RALEIGH, N.C., Sept. 21, 2020 — Visible yellow and green light are central to a process that researchers from North Carolina State University used to produce polymer gel objects from pure monomer solutions. After dissolving zinc meso-tetraphenylporphyrin (ZnTPP) into pure monomers trimethylolpropane triacrylate (TMPTA) and methyl acrylate (MA) and exposing the solutions to yellow light, light-generated energy created homomolecular triplets in ZnTPP. When the triplets combine, they create an extremely short-lived S2 excited state with enough energy to power the polymerization process.

That initial process is called homomolecular triplet-triplet annihilation. The work has application in the production of polymer gel objects and polymer products, such as plastics. It also highlights the potential and usability of low-energy photons in combining to produce high-energy excited states.

Spectroscopic analysis confirmed the existence of the S2 excited state in the presence of (low-energy) yellow and green light. The use of spectroscopy allowed the researchers to visually confirm polymer formation, a direct result of the S2 excited state, in their work.

“We used ZnTPP because it allows you to see light emission from two different excited states, and we could differentiate between lower energy S1 and higher energy S2 states,” said Felix Castellano, corresponding author of the article describing the process and Goodnight Innovation Chair of Chemistry at North Carolina State University. North Carolina State University graduate student Nancy Awwad is the study’s first author.

Though the application of homomolecular triplet-triplet annihilation is recent, Castellano and a team previously demonstrated the ability to combine the excited states of lower energy molecules to yield increasingly potent excited states capable of measurement.

“While triplets are really long lived in chemical terms — they live for milliseconds — the S2 excited state only lives for picoseconds, which is nine orders of magnitude less,” Castellano said. “One of the important facets of this work is demonstrating that if you have a pure liquid you can utilize this potent, short-lived state to facilitate important transformations.”

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A photoactivation mechanism supports polymer production. The method follows a North Carolina State University-led team's demonstration that the combination of the excited states of lower energy molecules can yield highly potent excited states. Courtesy of Anh Thy Bui.

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Currently, manufacturers can use a process known as free radical polymerization to produce certain polymers; the process relies on UV light to enable a polymer-forming reaction that occurs when a monomer solution is exposed to UV light. Though the method’s chemical waste byproducts are relatively few, the high-energy UV light is unsuitable for producing certain materials.

BioLEC, an Energy Frontier Research Center supported by the U.S. Department of Energy, funded the work, which appeared in Chem (www.doi.10.1016/j.chempr.2020.08.019).