N,N-Di aryl anilinosquaraines and their application to organic photovoltaics

Article abstract of DOI:10.1021/cm2020803

We report new derivatives of symmetric squaraine dyes with N,N-diarylanilino substituents that have high solubility and high absorptivity (ε = 0.71-4.1 ×10⁵ M^-1cm^-1) in the red solar spectral region (λ_max = 645-694 nm) making them promising candidates for application in organic photovoltaics (OPVs). Unsymmetrical N,N-diisobutylanilino- and N,N-diphenylanilino(diphenylamino) squaraines have also been prepared that give blue-shifted absorption spectra (λ_max = 529-535 nm) relative to their symmetric counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both symmetrical and unsymmetrical N,N-diarylanilino squaraines show markedly broader absorption bands in solution than their N,N-dialkylanilino squaraine counterparts: the full width at half-maximum (fwhm) for N,N-diarylanilino squaraines range from 1280-1980 cm^-1, while the fwhm value for the N,N-diisobutylanilino squarine is only 630 cm^-1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to 2500-3300 cm^-1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanilino)squaraine (φ_PL = 0.02-0.66 and 0.80, respectively). OPVs were prepared with solution processed squaraine layers using the following structure: ITO/squaraine (66-85 A)/C₆₀ (400 A)/BCP (100 A)/Al (1000 A), BCP = bathocuproine. Devices using thin films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepared with bis(N,N-dialkylanilino) squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage V_oc = 0.59 ± 0.05 V, short-circuit current J_sc = 5.58 ± 0.16 mA/cm², fill factor FF = 0.56 ± 0.03, and power conversion efficiency η = 1.8 ± 0.2% under 1 sun, AM1.5G simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: V _oc = 0.82 ± 0.02 V, J_sc = 6.71 ± 0.10 mA/cm², FF = 0.59 ± 0.01, and η = 3.2 ± 0.1%. Morphological studies of thin films suggest that the solubility of bis(N,N-diarylanilino)squaraines plays an important role in controlling the optoelectronic properties of the OPVs.

Full text of DOI:10.1021/cm2020803

ARTICLE
pubs.acs.org/cm
N,N-Diarylanilinosquaraines and Their Application to Organic
Photovoltaics
Siyi Wang,† Lincoln Hall,† Vyacheslav V. Diev,† Ralf Haiges,† Guodan Wei,‡ Xin Xiao,‡ Peter I. Djurovich,†
Stephen R. Forrest,‡,§ and Mark E. Thompson*,†
†Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States
‡Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States
§Departments of Electrical Engineering and Computer Science and Physics, University of Michigan, Ann Arbor, Michigan 48109,
United States
S Supporting Information
b
ABSTRACT:
We report new derivatives of symmetric squaraine dyes with N,N-diarylanilino substituents that have high solubility and high
absorptivity (ε = 0.71ꢀ4.1 ꢁ 105 Mꢀ1 cmꢀ1) in the red solar spectral region (λmax = 645ꢀ694 nm) making them promising
candidates for application in organic photovoltaics (OPVs). Unsymmetrical N,N-diisobutylanilino- and N,N-diphenylanilino-
(diphenylamino)squaraines have also been prepared that give blue-shifted absorption spectra (λmax = 529ꢀ535 nm) relative to their
symmetric counterparts. Compared to bis(N,N-diisobutylanilino)squaraine, both symmetrical and unsymmetrical N,N-diarylani-
lino squaraines show markedly broader absorption bands in solution than their N,N-dialkylanilino squaraine counterparts: the full
width at half-maximum (fwhm) for N,N-diarylanilino squaraines range from 1280ꢀ1980 cmꢀ1, while the fwhm value for the N,N-
diisobutylanilino squarine is only 630 cmꢀ1. The absorption bands for thin films of N,N-diarylanilino squaraines broaden further to
2500ꢀ3300 cmꢀ1. N,N-Diarylanilino squaraines are fluorescent, albeit with lower quantum yields than bis(N,N-diisobutylanili-
no)squaraine (ϕPL = 0.02ꢀ0.66 and 0.80, respectively). OPVs were prepared with solution processed squaraine layers using the
following structure: ITO/squaraine (66ꢀ85 Å)/C60 (400 Å)/BCP (100 Å)/Al (1000 Å), BCP = bathocuproine. Devices using thin
films of the bis(N,N-diarylanilino)squaraines as donor layers show improved performance relative to OPVs prepared with bis(N,N-
dialkylanilino)squaraines, i.e. bis(N,N-diisobutylanilino)squaraine: open-circuit voltage Voc = 0.59 ( 0.05 V, short-circuit current
Jsc = 5.58 ( 0.16 mA/cm2, fill factor FF = 0.56 ( 0.03, and power conversion efficiency η = 1.8 ( 0.2% under 1 sun, AM1.5G
simulated illumination, compared with bis(N,N-diphenylanilino)squaraine: Voc = 0.82 ( 0.02 V, Jsc = 6.71 ( 0.10 mA/cm2, FF =
0.59 ( 0.01, and η = 3.2 ( 0.1%. Morphological studies of thin films suggest that the solubility of bis(N,N-diarylanilino)squaraines
plays an important role in controlling the optoelectronic properties of the OPVs.
KEYWORDS: squaraine dyes, organic photovoltaic, solar cell,
’ INTRODUCTION
5.5%.22 However, there are several drawbacks associated with
the bis(N,N-dialkylanilino)squaraines that limit their perfor-
mance in OPVs: poor solubility unless substituted with large
alkyl groups, absorption bands mainly in the red and NIR
requiring other materials to achieve broadband response, and
Squaraine dyes give rise to intense absorption in the red
(600ꢀ700 nm) and near-infrared (NIR) spectral regions,1,2
making them excellent candidates for a range of potential appli-
cations such as xerographic photoreceptors,3ꢀ5 optical record-
ing,6 nonlinear optics,7 photodynamic therapy,8,9 bioimaging,10
and solar energy conversion.11ꢀ18 In previous work we have used
bis(N,N-dialkylanilino)squaraines as a donor material in organic
photovoltaics (OPVs)19ꢀ21 achieving efficiencies as high as
Received: July 20, 2011
Revised:
September 26, 2011
Published: October 18, 2011
r
2011 American Chemical Society
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|

Chemistry of Materials
ARTICLE
low charge carrier mobility. The presence of N,N-dialkyl groups,
while necessary to produce soluble squaraines, can hinder charge
carrier transport.14,23 Fortunately, incorporation of N-aryl
groups in alkylamines has been shown to improve the charge
and exciton transport properties in films due to enhanced
intermolecular πꢀπ overlap.24,25 By analogy, replacing the
N-alkyl groups of anilinosquaraines with N-aryl moieties should
improve both charge and exciton transport and, moreover, allow
for tuning of the absorption profile and the molecular frontier
orbital energy levels, which can result in tuning of the open circuit
voltage (Voc). To our knowledge there have not been any
reports to date of simple anilinosquaraines appended with
N-aryl substituents. Here, we demonstrate a facile synthesis of
both symmetric bis(N,N-diarylanilino)squaraines and unsym-
metrical anilino(diphenylamino)squaraines that are soluble and
have tunable absorption profiles and orbital energetics. Single
crystal X-ray analysis of one derivative, bis(N,N-diphenylanili-
no)squaraine 3, shows a π-stacking packing arrangement that is
conducive for intermolecular charge transport. Relative to N-
alkylanilino squaraines, the symmetric N-aryl analogs show sig-
nificant differences in their absorption and emission properties.
Bilayer photovoltaic devices fabricated using the symmetric
anilinosquaraines as solution-processed donors indicate that
the N-aryl substituents can improve the open-circuit voltage,
charge transport, and overall performance of OPVs.
and a Thermo VG Scientific CLAM4MCD electron energy analyzer
with a pass function full width at half max of 0.16 eV. Samples were
biased at ꢀ9 V to ensure collection of the lowest kinetic energy electrons
in order to accurately determine the photoemission cutoff and the
vacuum level at the sample. The HOMO level was determined by the
intersection of the background with a linear fit to the rising edge of the
shallowest binding energy states.
X-ray Crystallography. A metallic green prism-like crystal of 1
obtained from CH2Cl2/MeOH was used for the X-ray crystallographic
analysis. The X-ray intensity data were measured on a Bruker SMART
APEX diffractometer equipped with an APEX CCD system and a Mo
sealed-tube fine-focus source (λ = 0.71073 Å). A total of 1850 frames
were collected over a total exposure time of 10.3 h. The frames were
integrated with the Bruker SAINT V6.02A software package using a
narrow-frame algorithm. The calculated minimum and maximum trans-
mission coefficients (based on crystal size) are 0.782 and 0.983. The
X-ray crystallographic analysis for 3 was performed on a metallic green-
blue plate-like specimen obtained from toluene. The X-ray intensity
data were measured on a Bruker SMART APEX DUO diffractometer
equipped with a APEX II CCD system and a Cu ImuS microfocus source
(λ = 1.54178 Å). A total of 2764 frames were collected over a total
exposure time of 69.10 h. The frames were integrated with the Bruker
SAINT V7.68A software package using a narrow-frame algorithm. The
calculated minimum and maximum transmission coefficients (based on
crystal size) are 0.6087 and 0.7516. Both structures were solved and
refined using the Bruker SHELXTL V6.12 software package. Additional
refinement details and the resulting factors for 1 and 3 are given in
Table 1 and the Supporting Information.
’ EXPERIMENTAL SECTION
Device Fabrication. Photovoltaic cells were grown on ITO-coated
glass substrates that were solvent cleaned and treated in UV-ozone for 10
min immediately prior to loading into a high vacuum (base pressure
∼3 ꢁ 10ꢀ6 Torr) chamber. The C60 (MTR limited) and 2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP) (Aldrich) were purified by
thermal gradient sublimation in vacuum prior to use. The metal cathode
material, Al (99.999% pure, Alfa Aesar), was used as received. The
squaraine films were spin-cast in air at 3000 rpm for 40 s on precleaned
ITO substrates from 1 mg/mL chloroform solutions. The resultant films
had the following thickness: SQ 1 85 Å ((5 Å), SQ 3 85 Å ((5 Å), SQ 4
85 Å ((5 Å), and SQ 5 66 Å ((1 Å). The squaraine films were
transferred to a high vacuum chamber for deposition of C60 and BCP by
vacuum thermal evaporation at the following rates: C60 (2 Å/s) and
metals: 1000 Å thick Al (3 Å/s). The cathode was evaporated through a
shadow mask with an array of 1 mm diameter openings.
Currentꢀvoltage measurements were performed in ambient using a
Keithley 2420 SourceMeter in the dark and under corrected 1 kWmꢀ2
white light illumination from a 300 W Xe arc lamp (Newport Oriel Product
Line) equipped with an AM 1.5G filter. The currentꢀvoltage character-
istics are reported from the average performance of several contacts made
from individual devices. Spectral mismatch correction was performed as
described in the literature,26ꢀ28 using a Si photodiode (Hamamatsu
S1787-04; 8RA filtered or S1787-12; KG5 filtered), calibrated at the
National Renewable Energy Laboratory (NREL), and frequency modu-
lated illumination (250 Hz) from a Xe source coupled to a Cornerstone
260 1/4 M monochromator (Newport 74125) in conjunction with an
EG&G 7220 DSP Lock-In amplifier. This monochromatic system was also
used to collect all external quantum efficiency data.
Instrumentation. General. NMR spectra were recorded on Varian
Mercury 500 MHz spectrometers at room temperature and 60 °C. Mass
spectral analysis was performed on a Shimadzu Prominance-LCMS
2020 equipped with a column oven (T = 40 °C), a PDA photodetector
(200ꢀ800 nm), and an MS spectrometer (LCMS 2020; m/z range:
0ꢀ2000; ionization modes: ESI/APCI). Thermogravimetric analyses
were performed using a thermogravimetric analyzer TA Q50 (TA
Instruments) under a high-purity air flow (80 mL min‑1). Samples were
heated from 30 to 600 °C at a linear rate of 10 °C minꢀ1. UVꢀvisible
spectra were recorded on a Hewlett-Packard 4853 diode array spectro-
photometer. Steady-state emission experiments at room temperature
were performed using a Photon Technology International Quanta
Master Model C-60SE spectrofluorimeter. Quantum efficiency mea-
surements were carried out using a Hamamatsu C9920 system equipped
with a xenon lamp, calibrated integrating sphere, and model C10027
photonic multichannel analyzer.
Electrochemistry. Cyclic voltammetry (CV) and differential pulse
voltammetry (DPV) were performed using an EG&G potentiostat/
galvanostat model 263 under N2 atmosphere. Anhydrous DCM was used
as solvent for the scan from ꢀ2 to 2 V and 1.0 M tetrabutylammonium
hexafluorophosphate (TBAH) was used as the supporting electrolyte. A
glassy carbon rod was used as a working electrode, and a silver wire was
used as a pseudoreference electrode. Electrochemical reversibility and
redox potentials were determined using CV and DPV, respectively. The
redox potentials were calculated relative to an internal ferrocenium/
ferrocene (Fc+/Fc) reference.
Ultraviolet Photoemission Spectroscopy Measurements.
The highest occupied molecular orbital (HOMO) energy of squaraines
3, 4, 5, 9, and 10 were measured by ultraviolet photoemission spectros-
copy (UPS). Thin films of squaraines (20 Å) were spin-cast on the top of
ITO coated glass substrates in an inert N2 environment (H2O < 1 ppm,
O2 < 5 ppm). The samples were then transferred in a sealed N2-filled
case to the ultrahigh vacuum analysis chamber (base pressure <3 ꢁ 109
Torr) without exposure to air. UPS data of each squaraine sample were
collected using the 21.22 eV He(I) emission from a gas discharge lamp
Synthesis. All chemicals, reagents, and solvents were used as
received from commercial sources without further purification. All
glassware was oven-dried, and all reactions were performed under
N2. Compound 1 was made using the procedure described in the
literature.29
Symmetric Squaraines, 2ꢀ8. Compounds 2ꢀ8 were all made by
a similar synthetic procedure. The synthesis of 3 is detailed here and was
followed for the preparation of the other symmetric anilino-squaraines.
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Chemistry of Materials
ARTICLE
Table 1. Summary of Crystal Data, Data Collection, and Refinement Parameters for 1 and 3
compound
1
3
empirical formula
C
32H44N2O6
C40H28N2O6
formula weight
temperature
552.69
632.64
143(2) K
0.71073 Å
monoclinic
P 2(1)/n
100(2) K
1.54178 Å
triclinic
P-1
radiation wavelength
crystal system
space group
unit cell dimensions
a = 6.2034(16) Å
α = 90°
a = 11.1895(9) Å
α = 96.371(8)°
b = 11.7874(11) Å
β = 91.176(7)°
c = 11.9792(9) Å
γ = 92.485(7)°
1568.3(2) Å3
b = 16.478(4) Å
β = 92.406(4)°
c = 14.518(4) Å
γ = 90°
volume
1482.7(6) Å3
Z
2
2
density (calculated)
absorption coefficient
F(000)
1.238 Mg/m3
0.085 mmꢀ1
1.340 Mg/m3
0.738 mmꢀ1
660
596
crystal size
0.33 ꢁ 0.09 ꢁ 0.05 mm3
1.87 to 27.52°
ꢀ8 e h e 8
ꢀ21 e k e 20
ꢀ18 e l e 18
12527
0.10 ꢁ 0.09 ꢁ 0.02 mm3
3.71 to 59.29°
ꢀ12 e h e 12
ꢀ13 e k e 11
ꢀ13 e l e 13
14554
theta range for data collection
index ranges
reflections collected
independent reflections
completeness to theta = 27.52°
absorption correction
transmission factors
refinement method
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
wR2 = 0.1021
3349 [R(int) = 0.0508]
98.4%
4202 [R(int) = 0.0753]
99.0%
semiempirical
min/max 0.796
full-matrix least-squares on F2
3349/0/193
multiscan
min/max: 0.810
4202/6/437
1.084
1.014
R1 = 0.0477
R1 = 0.0819
wR2 = 0.2240
R indices (all data)
wR2 = 0.1147
R1 = 0.0890
R1 = 0.1154
wR2 = 0.2434
largest diff. peak and hole
0.218 and ꢀ0.184 e.Åꢀ3
0.619 and ꢀ0.322 e.Åꢀ3
The first synthesis given is for the preparation of N-(3,5-dihydroxyphe-
nyl)diphenylamine, and the second the reaction of this material with
squaric acid to form the N,N-diphenylanilino squaraine dye. The
characterization data for 2ꢀ8 are given below.
was then decanted to 30 mL of ice water, and after 30 min of hydrolysis
the organic layer was extracted twice with 100 mL of CH2Cl2. The
organic layer was washed twice with 100 mL of cold water to neutralize
any excess of BBr3. The solvent was removed under reduced pressure,
and the resulting solid was further purified on silica gel column chroma-
tography using hexane and ethyl acetate (6:1) to obtain 1.6 g (88%) of
N-(3,5-dihydroxyphenyl)diphenylamine.
N-(3,5-Dihydroxyphenyl)diphenylamine. Diphenylamine (3 g, 17.7
mmol), 1-bromo-3,5-dimethoxybenzene (4.6 g, 21 mmol), Pd2(dba)3
[tris(dibenzylidineacetone)dipalladium(0)] (0.78 g, 0.8 mmol), sodium
tert-butoxide (2.6 g, 25 mmol), and tri-tert-butylphosphine (0.52 g, 2.5
mmol) were dissolved in 100 mL of toluene and refluxed under N2 for
5 h. The reaction mixture was cooled down and passed through a plug of
silica gel with toluene to remove insoluble material. The crude product
was recrystallized from toluene/MeOH to obtain 4.5 g (83%) of N-(3,5-
dimethoxyphenyl)diphenylamine. Next, this triarylamine (2.0 g, 6.5
mmol) was added to 70 mL of anhydrous CH2Cl2 in a 250 mL
round-bottom flask equipped with a stir bar. Boron tribromide
(65 mL of 1 M solution in CH2Cl2, 65 mmol) was added, and the
solution was stirred under room temperature for 2 days. The solution
3. 2-[4-(N,N-Diphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-
diphenyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxocyclobut-
1-en-1-olate. A mixture of N-(3,5-dihydroxyphenyl)diphenylamine
(1 g, 3.6 mmol) and excess squaric acid (0.25 g, 2.2 mmol) in 1-butanol
(40 mL) and toluene (120 mL) were refluxed under N2 overnight. After
reaction was cooled down, the solvents were removed under reduced
pressure using a rotary evaporator leaving behind a green solid.
The crude solid was recrystallized from a 1:1 volume ratio of
dichloromethane methanol mixture to afford metallic green crystals of
1
3 (0.70 g, 61%). H NMR (CDCl3, 500 MHz): 11.00 (s, 4H), 7.41
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Chemistry of Materials
ARTICLE
(t, 8H, J = 7.5 Hz), 7.29 (t, 4H, J = 7 Hz), 7.23 (d, 8H, J = 8 Hz), 5.87
(s, 4H). 13C NMR (CDCl3, 125 MHz): 181.36, 163.50, 163.06, 159.51,
144.08, 129.81, 127.57, 127.05, 104.96, 98.75. MS (ESI+): m/z 633.3
126.71, 126.62, 126.09, 125.88, 125.84, 124.73, 122.07, 105.28, 98.79.
MS (ESI+): m/z 881.5 (M+); calcd: 884.97 amu. Elemental analysis for
C60H40N2O6 CH2Cl2 MeOH: calcd: C 80.16, H 4.52, N 3.06; found:
3
3
(MH+); calcd: 632.66 amu. Elemental analysis for C40H28N2O6
C 80.10, H 3.81, N 3.11.
3
CH2Cl2 MeOH: calcd: C 74.42, H 4.42, N 4.23; found: C 74.58, H
4.31, N 4.35.
Unsymmetrical Squaraines, 9 and 10. The synthesis of 9 and
10 were carried out by similar procedures. The first step involved the
synthesis of diphenylaminosquarate, followed its reaction with either
N-(3,5-dihydroxyphenyl)diisobutylamine or N-(3,5-dihydroxyphenyl)-
diphenylamine to give 9 or 10.
3
2. 2-[4-(N-Phenyl-N-methylamino)-2,6-dihydroxyphenyl]-4-[(4-(N-
phenyl-N-methyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-
oxocyclobut-1-en-1-olate. Yield = 17%. 1H NMR (CDCl3, 500 MHz,
60 °C): 10.95 (s, 4H), 7.44 (t, 4H, J = 8 Hz), 7.33 (t, 2H, J = 7.5 Hz), 7.19
(d, 4H, J = 8.5 Hz), 5.77 (s, 4H), 3.41 (s, 6H). 13C NMR (CDCl3, 125
MHz, 60 °C): 184.45, 163.25, 158.38, 144.08, 130.09, 127.68, 127.03,
104.96, 95.49, 86.48, 53.29. MS (ESI+): m/z 508.25 (M+); calcd: 508.52
3-Diphenylamino-4-hydroxycyclobut-3-ene-1,2-dione (diphenylamino-
squarate).30 A solution of diphenylamine (5.0 g, 30 mmol) in 100 mL of
propan-2-ol was added to a solution of 3,4-diisopropoxycyclobut-3-ene-
1,2-dione (4.7 g, 24 mmol) in 50 mL of the same solvent. Concentrated
HCl (1 mL) was then added, and the mixture was refluxed for
approximately 3 h. The solvent was removed on a rotary evaporator,
and the residue was dissolved in CHCl3 and filtered to remove any
squaric acid. The filtrate was then pumped down, and the residue was
dissolved in 150 mL of acetone and followed by 150 mL of 6 M HCl.
This solution was refluxed for 4 h with vigorous stirring followed by
evaporation under reduced pressure to give a residue that was subse-
quently dissolved in CHCl3 and filtered to remove any further quantities
of residual squaric acid. The solvent was removed from the filtrate under
reduced pressure, and the residue repeatedly was extracted with hot
water to give green crystals of the product. This material was recrys-
tallized once more from hot water to obtain 3.7 g (59%) of dipheny-
laminosquarate. 1H NMR (CDCl3, 500 MHz, 60 °C): 7.41
(t, 2H, J = 7.5 Hz), 7.35 (t, 1H, J = 7.5 Hz), 7.16 (d, 2H, J = 7.5 Hz),
5.61 (s, 1H), 3.41(s, 3H). 13C NMR (CDCl3, 125 MHz, 60 °C): 181.36,
163.06, 159.51, 144.08, 129.81, 127.57, 104.96, 98.75.
amu. Elemental analysis for C30H24N2O6 MeOH: calcd: C 68.88, H
3
5.22, N 5.18; found: C 69.44, H 5.48, N 5.18.
4. 2-[4-(N-Phenyl-N-1-naphthylamino)-2,6-dihydroxyphenyl]-4-
[(4-(N-phenyl-N-1-naphthyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-
1
ylidene]-3-oxocyclobut-1-en-1-olate. Yield = 51%. H NMR (CDCl3,
500 MHz, 60 °C): 10.91 (s, 4H), 7.91 (d, 2H, J = 7.5 Hz), 7.88 (t, 4H, J =
7.5 Hz), 7.47ꢀ7.54 (m, 8H), 7.39 (d, 2H, J = 7 Hz), 7.30ꢀ7.37 (m, 8H),
7.22 (m, 4H), 5.78 (s, 4H). 13C NMR (CDCl3, 125 MHz, 60 °C):
181.34, 163.78, 163.24, 160.31, 144.15, 140.10, 134.99, 130.48, 129.71,
128.83, 128.69, 127.57, 127.31, 126.77, 126.74, 126.40, 126.03, 123.08,
104.87, 98.27. MS (ESI+): m/z 733.2 (MH+); calcd: 732.78 amu.
Elemental analysis for C48H32N2O6: calcd: C 78.68, H 4.40, N 3.82;
found: C 78.07, H 4.28, N 3.85.
5. 2-[4-(N-Phenyl-N-2-naphthylamino)-2,6-dihydroxyphenyl]-4-
[(4-(N-phenyl-N-2-naphthyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-
1
ylidene]-3-oxocyclobut-1-en-1-olate. Yield = 38%. H NMR (CDCl3,
9. 2-[4-(N,N-Diisobutylamino)-2,6-dihydroxyphenyl]-4-(4-diphenyli-
minio)-2,5-dien-1-ylidene}-3-oxocyclobut-1-en-1-olate. A solution of
diphenylaminosquarate (1.5 g, 5.7 mmol) and N-(3,5-dihydroxyphenyl)
diisobutylamine (1.35 g, 5.7 mmol) in 45 mL of toluene and 15 mL of
1-butanol was refluxed under N2 overnight. The resulting reaction
mixture was dried over rotary evaporator to obtain a crude red solid.
The final product was purified by elution with CH2Cl2/hexane on a silica
gel column to obtain 0.76 g (28%) of 9. 1H NMR (CDCl3, 500 MHz,
60 °C): 12.00 (s, 2H), 7.43ꢀ7.47 (m, 4H), 7.36ꢀ7.39 (m, 2H),
7.21ꢀ7.23 (m, 4H), 5.76 (s, 2H), 3.21 (d, 4H, J = 7.5 Hz), 2.12 (sep,
2H, J = 6.7 Hz), 0.90 (d, 12H, J = 6.7 Hz). 13C NMR (CDCl3, 125 MHz,
60 °C): 175.59, 163.59, 139.98, 129.17, 128.07, 125.31, 93.82, 60.25,
27.56, 20.16. MS (ESI+): m/z 485.4 (MH+); calcd: 484.58 amu.
Elemental analysis for C30H32N2O4: calcd: C 74.36, H 6.66, N 7.78;
found: C 74.33, H 6.75, N 5.80.
10. 2-[4-(N,N-Diphenylamino)-2,6-dihydroxyphenyl]-4-(4-dipheny-
liminio)-2,5-dien-1-ylidene}-3-oxocyclobut-1-en-1-olate. A solution of
diphenylaminosquarate (0.5 g, 1.9 mmol) and N-(3,5-dihydroxyphe-
nyl)diphenylamine (0.56 g, 2.0 mmol) in 45 mL of toluene and 15 mL of
1-butanol was refluxed under N2 overnight. The resulting reaction
mixture was dried under reduced pressure in a rotary evaporator to
obtain a crude red solid. The final product was purified by elution with
CH2Cl2/hexane on a silica gel column to obtain 0.12 g (11%) of 10. 1H
NMR (CDCl3, 500 MHz, 60 °C): 11.88 (s, 2H), 7.48ꢀ7.51 (m, 4H),
7.42ꢀ7.46 (m, 2H), 7.35ꢀ7.39 (m, 4H), 7.26 (m, 2H), 7.24ꢀ7.26 (m,
4H), 7.21ꢀ7.23 (m, 4H), 5.84 (s, 2H). 13C NMR (CDCl3, 125 MHz,
60 °C): 175.34, 163.41, 144.36, 139.50, 129.56, 129.12, 128.51, 127.50,
126.42, 125.28, 99.14. MS (ESI+): m/z 525.35 (MH+); calcd: 524.56
500 MHz, 60 °C): 10.99 (s, 4H), 7.86 (d, 2H, J = 8.8 Hz), 7.82ꢀ7.86 (m,
2H), 7.73ꢀ7.77 (m, 2H), 7.66 (d, 2H, J = 2 Hz), 7.48ꢀ7.52 (m, 4H),
7.40 (t, 4H, J = 7.5 Hz), 7.32 (dd, 2H, J = 8.8, 2.1 Hz), 7.28 (t, 2H, J = 7.5
Hz), 7.27 (t, 4H, J = 7.5 Hz), 5.93 (s, 4H). 13C NMR (CDCl3, 125 MHz,
60 °C): 181.59, 164.10, 163.46, 159.84, 144.44, 141.78, 134.01, 132.14,
129.90, 129.86, 127.86, 127.81, 127.70, 127.12, 126.93, 126.59, 125.86,
125.58, 105.45, 99.33. MS (ESI+): m/z 733.3 (MH+); calcd: 732.78
amu. Elemental analysis for C48H32N2O6: calcd: C 78.68, H 4.40, N
3.82; found: C 78.74, H 4.33, N 3.84.
6. 2-[4-(N,N-4,40-Biphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-
4,40-biphenyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxo-
cyclobut-1-en-1-olate. Yield = 28%. 1H NMR (CDCl3, 500 MHz,
60 °C): 11.00 (s, 4H), 7.63 (d, 8H, J = 8.5 Hz), 7.60 (d, 8H, J = 7.8
Hz), 7.45 (t, 8H, J = 7.4 Hz), 7.36 (t, 4H, J = 7.4 Hz), 7.31 (d, 8H, J = 8.5
Hz), 6.00 (s, 4H). The solubility of 6 was too poor to obtain an
acceptable 13C NMR spectrum. Elemental analysis for C64H44N2O6:
calcd: C 82.03, H 4.73, N 2.99; found: C 83.9, H 4.31, N 1.75.
7. 2-[4-(N,N-2,20-Biphenylamino)-2,6-dihydroxyphenyl]-4-[(4-(N,N-
2,20-biphenyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-ylidene]-3-oxo-
cyclobut-1-en-1-olate. Yield = 19%. 1H NMR (CDCl3, 500 MHz,
60 °C): 11.00 (s, 4H), 7.24 (m, 4H), 7.22 (t, 8H, J = 7.5 Hz),
7.08ꢀ7.14 (m, 8H), 6.97 (d, 8H, J = 7.1 Hz), 6.82 (t, 4H, J = 7.1 Hz),
6.27 (d, 4H, J = 7.8 Hz), 5.89 (s, 4H). 13C NMR (CDCl3, 125 MHz,
60 °C): 181.44, 163.11, 163.06, 140.26, 140.15, 139.62, 131.43, 129.91,
128.55, 128.25, 127.38, 127.27, 127.02, 104.76, 98.00. MS (ESI+): m/z
937.4 (MH+); calcd: 937.04 amu. Elemental analysis for C64H44N2O6:
calcd: C 82.03, H 4.73, N 2.99; found: C 81.98, H 4.70, N 3.02.
8. 2-[4-(N-Phenyl-N-1-pyrenylamino)-2,6-dihydroxyphenyl]-4-
[(4-(N-phenyl-N-1-pyrenyliminio)-2,6-dihydroxyphenyl)-2,5-dien-1-
amu. Elemental analysis for C34H24N2O4 CH2Cl2: calcd: C 76.27, H
3
1
5.19, N 4.81; found: C 76.35, H 4.49, N 5.28.
ylidene]-3-oxocyclobut-1-en-1-olate. Yield = 60%. H NMR (CDCl3,
500 MHz, 60 °C): 10.89 (s, 4H), 8.24 (d, 2H, J = 7.5 Hz), 8.20 (d, 2H, J =
7.4 Hz), 8.19 (d, 2H, J = 8.2 Hz), 8.02ꢀ8.15 (m, 10H), 7.85 (d, 2H, J =
8.1 Hz), 7.32ꢀ7.39 (m, 4H), 7.20ꢀ7.24 (m, 2H), 5.81 (s, 4H). 13C
NMR (CDCl3, 125 MHz, 60 °C): 181.52, 164.17, 163.63, 160.78,
144.70, 137.32, 131.29, 131.01, 129.79, 129.44, 128.39, 127.07, 126.99,
’ RESULTS AND DISCUSSION
Synthesis. The synthesis of bis(N,N-alkylanilino)squaraines
is typically accomplished by the reaction between squaric acid
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Scheme 1
and an N,N-dialkylaniline.29,31,32 However, a synthesis using an
amine that has two different N-aryl groups, necessary for the
formation of N,N-diarylanilino squaraines, may encounter pro-
blems with respect to the selectivity of electrophilic substitution.33
To eliminate this possibility, we chose to substitute one aryl ring
with two activating meta-hydroxyl groups in order to direct
electrophilic attack by the squarate ion on this ring. The presence
of hydroxyl groups can also serve to impart higher thermal
stability and structural rigidity to the resultant squaraines.29
The N,N-diarylamines needed for the preparation of the
squaraines were obtained from commercial sources or synthe-
sized from aniline and aryl bromides using Buchwald-Hartwig
amination.34ꢀ36 N-(3,5-Dimethoxyphenyl)-N,N-diarylamines in
turn were prepared in yields up to 90% using a Buchwald-Hartwig
reaction between 1-bromo-3,5-dimethoxybenzene and the appro-
priate diarylamines (see the Supporting Information). The meth-
oxy groups of these triarylamines were then deprotected using
BBr3 in CH2Cl2.37 Symmetric squaraines 2ꢀ8 were obtained in
yields up to 60% by refluxing two equivalents of the correspond-
ing amine and one equivalent of squaric acid in a mixture of
1-butanol and toluene for 10 h (Scheme 1). The isolation of the
squaraines was carried out by simple crystallization methods
which allowed us to obtain high purity samples in gram quantities.
The composition of 2ꢀ8 was confirmed using standard char-
Figure 1. ORTEP diagrams for 1 (top) and 3 (bottom).
The π-system of 1 is nearly planar as there is only a small dihedral
twist (<4 degrees) between the dihydroxy-phenyl ring and
squarate core, whereas 3 displays a slightly twisted conformation
with dihedral angles of ca. 10° and 13° between the dihydroxy-
phenyl rings and squarate core. The amine groups of 1 and 3 are
planar; the sum of the three CꢀNꢀC bond angles for the amine
groups of both 1 and 3 are 360°. The amine groups of 3 show
larger dihedral twists with respect to the dihydroxyphenyl group
(12.2° and 13.2°) as compared to the corresponding dihedral
angles for the amine groups of 1 (0.95°).
Packing diagrams of 1 and 3 show significant differences in the
organization of squaraine molecules in the crystal lattice
(Figure 2). Squaraine molecules in crystals of 1 are organized in
staircase slip-stacked columns arranged in herringbone-like arrays
that alternate at skewed 90° angles with respect to the molecular
long axis (Figure 2a). To describe the angular inclinations away
from the molecular plane within the stacks, it is instructive to use
“pitch” and “roll” angles, terms coined by Curtis et al.41 The pitch
angle defines the displacement with respect to the long molecular
axis, while the roll angle identifies the displacement relative to the
short molecular axis. The squaraine planes are separated by ca. 3.2
Å and have pitch and roll angles of 45° and 36°, respectively. The
small rollangle in particular leads to a displacement alongthe short
axis by more than 3 Å such that there is no π-overlap between
aromatic rings in adjacent molecules (Figure 2b). In contrast,
crystals of 3 are composed of dimers separated by ca. 3.4ꢀ3.5 Å
between the molecular planes arranged in parallel columns. The
columns of 3 are also organized in a slip-stack staircase arrange-
ment with the long molecular axis having a pitch angle of ca. 38°
(Figure 2c). However, unlike 1, a large roll angle of 80° within the
stack leads to a small displacement along the short molecular axis
(Figure 2d). While there are no significant π-interactions between
the pendant phenyl rings of adjacent squaraine molecules, the
small lateral displacement leads to considerable π-overlap between
the anilinosquaraine ring systems within the stack. The small pitch
angle leads to a large slip offset in the long molecular axis such that
the squarate core is situated above the aromatic anilino ring in
adjacent molecules. Molecular solids with large roll angles and slip
offsets, and consequentcofacialπ-stacking asfoundincrystals of3,
have been proposed to have better charge and exciton transport
properties inordered films thanmaterials withsmall rollanglesand
poor π-overlap, as observed in 1.41
1
acterization methods: H NMR, 13C NMR spectroscopy, and
elemental analyses. Thermal gravimetric analysis (TGA) was
performed on squaraines 1, 3, and 4 to assess if thermal stability
was improved by the N-aryl substituents. The squaraines were
found to undergo 5% weight loss at 308 °C (1), 318 °C (3), and
326 °C (4). The modest increase in decomposition temperature
for the N-aryl derivatives suggests that the thermal breakdown
occurs at the squarate moiety. Consequently, attempts to sublime
samples of 3 and 4 led to extensive decomposition.
Single crystal X-ray structures were obtained for crystals of 1
obtained from CH2Cl2/MeOH and 3 obtained from toluene
(Figure 1). The metrical parameters of both derivatives are similar
to values reported for related bis[N,N-di(n-butyl)anilino]-
squaraines.38,39 A relatively short arylꢀsquarate bond distance
(C2ꢀC3 = 1.402(2) Å in 1, C16ꢀC19 = 1.409(7) Å in 3) is
consistent with strong conjugation between the anilino moi-
eties and squarate core. Likewise, the short nitrogenꢀanilino
bond distances (N1ꢀC6 = 1.359(2) Å in 1, N1ꢀC13 =
1.385(6) Å in 3) demonstrate strong conjugative stabilization
imparted by the amino substituent to the squaraine, particu-
larly when compared to the considerably longer nitrogen
ꢀphenyl bonds in 3 (average = 1.435(6) Å). The average
hydroxyꢀsquarate oxygen OꢀH O bond distances (O O =
3 3 3
3 3 3
2.651 Å in 1 and 2.652 Å in 3) are within values previously
assigned tointramolecular hydrogen bonds in related squaraines.40
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Figure 2. Crystal packing diagrams for 1 (top) and 3 (bottom). View of 1 showing herringbone structure (a) and molecular stacking arrangement (b).
Stacking arrangement of 3 viewed down the short (c) and long (d) molecular axes. Hydrogen atoms were removed for clarity.
squaric acid site available for further functionalization.30,43 Sub-
Scheme 2
sequent reaction between the monoamino squarate and either
N-(3,5-dihydroxyphenyl)diisobutylamine or N-(3,5-dihydroxy-
phenyl)diphenylamine forms the corresponding squaraines 9
and 10 in low to moderate yields. The reaction is complicated by
partial decomposition of the mono adduct to reform squaric acid
and diphenylamine, leading to the formation of the correspond-
ing symmetrical squaraines 1 or 3 as impurities. However, 1 and 3
can be readily removed by fractional crystallization or column
chromatography, giving pure samples of 9 and 10, respectively.
Like squaraines 3 and 4, compounds 9 and 10 are soluble in
organic solvents (>5 mg/mL) and form high quality films when
spun cast from solution.
Good solubility is essential to prepare smooth, uniform films
in OPVs.42 Typically, bis(N,N-dialkylanilino)squaraines have
poor solubility unless modified with bulky substituents such as
the isobutyl groups in 1. Similarly, the squaraine with N-methyl-
N-phenyl groups (2) has poor solubility. In contrast, most of the
bis(N,N-diarylanilino)squaraines, in particular 3 and 4, are
soluble in common organic solvents such as toluene, dichlor-
omethane, chlorobenzene, etc. and form pinhole-free film upon
spin-casting. Interestingly, the site of aryl substitution also affects
the solubility of bis(N,N-diarylanilino) squaraines. For example,
the derivative with N,N-(1-naphthyl)phenyl substituents (4) has
one of the highest solubilities in the studied series, whereas the
isomeric squaraine with N,N-(2-naphthyl)phenyl substituents
(5) is only moderately soluble (<1 mg/mL in CH2Cl2). Likewise,
squaraine 7, which has ortho N,N-2-biphenyl substituents, is
more soluble than the para N,N-4-biphenyl analog 6.
Photophysical and Electrochemical Characterization. The
absorption propertiesof squaraines 1ꢀ10 in CH2Cl2 solution and
in neat thin film are listed in Table 2, and representative spectra
are shown for 1, 3, and 10 in Figure 3a. In solution, squaraines
1ꢀ8 display an intense (ε ≈ 105 Mꢀ1 cmꢀ1) absorption band
with λmax = 650ꢀ700 nm. Relative to the absorption maximum of
1 (λmax = 645 nm) and other N,N-dialkyl derivatives,44 N-aryl
squaraines 2ꢀ8 show red-shifted peak maxima, Table 1. In
contrast, the unsymmetrical squaraines 9 and 10 have absorption
peaks that are blue-shifted (λmax ∼ 530 nm) relative to both
N-alkylanilino and N-arylanilino based symmetric squaraines.
The absorption bands of 1 and 2 in solution display narrow
full-width half maxima (fwhm) of 630 cmꢀ1 and 890 cmꢀ1
,
While squaraines 1ꢀ8 absorb in the red, derivatives that
absorb shorter wavelengths are also desired to achieve improved
overlap with the solar spectrum. Decreasing the extent of π-
conjugation in aromatic dye systems typically causes a hypso-
chromic shift in absorption and, to that end, unsymmetrical
squaraines having only one anilino group bound to the squarate
core were prepared (Scheme 2). Addition of one equivalent of
diphenylamine to diisopropoxysquarate, followed by acid hydro-
lysis, forms a monoamino squarate adduct that has a second
respectively, while the absorption bands of 3ꢀ10 are markedly
broader, with fwhm between 1280 and 1980 cmꢀ1. The absorp-
tion spectra for all the squaraines red-shift and undergo significant
broadening in thin films (Figure 3a). The broadening is important
for OPVs since it leads to improved spectral overlap with the solar
irradiance spectrum.
Squaraines 1ꢀ10 fluoresce in the wavelength range of λmax
=
658ꢀ758 nm (Table 2), and representative excitation and emis-
sion spectra for 1 and 3 are shown in Figure 3b. The excitation
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Table 2. Photophysical Data for Squaraines 1ꢀ10 in Solution and As Thin Films
absorption
emission
toluene solution
λem (nm)
solution in CH2Cl2 or (toluene)
thin film
ε (105 Mꢀ1 cmꢀ1
)
λmax (nm)
fwhm (cmꢀ1
)
λmax (nm)
fwhm (cmꢀ1
)
Φ
1
2
3
4
5
6
7
8
9
10
4.1
1.7
1.9
2.0
1.9
0.71
2.1
1.4
1.3
1.0
652 (649)
645 (645)
674 (677)
666 (669)
687 (689)
694 (693)
678 (673)
679 (682)
529 (529)
535 (539)
630
692
a
3140
a
658
672
750
726
768
786
721
738
582
625
0.80
0.14
0.29
0.36
0.07
0.02
0.66
0.02
0.01
0.02
890
1470
1280
1590
1790
1440
1530
1570
1980
717
691
717
697
686
712
552
560
3240
3280
2770
2910
2490
2520
2840
3160
a Solubility is too poor to form a uniform film.
spectra of 1ꢀ10 match the absorption profiles, although structure
seen in the excitation spectrum of 3 suggests small variations in
the luminescent response from conformational isomers. The
photoluminescent quantum efficiency (Φ) of 1 in toluene is
0.80, whereas 2ꢀ10 are lower, ranging from 0.01 to 0.66. The
Stokes shift of the N-alkylanilino-squaraine 1 is 9 nm in toluene
and is unaffected by solvents of higher polarity. The small,
solvent-insensitive Stokes shift is comparable to behavior ob-
served for other related N,N-dialkylanilino squaraines44,45 indi-
cating that only minor structural reorganization occurs in the
excited state.46 On the other hand, the N-arylanilinosquaraines
display much larger Stokes shifts that range between 27 nm for 2
to 79 nm for 5. The large Stokes shifts observed for 2ꢀ10 suggests
that excited state formation in the N-arylanilino derivatives is
accompanied by a greater degree of intramolecular distortion than
occurs in 1.47 In addition, the Stokes shift increases with increas-
ing solvent polarity (positive solvatochromism). For example, the
Stokes shift of 3 is 52 nm in cyclohexane, 73 nm in toluene, and
112 nm in 2-methyltetrahydrofuran (2MeTHF). The solvato-
chromic response indicates that the excited state of 3 is both more
polarizable than that of 1 and strongly stabilized by polar media.
The solvent-dependent luminescence properties of squaraines
1 and 3 were examined in more detail. The quantum yield of 1 is
largely independent of solvent polarity, ranging from 0.80 in
toluene to 0.73 in acetonitrile. This behavior differs from the
trend reported for bis[4-(N,N-dimethyl)anilino]squaraine (SQ-
DMA),48 an analogue of 1 without the meta-hydroxyl groups.
The quantum yield of SQ-DMA is also high in toluene (Φ =
0.85) but decreases dramatically in acetonitrile (Φ = 0.08). The
lower quantum yield of SQ-DMA in polar media was attributed
to the formation of a twisted intramolecular charge transfer
(TICT) state that acts as a nonradiative decay channel.48,49
Rotation around the N,N-dimethylanilinoꢀsquarate bond axis
was proposed to accompany formation of the TICT state. The
weak solvent dependence in the quantum yield of 1 suggests that
an analogous TICT state is not accessible in this compound. The
difference between 1 and SQ-DMA likely arises due to hydrogen
bonds formed between the four hydroxyl groups to the squarate
oxygens in 1 that suppress twisting along the phenylꢀsquarate
bond. Like SQ-DMA, the quantum yield of 3 is strongly affected
by solvent polarity, being highest in cyclohexane (Φ = 0.55) and
Figure 3. (a) Absorption spectra for 1, 3, and 10 in CH2Cl2 solution
(open symbols) and as neat films (solid lines). (b) Excitation (ex) and
emission (em) spectra of 1 and 3 in toluene.
decreasing sharply in 2-MeTHF (Φ = 0.05). This suggests that
an alternate TICT state may be accessible in 3. Since both
squaraine 1 and 3 have equivalent hydroxyl groups, twisting of
the diphenylamino moiety with respect to the molecular core
might account for the dependence of quantum yield on the
solvent polarity. Considering that the dipolar structure of 3
should lead to a nominally nonpolar ground state, the lumines-
cent quenching behavior implies that charge separation, along
with consequent breaking of the molecular symmetry, occurs
during formation of the TICT excited state.
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Table 3. Electrochemical Redox Potentialsa and Calculated
HOMO/LUMO Energiesb for Squaraines 1ꢀ10 along with
HOMO Energies from UPS Measurements for 1, 2ꢀ5, and 10
squaraines
Eox (V)
Ered (V)
HOMO (eV)c
LUMO (eV)
1
0.49, r
0.54, q
0.62, r
0.59, q
0.58, r
0.59, q
0.60, r
0.65, q
0.59, r
0.70, r
ꢀ1.23, i
ꢀ1.25, i
ꢀ1.08, i
ꢀ1.24, i
ꢀ1.11, i
ꢀ1.12, i
ꢀ1.10, r
ꢀ0.98, q
ꢀ1.40, r
ꢀ1.45, i
5.29 (5.1)
5.36
3.31
3.29
3.49
3.30
3.45
3.45
3.47
3.61
3.11
3.05
2
3
5.47 (5.3)
5.43(5.3)
5.41 (5.3)
5.43
4
5
6
7
5.44
8
5.51
9
5.43
10
5.58 (5.4)
a Recorded in 0.1 M Bu4N+PF6ꢀ in CH2Cl2, referenced to internal Fc+/
Fc; r = reversible, q = quasi-reversible, and i = irreversible. Values for
irreversible waves are cathodic peak potentials. b HOMO50 and LUMO51
valuescalculatedfrom redoxdata using methods described in the literature.
c HOMO values from UPS measurements are given in parentheses.
Electrochemical analysis of 1ꢀ10 using both cyclic and
differential pulse voltammetry were performed in CH2Cl2 and
referenced to Fc+/Fc as an internal standard, and redox data are
listed in Table 3. Squaraines 1ꢀ8 display reversible or quasi-
reversible oxidation waves in the range 0.49ꢀ0.65 V and
irreversible reduction waves between ꢀ0.98 V and ꢀ1.25 V.
Compared to squaraine 1 (Eox = 0.49 V), the oxidation potentials
of 2ꢀ8 are 0.05 to 0.16 V lower. Unsymmetrical squaraines 9 and
10 show reversible oxidation at comparable potentials (Eox
=
0.59 and 0.70 V, respectively) but display reversible reduction at
higher potentials (Ered = ꢀ1.40 V and ꢀ1.45 V, respectively),
which indicate that the blue shift in absorption is primarily due to
an increase in the energy of the LUMO. The energy of the
HOMO for films of 1, 3, 4, 5, and 10 has also been determined
using ultraviolet photoelectron spectroscopy (UPS). The first
ionization potential of squaraine 3, 4, and 5 are the same (IP1 =
5.3 eV) and 0.2 V higher than 1 (IP1 = 5.1 eV),21 while squaraine
10 has an ionization energy of 5.4 eV.
Photovoltaic Cell Properties. The electrochemical and UPS
data suggest that the N-aryl squaraines can serve as efficient
donor materials when paired with C60 as an acceptor in OPVs.
Thus, solar cells with the structure ITO/squaraine (66ꢀ85 Å)/
C60(400 Å)/BCP(100 Å)/Al(1000 Å) were fabricated using 1, 3,
4, and 5 to study the effects of N-aryl substitution on the
optoelectronic characteristics of OPVs. The performance of
these devices using layers of the squaraines spun cast from
chloroform is shown in Figure 4a. Currentꢀdensity versus
voltage (JꢀV) characteristics were measured in the dark and
under simulated one sun (1 kW/m2) AM1.5G illumination. The
power conversion efficiencies of squaraines 3 (η = 3.2 ( 0.1%)
and 4 (η = 2.5 ( 0.1%) were significantly higher than that of
1 (η = 1.8 ( 0.1%). Figure 4b clearly shows response from
the squaraines (550ꢀ850 nm), along with that from C60
(400ꢀ550 nm), in good agreement with the absorption spectra.
The Voc for devices made using 3 and 4 are also 0.23 V higher
than that of 1, consistent with the decrease of the HOMO energy
by 0.2 V. Further, devices made using 3 have an increased short
circuit currentꢀdensity (Jsc = 6.71 ( 0.1 mA/cm2). An increase
in fill factor (FF) from 0.51 ( 0.3 for 1 to near 0.60 for 3 and 4 is
consistent with improved charge carrier transport for the latter
Figure 4. (a) Potentialꢀcurrent density and (b) external quantum
efficiency (EQE) plots, along with tabulated OPV performance char-
acteristics of squaraine donor devices with the structure ITO/squaraine
(66ꢀ85 Å)/C60 (400 Å)/BCP (100 Å)/Al. (c) AFM images of films of 4
(rms = 0.51 nm) and (d) 5 (rms = 1.61 nm) spin-cast from chloroform.
The scale bar in the top right corner is 1 μm.
materials. The role of the N-aryl substituent position is illustrated
by comparing the performance of solar cells fabricated using
isomeric squaraines 4 and 5. The device made using 5 has a lower
Voc and Jsc that cannot be explained solely on the basis of differing
photophysical and electrochemical characteristics of the two
donor molecules. Atomic force microscopy (AFM) images
show that amorphous films of 4 (Figure 4c) are smooth
(rms = 0.51 nm), whereas films cast from the less soluble 5
(Figure 4d) are rough (rms = 1.61 nm). The AFM data suggest
that the aromatic substituents in isomeric squaraines 4 and 5
strongly influence the molecular packing and aggregation in the
films. Thus, the performance of OPVs made using 4 and 5 show a
strong correlation with the differences in squaraine solubility.
While the OPV efficiencies reported here are encouraging, higher
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efficiencies can be achieved by thermally annealing the squaraine
film and adding a buffer layer, resulting in an efficiency >5%.52
(4) Law, K. Y. J. Imaging Sci. 1987, 31, 83–93.
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’ SUMMARY
New derivatives of symmetric bis(N,N-diaryl)anilinosquaraine
dyes has been prepared using a facile and readily scalable
synthesis. Unsymmetrical mono(N,N-diarylanilino)squaraines
are also reported. The absorption spectra of the mono- and
bis(N,N-diarylanilino)squaraines, which span the green, red, and
near-infrared spectral regions, provide substantial overlap with
the solar spectrum. The nature of the N-aryl ring strongly affects
the solubility of the squaraines and consequently, the morphol-
ogy of solution cast thin films. The N-aryl groups lower the
HOMO energy levels relative to squaraines with N-alkyl groups;
thus OPVs incorporating bis(N-diarylanilino)squaraines as a
donor material show a significant increase in open-circuit voltage
as well as improved charge carrier transport compared to their
unsubstituted analogs. Previously, we have found that thermal
and solvent annealing of films of 1 can lead to significantly
enhanced OPV performance.20,22 We are currently exploring
these approaches to improve the OPV performance of devices
using bis(N,N-diarylanilino)squaraines as well as preparing other
N-aryl derivatives with enhanced optoelectronic properties. The
ability to tailor the molecular and solid state optoelectronic
properties of anilinosquaraines with N-aryl substituents opens up
a new avenue for exploration to exploit the unique characteristics
of these materials.
’ ASSOCIATED CONTENT
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S
Supporting Information. Synthesis and characterization
b
1
data for N-(3,5-dihydroxyphenyl)-N,N-diarylamines, H- and
13C NMR spectra for 2ꢀ10, thermogravimetric traces for 1, 3,
and 4, absorption spectra in CH2Cl2 and thin films for squaraines
1ꢀ10 reported here (only those for 1, 3, and 10 are given in the
text), electrochemical data for 1ꢀ10, and crystallographic details
for compounds 1 and 3 are given. This material is available free of
charge via the Internet at http://pubs.acs.org.
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Corresponding Author
*E-mail: met@usc.edu.
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’ ACKNOWLEDGMENT
Funding for this research was provided by the Center for
Advanced Molecular Photovoltaics (CAMP) (award KUS-C1-
015-21) of the King Abdullah University of Science and Tech-
nology (KAUST), the US Department of Energy Frontier
Center at the University of Southern California (award DE-
SC0001011, S.W. and M.E.T.), the US Department of Energy
EERE Beyond the Horizon program (DE-FG36-08GO18022, G.
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