Methylhydrosilyl Chemostructural Effects in Polyhydrosilanes Liviu Sacarescu,* ,† Angeliki Siokou, ‡ Gabriela Sacarescu, † Mihaela Simionescu, † and Ionel Mangalagiu § Inorganic Polymers Department, Institute of Macromolecular Chemistry “Petru Poni”, Aleea Gr. Ghica Voda 41A, 700487, Iasi, Romania, Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT), Stadiou Str. Platani Achaias, P.O. Box 1414, Rion, GR-26504, Patras, Greece, and Organic Chemistry Department, “Al. I. Cuza” UniVersity, Bd. Carol 11, 700506, Iasi, Romania ReceiVed August 16, 2007; ReVised Manuscript ReceiVed NoVember 9, 2007 ABSTRACT: The presence of the methylhydrosilyl segments within the poly(diphenylsilane) chain lead to important conformational changes and the formation of elemental silicon particles. To investigate these processes polyhydrosilanes with various contents of methylhydrosilyl groups were synthesized by homogeneous coupling of methyldichlorosilane with diphenyldichlorosilane. Spectral analysis techniques combined with atomic force and polarized light microscopy were used to elucidate the mechanism of the silicon particle formation within the polymeric matrix. Introduction In recent years, intense research has been dedicated to the modification of a wide variety of material properties simply by reducing the material domain size in order to obtain quantum non-negligible effects. Semiconductors have been the subject of the majority of this work, particular attention being paid to silicon. Silicon quantum size effects have been exploited to investigate properties such as photoluminescence, 1 melting and sintering, 2 band gap energy, 3 physical strength of derivative ceramics, 4 and phosphorescence. 5 Due to the importance of silicon in modern technology, modifications of its properties have a major impact on leading industrial sectors including electronics, aerospace, computers, energy, and sensors. Polysilanes are well-known merely as precursors for silicon carbide 6 and their exploitation as a possible source for opto- electronic materials is just beginning. Research in this field proved that UV irradiation or thermal processing of polysilanes under certain conditions, leads to a slight enrichment in elemental silicon especially when the material is deposited as a thin layer. 7,8 This property could be useful as a different approach to obtain polycrystalline silicon structures and layers by taking advantage of specially designed highly reactive soluble oligo- and polysilanes. This work presents the chemostructural effects induced by methylhydrosilyl groups enclosed within a poly(diphenylsilane) chain. It is shown that this specific structure is capable of producing small particles of elemental silicon with controllable dimension and possesses a long-ordered helical conformation with extended σ-electron delocalization. Discussion Polyhydrosilane structures with various reactive methylhy- drosilyl contents studied within this work were prepared by low- temperature homogeneous Wurtz coupling of diphenyldichlo- rosilane with controlled amounts of methyldichlorosilane. The homogeneous reaction system was obtained using THF solutions of crown ethers sodium metal complexes. The corresponding specific mechanism ensures a high molecular weight monomodal distribution avoiding destruction of the Si-H reactivity through side reactions 9 (Supporting Information Scheme S1). The composition of the resulted poly[diphenylsilane-co-methyl(H)- silane] (PDPHS) was controlled by selection of the CH 3 HSiCl 2 / (C 6 H 5 ) 2 SiCl 2 monomers ratios as 1/1 (a-PDPHS), 1/7 (b- PDPHS), and 1/20 (c-PDPHS). 9 To obtain comparative results, a poly(methylphenylsilane) homopolymer (PMPS) was prepared by the same procedure. The spectral characterization showed the specific details related to the synthesized polyhydrosilanes and confirms their chemical structures. Besides their functionality, the Si-H groups within the poly(diphenylsilane) backbone induced some intrigu- ing effects. Conformational Effects of the Methylhydrosilyl Segments. The conformational effects of the methylhydrosilyl fragments appeared when polysilanes were investigated by fluorescence spectroscopy (FL). First, it was observed that the FL spectral profile of PMPS with a full width at half-maximum, fwhm ) 25 nm, is very different from the mirror image of the absorption band at 340 nm indicating that the stiff helical chain conforma- tion of PMPS has frequent irregularities (Figure 1). 10 In contrast, a-PDPHS shows relatively narrow UV absorptions with fwhm ) 15 nm, and the FL spectral profile is closer to the mirror image of the 350 nm absorption band. This is somehow surprising because the fragmentation of the polysilane chain due to the high content of small methyl(H)silyl segments should produce a UV absorption maximum shift to lower wavelengths, widening of the bands and strong irregularities in the FL spectra. 10,11 The absence of such effects indicates that the long diphenylsilyl segments with a stiff and regular helical global conformation 11 are coupled through trans-planar flexible fragments which eliminate the internal conformational tensions inducing a highly ordered polymeric structure. The same conclusion could be drawn from the powder X-ray diffraction (XRD) analysis (Figure 2a). † Inorganic Polymers Department, Institute of Macromolecular Chemistry “Petru Poni”. ‡ Foundation of Research and Technology Hellas, Institute of Chemical Engineering and High Temperature Chemical Processes (FORTH/ICE-HT). § Organic Chemistry Department, “Al. I. Cuza” University. * Corresponding author. Telephone: +40 232 217454. Fax: +40 232 211299. E-mail: livius@icmpp.ro. 1019 Macromolecules 2008, 41, 1019-1024 10.1021/ma071853f CCC: $40.75 © 2008 American Chemical Society Published on Web 01/03/2008