Chiral mesoporous silica: Chiral construction and

TUTORIAL REVIEW

www.rsc.org/csr | Chemical Society Reviews

Chiral mesoporous silica: Chiral construction and imprinting via cooperative self-assembly of amphiphiles and silica precursors Huibin Qiu and Shunai Che* Received 5th June 2010 DOI: 10.1039/c0cs00002g Fabrication of chiral materials and revealing the mechanisms involved in their formation are crucial issues in scientific research. The combination of cooperative self-assembly routes and the chiral templating process favors the formation of inorganic chiral materials with highly ordered mesostructures. This tutorial review highlights the recent research on chiral mesoporous silica (CMS) of hierarchical helical constructions transcribed from organic templates. The rules and mechanisms involved in the synthesis of CMS and related materials, especially the novel expression of chirality and imprinting of helical micellar superstructure by the functional groups immobilized on the mesopore surface, provide us with a deeper insight into the chiral self-assemby process and new strategies for the design and application of chiral materials. This review is addressed to researchers and students interested in chiral chemistry, supramolecular chemistry and mesoporous materials (53 references).

1. Introduction Chirality is found universally in nature and performs as an inherent feature of the molecular and macromolecular components in organisms. Fabrication of chiral chemicals, chiral materials, and understanding the rules that govern their formation, are major topics in scientific research and contribute greatly to the fields of pharmacy, biochemistry, optical devices, etc. Generally, chiral chemistry has been considered to include three main courses: (i) creation, breaking the symmetry in achiral or racemic systems to raise chirality; (ii) transcription, transcribing the chirality of one School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Key Laboratory for Thin Film and Microfabrication Technology of the Ministry of Education, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai, 200240, P. R. China. E-mail: [email protected]

Huibin Qiu was born in 1981 and received his BS from Shanghai Jiao Tong University in 2004. He is now a doctoral candidate in Applied Chemistry under the supervision of Professor Che. His current research interests include studies of chiral mesoporous silica and novel mesostructured functional materials.

Huibin Qiu

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object to others to propagate chirality; and (iii) organization, organizing the asymmetric interactions between different units to recognize or amplify chirality. Recently, the combination of these processes has facilitated a fast development in supramolecular chiral construction. Variation of the organic building blocks and their connectivity has allowed chiral selfassemblies of various size, helical shape, chemical composition, and function to be obtained. Compared with organic assemblies, chirality is harder to impose on inorganic materials. A facile approach to solve this problem is to involve organic chiral elements in inorganic systems, through their use as ligands, intermediates, or templates. The templating route, which is prevalent in biomineralization, has recently been developed by Shinkai’s group and other groups to produce inorganic replicas of diverse chiral organogels, leading to the formation of nanotubes, ribbons, fibers, and other shaped inorganic

Shunai Che is a professor in the Department of Chemistry, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University. She was born in 1964 and received her PhD degree from the Yokohama National University. She was a guest researcher at Saitama University and worked as a postdoctoral fellow at the Yokohama National University. She moved to her current position in January, 2004. Her research interests encompass Shunai Che the development of mesoporous materials with novel structures and functions in view of applications in chiral imprinting, heterogeneous catalysis, controlled drug delivery, and nanofabrication. Chem. Soc. Rev., 2011, 40, 1259–1268

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materials with multiform chiral structures and tunable compositions.1–10 The unique chiral superstructures of the organic assemblies formed with noncovalent interactions can be permanently fixed in the inorganic materials through a sol–gel polycondensation. These inorganic chiral materials are highly capable of being used as heterogeneous catalysts for the asymmetric synthesis.4,11 It is well known that mesoporous silicas with various ordered mesostructures can be generated by the cooperative self-assembly of amphiphiles and silicates.12–15 Recently, the incorporation of this technology into the chiral templating process has favored the formation of inorganic chiral materials with highly ordered mesostructures. Among them, chiral mesoporous silica (CMS), which exhibits a novel helical mesostructure with hierarchical chirality transcribed from the organic templates, represents a new fashion for the design and application of chiral materials. This review describes the recent research advances on the synthesis and formation mechanism, synthetic controls, and supramolecular chiral imprinting of CMS. The expression of chirality on the supramolecular level, in a series of helices, will be focused on, aiming at a comprehensive understanding of the chiral self-assembling process and new hints for the investigation of chirality.

2. Synthesis and formation mechanism CMSs have been synthesized by cooperative self-assembly of chiral or achiral amphiphiles and the silica precursors, based on the electrostatic interactions between the head groups of amphiphiles and inorganic reagents (Fig. 1).16–27 For the cationic amphiphiles (13–15), the positively charged hydrophilic head groups may directly interact with the negatively charged silicates in alkaline solutions (Fig. 1a) or indirectly interact

with the positively charged silicates, mediated by the counter ions, in strong acidic conditions (Fig. 1b). For the anionic amphiphiles (1–12), co-structure-directing agents (CSDAs),28 such as 3-aminopropyltrimethoxysilane (APS) and Ntrimethoxysilylpropyl-N,N,N-trimethylammonium chloride (TMAPS), were applied to induce favorable electrostatic interactions; the positively charged ammonium ion of APS or TMAPS interacts with the negatively charged head group of anionic amphiphile through neutralization or double decomposition reactions, respectively (Fig. 1c and d). Meanwhile, the alkoxysilane sites of APS or TMAPS are polymerized with tetraethoxysilane (TEOS) to form the silica framework. Such cooperative interactions between the amphiphiles and the silica precursors drive the organization of silicates around the micellar superstructures of amphiphiles. Polymerization of the silicates simultaneously promotes the aggregation of amphiphiles, leading to an ordered liquid crystal-like mesostructure.14 Through the choice of appropriate amphiphiles, silicates and synthesis conditions, the formation of mesoporous silica crystals with highly ordered helical mesostructure have been achieved.16–27 Fig. 2 shows the morphology and mesostructure of a typical CMS, which was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM).16 This material was composed of particles uniform in shape: the particles have a well-defined twisted rodlike morphology with a hexagonal cross section (Fig. 2a). The handedness of each crystal can be easily distinguished by the six distinct surfaces. The XRD pattern (Fig. 2b) revealed three wellresolved peaks in the 2y range of 1.5–61, which can be indexed as 10, 11 and 20 reflections based on the two-dimensional (2D) hexagonal p6mm unit cell. TEM images (Fig. 2c and d) and a simulated image (Fig. 2e) confirmed that these particles contain hexagonally ordered helical channels twisted from 2D hexagonal

Fig. 1 Molecular structures of the amphiphiles used in the synthesis of CMS and the electrostatic interactions between the amphiphiles and the polymerized silica precursors.

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Fig. 2 (a) SEM image, (b) XRD pattern, and (c) TEM image of a typical CMS synthesized by using C14-L-Ala as template and TMAPS as CSDA at room temperature. Two types of fringes with different spacings were indicated by arrow and arrowhead, respectively. (d) Enlarged image of the selected area in (c). (e) A simulated TEM image in good correspondence with the observed image. Reproduced from ref. 16. Copyright 2004 Nature Publishing Group.

p6mm. Between two sets of (10) fringes (indicated by the arrows), the rod is twisted by 601, which means that the distance is one-sixth of the pitch length.

Fig. 3 Schematic Drawings: (a) a CMS Rod; (b) cross section of a CMS rod; (c) chiral channels within a CMS rod. Reproduced form ref. 17. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA.

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Fig. 3 shows a simple structural model of one pitch length of a CMS crystal.17 Compared with the straight channels in ordinary 2D mesoporous materials (such as MCM-41), the channels of CMS run in spirals along the rod. The curvature of each channel increases with increasing the distance from the rod center to the channel center and decreases with increasing the pitch length P, indicating a unique helical construction. The ordered arrangement of the amphiphiles in a helical form was thought to be crucial for the emergence of the helical mesostructure of CMS that formed with chiral amphiphiles. It can be simply speculated that, owing to the chiral configuration, the chiral amphiphiles in the rod-like micelles prefer a helical packing in the formation of CMS (Fig. 4).18 Aggregation of the rod-like micelles of single handedness forms longer assemblies, which adapt into helical forms because of the twisting power that originates from the helical packing and the handedness would be dominated by the handedness of helical micelles. The closest packing of these helices with the soft oligomeric frameworks derived from the silicates, in a 2D hexagonal arrangement, finally promotes the formation of a twisted CMS rod. However, the observation of the racemic CMSs formed with achiral amphiphiles18–27 indicated that the molecular chirality is not the only factor that initiates the helical construction but other driving forces also exist. So far, the exact mechanism for the origin of the helical mesostructure from achiral amphiphiles is still debatable. Lin et al. suggested that the tight intermolecular packing between the amphiphilic heads with planar groups, induced by the long alkyl chains, may induce a staggered wadding of achiral amphiphiles and hence force the micelles to twist into a helical structure.20 Some chiral intermediate species and chiral nucleis formed in the synthesis process were also presumed to account for the formation of the helical mesostructure.21 As shown in Fig. 5, Yu et al. proposed that the helical mesostructured materials were generated by a morphological transformation due to a reduction in surface free energy.23 On the other hand, Ying et al. claimed that the

Fig. 4 Molecular origin of helical mesostructure of CMS derived from the helical packing of chiral amphiphiles. Reprinted from ref. 18. Copyright 2008 American Chemical Society.

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3. Synthetic controls 3.1 Framework design

Fig. 5 Formation of helical mesostructured rods from hexagonally arrayed straight rod-like micelles with equal length to a helical rod with two rounded ends. Reproduced from ref. 23. Copyright 2006 American Chemical Society.

formation of the helical mesostructures could be explained by a entropically driven model.26 As illustrated in Fig. 1, the achiral N-acyl-amino acids (7–9) show very similar molecular structures to the chiral ones (1–6). Consequently, it is reasonable that the achiral amphiphiles may share some common mechanisms with the chiral amphiphiles to form the helical mesostructure of CMS. Recently, it is considered that the helical mesostructure may be induced by the instantaneous asymmetric shapes of achiral amphiphiles that survived in the micelles, which drives the achiral amphiphiles to adapt a helical micellar packing similar to that formed with the chiral amphiphilies.27 As illustrated in Fig. 6, take C16-PyrBr for example, different molecular conformations could be produced by rotating the C–C and C–N bonds during the micelle formation. It can be seen from the top views that these conformations reveal different asymmetric natures in x and/or y direction(s), along which neighboring amphiphiles pack together to form the rod-like micelles. Such instantaneous asymmetric shapes, similar to the intrinsic ones in chiral amphiphiles, would essentially induce the asymmetric interactions between neighboring achiral amphiphiles and drive the helical micellar packing, which finally leads to the formation of the helical mesostructure. This is quite similar to the case of banana-shaped compounds generated liquid crystals, in which helical structures are formed with achiral molecules due to the bent molecular shapes.29–31

Fig. 6 Illustration of the instantaneous asymmetric shapes of achiral amphiphiles. Reprinted from ref. 27. Copyright 2008 American Chemical Society.

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The functionalization of the mesostructured silica frameworks has attracted great attention from the standpoint of materials scientists. By varying the silica precursors, a variety of organic functional groups have been successfully modified on the mesopore surface or embedded in the framework as bridges. Surface functionalization of the CMS channels could be facilely accomplished through the use of CSDAs, which provides a homogenous distribution of the functional groups and simultaneously controls their arrangement (vide post). The co-condensation method is also applicable to modify the CMS mesopore surface with thiol, vinyl, phenyl and other organic groups.32 For the functional modification inside the framework, chiral periodic mesoporous organosilicas (PMOs) bridged with alkyl and aromatic groups have been synthesized by using cetyltrimethylammonium bromide (CTAB) as template and 1,2-bis(triethoxysilyl)ethane (BTEE) or 1,4-bis(triethoxysilyl)benzene (BTEB) as precursor.33 Wideangle XRD pattern and TEM image confirmed the existence of a crystal-like wall in the hybrid derived from BTEB. Recently, bi-functional chiral PMOs were also obtained by using chiral or achiral anionic amphiphiles as templates through the CSDA method (unpublished results). The obtained materials exhibited a bi-functional feature with quaternary ammonium groups on the mesopore surface and alkyl or aromatic groups inside the framework. Moreover, the chiral PMO formed with C16-L-Ala and BTEB revealed novel induced circular dichroism (ICD) signals in the adsorption region of 200–300 nm, implying a helical stacking of the benzene rings.34 3.2 Control of enantiopurity The enantiopurity, i.e. enantiomeric excess (ee), of the helical structures was seldom discussed for supramolecular chiral assemblies or inorganic chiral materials, since these entities are usually either of one handedness or racemic. Nevertheless, tunable enantiopurity (ee = 0 to 4 90%) has been revealed in the CMSs formed with a series of chiral anionic amphiphiles.18,35As plotted in Fig. 7, the ee of the CMSs formed with different chiral N-acyl-amino acids follows a linear relationship upon temperature, i.e., the absolute ee values decrease with increasing temperature.18 The maxima of the absolute ee values and the degree of temperature dependence depend on the substituent attached to the chiral center of amphiphiles and decrease in a sequence of C16-L-Phe B C16-L-Met 4 C16-L-Ile 4 C16-L-Val 4 C16-L-Ala. CMSs of over 90% ee (left-handed excess) were obtained by using C16-L-Phe and C16-L-Met as templates at lower temperatures. The remarkable temperature dependence of ee was interpreted thermodynamically by the equilibrium shift between two antipodal helical aggregates triggered by the temperature driven conformational changes of amphiphiles.18,36,37 By rotation of the Ca–N single bond of the chiral N-acyl-amino acid, a new conformer, which is diastereomeric to the original one, is generated with greatly altered topology around the head group. Usually, these rotational isomers (rotamers), which are diastereomeric to each other relative to the chiral center, are This journal is

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left- and right-handed CMS crystals are absolutely separated and each one is homogeneous in handedness, indicating that homochiral D–D and L–L interactions may be dominated in the micelles.40 3.3 Control of helicity

Fig. 7 Temperature dependence of ee for the CMSs formed with different chiral N-acyl-amino acids. Reprinted from ref. 18. Copyright 2008 American Chemical Society.

quickly equilibrated mutually at ambient temperatures, but in the stacked micellar structure the rate of equilibriation is decelerated and thus the two diastereomeric rotamers can survive to form the antipodal helical structures independently. For L-type N-acyl-amino acids, the conformer of lowest energy may possess the chiral sense upon packing, leading to lefthanded CMS, while the less stable conformer of smaller proportion gives right-handed CMS. Obviously, the proportion of the conformer with higher energy is increased at higher temperature, and hence the ee of CMS decreases. As mentioned above, both the absolute ee maxima and temperature dependence of ee are critically affected by the substituent attached to the chiral center. It is likely that the conformational freedoms of chiral amphiphiles in aggregate are determined by the intra- and/or intermolecular steric hindrance of the substituent attached to the chiral center. Thus, the introduction of a bulky substituent on the chiral center should reduce the conformational freedom through destabilization of less-favored diastereomeric conformations, leading to the formation of CMS with higher ee. At the same time, this destabilization makes the population of the less favored conformation more sensitive to the reaction temperature. For achiral amphiphiles, as shown in Fig. 6, mirror-image conformations (I and II), possibly with minimum energy in the micelles, can be generated in equal proportion by oppositely rotating the C–N bond to the same angle (take C16-PyrBr for example). These two conformations may have opposite handed helical packing senses and thus form CMS rods of opposite handedness in equal amounts. Recently, it was found that such an equilibrium can be effectively shifted by the addition of chiral dopants, which helps the formation of enantiomeric excessive CMSs.27,38,39 In the case of racemic amphiphiles, racemic CMSs were obtained as expected. However, it should be noted that the This journal is

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Helicity is a key parameter of the helical structures, such as double-stranded DNAs, a-helix of proteins, and single-walled carbon nanotubes. Controlling the helicity of CMS is of great significance for both formation mechanism investigation and material design. It was found for most CMSs, the pitch length of helical mesostructure, which is six times the distance between two sets of (10) fringes as estimated from the TEM images (see Section 2), increased linearly with increasing the diameter of rod-like crystal.19,23,27,39,41,42 As shown in Fig. 8, for the CMS rods formed with different amphiphiles but with the same diameter, the pitch length showed a dependence on the molecular geometry, exhibiting an increasing sequence of C16-2-AIBA o C14-GlyNa o SDS.27 On the other hand, for a certain amphiphile, it was found that such a linear relationship remained almost constant against varying the molar ratio of silica precursor/amphiphile and the pH, but alternated greatly with changing the reaction temperature, indicating a thermodynamic control on the helicity of CMS.42 Mechanical analysis has been carried out based upon the helical micellar packing to illustrate the above profile (Fig. 9).27 Considering the CMS rod with length of Dp and cross section diameter of D is straight at the initial stage, the relative rotation-angle between the two end cross sections is Df, the area of mesopore cross section and corresponding wall is S0 (unit area), the area of rod cross section is S, the number of mesopores in a rod is n, and the moment (formed due to the twisting power generated by the helical micellar pack) acting

Fig. 8 Plot of pitch length versus diameter for the CMSs formed with (a) C16-2-AIBA (b) SDS, and (c) C14-GlyNa. Reproduced from ref. 27. Copyright 2008 American Chemical Society.

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Fig. 9 Mechanical analysis of the helicity of CMS based upon the helical micellar packing of amphiphiles. Reprinted from ref. 27. Copyright 2008 American Chemical Society.

accelerated molecular movement, the amphiphiles become more loosely packed in the micelles when the temperature is increased, which subsequently releases the twisting power generated by the helical packing and makes the micellar moment (M0) smaller. Thus the P–D linear relationship is a steeper slope at higher reaction temperature and the helical mesostructure is less twisted. It should be noted that the above P–D linear function goes through the origin and is thus not in agreement with the experimental results, in which the P–D linear relationship has a non-zero interception (Fig. 8). This may be attributed to the over simplification involved in the mechanical analysis process. Recent, a new function of P = 1.89D1.5 was deduced by Yu et al. based upon a surface free energy reduction and energetic competition model, which satisfactorily fits the practical curves.43 This model makes it difficult to interpret the effects of reaction temperature and molecular structure on the P–D relationship due to its neglect of the cooperative selfassembling process. 3.4 Morphological and structural changes

on unit length of a pore is M0, then the whole moment M acted on the rod is given by: M ¼ nM0 Dp; n ¼

S S0

The resist moment M 0 of the rod accompanied by a forced twisting can be given by the following equation assuming the local inner resist force DF(r), which is tangential to the radii, is direct proportional to the local movement (Df"r) due to the twist and the local cross section area (DS): 0

M ¼ lim

Dr!0

X

DFðrÞ " r ¼

ZD=2

kDf " r2 dr ¼ KDfD3

0

where k and K are constant coefficients. It is reasonable that M 0 is equal to M when the twisting comes to an equilibrium and the pitch length of the helical mesostructure should be P¼

2pDp 2KpD3 8KDS0 ¼ ¼ nM0 M0 Df

It can be seen that the pitch length (P) is in direct proportion to the rod diameter (D) and unit area (S0) but inversely proportional to the moment of micelle (M0). Usually, the values of unit area (S0) and micellar moment (M0) keep constant under settled synthetic conditions. As a result, the pitch length experimentally increases linearly with the rod diameter (D). Since the unit area (S0) experimentally increased with the alkyl chain length of the amphiphiles, i.e., SDS o C14-GlyNa o C16-2-AIBA, the above inverse sequence of the pitch length indicated that the micellar moment (M0) increased in a sequence of SDSoC14-GlyNaoC16-2-AIBA. It is likely that the amphiphile with longer alkyl chain and/or larger head group forms a helical micelle of stronger moment and helps the formation of more twisted CMS. As mentioned above, the P–D linear relationship shifted with changing the reaction temperature, showing a larger slop at higher reaction temperature. It could be considered that, because of the 1264

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Compared with the conventional mesoporous silicas, the formation of CMS is extremely sensitive to the conditions of synthesis. CMSs of diverse size and shape have been synthesized by precise control of the reaction composition and experimental parameters, such as the temperature, pH and even stirring rate.41 As illustrated in Fig. 10, the inner part of SDS-templated CMS rods can be gradually carved by the addition of amino alcohols, such as (R) and (S)-(+)-2-amino3-phenyl-1-propanol, leading to the formation of nanotubes with helical mesostructured wall.38 Similar products were also obtained in CTAB-based systems when hexane was merged.44 It is likely that the addition of these dopants promotes the formation of vesicle-like structures, which hollows the CMS rod from inside or acts as a template for the construction of helical mesostructure.38,44 Structural changes of mesoporous silicas can usually be achieved by varying the micellar curvature, i.e., altering the g value of the templating amphiphiles.45 For anionic

Fig. 10 TEM images of calcined mesoporous silicas synthesized by using SDS as template and TMAPS as CSDA at (R)-(+)-2-amino-3phenyl-1-propanol/SDS molar ratios of (a) 0, (b) 0.2, (c) 0.4, and (d, e) 0.8. (f) Simulated chiral nanotubes shown in (d) and (e). Reprinted from ref. 38. Copyright 2007 American Chemical Society.

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Fig. 11 Synthesis-space diagram of C16-L-Ala templated mesoporous silicas. Each mixture had a constant H2O/C16-L-Ala molar ratio of 1722 and TEOS/C16-L-Ala molar ratio of 7. All the mesoporous silicas were synthesized under static conditions at 80 1C for 1 day. L: lamellar; I: bicontinuous Ia! 3d; P: 2D hexagonal p6mm; C: chiral mesophase. Reprinted from ref. 47. Copyright 2008 Elsevier Inc.

amphiphiles with weakly acidic head groups, e.g., N-acyl-amino acids with single alkyl chain, a structural change from lamellar to bicontinuous Ia!3d, to 2D hexagonal p6mm, with increasing micellar curvature, has been obtained by increasing the basicity.46 As shown in Fig. 11, the chiral mesophase usually locates in the region of 2D hexagonal p6mm.47 On the other hand, for cationic C18MIMBr-derived system, the mesostructure changes from 2D hexagonal chiral to 2D rectangular p2gg to lamellar with increasing the basicity.48 The lower condensation degree of silicates at higher basic solution leads to a higher negatively charged silicate network, which encourages closer packing of the hydrophilic head groups, resulting in a lower interfacial curvature of the micelles. Helical ribbons and nanotubes composed of chiral amphiphiles in a lamellar bilayer structure have been widely studied and applied as templates for chiral inorganic materials. It was found that, by using N-acyl-amino acids as templates and 3-aminopropyltriethoxysilane (APES) as a CSDA, mesoporous silica helical ribbons or nanotubes consisting of adhered double layers with a consistent thickness of about 15 nm can be readily produced by decreasing the temperature of the CMS reaction solution (Fig. 12).49 Unlike the CMS crystals, the mesoporous silica helical ribbons are uniform in handedness, i.e., completely left-handed or right-handed. This may be attributed to the fixed chiral molecular conformation frozen by the strong H-bonding network in the lamellar bilayers. Interestingly, while the L-type N-acyl-amino acid templated CMS crystals are predominantly left-handed, the mesoporous silica helical ribbons are completely right-handed. These findings unambiguously show that the chiral senses of the lamellar structured bilayer and the rod-like helical micelle formed with the N-acyl-amino acids may be expressed differently in the formation of diverse chiral assemblies though the mechanistic details are not clear yet. This journal is

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Fig. 12 SEM and TEM images of mesoporous silicas synthesized by using C14-L-Ala as template and APES as CSDA at (a and b) 0 1C and (c and d) 20 1C. Reproduced form ref. 49. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Otherwise, when TMAPS was employed as a CSDA in the N-acyl-amino acids templating system, mesoporous silica nanotubes with radially oriented mesopores perpendicular to the tube wall were obtained by increasing the amount of acid in the reaction mixture of CMS (Fig. 13a and b).50 The tube wall was composed of single layer of mesoporous silica and its thickness varied over the range of 27–45 nm. Fig. 13c shows a possible structural model of the nanotubes in which the chiral amphiphiles are arranged in a spring-like coiled lamellar

Fig. 13 (a) SEM and (b) TEM images of mesoporous silica nanotubes synthesized by using C14-L-Ala as template and TMAPS as CSDA with HCl/C14-L-Ala molar ratio of 0.3 at 30 1C and (c) schematic illustration of the arrangement of chiral amphiphiles in the nanotubes. Reproduced form ref. 50. Copyright 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

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bilayer. The silica precursors can thus easily penetrate into the tubular cylinder assembly and convert the lamellar bilayers into discrete mesopores through reassembly, forming the mesoporous silica nanotubes with thick tube walls. The above helical ribbon and nanotube were derived from the same chiral amphiphiles, both based on the lamellar bilayer structures. However, the distinct morphology and structure of the products imply a structural difference between the lamellar bilayers of chiral amphiphiles formed under different conditions. Unfortunately, our understanding of these processes has not yet reached the detailed molecular level. Further work on the synthesis of mesoporous silicas with diverse chiral structures and elucidation of the mechanism involved in the formation of chiral supramolecular selfassemblies is currently in progress.

linear polymers, e.g., poly(propiolic acid) sodium salt (PPAS), and the assembly of disk-like molecules, e.g., tetraphenylporphine tetrasulfonic acid (TPPS), when these molecules are introduced into the mesopores.51 Chiral conformation of PPAS and chiral column-like stacking of TPPS were induced by the electrostatic pairing between the negatively charged groups (–COO% or –SO3%) and the helically arranged quaternary ammonium groups (Fig. 14c and d), which was unambiguously detected by solid-state diffuse-reflectance circular dichroism (DRCD). As shown in Fig. 15a, the antipodal left-handed (L-) and right-handed (R-) CMS-PPAS complexes showed mirror-image ICD spectra. The PPAS loaded in the extracted L-CMS showed ICD with a positive sign in the UV-vis adsorption region owing to the polyacetylene main chain (about 300–600 nm) and a

4. Supramolecular chiral imprinting As described above, CMS of high ee has been synthesized by using chiral N-acyl-amino acids with large substitutes as templates at lower temperatures based upon the CSDA method. The amphiphiles are supposed to be helically packed in the rod-like micelles, their anionic head groups interact electrostatically with the cationic quaternary ammonium groups of the CSDA agent (TMAPS). Owing to the pairing effect, these cationic functional groups may be helically aligned on the mesopore surface surrounding the helical micelle, which is similar to the molecular imprinting process (Fig. 14a). The chirality of the primary superstructure of helical micelle is thus expected to be memorized and immobilized in the helical arrangement of functional groups on the surface of each mesopore upon removal of the template by extensive extraction (Fig. 14b).51 It was experimentally validated that such helical micelleimprinted chirality exists and can be delivered to the anionic

Fig. 14 (a) Helical arrangement of the quaternary ammonium groups (red spheres) induced by the helical packing of chiral amphiphiles (blue) due to the paired electrostatic interaction, (b) the chirality imprinted in a helical arrangement of the quaternary ammonium groups remained on the mesopore surface after removal of the chiral amphiphiles via extraction, (c) chiral supramolecular conformation of PPAS and (d) chiral supramolecular stacking of TPPS induced by the chirality memorized in the helical arrangement of the quaternary ammonium groups via electrostatic pairing, and (e) chiral supramolecular recognition of B-DNA. Reprinted form ref. 51. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

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Fig. 15 DRCD and UV-vis spectra of (a) PPAS and (b) TPPS loaded in the extracted L-CMS (blue) and R-CMS (red). Reproduced form ref. 51. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA.

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negative ICD around 210 nm probably due to the carboxylate groups, indicating that the adsorbed PPAS was in a chiral conformation. Contrarily, the R-CMS-PPAS complex showed exactly the opposite signals in these two regions, implying that the PPAS was in an antipodal chiral conformation. On the other hand, the antipodal L- and R-CMS-TPPS complexes showed mirror-image ICD spectra with a weak exciton couplet at the Soret band. The L-CMS-TPPS complex exhibited a positive exciton couplet, indicating TPPSs were right-handed stacked, while the R-CMS-TPPS complex reflected a left-handed stacking of TPPS. Interestingly, the handedness of such stacking is opposite to that of CMS. B-DNA was also used to detect the imprinted chirality on the mesopore surface of CMS. It was found the extracted R-CMS exhibited a faster adsorption rate of B-DNA than the left-handed counterpart.51 Such supramolecular chiral recognition was explained in terms of the matching between two sets of helices. As shown in Fig. 14e, the helix of the quaternary ammonium group array with an inner diameter of about 3 nm may be packed tightly with the B-DNA helix of an external diameter of ca. 2 nm. Despite the difference in pitch length and helical structure, B-DNA may still sense the imprinted chirality on the mesopore surface, forming more easily a complex with the same-handed helix of the functional groups.

5. Summary and outlook The cooperative interactions between the amphiphiles and silica precursors promote effective, precise, and systematic transcription and organization of chirality, leading to CMS with hierarchical chiral features. Ideally, CMS would possess chirality on three levels: (1) micrometre scale, i.e., the helical mesopores and twisted morphology; (2) supramolecular level, i.e., the helical packing of amphiphiles and its helical imprinting effect on the arrangement of the CSDA functional groups immobilized on the channel wall; and (3) molecular level, i.e., the molecular chirality imprinted on the channel wall and functional groups, which may be produced by the asymmetric head groups of chiral amphiphiles, and the chiral conformations of the silica or organoslica framework originated from the twisted rod-like shape. Because of their ordered mesostructure, controllable pore size, large pore volume, and huge surface area, mesoporous silicas have been extensively used in heterogeneous catalysis, chemical separation, drug delivery, nanofabrication, etc. The added benefit of chirality further enables CMS of advanced functions in chiral applications. For instance, based upon the well-known hard-template method, helical carbon or metal nanowires are capable of being fabricated by deposition of sugars or metal salts in the helical mesoporous channels, followed by a heat treatment for carbonization or recovery of metal. On the molecular level, with molecular chiral imprinting on the mesopore surface, CMS was thought to be highly useful for sorption and separation of chiral molecules, especially because it has a considerable loading amount of guest molecules. Meanwhile, the novel chiral imprinting revealed at the supramolecular level in CMS has undoubtedly paved a new way for the design of chiral This journal is

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materials. Various applications, such as asymmetric catalysis, chiral separation and recognition, are expected to be carried out through precise control of the functional groups on the mesopore surface of CMS and rational selection of guest molecules, including linear polymers, disk-like molecules, metalized molecules and metal ions (not published). The stability of the silica framework further ensures recylability of the materials. Although CMS shows a bright future for both fundamental research and applications, considerable challenges remain. The synthesis of non-siliceous chiral mesoporous materials, which is the most significance in view of material design, lags far behind that of CMS. Harmonizing the condensation rate of non-siliceous inorganic precursors with the self-assembling process should be a key process. Due to the poor diffusion of precursor into the mesopores, which is mainly induced by the small pore size of currently available CMS (less than 4 nm), it is still hard to prepare long helical nanowires. Studies on larger chiral assemblies, such as that formed by the chiral block copolymers,52 and their derived large-pore CMSs are required. Feasible strategies are also needed for imprinting the molecular chirality of chiral amphiphiles since most efforts on the separation of chiral molecules by CMS remain difficulty to be successful, probably because the cooperative selfassembling process occurs far beyond the molecular scale and the chirality of a single molecule is hardly memorized by the mesopore surface. This investigation is hopefully to be progressed by the introduction of multifunctional CSDAs.53 Moreover, calculation and simulation on the chiral packing of amphiphiles are of high urgency to be performed because the currently suggested mechanisms are rather hypothetical. It is important to integrate diverse experiment observations to build reliable theories for an accurate description of the chiral supramolecular assembling processes.

Acknowledgements The authors thank all their collaborators and coworkers whose names appear in the reference list. This work was supported by the National Natural Science Foundation of China (Grant No. 20890121 and 20821140537), the 973 project (2009CB930403) of China and Grand New Drug Development Program (No.2009ZX09310-007) of China.

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