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Point Controlled Diastereoselective Self-Assembly and Circularly Polarized Luminescence in Quadruple-Stranded Europium(III) Helicates Yanyan Zhou, Yuan Yao, Zhenyu Cheng, Ting Gao, Hongfeng Li,* and Pengfei Yan*

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ABSTRACT: Aromatic β-diketones have been extensively employed as highly effective sensitizers in luminescent lanthanide complexes. However, the difficulties to make the chiral modified groups effectively participate in the frontier molecular orbital (FMO) distributions limit their applications on lanthanide circularly polarized luminescence (CPL) fields. Considering the inherent chirality of the helical structure, a pair of enantiopure dinuclear europium quadruple-stranded ΔΔ ΛΛ ΔΔ ΛΛ ′ helicates, / -(HNEt3)2(Eu2L4)( / )-1;L=R/S-1,2-bis(4,4 -bis(4,4,4- trifluoro-1,3-dioxobutyl)phenoxyl)propane are assembled via a point chirality induced strategy. The comprehensive spectral characteristics combined with density functional theory (DFT) calculations demonstrate that the one point chirality at the spacer of the ligand successfully controls the Δ or Λ configuration around the Eu(III) ion center and the P or M helical patterns of the helicates. The mirror-image CPL and CD spectra further confirm the formation of the pairs. As expected, the helicate presents a higher luminescence quantum | | ff β yield (QY) of 68% and a large glum value (0.146). This study e ectively combines the excellent sensitization capability of -diketone and the helical chirality of helicates. This strategy provides an effective path for the synthesis of lanthanide material with excellent CPL performance.

■ INTRODUCTION dinuclear and multinuclear helicates with excellent optical properties have been developed by Piguet,12 Bünzili,13 and Chiral circularly polarized luminescence (CPL) materials have 14 attracted tremendous attention recently due to their potential Gunnlaugsson. However, most of the helicates are formed as 1 2 racemic mixtures because of the absence of applications in chiral recognition, CPL probes, three- 3 4 controls in self-assembly processes. To obtain an enantiopure dimensional (3D) displays, and optical storage. The helical structure, the introduction of a chiral element into the luminescence dissymmetry factor (g ) and luminescence lum ligand is the general strategy.15 However, compared with the quantum yields (QYs) are the two most important parameters Downloaded via UNIV OF GEORGIA on August 17, 2020 at 18:35:00 (UTC). transition-metal helicates, the larger radii and the labile to evaluate the performances of the CPL materials. Because of coordination geometries of the lanthanide ions make the the existence of the magnetic dipole transitions for some control the diastereoselectivity in self-assembly process rather lanthanide ions, the glum values of the lanthanide luminescent See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. − challenging. For instance, Law et al. reported that the extension materials (g =10 2−0.5) are obviously higher than those of lum of the distance between the coordination unit in a bis- other luminescent systems, such as organic molecules,5 6 7 (tridentate) ditopic ligand and chiral center caused a polymers, supramolecules, and transition-metal complexes 16 −5 −3 8 diastereoselective breaking of the helicate. (g =10 −10 ). For instance, a high g value of 1.38 is 17 18 16 lum lum Recently, Gunnlaugsson, Sun, and Law have success- observed in a cesium tetrakis(3-heptafluoro-butylryl-(+)-cam- 9 fully developed some enantiopure dinuclear lanthanide triple- phorato) europium complex, Cs[Eu((+)-hfbc)4]. However, stranded helicates, where the chiral modified groups at the unfortunately, the luminescence quantum yield is very low, 10 terminal end of the ditopic tridentate chelation ligands only reaching up to 0.63% in chloroform, which is much ff β e ectively controlled the diastereoselectivity during the self- lower than those of most of the aromatic -diketonate assembly processes. However, in comparison with tridentate lanthanide complexes (generally, QYs > 30%).11 Although aromatic β-diketones are the highly effective sensitizers for lanthanide ions luminescence, the lack of effective chiral Received: June 28, 2020 modifications limit their applications on lanthanide CPL materials. The helical structure is one of the most attractive and widespread structures in nature, which has intrinsical chirality, possessing P or M helical patterns. Recently, some lanthanide

© XXXX American Chemical Society https://dx.doi.org/10.1021/acs.inorgchem.0c01911 A Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article chelation ligands, the bidentate chelation of β-diketone units (R/S)-propanediol. The optimizations of all structures were carried brings a relatively loose coordination sphere around the metal out with the PBE/6-31g(d) with SDD functional (the SDD basis set center, which implies a larger difficulty to control the for Eu atom and 6-31g(d) for H, C, O, and F atoms). stereochemistry of the helicate. Herein, we employ the cheap Thermodynamic data were calculated under 298 K for all geometries. Equilibrium selectivity was determined from the difference between and commercially available chiral (R/S)-1,2-propanediol as the × β R the statistically corrected energy of each isomer at 298 K. RT ln(K) spacer to synthesize a pair of chiral bis( -diketone) ligands L (K is the number of ways a single isomer can be formed) was S ′ fl and L (R/S-1,2-bis(4,4 -bis(4,4,4-tri uoro-1,3-dioxobutyl)- subtracted from the isomer energies to obtain the statistically phenoxyl)propane, L), which self-assemble with Ln(III) ions corrected isomer energies A. These were converted to isomer in a 2:1 ratio, forming anion quadruple-stranded helicates, proportions P, where P = exp(−A/RT). R/S 2− ′ (Ln2L 4) . In comparison with the triple-stranded helicate, Experimental Details. R/S-1,2-Bis(4,4 -bis(acetylphenoxy))- the quadruple-stranded structure is favorable to enhance the isopropane (R/S-BAPI). A mixture of (R)-propanediol (1.00 g, 13.0 ′ fl rigid of the helix and make the control of the diastereose- mmol), 4 - uoroacetophenone (5.40 g, 39.0 mmol), and anhydrous potassium carbonate (3.63 g, 26.0 mmol) were added to a three- lectivity more easily performed in the assembly process. The fl necked round-bottom ask containing 10 mL of DMF under a N2 CD, CPL, and NMR spectra analyses as well as DFT ° fi atmosphere. The mixture was heated to 155 C for 19 h. After it calculation con rm the successful construction of the cooled to room temperature, the solution was filtered to remove the homochiral helicates. The luminescence quantum yields salt, and then the filtrate was evaporated in vacuum to remove the (QYs) and CPL measurements display that the helicates solvent. The final product was recrystallized from methanol to obtain | | 1 present relatively higher QY up to 68% and a moderate glum creamy-white crystals in 70% yield (2.87 g). H NMR (CD3CN, 400 value of 0.146. These results indicate that just one point MHz, ppm): 7.95−7.93 (dd, J = 8.9, 8.8 Hz, 4H), 7.05−6.99 (dd, J = chirality in the spacer could control the stereochemistry in this 8.9, 8.8 Hz, 4H), 5.00−4.93 (m, 1H), 4.25−4.24 (d, J = 4.7 Hz, 2H), − 13 quadruple-stranded lanthanide helicate. 2.52 (d, J = 1.2 Hz, 6H), 1.44 1.43 (d, J = 6.3 Hz, 3H). C NMR (CD3CN, 101 MHz, ppm): 197.41, 197.39, 163.46, 162.80, 131.51, 131.47, 116.22, 115.26, 73.50, 71.94, 26.70, 16.65. EI-MS m/z 312.14 ■ EXPERIMENTAL SECTION + [M] . The enantiomeric purity of R/S-BAPI (>99% ee) was General. Allchemicalsandsolventswereobtainedfrom determined by HPLC (CHIRALPAK IC column; hexane/i-propanol: commercial suppliers and were used without further purification. 75/75; flow rate: 1.5 mL/min) and compared with the retention Characterization. 1 19 The H, F, and DOSY NMR spectra were times of the (tR = 13.58 min and tS = 19.70 min). S- recorded with a Bruker Avance III 400 MHz spectrometer. Elemental BAPI was synthesized following the procedure for R-BAPI with the analyses were carried on an Elementar Vario EL cube analyzer. use of (S)-propanediol instead, in 72% yield (2.95 g). 1H NMR fl − Electrospray time-of- ight (ESI-TOF) mass spectra were measured (CD3CN, 400 MHz, ppm): 7.95 7.93 (dd, J = 8.9, 8.8 Hz, 4H), on a Bruker maXis mass spectrometer. EI mass spectra were recorded 7.05−6.99 (dd, J = 8.9, 8.8 Hz, 4H), 5.00−4.93 (m, 1H), 4.25−4.24 on an Agilent 5975N mass spectrometer. UV−vis spectra were (d, J = 4.7 Hz, 2H), 2.52 (d, J = 1.2 Hz, 6H), 1.44−1.43 (d, J = 6.3 13 performed on a PerkinElmer Lambda 25 spectrometer. CD, PL, and Hz, 3H). C NMR (CD3CN, 101 MHz, ppm): 197.41, 197.39, CPL spectra were collected on an Olis DM245 spectrometer with a 163.46, 162.80, 131.51, 131.47, 116.22, 115.26, 73.50, 71.94, 26.70, 150 W xenon lamp as the light source. In CPL measurements, the 16.65. EI-MS m/z 313.14 [M + H]+. The enantiomeric purity was fi integration time of 2.0 s and xed slits of 2.0 and 0.6 mm were determined to be >99% ee. selected. HPLC analyses were performed on a SHIMADZU LC-20AT R/S-1,2-Bis(4,4′-bis(4,4,4-trifluoro-1,3-dioxobutyl)phenoxyl)- chromatographic instrument (column: CHIRALPAK IC). The propane (LR/S). A solution of ethyl trifluoroacetate (1.82 g, 12.8 excitation spectra and luminescence lifetimes were measured on an mmol) and sodium methoxide (0.69 g, 12.8 mmol) in dry DME Edinburgh FLS 980 fluorescence spectrophotometer. The lifetime (ethylene glycol dimethyl ether) was stirred for 10 min, and then (R)- fitting curves were analyzed by software provided by Edinburgh 1,2-bis(4,4′-bis(acetyl phenoxy))isopropane (1.00 g, 3.2 mmol) was Instruments. The quantum yields of the Eu(III) center emissions were added and further stirred for 24 h at room temperature. Then, the determined by an absolute method using an integrating sphere resulting light yellow solution was poured into 100 mL of ice−water equipped on an Edinburgh FLS 980 fluorescence spectrophotometer. and acidified with hydrochloric acid (2.0 M) to pH 2−3. The The values of QYs are the average of three independent measure- precipitate was filtered and recrystallized from hexane to give yellow 1 ments per sample. The absolute quantum yield was calculated by the needle crystals in 75% yield (1.21 g). H NMR (CD3CN, 400 MHz, following formula: ppm): 8.03−8.01 (dd, J = 8.6, 8.6 Hz, 4H), 7.11−7.05 (dd, J = 8.6, 8.6 Hz, 4H), 6.73 (s, 2H), 5.04−4.98 (m, 1H), 4.30−4.28 (d, J = 4.8 ∫ Lemisson − 13 Φ= Hz, 2H), 1.46 1.44 (d, J = 6.3 Hz, 3H). C NMR (CD3CN, 101 ∫∫EE− MHz, ppm): 188.00, 187.97, 175.70, 175.68, 175.34, 175.33, 164.86, reference sample 164.30, 131.48, 131.43, 126.59, 126.40, 116.92, 116.05, 93.21, 93.17, + where L is the emission curve of the sample, collected by 73.76, 72.10, 16.55. ESI-MS m/z 505.10 [L + H] . The enantiomeric emission R/S integrating the sphere, E is the curve of incident light, and purity of L (>99% ee) was determined by HPLC (CHIRALPAK IC sample fl Ereference is the curve of the light used for excitation with only the column; hexane/dichloromethane: 40/40; ow rate: 0.8 mL/min) reference in the sphere. The accuracy of the method is within 10%. and compared with the retention times of the racemic mixture (tS = S Spectrophotometric Titrations. The UV−vis spectrophotomet- 6.49 min and tR = 7.48 min). L was synthesized following the ric titration used for calculating the conditional stability constant was procedure for LR with the use of (S)-1,2-bis(4,4′-bis(acetyl performed on an Olis DM245 spectrophotometer equipped with an phenoxy))isopropane instead, in 77% yield (1.24 g). 1H NMR − Olis 2 syringe titrator. In the experimental process, the deprotonated (CD3CN, 400 MHz, ppm): 8.03 8.01 (dd, J = 8.6, 8.6 Hz, 4H), [LR]2− (4.0 × 10−5 M, THF) were titrated with a solution of 7.11−7.05 (dd, J = 8.6, 8.6 Hz, 4H), 6.73 (s, 2H), 5.04−4.98 (m, × −3 − − Eu(CF3SO3)3 (4.0 10 M, THF) at 298 K. Mathematical 1H), 4.30 4.28 (d, J = 4.8 Hz, 2H), 1.46 1.44 (d, J = 6.3 Hz, 3H). 13 treatment and factor analysis of the spectral data were carried out by C NMR (CD3CN, 101 MHz, ppm): 188.00, 187.97, 175.70, 175.68, using the Jplus ReactLab Equilibria program. 175.34, 175.33, 164.86, 164.30, 131.48, 131.43, 126.59, 126.40, R Computational Details. The geometries of the H ΛΛ-1 and 116.92, 116.05, 93.21, 93.17, 73.76, 72.10, 16.55. ESI-MS m/z 505.10 R + H ΔΔ-1 were optimized by DFT calculations with the Gaussian 09 [L + H] . 19 R S program package. The construction of the original structure models (HNEt3)2(Ln2L 4)and(HNEt3)2(Ln2L 4)[Ln=Eu,La,Gd]. were in reference to our previously reported quadruple-stranded Triethylamine (0.20 g, 2.0 mmol) and LR/S (0.50 g, 1.0 mmol) 20 · helical structure, in which the spacers of ligands were replaced by were dissolved in 15 mL of MeOH. To this solution, LnCl3 6H2O

B https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article

R S R/S Scheme 1. Syntheses of the L ,L, and the Corresponding Lanthanide Complexes (HNEt3)2(Ln2L 4)

(0.5 mmol) in MeOH (5 mL) was added to the above solution and stirred for 24 h at room temperature. The solution was poured into ice−water, and the resulting white precipitate was filtered and dried in vacuum. R R -1 (HNEt3)2(Eu2L 4)(HΛΛ ). Yield: 86%. Anal. calcd for C104H94F24O24N2Eu2 (2515.43): C, 49.65; H, 3.77; N, 1.11. Found: R 2− C, 49.61; H, 3.82; N, 1.15. ESI-MS: m/z = 1156.5915 (Eu2L 4) . S S -1 (HNEt3)2(Eu2L 4)(HΔΔ ). Yield: 82%. Anal. calcd for C104H94F24O24N2Eu2 (2515.43): C, 49.65; H, 3.77; N, 1.11. Found: S 2− C, 49.62; H, 3.84; N, 1.16. ESI-MS: m/z = 1156.5954 (Eu2L 4) . R R -2 (HNEt3)2(La2L 4)(HΛΛ ). Yield: 80%. Anal. calcd for C104H94F24O24N2La2 (2489.40): C, 50.17; H, 3.81; N, 1.13. Found: R 2− 1 C, 50.15; H, 3.90; N, 1.15. ESI-MS: m/z = 1142.9779 (La2L 4) . H − NMR (CD3CN, 400 MHz, ppm): 7.88 7.85 (dd, J = 8.1, 9.7 Hz, 16H), 6.87−6.82 (dd, J = 13.6, 10.2 Hz, 16H), 6.32 (s, 8H), 4.81 (m, 4H), 4.11−4.10 (d, J = 4.0 Hz, 8H), 1.32−1.31 (d, J = 4.0 Hz, 12H). 19 − F NMR (CD3CN, 377 MHz, ppm): 0.27. R R -3 (HNEt3)2(Gd2L 4)(HΛΛ ). Yield: 88%. Anal. calcd for C104H94F24O24N2Gd2 (2526.32): C, 49.44; H, 3.75; N, 1.11. Found: R 2− C, 49.47; H, 3.81; N, 1.14. ESI-MS: m/z = 1161.9744 (Gd2L 4) .

■ RESULTS AND DISCUSSION Syntheses and Characterization of LR/S and the + R/S 2− Complexes (HNEt3) 2(La2(L 4) . The synthetic routes of the ligands (LR/S) and their corresponding complexes are outlined in Scheme 1. The intermediate R/S-BAPI was synthesized by the reaction of (R/S)-1,2-propanediol with ′ fl 4 - uoroacetophenone, following which the Claisen condensa- R R tion with ethyl trifluoroacetate gave the final ligands, LR/S. 1H Figure 1. (a) ESI-TOF mass spectrum of (HNEt3)2(Eu2L 4)(HΛΛ- 13 1) with insets showing the simulated (Sim.) and observed (Obs.) and C NMR spectroscopy and mass spectrometry analyses isotopic pattern. (b) 1H NMR (400 MHz) spectra of the affirmed the formation and purities of the intermediates and R R R 1 (HNEt3)2(La2L 4)(HΛΛ-2) and free ligand L in CD3CN. (c) H − R ligands (Figures S1 S12). Moreover, the enantiomeric purity DOSY NMR spectrum of H ΛΛ-2 in CD3CN. of LR and LS (>99% ee, Figure S13) were confirmed by HPLC analyses. R The quadruple-stranded helicates were isolated from the (H ΛΛ-2) was selected as the substitute for NMR experiments. R reaction solution of the corresponding ligand and LnCl3 salts H ΛΛ-2 shows a set of widened but differentiated signals with the ratio of 2:1 at a basic condition in methanol. The (Figure 1b), which almost undergo high-field shifts compared formation of quadruple-stranded binuclear helicates was first with those of free ligand. Additionally, the integral ratio of two fi + con rmed by high-resolution electrospray ionization mass counter cations (HNEt3) to one helicate is consistent with the + 2− spectrometry (ESI-TOF MS) analyses. In the negative expected molecular formula (HNEt3) 2(La2L4) . Further- ionization mode, the spectra show two clusters of peaks more, 1H NMR diffusion ordered spectroscopy (DOSY) of R corresponding to a doubly charged and a single charged the H ΛΛ-2 displayed one diffusion band with a diffusion 2− − −6 2 −1 species, [Ln2L4] and [Ln2L4 +H], respectively (Figure 1a coefficient of D = 3.31 × 10 cm s , which further affirmed and Figures S14−S16). Moreover, the populations of the the formation of a single species (Figure 1c). However, with experimental isotopic are consistent with the simulated the consideration of the absence of C2 symmetry of the ligand, isotopic patterns. Furthermore, the 1H and 19F NMR and the self-assembly processes would give rise to a series of DOSY NMR analyses also support the formation and stability mixtures of helicates. However, only a single set of signals was of the helicates in solution (Figures 1b, 1c, and S17). Due to observed in the 1H NMR spectrum, which was a contradiction the poor 1H NMR resolution of Eu(III) complex, the with the prediction of the presence of multicomponent + R 2− corresponding La(III) complex (HNEt3) 2(La2(L 4) mixtures. We suggest that it should originate from the

C https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article broadening of the peak caused by the partial paramagnetism of Table 1. DFT Calculated Energies of the Selected Isomers R R the Ln(III) ion as well as very similar magnetic environment in for H ΛΛ-1 and H ΔΔ-1 (298 K) the two aromatic β-diketonate moieties. Thus, we seek to estimate the possible assembly modes of the ligands in one complex isomer relative energy (kcal/mol) population (%) R ΛΛ helicate and the contents of helicates with different helical H ΛΛ-1 -HHHH 0.00 95.4 strand arrangement in solution by computational methods. ΛΛ-HHHT +1.89 4.4 Computational Studies. Obviously, each L in one ΛΛ-HTHT +3.98 0.2 R ΔΔ helicate exhibits two kinds of orientations relating to the H ΔΔ-1 -HHHH +13.12 0.0 helical axis, which are named as “H” (head) and “T” (tail) ΔΔ-HHHT +10.34 0.0 orientations, respectively (Figure 2a). Thus, there would exist ΔΔ-HTHT +7.27 0.0

By inspection of the optimized structures, it is suggested that the different system energies of three stable ΛΛ-isomers should be determined by the orientation of the methyl group in the spacer. In one helicate, methyl groups have two orientations; one is away from the central cavity, named as “exo”, and the other is toward into the cavity, named as “endo”. Notably, the “H” ligand in helicate always adopts exo form, while the “T” ligand has endo form. Taking the ΛΛ-HTHT as an example, the distance between Me and phenoxyl in a “T” ligand is obviously shorter than that in a “H” ligand (Figure S18). Obviously, the “H” orientation is favored to decrease the steric hindrance and lower the overall system energy. This observation is consistent with the calculated result that the ΛΛ-HHHH has the lowest energy among the helicate mixtures. Additionally, the Eu−Eu distances are ca. 16.4 Å in R three H ΛΛ-1, which are longer than the Eu−Eu distances of R ca. 14.0 Å in three H ΔΔ-1. Finally, the statistical populations of the six isomers were determined by reference method.21 The results show that the isomers ΛΛ-HHHH, ΛΛ-HHHT, and ΛΛ-HTHT account for 95.4%, 4.4%, and 0.2%, respectively, while the populations of ΔΔ isomers are negligible because of their significantly high statistically corrected energies. As reported previously, the coordination geometry around the Ln(III) ion has dramatic effect on the optical and chiroptical properties of the complex.22 Therefore, the coordination geometries of Eu(III) ions in three stable isomers, i.e., ΛΛ-HHHH, ΛΛ-HHHT, and ΛΛ-HTHT, are calculated with the SHAPE 2.1 software. In general, if the value trends to 0, a perfect polyhedron is indicated, while the greater deviation trends to 0, there is lower probability of the “ ” “ ” R corresponding coordination geometry. As shown in Table S1, Figure 2. (a) The H , head, and T , tail, orientations of L ligands the values for an eight-coordinated square antiprism structure relating to the helical axis. (b) The six optimized structures of HR-1. (8-SAPR) are obviously smaller than those for the other suggested coordination geometries. It indicates the coordina- ff three kinds of di erent combinations for four strand ligands in tion sphere is assigned to the square antiprism with D4d one helicate, named as HHHH, HHHT, and HTHT symmetry (Figure S19), which is similar to the symmetry fi 23 arrangements. If the con guration around the metal center observed from Cs[Eu((+)-hfbc)4]. (Δ and Λ) is further considered, there would exist six kinds of Speciation and Thermodynamic Behavior of the possible helical structures in an assembly process, named as Complexes. Considering the existence of dissociation ΔΔ or ΛΛ-(HHHH, HHHT, and HTHT). Thus, the six equilibrium, the thermodynamic stability and species compo- helicates with LR as helical strands are evaluated by density sition of the complexes in THF were studied by UV−vis functional theory (DFT) calculations (Figure 2b). The spectrophotometric titration. Conditional stability constants of ΔΔ optimization results show that the helicates with metal the possible species were determined by titrating Eu(CF3SO3)3 R −3 R 2− configurations (H ΔΔ-1) have the large torsional twist and solutions (4.0 × 10 M) into the deprotonated ligand [L ] steric strain in the helical strands, and the system energies are (4.0 × 10−5 M, obtained by reaction of ligand LR and obviously higher than that isomers with ΛΛ configurations stoichiometric triethylamine) at the ratios of R = [Eu]/[LR]2− R R (H ΛΛ-1, Table 1). In H ΛΛ-1, it is found that the steric strains =0−2(Figure 3a). Factor analysis indicates that there are five obviously decreased and that the pattern of the helix tended to absorbing species, i.e., EuL4,Eu2L4,Eu2L3,Eu2L2, and Eu2L, in be a “barrel shape”. The calculated results show that the ΛΛ- solution, and the data can be fittoeqs 1−5, respectively. The HHHH isomer has the lowest system energy, being stable by calculated stability constants of the complexes are listed in 1.89 and 3.98 kcal mol−1 compared to the ΛΛ-HHHT and Table 2. The species evolution and concentration calculated ΛΛ-HTHT isomers. during titration are shown in Figure 3(b). The distribution

D https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article

− R 2− × −5 Figure 3. (a) Variation of the UV vis absorption spectra of a [L ] in THF (4.0 10 M) upon addition of Eu(CF3SO3)3 from 0 to 2.0 equiv (THF, 4.0 × 10−3 M; 298 K). (b) Calculated distribution plots of the species.

Table 2. Species Distribution, Binding Constant Estimated by Fitting the Changes of UV-vis Spectra, and the Calculated Species Concentration of the Complex with the [Eu]/[LR]2− = 0.5

% species for M/ species concentration β R 2− −6 species log M/L [L ] (10 M) β log 14 26.012 0.52 0.051 β log 24 36.952 98.13 9.79 β log 23 30.248 1.35 0.13 β log 22 21.851 0.00 0.00 β log 21 13.946 0.00 0.00

R/S R S Figure 4. UV−vis and CD spectra of the L ,H ΛΛ-1, and H ΔΔ-1 in THF (1.0 × 10−5 M). curves show that the most predominant species are the R quadruple-stranded helicate Eu2L 4, being formed in 98.13% at Unlike the free ligand, an obvious exciton couplet effect with a stoichiometric ratio R = 0.5. The calculated stability Davydov splitting of 36 nm is observed in the helicate because constants and concentration for all the species are listed in of the spatial proximity of the quadruple-stranded chromo- Table 2. The calculated excellent thermodynamic stability is R phores. In H ΛΛ-1, a positive exciton couplet is observed, consistent with the results obtained from NMR analyses. which corresponds to the Λ configuration around the metal Eu4LEuLlog+ F 4 β14 (1) ion; on the contrary, a negative exciton couplet relating to the S Δ configuration around Eu(III) ion is observed in H ΔΔ-1. 2Eu+ 4LF Eu24 L log β24 (2) From some typical examples of transition-metal and lanthanide complexes, the relationship between the couplet sign and the + β 24 2Eu 3LF Eu23 L log 23 (3) absolute configuration have been well documented. With the empirical rule, the observed CD exciton coupling pattern is 2Eu+ 2LF Eu L log β 22 22 (4) consistent with the optimized structure of helicate. It is noted that the molar ECD (Δε) of the complexes improve 20 times 2Eu+ LF Eu L log β (5) 2 21 than that in the free ligands. It implies that the chirality of Chiroptical Properties. On the basis of the above NMR ligand is significantly amplified because of the formation of the analyses and DFT optimizational structures, it is suggested that helical structure. Combining the density functional theory the point chirality in the spacer of ligand could effectively (DFT) structure models and CD spectra, it is suggested that control the diastereoselectivity of the helicate in the self- the existence of one quaternary carbon in the assembly process. The resulting helical chirality of the spacer is sufficient to control the secondary homochiral helical enantiopure complex was first investigated by CD spectrosco- structure of the assembly and significantly enhance the optical py. As shown in Figure 4, the ligand and the helicate both show activities. R S an obvious Cotton effect in the corresponding UV−vis regions. As shown in Figure 5, the H ΛΛ-1 and H ΔΔ-1 show five Additionally, the exciton coupling cross is observed with the characteristic emission bands of Eu(III) ion in THF, which 5 → 7 − coupling cross points locating at 323 and 336 nm for the relate to D0 FJ (J =0 4) transitions, respectively. The 5 → 7 ligands and complexes, respectively. In comparison with the excitation spectrum obtained by monitoring the D0 F2 free ligand, the complex displays an about 24 nm transition at 612 nm (Figure S21). Irradiated with 365 nm UV hypsochromic shift (Figures 4 and S20), which could be light, the complexes emit very strong red light (Figure 5, attributed to the decrease of conjugation degree of the ligand insert), and the luminescence quantum yields (QYs) reach up arising from the twisting of the ligand in helicate. The CD to 68% in THF. In CPL spectra, the enantiomer pairs of spectra of free ligands show a very similar spectral pattern as its helicates show mirror-image CPL signals, which further absorption spectrum and give a positive exciton couplet for LR confirm their enantiomeric relationship. Two emission bands S 5 → 7 5 → 7 and a negative one for L . attributed to D0 F2 and D0 F1 transitions give rise to

E https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article

kr τobs Φ=Eu = kkrnr+ τrad (7)

1 3 Itot kAnr ==MD,0 τrad IMD (8) τ On the basis of the calculatedji zy radiative lifetimes ( rad) and τ j z Φ observed lifetimes ( obsj)(Figuresz S24 and S25), Eu is calculated to be 71%. Accordingk { to eq 7, the intrinsic quantum yield is the result of radiative transition competing with nonradiative transition. The calculated result shows that the −1 radiative decay rate constant (kr = 894 s ) is over two times −1 higher than nonradiative rate constants (knr = 358 s ). The R Figure 5. Total luminescence and CPL spectra of H ΛΛ-1 (red lines) lower knr should be contributed to by the fact that the anion S λ × −5 and H ΔΔ-1 (blue lines) in THF ( ex = 346 nm, 1.0 10 M). quadruple-stranded ligands saturate the coordination numbers of Eu(III) ion and exclude the possibility of solvent molecule to coordinate on the metal center. Additionally, the rigid helical structure is suggested to be effective to suppress the 7 the larger intensities than other deactivation F0,3,4 ground nonradiative transition originating from the thermal vibration states. Although having a lower total emission intensity for the of the framework. Furthermore, the helicates also show an 5 → 7 ffi η D0 F1 transition, the luminescence dissymmetry factor, ultrahigh energy transfer e ciency ( sens), up to 96%. This η fi Δ glum for this band is obviously higher than for other transitions high sens bene ts from the well energy level matching ( E = | | − 5 5 because of its magnetic-dipole feature. The glum values T1 D0) between the D0 energy level of Eu(III) ion (17500 −1 −1 corresponding to the transitions are listed in Table 3. The cm ) and triplet states of the ligand (T1 = 21739 cm , 5 → 7 magnetic-dipole transition ( D0 F1) displays a largest estimated from their phosphorescent spectrum, Figure S26). | 595 | −1 g lum = 0.146. This glum value is generally comparable to the Generally, ΔE > 2500 cm is thought to be a rational energy previously reported Eu(III) complexes, such as those of ffi Φ gap for high e ciency energy transfer. In all, the large Eu and pyridyldiamide-, 1-hydroxy-2-pyridinone-, chiral 2-hydroxyi- η 25 sens values are the essential factors to determine the higher sophthalamide-, and DOTA-based complexes. Additionally, luminescence quantum yields of the helicates. the CPL spectral signatures can also relate to the absolute configuration of the lanthanide complexes. As previously 26 5 → 7 ■ CONCLUSIONS observed by Muller and us, a positive signal at D0 F1 5 → 7 transition and a negative signal at D0 F2 transition indicate In conclusion, a pair of enantiopure Eu(III) quadruple- the existence of a Δ configuration around the Eu(III) ion stranded helicates with excellent circularly polarized lumines- center; conversely, it represents a Λ configuration. cence (CPL) properties were successfully synthesized. From In luminescent materials, the luminescence quantum yield the spectral analyses and density functional theory (DFT) (QY) is the essential parameter for estimating the performance calculations, it is confirmed that just one point chirality in the of the luminescence materials. Encouragingly, this pair of spacer of the bis(β-diketone) ligands could successfully control helicates show relatively higher values, reaching up to 68% the formation of the homochiral helical structure. Moreover, (Figures S22 and S23), which are much larger than the most of by combining the helical chirality and the highly efficient β-diketonate europium complexes (generally, 30%). In sensitization capability of the β-diketone, the quadruple- lanthanide complexes, the QY is generally determined by stranded helicates not only show high luminescence quantum Φ intrinsic quantum yield ( Ln) of the metal emission and by yields (up to 68%) but also present excellent CPL with |g | ffi η lum e ciency of the energy transfer ( sens)(eq 6). To clarify these values up to 0.146. In all, this work provides a method to two aspects, we first calculate the intrinsic quantum yield of resolve the difficulty of highly luminescent aromatic β- Φ Eu(III) ion emission ( Eu) by employing eq 7 and eq 8, where diketonate lanthanide complexes to be effectively modified τ τ rad represents the radiative lifetime and obs represents the by chirality elements. Additionally, the employment of the observed lifetime. cheap and commercially available chiral (R/S)-1,2-propanediol significantly decrease the cost on synthesizing chiral Φ=Φη overall sen Ln (6) luminescence materials.

τ Table 3. Rate Constants of Radiative (kr) and Nonradiative (knr) Decay, Observed Luminescence Lifetime ( Eu obs), ffi η Φ Φ 5 → 7 Sensitization E ciency ( sens), Intrinsic Quantum Yield ( Eu), Overall Quantum Yield ( overall), and glum values for D0 FJ of the Eu3+ iona

5 → 7 glum D0 FJ (J =0,1,2,3,4) −1 −1 τ μ Φ η Φ complex kr (s ) knr (s ) obs ( s) Eu (%) sens (%) overall (%) J =0 J =1 J =2 J =3 J =4 R H ΛΛ-1 894 358 798 71.4 96.5 68.9 0.008 0.146 −0.006 0.001 0.019 S H ΔΔ-1 892 361 798 71.2 96.5 68.7 −0.008 −0.146 0.006 −0.001 −0.019 a τ ± λ Error in Eu obs = 0.05 ms; 10% relative error in the other values; ex = 346 nm.

F https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX Inorganic Chemistry pubs.acs.org/IC Article ■ ASSOCIATED CONTENT Angew. Chem., Int. Ed. 2018, 57, 7488−7492. (b) Zinna, F.; Di Bari, L. *sı Supporting Information Lanthanide Circularly Polarized Luminescence: Bases and Applica- tions. Chirality 2015, 27,1−13. The Supporting Information is available free of charge at (2) (a) Shuvaev, S.; Starck, M.; Parker, D. Responsive, Water- https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01911. Soluble Europium(III) Luminescent Probes. Chem. - Eur. J. 2017, 23, 1H NMR, 13C NMR, and EI mass spectra of R/S-BAPI; 9974−9989. (b) Carr, R.; Evans, N. H.; Parker, D. Lanthanide 1H NMR, 13C NMR, and ESI-TOF mass spectra of LR/S; Complexes as Chiral Probes Exploiting Circularly Polarized HPLC chromatogram of LR/S and rac-L; ESI-TOF mass Luminescence. Chem. Soc. Rev. 2012, 41, 7673−7686. 19 R (3) Lim, D.-Y. Potential Application of Spintronic Light-Emitting spectra of the complexes; F NMR spectra of H ΛΛ-1 S Diode to Binocular Vision for Three-Dimensional Display Technol- and H ΔΔ-1; calculated structures of the complex R 2− ogy. J. Korean Phys. Soc. 2006, 49, S505−S508. (Eu2L 4) with HTHT arrangement; shape analysis R R (4) Huck, N. P. M.; Jager, W. F.; de Lange, B.; Feringa, B. L. data of H -1; coordination polyhedra of H ΛΛ-1;UV− R R Dynamic Control and Amplification of Molecular Chirality by vis absorption spectra of L and H ΛΛ-1; excitation Circular Polarized Light. Science 1996, 273, 1686−1688. spectra, quantum yield measurements, and decay curves (5) (a) Sun, Z.-B.; Liu, J.-K.; Yuan, D.-F.; Zhao, Z.-H.; Zhu, X.-Z.; R S R of H ΛΛ-1 and H ΔΔ-1; and emission spectrum of H ΛΛ- Liu, D.-H.; Peng, Q.; Zhao, C.-H. 2,2′-Diamino-6,6′-diboryl-1,1′- 3 (PDF) binaphthyl: A Versatile Building Block for Temperature-Dependent Dual Fluorescence and Switchable Circularly Polarized Luminescence. ■ AUTHOR INFORMATION Angew. Chem., Int. Ed. 2019, 58, 4840−4846. (b) Jiang, Q.; Xu, X.; Yin, P.-A.; Ma, K.; Zhen, Y.; Duan, P.; Peng, Q.; Chen, W.-Q.; Ding, Corresponding Authors − B. Circularly Polarized Luminescence of Achiral Cyanine Molecules Hongfeng Li Key Laboratory of Functional Inorganic Material Assembled on DNA Templates. J. Am. Chem. Soc. 2019, 141, 9490− Chemistry, Ministry of Education, School of Chemistry and 9494. Materials Science, Heilongjiang University, Harbin 150080, (6) (a) Lee, S.; Kim, K. Y.; Jung, S. H.; Lee, J. H.; Yamada, M.; China; orcid.org/0000-0003-4646-0515; Sethy, R.; Kawai, T.; Jung, J. H. Finely Controlled Circularly Polarized Email: [email protected] Luminescence of a Mechano-Responsive Supramolecular Polymer. Pengfei Yan − Key Laboratory of Functional Inorganic Material Angew. Chem., Int. Ed. 2019, 58, 18878−18882. (b) San Jose, B. A.; Chemistry, Ministry of Education, School of Chemistry and Yan, J.; Akagi, K. Dynamic Switching of the Circularly Polarized Materials Science, Heilongjiang University, Harbin 150080, Luminescence of Disubstituted Polyacetylene by Selective Trans- China; orcid.org/0000-0002-5124-1707; Email: Yanpf@ mission through a Thermotropic Chiral Nematic Liquid Crystal. − vip.sina.com Angew. Chem., Int. Ed. 2014, 53, 10641 10644. (7) (a) Yang, D.; Duan, P.; Zhang, L.; Liu, M. Chirality and Energy Authors Transfer Amplified Circularly Polarized Luminescence in Composite Yanyan Zhou − Key Laboratory of Functional Inorganic Nanohelix. Nat. Commun. 2017, 8, 15727. (b) Shen, Z.; Sang, Y.; Material Chemistry, Ministry of Education, School of Chemistry Wang, T.; Jiang, J.; Meng, Y.; Jiang, Y.; Okuro, K.; Aida, T.; Liu, M. and Materials Science, Heilongjiang University, Harbin Asymmetric Catalysis Mediated by a Mirror Symmetry-Broken Helical Nanoribbon. Nat. Commun. 2019, 10, 3976. 150080, China; orcid.org/0000-0001-6504-4205 ̈ Yuan Yao − Key Laboratory of Functional Inorganic Material (8) (a) Aoki, R.; Toyoda, R.; Kogel, J. F.; Sakamoto, R.; Kumar, J.; Kitagawa, Y.; Harano, K.; Kawai, T.; Nishihara, H. Bis(dipyrrinato)- Chemistry, Ministry of Education, School of Chemistry and zinc(II) Complex Chiroptical Wires: Exfoliation into Single Strands Materials Science, Heilongjiang University, Harbin 150080, and Intensification of Circularly Polarized Luminescence. J. Am. China 2017 − ́ − Chem. Soc. , 139, 16024 16027. (b) Jimenez, J.-R.; Doistau, B.; Zhenyu Cheng Key Laboratory of Functional Inorganic Cruz, C. M.; Besnard, C.; Cuerva, J. M.; Campaña, A. G.; Piguet, C. 3+ Material Chemistry, Ministry of Education, School of Chemistry Chiral Molecular Ruby [Cr(dqp)2] with Long-Lived Circularly and Materials Science, Heilongjiang University, Harbin Polarized Luminescence. J. Am. Chem. Soc. 2019, 141, 13244−13252. 150080, China (9) Lunkley, J. L.; Shirotani, D.; Yamanari, K.; Kaizaki, S.; Muller, G. Ting Gao − Key Laboratory of Functional Inorganic Material Extraordinary Circularly Polarized Luminescence Activity Exhibited Chemistry, Ministry of Education, School of Chemistry and by Cesium Tetrakis(3-heptafluoro-butylryl-(+)-camphorato) Eu(III) Materials Science, Heilongjiang University, Harbin 150080, Complexes in EtOH and CHCl3 Solutions. J. Am. Chem. Soc. 2008, − China; orcid.org/0000-0002-6981-1886 130, 13814 13815. (10) Kumar, J.; Marydasan, B.; Nakashima, T.; Kawai, T.; Yuasa, J. Complete contact information is available at: Chiral Supramolecular Polymerization Leading to Eye Differentiable https://pubs.acs.org/10.1021/acs.inorgchem.0c01911 Circular Polarization in Luminescence. Chem. Commun. 2016, 52, 9885−9888. Notes (11) Bünzli, J.-C. G. On the Design of Highly Luminescent The authors declare no competing financial interest. Lanthanide Complexes. Coord. Chem. Rev. 2015, 293−294,19−47. (12) (a) Zare, D.; Suffren, Y.; Nozary, H.; Hauser, A.; Piguet, C. ■ ACKNOWLEDGMENTS Controlling Lanthanide Exchange in Triple-Stranded Helicates: A This work is financially supported by the National Natural Way to Optimize Molecular Light-Upconversion. Angew. Chem., Int. Ed. 2017, 56, 14612−14617. (b) Suffren, Y.; Golesorkhi, B.; Zare, D.; Science Foundation of China (51773054 and 51872077). We Guenée,́ L.; Nozary, H.; Eliseeva, S. V.; Petoud, S.; Hauser, A.; Piguet, also thank the Key Laboratory of Functional Inorganic C. 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H https://dx.doi.org/10.1021/acs.inorgchem.0c01911 Inorg. Chem. XXXX, XXX, XXX−XXX