Materials Letters 63 (2009) 2032–2034

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Materials Letters

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Synthesis and investigation of the properties of a star-shaped polysilane

Ming Chen, Bo Zhou, Yijun Liao, Shiqiang Huang ⁎Faculty of Material Science and Engineering, Hubei University, Hubei Provincial Key Laboratory of Polymer Materials, 430062 Wuhan, China

⁎ Corresponding author. Tel.: +86 27 50865381.E-mail address: huangsq@hubu.edu.cn (S. Huang).

0167-577X/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.matlet.2009.06.049

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 April 2009Accepted 18 June 2009Available online 26 June 2009

Keywords:Star-shaped polysilaneBranched polysilaneFluorescenceUV absorption

The intermediate monomer of a star-shaped polymer's cyclosiloxane core, was prepared from CH3HSiCl2 and2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane(D4

vi) through the basic method of hydrosilylationreaction. Then, a star-shaped polysilane [poly(core-methylphenyl-diphenyl)silane] was synthesized by theclassical Wurtz coupling reaction. Futhermore, UV absorption, fluorescent spectrum, UV-resistant stabilityand thermal properties of the star-shaped polysilane were investigated. Results reveal that star-shapedpolysilane possesses good UV-resistant stability, favourable thermo-oxidative stability and strongfluorescence emitting near 400 nm.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Polysilanes consisting of a Si–Si main chain and organic side chainare of current interest because of their characteristic σ electrondelocalization along the Si chain [1,2]. The interest resides in thedelocalization of the σ-electrons over Si backbone providing electro-nically analogous properties to the π-conjugated polymers. Generally,polysilanes can strongly absorb UV light at 300–400 nm, whichoriginates from the σ–σ⁎ transition. The σ–σ⁎ transition requires alower vibration energy, as a result of lower electron density and dipole[3]. Therefore polysilane backbone is a chromophore itself and can beespecially suitable for a blue emission. This special characteristic givespolysilane unusual electronic and photo physical properties, as well astheir potential application especially in light emitting diodes (LED)and non-linear optical materials [2]. Besides, in order to realizeluminescence from devices based on polysilanes, the investigation ofperformance of polysilanes is necessary, because polysilanes areknown to be photo-reactive and thermo-reactive [4].

The opto-electronic properties of polysilanes are further tuned bytheir configurations of Si skeletons and the kind of attached pendantgroups, which influence the σ-conjugation system of polysilanes. Atpresent, lots of researchers have focused their research on linearpolysilanes bearing different groups, such as aryl, alkyl, alkoxy, amideand ester groups [5]. Recently, however, more and more attention hasbeen directed to polysilanes with various kinds of silicon skeleton, suchas branched polysilane [poly(silylene-co-silyne)], network polysilane(polysilyne), ladder polysilane, organosilicone nanocluster and poly-silane dendrimer [6]. But the correlative study on the properties of star-shaped polysilanes has rarely been reported, and its properties arepartially different from linear and branched polysilanes because of its

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unusual structure. In our work, we first prepared the core ofcyclosiloxane containing eight Cl atoms by hydrosilylation reactionand then synthesized the copolymer poly(core-methylphenyl-diphe-nyl)silane by Wurtz-type reductive coupling reaction. Moreover, theproperties of star-shaped polysilanewere discussed and contrastedwiththat of linear and branched polysilanes [7].

2. Experimental

2.1. General information

CH3HSiCl2 was purchased from JiangXi Xinghuo Chemical Works;MePhSiCl2 and Ph2SiCl2 were purchased from Dalian YuanyongOrganosilicon Plant; D4

vi was supplied by Hubei Wuhan UniversitySilicone New Material Company; chloroplatinic acid, toluene, iso-propanol, THF, methanol and sodium were provided by SinopharmChemical Reagent Company. Chlorosilanes were distilled in a streamof inert gas under reduced pressure; solvent toluene was dried oversodium for 24 h and distilled just before use.

Fourier Transform Infrared (FT-IR) spectra was collected onSpectrum One FT-IR spectrometer(Perkin-Elmer Cetus Instruments,Norwalk, CT); the polymer composition was analyzed by 1H NMR(Varian, I NOVA 600) in CDCl3, and thermogravimetric analysis (TGA)was carried out with a TGA Perkin-Elmer thermogravimetric apparatusat a heating rate of 10 °C/min in nitrogen. The flow rate of nitrogen is50 cm3min−1 and the scanning temperature ranges from0 °C to 800 °C.UV–vis spectra were obtained with a Japan RF-540 fluorescencespectrometer.

2.2. Synthesis of multifunctional core of cyclosiloxane

The core of cyclosiloxane was obtained through the addition of thevinyl to Si–H groups of CH3HSiCl2. According to the procedures

Fig. 1. UV-absorption spectrum of polysilane in toluene.

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outlined in Ref. [8], the 100 ml four-neck flask was first dried andpurged with dry nitrogen. Methylbenzene, D4

vi and chloroplatinic acid(catalyst)/isopropanol were then added to the flask in order, andCH3HSiCl2 was later added dropwise within an appropriate time.Then, the mixture was stirred for 4 h at 72 °C. After reaction, theproduct was purified by vacuum distillation, and the resulting yellowviscous liquid was collected.

2.3. Synthesis of star-shaped polysilane

The flask was dried and purged with dry nitrogen. 2.85 g sodiumwas dispersed in the boiling toluene under N2 in 30 min. Then,0.005 mol multifunctional core of cyclosiloxane, 0.02 mol MePhSiCl2,and 0.02 mol Ph2SiCl2 were added dropwise into the dispersionquickly in 13 min. The reacting mixture soon turned into blue purpleand was refluxed for 6 h under stirring, and then cooled to roomtemperature. The reaction was ended by quickly adding ethanol andwater to the mixture. We separated out the organic phase and thenwashed it until there were no Cl− ions. A viscous liquid andcomponents with low boiling points were obtained. The obtainedliquid was dissolved in 10 ml THF, and then 200 ml methanol wasadded to precipitate the polymers. The precipitates were dried in avacuum oven (0.01 MPa) at 50 °C for 6 h.

Fig. 2. Fluorescent spectrum of star-shaped polysilane. λex is 280 nm.

3. Results and discussion

3.1. Data of structure

Cyclosiloxane core IR (KBr) (cm−1): 2961.85 (C–H), 1406.36 (CH3),1268.74 (CH2), 537.40 (Si–Cl), 787.82 (Si–C), 1260.88 (Si–CH3), 1076.26(Si–O–Si). 1HNMR (DMSO-d6):δ:0.79(CH3), 0.14 (CH3 attached on thering), 1.0 (CH2), 0.65 (CH2 attached on the ring).

Polysilane IR (KBr) (cm−1): 3047 (Ph–H), 2960 (C–H),1427 (Si–Ph),780 (Si–CH3), 1069 (Si–O–Si), 781 (Si–C), 463 (Si–Si). 1HNMR (DMSO-d6): δ: 1.2–1.5 (SiCH2), 0–0.8 (CH3), 6.2–7.8 (Ph–H).

3.2. Fluorescent property of star-shaped polysilane

The UV absorption spectra of star-shaped polysilane in toluenesolution are given in Fig. 1. The absorption of polysilane consists of twomain peaks at about 276 and 338 nm, while the high-energy band andlow-energy band absorption are associated with π–π⁎ transition ofattached phenyl ring and σ–σ⁎ transition along the Si–Si chainrespectively [9]. Exact peak position depends upon factors such as thepolymer's molecular weight, the degree to which the extendedpolymer chain segment adopts an all-trans conformation, thesurrounding medium, and the nature of the side chain substituentsbound to the catenated silicon backbone [10]. It should be noted thatthe absorption spectra of star-shaped polysilane reflect its similarcharacteristic with linear polysilane bearing phenyl ring, but themaximal absorption wavelength and position of two peaks have a redshift also because of the increscent σ electron delocalization resultingfrom its increased chain branching. Fig. 2 displays fluorescenceemission spectrum of this polysilane in toluene at room temperature.

A new intense broad emission near 400 nm in near-UV light area isobserved, which indicates a promising nature toward light emittingapplication with lower excitation energy. This is the typical result dueto the influence of increased dimensions of Si skeletons attached onthe core or due to the backbone defects, most likely chain branchingintroduced by the presence of PhSiCl3 in the PhSiCl2 starting material[6,11]. Similar visible light emission has also been observed amongelectron acceptor-group substituted polysilanes, such as a new kind ofpolysilane, polymethyl-β-cyanoethylsilane [12]. Besides, anotherbroad emission near 367 nm was observed too, just as we can see inthe linear polysilane bearing alkyl and aryl [7]. This finding isencouraging and actually it has been firmly established that theemission spectra of star-shaped polysilane are characterized by thedual emission in the UV and near-UV regions and the large Stokes shiftbetween the absorption and emission spectra [13].

Fig. 3. UV spectra of star-shaped with different UV light irradiation. Black, red, green,and blue curves correspond to the irradiation times 0, 5, 15, 25 min respectively. (Forinterpretation of the references to colour in this figure legend, the reader is referred tothe web version of this article.)

Fig. 4. TG spectrum of star-shaped polysilane.

2034 M. Chen et al. / Materials Letters 63 (2009) 2032–2034

3.3. UV-resistant stability of star-shaped poysilane

The UV spectra of star-shaped polysilane with different irradiationtimes are shown in Fig. 3.

The absorption peak of polysilane around 276 nm and 338 nmundergoes a gradual change in absorption intensity but not in positionwith increased irradiation time. This property is analogous to thecommon branched polysilanes [7]. In contrast to linear polysilanes,the changeless position of emission peak in star-shaped polysilane canbe explained by the fact that the local photoscission in Si chaincouldn't affect the delocalization of the whole molecule because of itsstar-shaped structure. Another theory shows that the silyl radicals andsilylenes formed by a photo-scission of the network and branchedpolymers would have a tendency to recombine again before thereaction with oxygen because of the so-called cage effect [14].Moreover, in comparison with polysilanes containing exclusivelyaliphatic substituents, the presence of aryl groups in the polymer alsoenhances its stability, especially in solid state [15].This is probablybecause the ultraviolet absorption of polymer containing the Si–C6H4–

Si moiety, formed by a photo-induced rearrangement or the reactionof silyl radicals with phenyl groups, was observed in almost the sameultraviolet region of the original polysilanes [14,16].

3.4. Thermal stability of star-shaped polysilane

Fig. 4 shows the thermo-stability properties of star-shapedpolysilane investigated by TGA. The star-shaped polysilane wascharacterized with high-thermooxidative stability. One can observethat the sample has noweight loss from 0 °C to 300 °C or above 600 °C,and only a 35% weight loss between 300 °C and 600 °C. It is in

agreement with the fact that the activation energy Ea of a star-shapedpolysilane, just like common branched polysilane, is higher than thatof a linear polysilane [7]. Morever, the core of star-shaped polysilanealso contributes to its thermooxidative resistance because of theexistent Si–O–Si bonds. Furthermore, the high thermo-oxidativestability of the copolymer may be explained by the high resistanceto oxidation of phenyl groups and their inhibiting effect on theoxidation of methyl groups [15,17]. All these observations indicate thatstar-shaped polysilane has better thermal stability than linearpolysilane, and their effectiveness in increasing the production rateof SiC ceramics makes star-shaped polysilane a promising candidate inthe SiC ceramic industry.

4. Conclusion

We have reported a preliminary study on the preparation andoptical properties of a star-shaped polysilane. This polysilane has astrong UV absorption at 338 nm in toluene at room temperature and astrong fluorescence emission peak near 400 nm. Besides, it also hashigh thermo-oxidative stability and favorable UV-resistant stabilitymainly because of its special star-shaped structure. The resultsindicated a possibility of device application toward luminescencematerials.

Acknowledgement

The financial support from the HuBei Education DepartmentFundamental Key Research of Chain under Grant 020094110 is highlyappreciated.

References

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