Synthesis of polyamic acid possessing second-order nonlinear optical properties and its application to Langmuir-Blodgett films

Tetracarboxylic dianhydride containing the second-order nonlinear optical chromophore thiophene-type stilbene with diethylamino and tricyanoethenyl substituents on each terminal was newly synthesized. Polyamic acid prepared from the tetracarboxylic dianhydride and N,N′-diaminodiphenyl ether was soluble in aprotic polar solvents and had an inherent viscosity of 0.2 dL/g. Polyimide obtained by heating of the polyamic acid at 160°C was thermally stable to 240°C. Polyimide Langmuir-Blodgett (LB) films were prepared by the precursor method through polyamic acid alkyl amine salts (the precursor of polyimide) with three kinds of alkyl amine. The precursor LB film had a Y-type structure and a good linear relationship between thickness and absorption at 650 nm in both precursor and polyimide LB films. Fourier-transform IR spectra before and after imidization indicated that the nonlinear optical chromophore remained in the polyimide LB film. It was found from the observation of transmission UV-visible spectra that the nonlinear optical chromophore was oriented more nearly perpendicularly in polyimide LB film than in the precursor LB film.