Boron-dibenzopyrromethene-based organic dyes for application in dye-sensitized solar cells

Article abstract of DOI:10.1039/c3ta15340a

Novel boron-dibenzopyrromethene dyes with thienyl-cyanoacrylic acid units were synthesized and characterized for application in dye-sensitized solar cells (DSSCs); the dyes feature intense absorption bands in the longer wavelength region with λ_max values of 647 nm (ε = 1.57 × 10⁵ M^-1 cm^-1) for 1, which has two anchoring units, 660 nm (ε = 1.09 × 10⁵ M^-1 cm ^-1) for regioisomer 2, and 644 nm (ε = 1.39 × 10 ⁵ M^-1 cm^-1) for 3, which has a single anchoring unit. Density functional theory (DFT) analysis revealed that these absorption properties are mainly characterized by intramolecular charge transfer from the dibenzopyrromethene core to the thienyl-cyanoacrylic acid unit that depend on both the number of anchoring groups and the position of the thienyl-cyanoacrylic acid unit on the isoindole ring. The relationship between the chemical structures and cell properties of these dyes was investigated. Although the short circuit photocurrent density (J_sc) value of a 1-loaded cell is larger than that of a 3-loaded cell, which reflects the results of the incident photon-to-charge carrier efficiency (IPCE) spectra, the 1-loaded cell has a lower open circuit voltage (V_oc) and fill factor (FF). Accordingly, overall power-to-current conversion efficiencies of 5.24 and 5.48% were obtained for a 1-loaded cell and 3-loaded cell, respectively, under 100 mW cm ^-2 AM1.5G simulated light. On the other hand, the cell containing butterfly-shaped regioisomer 2, which showed improved intramolecular charge transfer, has an overall power-to-current conversion efficiency of 6.06%; this value is the highest published for BODIPY dyes even though the sensitizer does not contain any strong donor units, such as arylamines.

Full text of DOI:10.1039/c3ta15340a

Journal of
Materials Chemistry A
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Boron–dibenzopyrromethene-based organic
dyes for application in dye-sensitized solar cells†
Cite this: J. Mater. Chem. A, 2014, 2,
5204
a
Yuji Kubo, Daichi Eguchi,a Asaki Matsumoto,a Ryuhei Nishiyabu,a
*
Hidenori Yakushiji,b Koichiro Shigakib and Masayoshi Kanekob
Novel boron–dibenzopyrromethene dyes with thienyl-cyanoacrylic acid units were synthesized and
characterized for application in dye-sensitized solar cells (DSSCs); the dyes feature intense absorption
bands in the longer wavelength region with lmax values of 647 nm (3 ¼ 1.57 ꢀ 105 Mꢁ1 cmꢁ1) for 1, which
has two anchoring units, 660 nm (3 ¼ 1.09 ꢀ 105 Mꢁ1 cmꢁ1) for regioisomer 2, and 644 nm (3 ¼ 1.39 ꢀ
105 Mꢁ1 cmꢁ1) for 3, which has a single anchoring unit. Density functional theory (DFT) analysis revealed
that these absorption properties are mainly characterized by intramolecular charge transfer from the
dibenzopyrromethene core to the thienyl-cyanoacrylic acid unit that depend on both the number of
anchoring groups and the position of the thienyl-cyanoacrylic acid unit on the isoindole ring. The
relationship between the chemical structures and cell properties of these dyes was investigated.
Although the short circuit photocurrent density (Jsc) value of a 1-loaded cell is larger than that of a
3-loaded cell, which reflects the results of the incident photon-to-charge carrier efficiency (IPCE)
spectra, the 1-loaded cell has a lower open circuit voltage (Voc) and fill factor (FF). Accordingly, overall
power-to-current conversion efficiencies of 5.24 and 5.48% were obtained for a 1-loaded cell and
3-loaded cell, respectively, under 100 mW cmꢁ2 AM1.5G simulated light. On the other hand, the cell
containing butterfly-shaped regioisomer 2, which showed improved intramolecular charge transfer, has
an overall power-to-current conversion efficiency of 6.06%; this value is the highest published for
BODIPY dyes even though the sensitizer does not contain any strong donor units, such as arylamines.
Received 21st December 2013
Accepted 23rd January 2014
DOI: 10.1039/c3ta15340a
www.rsc.org/MaterialsA
promising dye candidates3 because of their outstanding prop-
erties, which include high molecular extinction coefficients of
Introduction
absorption, high uorescence quantum yield, and excellent
photostability. Thus, they have been applied in chemosensors,4
biological labels,5 organic light-emitting diodes,6 photodynamic
therapy,7 light-harvesting arrays,8 and solar-cell devices.9
However, despite the extensive research into the development of
DSSC devices based on BODIPY sensitizers,10 the power-to-
current conversion efficiency (PCE) value is currently limited at
less than 2.46%.10f The development of a new design for a
BODIPY-based p-system remains a major challenge despite
their excellent optical properties and facile structural modi-
cation via a variety of synthetic methods.
Our ongoing effort to prepare dye materials that absorb
longer wavelengths of radiation led us to modify the structure of
BODIPY by extending the p-conjugation to provide new types of
boron–dibenzopyrromethene dyes.11 The synthetic route that
we employ enables the development of hexylthiophene-conju-
gated boron–dibenzopyrromethenes with near-infrared light
absorption bands that serve as photosensitizers in bulk het-
erojunction solar cells.12 We realized that the synthetic
improvement of such a chromophoric system could enable it to
act as a potent sensitizer in DSSC devices. Accordingly, cya-
noacrylic acid, which acts as both an anchor to the TiO2-based
Currently, in response to the substantial demand for renewable
natural energy sources, devices based on dye-sensitized solar
cells (DSSCs) are being intensively investigated as low-cost
alternatives to traditional silicon-based photovoltaic devices.1
Dye molecules play a signicant role in the performance of
DSSCs, in which sensitization of the dye via the application of
solar energy onto the mesoporous photoanode excites electrons
to the electrode. These electrons transfer to the counter elec-
trode and are then shuttled back to the oxidized dye through an
ꢁ
Iꢁ/I3 redox couple. Therefore, sophisticated molecular design
of dyes with efficient light-harvesting capabilities and good
durability is required for the fabrication of practical cell
devices.2 4,4-Diuoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)
dyes composed of boron–dipyrrin complexes have emerged as
aDepartment of Applied Chemistry, Graduate School of Urban Environmental Sciences,
Tokyo Metropolitan University, 1-1 Minami-ohsawa, Hachioji, Tokyo, 192-0397,
Japan. E-mail: yujik@tmu.ac.jp; Fax: +81-42-677-3134; Tel: +81-42-677-3134
bNippon Kayaku Co., Ltd., 31-12, Shimo 3-Chome, Kita-ku, Tokyo, 115-8588, Japan
† Electronic supplementary information (ESI) available: Synthesis of
intermediates, variable temperature 1H NMR measurements, electrochemical
properties, ATR-FT-IR spectra, and characterization data of dyes. See DOI:
10.1039/c3ta15340a
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Journal of Materials Chemistry A
electrode and an electron acceptor, was incorporated into the 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane in the
core skeleton through the p-bridge of hexylthiophene. The hexyl presence of n-BuLi; 1H NMR of 5 (300 MHz, CDCl3) d (ppm) 0.78
group at the g-position of the thiophene ring weakens the (s, 3H), 0.87 (t, 3H, J ¼ 6.40 Hz), 1.25 (s, 3H), 1.25–1.36 (m, 18H),
intramolecular interactions of the dyes. In this study, two 1.50–1.59 (m, 2H), 2.81 (t, 2H, J ¼ 7.62 Hz), 3.61 (d, 2H, J ¼ 10.89
regioisomers with two anchoring units, i.e., 1 and 2, were Hz), 3.73 (d, 2H, J ¼ 11.01 Hz), 5.59 (s, 1H), 7.04 (s, 1H), FAB MS:
designed and synthesized by introducing an anchoring group at m/z 407 [M ꢁ 1]+, 408 [M]+. Details of synthetic procedures for
the 5 or 6 position of the isoindole ring of the dye and charac- the intermediate compounds 6–14 are described in the ESI.†
terized (Fig. 1). Further, during synthesis of 1, dye 3, which has a
single anchoring unit, was unexpectedly isolated (vide infra,
Scheme 1). The characterization of 3 as a sensitizer is important
Synthesis
to understand the DSSC performance of a 1-loaded cell. As
Diuoro[5-(3-hexyl-5-(2-carboxy-2-cyanovinyl)thiophen-2-yl)-
described below in detail, we discovered that both the number 1-[[5-(3-hexyl-5-(2-carboxy-2-cyanovinyl)thiophen-2-yl)-3-(2-
of anchoring groups and the position of the anchoring group methoxyphenyl)-2H-isoindole-1-yl]methylene]-3-(2-methoxy-
impact the cell properties. Accordingly, the DSSC device con- phenyl)-1H-isoindolate-N1,N2]boron (1) and diuoro[5-(3-hexyl-
taining buttery-shaped
performance.
2
has improved photovoltaic 5-(2-carboxy-2-cyanovinyl)thiophen-2-yl)-1-[[5-(3-hexyl-5-formyl-
thiophen-2-yl)-3-(2-methoxyphenyl)-2H-isoindole-l-yl]methylene]-
3-(2-methoxyphenyl)-1H-isoindolate-N1,N2]boron (3). To
a
solution of 7 (501 mg, 0.561 mmol) and piperidine (0.28 mL,
2.83 mmol) in THF (14 mL) was added a solution of 2-cyano-
acetic acid (242 mg, 2.84 mmol) and piperidine (0.28 mL, 2.83
mmol) in THF (14 mL). The resulting solution was reuxed for
5 h and then poured into H2O (100 mL). Aer extraction with
CH2Cl2 (100 mL), the organic layer [A] was washed with brine
(100 mL) and water (100 mL). Such a treatment led to precipi-
tation of a green solid being ltered off. Aer dissolving the
material in CH2Cl2 (400 mL), the resulting solution was washed
with 1% HCl aq. (100 mL) and was evaporated. In this way,
246 mg of 1 was obtained in 43% yield. On the other hand, the
ltrate of the organic layer [A] was dried with Na2SO4 and was
evaporated. The residue was chromatographed on silica gel
(Wacogel C-300) using a gradient of EtOH (0–10% v/v) in CH2Cl2
Experimental section
General
NMR spectra were taken by a JEOL JNM-ECS 300 (1H: 300 MHz)
or a Bruker Avance 500 (1H: 500 MHz, 13C: 125 MHz, 19F: 470
MHz) spectrometer. In 1H and 13C NMR measurements,
chemical shis (d) are reported downeld from the initial
standard Me4Si. While in 19F NMR measurement, hexa-
uorobenzene was used as an internal standard (ꢁ164.9 ppm).
In addition, fast atom bombardment (FAB) mass spectra were
obtained on a JEOL JMS-700 spectrometer where m-nitrobenzyl
alcohol was used as a matrix. The absorption spectra were
measured using a Shimadzu UV-3600 spectrophotometer.
Elemental analyses were performed on a Exeter Analytical, Inc.
CE-440F Elemental Analyzer. Infrared spectra were recorded on
JASCO FT/IR-4100.
1
as an eluent to give 3 in 40% yield. For 1; H NMR (500 MHz,
DMSO-d6) d (ppm) 0.79 (t, 6H, J ¼ 6.77 Hz), 1.14–1.20 (m, 12H),
1.48–1.59 (m, 4H), 2.62–2.66 (m, 4H), 3.69 (s, 3H), 3.76 (s, 3H),
7.03 (t, 1H, J ¼ 7.53 Hz), 7.10 (t, 1H, J ¼ 7.47 Hz), 7.20 (d, 1H, J ¼
8.30 Hz), 7.24 (d, 1H, J ¼ 8.45 Hz), 7.37 (s, 1H), 7.39 (s, 1H), 7.45
Materials
Reagents used for the synthesis were commercially available and (d, 1H, J ¼ 7.50 Hz), 7.51 (t, 3H, J ¼ 8.18 Hz), 7.71 (d, 2H, J ¼ 8.45
used as supplied. Dry-THF, EtOH, and toluene were prepared Hz), 7.94 (s, 2H), 8.30 (d, 1H, J ¼ 8.30), 8.31 (d, 1H, J ¼ 8.20 Hz),
according to a standard procedure. Diuoro[5-bromo-1-[[5- 8.43 (s, 2H), 8.84 and 8.85 (s, 1H), 13.75 (broad signal, 2H); 13
C
bromo-3-(2-methoxyphenyl)-2H-isoindole-l-yl]methylene]-3-(2- NMR (125 MHz, DMSO-d6) d (ppm): 163.5, 157.3, 157.2, 150.0,
methoxyphenyl)-1H-isoindolate-N1,N2]boron (4) was prepared 149.8, 148.8, 147.0, 146.4, 142.4, 140.2, 133.5, 132.9, 132.8,
from 4-bromo-2-hydroxyacetophenone through 4 steps.12 2-(5- 132.0, 131.7, 131.6, 131.3, 130.3, 130.2, 129.9, 129.4, 127.2,
(5,5-Dimethyl-1,3-dioxan-2-yl)-3-hexylthiophen-2-yl)-4,4,5,5-tet- 127.1, 123.4, 123.3, 120.7, 120.2, 120.0, 118.7, 118.5, 118.1,
ramethyl-1,3,2-dioxaborolane (5) was also prepared from 118.0, 117.7, 116.4, 111.5, 98.6, 55.6, 30.9, 30.0, 28.2, 27.8, 21.9,
2-bromo-3-hexyl-5-formylthiophene13 through protection of 13.8; 19F NMR (470 MHz, CDCl3: DMSO-d6 (3 : 7 v/v)) d (ppm)
2
aldehyde using neopentyl glycol, followed by borylation with ꢁ149.1 (JBF ¼ 29.0 Hz, JFF ¼ 94.6 Hz), ꢁ134.8 (JBF ¼ 30.3 Hz),
2
ꢁ121.1 (JBF ¼ 32.7 Hz, JFF ¼ 95.7 Hz); FAB MS: m/z 1026 [M]+;
elemental analysis: calcd for C59H53BF2N4O6S2$H2O: C, 67.81;
H, 5.30; N, 5.36, found: C, 67.70; H, 5.15; N, 5.34. For 3; 1H NMR
(500 MHz, DMSO-d6) d (ppm) 0.79 (t, 6H, J ¼ 6.90 Hz), 1.14–1.19
(m, 12H), 1.48–1.59 (m, 4H), 2.61–2.66 (m, 4H), 3.69 (s, 3H), 3.75
and 3.76 (s, 3H), 7.03 (t, 1H, J ¼ 7.45 Hz), 7.10 (t, 1H, J ¼ 7.53
Hz), 7.19 (d, 0.5H, J ¼ 8.70 Hz), 7.20 (d, 0.5H, J ¼ 8.25 Hz), 7.23
(d, 0.5H, J ¼ 8.70 Hz), 7.23 (d, 0.5H, J ¼ 8.40 Hz), 7.35–7.38 (m,
2H), 7.44 (t, 1H, J ¼ 6.05 Hz), 7.49–7.52 (m, 3H), 7.68–7.72 (m,
2H), 7.90 (s, 1H), 7.97 and 7.98 (s, 1H), 8.29 (d, 1H, J ¼ 8.35 Hz),
Fig. 1 Chemical structures of dyes 1, 2 and 3.
8.30 (d, 1H, J ¼ 8.45 Hz), 8.38 (s, 1H), 8.83 and 8.84 (s, 1H), 9.87
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Paper
Scheme 1 Synthesis of sensitizers 1 and 3.
(s, 1H), 13.77 (broad signal, 1H); 13C NMR (125 MHz, DMSO-d6)
d (ppm): 183.9, 163.4, 157.3, 157.2, 150.0, 149.7, 146.7, 140.8,
140.5, 140.2, 140.0, 133.8, 132.9, 132.8, 132.1, 132.0, 131.6,
131.3, 131.2, 130.3, 130.1, 130.0, 129.7, 129.5, 127.2, 127.1,
123.3, 123.2, 120.8, 120.7, 120.2, 120.0, 118.8, 118.5, 118.0,
117.9, 111.5, 55.6, 30.9, 30.0, 28.3, 28.0, 27.8, 21.9, 13.9; 19F
NMR (470 MHz, CDCl3: DMSO-d6 (3 : 7 v/v)) d (ppm) ꢁ148.0 (JBF
Electrochemical measurements
Cyclic voltammograms (CV) were recorded on a Princeton
Applied Research VersaSTAT 3 potentiostat operated at a scan
rate of 50 mV sꢁ1 and room temperature under a N2 atmo-
sphere. The solvent was DMF–CH2Cl2 (1 : 4 v/v) containing 0.1
M tetrabutylammonium hexauorophosphate (TBAPF6) as the
supporting electrolyte. The potentials were measured against
Ag/Ag+ (0.01 M of AgNO3) as a reference electrode; ferrocene/
ferrocenium (Fc/Fc+) was used as the internal standard and
measured to be 0.06 V under same conditions. The onset
potentials were determined from the intersection of two
tangents drawn at the rising and background currents of the
cyclic voltammogram.
¼ 28.1 Hz, 2JFF ¼ 95.2 Hz), ꢁ133.7 (JBF ¼ 30.9 Hz), ꢁ133.6 (JBF
¼
30.9 Hz), ꢁ120.8 (JBF ¼ 32.5 Hz, 2JFF ¼ 94.9 Hz); FAB MS: m/z 959
[M]+; elemental analysis: calcd for C56H52BF2N3O5S2$0.5H2O: C,
69.41; H, 5.51; N, 4.34, found: C, 69.08; H, 5.43; N, 4.33%.
Diuoro[6-(3-hexyl-5-(2-carboxy-2-cyanovinyl)thiophen-2-yl)-
1-[[6-(3-hexyl-5-(2-carboxy-2-cyanovinyl)thiophen-2-yl)-3-(2-
methoxyphenyl)-2H-isoindole-1-yl]methylene]-3-(2-methoxy-
phenyl)-1H-isoindolate-N1,N2]boron (2). To
a solution of
Theoretical calculation
compound 14 (266 mg, 0.298 mmol) and piperidine (0.2 mL, 2.03
mmol) in dry THF (7.8 mL) was added a solution of 2-cyanoacetic
acid (159 mg, 1.87 mmol) and piperidine (0.2 mL, 2.03 mmol) in
dry THF (7.8 mL). The mixture was reuxed for 2 days. The
addition of CH2Cl2 (30 mL) into the resulting solution gave a
precipitate, followed by extraction using CH2Cl2 (100 mL) and 1
N HCl aq. (100 mL). The organic layer was then dried with
Na2SO4 and evaporated in vacuo. The resulting residue was rep-
recipitated with CH2Cl2 (10 mL)/hexane (100 mL) to give 2 as a
All geometries of the dyes at the ground state were fully
optimized by means of the AM1 method. The structural
geometry was optimized by the AM1 method. Density func-
tional theory (DFT) calculations at the CAM-B3LYP/6-31G(d,p)
level were performed in the Gaussian 09 package. The
molecular orbitals in Fig. 4 were visualized using Gauss view
5.0.8 program.
1
conformation mixture (DGs ¼ 76.5 kJ molꢁ1) in 65% yield. H
Fabrication of a dye-sensitized solar cell device
NMR (500 MHz, DMSO-d6) d (ppm) 0.76 (t, 6H, J ¼ 6.93 Hz), 1.19–
1.26 (m, 12H), 1.60 (quint, 4H, J ¼ 7.24 Hz), 2.78 (t, 4H, J ¼ 7.35 The nanoporous TiO2 was deposited on a uorine-doped-tin-
Hz), 3.70 (s, 3H), 3.75 (s, 3H), 7.03 (t, 1H, J ¼ 7.75 Hz), 7.09 (t, 1H, oxide (FTO) substrate and sintered for 1 h at 500 ꢂC. The 10 mm
J ¼ 7.70 Hz), 7.20 (d, 1H, J ¼ 8.35 Hz), 7.24 (d, 1H, J ¼ 8.35 Hz), thick TiO2 electrode was immersed into acetone solution of dye
7.38–7.46 (m, 5H), 7.49 (d, 1H, J ¼ 7.25 Hz), 7.51 (t, 2H, J ¼ 8.02 sensitizers (1.6 ꢀ 10ꢁ4 M) and deoxycholic acid (10 mM) for 12 h
Hz), 8.03 (s, 2H), 8.32 and 8.33 (s, 2H), 8.51 (s, 2H), 8.90 and 8.91 at room temperature. The resultant dye-absorbed TiO2 electrode
(s, 1H), 13.77 (s, 2H); 13C NMR (125 MHz, DMSO-d6) d (ppm) was applied for the DSSC device with a platinized counter
163.4, 157.4, 157.3, 149.1, 148.8, 147.0, 146.5, 142.5, 140.7, 133.9, electrode and a liquid electrolyte comprising I2 (0.1 M), LiI (0.1
133.5, 131.9, 131.5, 131.2, 129.7, 127.4, 125.8, 124.0, 120.2, 120.0, M), and dimethylpropylimidazolium iodide (0.6 M) in methox-
119.9, 118.9, 118.7, 117.5, 116.2, 111.5, 98.7, 55.5, 30.7, 29.7, 28.1, ypropionitrile. The photocurrent–voltage characters were
27.7, 21.9, 13.7; 19F NMR (470 MHz, CDCl3 : DMSO-d6 (3 : 7 v/v)) measured on a sourcemeter (6243, ADC Co. LTD) under 100 mW
d (ppm) ꢁ148.4 (JBF ¼ 28.1 Hz, 2JFF ¼ 95.8 Hz), ꢁ134.4 (JBF ¼ 30.7 cmꢁ2 AM1.5G simulated solar light (WXS-155S-10, WACOM
2
Hz), ꢁ121.2 (JBF ¼ 34.1 Hz, JFF ¼ 95.8 Hz); FAB MS: m/z 1026 electric Co. LTD). The incident photon-to-charge carrier effi-
[M]+; elemental analysis: calcd for C59H53BF2N4O6S2$0.5H2O: C, ciencies (IPCEs) were measured on the IPCE Measurement
68.40; H, 5.25; N, 5.41, found: C, 68.24; H, 5.08; N, 5.33%.
System (SM-250, Bunkoukeiki Co., Ltd.)
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Results and discussion
The synthetic path that we employed in this study enables the
introduction of a bromo substituent at the 5 or 6 position of the
isoindole ring via the choice of the starting material; thus, two
regioisomers of the target sensitizers could be generated.
Accordingly, we investigated the regio-dependency of both the
optical properties and performance of DSSCs based on these
dyes. Indeed, the dipole moment (Dm) of 2 was determined to be
13.243 D from DFT calculations; this value is 3.0 times larger
than that of 1 (Table 1) and results in improved DSSC
performance.
The synthetic route to the target sensitizer, 1, is shown in
Scheme 1. The starting material, i.e., 5,5-dibromo- and bis(a-
nisole)-substituted boron–dibenzopyrromethene diuoride 4,12
coupled with acetal-protected borylthiophene derivative 5 fol-
Fig. 2 Proposed conformers of 1 and their 19F NMR spectra in DMSO-
d6 : CDCl3 (3 : 7 v/v) at room temperature.
lowed by deprotection with p-TsOH to afford 7. Finally, 7 comprise such a conformational mixture at room temperature.
underwent Knoevenagel condensation with ve equivalents of A similar phenomenon was observed for 3 (DGs ¼ 78.5 kJ molꢁ1
cyanoacetic acid in the presence of piperidine. Stirring the in Fig. S1(c)).†
mixture for 5 h under reux generated targets 1 and 3, which
Scheme 2 shows the synthetic route to 2: initially, 6,60-
contains an unreacted formyl group, in 43 and 40% yields, dibromo-substituted boron–dibenzopyrromethene 12, which
respectively. Structural assignments were performed via NMR, was prepared from 2-acetyl-4-bromophenol 8 via a four-step
FAB MS, and elemental analysis. The 1H NMR spectrum of 1 in synthesis, underwent Suzuki cross-coupling with 5, depro-
DMSO-d6 at room temperature contains two sets of signals tection with p-TsOH, and Knoevenagel condensation with cya-
assignable to the anisole segment: proton resonances arising noacetic acid to afford buttery-shaped regioisomer 2 as a
from the methoxy groups were detected at 3.69 and 3.76 ppm conformational mixture of anti- and syn-mixtures (DGs
(Fig. S1(a)†). VT-NMR analysis revealed a Tc value of 94 ꢂC, 76.5 kJ molꢁ1 in Fig. S1(b)).†
¼
which suggests that two conformers of 1 coexist under ambient
We were interested in how the slight differences in the
conditions. To gain further insight into the structures, we structures of these dyes would affect the electronic properties of
recorded a 19F NMR spectrum of 1 in DMSO-d6 : CDCl3 (3 : 7 the sensitizers. Our rst insight into this subject came from the
v/v), which revealed a quintet signal at ꢁ134.8 ppm ( JBF
¼
UV-vis absorption spectra shown in Fig. 3a, which includes the
spectrum of 5,5-dibromo derivative 4 (Scheme 1) as a reference:
30.3 Hz) and octet signals at ꢁ121.1 ppm (JBF ¼ 32.7 Hz, 2JFF
¼
2
95.7 Hz) and ꢁ149.1 ppm (JBF ¼ 29.0 Hz, JFF ¼ 94.6 Hz), as dye 1 absorbs visible light with a lmax of 647 nm and has a
shown in Fig. 2; this indicates that anti- and syn-isomers, in relatively large molecular extinction coefficient (3) of 1.57 ꢀ 105
which the anisole moieties are located on the opposite and Mꢁ1 cmꢁ1 as a result of the p-extended BODIPY chromophore.
same face of the dibenzopyrromethene core, respectively, are Comparison of the UV-vis spectra of 1 and 4 reveals that
stable under ambient conditions (DGs ¼ 77.4 kJ molꢁ1). This is insertion of cyanoacrylic acid as an accepting group via a thio-
intriguing because simple bis(anisole)-appended BODIPY does phene p-bridge causes a bathochromic shi of 16 nm and
not have conformational isomers.14 The isoindole ring in 1 may increased absorption intensity. The rst absorption band of 3,
restrict rotation about the two Canisole–Cisoindole bonds. Attempts which has an unreacted formyl group, is slightly blue-shied
at separating these isomers using silica gel chromatography and has a lower absorption intensity relative to that of 1. On the
were unsuccessful. The sensitizers prepared in this study other hand, regioisomer
2
exhibits distinct absorption
Table 1 Optical properties and theoretical data for dyes 1–4
c
Dye
1
lmaxa/nm (3/105 Mꢁ1 cmꢁ1
)
lmaxb (calcd)/nm
Calculated assignment
f
(calcd)
Dm/Debyec
4.486
647 (1.57), 457 (0.192), 371 (0.500)
660 (1.09), 406 (0.390), 369 (0.478)
529
542
HOMO / LUMOd (46.8%)
HOMO / LUMO+2 (1.6%)
HOMO / LUMOd (43.7%)
HOMO / LUMO+1 (1.2%)
HOMO / LUMO+2 (3.6%)
HOMO / LUMOd (45.3%)
HOMO / LUMO+1 (3.3%)
—
1.5550
1.0846
2
13.243
3
644 (1.39), 371 (0.332)
631 (1.11), 360 (0.212)
527
523
1.3939
0.9299
4.046
4
—
a
The absorption maxima (lmax) and molecular extinction coefficients (3) were measured in THF. b The longest wavelength absorption bands, which
were calculated using the CAM-B3LYP/6-31G(d,p) level. c Calculated using the CAM-B3LYP/6-31G(d,p) level. d (CI coefficient)2 ꢀ 100% (ref. 15).
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Scheme 2 Synthesis of sensitizer 2.
understanding of the absorption properties, the transition
energies and oscillator strengths ( f ) were calculated based on
time-dependent density functional theory (TD-DFT) at the CAM-
B3LYP/6-31G(d,p) level. The trend of the calculated lmax values
in the rst absorption band is almost consistent with those of
the spectra recorded in THF despite the twist in the structure
between the isoindole ring and the thiophene p spacer (dihe-
dral angle of ꢃ56ꢂ, Fig. 4a). The electron-density distributions
of these dyes are illustrated for the following molecular orbitals:
HOMO, LUMO, LUMO+1, and LUMO+2 (Fig. 4b). Although the
maximum absorption band is mainly ascribed to a HOMO–
LUMO transition, it is characterized as a mixture of several
congurations (Table 1). The electron-density distributions of
the HOMOs of the dyes are mainly localized on the dibenzo-
pyrromethene core. Although the LUMO and higher LUMO
orbitals of the dye core are populated with electron density, a
signicant amount of electron density extends toward the
thienyl-cyanoacrylic acid units. It appears that the LUMO and
Fig. 3 (a) Absorption spectra of dyes in THF (5 mM) and (b) normalized
absorption spectra of dyes on a 10 mm thin TiO2 film.
properties (lmax ¼ 660 nm, 3 ¼ 1.09 ꢀ 105 Mꢁ1 cmꢁ1) with a 13
nm red-shied lmax relative to that of 1. Also, dyes 1, 2, and 3
have a relatively small, but still signicant, absorption band at
320 to 500 nm. This spectral feature is valuable for DSSC
applications because it would contribute to their light-harvest-
Fig. 4 (a) Molecular geometries of 1, 2 and 3 with dihedral angles
between the isoindole ring and the thiophene p-spacer indicated, and
(b) surface plots, as estimated from DFT calculations (CAM-B3LYP/6-
ing efficiency as sensitizers (vide infra). For
a better 31G (d,p)).
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Journal of Materials Chemistry A
higher LUMO orbitals synchronize to inject electrons into the process enables application of these dyes as potential sensi-
conduction band of TiO2. Accordingly, the excitation is domi- tizers in DSSCs (Fig. 5). The absorption spectra of the dyes were
nated by intramolecular charge transfer (ICT) from the HOMO also measured on 10 mm thick TiO2 lms (Fig. 3b). Compared to
to the LUMO and higher LUMO orbitals, which is reected in the absorption bands observed in solution, the spectra on TiO2
the absorption properties of the dyes in the longer wavelength were broader with signicant red-shiing of the absorptions;
region. Further, the electron density on the cyanoacrylic acid this is presumably caused by J-aggregation of the dyes anchored
moiety in the LUMO of 2 is greater than that in the LUMO of 1. on the TiO2 lms.18 It is interesting to note that the absorption
The theoretical data indicate that the electronic properties are proles on the TiO2 lm are not affected by the number of
dependent on the position of the thienyl-cyanoacrylic acid thienyl-cyanoacrylic acid units. We investigated the interactions
moiety on the isoindole ring, which may be associated with between the carboxylic acid groups and TiO2 using infrared (IR)
the effectiveness of ICT. Indeed, 2 has a higher dipole moment absorption spectroscopy equipped with an attenuated total
(vide supra).
reection (ATR) apparatus. The characteristic signal at 1680
The electrochemical properties of the sensitizers were cmꢁ1 is attributed to the free carboxylic groups of 1; in contrast,
measured at room temperature using cyclic voltammetry (CV) in the IR absorption spectrum of the 1-loaded TiO2 lm features a
a DMF–CH2Cl2 (1 : 4 v/v) mixture containing 0.1 M tetrabuty- new absorption band at 1590 cmꢁ1 and an almost negligible
lammonium hexauorophosphate (TBAPF6) as the supporting peak at 1680 cmꢁ1 (Fig. S3†). This result suggests that both
electrolyte (Fig. S2†). The measurements were calibrated against cyanoacrylic acid moieties in 1 participate in binding to TiO2.
ferrocene (ꢁ4.8 eV) as the standard.16 The formal potential of On the other hand, the absorption prole of regioisomer 2
Fc/Fc+ was measured to be 0.06 V versus Ag/Ag+. Although the differs from those of 1 and 3, which suggests that the position of
voltammogram of 1 is almost identical to that of 3, the elec- the anchoring unit on the isoindole ring affects the orientation
trochemical properties of 1 differ from those of regioisomer 2 of the dyes on the TiO2 surface.
(Fig. S2†). The anodic scan revealed onset oxidation potentials
DSSCs based on these dyes were fabricated using a liquid
for 1 and 2 of 0.49 and 0.47 V, respectively, which correspond electrolyte comprising I2 (0.1 M), LiI (0.1 M), and dimethylpro-
to ionization potentials (HOMO levels) of ꢁ5.22 eV for 1 and pylimidazolium iodide (0.6 M) in methoxypropionitrile. The
ꢁ5.20 eV for 2 (Fig. 5); these values are more positive than the short circuit photocurrent density ( Jsc), open-circuit voltage
ꢁ
redox potential (ꢁ4.6 eV) of Iꢁ/I3 for dye regeneration. Simi- (Voc), and ll factor (FF) values under 100 mW cmꢁ2 AM1.5G
larly, the onset reduction potential of these dyes enabled esti- irradiation were determined from the photocurrent density–
mation of the electron affinities (LUMO level) for 1 and 2 voltage ( J–V) curves of the cells in the presence of deoxycholic
(ꢁ3.59 and ꢁ3.64 eV, respectively). The LUMO is more sensitive acid as a co-adsorbent (Fig. 6a). The DSSC based on 1, which has
than the HOMO to the position of the thienyl-cyanoacrylic acid two anchoring units, features Jsc, Voc, and FF values of 19.24 mA
moiety, which implies that the anchoring unit at the 6 position cmꢁ2, 0.53 V, and 0.514, respectively. The power-to-current
in 2 has a greater impact on the electron-withdrawing ability of conversion efficiency (h) was calculated to be 5.24% using the
the chromophore than that at the 5 position in 1, resulting in following equation: h ¼ Jsc ꢀ Voc ꢀ FF.19 It is worth emphasizing
improved ICT. The LUMO levels of these dyes are negative that the Jsc of the cell is greater than 19.0 mA cmꢁ2. In contrast,
relative to the conduction band of TiO2 (ꢁ4.0 eV),17 ensuring the cell based on 3, which has a single anchoring unit, has a
efficient electron injection from the excited state of the dyes into lower Jsc of 17.21 mA cmꢁ2. To verify the results, the incident
the TiO2 electrode. Accordingly, this energetically favorable photon-to-charge carrier efficiencies (IPCEs), which usually
depend on light-absorption photoinduced electron injection
into TiO2 and charge recombination, were measured. Fig. 6b
shows the IPCE intensity plotted as a function of the incident
wavelength for the DSSCs. All cells tested can convert the light
to photocurrents in the wide wavelength region up to about
800 nm. The cell containing 1 exhibited a considerably broader
and higher IPCE of 60% in the range of 500–700 nm, while the
3-based cell displays an IPCE spectrum with a similar spectral
pattern but slightly blue-shied and with a lower maximum.
The broader IPCE curves are consistent with the trend of the
absorption spectra on TiO2 lms, and occur because 1 can bind
to the surface of TiO2 in several ways. In contrast, the Voc of the
3-based cell (0.55 V) was slightly higher than that of the 1-based
cell, suggesting that introduction of two anchoring units does
not improve the Voc. The h of the 3-based cell was estimated to
be 5.48% (Table 2). In contrast, the 2-loaded cell has a Jsc of
19.02 mA cmꢁ2, Voc of 0.54 V, FF of 0.590, and h of 6.06%. The
Fig. 5 HOMO and LUMO energy levels of 1, 2 and 3. The energy levels
IPCE value of the 2-loaded cell is higher than that of the cell
were calculated from the CV measurements in DMF–CH2Cl2 (1 : 4 v/v)
loaded with 1 in the ranges of 300–490 and 580–760 nm. These
containing 0.1 M TBAPF6; HOMO [eV] ¼ ꢁ(Eox ꢁ EFc/Fc+ + 4.8) and
LUMO [eV] ¼ ꢁ(Ered ꢁ EFc/Fc+ + 4.8).
results demonstrate that cell performance is affected by the
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DSSC devices. The good light-harvesting abilities of the dyes led
to high Jsc values for these dye-loaded cells. As a result, the cell
containing buttery-shaped 2 provided a good power-to-current
conversion efficiency of 6.06%, which, to the best of our
knowledge, is the highest value for BODIPY-based DSSCs
reported to date. It is noteworthy that these sensitizers do not
contain strong donor units, such as arylamines; this is bene-
cial for the design of organic sensitizers because various aryl-
amines, such as triarylamine and carbazole, have been widely
employed as strong donors in D–p–A systems in DSSCs.20 We
believe that the use of boron–dibenzopyrromethene as a core
skeleton in metal-free organic sensitizers is a promising
approach to diverse structural modications to achieve good
light-harvesting properties. Further study on this subject is
currently underway in our laboratory.
Acknowledgements
This research was partially supported by a Grant-in-Aid for
Scientic Research from the Ministry of Education, Science,
Sports and Culture of Japan (no. 24655035).
Notes and references
¨
1 (a) B. O'Regan and M. Gratzel, Nature, 1991, 353, 737; (b)
¨
M. Gratzel, Acc. Chem. Res., 2009, 42, 1788; (c) Y. Ooyama and
Y. Harima, Eur. J. Org. Chem., 2009, 2903; (d) A. Hagfeldt,
G. Boschloo, L. Sun, L. Klooand and H. Pettersson, Chem.
Rev., 2010, 110, 6595; (e) S. K. Balasingam, M. Lee,
M. G. Kang and Y. Jun, Chem. Commun., 2013, 49, 1471.
2 B.-G. Kim, K. Chung and J. Kim, Chem.–Eur. J., 2013, 19,
5220.
Fig. 6 (a) Photocurrent density–voltage (J–V) curves and (b) IPCE
action spectra of dye-loaded cells.
3 (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891; (b)
G. Ulrich, R. Ziessel and A. Harriman, Angew. Chem., Int. Ed.,
2008, 47, 1184.
Table 2 Photovoltaic parameters of DSSC based on 1–3 under the
simulated AM1.5G sunlight (100 mW cmꢁ2 a
)
4 N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev., 2012, 41,
1130.
Dye
Jsc/mA cmꢁ2
Voc/V
FF
h/%
¨
5 (a) C. Peters, A. Billich, M. Ghobrial, K. Hogenauer, T. Ullrich
1
2
3
19.24
19.02
17.21
0.53
0.54
0.55
0.514
0.590
0.579
5.24
6.06
5.48
and P. Nussbaumer, J. Org. Chem., 2007, 72, 1842; (b) Z. Li
and R. Bittman, J. Org. Chem., 2007, 72, 8376.
6 L. Bonardi, H. Kanaan, F. Camerel, P. Jolinat, P. Retailleau
and R. Ziessel, Adv. Funct. Mater., 2008, 18, 401.
a
Electrolyte: I2 (0.1 M), LiI (0.1 M), and dimethylpropylimidazolium
iodide (0.6 M) in methoxypropionitrile.
7 (a) S. Erbas, A. Gorgulu, M. Kocakusakogullari and
E. U. Akkaya, Chem. Commun., 2009, 4956; (b) Y. Cakmak,
S. Kolemen, S. Duman, Y. Dede, Y. Dolen, B. Kilic,
Z. Kostereli, L. T. Yildirim, A. L. Dogan, D. Guc and
E. U. Akkaya, Angew. Chem., Int. Ed., 2011, 50, 11937; (c)
S. G. Awuah and Y. You, RSC Adv., 2012, 2, 11169; (d)
A. Kamkaew, S. H. Lim, H. B. Lee, L. V. Kiew, L. Y. Chung
and K. Burgess, Chem. Soc. Rev., 2013, 42, 77.
8 (a) O. A. Bozdemir, S. Erbas-Cakmak, O. O. Ekiz, A. Dana and
E. U. Akkaya, Angew. Chem., Int. Ed., 2011, 50, 10907; (b)
R. Ziessel, G. Ulrich, A. Haefele and A. Harriman, J. Am.
Chem. Soc., 2013, 135, 11330.
position of the thienyl-cyanoacrylic acid unit on the isoindole
ring. Considering the absorption proles on TiO2 (Fig. 3b), the
ICPE spectrum likely reects the orientation of the dye on the
surface. Thus, it is evident that the use of sensitizer 2, which has
a larger dipole moment than 1, results in improved Voc and FF
values under the investigated conditions.
Conclusions
In conclusion, we synthesized and characterized a series of
boron–dibenzopyrromethene dyes that contain thienyl-cyanoa-
crylic acid units, i.e., 1–3, for potential use as sensitizers in
9 (a) M. Benstead, G. H. Mehl and R. W. Boyle, Tetrahedron,
2011, 67, 3573; (b) H.-Y. Lin, W.-C. Huang, Y.-C. Chen,
H.-H. Chou, C.-Y. Hsu, J. T. Lin and H.-W. Lin, Chem.
5210 | J. Mater. Chem. A, 2014, 2, 5204–5211
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
Commun., 2012, 48, 8913; (c) T. Bura, N. Leclerc, S. Fall, 12 Y. Kubo, K. Watanabe, R. Nishiyabu, R. Hata, A. Murakami,
´ ˆ
P. Leveque, T. Heiser, P. Retailleau, S. Rihn, A. Mirloup
T. Shoda and H. Ota, Org. Lett., 2011, 13, 4574.
and R. Ziessel, J. Am. Chem. Soc., 2012, 134, 17404; (d) 13 Y. Zhao, K. Jiang, W. Xu and D. Zhu, Tetrahedron, 2012, 68,
¨
T. K. Khan, M. Broring, S. Mathur and M. Ravikanth,
9113.
Coord. Chem. Rev., 2013, 257, 2348.
10 (a) S. Hattori, K. Ohkubo, Y. Urano, H. Sunahara, T. Nagano,
14 N. Sakamoto, C. Ikeda, M. Yamamura and T. Nabeshima,
Chem. Commun., 2012, 48, 4818.
Y. Wada, N. V. Tkachenko, H. Lemmetyinen and 15 Y. Kubo, K. Yoshida, M. Adachi, S. Nakamura and S. Maeda,
S. Fukuzumi, J. Phys. Chem. B, 2005, 109, 15368; (b) J. Am. Chem. Soc., 1991, 113, 2868.
S. Erten-Ela, M. D. Yilmaz, B. Icli, Y. Dede, S. Icli and 16 J. Pommerehne, H. Vestweber, W. Guss, R. F. Mahrt,
¨
H. Bassler, M. Porsch and J. Daub, Adv. Mater., 1995, 7, 551.
E. U. Akkaya, Org. Lett., 2008, 10, 3299; (c) D. Kumaresan,
¨
R. P. Thummel, T. Bura, G. Ulrich and R. Ziessel, Chem.– 17 A. Hagfeldt and M. Gratzel, Chem. Rev., 1995, 95, 49.
Eur. J., 2009, 15, 6335; (d) C. Y. Lee and J. T. Hupp, 18 (a) K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga,
Langmuir, 2010, 26, 3760; (e) S. Kolemen, Y. Cakmak,
S. Erten-Ela, Y. Altay, J. Brendel, M. Thelakkat and
E. U. Akkaya, Org. Lett., 2010, 12, 3812; (f) S. Kolemen,
O. A. Bozdemir, Y. Cakmak, G. Barin, S. Erten-Ela,
M. Marszalek, J.-H. Yum, S. M. Zakeeruddin,
S. Tsukagoshi, Y. Abe, H. Sugihara and H. Arakawa, Chem.
Commun., 2000, 1173; (b) T. Horiuchi, H. Miura,
K. Sumioka and S. Uchida, J. Am. Chem. Soc., 2004, 126,
12218; (c) C. Teng, X. Yang, C. Yang, S. Li, M. Cheng,
A. Hagfeldt and L. Sun, J. Phys. Chem. C, 2010, 114, 9101;
(d) W. Lee, J. Y. Seng and J.-I. Hong, Tetrahedron, 2013, 69,
9175.
¨
M. K. Nazeeruddin, M. Gratzel and E. U. Akkaya, Chem.
Sci., 2011, 2, 949; (g) M. Mao, J.-B. Wang, Z.-F. Xiao,
S.-Y. Dai and Q.-H. Song, Dyes Pigm., 2012, 94, 224; (h) 19 A continuous thermal and light soaking stress test using 1-
¨
¨
¨
¨
K. Graf, T. Korzdorfer, S. Kummel and M. Thelakkat, New
J. Chem., 2013, 37, 1417; (i) Y. Ooyama, Y. Hagiwara,
T. Mizumo, Y. Harima and J. Ohshita, New J. Chem., 2013,
37, 2479; (j) Y. Ooyama, Y. Hagiwara, T. Mizumo,
Y. Harima and J. Ohshita, RSC Adv., 2013, 3, 18099.
11 Y. Kubo, Y. Minowa, T. Shoda and K. Takeshita, Tetrahedron
Lett., 2010, 51, 1600.
based cell was carried out under the irradiation of AM1.5G
full sunlight soaking at 40 ꢂC, where h values were
repeatedly measured at several times until about 500 h. As
a result, the h value remained at 78% of the initial value
for 504 h, meaning that cell has a relatively proper
reproducibility and stability for a long term.
20 Z. Ning and H. Tian, Chem. Commun., 2009, 5483.
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