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Intramolecular Torsion Based Molecular Switch Functionality Enhanced in π-Conjugated Oligomolecules by a π-Conjugated Pendant Group K. P. Dou,† Abir De Sarkar,‡ C. L. Wang,† and R. Q. Zhang*,‡ † ‡
School of Physics, Shandong University, Jinan 250100, People's Republic of China Department of Physics and Materials Science, City University of Hong Kong, Hong Kong Special Administrative Region, China ABSTRACT: Theoretical investigations of the variation of junction conductance with the intramolecular twist in π-conjugated oligomolecules induced by a pendant group and its effects on the enhancement in its molecular switch functionality are presented. The pendant group is found to introduce “through space” electronic coupling between the central phenyl group and its adjacent ethynylenes in the highest occupied molecular orbital, which opens an additional tunnel channel. Consequently, the π-conjugation in the backbone of the molecule is reinforced and the junction conductance along the molecular backbone gets elevated relative to the parent molecule. A 90° intramolecular twist angle of the molecule is regarded as the “OFF” state for its lowest conductance at this exactly perpendicular conformation, while the other twist angles including 0° are considered as “ON states” of the molecule. The pendant group raises the ratio of the junction conductance between the “ON” and “OFF” states by up to 2 orders of magnitude, thereby effectively enhancing its molecular switch functionality.
1. INTRODUCTION The continued trend for miniaturization of electronic devices will ultimately lead to electronics at the scale of atoms and molecules. Yet, the science governing molecular electronics as well as the architectural design of molecular or atomic circuits embodying the full device functionality is still in its infancy. The successful development of efficient molecular electronic devices necessitates a profound and a clear understanding of a multitude of interwoven factors which contribute to electron transport through molecules between the electrodes. Both theoretical and experimental studies on charge transport in molecular scale devices aim to gain useful insights into the chemistry between its active electronic components (e.g., molecular self-assembly or organization) and also the physics of the tunnel junction (e.g., coupling of these active components to the electrodes) in order to attain control over their operation. Eventually, efficient molecular devices are expected to be actualized by an optimal utilization of ability to tailor the structure and composition of its active device constituent, i.e., atoms/molecules. The aforementioned ability to tailor the molecular structure and composition can be exploited in various ways114 to fine tune the molecular conductance. For example, inducing conformational changes in a molecule in a tunnel junction, i.e., metal moleculemetal junction,16 is a commonly adopted technique to control or modulate the junction conductance. Moresco et al.1 reported an implementation using a porphyrin-based molecule functionalized with four bulky di-tert-butylphenyl groups. They demonstrated that a molecular electronic switch could be realized by a controlled rotation of the di-tert-butylphenyl legs of the porphyrin molecule. George et al.2 presented a large variation in conductance arising from the conformational changes in the r 2011 American Chemical Society
backbone of an oligosilane bridge, while the distinct conductance states in 4,40 -bipyridine molecules sandwiched between Au electrodes, observed in Quek et al.’s3 work, were switched by a mechanical manipulation of the contact geometry of the electrode molecule. Latha et al.4 studied a series of seven biphenyl molecules with different ring substituents that alter the twist angle of the molecules. They found that molecular conductance decreases with increasing twist angle, which is consistent with a cosinesquared relation between transmission coefficient and twist angle predicted for electron transport through π-conjugated biphenyl systems.5 Xia et al.6 investigated the effect of torsion angle on electronic transport through a 4,40 -biphenyl molecule coupled to Au(111) electrodes via dithiocarboxylate and found that the intramolecular torsion brought about a large variation in the currentvoltage (IV) characteristics. Another option frequently explored for the modulation of molecular conductance is by adding a pendant group. Collepardo-Guevara et al.7 obtained constructive and destructive interference patterns in the same alkene chain by adjusting the stick loop side group. Kalyanaraman et al.8 showed that for a molecular bridge with five sites along its main chain or backbone, the incorporation of a pendant group of higher generation brought about a substantial enhancement in the electron transfer rate through the wire, while for the one with six sites along the main chain, the electron transfer rate could be increased by nearly an order of magnitude when all six sites were linked to pendant groups. Ernzerhof et al.9 observed that for incident electrons with Received: March 19, 2011 Revised: June 14, 2011 Published: June 14, 2011 13911
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The Journal of Physical Chemistry C energy equal to the eigen energies of the pendant group, zero electron density was yielded at the intersection point of the side chain and the main chain and, consequently, no current passed through the main chain. Sun et al.10 also investigated this effect in simple quantum dot combinations at antiresonance, while Fowler et al.11 offered an intuitive theoretical explanation based on a graphical analysis for such an effect caused by the pendant group. Recently, Solomon et al.12 provided a successful model to account for the origin of quantum interference effects in crossconjugated molecules.1214 They indicated that the phases of the tunnel currents through the molecular orbitals differed by π leading to a complete destructive interference in their model with pendants. Oligomolecules operated by the two aforementioned modes of conductance modulation have been frequently studied to ascertain the mechanism behind their nonlinear IV characteristics, for instance, negative differential resistance (NDR),1517 resonant tunneling,18 and switching phenomena.1923 A great deal of research efforts have been directed to the role of electroactive groups such as electron-poor acceptor nitro (NO2) group and electron-rich donor amino (NH2) group in the oligomers. These electroactive groups can induce permanent dipole moment in the backbone of the molecule.24 Upon the application of bias voltage, the interaction between dipole moment of the oligomolecules and the electric field may induce electrostatic charging, conformational changes, and so on.23 These effects lead to nonlinear IV characteristics of the molecule. The electronic charge acceptor nitro group has been found to dominate over the charge donor amino group in governing electron transport through the oligomolecules. For instance, Chen et al.17 observed the NDR phenomenon in a phenylene ethynylene trimer with nitro moiety and Derosa et al.22 showed that the 3-nitro-2-(30 -nitro-20 ethynylpyridine)-5-pyridinethio (DNDP) molecule bearing nitro groups displayed a charge-induced controllable rectifying behavior. While Blum et al.23 verified that bipyridyl-dinitro oligophenylene-ethynylene dithiol (BPDN) containing nitro groups only exhibited both stochastic switching and voltage-triggered switching. However, nonlinear IV characteristics were not found in amino containing molecules only (20 -amino-4,40 -di(ethynylphenyl)-1-benzenethiolate) in Chen et al.’s investigations.17 Contemporary work on the Rose Bengal molecule indicated that the acceptor nitro group and donor amino group played different roles in influencing the molecular conductance. In absence of any donor group, the acceptor nitro groups surrounding the Rose Bengal molecule attract the π-electronic cloud toward themselves. As a result, the density of the π-electronic cloud in the molecular backbone is reduced, which largely affects its conjugation. Consequently, the molecule then behaves as an insulator. On the contrary, when the donor amino side groups are attached to the Rose Bengal molecule, the electronic distribution in its phenyl groups and hence the conjugation in the molecule are partially restored, which in turn raises the tunnel current through the molecule to some extent.25 On one hand, much research effort has been devoted to the exploration of the molecular electronic device functionality of oligomolecules enriched in strong electroactive groups, while on the other hand, reports on invoking conjugated, nondipolar pendant groups without contributing a significant dipole moment to the molecular backbone in such molecules are scarce. Our work presented in this paper aims at highlighting the crucial changes in the molecular switch functionality of the phenylene ethynylene oligomer induced by a conjugated pendant group, which does not
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impart a considerable dipole moment to the molecular backbone. The striking feature of our pendant group is its simplicity, as it is free from electroactive groups. The presence of electroactive groups in the pendant introduces additional complications, e.g., the electroactive pendant groups contribute a permanent dipole moment to the molecular backbone. Under the application of bias voltage, the molecular backbone attached to electroactive side group(s) may undergo electrostatic charging which may in turn induce conformational changes in the molecule. Due to the structure property relationship, the envisioned functionality of the molecule may be undesirably affected. Moreover, electroactive pendant groups and their effects have been extensively studied and reported, while simple π-conjugated side groups have been sparsely visited despite their relative simplicity and ease in scientific investigation. The pendant group reported in this paper is π-conjugated. Consequently, it imparts a minimal dipole moment to the molecular backbone and, therefore, it is free from all such complications arising from electroactive pendant groups. Yet, switch functionality is found to be retained and enhanced by employing this simple, π-conjugated pendant group, which is definitely an advantage over electroactive pendant groups. The influence of such a pendant group on the molecular conductance is naturally expected to be different from the ones caused by the electroactive pendant groups, e.g., electronic charge donor and acceptor groups.
2. THEORETICAL MODEL AND CALCULATIONS 2.1. Geometry Optimization Calculations. First, the molecular structures were optimized in the tunnel junction geometry shown in Figure 1 using density functional theory (DFT) implemented in the SIESTA package26 which employs norm-conserving pseudopotentials. The exchange and correlation functional was treated at the level of generalized gradient approximation (GGA),27 and an energy cutoff of 200 Ry was used. The geometries of the tunnel junctions investigated here are shown at the top of Figure 1: a linear phenylene ethynylene trimer as a reference molecule (Sl); a phenylene ethynylene trimer attached to a conjugated pendant group phenyleneethynylene (Sp). Structural relaxations were allowed until the force acting on each atom was less than 0.02 eV/Å. The positions of the metal atoms and the anchoring sulfur atoms shown in Figure 1 were fixed in their initial positions while all the atoms in the molecules were allowed to relax. The electrode setup and the AuS bond length are adapted from scientific literature.28 The distance between the S atoms along transport direction in both models was kept at 25.64 Å, which is the equilibrium length for Sl along its molecular axis. Double-ζ plus polarization (DZP) orbitals were used for the description of the electronic structure of the molecules (i.e., for the molecular constituents, C and H atoms) and the anchoring S atoms, while single-ζ plus polarization (SZP) orbitals were used for metal electrode atoms (Au atoms). 2.2. Electron Transport Calculations. Transport calculations as implemented in the TRANSIESTA package,29 based on the combination of DFT and nonequilibrium Green’s function (NEGF) were used in our calculations. The currentvoltage (IV) characteristics were calculated using LandauerButtiker formula.30 The bottom of Figure 1 is an illustration of internal conformational twist applied to the two models, Sl and Sp. The molecular structures in the tunnel junction optimized by SIESTA, as mentioned in the preceding subsection, were used here as the starting point. The central phenyl group was then rotated by θ about the molecular axis, out of the plane containing the other two phenyl 13912
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Figure 1. Top: A linear phenylene ethynylene trimer (Sl) and a phenylene ethynylene trimer attached to a conjugated pendant group phenyleneethynylene (Sp). Bottom: A pictorial representation of internal conformational twist in the two models (Sl and Sp), where θ is the internal conformational twist angle.
groups in model Sl, while in the other model Sp, the central phenyl group together with the pendant group was rotated by θ out of the initial plane.
3. RESULTS AND DISCUSSION The results in the present work support the original hypothesis that the introduction of a conjugated pendant group to the phenylene ethynylene oligomer influences the molecular switch functionality in some way. Here, we have gained a profound and a clear understanding of the effects induced by such a pendant group. The group is found to influence the molecular conductance by inducing changes in its π-conjugation along the molecular backbone. From different perspectives, we have uncovered the effect caused by the conjugated pendant group, which is discussed in the following five subsections. 3.1. Variation of π-Conjugation and Transmission Coefficient at the Fermi Level T(Ef) with Conformational Changes.
In the T(Ef) vs cos2 θ plot at zero bias shown in Figure 2a, we observe that the maximum in T(Ef) occurs at θ = 0° when the phenyl rings in the molecular backbone are coplanar as π-orbital overlap within the molecule is maximum for this conformation in both Sl and Sp. As θ increases, the π-overlap diminishes with the
Figure 2. The transmission coefficient T(Ef) vs cos2 θ plot (a) at zero bias voltage and (b) at an applied bias voltage, Vb = 0.6 V, where θ is the internal conformational twist angle, as shown at the bottom of Figure 1.
deviation from the planar arrangement of the phenyl rings, which effectively brings down the T(Ef). Furthermore, it can be noticed in Figure 2a that Sl is a weakly π-conjugated system as the points corresponding to it lie below 13913
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Figure 3. The IV characteristics of Sl and Sp for four conformational states: θ = 0°, 45°, 60°, and 90°, where θ is the internal conformational twist angle, as shown at the bottom of Figure 1.
the straight line with slope unity. Unity slope in the plot signifies strong π-conjugation, usually observed in biphenyl containing molecular structures.4,5 Lying below the straight line implies weakening in the π-conjugation in the molecule. The pendant group is clearly found to reinforce the π-conjugation in the molecule as the points for Sp move closer to the straight line and fall on it at θ = 0°. The T(Ef) vs cos2 θ at applied bias voltage, Vb = 0.6 V is plotted in Figure 2b. It is clear from both panels a and b of Figure 2 that the largest difference in T(Ef) and, correspondingly, π-conjugation between Sl and Sp occurs at θ = 45° at chemical equilibrium, corresponding to zero bias voltage. It is noteworthy that when a bias voltage is applied to a tunnel junction, nonequilibrium electron transport across the tunnel junction sets in and the Fermi level no longer remains relevant. The Ef then actually represents the average electrochemical potential of the two electrodes of the tunnel junction. It can be seen that the values of T(Ef) in both models increase in Figure 2b, as compared to the zero bias condition in Figure 2a and it even overshoots the straight line with unity slope for some molecular conformations. 3.2. IV Curve Analysis. Four conformations are taken into account for both Sl and Sp, which are characterized by an intramolecular torsion angle, θ = 0°, 45°, 60°, and 90°. The IV characteristics are shown in Figure 3. It can be seen that as θ increases from 0° to 90°, tunnel current in both models (Sl and Sp) drops by up to 2 orders of magnitude. Therefore, we can designate a current “ON” state and a current “OFF” state31 based on θ-dependence of tunnel current. We refer to models with θ = 0°, 45°, and 60° as “ON” states while the one at θ = 90° as the “OFF” state. Besides, from the IV characteristics, we notice that the ONOFF effect is more prominent in Sp than in Sl. For example, when the applied bias voltage, Vb equals 0.6 V, tunnel current, I (Sp) is 2.55, 8.0, and 2.75 nA larger in magnitude than I (Sl) for internal conformational twist angles, θ = 0°, 45°, and 60°, respectively. Interestingly, the maximum difference in tunnel current between Sl and Sp occurs at θ = 45°, which is consistent with Figure 2a, where the largest difference in conjugation is also
found to occur at θ = 45° at zero bias voltage. At the OFF state (when θ = 90°), both I (Sp) and I (Sl) are quite small. Therefore, we deduce from an analysis of the IV curves that Sp demonstrates a better ONOFF switch performance than Sl. 3.3. Transmission Coefficient, T(E) Spectral Analysis. In order to elucidate the difference between the two models (Sp and Sl) differing only in the pendant group, the transmission coefficient (T(E)) spectrum is plotted as a function of energy of incident electrons at zero bias voltage and at 0.6 V bias voltage in panels a and b of Figure 4, respectively. In Figure 4a, the transmission coefficients exhibit two main plateaus near the Fermi level for both models, Sp and Sl in all the four conformations defined by the intramolecular torsion angle. One of the plateaus extending between 0.5 and 0.08 eV is later referred to as p1, while the other ranging between 0.02 and 1.02 eV is denoted by p2. The p2 is partly shown here in the bias voltage window of our interest. It is observed that the magnitude of both p1 and p2 decreases when θ increases from 0° to 90°. This tendency indicates a gradual rise of tunnel barrier in both Sl and Sp with the increase of θ. The transition from electronic transparency at θ = 0°, 45°, 60° to electronic opacity at θ = 90° may be potentially utilized in switches for both Sp and Sl. In the following, we have focused on conductance at each conformation. For θ = 0°, the magnitude of p2 in Sp is slightly larger than that in Sl while almost no difference in p1 is noticed between Sl and Sp. Then, for θ = 45° and 60°, the magnitudes of both p1 and p2 in Sp are much larger than that in Sl. On the contrary, for θ = 90°, the magnitudes of both p1 and p2 in Sp are a bit smaller than that in Sl by an order of 6. When a positive bias is applied to Sp and Sl, both p1 and p2 shift to higher energies. As a result, a large part of p1 appears in the bias voltage window while p2 gradually wanes when moving from 0.0 to 0.6 V. At Vb = 0.6 V in Figure 4b, p1 totally moves into the bias voltage window while p2 contributes only by its tail for all the four conformations. Both I(Sp) and I(Sl) are found to increase linearly from 0.0 to 0.6 V due to contributions from the smooth plateaus of transmission coefficient, p1 and p2. 13914
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Figure 4. Transmission coefficient as a function of energy for Sl and Sp at their four conformations: θ = 0°, 45°, 60°, and 90°(a) at zero bias voltage, (b) at a bias voltage, Vb = 0.6 V. The electronic clouds are denoted by red and black dashed lines for Sp and Sl. The blue lines indicate the bias window.
Within this bias voltage range, I(Sp) is larger than I(Sl) for θ = 0°, 45°, and 60° while I(Sp) is smaller than I(Sl) for θ = 90°. Such IV pattern can be traced back to the transmission coefficient spectrum. Thus, Sp exhibits enhanced ONOFF effect relative to Sl under internal conformational twisting. 3.4. Interpretation Based on Electronic Cloud. In the following, the origin of enhancement in the switch functionality is explained further3234 by an analysis based on the spatial distribution of electronic cloud in the tunnel junction. Former work33,34 on oligomolecules had focused on the spatial distribution of an electronic cloud over the tunnel junction as a function of the applied bias voltage. In these two works,33,34 the electronic
clouds were usually reported to be localized in certain parts of the tunnel junction while they had distributed themselves continuously all over the tunnel junction in a very limited range of high applied bias voltage. The continuity in the spatial distribution of electronic cloud throughout the entire tunnel junction, except at its antinodes, ensures a tunnel current of considerable magnitude; otherwise the tunnel current is relatively small for discontinuities in the spatial distribution of the electronic cloud or for a spatial localization of electronic cloud in certain parts of the tunnel junction for the same isovalue used in the generation of electronic cloud (i.e., electronic wave function or charge density). Thus, attention is focused on the electronic cloud spatially and continuously 13915
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Figure 6. Isosurfaces of electronic wave functions mentioned in Figure 4b. The isovalue is 0.0075 e/Å3.
Figure 5. Isosurfaces of electronic wave functions mentioned in Figure 4a. The isovalue is 0.0075 e/Å3.
distributed over the whole tunnel junction at different bias voltages, which result in an appreciable tunnel current. In our study, the transmission coefficient of significant magnitude is found to plateau around the Fermi level, indicating the occurrence of tunneling in considerable proportion even at low bias voltages. We have classified the electronic clouds in the tunnel junction based on their contribution to transport into several groups at Vb = 0.0 V and Vb = 0.6 V. These orbitals are chosen based on the criteria for having significant electronic density both on the contacts of the tunnel junction and near the center of the junction. They are indicated by red and black dashed lines for Sp and Sl in panels a and b of Figure 4, respectively. First, electronic clouds with continuity in the spatial distribution all over the tunnel junction, except at their antinodes, arise at energies close to the Fermi level for both Sp and Sl. They are referred to as FOrt and occur only for
θ = 0° at zero bias voltage. Second, two groups of electronic clouds referred to as FO1 and FO2 are common to both Sp and Sl. They distribute over an interval of energy lower than the Fermi level at zero bias voltage and then move into the bias window at Vb = 0.6 V. Third, the exceptional electronic clouds which only appear in Sp are designated as FOd. They appear closer to the Fermi level than FO1 and FO2 at zero bias voltage and move even further closer to the Fermi level at Vb = 0.6 V. Isosurfaces of electronic wave functions and the energy levels corresponding to them are represented in Figures 5 and 6 at bias voltages 0.0 and 0.6 V, respectively. 3.4.1. Electronic Cloud Analysis at Zero Bias Voltage. The electronic cloud at zero bias voltage (Vb = 0.0 V) is discussed first for a systematic and meaningful analysis. Figure 5a presents the electronic cloud group, FOrt of Sp and Sl. For conformation at θ = 0°, the components of the molecular backbones in both models, Sp and Sl are coplanar and, consequently, the tunnel current is significantly large due to molecular conjugation, as discussed already in subsection 3.1. Once the internal conformational twisting is initiated, the continuity in the distribution of the electronic cloud all over the tunnel junction is progressively broken resulting in a gradual closure of the tunnel channels. It reaches its culmination at θ = 90° when the electronic cloud gets solely confined to the central part of the tunnel junction resulting in a complete closure of the tunnel channels and the smallest junction conductance or the 13916
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The Journal of Physical Chemistry C highest tunnel barrier. Thus, the group FOrt disappears in the other three twisted conformations. In panels c and d of Figure 5, the groups FO1 and FO2 are shown, which mainly induce the rise of p1. It can be seen that as θ increases from 0° to 90°, the spatial extent of the electronic cloud diminishes centrally in both Sp and Sl and finally disappears when θ = 90°. Consequently, electronic charge transport across the junction in these two groups decreases steadily as θ increases and stops completely at θ = 90°. The behavior of the two groups explains the whole ONOFF process. Finally, attention is put on the exceptional group, FOd which is borne only by Sp. This group is shown in Figure 5b. The additional group, FOd in Sp is attributed to the differences in charge transport between the two models, Sp and Sl. At θ = 0°, “through space” electronic coupling are found to occur between the positions marked 3 and 6 in the central benzene unit in Figure 1 and their adjacent ethynylenes. Such “through space” interactions were found to be responsible for quantum interferences in benzene systems induced by meta-connections.35 The “through space” interaction is also found to play a role in electron transport in our study. For this coplanar arrangement of the phenyl groups, the contribution from groups FOrt, FO1, and FO2 overwhelmingly outweighs that from FOd. Thus, the effect induced by FOd is not clearly evident. But for the twisted conformations, the role of group FOd cannot be ignored. Though the electronic cloud corresponding to group FOd gets localized in the central part of the tunnel junction as θ increases, the continuity in the distribution of the electronic cloud all throughout the tunnel junction in Sp lowers the effective tunnel barrier for θ = 45° and 60°. Eventually, at θ = 90° the electronic cloud centralizes completely in the tunnel junction. The total asymmetry in the distribution of electronic cloud between the central part and the rest of the tunnel junction heightens the tunnel barrier further. Therefore, the group FOd in Sp lowers the barrier at ON states and raises the barrier at OFF state, implying that it enhances the switch functionality. 3.4.2. Electronic Cloud Analysis at Applied Bias Voltages. Figure 6 shows the electronic clouds of the two models, Sl and Sp at a bias voltage, Vb = 0.6 V. They all appear in the bias window as depicted in Figure 4b. The group FO2 still distributes over the energy interval of p1 for θ = 0°. But for twisted conformations, the group FO2 distributes right below the energy range of p1. From panels b and c of Figure 6, it is observed that electronic clouds in FO1 and FO2 recede from the right lead as θ increases. For θ = 90°, the electronic clouds in the two groups centralize in the tunnel junction. This indicates that twisting progressively raises the tunnel barrier in both models. The tunnel barrier increases progressively with the increase in intramolecular torsion angle in these groups of electronic clouds and the tunnel channels close completely when θ = 90°. Similar to its role at Vb = 0.0 V, FOd still acts to lower the barrier in ON states while it raises the barrier in OFF states. 3.5. The Conformation Maximizing the Effect of the Pendant Group. It will be useful to ascertain the conformation for which effect induced by the π-conjugated pendant group reaches its maximum. The largest difference in π-conjugation between the two models (Sl and Sp) and, correspondingly, the largest difference in the absolute value (but not the relative value) of T(Ef) is found to occur at θ = 45° as shown by Figure 2a. However, interestingly enough, the largest enhancement in conductance in Sp with respect to Sl is found to occur at θ = 60°, as shown in the IV curve in Figure 3. I(Sp) increases nearly by 199% with respect to I(Sl) at a bias voltage of 0.6 V, as shown in Figure 3. Earlier
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findings36 indicated that at around θ = 60° in Sl, the ππ interaction between neighboring phenyl groups gets as weak as that of σσ interaction. However, in this work, the group FOd in Sp enhances conjugation between the central phenyl group and the pendant group. Consequently, it enhances the conductance of the Sp model relative to the Sl model at this twisting angle. As a result, the maximum difference in the switch functionality between these two models, Sl and Sp, is attained at a twist angle of θ = 60°. In other words, the most obvious effect of the pendant group in enhancing the switch functionality occurs at this internal conformational twist angle.
4. SUMMARY In this paper, a conjugated pendant group has been shown to reinforce conjugation in a nondipolar oligomolecule, which in turn enhances its switch functionality through the variation of its tunnel junction conductance with its internal conformational twist. The lack of experimental or technical control over this intramolecular torsion in oligomolecules apparently appears to be a hindrance to its actualization in molecular switch devices. However, systematic research on this37 has very recently shown this impediment to be potentially surmountable, demonstrating that intramolecular torsion can be controllably tuned by the application of external electrostatic fields. Our findings underline the possibility for a controlled manipulation of internal conformational twist in nondipolar oligomolecules, which can be gainfully exploited in designing molecular electronic switches. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (CityU 103609). K.D. would like to thank Dr. Fan Wei for useful discussions. ’ REFERENCES (1) Moresco, F.; Meyer, G.; Rieder, K-H; Tang, H.; Gourdon, A.; Joachim, C. Phys. Rev. Lett. 2001, 86, 672–675. (2) George, C. B.; Ratner, M. A.; Lambert, J. B. J. Phys. Chem. A 2009, 113, 3876–3880. (3) Quek, S. Y.; Kamenetska, M.; Steigerwald, M. L.; Choi, H. J.; Louie, S. G.; Hybertsen, M. S.; Neaton, J. B.; Venkataraman, L. Nat. Nanotechnol. 2009, 4, 230–234. (4) Venkataraman, L.; Klare, J. E.; Nuckolls, C.; Hybertsen, M. S.; Steigerwald, M. L. Nature 2006, 442, 904–907. (5) Woitellier, S.; Launay, J. P.; Joachim, C. Chem. Phys. 1989, 131, 481–488. (6) Xia, C.-J.; Fang, C.-F.; Zhao, P.; Xie, S.-J.; Liu, D.-S. Phys. Lett. A 2009, 373, 3787–3794. (7) Collepardo-Guevara, R.; Walter, D.; Neuhauser, D.; Baer, R. Chem. Phys. Lett. 2004, 393, 367–371. (8) Kalyanaraman, C.; Evans, D. G. Nano Lett. 2002, 2, 437–441. (9) Ernzerhof, M.; Zhuang, M.; Rocheleau, P. J. Chem. Phys. 2005, 123, 134704. (10) Sun, Z. Z.; Zhang, R. Q.; Fan, W.; Wang, X. R. J. Appl. Phys. 2009, 105, 043706. (11) Fowler, P. W.; Pickup, B. T.; Todorova, T. Z.; Pisanski, T. J. Chem. Phys. 2009, 130, 174708. 13917
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dx.doi.org/10.1021/jp202609x |J. Phys. Chem. C 2011, 115, 13911–13918
