1 Introduction

Ordered siloxane-based materials composed of regularly arranged SiO4 tetrahedral units are useful for various applications, including catalysis and adsorption [1]. Crystalline silicates such as zeolites [2] and layered silicates [3] are generally synthesized by hydrothermal treatments, but the complex crystallization mechanism makes it difficult to precisely control the structure and compositions. Controlled assembly of pre-designed molecular building blocks is a promising method to expand the structural and compositional diversity of molecularly ordered siloxane-based materials. Among various types of oligosiloxanes with well-defined structures [4,5,6,7,8,9,10], cage siloxanes have attracted particular attention as building blocks because of their rigid frameworks and various functional groups at the corners [7, 8]. Extensive studies have been made on cross-linking of cage siloxanes via Si–O–Si bonds and organic linkers [11, 12]; however, most of the materials have random network structures, and controlled assembly of cage siloxanes has been an important issue.

The use of intermolecular interactions is crucial to realize the assembly of oligosiloxanes into molecularly ordered structures. Hydrogen bonds (H-bonds) of silanol groups are quite effective for the regular assembly of cage siloxanes. Several different arrangements of octa-silanol modified cage siloxanes, from one-dimensional (1D) to three-dimensional (3D) assemblies, have been reported [13,14,15,16,17,18,19]. The silanol groups are also useful for intermolecular linking by the formation of Si–O–Si bonds either by dehydration condensation or by silylation. We have recently synthesized H-bonded molecular crystals of cage siloxanes modified with SiR2OH groups (R = Me and iPr) [16,17,18,19]. Furthermore, cross-linking of the H-bonded SiMe2OH groups of the cage siloxanes was achieved by silylation with trichlorosilane, resulting in the formation of a crystalline microporous material [17]. Nevertheless, the crystalline structures assembled from octa-silanol modified cage siloxanes have been still limited, and the formation of mesoscale structures remains a challenge.

Introducing organic substituents to the corner Si atoms of cage siloxanes allows mesoscale assembly. Polyhedral oligomeric silsesquioxanes (POSS, R8Si8O12, R = H or organic groups) modified with various organic functional groups and/or polymers were designed to drive hierarchical assembly into complex architectures [20,21,22]. However, the regularity of the cage siloxanes was not well-controlled in many cases. In addition, most of these POSS-based assemblies cannot be cross-linked by siloxane networks, except for those having SiH groups [20]. On the other hand, we reported the formation of lamellar and two-dimensional (2D) hexagonal mesostructures by the sol–gel reactions of cage siloxanes modified with a long-chain alkyl group and seven ethoxy groups [23, 24]. The key step for the self-assembly was hydrolysis of the SiOEt groups into SiOH groups, forming amphiphilic molecules containing silanol groups and a long alkyl group. However, the cage siloxanes were not regularly arranged at the molecular level probably because condensation of the silanol groups partially proceeded prior to self-assembly. Synthesis and isolation of similar amphiphilic siloxane cages modified with relatively stable organosilanol groups and a long alkyl chain are essential.

In this paper, we report the synthesis of a novel cage siloxane compound modified with seven organosilanol groups and a long-chain alkyl group that can form a hierarchical siloxane-based material. Cubic octameric silicate was modified with SiMe2H groups and a SiMe2C18H37 group by stepwise silylation (D4R-7SiH and D4R-7SiH-C18 in Scheme 1), followed by the conversion of the SiMe2H groups to SiMe2OH groups using a Pd/C catalyst (D4R-7SiOH-C18). Upon solvent evaporation, the molecule was assembled to form a crystalline solid with a regular arrangement of the cages induced by the intermolecular H-bonding of the silanol groups and a mesoscale periodicity induced by the long alkyl chains. Furthermore, cross-linking of the silanol groups by siloxane bonds was performed by silylation with dichlorodimethylsilane.

Scheme 1
scheme 1

Synthetic route to cage siloxane modified with a long alkyl chain and organosilanol groups (D4R-7SiOH-C18) starting from cubic octameric silicate

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2 Experimental

2.1 Materials

The following reagents were used as received: acetonitrile (FUJIFILM Wako Pure Chemical Co., >99.5%), chlorodimethylsilane (DMSCl, Sigma-Aldrich, >97.5%), chloroform (FUJIFILM Wako Pure Chemical Co., >99.0%), chloro(octadecyl)dimethylsilane (C18DMSCl, Tokyo Chemical Industry Co. Ltd., >97.0%), dichlorodimethylsilane (Tokyo Chemical Industry Co. Ltd., >98.0%), diethyl ether (FUJIFILM Wako Pure Chemical Co., >99.5%), ethanol (Junsei Chemical Co. Ltd., 99.5%), hexane (Kanto Chemical Co. Inc., >99.0%), methanol (FUJIFILM Wako Pure Chemical Co., >99.8%), magnesium sulfate (anhydrous, FUJIFILM Wako Pure Chemical Co., >98.0%), pyridine (dehydrated, FUJIFILM Wako Pure Chemical Co., >99.5%), tetraethylorthosilicate (TEOS, FUJIFILM Wako Pure Chemical Co., >95.0%), tetrahydrofuran (THF, stabilizer free, Wako Pure Chemical Industries Ltd., >99.5%), tetramethylammonium hydroxide pentahydrate (TMA hydroxide·5H2O, Tokyo Chemical Industry Co. Ltd., >97.0%), and Pd/C (type STD (Pd 5%) (wetted with water), Manufacturer: N.E. CHEMCAT, FUJIFILM Wako Pure Chemical Co.).

2.2 Synthesis of cubic octameric silicate

Tetramethylammonium silicate with a D4R structure (TMA8Si8O20·xH2O) was synthesized according to our previous report [25]. TEOS (61.8 mL) was added to the mixture of TMA hydroxide·5H2O (50 g), ethanol (160.6 mL), and H2O (24.8 mL). The molar ratio was TEOS: TMA hydroxide: ethanol: H2O = 1: 1: 10: 10. After stirring for 3 d, the solution was concentrated to about half of the original volume by heating on a hot plate. The hydrated crystals precipitated upon cooling were filtrated and vacuum dried at room temperature to obtain a white solid (54.1 g). 29Si nuclear magnetic resonance (NMR) (99.37 MHz, ethanol-d6): δ (ppm) −99.00 (Si(OSi)3O−). Thermogravimetry (TG)-differential thermal analysis (DTA) revealed that the weight loss was 72% when the final white solid was heated up to 500 °C. Assuming that the residue was SiO2, the hydration number (x) was calculated to be 32.

2.3 Synthesis of D4R-7SiH

It is reported that incompletely trimethylsilylated and dimethylvinylsilylated cage siloxanes can be obtained by silylation of Si8O208− using a THF solvent [26,27,28]. On the basis of these reports, incomplete dimethylsilylation of cage siloxane was conducted. TMA8Si8O20·32H2O (10.6 g) dissolved in methanol (40 mL) was added dropwise to the mixture of THF (100 mL), hexane (100 mL), and DMSCl (12 mL). After stirring for 10 min, H2O (20 mL) and hexane (200 mL) were added to the reaction mixture. After further stirring for 10 min, the hexane layer was extracted, and the volatile compounds were removed under reduced pressure to obtain a white solid. The solid was washed with acetonitrile (20 mL) by stirring for 10 min to remove cage siloxane containing six or less silylated vertices. Then, a white solid was recovered by filtration (3.1 g, yield: 51% based on the fact that 7.5 corners were silylated on average, as calculated by the 29Si NMR analysis). Although impurities such as Si8O12(OSiMe2H)6(OH)2 and Si8O12(OSiMe2H)8 were still contained, the product was used for the subsequent reaction without separation of D4R-7SiH.

2.4 Synthesis of D4R-7SiH-C18

D4R-7SiH containing impurities (1.5 g) was dissolved in hexane (40 mL). The solution was added dropwise to the mixture of C18DMSCl (0.54 g), pyridine (190 μL), and hexane (5 mL) in a Schlenk flask under nitrogen atmosphere. After stirring at room temperature for 1 d, DMSCl (3 mL) and pyridine (3 mL) were added. After further stirring at room temperature for 3 h, H2O (5.0 mL) was added to the reaction mixture and the hexane layer was extracted. Volatile compounds were removed under reduced pressure to obtain a white solid. The solid was dissolved in chloroform and D4R-7SiH-C18 (a white solid, 0.16 g, yield: 8% based on D4R-7SiH) was isolated by gel permeation chromatography (GPC). 1H NMR (500.16 MHz, chloroform-d): δ (ppm) 0.13 (s, 6H; OSi(CH3)2C18H37), 0.22–0.29 (br, 42H; OSi(CH3)2H), 0.56–0.62 (m, 2H; SiCH2(CH2)16CH3), 0.88 (t, J = 6.87 Hz, 3H; Si(CH2)17CH3), 1.20–1.40 (br, 32H; SiCH2(CH2)16CH3), 4.70–4.76 (m, 7H, SiMe2H); 13C NMR (125.77 MHz, THF-d8): δ (ppm) 0.39, 0.45, 0.48, 14.51, 18.07, 23.11, 23.35, 29.79, 29.82, 30.09, 30.13, 32.36, 33.86; 29Si NMR (99.37 MHz, chloroform-d): δ (ppm) 13.41, −1.47, −1.59, −108.57, −108.61, −108.66, −109.06.

2.5 Synthesis and self-assembly of D4R-7SiOH-C18

D4R-7SiH-C18 (0.30 g) was dissolved in THF (10 mL) under nitrogen atmosphere. Pd/C (20 mg) and H2O (50 μL) were added to the solution. After stirring for 1 d at 40 °C, anhydrous magnesium sulfate was added and filtered through Celite®. The volatile compound was removed using a rotary evaporator to give a white solid (0.32 g, yield: 98%). 1H NMR (500.16 MHz, THF-d8): δ (ppm) 0.10 (s, 42H; OSi(CH3)2OH), 0.14 (s, 6H; OSi(CH3)2C18H37), 0.60–0.65 (m, 2H; SiCH2(CH2)16CH3), 0.88 (t, J = 6.87 Hz, 3H; Si(CH2)17CH3), 1.22–1.38 (br, 32H; SiCH2(CH2)16CH3), 5.5–5.6 (br, 3H, SiMe2OH), 5.6–5.7 (br, 4H, SiMe2OH); 13C NMR (125.77 MHz, THF-d8): δ (ppm) 0.21, 0.27, 0.35, 14.61, 18.64, 23.69, 24.05, 30.44, 30.49, 30.74, 30.79, 30.85, 33.00, 34.61; 29Si NMR (99.37 MHz, THF-d8): δ (ppm) 12.77, −11.28, −11.37, −11.66, −108.67, −109.42, −109.45, −109.59.

D4R-7SiOH-C18 (0.13 g) was dissolved in diethyl ether (2 mL) and the solution was added over 5 mL of H2O in a 15 mL sample bottle. During evaporation of diethyl ether at room temperature for 3 d, crystalline solids were formed on the top of the H2O layer. The solids were collected by filtration.

2.6 Cross-linking of D4R-7SiOH-C18

A mixture of hexane (10 mL) and dichlorodimethylsilane (18 μL) was added to the crystal of D4R-7SiOH-C18 (50 mg) under nitrogen atmosphere. After standing at room temperature for 1 d, solids were collected by filtration (24 mg).

2.7 Characterization

Liquid-state 1H, 13C, and 29Si NMR spectra were recorded on a JEOL JNM ECZ 500 spectrometer at resonance frequencies of 500.0 MHz, 125.7 MHz, and 99.4 MHz, respectively, at ambient temperature using 5 mm glass tubes. Tetramethylsilane was used as an internal reference (set as 0 ppm). Chloroform-d, ethanol-d6, and THF-d8 were used to obtain lock signals. 29Si NMR spectra were measured with a 45° pulse and a recycle delay of 30 s. Solid-state 13C cross-polarization (CP)/magic-angle spinning (MAS) NMR and 29Si MAS NMR spectra were recorded on a JEOL JNM ECA 400 spectrometer at resonance frequencies of 100.53 MHz and 79.43 MHz, respectively, at ambient temperature. The sample was put in a 5 mm zirconia rotor. Fourier transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR-6100 spectrometer using the KBr method. X-ray diffraction (XRD) patterns were recorded on a Rigaku Ultima-III diffractometer with a parallel beam geometry equipped with a scintillation detector and parabolic multilayer mirror using Cu Kα radiation (λ = 1.5418 Å). GPC was carried out using a Japan Analytical Industry LC-7080 system equipped with a refractive index detector RI-700LA. JAIGEL-HR series columns (1HR, 2HR, and 2HR plus) were used with chloroform as the eluent at a flow rate of 7.0 mL min–1. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass analysis was conducted using a Bruker autoflex maX instrument with dithranol as the matrix and sodium trifluoroacetate as the ionization aids. Scanning electron microscopy (SEM) images were obtained on a Hitachi S5500 electron microscope with an accelerating voltage of 1–5 kV. Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2010 microscope with an accelerating voltage of 200 kV. Samples for TEM and SEM analyses were dispersed on a carbon-coated Cu grid (Okenshoji Co., Ltd.). Cross-sectional sample was prepared using a focused ion beam (FIB) milling and imaging system (JIB-4700) equipped with an FIB and a scanning ion microscope. D4R-7SiOH-C18 powder deposited on the sample fixing table was cut out using Ga ion beams, and the flake was transferred to a carbon-coated Cu grid. Cross-sectional high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image was obtained on a JEOL JEM-2100F microscope at an accelerating voltage of 200 kV. TG-DTA was conducted using a RIGAKU Thermo Plus EVO2 TG8121 under dry air at a heating rate of 10 K min–1.

3 Results and discussion

3.1 Synthesis of cage siloxanes modified with seven SiMe2OH and one SiMe2C18H37 groups (D4R-7SiOH-C18)

The first step for the synthesis of D4R-7SiOH-C18 was dimethylsilylation of Si8O208− to form cage siloxane modified with SiMe2H groups at 7 corners (D4R-7SiH). The progress of dimethylsilylation was confirmed by the 1H NMR spectrum of the silylated product (Fig. S1) showing the signals of the OSiMe2H groups (0.25 and 4.73 ppm) [16]. The 29Si NMR spectrum (Fig. 1A(a)) showed multiple signals at the M1 (SiOSiMe2H: −0.9 to −1.5 ppm), Q3 (Si(OSi)3OH: −100.4 to −100.6 ppm), and Q4 (Si(OSi)4: −108.4 to −108.9 ppm) regions. The integral ratio of the M1, Q3, and Q4 signals was 7.5: 0.5: 7.5, indicating that the average number of dimethylsilylated corners was 7.5 per Si8O20 and the products were mixtures of cage siloxanes with different numbers of silylated corners. Note that mono-/hepta-substituted cage siloxanes consist of four chemically inequivalent Si atoms [29, 30]. In the case of D4R-7SiH, the Q3 signal of the unsilylated SiOH group and the Q4 signals of the silylated corner Si atoms first, second, and third closest to the SiOH group should be observed with the 1: 3: 3: 1 ratio. The largest Q4 and M1 signals were probably derived from the symmetrical cage siloxane modified with eight SiMe2H groups, and the smaller signals were assumed to be derived from the cage siloxanes modified with six and seven SiMe2H groups. The MALDI-TOF mass spectrum (Fig. 1B(a)) showed the peaks corresponding to [Si8O12(OH)2(OSiMe2H)6 + Na]+, [Si8O12OH(OSiMe2H)7 + Na]+, and [Si8O12(OSiMe2H)8 + Na]+. These results indicated that the silylated product contained D4R-7SiH along with other cage siloxanes modified with 6 and 8 SiMe2H groups.

Fig. 1
figure 1

A Liquid-state 29Si NMR spectra and (B) MALDI-TOF mass spectra (the insets show the enlarged spectra) of (a) dimethylsilylated product containing D4R-7SiH (in chloroform-d), (b) D4R-7SiH-C18 (in chloroform-d), and (c) D4R-7SiOH-C18 (in THF-d8). The 29Si NMR signals of tetramethylsilane (δ = 0 ppm) are indicated with asterisks (*). The molecular structures of the products are shown in the insets of (Fig. 1A). One of the regioisomers is shown for [Si8O12(OH)2(OSiMe2H)6] (inset of Fig. 1A(a))

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The second step was the silylation of the residual silanol group of D4R-7SiH with C18DMSCl to form a cage siloxane modified with seven SiMe2H groups and one SiMe2C18H37 group (D4R-7SiH-C18). The crude product was separated by GPC (Fig. S2) to obtain D4R-7SiH-C18. The 1H NMR spectrum (Fig. S3) showed the signals for OSi(CH3)2C18H37 (a: 0.13 ppm), OSi(CH3)2H (b: 0.22–0.29 ppm), SiC18H37 (c: 0.56–0.62 ppm; d: 0.88 ppm; e: 1.20–1.40 ppm), and Si(CH3)2H (f: 4.70–4.76 ppm). The integral ratio of (a + b): (c + d + e): f was 48: 37: 7, which was consistent with the theoretical ratio for D4R-7SiH-C18 (48: 37: 7). The 29Si NMR spectrum (Fig. 1A(b)) showed the signals assigned to the M1 units of the silyl groups (OSiMe2C18H37: 13.41 ppm; OSiMe2H: −1.47 and −1.59 ppm) and the Q4 units of the cage framework (Si(OSi)4: −108.57, −108.61, −108.66, −109.06 ppm). The disappearance of the Q3 signals indicated that silylation was complete. The multiple signals observed for OSiMe2H and Si(OSi)4 can be ascribed to the slight differences in the chemical environments depending on the distance from the SiMe2C18H37 group. One of the M1 (SiOSiMe2H) signals is likely overlapping with another, resulting in the appearance of only two main signals. The integral ratio of the OSiMe2C18H37, OSiMe2H, and Si(OSi)4 signals was 1.0: 7.0: 8.0, indicating that the seven corners were modified with SiMe2H groups and one corner was modified with a SiMe2C18H37 group. The MALDI-TOF mass spectrum (Fig. 1B(b)) showed the peaks corresponding to [Si8O12(OSiMe2H)7(OSiMe2C18H37) + Na]+. These results indicated that D4R-7SiH-C18 was successfully obtained.

The final step was oxidation of the SiH groups of D4R-7SiH-C18 to form D4R-7SiOH-C18. The 1H NMR spectrum of D4R-7SiOH-C18 (Fig. S4) showed signals for OSi(CH3)2OH (a: 0.10 ppm), OSi(CH3)2C18H37 (b: 0.14 ppm), SiC18H37 (c: 0.60–0.65 ppm; d: 0.88 ppm; e: 1.22–1.38 ppm), and Si(CH3)2OH (f: 0.55–0.56 ppm; g: 0.56–0.57 ppm). The integral ratio of (a + b): (c + d + e): (f + g) was 48: 37: 7, which was consistent with the theoretical ratio for D4R-7SiOH-C18. The 29Si NMR spectrum (Fig. 1A(c)) showed a single M1 signal (OSiMe2C18H37: 12.77 ppm), three D1 signals (OSiMe2OH: −11.28, −11.37, −11.66 ppm), and four Q4 signals (Si(OSi)4: −108.67, −109.42, −109.45 (shoulder), −109.59 ppm). The numbers and the intensity ratios of the D1 and Q4 signals are consistent with those in D4R-7SiOH-C18. Although detailed signal assignments are difficult, the four Q4 signals can be attributed to the cage corner Si atoms first, second, third, and fourth closest to the SiMe2C18H37 group (1: 3: 3: 1 ratio). Similarly, the three D1 signals can be attributed to the Si(CH3)2OH groups first, second, and third closest to the SiMe2C18H37 group (3: 3: 1 ratio). The MALDI-TOF mass spectrum (Fig. 1B(c)) shows the peak corresponding to [Si8O12(OSiMe2OH)7(OSiMe2C18H37) + Na]+. The isotope pattern was consistent with that obtained by simulation. These results indicated that the cage siloxane was successfully modified with seven SiMe2OH groups and one SiMe2C18H37 group.

3.2 Self-assembly of D4R-7SiOH-C18

Self-assembly of D4R-7SiOH-C18 easily occurred upon solvent evaporation. The 29Si MAS NMR spectrum of the product (Fig. 2(a)) showed the M1, D1, and Q4 signals corresponding to D4R-7SiOH-C18. No signal was observed at the D2 region (around −20 ppm), indicating that the silanol groups remained intact without condensation to form siloxane bonds. This solid product was therefore resoluble in THF. The 13C CP/MAS NMR spectrum (Fig. 2(b)) shows the signals for SiCH3 (0.5 and 1.0 ppm) and SiC18H37 (15.2, 20.1, 24.8, 30.7, 33.7, and 36.9 ppm) [31, 32]. The interior methylene carbons of the C18H37 groups are observed at 30.4 ppm and 33.8 ppm, which correspond to mixed trans–gauche and all-trans conformations, respectively [33,34,35]. Because the latter signal (33.8 ppm) is mainly observed, it is likely that the alkyl chains are in an ordered all-trans state with some gauche defects. The FT-IR spectrum (Fig. 2(c)) showed bands due to alkyl chains, SiMe groups, SiOH groups, and SiOSi networks. The stretching vibration of the SiO–H groups was observed as a broad band centered at 3295 cm−1, which was attributed to the H-bonded silanol groups [16,17,18,19, 36].

Fig. 2
figure 2

a 29Si MAS NMR spectrum, (b) 13C CP/MAS NMR spectrum, and (c) FT-IR spectrum of D4R-7SiOH-C18 assembled by solvent evaporation

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The powder XRD pattern of self-assembled D4R-7SiOH-C18 (Fig. 3a) exhibited the most intense peak at d = 3.1 nm along with second- and third-order peaks. These peaks were assignable to a lamellar structure and differed from other mesophases such as 2D hexagonal and cubic phases [37]. In fact, when calcined in air at 250 °C for 1 d to remove alkyl chains, most of the diffraction peaks disappeared (Fig. S5), as in the previously reported lamellar alkylsiloxanes [9]. The d value of the first-order peak (3.1 nm) was slightly smaller than the molecular length of D4R-7SiOH-C18 when the alkyl group is assumed to be all-trans (~3.6 nm). Taking into account the relatively large occupying volume of the cage siloxane units, it is reasonable to consider that an interdigitated monolayer-type structure rather than a bilayer-type structure was formed. The other peaks observed at d = 1.1 nm and higher angles can be attributed to the arrangements of H-bonded cage siloxanes and closely packed long alkyl chains. Unfortunately, further information about the molecular arrangement has not been obtained because of the ill-defined peaks and difficulty in the preparation of large crystals for single-crystal X-ray analysis.

Fig. 3
figure 3

a XRD pattern, (b) SEM image, (c–e) TEM images, and (f) SAED pattern (the spots are marked with light blue circles) of self-assembled D4R-7SiOH-C18

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The SEM observation revealed that most of the particles have plate-like morphology (Fig. 3b). A stripe pattern characteristic of a lamellar structure was observed near the edge of the particles (Fig. 3c, d). The HAADF-STEM observation of the cross-section of the particle cut vertically by FIB processing also showed a stripe pattern oriented parallel to the particle surface (Fig. S6). The SAED pattern of another view (Fig. 3e, f) showed spots with a d value of about 0.95 nm. These spots are likely associated with the H-bonding arrangement of the cage molecules because similar square arrays of spots were reported for the H-bonded molecular crystals of D4R siloxanes modified with SiMe2OH groups at all corners [16, 17]. All the above results suggested that the cage siloxanes were arranged to form a molecularly and mesoscopically ordered structure.

3.3 Cross-linking of D4R-7SiOH-C18 by silylation

The self-assembled solids of D4R-7SiOH-C18 were treated with dichlorodimethylsilane to bridge the adjacent silanol groups by siloxane bonds. The solid-state 29Si MAS NMR spectrum of the product (Fig. 4a) showed the M1 (OSiMe2C18H37: 13.2 ppm), D1 (OSiMe2OH: −10.3 ppm), D2 (SiMe2(OSi)2: –17.5 ppm), and Q4 (Si(OSi)4: −108.9 ppm) signals. The sharp Q4 signal accompanying no Q3 signal suggested that the silylated cage siloxane units were almost retained without cleavage of the Si–O–Si bonds. The integral ratio of M1, D1, D2, and Q4 signals was calculated to be 1.0: 1.7: 7.7: 8.0. Assuming that one dichlrodimethylsilane reacts with two silanol groups, the theoretical ratio of M1, D2, and Q4 units is calculated to be 1.0: 10.5: 8.0. The difference between the theoretical and measured ratios indicated that ~75% of the silanol groups underwent cross-linking. FT-IR analysis also confirmed that the band assignable to Si–O–H deformation (887 cm−1) significantly decreased (Fig. 4b). After silylation, the XRD peaks observed for D4R-7SiOH-C18 became weak and/or broad, and no new peaks appeared (Fig. S7). Sheet-like morphology was still observed by SEM (Fig. 4c) and stripe patterns were observed by TEM (Fig. 4d). These results suggested that the lamellar structure was at least partially retained after the cross-linking. We expect that the structural regularity can be improved by optimizing the reaction conditions and/or by the choice of the silylating agent used.

Fig. 4
figure 4

a 29Si MAS NMR spectrum, (b) FT-IR spectrum (For comparison, the FT-IR spectrum of D4R-7SiOH-C18 before the treatment with dichlorodimethylsilane is shown as a dashed line in the enlarged figure.), (c) SEM image, and (d) TEM image of D4R-7SiOH-C18 treated by dichlorodimethylsilane

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The synthetic method of crystalline 2D silica-based materials presented in this study will be useful for the preparation of functional coatings and membranes. The structure of the siloxane frameworks is the critical factor for these applications. Until now, the top-down synthesis of crystalline silica nanosheets by exfoliation of 2D zeolites [38] and layered silicates [39], as well as their applications for barrier coatings and gas separation membranes, have been reported. However, the types of structures that can be obtained by exfoliation of those hydrothermally synthesized layered compounds are limited. The surfactant-directed self-assembly process allows for the preparation of lamellar silica- and organosiloxane-based materials [40,41,42], and a recent report has shown that colloidal silica nanosheets can be obtained by exfoliation [43]. However, these materials have amorphous siloxane networks unless post-synthetic hydrothermal treatment is employed [44]. The bottom-up self-assembly method using well-defined cage siloxanes has the potential to overcome these limitations, although further research is needed to retain the crystal structure upon cross-linking. The direct formation of single-layer nanosheets may also be possible by the Langmuir–Blodgett technique.

4 Conclusions

We have demonstrated the hierarchical assembly of cage siloxane building blocks by hydrophobic interactions and hydrogen bondings. A long-chain alkyl group and organosilanol groups were successfully introduced to cage siloxane with a D4R structure by stepwise silylation using chlorodimethylsilane and chloro(octadecyl)dimethylsilane followed by conversion of the SiH groups into SiOH groups. Self-assembly of the resulting compound, D4R-7SiOH-C18, occurred upon solvent evaporation. Furthermore, cross-linking of D4R-7SiOH-C18 using dichlorodimethylsilane proceeded while retaining the sheet-like morphology. This approach will allow for the creation of various hierarchically ordered siloxane-based materials by the molecular design of the building blocks.