Single molecule conductance of aromatic, nonaromatic, and partially antiaromatic systems

Article abstract of DOI:10.1016/j.tetlet.2015.06.076

We describe the syntheses of thiophene, cyclopentadienone and dimethylketalcyclopentadienone derivatives and their single molecule electrical conductivities. The ketone had the lowest conductivity of the three, although it has some antiaromaticity. The carbonyl group is electron attracting and its ionization potential outweighs any conductivity increase that the antiaromaticity effect might bring.

Full text of DOI:10.1016/j.tetlet.2015.06.076

Tetrahedron Letters 56 (2015) 4833–4835
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Single molecule conductance of aromatic, nonaromatic, and partially
antiaromatic systems
⇑
Adaickapillai Mahendran, Purushothaman Gopinath, Ronald Breslow
Department of Chemistry, Columbia University, 3000 Broadway, New York, NY 10027, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
We describe the syntheses of thiophene, cyclopentadienone and dimethylketalcyclopentadienone deriva-
tives and their single molecule electrical conductivities. The ketone had the lowest conductivity of the
three, although it has some antiaromaticity. The carbonyl group is electron attracting and its ionization
potential outweighs any conductivity increase that the antiaromaticity effect might bring.
Ó 2015 Published by Elsevier Ltd.
Received 28 May 2015
Revised 22 June 2015
Accepted 24 June 2015
Available online 29 June 2015
Keywords:
Single molecule conductance
Aromaticity
Antiaromaticity
Cyclopentadienone
STM break-junction
Introduction
1,1-dimethylcyclopentadiene ring with attached p-aminopheny-
lacetylene (Fig. 1) leads that then contacted gold electrodes in a
Miniaturization of photovoltaics and organic electronics is
advancing towards the molecular level.1,2 The goal is to replace
the interconnecting wires between transistors in electronics with
a conducting material as small as a single molecule or more likely
short oligomers, molecular wires.3–6 Conjugated organic oligomers
such as polythiophenes and polyaniline are a few examples of
highly conducting molecular systems that have been used in
nanoscaled devices.7,8
We have described evidence that the aromaticity of such com-
mon components of molecular wires as thiophene rings diminishes
their electrical conductivity.9 The reason is that the aromatic sys-
tems comprise six pi electrons cyclical delocalized in the rings,
while when the ring has connections to an anode and a cathode
the preferred electron distribution for maximum electrical conduc-
tivity has some pi electron density external to the ring. In a sense,
in order to maximize the conductivity of the system an aromatic
benzene ring in 1,4-diaminobenzene would shorten the external
C–N bonds and develop more quinoid character if the two amino
groups were placed between electrodes across a potential gap.
This diminishes the aromaticity, so there is an energy price com-
pared with similar nonaromatic systems.
gold scanning tunneling microscope break-junction (STM-BJ)
experiment.9 We saw that the poorest conductor was the thio-
phene ring, somewhat better was the oxazole ring (oxazole has less
aromatic stabilization than thiophene) and the best was the non-
aromatic dimethylcyclopentadiene ring. This raises the question
of whether the aromatic polythiophenes, commonly used as
molecular wires, are poorer electrical conductors than polymers
of less aromatic or anti aromatic compounds would be. It also
raises the question of whether an antiaromatic compound would
be a better electrical conductor. As we described,9 when aromatic
molecules are placed between electrodes there is a distortion of
the electronic structure of the aromatic system since the molecules
adopt a nonaromatic structure to some extent in which a benzene
ring partially resembles a quinone-like structure. Thus aromaticity
fights conductivity. However, antiaromatic systems diminish cyclic
conjugation as far as possible, for instant by making cyclobutadi-
ene an elongated rectangle, not a square. Thus antiaromatic com-
pounds should prefer the distortion that conductivity also
prefers, and become better conductors as a result.10
Results and discussion
Our first example of this effect was seen comparing the
conductivity of
a thiophene ring, an oxazole ring, and a
We wanted to examine the conductivity of a cyclopentadienone
ring, which is partially antiaromatic. Since such rings are very
reactive unless they have tetra-substitution, reflecting their
partial antiaromaticity, we have constructed three molecules
⇑
Corresponding author. Tel.: +1 212 854 2170.
E-mail address: rb33@columbia.edu (R. Breslow).
http://dx.doi.org/10.1016/j.tetlet.2015.06.076
0040-4039/Ó 2015 Published by Elsevier Ltd.

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A. Mahendran et al. / Tetrahedron Letters 56 (2015) 4833–4835
2,5-bis((4-aminophenyl)ethynyl)-3,4-bis(3,3-dimethylbut-1-yn-1-
yl)cyclopenta-2,4-dien-1-one (1), 4,40-((4,5-bis(3,3-dimethylbut-
1-yn-1-yl)-2,2-dimethoxycyclopenta-3,5-diene-1,3-diyl)bis(ethyne-2,
1-diyl))dianiline (2), and 4,40-((3,4-bis(3,3-dimethylbut-1-yn-1-
yl)thiophene-2,5-diyl)bis(ethyne-2,1-diyl))dianiline
(3)
with
p-aminophenylacetylene conducting links on carbons 2 and 5
and with tertbutylacetylene groups on positions 3 and 4 (Fig. 2).
In the synthesis (Scheme 1) we started with the known 1,2,3,
4-tetrabromo-5,5-dimethoxy cyclopentadiene 5,11,12 prepared
from a modified literature procedure using hexabromocyclopenta-
diene (4) as described in experimental section and coupled it with
two equivalents of tert-butylacetylene under Sonogashira coupling
conditions13 to afford intermediate 6. This was then coupled with
p-aminophenylacetylene to afford 2, the dimethylketal that has a
non-aromatic cyclopentadiene system. Aqueous acid catalyzed
hydrolysis of 2 afforded the ketone 1, the potentially antiaromatic
compound. For comparison we also synthesized the thiophene
derivative 3, with an aromatic central ring. Starting with commer-
cially available tetraiodothiophene 7, intermediate diiodo com-
pound 8 was synthesized by coupling with two equivalents of
aminophenylacetylene. Intermediate 8 was then coupled with
tert-butylacetylene to yield thiophene derivative 3 (see Scheme 2).
We then used the scanning tunneling microscope break-junc-
tion (STM-BJ) technique3,9 to determine the electrical conductance
of these three molecules. The conductance for the molecules 1, 2,
and 3 are given in Table 1 and their Log-binned one/two dimen-
sional histograms are shown in Figure 3. The comparison of mole-
cule 2 having a non-aromatic central core with molecule 3 having
an aromatic thiophene center shows 26% better conductance by
the non-aromatic species than the aromatic species. In our previ-
ous work the dimethylcyclopentadiene derivative was somewhat
better, ꢀ175% a better conductor than the thiophene. However,
Figure 1. Structures of thiophene, oxazole, and cyclopentadiene molecular wires.
Figure 2. Structures of three molecular wires investigated.
Scheme 1. Syntheses of compounds 1 and 2. Reagents and conditions: (a) tert-
butylacetylene, Pd(PPh3)4, CuI, iPr2NH, rt, 12 h; (b) 4-aminophenylacetylene,
Pd(PPh3)4, CuI, Et3N, THF, 50 °C, 12 h; (c) CF3COOH (5%), CH2Cl2, H2O, 0 °C–rt,
30 min.
the proposed anti-aromatic compound
conductor.
1 was the poorest
The poor conductance of 1 is related to the previous Letter14
that the diaminofluorenone derivative 9 is a poorer electrical con-
ductor than its oxime ether derivative 10 (Fig. 4). Our ‘antiaro-
matic’ molecule 1 also has a strong electron-attracting carbonyl
group.
Scheme 2. Syntheses of compound 3. Reagents and conditions: (a) 4-aminopheny-
lacetylene, Pd(PPh3)4, CuI, iPr2NH, THF, 0 °C–rt, 3 h; (b) tert-butylacetylene,
Pd(PPh3)4, CuI, Et3N, THF, 50 °C, 12 h.
Table 1
Molecular structures and their conductance
a
Molecule number
Conductance (G0 ꢁ 10ꢂ4
)
Relative conductance
1
2
3
1.25
3.16
2.51
0.50
1.26
1.00
a
G0 is the quantum of conductance, 2e2/h.
Figure 4. Structures of diaminofluorenone and its oxime derivative.
Figure 3. (A) Log-binned one-dimensional conductance histograms for molecules 1, 2, and 3 generated without any data selection, from 10,000 traces, using a bin size of 100/
decade. (B) Two-dimensional histograms for 1, 2, and 3 showing conductance peaks extending over a distance of 1 nm relative to the break of the G0 contact.

A. Mahendran et al. / Tetrahedron Letters 56 (2015) 4833–4835
4835
Conclusion
This work confirms the fact that aromaticity diminishes electri-
cal conductivity in single molecule conjugation, but it does not yet
furnish support for the idea that antiaromaticity should enhance
conductivity. This remains a challenge for future work.
Acknowledgments
We thank Dr. Latha Venkataraman and Brian Capozzi for
their generous help in conductivity measurements. We thank
Dr. Michael Steigerwald for his advice on theory, and for alerting
us to the relevant Letter cited as Ref. 13. We thank Dr. Wenbo
Chen and Dr. Vijayakumar Ramalingam for their valuable
discussion in synthesis.
Figure 5. Structures of compounds 11, 12, and 13.
The lower conductivity of the cyclopentadienone systems
apparently reflects the fact that with electron-rich systems, such
as those amines of the present and previous cases, placing the com-
pounds between anodes and cathodes will result in some partial
electron transfer to the anode (p-doping) to generate a better con-
ducting system. The conductivity is lower if the ionization poten-
tial for partial p-doping is more positive. Since the carbonyl
groups in compounds 1 and 9 are electron attracting, their effect
on the ionization potentials of the molecules outweigh anyhypo-
thetical increase in conductivity that the antiaromaticity effect
might bring.
Supplementary data
Supplementary data (experimental procedures and 1H, 13C NMR
and MS) associated with this article can be found, in the online
version, at http://dx.doi.org/10.1016/j.tetlet.2015.06.076.
References and notes
We have seen such counterbalancing effects previously.15 Few
years ago we synthesized compounds 11 and 12, (Fig. 5) and deter-
mined their single-molecule conductivities. We found that they
had essentially the same conductivity. Since the X-ray structure
for biphenylene 13 shows strong distortions (alternating short
and long lengths) induced by the antiaromaticity of biphenylene
itself in order to diminish any double-bond character in the four-
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the current case, where any conductivity increase because of
antiaromaticity was overwhelmed by the electron-withdrawing
effect of the carbonyl group. If aromaticity decreases conductivity,
as we have shown here and previously, the argument that antiaro-
maticity should increase it seems reasonable. However, the pro-
posed effect remains to be demonstrated.
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