New Homotrinuclear Lanthanide Complexes: Synthesis

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New Homotrinuclear Lanthanide Complexes: Synthesis, Characterization and Spectroscopic Study Wagner E. Silva,*,† Moˆnica Freire Belian,‡ Ricardo O. Freire,§ Gilberto F. de Sa´,† and Severino Alves Jr.† Departamento de Quı´mica Fundamental, UFPE, 50670-901 Recife, Pernambuco, Brazil, Departamento de Quı´mica, UFRPE, 52171-900 Recife, Pernambuco, Brazil, and Departamento de Quı´mica, UFS, 49100-000, Saõ Cristo´Vaõ, SE, Brazil ReceiVed: May 14, 2010; ReVised Manuscript ReceiVed: August 10, 2010

This work presents the synthesis and spectroscopic study of new homotrinuclear (TRI) systems for photonics applications. The luminescence spectroscopy shows characteristics transitions of Eu(III) and Tb(III) ions. For the Gd(III) complexes, the triplets states were determined by phosphorescence measurement. The complexes’ coordination geometries were calculated using the Sparkle/AM1 model. For the europium systems, the Sparkle/AM1 geometries were used to calculate all details involved in the energy transfer process, and the theoretical quantum yields were determined. From an energy diagram, that estimates triplet levels, it was possible to understand some experimental phenomenon, such as weak luminescence for precursor complex (without heterocyclics ligands), and ligands emission in terbium complexes. Some of these observations can also be explained by the Jablonski diagrams that describe, based on theoretical calculations, all luminescent process. The synthesized complexes showed high values of quantum yield in ethanolic environment: 50% for EuTRIDipy, 26% EuTRITerpy, and 56% for EuTRIPhen complexes. 1. Introduction The synthesis of the new coordination compounds, involving lanthanide ions, consist of a promising area inside of molecular chemistry‘s nanometric perspective (light conversion molecular devices - LCMD), electronic-optics (Lasers and Displays), and telecommunication.1-8 The probes based on europium(III) and terbium(III) ions are very interesting, particularly because of their suitable spectroscopic properties, such as large Stokes shift and narrow emission profiles.1 One of the most important properties of lanthanide compounds is the very narrow absorption and emission bands arising from the parity-forbidden intraconfigurational 4f-4f transitions.9 To gain insight into the factors that determine the quantum yield and other relevant properties of lanthanide complexes, many groups have been investigating the photophysical properties of a large number of new lanthanide complexes, using an approach based upon both theoretical and experimental work.10-19 The results have shown that the quantum yield of a lanthanide complex arises from a balance among the rates of several processes, for example, ligand to Ln(III) energy transfer, multiphonon relaxation, energy back-transfer, crossover to chargetransfer states, etc. The control of these rates and other relevant physical properties are accomplished by a thorough selection of ligands, allowing us to develop some promising light conversion molecular devices (LCMDs),12,20 with high quantum yields at room temperature, leading to new applications. The investigation of the luminescence properties of the Eu(III) and Tb(III) ions have a special relevance because their specific strong red (620 nm) and green (540 nm) emission, and also the long lifetimes of the excited 5D0 and 5D4 states respectively. In * To whom correspondence should be addressed. Fax: +55 81 21268442. E-mail: [email protected]. † UFPE. ‡ UFRPE. § UFS.

particular, the special attention given to the Eu(III) ion is mainly due to the nondegenerate emitting level (5D0), which means that the emission bands arising from the 5D0 f 7FJ transitions (J ) 0, 1, 2, 3, or 4) can provide information about the local symmetry.21 The 4f orbital in the lanthanide complexes shows its low contribution in the coordination chemical bonding generation, resulting in smaller overlap values.22 Recently, trinuclear lanthanide complexes, especially with luminescent properties, have attracted the interest of researchers.23 Our group, in current work, decided to investigate the coordination chemistry of lanthanide ions with β-diketonatebased ligands, which have strong coordination capability to the lanthanide ions. Ln(III)/β-diketonate complexes represent an important contribution within lanthanide coordination chemistry, since it enable a large number of versatile compounds, contributing in this case, for lanthanide attachment as well as intense luminescence allowance.24-26 In particular, at the same ligand it was possible to connect three acetylacetonate groups, which work like excellent complexing agents, once that was able to have three coordination sites for metal ions,27 besides presenting a high degree of covalency.28 The photophysical properties of Ln(III)-complexes containing diketonate anions have been extensively reported29 and have been shown to act as great luminescent sensitizers, especially for Eu(III) and Tb (III) ions, by an efficient intramolecular ligand-to-metal energy transfer. This article reports the synthesis, characterization, and photoluminescence properties of the luminescent trivalent lanthanide homotrinuclear complexes (Eu, Gd, and Tb) with tris[2(3′petane-2,4-dione)ethyl]amine (TRI). A detailed theoretical and experimental study of the photoluminescence properties of the Ln(III)-TRI complexes was made aiming to obtain information about the energy transfer process from the TRI ligand to the Ln(III) ions and for the luminescent systems. The intensity parameters Ωλ (λ ) 2, 4, and 6) for Eu(III) systems were

10.1021/jp104396k  2010 American Chemical Society Published on Web 08/25/2010

Homotrinuclear Lanthanide Complexes

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Figure 1. Synthesis of the Tris[2(3′-pentane-2,4-dione)ethyl]amine (TRI) Ligand.

calculated and compared with the experimental ones. In addition, the R02 parameters, radiative and nonradiative rates, experimental emission quantum efficiencies, and quantum yield were also investigated. 2. Experimental Section 2.1. Materials and Methods. The starting chemicals were TbCl3 · 6H2O, 2,5-petanedione, tris(2-chloroethyl)amine hydrochloride, 2,2′-dipyridine (dipy), 1,10-phenanthroline (phen), and 2,2′:6′,2′′-terpyridine (terpy) (Aldrich). The lanthanides salts EuCl3 · 6H2O and GdCl3 · 6H2O were prepared by dissolving their oxides in concentrated hydrochloric acid, and the solvents were evaporated to dryness. The lanthanide contents, in their chloride hydrate salt forms, were determined by EDTA titration. All others reagents were commercially available and were used without further purification. Carbon, nitrogen, and hydrogen percentages in the complexes were determined from elemental analyses, using a Perkin-Elmer Model 240 microanalyzer. The IR vibrational spectra were obtained in a Bruker IF 566 FTIR spectrophotometer (KBr discs). The absorption spectra were obtained on Perkin-Elmer spectrophotometer, lambda 6, with tungsten lamp for excitation in the visible wavelength range and deuterium lamp for excitation in the ultraviolet wavelength range. The excitation and emission spectra were obtained on Jobin Yvon Ramanor U1000 model H-10 equipment. The emission detection was performed by using a RCA C31034-02 photomultiplier; register and processing of the obtained signals were performed by using a Spectralink interface. The luminescence decay curves of the emitting levels were measured using a phosphorimeter SPEX 1934D accessories coupled to spectrophotometer. The NMR spectra were obtained in the VARIAN Unity Plus 300 equipment, frequency of the 300 MHz for 1H and 75 MHz for 13C, in CDCl3. 2.2. Experimental Methods: Synthesis of Ligand and Complexes. 2.2.1. Synthesis of Tris[2(3′-petane-2,4-dione)ethyl]amine (TRI). The tris(2-iodineethyl)amine (TIA) was synthesized from a mixture of 1:3 (mol/mol) of tris(2-chloroethyl)amine hydrochloride (1) (0.5 g, 2.1 mmol) and potassium iodide (1.38 g, 8.3 mmol) dissolved in 50 mL of acetonitrile for 12 h. The excess of potassium chloride was removed by a simple filtration. The reaction mixture was evaporated, and the resulting yellow solid was washed with saturate sodium bisulfite solution (1 mL), distilled water (1 mL), and dichloromethane (1 mL); and then dried under vacuum at room temperature. Tris[2(3′-petane-2,4-dione)ethyl] amine (2) was obtained from the reaction of the TIA (0.25 g, 0.41 mmol) with 2,4-pentanedione (126.3 µL, 1.23 mmol) catalyzed by potassium

carbonate (170 mg) in 60 mL of dry ethanol. The system was stirred and refluxed during 3 days. Finally, the unreacted carbonate and salt formed were removed by a simple filtration, and the final product was dried under vacuum (Figure 1). 2.2.2. Synthesis of Trinuclear Complexes. The precursor complex was synthesized from the TRI ligand (60 mg, 0.15 mmol) dissolved in ethanol (20 mL); then, potassium tertbutoxide was added. The mixture was stirred for 30 min. The lanthanide salt (0.45 mmol) was dissolved in ethanol and was added dropwise to the mixture. The system was kept under stir during 24 h. The solution obtained was dried under vacuum. After synthesis of the precursor complex, the water molecules were substituted by dipy, phen, and terpy ligands (Figure 2), dissolved in ethanol, in the ratio 1:6 (precursor complex mol/ ligand mol), under stirring for 24 h. The solvent was evaporated, and the obtained solid was washed with hexane (for dipy and terpy complexes cases) or chloroform (for terpy complex case). 2.3. Intensity Parameters: Theoretical and Experimental. For theoretical description of the intensities parameters (Ωλ, where λ) 2, 4, and 6 for Eu(III) ion) have been used mechanisms of forced electric dipole and dynamic coupling.30-32 Theoretical intensity parameters were calculated from the structural results obtained by Sparkle model and using the following equations:33

Ωλ ) (2λ + 1)

∑ t,p

|Bλtp | 2 2t + 1

(1)

ed dc Bλtp ) Bλtp + Bλtp

(2)

ed Bλtp ) θ(t, λ)γtp

(3)

where

and dc Bλtp )-

[

(λ + 1)(2λ + 3) 1/2 × (2λ + 1) 〈4f|rλ |4f〉(1 - σλ)〈f||C(λ) ||f〉Γtpδt,λ+1

]

(4)

The first and second terms, in the rhs of eq 2, correspond to forced electric dipole (ed) and dynamic coupling (dc) mechanisms, respectively. The quantities γpt (odd-rank ligand field parameters) and Γtp (ligand atom polarizability dependent terms) in eqs 3 and 4 contain a sum over the ligand atoms involving

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Figure 2. Structure of the (a) [Ln3(TRI)(dipy)6(H2O)6]Cl6, (b) [Ln3(TRI)(phen)6(H2O)6]Cl6, and (c) [Ln3(TRI)(terpy)6]Cl6 complexes.

a spherical harmonic of rank t (Yt,p). The nature of the chemical environment and structural aspects in the first coordination sphere of the europium ion are precisely taken into account in this sum. In eq 3, the numerical factor θ(t, λ) is a function of the lanthanide ion, and in eq 4 the quantities〈4f|rλ|4f〉, (1 - σλ), and 〈f|C(λ)|f〉 are a radial integral, a shielding factor, and a oneelectron reduced matrix element, respectively.34 The experimental intensity parameters (Ω2 and Ω4) were determined from the emission spectra, based on the 5D0 f 7F2 and 5D0 f 7F4 electronic transitions of the Eu(III) ion, and they are estimated according to the following eq 5:35

Ωλ )

3pc3A0λ 4e2ω3χ〈7Fλ ||U(λ) || 5D0〉2

(5)

where χ is the Lorentz local field correction term, given by χ ) (n(n2 + 2)2)/9, and 〈7Fλ|U(λ)|5D0〉2 is a squared reduced matrix element whose value is 0.0032 and 0.0023 for the 5D0 f 7F2 and 5D0 f 7F4 transitions, respectively.36 The refraction index, n, has been assumed as 1.5. The spontaneous emission coefficient, A01, is given by the expression A01 ) 0.31 × 10-11(n)3(ν01)3, leading to an estimated value around 50 s-1.12 In eq 5, the A0λ term, where λ ) 2 and 4, represents the spontaneous emission coefficients of the 5D0 f 7F2 and 5D0 f 7 F4 transitions, which can be calculated from the 5D0 f 7F1 reference transition (magnetic dipole mechanism), therefore this

transition is practically insensitive to chemical environment changing (eq 6).

A0λ ) A01

( )( ) S0λ ν01 S01 ν0λ

(6)

where S01 and S0λ are the areas under the curves of the 5D0 f F1 and 5D0 f 7Fλ transitions, with ν01 and ν0λ being their energy barycenters respectively. 2.4. Experimental Measure of R02 Parameter. The 5D0 f 7F0 transition presence for the europium(III), sufficiently discussed in the literature,33 would not have to inside occur of the configuration 4fN for the mechanisms of electric dipole, electric quadrupole, and vibronic; therefore, its element of reduced matrix of the operator U(0) is equal to zero. The pseudomultipolar mechanism determines when this transition occurs, due to J’s mixture, that describes the participation of the 7F2, 7 F4, and 7F6 states, in the formation of 7F0, having in this case, greatest participation of the 7F2 state. This mixture due to the representation of the terms pairs described byBqkCq(k), from the ligand field Hamiltonian in the eq 7. We can describe the single 7F0 state as a linear combination of states 7FJ, where J ) 2, 4, and 6: 7

| 7F0〉 )

∑ CJM | 7FJMJ〉 JMJ

J

(7)



TABLE 1: 1H and 13C NMR Chemical Shifts of TRI Ligand (ppm)a

a1

H NMR signal, on labeled carbons, means a single line. s, singlet; d, doublet; t, triplet; m, multiplet.

TABLE 2: Elemental Analysis Data of the Trinuclear Complexes %C

%H

%N

complexes

exp.

theo.

exp.

theo.

exp.

theo.

[Eu3(TRI)(H2O)18]Cl6 [Eu3(TRI)(dipy)6(H2O)6]Cl6 [Eu3(TRI)(phen)6(H2O)6]Cl6 [Eu3(TRI)(terpy)6]Cl6 [Tb3(TRI)(H2O)18]Cl6 [Tb3(TRI)(dipy)6(H2O)6]Cl6 [Tb3(TRI)(phen)6(H2O)6]Cl6 [Tb3(TRI)(terpy)6]Cl6 [Gd3(TRI)(H2O)18]Cl6 [Gd3(TRI)(dipy)6(H2O)6]Cl6 [Gd3(TRI)(phen)6(H2O)6]Cl6 [Gd3(TRI)(terpy)6]Cl6

18.16 46.14 49.60 58.52 17.90 45.71 49.15 58.05 17.99 45.78 49.24 58.17

18.19 46.17 49.62 58.56 17.92 45.72 49.16 58.06 18.00 45.80 49.29 58.20

4.75 4.25 3.97 3.88 4.70 4.16 3.94 3.86 4.70 4.23 3.96 3.88

4.76 4.27 4.00 3.90 4.70 4.23 3.96 3.87 4.71 4.25 3.97 3.88

0.99 8.65 8.10 10.86 0.97 8.58 7.99 10.70 0.98 8.59 8.01 10.77

1.01 8.65 8.09 10.82 0.99 8.56 8.02 10.73 1.00 8.58 8.04 10.75

When assume that the J’s mixture occurs inside of the 7F term and just the J ) 2 is possible, the eq 6 can be rewritten as (eq 8):

| 7F0〉 ) C00 | 7F00〉 +

∑ C2MJ| 7F2MJ〉

(8)

2.5. Experimental Emission Quantum Yield (q). The emission quantum yield (q) is defined as the ratio between the number of photons emitted by the lanthanide ion and the number of photons absorbed by the ligand. The quantum yields33 are determined according to eq 10:

MJ

The 5D0 f 7F0 transition can be considered pseudo-hypersensible, once that exist its dependence, with 5D0 f 7F2 transition. The obtained parameter from eq 9, can be used in the analysis of 5D0 f 7F0 intensity transition, and in the evaluation of the J’s mixture effect.

σ0,0A(0, 0) I(0, 0) S(0, 0) A(0, 0) R02 ) ) ) = I(0, 2) S(0, 2) σ0,2A(0, 2) A(0, 2)

(9) where S is the area under the curve of the corresponding transition, and σ is the average energy (cm-1) of the transition.

q ) qref

( )( )( )(

Sx Aref λref Iexc λref n2x Sref Ax λx Ix λx n2 ref

)

(10)

where ref is used to reference tris(2,2-bipyridyl)dichlororuthenium(II) ) 2,8%, in ethanolic solution; and x represents the samples; S are areas under the curve in the emission spectra; A(λ) is the absorption maximum in λ; I(λ) is maximum excitation in λ; n is the refraction index, and the quantum yield of the reference is represented by qref. The quantum yield measurements for all samples were performed in ethanolic solution (10-5 mol dm-3).

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Figure 3. Calculated ground-state geometry, using the Sparkle/AM1 model, for the [Ln3(TRI)(dipy)6(H2O)6]Cl6 system.

3. Theoretical Basis 3.1. The Ground State Geometries Calculation. The ground state geometries of all trinuclear systems were calculated with the Sparkle/AM137 model using the MOPAC2009 software.38 The Sparkle/AM135,37 and Sparkle/PM339 models are semiempirical approaches for lanthanide complexes calculations that present a level of accuracy useful for coordination compounds design. For this reason, these models have been very useful to elucidate many aspects in the luminescent process of lanthanide complexes. 3.2. Excited States Energies and Absorption Spectra. The Sparkle/AM1 geometries were used to calculate the singlet and triplet excited states using the configuration interaction single (CIS) based on the intermediate neglect of differential overlap/ spectroscopic (INDO/S) method40,41 implemented in the ZINDO software.42 We used a point charge of +3e to represent the trivalent europium ion. The CIS space was gradually increased until there were no further meaningful changes in the calculated transitions. In contrast, the energy levels of the Eu(III) metal ions were considered as being those of the free ion in the intermediate coupling scheme. The INDO/S accuracy is about 1000 cm-1. A Lorentzian line shape was fitted to the calculated singlet transitions, together with the relative intensities obtained from oscillator strengths. All the simulated spectra had halfheight bandwidth of 25 nm. 3.3. Photoluminescence Properties. The theoretical values of the energy transfer and back-transfer rates, radiative (Arad) and nonradiative (Anrad) decay rates, quantum efficiency (η), and quantum yield (q) values were calculated using the same procedure described in ref 44. 4. Results and Discussion 4.1. TRI Ligand. The TRI ligand presented as a yellow solid. The yield of the reaction was 85%. Elemental analysis, infrared

spectroscopy, and nuclear resonance magnetic were used in its characterization. Chemical analysis results (CHN) are all in good agreement with the theoretically expected values (%C ) 63.80 (63.77); %H ) 8.35 (8.35); %N ) 3.54 (3.54)). The data of the elemental analysis had been 0.5% error. The 1H NMR spectrum of TRI was recorded in D2O, and its 1 HNMR spectrum shows: a broad singlet at 1.85 ppm corresponds to 18 protons of methyl (CH3) parts (a). A triplet signal at 2.73 ppm corresponds to 3 protons between carbonyl groups (b). Other triplet observed at 3.55 ppm due to 6 protons of the ethylene part (d). A multiplet signal in 1.15 ppm is referring to others 6 protons (c). All signals of the 1H are summarized in the Table 1. The 13C NMR spectra of TRI ligand was taken in D2O, and the carbon assignments are given in Table 1. The carbonyl carbon is observed at 181.746 ppm (Cf). The signal of the methyl group is at 23.673 ppm (Ce). The carbon between carbonyl groups is at 53.352 ppm (Cg). The ethylene part presents in 14.486 ppm (Ch) and 67.72-66.756 ppm (Ci). The infrared spectrum of TRI ligand shows two signals, in the 1250-2000 cm-1 region, referring to CdO (strong band -1600 cm-1) and C-N stretching modes (medium band -1420 cm-1). The OH stretching mode appears at 3500 cm-1, referring to hydration water molecules. 4.2. Lanthanide Complexes. All the precursors complexes (Eu(III), Tb(III), and Gd(III)) presented as a yellow solid; after substitution of the water, molecules showed as a brownyellowish solid. The yields of the complexes were 92-94%. Chemical analysis results are all in good agreement (within an 1% error) with theoretically expected values (see Table 2). Carbonyl groups complexation to Ln(III) ions shifts the CdO vibration mode in the infrared spectrum (not shown) from 1600 cm-1, in the free ligand, to 1570 cm-1 (complexed form). The



TABLE 3: Spherical Atomic Coordinates Calculated via Sparkle/AM1 Coordination Polyhedron of the [Eu3(TRI)(dipy)6(H2O)6]Cl6 Compound, Charge Factors (g), and the Polarizability (r) of the Coordinated Atom atom

R (Å)

θ (deg)

φ (deg)

ga

R (Å3)a

Eu(III) O (β-diketone) O (β-diketone) N (dipy) N (dipy) N (dipy) N (dipy) O (H2O) O (H2O)

0.000 2.369 2.372 2.495 2.495 2.496 2.496 2.392 2.395

0.000 76.008 90.908 47.981 127.731 39.683 114.525 142.553 106.516

0.000 0.733 60.160 147.412 139.078 251.995 217.689 4.228 299.576

1.6974 1.6974 0.0974 0.0974 0.0974 0.0974 0.3203 0.3203

26 220 26 220 31 106 31 106 31 106 31 106 30 517 30 517

a

Obtained using a nonlinear minimization technique.

Figure 4. Experimental absorption spectrum of [Ln3(TRI)(dipy)6(H2O)6]Cl6 system and the calculated electronic absorption spectrum from the optimized geometry by the Sparkle/AM1 model.

dipy, phen, and terpy ligands complexation can be followed by a displacement of the νCdN band (1420 cm-1) of the pyridine ring to lower frequencies (1398 cm-1). The OH stretching mode appears at 3300 cm-1, which means that water is present in the Ln(III) coordination sphere, in the complexes. Figure 3 shows the optimized molecular structures of the [Eu3(TRI)(dipy)6(H2O)6]Cl6 complex calculated using the Sparkle/

AM1 model. The spherical atomic coordinates for the coordination polyhedra are summarized in Table 3. The optimized molecular structures and tables containing the spherical atomic coordinates for the systems [Eu3(TRI)(H2O)18]Cl6, [Eu3(TRI)(phen)6(H2O)6]Cl6 and [Eu3(TRI)(terpy)6]Cl6 can be found in the Supporting Information (Figures S1, S4, S7 and Tables S1 to S3). The experimental and theoretical electronic absorption spectra of the [Eu3(TRI)(dipy)6(H2O)6]Cl6 system are shown in Figure 4. A good agreement in the profile and number of absorption bands and relative intensities can be observed, although displaying a small systematic red shift, which may be due of the impossibility of considering all the symmetrical orbital in CI calculation and, in part, due to solvent effects, which have not been considered in the calculation. The theoretical spectrum was calculated for a molecule in vacuum, whereas the observed spectrum was obtained for ethanolic solutions of the complex. The relative intensities of the peaks have been qualitatively reproduced, as shown in Figure 4. The experimental and theoretical absorption spectrum for the systems [Eu3(TRI)(H2O)18]Cl6, [Eu3(TRI)(phen)6(H2O)6]Cl6 and [Eu3(TRI)(terpy)6]Cl6 can be observed in Figures S2, S5, and S8 presented in Supporting Information. The emission spectra of [Eu3(TRI)(H2O)18]Cl6 (EuTRI) complex is presented in Figure 5, in which the typical transitions from europium ion can be observed. Eu(III) ion transitions, observed in the emission spectrum, occur via three main mechanisms: forced electric dipole, magnetic dipole, and dynamic coupling.12 The relationship between the magnetic (5D0 f 7F1) and electric (5D0 f 7F2) parcel give informations about the symmetry surrounding ion. A high ratio (5D0 f 7F1/5D0 f 7F2 > 1) was observed in the case of the EuTRI complex, suggesting that the forced electric dipole mechanism does not present a strong contribution for Eu(III) ion luminescence in this complex (J’s minor mixing effect). In this case, it can be suggested that the europium ion is in a high symmetry environment or that the symmetry surrounding europium ion approaches a center of inversion. The complexes emission spectrum are shown in Figure 5. The terpy complex showed higher intensity of luminescence when compared with others complexes, this last observation also occurred in the terbium-terpy complex case (Figure 6). The systems compared with similar complexes, using acetylacetonate as ligand, observed change of electronic properties, due to the

Figure 5. Emission spectrum of the (a) [Eu3(TRI)(H2O)18]Cl6; (b) [Eu3(TRI)(dipy)6(H2O)6]Cl6; (c) [Eu3(TRI)(phen)6(H2O)6]Cl6; and (d) [Eu3(TRI)(terpy)6]Cl6 complexes.

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Figure 6. Emission spectrum of the (a) [Tb3(TRI)(H2O)18]Cl6; (b) [Tb3(TRI)(dipy)6(H2O)6]Cl6; (c) [Tb3(TRI)(phen)6(H2O)6]Cl6; and (d) [Tb3(TRI)(terpy)6]Cl6 complexes.

Figure 7. Energy states of the Eu(III), Tb(III), and TRI systems (theoretical and experimental).

influence of the metallic center above others ions and the structural changing in the R carbon at carbonyl groups. This last case, the terbium acetylacetonate complex shows a high intensity of luminescence because of its triplet state that is ressonant with 5D4 from the terbium(III) ion. In the case of terbium TRI and dipy complexes, it was observed the luminescence intensity decrease; for phen and terpy complexes, higher luminescence intensity was observed when compared with europium similar complexes (around 10-50 times). Energy states of the Eu(III), Tb(III), and ligands (gadolinum complexes phosphorescence) are shown in Figure 7. For the all complexes observed (except for precursor system), two triplete states localized in electronic structure of the ligands.

Due to of this last situation, the energy transfer process can occur for 5D0 and 5D1 states localized in Eu(III) ion, and for the 5D4 state of the Tb(III) ion. For the precursor system, triplet states with low energy was observed for the GdTRI complex, suggesting that energy tranfers process is more efficient for the europium ion than for others; this was observed by the comparison between the emission spectra of the EuTRI and TbTRI complexes. Table 4 presents the experimental and theorecical values for the intensity (Ωλ; where λ ) 2 or 4) parameters; radiative (Arad) and nonradiative (Anrad) rates, quantum efficiency (η), yield quantum (Φx), and experimental R02 parameter for [Eu3(TRI)(H2O)18]Cl6, [Eu3(TRI)(dipy)6(H2O)6]Cl6, [Eu3(TRI)(phen)6-



TABLE 4: Theoretical and Experimental Intensity Parameters (Ωλ), Experimental R02 Parameter, Radiative (Arad) and Nonradiative (Anrad) Rates, Quantum Efficiency (η), Quantum Yield (Φx), and Intensity (Ωλ; λ ) 2 or 4) Parameters of the Eu(III) Systems R02 Ω2 (10-20cm2) Ω4 (10-20cm2) Ω6 (10-20cm2) A(rad) (s-1) A(nrad) (s-1) η (%) Φx(%)

system

[Eu3(TRI)(H2O)18]Cl6 [Eu3(TRI)(dipy)6(H2O)6]Cl6 0.14 [Eu3(TRI)(dipy)6(H2O)6]Cl6 (Sparkle/AM1 geometry) [Eu3(TRI)(phen)6(H2O)6]Cl6 0.051 [Eu3(TRI)(phen)6(H2O)6]Cl6 (Sparkle/AM1 geometry) [Eu3(TRI)(terpy)6]Cl6 0.092 [Eu3(TRI)(terpy)6]Cl6 (Sparkle/AM1 geometry)

4.2 7.86 3.1 3.01 3.8 1.99

2.4 4.69 7.1 4.37 6.0 6.30

2.66 1.41 0.99

435.8 358.05 302.9 206.84 340.2 205.19

252.1 329.85 498.7 594.76 257.8 392.81

63.4 52.0 37.8 25.8 56.9 34.3

1.7 50.0 51.2 5.6 6.9 26.0 21.3

TABLE 5: Calculated Values of Intramolecular Energy Transfer and Back-Transfer Rates singlet f 5D4

triplet f 5D1 -1 b

-1 a

triplet f 5D0

system

WET1 (s )

WBT1 (s )

WET2 (s )

WBT2 (s )

WET3 (s )

WBT3 (s-1)b

[Eu3(TRI)(dipy)6(H2O)6]Cl6 [Eu3(TRI)(phen)6(H2O)6]Cl6 [Eu3(TRI)(terpy)6]Cl6

1.59 × 105 2.45 × 104 1.64 × 105

2.20 × 10-9 2.89 × 10-12 6.37 × 10-9

7.17 × 107 2.87 × 105 1.33 × 106

9.044 1.81 × 10-4 9.09 × 10-5

2.64 × 107 8.17 × 104 3.39 × 105

7.23 × 10-4 1.12 × 10-8 5.05 × 10-9

a

-1 a

-1 b

-1 a

WET - Transfer rate. b WBT - Back-transfer rate.

Figure 8. Energy level diagram for [Tb3(TRI)(dipy)6(H2O)6]Cl6 showing the most probable channels for the intramolecular energy transfer process.

(H2O)6]Cl6, and [Eu3(TRI)(terpy)6]Cl6 systems. First, we can observe the good agreement between theoretical and experimental values. We also observe that the EuTRIphen complex showed the highest nonradiative rate, resulting in lowest quantum efficiency. The EuTRIdipy complex present the best quantum efficient when compared to the terpy complex, indicating that the steric effect, in this case, increases the lanthanide ion interaction with chemical environment (minor radiative rate); however, it became possible to observe a minor nonradiative rate in the terpy complex (absence of water molecules). The intensity (Ω2 and Ω4) and R02 paramenters suggesting that high polarizability and greater effect in the J mixtures, causing a quantum yield increase, which corroborates with emission spectra of the europium complexes. The results of intensity parameters reinforce the importance of dinamic coupling mecanism for 4f-4f transitions.31 As the quantum efficiency usually modulates the quantum yield values, for the systems synthesized we observed this tendency. For example, the EuTRIdipy complex shows quantum yield of 50% and quantum efficiency of 63.4%.

For the EuTRI system, three β-diketones are connected in the same molecule, allowing a trinuclear system building with a single β-diketonate availability, for each lanthanide center; in this situation a different behavior was observed when compared with some analogous Ln(III)/β-diketonate complexes (ligand without any connection), such as higher quantum yield for EuTRI system.24-26,28 Table 5 presents the intramolecular energy transfer (WET) and back-transfer (WBT) rates for [Eu3(TRI)(dipy)6(H2O)6]Cl6, [Eu3(TRI)(phen)6(H2O)6]Cl6, and [Eu3(TRI)(terpy)6]Cl6 systems. As we can observe the triplet f 5D1 and triplet f 5D0 are the main energy transfer channels, considering WET only. The backtransfer rates shows high values in case of triplet f 5D1 and triplet f 5D0 compared with the values of singletf 5D4, and this suggest higher resonance between triplet and 5D1 states. These results agree with the experimentally observed on the quantum yield and with the position of the triplet states, the latter being obtained from the ligands’ phosphorescence (gadolinium(III) complexes). As we can observe in eqs 11-13, small values of the RL parameter, that is, the distance from the donor state located at

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Silva et al.

the organic ligands and the Eu(III) ion, generate high values of energy transfer rates. The RL value calculated for triplet state of the [Eu3(TRI)(dipy)6(H2O)6]Cl6 is only 3.85 Å, whereas the RL values for the [Eu3(TRI)(phen)6(H2O)6]Cl6 and [Eu3(TRI)(terpy)6]Cl6 are 10.34 and 7.39 Å. mm em WET ) WET + WET

(11)

where Wmm ET corresponds to the energy transfer rate obtained from the multipolar mechanism, given by:

mm WET

2 2π e SL ) F p (2J + 1)G

∑ γλ〈R′J′U(λ)RJ〉2 + λ

2

e SL 2π F p (2J + 1)GR6 L

∑ Ωedλ 〈R′J′U(λ)RJ〉2

(12)

λ

em corresponds to the energy transfer rate obtained from and, WET the exchange mechanism. This term is calculated by:

em WET ) 2 2 8π e (1 - σ0) F〈R′J′SRJ〉2 3p (2J + 1)R4 L

∑ |〈φ| ∑ µZ(k)sm(k)|φ′〉|2 m

(13)

k

This fact could explain the high quantum yield for [Eu3(TRI)(dipy)6(H2O)6]Cl6 compared with the values observed for [Eu3(TRI)(phen)6(H2O)6]Cl6 and [Eu3(TRI)(terpy)6]Cl6 systems. Figure 8 is the proposed energy diagram for the [Eu3(TRI)(dipy)6(H2O)6]Cl6. Full lines concern the radiative transitions, whereas the dashed lines concern those associated with nonradiative paths. The curved lines are related to the ligand f lanthanide energy transfer or back-transfer. The energy transfer rates from the ligand triplet state (T1) to the 5D1 and 5D0 levels, as well as the energy transfer rates from the singlet state (S1) and singlet and triplet values, are summarized in Table 5. The energy diagrams for the [Eu3(TRI)(phen)6(H2O)6]Cl6 and [Eu3(TRI)(terpy)6]Cl6 systems can be observed in Figures S3, S6, and S9 in the Supporting Information. 5. Conclusions All complexes showed good quantum efficiency, which is an expected result, since the Ln(III)/β-diketonates act as good antennas, due to the high extinction molar coefficient. The study was motivated by these novel ligands, trying to explore new properties and applications as luminescent devices. The [Eu3(TRI)(dipy)6(H2O)6]Cl6 system had a higher intensity of emission (about 50%). This is probably due to two factors: (i) the fact that the triplet states (of the ligands) present the best resonance among europium(III) ion and (ii) the small distance between the donor triplet state located at the organic ligands and the Eu(III) ion (small value of RL parameter). Acknowledgment. We appreciate the financial support from the Brazilian agencies, institutes and networks: CNPq, FACEPE, CAPES, FAPITEC-SE, INAMI, Instituto do Mileˆnio, and RENAMI. We are also grateful to Professor A. E. A. Paixaõ for the use of the software Statistic. Supporting Information Available: Additional figures (ground-state geometry, experimental and calculated absorption

spectrum, and energy level diagram) and tables (Sparkle/AM1 Spherical atomic coordinates) for [Eu3(TRI)(H2O)18]Cl6, [Eu3(TRI)(phen)6(H2O)6]Cl6, and [Eu3(TRI)(terpy)6]Cl6 systems. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Lanthanide Probes in Life: Chemical and Earth Sciences; Desreux, J. F.; Choppin, G. R.; Bu¨nzli, J.-C., Eds.; Elsevier: Amsterdam, 1989. (2) Armelao, L.; Quici, S.; Barigelletti, F.; Accorsi, G.; Bottaro, G.; Cavazzini, M.; Tondello, E. Coord. Chem. ReV. 2010, 254 (5-6), 487– 505. (3) Tang, S.; Shao, C.; Liu, Y.; Mu, R. J. Phys. Chem. Solids 2010, 71 (3), 273–278. ´ vila, L. R.; Nassor, E. C. O.; (4) Pereira, P. F. S.; Matos, M. G.; A Cestari, A.; Ciuffi, K. J.; Calefi, P. S.; Nassar, E. J. J. Lumin. 2010, 130 (3), 488–493. (5) Cui, X.; She, J.; Gao, C.; Cui, K.; Hou, C.; Wei, W.; Peng, B. Chem. Phys. Lett. 2010, 494 (1-3), 60–63. (6) Bacigalupo, M. A.; Meroni, G.; Secundo, F.; Scalera, C.; Quici, S. Talanta 2009, 80 (2), 954–958. (7) Liu, S.; He, P.; Wang, H.; Shi, J.; Gong, M. Inorg. Chem. Commun. 2009, 12 (6), 506–508. (8) Appavoo, I. A.; Zhang, Y. Emerg. Nanotec. Manuf. 2010, 15, 9– 175. (9) Teotonio, E. E. S.; Brito, H. F.; Felinto, M. C. F. C.; Thompson, L. C.; Young, V. G.; Malta, O. L. J. Mol. Struct. 2005, 751, 85–94. (10) Malta, O. L.; Brito, H. F.; Menezes, J. F. S.; Silva, F. R. G.; Alves, S., Jr.; Farias, F. S., Jr.; de Andrade, A. V. M. J. Lumin. 1997, 75, 255– 268. (11) Batista, H. J.; de Andrade, A. V. M.; Longo, R. L.; Simas, A. M.; de Sa´, G. F.; Ito, N. K.; Thompson, L. C. Inorg. Chem. 1998, 37, 3542– 3547. (12) de Sa´, G. F.; Malta, O. L.; Donega´, C.; de Mello; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr. Coord. Chem. ReV. 2000, 196, 165–195. (13) Duan, L.-M.; Lin, C.-K.; Wang, H.; Liu, X.-M.; Lin, J. Inorg. Chim. Acta 2010, 363 (7), 1507–1512. (14) Lozano, V. A.; Tauler, R.; Iban˜ez, G. A.; Olivieri, A. C. Talanta 2009, 77 (5), 1715–1723. (15) Lis, S.; Pawlicki, G. J. Lumin. 2010, 130 (5), 832–838. (16) Reisfeld, R.; Pietraszkiewicz, M.; Saraidarov, T.; Levchenko, V. J. Rare Earths 2009, 27 (4), 544–549. (17) Belian, M. F.; Freire, R. O.; Galembeck, A.; de Sa´, G. F.; de Farias, R. F.; Alves, S., Jr. J. Lumin. 2010, 130 (10), 1946–1951. (18) Sokolnicki, J.; Legendziewicz, J.; Muller, G.; Riehl, J. P. Opt. Mat. 2005, 27 (9), 1529–1536. (19) Armelao, L.; Quici, S.; Barigelletti, F.; Accorsi, G.; Bottaro, G.; Cavazzini, M.; Tondello, E. Coord. Chem. ReV. 2010, 254 (5-6), 487– 505. (20) Luo, Y.; Yan, Q.; Zhang, Z.; Yu, X.; Wu, W.; Su, W.; Zhang, Q. J. Photochem. Photobiol. A: Chem. 2009, 206 (1), 102–108. (21) Silva, F. R. G.; Malta, O. L. J. Alloys Compd. 1997, 250, 427– 430. (22) Garcia, D.; Faucher, M. J. Chem. Phys. 1985, 82, 5554–5565. (23) Carlos, L. D.; Fernandes, J. A.; Sa´ Ferreira, R. A.; Malta, O. L.; Gonçalves, I. S.; Ribeiro-Claro, P. Chem. Phys. Lett. 2005, 413, 22–24. (24) de Sa´, G. F.; Alves, S., Jr.; da Silva, B. J. P.; da Silva, E. F., Jr. Opt. Mat. 1998, 11, 23–28. (25) Wanga, F.; Fana, X.; Wanga, M.; Zhang, X. J. Lumin. 2005, 114, 281–287. (26) Imura, H.; Ebisawa, M.; Kato, M.; Ohashi, K. J. Alloys Compd. 2006, 408-412, 952–957. (27) Sabatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993, 123, 201–228. (28) Buono-Core, G. E.; Li, H. Coord. Chem. ReV. 1990, 99, 55–87. (29) Felinto, M. C. F. C.; Tomiyama, C. S.; Brito, H. F.; Teotonio, E. E. S.; Malta, O. L. J. Sol. State Chem. 2003, 171, 189–194. (30) Judd, B. R. Phys. ReV. 1962, 127, 750–761. (31) Ofelt, G. S. J. Chem. Phys. 1962, 37, 511–520. (32) Jørgensen, C. K.; Judd, B. R. Mol. Phys. 1964, 8, 281. (33) Malta, O. L.; Carlos, L. D. Quim. NoVa 2003, 26, 889. (34) Malta, O. L.; Ribeiro, S. J. L.; Faucher, M.; Porcher, P. J. Phys. Chem. Solids 1991, 52, 587. (35) Costa, N. B., Jr.; Freire, R. O.; Rocha, G. B.; Simas, A. M. Polyhedron 2005, 24, 3046–3051. (36) Energy LeVels, Structure and Transition Probabilities of the TriValent Lanthanides in LaF3; Carnall, W. T., Crosswhite, H. M., Eds.; Argonne National Laboratory: Illinois, 1977.

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JP104396K

New Homotrinuclear Lanthanide Complexes: Synthesis

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