Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb 1-x Eu x (dpm) 3 solid solutions

Welington F. MAGALHÃES 1§, F. FULGÊNCIO 1, D. WINDMÖLLER 1, J. C. MACHADO 1, F. C. de OLIVEIRA 2, H. F. BRITO 3, O. L. MALTA 4 and G. F. de SÁ 4 1 § Departamento de Química, ICEx, Universidade Federal de Minas Gerais - UFMG, Av Antônio Carlos, 6627, Belo Horizonte, Brazil Laboratório de Espectroscopia de Aniquilação de Pósitrons - LEAP 2 Centro Federal de Educação Tecnológica de Minas Gerais, Timóteo, MG, Brazil 3 Instituto de Química, Universidade de São Paulo, , São Paulo, SP, Brazil 4 Departamento de Química Fundamental, UFPE, Recife, PE, Brazil

Positron Annihilation Lifetime Spectroscopy – PALS at 294 K. Resolution: 220 ps Time resolved Photoluminescence Spectroscopy – TPhoS at 294 K and 77 K. Studied system: The molecular complexes Tb and Eu dipivaloylmetanates, Tb(dpm) 3 and Eu(dpm) 3, and their binary solid solutions, Tb 1-x Eu x (dpm) 3. Welington F. MAGALHÃES: 3

Studied systems: metal complexes Tb(dpm) 3, Eu(dpm) 3 and their solid solutions Tb 1-x Eu x (dpm) 3 (dpm = dipivaloylmethanate) Their molecular structures are similar to that of the acac, acetylacetonates, complexes shown in Fig. 1 from [Porto 1997]Porto 1997 Welington F. MAGALHÃES: 4 Fig. 1 Structures of the Al and other metal acetylacetonates.

Welington F. MAGALHÃES: 5 Fig. 2 – Photoluminescence emission spectra obtained at 77 K, excited at 340 nm for: a) Tb(dpm) 3, b) Eu(dpm) 3, c) Tb (0.9) Eu (0.1) (dpm) 3, d) Tb (0.7) Eu (0.3) (dpm) 3, and e) Tb (0.5) Eu (0.5) (dpm) nm 615 nm nm

Welington F. MAGALHÃES: 6 Fig. 3 – Luminescence excitation spectra Tb: 485 nm Tb  Eu Energy transfer Tb: emission = 544 nm Eu: emission = 615 nm

Fig. 3 – Luminescence excitation spectra obtained at 77K temperature for (a) Tb(dpm) 3, (b) Eu(dpm) 3, (c) Tb (0.5) Eu (0.5) (dpm) 3 ( emission = 544 nm) and (d) Tb (0.5) Eu (0.5) (dpm) 3 ( emission = 615 nm). Spectra (a) and c) were obtained under emission at 544 nm corresponding to the Tb( 5 D 4  7 F 5 ) transition. Excitation spectra b) and d) were recorded with emission monitored at 615 nm corresponding to the Eu( 5 D 0  7 F 2 ) transition. Welington F. MAGALHÃES:

Broad band observed from approximately 250 to 420 nm corresponds to the 1  1  * absorption in the dpm ligand. The energy transfer process 3  *  5 D 4 then takes places. The behaviour of the weak sharp band located around 485nm in the excitation spectra (b), (c) and (d) indicates the occurrence of a Tb(III)  Eu(III) energy transfer process. Welington F. MAGALHÃES: 8 6 6

Fig. 4 – Partial energy level diagram for the relevant photophysical process associated with photoluminescence in Tb 1-x Eu x (dpm) 3. Welington F. MAGALHÃES: 9

x Eu τ 3 / nsτ 2 / nsI 3 / %I 2 / %  LTb* / ms at 298 K  LTb* / ms at 77 K ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± Welington F. MAGALHÃES: 10 Table 1 – PALS parameters (lifetimes and intensities) at (294  1) K,  1 fixed at ns, and the luminescence lifetimes for Tb 1 ‑ x Eu x (dpm) 3 solid solutions.

Fig. 5 – Luminescence decay constants, LTb* = 1/  LTb*, for the luminescent 5 D 4 excited state level ( = 544 nm ) of Tb(III) ion in the Tb(dpm) 3 complex versus the mole fraction of Eu(dpm) 3 complex in the Tb 1-x Eu x (dpm) 3 solid solutions. Welington F. MAGALHÃES: 11   k Qlum

Fig. 6: Inhibition of the o-Ps intensity as a function of the mole fraction of Eu(III). The lines shows the fits (a), (c) and (d) of the equation (19) with parameters in Table 2.equation (19)Table 2 Welington F. MAGALHÃES: 12 (c) (d) (a)

Fig. 7: The strong linear correlation between the o-Ps intensity I 3 and the luminescence lifetime of the Tb(III) 5 D 4 energy level. I 3calc : 39.4, 23.2, 14.0, 13.2%. 13 Welington F. MAGALHÃES: x Eu = 0 x Eu = 0.1 x Eu = 0.3 x Eu = 0.5 Calculated data from eq. (16), eq. (19) and fit (a) in Table Table 2 Fitted line, R 2 = Experimental data: Table 1,.Table 1 Fitted line, R 2 =

Fig. 8 – Scheme for the kinetic mechanism of the Ps formation from ligand excited states in Tb 1-x Eu x (dpm) 3 solid solutions, showing the Ps inhibition formation and the luminescence quenching, due to energy transfer between Tb(III) and Eu(III) ions. Welington F. MAGALHÃES: 14 Direct Ps precursor Indirect Ps precursor

Kinetic reaction equations for the Tb complex branch of the mechanism (1Tb) (2Tb) (3Tb) (4Tb) Welington F. MAGALHÃES: 15 

Kinetic mechanism reaction equations (5Tb) (6Tb) (7Tb) (8Tb) Welington F. MAGALHÃES: 16

Kinetic mechanism reaction equations (8Eu) (9Tb) (10Tb) (11Tb) Welington F. MAGALHÃES: 17

Kinetic rate equations for Tb side of the mechanism (12) (13) (14) (15) (16) (17) Welington F. MAGALHÃES: 18 k Qlum Fig.5

Applying the steady-state hypothesis in the equations (12) and (13) leads to:equations (12) 13 (18) k’ 1Tb is a pseudo first order reaction rate constant for the ligand excitation by epithermal positrons: Welington F. MAGALHÃES: 19 2 nd parcel 1 st parcel

The probability of Ps formation, I 3, should be proportional to the probability of presence of the direct Ps formation precursor [{L * Tb e +* }], with its ligand 3  * triplet excited state : The proportionality constant k is dependent on k 3Tb, the reaction rate constant for the reaction (3Tb) of Ps formation. For simplicity we assume k = 1.reaction (3Tb) Welington F. MAGALHÃES: 20

The equation (18) can already explain our results, but it has many fitting parameters, and is coupled with the equations of the Eu reaction chain in the model presented in Fig. 8. The complete model solution is complicated.equation (18)Fig. 8 To simplify the equation (18) it is assumed:, negleting the fraction of Ps formed in the Eu reaction chain, leading to equation (19).equation (18) equation (19) Welington F. MAGALHÃES: 21

(19) Welington F. MAGALHÃES:

Table 2: Values of the fitted parameters of equation (19) on the positronium yields I 3 in Table 1 and shown in Fig. 6. The parameters without uncertainties are fixed values. The uncertainties were obtained by a numerical procedure [Bevington 2003]. k Qlum = k 8Tb + k 9Tb.equation (19)Table 1Fig. 6Bevington 2003 Welington F. MAGALHÃES: 23 Fit / ns –1 / ps/ s –1 / ms k´ 1Tb / ns –1 k 4Tb / ns –1 k 5Tb /  s –1 k 6Tb / ns –1 k Qlum / s –1 s fit / % (a)      (b)     –0.90  (c)     –0.90  (d)     –6.0   10 3  0.32  Fig.6

CONCLUSIONS From the proposed mechanism a equation was deduced, and it describes very well the inhibition of Ps formation performed by the Eu(dpm) 3 complex, as well as the linear correlation between the Ps formation probability, I 3, and the lifetime of the Tb 5 D 4 luminescent excited state of the Tb(dpm) 3 complex, the indirect Ps formation precursor. Welington F. MAGALHÃES: 24

CONCLUSIONS The proposed mechanism raises strong evidences of the participation of electronic excited states as precursors for the Ps formation, at the positron molecule scattering, what is a characteristic of the Ore and resonant models. As in the spur model the proposed mechanism presents various competitive reactions that can reduce the probability of positronium formation, in a way completely consistent with the Stern- Volmer behavior. Welington F. MAGALHÃES: 25

REFERENCES: A.O. Porto, W.F. Magalhães, N.G. Fernandes, J.C. Machado: Chem. Phys. V.221, n.1-2 (1997) p , “Positron annihilation study in bynary molecular solid solutions of metal acetylacetonate complexes using positron annihilation lifetime (PAL) and Doppler Broadening (DBS) spectroscopies. Welington F. MAGALHÃES: 26

REFERENCES: W. M. Faustino, G. F. de Sá, O. L. Malta, M. C. F. Soares, D. Windmöller, J. C. Machado. Chem. Phys. Lett., 2006, 424, “Positronium formation in europium(III) coordination compounds” Welington F. MAGALHÃES: 27

REFERENCES: W. M. Faustino, G. F. de Sá, O. L. Malta, W. F. Magalhães, J. C. Machado. Chem. Phys. Lett., 2008, 452, “Positron annihilation in triphenylphosphine oxide complexes: Positronium inhibition mechanism involving excitation of charge transfer states” Welington F. MAGALHÃES: 28

REFERENCES: W. M. Faustino, O. L. Malta and G. F. De Sá, Chem. Phys. Lett., 2006, 429, G. F. de Sá, O. L. Malta, C. D. Donegá, A. M. Simas, R. L. Longo, P. A. Santa-Cruz and E. F. Da Silva, Coord. Chem. Rev., 2000, 196, “Spectroscopic properties and design of highly luminescent lanthanide coordination complexes” Welington F. MAGALHÃES: 29

REFERENCES: W. M. Faustino, O. L. Malta and G. F. De Sá, J. Chem. Phys., 2005, 122, – W. M. Faustino, O. L Malta, E. Teotônio, H. F. Brito, A. M. Simas and G. F. de Sá, J. Phys. Chem. A, 2006, 110, Welington F. MAGALHÃES: 30

REFERENCES: L. D. Carlos, W. M. Faustino, O. L. Malta, J. Braz. Chem. Soc. 19 (2), “Comment on Trivalent Europium Lifetimes in the Presence of Intramolecular Energy Transfer Processes” Welington F. MAGALHÃES: 31

REFERENCES: P. R. Bevington, D. K. Robinson, “Data Reduction and Error Analysis for Physical Sciences”, third edition, McGraw-Hill, Boston, 2003, ISBN-13: See pages and Welington F. MAGALHÃES: 32

Welington F. MAGALHÃES: 33 Thanks for your attention ? ? ? ? ? ? ?

09Sep11 Friday 10h00: Evidence of the Participation of Electronic Excited States in the Mechanism of Positronium Formation in Tb 1-x Eu x (dpm) 3 solid solutions. Tb  Eu Energy transfer Luminescence excitation spectra Ps formation inhibition fitted by the deduced model: model prediction: Proposed kinetic model experimental values: Welington F. MAGALHÃES:

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