Hydrogen-Bonding-Induced Supramolecular Liquid Crystals and

Feb 4, 2009 - The present investigation provides an example for developing hydrogen-bonding-induced polyoxometalate-containing hybrid liquid crystal ...
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J. Phys. Chem. B 2009, 113, 2355–2364

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Hydrogen-Bonding-Induced Supramolecular Liquid Crystals and Luminescent Properties of Europium-Substituted Polyoxometalate Hybrids Shengyan Yin, Hang Sun, Yi Yan, Wen Li, and Lixin Wu* State Key Laboratory of Supramolecular Structure and Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: NoVember 21, 2008; ReVised Manuscript ReceiVed: December 28, 2008

Eu-containing polyoxometalates, Na9EuW10O36, K11Eu(PW11O39)2, and K13Eu(SiW11O39)2, were electrostatically canned by a cationic surfactant, N-[12-(4-carboxylphenoxy)dodecyl]-N-dodecyl-N,N-dimethylammonium bromide, through the replacement of counterions, and the resulting surfactant-encapsulated polyoxometalate complexes were characterized in detail by elemental analysis as well as IR and NMR spectra. The carboxyls bearing in the complexes were confirmed existing in the dimer state through intermolecular hydrogen bonding, which leads to stable and reversible thermotropic liquid crystal properties of these complexes. The results of differential scanning calorimetry, polarized optical microscopy, and X-ray diffraction revealed that these complexes underwent smectic mesophases during the heating and cooling cycles. These complexes displayed intrinsic luminescence both in the amorphous powder states and in their mesophases. The photophysical properties showed the dependence on the existing states of samples, and the quantum yields of the complexes in the liquid crystalline structures are higher than the corresponding amorphous powders. The present investigation provides an example for developing hydrogen-bonding-induced polyoxometalate-containing hybrid liquid crystal materials with intrinsic luminescence. Introduction Liquid crystal (LC) materials with luminescent property are of considerable interest over recent years due to their potential applications in the fields of anisotropic light emitters,1 photoconductors,2 LC display technology,3 and so forth. On one hand, LC structure can tune the luminescence and polarization due to the ordered arrays of luminescent groups, while on the other hand, the self-luminescence of LC materials can enrich the performance of LC display remarkably. To develop luminescent LC materials, researchers have exploited several methods such as designing molecules with organic luminescent mesogenic groups4 and doping a fluorescent dye into LC matrixes.5 Comparing with organic luminescent groups, inorganic ones possess higher luminescent stability and better color purity, and metallomesogens (lanthanidomesogens, especially)6 and semiconductor nanopaticles7 have been incorporated into the mesophases. However, the introduction of nanometer-sized inorganic materials into LC matrixes was less studied, and increasing miscibility and stability of inorganic components and promoting the quantum yield of the hybrid systems in the mesophases are still challenges. Therefore, it becomes significant to develop new luminescent LC materials which exhibit low quenching, high miscibility, and high stability. Polyoxometalates (PMs) are a kind of nanoscale polyanion clusters possessing potential applications in electrochemistry, proton conduction, magnetism, and optics.8 Some lanthanidesubstituted PMs, such as Eu3+, Tb3+, Sm3+, and Dy3+ derivatives, have attracted much attention owing to their characteristic luminescent properties.9 To utilize PMs in an organized way, various interesting methods have recently been reported,10 and among them, the encapsulation of PMs with organic molecules through electrostatic interaction has proven to be a highly effective route.10c,11 Through encapsulation, the luminescent * To whom correspondence should be addressed. E-mail: wulx@ jlu.edu.cn.

properties of PMs can be well-organized into various matrixes, such as polystyrene latex,12 poly(methyl methacrylate) matrix,11c and even ordered microporous films.13 We demonstrated the mesomorphic structures of these kinds of complexes as integrated building blocks by an appropriate selection of PMs and surfactants with mesogenic groups.14 However, the fluorescence of the complexes in LCs is usually quenched by the mesogen groups containing in the surfactants. To sustain the fluorescence of PMs in LC states, the employed surfactant has to be modified. Aromatic acid derivatives with a long alkyl chain are known to show mesomorphism because the dimerization of the carboxylic acids through hydrogen bonding plays a role of mesogen group.15 As the benzoic acid dimer is not a real conjugated group, it should not quench the luminescence of the inorganic PM core while it directs the formation of LC phases. Thus, combining the surfactant bearing benzoic acid group at the end of the alkyl chain and fluorescent PMs together, one can expect to bring a novel hybrid material which exhibits the luminescence of PMs in the mesophase. On the basis of this motivation, in this paper, we reported a representative investigation concerning PM-containing hybrid supramolecular LCs with intrinsic luminescence in the mesophases. A surfactant with two alkyl chains, one of which is modified by benzoic acid at the hydrophobic end, was designed. Three Eu-PMs, Na9EuW10O36 (PM-1), K11Eu(PW11O39)2 (PM2), and K13Eu(SiW11O39)2 (PM-3),16 which possess different topologic structures, surface negative charges, and chemical compositions, were selected to be encapsulated, as schematically represented in Figure 1. The resulting surfactant-encapsulated PM (SEP) complexes exhibit both the typical hydrogen-bonding LC characteristics and unique luminescence at LC states. More significantly, the LC structures can be applied to adjust the photophysical properties of PMs. As there are several carboxylic groups surrounded on each SEP, the present research provides an example of hydrogen-bonding supramolecular network LC hybrid materials with PMs.

10.1021/jp810262c CCC: $40.75  2009 American Chemical Society Published on Web 02/04/2009

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Figure 1. Coordination polyhedral representations of PMs, chemical structure of a benzoic acid-terminated surfactant, and schematic drawings of the hybrid complexes.

SCHEME 1: Synthetic Path of the Benzoic Acid-Terminated Surfactant

Experimental Section 1. Materials. PM-1, PM-2, and PM-3 were freshly prepared according to the literature procedures.16,17 4-Hydroxybenzoic acid and p-toluenesulfonic acid were purchased from Sinopharm Chemical Reagent Co., Ltd. 1,12-Dibromododecane and N,Ndimethyldodecylamine were obtained from Fluka and Aldrich, respectively, and used as received. Other starting compounds and solvents applied in the preparation were commercial products from local chemical reagent companies. Doubly distilled water was used in the experiments. Silica gel (100-200 mesh)wasemployedforthepurificationovercolumnchromatography. 2. Synthesis of Benzoic Acid-Terminated Surfactant. The synthesis of the specified surfactant was carried out following the modified routes by referencing the literatures,14b,18 as shown in Scheme 1. Detailed procedures are as follows. 4-Hydroxyethylbenzoate. 4-Hydroxybenzoic acid (10.0 g, 0.07 mol) and p-toluenesulfonic acid (49.8 g, 0.28 mol), with

the initial molar ratio controlled at 1:4, were dissolved in 150 mL of anhydrous ethanol. The reaction mixture was stirred under refluxing for 28 h and then cooled to room temperature. After the evaporation of solvent under the reduced pressure, the crude product was recrystallized from water and then washed with water several times. The obtained white powder was dried under vacuum, giving 10.7 g of 4-hydroxyethylbenzoate (yield: 89%). 1 H NMR (DMSO-d6, δ, ppm): 1.29 (t, 3H), 4.26 (q, 2H), 6.86 (d, 2H), 7.81 (d, 2H), 10.34 (s, 1H). 4-(12-Bromododecyloxyl)ethylbenzoate. A mixture of 4-hydroxyethylbenzoate (2.4 g, 14.0 mmol), 1,12-dibromododecane (9.2 g, 28.0 mmol), and anhydrous sodium carbonate (4.5 g, 42.0 mmol) with the initial molar ratio of 1:2:3 in 150 mL of acetone was stirred under refluxing for 26 h. After cooling to room temperature, the solvent was removed under the reduced pressure, the crude product was dissolved and extracted with three portions of chloroform (50 mL), and then the organic phases were combined. After the evaporation of solvent, the residue was further purified over column chromatography on silica gel using dichloromethane/cyclohexane (1:1 in v/v) as eluent, giving 4.2 g of 4-(12-bromododecyloxyl)ethylbenzoate (yield: 70%). 1H NMR (CDCl3, δ, ppm): 1.21-1.39 (m, 19H), 1.78 (m, 4H), 3.34 (t, 2H), 3.93 (t, 2H), 4.28 (q, 2H), 6.84 (d, 2H), 7.92 (d, 2H). N-12-(4-Ethylbenzoate)dodecyloxyl-N-dodecyl-N,N-dimethylammonium Bromide. Dodecyldimethylamine (0.6 g, 2.8 mmol) and 4-(12-bromododecyloxyl)ethylbenzoate (1.5 g, 3.6 mmol) with the initial molar ratio of 1:1.3 were dissolved in 150 mL of acetone, and the reaction mixture was stirred under refluxing for 72 h. After cooling to room temperature, the mixture was concentrated to 3-5 mL by removing excess solvent under reduced pressure. Then, 30 mL of cold diethyl ether was added dropwise to the residue, and the mixture was kept at 0 °C for 3 days. The formed white precipitate was filtered and washed with cold diethyl ether several times, giving 1.3 g

Europium-Substituted Polyoxometalate Hybrids of white pure product of N-12-[4-(ethylbenzoate)dodecyloxyl]N-dodecyl-N,N-dimethylammonium bromide (yield: 75%). 1H NMR (CDCl3, δ, ppm): 0.81 (t, 3H), 1.19-1.39 (m, 37H), 1.62-1.72 (m, 6H), 3.32 (s, 6H), 3.42 (m, 4H), 3.94 (t, 2H), 4.28 (q, 2H), 6.84 (d, 2H), 7.92 (d, 2H). N-[12-(4-Carboxylphenyl)dodecyloxyl]-N-dodecyl-N,N-dimethylammonium Bromide (CDDA). A mixture of N-12-(4ethylbenzoate)dodecyloxyl-N-dodecyl-N,N-dimethylammonium bromide (0.6 g, 0.96 mmol) and sodium hydroxide (0.5 g, 12.5 mmol) were dissolved in 25 mL of mixed solvent of water and methanol (3:22 in volume ratio). The reaction mixture was stirred under refluxing for 12 h and then cooled to room temperature. After the solvent was removed, the crude product was redissolved in chloroform, washed with dilute hydrochloric acid (pH ) 3) and pure water, and then dried over magnesium sulfate. The pure white powder (CDDA) was obtained after removing solvent and further drying under vacuum until the weight kept constant. 1H NMR (DMSO-d6, δ, ppm): 0.82 (t, 3H), 1.25-1.41 (m, 34H), 1.62-1.72 (m, 6H), 2.98 (s, 6H), 3.19 (d, 4H), 4.03 (t, 2H), 6.98 (d, 2H), 7.86 (d, 2H), 12.49 (s, 1H). 1H NMR (CDCl3, δ, ppm): 0.88 (t, 3H), 1.24-1.46 (m, 34H), 1.68-1.79 (m, 6H), 3.4 (s, 6H), 3.49 (d, 4H), 4.06 (t, 2H), 6.92 (d, 2H), 8.01 (d, 2H). IR (KBr, cm-1) for CDDA: ν ) 3402, 2955, 2921, 2852, 2579, 2457, 1702, 1608, 1583, 1511, 1494, 1469, 1419, 1388, 1249, 723. Anal. Calcd for CDDA (C33H60NO3Br, 598.7): C, 66.20; H, 10.10; N, 2.34. Found: C, 65.80; H, 10.11; N, 2.32. MALDI-TOF MS (MW ) 518.8), m/z ) 517.7 [M+ - 1]. 3. Preparation of Benzoic Acid Bearing SEPs. The composites of surfactant CDDA encapsulated PMs were prepared following the procedure reported previously, as exemplified by SEP-1.11,14 PM-1 was dissolved in water, and to the aqueous solution a chloroform solution of CDDA was added with stirring. The initial molar ratio of CDDA to PM-1 was controlled at 7:1. The organic phase was then separated and washed by dilute hydrochloric acid (pH ) 3.5). Then, the hybrid complex SEP-1 was obtained by evaporating the chloroform to dryness. The sample was further dried under vacuum until its weight remained constant. Following the similar procedures, other SEPs were prepared. All three complexes were characterized by IR spectrum, elemental analysis, and thermogravimetric analysis (TGA) as follows. SEP-1. IR (KBr, cm-1) for SEP-1: ν ) 3450, 2955, 2923, 2852, 2590, 2453, 1706, 1606, 1583, 1512, 1484, 1467, 1419, 1384, 1252, 944, 870, 849, 815, 782, 721. Anal. Calcd for SEP-1 (C264H485N8O62EuW10, 6772.1): C, 46.95; H, 7.24; N, 1.66. Found: C, 47.23; H, 7.36; N, 1.77. As a mass loss of 0.94% occurs in the range of 30-150 °C from thermogravimetric analysis (TGA), which arises from crystal water, the speculated chemical formula should be (CDDA)8H(EuW10O36)(H2O)3 (6772.1). SEP-2. IR (KBr, cm-1) for SEP-2: ν ) 3434, 2955, 2921, 2852, 2620, 2493, 1703, 1606, 1579, 1512, 1488, 1467, 1419, 1388, 1249, 970, 889, 846, 821, 783, 721. Anal. Calcd for SEP-2 (C297H552N9O110EuP2W22, 10267.9): C, 35.05; H, 5.37; N, 1.24. Found: C, 34.81; H, 5.13; N, 1.18. As a mass loss of 0.961% occurs in the range of 30-150 °C from TGA measurement, which arises from crystal water, the speculated chemical formula should be (CDDA)9H2[Eu(PW11O39)2](H2O)5 (10 267.9). SEP-3. IR (KBr, cm-1) for SEP-3: ν ) 3435, 2955, 2921, 2852, 2592, 2476, 1704, 1606, 1581, 1512, 1486, 1467, 1421, 1388, 1253, 960, 906, 846, 798, 773, 727. Anal. Calcd for SEP-3 (C363H672N11O116EuW22Si2, 11299.8): C, 38.58; H, 5.99; N, 1.36. Found: C, 38.37; H, 5.75; N, 1.56. As a mass loss of 0.778%

J. Phys. Chem. B, Vol. 113, No. 8, 2009 2357 occurs in the range of 30-150 °C from TGA measurement, which arises from crystal water, the speculated chemical formula should be (CDDA)11H2[Eu(SiW11O39)2](H2O)5 (11 299.8). 4. Sample Preparation for Characterizations. The SEPs were treated on a heating stage over their highest endothermic transition temperature and then cooling down to the certain temperature at which the LC phases formed. After holding the state at mesophase isothermally for 10 min, the sample was quenched in the liquid nitrogen for half an hour. The annealing temperature was carefully selected to get characteristic LC phases. The formed thin films covered on the quartzes were used for X-ray diffraction and fluorescence spectral measurements. Then, the thin films were removed from the quartz glass, floated on the water surface, and recovered using copper grids for transmission electron microscopic (TEM) observations. 5. Measurements. 1H NMR spectra were recorded on a Bruker Avance 500 instrument using CDCl3, CD3OD, and DMSO-d6 as solvents. Elemental analysis (C, H, N) was performed on a Flash EA1112 from ThermoQuest Italia SPA. FT-IR spectra were carried out on a Bruker IFS66V equipped with a DGTS detector with a resolution of 4 cm-1 from pressed KBr pellets. TGA was conducted with a Perkin-Elmer TG/ DTA-7 instrument, and the heating rate was set at 10 °C min-1. MALDI-TOF spectra were recorded on a LDI-1700 mass spectrometer. The phase behaviors were performed using a polarized optical microscope (POM) (Leica DMLP, Germany) equipped with a Mettler FP82HT hot stage and a Mettler FP90 central processor. Differential scanning calorimetric (DSC) measurements were performed on a Netzsch DSC 204 with scanning rate of 10 °C min-1. The samples that were heated over 20 °C higher than their melting temperatures were used for the DSC measurements.19b Variable-temperature X-ray diffraction (XRD) was carried out on a Philips PW 1700 X-ray diffractometer (using Cu KR1 radiation of a wavelength of 1.54 Å) with a TTK-HC temperature controller. Luminescence measurements were performed on a HORIBA Jobin Yvon FL3TCSPC fluorescence spectrophotometer. TEM observations were carried out on a JEOL-2010 electron microscope operating at 200 kV. All the measurements were operated at room temperature in ambient conditions. Results and Discussion Structural Characterization of CDDA and SEPs. In this paper, we designed a double-chain ammonium surfactant, CDDA, in which one of the chains is terminated with benzoic acid group (see Scheme 1), and we employed the surfactant to encapsulate luminescent PMs, PM-1, PM-2, and PM-3. The synthetic and encapsulated procedures were described in detail in the Experimental Section. The elemental analysis (C, H, N) and TGA reveal the expected chemical components of the complexes. The as-prepared complexes are no longer soluble in water, but easily dissolve in mixed organic media such as chloroform/methanol, chloroform/ethanol, etc., suggesting that the surfaces of PMs have been effectively covered by CDDA. As demonstrated through TGA (shown in the Supporting Information) and DSC in the following data, the complexes are thermally stable in air, even heated up to 200 °C. The spectroscopic measurements were used to examine the surfactant and the complexes. As a representative example, IR spectra of CDDA and SEP-1 are shown in Figure 2. For pure CDDA, the absorption band at 1702 cm-1, which is assigned to CdO stretching mode, and the weak double absorption bands at 2457 and 2579 cm-1 (normally called satellite bands), which represent the formation of the hydrogen bonding, indicate that the terminal

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Yin et al. TABLE 1: Summary of Phase Transition Temperatures (°C), Enthalpies (kJ/mol), and Assignments of Phase Transitions for All Complexesa second heating samples

transitions

CDDA

S-SmC SmC-SmA SmA-Iso S-SmA SmA-Iso S-SmA SmA-Iso S-SmC SmC-SmA SmA-Iso

SEP-1 SEP-2 SEP-3

Figure 2. FT-IR spectra of pure CDDA and SEP-1 in KBr pellets.

Figure 3. 1H NMR spectra of CDDA and SEP-1 in 3:1 of CDCl3/ CD3OD. Inset: local magnification of full spectra, where “a” marks the protons of N-methyl (N+-CH3) and “b” marks the protons of N-methylene (N+-CH2).

carboxylic acids exist in the intermolecular cyclic dimer state definitely.20 The detailed assignments of other absorption bands are summarized in the Supporting Information. For SEP-1, we can also see the characteristic vibration bands that confirm the hydrogen-bonding dimer of carboxylic acids appearing at about 2590 and 2453 cm-1 derived from OH stretching and at 1706 cm-1 from carbonyl stretching.21 Other bands are apparently from the characteristic vibrations of surfactant and PM-1. Thus, one can expect that the complexes exist in a hydrogen-bonding supramolecular network state through the intermolecular carboxylic acid dimer between adjacent SEP-1 units because such a combined dimer is difficult to occur in one SEP-1 unit due to the mismatched orientation. It should be noted that besides the hydrogen-bonding dimer, it is possible that the neighboring carboxylic groups of CDDAs on the same or adjacent PMs forming the traverse hydrogen bonding. However, in view of the unfavorable orientation of carboxylic groups and small hydrogen-bonding angle, the traverse hydrogen bonding should be quite weak. To identify the exact binding position of CDDA with PMs, the complexes were characterized by 1H NMR spectra. As shown in Figure 3, in contrast to that of CDDA alone, the proton chemical shifts of CDDA in SEP-1 show the following characteristics: (1) the proton peak of N-methyl has broadened significantly and shifted to the high field by 0.46 ppm, (2) the proton peak of N-methylene becomes a considerably broadened halo and has shifted toward high field by 0.31 ppm, and (3) other peaks maintain at the same positions as those of pure CDDA. The peak broadening implies the strong

first cooling

T (°C) ∆H (kJ/mol) T (°C) ∆H (kJ/mol) 113 143 174 137 160 117 155 105 126 153

4.02 3.44 7.61 26.75 16.25 49.71 26.88 2.67 9.24 2.21

97 167 116 157 94 147 103 143

-b

-b

3.74 7.44 17.99 13.21 4.36 7.44 1.31 1.43

a

S, SmC, SmA, and Iso denote solid, smectic C, smectic A, and isotropic phase, respectively. b Transition of SmC-SmA is observed under polarized optical microscopy, though it is not apparent in DSC thermogram.19b,c

electrostatic interaction between CDDA and PM-1 cluster, which restricts the mobility of the ammonium headgroup.11a,22 Considering the fact that the chemical shift and peak width of the proton signals are sensitive to the local physical and chemical environment, the cationic ammonium headgroup of CDDA is suggested binding to the negative charged cluster electrostatically. Supramolecular Mesomorphic Behavior of CDDA and SEPs. The thermal properties of CDDA and the complexes were investigated by DSC, POM, and XRD. The phase transition temperatures, enthalpies, and assignments of the phase transitions for all the samples are summarized in Table 1. DSC curves (Figure 4) display reversible phase transitions of CDDA and SEPs in the first cooling and second heating processes. Upon the second heating run, CDDA exhibits an exothermic transition at ca. 93 °C, which may be associated with the further ordering process for the low-temperature phase. On further heating, three endothermic peaks at 113, 143, and 174 °C, which can be attributed to the transitions of solid to LC phase, LC to LC phases, and LC phase to isotropic state, respectively, based on the following POM data. In contrast to the heating process, during the first cooling run, CDDA exhibits an exothermic transition at 167 °C, which can be attributed to the transition of isotropic state to LC phase, and a halo from 125 to 90 °C with the apex at 97 °C, which can be assigned to the transition of the LC phase to solid state, respectively. The phase transitions of CDDA emerging in the cooling run, determined by DSC measurement, are not fully in accordance with those found in the heating run, while the transition from a broken fan-shaped texture to a focal conic fan-shaped texture under POM can be seen visually at ca. 135 °C during the cooling run. This behavior often appears in a smectic C phase.19 One possible reason is that the needed energy to change is too small to be monitored by DSC.19c With the temperature decreasing, a broad halo occurs from 70 °C to room temperature, indicating that the crystallization of CDDA seems to be a slow process. Comparing with the phase transitions of CDDA, the thermal behavior of SEP-1 displays two endothermic transitions at 137 and 160 °C during the second heating run, in which the first one is attributed to the change from solid to LC phase and the second should correspond to the phase transition from LC phase to isotropic state, as supported by the following POM results. The first cooling curve of SEP-1 exhibits a transition at 157 °C, and a halo from 122 to 92 °C with the apex at 116 °C, similar to that found in CDDA. In accordance with the case of heating run,

Europium-Substituted Polyoxometalate Hybrids

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Figure 4. DSC curves of CDDA and SEPs on their (A) second heating and (B) first cooling cycles, respectively.

the peak at high temperature can be clearly assigned to the transition of isotropic state to LC phase, and the halo is ascribed to the change of LC phase to solid state. SEP-2 and SEP-3 also exhibit multipeaks during the heating and cooling cycles. Two transitions for SEP-2, one from solid state to LC phase at low temperature and the other from LC phase to isotropic phase at high temperature, emerge on both second heating and first cooling runs. SEP-3 shows an exothermic transition at ca. 91 °C on the heating run, which may be associated with the further ordering process as appeared in the case of CDDA. In the following heating, three endothermic peaks at 105, 126, and 153 °C emerge, continuously, while only two peaks appear at 103 and 143 °C in the cooling process. On the basis of the results of the POM, the change at 105 °C is the transition of solid state to LC phase, 126 °C indicates the change between the different LC phases, and 153 °C is attributed to the transition from LC phase to isotropic phase. In the cooling run, the peak at 143 °C can be clearly assigned to the transition of isotropic state to LC phase, and the transition at 103 °C is ascribed to the change of LC phase to solid state. Similar to the case of CDDA, the transition between different LC phases does not appear in the cooling DSC curve, while it can be observed under POM. The detailed assignments of the phase transitions of CDDA and SEPs are shown in Table 1. From the DSC curves, we see that the clearing point temperatures of SEPs are lower than that of pure CDDA and decrease gradually from SEP-1 to SEP-3. We consider that the main reason is that the incorporation of the PMs makes the arrangement of CDDA molecules in LC structures change consecutively. On one hand, CDDAs bond to PMs through electrostatic interaction in the complexes, which makes the CDDAs anchored on the surface of PMs, leading to a more ordered and tight packing of alkyl chains along with the surface of PMs. On the other hand, the surface curvature and limited area of PMs should decrease the packing order of CDDA in the LC structures, as have been confirmed in the literature,14c leading to the decrease of the phase transition temperature. As an example, PM-2 and PM-3 have different surface charges but have the same geometric shape and volume; thus, the occupied surface area per alkyl chain in SEP-2 is larger than that in SEP-3 (see Table S2 in the Supporting Information). This case yields the favorable rearrangement of alkyl chains and higher transition temperature for SEP-2 than SEP-3. Although the occupied area per alkyl chain in SEP-1 is almost the same as that of SEP-3, the phase transition temperature of the former is still higher than the latter due to the influence of the PM’s volumes. Therefore, the phase transitions of the SEPs should be dominated by the various conditions. From Table S2 in the Supporting Information, the alkyl chain density of SEP-3 is larger than that of SEP-2. As discussed above, the surfactant CDDAs will rearrange to adapt the curvature of the PM-3 cluster. More

Figure 5. Temperature-dependent IR spectra of CDDA at 160 °C and SEP-1 at 150 °C.

CDDAs covered on PM-3 generate a crowded array around the surface of PM-3, leading to a more distorted packing than that in SEP-2. This makes the stabilization of the layer structure of SEP-3 become worse. Considering this point, it is reasonable that the enthalpy of SEP-3 is significantly lower than SEP-2. To clarify the hydrogen bonding keeping at the LC states, we checked the samples by IR spectra (Figure 5). As a representative example, at the LC state, the carbonyl stretching vibrations of CDDA and SEP-1 appear at 1708 and 1710 cm-1, respectively, and the satellite double absorption bands emerge at 2400-2600 cm-1, obviously confirming that the benzoic acid groups still exist in the state of hydrogen-bonding dimer.20,21 However, the shifting of carbonyl stretching bands to high wavenumber and the weakening and broadening of the satellite absorptions imply that the intensity of the hydrogen bonding becomes weak at the LC states.20c The LC behaviors of CDDA and SEPs are also identified through POM in detail. During the cooling run, CDDA exhibits a typical focal conic fan-shaped texture at 160 °C (Figure 6A) and a broken fan-shaped texture at 130 °C (Figure 6B), which can be attributed to smectic A (SmA) and smectic C (SmC) phase, respectively. The two LC phases observed in POM support the assignment for the thermotropic transitions of CDDA during the cooling run. For SEP-1, only one LC phase has been found, and the focal conic fan-shaped texture (Figure 6C) suggests the formation of SmA phase. In the case of SEP-2, a like SmA phase (Figure 6D) was observed during the cooling run. Although the intermediate phase transition has not been found in DSC curve during the cooling run, SEP-3 exhibits two different textures in POM images, focal conic fan-shaped texture and broken fan-shaped texture, indicative of SmA and SmC phases (Figure 6E,F), respectively, which are in agreement with DSC results in the heating run. We also checked the esterized derivative, the non-hydrolyzed precursor of CDDA by using POM, whereas we did not observe any double refractions in

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Figure 6. POM images of CDDA at (A) 160 and (B) 130 °C, SEP-1 at (C) 148 °C, SEP-2 at (D) 140 °C, and SEP-3 at (E) 138 and (F) 120 °C during the cooling process (magnification: ×400).

Figure 7. X-ray diffraction patterns of SEPs quenched in liquid nitrogen at the temperatures of (A) 144 °C for SEP-1, (B) 130 °C for SEP-2, and (C) 140 and 110 °C for SEP-3, cooled from isotropic state. The insets display the corresponding diffractions in wide-angle region.

the heating and cooling runs. Apparently, the LC properties of SEPs source from the hydrogen-bonding dimers formed between CDDAs, but their characteristics are distinctly different from those of CDDA. And, SEPs, as a kind of integrated building blocks, represent new type of LC materials. Thus, the present strategy provides an optimal route to functionalize LC materials with PMs. The LC behaviors of CDDA and all the complexes were further investigated by variable-temperature XRD. Unfortunately, we have not observed any strong XRD diffractions at the temperatures of LC phases by directly heating all the samples. A possible reason may be the dynamic nature of the noncovalent interactions of the intermolecular carboxylic acid dimer directed by hydrogen bonding at the LC states.23 To identify the LC structures clearly, we fleetly froze the samples at the temperatures just in their LC phases using liquid nitrogen, as described in the Experimental Section, and then performed XRD measurements. The XRD data support the assignment of lamellar phases. As shown in Figure 7A, two equidistant diffractions for SEP-1 emerge in the small-angle region, just corresponding to a layered structure with d-spacing of 2.5 nm, calculated from Bragg equation. The halo at wide angle region, at ca. 22° (Figure 7A, inset), accompanied by the diffractions at small-angle region, suggests that the packing of the alkyl chains is disordered. As presented in the IR spectrum of SEP-1 at the temperature of LC state, CH2 symmetric and antisymmetric stretching vibrations appear at 2855 and 2927 cm-1 (Figure 5), respectively, indicating that the conformation of alkyl chains of surfactants around PM-1 is disordered.14b At the LC phase, the mesogenic group composed of benzoic acid dimer should stand perpendicularly to the layer surface based on the smectic phase assignment from POM results. Combining these results and analysis, we suggest a schematic LC packing model of SEP-1 as shown in Figure 8.

Figure 8. Schematic drawing of packing model of SEP-1 in the LC state.

SEP-2 and SEP-3 exhibit similar lamellar structures as SEP1. In the small-angle region, two equidistant diffractions for SEP-2 (Figure 7B) and one diffraction for SEP-3 at high temperature phase and two equidistant diffractions at lowtemperature phase emerge (Figure 7C), which can be assigned to the layered structures. Similar to SEP-1, both samples exhibit a halo at wide-angle region, suggesting the disordered packing of alkyl chains. The calculated layer spacings from the XRD data (Figure 7B,C) are 2.7 nm for SEP-2 and 2.9 and 2.6 nm for SEP-3 under different LC states. Meanwhile, it is reasonable that SEP-2 and SEP-3 exhibit a little bit larger layer spacings than SEP-1 because more CDDA covered on PM-2 and PM-3 induces the thicker layer of alkyl chains. Considering the similar

Europium-Substituted Polyoxometalate Hybrids

J. Phys. Chem. B, Vol. 113, No. 8, 2009 2361 transitions of Eu3+. The band near 579 nm is attributed to the D0 f 7F0 transition; the bands near 589 and 594 nm are assigned to the 5D0 f 7F1 transitions; the bands near 612 and 618 nm are ascribed to the 5D0 f 7F2 transitions; the band near 652 nm is derived from the 5D0 f 7F3 transition; and the bands near 692 and 700 nm are sourced from the 5D0 f 7F4 transitions.26 It is well-known that the 5D0 f 7F0 transition of Eu3+ is strictly forbidden in a symmetric field. Hence, the presence of the band near 579 nm suggests that Eu3+ in PM-1 and in the complex is in low symmetry and does not possess an inversion center.28 Furthermore, the 5D0 f 7F0 transition displays a single band in PM-1 and the corresponding complexes, suggesting the existence of one local site symmetry for the chemical environment of the Eu3+ ion.29 It is noted that the intensity of 5D0 f 7F0 transition becomes stronger from PM-1 to SEP-1, indicating that the microenvironment of Eu3+ is influenced by the ambient organic components.11c As the 5 D0 f 7F1 transition is a magnetic dipole transition, its intensity hardly changes with the microenvironments of Eu3+, and based on the fact, we normalized the band intensity for all the emission spectra to examine the changes of other bands. On the other hand, the 5D0 f 7F2 transition is attributed to the electric dipole transition and is sensitive to the chemical surroundings of Eu3+ ions: The transition intensity increases with the decrease of Eu3+ symmetry. Therefore, the intensity ratio of the 5D0 f 7F2 to 5 D0 f 7F1 transition, referred as I(0f2)/I(0f1), could be used to evaluate the change of Eu3+ symmetry under different conditions. The increased intensity ratio corresponds to a decrease of Eu3+ symmetry.26d,29b,30 Of course, it should be kept in mind that this ratio is also influenced by other factors, such as the polarizability of the ligands, and so on.29b Therefore, under the same band intensity of the 5D0 f 7F1 transition, the stronger luminescence of SEP-1 amorphous powder, especially for the LC structure, than pure PM-1 implies that the organic microenvironments and the LC structures exhibit a strong influence on the symmetry of Eu3+. The encapsulation and mesomorphic state result in a less symmetry. The photophysical data for all the samples are summarized in Table 2. The value of I(0f2)/I(0f1) changes from 0.24 for PM-1 to 2.36 for SEP-1 amorphous powder, implying that the symmetric environment of Eu3+ becomes poorer from unrestricted clusters to the surfactant-encapsulated complex. Similar results have been reported in organic and polymer matrixes previously.11c,26d In LC structure of SEP-1, the intensity ratio increases to 6.51, much larger than the SEP-1 amorphous powder, suggesting the higher asymmetry of Eu3+ due to the anisotropy of LC structure. The other two PMs and corresponding SEPs exhibit different photophysical properties from PM-1 and SEP-1, respectively, due to the change of PMs. As displayed in the excitation spectra (Figure 10B) of PM-2 and SEP-2, the O f W LMCT transition, which is not observed in PM-2 at room temperature,17a,31 appears at 273 nm in SEP-2, while the transitions of the 4f6 shell become weakened when PM-2 has been covered by CDDA. Therefore, it can be inferred that the energy transfer from the O f W LMCT band to Eu3+ ion is more efficient, and the communication between the O f W LMCT band and the excited Eu3+ electronic level is drastically increased in SEP-2. In addition, the excitation property of SEP-2 is well kept in the LC structure. The excitation spectra of PM-3 and SEP-3 amorphous powder also show the characteristic transitions of Eu3+. Similar to PM2, we cannot observe the O f W LMCT transition of PM-3.32 The intensity of O f W LMCT transition (250-300 nm) is pretty low for SEP-3 amorphous powder (Figure 10C, black line). Interestingly, the enhanced O f W LMCT band at ca. 5

Figure 9. TEM images of SEPs quenched by liquid nitrogen under different temperatures: (A) 144 °C for SEP-1, (B) 130 °C for SEP-2, and (C) 140 and (D) 110 °C for SEP-3, cooled from the isotropic states.

complex composition, the three complexes should possess similar packing forms. To further confirm the structure of the LC states, we investigated the frozen samples by TEM directly. From the TEM images (Figure 9), well-defined layer structures of the three complexes with the estimated distance of ca. 2.8 ( 0.3 nm were obtained, in perfect accordance with the layer spacings estimated from XRD. These TEM results strongly support the assignment of lamellar LC phases of all the three complexes. Luminescent Properties of SEPs. As SEPs are structurally stable at the temperature below 200 °C and the mesogenic groups formed via the hydrogen-bonding dimer of benzoic acids among complexes reveal no quenching for the fluorescence, the luminescent property of SEPs, sourced from Eu3+ in PMs, should be well kept in the LC state. And the change of photophysical property of PMs at different aggregated states can be well examined because of the sensitivity of Eu3+ to the external environment. Upon quick freezing the samples in the LC phases in liquid nitrogen, vitrified mesophase solids were obtained and the luminescent properties of the LC structures were then examined.24 From the fluorescent spectra of solid PM-1 and SEP-1 (Figure 10A,D), we see that the luminescence of PM-1 is well retained in SEP-1 amorphous powder and LC structure. All narrow peaks in the excitation spectrum of PM-1 are corresponding to the characteristic transitions of 4f6 shell of Eu3+ ion: 382 nm (7F0-5G3), 394 nm (7F0-5L6), 416 nm (7F0-5D3), and 465 nm (7F0-5D2).25 Because of the change of surface environment of Eu3+ after the encapsulation, the strong excitation band at 313 nm for PM-1, which is assigned to the ligand-to-metal charge-transfer (O f W LMCT) transition, moves to ca. 272 nm for SEP-1.26c,d In addition, the relative excitation intensities of the characteristic transitions of the 4f6 shell become weak in the complex, also indicative of the effective intramolecular energy transfer from O f W LMCT band to Eu3+.11c,27 It is possible that the organic matrix limits the delocalization of the d1 electron, leading to a more effective communication from LMCT band of PM-1 to Eu3+.26 The emission spectra obtained by exciting the O f W LMCT band show the characteristic 5D0 f 7Fj (j ) 0, 1, 2, 3, 4)

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Yin et al.

Figure 10. Excitation of (A) PM-1 and SEP-1, (B) PM-2 and SEP-2, and (C) PM-3 and SEP-3 and emission spectra of (D) PM-1 and SEP-1, (E) PM-2 and SEP-2, and (F) PM-3 and SEP-3 in PM solids (blue solid line), SEP amorphous powders (black dashed line), and LC structures (red dotted line; pink dashed and dotted line). The intensities of the emission spectra were normalized based on the transition at 5D0 f 7F1. Inset in (D, E, F): photographs of SEPs at LC states under the irradiation of 245 nm light.

TABLE 2: Summary of LMCT Band Wavelength, Intensity Ratio of 5D0 f 7F2 to 5D0 f 7F1 Transition (I(0f2)/I(0f1)), Experimental 5D0 Lifetime τ, Decay Rate ktot, Calculated Radiative Rate kr, Nonradiative Rate knr, and Absolute Quantum Yield η samples

states

PM-1 SEP-1 SEP-1 PM-2 SEP-2 SEP-2 PM-3 SEP-3 SEP-3 SEP-3

solid amorphous powder LC structure (SmA) solid amorphous powder LC structure (SmA) solid amorphous powder LC structure (SmA) LC structure (SmC)

LMCT I(0f2)/ τ ktot kr knr η [nm] I(0f1) [ms] [ms-1] [ms-1] [ms-1] [%] 313 272 274 273 275 279 269 267

0.24 2.36 6.51 0.96 5.37 6.96 0.90 2.52 6.53 6.49

3.08 0.83 0.73 2.41 0.86 0.81 2.48 0.51 0.58 0.57

0.32 1.20 1.37 0.41 1.16 1.23 0.40 1.96 1.72 1.75

0.11 0.31 0.47 0.15 0.46 0.51 0.15 0.29 0.52 0.50

0.22 0.89 0.09 0.27 0.70 0.72 0.25 1.67 1.20 1.26

34.38 25.83 34.31 35.59 39.66 41.46 37.50 14.80 30.23 28.57

269 nm in the excitation spectrum of SEP-3 can be found at its two LC sates (Figure 10C, red and pink lines). The results also mean that the energy transfer from the O f W LMCT band to Eu3+ ion is more efficient in the LC structures of SEP-3. The other two PMs and two SEPs display quite similar emission spectral features to those of PM-1 and SEP-1, respectively, demonstrating that the symmetric environment of Eu3+ ion in SEP-2 and SEP-3 changes to a low state after the encapsulation and even lower in the LC structures. Noticeably, the intensity ratios of I(0f2)/I(0f1) for SEP-1, SEP-2, and SEP-3 in LC structures are larger than those corresponding amorphous SEPs, indicating that the anisotropic structure strongly affects the local environments of Eu3+ ions and increases the fluorescence intensity of LC structures. Well then, according to the above results, we can learn that the luminescence properties of PMs can be adjusted through the formation of the mesophases. To identify the agreement of the luminescent characteristic of SEPs in LC structures frozen by liquid nitrogen and that of SEPs keeping at the in situ temperature, we examined the luminescent spectra during heating and the results clearly reveal the consistent spectral features. The temperature-dependent fluorescent spectrum of SEP-1 is shown in Figure 11. When the

Figure 11. Temperature-dependent emission spectra of SEP-1.

sample changes from solid state to SmA phase, the luminescent intensity decreases gradually and continues falling off upon further heating, as predicted for general materials due to the quenching from nonradiative transition. The other two complexes exhibit the consistent process as SEP-1 (see Supporting Information). Because of the intensity invariability, the 5D0 f 7F1 transition can be taken as a reference for the calculation of luminescent quantum yield. Based on the intensity parameters of the emission spectrum, the total radiative rate of 5D0 can be calculated by eq 1, where A0f1 denotes Einstein’s coefficient of spontaneous emission for 5D0-7F1 transition, which is commonly referenced to 50 s-1 for solid samples;33 pω0fj and S0fj represent the energy and the integration intensity of the 5D0-7Fj transition, respectively.17a,31 All the 5D0 decay curves for the samples fit well to a single-exponential function, indicating only one local symmetry of Eu3+ existing in mesophases (see Supporting Information).29b From the fitted lifetimes (τ) listed in Table 2, the total decay rate of 5D0 (ktot) can be estimated according to eq 2.34 Finally, the absolute emission quantum yield η is determined by eq 3. All the deduced fluorescent data of Eu3+ in different environments (PM1, amorphous powder of SEPs, and LC structures of SEPs) are summarized in Table 2.

Europium-Substituted Polyoxometalate Hybrids

kr ) A0f1

ktot )

J. Phys. Chem. B, Vol. 113, No. 8, 2009 2363

pω0f1 4 S0fj S0f1 J)0 pω0fj

(1)

1 ) kr + knr τ

(2)

∑

η)

kr kr + knr

(3)

After the encapsulation to PMs, both the lifetime and quantum yield show a remarkable decrease. Among the existing states of SEPs, there is a shorter lifetime but higher quantum yield in the LC structures than in the corresponding SEP amorphous powder. Since we have confirmed that the chemical components of the SEP complexes are well maintained, and the only difference is derived from the change of aggregated structure, the present results clearly show the possible adjustment of photophysical properties of PMs through the formation of LC structures. Conclusions In conclusion, we report a kind of intrinsic luminescent LC hybrid materials. The stable and reversible thermotropic LC properties are induced by intermolecular hydrogenbonding interaction. The benzoic acid-terminated surfactant encapsulated luminescent PM complexes, SEP-1, SEP-2, and SEP-3, form supramolecular network structures and lamellar LC phases upon heating, through intermolecular hydrogen bonding. The designed ionic surfactant itself also forms SmA and SmC phases. SEP-1 and SEP-2 exhibit SmA phase, while SEP-3 exhibits both SmA and SmC phases. All the complexes exhibit luminescent properties in the mesophases. The photophysical properties in the amorphous powder and in the mesophase are quite different. The quantum yields of SEPs at LC structures are proved to be higher than the amorphous samples. We believe that our studies show a specific approach to prepare intrinsic luminescent LC hybrid material, and the photophysical properties and the quantum yield of SEPs could be effectively adjusted by the LC phases. Acknowledgment. We acknowledge the financial support from National Basic Research Program (2007CB808003), National Natural Science Foundation of China (20703019, 20731160002), PCSIRT of Ministry of Education of China (IRT0422), and Open Project of State Key Laboratory of Polymer Physics and Chemistry of CAS. We thank Dr. T. Liu from Lehigh University for his fruitful discussion, supported by 111 project (B06009). Supporting Information Available: FT-IR and TGA data of CDDA and SEPs, temperature-dependent fluorescence spectra of SEP-2 and SEP-3, and fluorescence spectra of SEPs. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Grell, M.; Bradley, D. D. C. AdV. Mater. 1999, 11, 895. (b) O’Neill, M.; Kelly, S. M. AdV. Mater. 2003, 15, 1135. (2) Adam, D.; Schuhmacher, P.; Simmerer, J.; Ha¨ussling, L.; Siemensmeyer, K.; Etzbach, K. H.; Ringsdorf, H.; Haarer, D. Nature (London) 1994, 371, 141. (3) Bacher, A.; Erdelen, C. H.; Haarer, D.; Paulus, W.; Schmidt, H. W. AdV. Mater. 1997, 9, 1031.

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