Autoxidation of Poly(hydrosilane)s

A thermolytic route to a polysilyne

We report a safe and convenient method to prepare a new class of network polysilane, or polysilyne ([RSi]_n). Simple thermolysis of a readily accessible linear poly(phenylsilane), [PhSiH]_n, affords polysilyne [PhSi]_n with concomitant evolution of monosilanes. This new polymer shows a hyperbranched structure with unique features not observed in known polysilynes prepared via hazardous Wurtz coupling routes. Despite these differences, our soluble, yellow polysilyne exhibits some important properties associated with the traditional random network structure: it absorbs up to 400 nm in the UV spectrum, yet is stable to photolysis under inert atmosphere. This efficient new synthetic route opens the door to exciting applications for these hyperbranched polymers in materials and device technologies.

A metal-free radical technique for cross-linking of polymethylhydrosiloxane or polymethylvinylsiloxane using AIBN

A new method was developed for the metal-free cross-linking of silicone rubbers. This process uses azobisisobutyronitrile (AIBN) to selectively react with Si-H and vinyl groups as a free-radical initiator for the thermal curing of polymethylhydrosiloxane (PMHS) and polymethylvinylsiloxane (PMVS). The AIBN-initiated curing reaction between the Si-H groups of PMHS generated Si-O-Si and Si-Si cross-links. In contrast, PMVS was cured via the formation of C-C bonds through "methyl-vinyl" and "vinyl-vinyl" mechanisms. Curing reactions were performed at 80-120 °C in air and confirmed by 13C and 29Si solid state NMR analyses and swelling trials.

Platinum-catalyzed reactions between Si-H groups as a new method for cross-linking of silicones

The platinum-catalyzed self-cross-linking of polymethylhydrosiloxane at RT in air was performed for the first time and proved by 1H, 13C, and 29Si SSNMR and swelling measurements. Quantum chemical modeling of possible structures was investigated. Platinum (0) and (ii) complexes were used as catalysts between the Si-H groups of polymethylhydrosiloxane. Karstedt's catalyst leads to Si-O-Si and Si-Si bond formation, but cis-[PtCl2(BnCN)2] generates predominantly Si-O-Si cross-links. cis-[PtCl2(BnCN)2] allows creating high-quality silicone rubbers without visible mechanical defects. This cross-linking approach can be used to obtain new Si-H-containing silicone materials.

Borane-catalysed postpolymerisation modification of the Si–H bonds in poly(phenylsilane)

An unusually wide variety of new sidechains are introduced at a polysilane through chemoselective catalytic hydrosilation, heterodehydrocoupling, and demethanative coupling.

Water-Dispersible Fluorescent Silicon Nanoparticles and their Optical Applications

Fluorescent silicon nanoparticles (SiNPs) attract considerable attention owing to their intrinsic advantages, including relatively strong fluorescence coupled with robust photostability, rich resource support and relatively low cost, industrial maturity, and good biocompatibility. Extensive efforts are devoted to developing effective methods for the synthesis of hydrogen or halogen-terminated SiNPs, which nevertheless need further surface modification to improve their stability and solubility for wide-ranging applications. Notably, recent years have witnessed the development of various aqueous synthetic strategies for direct preparation of highly fluorescent and photostable SiNPs in the aqueous phase, facilitating the promotion of this promising material for myriad optical applications. Here, a concise discussion of the latest exciting research progress of the preparation of SiNPs is given, with a focus on water-dispersible SiNPs synthesized in the aqueous phase. In addition, representative optical applications of SiNPs in bioimaging and sensing are also summarized. Finally, the opportunities and challenges of fluorescent-SiNP-based optical applications are discussed.

Sticky or Slippery Wetting: Network Formation Conditions Can Provide a One-Way Street for Water Flow on Platinum-cured Silicone

In the course of studies on Sylgard 184 (S-PDMS), we discovered strong effects on receding contact angles (CAs), θrec, while cure conditions have little effect on advancing CAs. Network formation at high temperatures resulted in high θadv of 115-120° and high θrec ≥ 80°. After network formation at low temperatures (≤25 °C), θadv was still high but θrec was 30-50°. Uncertainty about compositional effects on wetting behavior resulted in similar experiments with a model D(V)D(H) silicone elastomer (Pt-PDMS) composed of a vinyl-terminated poly(dimethylsiloxane) (PDMS) base and a polymeric hydromethylsilane cross-linker. Again, network formation at high temperature (∼100 °C) resulted in high CAs, while low-temperature curing retained high advancing CAs but gave low receding CAs (θrec 30-50°). These changes in receding CAs translate to strong effects on water adhesion, wp, which is the actual work required to separate a liquid (water) from a surface: wp ∝ (1 + θrec). When the values θrec 84° for high-temperature and θrec 50° for low-temperature network formation are used, wp is ∼1.5 times higher for curing at low temperature. The origin of low receding contact angles was investigated by attenuated total reflectance IR spectroscopy. Absorptions for Si-OH hydrogen-bonded to water (3350 cm(-1)) were stronger for low- versus high-temperature curing. This result is attributed to faster hydrosilylation during curing at higher temperatures that consumes Si-H before autoxidation to Si-OH. Sharp bands at 3750 and 3690 cm(-1) due to isolated -Si-OH are more prominent for Pt-PDMS than those for S-PDMS, which may be due to an effect of functionalized nanofiller. To explore the impact of wp on water droplet flow, gradient coatings of S-PDMS and Pt-PDMS elastomers were prepared by coating a slide, maintaining opposite ends at high and low temperatures and thus forming a thermal gradient. When the slide was tilted, a droplet moved easily on the high-temperature end (slippery surface) but became pinned at the low-temperature end (sticky surface) and did not move when the slide was rotated 180°. The surface was therefore a "one-way street" for water droplet flow. Theory provides fundamental understanding for slippery/sticky behavior for gradient S-PDMS and Pt-PDMS coatings. A model for network formation is based on hydrosilylation at high temperature and condensation curing of Si-OH from autoxidation of Si-H at low temperatures. In summary, network formation conditions strongly affect receding contact angles and water adhesion for Sylgard 184 and the filler-free mimic Pt-PDMS. These findings suggest careful control of curing conditions is important to silicones used in microfluidic devices or as biomedical materials. Network-forming conditions also impact bulk mechanical properties for Sylgard 184, but the range that can be obtained has not been critically examined for specific applications.

Recent applications of the (TMS)3SiH radical-based reagent

This review article focuses on the recent applications of tris(trimethylsilyl)silane as a radical-based reagent in organic chemistry. Numerous examples of the successful use of (TMS)(3)SiH in radical reductions, hydrosilylation and consecutive radical reactions are given. The use of (TMS)(3)SiH allows reactions to be carried out under mild conditions with excellent yields of products and remarkable chemo-, regio-, and stereoselectivity. The strategic role of (TMS)(3)SiH in polymerization is underlined with emphasis on the photo-induced radical polymerization of olefins and photo-promoted cationic polymerization of epoxides.

Solution-processed silicon films and transistors

The use of solution processes-as opposed to conventional vacuum processes and vapour-phase deposition-for the fabrication of electronic devices has received considerable attention for a wide range of applications, with a view to reducing processing costs. In particular, the ability to print semiconductor devices using liquid-phase materials could prove essential for some envisaged applications, such as large-area flexible displays. Recent research in this area has largely been focused on organic semiconductors, some of which have mobilities comparable to that of amorphous silicon (a-Si); but issues of reliability remain. Solution processing of metal chalcogenide semiconductors to fabricate stable and high-performance transistors has also been reported. This class of materials is being explored as a possible substitute for silicon, given the complex and expensive manufacturing processes required to fabricate devices from the latter. However, if high-quality silicon films could be prepared by a solution process, this situation might change drastically. Here we demonstrate the solution processing of silicon thin-film transistors (TFTs) using a silane-based liquid precursor. Using this precursor, we have prepared polycrystalline silicon (poly-Si) films by both spin-coating and ink-jet printing, from which we fabricate TFTs with mobilities of 108 cm2 V(-1) s(-1) and 6.5 cm2 V(-1) s(-1), respectively. Although the processing conditions have yet to be optimized, these mobilities are already greater than those that have been achieved in solution-processed organic TFTs, and they exceed those of a-Si TFTs (< or = 1 cm2 V(-1) s(-1)).

Regioselective hydrosilylations of propiolate esters with tris(trimethylsilyl)silane

Lewis acid and substituent dependency on the regioselectivity of hydrosilylation of propiolate esters 1a-c with tris(trimethylsilyl)silane (2a) was found. The reaction of methyl and ethyl propiolate esters and 2a without Lewis acid and in the presence of EtAlCl2 and Et2AlCl gave beta-silicon-substituted Z-alkenes 3 selectively. On the other hand, reaction in the presence of AlCl3 in dichloromethane gave alpha-silicon-substituted alkenes 4. In the case of trifluoroethyl propiolate ester 1c, reaction with aluminum chloride-based Lewis acids gave alpha-silicon-substituted alkenes 4 exclusively. Two competitive mechanisms, free-radical and ionic, are proposed as the source of the complementary regioselectivity displayed in these reactions. A transition state of the radical-forming step was obtained computationally. The reaction of various reactive acetylene substrates and 2a without Lewis acid and without solvent at room temperature gave beta-silicon-substituted Z-alkenes 3 selectively.

Reactivity of tris(trimethylsilyl)silane toward diarylaminyl radicals

Absolute rate constants for the reaction of tri-tert-butylphenoxyl radical (ArO*) with (TMS)(3)SiH were measured spectrophotometrically in the temperature range 321-383 K. Rate constants for the hydrogen abstraction from (TMS)(3)SiH by diarylaminyl radicals of type (4-X-C(6)H(4))(2)N* were determined by using a method in which the corresponding amines catalyze the reaction of ArO* with (TMS)(3)SiH. At 364.2 K, rate constants are in the range of 2-50 M(-)(1) s(-)(1) for X = H, CH(3), CH(3)O, and Br, whereas the corresponding value for ArO* is 3 orders of magnitude lower. A common feature of these reactions is the low preexponential factor [log(A/M(-1)s(-1)) of 4.4 and 5.2 for ArO* and Ph(2)N*, respectively], which reflects high steric demand in the transition state. A semiempirical approach based on intersecting parabolas suggests that the observed reactivity is mainly related to the enthalpy of the reaction and allowed to estimate activation energies for the reaction of (4-X-C(6)H(4))(2)N* and ArO* radicals with a variety of silicon hydrides.