TTF and TCNQ analogues derived from a new benzo-fused thiopyranyl building block

Article abstract of DOI:10.1016/j.tetlet.2009.03.101

TTF-type and TCNQ-type analogues (9-12) have been prepared from a new benzo-fused thiopyranyl precursor 8. The synthetic conditions of 8 were optimized. An anomaly was observed in the ¹H NMR spectrum of 9. The crystal structures of 9 and 10 sho

Full text of DOI:10.1016/j.tetlet.2009.03.101

Tetrahedron Letters 50 (2009) 2597–2600
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
TTF and TCNQ analogues derived from a new benzo-fused thiopyranyl
building block
a,
Zhiming Duan a,b, Zhongming Wei a,b, Wei Xu a, , Daoben Zhu
*
*
a Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
TTF-type and TCNQ-type analogues (9–12) have been prepared from a new benzo-fused thiopyranyl pre-
cursor 8. The synthetic conditions of 8 were optimized. An anomaly was observed in the 1H NMR spec-
trum of 9. The crystal structures of 9 and 10 show distorted conformations and ordered packing
geometries connected by short contacts. Electronic and redox properties of 9–12 were investigated by
UV–vis spectroscopy and cyclic voltammetry. Both 9 and 10 exhibited p-type OFET performances.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 13 January 2009
Accepted 13 March 2009
Available online 19 March 2009
Over the past decades, there has been increasing interest in the
synthesis of -extended tetrathiafulvalene and tetracyano-p-
sonable design and synthesis of new p-spacers based on thiopyra-
nyl systems are necessary for the development of new exTTFs and
p
quinodimethane analogues (exTTFs and exTCNQs) due to their ver-
exTCNQs.
satile applications in many areas such as supramolecular and or-
The main structural feature of our compounds includes a thio-
pyrano[3,2-h]thiochromene moiety, which was introduced by the
Wittig–Horner or Knoevenagel reaction of the precursor 8 with
phosphonate ester 13 or malononitrile, respectively. As shown in
Scheme 1, 8 was obtained through eight steps. The synthesis began
with a nucleophilic aromatic substitution of o-dichlorobenzene
with thiolate ions in hexamethylphosphoramide (HMPA) at
100 °C. We had improved the yield of 1 to ca. 85% and shortened
the reaction time to ca. 14 h by using high concentration of the
freshly prepared sodium 2-propanethiolate (2 mol/L in HMPA).
The following dealkylation of 1 was conducted smoothly according
to the well-established methods reported by M. Tiecco’s group,9
then the intermediate 2 was added reversely into the benzene
solution of 3-bromopropionitrile to give 3 with a yield of ca. 60%.
3 was converted to the corresponding diacid (4) in a satisfactory
yield of 89% by boiling it in concentrated HCl (6 M) for 4–5 h.
Due to the failure of the direct phosphoric acid (PPA) ring-closure
of 4, it was firstly transformed into the diacyl chloride 5 by oxalyl
chloride, then 5 was cyclized to 6 quantitatively and neatly under
Friedel-Crafts condition (AlCl3 as a catalyst) at room tempera-
ture.10 The conventional dehydrogenation methods11 could not
succeed in the conversion of 6 into 8, and the halogenation-dehy-
drohalogenation sequence turned out to be the method of choice.12
ganic electronic materials chemistry.1 Different
p-spacers
spanning from planar2 to nonplanar3 conjugated units, have been
employed to construct new exTTFs and exTCNQs. Among numer-
ous examples, anthraquinone-derived exTTFs (such as TTFAQ)
and exTCNQs (such as TCAQ)4 are mostly studied, and exhibit
attractive redox and structural properties. In these systems, the
electroactive units, that is, the 1,3-dithiole rings or dicyanomethyl-
ene fragments are in a relative para-position, and the molecules
adopt twisted butterfly shaped geometries in neutral state due to
the steric interaction of the aromatic peri-hydrogens with the elec-
troactive units. Another interesting system, in which the electroac-
tive units are in a relative meta-position, is less studied, and only a
few examples, such as azulene-5 or pyridine-bridged6 exTTFs have
been reported. The meta-linkage would lead to electronic decou-
pling of the electroactive units in the ground states, and to elec-
tronic coupling in the excited states.
To shed more light on the relationship between the electronic
states and the different linking mode of the electroactive units rel-
ative to the p-spacer, we report herein on the synthesis and prop-
erties of a class of TTF and TCNQ analogues (9–12) derived from a
new benzo-fused thiopyranyl building block. As we know, 2-(thio-
pyran-40-ylidene)-1,3-dithiole (TPDT) was synthesized about
twenty years ago,7 and the
p
-extension of TPDT was achieved by
The a-dibromination of diketone 6 yielded a mixture of diastereo-
the benzo-fusing on the dithiole ring.8 However, the benzo-fusing
on the thiopyran ring was rarely reported, and novel structural
benzo-fused thiopyranyl systems are not at hand. Therefore, rea-
mers (7),13 which gave rise to 8 by treatment with sodium acetate
in refluxing ethanol. The overall yield was 34% for the whole syn-
thetic route.
The synthesis of TTF and TCNQ analogues (9–12) is shown in
Scheme 2. The TCNQ analogues (11 and 12) were prepared from
8 by Knoevenagel condensation using Lehnert’s reagent (malono-
nitrile, TiCl4, and pyridine).14 The conjugated olefinic structure
* Corresponding authors. Tel.: +86 10 6263 9355; fax: +86 10 6256 9349 (W.X.),
tel.: +86 10 6254 4083; fax: +86 10 6256 9349 (D.Z.).
E-mail addresses: wxu@iccas.ac.cn (W. Xu), zhudb@iccas.ac.cn (D. Zhu).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.03.101

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Z. Duan et al. / Tetrahedron Letters 50 (2009) 2597–2600
Scheme 1. Synthetic route of the precursor 8.
Figure 1. Variable-temperature 1H NMR of 9 in CDCl3.
Scheme 2. Synthesis of 9–12.
and the poor solubility of 8 may account for the low yield of 11 and
12.15
The TTF analogues (9 and 10) were synthesized by Wittig-Horn-
er reaction using in situ-generated carbanion of 13 in the presence
of n-BuLi at ꢀ78 °C. The precise addition of n-BuLi and the fine
grinding of 8 are necessary for the success of the procedure.16
Two vicinal olefinic hydrogens on one of the thiopyran rings of
9 are in an AB spin system, and should display a set of two skewed
doublets in the 1H NMR spectrum. However, the 1H NMR spectrum
of 9 in CDCl3 at room temperature showed a singlet peak at ca.
6.31 ppm corresponding to the hydrogen signal on thiopyran ring.
By performing the variable-temperature (VT) 1H NMR experiment,
the singlet peak gradually restored into two doublet peaks with the
decrease of the temperature (from 293 K to 233 K) as shown in Fig-
ure 1. This phenomenon may be explained by the random confor-
mation promoted by halogenated solvents in which the
interactions between solute (donor molecules) and solvent may
be involved.17
Figure 2. The crystal structures of 6, 9, and 10.
Single crystals of 6, 9, and 10 were obtained by slow evapora-
tion or diffusion method, and their crystal structures (as shown
in Fig. 2) were investigated by X-ray crystallographic analysis. 6
adopts almost planar conformation, and in the packing mode of
6, the molecules form two kinds of opposite-oriented chains
through the short contacts (see Figure S3 in SI-1). 9 displays a dis-
torted structure, in which two TPDT parts take boat conformations
with opposite orientations, and the dithiole ring is tilted to 36°
with respect to the C(1)S(4)C(8)S(5) plane. Compound 9 stacks in

Z. Duan et al. / Tetrahedron Letters 50 (2009) 2597–2600
2599
ꢀ2.06 and ꢀ2.58 V).1b These waves could be attributed to the for-
mation of radical anions. 12 gives two irreversible reduction waves
at ꢀ0.78 and ꢀ1.56 V corresponding to two one-electron reduction
steps located on the dicyanomethylidene and carbonyl groups
separately, and one irreversible oxidation wave at 1.98 V corre-
sponding to the one-electron oxidation of thiopyrano[3,2-h]thio-
chromene unit. The difference in the first reduction potentials of
11 and 12 may be ascribed to the worse accepting ability of car-
bonyl group compared to dicyanomethylidene group.
Preliminary results of spin coating and OFET device perfor-
mance of 9 and 10 (see page S13 and S14 in SI-1) indicated that
these
p-extended TTFs with thiopyrano[3,2-h]thiochromene moi-
ety can form homogeneous thin-films. Both 9 and 10 exhibited
p-type transistor performances, although the field-effect mobilities
and the current ratios are not high compared to those of other
attractive p
-extended TTFs.20
In summary, a new class of TTF and TCNQ analogues (9–12)
have been synthesized from the precursor 8. Compound 8 may
be a promising candidate for versatile synthesis of electron donors
and acceptors in the near future. The electrochemical study reveals
moderate donating ability of 9 and 10, and poor accepting ability of
11 and 12. The distorted geometries of 9 and 10 established by X-
ray single crystal analysis may account for the coalescence of their
oxidation process and p-type performances of their OFET devices.
Further experiments to optimize the device fabrication and to
Figure 3. Cyclic voltammograms of 9–12 measured in CH2Cl2 (9 and anodic part of
10), DMSO (11) and benzonitrile (12 and cathodic part of 10) at room temperature
(V vs Ag/AgCl, 100 mV/s, 0.1 M ðn-BuÞ4NþClO4ꢀ).
a row along a direction, and the face to face contacts only occur be-
tween two neighboring dithiole rings within the row (the average
distance between the mean planes of two neighboring dithiole
rings falls in the range of 3.55–3.65 Å, while the average distance
between the mean planes of two neighboring thiopyrano[3,2-
h]thiochromene moieties falls in the range of 4.06–4.08 Å) (see Fig-
ure S5 in SI-1). There exist interactions through SꢁꢁꢁS short contacts
(ca. 3.63 Å) between the methylthio groups (see Figure S4 in SI-1).
The boat conformation of the TPDT part of 10 is similar to that of 9,
and the existence of OꢁꢁꢁS short contacts (ca. 3.15 Å) may account
for the complicated packing mode (see Figure S6 in SI-1).
modify the p-extended structure are currently underway.
Acknowledgments
We thank the National Natural Science Foundation of China
(Grant 20421101 and 20572113), State Key Basic Research Pro-
gram (2006CB806200) and Chinese Academy of Sciences for finan-
cial support. We would like to thank Dr. Junfeng Xiang for
assistance in obtaining VT-NMR spectra and solid-state 13C NMR
spectra.
The solution UV–vis spectra of 9–12 were shown in Figure S7 and
the absorption maxima were collated in Table S3. All compounds
feature strong absorptions in the range between 350 nm and
500 nm, which could be assigned to the transitions involving the
thiopyrano[3,2-h]thiochromene moiety.7 For 10, the tail of absorp-
tion band above 500 nm indicated the existence of possible weak
intramolecular charge transfer (ICT) interactions between the 1,3-
dithiole ring and the thiopyrano[3,2-h]thio-chromene fragment.
Electrochemical redox properties of 9–12 were investigated by
cyclic voltammetry (CV) as shown in Figure 3, and the redox poten-
tials were listed in Table S3. 9 and 10 show irreversible oxidation
waves on anodic scanning, and the oxidation potentials are higher
than those of TTF (E1/2, +0.34 and +0.71 V)18 or TPDT (E1/2, +0.23
and +0.61 V).7 These waves could be attributed to the formation
of radical cations. Compound 10 also gives an irreversible reduc-
tion wave at –1.64 V corresponding to one-electron reduction of
carbonyl group. The coalescence of the oxidation waves of 9 and
10 could be resolved in THF as shown in Figure S8. Compound 9
gives two slightly resolved irreversible electron transfers corre-
sponding to a two-electron oxidation step on two 1,3-dithiole rings
and a one-electron oxidation step on the thiopyrano[3,2-h]thio-
chromene moiety. Compound 10 shows two well-resolved irre-
Supplementary data
Experimental procedures (SI-2); copies of 1H and 13C NMR spec-
tra for new compounds (SI-2); a scheme for donors and acceptors
involved in this Letter; crystal data (Table S2 in SI-1) for 6 (CCDC
702881), 9 (CCDC 702882) and 10 (CCDC 702883); details for the
OFET devices fabrication/characterization of 9 and 10. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.tetlet.2009.03.101.
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