Oxadiazoles and phenylquinoxalines as electron transport materials

ELSEVIER

Synthetic

Metals

91 ( 1997) 223-225

Oxadiazoles and phenylquinoxalines

as electron transport materials

J. Bettenhausen, M. Greczmiel, M. Jandke, P. Strohriegl *

Abstract

The synthesis of a numberof star-shaped oxadiazolesandphenylquinoxalines isdescribed.Theselow molarmasscompounds areableto form stableglasses with glass-transition temperatures up to 248“C. Thematerialshavebeentestedaselectrontransportlayersin light-emitting devices( LEDs) togetherwith poly( 1.4-phenylene vinylene) (PPV). The two-layerLEDsshowanimprovedquantumyield andbrightness comparedto PPV-monolayerLEDs. 0 1997ElsevierScienceS.A. Kepords:

Organic

glasses; Oxadiazole;

Phenylquinoxaline:

Light-emitting

1. Introduction To date, limited attention hasbeenpaid to low molecular weight organic compoundsthat are ableto form stableglasses [ 11. Therefore, only a smallnumber of suchmoleculeshave yet been described in the literature. Recently, this classof materials has attracted interest for applications in organic light-emitting devices (LEDs). One typical example is 4,4’,4”-tri( N-carbazolyl)-triphenylamine (TCTA) [ 21, a starburst molecule with a glass-transitiontemperature (T,) at 1.51“C. TCTA andsimilar materialshave beensuccessfully applied as hole transport layers ( HTLs) in organic LEDs [3,41. This gave us a strong motivation for the synthesisof starshapedelectron transport materials. One interesting classof suchmaterialsare oxadiazoles.Within the last years, various low molar massoxadiazole derivatives like 2-biphenyl-5-( 3tert-butylphenyl)- 1,3.4-oxadiazole (PBD) have been used as electron transport and emissionlayers in organic electroluminescence (EL) devices [5,6]. Thin films of these compounds are usually prepared by vacuum evaporation. Although these films are amorphous. they have a strong tendency to recrystallize. To overcome thesedifficulties the oxadiazoles are sometimes embeddedin a polymer matrix [7 1. Nevertheless,the concentration of active moleculesin the polymer host is limited due to the stability of the guestmolecule. Another alternative are polymers in which the oxadiazole units are covalently fixed in the main chain or as sidegroups [8,9]. The main disadvantage of polymers is the lack of * Corresponding

author.

0379-6779/97/$17.00 PllSO379-6779(97)01018-6

0 1997 Elsrvier

Science S.A. All rights reserved

devices

efficient purification proceduresresulting in structuraldefects and ill-defined end groups. In contrast to polymers, low molar massorganic glasses are well-defined materialswhich can be purified by standard methodsof organic chemistry like sublimation or chromatography. Therefore, starburstcompoundsanddendrimerswith their uniform molecular structure seemto be promisingcandidatesfor usein organic LEDs. In two preceding paperswe have describedthe synthesis and characterization of a large number of starburstoxadiazoles as well as a dendrimer of the first generation [ lo:1 11. Recently, it came to our knowledge that similar compounds have been preparedby the Toyota researchgroup [ 12,131. Here, we report on the synthesisof another dendrimer and the application of the star-shapedcompounds as electron transport layers (ETLs) in EL devices, together with poly( 1,4-phenylene vinylene) (PPV) Recently, we have extended our synthetic efforts to glass-forming phenylquinoxalines which will be presentedat the end of this paper.

2. Synthesis and characterization The usualsynthetic route to oxadiazolesinvolves the dehydration of bisacylhydrazides with dehydrating agents like phosphorousoxychloride. Unfortunately this reaction led to the formation of a large numberof byproducts and, hence,to a difficult purification in the caseof starburst oxadiazoles. Recently, we have developeda different andefficient synthesisof starburstoxadiazole compounds[ 10,111.The key step is the reaction of acyl chlorides with substituted tetrazoles which leadsto oxadiazoles via intramolecular ring transfor-

J. Bettenhmsen

224

Scheme 1. Synthesis

of the dendrimer

et a/. /Sythetic

6.

mation. The aromatic tetrazoles used as starting materials are easily obtained by the reaction of the corresponding nitriles with sodium azide and ammonium chloride. The synthetic pathway described above provides an easy access to starshaped derivatives and dendrimers with 1,3,4-oxadiazole units. In the following chapter the synthesis of a new dendrimer of the first generation is described. As shown in Scheme 1, the novel compound is prepared in three steps starting from 5-cyanoisophthaloyl chloride. Reaction of the acyl chloride 1 with the tetrazole 2 leads to 3. By reaction with sodium azide and ammonium chloride the latter is transferred into the corresponding tetrazole 4. The

Metals

91 (1997)

223-228

last step is the reaction of 4 with 1,3,5-tris( 4-phenylcarboxylic acid chloride) benzene 5 to the novel dendrimer 6. The acyl chloride 5 used as core is obtained by trimerization of 4-bromo acetophenone with trifluoro-methyl sulfonic acid, followed by the reaction with butyl lithium/carbon dioxide and subsequent reaction with thionyl chloride [ 14,151. The thermal behaviour of 6 was investigated by differential scanning calorimetry (DSC) and thermogravimetric (TGA) measurements. The experiments show that the dendrimer is able to form a stable glass. Although 6 showed a melting point at 388 “C during the first DSC heating, no recrystallization takes place upon cooling (20 K/min) . In further heating cycles the glass transition is observed at 248 “C which is even 25 “C higher than the TZ of a comparable dendrimer with a small benzene core [ 111. Besides the very high glass transition 6 exhibits excellent film-forming properties and high thermal stability. The onset of decomposition is detected at 390 “C in a thermogravimetric experiment (nitrogen, 10 K/min). The absorbance maximum of the novel dendrimer 6 in the solid state is at 313 nm. In addition, 6 exhibits strong violet fluorescence in the solid state with a maximum at 402 nm. The synthesis of a novel series of starburst oxadiazoles with an additional oxadiazole sidearm is pictured in Scheme 2. It starts with the reaction of 5-bromo isophthaloyl chloride and the corresponding aromatic tetrazoles which leads to the oxadiazole derivatives 9a-c. The next step is an intermolecular coupling of the aryl halides using bis( 1,5cycloocatadiene)nickel (Ni( COD) 2) to obtain the X-shaped oxadiazole compounds 1Oa-c with a biphenyl core. The thermal behaviour of lOa-c was determined by DSC and TGA. Compounds 10a with the tert-butyl and 10~ with CF, substituents show similar behaviour in the DSC experiments. In the first heating cycle (20 K/min) a melting point is detected. In the following cooling cycle no recrystallization is observed. In the second heating a glass transition is detected at 186 “C for compound 10a and at 144 “C for lOc, and on further heating recrystallization and melting occur. Compound lob with the naphthyl groups shows a melting point at 372 “C in the first heating cycle. In all subsequent cooling and heating curves no recrystallization or melting is detected anymore. Upon heating, a T,: = 152“C is found. The thermal data of all X-shaped oxadiazole derivatives are summarized in Table 1.

3. Polyphenylquinoxalines and trisphenylquinoxalines Recently, we have extendedour synthetic efforts to phenylquinoxalines. In polymer chemistry, polyphenylquinoxalines (PPQs) are known as temperature-resistant and mechanically tough polymers. Within the last years, they have attracted additional attention aspotential electron transport materialsfor organic LEDs [ 16-191. This is due to their

.I. Bmeihmsen

et 01. /Swrhrric

Mrrnls

91 11997) 223-228

225

clot

N-NH +

7P-c

8

2

J

Ni(COD)z

1RT

2.6O”C CHM,

1 Oa-c / CF3

b a Scheme 2. Synthesis of the novel X-shaped oxadiazole

Table 1 Thermal data of the X-shaped

+iFHf--

c

compounds

1Oa-c.

15b 15c

t-Bu H

Scheme 3. Synthesis oxadiarole

derivatives

of the PPQs.

lOa-c

Compound

T,’ (“C)

T,, a ( "C )

T&C h (“Cl

10a lob 1oc

186 152 114

385 377 331

390 380 370

a DSC, heating/cooling rate: 20 Kimin. h TGA, onset of decomposition, heating rate: IO KImin.

H CF3

Nz atmosphere

high electron affinity resulting from their electron deficient heterocyclic r-system. The establishedsynthetic route to polyphenylquinoxalines involves the polycondensation of bis( phenylglyoxaloyl) benzenes (bisbenziles) with 3,3’-diaminobenzidines in pncresol as solvent. The bis( phenylglyoxaloyl) benzenesare usually prepared by selenium dioxide oxidation of bis( phenylkcto)xylylenes [ 201. We used a different synthetic approach which is outlined in Scheme3. Substituted phenylacetylenes 11 and 1,3-dibromobenzenewere crosscoupled in a palladium-catalyzed Heck-reaction to 1,3-

bis( arylalkynes) 13 that could be oxidized to the tetraketones 14 with KMnO,/acetone in almost quantitative yields. This route to the tetraketones avoids the use of the highly toxic selenium dioxide. The last step is the polycondensation of the bisbenziles 14 and 3,3’-diaminobenzidine, leading to new substituted poly[ t?z-phenylen-3,3’-bis( 2,2’-diphenyl) quinoxaline-6,6’-diyls] (PPQs, Ha-c). The polycondensation is usually carried out in m-cresol or in mixtures of mcresol and chloroform as solvent. The useof the high boiling and polar m-cresol has several disadvantages.The most important is the poor removability of the solvent from cast films for LED applications. Since our bisbenzilesare substituted with solubilizing tert-butyl and CF, groups, the polycondensationcould be carried out in chloroform solution in which all three PPQsare soluble. In addition to the polymers mentioned above we have synthesized a number of new starburst 1,3,5-tris( 3-phenylquinoxaline-2-yl) benzenes (TPQs, 20a-d) . The thermal, optical and electrochemical properties have been examined and will be discussedin the following chapter.

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et al. /Syntheric

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91 (1997) 223-228

4. Thermal

Et,

and optical characterization

All starburst tris( phenylquinoxalines) revealed excellent film-forming properties. In the DSC experiment, no melting was observed for the TPQs 20a, c and d. Only 20b shows a melting point at 36 1“C in the first heating cycle. Upon cooling and in a second heating no recrystallization is observed. All four TPQs exhibit glass-transition temperatures in the range from 147 up to 179 “C (Table 2) and therewith represent a new class of low molecular weight organic glasses. The TPQs are thermally stable and show decomposition temperatures between 350 and 400 “C. Two absorptions can be seen in the UV-Vis spectra which are assigned to the phenyl ring and the quinoxaiine moieties, respectively. In chloroform solution, the TPQs show a pale blue fluorescence with a maximum at about 400 nm. Rather different features appear in the fluorescence spectra of the TPQ films. The maximum is slightly red-shifted. Beyond this a new peak appears at about 500 nm exceeding the former in intensity (except for 2d). This peak is attributed to the ability of the TPQs to form self-associated, stacked complexes. Only the spectrum of 20d, with its bulky tert-butyl substituents, shows no additional fluorescence in the 500 nm range. The trifluormethyl-substituted TPQs 20~ and d are soluble in acetonitrile, and were investigated by cyclic voltammetry (Table 3). They both exhibit a reduction potential at - 1.78 V (versus fc/fc+ ). Taking into account the energy level of ferrocene/ferrocenium and the optical bandgap [ 231 one can calculate HOMO and LUMO energies of 6.2 and 3.0 eV for both TPQs.

1 Pd(PPh,hCI,

Table 2 Thermal and optical data of the TPQs

TPQ 20a 20b 2Oc 20d

RI RZ H H H CH3 WCF3 H t-13x1 CF3/H

Scheme 3. Synthetic

pathway

R3 H CH3

CFy’H

PQ

rg = (“C)

Td,, b i”C)

Abs. ’ Lx (nm)

FL*, * A,,, (nm)

FIAlmcI Lx (nm)

20a 20b 2oc 20d

151 179 147 165

414 408 370 363

250/350 259/365 247713% 247/357

395/524 402 403 406

4151515 4121516 4171493 426

HKF3 to TPQs 20sd.

The synthetic route to the TPQs 20a-d (Scheme 4) starts with the palladium-catalyzed cross-coupling of the phenylacetylenes lla with the 1,3,5-tribromobenzene 16, yielding tris( phenylethinyl) benzenes 17 [ 2 I]. Unlike the 1,3bis(arylalkynes) they cannot be oxidized with KMnO, in acetone. Instead, the 1,3,5-tris(phenylglyoxaloyl)benzenes 18 were prepared via NBS oxidation in anhydrous dimethylsulfoxide [ 221. Subsequent condensation with the substituted 1,Zphenylenediamines 19 in CHC13 solution led to the 1,3,5-tris(3phenylquinoxaline-2-yl) benzenes (TPQs, 20a-d) .

’ Glass-transition temperature determined bp DSC, second heating min. h TGA; onset of the decomposition in Nz, 10 “C/min. ’ Films on quartz substrates, absorption (Abs.), fluorescence (PI). ’ Chloroform solution. Table 3 Results of the cyclic voltammetry

measurements

PQ

E,@d (vs. fc/fc + ) a (VI

LUMO (eV)

2oc 20d

- 1.78

-3.02 -3.02

-1.75

10 “C/

b

HOMO (eV) -6.20 -6.19

’ Measured in acetonitrile solution containing 0.1 mol/l of [ (H,C,)N] PF,. b Calculated with the assumprion that the energ level of ferroceneiferrocenium is -1.8 eV below vacuum [ 231.

J. Bertenlmmw~

et ui. /Synrheric

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91 (1997)

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227

5. Application of starburst oxadiazoles and phenylquinoxalines in organic LEDs In this chapter we describe the use of star-shaped oxadiazoles as ETLs in hererolayer LEDs together with poly( 1,4phenylene vinylene) ( PPV) as the active emitting layer. Monolayer EL devices from PPV usually show relatively low luminescence efficiencies (about 0.001% external quantum efficiency in the configuration ITO/PPV/Al). Theselow values are a consequence of the fact that these devices are majority carrier diodes (Schottky devices) and there is a large excess hole current leading to an unbalanced current flow. The electron transport capability of the starburst oxadiazoles has recently been demonstrated by time-of-flight measurements in which electron drift mobilities of 1 X IO-” cm’/ (V s) at an applied electric field of 7 X 10’ V/cm (273 K) have been detected [ 241. Our measurements with the PPV heterolayer LEDs show that all tested starburst oxadiazoles can act as efficient electron transport and injection layers. The devices show a considerable increase in the physiological brightness and quantum efficiency compared to PPV monolayer devices. Fig. 1 shows a comparison of the star-shaped oxadiazoles with a guest host system (PBD in polystyrene) and a mainchain polymer with oxadiazole units that has been described before [ 81. The structures of all oxadiazole units used in twolayer LEDs are shown in Scheme 5. In all cases the EL spectra of the multilayer LEDs are identical to those of monolayer PPV devices. i.e. the LEDs emit yellowish green light. The threshold voltage for EL only slightly increased from 2.5 to 3 V in the case of the starburst oxadiazole 21 and the dendrimer 22. In contrast. the devices using PBD in polystyrene or the main-chain polymer23 show threshold voltages of 5 and 11 V, respectively. The brightness and quantum efficiency of the LEDs with the starburst oxadiazole 21 as ETL are comparable to devices using known materials like PBD. External quantum efficiencies of 0.1% and a brightness of several hundred cd/m’ have been achieved. The best results have been obtained with the oxadiazole dendrimer22. In LEDs with the configuration ITO/PPV ( 150

z6

051-i 0.4-

5 g

03-

: $ J u-

02-

E d a

n n

Scheme 5. Structures devices.

of the oxadiazole

derivatives

tested in PPV heterolayer

nm) /dendrimer (30 nm) /Ag, external quantum efficiencies of 0.4% have been measured. This may be a first hint that dendrimers possess enhanced carrier mobilities compared to analogous linear polymers as it was predicted in a recent theoretical paper [ 251.

6. Phenylquinoxalines Due to their excellent film-forming properties homogenous transparent films could be fabricated from the TPQs 20a-d, which were applied in two-layer LED devices with the configuration ITO/PPV ( 1.50nm) /TPQ (40 nm) /Ag. The threshold voltage for EL ranges from 5.7 (20~) to 10 V. The EL spectra of the two-layer devices coincide with a PPV single-layer device, whereas the physiological brightness of the TPQ devices is two orders of magnitude higher than for PPV single-layer devices. These results support the electron-conducting and holeblocking properties of the TPQ derivatives. The recombination zone is moved into the PPV close to the PPV/TPQ interface, resulting in an improvement of quantum efficiency compared to the single-layer device. External quantum yields up to 0.11% and a brightness of about 200 cd/m2 (at 20 V, for 20b,c) were obtained in our first experiments with TPQs in two-layer LEDs. We believe that these values can be further enhanced by producing TPQ films of higher quality, either from solution or by vacuum deposition and by processing our LEDs in an inert environment. Furthermore, the quantum efficiency of our PPV has to be increased in order to obtain bright LEDs.

Acknowledgements

O.'. 0.01 i

0

.,

2

,

I

I

I

I

I

/

I

4

6

8

10

12

14

16

18

0

I

20

22

VoltageM

Fig. 1. External quantum efficicnciea ofthe two-layer uration: ITOiPPV ( I50 nm) ioxndiazole derivative

LEDs with the config(30 nm) /Ag.

The authors thank W. Briitting, M. Meier and E. Werner from M. Schwoerer’s Group for the LED measurements. This work was supported by the Bayerische Forschungsstiftung in the framework of the FOROPTO Program.

228

J. Bettenlmrsm

et 01. /.Sythetic

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