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Single-Step Dispersion of Functionalities on a Silica Surface Philippe Banet,† Nathalie Marcotte,* Dan A. Lerner, and Daniel Brunel Institut Charles Gerhardt Montpellier, UMR 5253 CNRS-UM2-ENSCM-UM1, Mate´riaux AVance´s pour la Catalyse et la Sante´, ENSCM, 8 rue de l‘Ecole Normale, 34296 Montpellier cedex 5, France ReceiVed March 10, 2008. ReVised Manuscript ReceiVed May 20, 2008 A new process for coating a mesoporous silica gel with a mixture of the grafting reagents para-aminophenyltrimethoxysilane and phenyltrimethoxysilane is thoroughly analyzed. The dilution of para-aminophenylsilane with phenylsilane at different ratios allows the density of the functional amino groups present on the silica surface to be controlled, while keeping constant the overall number of grafts. Furthermore, the choice of a rigid linker prevents undesirable interactions between the active function and the inorganic support that could alter the function reactivity. This simple and new method, which results in the improvement of the dispersion of a functionality in a one-pot synthesis, could be particularly interesting in the field of supported catalysis and molecular recognition. The dispersion of the functional groups of the synthesized hybrid solids is investigated using a pyrene derivative covalently linked to the free amino groups of the para-aminophenylsilanes by analyzing the excimer and monomer fluorescence properties of the probe.
Introduction The development of soft chemistry and particularly the sol-gel process in the past decades has led to the emergence of hybrid organic-inorganic materials (HOIM).1,2 Among these materials, porous HOIM are of particular importance for numerous applications in various domains such as sensing, (bio)molecular recognition, selective adsorption and separation, chromatography, and catalysis. Actually, these combine the properties of an organic moiety that can be tailored toward a particular application, with those of an inorganic component that makes the tailored organic group accessible through its texture. Moreover, the presence of the inorganic part ensures the mechanical stability of the hybrid materials.3–8 In most studies published, the inorganic part is a silica obtained via gelification of a sol consisting in a mixture of molecular silica and organosilane precursors suspended in a solvent. During the gelification process, the organic molecules are trapped in the inorganic network. This synthesis thus precludes the control of the distribution of the organic groups within the material and restricts the accessibility of an external agent to the organic molecules contained inside the hybrid solids.9–13 * Author to whom correspondence should be addressed. Tel.: (+33) 4 67 16 34 79; Fax: (+33) 4 67 16 34 70. E-mail
[email protected]. † Present address: P. Banet, Equipe Circuits, Instrumentation et Mode´lisation en Electronique/Laboratoire de Physico-chimie des Polyme`res et des Interfaces, Universite´ de Cergy Pontoise, 5 mail Gay Lussac - 95031 Neuville-sur-Oise cedex, France.
(1) Livage, J. New J. Chem. 2001, 25, 1. (2) Corriu, R. J. P. New J. Chem. 2001, 25, 2. (3) Boury, B.; Corriu, R. J. P. Chem. Commun. 2002, 8, 795–802. (4) Kickelbick, G. Angew. Chem., Int. Ed. 2004, 43, 3102–3104. (5) Innocenzi, P.; Lebeau, B. J. Mater. Chem. 2005, 15, 3821–3831. (6) Liu, J.; Yang, Q.; Zhang, L.; Guo, Y. Prog. Chem. 2005, 17(5), 809–817. (7) Hoffmann, F.; Cornelius, M.; Morell, J.; Fro¨ba, M. Angew. Chem., Int. Ed. 2006, 45(20), 3216–3251. (8) Nicole, L.; Boissiere, C.; Grosso, D.; Quach, A.; Sanchez, C. J. Mater. Chem. 2005, 15(35-36), 3598–3627. (9) Shea, K. J.; Loy, A. D.; Webster, O. J. Am. Chem. Soc. 1992, 114, 6700– 6710. (10) Corriu, R. J. P.; Moreau, J.; The´pot, P.; Wong Chi Man, M. Chem. Mater. 1992, 4, 121724C. (11) Sanchez, C.; Ribot, F. New J. Chem. 1994, 18, 1007–1047. (12) Corriu, R. J. P.; Hesemann, P.; Lanneau, G. Chem. Commun. 1996, 1845– 1846. (13) Sanchez, C.; Ribot, F.; Lebeau, B. J. Mater. Chem. 1999, 9, 35–44.
In order to improve the accessibility of the organic groups, Mann et al.14 and Macquarrie et al.15 have first mesostructured these hybrid materials by addition of structuring templates to the monophasic reacting mixture. Furthermore, this methodology has been developed intensively by many research groups which introduced various functional groups2,16–20 located either inside the silica framework or on the porous surface. Some work has also been reported on the simultaneous bifunctionalization of both the channel pores and the framework with different functional groups.21,22 Another method of improving the accessibility to the organic molecules simply consists of the impregnation of the porous silica by organic molecules by dipping the silica into an organic solution. The major drawback of this method results from a leaching of the adsorbed organics, highly probable under most conditions. To overcome this problem, the organic molecules have been anchored covalently to the surface of the porous silica network in a postsynthetic grafting step.23–25 The usual agents used in that grafting process are alkylorganosilanes (such as alkylalkoxysilane, alkylchlorosilane, alkylsilazane) that also possess reactive groups on the alkyl chain which in turn can react easily with various organic functionalities. In many applications such as chromatography or catalysis, total surface coverage by the functional organic moieties is aimed so as to prevent potential undesirable interactions between functional species and the inorganic support. In this respect, (14) Burkett, S. L.; Sims, S. D.; Mann, S. Chem. Commun. 1996, 1367–1368. (15) Macquarrie, D. J. Chem. Commun. 1996, 1961–1962. (16) Hall, R. S.; Fowler, C. E.; Mann, S.; Lebeau, B. Chem. Commun. 1999, 201–202. (17) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403– 1419. (18) Lu, Z.-L.; Lindner, E.; Mayer, H. A. Chem. ReV. 2002, 102, 3543–3578. (19) Li, C. Catal. ReV. 2004, 46, 419–492. (20) Macquarrie, D. J., Modified mesoporous materials as acid and base catalysts. In Nanoporous Materials-Science and Engineering, Zhao, G. Q. L. a. X. S., Ed.; Imperial College Press: London, 2004; Vol. 4, Chapter 18; pp 553-595. (21) Corriu, R. J. P.; Mehdi, A.; Reye, C.; Thieuleux, C. Chem. Commun. 2003, 1564–1565. (22) Mouawia, R.; Mehdi, A.; Reye, C.; Corriu, R. J. P. New J. Chem. 2006, 30, 1077–1082. (23) Nagel, U.; Kingel, E. J. Chem. Soc., Chem. Commun. 1986, 1098–1099. (24) Walcarius, A.; Etienne, M.; Sayen, S.; Lebeau, B. Electroanalysis 2003, 15, 414–421. (25) Corma, A.; Garcia, H. AdV. Synth. Catal. 2006, 348, 1391–1412.
10.1021/la800745g CCC: $40.75 2008 American Chemical Society Published on Web 07/11/2008
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Scheme 1. Schematic Representation of an Ideal Dispersion (Left) and of a Potential Clustering (Right) of Grafted Amino Functions
various coating techniques have been developed.26–31 In addition, mutual interactions between neighboring grafted organic functions can also alter the intrinsic properties of an isolated function. Those must then be avoided in various applications such as in the catalysis by transition metal complexes. Hence, control of the dispersion of the active sites is deemed of crucial importance for this type of application. This is also true from a more general point of view considering the higher selectivity of a single active site compared to a multisite one. The pioneering work in tuning the spacing between active functions was performed by Wulff et al.32 via a templating approach. It was based on an imprinting concept used initially to introduce functions at well-designed separations into cross-linked polymers, and was further developed by several groups.33,34 In the work of Shea et al.,34,35 the first step consisted of grafting the silica surface with a silylating molecule bearing two p-iminophenyltrialkoxysilane moieties separated by a large molecular fragment (biphenyl or triphenyl) acting as a template. Hence, the distance between the two anchored silyl groups was defined by the length of the template. In the second step, the template was split off, leaving the two grafted p-aminophenylsilane molecules in an unchanged position on the surface. A similar methodology was further applied by Hwang et al.36,37 using a multidentate template assemby (a Ru(II) trisbipyridyl complex bearing a formaldehyde function) in order to introduce three organized amino groups on the silica surface. Katz et al.38,39 have introduced spatially organized chemical functionalities during the synthesis process rather than using a grafting approach. The subsequent splitting off of the template in the second step left microcavities in the bulk silica with spatially isolated aminopropyl groups covalently anchored to the microporous walls. With a view to creating nanocavities inside organic-lined mesoporous silica, Liu et al.40 anchored some tripodal templating molecules on the pore surface of ordered mesoporous silicas. A monolayer coating with long chain silanes was then performed and the tripodal spacers were finally removed by acid hydrolysis. In a different approach based on a molecular (26) Aue, W. A.; C.R.H. J. Chromatogr. 1969, 42, 319–335. (27) Wirth, M. J.; Fatunmbi, H. O. Anal. Chem. 1993, 65, 22–826. (28) Tatsumi, T.; Koyano, K. A.; Igarashi, N. J. Chem. Soc., Chem. Commun. 1998, 325–326. (29) Brunel, D.; Blanc, A. C.; Galarneau, A.; Fajula, F. Catal. Today 2002, 73, 139–152. (30) Abramson, S.; Laspe´ras, M.; Galarneau, A.; Desplantier-Giscard, D.; Brunel, D. Chem. Commun. 2000, 1773–1774. (31) Martin, T.; Galarneau, A.; Brunel, D.; Izard, V.; Hulea, V.; Blanc, A. C.; Abramson, S.; Di Renzo, F.; Fajula, F. Stud. Surf. Sci. Catal. 2001, 135, 4621– 4628. (32) Wulff, G.; Heide, B.; Helmeier, G. J. Am. Chem. Soc. 1986, 108, 1089. (33) Becker, J. J.; Gagne´, M. R. Acc. Chem. Res. 2004, 37, 798–804. (34) Shea, K. J.; Dougherty, T. K. J. Am. Chem. Soc. 1986, 108, 1091–1093. (35) Shea, K. J.; Thompson, E. A.; Pandey, S. D.; Beauchamp, P. S. J. Am. Chem. Soc. 1980, 102, 3149–3155. (36) Hwang, K.-O.; Yakura, Y.; Ohuchi, F. S.; Sasaki, T. Mater. Sci. Eng., C 1995, 3, 137–141. (37) Hwang, K.-O.; Sasaki, T. J. Mater. Sci. 1998, 8, 2153–2156. (38) Katz, A.; Davis, M. E. Nature 2000, 403, 286–289. (39) Bass, J. D.; Katz, A. Chem. Mater. 2003, 15, 2757–2763. (40) Liu, J.; Shin, Y.; Wang, L.-Q.; Nie, Z.; Samuels, J. W. D.; Chang, H.; Fryxell, G.; Exarhos, G. J. J. Phys. Chem. A 2000, 104, 8328–8329.
patterning technique, large trityl-imine molecules were individually anchored on a silica surface.41–43 Two more steps were necessary to eliminate the protecting trityl molecules and passivate the residual silanols. Another strategy proposed by Bonneviot et al.44 consisted in a dual patterning onto micelle-templated silicas (MCM-41-type silica) by sequential graftings. MCM41-type silicas usually have been prepared by a cooperative assembly of silicates around micelles of long alkyl chain amphiphilic molecules, typically possessing a trimethyl ammonium cationic end function. The silica surface of the pore channels of the as-made MCM-41 silica could be silylated with organosilanes, which effected a partial substitution of the amphiphile and replaced the electrostatic interactions between cationic surfactant and charged silicates by strong covalent siloxane bonds.45 After total removal of the surfactant, the partially silylated material surface unveiled silanol groups originating from the silicates, which could then be silylated with a different functional organosilane. All the strategies so developed involve a multistep procedure, and require each step to be well controlled. It must be pointed out, especially in the field of catalysis, that a small amount of functional group is often required while ensuring total coverage of the surface so that undesirable interactions between the catalytic sites themselves or between the latter and the uncovered surface can be reduced. Total surface coverage can however be achieved by cografting the functional organosilane with nonfunctional ones. In that case, the control of the dispersion of the functional groups on the silica surface is of major importance to prevent those undesirable interactions. Furthermore, despite dilution of the functional organosilane in the reacting mixture, it is not possible to avoid the existence of stabilizing interactions (H-bonding, Van der Waals, etc.) between the anchored functional chains that could favor locally some packing together during the grafting process (Scheme 1). A critical question that we wish to address in the present work is how efficient would be the dilution of the functional organosilanes with the nonfunctional ones to result in the satisfactory dispersion of the functionalities on the silica surface. With this aim in mind, we have investigated the functionalization of a mesoporous silica gel by a mixture of para-aminophenylsilane and phenylsilane at different molecular ratios, the diluting agent, i.e., phenylsilane, being of the same nature as that the amino group linker. Moreover, the rigidity of this linker has previously been demonstrated to reduce possible undesirable interactions between the grafted function and the mineral support.46 Functionalization of the surface has been performed according to a published coating procedure giving very good surface cover(41) McKittrick, M. W.; Jones, C. W. Chem. Mater. 2003, 15, 1132–1139. (42) McKittrick, M. W.; Jones, C. W. J. Am. Chem. Soc. 2004, 126, 3052– 3053. (43) Hicks, J. C.; Jones, C. W. Chem. Mater. 2006, 18, 5022–5032. (44) Bonneviot, L.; Badiei, A.; Crowther, N. PCT Int. Appl. 2002, wo2002016267 A1. (45) Antochshuk, V.; Jaroniec, M. Chem. Mater. 2000, 12, 2496–2501. (46) Fiorilli, S.; Onida, B.; Barrolo, C.; Viscardi, G.; Brunel, D.; Garrone, E. Langmuir 2007, 23, 2261–2268.
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Scheme 2. Schematic Representation of Isolated Chromophores Exhibiting Pyrene Monomer Fluorescence Emission (Left) and Interacting Chromophores Inducing Excimer Fluorescence Emission (Right)
age.30,31 Actually, a high surface coverage is desirable to avoid lateral surface migration of the bound organosilane.47 A second question that arises is whether the degree of dispersion could be improved while maintaining the same dilution ratio. To answer this question, we have used methyl-urea as an additional compound during HOIM synthesis to take advantage of the ability of urea derivatives to form mixed supramolecular-like assemblies with amines. Actually, Moreau et al.48 have described the synthesis of tridimensional hybrid structures organized through the supramolecular self-association of urea-type linkers via multiple hydrogen bonds. The resulting supramolecular assemblies were able to direct the spatial organization of functional organic entities into elongated nanofiber-like structures.49 In our experiments, it was assumed that the presence of the assembly of amines and urea derivatives would induce a better dispersion of the functional groups on the surface of the support. We report for the first time on the synthesis and full characterization of silica gels coated with various ratios of paraaminophenylsilane using a one-pot synthesis strategy. The effect of the dilution of the para-aminophenylsilane in the reacting mixture on the effective dispersion of the aniline moieties is then investigated by fluorescence using pyrene as a probe. Pyrene derivatives are anchored to the amine sites by a sulfonamide bond as shown in Scheme 2. The pioneer study on the luminescence of pyrene molecules bound to a silica gel surface was undertaken by Lochmu¨ller et al.50 to assess the proximity and distribution of chemically bound molecules on this support. This methodology was used to analyze the clustering of pyrene derivatives adsorbed chemically on silica gels,51 to study the spacing and amine site isolation on silica surfaces,42,43,52,53 and to characterize the isolation of sites in imprinting bulk silica.38,39 In the present study, we have analyzed the fluorescence of pyrene covalently anchored on the grafted amine groups as a function of their dilution in the synthesis reaction mixture and as a function of the coupling reaction yield. We have furthermore investigated the effect of a possible supramolecular assembly induced by the (47) Wang, H.; Harris, J. M. J. Am. Chem. Soc. 1994, 116, 5754–5761. (48) Moreau, J. J. E.; Vellutini, L.; Wong Chi Man, M.; Bied, C. Chem. Eur. J. 2003, 9, 1594–1599. (49) Moreau, J. J. E.; Pichon, B. P.; Wong Chi Man, M.; Bied, C.; Pritzkow, H.; Bantignies, J.-L.; Dieudonne, P.; Sauvajol, J.-L. Angew. Chem., Int. Ed. 2004, 43, 203–206. (50) Lochmu¨ller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. Anal. Chem. 1983, 55, 1344–1348. (51) Brown, R.; Lacombe, S.; Cardy, H. Microporous Mesoporous Mater. 2003, 59, 93–103. (52) Hicks, J. C.; Jones, C. W. Langmuir 2006, 22, 2676–2681. (53) Hicks, J. C.; Dabestani, R.; Buchanan III, A. C.; Jones, C. W., Inorg. Chim. Acta 2008, in press.
presence of a urea derivative during the silica surface functionalization.
Experimental Section Materials. The G5H silica gel was from Grace Davison (see Figure 1 and Table 3 for porosity characteristics). Para-aminophenyltrimethoxysilane was purchased from ABCR. Phenyltrimethoxysilane was obtained from Fluka. Ammonium fluoride (NH4F), paratoluenesulfonic acid (TSA), trimethylsilylimidazole, methyl urea (Me-Urea), and 1-pyrenesulfonic acid were from Aldrich and were used as received. Thionyl chloride from Aldrich was freshly distilled prior to use. Triethylamine from Riedel-de Hae¨n and dimethylformamide (DMF) from Prolabo were purified over KOH and CaH2 respectively, and were distilled prior to use. Toluene (Carlo Erba) was dried over molecular sieves (4 Å). Methanol, diethyl ether (Riedel-de Hae¨n), and dichloromethane (Fluka) were used as received.
Figure 1. N2 adsorption-desorption isotherms of the starting silica gel (O) and of the S100 sample unlabeled (•) and labeled with pyrene (∆) recorded at 77 K. Inset: N2 adsorption-desorption isotherms of the S100 and S100-pyr samples with the porous volume calculated respective to the pure mineral oxide silica. Table 1. Composition of the Mixtures of the Grafting Agents Used for Coating the Silica sample GPhNH2 (mmol) GPh (mmol) GPhNH2 (%) Me-urea (mmol) S100 S033 S020 S010 S000 S033u S020u S010u
12.48 4.16 2.50 1.25 0 4.16 2.50 1.25
0 8.32 9.98 11.23 12.48 8.32 9.98 11.23
100 33 20 10 0 33 20 10
0 0 0 0 0 12.2 12.2 12.2
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Table 2a sample
org wt %(hybrid)(a) [org wt %(SiO2)](b)
S100 S033 S020 S010 S033u S020u S010u
19 [23.4] 20 [25.5] 18 [22.0] 18 [22.2] 18 [22.0] 17 [19.8] 17 [20.0]
nN(c) (molecule.nm-2) 2.50 ( 0.25 0.90 ( 0.09 0.45 ( 0.05 0.25 ( 0.03 0.90 (0.09 0.50 ( 0.05 0.25 ( 0.03
yield(d) (%) 50 18 9 5 18 10 5
a (a) Weight percentages of the organic part of the materials obtained from TGA, respective to the hybrid material (org wt %(hybrid)), (b) the same respective to the silica content (org wt %(SiO2)), (c) surface density of para-aminophenylsilane (nN) calculated from elemental analysis data using formula 1, and (d) yield of silica sites grafted by amino function respective to the surface density of silanols (5 silanol.nm-2).
Table 3. Surface Area (SBET), CBET Parameter, Pore Diameter (O), Porous Volume of the Hybrid Material (Vph), and Porous Volume Calculated Respective to the Pure Mineral Oxide Silica (Vps) from the N2 Adsorption-Desorption Isothermsa sample SBET (m2.g-1) CBET φ, (nm) Vhp (cm3.g-1) Vsp (cm3.g-1SiO2) G5H S100 S033 S033a S020 S010 S033ua S020ua S010ua a
513 325 376 380 342 362 432 438 462
103 46 41 70 25 25 64 54 53
16.5 14.3 13.3 13.4 14.2 13.5 14.1 14.2 14.2
1.80 1.00 1.10 1.20 1.10 1.10 1.15 1.00 1.30
1.80 1.23 1.40 1.50 1.35 1.35 1.40 1.20 1.55
Experiments performed on uncapped materials.
All solutions were prepared using deionized water produced by a Milli-Q (Millipore) purification system. Synthesis. 1-Pyrenesulfonylchloride (PSC). A solution of 1-pyrenesulfonic acid (0.41 g; 1.5 mmol) was poured into 16 mL of thionylchloride (SOCl2) containing a few drops of DMF and was heated at 80 °C until the release of SO2 and HCl stopped (∼30 min). Excess of thionylchloride was then distilled off under reduced pressure and the recovered solid was dissolved in dry DMF under argon for use in the next step without further purification. N-(Phenyl)-1-pyrenesulfonamide (PhPSN). A solution of triethylamine (0.3 mg; 3 mmol) in 5 mL of dry DMF was added under argon to a cooled (0 °C) solution of freshly distilled aniline (139 mg; 1.5 mmol) in dry DMF (30 mL). The mixture was then added dropwise to a solution of 1-pyrenesulfonyl chloride (0.41 g; 1.5 mmol) in dry DMF (20 mL) at 0 °C. The reaction was completed overnight at room temperature. After evacuation of DMF under vacuum (0.1 atm, 40 °C), the reaction mixture was extracted with CH2Cl2, and successively washed with a dilute HCl solution (5%), a Na2CO3 solution (5%), a dilute HCl solution (5%), and with water. After vacuum distillation of the solvent, the recovered solid was purified by thin layer chromatography on silica gel with ethyl acetate as the eluent. Yield: 40%. 1H NMR (d6-DMSO) δ (ppm): 4.0 (NH), 6.80 (t, 1H), 7.00-7.10 (m, 4H), 8.20-8.25 (m, 2H), 8.35-8.40 (m, 2H), 8.45-8.50 (m, 3H), 8.65 (d, 1H), 9.10 (d, 1H). 13C NMR (d6-DMSO) δ (ppm): 119.80 (CH), 123.35 (CH), 123.55 (C), 123.80 (CH), 124.50 (CH), 124.60 (C), 127.35 (CH), 127.55 (CH), 127.65 (CH), 127.75 (C), 128.05 (CH), 129.40 (CH), 130.00 (CH), 130.10 (C), 130.95 (C), 132.60 (C), 134.50 (C). Mass spectrometry (FAB, positive mode): m/z ) 358 (M+1); DRIFT (supported on G5H silica) ν (cm-1): 3270 (Ν-Η sulfonamide), 3085 and 3047 (aromatic C-Η stretching vibrations), 1625 (Ν-Η bending vibrations), 1600 and 1500 (phenyl ring CdC stretching vibration), 1480, 1415 and 1380 (C-H deformation bands). Hybrid Materials. Two sets of samples were prepared, one in the absence of methyl urea and the other in its presence. In a typical experiment, the silica was first heated under reduced pressure (10-2 mmHg) at 180 °C for 18 h and then allowed to cool at room temperature. The grafting agents para-aminophenyltrimethoxysilane
(GPhNH2) and phenyltrimethoxysilane (GPh) were mixed together at different molar ratios to reach a constant global concentration of 5 grafts per nm2 of silica surface (composition shown in Table 1). The grafting agents mixtures used for the surface coating of the suspended silica were added under argon to a toluene suspension (90 mL) containing 3.0 g of silica and stirred 1 h at room temperature. Thereafter, 224 µL of water, 23 mg of NH4F, and 118 mg of TSA (7.5:0.25:0.25 molecules.nm-2 SiO2) were added under argon. The suspension was stirred for one hour at room temperature, 6 h at 60 °C, and 1 h at 120 °C (Dean Stark). The powder was collected by filtration and rinsed successively with toluene, methanol, methanolwater (1:1), methanol, and diethyl ether. It was then extracted in a Soxhlet apparatus (CH2Cl2-Et2O). Finally, the following end-capping step was completed. The sample was heated under vacuum at 150 °C for 18 h and then cooled to room temperature. The trimethylsilylimidazole end-capping agent (2.5 mL, 7 silanes.nm-2 of SiO2) was added under argon to a suspension of the grafted silica (previously activated overnight at 120 °C under vacuum) in toluene (50 mL). The mixture was magnetically stirred for 16 h at 60 °C. The powder was recovered by filtration and washed successively with toluene, methanol, dichloromethane, and diethyl ether. It was then extracted in a Soxhlet apparatus and dried for 2 h in an oven at 50 °C. The hybrid materials synthesized using methyl urea as a dispersing agent were prepared following a similar protocol, except that methyl urea (0.9 g) was added to the grafting agents (5 silanes.nm-2 of SiO2) in a toluene-methanol solution (90:10, 100 mL), that was stirred 1 h at 60 °C. The silylation reaction was performed under magnetic stirring for 1 h at 60 °C, 1 h at 70 °C, 1 h at 85 °C, and then for 1 h at 120 °C (Dean Stark). Note that contrary to most of the studies dealing with silica functionalization in which a curing step is performed, this step must be eliminated when grafting an anilinelike molecule to avoid amine oxidation. Hybrid Materials Labeled with PSC. In a typical PSC anchoring experiment, the grafted silica was heated at 150 °C overnight, then allowed to cool down to room temperature. A solution containing 0.072 mol.L-1 of PSC in DMF and triethylamine as a base was added to a suspension of the grafted silica in DMF. The molar ratio PSC/base/GNH2 used in the mixture was 2:2:1. The suspension was stirred for 18 h at room temperature. The solid was then recovered by filtration and washed successively twice with dimethylformamide, methanol, dichloromethane, and diethyl ether. To avoid degradation over time, the labeled hybrid materials have to be stored in the dark under inert atmosphere condition. Characterization. The N2 adsorption-desorption isotherms were measured at 77 K on a Micromeritics ASAP 2010 apparatus on 40 to 50 mg samples previously heated overnight at 150 °C under vacuum. Thermogravimetric analyses (TGA) were measured on a Netzsch TG 209C system. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were collected on a Bruker Vector 22 spectrometer equipped with a MCT cryodetector. Elemental analyses were performed by the Service Central d’Analyse (CNRS, Solaize, France). Diffuse reflectance UV-visible (DRUV) spectra were measured in the 200-800 nm range using BaSO4 as the reference on a Perkin-Elmer Lambda 14 spectrometer equipped with an integrating sphere (Labsphere, North Sutton, USA). Quartz cells (Hellma) of 0.05 mm path length were used. The spectra were displayed as their Kubelka-Munk F(R) transform
F(R) ) (1 - R2) ⁄ 2R in which R was the measured reflectance. Before measurements, hybrid materials were diluted with the underivatized G5H silica to obtain reasonable reflectance values. Steady-state fluorescence emission spectra were measured on a spectrofluorimeter built around two Jobin Yvon M25 monochromators each equipped with a 1200 lines/mm grating (Czerny-turner, 1/4 m) and continuously variable slits. Detection was carried out with a R928 photomultiplier (Hamamatsu). The emission and excitation bandpass were set at 8 nm.
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Results and Discussion Synthesis and Characterization of the Hybrid Materials. Two sets of samples, one prepared in the absence of the H-bonding dispersing agent methyl urea and the other in its presence, were synthesized according to the general grafting procedure described in the Experimental Section. In our experimental protocol the total number of grafts, phenyltrimethoxysilane (GPh) and paraaminophenyl-trimethoxysilane (GPhNH2), was kept constant, giving a total surface concentration of 5 grafts.nm-2 SiO2. Different samples were prepared by varying the molar ratio of GPhNH2 grafts introduced. The concentrations used for the synthesis of the hybrid materials are displayed in Table 1. The compounds prepared in the presence of the dispersing agent (samples S033u, S020u, and S010u containing respectively 33%, 20%, and 10% GPhNH2) were synthesized at a methyl urea concentration (0.12 mol.L-1) such that supramolecular assemblies should be formed.48,49 These assemblies are expected to improve the dispersion of the grafted amino groups on the surface of the mesoporous material. However, the presence of the dispersing agent could also induce some side effects, such as a change in the efficiency of the first surface coverage step, which must then be controlled perfectly. The amount and composition of the organic material present in the hybrid materials were obtained by thermogravimetric (TGA) and elemental analyses, respectively. Table 2 displays the TGA results and the calculated number of nitrogen containing molecules per nm2 (nN) determined using formula 1.
nN)(N% × MSi×Na) ⁄ (Si% × MN × MSiO2×SBET)
(1)
In the formula, N% and Si% are the nitrogen and silicon weight percentages measured by elemental analysis (see Table 1 in Supporting Information); MSi, MN, and MSiO2 are the molecular weights of Si, N, and SiO2; Na is Avogadro’s number; and SBET (in nm2.g-1) is the specific surface of the nongrafted silica calculated from N2 adsorption-desorption measurements (see below). For sample S100, a value of 2.5 molecule.nm-2 is obtained for nN when the organic grafting is determined from TGA measurements instead of elemental analysis. This shows a good agreement between the two sets of data. The formula used for the calculation of nN from the TGA data is as follows:
nN)org wt % × Na ⁄ [100 - (org wt % H2O wt %)] × (Maniline-MH2O) × SBET (2) where org wt % is the weight fraction of the organic part per SiO2 measured on the nonpassivated sample, (Maniline - MH2O) represents the water lost per grafted molecule taking into account the formation of one silanol function following one Si-C bond oxidation. For all samples, the organic fraction is found to be almost constant (Table 2), varying only between 17% and 20%. As the total number of alkoxysilane (GPh + GPhNH2) introduced for coating the silica is kept constant during the synthesis and, considering that the molecular weights of phenylsilane and paraaminophenylsilane are roughly similar (108 vs 123 g.mol-1), the constant value of the organic fraction indicates that the same amount of alkoxysilane has been grafted on the materials, whatever the initial ratio of para-aminophenylsilane and phenylsilane. For all samples, the results are consistent with the introduction of 2.5 grafts.nm-2. This high surface density coverage obtained by coating the silica gel with aromatic silanes is similar to that previously reported with long alkylsilane chains using similar coating conditions.31 In addition, the nitrogen amount
found in the materials (Table 2) decreases in the same way as the molar ratio of GNH2 of the coating solution (Table 1). Consequently, when GNH2 decreases, the amount of grafted paraaminophenylsilane decreases, whereas the total amount of grafts is kept constant. This means that there is a linear dilution of grafted para-aminophenylsilane by grafted phenylsilane moieties (see Supporting Information). Hence, the average density of the functional amino grafts can be tuned simply by varying the molar fraction of the functional silane in the reacting mixture. The texture and porosity of the samples after the grafting and the end-capping steps were characterized by N2 adsorptiondesorption isotherms. As examples, isotherms of the starting silica gel (G5H) and of silica fully grafted with amino functions (S100) are shown in Figure 1. According to the Brunauer classification,54 the starting silica gel isotherm is of type IV and the hysteresis loop is of type A.55 The starting silica gel is thus of the mesoporous type with cylindrical pores. Specific surface area (SBET), CBET parameter, pore diameters (φ), and porous volume (Vp) were calculated using, respectively, the BrunauerEmmett-Teller model56 in which CBET ) exp[(E1 - EL)/RT], the nitrogen volume adsorbed when capillary condensation is ended, and the BdB model57 (Table 3). The isotherm shape of the silica fully grafted with amino functions is similar to that of the starting material (Figure 1), a fact which indicates that the porous texture is preserved after the grafting step and the endcapping treatment. The textural features of all samples were maintained after the coating procedure, the adsorption-desorption isotherms exhibiting similar characteristics to those presented in Figure 1. The porosity characteristics of the different samples calculated from the isotherm data are listed in Table 3. For all samples, specific surface area, pore diameter, porous volume, and CBET parameter all decrease compared to the starting material. The volume of the silica mesopores calculated with respect to the pure silica (Vsp) is decreased on grafting, indicating that the grafted functions are located inside the pores. This is also supported by the change in the pore diameter size, which decreases from 16.5 nm for the bare silica to an average value of 14 nm for the grafted material. The change in the pore size is consistent with a surface covered by a monolayer as previously reported.31 Moreover, it has been shown that the CBET coefficient depends on the surface polarity when the sole interaction energy E1 between the surface and the first adsorbed layer of polarizable nitrogen molecules is considered while neglecting lateral intermolecular interactions.31 Then, as the silica surface possessing polar silanol groups is substituted by less polar organosilane grafts, the CBET parameter is expected to decrease. This has been confirmed previously by indirect contact angle measurement during water intrusionextrusion studies on various hydrophobized porous silica gels58 and by probing the polarity of various lined surfaces with solvatochromic Reichardt dyes.59 In our experiments, the large CBET coefficient decrease observed after silanol passivation with the trimethylsilylation reagent suggests an almost total surface coverage by organic chains (Table 3). This high surface coverage (54) Brunauer, S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723–1732. (55) De Boer, J. H. The shapes of capillaries In The structure and properties of porous materials; Everett, D. H., Stone, F. S, Eds.; Butterworths: London, 1958; pp 68-94. (56) Brunauer, S.; Emmet, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309– 319. (57) Broekhoff, J. C. P.; De Boer, J. H. J. Catal. 1968, 10, 377–390. (58) Martin, T.; Brunel, D.; Galarneau, A.; Renzo, F. D.; Fajula, F.; B.; Lefevre; Gobin, P. F.; Quinson, J. F.; Vigier, G. Chem. Commun. 2002, 24–25. (59) Macquarrie, D. J.; Taverner, S. J.; Gray, G. W.; Heath, R. A.; Rafelt, J. S.; Saulzet, S. I.; Hardy, J. E.; Clark, J. H.; Sutra, P.; Brunel, D.; Renzo, F. D.; Fajula, F. New J. Chem. 1999, 23(7), 725–731.
Single-Step Functionalities Dispersion on Silica
Langmuir, Vol. 24, No. 16, 2008 9035 Table 4. Density of the Anchored Pyrene Molecules (npyr), and Amount of Pyrene Grafted Respective to the Number of Amino N Sites (npyr ) and Respective to the Total Number of Grafted T Molecules (npyr ) sample S100-pyr S033-pyr S020-pyr S010-pyr S033u-pyr S020u-pyr S010u-pyr
Figure 2. DRIFT spectra of the samples grafted at different aminophenyltrimethoxysilane ratio: 0% ( ×), 10% (O), 20% (0), 33% (4) and 100% (full line). For the sake of clarity the spectra are offset in absorbance, as shown.
associated with the decrease in both the pore volume and the pore diameter (without pore entrance plugging) reveals a monolayer-like lining of the silica surface by the organosilane60 independently of the silane composition. The CBET values obtained for the materials coated in the presence of methyl urea are slightly higher than those obtained without the dispersing agent. However, one has to mention that for the materials prepared without methyl urea the N2 adsorption-desorption experiments have been performed before the end-capping step, contrary to experiments done in the presence of methyl urea. The change observed in the CBET values is likely to be due to the end-capping step rather than to any actual effect of the dispersing agent itself. Actually, we observed such a discrepancy between the end-capped and the uncapped materials prepared without methyl urea. The data of an uncapped sample at 33% of GPhNH2 (S033*) are given in Table 3 as an example of the changes that result from the capping treatment, and should be compared to the S033 end-capped sample. DRIFT spectroscopy was also used to analyze the presence of the organic part in the hybrid materials. The spectra of the samples coated with GPh-only (S00) and GPhNH2-only (S100) in the absence of methyl-urea are illustrated in Figure 2. In the absence of amino functions, the spectrum shows the characteristic stretching vibrations of aromatic C-H (3000-3100 cm-1) and of conjugated CdC (1600 cm-1 bonds). Additional bands seen in the 2850-3000 cm-1 region are due to the stretching vibrations of the methyl groups of Si(CH3)3 introduced during the endcapping step. The presence of amino functions in the materials results in four main bands arising at 3390 and 3470 cm-1 for the N-H stretching vibrations, and at 1510 and 1625 cm-1 for the NH2 bending vibrations. As the number of amino grafts increases, growth in the intensity of the four main absorption bands of the NH2 group is observed. The spectra of the grafted materials prepared in the presence of methyl-urea showed similar behavior (data not shown). DRUV spectra of the hybrid materials prepared in the absence or in the presence of methyl urea exhibited three main bands peaking at 215, 260, and 295 nm, the latter band being characteristic of the ππ* transitions of aniline. Labeling of the Hybrid Materials with a Fluorescent Probe. The present study was meant to prepare the way toward efficient catalysts made by grafting selected active sites involved in catalytic reactions like transition metal complexes, organic (60) Cauvel, A.; Brunel, D.; Renzo, D.; Fajula, F. Am. Inst. Phys. 1996, 354, 477–484.
npyr (molecule.nm-2)
N npyr (%)
T npyr (%)
24 29 33 40 31 24 28
12.0 5.0 3.0 2.0 5.5 2.5 1.5
0.59 ( 0.06 0.26 ( 0.03 0.15 ( 0.02 0.10 ( 0.01 0.28 ( 0.03 0.12 (0.01 0.07 ( 0.01
bases,29 or mediators,61 on the amino groups of the best of the hybrid materials just described. Hence, control of the dispersion of the active sites is of crucial importance. The choice was made to monitor that dispersion using a classical method based on the eventual appearance of the excimer emission of a pyrene derivative grafted on the amine function as the catalytic ligands would be. To this end, the materials coated with varying amounts of para-aminophenyl silane were reacted with pyrenesulfonyl chloride to covalently link the fluorescent pyrene moiety to the amino sites (Scheme 2) in the conditions described by Gao et al.62 The infrared spectra of the fluorescently labeled materials exhibited a decrease in the intensities of the stretching and bending bands of NH2. The sulfonamide bond formed between the aniline moiety and the pyrene residue should exhibit a characteristic N-H vibration band at 3270 cm-1, as observed for the organic parent compound N-(phenyl)-1-pyrenesulfonamide. We were not able to locate this band in the spectra, and suspect it was buried under the broad and intense bands in the 3200-3700 cm-1 region. The DRUV spectra of the hybrid materials labeled with pyrene (prepared with or without methyl urea) displayed two additional bands compared to that of the unlabeled material: an intense band at 355 nm and a low intensity band at 380 nm (see Supporting Information Available). These bands were ascribed to the presence of grafted pyrene. As a matter of fact, the organic parent molecule in toluene solution exhibits these two characteristic bands in the 300-400 nm spectral range, plus another band at 360 nm appearing as a shoulder on the 355 nm band. Thermogravimetric analyses of these materials show an increase in the organic part after the reaction with pyrenesulfonyl chloride (Table 4). The density of the anchored pyrene molecules npyr (molecule.nm-2) was calculated from the TGA data using eq 3:
npyr ) Cl × Na ⁄ SBET
(3)
mol.g-1),
where Cl, the pyrene loading (in is determined from TGA data on samples taken before and after the coupling with pyrenesulfonyl chloride, as shown in eq 4:
Cl ) ∆organic wt % ⁄ (MpyrSO2 - 1)
(4)
in which ∆organic wt % refers to the difference in organic weight between the final pyrene grafted silica and the parent passivated aniline grafted silica, scaled to 1 g of pure silica, and (MpyrSO2 - 1) is equal to 264 g.mol-1. Table 4 also reports the amount of labeled sites with respect N to the number of grafted amino sites (npyr ) and the total number T of sites (npyr). It is found that the pyrene labeling yield decreases with increasing number of amino functionalized grafts. A similar trend was recently reported by Hicks et al.53 for the cooperative (61) Brunel, D.; Fajula, F.; B.Nagy, J.; Deroide, B.; Verhoef, M. J.; Veum, L.; Peters, J. A.; Van Bekkum, H. Appl. Catal., A 2001, 213, 73–82. (62) Gao, L.; Fang, Y.; Wen, X.; Li, Y.; Hu, D. J. Phys. Chem. B 2004, 108(4), 1207–1213.
9036 Langmuir, Vol. 24, No. 16, 2008
dilution of aminosilica with mixtures of methyltrimethoxysilane and 3-aminopropyltrimethoxysilane. The textural characteristics of the functionalized materials calculated from the N2 adsorptiondesorption isotherms are given in the Supporting Information provided. These data show that the structure of the materials is conserved after grafting pyrene, and are consistent with a pyrene anchorage inside the mesopores (decrease both in porous volume and in pore diameter). The covalent linking of the pyrene moiety on the hybrid materials bearing amino functions aims at investigating the dispersion of the grafted functional amino groups on the silica surface. At low concentrations, pyrene is known to exhibit a characteristic structured emission band in the 350-450 nm range ascribed to the monomer fluorescence emission. At higher concentrations, when two pyrene molecules can approach to a close distance, a new broad and structureless fluorescence band is seen at longer wavelengths and attributed to the formation of an excimer, i.e., a short-lived dimeric species involving one electronically excited pyrene and a second in its ground state. The existence of a preassociated ground-state dimer also gives rise to a fluorescence emission similar to that of the excimer and is referred as an excimer-like emission. In grafted solids, both types of excimer should exist: the ground-state “static” pairs, in which the two pyrene molecules would be in close contact and have a ground-state geometry near-fit for energy sharing and excimer-like emission; and the “dynamic” pairs, in which the covalent link (para-aminophenyl silyl grafts in our study) would leave some freedom of torsion and/or rotation so that some movement would be required to give a geometry allowing excimer emission. Hence, the spectra observed would reflect a distribution of slightly different emitting pairs, the critical intermolecular distance over which excimer fluorescence should be observed being of 3-10 Å.63 Owing to its peculiar fluorescence properties, pyrene has been used widely to probe the degree of site isolation in various heterogeneous media,38,42,50,51,64 with pyrene either covalently bound to silica50 or simply adsorbed onto silica gels.51 Pyrene photophysics can thus be used to probe whether the grafted amino functional groups of our materials are evenly distributed on the silica surface (pyrene monomer fluorescence should then be dominant) or are clustered into regions of high density (giving rise to pyrene excimer fluorescence; Scheme 2, right). The fluorescence emission spectra of the hybrid materials synthesized without methyl urea are displayed in Figure 3 for various molar ratios of grafted amino groups. At the lowest concentration of grafted amino functions (S010 sample), the fluorescence spectrum exhibits the structured emission bands expected for the monomer pyrene emission (Scheme 2, left). As a matter of fact, the three main bands at 380, 400, and 420 nm are centered at the same wavelengths as those of the parent molecule N-(phenyl)-1-pyrenesulfonamide in toluene solution. Furthermore, molecules bearing a similar pyrene sulfonamide chromophore displayed emission bands close to these maxima.65–67 Using the terminology of Thomas et al.68 also taken up by Gonza´lez-Benito et al.,65 the bands at 380 and 400 nm are referred to as band I and band V, respectively. In the silica materials, bands I and V were used as an index of the polarity65 (63) Winnik, F. M. Chem. ReV. 1993, 93, 587. (64) Lochmu¨ller, C. H.; Colborn, A. S.; Hunnicutt, M. L.; Harris, J. M. J. Am. Chem. Soc. 1984, 106, 4077–4082. (65) Gonzalez-Benito, J.; Aznar, A. J.; L. Lima, F. B.; Mac¸anita, A. L.; Baselga, J. J. Fluoresc. 2000, 10(2), 141–146. (66) Sandez Macho, M. I.; Gonzalez, A. G.; Varela, A. S. Langmuir 2000, 16, 9347–9351. (67) Ezzell, S. A.; Hoyle, C. E.; Creed, D.; McCormick, C. L. Macromolecules 1992, 25(7), 1887–1895. (68) Kalyanasundaram, K.; Thomas, J. K. J. Am. Chem. Soc. 1977, 99, 2039– 2044.
Banet et al.
Figure 3. Fluorescence spectra of the pyrene labeled samples grafted at different aminophenyltrimethoxysilane ratios in the absence of methyl urea: 10% (solid line), 20% ( ×), 33% (O), and 100% (4). λexc ) 330 nm.
Figure 4. Fluorescence spectra of the pyrene labeled samples grafted at different aminophenyltrimethoxysilane ratios in the presence of methyl urea: 10% (solid line), 20% ( ×), and 33% (O). λexc ) 330 nm. Table 5. Fluorescence Intensities Ratios of Bands I and V (I1/I5) and of the Monomer to the Excimer bands (I5/IE) for the Various Materialsa sample
I1/I5
I5/IE
sample
I1/I5
I5/IE
S100-pyr S033-pyr S020-pyr S010-pyr
0.50 0.65 0.75 0.85
0.55 1.00 1.40 1.95
S033u-pyr S020u-pyr S010u-pyr
0.70 0.90 1.05
0.95 1.50 2.45
a
Experimental errors on ratio are 5%.
by analogy with the well-known polarity py-scale in solutions.68 In addition to these structured bands, one observes a broad emission above 435 nm arising from pyrene excimers. This attribution is supported by an increase in the intensity of the excimer-like fluorescence relative to that of the monomer as the number of grafted pyrene molecules increases (Table 4). This can be seen clearly looking at the emission spectrum of the hybrid material fully grafted with amino functions (S100-pyr, Figure 3) exhibiting reduced monomer emission and an intense broad excimer-like band with a maximum at 470 nm, the intensity of which will be noted as IE. The fluorescence spectra of the series prepared in the presence of methyl urea are almost similar (Figure 4) to those measured in its absence. The intensity ratios of bands I and V (I1/I5) and the ratio of the monomer-to-excimer fluorescence intensities (I5/IE) for the various materials are reported in Table 5. The I1/I5 ratio of the materials prepared without methyl urea decreases from 0.85 to 0.50 as the number of grafted fluorescent molecules increases
Single-Step Functionalities Dispersion on Silica
from 0.10 to 0.59 molecule.nm-2 (Table 4). The I1/I5 decrease can be explained in part by an overlap of the pyrene monomer and excimer-like emission spectra (compare spectra S010-pyr and S100-pyr of Figure 3), the contribution of the excimer-like fluorescence being obviously higher at 400 nm (band V) than at 380 nm (band I). For the hybrid materials prepared in the presence of methyl urea, the same overall trend is observed. Comparing materials synthesized with and without methyl urea with the same amine composition, some discrepancies in the I1/I5 ratio are observed (Table 5). These discrepancies are slight and stand within the range of the overall measurements uncertainties, except for the lowest labeled material (samples S010-pyr and S010u-pyr). Even if for this material, methyl urea seems to have an effect on the polarity of the pyrene environment that remains unclear to us, its role as a potential dispersing agent could not be demonstrated. Indeed, it seems more reasonable to us to assume that the changes observed between sample S010pyr and sample S010u-pyr result from the differences in their labeling efficiency, respectively, of 40% and 28% (see Table 4). This decrease in the labeling efficiency could explain the apparent decrease of the excimer-like emission. The ratio of the monomerto-excimer intensities was calculated using the I5 band of the monomer rather than band I1, even if the contribution of the excimer emission is expected to be higher at that wavelength. Indeed, some reabsorption by pyrene molecules could distort the spectrum, especially in its high-energy region (band I) and strictly cannot be excluded in measurements at silica interfaces.69 Using the I5/IE ratio, these errors are minimized. For both type of synthesis (with and without urea), increasing the number of amino grafts reduces the monomer-to-excimer ratio (Table 5). Again, the slight differences between the two sets of materials could be explained by the disparities in the number of grafted pyrenesulfonamide molecules (Table 4). So, in the following, only the results obtained for materials synthesized in the absence of methyl urea will be discussed. At this stage one has to comment on the presence of the excimerlike fluorescence with respect to the surface density of the grafted fluorophores. It was considered that the pyrene derivatives were evenly distributed over the silica surface. For the most complete amino coating conditions used (S100-pyr sample), 2.5 amino graft.nm-2 were introduced (Table 2). Considering that these were distributed evenly over the silica surface in a 2-D compact packing, the shortest distance between two amino sites would be 0.63 nm. Any pair of such nearest neighbors bearing a pyrene chromophore would lead to excimer fluorescence. Considering that 24% of the amino sites were labeled with pyrene (Table 4), excimer fluorescence is expected to be observed with a high probability, as indicated by the I5/IE ratio. Decreasing the number of amino grafts results in a decrease in the pyrene excimer fluorescence (Table 5). Actually, as the number of amino grafts decreases, the shortest distance between two amino sites increases, going from 0.63 nm for S100-pyr to 2.0 nm for the sample S010pyr. For the latter sample, 40% of the amino functions were grafted with pyrene, which would clearly result in an almost null
Langmuir, Vol. 24, No. 16, 2008 9037
probability to observe excimer. Hence, only monomer fluorescence should have been observed, whereas a low excimer fluorescence emission was measured. It is known that silica surfaces are intrinsically hetereogenous in nature, particularly in terms of their surface chemical potential due to numerous defects, silanol clustering, surface roughness, and surface curvature changes.70 All of these would result in a wide distribution for the surface density and orientation of the grafts with respect to the idealized surface. The low excimer fluorescence emission measured on S010-pyr could then be due to a few clusters of amine functionalities resulting from the intrinsic behavior of the silica surface. Conversely, the intense monomer fluorescence of sample S010-pyr indicates a good dispersion of amine functionalities, even for a low amino grafts concentration.
Conclusion A one-step method for controlling the dispersion of active sites on the surface of a mesoporous silica gel has been described. It involves the dual functionalization of an inorganic framework with a mixture of a functional organosilane and a nonfunctional one. The functional group (aniline) plays the role of a linker, selected to allow the tethering of a catalytic or a fluorescent center to the surface. The rigidity of the aniline moiety is aimed at preventing interactions between the active sites and the surface of the silica, that could result in a decrease in the activity of the functional group. This is particularly important if heterogeneous catalysis applications are concerned. The density of the grafted aniline over the silica surface can be controlled by its dilution with phenyl silane during the synthesis step, and is found to be a linear function of the dilution. Analysis of the fluorescence of a pyrene derivative bound to the aniline moiety indicates that the distribution of the grafted aniline is dispersed evenly over the silica surface. At the lowest amino-grafting ratio (10% of the aniline moiety), a few clusters of grafted aniline functions are still present, possibly due to the intrinsically irregular nature of the silica surfaces. Although an attempt to control the spatial distribution of the amine groups with methyl urea as a potential dispersing agent was not successful, its role deserves further attention. The benefits of the method described rely mostly on getting an improvement of the dispersion of functionality from a one-pot synthesis, and a restriction of the active site/silica support interactions enforced by the presence of the rigid linker. Acknowledgment. Funding from CNRS and Total Petrochemicals were gratefully acknowledged. The authors thank Marie France Driole, Christine Biolley for technical assistance with nitrogen sorption volumetry, Robert Durand and Bich Chiche for technical assistance with FTIR spectroscopy. They are grateful to the Service Central d’Analyse (CNRS, Solaize, France) and particularly J-L Imbert for his efficiency. Supporting Information Available: Data derived from the N2 adsorption-desorption experiments of the hybrid materials functionalized with pyrene. This material is available free of charge via the Internet at http://pubs.acs.org. LA800745G
(69) Lochmu¨ller, C. H.; Wenzel, T. J. J. Phys. Chem. 1990, 94(10), 4230– 4235.
(70) Farin, D.; Peleg, S.; Yavin, D.; Avnir, D. Langmuir 1985, 1, 399–407.
