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Supramolecular metallogels with complex of phosphonate substituted carbazole derivative and aluminum(III) ion as gelator
Journal of Colloid and Interface Science 425 (2014) 102–109
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Supramolecular metallogels with complex of phosphonate substituted carbazole derivative and aluminum(III) ion as gelator Zicheng Ding, Bo Chen, Junqiao Ding, Lixiang Wang, Yanchun Han ⇑ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China University of the Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 December 2013 Accepted 1 March 2014 Available online 28 March 2014 Keywords: Metallogel Phosphonate Carbazole derivative p–p interactions Coordination interactions
a b s t r a c t Supramolecular metallogels can be gained from the phosphonate substituted 4,40 -bis(N-carbazolyl)biphenyl (PCBP) in the presence of aluminum chloride in alcohols, which can donate oxygen to aid proton transfer in the aluminum organophosphorus complexes. Inside the metallogels, three-dimensional fiber networks with nanofibers entangling and intersecting with each other inside are formed. The nanofibers show layered structures with a period thickness of 0.82 nm. As the content of aluminum(III) increases, the size of the fibers becomes smaller and the fibers pack more densely. It makes the transparent gel become turbid but nevertheless improves the stability of the metallogels. NMR, FT-IR and fluorescence spectroscopy show that the coordination interactions between the phosphonate groups of PCBP molecules and aluminum(III) ions as well as the p–p interactions among PCBP molecules are involved during the gel formation process. Ó 2014 Elsevier Inc. All rights reserved.
1. Introductions Supramolecular gels have attracted wide attention for their applications in various areas, including sensors, catalysts, templates, biomedicine, molecular electronics and photonics [1–4]. Through cooperating coordination interaction with other noncovalent interactions, such as hydrogen bonding, p–p interaction and van der Waals interaction, lots of metallogels have been exploited from organic–metal complexes and coordination polymers [5,6]. Taking advantage of the incorporated metallic elements, the metallogels show unique behaviors in luminescence, vapor sensing, redox stimuli, catalytic and magnetic properties. The relative works in these fields have also been reported recently [7–12]. Phosphate and phosphonate with oxygen atoms can bond to numerous metal ions to form organic–metal complexes [13,14]. Multiple aluminumphosphate and aluminophosphonate complexes have been synthesized and found applications in zeolite science for their ordered mesoporous structures [15–17]. Supramolecular structures such as nanoparticles, fibers and rods can also be gained from aluminum organophosphorus complexes driven by the coordination interactions between aluminum and phosphates as well as other non-covalent interactions [18–20]. Metallogels based on metal–ligand coordination from the complexes of aluminum and phosphate surfactant have also been ⇑ Corresponding author. Fax: +86 431 85262126. E-mail address: [email protected] (Y. Han). http://dx.doi.org/10.1016/j.jcis.2014.03.016 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
demonstrated. Lima et al. studied the thermoreversible gelation behavior of sodium polyphosphate and aluminum salt in aqueous solutions by 31P and 27Al NMR [21,22]. Warr et al. reported that aluminum isopropoxide and didodecyl phosphate can form gel in decane [23,24]. They found that the gel structure transition from independent wormlike micelles to a physical branched gel network takes place as the amount of aluminum increases, which makes the gel system displace Maxwellian rheology. Weiss et al. investigated the gelation behaviors of bisphosphonate esters, phosphonic acids, monophosphonate esters and aphosphoric acid ester in the presence of aluminum(III) and boron(III) in an organic liquid [25]. They found that the complex of boric acid with monophosphonate ester forms crystalline aggregates in the gel, whereas Al(iPrO)3– monophosphonate ester complex leads to nonbirefringent, amorphous aggregates. However, the metallogels from other aluminum organophosphorus complexes with functionalities are rare. In this work, we reported a new fluorescent metallogels from the complex of the phosphonate substituted carbazole derivative and aluminum chloride in alcohols, which are driven by the coordination bonding between phosphonate groups and aluminum(III) ions as well as the p–p interactions between the aromatic rings. The organic–metal complex forms fibrous networks inside the gel, and the fiber morphologies and stability are sensitive to the content of aluminum(III). As the content of aluminum(III) increases, the gel fibers become shorter and pack more densely, which makes the transparent gel become turbid and more stable.
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2. Experimental section 2.1. Materials The phosphonate substituted carbazole-based derivative, phosphonate substituted 4,40 -bis(N-carbazolyl)biphenyl (PCBP) (Scheme 1), was synthesized from Ref. [26]. Anhydrous aluminum chloride (AlCl3) was purchased from Alfa Aesar Company and used as received. The solvent ethanol-d6 was purchased from Sigma– Aldrich Company. The other solvents were purchased from Beijing Chemical Reagent Co., Ltd., China, and purified before used. The glass substrates were cleaned in a piranha solution (70/30 v/v of concentrated H2SO4 and 30% H2O2) at 70 °C for 30 min, thoroughly rinsed with deionized water and dried under nitrogen flow. 2.2. Preparation of metallogels and xerogel samples The solubilities of PCBP and AlCl3 in various solvents were tested separately at first. Then AlCl3 solution was added to PCBP solution. If AlCl3 is insoluble in the selected solvent, AlCl3 solids were added to PCBP solution and the mixtures were exerted ultrasonication for 1 min to obtain homogeneous system [27]. The concentration and the compositional ratio of PCBP–AlCl3 complex in the mixtures were controlled. After aging the mixture at room temperature for tens of minutes or several hours, gels or precipitates were formed. The gelation ability of the PCBP–AlCl3 complex in different solvents was investigated by the ‘‘stable to inversion in a test tube’’ method [28]. The sol–gel conversion process was studied by heating the gel in a water-bath and then cooling in room temperature. The samples for optical microscopy and XRD characterization were prepared by drop-casting the metallogels or the precipitates onto the glass substrate. For TEM characterization, the gels or the precipitates were dropped onto copper mesh with filter paper to absorb the solvent quickly from the back. All the samples were dried in a vacuum oven overnight to remove the solvent before characterization. 2.3. Characterization The microscopy morphologies of the samples were investigated by optical microscopy (OM) and transmission electron microscopy (TEM). OM characterization was carried out on a Zeiss Axio Imager A2m (Carl Zeiss, Germany). TEM experiments were performed on a JEM-1011 (JEOL Co., Japan) with an accelerating voltage of 100 kV. The out-of-plane XRD measurement was performed using a Bruker D8 Discover Reflector (Cu Ka, k = 1.540 56 Å) with generation power of 40 kV tube voltages and 40 mA tube current in locked
Scheme 1. Molecular structure of PCBP.
couple mode. The rheological properties of the metallogel were conducted on a controlled strain rate rheometer (ARES rheometer) at room temperature with frequency sweep from 0.05 to 100 rad/s. The UV–Vis absorption spectra and concentration dependent absorption spectra were recorded by a Lambda 750 spectrometer (Perkin–Elmer, Wellesley, MA) with 5.0 nm slit. The PL spectra were recorded using a Perkin–Elmer LS 50B spectrofluorometer with 5.0 nm slit for excitation and emission. The concentration of PCBP–AlCl3 complex in ethanol solution for the PL spectra was 1 105 M for PCBP. The quartz cells of 10 mm thickness were used to measure the spectra of the dilute solution. The amorphous films and xerogel films from PCBP–AlCl3 complex for the spectra characterization were fabricated on quartz substrates. The excitation wavelength was 270 nm for the PL spectra. 1H NMR and 31P NMR spectra were obtained using a Bruker AV400 spectrometer (Bruker BioSpin Corporation, USA) at 300 K with ethanol-d6 as the solvent. Deuterated solvent was used as internal standard in 1 H NMR. For 31P NMR test, the spectrometer was adjusted from concentrated phosphoric acid before the test. Fourier transform infrared (FT-IR) spectra were recorded at room temperature with a Bruker IFS 66V instrument in transmission mode. 3. Results and discussion 3.1. Gelation of PCBP–AlCl3 complex PCBP–AlCl3 complex prefers to form gel in alcohols rather than that in other solvent systems. The solubility and gelation properties of PCBP and AlCl3 are shown in Table 1. The carbazole derivative PCBP is soluble in aromatic solvents, halogenated solvents, alcohols, acetone, DMF, DMSO and propylene glycol, but is insoluble in hexane and cyclohexane. Compared with the alkyl group substituted carbazole compounds, the promoted solubility of PCBP in these polar solvents may result from the polar phosphonate as the surface group [26,29]. AlCl3 dissolves well in alcohols and ethers. After mixing the PCBP solution and the AlCl3 solution, solution, gels or precipitate can be gained, which depends on the solvents. It indicates that the phosphonate substituted carbazole derivative PCBP can interact with AlCl3 and a complex of PCBP– AlCl3 may form in the solvents. Alcohols are the selective solvents for the gelation of PCBP–AlCl3 complex though both of the single
Table 1 The solubility of PCBP, AlCl3 and the gelation ability of PCBP–AlCl3 in different solvents. Solvent
PCBPa
AlCl3b
PCBP–AlCl3c
Methanol Ethanol 1-Propanol Isopropanol 1-Butanol 1-Hexanol Ethylene glycol THF CHCl3 CHCl2 Ether Acetone DMF DMSO Propylene carbonate
S S S S S S S S S S I S S S S
S S Sd S S S S S S S S I S I S
S G Ge G G G P P P P P P P P S
S: soluble; I: insoluble; G: gel; P: precipitate. a The concentration of PCBP is 10 mg/ml. b The concentration of AlCl3 is 5 mg/ml. c The concentration of PCBP is 10 mg/ml (0.097 M) and the compositional ratio is 1:5 (n/n). d The concentration of AlCl3 is 2.5 mg/ml. e The compositional ratio is 1:2 (n/n).
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component are soluble in alcohols. This may be because that alcohols are amphoteric solvents, which can act as oxygen donors and aid proton transfer within acid–base pair system from the organic– inorganic species [30]. Methanol and propylene carbonate can dissolve the PCBM–AlCl3 complex well and thus results in a solution phase, which may be due to their higher polarity. The gelation process is also influenced by the compositional ratio and the concentration of the complex. As it is shown in Fig. 1A, when the ratio of PCBP–AlCl3 (n/n) is 4:3, the transparent gel with light blue color can be gained in ethanol. As the content of aluminum(III) increases, the metallogels become turbid (Fig. 1B–F). This phenomenon has also been observed in Pd(II) metallogels [11]. When in a much lower or higher content of aluminum(III) with PCBP concentration fixed at 10 mg/ml in ethanol, the gel formation fails. As shown in Fig. S1A, colorless solution was obtained with a compositional ratio of 2:1 (n/n), while partial gel and precipitates were gained from the PCBP–AlCl3 complex with a ratio of 1:20 and 1:50 (n/n), respectively (Fig. S1H and I). It agrees with the 4- and 6-coordinated aluminum phosphonate complexes in other works [31]. The critical gelation concentration (CGC) values of the PCBP–AlCl3 system are 9.6 mg/ml (ca. 9.3 mM), 9.2 mg/ml (ca. 8.9 mM), 7.4 mg/ml (ca. 7 mM), 6.0 mg/ml (ca. 5.8 mM), 4.1 mg/ml (ca. 4 mM) and 2.8 mg/ml (ca. 2.3 mM) for PCBP at the ratio of 4:3, 1:1, 1:3, 1:4, 1:5 and 1:10 in ethanol, respectively. As it shown in Fig. S2, the CGC value for PCBP decreases as the content of aluminum(III) increases. This is because that aluminum(III) ions can act as the linking center to coordinate with PCBP molecules and thus the gel networks form more easily with higher content of aluminum(III). The sol–gel conversion process is also related to the compositional ratio of the complex. The sol–gel conversion can be observed by heating the metallogels with a ratio of 1:10 (n/n) in ethanol and the gelation temperature (Tgel) is about 65 °C. However, solution is gained from the ratio of 4:3 (n/n) to 1:3 (n/n) and precipitates are formed from the ratio of 1:4 (n/n) and 1:5 (n/n) after heating the metallogels. So the gel formation process is dependent on the compositional ratio: The gels are obtained with the ratio from 4:3 to 1:10 by sonication, while the gels are gained only with the ratio from 1:10 by heat-cool cycle. The rheological properties of the metallogels are different as the compositional ratios change. The metallogels show different trends in the moduli which respect to frequency. As it shown in Fig. S3A, with the ratio of 1:4 and 1:10, the storage modulus (G0 ) is larger than the loss modulus (G00 ) and the gel-like behavior is observed. While with a ratio of 1:1, the gel-like behavior is observed at low-frequency and solution-like behavior is observed at high frequency [24]. The complex viscosities of the metallogels at different
Fig. 1. Photographic images of the metallogels from PCBP–AlCl3 complex at different ratios (n/n) in ethanol: (A) 4:3, (B) 1:1, (C) 1:3, (D) 1:4, (E) 1:5 and (F) 1:10. The concentration of PCBP is 10 mg/ml.
ratios show the shear-thinning behaviors (Fig. S3B), which is similar to the other gels [25,32]. As the content of aluminum(III) increases, the storage modulus, the loss modulus and viscosity increase at first and then decrease. It indicates that the interactions are strengthened at first and then destroyed by AlCl3 as the content of aluminum(III) increases. The stability of gel is not very good. After aging the metallogels at RT for a period of time, the gel phase changes. The gel only maintains for 3–5 days from the ratio of 4:3 (n/n) to 1:3 (n/n) and clear solution is gained, while the gel phase can last for about half month from the ratio of 1:4 (n/n) to 1:10 (n/n) and the mixture of solvent and precipitate are obtained at last. 3.2. Microstructure of the metallogels Generally, the gelators form entangled self-assembled fibrillar networks (SAFINs) in the supramolecular gels through the noncovalent interactions. While in the metallogels from the complex of aluminum–phosphonate surfactant, the complex forms entangled networks consisted of the amorphous cylindrical worm-like micelles through the coordination between phosphorus-containing molecule and aluminum ions [23,25]. In this work, fiber networks are formed inside the metallogels from the PCBP–AlCl3 complex. The fibrous networks of the metallogels from the PCBP–AlCl3 complex are observed by OM and TEM. As it is shown in Fig. 2, the morphologies of the assemblies from the complex change as the content of aluminum(III) increases. Birefringence is observed in the xerogel films from the metallogels with a ratio of 4:3 (n/n) to 1:3 (n/n) under polarized light, which means that the complex is anisotropic and crystallizes in the films (Fig. 2B0 –D0 ). The magnified images in Fig. S4B0 –D0 show that densely packed fibers are formed in the polycrystalline films. For the xerogel films from the metallogels and partial gel with a higher content of aluminum(III), spherulites with obvious Maltese cross patterns are viewed under polarized light (Fig. 2F0 –H0 ). The diameters of the spherulites are up to hundreds of micrometers to several millimeters. The magnified images in Fig. S4F0 –H0 show that the spherulites are made up of branched straight fibers. The unobvious spherulites in the relative OM images may be due to the low contrast in unpolarized light. For the transition ratio of 1:4 (n/n), the dendrites consisted of branched curled fibers are formed (Figs. 2E and E0 , S4E and E0 ). The film prepared from the solution with a compositional ratio of 2:1 (n/n) is dark and isotropic under polarized light (Figs. 2A and A0 , S4A and A0 ), which indicates that no obvious ordered structures are formed from the complex with low aluminum(III) content. The spherulites morphologies are also dominated in the casted films from the precipitates with a ratio of 1:50 (n/n) and the AlCl3 solution. So we speculate that the crystallization of AlCl3 molecules may affect the final xerogel morphologies of the PCBP–AlCl3 complex with high aluminum(III) content. The detailed structures of fiber networks inside the metallogel are characterized by TEM. As it is shown in Fig. 3, the single nanofibers with length up to several micrometers and diameters of several hundred nanometers can be distinguished with a ratio of 4:3 (n/n) (Fig. 3A). These fibers intersect and entangle with each other from different directions to form networks. As the content of aluminum(III) increases, the length and diameter of the fibers decrease (Fig. 3B and C). In the turbid metallogel with a ratio of 1:4 (n/n), fiber bundles are formed in the xerogel (Fig. 3D). In the bundles, the length and diameter of the fibers decrease to one micrometer and tens of nanometers, the small sized fibers pack densely and the discrete single fibers are not so easily to distinguish. The xerogel with a ratio of 1:5 (n/n) shows similar fiber morphology to that with the ratio of 1:4 (Fig. 3D and E). For the xerogel with a ratio of 1:10, more fiber bundles are formed. The
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Fig. 2. Optical microscopy images of the assemblies from PCBP–AlCl3 complex at different ratios (n/n) in ethanol after dropping on the glass substrates: (A) 2:1, solution; (B) 4:3, transparent gel; (C) 1:1, transparent gel; (D) 1:3, transparent gel; (E) 1:4, turbid gel; (F) 1:5, turbid gel; (G) 1:10, turbid gel; (H) 1:20, turbid partial gel; (I) 1:50, precipitate; (J) AlCl3 solution. (A0 –J0 ) are the relative polarized optical microscopy images of (A–J). The scale bar is 500 lm in each image.
Fig. 3. TEM images of the assemblies from PCBP–AlCl3 complex at different ratios (n/n) in ethanol: (A) 4:3, transparent gel; (B) 1:1, transparent gel; (C) 1:3, transparent gel; (D) 1:4, turbid gel; (E) 1:5, turbid gel; (F) 1:10, turbid gel; (G) 1:20, turbid partial gel; (H) 1:50, precipitate.
fibers pack more densely and the bundles compact with each other to form networks (Fig. 3F). We suppose that the small-sized densely packing fibers from the complex with high AlCl3 content leads to the turbid gel. In the partial gel with a ratio of 1:20, the length of the fibers decreased to hundreds of nanometers (Fig. 3G), which may result in the less connection points inside the fiber networks. While for the precipitates with a ratio of 1:50, the fiber assembles disappear and irregular plate-like aggregates with micrometersized diameter form (Fig. 3H). These large sized aggregates may lead to the precipitation.
From above results, we know that the spherulites observed in the OM images from the complex with high aluminum(III) content do not appear in the TEM images. For the similar spherulites are also formed in the crystallization film of AlCl3, we speculate that only part of the AlCl3 molecules coordinate with phosphonate groups to form organic–metal complex and the excess AlCl3 molecules crystallize during the solvent evaporation process. For AlCl3 molecules are capable to form highly crystallized spherulites, the fibrous networks of the metallogels are suppressed by the spherulites and cannot be observed by OM. For the AlCl3 solution among
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the fibrous networks is removed by the filter paper from the back of copper mesh when preparing the TEM samples, only the fibers can be observed by TEM. So the TEM images show the morphologies of the gel fibers, while the OM images are influenced by the crystallization morphology of excess aluminum chloride with high aluminum contents. The excess AlCl3 can form dimers, trimers, and tetramers with chlorine acting as a bridging ligand [23]. Both the densely packed fiber networks and the AlCl3 oligomers may improve the stability of the metallogels from the PCBP–AlCl3 complex with high content of aluminum(III). The XRD patterns from the assemblies of the PCBP–AlCl3 complex also demonstrate the crystallization of AlCl3 molecules. As it is shown in Fig. 4A, multiple diffraction peaks are observed in the samples from the xerogels and precipitates with high content of aluminum(III) (from the ratio of 1:5 to 1:50 (n/n)). The diffraction patterns are similar to the crystallized AlCl3 film, which indicates that the crystallization of AlCl3 is dominated in the films from these assemblies. For the xerogel films from the complex with a ratio of 4:3, 1:1 and 1:3 (n/n), the XRD patterns show the same diffraction peaks at 2h = 5.38° (d1 = 1.64 nm) and 2h = 10.76° (d2 = 0.82 nm), which means that the compositional ratio does not influence the molecular packing of the complex in the xerogel fibers. The d1 value is twice of the d2 value, which indicates that the PCBP–AlCl3 complex may form layered structures in the xerogel
Fig. 4. Out-of-plane XRD curves of assemblies from PCBP–AlCl3 complex at different ratios (n/n) after dropping on glass substrates: (A) PCBP crystals from CH2Cl2/hexane (black line) [26]; 2:1, solution from ethanol (red line); 4:3, transparent gel from ethanol (blue line); 1:1, transparent gel from ethanol (dark cyan line); 1:3, transparent gel from ethanol (magenta line); 1:4, turbid gel from ethanol (dark yellow line); 1:5, turbid gel from ethanol (navy line); 1:10, turbid gel from ethanol (wine line);1:20, turbid partial gel from ethanol (pink line); 1:50, precipitate from ethanol (olive line); AlCl3 film from ethanol (royal line). (B) Gels from the PCBP-AlCl3 complex at a ratio of 1:3 (black line), 1:4 (red line) and 1:5 (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
fibers. For these diffraction patterns of the xerogel samples are different from the PCBP crystals obtained from the mixed solvents and AlCl3 crystalline films, we suppose that new complex is formed from PCBP and AlCl3 [26]. The formation of layered supramolecular structures is common in aluminophosphates through coordination interactions [33]. The film from the solution of PCBP–AlCl3 complex with a ratio of 2:1 (n/n) does not show any diffraction peaks and it is consistent with the amorphous phase observed in the OM images. For the films with the ratio of 1:4 (n/n), the diffraction signals are more complicated, which contains the signal of the xerogel film with a ratio of 1:3 as well as the crystalline peaks of AlCl3 with a ratio of 1:5 (Fig. 4B). It indicates the complex is in a transition state at the ratio of 1:4. In a word, the XRD patterns of the films at different ratios agree well with the morphology evolution observed in the OM images. 3.3. The driving force for the metallogels The metallogels based on the coordination interactions have been demonstrated in the complexes of aluminum and phosphate surfactant recently [23–25,34]. We have also found that the carbazole derivatives can assemble into crystalline fibers in thin films through vapor annealing and form supramolecular gels in the mixture solvent, which are driven by the p–p interactions between the aryl units [29,35,36]. So we speculate that the coordination bonding and p–p interactions may promote the aggregation and gelation of the PCPB–AlCl3 complex in alcohols. The coordination interactions between Al(III) ions and phosphonate groups are studied by the NMR spectra. As it is shown in Fig. 5A, only single peak appears at 20.16 ppm for PCBP solution in d6-ethanol, which shows a little change compared with 31P signals observed in d6-DMSO [26]. The 31P NMR signal of PCBP is close to the chemical shift of the other acyclic phosphonates compound [37]. The signal of the PCBP–AlCl3 complex with a ratio of 2:1 (n/n) is similar to that of PCBP solution, which indicates that the content of AlCl3 is too low to change the chemical environment of PCBP molecules. As the compositional ratio changes, the signal of 31P NMR varies. The 31P NMR signals at 20.11 ppm, 15.42 ppm and 13.20 ppm can be observed from the metallogel with a ratio of 1:3 (n/n). It indicates that three types of phosphor atoms with nonequivalent environments are formed in the metallogel. When the compositional ratio changes to 1:10 (n/n), the 31P NMR signals shift to 19.81 ppm, 15.25 ppm and 13.22 ppm and the intensities of two peaks at higher field increase. This is because the paramagnetic contribution rather than the bonding electron distribution is dominant in the determination of chemical shift in 31P NMR spectra [38]. The difference in 31P signals in the metallogel from the PCBP–AlCl3 complex may result from the different coordination modes of aluminum(III) and phosphonate groups [31]. The enhanced intensity for the peaks in the high field with a ratio of 1:10 indicates that the content of the phosphonate groups with new coordination mode increase. There are also some changes in the 1H NMR spectra with different compositional ratios. As it shown in Fig. 5B, the intensity of proton signals changes for the solvent when gel forms. As the content of AlCl3 increases, the signal of –OH (5.25 ppm) resonance decreases, while the signal of –CH2 (3.56 ppm) and –CH3 (1.11 ppm) resonance increases. It may be because that part of the hydroxyl groups interact with the aluminum. In other words, the solvent takes part in gelation process. The proton signals of the aromatic rings are shifted downfield in the metallogel with a ratio of 1:10 (the inset in Fig. 5B), which may be due to the electron-withdrawing inductive effect on the proximate protons caused by the coordination of phosphonate and aluminum(III). In addition, the splitted peaks collapse to single broad peaks, which indicates that oligomerization or polymerization has taken place
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due to coordination interactions among aluminum ions and PCBP molecules [11]. The coordination interactions have also been investigated by FT-IR spectra. As it is shown in Fig. S5A, the broad absorption peak at 3462 cm1 in the PCBP solids can be ascribed to the vibration of OH from the adsorbed water molecules. This vibration is further broadened after the addition of AlCl3. The strong broad peaks at 3058 cm1 and 1630 cm1 in AlCl3 solid and the xerogel films are referred to the OH vibrations of the surface absorbed water molecules. When AlCl3 is added, the two peaks for C–C vibrations at 1243 cm1 and 1232 cm1 become a single peak and the peaks of P–O–R vibrations at 594 cm1 and 582 cm1 disappear in Fig. S5B. It indicates that the vibrations of phosphonate groups change. The two bands at 1209 cm1 and 592 cm1 formed in the xerogel can be regarded as the asymmetric stretching vibration of P–O–Al and the bending mode of Al–O [39]. It means
PCBP solution PCBP film 2-1 4-3 1-1 1-3 1-4 1-5 1-10 1-20 1-50
1.0
PL Intensity
Fig. 5. (A) 31P NMR and (B) 1H NMR spectra of PCBP–AlCl3 complex at different ratios (n/n) at room temperature: PCBP solution (black line); 2:1, solution (red line); 1:3, transparent gel (blue line); 1:10, turbid gel (dark cyan line). The concentration of PCBP is 10 mg/ml. The inset image in (B) shows an expanded portion of the spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
that the P–O–Al coordination bonding has formed in the metallogels. The p–p interactions among the aromatic groups are researched by fluorescence spectroscopy. Compared to the emission peak at 362 nm for PCBP solution, the film emits at 376 nm (Fig. 6). The red-shift of the spectra indicates that the interactions between PCBP molecules are enhanced in the film. The emission peak for the complex with a ratio of 2:1 (n/n) is also at 376 nm, which is identical to the PCBP film. It means that few amount of AlCl3 has little influence on the interactions of PCBP molecules, which is similar to that observed in 31P NMR spectra. As the content of AlCl3 increases in the metallogel, the emission peaks of the xerogel film shift from 379 nm (with a ratio of 4:3) to 384 nm (with a ratio of 1:10). We can ascribe this red-shift to the improved p–p interactions among PCBP molecules at the presence of AlCl3. However, as the content of AlCl3 further increases, a blue-shift of PL spectra takes place. The emission peaks shift from 376 nm (the partial gel with a ratio of 1:20) to 371 nm (the precipitate with a ratio of 1:50). We suppose that the PCBP–AlCl3 complex has been surrounded by the excess AlCl3 molecules at high aluminum(III) content conditions and the p–p interactions between the aromatic groups of PCBP molecules are disrupted, which is consistent with the moduli and viscosity change at high aluminum(III) content. The concentration dependent absorption spectra also can demonstrate the promoted p–p interactions at high-concentration conditions. As shown in Fig. S6A, the dilute solution shows absorption at 249 nm, 270 nm, 287 nm, 305 nm and 336 nm. The absorption at 305 nm and 336 nm can be ascribed to the absorption of carbazole groups [40]. As the concentration increases from 1 106 M to 2 105 M, the intensity of the absorption at 249 nm decreases and the intensity of absorption at longer wavelength increases. So we may figure that the p–p interactions between carbazole groups are improved as the concentration increases. When the concentration increases to 2 105 M, the absorption peaks are indistinguishable. The absorption spectra of the film show similar characterization to that of the highconcentration solutions except a little red-shift of the spectra edge. We think that the intermolecular interactions in the highconcentration solution and film are more complicated. For the PCBP–AlCl3 complex, the absorption spectra show similar change except the more pronounced red-shift of the spectra edge in the solid xerogel film (Fig. S6B). It indicates that the interactions between the PCBP molecules are stronger in xerogel films than those in PCBP films.
0.5
0.0 300
350
400
450
500
Wavelength (nm) Fig. 6. PL spectra of PCBP–AlCl3 assemblies from different ratios (n/n): PCBP solution (black line), PCBP film (red line), PCBP–AlCl3 complex film from the assemblies at different ratios: 2:1, solution (blue line), 4:3, transparent gel from ethanol (dark cyan line); 1:1, transparent gel from ethanol (magenta line); 1:3, transparent gel from ethanol(dark yellow line); 1:4, turbid gel from ethanol (navy line); 1:5, turbid gel from ethanol (wine line); 1:10, turbid gel from ethanol (pink line);1:20, turbid partial gel from ethanol (olive line); 1:50, precipitate from ethanol (royal line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Schematic illustrating possible arrangement of PCBP–AlCl3 complex within supramolecular aggregates at different ratios: (A) 1:1, (B) 1:2, (C) excess aluminum. In the schematic, X represents Cl or –OCH2CH3.
From the above results, we speculate that the coordination bonding between aluminum(III) and phosphonate groups of PCBP and the p–p interactions between the aromatic groups of PCBP may be the main driving force for the PCBP–AlCl3 complex to self-assemble into nanofibers. The possible arrangements of PCBP–AlCl3 complex within gel fibers from different ratios are shown in Fig. 7. The anhydrous aluminum chloride will react with ethanol as following: AlCl3 + CH3CH2OH = AlCl3x(OCH2CH3)x + x HCl (x P 1) [30]. When the compositional ratio is 1:1, the complex may form linear structure with coordination bonding to bridge the two components (Fig. 7A). At the ratio of 2:1, the content of aluminum is too low to crosslink PCBP to form one-dimensional coordination polymer. One PCBP molecule can coordinate with several aluminum ions (at most four), while one aluminum ion has four- or six-ordination sites. So as the content of aluminum increases, the two components can interact with each other at different directions to form three-dimensional crosslinking structures. Fig. 7B shows one possible crosslinking structure of the PCBP–AlCl3 complex from the ratio of 1:2. These crosslinking structures can further form gel networks. When the content of AlCl3 is high, all the phosphonate groups of PCBP molecule are coordinated with aluminum ions, and the excess aluminum
ions just bond with one phosphonate group or none. Thus the crosslinking structures are destroyed (Fig. 7C). When most of PCBP molecules are surrounded by aluminum ions, the crosslinking structures break up and the gel networks collapse, such as at the ratio of 1:20 and 1:50.
4. Conclusion In conclusion, we have reported the metallogels from the complex of phosphonate substituted carbazole derivative PCBP and AlCl3. The PCBP–AlCl3 complex can form gels in the selective solvent of alcohols. The fiber morphologies of the xerogel are highly influenced by the compositional ratio of the complex. As the content of aluminum(III) increases, the length and diameter of nanofibers decreases and the fibers pack more densely. It makes the transparent gel become turbid and more stable. The XRD patterns show that the complex forms layered structures in the fiber networks. We speculate that the coordination interactions between the phosphonate groups and aluminum(III) ions and the p–p interactions among the aromatic groups of PCBP dominate the gelation process in the alcohols.
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51273191, 51073151, 21334006). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.03.016. References [1] N.M. Sangeetha, U. Maitra, Chem. Soc. Rev. 34 (2005) 821–836. [2] S.S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew. Chem. Int. Ed. 51 (2012) 1766–1776. [3] A.R. Hirst, B. Escuder, J.F. Miravet, D.K. Smith, Angew. Chem. Int. Ed. 47 (2008) 8002–8018. [4] M. Llusar, C. Sanchez, Chem. Mater. 20 (2008). 782–780. [5] F. Fages, Angew. Chem. Int. Ed. 45 (2006) 1680–1682. [6] M.O.M. Piepenbrock, G.O. Lloyd, N. Clarke, J.W. Steed, Chem. Rev. 110 (2010) 1960–2004. [7] X. de Hatten, N. Bell, N. Yufa, G. Christmann, J.R. Nitschke, J. Am. Chem. Soc. 133 (2011) 3158–3164. [8] A. Kishimura, T. Yamashita, T. Aida, J. Am. Chem. Soc. 127 (2005) 179–183. [9] H. Lee, S.H. Jung, W.S. Han, J.H. Moon, S. Kang, J.Y. Lee, J.H. Jung, S. Shinkai, Chem. Eur. J. 17 (2011) 2823–2827. [10] S. Kawano, N. Fujita, S. Shinkai, J. Am. Chem. Soc. 126 (2004) 8592–8593. [11] Y.R. Liu, L.S. He, J.Y. Zhang, X.B. Wang, C.Y. Su, Chem. Mater. 21 (2009) 557– 563. [12] S.C. Li, V.T. John, G.C. Irvin, S.H. Rachakonda, G.L. McPherson, C.J. O’Connor, J. Appl. Phys. 85 (1999) 5965–5967. [13] R. Murugavel, A. Choudhury, M.G. Walawalkar, R. Pothiraja, C.N.R. Rao, Chem. Rev. 108 (2008) 3549–3655. [14] K.J. Gagnon, H.P. Perry, A. Clearfield, Chem. Rev. 112 (2012) 1034–1054. [15] M. Froba, M. Tiemann, Chem. Mater. 10 (1998) 3475–3483. [16] T. Kimura, Chem. Mater. 15 (2003) 3742–3744.
109
[17] T. Kimura, Chem. Mater. 17 (2005) 5521–5528. [18] Z. Florjanczyk, A. Wolak, M. Debowski, A. Plichta, J. Ryszkowska, J. Zachara, A. Ostrowski, E. Zawadzak, M. Jurczyk-Kowalska, Chem. Mater. 19 (2007) 5584– 5592. [19] Y. Huang, J. Ma, J. Yang, K. Cao, Z. Lu, J. Phys. Chem. C 116 (2012) 22518–22525. [20] J. Ma, J. Yang, Y. Huang, K. Cao, J. Mater. Chem. 22 (2012) 2007–2017. [21] E.C.D. Lima, F. Galembeck, J. Colloid Interf. Sci. 166 (1994) 309–315. [22] E.C.D. Lima, J.M.M. Neto, F.Y. Fujiwara, F. Galembeck, J. Colloid Interf. Sci. 176 (1995) 388–396. [23] M.G. Page, G.G. Warr, J. Phys. Chem. B 108 (2004) 16983–16989. [24] M.G. Page, G.G. Warr, Langmuir 25 (2009) 8810–8816. [25] M. George, G.P. Funkhouser, R.G. Weiss, Langmuir 24 (2008) 3537–3544. [26] B. Chen, J.Q. Ding, L.X. Wang, X.B. Jing, F.S. Wang, J. Mater. Chem. 22 (2012) 23680–23686. [27] C.D. Dou, D. Chen, J. Iqbal, Y. Yuan, H.Y. Zhang, Y. Wang, Langmuir 27 (2011) 6323–6329. [28] R. Mukkamala, R.G. Weiss, Langmuir 12 (1996) 1474–1482. [29] Z. Ding, Q. Zhao, R. Xing, X. Wang, J. Ding, L. Wang, Y. Han, J. Mater. Chem. C 1 (2013) 786–792. [30] B.Z. Tian, X.Y. Liu, B. Tu, C.Z. Yu, J. Fan, L.M. Wang, S.H. Xie, G.D. Stucky, D.Y. Zhao, Nat. Mater. 2 (2003) 159–163. [31] Z. Florjanczyk, A. Lasota, A. Wolak, J. Zachara, Chem. Mater. 18 (2006) 1995– 2003. [32] W.G. Weng, J.B. Beck, A.M. Jamieson, S.J. Rowan, J. Am. Chem. Soc. 128 (2006) 11663–11672. [33] M. Tiemann, M. Froba, Chem. Mater. 13 (2001) 3211–3217. [34] G.P. Funkhouser, N. Tonmukayakul, F. Liang, Langmuir 25 (2009) 8672–8677. [35] Z. Ding, R. Xing, L. Zheng, Y. Sun, X. Wang, J. Ding, L. Wang, Y. Han, J. Phys. Chem. B 115 (2011) 15159–15166. [36] Z. Ding, R. Xing, Y. Sun, L. Zheng, X. Wang, J. Ding, L. Wang, Y. Han, RSC Adv. 3 (2013) 8037–8046. [37] R.M. Twyman, Nuclear Magnetic Resonance Spectroscopy-Applicable Elements: Phosphorus-31, Elsevier Science, London, UK, 2005. pp. 278–286. [38] F. Ribasprado, C. Giessnerprettre, B. Pullman, J.P. Daudey, J. Am. Chem. Soc. 101 (1979) 1737–1742. [39] J. Xu, L. Chen, D.L. Zeng, J. Yang, M.J. Zhang, C.H. Ye, F. Deng, J. Phys. Chem. B 111 (2007) 7105–7113. [40] S.M. Bonesi, R. Erra-Balsells, J. Lumin. 93 (2001) 51–74.
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
Supramolecular metallogels with complex of phosphonate substituted carbazole derivative and aluminum(III) ion as gelator Zicheng Ding, Bo Chen, Junqiao Ding, Lixiang Wang, Yanchun Han ⇑ State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, China University of the Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 12 December 2013 Accepted 1 March 2014 Available online 28 March 2014 Keywords: Metallogel Phosphonate Carbazole derivative p–p interactions Coordination interactions
a b s t r a c t Supramolecular metallogels can be gained from the phosphonate substituted 4,40 -bis(N-carbazolyl)biphenyl (PCBP) in the presence of aluminum chloride in alcohols, which can donate oxygen to aid proton transfer in the aluminum organophosphorus complexes. Inside the metallogels, three-dimensional fiber networks with nanofibers entangling and intersecting with each other inside are formed. The nanofibers show layered structures with a period thickness of 0.82 nm. As the content of aluminum(III) increases, the size of the fibers becomes smaller and the fibers pack more densely. It makes the transparent gel become turbid but nevertheless improves the stability of the metallogels. NMR, FT-IR and fluorescence spectroscopy show that the coordination interactions between the phosphonate groups of PCBP molecules and aluminum(III) ions as well as the p–p interactions among PCBP molecules are involved during the gel formation process. Ó 2014 Elsevier Inc. All rights reserved.
1. Introductions Supramolecular gels have attracted wide attention for their applications in various areas, including sensors, catalysts, templates, biomedicine, molecular electronics and photonics [1–4]. Through cooperating coordination interaction with other noncovalent interactions, such as hydrogen bonding, p–p interaction and van der Waals interaction, lots of metallogels have been exploited from organic–metal complexes and coordination polymers [5,6]. Taking advantage of the incorporated metallic elements, the metallogels show unique behaviors in luminescence, vapor sensing, redox stimuli, catalytic and magnetic properties. The relative works in these fields have also been reported recently [7–12]. Phosphate and phosphonate with oxygen atoms can bond to numerous metal ions to form organic–metal complexes [13,14]. Multiple aluminumphosphate and aluminophosphonate complexes have been synthesized and found applications in zeolite science for their ordered mesoporous structures [15–17]. Supramolecular structures such as nanoparticles, fibers and rods can also be gained from aluminum organophosphorus complexes driven by the coordination interactions between aluminum and phosphates as well as other non-covalent interactions [18–20]. Metallogels based on metal–ligand coordination from the complexes of aluminum and phosphate surfactant have also been ⇑ Corresponding author. Fax: +86 431 85262126. E-mail address: [email protected] (Y. Han). http://dx.doi.org/10.1016/j.jcis.2014.03.016 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
demonstrated. Lima et al. studied the thermoreversible gelation behavior of sodium polyphosphate and aluminum salt in aqueous solutions by 31P and 27Al NMR [21,22]. Warr et al. reported that aluminum isopropoxide and didodecyl phosphate can form gel in decane [23,24]. They found that the gel structure transition from independent wormlike micelles to a physical branched gel network takes place as the amount of aluminum increases, which makes the gel system displace Maxwellian rheology. Weiss et al. investigated the gelation behaviors of bisphosphonate esters, phosphonic acids, monophosphonate esters and aphosphoric acid ester in the presence of aluminum(III) and boron(III) in an organic liquid [25]. They found that the complex of boric acid with monophosphonate ester forms crystalline aggregates in the gel, whereas Al(iPrO)3– monophosphonate ester complex leads to nonbirefringent, amorphous aggregates. However, the metallogels from other aluminum organophosphorus complexes with functionalities are rare. In this work, we reported a new fluorescent metallogels from the complex of the phosphonate substituted carbazole derivative and aluminum chloride in alcohols, which are driven by the coordination bonding between phosphonate groups and aluminum(III) ions as well as the p–p interactions between the aromatic rings. The organic–metal complex forms fibrous networks inside the gel, and the fiber morphologies and stability are sensitive to the content of aluminum(III). As the content of aluminum(III) increases, the gel fibers become shorter and pack more densely, which makes the transparent gel become turbid and more stable.
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2. Experimental section 2.1. Materials The phosphonate substituted carbazole-based derivative, phosphonate substituted 4,40 -bis(N-carbazolyl)biphenyl (PCBP) (Scheme 1), was synthesized from Ref. [26]. Anhydrous aluminum chloride (AlCl3) was purchased from Alfa Aesar Company and used as received. The solvent ethanol-d6 was purchased from Sigma– Aldrich Company. The other solvents were purchased from Beijing Chemical Reagent Co., Ltd., China, and purified before used. The glass substrates were cleaned in a piranha solution (70/30 v/v of concentrated H2SO4 and 30% H2O2) at 70 °C for 30 min, thoroughly rinsed with deionized water and dried under nitrogen flow. 2.2. Preparation of metallogels and xerogel samples The solubilities of PCBP and AlCl3 in various solvents were tested separately at first. Then AlCl3 solution was added to PCBP solution. If AlCl3 is insoluble in the selected solvent, AlCl3 solids were added to PCBP solution and the mixtures were exerted ultrasonication for 1 min to obtain homogeneous system [27]. The concentration and the compositional ratio of PCBP–AlCl3 complex in the mixtures were controlled. After aging the mixture at room temperature for tens of minutes or several hours, gels or precipitates were formed. The gelation ability of the PCBP–AlCl3 complex in different solvents was investigated by the ‘‘stable to inversion in a test tube’’ method [28]. The sol–gel conversion process was studied by heating the gel in a water-bath and then cooling in room temperature. The samples for optical microscopy and XRD characterization were prepared by drop-casting the metallogels or the precipitates onto the glass substrate. For TEM characterization, the gels or the precipitates were dropped onto copper mesh with filter paper to absorb the solvent quickly from the back. All the samples were dried in a vacuum oven overnight to remove the solvent before characterization. 2.3. Characterization The microscopy morphologies of the samples were investigated by optical microscopy (OM) and transmission electron microscopy (TEM). OM characterization was carried out on a Zeiss Axio Imager A2m (Carl Zeiss, Germany). TEM experiments were performed on a JEM-1011 (JEOL Co., Japan) with an accelerating voltage of 100 kV. The out-of-plane XRD measurement was performed using a Bruker D8 Discover Reflector (Cu Ka, k = 1.540 56 Å) with generation power of 40 kV tube voltages and 40 mA tube current in locked
Scheme 1. Molecular structure of PCBP.
couple mode. The rheological properties of the metallogel were conducted on a controlled strain rate rheometer (ARES rheometer) at room temperature with frequency sweep from 0.05 to 100 rad/s. The UV–Vis absorption spectra and concentration dependent absorption spectra were recorded by a Lambda 750 spectrometer (Perkin–Elmer, Wellesley, MA) with 5.0 nm slit. The PL spectra were recorded using a Perkin–Elmer LS 50B spectrofluorometer with 5.0 nm slit for excitation and emission. The concentration of PCBP–AlCl3 complex in ethanol solution for the PL spectra was 1 105 M for PCBP. The quartz cells of 10 mm thickness were used to measure the spectra of the dilute solution. The amorphous films and xerogel films from PCBP–AlCl3 complex for the spectra characterization were fabricated on quartz substrates. The excitation wavelength was 270 nm for the PL spectra. 1H NMR and 31P NMR spectra were obtained using a Bruker AV400 spectrometer (Bruker BioSpin Corporation, USA) at 300 K with ethanol-d6 as the solvent. Deuterated solvent was used as internal standard in 1 H NMR. For 31P NMR test, the spectrometer was adjusted from concentrated phosphoric acid before the test. Fourier transform infrared (FT-IR) spectra were recorded at room temperature with a Bruker IFS 66V instrument in transmission mode. 3. Results and discussion 3.1. Gelation of PCBP–AlCl3 complex PCBP–AlCl3 complex prefers to form gel in alcohols rather than that in other solvent systems. The solubility and gelation properties of PCBP and AlCl3 are shown in Table 1. The carbazole derivative PCBP is soluble in aromatic solvents, halogenated solvents, alcohols, acetone, DMF, DMSO and propylene glycol, but is insoluble in hexane and cyclohexane. Compared with the alkyl group substituted carbazole compounds, the promoted solubility of PCBP in these polar solvents may result from the polar phosphonate as the surface group [26,29]. AlCl3 dissolves well in alcohols and ethers. After mixing the PCBP solution and the AlCl3 solution, solution, gels or precipitate can be gained, which depends on the solvents. It indicates that the phosphonate substituted carbazole derivative PCBP can interact with AlCl3 and a complex of PCBP– AlCl3 may form in the solvents. Alcohols are the selective solvents for the gelation of PCBP–AlCl3 complex though both of the single
Table 1 The solubility of PCBP, AlCl3 and the gelation ability of PCBP–AlCl3 in different solvents. Solvent
PCBPa
AlCl3b
PCBP–AlCl3c
Methanol Ethanol 1-Propanol Isopropanol 1-Butanol 1-Hexanol Ethylene glycol THF CHCl3 CHCl2 Ether Acetone DMF DMSO Propylene carbonate
S S S S S S S S S S I S S S S
S S Sd S S S S S S S S I S I S
S G Ge G G G P P P P P P P P S
S: soluble; I: insoluble; G: gel; P: precipitate. a The concentration of PCBP is 10 mg/ml. b The concentration of AlCl3 is 5 mg/ml. c The concentration of PCBP is 10 mg/ml (0.097 M) and the compositional ratio is 1:5 (n/n). d The concentration of AlCl3 is 2.5 mg/ml. e The compositional ratio is 1:2 (n/n).
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component are soluble in alcohols. This may be because that alcohols are amphoteric solvents, which can act as oxygen donors and aid proton transfer within acid–base pair system from the organic– inorganic species [30]. Methanol and propylene carbonate can dissolve the PCBM–AlCl3 complex well and thus results in a solution phase, which may be due to their higher polarity. The gelation process is also influenced by the compositional ratio and the concentration of the complex. As it is shown in Fig. 1A, when the ratio of PCBP–AlCl3 (n/n) is 4:3, the transparent gel with light blue color can be gained in ethanol. As the content of aluminum(III) increases, the metallogels become turbid (Fig. 1B–F). This phenomenon has also been observed in Pd(II) metallogels [11]. When in a much lower or higher content of aluminum(III) with PCBP concentration fixed at 10 mg/ml in ethanol, the gel formation fails. As shown in Fig. S1A, colorless solution was obtained with a compositional ratio of 2:1 (n/n), while partial gel and precipitates were gained from the PCBP–AlCl3 complex with a ratio of 1:20 and 1:50 (n/n), respectively (Fig. S1H and I). It agrees with the 4- and 6-coordinated aluminum phosphonate complexes in other works [31]. The critical gelation concentration (CGC) values of the PCBP–AlCl3 system are 9.6 mg/ml (ca. 9.3 mM), 9.2 mg/ml (ca. 8.9 mM), 7.4 mg/ml (ca. 7 mM), 6.0 mg/ml (ca. 5.8 mM), 4.1 mg/ml (ca. 4 mM) and 2.8 mg/ml (ca. 2.3 mM) for PCBP at the ratio of 4:3, 1:1, 1:3, 1:4, 1:5 and 1:10 in ethanol, respectively. As it shown in Fig. S2, the CGC value for PCBP decreases as the content of aluminum(III) increases. This is because that aluminum(III) ions can act as the linking center to coordinate with PCBP molecules and thus the gel networks form more easily with higher content of aluminum(III). The sol–gel conversion process is also related to the compositional ratio of the complex. The sol–gel conversion can be observed by heating the metallogels with a ratio of 1:10 (n/n) in ethanol and the gelation temperature (Tgel) is about 65 °C. However, solution is gained from the ratio of 4:3 (n/n) to 1:3 (n/n) and precipitates are formed from the ratio of 1:4 (n/n) and 1:5 (n/n) after heating the metallogels. So the gel formation process is dependent on the compositional ratio: The gels are obtained with the ratio from 4:3 to 1:10 by sonication, while the gels are gained only with the ratio from 1:10 by heat-cool cycle. The rheological properties of the metallogels are different as the compositional ratios change. The metallogels show different trends in the moduli which respect to frequency. As it shown in Fig. S3A, with the ratio of 1:4 and 1:10, the storage modulus (G0 ) is larger than the loss modulus (G00 ) and the gel-like behavior is observed. While with a ratio of 1:1, the gel-like behavior is observed at low-frequency and solution-like behavior is observed at high frequency [24]. The complex viscosities of the metallogels at different
Fig. 1. Photographic images of the metallogels from PCBP–AlCl3 complex at different ratios (n/n) in ethanol: (A) 4:3, (B) 1:1, (C) 1:3, (D) 1:4, (E) 1:5 and (F) 1:10. The concentration of PCBP is 10 mg/ml.
ratios show the shear-thinning behaviors (Fig. S3B), which is similar to the other gels [25,32]. As the content of aluminum(III) increases, the storage modulus, the loss modulus and viscosity increase at first and then decrease. It indicates that the interactions are strengthened at first and then destroyed by AlCl3 as the content of aluminum(III) increases. The stability of gel is not very good. After aging the metallogels at RT for a period of time, the gel phase changes. The gel only maintains for 3–5 days from the ratio of 4:3 (n/n) to 1:3 (n/n) and clear solution is gained, while the gel phase can last for about half month from the ratio of 1:4 (n/n) to 1:10 (n/n) and the mixture of solvent and precipitate are obtained at last. 3.2. Microstructure of the metallogels Generally, the gelators form entangled self-assembled fibrillar networks (SAFINs) in the supramolecular gels through the noncovalent interactions. While in the metallogels from the complex of aluminum–phosphonate surfactant, the complex forms entangled networks consisted of the amorphous cylindrical worm-like micelles through the coordination between phosphorus-containing molecule and aluminum ions [23,25]. In this work, fiber networks are formed inside the metallogels from the PCBP–AlCl3 complex. The fibrous networks of the metallogels from the PCBP–AlCl3 complex are observed by OM and TEM. As it is shown in Fig. 2, the morphologies of the assemblies from the complex change as the content of aluminum(III) increases. Birefringence is observed in the xerogel films from the metallogels with a ratio of 4:3 (n/n) to 1:3 (n/n) under polarized light, which means that the complex is anisotropic and crystallizes in the films (Fig. 2B0 –D0 ). The magnified images in Fig. S4B0 –D0 show that densely packed fibers are formed in the polycrystalline films. For the xerogel films from the metallogels and partial gel with a higher content of aluminum(III), spherulites with obvious Maltese cross patterns are viewed under polarized light (Fig. 2F0 –H0 ). The diameters of the spherulites are up to hundreds of micrometers to several millimeters. The magnified images in Fig. S4F0 –H0 show that the spherulites are made up of branched straight fibers. The unobvious spherulites in the relative OM images may be due to the low contrast in unpolarized light. For the transition ratio of 1:4 (n/n), the dendrites consisted of branched curled fibers are formed (Figs. 2E and E0 , S4E and E0 ). The film prepared from the solution with a compositional ratio of 2:1 (n/n) is dark and isotropic under polarized light (Figs. 2A and A0 , S4A and A0 ), which indicates that no obvious ordered structures are formed from the complex with low aluminum(III) content. The spherulites morphologies are also dominated in the casted films from the precipitates with a ratio of 1:50 (n/n) and the AlCl3 solution. So we speculate that the crystallization of AlCl3 molecules may affect the final xerogel morphologies of the PCBP–AlCl3 complex with high aluminum(III) content. The detailed structures of fiber networks inside the metallogel are characterized by TEM. As it is shown in Fig. 3, the single nanofibers with length up to several micrometers and diameters of several hundred nanometers can be distinguished with a ratio of 4:3 (n/n) (Fig. 3A). These fibers intersect and entangle with each other from different directions to form networks. As the content of aluminum(III) increases, the length and diameter of the fibers decrease (Fig. 3B and C). In the turbid metallogel with a ratio of 1:4 (n/n), fiber bundles are formed in the xerogel (Fig. 3D). In the bundles, the length and diameter of the fibers decrease to one micrometer and tens of nanometers, the small sized fibers pack densely and the discrete single fibers are not so easily to distinguish. The xerogel with a ratio of 1:5 (n/n) shows similar fiber morphology to that with the ratio of 1:4 (Fig. 3D and E). For the xerogel with a ratio of 1:10, more fiber bundles are formed. The
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Fig. 2. Optical microscopy images of the assemblies from PCBP–AlCl3 complex at different ratios (n/n) in ethanol after dropping on the glass substrates: (A) 2:1, solution; (B) 4:3, transparent gel; (C) 1:1, transparent gel; (D) 1:3, transparent gel; (E) 1:4, turbid gel; (F) 1:5, turbid gel; (G) 1:10, turbid gel; (H) 1:20, turbid partial gel; (I) 1:50, precipitate; (J) AlCl3 solution. (A0 –J0 ) are the relative polarized optical microscopy images of (A–J). The scale bar is 500 lm in each image.
Fig. 3. TEM images of the assemblies from PCBP–AlCl3 complex at different ratios (n/n) in ethanol: (A) 4:3, transparent gel; (B) 1:1, transparent gel; (C) 1:3, transparent gel; (D) 1:4, turbid gel; (E) 1:5, turbid gel; (F) 1:10, turbid gel; (G) 1:20, turbid partial gel; (H) 1:50, precipitate.
fibers pack more densely and the bundles compact with each other to form networks (Fig. 3F). We suppose that the small-sized densely packing fibers from the complex with high AlCl3 content leads to the turbid gel. In the partial gel with a ratio of 1:20, the length of the fibers decreased to hundreds of nanometers (Fig. 3G), which may result in the less connection points inside the fiber networks. While for the precipitates with a ratio of 1:50, the fiber assembles disappear and irregular plate-like aggregates with micrometersized diameter form (Fig. 3H). These large sized aggregates may lead to the precipitation.
From above results, we know that the spherulites observed in the OM images from the complex with high aluminum(III) content do not appear in the TEM images. For the similar spherulites are also formed in the crystallization film of AlCl3, we speculate that only part of the AlCl3 molecules coordinate with phosphonate groups to form organic–metal complex and the excess AlCl3 molecules crystallize during the solvent evaporation process. For AlCl3 molecules are capable to form highly crystallized spherulites, the fibrous networks of the metallogels are suppressed by the spherulites and cannot be observed by OM. For the AlCl3 solution among
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the fibrous networks is removed by the filter paper from the back of copper mesh when preparing the TEM samples, only the fibers can be observed by TEM. So the TEM images show the morphologies of the gel fibers, while the OM images are influenced by the crystallization morphology of excess aluminum chloride with high aluminum contents. The excess AlCl3 can form dimers, trimers, and tetramers with chlorine acting as a bridging ligand [23]. Both the densely packed fiber networks and the AlCl3 oligomers may improve the stability of the metallogels from the PCBP–AlCl3 complex with high content of aluminum(III). The XRD patterns from the assemblies of the PCBP–AlCl3 complex also demonstrate the crystallization of AlCl3 molecules. As it is shown in Fig. 4A, multiple diffraction peaks are observed in the samples from the xerogels and precipitates with high content of aluminum(III) (from the ratio of 1:5 to 1:50 (n/n)). The diffraction patterns are similar to the crystallized AlCl3 film, which indicates that the crystallization of AlCl3 is dominated in the films from these assemblies. For the xerogel films from the complex with a ratio of 4:3, 1:1 and 1:3 (n/n), the XRD patterns show the same diffraction peaks at 2h = 5.38° (d1 = 1.64 nm) and 2h = 10.76° (d2 = 0.82 nm), which means that the compositional ratio does not influence the molecular packing of the complex in the xerogel fibers. The d1 value is twice of the d2 value, which indicates that the PCBP–AlCl3 complex may form layered structures in the xerogel
Fig. 4. Out-of-plane XRD curves of assemblies from PCBP–AlCl3 complex at different ratios (n/n) after dropping on glass substrates: (A) PCBP crystals from CH2Cl2/hexane (black line) [26]; 2:1, solution from ethanol (red line); 4:3, transparent gel from ethanol (blue line); 1:1, transparent gel from ethanol (dark cyan line); 1:3, transparent gel from ethanol (magenta line); 1:4, turbid gel from ethanol (dark yellow line); 1:5, turbid gel from ethanol (navy line); 1:10, turbid gel from ethanol (wine line);1:20, turbid partial gel from ethanol (pink line); 1:50, precipitate from ethanol (olive line); AlCl3 film from ethanol (royal line). (B) Gels from the PCBP-AlCl3 complex at a ratio of 1:3 (black line), 1:4 (red line) and 1:5 (blue line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
fibers. For these diffraction patterns of the xerogel samples are different from the PCBP crystals obtained from the mixed solvents and AlCl3 crystalline films, we suppose that new complex is formed from PCBP and AlCl3 [26]. The formation of layered supramolecular structures is common in aluminophosphates through coordination interactions [33]. The film from the solution of PCBP–AlCl3 complex with a ratio of 2:1 (n/n) does not show any diffraction peaks and it is consistent with the amorphous phase observed in the OM images. For the films with the ratio of 1:4 (n/n), the diffraction signals are more complicated, which contains the signal of the xerogel film with a ratio of 1:3 as well as the crystalline peaks of AlCl3 with a ratio of 1:5 (Fig. 4B). It indicates the complex is in a transition state at the ratio of 1:4. In a word, the XRD patterns of the films at different ratios agree well with the morphology evolution observed in the OM images. 3.3. The driving force for the metallogels The metallogels based on the coordination interactions have been demonstrated in the complexes of aluminum and phosphate surfactant recently [23–25,34]. We have also found that the carbazole derivatives can assemble into crystalline fibers in thin films through vapor annealing and form supramolecular gels in the mixture solvent, which are driven by the p–p interactions between the aryl units [29,35,36]. So we speculate that the coordination bonding and p–p interactions may promote the aggregation and gelation of the PCPB–AlCl3 complex in alcohols. The coordination interactions between Al(III) ions and phosphonate groups are studied by the NMR spectra. As it is shown in Fig. 5A, only single peak appears at 20.16 ppm for PCBP solution in d6-ethanol, which shows a little change compared with 31P signals observed in d6-DMSO [26]. The 31P NMR signal of PCBP is close to the chemical shift of the other acyclic phosphonates compound [37]. The signal of the PCBP–AlCl3 complex with a ratio of 2:1 (n/n) is similar to that of PCBP solution, which indicates that the content of AlCl3 is too low to change the chemical environment of PCBP molecules. As the compositional ratio changes, the signal of 31P NMR varies. The 31P NMR signals at 20.11 ppm, 15.42 ppm and 13.20 ppm can be observed from the metallogel with a ratio of 1:3 (n/n). It indicates that three types of phosphor atoms with nonequivalent environments are formed in the metallogel. When the compositional ratio changes to 1:10 (n/n), the 31P NMR signals shift to 19.81 ppm, 15.25 ppm and 13.22 ppm and the intensities of two peaks at higher field increase. This is because the paramagnetic contribution rather than the bonding electron distribution is dominant in the determination of chemical shift in 31P NMR spectra [38]. The difference in 31P signals in the metallogel from the PCBP–AlCl3 complex may result from the different coordination modes of aluminum(III) and phosphonate groups [31]. The enhanced intensity for the peaks in the high field with a ratio of 1:10 indicates that the content of the phosphonate groups with new coordination mode increase. There are also some changes in the 1H NMR spectra with different compositional ratios. As it shown in Fig. 5B, the intensity of proton signals changes for the solvent when gel forms. As the content of AlCl3 increases, the signal of –OH (5.25 ppm) resonance decreases, while the signal of –CH2 (3.56 ppm) and –CH3 (1.11 ppm) resonance increases. It may be because that part of the hydroxyl groups interact with the aluminum. In other words, the solvent takes part in gelation process. The proton signals of the aromatic rings are shifted downfield in the metallogel with a ratio of 1:10 (the inset in Fig. 5B), which may be due to the electron-withdrawing inductive effect on the proximate protons caused by the coordination of phosphonate and aluminum(III). In addition, the splitted peaks collapse to single broad peaks, which indicates that oligomerization or polymerization has taken place
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due to coordination interactions among aluminum ions and PCBP molecules [11]. The coordination interactions have also been investigated by FT-IR spectra. As it is shown in Fig. S5A, the broad absorption peak at 3462 cm1 in the PCBP solids can be ascribed to the vibration of OH from the adsorbed water molecules. This vibration is further broadened after the addition of AlCl3. The strong broad peaks at 3058 cm1 and 1630 cm1 in AlCl3 solid and the xerogel films are referred to the OH vibrations of the surface absorbed water molecules. When AlCl3 is added, the two peaks for C–C vibrations at 1243 cm1 and 1232 cm1 become a single peak and the peaks of P–O–R vibrations at 594 cm1 and 582 cm1 disappear in Fig. S5B. It indicates that the vibrations of phosphonate groups change. The two bands at 1209 cm1 and 592 cm1 formed in the xerogel can be regarded as the asymmetric stretching vibration of P–O–Al and the bending mode of Al–O [39]. It means
PCBP solution PCBP film 2-1 4-3 1-1 1-3 1-4 1-5 1-10 1-20 1-50
1.0
PL Intensity
Fig. 5. (A) 31P NMR and (B) 1H NMR spectra of PCBP–AlCl3 complex at different ratios (n/n) at room temperature: PCBP solution (black line); 2:1, solution (red line); 1:3, transparent gel (blue line); 1:10, turbid gel (dark cyan line). The concentration of PCBP is 10 mg/ml. The inset image in (B) shows an expanded portion of the spectra. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
that the P–O–Al coordination bonding has formed in the metallogels. The p–p interactions among the aromatic groups are researched by fluorescence spectroscopy. Compared to the emission peak at 362 nm for PCBP solution, the film emits at 376 nm (Fig. 6). The red-shift of the spectra indicates that the interactions between PCBP molecules are enhanced in the film. The emission peak for the complex with a ratio of 2:1 (n/n) is also at 376 nm, which is identical to the PCBP film. It means that few amount of AlCl3 has little influence on the interactions of PCBP molecules, which is similar to that observed in 31P NMR spectra. As the content of AlCl3 increases in the metallogel, the emission peaks of the xerogel film shift from 379 nm (with a ratio of 4:3) to 384 nm (with a ratio of 1:10). We can ascribe this red-shift to the improved p–p interactions among PCBP molecules at the presence of AlCl3. However, as the content of AlCl3 further increases, a blue-shift of PL spectra takes place. The emission peaks shift from 376 nm (the partial gel with a ratio of 1:20) to 371 nm (the precipitate with a ratio of 1:50). We suppose that the PCBP–AlCl3 complex has been surrounded by the excess AlCl3 molecules at high aluminum(III) content conditions and the p–p interactions between the aromatic groups of PCBP molecules are disrupted, which is consistent with the moduli and viscosity change at high aluminum(III) content. The concentration dependent absorption spectra also can demonstrate the promoted p–p interactions at high-concentration conditions. As shown in Fig. S6A, the dilute solution shows absorption at 249 nm, 270 nm, 287 nm, 305 nm and 336 nm. The absorption at 305 nm and 336 nm can be ascribed to the absorption of carbazole groups [40]. As the concentration increases from 1 106 M to 2 105 M, the intensity of the absorption at 249 nm decreases and the intensity of absorption at longer wavelength increases. So we may figure that the p–p interactions between carbazole groups are improved as the concentration increases. When the concentration increases to 2 105 M, the absorption peaks are indistinguishable. The absorption spectra of the film show similar characterization to that of the highconcentration solutions except a little red-shift of the spectra edge. We think that the intermolecular interactions in the highconcentration solution and film are more complicated. For the PCBP–AlCl3 complex, the absorption spectra show similar change except the more pronounced red-shift of the spectra edge in the solid xerogel film (Fig. S6B). It indicates that the interactions between the PCBP molecules are stronger in xerogel films than those in PCBP films.
0.5
0.0 300
350
400
450
500
Wavelength (nm) Fig. 6. PL spectra of PCBP–AlCl3 assemblies from different ratios (n/n): PCBP solution (black line), PCBP film (red line), PCBP–AlCl3 complex film from the assemblies at different ratios: 2:1, solution (blue line), 4:3, transparent gel from ethanol (dark cyan line); 1:1, transparent gel from ethanol (magenta line); 1:3, transparent gel from ethanol(dark yellow line); 1:4, turbid gel from ethanol (navy line); 1:5, turbid gel from ethanol (wine line); 1:10, turbid gel from ethanol (pink line);1:20, turbid partial gel from ethanol (olive line); 1:50, precipitate from ethanol (royal line). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Fig. 7. Schematic illustrating possible arrangement of PCBP–AlCl3 complex within supramolecular aggregates at different ratios: (A) 1:1, (B) 1:2, (C) excess aluminum. In the schematic, X represents Cl or –OCH2CH3.
From the above results, we speculate that the coordination bonding between aluminum(III) and phosphonate groups of PCBP and the p–p interactions between the aromatic groups of PCBP may be the main driving force for the PCBP–AlCl3 complex to self-assemble into nanofibers. The possible arrangements of PCBP–AlCl3 complex within gel fibers from different ratios are shown in Fig. 7. The anhydrous aluminum chloride will react with ethanol as following: AlCl3 + CH3CH2OH = AlCl3x(OCH2CH3)x + x HCl (x P 1) [30]. When the compositional ratio is 1:1, the complex may form linear structure with coordination bonding to bridge the two components (Fig. 7A). At the ratio of 2:1, the content of aluminum is too low to crosslink PCBP to form one-dimensional coordination polymer. One PCBP molecule can coordinate with several aluminum ions (at most four), while one aluminum ion has four- or six-ordination sites. So as the content of aluminum increases, the two components can interact with each other at different directions to form three-dimensional crosslinking structures. Fig. 7B shows one possible crosslinking structure of the PCBP–AlCl3 complex from the ratio of 1:2. These crosslinking structures can further form gel networks. When the content of AlCl3 is high, all the phosphonate groups of PCBP molecule are coordinated with aluminum ions, and the excess aluminum
ions just bond with one phosphonate group or none. Thus the crosslinking structures are destroyed (Fig. 7C). When most of PCBP molecules are surrounded by aluminum ions, the crosslinking structures break up and the gel networks collapse, such as at the ratio of 1:20 and 1:50.
4. Conclusion In conclusion, we have reported the metallogels from the complex of phosphonate substituted carbazole derivative PCBP and AlCl3. The PCBP–AlCl3 complex can form gels in the selective solvent of alcohols. The fiber morphologies of the xerogel are highly influenced by the compositional ratio of the complex. As the content of aluminum(III) increases, the length and diameter of nanofibers decreases and the fibers pack more densely. It makes the transparent gel become turbid and more stable. The XRD patterns show that the complex forms layered structures in the fiber networks. We speculate that the coordination interactions between the phosphonate groups and aluminum(III) ions and the p–p interactions among the aromatic groups of PCBP dominate the gelation process in the alcohols.
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Acknowledgments This work was financially supported by the National Natural Science Foundation of China (51273191, 51073151, 21334006). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2014.03.016. References [1] N.M. Sangeetha, U. Maitra, Chem. Soc. Rev. 34 (2005) 821–836. [2] S.S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew. Chem. Int. Ed. 51 (2012) 1766–1776. [3] A.R. Hirst, B. Escuder, J.F. Miravet, D.K. Smith, Angew. Chem. Int. Ed. 47 (2008) 8002–8018. [4] M. Llusar, C. Sanchez, Chem. Mater. 20 (2008). 782–780. [5] F. Fages, Angew. Chem. Int. Ed. 45 (2006) 1680–1682. [6] M.O.M. Piepenbrock, G.O. Lloyd, N. Clarke, J.W. Steed, Chem. Rev. 110 (2010) 1960–2004. [7] X. de Hatten, N. Bell, N. Yufa, G. Christmann, J.R. Nitschke, J. Am. Chem. Soc. 133 (2011) 3158–3164. [8] A. Kishimura, T. Yamashita, T. Aida, J. Am. Chem. Soc. 127 (2005) 179–183. [9] H. Lee, S.H. Jung, W.S. Han, J.H. Moon, S. Kang, J.Y. Lee, J.H. Jung, S. Shinkai, Chem. Eur. J. 17 (2011) 2823–2827. [10] S. Kawano, N. Fujita, S. Shinkai, J. Am. Chem. Soc. 126 (2004) 8592–8593. [11] Y.R. Liu, L.S. He, J.Y. Zhang, X.B. Wang, C.Y. Su, Chem. Mater. 21 (2009) 557– 563. [12] S.C. Li, V.T. John, G.C. Irvin, S.H. Rachakonda, G.L. McPherson, C.J. O’Connor, J. Appl. Phys. 85 (1999) 5965–5967. [13] R. Murugavel, A. Choudhury, M.G. Walawalkar, R. Pothiraja, C.N.R. Rao, Chem. Rev. 108 (2008) 3549–3655. [14] K.J. Gagnon, H.P. Perry, A. Clearfield, Chem. Rev. 112 (2012) 1034–1054. [15] M. Froba, M. Tiemann, Chem. Mater. 10 (1998) 3475–3483. [16] T. Kimura, Chem. Mater. 15 (2003) 3742–3744.
109
[17] T. Kimura, Chem. Mater. 17 (2005) 5521–5528. [18] Z. Florjanczyk, A. Wolak, M. Debowski, A. Plichta, J. Ryszkowska, J. Zachara, A. Ostrowski, E. Zawadzak, M. Jurczyk-Kowalska, Chem. Mater. 19 (2007) 5584– 5592. [19] Y. Huang, J. Ma, J. Yang, K. Cao, Z. Lu, J. Phys. Chem. C 116 (2012) 22518–22525. [20] J. Ma, J. Yang, Y. Huang, K. Cao, J. Mater. Chem. 22 (2012) 2007–2017. [21] E.C.D. Lima, F. Galembeck, J. Colloid Interf. Sci. 166 (1994) 309–315. [22] E.C.D. Lima, J.M.M. Neto, F.Y. Fujiwara, F. Galembeck, J. Colloid Interf. Sci. 176 (1995) 388–396. [23] M.G. Page, G.G. Warr, J. Phys. Chem. B 108 (2004) 16983–16989. [24] M.G. Page, G.G. Warr, Langmuir 25 (2009) 8810–8816. [25] M. George, G.P. Funkhouser, R.G. Weiss, Langmuir 24 (2008) 3537–3544. [26] B. Chen, J.Q. Ding, L.X. Wang, X.B. Jing, F.S. Wang, J. Mater. Chem. 22 (2012) 23680–23686. [27] C.D. Dou, D. Chen, J. Iqbal, Y. Yuan, H.Y. Zhang, Y. Wang, Langmuir 27 (2011) 6323–6329. [28] R. Mukkamala, R.G. Weiss, Langmuir 12 (1996) 1474–1482. [29] Z. Ding, Q. Zhao, R. Xing, X. Wang, J. Ding, L. Wang, Y. Han, J. Mater. Chem. C 1 (2013) 786–792. [30] B.Z. Tian, X.Y. Liu, B. Tu, C.Z. Yu, J. Fan, L.M. Wang, S.H. Xie, G.D. Stucky, D.Y. Zhao, Nat. Mater. 2 (2003) 159–163. [31] Z. Florjanczyk, A. Lasota, A. Wolak, J. Zachara, Chem. Mater. 18 (2006) 1995– 2003. [32] W.G. Weng, J.B. Beck, A.M. Jamieson, S.J. Rowan, J. Am. Chem. Soc. 128 (2006) 11663–11672. [33] M. Tiemann, M. Froba, Chem. Mater. 13 (2001) 3211–3217. [34] G.P. Funkhouser, N. Tonmukayakul, F. Liang, Langmuir 25 (2009) 8672–8677. [35] Z. Ding, R. Xing, L. Zheng, Y. Sun, X. Wang, J. Ding, L. Wang, Y. Han, J. Phys. Chem. B 115 (2011) 15159–15166. [36] Z. Ding, R. Xing, Y. Sun, L. Zheng, X. Wang, J. Ding, L. Wang, Y. Han, RSC Adv. 3 (2013) 8037–8046. [37] R.M. Twyman, Nuclear Magnetic Resonance Spectroscopy-Applicable Elements: Phosphorus-31, Elsevier Science, London, UK, 2005. pp. 278–286. [38] F. Ribasprado, C. Giessnerprettre, B. Pullman, J.P. Daudey, J. Am. Chem. Soc. 101 (1979) 1737–1742. [39] J. Xu, L. Chen, D.L. Zeng, J. Yang, M.J. Zhang, C.H. Ye, F. Deng, J. Phys. Chem. B 111 (2007) 7105–7113. [40] S.M. Bonesi, R. Erra-Balsells, J. Lumin. 93 (2001) 51–74.