Effect of Chalcogen Atom on the Properties of Naphthobischalcogenadiazole-Based π-Conjugated Polymers

Article abstract of DOI:10.1021/acs.chemmater.5b03124

We here discuss the effect of chalcogen atom on the properties of π-conjugated polymers based on electron deficient fused π-electron systems, naphtho[1,2-c:5,6-c']bis[1,2,5]oxadiazole (NOz), naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NTz), and naphtho[1,2-c:5,6-c']bis[1,2,5]selenadiazole (NSz). We synthesize new donor-acceptor polymers having NOz and NSz combined with a quaterthiophene donor unit, namely, PNOz4T and PNSz4T, respectively. Physicochemical properties, structures, and charge transport properties of PNOz4T and PNSz4T in comparison with the sulfur analogue (PNTz4T) are discussed. As both NOz and NSz have a higher electron deficient nature than NTz, PNOz4T and PNSz4T are found to possess deeper frontier orbital energy levels. In particular, PNOz4T have a sufficiently deep lowest unoccupied molecular orbital (LUMO) energy level which allows for ambient ambipolar behavior in transistor devices. Interestingly, whereas PNSz4T is mostly amorphous in the thin film, PNOz4T forms a highly crystalline structure that is similar to PNTz4T. This is probably due to the difference of the atomic size, where the large selenium may cause a larger steric repulsion between NSz and the adjacenet thiophene ring, resulting in a larger defect in the polymer backbone than the others. As a result, whereas PNSz4T exhibited p-channel behavior with modest hole mobilities of the order of 10^-2 cm² V^-1 s^-1, PNOz4T demonstrated p-channel behavior with high hole mobilities of ～1 cm² V^-1 s^-1 and ambipolar behavior with balanced hole and electron mobilities of ～0.5 and ～0.3 cm² V^-1 s^-1 depending on the surfactant used for the devices. These results show clear correlations between the chalcogen atom and the properties of the naphthobischalcogenadiazole-based polymers. Overall, in addition to NTz, NOz is found to be another fascinating acceptor unit for π-conjugated polymers.

Full text of DOI:10.1021/acs.chemmater.5b03124

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Article
Effect of Chalcogen Atom on the Properties of
Naphthobischalcogenadiazole-Based #-Conjugated Polymers
Kazuaki Kawashima, Itaru Osaka, and Kazuo Takimiya
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03124 • Publication Date (Web): 03 Sep 2015
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Chemistry of Materials
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Effect of Chalcogen Atom on the Properties of
NaphthobischalcogenadiazoleꢀBased πꢀConjugated
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Kazuaki Kawashima, Itaru Osaka, * and Kazuo Takimiya *
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Department of Applied Chemistry, Graduate School of Engineering, Hiroshima University, 1ꢀ4ꢀ1
Kagamiya, HigashiꢀHiroshima, Hiroshima 739ꢀ8527, Japan
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RIKEN Center for Emergent Matter Science (CEMS), Wako, Saitama 351ꢀ0198, Japan
Eꢀmail: itaru.osaka@riken.jp; takimiya@riken.jp
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Abstract
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We here discuss the effect of chalcogen atom on the properties of πꢀconjugated polymers based on
electron deficient fused πꢀelectron systems, naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]oxadiazole (NOz),
naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]thiadiazole (NTz), and naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]selenadiazole (NSz).
We synthesize new donor–acceptor polymers having NOz and NSz combined with quaterthiophene
donor unit, namely PNOz4T and PNSz4T, respectively. Physicochemical properties, structures, and
charge transport properties of PNOz4T and PNSz4T in comparison with the sulfur analogue (PNTz4T)
are discussed. As both NOz and NSz have higher electron deficient nature than NTz, PNOz4T and
PNSz4T are found to possess deeper frontier orbital energy levels. In particular, PNOz4T have a
sufficiently deep LUMO energy level which allows for ambient ambipolar behavior in transistor devices.
Interestingly, whereas PNSz4T is mostly amorphous in the thin film, PNOz4T forms highly crystalline
structure that is similar to PNTz4T. This is probably due to the difference of the atomic size, where large
selenium may cause larger steric repulsion between NSz and the adjacenet thiophene ring, resulting in a
larger defect in the polymer backbone than the others. As a result, whereas PNSz4T exhibited pꢀchannel
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behavior with modest hole mobilities of the order of 10 cm /Vs, PNOz4T demonstrated pꢀchannel
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behavior with high hole mobilities of ~1 cm /Vs and ambipolar behavior with balanced hole and
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electron mobilities of ~0.5 cm /Vs and ~0.3 cm /Vs depending on the surfactant used for the devices.
These results show clear correlations between the chalcogen atom and the properties of the
naphthobischalcogenadiazoleꢀbased polymers. Overall, in addition to NTz, NOz is found to be another
fascinating acceptor unit for πꢀconjugated polymers.
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Chemistry of Materials
Introduction
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πꢀConjugated polymers have been widely used in organic electronics due to the advantage in solutionꢀ
processability as well as the good electrical properties in spite of the low crystallinity compared with πꢀ
1ꢀ3
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conjugated molecular systems. Organic fieldꢀeffect transistors (OFETs) and organic photovoltaics
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(
OPVs) are important devices for the application of πꢀconjugated polymers, which a great number of
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research has been conducted in the past decades. One of the most important physical phenomena of πꢀ
conjugated polymers is the charge transport in thin films, which is strongly dependent on the ordering
structure and the electronic structure. High coplanarity of the polymer backbone and strong
6
intermolecular interaction between the backbones are necessary to have highly ordered structure. The
molecular orbital should be highly overlaped between the backbones and thus the geometry of the
highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) is a
4
key parameter. The energy levels also largely affect the charge carrier polarity. Therefore, the HOMO
and LUMO energy levels must be tuned to match the work function of the electrode of the devices for
efficient charge injection. In order to realize highly ordered structures and appropriate electronic
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structures, sulfurꢀcontaining πꢀextended fused heteroaromatic rings, e.g., thiopheneꢀbased acenes,
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thiazoleꢀfused systems,
and thiadiazoleꢀfused systems,
are often incorporated into the polymer
backbone. In the meantime, other chalcogen atoms such as oxygen and selenium have also been used as
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alternatives to sulfur for the aboveꢀmentioned heteroaromatic rings.
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,1,3ꢀBenzothiadiazole (BTz) is one of the most widely used heteroaromatic rings for semiconducting
1
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polymers (Figure 1). Due to the high electron deficiency, BTz acts as an acceptor unit for soꢀcalled
donor–acceptor (D–A) polymers, and largely deepens the energy levels of the polymers. The oxygen and
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selenium analogues of BTz, i.e., 2,1,3ꢀbenzoxadiazole (BOz) and 2,1,3ꢀbenzoselenadiazole (BSz)
(Figure 1), have also been used as an acceptor unit for D–A polymers. It has been reported that although
the crystallinity of the resulting polymers were similar, the replacement of BTz by BOz or BSz altered
the electronic structure. Whereas the replacement of BTz by BOz further deepened the energy levels of
the polymers, replacement by BSz shallowed the energy levels.
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Naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]thiadiazole (NTz, Figure 1) is a tetracyclic heteroaromatic where two
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BTz rings are fused. With its high planarity and high electron deficiency, NTz is a fascinating acceptor
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unit for D–A polymers.
Independently from other groups, we have been focusing on NTzꢀbased
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polymers.
We revealed that the NTzꢀbased polymers form high crystalline structures and
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exhibit high hole mobilities of around 0.5 cm V s in OFETs. NTz polymers also showed quite
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high power conversion efficiencies when used as the pꢀtype material for the bulk heterojunction solar
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cells.
These results indicated that NTz is promising building unit for D–A polymers.
Replacement of the sulfur atom of NTz by the oxygen and selenium atoms, resulting in naphtho[1,2ꢀ
c:5,6ꢀc′]bis[1,2,5]oxadiazole (NOz) and naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]selenadiazole (NSz) (Figure 1),
is expected to alter the electronic structure of the corresponding polymers. As seen from the
computation
of
model
compounds,
where
benzodichalcogenadiazoles
(BXz)
and
naphthobischalcogenadiazoles (NXz) are sandwiched with methylthiophenes, the effect of the chalcogen
atom seems to be even larger than in the case of BXz (Figure 2). In particular NOz is expected to bring
the corresponding polymers quite deep LUMO energy levels as well as deep HOMO energy levels,
which is important in terms of providing nꢀchannel (or ambipolar) behavior in OFETs and high openꢀ
circuit voltage (V ) in OPVs. In fact, during the course of our study, Zhang and coꢀworkers reported on
OC
an NOzꢀbased D−A semiconducting polymer incorporating a ladderꢀtype heptacyclic arene as the donor
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unit, which exhibits power conversion efficiencies of ~8.4% with high VOC of ~0.9 V in OPV devices.
Herein, we synthesize NOz and NSz, and the new polymers based on these building units. The
chemical structure of the polymers used here is composed of quaterthiophene as the donor units and
NXz as the acceptor units (PNOz4T, PNTz4T and PNSz4T, Figure 3). The use of the simple
quaterthiophene unit allows us to better understand the nature of the NXz units. We describe the
synthesis, characterization, and structures of PNOz4T and PNSz4T, as well as the corresponding
monomeric compounds, in comparison with the NTz analogues. We specifically focus on the charge
carrier transport properties of the polymers evaluated by OFET devices, and discuss the effects of
chalcogen atoms and the correlations with the electronic structure and the ordering structure.
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Chemistry of Materials
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Figure 1. Chemical structure of benzochalcogenadiazoles (BXz) and naphthobischalcogenadiazoles
NXz). BOz = 2,1,3ꢀbenzothiadiazole, BTz = 2,1,3ꢀbenzoxadiazole, BSz = 2,1,3ꢀbenzoselenadiazole,
NOz = naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]oxadiazole, NTz = naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]thiadiazole, NSz
= naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]selenadiazole.
(
Figure 2. Calculated HOMOs and LUMOs of the model compounds for BXz (BOz2T, BTz2T, and
BSz2T) and NXz (NOz2T, NTz2T, and NSz2T) using the DFT method at the B3LYP/6ꢀ31G(d) level.
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Figure 3. Chemical structures of the polymers based on NOz, NTz, and NSz studied in this work.
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Chemistry of Materials
Results and discussion
Synthesis and properties.
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Synthesis of NOz and NSz is shown in Scheme 1. NOz was synthesized as follows. An oxidation of
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,5ꢀdinitroꢀ2,6ꢀdiaminonaphtalene (1) by sodium hypochlorite and potassium hydroxide gave three
isomers of NOz Nꢀoxides (2a-c), which were then reduced by hydroxylamine in the presence of
potassium hydroxide, giving NOz in a 53% yield with two steps. For NSz, a reduction of 1 by Raney
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nickel gave 1,2,5,6ꢀtetraaminonaphthalene in a 70% yield, which was cyclized by using seleninyl
chloride to afford NSz in a 68% yield. Synthesis of monomers and polymers is shown in Scheme 2. Both
NOz and NSz were dibrominated at the 5,10 position to give 4 and 5, respectively. These were crossꢀ
coupled with 4ꢀ(2ꢀdecyltetradecyl)ꢀ2ꢀ(trimethylstannyl)thiophene (6) via the Stille coupling reaction to
afford NOz2TꢀDT (DT = 2ꢀdecyltetradecyl) and NSz2TꢀDT, which were then dibrominated with Nꢀ
bromosuccinimide (NBS) to yield the corresponding monomers (7 and 8). The monomers were
copolymerized with 5,5′ꢀbis(trimethylstannyl)ꢀ2,2′ꢀbithiophene (9) via the Stille coupling reaction
associated with microwave to give PNOz4T and PNSz4T, respectively. While PNOz4T was synthesized
at 140 °C, PNSz4T was synthesized at 180 °C. The lower polymerization temperature for PNOz4T is to
avoid “overꢀpolymerization”. In fact, when PNOz4T was synthesized at 180 °C, more than 80 wt% of
the resulted polymer sample was insoluble. Both PNOz4T and PNSz4T synthesized at 140 °C and
1
80 °C, respectively, were soluble in hot chlorobenzene (CB) and oꢀdichlorobenzene (DCB). Although
it is difficult to quantify the solubility, both polymers had less solubility than PNTz4T. The number
average molecular weight (M ) and weight average molecular weight (M ) were evaluated by highꢀ
n
w
temperature gelꢀpermeation chromatography (GPC) using DCB (140 °C) as the eluent. M and M were
n
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4
9.7 kDa and 240.8 kDa with the polydispersity index (PDI) of 4.8, and 28.7 kDa and 60.6 kDa with
PDI of 2.1 for PNOz4T and PNSz4T, respectively (Table 1). As the comparison, PNTz4T was also
synthesized by the reported procedure, where the polymerization was carried out at 180 °C (M = 52.9
n
kDa, M = 113.5 kDa, PDI = 2.1). It is noted that the chromatogram of PNOz4T exhibited bimodal
w
distributions, resulting in a high PDI of 4.8. We assume that this is due to the strong aggregation, as
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sometimes seen in D–A semiconducting polymers.
aggregation tendency than PNTz4T and PNSz4T.
This implies that PNOz4T has stronger
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Thermal properties of the polymers were evaluated by the differential scanning calorimetry (DSC)
Figure S27) and thermogravimetry (TG) (Figure S28). While the DSC curve of PNTz4T and PNSz4T
(
showed a melting point (T ) at 321 °C and 221 °C, respectively, in the heating process (302 °C and
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120 °C in the cooling process), that of PNOz4T showed no transition peaks below 350 °C. The DSC
results suggest that rigidity or the backbone is in order of PNOz4T > PNTz4T > PNSz4T. The
temperatures of 5% weight loss (T ) were 368 °C, 439, and 368 for PNOz4T, PNTz4T, and PNSz4T,
d5
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Chemistry of Materials
Scheme 1. Synthesis of NOz and NSz.
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Scheme 2. Synthesis of the monomers and polymers. (TBAB = tetrabutylammonium bromide)
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Table 1. Polymerization results and polymer properties.
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Polymer
M (kDa)
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w
PDI
DPn
Tm (°C)
Td5 (°C)
heating/cooling
PNOz4T
PNTz4T
49.7
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240.8
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0
1
2
3
4
5
6
7
8
9
0
PNSz4T
a
b
Determined by GPC using polystyrene standard and DCB as the eluent at 140 °C. Based on the
c
d
repeating unit. Melting point determined by DSC measurements at a scan rate of 10 °C/min.
Temperature of 5% weight loss.
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Chemistry of Materials
Properties and structures of the monomers
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
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2
2
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4
4
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4
4
4
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5
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5
5
5
5
5
5
5
6
We evaluated the electrochemical and optical properties of the NXzꢀbased compounds, i.e., NOz2Tꢀ
DT, NTz2TꢀDT and NSz2TꢀDT, by using cyclic voltammetry, UVꢀvis absorption and
photoluminescence (PL) spectroscopies. Figure 4a shows the cyclic voltammograms of the monomers
and the corresponding parameters are summarized in Table 2. Whereas the oxidation onset of NOz2Tꢀ
DT (1.40 V) was higher than that of NTz2TꢀDT (1.18 V), that of NSz2TꢀDT (1.09 V) was lower. These
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
onset potentials for NOz2TꢀDT, NTz2TꢀDT, and NSz2TꢀDT correspond to the HOMO energy levels
CV
(
EH ) of –5.76 eV, –5.54 eV, and –5.45 eV, respectively (Table 2, Figure 5). The reduction onset was
higher for both NOz2TꢀDT (–0.78 V) and NSz2TꢀDT (–0.98 V) than for NTz2TꢀDT (–1.08 V).
CV
Accordingly, the LUMO energy levels (EL ) were –3.58 eV, –3.28 eV, and –3.38 eV for NOz2TꢀDT,
CV
CV
NTz2TꢀDT, and NSz2TꢀDT, respectively (Table 2, Figure 5). These EH s and EL s were in good
agreement with the computation by the DFT method at the B3LYP/6ꢀ31g(d) level (Table 2), in which
NOz2T, NTz2T, and NSz2T with the methyl group were used instead of the DT group to simplify the
calculation. The computation also revealed that whereas HOMOs are delocalized over the molecule,
LUMOs are relatively localized on the NXz unit in all cases (Figure 2).
CV
It is noted that the order of EH of the present system follows the general trend observed in many
chalcogenopheneꢀfused
aromatic
systems,
such
as
benzodichalcogenophenes,
naphthodichalcogenophenes, and anthradichalcogenophenes, where it rises in the order of oxygen, sulfur,
40, 41
CV
and selenium.
On the other hand, EL
does not follow the trend observed in the above
CV
chalcogenopheneꢀfused aromatic systems, where it falls in the same order. In contrast, EL of NOz2Tꢀ
CV
DT was lower than that of NTz2TꢀDT, although EL of NSz2TꢀDT was accordingly lower than that of
NTz2TꢀDT, as shown above. This could be explained by taking into account the difference in
4
2
aromaticity of chalcogenadiazoles and electron negativity of chalcogens. Judging from the reported
nucleusꢀindependent chemical shifts (NICS) of 1,2,5ꢀoxadiazole (–11.11 ppm, NICS(0)), 1,2,5ꢀ
4
3
thiadiazole (–13.63 ppm), and 1,2,5ꢀselenadiazole (–12.48 ppm), aromaticity of the oxadiazole ring is
4
4
markedly smaller than those of the thiaꢀ and selenaꢀdiazole rings, which implies that the local atomic
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effect of oxygen is pronounced. In other words, the higher electronegativity of oxygen (3.44) than sulfur
2.58) and selenium (2.55) should be a dominant factor for determining the energy level of the frontier
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
(
orbitals, in particular, LUMO.
UVꢀvis absorption and PL spectra of the compounds are displayed in Figures 4b and 4c, respectively,
and the properties are summarized in Table 3. NOz2TꢀDT and NTz2TꢀDT showed similar spectra for
both absorption and PL with the absorption maxima (λmax) at around 500 nm and for emission maxima
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(PLmax) at around 580 nm. In contrast, NSz2TꢀDT showed a bathochromic shift for both the absorption
and PL spectra by roughly 50 nm compared with NOz2TꢀDT and NTz2TꢀDT. The difference is mostly
consistent with the computation. To better understand the fact that NOz2TꢀDT and NTz2TꢀDT provide
similar absorption spectra and NSz2TꢀDT provides slightly bathochromically shifted absorption
spectrum, TDꢀDFT calculations were carried out to simulate the electronic transistion using NOz2T,
NTz2T, and NSz2T (Figure S23). For all the compounds, the absorption bands mainly contributed by
the HOMO–LUMO transition and appear in the visible region with λmax of 547, 575, and 626 nm for
NOz2T, NTz2T, and NSz2T, respectively. These values are somewhat larger than the experimenatal
values. This is probably due to the overestimation of the coplanarity of the compounds in the
calculations. The actual molecules have various molecular motions including rotation around the
aromatic rings in solution at rt, which may reduce the orbital overlap between the adjacent rings. It is
also noted that the difference between experimental and theoretical λmaxs become larger with the heavier
chalcogen atoms, implying that the ringꢀsize expansion with the heavier chalocogen atom can reduce the
coplanarity, as will be discussed below, giving rise to the reduction of the πꢀelectron delocalization in
the actual molecules.
A higher PL quantum yield was observed for the oxygen analogue, NOz2TꢀDT, as similar to the other
4
0, 41
πꢀsystems.
Whereas in the chloroform solution, the quantum yields of NOz2TꢀDT and NTz2TꢀDT
were similar with 95% and 97%, respectively, in powder, that of NOz2TꢀDT was 30% which is about
three times higher than that of NTz2TꢀDT (12%). The quantum yield of selenium analogue, NSz2TꢀDT,
was much lower in both solution (52%) and powder (≈ 0%), most likely due to the heavy atom effect.
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Chemistry of Materials
2
8
We also investigated in depth the molecular structure of NOz2T, NTz2T, and NSz2T by means of
single crystal Xꢀray analysis. Note that these molecules have the methyl groups instead of the DT groups
on the flanking thiophene rings (see supporting information for the synthesis). As depicted in Figure 6,
the molecules were almost coplanar in all cases. It is also noted that the sulfur atom of the thiophene
ring points to the benzene substructure of NXz, resulting in the anti arrangement of the thiophene rings.
Inspection of bond lengths in the compounds indicates that they have basically similar bond length
alternation, representing the formal naphtho[1,2ꢀc:5,6ꢀc']bis[1,2,5]chalcogenadiazole structure. Subtle
but potentially important differences in the molecular structure are the N–X distance in the
chalcogenadiazole substructure (Figure 6). As the chalcogen atom becomes heavier from oxygen to
sulfur and selenium, the N–X distance increases from 1.38 Å (NOz2T) to 1.61 Å (NTz2T) and 1.79 Å
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
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9
0
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6
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0
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
(NSz2T). This increase apparently pushes out the nitrogen atom out of the chalcogenadiazole
substructure toward the adjacent thiophene ring, which is evident by the fact that θ becomes smaller
from 129.3° to 125.2°, as it goes from NOz2T to NSz2T (Figure 6). This suggests that the steric
repulsion between the NXz core and the thiophene rings becomes larger as the chalcogen atom becomes
heavier, although all the molecules remain coplanar in the single crystal: it is often the case and wellꢀ
known that the molecular structure is largely affected by the packing motif in the single crystal, which
can overcome the local steric repulsion. In parallel, α is smaller in NTz2T and NSz2T than in NOz2T.
This implies that the thiophene ring slightly bends toward the benzene substructure, which suggests that
NTz and NSz can cause more steric repulsion than NOz. In addition, the computation by the density
functional theory (DFT) method predictied that the energy barrier of the rotation around the NOz core
and flanking thiophene rings is largest among the NXz2T (Figure S25), indicating that NOz2T is
energetically favorable to take the coplanar structure than others. With these structural features in the
single crystal and computation, overall steric repulsion likely increases in the order of NOz2T < NTz2T
<
NSz2T. Thus, it is speculated that with the addition of the effect of the bulky side chains NXz2TꢀDT
can lose the coplanarity as the chalcogen atom becomes heavier in the solution, which can rationalize
the trend in the difference between the experimental and theoretical λmaxs as discussed above.
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a
ꢀ
b
ꢀ
c ꢀ
4
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
5x10
NOz2T-DT
NTz2T-DT
NSz2T-DT
NOz2T-DT
NTz2T-DT
NSz2T-DT
NOz2T-DT
NTz2T-DT
NSz2T-DT
4
4
4
4
4
3
2
1
x10
x10
x10
x10
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
0
-
1.5 -1 -0.5
0
0.5
1
1.5
2
300
400
500
600
700
500
600
700
800
900
Potential (V vs Ag/AgCl)
Wavelength (nm)
Wavelength (nm)
Figure 4. Cyclic voltammograms (a), UV–vis. absorption spectra (b), and photoluminescence spectra (c)
of NOz2TꢀDT, NTz2TꢀDT, and NSz2TꢀDT in the solution. The cyclic voltammetry were carried out in
dichloromethane, and the absorption and photoluminescence spectroscopies were carried out in
chloroform.
Figure 5. HOMO and LUMO energy levels of the monomers and polymers evaluated by cyclic
voltammetry.
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Chemistry of Materials
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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4
4
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5
5
5
5
5
5
5
5
5
5
6
a
ꢀ
b
ꢀ
c ꢀ
N–O
.38 Å
N–S
1.61 Å
N–Se
1.79 Å ꢀ
1
ꢀ
ꢀ
θ = 129.3°
ꢀ
θ = 126.5°
ꢀ
θ = 125.2° ꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
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8
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0
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3
4
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7
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9
0
1
2
3
4
5
6
7
8
9
0
NOz2T
ꢀ
NTz2T
ꢀ
NSz2T ꢀ
Figure 6. Molecular structures of NOz2T, NTz2T, and NSz2T determined by the single crystal Xꢀray
structural analysis.
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Table 2. Electrochemical properties and molecular orbital energy levels of the monomers and polymers.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CV
b
CV
PESA
c
opt
DFT
e
DFT
e
Compounds
Eox
Ered
EH
EL
EH
EL
EH
EL
a
a
b
d
(V)
(V)
(eV)
(eV)
(eV)
(eV)
–
(eV)
–5.64
–5.35
–5.25
–
(eV)
–3.13
–2.88
–2.93
–
NOz2TꢀDT
NTz2TꢀDT
NSz2TꢀDT
PNOz4T
1.40
1.18
1.09
1.13
0.79
0.87
–0.78
–1.08
–0.98
–0.70
–0.89
–0.80
–5.76
–5.54
–5.45
–5.48
–5.14
–5.22
–3.58
–3.28
–3.38
–3.65
–3.46
–3.55
–
–
–
–
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
–
–5.50
–5.15
–5.25
–3.98
–3.59
–3.69
PNTz4T
–
–
PNSz4T
–
–
a) Onset oxidation (E ) and reduction (E ) potentials observed in cyclic voltammograms. The
ox
red
CV
monomers and polymers were measured in solution and film, respectively. b) HOMO (EH ) and
LUMO (EL ) energy levels evaluated using E and E , respectively. c) HOMO energy level (E
evaluated by photoelectron spectroscopy in air. d) LUMO energy level (E ) calculated by the addition
CV
PESA
)
ox
red
H
opt
L
PESA
DFT
DFT
of the optical band gap (E ) to EH
. e) HOMO (EH ) and LUMO (EL ) energy levels calculated by
g
the DFT method at the B3LYP/6ꢀ31g(d) level. Note that the DT group of the side chain was replaced
with the methyl group.
Table 3. Optical properties of the monomer and polymers.
a
b
c
d
e
Compounds
λmax (nm)
λedge (nm)
Eg (eV)
PLmax (nm)
Φ (%)
PL
solution film solution film solution film solution solid solution solid
NOz2TꢀDT
NTz2TꢀDT
NSz2TꢀDT
PNOz4T
504
490
538
–
–
–
557
547
606
–
–
–
2.23
2.27
2.05
–
–
–
585
579
648
648
643
–
95
97
52
30
12
<1
749,
685
749,
680
7
98
84
815
1.55
1.52
–
–
–
–
PNTz4T
PNSz4T
717,
721,
658
7
793
792
1.58
1.56
1.56
1.56
–
–
–
–
–
–
–
–
660sh
686
706
796
a) absorption maximum, b) absorption onset, c) optical band gap calculated by using λedge, d) emission
maximum, e) photoluminescence quantum yield.
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Chemistry of Materials
Physicochemical properties of the polymers
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
CV
CV
EH s and EL s of the polymers were evaluated using the thin films. Figure 7a depicts the cyclic
voltammograms of the polymer thin films, in which all polymers showed clear oxidation and reduction
CV
CV
signals. The onset oxidation and reduction potentials and EH and EL calculated from these potentials
CV
are summarized in Table 2. EH s of PNOz4T and PNSz4T was –5.48 eV and –5.22 eV, respectively,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
both of which were deeper than that of PNTz4T (–5.14 eV) by 0.34 eV and 0.08 eV (Table 2, Figure 5).
CV
EL s of PNOz4T and PNSz4T were –3.65 eV and –3.55 eV, respectively, which were deeper by 0.19
eV and 0.09 eV than that of PNTz4T (–3.46 eV) (Table 2, Figure 5). It is noted that whereas the trend of
CV
CV
EL in the polymers was similar to that in NXz2TꢀDTs, the trend of EH in the polymers was different
CV
CV
from that in NXz2TꢀDT. Whereas EH of NSz2TꢀDT was shallower than that of NTz2TꢀDT, EH of
PNSz4T was deeper than that of PNTz4T. A plausible explanation is that the large steric repulsion
between NXz and the adjacent thiophene rings in the NSz2TꢀDT moiety compared with the NTz2TꢀDT
moiety, as discussed above, can lead to further large defect of the backbone in the polymer system and
thereby the packing structure of PNSz4T compared with PNTz4T. This may result in the reduction of
CV
the effective πꢀconjugation and wider bandgap, which in turn gives rise to the deeper EH in PNSz4T
than in PNTz4T. This explantion is consistent with the fact that λmax of PNSz4T was slightly
hypsochromically shifted compared to that of PNTz4T as shown below. Photoelectron spectroscopy in
air (PESA) was also carried out using the polymer thin films to evaluate the HOMO energy levels
PESA
PESA
(EH
) of the polymers (Figure S26). EH
of PNOz4T, PNTz4T, and PNSz4T were –5.50 eV, –5.15
CV
opt
eV, and –5.25 eV, respectively, which were mostly similar to EH . LUMO energy levels (EL ) were
PESA
calculated by the addition of optical band gap (E ) to EH
, which were –3.98 eV, –3.59 eV, and –3.69
g
CV
opt
eV for PNOz4T, PNTz4T, and PNSz4T, respectively. The difference between EL and EL can be
4
5
ascribed to the exciton binding energy.
UVꢀvis absorption spectra of the polymers in thin film and in CB solution are shown in Figures 7b and
8
, respectively. In the thin film, PNOz4T gave λmax at 749 nm, which was bathochromically shifted by ca.
3
0 nm compared with PNTz4T (λmax = 721 nm). E of PNOz4T was calculated to be 1.52 eV from the
g
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absorption edge (λedge = 815 nm), which was slightly narrower than that of PNTz4T (E = 1.56 eV, λedge
g
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
=
793 nm). λmax of PNSz4T (706 nm) was hypsochromically shifted by 15 nm compared with that of
PNTz4T, although λedge (793 nm) and E (1.56 eV) was similar due to the blunt absorption signal. It is
g
noted that while the bathochromic shift of PNOz4T against PNTz4T is consistent with that observed in
NXz2T, the hypsochromic shift of PNSz4T against PNTz4T does not follow the trend in NXz2TꢀDT.
We speculate that the larger steric repulsion in the NSz2TꢀDT moiety than in the NTz2TꢀDT moiety
may give rise to a further large defect of the backbone structure in PNSz4T than in PNTz4T, which in
turn gives rise to the reduction of the effective πꢀconjugation. In the solution, all the polymers exhibited
mostly identical spectrum to that in the film, implying the partial aggregation. The absorption coefficient
of the absorption band at 500–800 nm was lower for the polymers with heavier chalcogen atoms. This is
consistent with the trend observed in NXz2TꢀDT and the computation. The polymer solutions were
heated up to 110 °C. In PNTz4T, the spectrum became structureꢀless upon heating, and λmax showed
large hypsochromic shift to 597 nm, suggesting the disaggregation as well as the destruction of
coplanarity due to the molecular motion (Figure 8b). In PNOz4T, on the contrary, λmax did not show a
hypsochromic shift (Figure 8a). This is most likely due to the higher rigidigy and thus the stronger
intermolecular interaction of the PNOz4T backbone than PNTz4T, likely originating in the less steric
repulsion in the NOz2TꢀDT moiety than the NTz2TꢀDT moiety, as described above. Furthermore, the
large atom size in the heavier chalcogen can reduce the π–π interaction between the polymer backbones,
which also contributes to the difference in the aggregation behavior seen in the solution absorption
spectra. The spectrum of PNSz4T also became structureꢀless upon heating and hypsochromically shifted,
but the shift of λmax (λmax = 633 nm at 110 °C) was smaller than that in PNTz4T (Figure 8c). The smaller
hypsochromic shift is beause λmax of PNSz4T is already located at the slightly shorter wavelength region
at rt than that of PNTz4T.
0
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Chemistry of Materials
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9
a
ꢀ
b ꢀ
PNOz4T
PNTz4T
PNSz4T
PNOz4T
PNTz4T
PNSz4T
10
11
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-
1.5
-1
-0.5
0
0.5
1
1.5
300 400 500 600 700 800 900 1000
Wavelength (nm)
Potential (V vs Ag/AgCl)
Figure 7. Cyclic voltammograms (a) and UVꢀvis. absorption spectra (b) of the polymers in thin films.
a
ꢀ
b
ꢀ
c ꢀ
6
5
4
3
2
1
0
5
4
3
2
1
0
8
7
6
5
4
3
2
1
0
PNOz4T
20 ºC
0 ºC
0 ºC
50 ºC
0 ºC
70 ºC
0 ºC
0 ºC
00 ºC
20 ºC
30 ºC
40 ºC
50 ºC
60 ºC
70 ºC
80 ºC
90 ºC
100 ºC
110 ºC
20 ºC
30 ºC
40 ºC
50 ºC
60 ºC
70 ºC
80 ºC
90 ºC
100 ºC
110 ºC
PNTz4T
PNSz4T
3
4
6
8
9
1
110 ºC
300 400 500 600 700 800 900 1000
300 400 500 600 700 800 900 1000
300 400 500 600 700 800 900 1000
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Figure 8. UVꢀvis. absorption spectra of the polymers in chlorobenzene. (a) PNOz4T, (b) PNTz4T, and
c) PNSz4T.
(
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Thin film structure and morphology of the polymers
The grazing incidence wideꢀangle Xꢀray diffraction (GIXD) studies were carried out using the
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polymer thin films. Figure 9 shows the twoꢀdimensional (2D) GIXD patterns of the polymer thin films
on the ODTSꢀtreated Si/SiO substrate. The 2D GIXD patterns of the polymers on the FDTSꢀtreated
2
substrate were almost the same as that on the ODTSꢀtreated substrate (Figure S29). In PNOz4T,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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1
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3
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9
0
1
2
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9
0
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2
3
4
5
6
7
8
9
0
−
1
−1
diffractions assignable to the lamellar (q ≈ 0.28 Å ) and the π−π stacking structures (q ≈ 1.8 Å )
z
xy
mainly appeared on the q and q axes, respectively, indicating the predominant edgeꢀon orientation on
z
xy
the substrate surface as similar to PNTz4T, though some fraction of faceꢀon orientation also exist as the
lamellar and π–π stacking diffraction also but weakly appeared on the q and q axes, respectively. The
xy
z
π–πꢀstacking distance (d ) of PNOz4T was determined to be 3.5 Å, which was comparable to that of
π
PNTz4T. Crsytallinity of PNOz4T and PNTz4T was almost the same as the crystallite coherence length,
calculated by the Scherrer equation using the fullꢀwidthꢀatꢀhalfꢀmaximum of the lamellar peak in the qz
47, 48
axis and the π–π stacking peak in the q axis, showed similar values (Table S2).
On the contrary,
xy
only a weak diffraction in the smallꢀangle region was observed for PNSz4T, indicative of an amorphous
nature. The significantly reduced crystallinity of PNSz4T is attributable to the larger steric repulsion in
the NSz2TꢀDT moiety than in the oxygen and sulfur analogues, which may cause larger twist in the
polymer backbone, as well as the larger atomic size of selenium, which could diminish the π–π
interaction of the polymer backbones. This structural difference in the series of PNXz4T can be wellꢀ
correlated with the electrochemical and optical properties.
Morphologies of the polymer thin films were investigated by using the atomic force microscopy
(AFM). Figure 10 displays the topographic AFM images and the crossꢀsectional profiles of the polymer
thin films on the ODTSꢀtreated Si/SiO substrate. Seemingly the PNOz4T film consists of larger
2
domains compared with the PNTz4T and PNSz4T films, which would facilitate the charge carrier
transport. This could explain the slightly higher mobility of PNOz4T than PNTz4T.
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Chemistry of Materials
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50
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57
58
59
60
Figure 9. (a–c) 2D GIXD patterns of the thin films on the ODTSꢀtreated Si/SiO substrate for PNOz4T
2
(a), PNTz4T (b), and PNSz4T (c). (d, e) The crossꢀsectional profiles along the q and q axes,
z
xy
respectively.
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a
ꢀ
b
ꢀ
c
ꢀ
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PNOz4T
ꢀ
PNTz4T
ꢀ
PNSz4T ꢀ
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00 nm
ꢀ
ꢀ
500 nm
ꢀ
ꢀ
ꢀꢀ
500 nm ꢀ
25
10
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2
1
0
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1
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4
2
0
0
500
1000
nm
1500
2000
0
500
1000
nm
1500
2000
0
500
1000
nm
1500
2000
Figure 10. AFM height images (top) and cross section profiles (bottom) of the thin films on the actual
device treated with ODTS for PNOz4T (a), PNTz4T (b), and PNSz4T (c).
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Chemistry of Materials
Charge carrier transport property of the polymers
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OFET devices were fabricated to investigate the charge carrier transport property of the polymers. The
device used here has a bottomꢀgateꢀtopꢀcontact (BGTC) architecture using Si/SiO as the substrate. The
2
substrate surface was treated with surfactants such as 1H,1H,2H,2Hꢀperfluorodecyltriethoxysilane
(FDTS) or octadecyltriethoxysilane (ODTS), forming a selfꢀassembled monolayer (SAM). Spinꢀcoated
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0
polymer films were thermally annealed at 200 °C for 30 min, which refers to preꢀannealing. After the
deposition of the Au electrodes, the devices were again annealed at 200 °C, which refers to postꢀ
annealing. Figure 11 shows transfer (a,c,e,g,i,k) and output (b,d,f,h,j,l) curves of the polymer devices
measured in air. The OFET characteristics are summarized in Table 4. All the polymers showed pꢀ
channel characteristics in the FDTSꢀtreated devices. The hole mobility (µ ) of PNOz4T was as high as
h
2
2
–1 –1
1
.0 cm /Vs with the average of 0.55 cm V s , which were about twice as high as those of PNTz4T
2
–1 –1
2
–1 –1
(
maximum 0.56 cm V s , average 0.23 cm V s ). In contrast, µ of PNSz4T was one order of
h
2
–1 –1
2
–1 –1
magnitude lower than those of the others (maximum 0.040 cm V s , average 0.026 cm V s ).
6
7
Current on and off ratios (I /I ) were fairly high (10 –10 ). The difference of µ is quite in good
on off
h
agreement with the 2D GIXD results, where PNOz4T and PNTz4T form crystalline structures and
PNSz4T forms an amorphousꢀlike structure.
It is also important to note the effect of molecular weight on the device performance, in particular for
PNOz4T and PNTz4T with higher mobilities among the series. We have tested the PNOz4T devices
using two other batches with higher and lower molecular weights (higher one: M = 57.2 kDa, M =
n
w
416.8 kDa, and PDI = 7.3, lower one: M = 34.8 kDa, M = 152.1 kDa, and PDI = 4.4) (Figure S19,
n
w
2
–1 –1
2
Table S1). While the higher molecular weight batch exhibited µ of ~0.84 cm V s (average 0.55 cm
h
–
1
–1
2
–1 –1
2
–1 –1
V
s ), the lower molecular weight batch exhibited ~0.56 cm V s (average 0.34 cm V s )
(Figure S30, Table S3). Therefore, it could be mentioned that the molecular weight of the initial batch
of PNOz4T (M = 49.7 kDa, M = 240.8 kDa, see Table 1) is sufficiently high (in the optimal M and
n
w
n
M ranges) to maximize the transistor performance. In addition, M or PDI may not affect the device
w
w
performance as long as M is in the optimal range (> ca. 50 kDa). In the case of PNTz4T, the molecular
n
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Page 24 of 41
weight shown in Table 1 is also sufficiently high (Figure S19 and S30, Table S1 and S3). As the
polymer became insoluble when the molecular weight was increased (synthesized by more harsh
conditions), the PNSz4T sample shown here should be the highest level with sufficient solubility. With
these additional results, although the molecular weights of the polymers are quite different, the
performance of the polymers summarized in Figure 11 and Table 4 can be reasonably evaluated and
compared.
1
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3
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9
1
1
1
1
1
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1
1
2
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In the ODTSꢀtreated devices, it is interesting to note that whereas PNTz4T and PNSz4T showed pꢀ
2
–1 –1
2
–1 –1
channel characteristics with of 0.22 cm V s and 0.059 cm V s , respectively, PNOz4T showed
2
–1 –1
ambipolar characteristics with wellꢀbalanced µ and electron mobilities (µ ) of up to 0.45 cm V s
h
e
2
–1 –1
and 0.28 cm V s , respectively. The observed ambipolar behavior of PNOz4T is ascribed to its
deeper LUMO energy level. The difference in the charge carrier polarity between FDTS and ODTSꢀ
treated devices is probably due to the difference in the dipole between a perfluoroalkylsilane (FDTS)
49
and an alkylsilane (ODTS). A similar phenomenon was also seen in the thienothiophenedioneꢀbased
3
9
polymers.
In the transfer curves (Figures 11a, c, e, g, i, k) was also plotted the mobility as a function of the gate
voltage (V ). Whereas the mobilities were mostly independent of V above the threshold in the FDTSꢀ
g
g
treated devices, they were dependent on V and were somewhat higher in the lower V region in the
g
g
ODTSꢀtreated devices. It is also noted that, in particular, FDTSꢀtreated devices that use PNOz4T
1/2
without postꢀannealing gave a kinked V –(–I ) plot in the transfer characteristics (Figure 12) with a
g
d
larger threshold voltage in negative direction, resulting in the significantly V ꢀdependent mobility and
g
larger values. We speculate that, in the devices without postꢀannealing, the contact between the Au
electrode and the polymer film might be poor, which causes too high charge accumulation and the
charges would pour out at a higher V . The kink, however, disappeared by postꢀannealing, which is
g
probably due to the improved contacts.
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Chemistry of Materials
a
ꢀ
b
ꢀ
c
g
k
ꢀ
d ꢀ
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8
9
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0-3
1
-3.5x10-4
-3x10-4
10-3
1
0
3.5x10-4
-
3.5x10-4
1
1
1
1
1
1
1
0.025
10
10
10
10
10
0.025
0.02
0.015
0.01
0.005
0
10
PNOz4T
(FDTS)
PNOz4T
(FDTS)
n-operation PNOz4T
PNOz4T
(ODTS)
p-operation
0
-4
ꢀ
ꢀ
ꢀ
ꢀ
(ODTS)
ꢀ
ꢀ
3x10-4
ꢀ
ꢀ
3x10-4
-
10-4
10-5
0.02
0-5
0
10
p-operation
n-operation
-
2.5x10-4
-2.5x10-4
-2x10-4
2.5x10-4
2x10-4
0-6
0.015
2x10-4
-
0-7
0-8
0-9
-1
-2
-3
10-1
10-2
10-3
-
1.5x10-4
1x10-4
10-6
10-7
10-8
-1.5x10-4
-1x10-4
1.5x10-4
1x10-4
0.01
-
0.005
-5x10-5
0
-5x10-5
5x10-5
0-10
1
1
0-11
0
0
0
60
-60 -50 -40 -30 -20 -10
0
10 20
0
-10 -20 -30 -40 -50 -60
(V)
-60 -40 -20
0
20
40
60
-60 -40 -20
0
20
40
0
1
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0
1
2
3
4
5
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8
9
0
Vg (V)
V
d
V
g
(V)
Vd (V)
e
ꢀ
f
ꢀ
ꢀ
h ꢀ
-
-
-
-
-
-
-
3
4
5
6
7
8
9
1
3.5x10-4
10-3
101
100
10-1
10-2
10-3
-3.5x10-4
1
0
0
0
0
0
0
0
0.025
10
10
10
10
10
-
0.025
0.02
0.015
0.01
0.005
0
PNTz4T
(FDTS)
PNTz4T
(FDTS)
PNTz4T
(ODTS)
PNTz4T
ꢀꢀ
(ODTS) ꢀ
1
1
1
1
1
1
ꢀ
-3x10-4
ꢀ
10-4
ꢀ
-3x10-4
0.02
0
0-5
1
-2.5x10-4
-2x10-4
-2.5x10-4
10-6
0.015
-2x10-4
-1
-2
-3
0-7
1
1
1
1.5x10-4
-1.5x10-4
-
0.01
0.005
0
0-8
-1x10-4
-5x10-5
0
-1x10-4
-5x10-5
0
0-9
-
10
0-10
10
1
-
11
10-11
-60 -50 -40 -30 -20 -10
(V)
10
-60 -50 -40 -30 -20 -10
0
10 20
0
-10 -20 -30 -40 -50 -60
(V)
0
10 20
0
-10 -20 -30 -40 -50 -60
(V)
Vg (V)
V
V
V
d
d
g
i
ꢀ
j
ꢀ
ꢀ
l ꢀ
1
0-3
0.025
101
10-3
0.025
0.02
0.015
0.01
0.005
0
101
PNSz4T
(FDTS)
PNSz4T
(FDTS)
PNSz4T
(ODTS)
PNSz4T
ꢀꢀ
(ODTS) ꢀ
1
1
1
1
1
0-4
ꢀ
-4x10-5
ꢀ
10-4
10-5
ꢀ
-4x10-5
-3x10-5
-2x10-5
0.02
0
-5
10
0
10
0
-
-
-
3x10-5
2x10-5
0-6
10-6
10-7
0.015
0
-7
10-1
10-2
10-3
10-1
10-2
10-3
0-8
0.01
10-8
10-9
10-9
1x10-5
-1x10-5
0.005
0-10
10-10
1
0-11
10-11
1
0
0
0
-60 -50 -40 -30 -20 -10
0
10 20
0
-10 -20 -30 -40 -50 -60
-60 -50 -40 -30 -20 -10
0
10 20
0
-10 -20 -30 -40 -50 -60
Vg (V)
Vd (V)
Vg (V)
Vd (V)
Figure 11. Current–voltage characteristics of OFET devices fabricated with PNOz4T (a–d), PNTz4T (e–
h), and PNSz4T (i–l). (a, e, i) Transfer curves of OFET devices with the FDTS SAM. (b, f, j) Output
curves of OFET devices with the FDTS SAM. (c, g, k) Transfer curves of OFET devices with the ODTS
SAM. (d, h, l) Output curves of OFET devices with the ODTS SAM. The lightꢀcolored curves depicted
with the transfer curves are the dependence of the mobility on the V ; the hole mobility for pꢀoperation
g
and the electron mobility for nꢀoperation in the case of PNOz4T with the ODTS SAM modified device.
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1 -
0
3
0.025
01
1
1
1
1
1
pre-anealing
pre- and post-annealing
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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0-4
1
0.02
1 -
1 -
1 -
0
5
6
7
00
0
0.015
0
0-1
0-2
0-3
10-8
0.01
0-9
1
0.005
0-10
1
1 -
0
11
0
0
1
2
3
4
5
6
7
8
9
0
1
2
3
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1
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0
1
2
3
4
5
6
7
8
9
0
-60 -50 -40 -30 -20 -10
Vg (V)
0
10 20
Figure 12. I vs. V curves (solid and dotted lines) and µ vs. V curves (lightꢀcolored lines) of OFET
d
g
g
devices based on PNOz4T. Blue curves stand for the device with the preꢀannealed (200 °C) film. Red
curves stand for the device with the preꢀannealed films annealing at 200 °C after the deposition of the
source and drain Au electrodes.
Table 4. OFET characteristics of the polymers.
Polymer
FDTS
ODTS
pꢀchannel
pꢀchannel
µh
nꢀchannel
µe
µh
2
I /I
on off
I /I
on off
I /I
on off
2
2
(
cm /Vs)
(cm /Vs)
(cm /Vs)
7
3
2
PNOz4T
PNTz4T
PNSz4T
0.55 [0.22 ~ 1.0]
0.23 [0.18 ~ 0.56]
0.026 [0.016 ~
~10
0.27 [0.21 ~ 0.45]
0.15 [0.12 ~ 0.22]
~10
0.17 [0.10 ~ 0.28]
~10
7
7
~10
~10
–
–
–
–
6
5
~10
0.040 [0.008 ~
0.059]
~10
0
.040]
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Chemistry of Materials
Conclusion
1
2
3
4
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6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
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3
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4
4
4
4
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4
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5
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5
5
5
5
5
5
6
We have synthesized oxygen and selenium analogues of naphthobischalcogenadiazole, namely,
naphthobisoxadiazole (NOz) and naphthobisselenadiazole (NSz), and the corresponding polymers. We
found that NOz and NSz had high electron deficiency compared with the sulfur analogue,
naphthobisthiadiazole (NTz), which is different from the trend observed in acenedichalcogenophenes.
This is probably because the electron negativity of the chalcogen atom is the dominant factor for the
frontier orbital energy levels. It is also found that although all the model compounds, NXz2T, were
similarly coplanar in the single crystals, steric repulsion between NXz and the adjacent thiophene rings
seems to be larger as the chalcogen atom become heavier. The difference of the steric repulsion can
rationalize the difference in the electronic structure of NXz2TꢀDT and PNXz4T, and the ordering
structure of PNXz4T: whereas PNOz4T exhibited bathochromic shift in the absorption and similarly
high crystallinity in the film, PNSz4T exhibited hypsochromic shift and amorphousꢀlike structure as
compared to PNTz4T. Thanks to the higher electron deficiency, the NOz and NSzꢀbased polymers,
PNOz4T and PNSz4T, respectively had deeper LUMO energy levels than the NTzꢀbased polymer,
PNTz4T. Notably, the LUMO energy level of PNOz4T was found to be –3.65 eV by electrochemistry
and –4.00 eV by spectroscopies, which is around the borderline for allowing ambient nꢀchannel behavior
in OFETs. In fact, whereas PNSz4T showed only pꢀchannel behavior, as is the case in PNTz4T,
PNOz4T exhibited a pꢀchannel and an ambipolar behavior depending on the surfactant used in OFETs.
It is also mentioned that the mobilities of PNOz4T were significantly higher than those of PNSz4T,
which was consistent with the higher crystallinity of PNOz4T. The hole mobility of PNOz4T in the pꢀ
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
channel devices reached 1 cm /Vs, which was even higher than that of PNTz4T. In the ambipolar
devices, PNOz4T demonstrated wellꢀbalanced and relatively high hole and electron mobilities of ~0.5
2
2
cm /Vs and ~0.3 cm /Vs, respectively. We have shown the clear correlation between the chalcogen atom
in the NXz unit and the properties of the corresponding polymers. Furthermore, these results clearly
demonstrate that NOz is an interesting acceptor unit for πꢀconjugated materials.
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Page 28 of 41
Experimental section
Synthesis. 2,6ꢀDiaminoꢀ1,5ꢀdinitronaphthalene (1), 1,2,5,6ꢀtetraaminonaphthalene (3), 4ꢀ(2ꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
7
37
2
8
50
decyltetradecyl)ꢀ2ꢀtrimethylstannylthiophene (6), 5,5′ꢀbis(trimethylstannyl)ꢀ2,2′ꢀbithiophene (9) were
synthesized according to the reported procedure, respectively. Even in the hydrochloride form, 3
gradually changes the color from gray to bluish gray in air, which may affect the synthesis of NSz. It is
recommended to store in an inert atmosphere. All chemicals and solvents are of reagent grade unless
otherwise indicated. THF and toluene were purified by a Glass Contour Solvent System (Nikko Hansen
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
&
Co., Ltd.) prior to use. Polymerization was carried out with a microwave reactor (Biotage Initiator).
Nuclear magnetic resonance (NMR) spectra were obtained in deuterated chloroform and DCB (for
polymers) with TMS as internal reference by using a JNMꢀECS400 (JEOL RESONANCE). EIꢀMS
spectra were obtained with a GCMSꢀQPꢀ2010SE spectrometer (SHIMADZU) using an electron impact
ionization procedure (70 eV). The molecular ion peaks of the chlorineꢀ, bromineꢀ, sulfurꢀ, seleniumꢀ, or
tinꢀcontaining compounds showed a typical isotropic pattern, and all the mass peaks are reported based
3
5
32
79
80
120
on Cl, S, Br, Se, or Sn, respectively. Molecular weights were determined by gel permeation
chromatography (GPC) by calibrating with polystyrene standards using a TOSOH HLCꢀ8121GPC/HT at
1
2
40 °C with DCB as the solvent.
Naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]oxadiazole (NOz). 1,5ꢀDiaminoꢀ2,6ꢀdinitronaphthalene (1) (5.00 g,
0.1 mmol), 85 % potassium hydroxide (5.3 g, 80.3 mmol), tetrabutylammonium bromide (0.65 g, 2.02
mmol), dichloromethane (100 mL), and water (13.5 mL) were added to a 300 mL threeꢀneck flask. 10%
sodium hypochlorite solution (96.0 g, 129.0 mmol) was added slowly. After the mixture was stirred at
room temperature for 0.5 h, the mixture was refluxed for 2 h. While concentrating the solvent, 50 %
ethanol aqueous solution was added slowly. After cooling to 0 °C, the precipitate was filtered, and then
washed by water and 50% ethanol aqueous solution. The crude product (a mixture of 2aꢀc) as yellow
solid was dried in vacuo and used without further purification (3.59 g, 73%). The mixture of 2aꢀc (3.22
g, 13.2 mmol) was added to 80 mL of ethanol in a flask. After the mixture was cooled to 0 °C,
hydroxylamine hydrochloride (3.67 g, 52.8 mmol) and water (16 mL) was added, and then 85 %
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Chemistry of Materials
potassium hydroxide (7.41 g, 112.3 mmol) and water (16 mL) was added successively. The mixture was
stirred at room temperature for 1 h, and subsequently was refluxed for 3 h. Water was added slowly to
the reaction mixture, which was then cooled to 0 °C. The precipitate was collected by filtration. After
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
washing with water, the crude product was purified by recrystallization from chloroform/ethanol/water
1
to give NOz as orange solid (2.04 g, 73%). H NMR (400 MHz, CDCl ): δ 8.52 (d, J = 9.2 Hz, 2H), 8.09
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
3
1
3
(d, J = 9.2 Hz, 2H). C NMR (100 MHz, CDCl ): δ 148.9, 147.5, 128.6, 122.9, 117.4. EIꢀMS (70 eV)
3
+
m/z 212 (M ). Anal. Calcd for C H N O : C, 56.61; H, 1.90; N, 26.41. Found: C, 56.60; H, 2.09; N,
1
0
4
4
2
2
6.21.
Naphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]selenadiazole
(NSz).
1,2,5,6ꢀTetraaminonaphthalene
tetrahydrochloride (0.50 g, 1.50 mmol) and pyridine (7.8 g) were added to a 300 mL threeꢀneck flask.
After cooling to 0 °C, seleninyl chloride (1.10 g, 6.63 mmol) was added dropwise. The solution was
heated to 70 °C, and stirred for 2 h. Water was added dropwise into the reaction mixture, which was
then cooled to 0 °C. The resulting precipitate was filtered, washed by water and isopropanol, and dried
under vacuum. The crude product was purified by recrystallization from DCB to give NSz as dark
1
yellow solid (345 mg, 68%). H NMR (400 MHz, DCBꢀd , 150 °C): δ 8.63 (d, J = 9.3 Hz, 2H), 7.76 (d,
4
+
2
H, J = 9.3 Hz). EIꢀMS (70 eV) m/z 340 (M ). Anal. Calcd for C H N Se : C, 35.53; H, 1.19; N, 16.57.
1
0
4
4
2
Found: C, 35.37; H, 1.18; N, 16.45.
5
,10ꢀDibromonaphtho[1,2ꢀc:5,6ꢀc′]bis[1,2,5]oxadiazole (4). NOz (2.25 g, 10.6 mmol) was dissolved
in concentrated sulfuric acid (112.5 g) in a 200 mL threeꢀneck flask. NBS (5.66 g, 31.8 mmol) was
added in several portions over the period of 1 h, and then the reaction mixture was stirred for 24 h at
room temperature. The reaction mixture was poured into cold water, and the resulting precipitate was
filtered. The collected product was washed with water, NaHCO aq., ethanol and THF, which was then
3
1
purified by recrystallization from chloroform/isopropanol to give 4 as yellow solid (2.92 g, 75%). H
13
NMR (400 MHz, CDCl ): δ 8.68 (s, 2H). C NMR (100 MHz, CDCl ): δ 149.5, 146.7, 130.6, 122.6,
3
3
+
1
110.9. EIꢀMS (70 eV) m/z 370 (M ). Anal. Calcd for C H Br N O : C, 32.47; H, 0.54; N, 15.14.
10 2 2 4 2
Found: C, 32.63; H, 0.59; N, 15.09.
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