Synthesis and properties of polyimide with diazacrown ether moiety and the corresponding polymer barium complexes

A series of polyimides with diaza-18-crown-6 moieties on the main chain were synthesized by the reaction between 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and different macrocyclic diamine monomers. The polyimide barium complexes were also synthesized from the corresponding monomeric barium complexes. An X-ray crystal structure of one of the monomers was also determined. Good quality, free-standing polymer films were obtained by thermal imidization of the poly(amic acid) presursors. No decomposition of the barium complexes was found during the thermal imidization processes. The polyimides and their barium complexes exhibit different electronic absorption properties, which is attributed to the formation of charge-transfer excited states. The binding of barium ion to the polymer main chain may affect the energy level of the resulting charge transfer states. In addition, the emission properties of the polyimides with and without barium ion are also different. Changes in the emission spectrum were observed when the metal-free polyimide film was immersed in barium solution. These materials may serve as potential candidates as sensors for barium or other alkaline earth metals.