# Atmospherically Stable Poly(Heptazine Imide) Composites

## TL;DR

Researchers developed a stable composite material that maintains its light-induced conductivity in air, offering potential for new photoresponsive devices.

## Contribution

A novel HPHI:PVA composite was created to sustain the photoresponsive state in air by blocking oxygen.

## Key findings

- HPHI:PVA maintains a color-changed state for extended periods in air.
- The time constant for HPHI:PVA is six times longer than previous composites.
- Proton conduction in HPHI:PVA is due to percolation of poly(heptazine imide) particles.

## Abstract

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.