Synthesis and properties of thiophene-functionalized π-extended tetrathiafulvalenes

Article abstract of DOI:10.1016/j.tetlet.2009.09.150

Two molecular ensembles composed of an array of thiophene-extended tetrathiafulvalene-thiophene were synthesized using Stille coupling and Horner-Wittig reaction as the key steps. Electrochemical redox and electronic absorption properties were investigate

Full text of DOI:10.1016/j.tetlet.2009.09.150

Tetrahedron Letters 50 (2009) 6897–6900
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and properties of thiophene-functionalized
tetrathiafulvalenes
p-extended
*
Min Shao, Yuming Zhao
Department of Chemistry, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada A1B 3X7
a r t i c l e i n f o
a b s t r a c t
Article history:
Two molecular ensembles composed of an array of thiophene–extended tetrathiafulvalene–thiophene
were synthesized using Stille coupling and Horner–Wittig reaction as the key steps. Electrochemical
redox and electronic absorption properties were investigated by voltammetric and UV–vis spectroscopic
analyses.
Received 14 August 2009
Revised 21 September 2009
Accepted 24 September 2009
Available online 1 October 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Keywords:
Tetrathiafulvalene
Thiophene
Stille coupling
Horner–Wittig reaction
Electrochemistry
UV–vis
Sulfur-rich
p
-conjugated molecules constitute important build-
chemical polymerization of TTF-attached thiophenes comes from
the low oxidation potential of TTF, which hampers the formation
of suitable thiophene radical cation intermediates for polymeriza-
tion. Rational molecular design and tailoring based on in-depth
understanding of structure–property relationship should offer an
effective approach to address the challenges. To this end, synthetic
access to various new TTF–thiophene molecular architectures is
prerequisite.
Motivated by the issues mentioned above, a type of thiophene-
functionalized exTTFs 1 was targeted in our research. The general
motif of 1 represents a wide range of thiophene–exTTF–thiophene
triads, which are expected to give rise to beneficial features in the
context of molecular electronics; for example, tunableredox proper-
ties and flexible post-functionalization at the thiophene end groups.
In this work, two structures belonging to the family of 1 were ex-
plored as shown in Scheme 1. In compound 2, two thiophene groups
were directly attached to an anthraquinoid-type exTTF (namely
TTFAQ),27 while in compound 3 thiophene groups were fused with
the conjugated framework of the planar central exTTF core.
ing blocks in current research on molecular electronics. Among
them, thiophene and tetrathiafulvalene (TTF) are two extraordinary
candidates, which show remarkable electronic properties and ample
application potentials. Thiophene is the essential repeat unit for a
class of well-known conducting polymers (CPs), poly/oligo-thio-
phenes,1–3 while TTF and its derivatives are excellent organic elec-
tron donors widely applicable in various fields ranging from
charge-transfer complexes, nonlinear optical materials, organic
transistors, to molecular switches and sensors.4–7 Remarkable opto-
electronic performances have been found with thiophene and TTF-
containing organic materials, which has fueled the pursuit for a
broad range of thiophene–TTF hybrid materials since the early
1990s.7–20 In previous reports related to this topic, design motifs
were mainly focused on the use of thiophene or oligothiophenes as
p
-spacers to spatially expand the conjugation of TTF, affording the
so-called
p
-extended TTF derivatives (exTTFs).16–20 With such mod-
ifications, the molecular properties that are advantageous to device
fabrications, such as increased dimensionality, improved donor abil-
ity, intimated solid-state packing, and enhanced air stability, have
become attainable. Another benefit stemming from intermarrying
thiophene and TTF lies in the possibility of generating novel electro-
active CPs through straightforward electrodeposition means.8,21–26
Nevertheless, successes in producing TTF-functionalized polythio-
phenes are still quite limited.25,26 A major barrier to the electro-
* Corresponding author. Tel.: +1 709 737 8747; fax: +1 709 737 3702.
E-mail address: yuming@mun.ca (Y. Zhao).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.150

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M. Shao, Y. Zhao / Tetrahedron Letters 50 (2009) 6897–6900
Scheme 1. Synthesis of thiophene–exTTF–thiophene triads 2 and 3.
Figure 1. (A) Cyclic voltammogram of 2 (2.1 mM). (B) Differential pulse voltammogram of 2. (C) Cyclic voltammogram of 3 (2.3 mM). (D) Differential pulse voltammogram of
3. CV experimental conditions: electrolyte: Bu4NPF6 (0.1 M); working: glassy carbon; counter: Pt; reference: Ag/AgCl; scan rate: 100 mV/s. DPV experimental conditions:
step: 4 mV; pulse width: 250 mV; period: 200 ms.

M. Shao, Y. Zhao / Tetrahedron Letters 50 (2009) 6897–6900
6899
In Figure 1A, a prominent pair of redox waves (E1pa = 0.76 V,
E1pc = 0.26 V) is clearly seen, which can be attributed to a simulta-
neous two-electron transfer at the central TTFAQ unit, leading to
the formation of dication of 2. The nature of this redox wave pair
is quasi-reversible, which is in line with the typical redox behavior
of TTFAQ.34 In addition to this redox couple, two weak current
peaks (E2pa = 1.07 V, Ep1c = 1.01 V) are discernible in the CV of 2.
The DPV of 2 (Fig. 1B) shows the presence of a significant oxidation
peak at 0.63 V followed up by two weak peaks at 0.88 and 1.01 V in
the course of anodic scan. The two oxidation processes at relatively
higher voltages can be explained by that the formed [2]2+ species,
to some extent, might have yielded electrochemical byproducts
which were subjected to further oxidation.
In the cyclic voltammogram of 3 (Fig. 1C), there are three anodic
waves observable at 0.93, 1.15, and 1.33 V, along with a barely
noticeable cathodic wave at 0.98 V. The first oxidation presumably
leads to the formation of dication of 3. However, in notable con-
trast to the CV of 2, the redox pattern displayed by 3 is completely
irreversible. The irreversibility is likely due to an EC mechanism,
which results in the decomposition of [3]2+ at high voltage. The
DPV of 3 shows three oxidation peaks when scanned anodically
from 0 to 1.5 V (see Fig. 1D). This result agrees with its CV data,
suggesting the formation of multiple electrochemical products in
anodic scan. Compared with compound 2, compound 3 has higher
first oxidation potential, indicating that 3 is a weaker electron do-
nor than 2. For both compounds 2 and 3, no indication of the occur-
rence of electropolymerization was observed from repetitive CV
The synthesis of exTTF–thiophene triads 2 and 3 is outlined in
Scheme 1. 2,6-Diiodoanthraquinone 428 and thienylstannane 5
were subjected to a Stille coupling under the catalysis of Pd(0),
affording ketone 6, which is a key precursor to compound 2. Upon
a Horner–Wittig reaction with the phosphonate ylide generated
in situ from compound 13,28–30 ketone 6 was converted into
exTTF–thiophene 14 in a high yield of 95%. Treatment of 14 with
TFA removed the TMS groups, giving the desired product 2. The
presence of n-octyl groups on the thiophene units during the syn-
thesis of 2 was essential as they conferred satisfactory solubility to
the intermediates and the product.
The synthesis of exTTF–thiophene 3 requires a key precursor, ke-
tone 12, the preparation of which was implemented following the
synthetic route reported by Ng and co-workers.31,32 It began with a
Stille coupling between compound 5 and diiodoarene 7, which gave
compound 8 in 77% yield. Removal of the TMS groups in 8 by TFA
yielded compound 9, which was then subjected to saponification
and chlorination to form acyl chloride 11. Compound 11 underwent
a twofold intramolecular Friedel–Crafts cyclization to form the de-
siredketoneprecursor12. With12inhand, aHorner–Wittigreaction
was executed to furnish exTTF–thiophene hybrid 3 in 57% yield.
Molecular structures of compounds 2 and 3 were characterized
by IR, NMR, and MS analyses,33 while their electrochemical redox
properties were investigated by cyclic voltammetry (CV) and differ-
ential pulse voltammetry (DPV) techniques. Figure 1 illustrates the
voltammograms measured in CHCl3/CH3CN (4:1, v/v) at room
temperature.
Figure 2. UV–vis spectroscopic changes accompanying: (A) oxidative titration of 2 (3.6 lM). (B) Oxidative titration of 3 (3.3 lM). (C) Controlled-potential oxidation of 2. (D)
Controlled-potential oxidation of 3. Experimental conditions of spectroelectrochemistry: electrolyte: Bu4NPF6 (0.1 M); working: Pt mesh; counter: Pt wire; reference: Ag/
AgCl.

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M. Shao, Y. Zhao / Tetrahedron Letters 50 (2009) 6897–6900
scan experiments. In fact, the voltammograms were virtually un-
changed after more than 10 cycles of scans. The inability of 2
and 3 to undergo electropolymerization can be rationalized as fol-
References and notes
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able radical cations for electropolymerization.26
To investigate the electronic properties of compounds 2 and 3 in
neutral and oxidized forms, oxidative UV–vis titration and spectro-
electrochemistry measurements were conducted on their CHCl3
solutions (see Fig. 2). In Figure 2A, compound 2 shows three UV–
vis absorption bands at 453, 390, and 317 nm in neutral state.
Upon titration with an oxidant, PhI(OAc)2/CF3SO3H (note that
1 M equiv of the oxidant theoretically consumes two moles of elec-
trons),27,29 up to 3.0 M equiv, the absorption peak at 453 nm is ob-
served to steadily decrease in intensity, while the other two peaks
show rather insignificant change. An isosbestic point can be clearly
seen at 419 nm. Similar UV–vis spectroscopic changes can be ob-
served in Figure 2C, wherein compound 2 is subjected to con-
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a 1 mm quartz cuvette. These
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tween 2 and [2]2+, is formed during the oxidation.
Compound 3 shows absorption bands at 475, 357, and 301 nm
in its UV–vis profile (see Fig. 2B). Upon addition of oxidant up to
2.0 M equiv, the peak at 475 nm decreases considerably. In the
meantime, an absorption tail from ca. 550 to 750 nm increases
appreciably, the origin of which is assigned to [3]2+. There are
two isosbestic points at 538 and 441 nm observed during the titra-
tion of oxidant from 0 to 1.0 M equiv, and these two isosbestic
points drift slightly when the amount of oxidant is further in-
creased. More significant drift of isosbestic points can be seen in
the spectroelectrochemical measurements when the applied volt-
age is greater than 1.0 V (Fig. 2D). The drift of isosbestic points is
also accompanied by decreasing absorption tail from 550 to
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In summary, we have successfully developed synthetic routes to
two thiophene–exTTF hybrid systems 2 and 3. Electrochemical and
electronic absorption properties were elucidated by voltammetric
techniques in combination with UV–vis spectroscopic analysis. Ef-
forts toward electropolymerization of 2 and 3 have not been suc-
cessful; however, this barrier is envisaged to be circumvented by
incorporation of oligothiophenes to the design motif26 or through
a Ni-catalyzed polymerization35 of the brominated products of 2
and 3 in our future work. In this vein, further investigations on thi-
ophene–exTTF triads 2 and 3 as well as their analogues are worth-
while and should lead to appealing new electronic materials.
26. Skabara, P. J.; Roberts, D. M.; Serebryakov, I. M.; Pozo-Gonzalo, C. Chem.
Commun. 2000, 1005–1006.
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28. Shao, M.; Chen, G.; Zhao, Y. Synlett 2008, 371–376.
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32. Zhao, C.; Chen, X.; Zhang, Y.; Ng, M.-K. J. Polym. Sci., Part A: Polym. Chem. 2008,
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33. Characterization data for compound 2: IR (neat): 2921, 2851, 1638, 1617, 1558,
1530, 825, 668 cmÀ1
; d 7.77 (s, 2H), 7.55 (d,
1H NMR (500 MHz, CDCl3)
J = 7.99 Hz, 2H), 7.53 (d, J = 8.05 Hz, 2H), 7.21 (s, 2H), 6.90 (s, 2H), 2.64 (t,
J = 7.65 Hz, 4H), 2.41 (s, 6H), 2.40 (s, 6H), 1.66 (m, 4H), 1.40–1.25 (m, 20H), 0.89
(t, J = 6.76 Hz, 6H); 13C NMR (125 MHz, CDCl3) d 144.9, 143.9, 135.5, 133.7,
133.1, 132.0, 126.4, 126.3, 126.2, 125.2, 123.7, 123.6, 122.9, 120.2, 32.3, 31.1,
30.9, 29.9, 29.8, 29.7, 23.1, 19.6, 19.5, 14.6; HR-EIMS (+eV) m/z calcd for
C48H56S10 952.1589, found 952.1589 [M]+. Characterization data for compound
Acknowledgments
3: Mp 158–160 °C; IR (neat) 2941, 2878, 1560, 1507, 1474, 1337 cmÀ1 1H NMR
;
(500 MHz, CDCl3) d 7.71 (s, 2H), 6.84 (s, 2H), 3.08 (t, J = 7.73 Hz, 4H), 2.57 (s,
6H), 2.52 (s, 6H), 1.68 (m, 4H), 1.41 (m, 4H), 1.36–1.24 (m, 20H), 0.88 (t,
J = 6.85 Hz, 6H); 13C NMR (125 MHz, CDCl3) d 143.5, 140.5, 137.4, 137.3, 136.8,
132.1, 129.8, 128.1, 122.5, 121.0, 114.1, 33.0, 32.3, 32.0, 30.0, 29.84, 29.79, 23.1,
19.9, 19.7, 14.5; HR-CIMS (+eV) m/z calcd for C42H50S10 874.1120, found
875.1230 [M+H]+.
The authors thank NSERC, CFI, and Memorial University of New-
foundland for funding support.
34. Gruhn, N. E.; Macías-Ruvalcaba, N. A.; Evans, D. H. Langmuir 2006, 22, 10683–
10688.
Supplementary data
35. Skabara, P. J.; Berridge, R.; McInnes, E. J. L.; West, D. P.; Coles, S. J.; Hursthouse,
M. B.; Müllen, K. J. Mater. Chem. 2004, 14, 1964–1969.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2009.09.150.