Evidence of Specific Interactions between Laser-Polarized Xenon and

13564

J. Phys. Chem. C 2007, 111, 13564-13569

Evidence of Specific Interactions between Laser-Polarized Xenon and Arenesulfonic Acid Groups in Functionalized SBA-15 Materials Mirella H. Nader, Flavien Guenneau, Pe´ tra Salame, Franck Launay, Virginie Semmer, and Antoine Ge´ de´ on* Laboratoire des Syste` mes Interfaciaux a` l’Echelle Nanome´ trique (UMR 7142), UniVersite´ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 5, France ReceiVed: May 11, 2007; In Final Form: July 4, 2007

SBA-15 samples functionalized with different arenesulfonic organic contents and prepared at 373 K were characterized by hyperpolarized 129Xe NMR spectroscopy. Variable-temperature measurements provide information on the organic groups distribution in the materials and on Xe diffusion in the mesopores. The heat of adsorption of xenon on the surface as well as its chemical shift value in interaction with the surface are deduced from the temperature dependence of the 129Xe chemical shift. The arenesulfonic group surface coverage was determined, and consequently, the “organic phase” contribution to the shielding constant was estimated. The evolution of the chemical shift with pressure allowed us to evaluate the “xenon-organic phase” interactions and hence distinguish the different organic contents. The slope of the pressure chemical shift dependence reveals the way in which the functionalization process took place. For low organic content (0.05 and 0.10), the slope is negative, indicating the presence of shallow micropores, while for high organic content (0.15 and 0.20), the chemical shift remains unchanged with the pressure showing that the modified mesoporous surface is homogeneous and micropores exempt. Information about the vicinity of Xe-organic phase are obtained from 129Xe-1H CP and SPINOE experiments.

Introduction Organically modified mesoporous silica phases have attracted much attention in the field of adsorption,1 chemical sensing,2 and catalysis.3,4 Among the enormous functional variation of organic chemistry, materials bearing alkyl or arenesulfonic groups have received considerable interest in heterogeneous catalysis due to their high surface area and relatively important acid strength.5-17 Co-condensation routes6-9,11,12,14 are preferred over grafting procedures10,15,16 due to the better distribution of the functions throughout the resulting materials. In most of the cases, the sulfonic acid functions are introduced by the rather unselective oxidation of mercaptopropyl groups onto silica.6-15 Direct introduction of a stronger function by 2-(4-chlorosulfonylphenyl)ethyltrimethoxysilane (CSPTMS) is also considered with growing attention. In this work, SBA-15-type materials functionalized with different incorporation ratio 0.05 < R ) (CSPTMS/CSPTMS + TEOS) < 0.20 have been prepared by the co-condensation of CSPTMS and prehydrolyzed TEOS using Pluronic 123 as a template under acidic conditions. Thermally polarized xenon NMR spectroscopy has proved to be a powerful technique for the study of a diversity of materials such as zeolites,18 clathrates, mesoporous silicas, and silica glasses.18,19,20,21 Considerable improvement of this technique has been achieved by enhancing its signal sensitivity via optical pumping methods.22 The hyperpolarized (HP) xenon atoms produced can attain spin polarizations larger than thermal ones by 4 orders of magnitude. Thus, the HP 129Xe NMR technique has turned out to be particularly useful for systems with long spin lattice relaxation times (T1) and low surface areas such as molecular crystals23 or mesoporous thin films.24 This * To whom correspondence [email protected].

should

be

addressed.

E-mail:

high polarization of xenon can be transferred by crosspolarization (CP)25 or spin-polarization-induced nuclear Overhauser effect (SPINOE)26,27 to other nuclei in order to benefit from the sensitivity enhancements26 and/or obtain surfaceselective information.28 This paper is devoted to explore the internal structure of arenesulfonic-functionalized SBA-15 at different organic content by HP 129Xe NMR. Following the encouraging results of previous studies on pure silica SBA-15 materials,29 we present herein variable-pressure HP 129Xe NMR experiments in continuous flow conditions on organically modified samples. We also report the effect of the temperature on Xe NMR parameters, and finally, we discuss polarization transfer CP-SPINOE experiments. Experimental Section Synthesis Protocol. The synthesis of pure SBA-15 (373 K) used in this work was described elsewhere.30 Arenesulfonicfunctionalized mesoporous solids were synthesized according to the following procedure: 4.00 g of Pluronic 123 (EO20 PO70 EO20, Mav ) 5800, Aldrich) was dissolved in 125 mL of 1.9 M HCl. The mixture (solution A) was stirred at 313 K till the complete dissolution of the surfactant. Concurrently, the main silicon precursor, tetraethylorthosilicate (TEOS, Fluka), was prehydrolyzed for 45 min at room temperature in 10 mL of 2 M HCl and then added simultaneously with CSPTMS (ABCR) to solution A. The resulting mixture was maintained under stirring at 313 K for 24 h and then aged at 373 K for 24 h. The solution was then filtered, and the white solid was air-dried at room temperature overnight. Removal of the surfactant (Pluronic 123) was performed by a two-step procedure in order to avoid destruction of the sulfonic

10.1021/jp073609w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/22/2007

Functionalized SBA-15 Materials

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13565

TABLE 1: Chemical and Textural Characteristics of the As-Synthesized Pure and Acidic Materials sample

ABET (m2g-1)

Vp (cm3g-1)

D (BJH) (nm)

WS

F (nm-2)

SBA (0) SBA (0.05) SBA (0.10) SBA (0.15) SBA (0.20)

829 850 715 601 471

1.00 1.05 0.81 0.54 0.40

6.2 6.7 6.7 4.0 and 6.4 3.9 and 6.2

0 0.0228 0.0366 0.0437 0.0531

0 0.44 0.77 1.03 1.48

group. Most of the surfactant molecules were eliminated by extraction with pure ethanol under reflux for 24 h (1.5 g/400 mL) followed by a calcination step at 473 K for 24 h (air flow 9 L h-1). The efficiency of the two-step procedure has been recently investigated.31 The synthesized materials are denoted SBA (R). R is the incorporation ratio. The surface coverage, F (nm-2), of arenesulfonic groups for each sample was determined by elemental analysis and N2 sorption measurements according to the ref 32.

F)

[ ( )

WSNA MS

WS A 1(8MC + MS + 3MO + 9MH) MS

]

(1)

where WS is the sulfur content in gram per gram sample, NA is Avogadro’s number, MS, MH, MC, and MO indicate the atomic mass of sulfur, proton, carbon, and oxygen, respectively, and (A) is the specific surface area of the sample obtained by nitrogen adsorption data. N2 adsorption-desorption isotherm measurements for the different functionalized SBA-15 samples were published elsewhere.13b With increasing amounts of CSPTMS in the synthesis gel the specific surface and pore volume of the resulting materials were decreasing (Table 1). The isotherms of the pure SBA-15 sample and those of SBA (0.05) and SBA (0.10) were of type IV according to the Brunauer classification, indicating that the hexagonal structure is still maintained. The pore diameter was slightly larger for these two samples compared to the all-silica one (Table 1). However, co-hydrolysis of TEOS with higher amounts of CSPTMS induced significant changes to the structure. The isotherms of SBA (0.15) and SBA (0.20) were obviously different from type IV. Indeed, two hysteresis loops were detected as the result of a bimodal pore distribution. Smaller pores centered on roughly 3.9 nm existed besides the 6.2 nm ones, already present in the pure silica SBA15 material. It is noteworthy that most of the pores were of smaller size (3.9 nm) in sample SBA (0.20). Their volume corresponds to 78% of the total pore volume (Table 1). The different synthesized materials as well as their chemical and textural characteristics are presented in Table 1. NMR Spectroscopy. All powder samples were compressed by applying a pressure of 38 MPa. For flow experiments, the 0.2 mm thick pellets were broken into smaller pieces that would fit inside a 10 mm o.d. NMR tube equipped with two Young valves. The above procedure eliminates the effect arising from fast exchange between Xe adsorbed in the intra- and interparticle voids.33 The samples were subsequently dehydrated under vacuum at 473 K for 12 h prior to the HP 129Xe NMR experiments. The continuous-flow optical pumping apparatus for producing the HP 129Xe gas has been described elsewhere.34 All HP 129Xe NMR spectra were acquired on a Bruker Avance DSX300 NMR spectrometer at a Larmor frequency of 83.012 MHz using a single-pulse sequence with a π/2 pulse (15 µs). There were 64-

Figure 1. Variation of 129Xe NMR chemical shift at 298 K vs xenon pressure for SBA (0.05).

256 FIDs accumulated with a recycle delay of 1 s to ensure a sufficient signal-to-noise ratio. Hyperpolarized (HP) xenon was produced in the optical pumping cell, containing 1 g of Rb metal, placed under a magnetic field of ca. 200 G, and maintained at 423 K. The hyperpolarization is obtained by irradiating the gas mixture with a right-circularly polarized light (wavelength 794.8 nm) generated by a diode laser (Coherent; FAP-30). He-Xe gas mixtures containing 9-1000 Torr of Xe polarized to ca. 1% at total pressure of 1000 Torr were introduced at 100 cm3 min-1 via plastic tubing into the sample. Variable partial pressure of xenon is obtained by adding controlled doses of pure xenon in the gas flow. The actual partial pressure is determined using a MKS-750B gauge pressure connected to the circular apparatus. The chemical shifts are referenced to the signal of xenon gas. Variable-temperature experiments were carried out in the temperature range 143-413 K. Tuning and matching of the probe is carefully checked at each temperature step. This ensures that the π/2 pulse length and phase of the spectrum remain unaffected by the temperature change. Due to the design of the polarization setup, the HP and thermal signal have opposite phases and can be easily distinguished. An exponential multiplication of 50 Hz and a zero filling of 32K points were applied to the FIDs prior to the Fourrier transformation. Results and Discussion Variable-Pressure HP 129Xe NMR. HP 129Xe NMR spectra of xenon adsorbed at room temperature on arenesulfonic SBA (0.05) material are depicted in Figure 1 (for other incorporation ratios see Supporting Information). In addition to the gas peak near 0 ppm, all spectra show a single peak arising from the fast exchange between Xe adsorbed in different regions in the mesopores. The resonance lines are located at a higher chemical shift than for the pure silica SBA (0) (Figure 5). Pressuredependent 129Xe chemical shifts of the studied samples are shown in Figure 2. For SBA (0.05), the rise of the xenon partial pressure from 9 to 981 Torr leads to a decrease of the chemical

13566 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Figure 2. Evolution of Xe chemical shift versus Xe pressure at 298 K of SBA (0) (9), SBA (0.05) (2), SBA (0.10) (b), SBA (0.15) (O), and SBA (0.20) ([).

shift of xenon from 90 to 82 ppm. The pressure dependence of the chemical shift reveals the presence of an interaction between xenon and the organic phase acting as a strong adsorption site (SAS). At very low pressure, each xenon atom spends a relatively long time on these sites, which would increase the observed chemical shift. At high xenon loadings the relative population of xenon adsorbed on SAS decreases, leading to a decrease of the chemical shift. For the sample SBA (0.10), the curve δ ) f(P) is similar to the one obtained at R ) 0.05 with a constant global shift difference of 12 ppm observed. Since the two materials have a similar pore diameter (D ) 6.7 nm), we assume that “Xe-organic phase” interactions are solely responsible for this difference. On the other hand, at higher organic coverage (R ) 0.15 and 0.20), we observe that the relatively high chemical shift values do not vary significantly with pressure. This suggests that at high organic contents, the xenon atoms are exclusively in contact with the organic phase and the modified mesoporous surface is homogeneous from the NMR point of view. Similar results were observed with MCM41 during adsorption of water. The chemical shift independence was explained by the fact that water adsorbs on surface heterogeneities, leaving a more homogeneous surface accessible to xenon atoms.35 Variable-Temperature HP 129Xe NMR. Figure 3A shows the spectra at 403 K for three functionalized materials (R ) 0.05, 0.10, and 0.15) at a low partial pressure of xenon of ca. 9 Torr. For samples SBA (0.05) and SBA (0.10), the NMR spectra consist of two overlapped peaks at about 54 and 62 ppm.

Nader et al. The highest chemical shift (62 ppm) can be attributed to xenon atoms embedded in the organic phase, whereas the peak at 54 ppm corresponds to more mobile xenon atoms in fast exchange between the center of the mesopores and the bare silica surface. At this temperature (T ) 403 K) the Xe exchange between those two “adsorbed” environments is not fast enough to obtain a weighted average resonance peak. By decreasing the temperature xenon would preferentially reside in the organic phase at the expense of Xe adsorbed on the bare silica surface, eventually leading to the disappearance of the peak at low chemical shift (Figure 3B). A similar behavior was observed for dodecylsilanegrafted MCM-41 at high surface coverage32 as well as for multifunctional organoalkoxysilanes mesoporous silica materials.36 However, in both cases no clear explanation was proposed. By increasing the organic coverage (R ) 0.15) the spectrum shows one perfectly symmetric peak at 70 ppm corresponding to xenon atoms trapped in small voids surrounded by the arenesulfonic groups. The temperature-dependent spectra of the SBA (0.05) can be seen in Figure 4 (for other incorporation ratios see Supporting Information). Figure 5 shows the effect of temperature on the chemical shift of xenon adsorbed on the investigated SBA-15 materials. At 298 K, the chemical shift observed for the functionalized materials is higher than the one obtained for pure silica. By increasing the temperature and due to the increase of the rate of exchange with the xenon gas, the observed chemical shift moves in a continuous way toward the peak at 0 ppm (xenon gas). For the functionalized materials, the evolution of the curve δ ) f(T) for T > 190 K shows that the interaction between xenon and the organic phase slows down the exchange phenomenon with the xenon gas and consequently reduces the decrease of the chemical shift with temperature. This evolution (at T > 190 K) is consistent with the one recently reported by Zhang et al. for MCM-41 hosting gallium nanocrystals at different loadings.37 They attributed the smaller decrease of the chemical shift to a shorter average free path of xenon (λ) in the mesocomposites. At 190 K, all the curves δ ) f(T) are superimposed. Taking into account the very low Xe pressure (10 Torr) in the continuous-flow HP 129Xe NMR experiments we assume that in the adsorbed state the contribution of XeXe interactions to the observed chemical shift is very small.

Figure 3. (A) HP 129Xe NMR spectrum at 403 K of xenon adsorbed on SBA (0.05), SBA (0.10), and SBA (0.15) (PXe ) 9 Torr). (B) HP NMR spectrum at 298 K of xenon adsorbed on SBA (0.05), SBA (0.10), and SBA (0.15) (PXe ) 9 Torr).

129Xe

Functionalized SBA-15 Materials

J. Phys. Chem. C, Vol. 111, No. 36, 2007 13567 TABLE 2: K0, ∆Hads, and δa for SBA (0), SBA (0.05), SBA (0.10), SBA (0.15), and SBA (0.20) sample

1011 K0 (Torr-1 m-2)

∆Hads (kJ mol-1)

δa (ppm)

16.0 ( 0.2 14.4 ( 0.6 15.1 ( 0.4 12.9 ( 0.4

116.4 ( 1.0 123.0 ( 1.5 125.5 ( 1.0 124.9 ( 0.8

12.1 ( 0.7

130.5 ( 0.8

SBA (0) 3.9 ( 0.2 SBA (0.05) 32.5 ( 4.6 SBA (0.10) 24.7 ( 2.3 SBA (0.15) 44.2 ( 3.5 (D ) 4) 70.9 ( 5.6 (D ) 6.4) SBA (0.20) 54.8 ( 10.3 (D ) 3.9) 87.0 ( 1.6 (D ) 6.2)

As mentioned in previous studies,20,32,33,34 in the fast-exchange approximation and dilute adsorption limit (Henry’s Law) the temperature dependence of the observed chemical shift (δ) can be expressed as

δa

δ) 1+

D

(2)

(

4K0RT1/2 exp Figure 4. Temperature-dependent 129Xe NMR spectra of optically polarized xenon at partial Xe pressure of 9 Torr in SBA (0.05).

Figure 5. Effect of temperature on the chemical shift of xenon adsorbed on SBA (0) (9), SBA (0.05) (2), SBA (0.10) (b), and SBA (0.15) (O) and SBA (0.20) ([) (PXe ) 9 Torr).

However, this is not the case for T < 190 K because of the condensation of xenon on the mesopores surface which leads to the increase of the chemical shift for all materials.29,33,34,37,38,39 Usually the latter is confirmed by the strong increase of the signal intensity.39 However, in our grafted samples below a certain temperature (163 K for the low organic content and ∼203 K for the high organic content) Xe signal intensity decreases, reflecting that the HP Xe atoms face a diffusion problem (see Figure 4 and Supporting Information). A similar decrease of the 129Xe NMR peak intensity with decreasing temperature has been already observed and attributed to the inability of freshly hyperpolarized Xe atoms to enter the pores network.40,41 Increasing the repetition time of NMR experiments should give HP Xe more time to enter the mesopores and thus restore the signal intensity. Unfortunately, the high level of polarization cannot be maintained due to the fast longitudinal relaxation rate (R1) of xenon. In our case, the reduced mobility of the organic functions considerably hinders the entry of HP xenon atoms in the mesopores. This is confirmed by the disappearance of the HP signal and the simultaneous appearance of thermally polarized Xe, which in our optical pumping setup manifests itself as an inverted signal. The increase of the signal from thermally polarized Xe with lowering temperature can also be attributed to strong Xe condensation.

)

∆Hads RT

where δa is the characteristic chemical shift of xenon adsorbed on the surface, K0 the pre-exponential factor of Henry’s constant, R the universal gas constant, and ∆Hads the effective heat of adsorption for the pores under consideration. The linearization of eq 2 allowed Terskikh et al. to extract the ∆Hads values.20 The limitation of this method resides in the use of the same δa value for pure and organically modified silica samples. Indeed, this value should be influenced by the chemical composition of the surface. Therefore, we used an alternative method32,36 which consists in a nonlinear least-squares fitting of the observed chemical shift to eq 2. The pore diameter D was taken from N2 adsorption-desorption data. The optimized ∆Hads, K0, and δa values are presented in Table 2. In order to neglect Xe-Xe interactions, only chemical shifts in the high-temperature region (T > 190 K) were included in data fittings.32,33,34 The measured ∆Hads are in the same range of those obtained previously for other silica samples.20,21,29,32,34,37 One can observe that the adsorption enthalpy is not noticeably affected by the functionalization of the surface. In contrast, the obtained K0 values seem to be more sensitive to the amount of organic phases on the surface. The calculated chemical shift δa values are also found to increase with the organic content (R). This evolution can be explained by stronger interactions between Xe and organic moieties. Indeed, δa is the sum of Xe-silica wall and Xeorganic phase interactions, δa(SBA) and δa(org), respectively. Since δa(org) depends on the surface coverage F, δa can be expressed as δa ) δa(SBA) - Fσorg. -σorg represents the intrinsic shielding due to intermolecular van der Waals Xe-organic groups interactions32 and can be evaluated by studying the variation of δa versus the surface coverage F. From the linear variation of δa with F (Figure 6), a value of -σorg ) 8.65 ppm nm2 is obtained for arenesulfonicfunctionalized samples. In an attempt to further precise the nature of “Xe-organic phase” interactions, we applied the group contribution of -σorg proposed by Luhmer et al.42 From other results32 we deduced the -σ value arising from the phenylSO3H group -σ phenyl- SO3H ) 0.97 ppm nm2. However, due to the basic assumptions of this method, the obtained σorg values cannot be used to predict the preferential residence regions of xenon. Thus, it was necessary to have recourse to more demanding experiments consisting of polarization transfer from HP 129Xe to the nuclei of interest (1H in our case), CP and SPINOE.25,26,27 In a previous paper, we described a 7 mm MAS probe specifically designed for HP xenon experiments (XeMAS

13568 J. Phys. Chem. C, Vol. 111, No. 36, 2007

Nader et al. K the residence time of xenon on the surface is quite long and the transfer originates mainly from these adsorbed atoms. Since the alkyl groups are closer to the surface, their signal is preferentially enhanced in the CP experiments. Conclusion

Figure 6. Variation of δa with surface arenesulfonic coverage, F. The error bars were taken from fitting calculations and represent the minimal error in the fitted values.

Figure 7. 1H NMR spectrum and CP and SPINOE HP Xe spectra for SBA (0.10) sample at 200 K.

probe).34 A glass capillary delivers the gas to the sample through a hole drilled in the rotor cap. The powder samples carefully packed in 7 mm rotors can be spun at speeds up to 3.5 kHz under flowing gas conditions. 1H NMR, CP, and SPINOE experiments were carried out at 200 K. We found that the singlepulse 1H spectrum of SBA (0.10) sample consists of a large peak at 8 ppm, a shoulder at 4 ppm, and a large width hump (Figure 7). We attributed the 8 ppm peak to phenyl protons, the high-field shoulder has been assigned to alkyl protons, while the hump is probably due to the probe background, the surface hydroxyl and sulfonic protons. The SPINOE spectrum is nearly identical to the 1H spectrum, whereas the CP line shape is different. In both spectra the hump is less pronounced mainly because the proton signal comes from transfer of HP Xe which eliminates the probe signal. In the SPINOE spectrum, both the phenyl and the alkyl peaks benefit from the transfer process. This can be attributed to spin diffusion43 since the SPINOE acts through the second-order nuclear relaxation process associated with the nuclear Overhauser effect.44 Moreover, the SPINOE periods are on the order of seconds, and therefore, the polarization transfer affects all protons. Unlike SPINOE, CP relies on direct dipolar coupling mechanism, and therefore, the polarization transfer occurs over a period of time determined by the cross-polarization time (10 ms). Thus, the protons affected by the polarization transfer in CP are only those in the vicinity of Xe atoms. The CP spectrum of Figure 7 clearly shows that these protons are located in the alkyl groups (4 ppm). Indeed, at 200

Use of 129Xe NMR spectroscopy is confirmed to be a powerful tool to obtain fast and reliable information concerning the internal structure of organically modified mesoporous materials. In this paper, special attention has been placed on investigating how organic content is reflected in the spectra observed at different xenon pressures and temperatures. The character of the pressure dependence of the chemical shift provided valuable information regarding the presence of strong adsorption centers associated with modification of the chemical composition of the mesoporous materials and the presence of an interaction between xenon and the organic phase. The variable-pressure continuous-flow HP 129Xe NMR measurements on xenon adsorbed in SBA-15 materials containing arenesulfonic acid groups also allowed us to better understand the distribution of these organic groups in the pore structure. Indeed, at high organic contents we have shown that the xenon atoms are exclusively in contact with the organic phase and the modified mesoporous surface is homogeneous from the NMR point of view. The evolution of the 129Xe NMR chemical shift with temperature allowed us to evaluate the heat of adsorption of xenon as well as δa and K0 values and provided information on the organic groups’ distribution in the materials and on Xe diffusion in the mesopores. In fact, introduction of organic groups onto the pore walls of the mesoporous silica leads to an increase in both the chemical shift and the line width. With increasing grafting ratio, the resonance peak is shifted to lower field. The presence of the arenesulfonic group introduces a new interaction (Xe-organic phase) and thus adds a new contribution on the chemical shift. This organic phase could also hinder the diffusion of xenon in the mesopores, which also leads to an increase of the chemical shift. Moreover, for high grafting ratio the organic phase could completely block the entry of the Xe atoms in the mesopores. This approach opens a sensitive way to probe the distribution of high content functions in porous host materials. In this paper we have also shown that the variation of δa(org), arising from Xe- organic moieties interactions, with the organic surface coverage F allowed us to estimate the intrinsic shielding value, -σorg. Finally, evidence of selective enhanced proton NMR signal via cross polarization with hyperpolarized 129Xe has been obtained for the first time in functionalized SBA-15 materials. The results showed that only the alkyl protons are affected by the xenon polarization transfer. Supporting Information Available: Variable-temperature and pressure experiments for SBA (0.10, 0.15, and 0.20). References and Notes (1) (a) Dai, S.; Burleigh, M. C.; Shin, Y.; Morrow, C. C.; Barnes, C. E.; Xue, Z. Angew. Chem., Int. Ed. 1999, 38, 1235. (b) Brown, J.; Mercier L.; Pinnavaia, T. J. Chem. Commun. 1999, 69. (c) Feng, X.; Fryxell, G. E.; Wang, L. Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (2) (a) Descalzo, A. B.; Jimenez, D.; Marcos, M. D.; Martinez-Manez, R.; Soto, J.; El Haskouri, J.; Guillem, C.; Beltran, D.; Amoros, P.; Borrachero, M. B. AdV. Mater. 2002, 14, 966. (b) Lin, V. S.-Y.; Lai, C. Y.; Huang, J.; Song, S. A.; Xu, S. J. Am. Chem. Soc. 2001, 123, 11510. (c) Wirnsberger, G.; Scott, B. J.; Stucky, G. D. Chem. Commun. 2001, 119. (3) (a) Stein, A. AdV. Mater. 2003, 15, 763. (b) Stein, A.; Melde, B. J.; Schroden, R. C. AdV. Mater. 2000, 12, 1403.

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