It is worth noting that the signals in the VCD spectra of (
S)- and (
R)-HthBta-OH COFs were inverted compared to those of chiral Hth monomers, which intrigued us to further elucidate the formation process of (
S)- and (
R)-HthBta-OH COFs. Toward this end, the synthesis of (
S)-HthBta-OH COF as a representative example was undertaken in the identical solvothermal conditions with varied reaction times. The products were collected by filtration, washed with 1, 4-dioxane and THF, and then subjected to PXRD, VCD, and ECD measurements. As shown in
Fig. 3d, the PXRD pattern of the product obtained in 15 min exhibited a set of diffraction peaks arising from the (
S)-Hth monomer and two very weak peaks in the smaller angle region, indicating the incompleteness of Schiff-base reaction and the amorphous nature of (
S)-HthBta-OH COF. When the reaction time was extended to 30 min, the peak intensity of (
S)-Hth significantly decreased, while those of (
S)-HthBta-OH COF increased. Notably, the (
S)-Hth monomer completely disappeared after 45 min, whereas medium-strong diffraction peaks at 3.5°, 7.0° and 26.4° were observed, indicative of an amorphous-crystalline transformation in (
S)-HthBta-OH COF. The crystallinity of (
S)-HthBta-OH COF further increased in 75 min, which was comparable to that obtained in 72 h, revealing the rapid formation of (
S)-HthBta-OH COF, which was drastically faster than the conventional hydrazone-linked COFs [
12]. To gain deeper insights into the chiral signal change from monomers to chiral COFs, we conducted the time-dependent VCD measurements. As illustrated in
Fig. 3e, the VCD spectrum of (
S)-Hth exhibited three intense positive peaks at 1645, 1614 and 1526 cm
−1, which were ascribed to the C=O stretching vibration, N-H bending vibration of -NH
2 group, and the skeleton vibration of the aromatic ring, respectively. The intensities of the three positive peaks continuously decreased as the reaction times were kept at 15 and 30 min. These three positive peaks disappeared after 45 min, whereas new negative peaks at 1616, 1526 cm
−1 arose, indicating the complete consumption of (
S)-Hth and the formation of (
S)-HthBta-OH COF. In particular, the VCD spectrum of (
S)-HthBta-OH COF obtained in 75 min (similar to the one synthesized in 72 h) displayed three obvious negative peaks at 1652, 1616, 1526 cm
−1, corresponding to the C=O stretching vibration, the C=N stretching vibration, and the aromatic ring skeleton vibration of the resulting chiral COF. These VCD results were in line with the PXRD and IR analyses (Fig. S4 in Supporting information). Remarkably, the VCD spectra revealed a unique VCD chiral signal inversion from the positive Cotton effect of (
S)-Hth monomer to the negative Cotton effect of (
S)-HthBta-OH COF, which has never been documented in chiral COFs. Given that the VCD chiral signals could be profoundly affected by the conformations of chiral compounds [
37,
38], we postulated that such a rare chiral signal inversion from monomer to COF could be attributed to the conformation change of (
S)-Hth moieties in the resulting (
S)-HthBta-OH COF. While the conformation change of (
S)-Hth moieties was due to the
π-π stacking and hydrogen bonding interactions between the adjacent COF layers. Moreover, the time-dependent synthesis of (
S)-HthBta-OH COF was monitored using ECD spectroscopy (
Fig. 3f). The ECD spectra of the products obtained from 15 min to 72 h presented strong positive peaks centered at 416, 419, 421, 424, 426 nm, respectively. Such bathochromic shifts could be due to the formation of the more extended
π-systems in (
S)-HthBta-OH COF.