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Reaction Dynamics Control Molecular Imprints

Photonics Spectra
Jan 2001
Richard Gaughan

John C. Polanyi, professor of chemistry at the University of Toronto, has studied chemical reactions for years, sharing the 1986 Nobel Prize in chemistry for his insights into this field of reaction dynamics. Today he is investigating the localized reaction of chlorine atoms separating from dichlorobenzene molecules. The reaction dynamics of chlorobenzene adsorbed onto the face of a silicon crystal may enable researchers to manipulate the structures of molecules at the most fundamental scale.

Polanyi's team induced the reactions with 12-ns pulses from a 193-nm excimer laser. The chlorine atoms, once freed from their attachment to the benzene and with the 6.3-eV photon energy from the light, could have bonded to the crystal at random locations. The researchers found, however, that the atoms adhere to specific sites.

"The effect of the light is to induce reaction in which formation of a new Cl-Si bond occurs concurrently with the breaking of the old C-Cl bond," Polanyi said, offering a sociological analogy to explain the orderly effect. "The chlorine leads a sheltered existence, going straight from its parent, chlorobenzene, to its marriage bed, silicon, without intervening bachelordom. It's easy to understand why it generally marries the girl next door."

The parent molecule is the key to the phenomenon. The chlorine atoms in the isomers of dichlorobenzene are attached to different locations on the benzene ring. When the parent 1,2 isomer photoreacts, it attaches a pair of chlorine atoms to silicon atoms that are 8 Å ±3 apart. The 1,4 parent, on the other hand, whose chlorine molecules are farther apart, marries them to silicon atoms separated by 14 Å ±3. Essentially, the shape of the parent molecule determines the shape of the imprint on the surface.

Molecular Printing

Although Polanyi is most interested in the factors that influence the spatial localization of the photoreactions, he imagines future applications. "One can dream of a molecular-scale printing press in which the ink is the adsorbate, the print is the pattern present in the ink and the press is the light," he said.

The first step is to continue basic research. "We need first to understand the reaction dynamics better so that we can control it," he said.

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