Controlled Self-Assembly and Luminescence Characteristics of Eu(III

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Controlled Self-assembly and Luminescence Characteristics of Eu(III) Complexes in Binary Aqueous/Organic Media Masa-aki Morikawa, Shohei Tsunofuri, and Nobuo Kimizuka Langmuir, Just Accepted Manuscript • DOI: 10.1021/la403216e • Publication Date (Web): 30 Sep 2013 Downloaded from http://pubs.acs.org on October 8, 2013

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Controlled Self-assembly and Luminescence Characteristics of Eu(III) Complexes in Binary Aqueous/Organic Media Masa-aki Morikawa,*,†, ‡,§ Shohei Tsunofuri,† and Nobuo Kimizuka*,†,‡,§ †

Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu

University ‡

§

Center for Molecular Systems (CMS), Kyushu University

JST CREST, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan

ACS Paragon Plus Environment

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ABSTRACT

Luminescence of sodium tetrakis(naphthoyl trifluoroacetonato) europium(III) (Na[Eu(nta)4]) in binary aqueous-ethanol media is quenched continuously with increase in the water content, which is ascribed to commonly observed relaxation of photo-excited lanthanide complexes through vibrational coupling with coordinating water. Meanwhile, replacement of sodium ion with an ammonium amphiphile 1 gives a lipid complex 1[Eu(nta)4] which shows distinct changes: its luminescence quantum yield Φ is remarkably increased to ~0.6 above the water content of ~60 vol%. This unusual enhancement in luminescence intensity occurs in response to self-assembly of 1[Eu(nta)4] into nanoparticles. The lipid counterions provide hydrophobic atmosphere inside nanoparticles and they simultaneously form monolayers on the nanoparticle surface that enhance dispersion stability. The size of nanoparticles is tunable depending on the volume fraction of water in the binary media. The lipid-assisted self-assembly of lanthanide complexes provides a unique means to fabricate luminescent nanomaterials and this approach will be widely applied to fabricate functional coordination nanomaterials.


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INTRODUCTION Luminescent transition metal complexes are attracting much attention because of their unique properties that provide a wide variety of technological applications.1,2 Among them, lanthanide(III) complexes are most notable because of their sharp emission with large Stokes shift and long-lived excited state with typical luminescent lifetimes on the micro- to milli-second timescale. Although the f-f transition of lanthanide(III) ions are Laporte-forbidden, they are facilely activated via energy transfer from photoexcited π-conjugated antennae ligands.3,4 These unique luminescent features rendered lanthanide(III) complexes key building blocks for many applications such as luminescence-based bioassays5,6 and light energy conversion systems7,8 to name a few. Consequently, improvement of their luminescence performance has been a key issue and efforts have been devoted to control energy levels of donor ligands, coordination structures including ligand field symmetry and coordination numbers.9,10 The replacement of ligand C-H bonds with C-F bonding has been discovered to effectively prevent thermally induced nonradiative deactivation process. Despite all these developments, application of luminescence lanthanide complexes in aqueous environment has still remained a challenge because water molecules in the first coordination spheres quench luminescence of lanthanide ions through vibrational coupling with the higher O-H vibration overtones.11 To enhance their luminescence quantum yield in aqueous media, it is essential to remove water molecules from the coordination sphere. A straightforward approach for this is to create hydrophobic environment in aqueous media so that coordinating water molecules are stripped away from lanthanide ions. Self-assembly of lanthanide complexes is expected to provide simple and useful means to solve this issue. We have recently developed a cationic Tb(III) receptor complex which self-assembles to form


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vesicles in aqueous media.12 Dianionic bis(pyridine) arms were introduced to a hydrophilic head group, which quantitatively complexed with a Tb(III) ion. This cationic receptor complex was designed to show coordinative unsaturation and consequently water molecules occupy the rest of first coordination spheres. Upon adding aqueous adenosine-5’-triphosphate (ATP) to the receptor vesicles, their luminescence showed sigmoidal increase due to cooperative replacement of coordinating water molecules by triphosphate bonds.12 Thus, preorganized lanthanide receptors provided luminescent nanointerface to convert and amplify molecular information of highenergy phosphates. An alternative approach developed is self-assembly of coordination nanoparticles from lanthanide ions and nucleotides in water. In this approach, nucleotides serve as multidentate photosensitizer ligands.13 The amorphous coordination networks formed in water show adaptive inclusion of anionic guest molecules in the hydrophobic interior of nanoparticles. In this paper, we describe a new self-assembly strategy which brings out innate luminescence ability of Eu(III) complexes in aqueous-organic media. It is based on the lipid counterioninduced accumulation of Eu(III) complexes in nanoparticles. As a water-miscible organic solvent, ethanol was selected since it forms nano-sized clusters in aqueous solution.14,15 Such organic clusters in aqueous media provide a new basis to control nano-interfacial self-assembly and concomitant function of nanomaterials.

EXPERIMENTAL SECTION Materials. Ethanol, methanol, CHCl3, and toluene were purchased from commercial sources and used as received without purification. The water used in all experiments was purified with a Direct-Q system (Millipore Co.) and had a resistivity higher than 18.2 MΩcm. Preparation of


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tetrakis(naphthoyl trifluoroacetonato) europium(III) with lipid counterion (1[Eu(nta)4]) is shown in the Supporting Information. Characterization. UV-vis absorption spectra were recorded on a JASCO V-670 spectrophotometer at 20 oC. Quartz cell with 1-mm path length was used. Emission and excitation spectra were recorded on PerkinElmer LS55 spectrophotometer with 1-mm path length cell. Absolute photoluminescence quantum yields were determined by absolute PL quantum yield measurement system (Hamamatsu Photonics K.K., C9920-02). Luminescence decay curves were obtained by a Quantaurus-Tau (Hamamatsu, C11367-01) with a LED light (λex = 340 nm) as an excitation source. Fluorescence microscopy was conducted on a Nikon ECLIPSE 80i with a 20× objective lens. Transmission electron microscopy (TEM) was conducted on a JEOL JEM-2010 (acceleration voltage, 120 kV). A drop of the sample in aqueous EtOH was placed on a carbon-coated Cu grid. After standing for ca. 1 minute, the droplet was removed by adsorbing to filter paper and was dried in vacuo. Attention was paid not to evaporate the sample dispersion on the grid. Dynamic light scattering (DLS) and ζ-potential were measured by using the Malvern Zeta sizer Nano-ZS. X-ray diffraction (XRD) measurements were performed on a Rigaku SmartLab diffractometer with Cu Kα radiation (λ = 1.5406 Å). The viscous solid of 1[Eu(nta)4] was placed on glass substrate and XRD patterns were collected in the 2θ range of 2o−50o at room temperature.

RESULTS AND DISCUSSION As a lanthanide complex, anionic tetrakis(β-diketonate) Eu(III) complex was employed, since its luminescence is sensitive to the presence of water molecules.16 Naphthoyltrifluoroacetonate (nta) was chosen as a ligand sensitizer.17 The cationic synthetic amphiphile 1 was introduced as


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counter ion of [Eu(nta)4]–, in expectation that the lipid complex 1[Eu(nta)4] shows enhanced selfassembling characteristics in aqueous media (Figure 1).18 Three alkyl chains were introduced in amphiphile 1 since amphiphiles with large cross sectional area are desirable to counterbalance the large molecular volume of [Eu(nta)4]–.

Figure 1. Chemical structure of Eu(III) complex having triple chained ammonium lipid counter ion (1[Eu(nta)4]) and its CPK model.

Bottom, photographs of (a) pale yellow powder of

Na[Eu(nta)4], (b) viscous solid of 1[Eu(nta)4], and (c) 1[Eu(nta)4] under illumination by a UV lamp (λex = 365 nm). 1[Eu(nta)4] was obtained as a viscous solid with amorphous structure, as shown by X-ray diffraction analysis (Figure 1b, Figure S1, Supporting Information). This is in contrast to Na[Eu(nta)4] which was in the powdery form (Figure 1a). Presumably, the size of [Eu(nta)4]– complex is so large that it does not go together with the ordered molecular alignment of 1. Interestingly, 1[Eu(nta)4] showed strong red luminescence even in the solid state under UV irradiation (Figure 1c). In emission spectrum (λex = 330 nm), luminescence peaks are observed at 592 (5D0 → 7F1), 612 (5D0 → 7F2), 652 (5D0 → 7F3), and 702 nm (5D0 → 7F4), which are characteristics of Eu(III) ion (Figure S2, Supporting Information). Absolute photoluminescence quantum yields (Φ) determined for each solid sample (Φ = 0.72 for 1[Eu(nta)4], 0.79 for


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Na[Eu(nta)4]) are consistent with those reported for [Eu(nta)4]– complexes.19 These quantum yields are one of the highest values reported for solid state Eu(III) complexes. On the other hand, their luminescence are weakened in solution (Table S1 in Supporting Information), probably due to non-radiative loss of energy as heat to the environment. For example, 1[Eu(nta)4] dissolved in EtOH gave an emission with quantum yield of 0.30 (Figure S2, Table S1, Supporting Information). The excitation spectrum monitored at 612 nm exhibits a peak centered at around 330 nm, which coincides with the absorption band of naphthoyl trifluoroacetonate ligand. It confirms the energy transfer from naphthoyltrifluoroacetone ligands to Eu(III) ion. The [Eu(nta)4]– complexes were dissolved in EtOH since they are not soluble in pure water. By adding EtOH solutions of [Eu(nta)4]– to pure water under vigorous stirring, they were stably dispersed in binary EtOH-water mixtures. It provides opportunity to study the correlation between luminescence characteristics of [Eu(nta)4]– and their self-assembling characteristics. The self-assembly of 1[Eu(nta)4] in the binary media was then investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). It was found that 1[Eu(nta)4] was molecularly dissolved in EtOH-rich aqueous mixtures with the water content ranging from 0 to 30 vol%. On the other hand, self-assembly of 1[Eu(nta)4] started to occur when the water content is raised to 40–90 vol%. Figure 2a shows a TEM image of 1[Eu(nta)4] dispersed in aqueous EtOH (concentration, 100 µM, 60 vol% water). Spherical solid nanoparticles with diameters of 100–300 nm are abundantly seen. The observed diameters of nanoparticles are consistent with that obtained by DLS measurement (139 ± 15 nm, Figure 2b). As water is a poor solvent for 1[Eu(nta)4], the current nanoparticle formation in water-EtOH mixtures would be driven by hydrophobic interaction.

In fluorescence microscopy, these nanoparticles show red

luminescence under UV light excitation (λex = 365 nm, Figure S3 in Supporting Information).


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Figure 2. (a) TEM image and (b) size distribution of 1[Eu(nta)4] dispersed in aqueous EtOH (100 µM, 60 vol% water). The size of nanoparticles was tunable depending on the water content in the binary solvent. At the water content of 50 vol%, large nanoparticles with average diameter of 234 ± 43 nm were obtained (Figure S4 and S5, Supporting Information). Interestingly, further increase in the water content resulted in decrease in the particle size: small nanoparticles with diameter of 32 ± 2.5 nm were obtained at the high water content of 90 vol% (Figure S6, Supporting Information). It is likely that the internal structure of nanoparticles is not affected by the particle size, as they exhibit similar contrast in TEM images (Figure S7, Supporting Information). The decrease in nanoparticle size observed at higher water content indicates rapid nucleation and growth of nanoparticles, with the nanoparticle surface stabilized to avoid agglomeration. Although binary water-EtOH mixtures are macroscopically homogenous, it is known that they consist of solvent clusters at least on the nanometer scale.14,15 As 1[Eu(nta)4] is not soluble in pure water and was added to water as EtOH solution, it is natural to assume that 1[Eu(nta)4] is initially kept solvated in the clusters (or domains) of EtOH in binary mixtures. The size of EtOH domains would be amenable to change depending on the solvent composition, and it will become smaller at higher water content. This is consistent with the observed dependence of DLS-diameter on the solvent composition. It is noteworthy that nanoparticles of 1[Eu(nta)4] show sufficient stability and they


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are kept dispersed without agglomeration even in 90 vol% water. The nanoparticle showed positive ζ-potential of +56 mV (water content, 70 vol%), indicating that the surface of nanoparticles are covered by cationic lipid molecules. Such interfacial self-assembly stabilizes the interface between nanoparticles and the binary media, thereby securing the dispersion stability. Meanwhile, in the case of sodium salt Na[Eu(nta)4], it was molecularly dissolved in binary mixtures below the water content of 50 vol%. Further increase in the water content caused formation of aggregates (concentration, 100 µM. water content = 60–90 vol%). In DLS measurement, nanoparticles with an average diameter below 100 nm were observed (Figure S6, Supporting Information) with irregular morphology as observed by TEM (Figure S8, Supporting Information). These Na[Eu(nta)4] nanoparticles showed negative ζ-potential of –67 mV (water content, 70 vol%), indicating that the surface of nanoparticles is comprised of anionic [Eu(nta)4]– complexes and dissociated Na+ ions. The influence of nanoparticle formation on the photoluminescence characteristics of [Eu(nta)4]– complexes was then investigated. When 1[Eu(nta)4] is molecularly dissolved in the binary solvent at lower water content (at 0–30 vol%), the luminescence quantum yield showed decrease with increasing the water content (Φ = 0.30 at 0 vol% water, Φ = 0.15 at 30 vol% water). This is reasonably explainable by the quenching of Eu(III) luminescence by the vibrational coupling to coordinating water molecules.11 To our surprise, however, the luminescence intensity of 1[Eu(nta)4] significantly increased by further increasing the water content to 50–70 vol% (Figure 3a). The dependence of [Eu(nta)4]– luminescence intensity on the volume fraction of water in the binary mixture is shown in Figure 3b.


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Figure 3. (a) Photoluminescence spectra of 1[Eu(nta)4] at different water content in the binary aqueous-EtOH mixtures (λex = 330 nm). Inset shows photographs of the dispersion under illumination by a UV lamp (λex = 365 nm). (b) Photoluminescence quantum yield (Φ) as a function of the water content. Filled circle; 1[Eu(nta)4], open circle; Na[Eu(nta)4], concentration of the Eu(III) complexes = 100 µM, λex = 330 nm. The sodium salt Na[Eu(nta)4] gave a luminescence quantum yield Φ of 0.3 in pure EtOH, and it continuously decreased with increasing the water content (Φ = 0.04 at 90 vol% water, open circle in Figure 3b). Apparently in this case, the change in quantum yield Φ is not influenced by the formation of nanoparticles observed at higher water content (60–90 vol%). Meanwhile, the


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lipid complex 1[Eu(nta)4] gave luminescence quantum yield Φ of 0.3 in pure EtOH, which is almost identical with that observed for Na[Eu(nta)4]. It exhibits the same dependence in quantum yield up to the water content of 40 vol% (filled circle). However, further increase in water content caused a jump in quantum yield which reached Φ of 0.6 at 60 vol% water. The abrupt rise in quantum yield apparently occurred in response to the formation of nanoparticles at the water content of 40 vol%, indicating confinement of [Eu(nta)4]– in hydrophobic interior of nanoparticles. The observation that 1[Eu(nta)4] show the highest quantum yield at the water-rich environment (∼ 60–90 vol%) indicates that the coordination of water molecules to 1[Eu(nta)4] is effectively suppressed by self-assembly. To shed light on the mechanism of luminescence enhancement, the number of water molecules in the first coordination sphere of [Eu(nta)4]– complexes was examined. As described previously, the luminescence of [Eu(nta)4]– complex undergoes quenching by the energy transfer from excited state of Eu3+ to the higher O-H vibration overtones of coordinating water. It follows that the luminescence lifetime of [Eu(nta)4]– complex is increased upon replacing coordinating H2O molecules with D2O. This is because those O-D oscillators of coordinated D2O molecules are ineffective to deactivate excited states of Eu(III) complexes. By comparing the luminescence lifetime measured in aqueous EtOH (τH) with that obtained in the deuterated binary mixtures (τD), the number of water molecules coordinating to Eu3+ ion in the first coordination sphere (q value) can be empirically determined according to the following equation 1.20-22 q = 1.2(τH–1 – τD–1 – 0.25)

(1)

When 1[Eu(nta)4] is molecularly dispersed in the binary water-EtOH mixture (water = 30 vol%), its luminescence lifetime (τH = 207 µs) is significantly shortened compared to that observed in the corresponding deuterated binary media (τD = 458 µs, Table 1).


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Table 1. Luminescence Lifetime τ and the Number of Water Molecules Coordinated to Eu(III) Complexes qa sample

1[Eu(nta)4]

Na[Eu(nta)4]

FH2O (vol%)b

τH (µs)

τD (µs)

q

30

207

458

2.9

50

472

517

-0.1

70

514

538

-0.2

30

255

456

1.8

50

227

380

1.8

70

167

220

1.4

a

q values were estimated according to the eq 1. b FH2O denotes the volume fraction of water in the binary aqueous-EtOH mixtures. Under this condition, the q value was determined as 2.9, indicating that almost three water molecules are coordinated to the Eu(III) complex. Meanwhile, luminescence decay measurements for 1[Eu(nta)4] nanoparticles formed under higher water fractions gave longer luminescence lifetimes of 472 µs (water = 50 vol%) and 514 µs (water = 70 vol%), which are consistent with the observed increase in the luminescence intensity (Figure 3a). The q values obtained in these water-rich dispersions (water = 50 vol%; q = –0.1, water = 70 vol%; q = –0.2) indicate that the coordination of water molecules is significantly suppressed in 1[Eu(nta)4] nanoparticles. In contrast, luminescence lifetime determined for Na[Eu(nta)4] showed decrease from 255 to 167 µs upon increasing the water content from 30 to 70 vol%. The q value of 1.4 was obtained for Na[Eu(nta)4] nanoparticles under the water composition of 70 vol%, which is consistent with the shortened luminescence lifetime and the decreased quantum yield. These observations clearly indicate that formation of nanoparticles and hydrophobic microenvironment as provided by lipid 1 is essential to strip water molecules from [Eu(nta)4]–.


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The spontaneous self-assembly of luminescent nanoparticles from anionic Eu(III) complex and cationic amphiphiles in binary aqueous-EtOH binary mixtures is schematically shown in Figure 4.

Figure 4. A schematic representation for the self-assembly of 1[Eu(nta)4] in binary aqueousEtOH media. 1[Eu(nta)4] is molecularly dissolved in EtOH-rich binary solvent (water, 0–30 vol%), where water molecules facilely coordinate to the monomeric complex and consequently luminescence quenching was observed. On the other hand, 1[Eu(nta)4] self-assembled to form nanoparticles above the water content of 40 vol%. The positive ζ-potential observed for 1[Eu(nta)4] nanoparticles is explainable by adsorption of lipid 1 at the interface of water/EtOH domains, which may have served as interfacial templates for the accumulation of 1[Eu(nta)4] into ordered nanoparticles. The average size of EtOH domains seems to specify the size of nanoparticles, which are controllable depending on the solvent composition. Diffusion of EtOH from the domains to the bulk water phase naturally occurs with time, which makes 1[Eu(nta)4] concentrated in the droplets shrinking in size. The Eu(III) complexes are densely accumulated in the hydrophobic nanoparticle interior, which effectively strips water molecules from the


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coordination spheres and suppresses thermal deactivation process as well. As a consequence, unusual enhancement in luminescence intensity was achieved by specific self-assembly in aqueous media. The suppression of thermal deactivation process by self-assembly is a common feature to the aggregation-induced emission enhancement (AIEE) of aromatic molecules, such reported for tetraphenylethene (TPE).23 Indeed, 1[Eu(nta)4] and Na[Eu(nta)4] show larger luminescence quantum yields in the sold state (Table S1, Supporting Information). However, the selective enhancement of quantum yield upon self-assembly of 1[Eu(nta)4] above the water content of ∼40 vol% in the binary aqueous EtOH mixtures clearly indicates the importance in controlling the coordination environment of Eu(III) center by self-assembly.

CONCLUSION The electrostatic complexation of Eu(III) complex with suitably designed lipid counterion allows self-assembly of nanoparticles in aqueous EtOH media. Enhancement of Eu(III) luminescence occurred in response to self-assembly, which results from stripping of coordinating water molecules from the first coordination sphere of the Eu(III) complex accumulated in nanoparticles. The size of nanoparticles is controllable depending on the solvent composition, and simultaneous absorption of lipid monolayers on the nanoparticle surface secures their dispersion stability. The unique self-assembly features of Eu(III)/lipid complexes in aqueous organic media, together with the self-assembly-based control on photoluminescence properties, render the current self-assembly approach distinct from the existing techniques as exemplified by the encapsulation of lanthanide(III) complexes in silica nanoparticles.24-28 It provides a new perspective to synthesize functional nanomaterials and fills the gap between the previously reported self-assembly of lanthanide complex in water11,12 and the family of lipophilic


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coordination nano-assemblies in organic media.29-33 We envisage the solvent clusters in binary aqueous-organic systems would be widely applicable to prepare functional lipid-hybrid nanomaterials.

ASSOCIATED CONTENT Supporting Information. Preparation of 1[Eu(nta)4], table of luminescence quantum yields, UV absorption, emission, excitation spectra, fluorescence microscopy, XRD, DLS, SEM and TEM data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We thank Prof. Chihaya Adachi and Dr. Kenichi Goushi of Department of Applied Chemistry, Kyushu University for the luminescence lifetime measurement. This research was supported in part by a Grant-in-Aid for Challenging Exploratory Research (No. 23655130) and by JST, CREST.

REFERENCES


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(1) Costa, R. D.; Ortí, E.; Bolink, H. J.; Monti, F.; Accorsi, G.; Armaroli, N. Luminescent Ionic Transition-Metal Complexes for Light-Emitting Electrochemical Cells. Angew. Chem., Int. Ed. 2012, 51, 8178–8211. (2) Drewry, J. A.; Gunning, P. T. Recent Advances in Biosensory and Medicinal Therapeutic Applications of Zinc(II) and Copper(II) Coordination Complexes. Coord. Chem. Rev. 2011, 255, 459–472. (3) Weissman, S. I. Intramolecular Energy Transfer The Fluorescence of Complexes of Europium. J. Chem. Phys. 1942, 10, 214−217. (4) Moore, E. G.; Samuel, A. P. S.; Raymond, K. N. From Antenna to Assay: Lessons Learned in Lanthanide Luminescence. Acc. Chem. Res. 2009, 42, 542–552. (5) Parker, D.; Williams, J. A. G. Getting Excited about Lanthanide Complexation Chemistry. J. Chem. Soc., Dalton Trans. 1996, 18, 3613−3628. (6) Bünzli, J.-C. G.; Piguet, C. Taking Advantage of Luminescent Lanthanide Ions. Chem. Soc. Rev. 2005, 34, 1048–1077. (7) Mader, H. S.; Kele, P.; Saleh, S. M.; Wolfbeis, O. S. Upconverting Luminescent Nanoparticles for Use in Bioconjugation and Bioimaging. Curr. Opin. Chem. Biol. 2010, 14, 582–596. (8) Aboshyan-Sorgho, L.; Besnard, C.; Pattison, P.; Kittilstved, K. R.; Aebischer, A.; Bünzli, J.-C. G.; Hauser, A.; Piguet, C. Near-Infrared→Visible Light Upconversion in a Molecular Trinuclear d–f–d Complex. Angew. Chem., Int. Ed. 2011, 50, 4108–4112.


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(9) Nakamura, K.; Hasegawa, Y.; Kawai, H.; Yasuda, N.; Kanehisa, N.; Kai, Y.; Nagamura, T.; Yanagida, S.; Wada, Y. Enhanced Lasing Properties of Dissymmetric Eu(III) Complex with Bidentate Phosphine Ligands. J. Phys. Chem. A 2007, 111, 3029–3037. (10) Harada, T.; Tsumatori, H.; Nishiyama, K.; Yuasa, J.; Hasegawa, Y.; Kawai, T. NonaCoordinated Chiral Eu(III) Complexes with Stereoselective Ligand−Ligand Noncovalent Interactions for Enhanced Circularly Polarized Luminescence. Inorg. Chem. 2012, 51, 6476−6485. (11) Kimura, T.; Kato, Y. Luminescence Study on Hydration States of Lanthanide(III)– Polyaminopolycarboxylate Complexes in Aqueous Solution. J. Alloys Compounds 1998, 275– 277, 806–810. (12) Liu, J.; Morikawa, M-a.; Kimizuka, N. Conversion of Molecular Information by Luminescent Nanointerface Self-Assembled from Amphiphilic Tb(III) Complexes. J. Am. Chem. Soc. 2011, 133, 17370–17374. (13) Nishiyabu, R.; Hashimoto, N.; Cho, T.; Watanabe, K.; Yasunaga, T.; Endo, A.; Kaneko, K.; Niidome, T.; Murata, M.; Adachi, C.; Katayama, Y.; Hashizume, M.; Kimizuka, N. Nanoparticles of Adaptive Supramolecular Networks Self-Assembled from Nucleotides and Lanthanide Ions. J. Am. Chem. Soc. 2009, 131, 2151–2158. (14) Nishi, N.; Koga, K.; Ohshima, C.; Yamamoto, K.; Nagashima, U.; Nagami, K. Molecular Association in Ethanol-Water Mixtures Studied by Mass Spectrometric Analysis of Clusters Generated through Adiabatic Expansion of Liquid Jets. J. Am. Chem. Soc. 1988, 110, 5246– 5255.


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(15) Kimizuka, N.; Wakiyama, T.; Miyauchi, H.; Yoshimi, T.; Tokuhiro, M.; Kunitake, T. Formation of Stable Bilayer Membranes in Binary Aqueous−Organic Media from a Dialkyl Amphiphile with a Highly Dipolar Head Group. J. Am. Chem. Soc. 1996, 118, 5808–5809. (16) Kropp, J. L.; Windsor, M. W. Enhancement of Fluorescence Yield of Rare-Earth Ions by Heavy Water. J. Chem. Phys. 1963, 39, 2769–2770. (17) Gago, S.; Fernandes, J. A.; Rainho, J. P.; Ferreira, R. A. S.; Pillinger, M.; Valente, A. A.; Santos, T. M.; Carlos, L. D.; Ribeiro-Claro, P. J. A.; Gonçalves, I. S. Highly Luminescent Tris(β-diketonate)europium(III) Complexes Immobilized in a Functionalized Mesoporous Silica. Chem. Mater. 2005, 17, 5077–5084. (18) Kunitake, T.; Kimizuka, N.; Higashi, N.; Nakashima, N. Bilayer Membranes of TripleChain Ammonium Amphiphiles. J. Am. Chem. Soc. 1984, 106, 1978–1983. (19) Bruno, S. M.; Ferreira, R. A. S.; Paz, F. A. A.; Carlos, L. D.; Pillinger, M.; Ribeiro-Claro, P.; Gonçalves I. S. Structural and Photoluminescence Studies of a Europium(III) Tetrakis(βdiketonate) Complex with Tetrabutylammonium, Imidazolium, Pyridinium and Silica-Supported Imidazolium Counterions. Inorg. Chem. 2009, 48, 4882–4895. (20) Supkowski, R. M.; Horrocks, Jr., W. D. Displacement of Inner-Sphere Water Molecules from Eu3+ Analogues of Gd3+ MRI Contrast Agents by Carbonate and Phosphate Anions: Dissociation Constants from Luminescence Data in the Rapid-Exchange Limit. Inorg. Chem. 1999, 38, 5616–5619. (21) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker, D.; Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. Non-radiative Deactivation of the Excited States of


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Page 19 of 21

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Langmuir

Europium, Terbium and Ytterbium Complexes by Proximate Energy-Matched OH, NH and CH Oscillators: an Improved Luminescence Method for Establishing Solution Hydration States. J. Chem. Soc., Perkin Trans. 2, 1999, 493–503. (22) Supkowski, R. M.; Horrocks, Jr., W. D. On the Determination of the Number of Water Molecules, q, Coordinated to Europium(III) Ions in Solution from Luminescence Decay Lifetimes. Inorg. Chim. Acta 2002, 340, 44–48. (23) Hong, Y.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Phenomenon, Mechanism and Applications. Chem. Commun. 2009, 4332–4353. (24)

Comby,

S.;

Gunnlaugsson,

T.

Luminescent

Lanthanide-Functionalized

Gold

Nanoparticles: Exploiting the Interaction with Bovine Serum Albumin for Potential Sensing Applications. ACS Nano 2011, 5, 7184–7197. (25) Ai, K.; Zhang, B.; Lu, L. Europium-Based Fluorescence Nanoparticle Sensor for Rapid and Ultrasensitive Detection of an Anthrax Biomarker. Angew. Chem., Int. Ed. 2009, 48, 304– 308. (26) Wu, J.; Wang, G.; Jin, D.; Yuan, J.; Guan, Y.; Piper, J. Luminescent Europium Nanoparticles with a Wide Excitation Range from UV to Visible Light for Biolabeling and Time-Gated Luminescence Bioimaging. Chem. Commun. 2008, 365–367. (27) Yu, M.; Lin, J.; Wang, Z.; Fu, J.; Wang, S.; Zhang, H. J.; Han, Y. C. Fabrication, Patterning, and Optical Properties of Nanocrystalline YVO4:A = Eu3+, Dy3+, Sm3+, Er3+) Phosphor Films via Sol-Gel Soft Lithography. Chem. Mater. 2002, 14, 2224–2231.


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Page 20 of 21

(28) Yu, M.; Lin, J.; Fang J., Silica Spheres Coated with YVO4:Eu3+ Layers via Sol-Gel Process: A Simple Method To Obtain Spherical Core-Shell Phosphors. Chem. Mater. 2005, 17, 1783–1791. (29) Kimizuka, N.; Lee, S. H.; Kunitake, T. Molecular Dispersion of Chains in the MixedValence Complexes [M(en)2][MCl2(en)2] (M: Pt, Pd, Ni) and Anionic Amphiphiles in Organic Media. Angew. Chem., Int. Ed. 2000, 39, 389–391. (30) Matsukizono, H.; Kuroiwa, K.; Kimizuka, N. Lipid-Packaged Linear Iron(II) Triazole Complexes in Solution: Controlled Spin Conversion via Solvophobic Self-Assembly. J. Am. Chem. Soc. 2008, 130, 5622–5623. (31) Bodenthin, Y.; Pietsch, U.; Möhwald, H.; Kurth, D. G. Inducing Spin Crossover in Metallo-supramolecular Polyelectrolytes through an Amphiphilic Phase Transition. J. Am. Chem. Soc. 2005, 127, 3110–3114. (32) Bodenthin, Y.; Schwarz, G.; Tomkowicz, Z.; Geue, T.; Haase, W.; Pietsch, U.; Kurth, D. G. Liquid Crystalline Phase Transition Induces Spin Crossover in a Polyelectrolyte Amphiphile Complex. J. Am. Chem. Soc. 2009, 131, 2934–2941. (33) Schwarz, G.; Bodenthin, Y.; Tomkowicz, Z.; Haase, W.; Geue, T.; Kohlbrecher, J.; Pietsch, U.; Kurth, D. G. Tuning the Structure and the Magnetic Properties of Metallosupramolecular Polyelectrolyte-Amphiphile Complexes. J. Am. Chem. Soc. 2011, 133, 547–558.


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Controlled Self-Assembly and Luminescence Characteristics of Eu(III

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