Panchromatic squaraine compounds for broad band light harvesting electronic devices

Article abstract of DOI:10.1039/c2jm16240g

Squaraine compounds are currently investigated as high performance active components in both organic and hybrid photovoltaic devices as well as in photodetectors. Their most valuable features include a particularly efficient optical absorption in the Vis-NIR region, high polarizability, and a remarkable chemical stability. Their full exploitation is somewhat limited by a negligible absorption in the UV-Vis region (prototypical squaraines basically do not absorb below 500 nm). The aim of the present paper is the design and synthesis of truly panchromatic squaraines to be effectively employed as the photoactive materials in Vis operating optoelectronic devices. Our strategy involves the design of squaraines that are both nonsymmetric and core-substituted with suitable electron-withdrawing groups. We show the effect of such a design strategy by means of UV-Vis spectroscopy, cyclic voltammetry and prototypical device performances rationalization. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16240g

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PAPER
Panchromatic squaraine compounds for broad band light harvesting electronic
devices†
a
Luca Beverina, Riccardo Ruffo,a Matteo M. Salamone,a Elisabetta Ronchi,a Maddalena Binda,b
*
Dario Natalibc and Marco Sampietrobc
Received 29th November 2011, Accepted 19th January 2012
DOI: 10.1039/c2jm16240g
Squaraine compounds are currently investigated as high performance active components in both
organic and hybrid photovoltaic devices as well as in photodetectors. Their most valuable features
include a particularly efficient optical absorption in the Vis-NIR region, high polarizability, and
a remarkable chemical stability. Their full exploitation is somewhat limited by a negligible absorption
in the UV-Vis region (prototypical squaraines basically do not absorb below 500 nm). The aim of the
present paper is the design and synthesis of truly panchromatic squaraines to be effectively employed as
the photoactive materials in Vis operating optoelectronic devices. Our strategy involves the design of
squaraines that are both nonsymmetric and core-substituted with suitable electron-withdrawing
groups. We show the effect of such a design strategy by means of UV-Vis spectroscopy, cyclic
voltammetry and prototypical device performances rationalization.
pronounced electron-withdrawing nature of the squarylium core,
squaraines usually possess a low-lying HOMO level.24,27 In the
Introduction
1,3-Squaraines (or squarylium dyes) are the condensation product
Bulk Heterojunction configuration this is an additional value as
of squaric acid and two equivalents of a suitable electron rich
the maximum voltage that a solar cell can generate (the Open
precursor (anhydrobase or an activated arene).1–3 The use of two
Circuit Voltage) is proportional to the difference in energy
different electron rich precursors leads—through a variety of
between the HOMO of the donor material (the squaraine) and
different multi-step procedures4—to nonsymmetric squaraines.
the LUMO of the accepting one (in the most common embodi-
Both symmetric and nonsymmetric squaraines are highly delo-
ment the acceptor is a fullerene derivative).34
calized conjugated materials possessing a cyanine-like structure.5–7
So far, squaraines demonstrated moderate to high efficiencies
Their most notable features are a strong and localized absorption
in both Bulk Heterojunction Solar Cells (with top performances
in the Vis-NIR region, solubility in low polarity solvents,
averaging at 4.5%)25 and dye sensitized solar cells (most efficient
remarkablechemicalstability, photoconducting capabilitiesandin
derivatives approach 6.5%).30 Moreover, squaraines behave as
somecasesastrongNIRfluorescence.8,9 Squaraineshavethusbeen
very efficient active media, again in a Bulk Heterojunction
employed in a number of technologically relevant research fields
configuration along with [6,6]-Phenyl C61 butyric acid methyl
including: non-linear optics,10–12 highly stable NIR-fluorescent
ester (PC61BM) as the acceptor, in high performance and
dyes,13,14 photopatterning,15 bioimaging,16 fluorescent ratiometric
remarkably stable organic NIR photodetectors, a class of devices
bioprobes,17 photodynamic therapy,18–21 field effect transistors,22
working according to the same principle of solar cells.32,33,35,36 In
chemiluminescent materials,23 bulk-heterojunction24–28 and
this particular field, squaraines are unsurpassed in terms of
hybrid29–31 solar cells and organic photodetectors.32,33
absolute performances and environmental stability, even under
Concerning their application in optoelectronic devices, the
strong operating bias.33
main appeal of squaraines as active media is their extremely
One relevant issue affecting squaraines is that their extremely
efficient absorption. In addition, as a consequence of the
narrow absorption band leads to photon harvesting essentially
localized in the Vis-NIR region. Even in the solid state, when
a pronounced tendency to aggregation causes a relevant band
broadening, the absorption below 500 nm is negligible.37 Within
aDepartment of Materials Science and INSTM, University of
Milano-Bicocca, Via Roberto Cozzi 53, 20125 Milano, Italy. E-mail:
luca.beverina@mater.unimib.it
bCenter for Nano Science and Technology @Polimi, Istituto Italiano di
tecnologia, Via Pascoli 70/3, 20133 Milano, Italy
cPolitecnico di Milano, Dipartimento di Elettronica e Informazione, Piazza
Leonardo Da Vinci 32, 20133 Milano, Italy
this context, the use of nonsymmetric squaraines, instead of the
definitely more common (and synthetically accessible) symmetric
ones, is helpful as it introduces a certain charge-transfer contri-
bution in the otherwise completely delocalized HOMO–LUMO
optical transition. This translates into a remarkable broadening
of the absorption band.10
† Electronic supplementary information (ESI) available: Copy of the 1H
and 13C NMR for all new materials. See DOI: 10.1039/c2jm16240g
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Another valuable strategy to increase the squaraine photon
harvesting capabilities, particularly regarding the high-energy
portion of the solar spectrum, requires the introduction of an
electron withdrawing substituent directly on the squarylium core.
This functionalization strategy, originally introduced by Tatarets
and Terpetschnig,38 provides the squaraine with an additional
high-energy absorption, beneficial in both the DSSC39 and BHJ40
solar cell configurations. Recently, it has also been shown that
this strategy provides bright NIR emitting dyes.41
give the emisquarate 6 as a yellow solid. This can be easily
converted to the corresponding emisquaraine 9 by hydrolysis in
a 1 M LiOH THF solution, followed by acidification.10 The
emisquaraine 9 was then converted into the nonsymmetric
squaraine sq-CO in good yield, by reaction with the anhydrobase
generated in situ by the action of imidazole on the salt 11,
according to the previously published procedure (Scheme 2).10
The emisquarate 6 is also the common intermediate for the
preparation of the core-substituted derivatives sq-CN and sq-PY.
In detail, 6 was condensed with malononitrile in refluxing
ethanol in the presence of excess triethylamine. The substituted
emisquaraine 8 was precipitated by acidification of the reaction
mixture with excess 5% HCl to give the pure compound as
a yellow-orange precipitate. Derivative 8 is a strong acid and is
highly soluble in water; an excess of HCl is needed in order to
trigger its precipitation. Finally, 8 was reacted with 11 in the
presence of stoichiometric imidazole in a 3 : 1 refluxing mixture
For example, we have shown that the absorption spectrum of
the anhydrobase derived symmetric squaraine 1 (Scheme 1)
shows the typical strong and sharp absorption band arising from
the cyanine-like transition associated with the delocalization of
the nitrogen lone pair all over the main conjugation axis of the
molecule. The core substituted derivative 2 (Scheme 1) displays
the same low energy band and a new high energy one attributed
to the core functionalization.39
i
The aim of the present paper is the preparation of two
squaraines of general formula 3 that are at the same time
nonsymmetric (and thus possessing a broadened low energy
absorption) and core substituted (in order to observe the addi-
tional high energy absorption band) to be employed as
panchromatic sensitizers in organic light harvesting devices
operating in the BHJ configuration (Scheme 1).
of PrOH and toluene under a Dean–Stark trap. The nonsym-
metric squaraine sq-CN was isolated in 25% yield as a deep blue,
shiny solid (Scheme 2).
The reaction of 6 with 3-methyl-1-phenyl-pyrazol-5-one 7 in
refluxing ethanol in the presence of excess triethylamine gave,
after acidification, the substituted emisquaraine 10 in good yield
as a yellow-brown solid. In the last step, 10 was reacted with 12 in
the presence of stoichiometric imidazole in a iPrOH:toluene 1 : 1
refluxing mixture to give the squaraine sq-PY after chromato-
graphic purification (Scheme 2).
Experimental
1. Synthesis of the materials
Interestingly, the reaction of 7 with the dimethylindolenine
derived emisquarate 15, under the same conditions employed for
the preparation of 10, only gave the corresponding, core
unsubstituted, emisquaraine 16 (Scheme 3).
Symmetric squaraines can be prepared by the direct reaction,
under a variety of conditions, of two equivalents of the electron-
rich precursor and squaric acid.37 Conversely, the preparation of
a nonsymmetric squaraine is a multi-step process that involves
the isolation of the condensation product of one equivalent of the
2. UV-Vis characterization
first electron rich material with
derivative.4
a suitable squaric acid
Squaraine sq-CO was originally synthesized as a Second
Harmonic Generating molecule in the context of nonlinear
optics. Our previous investigation shows that sq-CO is a moder-
ately polar molecule possessing a Full Width at Half Maximum
(FWHM) of 894 cmꢀ1 in CHCl3 and moderate solvatochromic
effect.10
Particularly activated arenes such as 2,4-dimethylpyrrole (4)
are able to react with squaric acid esters to give the corre-
sponding emisquarate in good yield. Thus, derivative 4 was
reacted with stoichiometric ethylsquarate in refluxing ethanol to
Fig. 1 shows the comparison between the absorption spectra of
sq-CO and the corresponding symmetric squaraines 13 (derived
from the benzothiazolium anhydrobase)21 and 14 (derived from
2,4-dimethylpyrrole).1 The HOMO–LUMO gap of sq-CO is
intermediate between that of 14 and 13 but its FWHM is larger
than both of them (see Table 1).
These favourable features, along with a relatively simple and
high yield synthetic access, made sq-CO the ideal structure to
study the influence of core functionalization on optical and
optoelectronic properties. Fig. 2 shows the comparison between
the absorption spectra of squaraine sq-CO and the core-
substituted squaraine sq-CN. Both the position and the FWHM
of the main cyanine-like absorption band are only marginally
affected by the presence of the substituent.
Indeed, sq-CN possesses a second absorption band in the 350–
400 nm region. Such a band is connected with the presence of the
malononitrile group and can be described as the merocyanine-
like oscillator due to the delocalization of the core negative
charge between the oxygen atom and the malononitrile moiety.
Scheme 1
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Scheme 2
Scheme 3
This interpretation is coherent with our previous computational
investigation of a dimethylindolenine-based core substituted
squaraine.39
In order to both increase the relative intensity of the high-
energy peak and possibly further broaden the main absorption
band, we replaced the malononitrile residue with a more conju-
gated and nonsymmetric acceptor, the 3-methyl-1-phenyl-pyr-
azol-5-one 7, thus giving the derivative sq-PY.
Fig. 1 Comparison between the absorption spectra (CH2Cl2) of the
symmetric squaraines 13 and 14 and the nonsymmetric derivative sq-CO.
Fig. 3 shows the comparison between the absorption spectra of
squaraines sq-CN and sq-PY. The pyrazolone-substituted
derivative shows three distinct absorption bands. The main
cyanine-like transition falls at the exact same energy as that of sq-
CN, its FWHM however goes from the 1450 cmꢀ1 of the former
to a very remarkable (for a squaraine compound) 2121 cmꢀ1. At
the same time a new band is formed at 510 nm. sq-PY is thus able
to absorb all over the visible spectrum. To the best of our
knowledge, this is an unprecedented result for squaraine
compounds.
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Table 1 UV-Vis characterization data for squaraines sq-CO, sq-CN and
sq-PY
purpose. The tentative, unverified assignment of the specific
regioisomer we show is based on the chemical shift of the pyrrolic
N–H signal that falls at 6.05 ppm, very far from its usual position
between 10 and 9 ppm. We attributed this uncommon high-field
shift to the establishment of a hydrogen bond between the
pyrrolic N–H and the pyrazolone carbonyl residue.
lmax (1)/nm
lmax (2)/nm
3/l cmꢀ1 molꢀ1
FWHM/cmꢀ1
14
15
sq-CO
sq-CN
sq-PY
672
552
608
638
637
—
—
—
368
444, 514
295 000
79 000
230 000
118 000
81 000
668
797
894
1450
2121
The establishment of such a hydrogen-bond could be the
reason for both the high yield and the stereoselectivity of the
condensation reaction between the emisquarate 6 and the pyr-
azolone 7.
3. Electrochemical characterization
Cyclic voltammetry (CV) gives an immediate insight into the
electronic effects induced by the presence of the electron-with-
drawing group on the squarylium core.
Fig. 4 shows the comparison between the CV plots for the
three squaraines considered in the present work. The three vol-
tammograms show similar features. At positive potentials the
plots show two waves corresponding to the oxidation of the two
electron rich moieties: the anhydrobase and the dimethylpyrrole.
In the case of both sq-CO and sq-PY such processes are irre-
versible. Conversely, in the case of sq-CN the process is quasi
reversible.
Indeed, we recorded Differential Pulsed Voltammetry (DPV)
plots for all of the compounds, as this is the most appropriate
technique to characterize partially or completely irreversible
processes (Fig. 5).
Fig. 2 Comparison between the absorption spectra (CHCl3) of the
nonsymmetric squaraine sq-CO and that of the core substituted squar-
aine sq-CN.
The peaks of the first oxidation process in the DPV plots are
shifted progressively towards more positive potentials in the
series sq-CO, sq-PY, sq-CN. This is in agreement with the elec-
tron withdrawing capabilities of the substituent, contributing to
the enhancement of the accepting nature of the squarylium core.
The peaks of the first reduction process are shifted towards less
negative potentials in the series sq-CN, sq-PY, sq-CO. In the case
of sq-PY the reduction process becomes fully reversible.
From the standpoint of their use in photodetecting/photovol-
taic devices, both sq-CN and sq-PY show promising features,
regarding their low lying HOMO levels. However, we focused our
attention on derivative sq-PY as it ensures both a very efficient
photon harvesting all over the visible spectrum and a LUMO level
Fig. 3 Comparison between the UV-Vis absorption spectra of CHCl3
solutions of the cross-substituted nonsymmetric squaraines sq-CN and
sq-PY.
Derivative sq-PY could in principle be obtained as a mixture of
isomers having the pyrazolone ring oriented in both the possible
ways.
The NMR characterization clearly shows that the chromato-
graphic purification enables to isolate one single isomer (see
ESI†). NMR also shows that the precursor of sq-PY, the emis-
quaraine 10, is formed too as a single isomer. In this case no
purification step is involved and the particularly high reaction
yield ensures that the reaction is almost completely
stereoselective.
The unambiguous assignment of the structure of sq-PY would
require a single crystal X-ray characterization. Unfortunately,
we did not succeed in growing high quality crystals for this
Fig. 4 Cyclic voltammetry plots for squaraines sq-CO, sq-CN, sq-PY in
CH3CN with tetrabutylammonium hexafluorophosphate as the sup-
porting electrolyte.
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Fig. 6 Sketch of the device based on the sq-PY:PC61BM bulk-hetero-
junction active material.
The device is still working in the NIR-IR region with effi-
ciencies as high as 0.5% at 850 nm.
Fig. 5 DPV plots for squaraines sq-CO, sq-CNandsq-PY in CH3CN with
tetrabutylammonium hexafluorophosphate as the supporting electrolyte.
Materials. All chemicals and solvents were purchased from
Fluka or Aldrich and used as received unless explicitly stated. 1H
spectra were recorded using a Bruker AMX-500 spectrometer.
13C NMR spectra were recorded on a Bruker AMX-500 spec-
trometer operating at 125.70 MHz. Absorption spectroscopy was
performed using a Jasco V570 spectrophotometer. Melting
points are uncorrected.
that is slightly above that of PC61BM. Conversely, the sq-CN
LUMO is slightly below than that of PC61BM, thus in principle
unsuitable for the photoinduced electron transfer process.
Table 2 summarizes the most relevant electrochemical prop-
erties of sq-CO, sq-CN and sq-PY highlighting both the low lying
position of their HOMO levels with respect to P3HT and the
progressive shift of the LUMO energies towards that of PC61BM.
Derivatives 6,4 7,43 9,4 11,44 12,45 13,21 14 (ref. 1) and 15–16 (ref.
46) were prepared according to the published procedures.
2-(2-(3,5-Dimethyl-1H-pyrrol-2-yl)-3-hydroxy-4-oxocyclobut-
2-enylidene)malononitrile 8. A solution of the ethyl emisquarate 6
(1.55 g, 7.07 mmol), malononitrile (0.48 g, 7.27 mmol) and trie-
thylamine (2.8 ml, 15 mmol) in absolute ethanol (30 ml) was
refluxed for 4 h. The colour gradually turns from pale yellow to
4. Device assembly and testing
In order to test the capability of sq-PY to act as the photosen-
sitizer in light harvesting applications, we prepared a device
based on the classical sandwich-like structure depicted in Fig. 6,
where the squaraine is used as the donor material in bulk-het-
erojunction configuration with PC61BM as the acceptor.
A sq-PY:PC61BM 1 : 3 by weight ratio was chosen since this
particular composition gave the best performance for squaraine–
PC61BM bulk-heterojunction based active materials.27,32,33
Fig. 7 shows the device electrical characterization. In partic-
ular, Fig. 7a shows the dark currents of the device as a function
of the externally applied bias voltage. Clear rectification can be
observed, with a 103 ratio between current in forward bias
(negative voltages in the plot) and current in reverse bias (posi-
tive voltages in the plot), both evaluated at 1 V. Fig. 7b shows the
photon-to-electron conversion efficiency (External Quantum
Efficiency—EQE) of the device, reverse biased at 1 V, displayed
as a function of the illumination wavelength. The EQE curve is
always higher than 3% all over the visible spectrum (400–750
nm). This result clearly demonstrates that sq-PY can be used as
a truly panchromatic sensitizer, as all of its three absorption
bands are capable of generating a photocurrent.
Table 2 sq-CO, sq-CN and sq-PY in CH3CN with tetrabutylammonium
p-toluenesulfonate as the supporting electrolyte. Electrochemical HOMO
and LUMO levels
Compound
E
½
(1)/V
Epcred/V
HOMO
LUMO
sq-CO
sq-CN
sq-PY
P3HT
PC61BM42
0.11
0.22
0.15
—
ꢀ1.50
ꢀ1.19
ꢀ1.32
—
ꢀ5.31
ꢀ5.42
ꢀ5.35
ꢀ5.00
ꢀ5.93
ꢀ3.70
ꢀ4.01
ꢀ3.88
ꢀ3.30
ꢀ3.91
Fig. 7 (a) I–V characteristics under dark conditions and (b) External
Quantum Efficiency as a function of wavelength for the device based on
a 1 : 3 mixture of squaraine sq-PY and PC61BM.
—
—
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dark yellow-brown. The homogeneous dark solution was poured
into 100 ml of 5% HCl, to give a deep yellow clear solution. After
standing overnight at room temperature, a thick red precipitate
was formed. Suction filtration gave the title compound as red
needles (1.06 g, 4.43 mmol, 63% yield). Mp 140–142 ꢁC (dec); 1H
NMR (DMSO-d6) d: 9.98 (1H, s), 7.19 (s, broad), 5.78 (1H, s),
2.27 (3H, s), 2.16 (3H, s). 13C d: 193.4, 186.0, 176.6, 162.5, 134.3,
125.9, 120.5, 119.3, 118.6, 111.7, 13.5 (2C). Analysis calcd for
C13H9N3O2: C, 65.27; H, 3.79; N, 17.56; calcd for
C13H9N3O2$1H2O: C, 60.70; H, 4.31; N, 16.33; found: C, 60.92;
H, 4.40; N, 16.50%.
wax. The product was dissolved in 20 ml of n-hexane and cooled
at ꢀ78 ꢁC. The purple precipitate thus formed was filtered in
a cold Hirsh funnel and dried under vacuum (0.04 g, 0.07 mmol,
1
30% yield). Mp 90 ꢁC; H-NMR (acetone-d6) d: 8.30 (2H, d,
J ¼ 8.54), 7.61 (1H, d, J ¼ 7.43), 7.48 (1H, d, J ¼ 8.28), 7.46 (1H,
t, J ¼ 8.01), 7.39 (2H, t, J ¼ 8.01), 7.34 (1H, t, J ¼ 7.20), 7.11 (1H,
t, J ¼ 8.33), 6.19 (1H, s), 6.1–6.0 (1H, s-broad), 4.38 (2H, t,
J ¼ 8.00), 2.68 (3H, s), 2.41 (3H, s), 2.26 (3H, s), 1.87 (2H, q,
J ¼ 7.50), 1.70 (6H, s), 1.46 (2H, q, J ¼ 7.50), 1.36–1.25 (4H, m),
0.84 (3H, t, J ¼ 7.00). 13C-NMR (acetone-d6) d: 176.3, 175.0,
173.2, 164.0, 163.0, 161.7, 148.7, 146.3, 142.2, 142.0, 140.9, 128.3,
128.2, 125.5, 123.1, 122.6, 122.4, 118.8, 118.0, 111.7, 108.3, 95.2,
91.5, 50.3, 45.0, 31.2, 27.3, 26.2, 25.6, 22.3, 17.0, 13.8, 13.4. Anal
calcd for: C37H40N4O2: C, 77.59; H, 7.04; N, 9.78; calcd for
C37H40N4O2$½H2O: C, 76.39; H, 7.10; N, 9.63; found: C, 76.61;
H, 7.27; N, 9.27%.
(E)-4-(2-(3,5-Dimethyl-1H-pyrrol-2-yl)-3-hydroxy-4-oxocy-
clobut-2-enylidene)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-one 10.
A solution of the ethyl emisquarate 6 (1.53 g, 7.00 mmol), the
pyrazolone 7 (1.26 g, 7.23 mmol) and triethylamine (2.7 ml, 14
mmol) in absolute ethanol (20 ml) was refluxed for 3 h. The
colour gradually turns from pale yellow to dark yellow-brown.
The homogeneous dark solution was poured into 300 ml of 5%
HCl, to give a fine yellow precipitate. Suction filtration gave the
title compound as a yellow powder that was dried in the vacuum
at 50 ꢁC till constant weight (1.94 g, 5.58 mmol, 80% yield). Mp
180–182 ꢁC (dec); 1H NMR (DMSO-d6) d: 7.76 (2H, d, J ¼ 7.50),
7.55 (2H, t, J ¼ 8.00), 7.37 (1H, t, J ¼ 7.50), 5.98 (1H, s), 2.56
(3H, s), 2.48 (3H, s), 2.26 (3H, s). 13C d: 193.8, 193.1, 173.7, 168.9,
160.1, 148.7, 141.3, 135.9, 133.8, 129.8, 127.3, 122.2, 119.7, 114.9,
97.2, 14.4, 14.2, 13.9. Analysis calcd for C20H17N3O3: C, 69.15;
H, 4.93; N, 12.10; calcd for C20H17N3O3$½H2O: C, 67.40; H,
5.09; N, 11.79; found: C, 67.00; H, 4.96; N, 11.77%.
Details on electrochemical characterization. Squaraines were
dissolved (concentration about 10ꢀ4 M) in the supporting elec-
trolyte that was a 0.1 M solution of tetrabutylammonium hex-
afluorophosphate (Fluka, electrochemical grade, $99.0%) in
anhydrous acetonitrile (Aldrich, 99.8%). Differential Pulsed
Voltammetry (DPV) and Cyclic Voltammetry (CV) were carried
out at scan rate of 20 and 50 mV sꢀ1, respectively, using
a PARSTA2273 potentiostat in a single chamber three electrodes
electrochemical cell in a glove box filled with argon ([O2] #
1 ppm). The working, counter, and the pseudo-reference elec-
trodes were a Au pin, a Pt flag and a Ag/AgCl wire, respectively.
The Ag/AgCl pseudo-reference electrode was externally cali-
brated by adding ferrocene (1 mM) to the electrolyte.
Squaraine sq-CN. A suspension of the emisquaraine 8 (0.544 g,
1.90 mmol), the benzothiazolium salt 11 (0.710 g, 1.97 mmol) and
imidazole (0.272 g, 4.00 mmol) in a 1 : 1 n-BuOH:toluene
mixture (20 ml) was refluxed under a Dean–Stark trap. The
colour gradually turns from yellow to deep green. After 6 h at the
reflux temperature, the deeply coloured mixture was cooled at
room temperature and a shiny purple precipitate was formed.
Suction filtration gave a purple powder that was taken up with
5 ml of ethanol, stirred at rt for 24 h and filtered again to give the
title compound as aꢁdark precipitate (0.220 g, 0.48 mmol, 25%
Details on device preparation and characterization
Device
structure:
ITO-PEDOT:PSS-SQ:PCBM-Al.
PEDOT:PSS (Clevios P VP AI 4083) was spin-coated from
aqueous solution at 2000 rpm onto clean glass-ITO substrates
pre-treated with oxygen plasma. Following the deposition,
a thermal treatment at 100 ꢁC for 15 minutes under nitrogen was
applied. The sq-PY:PC61BM 1 : 3 by weight blend was dissolved
in chloroform (19.2 mg mlꢀ1) and spin-coated at 100 rpm in
a glovebox for 1 minute (followed by 1 minute at 1000 rpm). Al
was evaporated at 10ꢀ6 mbar. Device active area: 4 mm2.
All electrical measurements were performed in vacuum (below
10ꢀ6 mbar).
1
yield). Mp 215–217 C. H NMR (DMSO-d6) d: 10.36 (1H, s),
8.26 (1H, d, J ¼ 8.00), 8.06 (1H, d, J ¼ 8.50), 7.71 (1H, t,
J ¼ 8.50), 7.59 (1H, t, J ¼ 8.00), 6.76 (1H, s), 5.93 (1H, s), 4.49
(2H, t, J ¼ 8.00), 2.37 (3H, s), 2.22 (3H, s), 1.81 (2H, q, J ¼ 8.00),
1.44 (2H, q, J ¼ 8.00), 1.32–1.28 (4H, m), 0.87 (3H, t, J ¼ 7.00).
13C d: 206.9, 173.1, 167.8, 165.9, 161.8, 150.7, 140.9, 138.5, 130.7,
129.5, 129.1, 127.3, 124.3, 120.1, 119.6, 119.2, 115.7, 113.5, 92.2,
48.5, 31.2, 31.1, 28.3, 26.1, 22.4, 14.2, 13.7, 13.6. Anal calcd for
C27H26N4OS: C, 71.34; H, 5.76; N, 12.32; S, 7.05; found: C,
71.84; H, 5.75; N, 12.65; S, 7.20%.
Dark currents were recorded through a Keithley SCS4200. For
the quantum efficiency measurements, the device was reverse
biased at 1 V and irradiated by means of a set of light emitting
diodes (LEDs) providing 500 mss long light pulses. The resulting
photocurrent was read by connecting the device to a low-noise
transimpedance amplifier. The EQE was calculated as the ratio
of the number of collected charges, determined by integrating the
transient output photocurrent during the radiation pulse, and the
number of incident photons, obtained by calibrating the LEDs
(giving a light intensity of about 30 mW cmꢀ2) by means of
a silicon photodetector.
Squaraine sq-PY. A suspension of the emisquaraine 10 (0.08 g,
0.23 mmol), derivative 12 (0.09 g, 0.24 mmol) and imidazole
i
(0.017 g, 0.25 mmol) was refluxed in a 1 : 1 PrOH:toluene
mixture (20 ml) under a Dean–Stark trap. The colour slowly
turns from deep yellow to dark green. After 20 h at the reflux
temperature the solvent was removed under reduced pressure
and the residue was purified by chromatography (SiO2,
AcOEt:CH2Cl2 1 : 5) to give the pure title compound as a purple
Conclusions
We successfully designed and synthesized core substituted
squaraine compounds possessing a panchromatic absorption.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 6704–6710 | 6709

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20 D. Ramaiah, I. Eckert, K. T. Arun, L. Weidenfeller and B. Epe,
Photochem. Photobiol., 2004, 79, 99–104.
The electrochemical characterization shows that core substitution
is an efficient strategy for the tuning of the HOMO and LUMO
levels position in squaraine compounds. Dealing with optical
properties, core substitution leads to the formation of a high-
energy band within a region where standard squaraine sensitizers
do not absorb. In the case of sq-PY both absorption and photo-
current generation are efficient all over the visible spectrum.
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This paper is dedicated to the memory of Dr Miriam Ferrari. We
acknowledge INSTM (PRISMA2007) and MURST (FIRB
2003: Molecular compounds and hybrid nanostructured mate-
rials with resonant and non-resonant optical properties for
photonic devices) for financial support.
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6710 | J. Mater. Chem., 2012, 22, 6704–6710
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