Rod-like oligomers incorporating 2,6-dialkylamino core-substituted naphthalene diimide as acceptors for organic photovoltaic

Article abstract of DOI:10.1016/j.orgel.2013.03.039

Core-substituted naphthalene diimides (core-substituted NDIs) were incorporated into rod-like molecules and oligomers through reaction at the imide nitrogen positions. N,N′-Di(4-bromophenyl)-2,6-di(N-alkylamino)-1,4,5,8- naphthalenetetracarboxydiimide was synthesized in only three steps, and used as a versatile platform to prepare extended structures by reaction with thiophene substrates using Suzuki-coupling conditions. The optoelectronic properties of the new compounds were examined by UV/vis absorption spectroscopy, fluorescence spectroscopy, cyclic voltammetry and theoretical calculations. The imide substituents had little effect on the optical and electrochemical properties of core-substituted NDIs in solution. A bathochromic shift of the absorption was observed upon film formation, accompanied by quenching of fluorescence. These observations are consistent with increased inter-molecular interactions between core-substituted NDI moieties in the solid state. All compounds were tested in organic solar cells by blending with poly(3-hexylthiophene), and several showed a photovoltaic effect, demonstrating their potential as electron acceptors in organic solar cell. The best solar cell was observed for core-substituted NDI with 4-(thiophen-2-yl)phenyl imide substituents (5a), showing a power conversion efficiency of 0.57% and a large open circuit voltage of 0.87 V. This approach allows new structure-property relationship studies of non-fullerene acceptors in organic solar cells, where one can vary the imide substituent to optimize photovoltaic parameters while keeping the optical and electrochemical properties constant.

Full text of DOI:10.1016/j.orgel.2013.03.039

Organic Electronics 14 (2013) 1683–1692
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Rod-like oligomers incorporating 2,6-dialkylamino
core-substituted naphthalene diimide as acceptors for organic
photovoltaic
⇑
Roshan Fernando, Zhenghao Mao, Geneviève Sauvé
Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106-7078, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 11 April 2013
Core-substituted naphthalene diimides (core-substituted NDIs) were incorporated into
rod-like molecules and oligomers through reaction at the imide nitrogen positions. N,N -
0
Di(4-bromophenyl)-2,6-di(N-alkylamino)-1,4,5,8-naphthalenetetracarboxydiimide
was
Keywords:
synthesized in only three steps, and used as a versatile platform to prepare extended struc-
tures by reaction with thiophene substrates using Suzuki-coupling conditions. The opto-
electronic properties of the new compounds were examined by UV/vis absorption
spectroscopy, fluorescence spectroscopy, cyclic voltammetry and theoretical calculations.
The imide substituents had little effect on the optical and electrochemical properties of
core-substituted NDIs in solution. A bathochromic shift of the absorption was observed
upon film formation, accompanied by quenching of fluorescence. These observations are
consistent with increased inter-molecular interactions between core-substituted NDI moi-
eties in the solid state. All compounds were tested in organic solar cells by blending with
poly(3-hexylthiophene), and several showed a photovoltaic effect, demonstrating their
potential as electron acceptors in organic solar cell. The best solar cell was observed for
core-substituted NDI with 4-(thiophen-2-yl)phenyl imide substituents (5a), showing a
power conversion efficiency of 0.57% and a large open circuit voltage of 0.87 V. This
approach allows new structure–property relationship studies of non-fullerene acceptors
in organic solar cells, where one can vary the imide substituent to optimize photovoltaic
parameters while keeping the optical and electrochemical properties constant.
Ó 2013 Elsevier B.V. All rights reserved.
Core-substituted naphthalene diimide
Non-fullerene acceptor
Organic solar cell
Structure–property relationship
1
. Introduction
in optoelectronic applications. To increase absorption into
the visible range, one can either extend the conjugation of
Interest in core-substituted naphthalene diimides
NDIs) was revived recently, due to advances in their syn-
NDI through the core [12–22], or use electron-donating
core substituents [6]. In the later strategy, the combination
of electron-donating core substituents with an electron-
poor NDI core creates a push-pull system with tunable col-
or [6]. The core substituents also greatly impact the fluores-
cent properties of these molecules; alkoxy or alkylamino
groups have resulted in highly fluorescent molecules
whereas arylamino groups have given non-fluorescent
molecules [7]. Of special interest, 2,6-dialkylaminonaph-
thalene diimides have intense absorption around 610 nm
and red emission around 630 nm. They can also be easily
reduced and oxidized to their corresponding dianion and
(
thesis [1–5], their tunable optical and electrochemical
properties [6,7], their anion binding properties [4,8], and
their excellent electron transport properties [9,10]. Core-
substituted NDIs with electron-withdrawing core substitu-
ents were reported to have stable and high electron mobil-
ity in field-effect transistors [3,11]. However, these
compounds do not absorb visible light, limiting their use
⇑
Corresponding author. Tel.: +1 2163683665.
E-mail address: genevieve.sauve@case.edu (G. Sauvé).
1
566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.03.039

1
684
R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
dication, respectively [23,24]. However, only limited stud-
ies have explored these core-substituted NDIs for solar en-
ergy conversion applications [23,25].
In organic electronic applications, making films with
desirable properties is critical, and the molecular structure
of the organic compounds greatly affects the final opto-
electronic properties of the film. In the case of naphthalene
diimides, both core and imide substituents are known to
affect molecular packing and optoelectronic properties
thiophene substrates using Suzuki-coupling conditions. The
optoelectronic properties of the new compounds were stud-
ied by UV/vis absorption spectroscopy, fluorescence spec-
troscopy, cyclic voltammetry and theoretical calculations.
The compounds were also tested in organic solar cells by
blending with poly(3-hexylthiophene).
2
. Materials and methods
[
6,13,26,27]. Generally, core-substituents are used to tune
2
.1. Synthesis of materials
the HOMO and LUMO energy levels because they are con-
jugated with the NDI core, whereas imide substituents are
used as solubilizing groups, though they have been shown
to fine-tune the LUMO energy levels by influencing charge
density delocalization and by stabilizing the radical anions
All reagents and solvents were used as received unless
0
0
0
otherwise specified. 5,5 -Dibromo-4,4 -dihexyl-2,2 -bithi-
ophene and 2,6-dibromonaphthalene-1,4,5,8-tetracarb-
oxylic dianhydride (2) were synthesized according to the
literature [37,38].
[
28–30].
Efforts to incorporate NDI molecules into polymers are
limited. In most cases, NDI is copolymerized with aromatic
co-monomers through the core positions, leading to conju-
gated polymers with good electron transport properties
0
0
0
2
.1.1. 4,4 -Dihexyl-2,2 -bithiophene-5,5 -diboronic acid
bis(pinacol) ester
This synthesis was based on a literature procedure for a
[
9,14,31]. One study has incorporated NDI through the
imide nitrogen position, alternating NDI with an oligoester
32]. These copolymers had poor charge carrier transport
due to the inability of the NDI moieties to form supramo-
lecular organizations of NDI -stacks, although evidence
0
0
0
similar molecule [39]. 5,5 -dibromo-4,4 -dihexyl-2,2 -
bithiophene (0.98 g/2 mmol) was dissolved in dry tetrahy-
drofuran (20 mL) under nitrogen and cooled to ꢀ78 °C.
[
2
mL of butyllithium solution (2.5 M in hexane) was added
p
drop wise. The reaction was allowed to warm to room tem-
perature and stirred for 2 h. The mixture was cooled to
78 °C and a degassed solution of 2-isopropoxy-4,4,5,5-
tetramethyl-1,2,3-dioxaborolane (0.93 g/5 mmol) in THF
4 mL) was added drop wise. The mixture was then al-
lowed to warm to room temperature and stirred overnight.
The mixture was poured into an aqueous HCl (0.1 M) solu-
tion and extracted with CHCl
were dried with anhydrous MgSO
product was obtained as yellow oil after vacuum evapora-
tion of the solvents and was purified by column chroma-
tography using silica gel and chloroform. Yield = 0.60 g
51%) – a pale yellow oil. H NMR (400 MHz, CDCl
for aggregates of NDI was observed. These modified NDI
did not have any core substituents other than hydrogens.
Core-substituted NDIs were also recently incorporated into
supramolecular architectures [24]. In comparison, more
has been done with its larger analogue, perylene diimide
ꢀ
(
(
[
PDI), by incorporating it into polymers as pendant groups
33,34], through the core [35], and through the imide
3
. The combined organics
and filtered. The crude
nitrogen position [36]. An alternating copolymer of phen-
ylenevinylene with PDI, where the phenyl group is an
imide substituent of PDI, was reported to behave as an
acceptor in organic solar cells and as n-type material in
transistors [36].
4
1
(
3
, d,
One could create polymers where core-substituted NDI
moieties are connected to other aromatic units through
the imide positions, Fig. 1. This strategy would allow preser-
vation of the optical and electrochemical properties of core-
substituted NDI while improving film formation by solution
processing and/or self-assembly in films. Aromatic imide
substituents such as phenyl and phenyl-thiophene could
create rod-like structures that may favor order in films. Here,
ppm): 7.10 (s, 2H), 2.81 (t, 4H), 1.59 (m, 4H), 1.30 (m,
H), 1.32 (s, 24H), 0.88 (t, 6H).
4
0
2
.1.2. N,N -Di(4-bromophenyl)-2,6-dibromo-1,4,5,8-
naphthalenetetracarboxydiimide (3)
To a suspension of 2,6-dibromonaphthalene-1,4,5,8-tet-
racarboxylic acid, 2 (0.20 g, 0.5 mmol; assuming 100% con-
version of 1–2) in glacial acetic acid (10 mL), 4-
bromoaniline (0.69 g, 4 mmol) was added at once, under
0
we report the synthesis of N,N -Di(4-bromophenyl)-2,6-di(N-
alkylamino)-1,4,5,8-naphthalenetetracarboxydiimide,
4,
2
N atmosphere. The mixture was stirred at 120 °C for 2 h.
and its incorporation into larger molecules by reaction with
The mixture was cooled to room temperature and poured
into ice to precipitate the product. The solid was collected
using a Buchner funnel, washed with 0.1 M HCl and dis-
tilled water, dried under vacuum overnight and used in
the next step without further purification (Yield: 0.63 g).
1
H NMR (400 MHz, DMSO, d, ppm): 8.69 (s, 2H), 7.75 (d,
4
H), 7.42 (d, 4H).
0
2
1
.1.3. N,N -Di(4-bromophenyl)-2,6-di(N-cyclohexylamino)-
,4,5,8-naphthalenetetcarboxydiimide (4a)
The crude compound, 3 (0.63 g) was dissolved in cyclo-
hexylamine (20 mL) and heated to reflux at 130 °C under
for 2.5 h. The dark blue solution was allowed to cool
Fig. 1. General molecular structure of rod-like molecules and polymers
incorporating core-substituted NDI, where the structure is extended
through reaction at the imide positions.
N
2

R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
1685
to room temperature and concentrated under vacuum. The
concentrate was dissolved in chloroform (150 mL) and
washed with 1 M HCl and distilled water. The organic layer
the solvent under vacuum gave 5a as a deep blue solid.
Yield: 30 mg (29%). H NMR (600 MHz, CDCl , d, ppm):
3
1
9.28 (d, 2H), 8.17 (s, 2H), 7.75 (d, 4H), 7.27–7.32 (m, 8H),
7.06 (dd, 2H), 3.67 (m, 2H), 2.04 (m, 4H), 1.73 (m, 4H),
1.60–1.58 (m, 2H), 1.39–1.28 (m, 8H), 1.22–1.14 (m, 2H).
4
was dried with anhydrous MgSO and filtered. Removal of
the solvent under vacuum yielded a dark blue solid. The
crude product was purified by column chromatography
using silica gel and chloroform and followed by Soxhlet
extraction using ether, hexane and chloroform. Compound
1
3
3
C NMR (600 MHz, CDCl , d, ppm): 24.7, 25.5, 33.2, 51.4,
101.8, 119.3, 121.8, 123.9, 125.5, 126.3, 127.1, 128.1,
129.1, 134.4, 135.2, 143.5, 148.6, 163.4, 166.4. MALDI-
4
a was isolated from the chloroform fraction as a deep blue
TOF-MS: m/z calcd for
46 40 4 4 2
C H N O S 776.96, found
1
solid. Yield: 0.15 g (39% overall). H NMR (600 MHz, CDCl
d, ppm): 9.23 (d, 2H), 8.14 (s, 2H), 7.64 (d, 4H), 7.15 (d, 4H),
3
1
3
,
775.66. TGA: T = 215 °C at 5% weight loss. UV/vis (THF):
d
ꢀ
1
ꢀ1
k
max/nm
(
ꢀ
/M cm ) = 609 (19,896). Fluorescence
max = 639 nm. Fluorescence quantum yield:
= 0.55. Anal. Calcd for C46 : C, 71.11; H, 5.19;
.65 (m, 2H), 2.02 (m, 4H), 1.73 (m, 4H), 1.60–1.58 (m, 2H),
.39–1.28 (m, 8H), 1.21–1.17 (m, 2H). 13C NMR (600 MHz,
(THF):
U
k
fl
40 4 4 2
H N O S
CDCl
23.0, 126.2, 130.3, 132.8, 134.3, 148.6, 163.2, 166.2. MAL-
DI-TOF-MS: m/z calcd for C38 770.51, found
70.27. TGA: T = 407 °C at 5% weight loss. No melting
was observed below decomposition temperature of
3
, d, ppm): 24.6, 25.5, 33.1, 51.4, 101.6, 119.3, 121.7,
N, 7.21. Found: C, 70.89; H, 5.07; N, 7.14.
1
2 4 4
H34Br N O
0
7
d
2.1.6. N,N -Di(4-(3-hexylthiophen-2-yl)phenyl)-2,6-bis-(N-
cyclohexylamino)-1,4,5,8-naphthalenetetracarboxydiimide
(5b)
This compound was synthesized as 5a above, using 3-
hexylthiophene-2-boronic acid pinacol ester instead of thi-
ꢀ1
ꢀ1
4
07 °C. UV/vis (THF):
18,392). Fluorescence (THF): kmax = 639 nm. Fluorescence
2 4 4
quantum yield: = 0.56. Anal. Calcd for C38 34Br N O :
k
max/nm
(ꢀ/M cm ) = 611
(
U
fl
H
C, 59.23; H, 4.45; N, 7.27. Found: C, 58.95; H, 4.27; N, 7.08.
ophene-2-boronic acid pinacol ester. Yield: 90 mg (73%).
H NMR (600 MHz, CDCl , d, ppm): 9.27 (d, 2H), 8.19 (s,
3
1
0
2
1
.1.4. N,N -Di(4-bromophenyl)-2,6-di(N-2-ethylhexylamino)-
,4,5,8-naphthalenetetracarboxydiimide (4b)
2H), 7.58 (d, 4H), 7.32 (d, 4H), 7.21 (d, 2H), 6.96 (d, 2H),
3.67 (m, 2H), 2.68 (t, 4H), 2.05 (m, 4H), 1.74 (m, 4H),
1
3
The synthesis, separation and purification of 4b was
1.62–1.57 (m, 6H), 1.39–1.16 (m, 22H), 0.82 (t, 6H).
C
similar to that of 4a, by refluxing crude compound, 3
0.63 g) in 2-ethylhexylamine (15 mL) at 145 °C under N
NMR (600 MHz, CDCl , d, ppm): 14.1, 22.6, 24.7, 25.5,
3
(
2
28.8, 29.3, 31.1, 31.7, 33.2, 51.6, 101.8, 119.3, 121.8,
124.2, 126.3, 128.6, 129.5, 130.4, 134.3, 135.5, 136.8,
139.3, 148.6, 163.5, 166.5. MALDI-TOF-MS: m/z calcd for
for 4 h. Purification by silica gel column chromatography
was done using hexane/chloroform (4:6) mixture as the
eluent, which was gradually increased to 100% chloroform.
C
58
H
64
N
4
O
4
S
2
945.28, found 944.74. TGA: T
d
= 395 °C at
Yield: 0.05 g (12% overall). 1H NMR (600 MHz, CDCl
, d,
ppm): 9.22 (t, 2H), 8.15 (s, 2H), 7.65 (d, 4H), 7.16 (d, 4H),
.31 (t, 4H), 1.63 (m, 2H), 1.36–1.41 (m, 4H), 1.29–1.35
5% weight loss. UV/vis (THF): kmax/nm (
ꢀ
/M cm ) = 610
ꢀ1
ꢀ1
3
(20,686). Fluorescence (THF): kmax = 639 nm. Fluorescence
64 4 4 2
quantum yield: = 0.57. Anal. Calcd for C58H N O S :
3
U
fl
(
m, 4H), 1.19–1.26 (m, 8H), 0.85 (t, 6H), 0.80 (t, 6H). 13
C
C, 73.69; H, 6.82; N, 5.93. Found: C, 73.40; H, 6.69; N, 5.79.
3
NMR (600 MHz, CDCl , d, ppm): 10.9, 14.0, 23.0, 24.4,
2
1
8.8, 31.2, 39.2, 46.4, 101.9, 119.0, 121.7, 123.0, 126.2,
30.4, 132.8, 134.3, 149.7, 163.1, 166.2. MALDI-TOF-MS:
830.19, found 829.58. TGA:
= 390 °C at 5% weight loss. UV/vis (THF): kmax/nm (
2.1.7. 6a
0
0
m/z calcd for C42
H46Br
2
N
4
O
4
A
mixture of 2,2 -bithiophene-5,5 -diboronic acid
T
d
ꢀ
/
bis(pinacol)ester (63 mg, 0.15 mmol) and 4b (125 mg,
0.15 mmol) was dissolved in dry chlorobenzene (5 mL).
To this solution, tetrakis(triphenylphosphene)palladium
(9 mg, ꢁ5 mol%) was added under nitrogen. After stirring
the mixture for 30 min at room temperature, a degassed
aqueous solution (2 mL) of potassium carbonate (141 mg)
and tetrabutylammonium bromide (10 mg) was added.
The mixture was heated to 90 °C and allowed to react for
48 h. After cooling to room temperature, the mixture was
dissolved in chloroform and concentrated under reduced
pressure. The concentrated solution was poured into meth-
anol to precipitate the product. This precipitate was puri-
fied by a series of Soxhlet extractions (methanol, acetone,
hexane, and chloroform) and isolated from the chloroform
ꢀ1
ꢀ1
M
cm ) = 611 (18,248). Fluorescence (THF): kmax = 637 -
nm. Fluorescence quantum yield: = 0.56. Anal. Calcd for
: C, 60.73; H, 5.58; N, 6.74. Found: C, 60.91;
U
fl
42 2 4 4
C H46Br N O
H, 5.60; N, 6.82.
0
2.1.5. N,N -Di(4-(thiophen-2-yl)phenyl)-2,6-bis-(N-
cyclohexylamino)-1,4,5,8-naphthalenetetracarboxydiimide
5a)
Compound 4a (100 mg, 0.13 mmol), thiophene-2-boro-
(
nic acid pinacol ester (109 mg, 0.52 mmol), Palladium(II)
acetate (10 mg) and tris(o-tolyl)phosphine (34 mg) were
dissolved in toluene (5 mL) and stirred under nitrogen for
3
0 min at room temperature. An aqueous solution (2 mL,
1
degassed) of potassium carbonate (0.141 g, 1.02 mmol)
and tetrabutylammonium bromide (10 mg) was added to
the reaction mixture. The mixture was heated to reflux
and stirred for 36 h. After, the mixture was dissolved in
chloroform and filtered through a Celite plug. The filtrate
was concentrated under vacuum, passed through a silica
gel column and washed with hexane. The product was
recovered using chloroform as the eluent. Evaporation of
fraction as a blue green solid (70 mg, 56%). H NMR
(600 MHz, CDCl
(b, proton on NDI core), 7.94–6.98 (m, protons on benzene
and thiophene), 3.28 (b, CH on ethylhexyl), 1.62 (b, proton
on tertiary carbon of ethylhexyl), 1.57–1.04 (m, ethylhexyl
body protons), 0.85–0.74 (m, CH on ethylhexyl). GPC:
= 2.0 kDa, M = 3.2 kDa, PDI = 2.4. TGA: T = 219 °C at
5% weight loss.
3
, d, ppm): 9.26 (b, NH on NDI core), 8.14
2
3
M
n
w
d

1
686
R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
2
.1.8. 6b
ꢀ[E1/2,ox + 5.1] eV and LUMO = ꢀ[E1/2,red + 5.1] eV. Density
functional theory (DFT) calculations were done at the
B3LYP/6-31G(d,p) level using Gaussian 09 software. All
optimized geometries were identified as local minimum
with frequency calculations.
0
0
6
b was synthesized as 6a above, using 3,3 -dihexyl-2,2 -
0
bithiophene-5,5 -diboronic acid bis(pinacol)ester instead
of 2,2 -bithiophene-5,5 -diboronic acid bis(pinacol)ester.
0
0
1
(
100 mg, 87%). H NMR (600 MHz, CDCl
b, NH on NDI core), 8.21 (b, proton on NDI core), 7.70–
.74 (m, protons on benzene and thiophene), 3.33 (b, CH
on ethylhexyl), 2.68 (b, CH on hexyl), 1.65 (b, proton on
tertiary carbon of ethylhexyl), 1.40–1.19 (m, ethylhexyl
and hexyl body protons), 0.82 (b, CH on ethylhexyl and
hexyl). GPC: M = 4.7 kDa, M = 11.3 kDa, PDI = 2.4. TGA:
= 371 °C at 5% weight loss.
3
, d, ppm): 9.30
(
6
The core-substituted NDI compounds were tested in or-
ganic solar cells by blending with poly(3-hexylthiophene)
2
2
(P3HT, M
ring at 75 °C overnight in a glove box. The total concentra-
tion was 32 mg/mL. ITO-coated glass (R = 15 /sq)
n
= 30 kDa, PDI = 1.3) in dichlorobenzene and stir-
3
X
n
w
substrates were cleaned stepwise in detergent, water, ace-
tone and isopropanol under ultra-sonication for 30 min
each, and dried in an oven for several hours. A thin PED-
OT:PSS layer was spin-coated onto the ITO glass which
was pretreated by UV-ozone for 20 min, followed by drying
at 150 °C for 15 min in glovebox. The film was thermally an-
nealed at 100 °C for 30 min. Ca (25 nm) and Al (100 nm)
were deposited in sequence under vacuum. The devices
were measured under 1.5 AM solar illumination at an inten-
T
d
2
.1.9. Electropolymerization
For cyclic voltammetry measurements, compounds 5a
and 5b were electropolymerized on glassy carbon working
electrodes from a monomer solution in dry dichlorometh-
ane with 0.1 M Bu NPF by cycling the applied potential
4 6
between 0 and 1.8 V at 0.1 V/s, ten times, using a Pt wire
as pseudo reference and a Pt wire as counter electrodes.
The films were characterized by cyclic voltammetry in ace-
2
2
sity of 100 mW/cm . The total effective area was 0.12 cm .
4 6 3
tonitrile with 0.1 M Bu NPF using a Ag/AgNO reference
+
electrode (E1/2 = 0.091 V vs. Fc/Fc ) and a Pt wire counter
electrode. For absorption measurements, 5a and 5b were
electropolymerized on Indium Tin Oxide (ITO) conducting
glass by cycling the applied potential between 0 and
3. Results and discussion
3.1. Synthesis
1
.8 V at 0.1 V/s, 100 times, using a Pt wire as pseudo refer-
We synthesized 2,6-dialkylaminonaphthalene diimide
derivatives with 4-bromophenyl substituents at the imide
position according to Scheme 1; the synthesis of 4a-b was
done in only three steps. To further save time and re-
sources, only the desired final compounds (4a-b) were
purified by column chromatography. First, 1,4,5,8-naph-
thalenetetracarboxylic dianhydride was brominated fol-
lowing a literature procedure; several synthesis protocols
were tested and the procedure reported by Chaigon et al.
yielded the least number of by-products, with the desired
compound 2 as the main product [1,2,38,42]. Crude 2
was then reacted with 4-bromoaniline to give compound
3. This reaction was carried out in acetic acid to avoid
ence and a stainless steel plate as the counter electrode.
The films were washed with acetonitrile or methanol and
dried with a flow of N gas prior to analysis.
2
2.2. Methods
1H NMR and 13C NMR spectra were recorded using a
00 MHz spectrometer. MALDI-TOF MS spectra were ac-
6
quired in reflective mode (for small molecules) and linear
mode (for oligomers); samples were prepared from chloro-
form solutions using terthiophene as the matrix. Number-
average molecular weight (M
n
) and molecular weight dis-
tribution (PDI = M /M ) were estimated by gel permeation
w
n
N
the undesirable S Ar that would replace the core bromines
chromatography (GPC) with a ultraviolet detector. The sys-
tem was operated at a flow rate of 0.75 mL/min at 30 °C
using chloroform as eluent and calibrated using polysty-
rene standards. UV/vis and emission spectra were collected
in tetrahydrofuran or on 1 cm glass plates. Fluorescence
quantum yields in tetrahydrofuran solutions were calcu-
lated using the zinc phthalocyanine standard (quantum
yield is 0.23) in THF. The absorption for both the standard
and the analyte were similar and below 0.2 absorbance
units; the concentration was 0.008 mg/mL (or
by protonating 4-bromoaniline and thus lowering the reac-
tivity of the amine [1,5]. The core bromines of 3 were
substituted with alkylamine using a S Ar to produce com-
N
pounds 4a-b. Compounds 4a and 4b were blue and soluble
in common organic solvents. They were purified by column
chromatography to give overall yields from 1 of 39% and
12% for 4a and 4b, respectively.
The core-substituted NDI derivative 4a was successfully
coupled with thiophene-2-boronic acid pinacol ester and
3-hexylthiophene-2-boronic acid pinacol ester using Suzu-
ki coupling conditions to give 5a and 5b, respectively
(Scheme 2). Using 3-hexylthiophene instead of thiophene
resulted in higher yields, perhaps due to increased solubil-
ity or higher electron density on the thiophene ring. The
yield of 5a could be significantly increased by replacing
the Pd(OAc) /tri(o-tolyl)phosphine catalyst with tetra-
2
kis(triphenylphosphine)palladium(0); the later was there-
fore used in subsequent reactions.
To obtain polymers, we attempted to react 4a with
bithiophene boronic acid pinacol esters using Suzuki cou-
pling conditions. However, most of the product obtained
ꢀ5
1
ꢂ 10 M) for 5a and 0.002 mg/mL for 6b.
Cyclic voltammetry measurements were performed
4 6
using 0.1 M Bu NPF in dry dichloromethane as electrolyte
and ferrocene as internal standard. The solution was purged
with dry nitrogen for 10 min prior to the measurement.
Glassy carbon was used as working electrode and platinum
wires were used as the counter and reference electrodes.
The scan rate was 0.1 V/s. HOMO and LUMO energy levels
were estimated from the E1/2 values of the oxidation and
reduction waves, respectively, and the value of ꢀ5.1 eV
+
vs. vacuum was used for Fc/Fc [40,41], i.e. HOMO =

R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
1687
Scheme 1. Synthesis of core-substituted NDI molecules.
was insoluble in common organic solvents and the soluble
part had very low molecular weight with a degree of poly-
merization (DP) of at most 2. To increase solubility, the
cyclohexylamine core substituents were replaced with 2-
ethylhexylamine, 4b. This monomer was allowed to react
with bithiophene boronic acid pinacol esters using Suzuki
coupling conditions to give 6a-b, Scheme 2. The structural
elements of both co-monomers were observed by H NMR
spectroscopy (Supporting Information). Fig. 2 shows the
gel permeation chromatography (GPC) traces comparing
monomer 4b with oligomers 6a and 6b. Both 6a and 6b
eluted at shorter times than 4b, indicating that they have
higher molecular weights than the monomer. Molecular
weights estimated using polystyrene standards were
n n
M = 2.0 kDa, PDI = 1.6 for 6a and M = 4.7 kDa, PDI = 2.4
for 6b, consistent with the formation of oligomers. The
maximum m/z detected by MALDI-TOF MS corresponds
to a DP of 4 for 6a and 5 for 6b. Attempts to increase
molecular weight by improving stoichiometry precision
and by recrystallizing the diboronic ester monomer failed.
1
Scheme 2. Suzuki coupling reactions of core-substituted NDI molecules.

1
688
R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
Fig. 2. GPC traces (UV detector at 254 nm) comparing 4b with 6a and 6b.
Using toluene as solvent or lowering the reaction temper-
ature yielded lower molecular weights. Molecular weights
are most likely limited by the low conversion of the Suzuki
coupling and by the solubility of these rod-like materials in
organic solvents.
3.2. Optical properties
The optical properties of all compounds are summa-
Fig. 3. (a) Absorption and emission spectra of 4a and 5a in THF; (b)
Absorption spectra of 6b in THF and in film, and emission spectra in THF.
rized in Table 1, Fig. 3 and Figs. S18–S21. Monomer 4a-b
and 5a-b had similar optical properties than other 2,6-dial-
kylamino core-substituted NDI [23]; they were deep blue
ꢀ
1
ꢀ1
with high extinction coefficients (ꢁ20 k M cm ) and
displayed red fluorescence in solution with high quantum
yields (ꢁ0.56). The nature of the alkylamino core substitu-
ent, as well as the addition of thiophene or 3-hexylthioph-
ene on each benzene group, had little effect on the optical
properties. Oligomers 6a-b had similar absorption spectra
to 4b in solution, but the quantum yield decreased from
they were obtained by spin-coating from chloroform solu-
tions. Most compounds had a modest bathochromic shift
upon film formation, with 5a and 6b showing the largest
kmax shift of 18 nm and 20 nm, respectively. The absorption
onsets were between 1.75 eV and 1.87 eV. All films had no
detectable emission, consistent with aggregation.
0
.56 for 4b to 0.28 for 6a and 0.23 for 6b, partly because
3
.3. Electrochemical properties
of increased aggregation. Support for this hypothesis can
be found in the supporting information Figs. S22 and S23.
Films of 6a-b were made by drop-casting from a chloro-
form solution and the larger oligomer 6b formed more
homogeneous films than 6a. Uniform films of 4a-b and
The electrochemical properties were obtained in dichlo-
romethane and are summarized in Table 2 and Fig. 4. All
core-substituted NDI derivatives were easily and reversibly
reduced up to the dianion. While the oxidations were
reversible for 4a-b, they were quasi-reversible for 5a-b
and 6a-b. The values for the oxidation and reduction
potentials were all similar, despite different alkyl and
imide functionality. The estimated HOMO energy levels
ranged between ꢀ5.7 eV and ꢀ5.8 eV and LUMO energy
levels ranged between ꢀ3.7 eV and ꢀ3.8 eV. For compari-
son, the LUMO energy level of a common electron acceptor,
phenyl-C61-butyric acid methyl ester (PCBM), had a LUMO
level of ꢀ4.0 eV, measured in our laboratory under the
same conditions. We also attempted to obtain electro-
chemical information of films of 6a and 6b, but cyclic vol-
tammograms could not be obtained because the films
delaminated upon positive or negative applied potentials.
During the CV measurement of 5a-b in dichlorometh-
ane, we observed an increase in current density after the
5
a-b could not be made using the same method; instead,
Table 1
Optical properties.
a
a
a
Molecule
k
abs
e
k
em
U
f
k
film
(nm)
abs
E
g,opt
film
(eV)
a
ꢀ1
ꢀ1
sol.
nm)
(M cm
)
(nm)
(
b
4
4
5
5
6
6
a
b
a
b
a
b
611
611
609
610
610
612
18,400
18,200
19,900
20,700
–
639
637
639
639
636
634
0.56 622
0.56 614
0.55 626
0.57 602
1.85
1.75
1.83
1.78
1.87
1.85
b
b
b
0.28 617c
0.23 632c
–
a
b
c
In THF.
Spin-coated.
Drop-casted.

R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
1689
Table 2
Electrochemical properties in dichloromethane.
ox
(V vs. Fc/Fc+)
E
r1 e= d2 (V vs. Fc/Fc+)
HOMO (eV)a
LUMO (eV)a
Molecule
E
g
E (eV)
1
=2
4
4
5
5
6
6
a
b
a
b
a
b
0.60, 1.05
0.63, 1.08
0.60
0.60
0.69
ꢀ1.36, ꢀ1.77
ꢀ1.34, ꢀ1.75
ꢀ1.38, ꢀ1.78
ꢀ1.38, ꢀ1.78
ꢀ1.32, ꢀ1.71
ꢀ1.37, ꢀ1.74
ꢀ1.11, ꢀ1.50
ꢀ5.70
ꢀ5.73
ꢀ5.70
ꢀ5.70
ꢀ5.79
ꢀ5.76
ꢀ3.74
ꢀ3.76
ꢀ3.72
ꢀ3.72
ꢀ3.78
ꢀ3.73
ꢀ3.99
1.96
1.97
1.98
1.98
2.01
2.03
0.66
Not observed
b
PCBM
a
Estimated using the first oxidation and reduction E1/2 values, assuming Fc/Fc+ = ꢀ5.1 eV vs. vacuum.
b
Purchased from SES research (99% pure).
3.4. Theoretical calculations
To better understand the optical and electrochemical
properties, the molecular orbitals of 4a-b and 5a-b were
calculated using density functional theory (DFT) at the
B3LYP/6-31G(d,p) level, using the polarizable continuum
model (PCM) for solvent effects and the results are summa-
rized in Table 3. The frontier molecular orbitals and opti-
mized geometries for 4a and 5a are shown in Fig. 5. In
both molecules, the dihedral angle between the core-
substituted NDI and phenyl group is ꢁ90° in dichlorometh-
ane. Both the HOMO and LUMO orbitals are localized on
the core-substituted NDI unit, indicating that the optical
and electrochemical properties should be governed by
the core-substituted NDI unit, consistent with our experi-
mental data in solution. The results for 4b are very similar
to 4a, confirming that changing the nature of the alkyl
amine core substituent does not affect optoelectronic
properties of the molecules. Comparing 5a and 5b, the
dihedral angle between the phenyl and thiophene is 26°
and 48°, respectively. The thiophene and phenyl groups
are therefore more twisted in 5b due to the presence of
an alkyl substituent at the 3rd position on the thiophene.
In the solid state, the dihedral angle between core-
substituted NDI and the phenyl group is expected to be dif-
ferent than in solution because of the contribution of
molecular interactions. Specifically, the added stabilization
Fig. 4. Cyclic voltammograms in dichloromethane.
Table 3
energy obtained from p–p stacking could favor a co-planar
Calculated (DFT) parameters in dichloromethane.
arrangement, which would in turn favor increased conjuga-
tion and cause a bathochromic shift in the absorption spec-
tra. To determine how much energy would be required to
force the phenyl groups to be co-planar with core-substi-
tuted NDI, we performed calculations where the dihedral
angle between core-substituted NDI and the phenyl group
was varied. This was done starting from the optimized
geometry and scanning the dihedral angle between core-
substituted NDI and the phenyl group, keeping all other
parameters constant. The relative potential energy for the
co-planar geometry of 4a was found to be 62 kcal/mol in
the gas phase, which corresponds to a rotational barrier of
approximately 31 kcal/mol for each imide-linker. Although
Molecule HOMO
eV)
LUMO
(eV)
E
g
Dihedral
Dihedral
angle (°) Ph-
Th
(
(eV) angle (°)
cNDI-Ph
4a
4b
5a
5b
ꢀ5.3
ꢀ5.3
ꢀ5.3
ꢀ5.3
ꢀ2.9
ꢀ3.0
ꢀ3.0
ꢀ2.9
2.4
2.4
2.4
2.4
90.1
89.6
89.0
85.0
25.6
47.8
second oxidation cycle, indicating possible oxidative elec-
trochemical polymerization due to the presence of termi-
nal thiophenes. After several scans, the working
electrodes were covered with a film, confirming electro-
chemical polymerization. 5a-b were polymerized on glassy
carbon electrodes and on Indium Tin Oxide (ITO) coated
glass. The optical and electrochemical properties of the
electropolymerized 5a-b are reported in the Supporting
Information section (Figs. S24 and S25, Tables S1 and S2)
and were similar to those for 6a and 6b.
the
to-face
p
-acidity of core-substituted NDI strongly favors face-
-stacking, it is unlikely to overcome this energy
p
barrier in the solid state. The interaction energy of face-
to-face dimers of 2,6-ethoxy-core-substituted NDI were
calculated to be ꢀ27.2 kcal/mol [23], and this number is ex-
pected to be smaller for 2,6-diamino-core-substituted NDI

1
690
R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
Fig. 5. Molecular orbitals and optimized geometry for 4a and 5a in dichloromethane.
due to the stronger electron donating core substituents.
Experimentally, our compounds showed a bathochromic
shift upon film formation accompanied by fluorescence
quenching. Based on the above discussion, we think that
the red shift is unlikely due to planarization of the aro-
matic-core-substituted NDI-aromatic structure, but rather
due to intermolecular interactions between core-substi-
tuted NDI moieties that are likely to occur with the rod-like
geometry.
consistent with lamellar ordering. The absence of p–p
stacking order is not surprising, considering that the calcu-
lations show a 90° angle between core-substituted NDI and
the phenyl group and a 48° angle between the phenyl
group and 3-hexylthiophene group. Although these angles
were calculated in solution and not in the solid state, it is
reasonable to assume that they are probably not close to
0° in the solid state, as previously discussed, making it hard
Table 4
a
Performance parameters for best organic solar cells.
3.5. X-ray diffraction (XRD) studies
Donor:acceptor 1:2
ratio
J
sc (mA/
2
Voc (V) FF (%) PCE (%)
cm )
We attempted to crystallize 4a-b and 5a-b, but only
P3HT:4a
P3HT:4b
P3HT:5a
P3HT:5b
P3HT:6b
0.21
1.36
1.94
0.93
0.038
0.46
0.55
0.87
0.87
0.73
34
39
34
31
31
0.03
0.29
0.57
0.25
0.009
non-crystalline flakes could be formed. We then turned
to transmission XRD of flakes for the oligomers. Thick
flakes appropriate for a measurement could only be ob-
tained for 6b, and the spectrum is shown in the supporting
information. The spectrum had a broad amorphous peak
and two sharper peaks with d-spacings of 2.0 and 1.6 nm,
a
All films were annealed at 100 °C for 30 min prior to cathode
deposition.

R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
1691
Fig. 6. (a) Energy level diagram for P3HT, PCBM and 5a; (b) Current–voltage curve for a P3HT: 5a solar cell measured under AM 1.5 solar illumination at
ꢀ
2
1
00 mW cm .
for the conjugated parts to
evidence of liquid crystalline phase up to 240 °C, and DSC
shows no phase transition between 20 °C and 350 °C.
p
stack. Films of 6b showed no
and 4b suggests that using 2-ethylhexylamino core sub-
stituents is better than cyclohexylamino core substituents.
Comparing 4a with 5a suggests that the additional thio-
phene on the benzene imide substituents is beneficial,
while adding a hexyl group at the 3rd position on the thi-
ophene is detrimental to device performance (5a and 5b).
Further work is underway to establish structure–property
relationships for these types of acceptors, and to under-
stand how these structural changes affect device perfor-
mance. These studies are beyond the scope of this paper
and will be published separately.
3.6. Organic solar cells
Fullerene derivatives are currently the dominant accep-
tor used in organic solar cells [43,44]. However, fullerene
derivatives have drawbacks, including a low absorption
in the visible-near-IR range and limited control over their
HOMO and LUMO energy levels [13,45–47]. These draw-
backs could be overcome using non-fullerene acceptors
[
48,49]. We tested our core-substituted NDI-based materi-
4
. Conclusions
als by blending them with poly(3-hexylthiophene), a com-
mon electron donor that is readily available. To our
surprise, solar cell devices using 4a,b and 5a,b as acceptors
showed a photovoltaic effect (Tables 4 and S3), whereas
solar cells made with oligomers 6a and 6b showed negligi-
ble photovoltaic effect. A ratio of 1:2 P3HT:acceptor gener-
ally yielded higher efficiencies than a ratio of 1:1, and pre-
annealing at 100 °C for 30 min also improved efficiencies.
Surface analysis of P3HT:acceptor blends by AFM revealed
features of the order of 50 nm for the small molecules and
of micron-size for the oligomer 6b (Fig. S28). The later
observation explains the poor performance of 6b when
blended with P3HT. The annealed film of P3HT:5b also
showed larger domains, most likely due to 5b aggregation.
Compound 5a gave the largest power conversion effi-
ciency, 0.57%, shown in Fig. 6. Although this efficiency is
lower than those reported for oligothiophene core-substi-
tuted NDI (1.5%) [13] and conjugated polymers based on
NDI (1.4%) [16], our cell had a higher Voc, 0.87 V, due to
the higher LUMO level of the 2,6-alkylamino core-substi-
tuted NDI. This Voc is about 0.3 eV larger than typically ob-
tained for P3HT blended with PCBM [50], as expected since
Creating extended structures of core-substituted NDI
through the imide positions is an underexplored way to
design new molecules for structure–property relationship
studies. Here, phenyl and phenyl-thiophene imide substit-
uents were explored. In solution, optical and electrochem-
ical properties of the core-substituted NDI derivatives were
similar to the core-substituted NDI. This is because phenyl
imide substituents are not conjugated with the core-
substituted NDI, as expected from calculations that indi-
cate the phenyl to be perpendicular to the NDI plane. The
quantum yields were high for the small molecules, but
were lower for the oligomers due to the higher tendency
of the oligomers to aggregate. The UV/vis specta showed
a modest bathochromic shift upon film formation and
complete quenching of the emission. Both solution, film
and theoretical calculation results are consistent with in-
ter-molecular interactions between the core-substituted
NDI moieties as the molecules aggregate, despite having
bulky N-phenyl substituents. The compounds were tested
in organic solar cells by blending with poly(3-hexylthioph-
ene). Our results demonstrate that these compounds have
potential as acceptors in organic solar cells, and that this
approach merits further exploration.
the LUMO energy level of 5a is 0.27 eV higher than that of
PCBM. The Jsc and FF were low at 1.94 mA/cm2 and 0.34,
respectively. Lower Voc observed for devices with 4a and
4
b may result from higher reverse current and lower
photocurrent.
Acknowledgments
Using our preliminary solar cell results, Table 4, one can
start to hypothesize on structure–property relationships
for these types of acceptors. For example, comparing 4a
We are grateful to Case Western Reserve University for
providing start-up funds. The synthesis and theoretical

1
692
R. Fernando et al. / Organic Electronics 14 (2013) 1683–1692
work was funded by the American Chemical Society Petro-
leum Research Fund (SPN00045) and the subsequent pho-
tovoltaic work was funded by the National Science
Foundation (CHE 1148652). We are grateful to Saide Tang
and Dr. Lei Zhu (CWRU) for measuring the XRD spectrum
of 6b. We thank Prof. Daniel A. Scherson and Adriel Jebin
Jacob Jebaraj for their help with cyclic voltammetry, Dr.
Dale Ray (CWRU) for help with NMR and the Case High
Performance Computing Cluster for the allocation of com-
puting time. This material is based upon work supported
by the National Science Foundation under Grant No. MRI-
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