# Chiroptical Signal Amplification in Amorphous Siloxane Network by Asymmetric Ring Distortion

Yutaka Okazaki, Thierry Buffeteau, Naoya Ryu, Kyohei Yoshida, Ju-Yeon Jo, Kan Hachiya, Takashi Sagawa, Reiko Oda

► To cite this version:

Yutaka Okazaki, Thierry Buffeteau, Naoya Ryu, Kyohei Yoshida, Ju-Yeon Jo, et al.. Chiroptical Signal Amplification in Amorphous Siloxane Network by Asymmetric Ring Distortion. Journal of the American Chemical Society, 2025, 147 (31), pp.27552-27560. 10.1021/jacs.5c05168. hal-05361269

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Distributed under a Creative Commons Attribution 4.0 International License
Chiroptical signal amplification in amorphous
siloxane network by asymmetric ring distortion

Yutaka Okazaki¹*, Thierry Buffeteau², Naoya Ryu³, Kyohei Yoshida³,⁴, Ju-yeon Jo¹, Kan Hachiya¹, Takashi Sagawa¹ & Reiko Oda⁴,⁵*

¹ Graduate School of Energy Science, Kyoto University, Yoshida-Honmachi, Sakyo-ku Kyoto
606-8501, Japan

² Institut des Sciences Moléculaires (UMR5255 ISM), CNRS - Université de Bordeaux, 33405 Talence, France

³ Materials Development Department, Kumamoto Industrial Research Institute, 3-11-38 Higashimachi, Higashi-ku Kumamoto 862-0901, Japan

⁴ Univ. Bordeaux, CNRS, Bordeaux INP, CBMN, UMR 5248, F-33600 Pessac, France

⁵ WPI-Advanced Institute for Materials Research, Tohoku University, 2-1-1, Katahira, Aoba-Ku, Sendai 980-8577, Japan
ABSTRACT

Understanding the polymorphs and amorphous structures of silicon oxide is an essential issue not only for fundamental science but also for energy science and environmental science. In particular, the amplification mechanism of chiral signal in amorphous silicon oxide has drawn significant attention. Here, we explore how the chiroptical signal can be amplified in amorphous silicon oxide (silica) having a nano-scale helical structures. Our findings reveal that the observed vibrational circular dichroism (VCD) signals originate from the formation of symmetry-broken siloxane ring structures. Furthermore, we demonstrate that the amplification of siloxane VCD signals is driven by enthalpy-driven asymmetric ring distortion during volumetric shrinkage.

INTRODUCTION

Silicon oxides—among the most abundant materials on Earth and in cosmic space—have shaped human civilization for over 4000 years. Despite their ubiquity, sustainability, and technological potential, their amorphous structures remain enigmatic, with unresolved questions about their polymorphism and chiroptical properties due to their vast structural variability, which arises from a wide range of Si–O bond lengths, Si–O–Si angles, and O–Si–O angles. Investigating polymorphs and amorphous structures of silicon oxides is crucial, not only to uncover their fundamental properties, but also to explore their potential applications in generating the chiroptical signals. Such research holds promise for a various fields including telecommunications[1], optical communication[2], recording[3], enantioselective photosynthesis[4], photoelectric conversion[5,6], and spintronics[7,8]. The use of chiral molecular systems as
templates in the sol-gel reaction of alkoxysilane, first pioneered by Shinkai et al.[9], triggered extensive research into the synthesis of chiral silica nanomaterials and investigations of their chiroptical properties[10-14]. Several mechanisms have been proposed for generating chiroptical signals in silica, primarily based on central chirality concepts, such as chiral Si centers[15] and asymmetric tetrahedral SiO₄ units[16]. However, unlike well-studied chiral ordered structures such as quartz and chiral zeolites[17,18], no direct experimental evidence has yet clarified the structural origin of chiroptical signal in amorphous silica, let alone the mechanisms underlying their amplification. Here, we uncover the mechanism behind chiroptical signal amplification in amorphous silica during heating, as revealed by vibrational circular dichroism (VCD) measurements and quantum chemical calculation. Understanding the structural origins and amplification mechanism of chiroptical signals in amorphous materials paves the way for new design strategies in chiroptical materials – ones that do not rely on conventional ordered structure[11].

## RESULTS AND DISCUSSION

For this study, we selected nanometric silica helices (Sil-helix) synthesized via sol-gel transcription of amphiphilic self-assemblies. These helices exhibit well-defined three-dimensional chiral morphologies with precisely controlled handedness and dimensions such as helical pitch, helix diameter, and ribbon width (Figure S2a). As previously reported, gemini-type dialkyl diammonium amphiphiles with L- (and D-) tartrate anion (16-2-16 L- (and D-) tartrate, Figure S1) self-assemble into various cationic bilayer-based right-handed (RH-) and left-handed (LH-) supramolecular nano-assemblies. The morphologies of these assemblies can be finely tuned by controlling incubation time, e.g. 1-2 hours yields twisted ribbon, 2-3 days result in
helical ribbon, and over a week leads to tubular molecular assemblies[19-23]. These RH- (and LH-)
helical ribbon-like molecular assemblies of 16-2-16 L- (and D-) tartrate served as templates for
the sol-gel reaction of tetraethoxysilane (TEOS)[24-27]. This process led to the formation of helical
double walls of silica surrounding the ribbons on both sides[25-27]. When the 3-days-aged organic
molecular assemblies were used as templates (helical ribbons), the obtained **Sil-helix** was
calcined at 600, 900, 1000, and 1100 °C to eliminate the organic components, yielding **Sil-helix-600**, **Sil-helix-900**, **Sil-helix-1000**, and **Sil-helix-1100**, respectively. Transmission electron
microscopy (TEM) was used to investigate the morphology at the mesoscopic scale, while
vibrational circular dichroism (VCD) spectroscopy was employed for interatomic structural
analysis. In our previous study[27], we reported that such helical nanometric silica exhibits
distinct VCD signals corresponding to the transverse optical (TO) mode (around 1090 cm⁻¹) and
the longitudinal optical (LO) mode (around 1240 cm⁻¹) of Si-O-Si asymmetric stretching
vibration (νₐ Si-O-Si). Only a few examples of siloxane VCD signal, firstly reported by Y.
Kaneko et al.[28] and well summarized by H. Sato et al.[29], have been reported so far. Notably,
when calcined at 600 °C, the VCD dissymmetry factor (g = Δε/ε = ΔA/A) nearly doubled against
the one of uncalcined sample (**Sil-helix**), while the helical pitch decreased by ~15% from the
original. In the present study, as shown in Figure 1b, we observed that the helical morphology
was preserved upon heating up to 1000 °C while showing significant shrinkage (Figures S2). At
this temperature, the double silica walls fused into a single helical wall. While the bisignate VCD
signals observed at ~1090 cm⁻¹ (TO mode) and ~1240 cm⁻¹ (LO mode) were still present[27], the
g factor for the both modes drastically increased upon calcination (Figure 2a,b). In general,
chiroptical signal amplification is observed during the formation of “ordered structures” such as
chiral crystallization[30-34] chiral supramolecular organization and polymerization[35-37]. From this
perspective, the amplification of chiroptical signal in an amorphous structure is an uncommon
and intriguing phenomenon, warranting further investigation into its structural origin. Further
heating **Sil-helix** up to 1100 °C (above T_g) destroys the helical morphology, coinciding with the
total disappearance of VCD signals. A clear correlation between experimentally measured
|ΔA/A_max| areas for the ν_a Si-O-Si band (integrated over 1300-950 cm⁻¹ denoted as Σ|ΔA/A_max|)
and the size of helical nanometric silica, defined by its helical pitch, helix diameter, and ribbon
width (as modified through calcination at different temperatures) was observed in Figures 2c and
S4. The proposition for the origin of such chiroptical signals will be discussed in detail later.

<figure><img src="image_2.png" /><figcaption>**Figure 1.** Morphological evaluation of chirality of helical nanometric silica. (a) Synthesis scheme of helical nanometric silica from TEOS in an aqueous system. (b) TEM images of **Sil-helix**, **Sil-helix-600**, **Sil-helix-900**, **Sil-helix-1000**, and **Sil-helix-1100**.</figcaption></figure>
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 2.** Chirality on helical nanometric silica evaluated from chemical structural aspect. (a) VCD and IR spectra of **Sil-helix**, **Sil-helix-600**, **Sil-helix-900**, **Sil-helix-1000**, and **Sil-helix-1100**. All VCD spectra were normalized for absorbance peak of the $v_a$ Si-O-Si band equals to one. (b) Plots of helical pitch (black) and $g_{max}$-factor (red) versus calcination temperature. (c) Plots of $\Sigma|\Delta A/A_{max}|$ value versus helical pitch (since several positive and negative components appear in the VCD spectra for the $v_a$ Si-O-Si band, the $\Sigma|\Delta A/A_{max}|$ factor over 1300-950 cm$^{-1}$ range was calculated).</figcaption></figure>
The siloxane VCD signals of amorphous **Sil-helix** do not originate from the chiral crystalline (e.g., quartz) domains or from the chiral molecules, that was carefully confirmed by nano beam electron diffraction, powder X-ray diffraction, and IR spectra (Figure S5-S7). To investigate the chiral source in silica synthesis, we systematically synthesized various silica samples by sol-gel reaction of TEOS using the same cationic chiral molecular assemblies (16-2-16 L- (and D-) tartrate) as templates. However, by varying the aging duration, we controlled the morphology of the assemblies before silica formation. Fragmented chiral bilayers were obtained when the incubation time was only 10 minutes, whereas tubular aggregates formed after 4 months of incubation. These were directly used as template for silica synthesis resulting in **Sil-fragment** (ill-defined structure with no periodic morphology) and **Sil-tube** (double-wall tubular structure) (Figure 3a). Despite their different morphologies, all these silica nanostructures exhibited bisignate VCD signals at around $1090 \text{ cm}^{-1}$ (Figure 3b). However, their g-factor varied significantly. **Sil-helix**, which possesses a clear helical (morphologically chiral) structure, exhibited the highest g-factor, whereas the g-factors of **Sil-fragment** and **Sil-tube** were about the half the g-factor of **Sil-helix**. To further investigate the role of chiral sources, we synthesized silica using aqueous solutions of L- (or D-) tartaric acid (0.1 mM) mixed with TEOS, but without cationic molecular assemblies as template. The resulting gel-like materials (RH- (or LH-) **Sil-tart**, Figure 3a) showed much lower VCD signals ($g \approx 0.5 \times 10^{-4}$) at $\sim 1090 \text{ cm}^{-1}$ (Figure 3b). Furthermore, when hydrobromic acid (HBr), a completely achiral acid, was used instead of tartaric acid, the resulting silica (**Sil-HBr**) showed no VCD signal (Figure S9). This indicates that tartaric acid does induce some chirality by generating asymmetric Si centers during silica formation (**Sil-tart**),[15,16] but its effect on the final VCD signals intensity is much weaker than chiral molecular assemblies. The presence of a chiral molecular assembly-template as nano-
template during the sol-gel process is crucial for the amplification of siloxane VCD signals after calcination (3-5 times increase upon heating up to 900°C). In contrast, no significant amplification was observed in **Sil-tart** upon calcination at 900 °C (**Sil-tart-900**). This strongly suggests that the mesoscopic chiral information embedded in the supramolecular nano-templates during sol-gel transcription is crucial for the amplification of siloxane VCD signals upon heating.

<figure><img src="image_6.png" /><figcaption>b</figcaption></figure>

<figure><img src="image_7.png" /></figure>
**Figure 3.** Effect of organic templates on siloxane VCD generation and its amplification. (a) Illustrations of various templates and silica samples. TEM images of **Sil-fragment**, **Sil-helix**, **Sil-tube** and **Sil-tart**. (b) VCD and IR spectra of **Sil-fragment**, **Sil-helix**, **Sil-tube**, and **Sil-tart** before (blue lines) and after calcination at 900 °C (orange lines). Solid and dash lines indicate spectra of the LH- and RH- silica samples and the silica samples synthesized with D- and L-tartaric acid. All VCD spectra were normalized for absorbance peak of the $v_a$ Si-O-Si band equals to one.

To investigate the origin of the siloxane VCD signal and its amplification, we focused on the symmetry of siloxane ring structures. Amorphous silica is often conceptualized as a network polymer composed of various siloxane rings, mainly 3- to 9-membered rings.[38-40] It is well known that even achiral cyclic compounds can exhibit chiral conformations, such as twist-boat and half-chair of cyclohexane.[41,42] This suggests that mirror symmetry-breaking within siloxane ring structure could be the structural origin of siloxane chirality. To verify this hypothesis, we performed density functional theory (DFT) calculations to examine siloxane ring symmetry. We first selected 6-membered siloxane rings, known to be the most prevalent siloxane rings in silica[38-40]. The geometry of these rings was fully optimized in gas phase at the B3LYP/6-31G(d) level, and we calculated their potential energies, vibrational frequencies, IR and VCD intensities. As shown in Figure 4a, the 6-membered siloxane ring exhibited two optimized chiral conformations (**6r<sub>4</sub>-A** and **6r<sub>4</sub>-B**) with $D_2$ symmetry, akin to the twist-boat form of cyclohexane. The calculated VCD spectra of these chiral conformers showed clear mirror spectral patterns (Figure 4b). Vibrational motions of **6r<sub>4</sub>-A** for various Si-O-Si asymmetric stretching ($v_a$ Si-O-Si) modes are visualized in Figure S10 and Supplementary Video 1-15. To further probe the chiral
origin of siloxane VCD signals, we isolated six Si-O-Si substructural fragments of **6r₄-A** (named **6r₄-A1**, **-A2**, **-A3**, **-A4**, **-A5**, and **-A6**) and calculated their VCD spectra individually (Figure S11). For simplicity, terminal atoms were replaced with hydrogen. As shown in Figure S11, all six partial structures exhibited positive VCD signals for the $v_a$ Si-O-Si region, whereas the corresponding partial structures extracted from **6r₄-B** (named **6r₄-B1**, **-B2**, **-B3**, **-B4**, **-B5**, and **-B6**) showed negative VCD signals (Figure S12). These results confirm that siloxane chirality can be retained even in such simple Si-O-Si-based structures. Remarkably, the **6r₄-A** (and **6r₄-B**) ring exhibited 150-300 times larger VCD signals at newly appeared vibrational modes compared to their isolated partial structures. **6r₄-A** exhibited $|g| = 3.1 \times 10^{-4}$ at 1088 cm⁻¹ and $2.4 \times 10^{-4}$ at 1071 cm⁻¹ whereas the partial structures only showed $|g| = 1.5 \times 10^{-6}$ at 1091 and 1090 cm⁻¹ and $1.0 \times 10^{-6}$ at 1108 cm⁻¹. These results demonstrate that ring formation imposes structural constraints that stabilize chiral conformation and enhance siloxane VCD signals. To systematically evaluate the VCD sign dependence, we calculated VCD signals of model structures at different Si-O-Si bond angle ($\theta$) dihedral angle and Si-O-Si-H dihedral angle ($\omega$) (Figure S13). The results revealed that the signs of the VCD signal drastically changed by varying $\theta$ and $\omega$ values whereas no single $\theta$ and $\omega$ angles consistently determine the VCD sign.

The sign of VCD signal is governed by a combination of $\theta$ and $\omega$ values. While the partial **6r₄-A** or **-B** structures consistently showed positive or negative signals, respectively, the **6r₄-A** (or **6r₄-B**) ring structure exhibited bisignate signals due to the emergence of new mirror symmetry-broken vibrational modes. At 1071 cm⁻¹, a new VCD band appeared that exhibited the opposite sign compared to simple Si-O-Si vibrational modes, indicating a unique chiral contribution from ring formation.
a

<figure><img src="image_8.png" /></figure>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

b

<figure><img src="image_11.png" /></figure>

**Figure 4.** Chirality on siloxane ring structure. (a) Top (top), side (middle), and front (bottom) view of two optimized conformations of 6-membered siloxane ring structure. **6r<sub>4</sub>-A** and **6r<sub>4</sub>-B** are chiral conformers. (b) Calculated IR (top), VCD (middle), and g-factor spectra (bottom) of **6r<sub>4</sub>-A** (solid line) and **6r<sub>4</sub>-B** (dashed line).

In addition to the **6r<sub>4</sub>** conformers, other metastable conformers of the 6-membered ring were identified as global and local minima of the potential energy and shown in Figure 5a. While the conformation **6r<sub>1</sub>** is mostly planar, the other conformations **6r<sub>2</sub>**, **6r<sub>3</sub>** and **6r<sub>4</sub>** are mirror symmetry-broken chiral conformations. By carefully comparing with various ring structures in crystalline
silicon oxides[43], we found that the most stable and highly-folded ring structure, **6r**_4_, as
described above exhibits similar conformation (chiral *D*₂ symmetry) to the 6-membered ring
found in chiral β-quartz form[44] (Figure S14, Table S1-S4). Instead of the other approaches to
quantitatively characterize ring shapes[40,44-48], we used circumsphere volume (detailed in
Supplementary Information) as a suitable parameter to express the volume shrinkage of every
membered siloxane rings. Interestingly, we observe a strong correlation between potential
energy, relative to the most planar conformer (**6r**₁), occupied volume (*V*), defined using
circumsphere volume calculations, and summed dissymmetry factor (Σ|Δε/εmax|). The
energetically stable conformer shows a smaller *V* and larger Σ|Δε/εmax| (Figure 5b), and
subsequently, *V* and Σ|Δε/εmax| show significant correlation (Figure 5c), consistent with
experimental data (Figure 2c and S4). Other 5- to 9-membered siloxane ring structures also
showed that the highly-folded chiral conformers are the most energetically stable and can exhibit
the largest VCD signals (Figures S13-S16). These results strongly support the idea as follows: In
contact with mesoscopic chiral platform during sol-gel transcription, metastable mirror-
symmetry broken siloxane ring conformers are initially formed. Upon heating up to 1000 °C,
these rings then undergo an enthalpy-driven conformational transition to their most
thermodynamically stable form. However, 3- and 4-membered rings do not exhibit significant
difference in occupied volume or VCD signals (Figures S11-S12). This is due to the small
energy gain associated with transition from a planar to a folded structure as previously
reported[49]. As shown in Figure S24, the larger the siloxane rings, the larger gain in potential
energy and circumsphere volume shrinkage. Meanwhile, the 6-membered ring exhibits an
exceptionally large Σ|Δε/εmax| value (Figures 5d-5f). Such an exceptional VCD signal
amplification is attributed to the most stable 6-membered ring conformer (**6r**₄) having the
smallest occupied volume while having the higher symmetry ($D_2$) compared to the $C_1$ and $C_2$
symmetry as found in other ring sizes.

<figure><img src="image_12.png" /><figcaption>**Figure 5.** Correlation between chiral signal and volume shrinkage studied by DFT calculation. (a) Three-dimensional structures of each 6-membered ring conformers. All the atomic positions of Si (blue) and O (red) in each ring are summarized in the Table S12-15. (b) Plots of circumsphere volume (black circles) and $\Sigma|\Delta\theta/\epsilon_{max}|$ value (red circles) versus relative potential</figcaption></figure>
energy. (c) A $\Sigma|\Delta\epsilon/\epsilon_{max}|$ value versus circumsphere volume. Inset illustrations in (b) and (c) indicate 6-membered siloxane ring structures and its circumspheres. (d-f) Plots of largest $\Sigma|\Delta\epsilon/\epsilon_{max}|$ value at each ring number versus ring number (d), potential energy gain (e), and circumsphere volume shrinkage (f), respectively. Potential energy gains of each ring number were calculated as potential energy differences between the most stable and the most planer ring conformers. Number written in the solid circles indicates siloxane ring numbers.

Based on the experimental observation and the calculation results presented above, we now explore how silica helix shrinkage during calcination results in VCD signal amplification. When silica materials are heated, the shrinkage of the silica occurs (Figure 2b) along with the several modifications of the intimate silica structures. First, a condensation reaction between silanol groups (Si-OH + HO-Si $\rightarrow$ Si-O-Si + H₂O, Figure 6e) occurs, resulting in a weight loss as observed in **Sil-helix** during heating, with the disappearance of silanol groups confirmed by Raman and IR spectroscopies (Figures S25 and S26). To quantify primary structural modifications in the siloxane network, we performed thermogravimetry analysis (TGA) and $^{29}$Si DD/MAS NMR spectral measurement. The TGA showed that the weight loss of silica, adjusted to be 100 wt% at 600 °C, was 1.21 wt% for 600-900 °C, 0.06 wt% for 900-1000 °C, and 0.05 wt% for 1000-1100 °C, respectively (Figure 6a). Assuming all the weight loss above 600 °C was derived from the condensation reaction (Si-OH + HO-Si $\rightarrow$ Si-O-Si + H₂O) occurred in the silica skeleton, the molar fractions of the released H₂O/SiO₂ units were only 0.1090 (mol/mol) for 600-900 °C, 0.0045 (mol/mol) for 900-1000 °C, and 0.0045 (mol/mol) for 1000-1100 °C, respectively. The details of the calculation method are provided in the Supplementary Information. No significant change in the primary structure of silica above 900 °C was confirmed
by $^{29}$Si DD/MAS NMR spectral measurements (Figures S27-S28). This suggests that such minor structural changes alone cannot account for the drastic amplification of siloxane VCD signal or the significant shrinkage of helical dimensions, i.e. helical pitches decrease from 54.4 nm (600 °C) to 35.1 nm (1000°C) during calcination.

On the other hand, DFT calculations indicate that enthalpy-driven asymmetric ring distortion can occur without weight loss (Figures 5b and 5c). This conformational change as illustrated in Figure 6f involves volume shrinkage at molecular scale and structural reorganization of the siloxane rings. If this indeed occurs in the **Sil-helix** system, the accumulation of the molecular-scale volume shrinkage should lead to an increase in the density of silica and to shrinkage of the nano-scale morphology of the silica helices. Indeed, the densification of silica leads to the rigidification of Si-O-Si bonds, which was observed in the $v_a$ Si-O-Si band shift (from 1086.7 cm$^{-1}$ to 1108.9 cm$^{-1}$) upon heating to 1000 °C due to the increase of the spring constant of the harmonic oscillator (Figure 6b). Notably, a significant wavenumber increase indicating densification of siloxane networks was observed upon heating from 900 °C to 1000 °C. In addition, the peak position of $v_a$ Si-O-Si band ($v_{max}$) showed good correlation with VCD intensities (Figure 6c). Interestingly, this trend is also observed for **Sil-fragment** and **Sil-tube** (Figure S29), reinforcing the connection between siloxane ring distortion and VCD signal amplification. Upon heating up to 1100 °C, the peak position of $v_a$ Si-O-Si band decreases from 1108.9 cm$^{-1}$ to 1097.3 cm$^{-1}$ (Figure 6c). This can be explained by an entropy-driven conformational change known as enthalpy relaxation (Figure 6g), which is commonly observed with polymers$^{[50,51]}$.
<figure><img src="image_13.png" /><figcaption>**Figure 6.** Mechanistic studies of siloxane VCD amplification. (a) TGA thermogram of **Sil-helix** measured under air. The entire thermogram from 20 °C up to 1100 °C is shown in supplementary information. (b) Plots of $v_{max}$ versus calcination temperature. (c) Plots of g-factor versus $v_{max}$. (d) Schematic illustration of morphological change in nanometric scale. (e-g) Schematic illustrations of the condensation reaction between silanol groups (e), the distortion of siloxane rings by enthalpy-driven conformational change (f), and the racemization of siloxane rings by entropy-driven conformational change (g).</figcaption></figure>

CONCLUSION
In this study, we explore the origin and amplification mechanism of the chiroptical signal
observed in the $v_a$ Si-O-Si band of nanometric helical silica, particularly during heating up to
1000 °C. Combining the experimental results and DFT calculations, we demonstrate that the
observed siloxane VCD signals originate from mirror symmetry-breaking within siloxane rings.
We find that chiral conformers of siloxane rings exhibit lower potential energy than their planer
counterparts. Consequently, the key distinction between VCD-inactive **(Sil-HBr)** and the VCD-
active **(Sil-fragment, Sil-helix, and Sil-tube)** silica samples is that VCD-inactive silica contains
a racemic mixture of both enantiomers whereas VCD-active silica contains a higher proportion
of one chiral conformer over the other. The symmetry-breaking mechanism originates from the
sol-gel reaction of TEOS, catalyzed by supramolecular nano-templates of chiral molecules,
whose morphologies at mesoscopic level encode chiral information. The amplification of
siloxane VCD signals during the heating process is attributed to enthalpy-driven asymmetric ring
distortion, where molecular-scale volume shrinkage and ring distortion collectively manifest as
morphological shrinkage of **Sil-fragment, Sil-helix, and Sil-tube** at the nanometric scale. For
the first time, we establish a correlation between siloxane ring size (3- to 9-membered ring) and
amplifiable chiral signal. Notably, 6-membered ring, which constitute the largest proportion of
siloxane ring in silica, exhibit exceptional chiral signal amplification due to their higher
symmetry and energy gain during shrinking. These findings provide new insights into chirality
transmission from the molecular to mesoscopic scale and reveal structural relaxation behaviors in
amorphous polymer materials, paving the way to the design of new types of optically active
filters.

METHODS
**IR and VCD spectral measurements.** KBr disks were prepared by mixing the powder of silica samples (0.2-0.3 mg) with KBr powder (120 mg) and pressing at a pressure of 10 ton/cm² in vacuo. The infrared and VCD spectra were recorded with a ThermoNicolet Nexus 670 FTIR spectrometer equipped with a VCD optical bench[52]. In this optical bench, the light beam was focused on the sample by a BaF₂ lens (191 mm focal length), passing an optical filter (depending on the studied spectral range), a BaF₂ wire grid polarizer (Specac), and a ZnSe photoelastic modulator (Hinds Instruments, Type II/ZS50). The light was then focused by a ZnSe lens (38.1 mm focal length) onto a 1x1 mm² HgCdTe (ThermoNicolet, MCTA* E6032) detector. IR absorption and VCD spectra were recorded at a resolution of 4 cm⁻¹, by coadding 50 scans and 24000 scans (8h acquisition time), respectively. Since the sample preparation may induce orientation of the silica samples, vibrational linear dichroism (VLD) spectra were systematically measured before each VCD experiment. Only KBr disks exhibiting very low or no VLD were used for VCD experiments. The KBr disks of powdered materials were placed on a rotating sample holder and the VCD spectra were obtained with the KBr disks rotated at four angles around the light beam axis (0°, 45°, 90° and 135°). The VCD spectra of the two enantiomers of silica samples were obtained by averaging the four spectra. To eliminate spectral artefact of the optical setup, baseline corrections of the VCD spectra were performed by subtracting the two VCD spectra of enantiomers, with division by two. Raw VCD spectra without baseline corrections were shown in Figure S36 and S37. The photoelastic modulator was adjusted for a maximum efficiency in the mid-IR region at 1400 cm⁻¹. Calculations were performed via the standard ThermoNicolet software, using Happ and Genzel apodization, de-Haseth phase-correction and a zero-filling factor of one. Calibration spectra were recorded using a birefringent
plate (CdSe) and a second BaF₂ wire grid polarizer, following the experimental procedure previously published[53].

**Quantum chemical calculations.** The geometries, potential energies, vibrational frequencies, IR and vibrational circular dichroism (VCD) intensities were calculated by quantum chemical approaches. All calculations were carried out in the gas phase by Gaussian 16[54]. VCD calculations were performed using the method developed by Cheeseman et al. as implemented in Gaussian16's VCD package[55]. In order to evaluate the symmetry of siloxane rings, various (3- to 9-membered) siloxane ring compounds without having hydroxyl groups were used as model structures. The geometries of 3- to 9-membered ring structures were fully optimized at the B3LYP/6-31G(d) level. The geometries of various metastable conformers were also calculated at the same level. All potential energies of siloxane ring conformers were calculated in the single ring condition with no packing effect. Taking into account of the overestimation of vibrational frequency determined by the B3LYP/6-31G(d) level, the calculated frequencies were scaled by 0.9614[56,57].

**Calculation of occupied volume of siloxane ring structures.** The circumsphere volume of each siloxane rings were calculated and were used as the occupied volume of them. The detail calculation method was described as follows: The distances from the center of gravity of ring structure to the Si atoms (r) were calculated by the equation (1), where x, y, and z are the atomic position coordinates of the Si atom. The maximum distance (rmax) was selected for the calculation of circumsphere volume. The circumsphere volume (V) was calculated by the equation (2), where rSi is atomic radius of Si (1.11 Å). All the atomic position coordinates and the calculated circumsphere volumes were summarized in the Table S1-S28.

$$r = \sqrt{x^2 + y^2 + z^2} \quad \text{equation (1)}$$
$$V = \frac{4\pi(r_{max}+r_{si})^3}{3} \qquad \text{equation (2)}$$

## ASSOCIATED CONTENT

### Supporting Information.

The following files are available free of charge.

Materials details, synthesis details, methods (TEM, NBED, Zeta potential, Raman, TGA, NMR) and supplementary Figures and Tables (PDF)

Supplementary videos about vibrational motions of 6r₄-A (MP4)

## AUTHOR INFORMATION

### Corresponding Author

*E-mail: reiko.oda@u-bordeaux.fr

*E-mail: okazaki.yutaka.8c@kyoto-u.ac.jp

### Author Contributions

Y.O. synthesized chiral surfactants and silica samples. Y.O. and N.R. carried out TEM observations. N.R. carried out ζ potential analysis. Y.O. and K.Y. prepared KBr pellets for IR and VCD spectral measurements. T.B. measured IR, VCD and Raman spectra. Y.O. carried out DFT calculations and Y.O., T.B., J.J., and K.H. analyzed the calculated data. Y.O., T.B. and R.O. designed the study and wrote the paper. All authors discussed the results, commented on the manuscript, and contributed to the interpretation of the data.

### Notes

The authors declare no competing interests.
ACKNOWLEDGMENT

This work was supported by the France-Japan International Associated Laboratory on Chiral
Nanostructures for Photonic Applications (LIA-CNPA), and on Chiral Field-Matter Interaction
(IRP-ChiFiMI), the JSPS Overseas Research Fellowship, the JSPS KAKENHI Grant Number
19K15376, the JSPS Core to core Grant Number JPJSCCA20250005, Idex Bordeaux (Research
Program GPR LIGHT), the ANR under the France 2030 program, with reference ANR-23-
EXLU-0004, PEPR LUMA TORNADO, and the Center National de la Recherche Scientifique
(CNRS). We thank T. Kiyomura from Institute for Chemical Research, Kyoto University for the
nano beam electron diffraction measurements, which was supported by Kyoto University Nano
Technology Hub in "Nanotechnology Platform Project" sponsored by the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan, Grant number A-19-KT-0002. We are
grateful to Dr. T. Oishi from Faculty of Engineering, Kumamoto University for the $^{29}$Si
DD/MAS NMR spectral measurements, and to T. Kajiwara from NETZSCH Japan K.K. for
TGA.

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