Sn- and Pd-Free Synthesis of D–Π–A Organic Sensitizers for Dye-Sensitized Solar Cells by Cu-Catalyzed Direct Arylation

Article abstract of DOI:10.1002/cssc.201700421

variety of D–π–A-type functional organic dyes are facilely synthesized by direct C?H arylation catalyzed by inexpensive copper salts. Under optimized reaction conditions, a broad substrate scope with good functional group compatibility was demonstrated. Based on this synthetic strategy, three new dye sensitizers (CYL-5–7) were designed and fabricated for use in dye-sensitized solar cells (DSSCs). Photovoltaic characterization showed that these devices gave open-circuit voltages of 0.65–0.75 V, short-circuit currents of 5.90–12.60 mA cm^?2, and fill factors of 65.6–76.9 %, corresponding to power conversion efficiencies (PCEs) of 2.95–6.20 %. This work represents the first use of Cu-catalyzed C?H arylation for a step-saving, Sn-free synthesis of precursor dyes for DSSC applications.

Full text of DOI:10.1002/cssc.201700421

Accepted Article
Title: Greener Synthesis of D-pi-A Organic Sensitizers via Cu-
Catalyzed Direct Arylations: Development of Sn- & Pd-Free
Process for Dye-Sensitized Solar Cells
Authors: Jiung-Huai Huang, Po-Han Lin, Wei-Ming Li, Kun-Mu Lee,
and Ching-Yuan Liu
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To be cited as: ChemSusChem 10.1002/cssc.201700421
Link to VoR: http://dx.doi.org/10.1002/cssc.201700421
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10.1002/cssc.201700421
ChemSusChem
FULL PAPER
Greener Synthesis of D--A Organic Sensitizers via
Cu-Catalyzed Direct Arylations: Development of Sn- & Pd-Free
Process for Dye-Sensitized Solar Cells
Jiung-Huai Huang,[a] Po-Han Lin,[a] Wei-Ming Li,[a] Kun-Mu Lee,[b] and Ching-Yuan Liu*[a]
Abstract: A variety of DA type functional organic dyes have
been facilely synthesized via direct C-H arylations using inexpensive
copper salts as catalyst. Under well-optimized reaction conditions, a
broad substrate scope with good functional group compatibility was
achieved. Based on this synthetic strategy, we designed and
prepared three new dye sensitizers (CYL-5~7) that were fabricated
the first example applying Cu-catalyzed direct C-H arylations in
the facile preparation of DSSCs-oriented small molecules.
as
characterizations showed that these devices gave a Voc of 0.65~0.75
V, FF of 65.6~76.9%,
Jsc of 5.90~12.60 mA/cm2, and
dye-sensitized
solar
cells
(DSSCs).
Photovoltaic
a
a
corresponding to the power conversion efficiency (PCE) of
2.95~6.20%. This work represents the first example using Cu-
catalyzed C-H arylations as step-saving and Sn-free synthesis of
various precursor dyes for DSSCs applications.
Introduction
Scheme 1. Different synthetic routes and catalytic systems to access
DA type organic sensitizers.
Donoracceptor (DA) type metal-free organic dyes
constitute a prominent part in modern research of dye-sensitized
solar cells (DSSCs) because of their lower cost, flexible
molecular design, and efficient intramolecular charge transfer
(ICT).[1-5] Over the last decade, pioneers in this field have
disclosed promising power conversion efficiencies (over 10.0%)
based on a variety of organic sensitizers.[6-8] In general, these
DA or DAA’ type organic dyes were synthesized via
palladium-catalyzed cross couplings such as Stille- or Suzuki
reactions, which requires tedious synthetic transformations
including the pre-generation of organometallic reagents. Up to
date, only a few reports described the step-economical direct
arylation synthesis of DSSCs-oriented DA type molecules[9-14]
(Scheme 1). More importantly, we noticed that either by
traditional cross-coupling techniques or through above
Results and Discussion
In order to access the targeted DA type dye molecule via
copper-catalyzed C-H arylations, 4-iodotriphenylamine 1a (as
Donor) and 2-thiophenecarboxaldehyde 2a (as Acceptor)
were selected as starting materials and the optimization of their
reaction conditions was summarized in Table 1. Initially, the C-H
arylation was tested under an optimal reaction condition
disclosed previously by our group[22] [CuCl2, mono-dentate
phosphine ligand (PPh3 or PCy3), and Cs2CO3 in DMPU/m-
xylene], giving the desired product 3a in 40-52% yields (entries 1,
2). Using bidentate ligands (dppe, dppb) afforded 3a in relatively
lower yields (18-39%, entries 3-4). Xantphos and DPEphos, the
bidentate ligands with wide bite angles, showed better results
and 3a was produced in 60-72% yields (entries 5-6). C-H
arylation with N, N-bidentate ligands generally gave poor yields
(16-40%, entries 7, 9, 11, 12), except for phenanthroline (85%,
entry 8) and bathophenanthroline (66%, entry 10). In the
absence of ligand, 3a was obtained in only 31% yield (entry 13).
Based on above preliminary results, phenanthroline was used as
the optimal ligand for following base and (co)solvent screenings.
In contrast to Cs2CO3, less basic K2CO3 and Na2CO3 were
shown to be inefficient in C-H arylations (24%, trace; entries 14,
15). When the base was switched to the more basic K3PO4, the
arylation proceeded smoothly and 3a was isolated in moderate
yield (60%, entry 16). Without the addition of base, C-H arylation
mentioned
C-H
activation/arylation
strategies,[9-14],[15-21]
preparation of these organic dyes must involve the use of
palladium-based catalysts for the key carbon-carbon bond
formations. Hence, we anticipate that developing an equally
efficient while Pd- & Sn-free greener catalytic system for the dye
synthesis would be highly desirable. Accordingly, we
demonstrate herein a step-saving new synthesis of the DA
type sensitizers using inexpensive copper salts as catalyst
(Scheme 1). To the best of our knowledge, this work represents
[a]
Prof. Dr. C.-Y. Liu, J.-H. Huang, P.-H. Lin, W.-M. Li
Department of Chemical and Materials Engineering
National Central University
Jhongli District, Taoyuan, Taiwan 320 (R.O.C)
TEL: (+886) 3-422-7151ext.34217
E-mail: cyliu0312@ncu.edu.tw
[b]
Prof. Dr. K.-M. Lee
Department of Chemical and Materials Engineering
Chang Gung University
Guishan District, Taoyuan, Taiwan 333 (R.O.C)
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10.1002/cssc.201700421
ChemSusChem
FULL PAPER
Table 1. Synthesis of the D-π-A type dye molecule 3a via Cu-catalyzed
direct C-H arylation: Optimization of the reaction parameters.[a]
Subsequently, reaction scope with respect to the donor moiety
of the D-π-A dye molecule was investigated under the reaction
conditions obtained in entry 8 of Table 1. Initially, as shown in
Table 2, reaction of 2a with various triphenylamine (TPA)-based
aryl iodides (1b-g) bearing linear-/branched alkyl groups,
methoxyl, thiomethyl, or ethylenedioxy substituents led to the
Table 2. Substrate scope exploration concerned with donor moieties.[a]
Yield
Entry
Ligand
Base
Solvent
(%)[c]
40
52
18
39
60
72
36
85
40
66
16
33
31
24
trace
60
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24[b]
PPh3
PCy3
dppe
dppb
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
K3PO4
---
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
DMPU/m-xylene
diglyme/m-xylene
DMAc/m-xylene
DMSO/m-xylene
NMP/m-xylene
DMPU
Xantphos
DPEphos
2,2‘-bipyridyl
1,10-phenanthroline
neocuproine
bathophenanthroline
ethylenediamine
TMEDA
---
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
1,10-phenanthroline
74
66
13
25
70
45
25
m-xylene
DMPU/m-xylene
[a]
Unless specified, the C-H arylation was conducted with 4-
iodotriphenylamine 1a (2.00 mmol) and 2-thiophenecarboxaldehyde 2a (1.00
mmol) under N2 in the presence of CuCl2 (30 mol%, ultra-dry, 99.995% metals
basis), ligand (30 mol%), and base (2.00 mmol) in 2 mL solvent (v/v = 1:1 if a
[b]
cosolvent system is used) at 120 oC for 24 h.
used instead of 1a. [c] Isolated yield.
4-Bromotriphenylamine was
did not occur and both starting substrates (1a, 2a) could be
recovered (entry 17).
Next on, the (co)solvent system was examined. Combining m-
xylene with diglyme or DMAc gave 3a in moderate yields (74%,
66%; entries 18, 19). Reaction in other cosolvents such as
DMSO/m-xylene or NMP/m-xylene appeared to be much less
effective (13%, 25%; entries 20, 21). Importantly, using either
DMPU or m-xylene as sole reaction solvent led to lower yields of
3a (70%, 45%; entries 22, 23) when compared with the result of
entry 8 (85%), indicating DMPU/m-xylene as cosolvent was
essential to high conversion of the C-H arylation. Finally, under
optimum reaction conditions (CuCl2, phenanthroline, Cs2CO3,
DMPU/m-xylene, 120 oC, 24 h), we tested the C-H arylation with
4-bromotriphenylamine, however, it was found that product 3a
was generated in only 25% yield (entry 24).
[a]
Unless specified, the C-H arylation was conducted with (hetero)aryl iodides
1b-v (2.00 mmol) and 2-thiophenecarboxaldehyde 2a (1.00 mmol) under N2 in
the presence of CuCl2 (30 mol%, ultra-dry, 99.995% metals basis), 1,10-
phenanthroline (30 mol%), and Cs2CO3 (2.00 mmol) in DMPU/m-xylene (2 mL,
v/v = 1:1) at 120 oC for 24 h. [b] Isolated yields.
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10.1002/cssc.201700421
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formation of desired D-π-A type products in moderate to good
isolated yields (65-91%, 3b-3g). When the donor moiety was
changed to carbazole-based derivatives, we obtained relatively
lower yields of pure 3h and 3i (61%, 31%), due to the inefficient
separation of product 3h or 3i from the unreacted (4-
iodophenyl)- or (5-iodothienyl)carbazoles (1h-i). In addition, we
examined a series of 9-substituted-3-iodocarbazoles (1j-p) as
another important class of hole-transporting/donor-type
molecules. Under optimum reaction conditions, products 3j-3p
were isolated in 64-87% yields and we found ester (3n) and
The reaction scope was further explored using various (hetero)
-aryl aldehydes (Table 3). C-H arylation of aldehydes 2b or 2c
with different donor-type iodoarenes under optimum reaction
conditions generated dye molecules 4a, 4b in good isolated
yields (84%, 82%). Electron-deficient aldehydes bearing diester-
group (2d) or thieno[3,4-c]pyrrole-4,6-dione (TPD, 2e)
underwent arylation to provide products 4c and 4d in relatively
lower yields (51%, 35%), presumably owing to the steric effect
resulted from the ester or TPD moiety of 2d or 2e. Additionally,
we also investigated other heteroaryls including furan,
nitrile (3o) groups were well tolerated. Further, aryl iodides (1q-r) selenophene, and pyrrole, affording the corresponding products
containing electron-rich phenothiazines were also tested,
affording products 3q, 3r in 68%, 90% yields, respectively.
Donor substrates involving pyrene (1s) or alkyl bithiophene (1t)
proceeded to give the corresponding products in moderate
yields (76%, 63%; 3s, 3t). Interestingly, 4-iodophenyltriazene
in 50-72% yields (4e, 4f, 4g). Notably, desired molecule 4h was
isolated in trace amount, due to the poor reactivity of N-methyl-
2-formylpyrrole under present reaction conditions (starting
materials mostly recovered).
(1u)
underwent
C-H
arylation
smoothly
with
2-
In order to apply current synthetic methodology in modern
dye-sensitized solar cells (DSSCs), three D--A type organic
dyes (5a, 5b, and 5c) were facilely prepared using Cu-catalyzed
C-H arylation as key step. As shown in Scheme 2, under our
optimum reaction conditions employed in Table 2-3, 5a-c were
isolated in moderate yields (40-66%). Conversion of the
aldehyde group of 5a-c into cyanoacetic acid (as anchoring
groups for TiO2 attachment) by Knövenagel condensation
afforded desired dye-sensitizers CYL-5 ~ CYL-7.
thiophenecarboxaldehyde and 3u was obtained in good yield
(80%). Organic-electronically important naphthyl group (from 2-
iodonaphthalene, 1v) was also efficiently introdcued to D-π-A
system to furnish product 3v in 62%.
Table 3. Substrate scope exploration concerned with -acceptor
moieties.[a]
Scheme 2. [Cu]-catalyzed C-H arylations as key-step to synthesize new
sensitizers CYL-5, CYL-6, and CYL-7.
The UV-Vis absorption and electrochemical properties of
sensitizers CYL-5~7 were studied and the results were collected
in Table 4. Although CYL-5 and CYL-6 are structurally similar, in
absorption spectra they exhibited fairly different λmax and  in
THF solutions. Particularly, the furan-containing sensitizer (CYL-
6) showed much higher molar extinction coefficient in the intra-
[a]
Unless specified, the C-H arylation was conducted with aryl iodides (2.00
mmol) and various aldehydes 2b-h (1.00 mmol) under N2 in the presence of
CuCl2 (30 mol%, ultra-dry, 99.995% metals basis), 1,10-phenanthroline (30
mol%), and Cs2CO3 (2.00 mmol) in DMPU/m-xylene (2 mL, v/v = 1:1) at 120
oC for 24 h. [b] Isolated yields.
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10.1002/cssc.201700421
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molecular charge transfer (ICT) band (ɛ = 22703 M-1cm-1 for
CYL-5; ɛ = 49278 M-1cm-1 for CYL-6). On the other hand, CYL-7
was found to exhibit more red-shifted absorption band and
of 6.20 %. In terms of the photovoltaic performance and the
molecular structure of the dyes (CYL-5~7), we may conclude
that longer conjugation length could lead to a higher Jsc thus
giving a better PCE in the case of CYL-7. Besides, to further
enhance PCE, selection of an appropriate hole-transporting unit
(electron donors) would be of high importance. In three
sensitizers, we assumed that the electron-donating ability of
alkoxyl-substituted triphenylamine (in CYL-7) is greater than that
of triphenylamine-carbazole-hybrid type donors (in CYL-5~6),
which was consistent with the UV-Vis absorption data obtained
in Table 4.
opt
narrower optical band gap (Eg = 1.07~1.09 eV for CYL-5 &
opt
CYL-6; Eg
= 0.89 eV for CYL-7), indicating the electron-
donating ability of the alkoxyl-substituted triphenylamine (TPA) is
stronger than the TPA-Cbz-hybrid donor moieties of CYL-5 &
CYL-6 and a more efficient ICT effect in CYL-7. In addition,
cyclic voltammetry measurements were conducted in THF
solutions containing (n-C4H9)4NPF6 as supporting electrolyte.
Compared with CYL-5~6, the uplifted EHOMO of CYL-7 (EHOMO = -
5.58 eV) was also attributable to the stronger electron-donating
ability of its TPA unit.
Table 5. Photovoltaic performance of dye-sensitized solar cells
CYL-6, and CYL-7 [a], [b]
fabricated using CYL-5, .
Voc [V]
Jsc [mA/cm2]
FF [%]
PCE [%]
Table 4. UV-Vis absorption and electrochemical properties of sensitizers
DSSCs
CYL-5
,
CYL-6, and CYL-7
.
CYL-5
CYL-6
CYL-7
Z907
0.65
0.72
0.75
0.69
5.90
7.90
76.9
71.3
65.6
69.5
2.95
4.06
6.20
7.07
opt
λmax 
λmax on TiO2 Eg
E1/2 EHOMO/ELUMO
Dyes
[nm][a] [M-1cm-1][a]
[nm][a]
438
[eV][b] [V][c] [eV][d]
12.60
14.75
CYL-5 427
CYL-6 402
CYL-7 481
22703
49278
20645
2.08
2.06
1.71
1.07 -5.77/-3.69
1.09 -5.79/-3.72
0.89 -5.58/-3.88
[a]
Irradiation light source: AM 1.5G solar simulator (100 mW/cm2);
449
Photo-electrode (working electrode): TiO2 film was composed of 16 μm
thick mesoporous anatase-TiO2 nanoparticles with a diameter of 20 nm
and has an active area of 0.160 cm2; Electrolyte solution: a mixture of 0.6
M 1-butyl-3-methylimidazolium iodide ([bmim][I]), 0.1 M LiI, 0.05 M I2, 0.1
442
[a] Measured in THF solutions (~10-5 M); : extinction coefficient. [b] The optical
band gap, Egopt, was calculated by 1240/λonset (dyes on TiO2 films). [c] The half-
wave potential, E1/2, was calculated by (Epa+Epc)/2, where Epa and Epc are the
potential energy of anodic and cathodic peaks, respectively; The
measurements are conducted in THF solutions (~10-4 M) containing (n-
C4H9)4NPF6 as a supporting electrolyte under a scan rate of 100 mVs-1. [d] The
M guanidinium thiocyanate (GuNCS) and 0.5 M 4-t-butylpyridine (TBP)
[b]
in acetonitrile.
Chenodeoxycholic acid (CDCA) was added as co-
adsorbent (5 mM for CYL-5; 5 mM for CYL-6; 10 mM for CYL-7).
Optimization of the different amount of CDCA addition has been
summarized in Table S1 in Supporting Information.
HOMO energy level, EHOMO, was calculated by –[(E1/2 + 0.197)vs.NHE + 4.500]
opt
eV; ELUMO = EHOMO + Eg
.
CYL-5~7 were employed as sensitizers to fabricate dye-
sensitized solar cells and the photovoltaic
performance/parameters were summarized in Table 5 and
Figure 1 (detailed procedures for solar cells fabrication and
characterization were recorded in Supporting Information). Dye
sensitizer with a furan moiety (CYL-6) showed higher open-
circuit voltage (Voc) and short-circuit current (Jsc) than its
thiophene congener (CYL-5), thus leading to an enhanced PCE
of 4.06%. CYL-7 has the longest conjugation extension[23]
among three sensitizers, making it exhibit much higher Jsc (12.60
mA/cm2). This may also result from the superior electron-
donating ability of alkoxyl-substituted triphenylamine of CYL-7
thereby generating an efficient intramolecular charge transfer
(ICT). This result was in agreement with the outcome obtained
from the incident photon-to-current conversion efficiency (IPCE,
Figure 1b), showing that CYL-7 had the largest integral area of
IPCE curve. In addition, three dyes were also investigated by
electrochemical impedance spectroscopy (EIS). Figure 1c
demonstrated Nyquist plots concerned with DSSCs based on
CYL-5~7 in dark with a bias of 0.65~0.75 V. Under dark
conditions, CYL-7 exhibited the largest charge transfer
impedance between TiO2-electrolyte interface, thus leading to a
higher Voc while irradiated under AM 1.5G solar simulator.
Among three new sensitizers, CYL-7 achieved a Voc of 0.75 V, a
(a)
(b)
Jsc of 12.60 mA/cm2, and
corresponding to the highest power conversion efficiency (PCE)
a fill factor (FF) of 65.6%,
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10.1002/cssc.201700421
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(CDCl3, 75 MHz, ppm): δ 182.4, 156.5, 155.1, 150.0, 140.8, 139.9, 137.7,
127.2, 127.1, 124.2, 122.3, 119.3, 114.9, 55.5.
Acknowledgements
(c)
Financial support provided by the Ministry of Science and
Technology (MOST), Taiwan (MOST 103-2113-M-008-009-MY2)
and the National Central University (NCU) are gratefully
acknowledged. We also thank the staffs of Research Center for
New Generation Photovoltaics (RCNPV, NCU) for giving helpful
advices and guidance on the fabrication and characterization of
dye-sensitized solar cells.
Figure 1. Photovoltaic characteristics of CYL-5, CYL-6, and CYL-7:
(a) photocurrent density vs. photovoltage; (b) Incident photon-to-
current conversion efficiency (IPCE); (c) electrochemical impedance
spectroscopy (EIS).
Keywords: Copper • Direct C-H Arylations • Dye-Sensitized
Solar Cells (DSSCs) • Sn- & Pd-Free Process
[1]
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Conclusions
Unlike the traditional synthetic approaches for organic
electronics, we demonstrate herein a step-economical, Pd and
Sn-free new synthetic pathway to access various DA type
dye molecules employing inexpensive CuCl2 as catalyst. We find
that, after reaction condition optimizations, most desired
products targeting on dye-sensitized solar cells (DSSCs)
applications can be isolated in good to excellent yields (total 32
examples). Based on this Cu-catalyzed C-H arylation
methodology, new sensitizers (CYL-5~7) are facilely obtained
and fabricated as DSSCs devices. One of them (CYL-7) can
reach the power conversion efficiency (PCE) of 6.20%. We
expect this relatively greener synthetic protocol would turn into a
viable alternative for the efficient preparation of small-molecule
-functional materials.
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Experimental Section
Representative procedure for the Cu-catalyzed C-H arylation of 2-
thiophenecarboxaldehyde
(2a)
with
4-iodo-N,N-bis(4-
methoxyphenyl)aniline (1e) ─ Synthesis and characterization of
product 3e: To a solution of CuCl2 (40 mg, 0.30 mmol), phenanthroline
(54 mg, 0.30 mmol), Cs2CO3 (652 mg, 2.00 mmol) in the co-solvent of
N,N′-dimethylpropylene urea (DMPU) and m-xylene (total 2 mL, v/v =
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1:1) in
a flame-dried Schlenk tube were added 4-iodo-N,N-bis(4-
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methoxyphenyl)aniline (1e) (862 mg, 2.00 mmol) and 2-
thiophenecarboxaldehyde (2a) (112 mg, 1.00 mmol) under N2. The
reaction mixture was then heated at 120 °C under N2 for 24 h. After the
reaction mixture had cooled to room temperature, water (10 mL) was
added. The mixture was extracted with ethyl acetate (2 × 30 mL), and the
combined organic layers were washed with brine (50 mL), dried (Na2SO4)
and concentrated in vacuo. Purification by flash chromatography (ethyl
acetate : hexanes = 20 : 80) yielded the pure product 3e (378 mg, 91 %).
Red viscous liquid. 1H NMR (CDCl3, 300 MHz, ppm): δ 9.82 (s, 1 H), 7.67
(d, J = 4.0 Hz, 1 H), 7.46 (d, J = 8.8 Hz, 2 H), 7.25 (d, J = 4.0 Hz, 1 H),
7.05-7.15 (comp, 4 H), 6.80-6.97 (comp, 6 H), 3.81 (s, 6 H); 13C NMR
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10.1002/cssc.201700421
ChemSusChem
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Jiung-Huai Huang, Po-Han Lin, Wei-
Ming Li, Kun-Mu Lee, and Ching-Yuan
Liu*
00
Page No. – Page No.
Greener Synthesis of D--A Organic
Sensitizers via Cu-Catalyzed Direct
Arylations: Development of Sn- & Pd-
Free Process for Dye-Sensitized Solar
Cells
Cu-catalyzed direct C-H arylation is, for the first time, used as key step to synthesize
various D--A type organic dyes for efficient dye-sensitized solar cells (DSSCs).
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