Facile synthesis and characterization of a novel thiophene-fused polycyclic aromatics based on pyrene

Article abstract of DOI:10.1016/j.cclet.2010.12.054

A novel large thiophene-fused polycyclic aromatics 1 based on pyrene 5 has been synthesized, and its structure was confirmed by ¹H NMR, ¹³C NMR, MS, UV-vis and elemental analysis. The key steps of the synthesis involved the Stille cross-coupling reaction and followed by selective β-β oxidative cyclization of pendant thienyl rings by FeCl₃ under mild conditions.

Full text of DOI:10.1016/j.cclet.2010.12.054

Available online at www.sciencedirect.com
Chinese Chemical Letters 22 (2011) 819–822
www.elsevier.com/locate/cclet
Facile synthesis and characterization of a novel thiophene-fused
polycyclic aromatics based on pyrene
a,b,*
c
, Zhi Gang Yang , Dong Jie Liu , Li Cai , Daiki Hoshino ,
b
a
a
Zong Fan Duan
a
b
Tsuyoshi Morita , Gao Yang Zhao , Yasushiro Nishioka
a
a
College of Science and Technology, Nihon University, Narashinodai, Funabashi, Chiba 2748501, Japan
b
School of Materials Science & Engineering, Xi’an University of Technology, Xi’an 710048, China
c
College of Pharmacy, Nihon University, Narashinodai, Funabashi, Chiba 2748555, Japan
Received 28 October 2010
Available online 17 April 2011
Abstract
A novel large thiophene-fused polycyclic aromatics 1 based on pyrene 5 has been synthesized, and its structure was confirmed by
13
H NMR, C NMR, MS, UV–vis and elemental analysis. The key steps of the synthesis involved the Stille cross-coupling reaction
1
and followed by selective b-b oxidative cyclization of pendant thienyl rings by FeCl
#
3
under mild conditions.
2011 Zong Fan Duan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Pyrene; Thiophene; Stille coupling reaction; Oxidative cyclization
In the past decades, thiophene-containing aromatics electronic materials with well-defined geometry and desired
electronic properties have attracted considerable attention through applications such as organic light emitting diodes
(
OLEDs), solar cells and organic field effect transistors (OFETs) [1]. Research initially focused on thiophene-
containing aromatics because a ‘‘bridging sulfur’’ could effectively maximize the p-orbital overlap by a reduction of
the freedom of rotation in the molecule and possibly induce a densely packed crystal structure toward face-to-face p–
p stacking motifs. In addition to, thiophene moiety could effectively provide aromatic stability to aromatics [2].
Since both the molecular size and the shape of thiophene-containing aromatics play very important roles on their
electronic properties, energetics, and reactivity, the construction of novel nanosized, soluble and planar thiophene-
containing polycyclic aromatic compounds is of great interest. We reported herein a facile synthetic approach (Scheme
1) for novel large thiophene-fused polycyclic aromatics 1 based on pyrene 5. The process of the synthesis mainly
involved Friedel–Crafts alkylation reaction, bromination, Stille coupling reaction and followed by selective b-b
oxidative cyclization of pendant thienyl rings by FeCl under mild conditions. The target compound 1 with large
3
planar p-conjugated structure has the tendency to self-associate through p–p stacking between molecules, and may be
a good candidate material in organic electronic devices.
Electrophilic substitution of pyrene 5 occurred at the 1-,3-,6- and 8-position [3], but not at the other position
(
2,4,5,7,9 and 10). Therefore, pyrene 5 substituted at the latter position must be prepared in ways other than by direct
*
Corresponding author.
E-mail address: duanzf@xaut.edu.cn (Z.F. Duan).
1001-8417/$ – see front matter # 2011 Zong Fan Duan. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
doi:10.1016/j.cclet.2010.12.054

[()TD$FIG]
8
20
Z.F. Duan et al. / Chinese Chemical Letters 22 (2011) 819–822
C H
C H
C H
C H
6 13
6
13
6
13
6
13
S
S
S
S
S
S
S
S
f
C H
C H
C H
C H
6 13
6
13
6
13
6
13
1
2
a
b
C H
Bu3Sn
e (80%)
6
13
C H
S
S
6
13
S
6
7
8
Br
Br
Br
Br
c
d
5
4
3
b
Bu Sn
e (83%)
3
S
6
S
9
S
S
S
S
S
S
S
f
S
1
1
10
Scheme 1. (a) i: n-BuLi, THF, À78 8C; ii: bromohexane, room temperature, 86%; (b) i: n-BuLi, THF, À78 8C; ii: tributyltin chloride, room
temperature, 100%; (c) tert-butyl chloride, powdered AlCl , 0 8C, 88%; (d) bromine, CCl , Fe powder, 0 8C, 80%; (e) DMF, Pd(PPh , 100 8C,
3
4
3 4
)
8
0%, 83%; (f) i: FeCl , CH NO , CH Cl , 0 8C; ii: MeOH, 78%.
3
3
2
2
2
electrophilic substitution. Firstly, Friedel-Crafts alkylation of 5 with tert-butyl chloride can introduce tert-butyl group as
a positional protective group into the 2- and 7-positions of 5 [4]. After that the bromination of 2,7-di-tert-butylpyrene 4
with 6 mol equiv. of bromine in a carbon tetrachloride solution in the presence of iron powder afforded 4,5,9,10-
tetrabromo-2,7-di-tert-butylpyrene 3 [5]. The synthesis of 4,5,9,10-tetra(2-(5-hexyl)- thienyl)-2,7-di-tert- butylpyrene 2
was commenced with the Stille cross-coupling reaction between the 4,5,9,10- tetrabromo-2,7-di-tert-butylpyrene 3 and
2
-(tributylstannyl)-5-hexylthiophene 8 in the presence of Pd(PPh ) , which was prepared in situ from the lithiation of 2-
3 4
alkylthiophene 7 with n-BuLi followed by the addition of tributyltin chloride [6], in 80% yield. It is noteworthy that other
cross-coupling reactions such as Kumada and Negishi reaction were also attempted, but the very low product yields as
wellas the necessityofits separation from the reactionmixture make them unacceptable. As an effectiveway to construct
polycyclic aromatic hydrocarbons, The FeCl oxidative cyclization was then utilized to construct the extended
3
thiophene-fused oligoacene skeletonvia thienyl–thienyl carbon–carbon bond formation [7]. The oxidative cyclization of
2
using various equivalences of FeCl was performed at different temperature, and the progress was monitored by thin-
3
layerchromatography(TLC). Theoxidative cyclizationof2with8equivalencesofexcessiveFeCl carriedoutat0 8Cfor
3
30 min has the highest yield (78%) for 1. The new compounds 1 and 2 are all soluble in common solvents such as
chloroform, THF, and even hexane, and can be purified by column chromatography and recrystallization.

[()TD$FIG]
Z.F. Duan et al. / Chinese Chemical Letters 22 (2011) 819–822
821
1
Fig. 1. Comparison of H NMR spectra of compound 1 in CDCl
3
at different concentrations.
To assess the specificity of the oxidative cyclization reaction under FeCl , we synthesized and chose model
3
compound 4,5,9,10-tetra(2-thienyl)-2,7-di-tert-butylpyrene 10, which have the pendant 2-thienyl rings without
blocking groups on the reactive a- and b-sites. We envisioned that the b-b-sites of 10 would participate in the discrete
monomer cyclization to afford 11 selectively. However, attempts to do oxidative cyclodehydrogenation of 10 gave a
complicated mixture rather than 11 even a shortage amount of FeCl (1.8 equiv per C–C bond formation) and diluted
3
reaction concentration were used. The above results indicate that the FeCl -mediated cyclization usually could not be
3
applied to electron-rich precursors, further more the known higher spin density and electron-rich property at the a-
position of thiophene ring favors C–C coupling reaction at this site to form cross-linked oligomers and polymers rather
than discrete monomer cyclization compounds.
1
13
In addition to H and C NMR spectra and elemental analysis, FAB-MS we also employed to characterize the
structure and molecular weight of all new compounds [8]. We observed the exact loss of four hydrogen atoms from the
FAB-MS spectrum of 1 in comparison with that of 2. After cyclization, the resonance at about 6.81 ppm which can be
assigned to the b-H on the thiophene rings closer to the pyrene core in 2 disappeared and the signals at 6.71 ppm as a
doublet correlated to the other b-H of thiophene rings now appeared as a singlet with significant 0.87 ppm downfield
1
shift in the H NMR spectrum of 1.
The compound 1 with large p-conjugated structure has the tendency to self-associate through p–p stacking
1
between molecules, which was evidenced by its concentration-dependent H NMR spectra in CDCl and the
3
comparison of absorption spectra in solution and in solid state. It was observed that the single peak at a chemical shift
of 7.61 ppm assigned to the protons in the fused-thiophene rings significantly shifted to 7.47 ppm when the
concentration of the CDCl solution increased from 2 to 50 mg/mL at room temperature, as shown in Fig. 1. Such an
3
upfield shift was attributed to the intermolecular shielding from the neighboring aromatic molecules in concentrated
solution, implying their tendency to self-associate through p–p stacking between molecules [9]. In contrast, the other
compounds 2 and 10 showed no concentration-dependant chemical shift likely due to the relative small p-conjugated
system and the intermolecular steric repulsion between the twisted thiophene arms. Fig. 2 shows the absorption spectra
À5
in CH Cl solution (10 mol/L) and in thin film fabricated by vacuum evaporation on quartz glasses with a nominal
2
2
thickness of 50 nm. The absorption of 1 in dilute CH Cl solution showed the absorption maximum at 338 nm with a
2
2
small peak at 410 nm. Compared with that in solution, the absorption spectrum of 1 in thin film became less structured.
[()TD$FIG]
Fig. 2. Normalized UV–vis absorption spectra of 1 in dichloromethane and in thin film.

822
Z.F. Duan et al. / Chinese Chemical Letters 22 (2011) 819–822
The maximum absorption red-shifted about 6 nm from 338 to 344 nm, while the small peak at 410 nm red-shifted
about 5 nm–415 nm accompanied with obvious absorbance enhancement and the absorption edge underwent 13 nm
red-shifts. The compound 1 with the larger rigid core has strong p–p interactions in the solid state and may be a good
candidate material in organic electronic devices. Further studies on the application of this molecule to electronic
devices such as field effect transistors are currently underway.
Acknowledgments
The film investigated in this work was fabricated in the Micro Functional Device Research Center of the College of
Science and Technology, Nihon University.
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[
8] Spectral data of compound 1: a yellow solid. mp 185–187 8C. H NMR (600 MHz, CDCl
3
): d 9.48 (s, 4H), 7.58 (s, 4H), 3.11 (t, 8H, J = 7.4 Hz),
): d 149.0,
1
.90–1.91 (m, 8H), 1.78 (s, 18H), 1.50–1.53 (m, 8H), 1.35–1.41(m, 16H), 0.92 (t, 12H, J = 7.2 Hz). C NMR (150 MHz, CDCl
+
47.3, 135.5, 132.9, 128.4, 124.3, 122.5, 122.0, 119.2, 36.0, 32.1, 31.7, 31.3, 30.6, 28.9, 22.6, 14.1. FAB-MS, m/z: [M] 974.3. Anal. Calcd. for
3
1
3
1
1
C
64
H
78
S
4
: C, 78.79; H, 8.06. Found: C, 78.62; H, 8.17. Spectral data of compound 2: a pale yellow solid. mp = 113–15 8C. H NMR (600 MHz,
): d 8.24 (s, 4H), 6.81 (d, 4H, J = 3.4 Hz), 6.71 (d, 4H, J = 3.4 Hz), 2.83 (t, 8H, J = 7.2 Hz), 1.67–1.70 (m, 8H), 1.35(s, 18H), 1.32–1.35
CDCl
3
13
(
m, 24H), 0.91 (t, 12H, J = 6.9 Hz). C NMR (150 MHz, CDCl
3
): d 148.3, 146.4, 137.7, 132.9, 131.1, 128.8, 123.4, 122.3, 35.4, 31.8, 31.7, 31.6,
0.1, 28.5, 22.7, 14.1. FAB-MS, m/z: [M] 978.6. Anal. Calcd. for C64 : C, 78.46; H, 8.44. Found: C, 78.68; H, 8.20. Spectral data of
): d 8.17 (s, 4H), 7.40 (dd, 4H, J = 5.2, 1.4 Hz), 7.05–7.08 (m, 8H), 1.33
+
3
82 4
H S
1
compound 10: a white solid. Mp > 300 8C. H NMR (600 MHz, CDCl
3
1
3
+
(
s, 18H). C NMR (150 MHz, CDCl
Anal. Calcd. for C40 : C, 74.72; H, 5.33. Found: C, 74.96; H, 5.18
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34 4
H S
[
(
(
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