Important Effects of Lithium Carbonate on Stoichiometry and Property

Jan 10, 2011 - ABSTRACT: The present work reveals a significant influence of lithium carbonate (Li2CO3) on stoichiometry, yield, spectral property, an...
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Important Effects of Lithium Carbonate on Stoichiometry and Property of the Inclusion Complexes of Polypropylene Glycol and β-Cyclodextrin Xue Qing Guo,† Le Xin Song,*,†,‡ Fang Yun Du,† Zheng Dang,† and Mang Wang‡ †

CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, and ‡Department of Chemistry, University of Science and Technology of China, Hefei 230026, P. R. China

bS Supporting Information ABSTRACT: The present work reveals a significant influence of lithium carbonate (Li2CO3) on stoichiometry, yield, spectral property, and thermal behavior of the inclusion complex formed by polypropylene glycol (PPG) and β-cyclodextrin (CD). First, the presence of Li2CO3 in aqueous solution leads to the formation of an inclusion complex PPG-(β-CD)6, which is completely different from that precipitated from pure water. This finding is supported by the result of a similar experiment in the case of lithium chloride, demonstrating that the self-assembling behavior of PPG, a flexible oligomer, and β-CD, a rigid oligomer, in solution can be mediated by additions of the lithium salts. Second, powder X-ray diffraction patterns indicate that the lithium salts in solution play a considerable role in fabricating three-dimensional structures of the complex. Third, the difference in stoichiometry and microstructure of the complexes precipitated from different media is reflected by the difference of their thermal properties. Finally, the results of viscosity, surface tension, and conductivity measurements provide positive support on the effect of the lithium salts on the physical property of PPG solution. Taken together, these observations provide a novel framework for understanding functions of inorganic salts in designing and constructing supramolecules.

’ INTRODUCTION Cyclodextrins (CDs) can interact with many kinds of polymers, such as polyalkanes, polyethers, polyesters, and so on, to form inclusion complexes in solution.1-5 The inclusion complexes have been the subject of scientific interest in many different fields, such as biological simulations, drug-delivery systems, syntheses of functional nanoparticles, and so on.6-10 More and more researchers are interested in the effects of extrinsic factors, such as reaction times, temperatures, solvents, concentrations of reactants, and the like, on the formation of the inclusion complexes, and how they interact with perception of intrinsic product characteristics.11-15 For example, the effects of temperature, solvent, and dilution of two reactants, R-CD and polyethylene glycol, in solution on the aggregating process between them were reported in a previous paper.12b Also, the values of stoichiometric ratio (SR) of two components in the inclusion complexes formed by polypropylene glycol (PPG) with β-CD, as well as spectral performances and thermal behaviors of the complexes, were found to be associated with preparation conditions: hydrothermal treatment and magnetic stirring.16 In some cases, the formation reactions of inclusion complexes were carried out in highly polar media, such as aqueous solution of inorganic salts.12a,12c Attention is now focused on how r 2011 American Chemical Society

inorganic salts affect the threading process of CD molecules on a polymer chain. Also, recent studies on the Hofmeister effect showed that, instead of water structure making and breaking by inorganic salts, the direct ion-macromolecule interactions are largely responsible for the phenomenon of salt effect.17,18 However, there is no report about the contribution of salt effect to stoichiometric regulation of host and guest in an inclusion complex precipitated from solution. This prompts us to pay particular attention to the role of inorganic salts in modulating the formation of macromolecular complexes as well as their properties. Taking into consideration the positive response of lithium carbonate (Li2CO3) to a host-guest system,19 we select it as the component to be assayed here. Initially, the inclusion complex of PPG with β-CD in the presence of Li2CO3 was prepared and characterized by elemental analyses, nuclear magnetic resonance (NMR) measurements, and thermogravimetry (TG) analyses. Our results indicated that the existence of Li2CO3 in solution resulted in a stoichiometric variation of PPG and β-CD in their inclusion complex when compared with that obtained from pure water.16 This finding Received: July 16, 2010 Revised: November 4, 2010 Published: January 10, 2011 1139

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The Journal of Physical Chemistry B may open a new and fascinating possibility for study of the contribution of salt effect to the formation, stoichiometry, and structure of supramolecular complexes. Next, Fourier transform infrared (FTIR), TG, and gas chromatography coupled to time-of-flight mass spectrometry (GCTOF-MS) indicated that there was a considerable difference in stacking structures, hydrogen-bonding networks, and thermal stabilities of two binary aggregates of PPG and β-CD, prepared in different cases, one of which was obtained from pure water; the other was formed in the presence of Li2CO3. Finally, the results of viscosity, surface tension, and conductivity measurements provide a positive support on the effect of the lithium salts on the physical property of PPG solution. In short, the aim of the present survey is to assess the extent to which the presence of Li2CO3 influences the inclusion behavior between PPG and β-CD.

’ EXPERIMENTAL SECTION Materials. β-CD was purchased from Shanghai Chemical Reagent Co. and recrystallized twice from deionized water. PPG (Mn = 1000, Mw/Mn = 1.12) was kindly donated by Ms X. W. Wang. Li2CO3 and lithium chloride (LiCl) were purchased from Shanghai Chemical Reagent Co. and used without further purification. The inorganic salts were dried before use for 4 h under the same conditions: at 383.2 K in vacuum (76 Torr). DMSO-d6 used as a solvent in NMR measurements was obtained from Aldrich Chemical Co. All other reagents and chemicals used were of general purpose or Analar grade. Preparation of Solid Inclusion Complexes. Two inclusion complexes formed by β-CD with PPG in aqueous solutions of Li2CO3 and LiCl were marked as PPG-(β-CD)6-1 and PPG-(β-CD)6-2, respectively. They were prepared according to the following procedure. Liquid PPG of 180 mg (0.18 mmol) was added to an aqueous solution (200 mL) containing an excess amount of β-CD (1.8 mmol, 2.043 g) and an excess amount of one of the salts (1.8 mmol) at room temperature. This mixed solution was stirred first for 1 h at 353.2 K, and then for 48 h at room temperature. During stirring, the clear solution became turbid. Significantly, a white precipitate was gradually formed. This was then filtered, and washed repeatedly with deionized water until the effluents gave a negative flame test for Liþ ion. Prior to measurements, all samples were kept in an oven under vacuum (76 Torr) at 383.2 K for 24 h. The dried samples were obtained as a white powder, and weighed at room temperature. Their percentage yields were calculated based on the amount of the PPG employed. Results of metal content analyses indicated that there was no observable lithium in the complexes. PPG-(β-CD)6-1: yield, 64.1%; decomposition temperature, 693.0 K. The result of elemental analysis: Anal. Calcd. for C51H104O18 3 6C42H70O35 3 26H2O: C, 43.90; H, 6.95. Found: C, 43.77; H, 6.89. 1H NMR (300 MHz, DMSO-d6, 298.2 K): Chemical shift (δ) 5.65 (m, 84 H, O2H and O3H of β-CD), 4.82 (s, 42 H, C1H of β-CD), 4.42 (t, 42 H, O6H of β-CD), 3.63 (s, 42 H, C3H of β-CD), 3.61 (s, 84 H, C6H of β-CD), 3.55 (s, 42 H, C5H of β-CD), 3.35 (m, 84 H, C2H and C4H of β-CD), 1.03 (m, 51 H, methyl H of PPG). PPG-(β-CD)6-2: yield, 56.2%; decomposition temperature, 693.0 K. The result of elemental analyses: Anal. Calcd for C51H104O18 3 6C42H70O35 3 26H2O: C, 43.90; H, 6.95. Found: C, 43.93; H, 7.03. 1H NMR (300 MHz, DMSO-d6, 298.2 K): δ 5.71 (d, 42 H, O2H of β-CD), 5.66 (s, 42 H, O3H of β-CD),

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4.83 (d, 42 H, C1H of β-CD), 4.43 (t, 42 H, O6H of β-CD), 3.61 (t, 42 H, C3H of β-CD), 3.58 (m, 84 H, C6H of β-CD), 3.55 (m, 42 H, C5H of β-CD), 3.35 (m, 84 H, C2H and C4H of β-CD), 1.03 (m, 51 H, methyl H of PPG). Preparation of Solution Samples. Aqueous solutions of 1.00  10-4 mol 3 dm-3 of PPG in the absence and presence of Li2CO3 or LiCl with the concentrations from 0 to 1.00  10-2 mol 3 dm-3 were prepared to measure the change in surface tensions under the same conditions. Aqueous solutions of 1.00  10-4 mol 3 dm-3 of LiCl or Li2CO3 in the presence of different concentrations of PPG (0, 2, 4, 6, and 8.00  10-6 mol 3 dm-3) were prepared to determine the change in molar conductivities under the same conditions. Aqueous solutions were stirred at 298.2 K for 10 min before measurements. PPG of 500 mg (0.50 mmol) was dissolved in a mixture solvent (50 mL) of ethanol and water (volume ratio, 1:6). The concentration of the PPG solution is 1.00  10-2 mol 3 dm-3. Then, a series of mixed solutions of PPG and Li2CO3 or LiCl, in which the concentration of PPG is constant (1.00  10-2 mol 3 dm-3), and the concentration of Li2CO3 or LiCl changes from 0 to 1.00  10-2 mol 3 dm-3, were prepared to compare the change in viscosities under the same conditions. Instruments and Methods. 1H NMR spectra were recorded on a Bruker AV-300 NMR spectrometer at 300 MHz at room temperature. The δ values were referred to the solvent peaks (δ = 2.5 ppm for DMSO-d6) using TMS as an internal standard. Powder X-ray diffraction (XRD) measurements were carried out by a Philips X'Pert Pro X-ray diffractometer using a monochromatized Cu KR radiation source (40 kV, 40 mA) with a wavelength of 0.1542 nm and analyzed in the range 5° e 2θ e 40°. FTIR spectra were done with a Bruker Equinox 55 spectrometer in KBr pellets in the range 4000-400 cm-1. TG analyses were performed on a Shimadzu TGA-50 thermogravimetric analyzer at a constant heating rate of 10.0 K 3 min-1 under a nitrogen atmosphere with a gas flow of 25 mL 3 min-1. Elemental analyses were made using a Vario EL III elemental analyzer. The mass percentage of Liþ in the samples was measured by using a Thermo Jarrell Ash Atomscan Advantage spectrometer (AAS). GC-TOF-MS analysis was conducted using a Micromass GCT-MS spectrometer. The heating program of samples was the same as that previously reported in the literature.20 Viscosity measurements were done by a conventional Ubbelohde viscometer placed in a thermostatically controlled bath at 308.2 K. The experiments were initiated after approximately 5-10 min equilibrium time, and the flow times (t) were then measured at regular intervals. Surface tension was determined using the Wilhelmy plate method and a JK99B automatic surface tensiometer (Shanghai Zhongchen Co., China) at a constant temperature of 298.2 K. The molar conductivities of solutions were measured using a DDSJ-308 conductivity meter (Shanghai Leici Instrument Inc., China) with automatic temperature compensation and automatic calibration. A conductance cell with a cell constant of around 1.083 cm-1 was used, and the temperature was kept at 298.2 K.

’ RESULTS AND DISCUSSION Product Yields and Compositions. Based on the yields of the products obtained by the method described in the present paper, the precipitated percentages of PPG in the salt solutions 1140

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Figure 2. XRD patterns of β-CD (a), PPG-(β-CD)6-1 (b), and PPG-(β-CD)6-2 (c). Figure 1. 300-MHz 1H NMR spectra of PPG (A), β-CD (B), and samples a (C) and b (D) in DMSO-d6.

are far less than that of it in pure water (yield, 92.6%).16 Likewise, the precipitated percentages of β-CD are 38.46 and 33.72% for PPG-(β-CD)6-1 and PPG-(β-CD)6-2, respectively, both of which are lower than 46.37% in the case of the inclusion complex, PPG-(β-CD)5, precipitated from pure water at the same initial stoichiometric ratio, reaction temperature, and time.16 The result indicates that the existence of the lithium salts in solution plays a negative role in regulating the generation and precipitation of the complexes. The effect of the lithium salts in solution on the interaction between PPG and β-CD is reflected by change in the composition of the complexes precipitated. Element analysis and 1H NMR data suggest that the stoichiometric ratios of PPG to β-CD in the two inclusion complexes obtained in this work are both 1:6 for PPG-(β-CD)6-1 and PPG-(β-CD)6-2, and they contain the same number of water molecules. This is supported by two other observations from our study: (1) AAS measurements of mass percentage of Liþ and (2) TG measurements of water contents in the samples. This result provides two important information points regarding the effect of the lithium salts on the formation of the complexes: (1) The two complexes have the same compositions and neither of the lithium salts used here occurs in them. (2) The existence of the lithium salts in solution tends to make a higher inclusion stoichiometry between PPG and β-CD molecules when compared to a pure water medium.16 After the precipitates were collected by filtration, the filtrates were dried into a powder form (samples a and b) at room temperature under reduced pressure. Samples a and b correspond to the filtrates of PPG-(β-CD)6-1 and PPG-(β-CD)6-2 in this paper, respectively. The 1H NMR spectra of the two samples, as well as free β-CD and free PPG, are illustrated in Figure 1. The δ values of proton signals are presented in parts per million (ppm) with respect to TMS in DMSO-d6. As shown in Figure 1, in the presence of the lithium salts, all the signals of the hydroxyl protons (O2H, O3H, O6H) of β-CD shift to downfield (see green arrows in this figure), exhibiting chemical shift changes of more than 0.030 ppm. At the same time, no observable changes in chemical shifts of all other protons of β-CD and PPG are found in the existence of the lithium salts. This observation shows that the lithium salts in solution have an interaction with the hydroxyl groups in β-CD. A third inclusion complex of β-CD with PPG was precipitated in aqueous solution of 1.05  10-3 mol 3 dm-3 LiOH with the same pH value (10.81) as that of the aqueous solution of 9.00  10-3 mol 3 dm-3 Li2CO3. The results of element analysis (Anal. Calcd for C51H104O18 3 6C42H70O35 3 38H2O: C, 42.82; H, 7.07.

Found: C, 42.73; H, 7.01) and mass percentage of Liþ in the sample (no observable lithium was found in the complex, found: Li 0.031% 3 s-1), which is different from the case of PPG-(β-CD)5, because it exhibits a lower decomposition rate than β-CD (ΔVmax, -0.059% 3 s-1).24 Next, although there is no difference in the decomposition temperatures of β-CD and its inclusion complexes, the difference in their RM values at higher temperatures is obvious. For example, the RM values of β-CD, PPG-(β-CD)6-1, and PPG-(β-CD)6-2 at 750.0 K are 17.40, 8.37, and 6.72%, respectively. PPG begins to decompose at 648 K and shows a complete decomposition at temperatures of 661 K and above.16 The mass content of β-CD in its hydrate is 90.33%, and the mass contents of PPG and β-CD in the complexes are 12.08 and 82.27%. Hence, the values of RM at 750.0 K reflect that the complexation between PPG and β-CD leads to more decomposition of β-CD. Also, Figure 3 depicts the fitted theoretical curves (curve d) based on a mixed form of β-CD and PPG in a 6:1 molar ratio. Clearly, the performance of β-CD and the mixture in Figure 3A is

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Figure 5. Mass spectra of PPG-(β-CD)6 -1 at 25.41 (A) and 31.77 min (B).

rather similar. Figure 3B indicates that the absolute values of Vmax decrease in the following order: the complexes > β-CD > the mixture. These results effectively demonstrate the formation of the complexes. The difference in Vmax and RM between β-CD and its complexes may be ascribed to the structural transformation in molecular arrangements from β-CD to its complexes. In order to investigate whether there is a relationship between the composition of β-CD complexes and their decomposition products, the GC-TOF-MS measurement of PPG-(β-CD)6-1 was performed. Thermal Decomposition Products. Figure 4 shows the release signals in the decomposition process of PPG-(β-CD)61 in vacuum, which appear at 25.61 (very strong), 29.01 (very weak), and 32.09 min (very weak), respectively. The positions and shapes of these signals, especially the first signal corresponding to the main decomposition process, are quite different from those in of β-CD, PPG, and PPG-(β-CD)5.24 This phenomenon suggests there is a large change in their decomposition routes. Figure 5 indicates the mass spectra of PPG-(β-CD)6-1 at 25.41 (653.0 K) and 31.77 min (773.0 K). On the one hand, it is obvious that two main decomposition fragments of PPG(β-CD)6-1 at 25.41 min appear at m/z 29.002 (CHO þ, with the highest relative abundance, RA, 100%) and 43.019 (CH2CHOþ, 78.87%). Although the two signals also occur in β-CD and PPG-(β-CD)5, but in the two cases, their RA values are much lower than those of the two fragments at m/z 73.030 (CH2COCH2OHþ, 100%) and 60.021 (HCOCH2OHþ, >97%) at this moment.20,24 This shows that the inclusion complex isolated from the aqueous solution of Li2CO3 is ready to be cracked into smaller fragments. Further, the tropylium ion: C3H3þ at m/z 39.023 exhibits very high RA values of 68.38% and 33.37% at 25.41 and 31.77 min in the case of PPG-(β-CD)6-1, which are much higher than those obtained with β-CD (15.15% and 7.94%), PPG-(β-CD)5 (18.84% and 1.79%) or Li2CO3-β-CD (7.42% and 10.06%) at the same times.20,24,25 Additionally, the RA values of several signals at m/z 55.018 (C3H3Oþ, 62.42%), 68.025 (C4H4Oþ, 31.22%), 84.022 (C4H4O2þ, 27.88%), and 96.022 (C 5H4O2þ, 16.46%) increase in the order β-CD < PPG-(βCD)5 < PPG-(β-CD)6-1.20,24 The difference in decomposition products implies the importance and complexity of the complexation between PPG and β-CD. On the other hand, the characteristic signal at m/z 59.050 (C3H7Oþ), which is due to the degradation of the PPG chain, shows a low RA value of 4.05% at 25.41 min and disappears at 31.77 min in the mass spectra of PPG-(β-CD)6-1. This is different from the performance of the signal in the cases of PPG (e.g., 100% at 25.41 min) and PPG-(β-CD)5 (61.37% at 25.41 min and 8.71% at 31.77 min).24 1142

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Figure 6. Dehydration of PPG chains in the presence of lithium salts and formation of the complexes of PPG and β-CD.

Figure 7. Changes in viscosities of PPG solutions (1.00  10-2 mol 3 dm-3) with the addition of Li2CO3 (a) and LiCl (b) in the concentration range from 0 to 1.00  10-2 mol 3 dm-3.

Figure 8. Surface tensions of PPG solutions (1.00  10-4 mol 3 dm-3) with the addition of Li2CO3 (a) and LiCl (b) in the concentration range from 0 to 1.00  10-2 mol 3 dm-3.

In a word, there is a change in their decomposition processes of PPG-(β-CD)5 and PPG-(β-CD)6-1. This observation, together with those results we obtained in TG/DTG measurements, leads us to emphasize the important significance of medium effects on thermal stabilities of the complexes. Lithium Salt Effects on the Formation of the Complexes. The salting-out behavior of inorganic salts can be depicted by the Hofmeister series.26 However, it is not clear whether Hofmeister effects on polymers derive from direct interactions of inorganic ions with the polymers or indirectly with the bulk water.27 Here, a preliminary attempt to explain the effect of lithium salts on the conformation of the PPG chains is illustrated in Figure 6. In our opinion, the difference in chemical stoichiometries of the complexes is likely to be related to a change in conformation of PPG chains in solution. This change may be due to two types of mechanisms. One is the effect of the electrolytes on the dehydration of the PPG chains, which makes the PPG chains more prone to thread the cyclodextrin cavities. This effect is closely related to Liþ, Cl-, and CO32-. In view of the nature of

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Figure 9. Changes in molar conductivities of Li2CO3 (a) and LiCl (b) solutions (1.00  10-4 mol 3 dm-3) with the addition of PPG in the concentration range from 0 to 8.00  10-6 mol 3 dm-3.

the two electrolytes, such as the number and charge of the ions, it is reasonable that the effect induced by Li2CO3 is much larger than that promoted by LiCl. The other is a direct interaction between Liþ and the ethylene oxide units of the PPG chains, resulting in a completely different effect on the conformation of the polymer from the dehydration, and the magnitude of the interaction is also bigger in Li2CO3 than in LiCl based on the number of Liþ. In a word, there are two types of mechanisms that may operate differently with the two electrolytes. It should be reasonable that the more extended conformation the chains the higher stoichiometries the complexes. In order to demonstrate this, three independent experiments were performed to evaluate the effect of the lithium salts on the physical properties of PPG solution. Initially, Figure 7 illustrates the relationship between the concentrations of the lithium salts and the difference (Δt) in flow times of the PPG solutions caused by the salts. Obviously, the gradual decrease in flow times along with the increase of concentrations of the lithium salts reveals the decrease in viscosities of the PPG solutions. This may mean an increase in apparent molecular length28 of PPG chains. Although the addition of the lithium salts does not result in a significant decrease of the intrinsic viscosity of the solutions, the equivalent addition of them leads to different cases; namely, the effect of LiCl is much larger than that of Li2CO3, especially at higher concentrations. This phenomenon may be involved in the difference of moleculeion interaction between PPG chains and the lithium salts. Also, the figure indicates that the viscosities of the PPG solutions exhibit a good concentration dependence characteristic of a fast-to-slow transition. Next, the surface tension of pure water is 72.0 mN m-1 in our measurements, and it decreases sharply to 51.8 mN m-1 after an appropriate amount of PPG (1.00  10-4 mol 3 dm-3) is added to the water. In this case, the result means that the high hydrophobicity of PPG chains makes it preferable to form into a coiled shape in a polar solvent like water. However, as shown in Figure 8, the surface tension increases while Li2CO3 is added to the PPG solution. We also observe a good linear correlation (r = 0.99) between the surface tension of the solution and the concentration of Li2CO3. Similar phenomenon seems to appear in the case of LiCl, but the effect of LiCl is lower than that of Li2CO3 and does not present a linear concentration dependence. Finally, as displayed in Figure 9, the molar conductivity (Λm) of the lithium salts increases in the presence of PPG, and further increases with the increase of concentration of PPG. This implies a concentration dependence of the molecule-ion interaction between PPG chains and the lithium salts. This can be ascribed to two factors. (1) The interaction between water molecules is 1143

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The Journal of Physical Chemistry B weakened by the presence of PPG, which would be in favor of enhancing the mobility of ions. (2) A flexible PPG chain can be a good carrier for an ion delivery under our experimental conditions. Overall, we consider that the mutual effect of the lithium salts and PPG chains in solution is responsible for all the changes seen in the present report and that the different results produced by lithium chloride and carbonate in solutions were ascribed to different anions (Cl- and CO32-) and to the fact that the Li2CO3 is a 2:1 electrolyte, and therefore, it carries two Liþ per one anion. It is noteworthy to observe the similarity of the effect of LiCl and Li2CO3, although, apparently, there is no particular similarity to each other.

’ CONCLUSIONS In this study, two inclusion complexes, PPG-(β-CD)6-1 and PPG-(β-CD)6-2, were precipitated from aqueous solution of two lithium salts. Interestingly, the presence of the lithium salts caused them to produce the same stoichiometry, but different yields and properties. Our findings suggested that, despite a similar stacking behavior, there was a large difference in microstructures and thermal behaviors. The difference was ascribed to different effects of the lithium salts on the conformation of the PPG chains in solution. Thus, the present result provides an important clue to the possible significance of such a salt effect in modulating the self-assembly process of complexes through changing reaction media. ’ ASSOCIATED CONTENT

bS

(1) 1H NMR spectra, (2) FTIR spectra, (3) PXRD patterns, (4) field emission scanning electron microscopy images, and (5) TG/DTG curves of the complexes. This material is available free of charge via the Internet at http://pubs.acs.org. Supporting Information.

’ AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected].

’ ACKNOWLEDGMENT This project was supported by NSFC (No. 21071139) and Natural Science Foundation of Anhui Province (No. 090416228). ’ REFERENCES (1) (a) Harada, A.; Hashidzume, A.; Yamaguchi, H.; Takashima, Y. Chem. Rev. 2009, 109, 5974–6023. (b) Harada, A.; Okada, M. Polym. Adv. Technol. 1999, 10, 3–12. (2) (a) Wenz, G. Angew. Chem., Int. Ed. Engl. 1994, 33, 803–822. (b) Rusa, C. C.; Wei, M.; Bullions, T. A.; Shuai, X. T.; Uyar, T.; Tonelli, A. E. Polym. Adv. Technol. 2005, 16, 269–275. (3) Huang, F. H.; Gibson, H. W. Prog. Polym. Sci. 2005, 30, 982– 1018. (4) Lu, J.; Mirau, P. A.; Tonelli, A. E. Macromolecules 2001, 34, 3276–3284. (5) Song, L. X.; Bai, L.; Xu, X. M.; He, J.; Pan, S. Z. Coord. Chem. Rev. 2009, 253, 1276–1284. (6) Li, J.; Yang, C.; Li, H. Z.; Wang, X.; Goh, S. H.; Ding, J. L.; Wang, D. Y.; Leong, K. W. Adv. Mater. 2006, 18, 2969–2974. 1144

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