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.