Chiroptical Signal Amplification in Amorphous Siloxane Network by Asymmetric Ring Distortion

Abstract

Understanding the polymorphs and amorphous structures of silicon oxide is an essential issue not only for fundamental science but also for energy and environmental science. In particular, the amplification mechanism of chiroptical signals in amorphous silicon oxide has drawn significant attention. Here, we explore how the chiroptical signal can be amplified in amorphous silicon oxide (silica) having nanoscale 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 an enthalpy-driven asymmetric ring distortion during volumetric shrinkage.

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.

2

Chirality on helical nanometric silica evaluated from a 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 the absorbance peak of the ν_a Si–O–Si band equal to one. (b) Plots of helical pitch (black) and g _max-factor (red) versus calcination temperature. (c) Plots of ∑|ΔA/A _max| value versus helical pitch (since several positive and negative components appear in the VCD spectra for the ν_a Si–O–Si band, the ∑|ΔA/A _max| factor over 1300–950 cm^–1 range was calculated.

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 dashed 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 the absorbance peak of the ν_a Si–O–Si band equal to one.

4

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

5

Correlation between chiral signal and volume shrinkage studied by DFT calculation. (a) Three-dimensional structures of each 6-membered ring conformer. All of the atomic positions of Si (blue) and O (red) in each ring are summarized in Tables S15–S18. (b) Plots of circumsphere volume (black circles) and ∑|Δε/ε _max| value (red circles) versus relative potential energy. (c) ∑|Δε/ε _max| value versus circumsphere volume. Inset illustrations in (b) and (c) indicate 6-membered siloxane ring structures and their circumspheres. (d–f) Plots of the largest ∑|Δε/ε _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 planar ring conformers. Numbers written in the solid circles indicate siloxane ring numbers.

6

Mechanistic studies of siloxane VCD amplification. (a) TGA thermogram of Sil-helix measured under air. The entire thermogram from 20 up to 1100 °C is shown in Figure S31. (b) Plots of ν_max versus calcination temperature. (c) Plots of g-factor versus ν_max. (d) Schematic illustration of morphological change on the 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).