Abstract

Aryl substituted tetraazaphenanthrenes (9) were synthesized as electron transport material for electroluminescent-devices (EL-devices). Their synthesis and their physical properties are discussed. The results of an experimental EL-device using tetraphenyltetraazaphenanthrene (9a) as an electron transport material are presented.

Introduction

With the fast growing array of electronic equipment, a demand for novel types of displays has arisen over the last few years. These range from small, flat panel displays, useful for mobile telephones and for the ‘back-rest' movie and entertainment screens recently installed in certain aircraft, to large full-color displays for new generations of televisions, video players and perhaps even cinema viewing screens. The characteristics required of such displays are manifold, but difficult to achieve: long life-time, high brightness, high resolution, fast response time, low operating voltage. Anybody who has worked with flat panel displays realizes that a wide viewing angle is of great importance.

Against this background, the discovery of electroluminescence in non-crystalline organic material (in both ‘low-weight' and polymeric material) was propitious. While light-emitting diodes (LEDs) based on inorganic solids have been known for some time, it was shown in the 1960s that single crystals of anthracene sandwiched between two electrodes exhibit electroluminescence (Helfrich 1965, Pope 1963). The underlying principle of this is the injection of electrons into the semiconducting material from one electrode and a concomitant creation of migrating ‘positive' holes at the other electrode. The capture of the electron by the hole (recombination) is associated with the emission of radiative energy, produced by the so-called excited electron-hole state (exciton). Over 25 years later, Tang and Van Slyke demonstrated that organic polymers could be used in making LEDs (Tang 1987). These devices consisted of a hole-transporting layer of an aromatic diamine and an emissive layer of (8)-hydroxyquinoline aluminum (Alq₃). Although the quantum efficiency of the devices was on the order of 0.05% and the driving voltage was > 10 V, the potential advantages associated with organic materials versus inorganic materials in the variability of compounds that can be used and the processability of these materials to form uniform, thin layered coatings, made the use of organic compounds in LEDs an attractive alternative. Another 15 years have passed since then. The research effort devoted to organic luminescent material has been tremendous. (For a review of organic materials in modern display technology, see Ziemelis 1999, Friend 1999). Last year, a number of products were announced which feature electroluminescent displays based on organic materials (Philipps, Pioneer). Today, the research in this area is an interplay of chemistry (designing of new organic compounds), physics and engineering. In order to achieve a balanced injection and transport of electrons and holes into the emissive layer, additional organic layers have been introduced into the device structure, providing for the crucial separation of the emissive layer from the electrodes. Thus, much higher quantum efficiencies and much lower driving voltages have been achieved. The research, however, is far from over. The development of improved full-color LEDs is one of the major targets in the next few years.

For the fabrication of high-performance LEDs, hole and electron-transporting material, as well as light emitting material, are required. The aim of our work is the synthesis of electron-transporting material. Important for this type of material is a strong electron acceptor characteristic and the ability to form amorphous films. Pyrazino-annelated benzenes are known to accept an electron relatively easily. Thus, 1,4,5,8,9,12-hexaazatriphenylene (HAT) (see also: Nasielski-Hinkens 1981) has been studied extensively in our laboratory. Previously, it had been reported to be a strong electron acceptor. Albeit it crystallizes easily and has a high melting point. Hexaphenyl-HAT is also expected to be a good electron-acceptor. Moreover, due to the phenyl groups, hexaphenyl-HAT crystallizes less easily, thus a better result was expected for the formation of an amorphous film incorporating this material. Its preparation was carried out via hexaaminobenzene (2) as the key intermediate, using a known procedure. The synthesis of hexaaminobenzene (2) had been described via reduction of 1,3,5-triamino-2,4,6-trinitrobenzene (1) (Rogers 1986). As this compound is explosive, a new route had been devised, which takes the advantage of the reducability of nitrobenzothiadiazoles, e.g. (6) and (7), to the corresponding polyaminobenzenes (Mataka 1989). In a method similar to this route, it was also possible to prepare 1,4,5,8-tetraazaphenanthrene. In the following text, the synthesis and the physical properties of some substituted 1,4,5,8-tetraazaphenanthrenes (9) are described, with attention to the fact that substituents are needed to tailor the electronic properties and to facilitate the formation of amorphous films with this material at a later stage.

Experimental Procedure

A mixture of 1,2,3,4-tetraaminobenzene bishydrochloride (8) (211 mg, 1.0 mmol) and 1,2-bis-(4-chlorophenyl)-1,2-diketone (560 mg, 2.0 mmol) in ethanol (10 mL) and acetic acid (3 mL) was heated under reflux for 15 hours under an argon atmosphere. Thereafter the cooled reaction mixture was poured into water (xx mL) and the formed precipitate was filtered off. The precipitate was dried and purified by column chromatography (silica gel, eluant CHCl₃) and recrystallised from ethanol to give 2,3,6,7-tetrakis-(4-chlorophenyl)-1,4,5,8-tetraazaphenanthrene (9e) (150 mg, 24%) as yellow needles; mp > 300°C; IR (KBr/cm^-1) u 1594, 1492, 1365, 1250, 1097, 1014; ¹H NMR (270 MHz, CDCl₃) d 7.36 - 7.41 (m, 8H), 7.58 - 7.67 (m, 8H), 8.36 (s, 2H).

Synthesis of 2,3,6,7-tetraaryl substituted 1,4,5,8-tetraazaphenanthrenes (9)

The key intermediate in the synthesis of the substituted tetraazaphenanthrenes is the 1,2,3,4,-tetraaminobenzene bishydrochloride (8). Starting from o-phenylenediamine (4), which is reacted with thionyl chloride in the presence of pyridine to give 2,1,3-benzothiadiazole (5), 5-amino-4-nitro-2,1,3-benzothiadiazole (7) can be prepared in four steps as shown in Scheme 2. (7) can be reduced with ease to (8) with tin-hydrochloric acid in dioxane.

Physical properties

UV data

All the tetraazaphenanthrenes synthesized are colored materials. For the most part, they are yellow. The UV data of the TAP derivatives are listed in Table 1. Substituting tetraphenylazaphenanthrenes with electron withdrawing moities such as with fluoro, or chloro groups induces a hypochromic shift in the wavelength of the absorption maximum, when compared with molecules having one or more electron donors on the phenyl groups (e.g., methoxy-(9c), or trimethoxy-(9d)). Nevertheless, all substituted tetraphenylazaphenanthrenes experience a bathochromic shift in relation to the unsubstituted tetraphenylazaphenanthrene. This is in reasonable agreement with the expected data.