Synthetic Metals 156 (2006) 13–20

Synthesis and characterization of new electroluminescentmolecules containing carbazole and oxadiazole units

Valeria Bugattia, Simona Concilioa, Pio Iannellia,∗, Stefano P. Piottoa,Salvatore Belloneb, Manuela Ferrarab, Heinrich C. Neitzertb,

Alfredo Rubinob, Dario Della Salac, Paolo Vaccac

a Dipartimento di Ingegneria Chimica ed Alimentare, Universita di Salerno, via Ponte Don Melillo, I-84084 Fisciano (Salerno), Italyb Dipartimento di Ingegneria Elettronica, Universita degli Studi di Salerno, via Ponte Don Melillo, I-84084 Fisciano (Salerno), Italy,

c ENEA C.R. Portici, v. Vecchio Macello-loc. Granatello, 80055 Portici (Na), Italy

Received 12 April 2005; received in revised form 7 July 2005; accepted 19 July 2005Available online 18 November 2005

Abstract

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The synthesis of new molecules containing both electron and hole transporter units is reported. This class of compounds, nameOC, maye used for assembling electroluminescent devices made by a single organic layer. The active moieties are the carbazole, as the honit, and the oxadiazole, as the electron transporter unit. The chemical formulation and the complex geometry of the molecular frameolubility in chlorinate solvents and the preparation of homogeneous films by spinning technique.Photoluminescence of molecules, both in solution and in film, occurs in the blue region of visible spectra, the exact peak position o

epending on the pendants attached to the oxadiazole unit. The electroluminescence occurs in a higher wavelength region, with amission. The electroluminescent devices consist in the simple sequence ITO–OC–Al and ITO–PEDOT–OC–Al.2005 Elsevier B.V. All rights reserved.

eywords: Electroluminescent molecules; Carbazole; Oxadiazole; Single layer oled

. Introduction

In the last years a number of new low molecular massompounds and polymeric materials have been synthesizednd analyzed for applications in the field of organic light-mitting devices (OLEDs)[1–12]. For fabricating a LED it

s usually required the presence of very thin layers of bothole-transporting and electron-transporting molecules. The two

ayers are confined between a hole-injection electrode and aow work function electron-injection metal contact, respectively.lectroluminescence is induced by the recombination of holesnd electrons, when the resulting diode is biased in forwardirection.

The use of a single layer in the preparation of OLEDs isttractive for the easier technology approach, the less expensivepplication of only a single thin layer and the suppression of the

∗ Corresponding author. Fax: +39 089 964 057.E-mail addresses: piannelli@unisa.it, Iannelli@dica.unisa.it (P. Iannelli).

interface between the two-layers[13–17]. Multi-layer devicesare difficult to produce by spin-coating of the organic molecubecause the solvent used for the spinning of a second layegenerally dissolve also the previously deposited layers. Ositely, single layer has been considered less promising folower luminous efficacy because the balance between holelectron current is more difficult to control.

To prepare a single layer system it is necessary to incboth the active moieties, hole and electron transporting, ilayer composition. This may be obtained by mixing thecomponents, but problems may occur due to physical sepaand crystallization of components. A way to ensure high stabof composition is to chemically bond the two moieties, timposing a chemical constraint, which avoids any segregand suppresses crystallization. Moreover, the chemical linmay be chosen in order to tailor solubility, morphology and oproperties of the active molecules.

We have recently shown that new oxadiazole-contaicompounds are blue-photoluminescent materials[18,19], inagreement with that reported by several authors for sim

379-6779/$ – see front matter © 2005 Elsevier B.V. All rights reserved.

oi:10.1016/j.synthmet.2005.07.341

14 V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20

molecules[20–32]. We observed that the anisotropic shape ofconjugated unit promotes the appearing of high ordered smecticphases and, only in few cases, of the nematic phase. The liquidcrystalline (LC) order is particularly interesting when opticalactive systems are required to be anisotropic at the macroscopiclevel.

In this paper, we report on the synthesis and the characteriza-tion of a new class of photo- and electroluminescent moleculeshaving formula:

The insertion of the flexible aliphatic segments along andside attached to the aromatic molecular frame moderates melt-ing temperature and promotes solubility in common organicsolvents.

Electroluminescence and optical properties of OLEDs madeb andd

2

2

richad de,d .O .A yle uslyr

2

s utan( ofa EA,0 s let

tera Theo l of

water, dried over sodium sulfate and the solvent was removed atreduced pressure. The row amber-yellow product was purifiedby treatment in boilingn-heptane (200 ml), to remove the excessof 1,4-dibromobutane. The white solid product collected at roomtemperature was then crystallized form ethanol/water. TheN-(4-bromobutyl)carbazole was obtained as needle crystalline solid,with a 50% yield.Tm = 103.33◦C; �Hm = 75.231 J/g.

1H NMR (CDCl3): δ (ppm) = 8.15 (d, 2H), 7.46 (m, 4H),7.29 (m, 2H), 4.33 (t, 2H), 3.40 (t, 2H), 2.05 (m, 2H), 1.97(m, 2H).

2.1.2. Synthesis of N-(2-chloroethyl)carbazole

Following the procedure described in reference[36], in a100 ml round bottom flask, to a solution of carbazole (0.059 mol)in 1,2-dichloroethane (1.256 mol), TBEA (0.0020 mol), andNaOH 16 M (35 ml) were subsequently added. The mixturewas let to react for 12 h, at 60◦C. The reaction mixture wasdried under vacuum at 80◦C, and a solid was obtained. Thisraw product was purified by crystallization from ethanol, toeliminate the insoluble salt, and a second crystallization fromn eu shapec�

8(

2at

ol)w wasac dedd 0f ouss ctw rome s witha

( H),7 (t,2

ioni

y usingOC as single layer component is also reportediscussed.

. Experimental

.1. Materials

All reagents and solvents were purchased from Aldnd Carlo Erba.N,N-dimethylacetamide (DMAc) andN,N-imethylformamide (DMF) were refluxed on calcium hydriistilled in vacuum and stored on 4A molecular sievesther reagents were used without further purificationn-lkoxybenzylchloride,n-alkoxyterephthalic acid and its methster derivative were synthesized according to a previoeported procedure[33,34].

.1.1. Synthesis of N-(4-bromobutyl)carbazole

Following the procedure described in reference[35], auspension of carbazole (0.0870 mol) and 1,4-dibromob0.261 mol) in 60 ml of toluene was added to 44 mlqueous solution of benzyltriethylamonium bromide (TB.00362 mol) in 50% aqueous NaOH, and the reaction wa

o take place for 20 h, at room temperature.The final mixture was poured into 600 ml of distilled wa

nd the product extracted twice with 150 ml of chloroform.rganic layer was subsequently washed twice with 300 m

e

d

-heptane, to isolate theN-(2-chloroethyl)carbazole from thnreacted carbazole. The finale product was a needle-rystalline solid, obtained with a 30% yield.Tm = 128.3◦C,Hm = 114.3 J/g.1H NMR (CDCl3): δ (ppm) = 8.12 (d, 2H), 7.45 (m, 4H), 7.2

m, 2H), 4.66 (t, 2H), 3.86 (t, 2H).

.1.3. Synthesis of 2-(4-carbazol-9-yl-butoxy)-terephthaliccid dimethyl ester (1) and 2-(2-carbazol-9-yl-ethoxy)-erephthalic acid dimethyl ester (2)

2-Hydroxy-terephthalic acid dimethyl ester (0.0300 mas dissolved in 250 ml of DMF; potassium carbonatedded and a solution ofN-(4-bromobutyl)carbazole orN-(2-hloroethyl)carbazole (0.0360 mol) in DMF (50 ml) was adrop wise to the first solution. The mixture was stirred at 15◦C

or 16 h and then filtered into water (1200 ml). The aqueolution was placed at 4◦C for 4 h and the solid crude produas filtered under vacuum and purified by crystallization fthanol/water. The ester was colleted as pale yellow needlepproximately 70% final yield.

(1) Tm = 112.3◦C; �Hm = 55.2 J/g;1H NMR (CDCl3): δ

ppm) = 8.09 (d, 2H), 7.74 (d, 1H), 7.68 (s, 1H), 7.62 (d, 1.56 (m, 2H), 7.46 (m, 2H), 7.24 (m, 2H), 4.44 (t, 2H), 4.09H), 3.94 (s, 3H), 3.77 (s, 3H), 2.15 (m, 2H), 1.92 (m, 2H).

(2) Yellow oil; the compound was used without purificatn the following synthesis of the dihydrazide derivative.

V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20 15

2.1.4. Synthesis of 2-(4-carbazol-9-yl-butoxy)-terephthalicacid dihydrazide (3) and 2-(2-carbazol-9-yl-ethyl)-terephthalic acid dihydrazide (4)

Compound1 or 2 (3.00 g) was dissolved in 100 ml of abso-lute ethanol and 100 ml of hydrazine monohydrate were added.The solution was stirred at reflux for 2 h, then cooled downin an ice bath. The precipitated solid was collected, repeatedlywashed with boiling water, and purified by crystallization fromdioxane/water. The pure dihydrazide was obtained with approx-imately 60% yield.

(3) Tm = 231.3◦C, �Hm = 121.1 J/g;1H NMR (DMSO-d6):δ (ppm) = 9.84 (s, 1H), 9.17 (s, 1H), 8.13 (d, 2H), 7.64 (d, 1H),7.60 (s, 1H), 7.47 (d, 1H), 7.46 (m, 4H), 7.18 (m, 2H), 4.49 (m,4H), 4.47 (t, 2H), 4.14 (t, 2H), 1.94 (m, 2H), 1.81 (m, 2H).

(4) Tm = 235.4◦C, �Hm = 98.5J/g;1H NMR (DMSO-d6): δ

(ppm) = 9.78 (s, 1H), 9.20 (s, 1H), 8.11 (d, 2H), 7.68 (d, 1H),7.66 (s, 1H), 7.53 (d, 1H), 7.46 (m, 4H), 7.16 (m, 2H), 4.85 (t,2H), 4.46 (t, 2H), 4.44 (m, 4H).

2.1.5. General procedure for the syntheses of compoundsO• )

ectiontionthe

withculagree

,.65

H),

F dd

4.38 (t, 2H), 4.04 (t, 4H), 2.01 (m, 4H), 1.85 (t, 4H), 1.42 (m,12H), 0.88 (t, 6H).

• Step 2. The freshly synthesized azidePOC(n,m) (1.0 g) waspoured in 80 ml of phosphorous oxychloride (POCl3); thereaction was conducted at refluxing temperature for 5 h. Uponthe removal of the POCl3 under vacuum, 200 ml of aqueousNaOH 0.1 M were added and the crude pale blue residue waspurified by crystallization from DMF/water to obtain whitecrystals. The proton resonance data are in agreement withthe expected values. For example, forOC(6,4) (seeFig. 1) –1H NMR (DMSO):δ (ppm) = 8.14 (d, 1H), 8.08 (t, 4H), 7.88(d, 2H), 7.84 (m, 2H), 7.56 (d, 2H), 7.33 (t, 2H), 7.13 (m,4H), 7.05 (d, 2H), 4.49 (t, 2H), 4.34 (t, 2H), 4.05 (m, 4H),2.08 (m, 2H), 1.93 (m, 2H), 1.73 (t, 4H), 1.31 (m, 12H), 0.88(t, 6H).

2.1.6. Synthesis of compound O(6,4)The compound ofO(6,4), analogous toOC(6,4) but without

the carbazole pendant group, having formula

was synthesized following the procedure described in refer-ence[18].

2

erkinE

tru-m

am-e CuK ujiB forr

00S esid-u

lmerL werer

2

ates( uctivea e for-m , theI Thee aterf opyla thest

C(n,m) (Scheme 1)Step 1. To a suspension of compound3 or 4 (0.00198 molin dry DMAc (30 ml) at room temperature, the appropriatn-alkoxybenzylchloride (0.00435) was added and the reawas let to take place overnight under stirring. The reacmixture was then poured into cold water (350 ml) andresulting solid product was filtered and washed twicewater. The product was used in the subsequent intramolering closure reaction. The proton resonance data are in ament with the expected values. For example, forPOC(6,4)(seeFig. 1) – 1H NMR (DMSO): δ (ppm) = 10.58 (m, 2H)10.40 (s, 1H), 10.23 (s, 1H), 8.14 (d, 2H), 7.92 (m, 5H), 7(m, 4H), 7.36 (t, 2H), 7.15 (t, 2H), 7.04 (m, 4H), 4.50 (t, 2

ig. 1. 1H NMR spectra ofPOC(6,4) and OC(6,4) dissolved in deuterateimethylsulfoxide.

r-

.2. Characterization

Thermal measurements were performed by a DSC-7 Plmer calorimeter under nitrogen flow at 10◦C/min rate.Thermogravimetric analysis was performed with a TA Ins

ents SDT 2960 apparatus, in air at 20◦C/min.X-ray diffraction spectra were recorded using a flat c

ra with a sample-to-film distance of 90.0 mm (Ni-filtered� radiation). The Fujifilm MS 2025 imaging plate and a Fio-imaging Analyzer System, mod. BAS-1800, were used

ecording and digitizing the diffraction patterns.1H NMR spectra were recorded with a Brucker DRX/4

pectrometer. Chemical shifts are reported relative to the ral solvent peak.

UV–vis measurements were performed by a Perkin-Eambda 800 Spectrophotometer and photoluminescenceecorded by a Jasco FP-750 Spectrofluorometer.

.3. Devices preparation

Commercial indium tin oxide (ITO) (200 nm) glass substrDelta Technologies) have been used as transparent condnode. In order to create contact areas and to prevent thation of shorts during the top electrode contact soldering

TO layers were patterned by standard photolithography.tched ITO layers were treated with surfactant in boiling w

or 2 h in an ultrasonic bath and they were rinsed with isoprlcohol for 5 min and dried under a nitrogen flow. After that,ubstrates were annealed for 2 h at 200◦C in order to optimizehe electrical and optical properties.

16 V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20

Scheme 1.

Electronic grade PEDOT:PSS, 2.8 wt.% dispersion in water(from Aldrich), was filtered through a 0.40�m membrane andspin-coated at 6000 rpm for 10 s. The film was then allowedto dry by hot plate annealing at 120◦C for 15 min. A 25 g/lsolution ofOC(6,4) in chlorobenzene was filtered (0.2�m) andspin-coated at 1500 rpm for 30 s onto the processed PEDOT:PSSlayer. TheOC(6,4) layer was allowed to dry at 60◦C for 10 h invacuum. The spin-coating process has been developed using aBrewer Science Limited Model 100 Spin-coater.

To complete the device structure, Al contacts (200 nm) weredeposited onto the organic layers by vacuum evaporation atlow pressure (10–8 mbar) by a high vacuum chamber manu-factured by Elettrorava S.p.A. (Italy). The deposition rate was1 nm s−1 and a mask was used with a shape adapted to thedesired devices. The single device has circular shape, 3 mm indiameter.

Thus, the structure of the investigated OLEDs consists of a40 nm thick PEDOT layer spin-coated on top of the ITO cov-ered glass substrate, a 60 nm thick active oxadiazole/carbazoleemitting layer, and top 200 nm thick aluminum contacts of 3 mmdiameter. Each sample consists of 20 devices placed on two par-allel rows.

2.4. Devices characterization

For electroluminescence measurements the devices havebeen forward biased applying increasingly negative voltagesto the top aluminum electrodes with respect to the bottomITO contact, using a Keithley Model “2400” source meter,and the OLED diode current has been measured with thesame instrument. The measuring system for the luminescenceintensity consists of a mechanical chopper working at 230 Hz,

V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20 17

Table 1Thermodynamic data of virgin samples ofPOC(n,m)

POC(n,m) Tm (◦C) �Hm (J/g) Tc (◦C) �Hc (J/g) Td (◦C)

POC(0,2) 267.3 33.16 227.3a 12.41a 308.3POC(0,4) 258.0 51.37 218.3 60.05 294.4POC(4,6) 245.4 58.76 206.3 74.30 302.8POC(6,4) 229.0 66.12 180.6 59.84 327.8

Tm/�Hm = melting temperature/enthalpy, second heating run;Tc/�Hc =crystallization temperature/enthalpy, cooling run;Td = 5% weight loss temper-ature.

a Crystallization taking place during heating run.

a United Detector Technology “PIN10” large area siliconphotodiode and a Stanford Research “SR830” Digital Lock-inAmplifier. The use of the Lock-in measurement system permitsto obtain noise free measurements even for very low lightintensities and has been used to follow the degradation kineticsof the OLED, as will be reported in detail elsewhere.

The optical emission spectrum of the organic LED has beenmeasured using a fiber coupled “PC2000” UV–vis spectrometerfrom Ocean Optics.

For absolute measurements of the luminous efficacy an inte-grating sphere with a Hamamatsu “S2386–44 K” silicon pho-todiode has been used. A Hewlett-Packard HLMP-CB15 blueInGaN LED with a peak wavelength of 473 nm has been usedfor calibration.

3. Results and discussion

3.1. Calorimetric characterization and X-ray diffractionanalysis

Thermodynamic properties ofPOC and OC are given inTables 1 and 2, respectively.POC and OC as obtained fromthe synthesis are crystalline materials, with melting temperaturedepending on the length of aliphatic segments attached to theoxadiazole unit and on the length of aliphatic linkage betweenthe carbazole unit and the oxadiazole moiety. As expected,s turesw s,p e.

red,o y bea esti-g iazole

TT

O

OOOO

Tc r-a

Fig. 2. X-ray diffraction pattern recorded at room temperature on a powdersample ofOC(6,4), as obtained from the synthesis.

segment promotes the intermolecular packing, whilst the inser-tion of carbazole unit destabilizes the crystalline order. Theresult is an amorphous phase or a mesophase stable at roomtemperature.

The X-ray diffraction pattern of virgin samples, recordedat room temperature, shows one-two strong reflections at lowangle diffraction region (seeFig. 2). These reflections seem tobe not correlated, and a trend of the series cannot be extrapolated(Table 3).

In the thermogravimetric trace ofPOC, a shoulder isobserved starting at a temperature of about 300◦C, which corre-sponds to the loss of water related to the intramolecular cycliza-tion process (Table 1). OC are stable at temperatures lowerthan about 400◦C, while a degradation process occurs at about400–450◦C (Table 2).

3.2. Optical and photoluminescence characterization

As already pointed out, the use of oxadiazole moiety as activeunit for the synthesis of new materials, suitable for the fabrica-tion of OLED, has been widely investigated. The main reason isthe possibility to extend the electron-transport character of thismolecular frame to new materials and to set emission in the blueregion of the visible spectra. Similarly, the carbazole unit hasbeen widely used in the synthesis of hole-transporter materials.

Td

O

OOO

O

horter aliphatic segments bring to higher melting temperaith the exception ofOC(0,2) that is completely amorphourobably due to the lower mobility of the rigid molecular fram

After melt, in the cooling run, crystalline phase is not restor a large undercooling process is observed. This maccounted for the complex molecular geometry of the invated molecules: the flat shape of the conjugated bis-oxad

able 2hermodynamic data of virgin samples ofCO(n,m)

C(n,m) Tm (◦C) �Hm (J/g) Tc (◦C) �Hc (J/g) Td (◦C)

C(0,2) 273a – – – 345.8C(0,4) 207.0 32.68 136.3 16.74 372.2C(4,6) 185.0 66.24 128.6 47.79 375.0C(6,4) 175.7 40.91 114.0 39.64 390.9

m/�Hm = melting temperature/enthalpy, second heating run;Tc/�Hc =rystallization temperature/enthalpy, cooling run;Td = 5% weight loss tempeture.a Poorly defined isotropization range.

able 3-spacing of the strong low angle reflections forOC(n,m)

C(n,m) d (A)

C(0,2) n.o.C(0,4) 13.92C(4,6) 10.35

30.83C(6,4) 16.64

25.40

18 V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20

For instance, poly(vinyl-carbazole) (PVK) is a commerciallyavailable polymer and it has been used for the preparation oftwo- or multi-layers OLEDs.

To succeed in the preparation of OLEDs and to ensure goodelectroluminescence, it is important to balance the hole and theelectron injection. This may be obtained by either separate thetwo active components, the hole and the electron transporters,into two layers to be placed one on the top of the other in atight configuration, or placing them mixed together into a sin-gle layer. In the former case the occurring of the electron–holerecombination at the interface of the two layers may increasethe luminous efficacy, but the device preparation is more com-plicated and the probability to have defects at the interface isincreased. In the latter case, the structure of OLEDs is simplerand more attractive by the industrial point of view.

In the case of single-layer devices, the active hole and electrontransporter molecules should be mixed at molecular level andseparation, which may occur during the use, must be avoided:this may cause the loose of electroluminescent performances andthe device degradation. In our case, the oxadiazole and the car-bazole units are linked by chemical bonds to avoid the separationof the active units and to ensure a constant and homogeneouscomposition of the layer, at the molecular level. This is similarto that made by several authors for analogous systems, wheredifferent oxadiazole and carbazole moieties have been used asmonomers for the synthesis of side chain copolymers[37].

ofO ofO ate-r trongp ronga lengthr iazoleu enceo

am-p e ath

F inc

Fig. 4. Photoluminescence ofOC(6,4), O(6,4) andN-(2-chloroethyl)carbazole,irradiated at 330 nm in chloroform solution, are compared.

Fig. 5. Photoluminescence ofOC(6,4) samples, irradiated at 330 nm, are com-pared: (a) chloroform solution and (b) film sample.

3.3. Electroluminescence

Comparing the electroluminescence of devices with andwithout insertion of the PEDOT layer (Fig. 6), we observe, asexpected, a reduction of the threshold voltage for light emissionfrom approximately 12 to 10 V, as well as a stabilization of thecurrent and optical power, with insertion of the PEDOT layer.

Fig. 6. I–V andIPH–V traces of a ITO/PEDOT/OC(6,4)/Al device.

In Figs. 3 and 4the UV–vis and photoluminescenceC(6,4) in chloroform solution are compared with those(6,4) andN-(2-chloroethyl)carbazole, taken as reference m

ials. The carbazole moiety absorbs at lower wavelength (seak at 292 nm) with respect to the oxadiazole unit (two stnd broad peaks at 322 and 344 nm) and emits in the waveange (two strong peaks at 354 and 369 nm) where the oxadnit strongly absorbs. The result is that the photoluminescf OC(6,4) is identical to that ofO(6,4).

In Fig. 5the photoluminescence of a solution and a film sle of OC(6,4) are compared: a shift of photoluminescencigher wavelength is evident in the case of film sample.

ig. 3. UV–vis spectra ofOC(6,4), O(6,4) andN-(2-chloroethyl)carbazole,hloroform solution, are compared.

V. Bugatti et al. / Synthetic Metals 156 (2006) 13–20 19

Fig. 7. Photoluminescence of aOC(6,4) film, irradiated at 330 nm, and elec-troluminescence of a ITO/PEDOT/OC(6,4)/Al device. The image of a lighteddevice is shown.

The current–voltage (I–V) and optical power–voltage (L–V)characteristics of the device incorporating the PEDOT layer,are shown inFig. 6. We find an almost linear relation betweenOLED current and emitted optical power with a threshold volt-age slightly above 10 V and no light saturation within the appliedvoltage range. The observed threshold voltage for light emissionis still rather high, but it should be mentioned that so far no opti-mization of the balance between electron and hole injection habeen done. In particular, it is known that the aluminum top con-tact is a poor electron injector. In order to improve electron injec-tion, the insertion of a thin intermediate LiF layer is under way.

The EL emission spectrum, compared to the photoluminescence one inFig. 7, shows two peaks centered at 444 and 486 nmThe resulting color is blue. The slight red shift of the emis-sion peaks, respect to the photoluminescence, may be explainewith some degree of exciplex formation between oxadiazoleand carbazole units according to analogous reports[38–42]. Theluminous efficacy evaluated according to the experimental section is of 0.06 lm W−1.

In order to better understand the role played by carbazoleunit in setting the electroluminescence performance, particularlyregarding to its hole transporter character, we also prepared thITO/PEDOT/O(6,4)/Al device, without carbazole moiety. Thedevice fabricated with compoundO(6,4) instead ofOC(6,4)does not show EL activity. The current–voltage characteristicsof this device was very unstable and the data are not reported itm ith as layeb perf n thea

4

ec-u zole

unit [18,19,40,41]and to several reports on similar systems[23–27,29,30], OC are blue photoluminescent material. Takingadvantage of the steric constraints due to the side insertion ofcarbazol unit to the oxadiazole moiety,OC are very soluble inchlorinate organic solvents and they can be easily processed tohomogenous films.

Thin films obtained by spinning technique show strong pho-toemission in the blue region of the visible spectrum. OLEDsprepared by a single layer ofOC compounds are electrolumi-nescent in the blue region. All these features makeOC suitableto be used as active material in the fabrication of OLEDs withsingle layer structure.

The study of synthesis of polymeric materials containingOC moiety and the realization of analogous electroluminescentdevices are under scrutiny.

Acknowledgement

Support by FIRB “Micropolys” Project financed by the Min-istero dell’Istruzione, dell’Universita e della Ricerca (MIUR) isgratefully acknowledged.

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es,et.

rsson,

urn,

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01.nce

l,

.[[[ 98)

[[[[ o,

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[ ato,

[ H.15.

[ F.n. 21

[ nth.

[

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. Conclusions

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