Theoretical Spectroscopic Study of Europium Tris (bipyridine) Cryptates

Article pubs.acs.org/JPCA

Theoretical Spectroscopic Study of Europium Tris(bipyridine) Cryptates Júlio G. Santos,† José Diogo L. Dutra,† Severino A. Junior,‡ Ricardo O. Freire,*,† and Nivan B. da Costa Junior*,† †

Department of Chemistry, UFS, 49100-000, São CristóvãoSE, Brazil Department of Fundamental Chemistry, UFPE, 50590-470, RecifePE, Brazil

‡

ABSTRACT: A series of europium cryptates are studied, using semiempirical methods to predict electronic and spectroscopic properties. The results are compared with theoretical (DFT) and experimental results published by Guillaumont and co-workers (ChemPhysChem 2007, 8, 480). Triplet energies calculated by semiempirical methods have errors similar to those obtained by TD-DFT methodology but hundreds of times faster. Moreover, the semiempirical results not only reproduce well the experimental values but also help explain the low values of quantum efficiency observed for these complexes.

1. INTRODUCTION The application of luminescent properties of lanthanide ions in biological systems ranges from fluoroimmunoassays1,2 to the function of enzymes and proteins3 and labels of biological molecules.4 Among the trivalent lanthanide ions, Eu3+ and Tb3+ have been used extensively because these two ions are usually highly luminescent and typically present long lifetimes (in the millisecond range). Many of these applications commonly use lanthanide complexes, with the lanthanide ion coordinated to an organic chromophore that serves as an antenna.5−13 The use of an antenna is necessary due to the weak absorbance of lanthanide. The process of luminescence begins with the absorption of ultraviolet light by the organic chromophore. This energy is then transferred to the lanthanide ion, enabling it to emit, usually in the visible region. The efficiency of this process stems from the fact that the organic ligand is a good chromophore, with high energy transfer rates (WET) and low energy back transfer rates (WBT). The emission from lanthanide ions can often be quenched by high frequency vibrations of organic ligands such as those of CH, NH, or OH bonds.14 Theoretical tools can be helpful in the investigation of luminescent lanthanide systems. The development of the accurate semiempirical Sparkle model15−22 has allowed for the prediction of the ground state geometries of large lanthanide complexes. Based on the knowledge of this geometry and using different approaches, many spectroscopic properties can be predicted that shed light on numerous details of the luminescent process.23−25 Quantum chemical methods can be used today to calculate the ground state geometries and excited energy levels of lanthanide complexes. Two possibilities have been available since the early 1990s: effective core potential26,27 using ab initio © 2012 American Chemical Society

or DFT (density functional theory) methodologies and the semiempirical Sparkle model.16−22 The effective core potential (ECP) approach has been widely used to treat lanthanide trivalent ions. However, in a recent paper28 we showed that, contrary to what would normally be expected, an increase in the base set or an inclusion of an electron correlation, or both, consistently augmented the deviations and impaired the quality of predicted coordination polyhedron geometries.28 Thus, the RHF/STO-3G/ECP appears to be the most efficient chemical model for predicting coordination polyhedron crystallographic geometry in isolated lanthanide complex ion calculations. Moreover, semiempirical Sparkle/AM1 results show a similar accuracy when large lanthanide complexes are involved.28−30 Two methodologies have been employed to calculate the excited state: the time-dependent density functional theory methodology (TD-DFT)31 and the semiempirical INDO/S32,33 (intermediate neglect of differential overlap/spectroscopic) method. The former method is extensively used for organic compounds, but for lanthanide complexes, in which the ion is coordinated to a large organic ligand, it is still very computationally demanding and therefore practically infeasible. In contrast, the semiempirical methodology is computationally efficient, which makes it more recommendable for this type of system. The main question is this: Is the accuracy of the semiempirical INDO/S method comparable to TD-DFT ab initio results in studies of lanthanide complexes? In a recent paper, Guillaumont and co-workers described the synthesis of a series of lanthanide tris(bipyridine) cryptates [Ln⊂RBpy·RBpy·RBpy]3+·2H2O, where Ln = Eu, Gd and R = Received: January 20, 2012 Revised: March 23, 2012 Published: March 24, 2012 4318

dx.doi.org/10.1021/jp300707v | J. Phys. Chem. A 2012, 116, 4318−4322

The Journal of Physical Chemistry A

Article

H, COOH, COOCH3, and CONH(CH2)2NH2.34 Their results suggest that substitutions in the precursor compound where R = H do not significantly alter the photophysical properties of lanthanide cryptate. The authors also used DFT and TD-DFT to complete the experimental characterization. They concluded that the theoretical results can be considered accurate when compared with the experimental data. The main objective of present work was to study the luminescent process of europium cryptates in detail. A detailed comprehension of this process could contribute significantly to the synthesis of new luminescent europium cryptates from a systematic structure modification aiming to reduce the high nonradiative emission rates presented by those systems. Here we apply semiempirical methodologies to study the luminescent properties of the systems reported by Guillaumont and coworkers in 2007.34 We also attempt to determine if semiempirical methods can be used to predict the spectroscopic properties of europium cryptates with accuracy comparable to that shown by DFT and TD-DFT results.34

Ωλ = (2λ + 1) ∑ t ,p

|Bλtp|2 (2t + 1)

(1)

with Bλtp =

⎡ (λ + 1)(2λ + 3) ⎤ t 2 t+1 ⟨r ⟩θ(t , p)γpt − ⎢ ⎥⟨r ⟩ ⎣ ⎦ ΔE 2λ + 1 (1 − σλ)⟨f || C(λ) || f ⟩Γ tpδt , λ + 1

(2)

Details on the parameters of eqs 1 and 2 are widely discussed in the literature.39−41 2.4. Energy Transfer Rates and Quantum Yield. The models used to obtain the energy transfer rates between the ligands and the lanthanide ion, the numerical solution of the rate equations, and the emission quantum yield are described in refs 42 and 43.

3. RESULTS AND DISCUSSION Figure 1 illustrates the ground state geometries of the precursor compound optimized by the Sparkle/AM1 model.

2. METHODOLOGY The work of Guillaumont and co-workers published in 200734 involved a study of a series of lanthanide tris(bipyridine) cryptates Ln⊂RBpy·RBpy·RBpy, where Ln = Eu, Gd and R = H, COOH, COOCH3, and CONH(CH2)2NH2. Theoretical calculations using DFT and TD-DFT were done to evaluate the ability of these approaches to predict absorption maxima, triplet state energies, and structural parameters of lanthanide cryptates. In the present work, we used semiempirical methodologies such as the Sparkle model and INDO/S to calculate the ground state geometries and some of the spectroscopic properties of all europium cryptates. Luminescent properties were also calculated and compared with experimental values reported by Guillaumont et al.34 2.1. Sparkle/AM1 Calculations. The Sparkle/AM116 model implemented in the MOPAC2007 software35 was used to calculate the ground state geometries of all europium cryptates. The keywords used in the calculation reported here were AM1, PRECISE, BFGS, GNORM = 0.25, SCFCRT = 1.D-10 (to increase the SCF convergence criterion) and XYZ (for Cartesian coordinates). These calculations were also performed using the Sparkle/PM318 and Sparkle/PM622 models, and the results provided by the three versions of the Sparkle model were very similar. 2.2. Excited Energies and Absorption Spectra. We predicted the singlet and triplet excited states of all the calculated ground state geometries using configuration interaction with single excitations (CIS) based on the intermediate neglect of differential overlap/spectroscopic (INDO/S) methodology32,33 implemented in the ZINDO package,36 using a point charge of +3e to represent the trivalent lanthanide ion. The CIS space was increased gradually, and its effect on the triplet energies and absorption spectra was evaluated. A Lorentzian line shape was fitted to the calculated singlet transitions, together with the relative intensities obtained from oscillator strengths. All the simulated spectra, which had a half-height bandwidth of 25 nm, were compared with experimental spectra. 2.3. Calculation of Judd−Ofelt Intensity Parameters. The coordination interaction between a lanthanide ion and an L ligand can be described by the Judd−Ofelt theory,37,38 followed by the definition of the intensity parameters Ωλ (λ = 2, 4, and 6) by

Figure 1. Sparkle/AM1 ground state geometry of the [Eu⊂bp·bp·bp]3+·2H2O cryptate.

In all five compounds, the coordination polyhedron is formed by the nitrogen atoms of the three bipyridine ligands and two oxygen atoms of the water molecules, as estimated by Guillaumont and co-workers.34 To begin with, we gradually increased the CIS space to evaluate the resulting changes in the absorption spectra and triplet energies. Figure 2 depicts the eight different spectra generated by increasing the excitation window in steps of 5 orbitals at a time up to 20, followed by steps of 10 orbitals. We found that, in general, theoretical absorption spectra converged when 40 occupied and 40 unoccupied orbitals (40 × 40) were used in the CI (configuration interaction) calculation. When a CI space of 5 × 5 was used, the simulated spectra presented a single band at 271 nm. The increase in CI space to 20 × 20 resulted in a shift of about 20 nm. A subsequent increase to a 60 × 60 space caused a shift of only 7 nm. Similar conclusions were reached for the band at 350 nm. In both cases, stabilization occurred when a CI space of 40 × 40 was used. 4319



Article

Figure 4. Energy level diagram for [Eu⊂bp·bp·bp]3+·2H2O, showing the most probable channels for the process of intramolecular energy transfer.

Figure 2. Theoretical absorption spectra of the [Eu⊂bp·bp·bp]3+·2H2O cryptate calculated by varying the excitation window.

the explicit treatment of the europium trivalent ion.36 This analysis was performed using the AM1 and PM3 semiempirical methods. Table 1 compares the theoretical energies calculated by us against the experimental and theoretical energies obtained via TD-DFT methodologies reported by Guillaumont and coworkers.34 The semiempirical calculation of the energies of the five cryptates underestimated the experimental value, whereas the TD/DFT energies overestimated the experimental ones. Note that the accuracy of the semiempirical results is comparable to that obtained with the TD-DFT methodology. However, this type of calculation requires many hours, whereas the semiempirical INDO/S calculations are performed in a matter of minutes. Moreover, the accuracy of the unsigned mean error obtained by the semiempirical model is consistent with that obtained by the INDO/S method.32,33 The intensity parameters were then calculated based on Sparkle/AM1 geometry and using the INDO/S singlet and triplet energies. These values were used to calculate the energy transfer and back transfer rates and the radiative and nonradiative decay rates. Lastly, the quantum efficiency and quantum yield were calculated. Table 2 lists the spectroscopic properties of the five cryptates under study. The high values of the Ω2 intensity parameter can be attributed to the hypersensitive behavior of the 5D0 → 7F2 transition.44 The 5D0 →7F2 transition in europium complexes is hypersensitive and can therefore be used as a reference in the study of the influence of the ligand field on europium trivalent ions. The high intensity of this transition can be associated with a low symmetry around the europium ion and the high covalence of the metal−ligand bond. Hence, an analysis of the calculated Ω2 parameters of the cryptates suggest an increase in the symmetry of the coordination polyhedron in the following order: [Eu⊂COOCH3_Bpy·Bpy·Bpy]3+·2H2O; [Eu⊂Bpy·Bpy·Bpy]3+·2H2O; [Eu⊂COOH_Bpy·Bpy·Bpy]3+·2H2O; [Eu⊂COOH_Bpy·COOH_Bpy·COOH_Bpy] 3 + ·2H 2 O, and [Eu⊂CONH(CH2)2NH2 _Bpy·Bpy·Bpy]3+·2H2O. This same order can be used to interpret the reduction in the covalence of the metal−ligand bond. As can be seen in Table 2, the theoretical results of all the spectroscopic properties are highly consistent with the

Figure 3 shows that the energy of the triplet associated with the [Eu⊂bp·bp·bp]3+·2H2O cryptate changes with increasing

Figure 3. Variation of triplet energy as a function of the increase in the number of orbitals considered in the CI calculation of the [Eu⊂bp·bp·bp]3+·2H2O cryptate.

CI space. A detailed analysis of the orbitals was made to choose the main triplet state. As can be seen, the triplet energy decreases as the number of orbitals considered in the CI calculation increases. This reduction is substantial, as indicated by the decrease in energy from about 22 800 cm−1 for a CI space of 5 × 5 to 19 500 cm−1 for a CI space of 80 × 80. The energy transfer process shown in Figure 4 indicates that, for the triplet channel → 5D1 (located at about 19 000 cm−1), this difference can result in high back transfer rates, considerably compromising the accuracy of the theoretical results. Clearly, therefore, a CI space of 40 × 40 is insufficient for convergence of the triplet energy. The convergence occurs when a CI space of 60 × 60 is considered. Having defined a CI space of 60 × 60, several calculations were done to evaluate the influence of the representation of the europium ion. Two possibilities were investigated: (i) a point charge of +3e to represent the trivalent europium ion and (ii) 4320



Article

Table 1. Singlet Energies Calculated by the Semiempirical Method and Comparison of Theoretical and Experimental Triplet Energiesa triplet (cm−1)

a

−1

cryptate

singlet (cm ) Sparkle//INDO/S

Sparkle//INDO/S

TD-DFT

exptl

[Eu⊂Bpy·Bpy·Bpy]3+·2H2O [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOCH3 [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOH [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = CONH(CH2)2NH2 [Eu⊂R_Bpy·R_Bpy·R_Bpy]3+·2H2O R = COOH unsigned mean error

33490.7 28312.2 28271.8 28573.3 28097.0 −

19621.0 (2479.0) 19787.6 (2012.4) 19703.3 (1896.7) 20995.8 (904.2) 21197.9 (902.1) 1638.88

24370.0 (2270.0) 23309.0 (1509.0) 23244.0 (1644.0) 24361.0 (2461.0) 23187.0 (1087.0) 1675.25

22100.0 21800.0 21600.0 21900.0 22100.0 −

The errors between theoretical and experimental values are given in parentheses.

Table 2. Singlet Energies, Theoretical Intensity Parameters Ω2, Ω4, and Ω6, Radiative (Arad) and Nonradiative (Anrad) Decay Rates, Quantum Efficiency (η), and Quantum Yield (q) Values Derived from the Optimized Sparkle/AM1 Models; Related Experimental Data, Including the Lifetime (τ) of the Eu3+ Centera cryptate

Ω2

Ω4

Ω6

τ (ms)

Arad (s−1)

Anrad (s−1)

η (%)

[Eu⊂Bpy·Bpy·Bpy] ·2H2O (exptl) [Eu⊂Bpy·Bpy·Bpy]3+·2H2O (theor.) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOCH3 (exptl) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOCH3 (theor.) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOH (exptl) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOH (theor.) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = CONH(CH2)2NH2 (exptl) [Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = CONH(CH2)2NH2 (theor.) [Eu⊂R_Bpy·R_Bpy·R_Bpy]3+·2H2O R = COOH (exptl) [Eu⊂R_Bpy·R_Bpy·R_Bpy]3+·2H2O R = COOH (theor.)

− 14.81 − 17.10 − 13.37 − 11.74 − 13.12

− 7.88 − 9.99 − 10.12 − 8.90 − 7.98

− 0.16 − 0.19 − 0.19 − 0.17 − 0.17

0.34 − 0.34 − 0.34 − 0.34 − 0.34 −

588.0 619.1 667.0 718.0 599.0 607.5 595.0 539.7 602.0 567.3

2353.0 2322.1 2275.0 2223.2 2300.0 2333.7 2267.0 2401.5 2265.0 2373.9

20.0 21.1 22.7 24.4 20.7 20.7 20.8 18.4 21.0 19.3

3+

a

q (%) 20.7 24.1 20.4 18.2 19.1

The values of the intensity parameters are presented in 10−20 cm2.

Table 3. Calculated Intramolecular Energy Transfer and Back Transfer Rates of All the Cryptates under Studya ligand state (cm−1)

cryptate [Eu⊂Bpy·Bpy·Bpy] ·2H2O 3+

[Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOCH3

[Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = COOH

[Eu⊂R_Bpy·Bpy·Bpy]3+·2H2O R = CONH(CH2)2NH2

[Eu⊂R_Bpy·R_Bpy·R_Bpy]3+·2H2O R = COOH

4f state (cm−1)

RL (Å)

transfer rate (s−1)

back transfer rate (s−1)

singlet (33490.7) triplet (19621.0) triplet (19621.0)

→ → →

5

D4 (27586) D1 (19027) 5 D0 (17293)

3.885 3.885 3.885

WET1 = 1.6 × 10 WET2 = 4.3 × 1010 WET3 = 5.9 × 1010

WBT1 = 8.0 × 10−8 WBT2 = 3.0 × 109 WBT3 = 8.5 × 105


→ → →

5

D4 (27586) D1 (19027) 5 D0 (17293)

3.516 4.134 4.134

WET1 = 4.6 × 106 WET2 = 3.3 × 1010 WET3 = 4.4 × 1010

WBT1 = 1.5 × 105 WBT2 = 1.1 × 109 WBT3 = 2.8 × 105


→ → →

5

D4 (27586) D1 (19027) 5 D0 (17293)

3.517 3.789 3.789

WET1 = 4.5 × 106 WET2 = 4.7 × 1010 WET3 = 6.4 × 1010

WBT1 = 1.8 × 105 WBT2 = 2.3 × 109 WBT3 = 6.1 × 105


→ → →

5

D4 (27586) D1 (19027) 5 D0 (17293)

3.606 3.665 3.665

WET1 = 3.3 × 106 WET2 = 4.4 × 1010 WET3 = 4.4 × 1010

WBT1 = 3.1 × 104 WBT2 = 4.2 × 106 WBT3 = 8.4 × 102


→ → →

5

3.580 4.277 4.277

WET1 = 3.7 × 106 WET2 = 2.2 × 1010 WET3 = 2.1 × 1010

WBT1 = 3.4 × 105 WBT2 = 8.0 × 105 WBT3 = 1.5 × 102

5

5

5

5

D4 (27586) D1 (19027) 5 D0 (17293) 5

5

a

The RL value is the distance from the donor state located at the organic ligands to the Eu3+ ion nucleus. More information about this parameter can be found in ref 25.

experimental ones. Moreover, the unsigned mean error obtained for the quantum efficiency of the five cryptates is only 6.3%, suggesting that the semiempirical methodologies applied are suitable for studies of the lanthanide luminescent system. Figure 4 shows the most probable channels for the intramolecular energy transfer process. Guillaumont and coworkers attribute the low intensity of luminescence to

nonradiative deactivation via O−H vibrations due to the water molecules present in the coordination polyhedron. The high nonradiative rates of all the cryptates corroborate this assumption. Table 3 lists transfer and back transfer rates of other structures, showing back transfer rates that are similar to those obtained for the [Eu⊂bp·bp·bp]3+·2H2O cryptate, thus confirming the low values of quantum efficiency of the five cryptates. 4321



Article

(16) Freire, R. O.; Rocha, G. B.; Simas, A. M. Inorg. Chem. 2005, 44, 3299−3310. (17) Freire, R. O.; da Costa, N. B.; Rocha, G. B.; Simas, A. M. J. Chem. Theory Comput. 2006, 2, 64−74. (18) Bastos, C. C.; Freire, R. O.; Rocha, G. B.; Simas, A. M. J. Photochem. Photobiol., A 2006, 177, 225−237. (19) Freire, R. O.; Costa, N. B., Jr.; Rocha, G. B.; Simas, A. M. J. Phys. Chem. A 2006, 110, 5897. (20) Freire, R. O.; Monte, E. V.; Rocha, G. B.; Simas, A. M. J. Organomet. Chem. 2006, 691, 2584. (21) Freire, R. O.; Rocha, G. B.; Simas, A. M. J. Braz. Chem. Soc. 2009, 20, 1638−1645. (22) Freire, R. O.; Simas, A. M. J. Chem. Theory Comput. 2010, 6, 2019−2023. (23) de Sá, G. F.; Malta, O. L.; de Mello Donegá, C.; Simas, A. M.; Longo, R. L.; Santa-Cruz, P. A.; da Silva, E. F., Jr. Coord. Chem. Rev. 2000, 196, 165−195. (24) Freire, R. O.; Gonçalves e Silva, F. R.; Rodrigues, M. O.; de Mesquita, M. E.; da Costa, N. B. J. Mol. Modell. 2005, 12, 16−23. (25) Rodrigues, M. O.; Júnior, N. B. C.; Simone, C. A.; Araujo, A. A. S.; Brito-Silva, A. M.; Paz, F. A. A.; Mesquita, M. E.; Júnior, S. A.; Freire, R. O. J. Phys. Chem. B 2008, 112, 4204−4212. (26) Dolg, M.; Stoll, H; Preuss, H. J. Chem. Phys. 1989, 90, 1730− 1734. (27) Dolg, M.; Stoll, H; Savin, A.; Preuss, H. Theor. Chim. Acta 1989, 75, 173−194. (28) Freire, R. O.; Rocha, G. B.; Simas, A. M. J. Mol. Modell. 2006, 12, 373−389. (29) Freire, R. O.; Rocha, G. B.; Albuquerque, R. Q.; Simas, A. M. J. Lumin. 2005, 111, 81−87. (30) Rodrigues, D. A.; da Costa, N. B.; Freire, R. O. J. Chem. Inf. Modell. 2011, 51, 45−51. (31) Stratmann, R. E.; Scuseria, G. E.; Frisch, M. J. J. Chem. Phys. 1998, 109, 8218−8224. (32) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223− 236. (33) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; MuellerWesterhoff, U. T. J. Am. Chem. Soc. 1980, 102, 589−599. (34) Guillaumont, D.; Bazin, H.; Benech, J.-M.; Boyer, M.; Mathis, G. ChemPhysChem 2007, 8, 480−488. (35) Stewart, J. J. P. MOPAC2007, version 7.058; Colorado Springs, CO, 2007. (36) Zerner, M. C. ZINDO Manual, QTP; University of Florida: Gainesville, FL, 1990. (37) Judd, B. R. Phys. Rev. 1962, 127, 750−761. (38) Ofelt, G. S. J. Chem. Phys. 1962, 37, 511−520. (39) Malta, O. L.; Ribeiro, S. J. L.; Faucher, M.; Porcher, P. J. Phys. Chem. Solids 1991, 52, 587−596. (40) Malta, O. L.; Couto dos Santos, M. A.; Thompson, L. C.; Ito, N. K. J. Lumin. 1996, 69, 77−84. (41) Malta, O. L.; Brito, H. F.; Meneses, 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. (42) Malta, O. L.; Silva, F. R. G. Spectrochim. Acta, Part A 1998, 54, 1593−1599. (43) Malta, O. L.; Silva, F. R. G.; Longo, R. Chem. Phys. Lett. 1999, 307, 518−526. (44) Reisfeld, N. R.; Jörgensen, C. K. Handbook on the Physics and Chemistry Rare-Earths; North-Holland: Amsterdam, The Netherlands, 1987.

4. CONCLUDING REMARKS The systematic study of the excitation window revealed that an increase in the number of orbitals considered in CI calculations strongly influences the absorption spectrum and triplet energies. Our findings also suggest that experimental spectroscopic data are better reproduced in ZINDO calculations when the lanthanide ion is represented by a point charge. Triplet energies calculated by semiempirical methods have errors similar to those obtained by TD-DFT methodology but are hundreds of times faster. The semiempirical methods also enabled the calculation of luminescent properties. The results of these methods not only reproduced well the experimental values but also helped explain the low values of quantum efficiency observed for these complexes. Our expectation is that this set of semiempirical methodologies can be used in the theoretical design of new luminescent europium complexes.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +55 79 2105-6651. Tel.: +55 79 2105-6650. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We appreciate the financial support provided by the Brazilian agencies, institutes, and networks: CNPq, CAPES, FAPITECSE, INAMI, and RENAMI. We are also grateful to CENAPAD (Centro Nacional de Processamento de Alto Desempenho), at Campinas, Brazil, and to Prof. AEA Paixão for the use of the software Statistica. Finally, we gratefully acknowledge Cambridge Crystallographic Data Centre (CCDC) for allowing us access to the Cambridge Structural Database.

■

REFERENCES

(1) Mathis, G. Clin. Chem. 1993, 39, 1953−1959. (2) Mathis, G. Clin. Chem. 1995, 41, 1391−1397. (3) Hagan, A. K.; Zuchner, T. Anal. Bioanal. Chem. 2011, 400, 2847− 2864. (4) Rajasekhar, R. D.; Pedro, R. L. E.; Lawrence, W. M. Bioconjugate Chem. 2011, 22, 1402−1409. (5) da Costa, N. B.; Freire, R. O.; dos Santos, M. A. C.; de Mesquita, M. E. J. Mol. Struct. (Theochem) 2001, 545, 131−135. (6) Mesquita, M. E.; Junior, S. A.; Oliveira, F. C.; Freire, R. O.; Júnior, N. B. C.; de Sá, G. F. Inorg. Chem. Commun. 2002, 5, 292−295. (7) Mesquita, M. E.; Junior, S. A.; Costa, N. B., Jr.; Freire, R. O.; Silva, R. F. G.; de Sá, G. F. J. Solid State Chem. 2003, 171, 183−188. (8) Mesquita, M. E.; Gonçalves e Silva, F. R.; Albuquerque, R. Q.; Freire, R. O.; da Conceiçaõ , E. C.; da Silva, J. E. C.; Júnior, N. B. C.; de Sá, G. F. J. Alloys Compd. 2004, 366, 124−131. (9) Freire, R. O.; Albuquerque, R. Q.; Júnior, S. A.; Rocha, G. B.; Mesquita, M. E. Chem. Phys. Lett. 2005, 405, 123−126. (10) Pavithran, R.; Reddy, M. L. P.; Junior, S. A.; Freire, R. O.; Rocha, G. B.; Lima, P. P. Eur. J. Inorg. Chem. 2005, 20, 4129−4137. (11) Biju, S.; Reddy, M. L. P.; Freire, R. O. Inorg. Chem. Commun. 2007, 10, 393−396. (12) Yanmei, W.; Li, W. Prog. Chem. 2010, 22, 427−432. (13) Divya, V.; Freire, R. O.; Reddy, M. L. P. Dalton Trans. 2011, 40, 3257−3268. (14) Ermolaev, V. L.; Sveshnikova, E. B. Russ. Chem. Rev. 1994, 63, 905−922. (15) de Andrade, A. V. M; da Costa, N. B.; Simas, A. M.; de Sa, G. F. Chem. Phys. Lett. 1994, No. 227, 349−353. 4322


Theoretical Spectroscopic Study of Europium Tris (bipyridine) Cryptates

Recommend Documents