Figure 4 (a) CD and UV-vis spectra of the polymer composite films with different molecular weights of Azo polymers (entries 2, 8, 9). (b) Photographs of the corresponding composite films. Changes in the (c) CD spectra and (d) the maximum CD values of the CABu/PMMAzo composite films (entry 2).
The induced circular dichroism of the polymer films is slightly affected by the molecular weight of Azo polymers (entries 2, 8 and 9). Despite differences in molecular weight, all three polymers demonstrated strong CD signals (Figure 4a). And PAzoMA-1 film (M n = 4600 gmol−1) exhibited a slightly stronger CD intensity than the other polymers with varying M ns. This may be due to the decrease in the compatibility of the Azo polymer and CABu as well as the transparency of the polymer film as the molecular weight of the polymer increases (Figure 4b). Furthermore, the CABu induced film also showed good chiral memory properties. As presented in Figures 4c and 4d, the CD signal of the CABu/PMMAzo-1 composite film was essentially unchanged over two days. This implies that the well-organized supramolecular helical structures of Azo units in the film is stable at room temperature, which is helpful for the further regulation of the chiral film.
It is well-known that chiroptical switch of Azo-containing systems offers fascinating opportunities, because molecular or supramolecular chirality might be tunable by trans -cis process in a fully reversible manner (Figure 5a).32-34 Elegant chiroptical switches and photo-responsive polymer systems have been widely reported. In this system, CABu/PMMAzo film can act as carriers for Azo-based photochromic switch that undergoestrans -to-cis photoisomerization under irradiation with ultraviolet light. The results in Figures 5b and 5c show that the helical stacks of Azo units undergo an order-disorder-order transition with trans -cis -trans isomerization. Photo-switching of CABu/PMMAzo-1 film is, indeed, reversible, even after leaving the polymer film for several months, and there was nearly no fatigue effect on the sample after five cycles of alternating UV light irradiation (tans -to-cis ) and heating treatment (cis -to-trans ). The switch acts as a nanoscale energy converter to dynamically realize the chiroptical response of chiral signals without contact.