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Molecular hopscotch boosts light upconversion

A new molecule that lets energy hop around quickly within its structure makes the upcycling of light more efficient and tunable. The Kobe University development lays out a design strategy for better solar power harnessing as well as medical and sensor applications.

While low-energy light is abundant and harmless, many technical applications from solar power generation to medical treatments rely on high-energy light. To increase the efficiency of light harvesting and to avoid high-energy light as much as possible in treatment technologies, it is possible to fuse two low-energy light particles into a single high-energy particle in a process called "upconversion."

However, this requires two molecules that have each absorbed a light particle to bump into each other at just the right orientation, and researchers are still groping for a framework to improve and control this process.

The Kobe University photoscientist Kobori Yasuhiro has been studying the opposite process—an energized molecule transferring half of its excitation energy to another molecule—and says, "We have established an analytical method for clarifying how the absorbed energy transforms the molecule and how it moves around, and I believe that this can be useful when trying to achieve upconversion."

In principle, there are two ways of making it more likely that the excitation energy jumps from one molecule to the other, that is, either by allowing the molecules to bump into each other more frequently, by reducing the viscosity of the medium, or by increasing the area over which the reaction can happen. But there is a limit to how fluid a medium can be, and a larger reaction area often means that the energy that can be effectively transferred will be lower.

In the journal Angewandte Chemie International Edition, Kobori and his team that they have created a molecule that allows a 20% faster energy fusion rate than with previous materials.

"We used three anthracene molecules that are arranged like antennas in three different directions around a central boron atom. This allows the excited state, which we call a 'triplet exciton,' to quickly hop around the three anthracenes and essentially lets it sample a larger space and multiple configurations without losing its energy," explains Kobori.

The hopping is so quick, in fact, that it is faster than the usual duration of a molecular collision, thus making it highly likely that the excited triplet states are oriented right and the energy fusion process called "triplet-triplet annihilation" can take place.

When studying the mechanics of how this process occurs, the Kobe University researchers made another discovery. They found that they can control the luminescence by changing the viscosity of the medium, because more viscous materials lower both the encounter rate between molecules as well as the ability of the excited triplet state to hop between parts of one molecule.

Kobori says, "We think that with this, it might be possible to track the fluid environment in microscopic regions, for example within cells."

What Kobori and his team found is not only a better molecular configuration, but a molecular design strategy. They expect that incorporating such an intramolecular perspective can advance the development of high-efficiency upconverters.

Kobori says, "We expect that this development may contribute to solving global energy problems, as well as to expand into a wide range of fields such as cancer therapy and diagnostics, by using harmless low-energy light and in-situ upconversion."