SYNTHESIS, CHARACTERIZATION, AND 2ND-ORDER OPTICAL NONLINEARITY OF A POLYURETHANE STRUCTURE FUNCTIONALIZED WITH A HEMICYANINE DYE

A new polyurethane structure with a hemicyanine dye attached to the po lymer side chain was synthesized by the step growth polymerization in a reaction between bis(2-hydroxyethyl)amino]phenyl]ethenyl]pyridinium tetraphenylborate and 2,4-toluene diisocyanate. The molecular weight o f 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 processe d into optical quality films by spin casting. No evidence of melting w as detected by differential scanning calorimetry, suggesting that this polymer presents an amorphous phase. It shows a glass transition temp erature of 121 degrees C. The macroscopic second-order hyperpolarizabi lity chi((2)) of this polyurethane was determined by measuring the sec ond harmonic generation (SHG) for a thin polymer film. The chi((2)) va lue was in the range 1.8 x 10(-7) to 5.0 x 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 secon d-order properties of organic materials triggered by utilizing the str ong electron-accepting nature of the pyridinium group. In the presente d polymer, the alignment of the nonlinear chromophore moieties induced by electric poling exhibits an extended temporal stability, due to th e stabilizing function of the hydrogen bridges formed between the neig hboring polyurethane chains, preventing the relaxation of oriented mol ecular dipoles.