Discotic liquid crystal and organic electroluminescence...

Details Discotic liquid crystal and organic electroluminescence... Discotic liquid crystal and organic electroluminescence...

2000-09-20

2002-12-10

Wu, Shean C. (Department: 1756)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C349S069000, C570S183000, C570S187000, C544S245000, C552S208000, C552S224000, C552S266000

Reexamination Certificate

active

06491847

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a novel discotic liquid crystal having a polyfluorinated side chain and a broad and stable liquid crystal phase and to an organic electroluminescence device using the discotic liquid crystal.
Discotic liquid crystal phase is a liquid crystal phase discovered in 1977 by Chandrasekhar, et al. (Pramana, 9, 471 (1977)). As described in their paper entitled “Discotic Liquid Crystals” (Rep. Prog. Phys., 53, 57 (1990)) and in a paper entitled “Design and Synthesis of Discotic Liquid Crystal Molecules” by Shunsuke Takenaka (Japanese Chemical Society, Seasonal Publication, General Review, vol. 22, pp. 60+), the discotic liquid crystal phase is found in compounds having a disk-shaped core and a plurality of relatively long chains connected to the core. Such compounds may be classified into various types according to their core structure, inclusive of derivatives of hexa-substituted benzene and tri-substituted benzene; derivatives of phthalocyanine and porphyrin; derivatives of triphenylene, truxene and pyrylium, respectively; tribenzocyclononene derivatives, azacrown derivatives, and cyclohexane derivatives.
Based on the structural characteristic of a discotic liquid crystal, several reports have been made suggesting application thereof to devices. A systems including conjugated &pgr;-electrons, as found in derivatives of phthalocyanine or triphenylene, can provide a channel for electrons (or holes) (Piechocki, et al., J. Am. Chem. Soc., 104, pp. 5245 (1982)). Further, a system including an annular core, as found in an aza-crown derivative, can provide a molecular channel using the central spacing thereof as a selective molecular passage (Lehn, et al., J. Chem. Soc., Chem. Commun., pp. 1794 (1985)).
On the other hand, since 1987 when T. W. Tang et al. proved that a high luminance light emission was achieved by a low voltage drive of a laminate of their films of a fluorescent metal chelate complex and a diamine molecule, extensive research has been made on organic electroluminescence devices (hereinafter, the term “electroluminescence” is sometimes abbreviated as “EL” according to a common usage in the field) as luminescence or light emission devices having a high speed responsiveness and a high efficiency. An organic EL device is a carrier injection-type self-light emission device utilizing luminescence caused at the time of recombination of electrons and holes having reached the luminescence layer.
FIGS. 2 and 3
respectively illustrate a laminate structure of an ordinary organic EL device. Referring to
FIG. 2
(or FIG.
3
), an EL device includes a cathode metal electrode
21
(or
31
) and an anode transparent electrode
24
(or
35
) disposed on a transparent substrate
25
(or
36
) for taking out luminescent light. Organic compound layers, each having a thickness on the order of several hundred Å (angstoms), are sandwiched between the electrodes. The cathode may generally comprise a metal having a small work function, such as aluminum, aluminum-lithium alloy, magnesium-silver alloy, etc. The anode may comprise a conductive material having a large work function, such as indium tin oxide (ITO). The organic compound layers, may ordinarily have a two layer structure including a luminescence layer
22
and a hole-transporting layer
23
as shown in
FIG. 2
or a three layer structure including an electron-transporting layer
32
, a luminescence layer
33
and a hole-transporting layer
34
as shown in FIG.
3
.
The hole-transporting layer has a function of effectively injecting holes from the anode into the luminescence layer, and the electron-transporting layer has a function of effectively injecting electrons from the cathode into the luminescence layer. The hole-transporting layer also has a function of confining electrons, and the electron-transporting layer also has a function of confining holes, respectively, into the luminescence layer, i.e., carrier-blocking functions for enhancing the luminescence efficiency. For these carrier-transporting layers, inclusive of the hole-transporting layer and the electron-transporting layer, a charge-transporting performance, particularly a carrier mobility, may be regarded as an important property. An organic compound in an amorphous state may generally exhibit a carrier mobility on the order of 10
−5
cm
2
/V.sec, which cannot be said to represent a sufficient transporting performance. It is believed that if the mobility of a carrier-transporting layer is increased, a larger amount of carrier can be injected into the luminescence layer to enhance the luminescence efficiency, and simultaneously the thickness of the carrier-transporting layer (generally having a thickness on the order of several hundred Å) can be increased (to a thickness up to ca. 1 &mgr;m), so that it becomes possible to effectively prevent a short circuit between the electrodes sandwiching the organic layers and provide an improved productivity.
At present, in order to achieve a higher efficiency organic EL device, extensive work toward the development of various compound materials for the carrier-transporting layers has been made. Along with the activity, some proposal has been made to achieve a higher mobility by imparting mesomorphism to organic compounds forming carrier-transporting layers. Organic films generally used in organic EL devices are in an amorphous state and have no regularity regarding molecular alignment. In contrast thereto, some organic compounds in a liquid crystal state, i.e., having some order of molecular alignment, have been found to show a high mobility, thus calling attention.
For example, Haarer, et al. observed that a long chain triphenylene compound, a representative discotic liquid crystal material, exhibited a high hole mobility of 10
−1
cm
2
/V.sec (Nature, vol. 371, p. 141 (1994)). Further, Haarer, et al. examined a relationship between hole mobility and molecular alignment order in columnar phase for a series of triphenylene-type discotic liquid crystals and reported that a higher order provided a higher mobility (Adv. Mater., vol. 8, p. 815 (1996)). Thus, a molecular alignment control advantageous for carrier transportation is expected to be achieved by utilizing spontaneous alignment of mesomorphic organic compounds, thus providing excellent carrier-transporting materials. On the other hand, organic EL devices involve problems regarding durability, such as deterioration of luminescence performance due to moisture and due to reaction between organic compound layers.
Some discotic liquid crystal compounds having polyfluorinated side chains have been reported in Liquid Crystal, vol. 19, No. 6, pp. 759-764 (1995). More specifically, three species of triphenylene derivatives (
5
a,
5
b
and
5
c
), each having 6 polyfluorinated side chains, are shown in
FIG. 2
at page 760 of the above report. These compounds have an intermediate carboxyl group in their side chains and do not cause transformation from the discotic columnar phase to clarifying point (Iso), but reach a decomposition point on temperature increase as shown in a table at an upper left portion of page 761, so that they cannot be regarded as stable discotic liquid crystal compounds.
SUMMARY OF THE INVENTION
A generic object of the present invention is to solve the above-mentioned problems of the prior art.
A more specific object of the present invention is to provide a novel discotic liquid crystal compound having a stable and broad discotic liquid crystal phase.
Another object of the present invention is to provide an organic electroluminescence device exhibiting stable and good luminescence performance by using the liquid crystal compound.
According to the present invention, there is provided a discotic liquid crystal compound represented by formula (1) below:
Ar&Parenopenst;X—R)
n
  (1),
wherein Ar denotes a group of 2,3,5,6-benzoquinone-tetra-yl, 2,3,4,6,7,8-anthraquinone-hexa-yl, 2,3,6,7,10,1 1-triphenylene-hexa-yl, 2,3,7,8,12,13-truxene-hexa-yl, 2,3,6,7,10,11-tricycloquinazo

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