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. 2021 Apr 1;1(4):375–379. doi: 10.1021/jacsau.1c00098

Chiral Silica with Preferred-Handed Helical Structure via Chiral Transfer

Kei Manabe †, Sung-Yu Tsai §, Satoshi Kuretani †, Satoshi Kometani †, Katsuyuki Ando †, Yoshihiro Agata □, Noboru Ohta ‡, Yeo-Wan Chiang ⊥, I-Ming Lin ⊥, Syuji Fujii †, Yoshinobu Nakamura †, Yu-Ning Chang , Yuta Nabae □, Teruaki Hayakawa □, Chien-Lung Wang §, Ming-Chia Li ¶,○,*, Tomoyasu Hirai †,*
PMCID: PMC8395658  PMID: 34467302

Abstract

graphic file with name au1c00098_0005.jpg

A strategy to obtain chiral silica using an achiral stereoregular polymer with polyhedral oligomeric silsesquioxane (POSS) side chains is described herein. The preferred helical conformation of the POSS-containing polymer could be achieved by mixing isotactic polymethacrylate-functionalized POSS (it-PMAPOSS) and a chiral dopant. The array structure of POSS molecules, which are placed along the helical conformation, is memorized even after removing the chiral dopant at high temperatures, leading to a chiral silica compound with exclusive optical activity after calcination.

Keywords: polyhedral oligomeric silsesquioxane (POSS), stereoregularity, chiral transfer, living anionic polymerization, silica

Silica-based chiral materials such as helical mesoporous silica have attracted considerable attention in the fields of catalysis, templating, and chiral recognition.1−3 These materials are prepared by sol–gel transcription using an organic chiral structure as a template.4−7 This process is generally tedious, requiring many steps and long reaction times. Moreover, the chemical reaction leads to an aggregated structure approximately few μm to 100 nm in length with a 2–3 nm helical pore. Subnanometer-sized helical structures are difficult to form using sol–gel techniques. In a living organism, polymeric nucleotides such as DNA, RNA, and peptides provide important biofunctional properties through the formation of helical conformations. Helical conformations less than several nanometers in size and with the preferred handedness can simply be formed through noncovalent interactions involving achiral polymers with rigid main chains and chiral dopants.8 Numerous polymethacrylate derivatives with inorganic9 and metal10 precursors have been prepared; however, it is still difficult to form helical structures with the preferred handedness in polymethacrylate derivatives in the absence of complex formation. To the best of our knowledge, preparation of preferred-handed helical silica derivatives using an achiral polymer as a template has not been reported yet.

Polyhedral oligomeric silsesquioxane (POSS) is an organic–inorganic hybrid nanoparticle.11 Methacrylate-functionalized POSS (MAPOSS) is commercially available, and its polymers (PMAPOSS) with a well-controlled primary structure were obtained by controlled radical and living anionic polymerization.9,12,13 Mesoporous silica14,15 could be obtained by performing calcination using a mixture of a block copolymer and POSS derivatives.16 A simple and cost-effective preparation method for chiral silica materials can be designed if the helical conformation in PMAPOSS could be controlled and the POSS domains could maintain the helical structure during calcination. As POSS nanoparticles have diameters of about 0.5 nm, it should be possible to form a subnanometer-scale helical structure. Here, we report the fabrication of silica compounds with a helical structure using PMAPOSS preferred-handed helical conformation as the template prepared by mixing isotactic PMAPOSS (it-PMAPOSS) and a chiral dopant (Figure 1).

Figure 1.

Figure 1

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Schematic illustration of the fabrication of a silica compound with the preferred helical structure using it-PMAPOSS as the template.

A conventional living anionic polymerization method was used to control the stereoregularity in PMAPOSS (see the Supporting Information and Table S1).17,18 Atactic PMAPOSS and isotactic PMAPOSS, designated as at-PMAPOSS and it-PMAPOSS, respectively, were used. To control the preferred-handed helical conformation of the polymer, it-PMAPOSS and (S)-(−) or (R)-(+)-5,5′, 6,6′, 7,7′, 8,8′-octahydro-1,1′-bi-2-naphthol (BN) were mixed and drop-casted on a quartz substrate or silicon wafer. The helical conformation thus induced was evaluated by electronic circular dichroism (ECD) and vibrational circular dichroism (VCD), respectively. Figures 2a and b show the solid-state ECD spectra of it-PMAPOSS without and with (R)- or (S)-BN films, respectively. More importantly, the ECD signals for it-PMAPOSS with the enantiomeric BN showed a split-type Cotton effect in the wavelength range of 200–234 nm in the UV absorption spectrum, which can be assigned to the carbonyl group of PMAPOSS. Moreover, the Cotton effect resulted in mirror images of (R)- and (S)-BN.

Figure 2.

Figure 2

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ECD analyses of (a) it-PMAPOSS and (b) it-PMAPOSS with (R)- or (S)-BN. (c) VCD spectra of it-PMAPOSS with (R)- or (S)-BN.

To evaluate the effect of stereoregularity on the ECD signals of the carbonyl group, at-PMAPOSS with (R)- and (S)-BN films were also prepared and analyzed by ECD (Figure S7a). No Cotton effect was observed in the wavelength range of 200–234 nm, which implies that stereoregularity strongly affected the molecular aggregation. VCD measurements were performed to further investigate the induced preferred-handed conformation of the helices in PMAPOSS with main-chain chirality. Figures 2c and S7b show the VCD spectra of it-PMAPOSS and at-PMAPOSS with enantiomeric BN, respectively. Specific split-type and mirror-image VCD signals were observed in it-PMAPOSS with enantiomeric BN at ∼1730 cm–1 corresponding to carbonyl stretching in the ester group of PMAPOSS. In contrast, split-type VCD signals were not observed at 1730 cm–1 for at-PMAPOSS with enantiomeric BN. Although several IR peaks ranging from 1000 to 1250 cm–1 overlapped, including C–O–C (1050–1250 cm–1)19 and asymmetric Si–O–Si (1000–1240 cm–1)4 vibrations, clear split-type Cotton effects could be seen for it-PMAPOSS with BN. In contrast to at-PMAPOSS with BN, the VCD signals of it-PMAPOSS with BN in the range of 1000–1250 cm–1 showed mirror-image and split-type Cotton effects and were quite different from those of the (R)-BN and (S)-BN thin films (Figure S7c) because of the appearance of an induced split-type Cotton effect in the asymmetric Si–O–Si vibrational bands. These results are in good agreement with the ECD results. Thus, we suggest that the split-type Cotton effects of the induced VCD signals in the absorption bands of the C=O, C–O–C, and Si–O–Si vibrations can be attributed to the occurrence of intramolecular interactions with the adjacent POSS pendants twisted in a preferential direction along the polymer main chain, resulting in the induced preferred-handed conformation of the helixes in the it-PMAPOSS polymers with main-chain chirality. The morphology of it-PMAPOSS with BN films was evaluated by grazing-incidence wide-angle X-ray diffraction (GIWAXD) analysis. The results showed that the blending of the chiral dopant does not affect the thermodynamically stable helix-like structure of it-PMAPOSS (see the Figure S16 in Supporting Information).20

By taking advantage of the low melting temperature of chiral BN, free chiral BN can be removed by thermal annealing at 200 °C for 30 min. As observed, ECD signals assigned to the carbonyl group of it-PMAPOSS are still evident in the 200–234 nm wavelength range (Figure 3a). Next, it-PMAPOSS with enantiomeric BN was further annealed at 200 °C for 1 h. During annealing, the intensity of the IR peak at 1584 cm–1, which can be assigned to C=C stretching in the phenyl ring of BN, decreased. Finally, the peak vanished at the end of the annealing process (Figure 3b and see the Supporting Information), suggesting that sufficient thermal energy was present to overcome intermolecular hydrogen bonding, leading to the evaporation of enantiomeric BN at this temperature (Figure S10). Most importantly, split-type Cotton effects were evident in the annealed sample at 1730 cm–1 and between 1000 and 1250 cm–1. It is clear that the preferred-handed helical conformation remained even after BN was eliminated from it-PMAPOSS at a high temperature, showing a unique chirality memory effect in stereoregular PMAPOSS. The large steric hindrance associated with the POSS moieties restricts free rotation in the flexible polymethacrylate main chain and further assists in preserving chiral memory.

Figure 3.

Figure 3

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(a) ECD spectra of it-PMAPOSS with (R)- or (S)-BN films annealed at 200 °C for 30 min. (b) VCD spectra of it-PMAPOSS with (R)- or (S)-BN films annealed at 200 °C for 1 h. (c) TGA trace of a mixture of it-PMAPOSS and BN. (d) VCD spectra of calcined samples.

Figures 3c and S17 show the thermogravimetric analysis (TGA) measurements for the mixture of PMAPOSS and BN. The measurements were performed from 30 to 620 °C at a heating rate of 10 °C/min under air. The chirality of the residue thus obtained was evaluated using VCD measurements (Figure 3d). The peaks ranging from 1000 to 1240 cm–1, which can be assigned to Si–O–Si vibration, were dominant. The two bands centered at 1131 and 1047 cm–1 due to the cage and network Si–O bond stretching, respectively, broadened to a single unresolved band, showing a silica structure owing to cage cross-linking.21 Hence, it is clear that most of the organic segments were removed from the it-PMAPOSS.16 Interestingly, the split-type Cotton effect could still be observed in this region and was more striking than that of the sample annealed at 200 °C for 1 h, indicating that the distance between Si–O–Si decreased.

Figure 4 shows the transmission electron microscopy (TEM) image of the calcined sample prepared using a mixture of it-PMAPOSS and (R)- or (S)-BN. A helical structure with the preferred handedness was clearly observed in each calcined sample. The VCD and TEM results suggest that the induced preferred helical structure in the PMAPOSS and BN mixture can be used as a chiral template and maintained during the calcination process, leading to the formation of silica with exclusive optical activity (Figure 1). Hence, this work discusses the preparation of chiral silica using a polymer with well-controlled stereoregularity as a template.

Figure 4.

Figure 4

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TEM images of calcined samples prepared using a mixture of it-PMAPOSS and (a) (R)-BN or (b) (S)-BN.

In conclusion, at- and novel it-PMAPOSS were prepared through living anionic polymerization. The it-PMAPOSS formed a stable, dynamic, preferred-handed helical conformation, which could be retained even after the evaporation of the enantiomeric BN additives at high temperatures. Moreover, the helical structure was maintained during the calcinating process, leading to the formation of silica with exclusive optical activity. These results are important because they provide an important breakthrough in fabricating chiral silica materials that can be used as a promising material system for applications such as optics, asymmetric catalysts, chiral separation, and medicine.

Acknowledgments

We would like to express our gratitude to Mr. Ryohei Kikuchi, Mr. Katsuaki Hori, and Mr. Akira Genseki of the Open Facility Center, Materials Analysis Division, Tokyo Institute of Technology for assisting us with TEM measurements. This work was supported by JSPS KAKENHI Grant JP19K05590. Part of this work was supported by the International Collaborative Research Program of Institute for Chemical Research, Kyoto University (Grants 2019-43 and 2020-42) and JSPS KAKENHI Grant 20H02785. Part of this work was also obtained from a project, “Moonshot Research and Development Program” (JPNP18016), commissioned by the New Energy and Industrial Technology Development Organization (NEDO). X-ray diffraction measurements were performed at the BL40B2 beamline of SPring-8 under proposals 2019B1179, 2020A0627, 2020A0774, and 2020A1135. This work was financially supported in part by the “Center for Intelligent Drug Systems and Smart Bio-devices (IDS2B)” for the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) of Taiwan and the Ministry of Science and Technology of Taiwan for financial support under Contract 107-2221-E-009-044 -MY2.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacsau.1c00098.

  • Synthesis method, experimental methods, and data analysis (PDF)

Author Contributions

△ K. Manabe and S.-Y. Tsai contributed equally to this study. Conceptualization and methodology: K. Manabe, S.-Y. Tsai, S. Kuretani, S. Kometani, K. Ando, Y. Agata, N. Ohta, Y.-W. Chiang, I.-M. Lin, S. Fujii, Y. Nakamura, Y.-N. Chang, Y. Nabae, T. Hayakawa, M.-C. Li, C.-L. Wang, and T. Hirai. Writing original draft preparation: K. Manabe and T. Hirai. All authors have read and agreed to the published version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

au1c00098_si_001.pdf (5.7MB, pdf)

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Associated Data

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Supplementary Materials

au1c00098_si_001.pdf (5.7MB, pdf)

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