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.
Izumi et al. (Wed,) studied this question.