Synthesis and properties of an anthraquinone-based redox switch for molecular electronics

Article abstract of DOI:10.1021/ol0606278

The synthesis of a molecular wire bearing an anthraquinone core and thioacetyl end groups for gold electrode binding is described. A model anthraquinone system, substituted with tert-butylthio groups, can be reversibly switched electrochemically from cros

Full text of DOI:10.1021/ol0606278

ORGANIC
LETTERS
2006
Vol. 8, No. 11
2333-2336
Synthesis and Properties of an
Anthraquinone-Based Redox Switch for
Molecular Electronics
Elisabeth H. van Dijk, Daniel J. T. Myles, Marleen H. van der Veen, and
Jan C. Hummelen*
Molecular Electronics, Materials Science CentrePlus, UniVersity of Groningen,
Nijenborgh 4, 9747 AG, Groningen, The Netherlands
j.c.hummelen@rug.nl
Received March 14, 2006
ABSTRACT
The synthesis of a molecular wire bearing an anthraquinone core and thioacetyl end groups for gold electrode binding is described. A model
anthraquinone system, substituted with tert-butylthio groups, can be reversibly switched electrochemically from cross conjugated (low
conductance “off”) to linear conjugated (high conductance “on”) via two-electron reduction/oxidation reactions. This feature holds promise
for the anthraquinone-based wires to be used as redox-controlled switches in molecular electronic devices.
The understanding of electronic-transport properties through
single molecules between metal contacts has recently become
possible, for example, by the mechanically controllable break
junction (MCBJ) technique1 and STM.2 A large number of
conjugated molecular wires containing terminal sulfur groups
based on oligo(thiopheneethynylene)s, oligo(phenyleneethy-
nylene)s (OPEs), and oligo(phenylenevinylene)s (OPVs)
have been synthesized,3 some of which have been embedded
as passive elements in MCBJs.4 Mayor et al.5 have shown
that the electronic properties of bis-9,10-phenylethynyl-
anthracenes largely depend on whether the thiol anchors are
situated at the para or meta positions. An approximate 2-order
magnitude drop in the current was observed in going from
the para- to the meta-substituted compound. This example
clearly illustrates how the conductivity is influenced by the
molecular topology (i.e., linear vs cross conjugated). Active
elements such as multiterminal molecular junctions, switches,
and logic gates are also being investigated for molecular
electronics.6 In this vein, diarylethene-based photochromic
switches have been studied in MCBJs by the group of Van
Wees.7 The “closed” (linear conjugated) form of the switch
was assembled into a break junction and illuminated with
light (λ ) 546 nm), resulting in a 103 increase of the
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2006, 128, 1959.
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Wang, C. S.; Batsanov, A. S.; Bryce, M. R. J. Org. Chem. 2006, 71, 108.
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Magoga, M.; Joachim, C. Phys. ReV. B 1999, 59, 12505.
(5) Mayor, M.; Weber, H. B.; Reichert, J.; Elbing, M.; von Ha¨nisch, C.;
Beckmann, D.; Fischer, M. Angew. Chem., Int. Ed. 2003, 42, 5834.
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10.1021/ol0606278 CCC: $33.50
© 2006 American Chemical Society
Published on Web 05/04/2006

Scheme 1
resistance (i.e., from the MΩ to the GΩ range). This is due
By modification of the literature methods,5,10,13 the trimeth-
ylsilyl protecting group of 2 was then removed using TBAF
in THF affording key building block 3 in quantitative yield.
Compounds 4 and 7 were prepared according to the
literature.14 Acetylene 3 was then independently cross-
coupled to 4 and 7, yielding products 5 and 8, respectively.
Both 5 and 8 contain tert-butyl-protected thiol end groups,
which were converted to the more labile thioacetyl groups
by a modified bromine-catalyzed deprotection protocol that
was initially developed by the group of Mayor.13 Because
of the poor solubility of 5 and 8 in acetyl chloride, we found
it necessary to use chloroform as cosolvent for the reactions.
Target compounds 6 and 9 were obtained in quantitative
yields. The 1H and 13C NMR spectra are in agreement with
their symmetrical structures, and after the thiol protecting
group conversion (i.e., t-Bu to Ac), both displayed an
additional carbonyl stretch (1705 cm-1 for 6 and 1692 cm-1
for 9). The thioacetyl terminated compounds 6 and 9 can in
principle be assembled onto gold15 and studied, for example,
by the MCBJ technique or by STM. However, because the
thioacetyl compounds are much less soluble than their tert-
butyl congeners 5 and 8, the latter are used for the
spectroscopic and electrochemical measurements reported
herein.
to switching of the molecule from the closed to the open
state and thus from linear to cross conjugated. Upon
illumination of the open form in the break junction with 313
nm light, no reverse switching was observed. This is
attributed to quenching of the first excited state of the open
form by gold. However, it has been shown that reversible
switching can be achieved for a diarylethene containing
methylene spacers between the conjugated system and the
sulfur end groups.8
Very recently, Nielsen and co-workers reported an oligo-
(phenyleneethynylene)-tetrathiafulvalene cruciform for its use
in molecular electronics.9 The linear-conjugated pathway
along the OPE could be irreversibly switched electrochemi-
cally to a cross-conjugated one via a two-electron oxidation
of the flanking TTF moieties.
In this paper, we present the synthesis of an anthraquinone-
based molecular switch (6) that has thioacetyl end groups
for attachment to gold surfaces. The long-term objective is
to use these molecules in electronic devices whereby the
conductivity is reversibly switched from low (off) to high
(on) and vice versa.
Anthraquinone-based redox switch 6 and reference an-
thracene compound 9 were synthesized as shown in Scheme
1. Compound 2 was previously prepared by Mayor et al.
using different starting materials, although no experimental
details were reported.5 More recently, Nielsen and co-workers
developed a method for its preparation in 75% yield, but by
employing a rather elaborate catalytic system (Cl2Pd(PhCN)2/
P(t-Bu)3/CuI).10 We have found that 2 can be prepared in
85% by a standard11 (Cl2Pd(PPh3)2/CuI) Sonogashira cross-
coupling reaction of known 112 with trimethylsilylacetylene.
The UV-vis absorption spectrum of 5 shows maxima at
305, 334, and 373 nm, whereas the spectrum of 8 shows
maxima at 319, 331, 385, 406, and 430 nm (see Supporting
Information). The linear-conjugated compound 8 absorbs
more strongly in the UV region than 5 and has anthracene-
(11) Sonogashira, K.; Tohda, Y.; Hagihara, N. Tetrahedron Lett. 1975,
50, 4467.
(12) Stuhr-Hansen, N. Synth. Commun. 2003, 33, 641.
(13) Błaszczyk, A.; Elbing, M.; Mayor, M. Org. Biomol. Chem. 2004,
2, 2722.
(14) Yang, J. H.; Dass, A.; Rawashdeh, A.-M. M.; Sotiriou-Leventis,
C.; Panzner, M. J.; Tyson, D. S.; Kinder, J. D.; Leventis, N. Chem. Mater.
2004, 16, 3457.
(15) Tour, J. M.; Jones, L.; Pearson, D. L.; Lamba, J. J. S.; Burgin, T.
P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am.
Chem. Soc. 1995, 117, 9529.
(8) He, J.; Chen, F.; Liddell, P. A.; Andre´asson, J.; Straight, S. D.; Gust,
D.; Moore, T. A.; Moore, A. L.; Li, J.; Sankey, O. F.; Lindsay, S. M.
Nanotechnology 2005, 16, 695.
(9) Sørensen, J. K.; Vestergaard, M.; Kadziola, A.; Kilså, K.; Nielsen,
M. B. Org. Lett. 2006, 8, 1173.
(10) Stuhr-Hansen, N.; Sørensen, J. K.; Moth-Poulsen, K.; Christensen,
J. B.; Bjørnholm, T.; Nielsen, M. B. Tetrahedron 2005, 61, 12288.
2334
Org. Lett., Vol. 8, No. 11, 2006

like absorptions appearing at lower energies due to the
extended conjugation. Cross-conjugated compound 5 shows
a broad fluorescence emission peak at 675 nm. Conjugated
compound 8 is approximately 1000 times more fluorescent
and emits blue light (440 and 470 nm). These spectroscopic
experiments show that the electronic properties of the linear-
conjugated (on) and cross-conjugated (off) compounds show
substantial differences. These findings are in good agreement
with those found by Leventis et al. on similar types of
compounds.14
Figure 2. Differential absorption spectra of 5 in ODCB/CH3CN.
Cyclic voltammetry measurements on 5 showed two fully
reversible reduction waves at a scan rate of 10 mV/s (Figure
1). The anthraquinone core is first reduced to the semi-
ne, which has its principle absorptions at 470 and 622 nm
in DMF.16 The differences in the positions of absorption
maxima for 5 with respect to 9,10-anthraquinone are ascribed
to a combination of substituent and solvent polarity effects.
When the potential was subsequently set back at 0 V, the
original absorption spectrum was obtained. The experiment
proved that anthraquinone 5 could be switched reversibly
between its neutral and dianionic states.
The π-electron delocalization within the frontier orbitals
provides a suitable observable to obtain insight into the
charge-transport properties of single molecules.17 We have
performed molecular orbital calculations to qualitatively
determine whether the conductivity could change from low
to high when the anthraquinone system is switched from a
cross- to a linear-conjugated state. All molecular orbital
calculations and geometry optimizations were performed for
the single molecules in a vacuum, using the semiempirical
Austin Model 1 (AM1)18 level of theory implemented in the
HyperChem 7.5 program package of Hypercube Inc.19 The
conjugated hydroquinone dianion was simulated by imposing
a total charge of -2 on the structure during the semiempirical
calculations.
Figure 3 shows the electron probability distribution æ-
(x,y)2 within the frontier orbitals for the neutral and hydro-
quinone dianion of thiol (SH) end-capped 5. The frontier
orbitals of the neutral cross-conjugated state are mainly
localized to certain parts of the molecule (here, the HOMO
and HOMO-1 are close to degenerate, as expected for
symmetry reasons). On the other hand, the HOMO and
LUMO of the conjugated hydroquinone dianion are fully
delocalized across the molecule far into the terminals. Hence,
the spatial extent of the LUMO demonstrates that there is
enough delocalization to allow for efficient transport of
electrons between the termini of the dianionic conjugated
form. An even more enhanced π-electron delocalization was
observed within the frontier orbitals of the neutral conjugated
compound 9. The reference anthracene 9 would therefore
be an ideal charge-transport medium for holes and/or
Figure 1. Cyclic voltammogram of 5 in ODCB/CH3CN.
quinone at -1.27 V and then to the hydroquinone dianion
at -1.80 V vs Fc/Fc+. The potentials of anthraquinone 5 were
compared with the redox potentials of 9,10-anthraquinone,
2,6-dibromoanthraquinone, and 2,6-bis(phenylethynyl)-9,10-
anthraquinone by differential pulse voltammetry (DPV), and
these data can be found in the Supporting Information. The
measurements showed that the reduction potentials of these
2,6-disubstituted anthraquinones are nearly identical to each
other but considerably different when compared to unsub-
stituted 9,10-anthraquinone. This is a promising feature for
the switching behavior of compound 6 while attached to gold
because the nature of the substituents appears to have little
if any effect on the redox chemistry of the anthraquinone
core.
UV-vis spectroelectrochemical experiments on 5 were
performed in a thin cell setup with a path length of 1 mm
that was supported with a platinum gauze working electrode,
a platinum wire counterelectrode, and a SCE reference
electrode. A UV-vis spectrum was first recorded on a pale
yellow solution of neutral 5 at 0 V (Figure 2). Applying a
potential of -1.6 V (vs SCE) resulted in significant changes
in the absorption spectrum. The absorption at 373 nm
disappeared, and broad absorptions at 460 and 670 nm arose
with a concomitant color change of the solution to green.
These findings are qualitatively the same as those found for
the hydroquinone dianion of unsubstituted 9,10-anthraquino-
(16) Babaei, A.; Connor, P. A.; McQuillan, A. J.; Umapathy, S. J. Chem.
Educ. 1997, 74, 1200.
(17) (a) Seminario, J. M.; Zacarias, A. G.; Tour, J. M. J. Am. Chem.
Soc. 2000, 122, 3015. (b) Karzazi, Y.; Cornil, J.; Bre´das, J. L. J. Am. Chem.
Soc. 2001, 123, 10076. (c) Heurich, J.; Cuevas, J. C.; Wenzel, W.; Scho¨n,
G. Phys. ReV. Lett. 2002, 88, 256803.
(18) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902.
(19) HyperChem, release 7.5 for Windows, Molecular Modeling System;
Hypercube, Inc. and Autodesk, Inc.
Org. Lett., Vol. 8, No. 11, 2006
2335

mechanism solely determined by the topology (linear or cross
conjugated) of the conductive path.
In summary, we have prepared an anthraquinone-based
molecular wire that can be reversibly switched from cross
conjugated (low conductance “off”) to linear conjugated
(high conductance “on”). We are currently investigating the
electronic-transport properties of this redox-controlled switch
while attached to gold electrodes, for example, by electro-
chemistry, STM, and the MCBJ technique. Results of these
studies will be reported in due course.
Acknowledgment. The financial support of the Materials
Science CentrePlus is greatly appreciated. The authors wish
to thank Dr. Wesley Browne for help with the spectroelec-
trochemical measurements. The group of Professor Bart J.
van Wees and also Dr. Alex Sieval (University of Groningen)
are thanked for valuable discussions.
Figure 3. AM1-calculated representations of the electron prob-
ability distribution within the frontier orbitals of the neutral and
hydroquinone dianion of thiol end-capped 5.
Supporting Information Available: Experimental pro-
cedures and spectroscopic data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
electrons and could serve as an important model system for
MCBJ experiments. These results suggest that we have
designed a well-defined redox-active switch and a switching
OL0606278
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Org. Lett., Vol. 8, No. 11, 2006