Bulk heterojunction solar cells-tuning of the HOMO and LUMO energy levels of pyrrolic squaraine dyes

Article abstract of DOI:10.1002/ejoc.201100940

The optimization of organic photovoltaic (OPV) devices requires, amongst several factors mainly related to thin-film blend morphology, the capability to finely tune the characteristic energy levels of both the donor and the acceptor materials constituting

Full text of DOI:10.1002/ejoc.201100940

FULL PAPER
DOI: 10.1002/ejoc.201100940
Bulk Heterojunction Solar Cells – Tuning of the HOMO and LUMO Energy
Levels of Pyrrolic Squaraine Dyes
Luca Beverina,*[a] Martin Drees,[b] Antonio Facchetti,[b] Matteo Salamone,[a]
Riccardo Ruffo,[a] and Giorgio A. Pagani[a]
Dedicated to Professor Gianfranco Scorrano on the occasion of his 72nd birthday
Keywords: Solar cells / Organic photovoltaic devices / Heterocycles / Structure–activity relationships / Voltammetry / Dyes /
Squaraines
The optimization of organic photovoltaic (OPV) devices re-
quires, amongst several factors mainly related to thin-film
blend morphology, the capability to finely tune the character-
istic energy levels of both the donor and the acceptor materi-
als constituting the active layer. Squaraine compounds repre-
sent one of the most promising small molecule materials to
be employed as the donor in the bulk heterojunction (BHJ)
OPV configuration. In this study a series of arylhydrazone
characterized by optical spectroscopy (UV/Vis absorption
and IR transmittance) and electrochemical measurements
(cyclic voltammetry and differential pulsed voltammetry).
Our data enable the establishment of interesting structure–
property relationships relating the nature, position and
number of electron-withdrawing substituents to the position
of the donor HOMO and lowest unocupied molecular orbital
(LUMO) levels. The low energy shift effect on the HOMO
end-capped symmetric squaraine compounds bearing elec- level with respect to the parent unsubstituted squaraine can
tron-withdrawing substituents on the aryl ring has been syn-
thesized with the aim of increasing the device open circuit
voltage by shifting the highest occupied molecular orbital
(HOMO) energy level. These new semiconductors have been
be as high as 0.3 eV. The effect is so strong that the derivative
bearing the strongest acceptor no longer behaves as a donor
semiconductor. Preliminary results on OPV device prepara-
tion and characterization are also discussed.
tors,[9] chemiluminescent materials,[10] bulk heterojunc-
tion[11] and hybrid[12] solar cells and organic photodetec-
tors.[13]
Introduction
1,3-Squaraines (or squarilium dyes) are the condensation
products of squaric acid and two equivalents of a suitable
electron-rich precursor (heterocyclic anhydrobase or an ac-
tivated arene).[1] From the standpoint of their electronic
properties, squaraines are highly delocalized conjugated
materials possessing a cyanine-like structure.[2] Their most
notable features are a strong, narrow absorption in the Vis/
NIR region, good solubility in low polarity solvents, re-
markable chemical stability, well-documented photocon-
ducting capabilities and, in some cases, high redox revers-
ibility.[3] Squaraines have been employed in a number of
technologically relevant fields including: nonlinearoptics,[4]
highly stable NIR-fluorescent dyes,[5] photopatterning,[6]
bioimaging,[7] photodynamic therapy,[8] field-effect transis-
Concerning their application in photoconducting de-
vices, the main appeal of squaraines as an active media is
their strong absorption in the 500–700 nm region, where the
photon flux of the solar emission is higher. In the BHJ solar
cell configuration, the photocurrent is the consequence of
the photoinduced charge transfer between a molecular or
polymeric donor and acceptor semiconducting blend. The
maximum voltage that a BHJ cell can generate (the open
circuit voltage, Voc) is directly connected with the difference
in energy between the HOMO level of the donor material
and the LUMO level of the acceptor.[14] In its most
common embodiment, the acceptor of a BHJ cell is a
fullerene derivative, either methyl-[6,6]-phenyl-C61-butyrate
(PC61BM) or methyl-[6,6]-phenyl-C71-butyrate (PC71BM).
Squaraines possess an intrinsically low lying HOMO level.
This is a relevant advantage over most polymeric donor ma-
terials – most notably poly(3-hexylthiophene) (P3HT) – and
some small molecule materials.[15] So far, squaraine-based
solar cells have demonstrated moderate to high efficiencies
in both BHJ [power conversion efficiency (PCE) ca. 4.5%]
[a] Department of Materials Science, University of Milano-
Bicocca,
Via R. Cozzi, 53, 20125 Milano, Italy
Fax: +39-264485403
E-mail: luca.beverina@unimib.it
[b] Polyera Corporation
8045 Lamon Avenue, Suite 140, Skokie, IL 60077, USA
Fax: +1-847-677-7510
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201100940.
[11b]
and dye sensitized (PCE ca. 6.5%) architectures.[12]
Eur. J. Org. Chem. 2011, 5555–5563
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5555

L. Beverina
FULL PAPER
Scheme 1.
In particular, we have demonstrated that a 1:3 by weight
blend of the end-capped hydrazone squaraine sq-Ph
(Scheme 1) with PC71BM as the acceptor afforded a PCE
as high as 2%.[11a] In order to increase the low Voc value of
these cells, we present a series of new squaraine derivatives
closely related to sq-Ph but possessing a number of different
electron-withdrawing substituents on the hydrazone moiety.
The goal is to tune the HOMO level energies, at the same
time preserving the other favourable features of the parent
sq-Ph derivative. Accordingly, all of the new derivatives pos-
sess the hex-5-enyl solubilizing chain (Scheme 1). Indeed,
we have demonstrated that such functionalization increases
squaraine hole mobility in both BHJ OPV and thin-film
field effect transistor structures vs. conventional alkyl-func-
tionalized derivatives,[11a] which is the consequence of the
formation of a network of noncovalent interactions be-
tween the double bonds and the peripheral phenyl rings of
neighbouring molecules.[11a]
Results and Discussion
Synthesis of the Compounds
Symmetric squaraine compounds can be prepared by the
reaction of two equivalents of the corresponding activated
arene or anhydrobase with one equivalent of squaric acid
in a toluene/nBuOH mixture.[16] The water formed during
the condensation reaction was removed by a Dean–Stark
trap. We have shown that both phenyl and diphenyl hydra-
zones of pyrrole carbaldehyde readily react with squaric
acid to give the corresponding squaraines in moderate to
high yields.[17]
We prepared substituted aryl- and diarylhydrazones fol-
lowing a two-step procedure (Scheme 2). Firstly, we pre-
pared the monosubstituted hydrazones 2a–d by the reac-
tions of aldehydes 1a–c with the corresponding commer-
cially available 4-bromophenyl- and pentafluorophenyl hy-
drazines in ethanol with heating in the presence of imid-
According to the commercial availability of suitable ar- azole hydrochloride as the acid catalyst. Such hydrazones
ylhydrazine precursors, we exploited halogens as the pre- were subsequently either alkylated (3a–c) or arylated (3d)
ferred electron-withdrawing groups. Furthermore, our func- by the reaction of the corresponding nitranion, obtained by
tionalization strategy aims at reducing the energy of the deprotonation at 0 °C with tBuOK, with 6-bromohex-1-ene
HOMO levels without affecting the already favourable for 3a–c and pentafluoropyridine for 3d. Hydrazone 3d was
squaraine absorption properties. The use of different elec- obtained as a mixture of pyridyl isomers, and we success-
tron-withdrawing groups such as nitro or cyano groups fully isolated the 4-substutited isomer (major product) by
could have affected both the optical gap and the overall chromatographic purification.
stability. Finally, with sq-Py-F we exploited an acceptor that
The reaction of 3a–d with stoichiometric squaric acid in
is perfluorinated, thus accepting according to the inductive BuOH/toluene with heating gave, after the appropriate reac-
effect, and π deficient, thus accepting according to the reso- tion time, the corresponding squaraines in moderate to low
nance effect.
yields (Scheme 3). In particular, squaraine sq-Py-F was iso-
5556
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 5555–5563

Bulk Heterojunction Solar Cells
Figure 1. UV/Vis absorption spectra of 10–6 m solutions (CH2Cl2)
of sq-Ph (black), sq-Ph-Br (red), sq-Ph-F-CH3 (green), sq-Ph-F-Hex
(blue) and sq-Py-F (grey).
Scheme 2.
lated, after chromatographic purification, in only 7% yield.
This is probably because of the reactivity of the tetrafluo-
ropyridine residue, particularly towards nucleophiles (espe-
cially BuOH) in an acidic environment. TLC inspection of
the reaction mixture actually shows the formation of several
deep blue bands eluting very close to the product.[18]
lone pairs along the cyanine-like squaraine conjugation
axis. The presence of a strong electron-withdrawing substit-
uent, such as the pentafluorobenzene residue, limits the
availability of the hydrazonic lone pair to delocalize to the
squarylium core. This effect is also evident for sq-Py-F
where the blueshift of the absorption maxima is associated
with a reduction of the transition oscillator strength. This
latter effect is the consequence of the nature of the tetra-
fluoropyridine acceptor, which is electron withdrawing ac-
cording to both resonance and inductive effects, whereas
the pentafluorobenzene accepting capabilities are essentially
inductive in nature. Table 1 summarizes the most relevant
UV/Vis absorption features for all of the compounds.
Table 1. UV/Vis absorption properties of squaraines, where Eg
=
optical gap and full width at half maximum (FWHM).
Compound
λmax
[nm]
ε [l cm–1
Eg [eV]
FWHM
[cm–1]
mol–1]
sq-Ph
728
729
676
674
687
241000
222000
238000
236000
202000
1.70
1.70
1.83
1.83
1.80
850
742
525
525
562
Scheme 3.
sq-Ph-Br
sq-Ph-F-CH3
sq-Ph-F-Hex
sq-Py-F
UV/Vis Characterization
Figure 1 shows the UV/Vis absorption spectra of sq-Ph,
sq-Ph-Br, sq-Ph-F-CH3, sq-Ph-F-Hex and sq-Py-F as a
function of their extinction coefficients.
Electrochemical Characterization
Figure 2 shows the cyclic voltammetry (CV) plots (black
All of the compounds possess the characteristic strong,
narrow low energy absorption band typical of squaraine curves) for sq-Ph-Br (a), sq-Ph-F-CH3 (b), sq-Ph-F-Hex (c)
compounds. However, the effect of the electron-with- and sq-Py-F (d) in CH3CN solution with tetrabuthylamon-
drawing substituent is in general modest. In particular, the ium hexafluorophosphate (TBAPF6, 0.1 m) as the support-
sq-Ph-Br absorption can almost be superimposed on that of ing electrolyte. All of the new derivatives possess CV traces
the parent sq-Ph derivative.[11a] The two pentafluorophenyl- similar to that of sq-Ph. At positive (oxidative) potentials
substituted derivatives, sq-Ph-F-CH3 and sq-Ph-F-Hex, all of the compounds show two reversible oxidation waves
show identical UV/Vis features and are blueshifted with re- corresponding to the oxidation of the first and second hy-
spect to sq-Ph by 0.14 eV. This is in accordance with the drazonic nitrogen atoms.
nature of the main optical transition of squaraine com-
In all of the derivatives, the two redox active end-capping
pounds. In fact, the HOMO–LUMO transition in this class groups are identical. They can be either weakly or strongly
of dyes is dominated by the delocalization of the nitrogen coupled, according to the efficiency of the conjugation
Eur. J. Org. Chem. 2011, 5555–5563
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5557

L. Beverina
FULL PAPER
Figure 2. Cyclic voltammetry (black) and differential pulsed voltammetry (red) plots of 10–4 m solutions of sq-Ph-Br (a), sq-Ph-F-CH3
(b), sq-Ph-F-Hex (c) and sq-Py-F (d) with TBAPF6 (0.1 m) in CH3CN and glassy carbon as the working electrode.
along the chromophore backbone. The observation of two redox couple. The electrochemical LUMO levels (Ered) are
distinct one-electron processes with a peak potential separa- obtained as the peak potential of the first reductive wave of
tion is the consequence of the coupling between the two the DPV plot. In our experimental setup the Fc+/Fc couple
redox centres. The peak separation can be used as a qualita- is at –5.23 eV with respect to the vacuum. We corrected the
tive tool for the characterization of the coupling electrochemical values, according to the method proposed
strength.[19] Table 2 shows that the peak separation values by Janssen, by means of the definition of “effective” optical
opt
opt
are comparable for all of the squaraines, irrespective of the HOMO (E
) and LUMO (E
) levels.[20] This
HOMO
LUMO
number and nature of the electron-withdrawing substitu- method enables the correction of the experimental discrep-
ents.
ancy between electrochemical and optical energy gaps. The
optical HOMO and LUMO gaps were obtained according
to Equations (1) and (2), respectively.
Table 2. Peak potentials of Eox and Ered from the DPV plots and
effective optical HOMO and LUMO levels for sq-Ph, sq-Ph-Br, sq-
Ph-F-CH3, sq-Ph-F-Hex, sq-Py-F and PC71BM.
opt
opt
Compound
Eox [V]
Ered [V]
E
[eV]
E
[eV]
HOMO
LUMO
PC71BM[21]
sq-Ph
–
–
–5.87
–5.62
–5.53
–5.73
–5.74
–5.88
–3.91
–3.92
–3.83
–3.90
–3.90
–4.08
(1)
0.18
0.06
0.27
0.27
0.41
–1.10
–1.16
–1.09
–1.10
–0.92
sq-Ph-Br
sq-Ph-FCH3
sq-Ph-F-Hex
sq-Py-F
(2)
sol
Table 2 also shows the HOMO and LUMO energies for
Where E is the difference between the electrochemical
CV
all of the squaraines along with those of PC71BM. The elec- HOMO and LUMO levels and Eg is the optical gap.
trochemical HOMO levels (Eox) of the squaraines are con- The absolute position of the squaraine HOMO levels en-
sistently obtained from the peak potential of the first oxi- ables the ranking of the electron accepting capabilities of
dation wave of the corresponding differential pulsed vol- the electron-withdrawing substituents employed. In fact,
tammetry (DPV) trace (Figure 2, red traces) vs. the Fc+/Fc with the notable exception of sq-Ph-Br, their introduction
5558
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 5555–5563

Bulk Heterojunction Solar Cells
is associated with a progressive energy decrease of the vices were fabricated for the parent sq-Ph squaraine (con-
HOMO level with respect to the parent sq-Ph. In agreement trol) and the two derivatives sq-Ph-Br and sq-Ph-F-CH3
with our observations from the UV/Vis characterization, using PC71BM as the acceptor. Note that sq-Ph-F-Hex is a
tetrafluoropyridine proved to be the strongest acceptor.
liquid at ambient temperature and thus cannot be employed
The case of sq-Ph-Br is peculiar; the bromine atom be- for thin-film fabrication, and sq-Py-F was not taken into
haves as an acceptor only according to its high electronega- consideration as a donor for the reasons described in the
tivity, i.e. a purely inductive effect. Conversely, bromine is a previous section.
donor by the mesomeric effect. Overall, the HOMO of sq-
The device structure employed in this study was indium
Ph-Br lies (slightly) above that of sq-Ph thanks to the meso- tin oxide (ITO)/polyethylenedioxythiophene (PEDOT):po-
meric contribution, and the inductive contribution is com- lystyrenesulfonate (PSS)/squaraine:PCBM blend/LiF/Al
pletely counterbalanced.
with a ca. 6 mm2 illuminated areas. Our cells were fabri-
At negative (reductive) potentials, the CV traces show cated in air by spin-coating squaraine:acceptor 1:3 wt:wt
one peak associated with the reduction of the squarylium ratio solutions in CHCl3 on to ITO-coated glass substrates,
core. This reduction is almost irreversible and is a common which were modified by spin-coating with a PEDOT:PSS
feature of most squaraine compounds. Irreversible electro- layer as a hole extraction/electron-blocking layer. After dry-
chemical transformations are best characterized by means ing, the cells were then completed by sequential thermal
of DPV. Indeed, this technique enables clear identification vacuum deposition of LiF and Al as the cathode (see Sup-
of the oxidation and reduction potentials from the peaks of porting Information for a sketch of the cell structure). Typi-
the corresponding DPV traces (in red in Figure 2) even in cal Jsc/Voc plots for the sq-Ph-Br and sq-Ph-F-CH3 devices
the case of irreversible transformations. Interestingly in the are shown in Figure 4, along with cell external quantum
case of sq-Ph-F-Hex, we observed almost reversible re- efficiency (EQE) data. The preliminary OPV performance
duction behaviour. Such an uncommon feature could open data are summarized in Table 3.
the way for the exploitation of severely electron-poor squa-
raines as accepting materials instead of their common role
as donors in BHJ configurations.
The absolute position of the LUMO levels does not vary
significantly in the series, with the exception of sq-Py-F. In
this case the LUMO is considerably stabilized, again in
agreement with the strong accepting capabilities of the tet-
rafluoropyridine residue.
Figure 3 shows a schematic representation of the energy
level diagram for all of the squaraine compounds and
PC71BM. Indeed, the HOMO levels of the fluorinated
squaraines are progressively shifted towards lower energies.
In particular, in the case of sq-Py-F, the electron-with-
drawing effect is so pronounced that the squaraine LUMO
is shifted below the PC71BM LUMO. In principle, this me-
ans that the squaraine no longer behaves as a donor.[22]
Figure 3. Schematic representation of the HOMO and LUMO
levels of the sq-Ph, sq-Ph-Br, sq-Ph-F-CH3, sq-Ph-F-Hex and sq-
Py-F with respect to those of PC71BM.
Figure 4. Average Jsc/Voc response (a) and average external quan-
tum efficiency (EQE) and (b) of sq-Ph-Br (grey) and sq-Ph-F-CH3
(black) devices.
All devices were characterized under standard AM1.5G
1 Sun test conditions using instrumentation and analytical
OPV Device Preparation and Testing
We carried out preliminarily OPV measurements for procedures described previously[23] and PCE values are de-
some of the newly synthesized squaraines. Thus BHJ de- rived from the equation η p = (JscVocFF)/Po, where Jsc
=
Eur. J. Org. Chem. 2011, 5555–5563
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5559

L. Beverina
FULL PAPER
Table 3. Comparison of squaraine:PCBM BHJ photovoltaic cells.
vice, the morphology of the active layer is the least predicta-
ble. This is particularly crucial for compounds such as squa-
raines, which, bearing net charges, can form electrostatic
interactions leading to phase segregation. This study suc-
cessfully highlights a strategy for the fine-tuning of the do-
nor materials optoelectronic features. Further development
will be focused on the evaluation of suitable solubilizing
chains enabling the achievement of better morphologies.
Such development will not require further manipulation of
the squaraine HOMO and LUMO level energies as the re-
sults described here are general. In addition, it is known
that the nature and number of the solubilizing chains do
not affect the energy level position, which we have demon-
strated with sq-Ph-F-CH3 and sq-Ph-F-Hex.
Squaraine
Voc [V] Jsc [mA/cm2] FF [%]
PCE [%]
sq-Ph[11a]
0.57
0.44
0.56
9.32
2.75
2.35
37
46.7
31.7
1.99
0.56
0.41
sq-Ph-Br
sq-Ph-F-CH3
the short circuit current [mA/cm2], Voc the open circuit volt-
age [V], FF the fill factor and Po the incident light intensity
[mW/cm2].
Table 3 shows that the Voc value of the sq-Ph-F-CH3 de-
vice (0.56 V) is indeed higher than that of the sq-Ph-Br de-
vice (0.44 V). However, both values are lower than that pre-
viously obtained for the sq-Ph optimized devices.
This is likely the consequence of the different mor-
phology of the active layers of the corresponding OPV cells.
The AFM images shown in Figure 5 provide evidence that
both sq-Ph-Br (Figure 5, b) and sq-Ph-F-CH3 (Figure 5, c)
are strongly phase segregated with respect to PC71BM with
a texture in the range of several micrometers. The effect is
particularly clear if one compares the AFM images of the
new squaraines with that of the blend based on the parent
derivative sq-Ph (Figure 5, a). The phase separation effect
is also the reason for the rather small short circuit current
that characterizes both the sq-Ph-Br and sq-Ph-F-CH3 de-
vices. Indeed, the low Jsc the Voc values obtained can only
be considered as an estimate of the actual value.[24]
Conclusions
We successfully synthesized and characterized a series of
end-capped pyrrolic arylhydrazone squaraines bearing a
variety of electron-withdrawing substituents on the aryl
group. We demonstrated that it is possible to control the
HOMO level energy of such derivatives by careful selection
of the nature, number and position of the electron-with-
drawing residues. In the case of the most efficient electron-
withdrawing group we exploited, the low energy shift of the
squaraine levels is so strong that the dye is not expected to
behave as a donor with respect to PC71BM. Preliminarily
measurements for BHJ OPV cells based on the two most
promising derivatives, sq-Ph-Br and sq-Ph-F-CH3, using
PCBM as the acceptor, were carried out and exhibit PCE
values of about 0.5%. Consistent with the energy of the cor-
responding HOMO levels, the Voc value for the sq-Ph-F-
CH3 device (0.56 V) was higher than that for the sq-Ph-Br
device (0.44 V). However, both values are lower than that
of the parent derivative as a result of the very poor blend
morphology. We believe that this dramatic morphology
change with respect to sq-Ph-based blends is the conse-
quence of the sizeable increase in lipophilicity connected
with halogen substitution. We plan to counterbalance this
effect by changing the nature and branching of the squa-
raine solubilizing chains.
Experimental Section
Materials and Methods: PCBM was obtained from American Dye
Source Inc. All other chemicals and solvents were purchased from
Fluka or Aldrich and used as received unless stated. 1H and 13C
NMR spectra were recorded with a Bruker AMX-500 spectrometer
operating at 500 and 125.70 MHz respectively. FTIR spectra were
recorded with a Perkin–Elmer Spectrum 100-F operating in ATR
mode. Absorption spectroscopy was performed with a Jasco V570
spectrophotometer.
Figure 5. AFM images of OPV BHJ cells based on sq-Ph (a), sq-
Ph-Br (b) and sq-Ph-F-CH3 (c).
Derivative sq-Ph-F-Hex was originally prepared in order
to avoid such drastic phase segregation. However, its very
low melting point made it unsuitable for device preparation.
We plan to further optimize the active layer morphology by
changing both the squaraine substitution pattern and the
deposition solvent. However, it should be noted that, of the
Derivatives 1a, 1b, 1c, 2a[17] and sq-Ph[11a] were prepared according
to published procedures.
(E)-1-Methyl-2-[{2-(perfluorophenyl)hydrazono}methyl]-1H-pyrrole
(2b): A suspension of 1-N-methylpyrrole-2-carbaldehyde (1.102 g,
different factors influencing the efficiency of an OPV de- 10.10 mmol), pentafluorophenyl hydrazine (2.000 g, 10.10 mmol)
5560 Eur. J. Org. Chem. 2011, 5555–5563
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Bulk Heterojunction Solar Cells
and imidazolium hydrochloride (1.056 g, 10.10 mmol) in absolute
ethanol (20 mL) was heated to reflux for 2 h to give a brown homo-
geneous solution. The solvent was removed to give a red-brown
solid that was taken up in CH2Cl2 (100 mL) and filtered through
a plug of silica gel. Evaporation of the solvent under reduced pres-
sure gave a reddish solid that was further purified by crystallization
from iPrOH to give light brown needles (1.964 g, 6.77 mmol); yield
dissolved under a nitrogen atmosphere in anhydrous DMF
(30 mL). The pale yellow solution was cooled in an ice bath and
tBuOK (0.811 g, 7.23 mmol) was added portionwise over a period
of 10 min. The colour gradually darkened to a deep yellow-brown.
After 1.5 h at 0 °C, 6-bromohex-1-ene (1.178 g, 7.23 mmol) was
added dropwise, and the mixture warmed to room temperature and
stirred overnight. The mixture was poured into H2O (200 mL). The
resulting yellow solution was extracted into AcOEt (3ϫ50 mL).
The combined organic phase was washed with water (5ϫ100 mL),
dried with Na2SO4, and the solvents evaporated to dryness to give
a dark brown waxy solid, which was further purified by chromatog-
raphy (SiO2, hexane/AcOEt, 4:1) to give pure 3b as a yellow liquid
(0.976 g, 2.63 mmol); yield 40%. 1H NMR (500 MHz, [D6]DMSO,
25 °C): δ = 7.41 (s, 1 H), 6.80 (t, J = 2.20 Hz, 1 H), 6.22 (dd, J1 =
3.60, J2 = 1.80 Hz, 1 H), 6.00 (dd, J1 = 3.60, J2 = 2.50 Hz, 1 H),
5.79 (m, 1 H), 5.01 (dd, J1 = 17.3, J2 = 2.20 Hz, 1 H), 4.95 (dd, J1
= 10.4, J2 = 2.20 Hz, 1 H), 3.75 (s, 3 H), 3.59 (t, J = 7.20 Hz, 2
H), 2.05 (q, J = 1.40 Hz, 2 H), 1.62 (quint, J = 7.50 Hz, 2 H), 1.44
(quint, J = 7.5 Hz, 2 H) ppm.
1
67%. H NMR (500 MHz, [D6]DMSO, 25 °C): δ = 9.90 (s, 1 H),
8.09 (s, 1 H), 6.88 (t, J = 2.41 Hz, 1 H), 6.33 (dd, J1 = 3.95, J2 =
1.75 Hz, 1 H), 6.05 (dd, J1 = 3.49, J2 = 2.61 Hz, 1 H), 3.80 (s, 3
H) ppm; m.p. 149–150 °C.
(E)-1-(2-Ethylhexyl)-2-[{2-(perfluorophenyl)hydrazono}methyl]-
1H-pyrrole (2c): A suspension of 1c (2.000 g, 9.65 mmol), penta-
fluorophenyl hydrazine (1.056 g, 9.65 mmol) and imidazolium hy-
drochloride (1.008 g, 9.65 mmol) in absolute ethanol (30 mL) was
heated to reflux for 1.5 h to give a brown homogeneous solution.
The solvent was removed to give a red-brown solid that was taken
up in CH2Cl2 (100 mL) and filtered through a plug of silica gel.
Evaporation of the solvent under reduced pressure gave a reddish
oil that was further purified by column chromatography (SiO2, tol-
uene/hexane/CH2Cl2, 5:4:1) to give pure 2c as a light brown oil
(E)-1-(2-Ethylhexyl)-2-[{2-(hex-5-enyl)-2-(perfluorophenyl)-
hydrazono}methyl]-1H-pyrrole (3c): Hydrazone 2c (1.771 g,
4.57 mmol) was dissolved under a nitrogen atmosphere in anhy-
drous DMF (30 mL). The pale yellow solution was cooled in an
ice bath and tBuOK (0.564 g, 5.03 mmol) was added portionwise
over a period of 10 min. The colour gradually darkened to a deep
yellow-brown. After 1.5 h at 0 °C, 6-bromohex-1-ene (0.820 g,
5.03 mmol) was added dropwise, and the mixture warmed to room
temperature and stirred for 24 h. The mixture was poured into H2O
(100 mL). The resulting yellow solution was extracted into AcOEt
(3ϫ75 mL). The combined organic phase was washed with water
(5ϫ100 mL), dried with Na2SO4, and the solvents evaporated to
dryness to give a dark brown oil, which was purified by chromatog-
raphy (SiO2, hexane/AcOEt, 9:1) to give pure 3c as a yellow oil
(0.451 g, 0.96 mmol); yield 21%. 1H NMR (500 MHz, [D6]DMSO,
25 °C): δ = 7.36 (s, 1 H), 6.79 (t, J = 2.30 Hz, 1 H), 6.24 (dd, J1 =
3.89, J2 = 1.77 Hz, 1 H), 5.99 (m, 1 H), 5.78 (dd, J1 = 17.2, J2 =
10.3, J3 = 6.58 Hz, 1 H), 4.99 (dd, J1 = 17.1, J2 = 1.59 Hz, 1 H),
4.93 (dd, J1 = 10.2, J2 = 1.06 Hz, 1 H), 4.05 (m, 2 H), 3.57 (t, J1
= 7.24 Hz, 2 H), 2.04 (q, J = 7.18 Hz, 2 H), 1.73 (m, 1 H), 1.62
(quint, J = 7.42 Hz, 2 H), 1.43 (quint, J = 7.51 Hz, 2 H), 1.13 (m,
8 H), 0.81 (t, J = 7.06 Hz, 3 H), 0.75 (t, J = 7.42 Hz, 3 H) ppm.
1
(3.18 g, 8.21 mmol); yield 85%. H NMR (500 MHz, [D6]DMSO,
25 °C): δ = 9.79 (s, 1 H), 8.08 (s, 1 H), 6.88 (t, 1 H, broad), 6.34
(dd, J1 = 3.89, J2 = 1.77 Hz, 1 H), 6.06 (dd, J1 = 3.71, J2 = 2.65 Hz,
1 H), 4.12 (m, 2 H), 1.76 (m, 1 H), 1.17 (m, 8 H), 0.67 (m, 6 H)
ppm.
(E)-1-(Hex-5-enyl)-2-[(2-phenylhydrazono)methyl]-1H-pyrrole (2d):
A solution of 1b (1.000 g, 5.64 mmol), phenylhydrazine (0.610 g,
5.64 mmol) and imidazolium hydrochloride (0.590 g, 5.64 mmol) in
absolute ethanol (20 mL) was heated to reflux for 0.5 h to give a
yellow-brown homogeneous solution. The solvent was removed to
give a red-brown solid that was taken up in CH2Cl2 (200 mL) and
filtered through a plug of silica gel. Evaporation of the solvent
under reduced pressure gave a yellow oil that was not purified fur-
ther (1.434 g, 5.36 mmol); yield 95%. 1H NMR (500 MHz, [D6]-
DMSO, 25 °C): δ = 9.88 (s, 1 H), 7.82 (s, 1 H), 7.18 (t, J = 7.71 Hz,
2 H), 6.95 (m, 2 H), 6.68 (t, J = 7.00 Hz, 1 H), 6.28 (t, J = 1.75 Hz,
1 H), 6.04 (t, J = 3.07 Hz, 1 H), 5.80 (m, 1 H), 4.95 (m, 2 H), 4.27
(t, J = 7.10 Hz, 2 H), 1.99 (m, 2 H), 1.70 (m, 2 H), 1.34 (m, 2 H)
ppm.
(E)-2-[{2-(4-Bromophenyl)-2-(hex-5-enyl)hydrazono}methyl]-1-
methyl-1H-pyrrole (3a): Hydrazone 2a (1.900 g, 6.83 mmol) was
dissolved a under nitrogen atmosphere in anhydrous DMF
(30 mL). The pale yellow solution was cooled in an ice bath and
tBuOK (0.918 g, 7.51 mmol) was added portionwise over a period
of 10 min. The colour gradually darkened to a deep yellow-brown.
After 1 h at 0 °C, 6-bromohex-1-ene (1.224 g, 7.51 mmol) was
added dropwise, and the mixture warmed to room temperature.
After 40 min at ambient temperature, the solution was poured into
H2 O (150 mL). The mixture was extracted into CH2Cl2
(3ϫ50 mL). The combined organic phase was washed with water
(5ϫ100 mL), dried with Na2SO4, and the solvents evaporated to
dryness to give a brown solid, which was purified by chromatog-
raphy (SiO2 CH2Cl2) to give pure 3a as a light brown solid (2.349 g,
6.51 mmol); yield 95%. 1H NMR (500 MHz, [D6]DMSO, 25 °C):
δ = 7.74 (s, 1 H), 7.40 (d, J = 9.21 Hz, 2 H), 7.20 (d, J = 9.21 Hz,
2 H), 6.85 (t, J = 2.19 Hz, 1 H), 6.39 (dd, J1 = 3.95, J2 = 1.75 Hz,
1 H), 6.05 (dd, J1 = 3.51, J2 = 2.63 Hz, 1 H), 5.80 (m, 1 H), 5.04
(dd, J1 = 17.1, J2 = 1.75 Hz, 1 H), 4.97 (dd, J1 = 10.1, J2 = 1.75 Hz,
1 H), 3.87 (s, 3 H), 3.82 (t, J = 7.45 Hz, 2 H), 2.10 (m, 2 H), 1.51
(m, 4 H) ppm; m.p. 59–60 °C.
(E)-2,3,5,6-Tetrafluoro-4-[2-({1-(hex-5-enyl)-1H-pyrrol-2-yl}-
methylene)-1-phenylhydrazinyl]pyridine (3d): Hydrazone 2d (1.430 g,
5.35 mmol) was dissolved under a nitrogen atmosphere in anhy-
drous DMF (30 mL). The pale yellow solution was cooled in an
ice bath and tBuOK (0.660 g, 5.89 mmol) was added portionwise
over a period of 10 min. The colour gradually darkened to a deep
yellow-brown. After 1.5 h at 0 °C, pentafluoropyridine (2.698 g,
15.96 mmol) was quickly added under vigorous stirring, and the
mixture warmed to room temperature and stirred for 24 h. The
mixture was poured into H2O (100 mL). The resulting yellow solu-
tion was extracted into AcOEt (3ϫ100 mL). The combined or-
ganic phase was washed with water (5ϫ100 mL), dried with
Na2SO4, and the solvents evaporated to dryness to give a dark
brown oil, which was purified by chromatography (SiO2, hexane/
Et2O, 9:1) to give pure 3d as a yellow oil (0.881 g, 2.14 mmol); yield
40%. 1H NMR (500 MHz, [D6]DMSO, 25 °C): δ = 7.84 (t, J =
2.71 Hz, 1 H), 7.36 (dd, J1 = 15.4, J2 = 8.11 Hz, 2 H), 7.24 (d, J =
7.78 Hz, 2 H), 7.03 (m, 1 H), 6.44 (dd, J1 = 3.52, J2 = 1.52 Hz, 1
H), 5.77 (m, 1 H), 6.10 (m, 1 H), 4.94 (m, 2 H), 4.30 (t, J = 7.17 Hz,
2 H), 2.01 (m, 2 H), 1.73 (dt, J1 = 7.96, J2 = 6.20 Hz, 2 H), 1.32
(m, 2 H) ppm. 13C NMR (125.70 MHz, [D6]DMSO, 25 °C): δ =
145–143 (m), 144.37 (s), 140–137 (m), 138.93 (d), 138.38 (d), 131.42
(E)-2-[{2-(Hex-5-enyl)-2-(perfluorophenyl)hydrazono}methyl]-1-
methyl-1H-pyrrole (3b): Hydrazone 2b (1.900 g, 6.57 mmol) was
Eur. J. Org. Chem. 2011, 5555–5563
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5561

L. Beverina
FULL PAPER
(m), 129.85 (s), 128.22 (d), 126.57 (s), 123.96 (s), 118.00 (s), 116.83
(s), 115.35 (s), 108.89 (s), 48.67 (s), 33.38 (s), 31.22 (s), 25.78 (s)
ppm.
sphere, in a 1:1 iPrOH/toluene mixture (60 mL). The yellow suspen-
sion was heated to reflux with a Dean–Stark trap for 4 h, and the
colour gradually turned deep green. A second aliquot of squaric
acid was added (0.065 mg, 0.50 mmol), and mixture was heated for
an additional 4 h. The solvent was removed under reduced pressure
to give a dark green oil, which was purified by column chromatog-
raphy (SiO2, hexane/Et2O, 1:1). sq-Py-F was isolated as a dark
green powder (0.065 g, 0.07 mmol); yield 7%. 1H NMR (500 MHz,
CD2Cl2, 25 °C): δ = 7.85 (d, J = 4.59 Hz, 2 H), 7.56 (m, 4 H), 7.50
(s, 2 H), 7.43 (t, J = 7.60 Hz, 2 H), 7.32 (d, J = 7.77 Hz, 4 H), 6.87
(d, J = 4.59 Hz, 2 H), 5.78 (dt, J1 = 16.9, J2 = 10.2, J3 = 6.58 Hz,
2 H), 4.98 (dd, J1 = 17.1, J1 = 1.59 Hz, 2 H), 4.93 (m, 6 H), 2.05
(q, J = 7.3 Hz, 4 H), 1.76 (dt, J1 = 15.1, J2 = 7.68 Hz, 4 H), 1.44
(m, 4 H) ppm. 13C NMR (125.70 MHz, CD2Cl2, 25 °C): δ = 173.92
(s) 145.73 (m), 143.79 (m), 141.29 (s), 140.81 (s), 138.53 (s), 138.32
(m), 136.21 (m), 133.11 (m), 132.39 (s), 130.27 (s), 127.71 (s), 123.80
(s), 123.22 (s), 118.68 (s), 114.50 (s), 48.06 (s), 33.51 (s), 31.71 (s),
25.74 (s) ppm. C48H38F8N8O2·3/2H2O (910.86 + 27.0): calcd. C
61.47, H 4.41, N 11.95; found C 61.88, H 4.79, N 11.82; m.p. 50–
60 °C (dec.).
sq-Ph-Br: Hydrazone 3a (1.500 g, 4.16 mmol) and squaric acid
(0.237 mg, 2.08 mmol) were suspended, under a nitrogen atmo-
sphere, in a 1:1 BuOH/toluene mixture (60 mL). The yellow suspen-
sion was heated to reflux with a Dean–Stark trap for 2 h, and the
colour gradually turned deep green. The solvent was removed un-
der reduced pressure to give a dark green oil, which was taken up
in EtOH to give a green suspension. The precipitate was isolated
by suction filtration and washed with Et2O to give sq-Ph-Br as a
green powder (0.673 g, 0.84 mmol); yield 41%. 1H NMR
(500 MHz, CD2Cl2, 25 °C): δ = 7.71 (s, 2 H), 7.43 (d, J = 8.77 Hz,
4 H), 7.35 (s, 2 H) 7.20 (d, J = 8.33 Hz, 4 H), 6.82 (d, J = 3.95 Hz,
2 H), 5.88 (m, 2 H), 5.11 (d, J = 17.1 Hz, 2 H), 5.06 (d, J = 10.1 Hz,
2 H), 4.31 (s, 6 H), 3.87 (t, J = 8.11 Hz, 4 H), 2.20 (q, J = 7.31 Hz,
4 H), 1.70 (m, 4 H), 1.57 (m, 4 H) ppm. 13C NMR (125.70 MHz,
CD2Cl2, 25 °C): δ = 168.18, 145.27, 144.18, 138.26, 132.06, 131, 72,
123.11, 121.62, 116.86, 116.11, 115.13, 114.22, 45.33, 35.66, 33.29,
26.24, 23.94 ppm. C40H42Br2N6O2 (798.62): calcd. C 60.16, H 5.30,
N 10.52; found C 60.47, H 5.53, N 10.28; m.p. 209–210 °C.
Electrochemical Characterization: The squaraines were dissolved
(concentration about 10–4 m) in a 0.1 m solution of tetrabutyl-
ammonium hexafluorophosphate (Fluka, electrochemical grade,
Ն 99.0%) in anhydrous acetonitrile (Aldrich, 99.8%). DPV and CV
were carried out at scan rate of 20 and 50 mV/s, respectively, with
a PARSTA2273 potentiostat in a single chamber three-electrode
electrochemical cell in a glove box filled with argon ([O2] Յ 1 ppm).
The working, counter and pseudoreference electrodes were a Au
pin, a Pt flag and a Ag/AgCl wire, respectively. The Ag/AgCl pseu-
doreference electrode was externally calibrated by adding ferrocene
(1 mm) to the electrolyte.
sq-Ph-F-CH3: Hydrazone 3b (0.976 g, 2.63 mmol) and squaric acid
(0.150 mg, 1.31 mmol) were suspended, under a nitrogen atmo-
sphere, in a 1:1 BuOH/toluene mixture (60 mL). The yellow suspen-
sion was heated to reflux with a Dean–Stark trap for 2 h, and the
colour gradually turned deep green. The solvent was removed un-
der reduced pressure to give a dark green oil, which was purified
by column chromatography (SiO2, CH2Cl2). sq-Ph-F-CH3 was iso-
lated as a dark solid with a metallic lustre (0.246 g, 0.30 mmol);
1
yield 23%. H NMR (500 MHz, CD2Cl2, 25 °C): δ = 8.34 (d, J =
4.40 Hz, 2 H), 6.76 (broad s, 2 H), 6.55 (d, J = 4.8 0 Hz, 2 H), 5.68
(m, 2 H), 5.00 (dd, J1 = 17.1, J2 = 1.80 Hz, 2 H), 4.98 (dd, J1 =
10.1, J2 = 1.80 Hz, 2 H), 4.15 (s, 6 H), 3.25 (t, J = 7.20 Hz, 4 H),
1.88 (q, J = 7.30 Hz, 4 H), 1.43 (tt, J1 = 8.30, J2 = 7.40 Hz, 4 H),
1.20 (tt, J1 = 8.00, J2 = 4.80 Hz, 4 H) ppm. 13C NMR
(125.70 MHz, CDCl3, 25 °C): δ = 173.57 (s), 143.85 (m), 142.70 (s),
140.40 (m), 138.15 (m), 138.04 (s), 132.77 (s), 124.66 (s), 124.01 (s),
118.87 (s), 116.84 (s), 115.23 (s), 54.88 (s), 35.75 (s), 33.35 (s), 26.52
(s), 25.82 (s) ppm. C40H34F10N6O2 (820.73): calcd. C 58.54, H 4.18,
N 10.24; found C 58.06, H 4.20, N 9.96; m.p. 70–72 °C.
BHJ OPV Device Fabrication and Thin Film Characterization: De-
vices were fabricated by spin-coating blends of squaraine and
PCBM, sandwiched between a transparent anode and an Al cath-
ode. PCBM was purified as described elsewhere.[23] The anode con-
sisted of glass substrates precoated with ITO, modified by spin-
coating with a PEDOT:PSS layer (ca. 50 nm and ca. 75 nm) as
hole-transport, and the cathode consisted of lithium fluoride
(ca. 0.6 nm) capped with aluminum (ca. 120 nm). Details of these
procedures and the equipment have been described previously.[23]
sq-Ph-F-Hex: Hydrazone 3c (0.425 g, 0.91 mmol) and squaric acid
(0.052 mg, 0.45 mmol) were suspended, under a nitrogen atmo-
sphere, in a 1:1 BuOH/toluene mixture (40 mL). The yellow suspen-
sion was heated to reflux with a Dean–Stark trap for 4 h, and the
colour gradually turned deep green. The solvent was removed un-
der reduced pressure to give a dark green oil, which was purified
by column chromatography (SiO2, hexane/Et2O, 1:1). sq-Ph-F-Hex
was isolated as a dark green wax (0.064 g, 0.06 mmol); yield 14%.
1H NMR (500 MHz, CD2Cl2, 25 °C): δ = 7.78 (s, 2 H), 7.17 (s, 2
H), 6.83 (d, J = 4.53 Hz, 2 H), 5.85 (m, 2 H), 5.05 (dd, J1 = 17.15,
J2 = 1.89 Hz, 2 H), 5.00 (d, J = 10.21 Hz, 2 H), 4.82 (m, 4 H), 3.81
(t, J = 7.37 Hz, 4 H), 2.13 (q, J = 7.10 Hz, 4 H), 1.76 (quint, J =
7.53 Hz, 4 H), 1.67 (m, 2 H), 1.52 (quint, J = 7.36 Hz, 4 H), 1.1–
1.4 (m, 8 H), 0.86 (t, J = 7.16 Hz, 6 H), 0.82 (t, J = 7.33 Hz, 6 H)
ppm. 13C NMR (125.70 MHz, CD2Cl2, 25 °C): δ = 171.47 (s),
144.17 (m), 143.05 (s), 140.97 (m), 138.41 (m), 138.38 (s), 132.29
(s), 124.38 (s), 123.24 (s), 118.33 (s), 116.31 (s), 114.75 (s), 55.96
(s), 51.54 (s), 41.63 (s), 33.41 (s), 30.24 (s), 28.92 (s), 27.09 (s), 25.94
(s), 23.66 (s), 23.11 (s), 13.76 (s), 10.77 (s) ppm. C54H62F10N6O2
(1017.10): calcd. C 63.77, H 6.14, N 8.26; found C 63.30, H 6.40,
N 8.18.
Before device fabrication, the commercial ITO film on glass sub-
strates (1.1 mm) was patterned with aqueous HCl solution. The
ITO-coated (150 nm thick films) glass substrates were then cleaned
by ultrasonic treatment in aqueous detergent, deionized water, iso-
propyl alcohol, methanol and acetone sequentially, and finally in a
UV-ozone cleaner for 30 min under ambient atmosphere. In the
meantime, a solution of PEDOT:PSS (Baytron VP) was used as a
hole-transport layer, after filtration through a 0.45 μm Teflon filter,
and was used to spin-coat layers onto the ITO at 2000 rpm for 60
sec or 4000 rpm for 60 sec. The sample was cured at 135 °C for
15 min under a Petri dish in ambient atmosphere. All blend solu-
tions of squaraine and PCBM were stirred for 1 h at 40 °C and
then sonicated for 1 at 40 °C in the dark. The active layer was
then obtained by spin-coating the blends at 5000 rpm for 30 sec in
ambient atmosphere. Where necessary, sample areas for contacts
were cleaned with chloroform and a cotton swab, and then transfer-
red to the glove box under a nitrogen atmosphere for the electrode
evaporation. The cathode growth rates used were 0.1 Å/s for LiF
(Acros, 99.98%) and ca. 1.5 Å/s for Al (Sigma–Aldrich, 99.999%),
with a chamber pressure of 1.1ϫ10–6 Torr. The cathodes were de-
posited through a shadow mask with two 2 mm strips perpendicu-
lar to the two patterned ITO (ca. 3 mm) strips to make four devices
sq-Py-F: Hydrazone 3d (0.841 g, 2.02 mmol) and squaric acid
(0.115 mg, 1.01 mmol) were suspended, under a nitrogen atmo-
5562
www.eurjoc.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2011, 5555–5563

Bulk Heterojunction Solar Cells
[7] Z. Xiang, E. E. Nesterov, J. Skoch, T. Lin, B. T. Hyman, T. M.
Swager, B. J. Bacskai, S. A. Reeves, J. Histochem. Cytochem.
2005, 53, 1511–1516.
per substrate. Finally, the completed solar cells were encapsulated
with a glass slide using UV-curable epoxy (Electro-Lite ELC-2500),
which was cured inside a UV chamber in the glove box. In the case
of annealed samples, a hot plate at 50 °C was used for 30 min inside
the glove box, and samples were then encapsulated. AFM images
were obtained with a JEOL-5200 Scanning Probe Microscope with
silicon cantilevers in the tapping mode, using WinSPM Software.
[8] a) V. Rapozzi, L. Beverina, P. Salice, G. A. Pagani, M. Ca-
merin, L. E. Xodo, J. Med. Chem. 2010, 53, 2188–2196; b) D.
Ramaiah, I. Eckert, K. Arun, L. Weidenfeller, B. Epe, Pho-
tochem. Photobiol. 2004, 79, 99–104; c) P. F. Santos, L. V. Reis,
P. Almeida, J. P. Serrano, A. S. Oliveira, L. F. Vieira Ferreira,
J. Photochem. Photobiol. A: Chem. 2004, 163, 267–269.
[9] E. C. P. Smits, S. Setayesh, T. D. Anthopoulos, M. Buechel, W.
Nijssen, R. Coehoorn, P. W. M. Blom, B. de Boer, D. M.
de Leeuw, Adv. Mater. 2007, 19, 734–738.
BHJ OPV I-V Characterization: Device evaluation was performed
at 298 K with a Class A Spectra-Nova Technologies solar cell ana-
lyzer having a xenon lamp that simulates AM1.5G light from 400–
1100 nm. The instrument was calibrated with a monocrystalline Si
diode fitted with a KG3 filter to bring spectral mismatch to unity.
The National Renewable Energy Laboratory calibrated the cali-
bration standard. Data are corrected for spectral mismatch. Four-
point contacts were made to the substrate with copper alligator
clips. A mask isolated individual devices during testing to avoid
current collection from adjacent devices and edge effects.[23]
[10] J. M. Baumes, J. J. Gassensmith, J. Giblin, J.-J. Lee, A. G.
White, W. J. Culligan, W. M. Leevy, M. Kuno, B. D. Smith,
Nat. Chem. 2010, 2, 1025–1030.
[11] a) D. Bagnis, L. Beverina, H. Huang, F. Silvestri, Y. Zheng,
G. A. Pagani, T. J. Marks, A. Facchetti, J. Am. Chem. Soc.
2010, 132, 4074–4075; b) G. Wei, S. Wang, K. Sun, M. E.
Thompson, S. R. Forrest, Adv. Energy Mater. 2011, 1, 184–187.
[12] a) H. Yum, P. Walter, S. Huber, D. Rentsch, T. Geiger, F.
Nuesch, F. De Angelis, M. Grätzel, M. K. Nazeeruddin, J. Am.
Chem. Soc. 2007, 129, 10320; b) S. Paek, H. Choi, C. Kim, N.
Cho, S. So, K. Song, M. K. Nazeeruddin, J. Ko, Chem. Com-
mun. 2011, 47, 2874–2876.
Supporting Information (see footnote on the first page of this arti-
cle): FTIR, 1H and 13C NMR spectra of all new squaraine com-
pounds, schematic representation of the OPV cells structure.
[13] M. Binda, A. Iacchetti, D. Natali, L. Beverina, M. Sassi, M.
Sampietr, Appl. Phys. Lett. 2011, 98, 073303.
Acknowledgments
[14] J. Peet, A. J. Heeger, G. C. Bazan, Acc. Chem. Res. 2009, 42,
1700–1708.
We gratefully acknowledge the Consorzio Interuniversitario per la
Scienza e la Tecnologia dei Materiali INSTM (PRISMA2007) and
the Ministero dell’Istruzione dell’Università
(MIUR) (FIRB 2003: Molecular compounds and hybrid nano-
structured materials with resonant and nonresonant optical proper-
ties for photonic devices) for financial support. We thank Dr. Ric-
cardo Crippa for sample preparation and helpful discussion.
[15] a) B. Walker, C. Kim, T.-Q. Nguyen, Chem. Mater. 2011, 23,
470–482; b) J. Roncali, Acc. Chem. Res. 2009, 42, 1719–1730;
c) M. T. Lloyd, J. E. Anthony, G. G. Malliaras, Mater. Today
2007, 10, 34–41; d) H. Shang, H. Fan, Y. Liu, W. Hu, Y. Li,
X. Zhan, Adv. Mater. 2011, 23, 1554–1557; e) B. Walker, A. B.
Tamayo, X. D. Dang, P. Zalar, J. H. Seo, A. Garcia, M. Tanti-
wiwat, T. Q. Nguyen, Adv. Funct. Mater. 2009, 19, 3063–3069.
[16] For a comprehensive account of squaraine synthesis, see: D.
Keil, H. Hartmann, Dyes Pigm. 2001, 49, 161–179.
e della Ricerca
[1] a) A. Treibs, K. Jacob, Angew. Chem. 1965, 77, 680; Angew.
Chem. Int. Ed. Engl. 1965, 4, 694; b) A. Treibs, K. Jacob, Justus
Liebigs Ann. Chem. 1968, 712, 123–137; c) A. Treibs, K. Jacob,
[17] L. Beverina, A. Abbotto, M. Landenna, M. Cerminara, R. Tu-
bino, F. Meinardi, S. Bradamante, G. A. Pagani, Org. Lett.
2005, 7, 4257–4260.
Justus Liebigs Ann. Chem. 1966, 699, 153–167; d) K. Y. Law, [18] The structure of sq-Py-F, particularly regarding to the substitu-
Chem. Rev. 1993, 93, 449–486.
tion pattern at the tetrafluoropyridine residue, was confirmed
by the 13C NMR spectrum (see Supporting Information). The
spectrum shows the expected multiplicity structure for a 4-sub-
stituted tetrafluoropyridine derivative.
[2] a) L. Beverina, P. Salice, Eur. J. Org. Chem. 2010, 23, 1207–
1225; b) A. Ajayaghosh, Acc. Chem. Res. 2005, 38, 449–459; c)
S. Sreejith, P. Carol, P. Chithra, A. Ajayaghosh, J. Mater.
Chem. 2008, 18, 264–274.
[19] F. Barriere, N. Camire, W. E. Geiger, U. T. Mueller-Westerhoff,
R. Sanders, J. Am. Chem. Soc. 2002, 124, 7262.
[20] B. P. Karsten, J. C. Bijleveld, R. A. J. Janssen, Macromol. Rapid
Commun. 2010, 31, 1554–1559.
[3] a) K. Y. Law, J. Phys. Chem. 1987, 91, 5184–5193; b) K. Law,
F. Bailey, J. Org. Chem. 1992, 57, 3278–3286.
[4] a) L. Beverina, R. Ruffo, G. Patriarca, F. D. Angelis, D. Ro-
berto, S. Righetto, R. Ugo, G. A. Pagani, J. Mater. Chem. 2009,
19, 8190–8197; b) L. Beverina, M. Crippa, P. Salice, R. Ruffo,
C. Ferrante, I. Fortunati, R. Signorini, C. M. Mari, R. Bozio,
A. Facchetti, G. A. Pagani, Chem. Mater. 2008, 20, 3242–3244;
c) C.-T. Chen, S. R. Marder, L. T. Cheng, J. Am. Chem. Soc.
1994, 116, 3117.
[21] Y. He, G. Zhao, B. Peng, Y. Li, Adv. Funct. Mater. 2010, 20,
3383–3389.
[22] A full investigation of this effect is beyond the scope of this
paper, however, we are currently investigating the behaviour of
sq-Py-F and sq-Ph-F-Hex as acceptors in a BHJ blend with
P3HT as the donor.
[5] a) E. Arunkumar, P. Sudeep, P. Kamat, B. Noll, B. Smith, New
J. Chem. 2007, 31, 677–683; b) E. Arunkumar, C. C. Forbes,
B. C. Noll, B. D. Smith, J. Am. Chem. Soc. 2005, 127, 3288–
3289.
[6] L.-H. Liu, K. Nakatani, R. Pansu, J.-J. Vachon, P. Tauc, E.
Ishow, Adv. Mater. 2007, 19, 433–436.
[23] M. D. Irwin, B. Buchholz, A. W. Hains, R. P. H. Chang, T. J.
Marks, Proc. Natl. Acad. Sci. USA 2008, 105, 2783.
[24] M. Dolores Perez, C. Borek, S. R. Forrest, M. E. Thompson,
J. Am. Chem. Soc. 2009, 131, 9281–9286.
Received: June 29, 2011
Published Online: August 30, 2011
Eur. J. Org. Chem. 2011, 5555–5563
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5563