Point Chirality Regulated Diastereoselective Self-Assembly and Circularly Polarized Luminescence in Eu(III) Triple-Stranded Helicates

The supramolecular chirality of lanthanide helicate offers a structural base to synthesize the excellent lanthanide circularly polarized luminescent (CPL) materials. However, the larger radii and the labile coordination geometries of the Ln(III) ions make it difficult to control the diastereoselectivity of lanthanide helicates in the self-assembly process. Herein, a pair of chiral dinuclear europium triple-stranded helicates [Eu-2(L-RR)(3)](OTf)(6) and [Eu-2(L-SS)(3)](OTf)(6) (L-RR/SS = N,N'-(ethane-4,4'-diyl)bis[6-(R/S)-(1-phenethylcarbamoyl)-pyridine-2-dimethylamide]) were synthesized via point chirality induced strategy. The ligands L-RR/SS were composed of two chiral 2,6-dipicolinic amides as coordination units and ethylenediamine moieties as spacers. General procedure for the preparation of ligands L-RR/SS and corresponding europium complexes [Eu-2(L-RR/SS)(3)](OTf)(6): 6-(Methoxycarbonyl)picolinic acid 4a (1.50 g, 5.6 mmol) was dissolved in 5 mL SOCl2 and stirred for 6 h. The white precipitate was obtained after removed SOCl2, and then it was added to 30 mL CH2Cl2 containing 4-dimethylaminopyridine (0.12 g, 1.0 mmol) and triethylamine (0.48 g, 8.0 mmol). Ethylenediamine (0.12 g, 2.0 mmol) in 5 mL CH2Cl2 was added by dropwise to the above solution and further stirred for 10 h. After the solution was washed with 1 HCl, saturated sodium bicarbonate and water, dried over sodium sulfate, and evaporated to remove solvent. The crude product was purified by crystallization from CH2Cl2:n-hexane (V/V=1:20) to give L-RR/SS (0.54 g, 47%). L-RR/SS (0.10 g, 0.18 mmol) dissolved in 10 mL acetonitrile, Eu(OTf)(3) (0.07 g, 0.12 mmol) in 5 mL acetonitrile was added dropwise to the above solution and stirred for 24 h. After the slow volatilization of reaction solution, the white crystals were obtained, [Eu-2(L-RR/SS)(3)](OTf)(6) (0.12 g, 71%). Combination of the comprehensive spectral characteristics and semiempirical geometry optimization demonstrated that the point chirality at the terminal of the ligand successfully controlled the Delta or A configuration around the metal center and the P or M helical confonnation of the helicates. The mirror-image circular dichroism (CD) and CPL spectra further confirmed the formation of the enantiomer pairs. Notably, the helicates exhibit excellent CPL emission with the vertical bar glum vertical bar values reaching to 0.083 and the modest luminescence quantum yields (QYs=19%) in CH3CN. This study provides an effective strategy for the syntheses of chiral lanthanide CPL materials with excellent performance.