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“Rigid” Luminescent Soft Materials: Europium-Containing Lyotropic Liquid Crystals Based on Polyoxyethylene Phytosterols and Ionic Liquids Sijing Yi, Jiao Wang, Zhenyu Feng, and Xiao Chen J. Phys. Chem. B, Just Accepted Manuscript • Publication Date (Web): 06 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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The Journal of Physical Chemistry

“Rigid” Luminescent Soft Materials: Europium-Containing Lyotropic Liquid Crystals Based on Polyoxyethylene Phytosterols and Ionic Liquids Sijing Yi,

Jiao Wang, Zhenyu Feng,

Xiao Chen*

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China

*Corresponding author: Xiao Chen Address: Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan, 250100, China E–mail: [email protected]. Tel.: +86–531–88365420. Fax: +86–531–88564464.

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Abstract Soft materials of europium β-diketonate complexes constructed in lyotropic liquid crystals (LLCs) mediated by ionic liquids (ILs) are impressive for their excellent luminescence performance and stability. For the aim to further improve their mechanical processability and luminescent tunablility, the polyoxyethylene phytosterols (BPS-n) were introduced here as structure directing agents to prepare

relatively

“rigid”

lamellar

luminescent

LLCs

in

1-butyl-3-methyl-imidazolium

hexafluorophosphate by doping europium β-diketonate complexes with different imidazolium counter ions. As a result of the solvophobic sterol ring structure of BPS-n, the more effective isolation and confinement effects of europium complexes could be achieved. The longest fluorescence lifetime and the highest quantum efficiency reported so far for europium containing lyotropic organized soft materials were thus obtained. Changing the molecular structures of BPS-n with different oxyethylene chains or doped complexes with imidazolium counter ions of different alkyl chain lengths, the spacings of lamellar LLC matrices and position of dispersed complexes became tunable. The measured luminescent and rheological properties for such composite LLCs showed a dependence on the rigidity and isolation capability afforded by sterol molecules. It was also found that the increase of counter ion alkyl chain length would weaken LLC matrix’s confinement and isolation effects and therefore exhibit the deteriorated luminescence performance. The enhanced luminescence efficiency and stability of doped BPS-n LLCs reflected the excellent segregation of europium complexes from each other and therefore the reduced self-quenching process. The obtained results here present the designability of LLC matrices and their great potential to promote achieving the luminescence tunability of soft materials.

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1. Introduction Luminescent materials based on lanthanide β-diketonate complexes have been an attractive emerging research field due to their charming luminescence performance like sharp emission lines, long lifetime, and high quantum yield. They have been considered as an indispensable part for organic light emitting diodes (OLEDs),1,2 luminescent solar concentrators,3-5 and electroluminescent devices.6,7 However, their sensitive quenching effect to concentration and poor stability under UV-irradiation have seriously hindered their practical applications. To overcome these defects, the strategies through incorporating the complexes into matrices like zeolite, gel, polymer and silica materials have been widely adopted.8-11 The introduction of lanthanide complexes could be achieved by either covalently grafting

10,11

or non-covalently dispersing

12-14

them into matrices.

Compared to the complicated synthesis processes usually required for covalent strategy, the non-covalent method could be realized more conveniently and environment-friendly. However, such introduced lanthanide complexes were liable to be clustered due to the lack of specific sites to capture them and thereby to generate self-quenching. Therefore, the non-covalent dispersion strategy still faces the challenge to get effective site-specific separation of complex. Encouragingly, the studies have shown that the soft matrices like lyotropic liquid crystals (LLCs) could not only avoid the aggregation quenching of lanthanide complexes through homogeneously dispersing, but also improve the stability and energy transfer efficiency of antenna effect through effective confinement.15-18 For instance, Drummond et al. have found that the lanthanide-containing LLCs assembled by lanthanide phytanates and oleates exhibited an effective shielding effect, and the fluorescence quenching could be thus inhibited when they were dispersed in aqueous system.15,16 Researches from Galyametdinov et al. and our group have also shown that the ordered 3



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LLC structures were much suitable for dispersing and isolating the doped europium complex.17,18 In addition, compared to the solid matrices, LLCs exhibit attractive features owing to their rich and tailorable phase morphologies, which make it possible to realize the tunable luminescence of lanthanide materials.19 For examples, Huang et al. have fabricated luminescent LLCs through maleyl ethyl cellulose and acetic acid with the europium ions as a fluorescence probe.20 They found that the phase transformation and the molecular interaction variation within LLCs could be reflected by the spectral change of Eu3+. It has also been demonstrated by Faul et al. that the luminescence property of LLCs with europium-containing polyoxometalate inside could be affected by the LLC phase transition.21 Our previous studies have confirmed that the LLC phase structures could influence the antenna effect of doped complex, which might induce luminescence efficiency changes.22 Nevertheless, the real progress for such lanthanide-based soft materials has been reached with the help of ionic liquids (ILs).23 It has been reported that the considerable improvements of luminescence property and stability of lanthanide materials could be achieved from the excellent solvent properties of ILs (like the non-volatile/nonflammable, high thermal stability and no spectral interference for lanthanide),24 as well as the potential stablizing effect by hydrogen bond formation occurred in the complex containing imidazolium ILs.25 Meanwhile, ILs have been demonstrated as ideal self-assembling promotion solvents for LLCs and lyotropic aggregates, which usually exhibit better thermal stabilities than those constructed with water or other organic solvents.26 Among them, the ordered aggregates in imidazolium-based ILs could be realized by nonionic surfactants containing polyoxyethylene (PEO) chain, such as Pluronics (copolymers with ethylene and propylene oxide blocks) and Brij surfactants.27,28 The improvements of both luminescence efficiency and stability could be thus obtained through doping such LLC matrices with europium 4


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β-diketonate complexes.29 Recently, Abe, et al.30,31 and our group 32,33 have found that the polyoxyethylene phytosterol surfactants (BPS-n, n is the length of oxyethylene chain) could provide more effective isolation between hydrophilic and hydrophobic domains due to their rigid sterol ring structures, and thus promote the formation of highly organized liquid crystalline aggregates. In addition, the relatively low toxicity and high degree of “rigidity” of BPS-n also made it possible for the construction of LLC matrix with good biocompatibility and more stable structure. For this motivation, two LLC matrices with different lamellar spacings have been constructed using BPS-n (n = 5 and 10; Figure 1a) and an IL, 1-butyl-3-methyl-imidazolium hexafluorophosphate ([Bmim]PF6). After introducing the synthesized IL-based europium β-diketonate complexes with imidazolium counter ions of different alkyl chain lengths, 1-alkyl-3-methyl-imidazolium tetrakis-(thenoyltrifluoroacetonato) europate (alkyl = butyl, hexyl and octyl) (abbreviated as Bmim-Eu, Hmim-Eu and Omim-Eu; Figure 1b), the luminescence efficiencies of such soft hybrid materials have been systematically investigated. The selection of europium ions as central ions here is not only due to their better energy matching with the β-diketonate ligand, but also for their excellent luminescence properties, such as strong fluorescence intensity and good monochromaticity. Their sensitive coordination structures to the micro-chemical environment will be helpful to reveal the underlying enhancement mechanism in such LLC matrices, which should shed light for design novel soft luminescent materials with tunable capabilities.

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Figure 1. Molecular structures of BPS-5 or BPS-10 (a) and europium complexes with imidazolium counter ion of different alkyl chain lengths (b).

2. Experimental Section Materials Eu(NO3)3·6H2O (99.9%) and 2-thenoyltrifluoroacetone (99%) were purchased from Alfa Aesar and used as received. Polyoxyethylene phytosterols BPS-n (n = 5 and 10) were supplied as gifts by Nikko Chemicals Shanghai Corporation. Room-temperature ionic liquids, [Bmim]PF6, and 1-hexyl or 1-octyl-3-methyl-imidazolium hexafluorophosphates ([Hmim]PF6 and [Omim]PF6) (≥ 99.0%), were purchased from Center for Green Chemistry and Catalysis, LICP, CAS. They were both dried in a vacuum oven at 50 °C for 48 h before use. Synthesis of Europium β–Diketonate Complexes The europium complexes, Bmim-Eu, Hmim-Eu and Omim-Eu were prepared according to the procedures previously reported.29 First, an ethanol solution of 2-thenoyltrifluoroacetone (6 mmol, 4 eq.) was neutralized to pH = 7 by 1 M NaOH aqueous solution. Then, 1.5 mmol of ionic liquid (1.5 eq.) dissolved in 9 mL of ethanol was added and the mixture was stirred and heated to 50 ° C for half an hour. After that, 10 mL of europium nitrate (1.0 mmol, 1 eq.) aqueous solution was added dropwise at 50 °C and stirred for 2 h. After cooling the reaction solution overnight, the resulting precipitate was filtered and washed with ice water. Finally, the product was recrystallized from ethanol. The resulting pale yellow product was dried in a vacuum oven at 50 °C for 48 h and stored in the dry and dark place. 6


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Elemental analysis calculated for Bmim-Eu (C40H35N2F12O8S4·Eu, %): C, 40.72; H, 2.99; N, 2.37; S, 10.85. Found (%): C, 39.15; H, 2.58; N, 2.20; S, 10.86. Elemental analysis calculated for Hmim-Eu (C42H39N2F12O8S4·Eu, %): C, 41.76; H, 3.25; N, 2.32; S, 10.62. Found (%): C, 41.56; H, 2.84; N, 2.25; S, 10.81. Elemental analysis calculated for Omim-Eu (C44H43N2F12O8S4·Eu, %): C, 42.76; H, 3.54; N, 2.27; S, 10.38. Found (%): C, 42.93; H, 3.20; N, 2.25; S, 10.58. Sample Preparation To obtain the same concentration of europium complexes in the samples, the LLC matrices should keep the same volumes. Due to different polyoxyethylene chain lengths, however, the densities of BPS-5 and BPS-10 are different. Therefore, LLC samples with a certain weight percentage were firstly prepared by mixing the precise amounts of BPS-n and [Bmim]PF6. After repeated centrifugation and shaking, samples were mixed well and 1 mL of them was taken after heating by an injector. Then, 5 µmol europium complex was dispersed in it and equilibrated at 25 ± 0.1 °C for two weeks. For simplicity, various LLC matrices doped with different europium complexes were denoted as BPS-n-X (n = 5 and 10, X = B, H and O). Take BPS-5-B as an example, which represented the LLC matrix constructed by BPS-5/[Bmim]PF6 doped with Bmim-Eu complex. Small–Angle X–ray Scattering (SAXS). The specific phase type and structure parameter of LLC samples were determined by SAXSess MC2 high flux small-angle X-ray scattering instrument (Anton Paar, Austria). The wavelength of X-ray produced by a Ni-filtered Cu Kα radiation source was 1.542 Å. The X-ray tube voltage was 40 kV and a tube current of 50 mA was used. The working distance of the sample to the image board was 264.5 mm, and the scanning time was 5 min. Fluorescence Spectroscopy 7



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The excitation and emission spectra of LLC samples were recorded on a Hitachi F-7000 spectrofluorometer (xenon lamp, 150 W). To reduce the stray radiation in the spectra, a filter with a cutoff wavelength of 390 nm was used. The absolute luminescence quantum yield and lifetime were recorded on the Edinburgh Instruments FLS920. The absolute quantum yield was monitored through an integrating sphere coated with BaSO4. The luminescence lifetime was measured by using a µF920 microsecond flash lamp as excitation source and monitoring the luminescence intensity decay with time at 5D0→7F2 transition of europium. The absolute value errors of the quantum yield and lifetime were about 0.2% and 0.3 µs, respectively. The photostability tests were conducted under a UV–lamp (6 W) with the excitation wavelength at 365 nm. Fourier Transformed Infrared Spectroscopy (FTIR) FTIR spectra were recorded on an Alpha–T spectrometer (Bruker) with a resolution of 2 cm–1. For the measurement, an appropriate amount of sample was directly coated on a dried CaF2 crystal by a capillary. Rheological Measurement The rheological measurements were carried out with a Haake Rheostress 6000 rheometer equipped with a cone-plate (diameter 25 mm with cone angle of 0.1 rad) geometry. Dynamic frequency sweep measurements were performed in the linear viscoelastic regime, as determined by dynamic strain sweep measurements.

3. Results and Discussion 3.1 Structural Characters of BPS-n-X LLC Materials Based on the phase behaviors for BPS-n with different EO chain lengths, the lamellar (Lα) or hexagonal (H1) phases could be formed by BPS-n when with shorter (n = 5, 10) or longer (n = 20, 8


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30) EO chains.32 As matrices for europium complex doping, however, the Lα structure exhibited better luminescence efficiency and stability than those in H1 phase.29 For this reason, a BPS-n concentration (weight percentage) of 75 wt% was selected here to construct Lα phase in [Bmim]PF6 for both BPS-5 and BPS-10. Their phase structures and luminescence properties when doped with IL-based europium complexes were mainly studied. Different from LLCs prepared by conventional surfactants, the Lα phases constructed by BPS-n molecules presented effective segregation tendency between the solvophilic and solvophobic parts due to the existence of rigid sterol rings.30-33 The solvent domain could be thus effectively isolated due to the enhanced organization degree of aggregate structure. As shown in Figure 2 on SAXS patterns for LLC samples of two sterol surfactants, four scattering peaks could be observed with their q-value (scattering factor) ratios being obviously 1: 2: 3: 4. The relative multi-level and well-resolved scattering peaks here were characteristic for highly ordered Lα phases compared to that in LLCs constructed by Pluronics and Brij surfactants,27,28 which also demonstrated the maintenance of LLC structures after doping europium complexes.

Figure 2. SAXS patterns for LLC materials with or without europium complexes in BPS-5 (a) and BPS-10 (b) systems. The insets were enlarged views of first peak positions.

Based on Bragg equation, d = 2π/ q1, the repeat spacing of lamellar phase (d) could be calculated 9



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from the first scattering peak position (q1).34 As shown in Table 1 for d values of two LLC samples, the increase of EO chain length from BPS-5 to BPS-10 expanded the Lα structure for about 1 nm. Such expansions also occurred in europium complex-doped samples, suggesting the possible intercalation of europium complexes into the lamellar structure. The increased alkyl chain length in imidazolium counter ion of europium complex would also enlarge the LLC lamellar spacing. One intuitive reason came from the increased molecular volume of europium complex with longer alkyl chains. On the other hand, the longer alkyl chain might decrease molecular polarity of europium complex and thus weaken their interaction with EO chains of BPS-n and solvent, which made it possible to insert more complexes into solvophobic layer and thereby resulting an increase of the layer spacing.35 Such structure variations would affect the constraint abilities of LLC matrices for doped europium complexes, leading to different luminescence efficiencies as discussed in the following section. Table 1. Lamellar Repeat Spacings for Europium Complex-Doped (Y) or Undoped (N) LLCs Lamellar Repeat Spacing (d)/nm LLCs N

Bmim-Eu (Y)

Hmim-Eu (Y)

Omim-Eu (Y)

BPS-5

6.74

6.99

7.05

7.10

BPS-10

7.77

7.99

8.13

8.19

3.2 Luminescence Properties of BPS-n-X LLCs 3.2.1 Effect of Alkyl Chain Length in Imidazolium Counter ion The self-quenching process occurred between neighboring europium complexes could cause the nonradiative dissipation of excited state. The effective isolation and confinement of europium complexes inside LLC matrices are thereby crucial to improve their luminescence efficiency.22,29 As 10


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indicated above, the LLC matrices constructed by BPS-n could exhibit excellent separation effect between solvophilic and solvophobic region, and the increased alkyl chain length in the doped complexes might also influence the molecular interactions inside. Therefore, we compared the luminescence properties of LLCs doped with three different europium complexes here. As a comparison, the pure europium complexes were firstly explored. As shown in Figure 3a for the excitation spectra of three pure europium complexes, both the broad peaks at about 390 nm due to the energy transfer from ligands (TTA) to central metal ions, and the two relatively weak peaks at 450-550 nm corresponding to intra-4f transitions of Eu3+ could be observed.36 It was noteworthy that the maximum excitation peaks were broadened and gradually red-shifted from 379 nm (Bmim-Eu) to 384 nm (Hmim-Eu) and 389 nm (Omim-Eu) with the growth of alkyl chain length in counter ion, reflecting the reduction of covalency of Eu-O bonds in complexes.37 In addition, the intensity of intra-4f transition peak was noted to gradually increase in the normalized spectra, indicating the elevated probability of energy transfer from europium ion itself and the reduced energy transition efficiency of ligand to central ion. 37,38

Figure 3. Excitation (a) and emission (b) normalized spectra for Bmim-Eu, Hmim-Eu and Omim-Eu complexes.

Figure 3b presented the corresponding emission spectra of three complexes. Five emission bands centered at 580, 595, 612, 657 and 702 nm, could be distinguished in all complexes, which were 11



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assigned to the 5D0→7FJ (J = 0 ~ 4) transitions of Eu3+.39 These peak positions and relative intensities of three complexes did not change significantly. Because of the dependency or independency nature of chemical environment for the intensity of 5D0→7F2 or 5D0→7F1 transitions, their intensity ratio (K) could be used to assess the asymmetry degree of Eu3+ coordination environment.40 A relatively higher ratio usually denotes a relatively higher asymmetry degree and better luminescent monochromaticity. The K values for three complexes were thus calculated and listed in Table 2. The slightly decreased K value with the growth of alkyl chain suggested certain reduction in the asymmetry of coordination structure, 41 which, however, was still not enough to significantly change the luminescence properties of europium complexes. As shown in Table 2, the fluorescence lifetime (τ) for three complexes was also slightly decreased with longer alkyl chains. Based on the equations (1)-(3) listed in Supporting Information (SI), the nonradiative (knr), radiative (kr) rate constants and quantum efficiency (Q) could be calculated from the emission spectra. As seen from Table 2 for these parameters, the knr value of complexes increased with the growth of alkyl chain in counter ion, while the kr value changed in opposite way. Although the fluorescence lifetime and quantum efficiency have decreased in the complexes with longer alkyl chains, the τ and Q values of Hmim-Eu and Omim-Eu were still much higher than that of [Eu(TTA)3(H2O)2] complex (τ = 0.521 ms and Q = 29%),22 which denoted the dominating role of imidazolium ions for stabilizing the europium complex. It has been confirmed that the hydrogen bond (H-bond) formed between β-diketonate ligands and imidazolium ions played a key role on the stability of europium complex.25 The H-bond forming ability of imidazolium cation, however, was mainly determined by electron transfer and steric effects of the alkyl substituents.42 Obviously, the increasing of substituted alkyl chain length would weaken the electron-withdrawing ability of imidazolium ions, and therefore the H-bonds between 12


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imidazolium ring and ligands. The stabilizing effect of imidazolium ions to the complex was thus reduced, along with the relatively higher nonradiative transition rate and the declined fluorescence quantum efficiency. Table 2. Characteristic Luminescence Parameters for Three Complexes in Solid State Complexes

K

τ

Q

kr –1

knr

(ms)

(%)

(ms )

(ms–1)

Bmim-Eu

17.1

0.521

53.5

1.027

0.893

Hmim-Eu

16.9

0.517

52.6

1.019

0.915

Omim-Eu

16.8

0.515

52.2

1.013

0.928

When doping these europium complexes into BPS-n LLC matrices, however, relatively obvious changes in their luminescence properties could be observed for three complexes. Figure 4 presented the excitation and emission spectra of the complexes doped in LLC matrices by BPS-5. Compared to those for pure complexes, the disappearance of intra-4f transition peaks located at 450-550 nm in Figure 4a indicated more effective energy transfers from ligand to Eu3+.43 However, from Bmim-Eu to Omim-Eu, the energy transfer from ligand itself (ILCT) were gradually increased which meant decreased energy transfer from ligand to the central metal ions (LMCT). Such a change suggested a deteriorated antenna effect of the doped LLCs,36 indicating the weakened isolation and constraint effects on dispersed europium complexes. This should be attributed to the enhanced solvophobic effect by longer alkyl chains, which was not favorable for the constraint of europium complex by the solvophilic domain (PEO chains and ILs). In addition, the lack of active sites to form H-bond in sterol rings would decrease the possibility to distribute europium complexes in solvophobic domains of LLC, which was also not favorable for the stabilization of complex. Therefore, the energy transfer efficiency between the ligand and europium ions would be reduced. This point could be also reflected from the changes of their emission spectra. 13



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Figure 4. Normalized excitation (a) and emission (b) spectra for Bmim-Eu, Hmim-Eu and Omim-Eu complexes doped BPS-5 LLC matrices.

Figure 4b presented the emission spectra of luminescent LLCs excited by LMCT transition at 365 nm. It could be seen that the 5D0→7F2 hypersensitive transition peak at 612 nm still maintained relatively higher intensities, indicating the keeping of good monochromaticity of complexes after being doped in LLCs. Meanwhile, from these characteristic transitions of Eu3+, the detailed information of coordination structure for europium complexes encapsulated in LLCs could be mapped. The obtained results were listed in Table 3. Compared to the pure complexes, K values were observed to enhance in complex-doped LLCs, implying higher asymmetry degree of the coordination structure for europium complexes. This demonstrated that LLC matrices have interacted with the doped complexes to cause their structures deformed. It was noted that the K value for Bmim-Eu doped BPS-n LLC matrix (21.7) was higher than those of the same complex doped LLCs by P123/[Bmim]PF6.22 It should be indicated that a value higher than 20 when incorporating europium β-diketonate complex in matrices via non-covalent interaction was difficult to achieve, though they usually exhibited good monochromaticity. This also indicated that a relatively highly organized LLC structure with sterol molecules could provide more effective constraint of the europium complexes. In addition, the similar reduction tendency of intensity ratio from Bmim-Eu to Omim-Eu was observed in Table 3. This also suggested the gradually 14


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disappearance of constraint effect on europium complexes by LLC matrix, confirming our previous conclusions that the enhanced solvophobic effect of doped complexes was not favorable for their stabilization in LLCs. Table 3. Characteristic Luminescence Parameters for BPS-5/[Bmim]PF6 LLC Matrices Doped with Three Europium Complexes LLCs

K

τ

Q

kr –1

knr

(ms)

(%)

(ms )

(ms–1)

BPS-5-B

21.7

0.640

77.5

1.211

0.352

BPS-5-H

19.1

0.620

67.7

1.092

0.520

BPS-5-O

18.4

0.576

61.4

1.065

0.671

The observed single and symmetric peak at 5D0→7F0 transition in inset of Figure 4b manifested that only one europium site was present in LLC system. Meanwhile, the single exponential fitting curve for florescence decay (shown in Figure S1) also supported this result, where the lifetimes for three europium complex doped LLCs could be obtained. As seen in Table 3, the Bmim-Eu doped LLCs exhibited a quite long lifetime (0.640 ms), which was the longest one observed among other matrices with non-covalently dispersed europium complexes.32,14 The observed longer lifetime values of complex-doped LLCs than those in pure complexes reflected that a more stable micro-chemical environment could be provided by LLC matrices. It could also be noted that the lifetime was decreased with the elongation of alky chain in counter ion. By checking other specific luminescence parameters derived from the energy transfer equations, we could note that kr values of doped LLCs were observed to increase along with the decreased knr values in comparison with those in pure complexes. Thus, higher Q values have been obtained when the complexes were doped in LLC matrices. Moreover, the highest kr (1.211 ms-1) and lowest knr (0.352 ms-1) values were achieved in Bmim-Eu doped Lα phase constructed by BPS-5/[Bmim]PF6. This implied the most 15



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effective constraint and isolation to Bmim-Eu by this LLC matrix, which thus produced the most effective inhibition of nonradiative deactivation. Besides, the knr value was lower than that measured from the Lα phase by P123/[Bmim]PF6 (knr = 0.422 ms-1), indicating further improved stability and constraint effect by BPS-5 LLC matrix on europium complex. The quantum efficiency for Bmim-Eu complex here (77.5%) was much higher not only than those from europium containing polymers (48%) 44 and ionogels (53%),12 but also than all other lyotropic organized soft materials reported so far. Most importantly, the knr value here was almost comparable and even higher than those from matrices with imidazolium ions covalently bounded like nanozeolite (knr = 0.34 ms-1)

10

and mesoporous silica (knr = 0.643 ms-1).11 For the luminescent LLCs doped with

Hmim-Eu and Omim-Eu complexes, however, their kr values were relatively lower while knr values relatively higher due to the reason discussed above. 3.2.2 Effect of Polyoxyethylene Chain Length of Sterol As a comparison, the Bmim-Eu doped Lα phase by BPS-10/[Bmim]PF6 was also characterized to study the effects of lamellar spacing and rigidity of sterol packing on the luminescence properties of doped LLCs. As seen in the excitation spectra shown in Figure 5a, with the EO chain length increasing from n = 5 to 10, the absorption peak of ILCT transition was increased and that of LMCT transition was broadened. These variations reflected an enhanced energy transfer probability from the ligand’s triplet state to itself due to the shortened mutual distance of complex itself, which would lead to a decrease of energy transfer efficiency of the antenna effect.36

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Figure 5. The excitation (a) and emission (b) spectra of Bmim-Eu in two different LLC matrices.

The calculated K value for the luminescent LLC constructed by BPS-10 was 20.1, smaller than that in BPS-5 system, which indicated a relatively higher symmetric degree of the coordination environment and thus the decreased constrain of europium complex in BPS-10 based LLC system. However, it was still higher than that (19.4) measured in luminescent LLC constructed by P123, reflecting the better confinement of europium complex provided by BPS-n skeletons. By fitting the luminescence intensity decay curve for 5D0→7F2 transition in LLCs of BPS-10-B (Figure S2), the lifetime value was obtained as 0.612 ms, slightly shorter than that observed in BPS-5 LLCs. Using the similar method discussed before, the radiation transition rate kr in LLCs of BPS-10-B was obtained as 1.184 ms-1, which was close to that in LLCs of BPS-5-B. However, the knr value (0.450 ms-1) became higher. Therefore, it was clear that the increase of EO chain length resulted in a decrease of complex stability in LLC matrices. Also, the energy transfer efficiency of ligand to europium ion was reduced, which caused a lower Q value (72.5%) of Bmim-Eu complex in the LLC matrix of BPS-10/[Bmim]PF6. Such luminescence property changes reflected the different isolation and constraint effects between BPS-5 and BPS-10 LLC systems, which could be further disclosed by their structural and viscoelastic differences.

3.2.3 Isolation and Constraint Effects in LLCs with BPS-5 or BPS-10 17



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To depict the difference of site capture capability for europium complex between BPS-5 and BPS-10, the size of Bmim-Eu complex was estimated from its single crystal structure as 1.28 × 1.02 × 1.59 nm3.29 The geometry of BPS-n, however, was optimized by Gaussian 09 package at the B3LYP level. The length of a BPS-n molecule was evaluated by the density functional theory (DFT) calculation carried out with a mixed basis set of 6-31 G(d) for C, H and O atoms.45 As shown in Figure S3, the length of sterol ring part was about 1.58 nm, and the PEO lengths were 1.77 and 3.54 nm for BPS-5 and BPS-10, respectively. It had been demonstrated that PEO chains within LLCs could provide not only the driving force for assembly, but also the anchoring site for doped complex via H-bond interaction.22,29 It could be also noted that the size of one europium complex was matchable to the PEO chain length in BPS-5, while nearly half of that in BPS-10. Therefore, the LLC matrix constructed by BPS-5 could provide more effective isolation for doped europium complexes to ensure their homogeneous dispersion. The increased PEO chain length in BPS-10, however,

might

hold

more

doped

europium

complexes

together

and

thus

increase

the probability of collision quenching. The interaction between LLC matrix and europium complex could be also reflected from their infrared spectra and rheological properties. Figure S4 shows the FTIR spectra for luminescent LLCs constructed by BPS-n with different PEO chain lengths. The spectra for pure [Bmim]PF6 and BPS-n were also given for comparison. The hydrogen bonding interaction within LLCs doped with europium complexes could be manifested by the blue shifted peaks of C-H stretching in imidazolium ring (3155 cm–1) and C-O-C stretching in PEO chains of BPS-n (1061 and 1063 cm–1) when compared to the pure [Bmim]PF6 and neat BPS-n.29 Along with the fact that the hydrogen bond could be formed between TTA ligand and imidazolium ion in the europium complex, 25 it could be pointed out that the europium complexes were confined in the LLC matrix. 18


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As shown in Figure 6 for the dynamic rheological curves of BPS-5 and BPS-10 matrices, the elastic modulus (G’) kept higher than the viscous modulus (G’’) for LLC constructed by BPS-5 (Figure 6a), exhibiting an elastic behavior usually occurred in a reverse LLC phase. However, for BPS-10 sample, such an elastic property could only be seen in a narrow intermediate frequency region (Figure 6b), which was similar to the plastic rheological behavior of a typical lamellar phase reported previously.46

Figure 6. The dynamic rheological curves for LLCs of BPS-5-B (a) and BPS-10-B (b).

From Figure 6, it could be also observed that the complex viscosity (η*) values for BPS-5-B sample was ten times higher than that from BPS-10-B sample, reflecting more closely packed molecules in LLCs of BPS-5-B.47 In addition, due to the stronger interaction of rigid sterol rings, the η* values of BPS-n based LLCs were higher than those from P123 based LLCs.48,49 This explained why the LLC system based on BPS-5/[Bmim]PF6 exhibited the best matrix constraint effect on the europium complex among the systems reported up to now. A more quantitatively link between the microstructure of a flowing substance and its rheological properties could be provided by Bohlin cooperative flow theory,50 where the relationship between the dynamic moduli and angular frequency (ω) could be established concerning the interaction of molecules in LLCs by the following equation: 1

G* = G ' 2 +G ' ' 2 = Aω z 19


(1)


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where |G*| is the complex modulus, A is a parameter that can be regarded as the interaction strength between molecules in LLCs, and z is defined as the “coordination number” of the interactive flow units. Figure S5 displayed the logarithm curves plotted based on equation (1) and corresponding fitting curves. The derived parameters of cooperative flow units exhibited a much higher A value in LLCs of BPS-5-B (5680) than that in LLCs of BPS-10-B (87), which indicated a stronger cohesive interaction in former system, in accordance with the higher viscosity of BPS-5-B samples. As demonstrated by Abe and our group, the rigid and large sterol ring structure played a key role for the excellent aggregation ability of BPS-n, which induced the formation of highly ordered structure of LLCs. Here, the relatively weak interaction in LLCs constructed by BPS-10 could be attributed to the increased length of flexible EO chain that would change the solvophilic/solvophobic equilibrium, leading to the decrease of attraction between sterol rings.

3.3 Photostability of BPS-n luminescent LLC materials It has been found that the decomposition and quenching of β-diketonate complexes under the irradiation of UV light could be prevented via the interaction between ILs and β-diketonate ligands.25 Our previous studies also showed that the photo and thermal stabilities of europium complexes could be not only preserved in ILs-based LLCs, but also further improved as a result of the special organized structure of LLCs. To testify such properties here, the LLCs constructed by two BPS-n molecules and three europium complexes were monitored under the irradiation of an UV-lamp (6 W) with the excitation wavelength at 365 nm. The decayed percentage of quantum yields (φ) compared to their initial values was monitored by the Edinburgh Instruments FLS920 as described in the experimental section and has been recorded in Figure 7. For comparison, the decayed data for Bmim-Eu complex doped in LLCs of P123/[Bmim]PF6 (P123-B) and dissolved in CH3CN were also provided.

20


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Figure 7. Quantum yield decay profiles under UV exposure for BPS-5-B, BPS-5-H, BPS-5-O, BPS-10-B and P123-B, as well as Bmim-Eu complex dissolved in acetonitrile.

As shown in decay curves of Figure 7, the φ values of Bmim-Eu dissolved in acetonitrile decreased rapidly compared to those doped in LLC matrices. It could be observed that the best photostability was obtained in the sample of Bmim-Eu doped LLC matrix of BPS-5/[Bmim]PF6, in which the φ value could be kept above 91% of its initial one. Though the photostability decreased slightly in LLCs constructed by BPS-10, it was still better than those constructed by P123. Upon 20 hours exposure to UV light, the φ values of Hmim-Eu and Omim-Eu doped BPS-5/IL LLCs were observed to decrease gradually. Meanwhile, the quenching effect of BPS-5-O system was more obvious. After additional 40 hours UV irradiation, the φ values of three europium complex doped LLCs were decreased respectively to 91.5%, 70.6% and 62.4% of their initial ones. Such changes also reflected the influence of the molecular structure that might cause the dispersion position and interaction with LLC matrices changed owing to the increased solvophobic property of the doped complexes.

21



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4. Conclusions In conclusion, the europium β–diketonate complex doped LLC matrices by relatively “rigid” BPS-n molecules and [Bmim]PF6 ionic liquid have been investigated. The europium complex exhibited more excellent luminescent properties and photostability when doped in LLC matrices constructed by BPS-5/[Bmim]PF6, which were also the best results for europium containing lyotropic organized aggregate reported so far. It was found that such enhanced luminescence properties were resulted from the highly organized molecular arrangement and tightly packed sterol rings inside LLC matrices which favored the homogeneous dispersion and better constraint effects of the doped europium complexes. It is noteworthy that the observed advantages endow the soft luminescent LLCs with superior photophysical properties that could be comparable to those covalently grafted in solid matrices. Comparison between BPS-5 and BPS-10 reminds us that both the stronger solvophobic interaction and the suitable site capture of lanthanide complex in LLCs were important to get high luminescence efficiency and better photostability.

Supporting information Luminescence decay curves of all the LLC samples and FTIR spectra are provided. Details on the calculation method of luminescent parameters are also included.

Acknowledgements We are thankful for the financial supports from the National Natural Science Foundation of China (21373127 and 21673129). References 1. Carlos, L. D.; Ferreira, R. A.; Bermudez, V. D. Z.; Ribeiro, S. J. Lanthanide‐Containing 22


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Table of Contents Graphic

29



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Figure 1. Molecular structures of BPS-5 or BPS-10 (a) and europium complexes with imidazolium counter ion of different alkyl chain lengths (b). 36x13mm (300 x 300 DPI)


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Figure 2. SAXS patterns for LLC materials with or without europium complexes in BPS-5 (a) and BPS-10 (b) systems. The insets were enlarged views of first peak positions. 67x28mm (300 x 300 DPI)



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Figure 3. Excitation (a) and emission (b) normalized spectra for Bmim-Eu, Hmim-Eu and Omim-Eu complexes. 61x22mm (300 x 300 DPI)


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Figure 4. Normalized excitation (a) and emission (b) spectra for Bmim-Eu, Hmim-Eu and Omim-Eu complexes doped BPS-5 LLC matrices. 58x21mm (300 x 300 DPI)



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Figure 5. The excitation (a) and emission (b) spectra of Bmim-Eu in two different LLC matrices. 57x20mm (300 x 300 DPI)


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Figure 6. The dynamic rheological curves for LLCs of BPS-5-B (a) and BPS-10-B (b). 57x20mm (300 x 300 DPI)



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Figure 7. Quantum yield decay profiles under UV exposure for BPS-5-B, BPS-5-H, BPS-5-O, BPS-10-B and P123-B, as well as Bmim-Eu complex dissolved in acetonitrile. 57x46mm (300 x 300 DPI)


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Table of Contents Graphic 39x18mm (600 x 600 DPI)


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