J O U R N A L O F C H E M I S T R Y Materials Feature Article Molecular design of hybrid organic–inorganic nanocomposites synthesized via sol–gel chemistry† C. Sanchez, F. Ribot and B. Lebeau Chimie de la Matie`re Condense�e, UMR CNRS 7574, Universite� Pierre et Marie Curie 4, place Jussieu, 75252 Paris, France. E-mail: clems@ccr.jussieu.fr Received 12th May 1998, Accepted 16th July 1998 The design, synthesis and some optical properties of hybrid as organic polymerization and lead to hybrid organic–inororganic –inorganic nanocomposites materials are pre- ganic copolymers.6–8 This article reviews some of the previous sented.The properties that can be expected for such work we have performed on the design and synthesis of hybrid materials depend on the chemical nature of their compo- organic–inorganic nanocomposites in which organics are nents, but they also depend on the synergy of these simply embedded or chemically bonded to the inorganic components.Thus, the interface in these nanocomposites gel network. is of paramount significance and one key point of their synthesis is the control of this interface. These nanocom- 2 Siloxane based hybrid materials posites can be obtained by hydrolysis and condensation reactions of organically functionalized alkoxide precur- Organic groups can be bonded to an inorganic network as sors.Striking examples of hybrids made from modified network modifiers or network formers. Both functions have silicon, tin and transition metal alkoxides are presented. been achieved in the so-called ORMOSILS.3,9 The precursors Some optical properties (photochromic, luminescence, of these compounds are organo-substituted silicic acid esters NLO) of siloxane based hybrids are also discussed.of general formula R¾nSi(OR)4-n, where R¾ can be any organofunctional group. If R¾ is a simple non hydrolyzable group 1 Introduction bonded to silicon through a SiMC bond, it will have a network modifying eVect (SiMCH3). On the other hand, if R¾ can react Sol–gel chemistry is based on the polymerization of molecular with itself (R¾ contains a methacryl group for example) or precursors such as metal alkoxides M(OR)n.1,2 Hydrolysis and additional components, it will act as a network former.3c condensation of these alkoxides lead to the formation of metal Network modifiers and network formers can also introduce oxo-polymers.The mild characteristics oVered by the sol–gel other physical properties (mechanical, hydrophobic, electro- process allow the introduction of organic molecules inside an chemical, optical, etc.). Several examples related to some inorganic network.3 Inorganic and organic components can optical properties of siloxane based hybrids will be now then be mixed at the nanometric scale, in virtually any ratio described.leading to so-called hybrid organic–inorganic nanocomposites. 4 These hybrids are extremely versatile in their composi- 2-A Photochromic properties tion, processing and optical and mechanical properties.5 The nature of the interface between the organic and inorganic Spiropyrans and spirooxazines are two of the fascinating components has been used recently to classify these hybrids families of molecules exhibiting photochromic properties.into two diVerent classes.4h Class I corresponds to all the Upon irradiation, the colorless spiropyran or spirooxazine systems where there are no covalent or iono-covalent bonds undergoes heterolytic CMO ring cleavage, producing colored between the organic and inorganic components.In such mate- forms of merocyanines (Fig. 1). rials, the various components only exchange interactions such The merocyanines may interact with their environment, i.e. as van der Waals forces, hydrogen bondings or electrostatic solvent, matrix, etc. leading to diVerent photochromic forces. In contrast, in class II materials, some of the organic responses.Levy and Avnir10 first demonstrated the important and inorganic components are linked through strong chemical role played by the dye–matrix interactions on the photobonds (covalent or iono-covalent). Numerous hybrid organic– chromic response of spiropyrans. They studied the photochroinorganic materials have been developed in the past few years. mism of spiropyrans trapped in sol–gel matrices synthesized This development yields many interesting new materials, with via polymerization of Si(OCH3)4 or RSi(OEt)3 (R=ethyl, mechanical properties tunable between those of glasses and those of polymers, with improved optical properties (eYciency, stability, new sensors, etc.) and with improved catalytic or membrane based properties.4g This field of materials research mainly arises from chemists’ skills and demonstrates the major role played by chemistry in advanced materials.Siloxane based hybrids4 can be easily synthesized because SiMCsp3 bonds are rather covalent and therefore they are not broken upon hydrolysis. Similar chemistry can be developed from tin alkoxides. This is not aplicable to transition metals for which the more ionic MMC bond is easily cleaved by water; complexing organic ligands must be used.Such groups can be functionalized for any kind of organic reactions such †Basis of the presentation given at Materials Chemistry Discussion NO NO2 SP N O N SO N O– NO2 N N O hn1 hn2, D hn1 hn2, D + Fig. 1 Molecular structures of SO and SP photochromic dyes. No. 1, 24–26 September 1998, University of Bordeaux, France.J. Mater. Chem., 1999, 9, 35–44 35methyl, etc.) precursors, and observed two types of photochromic behavior. When the photochromic dye is trapped within a hydrophilic domain of the matrix (domain containing residual SiMOH groups), the open zwitterionic colored forms are probably stabilized through hydrogen bonding with the acidic silanol groups present at the pore surface.The result of this stabilization is the observation of the colored forms before irradiation. These colored forms can be bleached by irradiation in the visible range. This has been termed ‘reverse photochromism’. On the other hand spiropyran dyes embedded in a more hydrophobic hybrid network made by hydrolysis of RSi(OEt)3 exhibit direct photochromism, i.e.the colorless form is stable without irradiation. Such photochromic behavior has been reported for many spiropyran or spirooxazine doped sol–gel matrices.10–17 Moreover, for hybrid organic–inorganic matrices containing diVerent chemical environments (hydrophilic and hydrophobic domains) a competition between direct and reverse photochromisms can be observed.17 However many fundamental questions still need to be considered.Little is Fig. 2 Cartoon of the D/Zrx matrices. known concerning the role of the photochromic dye–matrix interactions in the kinetics of coloration and thermal fading. As far as photochromic devices are concerned the tuning between strong and fast photochromic coloration (high DA) and very fast thermal fading is needed. Usually spiropyran or spirooxazine doped sol–gel matrices or even spirooxazine doped polymeric matrices exhibit slow thermal fading.10–18 The photochromic behavior of a spiropyran SP (6-nitro- 1¾,3¾,3¾-trimethylspiro{2H-1-benzopyran-2,2¾-indoline}) and a spirooxazine SO (1,3,3-trimethylspiro{indoline-2,3¾(3H)- naph(2,1-b)(1,4)oxazine}) (Fig. 1) embedded within two new hybrid matrices have been recently studied.19 Photochromic properties of the SP or SO doped D/Zr hybrid matrices.The first kind of matrix obtained through hydrolysis and condensation of (CH3)2Si(OC2H5)2 (D) and appropriate amounts of Zr(OPrn)4 (Zr) is labelled D/Zrx, where Zr stands for the zirconium, x for the molar amount of zirconium. Fig. 3 Photocolouration and photobleaching of SP doped D/Zrx These D/Zrx matrices are hybrid nanocomposites made matrices19 [(a) x=10; (b) x=20; (c) x=30].from polydimethylsiloxane species (chains, cycles etc.) crosslinked by zirconium oxopolymers.20–22 The zirconium oxopolymers are hydrophilic domains that still contain some photochromism is partially reversed and be balanced by tuning the D/Zr ratio. hydroxo groups coming from residual ethanol or ZrMOH ligands.22 The size and the spacing between the ZrO2 based The thermal bleaching behavior of the D/Zr20 samples was fitted with a biexponential equation.The SP doped materials domains is about a few nanometers as evidenced by SAXS.22 17O MAS NMR and FTIR experiments show that inside these exhibited a very long bleaching time (about 24 h) while for the SO doped D/Zr20 materials the thermal fading was composites the hydrophobic polydimethylsiloxane species are interfaced with the zirconium oxo domains both through much faster.19 The kinetic data of the SP or SO doped D/Zr20 samples covalent ZrMOMSi bonds and through weak interactions (hydrogen and van der Waals bonds).19 The structure of the are similar to those reported for other modified sol–gel matrices or in organic polymers.17,23 As in organic polymers, the D/Zrx matrices is shown schematically in Fig. 2. D/Zrx matrices doped with SP or SO are lightly colored (pink with bleaching follows a biexponential equation which can be explained by an inhomogeneous distribution of free volumes SP or blue with SO) before irradiation. However the absorbance (A) in the visible region is weak in comparison in the gel.Moreover the presence of diVerent stereoisomers (cis or trans) could also account for this behavior. The diVerent with the total amount of embedded photochromic dyes. The amount of colored form depends on the x content. isomer–matrix interactions could explain the diVerent kinetics observed for SO and SP. Fig. 3 shows the photochromic behavior of SP doped D/Zrx gels for three compositions of zirconium.When the Zr amount The thermal fading is longer for SP doped hybrids than for SO doped ones. This phenomenon can be correlated to the increases, the A variation due to the coloration decreases while that due to the decoloration increases: there are more and fact that SP open forms are known for their tendancy to form zwitterionic species, while non charged quinonic species are more open forms in the gel. The amount of colored form increases proportionally with the x content.It is much higher usually favored for open SO molecules. Zwitterionic species can be strongly stabilized by hydrogen bonding with the for D/Zr30 samples than for D/Zr10 ones. This indicates that before irradiation the SO and SP dyes matrix, thus lowering the decay times of thermal fading.are roughly split into two populations. The colored merocyanine open forms of SO and SP are stabilized by hydrogen Photochromic properties of the SP or SO doped DH/TH hybrid matrices. The second kind of SP or SO doped matrix bonding within the hydrophilic regions made from the zirconium oxopolymers, while the closed SO and SP forms are prepared from the hydrolysis and cocondensation of (CH3)HSi(OC2H5)2 (DH) and HSi(OC2H5)3 (TH) precursors probably located in the environment of the hydrophobic polydimethylsiloxane chains.So for these D/Zrx matrices the is labelled DH70/TH30 (70/30 refers to the molar composi- 36 J. Mater. Chem., 1999, 9, 35–44few minutes under magnetic stirring. A solution of mixed alkoxides, Zr(OPrn)4 and a corresponding rare earth (M= Nd3+, Sm3 +, Dy3 +, etc.) methoxy ethoxide was then added to the previous solution in order to obtain a Zr5Si molar ratio of 159 and M5Si molar ratios of 159; 0.159; 0.0159.The sols were deposited onto glass sheets and allowed to gel and dry at room temperature. Such transparent coatings were deposited several times till a thin layer of about 50–100 mm was obtained.25 FTIR, 1H MAS NMR and 29Si CP MAS NMR experiments have shown that these hybrids can be described as nanocom- Fig. 4 Schematic representation of the environment experienced by posites built from siloxane polymers crosslinked by metal oxo the SO dye within the matrix DH70/TH30. species. The metal oxo domains are made of mixed zirconium–rare earth oxo species.tion). These hybrids can be described as strongly interpen- Absorption–emission properties of these Nd3+ doped hybrid etrated nanocomposites. The environment experienced by the coatings. The room temperature absorption spectrum of Nd3+ SO dye within the DH70/TH30 matrix is shown in Fig. 4. doped hybrid coatings presented in ref. 25 consisted of some The SO or SP DH70/TH30 doped matrices exhibit normal broad transitions.The 4I9/2A4F5/2,2H9/2 transition around photochromism. All the samples are colorless before 800 nm, particularly important for diode pumping systems, irradiation. For the two photochromic dyes, the thermal fading presents a FWHMof around 15 nm. The absorption coeYcient can be fitted with excellent agreement with a monoexponential at this wavelength is around 10 cm-1 for a coating containing equation.This may be related to the quasi-liquid mobility approximately 4×1020 Nd3+ ions cm-3. This high absorption observed by NMR for this matrix. coeYcient value indicates that a high Nd3+ concentration The time dependence of the absorption upon repeated could be introduced into the system without aVecting the irradiation with 365 nm light for SO doped DH70/TH30 synthesis and the transparency of the films.coatings is reported in Fig. 5. The photochromic behavior is For Nd3+ ions excited at 805 nm, the emission spectra of reversible, extremely fast (the rate constant is 0.2 s-1) and the 4F3/2A4I11/2 transition at room temperature is presented corresponds to a very high absorption jump (DA=1.2).The in Fig. 6. This emission is broad with a FWHM around 40 nm photochromic kinetics of these SO doped hybrid materials are and extends into the NIR range from approximately 1.045 to to the best of our knowledge much faster than those reported 1.095 mm. A broad emission has also been observed for the for SO in any other matrix (sol–gel matrices, organic polymers, 4F3/2A4I9/2 transition.25 Broad emissions for Nd3+ are charac- alcohols, etc.).11,14,15,17,23 teristic of wide sites distribution around the neodymium ions The very high reactivity of the DH/TH precursors towards in this hybrid materials.Such inhomogeneous broadening is hydrolysis–condensation reactions (SiMOH groups are immealso observed in glassy hosts.27,28 Experimentally, one can diatly consumed) and the strong hydrophobicity of the notice that the Nd3+ fluorescence intensity is weaker than resulting matrix are both responsible for the direct and very those reported in glassy or crystalline matrices where the fast photochromic behavior observed.quantum eYciency is relatively high.29 2-B Luminescent properties of rare-earth doped hybrids Fluorescence kinetics of Nd3+ in hybrid siloxane based coat- Neodymium doped sol–gel matrices are suitable luminescent ings.Fig. 7 presents the variation of the fluorescence intensity materials. However room temperature sol–gel derived matrices decay profiles as a function of the Nd3+ content. Lifetimes usually contain a large amount of hydroxy groups which are decrease from 160 ms for 0.4×1020 Nd3+ ions cm-3 to responsible for the quenching of the Nd3+ emission.Therefore approximately 200 ns for 4×1021 Nd3+ ions cm-3. the design of sol–gel matrices inside which the hydroxy content It is quite unusual to observe radiative emission of Nd3+ can be minimized24,25 and/or inside which the rare earth is ions in matrices prepared at room temperature by sol–gel protected via complexation26 or encapsulation is desirable.processing. Usually the numerous hydroxy groups present in Recent work has demonstrated fluorescence emission for the classical xerogels obtained at room temperature prevent any Nd3+, Sm3 +, Dy3 +, Er3 + and Tm3+ ions doped in hybrid Nd3+ radiative emission.5d,5e,32 A thermal treatment at high siloxane–oxide matrices.25 temperatures is necessary to allow the Nd3+ emission to be These hybrids were synthesized by the following procedure. observed.In all our samples fluorescence is detected, however Diethoxymethylsilane [DEMS=SiH(OEt)2(CH3)], absolute ethanol and water in a 15151 molar ratio were mixed for a Fig. 6 RT Nd3+ fluorescence spectra.30 Fig. 5 Photochromic response of the SO doped DH70/TH30 matrix.18 J. Mater. Chem., 1999, 9, 35–44 37case, phonons can interact with the atomic transitions of Nd3+ and this coupling, analogous to weak electric dipole coupling, is enhanced when hydroxy groups are close to the neodymium ions.These interactions may explain the low Nd3+ quantum eYciency values and the weak emission observed for high Nd3+ concentration. All these results need of course to be improved, however they open many possibilities in the field of room temperature processed luminescent films.Energy transfer between an organic dye and Nd3+ as inorganic chromophore. Codoped rhodamine 6G–Nd3+ hybrid samples shows that the rhodamine emission spectra exhibit some dips at wavelengths corresponding to the Nd3+ absorption bands.31 This feature indicates that radiative energy transfer occurs between the organic dye and the Nd3+ ions.33–35 A photon emitted by the rhodamine molecule can be trapped by the rare-earth ions in the hybrid siloxane network.Furthermore, when excited by an argon laser at 488 nm, a wavelength where only the organic dye molecules absorb, the codoped (R6G–Nd3+) hybrid coating exhibits a Nd3+ emission around 1.06 mm. No emission was detected around 1.06 mm under the same experimental conditions without the Fig. 7 Variation of the Nd3+ fluorescence decay31 (Nd3+ content in presence of the organic dye inside the hybrid coating. This atom cm-3, (a) 0.4×10,20 (b) 4×1020 (c) 4×1021. behavior reveals that energy transfer mechanisms may be favorable for Nd3+ emission. the Nd3+ fluorescence decay profiles lead to lifetime values New Eu2+ doped hybrid organic–inorganic nanocomposites lower than those usually recorded for Nd3+ in high temperasynthesized at room temperature.The Eu2+ ion is particularly ture processed glassy or crystalline matrices. These hybrid unique because its broad luminescence band 4f65d1A4f7 is materials can be described as a siloxane polymeric network strongly host dependent with emission wavelengths extending made from (CH3SiO3/2) units crosslinked by mixed nanoaggrefrom the UV to the red range of the electromagnetic spec- gates made from zirconium oxo and neodymium oxo species.trum.36 Therefore, the luminescent properties of Eu2+-doped Depending on the Nd3+ concentration, two diVerent optical solids have been intensively studied during the past three features are observed in these hybrid coatings. Both are well decades.These studies have led to the use of these compounds correlated with the structure. (1) For low Nd3+ concentration, as phosphors, notably blue-emitting Eu2+5BaMgAl10O17 in the initial non-exponential part of the decay profiles vary lamp and plasma display panels and UV-emitting linearly on a root-mean-square time scale showing the strong Eu2+5SrB4O7 for medical applications and skin tanning.cross-relaxation phenomena. Even if a few nOMH vibrations Crystalline or glassy Eu2+ doped materials are usually pro- are observed in the infrared spectrum, these hydroxy groups cessed at relatively high temperatures.36–39 Moreover, the are not located close to the Nd3+ ions and at low concentration synthesis and the stabilization of europium in the divalent the main non-radiative de-excitation mechanisms are the state under mild synthetic conditions is not an easy task.For Nd3+–Nd3+ interactions. The fact that these nanocomposites the first time, we reported recently the room temperature are made with segregated metal oxo species (the dispersion is synthesis of new Eu2+ doped hybrid materials together with not statistical all over the sample) leads to Nd3+–Nd3+ their absorption and emission properties.40 These hybrids are interactions even for low rare earth concentrations.The slope obtained though the hydrolysis and condensation of diethoxy- measured at long times give an indication of the Nd3+ methylsilane (MDES), methyltriethoxysilane (TREOS) and fluorescence quantum yield in this hybrid coating.The ratio g zirconium tetrapropoxide precursors in the presence of of the experimental lifetime at low concentration over the europium trichloride. calculated radiative lifetime (g=0.35) indicates that approxi- Dehydrocondensation of organic hydrosilanes with silanols mately 35% of the relaxation occurs radiatively at low Nd3+ is one of the common methods for the synthesis of the siloxane concentration. This is a high value for a room temperature linkage.41 This reaction, which occurs with the evolution of sol–gel processed material and this is in agreement with the hydrogen gas, has been described as follows:41 fact that, at low concentration, Nd3+ ions form clusters at relatively long distances from the remaining hydroxy groups.OSiMH+HOMSiOCA catalystOSiMOMSiO+H2 (2) When the Nd3+ concentration increases, interactions increase as the Nd3+–Nd3+ distance becomes shorter in the nanoaggregates. Simultaneously when the neodymium concen- In this sense, alkoxide precursors containing SiMH groups show the possibility of using the SiMH groups as an in situ tration increases (the zirconium concentration remaining constant) the probability of finding neodymium ions at the surface reducing agent which allows the formation of metal/silica nanocomposites.42 of the metal oxo species increases, rendering the rare earth prone to interactions with some remaining hydroxy groups In our study,40 the in situ formation of hydrogen provided by the cleavage of the SiMH bonds was used to generate, located close to the surface.As a consequence, short lifetimes are observed. For a concentration of 4×1021 Nd3+ ions cm-3 during the first step of hydrolysis and condensation reactions, europium in the divalent state. the quantum yield is less than 0.01%. This behavior characterizes non-radiative de-excitation processes due to strong A typical absorption spectra (Fig. 8) of these is constituted by a broad absorption band in the UV range (200–400 nm) energy transfers between Nd3+ but also non-radiative deexcitation occurring via the filling of the 4F3/2–4I15/2 energy attributed to the 4f75d0A4f65d1 (Eu2+) transition. The emission spectra (Fig. 9) of the corresponding hybrids gap by nOMH vibrations. Usually, according to the energy gap law, multiphonon non-radiative contributions do not exceed recorded under excitation at 355 nm show a broad emission corresponding to the interconfigurational 4f65d1A4f75d0 50% taking into account the energy diVerence around 5500 cm-1 between the 4F3/2 and 4I15/2 levels. In the present transition centered at 430 nm (ca. 23250 cm-1) and a intracon- 38 J. Mater. Chem., 1999, 9, 35–44are only achieved in a non-centrosymmetric environment, we first demonstrated that orientation of organic chromophores can be performed in hybrid sol–gel matrices43–46 by using electrical field induced second harmonic (EFISH) or corona electrical field poling techniques. Organic molecules such as N-(3-triethoxysilylpropyl )-2,4-dinitrophenylamine (TSDP) were chemically bonded to the oxide backbone of gels.The chemical bonding of the dye to the sol–gel matrix allowed dye concentration to be increased without any crystallization eVects.43–46 A first generation of sol–gel matrices was synthesized by copolymerization of silicon alkoxysilanes [TSDP and SiHCH3(OEt)2] and zirconium propoxide precursors.45 The sols were deposited as transparent coatings and exhibited after corona poling an SHG response of 1.6 pm V-1.45 Even if in this first generation of sol–gel matrices relaxation of the organic chromophores occurred over several hours, these results suggested the feasibility of poling techniques into hybrid inorganic sol–gel matrices more ionic than classical polymers.Fig. 8 Absorption spectra of europium doped hybrid xerogels. Consequently a range of opportunities for the synthesis of optical sol–gel devices with eYcient second harmonic properties was discovered. Since then, there has been increasing interest in second order NLO materials synthesized via sol–gel chemistry.47–60 The optimization of the second order NLO response of hybrid sol–gel matrices with grafted chromophores is currently under investigation by several research groups.Several strategies are used to improve the NLO response of the hybrid coatings.43–60 (i) The intrinsic NLO response of the dye can be increase by using chromophores such as N-(4- nitrophenyl )-L-prolinol (NPP) or disperse red one (DR1) derivatives which exhibit higher non linearities than nitroaniline ones. (ii) The chromophore relaxation can be controlled by increasing the matrix rigidity.This point is without doubt the most important in order to be able to make eYcient NLO devices. The modification of the binary composition (siloxane –crosslinker), the nature of theM(OR)4 crosslinking alkoxide [SiR¾x(OR)4-x–M(OR)4: R¾=any NLO chromophore; M=Zr, Si, Ti, etc.), and the processing of these hybrid Fig. 9 Emission spectrum of europium doped hybrid xerogel (lexc= materials in the presence of polymers with well known mechan- 355 nm).ical properties such as methyl methacrylates or polyimides, are the most commonly used strategies to minimize dye figurational 4f–4f Eu3+ emission in the longer wavelengths. relaxation. The strategies we have used to improve the NLO Several bands are obtained corresponding to the 5D0A7F0,1,2,3 response of hybrid materials will be llustrated in the two transitions. A Stokes’ shift value of the Eu2+ luminescence following sections.around 9000 cm-1 is obtained, this shift between the absorption and emission energies of Eu2+ located in an oxygen ligand TSPD/TMOS based hybrids with NLO properties.58,59 The field has been assigned to a combination of crystal field and second generation of hybrids investigated were made via nephelauxetic eVects.36 These hybrid structures contain oxygen hydrolysis and co-condensation of tetramethoxysilane atoms in higher coordination number environments (highly (TMOS) and N-(3-triethoxysilylpropyl )-2,4-dinitrophenylam- coordinated by metal atoms m3-O–Zr/Eu or m4-O–Zr/Eu) ine (TSDP) [Fig. 10(a)] precursors (T and Q are common which produce Eu2+ emission at longer wavelengths.Moreover notations referring to the oxo trifunctional R¾MSiO3 and distortion of the oxygen polyhedra from ideal coordination tetrafunctional SiO4 central units, respectively). FTIR, 17O geometry results in a large Stokes’ shift. First measurements and 29Si NMR experiments indicated the existence of linear indicate a Eu2+5Eu3+ concentration ratio of about 551.The and cyclic siloxane (T–T)a oligomers and silica (Q–Q)b units high Eu2+ content is probably related to the more eYcient reductive medium provided by the initial mixture of the europium trichloride with the MDES and TREOS precursors. Moreover, the intensity of the Eu2+ luminescence did not change when the xerogels were stored in air for several months, showing that Eu2+ ions are eYciently trapped inside the hybrid matrix. 2-C Quadratic NLO properties of siloxane based hybrids Most of the sol–gel optics research devoted to non-linear optic (NLO) materials was initially related to third order processes which are compatible with the isotropy of amorphous sol–gel matrices. Organic molecules inside amorphous sol–gel matrices are in general randomly oriented thus ruling out the emission O2N NO2 NH Si(OEt)3 TSDP O2N N N N CH2CH2OCONH-(CH2)3Si(OEt)3 CH2CH2OCONH-(CH2)3Si(OEt)3 CH3 (a) (b) Fig. 10 Graftable NLO dyes: (a)=TSDP (b)=ICTES-Red17. of second harmonic generation. As second order non-linearities J. Mater. Chem., 1999, 9, 35–44 39linked through stable T–O–Q bridges formed in the early also strongly depend on their thermal history.Chemical crosslinking is not complete at gelation or even after RT air stages of the process. Films of thickness 1–5 mm were easily obtained through spin-coating. In such systems gelation prob- drying as shown by 29Si NMR experiments.59 Upon ageing and curing the chemical reactions continue towards com- ably occurs through the crosslinking of siloxane polymers with Q silica based species.The degrees of condensation of T and pletion. Consequently, given suYcient time and temperature to allow mobility of the species a network forming system Q units measured in the solid state by 29Si MAS NMR spectroscopy are much higher in xerogels than in sols and this continues to crosslink long after gelation. The increase of the density of crosslinks modifies the thermomechanical properties diVerence demonstrates that a large number of condensation and crosslinking reactions still occur upon solvent removal.of the hybrid as illustrated by the changes observed in Tg upon thermal curing.57 The mobility of the NLO chromophores, as observed by high-resolution solid-state 13C NMR spectroscopy, is also Another processing parameter which has great importance is the electrical field used to poled the NLO chromophores.correlated with the glass-transition phenomenon of the matrix observed by DSC.57 This glass transition phenomenon corre- Accelerated field induced curing must occur in these hybrid TSDP/TMOS materials. The high electrical field provided sponds to the glass transition of the polysiloxane network.Tg, the glass-transition temperature, increases with the TMOS during poling must favor crosslinking and interpenetration of both polymeric T and Q networks. This was described by content, while the apparent variation of heat capacity corresponding to Tg decreases. These results, as well as the analysis Haruvy and co-workers,48b who have shown that greatly accelerated curing occurs under ambient conditions on thin of the polarization transfer in MAS/CP/DD 29Si NMR experiments, are consistent with the relatively high degree of inter- films processed from siloxane resins prepared by the sol–gel process when they are exposed to an intense corona-discharge penetration of T and Q units.Therefore, these hybrid TSDP/TMOS coatings can be described as nanocomposites field.The corona cured sol–gel films exhibited a more compact matrix as manifested by the lower mobility of the embedded made of silica rich domains and siloxane rich domains. Many Q and T species are mutually sequestered at the nanometer chromophores and a more hydrophilic surface than thermally cured ones. They suggest that the field induced removal of scale.Their microstructure is schematically pictured in Fig. 11. The white parts correspond to the silica-rich phase inside condensate small molecules and solvents allows better completion of the reactions and more eYcient crosslinking.48 which some T units (black dots) are sequestered. The black spheres correspond to the polysiloxane rich phase which Compared to the tremendous amount of work and time devoted to polymeric NLO materials, NLO materials made participates in the glass-transition phenomenon and contains Q units (white dots). The sizes of the polysiloxane and silica by sol–gel are still in their infancy. For these system, depending on chemical composition, the SHG values range between 2.5 domains depend not only on the chemical composition but also on the drying procedure and consequently on the solvent and 10 pm V-1.47,57 Moreover, the sol–gel materials described in this work have Tg values in the range of 30–70 °C, well and sample thickness.The TSDP/TMOS ratio, proton concentration, hydrolysis below the state of the art obtained with pure organic polymeric materials based on polyimides61 which are highly non linear ratio, sequence of mixing the reagents and ageing time of the sol are the chemical parameters that should directly influence and stable for hundred of hours at temperatures higher than 100 °C.However, the excellent knowledge of such systems a and b values characterizing the lengh of the constituent linear and cyclic siloxane (T–T)a oligomers and silica (Q–Q)b allowed us within a short period of time to design a third generation of hybrids with a highly improved NLO response.units respectively. However, it has been demonstrated that the mechanical properties of hybrid siloxane–oxide materials, and thus the ICTES-Red17/TMOS based hybrids with NLO properties. The third generation of hybrid organic–inorganic nanocom- relaxation behavior of chromophores grafted in these matrices, posites was designed on the basis of the following specifications: the NLO dye must have a high NLO response, it must be anchored by more than one trifunctional link and silica was kept as the crosslinking agent because coatings of better optical quality were usually obtained with binary silica–siloxane materials.56 In order to be able to perform double grafting of an NLO chromophore, the Red 17 [4-(amino-N,N-diethanol )-2-methyl-4¾-nitroazobenzene] with a very eYcient quadratic hyperpolarizability [b(0) (Red 17)=55×10-30 esu] was functionalized with two alkoxysilyl groups by a coupling reaction between the dye and 3-isocyanatopropyltriethoxysilane (ICTES).56,58 The resulting alkoxysilyl functionalized NLO precursor, ICTES-Red 17 [Fig. 10(b)] was hydrolyzed and co-condensed with TMOS in order to obtain the hybrid siloxane–silica nanocomposite.From the resulting sols, coatings with a thickness of a few mm can be deposited. The resulting hybrid materials do not exhibit Tg according to DSC results. Non-resonant second-order non-linearities as high as 150–200 pm V-1,58,62 measured on these hybrid systems, with significant long-term stability (10% of signal lost after 20 days) have been reported.58 The thermal stability at 80 °C has been shown to be excellent, making the ICTES-Red 17/TMOS systems competitive candidates for non-linear optics systems.Chemical characterization (FTIR, 29Si MAS NMR, UV–VIS spectroscopy) and thermal assisted in situ poling studies performed on these coatings revealed the importance of the processing and history of these systems.Three param- Fig. 11 Schematic representation of TSDP-TMOS based hybrids.52 eters are of paramount importance. (i) Aging of the solution 40 J. Mater. Chem., 1999, 9, 35–44has been shown to greatly influence the amplitude of the final Moreover, these clusters exhibit a high versality for the design of hybrids (Fig. 13).{(RSn)12O14(OH)6}2+2X- can non-linear signal. This results from improvement of crosslinking eYciency and from modifications of the distribution be assembled through organic networks by using the covalent interface provided by the SnKC bond or by using the ionic between cyclic and linear siloxane species. (ii) Thermal precuring of the samples at 150 °C was found to markedly improve interface associated with the charge compensating anions X- or even by using both interfaces.In the first case the organic the non-linear response as well as its stability. (iii) Optical poling recently tested in sol–gel derived matrices can also be moiety carried through the SnKCsp3 links should be polymerizable (R=butenyl, propylmethacrylate, propylcrotonate, 4- used to improve the chromophore anisotropy63 These very reproducible results58,62 are very promising, styrylbutyl, etc.).In the second case charge compensating organic dianions must be able to bridge the clusters. This can however as far as NLO devices are concerned they must be completed by measurements of electro-optical eYciency, be performed by using dicarboxylates,65 or a,v-telechelic macromonomers terminated by carboxylic or sulfonic groups.67 waveguiding properties and the evaluation of the optical losses.As an example, the coupling of these clusters by carboxymethyl terminated PEG macromonomers67 is schematically shown 3 Tin oxo species based hybrid materials in Fig. 14. Another strategy could be to use polymerizable anions Tin is a very interesting element because its characteristics make it intermediate between silicon and the transition metals.(methacrylate, 2-acrylamido-2-methylpropane-1-sulfonate, etc.) as monomers for organic polymerizations reactions.66,68a Like the latter, tin exhibits several coordination numbers (generally from 4 to 6) and coordination expansion makes By a simple acid-base reaction, the oxo-hydroxo butyltin macrocation, {(BuSn)12O14(OH)6}2+, was functionalized hydrolysis–condensation reactions of tin alkoxides fast.But, as for silicon, the Sn–Csp3 bond is stable, especially towards with 2-acrylamido-2-methylpropane-1-sulfonate, aVording nanobuilding blocks with two highly polymerizable groups.68a nucleophilic agents such as water. This last characteristic allows one to chemically link organic moieties to the tin oxo For the first time,68a the direct polymerization of such functionalized oxo-hydroxo butyltin nanoclusters has been polymers/oligomers but it also reduces the inorganic functionality of tin and therefore favors the formation of oxo successfully performed, yielding hybrid materials in which the nanosized inorganic component is perfectly defined.Two types clusters.These oxo clusters can be used as nanobuilding blocks in the design of new hybrid materials.64–72 of organic components are found in such materials. The butyl groups covalently bound onto tin atoms, and, more import- The nanobuilding block [(RSn)12(m3-O)14(m-OH)6]2+, the structure of which is shown in Fig. 12 can be obtained through antly, poly(2-acrylamido-2-methylpropane-1-sulfonate) chains which interact through electrostatic interactions with the several chemical pathways: hydrolysis of RSn(OPri)3 or RSnCl3 or by refluxing in toluene butyltin hydroxide oxide oxo-hydroxo butyltin macrocations and aVord the crosslinking. [BuSnO(OH)] in the presence of sulfonic acids (R¾SO3H)64–71 and more recently Jousseamme et al.opened a new route to Such an approach to the construction of tin-based hybrid materials from bifunctional nanobuilding blocks was pre- this cluster through hydrolysis of functionalized trialkynylorganotin precursors.72 viously attempted with pure {(BuSn)12O14(OH)6}- {O2CC(CH3)NCH2}2 but failed as its homopolymerization This compound is made of a tin oxo-hydroxo cluster with a equal numbers of six- and five-coordinate tin atoms.This appeared impossible.66 Addition of a co-monomer [CH3O2CC(CH3)NCH2] allowed the polymerization, but cage-like cluster is surrounded by twelve organic chains (butyl, butenyl, etc.) which prevent further condensation. Depending recent results have indicated that little crosslinking was achieved, the methacrylate charge compensating anions acting on the synthesis conditions the 2+ positive charge can be compensated by a large variety of anions (OH+,Cl-, sulfon- mainly as termination agents.66b These diYculties may be related to the fairly large molecular weight of the precursor ates, carboxylates, etc.).The position of the charge compensating anions in the structure indicates that the 2+ charge is (ca. 2600 g mol-1), but also to the shortness of the methacrylate functional anions which induce high steric hindrance.equally located at both cage poles, where six-coordinate tin atoms form hydroxylated [RSn(OH)]3O trimers. The second reason seems to prevail, as the use of AAMPS, where the polymerizable acrylamido group is more distant This cluster is confirmed both in solution by 119Sn NMR [it is characterized by two chemical shifts located at about -280 from the anionic anchoring head, allows the formation of a hybrid polymer by simple homopolymerization. More work is and -450 ppm (R=butyl or butenyl )] and a set of scalar tin–tin coupling satellites and in the solid state through 119MAS currently under way to obtain some information on the average length of the poly(2-acrylamido-2-methylpropane-1-sulfonate) NMR spectroscopy.69,71 Thus, this cluster can be followed easily throughout the polymerization or crosslinking reactions chains, which are probably short owing to strong steric hindrance.needed to tranform it into a hybrid material. As a consequence {(RSn)12O14(OH)6}2+ is a good nanobuilding block for the synthesis of well defined tin-oxo based hybrid materials that can be used as models. 4 Molecular design of transition metal alkoxides for the synthesis of hybrid organic–inorganic copolymers The chemical tailoring performed with systems containing a SiMC bond cannot be directly extended to pure transition metals because the more ionic MMC bond is broken down upon hydrolysis. Organic modification can however be performed by means of strong complexing ligands.The best are b-diketones and allied derivatives, polyhydroxylated ligands such as polyols, and a- or b-hydroxyacids. These ligands (HL) react readily with transition metal alkoxides M(OR)4 (M= Ce, Ti, Zr, etc.) to yield new precursors M(OR)3-x(L)x.73,74 Upon hydrolysing these new precursors, most of the alkoxy groups are quickly removed while all strong complexing ligands Fig. 12 Molecular structure of [(RSn)12(m3-O)14(m-OH)6 ]2+. cannot be completely removed. Complexing ligands appear to J. Mater. Chem., 1999, 9, 35–44 41Fig. 13 Schematic representation of the strategies that can be used from a hybrid [(RSn)12(m3-O)14(m-OH)6 ]2+2X- nanobuilding block. by partial hydrolysis of the alkoxy groups and radical polymerization of the allyl functions. However polymerization of allyl functions is slow and the degree of polymerization remains low.More reactive methacrylic acid can also be used as a polymerizable chelating ligand. The sol–gel synthesis of zirconium oxide based monoliths synthesized by UV copolymerization of zirconium oxide sols and organic monomers was recently reported.8 However, as carboxylic functions are weak ligands, they are largely removed upon hydrolysis2,75 and thus a large number of the chemical bonds between organic and inorganic networks is lost in the sol state.Therefore a new approach was chosen with diVerent ligands, such as acetoacetoxyethylmethacrylate (AAEM) and methacrylamidosalicylic acid (MASA), which contain both a strong chelating part and a highly reactive methacrylate group.6 Zirconium–oxo-PAAEM copolymers were synthesized from zirconium propoxide modified at the molecular level with AAEM.6 These hybrid organic–inorganic copolymers are made of zirconium oxo-polymers and polymethacrylate chains.The zirconium oxo species, in which zirconium is coordinated to seven oxygens, are chemically bonded to methacrylate chains Fig. 14 Schematical representation of the tin oxo-hydroxo clusters hybrids obtained through crosslinking of [(RSn)12O14(OH)6 ]2+ with through the b-diketo complexing function.The complexation a,v-PEG carboxylates.67 ratio (AAEM/Zr) is the key parameter which controls the structure and the texture of these hybrid materials (schematic structure Fig. 15). Careful adjustment of this parameter leads be quite stable towards hydrolysis because of chelate and steric to the tailoring of the ratio between organic and inorganic hindrance eVects.Thus, they allow organic groups to be components and also to zirconium oxo species with more or anchored to transition metal oxo-polymeric species and allow less open structures. The inorganic/organic ratio increases the synthesis of new hybrid organic–inorganic materials. when the complexation ratio decreases.For a high com- Organically modified TiO2 gels, which give photochromic plexation ratio (0.75) both networks interpenetrate intimately coatings, were synthesized from an allyl acetylacetone modified Ti(OBun)4 alkoxide.74 Double polymerization was performed at the nanometer scale, while for a low ratio (0.25) the size of 42 J.Mater. Chem., 1999, 9, 35–44References 1 C. J. Brinker and G. W. Scherrer, Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, CA, 1990. 2 (a) J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18, 259; (b) C. Sanchez, F. Ribot and S. DoeuV, Inorganic and Organometallic Polymers with Special Properties, ed.R. M. Laine, NATO ASI Series 206, Kluwer, New York, NY, 1992, p. 267. 3 (a) G. L. Wilkes, B. Orler and H. H. Huang, Polym. Prep., 1985, 26, 300; (b) G-S. Sur and J. E. Mark, Eur. Polym. J., 1985, 21, 1051; (c) H. Schmidt, A. Kaiser, H. Patzelt and H. Sholze, J. Phys., 1982, 43, C9-275. 4 (a) B.M. Novak, Adv. Mater., 1993, 5, 422; (b) Proceeding of the First European Workshop on Hybrid Organic–Inorganic Materials, ed.C. Sanchez and F. Ribot, New J. Chem., 1994, 18; Fig. 15 Schematic structures of class II hybrid materials made from (c) U. Schubert, N. Hu�sing and A. Lorenz, Chem.Mater., 1995, 7, zirconium n-propoxide complexed by acetoacetoxyethylmethacrylate 2010; (d) D. A. Loy and K. J. Shea, Chem. Rev., 1995, 95, 1431; (AAEM).6 (e) J. Sol–Gel Sci.Technol., 1995, 5; ( f ) P. Judenstein and C. Sanchez, J. Mater. Chem., 1996, 6, 511; (g) Mate�riaux Hybrides, Se� rie Arago 17, Masson, Paris, 1996; (h) C. Sanchez and F. Ribot, New J. Chem., 1994, 18, 1007; (i) U. Schubert, J. Chem. the inorganic domains increases to the sub-micrometer range. Soc., Dalton Trans., 1996, 3343; ( j) A. Morikawa, Y. Iyoku, M. Kakimoto and Y.Imai, J. Mater. Chem., 1992, 2, 679; Organic and inorganic growths are not independent and such (k) Y. Chujo and T.Saegusa, Ad. Polym. Sci., 1992, 100, 11; systems probably exhibit similar behavior to the so-called (l ) Hybrid Organic–Inorganic Composites, ed. J. E. Mark, interpenetrating polymer networks. C. Y. C. Lee and P. A. Bianconi, American Chemical Society, Washington, DC, 1995; (m) F.Ribot and C. Sanchez, Comments Inorg. Chem., 1998, in press. 5 (a) Better Ceramics Through Chemistry VI, ed. A. Cheetham, 5 Conclusions C. J Brinker, M. MacCartney, C. Sanchez, D. W. Schaefer and G. L. Wilkes, Mater. Res. Soc. Symp. Proc., Materials Research The combination at the nanosize level of inorganic and organic Society, Pittsburgh, PA, 1994, vol. 435; (b) Better Ceramics or even bio-active components in a single material makes Through Chemistry VII: Organic/Inorganic Hybrid Materials, ed.accessible an immense new area of materials science that has B. K. Coltrain, C. Sanchez, D. W. Schaefer and G. L. Wilkes, extraordinary implications for developing novel multi-func- Mater. Res. Soc. Symp. Proc., Materials Research Society, tional materials exhibiting a wide range of properties.This Pittsburgh, PA, 1996, vol. 435; (c) Sol–Gel Optics I, ed. J. D. Mackenzie and D. R. Ulrich, Proc. SPIE, Washington, 1990, fascinating new field of research brings together scientists vol. 1328; (d) Sol–Gel Optics II, ed. J. D. Mackenzie, Proc. SPIE, working in many diVerent domains. Among soft chemistry Washington, 1992, vol. 1758; (e) Sol–Gel Optics III, ed.processes, sol–gel chemistry oVers versatile access to the chemi- J. D. Mackenzie, Proc. SPIE, Washington, 1994, vol. 2288. cal design of new hybrid organic–inorganic materials. Many 6 (a) C. Sanchez and M. In, J. Non-Cryst. Solids 1992, 147–148, 1; new combinations between inorganic and organic or biological (b) M. In, C. Ge�rardin, J. Lambart and C.Sanchez, J. Sol–Gel Sci. Technol., 1995, 5, 101. components will probably appear in the future. Yet, better 7 (a) U. Schubert, E. Arpac, W. Glaubitt, A. Helmerich and understanding and control of the local and semi-local structure C. Chau, Chem. Mater., 1992, 4, 291; (b) C. Barglik-Chory and of these materials is an important issue, especially if tailored U. Schubert, J. Sol–Gel Sci.Technol., 1995, 5, 135. properties are sought. 8 R. Naß and H. Schmidt, in ‘Sol–Gel Optics I’, ed. J. D. Mackenzie To achieve such a control of the material structure, the and D. R. Ulrich, Proc. SPIE,Washington, 1990, vol. 1328, p. 258. 9 H. Schmidt and B. Seiferling, Mater. Res. Soc. Symp. Proc., 1986, assembly of well defined nanobuilding blocks is an interesting 73, 739.approach. The inorganic components are nanometric and 10 (a) D. Levy and D. Avnir, J. Phys. Chem., 1998, 92, 734; monodispersed. Their structures are perfectly defined, which (b) D. Levy, S. Einhorn and D. Avni, J. Non-Cryst. Solids, 1989, probably facilitates the characterization of the final materials. 113, 137. The variety found in the nanobuilding blocks (nature, struc- 11 Proceedings of the Second International Symposium on ture, functionality) and links, associated with diVerent assemb- Photochromism, ed.R. C. Bertelson, Chroma Chemicals Inc, Dayton, Ohio, USA, 1997. ling strategies, allow very diVerent types of architectures and 12 D. Preston, J. C. Pouxviel, T. Novinson, W. C. Kaska, B. Dunn organic–inorganic interfaces to be formed.ver, the and J.I. Zink, J. Phys. Chem., 1990, 94, 4167. step-by-step preparations of the materials usually allows some 13 H. Nakazumi, R. Nagashiro, S. Matsumoto and K. Isagawa, control over their semi-local structure. One can also combine Sol–Gel Optics III, ed. J. D. MacKenzie, Proc. SPIE,Washington, the nanobuilding blocks approach with the use of organic 1994, vol. 2288. 14 L. Hou, B.HoVmann, M. Menning and H. Schmidt, J. Sol–Gel templates that self-assemble and allow, through weak forces, Sci. Technol., 1994, 2, 635. control of the assembly step. With the aim of organizing/ 15 L. Hou, B. HoVmann, H. Schmidt and M. Menning, J. Sol–Gel structuring the nanobuilding blocks prior to their assembly, Sci. Technol., 1997, 8, 923, 927. one can also functionalize them with mesogenic groups and 16 L.Hou, M. Menning and H. Schmidt, Proc Eurogel’92, 1992, give them self-organizing properties. p. 173. 17 J. Biteau, F. Chaput and J. P. Boilot, J. Phys. Chem., 1996, 100, As described by Mann et al,76 the traditional view of 9024. inorganic solids as condensed matter is currently being 18 D. Levy, Chem. Mater., 1997, 9, 2666. reshaped by new insight in materials synthesis.77–79 A new 19 B.Schaudel, C. Guermeur, C. Sanchez, K. Keitaro and J. Delaire, paradigm, organized matter chemistry, is being formulated. In J. Mater. Chem., 1997, 7, 61. this field the sol–gel chemistry of organized matter is a very 20 S. Dire�, F. Babonneau, C. Sanchez and J. Livage, J. Mater. Chem., 1992, 2, 239. promising field of research which is just beginning to be 21 F.Babonneau, Polyhedron, 1994, 13, 1123. explored. Hybrids of both Class I and II4h and more particu- 22 C. Guermeur and C. Sanchez, forthcoming paper. larly those obtained through the ‘nanobuilding block 23 Applied photochromic polymers systems, ed. C. B. McArdle, approach’ will have paramount importance in the exploration Gordon and Breach Science Publishers, New York, 1991.the theme of ‘synthesis with construction’ of hierarchically 24 S. K. Yuh, E. Bescher, F. Babonneau and J. D. Mackenzie, Mater. Res. Soc. Symp. Proc., 1994, 346, 803. organized (structure and function) materials. J. Mater. Chem., 1999, 9, 35–44 4325 N. Koslova, B. Viana and C. Sanchez, J. Mater. Chem., 1993, M. Dumont, New J. Chem., 1996, 20, 13; (b) B. Lebeau, C.Sanchez, S. Brasselet and J. Zyss, Mater. Res. Soc. Symp. Proc., 3, 111. 1996, 435, 395; (c) B. Lebeau, C. Guermeur and C. Sanchez, 26 L. R. Mathews and E. T. Knobbe, Mater. Res. Soc. Symp. Proc., Mater. Res. Soc. Symp. Proc., 1994, 346, 315. 1993, 286, 259. 57 B. Lebeau, J. Maquet, C. Sanchez, E. Toussaere, R. Hierle and 27 I. M. Thomas, S. A. Payne and G. D. Wilke, J.Non-Cryst. Solids, J. Zyss, J. Mater. Chem., 1994, 4, 1855. 1992, 151, 183. 58 B. Lebeau, C. Sanchez, S. Brasselet and J. Zyss, Chem. Mater., 28 I. M. Thomas, S. A. Payne and G. D. Wilke, J. Non-Cryst. Solids, 1997, 9, 1012. 1992, 151, 183. 59 B. Lebeau, J. Maquet, C. Sanchez, F. Beaume and F. Laupre�tre, 29 A. A. Kaminskii, in Laser Crystals, Springer Verlag, Berlin, J. Mater. Chem., 1997, 7, 989.Heidelberg, New York, 2nd edn., 1981, pp. 361–433. 60 C. Sanchez and B.Lebeau, Pure Appl. Opt., 1996, 5, 689. 30 M. Lecomte, B. Viana and C. Sanchez, J. Chim. Phys., 1991, 61 R. F. Shi, M. H. Wu, S. Yamada, Y. M. Cai and A. F. Garito, 88, 39. Appl. Phys. Lett., 1993, 63, 1173. 31 B. Viana, N. Koslova, P. Aschehoug and C.Sanchez, J. Mater. 62 D. Blanc, P. Peyrot, C.Sanchez and C. Gonnet, Opt. Eng. Chem., 1995, 5, 719. Integrated Opt., 1998, 37, 1203. 32 C. Guizard, J.C. Achddou, A. Larbot, L. Cot, G. Le Flem, 63 C. Fiorini, F. Charra, J. M. Nunzi, I. D. W. Samuel and J. Zyss, C. Parent and C. L. Lurin, in ref. 5(c), p. 208. Opt. Lett., 1995, 20, 2469. 33 M. Genet, V. Brandel, M. P. LaHalle and E. Simoni, in ref. 5(c), 64 F. Ribot, F. Banse and C.Sanchez, Mater. Res. Soc. Symp. Proc., p. 194. 1994, 346, 121. 34 W. Nie, B. Dunn, C. Sanchez and P. Griesmar, Mater. Res. Soc. 65 F. Ribot, F. Banse, F. Diter and C. Sanchez, New J. Chem., 1995, Symp. Proc., 1992, 271, 639. 19, 1145. 35 M. Canva, P. Georges, G. Lesaux, A. Brun, F. Chaput and 66 (a) F. Ribot, F. Banse, C. Sanchez, M. Lahcini and J. P. Boilot, J. Non-Cryst.Solids, 1992, 147, 627. B. Jousseaumme, J. Sol–Gel Sci. Technol., 1997, 8, 529; 36 G. J. Dirksen and G. Blasse, J. Solid State Chem., 1991, 92, 591. (b) L. Angiolini, D. Caretti, C. Carlini, R. De Vito, F. T. Niesel, 37 A. Diaz and D. A Keszler, Chem. Mater. 1997, 9, 2071. E. Salatelli, F. Ribot and C. Sanchez, J. Inorg. Organomet. Polym., 38 A. Diaz and D. A Keszler, Mater. Res.Bull., 1996, 31, 147. 1998, 7, 151. 39 H. F. Folkerts and G. Blasse, J. Mater. Chem., 1995, 5, 1547. 67 (a) F. Ribot,C. Eychenne-Baron and C. Sanchez, Mater. Res. Soc. 40 E. Cordoncillo, P. Escribano, B. Viana and C. Sanchez, J. Mater. Symp. Proc., 1996, 435, 43; C. Eychenne-Baron, F. Ribot and Chem., 1998, 8, 1507. C. Sanchez, J. Organomet. Chem., 1998, 567, 137. 41 J. Chrusciel and Z.Lasocki, Pol. J. Chem., 1983, 57, 121. 68 F. Ribot, C. Eychenne-Baron and C. Sanchez, Mater. Res. Soc. 42 R. Campostrini and S. Dire�, Eurogel Proceedings, Advanced Symp. Proc., 1998, 519, in press. Materials and Processes by Sol–Gel Techniques, Colmar, 1992, ed. 69 F. Banse, F. Ribot, P. Tole�dano, J. Maquet and C. Sanchez, Inorg. S. Vilminot, Strasbourg, 1995, p. 307. Chem., 1995, 34, 6371. 43 G. Pucetti, I. Ledoux, J. Zyss, P. Griesmar and C. Sanchez, Polym. 70 H. PuV and H. Reuter, J. Organomet. Chem., 1989, 373, 173. Prepr., 1991, 32, 61. 71 D. Dakternieks, H. Zhu, E. R. T. Tiekink and R. J. Colton, 44 J. Zyss, G. Pucetti, I. Ledoux, P. Griesmar, J. Livage and J. Organomet. Chem., 1994, 476, 33. C. Sanchez, Fr. Pat., 1152, March 1991. 72 P. Jaumier, B.Jousseaume, M. Lahcini, F. Ribot and C. Sanchez, 45 (a) E. Toussaere, J. Zyss, P. Griesmar and C. Sanchez, Non Linear Chem. Commun., 1998, 369. Optics, 1991, 1, 349; (b) C. Sanchez, P. Griesmar, E. Toussaere, 73 (a) R.C.Mehrotra, R. Bohra and D. P. Gaur, Metal b-diketonates G. Puccetti, I. Ledoux and J. Zyss, Non Linear Optics, 1992, 4, 245. and Allied Derivatives, Academic Press, London, 1978; 46 P.Griesmar, C. Sanchez, G. Pucetti, I. Ledoux and J. Zyss, Mol. (b) D. C. Bradley, R. C. Mehrotra and D. P. Gaur, Metal Eng., 1991, 1, 205. Alkoxides, Academic Press, London, 1978; (c) L. G. Hubert- 47 (a) J. Kim, J. L. Plawsky, R. LaPeruta and G. M. Korenowski, Pfalzgraf, New J. Chem., 1987, 11, 663; ( f ) A. Leaustic, Chem. Mater., 1992, 4, 249; (b) J. Kim, J.L. Plawsky, E. Van F. Babonneau and J. Livage, Chem. Mater., 1989, 1, 248; Wagenen and G. M. Korenowski, Chem.Mater., 1993, 5, 1118. ( g) F. Ribot, P. Tole�dano and C. Sanchez, Chem. Mater., 1991, 3, 48 (a) Y. Haruvy and S. E. Weber, Mater. Res. Soc. Symp. Proc., 759; (h) C.Sanchez, M. In, P. Toledano and P. Griesmar, Mater. 1992, 271, 297; (b) Q. Hibben, E. Lu, Y. Haruvy and S.E.Weber, Res. Soc. Symp. Proc., 1992, 271, 669. Chem. Mater., 1994, 6, 761. 74 C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst. 49 F. Chaput, D. Riehl, Y Levy and J. P. Boilot, Chem.Mater., 1993, Solids, 1988, 100, 650. 5, 589. 75 S. DoeuV, M. Henry, C. Sanchez and J. Livage, J. Non-Cryst. 50 Y. Zhang, P. N. Prasad and R. Burzynski, Chem. Mater., 1992, Solids, 1987, 89, 206. 4, 851. 76 S. Mann, S. L. Burkett, S. A. Davis, C. E. Fowler, 51 Y. Nosaka, N. Tohriiwa, T. Kobayashi and N. Fujii, Chem. N. H. Mendelson, S. D. Sims, D.Walsh and N. T. Whilton, Chem. Mater., 1993, 5, 930. Mater., 1997, 9, 2300. 52 S. Marturunkakul, J. I. Chen, R. J. Jeng, S. Sengupta, J. Kumar 77 S. Mann and G. A. Ozin, Nature, 1996, 382, 313. and S. K. Tripathy, Chem. Mater., 1993, 5, 743; K.Izawa, 78 C. G. Go� ltner and M. Antonietti, Adv. Mater., 1997, 9, 431. N. Okamoto and O. Sugihara, Jpn. J. Appl. Phys., 1993, 32, 807. 79 (a) A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, 53 L. Kador, R. Fischer, D. Haarer, R. Kaseman, S. Bru� ck, R. S. Maxwell, G.D. Stucky, M. Krishnamurty, P. PetroV, H. Schmidt and H. Du� rr, Adv. Mater., 1993, 5, 270.A. Firouzi, M. Janicke and B. F. Chmelka, Scie54 (a) J. R. Caldwell, R. W. Cruse, K. J. Drost, V. P. Rao, A. K.- 1299; (b) A. Firouzi, D. Kumar, L. M. Bull, T. Besier, P. Sieger, Q. Hue, S. A. Walker, J. A. Zasadinski, C. Glinka, J. Nicol, D. Y. Jen, K. Y. Wong, Y. M. Cali, R. M. Mininni, J. Kenney, Margolese, G. D. Stucky and B. F. Chmelka, Science, 1995, 267, E.Binkley, L. R. Dalton, Y. Shi and W. H. Steier, Mater. Res. 1133; (c) D. Y. Zhao, P. D. Yang, Q. S. Huo, B. F. Chmelka and Soc. Symp. Proc., 1994, 328, 531; (b) Z. Yang, C. Xu, B. Wu, L. G. D. Stucky, Curr. Opin. Solid State Mater. Sci., 1998, 3, 111; (d) R. Dalton, S. Kalluri, W. H. Steier, Y. Shi and J. H. Bechtel, C. J. Brinker, Curr. Opin. Colloid Interface Sci., 1998, 3, 166; (e) Chem.Mater., 1994, 6, 1899; (c) H. W. Oviatt, K. J. Shea, S. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, Kalluri, Y. Shi, W. Steier and L. R. Dalton, Chem. Mater., 1995, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. 7, 493. Zink, Nature, 1997, 389, 364; ( f ) G. H. Mehl and J. W. Goodby, 55 (a) F. Chaput, D. Reihl, Y. Levy and J.-P. Boilot, Chem.Mater., Chem. Ber., 1996, 129, 521. 1993, 5, 589; (b) D. Reihl, F. Chaput, Y. Levy, J.-P. Boilot, F. Kajzar and P. A. Chollet, Chem. Phys. Lett., 1995, 245, 36. 56 (a) B. Lebeau, C. Sanchez, S. Brasselet, J. Zyss, G. Froc and Paper 8/05538F 44 J. Mater. Chem., 1999, 9, 35–44 J O U R N A L O F C H E M I S T R Y Materials Feature Article Molecular design of hybrid organic–inorganic nanocomposites synthesized via sol–gel chemistry† C.Sanchez, F. Ribot and B. Lebeau Chimie de la Matie`re Condense�e, UMR CNRS 7574, Universite� Pierre et Marie Curie 4, place Jussieu, 75252 Paris, France. E-mail: clems@ccr.jussieu.fr Received 12th May 1998, Accepted 16th July 1998 The design, synthesis and some optical properties of hybrid as organic polymerization and lead to hybrid organic–inororganic –inorganic nanocomposites materials are pre- ganic copolymers.6–8 This article reviews some of the previous sented.The properties that can be expected for such work we have performed on the design and synthesis of hybrid materials depend on the chemical nature of their compo- organic–inorganic nanocomposites in which organics are nents, but they also depend on the synergy of these simply embedded or chemically bonded to the inorganic components.Thus, the interface in these nanocomposites gel network. is of paramount significance and one key point of their synthesis is the control of this interface. These nanocom- 2 Siloxane based hybrid materials posites can be obtained by hydrolysis and condensation reactions of organically functionalized alkoxide precur- Organic groups can be bonded to an inorganic network as sors.Striking examples of hybrids made from modified network modifiers or network formers. Both functions have silicon, tin and transition metal alkoxides are presented. been achieved in the so-called ORMOSILS.3,9 The precursors Some optical properties (photochromic, luminescence, of these compounds are organo-substituted silicic acid esters NLO) of siloxane based hybrids are also discussed.of general formula R¾nSi(OR)4-n, where R¾ can be any organofunctional group. If R¾ is a simple non hydrolyzable group 1 Introduction bonded to silicon through a SiMC bond, it will have a network modifying eVect (SiMCH3). On the other hand, if R¾ can react Sol–gel chemistry is based on the polymerization of molecular with itself (R¾ contains a methacryl group for example) or precursors such as metal alkoxides M(OR)n.1,2 Hydrolysis and additional components, it will act as a network former.3c condensation of these alkoxides lead to the formation of metal Network modifiers and network formers can also introduce oxo-polymers.The mild characteristics oVered by the sol–gel other physical properties (mechanical, hydrophobic, electro- process allow the introduction of organic molecules inside an chemical, optical, etc.).Several examples related to some inorganic network.3 Inorganic and organic components can optical properties of siloxane based hybrids will be now then be mixed at the nanometric scale, in virtually any ratio described.leading to so-called hybrid organic–inorganic nanocomposites. 4 These hybrids are extremely versatile in their composi- 2-A Photochromic properties tion, processing and optical and mechanical properties.5 The nature of the interface between the organic and inorganic Spiropyrans and spirooxazines are two of the fascinating components has been used recently to classify these hybrids families of molecules exhibiting photochromic properties.into two diVerent classes.4h Class I corresponds to all the Upon irradiation, the colorless spiropyran or spirooxazine systems where there are no covalent or iono-covalent bonds undergoes heterolytic CMO ring cleavage, producing colored between the organic and inorganic components. In such mate- forms of merocyanines (Fig. 1). rials, the various components only exchange interactions such The merocyanines may interact with their environment, i.e. as van der Waals forces, hydrogen bondings or electrostatic solvent, matrix, etc. leading to diVerent photochromic forces. In contrast, in class II materials, some of the organic responses. Levy and Avnir10 first demonstrated the important and inorganic components are linked through strong chemical role played by the dye–matrix interactions on the photobonds (covalent or iono-covalent).Numerous hybrid organic– chromic response of spiropyrans. They studied the photochroinorganic materials have been developed in the past few years. mism of spiropyrans trapped in sol–gel matrices synthesized This development yields many interesting new materials, with via polymerization of Si(OCH3)4 or RSi(OEt)3 (R=ethyl, mechanical properties tunable between those of glasses and those of polymers, with improved optical properties (eYciency, stability, new sensors, etc.) and with improved catalytic or membrane based properties.4g This field of materials research mainly arises from chemists’ skills and demonstrates the major role played by chemistry in advanced materials.Siloxane based hybrids4 can be easily synthesized because SiMCsp3 bonds are rather covalent and therefore they are not broken upon hydrolysis. Similar chemistry can be developed from tin alkoxides. This is not aplicable to transition metals for which the more ionic MMC bond is easily cleaved by water; complexing organic ligands must be used.Such groups can be functionalized for any kind of organic reactions such †Basis of the presentation given at Materials Chemistry Discussion NO NO2 SP N O N SO N O– NO2 N N O hn1 hn2, D hn1 hn2, D + Fig. 1 Molecular structures of SO and SP photochromic dyes. No. 1, 24–26 September 1998, University of Bordeaux, France. J. Mater. Chem., 1999, 9, 35–44 35methyl, etc.) precursors, and observed two types of photochromic behavior.When the photochromic dye is trapped within a hydrophilic domain of the matrix (domain containing residual SiMOH groups), the open zwitterionic colored forms are probably stabilized through hydrogen bonding with the acidic silanol groups present at the pore surface. The result of this stabilization is the observation of the colored forms before irradiation.These colored forms can be bleached by irradiation in the visible range. This has been termed ‘reverse photochromism’. On the other hand spiropyran dyes embedded in a more hydrophobic hybrid network made by hydrolysis of RSi(OEt)3 exhibit direct photochromism, i.e. the colorless form is stable without irradiation. Such photochromic behavior has been reported for many spiropyran or spirooxazine doped sol–gel matrices.10–17 Moreover, for hybrid organic–inorganic matrices containing diVerent chemical environments (hydrophilic and hydrophobic domains) a competition between direct and reverse photochromisms can be observed.17 However many fundamental questions still need to be considered.Little is Fig. 2 Cartoon of the D/Zrx matrices. known concerning the role of the photochc dye–matrix interactions in the kinetics of coloration and thermal fading.As far as photochromic devices are concerned the tuning between strong and fast photochromic coloration (high DA) and very fast thermal fading is needed. Usually spiropyran or spirooxazine doped sol–gel matrices or even spirooxazine doped polymeric matrices exhibit slow thermal fading.10–18 The photochromic behavior of a spiropyran SP (6-nitro- 1¾,3¾,3¾-trimethylspiro{2H-1-benzopyran-2,2¾-indoline}) and a spirooxazine SO (1,3,3-trimethylspiro{indoline-2,3¾(3H)- naph(2,1-b)(1,4)oxazine}) (Fig. 1) embedded within two new hybrid matrices have been recently studied.19 Photochromic properties of the SP or SO doped D/Zr hybrid matrices. The first kind of matrix obtained through hydrolysis and condensation of (CH3)2Si(OC2H5)2 (D) and appropriate amounts of Zr(OPrn)4 (Zr) is labelled D/Zrx, where Zr stands for the zirconium, x for the molar amount of zirconium.Fig. 3 Photocolouration and photobleaching of SP doped D/Zrx These D/Zrx matrices are hybrid nanocomposites made matrices19 [(a) x=10; (b) x=20; (c) x=30]. from polydimethylsiloxane species (chains, cycles etc.) crosslinked by zirconium oxopolymers.20–22 The zirconium oxopolymers are hydrophilic domains that still contain some photochromism is partially reversed and can be balanced by tuning the D/Zr ratio.hydroxo groups coming from residual ethanol or ZrMOH ligands.22 The size and the spacing between the ZrO2 based The thermal bleaching behavior of the D/Zr20 samples was fitted with a biexponential equation.The SP doped materials domains is about a few nanometers as evidenced by SAXS.22 17O MAS NMR and FTIR experiments show that inside these exhibited a very long bleaching time (about 24 h) while for the SO doped D/Zr20 materials the thermal fading was composites the hydrophobic polydimethylsiloxane species are interfaced with the zirconium oxo domains both through much faster.19 The kinetic data of the SP or SO doped D/Zr20 samples covalent ZrMOMSi bonds and through weak interactions (hydrogen and van der Waals bonds).19 The structure of the are similar to those reported for other modified sol–gel matrices or in organic polymers.17,23 As in organic polymers, the D/Zrx matrices is shown schematically in Fig. 2. D/Zrx matrices doped with SP or SO are lightly colored (pink with bleaching follows a biexponential equation which can be explained by an inhomogeneous distribution of free volumes SP or blue with SO) before irradiation. However the absorbance (A) in the visible region is weak in comparison in the gel. Moreover the presence of diVerent stereoisomers (cis or trans) could also account for this behavior. The diVerent with the total amount of embedded photochromic dyes.The amount of colored form depends on the x content. isomer–matrix interactions could explain the diVerent kinetics observed for SO and SP. Fig. 3 shows the photochromic behavior of SP doped D/Zrx gels for three compositions of zirconium. When the Zr amount The thermal fading is longer for SP doped hybrids than for SO doped ones.This phenomenon can be correlated to the increases, the A variation due to the coloration decreases while that due to the decoloration increases: there are more and fact that SP open forms are known for their tendancy to form zwitterionic species, while non charged quinonic species are more open forms in the gel. The amount of colored form increases proportionally with the x content. It is much higher usually favored for open SO molecules.Zwitterionic species can be strongly stabilized by hydrogen bonding with the for D/Zr30 samples than for D/Zr10 ones. This indicates that before irradiation the SO and SP dyes matrix, thus lowering the decay times of thermal fading. are roughly split into two populations.The colored merocyanine open forms of SO and SP are stabilized by hydrogen Photochromic properties of the SP or SO doped DH/TH hybrid matrices. The second kind of SP or SO doped matrix bonding within the hydrophilic regions made from the zirconium oxopolymers, while the closed SO and SP forms are prepared from the hydrolysis and cocondensation of (CH3)HSi(OC2H5)2 (DH) and HSi(OC2H5)3 (TH) precursors probably located in the environment of the hydrophobic polydimethylsiloxane chains.So for these D/Zrx matrices the is labelled DH70/TH30 (70/30 refers to the molar composi- 36 J. Mater. Chem., 1999, 9, 35–44few minutes under magnetic stirring. A solution of mixed alkoxides, Zr(OPrn)4 and a corresponding rare earth (M= Nd3+, Sm3 +, Dy3 +, etc.) methoxy ethoxide was then added to the previous solution in order to obtain a Zr5Si molar ratio of 159 and M5Si molar ratios of 159; 0.159; 0.0159.The sols were deposited onto glass sheets and allowed to gel and dry at room temperature. Such transparent coatings were deposited several times till a thin layer of about 50–100 mm was obtained.25 FTIR, 1H MAS NMR and 29Si CP MAS NMR experiments have shown that these hybrids can be described as nanocom- Fig. 4 Schematic representation of the environment experienced by posites built from siloxane polymers crosslinked by metal oxo the SO dye within the matrix DH70/TH30. species. The metal oxo domains are made of mixed zirconium–rare earth oxo species. tion). These hybrids can be described as strongly interpen- Absorption–emission properties of these Nd3+ doped hybrid etrated nanocomposites.The environment experienced by the coatings. The room temperature absorption spectrum of Nd3+ SO dye within the DH70/TH30 matrix is shown in Fig. 4. doped hybrid coatings presented in ref. 25 consisted of some The SO or SP DH70/TH30 doped matrices exhibit normal broad transitions. The 4I9/2A4F5/2,2H9/2 transition around photochromism. All the samples are colorless before 800 nm, particularly important for diode pumping systems, irradiation.For the two photochromic dyes, the thermal fading presents a FWHMof around 15 nm. The absorption coeYcient can be fitted with excellent agreement with a monoexponential at this wavelength is around 10 cm-1 for a coating containing equation. This may be related to the quasi-liquid mobility approximately 4×1020 Nd3+ ions cm-3.This high absorption observed by NMR for this matrix. coeYcient value indicates that a high Nd3+ concentration The time dependence of the absorption upon repeated could be introduced into the system without aVecting the irradiation with 365 nm light for SO doped DH70/TH30 synthesis and the transparency of the films.coatings is reported in Fig. 5. The photochromic behavior is For Nd3+ ions excited at 805 nm, the emission spectra of reversible, extremely fast (the rate constant is 0.2 s-1) and the 4F3/2A4I11/2 transition at room temperature is presented corresponds to a very high absorption jump (DA=1.2). The in Fig. 6. This emission is broad with a FWHM around 40 nm photochromic kinetics of these SO doped hybrid materials are and extends into the NIR range from approximately 1.045 to to the best of our knowledge much faster than those reported 1.095 mm.A broad emission has also been observed for the for SO in any other matrix (sol–gel matrices, organic polymers, 4F3/2A4I9/2 transition.25 Broad emissions for Nd3+ are charac- alcohols, etc.).11,14,15,17,23 teristic of wide sites distribution around the neodymium ions The very high reactivity of the DH/TH precursors towards in this hybrid materials.Such inhomogeneous broadening is hydrolysis–condensation reactions (SiMOH groups are immealso observed in glassy hosts.27,28 Experimentally, one can diatly consumed) and the strong hydrophobicity of the notice that the Nd3+ fluorescence intensity is weaker than resulting matrix are both responsible for the direct and very those reported in glassy or crystalline matrices where the fast photochromic behavior observed.quantum eYciency is relatively high.29 2-B Luminescent properties of rare-earth doped hybrids Fluorescence kinetics of Nd3+ in hybrid siloxane based coat- Neodymium doped sol–gel matrices are suitable luminescent ings.Fig. 7 presents the variation of the fluorescence intensity materials. However room temperature sol–gel derived matrices decay profiles as a function of the Nd3+ content. Lifetimes usually contain a large amount of hydroxy groups which are decrease from 160 ms for 0.4×1020 Nd3+ ions cm-3 to responsible for the quenching of the Nd3+ emission. Therefore approximately 200 ns for 4×1021 Nd3+ ions cm-3.the design of sol–gel matrices inside which the hydroxy content It is quite unusual to observe radiative emission of Nd3+ can be minimized24,25 and/or inside which the rare earth is ions in matrices prepared at room temperature by sol–gel protected via complexation26 or encapsulation is desirable. processing. Usually the numerous hydroxy groups present in Recent work has demonstrated fluorescence emission for the classical xerogels obtained at room temperature prevent any Nd3+, Sm3 +, Dy3 +, Er3 + and Tm3+ ions doped in hybrid Nd3+ radiative emission.5d,5e,32 A thermal treatment at high siloxane–oxide matrices.25 temperatures is necessary to allow the Nd3+ emission to be These hybrids were synthesized by the following procedure.observed.In all our samples fluorescence is detected, however Diethoxymethylsilane [DEMS=SiH(OEt)2(CH3)], absolute ethanol and water in a 15151 molar ratio were mixed for a Fig. 6 RT Nd3+ fluorescence spectra.30 Fig. 5 Photochromic response of the SO doped DH70/TH30 matrix.18 J. Mater. Chem., 1999, 9, 35–44 37case, phonons can interact with the atomic transitions of Nd3+ and this coupling, analogous to weak electric dipole coupling, is enhanced when hydroxy groups are close to the neodymium ions.These interactions may explain the low Nd3+ quantum eYciency values and the weak emission observed for high Nd3+ concentration. All these results need of course to be improved, however they open many possibilities in the field of room temperature processed luminescent films.Energy transfer between an organic dye and Nd3+ as inorganic chromophore. Codoped rhodamine 6G–Nd3+ hybrid samples shows that the rhodamine emission spectra exhibit some dips at wavelengths corresponding to the Nd3+ absorption bands.31 This feature indicates that radiative energy transfer occurs between the organic dye and the Nd3+ ions.33–35 A photon emitted by the rhodamine molecule can be trapped by the rare-earth ions in the hybrid siloxane network. Furthermore, when excited by an argon laser at 488 nm, a wavelength where only the organic dye molecules absorb, the codoped (R6G–Nd3+) hybrid coating exhibits a Nd3+ emission around 1.06 mm.No emission was detected around 1.06 mm under the same experimental conditions without the Fig. 7 Variation of the Nd3+ fluorescence decay31 (Nd3+ content in presence of the organic dye inside the hybrid coating. This atom cm-3, (a) 0.4×10,20 (b) 4×1020 (c) 4×1021. behavior reveals that energy transfer mechanisms may be favorable for Nd3+ emission. the Nd3+ fluorescence decay profiles lead to lifetime values New Eu2+ doped hybrid organic–inorganic nanocomposites lower than those usually recorded for Nd3+ in high temperasynthesized at room temperature.The Eu2+ ion is particularly ture processed glassy or crystalline matrices. These hybrid unique because its broad luminescence band 4f65d1A4f7 is materials can be described as a siloxane polymeric network strongly host dependent with emission wavelengths extending made from (CH3SiO3/2) units crosslinked by mixed nanoaggrefrom the UV to the red range of the electromagnetic spec- gates made from zirconium oxo and neodymium oxo species.trum.36 Therefore, the luminescent properties of Eu2+-doped Depending on the Nd3+ concentration, two diVerent optical solids have been intensively studied during the past three features are observed in these hybrid coatings. Both are well decades. These studies have led to the use of these compounds correlated with the structure.(1) For low Nd3+ concentration, as phosphors, notably blue-emitting Eu2+5BaMgAl10O17 in the initial non-exponential part of the decay profiles vary lamp and plasma display panels and UV-emitting linearly on a root-mean-square time scale showing the strong Eu2+5SrB4O7 for medical applications and skin tanning.cross-relaxation phenomena. Even if a few nOMH vibrations Crystalline or glassy Eu2+ doped materials are usually pro- are observed in the infrared spectrum, these hydroxy groups cessed at relatively high temperatures.36–39 Moreover, the are not located close to the Nd3+ ions and at low concentration synthesis and the stabilization of europium in the divalent the main non-radiative de-excitation mechanisms are the state under mild synthetic conditions is not an easy task.For Nd3+–Nd3+ interactions. The fact that these nanocomposites the first time, we reported recently the room temperature are made with segregated metal oxo species (the dispersion is synthesis of new Eu2+ doped hybrid materials together with not statistical all over the sample) leads to Nd3+–Nd3+ their absorption and emission properties.40 These hybrids are interactions even for low rare earth concentrations.The slope obtained though the hydrolysis and condensation of diethoxy- measured at long times give an indication of the Nd3+ methylsilane (MDES), methyltriethoxysilane (TREOS) and fluorescence quantum yield in this hybrid coating. The ratio g zirconium tetrapropoxide precursors in the presence of of the experimental lifetime at low concentration over the europium trichloride.calculated radiative lifetime (g=0.35) indicates that approxi- Dehydrocondensation of organic hydrosilanes with silanols mately 35% of the relaxation occurs radiatively at low Nd3+ is one of the common methods for the synthesis of the siloxane concentration. This is a high value for a room temperature linkage.41 This reaction, which occurs with the evolution of sol–gel processed material and this is in agreement with the hydrogen gas, has been described as follows:41 fact that, at low concentration, Nd3+ ions form clusters at relatively long distances from the remaining hydroxy groups.OSiMH+HOMSiOCA catalystOSiMOMSiO+H2 (2) When the Nd3+ concentration increases, interactions increase as the Nd3+–Nd3+ distance becomes shorter in the nanoaggregates.Simultaneously when the neodymium concen- In this sense, alkoxide precursors containing SiMH groups show the possibility of using the SiMH groups as an in situ tration increases (the zirconium concentration remaining constant) the probability of finding neodymium ions at the surface reducing agent which allows the formation of metal/silica nanocomposites.42 of the metal oxo species increases, rendering the rare earth prone to interactions with some remaining hydroxy groups In our study,40 the in situ formation of hydrogen provided by the cleavage of the SiMH bonds was used to generate, located close to the surface.As a consequence, short lifetimes are observed.For a concentration of 4×1021 Nd3+ ions cm-3 during the first step of hydrolysis and condensation reactions, europium in the divalent state. the quantum yield is less than 0.01%. This behavior characterizes non-radiative de-excitation processes due to strong A typical absorption spectra (Fig. 8) of these is constituted by a broad absorption band in the UV range (200–400 nm) energy transfers between Nd3+ but also non-radiative deexcitation occurring via the filling of the 4F3/2–4I15/2 energy attributed to the 4f75d0A4f65d1 (Eu2+) transition.The emission spectra (Fig. 9) of the corresponding hybrids gap by nOMH vibrations. Usually, according to the energy gap law, multiphonon non-radiative contributions do not exceed recorded under excitation at 355 nm show a broad emission corresponding to the interconfigurational 4f65d1A4f75d0 50% taking into account the energy diVerence around 5500 cm-1 between the 4F3/2 and 4I15/2 levels.In the present transition centered at 430 nm (ca. 23250 cm-1) and a intracon- 38 J. Mater. Chem., 1999, 9, 35–44are only achieved in a non-centrosymmetric environment, we first demonstrated that orientation of organic chromophores can be performed in hybrid sol–gel matrices43–46 by using electrical field induced second harmonic (EFISH) or corona electrical field poling techniques.Organic molecules such as N-(3-triethoxysilylpropyl )-2,4-dinitrophenylamine (TSDP) were chemically bonded to the oxide backbone of gels. The chemical bonding of the dye to the sol–gel matrix allowed dye concentration to be increased without any crystallization eVects.43–46 A first generation of sol–gel matrices was synthesized by copolymerization of silicon alkoxysilanes [TSDP and SiHCH3(OEt)2] and zirconium propoxide precursors.45 The sols were deposited as transparent coatings and exhibited after corona poling an SHG response of 1.6 pm V-1.45 Even if in this first generation of sol–gel matrices relaxation of the organic chromophores occurred over several hours, these results suggested the feasibility of poling techniques into hybrid inorganic sol–gel matrices more ionic than classical polymers.Fig. 8 Absorption spectra of europium doped hybrid xerogels. Consequently a range of opportunities for the synthesis of optical sol–gel devices with eYcient second harmonic properties was discovered. Since then, there has been increasing interest in second order NLO materials synthesized via sol–gel chemistry.47–60 The optimization of the second order NLO response of hybrid sol–gel matrices with grafted chromophores is currently under investigation by several research groups.Several strategies are used to improve the NLO response of the hybrid coatings.43–60 (i) The intrinsic NLO response of the dye can be increase by using chromophores such as N-(4- nitrophenyl )-L-prolinol (NPP) or disperse red one (DR1) derivatives which exhibit higher non linearities than nitroaniline ones.(ii) The chromophore relaxation can be controlled by increasing the matrix rigidity. This point is without doubt the most important in order to be able to make eYcient NLO devices. The modification of the binary composition (siloxane –crosslinker), the nature of theM(OR)4 crosslinking alkoxide [SiR¾x(OR)4-x–M(OR)4: R¾=any NLO chromophore; M=Zr, Si, Ti, etc.), and the processing of these hybrid Fig. 9 Emission spectrum of europium doped hybrid xerogel (lexc= materials in the presence of polymers with well known mechan- 355 nm).ical properties such as methyl methacrylates or polyimides, are the most commonly used strategies to minimize dye figurational 4f–4f Eu3+ emission in the longer wavelengths. relaxation. The strategies we have used to improve the NLO Several bands are obtained corresponding to the 5D0A7F0,1,2,3 response of hybrid materials will be llustrated in the two transitions.A Stokes’ shift value of the Eu2+ luminescence following sections. around 9000 cm-1 is obtained, this shift between the absorption and emission energies of Eu2+ located in an oxygen ligand TSPD/TMOS based hybrids with NLO properties.58,59 The field has been assigned to a combination of crystal field and second generation of hybrids investigated were made via nephelauxetic eVects.36 These hybrid structures contain oxygen hydrolysis and co-condensation of tetramethoxysilane atoms in higher coordination number environments (highly (TMOS) and N-(3-triethoxysilylpropyl )-2,4-dinitrophenylam- coordinated by metal atoms m3-O–Zr/Eu or m4-O–Zr/Eu) ine (TSDP) [Fig. 10(a)] precursors (T and Q are common which produce Eu2+ emission at longer wavelengths. Moreover notations referring to the oxo trifunctional R¾MSiO3 and distortion of the oxygen polyhedra from ideal coordination tetrafunctional SiO4 central units, respectively). FTIR, 17O geometry results in a large Stokes’ shift.First measurements and 29Si NMR experiments indicated the existence of linear indicate a Eu2+5Eu3+ concentration ratio of about 551. The and cyclic siloxane (T–T)a oligomers and silica (Q–Q)b units high Eu2+ content is probably related to the more eYcient reductive medium provided by the initial mixture of the europium trichloride with the MDES and TREOS precursors.Moreover, the intensity of the Eu2+ luminescence did not change when the xerogels were stored in air for several months, showing that Eu2+ ions are eYciently trapped inside the hybrid matrix. 2-C Quadratic NLO properties of siloxane based hybrids Most of the sol–gel optics research devoted to non-linear optic (NLO) materials was initially related to third order processes which are compatible with the isotropy of amorphous sol–gel matrices.Organic molecules inside amorphous sol–gel matrices are in general randomly oriented thus ruling out the emission O2N NO2 NH Si(OEt)3 TSDP O2N N N N CH2CH2OCONH-(CH2)3Si(OEt)3 CH2CH2OCONH-(CH2)3Si(OEt)3 CH3 (a) (b) Fig. 10 Graftable NLO dyes: (a)=TSDP (b)=ICTES-Red17. of second harmonic generation. As second order non-linearities J. Mater. Chem., 1999, 9, 35–44 39linked through stable T–O–Q bridges formed in the early also strongly depend on their thermal history. Chemical crosslinking is not complete at gelation or even after RT air stages of the process.Films of thickness 1–5 mm were easily obtained through spin-coating. In such systems gelation prob- drying as shown by 29Si NMR experiments.59 Upon ageing and curing the chemical reactions continue towards com- ably occurs through the crosslinking of siloxane polymers with Q silica based species. The degrees of condensation of T and pletion.Consequently, given suYcient time and temperature to allow mobility of the species a network forming system Q units measured in the solid state by 29Si MAS NMR spectroscopy are much higher in xerogels than in sols and this continues to crosslink long after gelation. The increase of the density of crosslinks modifies the thermomechanical properties diVerence demonstrates that a large number of condensation and crosslinking reactions still occur upon solvent removal.of the hybrid as illustrated by the changes observed in Tg upon thermal curing.57 The mobility of the NLO chromophores, as observed by high-resolution solid-state 13C NMR spectroscopy, is also Another processing parameter which has great importance is the electrical field used to poled the NLO chromophores.correlated with the glass-transition phenomenon of the matrix observed by DSC.57 This glass transition phenomenon corre- Accelerated field induced curing must occur in these hybrid TSDP/TMOS materials. The high electrical field provided sponds to the glass transition of the polysiloxane network. Tg, the glass-transition temperature, increases with the TMOS during poling must favor crosslinking and interpenetration of both polymeric T and Q networks.This was described by content, while the apparent variation of heat capacity corresponding to Tg decreases. These results, as well as the analysis Haruvy and co-workers,48b who have shown that greatly accelerated curing occurs under ambient conditions on thin of the polarization transfer in MAS/CP/DD 29Si NMR experiments, are consistent with the relatively high degree of inter- films processed from siloxane resins prepared by the sol–gel process when they are exposed to an intense corona-discharge penetration of T and Q units.Therefore, these hybrid TSDP/TMOS coatings can be described as nanocomposites field. The corona cured sol–gel films exhibited a more compact matrix as manifested by the lower mobility of the embedded made of silica rich domains and siloxane rich domains.Many Q and T species are mutually sequestered at the nanometer chromophores and a more hydrophilic surface than thermally cured ones. They suggest that the field induced removal of scale. Their microstructure is schematically pictured in Fig. 11. The white parts correspond to the silica-rich phase inside condensate small molecules and solvents allows better completion of the reactions and more eYcient crosslinking.48 which some T units (black dots) are sequestered.The black spheres correspond to the polysiloxane rich phase which Compared to the tremendous amount of work and time devoted to polymeric NLO materials, NLO materials made participates in the glass-transition phenomenon and contains Q units (white dots).The sizes of the polysiloxane and silica by sol–gel are still in their infancy. For these system, depending on chemical composition, the SHG values range between 2.5 domains depend not only on the chemical composition but also on the drying procedure and consequently on the solvent and 10 pm V-1.47,57 Moreover, the sol–gel materials described in this work have Tg values in the range of 30–70 °C, well and sample thickness.The TSDP/TMOS ratio, proton concentration, hydrolysis below the state of the art obtained with pure organic polymeric materials based on polyimides61 which are highly non linear ratio, sequence of mixing the reagents and ageing time of the sol are the chemical parameters that should directly influence and stable for hundred of hours at temperatures higher than 100 °C.However, the excellent knowledge of such systems a and b values characterizing the lengh of the constituent linear and cyclic siloxane (T–T)a oligomers and silica (Q–Q)b allowed us within a short period of time to design a third generation of hybrids with a highly improved NLO response. units respectively.However, it has been demonstrated that the mechanical properties of hybrid siloxane–oxide materials, and thus the ICTES-Red17/TMOS based hybrids with NLO properties. The third generation of hybrid organic–inorganic nanocom- relaxation behavior of chromophores grafted in these matrices, posites was designed on the basis of the following specifications: the NLO dye must have a high NLO response, it must be anchored by more than one trifunctional link and silica was kept as the crosslinking agent because coatings of better optical quality were usually obtained with binary silica–siloxane materials.56 In order to be able to perform double grafting of an NLO chromophore, the Red 17 [4-(amino-N,N-diethanol )-2-methyl-4¾-nitroazobenzene] with a very eYcient quadratic hyperpolarizability [b(0) (Red 17)=55×10-30 esu] was functionalized with two alkoxysilyl groups by a coupling reaction between the dye and 3-isocyanatopropyltriethoxysilane (ICTES).56,58 The resulting alkoxysilyl functionalized NLO precursor, ICTES-Red 17 [Fig. 10(b)] was hydrolyzed and co-condensed with TMOS in order to obtain the hybrid siloxane–silica nanocomposite.From the resulting sols, coatings with a thickness of a few mm can be deposited. The resulting hybrid materials do not exhibit Tg according to DSC results. Non-resonant second-order non-linearities as high as 150–200 pm V-1,58,62 measured on these hybrid systems, with significant long-term stability (10% of signal lost after 20 days) have been reported.58 The thermal stability at 80 °C has been shown to be excellent, making the ICTES-Red 17/TMOS systems competitive candidates for non-linear optics systems.Chemical characterization (FTIR, 29Si MAS NMR, UV–VIS spectroscopy) and thermal assisted in situ poling studies performed on these coatings revealed the importance of the processing and history of these systems. Three param- Fig. 11 Schematic representation of TSDP-TMOS based hybrids.52 eters are of paramount importance.(i) Aging of the solution 40 J. Mater. Chem., 1999, 9, 35–44has been shown to greatly influence the amplitude of the final Moreover, these clusters exhibit a high versality for the design of hybrids (Fig. 13). {(RSn)12O14(OH)6}2+2X- can non-linear signal. This results from improvement of crosslinking eYciency and from modifications of the distribution be assembled through organic networks by using the covalent interface provided by the SnKC bond or by using the ionic between cyclic and linear siloxane species.(ii) Thermal precuring of the samples at 150 °C was found to markedly improve interface associated with the charge compensating anions X- or even by using both interfaces.In the first case the organic the non-linear response as well as its stability. (iii) Optical poling recently tested in sol–gel derived matrices can also be moiety carried through the SnKCsp3 links should be polymerizable (R=butenyl, propylmethacrylate, propylcrotonate, 4- used to improve the chromophore anisotropy63 These very reproducible results58,62 are very promising, styrylbutyl, etc.).In the second case charge compensating organic dianions must be able to bridge the clusters. This can however as far as NLO devices are concerned they must be completed by measurements of electro-optical eYciency, be performed by using dicarboxylates,65 or a,v-telechelic macromonomers terminated by carboxylic or sulfonic groups.67 waveguiding properties and the evaluation of the optical losses.As an example, the coupling of these clusters by carboxymethyl terminated PEG macromonomers67 is schematically shown 3 Tin oxo species based hybrid materials in Fig. 14. Another strategy could be to use polymerizable anions Tin is a very interesting element because its characteristics make it intermediate between silicon and the transition metals.(methacrylate, 2-acrylamido-2-methylpropane-1-sulfonate, etc.) as monomers for organic polymerizations reactions.66,68a Like the latter, tin exhibits several coordination numbers (generally from 4 to 6) and coordination expansion makes By a simple acid-base reaction, the oxo-hydroxo butyltin macrocation, {(BuSn)12O14(OH)6}2+, was functionalized hydrolysis–condensation reactions of tin alkoxides fast.But, as for silicon, the Sn–Csp3 bond is stable, especially towards with 2-acrylamido-2-methylpropane-1-sulfonate, aVording nanobuilding blocks with two highly polymerizable groups.68a nucleophilic agents such as water. This last characteristic allows one to chemically link organic moieties to the tin oxo For the first time,68a the direct polymerization of such functionalized oxo-hydroxo butyltin nanoclusters has been polymers/oligomers but it also reduces the inorganic functionality of tin and therefore favors the formation of oxo successfully performed, yielding hybrid materials in which the nanosized inorganic component is perfectly defined.Two types clusters. These oxo clusters can be used as nanobuilding blocks in the design of new hybrid materials.64–72 of organic components are found in such materials.The butyl groups covalently bound onto tin atoms, and, more import- The nanobuilding block [(RSn)12(m3-O)14(m-OH)6]2+, the structure of which is shown in Fig. 12 can be obtained through antly, poly(2-acrylamido-2-methylpropane-1-sulfonate) chains which interact through electrostatic interactions with the several chemical pathways: hydrolysis of RSn(OPri)3 or RSnCl3 or by refluxing in toluene butyltin hydroxide oxide oxo-hydroxo butyltin macrocations and aVord the crosslinking. [BuSnO(OH)] in the presence of sulfonic acids (R¾SO3H)64–71 and more recently Jousseamme et al.opened a new route to Such an approach to the construction of tin-based hybrid materials from bifunctional nanobuilding blocks was pre- this cluster through hydrolysis of functionalized trialkynylorganotin precursors.72 viously attempted with pure {(BuSn)12O14(OH)6}- {O2CC(CH3)NCH2}2 but failed as its homopolymerization This compound is made of a tin oxo-hydroxo cluster with a equal numbers of six- and five-coordinate tin atoms.This appeared impossible.66 Addition of a co-monomer [CH3O2CC(CH3)NCH2] allowed the polymerization, but cage-like cluster is surrounded by twelve organic chains (butyl, butenyl, etc.) which prevent further condensation.Depending recent results have indicated that little crosslinking was achieved, the methacrylate charge compensating anions acting on the synthesis conditions the 2+ positive charge can be compensated by a large variety of anions (OH+,Cl-, sulfon- mainly as termination agents.66b These diYculties may be related to the fairly large molecular weight of the precursor ates, carboxylates, etc.).The position of the charge compensating anions in the structure indicates that the 2+ charge is (ca. 2600 g mol-1), but also to the shortness of the methacrylate functional anions which induce high steric hindrance.equally located at both cage poles, where six-coordinate tin atoms form hydroxylated [RSn(OH)]3O trimers. The second reason seems to prevail, as the use of AAMPS, where the polymerizable acrylamido group is more distant This cluster is confirmed both in solution by 119Sn NMR [it is characterized by two chemical shifts located at about -280 from the anionic anchoring head, allows the formation of a hybrid polymer by simple homopolymerization.More work is and -450 ppm (R=butyl or butenyl )] and a set of scalar tin–tin coupling satellites and in the solid state through 119MAS currently under way to obtain some information on the average length of the poly(2-acrylamido-2-methylpropane-1-sulfonate) NMR spectroscopy.69,71 Thus, this cluster can be followed easily throughout the polymerization or crosslinking reactions chains, which are probably short owing to strong steric hindrance. needed to tranform it into a hybrid material.As a consequence {(RSn)12O14(OH)6}2+ is a good nanobuilding block for the synthesis of well defined tin-oxo based hybrid materials that can be used as models. 4 Molecular design of transition metal alkoxides for the synthesis of hybrid organic–inorganic copolymers The chemical tailoring performed with systems containing a SiMC bond cannot be directly extended to pure transition metals because the more ionic MMC bond is broken down upon hydrolysis.Organic modification can however be performed by means of strong complexing ligands. The best are b-diketones and allied derivatives, polyhydroxylated ligands such as polyols, and a- or b-hydroxyacids.These ligands (HL) react readily with transition metal alkoxides M(OR)4 (M= Ce, Ti, Zr, etc.) to yield new precursors M(OR)3-x(L)x.73,74 Upon hydrolysing these new precursors, most of the alkoxy groups are quickly removed while all strong complexing ligands Fig. 12 Molecular structure of [(RSn)12(m3-O)14(m-OH)6 ]2+. cannot be completely removed.Complexing ligands appear to J. Mater. Chem., 1999, 9, 35–44 41Fig. 13 Schematic representation of the strategies that can be used from a hybrid [(RSn)12(m3-O)14(m-OH)6 ]2+2X- nanobuilding block. by partial hydrolysis of the alkoxy groups and radical polymerization of the allyl functions. However polymerization of allyl functions is slow and the degree of polymerization remains low.More reactive methacrylic acid can also be used as a polymerizable chelating ligand. The sol–gel synthesis of zirconium oxide based monoliths synthesized by UV copolymerization of zirconium oxide sols and organic monomers was recently reported.8 However, as carboxylic functions are weak ligands, they are largely removed upon hydrolysis2,75 and thus a large number of the chemical bonds between organic and inorganic networks is lost in the sol state.Therefore a new approach was chosen with diVerent ligands, such as acetoacetoxyethylmethacrylate (AAEM) and methacrylamidosalicylic acid (MASA), which contain both a strong chelating part and a highly reactive methacrylate group.6 Zirconium–oxo-PAAEM copolymers were synthesized from zirconium propoxide modified at the molecular level with AAEM.6 These hybrid organic–inorganic copolymers are made of zirconium oxo-polymers and polymethacrylate chains. The zirconium oxo species, in which zirconium is coordinated to seven oxygens, are chemically bonded to methacrylate chains Fig. 14 Schematical representation of the tin oxo-hydroxo clusters hybrids obtained through crosslinking of [(RSn)12O14(OH)6 ]2+ with through the b-diketo complexing function.The complexation a,v-PEG carboxylates.67 ratio (AAEM/Zr) is the key parameter which controls the structure and the texture of these hybrid materials (schematic structure Fig. 15). Careful adjustment of this parameter leads be quite stable towards hydrolysis because of chelate and steric to the tailoring of the ratio between organic and inorganic hindrance eVects.Thus, they allow organic groups to be components and also to zirconium oxo species with more or anchored to transition metal oxo-polymeric species and allow less open structures. The inorganic/organic ratio increases the synthesis of new hybrid organic–inorganic materials. when the complexation ratio decreases.For a high com- Organically modified TiO2 gels, which give photochromic plexation ratio (0.75) both networks interpenetrate intimately coatings, were synthesized from an allyl acetylacetone modified Ti(OBun)4 alkoxide.74 Double polymerization was performed at the nanometer scale, while for a low ratio (0.25) the size of 42 J. Mater. Chem., 1999, 9, 35–44References 1 C.J. Brinker and G. W. Scherrer, Sol–Gel Science, The Physics and Chemistry of Sol–Gel Processing, Academic Press, San Diego, CA, 1990. 2 (a) J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18, 259; (b) C. Sanchez, F. Ribot and S. DoeuV, Inorganic and Organometallic Polymers with Special Properties, ed. R. M. Laine, NATO ASI Series 206, Kluwer, New York, NY, 1992, p. 267. 3 (a) G. L. Wilkes, B. Orler and H. H. Huang, Polym. Prep., 1985, 26, 300; (b) G-S. Sur and J. E. Mark, Eur. Polym. J., 1985, 21, 1051; (c) H. Schmidt, A. Kaiser, H. Patzelt and H. Sholze, J. Phys., 1982, 43, C9-275. 4 (a) B.M. Novak, Adv. Mater., 1993, 5, 422; (b) Proceeding of the First European Workshop on Hybrid Organic–Inorganic Materials, ed. C. Sanchez and F. Ribot, New J.Chem., 1994, 18; Fig. 15 Schematic structures of class II hybrid materials made from (c) U. Schubert, N. Hu�sing and A. Lorenz, Chem.Mater., 1995, 7, zirconium n-propoxide complexed by acetoacetoxyethylmethacrylate 2010; (d) D. A. Loy and K. J. Shea, Chem. Rev., 1995, 95, 1431; (AAEM).6 (e) J. Sol–Gel Sci. Technol., 1995, 5; ( f ) P. Judenstein and C. Sanchez, J. Mater. Chem., 1996, 6, 511; (g) Mate�riaux Hybrides, Se� rie Arago 17, Masson, Paris, 1996; (h) C.Sanchez and F. Ribot, New J. Chem., 1994, 18, 1007; (i) U. Schubert, J. Chem. the inorganic domains increases to the sub-micrometer range. Soc., Dalton Trans., 1996, 3343; ( j) A. Morikawa, Y. Iyoku, M. Kakimoto and Y. Imai, J. Mater. Chem., 1992, 2, 679; Organic and inorganic growths are not independent and such (k) Y.Chujo and T.Saegusa, Ad. Polym. Sci., 1992, 100, 11; systems probably exhibit similar behavior to the so-called (l ) Hybrid Organic–Inorganic Composites, ed. J. E. Mark, interpenetrating polymer networks. C. Y. C. Lee and P. A. Bianconi, American Chemical Society, Washington, DC, 1995; (m) F. Ribot and C. Sanchez, Comments Inorg. Chem., 1998, in press. 5 (a) Better Ceramics Through Chemistry VI, ed.A. Cheetham, 5 Conclusions C. J Brinker, M. MacCartney, C. Sanchez, D. W. Schaefer and G. L. Wilkes, Mater. Res. Soc. Symp. Proc., Materials Research The combination at the nanosize level of inorganic and organic Society, Pittsburgh, PA, 1994, vol. 435; (b) Better Ceramics or even bio-active components in a single material makes Through Chemistry VII: Organic/Inorganic Hybrid Materials, ed.accessible an immense new area of materials science that has B. K. Coltrain, C. Sanchez, D. W. Schaefer and G. L. Wilkes, extraordinary implications for developing novel multi-func- Mater. Res. Soc. Symp. Proc., Materials Research Society, tional materials exhibiting a wide range of properties. This Pittsburgh, PA, 1996, vol. 435; (c) Sol–Gel Optics I, ed. J. D. Mackenzie and D. R. Ulrich, Proc. SPIE, Washington, 1990, fascinating new field of research brings together scientists vol. 1328; (d) Sol–Gel Optics II, ed. J. D. Mackenzie, Proc. SPIE, working in many diVerent domains. Among soft chemistry Washington, 1992, vol. 1758; (e) Sol–Gel Optics III, ed. processes, sol–gel chemistry oVers versatile access to the chemi- J.D. Mackenzie, Proc. SPIE, Washington, 1994, vol. 2288. cal design of new hybrid organic–inorganic materials. Many 6 (a) C. Sanchez and M. In, J. Non-Cryst. Solids 1992, 147–148, 1; new combinations between inorganic and organic or biological (b) M. In, C. Ge�rardin, J. Lambart and C. Sanchez, J. Sol–Gel Sci. Technol., 1995, 5, 101. components will probably appear in the future. Yet, better 7 (a) U.Schubert, E. Arpac, W. Glaubitt, A. Helmerich and understanding and co the local and semi-local structure C. Chau, Chem. Mater., 1992, 4, 291; (b) C. Barglik-Chory and of these materials is an important issue, especially if tailored U. Schubert, J. Sol–Gel Sci. Technol., 1995, 5, 135. properties are sought. 8 R. Naß and H. Schmidt, in ‘Sol–Gel Optics I’, ed. J.D. Mackenzie To achieve such a control of the material structure, the and D. R. Ulrich, Proc. SPIE,Washington, 1990, vol. 1328, p. 258. 9 H. Schmidt and B. Seiferling, Mater. Res. Soc. Symp. Proc., 1986, assembly of well defined nanobuilding blocks is an interesting 73, 739. approach. The inorganic components are nanometric and 10 (a) D. Levy and D. Avnir, J.Phys. Chem., 1998, 92, 734; monodispersed. Their structures are perfectly defined, which (b) D. Levy, S. Einhorn and D. Avni, J. Non-Cryst. Solids, 1989, probably facilitates the characterization of the final materials. 113, 137. The variety found in the nanobuilding blocks (nature, struc- 11 Proceedings of the Second International Symposium on ture, functionality) and links, associated with diVerent assemb- Photochromism, ed.R. C. Bertelson, Chroma Chemicals Inc, Dayton, Ohio, USA, 1997. ling strategies, allow very diVerent types of architectures and 12 D. Preston, J. C. Pouxviel, T. Novinson, W. C. Kaska, B. Dunn organic–inorganic interfaces to be formed.4m Moreover, the and J. I. Zink, J. Phys. Chem., 1990, 94, 4167. step-by-step preparations of the materials usually allows some 13 H.Nakazumi, R. Nagashiro, S. Matsumoto and K. Isagawa, control over their semi-local structure. One can also combine Sol–Gel Optics III, ed. J. D. MacKenzie, Proc. SPIE,Washington, the nanobuilding blocks approach with the use of organic 1994, vol. 2288. 14 L. Hou, B. HoVmann, M. Menning and H. Schmidt, J. Sol–Gel templates that self-assemble and allow, through weak forces, Sci.Technol., 1994, 2, 635. control of the assembly step. With the aim of organizing/ 15 L. Hou, B. HoVmann, H. Schmidt and M. Menning, J. Sol–Gel structuring the nanobuilding blocks prior to their assembly, Sci. Technol., 1997, 8, 923, 927. one can also functionalize them with mesogenic groups and 16 L. Hou, M. Menning and H. Schmidt, Proc Eurogel’92, 1992, give them self-organizing properties.p. 173. 17 J. Biteau, F. Chaput and J. P. Boilot, J. Phys. Chem., 1996, 100, As described by Mann et al,76 the traditional view of 9024. inorganic solids as condensed matter is currently being 18 D. Levy, Chem. Mater., 1997, 9, 2666. reshaped by new insight in materials synthesis.77–79 A new 19 B. Schaudel, C. Guermeur, C. Sanchez, K. Keitaro and J.Delaire, paradigm, organized matter chemistry, is being formulated. In J. Mater. Chem., 1997, 7, 61. this field the sol–gel chemistry of organized matter is a very 20 S. Dire�, F. Babonneau, C. Sanchez and J. Livage, J. Mater. Chem., 1992, 2, 239. promising field of research which is just beginning to be 21 F. Babonneau, Polyhedron, 1994, 13, 1123. explored. Hybrids of both Class I and II4h and more particu- 22 C.Guermeur and C. Sanchez, forthcoming paper. larly those obtained through the ‘nanobuilding block 23 Applied photochromic polymers systems, ed. C. B. McArdle, approach’ will have paramount importance in the exploration Gordon and Breach Science Publishers, New York, 1991. the theme of ‘synthesis with construction’ of hierarchically 24 S.K. Yuh, E. Bescher, F. Babonneau and J. D. Mackenzie, Mater. Res. Soc. Symp. Proc., 1994, 346, 803. organized (structure and function) materials. J. Mater. Chem., 1999, 9, 35–44 4325 N. Koslova, B. Viana and C. Sanchez, J. Mater. Chem., 1993, M. Dumont, New J. Chem., 1996, 20, 13; (b) B. Lebeau, C. Sanchez, S. Brasselet and J. Zyss, Mater. Res. Soc. Symp. Proc., 3, 111. 1996, 435, 395; (c) B.Lebeau, C. Guermeur and C. Sanchez, 26 L. R. Mathews and E. T. Knobbe, Mater. Res. Soc. Symp. Proc., Mater. Res. Soc. Symp. Proc., 1994, 346, 315. 1993, 286, 259. 57 B. Lebeau, J. Maquet, C. Sanchez, E. Toussaere, R. Hierle and 27 I. M. Thomas, S. A. Payne and G. D. Wilke, J. Non-Cryst. Solids, J. Zyss, J. Mater. Chem., 1994, 4, 1855. 1992, 151, 183. 58 B. Lebeau, C.Sanchez, S. Brasselet and J. Zyss, Chem. Mater., 28 I. M. Thomas, S. A. Payne and G. D. Wilke, J. Non-Cryst. Solids, 1997, 9, 1012. 1992, 151, 183. 59 B. Lebeau, J. Maquet, C. Sanchez, F. Beaume and F. Laupre�tre, 29 A. A. Kaminskii, in Laser Crystals, Springer Verlag, Berlin, J. Mater. Chem., 1997, 7, 989. Heidelberg, New York, 2nd edn., 1981, pp. 361–433. 60 C. Sanchez and B.Lebeau, Pure Appl.Opt., 1996, 5, 689. 30 M. Lecomte, B. Viana and C. Sanchez, J. Chim. Phys., 1991, 61 R. F. Shi, M. H. Wu, S. Yamada, Y. M. Cai and A. F. Garito, 88, 39. Appl. Phys. Lett., 1993, 63, 1173. 31 B. Viana, N. Koslova, P. Aschehoug and C.Sanchez, J. Mater. 62 D. Blanc, P. Peyrot, C. Sanchez and C. Gonnet, Opt. Eng. Chem., 1995, 5, 719. Integrated Opt., 1998, 37, 1203. 32 C. Guizard, J.C. Achddou, A. Larbot, L. Cot, G. Le Flem, 63 C. Fiorini, F. Charra, J. M. Nunzi, I. D. W. Samuel and J. Zyss, C. Parent and C. L. Lurin, in ref. 5(c), p. 208. Opt. Lett., 1995, 20, 2469. 33 M. Genet, V. Brandel, M. P. LaHalle and E. Simoni, in ref. 5(c), 64 F. Ribot, F. Banse and C. Sanchez, Mater. Res. Soc. Symp. Proc., p. 194. 1994, 346, 121. 34 W. Nie, B. Dunn, C.Sanchez and P. Griesmar, Mater. Res. Soc. 65 F. Ribot, F. Banse, F. Diter and C. Sanchez, New J. Chem., 1995, Symp. Proc., 1992, 271, 639. 19, 1145. 35 M. Canva, P. Georges, G. Lesaux, A. Brun, F. Chaput and 66 (a) F. Ribot, F. Banse, C. Sanchez, M. Lahcini and J. P. Boilot, J. Non-Cryst. Solids, 1992, 147, 627. B. Jousseaumme, J. Sol–Gel Sci. Technol., 1997, 8, 529; 36 G. J.Dirksen and G. Blasse, J. Solid State Chem., 1991, 92, 591. (b) L. Angiolini, D. Caretti, C. Carlini, R. De Vito, F. T. Niesel, 37 A. Diaz and D. A Keszler, Chem. Mater. 1997, 9, 2071. E. Salatelli, F. Ribot and C. Sanchez, J. Inorg. Organomet. Polym., 38 A. Diaz and D. A Keszler, Mater. Res. Bull., 1996, 31, 147. 1998, 7, 151. 39 H. F. Folkerts and G. Blasse, J. Mater. Chem., 1995, 5, 1547. 67 (a) F. Ribot,C. Eychenne-Baron and C. Sanchez, Mater. Res. Soc. 40 E. Cordoncillo, P. Escribano, B. Viana and C. Sanchez, J. Mater. Symp. Proc., 1996, 435, 43; C. Eychenne-Baron, F. Ribot and Chem., 1998, 8, 1507. C. Sanchez, J. Organomet. Chem., 1998, 567, 137. 41 J. Chrusciel and Z. Lasocki, Pol. J. Chem., 1983, 57, 121. 68 F. Ribot, C. Eychenne-Baron and C. Sanchez, Mater. Res. Soc. 42 R. Campostrini and S. Dire�, Eurogel Proceedings, Advanced Symp. Proc., 1998, 519, in press. Materials and Processes by Sol–Gel Techniques, Colmar, 1992, ed. 69 F. Banse, F. Ribot, P. Tole�dano, J. Maquet and C. Sanchez, Inorg. S. Vilminot, Strasbourg, 1995, p. 307. Chem., 1995, 34, 6371. 43 G. Pucetti, I. Ledoux, J. Zyss, P. Griesmar and C. Sanchez, Polym. 70 H. PuV and H. Reuter, J. Organomet. Chem., 1989, 373, 173. Prepr., 1991, 32, 61. 71 D. Dakternieks, H. Zhu, E. R. T. Tiekink and R. J. Colton, 44 J. Zyss, G. Pucetti, I. Ledoux, P. Griesmar, J. Livage and J. Organomet. Chem., 1994, 476, 33. C. Sanchez, Fr. Pat., 1152, March 1991. 72 P. Jaumier, B. Jousseaume, M. Lahcini, F. Ribot and C. Sanchez, 45 (a) E. Toussaere, J. Zyss, P. Griesmar and C. Sanchez, Non Linear Chem. Commun., 1998, 369. Optics, 1991, 1, 349; (b) C. Sanchez, P. Griesmar, E. Toussaere, 73 (a) R.C.Mehrotra, R. Bohra and D. P. Gaur, Metal b-diketonates G. Puccetti, I. Ledoux and J. Zyss, Non Linear Optics, 1992, 4, 245. and Allied Derivatives, Academic Press, London, 1978; 46 P. Griesmar, C. Sanchez, G. Pucetti, I. Ledoux and J. Zyss, Mol. (b) D. C. Bradley, R. C. Mehrotra and D. P. Gaur, Metal Eng., 1991, 1, 205. Alkoxides, Academic Press, London, 1978; (c) L. G. Hubert- 47 (a) J. Kim, J. L. Plawsky, R. LaPeruta and G. M. Korenowski, Pfalzgraf, New J. Chem., 1987, 11, 663; ( f ) A. Leaustic, Chem. Mater., 1992, 4, 249; (b) J. Kim, J. L. Plawsky, E. Van F. Babonneau and J. Livage, Chem. Mater., 1989, 1, 248; Wagenen and G. M. Korenowski, Chem.Mater., 1993, 5, 1118. ( g) F. Ribot, P. Tole�dano and C. Sanchez, Chem. Mater., 1991, 3, 48 (a) Y. Haruvy ander, Mater. Res. Soc. Symp. Proc., 759; (h) C.Sanchez, M. In, P. Toledano and P. Griesmar, Mater. 1992, 271, 297; (b) Q. Hibben, E. Lu, Y. Haruvy and S.E. Weber, Res. Soc. Symp. Proc., 1992, 271, 669. Chem. Mater., 1994, 6, 761. 74 C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst. 49 F. Chaput, D. Riehl, Y Levy and J. P. Boilot, Chem.Mater., 1993, Solids, 1988, 100, 650. 5, 589. 75 S. DoeuV, M. Henry, C. Sanchez and J. Livage, J. Non-Cryst. 50 Y. Zhang, P. N. Prasad and R. Burzynski, Chem. Mater., 1992, Solids, 1987, 89, 206. 4, 851. 76 S. Mann, S. L. Burkett, S. A. Davis, C. E. Fowler, 51 Y. Nosaka, N. Tohriiwa, T. Kobayashi and N. Fujii, Chem. N. H. Mendelson, S. D. Sims, D.Walsh and N. T. Whilton, Chem. Mater., 1993, 5, 930. Mater., 1997, 9, 2300. 52 S. Marturunkakul, J. I. Chen, R. J. Jeng, S. Sengupta, J. Kumar 77 S. Mann and G. A. Ozin, Nature, 1996, 382, 313. and S. K. Tripathy, Chem. Mater., 1993, 5, 743; K. Izawa, 78 C. G. Go� ltner and M. Antonietti, Adv. Mater., 1997, 9, 431. N. Okamoto and O. Sugihara, Jpn. J. Appl. Phys., 1993, 32, 807. 79 (a) A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, 53 L. Kador, R. Fischer, D. Haarer, R. Kaseman, S. Bru� ck, R. S. Maxwell, G.D. Stucky, M. Krishnamurty, P. PetroV, H. Schmidt and H. Du� rr, Adv. Mater., 1993, 5, 270. A. Firouzi, M. Janicke and B. F. Chmelka, Science, 1993, 261, 54 (a) J. R. Caldwell, R. W. Cruse, K. J. Drost, V. P. Rao, A. K.- 1299; (b) A. Firouzi, D. Kumar, L. M. Bull, T. Besier, P. Sieger, Q. Hue, S. A. Walker, J. A. Zasadinski, C. Glinka, J. Nicol, D. Y. Jen, K. Y. Wong, Y. M. Cali, R. M. Mininni, J. Kenney, Margolese, G. D. Stucky and B. F. Chmelka, Science, 1995, 267, E. Binkley, L. R. Dalton, Y. Shi and W. H. Steier, Mater. Res. 1133; (c) D. Y. Zhao, P. D. Yang, Q. S. Huo, B. F. Chmelka and Soc. Symp. Proc., 1994, 328, 531; (b) Z. Yang, C. Xu, B. Wu, L. G. D. Stucky, Curr. Opin. Solid State Mater. Sci., 1998, 3, 111; (d) R. Dalton, S. Kalluri, W. H. Steier, Y. Shi and J. H. Bechtel, C. J. Brinker, Curr. Opin. Colloid Interface Sci., 1998, 3, 166; (e) Chem. Mater., 1994, 6, 1899; (c) H. W. Oviatt, K. J. Shea, S. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, Kalluri, Y. Shi, W. Steier and L. R. Dalton, Chem. Mater., 1995, W. Gong, Y. Guo, H. Soyez, B. Dunn, M. H. Huang and J. I. 7, 493. Zink, Nature, 1997, 389, 364; ( f ) G. H. Mehl and J. W. Goodby, 55 (a) F. Chaput, D. Reihl, Y. Levy and J.-P. Boilot, Chem. Mater., Chem. Ber., 1996, 129, 521. 1993, 5, 589; (b) D. Reihl, F. Chaput, Y. Levy, J.-P. Boilot, F. Kajzar and P. A. Chollet, Chem. Phys. Lett., 1995, 245, 36. 56 (a) B. Lebeau, C. Sanchez, S. Brasselet, J. Zyss, G. Froc and Paper 8/05538F 44 J. Mater. Chem., 1999, 9, 35&nd