High glass transition and thermal stability of new pyridine-containing polyimides: Effect of protonation on fluorescence

A new diamine monomer containing heterocyclic pyridine and triphenylamine groups, 4-(4,4′-diaminotriphenylamine)-2,6-bis(4-methylphenyl)pyridine (4), was synthesized by Chichibabin and nucleophilic fluoro-displacement reactions. The diamine was used to prepare a series of novel polyimides via polycondensation with various aromatic tetracarboxylic dianhydrides in N-methyl-2-pyrrolidinone. The polyimide 4a derived from the diamine 4 with 4,4′-hexafluoroisopropylidenediphthalic anhydride and having high T_g (313 °C), mechanical, and thermal properties was soluble in various organic solvents, such as N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, pyridine, chloroform, tetrahydrofuran, at room temperature. The polyimide (4a) could be cast into a self-standing film from DMAc solution and was thermally converted into tough and flexible film. The film had high tensile modulus of 2.2 GPa and exhibited excellent thermal stability in both nitrogen and air (T_d¹⁰ > 550 °C). The pristine polymer exhibited the UV-vis absorption bands in the region 240-400 nm and protonated polymer exhibited absorption in the region 390-500 nm. The protonated polymer possessed strong orange fluorescence (around 600 nm) in THF solution after protonation with acids as excited at 438 nm. The fluorescent intensity was influenced by the acid concentrations and the chemical structure of conjugated bases. The fluorescent intensity at 600 nm increased as acid concentration from a lower to a moderate concentration and decreased at higher concentrations.