Synthesis, characterization, and second-order optical nonlinearity of a polyurethane structure functionalized with a hemicyanine dye

A new polyurethane structure with a hemicyanine dye attached to the polymer side chain was synthesized by the step growth polymerization in a reaction between (E)-N-butyl-4-[2-[4-[bis(2-hydroxyethyl)amino]phenyl]ethenyl]pyridinium tetraphenylborate and 2,4-toluene diisocyanate. The molecular weight of the final product was determined to be M_n = 12 000 and M_w/M_n = 1.67. The polymer is soluble in dimethylformamide and can be processed into optical quality films by spin casting. No evidence of melting was detected by differential scanning calorimetry, suggesting that this polymer presents an amorphous phase. It shows a glass transition temperature of 121 °C. The macroscopic second-order hyperpolarizability χ⁽²⁾ of this polyurethane was determined by measuring the second harmonic generation (SHG) for a thin polymer film. The χ⁽²⁾ value was in the range 1.8 × 10^-7 to 5.0 × 10^-7 esu, depending upon poling conditions. This high second-order activity seems to prove the earlier prediction about a possible enhancement in the nonlinear second-order properties of organic materials triggered by utilizing the strong electron-accepting nature of the pyridinium group. In the presented polymer, the alignment of the nonlinear chromophore moieties induced by electric poling exhibits an extended temporal stability, due to the stabilizing function of the hydrogen bridges formed between the neighboring polyurethane chains, preventing the relaxation of oriented molecular dipoles.