Green alternative for light-emitting materials in displays uses plant waste and amino acids

Scientists have devised a way to create a green alternative to the light-emitting materials often used in TV, smartphones and other display technologies.

The research, led by a team at the Center for Green Chemistry and Green Engineering at Yale University in the US and involving Nottingham Trent University, aimed to address the challenge of “photoluminescent” solid-state materials, which often rely on non-renewable resources and toxic metals.

These materials are also often made in multi-step processes that produce a lot of chemical waste which can be hazardous.

The study is published in the journal Chem.

Photoluminescent solid-state materials work by absorbing UV light and re-emitting it as visible light, providing an ability to glow which makes them ideal for a range of applications such as display technologies, lighting, sensors, security inks, biomedical imaging through to glow-in-the-dark toys.

The challenge for researchers has been to develop these materials from sustainable sources that are environmentally-friendly and in a way that is less wasteful and less hazardous.

As part of the study, the team took lignin—a by-product of the wood pulping and paper industry and a natural substance found between and in the cell walls of plants and trees—and combined it with histidine, a simple amino acid, finding they could produce a range of solid-state materials that fluoresce under UV light.

In addition to easily tunable photoluminescent material properties, the preparation of the materials only uses green solvents in the form of water and acetone.

The fluorescence, or lighting, effect relies on specific parts of the lignin—”phenolic groups”—which become energized when they absorb the light.

In this energized state, they release protons to the histidine in the solid structure, a process known as “excited state proton transfer” (ESPT).

As the lignin relaxes back to its normal state, it releases light which can shine at room temperature. In some cases, the materials continued to glow very briefly even after the UV light was turned off.

“The concept of ESPT isn’t new, it is well known in pure phenolic molecules,” said first author Dr. Ho-Yin Tse, a researcher from the Center for Green Chemistry and Green Engineering at Yale University.

“But what is interesting is that lignin’s natural phenolic structures—present throughout the macromolecule—can inherently support this kind of photoacid behavior and this effect has rarely been examined in this context.”

“This is an excellent example of green and sustainable chemistry,” said study co-author Dr. Darren Lee, a researcher in sustainable chemistry in Nottingham Trent University’s School of Science and Technology.

He said, “Photoluminescent materials are vital for a range of everyday and smart technologies, but most rely on toxic metals and non-renewable resources.

“In this study we not only simplified the synthesis of these materials but also utilized abundant waste streams to produce tunable materials in a safer way.”

“Computational modeling revealed how molecular interactions between lignin and histidine enable this unique light-driven proton transfer,” said Dr. Chi-Shun Yeung, who led the computational analysis at The University of Hong Kong.

“These mechanistic insights explain how biopolymers can achieve efficient light emission without relying on metals.”