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.