Donor-acceptor-donor (D-A-D) molecules based on isoindigo as active material for organic solar cells

Article abstract of DOI:10.1039/c2nj40879a

Two D-A-D systems involving benzofuran and dithienopyrrole as donor blocks and isoindigo as an acceptor unit have been synthesized and their electronic properties have been characterized by UV-Vis absorption spectroscopy and cyclic voltammetry. A preliminary evaluation of these compounds as active material in organic solar cells shows that the presence or absence of substitution at the nitrogen atom of indigo strongly affects the properties of the material and thus the performances of the resulting solar cells.

Full text of DOI:10.1039/c2nj40879a

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Donor-acceptor-donor (D-A-D) molecules based on isoindigo as active material for
organic solar cells
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Ali Yassin, Philippe Leriche, Magali Allain and Jean Roncali*
Group Linear Conjugated Systems, CNRS UMR 6200, Moltech-Anjou, University of Angers, 2
Bd Lavoisier, 49045 Angers, France. jeanroncali@gmail.com
Abstract: Two D-A-D systems involving benzofuran and dithienopyrrole as donor blocks and
isoindigo as acceptor unit have been synthesized and their electronic properties have been
characterized by UV-Vis absorption spectroscopy and cyclic voltammetry. A preliminary
evaluation of these compounds as active material in organic solar cells shows that the
presence or absence of substitution at the nitrogen atom of indigo strongly affects the
properties of the material and thus the performances of the resulting solar cells.
Introduction
The design of active materials based on well-defined molecules is attracting
increasing interest motivated by the possibility to develop cost-effective and low
environmental impact organic photovoltaic cells (OPV).1-14 Compared to polydisperse
conjugated polymers,2 perfectly defined molecular donors present specific advantages in
terms of reproducibility of synthesis and purification and therefore of material composition
and device performances. Furthermore, from a more fundamental viewpoint, molecules allow
more reliable analyses of structure-properties relationships that remain a key tool for further
progress in material design.1
During the past few years many classes of molecular donors have been synthesized
and evaluated as donor material in both solution-processed and vacuum deposited OPV
cells.1,3-11 These include triphenylamines,3 cyanines,4 squaraines,5 oligothiophenes6
diketopyrrolopyrroles (DPP),7 borondipyrromethenes,8 and many tailored π-conjugated
molecules.1 The chemical structure of efficient molecular donor materials for OPV often
involves a combination of donor (D) and acceptor (A) groups leading to an internal charge
transfer (ICT) which produces at the same time a red shift extension of the absorption
spectrum and a decrease of the HOMO level resulting in an increase of the open-circuit
voltage (Voc) of the OPV cell. 3a

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While many D-A-D donors have been synthesized in the past few years, it is only
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recently that D-A-D donors involving isoindigo (IsI) as central acceptor block have been
described.10 Similarly to some other systems such as DPP,11 quinacridone12 or rylene
diimides,13 the nitrogen atoms of IsI can be substituted by labile solubilizing groups. The
thermal elimination of these labile groups allows to recovering an insoluble material
stabilized by intermolecular hydrogen-bonding. In this context we report here on the synthesis
and preliminary evaluation in OPV cells of two D-A-D donors involving IsI as central
acceptor block and benzofuran (BF) or dithienopyrrole (DTP) as side donor units (Chart).
N
H
N
N
O
O
S
S
O
S
S
O
O
O
N
N
H
N
IDTP
IBF
The BF donor group was selected on the basis of the high efficiency reported for OPV
cells based on the parent DPP system,7,14,15 while DTP is a strong donor group already used in
low band gap polymers or oligomers.2,16 On the other hand, the fixation of branched alkyl
chains at the nitrogen atom of DTP allows the solubilization of the molecule while keeping
free the NH group of IsI and thus allowing intermolecular hydrogen-bonding.
Experimental part
General
1
Solvents were purified and dried using standard protocols. H NMR and 13C NMR
spectra were recorded on a Bruker AVANCE DRX 500 spectrometer operating at 300.1 and
75.4 MHz; δ are given in ppm (relative to TMS) and coupling constants (J) in Hz. High
resolution mass spectra were recorded under FAB mode on a Jeol JMS 700 spectrometer. UV-
visible optical data were recorded with a Perkin Elmer lambda 19 spectrophotometer. Thermal
analyses were performed using a DSC 2010 CE (TA Instruments). Cyclic voltammetry was

Page 3 of 15
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3
performed on a EG&G PAR 273A potentiostat with a standard three-electrode cell with
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platinum electrodes and a SCE reference.
Synthesis
IDTP. 2-(trimethylstannyl)-N-(2-ethylhexyl)-dithieno[3,2-b:2’,3’-d]pyrrole (930 mg, 2.05
mmol) was added to a solution of 6,6′-dibromoisoindigo (110 mg, 0.26 mmol) and Pd(PPh3)4
(30 mg, 10 mol %) in 20 mL of DMF. The mixture was heated for 12 h at 105°C under inert
atmosphere and cooled later to room temperature. The reaction was quenched with water and
extracted with dichloromethane. The organic phase was dried over MgSO4 and the solvent
evaporated. The residue was precipitated several times with petroleum ether and
chromatographed (silica gel; eluent: THF) to give 130 mg (60%) of a shiny blue solid which
1
showed no melting point before decomposition. H NMR (500 MHz, DMSO) δ (ppm) 10.89
3
3
(s, 2H), 9.60 (d, 2H, J=8.59 Hz), 7.84 (s, 2H), 7.41 (d, 2H, J=5.31 Hz), 7.33 (dd, 2H,
3
4
3J=8.58 Hz, 4J=1.71 Hz), 7.22 (d, 2H, J=5.33 Hz), 7.08 (d, 2H, J=1.78 Hz), 4.17 (d, 4H),
13
2.00-1.89 (m, 2H), 1.34-1.17 (m, 16H), 0.87-0.77 (m, 12H). C NMR (500 MHz, DMSO):
169.55, 146.01, 145.57, 144.63, 139.67, 138.46, 130.99, 129.73, 125.28, 120.77, 117.29,
114.30, 113.89, 111.99, 110.02, 104.70, 50.45, 29.78, 27.73, 26.33, 23.26, 22.51, 13.81,
10.40. UV-Vis. λmax nm (logε) 606, 348 (4.58, 4.43). ESI+ HRMS cald for C48H48N4O2S4+Na
863.2552, found 863.2550 [M+Na]+.
IBF. 2-trimethylstannyl benzofuran (200 mg, 0.71 mmol) was added to a solution of 6,6’-
dibromo-N,N’-(2- ethylhexyl)-isoindigo (130 mg, 0.2 mmol) and Pd(PPh3)4 (65 mg, 20 mol
%) in 20 mL of toluene. The mixture was refluxed for 12 h under inert atmosphere and cooled
later to room temperature. The reaction was quenched with water and extracted with
dichloromethane. The organic phase was dried over MgSO4 and the solvent evaporated. The
residue was chromatographed (silica gel; eluent: 1:1 dichloromethane/petroleum ether) to give
130 mg (90%) of a deep red solid. M.p. 211°C (dec). 1H NMR (500 MHz, THF) δ (ppm) 9.41
3
(d, 2H, J=8.45 Hz), 7.60 (d, 2H), 7.55-7.52 (m, 4H), 7.31-7.28 (m, 6H), 7.23-7.20 (m, 2H),
3.84-3.72 (m, 4H), 1.99-1.94 (m, 2H), 1.51-1.33 (m, 16H), 1.00-0.93 (m, 12H). 13C NMR
(500 MHz, THF): 169.22, 156.59, 156.34, 147.04, 134.51, 133.01, 131.65, 130.38, 126.11,
124.26, 123.15, 122.26, 119.01, 112.07, 104.74, 104.60, 44.71, 38.87, 31.82, 29.78, 25.19,
24.21, 14.71, 11.24. ESI+ HRMS cald for C48H50N2O4+Na 741.3662, found 741.3661
[M+Na]+.

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Devices fabrication
Fullerene C60 (99+%) was purchased from MER and used as received. The ITO electrodVeisew(1A0rticle Online
Ohm/□, Kintec company) were cleaned in ultrasonic baths and then modified by a spun-cast
layer of PEDOT:PSS (Clevios P VP AI 4083” (HC Stark) (40 nm thick), which was dried at
130 °C for 15 min. The Clevios suspension was stirred and filtered through a 0.45µm
membrane (Minisart RC 15, Sartorius) just prior to casting. A 100 nm thick layer of Al was
thermally evaporated through a shadow mask, at a pressure of about 10-6 mbar. The mask
geometry defined a device’s area of 0.28 cm2. Each ITO coated-glass substrate supported two
individual devices and each batch involved a minimum of 4 devices (8 cells). After
preparation, the devices were stored and characterized in an argon glovebox (200B, MBraun).
The J-V curves of the devices were recorded in the dark and under illumination using a
Keithley 236 source-measure unit and a homemade acquisition program. The light source was
an AM1.5 Solar Constant 575 PV simulator (Steuernagel Lichttecknik, equipped with a metal
halogenide lamp). The light intensity was measured by a broad-band power meter
(13PEM001, Melles Griot). The efficiency values reported here are not corrected for possible
solar simulator spectral mismatch nor for the reflection/absorption of the
glass/ITO/PEDOT:PSS coated electrodes.
Results and discussion
Synthesis
6,6′-Dibromoisoindigo 1 and its derivatives 6,6’-disubstituted by ethylhexyl chains (2)
were synthesized according to literature procedures (Scheme 1).10 A Stille coupling with the
mono-stannylated derivative of dithienopyrrole 316 in DMF in presence of a palladium catalyst
gave the target IDTP compound in 60% yield. Despite the presence of alkyl chains on both
lateral DTP blocks, IDTP was insoluble in common organic solvents except for THF,
probably due to intermolecular hydrogen bonding. In contrast, IBF obtained in 90% by Stille
coupling of the trimethylstannyl-benzofuran 417 with 2 in refluxing toluene showed excellent
solubility in usual solvents.

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N
O
Br
H
N
O
Br
Br
Br
O
N
K2CO3/DMF
Br
O
N
H
1
2
Pd(PPh3)4
DMF
Pd(PPh3)4
Toluene
SnMe3
O
N
4
SnMe3
S
S
3
IDTP
IBF
Scheme 1. Synthesis of IDTP and IBF.
Deep red crystals of IBF obtained from a THF solution were analyzed by X-Ray
diffraction. Although the quality of the crystal was limited, the data for the triclinic system
show a quasi-planar molecular structure with torsion angles inferior to 10° for the central
double bond of the IsI unit. Thermogravimetric analysis (TGA) revealed that both compounds
are thermally stable under inert atmosphere with a 5% weight loss at 290°C and 357°C for
IDTP and IBF respectively. However, while IBF shows a clear melting point at 211°C,
IDTP does not melt before thermal decomposition.
Electronic properties of the donors
The electrochemical properties of the two compounds have been analyzed by cyclic
voltammetry in THF (reduction) and methylene chloride (oxidation) in the presence of 0.10 M
Bu4NPF6 as supporting electrolyte. For both compounds, the CV trace shows two reversible
reduction processes with cathodic peak potentials Epc1 and Epc2 associated to the formation of
the radical anion and dianion respectively (Fig. 1 and Table 1). The similarity of the reduction
potential for the two compounds suggests that the LUMO is essentially located on the IsI
moiety. In contrast, quite different behaviors are observed upon oxidation. Thus whereas IBF
undergoes a partially reversible oxidation process with an anodic peak potential Epa of 1.19 V,
the CV of IDTP exhibits two oxidation processes with a first anodic peak at 0.82 V. The
possible access to a second oxidation step reflects the more extended conjugated system

New Journal of Chemistry
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created by the DTP blocks. For both compounds, the first oxidation process which appears
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partially reversible at low scan rate becomes fully reversible at higher scan rate. For IDTP,
application of repetitive potential scans up to the onset of the second oxidation process leads
to the progressive emergence of a broad redox system at lower potential typical of the
electrodeposition of an electroactive material (not shown). This coupling process is illustrated
by the broad cathodic wave observed in the 0.20-0.60 V region in the reverse scan (Fig. 1).
4µA
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V vs SCE)
Fig. 1. Cyclic voltammograms of IBF (top) and IDTP (bottom) in 0.10 M Bu4NPF6 in CH2Cl2 (oxidation) and
THF (reduction), scan rate 25 mV sec-1.
The UV-Vis spectrum of IBF in THF presents a first maximum at 311 nm followed by
two bands with maxima at 439 and 551 nm (Fig. 2). The band of lowest energy can be
assigned to an internal charge transfer between the side donor block and the central IsI
electron acceptor.
Table 1. Data of UV-Vis spectroscopy (in THF) and CV in 0.10 M Bu4NPF6, scan rate 100 mV s-1, oxidation in
CH2Cl2, reduction in THF, ref. SCE.a determined on films
Eg (eV)a
1.48
Epa (V)
0.82
Epc1, Epc2 (V)
-1.12, -1.47
-1.05, -1.45
Compd
IDTP
IBF
λmax (nm)
Log ε (M-1 cm-1)
284, 348, 610
311, 439, 551
4.58
4.40
1.77
1.29
The spectrum of IDTP presents rather similar features with however a red shift of the
maxima reflecting the stronger donor effect of the DTP block compared to BF. While IBF

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does not fluoresce in THF solution, IDTP presents a weak fluorescence emission with
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maximum at 738 nm and a strong shoulder at 792 nm, (A quantum yield of 3.0 % was
measured against a cresyl violet perchlorate reference). The UV-Vis absorption spectra of thin
films spun-cast on glass from THF solutions are shown in Fig. 2. Compared to the solution
spectrum, the spectrum of IBF presents a broadening of the absorption bands while the
absorption edge extends to 700 nm corresponding to a band gap (Eg) of 1.77 eV. These effects
can be attributed to intermolecular π-stacking interactions in the solid-state. We note also the
emergence of a vibrational fine structure with maxima at 533, 570 and 670 nm suggesting the
presence of some crystallinity in the film. Compared to IBF, The spectrum of IDTP exhibits
a larger broadening of the solid state spectrum and a 37 nm red shift of λmax and a smaller
optical band gap (Eg = 1.48 eV).
1.2
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. 2. UV-Vis absorption spectra of IBF (top) and IDTP (bottom) in THF solution (dotted line) and as thin
films spun-cast on glass (solid line).
The effects of thermal annealing on films of the two compounds have been
investigated by UV-Vis spectroscopy and X-ray diffraction. For IBF, a 10 min thermal
treatment at 75°C produces a further red shift of the band maxima with an exaltation of the
resolution of the vibrational fine structure which suggests an increased crystallinity (Fig. 3).
This conclusion is confirmed by the emergence of a peak at 2θ = 5.55° in the diffractogram
whereas no discernible diffraction peak was observed for the as-cast film. Further heating to
130°C produces an increase of diffraction intensity. Using the Bragg equation, λ = 2d.sinθ

New Journal of Chemistry
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(λCu-α = 1.54 Å), the d-spacing was calculated to be 15.8 Å. While this distance does not
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correspond to the length of the molecule (21 Å), this result suggests the existence of
crystalline domains in the material. This thermally induced enhancement of crystallinity is
strongly reminiscent of previous observations on DPP parent systems containing BF side
groups, suggesting that this group plays a major role in this reorganization process.14,15
As expected a different behavior is observed for IDTP. The effects of thermal
treatment appear only after 10 min annealing at 150°C and results only a slight decrease of
absorbance indicative of a beginning of degradation. The X-ray diffractogram shows that the
film presents some crystallinity before thermal treatment with a diffraction peak at 2θ = 6.5°.
However, in agreement with UV-Vis results, thermal treatment does not enhance crystallinity
but produces a decrease of peak intensity consistent with the degradation of the material.
0.25
0.35
0.20
0.15
0.10
0.05
0.30
0.25
0.20
0.15
0.10
300
400
500
600
700
800
300
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
2100
1400
700
2100
1400
700
4
6
8
10
12
14
16
4
6
8
10
12
14
16
2 θ (degrees)
2 θ (degrees)
Fig. 3. Top: UV-Vis absorption spectra for IBF (left) and IDTP (right) films on glass. Dotted line: before
annealing; solid line: after annealing at 140oC. Bottom X-ray diffractograms for IBF (left) and IDTP (right).
Bottom trace: before annealing, middle trace: after annealing at 75oC, top trace: after annealing at 140oC.
Theoretical calculations based on density functional theory (DFT) methods were
performed for IDTP and IBF with Gaussian 09 program. Becke’s three-parameter gradient-
corrected functional (B3LYP) with 6-31G (d,p) basis in vacuum was used to optimize the
geometry and to compute the electronic structure at the minima found. The optimized
geometry of both compounds shows a quasi-planar conjugated system in agreement with the
X-ray structure of IBF. The ~ 15° dihedral twist angle of the central double bond of IBF is

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the only torsion in the structure while both side parts are coplanar. In contrast, the structure of
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IDTP, presents small dihedral angles between each donor block and the IsI core in addition to
the ~ 15° dihedral torsion around the central double bond.
Fig. 4. HOMO (bottom) and LUMO (top) distribution of IDTP (left) and IBF (right).
The molecular orbitals show that the LUMO is essentially localized on the IsI core, in
agreement with the observed minor influence of the end groups on the reduction potential
(Fig. 4). In contrast, as the HOMO is distributed over the entire π-system, its energy level
varies with the electron-donor effect of the end-groups. The calculated energy levels of the
HOMOs and LUMOs are in qualitative agreement with CV data and shows that IBF presents
lower HOMO and LUMO levels than IDTP.
A preliminary evaluation of the potentialities of the two compounds as donor material
in OPV cells has been carried out on two types of devices, solution-processed bulk
heterojunctions (BHJ) using PC61BM as electron acceptor and bilayer planar heterojunctions
(PHJ) based on spun-cast donor layer and vacuum deposited C60. BHJ cells were prepared by
spin-casting active films from a chlorobenzene solution with 1:2 donor/PC61BM mass ratio,
and a total concentration of 30 mg mL-1, on an ITO electrode pre-coated with 40 nm of
PEDOT:PSS. The devices were then completed by vacuum deposition of a 100 nm aluminum
electrode.
Despite multiple attempts with various donor/acceptor ratios, all devices based on
IDTP gave very poor performances. Under AM 1.5 simulated solar illumination with an
incident power light of 100 mW cm-2, the best BHJ cell delivers a short-circuit current density

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(Jsc) of only 0.15 mA cm-2 and an open-circuit voltage (Voc) of 0.39 V which combined with a
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poor filling factor (FF) of 0.16 results in a power conversion efficiency (PCE) of 0.01%
(Table 2). In the case of IBF the BHJ cells shows a Voc of 0.57 V, a Jsc of 0.20 mA cm-2 and a
PCE of 0.05%. As could be anticipated from UV-Vis and X-ray data, thermal treatment of
the cells based on the two donors leads to contrasting effects, For IDTP, only a degradation of
the already low performances is observed. In contrast, for IBF thermal treatment increases the
various photovoltaic parameters leading to a PCE of 0.26 % (Fig 6.).
The limited available amounts of donor materials were insufficient to carry out an
extensive optimization of the BHJ cells, therefore the two compounds have been evaluated in
bi-layer PHJ cells. Although often less efficient than BHJ, PHJ generally lead to more
reproducible results. PHJ cells have been fabricated by spin-casting a typically 20 nm donor
film on an ITO electrode pre-coated with a 40 nm film of PEDOT-PSS. A 50 nm layer of C60
was then thermally evaporated under vacuum and the cells were completed by a 100 nm thick
aluminium electrode.
Table 2. Photovoltaic characteristics of OPV cells based on IBF and IDTP donors under AM 1.5 simulated solar
irradiation
Compd Device Thermal Treatment
Jsc
Voc
FF PCE
(mA cm-2) (V) (%) (%)
IDTP
IDTP
IBF
BHJ
PHJ
BHJ
PHJ
-
0.15
0.03
1.20
0.60
0.20
1.00
0.93
1.90
0.35 16 0.01
0.33 14 0.002
0.51 27 0.19
0.49 24 0.08
0.57 36 0.05
0.60 38 0.26
0.80 20 0.15
0.90 35 0.65
100°C, 10 min
-
100°C, 10 min
-
100°C, 10 min
-
IBF
110°C, 10 min
As shown in Table 2. IBF gave Jsc of 0.93 mA cm-2 a Voc of 0.80 V and a PCE of
0.15%. A PCE of 0.19 % was obtained from IDTP with a Voc of 0.51 V and a Jsc of 1.20 mA
cm-2. As expected thermal treatment produces contrasting effects on the two types of cells.
For IDTP heating was always detrimental with a twofold decrease of PCE. For IBF, 10 min
thermal annealing at 110°C leads to an increase of Jsc, Voc and FF resulting in an PCE of
0.65% (Fig. 5). However, the S shape of the I/V curves suggests some interfacial problems

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suggesting that a judicious insertion of interfacial buffer layers could improve the cell
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performances.
3.0
2.0
1.0
0.0
-1.0
-2.0
1.0
0.0
-1.0
-2.0
-3.0
-0.50
0.00
0.50
1.00
1.50
Voltage (V)
Fig. 5. Current density vs voltage curves for OPV cells based on IBF. Black circles: in the dark, empty circles:
under AM 1.5 simulated solar illumination at 80 mW cm-2. Top: BHJ cell IBF/PC61BM (1:2) thermally annealed
at 100°C for 10 min. Bottom PHJ cell based on spun-cast IBF and vacuum deposited C60 after10 min thermal
treatment at 110°C.
The hole-mobility of the two donors has been determined by the space-charge limited
current method on hole-only devices of structure ITO/PEDOT:PSS/donor/gold. Mobility
values of 6.8 ×10-3 and 7.6×10-6 cm2 V-1 s-1 were obtained for IBF and IDTP respectively .
The higher mobility of IBF is consistent with the better efficiency of this donor.
Reynolds and coworkers have described an IsI based D-A-D system with two 2-hexyl-
bithienyl side donor groups10 and a PCE of 1.25 ± 0.37 % was reported in a recent paper.18 In
this context, the modest PCE obtained in our case seems to confirm that IsI-based systems
will not rank among the most efficient molecular donor materials for OPV. However, it
should be underlined that, as shown by recent works, for a given central acceptor block the
efficiency of D-A-D systems is subject to considerable variations depending on the nature of

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the side groups in a way impossible to predict on the basis of our present understanding of
structure-properties relationships.14,15
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Conclusion
Two D-A-D molecular donors based on a central IsI acceptor block have been
synthesized. Optical, thermal and X-diffraction studies provided consistent results showing
that the presence of a free imide group on the IsI block leads to intermolecular hydrogen-
bonding that renders the material insoluble in spite of the presence of two branched alkyl
chains. The low efficiency observed on the resulting OPV cells suggest that hydrogen
bonding locks the molecular packing into an organization unfavorable for efficient exciton
diffusion and/or charge transport. However, substitution of the nitrogen atom by a single
branched alkyl chain solubilizes the material and allows thermally induced reorganization and
thus improved efficiency. On the other hand, these results suggest that H-bonding could be an
interesting tool for the stabilization of active OPV materials provided that appropriate rules
for molecular engineering could be established, which of course implies further investigations
of structure-properties relationships in this class of compounds.
Acknowledgements
We thank CNRS for the PhD grant of A. Yassin.
1. Reviews: a) J. Roncali, Acc. Chem. Res., 2009, 42, 1719. b) B. Walker, C. Kim and T.-Q.
Nguyen, Chem. Mater. 2011, 23, 470. c) A. Mishra and P. Bäuerle, Angew. Chem. Int. Ed.
2011, 51, 2020. d) Y. Lin, Y. Li and X. Zhan, Chem. Soc. Rev. 2012, 41, 4245.
2. Y.-J. Cheng, S.-H. Yang and C.-S. Hsu, Chem. Rev. 2009, 109, 5868.
3. a) S. Roquet, A. Cravino, P. Leriche, O. Alévêque, P. Frère and J. Roncali, J. Am. Chem.
Soc., 2006, 128, 3459. b) H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li and X. Zhan, Adv.
Mater. 2011, 23, 1554.
4. a) N.M. Kroneneberg, M. Deppish, F. Würthner, H.W.A. Ledemann, K. Deing and K.
Meerholz, Chem. Commun. 2008, 6489. b) H. Bürckstümmer, E. V. Tulyakova, M.
Deppisch, M. R. Lenze, N. M. Kronenberg, M. Gsänger, M. Stolte, K. Meerholz and F.
Würthner, Angew. Chem. Int. Ed. 2011, 50, 11628. c) F. Würthner and K. Meerholz,
Chem. Eur. J. 2010, 16, 9366.

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5. a) F. Silvestri, M.D. Irwin, L. Beverina, A. Facchetti, G.A. Pagani and T.J. Marks, J. Am.
View Article Online
Chem. Soc. 2008, 130, 17640. b) S. Wang, L. Hall, V. V. Diev, R. Haiges, G. Wei, X.
Xiao, P. I. Djurovich, S. R. Forrest and M. E. Thompson, Chem. Mater. 2011, 23, 4789.
6. a) J. Roncali, P. Frère, P. Blanchard, R. de Bettignies, M. Turbiez, S. Roquet, P. Leriche
and Y. Nicolas, Thin Solid Films, 2006, 511–512, 567. b) C. Uhrich, R. Schueppel, A.
Petrich, M. Pfeiffer, K. Leo, E. Brier, P. Kilickiran and P. Bäuerle, Adv. Funct. Mater.
2007, 17, 2991. c) Y. Liu, X. Wan, F. Wang, J. Zhou, G. Long, J. Tian, J. You, Y. Yang
and Y. Chen, Adv. Energy Mater. 2011, 1, 771. d) Y. Sun, G.C. Welch, W.L. Leong, C.J.
Takacs, G.C. Bazan and A.J. Heeger, Nat. Mater. 2011, 11, 44.
7. B. Walker, A. B. Tamayo, X.-D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tantiwiwat and
T.-Q. Nguyen, Adv. Funct. Mater., 2009, 19, 3063.
8. a) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel and J. Roncali, Chem.
Commun. 2009, 1673. b) T. Rousseau, A. Cravino, T. Bura, G. Ulrich, R. Ziessel, J.
Roncali, J. Mater. Chem. 2009, 19, 2298. c) T. Rousseau, A. Cravino, E. Ripaud, P.
Leriche, S. Rihn, A. De Nicola, R. Ziessel and J. Roncali, J. Chem. Commun. 2010, 46,
5082.
9. A. Leliège, P. Blanchard, T. Rousseau and J. Roncali, Org. Lett., 2011, 13, 3098.
10. J. Mei, K. R. Graham, R. Stalder and J. R. Reynolds, Org. Lett. 2010, 12, 660.
11. a) J.S. Zambounis, Z. Hao and A. Iqbal, Nature 1997, 388, 131. b) K. Ichimura, K.
Arimitsu and M. Tahara, J. Mater. Chem. 2004, 14, 1164. c) B. Sun, W. Hong, H. Aziz
and Y. Li, J. Mater. Chem. 2012, 22, 18950. d) Y. Suna, J.-I. Nishida, Y. Fujisaki and Y.
Yamashita, Org. Lett. 2012, 14, 3356.
12. T.L. Chen, J. J.-A Chen, L. Catane and B. Ma, Organic Electronics, 2011, 12, 1126.
13. J.H. Oh, W-Y. Lee, T. Noe, W-C. Chen, M. Könemann and Z. Bao, J. Am. Chem. Soc.
2011, 133, 4204.
14. E. Ripaud, D. Demeter, T. Rousseau, E. Boucard-Cétol, M. Allain, R. Po, P. Leriche and
J. Roncali, Dyes and Pigments, 2012, 95, 126.
15. J. Liu, B. Walker, A. Tamayo, Y. Zhang and T.-Q. Nguyen, Adv. Funct. Mater. 2012,
DOI: 10.1002/adfm.201201599.
16. a) A. Yassin, P. Leriche and J. Roncali, Macromol. Rapid Comm. 2010, 31, 1467. b) A.
Yassin, G. Savitha, P. Leriche, P. Frère and J. Roncali, New J. Chem. 2012, 36, 2412.
17. V. Fargeas, F. Favresse, D. Mathieu, I. Beaudet, P. Charrue, B. Lebret, M. Piteau and J.-P.
Quintard, Eur. J. Org. Chem. 2003, 1711.

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18. K. R. Graham, J. ei, R. Stalder, J. W. Shim, H. Cheun, F. Steffy, F. So and J.R. Reynolds,
View Article Online
ACS Appl. Mater. Interfaces, 2012, dx.doi.org/10.1021/am2000328
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TOC
The effects of N-substitution of the isoindigo acceptor block on the photovoltaic
properties of the resulting materials are discussed
N
O
O
O
O
N
1.2
N
H
1.0
N
O
S
S
0.8
0.6
0.4
0.2
0.0
S
S
O
N
H
N
300
400
500
600
700
800
900
1000
Wavelength (nm)