Hydrogen-Bonding-Induced Supramolecular Liquid Crystals and Luminescent Properties

8/11/2019 Hydrogen-Bonding-Induced Supramolecular Liquid Crystals and Luminescent Properties

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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 electrostaticallycanned by a cationic surfactant, N-[12-(4-carboxylphenoxy)dodecyl]-N-dodecyl-N,N-dimethylammoniumbromide, through the replacement of counterions, and the resulting surfactant-encapsulated polyoxometalatecomplexes were characterized in detail by elemental analysis as well as IR and NMR spectra. The carboxylsbearing 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 ofdifferential scanning calorimetry, polarized optical microscopy, and X-ray diffraction revealed that thesecomplexes underwent smectic mesophases during the heating and cooling cycles. These complexes displayedintrinsic luminescence both in the amorphous powder states and in their mesophases. The photophysicalproperties showed the dependence on the existing states of samples, and the quantum yields of the complexesin the liquid crystalline structures are higher than the corresponding amorphous powders. The presentinvestigation provides an example for developing hydrogen-bonding-induced polyoxometalate-containing hybridliquid crystal materials with intrinsic luminescence.

Introduction

Liquid crystal (LC) materials with luminescent property areof considerable interest over recent years due to their potentialapplications in the fields of anisotropic light emitters,1 photo-conductors,2 LC display technology,3 and so forth. On one hand,LC structure can tune the luminescence and polarization due tothe ordered arrays of luminescent groups, while on the otherhand, the self-luminescence of LC materials can enrich theperformance of LC display remarkably. To develop luminescent

LC materials, researchers have exploited several methods suchas designing molecules with organic luminescent mesogenicgroups4 and doping a fluorescent dye into LC matrixes.5

Comparing with organic luminescent groups, inorganic onespossess higher luminescent stability and better color purity, andmetallomesogens (lanthanidomesogens, especially)6 and semi-conductor nanopaticles7 have been incorporated into the mes-ophases. However, the introduction of nanometer-sized inorganicmaterials into LC matrixes was less studied, and increasingmiscibility and stability of inorganic components and promotingthe quantum yield of the hybrid systems in the mesophases arestill challenges. Therefore, it becomes significant to develop newluminescent LC materials which exhibit low quenching, highmiscibility, and high stability.

Polyoxometalates (PMs) are a kind of nanoscale polyanionclusters possessing potential applications in electrochemistry,proton conduction, magnetism, and optics.8 Some lanthanide-substituted PMs, such as Eu3+, Tb3+, Sm3+, and Dy3+ deriva-tives, have attracted much attention owing to their characteristicluminescent properties.9 To utilize PMs in an organized way,various interesting methods have recently been reported,10 andamong them, the encapsulation of PMs with organic moleculesthrough electrostatic interaction has proven to be a highlyeffective route.10c,11 Through encapsulation, the luminescent

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 themesomorphic structures of these kinds of complexes as inte-grated building blocks by an appropriate selection of PMs andsurfactants with mesogenic groups.14 However, the fluorescenceof the complexes in LCs is usually quenched by the mesogengroups containing in the surfactants. To sustain the fluorescenceof PMs in LC states, the employed surfactant has to be modified.Aromatic acid derivatives with a long alkyl chain are known toshow mesomorphism because the dimerization of the carboxylicacids through hydrogen bonding plays a role of mesogengroup.15 As the benzoic acid dimer is not a real conjugatedgroup, it should not quench the luminescence of the inorganicPM core while it directs the formation of LC phases. Thus,combining the surfactant bearing benzoic acid group at the endof the alkyl chain and fluorescent PMs together, one can expectto bring a novel hybrid material which exhibits the luminescenceof PMs in the mesophase.

On the basis of this motivation, in this paper, we reported arepresentative investigation concerning PM-containing hybridsupramolecular LCs with intrinsic luminescence in the meso-phases. 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(PM-2), and K13Eu(SiW11O39)2 (PM-3),16 which possess differenttopologic structures, surface negative charges, and chemicalcompositions, were selected to be encapsulated, as schematicallyrepresented in Figure 1. The resulting surfactant-encapsulatedPM (SEP) complexes exhibit both the typical hydrogen-bondingLC characteristics and unique luminescence at LC states. Moresignificantly, the LC structures can be applied to adjust thephotophysical properties of PMs. As there are several carboxylicgroups surrounded on each SEP, the present research providesan example of hydrogen-bonding supramolecular network LChybrid materials with PMs.

* To whom correspondence should be addressed. E-mail: [email protected].

J. Phys. Chem. B 2009, 113,23552364 2355

10.1021/jp810262c CCC: $40.75 2009 American Chemical SocietyPublished on Web 02/04/2009


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Experimental Section

1. Materials.PM-1, PM-2, and PM-3 were freshly preparedaccording to the literature procedures.16,17 4-Hydroxybenzoicacid andp-toluenesulfonic acid were purchased from SinopharmChemical Reagent Co., Ltd. 1,12-Dibromododecane and N,N-dimethyldodecylamine were obtained from Fluka and Aldrich,respectively, and used as received. Other starting compoundsand solvents applied in the preparation were commercialproducts from local chemical reagent companies. Doublydistilled water was used in the experiments. Silica gel (100-200mesh)wasemployedforthepurificationovercolumnchromatography.

2. Synthesis of Benzoic Acid-Terminated Surfactant.Thesynthesis of the specified surfactant was carried out followingthe modified routes by referencing the literatures,14b,18 as shownin 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 150mL of anhydrous ethanol. The reaction mixture was stirred underrefluxing for 28 h and then cooled to room temperature. Afterthe evaporation of solvent under the reduced pressure, the crudeproduct was recrystallized from water and then washed withwater several times. The obtained white powder was dried under

vacuum, giving 10.7 g of 4-hydroxyethylbenzoate (yield: 89%).1H NMR (DMSO-d6,, 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-hy-droxyethylbenzoate (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 ofacetone was stirred under refluxing for 26 h. After cooling toroom temperature, the solvent was removed under the reducedpressure, the crude product was dissolved and extracted withthree portions of chloroform (50 mL), and then the organicphases were combined. After the evaporation of solvent, theresidue was further purified over column chromatography on

silica gel using dichloromethane/cyclohexane (1:1 in v/v) aseluent, giving 4.2 g of 4-(12-bromododecyloxyl)ethylbenzoate(yield: 70%). 1H NMR (CDCl3,, 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-dimeth-

ylammonium Bromide. Dodecyldimethylamine (0.6 g, 2.8mmol) and 4-(12-bromododecyloxyl)ethylbenzoate (1.5 g, 3.6mmol) with the initial molar ratio of 1:1.3 were dissolved in150 mL of acetone, and the reaction mixture was stirred underrefluxing for 72 h. After cooling to room temperature, themixture was concentrated to 3-5 mL by removing excesssolvent under reduced pressure. Then, 30 mL of cold diethylether was added dropwise to the residue, and the mixture was

kept at 0 C for 3 days. The formed white precipitate was filteredand washed with cold diethyl ether several times, giving 1.3 g

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

SCHEME 1: Synthetic Path of the BenzoicAcid-Terminated Surfactant

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of white pure product ofN-12-[4-(ethylbenzoate)dodecyloxyl]-N-dodecyl-N,N-dimethylammonium bromide (yield: 75%). 1HNMR (CDCl3, , 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-dim-

ethylammonium Bromide (CDDA). A mixture of N-12-(4-ethylbenzoate)dodecyloxyl-N-dodecyl-N,N-dimethylammoni-um 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 waterand methanol (3:22 in volume ratio). The reaction mixture wasstirred under refluxing for 12 h and then cooled to roomtemperature. After the solvent was removed, the crude productwas redissolved in chloroform, washed with dilute hydrochloricacid (pH ) 3) and pure water, and then dried over magnesiumsulfate. The pure white powder (CDDA) was obtained afterremoving solvent and further drying under vacuum until theweight kept constant. 1H NMR (DMSO-d6, , 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, , 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:) 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 com-posites of surfactant CDDA encapsulated PMs were preparedfollowing the procedure reported previously, as exemplified bySEP-1.11,14 PM-1 was dissolved in water, and to the aqueoussolution a chloroform solution of CDDA was added withstirring. The initial molar ratio of CDDA to PM-1 was controlledat 7:1. The organic phase was then separated and washed by

dilute hydrochloric acid (pH)

3.5). Then, the hybrid complexSEP-1 was obtained by evaporating the chloroform to dryness.The sample was further dried under vacuum until its weightremained constant. Following the similar procedures, other SEPswere prepared. All three complexes were characterized by IRspectrum, elemental analysis, and thermogravimetric analysis(TGA) as follows.

SEP-1.IR (KBr, cm-1) for SEP-1: )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 thermogravimetricanalysis (TGA), which arises from crystal water, the speculatedchemical formula should be (CDDA)8H(EuW10O36)(H2O)3(6772.1).

SEP-2.IR (KBr, cm-1) for SEP-2: )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 formulashould be (CDDA)9H2[Eu(PW11O39)2](H2O)5(10 267.9).

SEP-3.IR (KBr, cm-1) for SEP-3: )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%

occurs in the range of 30-150 C from TGA measurement,which arises from crystal water, the speculated chemical formulashould be (CDDA)11H2[Eu(SiW11O39)2](H2O)5(11 299.8).

4. Sample Preparation for Characterizations.The SEPswere treated on a heating stage over their highest endothermictransition temperature and then cooling down to the certaintemperature at which the LC phases formed. After holding thestate at mesophase isothermally for 10 min, the sample wasquenched in the liquid nitrogen for half an hour. The annealing

temperature was carefully selected to get characteristic LCphases. The formed thin films covered on the quartzes wereused for X-ray diffraction and fluorescence spectral measure-ments. Then, the thin films were removed from the quartz glass,floated on the water surface, and recovered using copper gridsfor transmission electron microscopic (TEM) observations.

5. Measurements. 1H NMR spectra were recorded on aBruker Avance 500 instrument using CDCl3, CD3OD, andDMSO-d6 as solvents. Elemental analysis (C, H, N) wasperformed on a Flash EA1112 from ThermoQuest Italia SPA.FT-IR spectra were carried out on a Bruker IFS66V equippedwith a DGTS detector with a resolution of 4 cm-1 from pressedKBr 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 massspectrometer. The phase behaviors were performed using apolarized optical microscope (POM) (Leica DMLP, Germany)equipped with a Mettler FP82HT hot stage and a Mettler FP90central processor. Differential scanning calorimetric (DSC)measurements were performed on a Netzsch DSC 204 withscanning rate of 10 C min-1. The samples that were heatedover 20 C higher than their melting temperatures were usedfor the DSC measurements.19b Variable-temperature X-raydiffraction (XRD) was carried out on a Philips PW 1700 X-raydiffractometer (using Cu KR1radiation of a wavelength of 1.54) with a TTK-HC temperature controller. Luminescencemeasurements were performed on a HORIBA Jobin Yvon FL3-

TCSPC fluorescence spectrophotometer. TEM observations werecarried out on a JEOL-2010 electron microscope operating at200 kV. All the measurements were operated at room temper-ature in ambient conditions.

Results and Discussion

Structural Characterization of CDDA and SEPs. In thispaper, we designed a double-chain ammonium surfactant,CDDA, in which one of the chains is terminated with benzoicacid group (see Scheme 1), and we employed the surfactant toencapsulate luminescent PMs, PM-1, PM-2, and PM-3. Thesynthetic 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 thecomplexes. The as-prepared complexes are no longer solublein water, but easily dissolve in mixed organic media such aschloroform/methanol, chloroform/ethanol, etc., suggesting thatthe surfaces of PMs have been effectively covered by CDDA.As demonstrated through TGA (shown in the SupportingInformation) and DSC in the following data, the complexes arethermally stable in air, even heated up to 200 C. Thespectroscopic measurements were used to examine the surfactantand the complexes. As a representative example, IR spectra ofCDDA and SEP-1 are shown in Figure 2. For pure CDDA, theabsorption band at 1702 cm-1, which is assigned to CdOstretching mode, and the weak double absorption bands at 2457

and 2579 cm-1 (normally called satellite bands), which representthe formation of the hydrogen bonding, indicate that the terminal

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carboxylic acids exist in the intermolecular cyclic dimer statedefinitely.20 The detailed assignments of other absorption bandsare summarized in the Supporting Information. For SEP-1, wecan also see the characteristic vibration bands that confirm thehydrogen-bonding dimer of carboxylic acids appearing at about2590 and 2453 cm-1 derived from OH stretching and at 1706cm-1 from carbonyl stretching.21 Other bands are apparentlyfrom the characteristic vibrations of surfactant and PM-1. Thus,one can expect that the complexes exist in a hydrogen-bondingsupramolecular network state through the intermolecular car-boxylic acid dimer between adjacent SEP-1 units because sucha combined dimer is difficult to occur in one SEP-1 unit due to

the mismatched orientation. It should be noted that besides thehydrogen-bonding dimer, it is possible that the neighboringcarboxylic groups of CDDAs on the same or adjacent PMsforming the traverse hydrogen bonding. However, in view ofthe unfavorable orientation of carboxylic groups and smallhydrogen-bonding angle, the traverse hydrogen bonding shouldbe quite weak. To identify the exact binding position of CDDAwith PMs, the complexes were characterized by 1H NMRspectra. As shown in Figure 3, in contrast to that of CDDAalone, the proton chemical shifts of CDDA in SEP-1 show thefollowing characteristics: (1) the proton peak ofN-methyl hasbroadened significantly and shifted to the high field by 0.46ppm, (2) the proton peak ofN-methylene becomes a consider-ably broadened halo and has shifted toward high field by 0.31

ppm, and (3) other peaks maintain at the same positions as thoseof pure CDDA. The peak broadening implies the strong

electrostatic interaction between CDDA and PM-1 cluster, whichrestricts the mobility of the ammonium headgroup.11a,22 Con-sidering the fact that the chemical shift and peak width of theproton signals are sensitive to the local physical and chemicalenvironment, the cationic ammonium headgroup of CDDA issuggested binding to the negative charged cluster electrostati-cally.

Supramolecular Mesomorphic Behavior of CDDA and

SEPs.The thermal properties of CDDA and the complexes wereinvestigated by DSC, POM, and XRD. The phase transitiontemperatures, enthalpies, and assignments of the phase transi-tions for all the samples are summarized in Table 1. DSC curves(Figure 4) display reversible phase transitions of CDDA andSEPs in the first cooling and second heating processes. Uponthe second heating run, CDDA exhibits an exothermic transitionat ca. 93 C, which may be associated with the further ordering

process for the low-temperature phase. On further heating, threeendothermic peaks at 113, 143, and 174 C, which can beattributed to the transitions of solid to LC phase, LC to LCphases, and LC phase to isotropic state, respectively, based onthe following POM data. In contrast to the heating process,during the first cooling run, CDDA exhibits an exothermictransition at 167 C, which can be attributed to the transitionof isotropic state to LC phase, and a halo from 125 to 90 Cwith the apex at 97 C, which can be assigned to the transitionof the LC phase to solid state, respectively. The phase transitionsof CDDA emerging in the cooling run, determined by DSCmeasurement, are not fully in accordance with those found inthe heating run, while the transition from a broken fan-shaped

texture to a focal conic fan-shaped texture under POM can beseen visually at ca. 135 C during the cooling run. This behavioroften appears in a smectic C phase.19 One possible reason isthat the needed energy to change is too small to be monitoredby DSC.19c With the temperature decreasing, a broad halo occursfrom 70 C to room temperature, indicating that the crystal-lization of CDDA seems to be a slow process. Comparing withthe phase transitions of CDDA, the thermal behavior of SEP-1displays two endothermic transitions at 137 and 160 C duringthe second heating run, in which the first one is attributed tothe change from solid to LC phase and the second shouldcorrespond to the phase transition from LC phase to isotropicstate, as supported by the following POM results. The firstcooling 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 thatfound in CDDA. In accordance with the case of heating run,

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 CDCl 3/CD3OD. Inset: local magnification of full spectra, where a marksthe protons of N-methyl (N+-CH3) and b marks the protons of

N-methylene (N+-CH2).

TABLE 1: Summary of Phase Transition Temperatures(C), Enthalpies (kJ/mol), and Assignments of PhaseTransitions for All Complexesa

second heating first cooling

samples transitions T(C) H(kJ/mol) T(C) H(kJ/mol)

CDDA S-SmC 113 4.02 97 3.74SmC-SmA 143 3.44 -b

SmA-Iso 174 7.61 167 7.44SEP-1 S-SmA 137 26.75 116 17.99

SmA-Iso 160 16.25 157 13.21SEP-2 S-SmA 117 49.71 94 4.36

SmA-Iso 155 26.88 147 7.44SEP-3 S-SmC 105 2.67 103 1.31

SmC-SmA 126 9.24 -b

SmA-Iso 153 2.21 143 1.43

a S, SmC, SmA, and Iso denote solid, smectic C, smectic A, andisotropic phase, respectively. b Transition of SmC-SmA is observedunder polarized optical microscopy, though it is not apparent inDSC thermogram.19b,c



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the peak at high temperature can be clearly assigned to thetransition of isotropic state to LC phase, and the halo is ascribedto the change of LC phase to solid state.

SEP-2 and SEP-3 also exhibit multipeaks during the heatingand cooling cycles. Two transitions for SEP-2, one from solidstate to LC phase at low temperature and the other from LCphase to isotropic phase at high temperature, emerge on bothsecond heating and first cooling runs. SEP-3 shows an exother-mic transition at ca. 91 C on the heating run, which may beassociated with the further ordering process as appeared in thecase of CDDA. In the following heating, three endothermicpeaks at 105, 126, and 153 C emerge, continuously, while onlytwo peaks appear at 103 and 143 C in the cooling process. Onthe basis of the results of the POM, the change at 105 C is thetransition of solid state to LC phase, 126 C indicates the changebetween the different LC phases, and 153 C is attributed tothe transition from LC phase to isotropic phase. In the coolingrun, the peak at 143 C can be clearly assigned to the transitionof isotropic state to LC phase, and the transition at 103 C isascribed to the change of LC phase to solid state. Similar tothe case of CDDA, the transition between different LC phasesdoes not appear in the cooling DSC curve, while it can be

observed under POM. The detailed assignments of the phasetransitions 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 anddecrease gradually from SEP-1 to SEP-3. We consider that themain reason is that the incorporation of the PMs makes thearrangement of CDDA molecules in LC structures changeconsecutively. On one hand, CDDAs bond to PMs throughelectrostatic interaction in the complexes, which makes theCDDAs anchored on the surface of PMs, leading to a moreordered and tight packing of alkyl chains along with the surfaceof PMs. On the other hand, the surface curvature and limitedarea 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. Asan example, PM-2 and PM-3 have different surface charges buthave the same geometric shape and volume; thus, the occupiedsurface area per alkyl chain in SEP-2 is larger than that in SEP-3(see Table S2 in the Supporting Information). This case yieldsthe favorable rearrangement of alkyl chains and higher transitiontemperature for SEP-2 than SEP-3. Although the occupied areaper alkyl chain in SEP-1 is almost the same as that of SEP-3,the phase transition temperature of the former is still higherthan the latter due to the influence of the PMs volumes.Therefore, the phase transitions of the SEPs should be dominatedby the various conditions. From Table S2 in the SupportingInformation, the alkyl chain density of SEP-3 is larger than that

of SEP-2. As discussed above, the surfactant CDDAs willrearrange to adapt the curvature of the PM-3 cluster. More

CDDAs covered on PM-3 generate a crowded array around thesurface of PM-3, leading to a more distorted packing than thatin SEP-2. This makes the stabilization of the layer structure ofSEP-3 become worse. Considering this point, it is reasonablethat 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 arepresentative example, at the LC state, the carbonyl stretchingvibrations of CDDA and SEP-1 appear at 1708 and 1710 cm-1,respectively, and the satellite double absorption bands emergeat 2400-2600 cm-1, obviously confirming that the benzoic acidgroups still exist in the state of hydrogen-bonding dimer.20,21

However, the shifting of carbonyl stretching bands to highwavenumber and the weakening and broadening of the satelliteabsorptions imply that the intensity of the hydrogen bondingbecomes weak at the LC states.20c

The LC behaviors of CDDA and SEPs are also identifiedthrough POM in detail. During the cooling run, CDDA exhibitsa typical focal conic fan-shaped texture at 160 C (Figure 6A)

and a broken fan-shaped texture at 130

C (Figure 6B), whichcan be attributed to smectic A (SmA) and smectic C (SmC)phase, respectively. The two LC phases observed in POMsupport the assignment for the thermotropic transitions of CDDAduring the cooling run. For SEP-1, only one LC phase has beenfound, and the focal conic fan-shaped texture (Figure 6C)suggests the formation of SmA phase. In the case of SEP-2, alike SmA phase (Figure 6D) was observed during the coolingrun. Although the intermediate phase transition has not beenfound in DSC curve during the cooling run, SEP-3 exhibits twodifferent textures in POM images, focal conic fan-shaped textureand broken fan-shaped texture, indicative of SmA and SmCphases (Figure 6E,F), respectively, which are in agreement withDSC results in the heating run. We also checked the esterized

derivative, the non-hydrolyzed precursor of CDDA by usingPOM, whereas we did not observe any double refractions in

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

Figure 5. Temperature-dependent IR spectra of CDDA at 160 C andSEP-1 at 150 C.



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the heating and cooling runs. Apparently, the LC properties ofSEPs source from the hydrogen-bonding dimers formed betweenCDDAs, but their characteristics are distinctly different fromthose of CDDA. And, SEPs, as a kind of integrated buildingblocks, represent new type of LC materials. Thus, the present

strategy provides an optimal route to functionalize LC materialswith PMs.

The LC behaviors of CDDA and all the complexes werefurther investigated by variable-temperature XRD. Unfortu-nately, we have not observed any strong XRD diffractions atthe temperatures of LC phases by directly heating all thesamples. A possible reason may be the dynamic nature of thenoncovalent interactions of the intermolecular carboxylic aciddimer directed by hydrogen bonding at the LC states.23 Toidentify the LC structures clearly, we fleetly froze the samplesat the temperatures just in their LC phases using liquid nitrogen,as described in the Experimental Section, and then performedXRD measurements. The XRD data support the assignment of

lamellar phases. As shown in Figure 7A, two equidistantdiffractions for SEP-1 emerge in the small-angle region, justcorresponding 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 diffractionsat small-angle region, suggests that the packing of the alkylchains is disordered. As presented in the IR spectrum of SEP-1at the temperature of LC state, CH2 symmetric and antisym-metric stretching vibrations appear at 2855 and 2927 cm-1

(Figure 5), respectively, indicating that the conformation of alkylchains of surfactants around PM-1 is disordered.14b At the LCphase, the mesogenic group composed of benzoic acid dimershould stand perpendicularly to the layer surface based on thesmectic phase assignment from POM results. Combining these

results and analysis, we suggest a schematic LC packing modelof SEP-1 as shown in Figure 8.

SEP-2 and SEP-3 exhibit similar lamellar structures as SEP-1. In the small-angle region, two equidistant diffractions forSEP-2 (Figure 7B) and one diffraction for SEP-3 at hightemperature phase and two equidistant diffractions at low-temperature phase emerge (Figure 7C), which can be assignedto the layered structures. Similar to SEP-1, both samples exhibita halo at wide-angle region, suggesting the disordered packingof alkyl chains. The calculated layer spacings from the XRDdata (Figure 7B,C) are 2.7 nm for SEP-2 and 2.9 and 2.6 nmfor SEP-3 under different LC states. Meanwhile, it is reasonablethat SEP-2 and SEP-3 exhibit a little bit larger layer spacings

than SEP-1 because more CDDA covered on PM-2 and PM-3induces the thicker layer of alkyl chains. Considering the similar

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 Cduring the cooling process (magnification: 400).

Figure 7. X-ray diffraction patterns of SEPs quenched in liquid nitrogen at the temperatures of (A) 144C 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.

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

state.



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complex composition, the three complexes should possesssimilar packing forms. To further confirm the structure of theLC states, we investigated the frozen samples by TEM directly.From the TEM images (Figure 9), well-defined layer structuresof the three complexes with the estimated distance of ca. 2.8 (0.3 nm were obtained, in perfect accordance with the layerspacings estimated from XRD. These TEM results stronglysupport the assignment of lamellar LC phases of all the threecomplexes.

Luminescent Properties of SEPs.As SEPs are structurally

stable at the temperature below 200 C and the mesogenicgroups formed via the hydrogen-bonding dimer of benzoic acidsamong complexes reveal no quenching for the fluorescence, theluminescent property of SEPs, sourced from Eu3+ in PMs,should be well kept in the LC state. And the change ofphotophysical property of PMs at different aggregated statescan be well examined because of the sensitivity of Eu3+ to theexternal environment. Upon quick freezing the samples in theLC phases in liquid nitrogen, vitrified mesophase solids wereobtained and the luminescent properties of the LC structureswere then examined.24 From the fluorescent spectra of solidPM-1 and SEP-1 (Figure 10A,D), we see that the luminescenceof PM-1 is well retained in SEP-1 amorphous powder and LCstructure. All narrow peaks in the excitation spectrum of PM-1are corresponding to the characteristic transitions of 4f6 shellof Eu3+ ion: 382 nm (7F0-5G3), 394 nm (7F0-5L6), 416 nm(7F0-5D3), and 465 nm (7F0-5D2).25 Because of the change ofsurface environment of Eu3+ after the encapsulation, the strongexcitation band at 313 nm for PM-1, which is assigned to theligand-to-metal charge-transfer (O f W LMCT) transition,moves to ca. 272 nm for SEP-1.26c,d In addition, the relativeexcitation intensities of the characteristic transitions of the 4f6

shell become weak in the complex, also indicative of theeffective intramolecular energy transfer from O fW LMCTband to Eu3+.11c,27 It is possible that the organic matrix limitsthe delocalization of the d1 electron, leading to a more effectivecommunication from LMCT band of PM-1 to Eu3+.26

The emission spectra obtained by exciting the O f W LMCTband show the characteristic 5D0 f 7Fj (j ) 0, 1, 2, 3, 4)

transitions of Eu3+. The band near 579 nm is attributed to the5D0 f 7F0 transition; the bands near 589 and 594 nm areassigned to the 5D0 f 7F1transitions; the bands near 612 and618 nm are ascribed to the 5D0 f 7F2transitions; the band near652 nm is derived from the 5D0 f 7F3transition; and the bandsnear 692 and 700 nm are sourced from the 5D0 f 7F4transitions.26 It is well-known that the 5D0 f 7F0transition ofEu3+ is strictly forbidden in a symmetric field. Hence, thepresence of the band near 579 nm suggests that Eu3+ in PM-1

and in the complex is in low symmetry and does not possessan inversion center.28 Furthermore, the 5D0 f 7F0 transitiondisplays a single band in PM-1 and the corresponding com-plexes, suggesting the existence of one local site symmetry forthe chemical environment of the Eu3+ ion.29 It is noted that theintensity of 5D0 f 7F0transition becomes stronger from PM-1to SEP-1, indicating that the microenvironment of Eu3+ isinfluenced by the ambient organic components.11c As the5D0 f 7F1transition is a magnetic dipole transition, its intensityhardly changes with the microenvironments of Eu3+, and basedon the fact, we normalized the band intensity for all the emissionspectra to examine the changes of other bands. On the otherhand, the 5D0 f 7F2transition 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 to5D0 f 7F1transition, referred as I(0f2)/I(0f1), could be used toevaluate the change of Eu3+ symmetry under different condi-tions. The increased intensity ratio corresponds to a decreaseof Eu3+ symmetry.26d,29b,30 Of course, it should be kept in mindthat this ratio is also influenced by other factors, such as thepolarizability of the ligands, and so on.29b Therefore, under thesame band intensity of the 5D0 f 7F1 transition, the strongerluminescence of SEP-1 amorphous powder, especially for theLC structure, than pure PM-1 implies that the organic microen-vironments and the LC structures exhibit a strong influence onthe symmetry of Eu3+. The encapsulation and mesomorphic stateresult in a less symmetry. The photophysical data for all thesamples are summarized in Table 2.

The value ofI(0f2)/I(0f1)changes from 0.24 for PM-1 to 2.36for SEP-1 amorphous powder, implying that the symmetricenvironment of Eu3+ becomes poorer from unrestricted clustersto the surfactant-encapsulated complex. Similar results have beenreported in organic and polymer matrixes previously.11c,26d InLC structure of SEP-1, the intensity ratio increases to 6.51, muchlarger than the SEP-1 amorphous powder, suggesting the higherasymmetry of Eu3+ due to the anisotropy of LC structure.

The other two PMs and corresponding SEPs exhibit differentphotophysical 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 appearsat 273 nm in SEP-2, while the transitions of the 4f6 shell becomeweakened when PM-2 has been covered by CDDA. Therefore,it can be inferred that the energy transfer from the O f WLMCT band to Eu3+ ion is more efficient, and the communica-tion 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 powderalso show the characteristic transitions of Eu3+. Similar to PM-2, 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, blackline). Interestingly, the enhanced O f W LMCT band at ca.

Figure 9. TEM images of SEPs quenched by liquid nitrogen underdifferent temperatures: (A) 144 C for SEP-1, (B) 130 C for SEP-2,

and (C) 140 and (D) 110C for SEP-3, cooled from the isotropic states.



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After the encapsulation to PMs, both the lifetime and quantumyield show a remarkable decrease. Among the existing states ofSEPs, there is a shorter lifetime but higher quantum yield in theLC structures than in the corresponding SEP amorphous powder.Since we have confirmed that the chemical components of the SEPcomplexes are well maintained, and the only difference is derivedfrom the change of aggregated structure, the present results clearlyshow the possible adjustment of photophysical properties of PMsthrough the formation of LC structures.

ConclusionsIn conclusion, we report a kind of intrinsic luminescent

LC hybrid materials. The stable and reversible thermotropicLC properties are induced by intermolecular hydrogen-bonding interaction. The benzoic acid-terminated surfactantencapsulated luminescent PM complexes, SEP-1, SEP-2, andSEP-3, form supramolecular network structures and lamellarLC phases upon heating, through intermolecular hydrogenbonding. The designed ionic surfactant itself also forms SmAand SmC phases. SEP-1 and SEP-2 exhibit SmA phase, whileSEP-3 exhibits both SmA and SmC phases. All the complexesexhibit luminescent properties in the mesophases. Thephotophysical properties in the amorphous powder and in

the mesophase are quite different. The quantum yields ofSEPs at LC structures are proved to be higher than theamorphous samples. We believe that our studies show aspecific approach to prepare intrinsic luminescent LC hybridmaterial, and the photophysical properties and the quantumyield of SEPs could be effectively adjusted by the LC phases.

Acknowledgment.We acknowledge the financial supportfrom 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 ofPolymer 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 dataof CDDA and SEPs, temperature-dependent fluorescence spectraof SEP-2 and SEP-3, and fluorescence spectra of SEPs. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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