Synthesis of side-chain regioregular and main-chain alternating poly(bichalcogenophene)s and an ABC-type periodic poly(terchalcogenophene)

Article abstract of DOI:10.1039/d0sc00404a

Three unsymmetrical diiodobichalcogenophenes SSeI₂, STeI₂, and SeTeI₂ and a diiodoterchalcogenophene SSeTeI₂ were prepared. Grignard metathesis of SSeI₂, STeI₂, SeTeI₂, and SSeTeI₂ occurred regioselectively at the lighter chalcogenophene site because of its relatively lower electron density and less steric bulk. Nickel-catalyzed Kumada catalyst-transfer polycondensation of these Mg species provided a new class of side-chain regioregular and main-chain AB-type alternating poly(bichalcogenophene)s - PSSe, PSTe, and PSeTe - through a chain-growth mechanism. The ring-walking of the Ni catalyst from the lighter to the heavier chalcogenophene facilitated subsequent oxidative addition, thereby suppressing the possibility of chain-transfer or chain-termination. More significantly, the Ni catalyst could walk over the distance of three rings (ca. 1 nm) - from a thiophene unit via a selenophene unit to a tellurophene unit - to form PSSeTe, the first ABC-type regioregular and periodic poly(terchalcogenophene) comprising three different types of 3-hexylchalcogenophenes.

Full text of DOI:10.1039/d0sc00404a

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DOI: 10.1039/D0SC00404A
ARTICLE
Synthesis of Side-Chain Regioregular and Main-Chain Alternating
Poly(bichalcogenophene)s
Poly(terchalcogenophene)
and
an
ABC-Type
Periodic
Received 00th January 20xx,
Accepted 00th January 20xx
Huai-Hsuan Liu,a Wei-Wei Liang,a Yu-Ying Lai,c Yen-Chen Su,a Hau-Ren Yang,c Kuang-Yi
Cheng,a Sheng-Cih Huanga and Yen-Ju Cheng*ab
DOI: 10.1039/x0xx00000x
Three unsymmetrical diiodobichalcogenophenes SSeI2, STeI2, and SeTeI2 and a diiodoterchalcogenophene SSeTeI2 were
prepared. Grignard metathesis of SSeI2, STeI2, SeTeI2, and SSeTeI2 occurred regioselectively at the lighter chalcogenophene
site, with its relatively lower electron density and less steric bulk. Nickel-catalyzed Kumada catalyst-transfer
polycondensation of these Mg species provided a new class of side-chain regioregular and main-chain AB-type alternating
poly(bichalcogenophene)s—PSSe, PSTe, and PSeTe—through a chain-growth mechanism. The ring-walking of the Ni catalyst
from the lighter to the heavier chalcogenophene facilitated subsequent oxidative addition, thereby suppressing the
possibility of chain-transfer or chain-termination. More significantly, the Ni catalyst could walk over the distance of three
rings (ca. 1 nm)—from a thiophene unit via a selenophene unit to a tellurophene unit—to form PSSeTe, the first ABC-type
regioregular and periodic poly(terchalcogenophene) comprising three different types of 3-hexylchalcogenophenes.
shifts in their absorption.42-50 The homopolymers 3-
alkylselenophene (P3HS)51, 52 and 3-alkyltellurophene (P3HTe)53, 54
have also been prepared using the KCTP. We were interested in
Introduction
studying
chalcogenophenes into a single polymer to generate a new class
of poly(bichalcogenophene)s. By integrating thiophene,
the
effects
of
incorporating
two
different
Conjugated polymers featuring continuous sp2/sp-hybridized
backbones for extensive delocalization of electrons have been
explored widely for several decades.1-5 Regioregular poly(3-
hexylthiophene) (P3HT) exhibiting high charge mobility has been
widely applied in organic light-emitting diodes,6, 7 transistors,8-14 and
selenophene, and tellurophene units in a single polymer in a
controlled sequence, the properties of the polymer could
presumably be tailored specifically. In particular, we envisaged that
positioning selenophene and tellurophene units, having five-
membered ring structures similar to thiophene, into the polymers
would allow fine-tuning of the optical and electronic properties
without substantially affecting the conformation of the main chain,
thereby maintaining high crystallinity. To date, the syntheses of
random55-59 and block poly(bichalcogenophene)s60-63 [e.g.,
poly(thiophene-block-selenophene)] have been realized simply by
controlling the sequence of addition of the monomer. Nevertheless,
the synthesis of main-chain alternating and side-chain regioregular
AB-type poly(bichalcogenophene)s remains a challenge, and these
materials have not been well explored.64-66 In particular,
tellurophene-incorporated alternating poly(bichalcogenophene)s
have never been described previously. In this present study, we
polymer solar cells15-18 due to its high crystallinity and well-ordered
20
packing structure in the thin film.19,
As a result, extensive
research have been focused on Kumada catalyst-transfer
polycondensation (KCTP) for the synthesis of highly regioregular
P3HT having a low polydispersity index (PDI).21-36 However, an
intrinsic drawback of P3HT and other thiophene-based polymers is
the limited absorption window from approximately 300 to 550 nm,
thereby hindering their various optoelectronic applications.
Structurally similar to thiophene, two other five-membered
heterocycles in the chalcogenophene family—selenophene and
tellurophene, featuring group-16 Se and Te elements,
respectively—have attracted considerable interest because (1) Se
and Te atoms are larger and have d-orbitals of higher polarizability
(relative to S) to induce strong Se···Se and Te···Te attractions,
potentially strengthening their interpolymer interactions,37-41 and (2)
as the chalcogen becomes heavier, the π-electrons in selenophene
and tellurophene tend to adopt more quinoidal character with higher
coplanarity, giving rise to narrower band gaps and bathochromic
developed
a
new
class
of
AB-type
alternating
poly(bichalcogenophene)s constructed from 3-hexylthiophene, 3-
hexylselenophene, and 3-hexyltellurophene units (Figure 1). After
careful design of the unsymmetrical bichalcogenophene monomers,
we synthesized poly(3-hexylthiophene–alt–3-hexylselenophene)
(PSSe), poly(3-hexylthiophene–alt–3-hexyltellurophene) (PSTe),
and poly(3-hexylselenophene–alt-3–hexyltellurophene) (PSeTe)
through KCTP operated in a chain-growth manner. Furthermore,
we report the first example of a regioregular ABC-type periodic
a. Department of Applied Chemistry, National Chiao Tung University, 1001
University Road, Hsin-Chu, Taiwan. E-mail: yjcheng@nctu.edu.tw
b. Center for Emergent Functional Matter Science, National Chiao Tung University,
1001 University Road, Hsinchu, 30010, Taiwan
poly(terchalcogenophene),
poly(3-hexylthiophene-per-3-
c. Institute of Polymer Science and Engineering, National Taiwan University, Taipei,
10617, Taiwan
hexylselenophene-per-3-hexyltellurophene) (PSSeTe).
Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
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ARTICLE
Journal Name
C6H13
C6H13
Te
C6H13
Te
C6H13
might occur with no selectivity. Consequently, we chose the
three monomers in model A for polymerization.
Table 1. Hirshfeld charge analysis of the charge distributions at atoms C2 and C5 of six
unsymmetrical diiodonated bichalcogenophenes, calculated at the cam-B3LYP/6-
311G(d,P), LanL2DZ(d,p) level of theory.
View Article Online
DOI: 10.1039/D0SC00404A
Se
S
Te
S
S
Se
Se
n
n
n
n
C6H13
C6H13
C6H13
C6H13
PSSeTe
C6H13
PSSe
PSTe
PSeTe
Figure 1. Chemical structures of the alternating poly(bichalcogenophene)s PSSe, PSTe,
and PSeTe and the periodic poly(terchalcogenophene) PSSeTe.
C2
C5
C6H13
X2
C5
I
C2
I
Results and discussion
X1
C6H13
Molecular design
X1 = S; X2 = Se
X1 = S; X2 = Te
X1 = Se; X2 = Te
X1 = Se; X2 = S
X1 = Te; X2 = S
X1 = Te; X2 = Se
–0.036
–0.042
–0.043
–0.030
–0.031
–0.038
–0.025
–0.026
–0.032
–0.030
–0.036
–0.036
To synthesize the three alternating polymers PSSe, PSTe, and
PSeTe through KCTP, we designed three corresponding
unsymmetrical diiodobichalcogenophene monomers having a
head-to-tail arrangement of their hexyl side chains. There are
two possible isomeric arrangements for the unsymmetrical
monomers (Figure 2): one with the heavier chalcogen placed at
X2 (i.e., model A: X1 = S, X2 = Se; X1 = S, X2 = Te; X1 = Se, X2 = Te)
and the other with the heavier chalcogen placed at X1 (i.e.,
model B: X1 = Se, X2 = S; X1 = Te, X2 = S; X1 = Te, X2 = Se). Achieving
high selectivity between the two reactive carbon atoms (C2 and
C5) during Grignard metathesis would be an important criterion
toward obtaining polymers of high head-to-tail regioregularity
after the nickel-catalyzed KCTP.27, 67 We used Hirshfeld charge
analysis to calculate the charge distributions at C2 and C5 of the
diiodobichalcogenophene monomers in models A and B (Table
1). Because of the proximity of the electron-donating aliphatic
substituent and the heavier chalcogen, C2 bore a more negative
charge than C5 in model A; in other words, a pronounced charge
difference existed between C5 and C2 in model A and,
furthermore, C5 is less sterically hindered than C2. Considering
both the electronic and steric effects, we envisioned that C5 in
model A would be more susceptible to Mg/I exchange (Grignard
metathesis) than C2, potentially leading to highly regioregular
polymers. In contrast, the C2 and C5 atoms in model B had very
similar charge densities, suggesting that Grignard metathesis
Synthetic procedures
Scheme 1 illustrates the synthesis of the three monomers SSeI2,
STeI2, and SeTeI2. Bromination of 3-hexylthiophene and 3-
hexylselenophene in the presence of N-bromosuccinimide
(NBS) afforded compounds 1 and 2, respectively. Stannylation
of 3-hexylselenophene and 3-hexyltellurophene using n-BuLi
and Me3SnCl furnished compounds 3 and 4, respectively. The
three unsymmetrical bichalcogenophenes SSe, STe, and SeTe
were obtained through Stille coupling of 1 and 3, 1 and 4, and 2
and 4, respectively. The three SSeI2, STeI2, and SeTeI2
monomers in model A were prepared through iodination of SSe,
STe, and SeTe, respectively, with N-iodosuccinimide (NIS).
Scheme
1
also presents the synthesis of the
terchalcogenophene monomer SSeTeI2. Iodination of SSe with
one equivalent of NIS afforded SSeI selectively; it was Stille
coupled with compound 4 to yield SSeTe. Iodination of SSeTe
with NIS provided SSeTeI2 in a yield of 78%.
C6H13
C6H13
Se
S
I
I
PSSe
I
I
S
Se
C6H13
C6H13
C6H13
C6H13
Te
C6H13
X2
1.Grignard metathesis
C5
I
S
I
C2
I
I
X1
I
I
or
I
I
PSTe
S
Te
Te
2.Nickel-catalyzed
C6H13
C6H13
C6H13
Kumada polymerization
Room temp. 16 hrs
C6H13
Te
C6H13
Se
I
I
Se
PSeTe
C6H13
C6H13
Model A
Model B
Figure 2. Design of six unsymmetrical monomers, in models A and B, for the synthesis of PSSe, PSTe, and PSeTe.
2 | J. Name., 2012, 00, 1-3
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Journal Name
ARTICLE
C6H13
Se
C6H13
Se
C6H13
View Article Online
DOI: 10.1039/D0SC00404A
1. iPrMgCl LiCl
S
S
Br
NIS
NBS
NBS
Se
3
I
I
S
S
2. Ni(dppp)Cl2
RT, 16 h
n
S
Pd(PPh3)4
C6H13
C6H13
C6H13
1
2
C6H13
C6H13
56% SSe
35% SSeI2
PSSe
Se
C6H13
Te
C6H13
Te
Se
Br
C6H13
1. iPrMgCl LiCl
Te
NIS
I
I
C6H13
S
S
C6H13
Se
1+4
2. Ni(dppp)Cl2
RT, 16 h
n
S
C6H13
Pd(PPh3)4
C6H13
C6H13
1. n-BuLi
Se
Te
32% STe
13% STeI2
PSTe
Sn
2. Me3SnCl
C6H13
C6H13
3
4
C6H13
Te
C6H13
Te
C6H13
Te
1. n-BuLi
Sn
Te
1. iPrMgCl LiCl
2. Ni(dppp)Cl2
2
I
NIS
I
Se
Se
2. Me3SnCl
Pd(PPh3)4
Se
n
C6H13
C6H13
95%
C6H13
C6H13
C6H13
RT, 16 h
66% SeTe
48% SeTeI2
S
PSeTe
C6H13
Te
C6H13
Te
C6H13
Te
C6H13
Se
C6H13
Se
1. iPrMgCl LiCl
2. Ni(dppp)Cl2
S
S
I
4
I
NIS
C6H13
Se
Se
I
Se
NIS
n
S
S
Pd(PPh3)4
C6H13
C6H13
C6H13
PSSeTe
RT, 16 h
C6H13
78% SSeTeI2
C6H13
79% SSeTe
C6H13
C6H13
SSe
95% SSeI
Scheme 1. Synthesis of the monomers SSeI2, STeI2, SeTeI2, and SSeTeI2 and their polymerization.
Selectivity of Grignard metathesis
limited solubility of the polymers, we used high-temperature
gel permeation chromatography (GPC), with trichlorobenzene
as the eluent, to determine their molecular weights. The GPC
data confirmed the excellent control over the molecular
weights with the relatively low polydispersity (Table 2). The
molecular weight of PSTe increased linearly (6800, 14,400, and
30,800 g/mol) upon increasing the monomer/catalyst molar
ratio (25, 50 and 100, respectively), confirming that the reaction
To confirm the regioselectivity of Grignard metathesis, we
conducted model studies prior to performing the
polymerizations. After treatment of SSeI2, STeI2, and SeTeI2
with one equivalent of isopropylmagnesium chloride lithium
chloride complex (iPrMgCl·LiCl) to induce Grignard metathesis,
the reactions were quenched with aqueous NH4Cl to replace the
reaction site with a proton. We used H NMR spectroscopy to
examine the crude products (Figure S1a). If the Grignard
metathesis had occurred at the C2 position, three singlets
1
had
the
characteristics
of
a
catalyst-transfer
polycondensation.27, 35 The polydispersity (PDI) of PSSe is a little
bit larger than the previous report.66 It should be noted that we
used diiodobichalcogenophene monomers for polymerization.
Compared to bromine or chlorine, the larger atomic radius of
iodine may increase the activation energy for transmetalation
during the polymerization, resulting in the higher PDI.68
1
should have appeared in the aromatic region of the H NMR
spectrum; if it had occurred at the C5 position, one singlet and
two doublets should have been expected. We observed one
singlet and two doublets in the spectra of each of the model
monomers, revealing that the Grignard metathesis had taken
place predominantly at the C5 atom. This phenomenon also
Table 2. Molecular weights, PDIs, and regioregularities of the polymers.
1
occurred for the SSeTeI2 monomer, the H NMR spectrum of
which featured two singlets and two doublets (Figure S1b),
implying that its Grignard metathesis also occurred at the C5
position, rather than at the C2 position. For reference, we
synthesized another isomeric SSeI2 monomer, of the model B
type, and performed the same experiment. Figure S2 reveals
two major products, resulting from protonation at both the C2
and C5 positions, after quenching with NH4Cl(aq), confirming that
the unsymmetrical bichalcogenophenes in model B did not
exhibit selectivity toward Grignard metathesis. Thus, we were
correct in choosing model A to achieve high selectivity in
Grignard metathesis.
a
polymer
PSSe
M/cat.
50
Mn
PDI
RR
16,500
4700
1.38
2.59
92%
85%
PSeS
20
25
50
100
6800
14,400
30,800
1.34
1.33
1.49
PSTe
94%
PSeTe
50
50
23,900
20,600
1.47
1.57
91%
94%
PSSeTe
aMeasured through high-temperature GPC at 160 °C, using polystyrene standards and
1,2,4-trichlorobenzene as the eluent. All the concentrations of polymeric solution are 1
mg/mL.
Polymer synthesis and structure identification
We used 1H and 13C NMR spectroscopy to investigate the
chemical structures and regioregularities of the new
poly(bichalcogenophene)s and poly(terchalcogenophene). The
1H NMR spectra of the polymers PSSe, PSTe, and PSeTe all
featured two well-defined aromatic singlets representing the
two kinds of protons on the two different 3-
hexylchalcogenophene rings (Figure S3). Similarly, the spectrum
of PSSeTe featured three singlets representing the three
We treated the monomers SSeI2, STeI2, SeTeI2, and SSeTeI2 with
1.0 equivalent of iPrMgCl·LiCl and then performed Ni(dppp)Cl2-
catalyzed KCTP at room temperature to yield the desired
polymers PSSe, PSTe, PSeTe, and PSSeTe, respectively (Scheme
1). All the polymers were precipitated using 6 M HCl/MeOH
solution and then they were washed with MeOH. Because of the
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J. Name., 2013, 00, 1-3 | 3
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ARTICLE
Journal Name
different 3-hexylchalcogenophene rings. The protons on the four unsymmetrical monomers in model A not only allowed
View Article Online
first carbon atoms of the hexyl groups appeared at 2.6–2.8 ppm; regioselective Grignard metathesis but DaOlsIo: 10fa.1v0o39re/Dd0SsCe0le0c4t0iv4Ae
those on the terminal carbon atoms of the hexyl groups ring-walking,
leading
to
highly
regioregular
appeared at 0.91–0.93 ppm. Through integration of the signals polychalcogenophenes through the KCTP mechanism. In
of the first sets of protons, we estimated the head-to-tail contrast, the polymer PSeS prepared from the isomeric model-
regioregularity to be greater than 90%.67 The 13C NMR spectra B monomer SSeI2 featured (Table 2) much higher polydispersity
(Figure S4) featured eight well-defined aromatic peaks for PSSe, (2.59) and lower regioregularity (85%), suggesting that the Ni
PSTe, and PSeTe and 12 for PSSeTe, suggesting that these new catalyst was more likely to dissociate from the polymer, leading
polychalcogenophenes featured alternating and periodic main to chain-transfer and chain-termination reactions.
reductive elmination & -complex
C6H13
chains characterized by regioregular head-to-tail hexyl groups.
Ni
C6H13
C6H13
S
Ni
C6H13
iPrMgCl
Cl
Cl
Te
S
Te
I
Te
C6H13
I
I
C6H13
Te
MgCl
I
Ni
S
S
S
Dependence of the number average molecular weight (Mn) and
PDI on monomer conversion for PSTe is shown in Table S1 and
Figure S5 where the Mn increased linearly with the conversion,
indicating the controlled polymerization process. MALDI-TOF
mass spectrometry was used to characterize the end-groups of
PSSeTe (Figure S6). The major peaks of PSSeTe matched that
expected for (SSeTe)n structure (where n is the number of
repeat units) terminated with an iodine and a hydrogen (I/H)
atom. Thus, the MALDI-TOF-MS was also in good agreement
with a mechanism involving catalyst-transfer polycondensation.
Te
C6H13
transmetallation
I
C6H13
C6H13
I
S
Te
C6H13
C6H13
I
C6H13
ring walking
C6H13
C6H13
Te
C6H13
I
I
C6H13
C6H13
Te
Ni
Te
I
Te
C6H13
S
C6H13
S
I
I
S
S
S
C6H13
S
Ni
Te
C6H13
S
oxidative addtition
C6H13
C6H13
Te
C6H13
Te
I
Ni
C6H13
C6H13
transmetallation
reductive elmination &
ring walking
C6H13
Te
C6H13
C6H13
C6H13
C6H13
Ni
Ni
H+
C6H13
Te
Te
S
I
C6H13
I
S
C6H13
S
S
Te
S
Te
C6H13
Te
S
I
S
n
C6H13
C6H13
n
I
S
I
oxidative addtition
C6H13
Te
C6H13
Te
C6H13
C6H13
C6H13
Scheme 2. Proposed mechanism for catalyst-transfer polycondensation of PSTe via
ring-walking.
Plausible mechanism
Table 3. Association energies (ΔG) for the interactions of Ni(dppp) with 2-
ethylthiophene, 2-ethylselenophene, and 2-ethyltellurophene.
According to the mechanism of catalyst-transfer
polycondensation proposed by Yokozawa et al.27 and research
through theoretical calculations, Scheme 2 presents our
suggested mechanism for the polymerization, taking STeI2 as an
example. Initially, STeI2 undergoes regioselective Grignard
metathesis to form a nucleophilic ITeS-MgCl intermediate,
which doubly attacks the Ni(dppp)Cl2 catalyst for ligand
transfer. Following reductive elimination, the Ni species is
coordinated to and stabilized by the π-electron
bichalcogenophenes.69 We suspect that the stronger chelation
ability of the heavier Te atom (relative to a S atom) drives the
Ni atom to shift from thiophene to tellurophene through a
selective “ring-walking” process. The chain grows through a
repetitive sequence of transmetalation, reductive elimination,
Ni -complexation, ring-walking, and oxidative addition.
Eventually, the polymerization is terminated by a proton to give
the final polymer chain. Based on this mechanism, the polymer
would contain repeating biaryl or triaryl units with I and H atoms
at the chain ends [I(biaryl)nH or I(triaryl)nH, respectively].
Notably, the successful synthesis of PSSeTe suggests that the Ni
catalyst could walk smoothly over three rings: from the
thiophene unit, via the selenophene unit, to the tellurophene
unit. To support this hypothesis, we performed theoretical
calculations to estimate the complexation energies of the Ni
catalyst with the independent chalcogenophenes. Table 3
reveals that the complexation energies of a Ni(dppp) species
with 2-ethylthiophene (complex 1), 2-ethylselenophene
(complex 2), and 2-ethyltellurophene (complex 3) were
approximately –15.1, –17.3, and –19.3 kcal/mol, respectively.
With increased complexation energy, the migration of the Ni
complex would be thermodynamically favored from the
thiophene unit via the selenophene unit to the tellurophene
unit, in turn facilitating the subsequent oxidative addition at the
iodotellurophene chain end. Thus, our rational design of the
complex 1
complex 2
complex 3
Ni(dppp)
S
Ni(dppp)
Se
Ni(dppp)
Te
ΔG
–15.1 kcal mol–1
–17.3 kcal mol–1
–19.3 kcal mol–1
Optical properties
To investigate the effect of the chalcogen on the optical
absorption behavior of the polychalcogenophenes, we
recorded the UV–Vis spectra of our four new polymers PSSe,
PSTe, PSeTe, and PSSeTe as well as those of the three
homopolymers PSS (P3HT), PSeSe (P3HS), and PTeTe (P3HTe).
Figure 3 presents the UV–Vis spectra of all of these polymers in
solution (o-dichlorobenzene) and in the solid state; Table 4
summarizes the absorption parameters and the band gaps. In
solution, the values of λmax of the polymers were highly
dependent on the chemical composition: PTeTe (631 nm) >
PSeTe (541 nm) > PSTe (511 nm) > PSeSe (494 nm) > PSSe (471
nm)
>
PSS (448 nm). Upon incorporating heavier
chalcogenophenes (selenophene and tellurophene) into the
polymers, the absorption red-shifted with concomitant
broadening of the full width at half maximum (FWHM) of the
4 | J. Name., 2012, 00, 1-3
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band. The ability of the chalcogenophene to red-shift the
absorption of the polymer followed the order tellurophene >
selenophene > thiophene. Thus, the absorption of the
View Article Online
DOI: 10.1039/D0SC00404A
Figure 4. CV plots of the polymer films, relative to ferrocene, revealing their oxidation
and reduction onsets.
Figure 3. Absorption spectra of PTeTe, PSeTe, PSSeTe, PSTe, PSeSe, PSSe, and PSS (a)
in oDCB solution and (b) as thin films.
Table 5. CV data of the polymers.
polychalcogenophene in the range from 300 to 800 nm could be
tuned subtly by carefully selecting the combination of
chalcogenophenes in a single polymer. Furthermore, these
polymers exhibited significant red-shifting and broadening of
their absorptions upon proceeding from solution to the solid
state, indicating that strong intermolecular interactions existed
between the chalcogenophene moieties. Notably, the
tellurophene-containing polymer PSeTe, without featuring any
electron-deficient units, possessed a relatively small band gap
LUMO (eV)
–2.46
HOMO (eV)
–4.89
Egele (eV)
2.43
PSS
PSSe
–2.70
–4.88
2.18
PSTe
–2.76
–4.82
2.06
PSeSe
PSSeTe
PSeTe
PTeTe
–2.78
–4.81
2.03
–2.80
–4.86
2.06
and had an onset wavelength of 800 nm.
Table 4. UV–Vis absorption spectral properties of the polymers.
–2.92
–4.81
1.89
–2.96
–4.77
1.81
λmax (nm)
oDCBa
(FWHM)
Film
λonset (nm)
Egopt (eV)
Thin film morphologies
PSS
448 (119)
471 (140)
494 (167)
511 (143)
497 (147)
541 (167)
631 (246)
514
523
547
576
593
589
632
643
711
746
770
777
811
877
1.93
1.74
1.66
1.61
1.60
1.53
1.41
We used grazing-incidence wide-angle X-ray scattering
(GIWAXS) to investigate the morphologies of the
polychalcogenophenes. Because of the relatively poor solubility
of the polychalcogenophenes, we selected o-dichlorobenzene
as the processing solvent. Figure 5 presents the two-
dimensional (2D) images and corresponding one-dimensional
(1D) patterns. All of the polymer films exhibited out-of-plane
(h00) signals corresponding to side-chain interdigitation. We
also observed obvious (010) in-plane peaks corresponding to
periodic π-stacking between the two facing conjugated
backbones. These in-plane (010) peaks revealed that the
polymers adopted predominately edge-on orientations with the
backbone plane approximately perpendicular to the substrate.
The out-of-plane lamellar spacing (dl) and the in-plane π-
stacking spacing (dπ) of the polymers were estimated using the
Bragg equation (Table 6). The π-stacking spacing of the
poly(bichalcogenophene)s increased upon increasing the
tellurophene content (dπ: 3.84 Å for PSSe, 3.87 Å for PSeTe, and
3.94 Å for PSTe), presumably because of the larger size of the
Te atom. In contrast, the lamellar spacing decreased upon
increasing the tellurophene content (dl: 15.95 Å for PSTe, 16.33
Å for PSeTe, and 16.51 Å for PSSe). It is likely that the increased
π-stacking distance provided more volume for the hexyl groups
to interdigitate, thereby contracting the lamellar spacing.70
PSSe
PSeSe
PSTe
PSSeTe
PSeTe
PTeTe
ao-Dichlorobenzene
Electrochemical properties
We used cyclic voltammetry to evaluate the electrochemical
properties of the polymers and determine their HOMO and
LUMO energy levels (Figure 4, Table 5). When a heavier
chalcogenophene was incorporated, the HOMO was slightly
elevated (higher-lying) while the LUMO had descended (lower-
lying) more significantly. Thus, the HOMO and LUMO energy
levels, along with the band gaps, could be controlled by
combining different chalcogenophenes into a single polymer.
Nevertheless,
it
is
interesting
that
the
poly(terchalcogenophene) PSSeTe, featuring the lowest
tellurophene content (one third), possessed the shortest π-
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stacking spacing (3.82 Å) and, thus, the largest lamellar spacing same as the direction of current flow. The mobility of the
View Article Online
DOI: 10.1039/D0SC00404A
(17.18 Å). After thermal annealing at 100 °C for 30 min, the poly(bichalcogenophene)s decreased when incorporating the
diffraction patterns were essentially unchanged, but with two heavier chalcogenophenes: 7.2  10–3 cm2 V–1 s–1 for PSSe
enhanced signal intensities and slightly lower values of dπ and > 2.7  10–3 cm2 V–1 s–1 for PSTe > 5.2  10–4 cm2 V–1 s–1 for
dl. Interestingly, PSeTe exhibited two lamellar interlayer PSeTe. Because of the larger radius of the Te atom, PSTe and
spacings (16.33 and 12.95 Å), indicating the formation of two PSeTe possessed larger π-stacking spacings, which were
crystal phases with different types of side-chain presumably related to their lower mobilities. Notably, however,
interdigitation.52, 71
the tellurophene-containing polymers had relatively poorer
solubilities, which might also have influenced their film quality
and, thus, mobility. Interestingly, the poly(terchalcogenophene)
PSSeTe exhibited the highest mobility (1.2  10–2 cm2 V–1 s–1),
consistent with it having the shortest π-stacking distance. This
mobility is one of the highest ever reported for a tellurophene-
chalcogenophene-based polymer.70
Table 7. Charge carrier mobilities, threshold voltages, and on/off ratios for OFETs
incorporating spin-coated PSSe, PSTe, PSeTe, and PSSeTe films.
Polymer
PSSe
Ion/Ioff
Vth (V)
–7.5
Mobility (cm2 V–1 s–1
7.2  10–3
)
8.5  103
8.1  102
4.9  103
8.0  103
PSTe
–3.6
2.7  10–3
PSeTe
PSSeTe
–19.6
–23.1
5.2  10–4
1.2  10–2
Figure 5. 2D GIWAXS images of the polymers and their corresponding 1D in-plane and
out-of-plane patterns, recorded before and after thermal annealing at 100 °C.
Conclusions
We have developed a new class of side-chain regioregular and
main-chain alternating polychalcogenophenes having precisely
controlled sequences. Our success in obtaining these highly
regular polychalcogenophenes relied on the synthesis of
Table 6. Lamellar spacings and π-stacking spacings of the polymers.
As-cast
Thermal annealing at 100 °C
corresponding
unsymmetrical
monomers:
the
lamellar
π-stacking
lamellar
π-stacking
diiodobichalcogenophenes SSeI2, STeI2, and SeTeI2 and the
diiodoterchalcogenophene SSeTeI2 (in model A). Because of
greater electron-deficiency and less steric hindrance, Grignard
metathesis of the monomers SSeI2, STeI2, SeTeI2 occurred
regioselectively at the lighter chalcogenophene unit (i.e., the
thiophene units for SSeI2, STeI2, and SSeTeI2; the selenophene
unit for SeTeI2). All of the regioselectively prepared Mg species
underwent Ni(dppp)Cl2-catalyzed KCTP. The key step during the
KCTP was the Ni complex undergoing thermodynamically
favorable ring-walking from the lighter to the heavier
chalcogenophene, with corresponding higher complexation
energy with the Ni catalyst. The ring-walking facilitated
subsequent oxidative addition, suppressing the possibility of
chain-transfer or chain-termination. Our first synthesis of an
ABC-type periodic poly(terchalcogenophene) PSSeTe also
reveals that the Ni catalyst could walk efficiently over a distance
of three rings (ca. 1 nm) from a thiophene unit via a
selenophene unit to a tellurophene unit, all with a gradual
increase in complexation energy, thereby preserving the
mechanism of catalyst-transfer polycondensation. The thin-film
morphologies and the optical, electrochemical, conformational,
and OFET properties of the polymers could be tailored
systematically by combining the different chalcogenophenes at
various ratios—a promising feature for a wide range of
spacing (Å)
spacing (Å)
spacing (Å)
spacing (Å)
PSSe
16.51
15.95
3.84
3.94
3.87
3.82
16.51
15.92
3.83
3.92
3.87
3.82
PSTe
PSeTe
PSSeTe
16.33, 12.95
17.18
16.28, 12.80
16.55
OFET devices
To evaluate the p-type mobilities of PSSe, PSTe, PSeTe, and
PSSeTe, we fabricated OFET devices incorporating ODTS-
treated SiO2/Si dielectric substrates in a bottom-gate/top-
contact configuration. The spin-cast polymer thin films were
thermally annealed at 100 °C for 10 min. When the applied gate-
to-source voltage (VGS) was ramped from 0 to –75 V in steps of
–15 V, the OFETs exhibited typical p-type behavior (Figure S7).
Table
7 summarizes the saturation organic field-effect
mobilities (μsat), threshold voltages (Vth), and current on/off
ratios (Ion/Ioff) calculated from the transfer characteristics
(Figure S8) of these devices. The edge-on π-stacking
orientations of the polymers were favorable for horizontal
carrier mobility, because the direction of π-stacking was the
6 | J. Name., 2012, 00, 1-3
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R. Durrant and I. McCulloch, Nat. Commun., 2016, 7,
potential applications. This paper provides a design concept for
the use of single unsymmetrical monomers to create new AB-
and ABC-type alternating and periodic conjugated polymers
with high regioregularity.
View Article Online
11585.
DOI: 10.1039/D0SC00404A
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work is supported by Ministry of Science and Technology,
Taiwan (grant No. MOST107-3017-F009-003) and Ministry of
Education, Taiwan (SPROUT Project-Center for Emergent
Functional Matter Science of National Chiao Tung University).
We thank the National Center of High-Performance Computing
(NCHC) in Taiwan for computer time and facilities. We also
thank the National Synchrotron Radiation Research Center
(NSRRC), and Dr. U-Ser Jeng and Dr. Chun-Jen Su at BL23A1
station for the help with the GIXS experiments.
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Table of content entry
Synthesis of Side-Chain Regioregular and Main-Chain Alternating
Poly(bichalcogenophene)s
Poly(terchalcogenophene)
and
an
ABC-Type
Periodic