Synthesis and properties of functionalized 4 nm scale molecular wires with thiolated termini for self-assembly onto metal surfaces

Article abstract of DOI:10.1021/jo800120n

(Chemical Equation Presented) We report the synthesis of new oligo(aryleneethynylene) molecular wires of ca. 4 nm length scale by palladium-catalyzed Sonogashira cross-coupling methodology. Key structural features are the presence of electron donor 9-(1,3-dithiol-2-ylidene)fluorene (compounds 13 and 14) and electron acceptor 9-[di(4-pyridyl)methylene]fluorene units (compound 16) at the core of the molecules. Terminal thiolate substituents are protected as cyanoethylsulfanyl (13 and 16) or thioacetate derivatives (14). The molecules display well-defined redox processes in solution electrochemical studies. The optical properties in solution are similar to those of the fluorenone analog 6: the strongest absorptions for 6, 13 and 16 are in the region λ_max = 387-393 nm, with 13 showing an additional shoulder at 415 nm which is not present for 6 and 16; this shoulder is assigned to a HOMO-LUMO transition from the dithiole to the fluorene unit. Molecules 6, 13, 14 and 16 form self-assembled monolayers on gold substrates which exhibit essentially symmetrical current-voltage (I-V) characteristics when contacted by a gold scanning tunelling microscope (STM) tip. The effects of the chemical modifications at the central unit of 6, 14 and 16 on the HOMO-LUMO levels and electron transport through the molecules in vacuum have been computed by an ab initio approach.

Full text of DOI:10.1021/jo800120n

Synthesis and Properties of Functionalized 4 nm Scale Molecular
Wires with Thiolated Termini for Self-Assembly onto Metal
Surfaces
Changsheng Wang,† Martin R. Bryce,*,† Joanna Gigon,‡ Geoffrey J. Ashwell,*,‡
Iain Grace,§ and Colin J. Lambert*,§
Department of Chemistry, Durham UniVersity, Durham DH1 3LE, U.K., The Nanomaterials Group, School
of Chemistry, Bangor UniVersity, Deiniol Street, Bangor, Gwynedd LL57 2UW, U.K., and Department of
Physics, Lancaster UniVersity, Lancaster LA1 4YB, U.K.
m.r.bryce@durham.ac.uk; g.j.ashwell@bangor.ac.uk; c.lambert@lancaster.ac.uk
ReceiVed January 18, 2008
We report the synthesis of new oligo(aryleneethynylene) molecular wires of ca. 4 nm length scale by
palladium-catalyzed Sonogashira cross-coupling methodology. Key structural features are the presence
of electron donor 9-(1,3-dithiol-2-ylidene)fluorene (compounds 13 and 14) and electron acceptor 9-[di(4-
pyridyl)methylene]fluorene units (compound 16) at the core of the molecules. Terminal thiolate substituents
are protected as cyanoethylsulfanyl (13 and 16) or thioacetate derivatives (14). The molecules display
well-defined redox processes in solution electrochemical studies. The optical properties in solution are
similar to those of the fluorenone analog 6: the strongest absorptions for 6, 13 and 16 are in the region
λmax ) 387-393 nm, with 13 showing an additional shoulder at 415 nm which is not present for 6 and
16; this shoulder is assigned to a HOMO-LUMO transition from the dithiole to the fluorene unit.
Molecules 6, 13, 14 and 16 form self-assembled monolayers on gold substrates which exhibit essentially
symmetrical current-voltage (I-V) characteristics when contacted by a gold scanning tunelling microscope
(STM) tip. The effects of the chemical modifications at the central unit of 6, 14 and 16 on the
HOMO-LUMO levels and electron transport through the molecules in vacuum have been computed by
an ab initio approach.
Introduction
memory elements and logic gates. Developing structure/property
relationships in wire-like molecules and probing their self-
assembly behavior at the electrode interface are key topics for
current investigations. Interconnection of the molecular structure
into electronic circuits is usually achieved via terminal thiol-
gold contacts.2 Scanning probe microscopy (SPM), mechanically
controllable break junction (MCBJ) and electromigration tech-
Nanometer-length conjugated organic molecules which can
be assembled onto metal or semiconductor surfaces are receiving
great attention due to their potential applications as molecular
wires in the development of molecular electronics and related
fields.1 A long-term aim of this technology is to overcome the
miniaturization threshold of silicon-based devices by utilizing
small assemblies of organic molecules, or even single molecules,
as components in conductors, rectifiers, switches, transistors,
(1) Reviews: (a) Tour, J. M. Acc. Chem. Res. 2000, 33, 791–804. (b) Carroll,
R. L.; Gorman, C. B. Angew. Chem., Int. Ed. 2002, 41, 4378–4400. (c) Maruccio,
G.; R.; Cingolani, R.; Rinaldi, R. J. Mater. Chem 2004, 14, 542–554. (d) Troisi,
A.; Ratner, M. A. Small 2006, 2, 172–181. (e) James, D. K.; Tour, J. M.
Aldrichimica Acta 2006, 39 (2), 47–56. (f) Weibel, N.; Grunder, S.; Mayor, M.
Org. Biomol. Chem. 2007, 5, 2343–2353.
† Durham University.
‡ Bangor University.
§ Lancaster University.
4810 J. Org. Chem. 2008, 73, 4810–4818
10.1021/jo800120n CCC: $40.75 2008 American Chemical Society
Published on Web 06/04/2008

Synthesis and Properties of Functionalized 4 nm Molecular Wires
CHART 1. Structures of Functionalized OPE Derivatives Reported Previously
niques have been used for the electrical characterization of the
resulting hybrid nanostructures.3 It is a major challenge to
distinguish whether properties arise from the molecule or from
the molecule-electrode interface, especially at the single-
molecule level.
groups have been synthesized for potential molecular electronics
applications. Examples include a derivative with a chiral 1,1′-
binaphthyl core,8 compound 1 with redox activity imparted by
pendant 1,3-dithiol-2-ylidene units,9 and compounds 2 and 3
with cross-conjugated anthraquinone and conjugated anthracene
cores, respectively (Chart 1).10 These molecules show interesting
optoelectronic properties in solution, but to the best of our
knowledge, their self-assembly and electrical properties in
devices have not yet been reported. Compound 4 in a MCBJ
device displays voltage-dependent conductance switching which
has been used as a single-molecule memory element by reading
and erasing bits by simple voltage pulses.11 Compound 5 is a
donor-acceptor system with two weakly coupled π-systems.
In a MCBJ device single molecules displayed weak diode-like
behavior with a rectification ratio of ca. 1:5 at (1.5 V.12 OPE
Efficient electronic communication over nanometer distances
requires molecules which possess delocalized π-systems with
low HOMO-LUMO gaps. Oligo(aryleneethynylene)s, (aryl-
CtCs)n, are an attractive class of molecules in this regard.4
They offer the advantages of rigid-rod, length-persistent struc-
tures, although it should be noted that the barrier to rotation
about the aryl-ethynyl bond is low (typically <1 kcal mol-1
for aryl ) phenylene, i.e., OPE derivatives),5 which means that
the phenylene rings are freely rotating at room temperature,
which will reduce conductance when they are rotated with
respect to each other. It has also been proposed that the short
ethynyl bond (CtC length ca. 1.22 Å) in OPE disrupts the
conjugation of the backbone more than the vinyl linkage (CdC
length ca. 1.35 Å) in OPV [oligo(phenylenevinylene)] systems,
i.e., there is increased bond alternation in OPE resulting in a
HOMO-LUMO gap larger than that in OPV.6 Several studies
have concerned OPE derivatives with terminal thiol groups6,7
and some of these molecules have been integrated into an
electronic circuit.
(6) (a) Kushmerick, J. G.; Holt, D. B.; Pollack, S. K.; Ratner, M. A.; Yang,
J. C.; Schull, T. L.; Naciri, J.; Moore, M. H.; Shashidhar, R. J. Am. Chem. Soc.
2002, 124, 10654–10655. (b) In this communication the OPE and OPV
derivatives studied experimentally have different end groups. The OPE has
terminal -C6H4-S units, whereas the OPV has terminal -C6H4-CH2-S units. For
the OPV with terminal -C6H4-S units, see: Seferos, D. W.; Trammell, S. A.;
Bazan, G. C.; Kushmerick, J. G. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8821–
8825.
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Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C. W.; Chen, J.; Wang, W. Y.;
Campbell, I. Chem. Eur. J. 2001, 7, 5118–5134. (b) Mayor, M.; Weber, H. B.;
Reichert, J.; Elbing, M.; von Ha¨nisch, C.; Beckmann, D.; Fischer, M. Angew.
Chem., Int. Ed. 2003, 42, 5834–5838. (c) Stuhr-Hansen, N.; Sørensen, J. K.;
Moth-Poulsen, K.; Christensen, J. B.; Bjørnholm, T.; Nielsen, M. B. Tetrahedron
2005, 61, 12288–12295. (d) Haiss, H.; Wang, C.; Grace, I.; Batsanov, A. S.;
Schiffrin, D. J.; Higgins, S. J.; Bryce, M. R.; Lambert, C. J.; Nichols, R. J. Nat.
Mater. 2006, 5, 995–1002. (e) Ashwell, G. J.; Urasinska, B.; Wang, C.; Bryce,
M. R.; Grace, I.; Lambert, C. J. Chem. Commun. 2006, 4706–4708. (f) Tam,
I. W.; Yan, J.; Breslow, R. Org. Lett. 2006, 8, 183–185. (g) Shi, Z.-F.; Shi;Wang,
L.-J.; Wang, H.; Cao, X.-P.; Zhang, H.-L. Org. Lett. 2007, 7, 595–598. (h)
Nilsson, D.; Watcharinyanon, S.; Eng, M.; Li, L.; Moons, E.; Johansson, L. S. O.;
Zharnikov, M.; Shaporenko, A.; Albinsson, B.; Mårtensson, J. Langmuir 2007,
23, 6170–6181. (i) Wang, C.; Batsanov, A. S.; Bryce, M. R.; Ashwell, G. J.;
Urasinska, B.; Grace, I.; Lambert, C. J. Nanotechnology 2007, 18, 044005/1-8.
(j) Huber, R.; Gonza´lez, M. T.; Wu, S.; Langer, M.; Grunder, S.; Horhoiu, V.;
Mayor, M.; Bryce, M. R.; Wang, C.; Jitchati, R.; Scho¨nenberger, C.; Calame,
M. J. Am. Chem. Soc. 2008, 130, 1080–1084.
Recently, thiol-terminated aryleneethynylenes with more
elaborate functionality either in the backbone or as pendant
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Low, P. J.; Rutter, S. R.; Beeby, A. J. Phys. Chem. A 2006, 110, 2114–2121.
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Org. Lett. 2006, 8, 355–358.
J. Org. Chem. Vol. 73, No. 13, 2008 4811

Wang et al.
SCHEME 1. Synthesis of 13 and 14
derivatives which contain a central redox-active arenediimide
unit (either pyromellitdiimide or naphthalenediimide) and are
terminated by isocyanide groups have been synthesized, al-
though their properties have not yet been reported.13 A thiol-
terminated OPV derivative with a cruciform geometry com-
prising a pyridyl-terminated OPE substructure has been studied
as a potential molecular switch in MCBJ experiments in a liquid
environment.14
In our initial studies we reported the synthesis and solution
electrochemistry of the aryleneethynylene derivative 6 with a
fluorenone core and a derivative incorporating three fluorenone
units (intramolecular S · · ·S distances of ca. 3.7 and 7.0 nm,
respectively.)15 We subsequently reported the electrical proper-
ties of a self-assembled monolayer (SAM) of a 7 nm analog
with a central 9-[di(4-pyridyl)methylene]fluorene unit.7e,i How-
ever, it is experimentally very challenging to make reliable
contact to single molecules of lengths greater than ca. 4 nm by
scanning probe or break junction techniques, thereby limiting
their applications in molecular electronics. The theoretical
modeling of longer molecules is also more time-consuming and
data are harder to interpret unambiguously. In this regard,
molecules of 1-4 nm lengths are more attractive candidates,
and we now present the synthesis of the new ca. 4 nm long
systems 13, 14 and 16 with different pendant substituents on
the central fluorene unit, namely, 1,3-dithiol-2-ylidene (electron
donor) and di(4-pyridyl)methylene (electron acceptor). This
strategy offers the possibility of adjusting the energy of the
frontier orbitals and hence the HOMO-LUMO gap, as dem-
onstrated previously for nonconjugated (diode-like) structures16
(9) Sørensen, J. K.; Vestergaard, M.; Kadziola, A.; Kilså, K.; Nielsen, M. B.
Org. Lett. 2006, 8, 1173–1176.
(10) Van Dijk, E. H.; Myles, D. J. T.; van der Veen, M.; Hummelen, J. C.
Org. Lett. 2006, 8, 2333–2336.
(11) Lo¨rtscher, E.; Ciszek, J. W.; Tour, J.; Riel, H. Small 2006, 2, 973–977.
(12) Elbing, M.; Ochs, R.; Koentopp, M.; Fischer, M.; von Ha¨nisch, C.;
Weigend, F.; Evers, F.; Weber, H. B.; Mayor, M. Proc. Natl. Acad. Sci. U.S.A.
2005, 102, 8815–8820.
(13) Mayr, A.; Srisailas, M.; Zhao, Q.; Gao, Y.; Hsieh, H.; Hoshmand-Kochi,
M.; Fleur, N. S. Tetrahedron 2007, 63, 8206–8217.
(15) Wang, C.; Batsanov, A. S.; Bryce, M. R.; Sage, I. Org. Lett. 2004, 6,
2181–2184.
(14) Grunder, S.; Huber, R.; Horhoiu, V.; Gonza´lez, M. T.; Scho¨nenberger,
C.; Calame, M.; Mayor, M. J. Org. Chem. 2007, 72, 8337–8344.
(16) Perepichka, D. F.; Bryce, M. R. Angew. Chem., Int. Ed. 2005, 44, 5370–
5373, and references therein.
4812 J. Org. Chem. Vol. 73, No. 13, 2008

Synthesis and Properties of Functionalized 4 nm Molecular Wires
SCHEME 2. Synthesis of 16, 17 and 18
and conjugated oligomers.17 We also report that 6, 13, 14 and
16 readily form SAMs on gold substrates which exhibit
symmetrical current-voltage (I-V) characteristics when con-
tacted by a gold scanning tunelling microscope (STM) tip.
Furthermore, ab initio calculations for the dithiol derivatives
of the series of molecules 6, 13 and 16 qualitatively support
the experimental data and show how the substituents modify
the HOMO-LUMO levels; the calculations also reveal that
Fano resonances can be controlled by modifying the side group.
Results and Discussion
Synthesis. A key feature of the new molecular wires 13 and
16 is the presence of the central 9-(1,3-dithiol-2-ylidene)fluorene
and 9-[di(4-pyridyl)methylene]fluorene units embedded in the
backbone which impart redox activity to the molecules (see
electrochemical section, below). Protected thiol anchor groups
are present at both terminal positions. Our synthetic strategy
was similar for both 13 (Scheme 1) and 16 (Scheme 2):
compound 10 served in both schemes as the key reagent for
2-fold couplings with the diiodo-derivatives of the core units,
viz., compounds 11 and 15, respectively. This represents a
different and more efficient protocol compared with that used
in the previous synthesis of 6.15
FIGURE 1. UV-vis absorption spectra of 6, 12, 13 and 16-18 in
chloroform solution.
Sonogashira reaction of 11. The terminal cyanoethylsulfanyl
groups of 13 were cleanly converted into the thioacetate groups
upon reaction with base and acetyl chloride7d,19 to give
analog 14.
The synthesis of 16 is outlined in Scheme 2. Compound 1518
reacted, as above, with reagent 10 to give the desired product
16 and the monosubstituted partner 17 in equal yields. A
significant side reaction on this occasion was oxidative homo-
coupling of 10 to give the symmetrical 1,3-butadiyne derivative
18 (39% yield). All three products from the reaction were
cleanly separated.
Reaction of 718 with 87d,19 under standard Sonogashira
conditions20 yielded the cross-coupled product 9 (Scheme 1).
The TMS-acetylene unit of 9 was cleanly deprotected to afford
the terminal alkyne 10 in the presence of the cyanoethylsulfanyl
group, under conditions used previously for analogs of 9.18
Reaction of 10 (2.1 equiv) with core reagent 1121 gave the
monosubstituted product 12 (29% yield) alongside the desired
symmetrical product 13 (41% yield) arising from 2-fold
Solution Optical and Electrochemical Properties. The
optical properties of the series of compounds 6, 12, 13, and
16-18 were studied in chloroform solution (Figure 1). The two
distinct bands in the absorption spectra of all of the compounds
are consistent with previous data for phenyleneethynylene
derivatives possessing dialkoxy substituents which have been
assigned to the electronic transitions from nearly identical
HOMO and HOMO-1/HOMO-2 to LUMO levels.5b A general
trend is that similar λmax values are observed for the wires of
comparable length, implying that the conjugation pathway along
the aryleneethynylene backbone is not disrupted by varying the
substituent at C(9) of the central fluorene unit. For compounds
(17) (a) Berlin, A.; Zotti, G.; Zecchin, S.; Schiavon, G.; Vercelli, B.; Xanelli,
A. Chem. Mater. 2004, 16, 3667–3676. (b) Zhang, G. L.; Ma, J.; Jiang, Y. S.
Macromolecules 2003, 36, 2130–2140.
(18) Wang, C.; Batsanov, A. S.; Bryce, M. R. J. Org. Chem. 2006, 71, 108–
116.
(19) Synthetic details are given in the supplementary information for ref 7d.
(20) (a) Sonogashira, K. J. Organomet. Chem. 2002, 653, 46–49. (b)
Chinchilla, R.; Na´jera, C. Chem. ReV. 2007, 107, 874–922.
(21) Amriou, S.; Wang, C.; Batsanov, A. S.; Bryce, M. R.; Perepichka, D. F.;
Ort´ı, E.; Viruela, R.; Vidal-Gancedo, J.; Rovira, C. Chem. Eur. J. 2006, 12,
3389–3400.
6, 13 and 16 the strongest absorptions are in the region λmax
)
387-393 nm, with 13 showing an additional shoulder at 415
nm which is not present for 6 and 16; this shoulder is assigned
J. Org. Chem. Vol. 73, No. 13, 2008 4813

Wang et al.
FIGURE 2. CVs of compounds 15, 16 and 17 in DMF solution
containing Bu4NPF6 (0.1 M), scan rate 100 mV s-1. Ered values are
shown for 15, 16, and 17. Ferrocene was added as internal reference
to the solution of 17.
FIGURE 3. Differential pulse voltammograms of compounds 12 and
13 in DMF solution containing Bu4NPF6 (0.1 M), scan rate 100 mV
s-1. Ferrocene was added as internal reference to the solutions.
to a HOMO-LUMO transition from the dithiole to the fluorene
unit.21 For the shorter (monocoupled) wires 12 and 17, the
strongest absorptions are blue-shifted to λmax ) 377 and 379
nm, respectively.
The reduction waves of the 9-[di(4-pyridyl)methylene]fluo-
rene unit in 16 showed solvent dependence. The second
reduction peak increased in intensity in THF compared to DMF,
although the reversibility was not improved (see Supporting
Information, Figure S1). Varying the scan rate (50-1600 mV
s-1) did not change the quasi-reversibility of the reductions.
The CVs of 17 obtained in THF at different scan speeds are
shown in Supporting Information (Figure S2). The relative
intensity of the second reduction wave on the reversed cathodic
scans remained unchanged at the different scan rates, which
suggests that the reductive electrochemical process of these wire
compounds is not kinetically controlled.
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) studies have probed the redox properties of the wire
molecules. We will first consider molecules 6, 15, 16 and 17
with electron-acceptor characteristics due to the fluorenone or
di(4-pyridyl)methylene-9-fluorene units. We have previously
reported15 that compound 6 displays a reversible single-electron
reduction wave at E1/2 ) -1.44 V (in DMF, vs Ag/Ag+) when
the cathodic scan was limited to -1.8 V. Scanning to more
negative potentials revealed two additional irreversible waves
at Ered ) -2.10 and -2.23 V, associated with the reduction of
the triple bonds. However, after scanning to such negative
potentials on the return scan the fluorenone reduction became
irreversible, presumably due to a chemical reaction changing
the molecular structure.
Figure 2 shows the CVs in DMF solution of 16, 17 and the
central building block 15. The reduction process for 15 is well-
defined: the two, one-electron reduction waves are reversible
with potentials at E1/2 ) -1.45 and -1.61 V, yielding the radical
anion and dianion species, respectively. This is in excellent
agreement with a diethynyl derivative of 15, which also
exhibited two reversible reduction waves.22 Similar to a 7 nm
long analog,7i the reduction of 16 gave a quasi-reversible wave
at Ered ) -1.51 V (vs Ag/Ag+) and an irreversible wave at
Ered ) -1.68 V, associated with the reduction of the 9-[di(4-
pyridyl)methylene]fluorene unit. However, the second reduction
wave of 16 is stronger (increased current) compared with those
of longer derivatives. The reduction associated with the
In contrast to the electron-accepting central units in the wires
6, 16 and 17, the 1,3-dithiol-2-ylidene unit imparted good
electron donor characteristics to molecules 12, 13 and 14. The
data for 13 and 14 were identical within experimental error,
showing that the protecting groups have no effect on the redox
chemistry. The DPVs of compounds 12 and 13 are shown in
Figure 3. (For these compounds DPV was preferable to CV
due to their limited solubility.) When the anodic scans were
limited to 0.8 V, a reversible oxidation wave corresponding to
radical cation formation was observed at ca. 0.53 V for both
compounds. (Reversibility was established in CV experiments.)
A second irreversible oxidation process, assigned to dication
formation was observed at 0.93-1.0 V: the potential for this
wave is positively shifted by 70 mV for 12 compared to 13
due either to the presence of the electron-withdrawing iodine
substituent in 12 or the extended conjugation in 13 stabilizing
its dication, which is a well-known phenomenon for π-extended
tetrathiafulvalenes. The first oxidation wave of 12 and 13
became irreversible when the anodic scans were extended
beyond 0.8 V, indicating decomposition of the dications. The
CVs of compounds 11-14 are shown in Supporting Information.
Self-Assembly and Electrical Characterization. To estab-
lish the potential of these molecules as candidates in electrical
circuits, we have investigated their self-assembly on gold-coated
substrates and performed preliminary STM experiments on the
resultant monolayer structures. We focus on the electrical
characterization of monolayers formed from 13 (cyanoethyl-
protected) and 14 (acetyl-protected); they were assembled under
mildly basic conditions, using sodium methoxide and ammonia,
respectively, to remove the protecting groups, and the process
was monitored from the frequency changes following deposition
on gold-coated 10 MHz quartz crystals. The plasma-cleaned
substrates were repeatedly immersed for 10-15 min intervals
triple bonds of 16 appeared as one irreversible peak at Ered
-2.40 V.
)
The CV profile for the monocoupled derivative 17 is in
between that of 15 (the noncoupled precursor) and 16 (the
dicoupled product). The second reduction wave of 17 (Ered
)
-1.65 V) is stronger on both the cathodic and anodic scans,
although the wave is still not reversible. However, the relative
intensity of the acetylenic reduction at -2.40 V decreased
compared to 16, agreeing with this wave arising from the
phenyleneethynylene (PE) sidearm(s) in 16 and 17. It seems
clear that the PE side-arms are responsible for the reduced
reversibility of the electrochemistry of the molecular wire 16.
(22) Wang, C.; Batsanov, A. S.; Bryce, M. R. Faraday Discuss. 2006, 131,
221–234.
4814 J. Org. Chem. Vol. 73, No. 13, 2008

Synthesis and Properties of Functionalized 4 nm Molecular Wires
FIGURE 5. I-V characteristics of SAMs of 14 formed under basic
conditions on gold contacted by a gold probe. The data were obtained
by averaging 10 scans on the same site for a set point current of 200
pA and bias of +0.2 V where the polarity is defined by the sign of the
substrate electrode.
FIGURE 4. Variation of the mean molecular area versus the total period
of immersion of gold-coated 10 MHz quartz crystals in a THF solution
of 14 (0.1 mg/mL) with (×) and without (O) addition of 2 drops of
ammonia solution.
in basic tetrahydrofuran solutions of the wires (0.1 mg mL-1
)
and thoroughly rinsed each time to remove any physisorbed
material. The frequency saturated to a constant value after ca.
2 h, and a Sauerbrey analysis23 of the data yielded areas of
1.2-1.5 nm2 molecule-1, which is consistent with the area
calculated from van der Waals’ dimensions of the chemisorbed
molecular wire assuming partial overlap of these tilted molecules
in the SAM. We note that assembly also occurs spontaneously
without the need for added base, and similar data have been
obtained for 14 in the absence of ammonia (Figure 4). The
deposition time is similar under both sets of conditions. The
areas were calculated for the molecular mass, and values at
saturation are consistent with those from molecular modeling.
It is, therefore, confidently assumed that deposition occurs to
give monolayers without the formation of disulfides, which
would significantly reduce the calculated area.
For the electrical characterization of 13 and 14 the self-
assembled monolayers (SAMs) were formed under basic condi-
tions to ensure removal of the protecting groups at both termini,
which if retained could act as a barrier to conduction. SAMs
on gold-coated highly oriented pyrolytic graphite (HOPG) were
investigated by scanning tunneling spectroscopy using a range
of set-point conditions to land the gold probe. The set-point
current and voltage have minimal effect on the profile of the
current-voltage (I-V) curves but affect the magnitude of the
tunneling current by influencing the distance between probe and
surface. Symmetrical or almost symmetrical I-V characteristics
have been obtained (Figure 5), this being expected from these
wire-like molecules, but slight electrical asymmetry is induced
for the more extreme set-point conditions.
SAMs were also investigated by the “current-jump” technique
of Haiss et al.24 Studies were performed with the gold probe
located at a fixed height above the surface and as the current
was monitored as a function of time. Films of the molecular
wires showed abrupt changes in current (Figure 6). These events
persist for ca. 50-100 ms with 30% of all jumps corresponding
to 0.12 ( 0.03 nA at 300 mV, this also being the lowest
FIGURE 6. Characteristic current jump obtained with a gold STM
probe located at a fixed height above a SAM of 14 at a bias at 300
mV. (The SAM was formed under basic conditions.)
observed current. The SAMs formed from 6 and 16 also exhibit
essentially symmetrical I-V curves as shown in Supporting
Information. The general shapes of the curves show some
differences which may be induced by different tip distances
above the monolayer surfaces as well as by the different
molecules being contacted. Sixty percent of the current jumps
of the SAMs from 6 and 16 measured at 300 mV were 0.10 (
0.05 nA and 0.17 ( 0.05 nA, respectively. Such behavior is
consistent with the formation of short-lived contacts between
the Au probe and the organic monolayer. The measured current
jumps are normally associated with a single-molecule event,24
although for molecules of this length it is unlikely that Au-S
contacts have been established between the probe and SAM.
Thus, the current jumps may not relate to the current along the
entire Au-S-wire-S-Au length but from contact via the gold
probe to the body of the chromophore which is tilted toward
the substrate with an Au-S-C angle of ca. 30°. The differences
in the HOMO and LUMO levels calculated for the series 6, 14
(23) Sauerbrey, G. Z. Phys. 1959, 155, 206–222.
(24) Haiss, W.; Nichols, R. J.; van Zalinge, H.; Higgins, S. J.; Bethell, D.;
Schiffrin, D. J. Phys. Chem. Chem. Phys. 2004, 6, 4330–4337.
J. Org. Chem. Vol. 73, No. 13, 2008 4815

Wang et al.
FIGURE 7. (Left) Energy levels of the isolated dithiol derivatives of molecule 6, 14, and 16. (Right) The spatial distribution of the HOMO and
LUMO orbitals for 6 (top) 14 (middle) and 16 (bottom) (with methoxy groups replacing the hexyloxy side chains).
and 16 (see below) did not give rise to any significant trends in
the experimental I-V profiles. Nonetheless, to have established
that this series of molecules which incorporate a significant level
of functionality can be self-assembled on gold and conductance
data can be obtained is an important first step in using these
molecules in electrical circuits.
substituent. These data are qualitatively consistent with the solution
electrochemical properties discussed above.
To calculate the transport properties of these molecules, they
were attached to gold leads; the zero bias transmission coef-
ficients are shown in Figure 8. For all three molecules the Fermi
energy (0 eV) lies closer to the HOMO resonance, and the
magnitude of the conductance is of the order 1-10 nS for all
three molecules, which broadly agrees with the STM measure-
ments which show similar conductance values for 6, 14 and
16. The main change in the magnitude occurs due to the decrease
in the gap between the HOMO and LUMO resonances. The
LUMO resonance for 6 and 16 displays a line shape typical of
a Fano resonance,28 with a resonance accompanied by an
antiresonance at ca. 1 eV, while the LUMO of 14 shows a
typical Breit-Wigner resonance. For 14 a Fano resonance
occurs at higher energy at ca. 1.8 eV. This behavior can be
attributed to the spatial distribution of the orbitals displayed in
Figure 7, with a Fano resonance occurring when the orbital is
localized on the central unit and pendant group of the molecule
(compounds 6 and 16). The Fano resonance is due to interfer-
ence between a bound state on the side group and the molecular
backbone. A Breit-Wigner resonance, on the other hand, arises
when an orbital is delocalized along the backbone of the
molecule, as seen for compound 14.
This behavior in the series 6, 14 and 16 opens up the
possibility of controlling transport through such molecules, since
Fano resonances are sensitive to geometrical and chemical
changes to the central and pendant groups.29 This sensitivity,
which is not present in Breit-Wigner resonances, suggest that
such structures, especially 6 and 16, could be used as future
single-molecule chemical sensors. To observe a dramatic change
in the conduction would require a change in the properties of
the side group causing the Fano resonance to shift closer to the
Fermi energy. This could be achieved by geometrical manipula-
tion which affected the conjugation or by a chemical change
such as protonation.
Theoretical Studies. To probe the electronic properties of 6,
14 and 16, the HOMO-LUMO levels and electron transport
through the molecules in vacuum have been computed by an ab
initio approach, using a combination of the density functional theory
(DFT) code SIESTA25 and a Green’s function scattering approach,
as encapsulated in the nonequilibrium molecular electronics
SMEAGOL code.26 Studying trends and making comparisons
between the types of orbitals displayed within a series of molecules
can provide a qualitative understanding of the experimentally
observed behavior. From a theoretical viewpoint the series 6, 14
and 16 are appealing systems where different side groups have
been chemically engineered into the central unit of the molecules.
The electron transport in many molecular devices is dominated by
conduction through broadened HOMO or LUMO states leading
to Breit-Wigner resonances.27
Figure 7presents the electronic properties of the series of
molecules 6, 14 and 16 (with methoxy groups replacing the
hexyloxy side chains); the level structure shows how the
HOMO-LUMO gap is modified by altering the side groups on
the central unit. Compounds 6 and 16 have values of ca. 1.4 eV,
whereas 14 is slightly larger at ca. 1.7 eV. The spatial distribution
of the HOMO orbital is very similar for each molecule with this
orbital delocalized along the backbone of the molecule. The main
difference occurs in the LUMO orbital: for 6 and 16 it is localized
on the central unit, whereas the LUMO of 14 is delocalized.
Compounds 6 and 16 have lower lying LUMOs than 14, which is
consistent with their stronger electron accepting properties. Com-
pound 14 has the highest lying HOMO in the series, consistent
with the electron-donating properties imparted by the dithiole
(25) Soler, J. M.; Artacho, E.; Gale, J. D.; Garcia, A.; Junquera, J.; Ordejon,
P.; Sanchez-Portal, D. J. Phys.: Condens. Matter 2002, 14, 2745–2779.
(26) Rocha, A. R.; Garcia-Suarez, V.; Bailey, S. W.; Lambert, C. J.; Ferrer,
J.; Sanvito, S. Phys. ReV. B 2006, 73, 085414.
(28) Fano, U. Phys. ReV. 1961, 49, 1866–1878.
(29) Papadopoulos, T. A.; Grace, I. M.; Lambert, C. J. Phys. ReV. B 2006,
74, 193306.
(27) Breit, G.; Wigner, E. Phys. ReV. 1936, 49, 519–531.
4816 J. Org. Chem. Vol. 73, No. 13, 2008

Synthesis and Properties of Functionalized 4 nm Molecular Wires
FIGURE 8. Zero bias transmission through dithiol derivatives of molecules 6, 14, and 16.
Conclusions
line shape typical of a Fano resonance, are potential candidates in
this regard. Attempts to experimentally demonstrate sensing of
analytes using these features as the read-out signal will also be
undertaken. Protonation of the pyridyl units of 16 is an attractive
proposition for sensing.
We have synthesized a series of oligo(aryleneethynylene)
molecular wires of ca. 4 nm length scale which are endowed with
functional groups at their cores and thiolated terminal substituents.
Their optoelectronic properties in solution have been characterized.
Systems 6, 13, 14 and 16 form self-assembled monolayers on gold
substrates which exhibit essentially symmetrical current-voltage
(I-V) characteristics when contacted by a gold scanning tunelling
microscope (STM) tip. This is an important first step in using these
molecules in electrical circuits. The central fluorenone, 9-(1,3-
dithiol-2-ylidene)fluorene and 9-[di(4-pyridyl)methylene]fluorene
units modify the HOMO-LUMO levels of the molecules. The
spatial distribution of the HOMO orbital is very similar for each
molecule with this orbital delocalized along the backbone of the
molecule. The main difference occurs in the LUMO orbital: for 6
and 16 it is localized on the central unit, whereas the LUMO of
14 is delocalized.
Future work, which is beyond the scope of this article, will
follow two themes. (i) Integrating these oligo(aryleneethynylene)
derivatives into a device architecture (e.g., a mechanically control-
lable break junction) and then applying a bias voltage to electro-
chemically charge the redox subunit may provide a means of
bringing molecular energy states into resonance with the Fermi
levels of the electrodes. If this process is reversible, an electro-
chemically driven conductance switch is obtained. However, these
are very intricate experiments, especially at a single-molecule level
and only a few experimental demonstrations of this behavior have
been reported.11,12,30 Different adsorption geometries, desorption
of the thiol from the gold surface and thermal fluctuations can be
induced by an applied bias; all these factors complicate the
interpretation of results. (ii) Ab initio calculations have shown that
the functional groups in 6, 14 and 16 impart interesting resonant
features which may open the possibility of single-molecule sensing.
Compounds 6 and 16, for which the LUMO resonance displays a
Experimental Section
General. Details of equipment and procedures are the same as
31
32
reported previously.15,18 Pd[PPh3]2Cl2 and Pd[PPh3]4 were
prepared by reported methods. N,N,N′,N′-Tetramethylethylenedi-
amine (TMEDA) was distilled over sodium metal under argon.
1-[4-(2-Cyanoethylsulfanyl)phenylethynyl]-2,5-dihexyloxy-4-
trimethylsilylethynylbenzene (9). To the solution of 87d,19 (1.01
g, 3.49 mmol) in dry THF (20 mL) were added Pd[PPh3]2Cl2 (120
mg, 5 mol%), CuI (40 mg) and triethylamine (80 mL). The mixture
was stirred at room temperature for 5 min followed by the addition
of 718 (1.40 g, 3.51 mmol) in one portion. The mixture was stirred
at room temperature for 6 h to afford a yellow suspension. The
solid was removed by suction filtration, and the filtrate was dried
by vacuum evaporation. The dark yellow solid residue was then
column chromatographed (silica gel, chloroform) and crystallized
from ethanol to afford 9 as yellow crystals (1.54 g, 79%): mp
91.3-92.3 °C; 1H NMR (400 MHz, CDCl3) δ 7.45 (d, J ) 8.4 Hz,
2H), 7.35 (d, J ) 8.4 Hz, 2H), 6.95 (s, 1H), 6.94 (s, 1H), 3.98 (m,
4H), 3.17 (t, J ) 7.2 Hz, 2H), 2.62 (t, J ) 7.2 Hz, 2H), 1.81 (m,
4H), 1.52 (m, 4H), 1.34 (m 8H), 0.90 (m, 6H), 0.26 (s, 9H); 13C
NMR (100 Hz, CDCl3) δ 154.2, 153.5, 133.7, 132.3, 130.4, 122.7,
117.7, 117.3, 116.9, 114.1, 113.8, 101.1, 100.3, 93.9, 87.2, 69.57,
69.54, 31.59, 31.54 29.8, 29.27, 29.23, 25.7, 22.6, 18.2, 14.1, 14.0,
-0.1. MS (EI) (m/z): 559.1 (M+, 100%). Anal. Calcd for
C34H45NO2SSi: C, 72.94; H, 8.10; N, 2.50. Found: C, 72.77; H,
8.08; N, 2.36.
4-[4-(2-Cyanoethylsulfanyl)phenylethynyl]-2,5-dihexyloxy-1-
ethynylbenzene (10). To a stirred solution of 9 (1.54 g, 2.75 mmol)
in dry DCM (50 mL) was added potassium carbonate powder (0.5
g) followed by methanol (30 mL) using a syringe. The mixture
was stirred for an additional 1.5 h and then suction filtered to
remove the solid K2CO3. Acetic acid (2 mL) was added to the
filtrate, and the mixture was evaporated under vacuum to yield a
yellow waxy solid, which was purified by column chromatography
(silica, chloroform) and recrystallization from methanol to afford
(30) (a) Haiss, W.; Van Zalinge, H.; Higgins, S. J.; Bethell, D.; Ho¨rbenreich,
H.; Schiffrin, D. J.; Nichols, R. J. J. Am. Chem. Soc. 2003, 125, 15294–15295.
(b) Kubatkin, S.; Danilov, A.; Hjort, M.; Cornil, J.; Bre´das, J. L.; Suhr-Hansen,
N.; Hedegård, P.; Bjørnholm, T. Nature 2003, 425, 698–701. (c) Xu, B.; Xiao,
X.; Yang, X.; Zang, L.; Tao, N. J. Am. Chem. Soc. 2005, 127, 2386–2387. (d)
Li, Z.; Pobelov, I.; Han, B.; Wandlowski, T.; Blaszcyk, A.; Mayor, M.
Nanotechnology 2007, 18, 044018. (e) Giacalone, F.; Herranz, M. A.; Gru¨ter,
L.; Gonza´lez, M. T.; Calame, M.; Scho¨nenberger, C.; Arroyo, C. R.; Rubio-
Bollinger, G.; Ve´lez, M.; Agra¨ıt, N.; Mart´ın, N. Chem. Commun. 2007, 4854–
4856.
(31) Heck, R. F. Palladium Reagents in Organic Synthesis; Academic Press:
London, 1985; p 18.
(32) Hegedus, L. S. In Organometallics in Synthesis; Schlosser, M., Ed.;
John Wiley & Sons: Chichester, 1994; p 448.
J. Org. Chem. Vol. 73, No. 13, 2008 4817

Wang et al.
1
10 as a pale yellow solid (1.13 g, 84%): mp 64.5-66.0 °C; H
31.6, 30.2, 29.5, 25.8, 22.6, 14.0, 13.1. MS (MALDI-TOF) (m/z):
1242.6 (M+). Anal. Calcd for C78H82O6S4: C, 75.32; H, 6.65. Found:
C, 75.20; H, 6.68.
NMR (300 MHz, CDCl3) δ 7.45 (d, J ) 8.5 Hz, 2H), 7.32 (d, J )
8.5 Hz, 2H), 6.97 (s, 1H), 6.96 (s, 1H), 3.96 (m 4H), 3.36 (s, 1H),
3.11 (t, J ) 7.3 Hz, 2H), 2.58 (t, J ) 7.2 Hz, 2H), 1.78 (m, 4H),
1.47 (m, 4H), 1.32 (m, 8H), 0.88 (m, 6H); 13C NMR (75 Hz, CDCl3)
δ 153.9, 153.2, 133.8, 132.0, 129.9, 122.2, 117.6, 117.4, 116.5,
114.0, 112.6, 93.9, 86.8, 82.4, 79.8, 69.4, 69.3, 31.35, 31.30, 29.4,
29.0, 28.9, 25.5, 25.4, 22.42, 22.37, 17.9, 13.85, 13.84. MS (EI)
(m/z): 487.2 (M+, 100%). Anal. Calcd for C31H37NO2S: C, 76.35;
H, 7.65; N, 2.87. Found: C, 76.29; H, 7.70; N, 2.66.
Compounds 16-18. To the solution of compound 1518 (117
mg, 0.2 mmol) in dry THF (20 mL) were added Pd[PPh3]4 (25
mg, 8.5 mol%), CuI (10 mg) and TMEDA (10 mL). The mixture
was stirred at room temperature for 5 min to afford a clear yellow
solution. A solution of compound 10 (205 mg, 0.42 mmol) in THF
(10 mL) was added dropwise at 50 °C under Ar. The mixture was
continuously heated and stirred for 48 h. The mixture was
evaporated in vacuo, and the residue was column chromatographed
on a silica column. Using hexane-DCM (1:9 v/v) as eluent yielded
compound 18 (79 mg, 39%, based on 10). Subsequent elution with
DCM-diethyl ether (4:1 v/v) afforded 16 (42 mg, 16%, based on
15) followed by 17 (31 mg, 16%, based on 15).
Compounds 12 and 13. To a suspension of compound 1121 (175
mg, 0.32 mmol) in THF (20 mL) and TMEDA (20 mL) were added
Pd[PPh3]2Cl2 (22 mg, 5 mol%) and CuI (8 mg), and the mixture
was stirred at room temperature for 5 min and then heated and
stirred in an oil bath at 70 °C, and compound 10 (0.320 g, 0.656
mmol) was added. These conditions were maintained for 3.5 h to
obtain a clear yellow solution which then turned gradually into a
yellow suspension after an additional 1 h of stirring. The solvents
were removed in vacuo, and the residue was boiled with ethanol
(50 mL) for 5 min and then suction filtered to afford an orange-
yellow solid which was columned chromatographed on silica (eluted
with chloroform) to yield two fractions. The first fraction was
crystallized from chloroform-ethanol to yield 12 as a yellow solid
(85 mg, 29%): mp 173.8-175.1 °C; 1H NMR (300 MHz, CDCl3)
δ 8.10 (s, 1H), 7.96 (s, 1H), 7.79 (d, J ) 7.9 Hz, 1H), 7.62 (d, J
) 8.1 Hz, 1H), 7.57 (d, J ) 7.9 Hz, 1H), 7.50 (d, J ) 8.4 Hz, 2H),
7.48 (d, J ) 7.8 Hz, 1H), 7.37 (d, J ) 8.5 Hz, 2H), 7.09 (s, 1H),
7.03 (s, 1H), 4.07 (t, J ) 6.5 Hz, 4H), 3.17 (t, J ) 7.3 Hz, 2H),
2.63 (t, J ) 7.3 Hz, 2H), 2.20 (s, 6H), 1.89 (m, 4H), 1.57 (m, 4H),
1.36 (m, 8H), 0.89 (m, 6H); 13C NMR (75 Hz, CDCl3) δ 153.7,
153.6, 136.2, 135.9, 133.6, 133.3, 132.3, 131.3, 130.4, 128.3, 125.5,
124.3, 122.7, 121.7, 121.2, 119.4, 117.8, 116.9, 116.8, 114.6, 113.3,
96.7, 93.9, 92.3, 87.4, 85.8, 69.63, 69.58, 31.59, 31.56, 29.8, 29.3,
25.7, 22.7, 18.2, 14.1, 13.1; λmax (CHCl3) 258, 336, 377, 455 nm.
MS (MALDI-TOF) (m/z): 905.2 (M+). Anal. Calcd for
C49H48INO2S3: C, 64.96; H, 5.34; N, 1.55. Found: C, 65.04; H,
5.35; N, 1.57.
The second fraction from the column was a mixture containing
compound 13 and the self-coupled product 18. Recrystallizing this
mixture from chloroform-toluene then from p-xylene gave 13 as a
yellow-orange solid (166 mg, 41%): mp 184.7-185.8 °C; 1H NMR
(300 MHz, CDCl3) δ 8.01 (s, 1H), 7.82 (d, J ) 7.9 Hz, 1H), 7.51
(d, J ) 7.9 Hz, 1H), 7.50 (d, J ) 8.4 Hz, 2H), 7.36 (d, J ) 8.4 Hz,
2H), 7.10 (s, 1H), 7.03 (s, 1H), 4.06 (m, 4H), 3.17 (t, J ) 7.3 Hz,
2H), 2.63 (t, J ) 7.3 Hz, 2H), 2.20 (s, 3H), 1.89 (m, 4H), 1.57 (m,
4H), 1.37 (m, 8H), 0.90 (m, 6H); 13C NMR (75 Hz, CDCl3) δ 153.7,
153.6, 137.4, 136.6, 133.7, 132.3, 130.4, 128.9, 128.4, 125.6, 124.2,
122.7, 121.6, 119.7, 117.8, 116.9, 116.8, 114.6, 113.3, 96.7, 93.9,
87.4, 85.9, 69.64, 69.57, 31.6, 29.7, 29.3, 25.7, 22.7, 18.2, 14.1,
13.1; λmax (CHCl3) 314, 387, 415(shoulder) nm. MS (MALDI-TOF)
(m/z): 1264.6 (M+). Anal. Calcd for C80H84N2O4S4: C, 75.91; H,
6.69; N, 2.21. Found: C, 75.86; H, 6.73; N, 2.27.
16: an orange oil which solidified into a yellow solid upon adding
1
hexane into its toluene solution; mp 119.6-120.8 °C; H NMR
(400 MHz, CDCl3) δ 8.76 (d, J ) 4.6 Hz, 2H), 7.66 (d, J ) 7.8
Hz, 2H), 7.49 (m, 3H), 7.36 (d, J ) 7.3 Hz, 2H), 7.33 (d, J ) 5.8
Hz, 2H), 6.97 (s, 1H), 6.91 (s, 1H), 6.84 (s, 1H), 4.01 (m, 4H),
3.16 (t, J ) 7.2 Hz, 2H), 2.62 (t, J ) 7.3 Hz, 2H), 1.83 (m, 4H),
1.53 (m, 4H), 1.36 (m, 8H), 0.90 (t, J ) 6.7 Hz, 6H); 13C NMR
(100 Hz, CDCl3) δ 153.7, 153.6, 150.9, 148.8, 140.2, 139.4, 137.6,
135.3, 133.7, 132.9, 132.3, 130.4, 127.8, 124.0, 122.7, 122.3, 119.8,
117.7, 116.9, 114.0, 113.7, 95.1, 94.0, 87.6, 86.8, 69.7, 69.6, 31.6,
29.8, 29.3, 29.2, 25.72, 25.66, 22.6, 18.2, 14.0; λmax (CHCl3) 316,
393 nm; MS (MALDI-TOF) (m/z): 1302.7 (M+). Anal. Calcd for
C86H86N4O4S2: C, 79.23; H, 6.65; N, 4.30. Found: C, 78.94; H,
6.65; N, 4.06.
18:abrightyellowsolidafterarecrystallizationfromDCM-ethanol;
1
mp 133.0-133.8 °C; H NMR (400 MHz, CDCl3) δ 7.48 (d, J )
8.3 Hz, 2H), 7.36 (d, J ) 8.3 Hz, 2H), 6.99 (s, 1H), 6.98 (s, 1H),
4.01 (t, J ) 6.8 Hz, 2H), 4.00 (t, J ) 6.8 Hz, 2H), 3.17 (t, J ) 7.3
Hz, 2H), 2.63 (t, J ) 7.2 Hz, 2H), 1.83 (m, 4 H), 1.52 (m, 4H),
1.36 (m, 8H), 0.91 (m, 6H); 13C NMR (100 Hz, CDCl3) δ 155.0,
153.5, 133.9, 132.3, 130.4, 122.6, 117.70, 117.66, 116.9, 114.8,
112.8, 94.6, 87.2, 79.5, 79.4, 69.8, 69.6, 31.6, 31.5, 29.8, 29.2, 29.1,
25.7, 25.6, 22.62, 22.58, 18.2, 14.0; λmax (CHCl3) 276, 323, 400
nm; MS (MALDI-TOF) (m/z): 972.4 (M+). Anal. Calcd for
C62H72N2O4S2: C, 76.50; H, 7.46; N, 2.88. Found: C, 76.34; H,
7.44; N, 2.80.
17: an orange solid after recrystallization from DCM-ethanol;
1
mp 112.3-114.1 °C; H NMR (400 MHz, CDCl3) δ 8.75 (d, J )
5.6 Hz, 4H), 7.62 (d, J ) 7.8 Hz, 2H), 7.47 (d, J ) 8.5 Hz, 3H),
7.41 (d, J ) 8.1 Hz, 1H), 7.35 (d, J ) 8.3 Hz, 2H), 7.30 (m, 4H),
6.96 (s, 1H), 6.91 (d, J ) 1.5 Hz, 1H), 6.90 (s, 1H), 6.84 (s, 1H),
4.00 (m, 4H), 3.16 (t, J ) 7.2 Hz, 2H), 2.62 (t, J ) 7.2 Hz, 2H),
1.81 (m, 4H), 1.52 (m, 4H), 1.35 (m, 8H), 0.89 (m, 6H); 13C NMR
(100 Hz, CDCl3) δ 153.7, 153.6, 150.9, 148.6, 148.5, 139.9, 139.8,
139.7, 139.5, 137.8, 136.7, 136.2, 134.7, 134.2, 133.7, 132.9, 132.3,
130.4, 127.7, 123.9, 123.8, 122.7, 122.5, 121.4, 119.7, 117.7, 116.8,
113.9, 113.8, 94.9, 94.0, 92.3, 87.3, 86.8, 69.65, 69.59, 31.6, 31.5,
30.3,29.8, 29.3, 29.2, 25.73, 25.65, 22.6, 18.2, 14.0; λmax (CHCl3)
321, 336, 379. MS (MALDI-TOF) (m/z): 943.3 (M+). Anal. Calcd
for C55H50IN3O2S: C, 69.98; H, 5.34; N, 4.45. Found: C, 70.16; H,
5.42; N, 4.37.
Compound 14. A solution of compound 13 (58 mg, 0.046 mmol)
in dry THF (20 mL) was cooled to -2 °C with an acetone-ice
bath. Tetrabutylammonium hydroxide solution (1.0 M in methanol,
0.1 mmol) was syringed dropwise under argon atmosphere to afford
a clear red-orange solution. Acetyl chloride (0.5 mL) was added,
and the resultant clear yellow solution was stirred at 0 °C for 5
min. The solvents were removed in vacuo and the yellow solid
residue was column chromatographed (silica, 1:1 DCM-chloroform)
to yield 14 as a yellow solid (54 mg, 94%): mp 182.4-185.0 °C;
1H NMR (300 MHz, CDCl3) δ 8.02 (s, 1H), 7.82 (d, J ) 7.9 Hz,
1H), 7.57 (d, J ) 7.9 Hz, 2H), 7.51 (d, J ) 7.9 Hz, 1H), 7.41 (d,
J ) 7.9 Hz, 2H), 7.10 (s, 1H), 7.04 (s, 1H), 7.09 (m, 4H), 2.44 (s,
3H), 2.22 (s, 3H), 1.90 (m, 4H), 1.58 (m, 4H), 1.39 (s, 8H), 0.90
(m, 6H); 13C NMR (75 Hz, CDCl3) δ 193.13, 154.1, 153.9, 142.7,
137.6, 136.8, 134.1, 132.1, 128.5, 128.2, 125.7, 124.9, 124.2, 121.9,
119.7, 117.6, 117.4, 117.3, 115.2, 96.8, 94.0, 88.0, 86.1, 70.1, 70.0,
Acknowledgment. We thank the EPSRC for funding this work.
Barbara Urasinska and Wayne D. Tyrrell are thanked for technical
assistance with the electrical measurements.
Supporting Information Available: Copies of NMR and
mass spectra; additional cyclic voltammograms and I-V data;
details of self-assembly procedures, electrical characterization and
theoretical studies. This material is available free of charge via the
Internet at http://pubs.acs.org.
JO800120N
4818 J. Org. Chem. Vol. 73, No. 13, 2008