Figure 5 (a) The isomerization of Azo units. (b) Changes in CD spectra and (c) the maximum g CD values of the first Cotton band of the CABu/PMMAzo-1 composite films (entry 2) during 365 nm light irradiation and heating-cooling treatment.
We further analysed the mechanism of molecular interaction and macromolecular geometry during chiral induction using NMR and FTIR instruments. The transfer of helicity and/or chirality, which induces left-right imbalance, is attributed to the vast amount of intermolecular C-H/π interactions between Azo and CTA. Since CABu is a mixture randomly containing methyl and propyl groups, we used CTA with a relatively well-defined repeating unit as the chirality inducing scaffolding (Figure 6a). The FTIR spectra were first recorded on a TENSOR-27 spectrophotometer using KBr as tablet. As presented in Figure 6b, numerous and dense absorption bands near 1600 cm-1were observed, corresponding to the benzene ring in PMMAzo polymers. After irradiation for 3 min with 365 nm UV light, we noticed a slight decrease in the absorption band of the benzene ring, which can be attributed to the trans -to-cis isomerization of Azo units. Furthermore, changes in NMR spectra of the CTA, PMMAzo and the CTA/PMMAzo composite mixtures were investigated (Figures 6c and 6d). The protic hydrogen atoms on the benzene ring are mainly located at 6.9 ppm and 7.8 ppm (Figure 6c). The proton hydrogen atoms of benzene moved before and after CTA addition. Meanwhile, clear shift was observed for the protic hydrogens (C-H) on the achiral methyl ester group near the chiral center of the glucose residue (Figure 6d), suggesting that the chirality transfer from celluloses to achiral PMMAzo polymers proceeds through the intermolecular C-H/π interaction. The chirality transfer mechanism are also reflected by the emergence of CD signals. Accordingly, this principle is also applicable to the chiral interaction between the CABu inducer and the achiral PMMAzo polymer.