Nanoscale aryleneethynylene molecular wires with reversible fluorenone electrochemistry for self-assembly onto metal surfaces

Article abstract of DOI:10.1021/ol0493608

Two rigid-rod conjugated molecules (11 and 12) of ca. 4 and 7 nm length, respectively, bearing protected terminal thiol groups have been synthesized via multistep Sonogashira coupling reactions and shown to possess reversible cathodic solution electrochem

Full text of DOI:10.1021/ol0493608

ORGANIC
LETTERS
2004
Vol. 6, No. 13
2181-2184
Nanoscale Aryleneethynylene Molecular
Wires with Reversible Fluorenone
Electrochemistry for Self-Assembly onto
Metal Surfaces
,†
Changsheng Wang,† Andrei S. Batsanov,† Martin R. Bryce,* and Ian Sage‡
Department of Chemistry, UniVersity of Durham, Durham DH1 3LE, United Kingdom,
and QinetiQ, St. Andrews Road, MalVern, Worcestershire WR14 3PS, United Kingdom
m.r.bryce@durham.ac.uk
Received April 8, 2004
ABSTRACT
Two rigid-rod conjugated molecules (11 and 12) of ca. 4 and 7 nm length, respectively, bearing protected terminal thiol groups have been
synthesized via multistep Sonogashira coupling reactions and shown to possess reversible cathodic solution electrochemistry arising from
reduction of the fluorenone units.
Single molecular (SM) electronics is attracting great attention
due to the potential applications in future computing technol-
ogy and related fields.1 The current molecular device
fabrication technique is de facto the preparation of metal-
molecule-metal junctions between macroscopic electrodes
and an individual organic molecule or a monolayer array of
molecules. Reliable molecular junctions require a bespoke
gap distance between the electrodes that is close to the length
of an active molecule. Due to the intrinsic roughness of noble
metal electrode surfaces, fabrication of devices with use of
small molecules2 has proved difficult and low yielding.
Scanning probe,3 break-junction,4 and electromigration5
methodologies are more reliable in terms of testing molecular
junctions. However, they are less likely to be industrialized
for large-scale production. Many issues still need to be
addressed as it becomes apparent that the behavior of
electrically active molecules in experimental devices is
critically dependent upon their immediate environment, e.g.
(2) (a) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999,
286, 1550-1552. (b) Chen, J.; Wang, W.; Reed, M. A.; Rawlett, A. M.;
Price, D. W.; Tour, J. M. Appl. Phys. Lett. 2000, 77, 1224-1226. (c)
Kratochvilova, I.; Kocirik, M.; Zambova, A.; Mbindyo, J.; Mallouk, T. E.;
Mayer, T. S. J. Mater. Chem. 2002, 12, 2927-2930. (d) Tour, J. M.; Cheng,
L.; Nackashi, D. P.; Yao, Y.; Flatt, A. K.; St. Angelo, S. K.; Mallouk, T.
E.; Franzon, P. D. J. Am. Chem. Soc. 2003, 125, 13279-13283.
(3) (a) Hersam, M. C.; Guisinger, N. P.; Lyding, J. W. Nanotechnology
2000, 11, 70-76. (b) Rawlett, A. M.; Hopson, T. J.; Nagahara, L. A.; Tsui,
R. K.; Ramachandran, G. K.; Lindsay, S. M. Appl. Phys. Lett. 2002, 81,
3043-3045. (c) Feldheim, D. Nature 2000, 408, 45-46. (d) Gittins, D. I.;
Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67-69.
(4) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M.
Science 1997, 278, 252-254.
(5) (a) Park, H.; Park, J.; Lim, A. K. L.; Anderson, E. H.; Alivisatos, A.
P.; McEuen, P. L. Nature 2000, 407, 57-60. (b) Park, J.; Pasupathy, A.
N.; Goldsmith, J. I.; Chang, C.; Yaish, Y.; Petta, J. R.; Rinkoski, M.; Sethna,
J. P.; Abrun˜a, H. D.; McEuen, P. L.; Ralph, D. C. Nature 2002, 417, 722-
725. (c) Liang, W.; Shores, M. P.; Bockrath, M.; Long, J. R.; Park, H.
Nature 2002, 417, 725-728. (d) Park, H.; Lim, A. K. L.; Alivisatos, A. P.;
Park, J.; McEuen, P. L. Appl. Phys. Lett. 1999, 75, 301-303.
† University of Durham.
‡ QinetiQ.
(1) (a) Feldheim, D. L.; Keating, C. D. Chem. Soc. ReV. 1998, 27, 1-12.
(b) Nitzan, A.; Ratner, M. A. Science 2003, 300, 1384-1389 (c) Joachim,
C.; Gimzewski, J. K.; Aviram, A. Nature 2000, 408, 541-548. (d) Tour, J.
M. Acc. Chem. Res. 2000, 33, 791-804. (e) Reed, M. A.; Tour, J. M. Sci.
Am. 2000, 6, 86-93. (f) Service, R. F. Science 2003, 302, 556-559.
10.1021/ol0493608 CCC: $27.50 © 2004 American Chemical Society
Published on Web 06/03/2004

Scheme 1 a
a Reagents and conditions: (i) (trimethylsilylacetylene) (TMSA) (4 equiv.), THF, Pd[PPh3]2Cl2 (3%), CuI, TEA, rt, 2 h, then 50 °C, 2
h, 98%; (ii) 1:1 (v/v) DCM-methanol, K2CO3, rt, 12 h, 76%; (iii) 3 and 4 (5 equiv), THF, Pd[PPh3]2Cl2 (3%), CuI, TEA, rt, 12 h, then 50
°C, 2 h, 43%; (iv) 6, 3-bromopropionitrile, dry DMF, K2CO3, 100 °C, 24 h, 93%; (v) 7, TMSA (1.5 equiv), THF, Pd[PPh3]2Cl2 (3%), CuI,
TEA, rt, 3 h, 88%; (vi) HOAc, Ac2O, TBAF, rt, 0.5 h, 76%; (vii) 5, 9, THF, Pd[PPh3]2Cl2, CuI, TEA, rt, 12 h, 50 °C, 2 h, 44% (for 10)
and 29% (for 11); (viii) 3, 10, THF, Pd[PPh3]4, CuI, TEA, rt, 0.5 h, then 50 °C, 3 h, 54%.
electrodes or other organic molecules. Earlier results on
conductivity changes and switching behavior are being
reappraised.1f Nevertheless, synthesizing functional molecules
with lengths of at least several nanometers, which will self-
assemble on metal electrodes, will no doubt permit easier
and more reliable fabrication (due to the larger gaps required)
and be of fundamental importance.
Monodisperse, π-conjugated oligomers based on rodlike
aryleneethynylene backbones are of considerable current
interest as optoelectronic materials and as model compounds
for related polymers.6 The arylene units in aryleneethynylene
structures include 2,5-dialkyl-p-phenylene,7 2,5-dialkoxy-p-
phenylene,8 2,6-diphenyl-p-phenylene,9 1,5-naphthalene,10
and 9,9-dihexylfluorene.11 Herein we report the first such
extended systems containing fluorenone units.12 The special
features of these new oligomers 11 and 12 are their reversible
solution electrochemistry and the presence of terminal
protected thiol groups. These molecular wires 11 and 12 are
designed for SM device fabrication.
The synthesis (Scheme 1) comprises an iterative Pd-
mediated Sonogashira strategy. The high degree of purity
of compounds 11 and 12 has been established by CHN
1
analysis, H and 13C NMR, and MALDI-TOF mass spec-
trometry (see Supporting Information). The molecules in-
corporate 2-cyanoethyl as the terminal thiol protecting group.
This protecting group is known for tetrathiafulvalene thiolates
but the original deprotection conditions13 are not suitable for
(6) Electronic Materials: The Oligomer Approach; Wegner, G., Mu¨llen,
K., Eds.; Wiley-VCH: Weinheim, Germany, 1998.
(7) Francke, V.; Mangel, T.; Mu¨llen, K. Macromolecules 1998, 31,
2447-2453.
(10) Rodr´ıguez, J. G.; Tejedor, J. L. J. Org. Chem. 2002, 67, 7631-
7640.
(8) (a) Schenning, A. P. H. J.; Tsipis, A. C.; Meskers, S. C. J.; Beljonne,
D.; Meijer, E. W.; Bre´das, J. L. Chem. Mater. 2002, 14, 1362-1368. (b)
Zhou, C.-Z.; Liu, T.; Xu, J.-M.; Chen, Z.-K. Macromolecules 2003, 36,
1457-1464.
(11) Lee, S. H.; Nakamura, T.; Tsutsui, T. Org. Lett. 2003, 3, 2005-
2007.
(12) A single 2,7-diethynylfluoren-9-one unit has been incorporated into
short wire-type molecules: Price, D. W.; Tour, J. M. Tetrahedron 2003,
59, 3131-3156.
(9) Kozaki, M.; Okada, K. Org. Lett. 2004, 6, 485-488.
2182
Org. Lett., Vol. 6, No. 13, 2004

Figure 1. Molecular structure of 11 determined by X-ray diffraction.
self-assembly. However, cyanoethyl-protected thiophenol
derivatives (such as 12) can be easily deprotected under much
milder conditions by either sodium methoxide or tetrabuty-
lammonium fluoride (TBAF) in THF solutions at room
temperature.14 A disadvantage of using an acetyl protecting
group for thiophenol derivatives is that it is too easily
removed, resulting in side reactions and difficult product
purification. The increased stability of the 2-cyanoethyl group
is advantageous for our Sonogashira reactions, enabling the
use of higher temperatures, longer reaction times, and an
increased amount of amines. In addition, the stability of the
final products is increased compared to that of acetyl
analogues.15 Starting from 4-iodothiophenol,15 the new
terminal phenylethynyl building block 9 bearing a protected
thiolate group was prepared in multigram quantities. This
compound provides an ideal substitute for 4-acetylthiophe-
nylacetylene developed by Tour and co-workers12,16 and the
(N,N-dimethylcarbamoyl)thio analogue synthesized by Mu¨llen
and co-workers.7 THF solutions of 11 and 12 were stored at
room temperature for a week without detectable decomposi-
tion.
additional Au‚‚‚S bonds in a Au‚‚‚11‚‚‚Au junction, a gap
of ca. 4 nm will be required for fabrication. Similarly, the
S‚‚‚S′ distance in 12 was predicted to be 69 Å,17 which allows
an electrode gap of ca. 7 nm.
Reversible cathodic solution electrochemistry was brought
about by reduction of the fluorene group(s) in 11 and 12
(Figure 2). As the irreversibility of a redox process means
The crystal structures of 5 and 11 have been determined
by X-ray diffraction.18 Molecule 11 (Figure 1) has crystal-
lographic C2 symmetry and an almost planar conformation,
with the average deviation of all non-H atoms of ca. 0.2 Å
and all ring planes parallel within 13°. The intramolecular
S‚‚‚S′ distance of 37.4 Å in 11 was in good agreement with
the MM+ calculated value of 36 Å.17 Considering the
Figure 2. Cyclic voltamograms of 11 (a and b) and 12 (c). Traces
a and b were recorded in dry DMF, and trace c in dry THF.
Supporting electrolyte: 0.1 M TBAPF6. Electrodes: working, Pt
disk (Φ ) 1.8 mm); counter, Pt wire; reference, Ag/AgNO3-
acetonitrile. Scan rate: (a, b) 100 mV/s; (c) 500 mV/s.
(13) Svenstrup, N.; Rasmussen, K. M.; Hansen, T. K.; Becher, J.
Synthesis 1994, 809-812.
either decomposition or subsequent chemical reactions of the
molecules under bias, it is likely that a molecular junction
could be more easily broken when a charge is injected into
an irreversible molecule, hence leading to failure of the
device, compared with those derived from reversible mol-
ecules. We note that at least two functioning SM transistors5a,b
were fabricated by using electrochemically reversible mol-
(14) Typical deprotection conditions are as follows: to methyl 4-[2-
cyanoethyl)thio]benzoate (0.221 g, 1 mmol) (see Supporting Information
for its preparation) in THF (40 mL) was added sodium methoxide-methanol
solution (0.5 M, 2.1 mL, 1.05 mmol) at 20 °C and the mixture was stirred
for 15 min. Iodomethane (0.2 mL, 3.2 mmol) was syringed in followed by
an additional 1 h of stirring, yielding methyl 4-methylthiobenzoate in 91%
yield after column chromatographic purification (silica, chloroform). For
the same reactions with tetrabutylammonium fluoride (1.0 M THF solution,
1.1 mmol) as deprotecting reagent, 2 h of stirring yielded methyl
4-methylthiobenzoate in 80% isolated yield.
(15) Wang, C.; Batsanov, A. S.; Bryce, M. R.; Sage, I. Synthesis 2003,
2089-2095.
(18) Crystal data: C53H62I2O5 (5), M ) 1032.83, triclinic, space group
P1h (no. 2), T ) 120 K, a ) 9.986(1) Å, b ) 15.134(3) Å, c ) 17.310(4)
Å, R ) 102.56(3)°, â ) 94.34(2)°, γ ) 107.62(2)°, U ) 2405.4(8) Å3, Z
) 2, systematic twinning, R ) 0.065 on 13154 unique data with I > 2σ(I);
CCDC-231672. C75H78N2O5S2 (11) M ) 1151.51, monoclinic, space group
C2/c (no. 15), T ) 120 K, a ) 8.541(1) Å, b ) 21.607(5) Å, c )
34.169(8) Å, â ) 95.17(2)°, U ) 6280(2) Å3, Z ) 4, R ) 0.056 on 2930
unique data with I > 2σ(I); CCDC-231673.
(16) (a) Pearson, D. L.; Tour, J. M. J. Org. Chem. 1997, 62, 1376-
1387. (b) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.; Jagessar, R.
C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C.-W.; Chen, J.; Wang,
W.; Campbell, I. Chem. Eur. J. 2001, 7, 5118-5134. (c) Flatt, A. K.; Yao,
Y.; Maya, F.; Tour, J. M. J. Org. Chem. 2004, 69, 1752-1755.
(17) HyperChem 6.03; 2000 Hypercube, Inc. The calculation terminated
at the RMS gradient value of 0.01 kcal/(Å mol).
Org. Lett., Vol. 6, No. 13, 2004
2183

ecules. As shown in Figure 2a, when the cathodic scan was
limited to -1.8 V, 11 had one reversible redox wave at E1/2
) -1.44 V, which was due to the reduction of the fluorenone
unit. The compound also showed two irreversible waves at
Ered ) -2.10 and -2.23 V, associated with the reduction of
the triple bonds. However, the current intensity on the reverse
anodic scan at -1.4 V (Figure 2b) was reduced, indicating
that at such negative potentials an electrochemical reaction
changed the molecular structure. In THF (it is poorly soluble
in DMF), 12 also showed a reversible wave at -1.40 V.
However, its acetylenic reductions could not be observed
with certainty in this solvent.
of other functional molecular wires of precise conjugation
lengths.
Acknowledgment. This work was supported by the
Materials Domain of the UK MoD Corporate Research
Programme. We thank Professor J. A. K. Howard for the
use of X-ray facilities and EPSRC for funding the improve-
ment of the X-ray instrumentation.
Supporting Information Available: Characterization
1
data for 12 including copies of the H NMR, MALDI-TOF
mass spectra, and HPLC trace; molecular structure of 5 and
packing diagram for 11; preparation of methyl 4-[2-cyano-
ethyl)thio]benzoate; cyclic voltamograms of fluorenone and
two model 2,7-diethynylfluoren-9-one derivatives. This
material is available free of charge via the Internet at
http://pubs.acs.org.
In summary, we have synthesized a new series of soluble
fluorenone-based molecular wires suitable for single molec-
ular device fabrication. The methodology provides efficient
access to versatile aryleneethynylene derivatives which
should serve as valuable building blocks for the construction
OL0493608
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Org. Lett., Vol. 6, No. 13, 2004