Synthesis, crystal structure, and application of an acenaphtho[1,2-k] fluoranthene diimide derivative

Article abstract of DOI:10.1007/s11426-014-5282-9

Organic electron acceptor materials play an important role in organic electronics. Recently, many organic electron acceptors have been developed, in which aromatic fused-imides have proved to be a promising family of excellent electron acceptors. We report the first synthesis of a novel aromatic fused-imide, acenaphtho[1, 2-k]fluoranthene diimide derivative (AFI), using lithium-halogen exchange and Diels-Alder reactions. The construction of a large conjugated plane and the introduction of electron-withdrawing imide groups endow AFI with a low lowest unoccupied molecular orbital (LUMO) level of -3.80 eV. AFI exhibits a regular molecular arrangement and strong ??-?? interactions in the single-crystal structure, which indicates its potential application in organic electronic devices. Solar cell devices that were fabricated using AFI as the electron acceptor and P3HT as the electron donor achieved an energy conversion efficiency of 0.33%.

Full text of DOI:10.1007/s11426-014-5282-9

SCIENCE CHINA
Chemistry
February 2015 Vol.58 No.2: 364–369
doi: 10.1007/s11426-014-5282-9
• ARTICLES •
· SPECIAL ISSUE · Organic photovoltaics
Synthesis, crystal structure, and application of an acenaphtho[1,2-k]
fluoranthene diimide derivative
Lin Ding1,2, Chiyuan Yang1, Zhongmin Su2* & Jian Pei1*
1College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
2College of Chemistry, Northeast Normal University, Changchun 130024, China
Received August 8, 2014; accepted August 25, 2014; published online January 4, 2015
Organic electron acceptor materials play an important role in organic electronics. Recently, many organic electron acceptors
have been developed, in which aromatic fused-imides have proved to be a promising family of excellent electron acceptors.
We report the first synthesis of a novel aromatic fused-imide, acenaphtho[1,2-k]fluoranthene diimide derivative (AFI), using
lithium-halogen exchange and Diels-Alder reactions. The construction of a large conjugated plane and the introduction of elec-
tron-withdrawing imide groups endow AFI with a low lowest unoccupied molecular orbital (LUMO) level of 3.80 eV. AFI
exhibits a regular molecular arrangement and strong - interactions in the single-crystal structure, which indicates its poten-
tial application in organic electronic devices. Solar cell devices that were fabricated using AFI as the electron acceptor
and P3HT as the electron donor achieved an energy conversion efficiency of 0.33%.
acenaphtho fluoranthene diimide, lithium-halogen exchange, Diels-Alder reaction, electron acceptor, solar cell
fluoranthene-fused imides as good electron acceptors [20].
1 Introduction
The introduction of imide unit in the fluoranthene-fused
core plays an important role in tuning their electronic struc-
tures. In particular, the lowest unoccupied molecular orbital
(LUMO) levels can be finely tuned by introducing other
electron-withdrawing groups. Herein, we facilely synthe-
sized a novel acenaphtho fluoranthene core using lithi-
um-halogen exchange and Diels-Alder reactions as the main
synthetic strategies. Two diimide substituents were intro-
duced to lower the LUMO level, and long-branched alkyl
chains were introduced at the nitrogen atoms to increase
solubility. The target acceptor AFI (Figure 1) was obtained
through six steps in a yield of 15% (Scheme 1).
Much attention has been paid to organic electron acceptor
materials [13], which are indispensable components of
organic electronics, due to their important role in p–n junc-
tion diodes, bipolar transistors, and complementary circuits
[46]. Recently, organic electron acceptor materials such as
fullerene and its derivatives [79], aromatic core-fused im-
ides [2,1012], perylene diimide derivatives [13,14], naph-
thalene diimide [15,16], quinoid derivatives [17], and ni-
trogen heterocyclics [18,19] have been intensively investi-
gated and have been shown to exhibit good performance in
optoelectronic devices. However, the structural diversity
and synthetic strategy of organic acceptors remain limited.
Therefore, it is still a challenge to develop novel air-stable
and solution-processable organic electron acceptors.
2 Experimental
In our previous contribution, we developed a series of
2.1 General method
All commercially available chemicals were used without
further purification unless otherwise noted. Dichloromethane
*Corresponding authors (email: zmsu@nenu.edu.cn; jianpei@pku.edu.cn)
© Science China Press and Springer-Verlag Berlin Heidelberg 2015
chem.scichina.com link.springer.com

Ding et al. Sci China Chem February (2015) Vol.58 No.2
365
All potentials were recorded using Ag/AgCl as a reference
electrode. The scan rate of cyclic voltammetry was 50
mV s1.
2.2 Synthesis of the acenaphtho[1,2-k]fluoranthene
diimide derivative (AFI)
2.2.1 6,7-Dihydroindeno[6,7,1-def]isochromene-1,3-dione
(2)
Compound 1 [21] (10 mmol) was dissolved in anhydrous
THF (50 mL) at hydrogen atmosphere. To the solution,
o
t-butyl lithium (40 mmol) was added slowly at 78 C and
Figure 1 Molecular structure of acenaphtho[1,2-k]fluoranthene diimide
derivative (AFI).
allowed to react for 30 min. The dianion was formed
through lithium-halogen exchange. Nucleophilic addition
reaction took place between dianion and excess dry CO2 for
20 min. After being warmed to room temperature, the sol-
vent was removed by reduced-pressure distillation. The
residue was dissolved by water, then concentrated hydro-
chloric acid was added and a white solid precipitated. The
precipitate was obtained by filtration and dried in vacuum.
After being treated with reflux acetic anhydride, the crude
product was used in the subsequent reaction without further
purification.
2.2.2 2-(2-Octyldodecyl)-6,7-dihydro-1H-indeno[6,7,1-def]-
isoquinoline-1,3(2H)-dione (4)
A mixture of compound 2 (2.24 g, 10.0 mmol) and com-
pound 3 (7.43 g, 25.0 mmol) was dissolved in acetic acid
(100 mL), and heated at 130 oC for 3 days. After cooling to
room temperature, the reaction mixture was extracted with
chloroform 3 times, washed with water, and dried with an-
hydrous sodium sulfate. The crude product was evaporated
under reduced pressure. The resulting solid was purified by
column chromatography (silica gel, petroleum ether/chloro-
form=4:1) to give product as a yellow solid. Yield: 4.68 g,
73% for the three steps. 1H NMR (CDCl3, 400 MHz, ppm):
δ 8.48 (2H, d, J=7.2 Hz), 7.55 (2H, d, J=7.2 Hz), 4.11 (2H,
d, J=7.2 Hz), 3.56 (4H, s), 1.98 (1H, m), 1.401.21 (32H,
m), 0.880.83 (6H, m); 13C NMR (CDCl3, 100 MHz, ppm):
δ 164.8, 153.6, 137.8, 132.8, 126.5, 120.9, 119.3, 44.4, 36.7,
31.9, 31.8, 31.7, 30.1, 29.63, 29.57, 29.3, 29.3, 26.6, 22.7,
14.1. Anal: calcd. for C34H49NO2: C, 81.06; H, 9.80; N, 2.78;
found: C, 80.93; H, 9.81; N, 2.67.
Scheme 1 Synthetic route to AFI.
was dried with calcium hydride and distilled under nitrogen.
Column chromatography was performed with silica gel.
Analytical thin-layer chromatography (TLC) was performed
on 0.2 mm silica-gel-coated glass sheets with an F254/365
indicator. All yields refer to isolated yields. Nuclear mag-
netic resonance (NMR) spectra were recorded on 400 MHz
Bruker spectrometers. Chemical shifts are reported in ppm
1
and coupling constants (J values) are reported in Hertz. H
NMR chemical shifts are referenced to TMS (0 ppm) or
CHCl3 (7.26 ppm) and 13C NMR chemical shifts are refer-
enced to CDCl3 (77.23 ppm). Absorption spectra were rec-
orded on a PerkinElmer Lambda 750 UV-Vis spectrometer.
Photoluminescence (PL) spectra were recorded on a Perki-
nElmer LS 55 Luminescence spectrometer. Cyclic voltam-
metry was performed using a BASI Epsilon workstation.
Cyclic voltammetry was measured in dichloromethane con-
taining 0.1 mol L1 n-Bu4NPF6 as a supporting electrolyte.
The glassy carbon electrode was used as a working elec-
trode and a platinum sheet was used as a counter electrode.
2.2.3 2-(2-Octyldodecyl)-1H-indeno[6,7,1-def]isoquinoline-
1,3(2H)-dione (5)
A mixture of compound 4 (1.51 g, 3.0 mmol), NBS (0.641 g,
3.6 mmol), and AIBN (0.098 g, 0.60 mmol) was dissolved
in CCl4 (50 mL), and heated at 80 oC overnight. After being
cooled to room temperature, the residue was removed by
filtrating and the filtrate was evaporated under reduced
pressure. The crude solid was used in the subsequent reac-
tion without further purification. The crude solid was mixed
with anhydrous lithium bromide (0.905 g, 10.4 mmol) and

366
Ding et al. Sci China Chem February (2015) Vol.58 No.2
dissolved in dry DMAc (20 mL) in nitrogen atmosphere,
and then heated at 80 C overnight. After being cooled to
141.3, 139.6, 136.8, 132.0, 132.0, 129.8, 129.1, 124.8,
124.2, 122.1, 115.4, 55.5, 44.4, 36.7, 31.9, 31.7, 30.0, 29.6,
29.6, 29.5, 29.3, 29.3, 26.5, 22.7, 14.1. HRMS (ESI): calcd.
for C84H105N2O6[M+H]+: 1237.7967; found: 1237.7967.
o
room temperature, the mixture was extracted with chloro-
form twice and washed with dilute hydrochloric acid. The
organic layer was combined and dried with anhydrous so-
dium sulfate. The crude product was purified by column
chromatography (silica gel, petroleum ether/dichlorome-
thane=6:1) to give product as an orange solid. Yield: 0.47 g,
2.3 Single-crystal X-ray diffraction
A single crystal suitable for X-ray structural analysis was
obtained by slow evaporation of the mixed solvents (ethyl
alcohol/dichloromethane) containing AFI. Diffraction data
were collected on a Ragaku mm007 diffractometer using the
ω-scan mode with graphite-monochromatic Mo-Kα radia-
tion (λ=0.71073 Å). The structure was solved by the direct
method and refined by the full-matrix least-squares method
on F2 using the SHELXTL-97 crystallographic software
package [23,24]. Non-hydrogen atoms were refined aniso-
tropically. The positions of the hydrogen atoms were calcu-
lated and refined isotropically.
1
31% for two steps. H NMR (CDCl3, 400 MHz, ppm): δ
8.31 (2 H, d, J=6.8 Hz), 7.71 (2 H, d, J=6.8 Hz), 7.07 (2 H,
s), 4.07 (2 H, d, J=7.2 Hz), 1.94(1 H, m), 1.39–1.21 (30 H,
m), 0.86 (6 H, m); 13H NMR (CDCl3, 100 MHz, ppm): δ
164.3, 144.3, 132.9, 131.6, 127.5, 125.0, 123. 3, 122.9, 44.4,
36.9, 31.93, 31.90, 31.7, 30.1, 29.64, 29.58, 29.34, 29.31,
26.6, 22.7, 14.1. HRMS (ESI): calcd. for C34H48NO2
(M+H)+: 502.3680; found: 502.3675.
2.2.4
2-(2-Octyldodecyl)-1H-indeno[6,7,1-def]isoquino-
line-1,3,6,7(2H)-tetraone (6)
Compound 5 (3.0 mmol) was dissolved in acetic anhydride
(100 mL), and heated to 110 oC. Chromium trioxide powder
(15.0 mmol) was slowly added into the solution in batches
and allowed to react for 20 min. Without prior cooling, the
reaction mixture was poured into ice (300 g), and dilute
hydrochloric acid (30 mL) was added slowly to quench the
reaction under stirring. The solid was filtrated and washed
with a little ethyl alcohol. A light yellow product was ob-
tained and used in the subsequent condensation reaction
without further purification.
3 Results and discussion
3.1 Photophysical property of AFI
The photophysical property of AFI was investigated in the
dilute CHCl3 solution (Figure 2). The absorption spectrum
of AFI reveals three main bands that peaked at 328 nm, 376
nm, and 484 nm. Compared with FAI we had synthesized
previously [25], the maximum absorption peak of AFI
red-shifted by 70 nm, which can be attributed to the addi-
tional imide group and the extension of the conjugated
backbone. The fluorescence spectrum of AFI reveals an
emission maximum at 613 nm, which red-shifted over 60
nm compared with FAI.
2.2.5 6,8-Bis(4-methoxyphenyl)-2-(2-octyldodecyl)cyclo-
penta[2,3]indeno[6,7,1-def]isoquinoline-1,3,7(2H)-trione (8)
A mixture of 1,3-bis(4-methoxyphenyl)propan-2-one (com-
pound 7) [22] (0.20 mmol) and compound 6 (0.20 mmol)
was dispersed in isopropyl alcohol (2 mL), and heated to
3.2 Electrochemical property
o
80 C. KOH sheets (0.10 mmol) were added into the mix-
ture and allowed to react for 30 min. After cooling in an ice
bath, the product as a reddish brown solid was obtained by
filtration and was used in the subsequent Diels-Alder reac-
tion without further purification.
The electrochemical property of AFI was measured by using
cyclic voltammetry (CV). AFI was dissolved in dichloro-
methane containing 0.1 mol L1 n-Bu4NPF6 as supporting
electrolyte at a concentration of 103 mol L1. Ag/AgCl was
used as reference electrode and ferrocene was the standard
compound. Three reversible reduction waves were observed
(Figure 3), which indicated its excellent electron-acceptor
property. The LUMO level of AFI estimated from the onset
of its reduction wave showed a value of 3.80 eV, which
indicated strong electron-withdrawing ability of AFI. The
introduction of the imide group and the extension of the con-
jugated skeleton obviously lowered the LUMO level of AFI.
2.2.6 AFI
A mixture of compound 5 (0.198 g, 0.26 mmol) and com-
pound 8 (0.130 g, 0.26 mmol) in nitrobenzene (2 mL) was
stirred at 180 °C overnight. After being cooled to room tem-
perature, the crude solution was then loaded onto a silica gel
column. The nitrobenzene was removed by petroleum ether
and dichloromethane with the ratio of 5:1 and the product as
an orange solid was eluted by petroleum ether and di-
chloromethane at a ratio of 2:1. Yield: 0.104 g, 20% for the
3.3 Single-crystal analysis
1
three steps. H NMR (CDCl3, 400 MHz, ppm): δ 8.21 (4H,
d, J=7.2 Hz), 7.55 (4H, d, J=8.8 Hz), 7.29 (4H, d, J=8.8 Hz),
6.88 (4H, d, J=7.2 Hz), 4.09 (6H, s), 4.00 (4H, d, J=3.2 Hz),
1.95–1.92 (2H, m), 1.37–1.19 (64H, m), 0.87–0.82 (12H,
m); 13C NMR (CDCl3, 100 MHz, ppm): δ 164.0, 160.2,
The molecular configuration of AFI is shown in Figure 4.
The two p-methoxyphenyl groups were nearly perpendicu-
lar to the flat acenaphtho fluoranthene-fused core, and the
torsion angle was measured to be 78.2°. Two acenaphtho

Ding et al. Sci China Chem February (2015) Vol.58 No.2
367
-diimide segments are exposed for intermolecular stacking.
The crystal data and structure refinement for AFI are sum-
marized in Table 1. Figure 5 illustrates the single-crystal
packing diagram of AFI. It adopts a brickwork packing
structure, which leads to slipped face-to-face - stacking
that is quite similar to rubrene [26]. No displacement along
the short axis of the molecular backbone was observed.
There is a close slipped - overlap between adjacent
acenaphtho-diimide cores with a distance of 3.72 Å. In this
crystal structure, sufficient and successive - orbital over-
lap is achieved. The - stacking distance may be further
decreased by using shorter linear alkyl chains at the nitro-
gen atoms.
3.4 Device performance in solar cells
In order to investigate the electron-acceptor property of AFI,
bulk-heterojunction solar cells were fabricated and tested in
Table 1 Crystal data and structure refinement for AFI
Parameters
empirical formula
formula weight
T (K)
Value
C86H106Cl6N2O6
1476.43
293 (2)
Figure 2 Absorption spectra (a) and photoluminescence (PL) spectra (b)
of AFI in CHCl3 solutions (1.0×105 mol L1).
λ (Å)
0.71073
triclinic
P1
crystal system
space group
a (Å)
10.229 (7)
12.374 (9)
16.608 (12)
78.82 (3)
80.15 (4)
81.91 (4)
2019 (2)
4
b (Å)
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
Z
Dc (g cm3)
4.857
μ (mm-1)
1.061
GOF on F2
R1a) [I > 2σ(I)]
wR2 b) [I > 2σ(I)]
θ range (°)
F(000)
1.113
0.1482
0.4166
1.69 to 24.88
3144
Figure 3 CV curve of AFI in CH2Cl2 (1.0×103 mol L1).
Data/restraints/parameters
7000/3/451
2
2
a) R1 = ∑||F0|-|FC||/∑|F0|; b) wR2 = ∑[w(F0 -FC2)2]/∑[w(F0 )2]1/2
.
Figure 5 Single-crystal packing diagram of AFI.
Figure 4 Molecular configuration of AFI in single crystal.

368
Ding et al. Sci China Chem February (2015) Vol.58 No.2
This work was financially supported by the National Basic Research Pro-
gram of China (2013CB933501), and the National Natural Science Foun-
dation of China. It was also supported by a General Financial Grant
(2013M530135) from the China Postdoctoral Science Foundation.
a configuration of ITO/PEDOT:PSS/P3HT:AFI/LiF/Al.
P3HT was chosen as the electron donor and AFI as the
electron acceptor. Devices with various P3HT/AFI weight
ratios were fabricated. Before the device fabrication, 40 mg
of AFI and 40 mg of P3HT were dissolved into 1 mL of
o-dichlorobenzene, respectively, to serve as the stock solu-
tions. Both solutions were heated to 100 °C for 1 h, filtered
through 0.2 μm filter, and then cooled to room temperature.
Mixing the stock solutions in different volume ratios pro-
vided various weight-blend ratios of donor/acceptor. After
spin-coating the blended solution onto the ITO/PEDOT:
PSS substrate, the solvent in the pristine wet film was al-
lowed to evaporate slowly in a petridish. All the devices
were measured in ambient conditions under 100 mW cm−2
AM 1.5G-simulated solar illumination. After careful opti-
mization, a weight ratio of 1:2 (w/w) for donor/acceptor was
adopted and all the devices underwent pre-annealing at 100
oC for 15 min. The devices yielded power conversion effi-
ciencies (PCEs) up to 0.33%, with an open circuit voltage
(Voc) of 0.65 V (Figure 6), a short circuit current (Jsc) of
1.37 mA cm2, and a fill factor (FF) of 0.45.
1
2
3
Usta H, Facchetti A, Marks TJ. n-Channel semiconductor materials
design for organic complementary circuits. Acc Chem Res, 2011, 44:
501–510
Zhan X, Facchetti A, Barlow S, Marks TJ, Ratner MA, Wasielewski
MR, Marder SR. Rylene and related diimides for organic electronics.
Adv Mater, 2011, 23: 268–284
Tang Q, Li H, Liu Y, Hu WP. High-performance air-stable n-type
transistors with an asymmetrical device configuration based on or-
ganic single-crystalline submicrometer/nanometer ribbons. J Am
Chem Soc, 2006, 128: 14634–14639
4
5
6
Streetman BG. Solid State Electronic Devices, 4th ed. Prentice Hall:
Englewood Cliffs, 1995
Zaumseil J, Sirringhaus H. Electron and ambipolar transport in or-
ganic field-effect transistors. Chem Rev, 2007, 107: 1296–1323
Dhar BM, Kini GS, Xia GQ, Jung BJ, Markovic N, Katz HE.
Field-effect-tuned lateral organic diodes. Proc Natl Acad Sci USA,
2010, 107: 3972–3976
7
Singh TB, Marjanovic N, Stadler P, Auinger M, Matt GJ, Günes S,
Sariciftci NS, Schwödiauer R, Bauer S. Correlation between mor-
phology and ambipolar transport in organic field-effect transistors. J
Appl Phys, 2005, 97: 083714-1-5
8
9
He YJ, Chen HY, Hou JH, Li YF. Indene-C-60 bisadduct: a new ac-
ceptor for high-performance polymer solar cells. J Am Chem Soc,
2010, 132: 1377–1382
Pei C, Li YF, Zhan XW. Efficient ternary blend polymer solar cells
with indene-C60 bisadduct as an electron-cascade acceptor. Energy
Environ Sci, 2014, 7: 2005–2011
4 Conclusions
We have developed a convenient synthetic strategy to
construct a new type of aromatic fused diimide AFI. The
construction of a large conjugated plane and the introduc-
tion of electron-withdrawing imide groups significantly
lowered its LUMO level, which makes AFI a promising
electron-acceptor material. The molecular configuration
and packing structure in solid state were investigated
through single-crystal analysis, which indicated strong
π-π interactions between adjacent molecules. BHJ solar
cells based on AFI/P3HT blend achieved PCEs up to
0.33%. Improved electron-acceptor performance is ex-
pected after rational structure modification and de-
vice-processing optimization.
10 Gsaenger M, Oh JH, Koenemann M, Hoeffken HW, Krause AM, Bao
Z, Würthner F. A crystal-engineered hydrogen-bonded octachlo-
roperylene diimide with a twisted core: an n-Channel organic semi-
conductor. Angew Chem Int Ed, 2010, 49: 740–743
11 Gao J, Li Y, Wang ZH. Synthesis and properties of
naphthobisbenzothiophene diimides. Org Lett, 2013, 15: 1366–1369
12 Wen YG, Liu YL, Di CA, Wang Y, Sun XN, Guo YL, Zheng J, Wu
WP, Ye SH, Yu G. Improvements in stability and performance of
N,N′-dialkyl perylene diimide-based n-type thin-film transistors. Adv
Mater, 2009, 21: 1631–1635
13 Zhou E, Cong J, Wei Q, Tajima K, Yang C, Hashimoto K.
All-polymer solar cells from perylene diimide based copolymers:
material design and phase separation control. Angew Chem Int Ed,
2011, 50: 2799–2803
14 Kamm V, Battagliarin G, Howard IA, Pisula W, Mavrinskiy A, Li C,
Müllen K, Laquai F. Polythiophene:perylene diimide solar cells – the
impact of alkyl-substitution on the photovoltaic performance. Adv
Energy Mater, 2011, 1: 297–302
15 Ahmed E, Ren G, Kim FS, Hollenbeck EC, Jenekhe SA. Design of
new electron acceptor materials for organic photovoltaics: synthesis,
electron transport, photophysics, and photovoltaic properties of oli-
gothiophene-functionalized naphthalene diimides. Chem Mater, 2011,
23: 4563–4577
16 Earmme T, Hwang YJ, Murari NM, Subramaniyan S, Jenekhe SA.
All-polymer solar cells with 3.3% efficiency based on naphthalene
diimide-selenophene copolymer acceptor. J Am Chem Soc, 2013, 135:
14960–14963
17 Handa S, Miyazaki E, Takimiya K, Kunugi Y. Solution-processible
n-channel organic field-effect transistors based on dicyanometh-
ylene-substituted terthienoquinoid derivative. J Am Chem Soc, 2007,
129: 11684–11685
18 Liang Z, Tang Q, Liu J, Li J, Yan F, Miao Q. n-Type organic semi-
Figure 6 Device performance of AFI in solar cells.

Ding et al. Sci China Chem February (2015) Vol.58 No.2
369
conductors based on pi-deficient pentacenequinones: synthesis, elec-
tronic structures, molecular packing, and thin film transistors. Chem
Mater, 2010, 22: 6438–6443
22 Bhandari S, Ray S. A novel synthesis of bisbenzyl ketones by DCC
induced condensation of phenylacetic acid. Synth Commun,1998, 28:
765–771
19 Ando S, Murakami R, Nishida JI, Tada H, ji Inoue Y, Tokito S,
Yamashita Y. n-Type organic field-effect transistors with very high
electron mobility based on thiazole oligomers with trifluoro-
methylphenyl groups. J Am Chem Soc, 2005, 127: 14996–14997
20 Ding L, Ying HZ, Zhou Y, Lei T, Pei J. Polycyclic imide derivatives:
synthesis and effective tuning of lowest unoccupied molecular orbital
levels through molecular engineering. Org Lett, 2010, 12: 5522–5525
21 Tesmer M, Vahrenkamp H. Sterically fixed dithiolate ligands and
their zinc complexes: derivatives of 1,8-dimercaptonaphthalene. Eur
J Inorg Chem, 2001, 1183–1188
23 Sheldrick GM, SHELXS-97. A program for automatic solution of
crystal structure. University of Goettingen, Germany, 1997
24 Sheldrick GM, SHELXL-97. A program for crystal structure refine-
ment. University of Goettingen, Germany, 1997
25 Zhou Y, Ding L, Shi K, Dai YZ, Ai N, Wang J, Pei J. A
non-fullerene small molecule as efficient electron acceptor in organic
bulk heterojunction solar cells. Adv Mater, 2012, 24: 957–961
26 Podzorov V, Pudalov VM, Gershenson ME, Field-effect transistors
on rubrene single crystals with parylene gate insulator. App Phys Lett,
2003, 82: 1739–1741