Solution-processable small molecule semiconductors based on pyrene-fused bisimidazole and influence of alkyl side-chain on the charge transport

Article abstract of DOI:10.1039/c6ra13864k

To explore the potential of pyrene-fused biimidazole as a building block for soluble small molecule semiconductors, we designed and synthesized PBI-L-Na and PBI-B-Na. Imidazole rings provided substitution positions for the solubilising groups at the K-region, however, DFT calculations revealed that the repulsive steric hindrance from the neighboring hydrogen atoms on pyrene forced alkyl side chains into a twisted conformation. Thus, side chains on this type of molecule exerted a prominent influence on the molecular packing and affected their optoelectronic properties intensively in the solid state. PBI-L-Na exhibited a more ordered packing of the large conjugated plane with a π-π stacking distance of 0.36 nm and showed a hole mobility up to 0.12 cm² V^-1 s^-1. Bulkier branched chains provided better solubility but impeded molecular packing of PBI-B-Na, thus giving a hole mobility of 4.6 × 10^-3 cm² V^-1 s^-1.

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Solution-processable small molecule semiconductors based on
pyrene-fused bisimidazole and influence of alkyl side-chain on the
charge transport
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
＊
a
b
＊
b
a
DOI: 10.1039/x0xx00000x
Hui Wen , Xiaohui Gong , Pei Han , Baoping Lin , Lei Zhang , Shanghui Ye , Ying Sun , Xueqin
a
a
Zhang and Hong Yang
www.rsc.org/
To explore the potential of pyrene-fused biimidazole as a building block for soluble small molecule semiconductors, we
designed and synthesized PBI-L-Na and PBI-B-Na. Imidazole rings provided substitution positions for the solubilising
groups at K-region, however, DFT calculations revealed that the repulsive steric hindrance from the neighboring hydrogen
atoms on pyrene forced alkyl side chains into a twisted conformation. Thus, side chains on this type of molecules exerted a
prominent influence on the molecular packing and affected their optoelectronic properties intensively in the solid state.
PBI-L-Na exhibited a more ordered packing of the large conjugated plane with a π-π stacking distance of 0.36 nm and
2
-1 -1
showed a hole mobility up to 0.12 cm V s . Bulkier branched chains provided better solubility but impeded molecular
-3
2
-1 -1
packing of PBI-B-Na, thus giving a hole mobility of 4.6 × 10 cm V s .
maintaining solubility, Pei et al. moved branch point away
Introduction
from the conjugated backbones and the polymers exhibited
promoted π-π interaction with moving branch positions, which
Conjugated semiconducting materials for the next-generation
flexible and printed electronics have attracted great attentions
due to their solution processability, superior mechanical and
7
was consistent with the improved charge mobility.
Pyrene,
a potential building block with the large
1
conjugated plane, has been widely investigated to develop
8
new π-conjugated systems. Electrophilic substitution of
tunable optoelectronic properties. Optimal molecular design
of organic semiconductors for improving the device
performance generally contains two parts: synthesis of new π-
conjugated systems and side chain engineering. Most efforts
have been focused on the design and synthesis of new π-
conjugated systems because the optoelectronic properties can
pyrene preferentially takes place at the active sites (1-, 3-, 6-,
and 8-positions). Thus, the most efforts to explore its potential
in organic semiconductors often start from 1-substituted
pyrenes due to the difficulties in synthesis of diverse
9
2
substitution patterns. An Ir-catalysed aromatic borylation at
be easily tuned by the modulation of conjugated moieties. In
2, 7 positions to produce pyrene-2,7-bis(boronate)ester was
particular,
π-conjugated
systems
constructed
by
3
diketopyrrolopyrole (DPP), isoindigo and fused thiophene
4
developed by Marder et al. which can be easily used in Suzuki
1
0
5
derivatives have been extensively investigated. Alkyl chains
coupling to synthesize new π-conjugated systems. Expansion
of π-system by fusing at pyrene core opens up a new way for
the more efficient utilization of pyrene conjugation. Recently,
a few of polycyclic aromatics fused at K-region and non-K-
region of pyrene were synthesized and they exhibited
are commonly attached to the π-conjugated systems as
solubilising groups to provide solution processability. However,
it has been demonstrated that side chains also play important
roles in the molecular packing and hence affect device
1
1
6
interesting properties. Rigid structure, large π-system and
strong stacking ability make this type of polycyclic aromatic
hydrocarbons received increasing attentions.
performance. Compared to linear chains, the commonly used
branched side chains (2-octyldodecyl, 2-decyltetradecyl)
provide better solubility, but π-π intermolecular interaction is
affected by the bulky chains due to the branch point is so close
to conjugated systems. To reduce such steric hindrance while
Herein, we developed a type of polycyclic building block
(
PBI-L and PBI-B, scheme 1) by fusing two imidazole rings onto
pyrene core at K-region. Fusion of five-membered imidazole
rings with a symmetrical pattern by Debus-Radziszewski
condensation offered several advantages: extended π-systems,
large conjugated plane can be easily used in metal-catalyzed
cross-coupling reactions for synthesis of new semiconductors
and alkyl chains can be readily introduced on N atoms to
afford satisfactory solubility for the purification and solution
processing. Considering that the soluble small molecules
a.
School of Chemistry and Chemical Engineering, Southeast University, Nanjing
2
11189, PR China.
E-mail: lbp@seu.edu.cn
b.
Institute of Advanced Materials (IAM), Nanjing University of Posts &
Telecommunications, Nanjing 210023, PR China.
E-mail: iamshye@njupt.edu.cn
1
†
Electronic Supplementary Informaꢀon (ESI) available: H spectra, electrochemical
and OFET data. See DOI: 10.1039/x0xx00000x
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display advantages of simple purification and well-defined
structures, we synthesized two compounds (PBI-L-Na and PBI-
B-Na, scheme 1) based on the pyrene-fused biimidazole to
explore the potential of this large conjugated system in small
molecule semiconductors. Linear and branched chains were
used to reveal side chain effects on the physical properties.
PBI-L-Na showed a more ordered structure with a hole
2
-1 -1
mobility of 0.12 cm V s , which is almost 30 times larger
-
3
2
than that of PBI-B-Na (4.6 × 10 cm V s ).
-1 -1
Results and discussion
Synthesis and characterizations
The synthesis of PBI-L-Na and PBI-B-Na, consisting of
pyrene-fused biimidazole, is shown in scheme 1. Pyrene-
4
3 2
,5,9,10-teraketone (1) was prepared by RuCl •xH O catalysed
Scheme 1: synthetic route for PBI-L-Na and PBI-B-Na: (a)
oxidation at K-region. Debus-Radziszewski condensation in the
presence of ammonium salt and 5-Bromothiophene-2-
NaIO
4
, RuCl
0 °C, 12 h, 6.8%; (b) NH
5%; (c) Br , triphenylphosphine, dichloromethane, 25 °C, 12
CO , DMF, 100 °C, 12 h, 67-41%; (e) Palladium-
tetrakis(triphenylphosphine), K O/THF, 65 °C, 12 h,
5%.
3
•xH
2
O, acetonitrile/dichloromethane/H
2
O, 30 °C-
4
7
4
OAc, glacial acetic acid, reflux, 10 h,
carboxaldehyde was successfully applied on
1 to construct
2
compound 2. This compound only has solubility in the strong
h, 90%; (d) K
2
3
polar solvents such as DMF and DMSO, which originates from
the active hydrogen atoms on the imidazole rings. Therefore,
2
CO3, H
2
6
the crude compound
2 was directly employed in the next
reaction without any further purification. In order to get the
satisfactory solubility and study influence of side chain on
physical properties, the long linear and branched alkyl chains
were attached on the nitrogen atoms, thus giving PBI-L and
PBI-B. Finally, the target compounds PBI-L-Na and PBI-B-Na
can be easily prepared using a palladium-catalyzed Suzuki
coupling reaction between the building blocks and
naphthalene-2-ylboronic acid.
Imidazole rings fused at K-region of pyrene provide
substitution positions for the side chains, thus, the steric
interaction between alkyl chains and the neighboring
hydrogen atoms on pyrene (indicated by red cycle in Fig. 1)
may exist to cause the twisted structure. Calculation on the
molecule based on naphthalene-fused bisimidazole (without
the neighboring hydrogen atoms) substantiated this kind of
steric hindrance (Fig. S2). PBI-L-Na and PBI-B-Na presented
almost the same LUMO/HOMO orbitals, indicating that
influence of alkyl chains on the electronic properties was
negligible in the single molecular state. Imidazole ring is
usually treated as an electron-deficient unit, however DFT
The decomposition temperatures were determined by
thermal gravimetric analysis (TGA). The 5% weight loss
temperatures were 392 °C and 387 °C for PBI-L-Na and PBI-B-
Na (Fig. S1), respectively, indicating their high thermal stability
which can be attributed to the robust conjugated system of
pyrene-fused biimidazole.
Density functional theory (DFT) calculations
To further understand structural and electronic
characteristics of pyrene-fused biimidazole based small
molecules, density functional theory (DFT) calculations were
performed at the B3LYP 6-31G* level using Gaussian 09
program. Note that calculations were performed in the single
molecular state. Alkyl chains were kept with a short length in
the calculations to figure out their architecture that can exert
an important influence on the molecular packing. As shown in
Fig. 1, pyrene-fused biimidazole unit, which is expected to
facilitate strong π-stacking, displayed a rigid and planar π-
skeleton in both molecules. PBI-L-Na and PBI-B-Na exhibited
almost identical geometry of aromatic core suggesting that
side chains do not affect conjugated systems. It is also should
be noted that alkyl chains extended out of the conjugated
plane. Especially for PBI-B-Na, the bulkier branched chains Fig. 1 Optimized geometry and calculated molecular orbitals for
may strongly impede intermolecular π-π interactions by the PBI-L-Na and PBI-B-Na. DFT calculations were performed at
extending out of the conjugated plane with two directions. the B3LYP/6-31G(d,p) level on Gaussian 09.
2
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Table 1 Electrochemical properties of PBI-L-Na and PBI-B-Na
ec
g
opt
LUMO
eV)
HOMO
(eV)
E
E
g
Compound
(
(eV)
(eV)
PBI-L-Na
PBI-B-Na
-3.15
-3.0
-5.64
-5.72
2.49
2.72
2.58
2.93
Fig. 2 Normalized UV-Vis absorption spectra of PBI-L-Na and
PBI-B-Na in chloroform solution and thin films.
commonly observed for conjugated materials due to the
stacking of π-systems. Furthermore, the shoulder at 455 nm
turned to an obvious peak after annealing at 125 °C, implying
calculations showed that LUMO orbitals were mainly that PBI-L-Na had an enhanced π-π interaction after thermal
delocalized over thiophene and naphthalene units and treatment. In contrast, spectra of PBI-B-Na films only exhibited
partially located on pyrene-fused bisimidazole. Additionally, more vibronic fine structures and a tiny red shift compared to
the HOMO orbitals were predominantly located on pyrene- that of solution, suggesting that the bulkier branched chains
fused bisimidazole. This distribution of molecular orbitals was disrupted the π-stacking in the solid state. As a result, the
opt
different from the typical D-A organic semiconductors optical energy gap (E
g
) of PBI-L-Na (2.58 eV) measured from
constructed with strong donor and acceptor. We suggested the onset of its absorption in the film was much narrower than
that imidazole ring is a weak electron-withdrawing unit due to that of PBI-B-Na (2.93 eV).
3
the electron-donating ability of the sp N in the five-membered
ring.
These results suggested that side chains on pyrene-fused
biimidazole based molecules can exert a prominent influence
on the molecular packing, thus affected optoelectronic
properties in the solid state. However, in the single molecular
state, the effect of side chains on π-conjugated system can be
neglected.
Optical and electrochemical properties
The UV-vis absorption spectra of PBI-L-Na and PBI-B-Na
3
were measured in CHCl solution and thin films (Fig. 2).
Electrochemical properties of PBI-L-Na and PBI-B-Na were
investigated using cyclic voltammetry measurements from
their drop-cast thin films on the platinum working electrode
and the data are summarized in Table 1. Both compounds
presented oxidation and reduction waves in their
voltammograms (Fig. 3). From the onset of reduction, the
LUMO energy level of PBI-L-Na was estimated to be -3.15 eV,
which is 0.15 eV lower than that of PBI-B-Na. On the other
hand, the HOMO energy levels were estimated to be -5.64 eV
and -5.72 eV for PBI-L-Na and PBI-B-Na, respectively. The
extremely deep HOMO energy levels of two molecules were
likely due to the pyrene unit consisting of four fused benzene
rings. Energy levels in solid state changed with different side
Solution of two compounds presented similar absorption
spectra, indicating that alkyl chains had no effect on the
photophysical properties in the single molecular state which is
consistent with the result from the DFT calculations.
Considering a small overlap between HOMO and LUMO
orbitals, the peak at around 400 nm in solution was attributed
to the intramolecular charge transfer (ICT) transition. The
other two characteristic bands located at ca. 349 nm and 293
nm can be assigned to the π-π* transition and imidazole-
centered n-π* transition, respectively. In solid state, PBI-L-Na
spectra showed
a notable bathochromic shift and a
pronounced peak at 423 nm with a small shoulder, which were
7
chains and similar tendency were observed in Jian Pei’s work.
ec
The electrochemical band gaps (E ) were found to be 2.49 eV
g
for PBI-L-Na and 2.72 eV for PBI-B-Na, respectively, within 0.4
eV of the optical energy gaps.
Characteristics of organic field-effect transistors
Large and coplanar conjugated plane may facilitate the
charge transport and it is believed that side chains also have a
great influence on the device performance. To investigate the
charge transport properties of PBI-L-Na and PBI-B-Na, top-
contact-bottom-gate OFET devices were fabricated by spin-
coating of their solutions on heavily doped n-type silicon
wafers with the 300 nm thermally grown SiO . The field-effect
2
Fig. 3 Cyclic voltammograms (CV) and schematic energy
diagrams of PBI-L-Na and PBI-B-Na films drop-casted onto
platinum working electrode.
mobilities were extracted from transfer curves in saturation
region tested in the ambient air conditions, and the
performance data are listed in Table 2. Two compounds are
typical P-type semiconductors according to their transfer and
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Table 2 Summary of performance of OFET devices based on
PBI-L-Na and PBI-B-Na
μ
Compound T (°C)
2
-1 -1
cm V S )
VT (V)
I /I
on off
(
4
4
5
5
3
-
-
0.032
0.069
0.12
-10.3
-25.1
-21.5
-15.6
-45.6
-46.1
-30.3
-48.1
3.2 × 10
5.9 × 10
1.0 × 10
5.3 × 10
100
125
150
PBI-L-Na
Fig. 5 XRD data obtained from PBI-L-Na and PBI-B-Na before
and after thermal annealing (125 °C).
0.080
-3
-3
-3
-3
corresponding to the lamellar structure with a d-spacing of
--
1.5 × 10
2.6 × 10
4.6 × 10
3.3 × 10
5.1 × 10
4.64 nm, was observed in pristine PBI-L-Na. Two weak peaks at
3
1
1
1
00
8.1 × 10
2θ = 2.8° and 4.0° also indicated a small number of more
closed lamellar structures. The π-π stacking resulted in a series
of peaks after 2θ = 20° in the pristine PBI-L-Na, suggested the
coexistence of different packing motifs of conjugated plane.
After thermal annealing, PBI-L-Na exhibited a stronger primary
peak and the peak red-shifted slightly to 2θ = 2.2° (d-spacing of
PBI-B-Na
4
25
50
1.9 × 10
3
4.2 × 10
4.01 nm), due to the drastically improved crystallinity. Most
output curves (Fig. 4, Fig. S3 and Fig. S4). Devices based on
importantly, thermal treatment also decreased distance of π-π
stacking to 0.36 nm as evidenced by the diffraction peak red-
shifted to the 2θ = 24.25°. On the other hand, Bulkier
branched chains on PBI-B-Na impeded aggregation of the
molecules, thus the effect of thermal annealing was limited. A
less intensive primary peak at 2θ = 5.7° was observed for PBI-
B-Na, indicating the reduced crystallinity compared to PBI-L-
-3
pristine films showed hole mobilities of 0.032 and 1.5 × 10
-1 -1
s for PBI-L-Na and PBI-B-Na, respectively. Thermal
2
cm V
annealing improved device performance significantly for PBI-L-
Na and it was found to exhibit the maximum hole mobility of
2
.12 cm V s with an on/off ratio of 1.0 × 10 after annealing
-1 -1
5
0
-3
at 125 °C. In contrast, the maximum hole mobility of 4.6 × 10
-1 -1
s for PBI-B-Na was dramatically lower than that of
2
cm V
Na
. The much smaller lamellar d-spacing (1.55 nm)
PBI-L-Na
.
corresponding to this diffraction was originated from the
interdigitation of the shorter branched chains. However, the
broad peak at 2θ = 22.80° showed a larger π-π stacking
distance of 0.39 nm.
Results from the OFET devices revealed that pyrene-fused
biimidazole is a potential core for the design of new small
molecule semiconductors. It is also interesting to note that
selection of side chains for this type of molecules is very
important as they can dramatically affect the charge transport
properties.
To further elucidate molecular packing in OFET devices and
correlate this important property with device performance, 2-
Microstructures and film morphology
To gain insight into above results, conventional X-ray
diffraction (XRD) measurements were performed to
investigate molecular aggregation of PBI-L-Na and PBI-B-Na
.
As shown in Fig. 5, an intense diffraction peak at 2θ = 1.9°,
Fig. 4 Output (left) and transfer (right) characteristics for PBI-
L-Na annealed at 125 °C. The hole mobility value calculated in
2 -1 -1
the saturation region is 0.12 cm V s .
Fig. 6 2-D GIXRD images (top) and out-of-plane GIXRD profiles
(bottom) for PBI-L-Na and PBI-B-Na films annealed at 125 °C.
4
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Two small molecule semiconductors, PBI-L-Na and PBI-B-
Na, were designed and synthesized based on pyrene-fused
biimidazole. According to the DFT calculations, steric
hindrance of neighboring hydrogen atoms on pyrene caused a
twisted structure of side chains. Thus, it was found that side
chains on this type of molecules had a prominent influence on
the molecular packing and intensively affected optoelectronic
properties in the solid state. Linear chains facilitated a more
ordered packing of the large conjugated plane with a π-π
stacking distance of 0.36 nm, thereby PBI-L-Na exhibited a
2
-1 -1
hole mobility up to 0.12 cm V s . While bulkier branched
chains impeded molecular packing thus giving a much lower
-3
2
-1 -1
hole mobility of 4.6 × 10 cm V s . This work showed the
potential of pyrene-fused biimidazole as a building block for
small molecule semiconductors and emphasized the
importance of side chain selection for this type of molecules.
Fig. 7 AFM height images for PBI-L-Na films before and after
Acknowledgements
thermal annealing at different temperatures
We gratefully acknowledge the financial support from
D grazing-incident X-ray diffraction (GIXRD) was used to National Natural Science Foundation of China (21304018
examine spin-coated films on OTS modified Si/SiO substrates 21374016 61106017) and Jiangsu Provincial Natural Science
2
annealed at 125 °C. The bright diffraction spot at around q = Foundation of China (BK20130619 BK20130617).
z
-
1
0
.5 nm which was partially blocked by the beam stop
originated from the intense diffuse reflectivity. As shown in
-
1
References
z
Fig. 6, PBI-L-Na exhibited an intensive peak at q = 1.53 nm
-
1
z
along with a weak diffraction at q = 3.18 nm , which were
consistent with the data obtained from the conventional XRD.
The clarity of diffraction peak indicated a high degree of
crystallinity in PBI-L-Na, whereas the arc shape suggested that
the crystalline regions were somewhat misaligned with respect
to the substrate. In contrast to PBI-B-Na, an arc diffraction
pattern with a drastically decreased intensity was observed at
1
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2
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2
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PBI-L-Na was uniform and featured with fine grains. Upon
thermal annealing, the grain size and surface roughness
increased obviously, suggestive of enhanced crystallinity
organized by the strong intermolecular packing force. Surface
morphology of PBI-B-Na looked quite different and no
apparent grains were found in the films thus giving the
smoother surface. This result was consistent with the lower
crystallinity nature of PBI-B-Na revealed by the conventional
XRD and GIXRD analysis.
2
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Conclusions
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RSC Advances
View Article Online
DOI: 10.1039/C6RA13864K
Highlights:
Soluble small molecule semiconductors based on pyrene-fused biimidazole and side-chain
effects on the semiconducting properties.