Optics

UV to red light conversion boosts plant growth

Nov. 3, 2022

A new europium complex-based thin-film coating has been shown to help accelerate the growth of plants by enhancing the properties of photosynthesis.

Justine Murphy

The new Eu3+-based WCM film converting UV light to red light.

The time involved in cultivating plants can have wide-reaching effects on food production, which in turn can impact food security and ultimately the global food supply chain. A conversion of light could alleviate, and potentially improve, such effects.

Researchers at the Institute for Chemical Reaction Design and Discovery (WPI-ICReDD) at Hokkaido University (Japan), together with a team from the school’s engineering and agriculture departments, have developed a wavelength-converting material (WCM) that when applied to sheets of plastic can convert UV light to red light to enhance the properties of photosynthesis.

Schematic of WCM film converting UV light to red light (left) and molecular structure of film components (right). (b) Photos of plastic sheets with and without WCM coating (top); a schematic of WCM film converting UV light to red light (bottom).

WCM is a thin-film coating based on a europium complex (Eu3+) mixed with a methoxy-substituted triphenylphosphine oxide that makes it amorphous. The Eu3+ complex luminophore exhibits strong red luminescence, says lead researcher Sunao Shoji, an assistant professor at WPI-ICReDD, based on 4f-4f transitions—a process in which UV light excites a certain electron state that then translates to the emittance of longer wavelength photons in the red spectral region—made by the attachment of organic ligands.

The Eu3+ luminophore with a photosensitizer—hexafluoroacetylacetonato—and a stabilizer—triphenylphosphine oxide (TPPO), [Eu(hfa)₃(TPPO)₂]—demonstrated a high conversion efficiency from UV to red light; specifically, an emission quantum yield of about 70%, and with almost no absorption bands in the visible light region.

Solar spectrum showing wavelengths absorbed and emitted by the Eu3+-based WCM film.

“This unique combination of properties significantly increases the amount of red light without blocking other visible light that is also used in photosynthesis,” Shoji says. Making effective WCM films requires a high concentration of the luminophore in the film. But Shoji’s team found difficulty in creating highly concentrated WCM films by incorporating the luminophore into the plastic itself.

“With our method, we coated the plastic with a paint that contains a high concentration of the luminophore complex, instead of incorporating the complex into the plastic itself,” he says.

Researchers tested the WCM’s capabilities on Swiss chard and Japanese larch trees, comparing the growth of each plant behind coated and non-coated plastic sheets. No significant differences were observed in the Swiss chard during the summer months when the WCM films were used. In winter months, when the sun is weaker and the days shorter, the researchers found that the Swiss chard plants grew 1.2 times larger with the film than without, and produced a 1.4-times greater biomass.

Japanese larch tree seedlings grown without (left) and with (right) the use of the Eu3+-based WCM sheet.

The WCM film also accelerated the larch tree growth with a higher relative growth rate in seedlings, prompting a 1.2-fold larger stem diameter and a total biomass that was 1.4-fold larger than the trees growing naturally. In just one year, the seedlings reached the standard planting size required for the city of Hokkaido’s forestry. Such growth without the WCM film typically takes twice that long.

“WCM technology can be used for a variety of plants,” Shoji says. “This advancement could improve the variety and growth of plants cultivated in challenging climates with less sunlight.”

Similar work with boosting plant growth is traditionally done with LEDs. But agricultural films using Eu3+-based WCMs are more promising and could be more sustainable candidates because unlike LEDs, the film doesn’t consume electric power (see video).

“Our method could be useful for a large-scale greenhouse building and for plant production in space where UV light is especially strong,” Shoji says. “We hope to expand on our findings by exploring how changing the molecular structures of the luminescent materials on the film affects plant growth and their production of bioresources.”

The team’s ultimate goal is “to contribute to society by developing eco-friendly and cost-effective sustainable plant production methods for crops, vegetables, trees, flowers, and so on.”

About the Author

Justine Murphy | Multimedia Director, Laser & Military

Justine Murphy is the multimedia director for the Laser & Military Group at Endeavor Business Media. In addition to Laser Focus World, the group includes Military & Aerospace Electronics and Vision Systems Design. She is a multiple award-winning writer and editor with more 20 years of experience in newspaper publishing as well as public relations, marketing, and communications. For nearly 10 years, she has covered all facets of the optics and photonics industry as an editor, writer, web news anchor, and podcast host for an internationally reaching magazine publishing company. Her work has earned accolades from the New England Press Association as well as the SIIA/Jesse H. Neal Awards. She received a B.A. from the Massachusetts College of Liberal Arts.