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

doi:10.6023/A21040185

Abstract

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 [Eu₂(L^RR)₃](OTf)₆ and [Eu₂(L^SS)₃](OTf)₆ (L^RR^/^SS=N,Nʹ-(ethane-4,4ʹ-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 [Eu₂(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 mol•L^-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, [Eu₂(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 Δ or Λ configuration around the metal center and the P or M helical conformation 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 |g_lum| 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.

Key words: lanthanide complex, chiral helicate, chiral induction, circularly polarized luminescence (CPL)

Cite this article

Longfei Song, Yanyan Zhou, Ting Gao, Pengfei Yan, Hongfeng Li. Point Chirality Regulated Diastereoselective Self-Assembly and Circularly Polarized Luminescence in Eu(III) Triple-Stranded Helicates[J]. Acta Chimica Sinica, 2021, 79(8): 1042-1048.

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Fig. & Tab. 7

Figure 1. Syntheses of the chiral ligands LRR/SS and the corresponding lanthanide complexes [Ln2(LRR/SS)3](OTf)6

Figure 2. (a) ESI-TOF mass spectrum of [Eu2(LSS)3](OTf)6 with insets showing the simulated (Sim.) and observed (Obs.) isotopic pattern. (b) 1H NMR (400 MHz) spectra of free ligand LSS and [Y2(LSS)3](OTf)6 in CD3CN. (c) 1H DOSY NMR spectrum of [Y2(LSS)3](OTf)6 in CD3CN

Figure 3. Sparkle/AM1 ground state geometries of the complexes [Eu2(LRR)3](OTf)6 (left) and [Eu2(LSS)3](OTf)6 (right)

Figure 4. (a) Variation of the UV-vis spectra of LSS in CH3CN (3×10-5 mol•L-1) upon addition of the Eu(CF3SO3)3 (CH3CN, 3×10-3 mol•L-1) from 0~2.0 equiv. (b) Calculated distribution plots of the species

Species	log β_M/L	% Species for M/[L^SS]	Concentration of Species/(10^-6 mol•L^-1)
log β₂₃	36.600	81.0	0.92
log β₂₂	26.636	9.5	0.11

Table 1. Binding constants, species distribution estimated by fitting the changes in absorption spectra, and the calculated species concentration of the complexes with the [Eu]/[LSS]=0.66

Figure 5. (a) UV-vis absorption (lower curves) and CD spectra (upper curves) of the chiral ligands LRR/SS and lanthanide helicates [Eu2(LRR)3](OTf)6 and [Eu2(LSS)3](OTf)6 in CH3CN (1.0×10-5 mol•L-1). (b) Total luminescence (lower curves) and CPL spectra (upper curves) of [Eu2(LRR)3](OTf)6 and [Eu2(LSS)3](OTf)6 in CH3CN (1.0×10-5 mol•L-1)

Complex	k_r/s^-1	k_nr/s^-1	τ_obs/μs	Φ_Eu/%	η_sens/%	Φ_overall/%	g_lum ⁵D₀→⁷F_J (J=0, 1, 2, 3, 4)
Complex	k_r/s^-1	k_nr/s^-1	τ_obs/μs	Φ_Eu/%	η_sens/%	Φ_overall/%	J=0	J=1	J=2	J=3	J=4
Eu₂(L^RR)₃	180	551	1372	24.8	75.0	18.6	–0.002	–0.083	0.026	0.037	0.011
Eu₂(L^SS)₃	184	553	1356	24.9	75.9	18.9	0.002	0.083	–0.024	–0.034	–0.013

Table 2. Rate constants of radiative (kr) and nonradiative (knr) decay, observed luminescence lifetime (τEu obs), sensitization efficiency (ηsens), intrinsic quantum yields (ΦEu), overall quantum yield (Φoverall), and glum values for 5D0→7FJ of the Eu3+ ion a

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