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Polaritons are quasiparticles emerging from strong interactions between light particles (i.e., photons) and matter excitations (e.g., excitons). Over the past few years, researchers have found that these quasiparticles can alter fundamental chemical and physical processes.

For instance, studies suggest that the formation of polaritons in molecular systems can alter photoinduced charge transfer, a process via which absorbed light prompts an electron from a donor molecule to shift to an acceptor molecule.

This process is known to be central to the functioning of various technologies, including energy harvesting systems and other devices for the clean production of fuels or for synthesizing specific chemical compounds.

In a recent paper in Nature Nanotechnology, researchers from the Advanced Science Research Center at the CUNY Graduate Center reported a direct, tunable and efficient polariton-driven charge transfer process experimentally for the first time.

Their study could open new exciting possibilities for the engineering of many chemistry-driven devices, including solar cells, photocatalysts, and optoelectronic systems.

During the charge transfer reaction, the process that the team tried to realize utilizing polaritons, electrons move from one molecule (i.e., the donor) to another (i.e., the acceptor). While this chemical reaction already supports the functioning of various existing technologies, typically it only involves light particles of a specific color (e.g., green or red).

"We show that when the molecules and the waves of light are confined at a small volume (e.g., the surface of a mirror) and strongly interact with each other, they form a new particle called a polariton which is a mix of light and matter," Matthew Y. Sfeir, senior author of the paper, told Â鶹ÒùÔº.

"In our work, we use these polaritons to tune photochemical charge transfer across a broader spectrum of light, including green, red, and infrared wavelengths. This idea had been widely discussed but hadn't been definitively shown to be possible before our study."

These researchers' study specifically leveraged a type of polaritons known as Bloch surface wave polaritons (BSWPs), known to propagate along the surface of layered optical structures. The unique properties of these quasiparticles, specifically their ability to prompt long-lived hybrid states, were found to be ideal for driving and tuning molecular charge transfer.

"The efficiency of many photochemical processes depends on the type of light available and on the details of the identity of the different molecules involved in the reaction," explained Sfeir. "The improvement and optimization of these processes usually require the synthesis of many different types of molecules with different properties. We were interested in whether we could tune these properties using the same molecule with different degrees of coupling to light fields."

Using a carefully engineered polaritonic platform, Sfeir and his colleagues were ultimately able to directly control charge transfer reactions. Their experiments also allowed them to delineate the conditions under which polaritons can reliably participate in charge transfer processes, which could inform the future development of other platforms for driving these reactions using polaritons.

"It turns out that it is very difficult to harness energy from polaritons," said Sfeir. "They want to release energy very quickly (much faster than a normal molecule) and this limits the overall efficiency. We play a lot of special tricks to make this work by engineering the way that we confined the light.

"Our conclusion is that while polariton-driven photochemistry is possible, it is very hard! Still, we did lower the amount of energy needed to drive charge photochemical transfer in a dye molecule by approximately 33%."

The findings of this recent study highlight the potential of polariton-based platforms for improving the efficiency and tunability of charge transfer reactions. In the future, they could contribute to the development of new types of photovoltaic, spintronic and optoelectronic devices that rely on polaritons to optimize molecular charge transfer.

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