Circularly polarized luminescence from Tb(iii) interacting with chiral polyether macrocycles

A straightforward two-step synthesis protocol affords a series of chiral amide-based bis-pyridine substituted polyether macrocycles. One ligand is particularly able to complex terbium(iii) ions spontaneously. Upon complexation, interesting chiroptical properties are observed both in absorbance (ECD) and in fluorescence (CPL). In ligand-centered electronic circular dichroism, a sign inversion coupled with a signal enhancement is measured; while an easily detectable metal-centered circularly polarized luminescence with a glum of 0.05 is obtained for the main 5D4 → 7F5 terbium transition. The coordination mode and structure of the complex was studied using different analysis methods (NMR analysis, spectrophotometric titration and solid-state elucidation).

Enantiomers of ligand 1a were resolved by chiral stationary phase HPLC on an Agilent 1260 Infinity II apparatus (quaternary pump, auto sampler, column thermostat and diode array detector) using a semi-preparative CHIRALPAK® IG column (250 x 10 mm, 5 mic). Mixtures of HPLC grade CH2Cl2 and MeOH (99:1, with 0.1% diethanolamine as additive) were used as mobile phase.

Optical properties
Optical properties were recorded in analytical grade solvent (acetonitrile). UV-Vis absorption spectra were recorded on a JASCO V-650 spectrophotometer at 20 °C. Electronic circular dichroism (ECD) spectra were recorded on a Jasco J-815 spectropolarimeter at 20 °C in a 1 cm cuvette.
Fluorescence spectra were measured using a Varian Cary 50 Eclipse spectrophotometer. All fluorescence spectra were corrected for the wavelength-dependent sensitivity of the detection. Fluorescence quantum yields φ were measured in diluted solutions (at least 5 different concentrations for each sample) with an optical density lower than 0.1 using the following equation: where A is the absorbance at the excitation wavelength (λ), n the refractive index and D the integrated intensity. "r" and "x" stand for reference and sample respectively. The fluorescence quantum yields were measured in acetonitrile relative to 9,10-diphenylanthracene (φ = 93% in cyclohexane). Excitation of reference and sample was performed at the same wavelength.
Circularly polarized luminescence (CPL) spectra were recorded with the home-made spectrofluoropolarimeter previously described. 1 The samples were excited with a 254 nm fluorescent mercury lamp, using a 90° geometry between excitation and detection.
Ba(ClO4)2 and Tb(OTf)3 salts used for titration experiments were purchased from commercial sources and used without purification.
Lifetimes were determined using the phosphorescence mode of a Fluorolog 3 spectrophotometer (Horiba Jobin Yvon) in which the lamp of the instrument is flashed. Excitation was performed at 305 nm (1 nm slit) and detection with a visible photomultiplier tube (220-850 nm, R928P, Hamamatsu) at 545 nm (3 nm slit) at 545 nm, with an initial time gate of 50 s.

S6
2 Synthesis and characterization of organic compounds 2.1 Synthesis of unsaturated ester macrocycle 2 Unsaturated ester macrocycle 2 was synthetized according to previously reported procedure from the literature 2 :

Resolution of ligand 1a 6
Compound 1a were resolved by chiral stationary phase HPLC using a semi-preparative CHIRALPAK® IG column using a mixture of CH2Cl2-MeOH (99:1, with 0.1% diethanolamine as additive) as mobile phase at 20 °C. It is worth mentioning that it is necessary to remove traces of diethanolamine present in the separated compounds. The residue was thus dissolved in CH2Cl2, the organic phase was washed three times with H2O, dried over anhydrous Na2SO4, filtered and concentrated under vacuum to afford the pure products.
In the Figure S2 is shown the HPLC traces of ligand 1a on analytical CHIRALPAK® IG column (left, test run) and on semi preparative CHIRALPAK® IG column (right, run for resolution) with CH2Cl2-MeOH (99:1, 0.1% diethanolamine) as mobile phase.  For the qualitative test, three solutions in three different vials (1 mL) were prepared and their emission was compared under UV irradiation (366 nm excitation wavelength). In the first one (reference 1), only the macrocycle of interest (<1 mg) is dissolved in acetonitrile. In the second one (reference 2), only terbium triflate (tip of a spatula) was dissolved in acetonitrile. In the third one, a mixture of the macrocycle of interest (<1 mg) and terbium triflate (tip of a spatula, excess) were dissolved in acetonitrile. In the reference 1 (1 st vial), only the fluorescence of the macrocycle can be observed when visible. In the reference 2, no emission of the terbium salt was observed at this wavelength, but for the third vial (macrocycle/Tb mixture) resulted in the characteristic green terbium emission (see below). Combination of ligand 1a and terbium presents the most efficient luminescence and were selected for this study.  Figure S5. Absorbance (red and blue lines) and fluorescence (pink and green lines) spectra of ligand 1a without (red and pink lines) or with 3.0 equivalents of Tb(III) (blue and green lines).

Titration of ligand 1a with Tb(III)
In a typical experiment, a known aliquot of a Tb(OTf)3 solution in acetonitrile (ca. 2·10 -3 M) was added to a solution of the ligand (ca. 0.5·10 -6 M) in acetonitrile. Spectra were recorded from 0 equivalent of Tb(III) to 4.0 equivalents.   The change in intensity in ECD is quantified using δΔε, which is the difference in normalized ECD intensity in presence and absence of tested metal ions: For CPL: The  To an NMR tube, LuCl3 was added (0, 0.5, 1 and 2 equivalents) as a solid. Just before the measurement 0.5 mL of a 15 mM solution of 1a was added and the tube was shaken, and the 1 H spectrum was recorded.
6.2 1 H-NMR spectra Figure S15, 1  A 10 -5 M solution of 1a (S1) and a 10 -3 M solution of Tb(OTf)3 (S2) both in MeCN or MeCN-d3 were prepared. The samples were prepared by mixing the 2 solutions directly in a cuvette.
The lifetime of complex 1a in acetonitrile was determined using the phosphorescence mode of a Fluorolog 3 spectrophotometer (Horiba Jobin Yvon) in which the lamp of the instrument is flashed. Excitation was performed at 305 nm (1 nm slit) and detection with a visible photomultiplier tube (220-850 nm, R928P, Hamamatsu) at 545 nm (3 nm slit) at 545 nm, with an initial time gate of 50 s.