Influence of Conformational Flexibility on Single ... - ACS Publications

Sep 30, 2009 - Richard J. Whitby,*,‡ and Richard J. Nichols*,†. Centre for Nanoscale Science and Department of Chemistry, UniVersity of LiVerpool,...
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J. Phys. Chem. C 2009, 113, 18884–18890

Influence of Conformational Flexibility on Single-Molecule Conductance in Nano-Electrical Junctions Santiago Martı´n,† Francesco Giustiniano,‡ Wolfgang Haiss,† Simon J. Higgins,† Richard J. Whitby,*,‡ and Richard J. Nichols*,† Centre for Nanoscale Science and Department of Chemistry, UniVersity of LiVerpool, Crown Street, LiVerpool, L69 7ZD, U.K., and School of Chemistry, UniVersity of Southampton, Southampton, Hampshire, SO17 1BJ, U.K. ReceiVed: July 17, 2009; ReVised Manuscript ReceiVed: September 5, 2009

The temperature dependence of the single-molecule conductance of conformationally flexible alkanedithiol molecular bridges is compared to that of more rigid analogues which contain cyclohexane ring(s). Molecular conductance has been measured with a scanning tunneling microscope (STM) at fixed gap separation by observing the stochastic formation of molecule bridges between a gold STM tip and substrate (the so-called “I(t)” technique). Under these conditions, the junction can be populated by a wide distribution of conformers of alkanedithiol molecular bridges and a strong temperature dependence of the single-molecule conductance is observed. By contrast the rigid analogues that contain cyclohexane ring(s), which cannot form the thermally accessible gauche rich conformers open to the alkanedithiols, show no dependence of the single-molecule conductance on temperature. This comparison demonstrates that it is the conformational flexibility and access to thermally populated higher energy conformers of the linear polymethylene (alkane) bridges which leads to the temperature dependence. By removing this possibility in the cyclohexane ring-containing bridges, this conformational gating is excluded and the temperature dependence is then effectively suppressed. 1. Introduction The measurement of the electrical properties of molecules, down to the single-molecule level, has become an experimental reality in recent years. A number of methods have been used to achieve the feat of trapping a single molecule between a pair of metallic contacts. These include the formation of break junctions by mechanical or electromechanical methods or through the use of an STM.1-4 Other methods use nanoparticles to form contacts to single molecules which can then be introduced between pairs of electrodes13b for electrical interrogation or electrically contacted using a conducting probe AFM.5 Using these methods it has been possible to study systematically the influence of key parameters upon singlemolecule conductance such as molecular electronic and geometric structure, the chemical contact group, the redox state of the molecular bridge, the effect of chemical substituents on the molecular bridge, molecular conformation, gap separation, solvent effects, and temperature.3,6-10 In many cases the influence of such parameters on single-molecule conductance mirrors similar observations for extended molecular films sandwiched in between electrical contacts. However, observations have also been made of behavior at the single-molecule level that have not been previously apparent in electrical data for large-area molecular devices. Two examples include the observation of multiple conductance groups for single-molecule junctions,4,7,11,12 and an apparently anomalous temperature dependence of the conductance of alkanedithiol single molecules.8 Alkanethiols and alkanedithiols are the most common testbed for molecular electronics and have featured strongly in the * To whom correspondence should be addressed. E-mail: [email protected] (R.J.N.); [email protected] (R.J.W.). † University of Liverpool. ‡ University of Southampton.

development of methods for recording single-molecule electrical properties.2,4,5,7,9-13 Alkanethiols, alkanedithiols, and certain endgroup-functionalized alkanethiols, form compact and wellordered self-assembling monolayers with low defect densities, which can be contacted with top metal electrodes to give reliable platforms for electrical characterization. In these metal-(molecular film)-metal structures many molecules are simultaneously contacted. On the other hand, in the case of alkanedithiols singlemolecular junctions can be formed with each thiol end group contacting metal electrodes separated by nanometer dimensions. The backbone of an alkanethiol or alkanedithiol molecule is composed of methylene groups (CH2), and the HOMO-LUMO separation is large, with molecular orbitals being energetically far removed from the Fermi levels of the enclosing metal electrodes. In this respect the expectation would be that such polymethylene bridges would provide a very simple model system for studying charge transport through molecular junctions. It may then be anticipated that tunneling or superexchange models would be sufficient for describing charge transport across such bridges. No temperature dependence would be expected given the large offset (.kT) between the Fermi levels of the contact and molecular levels or the bridge. Indeed, there are reports in the literature of temperature-independent electrical transport properties of alkanethiol self-assembled films.14,15 There are also reports of temperature-independent singlemolecule conductance of alkanedithiols which are stretched between gold electrodes in STM experiments.15 These reports would seem to justify the adequacy of basic tunneling or superexchange models in describing charge transport across polymethylene bridges. However, recent observations of temperature-dependent charge transport across polymethylene bridges, in both self-assembled molecular films16 and in single molecules,8 show that detailed consideration is needed to explain these phenomena.

10.1021/jp906763p CCC: $40.75  2009 American Chemical Society Published on Web 09/30/2009

Single-Molecule Conductance in Nano-Electrical Junctions Salomen et al. have observed temperature dependence in alkyl chain monolayers which are chemically grafted to silicon surfaces.16 They observed a gradual decrease in electrical junction conductance as the temperature increased, which was attributed to a temperature-dependent untilting of the adsorbed molecules, resulting in a concomitant increase in film thickness. They proposed that this leads to a decrease in “through space” charge transport between chains leading to a decreased conductance.16 The significance of this intermolecular chain-to-chain tunneling in metal-alkanethiol-metal junctions has been discussed in a number of publications where the total tunneling current through a molecular monolayer has been described in terms of the through-bond tunneling and the chain-to-chain tunneling.17 Clearly, intermolecular chain-to-chain tunneling cannot play a role in true single molecular measurements; nevertheless, temperature dependence has been observed for the conductance of single alkanedithiol molecules bridging a pair of gold electrodes.8 Haiss et al. measured significant temperature dependence for conductance of Au-alkanedithiol-Au single-molecule junctions in the temperature range between 293 and 353 K.8 These measurements were carried out using an STM in an ambient environment to form the single-molecular bridges which were recognized in a statistical analysis of the conductance data. They described the temperature dependence as a thermal gating of the single-molecular conductance. They observed that the conductance scales logarithmically with T-1, and that the proportionality factor in this relationship increased with increasing number of methylene groups in the carbon chain. It should be emphasized that this is not a small effect; for example, the conductance of nonanedithiol (NDT) increased from 0.52 nS at 20 °C to 2.90 nS at 67 °C. The origin of this temperature dependence was attributed to the change in distribution between molecular conformers, with the higher energy gauche-defectrich conformers becoming increasingly populated with rising temperature.8 Haiss et al. proposed that the more “folded” gauche conformers offer a lower barrier width for tunneling between the terminals.8 Using this assumption, a simple tunneling model was then employed to calculate the temperaturedependent conductance using this barrier width, the energy difference between the conformers, and their relative populations evaluated from their partition distribution. This model produced a linear dependence of the logarithm of conductance (ln σ) on reciprocal temperature (T-1) qualitatively similar to that experimentally observed.8 The slope of this calculated ln σ-T-1 response scaled with the number of CH2 groups in the alkanedithiol in a similar manner to the experimental observation. Jones and Troisi refined this model by computing the conductance of alkanedithiol conformers placed between a pair of gold leads.18 Density functional theory (DFT) with a Green’s function theoretical approach was used and conductance was computed as a function of conformer energy. On examining a large conformer space it was apparent that, on average, conductance increases with conformational energy. On the basis of the distribution of conformer energy and conductance values, thermally averaged conductance was then computed. In the experimentally relevant temperature range the computed conductance increased with temperature as a result of the general trend for conductance to increase with conformational energy (for conformational energies >2 kcal/mol).18 In this temperature range the conductance was seen to follow a logarithm of conductance versus T-1 relationship similar to the experimental results of Haiss et al. Although this behavior gives the appearance of an activated process, it is important to point out

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Figure 1. Molecules examined in this study of the temperature dependence of single-molecule conductance.

that it cannot arise from activation of charge transport across the bridge since alkanedithiols do not have low-lying levels that can be temporarily populated or depopulated in multiphonon electron transfer steps. Conversely, charge transport is by pure tunneling with the higher energy conformers offering more facile tunneling routes, and it is the conformer distribution that is “thermally activated”.8,18 In another theoretical study, Kornyshev and Kuznetsov have shown that the tunneling conductance in a flexible chain molecule (for instance, alkanedithiols) is controlled by fluctuations of the relative positions of nearest neighbor units of the chain molecule.19 Their model leads to a formula which reproduced the ln σ-T-1 response observed by Haiss et al. In this manuscript we explore further this issue of the higher energy conformers of polymethylene (alkane) bridges offering more facile tunneling routes. We have selected and synthesized polymethylene bridges which are conformationally less flexible to explore this issue. The single-molecule conductance of hydrocarbon-based molecular wires has been widely studied with alkanedithiols representing the most ubiquitously studied systems. Recently Yang et al. have used the STM break junction method to study the single molecule conductance of a range of rigid norbornyl-derived molecular bridges.20 Unlike the alkanedithiols, these are not conformationally flexible which brings clearer definition of the bridge conformation. They concluded that the norbornyl and alkyl systems possess electronically similar molecular junctions with a comparable exponential decay of the conductance with molecular length typical of through-bond electron tunneling. For such molecular bridges Yang et al. noted from this exponential length decay the correspondence between conductance measured at the singlemolecule level and the rate of photoelectron transfer across such bridges (between donor and acceptor groups as measured in photophysics experiments); both rely on a superexchange mechanism.20,21 However, temperature dependence was not investigated in the single-molecule conductance study of Yang et al. and this is the focus of the current study. The series of molecules examined in this comparative study are shown Figure 1. Two ring-containing compounds are shown alongside the linear chain hexanedithiol (Figure 1b) and decanedithiol (Figure 1d) in this figure. The presence of one cyclohexane ring in Figure 1a and two in Figure 1c prevents these molecules from forming the highly “folded” gauche rich conformers which are accessible to hexanedithiol (Figure 1b) and decanedithiol (Figure 1d). For each of these molecules, we measure the temperature dependence of the single molecule conductance and we discuss the relevance of the findings for electron transport in single molecule systems.

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SCHEME 1: Preparation of Compounds 2 and 7a

a (a) Tosyl chloride, pyridine, 0-5 °C, 16-65 h; (b) recrystallize from EtOH; (c) AcSK, acetone, reflux, 20 h; (d) 25 bar H2, PtO2, Pd/ C, AcOH, 50° C; (e) i. NaOH, EtOH/H2O, 75° C. ii. dry, 275 °C, 3 h; (f) LiAlH4, THF, 60° C.

2. Experimental Methods 2.1. Synthesis and Materials. Hexanedithiol and decanedithiol were used as received from Aldrich (reagent grade). S,S′((1r,4r)-cyclohexane-1,4-diylbis(methylene)) diethanethioate (2)22 (precursor to Ring-1) and S,S′-((1r,1′r,4r,4′r)-[1,1′-bi(cyclohexane)]-4,4′-diylbis(methylene)) diethanethioate (7) (precursor to Ring-2) were synthesized as shown in Scheme 1 and described in the supplementary material. The relative stereochemistries were confirmed by X-ray structures of 723 and of the bis-tosylate 1.24 2.2. STM Methods for Measurement of Molecular Conductance. Electrical measurements using an STM were performed with a low-coverage of the R,ω-dithiol molecule on a Au(111) surface using the “I(t) method”. The I(t) technique involves monitoring tunnelling current (I) in the STM as a function of time at constant tip height.4,9 In this technique the Au STM tip is held at a given distance above the substrate and current jumps are monitored as molecular wires form and subsequently break stochastically, bridging the tip and substrate. To record current jumps (I(t) events), the set-point current was fixed at a given value and then the STM feedback loop was temporarily disabled and the tunneling current was monitored for 0.1-1 s. Histograms were then constructed from the measured current jump values, enabling statistical analysis of the data and identification of molecular conductance values. For this histogram analysis, the jump height is analyzed by examining the difference between the average current before the jump and that of points after the abrupt current jump. The difference between these points then gives the current jump values (Ijump) that are plotted in current (or conductance) histograms.4,9 It has been found that the conductance values measured with the I(t) method are consistent with the low-current conductance groups (“A1 or Low”) seen in I(s) or STM-break junction methods.12 An Agilent STM running Picoscan 4.19 Software was used for all measurements. Commercially available gold on glass samples with a chromium adhesive layer (Arrandee) were flameannealed immediately prior to use to produce large Au(111) terraced regions, see Figure 2. During the flame-annealing process the gold slide was heated to produce a slight orange glow, and it was then kept in this state for around 30 s by repeatedly removing and reintroducing the sample into the flame to avoid overheating. The molecule under study was then

Figure 2. STM image of a flame-annealed gold on glass sample which exhibited large flat terraced regions separated by monatomic gold steps; 150 nm × 150 nm.

adsorbed from 5 × 10-4 M toluene solutions by immersion for 30 s. The gold samples were subsequently rinsed in ethanol and blown dry in a stream of N2 gas. Gold STM tips were freshly prepared for each experiment by etching of a 0.25 mm Au wire (99.99%) in a mixture of ethanol (50%) and HCl (50%) at +2.4 V. All the STM experiments presented here have been performed in air. Measurements at elevated temperatures were carried out by mounting the gold on glass samples on a metal plate that was heated with a variable-temperature resistive heating source. The temperature was monitored with two thermocouples simultaneously, one placed directly on the heating stage and the other pressed on top of the sample with a rubber seal ring. The thermocouples were calibrated with a precision thermometer. Using this temperature-control system, single-molecule conductance was determined in the temperature range from 25 to 90 °C. 2.3. Estimation of Separation between STM Tip and Gold Surface. The I(t) technique described above is the key method used in this study of single-molecule conductance. This method enables the separation between STM tip and substrate (“contact gap separation”) to be controlled through choice of the STM set-point current (I0); I0 is defined as the current flowing between tip and sample in the absence of a molecular wire. The contact gap separation is determined by calibration of the tip-sample distance as a function of I0 for a given bias voltage (Ut ) 0.6 V). This calibration is achieved by recording current-distance (I(s)) scans for the given sample in the absence of molecular wire formation. These current-distance (I(s)) scans are performed at regular intervals during the I(t) measurement. For these purposes I(s) scans were selected in which there were no signs of wire formation. Linear regression was then used to determine the slope of ln(I) versus distance (s). An average slope ((-8.9 ( 1.5) nm-1 for Ring-1; and (-8.4 ( 1.5) nm-1 for hexanedithiol) has been calculated within the range of I0 values relevant to the given experiment. In order to achieve an absolute separation between the STM tip and surface, we assume that the conductance at the point where the tip would make mechanical contact with the surface is the conductance quantum G0 (G0 ) 2e2/h). This assumption provides the basis for an absolute calibration of the gap separation at a given current according to eq 1:

Single-Molecule Conductance in Nano-Electrical Junctions

s(I0) )

ln(G0 × Ut /I0) d ln(I)/ds

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(1)

We calibrated the z piezo elongation factor by measuring the height of monatomic steps on Au(111) as are shown in figure 2 (0.236 nm per step was assumed). On the other hand, it is also noted that below a critical setpoint current (Ic), molecular wires can no longer span the gap, and hence current jumps for molecules bridging the gap are no longer observed in the I(t) experiment. It is assumed that the molecule is in its most extended upright orientation at the critical current set-point. This then provides an alternative absolute distance calibration if the headgroup to headgroup distance for the extended (all-trans) dithiol molecule is determined using a molecular modeling program (Spartan). 2.4. DFT Calculations. Molecular orbital energy level positions were calculated using DFT (Spartan B3LYP, 6-31+G*). 3. Results Figure 3a shows I(t) scans for Ring-1 recorded at a set-point current of 10 nA. The current jumps are assigned to the stochastic formation (upward jumps) or breaking (downward jumps) of molecular bridges. The current jump height is analyzed in histogram representations in Figure 3b, where only one conductance peak (group A1) is visible. Following previous reports this is assigned to the single molecule conductance.4 Figure 4 (top) shows the temperature dependence of molecular conductance for hexanedithiol measured by the I(t) technique at I0 ) 20 nA and Ut ) 0.6 V. Under these conditions, the measurements are conducted at intermediate gap separation where a range of conformations of the polymethylene backbone of this molecule can bridge the tunneling gap. Ring-1 is compared with hexanedithiol in Figure 4; the latter chosen since it has the same number of methylene groups in the shortest pathway between the two thiol contact atoms as Ring-1. Hexanedithiol shows significant temperature dependence with a linear dependence of the logarithm of conductance on reciprocal temperature (Figure 4, top), similar to that previously reported by Haiss et al.8 On the other hand Ring-1 shows no observable temperature dependence over this range (Figure 4, bottom) within the error limits of the determination. A similar behavior is seen when comparing Ring-2 with 1,10-decanedithiol (Figure 5). Note that the slope of the ln σ-T-1 plot is higher for decanedithiol (-(2384 ( 83) K-1) compared to hexanedithiol (-(1034 ( 126) K-1). Figure 4 also shows that the conductance of hexanedithiol at the higher temperature limit of our experiments approaches that of Ring-1 and similarly, the conductance of decanedithiol approaches that of Ring-2 at elevated temperatures (Figure 5). As discussed in the next section, the increasing conductance with temperature of the linear chain molecules is attributed to increasing population of gauche-rich conformers. However, at room temperature Ring-1 has a significantly higher conductance than hexanedithiol and Ring-2 has likewise a higher conductance than decanedithiol. This may be partly due to the shorter S-S distances of the former; the S-S distances are 0.839 (Ring-1) versus 0.930 nm (all-trans hexanedithiol) and 1.27 (Ring-2) versus 1.44 nm (alltrans dodecanedithiol), all distances from molecular mechanics minimized structures. It is also significant that although Ring-1 and Ring-2 incorporate gauche folds, these are so positioned that they do not compromise the ‘all trans’ conduction path through these molecules (Figure 8). Furthermore, there are parallel through-bond conduction paths which should further increase conductance for Ring-1 and Ring-2.

Figure 3. (a) I(t) method for compound Ring-1 showing typical current jumps. I0 ) 10 nA and Ut ) 600 mV (bias voltage). (b) The corresponding histogram constructed from 375 jumps as shown in (a). Solid line is a Gaussian fit.

Figure 4. Dependence of the logarithm of single molecule conductance on reciprocal temperature (T range: 25-90 °C) for hexanedithiol (HDT, top) and Ring-1 (bottom). Dashed lines are linear fits. I0 ) 20 nA, Ut ) 0.6 V.

4. Discussion The difference in the temperature dependence of the cyclohexane ring and the linear-chain molecules can be rationalized by examining the accessible conformers of the respective molecules. In the ambient temperature range the chair form of Ring-1 and Ring-2 is the predominant conformer in preference to the higher energy boat and twist conformers. Figure 6a shows molecular models for the chair conformer of Ring-1 with the

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Figure 7. Four selected conformers of hexanedithiol (H on thiol end groups omitted).

Figure 5. Dependence of the logarithm of single molecule conductance on reciprocal temperature (T range: 25-90 °C) for decanedithiol DDT, (top) and Ring-2 (bottom). Dashed lines are linear fits. I0 ) 6 nA, Ut ) 0.6 V.

Figure 6. Molecular models for the chair conformer of Ring-1 with the side chains in equatorial (a) and axial (b) positions (H on thiol end groups omitted). (c) Crystal structure of Ring-2 derivative, compound 7. S-S distance is 12.62 Å (thioacetate protecting groups are removed on surface adsorption).

side chains in energetically favored equatorial positions. A ring flip produces a higher energy conformer with the side chains in an axial position (Figure 6b). The X-ray structure of Ring-2 derivative, compound 7, shows the all-chair conformation (Figure 6c). However, most importantly, although Ring-1 and Ring-2 have a fixed 1 and 2 gauche folds respectively, together with 2 and 3 rotatable bonds which may adopt gauche conformations, the S-S distance varies little during rotation (7.6-8.4 Å for Ring-1 and 11.7-12.7 Å for Ring-2; from local minima conformers using molecular mechanics (MM2) calculations). This situation is quite different for the linear chain analogues as they are able to explore a much larger conformational space which is reflected in part by the much larger range of S-S separations (see Supporting Information for analysis of conformers of hexanedithiol and S-S separations). Figure 7 shows four of the many possible conformers of hexanedithiol. The lowest energy conformer is the all-trans conformer (Figure 7a). Figure 7b shows the addition of 1 gauche defect in the carbon chain and (c) and (d) are still higher energy gauche defect rich conformers. Indeed, the S-S distance in accessible conformers may vary from 9.4 Å (all trans) to close contact

with 4 or 5 gauche folds. Such gauche-defect-rich conformers and wide variations in the S-S distance found for the linear chain molecules do not exist for Ring-1 and Ring-2. By assuming that the higher energy conformers present a higher conductance modeled by a simple tunneling barrier representation, Haiss et al. have shown that the temperature dependence of the single-molecule conductance can be qualitatively reproduced by calculating the temperature-dependent partitioning of the conformers.8 Jones and Troisi have shown, through quantum mechanical transport calculations, that on average the higher energy conformers with a greater number of gauche conformations (for conformational energies >2 kcal/ mol) give rise to a higher conductance.18 They have proposed these “more folded” conformers provide more facile tunneling pathways which may be related to the presence of throughspace short-cuts. They noted that the particularly high conductance conformers are gauche defect rich with close nonbonded CH2 units as for the conformer in Figure 7d. The lack of temperature dependence for Ring-1 and Ring-2 supports this idea of the gauche-rich conformers of the linearchain molecules being implicit in the temperature dependence of the conductance, rather than electronic structural differences being the cause. The molecular backbone of all four molecules studied is composed of methylene units, so the differences observed are not likely to originate from differing electronic structures of the respective molecules. Like alkanedithiols Ring-1 and Ring-2 have molecular levels of the bridge which are far removed from the Fermi levels of the enclosing gold electrodes in the electrical junctions; for instance, the first spanning HOMO of Ring-1 (HOMO-2) lies at -8.1 eV (calculated by DFT, see Experimental Methods) which is similar to linear-chain analogues. On the basis of the molecular electronic structure, a superexchange mechanism thus seems most likely by analogy to alkanedithiols. It is also noted that the superexchange mechanism has been ascribed to the norbornyl molecular junctions that also have a rigid polymethylene backbone.20 These results, which show significant temperature dependence for hexanedithiol and decanedithiol single-molecule conductance would seem to disagree on first sight with the temperatureindependent molecule conductance measured by Chen et al.15 However, it should be noted that the latter measurements were made with the break junction technique in which the alkanedithiol molecular wire is stretched between two gold contacts until the junction cleaves. Our I(t) experiments presented above are different in that the gap separation between STM tip and gold substrate has been fixed at a value less than the length of the “stretched” trans conformer, so that a range of different conformers can span the gap, including higher energy conformers. We have measured the conductance of hexanedithiol at both r.t. (25 °C) and higher temperature (80 °C)

Single-Molecule Conductance in Nano-Electrical Junctions

J. Phys. Chem. C, Vol. 113, No. 43, 2009 18889 MTKD-CT-2005-029864), S.M. acknowledges a postdoctoral fellowship from Ministerio de Educacion y Ciencia of Spain. Supporting Information Available: Analysis of conformers of hexanedithiol and their S-S separations. Full experimental details of the preparation of 2 and 7 including compound characterization. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 8. All-trans conduction paths of Ring-1 and Ring-2.

References and Notes at larger separation between the tip STM and substrate (I0 ) 4 nA, Ut ) 0.6 V; d ln I/ds ) (-8.4 ( 1.5) nm-1; thus, using eq 1, we have a tip-sample distance of 1.11 nm) where only the all-trans conformer can effectively bridge the gap. Under these conditions, the measured conductance values are 2.38 ( 0.43 nS at 25 °C and 2.42 ( 0.45 nS at 80 °C (the threshold gap separation for alkanedithiol molecule junctions for the transition between temperature-dependent to temperature-independent behavior has been characterized in refs 8 and 9). As can be observed, there is no temperature dependence in this case for hexanedithiol when only the all-trans conformer can bridge the gap. It is also noted that there is no temperature dependence measured for the alkanedithiols with the break junction technique where the molecular wire is stretched between two gold contacts until the junction breaks.15 5. Concluding Remarks The current findings would seem at first sight to go against the well-established “all trans rule”. Through-bond coupling in saturated systems has been shown to be strongly influenced by the configuration and this rule gives the expectation that transport through the bridge occurs predominantly through antiperiplanar σ bonds.25 The 1,4-trans-substituted cyclohexyl systems Ring-1 and Ring-2 have the property that the key coupling paths are exclusively via antiperiplanar interactions, irrespective of rotation about the bonds a-c (Figure 8). Indeed, unexpectedly fast intramolecular electron transfer through, and large long-range coupling between radicals separated by, trans1,4-bis-substituted cyclohexanes, have been noted.26 Nevertheless, the conductance of hexanedithiol at the higher temperature limit of our experiments approaches that of Ring-1 (Figure 4), and likewise, the conductance of decanedithiol at elevated temperatures approaches that of Ring-2 (Figure 5). This indicates that alongside the all-trans rule, conformational gating of the alkanedithiol-bridge to produce the more highly folded gaucherich forms can greatly enhance electronic transmission. Jones and Troisi have contended that through space “short-cuts” for these gauche-rich forms greatly enhance conductance.18 The dynamics of the flexible alkanedithiol bridges and partitioning between differing conformers then means that electronic transmission across the bridge becomes temperature dependent as seen in Figures 4 and 5. This phenomenon of the conformational gating of the bridges leading to greatly enhanced rates of electron transport is a theme of contemporary interest in the literature, and our current investigation highlights that this needs to be taken into account when describing the single molecule conductance of flexible molecular junctions.27 Acknowledgment. This work was supported by EPSRC under Grants No. EP/C00678X/1 (Mechanisms of Single Molecule Conductance) and EP/D023645/1 (Directed Assembly of Functional Patterns), and the European Commission (for a Transfer of Knowledge Marie-Curie project, Contract No.

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