Electron-Deficient Poly(p-phenylene vinylene) Provides Electron Mobility over 1 cm2 V-1 s-1 under Ambient Conditions

Article abstract of DOI:10.1021/ja403624a

Poly(p-phenylene vinylene) derivatives (PPVs) are one of the most widely investigated p-type polymers in organic electronics. PPVs generally exhibit electron mobilities lower than 10^-4 cm² V^-1 s^-1, thus hindering their applications in high-performance polymer field-effect transistors and organic photovoltaics. Herein, we design and synthesize a novel electron-deficient PPV derivative, benzodifurandione-based PPV (BDPPV). This new PPV derivative displays high electron mobilities up to 1.1 cm² V^-1 s^-1 under ambient conditions (4 orders of magnitude higher than those of other PPVs), because it overcomes common defects in PPVs, such as conformational disorder, weak interchain interaction, and a high LUMO level. BDPPV represents the first polymer that can transport electrons over 1 cm² V^-1 s^-1 under ambient conditions.

Full text of DOI:10.1021/ja403624a

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Electron-Deficient Poly(p-Phenylene Vinylene) Provides
2
–1
–1
Electron Mobility over 1 cm V s under Ambient Conditions
Ting Lei, Jin-Hu Dou, Xiao-Yu Cao, Jie-Yu Wang, and Jian Pei
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja403624a • Publication Date (Web): 15 May 2013
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Electron-Deficient Poly(p-Phenylene Vinylene) Provides Elec-
tron Mobility over 1 cm2 V–1 s–1 under Ambient Conditions
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Ting Lei,† JinꢀHu Dou,† XiaoꢀYu Cao,‡,* JieꢀYu Wang,†,* and Jian Pei†,*
†Beijing National Laboratory for Molecular Sciences, the Key Laboratory of Bioorganic Chemistry and Molecular Engineerꢀ
ing of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China.
‡College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China.
Supporting Information Placeholder
ABSTRACT: Poly(pꢀphenylene vinylene) derivatives (PPVs)
are one of the most widely investigated pꢀtype polymers in
organic electronics. PPVs generally exhibit electron mobility
lower than 10–4 cm2 V–1 s–1, thus hindering their applications in
highꢀperformance polymer FETs and organic photovoltaics.
Herein, we design and synthesize a novel electronꢀdeficient PPV
derivative, benzodifurandioneꢀbased PPV (BDPPV). This new
PPV derivative displays high electron mobilities up to 1.1 cm2 V–1
s–1 under ambient conditions (4 orders of magnitude higher than
those of other PPVs), because it overcomes common defects in
PPVs, such as conformational disorder, weak interchain
interaction, and high LUMO level. BDPPV represents the first
polymer that can transport electrons over 1 cm2 V–1 s–1 under
Figure 1. (a) Double bond isomerization of the PPV segment
ambient conditions.
under UV irradiation. (b) Conformational isomers in PPV after
single bond rotation. (c) Design strategy for the electronꢀdeficient
PPV—BDPPV.
Poly(pꢀphenylene vinylene) derivatives (PPVs) are among the
Carrier mobility in conjugated polymer is determined by intraꢀ
earliest and the most widely investigated polymers in organic
chain and interchain transport.15 Increasing the effective conjugaꢀ
electronics.1 As typical pꢀtype polymers,2 PPVs have been used to
tion length and reducing torsional disorder along polymer backꢀ
fabricate the first polymer lightꢀemitting diode (PLED),3 the first
bone improve the intrachain transport.16 Good πꢀπ stacking and
bulk heterojunction (BHJ) solar cell,4 and the first optically
ordered polymer packing, on the other hand, increase the interꢀ
pumped solidꢀstate laser.5 Considerable efforts have been devoted
chain transport. Previously, we proved that the backbone curvaꢀ
to improving their emission color, efficiency, and life time in
ture of polymers has significant influence on their carrier mobiliꢀ
PLED,6 as well as power conversion efficiency in solar cells.7
ty, and polymers with linear backbones have mobilities several
Nonetheless, PPVs were seldom applied in fieldꢀeffect transistors
orders of magnitude higher than those with waveꢀlike backꢀ
(FETs) due to their low hole mobilities (10–5–10–2 cm2 V–1 s–1).8
bones.17 Thus, the low mobility of PPVs is presumably caused by
Moreover, their electron mobilities are lower (10–5–10–4 cm2 V–1
the following:
s–1), even when lowꢀworkꢀfunction metal (Ca) and hydroxylꢀfree
(1) The double bonds in PPVs readily undergo trans to cis conꢀ
gate dielectric were used to fabricate the device and all operations
formational transformation under UV light both in solution and in
were performed under inert atmosphere.9 Hence, one may ask,
solid state (Figure 1a).18
“can high carrier mobility be achieved in PPVꢀbased polymers?”
(2) Even for allꢀtrans backbones, the single bonds in PPVs roꢀ
Recently, the application of several electronꢀdeficient dyes,
tate freely under room temperature, creating many conformational
such as diketopyrrolopyrrole (DPP),10 isoindigo (II),11 naphthaꢀ
isomers (Figure 1b).
(3) PPVs generally have weak interchain interactions and are
lene diimide (NDI),12 and benzobisthiadiazole (BBT)13 in polymer
FETs afforded high hole mobilities under ambient conditions and
amorphous in solid state,19 thus resulting in slow interchain carrier
high electron mobilities in nitrogen. Nevertheless, high electron
transport.
mobilities over 1 cm2 V–1 s–1 under ambient conditions has not yet
(4) The LUMO levels of PPVs (–2.7 to –3.2 eV)9 are too high
been reported, although polymers with high electron mobilities
to achieve stable electron transport under ambient conditions.
under ambient conditions are important for both fundamental
The first two issues not only limit the intrachain carrier mobiliꢀ
understanding of carrier transport and advancing practical applicaꢀ
ties in PPVs, but also weaken the interchain πꢀπ interactions, thus
tions. The current scenario of new polymer design mainly focuses
further prohibiting the interchain carrier transport. Currently, the
on thiopheneꢀbased donorꢀacceptor copolymers (thirdꢀgeneration
most effective approach to obtain high electron mobility in polyꢀ
polymer named by Heeger), and PPVs (secondꢀgeneration polyꢀ
mer named by Heeger) are almost neglected.14
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mers, especially under ambient conditions, is to introduce elecꢀ
BDOPV displayed clearly four signals in aromatic region, agreeꢀ
tronꢀwithdrawing groups to lower their LUMO levels.20
ing well with its chemical structure and chemical environment.
The optimized structure of BDOPV displayed almost planar
backbone with small dihedral angles about 7.6º (Figure 2d). The
C–Ha–O and C–Hb–O angles are approximately linear and the C–
O bond distances (ca. 2.8 Å) are shorter than the combined van
der Waals radii (3.3 Å).24 These results indicate that four intramoꢀ
lecular hydrogen bonds are formed,25 which may contribute to the
planarity and shapeꢀpersistency of BDOPV.
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Herein, we try to tackle these problems in traditional PPVs by
synthesizing a novel PPV derivative, benzodifurandioneꢀbased
PPV (BDPPV) (Figure 1c). Firstly, strong electronꢀwithdrawing
carbonyl groups are introduced on the double bonds of PPV to
lower its LUMO level. Secondly, the carbonyl groups can form
intramolecular hydrogen bonds to prevent the conformational
transformation of the double bonds, affording a giant “locked”
aromatic plane. The enlarged aromatic backbone of BDPPV may
also contribute to the overlaps of intermolecular frontier orbitals,
thus facilitating interchain transport. Thirdly, 4ꢀoctadecyldocosyl
groups are used as side chains because we have demonstrated that
farther branching point of side chains can enhance interchain πꢀπ
stacking and thereby improve carrier mobilities.21 Using BDPPV
as active layer, FETs with electron mobilities up to 1.1 cm2 V–1 s–
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are obtained under ambient conditions, 4 orders of magnitude
higher than those of other PPVs! To our knowledge, this result is
among the highest in nꢀtype polymer FETs, and BDPPV repreꢀ
sents the first polymer that can transport electron over 1 cm2 V–1
s
–1 under ambient conditions.22
Scheme 1. Synthetic Approach to polymer BDPPVa
1
Figure 2. H NMR spectra of (a) 3, (b) 5, and (c) BDOPV in
CDCl3 (*: solvent peak). (d) The optimized molecular structure of
BDOPV (B3LYP/6ꢀ311+G(d,p)).
Under UV light irradiation (365 nm), the double bonds in PPVs
usually undergo trans to cis conformational change.18 Figure S3
displays the spectra change of a commercially available model
compound 1,4ꢀbis((E)ꢀ2ꢀmethylstyryl)benzene under UV irradiaꢀ
tion. Clearly, the compound showed decreased longꢀwavelength
and increased shortꢀwavelength absorption, indicating a conforꢀ
mational change from trans to cis happened under UV irradiation.
In contrast, BDOPV showed no spectra change under UV or visiꢀ
ble light irradiation, which may largely due to its “locked skeleꢀ
ton”. More importantly, both the solution and the film of polymer
BDPPV showed excellent stability under UV or visible light exꢀ
posure. Although one third of the double bonds in BDPPV are not
locked by intramolecular hydrogen bonds, conformational change
in BDPPV was not observed. This phenomenon might be attributꢀ
ed to largely conjugated BDOPV core, which has strong tendency
to undergo intermolecular stacking, thus creating large steric hinꢀ
drance to prevent the double bond adopting cisꢀconformation in
BDPPV.
Absorption spectra of BDOPV in solution and polymer
BDPPV in solution, thin film and annealed film are shown in
Figure 3. BDOPV displays two absorption peaks with an onset at
700 nm and a bandgap of 1.80 eV. After polymerization, the new
double bond in BDPPV further extends the conjugation length,
leading to strong nearꢀinfrared absorption with a bandgap of 1.42
eV. The main absorption peak of BDPPV in film only showed a
little redꢀshift compared to those in solution, and an obvious inꢀ
crease of the intensity of 0–0 vibrational peak was observed. Anꢀ
nealing the film led to a further increase of the intensity of 0–0
peak. These results indicate that the polymer backbone became
more planar in film and annealing was helpful for further adjustꢀ
a Reagents and conditions: (a) K2CO3, THF/DMF, 50 oC, 80%; (b)
Ac2O, toluene, 100 oC, 94%; (c) LTMP, THF, –78 oC, 30 min; (d)
3, THF,
0
oC,
2
h, two steps: 67%; (e) (E)ꢀ1,2ꢀ
bis(tributylstannyl)ethene, Pd2(dba)3, P(oꢀtol)3, chlorobenzene,
microwave, 83%.
Scheme 1 illustrates the synthetic route to polymer BDPPV.
The synthesis started from the alkylation of commercially
available 6ꢀbromoisatin (1), giving 3 in 80% yield. 2,2'ꢀ(2,5ꢀ
Dihydroxyꢀ1,4ꢀphenylene)diacetic acid (4) was synthesized from
1,4ꢀbenzoquinone in over 10ꢀgram scale.23 4 was subjected to
dehydration reaction using acetic anhydride to afford dilactone 5.
Lithium 2,2,6,6ꢀtetramethylpiperidine (LTMP) was used to
generate intermediate 6 and react with 3, providing the desired
benzodifurandioneꢀbased oligo(pꢀphenylene vinylene) (BDOPV)
in modorate yield. A Stille coupling polymerization between
BDOPV and (E)ꢀ1,2ꢀbis(tributylstannyl)ethene gave polymer
BDPPV in 83% yield. The molecular weight of BDPPV was
evaluated by high temperature gel permeation chromatography at
140 oC using 1,2,4ꢀtricholorobenzene as eluent. The polymer
displayed high molecular weight with Mn of 37.6 kDa and PDI of
2.38. The large PDI of BDPPV may be caused by its strong
aggregation tendency. The polymer showed excellent thermal
stability with decomposition temperature over 390 oC.
Figure 2 compares the 1H NMR spectra of compounds 3, 5, and
BDOPV. After the basic condensation reaction, the signal of Ha
of compound 3 obviously shifted from 7.46 to 8.92 ppm (Hh in
BDOPV). The signal of methylene (Hf) in reactant 5 disappeared,
and another signal of He on benzene ring significantly shifted
from 7.07 to 9.09 ppm (Ha in BDOPV). The 1H NMR spectrum of
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ment. For conjugated polymers, intermolecular stacking and conꢀ
formational change may significantly shift the absorption specꢀ
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tra,26 which is common for most PPVs.19 However, no significant
change was observed for BDPPV from solution to film. The abꢀ
sence of bathochromic effects indicates that the polymers probaꢀ
bly formed some preꢀaggregates in solution due to strong intermoꢀ
lecular interactions.27 In cyclic voltammetry (CV) measurement,
BDOPV displayed low HOMO/LUMO level of –6.21/–4.24 eV.
Compared with the HOMO/LUMO level of PPV (–5.2/–2.7 eV),9
BDPPV exhibited significantly lowered HOMO/LUMO level (–
6.12/–4.10 eV), which is also consistent with the HOMO/LUMO
level (–5.83/–4.41 eV) estimated from the photoelectron spectrosꢀ
copy (PES) and optical bandgap. Note that this LUMO level is
also lower than those of several ambientꢀstable nꢀtype polymers,
including NDI and PDI based polymers.28 Computational analysis
of the oligomer of BDPPV showed that the HOMO and LUMO
are well delocalized along the polymer backbone, and the HOMO
and LUMO distributions of BDPPV are similar to those of PPV,
due to their identical alternative phenylene vinylene skeletons
(Figure S5).
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Figure 4. (a) 2DꢀGIXD pattern and (b) AFM height images of
polymer BDPPV film prepared by spinꢀcoating its DCB solution
(3 mg/mL); (c) Transfer and (d) output characteristics of a
BDPPV device under ambient conditions (L = 10 ꢁm, W = 200
ꢁm, Ci = 3.7 nF cmꢀ2), measured VT = 5 V.
The lowꢀlying LUMO of BDPPV indicates that it could be
suitable for ambientꢀstable electron transport. Topꢀgate/bottomꢀ
contact (TG/BC) device configuration was used to fabricate polꢀ
ymer FETs, because this device configuration has better injection
characteristics and encapsulation effect.29 The semiconducting
layer was deposited by spinꢀcoating a polymer solution (3 mg/mL
in 1,2ꢀdichlorobenzene) on patterned Au/SiO2/Si substrate. After
thermal annealing the film for 10 min, a CYTOPTM solution was
spinꢀcoated as the dielectric layer, and an aluminum layer was
thermally evaporated as the gate electrode. FET devices of
BDPPV showed clearly nꢀtype transport characteristics under
ambient conditions (Figure 4c and 4d). Several annealing temperꢀ
Figure 3. (a) Normalized absorption spectra of BDOPV in CHCl3
(10–5 M, dashed line) and BDPPV in CHCl3 (10–5 M), in film and
in annealed film. (b) Cyclic voltammograms of BDOPV in DCB
solution (5 mg/mL) and BDPPV in dropꢀcasted film.
o
atures were tried, and annealing at 200 C gave the best device
performance. The polymer showed the highest electron mobilities
up to 1.1 cm2 V–1 s–1 with an average mobility of 0.84 cm2 V–1 s–1.
This result represents the first polymer FET that can transport
electrons over 1 cm2 V–1 s–1 under ambient conditions. Note that
the transfer and output characteristics showed negligible hystereꢀ
sis at high gate voltages, indicating that there are only a few traps
for electrons, presumably due to the significantly lowered LUMO
level. No contact resistance is observed in output curves, indicatꢀ
ing the ohmic contact between the polymer and gold electrode.
Furthermore, timeꢀdependent decay of the BDPPV device was
also performed by storing the devices under room light and ambiꢀ
ent conditions. The device could function effectively after being
stored for 30 days, affording an electron mobility of 0.31 cm2 V–1
s–1 (Figure S7), comparable to those of other highꢀperformance nꢀ
type polymer FETs.22 The slow degradation of the device was
probably caused by the further diffusion of oxygen and water into
the film.30 However, we believe that polymers with lower LUMO
levels may have better device performance and stability.
PPVs generally form amorphous films with weak Xꢀray scatterꢀ
ing, due to their weak interchain interactions and disordered packꢀ
ing.19 To investigate the molecular packing of BDPPV in thin
film, grazingꢀincident Xꢀray diffraction (GIXD) and tappingꢀ
mode atomic force microscopy (AFM) were used (Figure 4a and
4b). The polymer film of BDPPV showed a strong outꢀofꢀplane
o
diffraction peak (100) at 2θ of 2.20 , corresponding to a dꢀ
spacing of 32.3 Å (λ = 1.24 Å). Another three diffraction peaks
corresponding to (h00) were also observed, suggesting that a good
lamellar edgeꢀon packing was formed in the film. An inꢀplane
(010) peak attributed to the interchain πꢀπ stacking was also obꢀ
served with a distance of 3.55 Å, comparable to those of other
semiconducting polymers. The AFM height image of BDPPV
showed crystalline fiberꢀlike intercalating networks with a rootꢀ
meanꢀsquare (RMS) deviation of 1.30 nm. Such crystalline netꢀ
works are presumably caused by strong interchain interactions.
Unlike other PPVs, BDPPV showed clearly much stronger aggreꢀ
gation tendency and more ordered packing. Thus, better interchain
carrier transport is expected.
In conclusion, we have demonstrated that molecular engineerꢀ
ing of PPV skeleton can overcome factors that limit the electron
mobility in PPVs, such as conformational disorder, weak interꢀ
chain interactions, and high LUMO levels. The novel designed
electronꢀdeficient PPV derivative “locked” the polymer backbone
by intramolecular hydrogen bonds, endowing itself with excellent
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1859ꢀ1880; (c) Chen, H.; Guo, Y.; Yu, G.; Zhao, Y.; Zhang, J.; Gao, D.;
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UV and visible light stability, stronger aggregation tendency,
better crystallinity, lower LUMO level, and above all higher elecꢀ
tron mobilities up to 1.1 cm2 V–1 s–1 under ambient conditions.
Furthermore, FET devices based on BDPPV also showed good
ambient stability for at least 30 days. Our molecular engineering
strategy effectively avoids the defects of other PPVs and increase
the electron mobility of PPVs from 10–4 cm2 V–1 s–1 to over 1 cm2
V–1 s–1, which may trigger the renaissance of the secondꢀ
generation polymer. Through further molecular modulation, the
electronꢀdeficient BDOPV and its derivatives could find broad
applications for organic electronics requiring efficient electron
transport, such as ambipolar FETs and acceptors in organic solar
cells.
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ASSOCIATED CONTENT
Supporting Information
Experimental details and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*jianpei@pku.edu.cn (J. Pei); xcao@xmu.edu.cn (X.ꢀY. Cao );
jieyuwang@pku.edu.cn (J.ꢀY. Wang)
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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Soc. 2012, 134, 14877ꢀ14889.
This work was supported by the Major State Basic Research Deꢀ
velopment Program (Nos. 2009CB623601 and 2013CB933501)
from the Ministry of Science and Technology, and National Natuꢀ
ral Science Foundation of China. The authors thank beamline
BL14B1 (Shanghai Synchrotron Radiation Facility) for providing
the beam time.
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