Design and synthesis of a novel azobenzene-containing polymer both in the main- and side-chain toward unique photocontrolled isomerization properties

Azobenzene (Azo)-containing polymers have been widely exploited due to their unique reversible trans-cis photoisomerization properties driven by UV/Vis or all visible light. Herein, the copolymer AzoF8-alt-Azo was designed and synthesized, which contained Azo units both in the main- and side-chain. The -* band maximum absorption of the side-chain Azo (s-Azo) trans-isomer locates at 360 nm (UV region), and the n-* band maximum absorption is near 450 nm (Vis region). The -* band maximum absorption of the main-chain Azo (m-Azo) trans-isomer locates at 425 nm (Vis region), and the n-* band maximum absorption is near 520 nm (Vis region). The separation of absorptions of the -* bands of the trans-isomers and the n-* bands of the cis-isomer of the s- and m-Azo units endows the possibility to control the photoisomerization behaviour of different Azo units in the main- and side-chain. The structural changes were achieved upon alternating photoirradiation in the order of 365 nm light (cis-form of s-Azo and trans-form of m-Azo, SC-MT), 435 nm light (trans-form of s-Azo and cis-form of m-Azo, ST-MC), and 546 nm light (trans-form of s- and m-Azo, ST-MT), which were monitored by UV-Vis spectroscopy. This indicates that three forms, SC-MT, ST-MC and ST-MT in same polymer chain can be successfully obtained by the above strategy. Furthermore, upon the irradiation in the order of 313 nm, 405 nm, 435 nm and 536 nm light, four forms, ST-MT, SC-MT, SC-MC and ST-MC, were successfully achieved in the same AzoF8-alt-Azo chain. These interesting findings will not only expand the photoisomerization behaviours of Azo-containing polymers, but also give cues for the design of novel optical materials.