NEWS OF THE WEEK
SUPRAMOLECULAR EXTENSION CORD NANOELECTRONICS: Self-assembling photoinduced electron-transfer system mimics macroscopic device
T
HE BOTTOM-UP CONSTRUCTION of molecular computers has come one step nearer with the development of a solution-phase plug-and-socket molecular system that is an analog of a macroscopic electrical extension cord. The "wet" self-assembled system has three molecular components: a light-powered electron source, an electron drain, and a connecting cable, one end of which connects to the source and the other to the drain. It was designed and constructed by chemistry professors Vincenzo Balzani of the University of Bologna, in Italy, and J. Fraser Stoddart of the University of California, Los Angeles, and their coworkers (Proc. Natl. Acad. Sci. USA 2006,103,18411). "We carefully designed the system so that the connections between the three components are reversible," Balzani tells G&EN. "The system can therefore be programmed to perform signal processing in solution." The light-harvesting component is a ruthenium bipyridyl complex connected to a dibenzocrown ether unit that acts as a socket. The cable has a dialkylammonium ion center that can plug into the socket by hydrogen bonding. At the other end of the cable is a 7C-electron-rich benzonaphthocrown ether socket. The electron drain, a Tc-electron acceptor bipyridinium unit, plugs into the cable's socket by 7r-electron donoracceptor bonding. "In the fully assembled system, excitation with visible light of the electron power unit is followed by transfer of an electron, on the nanosecond timescale,
through the extension cable to the electron drain," Balzani explains. The system is switched on and off by connecting and disconnecting the molecular components. Acidbase and redox stimuli are used as switches for the hydrogen-bonding and 7t-donor-acceptor connections, respectively. Because the photoinduced electron flow in the fully connected system responds to two chemical inputs operating in series, the system can play the role of a logic gate. This socket-plug-socket-plug system is an improvement on a similar system that the same team reported four years ago (G&EN, Oct. 21,2002, page 17). That system did not mimic a real extension cord because it had a socket-plug-plug-socket configuration. In addition, the photoinduced electron transfer did not reach the last component of the assembly. The new system is a more accurate molecular analog of the macroscopic device, comments A. Prasanna de Silva, a chemistry professor at Queen's University, Belfast, Northern Ireland. The con
PLUGGED IN Assembled system consists of lightdriven electron source (purple) connected to socket (blue), cable with plug (orange) and socket (red), and electron drain plug (green).
Visible iight
struction molecular analogs of devices from the macro world is "a very profitable approach" to developing molecular nanosystems with demonstrable functions, he adds. The work is a "beautiful" extension of the 2002 concept, says Oxford University chemistry professor Harry L. Anderson. "One day, photoinduced electron transfer might be used in molecular-scale information processing devices," he says.— MICHAEL FREEMANTLE
°LVVJ
ATMOSPHERIC CHEMISTRY Air pollution controls nip NOx in the bud Using newly available satellite data, scientists in the U.S. and Germany have shown that atmospheric concentrations of NOx—the combination of NO and N0 2 central to the production of near-surface ozone and smog—have declined over the Ohio River Valley by about 4 0 % since 1999 (Geophys. Res. Lett, DOI: 10.1029/2006GL027749). In-stack NOx measurements from power plants fitted with emissions controls have been indicating declining NOx emissions, but it has taken atmospheric transport and chemistry models to get
pictures of the larger, regional consequences. Now, Si-Wan Kim, Stuart McKeen, Gregory Frost, and Michael Trainer of the National Oceanic & Atmospheric Administration and their colleagues have turned to orbiting sensors to measure those regional NOx trends more directly. "We have used satellite-borne measurement to demonstrate that the reduction of NOx by power plants has been effective" in the Ohio River Valley, Kim says. Similar declines in NOx levels did not show up along the Northeast Corri-
WWW.CEN-0NLINE.ORG
12
DECEMBER 4, 2 0 0 6
dor, where cars and other mobile sources, rather than power plants, are the dominant NOx sources. Computer models of near-surface ozone production suggest that the diminishing NOx levels should be reflected by 4 - 1 0 % reductions in ozone in the Ohio River Valley and nearby regions, the researchers report. Preliminary on-ground ozone measurements confirm that simulation result, comments Kenneth L. Schere of EPA's Atmospheric Sciences Modeling Division.—IVAN AMATO
