Synthesis and characterization of main‐chain,second‐order,nonlinear optical polyurethanes with isolation moieties and zigzag structures

In this study, two main‐chain second‐order nonlinear optical (NLO) polyurethanes were successfully prepared with indole‐based chromophores. The introduced phenyl isolation group and the continuous zigzag polymer backbone were found to be helpful for effectively decreasing the intermolecular dipole–dipole interactions and enhancing the NLO properties of the resulting polymers. The studied polymers exhibited good optical transparency, high thermal stability, and excellent NLO effects; this indicated that the nonlinearity–stability trade‐off and nonlinearity–transparency trade‐off could be alleviated by this newly designed polymer system. Poly{4‐anilinocarbonylN‐ethoxyl‐5‐phenyl‐3‐azo(2′‐oxyethylene‐4′‐nitrobenzene)indole]carbonylimino} with a zigzag backbone showed a large second harmonic generation coefficient (d₃₃) value of 88.4 pm/V. However, poly{5‐naphthyliminocarbonylN‐ethoxyl‐5‐phenyl‐3‐azo(2′‐oxyethylene‐4′‐nitrobenzene)indole]carbonylimino} (PUAZN) with a continuous zigzag structure exhibited a higher d₃₃ value of 116.2 pm/V, which was attributed to the unique rigid and zigzag linkage of 1,5‐naphthalene as the isolation spacer. The enhanced NLO efficiency and relatively longer term temporal stability made PUAZN as a promising candidate for practical applications in photonic devices. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42974.