Brightness and Color Tuning in a Series of Lanthanide-Based

Article pubs.acs.org/IC

Brightness and Color Tuning in a Series of Lanthanide-Based Coordination Polymers with Benzene-1,2,4,5-tetracarboxylic Acid as a Ligand Stéphane Freslon, Yun Luo,† Carole Daiguebonne,* Guillaume Calvez, Kevin Bernot, and Olivier Guillou* INSA, UMR 6226, Institut des Sciences Chimiques de Rennes, 20 Avenue des buttes de Coësmes, F-35708 Rennes, France S Supporting Information *

ABSTRACT: Reactions in water between the sodium salt of benzene-1,2,4,5tetracarboxylic acid (H4btec) and lanthanide ions (Sm−Dy) led to a series of isostructural lanthanide-based coordination polymers with the general chemical formula [Ln4(btec)3(H2O)12·20H2O]∞ with Ln = Sm−Dy. The family has been structurally characterized. From a luminescent point of view, the heterodinuclear strategy has been successfully applied and allows significant brightness enhancement (+35%) and color tuning from green to yellow to orange to red.

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diversity47); (iv) its conjugated π system can produce an efficient “antenna effect”48 for lanthanide-ion excitation. To date, numerous lanthanide-based coordination polymers involving pyromelitate as the ligand have been reported.47,49−57 57 Most of them have been obtained by hydrothermal methods. We want to report here a family of coordination polymers based on btec4− that can be obtained in high yield in water under atmospheric conditions.58,59 It presents some interesting and tunable luminescent properties that have been investigated in detail.

INTRODUCTION For almost 2 decades, lanthanide-based coordination polymers have been widely studied because of their potential application in various fields such as gas storage, 1−12 catalysis, 13 separation,14 luminescence15−24 or molecular magnetism.25−30 More recently, heteronuclear lanthanide-based coordination polymers have attracted much attention because of their tunable luminescent properties,31−35 and it has been proven that these compounds present a real technological interest as tags against counterfeiting.36−41 Our group is involved in this field of research,32,33,42−44 and the search for new ligands that could lead to new structural network and/or new physical properties is a continuous concern.45,46 Therefore, we have decided to study lanthanide-based coordination polymers with benzene-1,2,4,5-tetracarboxylic acid (or pyromellitic acid) as a ligand (Scheme 1). This ligand has attracted our interest because of its following characteristics: (i) it can coordinate with metal ions in various coordination modes because its four carboxylic functions can be partially or totally deprotonated; (ii) it can act as hydrogenbond donor or acceptor; (iii) all of its four carboxylate groups do not lie in the phenyl plane (a key point to favor structural

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EXPERIMENTAL SECTION

Syntheses of the Starting Salts. Benzene-1,2,4,5-tetracarboxylic acid (H4btec) was purchased from Sigma-Aldrich. Tetrasodium benzene-1,2,4,5-tetracarboxylate salt was prepared by the addition of 4 equiv of sodium hydroxide to a suspension of H4btec in deionized water. A limpid solution was obtained that was evaporated to dryness. The solid was digested in ethanol for 1 h. The addition of ethoxyethane provoked precipitation. After filtration and drying, a white powder of tetrasodium salt of H4btec was collected in 90% yield. Elem anal. Calcd (found) for C10H2O8Na4 (MW = 342.08 g mol−1): C, 35.1 (35.0); H, 0.6 (0.7); O, 37.4 (37.3); Na, 26.9 (27.0). Hydrated lanthanide chlorides were synthesized from the corresponding oxides as described elsewhere.60 Lanthanide oxides (99.99%) were purchased from AMPERE Company. Syntheses of the Single Crystals. Tetraethylorthosilicate (TEOS) was purchased from Acros Organics and jellified, as already described.57,61,62 Aqueous solutions of lanthanice(III) chloride (0.1 mol L−1) and tetrasodium benzene-1,2,4,5-tetracarboxylate (0.1 mol L−1) were allowed to slowly diffuse through a gel bridge (TEOS 7.5%

Scheme 1. Benzene-1,2,4,5-tetracarboxylic Acid, Hereafter Symbolized by H4btec

Received: October 1, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/acs.inorgchem.5b02242 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry by weight) in U-shaped tubes. After some weeks, single crystals were obtained. Syntheses of the Microcrystalline Powders. Microcrystalline powders of the homonuclear coordination polymers were obtained by mixing stoichiometric amounts of lanthanide chloride in water with the tetrasodium salt of H4btec. The precipitates were filtered and dried in air. The yields of the reactions were close to 90%. The obtained microcrystalline powders were assumed to be isostructural to [Sm4(btec)3(H2O)12·20H2O]∞ (the crystal structure is described hereafter) on the basis of their powder X-ray diffraction patterns. Experimental and simulated X-ray diffraction patterns are reported in Figure S1. The cell parameters of the compounds of the series have been refined on the basis of powder X-ray diffraction diagrams (see Figure S1). The results of elemental analyses are listed in Table S1. Microcrystalline powders of heteronuclear coordination polymers were obtained by a similar procedure by replacing the lanthanide chloride solution with a relevant mixture of lanthanide chloride solutions in the synthetic process. The microcrystalline powders that were obtained were assumed to be isostructural to the corresponding homonuclear compounds, on the basis of their powder X-ray diffraction diagrams. As examples, X-ray diffraction patterns for some heteronuclear compounds are reported in Figures S2−S4. The relative ratios between the two different lanthanide ions were measured by energy-dispersive spectrometry (EDS). Some results are listed in Tables S2−S4. Powder X-ray Diffraction. The diagrams were collected using a PanalyticalX’Pert Pro diffractometer equipped with an X’Celerator detector. Calculated patterns were produced using the POWDERCELL and WINPLOTR software programs.63,64 Pattern indexing was performed by the MACMAILLE program65 and refinement of the unit cell parameters by means of the CHECKCELL program from the CRYSTFIRE suite.66 Thermal-dependent X-ray diffraction diagrams were produced with the same diffractometer. The samples were heated from room temperature to 1000 °C using an Anton Parr HTK 1200 furnace under a nitrogen atmosphere. These experiments evidenced that the compounds present a poor thermal stability and the framework collapses after 100 °C (Figure S5). Single-Crystal X-ray Diffraction. Single crystals of [Sm4(btec)3(H2O)12·20H2O]∞ were mounted on a APEXII AXSBruker diffractometer with Mo Kα radiation (λ = 0.71073 Å). Crystal data collection was performed at room temperature. Data reduction and cell refinement were performed with the Denzo and Scalepack programs.67 The crystal structure was solved by direct methods using the SIR97 program68 and refined with full-matrix least-squares methods based on F2 (SHELX9769) with the WINGX program.70 All non-hydrogen atoms were refined anisotropically using the SHELXL program. Hydrogen atoms bound to the organic ligand were located at ideal positions. Hydrogen atoms of the water molecules were not located. Crystal and final structure refinement data of [Sm4(btec)3(H2O)12·20H2O]∞ are listed in Table 1. Full details of the X-ray structure determination of [Sm4(btec)3(H2O)12·20H2O]∞ are given in the Supporting Information and have also been deposited with the Cambridge Crystallographic Data Centre under the depository number CCDC 862207, which can be obtained free of charge upon request at http://www.ccdc.cam.ac.uk/conts/retrieving. html [or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 IEZ, U.K.; fax (int.) +44-1223/336-033; e-mail [email protected]]. Physical Measurements. Fourier transform infrared measurements, energy-dispersive spectroscopy (EDS), solid-state luminescent measurements, UV−vis absorption measurements, and colorimetric measurements were performed as described elsewhere.42,43 IR spectra support the crystal structure. Indeed, all spectra clearly show the expected strong characteristic absorptions for the symmetric and asymmetric vibrations of pyromelitate (1650−1550 and 1420− 1335 cm−1) and no absorption band of any protonated carboxylic group (1715−1680 cm−1). EDS measurements support the monophasic character of the samples.

Table 1. Crystal and Final Structure Refinement Data for [Sm4(btec)3(H2O)12·20H2O]∞ molecular formula cryst syst a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z formula weight (g mol−1) space group Dcalc (g cm−3) μ (mm−1) R (%) Rw (%) GOF CCDC

Sm2C15O28H35 triclinic 9.0978(3) 11.7166(5) 14.3709(7) 87.015(2) 80.229(2) 86.041(2) 1504.9(2) 2 963.72 P1̅ (No. 2) 2.128 3.974 4.59 13.53 1.242 862207

The solid-state luminescence (77 K) and UV−vis absorption (room temperature) spectra of the homonuclear gadolinium-containing compound were recorded for estimating respectively the lowest triplet and singlet states of the ligand energies (Figures S6 and S7). The molar absorption coefficient of the pyromelitate ligand was evaluated on the basis of solution-state UV−visible absorption spectra (Figure S8).

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RESULTS AND DISCUSSION Crystal Structure. The crystal structure has been solved on the basis of a samarium-containing single crystal. It is bidimensional. There are two independent btec4− ligands per unit cell. One of them is centered on an inversion center. It links four Sm1 ions in a bidentate manner. The second one binds four Sm2 and two Sm1 ions. It can be noticed that its four carboxylate clips present four different coordination modes (see Scheme 2). Both Sm3+ coordination polyhedra can best be described as slightly distorted tricapped trigonal prisms. Both Sm3+ ions are coordinated by nine oxygen atoms. Sm1 is bound to six oxygen atoms from three bidentate carboxylate clips, one oxygen atom from a monodentate carboxylate clip, and two oxygen atoms from coordination water molecules. Sm2 is bound to four oxygen atoms from coordination water molecules, three oxygen atoms from two different μ-oxo carboxylate groups, and four oxygen atoms from two ambidentate carboxylate groups (see Scheme 3). This leads to dinuclear species with short Sm2− Sm2 distances [3.9639(13) Å]. The crystal structure is 2D and can be described as a superimposition of wavy molecular layers that spread perpendicular to the b⃗ + c ⃗ direction (see Figure 1). In these planes, two alignments of Sm1 ions alternate with alignments of Sm2-based dinuclear species that spread parallel to the a⃗ axis. Except the short intermetallic distance between two Sm2 ions that belong to the same dinuclear entity, intermetallic distances between Sm3+ ions that belong to the same molecular wavy plane are quite long (between 6 and 10 Å). Shortest intermetallic distances between Sm3+ ions that belong to adjacent bidimensional molecular layers are quite long as well, at least 6.9 Å (Figure 2). The mean distance between the lanthanide ions in the crystal structure was estimated by a method that has already been B


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Inorganic Chemistry Scheme 2. Coordination Modes of the Two Independent btec4− Ligands

Scheme 3. Coordination Environment of Sm3+ Ions

described.71 In the present case, the mean volume is roughly v ̅ = 376 Å3 per metallic ion and corresponds to the volume of a sphere with a radius of 4.47 Å (r =

3

3v ̅ 4π

Figure 2. Intermolecular intermetallic distances: Sm1−Sm2i, 8.2570(28); Sm2−Sm2iv, 11.717(5); Sm1−Sm2ii, 8.6869(28); Sm1− Sm1v, 6.9842(23); Sm2−Sm2iii, 8.6206(35). Symmetry codes: (i) −1 + x, y, 1 + z; (ii) 2 − x, −y, 1 − z; (iii) 3 − x, −y, 1 − z; (iv) x, −1 + y, z; (v) 1 − x, 1 − y, 2 − z.

). According to this

quite rough calculation, the mean distance between two lanthanide ions is 8.94 Å. Crystallization water molecules are located between the 2D molecular layers. Crystal packing stability is ensured by a complex hydrogen-bonding network that involves crystallization and coordination of water molecules and oxygen atoms of the carboxylate groups. It is noticeable that these compounds are highly hydrated (32 water molecules per formula unit). This can be related to the synthetic method that has been used.

Indeed, compounds that are obtained by hydrothermal methods are usually less hydrated. This is actually the case in this system: previously reported lanthanide-based coordination polymers with pyromelitate that have been synthesized by h ydroth ermal m eth ods present a low hy drat ion rate.49,50,52−55,72

Figure 1. Left: Projection view along the a⃗ axis of [Sm4(btec)3(H2O)12·20H2O]∞. Right: Projection view of a molecular layer. Some intermetallic distances smaller than 10 Å are shown: Sm1−Sm1i, 6.7735(22); Sm1−Sm2iv, 10.1085(39); Sm1−Sm1ii, 9.098(5); Sm1−Sm1v, 9.098(5); Sm1− Sm2iii, 6.2323(17); Sm2−Sm2v, 9.098(5); Sm1−Sm2ii, 6.3662(22); Sm2−Sm2vi, 9.4282(41); Sm1−Sm2, 10.6519(44). Symmetry codes: (i) 1 − x, −y, 2 − z; (ii) −1 + x, y, z; (iii) 2 − x, 1 − y, 1 − z; (iv) 3 − x, 1 − y, 1 − z; (v) 1 + x, y, z; (vi) 4 − x, 1 − y, 1 − z. C


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Inorganic Chemistry

Figure 3. Excitation and emission spectra for [Ln4(btec)3(H2O)12·20H2O]∞, where Ln = Sm−Dy except Gd.

Table 2. Main Experimental Features of the Emission Spectra of [Ln4(btec)3(H2O)12·20H2O]∞, Where Ln = Sm− Dy except Gd, under UV Irradiation (λexc = 306 nm) observed transitions [Sm4(btec)3(H2O)12· 20H2O]∞

G5/2 → 6HJ

4

(J = 5/2 − 11/2) [Eu4(btec)3(H2O)12· 20H2O]∞ [Tb4(btec)3(H2O)12· 20H2O]∞ [Dy4(btec)3(H2O)12· 20H2O]∞

D0 → 7FJ

5

(J = 0−6) 5 D4 → 7FJ (J = 6−0) 4 F9/2 → 6HJ

main transitions

wavelength (nm)

G5/2 → 6 H7/2 4 G5/2 → 6 H9/2 5 D0 → 7F2

599

D4 → 7F5

545

4

5

Table 3. Overall Luminescent Quantum Yields and Observed Lifetimes for [Ln4(btec)3(H2O)12·20H2O]∞, Where Ln = Sm−Dy except Gd [Sm4(btec)3(H2O)12·20H2O]∞ [Eu4(btec)3(H2O)12·20H2O]∞ [Tb4(btec)3(H2O)12·20H2O]∞ [Dy4(btec)3(H2O)12·20H2O]∞

641 615

Qligand (%) Ln

τobs

0.10(1) 7.7(7) 31.1(3) 0.20(2)

10 (1) μs 0.286(3) ms 0.923(9) ms 22(2) μs

Table 4. Luminance and Colorimetric Coordinates under UV Irradiation (λexc = 312 nm) of [Ln4(btec)3(H2O)12· 20H2O]∞ with Ln = Sm−Dy Ln3+

4

F9/2 → 6 H13/2

3+

Sm

575

(J = 15/2 − 11/2)

x y

Luminescent Properties. Solid-state luminescence spectra have been recorded for lanthanide-based homonuclear compounds (Ln = Sm−Dy). Intramolecular energy transfer between the triplet state of the ligand and the lanthanide energy level is one of the most important processes that influence the luminescent properties of the LnIII ions in a lanthanide-based coordination compound. The lowest singlet and triplet excited states have been estimated by referring to the wavelength of the

L (Cd m−2)

Eu

3+

Gd3+

Colorimetric Coordinates 0.65(1) 0.21(1) 0.34(1) 0.27(1) Luminance 0.11(1) 5.1(2) 13.6(5) 0.38(1) 0.26(1)

Tb3+

Dy3+

0.33(1) 0.59(1)

0.64(1) 0.27(1)

49.3(5)

0.51(1)

UV−vis absorbance edge (325 nm ≈ 30750 cm−1) and to the shortest-wavelength phosphorescent band (400 nm ≈ 25000 cm−1) of the gadolinium-containing compound,73−76 respectively (see Figures S6 and S7). The gap between the singlet and triplet excited states (5750 cm−1) is favorable for an efficient D


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Inorganic Chemistry

Figure 4. Luminescence spectra versus x for compounds with the chemical formulas [Tb4−4xGd4x(btec)3(H2O)12·20H2O]∞ (left) and [Eu4−4xGd4x(btec)3(H2O)12·20H2O]∞ (right) with 0 ≤ x < 1. ligand Ln Q Ln = ηsensQ Ln = ηsens

τobs τrad

(1)

ligand QLn

where is the overall quantum yield upon ligand Ln excitation, QLn the intrinsic quantum yield upon direct excitation of the lanthanide ion, τobs the observed luminescent lifetime, and τrad the radiative luminescent lifetime.81,82 The luminescence lifetime and intrinsic quantum yield were measured for the europium-based coordination polymer83 under an excitation wavelength that corresponds to a f−f transition that does not overlap significantly with the absorption band of the ligand: 395 nm (5D3 energy level for Eu3+).84,85 This leads to QLn Ln = 11.6(5)% and ηsens = 66(5)%. For europium-containing compounds, a simplified equation leads to the radiative lifetime86 ⎛I ⎞ 1 = AMD,0n⎜ tot ⎟ τrad ⎝ IMD ⎠

Figure 5. Luminance versus x for [Tb4−4xGd4x(btec)3(H2O)12· 20H2O]∞ with 0 ≤ x < 1. The dotted line symbolizes the luminance of [Tb4(btec)3(H2O)12·20H2O]∞.

(2) −1

where AMD,0, a constant equal to 14.65 s , is the spontaneous emission probability of the magnetic dipole 5D0 → 7F1, n is the refractive index, Itot is the integrated emission of the 5D0 → 7FJ (J = 0−6) transitions, and IMD is the integrated emission of the 5 D0 → 7F1 transition. The refractive index was estimated as 1.50 on the basis of known refractive indexes of similar compounds.42 In this frame, QLn Ln = 11.3(5)% and ηsens = 70(5)% values were obtained. Both set of values are in perfect agreement and show that ligand-to-europium energy transfer is efficient. The ligand-to-terbium energy transfer was estimated by using the first method (eq 1) because eq 2 is only valid for Eu3+-based compounds. The f−f transition that has been used is the transition centered at 379 nm (5D3 energy level for Tb3+) that does not overlap significantly with the absorption band of the ligand. This leads to QLn Ln = 38.5(5)% and ηsens = 81(5)%. These results indicate that ligand-to-metal energy-transfer efficacy is not responsible of the significant difference between the overall quantum yields of the terbium- and europium-based compounds. This suggests that nonradiative deactivation of Eu3+ is important. This can be related to the presence of three and a half coordination water molecules per lanthanide ion. Indeed, it is known that deactivation of the luminescence by O−H oscillators is more efficient for Eu3+ ions than for Tb3+ ions.81

intersystem crossing process according to Reinhoudt’s empirical rules,77 which state that the intersystem crossing process becomes effective when ΔE(1ππ*−3ππ*) is at least 5000 cm−1. Compounds that involve lanthanide ions that present luminescence in the visible region (Sm3+, Eu3+, Tb3+, and Dy3+) can be excited at 306 nm (Figure 3), which indicates that H4btec exhibits an antenna effect with respect to these ions. Experimental features of the emission spectra are gathered together in Table 2. Overall luminescent quantum yields (Qligand Ln ) and observed lifetimes (τobs) are listed in Table 3. The luminescent decay rates are monoexponential. The small energy gaps of the samarium- and dysprosiumbased compounds induce weak luminescence.78 In contrast, the quantum yields of the europium- and terbium-based coordination polymers are quite sizable. The energy of the lowest triplet excited state of the ligand (25000 cm−1) is supposed to favor good ligand-to-metal energy transfer without significant backtransfer for both the europium- and terbium-based compounds according to Latva’s empirical rules.79,80 For a lanthanidecontaining compound, the efficiency of the overall ligand-tometal energy transfer (ηsens) is defined as the efficacy with which energy is transferred from the feeding levels of the ligand onto the excited states of the lanthanide ion: E


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Inorganic Chemistry

Figure 6. Luminescence spectra (top left), colorimetric coordinates (top right), luminance measurements (bottom left), and pictures under UV irradiation (bottom right) versus x of some [Tb4−4xEu4x(btec)3(H2O)12·20H2O]∞ compounds. λexc = 312 nm.

[Ln4(btec)3(H2O)12·20H2O]∞, where Ln = Sm−Dy (see Table 4). From these results, one can notice that the luminance of the terbium- and europium-based compounds are quite sizable but smaller than that of other coordination polymers that present comparable overall quantum yields.32 This discrepancy can be related to absorption of the ligand. Actually, the luminescence intensity depends on both the quantum yield and molar absorption coefficient:

Table 5. Radiative Lifetimes and Quantum Yields Measured for [Tb2Gd2(btec)3(H2O)12·20H2O]∞ and [Tb2Eu2(btec)3(H2O)12·20H2O]∞ (λexc = 306 nm) Qligand (%) Tb [Tb2Gd2(btec)3(H2O)12· 20H2O]∞ [Tb2Eu2(btec)3(H2O)12· 20H2O]∞

τobs (ms)

47(4) 1.2(1)

τ0 (ms)

Qligand Eu (%)

1.02(2) 0.08(1)

14(1)

ligand Φ lum ≈ ϵλQ Ln

Scheme 4. Schematic Representation of the Luminescence Mechanism for [Tb2Eu2(btec)3(H2O)12·20H2O]∞

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where Φlum is the luminescence intensity, is the overall quantum yield, and ελ is the molar absorption coefficient at the excitation wavelength.87 Because these coordination polymers are insoluble in the usual solvents, their molar absorption coefficients could not be measured. Therefore, to estimate their luminescence intensity, the molar absorption coefficient of the deprotonated ligand was calculated from dilute aqueous solutions of its sodium salt: ελmax = 1110 L mol−1 cm−1 (Figure S8). This moderate value32 explains the moderate luminance of these compounds. Luminescence of Heteronuclear Compounds. Luminescence of lanthanide-based coordination polymers can be tuned by mixing in the same compound two or more different lanthanide ions. The heterodinuclear strategy32,33,42−44,46,71 allows one to tune both the brightness (luminance) and the luminescence color (colorimetric coordinates) compared with the pure compounds: (i) The brightness is optimized by dilution of an optically active ion (Eu3+, Tb3+, Dy3+, etc.) with an inactive one (Gd3+, Y3+, etc.). (ii) The color is modified by associating different emitting ions. Typically, the series based on Eu3+ and Tb3+ affords a wide range of shades from red to green. Therefore, in order to evaluate the tunable character of the luminescence of this family of compounds, we studied three heterodinuclear series, two of them based on Gd 3+ ([Tb4−4xGd4x(btec)3(H2O)12·20H2O]∞ and Qligand Ln

Luminance and colorimetric coordinates under UV irradiation (λ e x c = 312 nm) were also measured for F


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Inorganic Chemistry [Eu4−4xGd4x(btec)3(H2O)12·20H2O]∞ with 0 ≤ x ≤ 1) and the third one based on Tb3+ and Eu3+ mixtures ([Tb4−4xEu4x(btec)3(H2O)12·20H2O]∞ with 0 ≤ x ≤ 1). Luminescence spectra of the two gadolinium-containing series are reported in Figure 4. As anticipated,32,33,71 these two sets of data clearly show that dilution of the optically active lanthanide ion (Tb3+ or Eu3+, respectively) by Gd3+ ions provokes enhancement of the luminescence intensity. The maximum is observed in both cases for x ≈ 0.30, that is, in the frame of the row model described above, when the mean Tb3+−Tb3+ (Eu3+−Eu3+, respectively) distance becomes bigger than 10 Å (10 Å is often considered to be the distance above which intermetallic energy transfer becomes less efficient88,89). More surprisingly, for both series of compounds, the luminance increases for x > 0.5 (see Figure 5). Generally, a decreasing trend is observed because there are less and less optically active ions in the compounds. This unusual behavior may be related to the crystal structure. Actually, the quite big mean Ln3+−Ln3+ distance does not reflect the existence of dimeric units with very short Ln3+−Ln3+ distances. The increasing trend of the luminance for x > 0.5 could be related to this structural feature: beyond x = 0.5, homonuclear dimeric units become less and less numerous.90 Last, it can be noticed that, in both series, the addition of Gd3+ ions leads to enhancement of the luminance no matter the addition rate is (see Figure 5). Color tuning is observed for compounds with the general chemical formula [Tb4−4xEu4x(btec)3(H2O)12·20H2O]∞ with 0 ≤ x ≤ 1 (Figure 6). These data clearly show that there is an efficient terbium-toeuropium intermetallic energy transfer (ηET) that can be estimated by using the relationship ηET = 1 −

τobs τ0

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address †

Y.L.: IC2MP, UMR-CNRS 7285, Université de Poitiers, 4 rue Michel Brunet, F-86022 Poitiers, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The China Scholarship Council Ph.D. program and a cooperation program with the French UT and INSA are acknowledged for financial support. The Center of Diffraction X of the University of Rennes is acknowledged for single-crystal X-ray diffraction data collection. Marie Delaflotte and Eva Marcel are acknowledged for their help during this study.

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REFERENCES

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where τobs and τ0 are respectively the lifetimes in the presence and absence of an acceptor.78 The Gd3+ ion with its first excited levels above the luminescent levels of the Tb3+ ion and above the ligand triplet state cannot act as an acceptor. Therefore, by replacement of Eu3+ ions with Gd3+ ions, it is possible to measure τ0 and therefore to calculate ηET. Some radiative lifetimes and quantum yields measured for [Tb2Gd2(btec)3(H2O)12·20H2O]∞ and [Tb2Eu2(btec)3(H2O)12·20H2O]∞ are listed in Table 5. On the basis of these values, ηET = 92% and an overall quantification of the different mechanisms can be suggested (see Scheme 4).

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CONCLUSION AND OUTLOOK To the best of our knowledge, compounds that are described herein constitute the first example of heterolanthanide-based coordination polymers with H4btec as the ligand. From a luminescent point of view, the heterodinuclear approach allows efficient brightness and color tuning. Therefore, these compounds could be of interest in the field of anticounterfeiting taggants.37

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Experimental and simulated powder X-ray diffraction diagrams, elemental analyses, relative ratios between Ln and Ln′, powder X-ray diffraction diagrams versus temperature, luminescence spectrum recorded at 77 K, solid-state UV−vis absorption spectrum, and solutionstate UV−vis absorbance versus concentration of the tetrasodium salt of H4btec in water (PDF) X-ray crystallographic data in CIF format (CIF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02242. G


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Brightness and Color Tuning in a Series of Lanthanide-Based

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