A soluble ladder-conjugated star-shaped oligomer composed of four perylene diimide branches and a fluorene core: Synthesis and properties

Article abstract of DOI:10.1002/chem.201402100

A new ladder-conjugated star-shaped oligomer electron-transporting material TetraPDI-PF, with four perylene diimide (PDI) branches and a fluorene core, was efficiently synthesized. The oligomer is highly soluble in dichlorobenzene with a solubility of 155 mg mL^-1, which is higher than those of PDI (35 mg mL^-1) and PDI-Phen (70 mg mL^-1). Demonstrated by thermogravimetric analysis (TGA), the oligomer exhibits excellent thermal stability with the decomposition temperature (T_d) of 291.2 °C, which is 65 °C higher than that of PDI. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were employed to investigate the electrochemical properties. Although the CV curves of TetraPDI-PF are successively scanned for 15 cycles, they still remain invariable reduction potentials. The oligomer also shows outstanding photostability, even better than PDI, which maintains 99 % fluorescence intensity after irradiation for 10 min using maximum laser intensity. In the steady-state space-charge-limited current (SCLC) devices, TetraPDI-PF exhibits higher intrinsic electron mobility of 2.22×10^-5 cm² V^-1 s^-1, three orders of magnitude over that of PDI (3.52×10^-8 cm² V^-1 s^-1). The bulk heterojunction (BHJ) organic solar cells (OSCs) using TetraPDI-PF as non-fullerene acceptors and P3HT as donors give optimum power conversion efficiency (PCE) of 0.64 %, which is 64 times that of the PDI:P3HT BHJ cells.

Full text of DOI:10.1002/chem.201402100

DOI: 10.1002/chem.201402100
Full Paper
&
Electron-Deficient Compounds
A Soluble Ladder-Conjugated Star-Shaped Oligomer Composed of
Four Perylene Diimide Branches and a Fluorene Core: Synthesis
and Properties
Youdi Zhang, Lingcheng Chen,* Kaichen Zhang, Helin Wang, and Yi Xiao*[a]
Abstract: A new ladder-conjugated star-shaped oligomer
electron-transporting material TetraPDI-PF, with four pery-
lene diimide (PDI) branches and a fluorene core, was effi-
ciently synthesized. The oligomer is highly soluble in dichlor-
obenzene with a solubility of 155 mgmLꢀ1, which is higher
than those of PDI (35 mgmLꢀ1) and PDI-Phen (70 mgmLꢀ1).
Demonstrated by thermogravimetric analysis (TGA), the oli-
gomer exhibits excellent thermal stability with the decom-
position temperature (Td) of 291.28C, which is 658C higher
than that of PDI. Cyclic voltammetry (CV) and differential
pulse voltammetry (DPV) were employed to investigate the
electrochemical properties. Although the CV curves of Tet-
raPDI-PF are successively scanned for 15 cycles, they still
remain invariable reduction potentials. The oligomer also
shows outstanding photostability, even better than PDI,
which maintains 99% fluorescence intensity after irradiation
for 10 min using maximum laser intensity. In the steady-state
space-charge-limited current (SCLC) devices, TetraPDI-PF ex-
hibits higher intrinsic electron mobility of 2.22ꢀ
10ꢀ5 cm2 Vꢀ1 sꢀ1, three orders of magnitude over that of PDI
(3.52ꢀ10ꢀ8 cm2 Vꢀ1 sꢀ1). The bulk heterojunction (BHJ) organ-
ic solar cells (OSCs) using TetraPDI-PF as non-fullerene ac-
ceptors and P3HT as donors give optimum power conver-
sion efficiency (PCE) of 0.64%, which is 64 times that of the
PDI:P3HT BHJ cells.
Introduction
reported soluble BFI-T2 that show high electron mobility up to
0.12 cm2 Vꢀ1 sꢀ1 in organic field-effect transistors and an effi-
ciency of 1.80% in BHJ Cells (Figure 1).[6] Another excellent ex-
ample is from the group of Mꢁllen who synthesized processa-
ble rylene diimide dyes of 4 nm in length.[7]
The design of new N-type (electron-transporting) organic semi-
conductors is an essential work for the fields related to organic
electronics, because the development of N-type materials lags
far behind P-type (hole-transporting) materials.[1] Although
there are plenty of excellent P-type organic semiconductors,[2]
the efficient and stable N-type counterparts are still rare,[3]
though both of them are indispensable for most electronic de-
vices.
From another perspective, quite a few star-shaped P-type
molecules exhibit good solubility, because, to some extents,
their flexible and non-planar branches inhibit the aggregation
effect caused by the aromatic cores.[8] In some cases, the coor-
dination of the branches and the core can even produce dis-
cotic or columnar liquid crystalline structures as the preferred
charge-transporting pathways.[9] However, there are rare re-
ports on the design of star-shaped N-type materials.[8a,10] Even
so, we still believe that the strategy to improve solubility by
the construction of star-shaped molecules should also be ap-
plicable for addressing the problem of ladder-conjugated N-
type conjugated materials.
A current trend is emerging to develop new soluble N-type
ladder-conjugated molecules.[4] Conventionally, several classes
of N-type ladder-conjugated polymers, such as BBL
(Figure 1),[5] are good organic semiconductors with satisfactory
electron mobility and stability, because of their large, rigid, and
planar frameworks with highly extended conjugation systems.
However, such ladder frameworks also result in extremely poor
solubility, which hampers the applications in solution-pro-
cessed organic electron devices. To address this problem, large
and flexible alkyl side chains have been introduced to enhance
the solubility. For example, very recently, Jenekhe et al. have
For several years, our group has been engaged in the devel-
opment of soluble N-type ladder-conjugated molecules based
on perylene diimide (PDI) derivatives that have outstanding
electron mobility, strong light absorption, and high photosta-
bility and thermal stability (Figure 1). In 2011, we synthesized
LCPT, the first PDI-based ladder conjugated polymer through
photocyclization within the corresponding PDI-thienothio-
phene single-stranded conjugated polymer.[11] In addition, in
2012, we reported Tr-PBI, a star-shaped oligomer containing
three PDI branches and using truxene as the core.[12] Further-
more, in 2013, we developed TriPBI, another three-PBI star-
shaped molecule with hexaazatriphenylene as the core.[13]
[a] Y. Zhang, Dr. L. Chen, K. Zhang, H. Wang, Prof. Y. Xiao
State Key Laboratory of Fine Chemicals
Dalian University of Technology, Dalian 116024 (P.R. China)
Fax: (+86)411-84986252
E-mail: lcchen@dlut.edu.cn
xiaoyi@dlut.edu.cn
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201402100.
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high yields of 75 and 90%, respectively. Secondly, the
coupling products, synthesized by Suzuki coupling
reaction between BrꢀPDI and phenanthrenequinone
boric acid ester with a 2,7-position or 3,6-position,
using [Pd0(PPh3)4] as the catalyst in a mixture of tolu-
ene and an aqueous solution of K2CO3 (2m), are
transformed into the uniform intermediate dg-PDI-
Phen containing the protection of diethylene glycol
by photoinduced cyclization in the presence of a cata-
lytic amount of iodine. As a result, dg-PDI-Phen is
obtained with a higher yield of 70% by the 3,6-posi-
tion coupling product, whereas a yield of 21% was
achieved by the one in the 2,7-position. Thirdly, PDI-
Phen, an enlarging conjugated pivotal intermediate,
is synthesized efficiently by deprotecting in anhy-
drous p-toluene sulfonic acid (PTSA). PDI-Phen con-
taining two adjacent carbonyl groups could fuse with
any kind of aromatic systems that have two adjacent
amino groups. Therefore, PDI-Phen, as a universal
building block, is a valuable intermediate that devel-
ops and explores more suitable organic N-type mate-
rials applied in organic electronics. Finally, a new
ladder-conjugated star-shaped molecule TetraPDI-PF
containing a TABEF core fused with two PDI-Phen
arms was synthesized efficiently in the mixing solu-
tion of glacial acetic acid and chloroform (CH3COOH/
CHCl3, v/v 1:1).
It should be noted that solubility plays an impor-
tant role in solution-processed organic electronic de-
vices. We measured their solubility in common or-
ganic solution, and found that the solubility of PDI is
65, 50, and 35 mgmLꢀ1 in chloroform, chlorobenzene,
and dichlorobenzene solution, respectively. The solu-
bility of PDI-Phen is 100, 85, and 70 mgmLꢀ1 in chloroform,
chlorobenzene, and dichlorobenzene solution, respectively.
Compared with PDI and PDI-Phen, TetraPDI-PF has excellent
solubility of 205, 170, and 155 mgmLꢀ1 in chloroform, chloro-
benzene, and dichlorobenzene solution, respectively. These re-
sults reveal that the improvement of solubility is mainly due to
the congestion and the rigidity of the conjugated framework
in which the steric hindrance results in a non-coplanar struc-
ture. As demonstrated by quantum-chemical calculations using
density functional theory (DFT) (Figure 2 and the Supporting
Information, Figure S1), there exists a maximum molecular
planar distortion of 13.318 between center core and PDI
branch. In addition, two 2-ethylhexyl chains, introduced to the
9, 9’-position of fluorene core, can form the definite steric hin-
drance, which efficiently inhibits the aggregation and further
enhances the solubility.
Figure 1. The structures of BBL, BFI-T2, LCPT, Tr-PBI, TriPBI, and TetraPDI-PF.
These soluble ladder-type conjugated molecules demonstrated
intriguing optoelectronic properties.
In this contribution, we have developed a new star-shaped
ladder-conjugated molecule TetraPDI-PF with four PDI branch-
es and a fluorene core (Figure 1). In our strategy, the center
core 2,3,6,7-tetraamino-9,9-bis(2-ethylhexyl)fluorene (TABEF)
can efficiently inhibit the aggregation and enhance the solubil-
ity.[14] In addition, we have designed a bulky branch containing
two PDI branches, as a universal building block framework, in
which the larger branch will extend conjugation for better
electron delocalization and higher charge mobility. The mole-
cule TetraPDI-PF is found to have high solubility of
155 mgmLꢀ1 in dichlorobenzene and excellent thermal, elec-
trochemical, and photostabilities. Meanwhile, we have investi-
gated the electron mobility and the photovoltaic performance
with TetraPDI-PF as electron acceptors and P3HT as electron
donors.
It is crucial to obtain the thermal stability of the star-shaped
molecule as they provide important reference information for
thermal annealing temperature in organic electronic devices.
The thermal stability is evaluated by thermogravimetric analy-
sis (TGA) in nitrogen with a heating rate of 108Cminꢀ1. As
shown in Figure 3, PDI, PDI-Phen, and TetraPDI-PF possess
thermal stability with the decomposition temperature (Td) of
225.8, 251.8, and 291.28C, respectively. Although the number
Results and Discussion
Scheme 1 shows two strategies to synthesize the target com-
pound TetraPDI-PF. Firstly, with phenanthrenequinone as the
starting material, 2,7-dibromo-phenanthrenequinone and 3,6-
dibromo-phenanthrenequinone are efficiently synthesized in
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Scheme 1. Synthesis of the target compound TetraPDI-PF and two synthetic pathways of dg-PDI-Phen.
of flexible chains within TetraPDI-PF is increased, in compari-
son with PDI and PDI-Phen, it does not accelerate the decom-
position, which means the rigid conjugated skeleton of this
large star-shaped oligomer plays a major role in the process of
the thermal decomposition. As a result, the Td of TetraPDI-PF
is 658C higher than that of PDI. The results reveal that the
star-shaped ladder-conjugated molecule has an excellent ther-
mal stability, which should be ascribed to the extension of
large rigid conjugated system.
The absorption and emission spectra of PDI, PDI-Phen, and
TetraPDI-PF in dichloromethane (1ꢀ10ꢀ5 molLꢀ1) and in films
are shown in Figure 4a–4d, and the relative data are shown in
Table 1. As shown in Figure 4a, they exhibit sharp peaks in
almost different multiple bands. PDI shows a narrow absorp-
tion spectra from 450 to 550 nm and a maximum absorption
peak at 525 nm (e=8320mꢀ1 cmꢀ1) and the maximum absorp-
tion peak of PDI-Phen is at 435 nm (e=7000mꢀ1 cmꢀ1). Com-
pared with the two ones, TetraPDI-PF shows stronger and
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Figure 2. Ground-state geometries of TetraPDI-PF calculated from density
functional theory.
broader absorption spectra. The absorption spectra cover
a broad wavelength range from 300 to 550 nm. The shorter
wavelength absorption bands at 325 nm is originated from the
p–p* transition and the maximum absorption peak is at
481 nm (e=25700mꢀ1 cmꢀ1). These results reveal that the en-
larged conjugated system benefits the efficient absorption of
the solar spectrum. And the optical energy band gaps (Egopt) of
PDI, PDI-Phen and TetraPDI-PF, deriving from the absorption
onset, are 2.3, 2.24, and 2.26 eV, respectively. In the films (Fig-
ure 4b), they show obvious redshifts relative to those in solu-
tion and exhibit significant broader absorption over 580 nm
with the maximum absorption peaks of 497, 492, and 488 nm
for PDI, PDI-Phen, and TetraPDI-PF, respectively. Compared
with the other twos, TetraPDI-PF shows blueshifts of the maxi-
mum peak and absorption edge in the film, which indicate the
large rigid structure could inhibit intermolecular aggregation.
The emission spectra of PDI have a peak at 534 nm in di-
chloromethane (Figure 4c). With the increase of the conjugat-
ed framework, the large redshifts of 36 and 50 nm for the
emission maximum of PDI-Phen and TetraPDI-PF are ob-
served. And as shown in Figure 4d, PDI displays a strong fluo-
rescence emission in thin films. However, the fluorescence in-
tensity of PDI-Phen is relatively weakened and TetraPDI-PF
shows strong fluorescence quenching. The weak fluorescence
emission of TetraPDI-PF will benefit to the charge separation
in the organic solar cells (OSCs).
Figure 3. a) TGA thermograms of PDI, b) PDI-Phen, and c) TetraPDI-PF mea-
sured under nitrogen flow (50 mLminꢀ1) with a heating rate of 108C minꢀ1
.
The cyclic voltammetry (CV) and differential pulse voltamme-
try (DPV) are employed to investigate the electrochemical
properties and determine the highest occupied molecular orbi-
tal (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels of PDI, PDI-Phen, and TetraPDI-PF (Fig-
ure 4e–4 g and the Supporting Information, Figure S2), and
the relative data are shown in Table 1. PDI shows two reversi-
ble reduction waves, whereas PDI-Phen and TetraPDI-PF show
three and four reversible reduction waves, respectively, indicat-
ing their ability to accept at least three electrons. More impor-
tantly, the first reduction potential of TetraPDI-PF is about
ꢀ1.20 V and obviously lower than those of PDI and PDI-Phen.
From the first reduction value, the LUMO energies of PDI, PDI-
Phen, and TetraPDI-PF are estimated to be ꢀ3.91, ꢀ4.05, and
ꢀ3.88 eV, respectively. The relatively higher LUMO energy level
of TetraPDI-PF is desirable for OSCs, according to the empirical
formula of calculating the open-circuit voltage (Voc).[10] In addi-
tion, to investigate the electrochemical stability of TetraPDI-PF,
the cyclic voltammetry curves are successively scanned for 15
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Figure 4. a) UV/Vis absorption spectra of PDI, PDI-Phen and TetraPDI-PF in dichloromethane; b) Normalized UV/Vis absorption spectra of PDI, PDI-Phen, and
TetraPDI-PF in thin films; c) Normalized fluorescence spectra of PDI, PDI-Phen, and TetraPDI-PF in dichloromethane; d) Fluorescence spectra of PDI, PDI-
Phen, and TetraPDI-PF in thin films; e–g) CVs of PDI, PDI-Phen and TetraPDI-PF, respectively; h) electrochemical stability of TetraPDI-PF.
Table 1. Photophysical and electrochemical properties of PDI, PDI-Phen, and TetraP-
DI-PF.
TetraPDI-PF, superior to other fluorophores of its
kind, possesses the outstanding photostability be-
cause of its large, rigid conjugated framework.
[a]
[b]
[a]
[b]
opt
[c]
Materials
labs
labs
lem
lem
Eg
E1r
LUMO[d]
[eV]
HOMO[d]
[eV]
The electron mobilities of PDI, PDI-Phen, and Tet-
raPDI-PF are investigated using the steady-state
space-charge-limited current (SCLC) model, which is
widely applied to evaluate the charge-transporting
ability in organic optoelectronics. Using the solution-
processed way, the electron-only device structure is
ITO/ZnO (10 nm)/PDI or PDI-Phen or TetraPDI-PF/
BCP (10 nm)/LiF (1.5 nm)/Al (100 nm) (ITO=indium
tin oxide, BCP=2,9-dimethyl-4,7-diphenyl-1,10-phe-
[nm]
[nm]
[nm]
[nm]
[eV]
[V]
PDI
PDI-Phen
TetraPDI-PF
525
435
481
497
492
488
534
570
584
600
593
–
2.30
2.24
2.26
ꢀ1.17
ꢀ1.03
ꢀ1.20
ꢀ3.91
ꢀ4.05
ꢀ3.88
ꢀ6.21
ꢀ6.29
ꢀ6.14
[a] Measured in dichloromethane. [b] Measured in thin films. [c] Measured in 0.05m
solution of Bu4NPF6 in dichloromethane with a scan rate of 100 mVsꢀ1. [d] Based on
the reference energy level of Fc/Fc+ (5.08 eV below vacuum).[15]
cycles, which still remain invariable reduction potentials (Fig-
ure 4 h). TetraPDI-PF with four PDI branches inherits the excel-
lent electrochemical stability of perylene diimides. These re-
sults reveal that TetraPDI-PF will be a potential N-type material
in organic electronics.
nanthroline). The SCLC in this case of mobility depending on
the field can be approximated by:[16]
9
8
E2
L
pffiffi
J ¼ ee0 m0 expð0:89g EÞ
The photostability of TetraPDI-PF is measured on a confocal
fluorescence microscope by a comparison study of PDI,
BODIPY, and fluorescein (the Supporting Information, Fig-
ure S3). Due to their stronger absorption in visible-light region
similar to TetraPDI-PF, these fluorophores are chosen for com-
parison. I is the fluorescence intensities (lex =488 nm) of these
compounds after certain periods of irradiation by blue light in
the range of 460–490 nm. I0 is the fluorescent intensity of com-
pounds without irradiation. With the increase of irradiation
time, the curves of I/I0 record the attenuation trend of these
compounds (Figure 5). The damped traces reveal that the pho-
tostability of TetraPDI-PF is the best and still maintain 99%
fluorescence intensity after irradiation for 10 min using maxi-
mum laser intensity. The fluorescence intensity of PDI and
BODIP are 98 and 95% of their initial intensity, which is close
to value of TetraPDI-PF. However, fluorescein shows poor pho-
tostability and its damped trace decreases to 10% fluorescence
intensity after irradiation for 9 min. These results reveal that
Figure 5. The changes of fluorescence intensity (lex =488 nm) of PDI, TetraP-
DI-PF, BODIPY, and fluorescein after certain periods of irradiation by the
light in the range of 460–490 nm.
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in which E is the electric field across the sample, e and e0 are
the relative dielectric constant and the permittivity of the free
space, respectively, and L is the thickness of the organic layer,
with m0 the zero-field mobility and g describing the field activa-
tion of the mobility. As shown in Table 2, TetraPDI-PF has
higher intrinsic electron mobility, compared with those of PDI
and PDI-Phen under the same condition. An electron mobility
of 3.52ꢀ10ꢀ8, 3.57ꢀ10ꢀ7, and 2.22ꢀ10ꢀ5 cm2 Vꢀ1 sꢀ1are ob-
tained for the film of PDI or PDI-Phen or TetraPDI-PF after an-
nealing at 1208C for 30 min, respectively. Obviously, the elec-
tron mobility of TetraPDI-PF is 3 orders of magnitude higher
than that of PDI. The reason is that the distortion of large rigid
conjugated framework suppresses strong aggregation trend
and excellent solubility is conductive to better film-forming
property.
Table 3. Device performance of the OSCs based on PDI, PDI-Phen, and
TetraPDI-PF.
Materials
Jsc
Voc
[V]
FF
PCEmax/av
[%]
[mAcmꢀ2
]
PDI
PDI-Phen
TetraPDI-PF
0.29
0.34
1.93
0.13
0.55
0.66
0.21
0.27
0.48
0.01/0.008
0.06/0.05
0.64/0.62
raPDI-PF. In addition, further studies on the BHJ OSCs are
under way and will be presented in a forthcoming paper.
Atomic force microscope (AFM) height images and phase
images are obtained to investigate the active layer morpholo-
gies of the OSCs. As shown in Figure 7, compared with those
of PDI (6.01 nm) and PDI-Phen (5.84 nm), the blended film of
TetraPDI-PF:P3HT with the high solubility and excellent film-
forming property has smoother surface roughness with the
root mean square (RMS) value of 4.28 nm. Corresponding to
RMS, the line profile value of TetraPDI-PF is 30 nm, whereas
the values of PDI and PDI-Phen are 65 and 50 nm, respectively.
From their phase images, the homogeneous phase separation
size of TetraPDI-PF can be clearly observed, and the phase
separation of TetraPDI-PF:P3HT is appropriate, which could
lead to a relative high Jsc, whereas the phase separation size of
PDI is larger than that of TetraPDI-PF, which generates more
geminate recombination and forms some alone islets. Howev-
er, the phase separation boundary of PDI-Phen is not very
clear. Therefore, the smoother morphology and suitable phase
separation of TetraPDI-PF:P3HT should be beneficial to charge
separation and transportation in the OSC devices.
Table 2. Performance of their electron-only devices.
Materials
m [cm2 Vꢀ1 sꢀ1
]
m0 [cm2 Vꢀ1 sꢀ1
]
g [cmVꢀ1
]
PDI
PDI-Phen
TetraPDI-PF
3.52ꢀ10ꢀ8
3.57ꢀ10ꢀ7
2.22ꢀ10ꢀ5
9.51ꢀ10ꢀ10
3.20ꢀ10ꢀ7
1.99ꢀ10ꢀ5
0.0066
0.0002
0.0002
To evaluate their potential as an electron acceptor in organic
solar cell devices, we fabricated bulk heterojunction (BHJ)
OSCs using P3HT as donor, and only investigate the weight
ratio of 1:1 at the thermal annealing of 1208C. As illustrated in
Figure 6 and Table 3, the device based on TetraPDI-PF exhibits
the best performance, overwhelming those of PDI-Phen and
PDI. The maximum power conversion efficiency (PCE) reaches
0.64% with a short-circuit current (Jsc) of 1.95 mAcmꢀ2, an
open-circuit voltage (Voc) of 0.67 V and a fill factor (FF) of 0.49
under AM 1.5 G simulated solar illumination (100 mWcmꢀ2).
However, the efficiencies of PDI-Phen and PDI cells are only of
0.06 and 0.01%, respectively. The PCE of TetraPDI-PF (0.64%)
is about 64 times that of the device based on PDI. We ascribe
the higher efficiency to the distortion, which can inhibit inter-
molecular strong aggregation, good solubility, which can form
the regular morphology, and higher electron mobility of Tet-
Figure 7. AFM height images (1 mmꢀ1 mm, left), line profiles (middle) and
phase images (1 mmꢀ1 mm, right) of the blended film of P3HT:PDI (A, D, G),
P3HT:PDI-Phen (B, E, H), and P3HT:TetraPDI-PF (C, F, I), respectively.
&
Figure 6. J/V-curves of photovoltaic devices based on P3HT:PDI ( ),
~
*
P3HT:PDI-Phen ( ), or P3HT:TetraPDI-PF ( ).
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(100 MHz, CDCl3, 258C): d=167.97, 134.98, 133.27, 131.33, 129.63,
125.64, 123.31, 92.19, 61.60 ppm; MS (LC/Q-TOF-MS, pos. mode):
m/z calcd for C18H14O4Na: 474.9157 [M+Na]+; found: 474.9150.
Conclusion
A new ladder-conjugated star-shaped molecule TetraPDI-PF
containing four PDI arms and a fluorene core has been effi-
ciently synthesized and characterized. Due to the congestion
and the rigidity of the conjugated framework in which the
steric hindrance results in a non-coplanar structure, TetraPDI-
PF exhibits good solubility of as high as 155 mgmLꢀ1 in di-
chlorobenzene solution, compared with those of PDI
(35 mgmLꢀ1) and PDI-Phen (70 mgmLꢀ1). Demonstrated by
TGA, the oligomer shows excellent thermal stability with the Td
of 291.28C, which is 658C higher than that of PDI. Although
the CV curves of TetraPDI-PF are successively scanned for 15
cycles, they still remain invariable reduction potentials. The oli-
gomer also shows outstanding photostability with the order of
TetraPDI-PF>PDI>BODIPY@fluorescein after irradiation for
10 min using maximum laser intensity. And the strong broad
absorption spectra range from 300 to 550 nm and high LUMO
level of ꢀ3.88 eV are beneficial for promoting the light absorp-
tion and enhancing Voc in the BHJ OSCs. In the electron-only
devices by SCLC model, TetraPDI-PF exhibits higher intrinsic
electron mobility of 2.22ꢀ10ꢀ5 cm2 Vꢀ1 sꢀ1, three orders of
magnitude over that of PDI (3.52ꢀ10ꢀ8 cm2 Vꢀ1 sꢀ1). The device,
using TetraPDI-PF as non-fullerene acceptors and P3HT as
donors, gives a maximum PCE of 0.64% in the BHJ OSCs.
Compound 4: A mixture of compound 3 (300 mg, 0.66 mmol), bis(-
pinacolato)diboron (504 mg, 1.98 mmol), postassium acetate
(389 mg, 3.96 mmol), [1,1’bis(diphenylphosphino)ferrocene]palla-
dium(II) chloride (24 mg) was stirred at 908C for 24 h in the tolu-
ene solution under argon atmosphere. Then, the crude product is
purified through silica gel column chromatography with CH2Cl2
and MeOH (v/v; 50:1) as eluent. Compound 4 was obtained as
1
white solid (200 mg, 55%). M.p.>3008C; H NMR (400 MHz, CDCl3,
258C, TMS): d=8.20 (s, 1H, CH), 7.92 (q, 2H, CH), 4.23 (s, 2H, CH2),
3.68 (s, 2H, CH2), 1.34 ppm (s, 12H, CH3); 13C NMR (100 MHz, CDCl3,
258C): d=136.29, 135.60, 132.68, 132.56, 132.32, 130.20, 129.61,
124.38, 123.52, 123.18, 92.88, 84.02, 83.64, 61.59, 53.58, 29.85,
25.05, 19.32 ppm; MS (MALDI-TOF-MS, pos. mode): m/z calcd for
C30H38O8B2Na: 571.2650 [M+Na]+; found: 571.2617.
Compound 5: A mixture of 1-bromoperylene diimide (728 mg,
0.635 mmol), compound 4 (179 mg, 0.327 mmol), K2CO3 (226 mg,
1.635 mmol), H2O (1.6 mL), and [Pd(PPh3)4] (42 mg) was heated to
708C for 16 h in THF under argon atmosphere. Then, the crude
product was purified through silica gel column chromatography
with CH2Cl2 and n-hexane (v/v; 2:1) as eluent. Compound 5 is ob-
tained as red solid (624 mg, 81%). M.p. 268.8–2708C; 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=8.73–8.57 (m, 3H), 8.36 (dd, 2H),
8.10 (dd, 4H), 7.69 (d, 1H), 5.37 (d, 2H), 4.00 (s, 2H), 3.56 (s, 2H),
2.43 (d, 4H), 2.08–1.89 (m, 4H), 1.54–1.12 (m, 84H), 0.82 ppm (dd,
15H); 13C NMR (100 MHz, CDCl3, 258C): d=164.96, 164.78, 163.86,
163.67, 147.91, 147.77, 147.54, 147.24, 146.98, 146.18, 145.31,
143.57, 141.52, 140.85, 138.70, 135.73, 135.02, 134.54, 132.82,
131.04, 130.39, 129.40, 128.83, 128.31, 127.75, 127.06, 126.36,
124.14, 123.79, 123.28, 122.97, 122.49, 92.66, 61.61, 54.89, 35.05,
34.70, 32.56, 32.07, 31.61, 30.48, 30.36, 29.77, 29.50, 27.13, 22.84,
14.26 ppm; MS (MALDI-TOF-MS, pos. mode): m/z calcd for
C158H216N4O12Na: 2384.6313 [M+Na]+; found: 2384.6470.
Experimental Section
General methods
All reactants were purchased from commercial sources. Com-
pounds 2,7-dibromophenanthraquinone, 3,6-dibromophenanthra-
quinone, and 7 were synthesized according to literature meth-
ods.[17] NMR spectra were measured with a 400 MHz Bruker spec-
Compound 8: A mixture of compound 7 (3 g, 6.61 mmol), bis(pina-
colato)diboron (4.36 g, 17.18 mmol), postassium acetate (3.89 g,
trometer using TMS as reference for H and 13C NMR spectroscopy.
1
39.64 mmol),
[1,1’-Bis(diphenylphosphino)ferrocene]palladium(II)
Accurate mass correction is measured with MALDI TOF mass spec-
trometer (MALDI micro MX). Film thicknesses are determined on
an Alphastep 500 surface profilometer. CV and DPV were per-
formed in 0.05m solution in CH2Cl2 with a standard commercial
electrochemical analyzer in a three electrode single-component
cell under argon with a scan rate of 100 mVsꢀ1. Working electrode:
glassy carbon; reference electrode: Ag/AgCl; auxiliary electrode: Pt
disk; internal standard: ferrocene (Fc). The energy of Fc/Fc+ is
5.08 eV relative to vacuum.[15] UV/Vis absorption spectra in chloro-
form (CHCl3) solution were recorded in a UV/Vis spectrophotome-
ter (HP8453) at room temperature using a glass cuvette with
a path length of 1 cm. TGA is carried out using a Mettler Toledo
TGA/SDTA 851e at a heating rate of 108C minꢀ1 under nitrogen
flow of 20 mLminꢀ1. The thin film morphology is investigated by
tapping mode AFM on the Nanoscope IV system commercialized
by Veeco.
chloride (248 mg) was stirred at 908C for 24 h in the toluene solu-
tion under argon atmosphere. Then, the crude product is purified
through silica gel column chromatography with CH2Cl2 and MeOH
(v/v; 50:1) as eluent. Compound 8 was obtained as pale-yellow
solid (3.35 g, 93%). M.p.>3008C; 1H NMR (400 MHz, CDCl3, 258C,
TMS): d=8.33 (d, 1H, CH), 7.79 (t, 1H, CH), 7.67 (d, 1H, CH), 4.12 (s,
2H, CH2), 3.57 (s, 2H, CH2), 1.32 ppm (s, 12H, CH3); 13C NMR
(100 MHz, CDCl3, 258C): d=135.60, 135.37, 132.66, 130.64, 125.43,
92.75, 84.20, 66.52, 61.64, 25.04 ppm; MS (MALDI-TOF-MS, pos.
mode): m/z calcd for C30H38B2O8: 548.2753 [M]+; found: 548.2726.
Compound 9: A mixture of 1-bromoperylene diimide (1.4 g,
1.26 mmol), compound 8 (344 mg, 0.628 mmol), K2CO3 (434 mg,
3.14 mmol), H2O (1.6 mL), and [Pd(PPh3)4] (98 mg) was heated to
908C for 13 h in toluene under argon atmosphere. Then, the crude
product was purified through silica gel column chromatography
with CH2Cl2 and n-hexane (v/v; 2:1) as eluent. Compound 9 was
obtained as red solid (933 mg, 63%). M.p.>3008C; 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=8.84 (m, 1H), 8.64–8.51 (m, 1H),
8.47–8.15 (m, 2H), 7.88 (t, 1H), 5.54 (t, 1H), 4.19 (t, 1H), 3.73 (d,
1H), 2.79–2.48 (m, 2H), 2.33–2.04 (m, 3H), 1.82–1.28 (m, 50H),
1.03 ppm (dd, 8H); 13C NMR (100 MHz, CDCl3, 258C): d=164.79,
163.72, 144.74, 140.70, 135.06, 134.50, 130.46, 129.59, 129.38,
128.80, 128.18, 127.65, 123.67, 122.91, 92.57, 54.86, 53.56, 32.48,
32.02, 29.73, 29.45, 27.09, 27.01, 22.79, 19.31, 19.11, 17.06,
Compound 3: A mixture of 2, 7-dibromophenanthraquinone (2 g,
5.46 mmol), ethylene glycol (6.1 mL, 109.83 mmol), p-toluene sul-
fonic acid (132 mg, 0.765 mmol) was heated to reflux by water sep-
arator in the toluene solution under argon atmosphere. Upon cool-
ing to RT, the white precipitate was collected by filtration and
washed with H2O, MeOH, and Et2O yielding compound 3 (1.268 g,
51%) as a white solid. M.p. 262.6–262.98C; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=7.88 (d, 1H, CH), 7.70 (d, 1H, CH), 7.58 (q,
1H, CH), 4.22 (s, 2H, CH2), 3.66 ppm (s, 2H, CH2); 13C NMR
Chem. Eur. J. 2014, 20, 1 – 10
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14.23 ppm; MS (MALDI-TOF-MS, pos. mode): m/z calcd for
C158H216N4O12Na: 2384.6313 [M+Na]+; found: 2384.6375.
shown in Scheme S2 (the Supporting Information). First, the solu-
tion of sample is sealed in a microscope concave slide with cover
slip (the concentration of all the compounds is 10ꢀ5 m; PDI, TetraP-
DI-PF, and BODIPY were dissolved in toluene; fluorescein was dis-
solved in an aqueous solution with pH 11; all solutions are air satu-
rated). Then it is placed on the objective lens of the confocal mi-
croscope, and irradiated by the light in the range of 460–490 nm
using a band pass filter that transmits from 460 to 490 nm with
the same light intensity. The fluorescent images are recorded with
Nikon C1 confocal fluorescence microscope exciting at 488 nm (Ar
Laser) at different lengths of time after exposure. The average fluo-
rescent intensity is calculated from the software EZ-C1 3.90 Free-
viewer.
Compound dg-PDI-Phen: The compound dg-PDI-Phen can be
prepared by two synthetic methods. Method 1: A mixture of com-
pound 5 (1.1 g, 0.465 mmol) and a catalytic amount of iodine was
heated to 1108C for 6 h in toluene under a sunlight atmosphere.
The mixture was evaporated and the crude product was purified
by silica gel column chromatography with dichloromethane and n-
hexane (v/v; 2:1) as eluent. Compound dg-PDI-Phen was obtained
as a red solid (235 mg, 21%). Method 2: A mixture of compound 9
(933 mg, 0.395 mmol) and a catalytic amount of iodine was heated
to 1108C for 6 h in toluene under sunlight atmosphere. The mix-
ture was evaporated and the crude product was purified by silica
gel column chromatography with dichloromethane and n-hexane
as eluent. Compound dg-PDI-Phen was obtained as red solid
Device preparation and characterization
1
(655 mg, 70%). M.p.>3008C. H NMR (400 MHz, CDCl3, 258C, TMS):
BHJ organic device preparation proceeds as follows. A 15 Wcmꢀ2
resistor of indium-doped tin oxide (ITO) substrate is purchased
from Xiangcheng Science and Technology Co., Ltd., and then the
substrates undergo a cleaned course in an ultrasonic bath with
acetone, ethanol, and ultrapure water and dried for 1 h under
a blast dry oven. Subsequently, the glasses were cleaned in oxygen
plasma treatment for 10 min, and a PEDOT:PSS (Clevios P VP AI
4083, H.C. Starck) solution is then spin-coated at 2000 rpm for 60 s
onto the cleaned ITO surface, resulting in a thickness of ꢁ40 nm,
as determined with a Dektak surface profilometer. The PEDOT:PSS
layer is annealed for 30 min at 1208C in air. A solution of active
layer P3HT (Rieke No. 4002-E) and PDI, PDI-Phen or TetraPDI-PF
were simultaneously dissolved in dichlorobenzene in a weight
ratio of 1:1 at a concentration of 16 mgmLꢀ1 and stirred for 1 h at
a temperature of 908C in an oil bath. Before deposition of the
active layer, the mixed solution of P3HT:PDI, PDI-Phen or TetraPDI-
PF is filtered through a polytetrafluoroethylene (PTFE) syringe filter
(0.45 mm pore size). The active layer is then spin-coated on top of
the PEDOT:PSS film at 1200 rpm (dichlorobenzene) for 18 s result-
ing in a film thickness of ꢁ100 nm. The films are annealed at
1208C for 30 min in the vacuum state. The 10 nm thick 2,9-dimeth-
yl-4,7-diphenyl-1,10-phenanthroline (BCP) and 1.5 nm thick LiF
layer is thermally deposited under a pressure of 2ꢀ10ꢀ4 Pa, fol-
lowed by aluminum counter electrodes are evaporated through
a shadow mask on top of the active layer with a thickness of
ꢁ100 nm. The active area of the pixels, as defined by the overlap
of anode and cathode area, is 0.20 cm2. The photovoltaic per-
formance was determined under simulated sunlight, using a com-
mercial solar simulator (Zolix ss150 solar simulator, China).
d=10.42 (s, 1H), 9.72 (s, 1H), 9.31–8.78 (m, 6H), 5.81 (s, 1H), 5.38
(s, 1H), 4.28 (s, 1H), 3.57 (s, 1H), 2.98 (s, 2H), 2.43 (d, 7H), 1.99–
0.66 ppm (m, 104H); 13C NMR (100 MHz, CDCl3, 258C): d=164.94,
163.84, 143.57, 140.84, 135.72, 135.02, 134.54, 132.82, 131.71,
131.05, 130.48, 129.40, 128.82, 128.31, 127.75, 127.11, 126.32,
123.79, 123.22, 122.97, 122.47, 92.66, 61.63, 55.01, 54.81, 32.56,
32.07, 29.77, 29.50, 27.14, 22.84, 14.28 ppm; MS (MALDI-TOF-MS,
pos. mode,): m/z calcd for C158H212N4O12Na: 2380.5999 [M+Na]+;
found: 2380.6194.
Compound PDI-Phen:
A mixture of compound dg-PDI-Phen
(735 mg, 0.312 mmol), p-toluene sulfonic acid (5 g, 26.3 mmol) was
heated to 1258C for 10 h. Then, the crude product was purified
through silica gel column chromatography with CH2Cl2 and n-
hexane (v/v; 1:1) as eluent. Compound PDI-Phen was obtained as
red solid (632 mg, 90%). M.p: 295.7–296.38C. 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=10.05–9.32 (m, 4H), 9.08 (d, J=66.8 Hz, 3H),
8.68 (s, 2H), 6.00 (s, 1H), 5.76 (s, 1H), 3.32–0.80 ppm (m, 129H);
13C NMR (100 MHz, CDCl3, 258C): d=178.80, 164.97, 164.25, 163.42,
162.57, 133.73, 132.63, 131.67, 130.89, 130.15, 128.59, 127.43,
126.85, 123.51, 123.00, 120.03, 55.56, 33.26, 32.09, 31.60, 29.86,
27.83, 22.83, 14.24 ppm; MS (MALDI-TOF-MS, pos. mode): m/z
calcd for C154H204N4O10Na: 2292.5475 [M+Na]+; found: 2292.5659.
Compound TetraPDI-PF:
A mixture of compound PDI-Phen
(100 mg, 0.044 mmol) and 2,3,6,7-tetraamino-9,9-bis(2-ethylhexyl)-
fluorene was heated to 1208C for 6 h in CHCl3 and glacial acetic
acid solution under argon atmosphere. The mixture is evaporated
and the crude product was purified by silica gel column chroma-
tography with dichloromethane and n-hexane (v/v; 3:1) as eluent.
Compound TetraPDI-PF was obtained as red solid (235 mg, 74%).
M.p.>3008C. 1H NMR (400 MHz, CDCl3, 258C, TMS): d=11.51 (d,
1H), 11.34–10.80 (m, 3H), 9.69–8.80 (m, 5H), 5.34 (d, 2H), 3.01–1.87
(m, 9H), 1.76–0.42 ppm (m, 98H); 13C NMR (100 MHz, CDCl3, 258C):
d=165.75, 165.48, 165.33, 165.04, 164.66, 164.35, 164.27–164.18,
155.43, 143.91, 143.20, 143.04–142.99, 142.96–142.93, 134.36,
131.60, 131.47, 130.88, 130.60, 130.11, 129.83, 129.50, 128.88,
128.14, 126.37–126.20, 126.09, 125.33, 125.00, 124.11, 123.61,
123.38–123.18, 122.84, 121.78, 119.76, 55.95, 55.86–55.69, 55.55,
48.11–47.83, 35.60, 33.48, 33.25, 33.00, 32.41, 32.22, 32.05, 31.80,
30.34, 30.12, 29.89, 29.81, 29.66, 29.51, 29.32, 28.28, 27.92, 27.68,
27.37, 27.10, 22.79, 22.61, 14.21, 14.08, 13.95–13.89, 13.87–13.83,
10.62 ppm; MS (MALDI-TOF-MS, neg. mode): m/z calcd for
C336H444N12O16: 4907.1914 [M]ꢀ; found: 4907.8926.
Acknowledgements
This work is supported by National Natural Science Foundation
of China (No. 21174022), National Basic Research Program of
China (No. 2013CB733702), Fundamental Research Funds for
the Central Universities (No. 1004852012) and Specialized Re-
search Fund for the Doctoral Program of Higher Education (No.
20110041110009).
Keywords: cyclic voltammetry · dyes/pigments · electron-
deficient compounds · electron transport · oligomers
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Published online on && &&, 2014
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FULL PAPER
Ladder to the stars: A new electron-
transporting material containing four
perylene diimide branches, which exhib-
its high solubility and excellent thermal
stability, electrochemical stability, and
photostability, was efficiently synthe-
sized. The intrinsic electron mobility and
the photovoltaic performance with Tet-
raPDI-PF as non-fullerene acceptors and
P3HT as electron donors were also in-
vestigated (see figure).
&
Electron-Deficient Compounds
Y. Zhang, L. Chen,* K. Zhang, H. Wang,
Y. Xiao*
&& – &&
A Soluble Ladder-Conjugated Star-
Shaped Oligomer Composed of Four
Perylene Diimide Branches and
a Fluorene Core: Synthesis and
Properties
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Chem. Eur. J. 2014, 20, 1 – 10
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ꢂ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!