COMMUNICATION
DOI: 10.1002/chem.201000700
Conductance Control in Stabilized Carotenoid Wires
Juwan Maeng,[a] Soo Bong Kim,[a] Nam Joo Lee,[b] Eunho Choi,[a] Se-Young Jung,[a]
Inseok Hong,[a] Sung-Hee Bae,[a] Jung Taek Oh,[a] Boram Lim,[a] Joon Woo Kim,[a]
Chi Jung Kang,[b] and Sangho Koo*[a]
A study of molecular electronics of organic nanowires has
been a popular research topic in the past decade not only
because of a booming atmosphere of nanoscience and engi-
neering but also a need for the smaller, faster, and flexible
substitutes for the conventional metallic wires.[1–5] The self-
assembly of organic molecules containing a terminal thiol
group on a gold substrate allows the measurement of elec-
tric conductance of the organic molecules using conducting
Atomic Force Microscopy (c-AFM).[6,7] Reproducible mea-
surement of molecular conductance is possible by through-
such as visual action and photosynthesis.[15,16] The potential
of these ideal molecular wires was not fully elucidated but
rather limitedly investigated under carefully controlled con-
ditions mainly due to their thermal and photochemical insta-
bilities.[12,13] The molecular conductance of the carotene di-
thiol 1 with typical nine C=C bond conjugation (Figure 1),
submerged in toluene under argon atmosphere was only
0.28 nS (nano-Siemens) even though it was still 6–7 orders
of magnitude higher than that of n-alkane of equivalent
chain length.[13,17] The molecular conductance of the carote-
noid wires decreased exponentially with a small decay con-
stant (b = 0.22 ꢀÀ1) as the number of double bonds in con-
jugation increased.[17] Our experience in synthesizing carote-
noids guided us to design the conceptually new and seem-
ingly stable carotenoids to challenge the possibility of im-
proving and diversifying the molecular conductance of
carotenoids of the same size, so that the electric circuits of
the stable carotenoid wires with various conductances (or
resistances) may be realized in practical sense.
À
bond contacts (e.g., S Au bond) between the organic mole-
cules and metal electrodes.[8,9] However, materialization of
high and controllable conductance in organic nanowires up
to the level of metallic ones has been an elusive dream.
Nevertheless, several nanometer-sized organic molecules
such as p-phenylene-ethylene oligomers,[10] p-phenylene–
ethynylene oligomers,[11] and especially carotenoids showed
somewhat promising properties as a conducting molecular
wire.[12,13] It was pointed out that the existence of conjugated
À
unsaturated carbon carbon bonds, reflecting delocalized p-
electron system, was essential for high conductance of or-
The instability of carotenoids is an inevitable consequence
of their antioxidant activities in quenching singlet oxygen
and scavenging reactive radical species.[18,19] The synergistic
protective effect of carotenoids in combination with vitamin
E containing the aromatic phenyl group against photody-
namic cell damage has been reported.[20] The enhanced anti-
oxidant efficiency of carotenoids was explained by the pre-
vention of free radical-mediated carotenoid degradation or
repair of the semi-oxidized carotenoid molecules by vitamin
E. We thus devised the novel carotenoid wires 2 so as to
provide the labile conjugated polyene chain with stability as
well as various conductances by attaching the aromatic
phenyl groups containing the para-substituent X (OMe, Me,
H, and Br) of diverse electronic natures to the polyene
chain at C-13 and C-13’ (Figure 1). The stability of carote-
noids 2 by the phenyl groups might be expected from the re-
pulsive steric interactions with attacking nucleophiles and/or
the reversible trapping of incoming radicals (e.g., reactive
oxygen species) that would cause fragmentations of the con-
jugate polyene chain. The benzene rings containing a para-
ganic molecular wires.[14]
Carotenoids, natural products known as a strong antioxi-
dant as well as a harmless red pigment, also play an impor-
tant role of transferring electrons in biological processes
[a] J. Maeng, S. B. Kim, E. Choi, S.-Y. Jung, I. Hong, S.-H. Bae, J. T. Oh,
B. Lim, J. W. Kim, Prof. Dr. S. Koo
Department of Chemistry; Department of Nano Science and
Engineering
Myong Ji University, San 38-2
Nam-Dong, Yongin, Kyunggi-Do, 449-728 (Korea)
Fax : (+82)31-335-7248
[b] N. J. Lee, Prof. Dr. C. J. Kang
Department of Physics, Department of Nano Science
and Engineering
Myong Ji University, San 38-2
Nam-Dong, Yongin, Kyunggi-Do, 449-728 (Korea)
Supporting information for this article is available on the WWW
Chem. Eur. J. 2010, 16, 7395 – 7399
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7395