Hydrophobic induced supramolecular self-organization of tetrahedral units based on van der Waals interactions

Abstract

Nanostructured hybrid materials were prepared by sol–gel hydrolysis–polycondensation of tetrakis(4-trisisopropoxysilylphenyl)germane PGe and tetrakis(4-trisisopropoxysilylphenyl) tin PSn which present a tetrahedral germanium or tin central atom. Both precursors presented twelve directions for solid formation oriented regularly on the four directions of the tetrahedron. Their organization was evaluated by X-ray diffraction studies (nanometric scale) and birefringence measurements (micrometric scale). It has been shown that the organization of these solids was influenced by the experimental parameters. The intensities of birefringence lay in the same range (1.6 × 10⁻³ to 2.1 × 10⁻³) for all the gels XGe and XSn. Regular parallel rectangular chunks were observed in all cases. The orientation of the optical axis was found perpendicular to the edges of the chunks in all cases. The existence of such optical axes regularly oriented towards the edges of the chunks corresponded to an anisotropic organization at the micrometric scale. The results presented here strengthen the fact that the auto-organization observed in nanostructured organic–inorganic hybrid materials occurs during the formation of Si–O–Si bonds, which transforms intermolecular interactions into intramolecular ones corresponding to the weak van der Waals-type interactions existing between the organic units. The role of hydrophilic–hydrophobic interactions is discussed.

---

Introduction

The supramolecular^1,2 self-organization of the organic spacers inside of the matrices of monophasic hybrid organic–inorganic materials obtained by sol–gel polymerization³ has been evidenced recently in the case of precursors containing at least two Si(OR)₃ groups (Scheme 1).^4–14

Scheme 1 Sol–gel process of nanostructured hybrid organic–inorganic materials.

In these solids the weak interactions (van der Waals and/or π–π stacking), the energy of which is lower than 1.5 kcal mol⁻¹, are inducing a self-organization of the organic units during the polycondensation at silicon. Hybrid materials involving stronger interactions by H-bonding and presenting supramolecular architectures have also been reported.^15–18 Such interactions cannot be involved in the solids that we consider here.

These materials are amorphous since X-ray scattering does not exhibit any Bragg peak. However, these materials have to be considered rather as non-crystalline organized materials. Indeed, all of the samples studied until now, whatever the geometry of the spacer^19–21 and the experimental conditions,²² exhibit X-ray diffraction diagrams presenting broad signals as well as birefringence properties. These properties are indicative of the unquestionable existence of organizations at both the nanometric scale evidenced by X-ray diffraction and at the micrometric scale as shown by birefringence¹⁹ (Scheme 2). This is the reason why we named them nanostructured hybrid materials.⁵

Scheme 2 Steps of material formation by inorganic polymerization. The birefringence appears during ageing whereas the auto-organization at the nanometric scale is induced during the first steps of polycondensation.

Another important feature of this process is the kinetic control of both the texture²³ (specific surface area, porosity) and the structure.²² Basically the general features of the organization are similar for a given geometry. However, a change in experimental conditions does not lead to the same X-ray diffraction and to the same birefringence data: the materials exhibit the same features, but they are not identical.

Many precursors have been studied which contain organic spacers having different geometries: linear rigid, linear with some flexibility,¹⁹ twisted linear,²⁰ planar.²¹ All of them exhibit X-ray diffraction and birefringence. The only exception corresponds to aliphatic spacers¹⁹ which exhibit an X-ray diffraction diagram but no birefringence properties. This might be explained by the formation of hydrophobic globular-type aggregates at the nanometric scale organization, the stacking of which does not present anisotropy at the micrometric scale.

It is interesting to point out that the birefringence pictures obtained for hybrid materials exhibit very different characteristics from those observed in the case of liquid crystals.²⁴ This is obviously due to the fact that all the materials studied here are cross-linked solids and not isolated molecules as in liquid crystals. In both cases, the intermolecular interactions are fundamental factors, however they are not involved in the same way. In the case of liquid crystals, the organization originates from intermolecular interactions in a static state depending on the thermodynamics. In contrast, the hybrid solids we are studying are kinetically controlled by the rate of polycondensation at silicon: both X-ray and birefringence measurements are highly dependant on kinetic parameters (solvent, catalyst, concentrations, etc).¹⁹ We assume that the organization is controlled by the van der Waals-type interactions between the organic units, taking place after the Si–O–Si bonds formation.

In this paper we report very surprising results concerning the auto-organization in solids obtained from precursors presenting a rigid tetrahedral geometry. This geometry is known to favor isotropy. For instance metallic alkoxides with the tetrahedral geometry such as Si(OR)₄ or Ti(OR)₄ lead always to amorphous oxides by hydrolytic polycondensation. The crystals are obtained only after thermal treatment (thermodynamic control). Here we describe the hydrolytic polycondensation of tetrakis(4-trisisopropoxysilylphenyl)germane PGe and tetrakis(4-trisisopropoxysilylphenyl)stannane PSn which present a tetrahedral germanium and tin central atom and twelve directions for solid formation oriented regularly on the four directions of the tetrahedron (Scheme 3).

Scheme 3

We have chosen these precursors because it has been recently described that rigid molecules containing Ph₄C and Ph₄Sn as a core and eight lipophilic long chains, explored as possible mesophases (Scheme 4), did not present any birefringence property.²⁵

Scheme 4

Results and discussion

Synthesis and sol–gel processing of precursors PGe and PSn

PGe and PSn were prepared by condensation of the Grignard reagent of (4-bromophenyl)trisisopropoxysilane²⁶ with respectively germanium tetrachloride and tin tetrachloride. They were isolated as white powders in 70 and 78% yield (experimental section). The hydrolysis–polycondensation reactions were performed in 0.5 M THF solutions in the presence of two types of catalyst, nucleophile (tetrabutylammonium fluoride (TBAF) or NH₄F) and acid (HCl) with the stoichiometric amount of water (Scheme 3). Gels formed within various periods of time. After ageing for 6 days, they were powdered, then washed with ethanol, acetone, diethyl ether and dried in vacuum (10⁻² mm Hg) at 120 °C for 2 h. The experimental conditions, the spectroscopic and textural data for the resulting solids are given in Table 1.

Table 1 Experimental conditions, ²⁹Si CP MAS NMR and textural data, birefringence values for xerogels obtained from PGe and PSn

Precursor	Xerogel	Catalyst (mol%)	tgel/h	^29Si CP MAS NMR (%)				LC^a (%)	BET		Birefringence Δn × 10^3
				T^0	T^1	T^2	T^3		SSA/m^2 g^−1	μ-pores (%) ^b
a Level of condensation calculated according the general equation LC = [0.5(T^1 area) + 1.0(T^2 area) + 1.5(T^3 area)]/1.5b Mesopores size: 20–120 Å
PGe	XGe1	NH4F (5)	48	0	22	53	25	68	483	80	1.6
PGe	XGe2	HCl (5)	144	0	29	56	15	62	424	79	1.7
PSn	XSn1	TBAF (5)	48	0	29	51	20	64	502	83	1.8
PSn	XSn2	TBAF (10)	36	0	17	52	36	75	463	76	1.7
PSn	XSn3	NH4F (5)	60	0	22	78	0	60	492	82	1.8
PSn	XSn4	HCl (5)	72	6	22	72	0	55	426	75	2.1
PSn	XSn5	HCl (10)	50	0	22	55	23	67	457	75	2.0

---

The solids were analyzed by ²⁹Si CP MAS NMR spectroscopy. The spectra displayed three resonances at ∼−70, ∼−79 and ∼−86 ppm in the case of XGe1 and XGe2, and at ∼−71, ∼−80 and ∼−89 ppm for XSn1, XSn2 and XSn5, assigned respectively to T¹, T² and T³ substructures. In the case of XSn3 only T¹ and T² substructures were present and for XSn4 it appeared an additional weak signal at ∼−62 ppm attributed to the presence of T⁰ units. The level of condensation was estimated in first approximation by deconvolution of spectra. It has been previously shown that no significant variation in relative peak intensity was observed in the case of such alkylene or arylene hybrid solids by comparison between CP MAS and single pulse experiments that allow a quantitative evaluation.^27,28 Moreover, since materials of the same precursor are compared, it can be assumed that relative peak intensity can be used as indicative of the order of magnitude for the level of condensation. It was found in the range 55–75% (Table 1). The specific surface areas of the xerogels^29,30 varied from 420 to 500 m² g⁻¹. The isotherms were of type II.³¹ The microporous volume^32–35 represented 75 to 83% of the total porous volume and the mesopores exhibited no narrow pore size distribution (20–120 Å).

X-Ray powder diffraction analysis and birefringence measurements for XGe and XSn

Like all the other nanostructured hybrid materials reported until now,³⁶ the X-ray diffraction diagrams did not exhibit any sharp Bragg signal. However, broad and intense signals were observed and it is well established that these broad signals correspond without any doubt to the existence of a nanometric scale order in the material.³⁷ This point is very important since it means that a tetrahedral precursor is able to induce a self-organization, at the nanometric scale, in the solid. As a first approximation assuming Bragg's law a priori, the distances associated with the q values were evaluated (d = 2π/q). The shape of the X-ray diffraction diagrams was the same for all the solids prepared in this study whatever the experimental conditions: two broad signals were observed at ∼7.1 Å (intense) and ∼4.8 Å (weak) for XGe1 and XGe2 and three ones at ∼11.9 Å (weak), ∼7.2 Å (intense) and ∼4.2 Å (weak and broad) for XSn1–XSn5. The diagrams are given in Fig. 1.

Fig. 1 X-Ray powder diffraction diagrams of the xerogels.

The signals centered at 4.2 and 4.8 Å corresponded to a broadening of the signal in the range of 4–5 Å always observed in the case of precursors already studied. It is, as usual, attributed to the contribution of the Si–O–Si units.³⁷ The stronger signal located at 7.1 (XGe) and 7.2 Å (XSn) might correspond to the distance between planes constituted of germanium or tin atoms on one hand, and planes containing the siloxane bridges on the other hand. The values obtained by molecular modelling were respectively 7.4 and 7.6 Å. The signal at 11.9 Å for XSn could fit with an average of the distances separating planes containing the silicon atoms. However, in the case of XGe such a signal did not appear as a clear shoulder.

Nevertheless, an interesting fact results from a comparison with the diffractograms obtained for linear and discotic precursors. Considering the signal lying in the range 3.5–4.5 Å as a reference (Si–O–Si units), we can observe that the signals attributed to the organic spacers in the case of XGe (7.1 Å) and XSn (7.2 Å) exhibit a relative intensity clearly higher than those obtained previously with the other precursors (linear, mesogen or discotic).

Birefringence measurements were performed under polarized light by introduction of colloidal solution into thin Teflon-coated cells a few minutes before the sol–gel transition.¹⁹

As always observed in all the previous cases the solution corresponded to an isotropic medium and the birefringence phenomenon occurred during ageing, after the sol–gel transition. Regular parallel rectangular chunks were observed (Fig. 2).

Fig. 2 Xerogels observed by polarizing optical microscopy.

The birefringence intensities lay in the same range (1.6 × 10⁻³ to 2.1 × 10⁻³) and were lower in the case of XGe than for XSn (Table 1). The width and the size of the rectangular chunks of gel were regular whatever the precursor and the catalyst. No changes occurred after several months. The orientation of the optical axis was determined using a Berek compensator. It was found to be perpendicular to the edges of the chunks in all cases which have ben studied (cf. Scheme 5). The existence of such optical axis regularly oriented towards the edges of the chunks corresponded to an anisotropic organization at the micrometric scale. In the case of liquid crystals the optical axis is connected with the orientation of independent molecules anisotropically organized. In our case this measurement is used as an experimental fact which corresponds to the presence of an orientation of the organized matter at the micrometric scale. However, it is not possible to propose any interpretation since it is the first time that tetrahedron units are showing this kind of organization. The general characteristic of the chunks was similar for all the xerogels XGe and XSn. However they were not identical since the formation of the solids is kinetically controlled (Scheme 5).

Scheme 5 Supramolecular organization induced by van der Waals interactions.

Moreover it is interesting to underline that the whole observation cells (2 cm × 2 cm) were organized by this type of chunks for all the xerogels XGe and XSn. Some differences were observed in the size, the spacing and the relative arrangement of the chunks. However, the orientation of the optical axis and the type of organization were similar in all types of arrangements.

The results presented here appear to be in agreement with the fact that the auto-organization observed during the hydrolytic polycondensation of nanostructured organic–inorganic hybrid materials is induced by the weak van der Waals-type interactions existing between the organic units during the polycondensation when the precursor is submitted to the hydrolytic conditions. Indeed, during the polycondensation, the irreversible formation of Si–O–Si bonds reduces the entropy by changing intermolecular interactions into intramolecular interactions. Thus, the extension of the auto-organization at the nanometric scale occurs by forcing the molecules to be linked one to each other. There is the formation of the bonds which permits to explain the organization in the case of PGe and PSn and the absence of liquid crystalline properties in the case of the compound reported in Scheme 4.

Furthermore it is important to underline the difference in behavior between the metallic alkoxides such as Si(OR)₄ or Ti(OR)₄ and the precursors PGe and PSn. This difference is due to the organic core of the hybrids which induces hydrophobic interactions in an hydrophilic medium. Thus during hydrolysis the metal alkoxides are leading to completely amorphous solids as expected. In contrast the behavior of PGe and PSn in the same conditions is very different because of the hydrophobic architecture of these precursors. When the molecules of H₂O are introduced, PGe and PSn will form aggregates in which the hydrophobic aromatic units are more or less overlapped. However, at present it is not possible to explain how the organization is extended at the micrometric scale, neither the shape of the chunks and nor the millimetric scale organization. All these transformations are occurring in solid state.

Conclusion

In this paper we have shown the drastic influence of the hydrophilic conditions on the organization in the solid. The organization of a symmetric tetrahedron was induced by the weak van der Waals interactions between the organic units. This organization into these amorphous systems is kinetically controlled and appears at three levels from the nanometric scale (X-ray diffraction) to the micrometric (birefringence) and even the millimetric one since the chunks are regularly distributed in the cell. In conclusion, these results have shown that the kinetic control of auto-organization of non-crystalline hybrid materials is a general phenomenon, in which hydrophilic–hydrophobic interactions and the formation of Si–O–Si bonds are playing a very important role.

Experimental

All reactions were carried out under argon using a vacuum line and Schlenk techniques. Solvents were dried and distilled just before use. The ¹H and ¹³C NMR spectra were recorded on a Bruker DPX-200 spectrometer and the ²⁹Si NMR spectra were recorded on a Bruker WP-200 SY spectrometer. The ²⁹Si CP MAS NMR spectra were recorded on a Bruker Avance 300 spectrometer operating at 59.6 MHz using a recycling delay of 10 s and a contact time of 5 ms. The spinning rate was 5 kHz. Chemical shifts are given relative to tetramethylsilane. The mass spectra at high resolution were recorded using mass spectrometer Varian MAT 311. The nitrogen adsorption–desorption isotherms at 77.35 K were recorded on a Micromeritics Gemini III 2375 apparatus. The specific surface area was determined using the BET equation. The pore size distribution was calculated using the BJH method, and the microporous volume was estimated by the t-plot method using the Harkins and Jura standard isotherm. The X-ray experiments were performed on powders of solids in a Lindeman tube with an imaging plate two-dimensional detector (Marresearch 2D “Image-Plate”) with a rotating anode apparatus (Rigaku RU 200). The radiation used was Cu Kα (λ = 1.5418 Å). Optical properties of the materials were observed with a Laborlux12POLS polarizing microscope. Photographs were taken using a Leica wild MPS28 camera. The birefringence Δn of the gels was obtained from the expression Δl = (Δn)d, where Δl is the optical path difference and d is the cell thickness which is evaluated by UV–vis spectroscopy (∼15 µm). Δl was measured by a Berek compensator. Elemental analyses were carried out by the “Service Central de Micro-Analyse du CNRS”.

Tetrakis(4-trisisopropoxysilylphenyl)germane PGe

The Grignard reagent of (4-bromophenyl)trisisopropoxysilane was prepared in THF from 24 g (66.42 mmol) bromo compound and 1.94 g (79.70 mmol) of magnesium according to the literature procedure.²⁶ The grey mixture was stirred during 3 h at room temperature, then 1.61 mL (13.84 mmol) of tetrachlorogermane were added. After an additional 12 h of stirring, THF was evaporated and the solid residue was taken up in pentane. After filtration was evaporated under vacuum to give a viscous white solid which was crystallized in isopropanol. PGe was obtained as white powder: 11.62 g (9.70 mmol, yield 70%, mp = 244–246 °C). ¹H NMR (δ/ppm, 200 MHz, CDCl₃): 1.25 (d, ³J_H–H = 6.1 Hz, 72 H); 4.31 (hept, ³J_H–H = 6.1 Hz, 12H); 7.63 (dd, ³J_ortho = 7.6 Hz, 16H). ¹³C NMR (δ/ppm, 50 MHz, CDCl₃): 25.2 (CH₃); 65.6 (OCH); 133.2 (Ar); 134.1 (Ar); 135.5 (Ar); 138.7 (Ar). ²⁹Si NMR (δ/ppm, 39 MHz, CDCl₃): −61.57. Electrospray (m/z): [M + Na⁺]_calcd. = 1221.5401 uma; [M+Na⁺]_found = 1221.5429 uma. Elemental analysis calcd.% for C₆₀H₁₀₀O₁₂GeSi₄: C 60.09, H 8.42, Si 9.41, Ge 6.12; found. C 60.29, H 8.26, Si 9.44, Ge 6.03.

Tetrakis(4-trisisopropoxysilylphenyl)stannane PSn

The precursor PSn was prepared similarly from 20 g (55.35 mmol) of (4-bromophenyl)trisisopropoxysilane, 1.61 g (66.42 mmol) of magnesium and 1.61 mL (11.53 mmol) of tintetrachloride. PSn was obtained as white powder: 11.20 g (9.00 mmol, yield 78%, mp = 288–289 °C). ¹H NMR (δ/ppm, 200 MHz, CDCl₃): 1.25 (d, ³J_H–H = 6.1 Hz, 72 H); 4.31 (Hpt, ³J_H–H = 6.1 Hz, 12H); 7.55 (dd, ³J_ortho = 7.6 Hz, 16H). ¹³C NMR (δ/ppm, 50 MHz, CDCl₃): 25.2 (CH₃); 65.5 (OCH); 134.4 (Ar); 135.3 (Ar); 136.8 (Ar); 140.1 (Ar). ²⁹Si NMR (δ/ppm, 39 MHz, CDCl₃): −61.49. Electrospray (m/z): [M + Na⁺]_calcd. = 1267.5212 uma; [M + Na⁺]_found = 1267.5236 uma. Elemental analysis calcd.% for C₆₀H₁₀₀O₁₂SnSi₄: C 57.90, H 8.11, Si 9.04, Sn 9.65; found C 57.93, H 8.06, Si 9.11, Sn 9.51.

Preparation of the xerogels XGe and XSn

The preparation of the xerogels was carried out according to the following general procedure. The yields were determined on the base of a totally polycondensed solid. The preparation of the xerogel XGe1 is given as an example: In a Schlenk tube 0.8 g (0.668 mmol) of PGe and 668 µL of dried THF were introduced. 668 µL of a solution containing 33 µL (33 µmol) of NH₄F (1 mol L⁻¹ in THF), 72 µL (4 mmol) of H₂O and 563 µL of dried THF, were added. After homogenization, the resulting solution was kept in the Schlenk tube and the sol–gel transition occurred after 48 h. A part of this mixture was introduced by capillarity into a Teflon-coated cell just before the sol–gel transition for birefringence measurements. After ageing for 6 days the gel was crushed and washed twice with acetone, ethanol and diethyl ether, and the resulting solid was dried at 120 °C in vacuo for 3 h yielding a white xerogel.

XGe1: 0.360 g (0.615 mmol); Yield 92%. ²⁹Si CP MAS NMR (δ/ppm, 60MHz): −70.9 (T¹); −78.9 (T²); −85.8 (T³). S_BET 483 m²g⁻¹.

XGe2: 0.800 g (0.668 mmol) of PGe in 1.336 mL of THF solution containing 33 µL (33 µmol) of HCl (1 mol L⁻¹in THF) and 72 µL (4 mmol) of H₂O gave a gel after 144 h. XGe2: 0.345 g (0.589 mmol); Yield 88%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −69.5 (T¹); −78.4 (T²); −86.5 (T³). S_BET 424 m²g⁻¹.

XSn1: 0.5 g (0.402 mmol) of PSn in 0.804 mL of THF solution containing 20 µL (20 µmol) of TBAF (1 mol L⁻¹ in THF) and 44 µL (2.41 mmol) of H₂O gave a gel after 48 h. XSn1: 0.245 g (0.388 mmol); Yield 96%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −70.8 (T¹); −80.1 (T²); −88.5 (T³). S_BET 452 m²g⁻¹.

XSn2: 1.0 g (0.803 mmol) of PSn in 1.606 mL of THF solution containing 80 µL (80 µmol) of TBAF (1 mol L⁻¹ in THF) and 87 µL (4.82 mmol) of H₂O gave a gel after 36 h. XSn2: 0.475 g (0.752 mmol); Yield = 94%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −71.0 (T¹); −79.5 (T²); −88.8 (T³). S_BET 463 m²g⁻¹.

XSn3: 1.0 g (0.803 mmol) of PSn in 1.606 mL of THF solution containing 40 µL (40 µmol) of NH₄F (1 mol L⁻¹ in THF) and 87 µL (4.82 mmol) of H₂O gave a gel after 60 h. XSn3: 0.468 g (0.741 mmol); Yield = 92%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −72.1 (T¹); −79.9 (T²); −89.8 (T³). S_BET 492 m²g⁻¹.

XSn4: 1.0 g (0.803 mmol) of PSn in 1.606 mL of THF solution containing 40 µL (40 µmol) of HCl (1 mol L⁻¹ in THF) and 87 µL (4.82 mmol) of H₂O gave a gel after 50 h. XSn4: 0.433 g (0.685 mmol); Yield = 85%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −62.1 (T⁰); −69.0 (T¹); −79.5 (T²). S_BET 426 m²g⁻¹.

XSn5: 0.3 g (0.241 mmol) of PSn in 0.482 mL of THF solution containing 24 µL (24 µmol) of HCl (1 mol L⁻¹ in THF) and 26 µL (1.45 mmol) of H₂O gave a gel after 72 h. XSn5: 0.143 g (0.226 mmol); Yield = 94%. ²⁹Si CP MAS NMR (δ/ppm, 60 MHz): −70.6 (T¹); −78.1 (T²); −87.8 (T³). S_BET 457 m²g⁻¹.

Acknowledgements

The authors are very grateful to Prof. Pierre Delord and Dr Philippe Dieudonné of the Department of Physics for fruitful discussions about the X-ray diffraction analysis.

References

1. J. M. Lehn, Supramolecular Chemistry, VCH Weinheim, 1st edn., 1995.
2. M. W. Hosseini, Coord. Chem. Rev., 2003, 240, 157.
3. C. J. Brinker and G. W. Scherer, Sol–Gel Science: The Physics and Chemistry of Sol–Gel Processing; Academic Press, San Diego, 1990.
4. D. A. Loy and K. J. Shea, Chem. Rev., 1995, 95, 1431.
5. R. J. P. Corriu, Angew. Chem., Int. Ed., 2000, 39, 1376.
6. G. J. de Soler-Illia, C. Sanchez, B. Lebeau and J. Patarin, Chem. Rev., 2002, 102, 4093.
7. C. Sanchez, G. J. de Soler-Illia, F. Ribot and D. Grosso, C. R. Chim., 2003, 6, 1131.
8. E. Toussaere, J. Zyss, P. Griesmar and C. Sanchez, Nonlinear Opt., 1991, 1, 349.
9. Z. Yang, C. Xu, B. Wu, L. R. Dalton, S. Kalluri, W. H. Steier, Y. Shi and J. H. Bechtel, Chem. Mater., 1994, 6, 1899.
10. K. J. Shea, D. A. Loy and O. Webster, J. Am. Chem. Soc., 1992, 114, 6700.
11. R. J. P. Corriu, P. Hesemann and G. F. Lanneau, Chem. Commun., 1996, 1845.
12. A. C. Franville, D. Zambon, R. Mahiou, S. Chou, Y. Troin and J. C. Cousseins, J. Alloys Compd., 1998, 275–277, 831.
13. A. C. Franville, D. Zambon, R. Mahiou and Y. Troin, Chem. Mater., 2000, 12, 428.
14. G. Dubois, C. Reye, R. J. P. Corriu, S. Brandes, F. Denat and R. Guilard, Angew. Chem., Int. Ed., 2001, 40, 1087.
15. A. Shimojima, Y. Sugahara and K. Kuroda, J. Am. Chem. Soc., 1998, 120, 4528.
16. J. J. E. Moreau, L. Vellutini, M. Wong Chi Man, C. Bied, J.-L. Bantignies, P. Dieudonne and J.-L. Sauvajol, J. Am. Chem. Soc., 2001, 123, 7957.
17. J. J. E. Moreau, L. Vellutini, M. Wong Chi Man and C. Bied, J. Am. Chem. Soc., 2001, 123, 1509.
18. J. J. E. Moreau, L. Vellutini, M. Wong Chi Man and C. Bied, Chem.—Eur. J., 2003, 9, 1594.
19. B. Boury and R. J. P. Corriu, Chem. Rec., 2003, 3, 120 and references therein.
20. G. Cerveau, R. J. P. Corriu and E. Framery, Chem. Mater., 2001, 13, 3373 and references cited.
21. B. Boury, F. Ben, R. J. P. Corriu, M. Nobili and P. Delord, Chem. Mater., 2002, 14, 730.
22. G. Cerveau, R. J. P. Corriu, E. Framery and F. Lerouge, Chem. Mater., 2004, 16, 3794.
23. G. Cerveau, R. J. P. Corriu, E. Framery and F. Lerouge, J. Mater. Chem., 2004, 14, 3019.
24. P. Collings and M. Hird, Introduction to Liquid Crystals: Chemistry and Physics; Taylor and Francis, London, 1997.
25. A. Pegenau, T. Hegmann, C. Tschierske and S. Diele, Chem.—Eur. J., 1999, 5, 1643.
26. J. P. Bezombes, C. Chuit, R. J. P. Corriu and C. Reye, J. Mater. Chem., 1998, 8, 1749.
27. G. Cerveau, R. J. P. Corriu, C. Lepeytre and P. H. Mutin, J. Mater. Chem., 1998, 8, 2707.
28. H. W. Oviatt, Jr., K. J. Shea and J. H. Small, Chem. Mater., 1993, 5, 943.
29. S. J. Gregg and S. W. Sing, Adsorption, Surface Area and Porosity; Academic Press: London, 1982.
30. S. Brunauer, P. H. Emmett and E. J. Teller, J. Am. Chem. Soc., 1938, 60, 309.
31. T. J. Barton, L. M. Bull, W. G. Klemperer, D. A. Loy, B. McEnaney, M. Misono, P. A. Monson, G. Pez, G. W. Scherer, J. C. Vartulli and O. M. Yaghi, Chem. Mater., 1999, 11, 2633–2656.
32. J. H. De Boer and B. C. Lippens, J. Catal., 1965, 4, 319.
33. W. D. Harkins and G. J. Jura, Chem. Phys., 1943, 11, 431.
34. E. P. Barrett, L. S. Joyner and P. P. Halenda, J. Am. Chem. Soc., 1951, 73, 373.
35. S. Brunauer, L. S. Deming, W. S. Deming and E. J. Teller, J. Am. Chem. Soc., 1940, 62, 1723.
36. B. Boury and R. J. P. Corriu, in Supplement Si: The Chemistry of Organic Silicon Compounds, ed. Z. Rappoport, Y. Apeloig, Wiley & Sons, Chichester, 2001, ch. 10, p. 565.
37. B. Boury, R. J. P. Corriu, P. Delord and V. Le Strat, J. Non-Cryst. Solids, 2000, 265, 41 and references therein.

---

Footnote

† Electronic supplementary information (ESI) available: Molecular modelling diagrams of PSn and PGe. See http://dx.doi.org/10.1039/b509017b

---