Low-Energy Conformational Gating in π-Conjugated Molecular

Letter pubs.acs.org/JPCL

Low-Energy Conformational Gating in π‑Conjugated Molecular Junctions Daijiro Nozaki,*,† Cormac Toher,†,∥ and Gianaurelio Cuniberti†,‡,§ †

Institute for Materials Science and Max Bergmann Center of Biomaterials, TU Dresden, 01062 Dresden, Germany Center for Advancing Electronics Dresden (cfAED), TU Dresden, 01062 Dresden, Germany § Dresden Center for Computational Materials Science (DCCMS), TU Dresden, 01062 Dresden, Germany ‡

ABSTRACT: Oligo(phenylene ethynelene) is widely used as a molecular conductor in molecular electronics. Phenylene units connected by two carbon−carbon triple bonds at the para position are known to have a low energy barrier (less than 10 kJ/mol) for rotation around the axis of the molecular wire. π orbital localization accompanied with the rotation of the phenylene unit due to the reduction of the π−π coupling strongly influences the electron transport through the molecular wires. Here we demonstrate low-energy conformational gating in oligo(phenylene ethynylene)-based molecular wires connected between contacts. We could achieve an on−off ratio of over 104 by rotating the phenylene units out of the plane of the molecule, which requires only 90 meV in total energy. This result implies that studies of electron transport using stationary geometries may require that special attention be paid to the relationship between molecular fluctuations and conductance. SECTION: Physical Processes in Nanomaterials and Nanostructures he class of π-conjugated organic molecules are attractive materials as molecular conductors for a wide range of applications such as organic light-emitting diodes and organic solar cells, due to their highly conducting electron transport channels, which are formed by delocalized π-orbitals.1 Among the many kinds of molecular wires, oligo(phenylene ethynylene) (OPE) and its derivatives are widely used for molecular wires and as the main building blocks of molecular devices due to their ease of synthesis and the controllability of their electronic structures.2−7 In addition, the controllability of the molecular structures by the choice of the positions of the linkers (ortho, meta, and para) is also another advantage of this class of molecules. In para-OPEs, the phenylene units have a low energy barrier for their rotation around the axis of the molecular wires.8−10 When a phenylene unit in the OPEs rotates out of the plane, the π−π coupling between the phenylene unit and the other parts of the molecule is reduced, decreasing the electron transmission through them.9 Since the electron transport through the molecules is highly sensitive to conformational fluctuations9,11−15 caused by thermal excitation, the design of molecular devices such as molecular switches13,16−22 and transistors16,23,24 has to take into account the relationship between the molecular conformation and the transmission. In this study we modeled OPE-based molecular junctions connected between contacts and investigated the electron transport properties of the junction as a function of rotational angles of phenylene units. We also have performed molecular dynamics (MD) simulations and analyzed how the conformational fluctuation influences the transport properties of the

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© XXXX American Chemical Society

OPE-based molecular junctions. We found that the electron transmission through the OPE-based junction is strongly modified by low energy conformational changes. This result implies that studies of electron transport using stationary geometries may need to account for the relationship between molecular fluctuation and its conductance. Figure 1 shows the prototypical molecular junctions considered in this study, where an OPE is connected between two leads consisting of carbon nanotubes (CNTs) via peptide linkers. Such molecular linkage to conducting CNTs via peptides has been realized in the study of the electron transport properties of DNA molecules.25 In order to determine the stable configuration of the molecule between

Figure 1. Relaxed structure of the molecular junctions considered in this work: oligo(para-phenylene ethynylene) is connected between electrodes made from carbon nanotube via peptide-linkers. Received: October 8, 2013 Accepted: November 21, 2013

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CNT contacts, we first performed a geometry optimization for the extended molecule, where the OPE molecule is connected to one surface layer of the CNT contacts on both the left and right, by means of the conjugate-gradient technique using the density- functional tight-binding (DFTB) method26 within periodic boundary conditions. The geometry optimization was carried out until the absolute value of the interatomic force reduced to less than 10−4 atomic units. We used the Slater− Koster parameters developed by Rauls et al. for the C, H, N, O atoms.27 The structure-optimized unit cell of the extended molecule is placed between semi-infinite CNTs, and then transmission calculations are carried out using the nonequilibrium Green’s function formalism as implemented in the gDFTB code.28 We first examined how the rotation of the central phenylene unit affects the total energy of the system shown in Figure 1 in the static configurations. For this purpose, we relaxed the extended molecule in Figure 1 under the restriction where the positions of a few atoms are kept fixed such that the rotational angle between two phenylene units is kept to certain values. Figure 2 presents the total energy of the extended molecule as a

not sterically hindered in the self- assembled monolayers. In order to examine the effect of the rotation of the phenylene units to transport properties, we have calculated transmission functions through the system in Figure 1 with different rotational angles using gDFTB code.28 The transmission function through the molecular junction as a function of electronic energy for different rotational angles is shown in Figure 3a. The electronic states originating from the OPE molecule correspond to the resonant peaks in the transmission, while the off-resonant tunneling regime spreads between them. Figure 3b shows the transmission spectra of the system in Figure 1 at the Fermi energy as a function of rotational angle of the phenylene unit in the middle. We can see that the transmission at the Fermi energy is reduced as the rotational angle of the central phenylene unit increases. This is due to the electronic separation between the central phenylene unit and the other part of the molecule. The reduction of the electronic interaction between phenylene units is also supported from the band gap between HOMO peaks (around −5.5 eV) and LUMO peaks (around −2.5 eV) in the transmission functions. For small rotational angles, the HOMO−LUMO gap is smaller, which means that the π-orbital hybridization between phenyl groups is large, while when the rotational angle is close to 90 degrees the gap gets larger. It is surprising that a rotation of the phenylene unit requiring less than 0.1 eV can considerably suppress the tunneling current yielding an on−off ratio at the Fermi energy of over 10 000, as shown in Figure 3b. The phenylene units in the OPE can rotate with low energy excitations. During the rotation, the π−π coupling between phenylene units is reduced. Thus, the relationship between transport properties and molecular conformation in dynamical situations needs to be assessed for the design of OPE-based molecular devices such as molecular switches and transistors. For this purpose, we have performed MD simulations for the molecular junction in Figure 1 with different temperatures using the DFTB method and analyzed the structures, energetics, and transport properties. It is worth mentioning that the MD trajectory should be calculated using the velocity calculated under nonequilibrium conditions including the effect of heating produced by tunneling current. In addition, the effect of electron transport on the vibrational state of the molecule should also be considered.30 Nevertheless, for simplicity, we assumed that the system is under equilibrium conditions within the linear scaling regime in the zero bias limit. Even under this simplification, it is possible to extract important insights into the changes of conductance due to modifications of the atomic structures of the molecules. The motion of the atoms during the MD simulations was calculated using the standard velocity Verlet

Figure 2. Total energy of the system in Figure 1 as a function of rotational angle of the middle phenylene unit. The energy barrier for the rotation is less than 0.1 eV.

function of rotational angle of the central phenylene unit. It is notable that the OPE requires less than 0.1 eV for the rotation of the central phenylene unit. This result was checked by comparing with full DFT calculations using the SIESTA method.29 These calculations indicated that the energy requirement to rotate the central ring through 90 degrees was of the order of 130 meV, which is of the same order as the DFTB results. From this result, it can be easily imagined that the phenylene units are freely rotating in the molecular junctions at room temperature as long as the rotation of the phenylene units is

Figure 3. (a) Transmission spectra with different torsional angles of the middle phenylene unit. (b) Transmission at the Fermi energy. 4193

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from the calculation of the DOS of an infinite carbon nanotube using the gDFTB code. Although the effect on the transport of the inelastic scattering of electrons by vibrational modes could also be significant,31 we do not include these effects in our work but instead focus only on coherent tunneling. The electron transmission through the system in Figure 1 at the Fermi energy along the MD trajectory with different temperature is shown in Figure 6. As expected from the static

algorithm with a time step of 1.0 fs and a total duration of 10 ps. For the investigation of the influence of the fluctuation of the molecular wires on the transport, only the atoms in the scattering region are allowed to move during the MD simulation. After the MD simulations, we first analyzed the total energies of the molecular system. Figure 4 presents the total energies of

Figure 4. Total energy as a function of time with different temperatures in DFTB-MD simulations. The MD calculation is performed under equilibrium conditions with the restriction that only atoms in the central molecule and the first layers of the left and right contacts are allowed to move.

Figure 6. Transmission at the Fermi energy as a function of time with different temperatures in the same run of the DFTB-MD simulation as in Figure 4

analysis in Figure 3, when the rotational angle of the phenylene unit is close to 90 degrees, the electron transmission is highly suppressed. At lower temperatures, this trend is rarely seen thus leading to stable current flows through the system, while the fluctuations are quite large in the case of room temperature. In summary, we have analyzed the effect of the rotation of the phenylene units on the electron transport, taking OPEbased molecular wires as an example in order to investigate the relationship between molecular conformation and electron transport. We have modeled OPE-based molecular junctions and calculated their conductance with different rotational angles of phenylene units. The phenylene units in OPE-based molecular wires can rotate with low energy excitations of less than 0.1 eV, and the rotation can suppress the electron transport through the wire considerably. The analysis of the transmission along the MD trajectory demonstrates the suppression of electron transport due to the rotation of the phenylene units caused by the thermal fluctuation of the molecular conformation. Thus, the design of molecular devices consisting of OPE components requires that special attention be paid to the relationship between molecular fluctuations and conductance under dynamic conditions.

the molecular system in Figure 1 as a function of time with different temperatures. We can see that the total fluctuations in the total energy are larger at higher temperatures and that the width of the fluctuation reaches 3 to 4 eV for the case of room temperature. From this result we can expect that the phenylene units in the OPE rotate freely at room temperature since the rotation requires an energy of less than 0.1 eV. In order to check the rotation of the central phenylene unit in the OPE due to the thermal fluctuations, the rotational angle of the central phenylene unit is plotted in Figure 5. We can observe that the dispersion of the rotation angle is wider at room temperature, while the dispersion is reduced at lower temperatures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected]. de.

Figure 5. Rotational angle as a function of time with different temperatures in the same run of the DFTB-MD simulation as in Figure 4. The middle phenylene ring rotates freely because of the low energy barrier for its rotation.

Present Address ∥

C.T. is now at the Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC. Notes

From this result, it can be easily imagined that the electron transport through the OPE is significantly modified along the MD trajectory due to the rotation of the phenylene units. In order to assess this, snapshots were extracted from the MD trajectories every 10 fs, and then the conductance at the Fermi energy for the 1000 selected geometries were calculated. The Fermi energy of the system was determined to be −4.544 eV

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the European project Synaptic Molecular Networks for Bioinspired Information Processing (SYMONE) under Contract No. 318597. We also 4194

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acknowledge the support by the German Research Foundation (DFG) within the Cluster of Excellence “Center for Advancing Electronics Dresden” (cfAED), the European Union (European Social Fund), and the Free State of Saxony (Sächsische Aufbaubank) in the young researcher group ‘InnovaSens’ (SABNo. 080942409). This work is also partially supported by the EU within the project Molecular Architectures for QCA-inspired Boolean Networks (MlArNet, project nr. 318516). We acknowledge the Center for Information Services and High Performance Computing (ZIH) at the Dresden University of Technology for computational resources.

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