Reverse Lyotropic Liquid Crystals from Europium Nitrate and P123

Article pubs.acs.org/JPCB

Reverse Lyotropic Liquid Crystals from Europium Nitrate and P123 with Enhanced Luminescence Efficiency Sijing Yi, Qintang Li, Hongguo Liu, and Xiao Chen* Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China S Supporting Information *

ABSTRACT: Fabrication of lyotropic aggregates containing the lanthanide ions is becoming a preferable way to prepare novel functional materials. Here, the lyotropic liquid crystals (LLCs) of reverse hexagonal, reverse bicontinuous cubic, and lamellar phases have been constructed in sequence directly from the mixtures of Eu(NO3)3·6H2O and Pluronic P123 amphiphilc block copolymer with increasing the salt proportion. Their phase types and structural characteristics were analyzed using polarized optical microscopy (POM) and small-angle X-ray scattering (SAXS) measurements. The driving forces of reverse LLC phase formation were investigated using Fourier-transformed infrared spectroscopy (FTIR) and rheological measurements. The hydrated europium salt was found to act not only as a solvent here, but also as the bridge to form hydrogen bonding between coordinated water molecules and PEO blocks, which played a key role in the reverse LLCs formation. Compared to those in aqueous solutions and solid state, the enhanced luminescence quantum yields and prolonged excited state lifetimes were observed in two europium containing reverse mesophases. The luminescence quenching effect of lanthanide ions was efficiently suppressed, probably due to the substitution of coordinated water molecules by oxyethyl groups of P123 and ordered phase structures of LLCs, where the coordinated europium ions were confined and isolated by PEO blocks. The optimum luminescence performance was then found to exist in the reverse hexagonal phase. The obtained results on such lanthanideinduced reverse LLCs should be referable for designing new luminescent soft materials construction to expand their application fields.

1. INTRODUCTION

drawbacks in past decades, including using nonaqueous solvents,7,8 using dendrimeric ligands,9 and introducing lanthanide ions into channels of mesoporous silica templates.10 From such a consideration, the directly construction of amphiphilic aggregates containing lanthanide complexes could be also a good choice because the highly ordered aggregate structure could not only provide confined environment of the lanthanides to reduce the vibrational radiationless deactivation, but also bring potential shielding to the possible quenching effect.11 It has been reported that the lanthanide complexes solubilized inside the micelle core or the micelles and vesicles self-assembled from the amphiphilic lanthanide complexes

Functional materials containing lanthanide ions have attracted much interest owing to their excellent luminescence properties, such as sharp emission lines and long (millisecond-order) lifetimes, which play increasingly important roles in fields like biological fluorescent probes,1 medical radiology images,2,3 polymeric optical amplifiers, and luminescent chemical sensors or molecular thermometers. 4 However, most of these technological applications required rather high local concentrations, and the aggregation of light-emitting compounds was found to result in a strong quenching of emission efficiency, which in turn, severely hampers their wide applications. Moreover, the luminescence quenching effect (radiationless deactivation) caused by the vibrational interaction between the hydroxyl groups (water, solvent, and silanol groups) and the excited lanthanide ions is another problem to be solved.5,6 Much effort has therefore been devoted to overcoming these © 2014 American Chemical Society

Received: July 31, 2014 Revised: August 30, 2014 Published: September 12, 2014 11581

dx.doi.org/10.1021/jp507745s | J. Phys. Chem. B 2014, 118, 11581−11590

The Journal of Physical Chemistry B

Article

For this motivation and based on our experiences for phase behaviors of amphiphilic triblock copolymers,33−36 we report here a highly ordered luminescent LLC system prepared by the introduction of europium nitrate hexahydrate (denoted as Eu(III)) into the block copolymer Pluronic P123. The latter was chosen to act not only as a ligand provider, but also as a LLC structure directing agent because of its excellent selfassembly property.37 By detailed analysis on LLC structure parameters and luminescence properties of different mesophases, the reasonable mechanism for luminescence enhancement could be established. The obtained results should contribute a better understanding on the novel phase behavior of metal salt−P123 mixtures and extend the application areas of lanthanide-containing soft materials.

could effectively inhibit the quenching process by improving the compound solubility or local concentration. The luminescence efficiency could thus be enhanced and further applied to in vivo bioimaging, biological sensing, and diagnostics.12−20 As examples, the electrostatic micelles formed by negatively charged coordination compound Eu-L2EO4 (1,11bis(2,6-dicarboxypyridin-4-yloxy)-3,6,9-trioxaundecane) and positively charged block polyelectrolytes would bring a significant luminescence enhancement.12−14 By incorporating dysprosium complexes into the micelles of Tween-80, their luminescence quenching effects were reduced and an exceptionally high absolute quantum yield could be established.15 The vesicles fabricated from the amphiphilic Tb3+ complexes also showed a significant elevation of luminescent intensity upon adding the adenosine triphosphate.20 With tailorable and rich phase morphologies, the more concentrated lyotropic liquid crystals containing rare earth ions are also attracting more and more attention.21−29 The lanthanide-containing LLCs were early reported in 1998 by Gin and co-workers.21 They fabricated the reverse hexagonal mesophases containing Eu3+ and Ce3+ as precursors for the nanostructured materials and this metallo-mesogen exhibited intense emission bands. A cholesteric LLC containing europium ions was formed by Huang et al. to investigate the molecular interactions during the aggregate formation and phase transformation by the luminescence spectra of Eu3+.22 More LLC structures were constructed from Galyametdinov’s or Binnemans’s groups by using the rare-earth tris-dodecylsulphates, Ln(C12H25SO4)3 (Ln = Y, La, Ce−Lu, except for Pm), with ethylene glycol or water.23,24 They also prepared a magnetic mesophase with low viscosity using a lanthanum salt and the N,N-dimethyldodecylamine oxide in mixed water− decanol solvent.25 Kurth and Charl et al., however, obtained a functional LLC structure exhibiting the visible and reversible electrochromism by complexing a series of quaternary ammonium salt cationic surfactants with the europium containing polyoxometalate, Eu(SiW 9 Mo 2 O 39 ) 2 ] 13− and [EuP5W30O110]12−.26,27 Recently, Drummond and his coworkers observed strong luminescence from Eu3+ coordinated with phytanates28 and oleates29 groups in aqueous hexagonal and lamellar LLCs, which would have the potential applications for luminescence imaging. Without a large amount of water as solvent, only the metal aqua complexes could also induce LLC phases, where the more suitable environment might be provided to avoid solvent quenching. For example, Dag and Celik assembled the hexagonal phases using the nonionic surfactants (CnH2n+1(CH2CH2O)mOH, denoted as CnEOm) with transition metal aqua complexes.30 They also found that the LLC mesophases from hydrated lithium salt−Pluronic copolymer mixture could exhibit high ionic conductivity.31 In this way, Galyametdinov et al. investigated the hexagonal and lamellar LLCs based on hydrated lanthanide salts and CnEOm. They found that the luminescence intensity was increased appreciably in both oriented mesophases.32 However, to our best knowledge, there is no report on such lanthanides containing LLC construction by using the amphiphilic block copolymers. The mechanism for luminescence enhancement in LLC phases is also not clear and remains to be explored. Therefore, designing more stable and performable LLC systems with enhanced luminescence intensity and prolonged lifetime is still an attractive and challengeable task.

2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. Pluronic P123 (PEO20PPO70PEO20), with an average molecular weight of 5800, was purchased from Sigma-Aldrich and used as received. Eu(NO3)3·6H2O (99.9%) was obtained from Alfa Aesar. The samples of different molar ratios of Eu(III)/P123 (R) were prepared by mixing appropriate amount of hydrated europium nitrates in P123 of a given weight. These mixtures were homogenized in sealed vials by repeated oscillation and centrifugation through several heating and cooling cycles between melting temperature and room temperature. Then, they were equilibrated at 25 ± 0.1 °C for at least 3 weeks before further investigation. The aqueous solution samples were prepared by dissolving europium nitrate into distilled water at the same molar concentrations as those in Eu(III)-P123 systems. 2.2. Characterization. Small-Angle X-ray Scattering. The obtained LLC phases were characterized by a SAXSess MC2 high flux small-angle X-ray scattering instrument (Anton Paar, Austrian) with a Ni-filtered Cu Kα radiation (0.154 nm), operating at 40 kV and 50 mA. The distance between the sample and detector was 27.8 cm. A standard temperature control unit (Anton-Paar TCS 120) connected with SAXSess was used to control the temperature at 25 °C. Polarized Optical Microscopy. Photographs of samples with birefringence were taken by a Motic B2 polarizing optical microscope (POM) with a CCD camera (Panasonic Super Dynamic II WVCP460). All samples were first heated to their isotropic status and then cooled to observation temperature. Fourier Transformed Infrared Spectroscopy. FTIR spectra were recorded by an Alpha-T spectrometer (Bruker) with a resolution of 4 cm−1. The liquid crystalline samples were measured by coating them on dried KBr plates. Luminescence Spectroscopy. The luminescence spectra were measured on an Edinburgh Instruments FLS920 flurescence spectrometer equipped with a Xe lamp (450 W). The LLC or aqueous solution samples were put into a selfmade quartz cell of 1 mm optical path length. All luminescence spectra were measured under the same conditions with the excitation wavelength of 395 nm, the slit width of 5 nm for excitation and 0.2 nm for emission. The absolute luminescence quantum yield and lifetime were recorded on the same instrument with the absolute value errors of about 0.2% and 0.3 μs, respectively. The quantum yield being defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal was measured using an integrating sphere (150 mm diameter, BaSO4 coating) of Edinburgh Instruments at room 11582



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get the more detailed structure information, the SAXS measurements have been carried out with the results discussed as following. Reverse Hexagonal Phase (H2). At low R values ( Li+ (2−4)), which induces more EO units to interact with each Eu3+ ion. 3.2. Luminescence Properties of LLC Systems. Based on above-mentioned LLC phases, the luminescence spectra of Eu(III)−P123 systems were measured to explore the effect of the organized LLC structures on the luminescence efficiency. Figure 4 illustrates the emission spectra of Eu(NO3)3·6H2O with P123 excited at 395 nm in different mesophases and also those measured in aqueous solutions without P123 but at the same Eu(III) concentrations. Because of the different concentrations of Eu(III) in three LLC phases, the observed luminescence intensity could not reflect the real luminescence efficiency. Therefore, the emission spectra measured here were mainly used for comparison on peak position and relative intensity changes. For all the LLC samples, no change of major emission peak positions was detected. The relative intensities, however, are significantly affected by the aggregation structure of LLC systems. The emission bands observed at 580, 593, 618, 650, and 686 nm correspond to the characteristic Eu3+ metal centered transition bands 5D0 → 7FJ (J = 0−4).49 The peak at 618 nm could be noted to dominate the emission spectra in Eu(III)−P123 systems and the same situation was observed for Eu(III) in the solid state (Figure S2). As a 5D0 → 7F2 transition (i.e., hypersensitive transition), it follows the selection rules for electric quadrupole transition. Its intensity is closely related to the local environment of the central europium ions and increases upon the reduction of site symmetry of Eu3+.49 The peak at 593 nm, corresponding to the 5D0 → 7F1 transition, is a magnetic-dipole transition of Eu3+ with its intensity almost

Table 2. Characteristic Luminescence Parameters for Eu(III) in Different States phase Eu(NO3)3 (aq) Eu(NO3)3·6H2O(s) H2

V2 Lα

R

η

ϕ (%)

τ (μs)

0.75 1 1.5 1.85 2 3 4 5 6

0.6−1.5 4.3 4.9 5.2 5.0 5.0 5.1 4.6 4.9 5.0 4.9

4.0a 8.4 8.2 14.0 12.8 9.9 8.4 3.8 4.1 4.1 3.5

114−121b 44(τ1), 166(τ2) 287 312 312 307 306 250 244 242 226

a

Data from ref 53 at Eu(III) of 0.5−1.0 M. bData from ref 7 at Eu(III) of 0.18−1.0 M.

parameters for Eu(III) in three different states. It is noted that the monochromaticity of Eu(III) has been enhanced 3−8 times in LLC systems compared to those in aqueous solution (η = 0.6−1.5), which is close to the level measured in solid salt. Obviously, the europium ions prefer an asymmetrical coordination sphere in LLC system as in its solid state than in aqueous solution. The average value of η in Eu(III)−P123 systems (η = 5.0) is comparable to those obtained from the hexagonal phase by C12EO10/Eu(III)/H2O (η = 4.1 and 3.1 at C12EO10/Eu(III) molar ratio of 1:2 and 1:1) and much higher than that from the lamellar phase by C12EO4/Eu(III)/H2O (η = 2.3).32 The high monochromaticity of Eu(III) has been kept in all three Eu(III)−P123 LLC systems, indicating a better structure stability of europium site for these copolymer aggregates. The relative lower η values in aqueous solutions indicate a relatively higher symmetry of the coordination sphere than those in LLC systems. Although the η value at higher concentrations was elevated (for example, 1.5 at 1.0 M Eu(III)) due to the penetration of few nitrate ions into Eu3+, it was still lower than those in Eu(III)−P123 systems.52 However, the asymmetric coordination sphere of Eu3+ can be kept in LLC systems as that 11585



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Figure 5. Dynamic rheological curves for Eu(III)−P123 system in different mesophases at 25 °C: (A) H2 (R = 1); (B) V2 (R = 2); (C) Lα (R = 4).

transitions were relatively weak. At the same time, the high frequency vibration from coordinated and free water molecules quenched such emissions of excited europium ions via loss of energy to hydrogen vibrations. What factors influence the luminescence properties in LLC phases? To explain the luminescence change in various phase structures, three points should be considered: (i) the interaction between Eu(III) and PEO blocks; (ii) the isolation effect originated from the aggregate structure; (iii) the changed coordination sphere of the europium ion. The interaction between hydrated salts and copolymers was manifested from the viscoelasticity properties of LLC systems. Figure 5 shows the dynamic rheological profiles for three representative samples of H2, V2, and Lα phases studied by oscillatory experiments, all of which exhibited a shear-thinning phenomenon within the studied angular frequencies (ω). In the whole angular frequency range investigated, the elastic moduli (G″) in H2 and V2 phases remain higher than the viscous moduli (G′), exhibiting an elastic behavior. In Lα phase, however, G′ is larger than G″ at low frequency. Then G″ increases faster than G′ until two curves intersect at 10.5 rad/s. This phenomenon suggests a plastic behavior, which is in agreement with the conventional rheological properties of lamellar phases.55 According to the theory of cooperative flow developed by Bohlin, the relationship between the system microstructure and its rheological properties can be established. 56 In the cooperative region, the observed flow response, which is determined by the “coordination number” of the interactive flow units (z), can be characterized by the following equation:

in solid state, which could be further distorted by interaction with PEO blocks. Therefore, a significantly enhancement on the emission intensity of 5D0 → 7F2 electric-dipole transition is induced to result in a higher monochromaticity than that in solid state. From Figure 4 and Table 2, it can be also seen that the emission spectra of Eu(III) in different mesophases present the similar patterns and intensity ratios, indicating the much similar asymmetric coordination spheres. To further discuss the Eu3+ coordination environment in all three mesophases, the luminescence lifetime (τ) as another characteristic parameter can be used because it is also very sensitive to the first coordination sphere of europium ion.53 It is noted that the luminescence intensity decays monoexponentially in Eu(III)− P123 systems (see Figure S3) and only one symmetrical band for the 5D0 → 7F0 transition (580 nm) was observed in their emission spectra. Both observations are evidence for only one local site symmetry for Eu3+ in LLC systems.54 In addition, the decay time is appreciably longer in LLC phases (monoexponential decay, 226−212 μs) than that in aqueous solutions (monoexponential decay, 114−121 μs) or in solid sample (biexponential decay, τ1 = 44 μs and τ2 = 166 μs). The relatively short lifetime in solid state should be originated from the interaction between two neighboring Eu3+ sites due to the smaller intermetallic distances as confirmed by its biexponential decay curve. The τ values in Eu(III)−P123 systems obtained here are also longer than those from the hexagonal phase by C12EO10/Eu(III)/H2O and the lamellar phase by C12EO4/ Eu(III)/H2O, which suggests a lower rate of non-nonradiative deactivation in Eu(III)−P123 systems to be discussed in section 3.3. To characterize the luminescence enhancement more quantitatively, the absolute luminescence quantum yield (ϕ) was also calculated with the results listed in Table 2. This parameter is found to increase significantly in the reverse hexagonal and bicontinuous cubic phases, but with similar or even lower values in lamellar phase than those in aqueous solutions. The maximum ϕ value of 14% was measured at R = 1, which is about three or two times higher than those in aqueous solution or in solid salt. 3.3. Luminescence Mechanism in LLC phases. Due to no absorption of light by P123 under the excitation wavelength at 395 nm, the europium ions in Eu(III)−P123 system were considered to excite directly to 5L6 level and then decay to the ground multiplet with radiative and radiationless transitions.53 The effective radiative transition from 5D0 to 7F2 level corresponds to the most intense band at 618 nm, presenting the red luminescence of the LLC systems. The other radiative

|G*| =

G′2 + G″2 = Aω1/ z

(1)

where |G*| is the complex modulus and A is a parameter interpreted as the interaction strength between the flow units. Through the log−log plots of |G*| versus ω, the values of A and z can be obtained by the intercept and slope, as seen in Figure S4. The z values for H2, V2, and Lα phases are therefore obtained as 5.6, 4.2, and 2.0, which are approximately equal to those observed in the colloidal structures and consistent with the previous reports on these LLC structures.56,57 The values of A, which can be defined as the amplitude of cooperative interactions, are higher in H2 and V2 phases (4878 and 3287) than in Lα phase (603), suggesting stronger cohesive interactions in reverse LLC structures. Then, the nonradiative vibronic deactivation (including vibrational and rotational energy transitions) of Eu(III) could be reduced more to result in higher energy transfer efficiencies and extended luminescence lifetimes in H2 and V2 phases. The relatively weak 11586



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extended, and finally, the lamellar phase was formed. The accommodation ability of PEO blocks to Eu(III) will reach a saturated state. Instead of being effectively isolated by the copolymers, the excess salts might aggregate directly between the adjacent PEO blocks. Therefore, the reduced shielding effect results in the decrease of quantum yield and the lifetime. Besides the structure influences mentioned above, the inherent or the dominating factor on the luminescence property comes from the changed coordination spheres of Eu3+ in LLC systems. It has been discussed in section 3.2 that there exists a single europium site in all three LLC structures. Moreover, the high intensity ratio η indicates the actual coordination polyhedron in Eu(III)−P123 systems is closer to a dodecahedron or a bicapped trigonal prism than to a square antiprism, which are two sorts of ideal coordination polyhedral of Eu3+ that allow for the 5D0 → 7F2 electric-dipole transition.60 Apart from the symmetry of the coordination sphere, the water molecules bonded with europium ion will be an important factor to consider. However, the actual number of the coordinated water molecules of Eu3+ in the Eu(III)−P123 system cannot be obtained because of the changed phase behavior if using additional ordinary or heavy water. Nevertheless, the data from FTIR spectra will be helpful to analyze the coordination sphere of Eu(III). As well-known, the PEO blocks have ability to coordinate with metal salts,61 the coordination sphere of europium ion is therefore much probably changed due to such coordinations. Such a speculation could be further confirmed by the conformation state change of PEO or PPO blocks in LLCs. The pure PEO chain is reported to adopt a helix (H) conformation in crystalline phase.62 In the metal ion/block copolymer hybrids, however, an amorphous structure based on trans (T) conformation is more preferable.62,63 As examinated on the FTIR spectra obtained here, the peaks at 1150, 1060, and 962 cm−1 corresponding to the vibrational modes of crystalline PEO blocks (amplified in Figure S6) were found to disappear gradually and become broad in LLC systems. The reduced intensities of CH2 twisting (1294 cm−1) and rocking (844 cm−1) vibrations indicated also the conformation transformation from H to T. All these demonstrate the amorphous state of PEO blocks and also the interaction between Eu3+ and copolymers. As we know, the rate of deactivation via energy transfer to O−H oscillators in the first coordination sphere of Eu(III) is directly proportional to the number of water molecules coordinated with central ion.64 The increased luminescence lifetime and quantum yield of europium salt in H2 and V2 phases are therefore benefited from the exchange of coordinated nitrate ions or even water molecules by oxyethyl groups of P123. In Lα phase, however, the luminescence of Eu(III) is quenched to a certain level due to less coordinated PEO blocks. In other words, more water molecules will be coordinated in the first coordination sphere of Eu3+. The relatively weaker coordination ability of PEO blocks in Lα phase can be explained by the weaker interaction between them and Eu3+ due to hydration. It is also noted that the peak intensity for free nitrate ion at 1347 cm−1 in LLC system was decreased, while those for coordinated nitrate ion at 1297 and 1489 cm−1 were increased. These variations indicate the increased amount of coordination nitrate ions during the phase transition from H2 to Lα, suggesting a relatively weaker coordination ability of PEO blocks in Lα phase. Moreover, data from rheological measurements have also demonstrated the stronger interaction strength in H2 and V2 phases than that in

interactions in the Lα phase can be attributed to increased hydration of PEO blocks with additional salts. As is well-known, the Eu3+ in hexahydrated europium nitrates is bonded with four water molecules and three nitrate ions. The remaining two free water molecules will interact with PEO chains to reduce their capacity to coordinate with europium ions. Though the cohesive interaction in the Lα phase here was relatively weak, it is still speculated to be stronger than that in the lamellar phase by C12EO4/Eu(III)/H2O because of the obtained longer luminescence lifetime. As for the isolation effect of LLCs, it is because of their ordered structures that the concentration and solvent (especially with water) quenching on excited rare earth ions could be shielded to reduce the nonradiative deactivation.58 The enhanced quantum yield and longer lifetime than those in aqueous solutions and solid sample have indicated the existence of such shielding “walls” formed by the organized LLC structures, which will prevent the rare earth ions from the water molecules and from each other. It has been seen from section 3.1 that, at small R values, the reverse hexagonal and bicontinuous cubic phases are formed with the rod-like reverse micelles containing Eu(III) either arranged into a hexagonal pattern or interconnected into infinite channel networks. In these two phases, the hydrated europium ions interact mainly with the hydrophilic PEO moieties twisted in the micellar core, but few with the hydrophobic PPO blocks extending to the micellar peripheries. Such molecular arrangements are established partly from the fact that only an expansion of packed PEO block domains were observed with increasing salt amounts, which could be also further clarified by FTIR spectra for Eu(III)−P123 systems in different phases (Figure S5). As seen from Table 3 on FTIR Table 3. Assignments of FTIR Bands of P123 and LLC Samples in Different Mesophases P123

H2

V2

Lα

assignmenta

3503 1373 1280 1150 1110 1060

3467 1373 1299 1096 -

3458 1373 1298 1091 -

3435 1373 1298 1091 -

O−H stretch CH3 symmetric deformation CH2 twist C−O−C stretch, C−C stretch C−O−C stretch C−O−C stretch, CH2 rock

Based on refs 59 and 63. “-” denotes peaks disappeared in LLC systems.

a

vibrational band assignments, the CH2 twisting vibration at 1280 cm−1 shifts to higher frequencies, while the symmetric deformation band of methyl groups at 1373 cm−1 remains unchanged in LLC aggregates, suggesting interactions mainly occurred between PEO blocks and Eu(III).59 The hydrogenbond formation between the oxyethyl groups of P123 and coordinated water molecules around Eu3+ could be confirmed by the changes of O−H and C−O−C stretching vibrations at 3503 and 1110 cm−1 in pure P123, which are shifted to lower frequencies with wide peaks in LLC systems. Considering the relatively low R values, the europium ions can be entirely surrounded by curved PEO block chains to form a better shielding structure to effectively isolate themselves from each other. Therefore, the relatively higher luminescence quantum yield and longer lifetime are obtained in H2 and V2 phases. At higher R values, the increased interactions between EO units and Eu(III) would induce the PEO chains being more 11587



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(5) Bunzli, J.-C. G.; Piguet, C. Taking Advantage of Luminescent Lanthanide Ions. Chem. Soc. Rev. 2005, 34, 1048−1077. (6) Feng, J.; Zhang, H. J. Hybrid Materials Based on Lanthanide Organic Complexes: A Review. Chem. Soc. Rev. 2013, 42, 387−410. (7) Kropp, J. L.; Windsor, M. W. Luminescence and Energy Transfer in Solutions of Rare-Earth Complexes. I. Enhancement of Fluorescence by Deuterium Substitution. J. Phys. Chem. 1965, 42, 1599−1608. (8) Binnemans, K. Lanthanides and Actinides in Ionic Liquids. Chem. Rev. 2007, 107, 2592−2614. (9) Giansante, C.; Ceroni, P.; Balzani, V.; Vogtle, F. Self-Assembly of a Light-Harvesting Antenna Formed by a Dendrimer, a RuII Complex, and a NdIII Ion. Angew. Chem., Int. Ed. 2008, 47, 5422−5425. (10) Vicinelli, V.; Ceroni, P.; Maestri, M.; Balzani, V.; Gorka, M.; Vogtle, F. Luminescent Lanthanide Ions Hosted in a Fluorescent Polylysin Dendrimer. Antenna-Like Sensitization of Visible and NearInfrared Emission. J. Am. Chem. Soc. 2002, 124, 6461−6468. (11) Strassert, C. A.; Mauro, M.; Cola, L. D. Photophysics of Soft and Hard Molecular Assemblies Based on Luminescent Complexes. In Adv. Inorg. Chem.; Van-Eldik, R., Stochel, G., Eds.; Elsevier: Amsterdam, 2011; Vol. 63, pp 47−103. (12) Yang, L.; Ding, Y.; Yang, Y.; Yan, Y.; Huang, J. B.; Keizer, A. D.; Stuart, M. A. C. Fluorescence Enhancement by Microphase Separation-Induced Chain Extension of Eu3+ Coordination Polymers: Phenomenon and Analysis. Soft Matter 2011, 7, 2720−2724. (13) Xu, L. M.; Jing, Y. Y.; Feng, L. Z.; Xian, Z. Y.; Yan, Y.; Liu, Z.; Huang, J. B. The Advantage of Reversible Coordination Polymers in Producing Visible Light Sensitized Eu(III) Emissions Over EDTA via Excluding Water from the Coordination Sphere. Phys. Chem. Chem. Phys. 2013, 15, 16641−16647. (14) Xu, L. M.; Feng, L. Z.; Han, Y. C.; Jing, Y. Y.; Xian, Z. Y.; Liu, Z.; Huang, J. B.; Yan, Y. Supramolecular Self-Assembly Enhanced Europium(III) Luminescence under Visible Light. Soft Matter 2014, 10, 4686−4693. (15) Debroye, E.; Laurent, S.; Elst, L. V.; Muller, R. N.; Parac-Vogt, T. N. Dysprosium Complexes and Their Micelles as Potential Bimodal Agents for Magnetic Resonance and Optical Imaging. Chem.Eur. J. 2013, 19, 16019−16028. (16) Cantuel, M.; Lincheneau, C.; Buffeteau, T.; Jonusauskaite, L.; Gunnlaugsson, T.; Jonusauskas, G.; McClenaghan, N. D. Enhanced Photolabelling of Luminescent EuIII Centres with a Chelating Antenna in a Micellar Nanodomain. Chem. Commun. 2010, 46, 2486−2488. (17) Wang, J. Y.; Velders, A. H.; Gianolio, E.; Aime, S.; Vergeldt, F. J.; Van As, H.; Yan, Y.; Drechsler, M.; de Keizer, A.; Stuart, M. A. C.; et al. Controlled Mixing of Lanthanide(III) Ions in Coacervate Core Micelles. Chem. Commun. 2013, 49, 3736−3738. (18) Li, Y. Y.; Cheng, H.; Zhang, Z. G.; Wang, C.; Zhu, J. L.; Liang, Y.; Zhang, K. L.; Cheng, S. X.; Zhang, X. Z.; Zhuo, R. X. Cellular Internalization and In Vivo Tracking of Thermosensitive Luminescent Micelles Based on Luminescent Lanthanide Chelate. ACS Nano 2008, 2, 125−133. (19) Xu, Q. S.; Tang, J. G.; Wang, Y.; Liu, J. X.; Wang, X. Z.; Huang, Z.; Huang, L. J.; Wang, Y. X.; Shen, W. F.; Belfiore, L. A. Eu3+-Induced Aggregates of Diblock Copolymers and Their Photoluminescent Property. J. Colloid Interface Sci. 2013, 394, 630−638. (20) Liu, J.; Morikawa, M.; Kimizuka, N. Conversion of Molecular Information by Luminescent Nanointerface Self-Assembled from Amphiphilic Tb (III) Complexes. J. Am. Chem. Soc. 2011, 133, 17370−17374. (21) Deng, H.; Gin, D. L.; Smith, R. C. Polymerizable Lyotropic Liquid Crystals Containing Transition-Metal and Lanthanide Ions: Architectural Control and Introduction of New Properties into Nanostructured Polymers. J. Am. Chem. Soc. 1998, 120, 3522−3523. (22) Dai, Q. Z.; Huang, Y. Formation of Lyotropic Liquid Crystals and Molecular Interactions in Maleyl Ethyl Cellulose/Acetic Acid System. Polymer 1998, 39, 3405−3409. (23) Galyametdinov, Y. G.; Jervis, H. B.; Bruce, D. W.; Binnemans, K. Lyotropic Mesomorphism of Rare-Earth Trisalkylsulphates in the Water−Ethylene Glycol System. Liq. Cryst. 2001, 28, 1877−1879.

Lα phase. Therefore, the coordination with europium ions by PEO blocks is considerably weak and the quenching effect due to penetrated water molecules could not be fully avoided.

4. CONCLUSIONS In summary, the lyotropic aggregation behavior of the Eu(III)− P123 system has been studied, accompanying with the characterization on luminescence properties of lanthanides in such LLC mesophases. Three main LLC phases including the reverse hexagonal, the reverse bicontinuous cubic, and the lamellar could be formed at various Eu(III)/P123 molar ratios. The appearance of reverse phase structures can be attributed to the coordination with europium ions and also the amphiphilic block structure of P123. Both reverse Eu(III) containing LLC structures present high fluorescent monochromaticities with the enhanced quantum yields and prolonged lifetimes. The optimum luminescence efficiency could be achieved in H2 phase as a consequence of comparative stronger molecular interaction between PEO blocks and Eu(III) and, thus, induced better isolation effect and less coordination water. The obtained results here will not only supplement the aggregation behavior of triblock copolymers with high metal salt concentration, but also provide a stable and biocompatible dispersion medium for the lanthanides fluorescent material. However, further studies are still needed to overcome the inherent quenching effect possibly caused by the water molecules in the first coordination sphere, which will be our research focus in future work.

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ASSOCIATED CONTENT

S Supporting Information *

Details on the calculation method of LLC structure parameters are provided. The photos of samples observed under the crossed polarizer, the emission spectrum of Eu(III) in solid state, the luminescence decay curves of LLC systems, and results of the Fourier-transformed infrared spectroscopy and Raman spectroscopy are also included. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-531-88365420. Fax: +86-531-88564464. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for the financial support from the National Natural Science Foundation of China (20973104, 21033005, 21373127, and 21273133) and the Specialized Research Fund for the Doctoral Program of Higher Education (20130131110010).

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REFERENCES

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Reverse Lyotropic Liquid Crystals from Europium Nitrate and P123

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