Introduction

There is a practical limit to how much can be packed onto a silicon chip. Present silicon chip technology is quickly closing in on this limit. "Physics may soon impose barriers that could slow the chips industry's blazing progress to a crawl."1 There is a need for the development of other technologies that will allow further miniaturization of electronics. One way this can be achieved is by using molecules to construct electronic circuits. Recently there have been numerous examples in the literature of nanoscale molecules with electronic properties.2-8 The work that is presented here proposes using molecular components, which have electronic properties, to construct a nanoscale molecular electronic logic circuit.' It is based on the concept of the molecular generator, which is one of the basic building blocks of this molecular circuit. The molecular nanomotor, from which we designed our molecular generator molecules, was first

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Figure 1. Schematic of a nanogenerator molecule.

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Figure 1. Schematic of a nanogenerator molecule.

f Author to whom correspondence should be addressed.

proposed by K.E. Drexler.10 Figure 1 shows the schematic of a nanogenerator molecule. The idea behind the nanogenerator molecule is a rotating benzene ring attached to conjugated polymers through o bonds. If the molecule is placed in a magnetic field perpendicular to the plane of the molecule, the rotating ring should produce an electrical potential that will induce a current in the overall molecule. Furthermore, different substituents such as halogens can be attached to each ring to alter the ring's resonant frequency. Circularly polarized light of different frequencies could then be used to stimulate rotation of the benzene ring on either one side of the molecule or the other This could be used to control the direction of the current in the molecular circuit. Figure 2 shows the schematic diagram of a potential logic circuit molecule, which incorporates the molecular generator molecule in Figure 1.

Figure 2. Nanoscale logic circuit molecule,

The logic circuit molecule is composed of two disubstituted phenyl rings connected to segments of polyaniline, polyphenylene vinylene, and polyacetylene polymer molecules. There is nothing unique about the conductive polymers chosen, and other good conductive polymer could be used. If each benzene ring is substituted with a different halogen at the X and Y positions (i.e. X=C1 and Y=F), then different frequencies of polarized light could be used to separately excite each of the molecular generators.

The frequencies of light required to excite rotation of the substituted benzene rings reside in the microwave frequency range. This will be demonstrated a little later. As an example, consider that microwave radiation of the particular frequency is used that only excites the substituted ring on the left side of the schematic in Figure 2. Let us also assume that there is a magnetic field coming perpendicular to and through the plane of the page directly at us. Rotation of the ring in the magnetic field in the indicated direction will cause a force to be exerted on the "free? electrons, to cause a small current to flow in the clockwise direction, as indicated by the arrow in Figure 2. This will cause electrons to flow to point A and then down the polyacetylene bridge to point B. It is important to note here that polyacetylene is used as a resistor in the circuit. This is based on the fact that the conductivity of bulk, undoped, trans-polyacetylene polymer is approximately 10-5 S/cm. The conductivity of the acid doped polyaniline polymer is approximately 10o to 101 S/cm.11 The resistance encountered by the electrons moving from A to B will cause more electrons to build up at point A, and less electrons at point B. Thus point A will be more negative, and point B will be more positive. Similarly if only the ring on the right side of the molecule in Figure 2 is excited, the rotation of the ring will cause a clockwise current in this side of the molecule. This in turn will cause current to flow from point B to point A across the polyacetylene bridge, which has the effect of making B more negative than A. This characteristic (the reversibility of charge on A and B according to frequency) will allow the circuit to function as a "yes" or "no" switch, or in other words as a basic logic component.

CALCULATIONS ElectricalProperties

We treated each benzene ring in the generator portions of the molecule in the approximation of a single wire loop, with the benzene ring's conjugated % system acting similar to a wire. If the benzene ring was placed in a magnetic field perpendicular to its plane as shown in Figure 3 then its rotation in this magnetic field would give rise to the following force, where N = number of turns in the loop, B is the magnitude of the magnetic field, A is the area of the loop, and to is the frequency of rotation for the loop. A benzene molecule was constructed using Hyperchem 5.0112 and then saved to a z-matrix file. This file was then imported into the Gaussian9413 program for geometry optimization. The molecule was then geometry optimized under Gaussian94 using the 6-31g* basis set, and the Berni optimization algorithm. The carbon bond lengths in the benzene ring where determined to be 1.385 A From this distance the area A in the formula for the benzene ring was calculated to be 9.967 x 10-20 m2. A value of N = 1 was used for the number of loops, and ax = 1.8 x 1012 sed-1 was the oscillation frequency used for the dichloro substituted benzene ring pictured in Figure 3. This frequency was obtained from dynamics simulations, which will be discussed in the next section. If the dichlorobenzene ring is placed in a 10 Tesla magnetic field, the rotation of the ring will produce a potential ( Eo) of 1.794 |V peak, 1.268 |V rms. Similarly when a value of to = 3.6 x 1011 sec-1 for the difluoro substituted benzene ring in a 10 T magnetic field will produce a potential Eo = 0.358 |V peak, and 0.253pV rms.

Figure 3. Dichloro benzene ring in a magnetic field coming out of the plane of the paper.

Figure 3. Dichloro benzene ring in a magnetic field coming out of the plane of the paper.

MolecularModelingandDynamics

The molecule was constructed and modeled using Sybyl14 on a Silicon Graphics Workstation. Figure 4 illustrates a wire diagram of the molecule without the hydrogen atoms after it was energy minimized using the interface to MOPAC, and the AMI parameter set under Sybyl.

Figure 4. Wire diagram of nanologic circuit molecule without hydrogen atoms.

Once the energy-minimized structure was obtained, it was used as a starting point for molecular dynamics (MD) calculations. We were interested in performing MD simulations on this molecule in order to assess the amount of flexibility that it possesses. In order for the molecule to perform as already described the polymers from which it is constructed have to stay fairly rigid and the molecule as a whole should be planar. MD simulations allowed us to observe the behavior of the system in vacuum at room temperature. The molecule was simulated in vacuum, using a timestep of 1 femptosecond - which corresponds to the period of a hydrogen bond stretch. The system was first heated from absolute zero to 298 K over a heating run of 50 picoseconds, where the temperature of the system was increased by 3 K every 500 timesteps. Then the system was equilibrated for another 10 picoseconds to allow the energy to dissipate evenly throughout the system. At this point the system was ready for a constant room temperature dynamics simulation. The dynamics simulation was run for 50 picoseconds, again with a timestep of 10-15 seconds. The system maintained its shape fairly well through approximately 25 picoseconds of dynamics. But after 25 picoseconds of dynamics the molecule started to fold over on itself (Figure 5). This folding over process continued during the next 25 picoseconds of dynamics to yield the conformation depicted in Figure 6.

One of the reasons for running the dynamics calculations was to observe how flexible the molecule is. The electronic properties that were described would only be observed if the molecule could maintain a fairly planar shape. Only a planar shape maintains enough % orbital overlap intramolecularly and in-between the polymers which allows conjugation and conductivity. The results as depicted in Figures 5 and 6 show that a single molecule is very flexible and that ways to impose planarity on the molecule need to be examined.

Figure 5. The nanologic molecule at 25 picoseconds.
Figure 6. The nanologic molecule at 50 picoseconds.

Further information about the properties of the molecule was also obtained from the dynamics calculations. The frequency of oscillation at room temperature for each of the dihalo - substituted benzene rings was obtained by plotting each ring's rotational dihedral angle versus time. Figure 7 is a plot of the difluorobenzene ring to vinylene dihedral angle versus timestep for the duration of the 50-picosecond constant room temperature dynamics run.

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Figure 7. Dihedral angle (in degrees) associated with the rotation of the difluorobenzene ring versus time in picoseconds (each tic represents 5 picoseconds).

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Figure 7. Dihedral angle (in degrees) associated with the rotation of the difluorobenzene ring versus time in picoseconds (each tic represents 5 picoseconds).

The frequency of rotation of the difluorobenzene ring was determined to be 1.8 x 1012 Hz. We also obtained a similar plot for the rotational dihedral angle of the dichlorobenzene ring versus time. The results are found in Figure 8. For the dichlorobenzene ring, the frequency of rotation is estimated to be 3.6 X 1011 Hz. The difference in the calculated frequencies between the two dihalo-substituted benzene rings demonstrates that different frequencies of electromagnetic radiation are required to stimulate rotation for each.

Time (5000timesteps =5 picoseconds

Figure 8. Dihedral angle (in degrees) associated with the rotation of the dichloro-benzene ring versus time in picoseconds (each tic represents 5 picoseconds).

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