Linear C2-symmetric polycyclic benzodithiophene: Efficient, highly diversified approaches and the optical properties

Article abstract of DOI:10.1016/j.tetlet.2005.09.107

Two facile approaches to two new series of the seven-rings fused benzodithiophene-based polycyclic aromatics are developed in good yields.

Full text of DOI:10.1016/j.tetlet.2005.09.107

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 8153–8157
Linear C2-symmetric polycyclic benzodithiophene: efficient,
highly diversified approaches and the optical properties
Cui-Hua Wang,a Rong-Rong Hu,a Shuang Liang,a Jia-Hua Chen,a
a,
Zhen Yanga,b, and Jian Pei
*
*
aKey Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry,
Peking University, Beijing 100871, China
bVivoQuest, Inc. 711 Executive Blvd, Valley Cottage, NY 10989, USA
Received 30 August 2005; revised 19 September 2005; accepted 20 September 2005
Available online 10 October 2005
Abstract—Two facile approaches to two new series of the seven-rings fused benzodithiophene-based polycyclic aromatics are devel-
oped in good yields.
Ó 2005 Elsevier Ltd. All rights reserved.
Organic molecules as semiconducting materials are of
great interest nowadays in electronic devices, particu-
larly in organic film effect transistors (OFETs),1 organic
light emitting diodes (OLEDs).2 Among p-conjugated
systems, polycyclic aromatics including acenes and
derivatives,3 heteroacenes,4 ladder-type oligo- and
poly(p-phenylene),5 polycyclic aromatic hydrocarbons
(PAHs),6 are most promising because of their rigid, pla-
nar, conjugated structures and ordered stacking proper-
ties. However, the application of acenes and
heteroacenes derivatives is limited due to their liability
and poor solubility.
available 1,4-dibromobenzene, and compound 4 then
underwent the Sonogashira reaction with four types of
terminal alkynes followed by methylthiolation to give
the corresponding products 5a–d in good yields. a-Posi-
tions of thiophene rings of 4c were also methylthiolated
to produce 5c. Such mercapto end groups of 5c were
useful for the assembly of the metal surface and the con-
jugated oligomers to construct the semiconducting
molecular wire. It was noteworthy that alkynes with var-
ious aromatic substituents could be introduced during
the Sonogashira reaction to realize the diversity. Thus,
the framework of compounds 3a–d8 was successfully
established by the iodine-induced cyclization developed
by Larock.9
Although benzodithiophene derivatives have been stud-
ied in OFETs,1a,7 few linear sulfur-incorporated fused
polycyclic aromatics were reported, probably owing to
the deficiency of the feasible synthetic methods. There-
fore, development of such new structures and efficient
synthetic methodologies for fused polycyclic aromatics
is imperative to content the requirement of new organic
materials in stability, solubility and processibility.
To improve the solubility of the desired compounds in
common organic solvents, 3b and 3d were then selected
to do the further synthetic transformation with regard to
their lipophilic R groups (hexyl and TIPS). Thus, 3b and
3d were first coupled with TMS acetylene by the Pd-cat-
alyzed Sonogashira reaction, and then underwent desily-
lation to give the terminal alkynes 6b and 6d in a
combined yield of 76% and 78%, respectively. As a
result, the poly-fused aromatic molecules 1a and 1b10
were eventually accomplished from 6b and 6d by the
Pt-catalyzed double cyclization (Scheme 2).11
Scheme 1 illustrates the synthetic route to the key
intermediate 3. 1,4-Dibromo-2,5-diiodobenzene 4 was
obtained by selective iodination from the commercially
For solubility reason, another type of polyaromatic
molecules was also designed based on the n-hexyl and
TIPS substituted substrates 3b and 3d. As described
in Scheme 3, 3b and 3d were first subjected to the
Keywords: Polycyclic; Cyclization; Conjugation; Acenes.
*
Corresponding authors. Tel.: +86 10 62758145; fax: +86 10
62759105; e-mail addresses: jianpei@pku.edu.cn; yangzhen@
pku.edu.cn
0040-4039/$ - see front matter Ó 2005Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.09.107

8154
C.-H. Wang et al. / Tetrahedron Letters 46 (2005) 8153–8157
Br
Ar
Br
Br
Br
Sonogashira
I
I2, H2SO4
reaction
120-135 0C
I
Ar
Br
Br
4
4a, Ar = Phenyl, 81%
4b, Ar = 4-C6H13O-C6H4, 70%
4c, Ar = 2-thienyl, 68%
4d, Ar = 4-TIPSO-C6H4, 60%
nBuLi, Et2O, -78 oC to 0 oC
then Me2S2, -78 oC to rt
I
SMe
SMe
Ar
S
I2, CH2Cl2
Ar
Ar
S
I
Ar
3a, Ar = Ph, 96%
3b, Ar = 4-C6H13O-C6H4, 85%
3c, Ar = 2-MeS-thienyl, 95%
3d, Ar = 4-TIPSO-C6H4, 81%
5a, Ar = Phenyl, 85%
5b, Ar = 4-C6H13O-C6H4, 84%
5c, Ar = 2-MeS-thienyl, 52%
5d, Ar = 4-TIPSO-C6H4, 81%
Scheme 1.
and the purity of all the final products were verified by
1H, 13C NMR and MALDI-TOF MS spectra. It is note-
worthy that 1a showed little solubility in most of the
organic solvents, which might be due to the strong inter-
molecular p–p interactions as well as the ordering aggre-
gation of the lipophilic groups.14 Compound 1b had
considerate solubility in common solvents, such as
CH2Cl2 and THF, because the bulky –OTIPS group in
1b effectively reduced the p–p interaction. For spiro-type
compounds 2c and 2d, we observed the characteristic
changes in 1H NMR spectra after the cyclization in com-
parison with those of 7c–d and 1. For 1b, the meta-cou-
pling between Hb and Hc split Hc into doublet
(J = 2.4 Hz) in 7.40–7.41 ppm, and Hc into quaterlet,
respectively. The singlet assigned to Ha was shifted
downfield from 8.30 to 8.72 ppm because of the forma-
tion of the larger p-conjugated system. However, the
singlet signal of Ha in 2d moved to upfield (6.88 ppm)
due to the effect of the circle current of the spiro-cyclic
structure (Scheme 3 and Fig. 1).
I
S
RO
OR
S
3b, R = -C6H13
3d, R = -TIPS
I
,
1. TMS
Pd(PPh3)4, CuI, iPr2NH
toluene, 80 0C
2. NaOH, MeOH
S
RO
OR
S
6b, R = -C6H13, 76%
6d, R = -TIPS, 78%
PtCl2, toluene, 80 oC
Ha
Hb
S
We also investigated the electric optical properties for
1a–b, 2a–d in dilute solutions. Normally, p-conjugated
oligomers and polymers exhibit the strong p–p* electron
absorption behaviors in the ultraviolet and visible
region. All the new compounds showed strong fluores-
cence under UV irritation. Table 1 summarizes the
results of the absorption and emission of compounds
1a–b and 2a–d at room temperature. Figures 2 and 3
illustrate UV–vis and fluorescent spectra of these six
compounds in dilute THF solutions, respectively. Both
absorption and emission spectra of all compounds
revealed a characteristic behavior because of the well-de-
fined structure of these compounds in solutions. Owing
to the same backbone, 1a showed the same absorption
features as 1b, in which the absorption peaked at
348 nm with several vibronic behaviors at 361 and
374 nm, respectively. 2a–d also showed the similar
OR
RO
S
Hc
1a, R = -C6H13, 45%
1b, R = -TIPS, 51%
Scheme 2.
lithium–halogen exchange by treatment with t-BuLi,
and then trapped with benzophenone and fluorenone
to afford the corresponding tertiary alcohols 7a–d. Even-
tually, the desired polycyclic molecules 2a–d12 were
achieved by the Friedel–Craft cyclization.13
As expected, compounds 2a–d were readily soluble in
common organic solvents, which provided us the conve-
nience to get the characterization data. The structure

C.-H. Wang et al. / Tetrahedron Letters 46 (2005) 8153–8157
8155
I
S
RO
OR
S
I
3b, R = -C6H13
3d, R = -TIPS
rBuLi, Et2O
then R'COR''
-78 0C to 0 0C
-78 0C to 25 0C
R''
R'
OH
S
RO
OR
S
HO
R'
7a, R = -C6H13, R' = R'' = -Ph, 84%
7b, R = -TIPS, R' = R'' = -Ph, 78%
R''
7c, R = -C6H13, R', R'' = 9, 9'-fluorene, 85%
7d, R = -TIPS, R', R'' = 9, 9'-fluorene, 72%
For 7c and 7d
BF3.Et2O, CH2Cl2
25 0C
For 7a and 7b
conc. H2SO4, HOAc
80 0C
Ha
S
Ph
Ph
OR
S
OR
RO
S
RO
S
Ph Ph
2a, R = -C6H13, R' = R'' = -Ph, 63%
2b, R = -TIPS, R' = R'' = -Ph, 68%
2c, R = -C6H13, R', R'' = 9, 9'-fluorene, 58%
2d, R = -TIPS, R', R'' = 9, 9'-fluorene, 61%
Scheme 3.
R''
R
S
S
S
R
R
Br
I
1
2
R''
S
Ar
Ar
R' R'
S
I
3
Br
R
S
R'
R'
Figure 1. Retrosynthetic analysis.
1a
1b
2a
2b
2c
2d
Table 1. Photophysical properties of compounds 1a–1b and 2a–d in
dilute THF solutions at room temperature (bold means the maximum
peak)
1
0.8
0.6
0.4
0.2
0
Compounds
kabs (nm)
kPL (nm)
1a
1b
2a
2b
2c
2d
348, 360, 372
348, 361, 374
349, 364, 384, 408
348, 363, 384, 407
348, 363, 383, 407
348, 362, 382, 406
362, 382.5, 407
363, 383.5, 407.5
403.5, 424, 45 2
402, 423.5, 451.5
401.5, 423, 45 1
403.5, 421.5, 45 0
300
350
400
450
500
Wavelength (nm)
absorption spectra and exhibited the maximum absorp-
tions at 407 and 383 nm, respectively. The aromatic sub-
stituents at methylene bridge did not play any role on
Figure 2. Normalized absorbance spectra of 1a–b, 2a–d recorded in
dilute THF solutions at room temperature.

8156
C.-H. Wang et al. / Tetrahedron Letters 46 (2005) 8153–8157
References and notes
1
0.8
0.6
0.4
0.2
0
1a
1b
2a
2b
2c
2d
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dilute THF solutions at room temperature.
the effective conjugation length of the backbone due
to sp3-hybrid carbon atoms.10 Moreover, the absorp-
tion maximum peaks of 2a–d red-shifted about 59 nm
in comparison with those of 1a–b, which meant
that the effective conjugation length of 2a–d were
longer than those of 1a–b. The absorption spectra of
2a–d obviously red-shifted in comparison with that of
the ladder oligo(p-aniline) (378 nm) were close to
those of ladder oligo(p-phenylene).5f In comparison
with the silicon-incorporated ladder oligo(p-phenylene),
the absorption of 2a–d slightly blue-shifted about
25nm. 5d
The emission behaviors of all compounds showed the
well-defined vibronic structures. The emission spectra
of compounds 1a and 1b exhibited the maximum at
about 383 nm with two peaks at 362 and 407 nm, respec-
tively. Compounds 2a–d peaked at about 424 nm with
two shoulders at about 402 and 451 nm, respectively.
We also observed the smaller StockÕs shift for 2a–d,
which indicated that no obvious aggregation or inter-
chain interactions for 2a–d were formed in the excited
states due to the steric structures.
Mullen, K. J. Am. Chem. Soc. 2004, 126, 6987–6995; (d)
¨
Wakim, S.; Bouchard, J.; Blouin, N.; Michaud, A.;
Leclerc, M. Org. Lett. 2004, 6, 3413–3416; (e) Haryono,
A.; Miyatake, K.; Natori, J.; Tsuchida, E. Macromolecules
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In summary, we have developed an efficient and highly
diversified route to construct novel, unique linear fused
p-conjugated polycyclic systems. These sulfur-incorpo-
rated fused polycyclic aromatics show interesting
optical properties. The maximum absorption peaks
of 2a–d obviously red-shifted compared to those of
1a–b. The properties in thin solid films including the
carrier mobility are still under investigation in our
laboratory. These linear polycyclic aromatic derivatives
should also be of potential application as semiconduct-
ing materials.
7. (a) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.;
Otsubo, T. J. Am. Chem. Soc. 2004, 126, 5084–5085; (b)
Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.;
Dodabalapur, A. Adv. Mater. 1997, 9, 36.
8. Compounds 3a–d were obtained as solids. 3b (yield: 87%):
1H NMR (300 MHz, CDCl3, ppm): d 0.91–0.95(t, 6H),
1.36–1.50 (m, 12H), 1.80–1.85 (m, 4H), 4.01–4.06 (t, 4H),
7.0–7.03 (d, J = 9.0 Hz, 4H), 7.66–7.69 (d, J = 9.0 Hz,
4H); 8.21 (s, 2H); 13C NMR (100.6 MHz, CDCl3, ppm): d
14.1, 22.6, 25.7, 29.2, 31.6, 68.1, 103.5, 114.4, 119.1, 126.5,
131.2, 136.8, 140.6, 143.3, 159.8; MS m/z (EI): 794 (M+,
Acknowledgements
1
100%). 3c: H NMR (300 MHz, CDCl3, ppm): d 2.60 (s,
6H), 7.10–7.12 (d, 2H), 7.49–7.51 (d, 2H), 8.14–8.16 (d,
The present research was financially supported by
NSFC, Chinese Academy of Sciences and State Key
Basic Research Program. Z. Yang and J. Pei thank the
National Science Fund for Distinguished Young
Scholars. The authors wish to thank the anonymous
referees for their comments and suggestion, which
enable us to improve the manuscript.
1
2H); MS m/z (EI): 698 (M+, 100%). 3d (yield: 80%): H
NMR (300 MHz, CDCl3, ppm): d 1.13–1.16 (d, 36H),
1.27–1.34 (m, 6H), 6.98–7.01 (d, J = 8.4 Hz, 4H), 7.60–
7.63 (d, J = 8.4 Hz, 4H), 8.21 (s, 2H); 13C NMR
(100.6 MHz, CDCl3, ppm): d 12.7, 17.9, 119.2, 119.9,
127.1, 131.2, 136.8, 140.7, 143.4, 157.0; MS m/z (EI): 938
(M+, 100%).

C.-H. Wang et al. / Tetrahedron Letters 46 (2005) 8153–8157
8157
9. Yue, D.; Larock, R. C. J. Org. Chem. 2002, 67, 1905–1909.
10. Compounds 1a–b were obtained as solids. 1b (yield: 51%):
1H NMR (400 MHz, CD2Cl2, ppm): d 1.12–1.16 (d, 36H),
1.32–1.34 (m, 6H), 7.27–7.30 (q, J = 2.4 Hz, J = 8.8 Hz,
2H), 7.40–7.41 (d, J = 2.4 Hz, 2H), 7.77–7.79 (d,
J = 8.8 Hz, 2H), 8.02–8.04 (d, J = 8.8 Hz, 2H), 8.19–8.21
(d, J = 8.8 Hz, 2H), 8.72 (s, 2H); 13C NMR (100.6 MHz,
CDCl3, ppm): d 12.8, 18.0, 114.7, 115.1, 116.2, 119.9,
122.4, 124.3, 124.7, 125.9, 130.3, 131.7, 134.0, 135.7, 136.1,
137.1, 138.0; MS m/z (EI): 734 (M+, 100%).
(M+, 100%). 2c (yield: 53%): 1H NMR (300 MHz, CDCl3,
ppm): d 0.81–0.86 (m, 6H), 1.23–1.34 (m, 12H), 1.54–1.63
(m, 4H), 3.72–3.77 (m, 4H), 6.23–6.24 (d, J = 2.1 Hz, 2H),
6.71–6.74 (d, J = 7.5Hz, 4H), 6.78–6.82 (q, J = 2.4 Hz,
4H), 6.85(s, 2H), 7.04–7.09 (t, 4H), 7.37–7.42 (m, 6H),
7.93–7.95(d, J = 7.5Hz, 4H); 13C NMR (100.6 MHz,
CDCl3, ppm): d 14.0, 22.5, 25.6, 29.2, 31.5, 63.7, 68.1,
110.8, 112.9, 114.9, 115.8, 120.1, 120.3, 123.9, 127.9, 128.0,
130.1, 131.6, 141.5, 141.7, 143.9, 146.0, 154.3, 158.7;
1
MALDI-TOF MS: 866 (M+, 100%). 2d (yield: 61%): H
11. Furstner, A.; Mamane, V. J. Org. Chem. 2002, 67, 6264–
NMR (300 MHz, CDCl3, ppm): d 0.91–0.94 (d, 36H),
1.04–1.09 (m, 6H), 6.17–6.18 (d, J = 2.4 Hz, 2H), 6.70–
6.73 (d, J = 7.8 Hz, 4H), 6.77–6.80 (q, J = 2.4 Hz, 4H),
6.88 (s, 2H), 7.03–7.08 (t, 4H), 7.32–7.42 (m, 6H), 7.93–
7.95(d, J = 7.8 Hz, 4H); 13C NMR (100.6 MHz, CDCl3,
ppm): d 12.5, 17.8, 63.6, 114.9, 116.2, 118.7, 119.9,
120.2, 123.8, 127.8, 128.0, 130.2, 132.1, 141.5, 141.6,
141.8, 144.0, 145.8, 154.2, 155.4; MALDI-TOF MS: 1010
(M+, 100%).
¨
6267.
12. Compounds 2a–b were obtained as solids. 2a (yield: 63%):
1H NMR (300 MHz, CDCl3, ppm): d 0.87–0.90 (t, 6H),
1.30–1.32 (m, 8H), 1.42–1.55 (m, 4H), 1.71–1.75 (m, 4H),
3.89–3.92 (t, 4H), 6.83–6.86 (q, 2H), 7.00 (d, 2H), 7.21–
7.38 (m, 20H), 7.38–7.40 (d, 2H), 7.87 (s, 2H); 13C NMR
(100.6 MHz, CDCl3, ppm): d 14.0, 22.6, 25.7, 29.2, 31.6,
64.4, 68.3, 112.9, 113.0, 116.4, 120.3, 126.9, 128.4, 128.5,
130.4, 131.0, 141.7, 142.3, 143.0, 145.2, 158.1, 158.8;
13. (a) Qiu, S.; Lu, P.; Liu, X.; Shen, F.; Liu, L.; Ma, Y.; Shen,
J. Macromolecules 2003, 36, 9823–9829; (b) Pei, J.; Ni, J.;
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Deloffre, F.; Servet, B.; Ries, S.; Alnot, P. J. Am. Chem.
Soc. 1993, 115, 8716–8721.
1
MALDI-TOF MS: 870 (M+, 100%). 2b (yield: 68%): H
NMR (300 MHz, CDCl3, ppm): d 0.99–1.05(d, 36H),
1.14–1.21 (m, 6H), 6.83–6.86 (q, J = 2.1 Hz, 2H), 6.95–
6.96 (d, J = 2.1 Hz, 2H), 7.21–7.29 (m, 22H), 7.90 (s, 2H);
13C NMR (100.6 MHz, CDCl3, ppm): d 12.5, 17.8, 64.3,
116.4, 117.8, 119.0, 120.3, 126.9, 128.3, 128.4, 130.7, 131.0,
141.7, 142.9, 145.3, 155.5, 158.0; MALDI-TOF MS: 1014