Synthesis and nonlinear optical properties of novel Y-type polyimides with enhanced thermal stability of dipole alignment

2,3-Bis-(3,4-dicarboxyphenylcarboxyethoxy)-4′-nitrostilbene dianhydride (4) was prepared and reacted with 1,4-phenylenediamine, 4,4′-oxydianiline, 4,4′-diaminobenzanilide and 4,4′-(hexafluoroisopropylidene)dianiline to yield novel polyimides 5–8 containing 2,3-dioxynitrostilbenyl groups as NLO-chromophores, which constituted parts of the polymer backbones. The resulting polyimides 5–8 were soluble in polar solvents such as acetone and DMF. Polymers 5–8 showed a thermal stability up to 300 °C in TGA thermograms with Tg values obtained from DSC thermograms in the range of 135–160 °C. The SHG coefficients (d33) of poled polymer films at the 1064 cm−1 fundamental wavelength were around 5.26 × 10−9 esu. The dipole alignment exhibited exceptionally high thermal stability even at 30 °C higher than Tg and there was no SHG decay below 170–190 °C due to the partial main chain character of polymer structure, which was acceptable for NLO device applications.