They are the first to engineer molecules that efficiently absorb two particles of light, or photons, at the same time and trigger acid production at any point in three dimensions in the materials.

The researchers say success at making these molecules demonstrates a new strategy that is broadly applicable in microfabrication, medicine and optical information technology.

They have illustrated how the new materials could dramatically change microfabrication. They made networks of microchannels, as well as free-standing microstructures, by exposing solid resins of the new materials to near-infrared laser light.

For the past few years, Seth R. Marder and Joseph W. Perry and colleagues in the UA chemistry department and Optical Sciences Center have been developing new "two-photon" absorbing molecules. These are molecules that each simultaneously absorb two photons. Now the team has developed molecules that once excited, efficiently activate chemical reactions by transferring electrons to another part of the molecule.

In Friday's issue of Science (May 10), Marder, Perry, Christopher Ober of Cornell University and their colleagues describe how they have synthesized two-photon-absorbing molecules that are efficient "photoacid generators."

Until now, so-called "photoacid generators" ­- materials that generate acid when exposed to laser light -­ did not efficiently absorb by a two-photon process. As a result, photoacid generators haven't been widely practical.

Conventional photoacid generators typically excite a one-photon absorption process with ultraviolet light. They thus require long exposure times and high-powered beams that often degrade or destroy materials used in the process.

In their Science article, the authors describe using new polymer materials that are between 20 to a few hundred times more sensitive for two-photon 3D microfabrication because they incorporate the new UA-developed two-photon-absorbing compounds.

The researchers demonstrate how the new materials can be used to make three-dimensional microstructures of any desired shape, simply by focusing the laser anywhere acid is to be produced.

"Acid is one of the most ubiquitous reagents in chemistry," Marder said. "Protons (the simplest acid) can be used to start reactions that string monomers together to make polymers, or rip polymers into smaller fragments, or change solubility. We have the ability to put protons anywhere in materials with three-dimensional pinpoint control afforded by the two-photon-absorption process."

"We typically add a low concentration -- something like one percent -- of the photoactive compounds to polymer materials," Perry said. "We can then use these photo-acid generating compounds to do chemistry in the material to give it certain properties." Some properties might include making the material transparent, or giving it mechanical strength or stiffness, or making it soluble in water rather than in organic solvents.

In earlier work, Marder and Perry developed two-photon-absorbing materials that could be scanned with a laser, then washed with solvent so that only the laser-written pattern remained intact. They could laser write stick-like structures in plastic, then dissolve the plastic so that only the stick structures remained.

"We can do that using these photoacid generators, too. But we can also do the reverse. We can use protons to chemically change polymers so they can be dissolved in a basic water solution," Perry said.

"Imagine that we start with a plastic, totally impervious to water. We can wash it all day and nothing happens. We scan with the laser, protons nip off parts hanging from the side of the polymer, and now there are polymer chains that can be dissolved in water. Instead of ending up with a little framework of stick-like structures, we have a bunch of little channels buried in the object.

"From an architectural point of view, it's the difference between erecting a building or digging a mine. If all you want to do is dig a mine, you don't want to have to build the mountain first just so you can tunnel."

The Science article describes how Perry's lab produced microchannels using two-photon-generated photoacid.

Tianyue Yu and others in Christopher Ober's group at Cornell University designed and synthesized the polymer used with the UA photoactive compounds in the experiments.

The joint research project has been funded by the Air Force Office of Scientific Research and the National Science Foundation.

Beyond microfabrication, the new two-photon-activated photoacid technology has broad and far-reaching potential applications in medicine and information technology, scientists say.

"Imagine the biological applications, by virtue of being able to deliver protons where you want them," Marder said. "Our bodies are driven by pH changes. The ability to trigger pH change at will with three-dimensional control could be very important."

"Their two-photon generated photoacid process could be used for photodynamic therapy, three-dimensional photolithography, fast optical switching, or any number of applications," said University of Washington chemistry Professor Larry Dalton. In photodynamic therapy, for example, two intersecting infrared laser beams could trigger the release of a drug capsule, he said.


[Contact: Seth R. Marder, Joseph W. Perry]

13-May-2002