An 11 nm molecular wire that switches electrochemically between an insulating and a fully conjugated conducting state

Article abstract of DOI:10.1021/ol052252g

Molecular wires consisting of oligothiophenes and oligothiophenylethynes, ranging in length from 2 to 11 nm, are synthesized and examined by spectroscopy and electrochemistry. UV spectroscopy shows that the longest wires are not fully conjugated, but twis

Full text of DOI:10.1021/ol052252g

ORGANIC
LETTERS
2006
Vol. 8, No. 2
183-185
An 11 nm Molecular Wire that Switches
Electrochemically between an Insulating
and a Fully Conjugated Conducting
State
Iris W. Tam, Jiaming Yan, and Ronald Breslow*
Department of Chemistry, Columbia UniVersity, 3000 Broadway,
New York, New York 10027
rb33@columbia.edu
Received September 16, 2005
ABSTRACT
Molecular wires consisting of oligothiophenes and oligothiophenylethynes, ranging in length from 2 to 11 nm, are synthesized and examined
by spectroscopy and electrochemistry. UV spectroscopy shows that the longest wires are not fully conjugated, but twisted. However, cyclic
voltammetry indicates that their monocations are now flattened and conjugated over the entire length range. In appropriate systems, the wires
will link to two electrodes across a nanoscale gap and conduct current.
For the construction of a molecular field effect transistor
(FET), it is important to have molecular wires that can switch
between an insulating and a conducting state. In pursuit of
this goal, we have examined a family of oligothiophenes
(OT) 1a-d and seen that, when terminated with an isocya-
nide group, such molecules could bind to a platinum surface
(Figure 1).1 This is a more desirable linkage between an
organic wire and a metal contact than the often-used thiol-
gold bond2 since high-melting platinum is less easily distorted
than gold and the isocyanide group is not as oxidizable as
the thiol group.3 The relatively small shifts in spectroscopic
and electrochemical data in the OT series 1a-d with
increasing molecular length (Table 1) suggested that there
could be sterically induced twisting of the direct thiophene-
to-thiophene linkage.
(1) Bong, D.; Tam, I.; Breslow, R. J. Am. Chem. Soc. 2004, 126, 11796-
11797.
Figure 1. Structures of OT monoisocyanide 1a-d and OTE
diisocyanides 3a-e, along with their respective calculated end-to-
end lengths, and bithiophenyl-ethynylene building unit 2.
(2) (a) Beebe, J. M.; Engelkes, V. B.; Miller, L. L.; Frisbie, C. D. J.
Am. Chem. Soc. 2002, 124, 11268-11269. (b) Cheng, L.; Yang, J.; Yao,
Y.; Price, D. W., Jr.; Dirk, S. M.; Tour, J. M. Langmuir 2004, 20, 1335-
1341. (c) Kushmerick, J. G.; Pollack, S. K.; Yang, J. C.; Naciri, J.; Holt,
D. B.; Ratner, M. A.; Shashidhar, R. Ann. N.Y. Acad. Sci. 2003, 1006
(Molecular Electronics III), 277-290.
We have now examined a series of oligomeric thiophen-
yl-ethynylenes (OTE) 3a-e terminated with isocyanide
(3) Zhitenev, N. B.; Erbe, A.; Bao, Z. Phys. ReV. Lett 2004, 92, 186805/
1-186805/4.
10.1021/ol052252g CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/27/2005

to cations. Most interestingly, the result is an electrochemical
ECE process, in which removal of the first electron from
the long wires induces a geometric change to full conjugation
that permits the second electron to be removed at the same
potential, overcoming the normal Coulombic repulsion in a
polycation. Thus the OTEs 3, including the longest molecule
3e, will apparently have the desired on/off switching proper-
ties that can be induced by a potential gap between source
and drain and perhaps modulated by the potential of the gate
in an FET.
Table 1. Absorption and Emission Maxima and First and
Observed (see text) Second Oxidation Potentials of 1a-d and
6a-e in Solutiona
UV-vis (nm)
Fl (nm)
λmax
Epa (mV)
(soln)b
Epa (mV)
(soln)c
λmax
1a1
1b1
1c1
1d1
6a
6b
6c
6d
6e
409
419
423
430
364
425
431
442
444
542
551
555
555
536
539
542
544
544
450
420
410
400
810
665
605
590
530
380
340
290
265
830d
655, 790
625, 880
525, 840
470, 745
Initial Sonagashira coupling of alkyne 4 to iodide 2 was
followed by iterative deprotection and coupling cycles to
yield a series of monoformamides 5a-d (Scheme 1).5
Subsequent dicoupling to 2,5-diiodo-3-hexylthiophene gave
diformamide intermediates 6a-e, which were readily dehy-
drated to the diisocyanide products 3a-e.6 The â-functional-
ity on each thiophene ring was changed from isobutyl in the
OT series to n-hexyl in the OTE series to confer better
solubility for the long oligomers in organic solvents.7
a UV and Fl maxima were acquired in CH2Cl2. b CV was measured at a
scan rate of 100 mV‚s-1 in CH2Cl2 with 0.1 M Bu4NPF6 using a Pt working
electrode and referenced to the Fc/Fc+ couple. c CV was measured at a scan
rate of 100 mV‚s-1 in CH2Cl2 with 0.1 M Bu4N[B(C6F5)4] using a Pt
working electrode and referenced to the Fc/Fc couple. d Irreversible
+
oxidation. For details, see Supporting Information.
Even with the addition of the acetylene spacers, OTEs
6a-e showed less than full conjugation in the longer wires
(Table 1). While the UV maxima exhibit a continuous red-
shift up to 16 subunits (7 nm) in the OT series 1, the UV
red-shift becomes saturated between 13 and 17 subunits
(8.8-11 nm) in the OTE series 6 (comparable to literature
values).4a,5a,8 Perhaps there is a less effective electronic
conjugation between thiophenes and triple bonds that coun-
teracts any π-conjugation improvement gained from the
decrease in steric interaction in 6a-e.4b,c,9
groups (Figure 1) and synthesized, via intermediates 4 and
5, their precursor formamides 6a-e (Scheme 1). Because
Scheme 1 a
One interesting finding is that the emission spectra in both
the OT and OTE series are almost independent of the lengths
of the wires. It is striking that lengthening the conjugated
systems moves the absorption maxima, at least until the
molecules are very long, but that this extra conjugation is
not reflected in the fluorescence maxima.
In contrast to the UV data for the neutral molecules, cyclic
voltammetry (CV) results indicate that OTEs 6a-e are better
able to extend the conjugation upon oxidation to the first
and higher cationic states. As the OTEs become longer, they
are oxidized more readily, as evident by the observed steady
decrease in the first oxidation potentials (Epa) in both
solutions with 6a-e (Table 1) and as a self-assembled
monolayer (SAM) of 3a-e (1040, 610, 565, 540, and 520
mV, respectively). The relative positions of the first Epa of
OTEs 6a-e are higher than OTs 1a-d, implying that the
acetylene spacers are slightly insulating electronically be-
a Reagents and conditions: (a) 2, Pd(PPh3)2Cl2, DIEA, THF; (b)
K2CO3, CH2Cl2/MeOH; (c) 3 equiv of 4 or 5a-d, Pd(PPh3)2Cl2,
2,5-diiodo-3-hexylthiophene, DIEA, THF; (d) triphosgene, CH2Cl2.
of their greater chemical stability, we have done our extensive
studies on the bis-amide series 6 rather than the isonitriles
3, and we list the results for 1 in Table 1.
The ethynyl units diminish the steric crowding that can
lead to twisting in the OT series 1.4 Even so, our studies
reveal that the longest of these molecules are twisted, so they
are not fully conjugated in their neutral states. However, they
become fully conjugated when electrochemically oxidized
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(b) Higuchi, F.; Ishikura, T.; Mori, K.; Takayama, Y.; Yamamoto, K.; Tani,
K.; Miyabayashi, K.; Miyaki, M. Bull. Chem. Soc. Jpn. 2001, 74, 889-
906. (c) Li, G.; Wang, X.; Li, J.; Zhao, X.; Wang, F. Synth. Commun. 2005,
35, 115-119.
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2003, 59, 2497-2518.
(7) (a) Roncali, J.; Garreau, R.; Yassar, A.; Marque, P.; Garnier, F.;
Lemaire, M. J. Phys. Chem. 1987, 91, 6706-6714. (b) McCullough, R.
D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58,
904-912.
(8) Meier, H.; Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379-
384.
(4) (a) Geiger, T.; Petersen, J. C.; Bjornholm, T.; Fischer, E.; Larsen, J.;
Deh, C.; Bredas, J.-L.; Tormos, G. V.; Nugara, P. N.; Cava, M. P.; Metzger,
R. M. J. Phys. Chem. 1994, 98, 10102-10111. (b) Samuel, I. D. W.;
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184
Org. Lett., Vol. 8, No. 2, 2006

tween the thiophene subunits. In-depth inspection of further
oxidation processes involving the removal of additional
electrons required the use of Bu4N[B(C6F5)4], owing to its
enhanced ability to solubilize positively charged oligomers
produced during anodic reactions in CH2Cl2.10
of 3a-e deposited on Pt films provided a positive correlation
between molecular length and SAM thickness. Assuming a
constant angle for all molecules, the OTEs deposit on Pt at
a tilt angle of ∼40° from the surface normal, similar to our
previous report with the OTs.1 Additionally, solution IR
spectroscopy of 3a-e provided unequivocal evidence for the
formation of isocyanide-platinum bonds upon treatment of
diisocyanides with Pt nanoparticles. The characteristic free
CN stretch at 2120 cm-1 was shifted to higher wave numbers
(2186 cm-1), indicating the formation of a Pt-CN bond.12
We have recently used the diamines derived by hydrolysis
of the 6 series to couple across a gap in a nanotube by direct
covalent formation of amide linkages.13 We saw that, indeed,
there is direct electrical conduction through the molecular
wires, not seen with saturated control molecules.
A striking finding concerns the second oxidation wave
observed in the 6a-e series. The ∆Ep (>60 mV) of the first
oxidation potential indicates only quasi-reversible behaviors
hence, the number of electrons transferred in each process
is not securely deducible. However, the position of the second
Epa of 6c is more positiVe than that of the shorter oligomer
6b, even though the shift of the first oxidation wave indicates
that 6c is more easily oxidized at its first wave than 6b, as
expected. Indeed, the second oxidation wave for 6c comes
between the second and third wave for the oxidation of 6b.
The only reasonable interpretation of this is that the second
oxidation wave of 6b involves its conversion to a dications
the normal processsbut the second wave for 6c corresponds
to its conversion from a dication to a trication. That is, the
first oxidation wave for 6c involves an ECE process: (1)
the first electron is removed; (2) the molecule then flattens
to induce better conjugation and delocalize the first positive
charge; and (3) the increased conjugation then permits a
second electron to be abstracted at the same potential,
producing a delocalized dication. This better delocalization
overcomes the Coulombic repulsion of the two charges, and
the same process must occur with all the longer wires.
However, in the short 6b, Coulombic repulsion wins, and
the second electron is abstracted only at a higher potential
than the first.
The next step is to use our wires in the construction of
molecular field effect transistors in suitable molecules with
three attached leads. This is underway.
Acknowledgment. This work was supported primarily
by the Nanoscale Science and Engineering Initiative of the
National Science Foundation (NSF) under NSF Award
Number CHE-0117752 and by the New York State Office
of Science, Technology, and Academic Research (NYSTAR).
We thank Prof. J. Norton for use of the CV equipment, and
Dr. E. de Poortere for providing the Pt films. I.W.T. thanks
the NSF for a predoctoral fellowship.
Supporting Information Available: Experimental details
for the synthesis of 2-6; UV, emission, solution, and SAM
CV, and nanoparticle IR experimental details. This material
is available free of charge via the Internet at http://pubs.acs.org.
The ability to form a self-assembled monolayer (SAM)
via chemisorption on the Pt working electrode confirms the
binding of the isocyanide group to Pt. SAM coverage was
determined to be >95% for 3a-e by comparing the oxidation
of K3[Fe(CN)6] by the Pt working electrode before and after
exposure to the diisocyanides.11 Ellipsometry measurements
OL052252G
(11) Cheng, L.; Yang, J.; Yao, Y.; Price, D. W., Jr.; Dirk, S. D.; Tour,
J. M. Langmuir 2004, 20, 1335-1341.
(12) Li, S.; McCarley, R. L. Langmuir 1999, 15, 151-159.
(13) Guo, X.; Small, J. P.; Klare, J.; Wang, Y.; Purewal, M. S.; Tam, I.;
Hong, B. H.; Caldwell, R.; Huang, L.; O’Brien, S.; Yan, J.; Breslow, R.;
Wind, S. J.; Hone, J.; Kim, P.; Nuckolls, C. Science, in press.
(10) Camire, N.; Nafady, A.; Geiger, W. E. J. Am. Chem. Soc. 2002,
124, 7260-7261 and references therein.
Org. Lett., Vol. 8, No. 2, 2006
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