Molecular design of hybrid organic-inorganic materials

Hybrid organic-inorganic materials offer a wide range of interesting applications especially in optics. Chemistry plays a major role in the elaboration and comprehension of such materials. Sol-gel process provide a method to blend, down to the molecular scale, organic and inorganic components. Examples of hybrid organic-inorganic materials for optics and optical characterizations of hybrid materials are given here. 1 INTRODUCTION The interest for sol-gel materials is not only related to their physical and chemical properties, but also to the wide possibilities of materials processing offer by the colloidal state. The viscosity of sols easily allows to prepare films by dip-coating, spraying or spin coating [I]. The chemistry involved in sol-gel process is based on inorganic polymerization [2,3]. In the organic route, precursors are usually metallo-organic compounds such as alkoxides : M(OR)n (M = Si, Ti, Zr, Al, ..., OR = OCnH2n+l). The initiation step of the polymerization corresponds to hydrolysis reactions where water molecules form reactive M-OH groups and alcohol molecules are released in the solution. The propagation step corresponds to polycondensation reactions which yield, by eliminating water or alcohol, 0x0 bridges (M-0-M) or hydroxo bridges (M-OH-M) from hydroxo and non hydrolysed alkoxo groups. An 0x0-hydroxo polymeric network is thus formed. The various characteristics of sol-gel process (metallo-organic precursors, organic solvents, low processing temperatures) allow to introduce "fragile" organic molecules inside an inorganic network. Such organic moIecules can improve the characteristics of the matrices. One can mention as examples : modification of the mechanical properties [4], easier processing of films and fibers, near shape moulding of various pieces for integrated optics [Sl, porosity control and adjustment of the hydrophilicfhydrophobic balance [6]. Moreover, the organic compounds may conmbute to a peculiar physical or chemical property (optical [7-101 or electrical [11] properties, electrochemical reactions [12], chemical or biochemical reactivity [13-171). On the other hand, the inorganic part of the material conmbutes to its mechanical and thermal strength 141, allows to modulate the optical index [18] and may lead to interesting electrochemical, electrical or magnetic properties. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:19937207 1350 JOURNAL DE PHYSIQUE IV This field of materials research mainly rises from chemists skills and demonstrates the major role played by chemistry in advanced materials. Sol-gel process has allowed to synthesise new hybrid organic-inorganic polymers which are composites on a molecular scale and exhibit novel properties. This paper deals with hybrid materials for optics elaborated by sol-gel process. The examples presented here mainly concern luminescence, non linear optical properties and photochromism. 2 MATERIALS FOR OPTICS The most commonly used precursors for hybrid materials are organically modified silicon alkoxides, SiRx(OR')4-,. The main reason comes from the stability of the Si-C bond, towards hydrolysis, which provides an easy way to introduce the organic part. In this area, we are involved in the synthesis of mixed siloxane-transition metal oxide materials (M = Ti, Zr, Al,.. . .). We are looking for properties such as easy film processing, transparency and adjustment of the hydrophobic/hydrophilic balance within the scope of matrices for optical applications [ZO]. Materials made from siloxane chains cross linked by transition metal 0x0-polymers yield hydrophobic films whose thickness can be easily adjusted from 1 to 50 yn. Various organic molecules can be incorporated or grafted in such films. The following examples illustrates the work currently under investigation. Absorption and emission properties Laser dyes such as Rhodamines 6G, Sulphorhodamine and Coumarine 4 (UV-Visible region) or HITC (near infra-red region) have been incorporated in hybrid organic-inorganic films prepared by hydrolysing and co-condensing diethoxydimethylsilane (DEDMS) and transition metal alkoxides (Zr, Ti, Al). The stability of R6G and C 4 dyes is good for most of the matrices [9,19]. Moreover, even for high concentrations (0.03 M) rhodamines have a niilch lower tendency to aggregate [10,20] (Fig.1). Emission spectra can be easily observed and Laser effect has been evidenced for some of these systems. R6G dyes exhibit, under irradiation, a better stability in such matrices than when included in organic polymers [9]. These results show that these materials are promising candidates for solid dye Lasers. The inorganic part also allows to incorporate various rare-earth's ( ~ d 3 + , Sm3+, ~ y 3 + , Er3+, ~ m 3 + ) in these hybrid polysiloxane-wansition metal 0x0-polymers materials which can be easily processed into films [21]. All these doped materials exhibit luminescence. Even neodymium fluorescence, usually quenched by the OH groups which remain in non-heated sol-gel systems, is clearly observed in these hybrid matrices (Fig.2). This effect is probably related to the hydrophobic nature of the polydimethylsiloxane chains which facilitate the elimination of residual hydroxyl