RTC Week 2015 – #5: Forging Molecules Using Light

This week (19th Oct–24th October) is Real Time Chem Week (if that means nothing to you,check out their FAQ page here!). As part of it, we’re featuring the RTC Week competition-winning entries of five different chemists here on Compound Interest, with a different feature every day this week. Today’s final feature takes a look at how we can use light to create giant molecules for a whole range of applications, from dental fillings to sticky coatings.

Dr. Andrew Davis is a senior researcher at 3M, where he works on UV and photo chemistry for a range of applications. Here, he details how light can be used to create new molecules, along with some of their uses.

“I turn light into work. Not the solar-powered light-into-electricity work that receives all the popular buzz. I turn light into the energy of chemical bonds.

It all starts with using light to shatter a molecule. Like breaking a glass with your voice. Or using the apocryphal earthquake engine of Nikola Tesla, striking the resonant frequency of a building’s very own structure to bring it crumbling to the ground. A single photon, the most fundamental particle of light, can completely fragment a molecule. If the molecule is built right.

Why shatter a molecule? Once broken, the residual fragments of a molecule are often just itching to cause a reaction, and cause that reaction quickly. They are reactive enough to initiate a chain reaction of nearby molecules, even if those molecules are insensitive to light themselves. A single photon, breaking a single chemical bond, can give such a reactive intermediate to cause the near spontaneous formation of thousands of new interconnected covalent molecular bonds. It’s like the flick which topples the first domino. If the molecule is built right.

Some fragments will only react with carbon-carbon double bonds, like in an acrylic molecule. Some fragments will only react with strained rings, like in an epoxy molecule. By carefully planning the molecular environment and composition surrounding a molecule about to be fragmented by light, a chain reaction can make massive macromolecular polymers, millions of times larger than the starting molecules. It would be as if a single building was toppled by Tesla’s earthquake machine, and its orchestrated freefall catalyzed the formation of a sprawling metropolis.

Many of these resulting photo-synthesized macromolecules are not new. Polymer scientists have been making rubber and glue and protective coatings for the better part of a century using just heat. Here the activation with light can produce these same materials, but they are manufactured much more rapidly and with much less intense energy input than is required with large thermal reactors.

But some of the resulting macromolecules are indeed new, solely enabled by the delicate interplay of light and chemistry. Light is surprisingly easy to manipulate – much easier than heat. Exposing a photo-active compound to a gradient of light, or a rapidly pulsing the light’s intensity, can make materials that are sticky on one side but non-sticky on the other. It can make them tougher in prescribed microscopically arranged patterns. These materials would be difficult, often impossible to synthesize by conventional thermal reactions.

My research investigates the balancing act of finding the right structure of photo-active molecules and watching how they fragment under just the right conditions of light energy. I use those carefully selected fragments, triggered at a precise moment like a soprano breaking a wine glass, to rapidly, reliably, and efficiently make all sorts of useful light-forged macromolecules: the famously sticky coating on tape, the liquid resins that turn into hard enamel at the flash of a dentist’s light, the complex micro-structured layers in electronic displays. All made possible if you build the molecules right.”

Further Reading:

• Photoinitiated polymerisation – Y Yagci & others
• Electroluminescent networks via photo ‘click’ chemistry – A R Davis & others

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