Benzidine derivatives and their preparation and use

Details Benzidine derivatives and their preparation and use Benzidine derivatives and their preparation and use

1999-08-17

2001-07-10

Yamnitzky, Marie (Department: 1774)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S917000, C428S704000, C313S504000, C313S506000, C546S018000

Reexamination Certificate

active

06258469

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to novel benzidine derivatives (benzidine=4,4′-diaminobiphenyl), to a process for their preparation, and to their use as hole transport materials in organic light emitting diodes (OLEDs).
Hole-transport materials based on tetraarylbenzidine are described, for example, in the dissertation by J. Simmerer “Ladungstransport in kolumnaren Phasen” [Charge transport in columnar phases], Bayreuth 1996, (pages 4 to 10); in this connection, see also: “Molecular Crystals and Liquid Crystals”, Vol. 183 (1990), pages 217 to 226, “Applied Physics Letters”, Vol. 65 (1994), pages 807 to 809, and “Chemical Physics”, Vol. 200 (1995), pages 245 to 255.
The materials disclosed hitherto absorb and emit exclusively in the UV region of the electromagnetic spectrum.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide novel benzidine derivatives, and a process for their preparation, that overcome the above-mentioned disadvantages of the prior art materials of this type and in particular emit in various regions of the visible spectral region and exhibit improved material properties such as higher glass transition temperatures than prior art materials.
It is a further object of the invention to provide improved organic light emitting diodes i.e. OLEDs incorporating benzidine derivatives of the invention as hole transport materials.
With the foregoing and other objects in view there are provided, in accordance with the invention, novel benzidine derivatives which are symmetrical fused and bridged spiro compounds.
The compounds of the invention are fluorescent chromophores. As a consequence of the limited twisting potential of the nitrogen and the fused ring structure, they are easier to oxidize than the benzidine derivatives disclosed hitherto, and hence they are highly suitable as hole-transport materials in organic light emitting diodes.
Owing to the presence of fused structural elements, further the longest-wavelength absorption of the compounds (1) is shifted into the visible spectral region. These compounds can thus—depending on the type of fusing—emit in various regions of the visible spectral region, enabling them to act simultaneously as emitter chromophore.
In accordance with a further feature of this invention, there are provided organic light emitting diodes (OLEDs) in which the novel fluorescent chromophore compounds of the invention serve as hole transport material, as well as pixel displays produced therewith for information monitors and light sources. An OLED in accordance with this invention comprises a layered construction including a transparent electrode, an organic chromophoric luminescent agent, an organic electron transport material, a metal electrode and a novel benzidine compound according to this invention. In such layered constructions the organic hole transport material, the chromophoric luminescent agent and the organic electron transport material can be combined into a single layer or in separate layers as desired. U.S. Pat. No. 4,539,507 can be consulted for various electron transport materials.
Compared with the materials used hitherto (based on tetraarylbenzidine), the benzidine derivatives of the invention exhibit improved material properties. These are, in particular, a higher glass transition temperature, which results in higher thermal stability of the light diodes, and a lower potential difference between the material and the ITO electrode (ITO=indium-tin oxide), which results in a higher light efficiency of the light diodes. This is because the HOMO potentials in the materials employed hitherto are lower in energetic terms than the potential of the ITO electrode, which produces a lower light efficiency.
The benzidine derivatives of the invention are symmetrical fused, and bridged spiro compounds of the following structure:
in which:
n=1, 2 or 3;
R
1
=H, phenyl or 1-naphthyl,
R
2
and R
3
together form a fused benzene ring, an a- or b-fused naphthalene ring or a fused 1,2,5-thiadiazole ring.
Preferably n=2, R
1
=H or phenyl, and R
2
and R
3
together form a fused benzene ring.
The novel benzidine derivatives are both fused, i.e. condensed, ring systems and bridged spiro compounds, i.e. compounds in which one carbon atom, the so-called spiro atom, is common to two rings. In addition, the amino groups in these benzidine derivatives are bridged, i.e. they resist being twisted, giving a very strong donor system. The assignment of R
1
in these benzidine derivatives is the same in each of the two rings to which R
1
is linked, and the fused ring formed by R
2
and R
3
together is the same in each of the two rings to which R
2
and R
3
are fused.
The benzidine derivatives (1) can be prepared in high purity in a simple manner from readily accessible starting materials, by condensing an amino-substituted aromatic or heteroaromatic compound, i.e. a heterocyclic compound, of structure (2) and a cycloaliphatic ketone of structure (3). The reactants are brought together, preferably in an equimolar ratio, without addition of a solvent, in the presence of a catalyst, in particular a catalytic amount of iodine, at elevated temperature, to give a bridged heterocyclic compound, i.e. an aromatic amine, of structure (4); the reaction of the two starting materials is preferably carried out at a temperature of from 200 to 250° C.
The resulting highly nucleophilic condensation product (4) is then oxidized to the benzidine derivative (1), preferably using hydrogen peroxide in a polar solvent, such as dimethylformamide. The oxidation can also be carried out using oxygen and other oxidants. It is advantageous to add a copper salt, such as copper (I) chloride (CuCl) in the oxidation using hydrogen peroxide.
The benzidine derivative (1) is also obtained when the condensation product (4) is isolated after its formation by distillation in a high vacuum, since under these conditions (4) is oxidized to the benzidine derivative owing to its high nucleophilicity. The honey-like product produced in the distillation contains a mixture of (1) and (4), which can easily be separated using diethyl ether. The benzidine derivative (1) is obtained as a flake-like precipitate, while the condensation product (4) remains in solution.
Compounds (1) in which R
1
=H, phenyl or 1-naphthyl can be prepared as described. However, the phenyl and naphthyl compounds can also be prepared by arylating the corresponding H compound. The arylation is carried out in a manner known per se by a reaction analogous to the Ullmann reaction or the Heck reaction, for example by reaction with bromobenzene in the presence of a palladium catalyst.
Owing to their spiro structure the compounds of structure (1) are difficult to crystallize; they are produced in the form of amorphous powders and have a glass transition temperature of >220° C.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in benzidine derivatives, and their preparation and use, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments.
The invention will be explained in greater detail below with reference to working examples.


REFERENCES:
patent: 4539507 (1985-09-01), VanSlyke et al.
patent: 227 434 A1 (1985-09-01), None
patent: 0 463 996 A2 (1992-01-01), None
S. Heun: “A comparative study of hole transport in vapor-deposited molecular glasses of N,N′, N″, N′″-tetrakis(4-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine and N,N′-diphenyl-N,N′-

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