Journal of Luminescence 134 (2013) 369–373

Contents lists available at SciVerse ScienceDirect

Journal of Luminescence

0022-23

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/jlumin

Molecular electrophosphorescence in (Sm, Gd)-b-diketonate complex blendfor OLED applications

R. Reyes a,n, M. Cremona b,c, E.E.S. Teotonio d, H.F. Brito e, O.L. Malta f

a Facultad de Ingenierıa Quımica y Textil, Universidad Nacional de Ingenierıa, UNI, Av. Tupac Amaru 210, Lima 31, Perub DIMAT – Divis ~ao de Metrologia de Materiais, Instituto Nacional de Metrologia, Normalizac- ~ao e Qualidade Industrial, INMETRO, Duque de Caxias, RJ, Brazilc Departamento de Fısica, Pontifıcia Universidade Catolica do Rio de Janeiro, PUC-Rio, C.P. 38071, Rio de Janeiro, RJ, CEP 22453-970, Brazild Departamento de Quimica, CCEN, Universidade Federal da Paraıba, UFPB, C.P. 5093, Jo~ao Pessoa, PB, CEP 5805-970, Brazile Instituto de Quımica, Universidade de S ~ao Paulo, USP, C.P. 26077, S ~ao Paulo, SP, CEP 05599-970, Brazilf Departamento de Quımica Fundamental, CCEN, Universidade Federal de Pernambuco, Cidade Universitaria, Recife, PE, CEP 50670-901, Brazil

a r t i c l e i n f o

Article history:

Received 18 April 2012

Received in revised form

31 July 2012

Accepted 17 August 2012Available online 27 August 2012

Keywords:

Samarium complex

Gadolinium complex

Diketonate

Electroluminescence

OLED

Electrophosphorescence

13/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.jlumin.2012.08.019

esponding author. Tel.: þ51 1 481 1070.

ail address: fisicaplic@hotmail.com (R. Reyes)

a b s t r a c t

In this work the preparation and characterization of the triple-layer organic light-emitting diode

(OLED) using a mixture of the samarium and gadolinium b-diketonate complexes [Sm0.5Gd0.5

(TTA)3(TPPO)2] as emitting layer is reported. The OLED’s devices contain 1-(3-methylphenyl)-1,2,3,

4-tetrahydroquinoline-6-carboxyaldehyde-1,1’-diphenylhydrazone (MTCD) as hole-transporting layer

and tris(8-hydroxyquinoline aluminum) (Alq3) as electron transporting layer. The electroluminescence

spectrum present emission narrow bands from the 4G5/2-6HJ transitions (where J¼5/2, 7/2 and 9/2)

characteristic of the Sm3þ ion. These sharp lines are overlapped with a broad band attributed to the

electrophosphorescence from the T1-S0 transition in the ligand TTA. The intramolecular energy

transfer is discussed and applied on the change of the emission color of the organic LEDs at different

bias voltages.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Organic light-emitting diodes (OLEDs) have attracted great inter-est owing to their potential applications in the development of newoptoelectronic components such as full-color and flat panel displays[1–3]. OLEDs have the advantage of ease of fabrication, low operatingvoltages, and the possibility of a wide selection of emission colorsthrough the molecular design of organic materials. Since the devel-opment of OLEDs a great number of studies have used organic dyes,metal complexes, and polymers as emitting materials. However, agreat part of these materials present broad emission bands, whichwould compromise the quality of colors in the display.

Trivalent rare earth ions (RE3þ) have been used as emittingcenters, in particular the complexes containing the Eu3þ [4–11] andTb3þ [12–15] ions that emit red and green quasi-monochromaticlight, respectively. The orange light was attained using the Sm3þ ion,yellow (Dy3þ), blue (Tm3þ) and infrared emission was achieved withEr3þ and Yb3þ , raising the interest on OLED devices for telecommu-nication applications [16–20]. In contrast, the Gd3þ ion has an energygap between the 8S7/2 ground state and first excited state 6P7/2 at

ll rights reserved.

.

around 32,000 cm�1, and its complexes are used to study the energyof the triplet states (T) of the organic ligand [21,22].

The major advantage in fabricating OLEDs devices using RE3þ-complexes is that their internal quantum efficiency can theoreti-cally rise up to 100% because both singlet and triplet excitons areinvolved in the emission process, and an efficient ligand-to-metalintramolecular singlet–triplet-rare earth ion energy transfer isgenerally operative [2,4]. For this reason, the value of electro-luminescence efficiency in devices based on rare earth complexesmay be four times higher than those for similar organic devicesusing other materials [4]. Among the RE-complexes studied bythe EL process, those based on b-diketonates have attractedspecial attention since they present high luminescence intensity.Furthermore, organic ligands can substitute water molecules inthese complexes that act as strong luminescence quenchers dueto OH oscillators, increasing the quantum yield [23].

The electroluminescence (EL) mechanism proposed in OLEDscontaining RE3þ complexes suggests a combination of hole andelectrons in the emitting layer, which are injected in this layer viathe highest occupied molecular orbital (HOMO) and the lowestunoccupied molecular orbital (LUMO), respectively [2].

In this paper, the RE3þ-b-diketonates complexes were used asthe emitting layer in the OLEDs. Typical OLED devices using MTCDas hole-transporting layer and Alq3 as electron transporting layer

R. Reyes et al. / Journal of Luminescence 134 (2013) 369–373370

were prepared and characterized. In order to investigate thebroad band at 570 nm observed in the EL spectra of the devicescontaining the (Sm0.5Gd0.5) mixed complex, devices with Gd3þ

and Sm3þ complexes as emitting layers were also prepared andstudied. The molecular electrophosphorescence from the TTAligand is also discussed.

2. Experimental

Thenoyltrifluoroacetonate (TTA) and triphenylphosphine oxide(TPPO) were purchased from Aldrich. The solid complexes[RE(TTA)3(H2O)2] (where RE¼Sm and Gd) were prepared by themethod described in Ref. [22], by dissolving the hydrated pre-cursors [RE(TTA)3(H2O)2] and triphenylphosphine oxide (TPPO) inethanolic solution in the molar ratio 1:2 (hydrated complex:-TPPO). The solution was then mixed until the precipitation ofpale-yellow compounds was obtained. Finally, the obtained solidwas twice purified with acetone and dried and desiccated invacuum. These complexes were mixed in powder form in the ratio1:1 (w/w). The homogeneity of the mixture was better achievedwith the initial fusion process of the complexes before the filmdeposition.

The OLED devices were manufactured using 1-(3-methylphenyl)-1,2,3,4,tetrahydroquinoline-6-carboxyaldehyde-1,1’-diphenylhy-drazone (MTCD) [11] as hole-transporting layer and tris(8-hydroxyquinolinato)aluminum (Alq3) as electron transportinglayer. Three different device configurations were obtained; thefirst one had [Gd(TTA)3(TPPO)2] as the emitting layer. In thesecond and third configurations the [Sm(TTA)3(TPPO)2] and[Sm0.5Gd0.5(TTA)3(TPPO)2] complexes, respectively, worked asemitting layers. Fig. 1 shows the molecular structures of the usedmaterials. The MTCD and the Alq3 were used as obtained. Theproduced OLEDs presented the following configurations:

Device 1: ITO/MTCD(50 nm)/[Gd(TTA)3(TPPO)2](50 nm)/Alq3(50 nm)/Al(120 nm)

NNHC

N

P O RE3+O

O

C

S

2

Fig. 1. Chemical structure of the MTCD (a), Alq3 (b) and [RE

Device 2: ITO/MTCD(50 nm)/[Sm(TTA)3(TPPO)2](50 nm)/Alq3(50 nm)/Al(120 nm)Device 3: ITO/MTCD(50 nm)/[Sm0.5Gd0.5(TTA)3(TPPO)2](50 nm)/Alq3(50 nm)/Al(120 nm)

The compounds were sequentially deposited by thermal eva-poration onto indium–tin oxide (ITO) glass substrates, at roomtemperature with a sheet resistance of 8.1 O/& supplied by AsahiGlass Co. The substrates were initially cleaned by ultrasonificationusing a detergent solution, followed by toluene degreasing andthen cleaned again by ultrasonification with pure isopropylalcohol. After drying on the spinner at 5000 rpm for 1 min, thesubstrates were loaded into the vacuum chamber. The basepressure was 5�10�6 Torr and during the evaporation, thepressure was �9�10�6 Torr. The rates of deposition of thecompounds were in the range of 0.1–0.3 nm/s. Tungsten crucibleswere used to evaporate the compounds. Finally, a 150 nm thickaluminum cathode was evaporated from a tungsten wire basketat a rate of approximately 2.0 nm/s in the same vacuum chamber.A shadow mask with 0.5�0.5 cm2 opening was used to shape thecathode. The layer thickness was controlled in situ through aquartz crystal monitor.

The absorption spectra of the RE3þ-complexes films wererecorded on a Perkin–Elmer Lambda 19 spectrophotometer. Onthe other hand, the photoluminescence and electroluminescencespectra were obtained in a Photon Technology International (PTI)fluorescence spectrophotometer. The brightness was measured byusing a calibrated radiometer/photometer by United DetectorTechnology (UDT-350).

3. Result and discussion

The Sm and Gd contents were determined by complexometrictitration with EDTA in methanol. The carbon, hydrogen andnitrogen contents were estimated by microanalytical procedures.The C, H, Sm and Gd percentage values calculated/found for the

N

N

N

O

O

O

Al3+

F33

(TTA)3(TPPO)2] (c), where RE¼Sm3þ and Gd3þ .

400 500 600 700

0

1

C

B

A

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

C EL Device 1B PL TTAA PL Gd(TTA)3(TPPO)2

Fig. 2. Photoluminescence spectrum of the [Gd(TTA)3(TPPO)2] complex (A line),

TTA free ligand excited at 348 nm (B line) and electroluminescent spectrum of the

device 1 at room temperature (C line).

400 500 600 700

0

1 6H9/24G5/2

6H7/24G5/2

6H5/24G5/2

B

A

C

EL

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

C Device 1 (20V)B Device 2 (22V)A Device 2 (10V)

Fig. 3. EL spectra obtained at room temperature for device 2 for bias voltages at

10 V (A line), device 2 for bias voltages at 22 V (B line) and device 1 recorded at

20 V (C line).

400 500 600 700

0

1E

D

C

BA

EL

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

E 22VD 20VC 18VB 16VA 14V

Fig. 4. Electroluminescence spectra of device 2 recorded at bias-voltages from 14

to 22 V.

R. Reyes et al. / Journal of Luminescence 134 (2013) 369–373 371

complexes with the respective ligands were (a) samarium com-plexes; H2O (C: 33.92/33.85; H:1.90/1.92; Sm3þ: 17.68/17.71),TPPO (C: 52.58/ 52.43; H: 3.09/3.39; Sm3þ: 10.97/10.39), and (b)Gadolinium complexes; H2O (C: 33.64/33.36; H: 1.88/1.96; Gd3þ:18.35/17.95), TPPO (C: 52.32/ 52.32; H: 3.08/3.23; Gd3þ: 11.42/11.25).

The photoluminescence (PL) spectra of the [Gd(TTA)3(TPPO)2](Fig. 2A) and TTA (Fig. 2B) compounds in powder forms wererecorded from 400 to 720 nm, at room temperature, with excita-tion monitored at 348 nm. The EL spectrum of the device 1, withthe gadolinium complex as emitting layer, is also shown inFig. 2C, with 20 V of bias voltage. As can be seen, in this figurethe EL spectrum of the device 1 presents a broad band that can beassigned to the phosphorescence from the TTA ligand [24,25],which is also consistent with the coincidence with TTA PLspectrum peaked at about 575 nm. On the other hand, the ELband is shifted about 33 nm (0.13 eV) to the red region ascompared with the PL spectrum of the [Gd(TTA)3(TPPO)2] andthis difference can be attributed to the exciton emission at theinterface between the Gd complex and the Alq3, while the relativeshifting between the PL spectra of TTA and [Gd(TTA)3(TPPO)2] canbe attributed to their difference in molecular structure [26]. Thepresence of the phosphorescence band in the EL spectrum of thisdevice 1 suggests that in the electroluminescent mechanism theposition of 4f-intraconfigurational transition arising from theGd3þ ion with respect to the triplet state (T) of the TTA ligandplays an important role in the intensification of the molecularphosphorescence from the Gd3þ-complex. Considering that ingeneral the Gd3þ ion cannot accept any energy from the firstexcited triplet state of the ligand due to the high energy of the 6PJ

manifolds, the electroluminescence observed in Gd3þ-complexgenerally arises from either the T-S0 or S1-S0 ligand centeredtransitions.

In Fig. 3A and B are shown the EL spectra of the device 2 at10 V (amplified for comparison) and 22 V of bias voltage respec-tively. These EL spectra present the characteristic emission bandsarising from the Sm3þ ion. The bands around 563, 598 and643 nm are assigned to the 4G5/2-

6H5/2, 4G7/2-6H7/2 and

4G5/2-6H9/2 transitions, respectively. The 4G5/2-

6H9/2 hypersensitivetransition shows the highest intensity (around 644 nm) [27]. TheEL spectrum at 10 V is similar to that of PL of the [Sm(TTA)3(TPPO)2]film reported in the Refs. [22,28]. On the other hand, the spectrum

recorded at 22 V exhibits one additional broad band from thephosphorescence of the TTA ligand attributed to the T-S0 transitionthat overlaps with the sharp lines from the 4G5/2-

6HJ transitions(J¼5/2, 7/2 and 9/2). This result is in agreement with the presence ofthe broad band in the spectral range from 400 to 700 nm in the ELspectrum recorded for the device 1 (Fig. 3C).

Fig. 4 presents the EL spectra of the device 2 in range of14–22 V of bias voltage. In the figure it is possible to observe thatwhen the bias voltage is increased up to 22 V, the EL spectrumexhibits clearly the phosphorescence from the TTA ligand, whichis also present in the EL spectrum of the device 1 (Fig. 3C) withthe gadolinium complex acting as emitting layer. The lumines-cence intensities of the sharp lines attributed to the 4G5/2-

6HJ

transitions (J¼5/2, 7/2 and 9/2) increase as the voltage isincreased from 14 up to 22 V.

Thus, in the device 2 (Fig. 4) are expected two antagonisticmechanisms: the electroluminescence one via energy transferfrom exciton to ligand and after Sm3þ ion, where the narrowbands observed in the EL spectra arise from the Sm3þ ion; and the

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

20 V

21 V10 V

22 V

10 V

y

x

Device 1Device 2Device 3

Fig. 7. CIE x,y chromaticity of the device 1 (D), 2(&) and 3(O) show the changes of

the colour of the emitted light as the bias voltage is varied.

R. Reyes et al. / Journal of Luminescence 134 (2013) 369–373372

mechanism of molecular electrophosphorescence with an initialenergy transfer from exciton to ligand and posterior intersystemcrossing from the excited singlet S1 to the triplet T state followedby molecular phosphorescence (T-S0), as occurs in the device 1(Fig. 3C). These results suggest that, in the samarium complex[Sm(TTA)3(TPPO)2], probably the energy transfer exciton–ligand–RE3þ ion is altered by the presence of electrons of the electriccurrent, showing a phosphorescence in the EL spectra. This is incontrast with the case of the same Sm3þ complex excited byphotons (PL) in which no phosphorescence from the TTA ligand isobserved [28].

The absorption and the photoluminescence spectra of the[Sm0.5Gd0.5(TTA)3(TPPO)2] blend thin film recorded at roomtemperature are shown in Fig. 5. The absorption spectrum(Fig. 5A) showed that the efficient excitation is in the ultravioletregion and the maximum is located at 348 nm. On the other hand,the PL spectrum (Fig. 5B) does not present any emission bandfrom the gadolinium complex, only the peaks corresponding tothe Sm3þ ion emission are observed.

Fig. 6 presents the electroluminescence spectra at roomtemperature of the device 3, with the mixed complex

200 300 400 500 600 700

0.0

0.3

0.6

Wavelength (nm)

Abs

orba

nce

(O.D

.)

0.0

0.5

1.0

BA

PL

Inte

nsity

(arb

. uni

ts)

Fig. 5. Spectral data of the film [Sm0.5Gd0.5(TTA)3(TPPO)2] complex, measured at

room temperature: absorption spectrum in the range of 220–700 nm (A line) and

photoluminescence spectrum under excitation at 348 nm band (B line).

400 500 600 700

0

1

H

G

F

E

D

C

BA

EL

Inte

nsity

(arb

. uni

ts)

Wavelength (nm)

H 19VG 18VF 17VE 16VD 15VC 14VB 13VA 12V

Fig. 6. Electroluminescence spectra of device 3 recorded at bias-voltages from 12

to 19 V.

[Sm0.5Gd0.5(TTA)3(TPPO)2] as emitting layer, at different biasvoltages (12 to 19 V). In this figure the EL spectra show thecharacteristic emission sharp lines of the Sm3þ ion. The peaksat around 563, 598 and 643 nm correspond to the 4G5/2-

6H5/2,4G7/2-

6H7/2 and 4G5/2-6H9/2 transitions, respectively. As can be

noted, the 4G5/2-6H7/2 transition presents a higher intensity that

of the hypersensitive 4G5/2-6H9/2 transition in contrast to the

situation observed for the EL spectra recorded for the device 2(Fig. 4). Similar to the EL spectra of the device 2 (Fig. 4), theluminescence of the rare earth complexes arises from 4f–4fintraconfigurational electronic transitions. The energy transferoccurs to the RE3þ ion through the triplet state (T) of the ligand[29,30]. The intensity of the broad band assigned to the electro-phosphorescence from the TTA ligand in the device 3 (Fig. 6)increases as the bias voltage is increased. When the molecularelectrophosphorescence of the device 3 is compared with that ofdevice 2 (Fig. 4) an increase in the intensity due to the presence ofthe gadolinium complex is observed.

Fig. 7 shows the x,y chromaticity coordinates according to theCommision Internationale D’Eclairage (CIE) for the devices 1,2 and 3. In the case of the device 1 it is observed that the emittedcolor practically remains unchanged with the applied bias vol-tages and the emissive color at the CIE coordinates x¼0.433 andy¼0.497 at 20 Vis green–yellow. On the other hand, the x,y CIEcoordinates of the devices 2 and 3 are dependent on the appliedvoltage. Thus, in device 2 the emitted color at 10 V (x¼0.630,y¼0.357) exhibits the red characteristic of the Sm3þ ion, while at22 V the CIE coordinates are x¼0.541, y¼0.427 and the colorbecomes orange due to the higher contribution of the molecularelectrophosphorescence from the TTA ligand. A similar behavior isobserved for the device 3 at 10 V, where the emitted color(x¼0.560, y¼0.413) is red, and at 21 V, where the CIE coordinatesare x¼0.496, y¼0.455 and the color becomes orange–yellow.Therefore, in the case of the devices fabricated with the samariumand gadolinium complexes, the molecular electrophosphores-cence presents an interesting potential for the fabrication ofvoltage color tunable organic LEDs.

4. Conclusion

Triple-layer electroluminescent organic devices using MTCD ashole-transporting layer, [Sm0.5Gd0.5(TTA)3(TPPO)2] or [RE(TTA)3

(TPPO)2] complexes (RE¼Sm and Gd) as emitting layer and Alq3 aselectron transporting layer were grown and characterized. The ELspectra of the devices exhibit emission characteristics of the sharp

R. Reyes et al. / Journal of Luminescence 134 (2013) 369–373 373

lines from the Sm3þ ion together with a broad band arising from theeletrophosphorescence of the TTA ligand in the RE system that is notobserved in the photoluminescence spectra of the film of the samecomplexes. This fact can be due to the presence of two antagonisticmechanisms: (1) the electroluminescence from the exciton–ligand–Sm3þ ion energy transfer, responsible for the EL narrow bands;and the molecular electrophosphorescence with an initial energytransfer exciton–ligand and posterior intersystem crossing fromthe excited singlet S1 to the triplet T state followed by molecularphosphorescence (T-S0).

Due to these peculiar characteristics, in these devices it waspossible to change the emission color depending on the presenceof the Sm3þ , Gd3þ and TTA species. The molecular electropho-sphorescence in these systems may be used to fabricate organicLEDs with light emitted color tunable voltage. Further investiga-tions are in progress to better understand the differences betweenphotonic and electrical excitation in these rare earth chelates.

Acknowledgments

The authors wish to acknowledge CNPq, RENAMI, FAPERJ andFAPESP for the financial support, Prof. Sonia R.W. Louro (PUC-Rio)for the use of the spectrofluorimeter, and Prof. Sung-Hoon Kim ofthe Department of Dyeing and Finishing, of the KyungpookNational University (Korea) for the MTCD material.

References

[1] J.R. Sheats, H. Antoniadis, M. Hueschen, W. Leonard, J. Miller, R. Moon,D. Roitman, A. Stocking, Science 273 (1996) 884.

[2] J. Kido, Y. Okamoto, Chem. Rev. 102 (2002) 2357.[3] Y.A. Ono, Electroluminescent Displays, World Scientific, Singapore, 1995.[4] E.E.S. Teotonio, H.F. Brito, M. Cremona, W.G. Quirino, C. Legnani,

M.C.F.C. Felinto, Opt. Mater. 32 (2009) 345.

[5] N. Takada, J. Peng, N. Minami, Synth. Met. 121 (2001) 1745.[6] H. Cao, X. Gao, C. Huang, Appl. Surf. Sci. 161 (2000) 443.[7] C.J. Liang, Z.R. Hong, X.Y. Liu, D.X. Zhao, D. Zhao, W.L. Li, J.B. Peng, J.Q. Yu,

C.S. Lee, S.T. Lee, Thin Solid Films 359 (2000) 14.[8] Y. Miyamoto, M. Uekawa, H. Ikeda, K. Kaifu, J. Lumin. 81 (1999) 159.[9] K. Okada, Y. Wang, T. Nakaya, Synth. Met. 97 (1998) 113.

[10] C. Liang, W. Li, Z. Hong, X. Liu, J. Peng, L. Liu, Z. Lu, M. Xie, Z. Liu, J. Yu, D. Zhao,Synth. Met. 91 (1997) 151.

[11] R. Reyes, C.F.B. da Silva, H.F. de Brito, M. Cremona, Braz. J. Phys. 32-2B (2002)535.

[12] D.G. Moon, O.V. Salata, M. Etchells, P.J. Dobson, V. Christou, Synth. Met. 123(2001) 355.

[13] V. Christou, O.V. Salata, T.Q. Ly, S. Capecchi, N.J. Bailey, A. Cowley,A.M. Chippindale, Synth. Met. 111–112 (2000) 7.

[14] O. Renault, O.V. Salata, M. Etchells, P.J. Dobson, V. Christou, Thin Solid Films379 (2000) 195.

[15] D. Ma, D. Wang, B. Li, Z. Hong, S. Lu, L. Wang, X. Zhao, N. Minami, N. Takada,Y. Ichino, K. Yase, H. Zhang, X. Jing, Synth. Met. 102 (1999) 1136.

[16] G.F. de Sa, O.L. Malta, C.M. Donega, A.M. Simas, R.L. Longo, P.A. Santa-Cruz,E.F da Silva Jr., Coord. Chem. Rev. 196 (2000) 165.

[17] R. Hong, C.J. Liang, R.G. Li, D. Zhao, D. Fan, W.L. Li, Thin Solid Films 391 (2001)

122.[18] G. Wang, Y. Ding, Y. Wei, Appl. Surf. Sci. 93 (1996) 281.[19] R.J. Curry, W.P. Gillin, Synth. Met. 111–112 (2000) 35.[20] O.M. Khreis, W.P. Gillin, M. Somerton, R.J. Curry, Org. Electron. 2 (2001) 45.[21] H.F. Brito, G.K. Liu, J. Chem. Phys. 112 (2000) 4334.[22] E.E.S. Teotonio, H.F. Brito, M.C.F.C. Felinto, C.A. Kodaira, O.L. Malta, J. Coord.

Chem. 56 (2003) 913.[23] O.L. Malta, H.F. Brito, J.F.S. Menezes, F.R.G. Silva, C.M. Donega, S. Alves Jr.,

Chem. Phys. Lett. 282 (1998) 233.[24] R. Reyes, M. Cremona, E.E.S. Teotonio, H.F. Brito, O.L. Malta, Thin Solid Films

469–470 (2004) 59.[25] R. Reyes, M. Cremona, E.E.S. Teotonio, H.F. Brito, O.L. Malta, Chem. Phys. Lett.

396 (2004) 54.[26] F.R.G. Silva, J.F.S. Menezes, G.B. Rocha, S. Alves, H.F. Brito, R.L. Longo,

O.L. Malta, J. Alloys Compd. 303–304 (2000) 364.[27] H.F. Brito, O.L. Malta, M.C.F.C. Felinto, E.E.S. Teotonio, J.F.S. Menezes,

C.F.B. Silva, C.S. Tomiyama, C.A.A. Carvalho, J. Alloys Compd. 344 (2002) 293.[28] R. Reyes, E.N. Hering, M. Cremona, C.F.B. Silva, H.F. Brito, C.A. Achete, Thin

Solid Films 420–421 (2002) 23.[29] R.E. Wan, G.A. Crosby, J. Mol. Spectrosc. 8 (1962) 315.[30] G.A. Crosby, R.E. Wan, R.M. Alire, J. Chem. Phys. 34 (1961) 743.