A Quinone Based Single-Molecule Switch as Building Block for Molecular Electronics

Azophenine has previously been identified as a controllable molecular switch when deposited on a Cu(110) surface, where it can be in two symmetry-equivalent states. Each of the two states can be set as well as read by means of a scanning tunneling microscope (STM). We propose a family of molecules based on the same quinone core, which show similar switching behavior without a supporting metal surface. Such a molecule could be an element in a molecular circuit or computer. Using the example of a simple hypothetical molecule, we show that it is possible to create molecules that show the necessary properties: two conformations with similar energy but different electric conductivity, and the possibility to switch between those by applying an external electric field.


INTRODUCTION
While the miniaturization of semiconductor-based circuits is still following Moore's Law, which describes a doubling of transistors per surface area every 18 months, this trend must reach its physical limits in the foreseeable future.One possible avenue of overcoming this barrier is the use of molecular electronics, where individual molecules would act as the building blocks of electronic devices, such as transistors or memory elements.A recent review article by Zhang et al. [1] demonstrates an active research area.Schaub et al. [2,3] have recently reported a controllable switch consisting of an azophenine molecule deposited on a Cu-(110) surface.If a voltage greater than 0.3 V is applied, one of two symmetry-related tautomers can be produced, depending on the position of an STM tip.A smaller voltage allows the current tautomeric state of the molecule to be determined without changing it.Translated into the language of computing, this constitutes a memory element that can be written and read.Unfortuntely, the need for an STM tip to be moved to the correct position above the molecule precludes the operation at a frequency that might be competitive with current microelectronics.An additional problem is that changes in conductance are only relevant in the direction perpendicular to the surface, because the supporting metal would short-circuit any voltage parallel to the surface.
In order to create a molecule that can be used in electronic devices, three prerequisites have to be in place: bistability, i.e. the existence of two states of the same or similar energy, a means of forcing the molecule to switch between these states, and a significant difference in conductance between those conformers.We will demonstrate that for a simple model system based on the same quinone core, with a coordinated iron atom, a similar aminoimino tautomerization mechanism as reported in the azophenine switch can be observed.
This makes it possible to address all three of these requirements.
If such a switch could be produced, it would be possible to create networks of switches, connected by molecular wires, in two or three dimensions.Figure 1 shows a possible example, using acetylene as a molecular wire, and for the sake of simplicity a quinone molecule with only one imino-amino switch.Assuming that the imino double bond is conducting and the amino single bond insulating, after switching the circled hydrogen atom as indicated by the arrow, the current will flow from the left bottom ring to the left upper ring, instead of from the left lower ring to the right upper ring.

METHODOLOGY
The molecular geometry and enery profile were determined using the PBE [4] hybrid functional with Grimme D3 dispersion correction and Becke-Johnson damping [5,6].The def2-TZVP basis set [7] was used for all calculations.As the aim of this study is a proof-ofprinciple on a model system, this level of theory was considered sufficient.All calculations were carried out using the ORCA [8] program.
The transition path was determined using a relaxed potential energy scan, reducing the distance of the switching hydrogen to the (initially imino) nitrogen from 2.2 Å to 0.9 Å in steps of 0.05 Å.As the energy changes more quickly in the later part of this scan, we reduced the step size to 0.025 Å in the range below 1.5 Å.The energies with an applied electric field were then calculated at the geometries determined during this scan.The molecule with a coordinated Fe atom was treated as an open-shell triplet, as this resulted in the lowest energy.Without a metal present, the molecule was treated as closed shell.Where an electric field was applied, this was oriented along the bond between the two carbon atoms furthest away from the switching hydrogen.This will be generally parallel to the switching direction, but the geometry of the carbon atoms defining the direction of the current will be less prone to geometric changes during the transition than atoms closer by.We did not take into account any changes in geometry that might be caused by the electric field.

RESULTS AND DISCUSSION
As discussed in the Introduction, we need to demonstrate the existence of two conformations of similar energy.This can be achieved by coordinating an iron atom above the quinone ring.Figure 2 shows the tautomerization barrier with and without this coordinated atom.
Without a metal atom, the quinone-like structure is, as expected, much more stable than the "switched" alternative.The latter is metastable, but with a barrier of 0.86 eV to reach, and a much smaller barrier (0.19 eV) for the backward transition.Note that the asymmetry in steepness of the curves, with the transition state appearing closer to the minimum at small N-H distance, is an artifact of the choice of reaction coordinate.With the coordinated Fe atom, the difference is reduced to 0.05 eV, which is much smaller than the barrier height.We believe that this difference in energy between the two minima may be further reduced by changes in the molecule or its surroundings.Thus, the first requirement for a molecular switch is fulfilled.
The second criterion is a considerable difference in conductivity between the two tautomers along the chosen bonds.This can be assumed to be the case, since the imino bond has double bond character and can function as the starting point of a molecular wire consisting of conjugated double or triple bonds, such as, e.g., [−CH−] n or [C 2 ] n , which are known to be conducting, whereas the amino nitrogen is connected to both of its neighbors via single bonds, which will break the conduction path.
The final requirement for a useable molecular switch is a means to trigger it.We tested the effect of an external electric field on the energy barrier.This would open up the opportunity to "clock" an electronic device through, for example, an oscillating external capacitor.We repeated our calculation of the energy barrier in the presence of a field of 1 V/ Å in the positive and negative direction along the switching path as described in the Methodology section.
Figure 3 shows the resulting barriers compared to the neutral case.All curves are calibrated to show their lowest point at zero.While not removing the barriers completely, an electric field introduces considerable asymmetry in the direction of the field.We have a difference of more than 0.5 eV between the minima (0.75 eV and 0.58 eV in the negative and positive direction, respectively), a large barrier of roughly 0.9 eV (0.86 eV and 0.98 eV for a negative and positive field, respectively) in the unfavored direction and less than a third of this (0.28 eV and 0.23 eV, respectively) towards the lower-energy minimum.While this asymmetry may not be sufficient to trigger an immediate transfer of hydrogen in case of a reversal of the electric field for our example molecule, it indicates that such a scenario should be possible, either by using slightly larger fields or lowering the barrier through a change in the chemical environment, such as electron donating or withdrawing side groups.

CONCLUSION AND OUTLOOK
In conclusion, the current work shows that a non-surface-based molecular switch satisfying our three criteria for good switching behavior is in principle feasible.It should be pointed out that this is a proof-of-principle study on a model system.In a realistic experiment, a means would have to be found to keep the coordinated metal atom in place, such as caging it in surrounding organic groups, or sandwiching it between two rings.A better understanding of the electronic structure features enabling the switch with a coordinated metal atom may facilitate the design of other molecules with similar properties but without the need for metal atoms.
If a network of switches was to be built, it would initially need to be constructed in two dimensions on a non-conducting surface or built into the third dimension as an organic or metal-organic framework.Besides an electric field, as explored in the current Letter, other switching mechanisms may include electronic excitations through photons or the current through the molecule.In the latter case, the network would be self-modifying and it would be possible to program various types of logic into it.

FIG. 1 .
FIG. 1. Network of molecular switches, showing the change in flow current (indicated by the solid lines), upon switching the circled hydrogen as indicated by the arrow .

6 FIG. 2 .
FIG.2.Energy profile along the switching coordinate for the molecule with and without a coordinated Fe.

FIG. 3 .
FIG.3.Energy profile along the switching coordinate in the presence of an external electric field.