Liquid-crystalline perylene diester polymers with tunable charge-carrier mobility

Article abstract of DOI:10.1002/adfm.201101694

New classes of liquid-crystalline semiconductor polymers based on perylene diester benzimidazole and perylene diester imide mesogens are reported. Two highly soluble side-chain polymers, poly(perylene diester benzimidazole acrylate) (PPDB) and poly(peryle

Full text of DOI:10.1002/adfm.201101694

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Liquid-Crystalline Perylene Diester Polymers
with Tunable Charge-Carrier Mobility
Mathis-Andreas Muth, Miguel Carrasco-Orozco,* and Mukundan Thelakkat*
to be overcome in designing novel n-type
polymer materials.[4] The successful inte-
gration of a naphthalenedicarboxyimide
based n-channel polymer into thin-film
New classes of liquid-crystalline semiconductor polymers based on perylene
diester benzimidazole and perylene diester imide mesogens are reported.
Two highly soluble side-chain polymers, poly(perylene diester benzimida-
zole acrylate) (PPDB) and poly(perylene diester imide acrylate) (PPDI) are
synthesized by nitroxide-mediated radical polymerization (NMRP). PPDB
shows n-type semiconductor performance with electron mobilities of 3.2 ×
10−4 cm2 V−1 s−1 obtained in a diode configuration by fitting the space-charge-
limited currents (SCLC) according to the Mott–Gurney equation. Interest-
ingly, PPDI performs preferentially as a p-type material with a hole mobility
of 1.5 × 10−4 cm2 V−1 s−1 , which is attributed to the less electron-deficient
perylene core of PPDI compared to PPDB. Optical properties are investigated
by UV-vis and fluorescence spectroscopy. The extended π-conjugation system
due to the benzimidazole unit of PPDB leads to a considerably broader
absorption in the visible region compared to PPDI. HOMO and LUMO levels
of the polymers are also determined by cyclic voltammetry; the resulting
energy band-gaps are 1.86 eV for PPDB and 2.16 eV for PPDI. Thermal
behavior and liquid crystallinity are studied by differential scanning calorim-
etry, polarized optical microscopy, and X-ray diffraction measurements. The
results indicate liquid-crystalline order of the polymers over a broad tempera-
ture range. These thermal, electrical, and optical properties make the perylene
side-chain polymers attractive materials for organic photovoltaics.
transistors, which leads to high electron
mobilities under ambient conditions, was
demonstrated recently.[5] In comparison,
in donor–acceptor bulk-heterojunction
solar cells, polymeric n-type materials
used as electron acceptors show signifi-
cantly lower performances than acceptors
based on small molecules such as phenyl-
C61-butyric acid methyl ester (PCBM).[6,7]
Polymers however, usually show better
film-forming properties than low mole-
cular weight compounds.[8] This property
is essential for solution processability,
illustrating the need for n-type polymers.
Perylene bisimides (PBI) are a relevant
class of n-type semiconductors due to
their relatively high electron affinity and
strong visible-light absorption, combined
with good photochemical and thermal sta-
bility.[9,10] Due to strong π–π interaction of
the perylene cores, electron mobilities of
0.1–1 cm2 V−1 s−1 for low molecular weight
PBIs and 1.2 × 10−3 cm2 V−1 s−1 for a solu-
tion processable PBI side-chain homo-
polymer in organic field-effect transistors were reported.[11,12]
It has been shown that, in terms of photoinduced charge gen-
eration, PBIs can be more effective electron acceptors than
PCBM.[13] Due to comparatively poor device performance, this
class of material has received limited attention in bulk hetero-
junction solar cells to date. The main issue is assumed to be an
unfavorable blend morphology due to the tendency of PBI to
form large crystals.[14] We reported recently on a series of highly
soluble, low-molecular-weight discotic-liquid-crystalline (LC)
PBI[15] and perylene diester benzimidazole (PDB) molecules.[16]
The latter exhibit mesophases even at room temperature, com-
bined with extended visible-light absorption compared to PBIs.
The self-organization of discotic LC materials can be exploited in
optoelectronic applications so that optimized morphology[17,18]
and good charge-carrier transport properties along the π–π
stacking axis[19] can be combined. The concept of utilizing LC
semiconducting polymers in organic photovoltaic cells,[20] as well
as in organic light-emitting diodes (OLEDs)[21] and organic field-
effect transistors (OFETs)[22] has been demonstrated success-
fully. In this context, to our knowledge, the polymerization of LC
perylene benzimidazole moieties has not yet been reported.
1. Introduction
The development of polymeric n-type semiconductors for elec-
tronic devices such as organic photovoltaic cells or organic
field-effect transistors is still challenging and less extensively
investigated, compared to the remarkable work done on p-type
polymers recently.[1–3] Poor processability, insufficient electron
mobilities, or lack of air-stability are current obstacles that have
M.-A. Muth, Prof. Dr. M. Thelakkat
Applied Functional Polymers
Department of Macromolecular Chemistry I
University of Bayreuth
Universitaetsstr. 30, 95440 Bayreuth, Germany
E-mail: mukundan.thelakkat@uni-bayreuth.de
Dr. M. Carrasco-Orozco
Merck Chemicals Ltd
Chilworth Technical Centre
University Parkway
Southampton SO16 7QD, UK
E-mail: miguel.carrasco@merckgroup.com
DOI: 10.1002/adfm.201101694
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Herein, we describe the synthesis of novel perylene side-
chain homopolymers, based on LC perylene diester benzimi-
dazole and perylene diester imide mesogens. Nitroxide-medi-
ated radical polymerization (NMRP) was found to be a suitable
method for the controlled synthesis of homopolymers and
block copolymers based on acrylate monomers with pendant
perylene moieties.[23–25] This radical polymerization method
allows a metal-free polymerization of acrylate monomers giving
narrow molecular-weight distributions. Additionally, a thorough
characterization of LC phases by differential scanning calor-
imetry (DSC), polarized optical microscopy (POM), and X-ray
diffraction measurements (XRD), and bulk electron-transport
properties by the space-charge-limited current (SCLC) method
are given.
carried out under microwave irradiation with adequate yield
(54%). Finally, to obtain monomer 6, 11-bromoundecyl acrylate
(5), bearing an acrylate moiety with alkyl spacer, was attached
to 4 in a nucleophilic substitution under alkaline conditions.
The acrylate unit of 5 serves as the polymerizable group and
the alkyl chain provides sufficient flexibility to facilitate the
polymerization in the presence of sterically hindering pendant
perylene cores. The monomer 6 was obtained in 75% yield;
the reaction conditions were similar to those in the perylene
bisimide acrylate monomer synthesis described by Lindner and
Thelakkat.[23]
A second perylene diester acrylate monomer 10, which
includes an imide group instead of a benzimidazole unit,
was synthesized to study the effect of the size of the conju-
gated π-system on optical, electronic and thermal properties.
Monoanhydride diester 2 and the primary amine 7 were reacted
to give the hydroxy-functionalized perylene diester imide 8. The
reaction was carried out in molten imidazole with an excess of
7 to give 8 in a high yield of 90%. The OH-group attached at the
alkyl spacer served as functional group for esterification with
acryloyl chloride (9). Finally, perylene diester imide acrylate 10
was obtained under basic conditions in a good yield of 68%.
A detailed description of the synthesis and purification of
monomers, polymers and reactants 3, 5, and 7, is given in the
Experimental Section. All compounds were characterized by
means of 1H NMR and IR spectroscopy.
2. Results and Discussion
2.1. Synthesis
In this section, the synthesis of perylene acrylate monomers
6 and 10, and their polymerization to the corresponding poly-
mers PPDB and PPDI (see Scheme 2 for structures) via NMRP
are described. The synthetic route to perylene acrylate mono-
mers 6 and 10 is shown in Scheme 1. Perylene diester mono-
anhydride 2 was obtained from perylene-3,4,9,10-tetracarboxylic
dianhydride (PTCDA) as the starting material according to a
procedure described recently.[16] The branched aliphatic ethyl
hexyl substituents linked to the ester groups give excellent sol-
ubility in various organic solvents. A benzimidazole unit was
introduced to the perylene core by condensation reaction of 2
with 3,4-diaminophenol (3) to give perylene benzimidazole 4
carrying an OH group. This zinc acetate catalyzed reaction was
NMRP was utilized to homopolymerize monomers 6 and 10.
The reactants and the resultant perylene homopolymers PPDB
and PPDI are depicted in Scheme 2. The ratio of monomer to
2,2,5-trimethyl-3-(1-phenylethoxy)-4-phenyl-3-azahexane (11),
which served as unimolecular initiator, was [M]:[I] = 100:1. To
shift the reaction equilibrium towards the dormant species,
0.1 equivalents of N-tert-butyl-α-isopropyl-α-phenylnitroxide
(12) were added to the reaction mixture. This addition slowed
i
ii
1
2
PTCDA
iii
v
7
3
4
8
vi
iv
5
9
6
10
Scheme 1. Synthesis of perylene acrylate monomers 6 and 10: i) 1. KOH/H2O, 0.5 h, 70 °C; 2. Aliquat 336, KI, 10 min, RT; 3. Br-R, 16 h, 100 °C.
ii) p-toluenesulfonic acid monohydrate, toluene/n-dodecane, 5 h, 95 °C. iii) Zn(OAc)2/DMAc, 25 min, 160 °C, 300 W (microwave). iv) K2CO3, KI, DMAc,
17 h, 85 °C. v) Imidazole, Zn(OAc)2, 4 h, 160 °C. vi) Et3N/DCM, 24 h, 0 °C.
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a)
b)
11
12
11
12
Trichlorobenzene ,
50h, 125°C
Trichlorobenzene ,
50h, 125°C
6
Poly(perylene diester benzimidazole acrylate)
PPDB
10
Poly(perylene diester imide acrylate)
PPDI
Scheme 2. Polymerization of a) 6 and b) 10 via NMRP.
T a b le 1 . Mn, Mw, PDI, highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy values and calcu-
lated band gap (Eg) of PPDB and PPDI.
the reaction rate and decreased the concentration of growing
polymer radicals, which led to better control over the poly-
merization. The initiator 11 and free nitroxide 12 were synthe-
sized according to a procedure described by Hawker and co-
workers.[26] Only small amounts of solvent were used to prevent
dilution of acrylate groups, so that the polymerization rate was
not further decreased.
Mn
[g mol−1
Mw
[g mol−1
PDI
HOMO
[eV]
LUMO
[eV]
Eg
[eV]
]
]
PPDB
PPDI
9400
13300
35300
1.4
1.7
1.86
2.16
−5.39
−5.68
−3.53
−3.52
20400
The reactions were quenched at a conversion of 35%, as
1
determined by H NMR, to prevent transfer reactions leading
to increased molecular weight distributions.[27] The molecular
weights of the polymers were identified by size-exclusion chro-
matography (SEC) in chlorobenzene, which was calibrated with
polystyrene standards. The refractive-index-detector signals of
the SEC traces of PPDB and PPDI are shown in Figure 1.
The number-average molecular weight (Mn) and weight-
average molecular weight (Mw) of PPDB were found to be 9400
and 13300 g mol−1 , respectively, with a polydispersity index
better solubility of PPDI in 1,2,4-trichlorobenzene. The high-
molecular-weight PPDI is still soluble, whereas PPDB becomes
insoluble after reaching an average molecular weight (Mn) of
above 10000 g mol−1 , which decreases the further polymeriza-
tion rate significantly. SEC data are summarized in Table 1.
2.2. Polymer Characterization
(PDI) of 1.4. PPDI has a higher molecular weight of Mn
=
20400 g mol−1 and Mw = 35300 g mol−1 , with a PDI of 1.7. Con-
sidering that a controlled radical-polymerization method was
used, the molecular-weight distributions are rather broad. This
result suggests that termination reactions took place, especially
after long reaction times of 50 h. The higher molecular weights
obtained for PPDI compared to PPDB are attributed to the
2.2.1. Thermotropic Properties
The thermotropic behavior of the two polymers was studied by
using DSC, POM, and XRD. The DSC traces shown in Figure 2a
and b were recorded at a scanning rate of 10 K min−1 . Phase-
transition temperatures and the corresponding enthalpies
ΔH are summarized in Table 2. The first heating cycles were
ignored to rule out an influence of the thermal history of the
samples. For PPDB a glass transition (Tg) at 151 °C and a revers-
ible phase transition to an isotropic state at 312 °C were found.
The comparatively small enthalpy of 3.8 J g−1 suggests a phase
transition from an LC mesophase to the isotropic melt. The cor-
responding crystallization peak upon cooling was detected at
279 °C. The existence of a LC phase was confirmed by POM,
which was equipped with a temperature-controlled hot stage.
Upon cooling from the isotropic state with a cooling rate of
3 K min−1 , birefringent textures under crossed polarizers for
PPDB were observed, which are shown in Figure 2c. The image
was obtained at a temperature of 300 °C. The sample was a
viscous liquid, which indicates an LC rather than a crystalline
1.2
PPDB
1.0
0.8
0.6
0.4
0.2
0.0
PPDI
1000
10000
100000
1000000
Molar mass / gmol-1
Figure 1. SEC traces (refractive-index detection) of PPDB and PPDI.
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0.4
a)
PPDI
PPDB
1000
100
PPDB 2nd heating cycle
PPDB 1st cooling cycle
°
312 C
0.2
0.0
Tg 151°C
4
8
12
16
20
q / nm-1
°
279 C
Figure 3. X-ray diffraction pattern of PPDI recorded at room temperature
after annealing at 140 °C and PPDB recorded at 250 °C.
-0.2
0
50 100 150 200 250 300
°
Temperature / C
-0.3
-0.4
-0.5
-0.6
-0.7
phase. The temperature difference observed for the phase
transition between POM (300 °C) and DSC (279 °C) is due to
the supercooling effect caused by a faster cooling rate in the
DSC measurement. Similar observations were made for PPDI,
where the melting point was identified at 132 °C by DSC and
POM. In the same way, the small transition enthalpy (2.8 J g−1 )
in DSC and birefringent textures under crossed polarizers in
POM (Figure 2d) indicate that PPDI also exhibits an LC mes-
ophase. Figure 2d was obtained after annealing PPDI at 110 °C
for 5 h. The absence of other phase transitions in the DSC
traces implies that PPDI is still in the liquid-crystalline state
at room temperature, however the viscosity of the compounds
increases with decreasing temperature and crystallization of the
materials might be thus kinetically hindered.
PPDI 2nd heating cycle
PPDI 1st cooling cycle
b)
°
132 C
°
69 C
0
50
100
150
°
Temperature / C
To further verify the thermotropic liquid-crystalline character
of both polymers, X-ray scattering experiments were carried
out. The instrument was equipped with a Guinier camera and
a hot stage. The XRD patterns of PPDB and PPDI are shown in
Figure 3. As reported recently,[16] for LC perylene derivatives a
columnar hexagonal ordering of the disclike mesogens is often
observed. According to the X-ray diffractogram (Figure 3), the
packing behavior of the perylene side-chain polymers PPDB
and PPDI could not be identified as columnar hexagonal. Here,
the Bragg reflections in the low q regime are indicative of a
2D columnar rectangular or columnar oblique ordering.[28] To
clarify the particular structure of the mesophases, additional
temperature-dependent small-angle X-ray scattering (SAXS)
experiments will be carried out at ESRF Grenoble. In the wide-
angle regime of the PPDB diffractogram a comparatively broad
reflection at a q value of 17.89 nm−1 corresponds to a short-
range repeat distance of 3.51 Å. This distance can be attrib-
uted to the π–π stacking distance of perylene cores of adjacent
polymer side-chains. For PPDI a clearly sharper reflection at
q = 18.21 nm−1 depicts a π–π stacking distance of 3.45 Å. The
more defined and sharper reflection and the closer packing dis-
tance of PPDI compared to PPDB demonstrate a slightly better
organization and more regular packing of the perylene cores.
It is important to mention that PPDB, in contrast to PPDI, was
not heated above the melting temperature before measurement
due to temperature limitations of our X-ray instrument. There-
fore the preconditions for self-organization were not similar
for the two polymers. More detailed temperature-dependent
wide-angle X-ray scattering (WAXS) experiments are currently
Figure 2. DSC thermogram (scan rate 10 K min−1 ) for a) PPDB and
b) PPDI showing the second heating and the first cooling cycle. Peak
values for the phase transitions are given. Optical microscopy images
(polarizers crossed) of the LC mesophases of c) PPDB at 300 °C
and d) PPDI at 110 °C. The textures were obtained upon cooling
the sample from the isotropic melt and annealing at the respective
temperature.
T a b le 2 . Phase behavior of PPDB and PPDI obtained from DSC meas-
urements. g: glassy, LC: liquid crystalline, I: isotropic.
Phase transitions and corresponding enthalpies ΔH
2nd heating cycle (T/ºC; ΔH/ Jg−1
g (151) → LC (312; 3.8) → I
LC (132; 2.8) → I
)
1st cooling cycle (T/ºC; ΔH/ Jg−1
I (279; 3.7) → LC
)
PPDB
PPDI
I (69; 1.6) → LC
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being performed. An amorphous halo and
the absence of further reflections at high q
values verify the liquidlike character of the
alkyl chains. In conclusion, PPDB and PPDI
are liquid crystalline; the latter even at room
temperature.
a)
b)
0.15
UV/vis PPDB solution
UV/vis PPDB film
UV/vis PPDI solution
UV/vis PPDI film
PL PPDI film
1.0
0.8
0.6
0.4
0.2
0.0
0.10
0.05
0.00
2.2.2. Optical and Electronic Properties
The two homopolymers PPDB and PPDI
were investigated in terms of their absorption
behavior in solution and in solid state, their
HOMO and LUMO levels, and their charge-
carrier mobilities. The UV-vis absorption
spectra of PPDB and PPDI in chlorobenzene
400 450 500 550 600 650 700 750
Wavelength / nm
500
550
600
650
700
750
Wavelength / nm
Figure 4. a) UV–vis absorption spectra of PPDB and PPDI measured in chlorobenzene solution
and in thin films and b) photoluminescence spectra of PPDI thin film (λex = 525 nm).
solution and in thin films are shown in Figure 4a. Characteristic
vibronic bands for perylene diester imide PPDI, similar to those
of PBI-based homopolymers,[29] are found. The three vibronic
transitions in solution at 510, 479, and 450 nm are hypsochro-
mically shifted by 20 nm compared to in the PBI homopolymer.
The fluorescence maximum of PPDI measured in film with an
excitation wavelength of 525 nm is at 573 nm (Figure 4b). The
presence of a benzimidazole unit in PPDB extends the absorp-
tion considerably, up to 670 nm in solution and 680 nm in film,
with an absorption maximum at 530 nm. Thus the absorption
edge of PPDB is red-shifted by almost 100 nm compared to in
PPDI. Neither vibronic fine structure in the absorption spectra
nor fluorescence could be observed for PPDB. A spectral broad-
ening for both polymers in thin films, compared to the solution
spectra is observed, which indicates a change in aggregation in
the solid state.
Cyclic voltammetry was carried out to estimate HOMO/
LUMO levels and study electrochemical stability of the com-
pounds. To determine LUMO energy values, each measure-
ment was calibrated with the ferrocene–ferrocenium couple
(Fc/Fc+) taking 4.8 eV as ferrocene’s HOMO level.[30] The cyclic
voltammograms (Figure 5) show two reversible reduction peaks
for both polymers, characteristic for the perylene unit. The first
reduction peaks occurring at –1.27 V for PPDB and –1.28 V for
PPDI result in almost identical LUMO energy levels of –3.53
and –3.52 eV, respectively. Up to five cycles were recorded and
no changes in the redox peaks were observed, which indicates
electrochemical stability and reversibility. HOMO values were
estimated from the optical band-gap and the LUMO levels.
The optical band-gap, obtained from the absorption edges of
absorption spectra in chlorobenzene solution (see Figure 4a),
was found to be 1.86 eV for PPDB and 2.16 eV for PPDI. The
resulting HOMO and LUMO energy levels and band-gaps are
summarized in Table 1.
prerequisites for use in organic electronic devices such as
photovoltaic cells. In addition, the charge-carrier mobility is a
crucial feature.
To determine the bulk charge-carrier mobilities of both poly-
mers, current–voltage I–V characteristics of each material sand-
wiched between two electrodes were measured and the space-
charge-limited currents (SCLC) were fitted according to the
Mott–Gurney equation,[31]
√
9
V2
L3
E
J = · εr · ε0 · μ0 · e0.89·γ ·
·
(1)
8
where J is the current density, εr is the dielectric constant of
the polymer (assumed to be 3 in our calculations[32]), ε0 is the
permittivity of free space, μ0 is the zero-field charge-carrier
mobility, γ is the field-activation parameter, E is the electric field,
L is the thickness of the polymer layer, and V is the voltage drop
across the device.
This method estimates the material’s charge-carrier mobility
in the bulk. In comparison, OFET measurements only measure
the charge transport within a very narrow sheet at the interface
with the gate insulator.[32–34] Charge-carrier densities in OFET
devices are orders of magnitude higher than in organic solar
cells or organic light-emitting diodes. Since charge-carrier
mobilities depend on charge-carrier densities, the mobility
values obtained from OFETs do not necessarily describe the
bulk carrier transport at low carrier densities like those in
0.0003
PPDB
PPDI
0.0002
0.0001
0.0000
-0.0001
UV-vis absorption measurements and cyclic voltammetry
both demonstrate the effect of the benzimidazole unit in PPDB
on optical and electronic properties. The π-conjugation system
of the perylene core is enlarged compared to PPDI, due to
the fused benzimidazole moiety in PPDB, which leads to an
increase in delocalized π-electrons. Therefore, less energy is
needed to excite an electron from the HOMO to the LUMO
in PPDB, which explains the bathochromic shift in absorp-
tion spectra and the lowered band-gap. Strong light harvesting
in the visible range and favorable energy levels are important
-2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6
+
Potential vs. Fc/Fc / V
Figure 5. Cyclic voltammograms of PPDB and PPDI showing the first
and second reduction peaks. The measurements were conducted in ace-
tonitrile with respect to Fc/Fc+ at a scan rate of 50 mV s−1
.
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organic photovoltaic devices. Furthermore, initial testing of the
two polymers under investigation in OFET devices show poor
performance, which we attribute to potential unfavorable align-
ment of the active material in the channel of the transistor; this
does not seem problematic for SCLC devices, since architec-
ture and underlying layers are different from those of OFET
devices. Two different types of SCLC devices were fabricated
to determine both electron and hole mobilities of the perylene
polymers. Electron-only devices based on PPDB and PPDI
the dominant charge carriers in these devices. The polymer
layer thicknesses of the single-carrier devices were varied from
300–650 nm. I–V characteristics of the devices were recorded at
up to 6 V at room temperature in an inert gas atmosphere. The
experimental data and the corresponding fits according to Equa-
tion 1 are shown in Figure 6c–f. For calculation, voltages were
corrected for a built-in potential of 2.2 eV from the difference
in work function of calcium (ca. 2.9 eV) and ITO/PEDOT:PSS
(ca. 5.1 eV) for electron-only devices.[35] At high voltages the
current is space-charge-limited only, assuming ohmic contacts
to the injecting electrode. In this regime, the current density
approximately scales with J ∼ V 2, so the charge-carrier mobility
at zero-field μ0 and the field-activation parameter γ can be
determined by fitting the curve according to Equation 1. The
values obtained for μ0 and γ for the best fits are summarized in
Table 3. Inserting these values into Equation 2[34]
comprise
a poly(3,4-ethylenedioxythiophene):poly(styrenesul
fonate) (PEDOT:PSS) covered indium–tin oxide (ITO) bottom
contact and a calcium top electrode capped with aluminum.
The device architecture is depicted in Figure 6a. Calcium has a
work function of 2.9 eV and serves as an ohmic contact for elec-
tron injection into the LUMO of PPDB and PPDI (ca. 3.5 eV).
Concurrently, the bottom contact does not hinder electrons
from leaving the device. Hole-only devices (Figure 6b) were fab-
ricated correspondingly with an ITO/PEDOT:PSS bottom con-
tact, but with a gold top electrode instead of calcium. The low
work function of gold (5.1 eV) provides a large mismatch with
the LUMO of PPDB and PPDI, which prevents electron injec-
tion and therefore holes injected from the bottom electrode are
√
E
μ = μ0 · eγ ·
(2)
yields the charge-carrier mobilities μ depending on the elec-
tric field within the device. Table 3 shows the charge-carrier
mobilities calculated for the maximum electric field.
10
d)
c)
PPDB Electron-only devices
100
PPDB Hole-only devices
1
0.1
10
0.01
1
L=320nm
L=440nm
L=590nm
L=350nm
L=490nm
L=610nm
1E-3
1E-4
0.1
2.0
2.0
2.5
3.0
(Vap-Vbi ) / V
3.5
2.5
3.0
3.5
(Vap-Vbi ) / V
1000
100
10
f)
e)
PPDI Hole-only devices
PPDI Electron-only devices
10
1
1
L=470nm
L=390nm
L=560nm
L=300nm
L=390nm
L=650nm
0.1
0.01
0.1
2.0
2.5
3.0
3.5
2.0
2.5
3.0
(Vap-Vbi ) / V
3.5
(Vap-Vbi ) / V
Figure 6. Scheme of a) an electron-only device with a calcium top electrode, capped with aluminum and b) a hole-only device with gold top electrode,
current density J vs. voltage V plots (data points) and fits according to Equation 1 (straight lines) at room temperature for c) PPDB electron-only
devices, d) PPDB hole-only devices, e) PPDI electron-only devices, and f) PPDI hole-only devices with varied layer thicknesses L. The voltage applied
(Vap) was corrected for a built in potential (Vbi) of 2.2 eV for electron-only devices resulting from the differences in work function of calcium and ITO/
PEDOT:PSS. Vbi in hole-only devices was estimated to be 0 eV.
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T a b le 3 . SCLC charge carrier mobilities and associated field activation parameters for PPDB and PPDI.
Electron mobility
Hole mobility
μ0e
γ
μe
μ0h
γ
μh
[cm2 V–1s–1
]
[m0.5 V–0.5
]
[cm2 V–1s–1
]
[cm2 V–1s–1
]
[m0.5 V–0.5
8.2 · 10–4
1.5 · 10–3
]
[cm2 V–1s–1
1.0 · 10–6
1.5 · 10–4
]
PPDB
PPDI
2.2 · 10–7
2.3 · 10–6
2.6 · 10–3
6.8 · 10–4
3.2 · 10–4
1.9 · 10–5
6.0 · 10–8
1.6 · 10–6
μ 0 and γ values were obtained by fitting the J–V curves of the single carrier devices according to equation 1. μ was determined from equation 2 and the maximum electric
field in the device.
behavior of PPDB and PPDI was studied by using polarized
optical microscopy, differential scanning calorimetry, and X-ray
diffraction. Both polymers exhibit liquid-crystalline mesophases
over a broad temperature range.
Using this method, the electron mobility μe was found to be
3.2 × 10−4 cm2 V−1 s−1 for PPDB and the hole mobility μh was
determined to be 1.0 × 10−6 cm2 V−1 s−1 . The hole mobility is
two orders of magnitude smaller than the electron mobility,
thus this polymer can be considered an n-type semiconductor.
PPDI has an electron mobility of 1.9 × 10−5 cm2 V−1 s−1 and a
hole mobility of 1.5 × 10−4 cm2 V−1 s−1 . The results from the
SCLC measurements are summarized in Table 3. Interest-
ingly, the hole mobility of PPDI is nearly one order of magni-
tude higher than its electron mobility. As a result, we assign
PPDI to a new class of p-type materials. In contrast to perylene
bisimides, which are well-known electron acceptors because of
their two electron-withdrawing imide groups,[36] the perylene
core of PPDI only contains one imide group. This imide con-
tent results in a less electron-deficient character and therefore
a more donor-type behavior. In comparison, the benzimidazole
unit of PPDB provides enough electron deficiency in the het-
erocyclic aromatic system to enhance the electron-accepting
properties of this material. We therefore demonstrate here that
the electronic character of perylene side-chain polymers can be
tuned by modifying the aromatic perylene core.
4. Experimental Section
Instrumentation: 1H NMR spectra were recorded using a Bruker AC
300 spectrometer (300 MHz) at 25 °C with deuterated chloroform as
solvent and tetramethylsilane as an internal standard. Chemical shifts
are reported in ppm, abbreviations used for splitting patterns are s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet. FTIR data
were recorded with a Perkin Elmer Spectrum 100 (FTIR) in the range
400–4000 cm−1 . Molecular-weight determinations were carried out in
chlorobenzene solution at 60 °C on an Agilent 1100 series GPC using
two Polymer Laboratories mixed B columns in series. The system was
calibrated against narrow weight PL polystyrene calibration standards.
DSC measurements were performed on a TA Q1000 under nitrogen
with a heating and cooling rate of 10 K min−1 . Phase transitions were
also examined using a POM Nikon Diaphot 300 with a Mettler FP
90 temperature controlled hot stage. XRD measurements were performed
on a Huber Guinier Diffraktometer 6000 equipped with a Huber quartz
monochromator 611 with Cu Kα1: 1.54051 Å. UV-vis spectra of solutions
in chlorobenzene with a concentration of 10−5 mol L−1 and thin films spin-
cast on quartz glass were recorded on a Hitachi 3000 spectrophotometer.
Photoluminescence spectra were acquired on a Shimadzu RF 5301 PC
spectrofluorophotometer upon excitation at 525 nm. Cyclic voltammetry
3. Conclusion
(CV) was recorded on
a Princeton Applied Research VersaSTAT
In summary, two novel liquid-crystalline perylene diester side
chain homopolymers were synthesized via nitroxide-mediated
radical polymerization. We have demonstrated that changing the
heterocyclic π-system permits access to materials with different
electronic character. PPDB has n-type semiconducting proper-
ties with an electron mobility of 3.2 × 10−4 cm2 V−1 s−1 , whereas
PPDI shows p-type behavior with a hole mobility of 1.5 ×
10−4 cm2 V−1 s−1 , as determined from SCLC measurements.
Altering the aromatic system of the perylene core not only
affects the electrical properties of the investigated polymers;
the absorption of PPDB in the visible range is notably broad-
ened up to 670 nm compared to PPDI (580 nm), which is due
to extension of π-conjugation of the perylene core by the benz-
imidazole unit of PPDB. The optical band-gaps as determined
by the absorption edge of UV-vis measurements and cyclic vol-
tammetry were 1.86 eV for PPDB and 2.16 eV for PPDI, with
similar LUMO energy levels of –3.53 and –3.52 eV, respectively.
Furthermore, the ethyl hexyl substituents attached to the ester
groups of the perylene core give excellent solubility in common
organic solvents. These findings qualify the two polymers as
potential materials for application in optoelectronic devices,
which will be the subject of future investigations. Thermotropic
4 Potentiostat/Galvanostat using platinum electrodes at a scan rate of
50 mV s−1 and an Ag/Ag+ (0.10 M AgNO3 in acetonitrile) reference
electrode at 25 °C in an anhydrous and nitrogen-saturated solution of 0.1 M
tetrabutylammonium tetrafluoroborate (Bu4NBF4) in acetonitrile. In these
conditions, the oxidation potential of ferrocene was 0.10 V versus Ag/Ag+,
whereas the oxidation potential of ferrocene was 0.41 V versus saturated
calomel electrode (SCE). The LUMO energy levels were determined from
the reduction peak taking into account the SCE level at –4.7 eV. Film
thicknesses were determined on an Alphastep 500 surface profilometer.
Chemicals: The starting materials, perylenetetracarboxylic acid
dianhydride (PTCDA), 3-(bromomethyl)heptane, Aliquat 336,
p-toluenesulfonic acid monohydrate, zinc acetate, 11-bromoundecanol,
sodium cyanide, Raney Ni, acryloyl chloride, and solvents were
purchased from Aldrich, Fluka, or Acros and used without any further
purification. Solvents used for precipitation and column chromatography
were distilled prior to use. Dimethyl acetate (DMAc; anhydrous with
crowncap, 99.5%), dimethyl sulfoxide (DMSO; anhydrous with crowncap,
99.5%) and 1,2,4 trichlorobenzene (anhydrous with crowncap, 99.0%)
were purchased from Aldrich. PEDOT:PSS (Clevios P VP Al 4083) was
purchased from Heraeus.
The preparation of 1 and 2 was described recently by Wicklein et al.[16]
5 was prepared according to Lang et al.[27] The initiator 11 and the free
nitroxide 12 were synthesized according to published procedures.[26]
Synthesis of 3,4-Diaminophenol (3) : 4-amino-3-nitrophenol (6.5 mmol,
1.00 g) and palladium on charcoal (0.10 g, 10%) were added to
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methanol (55 mL) under an inert gas atmosphere. The reaction flask
was flushed with hydrogen twice before stirring the reaction mixture for
3.5 h at room temperature under a hydrogen atmosphere. The solution
was filtered through Celite under an argon atmosphere. After the solvent
was evaporated under reduced pressure, the product was obtained in
quantitative yield as a brownish white solid, which was unstable in air
and used immediately upon preparation.
Synthesis
tetracarboxyl-monoimide (8) :
of
N-(dodecyl-1-ol)-bis(2-ethylhexyl)-perylene-3,4,9,10-
A mixture of 2 (6.0 mmol, 3.86 g),
imidazole (350 mmol, 24 g) and 7 (9.1 mmol, 1.84 g) was stirred for 2 h
at 130 °C. After cooling to room temperature the mixture was dissolved
in tetrahydrofuran (THF; 1 mL), precipitated in water (200 mL), and
filtered. The residue was washed with water (2 × 30 mL) and dried
overnight at 60 °C in vacuo. The crude product was purified using
column chromatography (silica flash gel, eluent chloroform:acetone 20:1
v/v). Yield: 4.47 g (90%) as an orange–red solid. 1H NMR (300 MHz,
CDCl3, 298 K, δ): 8.63 (d, J = 8.0 Hz, 2H, HAr), 8.47 (dd, 2J = 8.2 Hz, 3J =
4.6 Hz, 4H, HAr), 8.10 (d, J = 7.9 Hz, 2H, HAr), 4.38–4.27 (m, 4H, OCH2),
4.25–4.16 (m, 2H, OCH2), 3.70–3.62 (m, 2H, NCH2), 1.87–1.72 (m, 4H,
OCH2CH, OCH2), 1.61–1.22 (m, 35H, CH2, OH), 1.04–0.88 (m, 12H,
CH3); IR (ATR): ν = 3505 (w), 2924 (s), 2854 (m), 1695 (s), 1652 (s),
1593 (s), 1511 (m), 1356 (m), 1294 (s), 1261 (s), 1171 (s), 1076 (s),
SynthesisofBis(2-ethylhexyl)-perylene-3,4-(4-hydroxy-1,2-benzimidazole)-
9,10-dicarboxylate (4) : 2 (2.4 mmol, 1.50 g), zinc acetate (3.5 mmol,
0.65 g), and 3 (4.7 mmol, 0.59 g) were dissolved in dry DMAc (50 mL)
in a microwave pressure tube. The condensation reaction was carried
out under microwave irradiation conditions for 30 min at 150 °C and
100 W. The violet crude product was precipitated in methanol (300 mL)
and filtered. The residue was washed with water (2 × 30 mL) and
methanol (3 × 30 mL) and dried overnight at 60 °C in vacuo. The crude
product was purified using column chromatography (silica flash gel,
eluent chloroform:methanol 9:1 v/v). Yield: 0.90 g (53%) as a violet
solid. 1H NMR (300 MHz, CDCl3, 298 K, δ): 8.45–8.38 (m, 1H, HAr),
8.32–8.09 (m, 5H, HAr), 8.03–7.91 (m, 2H, HAr), 7.64–7.57 (m, 1H,
HAr), 6.62–6.59 (m, 1H, OH), 6.42–6.28 (m, 2H, HAr), 4.44–4.26 (m,
4H, OCH2), 1.96–1.80 (m, 2H, OCH2CH), 1.72–1.17 (m, 16H, CH2),
1.14–0.94 (m, 12H, CH3); IR (ATR): ν = 3360 (b), 2960 (m), 2930 (m),
2860 (m), 1691 (s), 1593 (s), 1445 (m), 1362 (s), 1260 (s), 1171 (s),
956 (m), 846 (m), 806 (m), 745 (s) cm−1
.
Synthesis of N-(Dodecylacrylic acid)-bis(2-ethylhexyl)-perylene-3,4,9,10-
tetracarboxyl-monoimide (10) : 8 (4.3 mmol, 3.5 g) was dissolved in dry
THF (120 mL) under an argon atmosphere and potassium carbonate
(8.6 mmol, 1.18 g) was added. Acryloyl chloride (19.3 mmol, 1.74 g) was
added dropwise at 0 °C and the reaction mixture was stirred for 20 h
at 60 °C. The mixture was poured into water (100 mL) and the crude
product was extracted with chloroform (3 × 100 mL). The combined
extracts were washed with water (3 × 100 mL), dried over sodium
sulfate, and the solvent was removed on the rotary evaporator. The crude
product was purified using column chromatography (silica flash gel,
eluent chloroform:acetone 20:1 v/v). Yield: 2.50 g (67%) as a orange–red
solid. 1H NMR (300 MHz, CDCl3, 298 K, δ): 8.62 (d, J = 8.0 Hz, 2H, HAr),
8.45 (dd, 2J = 8.2 Hz, 3J = 3.3 Hz, 4H, HAr), 8.09 (d, J = 7.9 Hz, 2H,
HAr), 6.41 (dd, 2J = 17.3 Hz, 3J = 1.6 Hz, 1H, acryl-CH2), 6.13 (dd, 2J =
17.3 Hz, 3J = 10.4 Hz, 1H, acryl-CH), 5.82 (dd, 2J = 10.4 Hz, 3J = 1.5 Hz,
1H, acryl-CH2), 4.38–4.26 (m, 4H, OCH2), 4.25–4.10 (m, 4H, acryl-OCH2,
NCH2), 1.89–1.74 (m, 4H, OCH2CH, acryl-OCH2CH2), 1.74–1.60 (m,
2H, NCH2CH2), 1.58–1.21 (m, 32H, CH2), 1.05–0.86 (m, 12H, CH3); IR
(ATR): ν = 2957 (m), 2925 (s), 2855 (s), 1723 (s), 1695 (s), 1652 (s),
1593 (s), 1511 (m), 1356 (s), 1294 (s), 1260 (s), 1171 (s), 1076 (s),
804 (s), 745 (s) cm−1
Synthesis of Bis(2-ethylhexyl)-perylene-3,4-(4-undecyloxyacrylic acid-
1,2-benzimidazole)-9,10-dicarboxylate (6) : (0.7 mmol, 0.51 g),
.
4
5
(1.4mmol,0.43g),potassiumcarbonate(1.4mmol,0.19g),andpotassium
iodide (0.14 mmol, 0.02 g) were dissolved in dry DMAc (20 mL) and
stirred for 24 h at 85 °C under an argon atmosphere. The crude product
was precipitated in a mixture of methanol and water (MeOH:H2O 3:1
v/v, 150 mL), filtered and washed (2 × MeOH:H2O 3:1 v/v, 30 mL).
The residue was dried over night at 60 °C in vacuo. The crude product
was purified using column chromatography (silica flash gel, eluent
chloroform:acetone 95:5 v/v). Yield: 0.59 g (88%) as a violet solid. 1H
NMR (300 MHz, CDCl3, 298 K, δ): 8.74–8.62 (m, 2H, HAr), 8.48–8.31
(m, 5H, HAr), 8.10–8.01 (m, 2H, HAr), 7.30 (m, 1H, HAr), 7.07–7.00 (m,
1H, HAr), 6.42 (dd, 2J = 17.3 Hz, 3J = 1.6 Hz, 1H, acryl-CH2), 6.14 (dd,
2J = 17.3 Hz, 3J = 10.4 Hz, 1H, acryl-CH), 5.83 (dd, 2J = 10.4 Hz, 3J =
1.5 Hz, 1H, acryl-CH2), 4.37–4.25 (m, 4H, OCH2), 4.18 (t, J = 6.8 Hz,
2H, acryl-OCH2), 4.07 (m, 2H, benzimidazole-OCH2), 1.94–1.77 (m,
4H, OCH2CH, benzimidazole-OCH2CH2), 1.75–1.63 (m, 2H, acryl-
OCH2CH2), 1.60–1.24 (m, 30H, CH2), 1.06–0.88 (m, 12H, CH3); IR
(ATR): ν = 2926 (s), 2856 (m), 1712 (s), 1688 (s), 1592 (m), 1466 (m),
984 (m), 961 (m), 847 (m), 806 (s), 745 (s) cm−1
.
Synthesis of Poly(perylene diester benzimidazole acrylate) (PPDB) : A
mixture of 6 (1.0 mmol, 740 mg), 11 (0.01 mmol, 3.2 mg), 12 (0.001 mmol,
0.22 mg), and 1,2,4-trichlorobenzene (1250 μL) was degassed using three
freeze/thaw cycles, sealed under argon and heated for 50 h at 125 °C. The
reaction mixture was cooled, dissolved in chloroform (4 mL), precipitated
in acetone (250 mL), and filtered. The residue was dried and further purified
by Soxhlet extraction with methyl ethyl ketone. Yield: 0.26 g (35%) as a
violet powder. GPC: Mn = 9400 g·mol−1 , Mw = 13300 g mol−1 , PDI = 1.4.
Synthesis of Poly(perylene diester imide acrylate) (PPDI) : A mixture of
10 (1.7 mmol, 1500 mg), 11 (0.017 mmol, 5.3 mg), 12 (0.0017 mmol,
0.37 mg), and 1,2,4-trichlorobenzene (915 μL) was degassed using three
freeze/thaw cycles, sealed under argon, and heated for 50 h at 125 °C. The
reaction mixture was cooled, dissolved in chloroform (3 mL), precipitated
in methanol (250 mL), and filtered. The residue was dried and further
purified by Soxhlet extraction with acetone. Yield: 0.48 g (32%) as an orange
powder. GPC: Mn = 20400 g mol−1 , Mw = 35300 g mol−1 , PDI = 1.7.
SCLC Devices: SCLC electron-only devices were fabricated using the
following structure: glass/ITO/PEDOT:PSS/polymer/Ca/Al. SCLC hole-
only devices were fabricated using the following structure: glass/ITO/
PEDOT:PSS/polymer/Au. Commercial ITO-coated glass substrates with
a sheet resistance of 13 Ohms per sq (Lumtec) were cleaned using the
following sequence in an ultrasonic bath: water, acetone, and 2-propanol.
Each ITO substrate was patterned using photolithography techniques.
After ozone treatment of the substrates for 5 min, PEDOT:PSS was spin-
coated onto the ITO surface and dried at 130 °C for 30 min. All following
steps were carried out under a nitrogen atmosphere with water and
oxygen levels ≤0.1 ppm. After cooling the substrate, the polymer layer
was blade-coated from chloroform solutions. The substrates were then
put in a thermal evaporation chamber to evaporate the top electrode
(30 nm Ca/100 nm Al for electron-only devices, 80 nm Au for hole-only
devices) under high vacuum (1 × 10−6 mbar) through a shadow mask
1268 (s), 1171 (s), 1062 (m), 804 (s), 742 (m) cm−1
.
Synthesis of 12-Hydroxydodecanenitrile: The synthesis was adapted
from the work of Jaeger et al.[37] A mixture of 11-bromoundecanol (20.0
mmol, 5.00 g), sodium cyanide (38.9 mmol, 1.91 g), and DMSO (75 mL)
was stirred for 24 h at 90 °C. Then the reaction mixture was added to
water (150 mL) and extracted four times with DCM (4 × 100 mL). The
combined extracts were washed six times with water, dried over sodium
sulfate, and the solvent was removed on the rotary evaporator to give
3.85 g (98%) product. 1H NMR (300 MHz, CDCl3, 298 K, δ): 3.65 (q, J =
6.3 Hz, 2H, CH2O), 2.35 (t, J = 7.1 Hz, 2H, CH2CN), 1.74–1.52 (m, 5H,
CH2CH2CN, OH), 1.51–1.40 (m, 2H, CH2CH2O), 1.38–1.24 (br s, 12H,
(CH2)6); IR (ATR): ν = 3383 (m), 2919 (s), 2851 (s), 2248 (m), 1470 (s),
1335 (m), 1053 (s), 1013 (m), 722 (s) cm−1
.
Synthesis of 12-Aminododecan-1-ol (7) : The synthesis was adapted
from the work of Jaeger et al.[37] A mixture of 12-hydroxydodecanenitrile
(19.3 mmol, 3.80 g), Raney Ni (38.9 mmol, 5.00 g), ethanol (80 mL, 95%),
and concentrated ammonium hydroxide (50 mL, 25%) was stirred under
a hydrogen atmosphere for 20 h at room temperature. Then the reaction
mixture was filtered through a pad of Celite, which was washed with
ethanol (50 mL, 95%). The combined filtrates were concentrated on the
rotary evaporator to give 3.6 g (95%) 7. 1H NMR (300 MHz, CDCl3, 298 K,
δ): 3.65 (t, J = 6.5 Hz, 2H, CH2O), 2.71 (br s, 2H, NH2), 1.64–1.52 (m, 2H,
CH2CH2NH2), 1.51–1.41 (m, 2H, CH2CH2O), 1.29 (br s, 19H, OH, (CH2)9);
IR (ATR): ν = 3329 (m), 3286 (m), 2917 (s), 2849 (s), 1614 (m), 1470 (s),
1372 (m), 1065 (s), 1044 (m), 999 (m), 855 (w), 828 (w), 719 (s) cm−1 .
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were measured using a Keithley 2420 (I–V) Digital SourceMeter at 25 °C.
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Acknowledgements
Financial support from Merck Chemicals Ltd. and EU-India project
“largecells” is kindly acknowledged.
Please note: This article was amended on December 6, 2011 to correct a
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