Structures and Photophysical Properties of Silicon-Containing

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Macromolecules 1998, 31, 2866-2871

Structures and Photophysical Properties of Silicon-Containing Phenyleneethynylene Polymers Hong Li and Robert West* Department of Chemistry, University of WisconsinsMadison, 1101 University Avenue, Madison, Wisconsin 53706 Received November 19, 1997; Revised Manuscript Received March 2, 1998

ABSTRACT: A series of silicon-containing phenylene-ethynylene polymers containing alkoxy groups (n-C6H13O- or n-C12H25O-), on some of the aromatic rings, has been synthesized by polycondensation using a palladium-catalyzed coupling reaction. The longer side chains led to greater solubility and higher crystallinity, as shown by wide-angle X-ray diffraction. The supramolecular structures of the polymers changed from ordered or semiordered to disordered structures as the Si component increased. A systematic blue shift was observed in the solution UV, solid-state UV, and solid-state emission spectra of the polymers with increasing silicon content. For all of the polymers except those with high silicon content, the emission spectra in dilute solution were quite similar, consistent with emission from localized excited states.

Polymers with linear, rigid conjugated chains have attractive properties as materials for optical applications, especially for large-area light-emitting displays.1 Several classes of conjugated organic polymers, poly(pphenylene),2 poly(p-phenylenevinylene),3 poly(phenyleneethynylene),4 and poly(thiophene),5 have been found to be promising for the generation of light by means of electron and hole injection. However, unsubstituted rigid rod polymers of high molecular weight are often insoluble and infusible, due to strong intermolecular interaction.6 Recently, the possibility of increasing the solubility and simultaneously the molecular weight of conjugated polymers was investigated by the attachment of long side chains, such as n-C12H25 and n-C16 H33 alkyl groups.7 These rod-coil polymers are expected to form films more easily, which may be crucial for the development of nonlinear materials for photonic applications. Polysilanes, silicon-based polymers, have one-dimensional delocalization of s electrons and exhibit photoconductivity and high hole mobility as well as nonlinear optical properties.9 Introducing the Si-Si bond into the main chain structure of poly(p-phenyleneethynylene)

will interrupt the planar structure. This combination provides alternating delocalized σ-bonds and π-bonds in the main chain, which may partially preserve electron delocalization over the polymer chain while increasing the flexibility of the polymers.10 Furthermore, colortunable luminescence might be realized by such a combination. Earlier, we investigated the properties of 2, 2b, 2c, 3a, and 3b as model compounds for polymers 1.11 The shortest contact distances between aromatic rings are around 3.1-3.5 Å in these molecules, indicating very strong intermolecular interactions. When the side chain R was changed from n-hexyl to n-dodecyl, a red shift of the solid-state emission was observed, due to the different intermolecular arrangement. In this paper we report the synthesis, structures, and photoproperties of polymers 1. Results and Discussion Synthesis and Characterization of Polymers. The polymers were synthesized by a palladium-catalyzed cross-coupling reaction of diiodobenzene deriva-

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Macromolecules, Vol. 31, No. 9, 1998

Si-Containing Phenyleneethynylene Polymers 2867

length of the side chain. The polymers with n-C6H13 side chains can be dissolved in toluene, chloroform, methylene chloride, and THF through the initial purification, and the NMR spectra for these polymers were taken at this stage. However, after being dried in a vacuum at room temperature, they are no longer soluble in common solvents, and only slightly soluble in hot toluene. For example, 1.0 mg of polymer 123C6 could not be completely dissolved in 1 L of THF. On the other hand, polymers with n-C12H25 side chains are soluble in THF, toluene, chloroform, benzene, and other solvents, even after being dried in a vacuum at 110 °C. Solid-State Structure. The supramolecular structure of the polymers was investigated by wide-angle X-ray diffraction (WAXD), on thin film samples. Figure 1 shows diffractograms for several of the polymers containing C12 side groups, along with that of the polymer 011C12 (which contains no silicon) for camparison. The width of the peaks indicates a low degree of crystallinity for all of these polymers. The diffraction pattern of 123C12 shows relatively intense peaks at 23.1°, 12.1°, and 5.9°, corresponding to d spacings of 3.9, 7.3, and 15.1 Å, respectively. The d spacing of 3.9 Å can be assigned as the distance of two aryl planes between the rigid rod chains and the d spacing of 15.1 Å may reflect the interchain distance in a layered structure. These results show that the supramolecular structure of polymer 123C12 is similar to that of 011C12, even though there is a Si-Si bond in the main chain of polymer 123C12. The side groups around the Si-Si bond are probably in a trans confor-

Scheme 1

tives, 1,4-diethynylbenzene, and 1,2-diethynyltetra-nbutyldisilane with different ratios of monomers, as shown in Scheme 1.12 For convenience, the monomer ratio and side chain length were used to name the polymers. The simplified names are shown in Table 1. The polymers were characterized by 1H, 13C, and 29Si NMR, as shown in Table 2. The chemical shifts and proton integration results fit the expected structure of the polymers. Only one sharp peak (around -30 ppm) was observed in the 29Si NMR of the polymers, indicating that all of the Si atoms have similar environments. The molecular weights depended greatly on the purity of monomers and the quality of the catalyst. Freshly recrystallized Pd(PPh3)2Cl2 catalyst was much better than catalyst that has been exposed to air. Molecular weights of the polymers are listed in Table 1. The solubility of the polymers was highly dependent on the

Table 1. Name of the Polymers and the Weight Average Molecular Weights (PDI ) 1.5-2.2)a I Bu Bu Si

a

RO

Si

polymer

Bu Bu

011C12 123C12 112C12 213C12 101C12 123C6 112C6 213C6 101C6

0 1 1 2 1 1 1 2 1

OR

R

M hw

n-C12H25 n-C12H25 n-C12H25 n-C12H25 n-C12H25 n-C6H13 n-C6H13 n-C6H13 n-C6H13

24 400 10 400 17 000 12 000 13 300 41 700 14 200 5 570 9 960

I

1 2 1 1 0 2 1 1 0

1 3 2 3 1 3 2 3 1

The polymer 123Si1C6 has the following structure (R ) n-C6H13): OR

OR C8H17 Si C6H13

RO

1

2

n

MW = 24, 200

RO

123Si1C6, R = C6H13

Table 2. NMR Data of Polymers polymer

NMR data (CDCl3, CD2Cl2, and C6D6; ppm), δ

123C12 [1H] 0.86 (26H, m), 1.1-1.6 (125H, br), 1.8 (12H, m), 4.0 (12H, br), 6.8-7.35 (6H, m), 7.5 (4H, br); [13C] 13.8, 14.1, 22.7, 26.1, 26.5, 27.0, 29.37, 29.42, 29.7, 31.9, 69.7, 94.8, 114.0, 116.9, 131.5, 153.7; [29Si] -30.2 112C12 [1H] 0.86 (32H, m), 1.1-1.6 (89H, br), 1.8 (6H, m), 4.0 (8H, br), 6.8-7.35 (3.9H, m), 7.5 (4H, br); [13C] 13.0, 13.8, 14.1, 22.7, 26.1, 26.6, 27.0, 29.4, 29.7, 31.9, 69.7, 116.9, 131.5, 153.7; [29Si] -30.2 213C12 [1H] 0.86 (32H, m), 1.1-1.6 (89H, br), 1.8 (6H, m), 4.0 (8H, br), 6.8-7.35 (3.9H, m), 7.5 (4H, br); [13C] 13.0, 14.0, 14.3, 23.1, 26.1, 26.4, 26.5, 27.0, 27.4, 29.4, 29.8, 30.1, 32.3, 70.0, 116.9, 131.8; [29Si] -30.0 101C12 [1H] 0.86 (6H, m), 1.1-1.6 (54H, br), 1.8 (4H, m), 4.0 (4H, br), 6.8-7.2 (2H, m); [13C] 12.7, 13.0, 13.7, 13.8, 14.1, 22.7, 26.0, 26.1, 26.5, 26.6, 26.9, 27.0, 29.4, 29.5, 29.7, 31.9, 69.4, 70.1, 117.3, 117.4, 153.9; [29Si] -30.2 123C6 [1H] 0.86 (38H, m), 1.1-1.6 (56H, br), 1.8 (12H, m), 4.0 (12H, br), 6.8-7.0 (3.6H, m), 7.5 (8H, br); [13C] 13.4, 14.0, 22.6, 25.8, 26.5, 27.0, 29.3, 29.3, 31.7, 70.0, 116.9, 131.8, 154.1; [29Si] -30.0 112C6 [1H] 0.86 (32H, m), 1.1-1.9 (52H, br), 4.0 (8H, br), 6.8-7.0 (4H, m), 7.5 (4H, br); [13C] 13.4, 14.0, 22.6, 25.8, 26.5, 27.0, 29.3, 29.3, 31.7, 70.0, 116.9, 131.8, 154.2; [29Si] -30.0 213C6 [1H] 0.86 (48H, m), 1.0-1.6 (84H, br), 1.8 (16H, m), 4.0 (12H, br), 6.8-7.8 (11H, m); [13C] 13.4, 14.0, 14.2, 23.1, 25.9, 26.0, 26.1, 26.9, 27.0, 27.3, 27.4, 29.7, 31.9, 32.0, 69.9, 70.0, 70.1, 117.2, 117.7,128.9,129, 130.3, 131.9, 134.4, 154.2; [29Si] -30.0

2868 Li and West


Figure 1. Film wide-angle X-ray diffraction of polymers with n-C12H25 side chains.

Figure 2. UV absorption spectra of polymers with an n-C12H25 side chain in THF solution.

Table 3. Thermal Properties of Polymers polymer

onset (°C)

peak (°C)

∆H (J/G)

123C12 112C12 213C12 101C12

397 241 215 -57.5 380 397 391 238 -27.6 384

402 244 219 -39.8

decomposed -43 -32 14 decomposed decomposed decomposed -55 3.1 decomposed

123C6 112C6 213C6 101C6

429 422 246 -9.4 420

mation in polymer 123C12, similar to that of model compound 3a. Thus, polymer 123C12 probably has a nearly rigid rod main chain structure. The diffraction patterns of 213C12 and 101C12 polymers show only two broad bands and no sharp peaks, indicating that these polymers are disordered. These polymers with larger amounts of the Si component may have coiled structures, resulting from the flexibility of the Si-Si bonds. The polymers with C6 side chains are all less crystalline than their C12 counterparts (diffractograms are given in the Supporting Information). However, polymer 123C6 exhibits a peak corresponding to a d spacing of 14.1 Å, not very different from that of the corresponding polymer with a C12 side chain, 123C12. Thus the interchain distance appears not to be strongly dependent on the length of the side chain in the solid state. It appears that the supramolecular structures of the polymers with both rigid rod segments and flexible SiSi bonds can change from an ordered to a disordered structure as the silicon content increases.8 This kind of variation and control in supramolecular structure and morphology should allow one to control various physical and photophysical properties, such as luminescence,

Figure 3. UV absorption spectra of polymers with an n-C6H13 side chain in THF solution.

photoconductivity, charge transport, energy transfer, and third-order nonlinear optical response.13 Thermal Properties. The thermal properties of the polymers were investigated by differential scanning



Table 4. Peak Wavelengths and Coefficients of Polymers in THF polymer

λmax (nm)

polymer

λmax (nm)

011C12 [, L/(cm‚g)] 123C12 [, L/(cm‚g)] 112C12 [, L/(cm‚g)] 213C12 [, L/(cm‚g)]

414 (58.7), 332 (23.8) 406 (61.9), 332 (30.8), 244 (25.2) 398 (36.8), 328 (23.3), 292 (20.4), 240 (29.3) 388 (26.2), 370 (30.1), 320 (24.1), 290 (37.8), 272 (40.7), 240 (42.1) 346 (13.1), 292, (17.0), 268 (17.2), 240 (25.3)

123Si1C6 123C6 112C6 213C6

406, 330, 240 404, 330, 330, 240 398, 328,292, 270, 240 388, 370, 328,290, 272, 240

101C6

350, 294, 272, 240

101C12 [, L/(cm‚g)]

calorimetry (DSC). The samples were dried at 110 °C under vacuum for 24 h. Otherwise, some low-temperature transitions could be observed, due to a trace amount of solvent. The results are listed in Table 3. For polymer 123C12, a large broad exothermic transition with onset temperature 397 °C and peak temperature 402 °C was observed, due to the decomposition of the polymer. Similar results were found for polymers 123C6 and 112C6. There is an exothermic transition for polymer 112C12 with an onset temperature at 241 °C and a peak at 244 °C. The small enthalpy change of -43 J/g and the fact that the resulting polymer after heating was insoluble suggest that the polymer became cross-linked. Similar results were observed for polymers 213C12 and 213C6. A low-temperature phase transition was recorded for polymer 101C12 with an onset temperature at -57.5 °C and a peak at -39.8 °C. This polymer decomposed around 380 °C. Similarly, polymer 101C6 exhibited a phase transition with onset temperature at -27.6 °C and a peak at -9.4 °C and decomposed exothermically at 384 °C. Photoproperties. The UV spectra of the polymers in THF solution are shown in Figures 2 and 3. For polymer 123C12, two absorption peaks (414 and 332 nm) were observed, which are the characteristic peaks of the rigid rod chromophore -(CtCsAr)n-. For polymer 101C12, additional peaks were observed, as shown in Table 4. The absorption peak at 346 nm is red shifed by about 87 nm compared to that of model compound 1,2-diphenylethynylene-1,1,2,2-tetramethyldisilane (3a).14 The copolymers 123C12, 112C12, and 213C12 have the absorption peaks of both 011C12 and 101C12. As the Si component is increased, the longest wavelength peaks of the polymers are systematically blue shifted, as shown in Figures 2 and 3. When the side chain changed from n-C12H25 to n-C6H13, no apparent difference was observed in the solution UV absorption spectra. Similar results were observed in the solid-state UV spectra (Figure 4), although the UV absorption bands of the polymers in the solid state are broad and red shifted compared to the corresponding solution spectra. A shoulder at 350 nm was observed in the absorption spectrum of polymer 101C12, while for 213C12 a stronger absorption band with a maximum at 394 nm appeared. The peak of the absorption band of 112C12 is further red shifted to 408 nm. With higher rigid rod moieties, the overall spectra are progressively red shifted, the absorption band of 123C12 having a maximum at 428 nm and that of homopolymer 011C12 occurring at 452 nm. The emission spectra of the polymers in THF solution are shown in Figures 5 and 6. The emission spectrum of polymer 101C12 has a broad band with a peak at 408 nm. The emission spectrum of polymer 101C6 has peaks at 408 and 445 nm and a shoulder at 375 nm. The emission spectra of polymers 101C12 and 101C6 depend somewhat on the excitation wavelength due to self-absorption, as shown in the Supporting Information.

Figure 4. UV absorption spectra of polymers with an n-C12H25 side chain in the solid state.

For polymer 123C12, a sharp and narrow emission spectrum with a peak at 446 nm and a shoulder at 470 nm was observed. Similar results were observed for polymers 123C6, 112C6, 213C6, 112C12, and 213C12. The emission spectra of these six polymers in solution are not dependent on the excitation wavelengths. The striking similarities of these emission spectra of polymers containing very different amounts of the disilane component indicate that the rigid rod chromophores have a dominant effect on the emission spectra. These polymers lack an emission band at 408 nm and do not show mirror image similarities between their absorption and emission bands, perhaps because energy transfer takes place, leading to emission from localized excited states (most likely after a migration of the exciton along the polymer chains to segments that represent lowenergy states).15 The fluorescence and UV spectra of polymer 123C12 were investigated in different solvents, such as toluene and chloroform. The results were similar to those in THF. The solid-state fluorescence was investigated using a thin film of the polymer as shown in Figure 7. The intensity of the emission spectra in the solid state are much lower than those in dilute solution. The emission peaks of the solid state are broad and red shifted compared to those in solution. It is evident that the emission peaks are systematically blue shifted, as the Si component is increased.

2870 Li and West

Figure 5. Fluorescence spectra of polymers with an n-C6H13 side chain in THF solution.

Figure 6. Fluorescence spectra of polymers with an n-C12H25 side chain in THF solution.

The emission spectra of these polymers are highly dependent on the concentration, due to excimer formation.16 The concentration dependent emission spectra of polymer 123C12 are shown in Figure 8. A red shift from 448 to 497 nm was observed, when the polymer 123C12 concentration increased from 0.91% in the polystyrene thin film to 100%. The quantum yield was measured at a UV absorbance less than 0.05 in THF solution. The excitation wavelength was fixed at 370 nm, and the standard was


Figure 7. Film emission spectra of polymers with an n-C12H25 side chain.

Figure 8. Film fluorescence spectra of 123C12 in a polystyrene solution.

quinine sulfate. The quantum yield of a sample in solution (Φs) relative to a reference sample of known quantum yield Φr was calculated by the following equation:17

Φs ) Φr(ArFs/AsFr)(ns2/nr2) where A is the absorbance, F is the integrated fluorescence intensity, and n is the refractive index. r is the reference and s is the sample. The refractive indices of pure 1 N H2SO4 (nr ) 1.339) and THF (ns ) 1.407) were used.18 The results are listed in Table 6. The fact that



Table 5. Solid-State UV Spectra of Polymers with n-C12H25 Side Chains polymer

011C12

123C12

112C12

213C12

λmax (nm)

428, 452

424, 445

408

394

Table 6. Quantum Yield of Polymers in THF Solvent with Absorbance at 0.02 and the Excitation Wavelength at 370 nm polymer

Φ (%) in THF

123C12 112C12 213C12 101C12

44 51 64 30

polymers 123C6 112C6 213C6 101C6

Φ (%) in THF 52 55 47 35

the quantum yields of 101C12 and 101C6 are lower than the other polymers is consistent with higher flexibility of the main chain in these two polymers. Conclusions We have synthesized a series of Si-containing polymers using the palladium-catalyzed cross-coupling reaction. These polymers contain both rigid rod segments and flexible Si groups. As the proportion of the siliconcontaining component increased, the supramolecular structures of the polymers changed from ordered or semiordered structures to disordered structures, and systematic blue shifts were observed in the solution and solid-state UV spectra. Similar results were observed in the solid-state emission spectra. However, except for polymers 101C12 and 101C6, no apparent difference was observed in the emission spectra of dilute solutions. Combining Si-Si bonds into the main chain structures of poly(phenyleneethynylene)s increases the flexibility and processability of the rigid rod polymers. The photoproperties can be tuned by varying the amount of the Si components. Experimental Section General Procedure. Proton, carbon, and silicon nuclear magnetic resonance (1H, 13C, and 29Si NMR) spectra were recorded in deuterated solvent on a Bruker WP-300 (300 MHz for 1H NMR) and Bruker AM-500 (500 MHz for 1H NMR, 125.3 MHz for 13C NMR, and 99.36 MHz for 29Si NMR). The chemical shifts are reported in parts per million (ppm), relative to TMS (δ, 0.00) or residual CHCl3 (δ, 7.24) and benzene (δ, 7.14). Molecular weight was measured by gel permeation chromatography using a Waters Associates Model 6000A liquid chromatograph equipped with three American Polymer Standards Corp. Ultrastyragel columns in series with porosity indices of 103, 104, and 105 Å and THF as eluent. The polymer was detected with a Waters Model 440 ultraviolet absorbance detector at a wavelength of 254 nm, and the data were manipulated using a Waters Model 745 data module. Molecular weight was determined relative to calibration from polystyrene standards. Films for X-ray diffraction of polymers were cast from a toluene solution and dried in a vacuum for 24 h. Measurement was done on a Nicolet I2/V diffractometer, with a Cu KR beam. Absorption spectra were measured with a Hewlett-Packard 8452A diode array spectrophotometer. Fluorescence emission and excitation spectra were recorded on a Hitachi Model F-4500 fluorescence spectrophotometer. DSC measurements were performed with a Perkin-Elmer DSC 7 differential scanning calorimeter with the 3700 data station using 4 mg of sample in a dry helium atmosphere. All reactions were performed under N2 or Ar atmosphere. Monomers 1,2-diethynyl-1,1,2,2-tetrabutyldisilane,12 1,4diethynylbenzene,19 1,4-diiodo-2,5-bis(n-hexyloxy)benzene, and 1,4-diiodo-2,5-bis(n-dodecyloxy)benzene20 were synthesized by following the procedures described in the literature. Synthesis. The synthesis of polymer 112C12 is described as a typical example. In a 250 mL three-necked flask equipped with condenser and magnetic stirring bar, 1,2-diethynyl1,1,2,2-tetra-n-butyldisilane (0.326 g, 9.74 × 10-4 mol), 1,4-

diethynylbenzene (0.123 g, 9.74 × 10-4 mol), 2,5-diiodo-1,4bis(dodecyloxy)benzene (1.361 g, 1.948 × 10-3 mol), NEt3 (10 mL), toluene (100 mL), and catalytic amounts of CuI, Pd(PPh3)2Cl2, and PPh3 were added. The mixture was stirred for 24 h, at room temperature, then warmed to 40 °C for 24 h and refluxed for 24 h. After the reaction mixture cooled to the room temperature, the salt precipitate was removed by filtration, and the solvent was pumped off. The residue was dissolved in THF, and the solution was passed through a silica gel chromatographic column. The solvent was removed, and the resulting material was dissolved in 40 mL of toluene. The toluene solution was added dropwise into 250 mL of methanol with stirring. The precipitate was collected and dried under vacuum for 3 days: 1.32 g (85% yield) of polymer 112C12 was obtained.

Acknowledgment. The authors acknowledge the financial support of the U.S Office of Naval Research. Supporting Information Available: Figures showing X-ray diffraction patterns for polymers with C6 side chains and the excitation dependence of the emission spectrum of polymer 101C6 in THF (4 pages). Ordering information is given on any current masthead page.

References and Notes (1) (a) Yu, L.; Dalton, L. R.; Macromolecules 1990, 23, 3439. (b) Agrawal, A. K.; Jenekhe, S. A.; Vanherzeele, H.; Meth, J. S. J. Phys. Chem. 1992, 96, 2837. (c) Kido, J.; Hayase, H.; Hongawa, K.; Nagai, K.; Okuyama, K. Appl. Phys. Lett. 1994, 65, 2124. (2) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Adv. Mater. 1992, 4, 36. (3) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (b) Braun, D.; Heeger, A. J.; Kroemer, H. J. Electron. Mater. 1991, 20, 945. (4) (a) Rutherford, D. R.; Stille, J. K.; Elliott, C. M.; Reichert, V. R. Macromolecules 1992, 25, 2294. (b) Sasabe, H.; Wada, T.; Hosoda, H.; Ohkawa, H.; Yamada, A.; Garito, A. F. Mol. Cryst. Liq. Cryst. 1990, 189, 155. (5) (a) Ohmori, Y.; Uchida, M.; Muro, K.; Yoshino, K. Jpn. J. Appl. Phys, 1991, 30, l1938. (b) Braun, D.; Gustafsson, G.; McBranch, D.; Heeger, A. J. J. Appl. Phys. 1992, 72, 564. (c) Greenham, N. C.; Brown, A. R.; Bradley, D. D. C.; Friend, R. H. Synth. Met. 1993, 55-57, 4143. (6) Wautelet, P.; Moroni, M.; Oswald, L.; Le Moigne, J.; Pham, A.; Bigot, J. Y.; Luzzati, S. Macromolecules 1996, 29, 446. (7) (a) Swager, T. M.; Gil, C. T.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886. (b) Moroni, M.; Le Moigne, J.; Luzzati, S. Macromolecules 1994, 27, 562. (8) Osaheni, J. A.; Jenekhe, S. A. J. Am. Chem. Soc. 1995, 117, 7389. (9) Miller, R. D.; Michl, J. Chem. Rev. 1989, 89, 1359. (10) Yuan, C.; West, R. Appl. Organomet. Chem. 1994, 8, 423. (11) (a) Li. H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31, 52. (b) Li, H.; Powell, D. R.; Firman, T. K.; West, R. Macromolecules, in press. (12) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press: New York, 1990. (13) (a) Osaheni, J. A.; Jenekhe, S. A. Chem. Mater. 1994, 6, 1906. (b) Osaheni, J. A.; Jenekhe, S. A. J. Bull. Am. Phys. Soc. 1994, 39 (1), 505. (c) Yang, C. J.; Jenekhe, S. A. Supramol. Sci. 1994, 1, 91. (14) Horn, K. A.; Grossman, R. B.; Thorne, J. R. G.; Whitenack, A. A. J. Am. Chem. Soc. 1989, 111, 4809. (15) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157. (16) Jenekhe, S. A.; Osaheni, J. A. Science 1994, 265, 765. (17) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991. (18) CRC Handbook of Chemistry of Chemistry and Physics, 77th ed.; CRC Press: Boca Raton, FL, 1996-1997; pp 8-77. (19) Iwahara, T.; Hayase, S.; West, R. Macromolecules 1990, 23, 1298. (20) (a) Takahashi, S.; Kuroyama, Y.; Sonogashira, K.; Hagihara, N. Synthesis 1980, 627. (b) Hay, A. S. J. Org. Chem. 1960, 25, 637. (c) Nuyken, O.; Siebzehnru¨bl, F. Macromol. Chem. 1988, 189, 541.

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Structures and Photophysical Properties of Silicon-Containing

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