Giving Molecular Computer Chemistry a Bit of Spin

Giving Molecular Computer Chemistry a Bit of Spin

Dr. John Emsley

WHAT'S HOT IN CHEMISTRY...

Rank	Paper	Citations This Period Nov-Dec 01	Rank Last Period Sep-Oct 01
1	G. Schaftenaar, J.H. Noordik, “Molden: a pre- and post-processing program for molecular and electronic structures,” J. Comp.-Aid. Molec. Des., 14(2):123-34, February 2000. [Nijmegen U., Netherlands] *279EG	20	6
2	Z. Yao, et al., “Carbon nanotube intramolecular junctions,” Nature, 402(6759):273-6, 18 November 1999. [Delft U., Netherlands; Bell Labs, Lucent Technol., Murray Hill, NJ] *257ZP	18	1
3	C.P. Collier, et al., “A [2]catenane-based solid state electronically reconfigurable switch,” Science, 289(5482):1172-1175, 18 August 2000. [Univ. Calif., Los Angeles] *346JE	15	†
4	A.K. Chatterjee, et al., “Synthesis of functionalized olefins by cross and ring-closing metatheses,” J. Amer. Chem. Soc., 122(15):3783-4, 19 April 2000. [Caltech, Pasadena] *308TT	12	†
5	Y. Shi, et al., “Radial capillary array electrophoresis microplate and scanner for high-performance nucleic acid analysis,” Anal. Chem., 71(23):5354-61, 1 December 1999. [U. Calif., Berkeley] *260YW	12	†
6	A.F. Littke, C. Dai, G.C. Fu, “Versatile catalysts for the Suzuki cross-coupling of arylboronic acids with aryl and vinyl halides and triflates under mild conditions,” J. Amer. Chem. Soc., 122(17):4020-8, 3 May 2000. [MIT, Cambridge] *312NZ	10	5
7	J.P. Wolfe, et al. “Simple, efficient catalyst system for the palladium-catalyzed amination of aryl chlorides, bromides, and triflates,” J. Org. Chem., 65(4):1158-74, 25 February 2000. [MIT, Cambridge, MA] *289DH	9	9
8	T.-Q. Nguyen, et al., “Controlling interchain interactions in conjugated polymers: The effects of chain morphology on exciton-exciton annihilation and aggregation in MEH-PPV films,” J. Phys. Chem., 104(2):237-55, 20 January 2000. [U. Calif., Los Angeles] *278TT	9	7
9	J. Grafenstein, et al., “An accurate description of the Bergman reaction using restricted and unrestricted DFT: Stability test, spin density, and on-top pair density,” J. Phys. Chem., 104(8):1748-61, 2 March 2000. [U. Goteborg, Sweden] *289VF	9	†
10	A. Furstner, et al., *“Ruthenium carbene complexes with N,N'-bis(mesityl)imidazol-2-ylidene ligands: RCM catalysts of extended scope,”* J. Org. Chem., 65(7):2204-7, 7 April 2000 [Max Planck Inst., Mulheim, Germany; U. New Orleans, LA] *302CV 8	8	†

SOURCE: ISI's Hot Papers Database. Read the full legend.

e all have access to a remarkable molecular computer: the human brain. If current research is successful, we may one day have access to electronic computers with molecular hardware. In fact many of the fundamental devices of today’s PCs, such as conducting wires, rectifiers, tunnel junctions, one-way switches, and logic gates, already have their molecular counterparts and, in August 2000, these were joined by the most sophisticated device yet: a nanoelectronic reconfigurable switch.

This was announced in Hot Paper #3 and came from the groups of J. Fraser Stoddart and James Heath based in the Department of Chemistry and Biochemistry at the University of California at Los Angeles, and its appearance in the list is an indication of how important this area of chemistry has become. Indeed the journal in which it appeared, Science, made molecular electronics the “breakthrough of 2001” and there are now expectations that within ten years this technology will dominate, judging by the research activity of computer hardware manufacturers.

Heath and Stoddart’s device consists of two interlinked cyclic components, the whole assembly being referred to as a [2]catenane. One of the rings is a crown ether in which there is incorporated a naphthalene component at one side and a tetrathiafulvalene (TTV) component on the opposite side. This ring is threaded through a cyclophane ring, which is made from two bipyridinium (BP) units joined together, and this carries four units of positive charge on the BP nitrogens. This second ring prefers to place itself around the TTV of the other ring, and this represents the “open” state of the molecular switch.

The switch can be “closed” by applying a positive charge to the TTV, which is then repelled by the cyclophane’s nitrogens. The result is that the crown ether ring spins around until its neutrally charged naphthalene segment is within the orbit of the cyclophane. Turn the charge off and the crown ether reverts to its preferred position. The switch is opened when +2 volts is applied and closed with –2 volts, and it can be “read” at 0.2 volts.

Heath and Stoddart’s molecular switches are stable and robust, and function under normal operating conditions. A device made from them was switched on and off several hundred times over a period of two months without any noticeable deterioration in their operation. It was fabricated from a single monolayer of the [2]catenane, which was sandwiched between an n-type silicon electrode at the bottom and a titanium/aluminium electrode at the top. The [2]catenane was anchored to the lower electrode by negatively charged phospholipids and the upper electrodes were deposited using electron-beam evaporation, first of a 0.5 micron layer of titanium and then a 10 micron layer of aluminium.

Fraser Stoddart has been active in molecular electronics for many years and began his research into this area when he was at the University of Sheffield, England, in the 1970s. In 1990 he moved to the University of Birmingham there, where his research was supported by specially targeted U.K. Government funding. Then in 1998 he moved to California, since when his research has gone from strength to strength.

Stoddart envisages a rosy future for his and Heath’s work. Referring to paper #3, he says: “Our catenane- and rotaxane-based molecular switches are going to be difficult to beat. It took us 20 years to develop them to their present level and they really are the nano equivalent of tiny Rolls-Royce engines.” He is keen to emphasize that what has been achieved could only have been done by close collaboration with Heath: “Neither of us could have made any significant progress toward the realization of a molecular computer without the other. United we have a chance of hitting the jackpot!”

A fuller version of paper #3 appeared last year in Journal of the American Chemical Society (see C.P Collier, et al., 123[50]:12632-41, 2001), and Stoddart has coauthored two major reviews of molecular electronics: one in Accounts of Chemical Research, (A.R. Pease, et al., 34[6]:433-44, June 2001), which deals with switching devices, and in Structure and Bonding, (A.R. Pease, J.F. Stoddart, 99:189-236, 2001), which covers all aspects of molecular computing. To Science Watch, Stoddart revealed that in December last year they were able to incorporate molecular switches into a 64-bit memory device with the ability to store 10-letter words in a molecular memory. “I may yet live to see the first molecular computer,” joked the 59-year-old chemist.

Dr. John Emsley is science writer in residence at the
Department of Chemistry, Cambridge University, U.K.