WO2012153083A1

WO2012153083A1 - Light emitting material, composition and device

Info

Publication number: WO2012153083A1
Application number: PCT/GB2012/000404
Authority: WO
Inventors: Annette Steudel; Jonathan Pillow
Original assignee: Cambridge Display Technology Limited; Sumitomo Chemical Company Limited
Priority date: 2011-05-12
Filing date: 2012-05-03
Publication date: 2012-11-15
Also published as: GB201321793D0; GB201107905D0; GB2505377A

Abstract

A light-emitting composition comprises a first fluorescent organic emitter and a phosphorescent organic emitter. The first fluorescent organic emitter has a peak wavelength in the range of 490-560 nm and a triplet excited state energy level is higher than a triplet excited state energy level of the phosphorescent organic emitter. The first fluorescent emitter is preferably selected from compounds of formulae (la)-(Id): wherein each Ar independently represents an aryl or heteroaryl group, R represents H or a substituent and wherein any of the rings of formulae (la) - (Id), including any ring of any Ar group, are unsubstituted or independently substituted with one or more substituents.

Description

LIGHT EMITTING MATERIAL, COMPOSITION AND DEVICE

Summary of the Invention

The present invention relates to a fluorescent emitter, a light-emitting composition comprising fluorescent emitter and phosphorescent emitters and OLEDs comprising the same.

Background

Electronic devices comprising active organic materials are attracting increasing attention for use in devices such as organic light emitting diodes (OLEDs), organic

photoresponsive devices (in particular organic photovoltaic devices and organic photosensors), organic transistors and memory array devices. Devices comprising organic materials offer benefits such as low weight, low power consumption and flexibility. Moreover, use of soluble organic materials allows use of solution processing in device manufacture, for example inkjet printing or spin-coating.

With reference to Figure 1 , an OLED may comprise a substrate 1 carrying an anode 2, a cathode 4 and an organic light-emitting layer 3 between the anode and cathode.

Holes are injected into the device through the anode 2 and electrons are injected through the cathode 4 during operation of the device. Holes in the highest occupied molecular orbital (HOMO) and electrons in the lowest unoccupied molecular orbital (LUMO) of a light-emitting material in the light-emitting layer combine to form an exciton that releases its energy as light.

Suitable light-emitting materials include small molecule, polymeric and dendrimeric materials. Suitable light-emitting polymers for use in layer 3 include poly(arylene vinylenes) such as poly(p-phenylene vinylenes) and polyarylenes such as polyfluorenes.

The light emitting layer may comprise a semiconducting host material and a light- emitting dopant wherein energy is transferred from the host material to the light-emitting dopant. For example, J. Appl. Phys. 65, 3610, 1989 discloses a host material doped with a fluorescent light-emitting dopant (that is, a light-emitting material in which light is emitted via decay of a singlet exciton) and Appl. Phys. Lett., 2000, 77, 904 discloses a host material doped with a phosphorescent light emitting dopant (that is, a light-emitting material in which light is emitted via decay of a triplet exciton).

The light-emitting layer may comprise more than one light-emitting material, for example to obtain white light.

WO 2009/093033 discloses a white light-emitting OLED wherein the light-emitting layer comprises a polymer comprising blue fluorescent repeat units, green fluorescent repeat units and red phosphorescent end-capping units. The preferred concentration for each of the green fluorescent monomer and red phosphorescent end -capping material in the polymerization feed used to form the polymer is 0.05-0.15 mol %.

WO 2004/037887 discloses a conjugated polymer comprising at least 1 mol % of a repeat unit comprising an amine repeat unit with a pendant condensed aromatic group.

Summary of the Invention

In a first aspect the invention provides a light-emitting composition comprising a first fluorescent organic emitter and a phosphorescent organic emitter wherein the first fluorescent organic emitter has a peak wavelength in the range of 490-560 nm and wherein a triplet excited state energy level of the first fluorescent organic emitter is higher than a triplet excited state energy level of the phosphorescent organic emitter.

Optionally, the fluorescent emitter has a singlet-triplet gap of less than 1 .2 eV, optionally less than 1 eV.

Optionally, the phosphorescent emitter has a peak wavelength in the range of 570-630 nm. Optionally, the first fluorescent emitter is selected from compounds of formulae (Ia)-(Id):

(la) (lb) (Ic) (Id) wherein each Ar independently represents an aryl or heteroaryl group, for example phenyl; R represents H or a substituent and wherein any of the rings of formulae (la) - (Id), including any ring of any Ar group, may independently be substituted with one or more substituents, for example one or more d^o alkyl groups.

Optionally, the phosphorescent organic emitter comprises a phosphorescent metal complex.

Optionally, the composition further comprises a second fluorescent organic emitter having a peak wavelength of less than or equal to 480 nm, optionally in the range 400- 480 nm.

In one optional arrangement, the first and second fluorescent emitters comprise separate light-emitting compounds blended together.

Optionally, the first fluorescent emitter is provided as repeats units of a polymer.

Optionally, the first fluorescent emitter comprises a repeat unit of formu la (Ie-Ih):

(le) (If) (Ig) (Ih) wherein each Ar independently represents an aryl or heteroaryl group, R represents H or a substituent and wherein any of the rings of formulae (la) - (Id), including any ring of any Ar group, may independently be substituted with one or more substituents. Optionally, the first and second fluorescent emitters are provided as repeat units of the polymer.

Optionally, the second fluorescent emitter comprises a repeat unit of formula (V):

(V)

wherein Ar¹, Ar² and R each independently represent a monocyclic aromatic group, preferably phenyl, each of which is optionally substituted by one or more substituents; n is greater than or equal to 1 , preferably 1 or 2; x and y are each independently 1 , 2 or 3; and any of the monocyclic aromatic groups may be linked by a direct bond or divalent linking group.

Optionally, the polymer comprises optionally substituted arylene or heteroarylene repeat units, optionally repeat units of formula (IV):

(IV)

* 1 2 1 2 wherein R and R are independently H or a substituent and wherein R and R may be linked to form a ring.

Optionally, the phosphorescent emitter is blended with the polymer.

Optionally, the composition emits white light.

In a second aspect the invention provides a formulation comprising a light-emitting composition according to the first aspect and a solvent.

In a third aspect the invention provides an organic light-emitting device comprising an anode, a cathode and a light-emitting layer between the anode and the cathode wherein the light-emitting layer comprises a light-emitting composition according to the first aspect.

In a fourth aspect the invention provides a method of forming an organic light-emitting device comprising the step of providing a first electrode on a substrate; forming an organic light-emitting layer by depositing a composition according to the first aspect over the first electrode; and depositing a second electrode over the organic light-emitting layer.

Optionally, the first electrode is an anode and the second electrode is a cathode.

Optionally, the step of forming the organic light-emitting layer comprises the step of depositing the formulation according to the second aspect and evaporating the solvent.

In a fifth aspect the invention provides a light-emitting material of formula (la), (lb), (Ic) or (Id):

wherein each Ar independently represents an aryl or heteroaryl group, and wherein each Ar group and the fluoranthene ring may each independently be substituted with one or more substituents.

Optionally, the light-emitting material of formula (la), (lb) or (Ic) is provided as repeat units of a polymer.

Description of the Drawings

The invention will now be described in more detail with reference to the drawings, in which: Figure 1 is a schematic illustration of an organic light-emitting device;

Figure 2 illustrates energy levels for a prior art white light emitting composition ;

Figure 3 illustrates energy levels for a white light emitting composition according to an example of the invention;

Figure 4 illustrates the photoluminescent spectra of a fluorescent material according to an example of the invention and a comparative material;

Figure 5 illustrates the photoluminescent spectra of a white light emitting composition accord ing to an example of the invention and a comparative composition;

Figure 6 illustrates the electroluminescent spectra of a white light emitting composition accord ing to an example of the invention and a comparative composition;

Figure 7 illustrates the photoluminescent spectra of polymers accord ing to embodiments of the invention and a comparative example; and

Figure 8 illustrates the electroluminescent spectra of a device accord ing to an embod iment of the invention and a comparative example .

Detailed Description of the Invention

Figure 2 i llustrates the energy levels for a prior art white-emitting composition comprising fluorescent blue, fluorescent green and phosphorescent red emitting components. Fluorescent blue and green emission (hv>) is illustrated by decay of an exciton from the singlet excited state S i to the ground state So from the respective emitters. Phosphorescent red emission (hv) is illustrated by decay of a triplet exciton from the triplet excited state T i to So.

The T i energy level of the blue fluorescent emitter is typically higher than that of the red phosphorescent emitter, and accordingly a blue fluorescent emitter wi ll typically not quench red phosphorescent emission. Longer wavelength fluorescent emitters such as green fluorescent emitters, on the other hand, usually have a T| energy level that is lower than that of a red phosphorescent emitter (the S i— T i energy gap for most organ ic materials comprising aryl groups is about 1 .2 eV or higher). For example, the green fluorescent emitting polymeric repeat unit illustrated in Figure 2 quenches red phosphorescence.

CD

White OLED compositions comprising a green fluorescent emitter and a red

phosphorescent emitter typically comprise very low concentrations of those emitters and accordingly it may be expected that significant quenching of red phosphorescence by the green fluorescent emitter will not occur, however the present inventors have surprisingly found that significant quenching of phosphorescence occurs even at those very low concentrations.

Referring to Figure 3, a fluorescent emitter having emission in the green region of the visible spectrum and a Ti level that is higher than that of the phosphorescent material avoids quenching of the phosphorescent red emitter, thereby a significant increase in device efficiency can be achieved.

Light-emitting composition

The light-emitting composition comprises at least one fluorescent emitter and at least one phosphorescent emitter. Red, green and blue emitters may be used in combination to obtain white light.

The light-emitting composition may further comprise hole transporting and / or electron transporting components.

The light emitters and any additional components, such as hole and electron transporting components, may each be provided as separate materials blended together in the composition. Alternatively, one or more of the emitters and / or one or more of the additional components may be components of the same molecule, such as components of a polymer. A polymer may comprise one or more of the fluorescent emitter, the phosphorescent emitter, the hole transporting component and the electron transporting component provided as repeat units in the backbone of the polymer; as substituents pendant from the polymer; or as end-capping groups of the polymer. Repeat units in the backbone of the polymer may be conjugated together to form an at least partially conjugated polymer backbone.

The light-emitting composition may emit white light, for example light having a CIE x coordinate equivalent to that emitted by a black body at 2500-9000K and a CE y coordinate within 0.05 of the CIE y co-ordinate of said light emitted by a black body, preferably a CE x coordinate equivalent to that emitted by a black body at 2700-4500K and a CE y coordinate within 0.025 of the CE y co-ordinate. In the case of a white light-emitting composition, the green fluorescent and red phosphorescent emitters are optionally present in an amount less than 1 mol %, preferably less than 0.5 mol %. The blue emitter may be present in an amount of 1 mol % or more. The concentration of the blue emitter is typically at least two times greater than that of the green and / or red emitters.

Reference is made above to green fluorescent emitters, which have a photoluminescent emission peak at around 510 nm, however it will be appreciated that the teaching of the present invention applies equally to any fluorescent emitter having a photoluminescent spectmm with a peak in the range of 490-560 nm, which encompasses blue-green, green, yellow-green and yellow emitters, for example, which may also be used in white-light emitting compositions as described in more detail below.

Fluorescent emitter

The fluorescent emitter has an emission peak in its photolu inescent spectrum in the range of 490-560 nm. In the case where the fluorescent emitter is a repeat unit conjugated to a polymer chain then the relevant emission peak is the peak of the emitter in the polymer.

Compounds that emit fluorescent light in this wavelength and yet do not have a T| level that quenches phosphorescence of other emitters, in particular emitters emitting in the range 570-630 nm, include compounds having an Si-T] gap less than 1 eV, optionally less than 0.9 eV. Si and Ti levels of organic compounds can be found in, for example, Dekker, Handbook of Photophysics, 2^nd Edition, 1993.

Exemplary compounds comprise optionally substituted fluoranthenes, acridones and phenazines. An exemplary substituent for these compounds is one or more amine groups of formula -NR⁷2 wherein each R⁷ independently represents H or a substituent, preferably a substituent.

R⁷ is preferably alkyl, optionally C₁.₂₀ alkyl, Ar, or a branched or linear chain of Ar groups, for example -(Ar)_r, wherein Ar in each occurrence is independently selected from aryl or heteroaryl, optionally phenyl or naphthyl, and r is at least 1 , optionally 1 , 2 or 3.

Exemplary compounds include compounds of formulae (la), (lb), (lc) and (Id):

(Ic) (Id)

Each Ar may independently be substituted with one or more groups R. wherein R independently in each occurrence is H or a substituent. Suitable substituents R include alkyl groups, for example one or more C i.₂o alkyl groups, wherein one or more non- adjacent C atoms may be replaced with O, S, substituted N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F. R is preferably alkyl, for example Ci.₂₀ alkyl. Each of compounds of formulae (la), (lb), (lc) and (Id) may be further substituted with one or more substituents. Exemplary substituents include alkyl, which may be CL20 alkyl, wherein one or more non-adjacent C atoms may be replaced with O, S, substituted N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F.

11

The fluorescent emitter may be a small molecule compound or it may be bound to a polymer. In one embodiment, the fluorescent emitter comprises at least one - Ar₂ group and the emitter is bound to a polymer through at least one of the Ar groups. The emitter may be present in a polymer sidechain or as a polymer end group, in which case one of the Ar groups may be bound to the backbone of the polymer or to the end of the polymer backbone respectively. Alternatively or additionally, the emitter may be present in a main chain repeat unit, in wh ich case both Ar groups may be bound to adjacent backbone repeat units of the polymer.

Exemplary repeat units in the case where the emitter is a repeat unit in a polymer backbone, include the following repeat units

Preferred Ar groups are phenyl, each of which may optionally be substituted with one or more alkyl groups. R is preferably alkyl.

 W

Exemplary endgroups in the case where the emitter is a endgroup in a polymer backbone, include the following units

Preferred Ar groups are phenyl, each of which may optionally be substituted with more alkyl groups. R is preferably alkyl.

21



Phosphorescent emitter

The phosphorescent emitter may have a peak in its photoluminescent emission spectrum at around 570-630 nm, in particular to provide red and / or orange light emission.

Materials that may be used as phosphorescent emitters include metal complexes comprising optionally substituted complexes of formula (II):

(Π)

wherein M is a metal; each of L¹, L² and L³ is a coordinating group; q is an integer; r and s are each independently 0 or an integer; and the sum of (a. q) + (b. r) + (c.s) is equal to the number of coordination sites available on M, wherein a is the number of coordination

1 2

sites on L , b is the number of coordination sites on L and c is the number of coordination sites on L³.

Heavy elements M induce strong spin-orbit coupling to allow rapid intersystem crossing and emission from triplet or higher states (phosphorescence). Suitable heavy metals M include d-block metals, in particular those in rows 2 and 3 i.e. elements 39 to 48 and 72 to 80, in particular ruthenium, rhodium, pallaidum, rhenium, osmium, iridium, platinum and gold. Iridium is particularly preferred.

These metals form complexes with carbon or nitrogen donors such as porphyrin or bidentate ligands of formula (IV):

(IV)

wherein Ar⁴ and Ar⁵ may be the same or different and are independently selected from optionally substituted aryl or heteroaryl; X ¹ and Y¹ may be the same or different and are independently selected from carbon or nitrogen; and Ar⁴ and Ar⁵ may be fused together. Ligands wherein X¹ is carbon and Y¹ is nitrogen are particularly preferred.

Examples of bidentate ligands are illustrated below:

Each of Ar⁴ and Ar⁵ may carry one or more substituents. Two or more of these substituents may be linked to form a ring, for example an aromatic ring. Particularly preferred substituents include fluorine or trifluoromethyl which may be used to blue-shift the emission of the complex as disclosed in WO 02/45466, WO 02/44189, US 2002- 1 17662 and US 2002-182441 ; alky, or alkoxy groups as disclosed in JP 2002-324679; carbazole which may be used to assist hole transport to the complex as disclosed in WO 02/81448; and dendrons which may be used to obtain or enhance solution processability of the metal complex as disclosed in WO 02/66552.

A light-emitting dendrimer may comprise a light-emitting core bound to one or more dendrons, wherein each dendron comprises a branching point and two or more dendritic branches. The dendron may be at least partially conjugated, and at least one of the branching point and dendritic branches may comprise an aryl or heteroaryl group, such as phenyl groups.

A dendron may have optionally substituted formula (VI):

(VI)

wherein BP represents a branching point for attachment to a core and Gi represents first generation branching groups.

The dendron may be a first, second, third or higher generation dendron. Gi may be substituted with two or more second generation branching groups G₂, and so on, as in optionally substituted formula (Via):

(Via)

wherein u is 0 or 1 ; v is 0 if u is 0 or may be 0 or 1 if u is 1 ; BP represents a branching point for attachment to a core and G], G? and G₃ represent first, second and third generation dendron branching groups.

BP and / or any group G may be substituted with one or more substituents, for example one or more C|.₂o alkyl or alkoxy groups.

The phosphorescent emitter may be used in combination with a host in a host-dopant arrangement. The host may be any material having an excited state energy level that is higher than that of the emitter. Preferably, the gap between the host and dopant excited state energy levels is at least kT in order to avoid back transfer ofexcitons from the dopant to the host material. The host material is optionally solution processable.

The host and the light-emitting dopant or dopants may be physically mixed.

Alternatively, the light-emitting dopant or dopants may be chemically bound to the host. If the host is a polymer, the phosphorescent emitter may be bound to the host polymer as a sidechain repeat unit, main chain repeat unit or end group (as disclosed in, for example, EP 1245659, WO 02/31 896, WO 03/18653, WO 03/22908 and WO 2009/093033). One or more ligands of the phosphorescent emitter may be functionalized by substitution with one or more reactive leaving groups such as bromine or iodine so that the phosphorescent emitter may be bound to the host polymer. The host material may be a fluorescent light-emitting material. For example, in use the host material may provide blue light emission.

Examples of phosphorescent compounds that are not bound to a host include the following:

Ligands substituted with 3,5-diphenylbenzene substituents are described in WO 2002/066552. Ligands substituted with 2,4-diphenyl-l ,3,5-triazine are described in WO 2009/157424

Examples of reactive materials that can be used to form polymer end-groups include the following

Examples of monomers that can be used to form repeat units of a polymer include the following:

32 These reactive materials and monomers may be incorporated into polymers using metal insertion polymerization described in detail below.

Hole transporting component

In one arrangement, suitable hole transporting components have a band gap at least 0.1 eV wider than the fluorescent blue emitter in the system, where present. In another arrangement, the hole transporting component can act as a blue emitter at the same time as transporting charges. The band gap can be determined by measuring the onset of the absorption spectrum.

A hole transporting layer preferably comprises a material having a low electron affinity (2 eV or lower) and low ionisation potential (5.8 eV or lower, preferably 5.7 eV or lower, more preferred 5.6 eV or lower). Electron affinities and ionisation potentials may be measured by cyclic voltammetry, as described in more detail below.

The ionisation potential (IP) of the hole transporting component is at least 0.1 eV lower than the IP of the electron transporting component, where present, more preferred at least 0.2 eV lower, even more preferred at least 0.3 eV lower.

One class of hole transporting materials are (hetero)arylamines. In the case where the hole transporting component is provided as a repeat unit of a polymer, suitable repeat units include repeat units of formula (V):

(V)

wherein Ar¹ and Ar² in each occurrence are independently selected from optionally substituted aryl or heteroaryl groups, n is greater than or equal to 1 , preferably 1 or 2, R is H or a substituent, preferably a substituent, and x and y are each independently 1 , 2 or 3.

R is preferably alky], Ar³, or a branched or linear chain of Ar³ groups, for example -

3 3

(Ar )_r, wherein Ar in each occurrence is independently selected from aryl or heteroaryl and r is at least 1 , optionally 1 , 2 or 3. Any of Ar¹, Ar² and Ar³ may independently be substituted with one or more substituents. Preferred substituents are selected from the group R^J consisting of:

alkyl wherein one or more non-adjacent C atoms may be replaced with 0, S, substituted N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F or aryl or heteroaryl optionally substituted with one or more groups R⁴,

aryl or heteroaryl optionally substituted with one or more groups R⁴,

fluorine, nitro and cyano;

wherein each R⁴ is independently alkyl in which one or more non-adjacent C atoms may be replaced with O, S, substituted N, C=0 and -COO- and one or more H atoms of the alkyl group may be replaced with F, and each R⁵ is independently selected from the group consisting of alkyl and aryl or heteroaryl optionally substituted with one or more alkyl groups.

R may comprise a crosslinkable-group, for example a group comprising a polymerisable double bond such and a vinyl or acrylate group, or a benzocyclobutane group.

Any of the aryl or heteroaryl groups in the repeat unit of Formula (V) may be linked by a direct bond or a divalent linking atom or group. Preferred divalent linking atoms and groups include O, S; substituted N; and substituted C.

Where present, substituted N or substituted C of R³, R⁴ or of the divalent linking group may independently in each occurrence be NR⁶ or CR⁶2 respectively wherein R⁶ is alkyl or optionally substituted aryl or heteroaryl. Optional substituents for aryl or heteroaryl groups R⁶ may be selected from R⁴ or R⁵.

In one preferred arrangement, R is Ar³ and each of Ar¹, Ar² and Ar³ are independently and optionally substituted with one or more Ci.₂o alkyl groups.

Particularly preferred units satisfying Formula 1 include units of Formulae 1 -3 :

1 2 3

* 1 2 3

wherein Ar and Ar are as defined above; and Ar is optionally substituted aryl or heteroaryl. Where present, preferred substituents for Ar³ include substituents as described forAr¹ and Ar², in particular alkyl and alkoxy groups.

Ar¹, Ar² and Ar³ are preferably phenyl, each of which may independently be substituted with one or more substituents as described above.

In another preferred arrangement, aryl or heteroaryl groups of formula (V) are phenyl, each phenyl group being optionally substituted with one or more alkyl groups.

In another preferred arrangement, Ar¹, Ar² and Ar³ are phenyl, each of which may be substituted with one or more Ci_₂₀ alkyl groups, and r = 1.

In another preferred arrangement, Ar¹ and Ar² are phenyl, each of which may be substituted with one or more C]_-2o alkyl groups, and R is 3,5-diphenylbenzene wherein each phenyl may be substituted with one or more alkyl groups.

Electron transporting component

Suitable electron transporting components have a electron affinity (EA) at least 0.1 eV higher than the EA of the hole transporting component, where present, preferably at least 0.2 eV higher than the EA of the hole transporting component.

Suitable electron transporting components preferably comprises a material having a high electron affinity (1 .8 eV or higher, preferably 2 eV or higher, even more preferred 2.2 eV or higher) and high ionisation potential (5.8 eV or h igher), as measured by cyclic voltammetry as described in more detail below. Suitable electron transport groups include groups disclosed in, for example, Shirota and ageyama, Chem. Rev. 2007, 107, 953- 1010.

Electron transport may be provided by a conjugated chain of arylene repeat units, for example a conjugated chain comprising one or more of fluorene. indenofluorene, and phenylene repeat units, each of which may optionally be substituted by, for example, alkyl or alkoxy.

Exemplary fluorene repeat units include repeat units of formula (IV):

(IV)

wherein R¹ and R² are independently H or a substituent and wherein R¹ and R² may be linked to form a ring.

R¹ and R² are optionally selected from the group consisting of hydrogen; optionally

3 3 ¾

substituted Ar or a linear or branched chain of Ar groups, wherein Ar is as described above; and optionally substituted alkyl wherein one or more non-adjacent C atoms of the alkyl group may be replaced with O, S, substituted N, C=0 and -COO-.

In the case where R¹ or R² comprises alkyl, optional substituents of the alkyl group include F, CN, nitro, and aryl or heteroaryl optionally substituted with one or more groups R⁴ wherein R⁴ is as described above.

In the case where R¹ or R² comprises aryl or heteroaryl, each aryl or heteroaryl group may independently be substituted. Preferred optional substituents for the aryl or heteroaryl groups include one or more substituents R³.

Optional substituents for the fluorene unit, other than substituents R¹ and R², are preferably selected from the group consisting of alkyl wherein one or more non-adjacent C atoms may be replaced with O, S, substituted N, C=0 and -COO-, optionally substituted aryl, optionally substituted heteroaryl, fluorine, cyano and nitro. Where present, substituted N in repeat units of formula (IV) may independently in each occurrence be NR⁵ or NR⁶.

In one preferred arrangement, at least one of R¹ and R² comprises an optionally substituted C1-C20 alkyl or an optionally substituted aryl group, in particular phenyl substituted with one or more C ₁-₂₀ alkyl groups.

Exemplary phenylene repeat units include repeat units of formula (VIII):

(VIII)

wherein R¹ is as described above with reference to formula (IV) and p is 1 , 2, 3 or 4, optionally 1 or 2. In one arrangement, the repeat unit is a 1 ,4-phenylene repeat unit.

The repeat unit of formula (VIII) may have formula (Villa):

(Villa)

Other groups suitable as electron transporting component are heterocyies with ionization potential and high electron affinity.

Particularly suitable are triazines, pyrimidines, pyridines, triazoles and oxadiazoles.

Triazine-containing materials may have formula (VII):

(Ar¹)z

(Ar²)₂ N (Ar³)₂

(VII)

wherein Ar , Ar and Ar are as described with reference to repeat units of formula (V) and z is at least 1 , optionally 1 , 2 or 3. Each of Ar', Ar² and Ar³ may independently be substituted with one or more substituents. In one arrangement, Ar¹, Ar² and Ar³ are phenyl in each occurrence. Exemplary substituents include R as described above with reference to repeat units of formula (V), for example Ci-20 alkyl or alkoxy.

Exemplary triazine repeat units of a polymer have formula (Vila):

(Vila)

wherein Ar¹, Ar² and Ar³ and z are as described above.

Other suitable electron transporting components include ketones (for example compounds disclosed in WO 2004/093207) and phosphine oxides (for example compounds disclosed in J. Phys. Chem. C 2008, 1 12, 7989).

Further emitters

The composition may comprise one or more further emitters to adjust the colour of light emitted from the composition, for example a fluorescent emitter having a peak photo luminescence wavelength less than or equal to 480 nm, such as in the range of 400- 480 nm. The further emitter may be a blue fluorescent emitter, in particular for a white light emitting composition. A blue fluorescent emitter may also function as a host for the phosphorescent emitter.

Exemplary blue fluorescent emitters include repeat units of formula (V) above wherein R is Ar³ and wherein Ar¹, Ar² and Ar³ are each an optionally substituted monocyclic aromatic group, in particular phenyl.

In one embodiment, Ar¹, Ar² and R are phenyl, each of which may be substituted with one or more Ci-20 alkyl groups, and Ar¹ and Ar² are linked by an 0 or S atom.

Polymers comprising phenoxazine repeat units are disclosed in, for example, WO 2004/060970.

Polymers comprising phenoxazine repeat units substituted with 3,5-diphenylbenzene are disclosed in WO 2010/001982. Monomers and polymerisation

Preferred methods for preparation of polymers comprise "metal insertion" reactions of monomers comprising a reactive leaving group bound to the terminal Ar group of a unit of formula (I). Exemplary metal insertion methods are Suzuki polymerisation as described in, for example, WO 00/53656 and Yamamoto polymerisation as described in, for example, T. Yamamoto, "Electrically Conducting And Thermally Stable □ - Conjugated Poly(arylene)s Prepared by Organometallic Processes", Progress in Polymer Science 1993, 17, 1 153-1205. In the case of Yamamoto polymerisation, a nickel complex catalyst is used; in the case of Suzuki polymerisation, a palladium complex catalyst is used.

For example, in the synthesis of a linear polymer by Yamamoto polymerisation, a monomer having two reactive halogen groups is used. Similarly, according to the method of Suzuki polymerisation, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine. As alternatives to halides, other leaving groups capable of participating in metal insertion include groups include tosylate, mesylate and triflate.

It will therefore be appreciated that repeat units illustrated throughout this application may be derived from a monomer carrying suitable leaving groups. Likewise, an end group or side group may be bound to the polymer by reaction of a suitable leaving group.

Suzuki polymerisation may be used to prepare regioregular, block and random copolymers. In particular, homopolymers or random copolymers may be prepared when one reactive group is a halogen and the other reactive group is a boron derivative group. Alternatively, block or regioregular, in particular AB, copolymers may be prepared when both reactive groups of a first monomer are boron and both reactive groups of a second monomer are halogen.

Hole injection layers

A conductive hole injection layer, which may be formed from a conductive organic or inorganic material, may be provided between the anode 2 and the light-emitting layer 3 to assist hole injection from the anode into the layer or layers of semiconducting polymer. Examples of doped organ ic hole injection materials include optionally substituted, doped poly(ethylene dioxythiophene) (PEDT), in particular PEDT doped with a charge- balancing polyacid such as polystyrene sulfonate (PSS) as disclosed in EP 0901 176 and EP 0947123, polyacrylic acid or a fluorinated sulfonic acid, for example Nafion ®;

polyan iline as disclosed in US 5723873 and US 5798170; and optionally substituted polythiophene or po ly(thienothiophene). Examples of conductive inorgan ic materials include transition metal oxides such as VOx, MoOx and RuOx as disclosed in Journal of Physics D: Applied Physics ( 1996), 29(1 1), 2750-2753.

Charge transporting layers

A hole transporting layer may be provided between the anode and the light-emitting layer. Likewise, an electron transporting layer may be provided between the cathode and the light-emitting layer.

Similarly, an electron blocking layer may be provided between the anode and the light- emitting layer and a hole blocking layer may be provided between the cathode and the light-emitting layer. Transporting and blocking layers may be used in combination. Depending on its HOMO and LUMO levels, a single layer may both transport one of holes and electrons and block the other of holes and electrons.

If present, a hole transporting layer located between anode 2 and light-emitting layer 3 preferably has a HOMO level of less than or equal to 5.8 eV, more preferably around 4.8- 5.6 eV. HOMO levels may be measured by cyclic voltammetry, for example. One suitable class of hole-transporting materials are amines, such as polymers comprising repeat units of formula (V), which may be a homopolymer or a copolymer, such as a copolymer comprising repeat units of formula (V) and one or more arylene co-repeat units such as repeat units of formula (IV).

If present, an electron transporting layer located between light-emitting layer 3 and cathode 4 preferably has a LUMO level of around 3-2 eV, more preferably of around 3- 2.5 eV For example, a layer of a silicon monoxide or silicon dioxide or other thin dielectric layer having thickness in the range of 0.2-2nm is provided between light- emitting layer 3 and layer 4. Suitable small molecule electron transport materials are disclosed in Shirota and Kageyama, Chem. Rev. 2007, 107, 953-1010 and references therein. Polymeric electron transport materials preferably comprise a high triplet level backbone monomer and electron transport unit as disclosed in, for example, US 2010/013377, for example:

Cathode

Cathode 4 is selected from materials that have a workfunction allowing injection of electrons into the electroluminescent layer. Other factors influence the selection of the cathode such as the possibility of adverse interactions between the cathode and the electroluminescent material. The cathode may consist of a single material such as a layer of aluminium. Alternatively, it may comprise a plurality of metals, for example a bilayer of a low workfunction material and a high workfunction material such as calcium and aluminium as disclosed in WO 98/10621 ; elemental barium as disclosed in WO

98/57381 , Appl. Phys. Lett. 2002, 81 (4), 634 and WO 02/84759; or a thin layer of metal compound, in particular an oxide or fluoride of an alkali or alkali earth metal, to assist electron injection, for example lithium fluoride as disclosed in WO 00/48258; barium fluoride as disclosed in Appl. Phys. Lett. 2001 , 79(5), 2001 ; and barium oxide. In order to provide efficient injection of electrons into the device, the cathode preferably has a workfunction of less than 3.5 eV, more preferably less than 3.2 eV, most preferably less than 3 eV. Work functions of metals can be found in, for example, Michaelson, J. Appl. Phys. 48(1 1), 4729, 1977.

The cathode may be opaque or transparent. Transparent cathodes are particularly advantageous for active matrix devices because emission through a transparent anode in such devices is at least partially blocked by drive circuitry located underneath the emissive pixels. A transparent cathode will comprises a layer of an electron injecting material that is sufficiently thin to be transparent. Typically, the lateral conductivity of this layer will be low as a result of its thinness. In this case, the layer of electron injecting material is used in combination with a th icker layer of transparent conducting material such as indium tin oxide.

It will be appreciated that a transparent cathode device need not have a transparent anode (unless, of course, a fully transparent device is desired), and so the transparent anode used for bottom -emitting devices may be replaced or supplemented with a layer of reflective material such as a layer of aluminium. Examples of transparent cathode devices are disclosed in, for example, GB 23483 16.

Encapsulation

OLEDs tend to be sensitive to moisture and oxygen. Accordingly, the substrate preferably has good barrier properties for prevention of ingress of moisture and oxygen into the device. The substrate is commonly glass, however alternative substrates may be used, in particular where flexibility of the device is desirable. For example, the substrate may comprise a plastic as in US 6268695 which d iscloses a substrate of alternating plastic and barrier layers or a laminate of thin glass and plastic as disclosed in EP 0949850.

The device is preferably encapsulated with an encapsulant (not shown) to prevent ingress of moisture and oxygen. Suitable encapsulants include a sheet of glass, films having suitable barrier properties such as silicon dioxide, silicon monoxide, silicon nitride or alternating stacks of polymer and dielectric as disclosed in, for example, WO 01 /81649 or an airtight container as disclosed in, for example, WO 01/19142. In the case of a transparent cathode device, a transparent encapsulating layer such as silicon monoxide or silicon dioxide may be deposited to micron levels of thickness, although in one preferred embodiment the thickness of such a layer is in the range of 20-300 nm. A getter material for absorption of any atmospheric moisture and / or oxygen that may permeate through the substrate or encapsulant may be disposed between the substrate and the encapsulant.

Solution processing

Light-emitting layer 3, and / or charge-transporting layer(s), where present, may be deposited by any process, including vacuum evaporation and deposition from a solution in a solvent. In the case where the polymer comprises arylene groups, in particular arylene groups substituted with alkyl groups such as alkylphenylenes or alkylfluorenes, suitable solvents for solution deposition include mono- or poly-alkyl, alkoxy and halobenzenes such as toluene, xylene, anisole, chlorobenzene, dichlorobenzene and similar. Particularly preferred solution deposition techniques including printing and coating techniques, preferably spin-coating and inkjet printing. Spin-coating is particularly suitable for devices wherein patterning of the

electroluminescent material is unnecessary - for example for lighting applications or simple monochrome segmented displays.

Inkjet printing is particularly suitable for h igh information content displays, in particular full colour displays. A device may be inkjet printed by providing a patterned layer over the first electrode and defining wells for printing of one colour (in the case of a monochrome device) or multiple colours (in the case of a multicolour, in particular full colour device). The patterned layer is typically a layer of photoresist that is patterned to define wells as described in, for example, EP 0880303.

As an alternative to wells, the ink may be printed into channels defined within a patterned layer. In particular, the photoresist may be patterned to form channels which, unlike wells, extend over a plurality of pixels and which may be closed or open at the channel ends.

Other solution deposition techniques include dip-coating, roll printing and screen printing.

If multiple layers of an OLED are formed by solution processing then the skilled person will be aware of techniques to prevent intermixing of adjacent layers, for example by crosslinking of one layer comprising one or more materials carrying crosslinking groups before deposition of a subsequent layer or selection of materials for adjacent layers such that the material from which the first ofthese layers is formed is not soluble in the solvent used to deposit the second layer. For example, where a polymer is used as described above, a crosslinkable group, such as a crosslinkable double bond or benzocyclobutane group, may be provided as a substituent of one or more repeat units, such as a substituent of repeat unit (IV) and / or (V) described above.

Applications

The fluorescent light-emitting material of the invention may be used in an organic light- emitting device, for example as a green emitter in a full colour display.

The while light-emitting composition may be used in a white light-emitting OLED, for example an OLED display, an LCD backlight or an area lighting device. W

lonisation Potential and Electron Affinity Measurement

The HOMO and LUMO energy levels of a material may be measured by cyclic voltammetry (CV) wherein the working electrode potential is ramped linearly versus time. When cyclic voltammetry reaches a set potential the working electrode's potential ramp is inverted. This inversion can happen multiple times during a single experiment. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram trace.

Apparatus to measure HOMO or LUMO energy levels by CV may comprise a cell containing a tert-butyl ammonium perc lorate/ or tertbutyl ammon ium

hexafluorophosphate solution in acetonitrile, a glassy carbon working electrode where the sample is coated as a film, a platinium counter electrode (donor or acceptor of electrons) and a reference glass electrode no leak Ag/AgCl. Ferrocene is added in the cell at the end of the experiment for calculation purposes. (Measurement of the difference of potential between Ag/AgCl/ferrocene and sample/ferrocene).

Method and settings:

3mm diameter glassy carbon working electrode

Ag/AgCl/no leak reference electrode

Pt wire auxiliary electrode

0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile

LUMO = 4.8 - ferrocene (peak to peak maximum average) + onset

Sample: 1 drop of 5mg/mL in toluene spun @3000rpm LUMO (reduction) measurement: A good reversible reduction event is typically observed for thick films measured at 200 mV/s and a switching potential of -2.5V. The reduction events should be measured and compared over 10 cycles, usually measurements are taken on the 3<rd> cycle. The onset is taken at the intersection of lines of best fit at the steepest part of the reduction event and the baseline. Exam les

Fluorescent small molecu le

Fluorescent emitter 1 was prepared by selective Buchwald coupling starting from commercially available 3-aminofiuoranthene according to the following synthetic methode:

Fluorescent Emitter 1

738 mg (1 eq, 3.4 mmol) 3-Aminofluoranthene (used as received from Acros Organics), 1.8 g (2.2 eq, 7.5 mmol) l -Bromo-4-n-hexylbenzene, 46 mg (0.05 mmol) Pd₂(dba)₃, 59 mg (0.2 mmol) ('Βυ)₃ΡΒΡ₄ and 979 mg (3 eq, 10 mmol) sodium tert-butoxide in 50 ml anhydrous toluene were heated to reflux for 16 hours. The reaction was quenched with water and filtered through Celite, followed by aqueous work-up and column

chromatography on silica using hexane/toluene 4: 1 to yield 1.8g orange oil, 96% pure by HPLC.

PLQY data of Fluoresent Emitter 1

The photo luminescence spectrum of Fluorescent Emitter 1 measured in 3 wt % PMMA gave an emission peak at 540 nm.

Figure 4 illustrates the photoluminescent emission spectrum of this material as compared to Comparative Emitter 1 , which is a green emitter of the type described in WO

2005/049546:

Comparative Emitter 1

With reference to Figure 4, Fluorescent Emitter 1 is a green-yellow emitter, i.e. having a longer peak wavelength than green-emitting Comparative Emitter 1.

PLOY data of small molecule white blends

Polymer A was prepared by Suzuki polymerisation as described in WO 00/53656 of the following monomers:

Monomer D

The polymer was prepared by polymerising 50 mol % Monomer A, 30 mol % Monomer B, 16.5 mol % Monomer C, 3.5 mol % Monomer D

GPC (relative to polystyrene standard) in Dalton: Mw 383,000, Mp 346,000, Mn 143,000, Pd 2.68

Polymer A is a blue light-emitting material that was blended with green and red light- emitting materials described below to provide a white light emitting composition.

Red Phosphorescent Emitter 2

Red Phosphorescent Emitter 1

1 . Comparison of blends with Red Phosphorescent Emitter 1

Blends of Polymer A, Fluorescent Emitter 1 or Fluorescent Emitter 2, respectively, and Red Phosphorescent Emitter 1 (98: 1 .1 % w/w) were spun from ortho-xylene on quartz substrates to achieve transmittance values of 0.3-0.4. The PLQY was measured using a integrating sphere connected to Hamamatsu C9920-02 with Mercury lamp E7536 and a monochromator for choice of exact wavelength. As can be seen in Table 1 and Figure 5, a significant increase in red emission is observed when using Fluorescent Emitter 1 .

2. Comparison of blends with Red Phosphorescent Emitter 2

Samples were prepared as described above, but using a ratio Polymer A: Fluorescent Emitter : Red Phosphorescent Emitter 2 of 97: 1 :2 % w/w. As can be seen in Table 1 and Figure 6, a significant increase in orange-red emission is observed.

Table 1

Monomer Example

Monomer 1 , for use as a green light-emitting monomer in a conjugated polymer, was prepared according to the following synthetic method:

C₁₆H,iN C₆H₄Brl C₂₈H, 7Br₂N

Mol. Wt: 217.27 Mol. Wt : 282 90 Mol. Wt.: 527.25

Monomer 1

Under nitrogen, 4g (1 eq., 1 8 mmol) 3-aminofluoranthene (used as received from Acros Organics), 1 3.68 g (2.6 eq, 48.4 mmol) 4-iodobromobenzene, 0.66 g (0.04 eq, 0.72 mmol) Pd₂(dba)₃, 0.9 g (0.08 eq, 1 .44 mmol) BINAP (racemic mixture) and 4.8 g (2.8 eq, 50 mmol) sodium iert-butoxide in 150 ml anhydrous toluene were heated to reflux for seven days. After cooling to RT, the mixture was filtered through Celite and eluted with toluene. The solvent was removed in vacuo and the residue purified via column chromatography using hexane/toluene (95:5 to 65 :35). Yield: 400 mg, 100% pure by GCMS, 98% pure by HPLC.

Polymer Examples

Polymer Examples 1 and 2 were prepared by Suzuki polymerisation as described in WO 00/53656 of the following monomers. For the purpose of comparison, Comparative Polymer 1 containing no repeat units derived from Monomer 1 was also prepared.

GPC data (relative to polystyrene standard) in Daltons:

Comparative 2,163,000 1 ,462,000 603,000 3.60 example 1

With reference to Figure 7, Polymer Example 1 has stronger emission between about 500-600 nm due to the green-yellow emission of the repeat unit derived from Monomer 1 as compared to the green emission of Comparative Polymer 1 .

Composition Example 1

A white light-emitting composition was prepared by mixing Polymer Example 2 with 0.1 mol % of Red Phosphorescent Emitter 1

Comparative Composition 1 was prepared as for Composition 1 except that Comparative Polymer 1 was used in place of Polymer Example 2.

Composition Example 2

A white light emitting composition was prepared according to Composition Example 1, except that Polymer Example 1 was used in place of Polymer Example 2.

Device Example 1

A device having the following structure was formed:

ITO / HfL / HTL / EL / MF / Al

wherein 1TO represents an indium-tin oxide anode; HIL is a hole-injection layer formed from Plextronics Inc; HTL is a hole transport layer of a polymer comprising fluorene repeat units of formula (IV) and amine repeat units of formula (V); EL is an

electroluminescent layer containing Composition Example 1 ; MF is a metal fluoride; and the bilayer of MF / Al forms a cathode for the device.

A substrate carrying ITO was cleaned using UV / Ozone. The hole injection layer was formed by spin-coating an aqueous formulation of a hole-injection material available from Plextronics, Inc. A hole transporting layer was formed to a thickness of 20 nm by spin-coating and crosslinked by heating. A light-emitting layer was formed by depositing a light-emitting formulation to a thickness of 75 nm by spin-coating from o-xylene W

solution. A cathode was formed by evaporation of a first layer of a metal fluoride to a thickness of about 2 nm, a second layer of aluminium to a thickness of about 200 nm and an optional third layer of silver.

Comparative Device 1

Comparative Device 1 was prepared in the same way, except that Comparative

Composition 1 was used in place of Composition Example 1.

Device Example 2

A device was prepared as described for Device Example 1 except that Composition Example 2 was used in place of Composition Example 1.

Figure 8 illustrates that Device Example 2 provides stronger emission in the range of 500-600 nm as compared to Comparative Device 1 due to the longer peak wavelength of the repeat unit derived from Monomer 1.

Furthermore, the intensity of red emission (with a peak at about 630-640 nm) is considerably more intense in Device Example 2 (79, as compared to 50 for Comparative Device 1 ) in the absence of quenching by the repeat unit derived from Monomer 1.

The maximum external quantum efficiency (EQE) of Device Example 2 is 12 %, as compared to 9 % for Comparative Device 1 , illustrating that a low Ti fluorescent emitter can cause significant quenching of phosphorescence even when the fluorescent emitter and phosphorescent material are present in low concentrations.

Device Example 3

A device was prepared as described with reference to Device Example 1 except that Red Phosphorescent Emitter 1 was replaced with Red Phosphorescent Emitter 2, which has a shorter peak wavelength than Red Phosphorescent Emitter 1.

Comparative Device 3

A device was prepared as described with reference to Comparative Device I except that Red Phosphorescent Emitter 1 was replaced with Red Phosphorescent Emitter 2.

The maximum external quantum efficiency of Device Example 3 was 6.3 %, compared to 5.2 % for Comparative Device Example 3. Although the present invention has been described in terms of specific exemplary embodiments, it will be appreciated that various modifications, alterations and/or combinations of features disclosed herein will be apparent to those skilled in the art without departing from the scope of the invention as set forth in the following claims.

Claims

1 . A light-emitting composition comprising a first fluorescent organic emitter and a phosphorescent organic emitter wherein the first fluorescent organic emitter has a peak wavelength in the range of 490-560 nm and wherein a triplet excited state energy level of the first fluorescent organic emitter is higher than a triplet excited state energy level of the phosphorescent organic emitter.

2. A light-emitting composition according to claim 1 wherein the fluorescent emitter has a singlet-triplet gap of less than 1 .2 eV, optionally less than 1 eV.

3. A light-emitting composition according to claim 1 or 2 wherein the

phosphorescent emitter has a peak wavelength in the range of 570-630 nm.

4. A light-emitting composition according to claim 1 , 2 or 3 wherein the first

lae (la)-(Id):

(Ic) (Id) wherein each Ar independently represents an aryl or heteroaryl group, R represents H or a substituent and wherein any of the rings of formulae (la) including any ring of any Ar group, are unsubstituted or independently substituted with one or more substituents.

5. A light-emitting composition according to any preced ing claim wherein the first fluorescent emitter is provided in the light emitting composition in an amount of up to 1 mol %, optionally up to 0.5 mol %.

6. A light-emitting composition according to any preceding claim wherein the

phosphorescent organic emitter comprises a phosphorescent metal complex. 7. A light-emitting composition according to claim any preceding claim further comprising a second fluorescent organic emitter having a peak wavelength of less than or equal to 480 nm, optionally in the range 400-480 nm.

8. A light-emitting composition according to any preceding claim wherein the

emitters are separate light-emitting compounds blended together.

9. A light-emitting composition according to any of claims 1 -3 and 5-8 wherein the first fluorescent emitter is provided as repeats units of a polymer.

10. A light-emitting composition according to claim 9 wherein the first fluorescent emitter comprises a repeat unit of formula (le-Ih):

(Ie) (If)

(Ig) (Ih) wherein each Ar independently represents an aryl or heteroaryl group, represents H or a substituent and wherein any of the rings of formulae (la) - (Id), including any ring of any Ar group, are unsubstituted or independently substituted with one or more substituents

A light-emitting composition according to claim 7 and claims 9 or 10 wherein the first and second fluorescent emitters are provided as repeat units of the polymer.

A light-emitting composition according to claim 1 1 wherein the second fluorescent emitter comprises a repeat unit of formula (V):

(V)

1 2

wherein Ar , Ar and R each independently represent a monocyclic aromatic group, preferably phenyl, each of wh ich is unsubstituted or substituted by one or more substituents; n is greater than or equal to 1 , preferably 1 or 2; x and y are each independently 1 , 2 or 3; and any of the monocyclic aromatic groups may be linked by a direct bond or divalent linking group.

3. A light-emitting composition according to any of claims 9-12 wherein the

polymer comprises unsubstituted or substituted arylene or heteroarylene repeat units, optionally fluorene repeat units and / or phenylene repeat units.repeat units of formula (IV):

(IV)

14. A light-emitting composition according to c laim 13 wherein the polymer comprises fluorene repeat units of formula (IV) and / or phenylene repeat units of formula

(IV) (VIII) wherein p is 1 , 2, 3 or 4; R¹ and R² are independently in each occurrence H or a substituent; and R¹ and R² may be linked to form a ring

15. A light-emitting composition according to any of claims 9-14 wherein the

phosphorescent emitter is blended with the polymer.

1 6. A light emitting composition according to any preceding claim wherein the

composition emits white light.

1 7. A formulation comprising a light-emitting composition according to any

preceding claim and a solvent.

1 8. An organic light-emitting device comprising an anode, a cathode and a light- emitting layer between the anode and the cathode wherein the light-emitting layer comprises a light-emitting composition according to any of claims 1 -16.

19. A method of forming an organic light-emitting device comprising the step of providing a first electrode on a substrate; forming an organic light-emitting layer by depositing a composition according to any one of claims 1 -16 over the first electrode; and depositing a second electrode over the organic light-emitting layer.

20. A method according to claim 19 wherein the first electrode is an anode and the second electrode is a cathode.

21. A method according to claim 19 or 20 wherein the step of forming the organ ic light-emitting layer comprises the step of depositing the formulation according to claim 17 and evaporating the solvent.

wherein each Ar independently represents an aryl or heteroaryl group, R represents H or a substituent and wherein any of the rings of formulae (la) - (Id), including any ring of any Ar group, may independently be substituted with one or more substituents.

A light-emitting material according to claim 22 wherein the light-emitting material of formula (la), (lb), (Ic) or (Id) is provided as repeat units or an end group of a polymer.