Solution-processable conjugated polymers containing alternating 1-alkyl-1,2,4-triazole and N=S=N links

Article abstract of DOI:10.1039/c0jm00254b

Poly(sulfur nitride), [SN]_x, is an infinite π-bonded system and only known polymeric superconductor composed of nonmetallic elements. The appealing electronic properties of [SN]_x, along with the safety issue involved in the synthesis

Full text of DOI:10.1039/c0jm00254b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Solution-processable conjugated polymers containing alternating
1-alkyl-1,2,4-triazole and N]S]N links†
*
Mingfeng Wang and Fred Wudl
Received 2nd February 2010, Accepted 5th May 2010
First published as an Advance Article on the web 4th June 2010
DOI: 10.1039/c0jm00254b
Poly(sulfur nitride), [SN]x, is an infinite p-bonded system and only known polymeric superconductor
composed of nonmetallic elements. The appealing electronic properties of [SN]x, along with the safety
issue involved in the synthesis, have motivated research on [SN]x analogues, for example, conjugated
polymers consisting of sulfur–nitrogen moieties separated by heterocyclic groups. In this paper, we
report an exploration for a new N]S]N-linked polymer, with a major objective to improve its
solubility for low-cost, large-area fabrication of molecular devices based on a solution process.
Specifically, we present the synthesis and characterization of a conjugated polymer based on alternating
1-alkyl-1,2,4-triazole and sulfur–nitrogen. We found that 4-dimethylamino pyridine is an effective
catalyst for the polymerization of bis-N-sulfinyl-3,5-diamino-1-dodecyl-1,2,4-triazole, resulting in
polymers that show a broad absorption band in the range of 400 to 600 nm. Moreover, this polymer
exhibits high solubility in a variety of non-polar solvents, including tetrahydrofuran, chloroform,
dichloromethane, and chlorobenzene.
electron affinity of triazoles and the established electron-injec-
1. Introduction
tion/transport properties in devices such as organic light emitting
diodes.4 The combination of triazole with N]S]N links can
lead to new n-type polymers with high electron affinity. The
primary focus of this paper is to describe effective synthetic
methodologies for highly soluble polymers containing N]S]N
links in the backbone.
Poly(sulfur nitride), [SN]x, was the first example of a polymeric
metal and superconductor.1 Thanks to its two-dimensional char-
acter with a large degree of anisotropy; the electronic conductivity
of [SN]x is largest along the backbone direction, where the over-
lapping of p-orbitalson sulfur and nitrogen leads to the formation
of a conduction band. Besides the electronic conductivity, [SN]x
shows high electronegativity, even greater than that of gold.
Despite these appealing properties,2 the exploitation of [SN]x in
electronic devices has been compromised by the explosive nature
of the S4N4 starting material and intermediate S2N2 molecule that
are key precursors in the synthesis of [SN]x. This safety issue has
prompted researches to investigate other synthetic strategies to
[SN]x or its analogues. To this end, polymers containing alter-
nating sulfur–nitrogen moieties separated by heterocyclic groups
have been synthesized.3 However, these polymers, to date, were
insoluble in common solvents, raising a significant challenge for
characterizing and processing these materials.
We have found that dimethylamino pyridine is a good
polymerization catalyst for bis-N-sulfinyl-3,5-diamino-1-decyl-
1,2,4-triazole, which was synthesized from the diamine with
N-sulfinyl-p-toluenesulfonamide. These polymers, showing
a broad absorption band in the range of 400 to 600 nm, are easily
soluble in a variety of non-polar solvents such as tetrahydro-
furan, chloroform, dichloromethane, and chlorobenzene.
2. Experimental section
2.1 Materials and instruments
To meet the need of low-cost, large-area fabrication of
molecular devices based on a solution process, it is important to
explore new sulfur–nitrogen-based polymers that show both
desired solubility and optoelectronic properties. Herein we
report the synthesis and characterization of a conjugated poly-
mer based on alternating 1-alkyl-1,2,4-triazole and sulfur–
nitrogen. Our initial objective of incorporating triazole units into
the backbone of polymer chains relies on the intrinsic high
Chemicals were purchased from Sigma-Aldrich or Across
Organics and used as received without further purification. All
dry solvents were freshly distilled under argon over an appro-
priate drying agent prior to use. H and 13C NMR spectra were
1
obtained on a Varian Unity Inova 500 MHz spectrometer and
referenced to the solvent peak. Mass spectrometry was
performed by UC Santa Barbara Mass Spectrometry Labora-
tory. UV-vis spectra were recorded on an Agilent 8453 spectro-
photometer using a THF solution in 1 cm quartz cuvettes or
a drop-cast film on a quartz-slide at room temperature.
Department of Chemistry and Biochemistry, Center for Polymers and
Organic Solids, Mitsubishi Chemical Center for Advanced Materials and
Materials Research Laboratory, University of California, Santa Barbara,
CA, 93106-6105, USA. E-mail: wudl@chem.ucsb.edu
2.2 Synthesis of 3,5-diamino-1-decyl-1,2,4-triazole (DDTA)
† Electronic supplementary information (ESI) available: 1H and 13C
The synthesis of 3,5-diamino-1-decyl-1,2,4-triazole followed
a literature procedure.5 To a 250 mL 3-neck round-bottom flask,
3,5-diamino-1,2,4-triazole (5.0 g, 0.05 mol) and methanol
NMR spectra of DDTA, UV-vis absorption spectra of PBSDDT-1 in
1
THF in air, FTIR spectra of PBSDDT-1 purified twice, H NMR and
FTIR spectra of PBSDDT-2. See DOI: 10.1039/c0jm00254b
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(80 mL) were added. The flask was equipped with a condenser,
a N2 inlet and an addition funnel. Metallic sodium (1.2 g, 0.05
mol) was added piece by piece to the mixture under stirring,
resulting in a clear dark red solution. (Note: 3,5-diamino-1,2,4-
triazole itself is not soluble in methanol at room temperature.)
overnight. Yield: 0.9 g (78%), dark brown solid. 1H NMR
(CDCl3, 500 MHz): d (ppm) 4.2–3.6 (m, 2H), 1.9–1.7 (m, 2H),
1.4–1.1 (m, 18 H), 0.9–0.8 (t, 3H).
2.6 Synthesis of poly(bis-N-sulfinyl-3,5-diamino-1-decyl-1,2,4-
triazole) (PBSDDT-2) via AlCl3
ꢀ
The mixture was then heated to 80–85 C for 1 h, followed by
dropwise addition of 1-bromododecane (13.0 g, 0.052 mol) over
3 h. The mixture turned from dark red to bright yellow with the
progress of the reaction. After 24 h of refluxing, the reaction was
stopped by cooling down to room temperature. The solvent was
removed under vacuum and the solid residue was re-dissolved in
a mixture of 2-propanol (70 mL) and distilled water (30 mL) and
was poured into 500 mL of distilled water. A pale yellow solid
was collected by filtration, followed by drying at 50 ꢀC for 3 days.
Yield: 7.66 g (57%). 1H NMR (DMSO-d6, 500 MHz): d (ppm) 5.8
(s, 2H), 4.7 (s, 2H), 3.6 (t, 2H), 1.6 (t, 2H), 1.2–1.3 (m, 18H), 0.8–
0.9 (t, 3H). 13C NMR (DMSO-d6, 500 MHz): d (ppm) 160, 154,
45, 32, 29, 28, 26, 22, 14. +ESI-TOF MS: Mw ¼ 267.41 (calcu-
lated), m/z 268.27 (measured). Elemental analysis: calculated for
C14H29N5: C 62.3, H 9.6, N 25.1%. Found: C 62.9, H 10.8, N
26.2%.
BSDDT-1 (0.51 g, 1.6 mmol), see above, AlCl3 (4.5 g, 33.8 mmol)
and freshly distilled toluene (10 mL) were mixed in a 50 mL
Schlenk flask. The mixture was allowed to stand in the dark at
room temperature for 2 days. The mixture was filtered under N2
atmosphere, followed by passing through diatomaceous earth to
remove excess AlCl3. Drying of the filtrate under vacuum led to
a dark brown solid. The 1H NMR (Fig. S5†) and FTIR (Fig. S6†)
spectra of this product are presented in the ESI†.
3. Results and discussion
Mainly two kinds of methods have been used so far to synthesize
analogues of poly(sulfur nitride). The first one involves reactions
of bis(trimethylsilyl)sulfur diimide with mono-7 or di-functional
arylsulfenyl chlorides.3b However, sulfur dichloride is no longer
commercially available from reliable sources. The second
method starts from the preparation of N-sulfinylamines that can
be converted to sulfodiimides using a catalyst such as pyridine,3
AlCl3,8 and Na metal.6b Despite the success of these synthetic
methods in making small molecules, very few conjugated poly-
mers containing –N]S]N– units have been synthesized so far.3
Herein we report the synthesis and characterization of
a conjugated polymer based on alternating 1-alkyl-1,2,4-triazole
and sulfur–nitrogen. The synthetic route is shown in Scheme 1.
In the exploration of soluble triazole molecules that are
amenable for further functionalization, we started by using
3,5-diamino-1,2,4-triazole which is commercially available and
low-cost. 3,5-Diamino-1,2,4-triazole itself is readily soluble in
polar solvents such as water, DMF and DMSO but insoluble in
either alcohols or non-polar solvents. To enhance the solubility
of this molecule in organic solvents, it is necessary to introduce
an alkyl chain. As a consequence, we started by refluxing 3,5-
diamino-1,2,4-triazole with either 1-bromohexane or 1-bromo-
dodecane in methanol in the presence of sodium metal, which
automatically leads to the connection of the alkyl chains to N-1
position of 1,2,4-triazole.5 In this paper, we mainly target effi-
cient synthetic methodologies towards polymerization of
monomers derived from 1-dodecyl-1,2,4-triazole. We expect that
similar synthetic approaches could be applicable to more inter-
esting triazole-based molecules such as 4-alkyl-1,2,4-triazoles.
2.3 Synthesis of N-sulfinyl-p-toluenesulfonamide
The synthesis of N-sulfinyl-p-toluenesulfonamide followed
a procedure reported by Kresze.6 p-Toluene sulfonamide (5.0 g,
0.029 mol), SOCl2 (21 mL, 0.29 mol, freshly distilled) and 30 mL
of anhydrous benzene were ꢀmixed in a 100 mL Schlenk flask. The
mixture was kept at 85–95 C for 3 days. The excess SOCl2 and
benzene were removed under vacuum, resulting in dark yellow
viscous liquid, which solidified upon cooling to ca. ꢁ5 ꢀC. (Note:
this product was quickly hydrolyzed into p-toluene sulfonamide
upon exposure to air.)
2.4 Synthesis of bis-N-sulfinyl-3,5-diamino-1-decyl-1,2,4-
triazole (BSDDT-1) using N-sulfinyl-p-toluenesulfonamide
N-Sulfinyl-p-toluenesulfonamide (6.0 g, 0.028 mol) was dissolved
in 25 mL of anhydrous benzene in a 100 mL Schlenk flask treated
with 3,5-diamino-1-decyl-1,2,4-triazole (3.8 g, 0.014 mol) under
a N2 atmosphere, resulting in a dark red mixture. The mixture
was stirred at room temperature in the dark for 5 days. A white
solid that formed during the reaction was removed by filtration
under N2 atmosphere. The filtrate was dried under vacuum,
resulting in a dark red gum. Yield: 2.1 g (48%). 1H NMR (CDCl3,
500 MHz): d (ppm) 4.2–4.4 (t, 2H), 1.7–2.0 (t, 2H), 1.1–1.4
(m, 18H), 0.7–0.9 (t, 3H).
2.5 Synthesis of poly(bis-N-sulfinyl-3,5-diamino-1-decyl-1,2,4-
triazole) (PBSDDT-1) via 4-dimethylaminopyridine
3.1 Monomer synthesis and characterization
We found that a hexyl group made 3,5-diamino-1,2,4-triazole
soluble only in water or methanol, but not in less polar solvents
such as 2-propanol and THF. Thus we synthesized 3,5-diamino-
1-dodecyl-1,2,4-triazole (DDTA). The dodecyl hydrocarbon
chain connected to the N-1 position of the triazole unit rendered
this molecule soluble in a range of solvents, including dimethyl
sulfoxide (DMSO), methanol, 2-propanol, acetone, tetrahydro-
furan (THF), pyridine and dichloromethane. DDTA was insol-
uble in hexane, toluene, ethyl ether, acetonitrile or ethyl acetate,
presumably due to the two polar –NH2 groups.
Bis-N-sulfinyl-3,5-diamino-1-decyl-1,2,4-triazole (1.45 g, 4.7
mmol) was charged in a 50 mL Schlenk flask and 4-dimethyla-
minopyridine (DMAP) (0.571 g, 4.7 mmol) dissolved in 6 mL of
freshly distilled CH2Cl2 was added to the flask with a syringe.
The mixture was allowed to stand under N2 atmosphere at room
temperature in the dark for 2 days and treated with 40 mL of dry
CH3CN. The mixture was filtered under N2 atmosphere and the
solid was washed with dry CH3CN twice (30 mL each portion),
followed by drying under vacuum at room temperature
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Scheme 1 Synthetic approach to a conjugated polymer based on alternating 1-dodecyl-1,2,4-triazole and sulfur–nitrogen.
The treatment of DDTA with N-sulfinyl-p-toluenesulfonamide
in benzene led to the formation of bis-N-sulfinyl-3,5-diamino-
1-dodecyl-1,2,4-triazole (BSDDT-1) under mild conditions,
accompanied by the formation of a white precipitate of p-tol-
uenesulfonamide. The 1H NMR spectrum (Fig. 1A) of BSDDT-1
showed a triplet at 4.2 ppm that corresponds to the first meth-
ylene group next to the triazole. In addition, some p-toluene-
sulfonamide contaminated the BSDDT-1 after filtration, despite
its marginal solubility in benzene. Further purification of
BSDDT-1 proved to be difficult due to its sensitivity to atmo-
spheric moisture.
In contrast to DDTA, BSDDT-1 wassoluble in most of the non-
polar solvents such as pentane, hexane, toluene, THF, chloroform
anddichloromethane. Theproduct, a darkredsolidthatwasstable
under inert atmosphere, was sensitive to moisture, hydrolyzing
immediately upon exposure to air with concomitant bleaching.
Fig. 1 1H NMR (CDCl3) of BSDDT-1 and PBSDDT-1. The BSDDT-1 monomer presented here was synthesized using N-sulfinyl-p-toluenesulfo-
namide as the sulfinylating agent.
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3.2 Polymer synthesis and characterization
asymmetric vibrations of –N]S]N– plus another strong peak
at 1039 cmꢁ1 that may correspond to N–S bonds. In addition, the
two peaks in the range of 3000–3600 cmꢁ1 corresponding to
–NH2– in PBSDDT decreased in intensity, compared to those of
DDTA.
We investigated two kinds of catalysts for the polymerization of
BSDDT-1. The first was 4-dimethylaminopyridine (DMAP).
There was no obvious change for the mixture of BSDDT-1 and
10 mol% DMAP in CH2Cl2, even over 24 h. In contrast, when the
amount of DMAP was increased to 1 equivalent (relative to
BSDDT), some black solid formed overnight, accompanied by
a color change from dark red to dark brown. The material
obtained in this way was insoluble in CH3CN. Considering the
good solubility of both DMAP and p-toluenesulfonamide in
CH3CN, we purified the final product by adding excess dry
CH3CN to a concentrated solution of the reaction mixture in
CH2Cl2, followed by filtration under N2 atmosphere and
a washing with CH3CN.
Fig. 3 shows the UV-vis absorption spectra of PBSDDT-1
(purified once by precipitation with CH3CN) and DDTA. The
latter shows a narrow absorption band at 216 nm, without
absorption in the visible range, whereas PBSDDT-1, dissolved in
dry THF under Ar atmosphere, showed a broad absorption band
centered at 476 nm with an onset around 630 nm. The absorption
of PBSDDT-1 in solid film state showed no obvious shift
compared to its absorption in THF. When the same sample was
prepared under ambient condition, the absorption peak in the
The final product, which appeared as a dark brown powder,
was soluble in most non-polar solvents such as THF, CHCl3,
CH2Cl2, chlorobenzene and toluene. The trace amount (ca. 2%)
of impurities could be further removed by repeated precipitation
of a CH2Cl2 solution with CH3CN. However, over-purification
resulted in a color change of the solid product from dark brown
to orange, indicating partial hydrolysis during handling. The
details of this hydrolysis will be discussed later in this section.
The 1H NMR spectrum of the solid dissolved in CDCl3
(Fig. 1B) shows that most of the impurities were removed from
the final product after precipitation with CH3CN. Compared to
1
the H NMR spectrum of BSDDT (Fig. 1A), interestingly, the
–CH2– resonance connected to the triazole N-1 position in the
polymer (denoted as PBSDDT) splits into four peaks at 4.2–3.6
ppm. The two peaks at 4.2 and 4.0 ppm, respectively, are
multiplets. The one at 3.9 ppm with relatively high intensity
appeared single and broad, while the one at 3.6–3.7 ppm was
triplet. In addition, there are two broad peaks at 5.3 and 4.5 ppm,
respectively, that correspond to the –NH2 group at the chain end
of PBSDDT. The integration ratio of the peak at 4.5 ppm over
that at 0.8–0.9 ppm (corresponding to –CH3) gives an average
value of 11 for the number of repeating units.
Fig. 3 UV-vis absorption spectra of PBSDDT-1 in THF and in solid
films as cast on a quartz slide. For comparison, the absorption spectrum
of DDTA is also shown. This polymer was synthesized from the poly-
merization of BSDDT-1 that was prepared by using N-sulfinyl-p-tolue-
nesulfonamide as the sulfinylating agent. DMAP was used as the catalyst
for the polymerization.
Fig. 2 shows the FTIR spectra of PBSDDT-1 and DDTA. One
can see two small peaks at 1215 and 1182 cmꢁ1, respectively, in
the sample of PBSDDT. Both peaks are assigned to the
1
Fig. 4 Evolution of H NMR (CDCl3) spectrum of PBSDDT-1 during
Fig. 2 FTIR spectra of DDTA and PBSDDT-1. This polymer was
synthesized from the polymerization of BSDDT that was prepared by
using N-sulfinyl-p-toluenesulfonamide as the sulfinylating agent. DMAP
was used as the catalyst for the polymerization. The inset shows the
aging and purification. This polymer was synthesized from the poly-
merization of BSDDT-1 that was prepared by using N-sulfinyl-p-tolue-
nesulfonamide as the sulfinylating agent. DMAP was used as the catalyst
for the polymerization.
spectra in the window of 1800–4000 cmꢁ1
.
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visible range shifted to 460 nm, accompanied by an increase in
the absorption at both 285 and 214 nm (Fig. S3†).
polymer solution in each of these solvents maintained its color
(dark red to brown) for a few hours at relatively high concen-
tration (5–10 mg mLꢁ1). But the color changed to pale yellow
quickly upon further dilution, accompanied by formation of
a white precipitate that was possibly from hydrolysis of AlCl3.
The 1H NMR spectrum (Fig. S5†) of this product showed
a significant amount of p-toluenesulfonamide impurity that was
difficult to remove. The FTIR spectrum (Fig. S6†) of this
product showed a new band at 1186 cmꢁ1, corresponding to the
asymmetric vibration of the –N]S]N– group.
In order to examine the stability of PBSDDT-1, we checked
1
the H NMR spectrum (Fig. 4B) of the same sample in CDCl3
after standing in a capped and sealed NMR tube for 24 h. One
can see that the intensity of the triplet peak at 3.6–3.7 ppm
increased, whilst the intensity of the peak at 4.2 ppm decreased
relative to the two peaks at 4.0 and 3.9 ppm, respectively. In
addition, the single peak at 1.8 ppm splits into two after 24 h of
aging. There is also a slight up-field shift (ca. 0.1 ppm) for the
peaks at 4.0 ppm and 3.7 ppm.
The instability of PBSDDT-1 toward hydrolysis could be
further discerned during the purification of PBSDDT-1 by
a second precipitation in CH3CN. The 1H NMR spectrum
(Fig. 4C) shows no DMAP or p-toluenesulfonamide in the
purified product. However, the two peaks at 4.2 and 4.0 ppm,
respectively, disappeared, whilst the intensity of the triplet peak
at 3.6–3.7 ppm increased. In addition, the two broad peaks at
1.6–1.9 ppm can be seen clearly, with a relative increase in the
intensity for the peak at 1.7 ppm.
4. Summary
We have synthesized and characterized a solution-processable
conjugated polymer containing alternating 1,2,4-triazole and
N]S]N units. We have investigated two reagents for the
polymerization of bis-N-sulfinyl-3,5-diamino-1-decyl-1,2,4-tri-
azole (BSDDT). While AlCl3 is effective for obtaining polymers
with an extended p-conjugated electronic structure, it is difficult
to remove AlCl3 from the polymer, possibly because it may form
complexes with the polymer.
1
The H NMR results (Fig. 1 and 4) described above imply
several structural characteristics of PBSDDT-1. First, the
multiple peaks at 4.2–3.6 ppm are presumably due to the rela-
tively low regioregularity of this polymer. Similar to the case of
regiorandom poly(3-alkylthiophene), one can imagine that
several coupling modes, including head-to-head (HH), head-to-
tail (HT) and tail-to-tail (TT), may exist during the polymeri-
zation of BSDDT-1. Second, the polydispersity of the polymer
molecular weights may also contribute to these multiple peaks of
the –CH2 at N-1 position. The results above indicate that the
chemical shift of the –CH2 at N-1 position is sensitive to the
electron affinity of the groups linked to C-3 and C-5 of the tri-
azole group. The peaks with larger chemical shifts (e.g. 4.2 and
4.0 ppm) presumably correspond to polymer chains with rela-
tively long chain lengths, in which the long conjugation lengths
lead to down-field shift of the proton signals for the first and the
second –CH2 groups linked to the triazole moiety.
We found that DMAP is effective to polymerize BSDDT. The
polymers (PBSDDT), appearing as dark brown or black solids,
are stable under inert atmosphere but hydrolyze upon exposure
to air. These polymers are easily soluble in a variety of non-polar
solvents, including tetrahydrofuran, chloroform, dichloro-
methane, and chlorobenzene. Visually smooth films can be
obtained by spin-coating of the polymer solution (ꢂ10 mg mLꢁ1
)
from chlorobenzene. Further research towards improving the
stability of these polymers is under investigation. Such polymers
containing N]S]N units may find applications in optoelec-
tronic devices fabricated from a solution process.
Acknowledgements
We are indebted to the Mitsubishi Center for Advanced Mate-
rials (MC-CAM) for support. M. Wang thanks the Natural
Sciences and Engineering Research Council (NSERC) of Canada
for support through a Postdoctoral Fellowship.
The hydrolysis of PBSDDT-1 during the second purification
was also reflected in the FTIR spectrum (Fig. S4†), in which the
peak at 1215 cmꢁ1 disappeared, while the peak at 1183 cmꢁ1 and
the one at 1038 cmꢁ1 were still present.
The cyclic voltammetry of PBSDDT-1 in dry CH2Cl2 (con-
taining 0.1 M tetrabutylammonium hexafluorophosphate, scan-
ning rate: 100 mV sꢁ1) showed an irreversible reduction peak at
ꢁ0.94 V, and an irreversible oxidation peak at 1.1 V vs.
ferrocene.
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