Hierarchical lanthanide-based self-assembly complexes that undergo reversible luminescent response to both temperature changes and chemical reactions within competitive media

Abstract

Here, we report hierarchical self-assembly formation of a luminescent Eu³⁺-1-DPA complex in mixed aqueous–organic (THF or ethanol) solutions. Its reversible responsive behaviour to temperature change and in situ chemical reactions were investigated. The ligands 1-DPA and 1-DPA-acetate as well as the Eu(III) complex of 1-DPA, Eu³⁺-1-DPA were synthesised, and their X-ray crystal structures were determined. The photophysical and material (morphological) properties of the ligands and Eu(III) complexes formed in situ of both 1-DPA and 1-DPA-acetate were probed in mixed aqueous–organic (THF or ethanol) solutions. The ligands as well as the complexes were shown to aggregate into hierarchical and homogenously formed microspheres and microrods, as demonstrated by carrying out SEM of the drop-cast solutions of these complexes in ethanol–water mixtures. The Eu³⁺-1-DPA complex gave rise to Eu(III)-centred emission [due to deactivation of the ⁵D₀ → ⁷F_J (J = 1–4)]. It showed reversible, thermo-responsive lanthanide-centred emission, while in situ esterification using acidic anhydride (forming Eu³⁺-1-DPA-acetate, which was not emissive), and the subsequent NaOH-mediated hydrolysis of the Eu³⁺-1-DPA-acetate assembly induced both Eu(III)-emission and morphological changes, as demonstrated using both spectroscopic and SEM studies.