Photoinduced conformational changes of azoaromatic polyaspartates containing octadecyl side chains

Copolymers of p‐(phenylazo)benzyl‐L‐aspartate and n‐octadecyl‐L‐aspartate exist as right‐ and left‐handed α‐helices in solution at 25°C depending on the copolymer composition: the reversal of helix sense from a right‐ to left‐handed one occurs with increasing the azobenzene content. The α‐helices of the copolymers are very sensitive to trifluoroacetic acid (TFA), and are converted into random coil below 2.0% of TFA. Among the copolymers, the copolymer containing 47% azobenzene groups is unique since it exhibits a TFA‐induced conformational change from right‐handed α‐helix to random coil via left‐handed α‐helix. Upon UV light irradiation at 25°C, the copolymers containing 68 and 89% azobenzene groups caused the reversal in helix sense from a left‐ to right‐handed one. The conformations of the copolymers were dependent on temperature, mostly right‐handed and left‐handed α‐helices at lower and higher temperatures, respectively. On this basis, the copolymer containing 47% azobenzene groups could be made to undergo a photoinduced helix reversal at high temperatures.