Molecular design and synthesis of copolymers with large photoinduced birefringence

In order to achieve both large photoinduced birefringence (ΔnPI) and good long-term stability, we designed and synthesized four series of side-chain-type copolymers consisting of a photoresponsive azo moiety and a long π-conjugated moiety. Chemical and physical characterization, such as the estimation of ΔnPI and its dependence on the copolymerization ratio of the two functional moieties, have been compared. Consequently, fairly large value (observed highest value was 0.244) and superior stability of ΔnPI have been achieved in a copolymer consisting of the 2-[4-(4-cyanophenylazo)phenoxy]ethyl moiety and the 2-[2-methyl-4-(4-phenylazophenylazo)phenoxy]ethyl moiety at a 50:50 molar ratio (PCDY50). Furthermore, an unusual dependence of ΔnPI on the copolymerization ratio has been found in some copolymer series. The dependence has been phenomenologically explained by assuming the formation of a supramolecular assembly between the different moieties, that might induce molecular cooperative photo-alignment resulting in a large macroscopic ΔnPI. In addition, durability of the copolymer with respect to ΔnPI has been examined using PCDY50. As a result, it was confirmed that the recording and erasing process could be repeated more than 105 times at certain optical conditions.