Kj. Moon et al., SYNTHESIS, CHARACTERIZATION, AND 2ND-ORDER OPTICAL NONLINEARITY OF A POLYURETHANE STRUCTURE FUNCTIONALIZED WITH A HEMICYANINE DYE, Macromolecules, 29(3), 1996, pp. 861-867
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.