Synthesis and Characterization of Fluorescent Polymer−Metal Nanocomposites Comprising Poly(silylene-co-silyne)s and Silver Nanoparticles

Branched organosilicon polymers of composition [(PhMeSi)x-co-(RSi)1-x]n [ x = 0.83−0.75, R = Et3SiCH2CH2 (1), 2-FuMe2SiCH2CH2 (Fu = Furyl, 2), n-Hex (3), Ph (4)] have been synthesized by reacting a mixture of dichloromethylphenylsilane and an appropriate trichloroorganosilane, RSiCl3 in 5:1 molar ratio with sodium dispersion in refluxing toluene (Wurtz coupling). A change in the feed ratio of Et3SiCH2CH2SiCl3 and PhMeSiCl2 monomers to 1:1 affords the polymer, [(PhMeSi)0.55-co-(Et3SiCH2CH2Si)0.45]n, 5, with increased domains of branching sites while the synthesis of polysilyne [Et3SiCH2CH2Si]n, 6, involves the corresponding organotrichlorosilane as the precursor. The polysilanes, 14 act as excellent precursors for the reduction of silver acetate [polymer repeat unit: Ag+ = 0.10, 0.25] under mild conditions (toluene, room temperature) and afford the formation of polymer−metal nanocomposites, 1a4a and 1b4b containing silver nanoparticles with an average diameter of 5−7 and 8−11 nm, respectively. The optical properties and fluorescent nature of these composites as green light emitters have been studied by UV−vis and photoluminescence (PL) spectroscopy as well as confocal microscopy. The results accord the formation of silver nanoclusters in conjunction with nanoparticles. The host polymer matrix devoid of silver nanoparticles is obtained in each case by addition of a donor solvent (THF) in the composites and subsequently characterized by GPC, PL and cyclic voltammetric studies. A substantial decrease in molecular weight (Mw = 4.8−10.4 × 103) with respect to that of the corresponding parent polysilane (Mw = 9.7−21.0 x103) suggests irreversible oxidative cleavage of the skeletal backbone. The results also provide a basis to conclude that reducing action of 14 toward Ag(I) ions is predominantly located in the polysilyne segments leading to partial inclusion of Si−O−Si bonds in the host polymer structures. The role of polymer composition on the optical properties of silver nanoparticles has been examined by studying the nanocomposites 5a and 6a derived from the polymers, 5 and 6 respectively. The results suggest that branched polysilanes, 15 are unique in acting as scaffolds for fluorescent silver nanoclusters.