PS

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Synthesis and Photophysical Properties of an Eu(II)-Complex/PS Blend: Role of Ag Nanoparticles in Surface-Enhanced Luminescence Jong-Moon Kim,† Yong-Kwang Jeong,† Youngku Sohn,‡ and Jun-Gill Kang*,† †

Department of Chemistry, Chungnam National University, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of Chemistry, Yeungnam University, Gyeongsan, Gyeongbuk 712-749, Republic of Korea

‡

S Supporting Information *

ABSTRACT: A novel Eu(II) complex with 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP or PC88A) was synthesized and blended with polystyrene polymer (PS). Both an independent complex and the Eu(II)/PS blend excited by near-UV light produced blue luminescence, arising from the 5d→ 4f transitions of Eu(II). Time-dependent density functional theory (TDDFT) calculations on electronic structures of the complex molecule indicated that the absorbing and emitting center was associated with the 2A(dz2) state under the C2 crystal field. We also synthesized silver nanoparticles (Ag NPs) with an average particle size of 4.48 nm (σ = 0.91 nm) using EHHEHP as a stabilizer. The effects of Ag NPs as a colloidal suspension and an interfacial layer on the luminescence intensity of the blend were investigated as functions of the concentration of Ag NPs and the thickness of the Ag NP layer, respectively. The critical concentration of the colloidal Ag NPs and the critical thickness of the interfacial Ag NP layer were ∼355 ppm and ∼0.16 μm, respectively. Under critical conditions, the Ag NPs increased the luminescence intensity by 4.4 times as a colloidal suspension in CH2Cl2 and 2.2 times as an interfacial-layer state.

1. INTRODUCTION Trivalent europium ions provide very unique red-luminescence properties, including hypersensitivity to the coordination environment, a narrow bandwidth, and a long lifetime.1−3However, due to the low f→f absorption cross section, the luminescence quantum yield of Eu(III) ion is very low, and the color tuning is invariant. To obtain high-efficient redluminescence, organic ligands were introduced as a sensitizer to these metal complexes.4Comparatively, the optical properties of divalent europium ions are preferable because the optical transition is characterized as the f → d transition allowed by the electric-dipole moment. The luminescence of Eu(II) can be tuned from blue to red depending on the crystal field strength.5,6 The preparation of phosphors doped with Eu(II) has been accomplished via a simple solid-state reaction at high energy under a reducing atmosphere. The Eu(II)-doped phosphors have been widely applied to the emitting devices such as color television, flat panel displays, and emitting devices. Alternatively, Eu(II) complexes with organic ligands offer advantages for incorporation into the processing and manufacturing of polymer-based solid-state devices. Recently, the incorporation of trivalent rare earth complexes into a polymer matrix has attracted great interest because of their typical applications to polymer optical fiber amplifiers (POFAs)7−9and organic light-emitting diodes (OLEDs).10−14 Most studies on Eu(II) complexes have examined their synthesis and structural properties.15−18 The luminescent properties of Eu(II) complexes have been limited to macro© XXXX American Chemical Society

cyclic complexes because most Eu(II) complexes are nonluminescent and very unstable toward oxygen and nitrogen possessing lone-pair electron.19−24Despite the impressive luminescence properties of Eu(II) in the solid state, progress in the luminescence spectroscopy of divalent europium complexes has been limited by obstacles in synthesis and luminescence materialization.25 Recently, the effects of metal nanoparticles on the luminescence properties have been investigated for fluorophors. Generally, their luminescence increased significantly upon interacting with silver nanoparticles (Ag NPs) as suspensions,26−28colloidal films,29or dopants.30−32Lee et al. fabricated EuCl2/poly(methyl methacrylate) on Ag NP-deposited glass substrate and investigate the emission intensity as a function of a thickness of Ag NP layer in the range of 1−12 nm.33 They observed that the emission intensity from Eu(II) was enhanced in proportion to the thickness of the Ag deposition. These phenomena, defined as surface-enhanced fluorescence (SEF), can be associated with dramatic changes in the electromagnetic field near an interface.34However, fluorescence quenching was observed when the NPs were located at a certain distance from the metal surface.35Eichelbaum and Rademann observed extraordinary luminescence enhancement in silicate glasses doped with gold and silver NPS, and trivalent lanthanide ions.36 Received: April 17, 2012 Revised: June 2, 2012

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dx.doi.org/10.1021/la301547z | Langmuir XXXX, XXX, XXX−XXX

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Figure 1. TEM photographs and particle-size distributions of Ag NPs prepared from AgNO3, EHHEHP, and hydrazine. synthesized Ag NPs were transferred to the organic phase. The organic phase was separated from the aqueous phase and dried over anhydrous magnesium sulfate. For transmission electron microscopy (TEM), we briefly dipped a lacey carbon-coated Cu grid into the Ag NP solution and removed it to dry the residual solvent. TEM images were recorded on a JEOL JSM-2000 EX electron microscope operated at 200 kV. Figure 1 shows the TEM image and size-distribution histogram of Ag NPs stabilized by EHHEHP in cyclohexane. The calculated mean particle size was 4.48 nm with the standard deviation of σ = 0.91 nm. Fabrication. The Eu(II)-complex-incorporated PS films were prepared simply by mixing the Eu(EHEHP)2 complex in PS in CH2Cl2. The complex to PS ratio was 1:2 by weight. After being stirred for 1 h to confirm homogeneity, evaporation of the solvent yielded blended films. After being dried in an electric oven at 80 °C, the blends were kept in the open air. As-prepared films were further used for photophysical studies. For the fabrication of thin film on quartz substrates, the substrate was cleaned with a KOH-saturated 2-propanol solution and then sonicated in 1.0 M HNO3 solution for 1 h. The cleaned substrate was washed with ethanol and acetone alternatively and then dried in an electrical oven at 80 °C. One side of the substrate was blocked by tape. Double thin-film layers were prepared by two steps. First, Ag NP layers were prepared by immersing the substrate in Ag NP-diffused cyclohexane solution.The thickness of Ag NP layer was controlled by varying the Ag NP concentration in cyclohexane from 10−9 to 10−3 M. Next, the thin film of the Eu(II)/PS blend was similarly casted on the Ag NP thin film using the blend solution. After drying in air, the blocked tape was removed. Thicknesses of the prepared thin-film layers were measured using field-emission scanning electron microscopy (FE-SEM; S-4800, Hitachi). Figure 2 shows a typical SEM image of the coated substrate; the thicknesses of the interfacial Ag NP film and the blended thin film were evaluated on the basis of the SEM image. The thickness of the Ag NP layer formed from 1.0 × 10−10 M of [Ag]0 was 0.07 μm. The thickness increased gradually with increasing concentration of [Ag]0 until [Ag]0 = 1.0 × 10−6 M. Above this concentration, the thickness increased and peaked rapidly at 1.64 μm for [Ag]0 = 1.0 × 10−3 M. The thickness of the blend casted on the Ag NP interfacial layer was nearly unaffected by the intermediate thickness of the interfacial layer: the thickness ranged from 1.19 to 1.39 μm. Spectroscopic Measurements. Electronic absorption spectra were recorded on a Hitach U-4100 UV−vis spectrophotometer. For photoluminescence (PL) and excitation spectra measurements, excited light from a 325 nm He−Cd laser or a Xe arc lamp (Oriel) passed through an Oriel MS257 monochromator was focused on the sample. Emission and excitation spectra were measured at 90° relative to the excitation beam path with an ARC 0.5 m Czerny−Turner monochromator equipped with a cooled Hamamatsu R-933-14 photomultiplier tube. The luminescence quantum yield of the blend dissolved in CH2Cl2, defined by

They attributed the enhancement to a classical energy transfer from novel metal particles to lanthanide ions. In this work, we synthesized Eu(II) complex with an organophosphorus compound, 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate (EHHEHP), by a convenient method and blended the complex with polystyrene polymer (PS). Acidic organophosphorus compounds have been effectively applied to separate rare earth metals as extractant.37,38Rare earth complexes with organophosporous compound have a linear rigid-chain coordination polymer structure that results from linkage via alkylphosphate bridges.39 We found that the EHEHP ligand prevented divalent europium ion from oxidizing and provided a good affinity for blending with PS. Here, PS exhibits excellent mechanical and optical properties because it has a low optical absorbance, low shrinkage, and provides a rigid environment for rare earth complexes. In addition, Ag NPs with a mean particle diameter of 4.48 nm (σ = 0.91 nm) were synthesized using EHHEHP as a stabilizer to maximize the luminescence intensity for use in molecular devices. The effects of Ag NPs on the luminescence intensity of the blend were investigated as a diffused state and an interfacial layer. In this study, we found that Ag NPs played two contrastive roles in the luminescence of the Eu(II)/PS blend.

2. EXPERIMENTAL SECTION Chemicals. Eu2O3 (99.99%) and hydrazine solution (80%) were purchased from Aldrich, and PC88A (reagent grade) was purchased from Daihaci. All solvents and chemicals were used without further purification. Synthesis of Eu(EHEHP)2·4C6H12. Divalent europium solution was obtained by dissolving Eu2O3 (0.25 g, 0.71 mmol) in concentrated HCl (0.741 mL). EHHEHP (0.905 mL, 2.8 mmol) dissolved in 100 mL of cyclohexane was added to the Eu(II) solution, and the mixture was refluxed at 30−40 °C for 1 week. The mixture then emitted blue luminescence under 365 nm irradiation. The organic layer was separated and dried over anhydrous magnesium sulfate. The solvent was evaporated and dried in an electric oven at 80 °C. Elementary analysis of the obtained gellous film was performed with an Automatic Elemental Analyzer (Flash EA 1112). Anal. Calcd for EuC32H68O6P2·4C6H12: Eu, 13.8; C, 61.2; H, 10.6; P, 5.6%. Found: Eu, 12.0; C, 61.4; H, 11.0; P, 5.0%. Synthesis of Diffused Ag NPs. Ag NPs were prepared by reduction of AgNO3 with hydrazine in the presence of EHHEHP. A total of 1 mL of 0.10 M silver nitrate solution was added to the molar equivalent of EHHEHP dissolved in 20 mL of cyclohexane and stirred for 5 min. Hydrazine (0.20 mmol) was added to the mixture and stirred for 1 min. The excess of hydrazine favors the formation of monodispersed particles through a rapid nucleation process. All of the B


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3. RESULTS AND DISCUSSION Absorption and TD-DFT Calculation. Figure 3 shows the absorption spectra of EHHEHP and Eu(EHEHP)2 dissolved in

Figure 3. Absorption spectra of EHHEHP (1) and Eu(EHEHP)2 (2) dissolved in CH2Cl2. The inset shows the extended A-absorption band.

CH2Cl2. The complex produced four absorption bands in the 200−450 nm region: hereafter, these four bands are referred to as A-, B-, C-, and D-absorption bands in the order of increasing energy. These four bands were attributed to the 4f7 (8S7/2) → 4f65d transitions. The excited states originate from 2D, because the 4f6 electronic configuration is isolated from the 5d (2D3/2,5/2) electronic configuration. The absorbance of the four bands was very weak because of the spin-forbidden transition. The extended A-absorption band was inserted in Figure 3. The “staircase” structure, A1−A3, was observed in the A-band. This structure can be associated with multiplet 4f6 states, which occasionally appeared in the 300−400 nm region. Quantum mechanical calculations were performed to characterize the observed optical properties of Eu(EHEHP)2. First, the molecular geometry of the complex molecule was optimized by the density functional theory (DFT) method at B3LYP using GEN basis functions (MWB52 for Eu; MWB2 for C and O; MWB10 for P; D95 for H).42 The coordinators of Eu, O, and P atoms, and their bond lengths and bond angles, are listed in Table S1 (Supporting Information). As shown in Figure 4, optimization for Eu(EHEHP)2 revealed that the europium ions achieved four-coordination state by binding to two molecules of bidentate EHEHP. The lengths of the two terminal Eu−O bonds are 2.3788 and 2.3975 Å, and the intraand interbond angles of O−Eu−O are 65.676° and 111.7°, respectively. As shown in Figure 5a, the four O atoms formed a slightly distorted rectangular geometry, and the Eu atom is located on the z-axis slightly above the planar. Optimization shows that the geometry of the complex is slightly distorted flat-pyramidal with C2 symmetry. Next, we performed timedependent self-consistent field (TD-SCF) calculation on the molecular orbitals of the complex to determine the splitting of the 5d orbitals of the central Eu atom due to the Eu−ligand interaction. Under the enforced C2 symmetry, the dz2, dx2−y2, and dyz orbitals mixed with the s, py, or other d orbitals. The mixing of the empty dz2 orbital with the s orbital resulted in the first lowest unoccupied molecular orbital (LUMO), l1, in which the d orbital was involved. Taking into account the participation of the Eu orbitals, the contribution of the dz2 orbital was 27%. The second LUMO l2 consisted of the dx2−y2 orbital as a main orbital with a contribution of 58% and dz2 as a

Figure 2. (a) A typical SEM image of a side view of the coated substrate (upper, blend layer; middle, interfacial layer; lower, substrate) and (b) the thicknesses of the interfacial Ag NP film (●) as a function of Ag NP concentration and the blend thin film (○) casted from a fixed blend concentration (0.3 g of the blend dissolved in 10 mL of CH2Cl2). Q=

number of photons emitted number of photons absorbed

was determined using a previously described method.40,41 The recorded spectra for the quantum yield were corrected for the spectral response of the system using an Oriel 45-W quartz tungsten halogen lamp standard. All measurements were repeated in triplicate. No significant experimental error was found. We determine the optimal experimental condition to obtain reliable quantum yield from integrating Labsphere measurements using a reference of quinine sulfate in diluted H2SO4. The slit’s aperture (0.25−1 mm) and the direction of the excitation beam (45° and 90°) with respect to a cuvette filled with the reference solution did not affect the obtained values within σ = 1.0. It indicated that the exciting and the emitting beam fluxes received at the aperture of the monochromator were proportional to the slit aperture, irrespective of their angular distributions, and were well compensated in the equation of Q. Furthermore, we measured Q of the reference solution excited at the He−Cd 325 nm line and five different excitation wavelengths in the 325−375 nm region using the Xe-arc lamp. The laser excitation source produced Q = 54.1% (σ = 1.85), well agreeing with the reported value of Q = 54.6%. Although the absorbance of the quinine sulfate solution at the excitation wavelength varied from 0.483 to 0.989, the obtained values of Q were identical as Q = 43.8% (σ = 1.20). It indicated that the quantum yield is independent of the cross section of the absorption, if the optical process of the excitation and luminescence is invariant in the excitation wavelength region. It was found that the excitation laser beam penetrated the solution thoroughly, while the excitation beam generated from the lamp did not. The 325 nm He− Cd laser, resulting in a more reliable Q value, was used in this work as an excitation source. C


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Figure 6. Emission (λexn = 350 nm) and excitation (λems = 450 nm) spectra of the as-prepared blend film measured at room temperature (1) and T = 10 K (2).

Figure 4. View of the Eu(EHEHP)2 molecule optimized by the DFT/ B3LYP method.

luminescence spectrum, spanning over 370−550 nm. Previously, the characteristic features in the luminescence of trivalent europium complex with EHEHP were found in the 5 D0→7F1 and 7F2 transitions, appearing in the 585−600 and 605−630 nm regions, respectively.38 Although the Eu(EHEHP)2 complex was exposed to air for a few months, the characteristic red emission from trivalent europium ion was not detected (Figure S2). This suggests that the Eu(III) complex was not formed from the employed synthetic method and the synthesized Eu(II) complex was stable in the presence of air and moisture. The emission and the excitation spectra were mirror-image, with multipeaks, indicating that the absorbing and the emitting levels were identical. The excitation spectrum corresponded to the A-absorption band; thus, the 2A(dz2) state accounts for the absorbing and the emitting centers. The wellsplit emission and excitation spectra of the Eu(EHEHP)2 complex could be due to the vibronic distortion of the 2A(dz2) state. We also measured the emission and excitation spectra of the as-prepared unblended film. There was no difference in the spectral shapes of the unblended and blended films.We also measured the absolute PL quantum yields (Q’s) of the unblended and blended samples dissolved in CH2Cl2. The Q of the unblended Eu(EHEHP)2 solution was 2.31%. PS

minor orbital with a contribution of 25%. For the l3 LUMO, the dzx orbital was predominant with a contribution of 97% (the rest was contributed from the px orbital). This suggests that the dzx orbital was almost unaffected by the C2-symmetry interaction. The mixing of the dyz orbital with the py orbital resulted in the l4 LUMO; the contribution of the dyz orbital was 72%. The l5 LUMO corresponded to the dxy orbital without mixing with other Eu orbitals. Figure S1 shows electron density isocontours of the l1−l5 LUMOs. On the basis of the TD-SCF calculation, we proposed the crystal-field splitting of the five d orbitals, as shown in Figure 5b. The observed four A-, B-, C-, and D-absorption bands were attributed to the transitions from the 8A(4f7; 8S7/2) ground state to the 2A(dz2), 2A(dx2−y2), 2 B(dzx), and 2B(dyz) states, respectively. The energies of the electronic states given in Figure 5b were determined from the barycenters of the observed absorption bands. Those values are comparable with the DFT results (−1.56, −0.10, 0.0, 0.41, and 1.24 eV). Luminescence of Eu(II)/PS Blend. The emission and the excitation spectra of the as-prepared Eu(II)/PS blend film were measured at T = 10 K and room temperature. As shown in Figure 6, the Eu(II) complex exhibited a characteristic

Figure 5. (a) Eu-centered geometry and (b) qualitative crystal-field splitting for d-manifold in the C2 symmetry. D


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blending, however, decreased the luminescence efficiency to Q = 0.74%. This may have occurred because the PS matrix weakly absorbed the 325 nm exciting light (Figure S2), and nonradiative energy loss occurred in the PS matrix. Ag-NP Enhancement. In this work, we attempted to enhance the luminescence of the Eu(II)/PS blend by applying surface-enhanced luminescence technique. We choose Ag NPs as an enhancer and investigated the effects of Ag NPs on the luminescence intensity of the blend as a diffuse state in CH2Cl2 and an interfacial thin-film layer. First, the Ag NP-diffused blend solutions were prepared by mixing different volumes of prepared Ag NP colloidal suspensions (29.0−132.0 μL) with fixed amounts of the blend dissolved in 10.0 mL of CH2Cl2. The critical concentration of Ag NPs was determined by monitoring the luminescence intensity. As shown in Figure 7a,

the Eu(II) blend luminescence as an interfacial layer. Figure 8a shows the emission spectra of the blend formulated on the Ag

Figure 8. (a) PL spectra and (b) relative PL intensity of the Eu(EHEHP)2/PS blend film versus thickness of the Ag NP interfacial layer. The effects of the Ag NP layer on the quantum yield were not taken into account due to an experimental lack of sensitivity.

NP interfacial layer. The vibrational distortion was removed by the Ag NP interfacial layer. The emission intensity of the blend thin film was significantly affected by the thickness of the Ag NP interfacial layer. As shown in Figure 8b, with increasing thickness of the interfacial layer, the emission intensity of the Eu(II)/PS blend film increased and peaked at t = 0.16 μm. The intensity increased 2-fold, as seen in the absence of the interfacial layer. Above this thickness, the intensity decreased. When the thickness was larger than 0.22 μm, the intensity decreased, as seen in the absence of the interfacial layer. Ag NPs as a suspension or an interfacial thin film played two contrastive roles in the luminescence of the Eu(II)/PS blend: as an enhancer or a quencher. Here, the luminescence enhancement due to the nonradiative energy transfer from Ag NPs to Eu(II) was ruled out, because the energy transfer band, spanning over the 250−400 nm region,36did not appear in the excitation spectrum of the 450 nm emission from the blend with the critical concentration of the colloidal Ag NPs or the Ag NP layer. Localized surface plasmon resonance (LSPR)34 formed on the Ag NPs provided very intense electromagnetic enhancement, resulting in the surface-enhanced luminescence of the Eu(II)/PS blend. Alternatively, the energy loss (EL) from the blend to the Ag NPs quenched the luminescence. Below the critical concentrations of the Ag NP colloid or the

Figure 7. (a) PL spectra and (b) Q’s of the Eu(EHEHP)2/PS blend (1.5 g) dissolved in 10 mL of CH2Cl2 versus amount of suspended Ag NPs (ppm is based on the weight of the Eu(EHEHP)2 complex).

the luminescence intensity was significantly affected by the Ag NPs. For accurate quantitative analysis of Ag NPs as diffused particles, the absolute Q of the blend was investigated as a function of the concentration of Ag NPs at a fixed concentration of the blend in CH2Cl2. As shown in Figure 7b, Q was 0.74% without Ag NPs. With increasing ppm of Ag NPs, Q increased and peaked (Q = 3.53%) at 350 ppm of Ag NPs. Above this ppm region, Q decreased with increasing the ppm of Ag NPs. The critical concentration of Ag NPs, resulting in the maximum intensity of the surface-enhanced luminescence of the blend, was ∼350 ppm. The Ag NPs increased the luminescence efficiency of the blend by a maximum of 4.8 times the diffuse state. Next, we investigated the effect of Ag NPs on E


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(11) Dumke, J. C.; El-Zahab, B.; Challa, S.; Das, S.; Chandler, L.; Tolocka, M.; Hayes, D. J.; Warner, I. M. Lanthanide-Based Luminescent NanoGUMBOS. Langmuir 2010, 26, 15599−15603. (12) Saleh, M. I.; Kusrini, E.; Sarjidan, M. A. M.; Majid, W. H. A. Study and Fabrication of Europium Picrate TriethyleneGlycol Complex. Spectrochim. Acta, Part A 2011, 78, 52−58. (13) Biju, S.; Reddy, M. L. P.; Cowley, A. H.; Vasudevan, K. V. 3Phenyl-4-acyl-5-isoxazolonate Complex of Tb3+ Doped into Poly-βhydroxybutyrate Matrix as a Promising Light-Conversion Molecular Device. J. Mater. Chem. 2009, 19, 5179−5187. (14) Natrajan, L. S. Hetero-Polymetallic Complexes Incorporating Luminescent Lanthanide Ions. Curr. Inorg. Chem. 2011, 1, 61−75. (15) Evans, W. J.; Shreeve, J. L.; Ziller, J. W. Synthesis and X-ray Structure of the First Divalent Lanthanide Acetylacetonate Complex, Bis(2,2,6,6-tetramethylheptane-3,5-dionato)bis-(dimethoxyethane)europium(II). Inorg. Chem. 1994, 33, 6435−6437. (16) Hitzbleck, J.; Deacon, G. B.; Ruhlandt-Senge, K. Structural Trends in Alkaline Earth and Rare Earth Metal 3,5-Diisopropylpyrazolates. Eur. J. Inorg. Chem. 2007, 592−601. (17) Daly, S. R.; Girolami, G. S. Synthesis, Characterization, and Structures of Divalent Europium and Ytterbium N,N-Dimethylaminodiboranates. Inorg. Chem. 2010, 49, 4578−4585. (18) Yao, S.; Chan, H.-S.; Lam, C.-K.; Lee, H. K. Synthesis, Structure, and Reaction Chemistry of Samarium(II), Europium(II), and Ytterbium(II) Complexes of the Unsymmetrical Benzamidinate Ligand [PhC(NSiMe3)(NC6H3Pri2-2,6)]−. Inorg. Chem. 2009, 48, 9936−9946. (19) Jiang, J.; Higashiyama, N.; Machida, K.-I.; Adachi, G.-Y. The Luminescent Properties of Divalent Europium Complexes of Crown Ethers and Cryptands. Coord. Chem. Rev. 1998, 170, 1−29. (20) Shinoda, S.; Nishioka, M.; Tsukube, H. In situ Generation of Fluorescent MacrocyclicEuropium(II) Complexes via Zinc Reduction. J. Alloys Compd. 2009, 488, 603−605. (21) Starynowicz, P. Europium(II) Complexes with Unsubstituted Crown Ethers. Polyhedron 2003, 22, 337−345. (22) Summerscales, O. T.; Jones, S. C.; Cloke, F. G. N.; Hitchcock, P. B. Anti-Bimetallic Complexes of Divalent Lanthanides with Silylated Pentalene and Cyclooctatetraenyl Bridging Ligands as Molecular Models for Lanthanide-Based Polymers. Organometallics 2009, 28, 5896−5908. (23) Christoffers, J.; Starynowicz, P. A Europium(II) Complex with Bis-pyridino-18-crown-6. Polyhedron 2008, 27, 2688−2692. (24) Pan, C.-L.; Pan, Y.-S.; Wang, J.; Song, J.-F. A Heterometallic Sandwich Complex of Europium(II) for Luminescent Studies. Dalton Trans. 2011, 40, 6361−6363. (25) Nief, F. Molecular Chemistry of the Rare-earth Elements in Uncommon Low Valent States. In Handbook on the Physics and Chemistry of Rare Earths; Gschneidner, Jr., Bünzli, J.-C. G., Pecharsky, V., Eds.; Elsevier Science: Amsterdam, 2010; Vol. 40, Chapter 246, pp 241−300. (26) Parfenov, A.; Gryczynski, I.; Malicka, J.; Geddes, C. D.; Lakowicz, J. R. Enhanced Fluorescence from Fluorophores on Fractal Silver Surfaces. J. Phys. Chem. B 2003, 107, 8829−8833. (27) Zhang, J.; Fu, Y.; Chowdhury, M. H.; Lakowicz, J. R. MetalEnhanced Single-Molecule Fluorescence on Silver Particle Monomer and Dimer: Coupling Effect between Metal Particles. Nano Lett. 2007, 7, 2101−2107. (28) Al-Kady, A. S.; Gaber, M.; Hussein, M. M.; Ebeid, E.-Z. M. Fluorescence Enhancement of Coumarin Thiourea Derivatives by Hg2+, Ag+, and Silver Nanoparticles. J. Phys. Chem. A 2009, 113, 9474− 9484. (29) Liu, F.; Nunzi, J.-M. Phosphorescent Organic Light Emitting Diode Efficiency Enhancement using Functionalized Silver Nanoparticles. Appl. Phys. Lett. 2011, 99, 123302. (30) Malta, O. L.; Santa-Cruz, P. A.; de Sá, G. F.; Auzel, F. Fluorescence Enhancement Induced by the Presence of Small Silver Particles in Eu2+ Doped Materials. J. Lumin. 1985, 33, 261−272. (31) Dong, G.; Liu, X.; Xiao, X.; Qian, B.; Ruan, J.; Ye, S.; Yang, H.; Chen, D.; Qiu, J. Photoluminescence of Ag Nanoparticle Embedded

Ag NP layer, the LAPR effect was more effective than the EL effect. Consequently, the luminescence of the Eu(II)/PS blend was enhanced.

4. CONCLUSIONS In this report, we presented an approach to quantitatively characterize the effect of Ag NP effects on the luminescence of the Eu(II)/PS blend. We determined the critical concentration of the colloidal Ag NPS in the organic solution and also investigated the dependence of the luminescence on the thickness of the Ag NP ultrafilm as an interfacial layer. The incorporation of Ag NPs into the Eu(II)/PS blend matrix can be applied to a POFA and molecular light-conversion devices.

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ASSOCIATED CONTENT

* Supporting Information S

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +81-42-821-6548. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Research Foundation (2009-0073199).

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REFERENCES

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