nr 7-8/2010 • tom 64 • 517

Synthesis of some heterocyclic compounds and application thereof inelectroluminescent cellsAndrzej DANEL, Krzysztof DANEL - Department of Chemistry and Physics, University of Agriculture in Kraków

Please cited as: CHEMIK 2010, 64, 7-8, 511-522

IntroductionUntil the late 60s of the last century, electronics were dominated

by materials of inorganic origin. This situation began to change when it was discovered that some organic materials are capable of conducting electricity. The importance of this discovery was accentuated in 2000 when Heeger, MacDiarmid and Shirakawa were awarded the Nobel Prize for their research on organic conductive polymers.

Nowadays organic materials are used in the construction of pho-toelements, capacitors, diodes, batteries, chemical sensors, polymer lasers, NLO devices and transistors [1]. One of the fastest developing disciplines is the electroluminescence (EL) of organic compounds. The phenomenon has been known since the early 60s of the last century, but it had not found practical application, as the organic EL diodes of that time could not compete with their inorganic counterparts. The break-through came in 1987, when results of work on a double-layered high performance EL cell were published [2]. The device comprised vacuum deposited films of an 8-hydroxyquinoline aluminium complex (AlQ3), which served as the electron transporting layer (ETL) and of an aromat-ic amine constituting the Hole Transporting Layer (HTL). In 1990 Friend reported on his work on a cell based on poly(p-phenylene vinylene) (PPV) [3]. One of the most important achievements in this area was the discovery of electrophosphorescence, which enables increasing the internal quantum yield up to nearly 100%, whereas the maximum yield in traditional cells, based on the host-guest design, reaches 25% only [4]. These and subsequent discoveries have indicated that EL displays can be constructed with the use of organic compounds.

These displays are now becoming increasingly common due to the high colour saturation they offer, low supply voltage, low weight, possibility of creating large luminous surfaces, wide angle of view, high contrast and the fact that they emit light - a feature for which liquid crystal displays require an additional layer. At present OLED (Organic Light Emitting Diode) displays are widely used in electronic equipment such as mobile telephones, digital cameras, car audio systems, electri-cal shavers, prototype 20-inch TV screens. However, it will take some time before TVs with OLED or PHOLED (Phosphorescent Organic Light Emitting Diode) screens arrive at shops. This is because there are problems with uneven ageing of luminescent materials used in their manufacture. University and industrial laboratories around the world are working on improving the materials used for the manufacture of these devices and on improving their performance, so that the require-ments of the industry and of everyday applications are met. Perhaps, traditional light bulbs will in the future be substituted by lighting planes emitting white light. One company, Universal Display Corporation, an-nounced that they have developed a PHOLED technology that allows 100% transformation of electrical energy into light [5].

Paths of developmentA wide range of organic materials are used in the manufacture of

EL cells. These materials include simple molecules, oligomers and poly-mers [6, 7]. Each of these groups include p-type materials for trans-porting holes, n-type materials for transporting electrons, and emission materials (n or p). Simple molecules of the p-type include aromatic amines, which were initially used in photocopiers, and later were for the first time applied in EL cells by Tang [2]. Other compounds of this

type include condensed aromatic hydrocarbons, such as anthracene, coronene or pentacene. The second group is represented by nume-rous heterocyclic compounds: derivatives of oxazoles, oxadiazoles, triazoles, quinoxalines, triazines, thiophene and other, where AlQ3 and its modifications play an important role [8÷10]. It is a representative of numerous complex compounds used in EL cells. Since the times of Tang’s pioneering work, complexes of this type have been the subject of intense research as ETL and emission materials and as a host sys-tem for various fluorescent dyes. Many complexes are recently applied in cells where use is made of the phenomenon of electrophosphore-scence (complexes of Ir, Os and Pt). These complexes have the form of dendrimers, on which efficient EL cells were built.

One of the disadvantages of vacuum deposited cells is the crystal-lization of the layer (ETL, HTL, emission layer), which reduces the operating life of the cell and deteriorates its performance. This is not observed in the case of polymers. Polymeric layers, deposited by wet-type spin-coating techniques, enable simpler and more economic manufacture and formation of such layers in cells. Techniques applied in the case of vacuum deposition of films require expensive and complex equipment and involve significant losses of raw material. The problem faced in the case of polymers used for fabricating multi-layer cells, is the damage done to previously deposited layer by the solvent used for applying the subsequent layer. However, there is much progress in this area and a growing number of reports on multi-layer polymeric cells are appearing. The most important polymers applied in EL cells include derivatives of poly(p-phenylene vinylene), polythiophenes, poli-(p-phenylenes), polifluorenes, polymers based on nitrogen-con-taining heterocycles (polypyridines, polyquinolines, polyquinoxalines, polycarbazoles, polyvinylcarbazole) [11, 12]. Polyvinylcarbazole (PVK) finds wide use not only as a hole-transporting material, but also as a matrix for various types of dopants.

The third group of compounds used in EL cells comprises oligo-meric systems. These materials can be deposited both by vacuum techniques as well as by spin-coating. One of the most important achievements in this area was the introduction of amines of the so-called star-burst structure, which form amorphous films of high glass transition temperature Tg [13]. The second type of compounds is the spiro type systems, the most important representatives of which are spiro-bis-fluorene and its derivatives (Fig. 1) [14]. Spirobisfluorene can be used to synthesize both p-type and n-type materials. It also serves as a structural component of many polymers.

Our investigations were focused on the synthesis of nitrogen-containing heterocyclic systems, such as: 1H-pyrazolo[3,4-b]quinoline PQ, 1H-pyrazolo[3,4-b]quinoxaline PQX, bis-pyrazolo[3,4-b;4’,3’-

Fig. 1. Examples of star burst and spiro type structures

scie

nce

• te

chni

que

518 • nr 7-8/2010 • tom 64

scie

nce

• te

chni

que e]pyridine BPP and benzoxazoles BOX (Fig. 2). All of these com-

pounds, except for the last one, have been applied in the fabrication of EL cells. The choice of PQ as the luminophore is supported by the ease of its synthesis, sometimes reduced to one step, which is an advantage in the industrial manufacture of luminophores. In addition, most of the PQ compounds synthesized by us demonstrated high emission in solutions and solids, which is one of the necessary conditions of using them as luminophores. Moreover, PQs are thermally stable enti-ties, the melting points of which sometimes reach 400°C and the glass transition temperatures of which are close to 280°C [15]. Measure-ments of electron mobility using the TOF (Time of Flight) method have shown that many of these compounds equal to 5-(4-biphenylyl)-2-(4-t-butylphenyl)-1,3,4-oxadiazole (PBD), widely used in EL devices.. The values for pyrazoloquinolines range from 10-6 to 3×10-5 cm2/Vs at field strength of (1-7)×105V/cm. For PBD the value is 1.9×10-5 cm2/Vs (1×106 V/cm) [16, 17]. PQs may be simultaneously used as an ETM system and as a dopant in polymer matrices.

The first pyrazoloquinoline described in the literature was ob-tained in 1928 by Friedländer condensation from anthranilic aldehyde and pyrazolone (Scheme 1); R1 = 2-Cl-C6H4, R

2 = Me, R3, 4 = H).

Substituted PQs are prepared from o-aminobenzophenone and o-aminoacetophenone (R3

= Me or Ph). The disadvantage of this syn-thesis is scarce availability of commercial o-aminocarbonyl systems. This problem was overcome by synthesizing PQ from derivatives of 5-aminopyrazoles and aromatic aldehydes in the presence of ZnCl2 (Scheme 2: route a) and in reactions between 4-benzylidenepyrazo-lones and aromatic amines (Scheme 2: route b) [18, 19].

The latter reaction led to the development of a single-step syn-thesis procedure, where a heated mixture of appropriate pyrazolone, aromatic aldehyde and amine yields PQ [20].

The yields of the reaction mentioned are quite low (ca. 20-33%). However, the advantage is that commercially available reagents can be used, and the reaction is the most efficient method of PQ syn-thesis among those described in the literature so far. The method is useful for obtaining various PQ (R1,2 = H, alkyl, aryl; R3,4

= F, Cl, Br, OMe, OAr, COOR, COR, CF3, NO2, CnH2n+1, aryl). The reaction takes a short time and in most cases is completed within 45-80 minutes. Moreover, in Friedländer condensation some pyrazolones do not yield PQ. This does not apply to the last reaction, which readily yields PQ unsubstituted in position 1 of the pyrazole ring. These PQ are formed particularly readily because of their low solubility, and they precipitate during the reaction. In a subsequent step the system obtained can be reacted with aromatic iodine or bromine derivatives to obtain PQs with substituents that are difficult to prepare by traditional methods due to unavailability of appropriate hydrazines required to produce pyrazolones (Scheme 4).

Another attempt at modifying PQ was to prepare derivatives sub-stituted in the phenyl ring (position 4) with N,N-aryl groups. These are obtained in reactions between bromine-substituted PQ and aromatic amines in the presence of palladium catalysts (Scheme 5).

The introduction of bulky substituents (e.g. CPh3) impedes the crystallization of vacuum-deposited layers or dopants in the matrix (Fig. 3). The thermal stability of the compounds is also improved.

Unsubstituted PQ (Fig. 2; R3 = Ph, R1, 2

= Me, Ph, R4 = H) are generally blue light emitters providing high quantum yield of emission (0.88-1.00) [21].

1H-pyrazolo[3,4-b]quinoxalines PQX, like PQ, have been studied mainly from the viewpoint of their biological properties, and as yet they have not been applied in optoelectronics. Synthesis thereof was based mainly on reactions of aromatic o-diamines with diones obtained from pyrazolones (Scheme 6) or on attaching a pyrazolone ring to a quinoxaline system.

Diones were obtained by reacting pyrazolones with p-nitroso-N,N-dimethylaniline and hydrolyzing the resulting compound. In the case of

Fig. 2. Molecular structures of luminophores used in OLED cells

Scheme 1. a) fusion or ethylene glycol/180 °C.24 hours

Scheme 2. a) aldehyde/ZnCl2/24 hours/180 °C; b) aromatic amine /glycol/180 °C/or MW

Scheme 3. a) ethylene glycol /180-190 °C/45-60 min

Scheme 4. a) I(Br) Ar, CuSO4, K2CO3, sulpholane, 190 °C

Scheme 5. a) Pd(0), P(t-Bu) 3, t-BuOK, toluene, 24 hours, 110 °C

Fig. 3. Examples of molecular structures of PQ with trityl groups

Scheme 6. a) p-nitroso-N,N-dimethylaniline/Na2CO3; H2SO4; b) o-phe-nylenediamine/CH3COOH

nr 7-8/2010 • tom 64 • 519

scie

nce

• te

chni

quesubstituted o-diamines, isomeric PQXs were formed. Therefore rea-

ctions between aromatic amines and 5-chloro-4-nitropyrazoles were carried out, followed by cyclization using P(OEt)3 or by heating for 3-5 min. with iron(II) oxalate in sulpholane at 250°C (Scheme 7) [22].

Currently other routes of synthesis and modifications of the structures are investigated in order to improve physical and chemical properties. Quinoxalines and polyquinoxalines are perfect electron-transporting materials.

Bis-pyrazolo[3,4-b;4,3-e’]pyridines BPP were synthesized by re-acting aminopyrazoles with aromatic aldehydes while heating them in sulpholane at 240-250°C for 4-5 h (Scheme 8) [23].

Some of the compounds were used in cell fabrication, other (fluo-rine and bromine derivatives) were used as a starting material for fur-ther modification and preparation of amorphous systems. Derivatives (scheme 8; R3

= p-Br) were reacted with aromatic amines to yield N,N-diarylamine BPP (Fig. 4).

The ultimate group of luminophores includes derivatives of ben-zoxazols BOX, which were obtained in a reaction between methyl-ated heterocycle derivatives and Shiff bases in a DMF/KOH/60-70æC arrangement [24].

Cells fabricated with the use of low molecular weight compounds often undergo degradation as a result of crystallization of individual layers. To avoid the luminophores mentioned were transformed into oligomeric systems. The most straightforward was the synthesis of aromatic ethers obtained through a reaction of amine derivatives of diphenyl ether (Scheme 10).

In the case of resorcinol, phloroglucinol and bisnaphthol, phenols were reacted with p-fluoronitrobenzene, and the di- or trinitroderiva-tives were reduced to corresponding amines. Reaction of amines with 5-chloro-4-formylopyrazoles yielded di- or trisubstituted PQ ( Fig. 5).

Further oligomeric systems were synthesized from derivatives of bisphenol A or AF. Fluoroderivatives of PQ and BPP were reacted with bisphenol A in a nucleophilic substitution reaction by heating the reagents for 3-4 hours in a mixture of toluene and 1-methyl-2-pyr-rolidone (NMP), while azeotropically removing water at 140-160°C, followed by heating at 190°C for 12 hours (Scheme 11). Bromomethyl derivatives, obtained by bromination with NBS of appropriate PQ methyl derivatives, were formed readily by heating with bisphenols in an acetone (or DMF)/K2CO3 arrangement. Products were precipi-tated by adding water, filtered, dried and purified by crystallization or column chromatography.

There is much evidence indicating that amorphous layers of high Tg values are less susceptible to morphological changes induced by heat generated during cell operation. The earlier mentioned star-burst structures, dendrimers and tetrahedral systems may provide amorphous films in many cases. This type of systems includes also cardo-type structures with bulky substituents bonded to carbon atom with sp3 hybridization. Introduction of cardo-type groups (e.g. fluo-rene) into polymers leads to amorphous dimeric systems with high Tg. Cardo systems containing PQ and BPP were synthesized starting from 9,9’-di(p-aminophenyl)fluorene and 9,9’-di(p-hydroxyphenyl)fluorene. The starting fluorene derivatives are commercially available or they can be readily prepared from fluorenone and aniline or phenol. Mo-

Scheme 7. a) K2CO3/DMF/120 °C b) P(OEt) 3 or FeC2O4/sulpholane

Scheme 8. Basic route of BPP synthesis

Fig. 4. Structure of 4-(p-N,N-diarylphenyl)-derivative of BPP

Scheme 9. a) PPA(polyphosphoric acid)/120 °C/12 hours; b) KOH/DMF/Schiff base

Scheme 10. a) sulpholane/140-210 °C

Fig. 5. PQ trimer synthesized from phloroglucinol

Scheme 11. a) NMP/toluene/K2CO3 or K2CO3/acetone for PQCH2Br

Fig. 6. Cardo molecules derived from fluorene

520 • nr 7-8/2010 • tom 64

scie

nce

• te

chni

que lecular structures of the cardo-type entities prepared in the syntheses

described are shown in Fig 6.A further step in synthesizing from bisphenols was the preparation

of spiro-type systems from spirobisindane, and namely of 6,6’-dihy-droxy-3,3,3’,3’-tetramethyl-1,1’-spirobisindane SBI and 5,5’,6,6’-tet-rahydroxy-3,3,3’,3’-tetramethyl-1,1’-spirobisindane SBI-tetr (Fig. 7).

In practice all spiro systems applied in EL cells are derived from spirobisfluorene. Synthesis of a spirobisfluorene arrangement is a quite costly and multistage process. SBI, on the other hand, can be obtained in a one-step synthesis from inexpensive reagents such as acetone, phenol, catechol or pyrogallol in the presence of HCl/CH3COOH. SBI may serve as a substrate for the synthesis of spiro-oligomeric systems in a reaction with fluoro- (or bromo-) derivatives of PQ, PQX and BPP [25].

Molecular structures of some of the compounds obtained are shown in Figure 8. The spiro core can be attached to PQ not only through an ether linkage (PQ-O-SBI; PQCH2-O-SBI; Fig. 7), but also via the nitrogen atom in the pyrazole ring. After substituting OH groups in SBI with bromine, the dibromo-derivative obtained can be attached to PQ via a C-N bond (Scheme 12).

The ultimate group of luminophores includes copolymers synthe-sized from PQ and carbazole. PVK is widely used in EL cells as a hole transporting system. In addition it is a thermally resistant polymer with glass transition temperature Tg of ca. 250°C. The first polymers were synthesized from methacrylic esters of PQ and N-vinylcarbazole. (Scheme 13). The ester derivative was prepared by demethylation of 6-methoxy-PQ with HBr/CH3COOH and reaction of 6-hydroxy-PQ with methacryloyl chloride in the presence of triethylamine. Polyme-rization was conducted in toluene in the presence AIBN (2,2’-azobis(isobutyronitryl)). Similar method was applied to prepare a polymer from BPP [26].

Another group of polymers comprises vinyl derivatives of PQ (Fig. 9) [27÷29].

Vinyl PQ monomers were synthesized by bromination of ap-propriate methyl derivatives, followed by reacting the products with triphenylphosphine to produce a phosphonium salt, which in Wittig re- action with formaldehyde yielded the vinyl system. The N-vinyl de-rivative was synthesized in a reaction of 1H-PQ with bromoethanol, then with TsCl. The ester obtained was transformed into the N-vinyl system by reacting it with t-BuOK in pyridine. Copolymerization was conducted using the following quantities of PQ: 0.01; 0.1; 1 mol%. No emission of the PQ group was observed in the case of (PQ = 0.01%) copolymer. The emission spectrum obtained corresponded to that of pure PVK. At PQ content of 1% and higher, PQ is the only source of emission. Polymers are thermally resistant systems. Thermogravimet-ric analysis (TG) demonstrated that they were stable up to 310 °C. Decomposition occurred at ca. 420°C.

Polymers with PQ in the main chain are represented by polyesters (Scheme 13) obtained by condensation polymerization of phenol-phthalein, terephthalic acid dichloride and dihydroxypyrazoloquinoline (Scheme 14) [26].

Electroluminescent cellsThe materials described above were used to construct OLED cells.

EL cells may in general be grouped into single-, double- and three-layer cells. Anode comprises an ITO (Indium-Tin Oxide) layer on a transpa-rent substrate, onto which charge-transporting and emission layers are applied. Cathode comprises metal of low work function (aluminium, magnesium, calcium). Calcium, because of its reactivity, requires coat-ing with aluminium. In addition, after completing its fabrication the cell may be processed to protect it against external factors. In the la- test OLEDs a flexible substrate (FOLED-Flexible OLED) is often used. This provides better resistance to mechanical damage. The earliest cells comprised single-layer devices with the luminophore sandwiched between two electrodes. However, due to the nature of the organic material used, which was usually of the p or, rarely, n type, the cells had low efficiency and brightness. Introduction of additional hole- and electron-transporting layers enabled significant improvement of cell operation [30]. The emission layer may be placed between HTL and ETL layers or it may also perform the function of one of these layers, then the cell has a two-layer structure. As mentioned, multilayer cells have higher efficiency, but are more difficult to fabricate. For that rea-son intense research is conducted on single-layer cells. One of the possible solutions is to use dopants in polymer matrices (e.g. PBD and luminophore in PVK) or type p and n polymer blends. One problem that can occur is the separation of layers. An ideal solution would be the creation of a material able to transport holes and electrons and to be at the same time an emitter. Materials of bipolar nature are known, but they do not come up to expectations yet, and still require addi-tional HTL or ETL layers [31].

Fig. 7. Molecular structures of hydroxy derivatives of spirobisindanes

Fig. 8. Molecular structures of spiro-PQ and spiro-BPP

Scheme 12. a) PPh3Br2/180-330 °C; b) CuI, K2CO3, nitrobenzene, 190 °C

Scheme 13. a) toluene/AIBN/65 °C

Fig. 9. Vinyl PQ monomers used for polymerization

Scheme 14. a) C2H4Cl2/Et3N/room temperature

nr 7-8/2010 • tom 64 • 521

scie

nce

• te

chni

que

The luminophores obtained (PQ, PQX, BPP, BOX) were used in the fabrication of EL cells. Two types of cells were fabricated. One type comprised cells with vacuum-deposited layers of general ITO/HTL/ETL configuration: EL/MgAg or ITO/HTL/EL/ETL/MgAg. Apart from these, two-layer ITO/PEDOT/PVK cells were used: EL: PBD/CaMg. In the second case aqueous solution of PEDOT was applied onto ITO by spin-coating. After drying and thermal hardening of the layer at 150°C, PVK solution, luminophore and PBD were applied. In the end a layer of calcium and magnesium was deposited.

Acronyms and molecular structures of the materials used in cell fabrication are shown in Figure10, where, from top to bottom, HTL, ETL and hole-transporting materials serving as matrices in single-layer cells are presented in sequence.

The first EL cell using PQ was constructed with the use of 1,3-di-phenyl-1H-pyrazo[3,4-b]quinoline (Fig. 1. R1,2

= Ph, R3, 4 = H) in a host-

guest arrangement with ITO/TPD/PBD configuration being PQ/MgAg, where the PQ layer was applied simultaneously with PBD.

Emission of blue light (λem = 470) was observed upon applying a

voltage of ca. 10 V. Its luminance, however, was very low at about 10 cd/m2. In order to improve the performance, a three-layer cell was constructed with an ITO/TPD/PQ/PBD/MgAg configuration, wherein PQ constituted the emission layer of varying thickness. The emission luminance increased to ca. 100 cd/m2. However, the increase in thick-ness of the PQ layer resulted in a shift of the emission towards longer wavelength. In addition, degradation of the cell occurred relatively early due to crystallization of the PQ layer. To block this process, bulky substituents (for instance R4

= N,N-diphenyl or CPh3) were introduced in position 6 and methyl and tert-butyl groups were introduced into phenol substituents at the pyrazole ring. PQ was deposited together with AlQ3, which was used instead of PBD. Cell specifications are pre-sented in Table 1. The constructed cells emitted blue and blue-green light with a luminance of up to 20,800 cd/m2. Duration of continuous light emission was extended to 120 hours (as opposed to 2 hours in the case of the first cell) [32].

In subsequent cells PQ (R3 = p-C6H4-OMe) was used in the con-figuration ITO/NPB/CBP/TPPBI: PQ/TPPBI/MgAg. It was possible to decrease the supply voltage to 4V and to increase external quantum yield to 3.4%. Cell luminance was 13,500 cd/m2 [33]. The PQ series (R1

= Me, R2 = Ph; R3

= OMe, t-Bu, H, F, CF3 , CN, NEt2) enabled the construction of cells that emitted blue and green light with a lumi-nance of 5000-6000 cd/m2 in the case of blue light [34]. PQ (R1

= Me, R2

= Ph, R3 = NEt2) was also used in a ITO/NPD/NPD: PQ/TPBI/MgAg

cell, where PQ content was 1 to 45%. Varying the dopant content provided emission colours ranging from blue-green to green. At PQ content of 16% and supply voltage 10V, the luminance was 36,000 cd/m2, while at 5V, the current efficiency, luminous efficiency and external quantum yield were 6.0 cd/A, 4.2 lm/W and 1.6%, respectively [35].

BPP was used in cells of similar configuration to those with PQ, attaining intensive blue luminance ranging from 1,000 to 11,200 cd/m2 [36, 37].

Mention should also be made of single-layer cells with a polymer PVK matrix [38÷40]. An ITO/PEDOT: PSS/PVK/PBD: PQ/CsF/Al cell emitted deep blue light (λem(EL) = 433 nm) with CIE values (0.15, 0.07), equalling some of the results recently published in the literature refer-ring to cells based on anthracene or quinoline derivatives, the molecu-lar structures of which are shown in Figure 11 [41÷43].

The specifications of these systems, TBADN, BDSA and BDB-PQ, are as follows: CIE(x, y) = (0.15, 0.14); (0. 14, 0.10) and (0.17, 0.16), respectively. In addition, PQ is synthesized in 1 or 2 steps only (Scheme 1 or 3). There are many publications describing very efficient cells. However, authors of these publications indicate that the mate-rials used for producing these cells are expensive, either because of the cost of the substrates, or of the number of steps needed to obtain these materials. This aspect, therefore, has to be taken into account when on the quest for new materials.

SummaryThe review is focused on the search for and synthesis of new

materials that could be used in electroluminescent devices. Research started in the nineties and concentrated primarily on finding blue lumi-nophores for OLEDs.

Such luminophores were found among pyrazoloquinolines and bis-pirazolopyridines. These compounds can readily be prepared from commercially available substrates, and there are many ways of modi-fying their structure (simple molecules, oligomers and polymers) and physicochemical properties to make them suitable for constructing EL cells.

Table 1

Specifications of some host-guest EL cells built with the use of PQ of ITO/TPD/PQ/AlQ/MgAg configuration

PQ R1 R2 R3 R4 ëema CIEx

b CIEyc çex

d çPe

1 p-tolyl p-tolyl H t-Bu 495 0.19 1.22 1.22 1.852 Ph Ph H NPh2 525 0.63 0.89 0.89 1.893 Ph Ph H CPh3 475 0.30 0.66 0.66 0.714 p-tolyl p-t-BuC6H4 H CPh3 490 0.43 0.98 0.98 1.355 Me Ph H CPh3 465 0.23 0.41 0.41 0.43

a) EL emission wavelength; b) and c) chromacity coordinates; d) external quantum yield (%); e) luminous efficiency (lm/W); CIE (Commission Internationale de l’Èclairage)

Fig. 10. Materials used in EL cells.

Fig. 11. Molecular structures of blue luminophores used as dopants in EL cells

522 • nr 7-8/2010 • tom 64

scie

nce

• te

chni

que Bbliograpy

1. Graja A.: Niskowymiarowe przewodniki organiczne. WNT Warszawa, 1989.2. Tang C.W., VanSlyke S. A.: Organic electroluminescent diodes. Appl. Phys.

Lett. 1987, 51, 913.3. Burroughes J. H., Bradley D. D. C., Brown A. R., Marks R. N., Mackay K.,

Friend R. H., Burn P. L., Holmes A. B.: Light-emitting diodes based on conju-gated polymers. Nature 1990, 347, 539.

4. Baldo M. A., O’Brien D. F., Yoy Y., Shoustikov A., Sibley S., Thompson M. E., Forrest S. R.: Highly efficient phosphorescent emission from organic electroluminescent devices. Nature 1998, 395, 151.

5. Williams E. L., Haavisto K., Li J., Jabbour G. E.: Excimer based white pho-sphorescent diodes with nearly 100% internal quantum efficiency. Adv. Mater. 2007, 19, 197.

6. Mitschke U., Bäuerle P.: The electroluminescence of organic materials. J. Ma-ter. Chem. 2000, 10, 1471.

7. Hung L. S., Chen C. H.: Recent progress of molecular organic electrolumine-scent materials and devices. Mater. Sci. Eng. 2002, R39, 143.

8. Hughes G., Bryce M. R.: Electron-transporting materials for organic electrolu-minescent and electrophosporescent device. J. Mater. Chem. 2005, 15, 94.

9. Kulkarni A. P., Tonzola C. J., Babel A., Jenekhe S. A.: Electron transport ma-terials for organic light emitting diodes. Chem. Mater. 2004, 16, 4556.

10. Thelakkat M., Schmidt H. W.: Low molecular weight and polymeric hetero-cyclics as electron transport/hole blocking materials in organic light emitting diodes. Polym. Adv. Technol. 1998, 9, 429.

11. Akcelrud L. Electroluminescent polymers. Prog. Polym. Sci. 2003, 28, 875.12. Kim, D.Y., Cho C.Y., Kim C.Y.: Blue light emitting polymers. Prog. Polym. Sci.

2000, 5, 1089.13. Shirota Y.: Photo-and electroactive amorphous molecular materials-molecu-

lar design, syntheses, reactions, properties and applications. J. Mater. Chem. 2005, 15, 75.

14. Saragi, T.P.I., Spehr T., Siebert A., Fuhrmann-Lieker T., Salbeck J.: Spiro com-pounds for organic optoelectronics. Chem. Rev. 2007, 107, 1011.

15. Chen C-H., Wu F-I., Shu C-F., Chien C-H., Tao Y.T.: Spirobifluorene based py-razoloquinolines:efficient blue electroluminescent materials. J. Mater. Chem. 2004, 14, 1585.

16. Tamaeev A.R., He Z., Milburn G.H.W, Kozlov A.A., Vannikov A.V., Danel A., Tomasik P.: Electron mobility in pyrazolo[3,4-b]quinoline doped polystyrene layers. Appl. Phys. Lett. 2000, 77, 322.

17. Tameev A.R., He Z., Danel A., Milburn H.G.W., Tomasik P., Vannikov A.V., Drift mobility of electron in pyrazole containing copolymers. Russ. J. Electro-chem. 2004, 40, 359.

18. Danel A., Chaczatrian K., Tomasik P.: Microwave-assisted, facile route to py-razolo[3,4-b]quinolines. Arkivoc 2000, 1, 51.

19. Chaczatrian K., Chaczatrian G., Danel A., Tomasik P.: The synthesis of 4-aryl-1H-pyrazolo[3,4-b]quinolines by cyclisation of 4-arylidenepyrazolin-5-ones with anilines. Arkivoc 2001, 6, 63.

20. Chaczatrian K., Chaczatrian G., Danel A., Tomasik P.: A Facile synthesis of 4-Aryl-1H-pyrazolo[3,4-b]quinolines. Polish J. Chem. 2003, 77, 1141.

21. Rechthaler K.,Rotkiewicz K., Danel A.,Tomasik P., Khatchatrian, K., Kohler G.: Emissive properties and intramolecular charge transfer of pyrazoloquinoline derivatives. J. Fluoresc. 1997, 7, 301.

22. Danel A., Szlachcic P., Wisła A,.Tochowicz A.: A new attempt to the synthe-sis of 1H-pyrazolo[3,4-b]quinoxalines-potential green luminophores for OLED. materiał niepublikowany.

23. He Z., Milburn G.H.W., Danel A., Puchała A., Tomasik P., Rasała D., Blue electroluminescence of novel pyrazoloquinoline and bispyrazolopyridine deriva-tives in doped polymer matrices. J. Mater. Chem. 1997, 7, 2323.

24. Ko Ch. W., Tao Y. T., Danel A., Krzemińska L., Tomasik P.: Organic light-emit-ting diodes based on 2-(stilben-4-yl)benzoxazole derivatives: An implication on emission mechanism. Chem. Mater. 2001, 13, 2441.

25. Kozyra D., Badanie własności luminescencyjnych pirazolochinolin w kontekście budowy diod organicznych. Praca magisterska, AGH, Kraków, 2003.

26. Danel A., He Z., Milburn G. H. W., Tomasik P.: Electroluminescence from novel pyrazole-based polymer systems. J. Mater.Chem. 1999, 9, 339.

27. Wrotniak M., Synteza nowych luminoforów w aspekcie zastosowania ich w dio-dach organicznych. Praca magisterska, AGH, Kraków, 2004.

28. Nizioł J. Danel A., Jarosz B., Armatys P., Wrotniak M., Kajzar F., Boiteux G.: Synthesis and electro-optic properties of pyrazolo[3,4-b]quinoline-PVK copo-lymers. J. Mol. Cryst. Liq. Crystals 2006, 447, 181.

29. Gondek, E., Kityk, I.V., Danel, A., Wisła, A., Sanetra, J.: Blue electrolumine-scence In 1H-pyrazoloquinoline derivatives. Synth. Met. 2006, 156, 1348.

30. Geffroy B., Le Roy P., Prat Ch.: Organic light emitting diode OLED technology: materials, devices and display technologies. Polym. Int. 2006, 55, 572.

31. Doi H., Kinoshita M., Okumoto K., Shirota, Y.: A novel class of emitting amorphous molecular materials with bipolar character for electroluminescence. Chem. Mater. 2003, 15, 1080.

32. Araki K., Funaki J., Danel A., Tomasik P.: Electroluminescent devices based on 1H-pyrazolo[3,4-b]quinolines. Polish. J. Chem. 2004, 78, 843.

33. Tao Y.T., Balsubramaniam B., Danel A., Wisła A., Tomasik P.: Pyrazoloquino-line derivatives as efficient blue electroluminescent materials. J. Mater. Chem. 2001, 11, 768.

34. Tao Y.T., Balasubramanian E., Danel A., Jarosz B., Tomasik P.: Organic light-emitting diodes based on variously substituted pyrazoloquinolines as emitting material. Chem. Mater. 2001, 13, 1207.

35. Tao, Y. T., Balasubramanian E., Danel A., Jarosz B., Tomasik P.: Sharp green electroluminescence from 1H-pyrazolo[3,4-b]quinoline-based light-emitting diodes. Appl. Phys.Lett. 2000, 77, 1575.

36. Tao Y.T., Balasubramaniam E., Tomasik P., Danel A.: Blue light-emitting dio-des based on bispyrazolopyridine derivatives. Chem.Mater. 2000, 12, 2788.

37. Tao Y.T., Balasubramanian E., Danel A., Tomasik P.: Dipyrazolopyridine deriva-tives as bright electroluminescent materials. Appl. Phys. Lett. 2000, 77, 933.

38. Łuczyńska B., Dobruchowska B., Głowacki I., Ulański J., Jaiser F., Yang X., Neher D., Danel A.: Poly(N-vinylcarbazole) doped with a pyrazoloquinoline dye: a deep blue light-emitting composite for light-emitting diode applications. J. Appl. Phys. 2006, 99, 024505.

39. Gondek E., Sanetra J., Armatys P., Nizioł J., Chaczatrian K., Danel A.: New pyrazoloquinolines and their application in organic light-emitting diodes with poly(N-vinylcarbazole) matrix. Polimery 2004, 49, 7.

40. He Z., Danel A., Milburn H. G. W.: Thin-layer photoluminescence and ele troluminescence observed from pyrazoloquinoline-doped polymer matrices. J. Luminescence 2007, 122–123, 605.

41. Shah B.K., Neckers, D.C.: Anthantrene derivatives as blue emitting materials for organic light emitting diode application. Chem. Mater. 2006, 18, 603.

42. Kim, Y.-H., Kwon, S.-K.: High-purity blue and high efficiency electrolumine-scent devices based on anthracene. Adv. Funct. Mater. 2005, 15, 1799.

43. Tonzola J.C., Kulkarni, A.P., Gifford P.A., Kaminsky W., Jenekhe SA: Blue-light emitting oligoquinolines:Synthesis, properties and high-efficiency blue light emitting diodes. Adv.Funct.Mater. 2007, 17, 863.

Dr Andrzej DANEL graduated from the Chemical Faculty of the Jagiellonian University (1984) and received his Ph.D. degree in 1996 in organic chemistry from the Jagiellonian University. He has completed 4 scientific internships in Austria (Institute für Organische Chemie, Karl Franzens Universität, Graz) and Scotland (Napier University, Edinburgh). Research interests: chemistry of het-erocycles, organic synthesis, organic electroluminescence. He is a co-author of 70 papers published in foreign scientific magazines and of 4 papers published in Polish magazines as well co-author of 2 handbooks and 2 textbooks for middle school students. Promotes chemistry among young people.

Dr Krzysztof DANEL graduated from the Chemical Faculty of the Jagiello-nian University (1989) and received his Ph.D. degree in 1998 in organic chem-istry from the Jagiellonian University. He has completed 4 scientific internships in Austria (Institute für Organische Chemie, Karl Franzens Universität, Graz), Denmark (University of Southern Denmark, Odense) and Taiwan (Academia Sinica, Taipei). Research interest: chemistry of nucleosides, inhibitors of re-verse transcriptase (HIV-1 RT), microbicides and electroluminescence. He is a co-author of 22 papers published in foreign scientific magazines and of 1 paper published in Polish magazine.