Organic Electronics 15 (2014) 798–808

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Organic Electronics

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White OLED based on a temperature sensitive Eu3+/Tb3+

b-diketonate complex

http://dx.doi.org/10.1016/j.orgel.2014.01.0091566-1199/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +351 234 378110; fax: +351 234 378197.E-mail address: patriciapereira@ua.pt (P.P. Lima).

P.P. Lima a,⇑, F.A.A. Paz b, C.D.S. Brites a, W.G. Quirino c, C. Legnani c, M. Costa e Silva d,R.A.S. Ferreira a, S.A. Júnior e, O.L. Malta e, M. Cremona f,d, L.D. Carlos a

a Department of Physics and CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb Department of Chemistry and CICECO, University of Aveiro, 3810-193 Aveiro, Portugalc LEO – Laboratório de Eletrônica Orgânica, Departamento de Física, Universidade Federal de Juiz de Fora, Juiz de Fora, MG 36036-330, Brazild LADOR – Laboratório de Dispositivos Orgânicos, Divisão de Metrologia de Materiais, Inmetro, Duque de Caxias, RJ 25250-020, Brazile Departamento de Química Fundamental, Universidade Federal de Pernambuco, Cidade Universitária, Recife, PE 50740-560, Brazilf Departamento de Física, Pontifícia Universidade Católica do Rio de Janeiro, PUC-Rio, Rio de Janeiro, RJ 22453-970, Brazil

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 October 2013Received in revised form 10 January 2014Accepted 12 January 2014Available online 24 January 2014

Keywords:b-Diketonate complexEmission quantum yieldColor coordinatesThermometryWOLED

A mixed lanthanide b-diketonate complex of molecular formula [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] (btfa– = 4,4,4–trifluoro–1–phenyl–1,3–butanedionate; 4,40-bpy = 4,40-dipyridyl;EtOH = ethanol) was synthesized and its structure was elucidated by single crystal X-raydiffraction. The temperature dependence of the complex emission intensity between 11and 298 K is illustrated by the Commission Internacionale l’Éclairage (CIE) (x,y) color coor-dinates change within the orange-red region, from (0.521, 0.443) to (0.658, 0.335). Theexistence of Tb3+-to-Eu3+ energy transfer was observed at room temperature and as thecomplex presents a relatively high emission quantum yield (0.34 ± 0.03) it was doped ina 4,40-bis(carbazol-9-yl)biphenyl (CBP) organic matrix to be used as emitting layer tofabricate a white organic light-emitting diode (WOLED). Continuous electroluminescenceemission was obtained varying the applied bias voltage showing a wide emission bandfrom 400 to 700 nm. The white emission results from a combined action between theEu3+ and Tb3+ peaks from the mixed Eu3+/Tb3+ complex and the other organic layers form-ing the device. The intensity ratio of the peaks is determined by the layer thickness and bythe bias voltage applied to the OLED, allowing us to obtain a color tunable light source.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction ligands used to coordinate to lanthanide (Ln3+) ions, popu-

A substantial increase in research regarding opticalproperties of lanthanide complexes with organic ligandshas been observed in the last two decades. The interestin investigating these kind of complexes is principally re-lated by the large field of applications of these compoundsin, for instance, luminescent solar concentrators [1], mag-netic resonance imaging (MRI) contrast agents [2], pHsensing [3] and UV dosimeters [4]. Among the organic

lar chelates known as b-diketones are distinguished essen-tially since they are in general commercially accessible andtheir respective lanthanide complexes are simply synthe-sized. Furthermore depending on the b-diketone ligand se-lected (namely on the resonance condition between thetriplet level of the ligand and the Ln3+ ion) the complexescan exhibit high emission quantum yield. Particularly ifthese Ln3+ b-diketonate complexes do not present mole-cules such water, for example, in its first coordinationsphere avoiding, therefore, the luminescence quenchingthrough non-radiative decay paths provided by OH oscilla-tors. For instance, very high emission quantum yields of

P.P. Lima et al. / Organic Electronics 15 (2014) 798–808 799

0.85, 0.75 and 0.65 where found, respectively, forEu(tta)3�2dbso (tta� = 2-thenoyltrifluoracetonate; dbso =dibenzyl sulphoxide) [5], Eu(nta)3�2dmso (nta� = naphtoil-trifluroacetonate; dmso = dimethyl sulphoxide) [6] andEu(btfa)3�bipy (btfa� = 4,4,4-trifluoro-1-phenyl-1,3-butan-edionate; bipy = 2,20-dipyridyl) [7].

In the last two decades special attention was devoted tofind out lanthanide complexes with high emission quan-tum yield to be exploited as efficient emitters in organiclight emitting devices (OLEDs) [8–10]. In this perspectiveseveral lanthanide b-diketonate complexes with highemission quantum yield have been selected for OLEDsdevelopment [8,10]. Interesting electroluminescent prop-erties can be achieved by the presence of more than onespecies of lanthanide ion as emitting layer [11–14]. For in-stance, Reyes et al. [13] developed OLEDs with voltage-dependent electroluminescence using a mixed Sm3+ andEu3+ b-diketonate complex, [Sm0.7Eu0.3(tta)3(tppo)2](TPPO = triphenylphosphine oxide), as emitting layer. Inthat case, a voltage-tunable color light source was obtainedby means of the emission intensity ratio of the peaks deter-mined by the bias voltage applied to the OLEDs, togetherwith the ligand electrophosphorescence. Following thisconcept, there has been an increasing interest in investi-gating luminescent materials containing more than onetype of Ln3+ ions (e.g. Eu3+ and Tb3+) whose tuning of theemission color can be achieved by variation of the lantha-nide content, wavelength excitation and temperature [15–20]. In certain cases the interest in investigating these typeof materials is partially dedicated to the study of Tb3+-to-Eu3+ energy transfer process [16,18]. More recently, hasbeen demonstrated that the mixing of Eu3+ and Tb3+ ionsin a single luminescent material may also be an importanttool to develop optical sensors, such as luminescent ther-mometer [19,21].

Herein, we investigate the electroluminescent proper-ties of a white organic light-emitting diode (WOLED) fabri-cated using a new [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]complex (Scheme 1), with an emission quantum yield of0.34 ± 0.03. The complex was used as a guest in a 4,40-bis(carbazol-9-yl)biphenyl (CBP) organic matrix. A wideelectroluminescence emission band from 400 to 700 nmwas obtained varying the device applied voltage. Althoughit is possible to detect the characteristics peaks from theEu3+ and Tb3+ ions, the white emission (CIE (0.353, 0.316)color coordinates) results from a combined action between

O

O

CF3

3

Ln

N

N

H-O-CH2CH3

Scheme 1. Chemical structure of [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]complex.

the single complex layer and the other organic compoundsforming the device. In addition the temperature depen-dence of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complexemission intensity between 11 and 298 K induces markedchanges on the CIE (x,y) color coordinates within theorange-red region opening the possibility to fabricatetemperature sensitive color tunable OLEDs.

2. Experimental

2.1. Materials

The chemicals lanthanide chloride hexahydrate(EuCl3�6H2O and TbCl3�6H2O, 99.9%, Aldrich), 4,4,4-trifluoro-l-phenyl-1,3-butanedione (Hbtfa, 99%, Aldrich), 4,40-dipyr-idyl (4,40-bpy, 98%, Aldrich), ethanol (EtOH, 99.9%, Merck),sodium hydroxide (NaOH, 98%, Merck), N,N0-bis(naphtha-len-1-yl)-N,N0-bis(phenyl)-benzidine (NPB, 99.5%, Lumtec),copper phthalocyanine complex (CuPC, 99%, Lumtec), 4,40-bis(carbazol-9-yl)biphenyl (CBP, 99.5%, Lumtec), 2,9-di-methyl-4,7-diphenyl-1,10-phenanthroline (BCP, Lumtec),and isopropyl alcohol (99.7%, Sigma Aldrich) were usedwithout prior purification.

2.2. Synthesis of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]complex

The synthetic procedure of the [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] complex is very similar to that of[Eu(btfa)3(4,40-bpy)(EtOH)] complex [22]. First, the precur-sor [Ln(btfa)3�2H2O] (Ln = Eu and Tb) complexes were pre-pared by the addition of 0.1 mmol of LnCl3�6H2O (Ln = Euand Tb) and 0.3 mmol of Hbtfa in 1 mL of EtOH. Then, thepH of the solution was adjusted to 6.5–7.0 with an etha-nolic solution of NaOH. The resulting mixture was stirredfor 24 h at room temperature and then the solvent wasslowly evaporated during an additional 48 h. The com-pound formed was washed with water and recrystallizedin EtOH and dried at 45 �C during 48 h. To obtain the[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex, an ethanolicsolution of 4,40-bpy (0.032 g, 0.2 mmol) was slowly addedto a solution of [Eu(btfa)3�2H2O] (0.083 g, 0.1 mmol) in eth-anol. After 30 min under stirring the [Tb(btfa)3�2H2O](0.087 g, 0.1 mmol) complex was added to this solution.Then, the reaction mixture was stirred for 24 h at roomtemperature. The solvent was slowly evaporated at roomtemperature. The compound formed was dissolved in eth-anol and recrystallized and dried at 45 �C under vacuumfor 48 h. Anal. Calcd. for C42H32Eu0.45F9N2O7Tb0.55:Eu, 6.82; Tb, 8.74; C, 50.25; H, 3.19; N, 2.79, found; Eu,6.85; Tb, 8.71; C, 50.53; H, 3.01; N, 2.68. IR (KBr): 3408,3066, 1639, 1614, 1599, 1411, 1001, 808 cm�1.

2.3. WOLED fabrication

The WOLED device was assembled using a heterojunc-tion between six organic molecular materials with thefollowing structure:

Table 1Crystal and structure refinement data for [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] (1).

Formula C42H32Eu0.45F9N2O7Tb0.55

Formula weight 1003.48Crystal system OrthorhombicSpace group Pbcaa (Å) 9.1950(18)b (Å) 21.863(4)c (Å) 39.386(8)Volume (Å3) 7918(3)Z 8Dc (g cm�3) 1.684l(Mo Ka) (mm�1) 1.792Crystal size (mm) 0.32 � 0.16 � 0.06Crystal type Colorless plateh range (�) 3.53–25.35Index ranges �11 6 h 6 11

�26 6 k 6 26�43 6 l 6 47

Reflections collected 65,099Independent reflections 7225 [Rint = 0.0841]Completeness to h = 25.35� 99.6%Final R indices [I > 2r(I)]a,b R1 = 0.0618

wR2 = 0.1190Final R indices (all data)a,b R1 = 0.0808

wR2 = 0.1261Weighting schemec m = 0.0

n = 113.8656Largest diff. peak and hole 2.190 and �4.502 eÅ-3

a R1 ¼PkFoj � jFck=

PjFoj.

b wR2 ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiP½wðF2

o � F2c Þ

2�=P½wðF2

oÞ2�

q.

c w ¼ 1=½r2ðF2oÞ þ ðmPÞ2 þ nP� where P ¼ ðF2

o þ 2F2c Þ=3.

800 P.P. Lima et al. / Organic Electronics 15 (2014) 798–808

2.3.1. Device 1Glass/ITO/CuPc(15)/NPB(25)/CBP: 8.3% [Eu0.45Tb0.55

(btfa)3(4,40-bpy)(EtOH)] (60)/BCP(11)/Alq3(15)/Al(100),where the thickness of each layer, in nanometer, are writ-ten between parentheses. The emitting molecular layerwas prepared from two individual sources by co-evapora-tion of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complexacting as a dopant in a bipolar organic CBP as the host mol-ecule matrix. The nominal concentration of complex wasabout 8.3% in the matrix. Furthermore, the CuPc wasdeposited as the hole-injection layer, the NPB as thehole-transporting layer, the BCP as hole-blocking layerand finally the aluminum was deposited as the cathode.All the different layers were sequentially deposited in ahigh vacuum environment (Angstrom Engineering tool)by thermal evaporation onto ITO substrates with a sheetresistance of 8.1 X/ square.

The ITO substrates were initially cleaned by ultrasonifi-cation using a detergent solution, followed by toluenedegreasing, and then cleaned again by ultrasonificationwith pure isopropyl alcohol. The base pressure was6.6 � 10�5 Pa and during the evaporation the pressurewas �10�4 Pa. The deposition rates for the organic com-pounds were in the range of 0.3–3.7 Å/s. The layer thick-ness was controlled in situ through a quartz crystalmonitor and confirmed with a profilometer measurement.The active area of the fabricated devices was around4 mm2 and they operated in forward bias voltage, withITO as the positive electrode and Al as the negative one.

2.4. Material characterization

2.4.1. Single-crystal X-ray diffraction studiesA suitable single-crystal of [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] (1) was mounted on a glass fiber usingperfluoropolyether oil [23]. Data were collected at100(2) K (at the Unidade de Raios-X, RIAIDT, Universityof Santiago de Compostela, Spain) on a Bruker SMART1000 charge-coupled device (CCD) area-detector diffrac-trometer (Mo Ka graphite-monochromated radiation,k = 0.7107 Å), controlled by the SMART software package[24]. Images were processed using the SAINTPlus softwarepackage [25], and data were corrected for absorption byusing the semi-empirical method of SADABS [26]. Thestructure was solved by the direct methods of SHELXS-97[27], and refined by full-matrix least squares on F2 usingSHELXL-97 [28]. Non-hydrogen atoms were directly lo-cated from difference Fourier maps and refined with aniso-tropic displacement parameters. The structure containstwo isostructural complexes, [Eu(btfa)3(4,40-bpy)(EtOH)]and [Tb(btfa)3(4,40-bpy)(EtOH)], which were found to oc-cupy the same crystallographic position in a 45:55 ratio.As a consequence, the crystal structure at 100(2) K in Pbcacalls for the presence of a crystallographically independentlanthanide site which was modelled as containing bothmetallic centers sharing the same crystallographic position(with a 45%:55% ratio) and identical anisotropic displace-ment parameters.

Hydrogen atoms bound to carbon were placed at theiridealized positions using appropriate HFIX instructions inSHELXL (43 for the aromatic groups, 23 for the methylene

and 137 for the methyl group) and included in subsequentleast-squares refinements with an isotropic displacementparameter (Uiso) fixed at 1.2 (for the aromatic and –CH2–hydrogen atoms) or 1.5 (for the –CH3 group) of Ueq of theparent carbon atom. The hydrogen atom associated withthe coordinated hydroxyl group of the ethanol residue wasplaced in a calculated position to give the best hydrogen-bonding fitting, and included in subsequent least-squaresrefinement cycles with the O–H distance restrained to1.00(1) Å and Uiso = 1.5 � Ueq(O).

The last difference Fourier map synthesis showed thehighest peak (2.190 e�3) and deepest hole(�4.502 e�3) located at 1.22 and 1.63 Šfrom O1 andH10, respectively. Information concerning crystallographicdata collection and structure refinement details for[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] is summarized inTable 1. Structural drawings have been produced usingthe software package Crystal Diamond [29].

2.4.2. Elemental analysesThe Europium and Terbium contents were obtained by

ICP-OES (Inductively Coupled Plasma Optical EmissionSpectroscopy) analysis on an Horiba-Jobin Yvon modelActiva-Mat the Central Analytical Laboratory of the Univer-sity of Aveiro. Elemental analyses for C, H and N were performedwith a CHNS-932 elemental analyser with standardcombustion conditions and handling of the samples at air.

2.4.3. Mid-Infrared spectra (FT-IR)The FT-IR spectra were recorded using a MATTSON

7000 FTIR Spectrometer. The spectra were collected over

P.P. Lima et al. / Organic Electronics 15 (2014) 798–808 801

the range 4000–400 cm�1 by averaging 128 scans at amaximum resolution of 2 cm�1. The compounds were fi-nely grounded (about 2 mg), mixed with approximately175 mg of dried KBr (Merck, spectroscopic grade) andpressed into pellets.

2.4.4. UV/Vis absorptionThe UV/Vis absorption spectra were recorded on a JAS-

CO V-560 instrument over the scan range 220–800 nm, at200 nm min�1 and a resolution of 0.5 nm by using a10�5 mol L�1 ethanol solution of the [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] complex and Hbtfa and 4,40-bpy ligands.

2.4.5. PhotoluminescenceThe luminescence spectra were recorded with a modu-

lar double grating excitation spectrofluorimeter with aTRIAX 320 emission monochromator (Fluorolog-3, HoribaScientific) coupled to a R928 Hamamatsu photomultiplier,using the front face acquisition mode. The excitationsource was a 450 W Xe arc lamp. The emission spectrawere corrected for detection and optical spectral responseof the spectrofluorimeter and the excitation spectra werecorrected for the spectral distribution of the lamp intensityusing a photodiode reference detector. The emission decaycurves were acquired with the same instrumentation,using a pulsed Xe–Hg lamp (6 ls pulse at half-width and20–30 ls tail).

2.4.6. Emission quantum yieldThe emission quantum yield were measured using a

quantum yield measurement system C9920-02 from Ham-amatsu with a 150 W Xe lamp coupled to a monochroma-tor for wavelength discrimination, an integration sphere assample chamber and a multichannel analyzer for signaldetection. Three measurements were made for each sam-ple, and the average values obtained are reported withaccuracy within 10% accorded to the manufacturer.

2.4.7. Device characterizationOptical and electrical properties of thermally-deposited

thin films and devices of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex were characterized at room temperature.The electrical measurements were taken simultaneouslywith the brightness using a LabVIEW�-based program, aKeithley 2240 and a calibrated radiometer/photometer(Newport Power Meter, model 1830-C). The photolumines-cence and electroluminescence spectra were obtained ona Photon Technology International (PTI) fluorescencespectrophotometer.

3. Results and discussion

3.1. Crystal structure description

The reaction between 4,4,4-trifluoro-1-phenyl-2,4-butanedione (Hbtfa) and 4,40-bipyridine (4,40-bpy) with amixture of Eu3+ and Tb3+ cations in ethanolic media, ledto the simultaneous formation of neutral [Eu(btfa)3(4,40-bpy)(EtOH)] and [Tb(btfa)3(4,40-bpy)(EtOH)] complexeswhich co-crystallized in a 9:11 ratio into a crystalline

material ultimately formulated as [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] (1) on the basis of single-crystal X-ray diffrac-tion (see Experimental Section). A search in the literatureand in the Cambridge Structural Database (CDS, Version5.33, November 2011 with four updates) [30,31] revealsthat only a few rare-earth complexes with btfa- and pyri-dine-based ligands have been reported to date. Most ofthese known structures contain in the rare-earth coordina-tion sphere N,N-chelated ligands such as 1,10-phenanthro-line [32], 2,20-bipyridine or derivatives [7,33], 2,4,6-tri(2-pyridyl)-1,3,5-triazine [34], chiral terpyridyl-based ligands[35] and 2,3-di(2-pyridyl)pyrazine [36]. Remarkably, solelyon the lanthanide-based complexes reported by us a fewyears ago [22] and those recently described by Mei et al.[37] the rare-earth coordination sphere is composed ofunidentate pyridine-based ligands. This structural evi-dence is clearly a consequence of the labile Ln–N bond thatcan easily be replaced by a ligand having oxygen atomsto act as donors. We note that crystal structure of[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] is isotypical withthose previously reported by us for the pure Eu3+ andGd3+ materials [22], with the difference of being insteada co-crystal of two independent complexes.

The crystal structure of [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] at 100 K in the orthorhombic Pbca space groupcalls for the presence of a single crystallographically inde-pendent lanthanide cation, which is coordinated to threebtfa- anionic ligands, one unidentate 4,40-bpy plus one eth-anol residue (coordinated via the hydroxyl group) (Fig. 1a).The coordination geometry of this eightfold environment,{EuNO7}, resembles a highly distorted square antiprismwith the two basal planes being defined byO3� � �O4� � �O7� � �N1 and O1� � �O2� � �O5� � �O5 (Fig. 1b): onethe one hand, the Ln–(N,O) bond lengths were found inthe wide 2.303(5)–2.594(6) Å range and, on the other, the(N,O)–Ln–(N,O) polyhedral angles range between69.98(18)� and 146.95(17)� (Table 2). Moreover, the distor-tion degree of the coordination polyhedron is also well vis-ible in the fact that the two average planes are not strictlyparallel, subtending instead a mutual dihedral angle of ca.4.7�.

Individual complexes close pack in the solid state med-iated by a number of cooperative supramolecular contacts.The most striking concern is the strong and highly direc-tional O–H� � �N hydrogen bonds connecting the ethanolmolecule of one complex to the pendant pyridine groupof the 2,20-bpy moiety of the adjacent complex. Thisarrangement leads to the formation of a one-dimensionalsupramolecular helical chain of individual complexes run-ning parallel to the b-axis of the unit cell, with a period ofthe length of this axis (Fig. 2). Various other intermolecularsupramolecular contacts such as C–H� � �O, C–H� � �F, C–H� � �p and p-p contacts help strengthen the robustness ofthe crystal structure (not shown; see Table 3 for geometri-cal details on these interactions).

3.2. Optical properties

The UV/Vis absorption spectrum of the 4,40-bpy ligandconsists of a broad band between 215 and 320 nm with amaximum occurring at 240 nm and a shoulder around

Fig. 1. (a) Mixed ball-and-stick and polyhedral representation of the isostructural [Eu(btfa)3(4,40-bpy)(EtOH)] and [Tb(btfa)3(4,40-bpy)(EtOH)] complexescomposing the crystal structure of [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]. The highly distorted square {LnNO7} antiprismatic coordination environment of theTb3+ and Eu3+ cations is represented as a semi-transparent green polyhedron. (b) Detailed view of the {LnNO7} coordination polyhedron showing thelabelling scheme for all atoms composing the coordination sphere. For selected bond lengths (in Å) and angles (in degrees) see Table 2.

Table 2Selected bond lengths (in Å) and angles (in degrees) for the crystalograph-ically independent lanthanide coordination environment present in[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] (1).

Ln1–O1 2.408(5) Ln1–O5 2.364(5)Ln1–O2 2.315(5) Ln1–O6 2.338(5)Ln1–O3 2.303(5) Ln1–O7 2.428(5)Ln1–O4 2.425(5) Ln1–N1 2.594(6)

O1–Ln1–O4 124.63(17) O3–Ln1–O7 89.42(18)O1–Ln1–O7 146.95(17) O3–Ln1–N1 74.41(19)O1–Ln1–N1 75.45(18) O4–Ln1–O7 72.84(17)O2–Ln1–O1 69.98(18) O4–Ln1–N1 130.02(18)O2–Ln1–O4 72.29(18) O5–Ln1–O1 82.84(17)O2–Ln1–O5 102.47(18) O5–Ln1–O4 144.72(17)O2–Ln1–O6 74.43(18) O5–Ln1–O7 96.13(18)O2–Ln1–O7 141.14(17) O5–Ln1–N1 73.89(19)O2–Ln1–N1 145.41(18) O6–Ln1–O1 129.59(17)O3–Ln1–O1 73.96(18) O6–Ln1–O4 73.86(17)O3–Ln1–O2 94.57(18) O6–Ln1–O5 71.21(17)O3–Ln1–O4 70.27(17) O6–Ln1–O7 79.89(17)O3–Ln1–O5 144.43(18) O6–Ln1–N1 132.32(19)O3–Ln1–O6 144.13(17) O7–Ln1–N1 72.60(17)

802 P.P. Lima et al. / Organic Electronics 15 (2014) 798–808

272 nm while for the btfa ligand a broad band between 220and 380 nm with the maximum occurring at 327 nm areobserved (Fig. S1 in Supporting Information). Smallchanges in the absorption spectra of the ligands after theirreaction with the Eu3+ and Tb3+ ions can be observed (seeFig. S1 in Supporting Information). The profile of the UV/Vis absorption spectrum of the [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] complex is very similar to that of the[Eu(btfa)3(4,40-bpy)(EtOH)] complex [22]. The absorptionspectrum of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] com-plex (Fig. S1 in Supporting Information) displays a largebroad band in the UV region with at least 2 main compo-nents at 245 and 328 nm and a shoulder around 270 nm.The changes aforementioned are evidence that complexa-tion has taken place.

Fig. 3a shows the room temperature excitation spectraof the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex moni-tored within the Eu3+ (700 nm, 5D0 ?

7F4 transition) andTb3+ (490 nm, 5D4 ?

7F6 transition) levels. Both spectra

exhibit a large broad band with two main components at270 and 370 nm, attributed to the ligand excited states. Theexcitation spectrum monitored within the 5D0 ?

7F4 tran-sition, besides the 7F0,1 ?

5D2,1 Eu3+ transitions, also exhib-its the 7F6 ?

5D4 Tb3+ transition evidencing the existence ofTb3+-to-Eu3+ energy transfer. The room temperature emis-sion spectrum of [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] com-plex excited at the ligand states, displays besides theemission lines from the Eu3+ (5D0 ?

7F0–4) ions (Fig. 3b),very low emission lines from the Tb3+ (5D4 ?

7F6–0) ions(inset in Fig. 3b). Moreover the energy and full width athalf-maximum (fwhm) of the single 5D0 ?

7F0 line and ofthe number of Stark components of the 5D0 ?

7F1,2 transi-tions resemble those observed for the emission spectrumof the [Eu(btfa)3(4,40-bpy)(EtOH)] complex [22]. Similaremission spectrum (Fig 3b) is obtained when the 5D4 man-ifold of the Tb3+ (485 nm) is excited supporting the Tb3+-to-Eu3+ energy transfer aforementioned. In contrast, theemission spectrum excited within the 5D2 level of theEu3+ ion displays only the typical 5D1 ? 7F1,2 and5D0 ?

7F0–6 emission lines from Eu3+ (Fig. 3b).The temperature dependence of the emission spectra of

[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] between 11 and 298 Kshows that whereas the 5D4 ? 7F6–0 (Tb3+) emission inten-sity decreases that of the 5D0 ?

7F0–4 (Eu3+) increases withincreasing of temperature (Fig. 4a and Fig. S2 in SupportingInformation). This dependence is well illustrated by thecalculus of the (x,y) CIE color coordinates (Fig. 4b): theemission colors coordinates change within the orange-red region from (0.521, 0.443) to (0.658, 0.335), as temper-ature increases in that range.

Fig. 5a and S3 in Supporting Information show the tem-perature dependence of the 5D0 and 5D4 lifetimes (the de-cay curves are well described by a single exponentialfunction as shown in Fig. S3 in the Supporting Informa-tion). While the 5D0 lifetime remains approximately con-stant (0.505–0.593 ms), the 5D4 lifetime increases within0.211–0.515 ms for temperature values between 11 and75 K, then decreases by approximately 12 times as thetemperature increases from 75 to 250 K (Fig. 5a). The

Fig. 2. Schematic representation of the strong and highly directional O–H� � �N hydrogen bonding interactions establishing physical connections betweenadjacent [Eu(btfa)3(4,40-bpy)(EtOH)] and/or [Tb(btfa)3(4,40-bpy)(EtOH)] complexes, leading to the formation of a supramolecular chain parallel to the [010]direction of the unit cell. Hydrogen bonds are depicted as dashed orange lines. For geometrical details on the existent supramolecular contacts in[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex see Table 3.

Table 3Supramolecular contacts present in compound [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] (1). Distances are given in Å and interaction angles indegrees.a,b

D–H� � �A d(D� � �A) <(DHA)

O7–H7A� � �N2i 2.772(8) 166

C9–H9� � �O2ii 3.246(9) 130C18–H18� � �F5iii 3.186(10) 131C19–H19� � �F3iv 3.245(11) 132C42–H42A� � �F9v 3.153(10) 131C42–H42B� � �F2v 3.181(11) 130

C–H� � �p Contacts d(D� � �Cg) <(CHCg)

C2–H2� � �Cg1ii 3.911(8) 163

C30–H30� � �Cg2ii 3.724(8) 143

p–p Contacts Inter-centroid distance

Cg3� � �Cg4ii 3.733(5)

a Symmetry transformations used to generate equivalent atoms:(i) 2.5 � x, ½ + y, z; (ii) 1.5 � x, �½ + y, z; (iii) 2 � x, 1 � y, �z;(iv) 1.5 � x, ½ + y, z; (v) 1 + x, y, z.

b Centres of gravity (Cg): Cg1 = C15, C16, C17, C18, C19 and C20;Cg2 = N2, C6, C7, C8, C9 and C10; Cg3 = N1, C1, C2, C3, C4 and C5;Cg4 = C35, C36, C37, C38, C39 and C40.

(a)

(b)

Fig. 3. (a) Excitation spectra of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]complex monitored at (1) 700 and (2) 490 nm. (b) Emission spectra of thecomplex excited at (3) 363, (4) 464 and (5) 485 nm.

P.P. Lima et al. / Organic Electronics 15 (2014) 798–808 803

behavior observed above 75 K may be explained by thepresence of a thermally active non-radiative mechanismassociated to the triplet state of the ligands. The non-radi-ative de-excitation probability of the Tb3+ 5D4 level(20,590 cm�1) may be approximately described by theMott–Seitz model, which expresses the temperaturedependence of the experimental lifetime as [20]:

s�1 ¼ s�1r þ k exp � DE

kBT

� �ð1Þ

where sr is the radiative lifetime, k is the migration energyrate, DE is the activation energy ascribed to the energetic

470 500 530 560 590 620 650 680 710

Nor

mal

ized

Inte

nsity

54

3

2

298250

225200

175150

125100

75

Wavelength (nm)

11

1

* Tempe

rature

(K)

(a)

0.3 0.4 0.5 0.6 0.70.3

0.4

0.5

0.6

0.7

100K

150K

12-75K

125K

175K

x

y

200-298K

(b)

Fig. 4. (a) Emission spectra of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex excited at 363 nm and recorded between 11 and 298 K. The sharp linesassigned to 1, 2, 3, 4 and 5 correspond to the 5D4 ?

7F6,5 (Tb3+) and 5D0 ?7F2–4 (Eu3+) transitions, respectively. In the area marked with an asterisk there is

an overlap between the Eu3+ (5D0 ?7F0,1) and Tb3+ (5D4 ?

7F4) emission lines. (b) Partial CIE chromaticity diagram showing the temperature dependence ofthe (x,y) color coordinates for the emission of the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)

804 P.P. Lima et al. / Organic Electronics 15 (2014) 798–808

difference between the triplet state of the ligand and the5D4 state, kB is the Boltzmann constant and T is the abso-lute temperature. The activation energy obtained is1022 ± 12 cm�1, a value compatible with the location ofthe zero-phonon transition of the triplet level at 450 nm(ca. 22,212 cm�1), in the low-wavelength emission edgeof the phosphorescence spectrum (14 K) of the[Gd(btfa)3(4,4-bpy)(EtOH)] complex [22]. Therefore, theactivation energy value obtained supports the aforemen-tioned suggestion that, in this case, the thermally activatednon-radiative mechanism involves the triplet state of theligands (see Fig. S4 in Supporting Information).

The measurement of the absolute temperature may beinferred using the temperature dependence of the

5D4 ?7F5 (ITb centered at 545 nm) and 5D0 ?

7F2 (IEu cen-tered at 612 nm) integrated areas transitions (Fig. S2 inSupporting Information) [19,38]. Whereas the integratedarea IEu increase to 30% the integrated area ITb decreaseto 0.5% of the initial value in the 11–298 K range. This lan-thanide complex presents a thermometric response thatcan be calibrated using the ratiometric D = ITb/IEu, parame-ter, where ITb and IEu are the Tb3+ 5D4 ? 7F5 and Eu3+

5D0 ?7F2 emission transitions areas, respectively [39,40].

Fig. 5b shows the temperature dependence of D for the[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex. The behav-ior is dominated by the decay in the ITb value, resultingin an S-shaped curve with maximum slope in the inflectionpoint (T = 130.3 ± 4.5 K). The maximum uncertainty of the

0 50 100 150 200 250 3000.00

0.15

0.30

0.45

0.60

5 D4 li

fetim

e (m

s)

Temperature (K)

(a)

0 50 100 150 200 250 3000.0

0.1

0.2

0.3

0.4

0.5

0.6

Temperature (K)

(b)

Δ

0 50 100 150 200 250 300

10-2

10-1

100

101

(c)(c)

Sens

itivi

ty (%

.K-1

)

Temperature (K)

Fig. 5. (a) Temperature dependence of the 5D4 lifetime (monitored at 490 nm and excited at 365 nm), (b) normalized D parameter and (c) sensitivity for the[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex (excitation wavelength of 363 nm). The line in (a) corresponds to the best fit of the experimental data usingEq. (1), while the lines in (b) and (c) are guides for the eyes.

P.P. Lima et al. / Organic Electronics 15 (2014) 798–808 805

thermometric parameter (dD = 1.2 � 10–3) (estimated fromthe deviation from the calibration curve) was used as a cut-off limit to define the operating range of such thermometer(T 6 225 K). This value agrees with the temperature for

which the uncertainty on ITb value is of the same order ofmagnitude of ITb. The relative sensitivity S ¼ dD

dt =D is a fig-ure of merit of this complex as a thermometer allowingto compare its performance with other thermometers

806 P.P. Lima et al. / Organic Electronics 15 (2014) 798–808

operating in this temperature range. The relative sensitivitywas computed for this complex in the temperature rangeits operating range resulting in a maximum sensitivity of4.0% K–1 at 225 K (Fig. 5c).

The maximum emission quantum yield value of[Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] (0.34 ± 0.03, underexcitation at 370 nm), similar to that of [Eu(btfa)3(4,40-bpy)(EtOH)] (0.38 ± 0.04, under excitation at 360 nm),[22] is substantially higher than that estimated for the pre-cursor aquocomplex, Eu(btfa)3�2H2O, (0.20 ± 0.02), [41]and then the [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex

Fig. 6. Electroluminescence spectra of the [Eu0.45Tb0.55(btfa)3

(4,40-bpy)(EtOH)] complex based OLED as a function of different biasvoltages. The inset shows the I � V curve for the device.

Fig. 7. The CIE color space chromaticity diagram of the emission of [Eu0.45Tb0.55(The inset shows a photo of the binuclear device with active area of 4 mm2. (For ireferred to the web version of this article.)

can be used as a single green- and red-emitting layer in aWOLED.

3.3. OLED device characterization

Fig. 6 exhibits the electroluminescence spectra of thedevice 1, at room temperature, by applying 16, 18 and20 V bias voltage. The electroluminescence spectra displaythe 5D0 ?

7F0–4 and 5D4 ?7F6–4 transitions from the Eu3+

and Tb3+ ions, respectively. It is worth mentioning thatthe 5D4 ? 7F6–4 (Tb3+) emission intensity increases withthe bias voltage applied to the device. The spectra alsopresent a peak around 410 nm and a broad band between350 and 600 nm. The peak around 410 nm can be attrib-uted to the NPB emission [42], while the broad band comesfrom the binuclear complex ligands. In this device the NPBacts as both charge transporting and emitting layer [43].The energy barrier (..0.55 eV) existing between the NPBHOMO level (..5.25 eV) and the ITO work function(..4.7 eV) induces the recombination to occur also insidethe NPB layer. Moreover, the broad emission in Fig. 6 alsoindicates that for the WOLED fabricated with the [Eu0.45-

Tb0.55(btfa)3(4,40-bpy)(EtOH)] complex, the energy transferfrom the ligand to the trivalent lanthanide ions is not veryefficient allowing the observation of the ligand electroph-osphorescence, as already observed in other systems [44].In the inset of the Fig. 6 the I � V characteristic for this de-vice is also shown. The conversion of the electrolumines-cence spectra of this WOLED in the (x,y) chromaticitydiagram of the Commission Internationale de l’Eclairage(CIE), leads to the x = 0.353, y = 0.316 color coordinates

btfa)3(4,40-bpy)(EtOH)] complex OLED (0.353, 0.316) at 18 V bias voltage.nterpretation of the references to colour in this figure legend, the reader is

P.P. Lima et al. / Organic Electronics 15 (2014) 798–808 807

(18 V bias voltage), which represents an intense white lightemission OLED, as can be seen in the picture of Fig. 7. Thedevice color at 16 V is pinkish-orange (0.309, 0.247) due tothe low intensity of the Tb3+ emission lines. It is worth-while to note that at least two factors can be responsiblefor the high bias voltage values used in this device: theuse of the aluminum as cathode and, the thickness of therare earth organic complexes were not fully optimized.

4. Conclusions

In this work we discussed the structural and opticalproperties of mixed [Eu0.45Tb0.55(btfa)3(4,40-bpy)(EtOH)]complex. From the emission spectra analysis it was ob-served that increasing the temperature from 11 to 298 Kthe emission colors coordinates change within the or-ange-red region from (0.521, 0.443) to (0.658, 0.335). Theemission spectra of the complex are temperature depen-dent and dominated by a decrease of the Tb3+ 5D4 ?

7F5

transition area to values below of the uncertainty of itsdetermination. This criterion limits the operating range ofsuch complex as a thermometer to temperatures below225 K, with maximum relative sensitivity value of4.0% K�1 at 225 K. Future developments on this unique in-ner thermometer will be focused in the extension of therange of operation of the device as a thermometer to therange of operation of the device as organic light-emittingdiode.

Moreover, at room temperature, the presence of Tb3+-to-Eu3+ energy transfer was observed. As a relatively highemission quantum yield was found for the complex itselectrical properties were investigated. Thus a white or-ganic light-emitting diode was fabricated using the com-plex here prepared as a dopant in a 4,40-bis(carbazol-9-yl)biphenyl organic matrix. It was found that the device colordepends on the electroluminescence emission of the triva-lent lanthanide ions via an intramolecular energy transfer,from the ligand electrophosphorescence and from the NPB.The white emission is simple to obtain in our devices andcan be useful to design new types of organic electrolumi-nescent devices.

Acknowledgments

We are grateful to the Fundação para a Ciência e a Tecn-ologia (FCT), Fundo Europeu de Desenvolvimento Regional(FEDER) and Mais Centro – PORC, under contracts Pest-C/CTM/LA0011/2013 and CENTRO-07-ST24-FEDER-002032.The funding by Brazilian agencies CAPES, CNPq, FAPEMIGand inct-INAMI is also acknowledged. PPL (SFRH/BPD/34365/2006) and CDSB (SFRH/BPD/89003/2012) thankFCT for grants.

Appendix A. Supplementary material

Supplementary material contains Figs. S1–S4. CCDC925796 contains the supplementary crystallographic datafor this paper. These data can be obtained free of chargevia www.ccdc.cam.ac.uk/conts/retrieving.html.

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.orgel.2014.01.009.

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