Synthesis and Properties of 9,9-Diarylfluorene-Based Triaryldiamines

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Synthesis and Properties of

9,9-Diarylfluorene-Based Triaryldiamines

**Ken-Tsung Wong,* Zi-Jien Wang, Yuh-Yih Chien, and Chien-Lung Wang**

Department of Chemistry, National Taiwan UniVersity, Taipei 106, Taiwan [email protected]

Received May 1, 2001

ABSTRACT

9,9-Diaryl-2,7-dibromofluorene was synthesized by a triflic acid promoted Friedel−Crafts reaction. Introduction of diarylamino groups at its C2 and C7 positions by a Pd-catalyzed amination results in the formation of a novel class of triaryldiamines. The 9,9-diaryl substituents at the central linkage play a less important role in the photophyscial properties but affect the oxidation potential and improve the morphological stability of these new triarylamines.

Amorphous triarylamines with high glassy state stability are suitable as hole-transporting materials in organic light-emitting devices (OLED). Many endeavors have been made to develop new amorphous triarylamines with high morpho-logical stability. For example, dendritic molecules with a triarylamine core have shown to form thin films of good quality with high glass transition temperatures (Tg)1. Intro-duction of a spiro linkage has also generated triarylamines with higher Tg.2 Another approach involves replacing the phenyl group of triarylamines by a bulkier aryl group. Asymmetric substituted triarylamines normally yield thin films of high thermal stability.3_{The primary structural feature} for an amorphous triarylamine to exhibit high morphological stability is a stable non-coplanar conformation, which can inhibit the crystallization efficiently. However, a chro-mophore without a rigid planar skeleton usually emits fluorescence with a low quantum yield.4_{To have an efficient} OLED device, it is important to improve the quantum yield of photoluminescence and also to retain the morphological stability of a triarylamine that is used as a hole transporter as well as a light emitter.

By introducing two bulky groups at the C9 position of polyfluorene, Mu¨llen and Leising not only blocked the

interchain interaction but also improved the thermal stability of polyfluorene.5_{Inspired by their results, we report here a} new synthetic route for the preparation of 9,9-diarylfluorene-based triaryldiamines. The rigidity of the fluorene core could be beneficial for improving the quantum efficiency. Sys-tematic tuning of the structure of the central linkage by introducing different aryl substituents at the C9 position of fluorene provides new possibilities for probing the properties of resulting triaryldiamines. According to Mu¨llen’s addition-cyclization strategy, only symmetric aryl groups can be introduced at C9 of fluorene as substituents. We developed another synthetic strategy for the synthesis of 2,7-dibromo-9,9-diarylfluorene, in which the two aryl groups can be different. The synthetic route to 9,9-diarylfluorene-based triaryldiamines is shown in Scheme 1.

Three different aryl Grignard reagents, i.e., C6H5MgBr, p-TolMgBr, and 1-NpMgBr (where Np is naphthyl) were selected for the reaction with 2,7-dibromofluorenone 16 _in diethyl ether at reflux temperature. The corresponding alcohol derivatives 2a,7_{2b, and 2c were isloated in 90%, 82%, and} 97% yields, respectively. The Friedel-Crafts reaction8 _of 2a with benzene was carried out in the presence of an excess

(1) Shirota, Y. J. Mater. Chem. 2000, 10, 1.

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(4) Nizhegorodov, N. I.; Downey, W. S. J. Phys. Chem. 1994, 98, 5639.

(5) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mu¨llen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123, 946.

(6) Hauser, A.; Thurner, J.-U.; Hinzmann, B. J. Prakt. Chem. 1988, 330, 367.

(7) Weber, E.; Dorpinghaus, N.; Csoregh I. J. Chem. Soc.; Perkin Trans.

2 1990. 2167.

(8) Ohta, T.; Shudo, K.; Okamoto, T. Tetrahedron Lett. 1983, 24, 71.

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amount of CF3SO3H for 6 h at 80°C to give

2,7-dibromo-9,9-diphenylfluorene (3a) with a yield of 88% (entry 1, Table 1). Under the same condition, the reaction of 2c with benzene

failed and led to the decomposition of the starting material. Only a low yield of 3b (less than 10%) was obtained when the same reaction mixture was stirred at room temperature overnight. However, 3b could be isolated in 56% yield after a reaction time of 30 min at 50°C (entry 4, Table 1). As a result of its electron richness, toluene reacted very efficiently with 2a, 2b, and 2c in a period of 10 min at 50°C. This led to the 9,9-diarylfluorene derivatives 3c, 3d, and 3e in good

yields (entries 2, 3, and 5, Table 1).9 _{The reactions of the}

9,9-diaryl-2,7-dibromofluorene derivatives 3a-3e with dif-ferent diarylamines were carried out in the presence of a catalytic amount of Pd(OAc)2and PtBu3in toluene at 100 °C with NaOt_{Bu as the base.}9,10 _{After column}

chromatog-raphy purification, the isolated yields for the products

4a-4g were good to excellent (Table 2). This synthetic strategy

allows us to tailor the desired properties of the products by the use of different combinations of aryl substituents at the C9 position and the diarylamino substituents at the C2 and the C7 positions of the central fluorene linkage.

The electronic absorption maxima (λmax), the emission

maxima (λem), and the quantum yields for the

9,9-diarylfluo-rene-based triaryldiamine derivatives 4 are summarized in Table 3. Triaryldiamines 4 with N-R-naphthyl-N-pheny-lamine as diarylamino substituents (4a-4e) have demon-Scheme 1

a_Et

2O, Ar1MgBr (2 equiv), reflux, 2-6 h.bCF3SO3H (2 equiv), Ar2_{H, 50}°_{C, 10-30 min; see text.}c_Pd(OAc)

2(5 mmol %), PtBu3 (20 mmol %), NaOt_{Bu (2 equiv), toluene, 100}°_{C, 4-8 h.}

Table 1. Isolated Yields for the CF3SO3H-Promoted Friedel-Crafts Reaction of the Alcohol Derivatives 2 with Benzene or Toluene

Table 2. Isolated Yields for the Pd-Catalyzed Reactions of 9,9-Diaryl-2,7-dibromofluorene 3 with Diarylamines

a_{R-Np is 1-naphthyl.}

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strated a similar photophysical behavior, irrespective of the different aryl substituents at the C9 position of the fluorene linkage. The second absorption bands of 4a-4g are signifi-cantly red-shifted with respect to that of R-NPD (N,N′

-di-R-naphthyl-N,N′-diphenyl-4,4′-diphenyl). This indicates that

the rigid and planar fluorenyl core plays an important role in reducing the HOMO-LUMO energy gap.

However, the triaryldiamine derivatives 4a-4e with N-R-naphthyl-N-phenylamine substituents, as well as R-NPD, exhibited similar emission maxima and quantum yields. The introduction of different diarylamino substituents into the 9,9-diaryl-substituted fluorenyl core (e.g., 4f, 4g) results in a substantial blue shift of the first absorption band and the

λemwith respect to that of 4a and R-NPD. A comparison of

the UV-vis spectra and the photoluminescent spectra of 4a,

4f, and R-NPD is shown in Figure 1. The photopysical

properties of triaryldiamines 4 are dominated by the stuctural feature of diamino substituents at C2 and C7 of the central fluorene linkage.

Cyclic voltammetry was performed in CH2Cl2 (with 0.1

M tetra-n-butylammonium hexafluorophosphate,TBAPF6, as

supporting electolyte). Similarly to the conventional triaryl-diamines, the 9,9-diarylfluorene-based triarydiamines 4 have two quasireversible anodic redox couples, corresponding to the removal of an electron from each triarylamine group. The results are summarized in Table 3, and a comparison of cyclic voltammogrammes of 4c, 4e, 4f, and R-NPD is shown in Figure 2. As a result of the rigidity and planarity of the

central fluorene linkage, the onset of the first oxidation step of triarydiamines 4 is evidently lower compared to that of Table 3. Physical Properties of 9,9-Diarylfluorene Bearing Triaryldiamines 4

compound λmax (nm, log )a λem (nm)a ΦF (%)b E1_{pa, E}1 pc [mV, (onset)] E2_{pa, E}2 pc (mV) ∆E (mV)c Tg (°C)d 4a 352, 4.55 457 15 740, 640 (625) 1080, 1000 340 129 379, 4.55 4b 378, 4.55 461 15 720, 620 (600) 1060, 980 340 125 356, 4.53 4c 353, 4.46 455 16 780, 700 (670) 1140, 1060 360 127 379, 4.46 4d 274, 4.40 450 18 740, 660 (600) 1100, 1000 350 109 381, 4.54 4e 350, 4.53 457 15 720, 640 (600) 1080, 980 360 134 379, 4.56 4f 308, 4.17 401, 420 58 700, 620 (600) 1040, 940 340 97 380, 4.38 4g 337, 4.55 410, 427 69 780, 700 (675) 1100, 1000 330 125 387, 4.62 R-NPD 335 449 16 840, 740 (730) 1100, 1020 280 100e

a_{In EtOAc, photoluminesence maximum was recorded upon irradiation at the absorption maximum.}b_{Quantum yield of PL, compared to coumarin I in}

EtOAc.c_{∆E ) E}2

pa- E1pa.dDetermined by differential scanning calorimetry (DSC) analysis of the liquid nitrogen quenched samples.eFrom ref 3.

Figure 1. A comparison of normalized absorption and

photolu-minescence specta of 4a, 4f, and R-NPD.

Figure 2. A comparison of normalized cyclic voltammograms of 4c, 4e, 4f, and R-NPD (0.1 M TBAPF6in CH2Cl2)

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R-NPD. These observations could be attributed to the better

π-conjugation along the molecular axis in 4 compared to

that in R-NPD. Significant potential differences between the first oxidation step and the second oxidation step also reveal that the radical cation could efficiently delocalize in the molecule once it is formed. For the case that the central linkage carries electron-rich aryl substituents (e.g., 4c), the first oxidation step is obviously more difficult. By introducing the electron-rich diarylamino substituents (e.g., 4f) both oxidation potentials were reduced. However, for 4e with the same diarylamino substituents as R-NPD, the second oxida-tion potential exhibited no significant changes.

The morphological stability of 4 was investigated by differential scanning calorimetry (DSC) analysis. The tri-aryldiamines derivatives 4 exhibited good glass forming properties with relative high glass transition temperatures (Table 3). The triaryldiamines 4 with the same diarylamino substituents as R-NPD exhibited a higher Tg compared to

that of R-NPD. The increase of the morphological stability could be explained by an increase of the molecular weight of the triaryldiamines 4, due to the introduction of the aryl substituents at the central linkage. However, the two aryl substituents at the C9 position of fluorene may hinder the further crystallization process, which could also be ascribed to the increase in Tgof triaryldiamines 4.

In summary, we have successfully established a new method for the synthesis of sterically hindered 9,9-diarylfluo-rene-based triaryldiamines. This new class of compounds has revealed that the planarity of the fluorenyl core has a significant effect on reducing the HOMO-LUMO energy gap, which is observed from the red shifts in the absorption spectra and the lower anodic oxidation onset. The diary-lamino substituents at C2 and C7 of 9,9-diarylfluorene, introduced by Pd-catalyzed amination, play a predominant role in the photophysical properties. The introduction of diaryl substituents at the C9 position of fluorene significantly enhances the morphological stability of these fluorene-based triaryldiamines. Further modifications of the central fluorene linkage by introducing various aryl substituents with a higher molecular weight and the application of these triaryldiamines in OLED are currently under investigation.

Acknowledgment. We thank Mr. Chung-Shen Kao for

the DSC analysis and Dr. Jo¨rn Wirsching for his help on the preparation of the manuscript. This work was supported by the National Science Council of Taiwan (NSC-89-2113-M002-053).

Supporting Information Available: Experimental

pro-cedures and characterization data of all new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

OL016051Y

(9) Please see Supporting Information for the experimental procedures. (10) Yamamoto, T.; Nishiyama, M.; Koie, Y. Tetrahedron Lett. 1998,

39, 2367.

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