Controlling nonlinear optical effects of polyurethanes by adjusting isolation spacers through facile postfunctional polymer reactions

With the aim to control nonlinear optical (NLO) effects of polyurethanes, especially to efficiently translate the fixed μβ values of the organic chromophore to possibly higher macroscopic NLO activities of polymers, a facile synthetic strategy, consisting of the postazo coupling and esterification reactions, was developed to prepare a series of second-order nonlinear optical polyurethanes (P2–P5). And in the polymers, different isolation groups were introduced to the sulfonyl-based chromophore moieties at the acceptor side. Thus, polyurethane P1 containing aniline groups was obtained from the copolymerization of 2,4-toluenediisocyanate (TDI) and N,N-2-(2-hydroxyethyl)aniline directly; then a postazo coupling reaction of p-(2-hydroxyethyl)sulfonylbenzenediazonium fluoroborate toward the aniline ring afforded the sulfonyl-based chromphore-functionalized polyurethane P2, which underwent the subsequent esterification reactions between the hydroxyl groups in P2 and different organic acids to link different isolation spacers (changing from small groups to much larger ones such as carbazolyl groups), to the chromophore moieties at the acceptor side, to yield polyurethanes P3, P4 and P5. The polymers exhibit good solubility in common organic solvents and are thermally stable. The maximum absorption appeared at about 436 nm with a cutoff at ca. 570 nm, resulting in a wider transparency window. The tested NLO properties demonstrate that the resonant d33 values of polymers could be improved about 1.5 times by attaching isolation spacers with suitable bulkiness.