Effect of Fluorination on the Radiative Properties of Er3+ Organic

Article pubs.acs.org/JPCC

Effect of Fluorination on the Radiative Properties of Er3+ Organic Complexes: An Opto-Structural Correlation Study H.-Q. Ye,† Y. Peng,†,‡ Z. Li,†,‡ C.-C. Wang,§ Y.-X. Zheng,§ M. Motevalli,‡ P. B. Wyatt,*,‡ W. P. Gillin,*,†,∥ and I. Hernández*,⊥ †

School of Physics and Astronomy and ‡School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS United Kingdom § State Key Laboratory of Coordination Chemistry, Nanjing National Laboratory of Microstructures, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093 People’s Republic of China ∥ College of Physical Science and Technology, Sichuan University, Chengdu, 610064 People’s Republic of China ⊥ MALTA Consolider Team, Dpto. CITIMAC, Universidad de Cantabria, Avda. Los Castros, s/n 39005, Santander Spain S Supporting Information *

ABSTRACT: It is expected that fluorinated organic erbium(III) complexes, of interest for optical applications at λ = 1.5 μm, have improved performance with respect to hydrogenated counterparts. However, the intrinsic radiative properties (including the absorption/emission line strengths) of organic Er3+ complexes have not been systematically studied and compared up to date. This has precluded the observation of opto-structural correlations as well as a proper characterization of the infrared f-f transitions and thus a lack of meaningful figures for the optical efficiency of these materials at the 1.5 μm emission. We have performed a complete opto-structural correlation study of the oscillator strengths of the f-f transitions of hydrogenated and fluorinated organic erbium(III) complexes, including a Judd-Ofelt analysis. The Judd-Ofelt analysis on the crystals has allowed the study of the interdependence of the chemical nature, structure, and spectroscopic behavior. We observe clear trends that can help the design and understanding of these important infrared emitters for phosphor and opto-electronic applications.

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fluorination (and, in particular, decoupling symmetry, distances, and chemical nature effects) has been achieved. What is more, a notable lack of measurements of the experimental oscillator strengths and radiative lifetimes of Er3+ organic complexes overall1,3,4 persists, despite more than 40 years of studies. This fact has impeded a proper comparison of radiative versus real lifetimes and, thus, the general determination and comparison of actual efficiencies. In this paper, we offer an opto-structural correlation study employing Judd-Ofelt (JO) analysis in solutions and solids of the following Er3+ complexes: Cs[Er(hfa)4] (1, hfa = 1,1,1,5,5,5-hexafluoroacetylacetonate), [Er(hfa)3(H2O)2] (2), [Er(acac)3(H2O)2] (3, acac = acetylacetonate), [Er(tpip)3] (4, tpip = tetraphenylimidodiphosphinate), [Er(ftpip)3] (5, ftpip = perfluorotetraphenylimidodiphosphinate). These materials are selected canonical and prototypical examples of hydrogenated or fluorinated complexes, and some of them show remarkably long emission lifetimes.7,10,12 JO theory is a model for describing the radiative processes of trivalent lanthanide ions by considering the effects of the admixture of electronic states through odd-parity ligand

INTRODUCTION Organic compounds with trivalent lanthanides and, in particular, erbium(III) complexes are promising materials to be employed as enhanced near-infrared (IR) emitters for optical fiber amplifiers and biological applications (including labeling and imaging).1−5 This is due to the potentially longlived monochromatic 4I13/2 → 4I15/2 (f-f) transition of the Er3+ ion at ∼1.5 μm (∼6500 cm−1) together with the possibility of photosensitization from organic chromophores in the organic ligands,1−6 which can increase excitation rates by orders of magnitude. However, their prospects for practical use are limited by the low emission efficiencies that are typically achieved due to the usual presence of C-H, N-H, and O-H groups.1−8 These groups cause very effective vibrational quenching of excited states, leading to emission lifetimes in the order of microseconds. Such short times are much smaller than the typical radiative lifetimes (in the order of milliseconds) that are achieved in highly efficient (inorganic) materials and required for applications.1−4,9 Exclusion of C-H and N-H bonds via deuteration or fluorination1−4 of the ligands is a known strategy to extend the lifetime and enhance the efficiency of the 1.5 μm PL.1−4,10,11 While it is evident that fluorination may have an effect on the radiative properties of these compounds, no general or indeed partial understanding of the effect of © 2013 American Chemical Society

Received: September 18, 2013 Revised: October 15, 2013 Published: October 15, 2013 23970

dx.doi.org/10.1021/jp4093282 | J. Phys. Chem. C 2013, 117, 23970−23975

The Journal of Physical Chemistry C

Article

fields.9,13−18 It can provide an unparalleled insight into the roles of the different surrounding species in the optical properties of the rare-earth in terms of the geometry, distances, bonds, and so on and, conversely, predict the radiative properties upon structural and chemical or crystallographic characterization of the complex.8,16−18 The theory offers an expression of the line strengths for the f-f transitions, Scalc(J → J′) as a function of phenomenological coupling parameters, Ωt (JO parameters): Scalc(J → J ′) =

∑

Ω t |⟨(S , L)J U (t ) (S′, L′)J ′⟩|2 (1)

t = 2,4,6

where |⟨(S,L)J∥U ∥(S′,L′)J′⟩| are the reduced matrix elements19 accounting for the electronic (including vibronic) interactions,19−22 and S, J, S′, J′ are the quantum numbers of final and initial states that specify the symmetries of the electronic wave function. Equation 1 gives a parametrization of the observable line strengths in terms of the (intrinsic) electronic and molecular (ligand field coupling) components. The line strengths S(J → J′) of specific electronic transitions are determined experimentally using the following expression: (t)

2

⎤ 3ch(2J + 1)n ⎡ 9 Smeas(J → J ′) = Γ ⎢ 2 2 2⎥ ⎣ (n + 2) ⎦ 8π 3λ ̅ e

Figure 1. Schematic excited states diagram for Er3+ in complexes in the 0−25000 cm−1 range. The exact corresponding values are specific for the different molecules or solids but not very sensitive.9 In contrast, the oscillator strengths of the electronic transitions between the states may be greatly affected by the surroundings of the Er3+.

(2)

where λ̅ is the mean wavelength of corresponding absorption bands; J′ and J are the quantum numbers of total angular momentum of the initial state and the excited state, respectively; n is the refractive index of the host material [in the solution, the refractive index of the specific solvent for the visible range was employed in the JO analysis (Table S2). The average refractive indexes of the crystals employed in the measurement of absorption spectra were estimated through the Becke line method and are given in Table S2]. Γ is the integrated absorption coefficient Γ = ∫ ε(λ̅)dλ̅, where ε is the absorption coefficient that we determine from the absorption spectra by ε = A/NL, where A denotes the corresponding absorbance, N is the Er3+ concentration, and L is the thickness of the sample. The factor [9/(n2 + 2)2] represents the local field correction of electric dipole transition. The radiative decay coefficient corresponding to each transition can be derived from the line strengths: A (J ′ ↔ J ) =

by the magnitude of the corresponding oscillator strengths. For instance, it can be shown that Ω2 is mostly influenced by the hypersensitive transitions9,20 and that the line strength of 4I15/2 → 4I13/2 and, therefore, its radiative lifetime, τr1.5 μm, are predominantly determined by Ω6.22,23 The application of JO analysis to organic Er3+ complexes has been limited by two factors. First, most reported instances have been done mostly in H-containing media that impede the direct observation of the 4I13/2 → 4I15/2 transition at 1.5 μm. Although information on this transition may be inferred to some extent by the measurements on the others, immediate comparison with the exact value is not usually possible. This is crucial in Er3+ compounds.3,16−18,24−26 Second, solutions do not grant direct access to intrinsic geometric/chemical properties due to solvation4 and, in some cases, the risk of having a mixture of species due to partial decomposition or displacement of ligands.27 The lack of systematic studies hampers the establishment of general rules to help in the design of efficient materials. This is particularly relevant in the case of fluorinated compounds, which are considered to show the highest efficiencies.1−3 In this work we obtain the JO parameters and related spectroscopic factors for both solutions and single crystals. Moreover, in the case of crystals, we also derive the corresponding values employing the 4I15/2 → 4I11/2 and 4I15/2 → 4I13/2 IR bands at 980 and 1540 nm, respectively, which allow us to test the validity of the JO analysis using only the visible lines.

⎡ n(n2 + 2)2 ⎤ ⎥Scalc(J → J ′) 9 ⎦ 3h(2J + 1)λ ̅ ⎣ 64π 4e 3

3⎢

(3)

and, therefore, the radiative lifetime τr = 1/A, represents another measure of the oscillator strength of the corresponding J ↔ J′ transition. The comparison of measured and calculated line strengths provides the phenomenological parameters Ω2, Ω4, and Ω6 through fitting. The Ωts describe the environment effect and account for a combination of many factors. They depend on the geometry and chemical composition of the lanthanidecontaining compound but comparison between materials must be carefully done (especially for some lanthanides and transitions),14,15,19,21 restricting them to specific systematic changes such as chemical series or substitutions. By developing eq 1 for the Er3+ level scheme (Figure 1) and analyzing the contributions for the transitions 4I15/2 → excited states, we can determine how the Ωt parameters contribute to the corresponding line strengths of the Er3+ f-f transitions and, accordingly, how the individual JO parameters are influenced

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RESULTS AND DISCUSSION Figure 2a−d depicts the structures of 2, 3, 4, and 5, as characterized by single crystal X-ray diffraction. Solutions of 1, 2, and 3 in methanol, dimethyl sulfoxide (DMSO), and tetrahydrofuran (THF), as well as 4 in chloroform and 5 in DMSO were studied spectroscopically (spectra are given in the Supporting Information). 23971



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Figure 2. Structure of studied complexes (a) Er(hfa)3(H2O)2, 2, (b) [Er(acac)3(H2O)2](H2O)(CH3CH2OH), 3, (c) Er(tpip)3, 4, and (e) Er(ftpip)3, 5, as determined from single crystal X-ray diffraction.

Figure 3a shows the absorption spectra of the diketonates, and Figure 3b shows the absorption of the imidodiphosphinates, all obtained on the single crystals. The figures (bottom graphs) show the low energy bands around 980 nm (4I15/2 → 4 I11/2) and 1540 nm (4I15/2 → 4I13/2), which are otherwise impossible to obtain due to the absorbance by the solvents. For the analysis, we have estimated an average refractive index and measured the thickness of the crystals (see Supporting Information). Our work gives access to a full set of calculated and measured line strengths for the f-f transitions of Er3+ and a corresponding set {Ω2, Ω4, Ω6} for each of the complexes in crystals and dissolved in a number of solvents (Tables 1−3). The values of the Ωts or τrad1.5 μm for Er3+ are strongly dependent on the nature and geometry of the compound, so this work cannot provide a general comparison of their magnitude with respect to other values for organic or inorganic hosts.3,4,9,14,15,23 However, our choice of materials permits studying the influence of the chemical composition, solvent, and structure within the set. We must note that 1 in THF represents an instance where the JO analysis fails, as the value obtained for Ω4 is negative. The reason for the failure likely reflects the decomposition of the complex as a result of the THF acting as a stronger Lewis base and causing the segregation of Cs(hfa), which is observed by the precipitation of this salt upon evaporation of the original solution. Solutions (Table 1) and crystals (Tables 2 and 3) provide significantly different values of the JO parameters and, particularly, Ω2 (which is the most sensitive to structural changes). It has been proposed that solvents can penetrate some complexes and achieve substantial coordination with the Er3+ ion.21 This has been reported to cause an increase in the value of Ω2, reflecting the associated distortion in the crystal field.9,21 Our work allows comparison of the exact values for unsolvated ions in crystals with the values from the solutions to demonstrate that Ω2 increases in all the studied cases except in the case of the 2 and 5 in DMSO. The fact that Ω2 differs in crystal and solution even in these cases suggests that DMSO displaces coordinated H2O and symmetrically coordinates to

Figure 3. Absorption spectra of the studied Er3+ complexes in solid state: (a) 1, 2, and 3; (b) 4 and 5. The peaks are labeled according to the states involved (see Figure 1).

Er3+ in 2 or that it causes a weakening of the Er-hfa or Er-ftpip bonds 27 via changes in covalent contributions to the bonds.9,10,14,15,22 Importantly, Tables 2 and 3 show similar values for Ω2, but considerably different values for Ω6. The JO analysis without the IR lines also results in a poor determination of Ω4, which has negative values with large errors. This proves that the actual measurement of the IR lines is critical for studying this parameter or, in other words, that a JO analysis without the IR lines may be not sufficiently reliable for determining the value of the oscillator strength and the radiative lifetime for the 4I15/2 ↔ 4I13/2 Er3+ transition in organic complexes. Table 3 shows that, for given ligands and geometries, fluorinated species show a higher Ω6, or reciprocally, increased oscillator strength for the 4I15/2 ↔ 4I13/2 line. This is, in principle, surprising as the chemical changes bear no major variations in the corresponding distances or geometries and the rigidity of the fluorinated compounds can be expected to be larger.13,22 However, Ω6 has been found to be greatly affected by the change of the magnitude of the ⟨4f |rs|5d⟩ radial integrals (where s = 1, 3, 5, as determined by the d symmetry and selection rules of 3-j and 6-j symbols)23−26,28 as the lanthanide 23972



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Table 1. JO Parameters Obtained by Fitting the Calculated Line Strengths to the Experimental Ones and Radiative Lifetime for the 4I15/2 ↔ 4I13/2 Transition, τr1.5μm, as Derived from the Line Strength in Solutions of the Corresponding Complexes in Methanol (CH3OH), DMSO ((CH3)2SO), THF ((CH2)4O), and Chloroform (CHCl3) material

solvent

Cs[Er(hfa)4] Er(hfa)3(H2O)2 Er(acac)3(H2O)2 Cs[Er(hfa)4] Er(hfa)3(H2O)2 Er(acac)3(H2O)2 Cs[Er(hfa)4] Er(hfa)3(H2O)2 Er(acac)3(H2O)2 Er(tpip)3 Er(ftpip)3

(1) (2) (3) (1) (2) (3) (1) (2) (3) (4) (5)

Ω2 × 10−20 (cm−1)

Ω4 × 10−20 (cm−1)

Ω6 × 10−20 (cm−1)

τrad1.5 μm (ms)

26.86 23.66 21.07 14.79 11.99 23.52 33.57 22.60 17.99 6.32 2.37

0.43 0.69 0.94 1.00 0.79 0.57 −0.24 0.47 3.52 0.82 0.30

0.89 1.05 0.72 0.79 1.32 0.80 1.11 1.12 0.43 0.34 1.00

18 17 22 16 11 15

methanol

DMSO

THF

chloroform DMSO

outer orbitals interact with those of the ligand.14,15,23,28 The exchange of H for F alters the electron density in the surroundings of the Er3+, the high electronegativity of F resulting in an electron withdrawal at the donor site;29 this influences the population of orbitals and, thus, these radial integrals. Our observation upon fluorination is consistent with the variation along the series, Ω6 [4] < Ω6 [3] < Ω6 [1] < Ω6 [2]; Ω6 [4] < Ω6 [5] as the electron density around Er3+ tends to decrease, in qualitative agreement with the nephelauxetic effect observed in the corresponding Eu3+ complexes.12,23,30−32 According to standard models, a higher Ω6 with a reduced electron density in the rare-earth coordination sphere indicates a smaller repulsion of the Er3+ 5d orbitals and a longer reach of the 6s orbitals toward the ligands.22,23,28 The increase of the oscillator strength of the 4I15/2 ↔ 4I13/2 transitions means a considerable reduction of the 1.5 μm radiative lifetime, which is nearly halved for the studied fluorinated chelates with respect to the hydrogenated counterparts. This is an important phenomenon that must be considered when studying the emission efficiency of organic Er3+ complexes (see Supporting Information).

Table 2. JO Parameters, Calculated Line Strengths and Radiative Lifetime for the 4I15/2 ↔ 4I13/2 Transitions of the Studied Chelates in Crystals as Derived from the UV-vis Absorption Spectra, Excluding the IR Transitions 4I15/2 → 4 I11/2 and 4I15/2 → 4I13/2 (Figure 3a,b, top) Ω2 × 10−20 (cm−1)

Ω4 × 10−20 (cm−1)

Ω6 × 10−20 (cm−1)

(1) (2) (3)

7.34 19.84 9.86

−0.17 −0.37 0.71

1.15 1.64 0.82

1.77 2.69 1.44

13.2 7.9 14.7

(4) (5)

4.11 6.82

−0.07 1.26

0.66 0.50

1.02 0.99

17.4 17.5

crystal sample Cs[Er(hfa)4] Er(hfa)3(H2O)2 [Er(acac)3 (H2O)2] (H2O) (CH3CH2OH) Er(tpip)3 Er(ftpip)3

Scalc1.5 μm τrad1.5 μm (cm−1) (ms)

Table 3. JO Parameters, Calculated and Measured Line Strengths and Radiative Lifetimes for the 4I15/2 ↔ 4I13/2 Transitions of the Studied Chelates in Crystals as Derived from the Absorption Spectra, Including the IR Transitions 4 I15/2 → 4I11/2 and 4I15/2 → 4I13/2 (Figure 3)a

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CONCLUSIONS This work provides direct measurement of the 4I15/2 → 4I11/2 and 4I15/2 → 4I13/2 transitions line strengths of Er3+ in fluorinated and nonfluorinated complexes and includes these transitions in a systematic JO analysis for the first time, allowing us to compare the results for solutions and crystals. Our results have strong methodological and quantitative implications on the determination of the emission efficiency, as they show that these bands are critical for the analysis. We show that fluorinated complexes show enhanced oscillator strengths for the technologically important 1.5 μm band. Hence, it opens new routes in their study and design for photonic and optoelectronic applications.

τrad1.5 μm

crystal samples Cs[Er(hfa)4]

Er(hfa)3(H2O)2

[Er(acac)3 (H2O)2] (H2O) (CH3CH2OH) Er(tpip)3

Er(ftpip)3

JO param. × 10−20 (cm−1) Ω2 Ω4 Ω6 Ω2 Ω4 Ω6 Ω2 Ω4 Ω6 Ω2 Ω4 Ω6 Ω2 Ω4 Ω6

7.11 0.35 0.65 19.59 0.15 1.19 9.77 0.90 0.63 4.06 0.04 0.58 6.93 1.00 0.73

Scalc1.5 μm Sm1.5 μm (cm−1) (cm−1)

calcd (ms)

measured (ms)

1.12

1.00

20.9

23.4

2.11

2.10

9.5

∼9.5

1.17

1.20

17.4

16.9

0.91

0.94

19.0

18.4

1.30

1.34

13.0

13 21 38 16

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EXPERIMENTAL SECTION Syntheses of the materials were performed according to published methods. Single crystals of up to hundreds of microns were obtained via slow evaporation from appropriate solvents at room temperature. UV−visible absorption spectra of solutions in quartz cuvettes were taken with a UV-3000 PerkinElmer spectrometer. Absorption spectra on crystals were obtained via a home- made single-beam microabsorption setup, similar to those employed for high pressure spectroscopy with the randomly oriented crystal flatly deposited on a quartz

12.6

a

The measured radiative lifetime has been derived from the corresponding line strength of the 1.5 μm absorption line (see Supporting Information).

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(8) Tan, R. H. C.; Motevalli, M.; Abrahams, I.; Wyatt, P. B.; Gillin, W. P. Quenching of IR Luminescence of Erbium, Neodymium, and Ytterbium β-Diketonate Complexes by Ligand C−H and C−D Bonds. J. Phys. Chem. B 2006, 110, 24476−24479. (9) Becke, P. C.; Olsson, N. A.; Simpson, J. R. Erbium Doped Fiber Amplifiers: Fundamentals and Technology; Academic Press: San Diego, 1999; pp131−149. (10) Mancino, G.; Ferguson, A. J.; Beeby, A.; Long, N. J.; Jones, T. S. Dramatic Increases in the Lifetime of the Er3+ Ion in a Molecular Complex Using a Perfluorinated Imidodiphosphinate Sensitizing Ligand. J. Am. Chem. Soc. 2005, 127, 524−525. (11) Hernández, I.; Tan, R. H. C.; Pearson, J. M.; Wyatt, P. B.; Gillin, W. P. Nonradiative De-excitation Mechanisms in Long-Lived Erbium(III) Organic Compounds ErxY1−x[(p-CF3-C6F4)2PO2]3. J. Phys. Chem. B 2009, 113, 7474−7481. (12) Glover, P. B.; Bassett, A. P.; Nockemann, P.; Kariuki, B. M.; Deun, R. V.; Pikramenou, Z. Fully Fluorinated Imidodiphosphinate Shells for Visible- and NIR-Emitting Lanthanides: Hitherto Unexpected Effects of Sensitizer Fluorination on Lanthanide Emission Properties. Chem.Eur. J. 2007, 13, 6308−6320. (13) Reid, M. F.; Richardson, F. S. Lanthanide 4f−4f Electric Dipole Intensity Theory. J. Chem. Phys. 1984, 88, 3579−3586. (14) Hehlen, M. P.; Brik, M. G.; Krämer, K. W. 50th Anniversary of the Judd-Ofelt Theory: An Experimentalist’s View of the Formalism and its Application. J. Lumin. 2013, 36, 221−239. (15) Görller-Walrand, C.; Binnemans, K. Handbook on the Physics and Chemistry of Rare Earths; North-Holland: Amsterdam, Netherlands, 1998; Vol. 25. (16) Judd, B. R. Optical Absorption Intensities of Rare-Earth Ions. Phys. Rev. 1962, 127, 750−761. (17) Ofelt, G. S. Intensities of Crystal Spectra of Rare-Earth Ions. J. Chem. Phys. 1962, 37, 511−520. (18) Judd, B. R. Configuration Interaction in Rare Earth Ions. Proc. Phys. Soc. 1963, 82, 874−881. (19) Carnall, W. T.; Fields, P. R.; Wybourne, B. G. Spectral Intensities of the Trivalent Lanthanides and Actinides in Solution. I. Pr3+, Nd3+, Er3+, Tm3+, and Yb3+. J. Chem. Phys. 1965, 42, 3797−3806. (20) Hatanaka, M.; Yabushita, S. Theoretical Study on the f-f Transition Intensities of Lanthanide Trihalide Systems. J. Phys. Chem. A 2009, 113, 12615−12625. (21) Karraker, D. G. Hypersensitive Transitions of Six-, Seven-, and Eight-Coordinate Neodymium, Holmium, and Erbium Chelates. Inorg. Chem. 1967, 6, 1863−1868. (22) Jørgensen, C. K.; Reisfeld, R. Judd-Ofelt Parameters and Chemical Bonding. J. Less-Common Met. 1983, 93, 107−112. (23) Tanabe, S.; Hanada, T.; Ohyagi, T.; Soga, N. Correlation between 151Eu Mössbauer Isomer Shift and Judd-Ofelt Ω6 Parameters of Nd3+ Ions in Phosphate and Silicate Laser Glasses. Phys. Rev. B 1993, 48, 10591−10594. (24) Song, L.; Wang, J.; Hu, J.; Liu, X.; Zhen, Z. Synthesis and Optical Properties of a New Fluorinated Erbium Complex/Polymer Composite Material. J. Alloys Compd. 2009, 473, 201−205. (25) Wang, H.; Qian, G.; Wang, Z.; Wang, M. Spectroscopic Properties and Judd−Ofelt Theory Analysis of Erbium Chelates. Spectrochim. Acta, Part A 2005, 62, 146−152. (26) Sun, L.-N.; Zhang, H.-J.; Fu, L.-S; Liu, F.-Y.; Meng, Q.-G.; Peng, C.-Y.; Yu, J.-B. A New Sol−Gel Material Doped with an Erbium Complex and Its Potential Optical-Amplification Application. Adv. Funct. Mater. 2005, 15, 1041−1048. (27) Hasegawa, Y.; Ohkubo, T.; Sogabe, K.; Kawamura, Y.; Wada, Y.; Nakashima, N.; Yanagida, S. Luminescence of Novel Neodymium sulfonylaminate Complexes in Organic Media. Angew. Chem., Int. Ed. 2000, 39, 357−360. (28) Ebendorff-Heidepriem, H.; Ehrt, D.; Bettinelli, M.; Speghini, A. Effect of Glass Composition on Judd−Ofelt Parameters and Radiative Decay Rates of Er3+ in Fluoride Phosphate and Phosphate Glasses. J. Non-Cryst. Solids 1998, 240, 66−78. (29) Lewis, F. D.; Salvi, G. D.; Kanis, D. R.; Ratner, M. A. Electronic Structure and Spectroscopy of Nickel(II), Palladium(II), and

substrate and focused by corresponding Cassegrain and Mitutoyo glass objectives. Absorption spectra of the 360−700 nm region were taken using a Xe lamp and a Hamamatsu 9113B PMT and IR absorption spectra in the 700−1600 nm range were taken using a tungsten lamp and a Hamamatsu R5509-72 nitrogen-cooled detector. A Triax 550 monochromator equipped with 1200 and 600 lines/mm gratings was employed for UV and IR bands, respectively. Signals were obtained by a 7265 DSP Perkin-Elmer lock-in amplifier and recorded on a PC with the resolution of 2 nm. The Becke line test with calibrated liquids (Cargille) was employed to estimate an averaged value for the refractive index of microcrystals. The crystal thicknesses were measured by a graduated microscope equipped with a micropositioning stage (accuracy = 5 μm). The crystals employed in XRD are the same as those employed for the absorption measurements.

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ASSOCIATED CONTENT

S Supporting Information *

CIF files of new structures, sample and crystal preparation, Xray crystallography, absorption in solution, line strength determination, extended discussion of the JO analysis, and experimental emission quantum yields for 1.5 μm emission of the studied complexes. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]; ignacio. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the UK National Crystallography Service, Southampton, for data collection and the Royal Academy of Engineering, EPSRC, China Scholarship Council (CSC), MALTA Ingenio (CSD2007-00045), and EU-FP7 (Marie Curie CIG Grant 303535) for financial support.

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REFERENCES

(1) Eliseeva, S. V.; Bünzli, J.-C. G. Lanthanide Luminescence for Functional Materials and Bio-Sciences. Chem. Soc. Rev. 2010, 39, 189− 227. (2) Bünzli, J.-C. G.; Piguet, C. Taking Advantage of Luminescent Lanthanide Ions. Chem. Soc. Rev. 2005, 34, 1048−1077. (3) Kuriki, K.; Koike, Y.; Okamoto, Y. Plastic Optical Fiber Lasers and Amplifiers Containing Lanthanide Complexes. Chem. Rev. 2002, 102, 2347−2356. (4) Koeppen, C.; Yamada, S.; Jiang, G.; Garito, A. F.; Dalton, L. R. Rare-Earth Organic Complexes for Amplification in Polymer Optical Fibers and Waveguides. J. Opt. Soc. Am. B 1997, 14, 155−162. (5) White, K. A.; Chengelis, D. A.; Gogick, K. A.; Stehman, J.; Rosi, N. L.; Petoud, S. Near-Infrared Luminescent Lanthanide MOF Barcodes. J. Am. Chem. Soc. 2009, 131, 18069−18071. (6) Pizzoferrato, R.; Francini, R.; Pietrantoni, S.; Paolesse, R.; Mandoj, F.; Monguzzi, A.; Meinardi, F. Effects of Progressive Halogen Substitution on the Photoluminescence Properties of an Erbium− Porphyrin Complex. J. Phys. Chem. A 2010, 114, 4163−4168. (7) Winkless, L.; Tan, R. H. C.; Zheng, Y.; Motevalli, M.; Wyatt, P. B.; Gillin, W. P. Quenching of Er(III) Luminescence by Ligand C−H Vibrations: Implications for the Use of Erbium Complexes in Telecommunications. Appl. Phys. Lett. 2006, 89, 111115/3. 23974



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Platinum(II) Acetylacetonate Complexes. Inorg. Chem. 1993, 32, 1251−1258. (30) Frey, S. T.; Horrocks, W. deW. On Correlating The Frequency of the 7F0 → 5D0 Transition in Eu3+ Complexes with the Sum of ‘Nephelauxetic Parameters’ for all of the Coordinating Atoms. Inorg. Chim. Acta 1995, 229, 390. (31) Albin, M.; Horrocks, W. d. Europium(III) Luminescence Excitation Spectroscopy. Quantitative Correlation Between the Total Charge on the Ligands and the 7F0 → 5D0 Transition Frequency in Europium(III) Complexes. Inorg. Chem. 1985, 24, 895−900. (32) Fan, X.; Wu, X.; Wang, M.; Qiu, J.; Kawamoto, Y. Luminescence Behaviors of Eu3+ β-Diketonate Complexes in Sol−Gel-Derived Host Materials. Mater. Lett. 2004, 58, 2217−2221.

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Effect of Fluorination on the Radiative Properties of Er3+ Organic

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