Oligoselenophenes (n and p Type): Synthesis and Properties

Article abstract of DOI:10.1134/S1070363219090287

An array of semiconducting oligoselenophenes (n and p types), up to hexamer units, has been synthesized by the double Stille coupling methods using tetrakis(triphenylphosphine)palladium(0) as a catalyst. A series of semiconducting oligomers (n and p types) containing mixed hetero-units (hexamers of thiophene and selenophene) have been also synthesized using the Stille coupling reaction. Their thermal properties are systematically studied and compared with those of π-conjugated thiophene based oligomers using DSC and TGA. The field-effect mobility of synthesized n and p type oligomers is analyzed.

Full text of DOI:10.1134/S1070363219090287

ISSN 1070-3632, Russian Journal of General Chemistry, 2019, Vol. 89, No. 9, pp. 1911–1922. © Pleiades Publishing, Ltd., 2019.
Dedicated to the Memory of Prof. M. Bendikov
Oligoselenophenes (n and p Type): Synthesis and Properties
a
b
a,b
B. Mondal , M. Bendikov , and U. Kanti Roy *
a
Department of Chemistry, Kazi Nazrul University, Asansol, 713340 India
*
e-mail: uroccu@gmail.com
b
Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, 76100 Israel
Received July 9, 2019; revised September 17, 2019; accepted September 19, 2019
Abstract―An array of semiconducting oligoselenophenes (n and p types), up to hexamer units, has been synthe-
sized by the double Stille coupling methods using tetrakis(triphenylphosphine)palladium(0) as a catalyst. A series
of semiconducting oligomers (n and p types) containing mixed hetero-units (hexamers of thiophene and seleno-
phene) have been also synthesized using the Stille coupling reaction. Their thermal properties are systematically
studied and compared with those of π-conjugated thiophene based oligomers using DSC and TGA. The field-effect
mobility of synthesized n and p type oligomers is analyzed.
Keywords: selenophene, oligomer, oligoselenophene, sexiselenophenes, Stille coupling, n and p type oligomers
DOI: 10.1134/S1070363219090287
INTRODUCTION
chain length of polymers, comparison of the properties
of polyselenophene and polythiophene does not provide
complete information on contributions of chalcogen at-
oms in electronic structures of the conjugated systems.
Comparison between oligoselenophenes and oligothio-
phenes having the same chain length provides some im-
portant information regarding contribution of chalcogen
atoms in the electronic structures. Taking in consideration
importance of oligomers (especially oligothiophenes)
for OFETs application [10–12], we were determined to
synthesize and study properties of n and p type oligomers
based on selenophene hetero-units.
Over past few years, conjugated oligomers (or poly-
mers) have been attracting close attention due to their
potential application in organic light-emitting diodes
(
OLEDs) [1, 2], photovoltaic cells [3, 4], organic field-
effect transistors (OFETs) [5–8], and electrochromic
devices [9]. Thiophene based π-conjugated systems
demonstrated significant characteristics in the field of
n and p type OFETs [10–12]. Selenophene is the heavy
member of chalcogenophenes series and its properties
are similar to some extend to those of thiophene [13, 14].
The electron-donating [15] and polarizable [16] proper-
ties of selenophenes are more pronounces than those of
thiophenes. The inter-chain charge transfer facilitated
by Se···Se contact is also higher than by S···S contacts.
These facts indicate that selenophene containing oligo-
mers and polymers should be particularly attractive as
electronic materials. Nevertheless, only recently some
new polyselenophenes were synthesized (chemically and
electrochemically) and their electronic properties were
studied systematically [9, 17]. Because of variation of
Bendikov and co-authors [18] have studied oligosele-
nophenes at B3LYP/6-31G(d) level for comparing those
with the corresponding oligothiophenes.It was indicated
that oligoselenophenes had more quinoid characters
than the corresponding oligothiophenes. The inter-ring
bond distances in oligoselenophenes are considerably
shorter than the related distances in oligothiophenes. The
HOMO-LUMO gap (band gap), bond length alternation
(BLA) and charge distribution of oligoselenophenes are
Scheme 1.
Se
Se
Se
Se
H
H
H
H
Se
n
Se
n
Aromatic
Quinoid
1
911

1912
MONDAL et al.
Scheme 2.
(a)
Se
S
Se
S
Se
Se
Se
S
Se
S
Se
S
6
Se
Se
4
S-2Se
Se-2S
Se
Se
S
Se
Se
S
S
4
Se
Se
S
Se
Se
S
C H
C H
6
13
6
13
Se
Se
Se
DH-6Se
C H
C H
6
13
6
13
Se
DH-4S-2Se
(b)
F
F
F
S
S
S
S
S
S
DF-6T
F
F
S
S
Se
Se
S
S
DF-4S-2Se
F
Se
S
Se
S
Se
DF-6Se
Se
Se
S
Se
S
S
S
6T
strongly dependant on twisting, which costs very little
energy. However, twisting of oligoselenophenes requires
higher energy than their corresponding thiophene ana-
logues. The energy required for twisting around inter-ring
bonds in 6Se is small, however, it is notably greater than
that in 6T. Twisting to a 36° inter-ring dihedral angle
requires only 3.4 kcal/mol per inter-ring bond for 6Se
(2.5 kcal/mol for 6T), and twisting to a 20° inter-ring di-
hedral angle requires only 0.7 kcal/mol per inter-ring bond
for 6Se (0.4 kcal/mol for 6T). These calculations suggest
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

OLIGOSELENOPHENES (n AND p TYPE): SYNTHESIS AND PROPERTIES
Scheme 3. Synthesis and reactions of unsubstituted oligomers [p-type].
1913
NBS (2 eq.)
NBS (1 eq.)
Br
X
Br
X
Br
X
CHCl /AcOH
X
X
CHCl /AcOH
X
3
3
5
1, 2
3, 4
Toluene
Reflux
Sn(n-Bu)3
Se
Pd(PPh3
)
4
n-BuLi
X
X
(
n-Bu) Sn
Sn(n-Bu)3
n-Bu SnCl
3
3
7, 8
X
X
X
X
6
Br
X
X
X
3
, 4
(
Ph P) Pd
3 4
+
Toluene
Reflux
X
X
Y
Y
X
X
9
ꢀ11
X
(
n-Bu) Sn
Sn(n-Bu)3
3
7, 8
X = Se (1), S (2), Se, 61% (3), S, 62% (4), Se, 75% (5), Se, 40% (6), Se, 78% (7), S, 80% (8), Se, 18% (9), S (10), Se (11) ; Y =
Se, 18% (9), Se, 32% (10), S, 23% (11).
that selenophene containing oligomers should maintain
planarity with a wider range of substituents (Scheme 1).
[24], nitro [25], dicyanomethylene [26], ester [27], per-
fluoroalkyl [8, 11, 28], perfluoroaryl [29], and perfluo-
roalkylphenyl [30] into the π-conjugated systems is an
efficient way of converting p-type into n-type organic
semiconductors. Suzuki and co-workers [31] described
perfluorinated oligo(p-phenylene) derivatives and perflu-
oro-p-sexiphenyl as an efficient n-type of semiconductors
for electron-transport layer of OLEDs.
Keeping in mind high importance of oligothiophenes
for organic electronic materials, and inspired by the struc-
tural differences between selenophenes and thiophenes,
we paid particular attention to the synthesis and properties
of electron-transporting as well as hole-transporting or-
ganic materials for OFETs applications. For the first time,
Inoune and co-workers [19] reported the synthesis of chal-
cogen selenophene series up to pentamer using the Stille
coupling methods. They also studied the electrochemi-
cal, spectroscopic properties and the crystal structure of
tetraselenophene [20, 21]. Millefiori and Alparone [22]
published theoretical investigation data on structures and
conformational behavior of small selenophene oligomers.
Zade and Bendikov [23] studied the hopping transport in
long oligothiophenes and oligoselenophenes.
Here, we report synthesis of p-type (Scheme 2a) and
n-type (Scheme 2b) oligomers.
RESULTS AND DISCUSSION
The Stille coupling reaction was used efficiently in
constructing a series of oligoselenophenes. Bromination
of dicyclic compounds 1 and 2 by N-bromosuccinimide
(NBS) yielded the corresponding monobrominated deri-
vates 3 (61%) and 4 (62%). Tributyltin derivatives 7 and
8 were accumulated in good yields upon lithiation of 1
and 2 followed by quenching with tributyltin chloride.
Synthesis of sexiselenophene (α-6Se) and mixed oligo-
A number of n-channel semiconductors developed
up to now is far less than that of p-channel ones. Incor-
poration of electron-withdrawing groups such as cyano
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

1914
MONDAL et al.
Scheme 4. Synthesis and reactions α,ω-alkyl substituted oligomers [p-type].
C H11COCl
5
LAH + AlCl3
Ether
C H
X
SnCl4
5
11
C H
X
X
5
11
X
X
X
O
1, 2
12, 13
14, 15
NBS/CHCl3
AcOH
7
, Pd(PPh3)4
Br
C H
5
11
R
X
Toluene
Reflux
X
X
R
X
Y
Y
X
X
1
6, 17
X = Se, 35% (12), S, 85% (13), Se, 78% (14), S, 64% (15), Se, 70% (16), S, 88% (17), Se, 22% (18), S, 27% (19); Y = Se, 22%
18), Se, 27% (19); R= C H .
1
8, 19
(
6
13
mers (α-4S-2Se, α-4Se-2S) was based on the double
Stille coupling reaction involving stannic derivatives
of bis-selenophene or bis-thiophene using monobromi-
nated derivatives of bis-selenophene or bis-thiophene
tetrakis(triphenylphosphine)palladium(0) as catalysts
tion of derivatives 1 and 2 followed by reduction of the
corresponding acylated derivates 12 and 13 by AlH3
(LAH+AlCl ). Bromo derivatives 16 and 17 were syn-
3
thesized in good yields by bromination of the correspond-
ing compounds 14 and 15 by NBS. The Stille coupling
reaction of compound 7 with bromoderivatives 16 and 17
using the Pd catalyst gave the corresponding oligomers
18 (DH-6Se) and 19 (DH-4S-2Se) in moderate yields.All
oligoselenophenes were purified by sublimation.
(Scheme 3). Similarly, synthesis of tetraselenophene 6
was accomplished in 40% yield by the Stille coupling
reaction of dibromo derivative 5 with 2-(n-tributyltin)-
selenophene using Pd catalyst.
It could be expected that introduction of alkyl groups
would support solubility of oligoselenophenes. For this
reason we have planned to synthesize DH-6Se and DH-
As presented in Scheme 5, the required starting ma-
terials were produced by lithiation of bithiophene 2 and
bi-selenophene 1 with an equivalent of n-BuLi followed
by treatment with (PhSO ) NF that yielded the respec-
tive 2-fluorobithiophene and 2-fluorobiselenophene in
moderate yields. Bromination of 2a and 2c with by NBS
afforded the respective monobromoderivatives 2b and 2d.
Tributyltin derivatives 8 and 7 were prepared in 80% and
4
2
S-2Se bearing hexyl groups at the terminal positions.
-Hexyl bisselenophene and 2-hexyl bisthiophene were
2
2
used as the starting materials for obtaining DH-6Se
and DH-4S-2Se. As presented in Scheme 4, these were
achieved in moderate yield by the Friedel–Crafts acyla-
7
8% yield by lithiation of compounds 2 and 1 followed
by quenching with n-Bu SnCl. The n-type oligomers
3
2
0–22 were synthesized by the double Stille coupling
reaction based on tin analog of bithiophene 8 or biseleno-
phene 7 and monobrominated derivatives 2b or 2d using
tetrakis(triphenylphosphine)palladium(0) as a catalyst.
All n-type oligomers were purified by sublimation.
UV-Vis spectra. UV absorbance maxima recorded for
the dimer (2Se 331 nm), trimer (3Se 389 nm), tetramer
(4Se 432 nm), and hexamer (6Se 452 nm) were higher
than their corresponding thiophene analog. UV absor-
bance maxima of 4S-2Se (441 nm) and 4Se-2S (438 nm)
were lower than that of sexi-selenophene (6Se 452 nm)
Fig. 1. UV-Vis spectra of sexi-selenophene and mixed
oligomers in 1,2-dichlorobenzene: (1) 6Se, (2) 4S-2Se, and
(Fig. 1). The profiles of absorption and emission spectra
(3) 4Se-2S.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

OLIGOSELENOPHENES (n AND p TYPE): SYNTHESIS AND PROPERTIES
Scheme 5. Synthesis and reactions of n-type oligomers.
1915
F
S
S
F
Se
Se
S
S
20
2
0%
7
Pd(PPh3 4
)
Toluene, reflux
NBS
n-BuLi
F
2
)
2NF
CHCl /AcOH
Br
F
S
S
S
(
PhSO
S
S
3
S
3
5%
5
6%
2
2a
2b
2
2%
8
Pd(PPh3 4
)
Toluene, reflux
F
S
S
F
S
S
S
S
21
NBS
CHCl /AcOH
n-BuLi
2NF
F
Br
F
Se
Se
(PhSO
5%
2
)
Se
Se
3
Se
Se
2
4
8%
1
2c
2d
20%
7
Pd(PPh3 4
)
Toluene, reflux
F
Se
Se
F
Se
Se
Se
Se
22
of alkylated oligomers (p-type) in 1,2-dichlorobenzene
and chloroform respectively were almost identical. The
absorption maximum of DH-6Se was recorded at 460 nm
and its emission maximum at 548 nm. UV-Vis spectral
data of synthesized oligoselenophenes are presented in
Table 1.
than those of the corresponding oligothiophenes, α-2Se,
α-3Se and α-4Se melt at 48, 170, and 296°C respectively,
whereas α-2S, α-3S and α-4S melt at 34, 92, and 208°C.
The higher melting points of α-oligoselenophenes indi-
cated stronger π–π intermolecular interaction among the
neighboring components in their molecules. The calcu-
lated enthalpy of fusion (ΔH) for α-3Se (5.14 kcal/mol)
and α-4Se (12.89 kcal/mol) were also higher than their
corresponding thiophene analogs [4.87 kcal/mol (α-3T)
and 12.55 kcal/mol (α-4T)]. Generally, melting points
became higher as the number of hetero rings in a given
Thermal properties. Thermal behavior of the oligo-
mers was determined by a repeated heating-cooling cycle
using DSC. Figures 2, 3 demonstrate the comparison
of DSC scans of sulfur and selenium analogs. Melting
points of oligoselenophenes were substantially higher
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

1916
MONDAL et al.
Fig. 2. DSC scans of 3S and 3Se under the atmosphere of N2:
1) 3S and (2) 3Se.
Fig. 3. DSC scans of 4S and 4Se under the atmosphere of N2:
(
(1) 4S and (2) 4Se.
oligomer chain increases. The melting point of α-4Se
296°C) was higher than its corresponding shorter chain
and 353°C, that were higher than those of α-6T. DSC
analysis of DH-α-6T demonstrated three phase transitions
at 296, 305, 316°C. The data were in accord with the ear-
lier studies [24]. DSC study of DH-4S-2Se exhibited two
phase transitions at 338 and 366°C. All phase transitions
were determined to be reversible in oligoselenophenes.
TGA data accumulated for DH-6Se and DH-4S-2Se re-
vealed inflections at 233 and 358°C that indicated their
decomposition processes.
(
analogue α-3Se (170°C) (Fig. 4). Longer chain oligo-
mers were expected to have stronger π–π intermolecular
interactions between the neighboring molecules than the
corresponding shorter chain oligomers. This could also
be the reason for the higher melting points of the former
compounds. DSC analysis of α-6T exhibited the liquid
crystalline phase at higher temperature [21–23]. Our DSC
studies of α-6T matched these results. DSC analysis of
α-4S-2Se demonstrated two phase transitions at 339°C
DSC scans of DF-6T and DF-4S-2Se demonstrated
sharp melting endotherms at 317 and 358°C, accordingly,
which was followed by crystallization at 313 and 352°C.
The melting endotherms of DF-6T and DF-4S-2Se were
higher than 6T. Similarly, the melting endotherm of DF-
Table 1. UV-Vis spectral data of oligoselenophenes
Absorbance,
λmax, nm
Emission,
λmax, nm
Compound
Se
4
S-2Se was also higher than DF-6T. The higher melting
endotherms of n-type oligomers indicated the stronger
π–π interaction between the molecules.
2
3
4
6
4
4
331
389
432
452
441
438
435
460
446
440
444
431
439
–
Se
–
The detailed shelf-live tests of hole mobility (μ),
threshold voltage (V ), on current (I ), and off current
Se
–
t
on
(Ioff) under atmospheric conditions of OTFTs based on
Se
–
α-6T, α-6-Se, α-4S-2Se, and α-4Se-2S indicated their
environmental stability over time (Table 2).
S-2Se
Se-2S
–
–
508
α – 6T
Table 2. Properties of sublimed oligomers
DH-6Se
DH-4S-2Se
DH-6T
548
Compound
α- 6T
Mobility
2.1×10–3
1.1×10–2
2.0×10–2
4.0×10–2
I /I
VT
on off
538
106
106
106
106
–20
–20
–20
–20
521, 551
508, 542
–
α -6Se
DF-6T
α -4S-2Se
α -4Se-2S
DF-6Se
DF-4S-2Se
–
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

OLIGOSELENOPHENES (n AND p TYPE): SYNTHESIS AND PROPERTIES
1917
EXPERIMENTAL
The chemicals used were purchased from Aldrich,
Lancaster, Fluka, Merck, SRL, and Spectrochem, and
distilled or recrystallized whenever required, or prepared
according to literature procedures. All manipulations
were performed under an inert atmosphere using standard
vacuum lines and Schlenk techniques. All solvents used
for the synthesis were deoxygenated, dried and distilled
by standard methods. Pre-coated silica gel 60F254
(Merck) was used for TLC and silica gel 60–120 and 100–
2
00 mesh (SRL) was used for column chromatography.
IR spectra were recorded on a Nicolet 6700 single beam
1
FTIR spectrophotometer. H NMR spectra were measured
Fig. 4. DSC scans of 2Se, 3Se and 4Se under the atmosphere
on a Brucker-AC 250 MHz and a Brucker-AC 400 MHz
spectrometers at 300 K. Chemical shifts are reported
in δ unit (ppm) from TMS with the solvent resonance
of N : (1) 2Se, (2) 3Se, and (3) 4Se.
2
5,5'-Dibromo-2,2'-biselenophene (5) was synthe-
sized according to the earlier developed procedure [19].
1
used as the internal standard (CDCl in H NMR spectra
3
7
.26 ppm and in 13C NMR spectra 77.0 ppm). EIMS
2
,2':5',2'':5'',2''' Quaterselenyl (6). A mixture
(
7
70 eV) spectra were measured on a VG MicroMass
070H and a VG Autospec M mass spectrometers.
of 2,5-dibromobiselenophene 5 (0.50 g, 1.20 mmol)
with 2-tributylstannylselenophene (1 g, 2.38 mmol)
and tetrakis(triphenylphosphine)palladium (0.14 g,
.12 mmol) in dry toluene (20 mL) was refluxed for
4 h under the atmosphere of nitrogen. The precipitate
Elemental analysis was carried out on a Perkin Elmer
Instruments 2400 Series II CHNS/O Analyzer and a
Vario EL, Elementar. Melting points were determined on
an Electrothermal 9100 melting point apparatus and are
uncorrected. DSC was run on a TAinstrument DSCQ200
analyzer at a heating/cooling rate of 10/-10°C/min under
nitrogen flow.
0
2
was filtered off under suction and sublimed at 220°C
to give compound 6 as an orange solid. Yield 40%, mp
–1
2
1
96°C. IR spectrum, ν, cm : 1503, 1456, 1437, 1204,
062, 1039, 823, 795, 754, 687, 675. MS (EI): m/z: 518
+
2
,2'-Biselenophene (1) and 2,2'-bithiophene (2)
[M] . Found, %: C 37.11, H 1.94. C H Se . Calculated,
16
10
4
were synthesized according to the earlier developed
procedure [19, 32].
%: C 37.09, H 1.95.
5,5'-Bis-tributylstannyl-2,2'-biselenophene (7). Into
2
-Bromo-5-(selenophene-2-yl)selenophene (3). Into
a stirred solution of bis-lithiobiselenophene, prepared
from biselenophene 1 (1 g, 3.84 mmol) and n-butyllithium
(3.4 mL, 8.43 mmol) in dry ether (50 mL) and cooled
down to –78°C, was added tributyltin chloride (2.75 g,
8.44 mmol) in the atmosphere of nitrogen, and the mixture
was stirred overnight at room temperature, then treated
with hexane (50 mL), washed with brine, and dried over
a stirred solution of biselenophene (1) (1 g, 3.84 mmol)
in chloroform-acetic acid (1 : 1 v/v 50 mL) was portion-
wise added NBS (0.68 g, 3.84 mmol), and the mixture
was stirred at room temperature for 1 h, then poured into
water (100 mL) and extracted with dichloromethane. The
extract was washed with water, 5% aqueous solution of
sodium hydrogen carbonate and brine solution. The mix-
MgSO . The solvent was evaporated, and the residue was
4
ture was dried over MgSO , the solvent was evaporated,
purified by column chromatography on silica gel (hexane
and few drops of TEA) to give product 7 as a pale yellow
oil. Yield 78%. H NMR (400 MHz) spectrum, δ, ppm:
4
and the residue was purified by column chromatography
on silica gel with hexane to give compound 3 as a white
1
1
solid. Yield 61%, mp 75°C. H NMR spectrum, δ, ppm:
0.89 t (J = 7.2 Hz, 18H), 1.06–1.10 m (12H), 1.30–1.37
m (12H), 1.52–1.60 m (12H), 7.31 d (J = 3.6 Hz, 2H),
7.34 d (J = 3.6 Hz, 2H). Found, %: C 45.91, H 6.94.
C H Se Sn . Calculated, %: C 45.86, H 6.98.
6
.94 d (J = 4.0 Hz, 1H), 7.14–7.25 m (3H), 7.87 d (J =
5
.2 Hz, 1H). Found, %: C 28.37, H 1.46. C H BrSe .
8
5
2
Calculated, %: C 28.35, H 1.49.
3
2
58
2
2
2
-Bromo-5-(thiophene-2-yl)thiophene (4) was syn-
5,5'-Bis-tributylstannyl-2,2'-bithiophene (8). Into a
stirred solution of bis-lithiobithiophene, prepared from
thesized according to the earlier developed procedure[32].
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

1918
MONDAL et al.
–
1
bithiophene 2 (2 g, 12.02 mmol) and n-butyllithium
11 mL, 27.65 mmol) in dry ether (50 mL) and cooled
down to –78°C, was added tributyltin chloride (8.23 g,
5.28 mmol) in the atmosphere of nitrogen, and the
mp > 300°C. IR spectrum, ν, cm : 1496, 1457, 1436,
(
1201, 1120, 1068, 1037, 818, 790, 763, 754, 723, 686,
+
675 cm–1. MS: m/z: 683 [M + 1] . Found, %: C 42.27,
2
H 2.06. C H S Se . Calculated, %: C 42.25, H 2.07.
2
4
14
2
4
mixture was stirred overnight at room temperature, then
treated with hexane (50 mL), washed with brine and
1
-[5-(Selenophen-2-yl)selenophen-2-yl]hexan-
1
-one (12). SnCl (1.05 g, 4.03 mmol) Was added to a
4
dried over MgSO . The solvent was evaporated, and the
4
mixture of 2,2'-biselenophene 1 (1 g, 3.84 mmol) with
hexanoyl chloride (0.54 g, 4.03 mmol) in anhydrous
benzene (50 mL) cooled down to 0°C. The reaction
mixture was stirred for 15 min at 0°C. After addition of
ice and dilution with CH Cl , the mixture was washed
successively with water and saturated aqueous solution
of NaHCO , dried over MgSO , and concentrated to dry-
ness affording crude yellow solid which was purified by
column chromatography on silica gel with hexane to give
compound 12 as a yellow solid. Yield 35%, mp 95°C.
residue was purified by column chromatography on silica
gel (hexane with few drops of TEA) to give compound
1
8
as a pale yellow oil. Yield 80%. H NMR (400 MHz)
spectrum, δ, ppm: 0.92 t (J = 7.2 Hz, 18H), 1.09–1.15 m
12H), 1.28–1.40 m (12H), 1.55–1.62 m (12H), 7.05 d (J =
.5 Hz, 2H), 7.29 d (J = 3.3 Hz, 2H). Found, %: C 51.66,
H 7.84. C H S Sn . Calculated, %: C 51.63, H 7.85.
2
2
(
3
3
4
32
58
2
2
2
, 2': 5', 2'': 5'', 2''': 5''', 2'''': 5'''', 2''''' Sexi-
selenophene (α-6Se) (9). A mixture of 2-bromo-5-
selenophene-2-yl)selenophene (3) (0.48 g, 1.43 mmol)
with 5,5'-bis-tributylstannyl-2,2'-biselenophene (7)
0.60 g, 0.71 mmol), and tetrakis(triphenylphosphine)
(
1
-[5-(Thiophen-2-yl)thiophen-2-yl]hexan-1-one
(
13). SnCl (4.94 mL, 18.9 mmol) Was added to a mixture
of 2,2'-bithiophene 2 (3.0 g, 18.0 mmol) with hexanoyl
4
(
palladium (0.16 g , 0.14 mmol ) in dry toluene (30 mL)
was refluxed for 24 h under the atmosphere of nitrogen.
The red precipitate was filtered off under suction. Its fol-
chloride (2.55 g, 18.90 mmol) in anhydrous benzene
(
50 mL) cooled down to 0°C, and the mixture was
stirred for 15 min. After addition of ice and dilution with
CH Cl , the mixture was washed successively with water
o
lowing sublimation at 320 C gave compound 9 as a dark
2
2
red solid. Yield 18% (first sublimation), 5% (second sub-
limation), mp 307°C. IR spectrum, ν, cm : 1500, 1456,
and saturated aqueous solution of NaHCO , dried over
3
–
1
MgSO and concentrated to dryness. The crude yellow
4
1
436, 1261, 1202, 1096, 1065, 1037, 818, 789, 752, 675.
solid was purified by column chromatography on silica
gel with hexane to give compound 13 as a pale yellow
solid. Yield 85%, mp 80°C.
+
MS (EI): m/z: 777 [M + 1] . Found, %: C 37.17, H 1.79.
C H Se . Calculated, %: C 37.14, H 1.82.
24
14
6
Oligomer (α-4S-2Se) (10). A mixture of 2-bromo-5-
thiophen-2-yl)thiophene (4) (0.20 g, 0.81 mmol) with
5-Hexyl-2,2'-biselenophene (14). A solution of
ketone intermediate 12 (0.60 g, 1.67 mmol) in toluene
(
5
0
,5'-bis-tributylstannyl-2,2'-biselenophene (8) (0.34 g,
.40 mmol) and tetrakis(triphenylphosphine)palladium
(40 mL) was added to a suspension of LiAlH (0.5 g,
4
13.15 mmol) withAlCl (0.44 g, 3.30 mmol) in anhydrous
3
(0.1 g , 0.09 mmol ) in dry toluene (20 mL) was refluxed
Et O (100 mL) under the atmosphere of N . After 1 h
2
2
for 24 h under the atmosphere of nitrogen. The precipitate
was filtered off under suction and sublimed at 260°C to
give compound 10 as a red solid. Yield 32% (sublima-
of stirring at 20°C, EtOAc (20 mL) and a 1 M aqueous
solution of HCl (30 mL) were successively added to the
reaction mixture. The organic phase was separated by
decantation and the aqueous phase was extracted with
–
1
tion), mp > 300°C. IR spectrum, ν, cm : 1495, 1451,
1
427, 1224, 1202, 1069, 838, 827, 798, 793, 702, 688.
Et O. The combined organic phases were dried over
2
+
MS (EI): m/z: 589 [M + 1] . Found, %: C 48.99, H 2.39.
MgSO and concentrated in vacuo. Purification by column
4
C H S Se . Calculated, %: C 48.98, H 2.40.
chromatography on silica gel with hexane gave compound
24
14
4
2
1
1
4 as a colorless oil. Yield 78%. H NMR (250 MHz)
Oligomer (α-4Se-2S) (11). A mixture of 2-bromo-
-(selenophene-2-yl)selenophene (3) (0.50 g, 1.28
mmol) with 5,5'-bis-tributylstannyl-2,2'-bisthiophene (8)
0.47 g, 0.64 mmol) and tetrakis(triphenylphosphine)pal-
spectrum, δ, ppm: 0.88 t (J = 6.6 Hz, 3H), 1.26–1.40 m
5
(
(
6H), 1.59–1.71 m (2H), 2.82 t (J = 7.5 Hz, 2H), 6.79 d
J = 3. 75 Hz, 1H), 7.03 d (J = 3.75 Hz, 1H), 7.14–7.24
(
m (2H), 7.78–7.81 m (1H). Found, %: C 48.85, H 5.27.
C H Se . Calculated, %: C:48.85, H 5.27.
ladium (0.15 g , 0.13 mmol ) in dry toluene (30 mL) was
refluxed for 24 h under the atmosphere of nitrogen. The
precipitate was filtered off under suction and sublimed
at 320°C to give compound 11 as a red solid. Yield 23%,
14 18
2
5-Hexyl-2,2'-bithiophene (15). A solution of ketone
intermediate 13 (2 g, 7.57 mmol) in toluene (40 mL) was
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

OLIGOSELENOPHENES (n AND p TYPE): SYNTHESIS AND PROPERTIES
1919
added to a suspension of LiAlH (2.3 g, 60 mmol) with
α,α'-Dihexylsexiselenophene (α-DH-6Se) (18).
A mixture of 5-bromo-5'-hexyl-2,2'-biselenophene
(16) (0.5 g, 1.18 mmol) with 5,5'-bis-tributylstan-
nyl-2,2'-biselenophene (7) (0.5 g, 0.59 mmol) and
tetrakis(triphenylphosphine)palladium (0.14 g,
0.12 mmol ) in dry toluene (30 mL) was refluxed for
24 h under the atmosphere of nitrogen. The precipitate
was filtered off by suction and subjected to soxhlet purifi-
4
AlCl (2.01 g, 15 mmol) in anhydrous Et O (100 mL)
3
2
under the atmosphere of nitrogen, and the mixture was
stirred for 1 h at 20°C. EtOAc (20 mL) And a 1 M aque-
ous solution of HCl (30 mL) were successively added to
the reaction mixture. The organic phase was separated by
decantation, and the aqueous phase was extracted with
Et O. The organic phases were combined, dried over
2
MgSO and concentrated in vacuo. Purification by column
cation (CHCl ). Sublimation of the insoluble precipitate at
4
3
chromatography on silica gel with hexane gave compound
290°C gave compound 18 as a dark red solid. Yield 22%,
mp > 300°C. IR spectrum, ν, cm : 2922, 1539, 1507,
1
–1
1
5 as a colorless oil. Yield 64%. H NMR (250 MHz)
spectrum, δ, ppm: 0.88 t (J = 6.5 Hz, 3H), 1.26–1.42 m
1456, 1200, 1065, 788, 766. Found, %: C 45.81, H 4.01.
C H Se . Calculated, %: C 45.78, H 4.06.
(
(
6H), 1.60–1.69 m (2H), 2.77 t (J = 7.6 Hz, 2H), 6.65 d
J = 3. 5 Hz, 1H), 6.95–6.98 m (2H), 7.07–7.09 m (1H),
3
6
38
6
Oligomer (α-DH-4S-2Se) (19). A mixture of 5-bro-
7
.13–7.15 m (1H). Found, %: C 67.17, H 7.23. C H S .
14 18 2
mo-5'-hexyl-2,2'-bithiophene (17) (0.50 g, 1.52 mmol)
with 5,5'-bis-tributylstannyl-2,2'-biselenophene (7)
(0.64 g, 0.76 mmol) and tetrakis(triphenylphosphine)pal-
ladium (0.18 g , 0.15 mmol) in dry toluene (30 mL) was
refluxed for 24 h under the atmosphere of nitrogen. The
precipitate was filtered off under suction and subjected to
soxhlet purification (CHCl3). Sublimation of the insoluble
precipitate at 270°C gave compound 19 as a red solid.
Calculated, %: C 67.15, H 7.24.
5
-Bromo-5'-hexyl-2,2'-biselenophene (16). A solu-
tion of NBS (0.38 g, 2.13 mmol) in DMF (15 mL) was
added dropwise in darkness to a solution of n-hexyl-
biselenophene (14) (0.7 g, 2.03 mmol) in DMF (10 mL)
cooled to –20°C, and stirred for 4 h.Addition of ice to the
reaction mixture was followed by extraction with CH Cl .
2
2
–1
The combined organic phases were washed with water,
dried over Na SO and concentrated in vacuo. The oily
Yield 27%, mp > 300°C. IR spectrum, ν, cm : 2922,
507, 1456, 1445, 1377, 1220, 1202, 1067, 857, 793.
Found, %: C 57.17, H 5.01. C H S Se . Calculated,
1
2
4
residue was purified by column chromatography on silica
gel with hexane to give compound 16 as a colorless solid.
36 38
4
2
%: C 57.13, H 5.06.
1
Yield 70%, mp 42°C. H NMR (400 MHz) spectrum, δ,
2-Fluoro-5-(thiophen-2-yl)thiophene (2a). To a
ppm: 0.87 t (J = 6.8 Hz, 3H), 1.26–1.40 m (6H), 1.62–1.66
m (2H), 2.80 t (J = 7.6 Hz, 2H), 6.78 d (J = 3.6 Hz, 1H),
stirred solution of mono-lithiobithiophene, prepared
from bithiophene 2 (1 g, 6.02 mmol) and n-butyllithium
(3.1 mL, 7.81 mmol) in dry THF (50 mL) and cooled to
–78°C, was added a solution of N-fluorobenzenesulfon-
amide (2.08 g, 6.59 mmol) in anhydrous THF under the
atmosphere of nitrogen. The mixture was stirred over-
night at room temperature and then treated with hexane
6
.83 d (J = 4.4 Hz, 1H), 6.94 d (J = 3.6 Hz, 1H), 7.09 d
(
J = 4.4 Hz, 1H). Found, %: C 39.77, H 4.03. C H-
1
4
BrSe . Calculated, %: C 39.74, H 4.05.
17
2
5
-Bromo-5'-hexyl-2,2'-bithiophene (17). In dark-
ness, a solution of NBS (0.74 g, 4.15 mmol) in DMF
15 mL) was added dropwise to a solution of n-hexylbi-
(
50 mL), washed with brine and dried over MgSO . The
(
4
solvent was concentrated, and the residue was purified
thiophene (15) (1 g, 4.00 mmol) in DMF (10 mL) cooled
down to –20°C, and stirred for 4 h. After addition of
ice to the reaction mixture and extraction with CH Cl ,
by column chromatography on silica gel with hexane to
1
give compound 2a as a colorless oil. Yield 20%. H NMR
2
2
400 MHz) spectrum, δ, ppm: 6.38 d.d (J = 2 Hz, JHF
( =
the organic phases were washed with water, dried over
Na SO and concentrated in vacuo. The oily residue
2
Hz, 1H), 6.74 t (J = 3.6 Hz, JHF = 3.6 Hz, 1H), 6.96–7.04
m (2H), 7.17–7.18 m (1H). Found, %: C 52.17, H 2.71.
C H FS . Calculated, %: C 52.15, H 2.74.
2
4
was purified by column chromatography on silica gel
with hexane to give compound 17 as a white solid. Yield
8
5
2
1
8
8%. H NMR (400 MHz) spectrum, δ, ppm: 0.89 t (J =
2-(5-Bromothiophen-2-yl)-5-fluorothiophene (2b).
6
.8 Hz, 3H), 1.28–1.38 m (6H), 1.62–1.67 m (2H), 2.76
Into a stirred solution of 2-fluoro-5-(thiophen-2-yl)thio-
phene (2a) (0.15 g, 0.815 mmol) in chloroform-acetic acid
(1 : 1 v/v 20 mL) was added portionwise NBS (0.159 g,
0.89 mmol). The mixture was stirred at room temperature
for 24 h, poured into water (100 mL) and extracted with
t (J = 7.6 Hz, 2H), 6.64 d (J = 3.6 Hz, 1H), 6.81 d (J =
3
.6 Hz, 1H), 6.89 d (J = 3.6 Hz, 1H), 6.91 d (J = 4.0 Hz,
1H). Found, %: C 51.07, H 5.16. C H BrS . Calculated,
14
17
2
%
: C 51.06, H 5.20.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

1920
MONDAL et al.
dichloromethane. The extract was successively washed
with water, 5% sodium hydrogen carbonate aqueous
gel with hexane to give compound 2c as a white solid.
o
1
Yield 25%, mp 71 C. H NMR (400 MHz) spectrum,
δ, ppm: 6.44 t (J = 4 Hz, JHF = 4.4 Hz, 1H), 6.77 t (J =
4.0 Hz, JHF = 3.6 Hz, 1H), 7.10 d (J = 3.6 Hz, 1H), 7.83 d
(J = 5.6 Hz, 1H). Found, %: C 34.57, H 1.80. C H FSe .
solution, and brine, and dried over MgSO . The solvent
4
was evaporated and the residue was purified by column
chromatography on silica gel with hexane to give com-
8
5
2
1
pound 2b as a white solid. Yield 56%, mp 78°C. H NMR
Calculated, %: C 34.56, H 1.81.
(
2
400 MHz) spectrum, δ, ppm: 6.37 d.d (J = 2.0 Hz, JHF
=
.0 Hz, 1H), 6.67 t (J = 4.0 Hz, JHF = 3.6 Hz, 1H), 6.76
2-(5-Bromoselenophen-2-yl)-5-fluoroselenophene
2d). Into a stirred solution of 2-fluoro-5-(selenophen-
(
d (J = 3.6 Hz, 1H), 6.92 d (J = 4.0Hz, 1H). Found, %:
C 36.53, H 1.51. C H BrFS . Calculated, %: C 36.51,
H 1.53.
2-yl)selenophene (2c) (0.25 g, 0.89 mmol) in chloroform-
8
4
2
acetic acid (1 : 1 v/v 20 mL) was added portionwise
NBS (0.16 g, 0.89 mmol). The mixture was stirred at
room temperature for 24 h, poured into water (100 mL)
and extracted with dichloromethane. The extract was
successively washed with water, 5% sodium hydrogen
carbonate aqueous solution and brine solution, and dried
over MgSO The solvent was evaporated, and the residue
DF-(4S-2Se) (20). Amixture of 2-(5-bromothiophen-
-yl)-5-fluorothiophene (2b) (0.10 g, 0.38 mmol) with
,5'-bis-tributylstannyl-2,2'-biselenophene (7) (0.16 g,
.19 mmol) and tetrakis(triphenylphosphine)palladium
2
5
0
(
0.044 g , 0.038 mmol ) in dry toluene (30 mL) was
4.
refluxed for 24 h under the atmosphere of nitrogen. The
precipitate was filtered off under suction and sublimed
at 270 C to give compound 20 as a dark red solid. Yield
2
1
8
was purified by column chromatography on silica gel with
hexane to give compound 2d as a white solid. Yield 48%,
o
o
1
mp 89 C. H NMR (400 MHz) spectrum, δ, ppm: 6.45 t
J = 4.8 Hz, JHF = 4.4 Hz, 1H), 6.69 t (J = 4.0 Hz, JHF
–
1
0%, mp > 358°C. IR spectrum, ν, cm : 1569, 1546,
(
=
513, 1480, 1447, 1271, 1244, 1225, 1198, 1066, 1030,
4
.0 Hz, 1H), 6.79 d (J = 4.0 Hz, 1H), 7.10 d (J = 4.0Hz,
58, 788, 756, 724, 712, 668, 584, 443. MS: m/z: 625
1
H). Found, %: C 26.95, H 1.11. C H BrFSe . Calculated,
8
4
2
+
[M + 1] . Found, %: C 46.17, H 1.91. C H F S Se .
2
4
12
2
4
2
%: C 26.92, H 1.13.
Calculated, %: C 46.16, H 1.94.
DF-6Se (22). A mixture of 2-(5-bromoselenophen-
DF-6T (21). Amixture of 2-(5-bromothiophen-2-yl)-
-fluorothiophene (2b) (2.0 g, 7.63 mmol) with 5,5'-bis-
2-yl)-5-fluoroselenophene (2d) (1.0 g, 2.80 mmol) with
5,5'-bis-tributylstannyl-2,2'-biselenophene (7) (1.17 g,
1.40 mmol) and tetrakis(triphenylphosphine)palladium
5
tributylstannyl-2,2'-bithiophene (8) (2.83 g, 3.80 mmol)
and tetrakis(triphenylphosphine)palladium (0.881 g,
0
2
was filtered off under suction and sublimed at 260°C to
give compound 21 as an orange red solid. Yield 22 %,
mp > 300°C. IR spectrum, ν, cm : 1569, 1546, 1509,
(
0.320 g , 0.28 mmol ) in dry toluene (30 mL) was refluxed
.76 mmol ) in dry toluene (30 mL) was refluxed for
4 h under the atmosphere of nitrogen. The precipitate
for 24 h under the atmosphere of nitrogen. The precipitate
was filtered under suction. Found, %: C 35.51, H 1.47.
C H F Se . Calculated, %: C 35.49, H 1.49.
2
4
12
2
6
–
1
CONCLUSIONS
1
1
5
480, 1442, 1353, 1273, 1246, 1228, 1199, 1072, 1067,
We have synthesized new p and n types oligosele-
031, 839, 788, 724, 712, 679, 626, 566, 465. MS: m/z:
+
nophenes by the double Stille coupling method using
tetrakis(triphenylphosphine)palladium(0) as a catalyst.
A series of semiconducting oligomers (n and p) consist-
ing of mixed hetero-units (hexamers of thiophene and
selenophene) are also synthesized by the Stille coupling
reaction. Thermal properties of n and p type oligosele-
nophenes are studied and compared with those of
π-conjugated thiophene based oligomers using DSC and
TGAmeasurements. Physical properties of the oligomers
are also studied.
30 [M] . Found, %: C 54.35, H 2.25. C H F S . Cal-
2
4
12 2 6
culated, %: C 54.31, H 2.28.
2
-Fluoro-5-(selenophen-2-yl)selenophene (2c).
Into a stirred solution of mono-lithiobiselenophene,
prepared from biselenophene 1 (1 g, 3.84 mmol)
and n-butyllithium (2.0 mL, 4.98 mmol) in dry ether
(50 mL) and cooled down to –78°C, was added a solution
of N-fluorobenzene sulfonamide (2.66 g, 8.43 mmol) in
anhydrous THF under the atmosphere of nitrogen. The
mixture was stirred overnight at room temperature and
then treated with hexane (50 mL), washed with brine, and
CONFLICT OF INTEREST
dried over MgSO . The solvent was evaporated and the
4
residue was purified by column chromatography on silica
No conflict of interest was declared by the authors.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 89 No. 9 2019

OLIGOSELENOPHENES (n AND p TYPE): SYNTHESIS AND PROPERTIES
1921
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