Photo-and Electroluminescent behavior of Eu 3 + Ions in Blends with Poly ( vinyl-carbazole )

Foi sintetizado o complexo tris(tenoiltrifluoracetonato)(2,2’-bipiridina)európio(III), Eu(tta) 3 bipy, e suas características luminescentes foram avaliadas. A emissão de blendas deste complexo com poli(9-vinil carbazol) (PVK-Eu), variando a composição do Eu(tta) 3 bipy de 1% até 8% em massa (Eu(tta) 3 bipy/PVK, m/m), apresentou picos de fotoluminescência em 425 nm e 613 nm, com intensidades dependentes da composição da blenda. Dispositivos emissores de luz (LED’s) de dupla camada, com configuração ITO/PEDOT-PSS/blends/Ca-Al apresentaram emissão de luz vermelha com voltagem mínima de 9 V e pico máximo em 617 nm; eficiência na luminescência foi de 0.04 cd A. Medidas resolvidas no tempo indicaram que a microestrutura da matriz polimérica não afetou as propriedades eletro-ópticas das blendas.


Introduction
The synthesis of appropriate electroluminescent polymers and of small organic molecules for the construction of RGB displays (designation for red, green and blue) is still a challenge, 1 because full color displays require pure hues and the luminescent compounds typically present spectra with wide emission, due to vibronic progression. 2n the red part of electromagnetic spectra filtering is usually necessary to sharpen the emission, because the human eye has a lower sensitivity in this region.Therefore, is important to minimize the loss in emission efficiency brought about by the broadening due to vibronic or transfer processes. 3In this context organic complexes of the trivalent europium ion (Eu 3+ ) are strong candidates to application in red light emission devices due to their high fluorescence at 613 nm, with essentially monochromatic emission, which minimizes the need for filtering.These systems also provide an enhanced theoretical yield.As is well known, spin statistics dictate that the singlet states which fluoresce provide a 25% maximum yield, whereas in these complexes with organic ligands, the quantum yield could reach 100%.3][4] The electronic transitions of lanthanides result from different electronic distributions of 4f orbitals that are effectively shielded from external influence by the overlap of 5s 2 and 5p 6 orbitals. 2,3,5he interaction of Eu 3+ ions with organic molecules and derived complexes have been well studied.The behavior of the ion in relation to polymer chains has been addressed by some authors, [6][7][8][9][10] particularly when solution and solid state properties are involved.][13] In this contribution we present some results related to the photophysical behavior of molecular complexes of Eu 3+ in solution and film form in the presence of poly(9vinyl carbazole) (PVK).The electroluminescent properties of the blends are also discussed.

Equipment
The complexes and the polymer were characterized by FTIR spectrometry and the data compared with the literature.Samples for FTIR measurements were prepared as KBr disks, the spectra were taken in a Biorad Excalibur Series spectrophotometer, FTS 3500 GX, in the spectral range of 400-4000 cm -1 , with resolution of 2 cm -1 and 32 scans.Absorption spectra were obtained using a Shimadzu UV 2401 PC, in THF solution and in film form.Fluorescence spectra were obtained using a spectrophotometer Shimadzu RF 5301 PC, in THF solution and in film form.Thermogravimetric analyses (TGA) were made on a Netzsch TG209 cell system with a TGA 2950 thermogravimetric analyzer at a heating rate of 20 °C min -1 and under a nitrogen flow rate of 15 mL min -1 .Differential scanning calorimetry (DSC) was performed using the Netzsch equipment series 204 F1 with a heating rate of 10 o C min -1 , under dry nitrogen flux of 15 mL min -1 in the range 25-300 o C.

Film
Optical properties in solid state were studied in cast films from THF solutions onto quartz plates, in appropriate thickness to obey Beer's law.

OLED
A hole injection layer of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) (Bayer Corporation) was first spin-coated on top of the ITO (OFC Corporation, 20 ohms sq -1 ), then dried at 100 °C for 1 h under vacuum.Solutions of polymer (20 mg mL -1 in chloroform) were filtered through 0.2 1/4 m Millex-FGS Filters (Millipore Co.) and were then spin coated onto PEDOT:PSS layer under nitrogen and speed 2000 rpm.Calcium layer (thickness ca.400 nm) was evaporated onto the polymer at about 10 -7 Torr, followed by a protective coating of aluminum.The devices were characterized using a system described elsewhere. 14The final configuration of the LED was ITO/PEDOT:PSS/blend/Ca/Al.

Time-resolved luminescence measurements
Luminescence spectra and luminescence decays were recorded with a Thermo Laser Science.VSL-337ND-S (Oriel 79070) pulsed nitrogen laser (λ em = 337.1 nm; < 4 ns Full-Width at Half-Maximum, FWHM) for the photoexcitation of the sample; a 1/4 m Oriel Cornerstone 260 emission monochromator for the spectral resolution of the luminescence emission, coupled to an Oriel 74063 diffraction grating (0.15 nm max.resolution at 546.1 nm, 1200 lines mm -1 , blaze at 350 nm), an Oriel mod.59470 (Schott GG385) color glass filter to block the excitation light at the micrometer-driven entrance slit, and an Oriel 77348 fast (2.2 ns rise-time) side-on photomultiplier tube set to -600 V or -700 V at the micrometer-driven exit slit, for acquisition of the organic emission or europium emission, respectively.The entrance and exit slitwidth were set to 50 μm and 5 μm, respectively, to reach a 0.4 nm spectral resolution.A Tektronix TDS3032B (300 MHz, 2 channels, 2.5 GSamples s -1 ) Digital Oscilloscope terminated to 255 Ω or 10 kΩ, was used to detected the organic emission or europium emission, respectively, with 500 points for each decay curve.With these settings, the measured RC time constants of the instrumentation, for acquisition of the organic emission or europium emission, were 15.94 ns and 642.0 ns, respectively.

Reagents
All reagents were purchased from Aldrich, Acros, Synth or VETEC.All reactions were carried out with dry, freshly distilled solvents under anhydrous conditions or in an inert atmosphere.Tetrahydrofuran (THF): First, the THF was dried with CaH 2 and distilled.It was further dried and fractionally distilled from sodium under a nitrogen atmosphere. 6Ethanol (Synth, 99,5% PA ACS) was distilled over calcium hydride under a nitrogen atmosphere.

Results and Discussion
The materials involved in blend preparation are illustrated in Figure 1.To obtain red emission, an efficient energy transfer from the PVK to the europium salt is needed.From Figure 2 it can be seen that this condition is fulfilled, due to the good overlap between PVK emission and Eu(tta) 3 bipy complex absorption.
The FT-IR spectral region comprised between 1750-500 cm -1 of the blends is displayed in Figure 3.A progressive increase in the absorptions of the europium complex bands bettwen 1630 and 1550 cm -1 (ν C=O diketone, ν C=C aryl rings and ν C=N bipyridine ring), in 1411 cm -1 (ν C-O diketone), in 1320 cm -1 (ν C-N bipyridine ring), 1189 cm -1 (ν C-F tta ligand), and 792 cm -1 (ν C-S tta ligand) is observed, as expected, with increasing amounts of the complex in the blends.A sharp endotherm was present in the DSC thermogram of the europium complex at 221-223.o C (lit.224 o C).These results along with the 1 H NMR and TGA profile, atested that the desired complex structure was obtained, with the necessary purity.

Spectroscopy in solution
The photophysical study was performed in the UV-Vis region using THF solutions (0.1 mg blend mL -1 , THF).The absorption spectra of PVK, of Eu(tta) 3 bipy and of the blends in THF solution are shown in Figure 4. Apart from the intrinsic differences among the absorption profiles it can be assumed that no changes in the electronic ground state or aggregate formation have occurred.On the other hand, the fluorescence emission spectra collected with excitation at 344 nm revealed progressive changes as shown in Figure 5.
For pure PVK the emission is centered at 375 nm, and for pure Eu(tta) 3 bipy at 613 nm, and the results of Figure 5 show that in solution, all the blends present incomplete   energy transfer, since both emission peaks were present in the spectra relative to every component ratio.

Spectroscopy in the solid state
The absorption profiles of cast films did not show any difference from those taken in solution, with the absorption peaking at 344 nm, Figure 6.The emission spectra, on the other hand, had some new features, Figure 7.The polymeric matrix emission showed a redshift to 405 nm, attributed to excimer formation 18 and was not seen in concentrations of Eu 3+ higher than 2%.As compared to the solution behavior described above, a more efficient energy transfer has taken place in the solid state. 19ere is no conclusive explanation for the complete transfer in the solid state but not in solution.It has been suggest that the solvent could act as an alternative nonradiative channel for triplet state decay, involving a spinorbit coupling with solvent molecules lowering the efficiency of the transfer process. 20

Electroluminescence characterization of the PVK-Eu blends
The turn on voltage, the electroluminescent peak and the device efficiency for each blend composition are presented in Table 1 and the corresponding EL spectra are depicted in Figure 8. Two findings are relevant here: the small redshift seen for the most concentrated blends, and the complete absence of PVK    Emission spectra of PVK and PVK-Eu blends in THF solutions (0.1 mg blend mL -1 THF) and Eu(tta) 3 bipy (10 -4 mol L -1 ) with λ exc in 344 nm: B 0 stands for pure PVK, B 1 , B 2 , B 4 and B 8 for blends containing 1%, 2%, 4% and 8% of Eu(tta) 3 bipy respectively.emission, noting however that in the film photoluminescence a subtle curvature could be seen at the PVK emission.These observations can be attributed to the different excitation mechanisms.In EL the ion can be directly excited, whereas in PL the emission depends of energy transfer from the ligand.In any case, the transfer from PVK is complete in EL emission; the most relevant feature of these devices. 3,,21,22The redsifted emission of the pure PVK (B 0 in Figure 8) was attributed to the luminescence of aggregate species as ground state and/or excimers as noted before in the literature. 23The occurrence of related forms deriving from the interaction of the complex with the matrix could not be detected, and needs more detailed work to be clarified.Devices built with the complex alone were unstable and it was not possible to get any information about their behavior with the LED configuration used.
The relationships between current vs. EL intensity and vs. voltage are shown in Figure 9.The turn on voltages are in the 7-9 V range (Table 1), and the highest emission intensity was observed for the B 8 sample (8% m/m Eu 3+ ) as found for the film photoluminescence, peaking at 617 nm, with a brightness of 66 cd m -2 at 17 V at a current density of 250 mA cm -2 .It is noteworthy that the blend containing 1% Eu 3+ (B 1 ) presented a more intense EL emission than the blends with 2 and 4% (B 2 and B 4 ).A plausible explanation for this effect is that in B 1 , the luminescence comes from both PVK and from the complex, whereas in the other blends, only the ion is the emitting species.Beyond that point, the intensity increases with the amount of the complex in the blends.

Excited state lifetime measurements and time-resolved photoluminescence spectra of the blends PVK-Eu
The determination of the excited state lifetimes (τ) of the PVK-Eu blends was performed to investigate if the polymeric matrix interfered with the optical properties of the ion.In Figure 10 the fluorescence emission spectra of the blends in the region 570-640 nm are shown, together with the corresponding decay curves, and that of the pure Eu 3+ complex as well.The time-resolved emission spectra of the Eu(tta) 3 bipy complex showed the highest intensity in the range 608-630 nm, which corresponds to the 5 D 0 → 7 F 2 electric dipole transition.A 5 D 0 → 7 F 2 transition with higher intensity than the 5 D 0 → 7 F 1 magnetic dipole could be due to the absence of a center of symmetry in the complex, 24 and also to the enhancement of the 5 D 0 → 7 F 2 hypersensitive transition by means of covalency of the ion and ligands.This was observed in the case of hydrated β-diketonate Eu(tta) 3 (OH 2 ) 2 complex, ,25,26 where the Judd-Ofelt Ω 2 intensity parameter indicated that the Eu 3+ ion are embedded in highly polarizable chemical environment.Higher values for Ω 2 intensity parameter have been reported when the water molecules in such complex are replaced by another ligands, and this has been ascribed to the enhancement of local asymmetry around the vicinity of europium. 27owever, for the Eu(tta) 3 .nLcomplexes dispersed in poly(methylmethacrylate) (PMMA), the Ω 2 values are nearly the same when the synergetic L ligands are substituted by others.The authors, therefore, assume that the chemical microenvironment is mainly controlled by the three tta -ligands when Eu 3+ ions are chelated with synergetic ligands in PMMA host.In our case, the relative intensities of the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 transitions are kept nearly the same with the dispersion of the Eu(tta) 3 bipy complex into PVK matrix, indicating that the Eu 3+ ions are experiencing similar microenvironments in PVK.
The lifetimes extracted from the decay curves are shown in Table 2.It was observed that τ increases with the Eu 3+ content in the blend, which can be attributed to an energy migration process among the Eu 3+ complexes.

Conclusions
The complex tris(thenoyltrifluoracetonate)(2,2'bipyridine)europium(III), Eu(tta) 3 bipy was prepared, showing PL at 613 nm.The emission of its blends with poly(9-vinyl carbazole), (PVK-Eu) varying in composition from 1 to 8% (Eu(tta) 3 bipy/PVK, m/m) showed PL at 613 nm and at 425 nm, with intensities which depended on the blend composition.The two most concentrated blends showed no blue emission, because of complete energy transfer from PVK to Eu 3+ .The EL emission originated only by from the Eu 3+ dopant at all blend compositions.Time resolved measurements indicate that the microstructure of the matrix had practically no effect on the electro-optical properties of the blends.

Figure 1 .
Figure 1. Materials used in the blends.

Table 1 .
EL λ max , turn on voltage and luminance for PVK-Eu blends