Luminescence tuning of MOFs via ligand to metal and metal to metal energy transfer by co-doping of 2∞[Gd2Cl6(bipy)3]·2bipy with europium and terbium

Abstract

The series of anhydrous lanthanide chlorides LnCl 3 , Ln ¼ Pr-Tb, and 4,4 0 -bipyridine (bipy) constitute (1-8), 0 # x, y # 0.5. The series of MOFs exhibits the opportunity of tuning the emission colour in-between green and red. Depending on the atomic ratio Gd:Eu:Tb, the yellow region was covered for the first time for an oxygen/carboxylate-free MOF system. In addition to a ligand to metal energy transfer (LMET) from the lowest ligand-centered triplet state of 4,4 0 -bipyridine, a metal to metal energy transfer (MMET) between 4f-levels from Tb 3+ to Eu 3+ is as well vital for the emission colour. However, no involvement of Gd 3+ in energy transfers is observed rendering it a suitable host lattice ion and connectivity centre for diluting the other two rare earth ions in the solid state. The materials retain their luminescence during activation of the MOFs for microporosity.

Interpretation of europium(III) spectra

“…has been studied in glasses [444,445], inorganic compounds [446][447][448], solid molecular compounds (including metal-organic frameworks) [449,450], and in solution [451][452][453][454][455][456][457][458][459][460][461][462].…”

Application of metal − organic frameworks

“…The MOF luminescence can be tuned through metal and organic ligand substitutions . The MOF [Ln 2 Cl 6 (bpy) 3 ].2bpy has been successfully used for an efficient tuning of the emission colour; this material is interesting as the luminescence is retained also during activation of the MOF for microporous materials . Different lanthanides can be incorporated into other frameworks such as [Ba 2 (H 2 O) 4 [LnL 3 (H 2 O) 2 ](H 2 O) n Cl] ∞ (L ≡ 4,4′‐disulfo‐2,2′‐bipyridine‐ N , N ′‐dioxide; Ln ≡ Sm 3+ , Eu 3+ , Gd 3+ , Tb 3+ , Dy 3+ ) (Fig.…”

Group 3 Elements and Lanthanide Metals

“…They can yield from reactions of anhydrous lanthanide halides like LnCl 3 and a melt of, for example, a linking amine like 4,4 ′ -bipyridine (bipy). Microporous twodimensional (2D) Ln-MOFs of the formula 2 ∞ [Ln 2 Cl 6 (bipy) 3 ] • 2bipy, C 10 H 8 N 2 , Ln = Nd-Yb (without Pm), were the first porous N-based MOFs for which statistic replacement of the lanthanide ions as connectivity centers was proven by formation of solid solutions [134]. They are also examples of MOFs for which reversible thermal formation and decomposition of the framework structure were shown [135].…”

“…The phenomenon is also observed for Ln-MOF nanocrystals like Tb(1,3,5-BTC)(H 2 O)⋅3H 2 O:Eu 3+ [64]. Suitable N-donor antenna ligands like bipy and lanthanide halides gave a series of solid solutions with three constituting lanthanide ions in 2 ∞ [Gd 2−x−y Eu x Tb y Cl 6 (bipy) 3 ] • 2bipy, 0 ≤ x, y ≤ 0.5, that exhibit a linear color tuning [134,135] (see Figure 9.4) with remarkable quantum yields. They are achieved by a reduction of quenching by concentration, as Gd ions as majority component of the host lattice reduce the proximity of the luminescent Eu and Tb ions.…”

“…Codoping of the MOF series with different Ln 3+ ions adapted for a linear tuning of the chromaticity of the luminescence (bottom). (Adapted with permission from[134]. Copyright 2012 Royal Society of Chemistry.…”