The effect of metal distribution on the luminescence properties of mixed-lanthanide metal-organic frameworks

A series of lanthanide metal-organic frameworks (MOFs) of the general formula [Ln(Hodip)(H 2 O)]· n H 2 O (Sm, 1 ; Eu, 2 ; Gd, 3 ; Tb, 4 ; Dy, 5 ; Er, 6 ; H 4 odip = 5,5′-oxydiisophthalic acid) have been prepared and shown crystallographically to have isostructural three-dimensional frameworks. The fluorescence emission spectra of the europium compound 2 , which is red, and the terbium compound 4 , which is green, show characteristic peaks for transitions involving the metal centres, whereas that for the gadolinium compound 3 is dominated by transitions involving Hodip. Using a 1 : 1 : 1 mixture of europium, gadolinium and terbium nitrates in the synthesis resulted in the mixed-metal MOF [Gd 0.17 Tb 0.19 Eu 0.64 (Hodip)(H 2 O)]· n H 2 O 7 , for which the ratio of the metal ions was determined using EDX spectroscopy. The fluorescence emission spectrum of 7 is dominated by europium emission bands reflecting the higher proportion of Eu 3+ centres and quenching of the terbium fluorescence by metal-to-metal energy transfer. A series of core-shell MOF materials based on the Ln(Hodip)(H 2 O) framework have been prepared in order to isolate the lanthanides in different domains within the crystals. The emission spectra for materials with Gd@Tb@Eu ( 8 ) and Tb@Eu@Gd ( 9 ) are dominated by terbium emissions, suggesting that physical separation from europium suppresses quenching. In contrast, the material with Eu@Gd@Tb ( 10 ) shows only broad ligand bands and europium emissions. This confirms that core-shell MOFs have different fluorescence properties to simple mixed-metal MOFs, demonstrating that the spatial distribution of the metals within a mixed-lanthanide MOF affects the fluorescence behaviour. The MOFs [Ln(Hodip)(H 2 O)]· n H 2 O (H 4 odip = 5,5′-oxydiisophthalic acid) have been prepared together with mixed-metal analogues, and the distribution of the metal centres has been shown to affect the luminescence properties.