Lanthanide appended rotaxanes respond to changing chloride concentration †

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 DJ 1⁄4 2 transition compared to the rest of the europium emission spectrum, combined with quenching of the overall intensity of emission as a consequence of nonradiative 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.


Synthetic procedures
Commercially available solvents and chemicals were used without further purification unless otherwise stated. Where dry solvents were used, they were degassed with nitrogen, dried by passing through an MBraun MPSP-800 column and then used immediately. Triethylamine was distilled from and stored over potassium hydroxide. Water was deionised and microfiltered using a Milli-Q Millipore machine.
Tetrabutylammonium (TBA) salts were stored under vacuum in a desiccator. Routine 300 MHz NMR spectra were recorded on a Varian Mercury 300 spectrometer, 1 H NMR operating at 300 MHz, 13 C at 75.5 MHz. All 500 MHz 1 H Spectra and all 1 H NMR titrations were recorded on a Varian Unity Plus 500 spectrometer. 700 MHz 1 H Spectra were recorded on a Varian VNMRS-700. All chemical shift (!) values are given in parts per million.
Low resolution mass spectra were recorded on a Micromass LCT Premier XE spectrometer. Accurate masses were determined to four decimal places using Bruker µTOF and Micromass GCT spectrometers.
Luminescence spectra were measured on a Horiba Jobin Yvon FluoroLog-3 equipped with a Hamamatsu R928 PMT detector and a double-grating emission monochromator. In the case of the ytterbium complex, the sample was excited using a pulsed nitrogen laser (PTI-3301-337nm. Light emitted at right angles to the excitation beam was focused onto the slits of the monochromator (PTI120), which was used to select the appropriate wavelength. The growth and decay of the luminescence at selected wavelengths was detected using a germanium photodiode (Edinburgh Instruments, EI-P) and recorded using a digital oscilloscope (Tektronix TDS220) before being transferred to the computer for analysis. Luminescence lifetimes were obtained by iterative reconvolution of the detector response (obtained by using a scatter) with exponential components for growth and decay of the metal centred luminescence for ytterbium complexes or by tail fit for europium complexes, using a spreadsheet running in Microsoft Excel.

Synthesis of azidobenzene 4
Aniline (204 mg, 2.2 mmol) was dissolved in anhydrous acetonitrile (6 mL) and this solution was left to cool in an ice-water bath for ten minutes. To this t-BuONO was added (340 mg, 3.3 mmol) followed by dropwise addition of TMSN 3 (305 mg, 2.6 mmol). After stirring under nitrogen atmosphere at room temperature for 1 h 30 minutes, the solvents were removed in vacuo and the product was purified by silica gel chromatography (hexane) to obtain a pale yellow oil (224 mg, 86%). 1

General Procedure for the Synthesis of Lanthanide Rotaxane
To a solution of azide rotaxane 4·PF 6 in CH 2 Cl 2 /MeOH (4:1, 5 mL) was added the appropriate lanthanide complex Ln·5 (3 molar eq.). The solution was stirred for 5 minutes before Cu I (MeCN) 4 PF 6 (0.1 molar eq.) and tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine (0.1 molar eq.) were added. The solution was stirred for 48 hours at room temperature in a nitrogen atmosphere after which the solvent was removed in vacuo. The crude solid was dissolved in DCM (10 mL) and the organic phase was washed with H 2 O (2 " 10 mL), with an NH 4 PF 6 (aq) solution (0.1M; 10 " 10 mL) and H 2 O (2 " 10 mL) to ensure the presence of only PF 6 as the counter ion. After removal of the organic solvent in vacuo the crude solid was redissolved in toluene and filtered. Purification by size-exclusion chromatography (Bio-Beads SX-3/ toluene) gave the compound as a yellow solid.         yellow; a = 13.2580 (9)

Fitting using Dynafit
Dynafit was used to fit the responses observed in the luminescence spectrum of the Eu complexes upon addition of anion to the solution. As the dilution method was used for the titrations, the concentration independent intensity ratio was used to determine the association constant. If intensities had to be used, only data obtained with a dilution of less than 5% were employed. As the binding behavior in most cases a simple 1:1 binding, the advanced capabilities of Dynafit were not used. The script used to determine all the association constants is given below.