David C.
Milan‡
a,
Maximilian
Krempe‡
b,
Ali K.
Ismael‡
cd,
Levon D.
Movsisyan‡
e,
Michael
Franz‡
b,
Iain
Grace
c,
Richard J.
Brooke
f,
Walther
Schwarzacher
f,
Simon J.
Higgins
*a,
Harry L.
Anderson
*e,
Colin J.
Lambert
*c,
Rik R.
Tykwinski
*b and
Richard J.
Nichols
*a
aDepartment of Chemistry, University of Liverpool, Crown St, Liverpool, L69 7ZD, UK. E-mail: shiggins@liverpool.ac.uk; nichols@liv.ac.uk
bDepartment of Chemistry and Pharmacy & Interdisciplinary Center for Molecular Materials (ICMM), University of Erlangen-Nürnberg (FAU), Henkestrasse 42, 91054 Erlangen, Germany. E-mail: rik.tykwinski@fau.de
cDepartment of Physics, University of Lancaster, Lancaster, LA1 4YB, UK. E-mail: c.lambert@lancaster.ac.uk
dDepartment of Physics, College of Education for Pure Science, Tikrit University, Tikrit, Iraq
eDepartment of Chemistry, University of Oxford, Chemistry Research Laboratory, Oxford, OX1 3TA, UK. E-mail: harry.anderson@chem.ox.ac.uk
fH. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK
First published on 29th November 2016
Oligoynes are archetypical molecular wires due to their 1-D chain of conjugated carbon atoms and ability to transmit charge over long distances by coherent tunneling. However, the stability of the oligoyne can be an issue. Here we address this problem by two stabilization methods, namely sterically shielding endgroups, and rotaxination to produce an insulated molecular wire. We demonstrate the threading of a hexayne within a macrocycle to form a rotaxane and report measurements of the electrical conductance of this single supramolecular assembly within an STM break junction. The macrocycle is retained around the hexayne through the use of 3,5-diphenylpyridine stoppers at both ends of the molecular wire, which also serve as chemisorption contacts to the gold electrodes of the junction. Molecular conductance was measured for both the supramolecular assembly and also for the molecular wire in the absence of the macrocycle. The threaded macrocycle, which at room temperature is mobile along the length of the hexayne between the stoppers, has only a minimal impact on the conductance. However, the probability of molecular junction formation in a given break junction formation cycle is notably lower with the rotaxane. In seeking to understand the conductance behavior, the electronic properties of these molecular assemblies and the electrical behavior of the junctions have been investigated by using DFT-based computational methods.
Oligoynes represent a prototypical molecular bridge system for charge transport studies.19–25 They consist of alternating CC and C–C bonds in 1-D molecular carbon chains and they have been described as “model substances for the hypothetical one-dimensional carbon allotrope carbyne C∞”.26 When appropriate anchoring groups are connected at either end of these chains they can be used to form single-molecule junctions using STM or break junction methods. Wang et al. first established the formation of oligoyne single-molecule electrical junctions in an STM with gold electrodes and pyridyl anchoring groups at both termini of the oligoyne chain.20 The conductance decay with length across a series of these oligomers was demonstrated to be low, in both experimental and theoretical studies.19,20 Further single-molecule experimental studies explored different anchoring groups at the chain termini and also the influence of the solvent medium on the molecular junction electrical properties.21,22
Oligoynes endcapped with pyridyl groups are viable candidates for single-molecular junction formation.20–22,27 To date, however, the synthesis and study of oligoynes bearing unfunctionalized pyridyl groups has been limited to the length of a hexayne,22,28 due to the increasing instability of oligoynes with length. Recently, we disclosed the synthesis of a series of dipyridyloligoyne derivatives kinetically stabilized by the presence of 3,5-diphenyl substitution of the pyridine.29 In this case, steric shielding of the endmost ethynyl moieties led to significant stabilization compared to the unsubstituted series.29 A similar approach has been used to synthesize the longest known polyynes.30 Aside from sterically demanding endgroups, an alternative approach toward the stabilization of sp-hybridized carbon chains has been described based on rotaxane formation, for examples see ref. 31–37. It is easy to envisage the combination of both stabilization methods, namely sterically shielding endgroups and rotaxination, to produce an insulated molecular wire. Equally interesting is the possible effect of isolating the molecular wire from the electrical influences of neighboring molecules.38–42
Given the interest in electrically conducting oligoyne molecular wires, we explore here the possibility of forming single-molecule junctions of an oligoyne threaded within a macrocycle. Rotaxane-based molecular assemblies have been considered as future candidates for molecular electronic devices,43 including concepts for nanoscale data recording.44 Single-molecule electrical conductance is determined for the hexameric oligoyne rotaxane and also for the unthreaded hexayne. The impact of the threading on the conductance is analyzed and discussed with complementary transport computations using DFT.
For conductance determination the STM-BJ tip was repeatedly brought into contact and then withdrawn from the surface. An STM set point current of 100 μA was used to drive the gold STM tip into the surface, followed by retracting the tip 4 nm away from the surface at a rate of 20 nm s−1. A small bias voltage of −300 mV was applied between the tip and substrate and the current versus retraction distance traces were recorded. If no molecules became bound between the tip and the surface while withdrawing the tip, a smooth exponential decay was observed in the current trace. However, when one or more molecules bridged between the tip and the surface, steps were observed. The cycle of driving the tip into the surface and retraction was repeated ∼4000 times for each sample, after which the current versus distance traces were analyzed. An automated procedure was employed to create conductance histograms from the raw current–distance data, which algorithmically rejects some curves with excessive noise or extremely long decay times.45 The peaks obtained in the histograms at G ≪ G0 (where G0 is the metallic point contact conductance) represent the most likely conductance values for molecular junctions.
The estimated maximum diameter of the 3,5-diphenylpyridyl endcapping group is ca. 10 Å,29 and thus the known phenanthroline-based macrocycle M1,31 which features a cavity size of ca. 8.5 Å, was chosen as second component for the rotaxane (Scheme 1). The diameters of the 3,5-diphenylpyridyl endcapping group and M1 were estimated from the X-ray crystallographic analyses reported in ref. 29 and 31 respectively. The phenanthroline-Cu(I) complex M1·Cu(I) was formed via the reaction of macrocycle M1 with CuI in CH2Cl2/CH3CN. Triyne 2 in THF was then added to the M1·Cu(I) complex, and the mixture was stirred under nitrogen at 60 °C in the presence of K2CO3 and I2 until the reaction was judged complete (24 h, as monitored by TLC). Cu(I) was removed with aqueous KCN, and rotaxane 3·M1 was isolated in 2% yield via column chromatography (over the three steps from 1a).
Although easier to synthesize and manipulate, i-Pr3Si-protected triyne 1a required desilylation with TBAF, which can lead to loss of material via decomposition. Thus, Et3Si-protected 1b was explored as an alternative, which can be desilylated under milder conditions with CsF. Using 1b, and an analogous protocol as described for 1a, the yield of rotaxane 3·M1 (13%) was indeed increased. As a model for comparison, the corresponding dumbbell 3 was synthesized as previously reported.29
Characterization by 1H and 13C NMR spectroscopy shows that the spectra of 3·M1 are essentially linear combinations of the spectra of the macrocyclic and polyyne components M1 and 3, respectively. No significant changes in the spectra of 3·M1 are observed. Previous studies of the mobility of phenanthroline-based macrocycles threaded around oligoynes (n ≥ 2) showed that macrocycle is mobile even at 200 K, and its rotation is faster than the NMR timescale (0.2 ms).48 Moreover, the macrocycle was found to be highly mobile in a hexayne [3]rotaxane where there is a significant restriction due to steric hindrance.32
Rotaxane 3·M1 was analyzed by differential scanning calorimetry (DSC) and showed a melting endotherm at 124 °C, followed immediately by decomposition of the rotaxane in the liquid phase. This behavior is similar to other reported hexayne rotaxanes, in which rotaxination leads to melting, rather than direct decomposition as often observed for the corresponding “naked” hexayne.32 For comparison, dumbbell 3 decomposes at 178 °C, without melting (see ref. 29 for details).
The lowest energy UV-vis absorption of 3·M1 (λmax = 478 nm) shows a slight redshift in comparison to the polyyne 3 (λmax = 474 nm). In the high energy region (280–380 nm), the absorption energies of the 3·M1 are unchanged relative to 3, although the relative absorption intensities change, due to absorptions by the macrocycle. No significant solvatochromism was observed for 3·M1, over a range of polar and apolar solvents (see ESI† for spectra). Interestingly, measurements in CHCl3 “out of the bottle” revealed that 3·M1 was readily protonated by residual acidic impurities, and the protonated rotaxane shows a red-shifted λmax value (λmax = 488 nm, see ESI†). Protonation could be effected completely by trifluoroacetic acid, and the process was reversible via addition of triethylamine (see ESI†). An analogous redshift of λmax is observed in the UV-vis spectrum of the protonated polyyne 3. Thus, the changes in the electronic absorption of 3·M1 are ascribed to protonation of the pyridyl groups (rather than the phenanthroline moiety within the macrocycle M1) and result from stabilization of the LUMO by protonation.49–52
Electrical conductance has been investigated here with the scanning tunneling microscope break junction technique (STM-BJ). This technique facilitates the repeated formation of molecular junctions and the determination of molecular conductance at the single-molecule level with large statistical datasets.3 The STM-BJ method and related STM techniques have been widely used to study conductive π-conjugated molecular wires such as oligo(phenylene ethynylenes), oligo(phenylene vinylenes) and oligoynes.53–55
STM-BJ measurements were carried out for 3, 3·M1 (Fig. 1), and as a control, bare gold slides. Fig. 2 shows 1- and 2-dimensional histograms for the dumbbell. The 1-D histogram shows a sharp 1G0 peak corresponding to the conductance of an Au point contact. Clear molecular conductance peaks can be seen at conductance values around 10−5G0 in both the 1- and 2-D histograms. In the 2-D histogram, a long conductance plateau is observed with a plateau length of around 1 nm. This could correspond to a rather invariant molecular conductance as the molecule is either pulled up in the junction from a tilted to upright orientation or as the molecule slides along the respective gold contacts until the molecular junction snaps. Fig. 3 compares the normalized 1-D histograms for 3 and a bare gold control sample, clearly showing the molecular conductance peak for 3.
Fig. 1 Schematic of the STM-BJ molecular junction for single molecular conductance measurements. Left dumbbell (3), right rotaxane (3·M1). |
The rotaxane-hexayne supramolecular assembly (3·M1) was measured using the same conditions, giving the histograms shown in Fig. 4. This figure shows a molecular conductance peak at slightly greater than 10−5G0 indicating that rotaxane formation only influences the conductance of the hexayne thread to a relatively small extent (Fig. 5); note however that the length of the conductance plateau in the 2D histogram in Fig. 4 is considerably shorter than the corresponding feature in Fig. 2. The two histograms are plotted together in Fig. 6 and the conductance peak values are summarized in Table 1. The conductance values here are broadly consistent with previous results,20–22 although an absolute comparison should be made with caution since different surface anchoring groups and different media are employed. For a pyridine capped oligoyne with 4 triple bonds conductance values in the range of 2 × 10−3G0 to 5 × 10−5G0, depending on the conductance group, have been reported.20 In another study significant dependence on solvent medium has been shown.22Ref. 21 showed that upon elongation of carbon chain from 1 to 4 triple bonds, also in the series of pyridine tethered oligoynes, the high conductance values obtained by STM-BJ varied from 4.0 × 10−4G0 (for n = 1) to 4.0 × 10−5G0 (for n = 4). By comparison, the conductance of the hexayne molecule in our experiments is (0.97 ± 0.17) × 10−5G0 (Table 1).
Conductance | |
---|---|
3 | (0.97 ± 0.17) × 10−5G0 |
3·M1 | (1.66 ± 0.25) × 10−5G0 |
From Table 1 it can be seen that the threaded macrocycle appears to increase the conductance of the hexayne molecular wire, but any effect is rather small. Since the y-scale is normalized in counts per trace, the lower peak for the 3·M1 indicates a lower probability for junction formation. This can be interpreted as the bulky phenanthroline-containing macrocycle positioned between the stoppers of the hexayne molecular wire inhibiting molecular bridge formation as illustrated schematically in Fig. 7. This model shows junction formation for 3 occurring by the compound sliding or diffusing along the contacts to attach to the respective electrodes through binding between the terminal pyridyl group and the gold contacts. Such a mechanism might be impeded with the bulky phenanthroline containing macrocycle in place in 3·M1, which is also consistent with the shorter plateau length observed for 3·M1 than 3. Although the phenanthroline macrocycle also contains pyridyl N atoms, their interaction with gold is expected to be extremely weak compared to the interaction with pyridine end-groups, due to the spatial geometry and steric hindrance.
Fig. 7 Schematic illustration of a possible sequence of events leading to junction formation with 3 (top sequence) and 3·M1 (bottom sequence), respectively. |
DFT computations have been performed to understand further the influence on the molecular conductance of threading the hexayne through the phenanthroline containing macrocycle. Fig. 8 shows the transmission coefficient for 3·M1 (red) and 3 (black), which are virtually identical, in agreement with the experimental measurements which show both molecules having a broadly similar conductance value of ∼10−5G0. Both curves show LUMO dominated transport with the DFT calculated Fermi energy (0 eV) sitting close to LUMO resonance and 3·M1 showing a slightly higher conductance value in the gap between the HOMO and LUMO. Examination of the orbitals of the isolated molecules can explain why the macrocycle of the rotaxane plays no part in the charge transport. Fig. 8 shows that the LUMO orbital of each molecule is identical. The LUMO of 3·M1 shows no weight on the ring and therefore plays no role in conductance. Further analysis of the frontier orbitals show that the HOMO of 3·M1 is localized on the ring (Fig. S5†) and it is the HOMO−5 orbital of 3·M1 which is delocalized along the oligoyne. The HOMO−5 of 3·M1 and the HOMO of 3 are identical.
Fig. 8 Left: Zero bias transmission coefficient curves, T(E) of 3 and 3·M1. Right: LUMO orbital of 3 (top) and 3·M1 (bottom). |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6nr06355a |
‡ These authors contributed equally to this work. |
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