In Millionths of a Second, "Photocrystallography" Captures Big Changes in Transient Molecular Species

BUFFALO, N.Y. -- While new technologies and methods have made the determination of many molecular structures in their stable states practically routine, capturing structures of transient species -- those that exist during reactions for fractions of an instant -- still poses a formidable challenge.

Now, University at Buffalo scientists have reported the first experimental measurements of structures of high-energy states of molecules that exist for just millionths of a second.

The UB research, published on the Web in Chemical Communications in August, and was cited by the journal as one of its "hot papers."

Led by Philip Coppens, Ph.D., co-author on the paper who is SUNY Distinguished Professor and Henry M. Woodburn Chair of Chemistry in the UB Department of Chemistry, the UB scientists used a technique they developed, for which they coined the term "photocrystallography," which uses intense laser light and X-ray diffraction to reveal the structure of highly reactive molecules in these transient states.

"In the time-resolved studies, we take very short snapshots to capture fleeting transient states of molecules," said Coppens. "Knowledge of the structure of such transient species is essential to the understanding of catalytic, photochemical and biological processes."

In their latest work at the Advanced Photon Source at Argonne National Laboratory, the UB scientists used intense laser light to excite a molecule containing two rhodium atoms. Almost simultaneously, they then hit the molecule for a matter of microseconds with a burst of X-rays about one million times brighter than a standard laboratory X-ray source.

The laser light excites the molecule, while the pattern of X-rays diffracting off of the excited molecules in the crystal provide the data from which the scientists can determine the structure of the transient species.

"We have to probe the transient state quickly because the intense laser beam will eventually destroy the crystal," explained Coppens.

And because the laser used is so powerful that it can burn a hole in metal, the sample must be cooled to 17 degrees Kelvin, almost absolute zero. Otherwise, Coppens says, "the crystal will become a puff of smoke."

The scientists found that the laser light induced an unusually large contraction between the rhodium atoms.

"The resulting high-energy species are highly reactive," said Coppens, "and the energy stored in such molecules for very brief times can be used to initiate chemical processes."

Similar species probed by the Coppens group at UB are used in studies of solar energy capture, an important focus for the Department of Energy, which, along with the National Science Foundation, is sponsoring Coppens' work.

The research paper was co-authored by Andrii Kovalevskyi, Milan Gembicky, Ivan Vorontsov, all UB postdoctoral fellows, Oksana Gerlits, a UB doctoral student in the Department of Chemistry, and Yu-Sheng Chen, a doctoral student at the University of Toledo, all of whom participated in the experiments at Argonne National Laboratory. Co-author Tim Graber of the University of Chicago provided high-level technical assistance. Irina V. Novozhilova, a postdoctoral fellow in the Department of Chemistry at UB and co-author, who conducted theoretical calculations of molecules.

The experimental research at Argonne is done in parallel with theoretical calculations on the high-energy molecular states conducted at UB's Center for Computational Research that are important for the full interpretation of the experimental findings.