Background Organic light-emitting diodes (OLEDs), also known as organic electroluminescent (EL) devices, are an emerging display technology. In essence an OLED comprises a thin organic layer or stack of organic layers sandwiched between two electrodes, such that when a voltage is applied visible or other light is emitted.

At least one of the electrodes must be transparent to light. For display applications the light must of course be visible to the eye, and therefore at least one of the electrodes must be transparent to visible light.

There are two principal techniques that can be used to deposit the organic layers in an OLED: thermal evaporation and solution processing. Solution processing has the potential to be the lower cost technique due to its potentially greater throughput and ability to handle large substrate sizes. Significant work has been undertaken to develop appropriate materials, particularly polymers. More recently dendrimers that are photoluminescent in the solid state have been shown to have great promise as solution processible light-emitting materials in OLEDs (S. -C. Lo, et al Adv. Mater., 2002, 13, 975; J. P. J. Markham, et al Appl. Phys. Lett., 2002, 80, 2645).

Dendrimers are branched macromolecules with a core and attached dendrons.

Dendrons are branched structures comprising branching units and optionally linking units. The generation of a dendron is defined by the number of sets of branching points ; see Figure 1. Dendrons with the same structure (architecture) but a higher generation, or order, may be composed of the same structural units (branching and linking units) but have an additional level of branching, i. e. an additional repetition of these branching and linking units. There can be surface groups on the periphery of the dendrons.

Light-emitting dendrimers typically have a luminescent core and in many cases conjugated dendrons. Further examples of light-emitting dendrimers include those found in P. W. Wang, et al Adv. Mater., 1996, 8, 237; M. Halim, et al Adv.

Mater., 1999, 11, 371; A. W. Freeman, et al J. Afra. Chem. Soc., 2000, 122, 12385 ; A.

Adronov, et al Chem. Coma., 2000, 1701 ; C. C. Kwok, et al Macromolecules, 2001, 34, 6821. Light-emitting dendrimers have the advantage over light-emitting polymers in that the light-emitting properties and the processing properties can be independently optimised as the nature of the core, dendrons and surface groups can be independently altered. For example the emission colour of a dendrimer with a light-emitting core can be changed by simply changing the core.

Other physical properties, such as viscosity, may also make dendrimers more easily tailored to the available manufacturing processes than polymers.

Organometallic dendrimers have previously been used in OLED applications as a single component in a film (i. e. a neat film) or in a blend with a molecular material or in a blend of more than one dendrimer of different type (e. g. different cores), e. g.

J. M. Lupton et al Adv. Funct. Mater., 2001, 11, 287 and J. P. J, Markham, et al Appl. Phys. Lett., 2002, 80, 2645.

Recently high efficiency OLEDs have been made with iridium based light- emitting organometallic dendrimers. The lifetimes of those devices that are based on existing materials needs improvement. The present invention is directed to dendrimers, which can have improved film forming properties compared to the known materials. The film forming properties and intrinsic nature of the new dendrimers may allow improved OLED lifetime.

Summary of the current invention According to the current invention there is provided an organometallic dendrimer possessing a core comprising a metal cation and one or more ligands which are attached to said metal cation, and one or more dendrons attached to one or more of said ligands, at least one dendron possessing a (fluorene) unit (Fl) of the formula wherein R, represents a substituent and R2 represents hydrogen or a substituent, at its distal end. Either of the benzene rings can be substituted but this is less preferred.

It is to be understood that, in the context of the present invention, an organometallic dendrimer is one in which at least one organic ligand is coordinated to the metal. Such dendrimers do not necessarily contain a metal-carbon bond, because the organic ligand may be coordinated to the metal through an atom other than carbon, e. g. a nitrogen atom. However, dendrimers which contain at least one metal-carbon bond are preferred. Preferably at least one of the dendrons is attached to a ligand that is bonded to the metal via at least one metal-carbon bond. For example the dendron may be attached to a ligand that is part of a cyclometallated ring. Also they are preferably neutral i. e. they do not require a counterion to balance the charge of the metal cation.

Typically the dendron possessing the distal unit consists of, or possesses at its distal end, a unit of the formula: -B (Lb-Fl) a (I) where B is a branching atom or group, especially a nitrogen atom or a (hetero) aryl group, which is attached, directly or indirectly, to said ligand (or further branching group or linking group in a higher order dendron), L is a divalent linking group, b is 0 or a positive integer, a is an integer of at least 2, and Fl is as defined above. In a second or higher generation dendron it will be appreciated that there will be further branching group (s) and optionally linking groups between the ligands and B. In a first generation dendrimer dendron B is either attached directly to the ligand or via an optional linking group or groups.

A second aspect of the invention is a process for forming the organometallic dendrimers of this invention. The process is described below.

Dendrimers of the invention, or dendrimers formed by the process of the invention, are preferably in solid form, and in particular in the form of a solid film.

According to the third aspect of the invention there is provided an OLED device comprising, in sequence, layers of an optional substrate, an electrode, a first optional charge-transporting layer, an emissive layer, a second optional charge- transporting layer and a counter electrode, wherein one of the emissive layer or the first or second charge-transporting layers if present, especially the emissive layer, is a solid film comprising a dendrimer of the current invention or one produced by a process of the invention.

In one embodiment the film or layer comprising the organometallic dendrimer of this invention contains one or more additional species such as light- emitting dopants, charge-transporting species and/or additional molecular, dendritic and/or polymeric materials.

Detailed description of the current invention The first generation dendron comprises just the distal unit -B (Lb-Fl) a (I) Thus the first generation organometallic dendrimer has the formula CORE- [B (Lb~Fl) a] x where x is the number of dendrons containing Fl of specified formula [B (Lb-Fl) a], and CORE is the metal cation and one or more ligands attached thereto, B being attached to at least one of the ligands of the CORE. It will be appreciated that there may be one or more linking groups which are part of the CORE between B and the components of the ligands that are directly attached to the metal cation.

As used herein the term"distal"means the part or parts of the molecule furthest from the core when following the bond sequence out from the core. It will be appreciated that due to the geometry of the bonds and moieties within a dendron a distal unit may be closer in space to the core than an earlier moiety in the dendron.

The second generation dendron comprises two sets of branching units, and attached to the end of at least one, and preferably each, branch, an Fl unit as defined above. In a preferred embodiment the second generation dendron comprises a first branching unit, and attached to the end of at least one, and preferably each, branch a distal unit as in formula (I) to give a second generation dendron as shown in formula (II) below -B'[L'b,{B(Lb-Fl)a}]a, (II) wherein Fl, L, a and b are as defined above, L'is as defined for L, b'is as defined for b and a'is as defined for a, B is a branching atom or group, especially a nitrogen atom or a (hetero) aryl group which is attached to L'if b'is an integer or to B'if b'is 0, and B'is a branching atom or group, especially a nitrogen atom or a (hetero) aryl group which is attached to said ligand (which forms part of the core of the dendrimer).

B'is the same as or different from B; all the comments regarding B and its branching unit (s) and optional linking group (s) apply equally to B'.

Hence a second generation dendrimer would have the following formula : CORE- (B'[L'b. {B (Lb-Fl) a}] a) x as shown in Figure 1. Third generation dendrons are formed by the attachment of the second generation dendrons to another branching atom or group via an optional linking unit. Higher generations follow accordingly. It is preferred that at least one of the dendrons of the dendrimer is of at least second generation.

It will be appreciated that, for all generations, not all the ligands that are part of the core necessarily have to have a dendron attached. There may also be more than one dendron attached to an individual ligand. It will also be appreciated that the dendrons need not all be the same although this is preferred. Also the dendrimer can contain one or more dendrons which are not of formula (I) although this is less preferred.

The number of distal groups depends on the number of arms branching out of each branching unit and the generation of the dendron (number of sets of branching groups B). It is preferred that each branching point has the same number of branches, so that a'= a. If at each branch point there are 2 new branches, i. e. a is 2, then in the second generation dendron there will be 2 distal units of formula (I) (therefore 4 fluorene Fl groups), in the third generation dendron there will be 2 x 2 = 4 distal units of formula (I) (therefore 8 fluorene Fl groups-see Figure 1), in the fourth generation dendron there will be 2 x 2 x 2= 8 distal units of formula (I). For a dendron with the same type of branches throughout, the number of distal units is a (n-') where a is the number of new branches at each point and n is the generation. It will be appreciated, though, that it is only preferred that all the distal groups are Fl groups. a is preferably 2. b and b'are preferably 0 or 1 or 2.

B is typically selected from phenyl (or more accurately trivalent benzene), carbazole, triazine, and a N atom. When B is phenyl, which is preferred, it is preferably bonded via the 1,3, 5 positions. Triazole is bonded via the 2, 4, 6- positions and carbazole is typically attached through the 3,6, N positions. The choice of branching or group, B, and the regiochemistry of attachment to B, is such that the attachment of Fl (or Lb F1) to B does not substantially change the emission colour of the individual dendrimer. In a preferred embodiment the linking group, L, is not present, so that the ring atom of B or N is directly attached to the 2 position of the Fl group. In an alternative embodiment L is a divalent group, preferably, chosen so that it is in conjugation with B and the Fl group, in this case L is typically divalent aryl, including phenyl, or fused aryl including fluorenyl, heteroaryl, including thiophene, vinyl, acetylenyl. In a less preferred embodiment L is a saturated group such as typically alkylene or 0-alkylen e. g. Cl-C6 such as-OCH2-. B'and L'are selected from the same list of groups as B and L. L'is preferably not present.

The substituents Rl and R2 that are part of the fluorene group, Fl, are the same or different and are typically alkyl (including cyclic alkyl and branched alkyl), alkoxy, ether, haloalkyl (especially perhaloalkyl or alkylhaloalkyl, especially alkylperhaloalkyl), aryl (including aryloxy, arylmethyleneoxy, alkylaryl, alkoxyaryl, arylthio, acetylenyl alkyl, alkenylalkyl, propargyl and heteroaryl), or a dendritic group or Rl or R2 together complete a ring. They can be straight, branched, or cyclic as appropriate and can be substituted. The alkyl groups generally have 1 to 15 carbon atoms, especially propyl, hexyl and octyl, the aryl group is typically phenyl and the heteroaryl group typically has 5 or 6 ring atoms. In a preferred embodiment R, and/or R2 represents CH2X where X represents straight or branched alkyl, haloalkyl, aryl, alkyloxy, aryloxy or arylmethylenoxy. Rl and R2 are preferably chosen so that the dendrimer is soluble in solvents suitable for solution processing, e. g. THF, toluene or alcoholic solvents such as Cl 6 alcohols such as methanol. As indicated above R2 can also be hydrogen. The R, and R2 groups can be easily changed over a wide range to modify the physical properties without appreciably altering the emissive properties of the core.

In a preferred embodiment the dendrimers are comprised of a metal ion containing core to which two or more ligands are attached with one or more of the ligands having at least one dendron attached in which the distal group or groups have formula (I). In a more preferred embodiment two or more of the ligands have dendrons attached one or more of which is such that the distal group or groups have formula (1). The ligands attached to the metal cation should be such that the co- ordination requirements of the metal cation are fulfilled. Thus the organometallic dendrimer is preferably neutral, i. e. , no extra counteranions are required to balance the charge of the dendrimer. The presence of counteranions can be detrimental to device performance.

The metal cation chosen can give rise to fluorescent or phosphorescent dendrimers ; phosphorescent dendrimers are preferred. Phosphorescence can be observed from metal complexes of some d and f block elements and dendrimers based on iridium, platinum, and rhenium are preferred. Other materials include rhodium, palladium, osmium and gold. It is also preferred that the metals are at the core of the dendrimers.

The ligands attached to the metal are generally mono-, bi-, tri-or tetra- dentate with bi-dentate being. preferred.

The dendrimers are preferably of the type disclosed in PCT/GB02/00750, to which reference should be made for further details, but possessing at least one of the specified fluorene (FI) at its distal end.

A preferred organometallic dendrimer of the invention has the formula (III) :- CORE [DENDRITE3n (III) in which: CORE represents a metal ion or a group containing a metal ion; n represents an integer of 1 or more; each DENDRITE, which may be the same or different, represents an inherently at least partially conjugated dendritic molecular structure comprising aryl and/or heteroaryl groups and optionally nitrogen and, optionally, vinyl or acetylenyl groups connected via spi or sp hybridised carbon atoms of said (hetero) aryl, vinyl and acetylenyl groups or via single bonds between N and (hetero) aryl groups; and CORE terminates in a single bond which is connected to an sp2 hybridised (ring) carbon atom of the first (hetero) aryl group or nitrogen to which more than one at least partially conjugated dendritic branch is attached, said ring carbon atom or N forming part of said DENDRITE. In particular the dendrimer can be phosphorescent and possess two or more coordinating groups as part of its core such that at least two said coordinating groups each have a dendron attached, at least one of which dendron comprises at least one nitrogen atom which forms part of an aromatic ring system or is directly bonded to at least two aromatic groups.

In an alternative embodiment not all the dendrons, also known as dendrites, in the dendrimer are the same. The dendrons which do not possess a fluorene distal unit typically possess surface groups such as those disclosed in PCT/GB02/00750, namely dendrimers having the formula: CORE- [DENDRITE'] n [DENDRITEz] m in which: CORE represents a metal ion or group containing a metal ion, n and m, which may be the same or different, each represent an integer of at least 1; each DENDRITE', which may be the same or different when n is greater than 1, and each DENDRITE2, which may be the same or different when m is greater than 1, represent dendritic structures, at least one of said structures being fully conjugated and comprising aryl and/or heteroaryl groups and, optionally, vinyl and/or acetylenyl groups, connected via spi or sp hybridized carbon atoms of said (hetero) aryl, vinyl and acetylenyl groups, and at least one branching point and/or link between the branching points in DENDRITE'being different from those in DENDRITE2 ; CORE terminates in a single bond which is connected to a sp2 hybridized (ring) carbon atom of the first (hetero) aryl group to which more than one conjugated dendritic branch is attached, said ring carbon atom forming part of said fully conjugated DENDRITE'or DENDRITE2 ; and CORE terminates at the single bond to the first branching point for the other of said DENDRITEt or DENDRITE2 ; at least one of CORE, DENDRITE' and DENDRITE2 being luminescent.

In a preferred embodiment CORE has the formula shown below: where M is a metal cation with a formal charge r+, Zt and Z2 are, independently, groups required to complete a 5 or 6 membered aryl or heteroaryl ring which can be optionally substituted, z is 0 or 1,2, or 3, t2 is a neutral or anionic ligand, such that each L2 can be the same or different if z is greater than 1, x is an integer, preferably at least 2, and the dendrimer is neutral such that r = (p. x) + (z. q) and at least one the (hetero) aryl rings formed by Zl or ZZ is attached by a single bond to a dendron that contains a unit Fl at its distal end.

It is preferred that Zt is such that the 5 or 6 membered aryl or heteroaryl ring which can optionally be part of a fused ring system is selected from phenyl, pyridyl, thiophenyl, naphthyl, anthryl, phenanthryl, benzamidazolyl, carbazolyl, fluorenyl, pyrimidinyl, pyrazinyl, pyridazinyl, quinolinyl, isoquinolinyl, quinoxalinyl, phthalazinyl, quinazolinyl, imidazolyl, pyrazolinyl, oxazolinyl, oxadiazolinyl, triazolyl, triazinyl, thiadiazolyl, benzimidazolyl, benzoxazolyl, phenanthridinyl, furyl and benzothiophenyl. It is preferred that Z2 is such that the 5 or 6 membered aryl or heteroaryl ring which can optionally be part of a fused ring system is selected from pyridine, pyrimidine, pyrazine, pyridazine, quinoline, isoquinoline, quinoxaline, phthalazine, quinazoline, naphtholidine, pyrimidine, phenanthroline, imidazole, pyrazole, oxazole, oxadiazole, triazole, thiadiazole, benzimidazole, benzoxazole and benzthiazole. Suitable optional substituents on the (hetero) aryl rings include halo, alkyl (C1 to 15), haloalkyl, especially perhaloalkyl (e. g. CF3, CF2CF3), alkyloxy, aryloxy, aryloxyaryl, alkyloxyalkyl, alkyloxyaryl, aryl, alkylaryl, cyano, amino, dialkylamino, diarylamino, alkylthio, arylthio, sulfinyl, sulfonyl, aryloxy, alkylarylamino, benzylic alcohol and aldehyde.

Preferred ligands Y include ligands of formula (IV): in which Z, and Z2 are as defined above and * represents a bond to M. Other preferred ligands Y include p-diketonates, 2-carboxylpyridines, such as picolinic acid, triarylphosphines, such as triphenylphosphine, trialkylphosphines, ethylenediamine, cyanide, carbon monoxide and carbon monosulfide.

The dendrons can be comprised of non-conjugated units, a combination of conjugated and non conjugated units as in Fréchet-type dendrons e. g. with-O-CH- linking groups, or conjugated units, provided at least one has a distal unit as in formula (I). A dendrimer that contains more than one dendron can have a combination of these types although it is preferred that all dendrons be conjugated.

Conjugated dendrons are also known as at least inherently partially conjugated dendrons, i. e. , whilst individual units within the dendron are in conjugation; due to the linking arrangement at the branching points the pi-system is not necessarily fully delocalised over the dendron. The conjugated dendrons are comprised of branching and, optionally, linking units. The branching units can be aryl and/or heteroaryl units and/or a nitrogen atom. Linking units can be aryl, heteroaryl, vinyl or acetylenyl.

When N is a branching point vinyl and acetylenyl linking groups are less preferred.

Conjugated dendrons are preferred. When conjugated dendrons are used the different bonding arrangements and/or generation can be used so that asymmetric dendrimers can be formed.

In a preferred embodiment the organometallic dendrimers are capable of emitting visible light. In an alternative embodiment the organometallic dendrimers have charge-transporting properties. It should be noted that the dendrimers that emit light can also transport charge. Also some organometallic dendrimers can emit light at wavelengths suitable for optical communications.

The properties of dendrimers make them ideal for solution processing.

Preferred dendrimers can be dissolved in a solvent, the solution deposited onto a substrate, and the solvent removed to leave a solid film. Conventional solution- processing techniques can be used, for example spin-coating, printing (e. g. ink-jet printing) and dip-coating. The resulting solid film containing the organometallic dendrimer can be either fluorescent or phosphorescent. The solid film is preferably formed on one side of a substrate ; the thickness of the solid film is preferably no greater than 2 microns.

The dendrimers of the present invention are typically prepared by: i) forming a core by making a complex between a metal cation and the desired ligands, at least one of said ligands having a reactive functionality, ii) forming one or more dendrons that have one or more fluorene (Fl)- containing distal units, with one or more reactive foci capable of reacting with the reactive functionality of the ligand, and iii) reacting the core with the dendron (s).

The dendrons can be formed in a convergent manner. This is the preferred process.

An alternative process for forming the dendrimers is to attach the dendrons to the ligand or ligands before subsequent complexation of the ligands to the metal cation to form a neutral dendrimer. Furthermore it is possible to have a strategy whereby some ligands have a dendron attached before complexation with the remaining ligands either not having a dendron attached or having a reactive moiety to allow subsequent dendron attachment.

The present invention also provides an OLED incorporating a solid film comprising dendrimers of this invention. In its simplest form, an organic light- emitting diode or electroluminescent device can be formed from a light-emitting layer sandwiched between two electrodes, at least one of which is transparent to the emitted light. More commonly there is one or more additional hole-transporting layers between the anode and the light-emitting layer and/or one or more additional electron-transporting layers between the light-emitting layer and the cathode. In one preferred embodiment the film comprising the organometallic dendrimer forms the light-emitting layer in an OLED. It is particularly preferred that the dendrimers are the light-emitting species in this light-emitting layer. In an alternative embodiment the film comprising the organometallic dendrimer constitutes at least part of a charge-transporting layer in an OLED.

Such a device can have a conventional arrangement comprising a transparent substrate layer, e. g. a glass or PET layer, a transparent electrode layer, a light- emitting layer and a back electrode. The anode, which is generally transparent is preferably made from indium tin oxide (ITO) although other similar materials including indium oxide/tin oxide, tin oxide/antimony, zinc oxide/aluminium, gold and platinum can also be used, as can conducting polymers such as PANI (polyaniline) or PEDOT/PSS. The cathode is normally made of a low work function metal or alloy such as Al, Ca, Mg, Li or MgAl or optionally with an additional layer of LiF. In an alternative configuration, the substrate may be made of an opaque material such as silicon and light is emitted through the opposing electrode. The OLED devices may be actively or passively addressed.

For a typical OLED device, as described above where the organometallic dendrimer is emissive, a solution of the dendrimer can be applied over a transparent electrode layer, the solvent evaporated and then subsequent charge-transporting layers can be applied. The thickness of the dendrimer layer in the OLED is typically lOnm to 1000nm, preferably no more than 200nm, more preferably 30nm to 120nm.

When a hole transport layer is incorporated between the anode and the emissive organometallic dendrimer containing layer, the hole transport material must not be removed to a significant extent during the solution deposition.

An OLED device incorporating an emissive layer comprising the organometallic dendrimer may optionally have an adjacent first and/or second charge-transporting layer. In our work on organometallic dendrimers, it has been found that it is particularly beneficial to have a hole-blocking/electron-transporting layer between the light-emitting dendrimer layer and the cathode. Suitable materials for such a hole-blocking/electron-transporting layer are known and include 2,9- dimethyl-4, 7-diphenyl-1, 10-phenanthroline (BCP), 1, 3,5-tris [2-N- phenylbenzimidazolyl) benzene (TPBI), 2-biphenyl-5 (4'-t-butylphenyl) oxadiazole (PBD), aluminium (8-hydroxyquinolate) (Alq) and aluminium (III) bis (2-methyl-8- quinolato) -4-phenylphenolate (BAlq). In this, and in other embodiments, the dendrimer-comprising layer may comprise mixtures or two or more dendrimer types, not all of which need be dendrimers of the present invention.

Furthermore, additional emissive (fluorescent or phosphorescent) or charge- transporting species may optionally be added to the layer of the organometallic dendrimer to improve device characteristics, e. g. efficiency and lifetime. It may further be of benefit to include with the present dendrimer one or more other molecular and/or dendrimeric and/or polymeric species to give improved performance. In one embodiment such additional components form a part of the total blend between 95 and 5 mol%. For example, additional charge-transporting components which can be used with dendrimers of the present invention include TPBI, PBD, BCP, 4,4'-bis (N-carbazolyl-) biphenyl (CBP), 4,4', 4"-tris (N- carbazolyl) triphenylamine (TCTA), and tris-4- (N-3-methylphenyl-N- phenyl) phenylamine (MTDATA).

The dendrimers of the current invention can also be used in other device applications such as photovoltaic cells which can contain one or more layers. When used in photovoltaic cells the dendrimer must be capable of absorbing light and/or transporting charge. The dendrimer may be used as a homogeneous layer in a photovoltaic device or blended with other molecular and/or dendritic and/or polymeric materials. Dendrimers may be used in one or more layers of the photovoltaic device. In photovoltaic applications the organometallic dendrimers need not necessarily be charge-neutral.

The present invention is further illustrated in the accompanying drawings in which: Figure 1 shows a reaction scheme for the preparation of a first generation iridium based dendrimer with the fluorene units attached to a carbazole group of the dendron (Examples 1-2; see below), Figure 2 shows a reaction scheme for the preparation of a first generation iridium based dendrimer with the fluorene units attached to a phenyl group of the dendron (Example 3), Figure 3 shows the current-voltage characteristics of an OLED containing a neat layer of dendrimer A5 (open circles o) and of an OLED containing a layer of A5 blended with CBP (solid circles ) (Example 4), Figure 4 shows the luminance-voltage characteristics of an OLED containing a neat layer of dendrimer A5 (open circles o) and of an OLED containing a layer of A5 blended with CBP (solid circles 9) (Example 4), Figure 5 shows the efficiency (cd/A) versus voltage (V) of an OLED containing a neat layer of dendrimer A5 (open Circles) and of an OLED containing a layer of A5 blended with CBP (solid circles) (Example 4).

The Examples that follow further illustrate the present invention.

Example 1 Synthesis of A-3 A mixture of 3,6-dibromocarbazole A-2 (1.59 g, 4.91 mmol) and boronic ester A-1 (4.80 g, 12.7 mmol), ethanol (16.8 mL), toluene (43.2 mL) and aqueous sodium carbonate (2 M, 16.8 mL) was deoxygenated and put under argon.

Tetrakis (triphenylphosphine) palladium (0) (0.14 g, 0.121 mmol) was added and the reaction mixture was further deoxygenated. The reaction mixture was heated for 20 h under argon at 90 °C. Brine (70 mL) was added to the reaction mixture after cooling and the resultant mixture was extracted with diethylether (3 x 75 mL). The organic phases were collected and dried over anhydrous magnesium sulfate, filtered and the solvent completely removed. The residue was purified by column chromatography over silica using a dichloromethane/light petroleum mixture (1: 2.5) as eluent. The main fraction was collected and the solvent removed. The residue was recrystallised from an ethylacetate/light petroleum mixture to yield A- 3 (1.12 g, 34%).

Example 2 Synthesis of A-5 A mixture of tris (dibenzylacetone) dipalladium (25 mg, 0.03 mmol), carbazole A-3 (0.465 g, 0.70 mmol), iridium complex A-4 (0.156, 0.175 mmol), and sodium t- butoxide (84 mg, 0.87 mmol) was deoxgenated and placed under argon. Toluene (20 mL) and then tri-t-butylphosphine in hexane (3 mL) were added with deoxygenation being carried after each step. The reaction mixture was heated at reflux under argon for =24 h. Water (30 mL) was added and the organic layer separated. The aqueous layer was extracted with diethylether (4 x 30 mL). The combined organic layers were dried over anhydrous magnesium sulfate, filtered and the solvent completely removed. The residue was purified by column chromatography over silica using a dichloromethane/light petroleum mixture (1: 2) as eluent to give almost pure dendrimer (462 mg). The mixture was further purified by column chromatography over silica gel using ethyl acetate-light petroleum (1: 100 to 1: 40) as eluent and then purified by chromatotron using dichloromethane-light petroleum (1: 30 to 1: 15), ethyl acetate-light petroleum (1: 10 to 1: 6) and then dichloromethane-light petroleum (1: 20) as eluent to give a yellow solid. The solid was washed with light petroleum (10 x 10 mL) and then dried over reduced pressure to give almost pure A-5 ("200 mg,-43%) ; AH (400 MHz; CDC13) 0.62-0. 86 (60 H, m, Me & CH), 1.94-2. 14 (24 H, m, CH), 7.03 (3 H, m, Ar), 7.30-8. 05 (72 H, m, ArH), and 8.55 (6 H, m, ArH): m/z [MALDI] 2640 (broad) tM+).

Example 3 Fac f2- (3'-DPFP-Ph) Py1Jr Fac tris (2- {3'-f3", 5"-di (9"', 9"'-dipropylfluoren-2"-yl) phenyllphenylTpyridine) iridium (III) Tert-butyl lithium (1.7 M, 12.7 mL, 21.7 mmol) was added to solution of 2-bromo- 9, 9-dipropylfluorene B-1 (4.46 g, 13.5 mmol) in anhydrous tetrahydrofuran (37 mL) which was under argon, and cooled in a dry ice/acetone bath. The mixture was stirred at-78 °C for 1 h and then tri-n-butylborate (21 mL, 77. 8 mmol) was added. The reaction was stirred at-78 °C for 2 h before being removed from the dry-ice/acetone bath. The mixture was then stirred at room temperature for a further 3 h before being quenched with hydrochloric acid (3 M, 20 mL). Water (10 mL) and ether (40 mL) were added and the two phases were separated. The aqueous layer was extracted with ether (3 x 50 mL). The organic layer and the ether extracts were combined, washed with brine (1 x 90 mL) and dried over anhydrous magnesium sulfate. The solvent was completely removed. The residue was purified by column chromatography over silica gel using dichloromethane-light petroleum (0: 1 to 1: 10), and then ethyl acetate- dichloromethane (0: 1 to 1: 4) as eluent, to give 3.22 g of the boronic derivative 1. A mixture of the boronic derivative 1 (3.22 g), 1,3, 5-tribromobenzene (1.50 g, 4.76 mmol), tetrakis (triphenylphosphine) palladium (0) (300 mg, 0.260 mmol), aqueous sodium carbonate (2 M, 4.5 mL), ethanol (4.5 mL) and toluene (13 mL) was deoxygenated and then heated at reflux (with bath temperature of 101 °C) under argon for 23 h. The mixture was allowed to cool to room temperature. Water (10 mL) and ether (10 mL) were added to the mixture. The two phases were separated and the aqueous layer was extracted with ether (3 x 8 mL). The organic layer and the ether extracts were combined, washed with brine (1 x 20 mL) and dried over anhydrous magnesium sulfate. The solvent was completely removed. The residue was purified by column chromatography over silica gel using dichloromethane-light petroleum (0: 1 to 1: 30) as eluent to give 766 mg (25%) of B-2 as a white solid ; (400 MHz; CDC13) 0.63-0. 83 (20 H, m, Me & CH2), 1.96-2. 11 (8 H, m, CH2), 7.34 (6 H, m, ArH), 7.59-7. 69 (4 H, m, ArH), and 7.74-7. 90 (7 H, m, ArH); c (101 MHz ; CDC13) 14.5, 17.3, 42.8, 55.5, 119.9, 120.1, 121.6, 122.9, 123.3, 125.1, 126.2, 126.9, 127.3, 128.7, 138.7, 140.5, 141.1, 144.3, 151. 0 and 151.6.

Tert-butyl lithium (1.7 M, 1.2 mL, 1.98 mmol) was added to a solution of B-2 (810 mg, 1.24 mmol) in anhydrous tetrahydrofuran (4 mL) under an argon atmosphere, and cooled in dry ice/acetone bath. The mixture was stirred at-78 °C for 1 h and then tri-n-butyl borate (2 mL, 7.43 mmol) was added to the cold mixture. The reaction was stirred at-78 °C for 2 h before being removed from the dry-ice/acetone bath. The mixture was then stirred at room temperature for further 2 h before being quenched with hydrochloric acid (3 M, 2 mL). Water (15 mL) and ether (10 mL) were added and then the two phases were separated. The aqueous layer was extracted with ether (3 x 8 mL). The organic layer and the ether extracts were combined, washed with brine (1 x 20 mL) and dried over anhydrous magnesium sulfate. The solvent was completely removed and the residue purified by column chromatography over silica gel using dichloromethane-light petroleum (0: 1 to 1: 30), and then ethyl acetate- dichloromethane (1: 4 to 1: 1) as eluent to give 644 mg of the boronic derivative 2 as a white solid. A mixture of the boronic derivative 2 (360 mg), fac tris [2- (3'- bromophenyl) pyridine] iridium (III) A-4 (86 mg, 0.097 mmol), tetrakis (triphenylphosphine) palladium (0) (20 mg, 0.010 mmol), aqueous sodium carbonate (2 M, 0.4 mL), ethanol (0.44 mL), tetrahydrofuran (0.8 mL) and toluene (0. 8 mL) was deoxygenated and then heated at reflux (with bath temperature of 100- 105 °C) under argon for 114 h. The mixture was allowed to cool and ether (10 mL) and water (3 mL) were added. The two phases were separated and the aqueous layer was extracted with ether (3 x 3 mL). The organic layer and the ether extracts were combined, washed with brine (1 x 10 mL) and dried over anhydrous magnesium sulfate. The solvent was completely removed and the residue was purified by column chromatography over silica gel using dichloromethane-light petroleum (1: 30) as eluent to give B-3 (70 mg, 30%) ; (400 MHz; CD3C1) 0.63-0. 80 (60 H, m, Me & CH2), 1.98-2. 15 (24 H, m, CH2), 7.01 (3 H, m, ArH), 7.10-7. 21 (3 H, m, ArH) 7.32- 7.51 (21 H, m, ArH), 7.67-7. 88 (33 H, m, ArH), and 7.91-8. 19 (12 H, m, ArH); m/z [MALDI] 2370,2371, 2372,2373, 2374,2375 (M+).

Example 4 Device results for dendrimer A5 The dendrimer A5 was used as both a neat material, and as a component in a blend, to make various OLEDs. The device structure of the OLEDs, which comprise a light emitting layer and a hole-blocking/electron-transporting layer, were: E1 : ITO/A5/TPBI (60 nm)/Ca (20 nm)/Al (100 nm) E2: ITO/A5 : CBP (20%: 80% w/w) /TPBI (60 nm)/Ca (20 nm)/Al (100 nm) E3: ITO/A5 : CBP: TPBI (20%: 40%: 40% w/w)/TPBI (60 nm)/Ca (20 nm)/Al (100 nm) The procedure for fabricating the OLEDs was as follows. The patterned ITO coated glass substrates were 02-plasma ashed prior to layer deposition using an Emitech plasma asher, in which the °2 flow rate was set to 10, RF power to 70 watts, and duration 4 min. The substrates were then immediately loaded into a spin coater. A solution of the active materials (10 mg/ml in chloroform for both neat and blend solutions) was spun onto the substrate at a spin speed of 2000 rpm for 60 s. The substrates were then transferred to the evaporator and layers of TPBI (60 nm),-Ca (20 nm) and Al (100 nm) were deposited to form the hole-blocking/electron-transporting layer and the cathode.

The devices E1, E2 and E3 emit green light and the CIE coordinates of the emission are (X= 0.36, Y=0.60), (X=0.33, Y=0.62) and (X=0.33, Y=0.62) respectively. The current voltage characteristics of the three devices are shown in Figure 3, the luminance-voltage characteristics in Figure 4 and the external efficiency in Figure 5.

As can be seen from the figures blending reduces the current at a given voltage, leading to higher efficiency than when the neat dendrimer A5 is used as the light emitting layer.