Alkoxysilylated Derivatives of Double Four Ring Silicate

Alkoxysilylated-Derivatives of Double-Four-Ring Silicate as Novel Building Blocks of Silica-Based Materials†Yoshiaki Hagiwara,‡Atsushi Shimojima,*,§,|and Kazuyuki Kuroda*,‡,§,#Department of Applied Chemistry,Waseda University,Ohkubo-3,Shinjuku-ku,Tokyo169-8555,Japan,Kagami Memorial Laboratory for Materials Science and Technology,Waseda University,Nishiwaseda-2,Shinjuku-ku,Tokyo169-0051,Japan,and CREST,Japan Scienceand Technology Agency(JST),JapanReceived June18,2007.Revised Manuscript Received July11,2007Novel oligomeric alkoxysilanes consisting of double-four-ring(D4R)units functionalized with mono-,di-,and trialkoxysilyl groups(designated as1,2,and3,respectively)were designed as nanobuilding blocks for synthesizing silica-based nanomaterials.Hydrolysis and polycondensation of these oligomers were performed under acidic conditions.The gelation occurred much faster than monomeric,dimeric,and cubic octameric alkoxysilanes,which should depend on both molecular size and siloxane structures.Structural analyses of the xerogels derived from1-3strongly suggested that the D4R units are at least partly retained in their frameworks.The number of alkoxy groups had a large effect on their porosities: the xerogel derived from2was a microporous solid with the BET surface area of600m2 g-1,whereas1-and3-derived xerogels had very low BET surface areas.Furthermore, mesostructured silica films were successfully synthesized from2and3by using amphiphilic triblock copolymer surfactant as a structure-directing agent.Such an approach will lead to the construction of molecularly designed siloxane networks.IntroductionSol-gel chemistry of alkoxysilanes represents one of the fundamental approaches for synthesizing silica-based materials under mild conditions.1The structural control of silica at various length scales is important not only for basic research but for many practical applications including catalysis and adsorption.The use of oligosiloxane units with well-defined structures as molecular building blocks is promising for the rational design of siloxane structures having at least a short-range order.Moreover,the precise control over the as sembly of those building blocks is a significant issue, leading to the construction of hierarchically ordered materials.Siloxane cage compounds,such as those having a double-four-ring(D4R)unit,are particularly useful as molecular building blocks because of their rigid and symmetrical frameworks.2,3It is known that D4R sili-cate anions(Si8O208-)are formed in high yield in alkaline silicate solutions containing tetramethylam-monium(TMA)cations.4Alternatively,organic deriva-tives(R8Si8O12,where R)H or organic groups)can be obtained by controlled hydrolysis and polycondensation of the corresponding trifunctional silanes.5Cross-linking of D4R units via Si-O-Si linkages is expected to offer a building block approach to molecularly ordered silica like zeolites.6However,only a few attempts have been reported so far,whereas a variety of hybrid materials have been prepared by covalent linking of D4R units via Si-C and C-C bonds.7Klemperer and co-workers reported the preparation of microporous silica xerogels by the sol-gel processes of cubic octameric alkoxysilane, an alkoxy-derivative of the D4R silicate((MeO)8Si8O12).8 The mesoscopic organization of D4R silicate anions has also been achieved by electrostatic interaction with cationic surfactants,forming lamellar,2D hexagonal, and cubic structures.9The molecular design of even larger building blocks with alkoxy-functionality should lead to the structural and morphological diversity of resulting materials.An interesting approach is to functionalize D4R units with†Part of the“Templated Materials Special Issue”.*To whom correspondence should be addressed.Phone&Fax:81-3-5286-3199.E-mail:kuroda@waseda.jp(K.K.);shimoji@chemsys.t.u-tokyo.ac.jp(A.S.).‡Department of Applied Chemistry,Waseda University.§CREST,Japan Science and Technology Agency.||Present address:Department of Chemical System Engineering, The University of Tokyo,Hongo-7,Bunkyo-ku,Tokyo113-8656,Japan #Kagami Memorial Laboratory for Materials Science and Technol-ogy,Waseda University.(1)Brinker, C.J.;Scherer,G.W.Sol-Gel Science;Academic Press:San Diego,CA,1990.(2)Laine,R.M.J.Mater.Chem.2005,15,3725.(3)Harrison,anomet.Chem.1997,542,141.(4)Hoebbel,D.;Garzo´,G.;Engelhardt,G.;Vargha,A.Z.Anorg. Allg.Chem.1982,494,31.(5)(a)Agaskar,P.A.Inorg.Chem.1991,30,2707.(b)Bassindale,A.R.;Liu,Z.;MacKinnon,I.A.;Taylor,P.G.;Yang,Y.;Light,M.E.; Horton,P.N.;Hursthouse,M.B.Dalton Trans.2003,2945.(6)Morris,R.E.J.Mater.Chem.2005,15,931.(7)(a)Hoebbel,D.;Endres,K.;Reinert,T.;Pitsch,I.J.Non-Cryst. Solids1994,176,179.(b)Harrison,P.G.;Kannengiesser,R.Chem. Commun.1996,415.(c)Morrison,J.J.;Love,C.J.;Manson,B.W.; Shannon,I.J.;Morris,R.E.J.Mater.Chem.2002,12,3208.(d)Zhang, C.;Babonneau,F.;Bonhomme,C.;Laine,R.M.;Soles,C.L.;Hristov,H.A.;Yee,A.F.J.Am.Chem.Soc.1998,120,8380.(8)(a)Day,V.W.;Klemperer,W.G.;Mainz,V.V.;Millar,D.M.J. Am.Chem.Soc.1985,107,8262.(b)Klemperer,W.G.;Mainz,V.V.; Millar,D.M.Mater.Res.Soc.Symp.Proc.1986,73,3.(c)Cagle,P.C.;Klemperer,W.G.;Simmons,C.A.Mater.Res.Soc.Symp.Proc. 1990,180,29.10.1021/cm0716194CCC:$27.50©xxxx American Chemical SocietyPAGE EST:6.7Published on Web09/08/2007additional eight alkoxysilyl groups,leading to spherical molecules with 16silicon atoms.Most recently,octavi-nyl-substituted D4R was hydrosilylated with HSi(OEt)3to design novel building blocks that formed mesoporous silsesquioxane-based materials.10In contrast to this strategy,we focus our efforts on designing a new type of siloxane-based oligomers,where all of the Si atoms are connected via Si -O -Si linkages.Such molecules can be directly synthesized by silylation of the corner SiO -groups of D4R silicates.Silylated-derivatives containing functional groups such as Si -H and Si -CH d CH 2have been prepared by silylation of D4R silicates with organochlorosilanes.11Also,porous solids have been prepared by covalent linking of D4R units by silylation with dimethyldichlorosilanes.12However,to the best of our knowledge,functionalization of D4R units with alkoxysilyl groups has been unprecedented,although such molecules are potentially useful for the sol -gel synthesis of new materials.In this paper,we report the synthesis of novel alkoxysilylated-derivatives of D4R silicates and their use as building blocks for constructing silica-based nanomaterials.Under well-regulated conditions,we succeeded in synthesizing three types of oligomeric alkoxysilanes,designated as 1,2,and 3(Scheme 1),by silylation of D4R silicate with dimethylethoxychlorosi-lane [Me 2(EtO)SiCl],methyldiethoxychlorosilane [Me-(EtO)2SiCl],and triethoxychlorosilane [Si(EtO)3SiCl],respectively.The reaction relies on the much higher reactivity of Si -Cl groups than the Si -OEt groups;therefore,the Si -Cl groups preferentially react with Si -OH (or O -)groups of silicate species.13These oligo-mers consist of a rigid D4R core and branched alkox-ysilyl groups with different numbers of alkoxy groups,which should affect the structure of the final products.The hydrolysis and polycondensation behaviors of 1,2,and 3under acidic conditions were compared with those of monomeric,dimeric,and cubic octameric alkoxysi-lanes.14Detailed characterization of the resulting xe-rogels was performed to understand their structural features.Furthermore,amphiphilic triblock copolymersurfactant (EO 20PO 70EO 20)was used as a structure-directing agent to form thin films with hierarchical structures.Experimental SectionMaterials.Dimethyldichlorosilane,methyltrichlorosilane,tetrachlorosilane (Tokyo Kasei Co.,>98%)and ethanol (dried,Wako Pure Chemical Co.,99.5%)were used for the synthesis of alkoxychlorosilanes.Tetramethylammonium hydroxide pen-tahydrate (Sigma-Aldrich,97%)and TEOS (Kishida Chemical Co.,>99%)were used to prepare D4R silicates.Poly(oxyeth-ylene)-poly(oxypropylene)-poly(oxyethylene)-type triblock co-polymer (EO 20PO 70EO 20,P123)was obtained from Sigma-Aldrich.Other chemicals,including pyridine (dried,Wako Pure Chemical Co.,99.5%),tetrahydrofuran (THF,dried,Wako Pure Chemical Industries,99.5%),hexane (Kanto Chemical Co.,96%),chloroform (Wako Pure Chemical Industries,99%),and 1N and 5N HCl (Wako Pure Chemical Industries)were used as received.Syntheses of Silylating Agents.Dimethylethoxychlorosi-lane [Me 2(EtO)SiCl],methyldiethoxychlorosilane [Me(EtO)2SiCl],and triethoxychlorosilane [(EtO)3SiCl]were synthesized by dropwise addition of ethanol to the corresponding chlorosilanes (molar ratios of ethanol/chlorosilanes were 1,2,and 3,respectively).The gaseous HCl generated by the reaction was allowed to go out of the vessel and was neutralized.The resulting liquids were the mixtures of silanes with various numbers of substituted alkoxyl groups,although monochloro-alkoxysilanes were dominant.After the removal of the species containing more than two Si -Cl groups under reduced pres-sure,the silylating agents [Me 2(EtO)SiCl,Me(EtO)2SiCl,and (EtO)3SiCl]containing 10,30,and 50%of completely alkoxy-lated species (Me 2Si(OEt)2,MeSi(OEt)3,and Si(OEt)4),respec-tively,were obtained as clear,colorless liquids.29Si NMR (99.3MHz,CDCl 3):δ13.3(Me 2(EtO)SiCl),-28.2(Me(EtO)2SiCl),and -70.3((EtO)3SiCl).Syntheses of Oligomeric Alkoxysilanes (1,2,and 3).D4R silicates (Si 8O 208-)were prepared from the mixture of TEOS/TMAOH/water/ethanol with the molar ratio of 1/1/10/10by vigorous stirring at room temperature for 3days.Hydrated crystals [(Me 4N)8Si 8O 20‚65H 2O]15were easily ob-tained by slow cooling of the concentrated D4R silicate solution.These crystals were dehydrated at 60°C under reduced pressure,yielding white powders.For alkoxysilylation,1g of this powder dissolved in 1.5mL of ethanol was added to the mixture of silylating agent,THF (100mL),and pyridine under vigorous stirring in an ice bath.The D4R/silylating agent/pyridine molar ratio was 1/150/10/0.Pyridine was added to neutralize HCl generated upon silylation.After the mixture was stirred for 30min,the unreacted silylating agents and solvents were removed under reduced pressure.The precipi-tates of pyridine hydrochloride were then removed by filtration to give a clear solution.Further removal of volatile components (mostly monomeric and dimeric alkoxysilanes)at 100°C under reduced pressure gave viscous liquids,from which the oligo-mers 1,2,and 3were finally isolated by gel permeation chromatography (GPC)using chloroform as the eluent.(9)(a)Fyfe,C.A.;Fu,G.J.Am.Chem.Soc.1995,117,9709.(b)Firouzi,A.;Kumar,D.;Bull,L.M.;Besier,T.;Sieger,P.;Huo,Q.;Walker,S.A.;Zasadzinski,J.A.;Glinka,C.;Nicol,J.;Margolese,D.;Stucky,G.D.;Chmelka,B.F.Science 1995,267,1138.(10)Zhang,L.;Abbenhuis,H. C.L.;Yang,Q.;Wang,Y.-M.;Magusin,P.C.M.M.;Mezari,B.;Santen,R.A.;Li,C.Angew.Chem.,Int.Ed.2007,46,5003.(11)(a)Hoebbel,D.;Reinert,T.;Endres,K.;Schmidt,H.Mater.Res.Soc.Symp.1994,346,863.(b)Hasegawa,I.;Ino,K.;Ohnishi,anomet.Chem.2003,17,287.(c)Hasegawa,I.;Motojima,anomet.Chem.1992,441,373.(12)Hasegawa,I.J.Sol -Gel Sci.Technol.1995,5,93.(13)Mochizuki,D.;Shimojima,A.;Kuroda,K.J.Am.Chem.Soc.2002,124,12082.(14)The synthesis procedures of dimeric and cubic octameric alkoxysilanes are contained in the Supporting Information.(15)Wiebcke,M.;Grube,M.;Koller,H.;Engelhardt,G.;Felsche,J.Microporous Mater.1993,2,55.Scheme 1.Novel Alkoxysilylated D4R Silicates (1,2and 3)as Nanobuilding Blocks of Xerogels (1G,2G,and 3G)and Mesostructured Films (1F,2F,and3F)B Chem.Mater.Hagiwara et al.Synthesis of Xerogels(1G,2G,and3G).Hydrolysis and polycondensation of the oligomers1,2,and3were conducted in the mixture of ethanol,H2O,and HCl.The mixtures were stirred at room temperature in closed vessels until gelation occurred.The molar ratio of oligomer/ethanol/H2O/HCl was 1/16/16/0.53.The xerogels1G,2G,and3G obtained by drying under a vacuum were pulverized before characterization.For comparison,xerogels were prepared by hydrolysis and poly-condensation of the1/1mixture of dimethyldiethoxysilane (DMDES)and TEOS,1/1mixture of methyltriethoxysialne (MTES)and TEOS,as well as TEOS alone.Synthesis of Mesostructured Films(1F,2F,and3F). The oligomers were partially hydrolyzed and polycondensed by stirring in the mixture of ethanol,H2O,and HCl at room temperature for1h.The mixtures were then added to the ethanol solution of a triblock copolymer surfactant(P123).The final molar ratio of the mixtures was1(or2or3)/ethanol/H2O/ HCl/P123)1/608/132/0.64/0.32.After being stirred for5min, the mixtures were spin-coated on glass substrates.Thin films were air-dried at room temperature for1day to afford as-synthesized films.The removal of the surfactant was carried out either by calcination at320°C for4h in air or by solventextraction(immersing the films in a stirred solution of ethanol (200mL)at room temperature for1day).Characterization.Liquid-state29Si NMR spectra were obtained on a JEOL Lambda-500spectrometer with resonance frequencies of99.25MHz.Solid-state29Si magic-angle spinning (MAS)NMR measurements were performed on a JEOL JNM-CMX-400spectrometer at a resonance frequency of79.42MHz, with a pulse width of45°and a recycle delay of100s. Deconvolution of the spectrum was performed by using a Gaussian function on Spinsight software,version4.3.2.Chemi-cal shifts for13C and29Si NMR were referenced to tetrameth-ylsilane at0ppm.Powder X-ray diffraction(XRD)patterns were recorded on a Mac Science M03XHF22diffractometer with Mn-filtered Fe K R radiation or on a Mac Science MXP3 diffractometer with Ni-filtered Cu K R radiation.Transmission electron microscopic(TEM)images of the mesostructured films were obtained on a JEOL JEM-2010microscope operated at 200kV.To prepare TEM samples,the films were scraped from substrates and ground with mortar and pestle and dispersed in ethanol.A carbon-coated copper grid was dipped in this dispersion and allowed to dry in air.FT-IR spectra of the products in KBr pellets were obtained using a Perkin-Elmer Spectrum One spectrometer with a nominal resolution of0.5 cm-1.Nitrogen adsorption measurements were performed by an Autosorb-1instrument(Quantachrome Instruments,Inc) at77K.Samples were preheated at120°C for3h under1×10-2Torr.Results and Discussion Alkoxysilylation of D4R silicate(Si8O208-).Hy-drated crystals of D4R silicate[(Me4N)8Si8O20‚65H2O]15 was dried at60°C under a vacuum prior to alkoxysi-lylation,which was crucial to minimize possible side reactions caused by water.Elemental analysis combined with thermogravimetry(TG)revealed that the H2O/D4R ratio decreased to about29.The solid-state29Si MAS NMR spectrum of these powders(see the Supporting Information,Figure S1)displayed a single signal at -96.3ppm(Q3unit),confirming that neither cleavage nor polycondensation of D4R units occurred during the drying process.Figure1shows the29Si NMR spectra of the oligomers 1,2,and3obtained by silylation of D4R silicate with alkoxychlorosilanes.The spectrum of1shows two signals at-9.9and-109.8ppm with the intensity ratio of1/1.These signals correspond to the D1and Q4units, respectively.16The oligomers2and3exhibit the T1 (-50.1ppm)and Q1(-89.4ppm)signals,16respectively,in addition to the Q4signals at around-110ppm.The presence of the Q4units strongly suggests that the (SiO)3SiOH(or O-)units of the D4R silicate(observed at-98ppm)were completely silylated without deterio-ration of the D4R units.The retention of the ethoxy groups without hydrolysis during silylation was con-firmed by13C NMR spectra(data not shown)that showed the signals of Si-O C H2C H3(59and18ppm). Thus the successful syntheses of1,2,and3are evident from these NMR data.Hydrolysis and Polycondensation Behavior.Hy-drolysis and polycondensation of1,2,and3led to gelation within several minutes,yielding transparent xerogels(1G,2G,and3G,respectively).The gelation time was much shorter than those of monomeric(Si-(OEt)4,TEOS)and dimeric((EtO)3Si-O-Si(OEt)3)alkox-ysilanes.14When these alkoxysilanes were reacted under the conditions at which the initial EtOH/Si,HCl/ Si,and H2O/SiOEt molar ratios were identical to those for synthesizing3G,gelation finally occurred after6and 3.5days of the reactions in the cases of monomeric and dimeric alkoxysilanes,respectively,even though13C NMR analysis revealed that ethoxy groups were almost completely hydrolyzed within1h(data not shown). Apparently,the larger molecular size of3allowed efficient growth of siloxane networks,resulting in extremely rapid gelation.The difference in the final EtOH/Si ratios in the systems after complete hydrolysis of ethoxy groups may also affect the gelation time. Actually,the ratio in the system of3(EtOH/Si)2.5) was relatively lower than those of monomer and dimer (EtOH/Si)5and4,respectively),thus favoring gela-tion.It should be noted,however,that gelation of3 occurred much faster(within6h)even when the molar ratio of ethanol was increased to the final EtOH/Si ratio of6.5.In addition to the molecular size,the siloxane struc-ture of the precursors had large effects on hydrolysis and polycondensation behavior.The reactivity of3 appeared to be much higher than the cubic octameric alkoxysilane(i.e.,ethoxy-derivative of D4R units).The(16)It is well-established that the29Si chemical shifts are largely influenced by the number of Si-C bonds and siloxane(Si-O-Si)bonds. ThesymbolsD n,T n,andQ n representC2Si(OSi)n(OX)2-n,C Si(OSi)n(OX)3-n, and Si(OSi)n(OX)4-n,where X)Et or H,respectively.Figure1.29Si NMR spectra of the oligomers:(a)1,(b)2, and(c)3in CDCl3.Building-Block Approach to Siloxane-Based Nanomaterials Chem.Mater.C13CNMR analysis of the hydrolysis process of cubic octameric alkoxysilane revealed that only 40%of alkoxy groups were hydrolyzed even after 1day of reaction.In this system,the amount of ethanol was increased to a final EtOH/Si ratio of 4,identical to that for the TEOS system,because of the lower solubility of octameric alkoxysilane (crystalline solids).Gelation did not occur even after 5days of reaction,which is in clear contrast to 3,the reaction of which proceeded so fast until gelation that we could not monitor by liquid-state NMR.Such a difference can be explained in terms of the hydrolysis mechanism;1,8hydrolysis of the cubic octamer should occur without inversion of the SiO 4tetrahedra,different from the S N 2type reaction potentially avail-able for 3.Thus we have demonstrated the specific feature of the oligomeric alkoxysilanes used in the present study,which is due to the large molecular size as well as the high reactivity of corner -Si(OEt)3groups.We expect that higher reaction rate of such oligomeric alkoxysi-lanes should contribute to minimization of the possible hydrolysis of Si -O -Si linkage during the reaction,which was previously reported for the methoxy-deriva-tive of D4R units.8Also important is that the use of oligomeric alkoxysi-lanes (1and 2),consisting of both SiO 4and methyl-substituted species (CSiO 3or C 2SiO 2),promises superior homogeneity of final xerogels if compared to that achieved by co-condensation of the mixture of mono-meric alkoxysilanes.Actually,the xerogel derived from the DMDES -TEOS mixture was visibly opaque,which suggests that phase separation occurred during the synthesis.In general,the reaction rate of alkoxysilanes is dominated by both inductive and steric effects.1Under acidic conditions,methyl-substituted alkoxysilanes such as DMDES and MTES are hydrolyzed faster than TEOS.It is therefore plausible to consider that each component is not homogeneously distributed in the final xerogel networks in the cases of the DMDES -TEOS and MTES -TEOS systems.Structures of Xerogels Prepared from 1,2,and 3.The formation of siloxane networks was confirmed by solid-state 29Si MAS NMR spectra of 1G ,2G ,and 3G (Figure 2).Note that structural modeling suggested that only intermolecular condensation was possible.Intramoleculer condensation between two alkoxysilyl groups to form a tetrasiloxane ring was strongly inhib-ited because of the large angle between the two Si -O bonds at the adjacent corners of a D4R unit.The spectrum of 1G (Figure 2a)mainly shows three signals corresponding to the D 1,D 2,and Q 4units.The D 2signal arises from the hydrolysis and condensation of -SiMe 2-(OEt)groups of 1to form siloxane linkages.From the D 2/(D 1+D 2)intensity ratio,it is estimated that 96%of the silyl groups at the cage corners are linked together.The spectrum of 2G consists of the signals correspond-ing to the T 1,T 2,and T 3units,along with the Q 4signal (Figure 2b).The (T 2+T 3)/(T 1+T 2+T 3)ratio indicates that 62%of the silyl groups of 2formed siloxane bonds.In the case of 3G ,broad Q 2,Q 3,and Q 4signals were observed (Figure 2c).The absence of the Q 1signal indicates that all of the terminal -Si(OEt)3groups of 3were hydrolyzed and polycondensed to form at least one siloxane bond.However,more detailed analysis of this spectrum is difficult because of the overlapping of the signals originating from the D4R units and the terminal -Si(OEt)3groups of 3.The Q 4signals observed for 1G and 2G are much narrower than those for the xerogels derived from the DMDES -TEOS and MTES -TEOS systems constituted of random siloxane networks (see the Supporting In-formation,Figure S2).This fact strongly suggests that the D4R units are retained in the networks of 1G and 2G .Although the small Q 3signals are indicative of the cleavage of Si -O -Si linkages during the reaction,their intensity ratios are smaller than those observed in Figure S2.In particular,the high Q 4/Q 3ratio of 2G is in clear contrast to the MTES -TEOS system where the large Q 3signals and even Q 2signal are observed.Also interesting is that the T 3/T 2ratios for 2G is much smaller than for the xerogels derived from the MTES -TEOS system.This indicates that the formation of three Si -O -Si linkage is sterically restricted.But unfortu-nately,we cannot see any difference in the spectra of 3G and TEOS-derived xerogel.Further information on the siloxane networks was obtained by FT-IR.Figure 3shows the FT-IR spectra of 1G ,2G ,and 3G .All of the samples show the absorption bands due to siloxane frameworks at around 400-450,800,and 1000-1200cm -1,1consistentwithFigure 2.Solid-state 29Si MAS NMR spectra of the xerogels:(a)1G ,(b)2G ,and (c)3G.Figure 3.FT-IR spectra of the xerogels:(a)1G ,(b)2G ,and (c)3G .D Chem.Mater.Hagiwara et al.the 29Si MAS NMR data.Also,the band due to Si -OH is observed at around 950cm -1.This peak becomes more prominent as the number of alkoxy groups of the oligomers increases (i.e.,1G <2G <3G ).This is in agreement with the 29Si NMR data showing that 3G has a certain amount of the Q 3units,whereas alkoxy groups in 1G were fully hydrolyzed and polycondensed to form the D 2units.The minor differences in the shapes and positions of these bands can be attributed to the differences in the arrangement of siloxane networks and to the number of methyl groups.It is important to note here that 1G ,2G ,and 3G exhibit characteristic bands at 550cm -1that are apparently larger than for the xerogels prepared from the mixture of the monomeric alkoxysilanes (see the Supporting Information,Figure S3).This band is reported to be associated with the vibration of D4R units.10,17This fact,together with the 29Si MAS NMR data showing the presence of the Q 4units as the dominant species,strongly suggests that D4R units are retained in the xerogel networks.Figure 4shows the powder XRD patterns of the xerogels 1G ,2G ,and 3G .All of the samples exhibit broad diffraction peaks at around 2θ)25+(d )0.3nm),which is typically observed for amorphous silica.Additional peaks also appear at lower scattering angle (d ≈1.1nm)in the patterns of 2G and 3G ,suggesting a certain structural periodicity in these products.Such diffraction peaks have previously been observed for the XRD patterns of xerogels prepared by the reaction of D4R silicates with dimethydichlorosilanes.12However,these peaks are also observable for the xerogels derived from the DMDES -TEOS and MTES -TEOS mixtures (see the Supporting Information,Figure S4).We there-fore suppose that the structural periodicities are not associated with the D4R units,but they may be at-tributed to some ring structures,such as tetrasiloxane rings and ladder polymers,although the details are still unclear.Porosity of Xerogels (1G,2G,and 3G).The num-ber of alkoxy groups in the oligomers is critical to the porosity of the resulting silica-based xerogels.Figure 5shows the nitrogen adsorption isotherms of 1G ,2G ,and 3G .The isotherm of 2G displays a type I curve,typicalof microporous silica.The Brunauer -Emmett -Teller (BET)surface area and Saito -Foley (SF)pore diameter were calculated to be 600m 2g -1and ca.1nm,respectively.In contrast,1G and 3G show no increase of the nitrogen uptake,suggesting that they are non-porous or have only closed pores.Such a difference should reflect the variation in the siloxane networks,and two Si -OEt groups at one cage corner are ap-propriate to generate pores in the products.In the case of 1G ,the oligomer 1is capable of forming only one siloxane bond at each corner of the D4R unit,resulting in the formation of relatively “flexible”net-works that possibly undergo large shrinkage during drying process.Previously,Harrison and co-worker reported the xerogel with high surface area (218.3m 2g -1)prepared from monobromosilylated-D4R units,which is similar to 1with respect to the number of reactive sites.7b Such a difference is possibly due to the difference in the reactivities of Si -Br and Si -OEt groups.On the other hand,the oligomer 3possesses 24functional groups (Si -OEt)and is therefore expected to form rather “dense”networks,leaving no void spaces between the D4R units accessible for N 2molecules.18This is an unusual phenomenon in sol -gel chemistry because acid-catalyzed hydrolysis and polycondensation of tetraalkoxysilanes generally yield porous silica xe-rogels.Actually,monomeric and dimeric alkoxysilanes afforded porous silicas with the BET surface area of 560m 2g -1and 490m 2g -1,respectively.Also,it is reported that cubic octameric alkoxysilane gave microporous silica by hydrolysis and polycondensation.8Thus these results open a way for the rational control of the porosity of silica-based materials by molecular design.Surfactant-Directed Synthesis of Mesostruc-tured Films (1F,2F,and 3F).The oligomeric alkox-ysilanes 2and 3can be used for producing hierarchical structures.Thin films (1F ,2F ,and 3F )obtained by hydrolysis and polycondensation of 1,2,and 3in the presence of a surfactant (P123)were transparent with the thickness of ca.200nm.Figure 6shows the XRD(17)Huang,Y.;Jiang,Z.Microporous Mater.1997,12,341.(18)It should be noted that the porosity of the xerogel derived from 3is not only dominated by the molecular structure of 3but also appears to be affected by reaction conditions (such as the amount of solvent and the drying conditions).Detailed study isunderway.Figure 4.Powder XRD patterns of the xerogels:(a)1G ,(b)2G ,and (c)3G.Figure 5.Nitrogen adsorption -desorption isotherms of (a)1G ,(b)2G ,and (c)3G .The open symbols and the filled symbols denote adsorption and desorption,respectively.Building-Block Approach to Siloxane-Based Nanomaterials Chem.Mater.Epatterns of the films.Although 1F has no ordered structure,2F and 3F exhibit sharp diffraction peaks with the d spacings of 9.3and 8.8nm,respectively,accompanying second-order reflections.Because these periodic structures were still retained even after calci-nation (d )5.1and 4.7nm,Figure 7),these films should possess three-dimensional siloxane frameworks rather than a lamellar structure that usually collapses upon surfactant removal.The TEM images of 2F after calcination mainly shows striped patterns suggestive of a 2D hexagonal structure (Figure 8a).The two XRD peaks are therefore indexed as 10and 20reflections.The lack of a (11)peak suggests that the mesochannels are oriented parallel to the substrate surface.19How-ever,the presence of the domain with a wormholelike structure was also confirmed (Figure 8b).The formation of mesostructured silica by using amphiphilic block copolymers having poly(ethylene oxide)(PEO)chains as hydrophilic blocks relies on the noncovalent interactions between silanol groups and PEO blocks.20,21The lack of structural order in 1F is possibly due to an insufficient interaction of hydrolyzed 1with PEO blocks of the surfactant,because of thesmaller number of Si -OH groups (only 0.5SiOH per SiO 4).Also,the relatively hydrophobic nature of 1having 16terminal methyl groups should be unfavor-able for the partitioning of the oligomeric units into the hydrophilic PEO regions of liquid-crystalline phases during the evaporation-induced self-assembly process.22To minimize the possible thermal rearrangement of the siloxane networks,we typically performed solvent extraction of the surfactants for the film 2F .Figure 9compares the IR spectra of as-synthesized 2F with those of 2F after calcination and after solvent extraction.The absorption bands due to the C -H stretching modes (2800-3000cm -1)considerably decreased by either calcination or solvent extraction.The molar ratio of Si -CH 3groups and -CH 3groups of the PPO blocks is roughly estimated to be 2/5from the 2/P123ratio in the starting solution.We suppose that the remaining bands observed after calcination or solvent extraction are mostly ascribed to the Si -CH 3groups.Upon surfactant removal,the d spacings of the (10)peaks significantly decreased because of the shrinkage of the siloxane frameworks (Figure 7).Nevertheless,the IR spectrum of the film after solvent-extraction clearly shows the band at 550cm -1,suggesting the retention of the D4R units,in contrast to the calcined film exhibiting a relatively ill-defined band at this region.Thus we clearly demonstrated the successful synthesis of hybrid meso-porous silica films containing D4R units in the frame-works.(19)Miyata,H.;Kuroda,K.Chem.Mater.1999,11,1609.(20)Soler-Illia,G.J.d.A.A.;Crepaldi,E.L.;Grosso,D.;Sanchez,C.Curr.Opin.Colloid Interface Sci.2003,8,109.(21)Soler-Illia,G.J.d.A.A.;Innocenzi,P.Chem.s Eur.J 2006,12,4478.(22)Brinker,C.J.;Lu,Y.;Sellinger,A.;Fan,H.Adv.Mater.1999,11,579.Figure 6.XRD patterns of the films:(a)1F ,(b)2F ,and (c)3F.Figure 7.XRD patterns of (a)2F after calcination and (b)2F aftersolvent-extraction.Figure 8.TEM images of 2F aftercalcination.Figure 9.FT-IR spectra of:(a)as-synthesized 2F ,(b)2F after calcination,and (c)2F after solvent-extraction.F Chem.Mater.Hagiwara et al.。