WO1998028296A1

WO1998028296A1 - Pyrazol-based organic electroluminescent compounds and their use in electroluminescent devices

Info

Publication number: WO1998028296A1
Application number: PCT/GB1997/003499
Authority: WO
Inventors: George Henry William Milburn; Piotr Tomasik; Andrzej Danel; Zhiqun He; Danuta Rasala
Original assignee: The University Court Of Napier University
Priority date: 1996-12-20
Filing date: 1997-12-19
Publication date: 1998-07-02
Also published as: AU5329998A; GB9626501D0

Abstract

A compound of formula (I), wherein A is a substituent selected from the group consisting of substituted or unsubstituted aromatic or heteroaromatic rings and a substituted or unsubstituted pyrazolic moiety; and R1 to R3 are substituent identical or different, chosen in the group consisting of hydrogen, halogens and alkyl, aryl, heterocyclic, alkenyl, alkynyl, alkoxy, allyoxy, aryloxy, benzyloxy, hydroxy, and amino groups. These compounds exhibit good photoluminescent quantum yield and are capable of use in electroluminescent devices. Advantageously, these compounds are doped into a polymer matrix in emitting layers or transporting layers in electroluminescent device.

Description

Pyrazol-based Organic Electroluminescent Compounds and their Use in Electroluminescent Devices .

The invention relates to organic blue, green and yellow light emitting compounds and more particularly to blue light emitting compounds, their use in electroluminescent devices, such devices and a method for providing the same.

Description of the prior art

Electroluminescent devices (EL) can be constructed using organic and polymeric materials (see [1] C.W Tang and S.A. VanSlyke, Appl . Phys Lett, 1987, 51, 913 and [2] J.H. Burroughs, D.D.C. Bradely, A.R. Brown, R.N. Marks, K Mackey, R.H. Friend, P.L. Burns and A.B. Holmes, Nature, 1990, 347, 539). The advantages of using organic materials are their high brightness, high efficiency, potential colour turning as well as their low cost of fabrication. Highly luminescent materials have the advantage in use as emitting layer of organic electroluminescent devices. Therefore, new compounds which can be more cost effective and/or exhibit relatively high photoluminescent quantum yield are always desirable in the industry. Organic electroluminescent devices can consist of a single layer or multiple layers of emitting materials provided between two electrodes . The emitting materials can be incorporated as a single layer with hole/electron transporting property; or as a hole/electron transporting layer in double-layer devices ; or in between the hole/electron transporting layer in a triple layer device as described in Kido, J. Bulletin of Electrochemistry, (1994), 10,1.

Blue is one of the principle colours which has to be used in devices applications in order to achieve, in combination with others, a white emission. However the common characteristic of blue emitter are their large optical band gap, as this is required in order to achieve an emission at relatively high energy. This may consequently restrict the injection characteristic and the conductivity as a result of limited delocalisation.

So far, no organic commercial blue light emitting devices have been reported which are sufficiently stable for general use. Therefore there is a particular need for a blue light emitting material which can overcome these drawbacks and offers good efficiency and stability.

Summary of the invention

This invention aims to provide a series of pyrazol based compounds and more particularly blue light emitting compounds which are capable of use in electroluminescent devices .

Accordingly the invention provides a compound having the general formula (I):

wherein :

A is a substituent selected from the group consisting of substituted or unsubstituted aromatic or heteroaromatic rings and a substituted or unsubstituted pyrazolic moiety; and

Rj to R₃ are substituents , identical or different, chosen from the group consisting of hydrogen, halogens and alkyl, aryl, heterocyclic , alkenyl, alkynyl, alkoxy, allyoxy, aryloxy, benzyloxy, hydroxy, and amino groups .

More particularly it is preferred that : the alkyl groups are -(CH₂)_nCH₃, (n=0-10), -C(CH₃)₃; the aryl groups are phenyl group, substituted phenyl group, naphthyl group; the heterocyclic groups are pyridine group; the alkenyl group are - ( CH₂) _nCH=CH₂ (n=0-8), the alkynyl group are -C≡CR" (R"=H, alkyl, aryl) the alkoxy groups are -OR' (R'=alkyl) the aryloxy groups are OAr (Ar=aryl or heterocyclic group) ; the halogens are -Cl or -Br;and the amino groups are -NR'jR'₂, (R'_n=H, alkyl, aryl or heterocyclic group) .

A first embodiment of the invention relates to a preferred first family of the electroluminescent compounds. This family is the pyrazolof 3 , 4-b] quinoline and its derivatives shown in structure (II) :

in which R has the same definition as R, to R₃.

More preferably,

Ri and R₂, identical or different, are aryl groups or heterocyclic groups; and

R₃, R₄, identical or different, are alkyl groups, aryl groups, heterocyclic groups, alkenyl groups, alkynyl groups, alkoxy groups, allyoxy groups, aryloxy groups, benzyloxy groups , hydroxy groups , halogen groups and/or amino groups .

Amongst this family the 1 , 3-biphenyl-4-methyl- pyrazolo[3, 4-b]quinoline (PAQ4) is a particularly preferred compound of the following structure :

An even more preferred second embodiment of the invention relates to the family of the pyrazolof 3 , 4- b;4 ' ,3 '-e]pyridine and its derivatives shown in structure (III) :

in which R₅ and R₅, identical or different, have the same definition as R_x to R₃.

It is further preferred that:

Ri, R₂, R₅, R₆ are aryl groups or heterocyclic groups; and

R₃, are alkyl groups, aryl groups, heterocyclic groups, alkenyl groups, alkynyl groups, alkoxy groups, allyoxy groups , aryloxy groups , benzyloxy groups , hydroxy groups, halogen groups and/or amino groups.

Amongst this last family the 4-phenyl-l , 3 , 5 , 7- tetramethyl-bis-pyrazolo-[3,4-b;4 ' , 3 '-ejpyridine of the following structure (Ilia) has shown particularly good results .

Another aspect of the invention is the use of these organic compounds in electroluminescent device.

In general, there are many approaches can be used to fabricate of organic EL devices;

(1) organic emitting materials can be vacuum deposited as a thin film in pure form as emitting layer;

(2) organic emitting materials can be vacuum deposited as a thin film together with other organic compounds to form organic doped emitting layer; [3] C.W. Tang, S.A. VanSlyke and CH. Chen, J. Appl . Phys , 1989, 65, 3610.

(3) organic emitting materials can be doped into a polymer matrix, which can be fabricated into thin film through wet coating process, such as spin-coating, dip- coating or casting. These coating process are described for example in the following publications: - J. Kido, K. Hongawa, K.Okuyama and K. Nagai, Appl . Phys . Lett , 1993, 63, 2627; and - J. Kido, K. Hongawa, K.Okuyama and K. Nagai, Appl . Phys . Lett , 1994, 64, 815.

Approaches (1) and (2) use low molecular weight organic compounds only and most are reported unstable due to chemical or morphological instability of the organic materials. Apart from that, the use of vacuum deposition for the thin film fabrication in these approaches means that cost of the fabrication is high.

The use of polymer materials will significantly reduce the cost of the device fabrication. In addition, the device performance can be modified through alternation or modification of the polymer matrices. Therefore, another aspect of the invention is the use of the compounds of the invention doped into a polymer matrix in emitting layers or transporting layers in electroluminescent device (EL) . The EL device preferably consists of a hole injection electrode and an electron injection electrode with single or multiple layer(s) sandwiched in between the electrodes.

A further aspect of the invention is an electroluminescent composition to be used in electroluminescent devices, said composition comprising at lest pyrazol-based compound according to the invention doped in a polymer matrix.

A further aspect of the invention is a method to manufacture an electroluminescent device comprising the steps of : a) affixing at least one layer of a polymer matrix doped with a compound of the invention as above described, between at least one hole injection electrode and at least one electron injection electrode.

A further aspect of the invention provides an to electroluminescent device comprising as a light emitting layer, at least one layer of the above mentioned compounds.

More particularly, said device comprises at least one hole injection electrode and at least one electron injection electrode with at least one layer of said compound doped in the polymer matrix doped with said compound sandwiched in between said electrodes.

A further aspect of the invention is to use the electron transporting property of the compounds of the invention in a electroluminescent device.

The compounds of the invention are synthesized according to any known method, for example Friedlander condensation.

For example, when R₄ is hydrogen, the pyrazolof 3 , - b]quinoline and its derivatives can be prepared through Friedlander condensation of substituted pyrazolin-5- ones and o-aminocarbonyl compounds at elevated temperature with or without a solvent.

The reactants are preferably, 2-aminobenzophenone, 5- chloro-2-aminobenzophenone, 2-aminobezaldeyde, 2- aminoacetophenone and methyl, phenyl, hydrogen substituted pyrazolin-5-ones in ethylene glycol in the range 120-200°C,

When R_A is not hydrogen, the pyrazoloquinoline and its derivatives are also prepared through cyclisation reaction of o,m,p-substituted aromatic amines with 1, 3-disubstituted-5-chloro-4-formylopyrazoles in elevated temperature 150-250°C in a high boiling solvent or in melt.

The reactants are preferably, 5-chloro-4-formylo pyrazoles substituted with alkyl and aromatic groups and p-substituted aromatic amines in diphenyl either, ethylene glycol, propylene glycol, sulpholane.

Symmetrical bispyrazolopyridine (i.e. Ri=R₆ and R₂=R₅) and its derivatives are prepared through thermal cyclisation of 1, 3-disubstituted-5-aminopyrazoles with aromatic acids and aldehydes in high boiling solvent or in a melt.

Rι=Rθ , R2=Rs

Preferable reactants are 1,3-dialkyl, l-aryl-3-alkyl-5- aminopyrazoles with aromatic aldehydes by melting reactants in the range 120-240°C. Unsymmetrically 1, 3 , 5 , 7-substituted bispyrazolopyridine and its derivatives are prepared through thermal condensation of 1 , 3-disubstituted-5-aminopyrazoles and 1 , 3-disubstituted-5-chloro-4-aroylpyrazoles . Preferably they are l-aryl-3-alkyl-5-chloro-4- benzoylpyrazoles and 1,3-dialkyl or l-aryl-3-alkyl or l-alkyl-3-aryl-5-aminopyrazoles at elevated temperature .

A group of particular interest for blue electroluminescent device application has the structure of the formula (II) wherein R, to R₃ have the same definition as previously described and wherein R_A is hydrogen or an alkyl group.

When R, and R₂ are also alkyl groups, the emission is violet-blue; when R, is an alkyl and R₂ an aryl group, the emissions are from violet blue to deep blue; when R_: is an aryl and R₂ an alkyl group, the emissions are blue; when R, and R₂ are both an aryl group, the emissions are greenish blue.

The introduction of various R groups will generally enhance the quantum efficiency of the photoluminescence . Preferably, these groups are methyl and aryl groups .

The photoluminescence quantum efficiencies of these compounds in solutions were measured using a relative method and quinine sulphate as standard on Perkin- Elm.er._rw LS-50B fluorescent spectrophotometer . The Blue emitters mentioned above generally have high quantum efficiencies, which is about 0.7-1.0 in solution.

For example, the following table A shows the compounds of the structure shows in formula (II), where R, is phenyl and R₄ is hydrogen.

TABLE A

With respect to the compounds of the structure (III), the blue emitters are particularly those where R^g have the same definition as described before.

Preferably, R_x and R₆ are alkyl groups or aryl groups; R₂ and R₅ are alkyl or aryl groups. The emissions from these compounds are from violet blue to greenish blue.

The photoluminescence quantum efficiencies of these compounds in solutions were measured using a relative method and quinine sulphate as standard on Perkin- Elmerxw LS-50B fluorescent spectrophotometer. The Blue emitters mentioned above generally have high quantum efficiencies, which is about 0.7-1.0 in solution. For example, the compounds of structure (II), where R, = R₆ are phenyl groups and R₂ = R₅ are methyl groups . The emissions wavelengths and the photoluminescence quantum efficiencies from these compounds (in chloroform) with different R₃ groups are listed in the following Table B.

TABLE B

Advantageously the electroluminescent device according to the invention has a sandwich structure consisting of doped polymer layer (s) in between a transparent electrode and a metal or metal alloy electrode.

The device performance may be optimised by balancing the hole/electron transporting properties of the pyrazol-based doped matrices. Well known hole transporting materials, such as poly( N-vinylcarbazole ) (PVK), triphenylamine derivative (TPD) and/or electron transporting materials such as 2- ( 4-Biphenylyl ) -5- ( 4- tert-butylphenyl) -1, 3 , 4-oxadiazole (PBD) or 8- hydroxyquinoline aluminium complex (Alq₃) may also be used for the optimisation of device performance although it is not necessary.

It should be noted that pyrazol-based compounds which show electron transporting properties may be mixed with hole transporting materials and used as an the emitting layer in a single layer Electroluminescent device.

Also, in multiple layer devices they can be used as electron transporting layer and emitting layer.

The transparent electrode is preferably an indium-tin- oxide (ITO) layer coated on a glass substrate, which is used as a hole injection electrode. The ITO coated glass should be thoroughly cleaned using a solution with or without a ultrasonic bath. Preferably the solution is an organic solvent, such as ethanol, propanol or acetone. The electron injection electrode may be of the type of a metal or metal alloy electrode of Al, Ag, Mg, In, Ca, or Mg/Ag vacuum deposited at ( 8 x 10"⁶ mbar. The alloy may be prepared through a co- evaporation process.

The polymer layers are preferably spin-coated from a solution in which either of the compounds of the invention has been previously doped. The polymers which can be used for this purpose are , for example, the poly(N-vinyl carbazole) (PVK), the poly(methyl methacrylate) (PMMA) , the polystyrene (PS) and/or the polycarbonate (PC) . The solvents used to prepare polymer solution depend on the polymers and depend on the availability of commercial solvents. Preferably the polymer is PVK.

The polymer layers thus formed have preferably a thickness of no more than 250 nm. The polymer layer can be a single layer or multiple layers. In the case of multiple layers, the total thickness of the polymer layers should be no more than 250nm, preferably no more than 150nm. In the case of multiple layers, either pyrazol-based compounds or the other transporting materials such as the one previously mentioned in the description may be used as transporting layer in the device optimisation either in pure form or doped in a polymer matrix. The spin-coating process is carried out in air but in a clean environment, preferably in a clean room.

Electroluminescent devices prepared as described herein were tested in a clean environment. The devices were driven using a forward bias DC voltage (or current) . The current-voltage characteristics and Light-Current (or Voltage) characteristics were measured using both linear stair case sweep and linear stair case pulsed swee .

Some of the compounds of the invention described herein above emit light other than blue. Of course they can also be used in electroluminescent devices based on the same principle.

Particular examples of the invention will now be described with reference to the figures in which :

Fig.l is a schematic view of a typical electroluminescent display device made according to Example 1.

Fig.2 is a diagram showing the variation of the light intensity and the current with respect to the voltage applied to the electroluminescent device of the Example 1. Fig.3 is a diagram showing the variation of light intensity of the electroluminescent device of the Example 1 with respect to the current.

Fig.4 shows the electroluminescent spectrum of the EL device of the Example 1.

Fig.5 shows the electroluminescent spectrum of an EL device made according to Example 2.

EXAMPLE 1

Preparation of 1, 3-biphenyl-4-methyl-pyrazolo[3,4- b]quinoline (PAQ4)

A mixture of equal molar amounts (0.01M) of 1,3- bisphenylpyrazoline-5-one and 2-aminoacetophenone was heated at 180°C in ethylene glycol (5ml) for 24 hours. After the mixture was cooled, EtOH was added. The crystalline precipitate was filtered out. It was then washed with EtOH recrystallised from toluene. Yellow crystals with a melting temperature of 174-5°C were obtained. (Yield 63%)

Elemental Analysis: Calc . for C₂₃H₁₇N₃: C: 82.37%; H: 5.11%; N:12.52%; found:C: 82.28%; H: 4.98%; N: 12.48%

^lE NMR (CDC1₃) δ(ppm): 2.78(s,3H); 7.29(t,lH); 7.44(t,lH); 7.50-7.56 (m, 5H) ; 7.71-7.47 (m, 3H) ; 8.11(d,J=8,26Hz,lH) ; 8.14(d,J=8,62Hz,lH) ; 8.56 (d, J=7 , 59Hz , 2H) ¹³C NMR(CDC1₃) δ(ppm): 15.48, 116.32, 120.73, 123.81, 124.36, 125.22, 128.35, 128.94, 129.47, 130.06, 130.24, 133.96, 139.89, 141.36, 146.81,148.05, 150.03.

Device Fabrication An electroluminescent device consisting of a single- layered sandwich structure: a thin layer of doped polymer film on top of an indium-tin-oxide (ITO) glass and an aluminium top electrode. The ITO glass has a sheet resistance of 30 Ohm/sq. The thin polymer film was spin-coated from a chloroform solution which was previously doped with 1, 3-diphenyl-4-methyl-lH- pyrazolo[3,4-b]quinoline (PAQ4) (4% to the polymer weight) onto the ITO glass with a thickness within 150 nm. Aluminium electrode was vacuum deposited at a pressure <8xl0^"6 mbar and had a final thickness within 150nm. The final device has an emitting area in the range of 10-20mm².

The device was driven using a forward bias pulsed voltage or current. The turn-on voltage of the device is about 15 V. The electroluminescence from device is blue and its emission spectra has a maximum centred at 442nm.

Figure 2 shows that the light output has a linearly relationship with the forwards bias current. Figure 3 shows the turn-on voltage of this device is above 15V.

The electroluminescent spectrum of this device is shown in Figure 4. Its electroluminescence has about 20nm from its photoluminescence.

EXAMPLE 2

Preparation of 4-phenyl-l, 3,5, 7-tetramethyl-bis- pyrazolo-[3,4-b;4' ,3'-e]pyridine (PAP1)

A mixture of 5-amino-l , 3-dimethylpyrazole (0.05 mole) and benzaldehyde (0.025 mole) was heated initially at 120-140°C. After about 30 minutes, gas evolution had ceased and the reaction was allowed to continue for an additional 3 hours at 240°C It was then cooled to room temperature, the resulting solid was recrystallised from ethanol/water (4:1) to give 2.3g (42%) of 4- phenyl-1, 3,5, 7-tetramethyl-bis-pyrazolo- [ 3 , 4-b; 4 ' , 3 ' - e]pyridine (m.p. 164-l65^-5oC) . The sample was further purified by column-chromatography on Merk silica gel using CH₂,C1₂ as a solvent. The purified sample has a melting temperature of 165-6°C.

Elemental Analysis: Calc . for C₁₇H₁₇N₅: C: 70.07%; H: 5.90%; N: 24.03%; found: C: 70.25%; H:6.01%; N: 24.12%.

^XH NMR (CDC1₃) δ(ppm): 7.49-7.52 (m, 3H) ; 7.38-7.39 (m, 2H) ; 4.06(s,3H) ; 2.00(s,3H) .

¹³C NMR (CDCI₃) δ(ppm): 151.71, 141.88, 140.77, 134.62, 128.61, 128.53, 127.69, 111.20, 33.17, 14.52.

Device Fabrication

An EL device can be fabricated using the above mentioned compound (PAPl) using the method described in Example 1. Fig.5 shows the electroluminescent spectrum of such a device. Its electroluminescence is within the same spectral range of its photoluminescence.

Claims

1. A compound of the formula I

wherein :

Ri to R₃ are substituents , identical or different, chosen in the group consisting of hydrogen, halogens and alkyl, aryl, heterocyclic, alkenyl, alkynyl, alkoxy, allyoxy, aryloxy, benzyloxy, hydroxy, and amino groups .

2. A compound as claimed in Claim 1, wherein R, to R₃, identical or different, are chosen in the group consisting of linear alkyl groups having 1 to 11 carbon atoms; tert-butyl; phenyl groups, substituted or not; naphthyl group; pyridine group; alkenyl groups of the formula - (CH₂)_nCH=CH₂ wherein m is an integer ranging from 0 to 8 ; alkynyl groups of the formula -C≡C-R' ' wherein R' ' is hydrogen, alkyl or aryl; alkoxy groups of the formula -OR' wherein R' is alkyl; aryloxy groups of the formula -OAr wherein Ar is an aryl or an heterocyclic group; -Cl; -Br and amino groups of the formula -NR'iR'z, wherein R', and R'₂, identical or different, are an alkyl, aryl or heterocyclic group .

The compound as claimed in any Claim 1 or 2 , said compound having the following formula (II) :

wherein R₄ has the same meaning as R_x to R₃.

The compound as claimed in Claim 3, wherein R_: and R₂, identical or different, are an aryl or heterocyclic group; and

R₃ and R^,, identical or different, are an halogen or an alkyl, aryl, heterocyclic, alkenyl, alkynyl, alkoxy, allyoxy, aryloxy, benzyloxy, hydroxy, and amino groups .

The compound as claimed in any of Claims 1 to 3 , wherein R_x is phenyl.

The compound as claimed in any of Claims 1 to 3 and 4, wherein R₂ and R_3/ identical or different are selected from the group consisting of hydrogen, phenyl and Cι-_n alkyl group.

The compound as claimed in Claim 3, said compound having the following formula (Ila) :

The compound as claimed in Claims 1 or 2 , said compound having the following general formula

wherein R₅ and R₆, identical or different have the same meaning as R, to R₃.

9. The compound claimed in Claim 8, wherein R R₂, R₅ and R₆, identical or different, are an aryl or an heterocyclic group and R₃ is an halogen or an alkyl, aryl, heterocyclic, alkenyl, alkynyl, alkoxy, allyoxy, aryloxy, benzyloxy, hydroxy, and amino group .

10. The compound as claimed in Claim 6, wherein R_i=R₆ and R₂=R₅.

11. The compound as claimed in Claim 10, wherein R, is a phenyl group and R₂ is a methyl group.

12. The compound claimed in Claim 6, and having the following formula (Ilia):

13. The compound as claimed in any of Claims 1 to 12, wherein said compound has electron transporting property.

14. Use of a compound as claimed in any of Claims 1 to 13 in an electroluminescent device.

15. Use as claimed in Claim 14, wherein said compound is doped into a polymer matrix.

16. Use as claimed in Claim 15, wherein the polymer is chosen from the group consisting of poly(N-vinyl carbazole) , poly(methyl methacrylate) , polystyrene, polycarbonate and a mixture thereof.

17. Use as claimed in Claim 14, wherein the electroluminescent device comprises at least one hole injection electrode and at least one electron injection electrode with at least one layer comprising said compound, said layer being sandwiched in between said electrodes .

18. Use as claimed in Claim 17, wherein said compound is doped in a polymer matrix.

19. Use as claimed in any of Claims 17 or 18, wherein the device is a multiple layers device and wherein said compound is used as an electron transporting material .

20. Method to manufacture an electroluminescent device comprising the steps of : a) affixing at least one layer of a polymer matrix doped with a compound as claimed in any of the Claims 1 to 9 between at least one hole injection electrode and at least one electron injection electrode.

21. Electroluminescent device comprising as a light emitting layer, at least one layer of a compound as described in any of Claims 1 to 13.

22. The device claimed in Claim 21, wherein said device comprises at least one hole injection electrode and at least one electron injection electrode with at least one layer of said compound doped in the polymer matrix doped with said compound sandwiched in between said electrodes.

22. Electroluminescent composition to be used in electroluminescent devices, said composition comprising a compound as described in any of Claims 1 to 13 doped in a polymer matrix.