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-anilinocarbonyl[N-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-naphthyliminocarbonyl[N-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.