Novel Luminescent Eu 3 +-Indandionate Complexes Containing Heterobiaryl Ligands

Novos complexos indandionatos de fórmulas [Ln(aind)3L(H2O)] e [Ln(bind)3L]∙H2O, (L: 1,10-fenantrolina (phen) ou 4,7-dimetil-1,10-fenantrolina (dmphen), Ln: Eu ou Gd, aind: 2-acetil-1,3-indandionato e bind: 2-benzoil-1,3-indandionato) foram sintetizados e caracterizados por análise elementar, espectroscopia de absorção na região do infravermelho e análise termogravimétrica. Os dados de caracterização são consistentes com a presença de uma molécula de água de cristalização nas estruturas dos compostos [Ln(bind)3L]∙H2O, enquanto que nos complexos [Ln(aind)3L(H2O)] a molécula de água atua como ligante. As geometrias dos complexos do íon Eu, otimizadas pelo modelo SPARKLE/AM1, foram consistentes com os resultados experimentais de luminescência. As propriedades luminescentes dos compostos de Eu são discutidas em termos de parâmetros de intensidade experimentais (Ω2 e Ω4), taxas radiativas (Arad) e não-radiativas (Anrad) e eficiência quântica de emissão (η). Os maiores valores de η foram obtidos para os compostos [Eu(bind)3L]∙H2O, refletindo a ausência de moléculas de água na primeira esfera de coordenação do íon Eu.

5][16][17] Although the great coordinating ability of bidentate 1,10-phenanthroline and 4,7-dimethyl-1,10-phenanthroline ligands through the two nitrogen atoms to the Ln 3+ ions, up to now, to our knowledge, no work has been reported on luminescent properties of complexes containing indandionate and heterobiaryl as ancillary ligands.Furthermore, the relatively rigid structure of the heteroaryl ancillary ligands may contribute to minimize the luminescence quenching of the europium ion via thermal vibrations.

Materials and methods
The solvents (acetone and ethanol), 1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline and lanthanide oxides (Ln 2 O 3 ) were purchased from Aldrich Co. and used without previous treatment.Hexahydrated lanthanide chlorides, LnCl 3 ⋅6H 2 O, were prepared by reaction between Ln 2 O 3 and hydrochloric acid purchased from Aldrich Co. 18 2-acetyl-1,3-indandione (aind), 2-benzoyl-1,3-indandione (bind) ligands, and [Ln(aind precursor complexes were synthesized according to the procedures as reported in the literature. 4,11lemental analyses (C, H, and N) were carried out using a Perkin Elmer 2400 elemental microanalyzer.Infrared (IR) spectra were recorded from 400 to 4000 cm −1 on a Shimadzu FT-IR spectrophotometer model IRPRESTIGE-21 using KBr pellets.Excitation and emission spectra of the Ln 3+ -complexes in solid state were recorded at liquid nitrogen temperature on a Fluorolog-3 spectrofluorometer (Horiba) with excitation and emission double grating 0.22 m monochromators, and a 450 W Xenon lamp as excitation source and R928P PMT as detector.All spectra were recorded using a detector mode correction.The second-order diffraction of the source radiation was eliminated using a cut-off filter.Luminescence decay curves of the Eu 3+ -complexes in the solid state were recorded at low temperature (77 K) using the same equipment but operating on the phosphorescence mode with pulsed Xenon lamp as excitation source.The luminescence instruments were fully controlled by the FluorEssence program.All the luminescence data of the samples were measured in quartz tube of 2 mm in diameter.TG curves were recorded from 30 to 900 °C using a Shimadzu DTG-60H thermobalance with a heating rate 10 °C min −1 under a nitrogen atmosphere.

Synthesis of the Ln 3+ -complexes with heteroaromatic ligands
Lanthanide indandionate complexes containing ancillary ligands were synthesized by mixing acetonic solutions of the corresponding hydrated complexes, [Ln(aind) 3 (H 2 O) 2 ] or [Ln(bind) 3 (H 2 O) 2 ], with phen or dmphen ligands in the molar ratio of complex:ligand (1:1.3).The resulting solutions were stirred for 1 h at room temperature (ca.27 °C).Later on, these solutions were stand up until the total evaporation of solvent.The prepared Novel Luminescent Eu 3+ -Indandionate Complexes Containing Heterobiaryl Ligands J. Braz.Chem.Soc.2082 yellow solids were washed thoroughly with cold ethanol and dried under reduced pressure.

Characterization
The results of the elemental analysis (C, H, and N) are consistent with the monohydrated formulas of Ln(aind) 3 L⋅H 2 O and Ln(bind) 3 L⋅H 2 O complexes, where Ln = Eu 3+ or Gd 3+ .
The infrared spectra of the Ln 3+ -complexes (Figure 2) exhibit two bands at around 1560 and 1690 cm −1 assigned to the ν(C=O) stretching modes, due to the coordinated and noncoordinated carbonyl groups, respectively.These data are in agreement with the FT-IR spectral data of precursor-hydrated complexes, 11 suggesting that the 2-acyl-1,3-indandione ligands are coordinated to Ln 3+ ions through oxygen atoms of the carbonyl groups in bidentate mode.Moreover, the absorption bands at around 1468 and 1423 cm −1 indicate that the phen and dmphen ligands act as chelating agents, coordinating to the Ln 3+ ion through the nitrogen atoms. 19,20he complexes also exhibit absorption bands in the spectral range 4000-3200 cm −1 assigned to ν(O−H) stretching of the water molecule (Figure 2).Interestingly, FT-IR spectra of the [Ln(aind) 3 L⋅H 2 O] compounds exhibit narrow peaks at around 3560 cm −1 , while the [Ln(bind) 3 L]⋅H 2 O compounds show only one broad absorption band.These data give evidences that the water molecule is coordinated to the Ln 3+ ion only in the [Ln(aind) 3 L⋅H 2 O] complexes.Similar spectral profiles have also been observed for La 3+ and Tb 3+ complexes using EDTA as ligand that present coordinated and non-coordinated water molecules, respectively. 21,22hermogravimetric curves of the [Eu(aind) 3 (dmphen) (H 2 O)] and [Eu(bind) 3 (dmphen)]•H 2 O compounds (Figure 3) show that these systems undergo thermal decomposition in consecutive steps.The curves show the first weight loss in the temperature range from 60 to 110 o C that correspond to 1.6% for [Eu(bind) 3 (dmphen)]•H 2 O and 2.1% for [Eu(aind) 3 (dmphen)(H 2 O)], which are in agreement with the release of water molecules.However, it can be observed in Figure 3 that the [Eu(aind) 3 (dmphen)(H 2 O)] complex exhibits a shorter dehydration interval (60-130 o C) than the [Eu(bind) 3 (dmphen)](H 2 O) complex, in which the dehydration process occurs from 40 to 150 o C (see insert Figure 3).These data corroborate with the FT-IR results, indicating that the water molecules are acting as ligand in the former complex.Similar behavior has been observed in the thermal and FT-IR studies of the Ln 3+ -EDTA complexes, as reported by Gigante et al.. 22 Figure 3   model for Eu(III) and other lanthanide ions implemented in the software package MOPAC2012. 23,24ater molecule was not considered in the last complexes, since the FT-IR and thermogravimetric data indicate that the water molecule is out of the first coordination sphere.The calculated structures of the [Eu(aind) 3 (dmphen)(H 2 O)] and [Eu(bind) 3 (dmphen)]•H 2 O complexes are shown in Figure 4, while those ones for the similar complexes with phen ligand are presented in Supporting Information (Figure S1).
The optimized structures reveal that the indandionate ligands are characterized by the planarity of 1,3-indandione moieties.In addition, the substituent phenyl groups in the bind ligands are twisted from the 1,3-indandione moieties at around angles of 29 and 40°.The less pronounced twist is observed in the phenyl group that belongs to the bonded molecule more distant from heteroaromatic ligand.This behavior reflects the steric hindrance on the ligands due to the heteroaromatic one in the first coordination sphere.Thus, the phenyl groups are orientated to minimize steric repulsion among ligands.Therefore, the steric hindrance due to the substituent phenyl groups of the ligand is enough to avoid water coordination to the lanthanide ion compared to the [Eu(aind) 3 L(H 2 O)] complexes.
The coordination polyhedron, EuN 2 O 6 , for the [Eu(bind) 3 L]⋅H 2 O complexes can be described as distorted  Novel Luminescent Eu 3+ -Indandionate Complexes Containing Heterobiaryl Ligands J. Braz.Chem.Soc.2084 square antiprismatic geometries (Figure 4b, Figure S1).A square face contains four oxygen atoms from indandionate ligands, another one presenting two oxygen atoms from indandionate and two nitrogen atoms belong to phen or dmphen ligand.
The coordination polyhedron of the [Eu(aind) 3 L(H 2 O)] complex, EuN 2 O 7 , is better described as a distorted monocapped square antiprismatic geometry, with oxygen atom from water molecule occupying the capping position (Figure 4a, Figure S1).The bond lengths of Eu 3+ -O(indandionate), and Eu 3+ -N(heteroaromatic) in the complexes are around 2.38 and 2.51 Å, respectively.[27][28] Photoluminescent properties of the Gd 3+ -compounds The phosphorescence spectra of the analogous [Gd(aind) 3 L(H 2 O)] and [Gd(bind) 3 L]•H 2 O compounds were recorded at 77 K with excitation monitored at 370 nm for S 0 →S 1 transition, in order to estimate the triplet state energies (T 1 ) of the indandionate ligands.The Gd 3+ -compounds were used for this propose because the lowest excited energy level of Gd 3+ ion ( 6 P 7/2 ) is too high to receive energy from the ligands.In addition, the Gd 3+ radius is similar to the Eu 3+ ion, therefore, Gd 3+ -complexes mimetize both geometry and intraligand energy level structures of Eu 3+ -complexes.Figure S2 shows the phosphorescence spectra of the Gd 3+ -compounds, which exhibit broad emission bands in the spectral range from 440 to 650 nm assigned to the T→S 0 transition centered on the indandionate ligands.
The first T 1 excited state energies of the ligands were estimated as the energies correspond to the 0-0 phonon transitions from the phosphorescence spectra of the [Gd(aind) 3 L(H 2 O)], and [Gd(bind) 3 L]•H 2 O compounds with values of 21906 cm −1 (456.5 nm), and 22371 cm −1 (447.0 nm), respectively.These results suggest that both the aind and bind ligands have appropriated T 1 state energies, which play most important role in intramolecular ligand-tometal energy transfer process.Although, the S 1 and T 1 energy levels of the 1,10-phenanthroline ligand and its derivative are usually located at around 30000 and 22000 cm −1 , respectively, 29 the main role of these ligands in lanthanide diketonate complexes is only to act as ancillary ones. 30otoluminescent properties of the Eu 3+ -compounds recorded at 77 K in the spectral range 250-590 nm with emission monitored on the 5 D 0 → 7 F 2 hypersensitive transition at 611 nm.These spectra are dominated by two intense broad absorption bands in the spectral range from 270-470 nm, which are assigned to the S 0 →S 1 and S 0 →S 2 transitions centered on the acind ligands.These bands are overlapped with the absorption bands assigned to the S 0 →S 1 transition of the phen and dmphen ligands at 300 nm.The spectral profiles of the Eu 3+ -compounds (Figure 5) indicate that the S 0 →S 1 and S 0 →S 2 transitions are approximately located in the 1,3-indandionate moieties.These results are in agreement with those ones observed for similar complexes, as reported in the literature. 11igure 5 also presents some characteristic narrow absorption bands assigned to the 7 F 0 → 2S+1 L J intraconfigurational transitions of the Eu 3+ ion.These bands exhibit lower intensity than those ones, which correspond to the intraligand transitions, corroborating the efficient sensitization process via antenna effect.
The emission spectra of the [Eu(aind) 3 L(H 2 O)] and [Eu(bind) 3 L]•H 2 O compounds were recorded at 77 K in the spectral range from 420 to 730 nm, under excitation centered on indandionate ligand, S 0 →S 1 transitions (at 370 nm), as shown in Figure 6.The luminescent spectra display only the characteristic emission narrow bands that are assigned to the 5 D 0 → 7 F J transitions of the Eu 3+ ion, where J = 0, 1, 2, 3, and 4. 31,32 The local ligand field effect of the Eu 3+ -complexes has been qualitatively investigated based on the splitting and relative emission intensities of the 5 D 0 → 7 F J(0-4) transitions.Table 1 presents the intraconfigurational-4f 6 transitions and their Stark (2J + 1)-components.The presence of the only one emission band assigned to 5 D 0 → 7 F 0 , as well as three stark components for 5 D 0 → 7 F 1 indicate that the Eu 3+ is located at low symmetry environment (C n , C nv or C s ). 33These optical data corroborate with the fact that the intensity of the band assigned to 5 D 0 → 7 F 2 transition is higher than that of the assigned to 5 D 0 → 7 F 1 one, 27 which are in close agreement with the distorted coordination polyhedra obtained from theoretical modeling for both Eu 3+ -complexes (Figure 4, Figure S1).
The absence of broad phosphorescence bands arising from the indandionate ligands (Figure 6) emphasizes the presence of an efficient luminescence sensitization occurring via antenna effect.] Judd-Ofelt intensity parameters (W λ = 2.4), radiative (A rad ), non-radiative (A nrad ) rates, and emission quantum efficiency (η) of the 5 D 0 emitting level of the Eu 3+ complexes have been quantitatively obtained from the luminescence decay curves and emission spectral data (Table 2).The luminescence decay curves (Figure S3) were recorded by monitoring the emission intensity of the hypersensitive  2), respectively.While using these data, the total decay rates were determined according to the expression: A total = 1/τ.The radiative rates (A rad ) were determined using the 5 D 0 → 7 F 2,4 transitions (forced electric dipole) and the 5 D 0 → 7 F 1 transition (magnetic-dipole) as internal reference. 36,37The values of both radiative and noradiative rates for [Eu(aind) 3 L(H 2 O)], and [Eu(bind) 3 L]•H 2 O complexes are presented in Table 2.When τ values of these complexes were compared with the corresponding ones of precursor hydrated complexes, it was observed a considerable decrease in both A rad and A nrad rates.The decrease in A nrad values might be attributed to the water molecules substitution by the auxiliary phen and dmphen ligands in the first coordination sphere of the Eu 3+ ion (Table 2), that reduce the multiphonon decays, Table 1.Energy levels of the Eu 3+ -complexes assigned to the transitions ( 5 D 0 → 7 F J , J = 0-4) (in cm −1 )  also observed for the other Eu 3+ -indandionate complexes, as reported in the literature.Moreover, the luminescence quantum efficiency was determined using the expression η = A rad /(A rad + A nrad ).Table 2 shows that the higher values were observed for the [Eu(bind) 3 L]⋅H 2 O compounds than the [Eu(aind) 3 L(H 2 O)] ones, indicating the absence of water molecules in the first coordination of Eu 3+ ion, which decrease significantly the luminescence quenching effect.
The experimental intensity parameters W 2 and W 4 for the complexes were directly determined from the A rad values of the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 transitions, respectively. 35,36These parameters depend on the local geometry and the ligating atoms polarizabilities in the first coordinating sphere around the metal central ion. 37As observed in Table 2, the values of W 2 for the complexes containing heteroaromatic phen and dmphen ligands are lower than those ones calculated for the hydrated precursor compounds. 11This result suggests that the chemical environment polarizabilities have decreased when phen and dmphen ligands water molecules in the first coordination sphere of the Eu 3+ ion, which have contributed to a significant decrease in the emission rate assigned to the 5 D 0 → 7 F 2 transition.These values are similar to those found for europium 2-acyl-1,3-indandionate complexes containing triphenylphosphine oxide ligand. 11he values of W 4 exhibit similar behavior, suggesting that the sensibility of the 5 D 0 → 7 F 4 transition has also been changed with the polarizability and chemical environment.

Conclusions
In the current work, we have reported the synthesis, characterization, and photoluminecent properties of

Figure 2 .
Figure 2. FT-IR absorption spectra of the Eu 3+ -complexes recorded in the range of 4000-400 cm −1 in KBr pellets.
Theoretical structural studies T h e o r e t i c a l m o l e c u l a r s t r u c t u r e s o f t h e [Eu(aind) 3 L(H 2 O)] and [Eu(bind) 3 L]•H 2 O complexes, where L = phen or dmphen, were optimized using the reparameterized latest version of the SPARKLE/AM1

Figure 6 .
Figure 6.Emission spectra of the Eu 3+ -complexes recorded at 77 K in the spectral range of 420-720 nm, under excitation at 370 nm.

[
Ln(aind) 3 L(H 2 O)] and [Ln(bind) 3 L]•H 2 O coordination compounds (Ln : Eu 3+ or Gd 3+ ).The FT-IR data suggest that the water molecule is coordinated to the Ln 3+ ion in the case of 1, 3, 5, and 7 complexes.The photoluminescence data are consistent with coordination polyhedra calculated via SPARKLE/AM1 model for the [Ln(aind) 3 L(H 2 O)] and [Ln(bind) 3 L]•H 2 O complexes.The structural differences and the absence of water molecules in the first coordination sphere have great impact on the luminescence quantum efficiency (η) of the Eu 3+ -complexes, decreasing the luminescence quenching in the intramolecular energy transfer process.
11,12The lower values of A nrad for [Eu(bind) 3 L]•H 2 O compounds compared to those observed for [Eu(aind) 3 L(H 2 O)] are a direct consequence of the multiphonon decay, because the water molecule is not coordinated to the Eu 3+ ion.