Asma
Alanazy‡
a,
Edmund
Leary‡
*bcd,
Takayuki
Kobatake‡
e,
Sara
Sangtarash‡
*f,
M. Teresa
González
g,
Hua-Wei
Jiang
e,
Gabino Rubio
Bollinger
d,
Nicolás
Agräit
dg,
Hatef
Sadeghi
f,
Iain
Grace
f,
Simon J.
Higgins
*c,
Harry L.
Anderson
*e,
Richard J.
Nichols
*bc and
Colin J.
Lambert
*f
aThe Department of Mathematics, Lancaster University, LA1 4YF, UK
bSurface Science Research Centre and Department of Chemistry, University of Liverpool, Oxford Street, Liverpool L69 3BX, UK. E-mail: E.Leary@liverpool.ac.uk
cDepartment of Chemistry, Donnan and Robert Robinson Laboratories, University of Liverpool, Liverpool L69 7ZD, UK. E-mail: shiggins@liverpool.ac.uk
dDepartamento de Física de la Materia Condensada, IFIMAC and Instituto “Nicolás Cabrera”, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
eDepartment of Chemistry, Oxford University, Chemistry Research Laboratory, Mansfield Road, Oxford OX1 3TA, UK
fPhysics Department, Lancaster University, Lancaster LA1 4YB, UK. E-mail: s.sangtarash@lancaster.ac.uk
gFundación IMDEA Nanociencia, Calle Faraday 9, Campus Universitario de Cantoblanco, 28049 Madrid, Spain
First published on 12th July 2019
Charge transport is strongly suppressed by destructive quantum interference (DQI) in meta-connected 1,1′-biphenyl-containing molecules, resulting in low electrical conductance. Surprisingly, we have found that DQI is almost entirely overcome by adding a bridging carbonyl, to yield a cross-conjugated fluorenone. This contrasts with other π-systems, such as para-connected anthraquinone, where cross-conjugation results in low conductance.
We compared the single-molecule conductance of eight compounds with the molecular structures illustrated in Fig. 1: fluorenes p-CMe2-S, m-CMe2-S, p-CMe2-N and m-CMe2-N, and fluorenones p-CO-S, m-CO-S, p-CO-N and m-CO-N. The synthesis and chemical characterization of these compounds are reported in the ESI.† Molecules with suffixes p- and m- are referred to as para- and meta-connected respectively, which reflects the connectivity of the acetylene linkers to their biphenyl cores in the absence of bridging moieties. Scanning tunneling microscopy-break junction (STM-BJ) measurement of these eight compounds are summarized in Fig. 2, which shows the low-bias conductance of each compound at fixed applied bias voltage of 0.2 V. For the meta-connected m-CMe2-N we had to increase the bias to 0.8 V in order to resolve the full conductance distribution due to its very low conductance. We performed thousands of open-close cycles on each sample and focused on the opening stage of the measurement. For more details about the sample preparation and the measurement methodology, see the ESI.† The precise number and percentage of molecular junctions observed in each case is given in the caption to Fig. 2.
Fig. 1 Chemical structures of the eight compounds investigated. The biphenyl core is common throughout; fluorenones have a carbonyl bridge (red) and fluorenes have a –CMe2 bridge (green). |
We processed the data by first separating the molecular junctions from the pure-tunneling junctions (containing no molecule) using an algorithm which searches for plateaus (details in the ESI†). The resulting 1D and 2D ‘molecular-junction’ histograms from this procedure are shown in Fig. 2 (examples of selected conductance–distance, G–z, traces for each compound are shown in Fig. S4.1†).§ Histograms of the ‘tunneling-only’ traces are shown in Fig. S4.2.† The 1D histograms are normalized according to the procedure described in ref. 9 allowing facile comparison of the average length of the plateau region for each compound (i.e. from the point where the plateau begins). To extract a conductance for each compound, we fit a single Gaussian curve to each 1D histogram and extract the peak position. A summary of the low-bias conductance values is presented in Table 1.
Molecule | Measured low-bias conductance (log(G/G0)) | Measured 95th percentile (L95) + 0.4 nm | Theoretical Au–Au distance (nm) | DFT-predicted HOMO–LUMO gaps (eV) |
---|---|---|---|---|
p-CMe 2 -S | −4.5 (0.9) | 2.6 | 2.5 | 2.13 |
m-CMe 2 -S | −6.4 (0.8) | 2.4 | 2.2 | 2.63 |
p-CO-S | −4.6 (0.8) | 2.6 | 2.5 | 1.65 |
m-CO-S | −5.0 (0.8) | 2.3 | 2.2 | 1.81 |
p-CMe 2 -N | −5.6 (0.7) | 2.2 | 2.4 | 2.30 |
m-CMe 2 -N | −7.4 (0.6) | 1.9 | 1.8 | 2.95 |
p-CO-N | −5.7 (0.6) | 2.2 | 2.4 | 1.96 |
m-CO-N | −6.1 (0.7) | 1.7 | 1.8 | 2.08 |
The most surprising outcome of the STM-BJ measurements is that for the meta-compounds with both thiol and pyridyl anchor groups, replacing the CMe2 bridge by a CO leads to a dramatic increase in conductance by approximately a factor 30. The same replacement in the para-compounds has a negligible effect (both p-CO compounds actually appear to be fractionally lower in conductance than their p-CMe2 counterparts). This agrees with our previous results on para-connected OPE3 molecules, where different substituents on the central phenyl ring have little influence on electrical conductance.4d,10 Viewed another way, switching from para to meta connectivity when the bridge is CMe2 causes the conductance to drop two orders of magnitude. In contrast, when the bridge is CO, the same operation causes the conductance to drop by only a factor 2–3. This behavior is remarkable, because from a valence-bond perspective, each terminal S/N atom is formally cross-conjugated via the carbonyl group, as noted for similar structures by Estrada et al. and Homnick et al.,11 and therefore no direct alternating single/double bond path exists for meta-connectivity.
The junction length distributions (shown in Fig. S4.3,† with mean values from Gaussian fits quoted in Table 1) confirm that we measure fully-stretched junctions at the upper extreme of the distribution. After correcting for the initial jump-out-of-contact (JOC) by adding 0.4 nm to the raw junction lengths (L), we find very good agreement between the calculated Au–Au distance (for gold atoms attached to the terminal S/N atoms) and the 95th percentile length (L95). We find that the longest plateaus for thiol-terminated junctions tend to exceed the predicted maximum value by 1–2 Å, whereas the pyridyls tend to be shorter than this value by 1–2 Å. This behavior agrees with our previous observations, which for thiols12 is a result of the strength of the Au–S bond which produces more significant deformation of the electrodes compared with weaker binding groups like pyridyl. For both para fluorene and fluorenone compounds with the same anchor group, the break-off histograms almost coincide (Fig. S4.3†). In contrast, for the meta compounds, the histograms for m-CO compounds are noticeably centred towards lower values compared to the m-CMe2 counterparts, indicating plateaus are on average 1–2 Å shorter. This indicates that conjugation slightly affects junction binding strength.
Comparison between the thiol and the pyridyl anchor groups reveals that for any given backbone, the conductance is about 10 times lower for pyridyls compared to thiols. A few published reports directly compare thiol anchors with pyridyls, and in general the pyridyls all display lower conductance than the corresponding thiols.13 In ref. 14 the benzenethiol, PhS, groups in an OPE3 wire were exchanged for Py, resulting in a 30-fold drop in conductance. In two independent studies of oligophenyls 1,4-di(pyridin-4-yl)benzene was measured to have a conductance of log(G/G0) = −4.7,15 whereas structurally-analogous p-terphenyl dithiol has a conductance of log(G/G0) = −3.2 (ref. 12) (also about a factor 30 difference). Therefore, our results are consistent with these previous measurements. We also studied the voltage dependence of the conductance, log(G/G0) vs. V, presented in Fig. S4.5 (section 4 of the ESI†). In short, all compounds tested showed a moderate increase in conductance between 0 V and 1.0 V, but no major differences in log(G/G0) vs. V behaviour were found between the para and the meta compounds. Finally, for the thiol measurements we observed a faint low-conductance group after the main plateau, which we have previously shown arises from junctions involving gold adatoms and sulfur groups from neighboring molecules.12
To calculate the conductance of each molecule connected to two gold electrodes, the optimal geometry and ground state Hamiltonian were obtained using the SIESTA16 implementation of density functional theory (DFT) and the room-temperature electrical conductance was calculated using the Gollum17 code (see computational methods in the ESI). Fig. 3 shows the calculated conductance of the molecules in para and meta connectivities for fluorene and fluorenone with thiol (a and c) and pyridine anchors (b and d) respectively (see relaxed structure of the molecules between leads in Fig. S5-1 and S5-2†). Since the energy difference between the molecular orbitals and the DFT-predicted Fermi energy (EF, chosen to be zero in Fig. 3) and is usually unreliable, resulting in EF being placed too close to the HOMO (LUMO) for thiol (pyridyl) anchors, results are shown for a range of Fermi energies in the vicinity of the middle of the HOMO–LUMO gap (see section 8 in the ESI† for further discussion). For comparison, the horizontal bands in Fig. 3 show the measured conductance values in the second column of Table 1. The widths of the horizontal bands correspond to the experimental full width at half maximum (FWHM) of the conductance peak.
Fig. 3 The calculated room-temperature conductances of (a) p-CMe2-S and m-CMe2-S; (b) p-CMe2-N and m-CMe2-N; (c) p-CO-S and m-CO-S; (d) p-CO-N and m-CO-N; connected to gold electrodes, obtained from DFT. Results are plotted against the Fermi energy EF, where EF = 0 corresponds to the DFT-predicted Fermi energy. For comparison, the horizontal bands show the measured conductance values in the second column of Table 1. The widths of the horizontal bands are equal to the FWHM quoted in the second column of Table 1. |
Fig. 3 shows that there is qualitative agreement between calculated and measured conductance trends of the molecules, for a range of Fermi energies near the gap centre. For both thiol and pyridyl anchors, there is a large ratio (about 2 orders of magnitude) between the conductances of para vs. meta-connected fluorene molecules and a significantly smaller ratio between the conductance of the para and meta-connected fluorenones. The magnitude of the conductance with pyridyl anchors is about one order of magnitude lower than with thiol anchors. Furthermore, the conductance of the meta-connected fluorenone with thiol anchors is surprisingly high. From these results, we conclude that the bridge moiety strongly enhances the conductance of the meta-connected molecules, but does not significantly influence para-connected molecules.
Table 1 shows that there is a correlation between the DFT-predicted HOMO–LUMO gaps and the measured conductances. When switching from para to meta connectivity, the HOMO–LUMO gaps always increase. However, the increase is small for the fluorenones and significantly larger for the fluorenes. This correlates with the smaller reduction in conductance for the fluorenone core compared with fluorene, and can be attributed to conjugation between the anchor groups and the CO in the meta-fluorenones. The gap for para-fluorenes, however, is always larger than the corresponding meta-fluorenones, yet the conductance is lower for the meta-fluorenones than the para-fluorenes. This demonstrates that HOMO–LUMO gaps are not absolute predictors of molecular conductance, and that quantum interference due to scattering from the bridge moiety plays a significant role.
To demonstrate the role of the bridge in the core of the molecule, we considered the series of tight binding models (Fig. S6.1†), which demonstrate that the main effect of the bridge moiety is to alleviate DQI from the middle of HOMO and LUMO of the meta-connected biphenyl core and increase the conductance of the resulting meta-connected fluorene and fluorenone cores.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nr01235d |
‡ These authors contributed equally to this work. |
§ All individual traces (G–z and I–V) for the STM experiments are freely available after an embargo date of 01/10/19 at the following repository address: http://dx.doi.org/10.17638/datacat.liverpool.ac.uk/584. |
This journal is © The Royal Society of Chemistry 2019 |