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Metal-centered polyoxometalates encapsulated by surfactant resulting in the thermotropic liquid crystal materials
Inorganic Chemistry Communications 43 (2014) 5–9
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Metal-centered polyoxometalates encapsulated by surfactant resulting in the thermotropic liquid crystal materials Yue Jia a, Jun Zhang a, Zhi-Ming Zhang a,⁎, Qiu-Yu Li b, En-Bo Wang a,⁎ a b
Key Laboratory of Polyoxometalate Science of Ministry of Education, College of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin, 130024, China Key Laboratory of Grain and Oil Processing of Jilin Province, Jilin Business and Technology College, Xi An Road No. 4728, Changchun, Jilin, 130062, China
a r t i c l e
i n f o
Article history: Received 12 December 2013 Accepted 19 January 2014 Available online 28 January 2014 Keywords: Polyoxometalate Surfactant Thermotropic liquid crystal Lamellae structure Smectic phase
a b s t r a c t Waugh- and Silverton-type polyoxometalates (POMs) K3(NH4)3[MnMo9O32] and H8[CeMo12O42] are enwrapped by dioctadecyldimethylammonium (DODA+) forming the surfactant-encapsulated POMs (SEPs) (DODA)6 [MnMo9O32]·16H2O (DODA-MnMo9) and (DODA)8[CeMo12O42]·9H2O (DODA-CeMo12), which exhibit typical thermotropic liquid-crystalline properties. Here, the Waugh- and Silverton-type polyoxoanions centered by metal cation were firstly introduced into liquid crystal materials. The chemical composition of these SEP complexes was determined by IR spectra, elemental analysis and TG analysis. Also, the polarized optical microscopy, differential scanning calorimetry (DSC), variable temperature X-ray diffraction (VT-XRD) and transmission electron microscopes (TEM) were performed to characterize their liquid-crystalline behavior. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
POMs, as a unique class of nanosized metal oxide clusters with abundant topologies and oxygen-rich surface, have exhibited remarkable properties in the field of catalysis, medicine and optics, etc. [1–6]. Despite the unparalleled success in POM synthetic chemistry [7–12], the application of POMs in various functional architectures, devices and materials still requires a progressive step for constructing POMbased functional systems. To modify the weak compatibility between POMs and organic solvents, several issues, such as the low mass transfer and high lattice energy as well as improving the surface of POMs, need to be solved. Recently, POM-based functional materials were efficiently prepared by the exchange of counterions with cationic surfactants, which was regarded as an effective and highly economic procedure for constructing the POM-based functional materials. In these POM-based functional materials, the solubility of POMs in regular organic solvents and the structural stability in diverse chemical environments are effectively improved. Furthermore, the obtained SEPs exhibit enhanced catalytic efficiencies with easy and fast catalyst recovery from the reaction system. In addition, it is possible to predict and control, to some extent, the structure and functionality of the hybrid materials by selecting the surfactants and POMs with different physical–chemical properties. In recent years, the smart self-assembly of surfactants and POMs has resulted in remarkable nanostructured organic–inorganic hybrid materials that exhibit attractive prospects in the field of catalysis, Langmuir– Blodgett films and dynamic transformations under control of external
⁎ Corresponding authors. Tel./fax: +86 431 85098787. E-mail addresses: [email protected] (Z.-M. Zhang), [email protected] (E.-B. Wang).
stimulus. Up to date, various morphology self-assemblies and liquid crystals have been achieved in the POMs/surfactant system [13–21]. Among them, liquid crystals constructed from cationic surfactant and POMs have attracted much attention. The liquid crystal materials synthesized in this way has overcome the complex multiple-step synthetic process of the traditional organic liquid crystals. Wu and Dietz succeeded in introducing the POMs into ammonium or phosphonium derivatives by the ionic self-assembling route resulting in several SEPs with characteristic thermotropic liquid-crystalline behavior [22–26]. The giant Mo clusters [Mo132O372(H2O)72(CH3COO)30]42− enwrapped by cationic surfactants display ionic liquid-crystalline properties [27]. By reasonable modulation of the surfactant, Faul and coworkers [28] described the liquid-crystalline behaviors built from [EuPW5W30O110]12− and [Eu(SiW9Mo2O39)2]13−. Liu et al. [29] demonstrated that Keggintype polyoxoanions enwrapped by tetra-n-octylammonium counterions through ionic self-assembly exhibited thermotropic liquid-crystalline behavior. Obviously, part of the thermotropic liquid crystals were obtained by using the mesomorphic cationic surfactant to encapsulate POMs. However, this method cannot be widely adopted because of its complex multiple-step synthetic process. Besides, these liquidcrystalline materials are mostly constructed from Keggin-type and sandwich-type POMs. Herein, the Waugh- and Silverton-type POMs K3(NH4)3[MnMo9O32] and H8[CeMo12O42], centered by the MnO6 octahedron and CeO12 icosahedron, respectively, were firstly introduced into the POM/surfactant system to construct thermotropic liquid-crystalline materials (DODA)6[MnMo9O32] (DODA-MnMo9) and (DODA)8[CeMo12O42] (DODA-CeMo12) [30–32]. Further, the Cisterminal oxygen atoms of MoO6 octahedra were observed in both K3(NH4)3[MnMo9O32] and H8[CeMo12O42], which are rarely observed
http://dx.doi.org/10.1016/j.inoche.2014.01.015 1387-7003 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
Fig. 1. Schematic view of the synthesis of DODA-MnMo9 and DODA-CeMo12. (a) K3(NH4)3[MnMo9O32], (b) H8[CeMo12O42], (c) surfactant (DODA+), (d) DODA-MnMo9 and (e) DODA-CeMo12.
Fig. 2. (a) DSC curves of DODA-MnMo9 during its first cooling and second heating process; (b) DSC curves of DODA-CeMo12 during its first cooling and second heating process.
in the typical Keggin and Wells–Dawson POMs and their derivatives. These studies are of significance because they will offer opportunities for introducing new POMs into the SEPs system for constructing functional liquid crystal materials (Fig. 1). DODA-MnMo9 and DODA-CeMo12 were synthesized by a straightforward ion exchange reaction, respectively. The phase transitions of DODA-MnMo9 were determined by DSC during its first cooling and second heating cycles. As shown in Fig. 2a, DODA-MnMo9 displays three phase transitions at 31 °C, 49 °C and 148 °C during its second heating. We notice a halo with a peak at 65 °C, but the present evidence cannot further indentify the transition exactly. Upon cooling, three transitions were observed at 126 °C, 43 °C and 24 °C, respectively. The peak at 31 °C during the second heating process should be assigned to the transition between two kinds of solids, and the VT-XRD results could further confirm the transition [22]. The peak that appeared at 49 °C can be attributed to the transition from solid state to mesophase; the broad peak at 148 °C corresponds to the transition between liquid crystal phase and isotropic phase. Fig. 2b displays two transitions of DODACeMo12 during its first cooling and second heating process. Upon its second heating run, DODA-CeMo12 displays an endothermic halo from 40 °C to 55 °C with the apex at 50 °C, which can be attributed to the transition between the solid state and the liquid crystal phase. With the increase of the temperature, the transition from liquid crystal phase to isotropic phase can be observed at 137 °C. When DODACeMo12 was cooled from its isotropic phase, the transition of isotropic phase to liquid crystal phase can be seen at 126 °C. On further cooling, the halo from 52 °C to 32 °C with the peak at 43 °C can be assigned to
the transition between liquid crystal phase and solid state. The phase transition temperature, enthalpies and assignments of the transition between different states of DODA-MnMo9 and DODA-CeMo12 are summarized in Table 1. Polarizing optical microscopy was employed to identify the liquid crystal phases of DODA-MnMo9 and DODA-CeMo12. During the cooling process from isotropic phase, we observed clear liquid birefringence phenomenon of DODA-MnMo9 with fan-shaped textures at 95 °C (Fig. 3a). For DODA-CeMo12, the phenomenon of liquid birefringence is clearly observed at 90 °C during its cooling process with a grain texture, although the optical texture is not typical (Fig. 3b). TEM was employed to confirm the lamellae structures of DODAMnMo9 and DODA-CeMo12 at room temperature. Fig. 4a shows the lamellar structure of DODA-MnMo9, and the spacing between two layers is approximately 4.2 ± 0.3 nm. For DODA-CeMo12 (Fig. 4b), the layer distance is estimated to 4.0 ± 0.3 nm. VT-XRD was employed to Table 1 The phase transition temperatures, enthalpies and assignments of the phase transition for DODA-MnMo9 and DODA-CeMo12, S = Solid, S1 = Solid 1, S2 = Solid 2, SmA = Smectic A, SmX = Smectic X, Iso = isotropic phase; the data were given on its second heating process. Compound
Transition
Temperature/°C
ΔH/kJ mol−1
DODA-MnMo9
S1–S2 S2–SmA SmA–Iso S–SmX SmX–Iso
31 49 148 50 137
140.5 160.7 — 182.7 —
DODA-CeMo12
Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
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Fig. 3. (a) The optical textures of DODA-MnMo9 at 95 °C during its first cooling processes (100×); (b) the optical textures of DODA-CeMo12 at 90 °C during its first cooling process.
Fig. 4. (a) TEM of DODA-MnMo9 at room temperature; (b) TEM of DODA-CeMo12 at room temperature.
Fig. 5. (a) The VT-XRD of DODA-MnMo9 at 25 °C, 40 °C and 90 °C; (b) the VT-XRD of DODA-CeMo12 at 25 °C and 100 °C.
further demonstrate the liquid-crystalline behaviors of DODA-MnMo9 and DODA-CeMo12. From Fig. 5a, it can be concluded that DODAMnMo9 in its solid state exhibits the layer structure at room temperature according to the three equidistant diffractions in the small-angle region, and the layer distance is 4.53 nm, which is in accordance with the layer distance estimated from TEM at room temperature. When the temperature increases to 40 °C, we can clearly see the equidistant diffractions in the small-angle region and several sharp peaks in the wide-angle region. This heating process leads to a solid–solid transition as the layer distance decreases to 3.67 nm. These results could further confirm the transition between two solids observed from the DSC. With the increase of temperature, the equidistant diffractions were clearly observed in the small-angle region as well as a broad halo in the wide-angle region centered at 2θ ≈ 20°, which indicated the formation of typical lamellar structure, and the distance between the two neighboring layers is 3.15 nm. Combining the fan-shaped textures, the
phase can be identified as SmA phase. Fig. 5b shows the VT-XRD results of DODA-CeMo12 at 25 °C and 100 °C. The equidistant peaks in the lowangle region of the diffraction pattern illustrate the layered structure of DODA-CeMo12 both in its solid state and liquid crystal phase. The lamellar structure in its liquid crystal phase indicates the formation of a smectic X phase [23].
Table 2 Layer spacings (d) of DODA-MnMo9 and DODA-CeMo12 calculated from X-ray diffractions. Compound
T (°C)
d001 (Å)
d002 (Å)
d003 (Å)
DODA-MnMo9
25 40 90 25 100
45.3 36.7 31.5 43.2 36.4
22.7 18.7 16.1 22.1 18.7
15.4 12.4
DODA-CeMo12
14.7 12.6
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Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
Fig. 6. (a) Schematic drawing of packing model of DODA-MnMo9 at liquid crystal state; (b) Schematic drawing of packing model of DODA-CeMo12 at liquid crystal state.
The calculated layer spacing for DODA-MnMo9 and DODA-CeMo12 is listed in Table 2. In fact, the diameter of anion [MnMo9]6− is about 0.7 nm, the normal length of DODA+ is 2.25 nm, the ideal thickness of a single complex should be 5.2 nm. The measured layer distance of DODA-MnMo9 in its mesophase is shorter than the theoretical molecular length, indicating that the molecules in the mesophase are partly interdigitated or tilted [24,25]. For DODA-CeMo12, the layer distance (3.64 nm) calculated from the VT-XRD at 100 °C is not consistent with the theoretical layer 5.4 nm (the diameter of [CeMo12O42] is 0.9 nm), which also indicates the molecules in DODA-CeMo12 are interdigitated or titled in its liquid crystal phase. Possible packing structures of DODA-MnMo9 and DODA-CeMo12 are shown in Fig. 6a and b, respectively. In summary, a simple surfactant DODA+ was used to encapsulate metal-centered POMs (Waugh- and Silverton-type POMs), resulting in two SEPs complexes. The Waugh- and Silverton-type POMs with metal cation as their heteroatoms were firstly introduced into the POMs/surfactant system, and the obtained SEP materials exhibited thermotropic liquid crystal behaviors. The results of polarized optical microscopy, VT-XRD and DSC reveal the smectic mesomorphic behaviors of DODA-MnMo9 and DODA-CeMo12 in a relatively wide temperature range. Compared with the thermotropic liquid crystal materials built from Keggin-type POMs and cationic surfactant [29], DODACeMo12 shows reversible phase transitions during their first heating and second heating cycles, and the smectic-type phase of Keggin-type POMs liquid crystal materials is more structurally ordered than the SmA phase of DODA-MnMo9. The research on thermotropic liquidcrystalline properties of these materials will provide access for constructing new POMs-based functional materials. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no. 21101022). Appendix A. Supplementary material Synthesis of the compounds, IR spectra and TG curves. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche.2014.01.015. References [1] M.T. Pope, A. Müller, Polyoxometalate chemistry: an old field with new dimensions in several disciplines, Angew. Chem. Int. Ed. 30 (1991) 34–48. [2] A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh, G. Izzet, Functionalization and post-functionalization: a step towards polyoxometalate-based materials, Chem. Soc. Rev. 41 (2012) 7605–7622. [3] D.-L. Long, R. Tsunashima, L. Cronin, Polyoxometalates: building blocks for functional nanoscale systems, Angew. Chem. Int. Ed. 49 (2010) 1736–1758. [4] P. Yin, J. Zhang, T. Li, X. Zuo, J. Hao, A. Warner, S. Chattopadhyay, T. Shibata, Y. Wei, T. Liu, Self-recognition of structurally identical, rod-shaped macroions with different central metal atoms during their assembly process, J. Am. Chem. Soc. 135 (2013) 4529–4536.
[5] S.-T. Zheng, G.-Y. Yang, Recent advances in paramagnetic-TM-substituted polyoxometalates (TM = Mn, Fe, Co, Ni, Cu), Chem. Soc. Rev. 41 (2012) 7623–7646. [6] F. Song, Y. Ding, B. Ma, C. Wang, Q. Wang, X. Du, S. Fu, J. Song, K7[CoIIICoII(H2O) W11O39]: a molecular mixed-valence Keggin polyoxometalate catalyst of high stability and efficiency for visible light-driven water oxidation, Energy Environ. Sci. 6 (2013) 1170–1184. [7] Z. Zhou, D. Zhang, L. Yang, P. Ma, Y. Si, U. Kortz, J. Niu, J. Wang, Nonacopper(II)-containing 18-tungsto-8-arsenate(III) exhibits antitumor activity, Chem. Commun. 49 (2013) 5189–5191. [8] M. Ibrahim, Y. Lan, B.S. Bassil, Y. Xiang, A. Suchopar, A.K. Powell, U. Kortz, Hexadecacobalt(II)-containing polyoxometalate-based single-molecule magnet, Angew. Chem. Int. Ed. 50 (2011) 4708–4711. [9] H.N. Miras, G.J.T. Cooper, D.-L. Long, H. Bögge, A. Müller, C. Streb, L. Cronin, Unveiling the transient template in the self-assembly of a molecular oxide nanowheel, Science 327 (2010) 72–74. [10] A. Müller, P. Gouaerb, From linking of metal-oxide building blocks in a dynamic library to giant clusters with unique properties and towards adaptive chemistry, Chem. Soc. Rev. 41 (2012) 7431–7463. [11] Y. Hou, T.M. Alam, M.A. Rodriguez, M. Nyman, Aqueous compatibility of group IIIA monomers and Nb-polyoxoanions, Chem. Commun. 48 (2012) 6004–6006. [12] Z.-M. Zhang, S. Yao, Y.-G. Li, H.-H. Wu, Y.-H. Wang, M. Rouzières, R. Clérac, Z.-M. Su, E.-B. Wang, A polyoxometalate-based single-molecule magnet with a mixed-valent MnIV2MnIII6MnII4 core, Chem. Commun. 49 (2013) 2515–2517. [13] A. Nisar, J. Zhuang, X. Wang, Construction of Amphiphilic Polyoxometalate Mesostructures as a Highly Efficient Desulfurization Catalyst, Adv. Mater. 23 (2011) 1130–1135. [14] T. Ito, H. Yashiro, T. Yamase, Regular two-dimensional molecular array of photoluminescent Anderson-type polyoxometalate constructed by Langmuir − Blodgett technique, Langmuir 22 (2006) 2806–2810. [15] Y. Yan, H. Wang, B. Li, G. Hou, Z. Yin, L. Wu, V.W. Yam, Smart self-assemblies based on a surfactant-encapsulated photoresponsive polyoxometalate complex, Angew. Chem. Int. Ed. 49 (2010) 9233–9236. [16] Y. Wang, H. Li, C. Wu, Y. Yang, L. Shi, L. Wu, Chiral heteropoly blues and controllable switching of achiral polyoxometalate clusters, Angew. Chem. Int. Ed. 52 (2013) 4577–4581. [17] J. Zhang, W. Li, C. Wu, B. Li, J. Zhang, L. Wu, Redox-controlled helical self-assembly of a polyoxometalate complex, Chem. Eur. J. 19 (2013) 8129–8135. [18] W. Bu, H. Li, H. Sun, S. Yin, L. Wu, Polyoxometalate-based vesicle and its honeycomb architectures on solid surfaces, J. Am. Chem. Soc. 127 (2005) 8016–8017. [19] A. Nisar, J. Zhuang, X. Wang, Cluster-based self-assembly: reversible formation of polyoxometalate nanocones and nanotubes, Chem. Mater. 21 (2009) 3745–3751. [20] B. Liu, J. Yang, M. Yang, Y. Wang, N. Xia, Z. Zhang, P. Zheng, W. Wang, I. Lieberwirth, C. Kübel, Polyoxometalate cluster-contained hybrid gelator and hybrid organogel: a new concept of softenization of polyoxometalate clusters, Soft Matter 7 (2011) 2317–2320. [21] Y. Jia, H.-Q. Tan, Z.-M. Zhang, E.-B. Wang, Thermotropic liquid crystals built from organic–inorganic hybrid polyoxometalates and a simple cationic surfactant, J. Mater. Chem. C 1 (2013) 3681–3685. [22] S. Yin, W. Li, J. Wang, L. Wu, Mesomorphic structures of protonated surfactantencapsulated polyoxometalate complexes, J. Phys. Chem. B 112 (2008) 3983–3988. [23] W. Li, W. Bu, H. Li, L. Wu, M. Li, A surfactant-encapsulated polyoxometalate complex towards a thermotropic liquid crystal, Chem. Commun. (2005) 3785–3788. [24] W. Li, S. Yin, Y. Wu, L. Wu, Thermotropic mesomorphic behavior of surfactantencapsulated polyoxometalate hybrids, J. Phys. Chem. B 110 (2006) 16961–16966. [25] H. Wang, R. Shao, C. Zhu, B. Bai, C. Gong, P. Zhang, F. Li, M. Li, N.A. Clark, Synthesis and liquid crystalline properties of hydrazide derivatives: hydrogen bonding, molecular dipole, and smectic structures, Liq. Cryst. 35 (2008) 967–974. [26] P.G. Rickert, M.R. Antonio, M.A. Firestone, K.-A. Kunatko, T. Szreder, J.F. Wishart, M.L. Dietz, Tetraalkylphosphonium polyoxometalate ionic liquids: novel, organic– inorganic hybrid materials, J. Phys. Chem. B 111 (2007) 4685–4692. [27] S. Floquet, E. Terazzi, A. Hijazi, L. Guénée, C. Piguet, E. Cadot, Evidence of ionic liquid crystal properties for a DODA + salt of the keplerate [Mo 132 O 372 (CH3COO)30(H2O)72] 42−, New J. Chem. 36 (2012) 865–868. [28] T. Zhang, S. Liu, D.G. Kurth, C.F.J. Faul, Organized nanostructured complexes of polyoxometalates and surfactants that exhibit photoluminescence and electrochromism, Adv. Funct. Mater. 19 (2009) 642–652.
Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9 [29] Y. Jiang, S. Liu, S. Li, J. Miao, J. Zhang, L. Wu, Anisotropic ionic liquids built from nonmesogenic cation surfactants and Keggin-type polyoxoanions, Chem. Commun. 47 (2011) 10287–10289. [30] C.-D. Wu, C.-Z. Lu, H.-H. Zhuang, J.-S. Huang, Hydrothermal assembly of a novel three-dimensional framework formed by [GdMo12O42]9− anions and nine coordinated GdIII cations, J. Am. Chem. Soc. 124 (2002) 3836–3837.
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[31] Y. Lu, B. Keita, L. Nadjo, G. Lagarde, E. Simoni, G. Zhang, G.A. Tsirlina, Excited state behaviors of the dodecamolybdocerate (IV) anion: (NH4)6H2(CeMo12O42) · 9H2O, J. Phys. Chem. B 110 (2006) 15633–15639. [32] H. Tan, Y. Li, Z. Zhang, C. Qin, X. Wang, E. Wang, Z. Su, Chiral polyoxometalateinduced enantiomerically 3D architectures: a new route for synthesis of highdimensional chiral compounds, J. Am. Chem. Soc. 129 (2007) 10066–10067.
Contents lists available at ScienceDirect
Inorganic Chemistry Communications journal homepage: www.elsevier.com/locate/inoche
Metal-centered polyoxometalates encapsulated by surfactant resulting in the thermotropic liquid crystal materials Yue Jia a, Jun Zhang a, Zhi-Ming Zhang a,⁎, Qiu-Yu Li b, En-Bo Wang a,⁎ a b
Key Laboratory of Polyoxometalate Science of Ministry of Education, College of Chemistry, Northeast Normal University, Renmin Street No. 5268, Changchun, Jilin, 130024, China Key Laboratory of Grain and Oil Processing of Jilin Province, Jilin Business and Technology College, Xi An Road No. 4728, Changchun, Jilin, 130062, China
a r t i c l e
i n f o
Article history: Received 12 December 2013 Accepted 19 January 2014 Available online 28 January 2014 Keywords: Polyoxometalate Surfactant Thermotropic liquid crystal Lamellae structure Smectic phase
a b s t r a c t Waugh- and Silverton-type polyoxometalates (POMs) K3(NH4)3[MnMo9O32] and H8[CeMo12O42] are enwrapped by dioctadecyldimethylammonium (DODA+) forming the surfactant-encapsulated POMs (SEPs) (DODA)6 [MnMo9O32]·16H2O (DODA-MnMo9) and (DODA)8[CeMo12O42]·9H2O (DODA-CeMo12), which exhibit typical thermotropic liquid-crystalline properties. Here, the Waugh- and Silverton-type polyoxoanions centered by metal cation were firstly introduced into liquid crystal materials. The chemical composition of these SEP complexes was determined by IR spectra, elemental analysis and TG analysis. Also, the polarized optical microscopy, differential scanning calorimetry (DSC), variable temperature X-ray diffraction (VT-XRD) and transmission electron microscopes (TEM) were performed to characterize their liquid-crystalline behavior. © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
POMs, as a unique class of nanosized metal oxide clusters with abundant topologies and oxygen-rich surface, have exhibited remarkable properties in the field of catalysis, medicine and optics, etc. [1–6]. Despite the unparalleled success in POM synthetic chemistry [7–12], the application of POMs in various functional architectures, devices and materials still requires a progressive step for constructing POMbased functional systems. To modify the weak compatibility between POMs and organic solvents, several issues, such as the low mass transfer and high lattice energy as well as improving the surface of POMs, need to be solved. Recently, POM-based functional materials were efficiently prepared by the exchange of counterions with cationic surfactants, which was regarded as an effective and highly economic procedure for constructing the POM-based functional materials. In these POM-based functional materials, the solubility of POMs in regular organic solvents and the structural stability in diverse chemical environments are effectively improved. Furthermore, the obtained SEPs exhibit enhanced catalytic efficiencies with easy and fast catalyst recovery from the reaction system. In addition, it is possible to predict and control, to some extent, the structure and functionality of the hybrid materials by selecting the surfactants and POMs with different physical–chemical properties. In recent years, the smart self-assembly of surfactants and POMs has resulted in remarkable nanostructured organic–inorganic hybrid materials that exhibit attractive prospects in the field of catalysis, Langmuir– Blodgett films and dynamic transformations under control of external
⁎ Corresponding authors. Tel./fax: +86 431 85098787. E-mail addresses: [email protected] (Z.-M. Zhang), [email protected] (E.-B. Wang).
stimulus. Up to date, various morphology self-assemblies and liquid crystals have been achieved in the POMs/surfactant system [13–21]. Among them, liquid crystals constructed from cationic surfactant and POMs have attracted much attention. The liquid crystal materials synthesized in this way has overcome the complex multiple-step synthetic process of the traditional organic liquid crystals. Wu and Dietz succeeded in introducing the POMs into ammonium or phosphonium derivatives by the ionic self-assembling route resulting in several SEPs with characteristic thermotropic liquid-crystalline behavior [22–26]. The giant Mo clusters [Mo132O372(H2O)72(CH3COO)30]42− enwrapped by cationic surfactants display ionic liquid-crystalline properties [27]. By reasonable modulation of the surfactant, Faul and coworkers [28] described the liquid-crystalline behaviors built from [EuPW5W30O110]12− and [Eu(SiW9Mo2O39)2]13−. Liu et al. [29] demonstrated that Keggintype polyoxoanions enwrapped by tetra-n-octylammonium counterions through ionic self-assembly exhibited thermotropic liquid-crystalline behavior. Obviously, part of the thermotropic liquid crystals were obtained by using the mesomorphic cationic surfactant to encapsulate POMs. However, this method cannot be widely adopted because of its complex multiple-step synthetic process. Besides, these liquidcrystalline materials are mostly constructed from Keggin-type and sandwich-type POMs. Herein, the Waugh- and Silverton-type POMs K3(NH4)3[MnMo9O32] and H8[CeMo12O42], centered by the MnO6 octahedron and CeO12 icosahedron, respectively, were firstly introduced into the POM/surfactant system to construct thermotropic liquid-crystalline materials (DODA)6[MnMo9O32] (DODA-MnMo9) and (DODA)8[CeMo12O42] (DODA-CeMo12) [30–32]. Further, the Cisterminal oxygen atoms of MoO6 octahedra were observed in both K3(NH4)3[MnMo9O32] and H8[CeMo12O42], which are rarely observed
http://dx.doi.org/10.1016/j.inoche.2014.01.015 1387-7003 © 2014 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).
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Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
Fig. 1. Schematic view of the synthesis of DODA-MnMo9 and DODA-CeMo12. (a) K3(NH4)3[MnMo9O32], (b) H8[CeMo12O42], (c) surfactant (DODA+), (d) DODA-MnMo9 and (e) DODA-CeMo12.
Fig. 2. (a) DSC curves of DODA-MnMo9 during its first cooling and second heating process; (b) DSC curves of DODA-CeMo12 during its first cooling and second heating process.
in the typical Keggin and Wells–Dawson POMs and their derivatives. These studies are of significance because they will offer opportunities for introducing new POMs into the SEPs system for constructing functional liquid crystal materials (Fig. 1). DODA-MnMo9 and DODA-CeMo12 were synthesized by a straightforward ion exchange reaction, respectively. The phase transitions of DODA-MnMo9 were determined by DSC during its first cooling and second heating cycles. As shown in Fig. 2a, DODA-MnMo9 displays three phase transitions at 31 °C, 49 °C and 148 °C during its second heating. We notice a halo with a peak at 65 °C, but the present evidence cannot further indentify the transition exactly. Upon cooling, three transitions were observed at 126 °C, 43 °C and 24 °C, respectively. The peak at 31 °C during the second heating process should be assigned to the transition between two kinds of solids, and the VT-XRD results could further confirm the transition [22]. The peak that appeared at 49 °C can be attributed to the transition from solid state to mesophase; the broad peak at 148 °C corresponds to the transition between liquid crystal phase and isotropic phase. Fig. 2b displays two transitions of DODACeMo12 during its first cooling and second heating process. Upon its second heating run, DODA-CeMo12 displays an endothermic halo from 40 °C to 55 °C with the apex at 50 °C, which can be attributed to the transition between the solid state and the liquid crystal phase. With the increase of the temperature, the transition from liquid crystal phase to isotropic phase can be observed at 137 °C. When DODACeMo12 was cooled from its isotropic phase, the transition of isotropic phase to liquid crystal phase can be seen at 126 °C. On further cooling, the halo from 52 °C to 32 °C with the peak at 43 °C can be assigned to
the transition between liquid crystal phase and solid state. The phase transition temperature, enthalpies and assignments of the transition between different states of DODA-MnMo9 and DODA-CeMo12 are summarized in Table 1. Polarizing optical microscopy was employed to identify the liquid crystal phases of DODA-MnMo9 and DODA-CeMo12. During the cooling process from isotropic phase, we observed clear liquid birefringence phenomenon of DODA-MnMo9 with fan-shaped textures at 95 °C (Fig. 3a). For DODA-CeMo12, the phenomenon of liquid birefringence is clearly observed at 90 °C during its cooling process with a grain texture, although the optical texture is not typical (Fig. 3b). TEM was employed to confirm the lamellae structures of DODAMnMo9 and DODA-CeMo12 at room temperature. Fig. 4a shows the lamellar structure of DODA-MnMo9, and the spacing between two layers is approximately 4.2 ± 0.3 nm. For DODA-CeMo12 (Fig. 4b), the layer distance is estimated to 4.0 ± 0.3 nm. VT-XRD was employed to Table 1 The phase transition temperatures, enthalpies and assignments of the phase transition for DODA-MnMo9 and DODA-CeMo12, S = Solid, S1 = Solid 1, S2 = Solid 2, SmA = Smectic A, SmX = Smectic X, Iso = isotropic phase; the data were given on its second heating process. Compound
Transition
Temperature/°C
ΔH/kJ mol−1
DODA-MnMo9
S1–S2 S2–SmA SmA–Iso S–SmX SmX–Iso
31 49 148 50 137
140.5 160.7 — 182.7 —
DODA-CeMo12
Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
7
Fig. 3. (a) The optical textures of DODA-MnMo9 at 95 °C during its first cooling processes (100×); (b) the optical textures of DODA-CeMo12 at 90 °C during its first cooling process.
Fig. 4. (a) TEM of DODA-MnMo9 at room temperature; (b) TEM of DODA-CeMo12 at room temperature.
Fig. 5. (a) The VT-XRD of DODA-MnMo9 at 25 °C, 40 °C and 90 °C; (b) the VT-XRD of DODA-CeMo12 at 25 °C and 100 °C.
further demonstrate the liquid-crystalline behaviors of DODA-MnMo9 and DODA-CeMo12. From Fig. 5a, it can be concluded that DODAMnMo9 in its solid state exhibits the layer structure at room temperature according to the three equidistant diffractions in the small-angle region, and the layer distance is 4.53 nm, which is in accordance with the layer distance estimated from TEM at room temperature. When the temperature increases to 40 °C, we can clearly see the equidistant diffractions in the small-angle region and several sharp peaks in the wide-angle region. This heating process leads to a solid–solid transition as the layer distance decreases to 3.67 nm. These results could further confirm the transition between two solids observed from the DSC. With the increase of temperature, the equidistant diffractions were clearly observed in the small-angle region as well as a broad halo in the wide-angle region centered at 2θ ≈ 20°, which indicated the formation of typical lamellar structure, and the distance between the two neighboring layers is 3.15 nm. Combining the fan-shaped textures, the
phase can be identified as SmA phase. Fig. 5b shows the VT-XRD results of DODA-CeMo12 at 25 °C and 100 °C. The equidistant peaks in the lowangle region of the diffraction pattern illustrate the layered structure of DODA-CeMo12 both in its solid state and liquid crystal phase. The lamellar structure in its liquid crystal phase indicates the formation of a smectic X phase [23].
Table 2 Layer spacings (d) of DODA-MnMo9 and DODA-CeMo12 calculated from X-ray diffractions. Compound
T (°C)
d001 (Å)
d002 (Å)
d003 (Å)
DODA-MnMo9
25 40 90 25 100
45.3 36.7 31.5 43.2 36.4
22.7 18.7 16.1 22.1 18.7
15.4 12.4
DODA-CeMo12
14.7 12.6
8
Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9
Fig. 6. (a) Schematic drawing of packing model of DODA-MnMo9 at liquid crystal state; (b) Schematic drawing of packing model of DODA-CeMo12 at liquid crystal state.
The calculated layer spacing for DODA-MnMo9 and DODA-CeMo12 is listed in Table 2. In fact, the diameter of anion [MnMo9]6− is about 0.7 nm, the normal length of DODA+ is 2.25 nm, the ideal thickness of a single complex should be 5.2 nm. The measured layer distance of DODA-MnMo9 in its mesophase is shorter than the theoretical molecular length, indicating that the molecules in the mesophase are partly interdigitated or tilted [24,25]. For DODA-CeMo12, the layer distance (3.64 nm) calculated from the VT-XRD at 100 °C is not consistent with the theoretical layer 5.4 nm (the diameter of [CeMo12O42] is 0.9 nm), which also indicates the molecules in DODA-CeMo12 are interdigitated or titled in its liquid crystal phase. Possible packing structures of DODA-MnMo9 and DODA-CeMo12 are shown in Fig. 6a and b, respectively. In summary, a simple surfactant DODA+ was used to encapsulate metal-centered POMs (Waugh- and Silverton-type POMs), resulting in two SEPs complexes. The Waugh- and Silverton-type POMs with metal cation as their heteroatoms were firstly introduced into the POMs/surfactant system, and the obtained SEP materials exhibited thermotropic liquid crystal behaviors. The results of polarized optical microscopy, VT-XRD and DSC reveal the smectic mesomorphic behaviors of DODA-MnMo9 and DODA-CeMo12 in a relatively wide temperature range. Compared with the thermotropic liquid crystal materials built from Keggin-type POMs and cationic surfactant [29], DODACeMo12 shows reversible phase transitions during their first heating and second heating cycles, and the smectic-type phase of Keggin-type POMs liquid crystal materials is more structurally ordered than the SmA phase of DODA-MnMo9. The research on thermotropic liquidcrystalline properties of these materials will provide access for constructing new POMs-based functional materials. Acknowledgment This work was supported by the National Natural Science Foundation of China (grant no. 21101022). Appendix A. Supplementary material Synthesis of the compounds, IR spectra and TG curves. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.inoche.2014.01.015. References [1] M.T. Pope, A. Müller, Polyoxometalate chemistry: an old field with new dimensions in several disciplines, Angew. Chem. Int. Ed. 30 (1991) 34–48. [2] A. Proust, B. Matt, R. Villanneau, G. Guillemot, P. Gouzerh, G. Izzet, Functionalization and post-functionalization: a step towards polyoxometalate-based materials, Chem. Soc. Rev. 41 (2012) 7605–7622. [3] D.-L. Long, R. Tsunashima, L. Cronin, Polyoxometalates: building blocks for functional nanoscale systems, Angew. Chem. Int. Ed. 49 (2010) 1736–1758. [4] P. Yin, J. Zhang, T. Li, X. Zuo, J. Hao, A. Warner, S. Chattopadhyay, T. Shibata, Y. Wei, T. Liu, Self-recognition of structurally identical, rod-shaped macroions with different central metal atoms during their assembly process, J. Am. Chem. Soc. 135 (2013) 4529–4536.
[5] S.-T. Zheng, G.-Y. Yang, Recent advances in paramagnetic-TM-substituted polyoxometalates (TM = Mn, Fe, Co, Ni, Cu), Chem. Soc. Rev. 41 (2012) 7623–7646. [6] F. Song, Y. Ding, B. Ma, C. Wang, Q. Wang, X. Du, S. Fu, J. Song, K7[CoIIICoII(H2O) W11O39]: a molecular mixed-valence Keggin polyoxometalate catalyst of high stability and efficiency for visible light-driven water oxidation, Energy Environ. Sci. 6 (2013) 1170–1184. [7] Z. Zhou, D. Zhang, L. Yang, P. Ma, Y. Si, U. Kortz, J. Niu, J. Wang, Nonacopper(II)-containing 18-tungsto-8-arsenate(III) exhibits antitumor activity, Chem. Commun. 49 (2013) 5189–5191. [8] M. Ibrahim, Y. Lan, B.S. Bassil, Y. Xiang, A. Suchopar, A.K. Powell, U. Kortz, Hexadecacobalt(II)-containing polyoxometalate-based single-molecule magnet, Angew. Chem. Int. Ed. 50 (2011) 4708–4711. [9] H.N. Miras, G.J.T. Cooper, D.-L. Long, H. Bögge, A. Müller, C. Streb, L. Cronin, Unveiling the transient template in the self-assembly of a molecular oxide nanowheel, Science 327 (2010) 72–74. [10] A. Müller, P. Gouaerb, From linking of metal-oxide building blocks in a dynamic library to giant clusters with unique properties and towards adaptive chemistry, Chem. Soc. Rev. 41 (2012) 7431–7463. [11] Y. Hou, T.M. Alam, M.A. Rodriguez, M. Nyman, Aqueous compatibility of group IIIA monomers and Nb-polyoxoanions, Chem. Commun. 48 (2012) 6004–6006. [12] Z.-M. Zhang, S. Yao, Y.-G. Li, H.-H. Wu, Y.-H. Wang, M. Rouzières, R. Clérac, Z.-M. Su, E.-B. Wang, A polyoxometalate-based single-molecule magnet with a mixed-valent MnIV2MnIII6MnII4 core, Chem. Commun. 49 (2013) 2515–2517. [13] A. Nisar, J. Zhuang, X. Wang, Construction of Amphiphilic Polyoxometalate Mesostructures as a Highly Efficient Desulfurization Catalyst, Adv. Mater. 23 (2011) 1130–1135. [14] T. Ito, H. Yashiro, T. Yamase, Regular two-dimensional molecular array of photoluminescent Anderson-type polyoxometalate constructed by Langmuir − Blodgett technique, Langmuir 22 (2006) 2806–2810. [15] Y. Yan, H. Wang, B. Li, G. Hou, Z. Yin, L. Wu, V.W. Yam, Smart self-assemblies based on a surfactant-encapsulated photoresponsive polyoxometalate complex, Angew. Chem. Int. Ed. 49 (2010) 9233–9236. [16] Y. Wang, H. Li, C. Wu, Y. Yang, L. Shi, L. Wu, Chiral heteropoly blues and controllable switching of achiral polyoxometalate clusters, Angew. Chem. Int. Ed. 52 (2013) 4577–4581. [17] J. Zhang, W. Li, C. Wu, B. Li, J. Zhang, L. Wu, Redox-controlled helical self-assembly of a polyoxometalate complex, Chem. Eur. J. 19 (2013) 8129–8135. [18] W. Bu, H. Li, H. Sun, S. Yin, L. Wu, Polyoxometalate-based vesicle and its honeycomb architectures on solid surfaces, J. Am. Chem. Soc. 127 (2005) 8016–8017. [19] A. Nisar, J. Zhuang, X. Wang, Cluster-based self-assembly: reversible formation of polyoxometalate nanocones and nanotubes, Chem. Mater. 21 (2009) 3745–3751. [20] B. Liu, J. Yang, M. Yang, Y. Wang, N. Xia, Z. Zhang, P. Zheng, W. Wang, I. Lieberwirth, C. Kübel, Polyoxometalate cluster-contained hybrid gelator and hybrid organogel: a new concept of softenization of polyoxometalate clusters, Soft Matter 7 (2011) 2317–2320. [21] Y. Jia, H.-Q. Tan, Z.-M. Zhang, E.-B. Wang, Thermotropic liquid crystals built from organic–inorganic hybrid polyoxometalates and a simple cationic surfactant, J. Mater. Chem. C 1 (2013) 3681–3685. [22] S. Yin, W. Li, J. Wang, L. Wu, Mesomorphic structures of protonated surfactantencapsulated polyoxometalate complexes, J. Phys. Chem. B 112 (2008) 3983–3988. [23] W. Li, W. Bu, H. Li, L. Wu, M. Li, A surfactant-encapsulated polyoxometalate complex towards a thermotropic liquid crystal, Chem. Commun. (2005) 3785–3788. [24] W. Li, S. Yin, Y. Wu, L. Wu, Thermotropic mesomorphic behavior of surfactantencapsulated polyoxometalate hybrids, J. Phys. Chem. B 110 (2006) 16961–16966. [25] H. Wang, R. Shao, C. Zhu, B. Bai, C. Gong, P. Zhang, F. Li, M. Li, N.A. Clark, Synthesis and liquid crystalline properties of hydrazide derivatives: hydrogen bonding, molecular dipole, and smectic structures, Liq. Cryst. 35 (2008) 967–974. [26] P.G. Rickert, M.R. Antonio, M.A. Firestone, K.-A. Kunatko, T. Szreder, J.F. Wishart, M.L. Dietz, Tetraalkylphosphonium polyoxometalate ionic liquids: novel, organic– inorganic hybrid materials, J. Phys. Chem. B 111 (2007) 4685–4692. [27] S. Floquet, E. Terazzi, A. Hijazi, L. Guénée, C. Piguet, E. Cadot, Evidence of ionic liquid crystal properties for a DODA + salt of the keplerate [Mo 132 O 372 (CH3COO)30(H2O)72] 42−, New J. Chem. 36 (2012) 865–868. [28] T. Zhang, S. Liu, D.G. Kurth, C.F.J. Faul, Organized nanostructured complexes of polyoxometalates and surfactants that exhibit photoluminescence and electrochromism, Adv. Funct. Mater. 19 (2009) 642–652.
Y. Jia et al. / Inorganic Chemistry Communications 43 (2014) 5–9 [29] Y. Jiang, S. Liu, S. Li, J. Miao, J. Zhang, L. Wu, Anisotropic ionic liquids built from nonmesogenic cation surfactants and Keggin-type polyoxoanions, Chem. Commun. 47 (2011) 10287–10289. [30] C.-D. Wu, C.-Z. Lu, H.-H. Zhuang, J.-S. Huang, Hydrothermal assembly of a novel three-dimensional framework formed by [GdMo12O42]9− anions and nine coordinated GdIII cations, J. Am. Chem. Soc. 124 (2002) 3836–3837.
9
[31] Y. Lu, B. Keita, L. Nadjo, G. Lagarde, E. Simoni, G. Zhang, G.A. Tsirlina, Excited state behaviors of the dodecamolybdocerate (IV) anion: (NH4)6H2(CeMo12O42) · 9H2O, J. Phys. Chem. B 110 (2006) 15633–15639. [32] H. Tan, Y. Li, Z. Zhang, C. Qin, X. Wang, E. Wang, Z. Su, Chiral polyoxometalateinduced enantiomerically 3D architectures: a new route for synthesis of highdimensional chiral compounds, J. Am. Chem. Soc. 129 (2007) 10066–10067.