b School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China
Chirality exists widely in nature and expresses hierarchically in plenty of materials. Generally, the chirality of a supramolecular entity is controlled specifically by the chirality of its molecular building blocks. Nevertheless, such chiral sense upon forming helical structures usually changes with chemical or physical variations, such as salt [1, 2], solvent [3, 4], temperature [5, 6], light [7, 8], etc. [9-11]. It is known that the enantiomerically pure agent often gives rise a mixture of left- and right-handed helical assemblies . These findings suggest that the molecular chirality is not only driving force but some other factors also exist to cancel or reverse the inherent chiral sense. Controlling the chirality and investigating the mechanism is a challenging task, but is of great fundamental significance for understanding origin of chirality and for creating new functional metamaterials.
Amongst the experimental parameters, temperature has been considered to be dominative in most assembling processes as it controls both the kinetics and thermodynamics. In our N-acylamino acid-templated chiral mesoporous silicas synthesis system , the enantiopurity decreases linearly with increasing temperature, which was interpreted thermodynamically by the equilibrium shift between two antipodal helical cylinder micelles triggered by the temperature driven conformational changes of amphiphiles . Meanwhile, the twisted silica rods can be formed at higher temperatures, while helical mesoporous silica ribbons were favored by a lower temperature [15, 16]. The expression of chirality probably involves a complicated recognition, transcription and amplification of the initial molecular chirality during assembling. However, few evidence has been successfully gained so far to illustrate these processes, mainly due to a poor understanding of the packing styles of molecular building blocks.
Herein, chiral mesoporous silica ribbons and rods with inversed handedness have been synthesized with N-myristoylalanine (C14- L/D-Ala) amphiphile as templates and 3-aminopropyltriethoxysilane (APES) as a co-structural directing agent (CSDA) at different temperatures according to the previous report . A detailed investigation was by involving a well detectable probe to explore the chiral molecular stacking in these materials, and a diffusereflectance circular dichroism (DRCD) study in the formation process to explore the evolution of chiral assemblies of amphiphiles and the plausible mechanism of the temperature-depended handedness inversion (see details in Supporting information). The findings of this work explored a deeper understanding of the transcription and expression of chirality, regarding the formation of a variety of chiral assemblies.
The samples synthesized at 0 ℃ were consisted of tubular helical ribbons, which were absolutely right-handed for C14-L-Ala and vice versa (Fig. 1a). On the contrary, the samples formed at 20 ℃ were comprised uniformly by twisted hexagonal rods, which were left-handed excessive for C14-L-Ala and right-handed excessive for C14-D-Ala (Fig. 1a) with the ee value of ~50%, which was estimated by counting the characteristic morphologies from 500 randomly chosen crystals in the SEM images. The helical ribbons consisted disordered cylindrical mesoporous channels and distributed uniformly in the ribbon wall. On the other hand, the twisted rods showed a typical chiral mesostructure with the twodimensional hexagonally cylindrical mesoporous channels running in spirals along the central axis of the rod (Fig. S1 in Supporting information). Both two types of materials showed a narrow pore size distribution with an average pore diameter of about 3.5 nm, indicating a comparable size of the rod-like micelles of chiral amphiphiles inside the mesoporous channels. As shown by the solid-state 13C CP/MAS NMR spectra (Fig. S3 in Supporting information), the chiral amphiphiles can be effectively removed from the channels after extraction and the propylamino groups of APES were well preserved on the mesoporous walls. Elemental analysis (Table S1in Supporting information) revealed that the concentration of propylamino groups were approximately equal in the chiral mesoporous silica ribbons and rods, which enabled a reliable compare of their arrangement as a supramolecular imprinting of the primary structure of rod-like micelles. Consequently, ribbons and rods formed with the same enantiopure amphiphile revealed opposite handedness, but identical mesoporous channels size and amino functionalization. It is clear that chiral molecular stacking of C14-L-Ala are inversed at different temperatures.
|Fig. 1. SEM images of antipodal extracted chiral mesoporous silica ribbons and rods (a), DRCD and DRUV spectra after loaded with Cu2+ (b) and TPPS (c).|
During the CSDA promoted formation process of mesoporous silicas, the primary supramolecular structure of the cylinder micelles can be precisely memorized by the array of CSDA functional groups as an imprinting due to the pairing effect of electrostatic interaction , which provides an access to study the structural details of chiral molecular stacking in the cylinder micelles by involving well detectable molecular probes. Herein, a small ion, Cu2+, and a large conjugated discotic molecule, TPPS, were introduced to the extracted chiral mesoporous silica ribbons and rods. After loaded with Cu2+or TPPS, both antipodal ribbons and rods reveal mirror-image DRCD spectra (Figs. 1b and c). However, the C14-L-Ala-ribbon@Cu2+ complexes showed a positive Cotton effect around 230 nm (Fig. 1b, solid black line), indicating the Cu2+ ions were right-handed helically stacked in the mesoporous channels. On the contrary, the C14-L-Ala-rod@Cu2+ complexes exhibited absolutely a negative Cotton effect (Fig. 1b, solid red line) corresponding to a left-handed stacking of Cu2+. In a similar vein, the C14-L-Ala-ribbon@TPPS and C14-L-Ala-rod@TPPS, which are from the same amphiphile showed completely opposite Cotton effects in the Soret band of TPPS (Fig. 1c, solid lines), indicating the TPPS molecules were also opposite helically stacked in the mesoporous channels. The weaker intensity in rods might be attributed to the lower enantiomeric excess. The experimental data of calcined ribbons and rods loading with Cu2+ (Fig. S5 in Supporting information) and chiral mesostructured porphyrinsilica hybrid  indicate that the CD signals originated from Cu2+ and TPPS chiral stacking rather than chiral surrounding.
As to track the evolution of chiral assemblies of amphiphiles and the differentiation for the growth of chiral mesoporous silica ribbons and rods, the SEM, XRD and DRCD of the products formed at 0 ℃ and 20 ℃, were monitored as a function of the reaction time. It can be seen that the freeze-dried initial organic gels showed similar sheet-like morphologies. Whereas morphologies of the products were quite different after adding silica precursors. Meanwhile, DRCD signal (Fig. 2a) of organic gel with an exciton couplet centered at 215 nm faded away. At 0 ℃, twisted ribbons were initially formed and gradually grew and coiled into tubular helical ribbons (Figs. S6 and S7 in Supporting information). The DRCD spectra of these products (Fig. 2b) show that the exciton couplet decreased and a new Cotton effect at around 210 nm increased gradually with the same chiral sense to the exciton couplet, indicating that the chiral assembly structure of amphiphiles has been transformed but the chiral orientation maintained. At 20 ℃, however, the lamellar sheets were quickly dissolved and completely converted into twisted hexagonal rods in 20 min (Figs. S6 and S7). Shown as Fig. 2c, the exciton couplet decreased and a Cotton effect at around 210 nm increased but with opposite chirality to the signals of the final sample formed at 0 ℃, exhibiting a transformation amphiphiles assembled structures and inversion of chirality. It seemed that, during the transformation from lamellar structures in the organic gels to cylinder micelles in the mesoporous silicas, the exiton couplet between adjacent amphiphiles decreased, and the Cotton Effect of helical molecular stacking of amphiphiles were emerged. In the process, amphiphiles maintained the same helical orientation to the organic gels at 0 ℃, while rearranged into opposite orientation at 20 ℃.
|Fig. 2. DRCD spectra of C14-Ala gel (a) and the corresponding intermediate products sampled at different time (b and c).|
Based on the results, a plausible mechanism for the handedness inversion is proposed. Scheme 1 illustrates two possible chiral molecular stacking models of C14-L-Ala for the cylinder micelles presented in the chiral mesoporous silica ribbons and rods, respectively, regarding the inversed handedness and distinct molecular orientation. Actually, the variation of molecular orientation would be a means of a single enantiopure chiral amphiphile to overcome the energy barrier to adapt more than one chiral molecular stacking. The handedness inversion process thus can be analysed in terms of the H-bonding-depended molecular orientation and the corresponding chiral sense upon forming chiral molecular stacking. As shown in Scheme 1, the chiral amphiphiles are initially aligned in a lamellar structure with their orientation restricted by the H-bonding networks. With the addition of silica precursors, the carboxylic acid group is neutralized by APES and the electrostatic repulsion between adjacent amphiphiles is subsequently increased, leading to a molecular stacking transformation from lamellar bilayers to cylinder micelles with a higher curvature. At a lower temperature of 0 ℃, the H-bonding is presumedly maintained when the distance between adjacent amphiphiles is increased. The orientation of chiral amphiphiles is thus locked during the evolution process and the right-handed helical molecular stacking are specifically produced through a local phase transformation. At a higher temperature of 20 ℃, however, as indicated by Scheme 1, the original lamellar layers together with the H-bonding networks may be fully destroyed by the silica precursors. The amphiphiles are thus dissolved and become randomly orientated in the solution and subsequently reformed into cylinder micelles through cooperative assembling with the silica precursors. During the evolution process, the steric effect should be resiponsible for the left-handed preferring of helical molecular stacking dynamically selected. The reason why the different of ee values between samples at different temperatures could be that the steric effect was weaker than the H-bonding interactions.
|Scheme 1. Evolution of molecular stacking of C14-L-Ala during the formation of chiral mesoporous silica ribbon and rod, respectively.|
In summary, the chiral mesoporous silica ribbons and rods formed with a single enantiopure chiral amphiphile at 0 ℃ and 20 ℃, respectively, revealed inversed morphological handedness and inversed chiral molecular stacking of amphiphiles in the mesoporous channels. The connection between vicinal chiral amphiphiles (e.g., H-bonding) essentially controls their molecular orientation and the preferred style towards to chiral molecular stacking. The packing style of chiral molecular building blocks is thought to be the key factor to resolve this procedure and this work undoubtedly provides a reliable route to study it by employing the DRCD technology and the supramolecular chiral imprinting method. The present work is also helpful for the development of sol-gel method and the synthesis of chiral porous inorganic materials.Acknowledgments
This work was supported by the National Natural Science Foundation of China (Nos. 21471099, 21601123). We thank the Instrumental Analysis Centre of Shanghai Jiao Tong University for CD measurements.Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.cclet.2018.01.008.
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