Star-shaped triazine-thiophene conjugated systems

Article abstract of DOI:10.1016/j.tetlet.2009.07.126

Star-shaped molecules consisting of short-chain oligothiophenes attached onto an electron acceptor triazine core have been synthesized. Results of the analysis of the electronic properties of these compounds by UV-vis spectroscopy and cyclic voltammetry are used to discuss the impact of the electron-withdrawing node on the properties of the compounds. The electrochemical oxidation of some of these systems into the corresponding electroactive polymers is briefly discussed.

Full text of DOI:10.1016/j.tetlet.2009.07.126

Tetrahedron Letters 50 (2009) 5673–5676
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Star-shaped triazine–thiophene conjugated systems
Philippe Leriche a,*, Flavia Piron a,b, Emilie Ripaud , Pierre Frère , Magali Allain , Jean Roncali
a
a
a
a
a
University of Angers, CNRS, CIMA, Group Linear Conjugated Systems, 2 Bd Lavoisier 49045, Angers, France
Organic Chemistry Department and CCOCCAN, ‘Babes-Bolyai’ University, 11 Arany Janos str., 400028, Cluj-Napoca, Romania
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Star-shaped molecules consisting of short-chain oligothiophenes attached onto an electron acceptor tri-
azine core have been synthesized. Results of the analysis of the electronic properties of these compounds
by UV–vis spectroscopy and cyclic voltammetry are used to discuss the impact of the electron-withdraw-
ing node on the properties of the compounds. The electrochemical oxidation of some of these systems
into the corresponding electroactive polymers is briefly discussed.
Received 22 June 2009
Revised 16 July 2009
Accepted 22 July 2009
Available online 26 July 2009
Ó 2009 Published by Elsevier Ltd.
Keywords:
Triazine
Ethylenedioxythiophene
Electropolymerization
Stille reaction
Star-shaped conjugated systems are subject to much current
interest as basic unit for discotic liquid crystals1 and as active
materials for organic electronic devices such as field-effect transis-
branches (Scheme 1). Compounds 1–5 have been synthesized by
Stille coupling reaction between the triazine core and various stan-
nic derivatives. The syntheses as well as preliminary results on the
characterization of the electronic properties of the compounds by
UV–vis spectroscopy and cyclic voltammetry are presented.
Tris-2,4,6-thienyl-1,3,5-triazine 1 has been previously synthe-
sized by Chérioux et al. by aromatic substitution of 2,4,6-trichloro-
2
tors or solar cells. On the other hand, star-shaped systems pos-
sessing donor–acceptor characters can present an intense
internal charge transfer band at low energy and have found appli-
3
cations in organic solar cells. In this context, we present here the
4
synthesis of a series of planar donor–acceptor systems 1–5 based
on an electron-withdrawing triazine core substituted at the 2, 4
and 6 positions with various electron-donating bithiophene
2,4,5-triazine namely cyanuric chloride with (bis)thienyllithium.
Such compounds can also be synthesized by cyclotrimerization of
5
thiophene-2-carbonitrile in trifluoromethylsulfonic acid. Recently,
O
O
O
S
O
S
S
S
S
O
O
O
S
S
S
S
O
N
N
N
O
S
S
S
S
S
N
O
N
N
S
O
N
N
S
O
N
N
S
O
S
O
O
N
S
N
N
S
O
O
N
N
S
O
O
O
O
S
S
S
O
O
O
1
2
3
4
5
Scheme 1. Structures of compounds 1–5.
*
Corresponding author. Tel.: +33 (0)2 41 73 50 10; fax: +33 (0)2 41 73 54 05.
E-mail address: philippe.leriche@univ-angers.fr (P. Leriche).
0
040-4039/$ - see front matter Ó 2009 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2009.07.126

5
674
P. Leriche et al. / Tetrahedron Letters 50 (2009) 5673–5676
S6
C31
C27
C26
using the first method Avarvari et al. have synthesized TTF-triazine
compounds and shown that the tris-functionalization of triazine by
this route was not so straightforward.
We used the same method in a first attempt to synthesize com-
pound 1. The first mild synthetic pathway using lithiated thio-
phene and cyanuric chloride4 led to the target compound 1 in
C30
6
C29
C24
C25
C23
C22
N4
C28
S7
C21
2
0% yield together with 25% of the bis-derivatized compound 6.
C4
C3
C32
7
S5
N6
S3 C15
In contrast, reaction of cyanuric chloride with 2-tributylstannyl-
thiophene in the presence of Pd(PPh as catalyst led to the target
C16
N5
C5
3 4
)
N3
C2
C12
compound 1 in 80% yield after recrystallization in petroleum ether.
The same reaction starting from the stannic derivatives of EDOT 7
and bisEDOT 8 gave compounds 2 and 5 in 90% and 30% yield,
respectively. The presence of the triazine electron-withdrawing
moiety makes aromatic electrophilic substitution at the thiophene
units of compound 1 difficult and attempts to brominate this com-
pound using NBS remained unsuccessful. Bromination of 1 was
achieved in the presence of 30 equiv of bromine in refluxing chlo-
roform for 4 days. The tribromo compound 9 was isolated as single
product in 74% yield after recrystallization in a mixture of methy-
lene chloride and petroleum ether. Compound 9 was then reacted
in a Stille reaction with the stannic derivative of thiophene or EDOT
C17
C1
C14
3
.512(2) Å
C13
S8
S1
N1
C11
C18
S4
N2
C6
C19
C7
C20
S2
C8
C10
C9
c
a
7
leading to 3 and 4 in 60% and 83% yield, respectively. All com-
pounds were satisfactorily characterized by NMR and mass spec-
trometry (Scheme 2).
b
8
3.40 Å
The crystallographic structure of compound 1 is presented on
Figure 1. This compound crystallizes as two independent mole-
9
cules in the orthorhombic Pna2
1
space group. The S1. . .S4 contacts
at 3.512(2) Å and 3.534(2) Å are shorter than the sum of Van der
Waals radii for two sulfur atoms (3.70 Å) suggesting intermolecu-
lar interactions (Fig. 1, top). For each molecule, positional disorder
affects one of the three thiophene rings and both the sulfur atom
and the carbon atom in 3-position of the thiophene ring are located
at two positions. As shown in Figure 1 (bottom), the system is per-
fectly planar which is propitious for a good conjugation between
the thiophene moieties and the triazine core. Moreover, the
distances between two planar molecules are either 3.40 or
3.46 Å
3.40 Å
3
.46 Å. The triazine core from one molecule stacks with a little shift
to the thiophene ring of the other molecule.
3.46 Å
Ar-SnBu3
Pd(PPh3)4
Ar
N
Ar
Ar=Thiophene,1,80%
Ar=EDOT,2,90%
N
N
C
Ar=bisEDOT,5,30%
H
N
S
Ar
Figure 1. Top. Ortep view of derivative 1, ellipsoids drawn at 50% probability level.
Bottom. Part of the packing diagram in the unit cell.
S
N
Cl
N
Li
S
S
Cl
N
Cl
S
N
N
S
N
N
S
N
O
N
Et2O
Cyclic voltammetric and UV–vis spectroscopic data of com-
Cl
pounds 1–5 are gathered in Table 1. All compounds present a
*
O
O
O
O
6
, 25%
1
, 20%
large
pÀp
absorption band with a kmax which depends on the
S
length and composition of the side arms. Thus, extending the
branches by one thiophene ring (1 and 3) produces a red shift,
of kmax from 316 to 390 nm. On the other hand, the replacement
of thiophene by EDOT also produces a bathochromic shift of kmax
(Fig. 2.). Note that the absorption maxima of compounds 3–5 are
bathochromically shifted compared to those of the respective
branches namely bithiophene, thienyl-EDOT and bis-EDOT,10 an
effect attributed to the presence of the electron-withdrawing tri-
azine core. For compound 4 a weakly intense absorption tail is
observed around 500 nm (Fig. 2). The dependence of the position
of this band on solvent polarity is consistent with a charge-
transfer origin.
SnBu3
S
SnBu3
S
8
O
7
Br2, CHCl3
Br
Ar
S
S
Br
N
Ar
S
N
N
S
Ar-SnBu3
N
S
N
N
Pd(PPh3)4
Ar=Thiophene,3,60%
Ar=EDOT,4,83%
9,74%
S
Br
Ar
Scheme 2. Syntheses of compounds 1–5.

P. Leriche et al. / Tetrahedron Letters 50 (2009) 5673–5676
5675
Table 1
Electrochemical and UV–vis spectroscopic data of compounds 1–5 and related
polymers
a
b
Eox b
2
Entry
k
max (nm)
Eox
1
1
2
3
4
316
323
390
423
438
—
1.93
1.38
0.98
0.80
1.27
0.79
0.54
—
1.65
1.17
1.10
1.6
1.26
0.94
5
Poly(3)
Poly(4)
Poly(5)
—
—
a
À5
In CH
2
Cl
2
10 M.
b
Versus SCE, 100 mV/s.
4 6
Figure 4. CV trace of poly(5) in 0.1 M Bu NPF /ACN, scan rate 100 mV/s.
EDOT, which confirms the electron-withdrawing effect of the tri-
azine core.
As expected the increase in the EDOT content of the branches
from 3 to 5 leads to a decrease in the oxidation potential of the
molecule. The CV of the most easily oxidized compounds 4 and 5
present their two successive oxidation waves at 0.98 and 1.17 V
for 4 and 0.80 V and 1.10 V for 5. Application of recurrent potential
scans to electrolytic solutions of compounds 3–5 leads to the pro-
gressive grow of a reversible broad redox system corresponding to
the electrodeposition of an electroactive material on the electrode
surface (Fig. 3). Electrodeposition was found less efficient for com-
pound 3 than for compounds 4 and 5 that both possess a terminal
11
Figure 2. UV–vis spectra of compounds 3–5 in methylenechloride (normalized).
EDOT unit well known to facilitate electropolymerization. The
most efficient polymerization process was observed for compound
4
probably because the density of unpaired electron is maximal on
the terminal EDOT whereas it is more delocalized in the case of
compound 5. As these polymers form highly cross-linked net-
works, their solubility is too low to envision structural character-
ization in solution.
As for the precursor molecules, the oxidation potential of the
polymers decreases with the increase in the number of EDOTs.12
The CV of poly(3) presents a single quasi-reversible broad oxida-
tion wave peaking at 1.27 V the intensity of which rapidly de-
creases upon cycling. The CV of poly(4) presents two redox
processes at 0.79 and 1.26 V, respectively. The first redox process
is reversible but cycling up to the second oxidation potential leads
to a rapid decrease of electroactivity. Finally, poly(5) presents two
reversible redox waves at 0.54 and 0.94 V (Fig. 4). Upon scanning in
the negative potential region an irreversible reduction wave is ob-
served at ca. À2.0 V for poly(5) while poly(4) presents a quasi-
reversible reduction wave at À1.8 V assigned to the reduction of
the triazine core.
To summarize, a series of star-shaped molecules based on a
triazine core substituted with short-chain oligothiophene has
been synthesized. The UV–vis and cyclic voltammetric data of
these compounds show, in agreement with expectations, that
the increase in the number of EDOTs in the branches leads to
a red shift of the absorption spectrum and to a decrease in the
oxidation potential. On the other hand, compounds with a termi-
nal EDOT unit can be electropolymerized to produce electroac-
tive materials.
4 6
Figure 3. Potentiodynamic polymerization of 5, <1 mM in 0.1 M Bu NPF /DCM,
scan rate 100 mV/s.
All compounds are sparingly soluble and therefore electro-
chemical experiments were carried out with saturated solutions
À3
À1
of 1–5 (C <10 mol L ) in methylene chloride (spectro grade)
containing 0.10 M tetrabutylammonium hexafluorophosphate
(
Fluka puriss, used as received) as electrolyte.
Because of the presence of the electron-withdrawing triazine
moiety, oxidation of 1 was not observed in the potential range
accessible with the used electrolytic medium. Compound 2 with
three EDOT units is more easily oxidized and presents an irrevers-
ible oxidation wave at 1.93 V namely 0.50 V more positive than
Acknowledgements
The authors thank Dr. N. Avarvari for fruitful discussions and
M. Lagrée for MS.

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P. Leriche et al. / Tetrahedron Letters 50 (2009) 5673–5676
refluxed for 5 h under nitrogen atmosphere. After cooling, 100 mL of petroleum
Supplementary data
ether is added. After flocculation and filtration of the precipitate, it is dissolved
in methylenechloride and filtered on silica gel leading to 540 mg (90% yield) of
1
Potentiodynamic polymerizations of derivatives 3 and 4 and CV
traces of corresponding polymers. Supplementary data associated
with this article can be found, in the online version, at
doi:10.1016/j.tetlet.2009.07.126.
compound 2 as a white powder. H NMR (CDCl ): 6.65 (s, 1H); 4.51 (m, 2H);
3
4.29 (m, 2H) 13C NMR (CDCl
): 166.6; 145.2; 142.3; 115.9; 107.2; 65.7; À64.0—
3
+
MS-Malditoff (calcd), (501.0) 502.0 (M+H) —MP > °C. Compound 7. Hundred
milligrams of compound are dissolved in 10 mL of 1/1 mixture of
1
a
chloroform and acetic acid. Then, 30 equiv of bromine is added and the
mixture is refluxed for 4 days. The reaction is monitored by TLC and stopped
when only one spot is observed. After cooling, the organic phase is washed
thrice with sodium sulfite, twice with sodium hydrogenocarbonate and once
with water, and dried on magnesium sulfate. After evaporation of toluene, the
residue is dissolved in hot methylenechloride and precipitated by addition of
petroleum ether and cooling at 0 °C yielding 127 mg (74%) of creamy powder.
References and notes
1
2
.
.
Sergeyev, S.; Pisula, W.; Geerts, Y. H. Chem. Soc. Rev. 2007, 36, 1902–1929.
Schmidt-Mende, A.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R.;
MacKenzie, D. Science 2001, 293, 1119–1122; de Bettignies, R.; Nicolas, N.;
Blanchard, P.; Levillain, E.; Nunzi, J.-M.; Roncali, J. Adv. Mater. 2003, 15, 1939–
1
): 7.96 (d, 1H, 3J = 4 Hz); 7.17 (d, 1H, 3J = 4 Hz)— C NMR
13
H NMR (CDCl
CDCl ): 166.7; 142.1; 132.0; 131.6; 120.8—MS-EI (calcd), (562.7; 564.7),
62.5; 564.6 (100%)—MP: 250 °C (decomp).
3
(
5
3
1
943; Kanibolotsky, A. L.; Berridge, R.; Skabara, P. J.; Perepichka, I. F.; Bradley,
D. D. C.; Koeberg, M. J. Am. Chem. Soc. 2004, 126, 13695–13702; Cravino, A.;
Roquet, S.; Alévêque, O.; Leriche, P.; Frère, P.; Roncali, J. Chem. Mater. 2006, 18,
2
9.
Data collections were performed at 150K on a STOE IPDS diffractometer
equipped with graphite monochromator utilizing MoK radiation
k = 0.71073 Å). The structure was solved by direct methods using SIR92
a
a
584–2590; Luo, J.; Zhou, Y.; Niu, Z.-Q.; Zhou, Q.-F.; Ma, Y.; Pei, J. J. Am. Chem.
(
(
Soc. 2007, 129, 11314–11315; Zhou, Y.; Wang, L.; Wang, J.; Pei, J.; Cao, Y. Adv.
Mater. 2008, 20, 3745–3749; Alévêque, O.; Leriche, P.; Cocherel, N.; Frère, P.;
Cravino, A.; Roncali, J. Sol. En. Mat. Sol. Cells 2008, 92, 1170–1174; Yuan, S.-C.;
Sun, Q.; Lei, T.; Du, B.; Li, Y.-F.; Pei, J. Tetrahedron 2009, 65, 4165–4172.
Kronenberg, N. M.; Deppisch, M.; Würthner, F.; Lademann, H. W. A.; Deing, K.;
Meerholz, K. Chem. Commun. 2008, 6489–6491; He, C.; He, Q.; Yang, X.; Wu, G.;
Yang, C.; Bai, F.; Shuai, Z.; Wang, L.; Li, Y. J. Phys. Chem. C 2007, 111, 8661–8666;
Roquet, S.; Cravino, A.; Leriche, P.; Alévêque, O.; Frère, P.; Roncali, J. J. Am. Chem.
Soc. 2006, 128, 3459–3466; Leriche, P.; Frère, P.; Cravino, A.; Alévêque, O.;
Roncali, J. J. Org. Chem. 2007, 72, 8332–8336.
2
Altomare et al., 1993) and refined on F by full-matrix least-squares method,
using SHELXL97 (G. M. Sheldrick, 1998). Non-hydrogen atoms were refined
anisotropically and absorption was corrected by gaussian technique. All
hydrogen atoms were treated with a riding model. The refinement reveals a
positional disorder for one thiophene ring on each independent molecule. The
treatment of disorder leads to define two cycles for the thiophene, the
component being, after refinement, in a 65% majority for the first independent
3
.
.
molecule and equal to 50% for the second. C15
H
9
N
3
S
3
, Mw = 327.43, crystal size
3
0
.38 Â 0.27 Â 0.08 mm , orthorhombic, Pna2
1
, a = 19.537(1) Å, b = 5.0744(3) Å,
3
3
c = 29.532(2) Å, V = 2927.8(3) Å , Z = 8,
collected in the 2–26° h range, 5128 independent reflections (Rint = 0.09),
= 0.0540 and wR = 0.1429 using 4204 reflections with I >2 (I), R = 0.0664
qcalc = 1.486 g/cm , 15,842 reflections
4
Cherioux, F.; Guyard, L.; Audebert, P. Chem. Commun. 1998, 2225–2226;
Cherioux, F.; Guyard, L.; Audebert, P.; Zyss, J. Chem. Commun. 1999, 2083–2084;
See also: Ak, M.; Ak, M. S.; Toppare, L. Macromol. Chem. Phys. 2006, 207, 1351–
R
1
2
r
1
and wR
2
= 0.1564 using all data, absolute structure parameter 0.2(1),
1
358.
À3
GOF = 1.047, À0.475 <
D
q
< 0.607 e Å
.
5
6
7
8
.
.
.
.
Hayami, S.; Inoue, K. Chem. Lett. 1999, 7, 545–546; Li, S.-H.; Luang, H.-P.; YU, S.-
Y.; Li, X.-P. Chinese J. Chem. 2006, 24, 1225–1229.
Riobé, F.; Grosshans, P.; Sidorenkova, H.; Geoffroy, M.; Avarvari, N. Chem. Eur. J.
1
1
1
0. Akoudad, S.; Roncali, J. Synth. Met. 1998, 93, 2, 111–114; Hergé, N.; Leriche, P.;
Blanchard, P.; Gallégo-Planas, N.; Allain, M.; Frère, P.; Roncali, J. New J. Chem.
2
008, 32, 932–936.
2
009, 15, 380–387.
1. Roncali, J.; Blanchard, P.; Frère, P. J. Mater. Chem. 2005, 15, 1589–1610; Piron,
F.; Leriche, P.; Mabon, G.; Grosu, I.; Roncali, J. Electro. Chem. Commun. 2008, 10,
Fujita, M.; Oka, H.; Oguro, K. Tetrahedron Lett. 1995, 36, 5247–5250; Faust, R.;
Göblet, B. Tetrahedron Lett. 1997, 38, 8017–8020.
Examples of procedures and spectroscopic data. Compound 2. Hundred milligrams
of 2,4,6-trichloro-2,4,5-triazine are suspended in 30 mL of dry toluene. Four
equivalents of the stannic derivative of EDOT are added and the mixture is
1
427–1430.
2. Turbiez, M.; Frère, P.; Roncali, J. J. Org. Chem. 2003, 68, 5357–5360; Turbiez, M.;
Frère, P.; Allain, M.; Videlot, D.; Ackermann, J.; Roncali, J. Chemistry Eur. J. 2005,
1
1, 12, 3742–3752.
3 4
degassed with nitrogen. Then, Pd(PPh ) (5%) is added and the mixture is