Field of the Invention

The present invention relates to organic electroluminescent devices, which are sometimes otherwise referred to as organic light emitting diodes (OLED). Organic electroluminescent devices emit light on the application of an electric field to one layer or multiple layers of an organic compound (organic material layer). The present invention also relates to organic compounds suitable for use in such devices.

Background

An organic electroluminescent device is generally comprised of a pair of electrodes forming an anode and a cathode, and one layer or multiple layers comprising a hole injection layer, emission layer (with either fluorescent or phosphorescent material) and electron transporting layer. Into the organic layer(s), holes and electrons are injected from the anode and the cathode, respectively, thus resulting in excitons within the emission material. When the excitons transition to ground state, the organic luminescence device emits light.

According to the first study by Eastman Kodak Co. ("Appl. Phys. Lett", vol. 51 , pp. 913 (1987), an organic electroluminescent device which comprised a layer of an aluminium quinolinol complex (as electron transporting and luminescent material) and a layer of a triphenylamine derivative (as a hole transporting material) resulted in luminescence of about 1 ,000 cd/m ² under an application of a voltage of 10 V. Examples of related U. S. Patents include U. S. Pat. Nos. 4,539,507; 4,720,432 and 4,885,211.

Further studies by Baldo et al. revealed a promising OLED using phosphorescent material as dopant. The quantum yield of the phosphorescent OLED was significantly improved (U. S. Pat. No. 6,830,828).

In addition to the above-mentioned OLED, polymer organic electroluminescent device (PLED) using a conjugated polymer material has been reported by a group from Cambridge University (Nature, vol. 347, pp. 539- (1990), U.S. Pat. Nos. 5,247,190; 5,514,878 and 5,672,678).

PLED has an advantage in terms of device fabrication as a printing methodology may be adopted for soluble polymer materials.

Although in the past twenty years OLED and PLED have shown significant progress in their performance, there still remain problems that need to be solved.

For instance, organic electroluminescent devices described above still show insufficient performance in terms of durability when used for a long time. The performance of organic electroluminescent devices can be further improved by studying new materials such as hole injection materials, hole transporting materials, host materials, emission materials and some others. Additionally, improvement of the device fabrication process is required.

An important consideration to improving organic electroluminescent device performance is to further decrease the driving current within the device in order to enhance device lifetimes. For example, some materials that address this concern have been proposed (US 6,436,559, JP 3571977, JP 3614405). Despite these advances in organic electroluminescent devices using such materials, further improvements regarding stability and the performance are still required.

Summary of the Invention

The present invention either provides improvements to the problems encountered in organic electroluminescent devices as mentioned above, or provides a useful alternative. Specific embodiments may provide an organic electroluminescent device with high efficiency and longer life time. Specific embodiments may also provide a stable device which has reduced current leakage at low current range.

An organic electroluminescent device comprising: (i) an anode; (ii) a cathode; and (ii) one or more layers arranged between the anode and the cathode; wherein at least one of the one or more layers is a light emitting layer and wherein the one or more layers, comprises an organic compound represented by the formula:

Formula 1

wherein the substituents Ri and R2 are the same or different and are selected from the group consisting of: (a) hydrogen, (b) a substituted or unsubstituted heterocyclic group, (c) a substituted or unsubstituted alkyl group, (d) a substituted or unsubstituted aralkyl group, (e) a substituted aryl group containing at least one atom selected from the group consisting of N, O, S, Si, P, F, CI, Br; and/or comprising at least three organic rings, the organic rings being fused or non-fused, or (f) each of Ri or R ₂ may together form a substituted or unsubstituted cyclic group; and

the substituents R3, R4, R5 and Re are the same or different and are each selected from the group consisting of: hydrogen, deuterium, a substituted or unsubstituted alkyl group, a halogen atom or a cyano group. In certain embodiments, the compound represented by formula 1 is suitable for use in an electroluminescent device as any one of an electron transport layer, an emission layer, a host layer, a hole transporting layer, or a hole injection layer.

Brief Description of the Drawings

Figure 1 is a schematic illustration of a basic structure of an organic electroluminescent device according to an embodiment of the invention.

Figure 2 is a schematic illustration of a basic structure of an organic electroluminescent device according to another embodiment of the invention.

Figure 3 is a schematic illustration of a basic structure of an organic electroluminescent device according to another embodiment of the invention.

Figure 4 is a schematic illustration of a basic structure of an organic electroluminescent device according to another embodiment of the invention.

Figure 5 is a schematic illustration of a basic structure of an organic electroluminescent device according to another embodiment of the invention. Figure 6 is a schematic illustration of a basic structure of an organic electroluminescent device according to another embodiment of the invention.

Detailed Description

The organic electroluminescent device according to the present invention is composed of organic compounds layer(s) aligned between an anode and a cathode. The layer(s) may comprise an electron transport layer, an emission layer, a hole transporting layer, a hole injection layer, and insulation layers. At least one of these layers comprises a compound of formula 1.

The organic layer(s) may be constituted by (i) a single layer doped with a compound of formula 1 ; or (ii) multiple layers of which at least one layer may be doped with a compound of formula 1 ; or (iii) at least one layer is a layer comprised of a compound of formula 1 doped with a separate dopant; or (iv) multiple layers of which at least one layer may be comprised entirely of a compound of formula 1. The situation where a particular layer comprises primarily the compound of formula 1 , and the layer is doped with at least one dopant may also be referred to as a host-guest material. In this situation, the compound of formula 1 is the host material and the dopant is the guest material.

The term "compound" is used in its broadest sense to refer to any chemical substance of formula 1.

The compounds of formula 1 have a biimidazole ring structure and in particular a 2,2'-biimidazole structure. Ri and F¾ are the same or different and are selected from the group consisting of: hydrogen, a substituted or unsubstituted heterocyclic group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted aralkyl group, a substituted aryl group, or each of Ri or R ₂ may together form a substituted or unsubstituted cyclic; and wherein the substituted aryl group contains at least one atom selected from the group consisting of N, O, S, Si, P, F, CI, Br; and/or wherein the substituted aryl group comprises at least three organic rings, wherein the organic rings are fused or non-fused. Where the substituted aryl group contains at least one atom that is N, suitable substituents include: -NH _2> -NHR -NRR', -N=R, -ONRR', -C(=NH)R, -C(=NRJR\ -C(=NH)H, -C(=NR)H, (RCO) ₂ N-, -N ₃ , -N ₂ R, -OCN, -NCO, -ON0 _2l -CN, -NC, -ONO, -N0 ₂ , -NO, -C ₅ H ₄ N.

Where the substituted aryl group contains at least one atom that is O, suitable substituents include: -OH, -COR, -CHO, -COX where X is a halide, -OCOOR, -COOH, -COOR, -OOH, -OOR, -OR, -CH(OR)(OH), -C(OR')(OH)R, -CH(OR)(OR'), -C(OR')(OR")R, -C(OR)(OR')(OR"), -OC(OR)(OR')(OR").

Where the substituted aryl group contains at least one atom that is S, suitable substituents include: -SH, -SR, -SSR, -SOR, -S0 ₂ R, -S0 ₂ H, -S0 ₃ H, -SCN, -NCS, -CSR, -CSH.

Where the substituted aryl group contains at least one atom that is Si, suitable substituents include: SiRR'R"

Where the substituted aryl group contains at least one atom that is P, suitable substituents include: -PRR', -PORR', -PSRR', -P(=0)(OH) ₂ , -OP(=0)(OH) ₂ , -OPO(OR)(OH), -OPO(OR)(OR").

R, R', R" are further substituents that may be the same or different.

R3, R4, R5 and R6 are the same or different and are each selected from the group consisting of: hydrogen, deuterium, a substituted or unsubstituted alkyl group, a halogen atom or a cyano group.

The term "aryl" is well understood in the art of chemistry, and is used to refer to any aromatic substituent. The aryl group may be carbocyclic (i.e. contain carbon and hydrogen only) or may be heteroaromatic (i.e. contain carbon, hydrogen, and at least one heteroatom). The aryl group may be monocyclic such as a phenyl, or a polycyclic aryl group such as naphthyl or anthryl. Examples of aryl groups include a phenyl group, biphenyl group, terphenyl group, naphthyl group, anthryl group, pyrenyl group, etc.

The term "alkyl group" is well understood in the art of chemistry,. The alkyl group may be unsubstituted or substituted by a suitable substituent. The. alkyl group may be a linear or branched alkyl group or cyclic alkyl group, preferably comprising of between (and including) 1 and 20 carbon atoms. Examples of linear alkyl groups include methyl, propyl or decyl, and examples of branched alkyl groups include iso-butyl, tert- butyl or 3-methyl-hexyl. Examples of cyclic alkyl groups include mono cyclohexyl and fused alkyl cyclic ring systems.

The term "aralkyl" or "arylated alkyl" refers to an aryl group which is substituted for the hydrogen atom of an alkyl group.

The term "heterocyclic", and similarly "heterocyclic group" or "heterocyclic ring" is well understood in the art of organic chemistry, and is used to refer to any cyclic groups containing one or more rings, such as between one and four rings, and between 5 to 50 (preferably 5 to 20) ring atoms, of which at least one atom is a heteroatom. The heteroatoms may be selected from one or more of O, N, S and Si. The heterocyclic group for the formula 1 may be a 5 or 6 membered heterocyclic ring comprising carbon atoms with one or more of any of the following atoms: nitrogen, oxygen, sulphur and silicon, such as pyrrolyl, thienyl, pyridyl or pyridazinyl. The heterocyclic group may comprise a single heterocyclic ring, or more than one linked or fused rings, with at least one ring containing a heteroatom. One subclass of heterocyclic groups are the heteroaromatic (or heteroaryl) groups, which are aromatic groups containing one or more heteroatoms selected from one or more of O, N and S. Such heteroaromatic groups also fall within the definition of aryl group. Some specific examples for heterocyclic groups are pyrrole, triazole, imidazole, pyrazole, 1 ,2,5- oxathiazole, isoxazole, oxazole, furan, pyran, pyrone, thiazole, isothiazole, pyrrolidine, pyrroline, imidazolidine, pyrazolidine. Other examples include moieties of benzimidazole, thiophene, benzothiophene, oxadiazoline, indoline, carbazole, pyridine, quinoline, isoquinoline, benzoquinone, pyrazoline, imidazolidine, piperidine, etc. The heterocyclic group may be monocyclic or polycyclic. According to some embodiments, the heterocyclic group is polycyclic. An example of a polycyclic heterocyclic group within this class is carbazole.

In formula 1 , the aryl, alkyl, aralkyl, or heterocyclic group may additionally have one or more substituents selected from any suitable substituents known in the art. Suitable substituents may be selected from the group consisting of: halogen atom, nitro group, carbonyl group, amide group, cyano group, carboxylate group, sulfonate group, alkoxy group, aryloxy group, amino group, alkylamino group, arylamino group, another aryl, alkyi or heterocyclic group, in which each of the aryl, alkyi or heterocyclic groups may be further substituted by one or more further substituents. Thus, the further substituents on the aryl, alkyi or heterocyclic group substituents may be selected from one or more of a halogen atom, nitro group, carbonyl group, amide group, cyano group, carboxylate group, sulfonate group; alkoxy group, aryloxy group, amino group, alkylamino group and arylamino group.

The substituents may be linked to the aryl, alkyi, aralkyi or heterocyclic group directly by one atom or fused by more than one atom or via a heteroatom such as nitrogen, oxygen, sulphur and silicon. In some cases the substituent may be linked by two or more points of attachment, as in the case of a divalent alkyi group, or an ethylenedioxy group (i.e. -0-CH ₂ CH ₂ -0-).

The naming of ring systems and substituents is as per lUPAC nomenclature.

Specific examples of the compound represented as formula 1 may include the examples shown below or shown in the examples, but are not limited to these

*

25 26

It is preferred that the compound represented as formula 1 has a melting point preferably between 150°C and 500 °C, and more preferably between 200 °C and 400°C.

As described above, the organic layer(s) may be constituted by (i) a single layer doped with a compound of formula 1 ; or (ii) multiple layers of which at least one layer may be doped with a compound of formula 1; or (iii) at least one layer is a layer comprised of a compound of formula 1 doped with at least one dopant; or (iv) multiple layers of which at least one layer may be comprised entirely of a compound of formula 1

In the organic luminescence device of the present application, the organic compound layer comprising the above-mentioned compound of the formula 1 may be formed separately, or together, with the other layers (if any other layers are present) between the pair of electrodes (cathode and anode). Suitable formation techniques include vacuum deposition or solution process.

The thickness of the organic compound layer may be preferably less than at most 10 pm, more preferably less than 0.5 μιτι, even more preferably 0.001 - 0.5 pm.

Specific embodiments of the invention will now be described in further detail with reference to the accompanying figures, which illustrate a range of possible arrangements for the device of the present invention. It will be understood that these embodiments are provided by way of example only, and are not intended to limit the scope of the invention.

The electroluminescent device of embodiments of the present application may have a single layer structure comprised of only compound as defined by formula 1 as shown in Figure 1 or be a multiple layered structure of two or more layers as shown in Figures 2 and 3. More specifically, Figure 1 is a schematic cross section of a first embodiment of the organic electroluminescent device of the present invention. In Figure 1 , the organic electroluminescent device includes a- substrate 1 ,. an anode 2 (deposited on the substrate 1), an emission layer 3 (deposited on the anode 2) and a cathode 4 (deposited on the emission layer 3). In this embodiment, the emission layer 3 forms a single organic compound type-layer. This single layer may be composed entirely of a compound having hole transporting ability, electron transporting ability and luminescence ability (associated with the re-combination of electrons and holes) based on its own properties, or through combination with a dopant that enhances the performances of the hole transporting ability, the electron transporting ability and luminescence ability of the host compound. According to some embodiments, the compound of formula 1 can serve as a host layer with a dopant.

In Figure 1, the emission layer 3 may preferably have a thickness of 5 nm to 1 pm, preferably 5 to 50 nm and more preferably 30 to 45 nm.

Figure 2 shows another embodiment of the organic electroluminescent device of the present invention in the form of a multiple layer-type device comprised of a hole transporting layer 5 and an electron transporting layer 6.

Referring to Figure 2, the organic luminescent device includes a substrate 1 , an anode 2 (deposited on the substrate 1), the hole transporting layer 5 (deposited on the anode 2), the electron transporting layer 6 (deposited on the hole transporting layer 5) and a cathode (deposited on the electron transporting layer 6). In this embodiment, either one or both of the hole transporting layer 5 and the electron transporting layer 6 may contain an emissive compound as dopant(s) for forming an emission layer 3. In this case, the hole transporting layer 5 and the electron transporting layer 6 may be comprised of non-luminescent compound(s), respectively. The compound of formula 1 can form the hole transportin layer 5, or a component of the hole transporting layer.

In the embodiment of Figure 2, each of the hole transporting layer 5 and the electron transporting layer 6 may have the thickness. of 5 nm to 1 pm, more preferably 5 nm to 50 nm. Figure 3 shows another embodiment of the organic electroluminescent device of the present invention in the form of a multiple layer-type device comprising a hole transporting layer 5, an emission layer 3, an electron transporting layer 6. In Figure 3, the organic luminescent device includes a substrate 1 , an anode 2 (deposited on the substrate 1), the hole transporting layer 5 (deposited on the anode 2), the emission layer 3 (deposited on the hole transporting layer 5), the electron transporting layer 6 (deposited on the emission layer 3) and a cathode (deposited on the electron transporting layer 6). In this embodiment, each of the hole transporting layer, the emission layer and the electron transporting layer may be formed by use of a hole transporting compound, an emissive compound (or an emissive compound and a host compound) and an electron transporting compound, respectively or as a mixture of these kinds of compounds. The compound of formula 1 can form the emission layer 3, or a component of the emission layer.

Figure 4 shows another embodiment of the organic electroluminescent device of the present invention with multiple layers comprising a hole injection layer 7, a hole transporting layer 5, an emission layer 3 and an electron transporting layer 6. In Figure 4, the organic luminescent device includes a substrate 1, an anode 2 (deposited on the substrate 1), the hole injection layer 7 (deposited on the anode 2), the hole transporting layer 5 (deposited on the hole injection layer), the emission layer 3 (deposited on the hole transporting layer 5), the electron transporting layer 6 (deposited on the emission layer 3) and a cathode (deposited on the electron transporting layer 6). In this embodiment, each of the hole injection layer, the hole transporting layer, the emission layer and the electron transporting layer may be formed by use of a hole injection compound, a hole transporting compound, an emissive compound (or an emissive compound and a host compound) and an electron transporting compound, respectively or as a mixture of these kinds of compounds. The compound of formula 1 can form the emission layer 3, or a component of the emission layer. '

Figure 5 shows a further embodiment comprising layered structure that includes a cathode, an electron injection layer, an electron-transport layer, a hole-blocking layer, and emission (or emissive) layer, a hole transport layer and an anode, and a substrate layer. The anode is deposited on top of the glass substrate, and each of the other layers is further deposited on a preceding layer as shown in Figure 5, with the top layer being the cathode. In this particular figure, the cathode is aluminium with a thickness of 100nm, the electron-injection layer is LiF with a thickness of 1nm, the hole-blocking/electron transporting layer is BCP (i.e. 2,9-dimethyl-4,7-diphenyl- 1 ,10-phenanthroline) with a thickness of 40nm, the emission layer is a compound according to formula 1 (i.e. ^ ^'-b ^' \s(9-pheny -9H-carbazo -^-y\)-^H,^Ή-2,2'- biimidazole) with a thickness of 40nm and doped at 8wt% with Flrpic (i.e. bis(4,6- difluorophenylpyridinato-N,C2)picolinatoiridium), the hole-transporting layer is a-NPD (i.e. N,N'bis(napthalen-1-yl)-N,N'-bis(phenyl)-2,2'-dimethylbenzid ine) with a thickness of 50nm, the anode is ITO (i.e. indium tin oxide) with a thickness of 150nm. These layers are mounted on a glass substrate.

Figure 6 shows a further embodiment comprising layered structure that includes a cathode, an electron injection layer, an electron-transport layer, and emission (or emissive) layer, a hole transport layer and an anode, and a substrate layer. The anode is deposited on top of the glass substrate, and each of the other layers is further deposited on a preceding layer as shown in Figure 6, with the top layer being the cathode. In this particular figure, the cathode is aluminium with a thickness of 100nm, the electron-injection layer is LiF with a thickness of 1nm, the electron transporting layer is TmPyPB (i.e. 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene) with a thickness of 40 nm, the emission layer is a compound according to formula 1 (i.e. l .l'-bisiQ-phenyl-QH-carbazol-S-y -I H.rH^^'-biimidazole) with a thickness of 40 rim and doped at 6 wt% with Flrpic (i.e. bis(4,6-difluorophenylpyridinato- N,C2)picolinatoiridium), the hole-transporting layer is TcTa (i.e. 4,4',4"-tris(carbazol- 9-yl)triphenylamine) with a thickness of 70 nm, the anode is ITO (i.e. indium tin oxide) with a thickness of 145 nm. These layers are mounted on a glass substrate.

In Figures 1 , 2, 3, 4, 5 and 6, each of the layers between the anode and cathode may be formed by either vacuum deposition or wet process using a low molecular weight compound or polymeric compound or mixture of a low molecular weight compound and a polymeric compound. Each thickness of the layers between the anode and cathode may preferably be ranging from 1 nm to 1 pm. Each of the thickness of the cathode and the anode may be preferably 100 - 200 nm.

The organic layer structures in the devices shown in Figures 1 , 2, 3, 4, 5 and 6 represent the basic structure, respectively, so that the structure may be appropriately optimized depending on characteristics demanded. Examples of suitable modifications include the incorporation of one or more additional layers. For example, the hole transporting layer may be altered to comprise a hole injection layer (deposited on the anode) and hole transporting layer (deposited on the hole injection layer).

The term electron-injection layer refers to a layer of electron-injecting material. An electron-injecting material is one that promotes the injection of electrons from the cathode and through the electron-injection layer in the direction of cathode toward the anode.

The term electron-transport layer refers to a layer comprising an electron-transport material. An electron-transport material is one which promotes the transport of electrons from the cathode (or from the layer or layers in between the cathode and electron-transporting layer) and through the electron-transporting layer in the direction of cathode toward the anode.

The term hole-blocking layer refers to a layer comprising a hole-blocking material. A hole-blocking material is one which blocks the movement of "holes" from the emission layer (or from the layer or layers in between the emission layer and hole- blocking layer) in the direction of anode toward the cathode. Additionally, the hole- blocking material should be able to function as an electron-transporting material.

The term "emission layer" or "emissive layer" refers to the layer from which light is emitted. The recombination of holes and electrons within this layer produces light. The emission layer is comprised of at least one emissive material (or emitter). In some instances an emission layer may comprise a host material with an emitter or emitters. The term "guest" or "dopant" is also used to describe an emitter. The term "electron-blocking layer" refers to a layer comprising an electron-blocking material. An electron-blocking material is one which blocks the movement of electrons from the emission layer (or from the layer or layers in between the emission layer and electron-blocking layer) in the direction of cathode toward the anode. Additionally, the electron-blocking material should be able to function as a hole- transporting material.

The term hole-transporting layer refers to a layer comprising a hole-transporting material. A hole-transport material is one which promotes the transport of holes from the anode (or from the layer or layers in between the anode and hole-transporting Jayer) and through the hole-transporting layer in the direction of anode toward the cathode.

The term hole-injection layer refers to a layer of hole-injecting material. A hole- injecting material is one that promotes the injection of holes from the anode and through the hole-injection layer in the direction of anode toward the cathode.

More specific embodiments of the device structure other than those of Figures 1, 2, 3, 4, 5 and 6 are shown below, but not restricted to these device structures.

(1) Anode/hole transporting layer/emission layer/electron transporting layer/electron injection layer/cathode

(2) Anode/hole injection layer/emission layer/electron transporting layer/electron injection layer/cathode

(3) Anode/insulating layer/hole transporting layer/emission layer/ electron transporting layer/cathode

(4) Anode/hole transporting layer/emission layer/electron transporting layer/insulating layer/cathode

(5) Anode/inorganic semiconductor/insulator/hole transporting layer/emission layer/insulator/cathode

(6) Anode/insulating layer/hole transporting layer/emission layer/electron transporting layer/insulating layer/cathode

(7) Anode/insulating layer/hole injection layer/hole transporting layer/emission layer/electron transporting layer/electron injection layer/cathode (8) Anode/insulating layer/hole injection layer/hole transporting layer/emission layer/electron transporting layer/electron injection layer/insulating layer/cathode In the embodiments described above, more preferable device structures are (1), (2), (3), (7) and (8), although this is not a restriction. Furthermore, a hole blocking layer may be present between the emissive layer and the electron transport layer in any of the embodiments listed above, or in Figures 3 and 4.

The electron injection layer has a thickness of 1 nm to 1 pm, more preferably 1 - 50 nm. The electron transport layer has a thickness of 1 nm to 1 pm, preferably 1 - 50 nm and more preferably 25 - 35nm. The hole blocking layer has a thickness of 1 to 50 nm, preferably 1 to 20 nm and most preferably about 10 nm. The emissive layer has a thickness of 5 nm to 1 pm, preferably 5 to 50 nm and more preferably 30 to 45 nm. The hole transport layer has a thickness of 1 nm to 1 μιτι, more preferably 1 - 50 nm. The hole injection layer has a thickness of 1 nm to 1 pm, more preferably 1 - 50 nm.

According to some embodiments, the compound of the formula 1 may be formed as a hole injection layer, hole generation layer or hole transport layer. According to some embodiments, there is provided the use of the compound of formula 1 as a hole injection material, hole generation material, or hole transport material as a hole injection layer, a hole generation layer, a hole transporting material, or as a dopant in a hole transporting layer.

According to other embodiments, the compound of the formula 1 may be formed as an electron injection layer, electron generation layer or electron transport layer. According to some embodiments, there is provided the use of the compound of formula 1 as an electron injection material, electron generation material, or electron transport material as an electron injection layer, an electron generation layer, an electron transporting material, or as a dopant in an electron transporting layer.

In some embodiments, the compound of the formula 1 may be used in combination with a hole transporting compound (or material), an electron transporting compound and/or an emission compound. The situation where the compound of formula 1 is used in conjunction with a dopant is also referred to as a host-guest complex. In this situation the organic compound is present as the host or the matrix phase, and it contains a dopant or guest molecule dispersed throughout.

Examples of suitable dopants or guest materials are dependent on whether the organic compound of formula 1 is being used as a hole transporting material, an electron transporting material, or an emissive material. Suitable dopants or guest materials are not limited to, but may include the following:

Hole transporting materials/compounds:

comprises an iridium, osmium, platinum, rhodium, ruthenium, or palladium constituent

As a material for the anode (e.g. 2 in Figures 1 - 4), it is preferred to use one having a large work function, examples of which may include metals, such as gold, platinum, nickel, palladium, cobalt, selenium, vanadium and their alloys; metal oxides, such as tin oxide, zinc oxide, indium zinc oxide (IZO) and indium tin oxide (ITO) and electroconductive polymers, such as polyaniline, polypyrole and polythiophene and derivatives thereof. These compounds may be used singly or in combination of two or more species.

As a material for the cathode (e.g. 4 in Figures 1 - 4), it is preferred to use one having a smaller work function, usually under 4.0 eV, examples of which may include; metals such as sodium, magnesium, lithium, potassium, aluminium, indium, silver, lead, chromium and their alloys, or oxides. In the case of metal oxide, those such as indium oxide (ITO), indium zinc oxide (IZO) and zinc oxide are suitable.

The insulating layer may be deposited adjacent to either electrode to avoid current leakage as mentioned in embodiments (3) to (8). As the insulating material, it is preferred to use an inorganic compound, examples of which may include aluminium oxide, lithium fluoride, lithium oxide, caesium oxide, magnesium oxide, magnesium fluoride, calcium oxide, calcium fluoride, aluminium nitride, titanium oxide, silicon oxide, silicon nitride, boron nitride, vanadium oxide.

The substrate (e.g., 1 shown in Figures 1 to 4) for the organic electroluminescence device of the present invention may include an opaque substrate made from any suitable material, such as metal or ceramics, or a transparent substrate made from any suitable transparent material such as glass, quartz, plastics, etc. It is possible to form the substrate with a colour filter film, a fluorescent colour conversion film, dielectric reflection film, etc., thus controlling such aspects of the emitted luminescent light.

The devices of the present application can be provided in the form of a stacked organic electroluminescent (EL) device. The stacked organic EL device may contain a number of stacked emission layers, wherein each emission layer can comprise multiple emitters. In one embodiment, the stacked EL device is for emission of white light and contains three stacked emission layers, each layer containing a different single emitter. In another embodiment, the stacked EL device is for white emission and contains two stacked emission layers wherein at least one of the layers contains two emitters.

The present application also extends to electronic devices comprising the organic electroluminescent device of the present invention, including displays and light sources.

The present invention will be described below in detail with preparation examples and the device examples, but the present invention is not intended to be restricted to these examples.

As used herein, except where the context requires otherwise, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude other additives, components, integers or steps.

Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment, or any form of suggestion, that this prior art forms part of the common general knowledge in any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art. . . ^' ,

Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value and each potential range encompassed within said range, unless otherwise stated. Furthermore, each separate value and each potential range is incorporated into the specification as if it were individually recited herein. Examples

l .r-bisiO-phenyl-gH-carbazol-S-y -IH.rH^^'-biimidazole (980 mg, 36%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene/petrol) and then by sublimation (230 °C, ΚΓ ⁶ mBar): m.p. 235 - 242 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 6.76 (dd, J 2.1 , 8.6 Hz, 2H), 7.06 - 7.01 (m, 4H), 7.14 (ddd, J 1.4, 6.8, 8.0 Hz, 2H), 7.22 - 7.38 (m, 8H), 7.42 - 7.50 (m, 6H), 7.55 - 7.61 (m, 4H), 7.68 - 7.72 (m, 2H); ¹³ C NMR (CDCI ₃ , 100 MHz) δ 109.9, 110.1, 116.2, 120.2, 120.5, 122.0, 122.1 , 123.5, 126.8, 126.9, 127.9, 128.6, 130.0, 135.4, 136.8, 139.8, 141.4; HRMS (El) m/z 616.2350 C42H28N6 [Mf requires 616.2370. Synthetic example 2:

4,441H 'H-2 _> 2 ^, -biimidazole-1 vdiyl)bis(/V _l /V-diphenylaniline)

A mixture of IH.I'H^'-biimidazole (1.02 g, 7.63 mmol), 4-bromo-A/,A/- diphenylaniline (9.89 g, 30.5 mmol) and Cs ₂ C0 ₃ (12.4 g, 38.2 mmol) in DMF (150 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (440 mg, 3.0 mmol) was added and the mixture was heated (140 °C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH ₂ CI ₂ . The combined filtrate and washings were concentrated. The mixture was diluted with CH ₂ CI ₂ and H ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/Et-jN/MeOH 100:1 :0 then 100:1 :2 then 100:1:5) to give 4,4'- (1H _l 1 ^, H-2,2 ^, -biimidazole-1,r-diyl)bis(A/,A/-diphenylaniline) (1.57 g, 33%) as a colourless solid. A. portion of this material was further purified firstly, by recrystallisation (petrol/toluene) and then by distillation (sublimation apparatus 215 °C, 10 ^-6 mBar): m.p. 174 - 182 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 6.67 (d, J 8.8 Hz, 4H), 6.88 (d, J 8.8 Hz, 4H), 7.00 - 7.06 (m, 12H), 7.15 (br s, 2H), 7.18 - 7.25 (m, 8H), 7.34 (br s, 2H); ¹³ C NMR (CDCI ₃ , 100 MHz) δ 121.8, 122.7, 123.8, 124.6, 124.7, 128.9, 129.4, 130.0, 146.9, 147.8; HRMS (El) m/z 619.2598 C 2H ₃ iN ₆ [M - H] ^+* requires 619.2605. Synthetic example 3:

1 , 1 '-bis(4-(9W-carbazol-9-yl)phenyl)-1 H, 1 'H^'-biimidazole

A mixture of IH.I'H^'-biimidazole (610 mg, 4.5 mmol), 3-bromo-9-phen ^" yl-9H- carbazole (5.1 g, 15.8 mmol) and Cs ₂ C0 ₃ (6.62 g, 20.4 mmol) in DMF (100 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (260 mg, 1.8 mmol) was added and the mixture was heated (140 "C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH ₂ CI ₂ . The combined filtrate and washings were concentrated. The mixture was diluted with CH2CI2 and H ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH2CI2) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 40:60:0 then 40:60:2 then 40:60:4) to give 1,r-bis(4-(9H-carbazol-9-yl)phenyl)-1H,1'H-2,2 ^, -biimidazole (1.13 g, 41%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation and then by distillation (sublimation apparatus 300 °C, KT ⁶ mBar): m.p. 270 - 274 °C (DSC); H NMR (CDCI ₃ , 400 MHz) <57.17 (d, J 8.7 Hz, 4H), 7.25 - 7.39 (m, 10H), 7.42 (br s, 2H), 7.50 (d, J 8.7 Hz, 4H), 8.13 (dd, J 1.4, 7.7 Hz, 4H); ¹³ C NMR (CDCI ₃ , 100 MHz) δ 109.3, 120.5, 120.5, 121.4, 123.6, 125.3, 126.1 , 127.5, 130.6, 135.8, 137.2, 140.3; HRMS (El) m/z 616.2347 C42H28N6 [Mf requires 616.2370. Synthetic example 4:

1 , 1 '-bis(9-(pyridin- - - H-ca zol- - - ' - ,2'-biimidazole

A mixture of 1 H,1'H-2,2'-biimidazole (800 mg, 6.0 mmol), 3-bromo-9-(pyridin-2-yl)- 9W-carbazole (6.7 g, 20.8 mmol) and Cs ₂ C0 ₃ (9.66 g, 29.7 mmol) in DMF (100 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (340 mg, 2.4 mmol) was added and the mixture was heated (140 'C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH ₂ CI ₂ . The combined filtrate and washings were concentrated. The mixture was diluted with CH2CI2 and H2O and the organic phase was separated. The aqueous phase was re-extracted (CH2CI2) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 40:60:0 then 40:60:3 then 40:60:5) to give 1 ,1^bis(9-(pyridin-2-yl)-9H-carbazol-3-yl)-W (1.44 g, 39%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene/petrol) and then by distillation (sublimation apparatus 280 °C, 10 ^"6 mBar): m.p. 215 - 227 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 6.78 (dd, J 2.1 , 8.7 Hz, 2H), 7.02 - 7.12 (m, 4H), 7.18 (d, J 1.9 Hz, 2H), 7.28 - 7.34 (m, 6H), 7.47 (d, J 8.1 Hz, 2H), 7.54 (d, J 8.7 Hz, 2H), 7.60 (d, J 7.5 Hz, 2H), 7.66 (d, J 8.3 Hz, 2H), 7.88 (dt J 2.0, 7.5 Hz, 2H), 8.69 (ddd, J 0.7, 1.9, 4.9 Hz, 2H); ¹³ C NMR (CDCI _3> 50 MHz) δ 111.0, 111.3, 115.9, 118.8, 120.0, 121.0, 121.4, 122.4, 123.2, 124.4, 126.7, 129.8, 130.7, 138.1 , 138.5, 139.8, 149.6, 151.2; HRMS (El) m/z 617.2175 C40H25N8 [M - Η requires 616.2197. Synthetic example 5:

1 , 1 '-di(9,9'-spirobi[fluorene]-2-yl)-1 H, 1 'H-2,2'-biimidazole

A mixture of 1H,1 mmol), 2-bromo-9,9'- spirobi(fluorene) (3.2 g, 8.1 mmol) and Cs ₂ C0 ₃ (4.12 g, 12.6 mmol) in DMF (60 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (140 mg, 0.98 mmol) was added and the mixture was heated (140 °C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH2CI2. The combined filtrate and washings were concentrated. The mixture was diluted with CH ₂ CI ₂ and H ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 40:60:0 then 40:60:2 then 40:60:4) to give l. -d g.g'-spirobilfluorenel^-y -IH.i'H^^'-biimidazole (560 mg, 29%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene) and then by sublimation (320 °C, 10 ^" * mBar): m.p. 404 - 412 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 5.84 (d, J 1.9 Hz, 2H), 6.33 (d, J 7.6 Hz, 4H), 6.60 (s, 2H), 6.64 - 6.71 (m, 4H), 6.84 (s, 2H), 6.92 (t, J 7.4 Hz, 4H), 7.14 (t, J 7.4 Hz, 2H), 7.25 (t, J 7.5 Hz, 4H), 7.40 (t, J 7.4 Hz, 2H), 7.49 (d, J 8.2Hz, 2H), 7.72 - 7.80 (m, 6H); ³ C NMR (CDCI3, 100 MHz) δ 65.6, 118.2, 120.1 , 120.1 , 120.2, 120.3, 122.1 , 123.8, 124.2, 127.8, 127.9, 128.2, 129.3, 136.0, 136.5, 140.4, 140.6, 141.6, 147.7, 148.8, 150.0; HRMS (El) m/z 761.2662 C56H33N4 [M - Hf requires 761.2700. Synthetic example 6:

1 , 1 ^, -bis(4-(pyridin-3-yl)phenyl)-1 H,1 'H-2,2'-biimidazole

6

A mixture of 1H,1'H-2,2'-biimidazole (950 mg, 7.1 mmol), 3-(4-bromophenyl)pyridine (4.95 g, 21.3 mmol) and Cs ₂ C0 ₃ (9.24 g, 28.4 mmol) in DMF (80 mL) was degassed ( 2 bubbling, 15 min). Cu ₂ 0 (410 mg, 2.9 mmol) was added and the mixture was heated (140 "C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH ₂ CI ₂ . The combined filtrate and washings were concentrated. The mixture was diluted with CH ₂ CI ₂ and H ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 40:60:3 then 35:60:5 then 30:60:10) to give 1 ,1 ^, -bis(4-(pyridin-3-yl)phenyl)-1 H,1'H-2,2'-biimidazole (1.05 g, 34%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene/petrol) and then by distillation (sublimation apparatus 260 ^D C, 10 ^"6 mBar): m.p. 218 - 223 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 6.95 - 7.00 (m, 4H), 7.15 (d, J 1.2 Hz, 2H), 7.29 (d, J 1.2 Hz, 2H), 7.38 (ddd, J 0.6, 4.8, 7.9 Hz, 2H), 7.40 - 7.45 (m, 4H), 7.82 (ddd, J 0.6, 1.7, 7.9 Hz, 2H), 8.62 (dd, J 1.4, 4.8 Hz, 2H), 8.79 (d. J 1.9 Hz, 2H); ¹³ C NMR (CDCI3, 100 MHz) δ 121.4, 123.7, 124.8, 127.7, 130.2, 134.1 , 135.2, 137.0, 137.2, 137.3, 148.1 , 149.0; HRMS (El) m/z 439.1665 CzeHig e [M - H] ^+' requires 439.1666. Synthetic example 7: yl)pyridin-2-amine)

7

A mixture of 1H,17-/-2,2'-biimidazole (840 mg, 6.3 mmol), A/-(4-bromophenyl)-A/- (pyridin-2-yl)pyridin-2-amine (6.59 g, 20.3 mmol) and Cs ₂ C0 ₃ (10.14 g, 31.2 mmol) in DMF (70 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (360 mg, 2.5 mmol) was added and the mixture was heated (140 "C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH2CI2. The combined filtrate and washings were concentrated. The mixture was diluted with CH2CI2 and H ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI2) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 35:60:5 then 32:60:8 then 28:60:12) to give Λ/,Λ ·(4,4'-(1Η, 1 "H^'-biimidazoie-l , 1 '-diyl)bis(4, 1 -phenylene))bis(A/-(pyridin- 2-yl)pyridin-2-amine) (1.36 g, 35%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /EtOAc/petrol) and then by distillation (sublimation apparatus 260 °C, 10 ^~6 mBar): m.p. 218 - 222 °C (DSC); 'H NMR (CDCI _3l 400 MHz) δ 6.75 - 6.80 (m, 4H), 6.90 (ddd, J 0.8, 4.9, 7.3 Hz, 4H), 6.93 - 7.00 (m, 8H), 7.09 (d, J 1.0 Hz, 2H), 7.26 (s, 2H), 7.52 (ddd, J 2.0, 7.3, 8.3 Hz, 4H), 8.24 (ddd, J 0.7, 1.9, 4.9 Hz, 4H); ¹³ C NMR (CDCI ₃ , 100 MHz) δ 117.0, 118.6, 121.1 , 125.2, 127.5, 130.0, 133.7, 137.6, 144.3, 148.6, 157.7; HRMS (El) m/z 623.2420 C38H ₂ 7 ₁₀ [M - H] ^+' requires 623.2415. Synthetic example 8:

1 ,1 '-bis(4-(1 OH-phenothiazin-10-yl)phenyl)-1 H,1 'H^'-biimidazole

8

A mixture of 1H,1'H-2,2'-biimidazole (480 mg, 3.6 mmol), 10-(4-bromophenyl)-10H- phenothiazine (3.80 g, 10.8 mmol) and Cs ₂ C0 ₃ (5.85 g, 18.0 mmol) in DMF (60 mL) was degassed (N ₂ bubbling, 15 min). CU ₂ O (210 mg, 1.5 mmol) was added and the mixture was heated (140 °C, 72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH ₂ CI ₂ . The combined filtrate and washings were concentrated. The mixture was diluted with CH2CI2 and H ₂ O and the organic phase was separated. The aqueous phase was re-extracted (CH2CI2) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 40:60:0 then 38:60:2 then 35:60:5) to give 1 ,1'-bis(4-(10H-phenothiazin-10-yl)phenyl)-1H,1'H-2,2 ^, -biimidazole (1.12 g, 46%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene/petrol) and then by distillation (sublimation apparatus 260 °C, 10 ^"6 mBar): m.p. 227 - 232 °C (DSC); ¹ H NMR (CDCI ₃ , 400 MHz) δ 6.36 - 6.42 (m, 4H), 6.86 - 6.97 (m, 12H), 7.08 - 7.16 (m, 8H), 7.20 (br s, 2H), 7.35 (br s, 2H); ¹³ C NMR (CDCI3, 100 MHz) δ 118.4, 123.6, 123.8, 125.4, 126.8, 127.4, 128.4, 130.5, 135.2, 141.5, 143.3; HRMS (El) m/z 679.1710 C^Hzr^ [M - Hf requires 679.1733. Synthetic example 9:

S.S^^'-dimethyl-IH.I'H^^'-biimidazole-l.r-diy bisig-phenyl-gH-carbazole)

9

A mixture of 3-(2-iodo-4-methyl-1H-imidazol-1-yl)-9-phenyl-9H-carbazole (5.27 g, 11.7 mmol), 1,10-phenanthroline (2.11 g, 11.7 mmol) and Cs ₂ C0 ₃ (7.63 g, 23.5 mmol) in DMF (60 mL) was degassed (N ₂ bubbling, 15 min). Cul (2.23 g, 11.7 mmol) was added and the mixture was heated (140 "C, 24h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH2CI2. The combined filtrate and washings were concentrated. The mixture was diluted with CH ₂ CI ₂ and H ₂ 0 and the organic phase was separated. The aqueous phase was re- extracted (CH ₂ CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography, firstly, (EtOAc/CH ₂ CI ₂ /MeOH 49:60:1 then 37:60:3 then 34:60:6) and secondly, (EtOAc/MeOH 94:6) to give 3,3 ^, -(4,4 ^, -dimethyl- 1H _> rH-2,2 ^, -biimidazole-1,r-diyl)bis(9-phenyl-9H-carbazole) (2 g, 53%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (EtOAc/petrol) and then by distillation (sublimation apparatus 280 °C, KT ⁶ mBar): m.p. 245 - 253 °C (DSC); ¹ H NMR (CDCI _3l 400 MHz) δ 2.37 (d, J 0.7 Hz, 6H), 6.67 (dd, 2.1 , 8.6 Hz, 2H), 6.78 (d, J 0.9 Hz, 2H), 7.01 (d, J 8.6 Hz, 2H), 7.10 (dt, J 1.2, 7.0 Hz, 2H), 7.14 (d, J 1.9 Hz, 2H), 7.23 - 7.33 (m, 4H), 7.35 - 7.40 (m, 4H), 7.46 (tt, J 1.2, 6.7Hz, 2H), 7.53 - 7.59 (m, 4H), 7.61 (d, J, 7.8 Hz, 2H); ¹³ C NMR (CDCIs. OO MHz) δ 13.7, 109.4, 109.8, 115.6, 118.2, 119.8, 120.0, 122.0, 122.4, 123.3, 126.4, 126.7, 127.6, 129.9, 130.2, 137.2, 137.4, 138.8, 139.0, 141.1 ; HRMS (El) m/z 644.2664 C44H ₃₂ N ₆ [Mf requires 644.2683. Synthetic example 10:

1 , 1 '-bis(6-methyl-9-p-tolyl-9H-carbazol-3-y!)-1 H, 1 ^, H-2,2 ⁱ -biimidazole

10

A mixture of 1W, W-2,2*-biimidazole (890 mg, 6.6 mmol), 3-bromo-6-methyl-9-p-tolyl- 9H-carbazole (6.49 g, 18.6 mmol) and Cs ₂ C0 ₃ (7.55 g, 23.2 mmol) in D F (80 mL) was degassed (N ₂ bubbling, 15 min). Cu ₂ 0 (380 mg, 2.7 mmol) was added and the mixture was heated (140 °C, 96h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH2CI2. The combined filtrate and washings were concentrated. The mixture was diluted with CH2CI2 and H2O and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc CH ₂ CI ₂ / eOH 39:60:1 then 37:60:3 then 35:60:5) to give l.r-bisie-methyl-g-p-tolyl-gH-carbazol-S-y -IH.rH^^'-biimidazole (1.42 g, 32%) as a colourless solid. A portion of this material was further purified firstly, by recrystallisation (CH ₂ CI ₂ /toluene/petrol) and then by distillation (sublimation apparatus 270 °C, 1CT ⁶ mBar): m.p. 208 - 214 °C (DSC); ¹ H N R (CDCI ₃ , 400 MHz) δ 2.38 (s, 6H), 2.49 (s, 6H), 6.71 (dd, J 1.8, 8.7 Hz, 2H), 7.03 (d, J 8.6 Hz, 2H), 7.05 - 7.20 (m, 8H), 7.23 - 7.40 (m, 12H); ¹³ C NMR (CDCI3, 100 MHz) 5 21.2, 21.3, 109.4, 109.4, 115.7, 119.7, 121.8, 122.4, 123.1, 126.5, 127.8, 129.3, 129.6, 130.4, 134.6, 137.4, 139.6, 139.7; HRMS (El) m/z 671.2916 C46H35 ₆ [M - Hf ^* requires 671.2918. Synthetic example 11 :

1 ,r-bis(4-bromobenzyl)-1H,1 'H^'-biimidazole

11

To a mixture of 1H,1 W-2,2'-biimidazole (2.1 g, 15.7 mmol) in DMF at 0 "C was added NaH (1.32 g of a 60% dispersion in mineral oil, 32.8 mmol). The mixture was allowed to warm to room temperature and stirred for 30 min. 1-bromo-4- (bromomethyl)benzene (11.7 g, 46.8 mmol) was added and the mixture was heated to 80 "C and stirred (2h). The mixture was cooled to room temperature and saturated aqueous NH ₄ CI (2 mL) was added and the mixture was concentrated. The mixture was diluted with CH ₂ CI ₂ and Η ₂ 0 and the organic phase was separated. The aqueous phase was re-extracted (CH2CI ₂ ) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ 0:100 then 50:50 then 80:20) to give l .l'-bisi^bromobenzy -I ^^'-biimidazole (4.12 g, 56%) as a colourless solid. ¹ H NMR (CDCI ₃₎ 400 MHz) δ 5.66 (s, 4H), 6.86 -.6.91 (m, 4H), 6.93 (d ₍ J 1.2 Hz, 2H), 7.11 (d, J 1.2 Hz, 2H), 7.33 - 7.39 (m, 4H); ¹³ C NMR (CDCI3, 100 MHz) 5 50.2, 121.5, 121.7, 128.5, 129.1 , 131.8, 136.3, 138.0; HRMS (El) m/z 469.9741 C ₂ oHi ₆ 4Br2 [M] ^+" requires 671.2918.

Synthetic example 12:

1 , 1 '-bis(4-(9H-carbazol-9-yl)benzyl)-1 H, 1 'H^'-biimidazole

A mixture of I . is^-bromobenzy -IH.IW^'-biimidazole (1.95 g, 4.13 mmol), 9H-carbazole (2.76 g, 16.5 mmol), 1 ,2-diaminocyclohexane (mixture of cisltrans, 220 μΐ_, 1.8 mmol) and K ₃ P0 ₄ (2.63 g, 12.4 mmol) in toluene (120 mL) was degassed (N ₂ bubbling, 15 min). Cul (240 mg, 1.2 mmol) was added and the mixture was heated under reflux (72h). The mixture was allowed to cool to room temperature and filtered through Celite washing with CH2CI2. The combined filtrate and washings were concentrated. The mixture was diluted with CH ₂ CI2 and H2O and the organic phase was separated. The aqueous phase was re-extracted (CH ₂ CI2) and the combined organics were washed (saturated aqueous NaCI), dried (MgS0 ₄ ), filtered and concentrated to give a solid residue. The residue was purified by flash chromatography (EtOAc/CH ₂ CI ₂ /MeOH 39:60:1 then 38:60:2) to give 1 ,1'-bis(4-(9H- carbazol-9-yl)benzyl)-1H,1W-2,2 >iimidazole (1.05 g, 39%) as a colourless solid. ^r H NMR (CDC , 400 MHz) δ 5.96 (s, 4H), 7.10 (d, J 1.2 Hz, 2H), 7.22 - 7.28 (m, 6H), 7.30 - 7.38 (m, 8H), 7.42 - 7.47 (m, 4H), 7.48 - 7.53 (m, 4H), 8.11 (dt, J 7.6, 0.9 Hz, 4H); ¹³ C NMR (CDCI ₃ , 100 MHz) 5 50.6, 109.6, 120.0, 120.3, 121.6, 123.4, 125.9, 127.2, 128.7, 129.1 , 136.6, 137.3, 138.4, 140.6. Device Fabrication

OLEDs were fabricated in a Kitano Seiki KVD OLED Evo II system (base pressure of 1CT ⁶ Pa) on commercially patterned indium tin oxide (ITO) coated glass (NA-32R Alumino Silicate, 25 mm ^χ 25 mm) having a sheet resistance≤ 12 Ω/α (Sanyo Vacuum) using stainless steel shadow masks to define the device area (10 mm ² ).

Substrates were ultrasonically cleaned with a standard regimen of de-ionised water (Millipore Direct-Q 3) with 5% concentration of deconex FPD-211 (Borer Chemie) , de-ionised water , isopropyl alcohol (Aldrich), de-ionised water and isopropyl alcohol followed by drying with nitrogen gas (Entegris Wafergard GN Gas Filter Gun) and a UV-ozone treatment (Novascan) for 15 minutes.

Following the deposition of the organic layers, lithium fluoride (LiF) and aluminium (Al), the devices were encapsulated with a desiccant (Dynic) in a dry nitrogen atmosphere (MBraun MB20/200, < 1 ppm H ₂ 0/0 ₂ ) using a recessed glass can (IND26) and ultraviolet curable resin (Nagase Chemtex XNR5516Z-B1) perimeter seal.

Device example 1

The device structure for the OLED is shown in Figure 5 and is as follows:

Device A

Device A comprises 150 nm ITO (anode) / 50 nm a-NPD (hole-transport layer) / 40 nm CBP doped with Flrpic [8% by weight] (emissive layer) / 40nm BCP (hole blocking/electron transport layer) / 1 nm LiF (electron-injection layer) / 100nm Al (cathode), where a-NPD is A/,A/'bis(napthalen-1-yl)-/\/,/\r-bis(phenyl)-2,2'- dimethylbenzidine, CBP is 4,4'-di(9H-carbazol-9-yl)biphenyl, Flrpic is bis(4,6- difluorophenylpyridinato-N,C2)picolinatoiridium and BCP is 2,9-dimethyl-4,7- diphenyl-1 ,10-phenanthroline. a-NPD, CBP, Flrpic and BCP were purchased from Luminescence Technology Corp. The thickness of each layer was measured by a quartz crystal microbalance (ULVAC CRTM-9000) calibrated by a stylus profiler (Bruker Dektak 150). Device B

Device B was fabricated using the compound of synthetic example 1 (1 ,1'-bis(9- phenyl-9H-t¾rbazol-3-yl)-1H,1W-2,2'-biimidazole, synthesized by the authors and purified by gradient sublimation prior to use) in place of CBP in conditions directly analogous to Device A.

Device Characterisation

Voltage-current-luminance (V-l-L) measurements were taken using a Keithley 2400 SourceMeter and TOPCON BM-7A luminance colormeter (0.2° measurement angle).

The Commission International d'Eclairage (CIE) chromaticity coordinates (x,y) and peak emission wavelength (λΡ) were calculated from the electroluminescent (EL) spectral power distribution (SPD) measured using a Sphere Optics LCS-100 Characterisation System.

Calculation of the external quantum efficiency (EQE) was determined from the forward intensity assuming a Lambertian distribution of the EL emission.

The device performance is summarised in Table 1.

Table 1: Device performance data for both host materials CBP (Device A) and the compound of example 1 (Device B).

At a current density of 1 mA/cm2, Device A (CBP) showed a current efficiency of 10.1 cd/A and a brightness of 101 cd/m2. The colour was blue and the CIE coordinates were (0.17, 0.32). At a current density of 1 mA/cm2, Device B (1) showed a current efficiency of 25.5 cd/A and a brightness of 255 cd/m2. The colour was blue and the CIE co-ordinates were (0.18, 0.40).

Device example 2

The device structure for the OLED is shown in Figure 6 and is as follows:

Device C

Device C comprises 145 nm ITO (anode) / 70 nm TcTa (hole-transport layer) / 40 nm MCP doped with FlrPic [6% by weight] (emissive layer) / 40 nm TmPyPB (electron transport layer) / 1 nm LiF (electron-injection layer) / 100nm Al (cathode), where TcTa is 4,4',4"-tris(carbazol-9-yl)triphenylamine, MCP is 1,3-bis(carbazol-9- yl)benzene, FlrPic is bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium and TmPyPB is 1 ,3,5-tri[(3-pyridyl)-phen-3-yl]benzene. TcTa, MCP, FlrPic and TmPyPB were purchased from Luminescence Technology Corp. The thickness of each layer was measured by a quartz crystal microbalance (ULVAC CRTM-9000) calibrated by a stylus profiler (Bruker Dektak 150).

Device D

Device D was fabricated using the compound of synthetic example 1 (1,1'-bis(9- phenyl-9H-carbazol-3-yl)-1H,1'H-2,2'-biimidazole, (synthesized by the authors and purified by gradient sublimation prior to use) in place of MCP in conditions directly analogous to Device C.

Device Characterisation

Device characterisation is as for Device example 1.

The device performance for Device example 2 is summarised in Tables 2 and

3.

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Table 2: Device performance data for both host materials MCP (Device C) and the compound of example 1 (Device D) at 1000 cd/m ² .

At a brightness of 1000 cd/m ² , Device C (MCP) showed a current efficiency of 37.1 cd/A and a half life of 0.2 hours. The colour was blue and the CIE co-ordinates were (0.16, 0.30).

At a brightness of 1000 cd/m ² , Device D (1) showed a current efficiency of 26.2 cd/A and a half life of 4.6 hours. The colour was blue and the CIE co-ordinates were (0.16, 0.32).

Table 3: Device performance data for both host materials MCP (Device C) and the compound of example 1 (Device D) at 15,000 cd/m ² .

At a brightness of 15000 cd/m ² , Device C (MCP) showed a current density of 124.5 mA/cm ² and a current efficiency of 12.0 cd/A. The colour was blue and the CIE coordinates were (0.16, 0.31).

At a brightness of 15000 cd/m ² , Device D (1) showed a current density of 84.8 mA/cm ² and a current efficiency of 17.7 cd/A. The colour was blue and the CIE coordinates were (0.16, 0.31).