Combined effect of polar substituents on the electronic flows in the carotenoid molecular wires

Article abstract of DOI:10.1002/chem.201300984

Carrot wires: Carotenoid molecular wires containing a pair of polar aromatic substituents have been synthesized to probe the combined effect of electron-withdrawing and -donating groups as conductance modulators (see figure). The total conductance values of the carotenoid wires are mostly controlled by electron-withdrawing substituents, whereas fine tuning is possible by way of the electron-donating substituent. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Full text of DOI:10.1002/chem.201300984

DOI: 10.1002/chem.201300984
Combined Effect of Polar Substituents on the Electronic Flows in the
Carotenoid Molecular Wires
Yanan Zhao,[a] Stuart Lindsay,*[a] Sunhwa Jeon,[b] Hyung-Jun Kim,[b] Liang Su,[b]
Boram Lim,[b] and Sangho Koo*[b]
Fabrication of the electric circuits of organic molecular
wires with variable conductance is one of the urgent goals in
molecular electronics. A plethora of organic molecular wires
has been devised, and the measurements of their conduc-
tance values were reported.[1] Even though the conductance
of a single molecule varies from measurement to measure-
ment, being especially dependent on the geometry of con-
tacts between metal electrodes and the organic molecule,
the peak values of the conductance distribution generally
agree reasonably when adequate statistical data are collect-
ed.[2] The conductance value of the organic molecular wire is
a manifestation of the intrinsic bonding nature within the or-
ganic molecule. In this context, the carotenoid, which con-
sists of extensive p conjugation, is an ideal organic molecu-
lar wire of potentially high electric conductance.[3] The expo-
nential decay of the conductance according to the increased
chain length of the carotenoids indicates that the molecular
Scheme 1. a) Conceptual design of the circuit consisting of the carotenoid
wires with polar substituents R1 and R2 as conductance modulators.
b) Carotenoid molecular wires 1 and 2 with the aromatic substituent(s) X
of different electronic nature.
wires with variable conductance could be obtained by the
use of carotenoids of different chain length.[4] However, we
are concerned herein with the potentially more useful ap-
proach of modulating the conductance of a wire of fixed
length by inserting conductance modulators (R1, R2, etc.)
into a carotenoid molecule of a specific chain length (Sche-
me 1a). Some theoretical and experimental reports on the
variation of conductance of molecular wires by a substituent
have appeared in the literature;[5] however, no systematic
study on the combined effects of the substituent groups on
the conductance modulation of organic molecular wires was
reported to date. Herein, we report our efforts for the prep-
aration and the conductance measurement of the carotenoid
wires with variable conductance by implantation of the
polar aromatic substituents to the conjugated polyene chain,
and most importantly the combined effect on conductance
exerted by the implanted conductance elements.
Recently, we reported the carotenoid wires 1 (Y=X) and
2 containing the polar aromatic substituent(s) (p-X-C6H4-)
of different electronic nature (X=OMe, Me, H, and Br) as
the conductance (or resistance) element(s) to provide the
carotenoid wires with diverse conductance values (Sche-
me 1b).[6] The difference of the conductance values between
1 (Y=X) and 2 is the quantitative contribution of each aro-
ꢀ
matic substituent (p-X C6H4-) to the conductance of the
molecular wire, which may give an insight in designing the
carotenoid wires of any specific conductance values within
the range over which conductance can be modulated. The
initial conductance measurement has been carried out by
conducting atomic force microscopy (cAFM) with a gold-
coated probe on the self-assembled carotenoid wires be-
[a] Y. Zhao, Prof. Dr. S. Lindsay
Biodesign Institute
Department of Chemistry and Biochemistry
Department of Physics
Arizona State University Tempe, Arizona, 85287 (USA)
E-mail: stuart.lindsay@asu.edu
tween AuACHTNUTRGNE(UNG 1,1,1) substrate and an Au (2 nm diameter) parti-
[b] S. Jeon, H.-J. Kim, L. Su, B. Lim, Prof. Dr. S. Koo
Department of Nano Science and Engineering
Department of Chemistry
cle.[7] The methylsulfide terminal group was utilized as an
anchor to gold electrodes for the reasons of stability and
synthetic convenience.[6] The measurements of the current/
voltage (I/V) characteristics by this method were consistent,
and the conductance of each carotenoid wire was obtained
as the gradient of the fundamental I/V curve at a low bias
region.[7] The quantitative contribution of each polar group
Myong Ji University, San 38-2
Nam-Dong, Yongin, Kyunggi-Do, 449-728 (Korea)
Fax : (+82)31-335-7248
E-mail: sangkoo@mju.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300984.
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COMMUNICATION
to the conductance of the carotenoid wire 1 was measured
to be 15.38 nS for anisyl (X=OMe), 3.56 nS for tolyl (X=
Me), 1.53 nS for phenyl (X=H), and ꢀ0.90 nS for bromo-
phenyl (X=Br) groups.[6] However, it is crucial to figure out
pling products 5 were protected under a solvolytic condition
in dimethoxymethane with P2O5 to give the methoxymethyl
(MOM) ethers 6. A couple of the double elimination of
MOM and SO2Ph groups produced the desired carotenoid
wires 1, which were easily purified by recrystallization from
CH2Cl2 and MeOH to give all-trans-carotenoid chains (7-
(Z),12-(Z)-carotenoids according to the IUPAC naming
rule).[9]
Even though the measurement of I/V curves of the molec-
ular wires by using cAFM was consistent, it was somewhat
tedious and laborious, because it required not only to pre-
pare well-defined self-assembled monolayers of the molecu-
lar wires, but also the statistical treatments of several thou-
sands of data sets for each molecular wire to find out the
fundamental I/V curve.[7] Instead, we decided to use the re-
peated break-junction method developed by Xu and Tao for
the fast and efficient measurement of the conductance of
the carotenoid molecular wires.[10]
the combined effect on conductance of each element (R1
2
ꢀ
and R or p-X C6H4-) to design the carotenoid molecular
wires with a specific conductance value. It was thus necessa-
ry to prepare the carotenoid wires 1 (X¼Y) with different
combinations of the two polar aromatic substituents and to
measure the conductance values to find out the underlying
trend of conductance modulation.
The synthesis of the carotenoid molecular wires 1 (X¼Y)
containing two different polar aromatic substituents is pre-
sented in Scheme 2. The allylic sulfone 3, which has been
A highly diluted solution (0.5 mm) of the carotenoid mole-
cule in mesitylene was prepared and placed on a gold sub-
strate. When the gold STM probe was pushed into the gold
substrate (current set point 300 pA; bias ꢀ0.3 V; ramp rate
8 Vsꢀ1; pre-amplification 10 nAVꢀ1), it makes contacts with
the carotenoid molecules in the solution; as it is pulled away
from the substrate, the contacts are subsequently broken
(Scheme 3a). Several control curves in mesitylene and the
resulting control histogram are shown in Scheme 3b. Dis-
crete steps were observed in the sample current curves over
time, which correspond to an integer number of molecular
Scheme 2. Synthesis of the carotenoid molecular wires 1e–j (X¼Y) with
the aromatic substituents of different electronic nature as the conduc-
tance (or resistance) elements.
specially designed for the terminal part of the carotenoid
wires, was coupled with the key 4-octenedials 4 that con-
tained two different polar aromatic groups at 2- and 7-posi-
tions.[8] Six 4-octenedials 4e–j can been prepared by the se-
lection of two groups out of the four different X substituents
(MeO, Me, H, Br). The detailed synthesis of dialdehydes 4
is described in the Supporting Information. The coupling
has been carried out at ꢀ788C by deprotonation of allylic
sulfone 3 with n-butyllithium, followed by the addition of di-
aldehydes 4, and then quenching with 1m HCl at that tem-
perature to prevent the retro reaction. It is neither sugges-
tive nor necessary to separate the stereoisomeric coupling
products, because six new chiral centers are generated by
the coupling reaction, which will be destroyed eventually in
the final elimination step. The hydroxyl groups of the cou-
Scheme 3. Conductance measurement of the carotenoid wire 1c (X=Y=
H in Scheme 2) by the repeated break-junction method. a) Making and
breaking contacts between the STM probe and the carotenoid wires
anchored to the gold substrate when the probe is pushed into and pulled
away from the substrate. b) Control conductance curves over time and
the control histogram. c) Steps in the sample conductance curves over
time, which indicate the number of carotenoid molecules that make con-
tacts to the STM probe and the histogram of the conductance value of
the carotenoid wire (Gaussian fit to the conductance histogram is also
shown above).
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S. Lindsay, S. Koo et al.
wires bridging the substrate and the probe (Scheme 3c).
Thousands of measurements can be completed within an
hour, and the results were plotted in a histogram to give the
conductance value of the single-carotenoid wire in G0. The
conductance value at the maximum count of the Gaussian
fit around the histogram of the first local maximum after
that from the base line in the conductance curve was select-
ed as the conductance of a single-carotenoid wire in G0. The
dispersion of the conductance values is presumably due to
the different contact geometries between the molecular
wires and the probe (and the substrate).[11]
The conductance values of the original four carotenoid
wires 1a–d (X=Y) were first measured by the break-junc-
tion method to compare with those obtained from the I/V
measurement on the carotenoids between metal contacts
(Table 1, entries 1–4). Once again, the electron-releasing
as the lower conductance element, conductance values of
which were not much different from the symmetric wire 1d
with two bromophenyl groups. The total conductance of the
carotenoid wires containing two polar aromatic substituents
is controlled mainly by the more electron-withdrawing sub-
stituent, whereas the more electron-donating one contrib-
utes to the fine tuning: the conductance values of 1 f (MeO,
H) and 1h (Me, H) are 3.44 and 3.09 nS, respectively, and
those of 1g (MeO, Br), 1i (Me, Br), and 1j (H, Br) are 1.50,
1.49, and 1.37 nS, respectively.
It may not be easy to explain the theoretical background
for the combined effect of the two different polar aromatic
substituents on the conductance of the carotenoid wire. The
orthogonal disposition of the aromatic substituents to the
conjugated polyene chain surely does not alter the HOMO–
LUMO gap of the carotenoid series 1,[6,12] which is support-
ed by the almost identical UV absorption data (see
the Supporting Information). The electron-releasing
Table 1. The conductance values of the carotenoid wires 1 with the substituents
anisyl substituent would increase the near-by p-
electron density along the conjugated polyene
chain, whereas the electron-withdrawing bromo-
phenyl group would decrease it (Scheme 4). The
flow of p electrons along the carotenoid wire is con-
trolled by the more electron-withdrawing group of
the two. In other words, there is a “bottle-neck
effect” by an electron-withdrawing substituent on
the conductance of the carotenoid wires: the more
resistive contributor controls the total conductance
of the carotenoid wires. The overall effect after
quantum interference of the two polar substituents
was lowering the HOMO of the carotenoid relative
to the gold Fermi level by the resistive modulator,
thereby lowering the total conductance of the wire.
ꢀ
ꢀ
C6H4 X and C6H4 Y.
Entry 1 (X, Y)
Conductance [I/V][a] Conductance (break junction)[b]
[nS] [nS] Go (stand. error)
1
2
3
4
5
6
7
8
1a (MeO, MeO) 33.46ꢁ5.19
17.07ꢁ0.17 2.202ꢁ10ꢀ4 (2.151ꢁ10ꢀ6
5.56ꢁ0.07 7.174ꢁ10ꢀ5 (8.721ꢁ10ꢀ7
2.86ꢁ0.03 3.692ꢁ10ꢀ5 (3.318ꢁ10ꢀ7
1.15ꢁ0.01 1.483ꢁ10ꢀ5 (1.194ꢁ10ꢀ7
5.63ꢁ0.10 7.257ꢁ10ꢀ5 (1.230ꢁ10ꢀ6
3.44ꢁ0.04 4.442ꢁ10ꢀ5 (5.370ꢁ10ꢀ7
1.50ꢁ0.04 1.937ꢁ10ꢀ5 (4.565ꢁ10ꢀ7
3.09ꢁ0.02 3.987ꢁ10ꢀ5 (3.079ꢁ10ꢀ7
1.49ꢁ0.03 1.919ꢁ10ꢀ5 (4.435ꢁ10ꢀ7
1.37ꢁ0.02 1.768ꢁ10ꢀ5 (2.961ꢁ10ꢀ7
)
)
)
)
)
)
)
)
)
)
1b (Me, Me)
1c (H, H)
10.41ꢁ0.94
7.45ꢁ2.17
1d (Br, Br)
1e (MeO, Me)
1 f (MeO, H)
1g (MeO, Br)
1h (Me, H)
1i (Me, Br)
1j (H, Br)
3.37ꢁ1.30
–
–
–
–
–
–
9
10
[a] Conductance calculated as the gradient of the fundamental current/voltage (I/V)
curve at a low bias region. [b] Conductance measured by the break-junction method
at the maximum peak height of the Gaussian fit to the conductance histogram.
anisyl group provided the highest conductance value
(17.07 nS), whereas the electron-withdrawing bromophenyl
group gave the lowest one (1.15 nS) among the series. There
was a good correlation between the two measurement meth-
ods, and the values obtained by the break-junction method
were roughly half of those by the I/V measurement, which
presumably reflected the geometrical differences of the
same carotenoid molecule according to the measurement
method: compression of the chain molecule in the AFM
versus elongation of the chain molecule in the STM. The
conductance measurement on the new carotenoid wires 1e–
j with two different substituent combinations was then car-
ried out by the break-junction method (Table 1, entries 5–
10). We found a very interesting relationship between the
conductance of the asymmetric wires 1e–j and that of the
symmetric ones 1a–d. The conductance value of the wire 1e
containing anisyl and tolyl substituents was very close to
that of the symmetric wire 1b containing two tolyl groups.
The conductance values of the asymmetric wires 1 f and 1h
containing a phenyl group, as the lower conductance ele-
ment, were almost same as that of the symmetric wire 1c
containing two phenyl groups. The same trend was observed
for the wires 1g, 1i, and 1j containing a bromophenyl group
Scheme 4. Control of the electronic flow in the carotenoid wire by the re-
sistive substituent: a “bottle-neck” effect in the p-electron density by the
electron-withdrawing substituent.
The conductance of the carotenoid wire containing the
anisyl and the bromophenyl groups is obviously controlled
by the more resistive bromophenyl substituent.
In summary, the carotenoid molecular wires containing
two different polar aromatic substituents as the conductance
(or resistance) modulators were synthesized, and the electric
conductance of the wires was measured by the repeated
break junction method. The combined effect on the elec-
tronic flows by each polar substituent of the carotenoid wire
was disclosed. The conductance of the carotenoid molecular
wire is mainly controlled by the electron-withdrawing sub-
stituent, whereas a fine tuning is allowed by the electron-do-
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Chem. Eur. J. 2013, 19, 10832 – 10835

Combined Effect of Polar Substituents
COMMUNICATION
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Acknowledgements
We thank Professor Nongjian Tao and Dr. Ismael D. Perez at A.S.U. for
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wires. This research was supported by Basic Science Research Program
(Grant No. 2012-045344) and by Priority Research Centers Program
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Keywords: carotenoids · conductance · electrochemistry ·
molecular electronics · molecular wire
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Received: March 14, 2013
Published online: July 2, 2013
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10835