Air stable electron-transporting and ambipolar bay substituted perylene bisimides

Article abstract of DOI:10.1039/c1jm11091h

Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the bay region have been designed, synthesized and characterized. The materials possess high thermal stability and form uniform films. They display a wide absorption window extending to the near infrared region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer. Ionization potential values of these perylene bisimide derivatives measured by photoelectron spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino substituted material exhibited competent electron drift mobility (>10^-3 cm² V^-1 cm^-1) under ambient conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay substituted perylene bisimides in order to understand the complementary ambipolar charge transport as well as the difference in the optical properties.

Full text of DOI:10.1039/c1jm11091h

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PAPER
Air stable electron-transporting and ambipolar bay substituted perylene
bisimides
a
a
a
Renji R. Reghu, Hari Krishna Bisoyi, Juozas V. Grazulevicius, Padmesh Anjukandi,b Valentas Gaidelisc
*
and Vygintas Jankauskasc
Received 13th March 2011, Accepted 23rd March 2011
DOI: 10.1039/c1jm11091h
Four new perylene bisimides containing carbazolyl and triphenylamino electron-donor groups in the
bay region have been designed, synthesized and characterized. The materials possess high thermal
stability and form uniform films. They display a wide absorption window extending to the near infrared
region of the spectrum and demonstrate efficient photoinduced intramolecular electron transfer.
Ionization potential values of these perylene bisimide derivatives measured by photoelectron
spectroscopy range from 5.8 eV to 6 eV. Charge-transporting properties were investigated by the
xerographic time of flight (XTOF) technique. Complementary ambipolar charge-transport was
observed in differently linked carbazolyl substituted perylene bisimides while the triphenylamino
substituted material exhibited competent electron drift mobility (>10ꢀ3 cm2 Vꢀ1 cmꢀ1) under ambient
conditions. Density functional theory (DFT) calculations were performed for carbazolyl bay
substituted perylene bisimides in order to understand the complementary ambipolar charge transport
as well as the difference in the optical properties.
Perylene bisimide derivatives form a class of interesting n-type
1. Introduction
organic semiconductors owing to excellent charge carrier trans-
Recently there has been an unprecedented growth of interest in
port in conjunction with their outstanding chemical, thermal and
photochemical stability.13–16 Very high electron mobility of per-
organic semiconductors i.e. organic materials for application in
electronic and optoelectronic devices such as light emitting
diodes, photovoltaic devices and field-effect transistors etc.1–5
ylene bisimide derivatives has qualified them to be widely
employed in electronic and optoelectronic devices.6–8 Accord-
The superiority of organic semiconductors over their inorganic
ingly numerous perylene bisimide derivatives carrying different
counterparts resides in their tremendous architectural flexibility,
substituents on the imide positions as well as on the carbocyclic
inexpensiveness and ease of processing and device fabrication.
core, the so-called bay region, have been synthesized and inves-
However, one of the major challenges in the field is the devel-
tigated.13–16 However stable ambipolar molecular materials based
opment of high-mobility and stable electron-transporting
on p-conjugated perylene bisimides have not been explored well
(n-type) organic semiconductors for thin film device structures to
and are still interesting in the perspective of cost effective single
complement the high efficiency of current generation p-type
layer organic light emitting diodes and field-effect transistors.17–20
materials which would enable durable devices with high oper-
On the other hand, redox-active carbazole and triphenylamine
ating speeds and lower power consumption.6–8 The disadvanta-
derivatives are well known hole-transporting materials and have
geous low charge carrier mobility in most of the known n-type
been exploited extensively for various optoelectronic applica-
materials at ambient conditions is mainly due to the trapping of
tions.21–24 Additionally carbazole derivatives possess excellent
charge carriers (i.e., electrons) by common atmospheric oxidants
photorefractive properties while triphenylamines act as organic
such as oxygen, water etc.9,10 In this context, appropriate
photoconductors in the xerographic industry.21–24 It is interesting
molecular design towards efficient charge transport seems to be
the first stepping stone.11,12
to note that the optoelectronic devices fabricated from electro-
active materials derived from 2,7-substituted carbazoles exhibit
better performances than 3,6-substituted carbazole deriva-
tives.25–27 Moreover, molecular materials based on 2-substituted
carbazolyl derivatives have been less extensively studied because
of the much easier synthetic protocol of its regioisomeric coun-
terpart; i.e. 3-substituted carbazoles. To the best of our knowl-
edge, the donor–acceptor hybrids combining perylene bisimides
and carbazoles at the bay region are hitherto unknown.
aDepartment of Organic Technology, Kaunas University of Technology,
Radvilenu pl. 19, LT-50254 Kaunas, Lithuania. E-mail: juozas.
grazulevicius@ktu.lt; Fax: +37037 300152; Tel: +37037 300193
bInstitute of Fundamental Sciences, Massey University, Palmerston North,
New Zealand
cDepartment of Solid State Electronics, Vilnius University, Sauletekio al. 9,
LT-10222 Vilnius, Lithuania
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Furthermore, the combination of hole-transporting and elec-
tron-donating planar carbazole and non-planar triphenylamine
derivatives with electron-transporting perylene bisimides at
molecular level may lead to novel multichromophoric donor–
acceptor molecular materials with interesting and useful optical
and charge-transporting properties important for various device
applications. Keeping this in mind we have designed and
synthesized new 1,7-bay substituted perylene bisimide derivatives
directly linked to electron-donating triphenylamino and different
carbazolyl moieties furnishing efficient electron-transporting and
ambipolar materials possessing excellent photoinduced charge
transfer properties. Herein we present the thermal, optical,
photophysical, electrochemical and photoelectrical character-
izations and preliminary quantum chemical calculations of the
newly synthesized molecular materials based on bay substituted
perylene bisimides.
Scheme 1 Synthetic route for the preparation of compounds 1–4.
Reagents and conditions: (a) Br2, conc. H2SO4/I2, 80 ꢁC, 24 h; (b) dode-
cylamine or 2-ethylhexylamine, CH3COOH/N-methylpyrrolidone,
120 ꢁC, 12 h; (c) Pd(Ph3)2Cl2, KOH, THF–H2O, 80 ꢁC, 8–12 h.
subsequent borylation as reported elsewhere.31,32 The synthesis of
key boronic acid pinacol esters is described in Scheme 2.
The synthesized compounds were purified by column chro-
matography and characterized by 1H NMR, 13C NMR, MALDI-
TOF, IR spectroscopy and elemental analysis. The spectral and
elemental analysis data are in good agreement with their
chemical structures.
2. Results and discussion
2.1. Synthesis
The chemical structures of the newly designed target materials
are shown in Fig. 1. 1,7-Bis[4-(diphenylamino)phenyl] N,N0-2-
ethylhexyl perylene bisimide (1) was synthesised by the Suzuki-
Miyaura coupling reaction of 1,7-dibromoperylene bisimide with
2.2. Thermal characteristics
The glass transition temperatures (Tg) of the synthesized perylene
bisimide deriꢁvatives are summarised in Table 1. They range from
4-(diphenylamino)phenylboronic
acid
(4-DPB).
Other
ꢁ
compounds, 1,7-bis(9-ethyl-2-carbazolyl) N,N0-2-ethylhexyl
perylene bisimide (2), 1,7-bis(9-ethyl-3-carbazolyl) N,N0-2-ethyl-
hexyl perylene bisimide (3) and 1,7-bis(9-ethyl-3-carbazolyl) N,
N0-dodecyl perylene bisimide (4) were also synthesized by the
Suzuki–Miyaura coupling between 1,7-dibromoperylene bisi-
mide and the corresponding mono boronic acid pinacol ester
derivatives of carbazole (Scheme 1). 1,7-Dibromo N,N0-2-ethyl-
hexyl perylene bisimide (5) and 1,7-dibromo N,N0-dodecyl per-
ylene bisimide (6) were in turn prepared by the bromination of
commercially available 3,4:9,10-perylene tetracarboxylic
dianhydride followed by the condensation with corresponding
alkyl amines, 2-ethylhexylamine and n-dodecylamine, respec-
tively. 9-Ethyl-2-carbazolyl boronic acid pinacol ester (2-CZ)
was obtained from 4-bromobiphenyl by nitration followed by
ring closure with triphenylphosphine and the consequent N-
alkylation and borylation using 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane.28–30 The precursor for compounds 3
and 4, 9-ethyl-3-carbazolyl boronic acid pinacol ester (3-CZ),
was prepared from 3-bromocarbazole by N-alkylation and
81 C to 95 C. The linking topology of perylene bisimide and
carbazole derivatives seems not to have any substantial influence
on their Tg. Tg of 2-carbazolyl substituted derivative (2) is
comparable to that of its 3-carbazolyl substituted isomer (3). The
effect of the branched but shorter alkyl chain and longer but
linear alkyl chain on the glass transition of 3-carbazolyl
substituted derivatives is also comparable. Tg of 3 is close to that
of 4.
Thermal stability of compounds 1–4 was estimated by ther-
mogravimetric analysis (TGA). All the derivatives demonstrate
high thermal stability. The temperatures of the onsets of thermal
decomposition (TID) of perylene bisimide derivatives are
collected in Table 1. They range from 438 ꢁC to 471 ꢁC. The TID
Scheme 2 Synthetic route for the preparation of carbazolyl boronic acid
pinacol esters (2-CZ and 3-CZ). Reagents and conditions: (a) Br2, pyri-
dine, 0 ꢁC, 2 h; (b) C2H5I, KOH, DMF, RT, 24 h; (c) n-BuLi, 2-iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane,THF, ꢀ78 ꢁC to RT,
12 h; (d) CH3COOH/HNO3, reflux, 5 h; (e) PPh3, ortho-dichlorobenzene,
180 ꢁC, 12 h.
Fig. 1 Chemical structures of compounds 1–4.
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Table 1 Thermal characteristics of 1–4a
Table 2 Optical characterization data of 1–4a
Compound
Tg/ꢁC
Tm/ꢁC
Tcr/ꢁC
TID/ꢁC
Absorption
Compound lmax/nm
Emission
lmax/nm
(h)b
Egopt/eV c
1
2
3
4
95
82
81
82
251
—
—
—
194
—
—
—
455
441
438
471
1
2
3
4
211, 307, 496, 617 Non-fluorescent
224, 311, 430, 574 705
226, 301, 466, 599 756
— 1.58
0.004 1.85
0.03
0.03
1.74
1.74
226, 301, 466, 599 756
a
Tg – Glass transition temperature, Tm – Melting transition, Tcr
Crystallization temperature, TID – Thermal decomposition onset.
–
a
h – Fluorescence quantum yield. b Fluorescence was excited at 570 nm.
c
opt
Eg – Optical band gap. Calculated from solution absorption edges.
values of compounds 1–3 are comparable irrespective of the bay
substituents. The difference in the TID values of 3 and 4 is
pointing towards the influence of N-alkyl substituents on their
thermal stability.
results on the potential of these multichromophoric donor–
acceptor hybrids. There is a complete self-quenching of the
fluorescence of compound 1 whereas compounds 2–4 are almost
non-fluorescent with trivial fluorescence quantum yields
(Table 2). These results suggest that very efficient photo-induced
intramolecular electron transfer occurs in these compounds.
Since all these molecules absorb well in the visible–near IR region
with efficient photo-induced intramolecular charge transfer, they
can be promising candidates for application in photovoltaic
devices.37
2.3. Optical and photoluminescent properties
The synthesized compounds are well soluble in common organic
solvents like tetrahydrofuran, dichloromethane, chloroform etc.,
facilitating their optical characterization in solution state. UV-
Vis absorption spectra of dilute solutions are shown in Fig. 2.
The wavelengths of absorption maxima are collected in Table 2.
The low energy absorption edges of the newly synthesized
compounds range from 669 nm to 787 nm and as anticipated for
bay substitution by aromatic groups, are red shifted with respect
to the unsubstituted parent compound, perylene bisimide (PBI),
owing to their greater conjugation.33–35 Moreover, it is found that
the least energy absorption band of 1 carrying triphenylamino
moiety at the bay region is bathochromically shifted with respect
to that of compounds 2–4 possessing carbazole units. This
suggests distortion of the perylene core, known to occur upon
bay substitution, to a lesser degree upon substitution by triphe-
nylamino moiety as well as greater conjugation leading to rela-
tively low optical band gap compared to those of carbazole
derivatives.34,36 Furthermore, surprisingly the absorption bands
of 3-carbazolyl substituted derivatives (3&4) are shifted both
bathochromically and hyperchromically with respect to that of 2-
carbazolyl substituted derivative (2), clearly indicating a different
degree of perylene core distortion and/or different torsional
angles of bay substituents in these topological isomers. Investi-
gation of photoluminescent properties of 1–4 yields interesting
2.4. Electrochemical properties
Electrochemical properties of selected compounds were studied
by cyclic voltammetry in order to elucidate the electronic energy
levels which determine the energy and electron transfer processes
and the reversibility of redox processes.38 Cyclic voltammograms
of compounds 2 and 3 are given in Fig. 3. HOMO and LUMO
energy levels of these compounds were calculated using ferrocene
as the standard redox system.39 The half wave potentials and
HOMO–LUMO energy values are summarized in Table 3. Both
the carbazolyl substituted perylene bisimides exhibit dianionic
behaviour similar to other bay-substituted perylene derivatives.40
Moreover they also undergo oxidation and display cationic
behaviour which is believed to arise due to the influence of the
electron donating carbazolyl bay-substituents. As can be seen
from Table 3, compound 2 possesses higher electrochemical band
gap than compound 3 and the trend is comparable with the
optical band gaps. Relatively high LUMO levels of compounds 2
and 3 compared to other related materials may be attributed to
the twisted structure and rather high electron density in the
Fig. 3 Cyclic voltammograms of compounds 2 and 3. Electrolyte: n-
Bu4NPF6 in DCM (0.01 mol Lꢀ1). Working, counter and reference
electrodes: glassy carbon, platinum wire and Ag/AgNO3 (0.01 mol Lꢀ1 in
Fig. 2 UV-Vis absorption spectra of compounds 1–4 in THF (10ꢀ4 mol
Lꢀ1).
acetonitrile) respectively. Scan rate: 100 mV sꢀ1
.
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Table 3 Electrochemical characteristics of compounds 2 and 3a
Compound
Ered/V
Eoxi/V
Eredonset/V
Eoxionset/V
EHOMO/eV
ELUMO/eV
Egele/eV
2
3
ꢀ1.13, ꢀ1.24; ꢀ0.94, ꢀ1.05
ꢀ0.96, ꢀ1.05; ꢀ0.81, ꢀ0.87
ele
Eg – Electrochemical band gap.
0.94, 0.87
1.37, 1.18; 1.12, 0.86
ꢀ1.18; ꢀ0.97
ꢀ1.05; ꢀ0.84
0.90
1.27; 0.99
ꢀ5.53
ꢀ5.62
ꢀ3.66
ꢀ3.79
1.87
1.83
a
perylene core induced by bulky electron-donating bay
substituents.40,41
compounds 1–4 in air. Compounds 1 and 2 provide good neat
thin films. However, thin film preparation for XTOF experiment
was not successful for 3 and therefore, a solid solution (1 : 1; wt.)
was prepared with an inert polymer host, bisphenol Z-poly-
carbonate (PC-Z). In order to have a fair comparison of charge-
transporting properties in neat films between 2- and 3-carbazolyl
substituted derivatives, compound 4 was synthesized and found
to form good neat film.
2.5. Photoelectrical properties
Ionization potentials (Ip) of solid layers of the synthesized
compounds were measured by photoelectron spectroscopy
(PES). Photoelectron spectra of 1–4 are given in Fig. 4. The
intersection points of the linear parts of the spectra drawn with
the abscissa axis give the ionization potential values. Ip values are
rather close and range from 5.8 eV to 6.0 eV. The highest Ip value
was observed for 2-carbazolyl substituted derivative (2). A small
difference is observed in the values of HOMO (Ip) energy levels of
carbazolyl derivatives (2 and 3) obtained by PES and electro-
chemical studies. This is due to the difference in molecular
interactions and molecular arrangements in thin solid layers and
in dilute solutions of these derivatives.
All the compounds (1–4) exhibited capability of transporting
both electrons and holes. This is evidenced by the practically
equal amounts of positive and negative charges extracted after
the impulse of strongly absorbed irradiation with the wavelength
of 337 nm. However, due to the enhanced dispersity of hole-
transport we were not able to estimate the hole drift mobilities in
the layer of 1. XTOF charge drift mobility data for 1–4 are
summarized in Table 4 and representative dU/dt transients for
the neat films of 1 and 4 are shown in Fig. 5. A distinct inflection
point indicating transit time with the sign of non-dispersive
electron transport was observed in air for the neat film of 1 which
was unaltered even after ambient storage for several weeks.
Compound 2 also exhibited a non-dispersive electron transport
with significantly high electron drift mobility whereas a disper-
sive and rather slow hole-transport was observed. For
compounds 3 and 4, charge-transport was dispersive; however,
the drift mobility values were sufficiently high.
The xerographic time of flight (XTOF) technique was used for
the evaluation of charge-transporting properties of thin layers of
The room temperature electric field dependencies of hole and
electron drift mobility (m) values for the thin films of perylene
bisimide derivatives 1, 2 and 4 are given in Fig. 6. The linear
dependencies of hole/electron-drift mobilities on the square root
of electric field E are observed. In all cases charge drift mobility
may be well approximated by the Poole–Frenkel relationship:
m ¼ m0 exp (aOE), where a is the field dependence parameter and
m0 is the zero field mobility obtained by extrapolating the linear
dependencies to zero electric field.42,43 The highest electron
mobility is observed for 1 containing triphenylamino moiety at
the bay region and its value well exceeded 10ꢀ3 cm2 Vꢀ1 sꢀ1 at high
electric fields. Electron drift mobility values of 1 have also been
Fig. 4 Photoelectron spectra and ionization potentials (in eV) of the thin
films of compounds 1–4.
Table 4 XTOF charge mobility data of 1–4a
Electrons
Holes
Compound
m0
m
a
m0
m
a
1
2.8 ꢂ 10ꢀ4
9.2 ꢂ 10ꢀ5
1.4 ꢂ 10ꢀ8
7.3 ꢂ 10ꢀ7
2.3 ꢂ 10ꢀ3
1.4 ꢂ 10ꢀ3
1.2 ꢂ 10ꢀ6
8.9 ꢂ 10ꢀ5
0.0021
0.0027
0.0045
0.0021
—
—
—
2
8.1 ꢂ 10ꢀ11
1.2 ꢂ 10ꢀ7
5.8 ꢂ 10ꢀ5
1.2 ꢂ 10ꢀ7
9.2 ꢂ 10ꢀ6
1.7 ꢂ 10ꢀ3
0.007
0.0044
0.0034
b3
4
a
Zero-field mobilities (m0) in cm2 Vꢀ1 sꢀ1, mobilities (m) at an electric field of 106 V cmꢀ1 in cm2 Vꢀ1 sꢀ1 and field dependences (a) in (cm Vꢀ1
)
1/2 of holes and
b
electrons in solid layers. Solid solution [(3 + PC-Z) 1 : 1; wt.].
7814 | J. Mater. Chem., 2011, 21, 7811–7819
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Fig. 5 XTOF transients for the neat films of perylene derivatives 1 (a) and 4 (b). 1 ns laser operating at 337 nm was used, T ¼ 25 ꢁC. Arrows on insets (a)
and on main layout (b) indicate transit time of electrons and holes respectively at corresponding surface voltages.
measured after several weeks in order to evaluate the charge-
transporting stability in ambient conditions and it was found
that they are almost insensitive to ambient storage. The combi-
nation of efficient charge carrier mobility and its ambient
stability renders 1 particularly attractive for the fabrication of
field effect transistors.11,12 Compounds 3 and 4 exhibit a similar
pattern of ambipolar charge transportation. However, the
magnitude of carrier mobility in the layer of compound 3 is less
compared to that of 4 and it may be due to the less amount of
chromophores in the solid solution layer of 3 (Table 4). It is
interesting to compare the electron and hole drift mobilities in
compounds 2 and 4 since they exhibit reverse trends in charge
transportation. Compound 2, in which carbazolyl groups are
linked to perylene bay via their 2-position, exhibits a higher order
of electron mobility (>10ꢀ3 cm2 Vꢀ1 sꢀ1) than 4 (>10ꢀ5 cm2 Vꢀ1
sꢀ1). However compound 4, in which carbazolyl groups are
linked to perylene bay via their 3-position, displays a higher order
of hole mobility (>10ꢀ3 cm2 Vꢀ1 sꢀ1) than 2 (>10ꢀ7 cm2 Vꢀ1 sꢀ1). It
should be noted that 2 and 4 differ in the position of covalent
linking of carbazolyl groups to the perylene bisimide core.
Therefore the observation of these complementary charge carrier
mobilities just by changing the linking topology of the carbazolyl
groups to the perylene core is quite remarkable.
band-gap energies, twisting of the perylene core (dihedral angle,
a) and that of the bay substituted carbazoles with respect to the
perylene core (torsional angle, b1 and b2), in accordance with the
previous works.33,44 DFT calculations were as implemented in
the Gamess US package.45 Perylene bisimide derivatives were first
geometry optimised using a semi-empirical PM6 basis set as
implemented in MOPAC46 and these PM6 optimized geometries
were further subjected to complete geometry optimization using
DFT calculations at the basis B3LYP/6-311G*. These optimized
structures were then used to perform single point energy calcu-
lations at B3LYP/6-311++G** basis to analyse the above
mentioned properties.
Fig. 7 (A & B) demonstrates the calculated frontier orbitals for
molecules 2 and 3. HOMO and LUMO energy levels of these
molecules established by quantum chemical calculations are
summarised in Table 5. As seen earlier with the perylenes, these
molecules behave in a similar way with their LUMO mainly
localised on the perylene bisimide core.44 However, there is
a considerable difference observed between the HOMO orbitals
of species 2 and 3. In compound 3, it is seen that the HOMO is
distributed evenly on the perylene ring as well as on both the
carbazole units. However in 2, the HOMO is more localised on
one of the carbazole units unlike in 3. Fig. 7(C) demonstrates the
FMO for molecule 2 at two different isosurface values; (a) [same
as in 7(A)] and (b). It clearly shows the HOMO being spanned
throughout the molecule; but not evenly like in 3 and centred
mainly on one of the carbazoles. Thus it is evident that the
carbazole substitutions enhance a considerable difference in the
HOMO orbitals of 2 and 3, when there is a difference in their
2.6. DFT calculations
Quantum chemistry studies were performed for carbazolyl-
substituted perylene bisimides 2 and 3 in order to acquire
a deeper insight into their frontier molecular orbitals (FMOs),
Fig. 6 The electric field dependencies of charge mobilities of 1, 2 and 4 in air.
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Fig. 8 Schematic to demonstrate different twist angles in carbazole
substituted perylene bisimides. For clarity instead of carbazolyl groups
only phenyl groups are shown.
in Table 5. The calculations imply that the dihedral angle (a) for
both 2 and 3 is ꢃ20ꢁ as reported with perylenes before.44 As far as
the torsional angle (b) is concerned, it is seen that 3 possesses
a twisting angle of ꢃ57ꢁ (both b1 and b2) and that of 2 is found to
be ꢃ56ꢁ (b1) and ꢃ62ꢁ (b2). Thus here, it can be seen that there is
a considerable difference in b angles between 2 and 3. It is clearly
noticeable that b1 for 2 and 3 are similar, but b2 shows a devia-
tion by ꢃ6ꢁ. This is also quite visible in their FMOs, as in 2, for
which it can be seen that the electron cloud is much concentrated
on one of the carbazoles where it makes more inclination with the
perylene core. This might be due to the existence of strong twist
between two chromophoric sub units that in turn is limiting the
spatial orbital overlap.47 This could be the reason for the
different optical and complementary charge transporting prop-
erties of these regioisomers.
Fig. 7 FMOs obtained from quantum chemistry analysis. Comparison
of HOMO and LUMO orbitals for the molecules 2 (A) and 3 (B) as
obtained from DFT calculations. (C) HOMO orbitals for molecule 2 at
two different isosurface values.
Table 5 HOMO, LUMO and various angles of 2 and 3 from DFT
calculationsa
3. Experimental
ꢁ
ꢁ
b2
Compound EHOMO/eV ELUMO/eV Eg/eV aꢁ
b1
3.1. Materials and methods
2
3
ꢀ5.77
ꢀ5.61
ꢀ3.36
ꢀ3.28
2.41
2.33
20.66 56.77 61.73
19.52 57.69 56.03
All chemicals and solvents are of reagent grade and were used as
received unless otherwise indicated. Dichloromethane and THF
were purified by standard distillation procedures prior to use.
Nuclear magnetic resonance spectra were obtained in deuterated
chloroform with a Varian Unity Inova spectrometer operating at
300 and 75.5 MHz for 1H and 13C nuclei respectively. All the data
are given as chemical shifts d (ppm) downfield from TMS. IR-
spectroscopy measurements were performed on a Perkin Elmer
Spectrum GX spectrophotometer, using KBr pellets. The
absorbance and fluorescence spectra were recorded on a Shi-
madzu 3101 PC spectrophotometer and an RF-5301PC
Shimadzu spectrofluorometer, respectively. Mass spectra were
obtained on a Waters Micromass ZQ mass spectrometer. The
molecular weights of the target molecules were determined by
MALDI-TOF using a Shimadzu Biotech Axima Performance
system and 2,5-dihydroxybenzoic acid (DHB) was used as the
matrix under reflector mode of operation. Differential scanning
calorimetry measurements were performed on a Perkin Elmer
a
Eg – Band gap. (a) – Perylene core twisting (dihedral) angle. (b1, b2) –
torsional angles of carbazoles with perylene core.
attachment at the bay position of perylene bisimides. Calculated
band-gaps for these molecules were found to be 2.41 eV and 2.33
eV respectively for species 2 and 3. The band gap energies
calculated seem to deviate by a small factor from the experi-
mentally determined ones, but this is owing to the fact that the
quantum chemistry calculations are performed at vacuum and
the crystalline nature of the model. Another important feature to
address here is the twisting of the perylene core (a) and the
twisted bay substitution (b) of carbazoles around the perylene
bisimide as these parameters play a significant role in defining the
properties of these materials. Fig. 8 demonstrates different
twisting angles in these molecules and their values are described
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Pyris Diamond DSC apparatus at a heating/cooling rate of 10 ꢁC
minꢀ1 under nitrogen atmosphere and thermogravimetric anal-
ysis was executed on a TA Instruments Q100 under nitrogen
atmosphere at a heating rate of 20 ꢁC minꢀ1. Cyclic voltammetry
measurements were carried out with a glassy carbon working
electrode in a three-electrode cell using a m-Autolab Type III
(EcoChemie, Netherlands) potentiostat.
by dropwise addition of 8 mL of bromine for a period of 30 min
and stirred for 24 h. The reaction mixture was cooled to room
temperature and water was added into it. The precipitated
product was filtered, washed with acetone and dried in vacuum.
Thus obtained bromo substituted perylene dianhydride (2 g, 3.65
mmol) was suspended in 40 mL of N-methylpyrrolidone and
mixed with 20 mL of glacial acetic acid. It was heated at 60 ꢁC for
20 min and 2-ethylhexylamine (1.5 mL, 9.12 mmol) or n-dode-
cylamine (2.1 mL, 9.12 mmol) was added. The temperature was
raised to 120 ꢁC and stirring was continued for 12 h under
nitrogen atmosphere. The mixture was poured into 300 mL of
water and the precipitated product was filtered. It was washed
with methanol, subjected to column chromatography (silica gel,
eluent hexane–ethyl acetate, 8 : 2) and the desired product was
isolated.
The ionization potentials (Ip) were measured by photoelectron
spectroscopy in air. The materials were dissolved in THF and
coated onto Al plates pre-coated with ꢃ0.5 mm thick methyl
methacrylate and methacrylic acid copolymer (MKM) adhesive
layer. The function of the MKM layer is not only to improve
adhesion, but also to eliminate the electron photoemission from
the Al layer. In addition, this layer is conductive enough to avoid
charge accumulation on it during the measurements. The thick-
ness of the layers was 0.5–1 mm.
Charge drift mobilities of the neat materials or molecular
mixtures with a polymer host bisphenol Z polycarbonate (PC-Z)
were estimated by the xerographic time-of-flight method. The
samples for the measurements were prepared by casting the
solutions of the compounds or solutions of the mixtures of these
compounds with binder materials PC-Z at mass proportion 1 : 1
in THF. The substrates were glass plates with a conductive SnO2
layer or polyester film with Al layer. After coating the samples
were heated at 80 ꢁC for 1 h. Thus the transporting layers of the
samples were prepared. In some cases, in order to avoid crys-
tallization, the layers were dried at room temperature for several
hours. The thickness of the transporting layer varied in the range
of 1–7 mm. Electron drift mobility (m) was measured in the
xerographic mode. The electric field was created by negative
corona charging. The charge carriers were generated at the layer
surface by illumination with pulses of nitrogen laser (pulse
duration was 1 ns, wavelength 337 nm). The layer surface
potential decrease as a result of pulse illumination was up to 1–
5% of initial potential before illumination. The capacitance
probe connected to the wide frequency band electrometer
measured the speed of the surface potential decrease dU/dt. The
transit time tt for the samples with the transporting material was
determined by the kink on the curve of the dU/dt transient in
linear scale. In other cases, the dispersion of the transient current
was larger and log–log scale was used. The drift mobility was
calculated by using the formula m ¼ d2/U0tt, where d is the layer
thickness, and U0 the surface potential at the moment of
illumination.
1,7-dibromo N,N0-2-ethylhexyl perylene bisimide (5). Yield ¼
52%, red solid. 1H NMR (300 MHz, CDCl3, d ppm): 9.48 (d, J ¼
2.8 Hz, 2H), 8.91 (s, 2H), 8.70 (d, J ¼ 2.7 Hz, 2H), 4.23–4.11 (m,
4H, NCH2), 2.03–1.91 (m, 2H, CH), 1.47–1.28 (m, 16H, CH2), 1–
0.91 (m, 12H, CH3); MS (EI) m/z ¼ 773 (M+).
1,7-dibromo N,N0-dodecyl perylene bisimide (6). Yield ¼ 45%,
1
red solid. H NMR (300 MHz, CDCl3, d ppm): 9.49 (d, J ¼ 2.7
Hz, 2H), 8.91 (s, 2H), 8.71 (d, J ¼ 2.7 Hz, 2H), 4.27–4.18 (m, 4H,
NCH2), 1.78 (b, 4H, CH2), 1.44 (b, 4H, CH2), 1.29 (b, 32H,
CH2), 0.9 (t, J ¼ 4.4 Hz, 6H, CH3); MS (EI) m/z ¼ 885 (M+).
3.3. General procedure for Suzuki–Miyaura reactions
1,7-dibromo N,N0-2-ethylhexyl perylene bisimide (300 mg, 0.39
mmol) or 1,7-dibromo N,N0-dodecyl perylene bisimide (300 mg,
0.34 mmol) and the corresponding mono boronic acid or esters
(2.2 molar equiv.) were dissolved in a solvent mixture of 15 mL of
THF and 2 mL of water. Powdered potassium hydroxide (3.3
molar equiv.) was added and the reaction mixture was purged
with nitrogen for 10 min. Then the reaction vessel was degassed.
Bis(triphenylphosphine) palladium(II) dichloride (0.03 molar
ꢁ
equiv.) was added into it and stirred for 8–12 h at 80 C under
nitrogen. After completion, the reaction mixture was diluted with
water and extracted with ethyl acetate. The organic layer was
dried over sodium sulfate and evaporated. The crude product
was purified by column chromatography using silica gel as
stationary phase and hexane–ethyl acetate mixture (97 : 3 for 1;
95 : 5 for 2&3; 9 : 1 for 4) as eluent.
DFT calculations were as implemented in the Gamess US
package. Compounds were first geometry optimised using
a semi-empirical PM6 basis set as implemented in MOPAC and
these PM6 optimized geometries were further subjected to
complete geometry optimization using DFT calculations at the
basis B3LYP/6-311G*. Optimized structures were then used to
perform single point energy calculations at B3LYP/6-311++G**
basis set.
1,7-bis[4-(diphenylamino)phenyl] N,N0-2-ethylhexyl perylene
1
bisimide (1). Yield ¼ 24%; black solid. H NMR (300 MHz,
CDCl3, d ppm): 8.63 (s, 2H), 8.24 (d, J ¼ 2.8 Hz, 2H), 8.06 (d, J ¼
2.7 Hz, 2H), 7.42–7.34 (m, 12H), 7.24–7.1 (m, 16H), 4.26–4.1 (m,
4H, NCH2), 1.99–1.94 (m, 2H, CH), 1.47–1.31 (m, 16H, CH2),
0.99–0.92 (m, 12H, CH3). 13C NMR (75.5 MHz, CDCl3, d ppm):
164.1, 148.7, 147.4, 141.0, 135.6, 135.4, 135.3, 132.5, 130.2, 129.8,
129.6, 129.3, 125.4, 124.0, 123.9, 122.3,122.0, 44.5, 38.2, 31.0,
29.0, 24.3, 23.3, 14.4, 10.9. IR (KBr), n/cmꢀ1: (arene C–H) 3061,
3034; (aliphatic C–H) 2955, 2924, 2856; (imide C]O) 1698; (Ar
C]C) 1659, 1586; (imide C–N) 1326, 1270. Anal. Calc. for
C76H68N4O4: C, 82.88; H, 6.22; N, 5.09; O, 5.81%. Found: C,
3.2. General procedure for the synthesis of 1,7-dibromo
perylene bisimides
3,4:9,10-perylenetetracarboxylic anhydride (5 g, 12.7 mmol) was
suspended in 100 mL of concentrated sulfuric acid and 250 mg of
iodine was added to it. It was heated at 80 ꢁC for 45 min followed
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View Article Online
82.39; H 6.44; N, 4.85%. MS (MALDI-TOF) m/z ¼ 1100.29
(exact mass ¼ 1100).
Triphenylamino substituted perylene bisimides exhibit excellent
n-type characteristics with good air stability whereas bay car-
bazolyl substituted perylene bisimide derivatives can transport
both electrons and holes in ambient conditions. Changing the
linking topology of carbazolyl moieties to the core of perylene
bisimides has furnished materials with complementary semi-
conducting properties. These new electro-active multi-
chromophoric organic semiconductors with wide absorption
window, excellent photo-induced intramolecular charge transfer
and efficient charge carrier mobilities may find application in
various organic electronic and optoelectronic devices.
1,7-bis(9-ethyl-2-carbazolyl) N,N0-2-ethylhexyl perylene bisi-
mide (2). Yield ¼ 41%; black solid. 1H NMR (300 MHz, CDCl3,
d ppm): 8.64–8.51 (m, 2H), 8.23–7.96 (m, 7H), 7.74–7.46 (m, 7H),
7.34–7.02 (m, 4H), 4.83–4.25 (m, 8H, NCH2), 1.98–1.94 (m, 2H,
CH), 1.39 (b, 16H, CH2), 1.28 (b, 6H, CH3), 1.01–0.89 (m,
12H, CH3). 13C NMR (75.5 MHz, CDCl3, d ppm): 164.0, 163.7,
141.9, 140.9, 139.4, 135.5, 131.8, 130.4, 128.9, 126.5, 125.8, 122.6,
122.1, 121.7, 119.9, 119.6, 109.0, 44.6, 38.4, 31.0, 30.5, 29.9, 29.7,
29.0, 25.0, 24.4, 23.3, 14.4, 10.9. IR (KBr), n/cmꢀ1: (arene C–H)
3051; (aliphatic C–H) 2955, 2924, 2855; (imide C]O) 1695; (Ar
C]C) 1652, 1584; (imide C–N) 1326, 1259. Anal. Calc. for
C68H64N4O4: C, 81.57; H, 6.44; N, 5.60; O, 6.39%. Found: C,
81.68; H, 7.21; N, 5.03%. MS (MALDI-TOF) m/z ¼ 1000.30
(exact mass ¼ 1000).
Acknowledgements
This research work was supported by FP-7 PEOPLE PRO-
GRAMME, Marie Curie Actions – ITN Grant No. 215884.
A. Swinarew is thanked for MALDI-TOF measurements.
K. Kazalauskas is thanked for the measurements of fluorescence
quantum yields. Asta Sakalyte and Jurate Simokaitiene are
thanked for the TGA and DSC measurements.
1,7-bis(9-ethyl-3-carbazolyl) N,N0-2-ethylhexyl perylene bisi-
mide (3). Yield ¼ 55%; black solid. 1H NMR (300 MHz, CDCl3,
d ppm): 8.69 (s, 2H), 8.33 (b, 4H), 8.01 (d, J ¼ 2.8 Hz, 2H), 7.79–
7.73 (m, 2H), 7.59–7.51 (m, 5H), 7.38 (b, 5H), 4.49–4.42 (m, 4H,
NCH2), 4.27–4.12 (m, 4H, NCH2), 1.99 (b, 2H, CH), 1.55 (t, J ¼
4.8 Hz, 6H, CH3), 1.49–1.28 (m, 16H, CH2), 0.99–0.89 (m, 12H,
CH3). 13C NMR (75.5 MHz, CDCl3, d ppm): 164.1, 163.8, 141.9,
140.6, 140.1, 135.9, 134.9, 132.7, 129.8, 129.3, 128.9, 127.1, 126.9,
126.5, 121.9, 121.5, 109.1, 44.6, 38.3, 38.1, 31.1, 29.1, 24.4, 23.3,
14.4, 14.2, 10.9. IR (KBr), n/cmꢀ1: (arene C–H) 3050; (aliphatic
C–H) 2954, 2925, 2856; (imide C]O) 1695; (Ar C]C) 1656,
1585; (imide C–N) 1327, 1256. Anal. Calc. for C68H64N4O4: C,
81.57; H, 6.44; N, 5.60; O, 6.39%. Found: C, 81.46; H 6.75; N,
5.35%. MS (EI) m/z ¼ 1001(M+). MS (MALDI-TOF) m/z ¼
1000.40 (exact mass ¼ 1000).
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