In-situ poling and synthesis of NLO chromophore-bearing polyurethanes for second harmonic generation

The preparation of six diol and one triol monomers bearing donor–acceptor chromophores is described. The monomers contain the N,N′‐bis(2‐hydroxyethyl)aniline unit with various acceptor groups attached in some cases via azo or olefin linkages, at the para position. Acceptors studied include nitro, tricyanovinyl, cyclobutene‐1,2‐dione, and imidazolidine‐2,4‐dione. Poled polymer films were prepared by thermal polymerization of these with 2,4‐toluenediisocyanate (TDI) in the presence of an electric field generated by a corona tip. The resulting thin (ca. 1 μm), glassy polymer films were shown to exhibit reasonably good second harmonic generation efficiencies. Preliminary results show that the polymers with higher Tg's have the best temporal stability. The polymer derived from the diol bearing an imidazolidine‐2,4‐dione acceptor shows only a ca. 30% decrease in its second harmonic generation (SHG) signal over 200 days at room temperature, even though it is not crosslinked. We attribute this to hydrogen bonding interactions from the urethane and imidazolidine‐2,4‐dione groups. A crosslinked film derived from the triol and TDI has an exceedingly stable SHG response at room temperature, and no decrease in the SHG response is observed at 100°C for a few hours. © 1993 John Wiley & Sons, Inc.