Design and preparation of novel fluorescent polyimides containing <i>ortho</i>-linked units and pyridine moieties

A novel pyridine-containing aromatic diamine monomer, 4-[4-hydroxyphenyl]-2,6-bis[4-(2-aminophenoxy)phenyl]pyridine (<i>p</i>,<i>o</i>-HAPP), was synthesized by a modified Chichibabin reaction of <i>p</i>-Hydroxybenzaldehyde and a substituted acetophenone, 4-(2-nitrophenoxy)acetophenone (<i>p</i>,<i>o</i>-NPAP), followed by a reduction of the resulting dinitro compound 4-[4-hydroxyphenyl]-2,6-bis[4-(2-nitrophenoxy)phenyl]pyridine with Pd/C and hydrazine monohydrate. The aromatic diamine was employed to prepare a series of pyridine-containing polyimides (PIs) by polycondensation with various aromatic dianhydrides in <i>N</i>,<i>N</i>-dimethylformamide (DMF) via the conventional two-step method. The inherent viscosities of the resulting poly(amic acids) and PIs were in range of 0.62-0.76 and 0.52-0.64 dL/g, respectively. The obtained novel PIs exhibited high solubility in common organic solvents, such as <i>m</i>-Cresol, DMF, <i>N</i>,<i>N</i>-dimethylacetamide, dimethylsulfoxide, <i>N</i>-methylpyrrolidone (NMP), tetrahydrofuran, and chloroform. Meanwhile, flexible PI films were obtained, which had excellent thermal stability, with the glass transition temperature (<i>T</i> _g) of 259.8-323.4 °C and the temperature at 10% weight loss of 485.5-576.4 °C in nitrogen atmosphere. The protonated polymer showed UV-vis absorption in the region 200-400 nm and displayed strong fluorescence intensity (470 nm) in NMP solution.