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Eu3+-and Tb3+-Dipicolinate Complexes Covalently Grafted into Kaolinite as Luminescent Functionalized Clay Hybrid Materials Denis Talarico de Araujo, Katia Jorge Ciuffi, Eduardo José Nassar, Miguel Angel Vicente, Raquel Trujillano, Paulo Sergio Calefi, Vicente Rives, and Emerson Henrique de Faria J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b12308 • Publication Date (Web): 16 Feb 2017 Downloaded from http://pubs.acs.org on February 18, 2017

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The Journal of Physical Chemistry

Eu3+-and

Tb3+-Dipicolinate

Complexes

Covalently

Grafted into Kaolinite as Luminescent Functionalized Clay Hybrid Materials

Denis Talarico de Araujo,†,* Katia J. Ciuffi,† Eduardo J. Nassar,† Miguel A. Vicente,‡,* Raquel Trujillano,‡ Paulo S. Calefi,† Vicente Rives,‡ Emerson H. de Faria†,*

†

Universidade de Franca, Parque Universitário, 201, 14404-600, Franca,

SP, Brazil ‡

GIR-QUESCAT, Departamento de Química Inorgánica, Universidad de

Salamanca, 37008 Salamanca, Spain

AUTHOR INFORMATION Corresponding Authors * D.T. De Araújo: E-mail: [email protected] * M.A. Vicente: E-mail: [email protected] * E.H. De Faria: E-mail: [email protected] Phone: +55 16 3711 8969. Fax: +55 16 3711 8878.

1 Environment ACS Paragon Plus


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ABSTRACT: The luminescence properties of Eu3+- and Tb3+-dipicolinate (pyridinedicarboxylate) complexes covalently grafted into a kaolinite matrix were studied. The stability of the grafted lanthanide complexes as a function of the thermal treatment was also investigated. Kaolinite intercalated with dimethylsulfoxide was heated in the presence of melted dipicolinic acid to form dipicolinate-intercalated kaolinite. The luminescent hybrid solids were obtained by complexation of Eu3+ or Tb3+ cations with this intercalated solid, at cation/ligand molar ratios of 1:1, 1:2, or 1:3. The resulting materials were characterized by thermal analysis, CHN element analysis, powder X-ray diffraction, infrared absorption spectroscopy, and photoluminescence. The lanthanide complexes covalently grafted into kaolinite were thermally more stable than the isolated lanthanide complexes. The hybrid materials exhibited more intense Eu3+ and Tb3+ emissions than the isolated complexes. The excitation spectra of the hybrid materials showed a broad band at 277 nm, assigned to a ligand-to-metal charge transfer, whereas the emission spectra showed bands related to the typical electronic transitions of Eu3+ and Tb3+ ions from the excited states 5D0 and 5D4 to the 7FJ fundamental states. The (4→5 and 4→4) and (0→2) transitions were the most intense ones and corresponded to green and red emissions, respectively.


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INTRODUCTION Lanthanide-pyridinecarboxylic (picolinic) acid complexes are important to develop advanced luminescent materials with applications in photoelectronics, laser, luminescence labels, fluorescent probes, luminescent sensors, light-conversion molecular devices (LCMDs), organic light-emitting devices (OLEDs), fluorescent lamps and cathode-ray tubes, as well as plasma display panels (PDPs).1-10 Lanthanidearomatic carboxylate complexes display interesting photophysical properties, due to the antenna effect, i. e., the organic ligand in the complex absorbs energy more intensely than the isolated organic ligand.11 However, these complexes exhibit low thermal and chemical stabilities. One strategy to render more stable lanthanide-aromatic carboxylates is to immobilize them on inorganic matrixes. Hybrid materials obtained by modification of the surface of clay minerals with luminescent organic complexes have attracted the attention of several scientists, as the resulting materials possess new properties and functionalities absent in the isolated organic complex.12 Layered clays can be modified by intercalation of luminescent organic complexes through intermolecular forces. Alternatively, the interlayer space of these clays can be functionalized with luminescent organic complexes via covalent bonding. Both intercalation and functionalization processes produce ordered assemblies at the nanometric level.13 Recently, matrices such as zeolites, silica, alumina, and natural or synthetic clays (e.g., montmorillonite,14,15 hectorite, laponite,16,17 and saponite) have been combined with organic compounds like pyridinecarboxylic acids, porphyrins, aminoalcohols, polyalcohols, polymers, and alkoxides, among others, to yield organic-inorganic hybrids that can further accommodate luminescent ions.18,19 Smectite clays belong to the tetrahedral-octahedral-tetrahedral (TOT) type clays. Weak interactions between luminescent organic species and smectite allow only a



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partial modification of this clay with luminescent organic complexes. Indeed, the resulting hybrid material has low chemical and thermal stability, and the luminescent organic complex is easily leached from the clay.20 Other factor that limits the use of TOT clays to immobilize luminescent organic complexes is the presence of water molecules in the interlayer space of the inorganic matrix,21 the water molecules interact with the lanthanide ions, leading to non-radiative decays between the excited and fundamental levels and quenching the luminescence of the lanthanide complex.22 Kaolinites with the theoretical formula Al2Si2O5(OH)4 and a basal interlayer distance of 7.1 Å are clay minerals belonging to the tetrahedral-octahedral (TO) type. These clays consist of a 1:1 dioctahedral layered aluminosilicate bearing two types of interlayer surfaces —SiO6 macro-rings and aluminol groups— where luminescent organic complexes can be intercalated.23 Insertion of luminescent organic complexes into the interlayer spaces of TO clays has deserved little attention, probably because these matrices display a more difficult intercalation chemistry than smectite clays. Only a few authors24-28 have reported grafting of organic and inorganic units into the interlayer spaces of kaolinite. We have previously described the synthesis and characterization of kaolinite containing pyridinecarboxylic acids in the interlayer space24, evidencing the grafting of dpa species to kaolinite layers, although coexisting with intercalated dpa species. These solids have been used for catalysis in hydrocarbon oxidation reactions,25 dye degradation29 and luminescent materials coordinated with Tb3+ ions30 and co-ligand acetylacetonate with intense green emission. Lanthanide-pyridinecarboxylate complexes grafted into the interlayer space of clays exhibit enhanced photophysical properties due to the interaction between the host (the clay matrix) and the guest (the lanthanide complexes) units. Compared to isolated carboxylic acids, carboxylates coordinated in lanthanide complexes have more intense


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absorptions (antenna effect); however, these complexes have low thermal stability, which restricts their technological application.3,6 It has been shown that immobilization of luminescent organic complexes on inorganic matrixes like silica, alumina, zeolites, and clays avoids this drawback.31,32 Interest has recently increased on organic ligands derived from pyridine (pyridinecarboxylic acids), such as picolinic and dipicolinic acids,33,34 which rather easily form complexes with many transition metals. In particular, aromatic carboxylates can mediate significant magnetic exchange coupling between lanthanide(III) ions. The metal ions usually coordinate to picolinate and dipicolinate ligands via the N or O atoms.35 With respect to our previous work on the Tb3+-picolinate-kaolinite system,30 a dipicolinic acid (namely, pyridine-2,6-dicarboxylic acid) is now used; the best coordination properties of dipicolinate, due to the presence of two carboxylate groups, allows to study the luminescent properties of the lanthanide containing solids without the simultaneous use of a co-ligand (Figure S1 shows the structure of the dipicolinic acid used). The study of the luminescent properties was extended to Eu3+, in addition to Tb3+. This paper reports on the functionalization of a natural Brazilian kaolinite (from the city of São Simão) with Eu3+ or Tb3+ dipicolinates, and the structural and spectroscopic properties of the resulting hybrid materials, investigated by element chemical analysis, thermal analysis, X-ray diffraction, and infrared and luminescence spectroscopies.

EXPERIMENTAL SECTION Preparation of the Materials. The kaolinite clay used in this work came from the municipality of São Simão, State of São Paulo, Brazil, and was kindly supplied by the mining company Darcy R.O. Silva & Cia. Kaolinite (Ka) was grafted with 2,6-



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dipicolinic acid (dpa) by the soft-guest method, as reported elsewhere,24,25 leading to the kaolinite-dipicolinate hybrid (Ka-dpa). To prepare the luminescent hybrid materials, Ka-dpa was suspended in 0.1 mol/L solutions of Eu3+ or Tb3+ chlorides using cation/ligand molar ratios of 1:1, 1:2, and 1:3 in the case of Eu3+ and 1:1 in the case of Tb3+. The suspensions were stirred for 3 h at room temperature (near the 25 °C) and centrifuged. The resulting solids were washed five times with ethanol and dried overnight at 80 °C. The resulting materials were designated as Eu(Ka-dpa)-x (x being the cation/ligand molar ratio) and Tb(Ka-dpa). The amount of ligand in kaolinite was quantified by thermal analysis and element chemical analysis and, using these values, the cation/ligand molar ratio used was based on the actual amount of dpa per unit cell of kaolinite. The flowchart shown in Figure 1 summarizes the procedure followed in the preparation of the luminescent hybrid materials. The amount of Eu3+ and Tb3+ cations incorporated into Ka-dpa was quantified by the Arsenazo(III) method, as described by Marczenko36. Eu(Ka-dpa) and Tb(Ka-dpa) were digested with concentrated hydrochloric acid, and the remaining solid was heated at 50 °C for 3 h and removed by filtration. The resulting solution was transferred into 25-mL flat-bottom flasks, and the volume was completed with distilled water, and the amount of lanthanide was determined spectrophotometrically.


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Figure 1. Schematic representation of the preparation of luminescent hybrid materials.

Characterization Techniques. The X-ray diffractograms of the solids were recorded in a Siemens D-500 diffractometer operating at 40 kV and 30 mA (1200 W), with filtered Cu Kα radiation (λ=1.5418 Å); 2θ varied from 2º to 65º at a scan speed of 2º/min. The infrared absorption spectra were recorded in a FTIR Perkin-Elmer SpectrumOne spectrometer; the samples were pressed into KBr pellets. The thermal analyses (TG/DSC) were carried out on a TA Instruments SDT Q600 Simultaneous DTA-TGA thermal analyzer, from 25 to 1100 °C, at a heating rate of 20 °C/min, with an air flow of 100 mL/min.



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Carbon and nitrogen element analysis of the functionalized materials was carried out in a Perkin-Elmer CHN 2400 analyzer. The emission and excitation spectra were recorded at room temperature in a Jobin Yvon SPEX TRIAX 550 FLUOROLOG III spectrofluorometer. By using the provided apparatus software, all the spectra were corrected for the lamp intensity and for the sensitivity of the photomultiplier at the monitored wavelength ranges. The luminescence lifetimes were measured on a SPEX 1934D phosphorimeter equipped with a pulsed xenon lamp. To minimize instrumental interferences, all the emission, excitation, and lifetime measurements were accomplished by using a filter with absorption below 450 nm at the exit (detection) of the light beam. The emission spectra were recorded with 0.2-nm emission bandpass; the excitation spectra and decay curves were recorded with 1-nm emission bandpass. The values of the integrated emission intensities and the exponential decay fittings for the lifetimes were calculated with Microcal Origin® 6.0 software.

RESULTS AND DISCUSSION Figure 2 shows the powder X-ray diffractograms of the original clay and of the intercalated solids derived from kaolinite. Natural kaolinite had a basal spacing of 7.14 Å, which increased to 11.20 Å after treatment with DMSO (diagram not shown),26-28 further increasing to 11.90 Å when the DMSO-intercalated intermediate was treated with dpa. Thus, dpa entities occupied a height of 4.76 Å in the interlayer region of kaolinite. This is compatible with dpa located parallel or perpendicular to the layers of the clay, as this height is close to that of a monolayer of dpa molecules, as calculated by using

the

free

ChemSketch

12.0

software

(downloaded

from

http://www.acdlabs.com/download/). It may be remarked that the 001 reflection peak is


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intensified from natural kaolinite to the intercalated one, suggesting that the accommodation of dpa in the interlayer region improved the stacking of the layers, increasing the crystallinity of the solids. The 002 reflection was clearly observed in these solids at 5.9 Å, confirming their high crystallinity. The diffraction effects that do not depend on stacking of the layers in the c-dimension remained unaffected, indicating that the intercalation process did not alter the structure of the kaolinite layers.

500 c.p.s.

Intensity (c.p.s.)

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f e d c b a 10

20

30

40

50

60

2θ (Degrees)

Figure 2. Powder X-ray diffractograms of natural kaolinite, intercalated with dpa, and after complexation with Eu3+ or Tb3+ ions: (a) Ka, (b) Ka-dpa, (c) Eu(Ka-dpa)-1, (d) Eu(Ka-dpa)-2, (e) Eu(Ka-dpa)-3 and (f) Tb(Ka-dpa).

Basal spacing remained the same in all intercalated samples, 11.90 Å, corresponding to a swelling of 4.76 Å with respect to raw kaolinite. The reaction ratios for each solid are given in Table S1; this magnitude represents the percentage of kaolinite layers that are effectively intercalated in each solid, although it may be taken into account that drastic changes in the stacking of the kaolinite layers may significantly affect the validity of the calculation. Eu(Ka-dpa)-2 and -3 and Tb(Ka-dpa) had lower reaction



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ratios than Ka-dpa, indicating that a portion of the dpa units intercalated into the interlayer space in sample Ka-dpa leached from the matrix upon complexation with the lanthanide cation. However, for Eu(Ka-dpa)-1, the intercalated degree increases. This suggests that a lower amount of Ln3+ cation can be easily incorporated into the clay interlayer region, but that on increasing the Ln3+ amount the dpa entities leave the interlayer region, probably by the strong affinity of the cations for ligands, and the difficulties for the access of a large number of cations to the interlayer region. Figure 3 shows the infrared absorption spectra of the Eu3+ and Tb3+ luminescent samples, as well as that of the Ka-dpa precursor; the assignments of the bands recorded is summarized in Table S2, and the spectrum of original kaolinite is also given in Figure S2. The bands centered at 3635 and 3660 cm-1 correspond to intra- and interlamellar OH vibrations, respectively37. Complexation with Eu3+ or Tb3+ cations did not affect these vibrations, that is, the cations did not interact with the hydroxyls groups, but only with dpa entities. The fact that hydroxyl vibrations do not change suggests that the dpa molecules lixiviated during complexation were probably hydrogen-bonded and not covalently grafted to kaolinite.


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a b c d e

e d

Transmittance (%)

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c

b a

5% 4000

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-1

Wavenumber (cm )

Figure 3. FTIR absorption spectra of the Eu3+ and Tb3+ containing luminescent samples, and of the Ka-dpa precursor: (a) Ka-dpa, (b) Eu(Ka-dpa)-1, (c) Eu(Ka-dpa)-2, (d) Eu(Ka-dpa)-3 and (e) Tb(Ka-dpa).

The typical bands of carboxylic/carboxylate groups changed after incorporation of the lanthanide cations, proving the coordination of these groups to the cations.38 The spectrum of the Ka-dpa precursor showed two intense bands at 1672 and 1340 cm-1; while the last band was assigned to –COO- groups, that at 1672 cm-1 clearly corresponds to the C=O stretching mode of the acid (–COOH) groups, proving that part of the acid remained in the solid in protonated form (this band masked the bending band of water, which should be recorded as a shoulder at 1616 cm-1). After incorporation of the lanthanide cations, the band at 1672 cm-1 disappeared, becoming more evident the band of water at 1616 cm-1, while the carboxylate band shifted to 1400 cm-1, as a result of the coordination of carboxylate groups to the cations.24,25,29,30



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Figure 4 shows the results of the thermal analyses of one of the samples prepared from Ka-dpa after complexation with Eu3+ cations. Curves from other solids are included in Figure S3. The curves revealed mass loss steps centered between 75 and 120 °C, attributed to removal of water and perhaps traces of ethanol, remaining from the synthesis and washing processes; unfortunately we could not monitor the gases evolved during the thermal decomposition. A second mass loss step was detected at 295-300 °C, associated to a weak endothermic process, which can be reasonably attributed to the removal of dpa entities adsorbed on the external surface of the clay. The third mass loss step, associated to a strongly exothermic effect and centered at 490 °C, is assigned to the combustion of the dpa units, masking the endothermic effect due to dehydroxylation of kaolinite. In the case of the Tb3+ solid, this peak splits into two effects, centered at 489 and 517 °C, respectively, suggesting that dpa should exist in two different environments in the interlayer of kaolinite. The high temperature effect, close to 1000 ºC, is composed of an endothermic process immediately followed by an exothermic one. The most accepted explanation is that the endothermic effect corresponds to the removal of remaining hydroxyls from the kaolinite structure, which is followed by a phase change with nucleation of mullite and segregation of silica.39 In all cases, the solids after complexation with the lanthanide ions became thermally more stable that the precursor Ka-dpa, for which the decomposition of dpa unities took place at 370-375 °C.24


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1.00 TG DTG DTA

0.95 0.90

86 °C

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Temperature (°C)

Figure 4. TG, DTG, and DTA curves of Eu(Ka-dpa)-3 solid, recorded under air flow at a heating rate of 20 °C/min.

The number of ligands coordinated to the emitting ion is one of the characteristics that determine whether a complex is a good luminophore.30,34,40 Using the Arsenazo III method, the amount of lanthanide(III) ions existing in the luminescent hybrid complexes was determined and related to the results obtained by elemental analyses, which allowed to determine the stoichiometry of the Eu(Ka-dpa) and Tb(Ka-dpa) hybrid materials (Table 1).



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Table 1. Results from C and N Element Chemical Analyses, Structural Formulas, and Lanthanide(III) Content of Ka-dpa Solids before and after Complexation with Eu3+ or Tb3+ Ions Experimental Ln3+ Structural Ligand/Ln3+ b b C/N (mol/g)c formulad Ka-dpa 11.5 1.67 6.89 Ka-(dpa)0.37 -4 Eu(Ka-dpa)-1 7.1 1.14 6.52 5.69 10 0.70 Ka-(dpa)0.21-Eu0.30 Eu(Ka-dpa)-2 8.4 1.32 6.34 7.79 10-4 0.83 Ka-(dpa)0.26-Eu0.31 -4 Eu(Ka-dpa)-3 7.60 1.19 6.41 7.27 10 0.86 Ka-(dpa)0.24-Eu0.28 -4 Tb(Ka-dpa) 7.31 1.23 5.94 6.74 10 0.76 Ka-(dpa)0.22-Eu0.29 a mass percentage; bmolar ratio; cLn3+ = Eu3+ or Tb3+. din these formulae, Ka designates here the “molecular formula” of kaolinite, referred to 2 Si atoms Sample

Ca

Na

The Ln(Ka-dpa) samples contained lower amounts of dpa in the interlayer space of the clay, suggesting that part of the dpa unities intercalated in the Ka-dpa were leached during incorporation of the Ln3+ cations. The theoretical C/N ratio in dpa is 6.00. In the Ka-dpa precursor and in the Ln-samples, this ratio was acceptably close to this value, but in most of the cases slightly higher, probably due to the presence in the solids of traces of ethanol from the preparation process. In any case, these ratios suggest no degradation in dpa structure. The ligand/Ln3+ molar ratio ranges from 0.70 to 0.86; despite the dpa anion can act even as a pentadentate ligand, the rigidity of the molecule, because of the presence of the aromatic ring, introduces severe hindrances for a pentacoordination to a single metal cation; probably only some of the coordination positions of the metal cation are occupied by the anion donor atoms, and the remaining positions are occupied by aquo ligands and/or hydroxyl groups from the clay network, although the presence of residual chloride anions (from the lanthanide cation precursors used) acting as ligands cannot be either completely discarded. Moreover, the dpa anion could also act as a bridging ligand between two lanthanide cations. The content of dpa ligands in the final solids was a confirmation of their grafting to kaolinite. The preparation of the final solids involved suspending the Ka-dpa sample in lanthanide aqueous solutions for 3 hours (see Experimental Section), and dpa ligands


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were not removed from the solid during this treatment, that can be considered similar to the grafting efficiency test based on the washing of the solids with water.41 The excitation spectra of the Ln(Ka-dpa) hybrid samples showed broad bands with a maximum centered close to 310 nm (Figures 5 and 6). The position of this excitation maximum is shifted from those for the maximum absorption of the free dpa ligand (220 nm) and of the free Eu3+ (394 nm) and Tb3+ (368 nm) cations. This fact confirmed the formation of the Ln3+-dpa complexes, with charge transfer from the ligand to the metal (LMCT), as reported previously for comparable systems.4,40,42

60000

281 nm

Eu(Ka-dpa)-1 (a) Eu(Ka-dpa)-2 (b) Eu(Ka-dpa)-3 (c)

c

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b a

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Intensity (a. u.)

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Figure 5. Excitation spectra (λem = 612 nm), with fex = 4.0 nm and fem = 0.5 nm, of the luminescent hybrids Eu(Ka-dpa) at room temperature.



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Figure 6. Excitation spectra (λem = 543 nm), with fex = 4.0 nm and fem = 0.5 nm, of the luminescent hybrid Tb(Ka-dpa) at room temperature.

The emission spectra showed the typical transitions of the lanthanide ions: from the excited state 5D0 to the fundamental states 7FJ (J = 0, 1, 2, 3 and 4) for the Eu3+ cation, and from the excited state 5D4 to the fundamental states 7FJ (J = 5, 4, and 3) for the Tb3+ ion (Figures 7 and 8).


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5

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D0-> F2

Eu(Ka-dpa)-1 (a) Eu(Ka-dpa)-2 (b) Eu(Ka-dpa)-3 (c)

Intensity (a. u.)

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Figure 7. Emission spectra (λex = 277 nm), with fex = 4.0 nm and fem = 0.5 nm, of the luminescent solids Eu(Ka-dpa)-1 (a), Eu(Ka-dpa)-2 (b) and Eu(Ka-dpa)-3 (c) at room temperature. 35000

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Figure 8. Emission spectra (λex = 275 nm), with fex = 4.0 nm and fem = 0.5 nm, of the luminescent hybrid Tb(Ka-dpa) at room temperature.



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The emission spectra of the Eu(Ka-dpa) hybrids shows a single broad band due to the 5D0 7F0 transition (width at half height = 1 nm), assigned to the presence of polymeric materials, as previously reported for Eu3+—2,2´-bipyridine-4,4´-dicarboxylic acid complexes34. The presence of this relatively intense transition indicated that the Eu3+ ion was in a low-symmetry environment, justified by the formation of a complex with different stoichiometry as compared to isolated Ln(III)-pyridinecarboxylate complexes, and probably the presence of other ligands (e.g., water, hydroxyl or even chloride). The emission of the Eu(Ka-dpa) samples was less weaker than that of the precursor Ka-dpa.34 The strengthening of the emission corresponding to the 5D0 7F0 transition proved that the cation occupied different environments, as previously reported for lanthanide cations in silica gel and β-diketone hybrids.43 The relative intensity of the bands due to the 5D0→7F0, 5D0→7F1, and 5D0→7F2 transitions in the spectra of isolated complexes greatly differed from the relative intensities of the same bands in the spectra of the hybrid materials containing coordinated Eu3+ ions.42 The intensities of these bands also depended on the nature of the precursor. The ratio between the areas of the bands due to the 5D0→7F0 and 5

D0→7F2 transitions relative to intensity of the band due to the 5D0→7F1 transition

provide further information about the changes in the environment around the ion. The 5

D0→7F0 and 5D0→7F2 transitions have a electric-dipole character (EDT), and the

intensities of the bands due to these transitions heavily depend on the bonds established by the ion.44,45 On the contrary, the 5D0→7F1 transition is allowed by a magnetic-dipole mechanism (MDT), and its intensity is scarcely influenced by the coordination environment of the cation. Therefore, the latter band can be used as a sort of standard to measure the relative intensities of the other bands in the emission spectrum.4,40,42 The


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ratio between the bands of the EDT/MDT transitions allows to estimate the symmetry of the system, the higher the value of EDT/MDT, the lower the symmetry, and vice versa. In Eu3+ complexes, the higher the 5D0→7F2/5D0→7F1 ratio, the lower the symmetry of the complex, and the higher the covalent character of the Eu3+-ligand interaction. Table 2 lists the results concerning these ratios.

Table 2. Ratio between the Areas of the Bands Corresponding to the 5D0→7F2 and 5 D0→7F0 Transitions Relative to the 5D0→7F1 Transition for Eu(Ka-dpa) Samples Transition 5

7

5

7

D0→ F0/ D0→ F1 D0→7F2/5D0→7F1

5

Eu(Ka-dpa)-1

Eu(Ka-dpa)-2

Eu(Ka-dpa)-3

0.07 2.86

0.10 2.67

0.06 3.05

Sokolnicki et al.46 reported the sol-gel synthesis of silica functionalized with Eu3+ and Tb3+-dpa complexes containing different cation/ligand ratios, observing that the transitions had very similar emission intensities. These authors suggested that the cation/ligand ratio did not modify the structure of the complexes significantly, as confirmed by element chemical analysis. In the current case, the variation of the dp/Ln3+ molar ratio did not modify the chemical environment around the cations. However, the hybrids based on dpa showed a higher relative intensity area ratio and lower symmetry due to the low mobility of the functionalized ligand, bounded to kaolinite layer; this restricted environment may generate Ln3+ complexes with lower symmetry. In the case of the Tb(Ka-dpa) sample, all the bands due to the 5D4→7FJ (J = 5, 4, and 3) transitions were narrower and better defined than for the Eu3+-containing sample (Figure 8). The larger the number of carboxylate ligand groups, the lower the number of water molecules coordinated to the metal cation and the smaller the losses by nonradiative mechanisms. Lifetimes of the excited state of Tb3+ cations, discussed in the next paragraph, will confirm these results.



The emission lifetimes of Eu3+ and Tb3+ ions were calculated by using the software Microcal Origin® 6.0. The fitting analysis of the radiative decay of all the samples evidenced that the first-order exponential fit was not perfect, which confirmed that more than one coordination site existed in the hybrids. Thus, the second-order fit was applied; the results thus obtained are included for two of the samples in Figure 9, together with the corresponding radiative decay experimental curves.

Experimental Calculated

Intensity (a. u.)

Experimental Calculated

Intensity (a. u.)

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0.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

Time (ms)

1.5

2.0

2.5

3.0

Time (ms)

Figure 9. Experimental and second-order adjusted radiative decay curves of the samples Eu(Ka-dpa)-3, (left, λex = 282 nm and λem = 616 nm), and Tb(Ka-dpa) (right, λex = 282 nm and λem = 545 nm), both obtained at ambient temperature.

To investigate the photophysical properties of the materials, the following parameters were determined (Table 3): Lifetime (τ) of the excited state 5D0 of Eu3+ and 5

D4 of Tb3+ cations, quantum efficiency (φ), and number of water molecules (q)

coordinated to the cations in the complexes.


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Table 3. Lifetime (ττ) of the Excited States 5D0 of Eu3+ and 5D4 of Tb3+ Cations, Quantum Efficiency (φ φ), and Number of Water Molecules Coordinated to the Cations (q) in the Luminescent Materials τ1 τ2 φ1 φ2 Sample q1 q2 (ms)(%) (ms)(%) (%) (%) Eu(Ka-dpa)-1 0.198 (71) 0.482 (29) 4.98 11.88 5 2 Eu(Ka-dpa)-2 0.225 (73) 0.686 (27) 5.48 33.05 5 1 0.164 (37) 0.389 (63) 4.26 20.08 6 3 Eu(Ka-dpa)-3 Tb(Ka-dpa) 0.255 (74) 0.722 (26) 1 0

The quantum efficiency results agreed with the lifetime data, which in turn were compatible with the number of water molecules coordinated to the Ln3+ ion; i.e., the lifetime increased and the number of water molecules decreased, to minimize the energy loss by non-radiative mechanisms. The quantum efficiencies were higher than the usual values reported for Ln3+ complexes immobilized on other matrixes, as reported by Li and Yan.47 For instance, Bruno et al.48 reported a quantum efficiency of 15% for Eu3+ and Tb3+ cations incorporated into mesoporous supports consisting of MCM-41 silica impregnated with β-diketones, a value which was significantly lower than the quantum efficiency of the same isolated lanthanide complexes (35%). Our results resembled the data reported by Sarakha et al.49 who immobilized dpa complexes on layered double hydroxides, and achieved quantum efficiencies ranging from 20 to 30%. The variation in the ligand/cation ratio during the preparation of the solids did not affect the quantum efficiency of the resulting materials. In fact, this agreed with the results from the element chemical analyses, which proved that the Eu(Ka-dpa)-1, -2, and -3 samples actually contained very close ligand/Eu ratios, thus leading to similar coordination spheres for the Eu3+ cations. For all the solids here studied, the existence of two different complex species can be concluded, one with a short lifetime, and consequently a large number of coordinated water molecules, and another with longer



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lifetime, that is, with a lower amount of coordinated water, and consequently having more dpa anions coordinated to the cations.

CONCLUSIONS Kaolinite functionalized with dpa ligands led promising organic-inorganic hybrids. When immobilized on kaolinite, these ligands efficiently coordinated to Eu3+ and Tb3+ cations. Thus, grafting of the ligand to the matrix followed by complexation with the lanthanide cations provided highly luminescent materials with improved thermal stability as compared to the isolated complexes. The photo physical properties (intensity of luminescence emission, lifetime) and also the number of water molecules coordinated to the emitting ion of the hybrid materials indicated that the complexes located in the interlayer space of kaolinite showed a high quantum efficiency, larger to most of the hybrid materials studied in the literature. The reported kaolinite-lantanide complexes materials are easy to obtain by simple, viable, and reproducible synthetic routes, which opens new perspectives for the use of this type of materials for different applications.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX. Structure of 2,6-dipicolinic acid, Figures and Tables with additional characterization results.


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ACKNOWLEDGMENTS This work was carried out in the frame of a Spain–Brazil Interuniversity Cooperation Grant, funded by MEC (PHBP14/00003) and CAPES (317/15), and a Cooperation Grant Universidad de Salamanca–FAPESP (2013/50216–0). Spanish authors thank additional financial

support

from

the

Spanish

Ministry

of

Economy

and

Competitiveness (MINECO) and the European Regional Development Fund (ERDF) (grant MAT2013–47811–C2–R). The Brazilian group thanks support from Brazilian Research funding agency FAPESP (2016/01501-1 and 2013/19523-3). E.H. de Faria and K.J. Ciuffi thank CNPq Grants (311767/2015–0 and 305398/2015–6, respectively).

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and Tb3+-Dipicolinate Complexes Covalently ... - ACS Publications

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