Synthesis and Properties of Triphenodioxazine-Based Conjugated Polymers for Polymer Solar Cells

Article abstract of DOI:10.1002/ejoc.201700393

As a fused-ring conjugated unit, triphenodioxazine, has been applied in two series of thiophene-free conjugated polymers for polymer solar cells. The polymers were synthesized through a multi-step synthetic route, and the substituents had a great effect on the reactivity of the reactants. Relationships among the polymer structures as well as thermal, optical, and electrochemical properties were investigated in detail by experimental data analysis and theoretical simulation. The polymers possess highly planar backbones, broad Vis/NIR absorption bands, suitable frontier molecular orbital energies, and low band gaps ranging from 1.3 to 1.8 eV. The power conversion efficiencies of their photovoltaic devices are about 1 %. Introducing longer alkyl side chains into a polymer can bring about a better film-forming ability and improve the photovoltaic performance.

Full text of DOI:10.1002/ejoc.201700393

A Journal of
Accepted Article
Title: Synthesis and properties of triphenodioxazine-based conjugated
polymers for polymer solar cells
Authors: Xiaohui Gong, Pei Han, Hui Wen, Ying Sun, Xueqin Zhang,
Hong Yang, and Baoping Lin
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201700393
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10.1002/ejoc.201700393
European Journal of Organic Chemistry
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Synthesis and Properties of Triphenodioxazine-Based
Conjugated Polymers for Polymer Solar Cells
Xiaohui Gong,[a] Pei Han,[a] Hui Wen,[a] Ying Sun,[a] Xueqin Zhang,[a] Hong Yang,[a] and Baoping Lin*[a]
Abstract: As a fused-ring conjugated unit, triphenodioxazine has
been developed as two series of thiophene-free conjugated
polymers for polymer solar cells in this article. The polymers were
synthesized via a multi-step synthetic route, and it is found that the
substituent groups had a great effect on the reactivity of the
reactants. The relationships among polymer structures, thermal,
optical and electrochemical properties were investigated in detail by
experimental data analysis and theoretical simulation. The polymers
possess highly planar backbones, broad visible-near-infrared
absorption bands, suitable frontier molecular orbital energies and
low bandgaps ranging from 1.3 to 1.8 eV. The power conversion
efficiencies of their photovoltaic devices are about 1%. Introducing
longer alkyl side-chains into a polymer can bring about a better film-
forming ability and improve the photovoltaic performance.
century[5], is an electron-rich fused heterocycle with 22 atoms
and 24 π-electrons containing three aromatic benzene rings and
two nonaromatic oxazine rings (Scheme 1). Compared with
heavy-atom-containing
compounds,
triphenodioxazine
is
composed entirely of light atoms, so it is a more stable system
due to its tighter structure and stronger bonds; the p orbitals of
all the conjugated atoms have similar energies and sizes,
leading to larger orbital overlapping and better π-electron
conjugation. Derivatives based on triphenodioxazine show the
advantages of outstanding fluorescence characteristic, good
heat stability and chemical stability, developed synthetic routes
and processes[5d, 5e]. Furthermore, the planar structure of this
fused-ring compound is probably beneficial to charge conduction
and molecule π-π stacking.
Introduction
Benefiting from structural variety, high solubility and film-forming
ability, conjugated polymers are key components to achieve
lightweight, low-cost, and flexible polymer solar cells (PSCs)[1],
which are an attractive alternative to the traditional silicon
photovoltaics. Although there are less systematic and accurate
theories about the mechanism of organic photovoltaic
conversion, some qualitative empirical conclusions have been
developed to improve the performances of PSCs[2]. For example,
a photovoltaic polymer serving as an electron donor (D) and light
absorber should has a low bandgap below 1.8 eV and matching
frontier orbitals for electrode materials; appropriate compatibility
between the polymer and the electron acceptor (A) is essential
to form favorable inter-penetrating D/A network structure.
Thiophene-based conjugated polymers are a major part of
organic photovoltaic materials nowadays.[3] Thiophene ring is
more stable in synthetic reactions than other penta-heterocyclic
compounds since it is more difficult to be oxidized, added or
ring-opened[4], therefore various substituted monomers can be
derived from thiophene core; on the other hand, the bandgap of
a polymer based on six-membered aromatic rings, like benzene,
is too large for high-efficiency PSC devices, and the significant
steric hindrance of ortho substituents in six-membered-ring units
will lengthen the chemical bonds between adjacent units, or
even twist the polymer, both of these effects will weaken the π-
conjugation.
Scheme 1. Chemical structure of triphenodioxazine and its position numbers.
In spite of this, there are few reports[6] on triphenodioxazine
derivatives used as potential materials for organic electronics.
Here, we investigate two series of triphenodioxazine-based
conjugated
dialkoxytriphenodioxazine)
polymers,
and
poly(2-vinyl-3,10-
poly(2-ethynyl-3,10-
dialkoxytriphenodioxazine) (PVnT and PEnT, n is the number of
carbon atoms in a alkyl chain). The synthesis, optical and
electrochemical properties of the eight polymers as well as the
photovoltaic performances are described.
Results and Discussion
Synthesis
The synthetic route of the polymers is depicted in Scheme 2. In
Route I, the bromination and nitration proceed smoothly since
the steric hindrance of a shorter alkyl group (n = 8, 10, 12) was
not large enough to hinder the ortho-substitution reactions. The
alkyl group at the meta-position to the bromine atom played a
role in protecting the hydroxyl group and in directing subsequent
electrophilic aromatic substitution. It was attached to
hydroquinone at the very beginning of the route together with
another same alkyl group using for increasing the polymer
solubility, which saved an additional protecting reaction step.
Later, aluminium chloride was used to carry out nitro-group-
directed selective dealkylation and restore the hydroxyl group.
However, the bromination process was too slow to be monitored
by TLC when an n-hexadecyl group was introduced instead of a
shorter alkyl group. Obviously, the larger steric hindrance of the
hexadecyl group inhibited the reaction. In view of this, 2-bromo-
Triphenodioxazine, whose micromolecule derivatives have
been developed as pigments and dyes for more than half a
[a]
School of Chemistry and Chemical Engineering, Southeast
University
Nanjing 211189, P. R. China
E-mail: lbp@seu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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1,4-dihydroxybenzene was synthesized by bromination of
hydroquinone at first, alkylation was followed to give 2,5-
dihexadecylbromobenzene, and then the product was nitrated.
But also because the large steric hindrance of the alkyl group,
that attempt failed when we found the nitration reaction had not
occurred even after 24 hours.
Scheme 2. Synthetic route of PVnT and PEnT.
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Thus, the hexadecyl derivative was prepared following
Route II. The experiments shown that: due to the very small
steric hindrance of the alkyl group, the bromination,
hexadecylation, and nitration reactions were fast and complete
when commercial 4-methoxyphenol or 4-ethoxyphenol was used
as a starting compound instead of experimental synthetic 4-n-
butoxyphenol. But as we attempted to dealkylate selectively, the
reaction did not occurred with equivalent aluminium chloride.
Unexpectedly, the hexadecyl group was preferentially removed
after increasing the amount of aluminium chloride. As a result,
messy hydrosoluble products were formed while the target
compound should be water-insoluble. However, when 4-n-
group, 2-n-hexadecoxy-4-nitro-5-n-butoxybromobenzene could
be successfully dealkylated by equivalent amounts of aluminium
chloride without affecting the para hexadecoxy group.
In deed, monomers with octyl, decyl, and dodecyl groups (n
= 8, 10, 12) also can be prepared by Route II, but Route I is the
better way to synthetize these derivatives because Route II has
an additional protecting reaction and a lower yield.
Several features of the monomer synthesis are listed below.
a. The raw materials were inexpensive and plentiful, and the
reaction conditions were mild with no need for special
equipment;
b. Convenient purification operations, including filtration,
extraction and recrystallization, were the major aftertreatment
strategies , while relatively tedious column chromatography was
required just once, which saved a lot of time and effort;
c. Several new asymmetric multi-substituted compounds,
intermediate products (7) ~ (10), have been prepared by
determining a right protecting group and selective deprotection;
d. The last reaction generating monomers proceed in low
yield because the starting compounds (10) were easily
oxidizable, even though hydrogen chloride had been used to
stabilize the aniline derivatives.
butoxyphenol served as
a starting compound, aluminium
chloride removed the butyl group successfully and had less
impact on the hexadecyl group.
Eight polymers were synthesized by Stille coupling
reactions using a Pd(0)-based catalyst. At room temperature,
these polymers show acceptable solubility in chloroform and 1,2-
dichlorobenzene (see Table 1), poor solubility in toluene and
tetrahydrofuran, and cannot be dissolved by highly polar
solvents. A polymer with longer alkyl chains is more soluble. The
polymer solutions are dark blue, and the films are almost black
with conspicuously metallic luster like copper.
Theoretical Calculations
Scheme 3. Proposed dealkylation mechanism of ortho-substituted alkoxy
benzene with AlCl3.
Calculations of density functional theory with periodic boundary
conditions (DFT-PBC) were carried out for a better evaluation of
structure properties of PVnT and PEnT. All of the alkyl groups
were replaced by methyl groups to simplify the calculations. The
molecular geometries and surface plots are displayed in Figure
1.
The probable reaction mechanism of selective dealkylation
using aluminium chloride[7] is shown in Scheme 3. The
coordination of the aluminium atom in aluminium chloride with
the heteroatom of the substituted alkoxy benzene induces the
aluminium atom to bond with the oxygen atom in the alkoxy
group at the ortho-position, which activates the α-carbon atom in
the alkoxy group for the nucleophilic substitution reaction. Since
a six-membered-ring intermediate is more stable and easily
formed than
a five-membered-ring intermediate, a nitro-
substituted compound is more reactive than a bromo-substituted
compound containing a same alkyl group. However, the alkyl
group also has a great effect on the dealkylation activity. In our
synthetic experiments, methoxy- and ethoxy-substituted
compounds showed a very weak reactivity even with a highly
reactive ortho nitro group, which agreed with the reference[7].
When
2-n-hexadecoxy-4-nitro-5-methoxy-
or
ethoxy-
bromobenzene was treated with equivalent amounts of
aluminium chloride, aluminium chloride was preferentially
coordinated with the two oxygen atoms in the nitro and methoxy
or ethoxy groups, but could not dealkylate the compound. After
adding more aluminium chloride, the excessive aluminium
chloride was coordinated with the bromine atom and the oxygen
atom in the hexadecoxy group, resulting in the dealkylation of
the highly electrophilic active hexadecoxy group. With a highly
reactive butoxy group instead of an inert methoxy or ethoxy
Figure 1. DFT optimized geometries and frontier molecular orbitals of PVnT
and PEnT (red middle lines indicate extensional direction of backbones).
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These results suggest that the backbone of both PVnT and
PEnT are highly planar in their most stable conformations.
Because the small vinylene and ethynylene units increase the
distance between triphenodioxazine units, the large alkoxy
groups hardly create steric hindrance which will break the
planarity of the backbone. The bond between triphenodioxazine
and the sp hybridized carbon atom in adjacent vinylene units in
PVnT is 0.145 nm, longer than the bond between
triphenodioxazine and the sp2 hybridized carbon in ethynylene
units in PEnT (0.141 nm), so the rotational energy barriers of
PEnT units are higher than that of PVnT.
Although the steric hindrance of the side-chains tries to break
the backbone planarity, the effect is not powerful enough to
counter the large rotational energy barriers of comparatively rigid
PEnT. Therefore, longer alkyl groups increase the Tg of PEnT by
their larger Van der Waals forces. With a lower rotational energy
barriers of units (see Section “Theoretical Calculations”), PVnT
is more likely to be twisted by the steric hindrance of the side-
chains, so longer alkyl groups decrease the Tg of PEnT due to
the loose polymer stacking.
In a D-A alternating polymer[8], the electron density of the
HOMO is mainly localized on the donor units, while the electron
density of the LUMO is mainly localized on the acceptor units.
However, the electron density of the HOMO and LUMO of the
triphenodioxazine-based polymers are spread over the whole
backbone. The higher degree of orbital overlap between HOMO
and LUMO implies a stronger electronic transition between
HOMO and LUMO[8b], leading to a stronger optical absorbance.
The two-dimensional structures of the π-π stacked
polymers are shown in Figure 2. The distances between
adjacent planes of the backbone of PVnT and PEnT are 0.387
nm and 0.374 nm, respectively. The energy of a π-π stacked
repeating unit is lower than that of a gaseous repeating unit (the
structures in Figure 1), the energy differences (ΔE(RB+HF-LYP))
of PVnT and PEnT are -7.559 kJ mol-1 and -7.207 kJ mol-1,
respectively, so the two polymers tend to form stable π-π stacks
along out-of-plane direction.
Figure 3. TGA curves of the polymers under nitrogen.
Figure 2. DFT optimized geometries of π-π stacked PVnT and PEnT (red and
green lines indicate extensional direction of backbones and π-π stacking,
respectively).
Thermal Properties
The tight structures of triphenodioxazine derivatives have a
stabilizing effect against thermal. Figure
thermogravimetric analysis (TGA) spectra
triphenodioxazine-based conjugated polymers. The
3
shows the
of
the
5
wt%
weight loss temperatures of all polymers are above 300 °C
under nitrogen atmosphere, respectively. PEnT is a little more
thermostable than PVnT.
Figure 4. DSC curves of the polymers under nitrogen (dotted arrows indicate
direction of change in Tg).
The thermal transition temperatures of the polymers were
investigated by differential scanning calorimetry (DSC). The
glass transition temperatures (Tg) of PVnT decrease with the
growth of the alkyl group, the opposite tendency is apparent
upon comparison of the Tg of PEnT (Figure 4). The alkyl chains
have two opposite effects on the Tg: their steric hindrance tries
to twist the polymer backbones, which will weaken the polymer
stacking and decrease Tg; while their Van der Waals forces try to
strengthen the polymer interaction, which will increase Tg.
Optical and Electrochemical Properties
The polymers are not photoluminescent in solution or as films at
room temperature in air, so only the absorption spectra of the
polymers in chloroform solutions and as spin-coated films were
measured (Figure 5). The optical properties of the polymers are
summarized in Table 1.
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Figure. 5. Normalized UV-Vis-NIR absorption spectra of the polymers in chloroform solution and as thin films (annealed at 150 °C for 10 min).
The absorption bands of the polymer films are significantly
broader than the solutions, probably due to the better
planarization of the backbone and increased packing ability in
the solid state[9]. Although both the solution and the film of a
same polymer have similar wavelengths of maximum
absorbance, a conspicuous red-shift of the absorbance onset is
apparent in going from a solution to a corresponding thin film,
and a weak shoulder peak appears at about 700 nm in the
spectrum of the film, which can be interpreted as the effect of
interchain interaction between planar π-systems. In addition, the
polymerization degree grows with the increase of the polymer
solubility arising from longer alkyl groups, and the extension of
electron delocalization reduces the bandgap and shifts the peak
to longer wavelength. The optical bandgaps (Egopt) of PVnT and
PEnT estimated by the absorption onset are about 1.35 and 1.5
eV, respectively, which values meet the requirement of being
low-bandgap polymers for efficient PSC devices.
The electrochemical properties of the polymers were
investigated by using cyclic voltammetry to determine the
HOMO and LUMO energy levels. The cyclic voltammetry (CV)
curves are shown in Figure 6, and the data are also summarized
in Table 1. The results show that the electrochemical bandgap
(EgCV) of PVnT is about 1.45 eV, smaller than that of PEnT
(about 1.8 eV), in line with the trend of the optical bandgaps.
The frontier molecular orbital energies of the polymers match
well with those of the electron acceptor (PCBM) in PSC
Figure 6. CV curves of the polymer films.
devices[10]
.
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Table 1. Solubility, optical and electrochemical properties of the polymers.
Polymer
PV8T
5
PV10T
7
PV12T
9
PV16T
13
PE8T
4
PE10T
6
PE12T
9
PE16T
14
Solubility in CHCl3 (mg mL-1)[1]
λmax (solution, nm)
639
665
675
683
602
648
649
645
Egopt (solution, eV)
1.63
629
1.63
644
1.53
659
1.42
678
1.79
583
1.79
620
1.74
621
1.51
632
λmax (film, nm)
Egopt (film, eV)
1.41
-3.82
-5.19
1.37
-3.07
-4.36
1.29
1.39
-3.83
-5.24
1.41
1.36
-3.84
-5.29
1.45
1.30
-3.69
-5.28
1.59
1.56
-3.80
-5.47
1.67
-3.11
-4.52
1.41
1.55
-3.76
-5.54
1.78
1.53
-3.68
-5.48
1.80
1.46
-3.65
-5.49
1.84
LUMOCV (film, eV)
HOMOCV (film, eV)
EgCV (film, eV)
LUMODFT (methyl derivative, eV)
HOMODFT (methyl derivative, eV)
EgDFT (methyl derivative, eV)
[1] The rough solubility was determined by weighing the residue after evaporation of chloroform from 1mL polymer solution (20 mg/mL) which had been filtered at
0.45 μm.
Structural defects in a polymer molecule, like irregularity
and twisting, weaken the π-conjugation and intermolecular π-π
interaction, which enlarge the bandgap. But the weak π-π
interaction also enlarges the distance between adjacent polymer
molecules, leading to a high contact probability between the
molecule and the electrolyte, and making the molecule more
reactive in an electrochemical reaction and outputting stronger
electrochemical signals than well structured polymer molecules.
As a result, the large steric hindrance of long alkyl chains
creates more defects than that of short alkyl chains, so the
electrochemical bandgap are bigger than the corresponding
optical bandgaps, and a longer alkyl group leads to a larger
electrochemical bandgap, which is contrary to the results
obtained from the optical absorption spectra.
voltage (J-V) curves of PSCs are shown in Figure 7, and the
device performances are summarized in Table 2.
The PSCs with different polymers have similar open circuit
voltage (VOC) and short circuit current density (JSC), which values
are about 0.7 V and about 4 mA cm-2, respectively. The fill factor
(FF) and power conversion efficiency (PCE) are low.
The experimental bandgaps are generally in consistent with
the results from the theoretical calculations. The EgDFT is smaller
than the experimental Eg, because the theoretical models are
absolutely regular polymers with infinite degrees of
polymerization, whereas the actual polymers are not so perfect,
structure defects and limited degrees of polymerization weaken
their conjugation and enlarge the bandgaps. Both the
experimental and theoretical data show that the LUMOs of PVnT
and PEnT are similar, and the HOMOs of PVnT are higher.
Since the vacuum levels (zero reference energy levels) of
electrochemical and DFT potentials are different, there is no
comparability between an electrochemical level and a DFT level.
Figure 7. J-V characteristics of PVnT and PEnT-based PSC devices under
illumination of AM 1.5G, 100 mW cm-2.
Photovoltaic Performance
During the spin-coating process, it is found that the
polymer/PC71BM blend films were more or less discontinuous
and uneven, even tended to detach from the substrates. A
probable reason for the low film-forming ability of the polymers is
the high planarity and rigidity of the polymer molecules. Unlike
most reported conjugated polymers[1, 11], the conjugated units of
Unoptimized PSC devices were fabricated as an attempt to
investigate the photovoltaic properties of the triphenodioxazine-
based conjugated polymers, in which PVnT and PVnT served as
electron donors, while [6,6]-Phenyl-C71-butyric acid methyl ester
(PC71BM) served as a electron acceptor. The current density-
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PVnT and PEnT are completely confined to the center axes of
the backbones, while there are only flexible alkyl chains outside
the backbones, which make it relatively difficult to twist the
molecules around their center axes, just like moving a lever with
its short arm. Consequently, the perfect planarity of the polymers
causes excessively strong π-π interaction among the polymer
molecules, leading to strong self-aggregation tendency and low
appetency with other materials.
show that with the growth of the alkyl chains, the films become
smoother, and irregular large particles gradually disappear and
are replaced with regular small particles or wrinkles, which are
uniform in size and shape. Moreover, the phases in the phase
images become more compatible and organized as the alkyl
chains grow, which suggests that longer alkyl chains are
beneficial to the compatibility of polymer with PC71BM. Strangely,
there are many doughnut-like cavities distributing on the film
surface, which may caused by a certain kind of molecular
clusters.
The surface morphology of the blend films on glass
substrates were measured by atomic force microscope (AFM)
and the images are depicted in Figure 8. The height images
Table 2. Photovoltaic performances of the PSCs
Polymer
VOC (V)
PV8T
0.18
3.97
0.27
0.20
PV10T
0.56
PV12T
0.74
PV16T
0.69
PE8T
0.18
3.24
0.27
0.21
PE10T
0.73
PE12T
0.76
PE16T
0.69
JSC (mA cm-2)
FF
3.77
4.03
4.40
3.43
4.03
6.07
0.33
0.32
0.36
0.28
0.39
0.33
PCE (%)
0.77
0.90
1.04
0.82
1.19
1.33
Figure 8. AFM height and phase images of the polymer/PC71BM blend films (1:2, w/w, annealed at 150 °C for 10 min) on glass substrates. The units of the height
and phase scales are nm and degree, respectively.
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Bruker Reflex-TOF-MS instrument. Elementary analyses were performed
on a vario EL cube elementar. TGA and DSC data were collected on a
thermal analysis instrument (SDT-Q600 and STA 409, TA Instruments)
with a heating rate of 10 °C min-1 and a nitrogen flow of 100 mL min-1.
AFM images were obtained on an Asylum Research MFP-3D microscope
under the tapping mode. Gel permeation chromatography (GPC)
measurements were performed on a GPC-220 chromatograph using a
differential refractometer at room temperature with tetrahydrofuran as an
eluent at a flow rate of 1 mL min-1. And because the polymers dissolved
slightly in tetrahydrofuran, an indirect measurement was carried out,
which is described in Supplementary data. DFT-PBC calculations were
performed at the B3LYP/6-31G(d) level using Gaussian 09 program
The low film-forming properties and strong exclusiveness of
the polymers greatly limited the performances of corresponding
PSC devices, especially the FF. Since the shorter the alkyl
group a polymer has, the more compactly the molecules stack
together, the film quality and the device performance improves
with the alkyl chain lengthening. The difference between vinyl
and ethynyl units has little effect on the performance. By
comparison, PVnT slightly outperforms PEnT with short alkyl
groups, whereas long alkyl-substituted polymers exhibit an
opposite trend. That is because PVnT is more flexible than
PEnT, relaxing the structural rigidity is more beneficial to the
performance of a rigid polymer containing short alkyl groups,
while maintaining the structural planarity and π-conjugation is
beneficial to the performance of a flexible polymer containing
long alkyl groups.
package[12]
.
The UV-Vis-NIR spectra were obtained using an Agilent Cary 5000
spectrophotometer. The following equation[13] is used to calculate a
optical bandgap energy (Egopt):
1240
Egopt(eV) 
Conclusions
onset(nm)
where λonset is the onset of the long wavelength band in an absorption
spectrum.
Two series of triphenodioxazine-based conjugated polymers
without any thiophene units have been synthesized via an
optimized route.
CV measurements were carried out on an electrochemical
workstation (CHI660E, Shanghai Chenhua Co. Ltd, China) with a three-
The theoretical simulation shows that the introduction of
vinyl and ethynyl eliminates the steric hindrance between units
and endows these triphenodioxazine-based polymers highly
planar structures, which ensures continuity of the backbone
conjugation. The polymers are thermostable, and PVnT is more
flexible than PEnT. Their optical absorption bands range from
the visible to the near-infrared. The optical and electrochemical
bandgaps of a polymer are similar, to which the length of the
alkyl groups make little difference. The values of PEnT and
PEnT are about 1.4 eV and 1.6 eV, respectively.
Although the planar fused-ring structures, broad optical
absorption bands, ideal bandgaps and distribution of the frontier
molecular orbitals of the triphenodioxazine-based polymers
should be beneficial to the photovoltaic performances of the
PSC devices, the low quality of the blend films was adverse to
achieving high FF and PCE, which could be partially solved by
introducing long alkyl side-chains.
electrode
cell
in
tetrabutylammonium
tetrafluoroborate
(TBABF4)/acetonitrile electrolyte (0.1 mol L-1) using a scan rate of 100

polymer
onset
Fc/Fc
onset
EHOMO
/
ELUMO(eV)  
E
(eV) 
E
(eV)  4.8eV
mV s-1. A Pt electrode coated with a polymer film, a Pt wire, and an
Ag/AgCl (with 0.1 mol L-1 KCl aqueous solution) electrode served as a
working electrode, a counter electrode and
a reference electrode,
respectively. The potentials were internally calibrated using the
ferrocene/ferrocenium redox couple (Fc/Fc+), which is estimated to have
an oxidation potential of -4.8 eV vs vacuum. The redox potential of
Fc/Fc+, measured under the same condition as polymer samples, was
located at 0.33 V related to the Ag/AgCl electrode. The energies of
HOMO and LUMO (EHOMO and ELUMO) were calculated according to the
following equation[14]
:

Fc/Fc
E
onset
polymer
onset
E
where
and
are the onset potentials of a polymer film and
Fc/Fc+, respectively.
Synthesis
Experimental Section
1,4-Dialkoxybenzene (n = 8, 10, 12) (1) was prepared according to
the literature[15]. The products were further purified by recrystallization
from ethanol to afford compound 1 as white sheets (~80% yield). n = 8:
1H NMR (CDCl3, δ) 6.81 (s, 4 H), 3.89 (t, 4 H), 1.74 (m, 4 H), 1.28 (m, 20
H), 0.88 (t, 6 H); n = 10: 1H NMR (CDCl3, δ) 6.81 (s, 4 H), 3.89 (t, 4 H),
1.75 (m, 4 H), 1.27 (m, 28 H), 0.88 (t, 6 H); n = 12: 1H NMR (CDCl3, δ)
6.81 (s, 4 H), 3.89 (t, 4 H), 1.74 (m, 4 H), 1.26 (m, 36 H), 0.88 (t, 6 H).
Materials
2,5-Dihydroxy-1,4-benzoquinone, trans-1,2-bis(tri-n-butylstannyl)ethylene,
1,2-bis(trimethylstannyl)acetylene,
and
tetrakis(triphenylphosphine)palladium were purchased from Acros.
PEDOT:PSS water solution (Baytron P VP AI 4083) and PC71BM was
purchased from H. C. Starck and American Dye Source, respectively. All
chemical reagents, unless otherwise specified, were used as received.
Chloroform and toluene were freshly distilled over appropriate drying
agents prior to use and stored under nitrogen.
2,5-Dialkoxybromobenzene (n
according to the literature[16] without further purification. The product was
obtained as white solid and contained small amount of 1,4-
= 8, 10, 12) (2) was prepared
a
dialkoxybenzene and 2,5-dialkoxy-1,4-dibromobenzene (~95% yield).
Measurements and characterizations
4-Nitro-2,5-dialkoxybromobenzene (n = 8, 10, 12) (3): Compound 2
(24.0 mmol) was dissolved in propionic acid (80 mL) and the solution was
heated to 45 °C. Concentrated nitric acid (66 % w/w, 6 mL) was added
dropwise in 2 h and stirring was continued for 8 h at the same
temperature. The mixture was poured into ice water (100 mL), and then
filtered. The filter cake was washed with water several times. The product
was dried under vacuum and obtained as a light yellow solid (~0.022 mol,
Proton nuclear magnetic resonance (1H NMR) spectra were recorded on
a
Bruker AV-500 spectrometer. The Fourier transform infrared
spectroscopy (FT-IR) spectra were recorded at Nicolet 5700
spectrometer from KBr pellets. ESI-MS were run on an Agilent 1100
Series LC/MSD instrument. MALDI-TOF-MS spectra were acquired on a
a
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10.1002/ejoc.201700393
European Journal of Organic Chemistry
FULL PAPER
~90% yield), which was used directly for the next step without further
purification.
calcd. for C22H35BrNO4 [M-Na]- 456.2, found: 456.2. n = 8, 10, 12, 16: FT-
IR (νmax, cm-1) 3500 (s), 2920 (s), 2850 (m), 1530 (m), 1230 (s), 1180
(m), 860 (m).
4-n-Butoxyphenol (4): Hydroquinone (26.4 g, 0.24 mol), 1-
bromobutane (27.4 g, 0.2 mol), and sodium hydroxide (8.8 g, 0.22 mol)
were suspended in N,N-dimethylformamide (125 mL) and the mixture
was stirred at 60 °C for 12 h under nitrogen atmosphere. The reaction
mixture was poured into water (500 mL) and extracted with ethyl acetate
(150 mL), washed with water several times. The residue of the organic
phase after evaporation was purified by recrystallization from petroleum
ether to afford Compound 4 as white sheets (29 g, 0.174 mol, 87% yield).
1H NMR (DMSO-d6, δ) 6.76 (d, 4 H), 3.90 (t, 2 H), 1.74 (m, 2 H), 1.47 (m,
2 H), 0.97 (t, 6 H). Elemental analysis calcd. (%) for C10H14O2: C, 72.26;
H, 8.49; found: C, 72.62; H, 8.57. ESI-MS: calcd. for C10H13O2 [M-H]-
165.1, found 165.1.
2-Amino-4-alkoxy-5-bromophenol hydrochloride (n = 8, 10, 12, 16)
(10): Compound 9 (5 mmol) and nickel dichloride (0.8 g, 3.4 mmol) were
suspended in a mixed solvent of diethyl ether and ethanol (1:1 v/v, 75
mL). The mixture was cooled to 0 °C, then a suspension of sodium
borohydride (1.5 g, 40 mmol) in ethanol (20 mL) was added dropwise
with vigorous stirring in 10 min under nitrogen atmosphere. The stirring
was continued for 3 h at room temperature. The mixture was slowly
poured into concentrated hydrochloric acid (100 mL) and ice (50 g), then
filtered. The filter cake was washed with diluted hydrochloric acid and
dried under vacuum. The product was obtained as a white solid (~4 mmol,
~80% yield). n = 8: 1H NMR (DMSO-d6, δ) 10.46 (br s, 1 H), 9.22 (br s, 3
H), 7.21 (s, 1 H), 7.06 (s, 1 H), 3.90 (t, 2 H), 1.70 (m, 2 H), 1.26 (m, 10 H),
0.86 (t, 6 H). n = 10: 1H NMR (DMSO-d6, δ) 10.42 (br s, 1 H), 9.20 (br s,
3 H), 7.18 (s, 1 H), 7.04 (s, 1 H), 3.90 (t, 2 H), 1.69 (m, 2 H), 1.25 (m, 10
H), 0.85 (t, 6 H). n = 12: 1H NMR (DMSO-d6, δ) 10.40 (br s, 1 H), 9.44 (br
s, 3 H), 7.18 (s, 1 H), 7.03 (s, 1 H), 3.91 (t, 2 H), 1.70 (m, 2 H), 1.25 (m,
10 H), 0.86 (t, 6 H). n = 16: 1H NMR (DMSO-d6, δ) 10.23 (br s, 1 H), 8.83
(br s, 3 H), 7.13 (s, 1 H), 6.95 (s, 1 H), 3.89 (t, 2 H), 1.69 (m, 2 H), 1.24
(m, 10 H), 0.85 (t, 6 H).
2-Bromo-4-n-butoxyphenol (5): Compound 4 (29.9 g, 0.18 mol) was
dissolved in dichloromethane (125 mL) and bromine (30.4 g, 0.19 mol)
was added dropwise with stirring in 1 h at 0 °C. The solvent and
redundant bromine were removed under reduced pressure to afford a
colorless solid, which contained a small amount of unreacted raw
materials and dibromide (44 g, 0.18 mol, ~99% yield). The product was
used directly for the next step without further purification.
2,9-Dibromo-3,10-dialkoxytriphenodioxazine (n = 8, 10, 12, 16) (11):
Compound 10 (7.5 mmol), 2,5-dihyroxy-1,4-benzoquinone (0.50 g, 3.6
mmol), and anhydrous sodium acetate (0.62 g, 7.5 mmol) were
suspended in acetic acid (150 mL). The mixture was reacted at 80 °C for
2-n-Hexadecoxy-5-n-butoxybromobenzene (6): Compound 5 (24.5 g,
0.1 mol) and n-Hexadecyl bromide (32.0 g, 0.105 mol) were dissolved in
acetone (150 mL). After adding potassium carbonate (14.5 g, 0.105 mol),
the mixture was refluxed for 36 h under nitrogen atmosphere. Then it was
poured into water (500 mL) and extracted with petroleum ether (300 mL).
The organic phase was washed with dilute aqueous sodium hydroxide
solutions and water several times. The solvent was removed under
reduced pressure to afford a colorless solid (40 g, 0.085 mol, 85% yield).
The product was used directly for the next step without further purification.
18
h under nitrogen atmosphere, and then was cooled to room
temperature. Water (500 mL) was added and the mixture was extracted
with dichloromethane. The solvent was removed under reduced pressure
and the resulting crude product was purified by column chromatography
eluting with dichloromethane/hexane (3/1) to afford deep purple needles
(~1 mmol, ~15%). n = 8: 1H NMR (CD2Cl2, δ) 7.46 (s, 2 H), 7.17 (s, 2 H),
7.00 (s, 2 H), 4.09 (t, 4 H), 2.17~0.89 (m, > 30 H). Elemental analysis
calcd. (%) for C34H40Br2N2O4: C, 58.30; H, 5.76; N, 4.00; found: C, 58.72;
H, 5.85; N, 3.74. MALDI-TOF-MS: calcd. for C34H40Br2N2O4 700.13,
found: 700.50. n = 10: 1H NMR (CDCl3, δ) 7.40 (s, 2 H), 7.07 (s, 2 H),
6.83 (s, 2 H), 4.05 (t, 4 H), 1.85~0.89 (m, > 38 H). Elemental analysis
calcd. (%) for C38H48Br2N2O4: C, 60.32; H, 6.39; N, 3.70; found: C, 60.66;
H, 6.46; N, 3.44. MALDI-TOF-MS: calcd. for C38H48Br2N2O4 756.20,
found: 756.54. n = 12: 1H NMR (CDCl3, δ) 7.40 (s, 2 H), 7.08 (s, 2 H),
6.81 (s, 2 H), 4.06 (t, 4 H), 1.85~0.88 (m, > 46 H). Elemental analysis
calcd. (%) for C42H56Br2N2O4: C, 62.07; H, 6.95; N, 3.45; found: C, 62.44;
H, 7.07; N, 3.02. MALDI-TOF-MS: calcd. for C42H56Br2N2O4 812.26,
found: 812.60. n = 16: 1H NMR (CDCl3, δ) 7.40 (s, 2 H), 7.08 (s, 2 H),
6.82 (s, 2 H), 4.06 (t, 4 H), 1.57~0.88 (m, > 62 H). Elemental analysis
calcd. (%) for C50H72Br2N2O4: C, 64.93; H, 7.85; N, 3.03; found: C, 65.44;
H, 8.01; N, 2.87. MALDI-TOF-MS: calcd. for C50H72Br2N2O4 924.38,
found: 924.69. n = 8, 10, 12, 16: FT-IR (νmax, cm-1) 2920 (s), 2850 (m),
1630 (m), 1560 (s), 1460 (m), 1270 (m), 1180 (s), 860 (m), 580 (m). 13C
NMR could not be determined due to low solubility in deuterated solvents.
2-n-Hexadecoxy-4-nitro-5-n-butoxybromobenzene (7): Compound 6
(47.0 g, 0.1 mol) was dissolved in propionic acid (150 mL) and the
solution was heated to 45 °C. Concentrated nitric acid (66 % w/w, 20 mL)
was added dropwise in 1 h and stirring was continued for an additional 4
h at the same temperature. The mixture was poured into water (500 mL)
and extracted with petroleum ether (500 mL), washed with water several
times. The solvent was removed to obtain a light yellow solid (46 g, 0.09
mol, 90% yield). The product was used directly for the next step without
further purification.
Sodium 2-nitro-4-alkoxy-5-bromophenolate (n = 8, 10, 12, 16) (9):
Compound 3 or Compound 7 (0.08 mol), and anhydrous aluminium
chloride (11.7 g, 0.088 mol) were suspended in dry, ethanol-free
chloroform (500 mL) and the mixture was refluxed for 6 h. The resulting
red solution was poured into hydrochloric acid (15% w/w, 500 mL) and
extracted with chloroform. After evaporation of solvent, the residue of the
organic phase, mainly containing Compound 8, was dissolved in
petroleum ether (500 mL), and a solution of sodium ethoxide in ethanol
(~2.5 mol L-1, 50 mL) was added dropwise. The mixture was stirred for 15
min, then water (100 mL) was added. The stirring was continued for an
additional 30 min. The precipitate formed was collected by filtration,
washed with water and petroleum ether. The product was dried under
vacuum and obtained as a red solid (~0.06 mol, ~75% yield). n = 8: 1H
NMR (DMSO-d6, δ) 7.26 (s, 1 H), 6.69 (s, 1 H), 3.80 (t, 2 H), 1.66 (m, 2
H), 1.27 (m, 10 H), 0.86 (t, 6 H). ESI-MS: calcd. for C14H19BrNO4 [M-Na]-
344.1, found: 344.1. n = 10: 1H NMR (DMSO-d6, δ) 7.26 (s, 1 H), 6.69 (s,
1 H), 3.81 (t, 2 H), 1.67 (m, 2 H), 1.25 (m, 14 H), 0.85 (t, 6 H). ESI-MS:
calcd. for C16H23BrNO4 [M-Na]- 372.1, found: 372.1. n = 12: 1H NMR
(DMSO-d6, δ) 7.25 (s, 1 H), 6.68 (s, 1 H), 3.80 (t, 2 H), 1.66 (m, 2 H),
1.24 (m, 18 H), 0.85 (t, 6 H). ESI-MS: calcd. for C18H27BrNO4 [M-Na]-
400.1, found: 400.1. n = 16: 1H NMR (DMSO-d6, δ) 7.25 (s, 1 H), 6.64 (s,
1 H), 3.79 (t, 2 H), 1.66 (m, 2 H), 1.24 (m, 26 H), 0.85 (t, 6 H). ESI-MS:
Polymer: Compound 11 (0.375 mmol), trans-1,2-bis(tri-n-
butylstannyl)ethylene
bis(trimethylstannyl)acetylene
(0.24
g,
(0.139
0.375
g,
mmol)
0.375
or
mmol),
1,2-
and
tetrakis(triphenylphosphine)palladium (0.0347 g, 0.03 mmol) were
suspended in toluene (25 mL). The mixture was subjected to five cycles
of evacuation and admission of nitrogen, and then heated to 70 °C for
several hours (n = 8: 8 h; n = 10: 10 h; n = 12: 12 h; n = 16: 16 h). After
cooling to room temperature, the mixture was poured into methanol
(100mL) and stirred for 30 min. A black precipitate was collected by
filtration. The product was purified by washing with methanol and hexane
in a Soxhlet extractor for 48 h each. It was extracted with hot chloroform
in the extractor for 36 h. After removing solvent, a black solid was
collected (~45%). PV16T: 1H NMR (CDCl3, δ) 7.62~6.51 (br m, 7 H), 4.04
(br s, 4 H), 1.58~0.90 (m, > 62 H). PE16T: 1H NMR (CDCl3, δ) 7.62~6.51
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201700393
European Journal of Organic Chemistry
FULL PAPER
(br m, 6 H), 4.05 (br s, 4 H), 1.58~0.90 (m, > 62 H). PVnT: FT-IR (νmax,
cm-1) 2920 (s), 2850 (m), 1560 (s), 1430 (m), 1270 (s), 1180 (s), 850 (m),
580 (s). PEnT: FT-IR (νmax, cm-1) 2920 (s), 2850 (m), 1630 (m), 1560 (s),
1430 (m), 1280 (m), 1170 (s), 850 (m), 580 (s).
[5]
[6]
a) R. Mital, S. Jain, J Chem Soc: Organic 1971, 1875-1878; b) K. W.
Butz, B. L. Justus, G. W. Scott, J Phys Chem 1983, 87, 2578-2581; c)
M. Ikeda, K. Kitahara, H. Nishi, J Heterocyclic Chem 1992, 29, 289-
294; d) T. Hihara, Y. Okada, Z. Morita, Dyes Pigments 2000, 45, 131-
143; e) K. Hunger, Industrial dyes: chemistry, properties, applications,
John Wiley & Sons, 2007.
PSC device fabrication
a) J. Stille, M. Freeburger, J Polym Sci Pol Chem 1968, 6, 161-169; b)
L. Yu, L. R. Dalton, J Am Chem Soc 1989, 111, 8699-8702; c) L. Guo, J.
Quinn, J. Wang, C. Guo, X. Li, J. Wang, Y. Li, Dyes Pigments 2016,
132, 329-335.
PSC devices were fabricated on ITO glass substrates with
a
configuration of ITO/PEDOT:PSS/polymer:PC71BM/Ca/Al. The ITO glass
substrates were sequentially cleaned with detergent, deionized water,
acetone, and isopropyl alcohol. A PEDOT:PSS layer (45 nm) was spin-
coated from its water solution (filtered at 0.45 μm) onto the ITO glass,
and then baked at 150 °C for 10 min. The blend film was spin-coated in a
nitrogen filled glove box from a blending solution of the polymer with
PC71BM in 1,2-dichlorobenzene (1:2, w/w, filtered at 0.45 μm), and then
annealed at the temperature of 150 °C for 10 min. Subsequently, calcium
(30 nm) and aluminum (100 nm) were thermally deposited successively
under high vacuum (< 10-4 Pa) with a shadow mask to define the active
area of the devices (10.0 mm2). The J-V characteristics were measured
using a Keithley 2400 source meter under a simulated AM 1.5G
conditions.
[7]
[8]
X. Ji, B. Cheng, J. Song, C. L. Wang, Yufei, Synthetic Commun 2009,
volume 39, 2053-2057.
a) L. Zhang, K. Pei, M. Yu, Y. Huang, H. Zhao, M. Zeng, Y. Wang, J.
Gao, J Phys Chem C 2012, 116, 26154-26161; b) W. C. Yen, B. Pal, J.
S. Yang, Y. C. Hung, S. T. Lin, C. Y. Chao, W. F. Su, J Polym Sci Pol
Chem 2009, 47, 5044-5056.
[9]
a) Z. Guo, D. Lee, R. D. Schaller, X. Zuo, B. Lee, T. Luo, H. Gao, L.
Huang, J Am Chem Soc 2014, 136, 10024-10032; b) Y. Sun, C. Zhang,
B. Dai, B. Lin, H. Yang, X. Zhang, L. Guo, Y. Liu, J Polym Sci Pol Chem
2015, 53, 1915-1926.
[10] a) M. C. Scharber, D. Mühlbacher, M. Koppe, P. Denk, C. Waldauf, A. J.
Heeger, C. J. Brabec, Adv Mater 2006, 18, 789-794; b) N. Blouin, A.
Michaud, D. Gendron, S. Wakim, E. Blair, R. Neagu-Plesu, M. Belletete,
G. Durocher, Y. Tao, M. Leclerc, J Am Chem Soc 2008, 130, 732-742.
[11] X. Zhan, D. Zhu, Polym Chem 2010, 1, 409-419.
Acknowledgements
[12] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A.
Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F.
Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr., J. E. Peralta, F.
Ogliaro, M. J. Bearpark, J. Heyd, E. N. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam,
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R.
Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C.
Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels,
Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox,
Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford, CT, USA,
2009.
The authors thank the financial support from National Natural
Science Foundation of China (21304018 21374016), Jiangsu
Provincial Natural Science Foundation of China (BK20130619
BK20130617) and the Priority Academic Program Development
of Jiangsu Higher Education Institutions.
Keywords: triphenodioxazine • conjugated polymers • synthesis
• optical and electrochemical properties • polymer solar cells
[1]
[2]
a) H. Zhang, L. Ye, J. Hou, Polym Int 2015, 64, 957-962; b) X. Liu, H.
Chen, S. Tan, Renewable and Sustainable Energy Reviews 2015, 52,
1527-1538.
a) C.-X. Sheng, Z. V. Vardeny, in Progress in High-Efficient Solution
Process Organic Photovoltaic Devices, Springer, 2015, pp. 3-41; b) Y.
Li, Accounts Chem Res 2012, 45, 723-733; c) H. Kang, W. Lee, J. Oh,
T. Kim, C. Lee, B. J. Kim, Accounts Chem Res 2016, 49, 2424-2434.
a) A. Facchetti, Chem Mater 2011, 23, 733-758; b) U. Mehmood, A. Al-
Ahmed, I. A. Hussein, Renewable and Sustainable Energy Reviews
2016, 57, 550-561; c) C. Liu, K. Wang, X. Gong, A. J. Heeger, Chem
Soc Rev 2016, 45, 4825-4846.
[13] W. Cai, V. Shalaev, Optical Metamaterials: Fundamentals and
Applications, Springer New York, 2009.
[14] J. Y. Lee, S. W. Heo, H. Choi, Y. J. Kwon, J. R. Haw, D. K. Moon, Sol
Energ Mat Sol C 2009, 93, 1932-1938.
[3]
[4]
[15] A. Loupy, J. Sansoulet, E. Díezbarra, J. R. Carrillo, Synthetic Commun
1991, 21, 1465-1471.
[16] A. Sergawie, S. Admassie, W. Mammo, T. Yohannes, T. Solomon,
Synthetic Met 2008, 158, 307-314.
a) J. A. Joule, K. Mills, Heterocyclic chemistry, John Wiley & Sons,
2008; b) T. Eicher, S. Hauptmann, A. Speicher, The Chemistry of
Heterocycles: Structures, Reactions, Synthesis, and Applications 3rd,
John Wiley & Sons, 2013.
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10.1002/ejoc.201700393
European Journal of Organic Chemistry
FULL PAPER
Entry for the Table of Contents (Please choose one layout)
Layout 2:
FULL PAPER
Triphenodioxazine, conjugated
polymers
Xiaohui Gong, Pei Han, Hui Wen, Ying
Sun, Xueqin Zhang, Hong Yang, and
Baoping Lin*
Page No. – Page No.
A series of triphenodioxazine-based conjugated polymers were synthesized via an
optimized route. The relationships among polymer structures, thermal, optical,
electrochemical and photovoltaic properties were investigated in detail by
experimental data analysis and theoretical simulation.
Synthesis and Properties of
Triphenodioxazine-Based Conjugated
Polymers for Polymer Solar Cells
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