Gating of Quantum Interference in Molecular Junctions by Heteroatom Substitution

Article abstract of DOI:10.1002/anie.201609051

To guide the choice of future synthetic targets for single-molecule electronics, qualitative design rules are needed, which describe the effect of modifying chemical structure. Here the effect of heteroatom substitution on destructive quantum interference (QI) in single-molecule junctions is, for the first time experimentally addressed by investigating the conductance change when a “parent” meta-phenylene ethylene-type oligomer (m-OPE) is modified to yield a “daughter” by inserting one nitrogen atom into the m-OPE core. We find that if the substituted nitrogen is in a meta position relative to both acetylene linkers, the daughter conductance remains as low as the parent. However, if the substituted nitrogen is in an ortho position relative to one acetylene linker and a para position relative to the other, destructive QI is alleviated and the daughter conductance is high. This behavior contrasts with that of a para-connected parent, whose conductance is unaffected by heteroatom substitution. These experimental findings are rationalized by transport calculations and also agree with recent “magic ratio rules”, which capture the role of connectivity in determining the electrical conductance of such parents and daughters.

Full text of DOI:10.1002/anie.201609051

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201609051
German Edition: DOI: 10.1002/ange.201609051
Organic Electronics
Gating of Quantum Interference in Molecular Junctions by
Heteroatom Substitution
Xunshan Liu+, Sara Sangtarash+,* David Reber+, Dan Zhang+, Hatef Sadeghi, Jia Shi,
Zong-Yuan Xiao, Wenjing Hong,* Colin J. Lambert,* and Shi-Xia Liu*
Abstract: To guide the choice of future synthetic targets for
single-molecule electronics, qualitative design rules are needed,
which describe the effect of modifying chemical structure. Here
the effect of heteroatom substitution on destructive quantum
interference (QI) in single-molecule junctions is, for the first
time experimentally addressed by investigating the conduc-
tance change when a “parent” meta-phenylene ethylene-type
oligomer (m-OPE) is modified to yield a “daughter” by
inserting one nitrogen atom into the m-OPE core. We find that
if the substituted nitrogen is in a meta position relative to both
acetylene linkers, the daughter conductance remains as low as
the parent. However, if the substituted nitrogen is in an ortho
position relative to one acetylene linker and a para position
relative to the other, destructive QI is alleviated and the
daughter conductance is high. This behavior contrasts with that
of a para-connected parent, whose conductance is unaffected
by heteroatom substitution. These experimental findings are
rationalized by transport calculations and also agree with
recent “magic ratio rules”, which capture the role of con-
nectivity in determining the electrical conductance of such
parents and daughters.
between QI and chemical structure remains ambiguous. For
example, destructive QI has been observed in linearly
conjugated molecules, while constructive QI occurs in cross-
conjugated molecules of the close-loop type.[5] Amongst all
molecules investigated to date, nonalternant and alternant
hydrocarbon systems have been intensively investigated.[6]
Although heterocyclic aromatics are particularly interesting
due to the presence of spatially separated conductance
pathways, which offer opportunities for varying QI, only
a few papers about the effect of heteroatoms on single-
molecule junction conductances have appeared in the liter-
ature.[7] Recently, we developed a new magic ratio rule
(MRR) showing that the conductance ratio of two molecules
through the same aromatic core with different connectivties is
equal to the square of the ratio of their magic integers.[8] The
accuracy of the MRR for polycyclic aromatic hydrocarbons
(PAHs) is demonstrated in Ref. [8b], in which MRR pre-
dictions for the ratios of conductances for a range of PAHs are
compared with experiments. A remarkable feature of the
MRR is that the associated most-probable conductance ratios
are independent of the binding geometry, elongation etc.,
even though the conductances themselves do depend on such
features. As discussed in Ref. [7c] and in the Supporting
Information (SI), this insensitivity can be expected, provided
the statistical distribution of contact geometries is independ-
ent of connectivity. A generalization of this theory[7c] that
predicts three simple rules governing the effect of heteroatom
substitution in alternate hydrocarbon parents (see also SI),
has not yet been verified experimentally. As a consequence,
we set ourselves the task of investigating how destructive QI
in the “parent” meta-phenylene ethylene-type molecule (m-
OPE) changes when one nitrogen atom is inserted into the
central benzene ring at different positions, to yield the
“daughter” molecules M1, M2 and M3 in Scheme 1. To
assess the heteroatom effect on QI and to gain insight into the
correlation between connectivity and electrical conductance,
the parent para-OPE (p-OPE) and its daughter molecule P
(Scheme 1) were also studied.
R
ecent experiments demonstrating room temperature quan-
tum interference (QI) in single-molecule junctions have
stimulated intense interest in the development of molecular
switches,[1] transistors[2] and thermoelectric devices[3] with
improved performance. Much effort has been devoted to
controlling QI effects by modifying electronic structures or
molecular topologies.[2b,4] However, a direct correlation
[*] X. Liu,[+] D. Reber,[+] Dr. S.-X. Liu
Department of Chemistry and Biochemistry, University of Bern
Freiestrasse 3, 3012 Bern (Switzerland)
E-mail: liu@dcb.unibe.ch
S. Sangtarash,[+] Dr. H. Sadeghi, Prof. C. J. Lambert
Quantum Technology Centre, Physics Department
Lancaster University
Lancaster LA1 4YB (UK)
E-mail: s.sangtarash@lancaster.ac.uk
c.lambert@lancaster.ac.uk
The target compounds m-OPE, p-OPE, P and M1–M3
were synthesized via Sonogashira cross-coupling reactions of
S-4-iodophenyl ethanethioate with the corresponding dieth-
ynylbenzene/pyridine. All of them were purified using column
chromatography and characterized by NMR and high-reso-
lution mass spectrometry (see SI). Charge transport measure-
ments of single-molecule junctions with molecules shown in
Scheme 1 were performed using a mechanically controllable
break junction (MCBJ).[9] Figure 1A displays typical con-
ductance (G) versus distance (Dz) stretching traces, as plotted
in a semi-logarithmic scale. The single-molecule conductance
D. Zhang,[+] Dr. J. Shi, Dr. Z.-Y. Xiao, Prof. W. Hong
Collaborative Innovation Center of Chemistry for Energy Materials,
Department of Chemical and Biochemical Engineering, College of
Chemistry and Chemical Engineering, Xiamen University
Xiamen 361005 (China)
E-mail: whong@xmu.edu.cn
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201609051.
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substituted nitrogen is ortho to both contacts or para to one
connection and ortho to the other. On the other hand, there is
a negligible change in electrical conductance when the
nitrogen is meta to both contacts, in agreement with
Ref. [7c]. As depicted in Figure 1C, the presence of the
nitrogen atom shows no distinguishable effect on the con-
structive QI in the junctions through the para-coupled
molecules p-OPE and P, which is in good agreement with
Ref. [7c] and the results obtained by Gonzalez et al.[7b]
The two-dimensional (2D) conductance versus distance
histogram of molecule P demonstrates a clear molecular
density cloud at around 10À4.0 G0 (Figure 1D), which is quite
similar to that of p-OPE.[6c] Interestingly, there is no
significant conductance feature of the molecular junctions
with one of Au electrodes connected directly to the central
À
pyridine ring via the Au N bonds, as observed in the
analogous system containing a pyrimidine ring.[7b] This
observation can be attributed to the significant binding
Scheme 1. Chemical structures of the investigated molecules.
À
force difference between the Au S covalent bond
[11]
(1.2 nN)[10] and the Au N bond (0.8 nN) that is close to
À
the Au S (methylsulfide) bond (0.7 nN),[12] leading to a pref-
À
À
À
erence for the covalent Au S over the Au N binding motif in
the present case. Furthermore, the conductance plateau is ca.
0.8 nm (after adding the snap back distance of 0.5 nm it will be
1.3 nm), suggesting that the most pronouncing feature stems
À
from the molecules trapped only via Au S bonds. However,
the 2D histograms of the meta-coupled molecules (Figure S9,
SI) show that the conductance cloud becomes less clear as
previously reported in the literature.[4d,13] Apparently, the
bent molecular geometry strongly affects the evolution of
molecular junctions, resulting in a slanted conductance
region. Thus, it is impossible to plot the plateau-length
distribution and further calculate the molecular length in the
meta-coupled systems.
To further verify our hypothesis of an end-to-end config-
À
uration via the Au S binding motif and to completely rule out
the possibility of the conductance plateaus being due to the
molecules bonded to one of the electrodes through the N
atom at the central ring, we have performed control experi-
ments of the reference compounds R1–R3 containing only
one SAc group (Figure 2).
Figure 1. A) Typical individual conductance–distance traces of blank
experiment in TMB/THF (gray) and molecules m-OPE (black), p-OPE
(brown), P (orange), M1 (green), M2 (blue) and M3 (red). B,C) All-
data-point 1D conductance histograms constructed from 1000 MCBJ
traces of each molecule. D) All-data-point 2D conductance versus
relative distance (Dz) of the molecule P. The stretching distance
distribution is determined from the range of 0.7 G0 to 10À5 G0.
All of them provide clear evidence for creation of
molecular junctions with distinctly higher conductance
values compared to those of M1–M3. They are arranged in
the following order of their decreasing conductance, the para-
coupled molecule R2 (10À3.2Æ0.1 G0) > the ortho-coupled
molecule R3 (10À3.6Æ0.1 G0) > the meta-coupled molecule R1
(10À3.9Æ0.1 G0), which is a clear signature of the QI effect. As
illustrated in Figure 2B–D, a clear intensity cloud is observed
for each case and the plateau length for R2 is longer than for
R1 and R3. The most probable stretched distance Dz* is
determined from the plateau distribution histograms to be
0.8 nm (R2), 0.7 nm (R1) and 0.6 nm (R3), respectively. As
a results, the most probable absolute distance z* (z* = Dz* +
0.5 nm)[9c] between two gold tips is 1.3 nm for R2, 1.2 nm for
R1 and 1.1 nm for R3, which agrees well with theoretically
determined junction lengths (1.16 nm for R2 1.09 nm for R1
and 0.96 nm for R3, Table S3). It can therefore be deduced
that the distinct variations in conductance values of M1–M3
plateaus of 6 molecules vary from 10À3.5 to 10À6 G0 besides the
plateau presenting the gold–gold atomic contact located at
G0, while the blank experiment using THF/TMB solvent
without molecules shows no single-molecule plateau.
Figure 1B,C show the 1D conductance histograms of
molecules, constructed from 1000 experimental conductance–
distance traces for each compound without data selection.
After the rupture of gold–gold atomic contacts at G0, a single
conductance peak, corresponding to the most probable
conductance for each molecule was detected (also summar-
ized in Table 1). Briefly, the conductance values follow the
trend M2 > M3 > M1 = m-OPE, suggesting that heteroatom
substitution leads to an increase in conductance only when the
2
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Figure 3. A) Core transmission coefficients tij(E) of the molecules m-
OPE, p-OPE, P, M1–M3 against E/d, where d is half of the HOMO–
LUMO gap of the parental core; i.e. d=1, using DFT transport
approach implemented in Gollum.[15] B) The calculated transmission
coefficients Tij(E) of the molecules m-OPE, p-OPE, P, M1–M3, con-
nected to gold electrodes using the mean-field Hamiltonian from
Siesta.[14] Dashed lines correspond to “parents” and solid lines to
“daughters”.
Figure 2. A) 1D conductance histograms of R1 (black), R2 (blue) and
R3 (red), constructed from 1000 MCBJ traces of each molecule. B–
D) 2D conductance histograms of R1–R3, respectively, and stretching
distance Dz distributions (inset). The stretching distance distribution
is determined from the range of 0.7 G0 to 10À5 G0.
OPE, p-OPE, P and M1–M3, which are divided by con-
ductance value of molecule M3. Comparison with the
corresponding core-transmission ratios of Figure 3A also
reveals that the best agreement with experimental data is
obtained with a daughter Fermi energy of EFd = 0.211, which
differs from the mid-gap of the daughter (EHLd = À0.0795)
and parent (EHLp = 0), indicating the variation of the align-
ment of intrinsic molecular orbital (MO) levels due to
heteroatom substitution. Although the lone-pair electrons
of the N atom are held in an sp2 hybrid orbital perpendicular
to the orbitals in the p-system, its presence has an unambig-
uous effect on the energy levels and nodal properties of the
MOs (Figure S6, SI). Both experiment and DFT-based theory
reveal that the presence of the nitrogen atom shows no
discernible effect on the constructive quantum interference in
the junctions through the para-coupled molecules as a result
of the fact that the core transmissions are only slowly varying
functions of energy. In the absence of a Fermi energy shift, the
ratio of conductances for M1 and M3 should vanish. Taking
a shift of EFd = 0.211 into consideration, M-theory yields
a conductance ratio of 0.37 that compares well with the
experimental ratio of 0.32. The same holds true for the
conductance ratio of M2/M3. All of these results are captured
by the minimal theory,[7c] which identifies the conductance
and R1–R3 are attributed to the different electronic transport
pathways whereby the former molecules are trapped between
À
two Au leads via the Au S bonds while the latter ones
À
are connected on the one side via the Au S bonds, on the
À
other side via the Au N bonds. All these results provide
direct evidence for an unambiguous preference for the
À
À
covalent Au S over the Au N binding motif in the junctions
through M1–M3, which verifies the proposed junction con-
figurations.
The above heteroatom effect can be understood by
applying the theory of Ref. [7c], which identifies the ratio of
two molecular conductances with the ratio of two “energy-
dependent core transmission functions”, evaluated at the
Fermi energy (see SI). As discussed in section 4 of the SI,
these energy-dependent core transmission functions are
proportional to the Greenꢀs function of the core of the
molecule and are independent of the nature of the coupling to
the electrodes.
For the three-ring molecules of Scheme 1, the energy
dependences of the core electron transmission coefficients are
shown in Figure 3A. For m-OPE, the parental M-function
and the corresponding core transmission function
vanish at E = 0, due to destructive QI in the middle
Table 1: A summary of an experimental and theoretical study on the single-molecule
junction conductances of m-OPE, p-OPE, P and M1–M3.
of the HOMO–LUMO gap. However, by adding
a heteroatom to the central benzene ring, core
transmission coefficients of the molecules M2 and
M3 become non-zero while the core transmission
coefficient of the molecule M1 remains zero at E = 0.
On the other hand, the transmission functions of
para-coupled systems p-OPE and P are non-zero at
E = 0 and overlap with each other, indicating that the
heteroatom effect is negligible in the constructive QI
systems. Illustrated in Figure 3B are the full electron
transmission coefficients obtained from DFT mean
field Hamiltonian.[14,15]
Molecule Experimental Conductance DFT
Core
Core
conductance ratio[a]
conductance transmission transmission
ratio[b]
ratio[e]
m-OPE
p-OPE
P
M1
M2
10À5.5Æ0.05
10À4.0Æ0.1
10À4.0Æ0.1
10À5.55Æ0.05
10À4.35Æ0.05
10À5.1Æ0.05
0.32
12.59
12.59
0.32
5.18
1
0.02
13.40
14.00
0.04
5.12
1
0[c]
0
4[c]
30.77
36.6
0.37
5.15
1
4.75[d]
0.048[d]
0.67[d]
0.13[d]
M3
[a] Experimental conductance divided by experimental conductance of M3. [b] DFT
conductance divided by DFT conductance of M3 at EF =0.9. [c] Core transmission of
parents at EFp =0. [d] Core transmission of daughters at EFd =0.211. [e] Core
transmission divided by core transmission of M3.
Table 1 shows the experimental values for the
most probable conductances of the molecules m-
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In conclusion, it has been demonstrated that the destruc-
tive QI can be alleviated by the heteroatom substitution
whereas constructive QI is almost unaffected. The increase in
conductance for meta-coupled OPEs originates from two
distinct effects. First, the change in the Hamiltonian modifies
the interference pattern within the core, the energy depend-
ence of the associated M-functions and core transmission
coefficients. Secondly, heteroatom substitution shifts the
Fermi energy relative to the parent, which is also analyzed
in the MO viewpoint. Our results provide a qualitative guide
for tuning QI effect in single-molecule junctions that is of
prime importance in designing molecular devices with
desirable functions.
Acknowledgements
We thank Martin Bryce for stimulating discussions regarding
organic chemistry intuition. This work was supported by the
European Commission (EC) FP7 ITN “MOLESCO” (project
no. 606728), UK EPSRC (grant nos. EP/M014452/1 and EP/
N017188/1), the Swiss National Science Foundation (grant
No. 200020-144471) and Thousand Youth Talents Plan of
China.
Keywords: density functional calculations · heteroatom effects ·
molecular electronics · quantum interference ·
single-molecule transport
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Manuscript received: September 15, 2016
Revised: October 25, 2016
Final Article published: && &&, &&&&
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Communications
Organic Electronics
A significant effect of heteroatom substi-
tution on destructive quantum interfer-
ence (QI) in meta-coupled molecules was
observed for the first time. It is demon-
strated both experimentally and theoret-
ically that destructive QI can be alleviated
by the heteroatom effect, whereas con-
structive QI remains almost unaffected.
X. Liu, S. Sangtarash,* D. Reber,
D. Zhang, H. Sadeghi, J. Shi, Z.-Y. Xiao,
W. Hong,* C. J. Lambert,*
S.-X. Liu*
&&&& — &&&&
Gating of Quantum Interference in
Molecular Junctions by Heteroatom
Substitution
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
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