Chirality induction on non-chiral dye-linked polysilsesquioxane in nanohelical structures

Naoya Ryu a, Tsutomu Kawaguchi a, Hiroshi Yanagita b, Yutaka Okazaki c, Thierry Buffeteau d, Kyohei Yoshida e, Tomohiro Shirosaki a, Shoji Nagaoka *ab, Makoto Takafuji b, Hirotaka Ihara *b and Reiko Oda *e
aMaterials Development Department, Kumamoto Industrial Research Institute, 3-11-38 Higashimachi, Higashi-ku, Kumamoto 862-0901, Japan. E-mail: nagaoka@kmt-iri.go.jp
bDepartment of Applied Chemistry and Biochemistry, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. E-mail: ihara@kumamoto-u.ac.jp
cInternational Research and Education Centre of Advanced Energy Science, Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606-8501, Japan
dInstitut des Sciences Moléculaires (UMR5255 ISM), CNRS, Université de Bordeaux, 351 Cours de la Libération, 33405 Talence, France
eInstitut de Chimie & Biologie des Membranes & des Nano-objets (UMR5248 CBMN), CNRS, Université de Bordeaux, Institut Polytechnique Bordeaux, 2 rue Robert Escarpit, 33607 Pessac, France. E-mail: r.oda@iecb.u-bordeaux.fr

Received 26th March 2020 , Accepted 26th May 2020

First published on 27th May 2020


We demonstrate the direct induction of chirally arranged organic dye-linked polysilsesquioxane through a sol–gel transcription using a chiral supramolecular template. The chiral arrangement was confirmed by using electronic and vibrational circular dichroism and circularly polarized luminescence spectroscopies.


For the fabrication of chiral inorganic materials, one of the representative approaches is the sol–gel transcription method using the imprinting of chiral organic templates with achiral inorganic precursors. Such inorganic structures can then be used as a template to organize the achiral dyes to induce chiroptical properties.1 We have previously established the preparation procedure of chirally twisted and helical nanoribbons of inorganic silica from self-assembled gemini surfactant1a,1b,2 and have found that they show a large vibrational circular dichroism (VCD) signal at around 1000–1150 cm−1, indicating that the siloxane network has been chirally arranged.2d This is a rare example in which the chiral arrangement of the inorganic siloxane network was clearly evidenced by using VCD spectroscopy. This chirality induction comes from the chiral surface of the organic templates to the originally achiral inorganic ones.

Such chiral silica can potentially be used for the enantioselective separation/recognition3 and has the ability to induce the chiral arrangement to the achiral molecules adsorbed on their surface.4 To further enhance its functionality and broaden its applicability, the introduction of organic functional groups or molecules is an effective strategy. They are generally introduced using the monosilylated achiral organic molecules [X–Si(OR)3: X = organic functional group/molecule, R = CH3 or C2H5] by post-synthesis grafting on the surface of chiral silica. Such organic functional groups/molecules can show chiral properties.5 The organic-functionalized moieties have been, however, limited to only the silica surfaces, and the concentration of the functional groups/molecules is thus low on the whole.

Multisilylated organic molecules [X–(Si(OR)3)n: n ≥ 2] are interesting alternatives as precursors to fabricate the organic functionalized silica (strictly, polysilsesquioxane) with a high concentration of the organic groups/molecules. In particular, the organic dye-functionalized silica show interesting optoelectronic properties6 due to the densely ordered and immobilized organic dye molecules inside the polysilsesquioxane networks. However, the organic dye-linked polysilsesquioxane structures from an achiral precursor alone without any chiral dopants that show chiroptical signals have never been reported.

In this paper, we demonstrate the fabrication of organic dye biphenyl-linked polysilsesquioxane (hereafter, abbreviated as BP-PSQ) through sol–gel transcription using the achiral precursor, bis-triethoxysilylated biphenyl (hereafter, abbreviated as BS-BP, Fig. 1a), with a chiral organic template. For this procedure, a water-miscible organic liquid containing a small amount of water is used for the sol–gel transcription to dissolve water-insoluble BS-BP and to hydrolyse its ethoxysilyl groups. We have previously shown that the cationic gemini surfactant with an enantiomeric tartrate counterion (hereafter, abbreviated as the 16-2-16 tartrate, Fig. 1b) retains its chiral organization both in water and in pyridine as the main solvent,2a and furthermore, can be used as a template for the sol–gel transcription.1a,b,2


image file: d0cc02224a-f1.tif
Fig. 1 Chemical structure of (a) biphenyl-linked silsesquioxane precursor and (b) cationic gemini surfactant with enantiomeric tartrate counterion used in this study.

We first investigated the assembling morphology of the 16-2-16 tartrate in pyridine containing water. The pyridine[thin space (1/6-em)]:[thin space (1/6-em)]water ratio was set at 19[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v. The 16-2-16 tartrate (20 mM) was dissolved in this mixture by heating at 80 °C. After cooling to 20 °C, the solution immediately became a transparent gel (Fig. S1a, ESI) similar to that in pure pyridine. The gel–sol transition was visually observed upon heating and the transition temperature (Tgel) was detected using differential scanning calorimetry (DSC) and was estimated to be 52 °C (transition enthalpy, ΔH = 1.3 kJ mol−1) in the heating process (Fig. S1c, ESI). When the concentration was decreased to 5.0 mM, only a weak gel (Fig. S1b, ESI) was observed. Low-voltage scanning transmission electron microscopy (LV-STEM, 30 kV) observations revealed that the 16-2-16 tartrate self-assembled and formed the twisted ribbon-like nanostructures in the pyridine–water mixture (19[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v). The 16-2-16 L-tartrate formed exclusively right-handed twisted ribbons (Fig. 2a, left), and its enantiomer, the 16-2-16 D-tartrate, formed left-handed ones (Fig. 2a, right). These nanoribbons had an average width of 18 ± 2 nm and an average pitch of 107 ± 12 nm and were strongly entangled and bundled. The handedness and width are similar to their assemblies in water.7


image file: d0cc02224a-f2.tif
Fig. 2 LV-STEM images of (a) 16-2-16 tartrate self-assemblies in the cast film prepared from a 5.0 mM pyridine–water mixture (19[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) post-stained with OsO4, (b) unstained BP-PSQ containing 16-2-16 tartrate assemblies, and (c) unstained BP-PSQ after washing and sonication; left: prepared from 16-2-16 L-tartrate, right: prepared from 16-2-16 D-tartrate. Inset shows schematic illustrations of each cross-sectional surface.

These twisted nanoribbons were then used as the templates in the sol–gel reaction of BS-BP. In a typical procedure, BS-BP (20 μL) and n-butylamine (20 μL, as a catalyst) were added to the partial gel of the 16-2-16 tartrate (5.0 mM, aged for 24 h at 20 °C) in a pyridine–water mixture (4 mL, 19[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v) and thoroughly mixed using a vortex mixer. After keeping at 20 °C for 24 h, precipitates were formed which consist of BP-PSQ containing 16-2-16 tartrate. The LV-STEM observation of these precipitates revealed that they maintained twisted ribbon-like nanostructures (width: 23 ± 4 nm, pitch: 123 ± 10 nm, Fig. 2b and Fig. S2, ESI). The slight increase in the pitch may be due to the destabilisation of the chiral structure upon transcription as observed previously.2b The BP-PSQ containing 16-2-16 tartrate was then washed with hot methanol (55 °C) in order to remove the template molecules along with the excess precursor, catalyst, and solvent. The last washing solution hardly showed any peak in the UV-vis absorption spectra (Fig. S3, ESI), indicating that the template and other molecules were almost completely removed from BP-PSQ. The washed BP-PSQ was sonicated in ethanol for good dispersion. Fig. 2c shows LV-STEM images of the dispersed BP-PSQ where only the poorly structured nanofiber-like structures can be seen and the twisted ribbon-like nanostructures are destroyed after washing and sonication, unlike the silica nanoribbons prepared from tetraethyl orthosilicate using the same gemini surfactant chiral self-assemblies in water1a,2c,d and other organosilica materials prepared from BS-BP by other researchers.6b,8 This suggests that the BP-PSQ walls prepared using this method are more fragile than silica. Indeed, the twist ribbon-like nanostructures did not survive even after simply washing under “soft” conditions without sonication or a simple dilution with pyridine. Fig. 3a shows the powder X-ray diffraction (XRD) pattern of the washed and sonicated BP-PSQ after freeze-drying. Five diffraction peaks are clearly visible. The four peaks at 7.6, 15.6, 22.8, and 30.6° had the ratio of about 1[thin space (1/6-em)]:[thin space (1/6-em)]2[thin space (1/6-em)]:[thin space (1/6-em)]3[thin space (1/6-em)]:[thin space (1/6-em)]4, indicating a periodic structure with ∼1.17 nm periodicity.8a This shows that BP-PSQ forms a molecular-scale lamellar structure as illustrated in Fig. 3b. The diffraction peak at 19.9° (∼0.45 nm) may exhibit the distance between the neighbouring biphenyl moieties which are stacked with each other. These values are consistent with the result of the simulated structure (Fig. S4, ESI). The broad peak centred at around 20° is probably due to the amorphous silsesquioxane.


image file: d0cc02224a-f3.tif
Fig. 3 (a) Powder XRD pattern and (b) schematic illustration of BP-PSQ prepared from 16-2-16 L-tartrate.

These results indicate the much lower cross linking density of BP-PSQ than that of silica, which may lead to a more fragile network, which may explain why the mesoscopic helical structure does not resist washing and sonication. The washed and sonicated BP-PSQ dispersed in ethanol (0.05 mM as biphenyl-bis(silsesquioxane) unit) was subjected to ultraviolet-visible (UV-vis) absorption and fluorescence, electronic circular dichroism (ECD), and circularly polarized luminescence (CPL) measurements, and the obtained spectra are shown in Fig. 4. Compared to its precursor, BS-BP (0.05 mM) molecularly dispersed solution in ethanol which has an absorption peak at 261 nm (Fig. 4a), the absorption peak of BP-PSQ decreased and slightly red-shifted to 263 nm (Fig. 4a). These results imply that the biphenyl moieties of BP-PSQ interact with each other in the ground state which is in agreement with the XRD result. When excited at an absorption maximum wavelength, compared to the emission band of BS-BP (emission maximum wavelength, λem = 312 nm; full width at half maximum, FWHM = 3360 cm−1 (34 nm); Stokes shift = 6260 cm−1 (51 nm)), BP-PSQ showed a broader emission band at longer wavelength (λem = 384 nm, FWHM = 4840 cm−1 (71 nm), Stokes shift = 12[thin space (1/6-em)]000 cm−1 (121 nm)) without any peaks and shoulders around 312 nm (Fig. 4b). In addition, the emission of BP-PSQ has a longer lifetime (τ) than that of BS-BP (τBP-PSQ = 27 ns, τBS-BP = 5 ns, Fig. S5, ESI). A significant red shift, broadening of an emission band and a lengthened fluorescence lifetime are typical behaviours that appear when polycyclic aromatic dyes (pyrene, anthracene, and so on) form excimers (excited dimers); however, excimer formation is generally difficult for typical biphenyl species because of the steric hindrance of the twisted phenyl rings. Thus, it seems that the excimer formation of the biphenyl moieties of BP-PSQ is enabled due to the dense packing structure through the polysilsesquioxane network. On the other hand, in general, excimer formation causes a decrease in the fluorescence quantum yield (ϕ) because of the strong interaction between dye molecules in the excited state. In the present case, it is worth noting that in spite of the excimer formation, BP-PSQ has a higher quantum yield than that of its precursor (ϕBP-PSQ = 0.31, ϕBS-BP = 0.23). This enhancement of the quantum yield can be explained by the restricted internal rotation of the densely packed biphenyl moieties, caused by the intermolecular steric hindrance. Interestingly, BP-PSQ showed a cotton effect (ECD signal) in the region of the absorption band and CPL in the region of the emission band with a mirror image between the BP-PSQ prepared from 16-2-16 L- and D-tartrate (Fig. 4c and d) even though it has no asymmetric atom in the molecule and chiral shape. The dissymmetry factors (g-factors) were estimated to be |7.8 × 10−4| and |5.6 × 10−4| for the ECD signals at 246 and 271 nm, respectively, and |2.1 × 10−4| for the CPL at 378 nm. These results indicate that in both the ground and excited states, the biphenyl moieties of BP-PSQ were chirally arranged as illustrated in the inset of Fig. 4c, that is, right-handed arrangement for the BP-PSQ prepared from 16-2-16 L-tartrate and left-handed arrangement for the ones prepared from 16-2-16 D-tartrate according to exciton chirality theory,9 which means that the gemini surfactant-based chirality was successfully transferred to BP-PSQ at the bonding network level. While longer aging time (3 d) during the sol–gel process caused an increase in the yield of BP-PSQ, the ECD intensity was decreased (Fig. S6, ESI). This suggests that as the BP-PSQ wall grows, the internal chiral order is gradually lost.


image file: d0cc02224a-f4.tif
Fig. 4 (a) UV-vis absorption, (b) fluorescence, (c) ECD and (d) CPL spectra of BS-BP (0.05 mM) and BP-PSQ (0.05 mM as biphenyl-bis(silsesquioxane) unit) prepared from 16-2-16 L- and D-tartrate in ethanol at 20 °C; path length = 1.0 cm (except for (b)). For (b and d), BS-BP and BP-PSQ were excited at 261 and 263 nm, respectively. The CPL spectra in (d) were fitted using Voigt functions and the original spectra are shown as dotted lines. Inset in (c) shows schematic illustration of chirally arranged biphenyl moieties of BP-PSQ.

In general, chirally arranged dye molecules in media tend to be disrupted by heating because in many cases, the chiral structures are formed and maintained by weak intermolecular forces such as van der Waals force, hydrogen bonds, and hydrophobic effect.10 In the present system, the chiral arrangement showed very little modification upon heating up to 70 °C (Fig. S7, ESI) as demonstrated by the ECD signal of the biphenyl moieties of BP-PSQ. Moreover, the chiral arrangement remained almost unaltered even after standing at 20 °C in the dark for a month (Fig. S8, ESI). The chiral arrangement of the biphenyl moieties is clearly stabilized into the inorganic polysilsesquioxane network although the mesoscopic chiral shape is lost.

The chirality of the polysilsesquioxane network moiety was also investigated. Fig. 5 shows infrared (IR) and VCD spectra of the freeze-dried BP-PSQ. The IR spectrum showed a complex large absorption band in the 1250–950 cm−1 spectral range, with peaks at 1045 and 1143 cm−1 (Fig. 5a, right), which are associated with the Si–O–Si asymmetric stretching vibration (νasSi–O–Si).11 This multicomponent broad band reveals the VCD signatures, which are mirror images of the two enantiomeric structures (Fig. 5b, right). These results suggest that the polysilsesquioxane network is also chirally arranged. Furthermore, a small, but clear bisignate Cotton effect (VCD signal) appeared around 1600 cm−1, related to the νC[double bond, length as m-dash]C stretching vibration of the phenyl rings, with a mirror image between the BP-PSQ prepared from 16-2-16 L- and D-tartrate (Fig. 5b, left), indicating the chiral arrangement of the biphenyl moieties. Importantly, the sign of the VCD signal, namely the direction of the chiral arrangement of the biphenyl moieties,12 is consistent with the results in the ECD spectra.


image file: d0cc02224a-f5.tif
Fig. 5 (a) IR and (b) VCD spectra of freeze-dried BP-PSQ prepared from 16-2-16 L- and D-tartrate. Inset in (b, left) shows schematic illustration of the chirally arranged biphenyl moieties of BP-PSQ.

In conclusion, we succeeded in inducing the chiroptical signals from organic dye-linked polysilsesquioxane through chirality transfer from supramolecular templates through a sol–gel transcription process. Although washing and sonication destroy the macroscopic twisted morphology, local chiral organization of the organic dye moieties inside the polymerized network was well preserved. This is the first example of an organic-linked polysilsesquioxane fabricated from an achiral precursor alone (without any chiral dopants) that shows obvious chiral optical signals (ECD, CPL, and VCD). These findings provide a new promising strategy for the design of organic–inorganic chiral hybrid materials. The details on the mechanism of the chirality induction from chiral self-assemblies to BP-PSQ polymerisation is under investigation.

All authors discussed the results, commented on the manuscript and contributed to the interpretation of the data. The gemini surfactant was synthesized by N. R. and H. Y. The preparation condition of BP-PSQ was established by N. R. and T. K. DSC was measured by N. R. STEM and SEM observations were carried out by N. R. and H. Y. XRD was measured by H. Y. and S. N. Molecular simulation was carried out by T. S. UV-vis absorption, fluorescence and ECD spectra were measured by N. R. and T. K. CPL spectra was measured by Y. O. τ was measured by K. Y. ϕ was measured by H. Y. VCD spectra were measured by T. B., K. Y. and N. R. H. I. and R. O. designed the study. S. N. and M. T. were involved in study design. N. R. drafted the manuscript and Y. O., H. I. and R. O. assisted in the preparation of the manuscript.

This work was supported by “Nippon Sheet Glass Foundation for Materials Science and Engineering” and “French-Japanese International Associated Laboratory, Chiral Nanostructures for Photonic Applications (LIA-CNPA)”.

Conflicts of interest

There are no conflicts to declare.

Notes and references

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0cc02224a

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