Effects of Conjugated Substituents on the Optical, Electrochemical

Joji Ohshita , Kazuya Murakami , Daiki Tanaka , Yousuke Ooyama , Tomonobu Mizumo ..... Joji Ohshita , Daisuke Hamamoto , Keisuke Kimura , Atsutaka Kun...
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Organometallics 2001, 20, 4800-4805

Effects of Conjugated Substituents on the Optical, Electrochemical, and Electron-Transporting Properties of Dithienosiloles Joji Ohshita,*,†,‡ Hiroyuki Kai,† Atsuhiro Takata,† Toshiyuki Iida,† Atsutaka Kunai,*,† Nobuaki Ohta,† Kenji Komaguchi,† Masaru Shiotani,† Akira Adachi,§ Koichi Sakamaki,§ and Koichi Okita§ Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan, and Japan Chemical Innovation Institute, Tsukuba Research Center, 2-1-6 Sengen, Tsukuba 305-0047, Japan Received April 19, 2001

Dithienosiloles (DTSs) bearing conjugated aryl substituents on the thiophene rings (1 and 2) were prepared and their optical, electrochemical, and electron-transporting properties were investigated in comparison with those of simple DTSs having no conjugated substituents on the thiophene rings (4-6). UV absorption bands of 1 and 2 are red shifted from those of 4-6 by 40-80 nm, reflecting the expanded π-conjugation, whereas the first oxidation peaks in the cyclic voltammograms of 1 and 2 appear at potentials a little lower or almost the same energies relative to those of 4-6, depending on the nature of the substituents. The electron-transporting properties of 1 and 2 were evaluated by the performance of electroluminescent (EL) devices having vapor-deposited DTS, Alq, and TPD layers, as the electrontransport, emitter, and hole-transport, respectively. The results indicated that introduction of aryl substituents to DTSs led to inferior performance of the devices in most cases, while the device with 1c bearing trimethylsilylpyridyl substituents exhibited high efficiency of current-luminance energy conversion and emitted a green light with a maximum luminance of 16 000 cd/m2. A trap-controlled electron transporting model is proposed to explain their performance. Introduction There has been current interest in the chemistry of silole (silacyclopentadiene) ring systems.1 The orbital interaction between the σ*-orbital of the silole silicon atom and the π*-orbital of the butadiene fragment, namely, σ*-π*-conjugation, lowers the LUMO energy level,2 to provide opportunities to use silole-containing compounds as functionality materials.1 Tamao et al. have demonstrated that siloles having pyrrol-2-yl substituents at the 2,5-positions exhibit highly electrontransporting properties, as being applicable to the efficient electron-transport in electroluminescent (EL) devices.3 It has been also reported that silole-containing conjugated polymers have small band gap energies.4 Recently, we have synthesized dithienosiloles (DTSs) in which a bithiophene system is bridged intramolecularly by a silylene unit at the β,β′-positions, forming a †

Hiroshima University. Present address: Institute for Fundamental Research of Organic Chemistry, Kyushu University, Fukuoka 812-8581, Japan. § Japan Chemical Innovation Institute. (1) For a review, see: (a) Yamaguchi, S.; Tamao, K. J. Chem. Soc., Dalton Trans. 1998, 3693. For recent works see: (b) Yamaguchi, S. Jin, R.-Z.; Tamao, K. J. Am. Chem. Soc. 1999, 121, 2937. (c) Yamaguchi, S. Jin, R.-Z.; Itami, Y.; Tamao, K. J. Am. Chem. Soc. 1999, 121, 10420. (d) Sanji, T.; Funaya, M.; Sakurai, H. Chem. Lett. 1999, 547. (e) Sohn, H.; Huddleston, R. R.; Powell, D. R.; West, R.; Oka, K.; Yonghua, X. J. Am. Chem. Soc. 1999, 121, 2935. (f) Kanno, K.; Ichinohe, M.; Kabuto, C.; Kira, M. Chem. Lett. 1998, 99. (2) (a) Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69, 2327. (b) Yamaguchi, Y. Synth. Met. 1996, 82, 149. (c) Hong, S. Y.; Song, J. M. Chem. Mater. 1997, 9, 297. ‡

silole ring. As expected, the silylene bridge enhances the conjugation in the system, not only by the σ*-π*conjugation in the silole system, but also by forcing the bithiophene system to retain high planarity.5,6 Interestingly, vapor-deposited films of DTSs show highly electrontransporting properties. In fact, devices with the structure of ITO (indium tin oxide)/TPD (40 nm)/Alq (50 nm)/ DTS (10-20 nm)/Mg-Ag, where TPD (N,N′-diphenylN,N′-di(m-tolyl)biphenyl-4,4′-diamine), Alq (tris(8-quinolinolato)aluminum (III)), and DTS are the hole-transport, emitter, and electron-transport, respectively, emit strong electroluminescence (EL). Of these, the maximum luminance of 8000 cd/m2 was obtained with the device having a 4,4-di(p-tolyl)-2,6-bis(trimethylsilyl)(3) (a) Tamao, K.; Ohno, S.; Yamaguchi, S. Chem. Commun. 1996, 1873. (b) Tamao, K.; Uchida, M.; Izumikawa, T.; Furukawa, K.; Yamaguchi, S. J. Am. Chem. Soc. 1996, 116, 11974. (4) (a) Tamao, K.; Yamaguchi, S.; Shiozaki, M.; Nakagawa, Y.: Ito, Y. J. Am. Chem. Soc. 1992, 114, 5867. (b) Tamao, K.; Yamaguchi, S.; Ito, Y.; Matsuzaki, Y.; Yamabe, T.; Fukushima, M.; Mori, S. Macromolecules 1995, 28, 8668. (c) Yamaguchi, S.; Iimura, K.; Tamao, K. Chem. Lett. 1998, 89. (d) Yamaguchi, S.; Goto, T.; Tamao, K. Angew. Chem., Int. Ed. 2000, 39, 1695. (e) Ohshita, J.; Mimura, H. Arase, N.; Nodono, M.; Kunai, A.; Komaguchi, K.; Shiotani, M.; Ishikawa, M. Macromolecules 1998, 31, 7985. (f) Ohshita, J.; Hamaguchi, T.; Toyoda, E.; Kunai, A.; Komaguchi, K.; Shiotani, M.; Ishikawa, M. Naka, A. Organometallics 1999, 18, 1717. (5) Ohshita, J.; Nodono, M.; Watanabe, T.; Ueno, Y.; Kunai, A.; Harima, Y.; Yamashita, K.; Ishikawa, M. J. Organomet. Chem. 1998, 553, 487. (6) Ohshita, J.; Nodono, M.; Kai, H.; Watanabe, T.; Kunai, A.; Komaguchi, K.; Shiotani, Adachi, A.; Okita, K.; Harima, Y.; Yamashita, K.; Ishikawa, M. Organometallics 1999, 18, 1453.

10.1021/om0103254 CCC: $20.00 © 2001 American Chemical Society Publication on Web 10/13/2001

Properties of Dithienosiloles Scheme 1

Organometallics, Vol. 20, No. 23, 2001 4801 Table 1. Properties of Diaryldithienosiloles and Diarylbithiophenes compd

dithienosilole layer.6,7 On the basis of the performance of EL devices, we concluded that the electron-transporting properties of the DTSs are comparable to Alq, which is known as a typical electron-transporting material. We have also synthesized polymers having DTS units in the backbone and found that they can be used as both holeand electron-transporting materials.8 To obtain DTS-based materials with higher electrontransporting properties, we synthesized DTSs bearing conjugated aryl substituents, such as thienyl, phenyl, and pyridyl groups, at the R-positions of the thiophene rings (Ar2DTS). In these compounds, extended π-conjugation is anticipated to lead to an even lower LUMO level. In addition, extended π-conjugation would enhance the intermolecular packing by π-π stacking in the solid states, which may facilitate the intermolecular electron-hopping and stabilize the DTS layer thermally.

UV λmax/nma

emission λmax/nma

CV oxidation peak potential/Vb

mp/°C

1a 1b 1c 2a 2b 4c 5c 6c

435 417 418 426 409 356 356 358

Dithienosilole 532, 505 500, 479 498, 480 518 495 425 420 420

0.90 0.95 1.16 0.80 0.99 1.23 1.16 1.25

213-215 214-215 295 217-219 240-241 158-160 140-143 133-135

7a 7b 7c

405 386 392

Bithiophene 491, 461 1.02 465 1.00 462, 434 1.08

184-187 236-237 184-187

a

In THF. b Versus SCE. c Ref 6.

Chart 1

Scheme 2

Results and Discussion Synthesis and Optical and Electrochemical Properties of Diaryldithienosiloles. Diaryldithienosiloles (Ar2DTSs) 1 and 2 were synthesized as shown in Scheme 1. Thus, the nickel-catalyzed coupling reactions9 of 2,6-dibromo-4,4-diphenyldithienosilole (3) with the corresponding Grignard reagents gave Ar2DTSs, 1a,b, and 2a,b. Bis[2-(6-trimethylsilyl)pyridyl]dithienosilole (1c) was obtained by the Pd-catalyzed dehalostannylation10 of 3 with 2-tributylstannyl-6-(trimethylsilyl)pyridine. Table 1 summarizes UV absorption and emission maxima, oxidation peak potentials in the cyclic voltammograms (CVs), and melting points of 1 and 2, in comparison with those of simple DTSs 4-6 reported previously (Chart 1) and similarly substituted bithiophenes 7a-c, prepared as shown in Scheme 2. The absorption and emission maxima of Ar2DTSs 1 and 2 were markedly red-shifted from those of 4-6 and 7a-c, indicating that introduction of both conjugated substituents on the DTS system and the intramolecular silylene bridge into the diarylbithiophene unit lead to smaller band gap energies. (7) (a) Adachi, A.; Ohshita, J.; Kunai, A.; A. Okita, K.; Kido, J. Chem. Lett. 1998, 1233. (b) Adachi, A.; Ohshita, J.; Kunai, A.; Okita, K. Jpn. J. Appl. Phys. 1999, 38, 2148. (8) Ohshita, J.; Nodono, M.; Takata, A.; Kai, H.; Adachi, A.; Sakamaki, K.; Okita, K.; Kunai, A. Macromol. Chem. Phys. 2000, 201, 851. (9) Tamao, K.; Kodama, S.; Nishimura, I.; Kumada, M.; Minato, A. Tetrahedron 1982, 38, 3347. (10) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.

The CVs of Ar2DTSs measured in acetonitrile showed irreversible oxidation peaks at 0.80-1.16 V vs SCE, as listed in Table 1. No cathodic counterparts corresponding to the anodic peaks were observed even by rapid scanning (200 mV/s), similar to 4-6.6 Although we could not determine actual redox potentials because of the irreversibility of the anodic process, structural similarity for these compounds would permit us to estimate the relative order of HOMO levels of these compounds by comparing the oxidation peak potentials, as described in the preceding paper.6 Thus, the peak potential of 1a that bears (trimethylsilyl)thienyl groups at the 2,6-positions of DTS ring shifted negatively by -330 mV, relative to DTS 4 with simple trimethylsilyl groups. Introduction of thienyl groups to the 2,6-positions of 5 to give 2a also resulted in a -360 mV shift. These results indicate that extension of π-conjugation in the DTS system with electronrich thienyl groups raises the HOMO energy levels of 1a and 2a mainly, causing red-shifted UV maxima. In contrast, only a small shift (-70 mV) was observed for 1c from 4, indicating that introduction of electrondeficient pyridyl groups to DTS does not significantly affect the HOMO level, and hence the red-shifted UV

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Figure 1. Plots of operating voltage vs current density for type I device with (b) compound 1c, (2) 1a, (0) 7c, (4) 2a, and (×) 1b, and (O) type II device.

Figure 2. Plots of operating voltage vs luminescence for type I device with (b) compound 1c, (2) 1a, (0) 7c, (4) 2a, and (×) 1b, and (O) type II device.

absorption may be primarily due to the lowering of the LUMO level by the pyridyl substituents. Moderate potential shifts for 1b (-280 mV) and 2b (-170 mV) relative to 4 and 5, respectively, suggest that the redshifted UV maxima of 1b and 2b are due to both lowering LUMO and elevating HOMO levels by the phenyl substituents. Similar substitution effects were observed for bithiophene derivatives 7a-c; that is, the absorption maxima moved to longer wavelength in the order 7b < 7c < 7a, and the CV of 7c showed the anodic peak at higher voltage as compared to 7b and 7a. Performance of EL Devices with an Ar2DTS Layer as the Electron-Transport. To evaluate electron-transporting properties of the present Ar2DTSs, we fabricated multilayer EL devices having a Ar2DTS layer as the electron-transport, by vacuum vapor deposition (type I devices). The structure of the type I devices and the thickness of the layers are ITO/TPD (40 nm)/Alq (50 nm)/Ar2DTS (10-20 nm)/Mg-Ag, in which TPD, Ar2DTS, and Alq layers are used as the hole- and electrontransport and emitter, and ITO and Mg-Ag are the anode and cathode, respectively. We also examined a device of ITO/TPD (40 nm)/Alq (50 nm)/Mg-Ag (type II device) having an Alq layer as the electron-transporting emitter, for comparison. Figure 1 depicts the current density-voltage (I-V) characteristics of the type I devices with compounds 1a-c, 2a, and 7c, together with those of the type II device. The carrier-transporting properties, estimated on the basis of the comparison of I-V characteristics, were improved in the order 1b < 2a < 7c < 1a < 1c, and the device with 1c gave the best results among the present type I devices. This is in accordance with the results of optical and electrochemical analyses described above, which indicate that compound 1c would have a low-lying LUMO. The superior I-V characteristics of the device with 1c relative to that with 7c clearly indicate that the existence of the silole ring is essential for the highly carriertransporting properties of the film of 1c. The I-V characteristics of the type I device with 1c were quite similar to those of the type II device in the range of applied voltage of 5-13 V. This indicates that the vapor-

deposited film of compound 1c has carrier-transporting properties comparable with the Alq film, which is known as a typical and efficient electron-transporting material. As shown in Figure 1, the current of the device with 1c increased along the applied voltage and reached the maximum value at 14 V. Applying a voltage higher than 14 V on the device, however, resulted in a rapid decrease of the current density, probably due to the degradation of the Ar2DTS layer in the device. Similar behavior has been reported for the devices with the same structure bearing a vapor-deposited layer of simpler DTS (4-6) as the electron-transport.6 Figure 2 depicts the luminance-voltage (L-V) characteristics of the type I and type II devices. From both types of the present devices, green EL was observed and the spectra were identical with the photoluminescence (PL) spectrum of a vacuum-deposited film of Alq, implying that the EL originated from the emission of the Alq layer. As shown in Figure 2, among the type I devices, the highest luminance was attained by the device with 1c. The luminance of the device with 1c increased along the applied voltage and reached the maximum value of 16 000 cd/m2 at 13 V, which is much higher than those of the other type I devices and the type II device used as the reference. The luminance then decreased at voltages beyond 13.5 V, in accordance with the I-V characteristics. The maximum luminance is higher also than those from the devices with simpler DTS. Namely, the maximum luminance from the device with the layer of 47a or 66 was previously reported to be 8000 cd/m2 (at 13 V). Figure 3 shows luminancecurrent density plots for the type I devices with 1c or 6 as the electron transport, together with those of the type II device, indicating the much higher efficiency of current-luminance energy conversion of the device with 1c than that with 6. Thermal stability of the Ar2DTS layer at around 13 V as indicated by the higher melting point of 1c (295 °C) than those of 4 and 6 (130-160 °C) would also be the reason for the higher maximum luminance of the device with 1c. Trap-Controlled Model. Since DTSs 47a and 66 exhibit electron-transporting properties comparable

Properties of Dithienosiloles

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Figure 3. Plots of current density vs luminance for type I devices with (b) compound 1c and (4) 6, and (O) type II device.

with Alq as reported previously, introduction of π-conjugated substituents to the DTS ring in 1 and 2 seems to rather suppress the carrier-transport in the DTS layers. Even for the device with 1c, which shows the best EL performance among Ar2DTS examined, the I-V characteristics are a little inferior to those of DTSs 4 and 6. This disagrees with the results of optical and electrochemical analysis of DTSs and Ar2DTSs, which suggest that Ar2DTSs would have the LUMOs at lower energies as compared with DTSs. There might be, therefore, operating some other factors such as energy barrier height for electron-injection on the interface between the cathode and the Ar2DTS layer, as observed previously,7b or energy barrier for electron-transport in the Ar2DTS layer. To learn more about this disagreement, we examined 1a, as the example, with respect to the UV spectra in a 2-methyltetrahydrofuran (MTHF) and 3-methylpentane (MP) matrix at 77 K, as well as in a vapor-deposited film. Interestingly, the UV absorption band at 435 nm at room temperature split into three bands at 77 K, as can be seen in Figure 4A,B. Furthermore, in MP the intensities of absorptions at higher energies decreased. The splittings (ca. 27 nm) were too large to be ascribed to the vibration splittings, and compound 4 did not exhibit such temperature dependence in UV spectra in MTHF and MP. It is, therefore, most likely that the temperature dependence of the UV profile of 1a is due to the structural fluxionality arising from the rotation around the DTS-thienyl single bonds. Although the UV spectrum of the vapor-deposited film of 1a revealed a profile similar to that in THF at room temperature, the absorption peak is slightly broadened and two shoulders at higher and lower energies of the maximum appeared, as shown in Figure 4C. We also carried out ab initio molecular orbital calculations on conformers of 8a (Chart 2). In these calculations, one of the torsion angles of S-C(R)-C(R)-S was fixed to 180° and the other was rotated from 180° to 0° by 20°. Bond lengths and angles and other torsion angles in the conformers were optimized using the restricted-Hartree-Fock (RHF) method at the 6-31G

Figure 4. UV spectra of 1a (A) in MTHF, (B) in MP, and (C) in a vapor-deposited film at room temperature. Chart 2

level. As can be seen in Figure 5, the heat of formation increases gradually with decreasing the torsion angle from 180°, passing through the maximum energy at 80°, then decreases to the minimum at 40°. These results clearly suggest that compound 8a would have four conformers with the energy minimum shown in Chart 2. The (E,E)-conformer would have the most extended π-conjugation, whereas (E,Z)- and (Z,Z)-conformers seem to possess relatively restricted conjugation due to the twisted structures, and hence, the HOMOs rise and the LUMOs lower in the order of (Z,Z), (E,Z), and (E,E). In the solution phase at room temperature, rapid rotation around the aryl-DTS bonds would lead to smooth interconversion of the conformational isomers,

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simple DTSs without conjugated substituents, in most cases, while the device with Ar2DTS 1c (Ar ) trimethylsilylpyridyl) provided higher efficiency of currentluminance energy conversion as compared with the devices having 4 and 6, although the origin of the high efficiency is still unclear. The trap-controlled model is considered to explain the inferior electron-transporting properties of Ar2DTSs. Conformational fixation of the conjugated substituents may improve the electrontransporting properties of DTS-based materials. Experimental Section

Figure 5. Heat of formations of conformational isomers of compound 8a.

Figure 6. Schematic representation of electron transport via intermolecular hopping (A) in a film of 1 and 2, and (B) 4 and 6.

while the rotation may be frozen in the matrix at low temperature, as well as in the vapor-deposited films. The films, therefore, would consist of a mixture of conformational isomers with different electronic states, i.e., different LUMO levels. Once the electron, being transported in the film, is “trapped” by a molecule with low lying LUMO, the electron may not be readily transported to the neighboring molecules with higher LUMO energy levels, as represented in Figure 6A. On the other hand, electrons would be transported more smoothly in the film of 4 and 6, in which all of the molecules must have the LUMO at essentially the same energy level, although the LUMO lies at higher energy level than that of 1a (Figure 6B). Similar phenomena have been reported for PVK (poly(vinylcarbazole)) films doped with TPD.11 Contamination of TPD, which has lower lying HOMO than PVK, in a certain ratio leads to a drastic drop in the hole-transporting efficiency of the film. Conclusions We have demonstrated that introduction of conjugated aryl substituents on the DTS system leads to smaller band gaps. However, the present Ar2DTSs exhibited electron-transporting properties inferior to

General Procedures. All reactions were carried out under an atmosphere of dry nitrogen. Usual workup mentioned in the following involves the hydrolysis of the reaction mixture, separation of the organic layer, extraction of the aqueous layer, drying the combined organic layer and the extracts, and evaporation of the solvents, in this order. NMR spectra were recorded on JEOL Model JNM-EX 270 and JEOL Model JNM-LA 400 spectrometers. Mass spectra were measured with a Hitachi M80B spectrometer. UV spectra were measured on a Hitachi U-3210 spectrophotometer, and emission spectra were obtained by a Shimadzu RF5000 spectrophotometer. IR spectra were measured on a Perkin-Elmer FTIR Model 1600 spectrometer. Materials. THF was dried over sodium-potassium alloy12 and distilled just before use. Acetonitrile was distilled from P2O5 and stored in dark under an argon atmosphere at 4 °C before use. The starting compound 3 was prepared bromination of 4,4-diphenyl-2,6-bis(trimethylsilyl)dithienosilole13 as reported in the literature.6 Nickel-Catalyzed Grignard Coupling Reactions for the Preparation of Compounds 1a,b, 2a,b, and 7a,b. An illustrative procedure is as follows. A mixture of a Grignard reagent prepared from 0.470 g (2.00 mmol) of 2-bromo-5trimethylsilylthiophene and 0.048 g (2.00 mmol) of magnesium in 5 mL of THF, 0.453 g (0.90 mmol) of 3, and 10 mg (1.8 mol %) of NiCl2(dppe) was placed in a sealed glass tube (10 mm φ, wall thickness ) 2 mm) and the tube was heated at 150 °C for 80 h. After the usual workup, the resulting mixture was chromatographed on a silica gel column eluting 50:1 with hexane/benzene to give a crude dark yellow solid, which was recrystallized from hexane, to afford 0.334 g (57% yield) of 1a as yellow solid: mp 213-215 °C; 1H NMR (δ in CDCl3) 0.32 (s, 18H, SiMe), 7.13 (d, 2H, J ) 3.3 Hz, thienylene), 7.22 (d, 2H, J ) 3.3 Hz, thienylene), 7.31 (s, 2H, H on C3, C5), 7.377.44 (m, 6H, m- and p-Ph), 7.65 (dd, 4H, J ) 7.8, 1.4 Hz, o-Ph); 13C NMR (δ in CDCl ), -0.12 (SiMe ), 124.8, 126.4, 128.3, 3 3 130.5, 131.2, 134.8, 135.4, 139.1, 139.7 (Ph, thienylene, and C3, C5), 141.0 (C2, C6), 142.3 (C7a,b), 148.8 (C3a, C4a); 29Si NMR (δ in CDCl3) -6.5 (SiMe3), -20.1 (silole); MS m/z 654 (M+). Anal. Calcd for C34H34S4Si3: C, 62.33; H, 5.23. Found: C, 62.33; H, 5.23. Compounds 1b, 2a,b, and 7a,b were prepared as above using appropriate starting compounds as shown in Schemes 1 and 2. Data for 1b: 34% yield; yellow solid; mp 214-215 °C; 1H NMR (δ in CDCl3) 0.27 (s, 18H, SiMe), 7.34 (s, 2H, H (11) Pai, D. M.; Yanus, J. F.; Stolka, M. J. Phys. Chem. 1984, 88, 4714. (12) For the preparation of the alloy, sodium and potassium were place in a flask in an approximate weight ratio of Na/K ) 1:10 under a dry argon atmosphere, and the flask was heated above the melting point of sodium (98 °C). When both of the metals melted, the flask was slowly swirled to prepare the alloy. After the flask was cooled to room temperature, THF predried by distillation from sodium was added into the flask. (13) In ref 6, we described that the reaction of 3,3′-dilithio-5,5′-bis(trimethylsilyl)-2,2′-bithiophene with diphenyldichlorosilane in ether afforded 4,4-diphenyl-2,6-bis(trimethylsilyl)dithienosilole. However, it should be carried out in a mixed solvent of ether/THF ) 1:1-3:1 at the refluxing temperature.

Properties of Dithienosiloles on C3, C5), 7.37-7.65 (m, 18H, Ph); 13C NMR (δ in CDCl3) -1.2 (SiMe3), 124.9, 125.8, 128.3, 130.4, 131.5, 133.9, 134.7, 135.5, 139.7 (phenyl, phenylene, and C3, C5), 140.9 (C2, C6), 146.2 (C7a,b), 149.6 (C3a, C4a); 29Si NMR (δ in CDCl3) -4.0 (SiMe3), -20.0 (silole); MS m/z 642 (M+). Anal. Calcd for C38H38S2Si3: C, 70.97; H, 5.96. Found: C, 70.94; H, 6.12. Data for 2a: yellow solid; 20% yield; mp 217-219 °C; 1H NMR (δ in CDCl3) 7.00 (dd, 2H, J ) 5.0, 3.6 Hz, thienyl ring protons), 7.16-7.20 (m, 4H, thienyl ring protons), 7.29 (s, 2H, H on C3, C5), 7.37-7.47 (m, 6H, m- and p-Ph), 7.68 (dd, 4H, J ) 7.9, 1.4 Hz, o-Ph); 13C NMR (δ in CDCl3) 123.6, 124.2, 126.3, 127.9, 128.3, 130.5, 131.1, 135.4, 137.3 (thienyl, thienylene, and C3, C5), 139.1 (C2, C6), 141.0 (C7a,b), 148.7 (C3a, C4a); 29Si NMR (δ in CDCl3) -21.1; Ms m/z 510 (M+). Anal. Calcd for C28H18S4Si: C, 65.84; H, 3.55. Found: C, 65.83; H, 3.55. Data for 2b: yellow solid; 20% yield; mp 240-241 °C; 1H NMR (δ in CDCl3) 7.24 (s, 2H, thienyl ring protons), 7.34-7.43 (m, 12H, m-, p-phenyl and m-, p-Ph silole Si), 7.60 (dd, 4H, J ) 8.46, 0.21 Hz, o-Ph), 7.68 (dd, 4H, J ) 7.97, 1.54 Hz, o-Ph); 13C NMR (δ in CDCl3) 125.7, 127.4, 128.3, 128.9, 130.4, 131.5, 134.4, 135.5 (2C) (Ph, phenylene, and C3, C5), 140.9 (C2, C6), 146.1 (C7a,b), 149.6 (C3a, C4a); 29Si NMR (δ in CDCl3) -19.9; MS m/z 498 (M+). Anal. Calcd for C32H22S2Si: C, 77.06; H, 4.45. Found: C, 77.14; H, 4.44. Data for 7a: yellow solid; 36% yield; mp 184-187 °C; 1H NMR (δ in CDCl3) 0.33 (s, 18H, SiMe), 7.06 (dd, 4H, J ) 6.2, 0.6 Hz, ring protons), 7.13 (d, 2H, J ) 3.6 Hz, ring protons), 7.21-7.22 (m, 2H, ring protons); 13C NMR (δ in CDC3) -0.12 (SiMe3), 124.2, 124.4, 125.0, 134.8, 135.9, 136.3, 140.1, 142.0 (sp2-C); 29Si NMR (δ in CDCl3) -6.4; MS m/z 474 (M+). Anal. Calcd for C22H26S4Si2: C, 55.64; H, 5.52. Found: C, 55.63; H, 5.48. Data for 7b: yellow solid; 36% yield; mp 236-237 °C; 1H NMR (δ in CDCl3) 0.27 (s, 18H, SiMe), 7.16 (d, 2H, J ) 3.9 Hz, thienylene), 7.25 (d, 2H, J ) 3.9 Hz, thienylene), 7.51-7.58 (m, 8H, phenylene); 13C NMR (δ in CDCl3) -1.0 (SiMe3), 124.0, 124.7, 125.0, 134.1, 134.5, 136.9, 140.2, 143.3 (sp2-C); 29Si NMR (δ in CDCl3) -4.0; MS m/z 462 (M+). Anal. Calcd for C26H30S2Si2: C, 67.47; H, 6.53. Found: C, 67.35; H, 6.46. Dehalostannylation for the Preparation of 1c and 7c. An illustrative procedure is as follows. A mixture of 0.880 g (2.00 mmol) of 2-tributylstannyl-6-(trimethylsilyl)pyridine, 0.504 g (1.00 mmol) of 3, and 20 mg (1.7 mol %) of Pd(PPh3)4 in 5 mL of THF was placed in a sealed glass tube and heated at 150 °C for 80 h. After the usual workup, the resulting mixture was chromatographed on a silica gel column eluting with 10:1 hexane/benzene to give crude dark yellow solids, which were recrystallized from hexane to afford 0.200 g (12% yield) of 1c as yellow solid: sub 295 °C; 1H NMR (δ in CDCl3) 0.35 (s, 18H, SiMe), 7.32 (s, 2H, thienyl ring protons), 7.377.44 (m, 6H, m- and p-Ph), 7.52-7.66 (m, 6H, pyridyl), 7.69 (dd, 4H, J ) 7.7, 1.5 Hz, o-Ph); 13C NMR (δ in CDCl3) -1.8 (SiMe3), 117.3

Organometallics, Vol. 20, No. 23, 2001 4805 (pyridylene), 126.5, 128.2, 130.4, 131.6, 134.3, 135.5 (Ph, pyridylene, C3, C5), 141.2 (C2, C6), 148.6 (C3a, C4a), 152.2 (pyridyl), 152.7 (C7a,b); 168.4 (pyridylene); 29Si NMR (δ in CDCl3) -5.0 (C-SiMe), -20.1 (silole Si); MS m/z 644 (M+). Anal. Calcd for C36H36S2N2Si3: C, 67.03; H, 5.62; N, 4.34. Found: C, 66.90; H, 5.53; N, 4.23. Compound 7c was prepared as above using appropriate starting compounds shown in Scheme 2. Data for 7c: yellow solid; 16% yield; mp 184-187 °C; 1H NMR (δ in CDCl3) 0.35 (s, 18H, SiMe), 7.24-7.59 (m, 10H, ring protons); 13C NMR (δ in CDCl3) -1.79 (SiMe3), 117.3 (pyridylene), 124.7 (2C), 126.7, 134.4, 139.5, 151.9 (sp2-C), 168.3 (pyridylene); 29Si NMR (δ in CDCl3) -5.1; MS m/z 464 (M+). Anal. Calcd for C24H28S2N2Si2: C, 62.02; H, 6.07; N, 6.03. Found: C, 62.01; H, 5.95; N, 6.00. CV Measurements. CV measurements were carried out using a three-electrode system in acetonitrile solutions containing 100 mM of lithium perchlorate as the supporting electrolyte and 4 mM of the substrate. A glassy carbon electrode, platinum plat, and SCE were used as the working, counter, and reference electrodes, respectively. The currentvoltage curves were recorded at room temperature on a Hokuto Denko HAB-151 potentiostat/galvanostat. Preparation of EL Devices. Each layer of the EL devices was prepared by vacuum deposition at 1 × 10-5 Torr in the order of TPD, Alq, and a DTS (for type I device only), on ITO coated on a glass substrate with a sheet resistance of 15 Ω/cm (Asahi Glass Company). Finally a layer of magnesium-silver alloy with an atomic ratio of 10:1 was vacuum deposited as the top electrode. The thickness of each layer of the EL devices was measured with a Sloan Dektak 3030 surface profiler. The emitting area was 0.5 × 0.5 cm2. Luminance was measured with a Topcon BM-7 luminance meter at room temperature. MO Calculations. Geometry optimizations were carried out using the restricted-Hartree-Fock (RHF) method at the 6-31G level within the Gaussian 98 suite of programs14 on a Dell Precision 410 workstation.

Acknowledgment. The basic part of this work was supported by the Ministry of Education, Culture, Sports, Science and Technology, Japan (Grant-in-Aid for Scientific Research, Nos. 11120234, 12555245, and 13029080). Applications to EL devices were studied as a part of the Industrial Science and Technology Frontier Program by the New Energy and Industrial Technology Development Organization. We thank Sankyo Kasei Co. Ltd. and Sumitomo Electric Industry for financial support, and Shin-Etsu Chemical Co. Ltd. for gifts of chlorosilanes. OM0103254 (14) Gaussian 98, revision A.1; Gaussian, Inc.: Pittsburgh, PA, 1998.