Article pubs.acs.org/JACS
Evolution of Luminescent Supramolecular Lanthanide M2nL3n Complexes from Helicates and Tetrahedra to Cubes Xiao-Zhen Li,†,§,‡ Li-Peng Zhou,†,‡ Liang-Liang Yan,† Da-Qiang Yuan,† Chen-Sheng Lin,† and Qing-Fu Sun*,† †
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China § University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *
ABSTRACT: Lanthanide-containing molecules have many potential applications in material science and biology, that is, luminescent sensing/labling, MRI, magnetic refrigeration, and catalysis among others. Coordination-directed self-assembly has shown great power in the designed construction of welldefined supramolecular systems. However, application of this strategy to the lanthanide edifices is challenging due to the complicated and greatly labile coordination numbers and geometries for lanthanides. Here we demonstrate a sensitive structural switching phenomenon during the stereocontrolled self-assembly of a group of Ln2nL3n (Ln for lanthanides, L for organic ligands, and n = 1, 2, 4) compounds. Systematic variation of the offset distances between the two chelating arms on the bis(tridentate) ligands dictated the final outcomes of the lanthanide assembly, ranging from Ln2L3 helicates and Ln4L6 tetrahedra to Ln8L12 cubes. Remarkably, the borderline case leading to the formation of a mixture of the helicate and the tetrahedron was clearly revealed. Moreover, the concentration-dependent self-assembly of an unprecedented cubic Ln8L12 complex was also confirmed. The luminescent lanthanide cubes can serve as excellent turn-off sensors in explosives detection, featuring high selectivity and sensitivity toward picric acid. All complexes were confirmed by NMR, ESI-TOF-MS, and single crystal X-ray diffraction studies. Our results provide valuable design principles for the coordination self-assembly of multinuclear functional lanthanide architectures.
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INTRODUCTION Coordination-directed self-assembly toward well-defined 3D supramolecular architectures has been explosively developed in the past several decades.1 Among pervasive supramolecular constructs, polyhedral coordination cages2 are the most attractive targets owing to their intrinsic cavities, which have shown promising applications in the recognition,3 manipulation,4 and transformations5 of guest molecules. The coupling between the metal’s coordination geometry and the ligand’s orientation provides the instructions or “blueprints” for the proposed polyhedral cages. To avoid unwanted cluster stoichiometries or geometries, the orientations of the multiple binding units within a ligand must be rigidly fixed.6 From another perspective, a slight ligand variation can give rise to distinct structures, which has been proven by Fujita,7 Nitschke,8 Raymond,3b,9 and many others.10 Moreover, supramolecular transformations are also reported with stimuli-responsive features in the concentration,10a,11 pH,12 solvent,13 light,14 metal/ligand ratio,15 guests,16 and so on.17 By contrast to the most often exploited transition metals, rare earth elements are less utilized in coordination-directed selfassembly processes. Due to their unique chemical properties, rare earth elements are essential in modern chemistry and are © 2017 American Chemical Society
widely used in applications ranging from chemosensors, biolabling, electronics, and optical devices to powerful magnets and superconductors.18 However, on account of large ionic radius and complicated and volatile coordination numbers and geometries for lanthanides, controlled self-assembly of predetermined lanthanide architectures still represents a big challenge.19 Chiral cage compounds, which have great potential in stereoselective recognition and catalysis, are gaining increasing attention in coordination directed self-assemblies.20 However, stereocontrol in the assembly process of lanthanide compounds,21 particularly in polyhedral cages,22 is not easy to achieve in view of large lability of the LnIII ions. We have recently reported the first stereoselective self-assembly of chiral luminescent lanthanide coordination tetrahedral cages with both M4L6 and M4L4 topologies, where the periphery point chirality of the ligand is transferred during the self-assembly process to give either cooperative Δ or Λ metal stereochemistry.22 Herein, we report a systematic structure evolution of the chiral lanthanide coordination edifices from helicates and Received: March 20, 2017 Published: May 30, 2017 8237
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Scheme 1. Structural Evolution of Chiral Lanthanide Supramolecular Ln2nL3n Complexes from Ln2L3 Helicates and Ln4L6 Tetrahedra to Ln8L12 Cubes by Systematic Variation of the Offset Distances between the Two Chelating Arms on Bis(tridentate) Ligands L1−4
Figure 1. Self-assembly of L2aR with Eu(OTf)3. 1H NMR (400 MHz, 293 K) spectra of (A) free ligand L2aR (d6-DMSO, a small portion of the cisisomer was star-labeled) and (B) the self-assembled complexes (in CD3CN). (C) ESI-TOF-MS spectra showing the coexistence of the Eu2(L2aR)3 (red triangle) and the Eu4(L2aR)6 (blue triangle) complexes with insets showing the observed (Obs.) and simulated (Sim.) isotopic patterns of the peaks at m/z 1375.2949 corresponding to a mixture of [Eu2(L2aR)3(OTf)2−2H]2+ (Sim. 1) and [Eu4(L2aR)6(OTf)4−4H]4+ (Sim. 2). (D) X-ray crystal structure of Eu2(L2aR)3. For clarity, only the helicate framework is shown. Eu, purple sphere; C, white; N, dark blue; O, red; H, cyan.
tetrahedra to cubes via subtle ligand variations (Scheme 1). Remarkably, a borderline case between the helicate and tetrahedral complexes was clearly revealed by eutectic crystal
structures, which contain both the helicate and tetrahedral complexes from the same ligand. Moreover, M8L12-type supramolecular cubes were also obtained for the first time by 8238
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490.7386 ([Eu2(L2aR)3−H]5+), 613.1714 ([Eu2(L2aR)3−2H]4+), 650.6620 ([Eu2(L2aR)3(OTf)1−H]4+), 867.2132 ([Eu2(L2aR)3(OTf)1−2H]3+), 917.1996 ([Eu2(L2aR)3(OTf)2− H]3+), and 1375.2949 ([Eu2(L2aR)3(OTf)2−2H]2+)) and a tetranuclear species [Eu4(L2aR)6(OTf)12−n−m−mH]n+ (n = 3−5, m = 3−4, at m/z 1070.2453 ([Eu4(L2aR)6(OTf)3−4H]5+), 1100.2377 ([Eu 4 (L 2 a R ) 6 (OTf) 4 −3H] 5 + ), 1375.2949 ([Eu4(L2aR)6(OTf)4−4H]4+), and 1883.3753 ([Eu4(L2aR)6(OTf)5−4H]3+)), respectively. Similar spectroscopic data was obtained when L2aS was used. The formation of the Eu2(L2aR)3 helicate structure was also confirmed by X-ray single-crystal diffraction studies. Single crystals of phase-pure Eu2(L2aR)3 complex were obtained by slow diffusion of tetrahydrofuran vapor into a solution of the complexes (self-assembled at [L2aR] = 20 mg/mL). The Eu2(L2aR)3 triple helicate crystallized in the chiral P21 space group. In the crystal structure (Figure 1D), the distance between the two EuIII centers is measured to be 17.19 Å. Both EuIII centers adopted the same Λ configuration, and three transligand strands wrap around two EuIII centers, leading to an overall M helical sense. Such R,R-ligand point chirality induction of the metal-centered Λ chirality, and consequently the M helicity of the whole complex are consistent with the reported structures using the same strategy.21a,b,23 Complexes of L2b. Similar to L2a, a mixture of Eu2(L2bR)3 and Eu4(L2bR)6 were also obtained by treatment of 3 equiv of L2bR with 2 equiv of Eu(OTf)3 or Eu(ClO4)3 in CD3CN. The 1 H NMR spectra also displayed only a single set of signals (Figure S17−S21), indicating the fast equilibrium between the two species on the NMR time scale. ESI-TOF-MS analyses again showed two sets of peak series (Figure S22), corresponding to binuclear species [Eu2(L2bR)3(OTf)6−n−m− mH]n+ (n = 3−5, m = 0−2) and tetranuclear species [Eu4(L2bR)6(OTf)12−n]n+ (n = 4, 5), respectively, indicating the coexistence of the two complexes in solution. Fortunately, the coexistence of the helicate and the tetrahedral complexes in this case was confirmed by X-ray crystallographic analysis (Figure 2). Suitable single crystals were obtained by slow diffusion of dichloromethane vapor into the complex solution (perchlorate). The molecular structures revealed a 3:1 mixture of the Eu2(L2bR)3 helicate and the Eu4(L2bR)6 tetrahedral complexes in the solid state, which crystallized in the chiral I213 space group. Though the crystal is poorly diffracting due to a vast amount of amorphous counterions and solvents existing in the huge unit cell of 232 416 Å3, the connectivity could be reliably determined. In both two architectures, all the metal atoms cooperatively adopted the same Λ-configuration, demonstrating the reliability of the chiral-induction strategy. In the Eu2(L2bR)3 triple helicate, three ligands wrap around two EuIII centers resulting in an M helicity. In the Eu4(L2bR)6 tetrahedron, four metal atoms occupy the vertices, and six C2-symmetrical bis(tridentate) ligands are disposed along the edges. In contrast with the linear ligands on the helicate, which have to bend obviously due to the steric repulsion in the middle, the ligands on the tetrahedron are flatter. This leads to a longer EuIII−EuIII distance (18.28 Å) in the tetrahedra compared to that in the helicate (17.26 Å). Moreover, a central cavity of 732 Å3 is calculated for the tetrahedral cage, suggesting its potential applications in host−guest chemistry. Based on the above observations, we propose that the Eu2(L2)3 helicate and Eu4(L2)6 tetrahedral frameworks undergo rapid ligand-exchange in solution, which results in an averaged
taking advantage of a concentration-dependent self-assembly process. The photophysical properties of these chiral lanthanide complexes were also studied, as well as highly efficient luminescent sensing toward picric acid at ppb level in explosives detection.
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RESULTS AND DISCUSSION Ligand Design and Synthesis. Based on geometrical considerations, coordination complexes with empirical formula of M2nL3n are expected to form when C2-symmetric bis(tridentate) ligands are combined with C3-symmetric metal nodes. Regarding this design principle, Raymond et al. has pointed out that the offset arrangement of the chelating groups in the ligand is crucial to force the formation of M4L6 tetrahedron over M2L3 helicate when octahedral metal ions were employed.3b Concerning the lanthanide assemblies, pyridine-2,6-dicarboxamide (pcam) has been proven to be an efficient tridentate cheating group toward stable C3-symmetric nine-coordinated lanthanide centers. Moreover, stereochemistry around the lanthanide centers (Δ or Λ) could be easily controlled by chiral-inducing substituent on pcam. Indeed, it has been previously reported that ligands L1 and L3, which contain two chiral pcam arms bridged by either linear biphenyl or offset naphthyl linkers, can lead to the formation of Ln2L3type helicate or Ln4L6-type tetrahedral cages with strict stereoselectivity.21c,22 Herein, we find that the final outcome of the self-assembly is also sensitive to the offset distances between the two parallel arms. That is, when the spacers are changed into azophenyl (L2a) or stilbenyl (L2b), mixtures of Ln2L3 and Ln4L6 are obtained. More surprisingly, if two arms are bridged by an anthrathene (L4), new Ln8L12-type cubic complexes are evolved, whose formation are also concentrationdependent. Ligand Synthesis. Three pairs of enantiopure ligands L2aR/S, L2bR/S, and L4aR/S were synthesized by stepwise amide formation reactions according to an established method,21c,22 starting from dimethylpyridine-2,6-dicarboxylate, where the peripheral chiral amide groups were introduced first, followed by coupling of the central diamine spacers. The achiral ligand L4b was synthesized in a similar procedure, with replacing the chiral amine with isopropamide. All seven ligands were fully characterized by NMR and ESI-TOF-MS spectra. (See experimental section in the Supporting Information for details.) Complexes of L2a. When 3 equiv of L2aR was treated with 2 equiv of Eu(OTf)3 or Eu(ClO4)3 in CD3CN at 40 °C for several minutes, the turbid suspension of ligands quickly turned clear. No change is observed in the 1H NMR spectra after prolonged heating of the mixture, suggesting the process of coordination is very fast and thermodynamically stable. Compared with the free ligand, most signals arising from the final assembly are shifted, in line with coordination to paramagnetic EuIII ions (Figures 1B and S8). The high symmetry of the product is further confirmed by 1H−1H COSY spectra indicating that the ligands experience identical magnetic environments (Figure S10). It should be noted that the ligands on the complexes adopted a single transconformation, though both the trans- and cis-L2aR coexisted in the solution (Figure 1A). This indicated that the cis-to-trans transformation occurred during the coordination process. Though 1H NMR displayed only a single set of signals, ESITOF-MS analyses revealed two sets of independent peak series (Figures 1C and S12), corresponding to a dinuclear species [Eu2(L2aR)3(OTf)6−n−m−mH]n+ (n = 2−5, m = 0−2, at m/z 8239
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was lowered down to −40 °C (Figure S25). Further concentration-dependent complexation experiments were carried out to determine the apparent equilibrium constant for 2 × Eu2(L2bR)3 ⇌ Eu4(L2bR)6. Small shifts of the signals on the chelating pyridine ring were observed when concentration of L2bR varied, from which the equilibrium constant was estimated to be 278 M−1 (Figure S26). So, with the increase in the concentration of L2bR, the equilibrium will favor the Eu4(L2bR)6 species. Computational studies were also performed in order to gain further insight into the stability differrence between the helicate and the tetrahedral structures. To our delight, both molecular mechanical simulations and quantum mechanical single-point energy caculations all confirmed that the tetrahedral configuration is more stable than the helicate. The preferred tetrahedral geometry was possibly due to the obviously bent conformation of the ligands as a result of steric repulsion existing in the middle of the triple helicate (Figure S100 and Table S5). Complexes of L4a. Similar to L2, treatment of 3 equiv of L4aS with 2 equiv of EuIII (perchlorate or triflate) in CD3CN at low concentration also afforded the Eu2(L4aS)3 helicate complexes, as first confirmed by 1H NMR spectroscopy (Figures 3A,B,D and S30−S32). There was only one single set of ligand signals at a diffusion coefficient of 7.60 × 10−10 m2· s−1 (corresponding to a diameter of 16.56 Å) measured by DOSY, indicating that a highly symmetrical small complex was formed. ESI-TOF-MS spectra confirmed the formation of dinuclear Eu2(L4aS)3 species with the observation of a sequence of prominent peaks corresponding to [Eu2(L4aS)3(OTf)6−n−m−
Figure 2. X-ray crystal structure showing the coexistence of a mixture of Eu2(L2bR)3 and Eu4(L2bR)6 (only one helicate and one tetrahedron are shown for clarity, which are highlighted in blue and green, respectively).
NMR spectra of the two. This is further proved by lowtemperature 1H NMR measurements, where no distinguishable splitting of the signals was observed even when the temperature
Figure 3. Self-assembly of L4aS with EuIII. 1H NMR (400 MHz, 293 K) spectra of (A) the free ligand L4aS (d6-DMSO), (B) the Eu2(L4aS)3(OTf)6 complex ([L4aS] = 7.70 mg/mL in CD3CN), and (C) the redissolved Eu8(L4aS)12(ClO4)24 crystals (in CD3CN). 1H DOSY spectra of the (D) Eu2(L4aS)3 and (E) Eu8(L4aS)12 complexes. ESI-TOF-MS spectra of (F) Eu2(L4aS)3(OTf)6 and (G) Eu8(L4aS)12(ClO4)24 with insets showing the observed and simulated isotopic patterns of the peaks corresponding to [Eu2(L4aS)3-2H]4+ and [Eu8(L4aS)12(ClO4)16]8+, respectively. 8240
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Journal of the American Chemical Society mH]n+ (where n = 2−4 and m = 0−3) (Figure 3F). To our delight, a helicate-to-cube transformation occurred during the crystallization of Eu2(L4aS)3 (perchlorate) when vapor of dichloromethane was slowly diffused into the complex solution. Though the quality of the X-ray data was not of publication quality, the formation of Eu8(L4aS)12 could be established. (See SI for details) By dissolving the crystalline samples, a clearly distinct 1H NMR pattern was observed, with significant complexity compared to the Eu2(L4aS)3 species (Figure 3C). Although detailed assignment was impossible due to substantial line-broadening and overlapping of the signals, DOSY spectra distinctly demonstrated the formation of a single species with larger size (Figure 3E). The formation of Ln8L12-type cubic structure is confirmed by the ESI-TOF-MS spectrum (Figures 3G), which showed a clear sequence of cationic peaks arising from the consecutive loss of anions from the octanuclear assembly (such as m/z 1005.6415 ([Eu8(L4aS)12(ClO4)13]11+), 1116.1009 ([E u 8 (L 4 a S ) 1 2 (ClO 4 ) 1 4 ] 1 0 + ), 1251.2175 ([Eu8(L4aS)12(ClO4)15]9+), 1419.9882 ([Eu8(L4aS)12(ClO4)16]8+), 1637.1216 ([Eu8(L4aS)12(ClO4)17]7+), 1926.4662 ([Eu8(L4aS)12(ClO4)18]6+), 2331.7481 ([Eu8(L4aS)12(ClO4)19]5+). It is worth noting that once formed, the Eu8(L4aS)12 structure tends to be stable, and 1H NMR showed no equilibrium back in dilute solution after 3 days (Figure S37). In situ synthesis of Eu8(L4aS)12 can be accomplished through the self-assembly of L4a with EuIII at high concentration ([L4a] = 40 mg/mL) in CH3NO2 (Figures S38−S40). By replacing the metal source of EuIII with LaIII, with larger ionic radius and less paramagnetic character to form more labile complexes, the concentration-dependent formation of the cubic La8(L4aS)12 was well confirmed by varying the concentration of L4aS from 2.5 to 40 mg/mL (Figure S50). From the integration of 1H NMR signals associated with La2(L4aS)3 and La8(L4aS)12 at [L4aS] = 20 mg/mL, the apparent equilibrium constant (K) for 4 × La2(L4aS)3 ⇌ La8(L4aS)12 was calculated to be 9.42 × 109 M−3 (Figure S50). Considering the Le Chatelier’s principle, higher concentration of the reaction system strongly forces the formation of large-component La8(L4aS)12 complex, following the similar trend as observed on the helicate to tetrahedra equilibrium. Indeed, when [L4aS] was increased to 40 mg/mL, only the La8(L4aS)12 complex was obtained. However, different from Eu8(L4aS)12, this complex will gradually rearrange back to the La2(L4aS)3 helicate structure after dilution, possibly due to the rather labile nature of coordination interactions associated with LaIII (Figure S51). Similar to our presious observations on the Eu4L6-type tetrahedral cage,22 no self-sorting behavior was observed when racemic ligands of L2a, L2b, or L4a were used for self-assembly. Based on 1H NMR, ESI-TOF-MS, and CD spectra (Figure S60−S65), we conclude that complexes of the same formula but scrambled ligands and statistically distributed Δ and Λ metal stereocenters are formed. Finally, a high-quality crystal structure of Ln8L12 cubic complex was obtained using the achiral ligand L4b, where the peripheral chiral benzylamide was substituted by isopropamide in order to increase the symmetry of the complex. Suitable single crystals were obtained by slow diffusion of ethyl acetate vapor into the acetonitrile solution of the Nd2(L4b)3 complex (See SI for details). The octanuclear compound crystallizes in the R3̅ space group. As shown in Figure 4, this cubic structure of Nd8(L4b)12 is of the same topological connectivity with the M8L12 structures reported by Ward et al.10b The eight Nd
Figure 4. X-ray crystal structure of Nd8(L4b)12. For clarity, only the cubic cage framework is shown. Nd, purple sphere; C, white; N, dark blue; O, red; H, cyan.
centers occupy the vertices of an approximate cube, with an average Nd···Nd separation of 12.02 Å. Possibly due to the crystal packing effects, the cubic structure becomes a little distorted and the Nd···Nd···Nd angles on the cube range from 84.19° to 95.55°. The 12 C2 symmetrical bi(tridentate) ligands are disposed along the edges. Despite all eight metal centers having typical pseudo-octahedral tris-chelate coordination geometries, there are two opposite absolute configurations coexisting in the cube (four Δ and four Λ). Among them, two Nd centers located at the diagonal corners have facial coordination environments, while the other six are meridional. This suggests that the cage has (noncrystallographic) S6 symmetry with the principal rotation axis through the two facial-coordinating Nd centers. The average distance (11.99 Å) between the two meridionally coordinated metal centers is slightly shorter than that between meridionally and facially coordinated metal centers (12.05 Å). The two metal centers spanned by a ligand adopt opposite stereochemical configurations (Δ and Λ). The meridional metal centers with different stereochemical configurations were alternately connected into a ring by six ligands. Extensive π−π stacking interactions between ligands were observed in the crystal structure. Similar to the previous cube reported by Ward and co-workers,10b there are six sets of five-component ADADAtype (A = electron acceptor, D = electron donor) π-stacking, each with three electron-deficient pyridine-2,6-dicarboxamide (pcam) chelating moieties and two electron-rich anthryl components arranging alternately around the periphery of the cube. The anthryl spacer is sandwiched between two pcam units of different ligands. The existence of 24 such interactions in the structures of M8L12 provides substantial driving force for the formation of the cubic complex. Photophysical Studies. Though being a mixture of Eu2(L2)3 helicate and Eu4(L2)6 tetrahedral species, the enantiopurities of self-assembled complexes of both L2a and L2b were confirmed by solution circular dichroism (CD) measurements. Compared with free ligands L2a/2b, with only two peaks (275 and 382 nm for L2a; 273 and 367 nm for L2b) arising from π → π* transitions being observed, the self8241
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Journal of the American Chemical Society assembled mixtures exhibit multiple CD signals (240, 310, 351, 404, and 470 nm for assembly complexes of L2a; 214, 240, 277, 300, 362, and 413 nm for assembly complexes of L2b) and display mirror images of CD spectra, which confirms the strong circular dichroism induced by the chiral amide groups on the periphery of the ligands (Figures S59, S61, S63 and S65). As for ligand L4a, two CD signals were detected at 306 and 415 nm for the free ligand and seven CD signals were detected at 246, 264, 284, 302, 333, 408, and 452 nm for the Eu2(L4a)3 complexes, displaying strong Cotton effects (Figures S67 and S69). In contrast to the delicate mirror image of CD spectrum of the Eu2(L4a)3 enantiomers, only weak mirrored peaks at 400−470 nm deriving from the peripheral chiral amide groups appeared in the CD spectrum of Eu8(L4a)12, owing to the counteraction of the opposite absolute configurations in Eu8(L4a)12 cubic structures (Figure S71). Luminescent properties were also investigated to explore their potential in chemosensing or labeling materials. In general, emission spectra of all self-assembled complexes displayed characteristic EuIII red emission lines attributed to antenna effect of the ligand strands. Self-assembled complexes of both L2a and L2b exhibited poor luminescent properties, which is quite different from L4a (Figures S62 and S66). A new shoulder peak in 420−600 nm and a slight red shift were observed in the UV/vis absorption spectra of Eu2(L4a)3 and Eu8(L4a)12 compared with free ligand L4a, which suggest extensive π−π stacking interactions between ligands in the assembled structures (Figure 5A). Upon excitation in the ligand levels at 313 and 346 nm, respectively, Eu2(L4a)3 and Eu8(L4a)12 exhibited similar luminescent spectra, with the same characteristic emissions featuring 5D0 → 7FJ (J = 0−4) transitions of EuIII. The highest intensity peak occurs at 615 nm, corresponding to the 5D0 → 7F2 band, and no obvious difference in the relative spectral intensity upon excitation in different wavelength was observed (Figures 5B, S73, and S74). Similar to the complexes formed by L1 and L3,21c,22 quantum yields of both Eu2(L4a)3 and Eu8(L4a)12 remain modest; the latter (Φ = 0.834%) exhibited an significant increase compared with the former (Φ = 0.131%) (Table S1). Nitroaromatic compounds are widely used in industrial construction and military equipment, and their rapid, sensitive, and selective detection is of great importance and emergency due to health and security concerns.24 Fluorescent platinum/ lanthanide-based metallocycles and metallocages have been demonstrated to have good performance in explosives detection.25 As expected, the cubic lanthanide supramolecular Eu8(L4a)12 cages exhibit high sensitivity and selectivity in the explosives detection of some common nitroaromatic compounds. Quenching effects on the europium fluorescence of the cage were observed in the fluorescence quenching titration experiments of picric acid (PA), 4-nitrophenol, 4-nitroacetophenone, 2,4-dinitrochlorobenzene, and 1,4-dinitrobenzene, using Eu8(L4a)12 (1.25 × 10−5 M) cage in CH3CN (Figures S76−S85). Especially for picric acid, the initial fluorescence intensity quenched rapidly upon the addition of various equivalents of PA to the Eu8(L4a)12 solution, exhibiting more than 2 times higher quenching efficiency than all others (Figures 5 and S86). Based on the Stern−Volmer equation, I0/I = KSV[PA] + 1, the quenching constants (KSV) and the detection limits were determined to be 1.130 × 104 M−1 and 1.030 × 10−6 M (53.5 ppb, S/N = 3), respectively, suggesting that the europium cubic cage is an efficient chemosensor for PA detection at ppb level.
Figure 5. Photophysical characterization of the self-assembled complexes of L4aS with EuIII. (A) UV−vis absorption spectra and (B) excitation (dashed lines) and fluorescence (solid lines) spectra of L4a, Eu2(L4a)3(OTf)6, and Eu8(L4a)12(OTf)24. (C) Fluorescence emission spectra of Eu8(L4a)12(OTf)24 (1.25 × 10−5 M) with different amounts of picric acid (PA) (λex= 355 nm, in CH3CN) (Insets, Stern− Volmer plots).
UV−vis and 1H NMR titration experiments were conducted to identify the quenching mechanism. As there is no spectral overlap between the absorption spectrum of PA and the emission spectra of Eu8(L4a)12, an energy-transfer quenching mechanism can be excluded (Figure S86). A significant enhancement of a new absorption bands at 327 nm was found during the initial titration of PA (Figure S84−S85). The 1 H NMR titration also displayed slight upfield shift of aromatic protons of picric acid upon the addition into the Ln8(L4a)12 system (Figure S59). Accordingly, we infer that ground-state nonemissive charge-transfer complexes between the electronrich ligands and the electron-deficient PA were formed, 8242
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suggesting the contribution of a static quenching mechanism. Furthermore, decrease in fluorescence lifetimes of Eu8(L4a)12 upon addition of PA also indicated the existence of collisional quenching (Figure S87). As a result, static and dynamic mechanisms contribute cooperatively to the quenching process, with higher KS (1.035 × 104 M−1, Stern−Volmer constant for the static quenching process) value over KD (9.480 × 102 M−1, Stern−Volmer constant for the dynamic quenching process), implying a dominant static quenching mechanism.25a−c,e,26
CONCLUSIONS In conclusion, we have shown that rather small variation in ligands conformation will lead to distinct outcomes in the lanthanide coordination structures, highlighting the difficulties in taming the lanthanide supramolecular assembly process. For the first time a borderline case between the helicate and tetrahedral structural switch was revealed. Moreover, unprecedented supramolecular lanthanide coordination cubes were obtained through a concentration-dependent self-assembly approach, which exhibit high luminescent sensing selectivity and sensitivity toward PA at ppb level in explosives detection. Such a structural evolution of the M2nL3n-type lanthanide compounds provides new design principles for functional lanthanide-containing molecular materials. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02764. Additional synthetic and structural details and additional figures and tables as described in the text (PDF) Crystallographic data for Eu2(L2aR)3 (CIF) Crystallographic data for mixed Eu 2 (L 2b R ) 3 and Eu4(L2bR)6 (CIF) Crystallographic data for Eu8(L4aS)12 (CIF) Crystallographic data for Nd8(L4b)12 (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Da-Qiang Yuan: 0000-0003-4627-072X Qing-Fu Sun: 0000-0002-6419-8904 Author Contributions ‡
X.-Z.L. and L.-P.Z. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB20000000), the National Natural Science Foundation of China (Grant Nos. 21521061, 21402201, 21471150, and 21601183), and Natural Science Foundation of Fujian Province (Grant Nos. 2016J06005, and 2016J05051). We thank the staff of BL17B/BL18U1/BL19U1 beamlines at National Centre for Protein Sciences Shanghai and Shanghai Synchrotron Radiation Facility, Shanghai, People’s Republic of China, for assistance during data collection. Q.-F.S. is grateful for the award from ‘The Recruitment Program of Global Youth Experts’. 8243
DOI: 10.1021/jacs.7b02764 J. Am. Chem. Soc. 2017, 139, 8237−8244
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