An alternate synthetic approach for soluble nonlinear optical polyimides

Three functionalized second-order nonlinear optical polyimides were synthesized by the step growth polymerization reaction between a chromophore-containing dianhydride and three different diamine compounds. This alternate approach (i.e., incorporating a nonlinear chromophore into a dianhydride monomer) may avoid troublesome syntheses of the chromophore-containing diamine and can provide easy preparation of the polyimides with various chromophores. The number-average molecular weights (M_n) of final polymers determined by GPC ranged between 10 000 and 16 800 (M_w/M_n = 1.59-2.28). No melting was detected and they showed relatively low glass-transition temperatures ranging from 161 to 181 °C. They are readily soluble in aprotic polar solvents such as tetrahydrofuran, N,N-dimethylformamide, N-methylpyrrolidinone, and so forth. High optical quality films were prepared by spin casting from a 10 wt % cyclohexanone or N-methylpyrrolidinone solution. The second harmonic generation (SHG) coefficients, d₃₃, of the polymers were determined to be 24-37 pm/V (d₃₃(∞) = 7-10 pm/V) at a fundamental wavelength of 1064 nm. All of the poled polymer samples showed outstanding orientational stability up to 125 °C without any measurable decay of the SHG signal. In particular, the polyimide with a 4,4′-(hexafluoroisopropylidene)dianiline unit (PI-2-DANS) showed the best dynamical thermal stability of the poling-induced dipole alignment up to ca. 170 °C.