Charge Transport and Quantum Interference Effects in Oxazole-Terminated Conjugated Oligomers

Article abstract of DOI:10.1021/jacs.9b08427

Charge transport in single molecule junctions critically depends on the chemical identity of anchor groups used to connect molecular wires to electrodes. In this work, we report the charge transport properties of conjugated oligomers with oxazole anchors,

Full text of DOI:10.1021/jacs.9b08427

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Article
Charge Transport and Quantum Interference Effects
in Oxazole-Terminated Conjugated Oligomers
Songsong Li, Hao Yu, Kenneth Schwieter, Kejia Chen, Bo
Li, Yun Liu, Jeffrey S. Moore, and Charles M. Schroeder
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b08427 • Publication Date (Web): 13 Sep 2019
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Charge Transport and Quantum Interference Effects
in Oxazole-Terminated Conjugated Oligomers
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Songsong Li1,4†, Hao Yu2†, Kenneth Schwieter3, Kejia Chen2, Bo Li2, Yun Liu4,
Jeffrey S. Moore1,3,4, and Charles M. Schroeder1,2,3,4
*
1Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign,
Urbana, Illinois, 61801, United States
2Department of Chemical and Biomolecular Engineering, University of Illinois at Urbana-
Champaign, Urbana, Illinois, 61801, United States
3Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801,
United States
4Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana-
Champaign, Urbana, Illinois, 61801, United States
†Contributed equally to this work.
*To whom correspondence should be addressed. Email: cms@illinois.edu
Keywords: single molecule conductance, oxazole, molecular electronics, quantum interference,
scanning tunneling microscope break-junction (STM-BJ)
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Abstract
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Charge transport in single molecule junctions critically depends on the chemical identity of anchor
groups used to connect molecular wires to electrodes. In this work, we report the charge transport
properties of conjugated oligomers with oxazole anchors, focusing on the role of the heteroatom
substitution position in terminal oxazole groups. Our results show that oxazole serves as an
efficient anchor group to form stable gold-molecule-gold junctions. We further observe quantum
interference (QI) effects in oxazole-terminated phenylene molecular junctions, including
destructive QI in meta-substituted phenyl rings and constructive QI in para-substituted phenyl
rings containing terminal oxazole groups with the same chemical constitution on both termini (i.e.
O5O5 (5-oxazolyl) or O4O4 (4-oxazolyl) linkages on both termini). Surprisingly, meta-substituted
phenyl rings with non-equivalent constitutions (i.e. O4O5 oxazole terminal linkages) show
unexpectedly higher conductance compared to para-substituted analogs. These results suggest that
charge transport in oxazole-terminated molecules is determined by the heteroatom substitution
position of the oxazole anchor in addition to the aryl substitution pattern of the pi-conjugated core.
Our results further show that conjugated molecules with homogeneous oxazole linkages obey a
quantum circuit rule such that GO4-p-O4/GO4-m-O4 = GO5-p-O5/GO5-m-O5, where G is molecular
conductance. Overall, our work provides key insight into the development of new chemistries for
molecular circuitry in the rapidly advancing field of single molecule electronics.
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INTRODUCTION
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Recent advances in single molecule electronics have uncovered new chemistries and molecular
systems that offer new fundamental insight into molecular electronic structure and charge
transport.1-6 Single molecule junctions generally contain a central bridge terminated by anchor
groups that are used to connect molecular wires to electrodes.7 Charge transport in molecular
junctions significantly depends on the chemical identity of the anchor groups and on the nature of
the chemical interactions at the electrode-molecule interface.8 From this view, anchor groups are
conveniently divided into two categories based binding interactions with metal electrodes: dative
anchors bind to metal electrodes via coordination interactions, whereas covalent anchors provide
direct metal-molecule contacts.7 Prior work has explored a wide array of anchor groups for single
molecule junctions, including dative anchors with -donor groups such as fullerene,9 dative
anchors with lone pair donors such as primary amine (-NH2),10 pyridine11,12, cyano (-CN),13
isocyano (-NC),14 isothiocyanate (-NCS),15 selenol (-Se),16 methyl thio (-SCH3),17 phosphine,18
nitro (-NO2),19 and carboxylic acid (-COOH),20 and covalent anchors such as thiol (-SH),20
trimethyltin,21 trimethylsilyl-terminated alkynes,22 and diazonium.23
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Chemical anchors are a critical component of single molecule junctions and often play a key
role in determining molecular conductance in these systems. In particular, the chemical identity of
dative linker groups often determines whether charge transport occurs by a highest occupied
molecular orbital (HOMO)-dominated or lowest unoccupied molecular orbital (LUMO)-
dominated pathway.7 In addition, chemical anchors have been used to tune symmetric or
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asymmetric coupling interactions between molecular wires and electrode surfaces. In one case,
molecular design was used to install dative and covalent anchor groups at two termini of a junction,
thereby generating a molecular rectifier.24
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Quantum interference (QI) effects play an essential role in the charge transport properties of
molecular electronics.25,26 Single molecule conductance measurements have shown that meta-
substituted phenyl groups generally exhibit lower conductance than para-substituted phenyls.27 In
a meta-substituted phenyl ring, the de Broglie electron waves emerge out-of-phase after traversing
different molecular conduction pathways, thereby resulting in destructive QI. On the other hand,
para-substituted phenyl rings give rise to constructive QI and consequently higher levels of
conductance compared to the meta-substituted analog.25 QI effects also arise in anchor groups with
conjugated rings, though only a few chemical anchors for molecule-metal junctions feature a fused
ring structure. Anchor groups containing a conjugated ring structure have several potential
molecular conduction pathways, complicating the molecular design of quantum circuits due to
quantum effects.28 Despite recent progress, the effect of heteroatom substitution position on QI
effects is not fully understood in the context of anchor groups with conjugated ring structures.
In this work, we show that oxazole serves an efficient anchor group to form stable gold-
molecule-gold junctions, resulting in conductive molecular wires. We systematically explore the
role of heteroatom substitution position on QI in oxazole-terminated molecular junctions using the
scanning tunneling microscope break-junction (STM-BJ) technique and density functional theory
(DFT) simulations. Our results show that oligophenyls terminated with oxazole exhibit an
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Figure 1. Charge transport in oxazole-terminated molecular wires. (a) Schematic of metal-
molecule-metal junction and structures of compounds OX1-OX4. (b) Characteristic single
molecule conductance traces for OX1-OX4 molecular junctions. (c) Conductance histograms
for OX1-OX4, with each constructed from >10,000 traces.
exponential conductance decay constant (β = 0.40 Å-1), comparable to amine-terminated analogs
(β = 0.42 Å-1). Combined results from single molecule experiments and molecular modeling show
that the nitrogen atom in the oxazole ring is the point-of-contact to the metal surface through the
lone electron pair, similar to pyridine-based anchor groups.11 Our results further reveal unexpected
QI phenomena in oxazole-terminated molecular junctions. For oxazole-terminated molecules with
the same chemical constitution on both termini, including O5O5 (5-oxazolyl) or O4O4 (4-oxazolyl)
linkages, destructive QI is observed in meta-substituted phenyl molecular junctions. Surprisingly,
our results show that meta-substituted phenyl rings with non-equivalent constitutions (O4O5
oxazole linkages) exhibit constructive QI. Taken together, these results suggest that the heteroatom
position in oxazole rings plays a key role in conductance, in addition to the arene substitution
pattern in conjugated phenyl bridges. This work expands the library of chemical anchor groups
and explores the effect of anchors with conjugated ring structures on QI, thereby opening new
opportunities for molecular electronics.29
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Figure 2: Two-dimensional (2D) conductance histograms for OX1-OX4, determined over a
large ensemble of >10,000 individual molecules.
RESULTS AND DISCUSSION
A series of oxazole-containing molecules was synthesized by van Leusen oxazole synthesis30
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and characterized by H and 13C NMR spectroscopies and mass spectrometry (Supporting
Information, Figures S1-S21). We began by synthesizing oxazole-terminated oligomers with
homogenous linkages at both termini with respect to heteroatom position on the oxazole ring. In
particular, we synthesized 5,5’-bioxazole (OX1), (E)-1,2-bis(oxazol-5-yl)ethene (OX2), 1,4-
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bis(oxazol-5-yl)benzene (OX3), and 4,4’-bis(oxazol-5-yl)-1,1’-biphenyl (OX4), all of which
contain homogeneous linkages to oxazole anchors at the 5-position on the oxazole ring (Figure
1a). We further synthesized an expanded library of oxazole-terminated phenyl compounds with
varying arene substitution patterns (meta or para-linked) and defined linkages to the conjugated
ring oxazole anchors (Table 1). Additional details on the synthesis and chemical characterization
for all compounds can be found in the Supporting Information (Figures S1-S21).
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Single molecule conductance measurements were performed using a custom-built scanning
tunneling microscope break-junction (STM-BJ) technique under constant applied bias (250 mV)
in a liquid solvent (1,2,4-trichlorobenzene) (Figure 1a), as previously described.31-33 In brief, the
STM-BJ experiment is performed by measuring molecular conductance at a constant applied bias
while pulling a gold tip away from a gold electrode surface, eventually leading to a break in the
molecular junction. Figure 1b shows characteristic single molecule conductance traces of OX1-
OX4, together with a break junction measurement in the absence of organic molecules (denoted
as Au). Plateaus in molecular conductance G at values less than the quantum unit of conductance
(G0 = 2e2/h = 77.5 μS) correspond to charge transport through organic molecules bridging the
junction between the gold electrodes. Single molecule pulling experiments are generally repeated
for thousands of trials to enable robust statistical analysis. In this way, one-dimensional (1D)
conductance histograms are generated over a large ensemble of approximately 104 molecules for
OX1-OX4 (Figure 1c). Interestingly, we observe two peaks in the conductance histograms for
OX1 but only a single peak for OX2, OX3, and OX4. We conjecture that the two distinct
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Figure 3. Charge transport behavior of oxazole-terminated oligomers determined by single
molecule experiments and density functional theory (DFT) simulations. (a) Conductance peak
values for amine-terminated (green, n = 1, 2, 3), pyridine-terminated (blue, n = 0, 1), and
oxazole-terminated oligophenyls (red, n = 0, 1, 2) plotted on a semi-log scale as a function of
distance between N-N atoms on terminal anchors. Lines show fits to experimental data
following a quantum tunneling model G=G0e-βL. (b) Transmission spectra of OX1-OX4
determined by DFT simulations. (c) Calculated conductance values for OX1-OX4 determined
from DFT simulations.
conductance states in OX1 are due to two binding geometries.12 On the other hand, OX2, OX3,
and OX4, adopt a single favored binding geometry, thereby resulting in distinct conductance peaks
in the 1D histograms.
Two-dimensional (2D) conductance histograms of OX1-OX4 are shown in Figure 2. 2D
histograms show that average displacements during molecular pulling experiments tend to increase
with increasing molecular length, such that OX4 junctions can be stretched to approximately 0.8
nm. At an applied bias of 0.25 V, we found that the signal of OX4 merged with the amplifier
background noise. Therefore, we repeated the experiments and collected 2D conductance
histograms of OX4 at high bias (1 V) and observed a clear separation between signal and
background (Figure S22). Average molecular conductance values for OX1, OX3, and OX4 were
determined from the peak values in the corresponding 1D histogram data and plotted on a semi-
log scale versus the distance between nitrogen atoms in terminal anchor groups for each respective
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molecule (Figure 3a). In all cases, N-N distance was determined by density functional theory
(DFT) and corresponds to the distance between nitrogen atoms in terminal amines for amine-
terminated oligophenyls or N-N heteroatom distance between terminal oxazole rings. Our results
show that the average molecular conductance follows an exponential decay as a function of
molecular length, which suggests that charge transport follows a quantum tunneling model such
that ln(G/G0) = -βL, where β is the molecular decay constant β and L is the molecular length
between terminal nitrogen atoms. Using this approach, we determine a molecular decay constant
β = 0.40 ± 0.01 Å-1 for oxazole-terminated oligophenyls, which is consistent with amine-
terminated (β = 0.42 ± 0.01 Å-1) (Figure S23) and pyridine-terminated oligophenyls (β = 0.5 Å-
1).11,31 These results suggest that oxazole anchors form robust molecular contacts with metallic
gold electrodes and enable stable measurements of conductance. To identify the specific anchoring
site linking oxazole rings to gold electrodes, we synthesized a control molecule 1,4-di(furan-2-
yl)benzene (FU1) for single molecule conductance experiments (Figure S24). In contrast to
oxazole-terminated OX3, no conductance signal was observed for furan-terminated compound
FU1, which confirms that the nitrogen heteroatom serves as the anchoring site in oxazole rings.
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To further understand the charge transport properties of oxazole-terminated molecules, we
performed molecular modeling using nonequilibrium Green’s function-density functional theory
(NEGF-DFT) via the Atomistix Toolkit package.34 Molecular geometries for OX1-OX4 are
optimized using DFT calculations performed on Spartan’16 Parallel Suite using the B3LYP
functional with a 6-31G (d,p) basis set. Following determination of geometry-optimized structures,
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Table 1. Molecular conductance properties of oxazole-terminated phenyls, including
average conductance (expressed as log(G/Go)) and Gpara/Gmeta from STM-BJ experiments
and DFT simulations. Molecule names refer to amine anchors (A), oxazole anchors linked
at the 5-oxazolyl (O5) or 4-oxazolyl (O4) position, and arene substitution pattern (p,m).
transmission functions are determined for relaxed molecules for OX1-OX4 using NEGF-DFT
(Figure 3b). For oxazole-terminated molecules OX1-OX4, the peaks in the transmission function
appear between +0.1 to +0.5 eV, which corresponds to charge transport through the LUMO. We
found that the slope of the transmission spectrum is positive at the Fermi level which suggests that
the conduction orbital is in an unoccupied state, the thermopower becomes negative, and electrons
are transported through the LUMO in OX1-OX4.35 We also found that the magnitude of the
transmission values for OX1-OX4 close to Fermi energy are consistent with conductance trends
in the experimental results (Figure 3c).
We next characterized the charge transport properties of a series of oxazole-terminated
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Figure 4. Two-dimensional (2D) molecular conductance histograms for a series of oxazole-
terminated phenyl compounds at 0.25 V bias, each constructed from over 10,000 traces.
phenylene compounds with varying arene substitution patterns (para, meta) and different linkages
to terminal oxazole groups (5-oxazolyl or 4-oxazolyl, denoted as O5 and O4, respectively) (Figure
4, Figures S25-S28 and Table 1). To begin, we synthesized and studied the single molecule
conductance of 1,4-bis(4-oxazolyl)benzene (O4-p-O4), 1,3-bis(4-oxazolyl)benzene (O4-m-O4),
1,4-bis(5-oxazolyl)benzene (O5-p-O5, also denoted as OX3), and 1,3-bis(5-oxazolyl)benzene (O5-
m-O5). Our results show that meta-substituted oxazole-terminated phenylenes with homogeneous
O5O5 and O4O4 linkages show lower conductance than para-substituted analogs with homogeneous
O5O5 and O4O4 linkages, consistent with a destructive quantum interference (QI) effect
analogously observed in amine-linked,36 methyl sulfide-linked,28 and thiophene-linked27
molecular junctions. Experimental results and NEGF-DFT simulation data in Table 1 clearly show
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that the QI ratio of meta/para-linkage satisfies a quantum circuit rule such that GO4-p-O4/GO4-m-O4
=
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GO5-p-O5/GO5-m-O5 ≈ 6. 37,38 As a comparison, pyridine-terminated molecules also follow a quantum
circuit rule such that Gppp/Gpmp = Gmpm/Gmmm, where the contribution to the conductance from the
central phenylene ring is independent of the anchor groups with para or meta-linkages.27 From this
view, oxazole anchors offer an interesting opportunity to probe QI effects due to the asymmetric
nature of the five-membered ring associated with terminal oxazole groups. For oxazole-terminated
junctions with equivalent constitutions on both termini, we found that the conductance of
molecules with O4O4 linkages is higher compared to their counterparts with O5O5 linkages based
on most-probable conductance results and NEGF-DFT simulations, which further supports the
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quantum circuit rule GO4-p-O4/GO5-p-O5 = GO4-m-O4/GO5-m-O5
.
To further examine QI effects through oxazole-terminated molecular junctions with non-
equivalent constitutions on both termini, we synthesized and studied the molecular conductance
of 5-(4-(oxazol-4-yl)phenyl)oxazole (O4-p-O5) and 5-(3-(oxazol-4-yl)phenyl)oxazole (O4-m-O5).
From most-probable conductance results and NEGF-DFT simulations, our results show that the
conductance of O4-p-O5 is lower than O4-m-O5, such that GO4-p-O5/GO4-m-O5 < 0.5. Surprisingly,
these results suggest that the meta-substituted phenylene exhibits a higher conductance than the
para-substitute phenylene in oxazole-terminated molecules with heterogeneous O4O5 linkages.
Recently, Li et al.39 and Huang et al.40 used electrochemical gating to tune QI, which revealed that
meta-substituted phenyl groups show higher conductance than para-substitute phenyls. On the
other hand, our results show that a meta-substituted phenylene group can exhibit a higher
molecular conductance relative to the para-substituted analog without external electrochemical
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gating41. In prior work, Borges et al. reported that meta-coupled bipyridine molecules show higher
conductance through a σ-bonded system compared to the para-coupled analog.42 For O4-m-O5, the
electronic coupling occurs across seven bonds, whereas for O4-p-O5 the coupling is through eight
bonds. Therefore, we posit that O4-m-O5 has a higher σ contribution to charge transport which
gives rise to a larger total transmission for O4-m-O5 compared to O4-p-O5. Similar observations
were reported by Gorczak and coworkers,43 who showed that meta-substitute biphenyl bridges
exhibit a higher hole transfer rate compared to para-substitute analogs. Interestingly, this
phenomenon was attributed to the asymmetric donor and acceptor states which leads to asymmetry
in the molecular orbitals. In our work, we studied the molecular conductance of phenylenes
containing heterogenous anchor groups based on oxazole-terminated O4O5 linkages. For such
heterogeneous linked molecules, we conjecture that one anchor group contributes constructive QI
and the second anchor results in destructive QI. When coupled with a meta-substituted phenylene
bridge with destructive QI, heterogeneous oxazole O4O5 linkages can alter the overall QI in the
molecular junction, thereby giving rise to unexpectedly high conductance in the meta-linked
molecular bridge.
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CONCLUSION
In this work, we study the charge transport properties of a series of oxazole-terminated phenylene
compounds with varying arene substitution patterns (para, meta) and different linkages to terminal
oxazole rings using a combination of single molecule experiments and molecular models. Our
results show that oxazole serves as a stable chemical anchor group for facilitating molecular
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connections to gold metal electrode surfaces. Oxazole-terminated (O5 linkage) oligophenyls
exhibit a decay constant β = 0.40 ± 0.01 Å-1, which is comparable with amine-terminated and
pyridine-terminated equivalents. In addition, we systematically studied QI phenomena through the
central phenyl ring and terminal oxazole ring. In particular, for oxazole-terminated junctions with
equivalent constitutions on both termini, which include O4O4 and O5O5 oxazole-terminated
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molecules, the QI of the central phenylene follows a quantum circuit rule that GO4-p-O4/GO4-m-O4
=
GO5-p-O5/GO5-m-O5. For oxazole-terminated junctions with non-equivalent constitutions on both
termini, including O4O5 oxazole-terminated molecules, meta-substituted phenylene exhibits a
higher conductance compared to para-substitute phenylene. These results reveal the role of the
anchor group in controlling QI which facilitates an increased understanding of molecular
conductance in junctions with complex heterogeneous structures. Moving forward, this work could
aid in the design and development of new chemistries for molecular electronics.
Associated Content
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI:
XXX. Description of chemical synthesis, chemical and physical characterization of oxazole-
terminated molecules, experimental details on STM-BJ, simulation methods, supporting text,
supporting figures.
Corresponding Author
cms@illinois.edu
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ORCID
Songsong Li: 0000-0001-9784-2877
Hao Yu: 0000-0002-1594-769X
Yun Liu: 0000-0001-7077-363X
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Charles M. Schroeder: 0000-0001-6023-2274
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Notes
The authors declare no competing financial interests.
Acknowledgements
We thank Prof. Latha Venkataraman and Dr. Yaping Zang for generously providing advice in
building the STM-BJ instrument. This work was supported by the U.S. Department of Defense by
a MURI (Multi-University Research Initiative) through the Army Research Office (ARO) through
Award W911NF-16-1-0372 to C.M.S. and J.S.M.
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