Structural and optical properties of pillared Eu3+-containing layered double hydroxides intercalated by 2- to 12-carbon aliphatic dicarboxylates

Abstract

While the versatile composition of layered double hydroxides (LDHs) allows the introduction of almost any polyvalent cation in their structure, the exchangeable negatively charged ions intercalated between the plates increase their vast range of functionalities. Here we report on the preparation and optical properties of pillared Eu3+-substituted ZnAl LDHs intercalated by aliphatic dicarboxylates: ‒OOC‒(CH2)n–2‒COO‒. The basal distance in these materials is dependent on the size and packing of the intercalated anions. By varying the number (n) of carbon atoms in the aliphatic chain from 2 to 12, the interlayer gallery of these ZnAlEu LDHs is considerably expanded from 0.9 to 2.1 nm. In the interlayer gallery, the aliphatic dicarboxylates form a monolayer, with the aliphatic chain inclined by an angle α ≈ 63° with the hydroxide layers. The carbon atom in the COO– group is 0.43 nm far from the metal plane, showing that these carboxylates are not grafted in the brucite-like layers. These LDHs are thermally stable up to around 150 °C, after which dehydroxylation of the hydroxide layers is observed. The Judd-Ofelt intensity parameters were obtained and compared to the Eu3+-containing complexes with the same ligand series indicating a more symmetrical and less polarizable chemical environment around the rare earth ion.