Functionalized carbon nitride nanomaterials that can store light energy in the form of long-lived radicals have enabled photoreactions in the dark, new research results show.1

A picture with chemical structures and calculations

Photosynthesis, the process of converting light energy into an electrochemical potential in plants and using it to drive reactions in the dark, has proven difficult to replicate in artificial systems. This is because photogenerated charge pairs tend to recombine after exposure, which prevents photoreactions from occurring in the absence of light.

Because of its ability to promote electron-hole separation, a team led by Ze Zhang from the University of Science and Technology of China predicted that C3No4th-NH2 could prevent the recombination of charge pairs by trapping photogenerated electrons. While electron-hole separation occurred under irradiation when the team was studying the material experimentally, electron paramagnetic resonance spectroscopy (EPR) confirmed that C3No4th-NH2 has no electrons stored. Density functional theoretical calculations to determine the charge distribution of C3No4th-NH2 showed that the modification of the structure by exchanging a proton in the amino (NH2) Group with a cyano (CN) resulted in a more positive charge distribution due to the increased interaction of the heptazine rings with electrons.2 Heptazines are nitrogen-rich aromatic systems, which means that they are highly electron deficient and likely to trap electrons, making them ideal candidates for electron storage.

Irradiation of the C3No4th–NH-CN species resulted in a blue suspension and a G Value of 2.0021 using EPR which is characteristic of stored electrons. In the air, the electrons reacted immediately and the suspension changed its color. Neither the C3No4th–NH2 and C3No4th-NoCN suspensions were blue in color or showed an EPR signal.

A scheme showing electron transfer

To determine the electron storage capacity of C. to investigate3No4th–NH-CN, the team varied the proportion of protonated and non-protonated units by changing the pH value. This led to the conclusion that the more -NH-CN groups, the stronger the EPR signal strength and the stronger the color of the suspension. Further analysis by C3No4th–NH-CN was involved in methylene blue, which Zhang can attribute to his “significant absorption changes before and after the uptake of electrons, [meaning] it is helpful to determine the amount of electron storage. ”This investigation found that C3No4th-NH-CN with five layers was able to absorb all available methylene blue after 10 minutes of irradiation, which was confirmed by EPR. The EPR signal intensity indicated that after one month 50% of the stored electrons were still present and available to participate in the reversible addition-fragmentation-chain transfer (RAFT) polymerization. This process is made possible by electron transfer from C3No4th–NH-CN to diphenyliodonium (DPI), which generates phenyl radicals that can react with the RAFT reagent to promote photopolymerization. Compared to polymerization under continuous light irradiation, fewer side reactions take place and the RAFT agent is not degraded, which leads to polymers with higher molecular weights.

Athina Anastasaki of ETH Zurich in Switzerland, whose research focuses on radical polymerization, notes that the work “offers a new tool for the synthesis of advanced functional polymers that can be used for a wide range of applications”. Zhang says the team hopes “to continue developing materials for converting and storing light energy and expanding the scope of stored electrons.”

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