Synthesis and photophysical properties of poly(aryleneethynylene)s bearing dialkylsilyl side substituents

European Polymer Journal 45 (2009) 1092–1097

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Synthesis and photophysical properties of poly(aryleneethynylene)s bearing dialkylsilyl side substituents Lei Fang a,b, Ying Li c, Rui Wang a,b, Caihong Xu a,*, Shuhong Li c a

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, No. 2, Beiyijie, Zhongguancun, Beijing 100190, China Graduate School of the Chinese Academy of Sciences, Beijing 100049, China c School of Chemical and Environmental Engineering, Beijing Technology and Business University, Beijing 100037, China b

a r t i c l e

i n f o

Article history: Received 19 November 2008 Received in revised form 9 January 2009 Accepted 11 January 2009 Available online 18 January 2009

Keywords: Synthesis Photophysics Poly(aryleneethynylene)s Dialkylsilyl substituents Sonogashira cross-coupling

a b s t r a c t A series of poly(aryleneethynylene)s bearing dialkylsilyl (–SiR2H) side substituents has been synthesized by Sonogashira cross-coupling reactions of 1,4-diethynyl-2,5-bis(dialkylsilyl)benzene and diiodoarylene compounds. Their photophysical properties in solution have been studied. All of the polymers showed intense fluorescence with high quantum yield. Compared with their analogues containing para-phenylene units, the polymers with meta-phenylene units in the main chain showed absorption and emission maxima at shorter wavelengths, whereas the polymers having thiophenylene units in their backbones showed bathochromically shifted spectra. For polymers having the same conjugated parent backbone, silyl substituents have been found to exert negligible effect on their photophysical properties. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Since the first synthesis was reported by Yamamoto and co-workers in 1984, poly(aryleneethynylene)s (PAEs) have attracted intense attention over the past two decades [1]. This family of conjugated polymers has become established as an important class of materials with interesting optical and electronic properties, and many potential applications that utilize these polymers have been spurred. Hundreds of different PAEs have been designed and studied to date [2]. To obtain optimal properties and functions, much attention has been paid to modifying the PAE skeletons, both sterically as well as electronically, by variation of the side chains [3] or introducing different heteroatom and/or arene units into the main chain [4]. Among such modifications, the incorporation of silicon into PAE skeletons has been a particularly interesting subject. For example, the replacement of a benzene unit with a silylene unit * Corresponding author. E-mail address: [email protected] (C. Xu). 0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2009.01.010

leads to a new class of silyleneethynylene copolymers. In these, the lower ionization energy of the silicon atom compared with carbon can enhance the through-bond interaction of ethynyl units along the backbone, and thus materials with unusual electronic and optical properties can be obtained [5]. By introducing silole units into the backbones of PAEs, polymers with significantly narrower bandgaps were prepared [6]. Although a number of researchers have contributed to the areas of PAEs containing silicon moieties in their backbones, little work on PAEs bearing silyl side chains has been carried out [7]. To the best of our knowledge, there has been no report on PAEs containing functional Si–H groups in side silyl substituents. In view of the findings that silyl groups attached to p-conjugated systems can contribute to the electronic structure and thereby influence photophysical properties through important orbital interactions [8], we envisaged that the introduction of side dialkylsilyl groups on the backbone of PAEs not only contribute to the electronic structure, but also improve the solubility and inhibit the strong tendency for aggregation of rigid PAEs. In addition,

L. Fang et al. / European Polymer Journal 45 (2009) 1092–1097

the existence of reactive Si–H bonds in the dialkylsilyl groups also provides other possibilities for further structure modification through hydrosilylation, oxidation, or dehydrogenative silylation reactions [9]. In this paper, we report the synthesis of a series of PAEs bearing dialkylsilyl substituents and initial studies of their photophysical properties.

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chromatography (petroleum ether, Rf = 0.61) to give 0.94 g (3.15 mmol) of 2,5-bis(diethylsilyl)-1,4-diethynylbenzene (3a) in 94% yield as colorless solid: mp 29– 30 °C. 1H NMR (400 MHz, acetone-d6): d 7.67 (s, 2H), 4.32 (m, 2H), 4.00 (s, 2H), 0.961.04 (m, 20H); 13C NMR (400 MHz, acetone-d6): d 140.44, 140.32, 128.39, 84.56, 84.09, 8.57, 3.57. Anal. Calcd. for C18H26Si2: C, 72.41; H, 8.78. Found: C, 71.84; H, 9.11.

2. Experimental 2.2. 2,5-Bis(dihexylsilyl)-1,4-diethynylbenzene (3b) All reactions were carried out under nitrogen atmosphere unless otherwise noted. Anhydrous diethyl ether (Et2O) was distilled over sodium and benzophenone under nitrogen atmosphere. 2,5-dibromo-1,4-bis(trimethylsilylethynyl)benzene [10], diethylchlorosilane [11], dihexylchlorosilane [12] were prepared according to procedures described in the literatures, respectively. All other chemicals were purchased from Alfa Aesar or Aldrich and used as received. 1H and 13C NMR spectra were obtained using a BRUKER AVANCE 400 spectrometer with CDCl3 or acetone-d6 as solvent. Melting point (mp) determination was performed using a WRS-1A digital melting point apparatus. Elemental analyses were obtained with the FLASH EA1112 Microanalytical facility. UV–Vis absorption and fluorescence spectra were measured with a Shimadzu UV1601PC spectrometer and a Hitachi F4500 spectrometer, respectively. Thermal analysis was performed on an EXSTAR6000 TG/DTA6300 thermal analyzer at a heating rate of 10 °C/min in nitrogen at a flow rate of 200 cm3/ min for thermogravimetric and differential thermal analysis (TGDTA). Molecular weights were determined by a gel permeation chromatography (GPC) with polystyrene calibration using a WATERS 2690D Separation Module and WATERS 2410 Refractive Index Detector equipped with TSKGEL GMHHRM and TSK column at 30 °C in toluene. 2.1. 2,5-Bis(diethylsilyl)-1,4-diethynylbenzene (3a) To a solution of 2,5-dibromo-1,4-bis(trimethylsilylethynyl) benzene (1, 2.00 g, 4.67 mmol) in anhydrous Et2O (17 mL) was added a hexane solution of n-BuLi (2.5 M, 3.85 mL, 9.60 mmol) dropwise at 78 °C. After stirred for 1 h, diethylchlorosilane (1.20 g, 9.80 mmol) was added via syringe at the same temperature and the mixture was allowed to warm to room temperature over 14 h with stirring. The mixture was filtered and concentrated, and then passed through a silica gel short column (hexane, Rf = 0.52) to give 1.48 g (3.35 mmol) of 2,5-bis(diethylsilyl)-1,4-bis-(trimethylsilylethynyl)benzene (2a) in 72% yield as colorless solid, which was directly used for next desilylation reaction without further purification. 1H NMR (400 MHz, acetone-d6): d 7.61 (s, 2H), 4.29 (m, 2H), 0.961.05 (m, 20H), 0.240.26 (m, 18H). A solution of 2a (1.48 g, 3.35 mmol) and K2CO3 (90 mg, 0.67 mmol) in a 1/1 THF/EtOH mixed solvent (30 mL) was stirred for 3 h at room temperature. After quenched with water, the reaction mixture was extracted with Et2O. The organic layer was washed with brine, dried over MgSO4, and concentrated under reduced pressure. The mixture was subjected to a silica gel column

This compound was synthesized essentially in the same manner as described for 2,5-bis(diethylsilyl)-1,4-diethynylbenzene using dihexylchlorosilane in 93% yield as colorless oil based on 2b. 1H NMR (400 MHz, acetone-d6): d 7.68 (s, 2H), 4.39 (m, 2H), 3.99 (s, 2H), 1.261.39 (m, 32H); 0.981.09 (m, 8H); 0.830.94 (m, 12H); 13C NMR (400 MHz, acetone-d6): d 140.59, 139.98, 127.95, 84.29, 83.78, 33.05, 31.79, 25.00, 22.83, 13.98, 11.54. Anal. Calcd. for C34H58Si2: C, 78.08; H, 11.18. Found: C, 77.64; H, 10.93. 2.3. A Typical procedure for the synthesis of the polymers: Polymer P1 A mixture of 3a (80 mg, 0.27 mmol), 1,4-diiodobenzene (4a, 89 mg, 0.27 mmol), Pd(PPh3)2Cl2 (3.8 mg, 5.3 lmol), CuI (2.1 mg, 10.6 lmol) in a 3/1 toluene/Et3 N mixed solvent (28 mL) was stirred at room temperature for 22 h. After addition of 5% NH4Cl aqueous solution, the reaction mixture was extracted with CHCl3, and the organic layer was washed with brine, dried over MgSO4, filtered, and concentrated under reduced pressure. The yellow residue was dissolved with a small amount of CHCl3 and poured into a vigorously stirred MeOH to form yellow precipitates. The reprecipitation procedure was repeated twice to give 70 mg of polymer P1 in 70% yield as a yellow powder. 1H NMR (400 MHz, CDCl3): d 7.71 (s, 2H), 7.54 (s, 4H), 4.42 (m, 2H), 1.051.06 (m, 20H); 13C NMR (400 MHz, CDCl3): d 139.6, 139.2, 131.6, 127.9, 123.4, 93.5, 92.8, 8.6, 3.5. 2.4. Polymer P2 This polymer was synthesized essentially in the same manner as described for P1 by the reaction of 3b and 1,4-diiodobenzene in 86% yield as a yellow powder. 1H NMR (400 MHz, CDCl3): d 7.71 (s, 2H), 7.53 (s, 4H), 4.45 (m, 2H), 1.241.42 (m, 32H); 1.011.03 (m, 8H); 0.85 (t, J = 6.4 Hz, 12H); 13C NMR (400 MHz, CDCl3): d 140.2, 139.2, 131.5, 127.8, 123.4, 93.5, 92.9, 32.9, 31.6, 24.9, 22.7, 14.3, 11.8. 2.5. Polymer P3 P3 was synthesized essentially in the same manner as described for P1 by the reaction of 3a and 1,3-diiodobenzene in 71% yield as a bright yellow powder. 1H NMR (400 MHz, CDCl3): d 7.717.75 (m, 3H), 7.52 (d, J = 7.8 Hz, 2H), 7.38 (t, J = 7.8 Hz, 1H), 4.44 (m, 2H), 0.991.11 (m, 20H); 13C NMR (400 MHz, CDCl3): d 139.7, 139.2, 134.2, 131.4, 128.9, 127.8, 123.9, 92.8, 91.4, 8.6, 3.5.

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Scheme 1. Synthesis of 2,5-bis(dialkylsilyl)-1,4-diethynylbenzene. Reaction conditions: (a) n-BuLi, Et2O, 78 °C; (b) dialkylchlorosilane 78 °C to RT; (c) K2CO3, EtOH/THF, RT.

2.6. Polymer P4 P4 was prepared in the same manner as described for P1 from the reaction of 3b and 1,3-diiodobenzene in 65% yield as a waxy yellow substance. 1H NMR (400 MHz, CDCl3): d 7.717.76 (m, 3H), 7.52 (d, J = 8.3 Hz, 2H), 7.38 (t, J = 7.6 Hz, 1H), 4.46 (m, 2H), 1.231.41 (m, 32H); 1.011.06 (m, 8H), 0.82 (d, J = 6.8 Hz, 12H); 13C NMR (400 MHz, CDCl3): d 140.2, 139.2, 134.1, 131.4, 128.8, 127.6, 123.9, 92.7, 91.5, 32.9, 31.6, 24.9, 22.7, 14.2, 11.8. 2.7. Polymer P5 This polymer was synthesized essentially in the same manner as described for P1 by the reaction of 3a and 2,5diiodothiophene in 86% yield as a yellow powder: 1H NMR (400 MHz, CDCl3): d 7.70 (s, 2H), 7.19 (s, 2H), 4.37 (m, 2H), 1.011.08 (m, 20H); 13C NMR (400 MHz, CDCl3): d 139.7, 138.8, 132.0, 127.5, 125.1, 95.9, 87.0, 8.6, 3.5. 2.8. Polymer P6 This polymer was synthesized essentially in the same manner as described for P1 by the reaction of 3b and 2,5-diiodothiophene in 69% yield as a waxy yellow substance. 1H NMR (400 MHz, CDCl3): d 7.71 (s, 2H), 7.19 (s, 2H), 4.39 (m, 2H), 1.251.34 (m, 32H); 0.99 (m, 8H), 0.84 (t, J = 6.7 Hz, 12H); 13C NMR (400 MHz, CDCl3): d 140.3, 138.9, 131.9, 127.4, 125.1, 95.9, 86.9, 32.9, 31.6, 24.8, 22.7, 14.2, 11.8. 3. Results and discussion 3.1. Synthesis and characterization The two key monomers, 2,5-bis(diethylsilyl)-1,4-diethynylbenzene (3a) and 2,5-bis(dihexylsilyl)-1,4-diethynyl-

benzene (3b), were synthesized in two steps as indicated in Scheme 1, according to a literature method [13]. Lithium–halogen exchange of 1,4-dibromo-2,5-bis(trimethylsilylethynyl)benzene (1) with n-BuLi followed by treatment with diethylchlorosilane or dihexylchlorosilane produced compounds 2a and 2b, respectively. Subsequent alkaline desilylation of the terminal trimethylsilyl groups produced the Si–H– containing disilylbenzenes 3a and 3b in high yields. All of the polymers were synthesized by Sonogashira cross-coupling reactions between the 2,5-bis(dialkylsilyl)1,4-diethynylbenzene (3a, b) and the requisite diiodoarylene compounds. The reactions using Pd(PPh3)2Cl2 and CuI as co-catalysts in toluene/Et3 N proceeded well. Thus, the reactions of 2,5-bis(dimethylsilyl)-1,4-diethynylbenzene (3a) with 1,4-diiodobenzene, 1,3-diiodobenzene, and 2,5diiodothiophene produced polymer P1 containing paraphenylene units, polymer P3 containing meta-phenylene units, and polymer P5 containing thiophone units, respectively. Similarly, cross-couplings of 3b gave P2, P4, and P6, respectively (Scheme 2). All of the polymers were obtained in moderate to good yields. These polymers are freely soluble in common organic solvents, such as chloroform, toluene, and tetrahydrofuran. In addition, these polymers are quite stable in air though the existence of active Si–H groups, no variation in their 1H NMR spectra was observed after storing them for six months under ambient laboratory conditions. Table 1 lists the reaction conditions used to obtain the synthesized PAEs bearing dialkylsilyl substituents and their average molecular weights. The molecular weights were determined by GPC with polystyrene standards. The number average molecular weights (Mn) of the polymers varied from 3.35  104 to 8.42  104 and the degrees of polymerization (DP) ranged from 55 to 191. The molecular weights of the polymers containing para- or meta-phenylene co-units (P1–P4) were higher than those of P5 and

Scheme 2. Synthesis of polymers P1–P6.

L. Fang et al. / European Polymer Journal 45 (2009) 1092–1097 Table 1 Reaction conditions and average molecular weights of polymers. Polymer Monomer1 Monomer2 Yielda (%)

Mnb

Mwb

Mw/ Mnb

DPc (Mn)

P1 P2 P3 P4 P5 P6

71,100 84,200 63,200 66,500 41,300 33,500

108,900 111,700 98,400 103,000 97,200 73,900

1.53 1.33 1.56 1.55 2.35 2.20

191 141 169 111 109 55

a b c

3a 3b 3a 3b 3a 3b

4a 4a 4b 4b 4c 4c

70 86 71 65 86 69

After reprecipitation from methanol. Estimated by GPC in toluene on the basis of a polystyrene calibration. Degree of polymerization.

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The structures of the polymer were characterized by 1H and 13C NMR spectroscopy. The thermal properties of the synthesized polymers were evaluated by the means of TGA under nitrogen atmosphere. All of the polymers exhibited good thermal stability, with their temperatures at 5% weight loss (Td5) being higher than 350 °C (Fig. 1). For polymers having the same conjugated parent backbone, an increase in the alkyl chain length leads to a decrease in the resistance of the polymers to thermolysis. This is probably due to decomposition of the alkyl chains at high temperatures. 3.2. Photophysical properties

P6 containing thiophenylene units. Also, the DPs of the former were somewhat narrower than those of the latter. These differences may arise from variation in the monomeric reactivity under the reaction conditions employed.

The UV–Vis absorption and fluorescence spectra of the polymers in CHCl3 are shown in Fig. 2, and their photophysical data are summarized in Table 2.

Fig. 1. TGA plots of polymers P1–P6.

Fig. 2. Normalized (a) UV–Vis absorption and (b) fluorescence spectra of polymers P1–P6 in chloroform.

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There are several notable points: (1) all of the polymers show intense fluorescence with high quantum yield. The quantum yields of P1, P2, P3, and P4 were found to be around 0.60–0.70, all of these values being higher than those of their alkoxy substituted analogues 7 and 8, respectively [14]. Similarly, the quantum yields of P5 and P6, containing 2,5-thiophenylene units in their main chains, were also higher than that of the alkoxy substituted polymer 9 [15]. This fact is in accordance with the finding that the incorporation of silyl groups into poly(aryleneethynylene)s leads to intense fluorescence, as reported by Yamaguchi [7]. (2) The introduction of meta-phenylene units into the polymer main chain results in significant shifts of the absorption and emission maxima to shorter wavelengths. As shown in the absorption spectra, polymers P1 and P2 containing para-phenylene units display absorption maxima at about 390 nm, while the absorption maxima of meta-phenylene unit linked polymers P3 and P4 are blue-shifted to 340 nm. The lower kmax values for P3 and P4 indicate that meta-phenylene units interrupt the conjugation and thereby affect the energy band gaps of the corresponding PAEs. (3) The incorporation of 2,5thiophenylene units into the main chain shifts the absorption and emission maxima to longer wavelengths. It can be seen in Fig. 1, that P5 and P6 have their absorption maxima at 428 and 426 nm, i.e. at wavelengths longer than those of P1 and P2, their analogues containing para-phenylene units, by 35 and 40 nm, respectively. However, the fluorescence quantum yields of P5 and P6 are somewhat lower than those of P1 and P2, respectively. (4) For polymers having the same conjugated parent backbone, viz. P1 vs. P2, P3 vs. P4, and P5 vs. P6, the silyl substituents have negligible effect on the photophysical properties. The absorption and emission maxima of dihexylsilyl-substituted polymers P1, P3, and P5 appear at only slightly longer wavelengths than those of P2, P4, and P6, respectively. The two polymers of each pair have close quantum yields. Table 2 Photophysical properties of polymers. Polymer

P1 P2 P3 P4 P5 P6 a

UV–Vis kmax (nm)b

393 386 343 341 428 426

Absorptiona log e

4.54 4.32 4.73 4.61 4.69 4.55

Fluorescencea kmax (nm)b,c

UFd

425 423 368 367 467 466

0.60 0.68 0.69 0.70 0.54 0.53

In CHCl3. All polymers show vibronic absorption spectra. Only the longest absorptions are listed. c Excited at 340 nm. d Determined with 9,10-diphenylanthracene for P1, P2, anthracene for P3, P4, and quinine bisulfate for P5, P6 as a standard, respectively. The UF is the average values of repeated measurement within ±5% errors. b

4. Conclusions We have synthesized a series of new poly(aryleneethynylene)s bearing dialkylsilyl (–SiR2H) side substituents. All of the polymers display good thermal stability and good solubility in common organic solvents. The incorporation of the silyl groups was effective in increasing the fluorescence quantum yields of the PAE polymers. The new polymers containing dialkylsilyl groups show higher quantum yields than their alkoxy- or alkyl-substituted analogues. For polymers having the same conjugated parent backbone, the silyl substituents exerted negligible effect on the photophysical properties. Further studies involving modification of the structures of the produced polymers by exploiting the reactivity of the Si–H groups are underway in our laboratory. Acknowledgment This work was supported by the National Science Foundation of China (NSFC, Nos. 50673094 and 20774102). References [1] Sanechika K, Yamamoto T, Yamamoto A. Palladium catalyzed CC coupling for synthesis of p-conjugated polymers composed of arylene and ethynylene units. Bull Chem Soc Jpn 1984;57:752–5. [2] (a) For recent reviews, see Yamamoto T, Koizumi T. Synthesis of pconjugated polymers bearing electronic and optical functionalities by organometallic polycondensations and their chemical properties. Polymer 2007;48:5449–72; (b) Bunz UHF. Synthesis and structure of PAEs. Adv Polym Sci 2005;177:152; (c) Yamamoto T, Yamaguchi I, Yasuda T. PAEs with heteroaromatic rings. Adv Polym Sci 2005;177:181–208; (d) Bunz UHF. Poly(p-phenyleneethynylene)s by alkyne metathesis. Acc Chem Res 2001;34(12):998–1010; (e) Bunz UHF. Poly(aryleneethynylene)s: syntheses, properties, structures, and applications. Chem Rev 2000;100:1605–44; (f) Giesa RC. Synthesis and properties of conjugated poly(aryleneethynylene)s. J Macromol Sci-Rev Macromol Chem Phys 1996;C36:631–70. [3] (a) Sato T, Jiang D, Aida T. A blue-luminescent dendritic rod: poly(phenylene–ethynylene) within a light-harvesting dendritic envelope. J Am Chem Soc 1999;121(45):10658–9; (b) Masuo S, Yoshikawa H, Asahi T, Masuhara H, Sato T, Jiang D, et al. Fluorescence spectroscopic properties and single aggregate structures of p-conjugated wire-type dendrimers. J Phys Chem B 2003;107(11):2471–9; (c) Zhao C, Wakamiya A, Yamaguchi S. Highly emissive poly(aryleneethynylene)s containing 2,5-diboryl-1,4-phenylene as a building unit. Macromolecules 2007;40:3898–900. [4] (a) Itoh M, Inoue K, Iwata K, Mitsuzuka M, Kakigano T. New highly heatresistant polymers containing silicon: poly(silyleneethynylenephenyl– eneethynylene)s. Macromolecules 1997;30(4):694–701; (b) Itoh M, Iwata K, Ishikawa JI, Sukawa H, Kimura H, Okita K. Various silicon-containing polymers with Si(H)AC@C units. J Polym Sci A: Polym Chem 2001;39:2658–69; (c) Xu J, Wang C, Lai G, Shen Y. Synthesis and characterization of phenylacetylene-teriminated poly(silyleneethynylene-4,4’phenyletherene-ethynylene)s. Eur Polym J 2007;43:668–72. [5] (a) Kwak G, Masuda T. Photoisomerization and emission properties of transand cis-poly(dimethylsilylenephenylenevinylene)s.

L. Fang et al. / European Polymer Journal 45 (2009) 1092–1097 Macromol Rapid Commun 2001;22(11):846–9; (b) Kwak G, Masuda T. Synthesis and thermal properties of regioand stereoregular poly(silylene-1,4-phenylenevinylene)s. Macromol Rapid Commun 2001;22(15):1233–6; (c) Kwak G, Masuda T. Synthesis, characterization and optical properties of a novel Si-containing r-p-conjugated hyperbranched polymer. Macromol Rapid Commun 2002;23(1):6872. [6] Yamaguchi S, Iimura K, Tamao K. Silole-acetylene p-electronic systems with low bandgaps: synthesis, structure, and UV–Vis absorption spectra of 2,5-diethynylsiloles derivatives and their polymers. Chem Lett 1998;27:8990. [7] During our preparation of this manuscript, we found the report on highly fluorescent poly(aryleneethynylene)s with disilylphenylene and tetrasilylphenylene as new building unit. Iida A, Nagura K, Yamaguchi, S. Disilylphenylene and tetrasilylphenylene as a new building unit for highly fluorescent poly(aryleneethynylene)s. Chem Asian J 2008; 3:1456–1464. [8] (a) Shimizu M, Tatsumi H, Mochida K, Oda K, Hiyama T. Siliconbridge effects on photophysical properties of silafluorenes. Chem Asian J 2008;3:1238–47; (b) Kyushin S, Ishikita Y, Matsumoto H, Horiuchi H, Hiratsuka H. Yellow–green fluorescence of 5,11- and 5,12-bis(diisopropylsilyl)naphthacenes. Chem Lett 2006;35:6465; (c) Kyushin S, Takemasa N, Matsumoto H, Horiuchi H, Hiratsuka H. 2,3,6,7,10,11-Hexakis(dimethylsilyl)triphenylene. Chem Lett 2003;32:1048–9; (d) Kyushin S, Izumi Y, Tsunakawa S, Matsumoto H. Dibenzo[c,h]-1,6diisopropyl-1,6-disilabicyclo[4.4.0]deca-3,8-dienes. Chem Lett 1992;21:1393–6; (e) Kyushin S, Ikarugi M, Tsunakawa S, Izumi Y, Miyake M, Sato M, et al. Molecular structures and charge-transfer complexes of cisdibenzo[c,h]-1,6-disilabicyclo[4.4.0]deca-3,8-dienes and bi(benzo[c]silacyclopent-3-ene-1-yl)s. J Organomet Chem 1994;473:1927; (f) Kyushin S, Ikarugi M, Takatsuna K, Goto M, Matsumoto H.

[9]

[10]

[11]

[12]

[13]

[14]

[15]

1097

Dibenzo[b,e]-7,7,8,8-tetraneopentyland dibenzo[b,e]-7,7,8,8tetraisopropyl-7,8-disilabicyclo[2.2.2]octa-2,5-dienes: interaction with tetracyanoethylene and transition metal chlorides. J Organomet Chem 1996;510:121–33; (g) Kyushin S, Ikarugi M, Goto M, Hiratsuka H, Matsumoto H. Synthesis and electronic properties of 9,10-disilylanthracenes. Organometallics 1996;15(3):1067–70. Kwak G, Takagi A, Fujiki M, Masuda T. Facile preparation of transparent, homogeneous, fluorescent gel film based on r-pconjugated, hyperbranched polymer with siloxane linkages by means of hydrosilylation and aerial oxidation. Chem Mater 2004;16(5):781–5. Tovar JD, Swager TM. Exploiting the versatility of organometallic cross-coupling reactions for entry into extended aromatic systems. J Organomet Chem 2002;653:215–22. Pang Y, Ijadi-Maghsoodi S, Barton TJ. Catalytic synthesis of silylenevinylene preceramic polymers from ethynylsilanes. Macromolecules 1993;26:5671–5. (a) Kunai A, Kawakaml T, Toyoda E, Ishlkawa M. Highly selective synthesis of chlorosilanes from hydrosilanes. Organometallics 1992;11(7):2708–11; (b) Tannenbaum S, Kaye S, Lewenz GF. The solubility of phosphine in aqueous solutions. J Am Chem Soc 1954;76:1027–8. Yamaguchi S, Xu C, Yamada H, Wakamiya A. Synthesis, structures, and photophysical properties of silicon and carbon-bridged ladder oligo(p-phenylenevinylene)s and related p-electron systems. J Organomet Chem 2005;690:5365–77. Pang Y, Li J, Hu B, Karasz FE. A Processible poly(phenyleneethynylene) with Strong photoluminescence: synthesis and characterization of poly[(m-phenyleneethynylene)-alt-(p-phenyleneethynylene)]. Macromolecules 1998;31(19):6730–2. Pang Y, Li J, Barton TJ. Processible poly[(p-phenyleneethynylene)-alt(2,5-thienyleneethynylene)]s of high luminescence: their synthesis and physical properties. J Mater Chem 1998;8(8):1687–90.