Tunable white-light emission PMMA-supported film materials containing lanthanide coordination polymers: preparation, characterization, and properties

Abstract

A series of lanthanide coordination polymers(LnCPs) containing both light and heavy rare-earth elements, namely {[Eu₂(pydc)₃(H₂O)]·2H₂O}_n (1-Eu, H₂pydc = pyridine-2,3-dicarboxylic acid), [Ln(pyc)₂(Hpyc)(NO₃)]_n (Ln = Nd (2-Nd), Sm (3-Sm), Eu (4-Eu), Gd (5-Gd), Tb (6-Tb), Ho (7-Ho), and Er (8-Er), Hpyc = pyridine-3-carboxylic acid), has been synthesized under hydro(solvo)thermal conditions and fully characterized. The crystal structure analysis indicates that in situ decarboxylation of H₂pydc occurred in the synthesis process of 2-Nd–8-Er. Coordination polymer 1-Eu displays a 3-D pcu network with central-symmetric quad-core structural units [Eu₄(COO)₆] linked by 1-D chains. 2-Nd–8-Er are of triple helical chain enantiomeric pair 6₁/6₅ axis, and can be further linked through two separate kinds of H⋯bonding interaction to form a mirror symmetrical 3-D framework; the final topological symbol of the jointly connected network is rare {4⁷·6⁸}. Solid-state luminescence studies show that the emission spectra of these LnCPs cover both the visible and near-infrared luminescence region. 2-Nd exhibits characteristic ⁴F_3/2 → ⁴I_J/2 (J = 9, 11, 13) transition NIR emission. 1-Eu and 4-Eu provide characteristic ⁵D₀ → ⁷F_J intense and bright red luminescence, while 4-Eu exhibits better luminescence performance because of the presence of the O–H oscillators within 1-Eu. 6-Tb exhibits characteristic ⁵D₄ → ⁷F_J intense and bright green luminescence. Furthermore, through doping with PMMA polymer, the luminescence properties of 4-Eu and 6-Tb are all improved. The results show the best doping concentration is 4%. The thermal stabilities of 4-Eu-PMMA and 6-Tb-PMMA increased from 270 to 315 °C when compared with single coordination polymers 4-Eu and 6-Tb. The co-doping of 4-Eu, 5-Gd, and 6-Tb (0.92/0.04/0.04) with PMMA at a total concentration of 4% resulted in a tunable luminescence material W_{(4-Eu,5-Gd,6-Tb)}-PMMA film. When excited at 285 nm, W_{(4-Eu,5-Gd,6-Tb)}-PMMA shows white-light emission with the CIE chromaticity coordinates of (0.33, 0.35). The lifetime of Eu³⁺ ions in W_{(4-Eu,5-Gd,6-Tb)}-PMMA increased from 1032.24 μs to 1272.26 μs when compared with 4-Eu-PMMA, and the efficiency of energy transfer (η_ET) from Tb³⁺ to Eu³⁺ within the co-doping PMMA film could be estimated to be 29.9%.