Atmospherically Stable Poly(Heptazine Imide) Composites

Metal poly-(heptazine imide) (MPHI), a two-dimensional carbon nitride polymer containing monovalent metal ions (M⁺), has recently attracted attention as a novel visible-light-driven photocatalyst. It exhibits photochromism, changing from yellow to blue-green upon light irradiation, regardless of the metal species, and is known to enhance ionic conductivity. Consequently, it has the potential to serve as a novel photoresponsive ionic conductor. However, the excited (color-changed) state that exhibits ionic conductivity is easily deactivated by atmospheric or dissolved oxygen in solution, making its application in actual devices challenging. Therefore, in this study, we developed a composite, protonated poly-(heptazine imide) (HPHI):poly-(vinyl alcohol) (PVA), by dispersing HPHI prepared by the acid treatment of potassium poly-(heptazine imide) into a matrix of the insulating polymer PVA, which possesses high oxygen-blocking properties. HPHI:PVA can maintain a color-changed state for extended periods, even in air, while sustaining a low electrical resistance state. The time constant derived from the decay curve of HPHI:PVA's absorbance over time is six times longer than that reported for HPHI composites using poly-(methyl methacrylate) in previous studie. The duration of this color-changed state can be controlled by varying the degree of PVA saponification or temperature. Furthermore, a detailed investigation of the dependence of the electrical properties of HPHI:PVA on the percentage of HPHI revealed that proton conduction in HPHI:PVA arises from the percolation of poly-(heptazine imide) particles within the composite. This finding also provides fundamental information regarding the ion-conduction mechanism in other MPHI composites. This study serves as an important guideline for the future development of new MPHI composites and applied research.