WO2002100975A2

WO2002100975A2 - (electro)luminescent device with an organic dye as an acceptor

Info

Publication number: WO2002100975A2
Application number: PCT/IB2002/002194
Authority: WO
Inventors: Klemens Brunner; Albert Van Dijken; Johannes W. Hofstraat; Hermannus F. M. Schoo; Johannes A. E. H. Van Haare; Bea M. W. Langeveld
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2001-06-12
Filing date: 2002-06-10
Publication date: 2002-12-19
Also published as: WO2002100975A3; AU2002309147A1

Abstract

The invention pertains to an (electro)luminescent device comprising an organic dye as an acceptor wherein the dye has the general formula I: (I), wherein R1 is H, alkyl, or substituted or unsubstituted phenyl; R2 and R3 are independently selected from phenyl, naphthyl, and thienyl, which is unsubstituted or substituted with an alkyl, alkoxy, or halogenated alkoxy group.

Description

(Electro)luminescent device with an organic dye as an acceptor

The invention pertains to an (electro)luminescent device with an organic dye as an acceptor, to these organic dyes, and to the use of said dye in an (electro)luminescent device.

(Electro)luminescent devices (such as LEDs) comprising a dye are known in the art. (Electro)luminescent is short for luminescent and electroluminescent in particular. These devices are usually not very efficient for giving light. An improvement was found with polymer light emitting diodes (PLED) wherein polymeric luminescent substances are used to generate a light. Such devices are known, for instance from EP 1,043,382. In such devices the polymeric material is used as such for emitting light, or the polymer is mixed with other materials, such as organic dyes, in a light emitting layer. A disadvantage of such devices is that for the purpose of obtaining full color emission a large variety of specific luminescent polymers must be made. Such specific polymers are disclosed, for instance, in US 5,712,361. It is known that the luminescence character of such polymers can be changed by mixing the polymer with a luminescent dye, see for instance EP 892,028. However, such mixtures are obtained by trial and error and it is not known in advance which mixtures lead to efficient luminescence. Further, there is no reason that such arbitrary mixtures contribute to the stability of the luminescent system. It is an object of the present invention to use organic dyes as an acceptor in (electro)luminescent devices that can efficiently be used^'in acceptor systems without a polymer and in polymer-acceptor systems with optimum efficiency and stability.

It has now be found that a specific class of organic dyes is very efficient in

(electro)luminescent devices with and without comprising polymers. The invention therefore pertains to an (electro)lu inescent device comprising an organic dye as an acceptor, characterized in that the dve has the seneral formula I

wherein

Rl is H, alkyl, or substituted or unsubstituted phenyl;

R2 and R3 are independently selected from phenyl, naphthyl, and thienyl, which are unsubstituted or substituted with an alkyl, alkoxy, or halogenated alkoxy group.

The term alkyl means a branched or unbranched alkyl group with 1 to 18 carbon atoms, preferably 1 to 8 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl, isobutyl, pentyl, hexyl, decyl, dodecyl, and the like. The term alkoxy stands for a branched or unbranched alkoxy group with 1 to

18 C atoms, such as methoxy, ethoxy, octyloxy, dodecyloxy, and the like. The alkoxy group may be halogenated, preferably with chloride, bromide, or iodide. Preferably, the halide is bonded to the end-standing carbon atom.

The term substituted phenyl, naphthyl, and thienyl means a phenyl, naphthyl, or thienyl group that is substituted with a halogen, alkyl, alkoxy, or halogenated alkoxy group, as defined previously. In groups that are substituted with a halogen atom, the halogen atom can be used as an anchor to attach this group to a polymer, preferably to an (electro)luminescent polymer.

It is stressed that similar organic dyes are known in the art. For instance, in US

5,248,782 a compound was disclosed with the formula according to I, wherein Rl is hydrogen or methyl, and R2 and R3 are 2-thienyl or phenyl. These dyes are used in immunoassays, labeling nucleotides and oligonucleotides for hybridizing studies and staining of cells in imaging studies. The use of these dyes in luminescent devices such as LEDs is not disclosed. Other organic dyes with related chemical structures are disclosed as dyes for optical recording media, such as in CD-R. In EP 822,544 such dyes are used to absorb laser energy after which the dye decomposes to form a recordable pit in the absorption layer of the CD-R. The dyes disclosed in this reference differ with the present dyes in that they do not comprise two aromatic groups at the sites juxtapositioned to the imidazole nitrogen atoms. It is therefore a further object of the invention to obtain novel organic dyes that are suitable for use in (electroluminescent devices. These dyes have the general formula

wherein

Rl is H, alkyl, or substituted or unsubstituted phenyl;

R2 and R3 are independently selected from phenyl, naphthyl, and thienyl, which are unsubstituted or substituted with an alkyl, alkoxy, or halogenated alkoxy group, with the proviso that Rl is not hydrogen or alkyl when R2 and R3 are unsubstituted 2-thienyl or phenyl. R1-R3 have the same meanings as previously described.

Preferably, the (electro)luminescent device of the invention further comprises a polymer. These devices usually show the highest efficiency for luminescence. Moreover, (electro)luminescent devices comprising polymers are more easy to manufacture, because the polymer-dye combination can be applied directly from a solution. These polymers preferably comprise a phenylene-vinylene, phenylene, phenylene-ethyne, triphenylamine, thiophene, vinylcarbazole, fluorene, or a spirofluorene repeating unit or combinations thereof, which is optionally substituted such as with a group selected from aryl, alkyl, alkoxy, and unsaturated groups. In a preferred embodiment according to the invention the polymer is chemically bonded to the organic dye through a spacer.

The process of excitation energy migration in a polymer doped with a dye is not yet fully understood. The most pertinent reference in this respect is a publication by List et al., Chemical Physics Letters. 325 (2000), 132-138. These authors come to the conclusion that the excitation energy migration between a polymeric host and guest has to be explained by the sum of at least two processes. First a temperature-dependent migration process of singlet excitons which can be of Dexter or Coulomb type and a second temperature- independent transfer of the singlet exciton, which is of Fδrster dipole-dipole interaction type. However, these authors do not disclose the requirements that are necessary to obtain optimum luminescent efficiency at maximum stability. Applying the Fόrster theory, for instance as suggested by List et al, the skilled person comes to the conclusion that maximum overlap of the emission spectrum of the donor and the absorption spectrum of the dye is necessary to obtain the highest energy transfer efficiency. However, experiments performed by us unexpectedly showed that this is not the case. On the contrary, under these conditions the efficiency appeared to be very low. According to the present invention, in (electro)luminescent devices comprising polymers, the main issues in polymer luminescent materials, including (electro)luminescent materials, i.e. the stability, efficiency, and the color gamut of the presently known substances, can be addressed by using dyes in luminescent polymers. The stability is effectively increased by incorporation of an organic dye, which depopulates in a highly efficient manner the (reactive) excited state of the luminescent polymer. When a very stable emissive dye is applied, such as a laser dye, the stability of the material is significantly improved. The second important advantage of the present invention is that luminescent dyes can be used for the realization of a full color display. According to the invention the dyes are chosen to provide the optimum emission wavelength with respect to the desired color purity. The fact that several emissive dyes can optionally each be incorporated into a particular polymer for obtaining different emission wavelengths is an additional advantage of the invention. Preferably, the different dyes are incorporated in the same polymer. In this way for each of the three basic colors required for a full color display the same device structure can be used. The same full color wavelengths can, in principle, also be obtained by using different dyes without being doped onto a polymer.

If used in an electroluminescent device the polymer has to be capable of transporting charge carriers, such as holes and/or electrons. Suitably, the polymer may be an, at least partially, conjugated polymer.

The invention also pertains to the use of an organic dye as an acceptor in an (electro)luminescent device, including a device comprising a polymer-acceptor system wherein a polymer with a plurality of chromophores is doped with the dye for transferring excitation energy from the polymer to the dye wherein at least one wavelength of the polymer emission is a wavelength at which the dye absorbs energy, and for emitting energy as photons, characterized in that the dwell time of an exciton that is to be transferred from the polymer to the dye is longer than the time for transferring said exciton from the polymer to the dye, by satisfying the equations

PP -I ≠ k ET < / and

PP wherein ∞ is the rate constant of the energy transfer between the chromophores of the

polymer, ^^ is the rate constant of the energy transfer between the polymer and the dye,

^~ ^■*ϊ?*•^pd is the mean distance between the polymer and the dye, ^Λ J?^opd is the Fδrster radius, and pp ~ zr and ^ » are the experimental lifetimes of a single chromophore and of the plurality of chromophores of the polymer, respectively. The Fδrster radius is thereby defined as the separation between a donor and a dye for which the rate of energy transfer between the excited donor and the ground state dye and the inherent rate of deactivation of the excited donor are equal. The dwell time is the time that an exciton spends on a certain polymeric chromophore.

A way to tune the color of a polymer-based (electro)luminescent device is to incorporate luminescent dyes into the polymer. When the dye and the polymer satisfy the above conditions, upon excitation of the polymer the energy will be transferred to the dye, which process is known as excitation energy transfer (EET), followed by luminescence from the dye. By using different dyes, different colors can be obtained. A major advantage of such suitable combinations of polymer and dyes is that the emission properties are decoupled from the charge transporting and excitation properties of the polymer. The principle of this invention is based on the known Fδrster theory for Coulomb dipolar interaction. The standard formulation of this theory is given by the Fδrster equation:

In this relation k_ETis the rate constant for energy transfer, τ_Dthe experimental lifetime of the donor in the absence of a dye, and R the distance between the donor and the dye. Ro is the Fδrster radius, which for this type of polymer-acceptor systems is approximately 15 A.

Using Fδrster' s theory the rate constants of the energy transfer between the chromophores, which constitute a disordered polymer (pp transfer) and the energy transfer between a chromophore of the polymer and the dye (pd transfer) is given by:

wherein K is the mean distance between the species involved in the energy transfer process.

The rate for the energy transfer ( _ET) is:

Φ_ET= k_ET.[D*][A] (3) wherein D* stands for the concentration of excited donors and A for the concentration of dyes. Thus formula (2) can be written as:

wherein p and d denote the concentrations of the polymeric chromophores and the dye (acceptor) molecules, respectively. The pd transfer is more efficient than the pp transfer when φ- <φ , thus:

in which formula d and p are not included as they are constant for all samples

By definition fc PP . 1

ET ^' PP (6) V ET

Thus formulae (5) and (6) give:

The term ηr^?P describes the lifetime of the intra-polymer exciton transfer, or in other words, the transfer time of an exciton from chromophore I to chromophore j. This time is also called the dwell time of chromophore i ^^dweli ). Thus the dwell time is the time that an exciton stays on a certain chromophore.

A disordered polymer can be described as an ensemble (plurality) of chromophores differing in conjugation length and/or chemical surroundings. The dwell time of an exciton on a certain chromophore depends on the excited state energy of this particular chromophore. The dwell time increases when the energy of the excited state decreases. Equation (7) can now be transformed to:

Since the dwell time cannot be longer than the experimental lifetime τ_n of the polymer, the above ratio is always smaller than 1. This is shown in Fig. 1 where the mean polymer-dye distance (<R>^pd) relative to the Fδrster radius as a function of the ratio between the dwell time and the experimental lifetime of the polymer is given.

The value for the Fδrster radius ( J^^d ) can be obtained from the steady state emission spectrum of the undoped polymer and the absorption spectrum of the dye. This value is approximately the same (about 15 A) for similar systems as described herein. The value of τ_n can be obtained from time-resolved measurements on the emission from an undoped polymer. τ_n Is the lifetime of this emission which is dependent on the photon energy and which can vary by two orders of magnitude between high and low energy photons that are emitted from the polymer.

The energy transfer from the polymer to a dye is very inefficient when the dwell time is much shorter than the lifetime of the polymer. Energy transfer from the polymer to the dye can only compete with the intra-polymer exciton transfer when the dwell time and transfer time become comparable.

Experimental procedure.

The dyes of the invention can be used in light emitting diodes (LED) and polymer light emitting diodes (pLED), light emitting cells (LEC), polymer light emitting cells (pLEC), displays in general, and in plastic electronics (such as FETS).

The invention is illustrated by means of the following non-restrictive examples.

Example 1

A polymer which is a green emitting conjugated polymer with a repeating unit of the structural formula:

and a red emitting organic dye (dopant) with the structural formula:

[4,4-difluoro-3,5-bis[2-(5-methylthiophene)]-4-bora-3a,4a-diaza-s-indacene] were used for making the polymer-acceptor system of the invention.

First, a solution of the polymer was prepared by dissolving a specific amount of the polymer in toluene to yield a solution which contains 4 g polymer per 1 1 of toluene (0.4% weight-to-volume ratio). This solution was stirred overnight at room temperature. Secondly, a small amount of the dye was dissolved in toluene. The concentration of this dye solution was chosen such that only a few μl had to be added to about 5 ml of the polymer solution to give a 0.75% dye-to-polymer weight ratio. The dye-polymer solution was spin coated onto a glass substrate giving a layer thickness of about 70 nm.

Photoluminescence emission spectra of the undoped green emitting polymer (dashed line) and of the same polymer doped with the red emitting organic dye (full line) upon excitation with light of 410 nm are shown in Fig. 2. The photoluminescence excitation spectra as recorded at the maximum of the emission band were identical for both samples indicating that only the polymer is photoexcited and that the emission from the dye is due to energy transfer from the polymer to the dye. From Fig. 2 it can also be seen that in the dye- doped polymer still a remainder of the polymer emission is visible. If this remaining emission band is compared to the original emission band it is clear that the former is shifted to higher energies with respect to the latter. This means that in the dye-doped polymer, the polymeric chromophores with a relatively high HOMO-LUMO distance are still emitting, although an energy acceptor is present. Apparently, only the polymeric chromophores with a lower HOMO-LUMO distance are transferring their energy to the organic dye because the dwell time of the exciton on these chromophores is long enough to enable transfer of the exciton to the organic dye.

As can be seen in Fig. 3, the absorption spectrum of the red emitting organic dye overlaps the emission spectrum of the green emitting polymer (dashed line) at its low energy side. This is in agreement with the observations made from Fig. 2. The only polymeric chromophores that are capable of transferring energy to the organic dye are situated at the low-energy side of the polymer emission band, which is at the same point where the organic dye has its highest absorbance.

Example 2

S ynthesis of 4,4-difluoro-3 , 5-bis [2-(5-methylthiophene)] -4-bora-3 a,4a-diaza- s-indacene.

N-bromosuccinimide (9.08 g) was added to a mixture of 20-methylthiophene (5.00 g) in 25 ml of THF (tetrahydrofuran) at 0°C. The mixture was kept in the dark and stirred overnight. The solvent was evaporated, the precipitate was dissolved in 30 ml of diethyl ether, and washed with a saturated sodium hydrogencarbonate solution. The organic solution was dried over magnesium sulfate, filtered, and concentrated to give a crude oily product, which was purified with vacuum distillation to give a colorless oil (5-bromo-2- methylthiophene) in 82% yield. A mixture of 6-bromo-2-naphthol (10.0 g), 3-bromopropanol (9.4 g) and potassium hydroxide (3.0 g) in 50 ml of ethanol was refluxed for 16 h. The mixture was washed with diethyl ether and water. The combined organic layers were dried, filtered, and concentrated. The crude product was crystallized from ethanol to give 6-bromo-2-(3- hydroxyρropyloxy)-naphthalene in 40% yield. To N-tert-butoxycarbonyl-2-trimethylstannylpyrrole (2.0 g; prepared according to S. Martina et al., Synthesis. 1991, 613) and 5-bromo-2-methylthiophene (1.07 g) in 20 ml DMF (N,N-dimethylformamide) was added dichloro bis(triphenylρhosphino)- palladium(ϋ) (87 mg) under an argon atmosphere. The mixture was heated at 70°C for 16 h. After cooling the mixture was washed with diethyl ether and water. The combined organic layers were dried (MgSO ), filtered, and concentrated. The crude product was purified by column chromatography (silica, hexane/dichloromethane, 66/34,v/v) to give a pure yellow oil in 32% yield [2-(2-(5-methylthiophene)-N-tert-butoxycarbonylpyrrole].

To N-tert-butoxycarbonyl-2-trimethylstannylpyrrole (3.0 g) and 6-bromo-2-(3- hydroxypropyloxy)naphthalene (2.55 g) in 20 ml of DMF was added dichloro bis(triρhenylphosphino)palladium(II) (64 mg) under an argon atmosphere. The mixture was heated at 70°C for 16 h. After cooling the mixture was washed with diethyl ether and water. The combined organic layers were dried (MgSO₄), filtered, and concentrated. The crude product was purified by column chromatography (silica, hexane/dichloromethane, 2/98, v/v) to give a pure oil in 60% yield [2-(6-(3-hydroxypropyloxy)-naphthyl)-N-tert- butoxycarbonylpyrrole]. To this oil (0.5 g) in 10 ml of DMF was added dropwise phophorusoxychloride (0.42 g) at 0°C under an argon atmosphere. The mixture was heated at 60°C for 2 h. After cooling the mixture was neutralized with aqueous sodium hydroxide (1 M) and heated at 80°C for 1 h. After cooling the precipitate was filtered off and purified by column chromatography (silica, methanol/dichloromethane, 1/99 ,v/v) to give a 75% yield of 5-formyl-2-(6-(3-chloropropyloxy)-naphthyl)-N-H-pyrrole.

2-(2-(5-methylthiophene)-N-tert-butoxycarbonylpyrrole (0.50 g) was converted to 2-(2-(5-methylthiophene)-N-H-pyrrole by heating at 190°C for 15 min in an argon atmosphere. After cooling to room temperature dichloromethane (40 ml) and 5-formyl- 2-(6-(3-chloropropyloxy)-naphthyl)-N-H-pyrrole (0.60 g) were added.

Phosphorusoxychloride (180 μl) was added dropwise. After stirring at room temperature for 16 h, N,N-diisopropylethylamine (1.35 ml) and boron trifluoride diethyl etherate (1.0 ml) were added and the mixture was stirred for another 2 h. The reaction mixture was washed with brine, dried, filtered, concentrated, and purified by column chromatography (silica; hexane/chloroform 40/60 v/v) giving 14% yield of 4,4-difluoro-3,5-bis[2-(5- methylthiophene)]-4-bora-3a,4a-diaza-s-indacene.

Characteristic 1H-NMR (CDC1₃) signals (ppm) are: 7.98 (d, 2H, J=3.8Hz); 7.01 (s, 1H); 6.96 (d, J=4.2Hz, 2H); 6.85 (dd, J=3.8Hz, 2H); 6.74 (d, 2H, J=4.2Hz); 2.55 (s, 6H).

Example 3

Synthesis of 4,4-difluoro-8-phenyl-3,5-bis[6-(3,7-dimethyloctyloxy)-naphthyl]-4-bora-3a,4a- diaza-s-indacene:

A mixture of 6-bromo-2-naphtol (50 g), 3,7-dimethyloctyloxytosylate (66.2 g), and potassium carbonate (57 g) in methylisobutylketone (550 ml) was refluxed for 16 h. The mixture was concentrated, aqueous sodium hydroxide (1M) was added, and the reaction mixture was washed with dichloromethane. The organic layer was dried over magnesium sulfate, filtered, and concentrated to an oily product [6-bromo-2-(3,7-dimethyloctyloxy)- naphthalene] that was further used without purification. To a mixture of 55.06 g of this crude product and 50.02 g of N-tert- butoxycarbonyl-2-trimethylstannylpyrrole (prepared according to S. Martina et al., Synthesis. 1991, 613) in 300 ml of DMF (N,N-dimethylformamide) were added 1.50 g of dichloro bis(triphenylphosphino)palladium (13) under an atmosphere of argon. The mixture was heated at 80°C for 16 h. After cooling to room temperature, water was added and the reaction mixture was washed with diethyl ether. The organic layer was dried over magnesium sulfate, filtered, and concentrated. After column chromatography (silica; hexane/dichloromethane 60/40 v/v) 72% yield of pure yellow oily 2-(6-(3,7-dimethyloctyloxy)-naρhthyl)-N-tert- butoxycarbonylpyrrole.

This product (7.01 g) was converted to 2-(6-(3,7-dimethyloctyloxy)-naphthyl)- N-H-pyrrole by heating it at 190°C for 45 min under an argon atmosphere. After cooling to about 50°C, 100 ml of 1,2-dichloroethane and benzoyl chloride (4.36 g) were added and the mixture was refluxed for 16 h. After cooling to room temperature N,N-diisopropylethylamine (13.6 ml) and boron trifluoride diethyl etherate (15.6 ml) were added and the mixture was heated at 80^αC for 16 h. The reaction mixture was cooled to room temperature, concentrated, and purified by column chromatography (silica; hexane/dichloromethane 65/35 v/v) followed by crystallization from dichloromethane and hexane to obtain in 23% yield 4,4-difluoro-8- phenyl-3,5-bis[6-(3,7-dimethyloctyloxy)-naphthyl]-4-bora-3a,4a-diaza-s-indacene.

Characteristic 1H-NMR (CDC1₃) signals (ppm) are: 7.67-7.57 (m, 5H, phenyl); 6.94 (d, J=4.5Hz, 2H, pyrrole); 6.77 (d, J=4.5Hz, 2H, pyrrole).

Example 4

Synthesis of 4,4-difluoro-3,5-bis[6-(3,7-dimethyloctyloxy)-naphthyl]-4-bora- 3 a,4a-diaza-s-indac

To 2-(6-(3,7-dimethyloctyloxy)-naρhthyl)-N-tert-butoxycarbonyl-pyrrole (11.24 g) in 200 ml of DMF was added dropwise phosphorusoxychloride at 0°C under an argon atmosphere. After addition the mixture was heated at 60°C for 3h, after which the mixture was neutralized with aqueous sodium hydroxide (1 M) and heated at 80°C for 3 h.

After cooling, the reaction mixture was extracted with dichloromethane and the organic layer was dried over magnesium sulfate, filtered, and concentrated. After precipitation with hexane 43% yield of a white powder [5-formyl-2-(6-(3,7-dimethyloctyloxy)-naphthyl)-N-H-pyrrole] was obtained.

To 2-(6-(3,7-dimethyloctyloxy)-naphthyl)-N-H-pyrrole in dichloromethane was added at 50°C 5-formyl-2-(6-(3,7-dimethyloctyloxy)-naphthyl)-N-H-pyrrole (4.15 g).

Phosphorusoxychloride (1.10 ml) was added dropwise. After stirring at room temperature for 16 h N,N-diisopropylethylamine (6.3 ml) and boron trifluoride diethyl etherate (7.4 ml) were added and the mixture was refluxed for 16 h. The reaction mixture was cooled to room temperature, concentrated, and purified by column chromatography (silica; hexane/dichloromethane 80/20 v/v) giving 46% yield of 4,4-difluoro-3,5-bis[6-(3,7- dimethyloctyloxy)-naphthyl]-4-bora-3a,4a-diaza-s-indacene. Characteristic 1H-NMR (CDC1₃) signals (ppm) are: 7.24 (s, IH, bridge between the pyrrole moieties); 6.75 (d, J=3Hz, 2H, pyrrole).

Example 5 In a similar manner as described in Examples 2-4 the following dyes are prepared.

Example 6

An electroluminescent device was made by using the polymer-dye combination of Example 1, however, with a 1% dye-to-polymer weight ratio. A PEDOT (polyethylene dioxythiophene, Baytron™ P, ex Bayer) layer was spin coated onto an ITO (indium-tinoxide) covered glass substrate giving a layer thickness of about 100 nm. Onto this structure the dye-polymer solution was spin coated to give a layer thickness of about 70 nm. On top of this layered structure a thin layer of barium, followed by a thin layer of aluminum was evaporated to provide a cathode. Upon application of a voltage a bright red electroluminescence was observed with a threshold value of about 2.2 Volt, which is similar to the threshold value of the undoped polymer, which gives green electroluminescence. The brightness is 100 cd/m²at 3.3 Volt.

This experiment was repeated with the dye of Example 5, entry 3, to give a bright red electroluminescence with a threshold value of about 2.2 Volt. The brightness is 100 cdm² at 4.2 Volt.

Claims

CLAIMS:

1. An (electro)luminescent device comprising an organic dye as an acceptor, characterized in that the dye has the general formula I

wherein

Rl is H, alkyl, or substituted or unsubstituted phenyl;

R2 and R3 are independently selected from phenyl, naphthyl, and thienyl, which is unsubstituted or substituted with an alkyl, alkoxy, or halogenated alkoxy group.

2. The (electro)luminescent device of claim 1, further comprising a polymer with a plurality of chromophores, which is doped with the dye of claim 1 for transferring excitation energy from the polymer to the dye wherein at least one wavelength of the polymer emission is a wavelength at which the dye absorbs energy.

3. The (electro)luminescent device of claim 2 wherein the polymer comprises a substituted or unsubstituted phenylene-vinylene, phenylene, phenylene-ethyne, triphenylamine, thiophene, vinylcarbazole, fluorene, or a spirofluorene repeating unit.

4. The (electro)luminescent device of claim 2 or 3 wherein the polymer is chemically bonded to the dye through a spacer.

5. The (electro)luminescent device of any one of claims 2-4, wherein the dwell time of an exciton that is to be transferred from the polymer to the dye is longer than the time for transferring said exciton from the polymer to the dye, by satisfying the equations pd

Λ. CT l\ΕT

wherein fc is the rate constant of the energy transfer between the chromophores of the polymer, fc^p is the rate constant of the energy transfer between the polymer and the dye,

j ^pd is the mean distance between the polymer and the dye, Rζ^d is the Fδrster radius, and

<_j^ and _π are the experimental lifetimes of a single chromophore and of the plurality of chromophores of the polymer, respectively.

6. An organic dye having the general formula I of claim 1 wherein R1-R3 have the meanings as defined in claim 1, with the proviso that Rl is not hydrogen or alkyl when R2 and R3 are unsubstituted 2-thienyl or phenyl.

7. Use of an organic dye in the (electro)luminescent device of claim 1-5, characterized in that the dye has the general formula I of claim 1.