Spirobifluorene compounds for light emitting devices

[0001] The present invention relates to compounds based on spirobifluorene and light emitting devices comprising said compounds.

[0002] Various organic light emitting devices have been under active study and development, particularly those based on electroluminescence (EL) from small organic materials. For such organic devices, the ability to form morphologically stable amorphous films is a key requirement for the development of small materials for organic light emitting diodes (OLEDs). That is because when a small molecule compound is used in the organic light-emitting layer, crystallization usually occurs if the molecule of the compound is too small and its structure is too symmetrical. Therefore, when applied in an organic emission layer, the small molecule compound is vulnerable to morphological change such as crystallization, and once the crystal is formed, it yields negative impacts upon the light-emitting nature and service life of the OLED.

[0003] Thermal stress during device operation can lead to such phase transitions from the amorphous state to the thermodynamically stable polycrystalline state leading to dramatic degradation of the device. As a result it is crucial to design materials featuring high glass transition temperature (Tg

>150°C) in order to stabilize the amorphous state. For improving the stability of devices in order to increase operational lifetime, several host materials have been reported. Especially, designing materials having a spiro linkage has been a very successful strategy to obtain OLEDs materials with enhanced morphological stability while keeping their electro- optical functionality.

[0004] US2006/0141287 discloses light-emitting layers which include a solid

organic material containing a mixture of at least two components. The first host component is an organic compound capable of transporting electrical charges and also forms an aggregate. The second component of the mixture is an organic compound capable of transporting electrical charges and, upon mixing with the first host component, is capable of forming a continuous and substantially pin-hole-free layer. In the reference, as the second component, various compounds such as substituted fluorene derivatives, and spirobifluorene derivatives, etc. are used.

[0005] Spirobifluorene , as used herein denotes a structural element of formula (S1 ) and is referred to as SBF hereinafter, whereas Open SBF denotes a system of formula (S2) below.

[0007] Substituted spirobifluorene compounds have been extensively described in the prior art, in particular with a substitution of the SBF ring system in "para-position (i.e. the 2, 7, 2' or T position of the SBF) by a heteroatom.

[0008] Substituted spirobifluorene of this type where the heteroatom bound to the spirobifluorene unit is part of a ring system have also been described.

[0009] Salbeck et al., Chem.Rev. 2007, 107, 101 1 -1065 provides a good

overview of spiro compounds useful in organic optoelectronics. Thus, compounds (16) and (17) in Salbeck are the following

[0012] These compounds are reported to have absorption maxima below 400 nm and emission maxima of 491 and 51 1 nm, respectively. No data on efficiency are given.

[0013] Salbeck further reports that replacement of diphenylamino substituents like in the compounds above by carbazole ligands results in a distinct hypsochromic shift of absorption and emission. As an example the compound where the substitutents are replaced by N-carbazole shows an absorption maximum below 350 nm and an emission maximum at 372 nm.

[0014] Spirobifluorene compounds with diphenylamino substituents in para- position of the SBF unit are again disclosed in Salbeck et al., see e.g. compounds 42 to 48 thereof.

[0015] WO 201 1/06574 discloses 4 and 4,4'diphenylamino-substituted SBF

compounds (which may be referred to as ortho-substituted compounds relative to the direct bond linking the phenyl rings of the SBF unit).

[0016] European Patent Application 2 312 667 discloses compositions for organic electroluminescence elements comprising at least two different materials fulfilling a certain mathematical equation related to the solubility of the materials. Among an extended list of suitable materials having as a common structural feature substituted diphenylamino groups, 3,6-Bis-N,N'- di(4-tert.butylphenyl)amino-spirobifluorene as well as the respective Open SBF derivative are mentioned. [0017] Also, US Patent 6,893,743 discloses in table 1 compounds compounds H- 151 and H-152 in which a central SBF unit carries two substituents in 2 and 7 position, wherein the substituent is bound to the SBF through a nitrogen atom not forming part of an aromatic ring..

[0018] None of the above-disclosed materials meets all the requirements

necessary for OLED application, particularly suitable energy level for high phosphorescent efficiency (high triplet energy), high morphological stability, while maintaining other electro-optic and processing properties under operational conditions of the device, such as emission colour, dimensional stability, etc, in a fully satisfactory manner. Thus, there has been a need to develop new host materials, which are capable of satisfying all of the requirements indicated above.

[0019] Surprisingly, it has been found that spirobifluorene compounds as defined in claim 1 possess a property spectrum which makes them particularly suitable for use in organic electroluminescent devices.

[0020] Preferred compounds in accordance with the instant invention are

described in the dependent claims and the detailed specification hereinafter.

[0021] The compounds of the present invention are characterized by formulae 1 to 10 below

[0022] (1 ) (2)

[0025]

[0026] wherein n, m and o may be the same or different and represent an integer of from 0 to 3,

each of the phenyl rings may carry no ligands other than L ¹ or may be substituted by ligands other than L ¹ ,

[0027] L ¹ , which may be the same or different in each position, has the formula A [0028]

—

[0029] [0030] wherein

Y is selected from the group consisting of O, S, C=O, -CR ¹ R ² , N-R ³ , S=O, S(=O) ₂ , PR ⁴ and P(=O)R ⁵ , R ¹ to R ⁵ may be the same or different in each position and represent hydrogen or an aliphatic, carbocyclic, aromatic or heterocyclic group with 1 to 20 carbon atoms, and Ar ¹ and Ar ² represent optionally substituted aromatic or heteroaromatic ring systems comprising 4 to 20 ring atoms (the two ring atoms of the heterocyclic ring shown in formula I being part of the aromatic or heteroaromatic ring system for the purpose of counting ring atoms).

[0031] For the purpose of the present invention, the term "aliphatic" is intended to denote any group which is attached via a carbon atom or a heteroatom which carbon atom or heteroatom is not part of a ring whereas the term aromatic is intended to denote aryl groups as well as heteroaryl groups attached through a carbon atom or a heteroatom forming part of an aryl or heteroaryl ring.

[0032] In accordance with a first preferred embodiment R ¹ and R ² , which may be the same or different, represent hydrogen, an alkyl group, a carbocyclic group, an aryl group or a heteroaryl group, which groups are substituted or unsubstituted.

[0033] The compounds in accordance with the present invention share the

common feature that the SBF or Open SBF unit is substituted by a nitrogen atom, which is part of a ring system comprising two aromatic or heteroaromatic rings.

[0034] The substitution in the SBF system may be para, meta or ortho to the bond linking the phenyl rings in the SBF unit or in the analogous positions of the Open SBF unit.

[0035] It has been found that for certain purposes compounds having at least one substituent L ¹ in meta position can be advantageous in terms of efficiency when used in organic electronic devices.

[0036] A first preferred group of compounds are those where L ¹ has the formula A 1

[0038] where Y is as defined above.

[0039] Y is preferably selected from the group consisting of O, S, C=O, CR ¹ R ² , N- R ³ and S=O, particularly preferred from O, S and N-R ³ , even more preferably from O or S, most preferably Y is O, wherein R1 to R ³ have the meaning as defined above.

[0040] If R ¹ to R ⁵ represents an alkyl group, same has preferably 1 to 20,

especially 1 to 8 carbon atoms and may be straight chain or branched. Particularly preferred alkyl groups are Ci to C ₄ alkyl like methyl, ethyl, i- or n-propyl and i-, n- and t-butyl. The alkyl groups may themselves be substituted or unsubstituted.

[0041] Preferred carbocyclic groups for R ¹ to R ⁵ are 5 to 7 membered carbocyclic ring systems, which may be saturated or unsaturated like e.g.

cyclopentane, cyclohexane or cyclohexene, to give only three examples. As for the alkyl groups, the carbocyclic groups may be substituted or unsubstituted.

[0042] Preferred aryl groups for R ¹ to R ⁵ are are phenyl, naphthyl, anthracenyl, biphenyl or terphenyl, which may be unsubstituted or substituted by substituents selected from the group consisting of halogen, alkyl, alkoxy, amino, cyano, alkenyl, alkynyl, arylalkyl, aryl and heteroaryl groups or the aryl group may be part of an annealed ring system.

[0043] Especially preferred aryl substituents are derived from the following

substituted or unsubstituted aryl systems

[0044]

[0046] of which phenyl and biphenyl are especially preferred.

[0047] A particularly preferred group of heteroaryl groups for R ¹ to R ⁵

following:

2H-pyrrole 3H-pyrrole 1 -substituted 1H-imidazole 2H-imidazole 4H-imidazole

In all these ring systems one or more of the nitrogen atoms may be replaced by another heteroatom like O or S, to name only two exam as shown below:

oxazole isoxazole thiazole isothiazole

1 ,2,3-oxadiazole 1 ,2,5-oxadiazole 1 ,2,3-thiadiazole 1 ,2,5-thiadiazole

[0049] Still another preferred group of heteroaryl substituents comprises the 6- membered ring systems shown below:

pyridine pyridazine pyrimidine pyrazine 1 ,2,3-triazine

1 ,2,4-triazine 1 ,3,5-triazine 1 ,2,3,4-tetrazine 1 ,2,3,5-tetrazine 1 ,2,4,5-tetrazine

[0050] In the same manner as the aryl groups the aforementioned heteroaryl groups may be substituted, preferably by substituents selected from the group consisting of halogen, alkyl, alkoxy, amino, cyano, alkenyl, alkynyl, arylalkyl, aryl and heteroaryl groups or the heteroaryl group may be part of an annealed ring system.

[0051] It is apparent to the skilled person that steric reasons may exclude or render difficult a certain combination of R ¹ and R ² in the compounds of the present invention, no further explanations n this regard being necessary here.

[0052] Ar ¹ and Ar ² , which may be the same or different can be selected from the aromatic or heteroaromatic ring systems described above for substituents R ¹ to R ⁵ and thus reference thereto is made at this point. Preferably, Ar ¹ and or Ar ² , which may be the same or different, are aryl ring systems as defined above, preferably phenyl or naphthyl, which may be substituted or unsubstituted.

[0053] A further group of preferred compounds in accordance with the present invention are those wherein at least one of n, m or o represents an integer of from 1 to 3.

[0054] Also preferred are compounds wherein n, m and o are all zero.

[0055] Further preferred amongst such compounds (where n, m and o are zero are compounds of general formula

[0057] where L ¹ can have any of the meanings defined above.

[0058] Accordingly a particularly preferred group of compounds of the formulae above carries substituents of formula A-1 and in particular substituents A-1 wherein Y is O or S.

[0059] The SBF or open SBF ring system may or may not comprise further

substituents in addition to substituents L ¹ . If present, such additional substituents, which may be the same or different in each position they occur, are generally selected from of halogen, alkyl, alkoxy, amino, cyano, alkenyl, alkynyl, arylalkyl, aryl and heteroaryl groups.

[0060] The compounds in accordance with the present invention may be

synthesized by any known and suitable method. The skilled person is aware of suitable manufacturing processes.

[0061] Generally, the compounds of the present invention with meta-substituents may be prepared by the following general reaction schemes, which show an exemplary way for compounds carrying one or two ligands L ¹

[0062] wherein X is a leaving group selected from known leaving groups for such reactions such as halogen, OH, OR, SR , OCN, SCN or CN, especially preferably halogen, in particular chlorine or bromine.

[0063] The skilled person will select the suitable reactants and reaction conditions based on the individual needs of a specific synthesis.

[0064] The starting materials for such synthesis with at least one leaving group in a meta-position of the SBF or Open SBF ring system may be synthesized in accordance with various process routes which the skilled person will select in accordance with the specific needs. Generally, such compounds are not easily accessible through introduction of the substituents directly into a SBF or Open SBF core as these routes generally yield the para- substituted products preferably due to their higher reactivity. Accordingly, the substituents X have to be introduced through suitable precursor substances e.g. fluorene derivatives, benzophenone derivatives or biphenyl derivatives, to mention only three examples, which are thereafter reacted to yield the SBF or Open SBF structure.

[0065] Thus respective compounds may for example be obtained from substituted fluorenone derivatives of formula

[0067] with suitable biphenyl compounds.

[0068] Another possibility is the reaction of fluorenones with suitable substituted bi hen l com ounds in accordance with the eneral reaction scheme

[0069]

[0070] which is described in more detail in JP 2006/089585 for X= OH and which may be adopted for other substituents X.

[0071] Another embodiment of the present invention is directed to the use of the compounds of the present invention in an organic light emitting device, especially an organic light emitting diode (OLED).

[0072] The compounds in accordance with the present invention may

advantageously be used, together with an emitting material, in the emissive layer of an organic light emitting device.

[0073] Furthermore, the compounds are also suitable for other layers of organic electronic devices, in particular for other layers of organic electronic diodes.

[0074] According to another preferred embodiment, the present invention is also directed to an organic light emitting device (OLED) comprising an emissive layer (EML), said emissive layer comprising the compounds of the present invention as host material, said host material being notably suitable in an emissive layer (EML) in an OLED.

[0075] An OLED generally comprises :

a substrate, for example (but not limited to) glass, plastic, metal;

an anode, generally transparent anode, such as an indium-tin oxide (ITO) anode;

a hole injection layer (HIL) for example (but not limited to) PEDOT/PSS; a hole transporting layer (HTL);

an emissive layer (EML);

an electron transporting layer (ETL);

an electron injection layer (EIL) such as LiF, CS2CO3

a cathode, generally a metallic cathode, such as an Al layer.

[0076] For a hole conducting emissive layer, one may have a hole blocking layer (HBL) that can also act as an exciton blocking layer between the emissive layer and the electron transporting layer. For an electron conducting emissive layer, one may have an electron blocking layer (EBL) that can also act as an exciton blocking layer between the emissive layer and the hole transporting layer. The emissive layer may be equal to the hole transporting layer (in which case the exciton blocking layer is near or at the anode) or to the electron transporting layer (in which case the exciton blocking layer is near or at the cathode).

[0077] The compounds of the present invention may be used preferably used as hosts in an emissive layer.

[0078] Optionally, the emissive layer may also contain a polarization molecule, present as a dopant in said host material and having a dipole moment that generally affects the wavelength of light emitted.

[0079] A layer formed of an electron transporting material is advantageously used to transport electrons into the emissive layer comprising the light emitting material and the (optional) host material. The electron transporting material may be an electron-transporting matrix selected from the group of metal quinoxolates (e.g. Alq3, Liq), oxadiazoles, triazoles and ketones (e.g. Spirobifluorene ketones SBFK). Examples of electron transporting materials are tris-(8-hydroxyquinoline)aluminum of formula ["Alq3"] and spirobifluoreneketone SBFK:

SBFK

A layer formed of a hole transporting material is advantageously used to transport holes into the emissive layer comprising the light emitting material as above described and the (optional) host material. An example of a hole transporting material is 4,4'-bis[N-(1 -naphthyl)-N- phenylamino]biphenyl ["a-NPD"].

The use of an exciton blocking layer ("barrier layer") to confine excitons within the luminescent layer ("luminescent zone") is greatly preferred. For a hole-transporting host, the blocking layer may be placed between the emissive layer and the electron transport layer. An example of a material for such a barrier layer is 2,9-dimethyl-4,7-diphenyl-1 ,10-phenanthroline (also called bathocuproine or "BCP"), which has the formula

BCP

[0082] The OLED has preferably a multilayer structure, as depicted in Figure 1 , wherein 1 is a glass substrate, 2 is an ITO layer, 3 is a HIL layer comprising PEDOT/PSS, 4 is a HTL layer comprising a-NPD, 5 is an EML comprising mCBP as host material and the light emitting material or mixture of these materials as above defined as dopant in an amount of about 15 % wt with respect to the total weight of host plus dopant; 6 is a HBL comprising BCP; 7 is an ETL comprising Alq3; 8 is an EIL comprising LiF and 9 is an Al layer cathode.

[0083] An example of the present invention is reported hereinafter, whose

purpose is merely illustrative but not limitative of the scope of the invention itself.

[0084] Example 1

[0085] Synthesis of 3-Bromo-SBF

3-Bromofluorenone

[0088] In a three ways flask 60 ml of water were added to 8.9 ml of hydrochloric acid (HCI, 37 % w/w, 2.1 molar equivalents) and the medium was cooled to 0°C. NaNO2 (1.5 molar equivalents), dissolved in 50 ml of water, was added dropwise at 0°C. At the end of the addition, 4-amino-2- bromobenzophenone (one equivalent, 15.0 g, 51.6 mmole) solubilised in a mixture of acetone/water (400/230 ml), was added carefully. After 30 minutes at room temperature, the mixture was warmed up and kept at 60 °C for 3 hours.

[0089] After extraction with methylene chloride and evaporation of the organic phase, a brown solid was recovered (17.4 g) and a flash chromatography is realized. The pure compound was recovered after crystallization with hexane (4.2 g, 32 % yield).

[0090] Step 2: 3-bromo-SBF

[0091] ^^^^^^^Br

[0092] This compound was made in two steps from 3-bromofluorenone obtained in step 1. First, 2-bromobiphenyl (1.05 equivalents, 4.0 g, 16.5 mmol) is solubilised in 102 ml of anhydrous diethyl ether. This solution is cooled to -60°C and n-BuLi (1.16 eq.) is added dropwise. After 10 min at this temperature, a white precipitate appeared which was redissolved while the medium was warmed to room temperature. 3-Bromofluorenone was then added and the reaction mixture was kept at 45 °C for one night.

[0093] After addition of NH ₄ CI (5% aq., 260 ml) and extraction with diethyl ether, 7.0 g of an alcohol was obtained. This solid was solubilised in 141 ml of acetic acid and hydrolyzed by the addition of 78 ml of HCI/dioxane (10 % mol., 20 eq.). After evaporation of the solvents, the solid was subjected to normal phase flash chromatography to afford 5.86 g of the target compound (94 % yield).

[0094] Step 3: 3-phenoxazyl -SBF

[0096] In a 500 ml flask, 320 ml of toluene were degassed during 2 hours. 3-

Bromo SBF (9.9 g, 0.025 mol), sodium tert. butylate (tBuONa, 4.92 g, 0.05 mol) and phenoxazine (4,6g, 0.025 mol) were added and after 30 min, a mixture of Bis-dibenzylideneacetone palladium (Pd(dba)2) and Tris-tert.- butylphosphine (P(tBu)3) previously prepared was slowly added.

[0097] The solution was warmed to 1 10 °C for one hour and then filtrated on a celite pad. After evaporation of the solvent, the solid recovered was recrystallised in ethanol to afford 9.4 g of a white solid (77 % yield).

[0098] Example 2

[0099] Synthesis of 3-Chloro-SBF

[00100] Step 1 : Synthesis of 1 -Bromo-7-chloro-biphenyl

[00101] A 50-ml round-bottom flask under nitrogen atmosphere was charged

sequentially with Pd(OAc) ₂ (1.07 g, 0.0047 mol), PPh ₃ ( 5,0 g, 0.0032 mol) and dioxane (35 ml). This mixture was added to a 500 ml round bottom flask already filled with 1-Chloro-3-iodobenzene (13.6g, 0.056 mol) in dioxane (150 ml), 2N aqueous sodium carbonate (180 ml) and 2- Bromophenylboronic acid (12.3 g, 0.059 mol). This mixture was heated at reflux under N2 for 1.5 h and cooled to room temperature. The reaction medium was partitioned between water and ethyl acetate. The combined organic layers were washed with brine, dried over MgS04, and

concentrated. The residue was purified by column chromatography (Ch C /hexane) to afford the desired product with 76.6 % yield. -SBF

[00104] To a solution of 1-Bromo-7-Chloro-biphenyl (10 g, 0.037mol) in anhydrous THF (100 ml) cooled to -78°C a solution of 1.6 M n-BuLi in hexane (0.037 mol, 23.2 ml) was added dropwise. The reaction mixture was stirred at - 78°C for 1 h and a solution of fluorenone (0.031 mol, 5.58g) in anhydrous THF (25 ml) was added dropwise. After addition, the mixture was allowed to warm to room temperature and stirred for 2h. The mixture was quenched with saturated NH ₄ CI (200 ml) and extracted with ethyl acetate (3 ^* 125 ml). The combined organic layers were washed with brine, dried over Na2SO4 (or MgSO ₄ ) and concentrated in vacuo. The residue was purified by flash chromatography to afford the target compound with -20% yield.

[00105] Step 3: Preparation of 9,9-dimethyl-9,10-dihydroacridine

[00106] 2-(phenylamino)benzoic acid (50 g, 0.23 mol) was dissolved in methanol (1 L), put in an ice bath and stirred for ten minutes. After slowly adding SOCI2 (60 ml_, 0.58 mol) thereto at 0°C, the mixture was stirred under reflux for 12 hours at 90°C.Upon completion of the reaction, the reaction mixture was washed with distilled water and extracted with ethyl acetate. After drying the organic layer with magnesium sulfate and removing the solvent by a rotary type evaporator, 2-(phenylamino) methyl benzoate (47 g, 92%) was obtained through purification by column chromatography using ethyl acetate as developing solvent.

[00107] 90 g 2-(phenylamino)methyl benzoate (90g, 0.3 mole) was added to THF (1.5 L) and methyl magnesium bromide (462 ml_, 1.38 mole) was slowly added to the mixture, which was thereafter stirred at room temperature for 12 hours. Upon completion of the reaction, the reaction mixture was neutralized with distilled water and extracted with ethyl acetate. After drying the organic layer with magnesium sulfate and removing the solvent by a rotary type evaporator, 80 g (90%) of

[00108]

[00109] was obtained through purification by column chromatography using ethyl acetate as developing solvent

[001 10] 80 g (0.35 mole) of the compound obtained in the previous step was

added to 1.7L of phosphoric acid and the mixture was stirred for 12 hours at room temperature. Upon completion of the reaction, the reaction mixture was neutralized with distilled water and the produced solid was filtered while being washed with water. The solid was dissolved in

dichloromethane, extracted and neutralized with sodium hydroxide. After drying the organic layer with magnesium sulfate and removing the solvent by a rotary type evaporator, 64 g 9,9-dimethyl-9,10-dihydroacridine (87%) was obtained via recrystallization in hexane.

[001 1 1] : 3-(9,9-dimethyl-9,10-dihydroacridine)-SBF

[001 12] Catalyst Pd(dba) ₂ (5 % mol, 490 mg) and the phosphine P(tBu) ₃ (4% mol, 0.675 ml_ of 1 M P(tBu)3 in toluene) were introduced at room temperature in toluene (10 ml_, anhydrous and well degassed) in a two way flask. After 15 min under nitrogen, the other reagents 3 CI-SBF (1eq, 5.98 g, 16.9 mmol), 9,9-dimethyl-9,10-dihydroacridine (1eq, 3.53 g, 16.9 mmol) and tBuONa (3 eq, 5.0 g, 60.6 mmol) were introduced and the reaction medium was warmed at 90 °C for 3 hours. At the end of the reaction, the medium was filtrated on diatomaceous earth (celite) and solvent was evaporated under vacuum. The solid was absorbed on silica gel and a dry flash chromatography was realized (methylene chloride / hexane). After solvent evaporation, the solid was recrystallized in hexane (m = 7.66 g, yield = 87%).

[001 13] The HOMO level was determined to -5.29 eV, the LUMO level to -1.49 eV and the triplet energy to 2.88 eV.

[001 14] The HOMO level (EHOMO) has been calculated from the half wave potential obtained from the first oxidation (Ei _ox ^1/2 ) using the following equation:

EHOMO - (- 4.8) = - [Ei _ox ^{1 2} - E ₀ x ^{1 2} (Fc/Fc ⁺ )] where EHOMO (ferrocene) has been taken equal to 4.8 eV below the vacuum level.

[001 15] The E LUMO (CV) have been calculated from the 1 st reduction potential.

[001 16] The triplet energy has been calculated from the highest energy

phosphorescence peak in 2-MetTHF at 77 K.

[001 17] Device Examples

[001 18] All device examples were fabricated by high vacuum thermal evaporation, except for the hole injecting layer which was deposited by spin-coating technique. The anode electrode is 120 nm of indium tin oxide (ITO). All devices were encapsulated with a glass lid sealed with an epoxy resin in a nitrogen glovebox (<1 ppm of H2O and O2) immediately after fabrication, and a moisture getter was incorporated inside the package. The devices were characterized optically and electrically with a C9920-12 External Quantum Efficiency Measurement System from HAMAMATSU. EQE refers to external quantum efficiency expressed in %, while operational stability tests were done by driving the devices at continuous current at room temperature. LT50 is a measure of lifetime and corresponds to the time for light output to decrease by 50% of the initial value, when the device is driven at a constant current.

[001 19] The OLED stack consisted of sequentially, from the ITO surface, 30nm of Plexcore OC (a self-doping polymer poly(thiophene-3-[2[(2- methoxyethoxy)ethoxy]-2,5-diyl), supplied from Plextronics Inc.) deposited by spin-coating and dried on a hot plate at 200 °C for 20 min. On top of the HIL, 15 nm of NPB were deposited by by vacuum-thermal evaporation as hole transporting layer (HTL).

[00120] Then a 30nm layer of Compound B doped with different amounts of dopant Compound C was deposited by vacuum-thermal evaporation as the emissive layer (EML). Then a 5nm layer of Compound A was deposited by vacuum-thermal evaporation as the hole blocking layer (HBL). Then, a 40 nm layer of Alq3 was deposited by vacuum-thermal evaporation as the electron transporting layer (ETL). The cathode consisted of 1 nm of LiF followed by 100 nm of Aluminum.

[00121] As used herein, a-NPD, Compound A, Compound B, Compound C and Alq3 have the following structures

Compound A

Compound B

Compound C

[00123]

[00124] The device structure is summarized in Figure 2 while Table 1 shows the results measured for the fabricated devices. Compound B was used as host and compound C was used as dopant.

[00125] Table 1

[00126] As can be seen from the Examples, power efficiency increases with

increasing dopant concentration. The lifetime of the devices shows a maximum in the concentration range of from 15 to 20 wt% dopant, relative to the amount of host. Colour coordinates remain nearly constant whereas EQE is lower for 10 Wt% dopant compared to higher concentrations. The foregoing examples show that the compounds in accordance with the present invention are suitable as host materials in organic light emitting diodes.