Mechanism of charge accumulation of poly(heptazine imide) gel

Abstract

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.

Formable Optoelectronic Functional Composites with Potassium Poly(heptazine imide)

“…The wavelength of maximum absorbance in the absorption band on the long-wavelength side that appears by light irradiation is approximately 640 nm for both K-PHI:PMMA and K-PHI:IL. The change in the absorption band of K-PHI due to photochromism was also consistent with that of the K-PHI solution. ,,, The previous study has reported that isolated crystallites of K-PHI tend to aggregate upon freeze-drying. The optical gap of such solid K-PHI was found to increase substantially over the original K-PHI due to a decrease in the conjugation length of the PHI framework and partial detachment .…”

“…K + would undergo repeated collisions with ions in EmimPF 6 and polymer chains in PMMA. In fact, our previous report showed that the more viscous the ionic liquid is, the slower the rate of color recovery after the K-PHI composite was discolored by photochromism . This suggests a decrease in the mobility of K + in the composite owing to its interaction with the ionic liquid.…”

“…The “channel” is a pathway running in the stacking direction formed by overlapping these pores, allowing hydrated metal ions to move through. A previous report showed that a part of K + , bound to specific positions in the pore of K-PHI in the ground state, is released by a change in the charge distribution within the PHI framework after light absorption . Thus, K-PHI after light irradiation would be expected to show ionic conduction of K + by photostimulation, because K + that is now able to move within the pore is able to pass through the channel.…”

“…The photochromism of K-PHI is thought to be caused by the desorption of K + coordinated near the nitrogen at the bridge site in the periphery of the pore of the PHI layer that occurs after light absorption . To see if desorbed K + can migrate through the matrix of the composites, Figure a shows the direct observation of K + movement initiated by light irradiation using SEM/EDX and the results of the electrical measurements.…”

“…This clearly shows that some potassium is released from K-PHI and diffuses into EmimPF 6 after light irradiation. This result is consistent with the conclusion from the X-ray photoemission spectroscopy analysis reported previously, namely that light irradiation desorbs K + from the pore in the PHI layer and that the photochromism of K-PHI is caused by a change in energy band structure associated with K + desorption …”