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PHYSICAL SCIENCES / APPLIED PHYSICAL SCIENCES
A 16-bit parallel processing in a molecular assembly
International Center for Young Scientists, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Edited by Mark A. Ratner, Northwestern University, Evanston, IL, and approved January 15, 2008 (received for review April 4, 2007)
A machine assembly consisting of 17 identical molecules of 2,3,5,6-tetramethyl-1–4-benzoquinone (DRQ) executes 16 instructions at a time. A single DRQ is positioned at the center of a circular ring formed by 16 other DRQs, controlling their operation in parallel through hydrogen-bond channels. Each molecule is a logic machine and generates four instructions by rotating its alkyl groups. A single instruction executed by a scanning tunneling microscope tip on the central molecule can change decisions of 16 machines simultaneously, in four billion (416) ways. This parallel communication represents a significant conceptual advance relative to today's fastest processors, which execute only one instruction at a time.
multilevel logic | parallel communication | self-assembly
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/cgi/content/full/0703105105/DC1.
DRQ alkyls rotate like machines with change in logic states 0 
1 (120°), 1 2 (60°), and 2 3 (60°). DRQ single molecule demonstrates double negative differential resistance (NDR) peaks, which creates four different states; two neutral (state 0 and state 2) and two negatively charged either by one (state 1) or by two electrons (state 3). These multiple states of quinones are well known since long time. The previous reports can be found in ref. 18. For detailed RAM, ROM operations of DRQ, see ref. 19. The structural details and STM images of the four states can be found in ref. 20. Similar to DRQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (or DDQ) switches between any four conformers in 
1 µs and remembers a state for 8 min in an isolated form. Four states in quinones are also confirmed by cyclic voltammetry measurements and simultaneous electronic and optical studies (21). The DRQ is deposited on a reconstructed Au(111) substrate by using a single-crystal gold (99.99+%) substrate reconstructed by annealing the crystal at 700 K for 30 min, followed by Ar sputtering for 30 min (the cycle was repeated five to six times). The DRQ was evaporated from a K-cell at a 100-K substrate temperature in an ultrahigh vacuum STM (10–8 torr) by thermal evaporation from a source kept at 383 K (the melting point for DRQ is 383 K) with an exposure of 15 min.
The minimum energy structure and conformational search for the single molecule, dimer, linear, and circular chains, orientation of the molecules on a gold surface, and electronic properties were studied following ab initio Hartree–Fock and DFT computation at the B3LYP level with a 6-311G** basis set. The repulsion during architecture formation is modeled by combining an exponential repulsion with an attractive dispersion interaction (1/R6), EvanderWaals = 
ij
(290000e–12.5/R – 2.25R–6, where R = rij/(R*i + R*j). The parameters are as follows: R*i and R*j are the van der Waals radii for the atoms, epsilon () determines the depth of the attractive potential energy well, and rij is the actual distance between the atoms. At short distances, the above equation favors the repulsive interaction over a dispersive interaction. To compensate for this favor at short distances (R = 3.311), the term EvanderWaals is replaced with EvanderWaals = 336.176ijR–2. The architecture minimization is repeated until the simulation converges into well observed architecture.
¶ Once architecture is determined, the relative 3D spatial distribution of relative charge density is calculated by DFT, using numerical orbitals double zeta potentials (DZP), with local density approximation (LDA-PZ) and mesh cut-off at 150 Ry. Strict convergence of Hamiltonian at 10–5 critical limit was employed during computation.
|| We fix the STM tip as suggested in ref. 22 and then the point-contact measurements as suggested in ref. 23.
To whom correspondence should be addressed. E-mail: anirban.bandyopadhyay{at}nims.go.jp or anirban.bandyo{at}gmail.com
© 2008 by The National Academy of Sciences of the USA
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