A team of scientists from North Carolina State University has demonstrated a way to use low-energy, visible light to produce polymer gel objects from pure monomer solutions. The work not only poses a potential solution to current challenges in producing these materials, but it also sheds further light on how low energy photons can combine to produce high energy excited states.
Polymer products – primarily plastics – are used in everything from water bottles to medical applications, with billions of pounds of these materials being produced annually. Select polymers can be produced via a process called free-radical polymerization, in which a monomer solution is exposed to UV light. The high energy of UV light enables the reaction, forming the polymer. The advantages of this method include fewer chemical waste byproducts and less environmental impact.
However, this method is not without drawbacks. The high energy UV light used in generating these polymers can also degrade plastics and is unsuitable for producing certain materials.
Felix N. Castellano, Goodnight Innovation Distinguished Chair of Chemistry at NC State, had previously shown that it was possible to combine lower energy molecules’ excited states to achieve more potent excited states. In a new contribution, Castellano and his team applied a process – called homo-molecular triplet-triplet annihilation – to polymer production, by using lower energy yellow or green light to create polymer gels.
The team dissolved zinc(II) meso-tetraphenylporphyrin (ZnTPP) into 2 different pure monomers – trimethylolpropane triacrylate (TMPTA) & methyl acrylate (MA) – then exposed the solutions to yellow light. Energy from the light creates the homo-molecular triplets in ZnTPP, & when those triplets combine, they create an extremely short-lived S2 excited state that has enough energy to power the polymerization process.
“While triplets are 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 says.
“One of the important facets of this work is demonstrating that if you have a pure liquid you can utilize this potent, short-lived excited state to facilitate important transformations. The neat liquid ensures that electrons are transferred efficiently.”