Nematic main chain polymers with head-to-tail structure: Synthesis and enhanced NLO response

NLO-active semiflexible main chain polymers with a head-to-tail structure were synthesized from 2-nitro-4-carboxy-4'-[N-(11-hydroxyundecyl)-N-methylamino]azobenzene (CHRI) and 4-carboxy-4'[N-(11-hydroxyundecyl)-N-methylamino]azobenzene (CHRII). The CHRI polymers, with molecular weight approximately 40 000, were not Liquid crystalline. The electrooptical coefficient difference r(33) -r(13) ranged between 0.75 and 1.36 pm/V depending on the particular sample. Theory suggests that nematic head-to-tail polymers can exhibit greatly enhanced electrooptical (EO) response. The 50/50 copolymers from CHRII and p-hydroxybenzoic acid (molecular weight 38 000) were Liquid crystalline. r(33) -r(13) was enhanced by up to a factor of 20 to r(33)-r(13) similar to 21 pm/V, with a comparable poling field around 8 x 10(6) V/m. The corresponding chi((2)) are around 62 pm/V. The dynamics of the induction of NLO response by a poling field was clearly high polymer-like. Relaxation of the EO response below T-g is imperceptible. Considering the smaller hyperpolarizability and lower number density of CHRII (compared to in the CHRI polymer), this enhancement is effectively x40. Powerful chromophores, less modest poling fields, and longer chains should yield much larger chi((2)) using such polymers.