 3H6, 1G4 → 3F4 and 3F2,3 ​→ ​3H6 of the Tm3+ ion. An energy transfer mechanism is proposed by an energy level diagram of the Sm3+, Tm3+ ions and the isonicotinic acid ligand. Compounds 1 and 2 have remarkable CIE chromaticity coordinates of (0.5261, 0.4615) and (0.2329, 0.218). The solid-state UV/visible diffuse reflectance spectra shows that compounds 1 and 2 have wide optical band gaps of 3.22 ​eV and 3.15 ​eV, respectively.