Enhancing Molecular Conductance of Oligo(p-phenylene ethynylene

Article pubs.acs.org/JPCC

Enhancing Molecular Conductance of Oligo(p‑phenylene ethynylene)s by Incorporating Ferrocene into Their Backbones Qi Lu, Chuan Yao, Xianhong Wang,* and Fosong Wang Key Laboratory of Polymer Eco-materials, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, People’s Republic of China S Supporting Information *

ABSTRACT: Designing and preparing the molecular wires with good charge transport performance is of crucial importance to the development of molecular electronics. By incorporating ferrocene into molecular backbones, we successfully enhanced the molecular conductance of OPEs in both tunneling and hopping conduction regimes. Furthermore, we found that the increase degree of molecular conductance in the hopping regime is much more than that in the tunneling regime. Via this approach, the molecular conductance of a long molecule exceeds the molecular conductance of a short one at room temperature. A theoretical calculation provided a possible and preliminary explanation for these novel phenomena in terms of molecular electronic structures. The current work opens the opportunity for designing excellent charge transport performance molecules. An increasing number of new types of molecular wires with this unusual phenomenon are expected to be discovered in the future.

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INTRODUCTION The ultimate goal of molecular electronics is to replace the traditional silicon integrated circuits with nanocircuits based on single-molecular devices.1−8 Therefore, designing and preparing molecular wires with good charge transport performance as the connecters of the nanocircuits is of crucial importance to the achievement of this ambitious blueprint.9−14 An effective approach to enhance the molecular conductance is to modulate its structure properly under the guidance of the conduction mechanisms. Analyzing the different charge transport behaviors in various molecular wires, two distinct conduction mechanisms were proposed: the nonresonant tunneling mechanism and the hopping mechanism.15−18 The former one is predominant in short molecular wires. In this mechanism, an electron crosses molecular junctions in a single step, without appreciable residence time on the molecules. The molecular resistance depends greatly on the totally average potential barrier, namely, the alignment of two frontier molecular orbitals to the Fermi level of the metal electrode. And it varies exponentially with molecular length,19−21 as shown in eq 1: R = R 0 exp(βL)

hopping barrier and it has a relatively weak length dependence,25 as shown in eq 2: R = R 0 + αL = R 0 + α∞L exp[Ea /(kT )]

where again R0 is the contact resistance, L is the molecular length as above, α = α∞ exp[Ea/(kT)] is a molecular intrinsic parameter with unit resistance per unit length, Ea is the hopping activation energy, T is the temperature in Kelvin, and k is the Boltzmann constant. It can be concluded from the respective characteristics of the two conduction mechanisms that any changes of the molecular structure that would reduce the barrier height (no matter in tunneling conduction or hopping conduction) can enhance its conductance effectively.9,23,25,26 Although a large number of experiments have been carried out in this research field, few of them are related to the long molecular wires in the hopping regime because of the difficulty in synthesis and measurement. Recently, Frisbie et al. have reported a very interesting result involving both transport regimes.23 They incorporated a nonconjugated structure, i.e., a negative factor for the charge transport, into conjugated oligophenyleneimine molecules with different lengths and found that there was much more increase in molecular resistance for hopping conduction than for tunneling. For this novel phenomenon, they gave an explanation from the view of intrinsic difference between the two conduction mechanisms. This work inspired us to take a bold hypothesis from the opposite direction: incorporating a positive factor into the molecular backbone should take more decrease in molecular

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where R0 is the contact resistance, β is the tunneling decay constant, and L is the molecular length. When the molecular length exceeds a critical distance, usually 3−5 nm, the leading position of nonresonant tunneling conduction in molecular junction is replaced by hopping conduction, where the electron first enters into the molecular orbitals from the source electrode, and then drifts through a series of discrete steps to reach the drain electrode.22−24 In hopping conduction, the molecular resistance depends greatly on each site-to-site © 2012 American Chemical Society

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Received: December 13, 2011 Revised: July 16, 2012 Published: July 19, 2012 17853

dx.doi.org/10.1021/jp2119923 | J. Phys. Chem. C 2012, 116, 17853−17861

The Journal of Physical Chemistry C

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Figure 1. Structures of the molecular wires with and without positive incorporation.

ferrocene-based oligo(p-phenylene ethynylene)s illustrated in Figure 1 were described in the Supporting Information. Determination of Molecular Optical Band Gaps. The ultraviolet−visible (UV−vis) absorption spectra that used to determine the molecular optical band gaps were recorded from a Varian 50 Bio spectrometer at room temperature in dichloromethane with conventional 1.0 cm quartz cells. Preparation and Characterization of Monolayer. A self-assembled monolayer (SAM) of each molecular wire was formed by immersing the gold coated substrate into 0.1 mM solution in THF for 48 h in the absence of light and oxygen, then rinsing with THF, dichloromethane, and ethanol, and drying under a bland argon stream. The CV curves were acquired on a Solartron SI 1287 electrochemical interface using a standard three-electrode cell containing a gold electrode, a platinum flag counter electrode, and a Ag/AgCl reference electrode. A 0.5 mM K3Fe(CN)6/ K4Fe(CN)6 solution with 0.5 M KCl as supporting electrolyte was prepared freshly prior to use. Under argon protection, molecular wire was dissolved in mixture solvents of ethanol/ dichloromethane (9:1, v/v) to form a concentration of 0.5 mM. A clean gold electrode was then immersed into the solution in the absence of light and oxygen. The SAM was formed on the gold surface via an amine−Au link, and then the SAM-modified gold electrode was rinsed subsequently with THF, dichloromethane, and ethanol and dried under an argon stream. The XPS spectra of SAMs were carried out on a VG Scientific ES-CALAB 250 spectrometer with Al Kα X-ray source (1486.5 eV) using a pass energy of 20 eV at takeoff angles of 90° in an ultrahigh vacuum system. The surface charge effect was compensated by referencing the adventitious C1s peak at 284.6 eV. Electrical Measurements. The CP-AFM experiments were performed under an argon atmosphere on a Digital Instruments (Santa Barbara, CA) Nanoscope IIIA Multimode equipped with a J-scanner (containing a silicone gas chamber) and a current sensitive attachment. A gold coated conductive tip was brought into contact with SAM as the top electrode, and the Au substrate served as the bottom electrode. Each I−V curve was recorded at an applied load of 2 nN using the same tip. The I− V curves were collected at five different places for each SAM, and more than 10 measurements were conducted at each place. The STM-break junction technique employed to measure the single molecular resistance was first developed by Xu and Tao, and utilized by many other research groups extensively.29−38 This technique was based on the repeated forming

resistance for hopping conduction than for tunneling. If this assumption can be borne out, more attention should be paid for the exploration of long molecular wires with potential excellent electrical properties.27,28 As one of the typical molecular wires had been investigated in our earlier study,18 here we chose the oligo(p-phenylene ethynylene)s (OPEs) as the research object again. Considering the known favorable physical and electronics properties of ferrocene, we chose it as the positive factor to incorporate into the OPEs. The synthesis and electrical characterization of OPEs with or without positive factor incorporation are reported in this article. Among the 11 OPEs, 7 are all-organic molecules (OPE1−OPE7) ranging in length from 0.98 to 5.11 nm and the other 4 are ferrocene-based molecules (Fc1, Fc3, Fc5, and Fc7) ranging in length from 1.08 to 5.14 nm (Figure 1). Considering the comparability between our present and previous experiments, conductive probe-atomic force microscopy (CP-AFM) and scanning tunneling microscopy-break junction (STM-break junction) were employed to investigate the charge transport behaviors. Furthermore, to obtain more information on hopping conduction, all STM-break junction experiments were operated under variable temperature. Experimental results show that (1) incorporating ferrocene into molecular backbones could enhance the molecular conductance in both the tunneling and hopping regimes, (2) the increase degree of molecular conductance in the hopping regime is much more than that in the tunneling regime as we had anticipated, and (3) via this approach, the molecular conductance of Fc5 exceeds the molecular conductance of Fc3 at room temperature; namely, the longer molecular wire in the hopping regime could possess a higher conductance than the short molecular wire in the tunneling regime at room temperature. Finally, the theoretical calculations provided a possible and preliminary explanation for these phenomena in terms of molecular electronic structures.

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EXPERIMENTAL SECTION Materials. Contact mode AFM tips coated with gold were purchased from Mikro Masch. Gold wire of 99.9985% purity with a diameter of 0.25 mm as the STM tip was purchased from Alfa Aeser. Gold coated mica substrates (1.4 × 1.1 cm2) were purchased from Agilent Technologies. Tetrahydrofuran (THF) was purchased from Acros Organics and distilled under argon over sodium benzophenone ketyl prior to use. Other reagents and solvents were purchased from commercial suppliers and used without further purification. The synthesis details of four 17854



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Figure 2. (a) CV curves of KCl/K3[Fe(CN)6] (0.5 M/5 mM) on gold electrode before (■) and after immersion in an Fc5 solution for 1 (●), 10 (▲), 40 (▼), 120 (◀), 360 (▶), 1080 (◆), and 2880 min (⬟). (b) Growth rate for the SAM of Fc5 on a gold electrode. (c) XPS spectra of the Au4f region for a bare gold electrode and SAMs of an Fc1−Fc7-modified gold electrode. (d) XPS spectra of the N1s region for the SAM of Fc5 with two distinct peaks before and after the temperature-dependent measurement (inset).

SAM characterization. Therefore, prior to the electrical measurements, all the SAMs were characterized by cyclic voltammogram (CV) and X-ray photoelectron spectroscopy (XPS) to determine basic properties, such as coverage, rate constant of adsorption, and film thickness. The surface coverage (Γ) of the SAM-modified gold electrode is defined as shown in eq 3:

and breaking of the molecular junctions between the tip and substrate. The gold STM tip could penetrate into the SAM at −50 mV with the current set-point in the range of 0.5−20 nA, and the N−Au bond was therefore formed. The feedback was then disabled, and the tip was lifted at a vertical rate of 4 nm/s while keeping the X−Y position constant. The current was recorded as a function of the traveling distance when the tip moved from the substrate to break the molecule junctions. This procedure was repeated thousands of times for each sample, and statistical analysis was constructed to extract single molecular resistance from the staircase-like current−distance (I−S) curves. Variable temperature STM-break junction measurements were performed with a Digital Instrument environment controlled terminal. We have checked all the SAMs from room temperature (298 K) to 338 K with 10 K increments every time while keeping the relative humidity in the chamber below 5%. For each temperature setting, the system was allowed to stabilize for more than 30 min before measurements to minimize thermally induced drift. Calculation Methods. The optimization conformations and electronic structures of four ferrocene-based molecular wires were calculated within the density functional theory approximation. The density functional theory calculations were performed by using the B3LYP functional coupled with the LANL2DZ basis set for all atoms.39−41

Γ(t ) = 1 − It /I0

(3)

where Γ(t) is the surface coverage as a function of time and I0 and It refer to the initial redox currents of the bare gold electrode and the SAM-modified gold electrode at various times, respectively. By collecting and analyzing the surface coverage data at different times, the entire adsorption process for each molecular wire on the gold surface could be observed. Figure 2a shows the redox current change after the gold electrode was immersed in a solution of Fc5 for 1, 10, 40, 120, 360, 1080, and 2880 min, respectively. The current gradually decreased with the increasing assembly of the molecular wires on the electrode surface. This modified gold electrode obviously showed a similar trend of current variations to that immersed in the OPE5 solution [Figure 2a (inset)]. The growth process for the SAM of Fc5 on a gold electrode is presented in Figure 2b. The entire absorption process followed the Langmuir equation, Γ = 1 − exp(−kt), where the rate constant k of the adsorption at (1.72 ± 0.09) × 10−4 s−1 was resolved by a nonlinear simulation. This value is very close to (1.6 ± 0.2) × 10−4 s−1, the rate constant of OPE molecules without ferrocene obtained in our earlier work.18

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RESULTS AND DISCUSSION Self-Assembled Monolayer (SAM) Characterization. One important requirement for successful electrical measurement, especially for the CP-AFM test, is an ordered and dense 17855



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Figure 3. (a) Semilog plot of the average current of 10 I−V traces for Fc1−Fc7. The inset is a semilog plot of current versus electric field for Fc5 and Fc7. (b) Log−log plot of the average of 10 I−V traces for the Au−Fc1−Au and Au−Fc5−Au junctions, where fits are shown in different transport regimes. (c) Fowler−Nordheim plot for the Fc1 I−V data, where two distinct regimes are clearly observed. (d) Fowler−Nordheim plot for the Fc5 I−V data, where three distinct regimes are shown.

down. All of these findings prove that the OPE molecules, whether with or without a ferrocene unit, had the same assembly behavior on a gold surface and the anchoring-group amine was protected by argon while undergoing the hightemperature test. I−V Measurements by CP-AFM. The semilog plots of I versus V for all ferrocene-based OPE molecules are shown in Figure 3a. The low-voltage resistance of each SAM can be determined from the linear I−V relationship within the range of ±0.1 V, and compared with one another quantitatively along with some proper corrections. In Figure 3a, the current decreases as molecular length increases. However, the current of Fc3, Fc5, and Fc7 overlaps with one another in the low-bias region, indicating that their molecular resistance is very close to one another. Fc5 and Fc7 contained methoxy groups, so their single-molecular occupying areas on the gold substrate surface were larger than that of Fc3; that is, the similar current of the three molecules corresponded to different numbers of molecules connected between the tip and the substrate. Thus, Fc5 and Fc7, which were longer than Fc3, could possess lower single molecular resistance. A more accurate measurement based on STM-break junction will be given in the next section. More conduction details on the charge transport mechanisms can be acquired through a comprehensive and systematic analysis of the data in the entire bias range. In our earlier study, the transition from tunneling conduction to hopping occurred between OPE3 and OPE4 at room temperature. Here we estimate that the same transition should occur between Fc3 and Fc5. On the basis of the plot of log I versus electric field E [Figure 3a (inset), the larger size plot could be seen in the Supporting Information], the charge transport had field-driven

The thickness of the SAM can be estimated using the attenuation of the Au 4f signal from the gold substrate in the XPS measurement based on eq 4: Isubstrate = ISAM exp(d /λ sin θ)

(4)

where d is the film thickness; Isubstrate and ISAM are the average intensities of the Au 4f5/2 and 4f7/2 peaks before and after monolayer assembly, respectively; θ is the angle of photoelectron detection; and λ is the effective attenuation length of the photoelectron, which is approximately equal to 4.2 ± 0.1 nm. On the basis of the XPS data illustrated in Figure 2c, the SAM film thicknesses of the ferrocene-based molecular wires are calculated and listed in Table 1 in the Supporting Information. To ensure that the terminal amines of the molecular wires were not destroyed at high temperature, all the SAMs were again characterized by XPS in the N1s region after the temperature-dependent measurements. Figure 2d illustrates the XPS scans of the N1s region in the SAM of Fc5 before and after (inset) the temperaturedependent measurement. Two types of N atoms with binding energies of 399.8 and 400.9 eV were observed in these two scans with binding energies of 399.8 and 400.9 eV, respectively. Comparing the XPS scans of Fc5 powder with those in our earlier study, the peak at 399.8 eV was assigned to the nitrogen atoms not bonded to the gold substrate, whereas the highenergy peak was related to nitrogen atoms in strong interaction with the gold substrate. This binding energy cleavage was also true when amine-terminated molecules were anchored on Ag, W, and Fe surfaces.42 The similar intensity of the two peaks indicated that the amine-terminated ferrocene-based molecules adsorbed on the gold substrate were also standing up, not lying 17856



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Figure 4. (a) Plots of current versus stretching distance for Fc3 at a bias of 50 mV. (b) Histogram of recorded currents with peaks at ca. 1.15 and 2.3 nA. (c) Plots of current versus stretching distance for Fc5 at a bias of 50 mV. (d) Histogram of recorded currents with peaks as ca. 1.45, 2.9, and 4.3 nA. (e) Histograms of recorded currents for Fc5 measured at different temperatures. (f) Semilog plot of single-molecular resistance versus molecular length for all of the Au−molecular wire−Au junctions. The green and magenta lines are linear fits according to eq 1.

characteristic for Fc5 and Fc7, that is, the I−E traces collapsed on top of one another. In contrast, no collapse occurred for Fc1 and Fc3, indicating that the tunneling transport with voltagedriven characteristic was predominant.43−45 Figure 3b displays a log−log plot of the I−V characteristics for Fc1 and Fc5. For Fc1, an obvious transition occurred at 0.61 V, and the whole transport region was divided into two regimes, labeled I and II. The slope in regime I for Fc1 demonstrated that the current approximately scaled linearly with voltage, which is a typical I−V behavior for tunneling transport in the low-bias regime based on the Simmons approximation.46−50 The Simmons model is a representative quantum mechanical model for the tunneling mechanism, as shown in eq 5:

I=

⎛ 2d 2m Ae ⎧⎛⎜ eV ⎞⎟ ⎨ ϕ− exp⎜ − 2 2 ⎝ 2 ⎠ ℏ ⎝ 4π ℏd ⎩

ϕ−

⎛ 2d 2m ⎞ ⎛ eV ⎞⎟ eV ⎫ ⎬ exp⎜ − − ⎜ϕ + ⎟ ϕ+ ⎝ ⎝ 2 ⎠ 2 ⎭ ℏ ⎠

eV ⎞ ⎟ 2 ⎠

(5)

where A is the junction area, d is the molecular length, m is the electron effective mass, Φ is the barrier height, and e is the electronic charge. At low bias, this equation can be approximately reduced to eq 6: ⎛ 2d 2mϕ ⎞ ⎟⎟ I ∝ V exp⎜⎜ − ℏ ⎝ ⎠

(6)

This equation represents a direct tunneling mechanism with a linear I−V relation, similar to that in Figure 3b. At the opposite 17857



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Figure 5. Arrhenius plots for OPE1, OPE3, OPE5, OPE7, and Fc1−Fc7. Straight lines are the linear fits to the data.

and the substrate. The histogram was fitted with a Gaussian distribution to determine the peak centers and full widths at half-height (fwhh). The single-molecular resistance of Fc3 could be calculated from the first peak center value using the fwhh as the error bar. The same analysis was used for Fc5. The step lengths of Fc3 and Fc5 in the I−S curves were less than the theoretical length of the molecules. This is a typical characteristic of the STM-break junction technique. Although a number of studies found that a longer molecular wire had a shorter step length,11,53,54 the opposite result was obtained in the present study, which may result from the different anchoring groups used in respective experiments. The second current peak of Fc5 was not as obvious as the first because of the stronger intermolecular effect, which could prevent the molecules connecting between the tip and substrate simultaneously.55−58 At room temperature, the Fc5 had a lower single-molecular resistance compared with Fc3. To compare the low bias resistance of Fc3 and Fc5 obtained in the CP-AFM experiments with the single-molecular resistance obtained in STM-break junction, the numbers of respective molecules connected between the tip and the substrate can be assessed. As expected, the number of Fc3 connected in the molecular junctions was 1.2 times that of the number of Fc5, which is in agreement with our earlier research.18 Furthermore, the consistency of the two measurement techniques will be discussed in the Supporting Information. Figure 4e shows that the current distribution of Fc5 varied with temperature ranging from room temperature (298 K) to 328 K. The obvious increase in the current reveals that Fc5 had a typical hopping conduction characteristic. The peak height became lower, and the fwhh grew wider with increasing temperature. These findings resulted from the more intense perturbation of molecular junctions at high temperature.59−61 All values of single-molecular resistance were plotted against their molecular lengths on a natural log scale, as illustrated in Figure 4f. Fc5 and Fc7, which are temperature-dependent, should be hopping conduction. Comparing Fc1−Fc3 with OPE1−OPE3 and Fc5−Fc7 with OPE5−OPE7, similar length dependence of molecular resistance in tunneling and hopping regimes was observed, respectively. And it should be noted that, in the boundary region, the molecular resistance of Fc5 fell below the molecular resistance of Fc3, namely, a longer molecule had higher single molecular conductance. Ferrocene, as a positive factor,62−64 enhanced the molecular conductance of all molecules, but the amount of increase was

limit, as the bias exceeded the barrier height, the relationship between current and voltage can be described as eq 7: ⎛ 4d 2mϕ3 ⎞ ⎟ I = V 2 exp⎜⎜ − ⎟ ℏ 3 eV ⎝ ⎠

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This relationship is also called the field emission mechanism. To simplify the analysis of I−V characteristics, eq 5 can be transformed into the Fowler−Nordheim relation, as shown in eq 8: −4d 2mϕ3 ⎛ 1 ⎞ ⎛ I ⎞ ⎜ ⎟ ln⎜ 2 ⎟ ∝ ⎝V ⎠ ⎝V ⎠ 3ℏe

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The Fowler−Nordheim plot of Fc1 in Figure 3c revealed that a distinct transition occurred in transport behavior. The transition voltage corresponding to the inflection point in Figure 3b was equal to the low-bias barrier height. The barrier height values of Fc1 and Fc3 are listed in Table 1 in the Supporting Information. In regime II, the current scaled linearly with 1/V, with a negative slope characteristic of field emission. The logos on top of Figure 3c are representations of the barrier shapes, which are rectangular, trapezoidal, and triangular from right to left (arranged from low bias to high bias).51 For Fc5, the log−log plot was more complicated, with two inflection points at 0.62 and 1.17 V dividing the transport region into three regimes, labeled I′, II′, and III′, respectively. Compared with Fc1, the new transitional zone that appeared in the Fc5 plot was not a simple transition between direct tunneling and field emission because the conduction mechanism in Fc5 transformed into hopping. The most reasonable explanation for this occurrence is that, in the lowbias region, the charge transport was dominated by an ohmic hopping mechanism, that is, a field-driven conduction having a linear relationship with voltage. In contrast, in the high-bias region, field emission was still dominating, as indicated by the negative slope. As for the transitional region, no suitable theoretical explanation is currently available.52 Thus, further studies must be conducted in the future. Single-Molecular Measurements by STM-Break Junction. Figure 4a shows five traces of current versus stretching distance (I−S curves) for the SAM of Fc3 at room temperature, with well-defined steps appearing at an integer multiple of ca. 1.15 nA. Five hundred of these current data comprise the histogram displayed in Figure 4b, which shows two distinct peaks corresponding to one and two molecules between the tip 17858



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Fc5 ≈ Fc7 > Fc1 > Fc3. This result is attributable to the different conduction mechanisms. For Fc1 and Fc3 in the tunneling region, the incorporation of ferrocene reduced their average tunneling barriers by 0.45 and 0.25 eV, respectively (see Figure S10 in the Supporting Information). The greater decrease in barrier height of Fc1 than that of Fc3 is due to the difference in molecular structures. Fc1 was built up of ferrocene with two amines as anchoring groups, while Fc3 was built up of ferrocene and two phenylacetylene fragments. From this viewpoint, the molecular electronic structure of Fc1 would be inclined to that of pure ferrocene, while the molecular electronic structure of Fc3 would lie between that of pure ferrocene and OPE3. For Fc5 and Fc7 in the hopping region, ferrocene changed the site-to-site hopping barriers, which could enhance the total molecular conductance because of the special discrete steps of the transport mode of hopping. During this process, the advantage of ferrocene could be fully expressed when the electron jumped across it along the molecular backbone; namely, the conduction in long molecular wires was much more sensitive to their architectures than that in the short one. Although the site-to-site hopping barrier in a special location could not be directly observed in experiment, the hopping activation energy Ea obtained from the variable temperature experimental data would give us some valuable information in this respect. More temperature-dependent measurements are shown in Figure 5. The values of resistance for short molecular wires, including Fc1, Fc3, OPE1, and OPE3, were independent of temperature from 298 to 338 K. However, Fc5, Fc7, OPE5, and OPE7 displayed strong, thermally activated transport behavior of hopping conduction. The hopping activation energies determined from the slopes of the data were identical at 0.31 and 0.32 eV for Fc7 and Fc5 and 0.57 eV for both OPE5 and OPE7. The small activation energies of Fc5 and Fc7 result in excellent long-range charge transport performance. The activation energies of other molecular wires in the hopping region are listed in Table 1 in the Supporting Information, clearly showing that all the OPE molecules without a ferrocene unit possessed relatively larger activation energies. Theoretical Calculation. The charge transport mechanism is known to be affected by molecular electronic structures, including the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) and the alignment of two frontier molecular orbitals to the metal Fermi level.65−68 The electronic structures of four ferrocene-based OPE molecules (Figure 6)

were calculated to try to explore the origin of their excellent electrical properties. The energy gaps (Eg) obtained from quantum chemistry calculations and UV−visible spectroscopy were also given and were found to be close to one another, indicating the suitability of the theoretical simulation. A viewpoint in theory is that, when the difference between the LUMO and the Au Fermi level (−5.3 eV) is large, the charge transport occurred by tunneling, and the tunneling barrier is determined by the difference between the HOMO and Fermi level. When the LUMO decreased to a limited extent, hopping conduction would occur. Obviously, the limited extent is a characteristic value for different molecular wires. In Figure 6, it shows that the ferrocene-based OPE molecules had HOMO levels with small fluctuations, whereas the LUMO level decreased significantly from Fc1 to Fc7. To compare it with the all-organic OPE system in our earlier study, we found that the most significant difference between them is the variation trend of the LUMO level (see details in the Supporting Information). The LUMO of all-organic OPE molecules kept stable in the hopping regime, while the LUMO in ferrocene-based OPE molecules kept on decreasing in the whole length range. We hypothesize that the continual decrease in the LUMO level may result in the peculiar phenomenon observed in the present study. Due to the lack of correlative experimental data and theory, we could not make a further and systematical discussion, and we deeply hope that more research in this field could be done in the future.

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CONCLUSIONS By comparing two series of OPE molecules, we have found that the incorporation of ferrocene has a larger impact on the conduction of long molecular wire than short molecular wire due to the different transport mechanisms. Via this approach, the molecular conductance of a long molecule in the hopping regime exceeds the molecular conductance of a short one in the tunneling regime at room temperature. A theoretical calculation provided a possible and preliminary explanation for these novel phenomena in terms of molecular electronic structures. The current work opens the opportunity for designing excellent charge transport performance molecules. An increasing number of new types of molecular wires with this novel phenomenon are expected to be discovered in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details for the synthesis of molecular wires, UV− visible spectra, CP-AFM data, quantum chemistry calculation, and comparison between the two electrical measurement techniques. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (Grant No. 51021003). We gratefully thank Professor Liyan Wang in Jilin University for support in the temperature dependent measurement and theoretical calculation. We also

Figure 6. HOMO, LUMO, and Eg of Fc1−Fc7 obtained from quantum chemistry calculation and UV−visible spectroscopy. 17859



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gratefully thank editor and reviewers for the insightful and helpful comments.

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