Synthesis of thiophene/phenylene co-oligomers. III [1]. Thienyl-capped oligophenylenes

Article abstract of DOI:10.1002/jhet.5570380417

The author reports the synthesis of thienyl-capped oligophenylenes via improved synthetic schemes. These schemes are based on either the Grignard or Suzuki coupling reaction and enable the author to obtain the target compounds at appreciably high yields. Regarding several of these compounds, their synthesis and characterization are believed to be reported for the first time. The resulting materials have been fully characterized through the nmr and ir spectroscopy. The ir analysis is particularly useful in characterizing the materials of higher molecular weight, since those materials are difficult to dissolve in organic solvents.

Full text of DOI:10.1002/jhet.5570380417

Jul-Aug 2001
Synthesis of Thiophene/phenylene Co-oligomers. III [1].
Thienyl-capped Oligophenylenes
S. Hotta*
923
Joint Research Center for Harmonized Molecular Materials (JRCHMM)–Japan Chemical Innovation Institute
(JCII), c/o National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1
Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received December 19, 2000
The author reports the synthesis of thienyl-capped oligophenylenes via improved synthetic schemes.
These schemes are based on either the Grignard or Suzuki coupling reaction and enable the author to obtain
the target compounds at appreciably high yields. Regarding several of these compounds, their synthesis and
characterization are believed to be reported for the first time. The resulting materials have been fully charac-
terized through the nmr and ir spectroscopy. The ir analysis is particularly useful in characterizing the mate-
rials of higher molecular weight, since those materials are difficult to dissolve in organic solvents.
J. Heterocyclic Chem., 38, 923 (2001).
Introduction.
EXPERIMENTAL). Regarding its methyl derivative
(T2P-Me) neither the synthesis nor characterization has
yet been presented.
Various oligothiophenes and oligophenylenes are cur-
rently synthesized and developed, since these materials are
potentially useful as electroactive and/or photoactive
organic semiconductors in thin film devices. Using the for-
mer materials, for instance, field-effect transistors have
been proposed and fabricated [2,3]; the latter have been
studied on electroluminescent devices [4,5].
Preparation of the Materials.
Scheme 1 shows synthetic routes of T1P, T2P, and
T2P-Me based on the Grignard coupling reaction. All
these compounds were synthesized in diethyl ether. In the
case of the syntheses of T2P and T2P-Me, 2 was nearly
insoluble in diethyl ether, however this compound gradu-
ally disappeared during overnight stirring, followed by the
generation of strongly luminescent precipitates. Thus a
yield of 22% (after purification through recrystallization)
was attained for T2P. This is two and a half times as large
as the previously reported yield [10]. This is perhaps due
to the fact that iodine in 2 is more reducible in coexistence
with 2-thienylmagnesium iodide than with 2-thienylmag-
nesium bromide [11]. Note that the combination of 2 with
the latter reagent was adopted in the previous report [10].
In view of such promising aspects, Hotta and coworkers
[6] and Samulski and coworkers [7] have most recently
and independently developed thiophene/phenylene co-
oligomers (i.e. the hybridized oligomers comprising thio-
phenes and phenylenes) in which the electronic structure
and the extension of the π-conjugation can be tuned as
desired. This can be done by changing the total ring num-
ber of the thiophenes and phenylenes and their mutual
arrangement in the molecule [6]. Thus Hotta et al. have
presented in previous papers [1,8] the syntheses and char-
acterization of a variety of co-oligomers. Those include the
phenyl-capped oligothiophenes and block and alternating
co-oligomers. They have found out that these compounds
exhibit unique molecular alignments in thin films [9].
In this article the author reports syntheses of a series of
thienyl-capped oligophenylenes TnP (n = 1–3) and hTnP
(n = 2, 3), which comprise a specific class of thiophene/
phenylene co-oligomers; see their molecular structure in
Schemes 1–3. Of these, hTnP denote hemithienyl-capped
compounds. A methyl derivative (T2P-Me) and an alter-
nating co-oligomer (12) are also included. As compared
with the phenyl-capped oligothiophenes, literature that
deals with the syntheses of the co-oligomers possessing
the oligophenylene sequence (e.g. biphenyl and terphenyl
backbones) has been scarce up to this date. Most of the tar-
get compounds of the present studies are typical of such
co-oligomers. Furthermore, some of them possess novel
molecular structures that have not yet been reported.
Although T2P is a known compound, its synthetic yield
was very low (8.7%) [10]. The author found out, however,
that a combination of iodine compounds for the Grignard
coupling reaction improved the yield of T2P (see
To synthesize e.g. alkyl derivatives of the target com-
pounds intermediates like 4 are useful. For the purpose of
the iodination of 3, N-iodosuccinimide (NIS) can effectively
be used in combination with glacial acetic acid equimolar to
NIS [8]. The iodination takes place preferentially at the
α-position of thiophene as in the bromination with N-bro-
mosuccinimide [12] (see Scheme 1). This allows the author
to avoid using toxic reagents such as mercury(II) oxide [13].

924
S. Hotta
Vol. 38
Schemes 2 and 3 utilize the Suzuki coupling reactions
soluble in chloroform or acetone, and so their molecular
structure can be determined by H nmr. In addition to the
1
[14] that produce various thienyl-capped oligophenylenes.
In these Schemes the boronic acid compounds 6, 7, and 11
are effectively coupled with bromides and iodides (1, 5a,b,
and 8–10). In particular, 11 constitutes a co-oligomer itself
and can be a useful building block of the co-oligomers.
This boronic acid compound has been specially designed
so that it can produce variations in the co-oligomer com-
pounds. Its synthesis was carried out taking literature
methods [15] into account. Regarding the target com-
pounds hT3P, T3P, and 12, to the best of my knowledge,
their synthesis and characterization have been disclosed
for the first time. Although the synthesis of hT2P was
reported [16], its characterization remained insufficient.
In all the cases represented in Schemes 2 and 3, the reac-
tion solutions yield precipitates that are strongly lumines-
cent blue to green. The as-precipitated crude materials
were obtained nearly in a quantitative manner (88–100%).
After purifying these the overall yields were moderately
high (> 25%).
routine assignments, the location of nmr lines provide
interesting information about the local chemical environ-
ments in the vicinity of the individual hydrogen atoms. If
the said chemical environments are closely related around
those hydrogen atoms, the relevant lines also arise at
closely related positions and sometimes coalesce. This is
actually the case with hT3P and T2P-Me (see EXPERI-
MENTAL), and is also true of 1-(2-thienyl)-4-(5-phenylth-
iophen-2-yl)benzene, whose synthesis was reported previ-
ously [1]. Since a large number of co-oligomers are avail-
able, one expects that some of them will possess a suitable
molecular structure such that specific hydrogen atoms
show the coalesced lines in the spectra. Such spectroscopic
features are important from a point of view of the confor-
mational studies of molecules [17].
The five-ring compounds of T3P and 12 are extremely
difficult to dissolve at room temperature in any common
organic solvents. For their characterization, therefore, ir
Spectroscopic Characterization of the Materials.
1
spectroscopy is a powerful tool instead of H nmr. This is
The compounds in which the total number of the thio-
phene and phenylene rings is up to four are sufficiently
because the target compounds exhibit characteristic
intense lines in the ir region around 1400–1500 and

Jul-Aug 2001
Synthesis of Thiophene/phenylene Co-oligomers. III
925
–1
sium iodide in combination with 4,4'-diiodobiphenyl (2).
The overall yield after purification turned out to be two
and a half times higher than previously reported. To
achieve further variations in syntheses, the author used
three kinds of boronic acid compounds (11) one of which
was purposely designed. These compounds were effec-
tively coupled with bromides and iodides. The coupling
reactions proceeded in a nearly quantitative manner to pro-
duce the purified target compounds in moderately high
yields. All the materials prepared above have been satis-
700–800 cm that result from the ring stretching and CH
out-of-plane deformation vibrations, respectively.
Furthermore, the locations of these vibrations unambigu-
ously determine the substitution modes of the thiophenes
and phenylenes [18,19].
A series of TnP (n = 1–3) exhibit closely related ir pro-
files. For these compounds distinct bands around 1490 and
1430 cm are due to the 1,4-disubstituted benzene ring
[18] and the 2-substituted thiophene ring [20], respec-
tively. Sharply resolved peaks around 810 and 700 cm
are again assigned to the 1,4-disubstituted benzene [18]
and the 2-substituted thiophene [21], respectively. The
compound 12 additionally exhibits well-resolved bands at
–1
–1
1
factorily characterized by H nmr and ir spectroscopy.
EXPERIMENTAL
–1
1450 and 801 cm characteristic of the 2,5-disubstituted
Melting points were measured on a Seiko Instruments
thiophene [21,22]. On the basis of these assignments, the
author concludes that the molecular structures of T3P and
12 are identical to those depicted in Scheme 3.
The compounds hTnP (n = 2, 3) indicate sharply
resolved lines around 760 and 690 cm that are attributed
1
DSC6200 thermal analysis system. The H nmr spectra were
recorded on a Varian Gemini 300BB spectrometer in deuteri-
ochloroform solution and chemical shifts are reported in ppm (δ)
relative to tetramethylsilane as an internal standard. As an excep-
–1
1
tion the H nmr spectrum of 11 was measured in a deuterioace-
to the monosubstituted benzene [18], the latter line involv-
ing a thiophene-associated mode as a shoulder. A sharp
line occurring at 1400–1410 cm is specific to the 4-sub-
tone solution to obtain good resolution. The ir spectra were taken
on a Perkin-Elmer System 2000 FT-IR spectrophotometer with
finely pulverized particles dispersed and embedded in a potas-
sium bromide matrix. Elemental analyses (for carbon, hydrogen,
and sulfur) were carried out on a CEInstruments EA 1110
CHNS-O apparatus.
Dehydrated diethyl ether was purchased from Wako Pure
Chemical and used without further purification or desiccation.
Other chemical reagents were purchased from standard sources
and used as received unless otherwise specified. The formation of
the Grignard reagents and the subsequent Grignard coupling
reaction were carried out in dry glassware and the reaction sys-
tem was kept under dry nitrogen throughout.
–1
stituted biphenyl or p-terphenyl [1,23]. The CH out-of-
plane deformation mode for the 2,5-disubstituted thio-
–1
phene of T2P-Me is split into two at 799 and 792 cm .
The benzene-ring stretching mode of this compound is
–1
also split into two at 1506 and 1500 cm . These splittings
are probably showing the coexistence of two different
kinds of conformers [17]. The major characteristic fre-
quencies of the compounds are summarized in Table 1.
Thus the above-mentioned spectroscopic characteristics
demonstrate that all the co-oligomers in the present studies
possess the well-defined molecular structure, as repre-
sented in Schemes 1–3.
1,4-Bis(2-thienyl)benzene (T1P).
Equimolar amounts of distilled 2-iodothiophene (9.51 g, 45.3
mmoles) and magnesium (1.101 g, 45.3 mmoles) were mixed in
anhydrous diethyl ether (100 mL) to prepare a Grignard reagent.
To the resulting Grignard reagent were added [1,3-
bis(diphenylphosphino)propane]nickel(II) chloride [abbreviated
Conclusion.
The author has presented systematic synthetic methods
for the thienyl-capped oligophenylenes that are based upon
the Grignard coupling reaction and the Suzuki reaction. To
improve the yield of T2P the author used 2-thienylmagne-
as Ni(dppp)Cl in Scheme 1; 320 mg, 0.590 mmole] and 1 (5.98
2
g, 18.1 mmoles) successively. After stirring overnight at room
temperature the reaction mixture was refluxed for 6 hours. When
Table 1
Infrared Band Positions (cm ) and Their Assignment of Various Thiophene/phenylene Co-oligomers
–1
CH stretching
Ring stretching
CH out-of-plane deformation
Compound
Aromatic
Aliphatic
Phenyl(ene)
Thiophene
Phenyl(ene)
Thiophene
T1P
T2P
T2P-Me
T3P
hT2P
hT3P
12
3067
3066
3071
3067
3065
3066
3069


1496
1488
1506 [a], 1500 [a]
1486
1428
1426
1445
1429
1427
815
813
819
699
699
2914

799 [a], 792 [a]
811
697



1485, 1408
1482, 1401
1493
820, 762, 687 [b]
817, 764, 688 [b]
817

1427

1450, 1427
801, 697
[a] Split lines; [b] A thiophene-associated mode is involved as a shoulder.

926
S. Hotta
Vol. 38
cooled over an ice/water bath and subsequently hydrolyzed with
2 N hydrochloric acid (24 mL), the reaction mixture produced
precipitates, which were collected by filtration and washed with
cold methanol. The precipitates were recrystallized from
methanol to give pale yellow solid T1P, yield 23%, mp 210°, lit
ton, J = 5.2, 3.6 Hz), 7.30 (dd, 1H, a thienyl proton, J = 5.2, 1.2
Hz), 7.36 (dd, 1H, a thienyl proton, J = 3.6, 1.2 Hz), 7.33–7.64 (m,
5H, phenyl protons), 7.62 (dd, 2H, phenylene protons, J = 8.9, 2.1
Hz), 7.70 ppm (dd, 2H, phenylene protons, J = 8.9, 2.4 Hz).
Anal. Calcd. for C
H S: C, 81.31; H, 5.12; S, 13.57. Found:
16 12
1
[24] mp 202–204°, lit [13a] mp 204–206°; H nmr: δ 7.10 (dd,
C, 81.32; H, 5.17; S, 12.79.
2H, thienyl protons, J = 5.3, 3.6 Hz), 7.30 (dd, 2H, thienyl pro-
tons, J = 5.3, 1.1 Hz), 7.34 (dd, 2H, thienyl protons, J = 3.6, 1.1
Hz), 7.63 ppm (s, 4H, phenylene protons).
4-(2-Thienyl)-1,1':4',1"-terphenyl (hT3P).
The author applied the same procedure as the case of the hT2P
synthesis, except for using 4-biphenylboronic acid (7) in place of
6. The reaction mixture was then refluxed for 4 hours. The pre-
cipitates generated after the course of reaction were collected by
filtration and washed with methanol. These precipitates under-
went Soxhlet extraction with acetone and the extracted material
was recrystallized from 2-butanone to give white solid hT3P,
Anal. Calcd. for C
H S : C, 69.38; H, 4.16; S, 26.46. Found:
14 10 2
C, 69.36; H, 4.17; S, 26.31.
4,4'-Bis(2-thienyl)biphenyl (T2P).
The Grignard reagent was prepared using 2-iodothiophene
(8.68 g, 41.3 mmoles) in the same manner as the synthesis of
T1P. To the resulting Grignard reagent were added [1,3-
bis(diphenylphosphino)propane]nickel(II) chloride (320 mg,
0.590 mmole) and 2 (6.50 g, 16.0 mmoles) successively. The
reaction mixture was then treated similarly to the case of the T1P
synthesis. The resulting precipitates were recrystallized from
toluene to give pale yellow solid T2P, yield 22%, mp 311°, lit
1
yield 43%, mp 298°; H nmr: δ 7.11 (dd, 1H, a thienyl proton, J
= 5.0, 3.5 Hz), 7.31 (dd, 1H, a thienyl proton, J = 5.0, 1.2 Hz),
7.38 (dd, 1H, a thienyl proton, J = 3.5, 1.2 Hz), 7.34–7.68 (m,
5H, phenyl protons), 7.67 (d, 2H, phenylene protons, J = 8.7
Hz), 7.70 (s, 4H, phenylene protons), 7.72 ppm (d, 2H, phenylene
protons, J = 8.7 Hz).
1
[10] mp 310–315°; H nmr: δ 7.11 (dd, 2H, thienyl protons, J =
Anal. Calcd. for C H S: C, 84.58; H, 5.16; S, 10.26. Found:
22 16
5.2, 3.6 Hz), 7.31 (dd, 2H, thienyl protons, J = 5.2, 1.1 Hz), 7.37
(dd, 2H, thienyl protons, J = 3.6, 1.1 Hz), 7.65 (dd, 4H, pheny-
lene protons, J = 8.6, 2.1 Hz), 7.71 ppm (dd, 4H, phenylene pro-
tons, J = 8.6, 2.1 Hz).
C, 84.98; H, 5.30; S, 9.14.
4,4"-Bis(2-Thienyl)-1,1':4',1"-terphenyl (T3P).
1,4-Diiodobenzene 1 (247 mg, 0.75 mmole) and 4-(2-
thienyl)benzeneboronic acid (11) of 612 mg (3.00 mmoles) as
well as tetrakis(triphenylphosphine)palladium(0) (104 mg, 0.090
mmole) were dissolved in benzene (60 mL). Nitrogen was bub-
bled through this reaction solution for 30 minutes to remove dis-
solved oxygen. To the reaction solution was added 10 mL of an
aqueous solution of sodium carbonate (636 mg, 6.00 mmoles),
and then the solution was refluxed for 26 hours under nitrogen
environment. The precipitates generated after the course of reac-
tion were collected by filtration and washed by turns with
methanol and acetone. These precipitates were recrystallized
from 1,2,4-trichlorobenzene to give pale yellow solid of T3P,
yield 38%, mp 387°.
Anal. Calcd. for C
H S : C, 75.43; H, 4.43; S, 20.14. Found:
20 14 2
C, 74.54; H, 4.29; S, 19.82.
4,4'-Bis(5-methylthiophen-2-yl)biphenyl (T2P-Me).
The Grignard reagent was prepared using 2-iodo-5-methylth-
iophene (1.03 g, 4.60 mmoles) in a manner similar to the synthe-
sis of T1P. The same procedure as described for the T2P synthe-
sis was applied afterward. The resulting precipitates were recrys-
tallized from 2-butanone to give pale yellow solid T2P-Me, yield
1
17%, mp 305°; H nmr: δ 2.53 (d, 6H, methyl protons, J = 1.1
Hz) [25], 6.75 (dd, 2H, thienylene protons, J = 3.6, 1.1 Hz) [25],
7.16 (d, 2H, thienylene protons, J = 3.6 Hz), 7.62 ppm (s, 8H,
phenylene protons).
Anal. Calcd. for C
H S : C, 79.15; H, 4.60; S, 16.25. Found:
26 18 2
C, 79.04; H, 4.56; S, 15.88.
Anal. Calcd. for C
H S : C, 76.26; H, 5.23; S, 18.51. Found:
22 18 2
C, 76.08; H, 5.55; S, 18.35.
2,5-Bis[4-(2-thienyl)phenyl]thiophene (12).
4-(2-Thienyl)biphenyl (hT2P).
The author applied the same procedure as the case of the T3P
synthesis, except for using 2,5-diiodothiophene (9) in place of 1.
The resulting precipitates were recrystallized from 1,2,4-
trichlorobenzene to give yellow solid of 12, yield 63%, mp 284°.
2-(4-Bromophenyl)thiophene (5b) was synthesized following
the literature method [1]. Of this, 246 mg (1.03 mmoles) was
taken and dissolved in benzene (20 mL) together with ben-
zeneboronic acid (6) of 244 mg (2.00 mmoles) and tetrakis(triph-
Anal. Calcd. for C
H S : C, 71.96; H, 4.03; S, 24.01. Found:
24 16 3
C, 72.14; H, 3.84; S, 23.39.
enylphosphine)palladium(0) [abbreviated as Pd(PPh ) in
3 4
Schemes 2 and 3; 69.3 mg, 0.060 mmole]. Nitrogen was bubbled
through this reaction solution for 30 minutes to remove dissolved
oxygen. To the reaction solution was added 5 mL of an aqueous
solution of sodium carbonate (424 mg, 4.00 mmoles), and then the
solution was stirred for 20 hours at room temperature under nitro-
gen environment. The resulting solution was then cooled with an
ice/water bath and oxidized with 1 mL of an aqueous solution of
hydrogen peroxide (30%). This solution was washed in turn with
water, saturated solution of aqueous sodium bicarbonate, and
water and dried with anhydrous calcium chloride. White precipi-
tates obtained after evaporating benzene were washed with cold
methanol and recrystallized from hexane to give white solid
2-Iodo-5-methylthiophene (4).
2-Methylthiophene 3 (9.82 g, 0.10 mole) and N-iodosuccin-
imide (27.0 g, 0.12 mole) were dissolved in methanol (240 mL).
To this solution was added acetic acid (6.87 mL, 0.12 mole), and
the reaction solution was stirred for 3 hours over an ice/water
bath, making the resulting solution dark orange. To this solution
were added 240 mL water and 360 mL diethyl ether, the reaction
products obtained above were transferred to the diethyl ether
layer in a separating funnel. This layer was separated from the
water layer and washed successively with water, 10% aqueous
solution of sodium hydroxide (four times), and water (two times).
The resulting diethyl ether solution was dried with anhydrous
1
hT2P, yield 26%, mp 172°; H nmr: δ 7.11 (dd, 1H, a thienyl pro-

Jul-Aug 2001
Synthesis of Thiophene/phenylene Co-oligomers. III
927
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calcium chloride and diethyl ether was removed with a rotary
evaporator to give 15.4 g of a pale yellow liquid (in a yield of
69%). This compound was distilled under reduced pressure to
obtain a colorless liquid 4; H nmr: δ 2.46 (d, 3H, methyl protons,
J = 1.1 Hz) [25], 6.45 (dd, H, a thienylene proton, J = 3.6 , 1.1
Hz) [25], 7.01 ppm (d, H, a thienylene proton, J = 3.6 Hz).
1
4-(2-Thienyl)benzeneboronic acid (11).
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2-(4-Bromophenyl)thiophene (5b) was synthesized following
the literature method [15a]. The crude product of this compound
was purified by column chromatography on silica gel with
hexane as eluent.
Equimolar amounts of the purified material of 5b (40.0 g, 0.167
mole) and magnesium (4.07 g, 0.167 mole) were mixed in anhy-
drous diethyl ether (100 mL) to prepare a Grignard reagent.
Meanwhile, trimethyl borate (17.4 g, 0.167 mole) was dissolved in
anhydrous diethyl ether (100 mL) and kept at -78°. To this solution
of trimethyl borate, the above Grignard reagent was added slowly,
while stirring vigorously. After being stirred overnight at room
temperature the reaction mixture was slowly poured into 10% sul-
furic acid (150 mL) over an ice/water bath. The diethyl ether layer
was separated from the water layer in a separating funnel. The for-
mer layer was washed with water, while the latter was extracted
with diethyl ether. The combined diethyl ether layers were dried
with anhydrous calcium chloride and diethyl ether was removed
with a rotary evaporator to give a crude material.
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1459 (1998).
This material was purified by column chromatography on sil-
ica gel with ethyl acetate-tetrahydrofuran (10:1, v/v) as eluent.
The material obtained through evaporating the eluent was dis-
solved in refluxing ethyl acetate (five times the said material in
weight) and the resulting solution was subsequently reprecipi-
tated in hexane to yield pale gray powder. This was recrystallized
from a mixture solvents methanol:water (2:1, v/v) to give color-
less crystals 11, yield 45%, mp 252°; ir: ν (OH) 3329, (CH) 3062,
-1
1
(CH) 822, 702 cm ; H nmr: δ 7.14 (dd, 1H, a thienyl proton, J =
5.0, 3.7 Hz), 7.23 (s, 2H, hydroxyl protons) [26], 7.48 (dd, 1H, a
thienyl proton, J = 5.0, 1.1 Hz), 7.52 (dd, 1H, a thienyl proton,
J = 3.7, 1.1 Hz), 7.68 (dd, 2H, phenylene protons, J = 8.6, 2.1
Hz), 7.93 ppm (dd, 2H, phenylene protons, J = 8.6, 2.0 Hz).
Anal. Calcd. for C H O SB: C, 58.86; H, 4.45; S, 15.71.
[18] R. M. Silverstein, G. C. Bassler and T. C. Morrill,
Spectroscopic Identification of Organic Compounds, 5th edition,
John Wiley & Sons, New York, NY, 1991.
10
9 2
Found: C, 58.18; H, 4.37; S, 15.67.
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The author thanks Dr. M. Yoshida, National Institute of Advanced
Industrial Science and Technology (AIST), for his helpful discus-
sions and suggestions. This work was supported by NEDO for the
Harmonized Molecular Materials theme funded through the project
on Technology for Novel High-Functional Materials.
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[*] Author to whom correspondence should be addressed.
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