Blue electroluminescence of silyl substituted anthracene derivatives
Introduction
Recently, efficient light emitting materials applicable for organic electroluminescent (EL) materials have been actively investigated. Especially, electrophosphorescent iridium and platinum complexes have attracted much attention as green and red emitting materials because of their marvelous device performance [1], [2], [3], [4], [5], [6]. However, for a blue device, the materials for such a device are still under development, and low molecular weight materials are still useful for blue-emission.
In this study, the photophysical characteristics of the silylanthracenes were investigated to determine their suitability as candidates for such materials. Anthracene (Ant) was the first material producing an EL. However, it has problems with its photophysical and photochemical properties. Therefore, its performance is inferior for practical applications [7], [8], [9], [10], [11]. However, Ant and its derivatives have still been attracting attention from the viewpoint of application and basic science. For the recent rapid developments in organic electronic devices, the importance of molecular stacking control and adding a new functionality, such as charge transporting abilities, etc., are increasing [12], [13], [14], [15], [16], [17], [18].
The lowest singlet excited state, Ant undergoes about a 30% fluorescence and 70% intersystem crossing to the triplet state [19]. The excited state Ant has the potential to react with another Ant molecule through the excimer to produce the [4 + 4] cyclization adduct [20], react with oxygen to give the endoperoxide and then produce anthraquinone and other products [21]. Those products are usually non-emissive.
Our strategies to design materials for an OLED are very simple, i.e., the introduction of bulky substituent on the 9,10-position of Ant. This strategy produces a significant increase in the fluorescence quantum yield (Φf) as well as preventing intermolecular stacking that decreases the device lifetime and color purity. It has been reported that the introduction of silyl groups at the 9,10-positions of Ant results in a significant increase of Φf [22], [23], [24], [25], [26], which is rationally explained by the alteration of the relative position of the energy levels of S1 and T2 governing the efficiency of the intersystem crossing (Φisc) and Φf [27]. In addition to the energy level positioning, these silyl groups are useful for introducing a bulky substituent to control the molecular stacking since the typical Si–C bond (1.90 Å) is longer than the C–C bond (1.54 Å) [28]. There have been many reports using twisted structures to design hole transporting aryl amines, and there have been some similar attempts including the introduction of bulky substituents as the emitting dopant [17], [18]. However, the introduction of bulky substituents through the C–C bond results in bending of the Ant plane due to steric hindrance, which in turn induces intramolecular cyclization between the 9- and 10-positions of the Ant that produces a non-emissive Dewer-benzene-type product [29]. A spherical shape prevents stacking of the molecules, intermolecular exciplex/electroplex formation, and therefore, photochemical cycloaddition. The formation and emission of the electroplex formed between the dopant molecule and host molecule or polymers, and their formation affect the color purity and lifetime of the device [30], [31], [32], [33], [34], [35], [36], [37], [38], [39]. The molecular design proposed here would be one of the methods to control the formation of the electroplex. Anthony and us have already reported that an OLED device consisting of ethynylanthracene derivatives emit a green EL [40], [41]. X-ray crystallography showed perpendicular alignment of the silylethynylanthracene molecules [41a] which decreased the stacking. Silylethynyl groups produce a higher radiative rate constant, but a decreased EL efficiency by self-absorption due to a small Stokes shift. In this paper, their photophysical characteristics are compared to those of the silylanthracenes. In addition, after changing the bulkiness of the substituents by four silylanthracenes, the performance of the OLEDs were compared.
The compounds examined in this study were anthracenes having silyl or silylethynyl groups, trimethylsilyl-(Me3), dimethylphenylsilyl-(Me2Ph), methyldiphenylsilyl-(MePh2), triphenylsilyl-(Ph3), trimethylsilylethynyl-(Me3E), and triphenylsilylethynyl-(Ph3E) on their 9,10-positions (Scheme 1). Compounds containing silane atoms have been widely investigated, from the interest in their charge transportability and high triplet energies, those are important for the host of the electrophosphorescent device [42], [43], [44], [45].
Section snippets
Photophysical characteristics in solution
Fig. 1 shows the absorption and emission spectra of Me3, Me2Ph, MePh2, Ph3, Me3E and Ph3E in cyclohexane. In the absorption spectra of the Me3–Ph3 series, the molar extinction coefficients (ε) of 1La band around 400 nm were not affected by the substituents. The ε of the 1Bb band around 260 nm decreased and the 1Cb band around 230 nm increased upon increasing the bulkiness of the substituent. The absorption maximum of the 1La band of Ph3E shifts by 6 nm in comparison with that of Me3E, and much
Materials
In a typical run, 2.988 g (8.893 mmol) of 9,10-dibromoanthracene to which 8.0 ml (53.0 mmol) of tetramethylenediamine (TMEDA) was added in 72 ml of dry THF and 24 ml of dry ether, was lithiated with 17.9 mmol of n-BuLi at −50 to −60 °C. A 2.25 ml (17.9 mmol) aliquot of chlorosilanes was then added to give 9,10-bissilylanthracenes using a reported method [23]. The reaction mixture was poured into benzene and washed with water, and the organic layer was isolated and dried over sodium sulfate. After
Conclusions
In conclusion, the 9,10-bissilylanthracenes have a significant potential as fluorescent materials, and a pure-blue electroluminescence was obtained for the device fabricated using Ph3 as a dopant. The substitution by silyl groups produced a drastic increase in the fluorescence efficiency. The large Stokes shift of the fluorescence λmax by the substitution of the silyl groups are a result of the combination of destabilization of the ground state energy levels by steric hindrance of the bulky
Acknowledgements
The authors thank Prof. Shogo Shimadzu for the TGA measurements. This work was supported by a Grant-in-Aid for Scientific Research (No. 17350090) and a Scientific Research on Priority Areas (417, No. 17029013) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of the Japanese Government, and Chiba University President’s Grant to T.K.
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Present address: Faculty of Science, Toho University, Miyama 2-2-1, Funabashi, Chiba, 274-8510 Japan.