Figure 8 (a) FTIR spectra of D-Glu, PMMAzo-1 polymer and
D-Glu/PMMAzo-1 composite mixtures. (b) Magnified spectra in the range
from 2000 to 1000 cm-1. (c) 1H NMR
spectra of D-Glu, PMMAzo-1 polymer and D-Glu/PMMAzo-1 composite
mixtures. (d) Magnified spectra in the range from 2.2 to 2.0 ppm.
The chiral interaction between glucose and the achiral Azo polymer is
easier to be characterized by FTIR and NMR because glucose is the most
basic and well-defined repeating unit of cellulose. Figures 8a and 8b
show the FTIR spectra of D-Glu, PMMAzo-1 polymer and D-Glu/PMMAzo-1
composite polymer mixtures. Three different characteristic absorption
peaks of polymer mixtures were observed during the addition of D-Glu
(Figure 8b). Herein, strong absorption bands near 1750
cm-1 were observed, corresponding to the stretching
vibrations of carbon-oxygen (C=O) bonds. And the absorption band around
1600 cm-1 is characteristic of the benzene ring.
Additionally, asymmetric stretching vibration of the C-O-C bond was
observed around 1200 ~ 1250 cm-1.
After mixing the polymer with D-Glu, we noted slightly significant blue
shift and red shift of v(C=O) band and v(C-O-C) band, respectively.
Meanwhile, the characteristic absorption peak of the benzene ring
basically does not change in this case. These changes indicate that the
steric environment of the carbonyl group has changed before and after
D-Glu addition.
Furthermore, the 1H NMR spectra of D-Glu, PMMAzo-1
polymer and D-Glu/PMMAzo-1 composite mixtures were presented in Figures
8c and 8d. After mixing the polymer with D-Glu, pronounced upfield
shifts was observed for the protic hydrogens (C-H) on the achiral methyl
ester group near the chiral center of the D-Glu. Combining CD, FTIR and
NMR data, the chiral transfer can be attributed to be the result of
pseudochiral intermolecular C-H/O=C hydrogen bonds between D-Glu and the
carbonyl group of PMMAzo segments. As a result, the induction of
circular dichroism from achiral azobenzene-containing polymers through
complexation with CTA (regarded as D-Glu polymer) derivative, which
reflects the transfer of helicity and/or chirality and the imbalance of
left or right π-π stacks, is due to a vast number of intermolecular
C-H/O=C and C-H/π interactions between glucose repeating units and
side-chain Azo units (Scheme 2). This principle also applies to the
interaction between CABu and PMMAzo. However, this capability is
expected to be weaker with CABu and esters in PMMAzo due to the
suppression of C-H/O=C interactions by the bulkier n -butylate
groups and the mixture of n -butylate and acetyl groups.
Nevertheless, the interactions between chiral inducers and achiral
polymers still need to be deeply explored due to the infinite
intramolecular and intermolecular rotational freedom.