Synthesis and properties of air-stable n-channel semiconductors based on MEH-PPV derivatives containing benzo[c]cinnoline moieties

Article abstract of DOI:10.1039/c3tc32505a

In this study we synthesized three novel poly[2-methoxy-5-(2-ethylhexyloxy) -1,4-phenylenevinylene] (MEH-PPV) derivatives that differ in terms of their ratios of heterocyclic benzo[c]cinnoline moieties: P50 containing only MEH-PPV/benzo[c]cinnolinevinylene (BZCV) alternating segments and P25 and P10 containing both MEH-PPV/BZCV alternating segments and MEH-PPV block segments. UV-Vis and PL spectra of these polymers revealed values of λUVmax and λPLmax that ranged from 440 to 489 and from 492 to 551 nm, respectively; these wavelengths blue-shifted upon incorporating additional benzo[c]cinnoline moieties. We also observed energy transfer from the MEH-PPV/BZCV alternating segments to the MEH-PPV block segments. P50, which featured an alternating structure in its main chain, displayed a solvatochromic effect, emitting green light in non-polar solvents and yellow light in polar solvents. The HOMO and LUMO energy levels of these polymers, measured using cyclic voltammetry, ranged from -5.11 to -5.62 and from -3.08 to -3.31 eV, respectively. The incorporation of electron-withdrawing benzo[c]cinnoline moieties enhanced the electron affinity and improved the oxidative stability of the polymers. A bottom-gate, top-contact organic field-effect transistor (OFET) based on P50 exhibited n-channel behavior under ambient conditions, with an electron mobility of 7.8 × 10^-3 cm² V^-1 s^-1 and an on/off ratio greater than 10⁴. No significant variation in electron mobility can be observed after this OEFT was stored under ambient conditions up to 30 days. To the best of our knowledge, this is the first time that air-stable n-channel MEH-PPV derivatives have ever been reported. It indicated that p-type MEH-PPV can be transformed into n-type materials upon incorporation of benzo[c]cinnoline moieties. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3tc32505a

Journal of
Materials Chemistry C
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Synthesis and properties of air-stable n-channel
semiconductors based on MEH-PPV derivatives
containing benzo[c]cinnoline moieties†
a
Cite this: DOI: 10.1039/c3tc32505a
Jyh-Chien Chen, Hsin-Chung Wu,a Chi-Jui Chiang,a Tuo Chena and Li Xingb
*
In this study we synthesized three novel poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]
(MEH-PPV) derivatives that differ in terms of their ratios of heterocyclic benzo[c]cinnoline moieties: P50
containing only MEH-PPV/benzo[c]cinnolinevinylene (BZCV) alternating segments and P25 and P10
containing both MEH-PPV/BZCV alternating segments and MEH-PPV block segments. UV-Vis and PL
UV
max
PL
max
spectra of these polymers revealed values of l
and l
that ranged from 440 to 489 and from 492
to 551 nm, respectively; these wavelengths blue-shifted upon incorporating additional benzo[c]cinnoline
moieties. We also observed energy transfer from the MEH-PPV/BZCV alternating segments to the MEH-
PPV block segments. P50, which featured an alternating structure in its main chain, displayed a
solvatochromic effect, emitting green light in non-polar solvents and yellow light in polar solvents. The
HOMO and LUMO energy levels of these polymers, measured using cyclic voltammetry, ranged from
ꢀ5.11 to ꢀ5.62 and from ꢀ3.08 to ꢀ3.31 eV, respectively. The incorporation of electron-withdrawing
benzo[c]cinnoline moieties enhanced the electron affinity and improved the oxidative stability of the
polymers. A bottom-gate, top-contact organic field-effect transistor (OFET) based on P50 exhibited n-
channel behavior under ambient conditions, with an electron mobility of 7.8 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1 and an
on/off ratio greater than 104. No significant variation in electron mobility can be observed after this OEFT
was stored under ambient conditions up to 30 days. To the best of our knowledge, this is the first time
that air-stable n-channel MEH-PPV derivatives have ever been reported. It indicated that p-type MEH-
PPV can be transformed into n-type materials upon incorporation of benzo[c]cinnoline moieties.
Received 18th December 2013
Accepted 1st April 2014
DOI: 10.1039/c3tc32505a
www.rsc.org/MaterialsC
mobility of greater than 1 cm2 Vꢀ1 sꢀ1
,
with some even
13–17
Introduction
exceeding 8 cm2 Vꢀ1 sꢀ1
.
Although progress has also been
17
made in the development of n-type electron-transporting
conjugated polymers to enhance the performance of n-channel
OFETs,4–11,18 air-stable n-type conjugated polymers exhibiting
high electron mobility remain rare. In most cases, their electron
mobility must be measured in a dry box.
Organic conjugated polymers exhibit unique optoelectronic
properties that make them applicable in organic light-emitting
diodes (OLEDs), organic photovoltaic cells (OPCs), and organic
eld-effect transistors (OFETs).1–11 Unlike traditional inorganic
materials, the structures of organic conjugated polymers are
readily tunable for various purposes through chemical reac-
tions. They also offer attractive advantages such as exibility,
light weight, inexpensive preparation, and solution-process-
ability. Since the rst report of a p-channel OFET based on a
polythiophene lm grown electrochemically in 1986,11,12
impressive advances have been made in enhancing the hole
mobility through developments in both chemical structures
and fabrication techniques. Several materials exhibit a hole
Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene]
(MEH-PPV) is a p-type conjugated polymer possessing a low
ionization potential. Electron-withdrawing groups have been
incorporated into the backbones or side chains of MEH-PPVs
with the goal of enhancing the electron affinity, the electron
injection/transporting ability, and the corresponding OLED
performance.19–24 In addition, intra- or intermolecular charge
transfer, energy transfer, lower-lying energy levels for the
highest occupied molecular orbitals (HOMO) and the lowest
unoccupied molecular orbitals (LUMO), and changes in emis-
sive colors have also been observed for various MEH-PPV
derivatives.19–24 The hole mobility of MEH-PPVs has been
investigated using various methods, including the space-charge
limited current (SCLC), the time of ight (TOF), and OFETs.25–30
OFETs based on pristine MEH-PPV can exhibit ambipolar
properties under a N2 atmosphere when MEH-PPV has been
aDepartment of Materials Science and Engineering, National Taiwan University of
Science and Technology, No. 43 Sec. 4 Keelung Road, Taipei, 10607, Taiwan.
E-mail: jcchen@mail.ntust.edu.tw; Fax: +886-2-27376544; Tel: +886-2-27376526
bPzer Worldwide Research and Development, 200 Cambridge Park Drive, Cambridge,
MA 02140, USA
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc32505a
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annealed, a crosslinked polymer has been used as the dielectric, NMR) nuclear magnetic resonance spectra were recorded at
and calcium has been used as the contact electrodes.31 Never- 500 and 125 MHz, respectively, on a Bruker Avance-500
theless, several following examples of OFETs based on electron- spectrometer. Infrared spectra were obtained with a Digilab-
rich MEH-PPVs, or MEH-PPV derivatives containing electron- FTS1000 FTIR. Mass spectroscopy was conducted using a
withdrawing 1,3,4-oxadiazole moieties, have exhibited only Finnigan TSQ 700 mass spectrometer. Molecular weights
p-channel characteristics, with hole mobility in the range from were measured on a JASCO gel permeation chromatography
10ꢀ4 to 10ꢀ5 cm2 Vꢀ1 sꢀ1
MEH-PPV based OFETs have ever been reported to exhibit n- and a Jordi Gel DVB mixed bed column (250 mm ꢁ 10 mm),
type characteristics under ambient conditions. using dimethylacetamide (DMAc) as the eluent, and cali-
.
32,33 To the best of our knowledge, no (GPC) system (PU-980) equipped with an RI detector (RI-930)
Benzo[c]cinnoline and its derivatives are planar, nitrogen- brated with polystyrene standards. Thermal gravimetric
containing heterocyclic compounds that can be physiologically analyses (TGA) were performed in nitrogen with a TA TGA
active.34 The synthesis of benzo[c]cinnoline typically involves Q500 thermogravimetric analyzer operated at a heating rate
reductive cyclization of 2,20-dinitrobiphenyl.35 Their optical of 10 C minꢀ1. Differential scanning calorimetry (DSC) was
ꢂ
properties are sensitive to solvent environments. For example, conducted under a N2 atmosphere using a PerkinElmer DSC
enhanced photoluminescence quantum yields and phospho- 4000 analyzer operated at a heating rate of 20 C minꢀ1. UV-
ꢂ
rescence have been observed in alcoholic solvents.36–38 It has Vis spectroscopy was performed using a JASCO V-670 UV-Vis/
also been calculated that benzo[c]cinnoline moieties can lower NIR spectrophotometer. Photoluminescence (PL) was
HOMO and LUMO energy levels more effectively than phenan- measured using a JASCO FP-8500 spectrouorometer. The PL
threne moieties.39 From a previous study of the electrochemical quantum yields (FPL) of the polymers in tetrahydrofuran
properties of poly(1,3,4-oxadiazole)s, we reported that benzo[c] (THF) were measured using 10ꢀ5 M quinine sulfate in 1 N
cinnoline can indeed lower the HOMO and LUMO energy levels H2SO4 as the reference standard (FPL ¼ 0.546). Cyclic vol-
of conjugated polymers.40
tammetry (CV) was conducted at room temperature using a
In this present study, we used the Horner–Wadsworth– CH Instrument 611C electrochemical analyzer and a three-
Emmons reaction to synthesize the MEH-PPV derivatives P50, electrode electrochemical cell comprising a working elec-
P25, and P10, which differ in terms of the ratios of benzo[c] trode (polymer lm coated on ITO glass), a reference elec-
cinnolinevinylene (BZCV) moieties. We recorded UV-Vis and trode [Ag/Ag+, referenced against ferrocene/ferrocenium (Fc/
photoluminescence spectra of these polymers to investigate the Fc+), 0.09 V], and a counter electrode (Pt gauze)-operated at a
energy transfer between the MEH-PPV block segments and the scan rate of 100 mV sꢀ1. CV of polymer lms was performed
MEH-PPV/BZCV alternating segments. We also employed using an electrolyte solution of 0.1 M tetrabutylammonium
solvents of different dielectric constants to examine sol- perchlorate (TBAP) in MeCN. The potential windows for the
vatochromism. We measured, through cyclic voltammetry, the oxidative and reductive scans were from 0 to +2.0 and from
effect of introducing the electron-withdrawing benzo[c]cinno- 0 to ꢀ2.0 V, respectively.
line moieties on the polymers' HOMO and LUMO energy levels.
Finally, to investigate the effects of the benzo[c]cinnoline
moieties on the carrier mobility, we fabricated OFETs and
measured their performance.
Synthesis of the monomer
3,8-Dicarbaldehydebenzo[c]cinnoline (4).
Experimental
Materials
4,40-Dibromo-2,20-dinitrobiphenyl (1), 3,8-dibromobenzo[c]cin-
noline (2), and 3,8-dicyanobenzo[c]cinnoline (3) were synthe-
sized according to our previous report.40 2-Methoxy-5-(2-
ethylhexyloxy)-1,4-phenyldialdehyde (5) and 2-methoxy-5-(2-
ethylhexyloxy)-1,4-bis(diethoxyphosphinyl-methyl)-benzene (6)
were prepared following literature procedures.41,42 Tetrabuty-
lammonium perchlorate (TBAP) used in cyclic voltammetric
measurements was recrystallized twice with ethyl acetate and
dried at 120 ꢂC under reduced pressure overnight. The other
reagents were purchased from commercial companies and used
as received. All of the solvents used in this study were puried
according to standard methods prior to use.
To a 250 mL, three-necked, round-bottomed ask equipped
with a condenser was added 3,8-dicyanobenzo[c]cinnoline (3)
(1.00 g, 4.35 mmol) in chlorobenzene (150 mL) under a
nitrogen atmosphere. Aer 9 mL (10.09 mmol) of diisobuty-
laluminium hydride (DIBAL-H) (20% in hexane) was slowly
added with a syringe, the reaction mixture was further stirred
at 0 ꢂC overnight. The reaction mixture was poured into a
vigorously stirred 300 mL solution of methanol–water (1 : 1, v/
v) with an ice bath for 30 min. Aer 100 mL of 3.0 M HCl
solution was added and stirred for 10 min, the reaction
mixture was extracted with ethyl acetate. The combined
organic phase was washed with water, dried over anhydrous
MgSO4. Aer the solvent was evaporated, the collected solid
was puried by column chromatography using ethyl acetate–
Measurements
Melting points were determined on a Mel-Temp capillary chloroform (2 : 8) as the eluent to afford 0.22 g (yield: 22%) of
melting point apparatus. Proton (1H NMR) and carbon (13
C
yellow powder. mp > 275 ꢂC (decompose). 1H NMR (500 MHz,
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DMSO-d6, d, ppm): 10.41 (s, 2H, aldehyde-Hd), 9.34 (d, J ¼ 1.5
Hz, 2H, Ar-Ha), 9.17 (d, J ¼ 8.5 Hz, 2H, Ar-Hc), 8.49 (dd, J1 ¼ 8.5
Hz, J2 ¼ 1.5 Hz, 2H, Ar-Hb). 13C NMR (125 MHz, DMSO-d6, d,
ppm): 192.54, 145.00, 137.54, 134.55, 129.50, 124.63, 123.80.
Anal. calcd: C, 71.18; H, 3.41; N, 11.86. Found: C, 70.65; H,
3.66; N, 11.51. EIMS (m/z): calcd for C14H8N2O2, 236.1; found,
236.1 [M+].
Synthesis of polymers
P50.
The polymer was synthesized following the same procedure as
P50 with compound 4 (0.0472 g, 0.2 mmol), compound 5
(0.2337 g, 0.8 mmol), and compound 6 (0.5363 g, 1.0 mmol) to
afford a red solid 0.3440 g (yield: 64%).
Fabrication of organic eld-effect transistors
OFET devices were fabricated with a bottom-gate, top-contact
conguration. A heavily doped n+-Si/SiO2 wafer was used as the
substrate, wherein n+-Si and thermally grown SiO2 functioned as
the gate electrode and dielectric, respectively. The wafer was
washed sequentially with acetone and isopropanol. The SiO2
dielectric surface was treated with a monolayer of n-octadecyl-
trichlorosilane (OTS) prior to spin-coating to prevent electron-
trapping by the OH groups. An anhydrous solution of P50 (1 mg
mLꢀ1) in N-methyl-2-pyrrolidinone (NMP) was spin-coated
(1000 rpm, 40 s) onto the modied SiO2/n+-Si substrate at room
temperature and then the sample was dried on a hot plate under
N2 in a glove box to afford a thin lm of P50 (ca. 50 nm). The
drain and source electrodes were deposited by thermally evap-
orating gold (Au) through a shadow mask. The channel width
(W) and length (L) were 2000 and 50 mm, respectively. An OFET
device based on MEH-PPV was also fabricated for comparison.
To a 50 mL, three-neck, round-bottom ask were added
3,8-dicarbaldehydebenzo[c]cinnoline 4 (0.2361 g, 1.0 mmol), 2-
methoxy-5-(2-ethylhexyloxy)-1,4-bis(diethoxyphosphinyl-methyl)-
benzene 6 (0.5363 g, 1.0 mmol), and 40 mL of dry DMF. 1.0 M t-
BuOK (6 mL, 6.0 mmol, in THF solution) was slowly added with
a syringe under a nitrogen atmosphere. The mixture was stirred
at room temperature for 48 h under N2 before being poured into
water. The precipitate was ltered off, washed with hot MeOH
(Soxhlet apparatus) for 12 h, and then dried at 100 ꢂC overnight
under reduced pressure to afford a deep-red solid 0.2118 g
(yield: 43%).
Results and discussion
Synthesis of the monomer
P25.
We prepared the monomer 3,8-dicarbaldehydebenzo[c]cinnoline
(4) through a four-step synthesis (Scheme 1). 4,40-Dibromo-2,20-
dinitrobiphenyl (1) was prepared from 2,5-dibromonitrobenzene
through an Ullmann coupling reaction; it was then reduced by
LiAlH4 to form heterocyclic 3,8-dibromobenzo[c]cinnoline (2).
We obtained 3,8-dicarbaldehydebenzo[c]cinnoline (4) aer con-
verting the bromo groups of 2 to cyano groups40 and then
reducing them in the presence of DIBAL-H. Fig. S1 in the ESI†
1
displays the H NMR spectrum of 4. The signal at 10.41 ppm
indicates that the cyano groups had been successfully converted
into aldehyde groups. The 1H NMR spectrum combined with the
results from mass spectroscopy and elemental analysis conrms
the formation of highly pure 3,8-dicarbaldehydebenzo[c]cinno-
line (4). Compounds containing dialdehyde groups might be
The polymer was synthesized following the same procedure as utilized for polymerization of conjugated polymers containing
P50 with compound 4 (0.1180 g, 0.5 mmol), compound 5 carbon–carbon double bond or carbon–nitrogen double bond
(0.1461 g, 0.5 mmol), and compound 6 (0.5363 g, 1.0 mmol) to linkages. In this research, a series of polymers containing
afford a red solid 0.2291 g (yield: 44%).
carbon–carbon double bond linkages were synthesized by the
Horner–Wadsworth–Emmons reaction.
P10.
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Scheme 1 Synthesis route to 3,8-dicarbaldehydebenzo[c]cinnoline (4).
UV
The maximum absorption wavelengths (l ) appeared in the
Synthesis of polymers
max
PL
range from 440 to 489 nm, with the emission peaks (l
)
max
Scheme 2 presents our approach for the synthesis of the
conjugated polymers P50, P25, and P10 through Horner–
Wadsworth–Emmons reactions of compounds 4, 5, and 6 in
ratios of 1 : 0 : 1, 0.5 : 0.5 : 1, and 0.2 : 0.8 : 1, respectively. The
number-average molecular weights (Mn) of P50, P25, and P10
were all greater than 14 000 g molꢀ1, as measured through GPC
using DMAc as the eluent (Table 1). These conjugated polymers
contain both 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylvinylene
(MEH-PPV) and benzo[c]cinnolinevinylene (BZCV) segments.
Based on the monomer feed ratios, P50 is a complete alter-
nating polymer of MEH-PPV and BZCV segments, while P25 and
P10 contain both MEH-PPV/BZCV alternating segments and
MEH-PPV block segments.
ranging from 492 to 551 nm. The shoulders evident in the PL
spectra indicate the vibronic features of these polymers.43–45 The
absorption wavelengths and emission peaks underwent blue-
shi upon increasing the content of BZCV segments in the
UV
conjugated copolymers. The maximum absorption (l ) and
max
PL
max
emission (l ) of the P50 solution (in THF) appeared at 440
and 492 nm, respectively. Because P50 is a completely alter-
nating conjugated polymer, consisting of MEH-PPV and BZCV
segments, we can consider these two peaks as indicators of the
absorption and emission from the MEH-PPV/BZCV alternating
segments. On the other hand, we consider those peaks
(absorption at 497 nm, emission at 554 nm) for the pristine
MEH-PPV polymer to be indicators of the presence of the MEH-
PPV block segments. We also characterized the polymers in
terms of their optical properties in the solid state [Fig. 1(b) and
Table 3]. The absorption edges had red-shied to 700 nm or
beyond, relative to those of the solution samples. Thus, the
effective conjugation lengths of the polymers in the form of
their solid lms were longer than those in solution. The emis-
sion peaks were also red-shied within the range from 584 to
604 nm.
Conjugated polymers with donor–acceptor systems oen
exhibit red-shied absorption wavelengths and lower band gaps
as a result of intramolecular (or intermolecular) charge transfer
(ICT) between their electron-withdrawing and electron-
donating moieties.2 ICT is an important effect that can be used
to harvest more light energy and generate excitons more effi-
ciently in photovoltaic applications. Although this effect can be
related to planarity in the main chain or to the formation of
quinoid structures,46 such behavior is not always observed in
every donor–acceptor conjugated system. In this present study,
for example, we could not detect any donor–acceptor ICT effect.
Incorporating electron-withdrawing BZCV moieties into MEH-
PPV did not lead to red-shied absorption wavelengths or lower
band gaps. Likewise, the ICT effect was not observed either in
other MEH-PPV systems containing various electron-with-
drawing moieties.19,20,22,24,47 It is possible that MEH-PPV
segments are not appropriate donors for the ICT effect. In
addition, because most donor–acceptor conjugated polymers
contain thiophene moieties (or their derivatives) as their
donors, it is the thiophene–acceptor–thiophene structures that
Assuming that all of the monomers were consumed, the
theoretical ratios of the BZCV and MEH-PPV segments in P25 and
P10 should be 25 : 75 and 10 : 90, respectively. The actual
contents of BZCV segments in the main chains of P25 and P10,
determined using elemental analysis, were, however, 20 and 6%,
respectively, presumably because of the low yields (43–64%) of
these polymerization processes. This nding also suggests that
the reactivity of 3,8-dicarbaldehydebenzo[c]cinnoline (4) is lower
than that of 2-methoxy-5-(2-ethylhexyloxy)-1,4-phenyldialdehyde
1
(5). The H and 13C NMR spectra of P10 are shown in the ESI
(Fig. S2 and S3).† Although we did not observe (DSC) a glass
ꢂ
transition for P50 at temperatures of up to 350 C, P25 and P10
underwent glass transitions at 148 and 117 ꢂC, respectively. These
polymers also exhibited good thermal stability, with the decom-
position temperatures at 5% weight loss (Td) all higher than 340
ꢂC (Table 1). In the chosen solvents [NMP, DMF, DMAc, 1,2-
dichlorobenzene (DCB), chloroform (CF), and THF], we deter-
mined the qualitative solubilities (Table 2) using 0.001 g of
polymer in 1 mL of the solvent. P50 was soluble upon heating in
NMP, but only partially soluble in other tested solvents. P10 was
soluble in all solvents. The polymers became less soluble upon
increasing the content of BZCV, presumably because the planarity
and rigidity of the benzo[c]cinnoline moieties induced closer
interchain packing and stronger intermolecular interactions.
Optical properties
Fig. 1(a) presents UV-Vis and PL spectra of P50, P25, P10, and
MEH-PPV (each 10ꢀ5 M in THF); Table 3 summarizes the data.
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Scheme 2 Synthesis route to polymers.
Table 2 Solubility of polymersa
generate charge transfer most effectively.48–52 In some cases, the
absorption edge can extend into the near-infrared region.50–52
We hypothesize that polymerization of electron-withdrawing
benzo[c]cinnoline (BZC) moieties with thiophene derivatives, or
the synthesis of thiophene–BZC–thiophene structures, might
induce ICT effects; accordingly, we are currently investigating
the development of BZC-containing thiophene-based conju-
gated polymers for photovoltaic applications.
Solventb
Polymer
NMP
DMAc
DMF
DCB
CF
THF
P50
P25
P10
+
++
++
ꢃ
ꢃ
+
++
ꢃ
ꢃ
ꢃ
++
++
ꢃ
ꢃ
ꢃ
++
++
++
a
Abbreviations:
NMP,
DMAc,
N-methyl-2-pyrrolidinone;
dimethylacetamide; DCB,
DMF,
1,2-
It has been investigated that conjugated polymers with two
different segments showed dual emissions from these two
dimethylformamide;
b
dichlorobenzene; CF, chloroform; THF, tetrahydrofuran. Qualitative
segments separately.53,54 Fig. 1(a) reveals that P25 exhibited two solubility was determined using 0.001 g of polymer in 1 mL of
solvent. ++ (soluble at room temperature); + (soluble on heating at 60
emission peaks at 549 and 500 nm. From a comparison with the
emission peaks of pristine MEH-PPV and P50, we attribute
ꢂC); and ꢃ (partially soluble on heating at 60 ꢂC).
these two emissions as arising from the MEH-PPV block
segments (549 nm) and the MEH-PPV/BZCV alternating
segments (500 nm). In addition, we observe a spectral overlap
suspect that the emission peak at 549 nm, which we attribute to
the emission of the MEH-PPV block segments, resulted from the
between the absorption of MEH-PPV and the emission of P50. It
MEH-PPV block segments directly absorbing the excitation
is speculated that the emissive energy from the MEH-PPV/BZCV
radiation at 440 nm. It is also possible that the MEH-PPV block
alternating segments might be transferred to the MEH-PPV
segments received the energy transferred from the emission of
the MEH-PPV/BZCV alternating segments, because of the over-
lap between the emission peak of the MEH-PPV/BZCV
block segments.55,56 Thus, we used light at the value of lmUVax of
P50 (440 nm) to excite P25 and P10 in THF; Fig. 2(a) reveals a
major emission peak at 549 nm and a shoulder at 490 nm. We
Table 1 Molecular weights and thermal properties of polymers
Polymer
Mn
PDIa
Yield (%)
BZCVb (%) actual
BZCVc (%) theoretical
Char yieldf (wt%)
a
d
Td (ꢂC)
e
Tg (ꢂC)
g
P50
P25
P10
54 000
14 000
30 000
1.51
1.64
1.98
43
44
64
50
20
6
50
25
10
372
363
348
—
57
32
20
148
117
a
b
Obtained from GPC using DMAc as solvent and calibrated with polystyrene standards. Actual percentage of BZCV, calculated by elemental
analysis. Theoretical percentage of BZCV, calculated from feed ratio. Measured by TGA at a heating rate of 10 ꢂC minꢀ1 in nitrogen.
c
d
e
Measured by DSC at a heating rate of 20 ꢂC minꢀ1 in nitrogen. f Residual weight percentage at 800 ꢂC in nitrogen. g Not observed.
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block segments was too low to absorb the emission energy
completely.
In the case of P10, we do not observe the emission shoulder
at 490 nm in Fig. 2(a), presumably because of the low concen-
tration of the MEH-PPV/BZCV alternating segments relative to
the MEH-PPV block segments.
The optical properties of small molecules and polymers
containing electron-donating and electron-withdrawing
groups can be sensitive to the solvent polarity, that is, they
display solvatochromism, as a result of intramolecular charge
transfer.24,58–62 In this study, we investigated the sol-
vatochromism of P50, which consists of an alternating struc-
ture, with the dielectric constants of the solvents used as an
index of polarity. Fig. 3(a) displays UV-Vis and PL spectra of
P50 in the non-polar solvents THF and DCB (dielectric
constants: <15) and in the polar solvents DMAc and NMP
(dielectric constants: >15). The absorption spectra of P50 in
these different solvents are similar, with no perceptible red-
shis of the peak maximum. In contrast, the PL emission
spectra reveal not only red-shied peaks but also gradually
broadening curves upon increasing the solvent polarity. In
addition, Fig. 3(b) reveals that P50 in the different solvents
emitted light of different colors aer excitation at 365 nm:
green in THF and DCB and yellow in DMAc and NMP. We
attribute this positive uorosolvatochromism to the higher
dipole moment in the excited state than in the ground state,
due to photoexcited charge transfer. Thus, the excited state
would be stabilized more in polar solvents, leading to red-
shied peaks in the PL emission spectra.63–67
In general, molecules having larger dipole moments in the
excited state than in the ground state also exhibit more red-
shied absorptions upon increasing the solvent polarity. In this
study and in some other cases,68 however, only the signals in the
PL spectra were red-shied. This phenomenon might be
explained by differences in the times required for absorption,
emission, and molecular reorientation in the solvents. The
absorption of light by molecules occurs within a period of
approximately 10ꢀ15 s. It takes approximately 10ꢀ10 s for
solvents to reorient. Thus, no red-shi in absorption spectra
would be observed if a solvent did not have sufficient time to
reorient. In contrast, the uorescence lifetime typically ranges
from 10ꢀ8 to 10ꢀ9 s, much longer than the time required for
reorientation. Such molecules in the excited state will be
exposed to the reoriented solvents and, therefore, the energy of
the excited state will be lower as a result of stabilization
provided by the reoriented solvents, leading to red-shis in PL
spectra.68
Fig. 1 UV-Vis absorption and PL emission spectra (upon excitation by
l
UV
max
radiation) of the tested polymers (a) in THF at 10ꢀ5 M and (b) in the
solid state.
Table 3 Optical properties of polymers
Solutiona (nm)
Film
UV
max
PL
max
UV
max
UV
onset
PL
Polymer
l
l
FWHM FPLb (%)
l
l
l
max
P50
P25
P10
440
476
489
492
549
551
554
72
59
40
40
23
18
18
25
428
469
510
511
550
610
610
600
584
604
604
600
MEH-PPV 497
a
b
Measured in THF at a concentration of 10ꢀ5 M. Estimated using
quinine sulfate (dissolved in 1 N H2SO4(aq), assuming a value of FPL of
0.546) as the standard.
To further investigate the solvatochromism of P50 in polar
alternating segments and the absorption peak of the MEH-PPV solvents, we tested its behavior in mixed solvents of NMP and
block segments. Fig. 2(b) presents our proposed mechanism.57 water at volume ratios from 5 : 1 to 1 : 1. We chose water for this
The emission shoulder peak of P25 at 490 nm in Fig. 2(a) study because it has a very high dielectric constant (80.1;
suggests incomplete energy transfer between the MEH-PPV/ thereby allowing tuning of the solvent polarity upon mixing
BZCV alternating segments and the MEH-PPV block segments. with NMP) and because the optical properties of heterocyclic
We attribute this behavior to the short lifetime of the exciton of benzo[c]cinnoline rings are sensitive to solvents capable of
the MEH-PPV/BZCV alternating segments, leading to extinction hydrogen bonding37,38 (i.e., we suspected that hydrogen bonding
prior to absorption by the MEH-PPV block segments; it might between water and the benzo[c]cinnoline ring might also
also be possible that the extinction coefficient of the MEH-PPV inuence the optical properties of P50).
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Fig. 2 (a) PL emission spectra of P50, P25, and P10 in THF at 10ꢀ5
M
Fig. 3 (a) UV-Vis absorption and PL emission spectra of P50 in THF,
DCB, DMAc, and NMP at 10ꢀ5 M. (b) Colors emitted from dilute
solutions of P50 on exposure to light (365 nm) from a UV lamp.
excited by 440 nm radiation. (b) Proposed model of the excitation and
emission mechanism with intramolecular energy transfer between the
MEH-PPV/BZCV alternating segments and the MEH-PPV block
segments.
Electrochemical properties
We used CV to characterize the electrochemical redox behavior
of our polymers. Fig. S4† presents cyclic voltammograms of P50,
P25, and P10 in the form of thin lms. We observe irreversible
anodic and cathodic scans for each polymer. The onset poten-
tials for oxidation and reduction of these polymers ranged from
0.40 to 0.91 and from ꢀ1.4 to ꢀ1.63 eV, respectively. We
calculated the HOMO [ionization potential (IP)] and LUMO
[electron affinity (EA)] energy levels from the onset potentials of
oxidation and reduction, respectively. The absolute energy level
of ferrocene/ferrocenium (Fc/Fc+) is assumed to be 4.8 eV below
the vacuum level. The external Fc/Fc+ redox standard E1/2 was
determined to be 0.09 V versus Ag/Ag+ in acetonitrile by our
electrochemical measuring system.71 Table 5 summarizes the
CV data. The calculated HOMO and LUMO energy levels of P50,
P25, and P10 ranged from ꢀ5.62 to ꢀ5.11 and from ꢀ3.31 to
ꢀ3.08 eV, respectively. The energy band gaps calculated from
these electrochemical measurements ranged from 1.94 to
2.31 eV. Among these polymers, P50 had the lowest-lying HOMO
and LUMO energy levels (ꢀ5.62 and ꢀ3.31 eV, respectively).
In a previous study,40 we found that nitrogen-containing
heterocyclic benzo[c]cinnoline has good electron affinity;
accordingly, the introduction of benzo[c]cinnoline moieties into
Table 4 lists the dielectric constants we calculated for mixed
solvents containing NMP and water at various volume ratios.
The absorption spectra of P50 in these NMP–water mixtures
changed only slightly, similar to the behavior displayed in
Fig. 3(a). Fig. 4(a) presents PL spectra of P50 in the various
PL
max
NMP–water mixtures; we observe red-shiing of l
and peak
broadening upon increasing the dielectric constant and water
content of the mixed solvent. This effect might be also due to
hydrogen bonding between heterocyclic benzo[c]cinnoline and
water. Accordingly, we tested another mixed solvent system –
NMP and MeOH – to investigate the PL solvatochromism of P50.
We chose MeOH because it has a dielectric constant (32.7)
similar to that of NMP (31.7) while also allowing hydrogen
bonding with the benzo[c]cinnoline moieties. Fig. 4(b) displays
PL spectra of P50 in NMP/MeOH mixed solvents at volume
PL
max
ratios of 3 : 1 and 1 : 1. The values of l
and the peak shapes
in these PL spectra barely changed. Even though hydrogen
bonding existed in the NMP–MeOH mixtures, the dielectric
constants hardly changed at all. Thus, a comparison of Fig. 4(a)
and (b) allows us to conclude that the PL solvatochromism of
P50 resulted only from the solvent polarity and was indepen-
dent of hydrogen bonding.
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conjugated polymers would lower their HOMO and LUMO
energy levels. In this study, we found that P50, P25, and P10 all
had HOMO and LUMO energy levels (Table 5) lower than those
of MEH-PPV (EHOMO ¼ ꢀ5.07 eV; ELUMO ¼ ꢀ2.90 eV).72 In
addition, their HOMO and LUMO energy levels were more low-
lying upon increasing the content of benzo[c]cinnoline.
To provide further evidence for the observed electro-
chemical properties of these polymers, we performed theo-
retical calculations. The method was described in the ESI.†
Fig. 5 displays the calculated HOMO and LUMO contours of
MEH-PPV and P50. For MEH-PPV, the HOMO and LUMO
orbitals are localized mainly on the phenyl rings. For P50, the
electron density of the HOMO is localized mainly on the
phenylenevinylene segment, while that of the LUMO is local-
ized mainly on the BZCV segment. The simulation suggests
that the rst electron added to P50 is most likely received by
the electron-withdrawing benzo[c]cinnoline moieties and
adjacent vinylene bonds.
Several nitrogen-containing heterocyclic moieties have also
been incorporated into MEH-PPV to adjust the polymer's
HOMO and LUMO energy levels. For example, MEH-PPV
derivatives containing quinoxaline (PPV-1),20 triazole (PPV-2),47
and diketopyrrolopyrrole (PPV-3)73 with an alternating structure
have been reported. Table 6 summarizes their electrochemical
properties. The LUMO energy level of P50 (ELUMO ¼ ꢀ3.31 eV) is
comparable with those of PPV-2 (ELUMO ¼ ꢀ3.35 eV) and PPV-3
(ELUMO ¼ ꢀ3.36 eV), but lower than that of PPV-1 (ELUMO
¼
ꢀ2.63 eV). It indicates that the electron affinity of benzo[c]cin-
noline is stronger than that of quinoxaline (PPV-1) and
comparable with those of triazole (PPV-2) and diketopyrrolo-
Fig. 4 (a) PL emission spectra of P50 in NMP–H2O (volume ratios:
5 : 1, 4 : 1, 3 : 1, 2 : 1, and 1 : 1) at 10ꢀ5 M. (b) PL emission spectra of P50
in NMP/CH3OH (volume ratios: 3 : 1 and 1 : 1) at 10ꢀ5 M.
pyrrole (PPV-3). The HOMO energy level of P50 (EHOMO
¼
ꢀ5.61 eV) is the lowest among these reported MEH-PPV deriv-
atives containing nitrogen-containing heterocyclic rings. It
suggests that P50 would have good oxidative stability.
calculated the carrier mobility (m) from the saturation regime in
the transfer curves, according to the equation
Organic eld-effect transistor performance
IDS ¼ m(WC/2L)(VG ꢀ VT)2
The carrier mobility of polymers was measured by the OFET
method. OFET devices based on P50, P25, P10, and MEH-PPV
were fabricated in a bottom-gate, top-contact conguration
using gold as the source and drain electrodes, and heavily
doped n+-Si as the gate electrode. The dielectric layer was OTS-
treated SiO2. The spin-coated polymer lms were all annealing
free. All of the OFET measurements were performed under
ambient conditions. Fig. 6 displays the typical output and
transfer characteristics of the OFET based on P50. We
where W and L are the semiconductor channel width and
channel length, respectively. C (17 nF cmꢀ2) is the capacitance
of the gate dielectric layer. VT and VG are the threshold and gate
voltages, respectively. We determined the threshold voltage
1/2
from the linear relationship between IDS versus VG by extrap-
olating to IDS ¼ 0. Table 7 shows the carrier mobility, on/off
ratios, and threshold voltages of MEH-PPV and its derivatives.
Table 4 Optical properties of P50 in different solvents
Optical property THF (7.6)a DCB (9,9) DMAc (37.8) NMP (31.7) 5 : 1b (37.9)c 4 : 1 (39.23) 3 : 1 (41.24) 2 : 1 (44.74) 1 : 1 (52.31)
PL
l
492
23.26
496
4.75
500
2.99
516
9.42
545
0.57
560
0.46
562
0.35
577
0.25
583
0.16
max
FPLd (%)
a
b
c
1/3
Dielectric constant of the solvent. Volume ratio of NMP and water. Dielectric constant is calculated using the equation: 3mix ¼ [(3NMP
ꢀ
3water1/3)FNMP + 3water1/3]3, where 3NMP and 3water are the dielectric constants of NMP and water, respectively, and FNMP is the volume fraction of
d
NMP.69 Estimated using quinine sulfate (dissolved in 1 N H2SO4(aq), assuming a value of FPL of 0.546) as the standard. Refractive indices of
mixed solvents were calculated using the Lorentz–Lorenz equation.70
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Table 5 Electrochemical properties of polymers
ox
onset
red
onset
b
c
Polymer
E
a (V)
E
a (V)
EHOMO (eV)
ELUMO (eV)
Eegcd (eV)
Eogpte (eV)
P50
P25
0.91
0.52
0.40
—
ꢀ1.40
ꢀ1.63
ꢀ1.54
—
ꢀ5.62
ꢀ5.23
ꢀ5.11
ꢀ5.07
ꢀ3.31
ꢀ3.08
ꢀ3.17
ꢀ2.90
2.31
2.15
1.94
2.17
2.25
2.21
2.21
2.21
P10
MEH-PPVf
a
ox
red
ox
onset
red
Measured by cyclic voltammetry. b EHOMO ¼ ꢀ(E
+ 4.80 ꢀ 0.09) (eV). c ELUMO ¼ ꢀ(E
+ 4.80 ꢀ 0.09) (eV). d Egec ¼ IP(E
) ꢀ EA(E
) (eV).
onset
Ref. 72.
onset
onset
e
f
Calculated using the equation: 1240/lonset
.
those reported in the literature.27,30 When the gate voltage was
positive, the OFET based on MEH-PPV was inactive under
ambient conditions, consistent with the p-type nature of MEH-
PPV. The OFET based on P10 only exhibited p-channel charac-
teristic with a hole mobility of 7.7 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1. In
contrast, we could not determine the hole mobilities of P25 and
P50. The OFETs of these two polymers only exhibited n-channel
characteristics with electron mobilities of 1.6 ꢁ 10ꢀ3 and 7.8 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1, threshold voltages of 12 and 10 V, respec-
tively. Their on/off ratios were both greater than 104. These data
were measured in air without annealing. These results indicate
that p-type MEH-PPV can be transformed into n-type materials
by introducing an appropriate amount of electron-withdrawing
benzo[c]cinnoline moieties on the main chain. This can be
explained by the lower LUMO and HOMO energy levels resulted
from the presence of the heterocycle. The lower LUMO energy
level could facilitate electron injection, due to a lower energy
Fig. 5 Calculated HOMO and LUMO contours of MEH-PPV and P50.
The OFET based on MEH-PPV, with the above-mentioned
conguration, exhibited only p-channel characteristics, with a
hole mobility (2.5 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) on the same order as
Table 6 Electrochemical properties of MEH-PPV derivatives containing electron-withdrawing moieties
ox
onset
red
onset
E
E
Eegc
-Ar-
(V)
(V)
EHOMO (eV)
ELUMO (eV)
ꢀ3.31
(eV)
Ref.
0.91
ꢀ1.40
ꢀ5.62
2.31
2.65
This study
0.88
0.32
ꢀ1.77
ꢀ1.45
ꢀ5.28
ꢀ5.12
ꢀ2.63
ꢀ3.35
20
47
PPV-1
1.77
1.94
PPV-2
PPV-3
0.79
ꢀ1.18
ꢀ5.19
ꢀ3.36
73
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Table 7 Performance of OFETs based on polymersa
barrier between the gold electrodes and the polymers, thereby
facilitating electron mobility. In addition to enhanced air-
stability, the lower HOMO energy level decreased the hole-
transporting ability, such that P25 and P50 did not exhibit the
characteristics of p-type materials.
In most cases, n-type OFETs reported previously have dis-
played complete inactivity in air or when prepared without
annealing.4,74,75 To the best of our knowledge, this paper reports
the rst example of OFET devices based on n-channel MEH-PPV
derivatives that functions under ambient conditions. The elec-
tron mobility measured in air without annealing (1.6–7.8 ꢁ
10ꢀ3 cm2 Vꢀ1 sꢀ1) is at least two orders of magnitude higher
than the only comparable value (3 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1) reported
in the literature, although that system used calcium electrodes,
a crosslinked polymer as the dielectric, and was measured
under a N2 atmosphere.31
Polymer
mh (cm2 Vꢀ1 sꢀ1
)
me (cm2 Vꢀ1 sꢀ1
)
Ion/Ioff
VT (V)
b
P50
P25
P10
MEH-PPV
—
7.8 ꢁ 10ꢀ3
104
104
104
104
10
12
—
1.6 ꢁ 10ꢀ3
7.7 ꢁ 10ꢀ4
—
—
ꢀ5
2.5 ꢁ 10ꢀ4
ꢀ6
Measured under ambient conditions. b Not available.
a
In order to further investigate the long-term air-stability, the
performance of the OFET based on P50 was monitored during a
30 day period under ambient conditions. Fig. 7 shows the
electron mobility on 10, 20, and 30 days aer the device was
fabricated. The electron mobility barely changes aer 30 days
from 7.8 ꢁ 10ꢀ3 to 6.5 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1, which are in the
same order of magnitude without signicant variation. It indi-
cates that the OFET based on P50 can exhibit excellent air-
stability.
Fig. 7 Air-stability of the OFET based on P50.
Conclusions
We have successfully synthesized MEH-PPV derivatives con-
taining different amounts of benzo[c]cinnolinevinylene (BZCV)
segments on the main chain by the Horner–Wadsworth–
Emmons reaction. These new conjugated polymers exhibited
blue-shis in their UV-Vis and PL spectra. P50, with completely
alternating MEH-PPV/BZCV segments, exhibited PL sol-
vatochromism that resulted solely from solvent polarity. The
HOMO and LUMO energy levels were lower because of the
introduction of electron-withdrawing benzo[c]cinnoline moie-
ties. We also demonstrate that p-type MEH-PPV can be trans-
formed into n-type materials (P25 and P50) when electron-
withdrawing benzo[c]cinnoline moieties were introduced. The
OFET electron mobility (1.6–7.8 ꢁ 10ꢀ3 cm2 Vꢀ1 sꢀ1) measured
under ambient conditions without annealing is two orders of
magnitude higher than that of pristine MEH-PPV reported in
the literature. The OFET based on P50 also showed good air-
stability up to 30 days. To the best of our knowledge, this is the
rst time that n-channel MEH-PPV derivatives with excellent
air-stability have ever been reported.
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