Gelation-Induced Visible Supramolecular Chiral Recognition by

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Gelation-Induced Visible Supramolecular Chiral Recognition by Fluorescent Metal Complexes of Quinolinol−Glutamide Wangen Miao,†,‡ Li Zhang,† Xiufeng Wang,† Long Qin,† and Minghua Liu*,† †

Beijing National Laboratory for Molecular Science, CAS Key Laboratory of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China ‡ Chemistry Science and Technology School, Zhanjiang Normal University, Zhanjiang 524048, PR China S Supporting Information *

ABSTRACT: Three metal complexes consisting of Li+, Zn2+, and Al3+ and quinolinol-functionalized L-glutamides (HQLG), (abbreviated as LiHQLG, Zn(HQLG)2, and Al(HQLG)3) were found to form fluorescent metallogels in several organic solvents. In solution, these chiral complexes showed neither any CD signal in the chromophore region nor chiral recognition of the chiral species. However, upon gel formation, the supramolecular chirality emerged because of the self-assembled nanostructures, which provided an opportunity for the chiral recognition of enantiomeric ligands. The metallogels showed different fluorescence changes when they met with enantiomeric (R,R)- or (S,S)-1,2-diaminocyclohexane. Among them, the Al(HQLG)3 metallogels did not show any change whereas the LiHQLG gels exhibited the same decrease in fluorescence. Interestingly, the Zn(HQLG)2 gels showed obviously different fluorescent color with respect to (R,R)- and (S,S)-1,2-diaminocyclohexane, thus providing visible chiral recognition via the naked eye. Such different recognition ability was discussed on the basis of the assembled chiral nanostructures and the primary molecular structures of the metal complexes. It was shown that both of them played important roles in chiral recognition.

1. INTRODUCTION

Recently, metallogels have attracted great interest because of their tunable structures and their functions.28−34 Among the various functions of the metallogels, some of them are amplified on the supramolecular level. For example, Uozumi et al. prepared a metallogel catalyst via self-assembly from PdCl2(CH3CN)2 and a trisphosphine hub with three flexible alkyl-chain linkers,35 which could efficiently catalyze the Suzuki−Miyaura reaction under atmospheric conditions in water. Tu et al. synthesized an air-stable palladium complex metallogel that revealed promising catalytic activity upon Michael reaction in the gel state.36 Naota and co-workers found an aggregation-induced photoluminesence enhancement in a chiral binuclear salicylaldiminato platinum metallogel.37 Yi’s group also presented a luminescence enhancement in a cholesterol-based terpyridyl platinum metallogel via sonica-

Chiral recognition has attracted continuous attention because it is the fundamental process in nature and life.1−4 The human body could exhibit different physiological responses to enantiomers, and thus the synthesis of enantiomerically pure drugs is required.5 Generally, one isomer can produce the desired therapeutic activities whereas the other can be inactive or have serious side effects. Thus, the discrimination and separation of R and S enantiomers are of great importance in pharmacology, biology, and drug development.6−11 The synthesis of enantiomeric sensors and the development of analytical methods are also crucial. In past decades, many chiral host molecules have been developed for the recognition of enantiomeric guests.12−18 Simultaneously, analytical methods such as microcalorimetry,19 infrared spectroscopy,20,21 chromatography,22 capillary electrophoresis,23 mass spectrometry,24 NMR,25 CD,26 and fluorescence spectroscopy27 have also been developed. © XXXX American Chemical Society

Received: February 12, 2013 Revised: April 5, 2013

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tion.38 In another case, it has been found that some metallogels could show new properties that the corresponding molecule cannot. That is, new functions emerged in the supramolecular system such as the chiral recognition of the supramolecular gels.39−43 For example, Pu and co-workers prepared a chiral molecular gel based on the (R)-binol-terpyridine Cu(II) complex, which could be used for the recognition of enantiomeric amino alcohols.39 Tu et al. designed an ALStype pincer metallogelator to realize the chiral recognition between (R)- and (S)-binap.40 Ihara et al. employed a gfunctionalized zinc porphyrin (g-TPP/Zn) to form a chiral gel and recognized the enantiomeric amino acids.41 We have reported that the metallogels of Schiff’s base and quinolinol Cu(II) complexes could recognize tartaric acid and aromatic amino acids, respectively.42,43 Such chiral recognition is exhibited only by the gel nanostructures but not for the molecular solution system. During all of these chiral recognitions of the metallogels with respect to the chiral species, it seems that the competitive coordination between the metal ions with the gelators and the targeted chiral ligands is of great importance. Because the metal complexes are very different in their coordination structures depending on the central metal ions, it is necessary to show which kind of coordination mode would be favored for chiral recognition. However, this kind of work has not been disclosed. In this Article, we designed three fluorescent quinolinol metal complexes with different coordination structures and investigated their supramolecular chiral recognition of the enantiomeric species via fluorescence changes. It is well known that 8-quinolinol is an important fluorescent ligand and can form many kinds of metal complexes.44 Because fluorescent tris-(8-hydroxyquinolinato) aluminum (Alq3) was used as an OLED material by Tang et al.,45 various 8-quinolinol derivatives and their complexes have been prepared.46−51 Herein, by attaching the quinolinol moiety to the glutamatebased amphiphile, we found that the ligands and their metal complexes could easily form organogels. Previously, we have found that their Cu(II) complexes could form dual functional gels that showed chiral recognition and redox-responsive properties.43 Even though the Cu complex exhibited weak fluorescence, it showed emission as it reacted with the amino acids because of the release of partial ligands. Here, we extended our work to the fluorescent complexes, as shown in Figure 1. We prepared three typical emissive complexes of HQLG (i.e., LiHQLG, Zn(HQLG)2, and Al(HQLG)3). These compounds could form organogels in several organic solvents and showed strong fluorescence. The metallogels could show different fluorescence changes upon reacting with the chiral

species and provided the possibility for chiral recognition. Interestingly, we have also found that the Zn(HQLG)2 gels could realize a visible chiral recognition of enantiomeric 1,2diaminocyclohexane under irradiation at 365 nm. However, the LiHQLG and Al(HQLG)3 gels could not achieve such recognition under the same conditions although they also exhibited supramolecular chirality via self-assembly. It was disclosed that during supramolecular chiral recognition the molecular structure of the metal complex is as important as the supramolecular self-assembly structure.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the starting materials were used as received. Solvents were purified and dried according to standard methods. Mass spectra were determined with BEFLEXIII for the MALDI-TOF mass spectrometer. Elemental analyses were performed on a Carlo-Erba1106 instrument. 2.2. Synthesis of LiHQLG (HQLG−Lithium Ion Complex). One gram of HQLG43 was dissolved in 50 mL of CH3OH/CHCl3 (1:1 v/ v). Then, 5 mL of a LiOH methanol solution containing 50 mg of LiOH was dropped into the above solution. The reaction mixture was refluxed for 2 h and then cooled to room temperature. After 200 mL of methanol was poured into this solution, the obtained yellow precipitate was collected and then recrystallized from CH3OH/ CHCl3 (2:1 v/v) twice to afford 850 mg of LiHQLG. MALDI-TOFMS (m/z): 855.2 (M+). Anal. Calcd for C53H91N4O4Li: C, 74.43; H, 10.72; N, 6.55. Found: C, 74.62; H, 10.88; N, 6.57. In the preparation of Zn(HQLG) 2 and Al(HQLG) 3, Zn(NO3)2·6H2O and Al(NO3)3·9H2O were employed, respectively. Others were similar to that of LiHQLG. Zn(HQLG)2: MALDITOF-MS (m/z) : 1764.0 (M + Na) + . Anal. Calcd for C106H182N8O8Zn: C, 72.25; H, 10.41; N 6.36. Found: C, 72.41; H, 10.49; N, 6.45. Al(HQLG)3: MALDI-TOF-MS (m/z): 2571.1 (M+). Anal. Calcd for C159H273N12O12Al: C, 74.25; H, 10.70; N, 6.54. Found: C, 74.32; H, 10.92; N 6.65. 2.3. Formation of the Metallogels. Ligand HQLG or three metal complexes LiHQLG, Zn(HQLG)2, and Al(HQLG)3 were dispersed in a 5 mL seal-capped vial. The dispersion was heated until a uniform transparent solution was obtained. After the solution was cooled to room temperature, either a gel or a precipitate was formed. Gel formation was confirmed by the inverted test tube method. If there was no fluid in the upside-down tube, then a gel had formed. The LiHQLG metallogels were used by adding 6 mg of gelators to 1 mL of THF. As for the Zn(HQLG)2 and Al(HQLG)3 gels, the amounts were 4 and 5 mg, respectively. For the UV−vis, CD, and fluorescence measurements, the gels formed in a 1 cm quartz cell were employed for all measurements. 2.4. Characterization. SEM was performed on a Hitachi S-4300 FE-SEM microscope. The fully aged gels were cast onto single-crystal silica plates (Pt-coated). The trapped solvent in the gels was evaporated first under ambient conditions and then under vacuum for 12 h for SEM measurements. FTIR spectra were recorded on a Bruker Tensor 27 FTIR spectrometer. The gels were cast onto CaF2 substrates for the preparation of the vacuum-dried xerogels. These were used for the FTIR measurements. Quartz-plate-sustained xerogel films were used for X-ray diffraction (XRD) measurements on a Rigaku D/Max-2500 X-ray diffratometer (Japan) with Cu Kα radiation (λ = 1.5406 Å), which was operated at 45 kV, 100 mA. UV and CD spectra were obtained on Jasco UV-550 and Jasco J-810 CD spectrophotometers, respectively. Fluorescence emission spectra were recorded on an F-4500 fluorescence spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Gelation Properties of the HQLG Metal Complex. The gelation abilities were tested in various organic solvents by using the “stable to the inversion of a test tube” method. The ligand, HQLG, had good solubility in lower-polarity solvents.43

Figure 1. Structures of the HQLG ligand molecule and its metal complexes. Photographs of (a) LiHQLG, (b) Zn(HQLG)2, and (c) Al(HQLG)3 gels from THF and corresponding SEM images of their xerogels. B

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It could gel only THF, benzene, and DMSO. However, its metal complexes, LiHQLG, Zn(HQLG)2, and Al(HQLG)3, could gel a majority of low-polarity solvents such as cyclohexane, toluene, and petroleum ether. Some mediumpolarity solvents (THF and dichloromethane) and the higherpolarity solvent, DMSO, were also found to be immobilized. The results of gelation properties in various solvents are listed in Table 1. It can be seen that the critical gelation concentration Table 1. Gelation Properties of HQLG Metal Complex Gelators in Various Solventsa CGC/mg·mL−1 solvents

LiHQLG

Zn(HQLG)2

Al(HQLG)3

CH2Cl2 CHCl3 CCl4 ClCH2CH2Cl benzene toluene n-hexane cyclohexane THF 1,4-dioxane petroleum ether acetonitrile acetone i-propanol ethyl acetate ethanol methanol DMF DMSO

7 S 8 7 3 4 P 5 6 S 5 I I P I P P P 10

5 S 6 5 4 5 P 4 4 8 5 I I P I P P P 6

8 S 6 6 4 8 P 5 5 S 6 I I P P P P P 5

Figure 2. (A) XRD patterns of the (a) LiHQLG, (b) Zn(HQLG)2, and (c) Al(HQLG)3 xerogels from THF and possible arrangements of the (B) LiHQLG, (C) Zn(HQLG)2, and (D) Zn(HQLG)2 gelator molecules with the interdigitation of alkyl chains.

a

CGC, critical gelation concentration; P, precipitate; S, solution; I, insoluble.

(CGC) of metallogels was lower than that of HQLG. For example, in THF the CGCs of LiHQLG, Zn(HQLG)2, and Al(HQLG)3 were 6, 4, and 5 mg·mL−1, respectively. These revealed that the gellation abilities of the three metal gelators should be stronger than that of the HQLG ligand. The gelation tests in THF showed that the gel−sol transition temperatures of the LiHQLG, Zn(HQLG)2, and Al(HQLG)3 metallogels were 42, 51, and 38 °C, respectively. Figure 1 displays photographs of the metallogels in THF. The LiHQLG and Al(HQLG)3 gels are yellow-green, and the Zn(HQLG)2 gels are straw yellow. All metallogels are transparent. 3.2. Morphology and Self-Assembled Metallogels. To disclose the supramolecular structures and morphologies of metallogels, we recorded the XRD patterns and SEM images of the xerogels in THF. The SEM images (Figure 1) revealed that all metal gelator molecules self-assembled into fine nanofibers in THF. The diameter of the fibers was about 150−200 nm. The stacking mode in gels was further provided by the XRD measurement. Figure 2A shows the XRD patterns of the xerogels of LiHQLG, Zn(HQLG)2, and Al(HQLG)3 from THF. In the LiHQLG gels, the XRD diffraction peak appeared at 2θ = 2.10°, corresponding to 4.2 nm of the d space. Because the theoretical length of the LiHQLG molecule was estimated to be 3.2 nm by the CPK model, this value suggested that LiHQLG possibly formed bilayers with the interdigitation of the alkyl chains, as did many other L-glutamide gels.43,52 In the case of the Zn(HQLG)2 gels, the diffraction appeared at 2θ =

1.94°, corresponding to a d space of 4.6 nm. As for the Al(HQLG)3 gels, a peak appeared at 2θ = 1.77°, which corresponded to a d space of 5.0 nm. Previously, we showed that in THF the Cu(HQLG)2 gel formed a layered structure in which the amide moieties organized into a well-defined arrangement by the strong hydrogen bonding interaction, the π−π stacking of the quinolinol rings, and the hydrophobic interaction from the interdigitated adjacent aliphatic alkyl tails.43 Similarly, these XRD data suggested that in the metallogels the metal complexes, Zn(HQLG)2 and Al(HQLG)3, formed essentially a layered structure, as shown in Figure 2C,D, respectively. However, the LiHQLG formed a bilayer structure (Figure 2B). The arrangement of the Zn(HQLG)2 molecules was similar to that of Cu(HQLG)2 in the gels for the extended length of the Zn(HQLG)2 molecule of about 6.1 nm. Although an Al(HQLG)3 molecule contained triple HQLG ligands, the octahedral coordination structure determined that the extended length of a Al(HQLG)3 molecule would be no more than 6.1 nm, which was a theoretical length. Therefore, we could suggest that the Al(HQLG)3 molecules were arranged in modes similar to that of Zn(HQLG)2 gelators in metallogels. Such a basic unit could stack further to form the nanofiber. These results revealed that the coordination interaction, hydrogen bonding, hydrophobic interactions, and π−π stacking make a contribution to gel fomation. C

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3.3. Spectroscopic Analysis. Figure 3A shows the UV absorption of HQLG and its metal gels in THF. The HQLG

and nitrogen atoms of the quinolinol rings participated in the coordination interaction to form chelate complexes. Moreover, the coordination interaction of HQLG to zinc and aluminum ions was much stronger than that of lithium ions. These were in good agreement with the UV investigation. In addition, the peak at 1023 cm−1, assigned to the C−O stretching band,56,57 showed a change similar to that of the C−N mode. 3.4. Supramolecular Chirality and Photoluminescence. It is clear that the HQLG gelator has a chiral center at the L-glutamide moiety. However, the solution of HQLG or its metal complexes did not show any CD signal in the range of 220−600 nm. It revealed that the chirality could not be transferred to the quinolinol chromospheres in solution. Upon gelation, the chirality could be easily transferred to the quinolinol moiety. These phenomena have been elucidated in our recent study on the chiral recognition of Cu(HQLG)2 gels with respect to enantiomeric aromatic amino acids.43 As shown in Figure 4A, the Zn(HQLG)2 gels showed a positive signal at

Figure 3. (A) UV spectra of the (a) HQLG, (b) LiHQLG, (c) Zn(HQLG)2, and (d) Al(HQLG)3 gels and (B) FTIR spectra of their xerogels from THF.

gels exhibited two absorptions at 246 and 336 nm, which were assigned to 1 B b and 1 L a bands of quinolinol rings, respectively.53,54 When the HQLG ligands coordinated to metal ions, these two peaks would be red shifted. For example, in the Zn(HQLG)2 gels, they shifted to 266 and 362 nm, whereas these two absorptions shifted to 266 and 392 nm in the case of the Al(HQLG)3 gels. These results suggested the coordination interaction of HQLG to zinc and aluminum ions. As for the LiHQLG gels, both of the characteristic absorptions were retained. Moreover, the red-shift phenomena were also detected, indicating that HQLG exhibited the coexistence of two states: the ionic salts and coordinate covalent complexes. This is also indicative of weak coordination in LiHQLG gels.55 FTIR is a powerful tool for the characterization of molecular structures and supramolecular assemblies. Figure 3B shows the FTIR spectra of the xerogels of HQLG and its metal complexes from THF. A strong band at 3294 cm−1, which was attributed to the N−H stretching mode of the amide bonds, did not change when the HQLG ligands were coordinated to the metal ions. It revealed that the N−H of amide formed hydrogen bonds between neighboring molecules and was not involved in coordination, whereas the O−H bending mode, located at 1268 cm−1, disappeared after coordination.56,57 The C−N stretching mode at 1326 cm−1 in quinolinol rings displayed weak absorption in the HQLG xerogels. If coordinated to metal ions, it would be enhanced.56,57 In the case of the LiHQLG xerogels, the intensity of the C−N stretching vibration was a little stronger than that of HQLG. Thus, it could be concluded that both oxygen and nitrogen atoms of the quinolinol rings were involved in the coordination interaction. As for the Zn(HQLG)2 and Al(HQLG)3 gels, the intensities of the C−N modes were enhanced greatly. It also indicated that the oxygen

Figure 4. (A) CD and (B) fluorescence spectra of the (a) HQLG, (b) LiHQLG, (c) Zn(HQLG)2, and (d) Al(HQLG)3 gels in THF. (λexc = 340 nm, slit = 5.0/5.0 nm).

273 nm and a negative one at 249 nm with a crossover at 261 nm, whereas in the Al(HQLG)3 gels a positive signal at 276 nm and a negative one at 254 nm occurred with a crossover at 268 nm. Moreover, they would provide single positive bands at 421 and 375 nm, respectively. These results were in good agreement with the UV spectra. In the case of the LiHQLG gels, a negative peak at 277 nm and a positive one at 264 nm occurred with a crossover at 270 nm. Besides, another negative signal at 254 nm and positive one at 243 nm occurred with a crossover at 249 nm. Obviously, the former was concerned with the absorption at 266 nm of the LiHQLG gels whereas the latter was related to absorption at 246 nm. All of these were in good agreement with the UV observation. These results confirmed that the chirality had been transferred to chromospheres of quinolinol rings because of self-assembly in gels. Although the quinolinol is a weakly fluorescent material itself, its metal complexes have been confirmed to show strong D

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fluorescence. Therefore, we recorded the fluorescence spectra of the HQLG metallogels in THF. As shown in Figure 4B, it could be seen that the emission of the LiHQLG, Zn(HQLG)2, and Al(HQLG)3 metallogels appeared at 491, 505, and 497 nm, respectively, suggesting that these three metallogels emitted cyan light under UV irradiation. 3.5. Visible Recognition of Enantiomeric 1,2-Diaminocyclohexane. It was found that the Zn(HQLG)2 gels could achieve the visible recognition of enantiomeric 1,2-diaminocyclohexane. Figure 5 showed the fluorescence spectra of the

(HQLG)2 gels, no obvious spectral changes were found upon addition of the (R,R) enantiomers whereas the addition of (S,S) enantiomers would give rise to a new peak at 246 nm, indicating that some HQLG ligands of Zn(HQLG)2 in the gels might be replaced by (S,S) enantiomers. Therefore, it could be suggested that the self-assembled chiral supramolecular nanostructures induced such a different interaction, and thus we obtained visible enantioselective recognition. When the LiHQLG gels were used for the trial of chiral recognition of enantiomeric 1,2-diaminocyclohexane, we found that the cyan gels would become faint, as shown in Figure 6A.

Figure 5. Photograph of (a) the Zn(HQLG)2 gels and those in addition to equivalent (b) (R,R)- and (c) (S,S)-1,2-diaminocyclohexane in THF under irradiation at 365 nm and their fluorescence spectra (λexc = 340 nm, slit = 5.0/5.0 nm).

Zn(HQLG)2 gels upon addition of (R,R)- or (S,S)-1,2diaminocyclohexane and their fluorescent colors. The lightyellow Zn(HQLG)2 gels became cyan under irradiation at 365 nm. After the addition of an equivalent (R,R)-1,2-diaminocyclohexane, the gels were collapsed to a sol. Apart from this, no fluorescent color changes occurred. However, if the (S,S) enantiomers were used for trial, then the cyan dispersed gels would become yellow under irradiation at 365 nm (Figure 5). It is noted that the color change could be observed at an (S,S)1,2-diaminocyclohexane concentration of as low as 0.1 equiv (Figure S2B, top). The concentration of the used Zn(HQLG)2 gels in THF was 4 mg·mL−1, which was equal to 1.5 mmol·L−1. Thus, we could easily detected the (S,S)-1,2-diaminocyclohexane at a concentration of as low as 0.15 mmol·L−1 by naked eye. Such a detection limit was much lower than those from other reports.39−41 The fluorescence spectra showed that the emission wavelength would undergo a red shift from 505 to 525 nm when (S,S) enantiomers were added, and the emission intensity decreased simultaneously. When (R,R) enantiomers were added, the emission wavelength showed no change. However, the intensity was obviously enhanced. To clarify such differences further, the fluorescence spectra of Zn(HQLG)2 at different concentrations from the solution to gels (Figure S1), the fluorescence spectra of the Zn(HGLG)2 solution after the addition of various concentrations of (R,R)or (S,S)-1,2-diaminocyclohexane (Figure S2), and the UV−vis spectral changes in the Zn(HQLG)2 gels upon addition of the two enantiomers were recorded (Figure S3B). It could be seen that there was no fluorescent color change upon gel formation (Figure S1). The Zn(HQLG)2 solution showed a similar fluorescence change upon the addition of enantiomeric 1,2diaminocyclohexane, suggesting no chiral recognition in solution (Figure S2). In the UV−vis spectra of the Zn-

Figure 6. Photographs and fluorescence of the (A) LiHQLG and (B) Al(HQLG)3 gels and those in addition to equivalent (a) (R,R)- and (b) (S,S)-1,2-diaminocyclohexane in THF under irradiation at 365 nm and (c) their corresponding fluorescence spectra. (λexc = 340 nm, slit = 5.0/5.0 nm) .

The same changes were observed for both (R,R)- and (S,S)-1,2diaminocyclohexane. The fluorescence spectra confirmed these similar changes, which showed no discrimination between (R,R)- and (S,S)-1,2-diaminocyclohexane for the LiHQLG gels. It should be noted that this kind of nondiscrimination was due to the same reaction capacity of the LiHQLG with both of the (R,R) and (S,S) enantiomers. The peak at 268 nm in the UV spectra suffered a similar decrease upon addition of (R,R)- or (S,S)-1,2-diaminocyclohexane (Figure S3A). If the Al(HQLG)3 gels were employed to interact with the enantiomeric (R,R)- or (S,S)-1,2-diaminocyclohexanes, then no change was observed. As shown in Figure 6B, the color of the gels showed no change upon addition of either (R,R)- or (S,S)1,2-diaminocyclohexane under irradiation at 365 nm. Further investigation by fluorescence (Figure 6B) and UV spectra (Figure S3C) revealed that no interaction between the Al(HQLG)3 molecules and 1,2-diaminocyclohexanes occurred. The CD spectra, as shown in Figure S4, confirmed that the same changes occurred in the LiHQLG gels upon addition of 1,2-diaminocyclohexane, which gave rise to no discrimination. As for the Al(HQLG)3 gels, there were no changes upon addition of 1,2-diaminocyclohexane. However, a significant E

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the added ligands was consistent with supramolecular chirality from the self-assembled Zn(HQLG)2 gelators, such as (S,S)1,2-diaminocyclohexane, then the ligands could penetrate the supramolecular skeleton easily and then interact with gelators because of less steric hindrance between them, whereas for (R,R)-1,2-diaminocyclohexane the occurrence of a greater amount of steric hindrance due to inconsistent chirality would lead to less interaction. These similar phenomena have been reported in our recent research.43 Thus, we could obtain the visible chiral recognition of the Zn(HQLG)2 metallogels with respect to enantiomeric 1,2-diaminocyclohexanes. However, it is clear that the primary molecular structures of the metal complexes are also very important in the chiral recognition as the supramolecular chirality. In the case of the LiHQLG gels, the gelator molecules stacked as bilayers with the interdigitation of alkyl chains. The outside coordinated lithium ions and quinolinol chromophores provided open space for the approach of the other ligands. Because there is a vacancy for the lithium ion and no obstacle hindrance, it is easy for the additional ligand to access. Therefore, the same change was observed when either (R,R)- or (S,S)-1,2-diaminocyclohexane was added (Figure 6A). Thus, the expected chiral recognition could not be achieved although the gel could easily react with both of them and lead to the fluorescence change. However, the Al(HQLG)3 molecule adopted an octahedral coordination structure in which triple HQLG ligands enclosed the central aluminum ions completely. There is no additional site for the other ligand to access. Therefore, no interaction was observed after the addition of either (R,R)- or (S,S)-1,2-diaminocyclohexane. This clearly indicated that the coordination structure of the metal complexes also played the important role of supramolecular chirality in enantiomeric recognition in gels.

enhancement of the CD signal was obviously detected in the Zn(HQLG)2 gels upon addition of (S,S)-1,2-diaminocyclohexane. These results are in good agreement with the UV−vis and fluorescence results. 3.6. Discussion. Quinolinol glutamate-based amphiphile HQLG was a weak gelator. However, when it was coordinated to metal ions, the gelation ability was improved. This revealed that the coordination interaction also play an important role in gelation. In THF, the ordered arrangement of molecules of HQLG metal complexes was formed, in which the coordination interaction between metal ions and the HQLG ligands, the hydrophobic interactions from the alkyl chains, the hydrogen bond between the amide groups, and the stacking of the quinolinol rings made the amphiphiles self-assemble into metallogels. When the metallogels were formed, the chirality of the local gelator molecules was transferred to the whole nanostructure as supramolecular chirality, which provided an opportunity for chiral selectivity when other enantiomeric ligands were introduced, whereas such chiral recognition could not happen on the molecular level. The supramolecular chiral recognition for the organogels is illustrated in Figure 7. In solution or on

4. CONCLUSIONS Three fluorescent emissive gelatorsLiHQLG, Zn(HQLG)2, and Al(HQLG)3were designed and were found to form metallogels in several organic solvents. It was found that upon gel formation chiral supramolecular structures were formed. When interacting with enantiomeric 1,2-diaminocyclohexane, the Zn(HQLG)2 metallogels showed clear fluorescence changes with different enantiomers and produced visible chiral recognition. That is, when the Zn(HQLG)2 metallogels interacted with (S,S)-1,2-diaminocyclohexane, the cyan dispersed gels would become yellow under irradiation at 365 nm, and such a color change could be observed at a concentration of (S,S)-1,2-diaminocyclohexane of as low as 0.1 equiv. However, no similar chiral recognition was detected for the LiHQLG and Al(HQLG)3 gels. The results indicated that both the primary molecular structures of the complexes and the selfassembled chiral nanostructures played key roles in the supramolecular chiral recognition.

Figure 7. Possible chiral recognition brought about by the metallogels. The Zn(HQLG)2 metallogels could achieve a visible chiral recognition of enantiomeric 1,2-diaminocyclohexane because of its chiral nanostructure and the vacant coordinate site. In the LiHQLG gel, the coordination site is open but without steric hindrance. The Al(HQLG)3 metallogel has a chiral structure but without a vacant coordination site. The latter two gels could not show supramolecular chiral recognition.

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ASSOCIATED CONTENT

S Supporting Information *

Fluorescence change of the Zn(HQLG)2 gels and solution upon addition of the enantiomeric ligands. UV and CD spectra of the LiHQLG, Zn(HQLG)2, and Al(HQLG)3 metallogels for the recognition of enantiomeric 1,2-diaminocyclohexane. Fluorescence change of the Zn(HQLG)2 gelators with concentration from solution to gel. This material is available free of charge via the Internet at http://pubs.acs.org.

the molecular level, the complexes have the same accessibility for the second ligand, such as 1,2-diaminocyclohexane, because the chirality was essentially localized at the chiral center. There is no need for the chiral match to the second ligand. Thus, there is no difference for either (R,R)- or (S,S)-1,2diaminocyclohexane. However, in gels, the supramolecular chirality would influence such an interaction. If the chirality of F

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected];. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (nos. 91027042, 21021003, and 11072244) and the Fund of the Chinese Academy of Sciences.

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