Porphyrinated polyimide honeycomb films with high thermal stability for HCl gas sensing

We report thermally stable films with ordered pores from porphyrinated polyimides (PPIs) for HCl gas sensing. PPIs were synthesized by copolymerizing porphyrin units into the polyimide backbone. Thermal analyses reveal that the glass transition temperature and the thermal stability of PPIs are similar to that of the homopolyimide (HPI) without porphyrin units. The PPIs also show typical fluorescence emission behavior of the porphyrin units. Honeycomb films were prepared from PPIs and HPI via the breath figure method. The porous films show decreased pore diameters and change from monolayer to multilayer structures with the incorporation of porphyrin units. These are due to the interaction of porphyrin units with water molecules, as demonstrated by theoretical calculations based on density functional theory (DFT). The well-ordered porous PPI films have outstanding thermal stability. Furthermore, the films treated at different temperatures show remarkable fluorescence quenching behavior toward HCl gas with quenching ratios higher than 72%, whereas it is only about 50% for the corresponding dense film. Moreover, recovery experiments using ammonia gas demonstrate excellent reusability of the films. The PPI honeycomb films provide new opportunities in chemical sensing at high temperature.