Quantifying the formation of chiral luminescent lanthanide assemblies in an aqueous medium through chiroptical spectroscopy and generation of luminescent hydrogels. (25th September 2015)

• Record Type: Journal Article
• Authors: Bradberry, Samuel J.; Savyasachi, Aramballi Jayant; Peacock, Robert D.; Gunnlaugsson, Thorfinnur
• Is Part Of: Faraday discussions. Volume 185(2015)
• Page Start: 413
• Page End: 431
• Publication Date: 2015-09-25
• Subjects: Chemistry -- Periodicals; Metallurgy -- Periodicals; Electrochemistry -- Periodicals; 540
• DOI: 10.1039/c5fd00105f
• Languages: English
• ISSNs: 1359-6640

Abstract

Abstract : Herein we present the synthesis and the photophysical evaluation of water-soluble chiral ligands (2 ·( R, R ) and2 ·( S, S )) and their application in the formation of lanthanide directed self-assembled structures. These pyridine-2, 6-dicarboxylic amide based ligands, possessing two naphthalene moieties as sensitising antennae, that can be used to populate the excited state of lanthanide ions, were structurally modified using 3-propanesultone and caesium carbonate, allowing for the incorporation of a water-solubilising sulfonate motif. We show, using microwave synthesis, that Eu(iii ) forms chiral complexes in 1 : 3 (M : L) stoichiometries (Eu·[2 ·( R, R )]3 and Eu·[2 ·( S, S )]3 ) with these ligands, and that the red Eu(iii )-centred emission arising from these complexes has quantum yields ( Φ tot ) of 12% in water. Both circular dichroism (CD) and circular polarised luminescence (CPL) analysis show that the complexes are chiral; giving rise to characteristic CD and CPL signatures for both the Λ and the Δ complexes, which both possess characteristic luminescence dissymmetry factors ( g lum ), describing the structure in solution. The self-assembly process was also monitored in situ by observing the changes in the ligand absorption and fluorescence emission, as well as in the Eu(iii ) luminescence. The change, fitted using non-linear regression analysis, demonstrated high binding affinity for Eu(iii ) which in part can be assigned to being driven by additional hydrophobic effects. Moreover, using CD spectroscopy, the changes in the chiroptical properties of both (2 ·( R, R ) and2 ·( S, S )) were monitored in real time. Fitting the changes in the CD spectra allowed for the step-wise binding constants to be determined for these assemblies; these matched well with those determined from both the ground and the excited state changes. Both the ligands and the Eu(iii ) complexes were then used in the formation of hydrogels; the Eu(iii )-metallogels were luminescent to the naked-eye.