Abstract
Lanthanide appended rotaxanes have been prepared by the CuAAC ‘click’ reaction between an azide appended rotaxane and lanthanide complexes of propargyl DO3A. The resulting complexes are luminescent, and exhibit chloride responsive luminescence behavior consistent with the existence of two independent halide binding pockets, one in the rotaxane cavity and one on the ninth (axial) coordination site of the lanthanide. Strong halide binding to europium gives rise to changes in the relative intensity of the hypersensitive ΔJ = 2 transition compared to the rest of the europium emission spectrum, combined with quenching of the overall intensity of emission as a consequence of non-radiative quenching by the bound halide. The weaker interaction with the rotaxane pocket mediates a subsequent recovery of intensity of the europium centered luminescence despite the considerable separation between the lanthanide and the rotaxane binding pocket.
Highlights
In 2006 we reported the synthesis and properties of a d–f hybrid pseudorotaxane that was assembled through anion templation in non-competitive solvent media,[13] but which dissociates in competitive solvents
[298 900–1 014 000]c [264 200–354 600]c [52 520–64 300]c [87 554–102 540]c [54 780–86 250]c [20 290–26 310]c a Obtained from DJ ratios binding plots. b Modelling the observed data to an independent binding model for the two binding sites gave two association constants (K1 1⁄4 750 000 MÀ1 and 1360 MÀ1) that can be assigned to binding at the lanthanide and in the rotaxane cavity. c Con dence intervals determined as the values of K that are possible while keeping c2 with 99% of the minimum value
NMR studies on 4$PF6 were used to probe the affinity of the rotaxane for anions, revealing that the association constant with chloride (K1 1⁄4 1460 MÀ1) (CDCl3, MeOD, D2O, 45 : 45 : 10) was signi cantly greater than that for dihydrogenphosphate or acetate (K1 1⁄4 188 MÀ1 and K1 1⁄4 115 MÀ1 respectively). These observations lend further credence to the observation of two binding events in the titration of Eu$1 with chloride. These results show that lanthanide containing rotaxanes can be constructed via a modular approach, where an appropriately functionalized rotaxane can be coupled to a lanthanide centre
Summary
Comparison of the 1H spectra for Ln$1 with published data for other related systems15a clearly indicates an octadentate ligand bound to the metal centre (as opposed to the heptadentate ligand in Ln$5), with a square antiprismatic geometry predominating at the lanthanide centre This is in accordance with previous observations on the triazoloDO3A system.[10,15] Though the protons on the rotaxane are too far from the paramagnetic lanthanide centre to be shi ed signi cantly as a consequence of the presence of the lanthanide, the NMR spectrum of the Lu$1 complex is entirely consistent with the structure of the rotaxane domain being closely related to 4$Cl. Luminescence spectroscopy was used to probe the association of Ln$1 with chloride, dihydrogen phosphate and acetate in dichloromethane solution. Involves a substantial change in the intensity of the hypersensitive DJ 1⁄4 2 transition at 617 nm relative to the other europium centered transitions This is clear evidence for binding of chloride to the ninth (axial) donor site on the lanthanide complex.[16] For the rst strong binding event, the association constant could be determined from the luminescence titration data. The absence of a second binding event in the rotaxane cavity is to be expected given the known selectivity of response to chloride for related rotaxane
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