Photo-stimuli response in materials is a fascinating feature with many potential applications. A photoresponsive gel of poly(heptazine imide), PHI, termed PHIG, exhibits photochromism, photoconductivity, and photo-induced charge accumulation, and is generated using ionic liquids and PHI. Although there are several examples of ionic liquid gels that exhibit photochromism and photoconductivity, this is the first report of an ionic liquid gel that exhibits both these properties as well as charge accumulation. We conducted experimental and theoretical investigations to understand the mechanism of the photostimulus response of PHIG, especially charge accumulation. The proposed model explains both the mechanism of charge accumulation and dark photocatalysis by PHI and provides new concepts in the field of photofunctional materials.

Metal poly(heptazine imide) (MPHI) exhibits various optoelectronic functions by accommodating various metal ions in its heptazine-based graphite-like two-dimensional layer. Although potassium PHI (KPHI) is the most well-studied in MPHI series, there are unresolved issues regarding the mechanism responsible for its unique properties, particularly its high photocatalytic activity. The mechanisms underlying other diverse properties, such as photochromism and photoconduction, also remain unknown. Herein, we focused on elucidating the mechanisms of photochromism and photoconduction in KPHI. We developed a method to gradually replace K + in KPHI with H + and successfully prepare samples with tunable K + concentrations. The structural, chemical, optical, electronic, and electrical properties of samples with continuously varying K + concentrations were thoroughly investigated through various experiments and theoretical calculations. First, the gradual substitution of K + for H + in KPHI was investigated using X-ray photoelectron spectroscopy, X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy. XRD analysis revealed that K + in KPHI was not randomly substituted with H + , but even small amounts of K + substitution led to the formation of the long-range ordered region of HPHI. Absorbance measurements revealed a continuous blue shift of the absorption edge as the ratio of K + decreased from that of KPHI. Energy band calculations revealed that the energy gap of HPHI was more than 14% larger than that of KPHI. The dependence of the K + concentration measurements on the photochromism and photoconductive properties showed that in the K +rich samples, the K + released from the PHI layer upon white light irradiation had a significant effect on the electrical conduction properties. K + conduction was dominant in the electrical conduction of KPHI, including photoconduction, under ambient conditions. However, the mobility of H + in HPHI was very low, which was responsible for the lethargic return to the ground state after photoexcitation and minimal electrical conductivity.

Potassium poly(heptazine imide) (K-PHI) is a two-dimensional carbon nitride polymer that has recently attracted attention as a visible-light-driven photocatalyst. We developed a solid material that does not require a liquid electrolyte as a composite of K-PHI with an ionic liquid or insulating polymer matrix. The obtained K-PHI composites retained some of the unique optoelectronic properties of K-PHI and were in a semisolid state that could be easily shaped in a warm state. Simple devices using these K-PHI composites exhibited photoconductivity by the ionic conduction of potassium cations activated by white light irradiation.