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.