Synthesis of oligo(phenyleneethynylene)s with vertically disposed tetrathiafulvalene units

Article abstract of DOI:10.1055/s-0030-1258385

Functionalized oligo(phenyleneethynylene)s (OPEs) show potential as molecular wires for molecular electronics. A selection of OPEs with vertically disposed extended tetrathiafulvalene (TTF) units has been synthesized by a combination of metal-catalyzed cr

Full text of DOI:10.1055/s-0030-1258385

FEATURE ARTICLE
539
Synthesis of Oligo(phenyleneethynylene)s with Vertically Disposed
Tetrathiafulvalene Units
T
etrathiafulvalene
a
C
ruciform
r
Molecul
s
es ten Jennum, Mikkel Vestergaard, Anders Holmen Pedersen, Jeppe Fock, Jonas Jensen, Marco Santella,
Jens Jørgen Led, Kristine Kilså, Thomas Bjørnholm, Mogens Brøndsted Nielsen*
Department of Chemistry & Nano-Science Center, University of Copenhagen, Universitetsparken 5, 2100 Copenhagen Ø, Denmark
Fax +45 35320212; E-mail: mbn@kiku.dk
Received 30 October 2010; revised 13 December 2010
benzene core that also incorporates acetylenic units to al-
low for further scaffolding.1a,f Selective removal of the tri-
methylsilyl group in 2 followed by oxidative dimerization
of the terminal alkyne provided access to the cruciform
Abstract: Functionalized oligo(phenyleneethynylene)s (OPEs)
show potential as molecular wires for molecular electronics. A se-
lection of OPEs with vertically disposed extended tetrathiafulvalene
(TTF) units has been synthesized by a combination of metal-cata-
lyzed cross-coupling and Wittig reactions. Two general synthetic dimer 3.1f It is worth noting that the intermediate terminal
routes were developed. In one route, the OPE scaffold was first con-
alkyne showed good stability. In previous attempts to con-
structed and finally the extended TTF units were incorporated by
struct oligomers functionalized with dithiafulvene (DTF)
Wittig reactions. In the second route, the extended TTF module was
groups, we had employed di- or tetraethynylethenes as
first prepared and subsequently incorporated into an OPE backbone
central cores. However, acetylenic scaffolding with these
by palladium-catalyzed cross-coupling reactions. The latter route
acetylenic compounds turned out troublesome on account
of the limited stability of the terminal alkyne intermedi-
ates.6
was employed for functionalization with protected thiol end-
groups. Owing to the shape of these redox-active molecules, they
are termed ‘OPE-TTF cruciforms’. The electronic properties were
investigated by UV-Vis spectroscopy and by cyclic voltammetry.
R
Key words: alkynes, conjugation, cross-coupling, sulfur, Wittig re-
action
R
S
n
S
OPE
Cruciform molecules based on two perpendicularly dis-
posed p-systems are attractive as molecular wires and
switches for molecular electronics and advanced
materials1 owing to their multiple linear and cross-conju-
gated pathways for p-electron delocalization. Calcula-
tions have revealed that cross-conjugated molecules
exhibit distinctly different conductance characteristics to
both linearly conjugated and saturated molecules.2 By in-
corporation of redox-active units as vertical units in cruci-
form molecules, it may be possible to swap between
linearly and cross-conjugated pathways along the wire via
the redox state of these units. Oligo(phenyleneethy-
nylene)s (OPEs) are an important class of molecules that
have been employed as wires for molecular electronics.3
We aim to develop synthetic protocols for functionalizing
OPEs with vertically disposed tetrathiafulvalene (TTF)
units, hereby generating cruciform-like OPE-TTFs.1a,f
TTF is a good electron donor that can be oxidized revers-
ibly in two one-electron oxidation steps, and it has, for this
reason, been widely explored in molecular sensors,
switches, and devices.4
X
Y
S
S
S
S
S
TTF
S
S
R
R
R = CO2Me
1 X = Y = SiMe3
S
DTF
2 X = SiMe3, Y = Sii-Pr3
R
R
R
R
S
S
S
S
i-Pr3Si
Sii-Pr3
S
S
S
S
R
R
The construction of OPE-TTF scaffolds requires suitable
molecular building blocks. Benzene-extended TTFs were
reported previously in 1991 by Sallé and co-workers.5 Re-
cently we developed efficient synthetic protocols to ob-
tain extended TTFs 1 and 2 (Figure 1) based on a central
R = CO2Me
3
R
R
Figure 1
In an alternative strategy, the dimer 3 was prepared by a
fourfold Wittig reaction from the corresponding tetraalde-
hyde.1f In this protocol the central wire is first constructed
with suitable functional groups, and in the final step these
SYNTHESIS 2011, No. 4, pp 0539–0548
x
x
.
x
x
.2
0
1
1
Advanced online publication: 23.12.2010
DOI: 10.1055/s-0030-1258385; Art ID: E28610SS
© Georg Thieme Verlag Stuttgart · New York

540
K. Jennum et al.
FEATURE ARTICLE
Biographical Sketches
Karsten Jennum completed his M.Sc.
in 2009 in organic synthesis under the
supervision of Prof. Mogens Brøndst-
ed Nielsen. He is currently pursuing a
Ph.D. aimed at new molecules for mo-
lecular electronics.
Mikkel Vestergaard is currently em-
ployed as a research scientist at Nuevo-
lution a/s. He completed his M.Sc. in
2009 in organic synthesis at the Uni-
versity of Copenhagen under the super-
vision of Prof. Mogens Brøndsted
Nielsen.
Anders Holmen Pedersen completed
his Ph.D. in 2004 at the Department of
Chemistry, University of Copenhagen
under the supervision of Prof. Steen
Hammerum, followed by postdoctoral
work at the Faculty of Pharmaceutical
Science and at Department of Chemis-
try, University of Copenhagen.
Jeppe Fock completed his M.Sc.
project in 2008 in physical chemistry
and spectroscopy under the supervi-
sion of Prof. Kristine Kilså. He is cur-
rently pursuing a Ph.D. investigating
the properties of molecules for molec-
ular electronics in solution and in de-
vices.
Jonas Jensen completed his B.Sc. in
2008 in NMR spectroscopy under su-
pervision of Profs Jens Jørgen Led and
Mogens Brøndsted Nielsen. He is cur-
rently pursuing his M.Sc. project in
NMR spectroscopy at the University of
Copenhagen.
Marco Santella completed his B.Sc. in
2009 at the University of Florence. He
is currently pursuing his M.Sc. project
in organic synthesis at the University
of Copenhagen under the supervision
of Prof. Mogens Brøndsted Nielsen,
aimed at new molecules for molecular
electronics.
Jens Jørgen Led is emeritus docent at
the University of Copenhagen. He re-
ceived his Ph.D. in chemistry from the
University of Copenhagen 1968. He
was associated professor at the Depart-
ment of Chemistry University of
Copenhagen from 1968–1988, and do-
cent from 1988–2008. He was a visit-
ing research scientist at the University
of Utah and California Institute of
Technology 1971–1974, and at the
Australian National University in
1986. He was Chairman of the Ph.D.
board at the Department of Chemistry
1987–1996, the Computer Center at the
Department of Chemistry 1982–1987,
director of the Protein NMR Center at
the Department of Chemistry 1987–
1993 and the user group at the Danish
Instrument Center for NMR Spectros-
copy of Biological Macromolecules
1996–2008. His research focuses on
biophysical NMR studies of the stabil-
ity, aggregation, and function of pro-
tein hormones and blue copper
proteins, in particular human growth
hormone, insulin, and plastocyanin.
Kristine Kilså did her undergraduate
chemistry studies at University of
Copenhagen, but moved to Chalmers
University of Technology, Göteborg,
Sweden for her Ph.D. (2000), which fo-
cused on light-induced energy and
electron transfer. She worked at the
California Institute of Technology and
at Uppsala University, Sweden, before
returning to the University of Copen-
hagen in 2004 as Associate Professor.
She is also currently Deputy Head for
teaching at the Department of Chemis-
try.
Thomas Bjørnholm completed his
Ph.D. in materials chemistry in 1990
from the University of Copenhagen. In
1992 he became affiliated with the De-
partment of Chemistry at the Universi-
ty of Copenhagen as Associate
Professor, in 2000 as Research Profes-
sor and in 2005 as full Professor. In
1997 he was a visiting professor at the
University of Texas at Austin. He was
Director of the Nano-Science Center at
the University of Copenhagen 2001–
2010. In 2010 he was appointed as Pro-
rector of the University of Copen-
hagen. His main research interests lie
within the area of organic electronics,
single molecule science, nanochemis-
try, and bionanotechnology.
Mogens Brøndsted Nielsen is Profes-
sor of organic chemistry at the Univer-
sity of Copenhagen. He received his
Ph.D. in 1999 from the University of
Southern Denmark under supervision
of Prof. Jan Becher within the field of
supramolecular chemistry. During his
Ph.D. he spent one year with Prof. J.
Fraser Stoddart at UCLA. He did two
years of postdoctoral work with Prof.
François Diederich at ETH in Zürich
and returned in 2002 to the University
of Southern Denmark as Assistant Pro-
fessor. In 2004 he moved to the Univer-
sity of Copenhagen as Associate
Professor and was promoted in 2008 to
Professor. His current research inter-
ests cover acetylenic scaffolding with
redox-active tetrathiafulvalenes and di-
hydroazulene photoswitches, targeting
molecules for molecular electronics.
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

FEATURE ARTICLE
Tetrathiafulvalene Cruciform Molecules
541
CO2R
R = Et
groups are converted into dithiafulvene (DTF) units. We
have now employed these two synthetic approaches for
the preparation of different OPE3- and OPE5-TTFs,
which may be used as redox-controlled wires for molecu-
lar electronics. Scheme 1 depicts the two general routes
(A and B) for building the OPE-TTFs that were pursued.
The optical and electrochemical properties of the com-
pounds were also investigated.
Y = SiMe3: 4
i-Pr3Si
Y
i)
R = Me
Y = H: 5
RO2C
ii)
CO2Me
CO2Me
i-Pr3Si
Sii-Pr3
O
R
R
MeO2C
MeO2C
6
S
S
n
PR'3
iii)
O
route A
CH2OH
CH2OH
R
R
i-Pr3Si
Sii-Pr3
S
S
HOH2C
HOH2C
7
iv)
n
CHO
CHO
S
OPE-TTF
route B
S
i-Pr3Si
Sii-Pr3
R
OHC
OHC
R
8
R
v)
R
S
R
R
S
R
R
S
S
I
I
S
S
i-Pr3Si
Sii-Pr3
S
S
S
S
R
9
S
S
R
R
R
R
R
Scheme 1 Retrosynthetic analysis of OPE-TTFs
Scheme 2 Reagents and conditions: (i) K2CO3, MeOH, THF, 80%;
(ii) 4-IC6H4I, PdCl2(PPh3)2, CuI, Et3N, THF, 73%; (iii) DIBAL-H,
THF, 96%; (iv) PCC, CH2Cl2, 62%; (v) 10, Et3N, MeCN, THF, 63%.
Firstly, we synthesized an OPE3 with two vertically dis-
posed TTFs according to the general route A described
above (Scheme 2). The differentially protected diethynyl-
benzene derivative 41f was selectively desilylated using
potassium carbonate in methanol–tetrahydrofuran to pro-
vide the terminal alkyne 5. The conditions also led to
transesterification of both ethyl ester groups into methyl
esters. A twofold Sonogashira cross-coupling reaction7
between this alkyne and 1,4-diiodobenzene furnished
OPE3 6. The four ester groups were then reduced with di-
isobutylaluminium hydride, affording the tetraol 7 that
was subsequently oxidized with pyridinium chlorochro-
mate to afford the tetraaldehyde 8. Finally, fourfold Wittig
reaction provided OPE3-TTF 9, employing the phospho-
rus ylide generated from the readily obtainable phospho-
nium salt 108 (Figure 2).
MeO2C
H
S
BF4
PBu3
S
MeO2C
10
Figure 2
The next objective was to synthesize an OPE5 with two
vertically disposed TTF units. Here we chose the general
route B (vide supra), starting from the extended TTF
building block 2 (Scheme 3). Selective desilylation fol-
lowed by Sonogashira cross-coupling reaction with ex-
cess 1,4-diiodobenzene afforded OPE2 11. A twofold
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

542
K. Jennum et al.
FEATURE ARTICLE
Sonogashira cross-coupling reaction between this iodide
and 1,4-diethynylbenzene9 gave OPE5-TTF 12. This
compound showed good solubility in chlorinated sol-
vents. It was not stable, however, for a prolonged time in
solution, which limited its detailed characterization.
1
i), ii)
R
R
R
S
R
S
S
S
AcS
SAc
i), ii)
2
i-Pr3Si
I
S
R = CO2Me
S
S
R
R = CO2Me
S
13
i-Pr3Si
R
S
R
R
11
R
R
Scheme 4 Reagents and conditions: (i) K2CO3, MeOH, THF; (ii) 4-
BrC6H4SAc, Pd(Pt-Bu3)2, CuI, i-Pr2NH, THF, toluene, 32% (2 steps).
S
S
extended to longer alkyl esters (Table 1). These molecules
were made under standard conditions for desilylation
(K2CO3, alcohol, THF), but the scope of the reaction de-
pended strongly on the size of the alcohol. By using small,
non-bulky alcohols, we were able to desilylate compound
1 within two hours at room temperature (entries 1 and 2).
Treatment with propan-1-ol resulted additionally in trans-
esterification to provide the final product 14 (Figure 3).
When using longer alcohols (entries 3 and 4), the desily-
lation reaction was retarded significantly or even ceased.
Thus, using decan-1-ol the transesterified, but non-desily-
lated, product 15 (Figure 3) was obtained. Yet, desilyla-
tion of this compound was conveniently accomplished
simply by adding tetrabutylammonium fluoride to the re-
action mixture.
S
iii)
R
R
R = CO2Me
12
R
S
R
S
CO2Pr
S
CO2C10H21
S
PrO2C
C10H21O2C
S
S
S
S
R
R
Sii-Pr3
Me3Si
SiMe3
Scheme 3 Reagents and conditions: (i) K2CO3, MeOH, THF; (ii) 4-
IC6H4I, PdCl2(PPh3)2, CuI, Et3N, THF, 30% (two steps); (iii) 4-
HC≡C–C6H4–C≡CH, PdCl2(PPh3)2, CuI, Et3N, THF, 26%.
S
S
S
S
CO2Pr
CO2C10H21
For application of OPE-TTF cruciforms as redox-con-
trolled wires for molecular electronics, protected thiol
end-groups were incorporated. First, we subjected 1 to
desilylation followed by cross-coupling reaction with an
S-acetyl-protected 4-bromobenzenethiol, which gave the
cruciform wire 13 incorporating two protected thiol
groups (Scheme 4). This compound was previously syn-
thesized in a slightly different manner,1a namely by treat-
ing the related cruciform containing tert-butyl-protected
thiol groups with boron tribromide and acetyl chloride.
PrO2C
C10H21O2C
14
15
Figure 3
Compound 14 was subjected to a twofold cross-coupling
reaction with a large excess of 1,4-diiodobenzene
(Scheme 5), providing the OPE3 16 with two iodophenyl
functionalities for further scaffolding.
In addition, compound 14 was treated with excess 1-iodo-
4-[(trimethylsilyl)ethynyl]benzene in a Sonogashira reac-
tion (Scheme 6) to provide OPE3 17 incorporating two
To increase the solubility of the product obtained from
desilylation of compound 1, we developed a small series
of extended TTFs for which the methyl ester groups were
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

FEATURE ARTICLE
Tetrathiafulvalene Cruciform Molecules
543
Table 1 Desilylation and Transesterification of Compound 1 with Different Alcohols (K2CO3, THF)
Alcohol
MeOH
Desilylation
yes
Transesterification
Temp, time
r.t., 45 min
r.t., 2 h
Yield (%)
98a
–
PrOH
yes
yes
90 (14)
56b
BuOH
yes (slow)
no
yes
40 °C, 5 h
40 °C, 18 h
Me(CH2)9OH
a Ref. 1a.
yes (slow)
47 (15)
b The yield was estimated from NMR data and corresponds to both transesterification and desilylation.
14
1
i)
i)
R
SiMe3
R
S
R
R
S
S
S
S
R
S
S
R = CO2Pr
17
R = CO2Pr
S
Me3Si
R
R
14
R
Scheme
6
Reagents and conditions: (i) 4-IC6H4CCSiMe3,
PdCl2(PPh3)2, CuI, Et3N, THF, 62%.
ii)
R
17
SAc
R
S
i), ii)
S
I
I
R
S
R
S
S
R = CO2Pr
S
R
16
R
S
R
Scheme 5 Reagents and conditions: (i) K2CO3, PrOH, THF, 90%;
(ii) 4-IC6H4I, PdCl2(PPh3)2, CuI, Et3N, THF, 53%.
S
R
protected alkyne units. Compounds 16 and 17 are useful
building blocks for further scaffolding. Thus, desilylation
of 17 followed by cross-coupling reaction with excess 1-
(acetylthio)-4-iodobenzene (18) provided OPE5 cruci-
form 19 with two acetyl-protected thiol end-groups
(Scheme 7). Thereby we succeeded in developing two
OPE wires (13 and 19) with a single vertically disposed
TTF unit and with different lengths between the two sul-
fur anchoring groups.
R = CO2Pr
19
AcS
Scheme 7 Reagents and conditions: (i) TBAF, CH2Cl2, r.t.; (ii) 4-
IC6H4SAc (18), PdCl2(PPh3)2, CuI, i-Pr2NH, THF, 36% (2 steps).
from the commercially available 4-iodobenzenesulfonyl
chloride (20) (Scheme 8). Reduction of 20 with triphe-
nylphosphine in toluene10 provided 4-iodobenzenethiol
(21) in moderate yield. This method is a convenient alter-
native to monolithiation of 1,4-diiodobenzene with tert-
butyllithium followed by addition of elementary sulfur.11
Compound 21 was then converted quantitatively into the
Finally, we became interested in developing an improved
synthesis of the known compound 18, which is extensive-
ly employed for incorporating molecular ‘alligator clips’
into molecular wires. We found that its synthesis could be
carried out in two steps under mild conditions starting
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

544
K. Jennum et al.
FEATURE ARTICLE
Table 2 Assignment of 13C NMR Resonances for Compound 9
(CDCl3)
more stable acetylthio compound 18. In addition the syn-
thesis of 18 is shortened by two steps compared to the lat-
est synthetic improvement involving a Newman–Kwart
thiocarbamate rearrangement at high temperature.12
13C NMR (d)
Assignmenta
CH(CH3)3
CH(CH3)3
CO2CH3
11.5
O
18.9
SO2Cl
SH
S
53.5, 53.6b
i)
ii)
89.7
C≡C 2 or 9
C≡C 10
96.3
I
20
I
21
I
18
99.4
C≡C 1
Scheme 8 Reagents and conditions: (i) Ph3P, toluene, 60 °C, 48%;
(ii) AcCl, py, 99%.
104.7
C≡C 2 or 9
C=CH 13 or 14
Ar 5 or 8
Ar 5 or 8
Ar 11
112.7
We subjected the cruciform 9 to detailed NMR spectro-
scopic analysis employing Heteronuclear Single Quantum
Coherence (HSQC), Heteronuclear Multiple Quantum
Coherence (HMQC), and Attached Proton Test (APT)
methods. All chemically inequivalent nuclei in the cruci-
form 9 gave rise to separate signals in both the 1H and 13C
NMR spectra. The result of the analysis is listed in Table 2
and Figure 4. The signals for the two different carbon nu-
clei of the central phenylene ring were readily assigned.
While the signals originating from the six different carbon
nuclei of the outer C6H2 rings were unequivocally identi-
fied, they could only be partly assigned to the individual
nuclei. The six different carbon nuclei of the two DTF
rings were assigned to the resonances at d = 129.7, 129.8,
131.5, 131.7, 133.5, and 133.8. The two latter signals pre-
sumably correspond to the C2 of the 1,3-dithiole rings,
even though no HMQC cross-peaks to the fulvene protons
were observed.
121.3
122.2
123.2
128.5
Ar 4 or 7
Ar 4 or 7
DTF ring-C
Ar 12
129.5
129.7, 129.8, 131.5, 131.7
131.8
133.5, 133.8
DTF ring-C
Ar 3, 6
134.8
159.8, 160.2
CO2CH3
a For labelling, see Figure 4.
b Four different carbon nuclei (i.e., overlapping signals).
The absorption spectra of OPE-TTF cruciforms 9, 14, and
19 dissolved in CH2Cl2 are revealed in Figure 5. OPE3 9
shows a broad absorption in the range 350–500 nm with
maxima at 354, 382 (e = 60,300 M–1 cm–1), and 416 nm
(broad). By comparison with the extended TTF 14
(lmax = 424 nm, e = 67,400 M–1 cm–1), the broad absorp-
tion at 416 nm is attributed to the extended TTF units.
OPE5 19 shows a very strong absorption at 365 nm with
a molar absorptivity of 145,000 M–1 cm–1 ascribed to the
OPE scaffold which is in accordance to previously report-
ed data for the OPE5 system.13 In addition, compound 19
exhibits a broad shoulder absorption at 426 nm which is
assigned to the TTF moiety.
CO2Me
S
CO2Me
S
MeO2C
DTF
MeO2C
S
S
Figure 5 UV-Vis absorption spectra (CH2Cl2) of compound 9, 14,
and 19
14
5
12
12
4
3
11
i-Pr3Si
Sii-Pr3
6
7
The electrochemistry of compound 9 in 0.1 M tetrabutyl-
ammonium hexafluorophosphate in dichloromethane was
studied by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV). The compound underwent irrevers-
ible oxidations at 0.48 and 0.86 V vs Fc+/Fc (Figure 6,
curves A). Electrolysis at 1.2 V followed by electrolysis at
0 V vs Fc+/Fc provided a new species exhibiting quasi-re-
1
2
9
10
8
13
S
S
DTF
S
S
CO2Me
CO2Me
MeO2C
MeO2C
Figure 4
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

FEATURE ARTICLE
Tetrathiafulvalene Cruciform Molecules
545
melting point apparatus or a silicone oil bath apparatus and are un-
corrected. 1H, 13C, APT, HMQC, HMBC NMR spectra were record-
ed on 300 or 500 MHz instruments using an internal deuterium lock.
UV-Vis spectra were obtained using a Varian CARY50 spectropho-
tometer, and cuvette path lengths of 1 cm. Spectra are corrected for
solvent absorption (background). Calculated extinction coefficients
have uncertainties estimated to 10% due to limited sample quanti-
ties. Cyclic voltammograms and differential pulse voltammograms
were measured in a 3-electrode setup using a CHI-potentiostat, with
platinum as the working electrode, a Pt wire as the counter elec-
trode, and Ag/Ag+ was used as reference electrode. However, all
potentials are reported against ferrocene (II/III) measured under the
same conditions. All voltammograms are measured in CH2Cl2 with
0.1 M Bu4NPF6 as supporting electrolyte. Spectra of oxidized spe-
cies (spectroelectrochemistry) were measured using a Varian
versible electrochemistry (Figure 6, curves B). According
to DPV the new species is oxidized at 0.23 V. The UV-Vis
absorption spectrum of the product (Figure 6, inset)
showed a redshifted absorption maximum relative to that
of 9 and an increase in the molar absorptivity, which sug-
gests the formation of a more conjugated system. Com-
pound 13 showed similarly irreversible oxidations (at ca.
0.50 and 0.85 V vs Fc+/Fc). Electrolysis at 0.9 V followed
by electrolysis at 0 V vs Fc+/Fc provided again a new spe-
cies with a more reversible electrochemical nature. This
product of electrolysis had a similar first oxidation poten-
tial (0.20 V) as that formed from 9. In all, the electrochem-
istry of the cruciform molecules is rather complicated and
an interpretation of the results requires further experi- CARY50 spectrophotometer, and cuvette path lengths of 1 mm. The
working electrode was here a platinum grid immersed in the cu-
ments, which are outside the scope of this paper.
vette, and the potential was kept fixed during the measurement,
again using a platinum wire as the counter electrode, and Ag/Ag+ as
the reference electrode.
Compound 5
To a soln of 4 (0.563 g, 1.13 mmol) in THF (20 mL) and MeOH (10
mL) was added K2CO3 (0.138 g, 1.00 mmol). The mixture was
stirred under argon at r.t. for 8 h and then poured into CH2Cl2 (150
mL). The mixture was washed with H2O (3 × 100 mL) and sat. aq
NaCl (75 mL), dried (MgSO4), filtered, and concentrated in vacuo
to furnish the product (0.360 g, 80%) as a yellow solid; mp 78–79
°C.
1H NMR (300 MHz, CDCl3): d = 8.10 (s, 1 H), 8.08 (s, 1 H), 3.96
(s, 3 H), 3.92 (s, 3 H), 3.50 (s, 1 H), 1.15 (m, 21 H).
13C NMR (75 MHz, CDCl3): d = 165.7, 165.4, 136.7, 136.6, 135.3,
134.8, 123.5, 122.0, 103.6, 100.3, 84.7, 81.0, 52.8, 18.8, 11.4.
MS (FAB): m/z = 399 [M + H+].
Anal. Calcd for C23H30O4Si: C, 69.31; H, 7.59. Found: C, 69.34; H,
7.66.
Figure 6 Cyclic voltammetry (top) and differential pulse voltam-
metry (bottom) of 9 (labeled A). Electrolysis at 1.2 V followed by
OPE3 Tetraester 6
electrolysis at 0 vs Fc+/Fc provided the voltammograms labeled B.
The inset shows the absorption spectra of 9 before and after electro-
lysis. Counter electrode: Pt wire; Working electrode Pt disk; Refe-
rence electrode: Ag/Ag+ (10mM). Scan rate for cyclic voltammetry:
0.1 V/s. Solvent: CH2Cl2. Counter electrolyte: 0.1 M Bu4NPF6.
Compound 5 (0.447 g, 1.120 mmol), 1,4-diiodobenzene (0.123 g,
0.374 mmol), PdCl2(PPh3)2, and CuI (0.046 g, 0.241 mmol) were
dissolved in degassed THF (20 mL), whereafter degassed Et3N (10
mL) was added under argon. The mixture was stirred under argon
for 4 h and then poured into CH2Cl2 (200 mL). The mixture was
washed with sat. aq NH4Cl (2 × 150 mL), dried (MgSO4), filtered,
and concentrated in vacuo. The residue was subjected to column
In conclusion, stepwise protocols for the synthesis of sev-
eral novel OPE-TTFs have been developed. In one route, chromatography (silica gel, 10% EtOAc–heptane) to provide the
product (0.237 g, 73%) as a yellow solid; mp 175–176 °C.
1H NMR (300 MHz, CDCl3): d = 8.13 (s, 4 H), 7.57 (s, 4 H), 4.00
(s, 6 H), 3.94 (s, 6 H), 1.16 (s, 42 H).
13C NMR (75 MHz, CDCl3): d = 165.9, 165.5, 136.9, 135.8, 135.3,
134.0, 132.0, 123.5, 123.0, 122.9, 103.9, 100.1, 96.5, 89.7, 52.8
(two overlapping signals), 18.8, 11.2.
the OPE is first constructed with aldehyde groups along
the wire, which in a final step are subjected to Wittig re-
actions to furnish the dithiafulvene functionalities. In an-
other route, extended TTFs incorporating acetylenic units
are linked together via palladium-catalyzed cross-cou-
pling reactions. While an OPE3 scaffold with two verti-
cally disposed TTFs displayed good stability, the related
OPE5 decomposed after prolonged time in solution. In
contrast, OPE3 and OPE5 scaffolds with one central TTF
unit and protected thiol end-groups exhibited good stabil-
ity and are currently subject to single-molecule conductiv-
ity studies.
MS (FAB): m/z = 872 [M + H+].
Anal. Calcd for C52H62O8Si2: C, 71.69; H, 7.17. Found: C, 71.25; H,
7.34.
OPE3 Tetraol 7
A soln of 6 (0.210 g, 0.241 mmol) in THF was cooled to 0 °C. 1 M
DIBAL-H in hexane (2.41 mL) was added dropwise over 5 min, and
the mixture was then allowed to reach r.t. After stirring for 2 h, the
mixture was poured into CH2Cl2 (200 mL). The mixture was
washed with sat. aq NH4Cl (2 × 100 mL), dried (MgSO4), filtered,
and concentrated in vacuo to provide the product (0.175 g, 96%) as
a white solid; mp 191–195 °C.
All palladium-catalyzed reactions were performed in degassed sol-
vents, by sonicating the solvent with argon bubbling. TLC was car-
ried out on commercially available pre-coated plates (Merck 60
F
254). Melting points were measured on a microscope hot stage
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

546
K. Jennum et al.
FEATURE ARTICLE
1H NMR (300 MHz, DMSO-d6): d = 7.65 (s, 4 H), 7.64 (s, 2 H), 7.56
(s, 2 H), 5.44 (t, J = 5.6 Hz, 2 H), 5.43 (t, J = 5.9 Hz, 2 H), 4.71 (d,
J = 5.9 Hz, 4 H), 4.66 (t, J = 5.6 Hz, 4 H), 1.12 (s, 42 H).
13C NMR (75 MHz, DMSO-d6): d = 142.9, 142.5, 131.8, 129.7,
129.0, 122.6, 119.7, 119.6, 104.1, 96.9, 94.9, 89.0, 60.7 (two over-
lapping signals), 18.5, 10.7.
to provide the product (0.093 g, 30%) as a red solid; mp 142–144
°C.
1H NMR (300 MHz, CDCl3): d = 7.59 (d, J = 8.3 Hz, 2 H), 7.54 (2
s, 2 H), 7.28 (d, J = 8.3 Hz, 2 H), 6.95 (s, 1 H), 6.85 (s, 1 H), 3.95–
3.81 (m, 12 H), 1.21–1.01 (m, 21 H).
13C NMR (75 MHz, CDCl3): d = 160.1 (three overlapping signals),
159.9, 137.8, 135.2, 134.8, 133.8, 133.5, 133.2, 131.7, 131.5, 129.7
(two overlapping signals), 129.4, 128.4, 122.4, 122.1, 121.2, 112.7
(two overlapping signals), 104.7, 99.4, 95.6, 94.9, 88.9, 53.6 (two
overlapping signals), 53.5, 53.5, 18.9, 11.5.
MS (FAB): m/z = 759 [M + H+].
Anal. Calcd for C48H62O4Si2: C, 75.94; H, 8.23. Found: C, 75.58; H,
8.55.
OPE3 Tetraaldehyde 8
MS (MALDI-TOF): m/z = 944 (M+).
To a mixture of 7 (0.160 g, 0.211 mmol) in anhyd CH2Cl2 (40 mL)
were added PCC (0.318 g, 1.46 mmol), 4 Å molecular sieves (ca. 30
spheric pieces), and Celite (2 g). The mixture was stirred at r.t. for
3 h under argon and then filtered through a short plug of silica gel
(under suction). The silica gel was washed with CH2Cl2 (120 mL),
and the combined organic filtrates were concentrated in vacuo to
provide the product (0.098 g, 62%) as a yellow solid.
1H NMR (300 MHz, CDCl3): d = 10.63 (s, 2 H), 10.62 (s, 2 H), 8.18
(s, 2 H), 8.13 (s, 2 H), 7.61 (s, 4 H), 1.16 (s, 42 H).
13C NMR (75 MHz, CDCl3): d = 190.5, 190.3, 138.9, 138.3, 133.4,
132.3, 132.2, 126.7, 125.7, 123.0, 103.3, 100.7, 98.1, 86.5, 18.8,
11.3.
Anal. Calcd for C41H41IO8S4Si: C, 52.11; H, 4.37. Found: C, 52.30;
H, 4.45.
OPE5-TTF 12
Compound 11 (0.043 g, 0.045 mmol) was mixed with 1,4-dieth-
ynylbenzene (2.5 mg, 0.022 mmol), PdCl2(PPh3)2 (5 mg, 0.0011
mmol, 5 mol%), and CuI (1 mg, 0.005 mmol, 20 mol%) under ar-
gon. Then argon-degassed THF (4 mL) and argon-degassed Et3N (1
mL) were added. After stirring at r.t. for 20 h, the mixture was
poured into CH2Cl2 (30 mL). The mixture was passed through a
short plug of silica gel and then subjected to repeated column chro-
matography (silica gel, CH2Cl2 to 5% EtOAc–CH2Cl2) to provide
the product (10 mg, 26%) as a red solid; mp 184–186 °C.
MS (FAB): m/z = 752 [M + H+].
1H NMR (300 MHz, CDCl3): d = 7.67–7.51 (m, 12 H), 7.49 (d,
J = 2.7 Hz, 4 H), 7.02 (s, 2 H), 6.95 (s, 2 H), 3.97–3.68 (4 s, 24 H),
1.21 (2 s, 42 H).
Anal. Calcd for C48H54O4Si2: C, 76.40; H, 7.25. Found: C, 76.38; H,
7.79.
MS (MALDI-TOF): m/z = 1758/1759/1760/1761/1762/1763/1764/
OPE3-TTF 9
1765 [M+/M + H+/isotopologues].
Compound 8 (0.090 g, 0.120 mmol) and the phosphonium salt 10
(0.304 g, 0.600 mmol) were dissolved in anhyd THF (10 mL) and
anhyd MeCN (3 mL). Then Et3N (0.117 mL, 0.725 mmol) was add-
ed, and the mixture was stirred at r.t. for 3 h, whereafter it was
poured into CH2Cl2 (200 mL). The mixture was then washed with
H2O (2 × 100 mL) and sat. aq NaCl (100 mL), dried (MgSO4), fil-
tered, and concentrated in vacuo. The residue was subjected to col-
umn chromatography (silica gel, CH2Cl2 to 5% EtOAc–CH2Cl2) to
furnish the product (117 mg, 63%) as a red solid; mp 130–132 °C.
OPE3-TTF 13
Desilylated 1 (20.6 mg, 0.035 mmol), 1-(acetylthio)-4-bromoben-
zene (40 mg, 0.17 mmol), CuI (1.3 mg, 0.007 mmol), and
Pd(PtBu3)2 (1.9 mg, 0.004 mmol) were dissolved in degassed i-
Pr2NH (0.6 mL), toluene (1 mL), and THF (1 mL) under argon.
Then the mixture was subjected to ultrasonication at 50 °C for 90
min. The mixture was evaporated onto Celite and purified by anhyd
column chromatography (1. 0 to 100% CH2Cl2–heptane; 2. 0 to
10% MeOH–CH2Cl2), providing the product (10 mg, 32%) as a red
solid. The analytical data were in accordance to those previously re-
ported.1a
1H NMR (300 MHz, CDCl3): d = 7.57 (s, 4 H), 7.48 (d, J = 0.6 Hz,
2 H), 7.47 (d, J = 0.6 Hz, 2 H), 6.98 (s, 2 H), 6.92 (s, 2 H), 3.88 (s,
3 H), 3.87 (2 s, 6 H), 3.85 (s, 3 H).
13C NMR (125 MHz, CDCl3): d = 160.2, 159.8, 134.8, 133.8, 133.5,
131.8, 131.7, 131.5, 129.8, 129.7, 129.5, 128.5, 123.2, 122.2, 121.3,
112.7, 104.7, 99.4, 96.3, 89.7, 53.6, 53.5, 53.5 (two overlapping sig-
nals), 18.9, 11.5.
Extended TTF 14
To a soln of 1 (0.290 g, 0.397 mmol) in PrOH (60 mL) and THF (15
mL) was added K2CO3 (1.01 g). After stirring at r.t. for 90 min, the
mixture was poured into CH2Cl2 (150 mL). The organic soln was
passed through a short plug of silica gel. After concentration in
vacuo, the product (0.251 g, 90%) was obtained as a dark red solid;
mp 166–168 °C.
MS (FAB): m/z = 1559 [M + H+].
Anal. Calcd for C76H78O16S8Si2: C, 58.51; H, 5.04. Found: C, 58.16;
H, 5.06.
1H NMR (300 MHz, CDCl3): d = 7.40 (s, 2 H), 6.85 (s, 2 H), 4.29
(t, J = 6.8 Hz, 8 H), 3.45 (s, 2 H), 1.72 (m, 8 H), 1.06 (t, J = 7.4 Hz,
12 H).
13C NMR (75 MHz, CDCl3): d = 159.7, 159.6, 135.2, 134.3, 131.4,
129.9, 129.4, 120.9, 111.9, 84.5, 81.4, 68.5, 68.5, 21.9 (two overlap-
ping signals), 10.4 (two overlapping signals).
OPE2-TTF 11
To a mixture of 2 (0.271 g, 0.333 mmol) in THF (10 mL) and
MeOH (10 mL) was added K2CO3 (0.10 g). After stirring at r.t. for
20 min, Et2O (200 mL) was added, and the mixture was washed
with H2O (100 mL) and sat. aq NaCl soln (100 mL). The organic
phase was dried (MgSO4), filtered, and concentrated in vacuo. The
red residue was mixed with 1,4-diiodobenzene (0.44 g, 1.33 mmol),
PdCl2(PPh3)2 (0.012 g, 0.0167 mmol, 5 mol%), and CuI (0.003 g,
0.0167 mmol, 5 mol%). Then degassed THF (12 mL) and degassed
Et3N (4 mL) were added under argon. After stirring at r.t. for 20 h,
the mixture was poured into CH2Cl2 (50 mL), washed with sat. aq
NH4Cl (50 mL), and sat. aq NaCl. The organic phase was dried
(MgSO4), filtered, and concentrated in vacuo. The red-brown resi-
due was subjected to column chromatography (silica gel, CH2Cl2)
MS (FAB): m/z = 698 [M + H+].
Anal. Calcd for C34H34O8S4: C, 58.43; H, 4.90. Found: C, 58.33; H,
4.99.
Extended TTF 15
To a soln of compound 1 (0.52 g, 0.71 mmol) in decan-1-ol (8 mL)
and THF (10 mL) was added K2CO3 (0.70 g). After stirring at 40 °C
for 18 h under argon, the mixture was poured into Et2O (100 mL)
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

FEATURE ARTICLE
Tetrathiafulvalene Cruciform Molecules
OPE5-TTF 19
547
and washed with H2O (4 × 50 mL), dried (MgSO4), and concentrat-
ed in vacuo to a red residue. The residue was purified (silica gel,
CH2Cl2). After concentration in vacuo, the product (0.41 g, 47%)
was obtained as dark red solid; mp 52–55 °C.
1H NMR (500 MHz, CDCl3): d = 7.39 (s, 2 H), 6.84 (s, 2 H), 4.28–
4.19 (m, 8 H), 1.75–1.63 (m, 8 H), 1.42–1.20 (m, 56 H), 0.92–0.83
(m, 12 H), 0.29 (s, 18 H).
13C NMR (126 MHz, CDCl3): d = 159.8, 159.7, 135.1, 133.6, 131.3,
130.1, 128.7, 121.6, 112.6, 102.9, 102.5, 67.1, 67.1, 32.0, 29.8,
29.7, 29.7, 29.7, 29.6, 29.5, 29.3, 29.3, 28.5, 28.5, 26.0, 25.9, 22.8,
14.3, 0.1.
The OPE3 17 (0.120 g, 0.115 mmol) was desilylated with TBAF·3
H2O (0.110 g, 0.35 mmol) dissolved in CH2Cl2 (15 mL) at r.t. for 5
min. The mixture was then diluted by CH2Cl2 (50 mL) and the or-
ganic layer was washed with sat. aq NH4Cl. The organic layer was
poured through a short column of silica gel using additional CH2Cl2
(100 mL) and concentrated in vacuo to a red solid. The red crude
product was redissolved in anhyd THF (10 mL), then Pd(PPh3)2Cl2
(6 mg, 0.017 mmol, 6 mol%) and CuI (5 mg, 0.044 mmol, 15 mol%)
were added. i-Pr2NH (1 mL) was then added to the mixture and it
was allowed to stand for 5 min before 1-(acetylthio)-4-iodobenzene
(18, 0.192 g, 0.69 mmol) was added. The mixture was stirred under
argon at r.t. for 2 h. The mixture was then diluted with CH2Cl2 (50
mL) and washed with sat. aq NH4Cl (25 mL) followed by brine (25
mL). The organic layer was dried (Na2SO4) and concentrated in
vacuo to a red-brown solid. Column chromatography (silica gel,
CH2Cl2) gave the product (0.050 g, 36%) as a red solid; mp 204–206
°C.
MS (FAB): m/z = 1235 [M + H+].
Anal. Calcd for C68H106O8S4Si2: C, 66.08; H, 8.64. Found: C, 65.65;
H, 8.94.
OPE3-TTF 16
To a soln of 14 (0.110 g, 0.157 mmol) in degassed THF (25 mL) and
degassed Et3N (5 mL) were added 1,4-diiodobenzene (1.23 g, 3.73
mmol), PdCl2(PPh3)2 (6.0 mg, 0.0086 mmol, 5 mol%), and CuI (1.8
mg, 0.0086 mmol, 5 mol%) under argon. After stirring for 19 h un-
der argon, the mixture was poured into CH2Cl2 (150 mL). The mix-
ture was washed with sat. aq NH4Cl (100 mL) and sat. aq NaCl (100
mL), dried (MgSO4), filtered, and concentrated in vacuo to a red
residue. Column chromatography (silica gel, heptane–CH2Cl2,
100:0 to 0:100) gave the product (0.092 g, 53%) as orange crystals;
mp 192.5–194.0 °C.
1H NMR (300 MHz, CDCl3): d = 7.72 (d, J = 8.4 Hz, 4 H), 7.48 (s,
2 H), 7.36 (d, J = 5.5 Hz, 4 H), 6.92 (s, 2 H), 4.21 (t, J = 6.7 Hz, 8
H), 2.02–1.53 (m, 8 H), 0.98 (t, J = 7.4 Hz, 12 H).
13C NMR (75 MHz, CDCl3): d = 159.7 (two overlapping signals),
137.8, 134.9, 134.0, 133.3, 131.2, 130.2, 128.8, 122.4, 121.5, 112.3,
95.8, 94.9, 88.9, 68.5, 68.5, 21.9 (two overlapping signals), 10.5
(two overlapping signals).
IR (neat): 3062, 2970, 1755, 1628, 1553, 1272, 1115, 1038, 849,
627, 546 cm–1.
1H NMR (500 MHz, CD2Cl2): d = 7.57–7.47 (m, 14 H), 7.35 (d,
J = 8.5 Hz, 4 H), 6.92 (s, 2 H), 4.16–4.09 (m, 8 H), 2.35 (s, 6 H),
1.67–1.61 (m, 8 H), 0.93–0.86 (m, 12 H).
13C NMR (126 MHz, CD2Cl2): d = 193.8, 160.1, 159.9, 135.4,
135.0, 134.8, 132.7, 132.3, 132.3, 131.9, 130.3, 129.4, 129.3, 124.6,
123.8, 123.4, 122.0, 112.6, 96.7, 91.2, 91.1, 89.8, 69.0, 68.9, 30.7,
22.4, 22.3, 10.7 (two overlapping signals).
MS (MALDI-TOF): m/z = 1197 (M+).
Anal. Calcd for C66H54O10S6: C, 66.09; H, 4.54; S,16.04. Found: C,
66.28; H, 4.56; S, 16.05.
4-Iodobenzenethiol (21)
4-Iodobenzenesulfonyl chloride 20 (4.00 g, 13.2 mmol) was dis-
solved in anhyd toluene (60 mL) in a 3-neck round-bottom flask
with an N2 inlet, reflux condenser, and CaCl2 guard tube. Ph3P (10.4
g, 39.7 mmol) was added in portions and the mixture was stirred for
10 min (if the temperature had not reached 60 °C after addition of
Ph3P, then the mixture should be heated for an additional 10 min at
60 °C). The mixture was then cooled below 50 °C and H2O (15 mL)
was added. The mixture was allowed to stand for an additional 10
min. The aqueous layer was discarded, and the organic layer was
extracted with aq 10% NaOH (2 × 40 mL). The alkaline aqueous
extract was washed with toluene (2 × 30 mL), acidified with aq 4 M
HCl and extracted with CH2Cl2 (2 × 40 mL). The organic layer was
dried (Na2SO4) and the solvent was removed under reduced pres-
sure to obtain nearly pure 4-iodobenzenethiol. Column chromatog-
raphy (silica gel, heptane–EtOAc, 9:1) gave the product (1.50 g,
48%) as white crystals. The analytical data were in accordance to
those previously reported.11
MS (MALDI-TOF): m/z = 1104 [M + H+].
Anal. Calcd for C46H40I2O8S4: C, 50.10; H, 3.66. Found: C, 50.36;
H, 3.67.
OPE3-TTF 17
To a soln of the dialkyne 14 (0.200 g, 0.29 mmol) in anhyd THF (12
mL) and Et3N (5 mL) were added Pd(PPh3)2Cl2 (12 mg, 0.017
mmol, 6 mol%) and CuI (10 mg, 0.044 mmol, 15 mol%). After stir-
ring for 2 min, 1-iodo-4-[(trimethylsilyl)ethynyl]benzene (0.515 g,
1.72 mmol) was added. The mixture was allowed to stir at r.t. for 6
h under an inert atmosphere (argon). The mixture was then diluted
with CH2Cl2 (50 mL) and washed with sat. aq NH4Cl (25 mL). The
organic layer was dried (Na2SO4) and concentrated in vacuo to a
red-brown solid. The red-brown residue was subjected to column
chromatography (silica gel, CH2Cl2) to provide the product (0.188
g, 62%) as a red solid; mp 208–209 °C.
1H NMR (500 MHz, CDCl3): d = 7.53–7.46 (m, 10 H), 6.92 (s, 2 H),
4.25–4.18 (m, 8 H), 1.76–1.70 (m, 8 H), 1.02–0.94 (m, 12 H), 0.29–
0.22 (m, 18 H).
13C NMR (126 MHz, CDCl3): d = 159.7, 159.7, 134.9, 134.0, 132.1,
131.6, 131.3, 130.2, 128.9, 123.6, 122.9, 121.5, 112.4, 104.7, 96.8,
96.4, 89.4, 68.5, 68.5, 21.9 (two overlapping signals), 10.5 (two
overlapping signals), 0.06.
1-(Acetylthio)-4-iodobenzene (18)
4-Iodobenzenethiol (21, 1.50 g, 6.35 mmol) was dissolved in pyri-
dine (20 mL, dried over NaOH pellets) and under argon. AcCl (0.99
g, 0.90 mL, 12.7 mmol) was slowly added via syringe with vigorous
stirring. The mixture was then stirred for 25 min and poured into
H2O with crushed ice (approx. 100 mL). The cold H2O was extract-
ed with heptane (2 × 50 mL) and the organic layer was dried
(Na2SO4) and the solvent removed under reduced pressure to yield
a colorless oil that slowly solidified to white crystals (1.76 g, 99%).
The analytical data were in accordance to those previously report-
ed.11,12
MS (ESP+): m/z = 1043.4 [M + H+].
Anal. Calcd for C56H58O8S4Si2: C, 64.64; H, 5.60; S, 12.29. Found:
C, 64.38; H, 5.65; S, 12.12.
Synthesis 2011, No. 4, 539–548 © Thieme Stuttgart · New York

548
K. Jennum et al.
FEATURE ARTICLE
G. C.; Colvin, M. T.; Scott, A. M.; Chen, K.; Ratner, M. A.;
Wasielewski, M. R. J. Am. Chem. Soc. 2010, 132, 15427.
(3) (a) Tour, J. M. Acc. Chem. Res. 2000, 33, 791.
(b) Nørgaard, K.; Nielsen, M. B.; Bjørnholm, T. Functional
Organic Materials; Müller, T. J. J.; Bunz, U. H. F., Eds.;
Wiley-VCH: Weinheim, 2007, 353–392.
(4) (a) Bryce, M. R. Adv. Mater. 1999, 11, 11. (b) Nielsen, M.
B.; Lomholt, C.; Becher, J. Chem. Soc. Rev. 2000, 29, 153.
(c) Bryce, M. R. J. Mater. Chem. 2000, 10, 589. (d)Segura,
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(e) Stoddart, J. F. Chem. Soc. Rev. 2009, 38, 1802.
(f) Canevet, D.; Sallé, M.; Zhang, G. X.; Zhang, D.; Zhu, D.
Chem. Commun. 2009, 2245.
Acknowledgment
The research leading to these results has received funding from the
European Community’s Seventh Framework Programme (FP7/
2007-2013) under the grant agreement ‘SINGLE’ no 213609. The
Danish Research Council for Independent Research/Natural Sci-
ences is gratefully acknowledged for financial support (instrument
grant #09-066663).
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