Photoinduced cooperative molecular reorientation on azobenzene side-chain-type copolymers

In order to improve the value of photoinduced birefringence and the long-term stability of azobenzene copolymer, two series of azobenzene side-chain-type copolymers consisting of two repeating units, one bearing cyanoazobenzene (or cyanoazopyridine) and the other bearing a long π-conjugated chromophore (bisazobenzene), have been synthesized and compared with respect to their photoinduced birefringence characteristics. Consequently, a fairly large value (observed highest value was 0.244) and superior stability (initial relaxation value was less than 0.2%) of photoinduced birefringence have been achieved. Furthermore, unusual behavior of the photoinduced birefringence depending on the copolymerization ratio of the two chromophores has been found, i.e., a sharp peak of the photoinduced birefringence has been observed at a copolymerization ratio of around 50:50 mol%. This phenomenon could be attributed to mutual promotion of the photoinduced molecular reorientation process, that is cooperative molecular motion, taking place at a specific stoichiometric composition ratio of the two chromophores.