Twisted Polymers Fade with Taggant

Conjugated polyphenylenes may be the key to practical detection of 2,3-dimethyl-2,3-dinitrobutane (DMNB), according to new work by Samuel W. Thomas III, a graduate student of Massachusetts Institute of Technology chemistry professor Timothy M. Swager.

DMNB is a required taggant additive in commercially manufactured plastics explosives. Because of its high vapor pressure, DMNB signals the presence of the explosives, which generally have very low vapor pressures. Current technologies for homeland security, however, do not detect DMNB with sufficient sensitivity.

Thomas presented his findings on Aug. 28 during a poster session sponsored by the Division of Organic Chemistry at the American Chemical Society national meeting in Washington, D.C.

Swager’s lab has been developing sensors for explosives. A conjugated polymer developed there for sensing TNT has been commercialized by Nomadics, Stillwater, Okla., a company specializing in technologies for homeland security. That polymer, a poly(phenylene ethynylene), might have been the starting point for the work with DMNB, Thomas told C&EN, but it wasn’t.

DMNB and TNT are too dissimilar, Thomas said. The biggest difference is the ease with which TNT accepts excited-state electrons from the poly(phenylene ethynylene), which is the basis for its detection by the polymer. The transfer dims the polymer’s natural fluorescence, he explained.

It is much harder to stick electrons to DMNB, according to Thomas. “What’s needed is a polymer in which excited-state electrons are in a high enough energy state that they would want to go down to DMNB,” he explained.

Thomas turned to polyphenylenes, polymers with an uninterrupted string of aromatic rings that causes the backbone to twist. Polyphenylenes are well-known materials for organic light-emitting diodes, Thomas said, but they have not been widely explored for vapor sensing. The excited-state electrons in these polymers are in much higher energy states than those in the TNT sensor.

Experiments carried out in solution proved that DMNB can accept electrons from several polyphenylenes—and dim the polymers’ fluorescence—but not from poly(phenylene ethynylene)s. “The solution experiment showed us that twisting the backbone is good” for detecting DMNB, Thomas said. “But we didn’t know if that would work in the solid phase.”

Testing in the solid phase turned out to be easy. Thomas simply used a Nomadics device for TNT sensing and installed polyphenylene films in place of poly(phenylene ethynylene). The experiment revealed that, among the polyphenylenes tested, one that has been proposed as a blue light source for light-emitting devices works best.

“We have taken this problem as far as an academic lab wants to. We’ve learned how to make conjugated polymers more versatile, and we’re hoping to apply the lessons learned to other important and challenging analytes,” Thomas said. “As far as the future of DMNB detection, Nomadics is negotiating to license this technology.”