Synthesis of Hybrid Inorganic-Organic Sol-Gel Coatings for Optics
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ABSTRACT The synthesis of transparent hybrid organic-inorganic coatings obtained from condensation between alkoxysilane containing grafted dyes (azo dyes or phthalocyanines) and metal alkoxides precursors has been performed. The Red 17 azo dye, known for its high second order polarisability, was bonded to the silicon oxide network following two procedures: a silylation reaction between Red 17 and triethoxysilane; a coupling reaction between Red 17 and 3isocyanatopropyltriethoxy silane. Silicon phthalocyanines (known for their third order polarisability and inverse saturable absorption properties) were incorporated into the silicon oxide network by a silylation reaction. Phthalocyanine doped transparent hybrid organic-inorganic coatings were obtained from condensation between diethoxymethylsilane and zirconium propoxide precursors. INTRODUCTION The various characteristics of the sol-gel process (alkoxide precursors, organic solvents, low processing temperatures) allow one to introduce "fragile" organic molecules inside an inorganic network[I-4]. This chemistry generates hybrid organic-inorganic materials which are nanocomposites extremely versatile in their composition, processing and optical and mechanical properties. Since the pioneering work performed by Avnir et al [1], many organic dyes, have been incorporated into silica, aluminosilicates or transition metal oxide based matrices [1,3,5-15]. These dyes can play an important role for many applications in the field of optics [15-171. Azo dyes have been widely used to prepare materials for second order nonlinear optics [1820,14] because of their large optical nonlinearities and their electro-optic properties. Second order nonlinearities require noncentrosymmetric alignment of the NLO chromophores. It has been demonstated that chromophore alignment in sol-gel matrices can be performed by using electrical field poling processes [21,22]. However when the dye is simply embedded in the sol-gel matrix, in the absence of electrical field, relaxation of the dipole alignment to a random configuration is generally observed [22]. The chemical bonding of the chromophore to the polymeric backbone leads to a better orientational stability of the poled NLO dye and higher concentrations of organic dye in the sol-gel film [22,23,24]. Phthalocyanines are also known for their non-linear properties: They are efficient reverse saturable absorbers and have a high cubic susceptibility [25]. Aluminium [26], silicium, germanium, and tin [27] metallophthalocyanines had been demonstrated to be good inverse saturable absorbers in solution. Moreover, these molecules embedded in sol-gel matrices retain their properties [27,28]. However, solubilities of phthalocyanines dyes in organic solvants and sol-gel matrices are low, typically about 10-5 to 10-6 mol/l. Then, if processed as coatings these materials may not have sufficient absorption due to the low concentration of the active molecules. The grafting of dyes on to sol-gel matrices could provide the opportunity to solve this problem. This pa
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