Effect of Thiophene Spacer Position in Carbazole-Based Dye-Sensitized Solar Cells on Photophysical, Electrochemical and Photovoltaic Properties

Article abstract of DOI:10.1002/ejoc.201600353

The synthesis, photophysical and electrochemical properties, and photovoltaic performances in dye-sensitized solar cells (DSSCs) of five new metal-free organic sensitizers with a carbazole moiety as the electron donor are reported. The compounds contain a thiophene at various positions and a cyanoacrylic acid unit as the electron acceptor. The position of the thiophene moiety affected both the photophysical and electrochemical properties, leading to conversion efficiencies of 1.44 to 4.57 % under AM 1.5 solar conditions (100 mW cm^–2). The best performance and longest electron lifetime were found when a thiophene was located on both donor and acceptor sites. The DSSC showed a short-circuit current (J_sc) of 8.59 mA/cm², an open-circuit photovoltage (V_oc) of 0.75 V, and a fill factor (FF) of 0.71. The position of the π-conjugated bridges not only affects the absorption spectra and the energy levels of the sensitizers, but also adjusts the electron lifetime.

Full text of DOI:10.1002/ejoc.201600353

DOI: 10.1002/ejoc.201600353
Full Paper
Sensitizers
Effect of Thiophene Spacer Position in Carbazole-Based Dye-
Sensitized Solar Cells on Photophysical, Electrochemical and
Photovoltaic Properties
Ruslan Samae,[a] Panida Surawatanawong,[a,b] Utt Eiamprasert,[a] Songyos Pramjit,[c]
Laksana Saengdee,[a] Pramuan Tangboriboonrat,[a] and Supavadee Kiatisevi*[a,b]
Abstract: The synthesis, photophysical and electrochemical 1.44 to 4.57 % under AM 1.5 solar conditions (100 mW cm–2).
properties, and photovoltaic performances in dye-sensitized so- The best performance and longest electron lifetime were found
lar cells (DSSCs) of five new metal-free organic sensitizers with when a thiophene was located on both donor and acceptor
a carbazole moiety as the electron donor are reported. The sites. The DSSC showed a short-circuit current (Jsc) of 8.59 mA/
compounds contain a thiophene at various positions and a cm2, an open-circuit photovoltage (Voc) of 0.75 V, and a fill fac-
cyanoacrylic acid unit as the electron acceptor. The position tor (FF) of 0.71. The position of the π-conjugated bridges not
of the thiophene moiety affected both the photophysical and only affects the absorption spectra and the energy levels of the
electrochemical properties, leading to conversion efficiencies of sensitizers, but also adjusts the electron lifetime.
Introduction
and perylene[14] have been utilized successfully in DSSCs. The
strong electron-donating ability and facile structural modifica-
tion mean that carbazole,[15] triphenylamine,[16] and hetero-
antracene (phenothiazine and phenoxazine)[17] have also been
widely explored for application in DSSCs.
Dye-sensitized solar cells (DSSCs) are regarded as one of the
most promising photovoltaic devices because of their high effi-
ciency, low production cost, and facile fabrication.[1] For DSSCs,
the sensitizer is one of the most important components affect-
ing the solar cell performance. Metal complex structures such
as black dye[2] and N719[3] have been reported as highly effi-
cient light-harvesters for DSSCs, with overall power conversion
efficiencies (η) of over 11 %. A significant shortcoming of Ru
In general, metal-free organic sensitizers have a push–pull
D–π–A structural motif, in which D, π, and A represent electron
donor, π-conjugated bridge, and electron acceptor, respec-
tively.[1,18] Upon light absorption, this structure can induce in-
tramolecular charge transfer (ICT) from the donor to the ac-
ceptor through the π-bridge. Understanding the structure–
property relationships is crucial to construct efficient DSSC devi-
ces. Important factors that influence DSSC performance are
(i) light-harvesting properties of the dyes, (ii) the alignment of
the lowest unoccupied molecular orbital (LUMO) near the an-
choring group and above the conduction band edge of TiO2,
(iii) tuning energy levels by introducing π-bridge units, and
(iv) the properties of the redox couple in the electrolyte.[19]
polypyridyl-based dyes is their low molar extinction coefficient
(εmax ca. 1.4 × 104
M
cm–1).[4] Compared with the Ru com-
–1
plexes, organic dyes provide higher molar extinction coeffi-
cients (> 2.5 × 104
M
cm–1),[5] facile modification of molecular
–1
structures and require less expensive synthetic methods.[1,6] Nu-
merous organic molecules have been developed and demon-
strated as promising sensitizers in photovoltaic devices to
date;[7] for example, coumarin,[8] indoline,[9] tetrahydroquin-
oline,[10] N,N-dialkylaniline,[11] hemicyanine,[12] merocyanine,[13]
Among the metal-free organic dyes, carbazole and its deriva-
tives have been widely employed as a functional building block
in the fabrication of highly efficient DSSCs because they show
good light-harvesting capacity and high efficiency of up to
7.13 %.[20] The advantages of using carbazole are the strongly
donating nature of the nitrogen atom and the suppression of
dye aggregation upon to introduction of a bulky alkyl substitu-
ent at the nitrogen atom.[6,15,21] These studies, however, con-
centrated on variation of π-conjugation,[6,22] number of donor
units,[20] functionalization of carbazole at different positions,[23]
and variation of anchoring groups,[24] whereas a systematic ex-
tension of the π-system on the carbazole dyes has not been
explored. Given that the introduction of thiophene moieties
generally leads to a redshift and broadening of the absorption
[a] Department of Chemistry, Faculty of Science, Mahidol University,
272 Rama VI Road, Ratchathewi, Bangkok 10400, Thailand
http://chemistry.sc.mahidol.ac.th/
[b] Center of Sustainable Energy and Green Materials, and Center of
Excellence for Innovation in Chemistry (PERCH-CIC), Faculty of Science,
Mahidol University,
272 Rama VI Road, Ratchathewi, Bangkok 10400, Thailand
E-mail: supavadee.mon@mahidol.ac.th
http://www.sc.mahidol.ac.th/academics/staff/AC_s/Supavadee_M.htm
[c] Department of Chemistry, Faculty of Science and Technology,
Phuket Rajabhat University,
Ratsada, Muang, Phuket 83000, Thailand
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600353.
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Figure 1. Chemical structures of dyes RL1–5 based on carbazole.
spectra, we thus wished to investigate the effect of introducing pathways and characterization of all the organic dyes devel-
thiophene units at various positions of the carbazole-based dye oped herein. Molecular orbital calculations, photochemical
on DSSC properties. We reasoned that such structural changes properties, electrochemical characteristics, photovoltaic per-
could influence the charge-transfer properties of the carbazole formances and electrochemical impedance spectroscopy of the
dyes differently upon extension of the π-conjugation at various five new sensitizers were investigated and are discussed in de-
positions of the dye architecture.
tail.
In this study, we focused on an investigation of the light-
harvesting and electron transfer in the DSSCs and correlate
them with the structural changes and the electronic properties
of the dyes. We herein report on the design and synthesis of a
new series of carbazole dyes (RL1–5 as shown in Figure 1) con-
sisting of carbazole primary donor, 4-methoxyphenyl secondary
donor, 2-cyanoacetic acid acceptor, and thiophene spacers. Alk-
oxyphenyl moieties not only act as the electron donor but they
can also reduce the extent of aggregation of dyes on TiO2
films.[25]
Results and Discussion
Synthesis
The synthetic route to dyes RL1–5 is shown in Scheme 1. The
key intermediates in all syntheses were 3-bromocarbazole de-
rivatives; that is, compounds 5 and 6. 3,6-Dibromo-9-butyl-9H-
carbazole (5) was prepared from carbazole by a sequence of N-
alkylation and bromination in a total yield of 92 %. Compound
Starting with RL1 as a reference, four approaches were car- 6 was made from (4-methoxyphenyl)boronic acid and 5 by a
ried out to extend the π-conjugation as follows: (i) the exten- Suzuki coupling reaction catalyzed by Pd(PPh3)4. Aldehydes 7
sion of the D–A distance by introducing a thiophene between and 9 were obtained in reasonable yields of 60 % by the reac-
carbazole and the acceptor (RL2), (ii) the extension of the donor tion of 6 and 5, respectively, with nBuLi in tetrahydrofuran (THF)
system by inserting a thiophene between carbazole and 4- at –78 °C and subsequently with N,N-dimethylformamide
methoxyphenyl group (RL3), (iii) combining approach (i) and (DMF). Suzuki coupling reaction between 9 and 3, which was
(ii) by inserting a thiophene in both the donor and acceptor accomplished by the reaction of 2 with nBuLi at –78 °C and
sites (RL4), and (iv) enhancing the electron-donating ability of subsequently with triisopropylborate, was carried out using
the donor system by attachment of two 4-methoxyphenyl Pd(PPh3)4 as the catalyst to afford 10 in 75 % yield. Aldehyde 12
groups at the 3- and 5-positions of the thiophene on the donor was synthesized from 5 by a sequence of two Suzuki coupling
site (RL5). Finally, all the molecules were applied in nanocrystal- reactions with 3 and 5-formyl-2-thiopheneboronic acid, respec-
line TiO2-based solar cells. In this paper, we report the synthetic tively. Aldehyde 13 was synthesized by Suzuki coupling reac-
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tion of 9 with 2-thienylboronic acid using Pd(PPh3)4 as catalyst. thiophene group between the carbazole primary donor and ac-
Compound 13 was then reacted with N-bromosuccinimide ceptor moiety, i.e., RL2 and RL4, also display a more extended
(NBS) and (4-methoxyphenyl)boronic acid, respectively, to pro- absorption than those without.
duce 15. Finally, 7, 8, 10, 12, and 15 were converted into dyes
RL1–5, respectively, by Knoevenagel condensation reaction in
good yields of 63–89 %. The structures of all intermediates and
target dyes were confirmed by standard spectroscopic meth-
ods.
Figure 2. Absorption spectra of RL1–5 (a) in CH2Cl2 solution and (b) on nano-
crystalline TiO2 film, using a base TiO2 film as a reference, obtained by soak-
ing the film in a CH2Cl2 solution containing 1 × 10–5
M sensitizer overnight.
Table 1. Photophysical and electrochemical properties of dyes RL1–5.
Dye
λmax [nm][a]
(ε × 104 –1 cm–1
λmax
HOMO[c]
[V]
Gap[d]
[eV]
LUMO[e]
[V]
[b]
M
)
[nm]
Scheme 1. Syntheses of dyes RL1–5. Reaction conditions: (a) Pd(OAc)2, K2CO3,
DMF, 90 °C, 2.5 h; (b) NBS, DMF, 0 °C to room temp., 2 h; (c) 1. nBuLi, THF,
–78 °C, 15 min; 2. B(OiPr)3, –78 °C to room temp., 2 h; 3. HCl (aq.); (d) C4H9I,
KOH, DMF, room temp., 16 h; (e) NBS, THF, room temp., 2 h, dark; (f) Pd(PPh3)4,
K2CO3, THF/H2O, 80 °C, 24 h; (g) 1. nBuLi, THF, –78 °C, 1 h; 2. DMF, –78 °C,
30 min; 3. HCl (aq.); (h) CNCH2COOH, piperidine, CH3CN, reflux, 16 h;
(i) CNCH2COOH, NH4OAc, CH3COOH, 120 °C, 24 h.
RL1
RL2
RL3
RL4
RL5
338 (1.61), 407 (1.83)
369 (0.82), 459 (2.40)
339 (3.54), 408 (1.87)
351 (3.21), 436 (2.07)
337 (5.27), 395 (3.19)
389
424
384
421
383
1.07
0.99
0.84
0.77
0.86
2.70
2.34
2.62
2.32
2.58
–1.63
–1.35
–1.78
–1.55
–1.72
[a] Absorption maximum in CH2Cl2 solution (1 × 10–5
mum on TiO2 film. [c] HOMO energies were measured in CH2Cl2 with 0.1
M). [b] Absorption maxi-
M
nBu4NPF6 as electrolyte, potentials measured vs. Ag/Ag+ were converted into
NHE by addition of +0.63 V. [d] The gap was derived from the absorption
onset wavelength of the dye solution. [e] LUMO energies were calculated by
using: LUMO = HOMO – gap.
Photophysical Properties
UV/Vis absorption spectra of dyes RL1–5 in CH2Cl2 (1 × 10–5
solution and adsorbed on a thin TiO2 film are shown in Figure 2
M)
In Figure 2 (b) and Table 1, the absorption spectra of all
and their absorption data are listed in Table 1. In CH2Cl2 solu- dyes on TiO2 films show a blueshift relative to those in solution.
tion, all the dyes exhibit two major absorption bands, appear- Although the broader absorptions of dyes RL2 and RL4 on the
ing at 340–370 nm and 400–460 nm. The former absorption TiO2 films are favorable for light harvesting, their absorption
band can be ascribed to π–π* electron transitions, whereas the maxima were shifted to shorter wavelengths. This blueshift hap-
absorption peaks at longer wavelengths are attributed to intra- pens to be a common phenomena for such types of organic
molecular charge transfer (ICT) transitions. Compounds with a dyes,[26] resulting from H-aggregation and/or deprotonation of
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the carboxylic acid units, indicating that the carboxylic moieties compared with the reference dye without thiophene, whereas
are stronger electron acceptors compared with carboxylate- the thiophene on the donor site affects the spectral shift of the
titanium units.[19,23] However, such a spectral blueshift (ca. 10– dyes to a much greater extent (redshift of 30 nm). Thus, spectral
15 nm) is minimal in the case of RL4 and RL5, suggesting their positions of the maxima and minima are also influenced by
relatively weak H-aggregation compared with the other RL core donor units.
dyes.
Moreover, upon inserting a thiophene unit between primary
and secondary donors, RL3 and RL5 exhibited similar absorp-
tion patterns both in CH2Cl2 solution and on TiO2 film. The
Electrochemical Properties
maximum absorption peaks for RL1, RL3, and RL5 were 407,
408, and 395 nm, respectively. Compared with RL1 (dye without
any spacer), the λmax of RL5 is blueshifted by 12 nm, indicating
less extended π-conjugation caused by an increased twist angle
between the carbazole unit and the thiophene group contain-
ing two 4-methoxyphenyl rings, which has been supported by
DFT calculations. In other words, the presence of two secondary
donor groups on the thiophene ring of RL5 appears to reduce
the extent of π conjugation. Furthermore, the thiophene as part
of the donor moieties impacts the spectral shift of the dye RL3
very little compared with RL1 (shifts of 1 nm for both π–π*
electron and ICT transitions).
To investigate the possibilities of electron transfer from the ex-
cited state of RL1–5 dyes to the conduction band of TiO2 and
regeneration of the dyes, the redox potentials of all dyes in
CH2Cl2 were measured by cyclic voltammetry (CV) (Figure 3);
the results are summarized in Table 1. CV was carried out in
0.1
M tetrabutylammonium hexafluorophosphate (TBAPF6) as
supporting electrolyte, Ag/Ag+ as reference electrode, platinum
wire as counter electrode, and glassy carbon as working elec-
trode. The Ag/Ag+ reference electrode was calibrated by using
ferrocene/ferrocenium (Fc/Fc+). The oxidation potentials (Eox)
measured vs. Ag/Ag+ were converted into normal hydrogen
electrode (NHE) by addition of +0.63 V.[28] In addition, the re-
In contrast, we observed that insertion of a thiophene spacer duction potential vs. NHE (Ered) can be obtained from the differ-
between the carbazole primary donor and acceptor moiety ence between the first oxidation potential and the E0–0 value
makes the dominant absorption maximum of the sensitizer red- estimated from the absorption onset. All the dyes demon-
shifted ca. 30–50 nm, leading to a preferred absorption spec- strated HOMO values of RL1–5 corresponding to their first
trum for efficient light harvesting of solar energy. The λmax val- redox potentials of 1.07, 0.99, 0.84, 0.77, and 0.86 V vs. NHE,
ues both in solution and TiO2 surface are 459 and 424 nm for respectively (Figure 4). The HOMO potentials of all five dyes
RL2 and 436 and 421 for RL4, as compared with 407 and were more positive than that of iodide/triiodide redox potential
389 nm for RL1, respectively. These results show that increasing (0.40 V vs. NHE), indicating that the electronic matching can
the spectral coverage in the ICT region of a dye can be achieved occur to accept electrons from electrolyte. Thus, dye regenera-
by addition of a π spacer on the acceptor site and spectral tion should be thermodynamically favorable. Furthermore, we
changes on the film state are clearly affected by the position of observed all excited oxidation potentials (ELUMO) in the range of
the π spacer. Notably, different results were found from the –1.35 to –1.78 V vs. NHE for RL1–5. Based on the ELUMO values
investigation previously reported on dyes based on triphenyl- obtained above the conduction band edge of TiO2 (–0.50 V vs.
amine core donor,[27] in which the redshift in solution was only NHE), there is enough driving force for electron injection into
5 nm upon insertion of one thiophene unit on the acceptor site the conduction band of TiO2.
Figure 3. Cyclic voltammograms of RL1–5. The onset potentials of the first oxidation of the dyes were determined from intersection of the tangents between
the baseline and the signal current and measured in CH2Cl2 solution with 0.1
M
nBu4NPF6 used as the supporting electrolyte.
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Figure 4. Molecular orbital energy diagram of HOMO and LUMO of RL1–5.
It was also found that the introduction of two π spacers in
RL4 had a greater effect on the shift in HOMO than that of one
π spacer in RL2 and RL3 owing to higher π conjugated system
of RL4. Similar evidence has been reported previously.[29] The
oxidation potentials in decreasing order are RL4 < RL3 < RL5
< RL2. Attaching the thiophene on both sides of the carbazole
donor significantly extends the π-conjugation of the donor in
RL4, which raises the HOMO energy and facilitates its oxidation.
Interestingly, varying the position of the thiophene spacer
had a greater effect on the LUMO level than on the HOMO
level. The D′′–D′–π–A architecture of RL2 shifted the LUMO
positively by 0.43 and 0.37 V relative to the D′′–π–D′–A and
2D′′–π–D′–A structural motifs of RL3 and RL5, implying that
the introduction of a thiophene spacer on the acceptor site
decreases the energy gap between HOMO and LUMO, resulting
in a redshifted spectrum of RL2. Likewise, this phenomenon
was also found in RL4, which contains the D′–π–A moiety in
common. The results discussed above indicate that the photo-
electrochemical properties of the dye series could easily be
tuned by changing the position of the thiophene unit.
Figure 5. Dihedral angle of neighboring rings for RL1–5.
Table 2. Dihedral angles of RL1–5.
Dye
Dihedral angle [°][a]
D′′–Cbz D′′–3π2 D′′–5π2 π2–Cbz Cbz–π1 π1–A
Cbz–A
RL1
RL2
RL3
RL4
RL5
38.4
38.0
–
–
–
–
–
–
–
–
–
24.4
25.5
26.0
–
–
24.7
27.7
42.8
–
22.8
–
23.9
–
–
1.26
–
1.04
–
0.02
–
0.29
–
45.1
0.07
[a] Cbz = carbazole, D′′ = 4-methoxyphenyl, π1 = thiophene located at the
acceptor site, 3π2 = thiophene that is located at the donor site and linked to
the secondary donor at the 3-position, 5π2 = thiophene that is located at the
donor site and linked to the secondary donor at the 5-position.
The Optimized Ground-State Structures
The ground-state structures were optimized at the B3LYP/6-
31G(d) level. The optimized ground-state geometries of RL1–5
dyes are shown in Figure 5, and their dihedral angles are listed
in Table 2. We classify each group in a dye molecule into four tached to 4-methoxyphenyl group as secondary dihedral angle
fragments; the carbazole (Cbz) core, acting as primary electron (SDA); (iii) between the secondary donor and the 5-position of
donor (denoted as D), the 4-methoxyphenyl, acting as auxiliary the thiophene as tertiary dihedral angle (TDA) and; (iv) between
or secondary electron donor (denoted as D′′), the thiophene the secondary donor and the 3-position of the thiophene as
moiety, acting as π spacer (denoted as π), and cyanoacrylic acid quarternary dihedral angle (QDA).
(denoted as A). Considering two dihedral angles (i) between π
spacer and electron acceptor in RL2 and RL4 and (ii) between
carbazole primary donor and electron acceptor in RL1, RL3,
and RL5, the optimized structures of RL1–5 dyes in the ground
electronic state showed that both dihedral angles in columns
π1–A and Cbz–A as shown in Table 2 were calculated as 0.02–
1.26°, indicating that the cyanoacrylic acid can adopt a coplanar
orientation relative to both neighboring thiophene spacer and
carbazole.
RL1, as the reference dye containing no thiophene spacer,
displayed the most highly twisted dihedral angle of 38.4° be-
tween the carbazole core and the 4-methoxyphenyl auxiliary
donor. A comparable value of this dihedral angle at the same
position was calculated for RL2. Introducing thiophene spacers
at different positions showed slightly different dihedral angles;
for example, PDAs (RL2 and RL4) were calculated to be about
23°, whereas SDAs (in RL3 and RL4) were 26°. These results
suggest that the thiophene groups at the acceptor site tend to
For comparison, we name the dihedral angles (i) between be less twisted to the carbazole primary donor than those at
the carbazole primary donor and the thiophene spacer at- the donor part. The extension of the π-conjugation corresponds
tached to cyanoacrylic acid as primary dihedral angle (PDA); to the redshift of the UV absorption bands in RL2 and RL4. In
(ii) between the primary donor and the thiophene spacer at- contrast, inserting the thiophene at the donor part, which in-
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duced a twist between the two units with SDA of 24.7° (RL3),
Therefore, dihedral angle analysis provides a correlation be-
seems to have no effect on the extension of the π-conjugated tween the physical properties of the carbazole-based dyes and
system. This statement is supported by maximum absorptions thiophene positions as follows: (i) inserting a thiophene at the
of RL3 appearing at wavelengths similar to those of RL1.
Moreover, the PDA and SDA in the D′′–π–D–π–A system of
RL4 were calculated to be 23.9° and 27.7°, respectively, similar
to those of RL2 and RL3. It is interesting to note that the inser-
tion of one thiophene unit at both the acceptor and donor sites
in RL4 did not increase the extent of π conjugation, as ex-
pected according to its spectral absorption which were in the
same redshift region as that of RL2.
donor site induces the formation of two large twist angles (SDA
and TDA) and a nonplanar geometry of the dye molecule, and;
(ii) inserting a thiophene at the acceptor site results in small
dihedral angles (PDAs) and, as a consequence, an extension of
π-conjugation length because the acceptor unit adopts a co-
planar orientation relative to both neighboring thiophene spa-
cer and carbazole. This conclusion can be used as an appropri-
ate approach for the rational design of carbazole-based dyes.
In case of RL5, it is clear that inserting two 4-methoxyphenyl
units at the 3- and 5-positions of the thiophene on the donor
site led to large SDA and QDA of up to 42.8° and 45.1°, respec-
Computational Analysis
tively, because of the greater steric hindrance. These large di- To calculate the frontier orbitals and excited-state features of
hedral angles are thus the major factor contributing to decreas- dyes RL1–5, time-dependent (TD) DFT methods using the
ing the π conjugation length of the dye molecule, which is in Gaussian 09 package were employed. The frontier molecular
good agreement with the observed absorption spectra.
orbitals are shown in Figure 6. RL1 is considered as reference.
Figure 6. Frontier orbital representation with the corresponding energies and HOMO–LUMO energy gap of all RL dyes are computed by using the B3LYP/6-
31G(d) TD-DFT method in simulated dichloromethane environment.
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Its HOMO is well-delocalized over the carbazole and 4-methoxy- havior of the dyes that have strongest absorptions in the violet
phenyl moiety, whereas the HOMO–1 is clearly delocalized over region. Moreover, the orbital representations of RL4 suggest
the entire molecular structure. The HOMO energies of the four the strongest decoupling between HOMO and LUMO corre-
dyes RL2–5 are higher than that of RL1 by the increasing order sponding to the longest π-conjugation path and its lowest
of RL2 (0.37 eV), RL3 (0.48 eV), RL5 (0.49 eV), and RL4 (0.54 eV), HOMO–LUMO gap.
which is in good accordance with the experimental results.
Additionally, the lowest energy excitations in all compounds
Upon inserting the thiophene between donor and acceptor can be assigned to HOMO → LUMO transitions with relatively
in RL2, the HOMO of RL2 is distributed over the entire mol- high changes of the dipole moments of between 25 and 42 D.
ecule, whereas its LUMO is delocalized from the anchoring part The corresponding energies and excitation wavelengths of the
entirely into the adjacent thiophene. Clearly, such a localization lowest transitions are similar to the outcome of the absorption
of the electron density on the spacer and anchoring groups studies in solution and represent the trends in the HOMO–
results in increasing of the HOMO energy by 0.37 eV and de- LUMO energy gaps similar to the experimental results shown in
creasing of the LUMO energy by 0.43 eV. This leads to signifi- Figure 4. Thereby, the oscillator strengths increase from RL5 to
cant lowering of the HOMO–LUMO energy gap, which is in RL3 to RL1 to RL4 to RL2, indicating the highest extinction
good agreement with the redshift of the ICT band from the coefficients for RL4 and RL2.
experiment. Furthermore, the enhancement of its HOMO–1 en-
ergy contributes to the second electronic transition, which is
caused by the HOMO–1 → LUMO transition.
Based on the results of these calculations, we can conclude
that it is possible to decrease the HOMO–LUMO energy gap and
enhance the π-conjugation length by inserting the thiophene
spacer between the acceptor and donor parts. This leads to
a broadening and shift to longer wavelength of the spectral
absorption. Insertion of the thiophene spacer between the pri-
mary and secondary donor sites results in lowering of the sec-
ond excited state, LUMO+1 due to the extension of donor con-
jugation by thiophene and enabling the dominant absorption
in the short wavelength region.
When a thiophene is inserted between the primary and sec-
ondary donors in RL3, the HOMO is delocalized mainly on the
donor site (carbazole and 4-methoxyphenyl) and the LUMO is
distributed over the accepting moieties of the molecules and
partly on the carbazole moiety similar to RL1. The HOMO en-
ergy in RL3 increases by 0.48 eV compared with RL1, whereas
their LUMO energies are almost the same. Moreover, the
LUMO+1 of RL3 decreases in energy by 0.23 eV, which is proba-
bly due to the extension of the donor conjugation by the thio-
phene, indicating the higher probability of absorption for
HOMO → LUMO+1 transition in this dye than those in RL1 and
RL2. This transition corresponds to the secondary electronic
transition in spite of the highest oscillator strength (see
Table S1) because the LUMO+1 of this dye is localized on the
donating site. This implies that the consequent electron injec-
tion process in RL3-based DSSCs should not occur through this
transition. Therefore, the electronic transition in RL3 is mainly
caused by the HOMO → LUMO transition. Interestingly, RL3 and
RL5, which have a thiophene unit at the donor part, show simi-
larities in the ground- and excited-state properties.
Photovoltaic Properties of the DSSCs
The parameters of DSSCs fabricated with these dyes, i.e., short
circuit current (Jsc), open-circuit photovoltage (Voc), fill factor
(FF), and power conversion efficiency (η), measured under
AM 1.5 solar light (100 mW/cm2), are summarized in Table 3,
and the photocurrent–voltage (J–V) plots are shown in Figure 7.
The basic dye structure of RL1 gave a total power conversion
efficiency (η) of 1.87 % (Jsc = 3.69 mA/cm2, Voc = 0.72 V, and
FF = 0.70). Inserting a thiophene moiety on two different sites
of this D′′–D′–A dye structure influences the DSSC efficiency
differently. In comparison to RL3 and RL5, with the thiophene
spacer attached on the donor site, the efficiency of the device
based on RL2, with the thiophene inserted to the acceptor site,
is notably higher. This is probably because of the higher Jsc
value and wider absorption region.
Considering RL4, the upward shift of the HOMO in RL4 com-
pared with RL1 is 0.54 eV, which is larger than those of RL2
(0.37 eV) and RL3 (0.48 eV), whereas LUMO and LUMO+1 are
the same as RL2 and RL3, respectively. Thus, RL4 has
HOMO → LUMO and HOMO → LUMO+1 transition energies
similar to RL2 and RL3, respectively. These comparable elec-
The power conversion efficiencies (η) of RL2, RL3, and RL5
tronic structures in RL3–5 explain the similar photophysical be- were 4.54 % (Jsc = 8.47 mA/cm2, Voc = 0.76 V, and FF = 0.70),
Table 3. Photovoltaic parameters of DSSCs based on RL1–5 and N719 in full sunlight (AM 1.5 G, 100 mW/cm2).[a]
Jsc [mA/cm2]
Voc [V]
FF
η (%)
Loading amount[b] (1 × 10–6 mol [cm2])
Jsc [mA/mol]
Rrec [Ω]
τe [ms]
[c]
Dye
RL1
RL2
RL3
RL4
RL5
N719
3.69
8.47
4.70
8.59
3.30
15.17
0.72
0.76
0.74
0.75
0.65
0.74
0.70
0.70
0.73
0.71
0.67
0.57
1.87
4.54
2.52
4.57
1.44
6.41
0.76
1.26
1.20
1.22
0.55
–
4.86 ×106
6.72 ×106
3.92 ×106
7.04 ×106
6.00 ×106
–
137.9
137.0
165.2
148.8
50.3
–
12.85
12.85
12.85
16.77
1.51
–
[a] The effective areas of all the DSSCs were 0.25 cm2. Dyes were maintained at 0.2 m
1-methyl-3-propylimidazolium iodide (PMII), 0.03 I2, 0.05 LiI, 0.5 4-tert-butylpyridine (4-TBP), 0.1 M
M
in CH2Cl2 solution for dyes RL1–5 and in ethanol for N719. Electrolyte:
guadinium thiocyanate (GNCS) in a mixture of
1
M
M
M
M
acetonitrile and valeronitrile (8.5:1.5, v/v). [b] Amount of dye adsorbed on TiO2 film was estimated from the concentration difference of before and after TiO2
electrode soaking in the dye solution by means of UV/Vis absorption spectroscopy. [c] The short-circuit photocurrent generated per mol of dye molecules.
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showed the lowest FF, as reflected from the decrease of ad-
sorbed dye amount on the TiO2 electrode, compared with a
DSSC based on the other RL dyes. The relatively low quantity
of adsorbed dye RL5 on TiO2 (0.55 × 10–6 mol/cm2) is probably
caused by the bulky structure of the thiophene unit, which may
bend the molecule. This effect makes the molecule occupy a
larger surface area, resulting in its low solar-cell performance.
Electrochemical Impedance Spectroscopy Analysis
Electrochemical impedance spectroscopy (EIS) is a powerful
technique to characterize the important interfacial charge re-
combination process in DSSCs based on these dyes. Figure 8a
shows the EIS Nyquist plots for DSSCs based on RL1–5 under
a forward bias of –0.71 V in the dark with a frequency range of
0.1 to 105 Hz. The small and large semicircles indicate the resist-
ance of electron transport at Pt/electrolyte and TiO2/dye/elec-
trolyte interface, respectively. The radius of the middle semicir-
cle in the Nyquist plots increases in the order RL5 < RL1 <
RL2 < RL4 < RL3, indicating that the electron recombination
resistance increases accordingly.
Figure 7. The J–V curves of DSSCs based on RL1–5 and N719.
2.52 % (Jsc = 4.70 mA/cm2, Voc = 0.74 V, and FF = 0.73), and
1.44 % (Jsc = 3.30 mA/cm2, Voc = 0.65 V, and FF = 0.67), respec-
tively. Increasing molecular length by addition of two thio-
phene units to each side of carbazole in RL4 gave the η of
4.57 % (Jsc = 8.59 mA/cm2, Voc = 0.75 V, and FF = 0.71), which
Two peaks in Figure 8 (b) located at high frequency (right)
is comparable to the efficiency of the DSSC based on RL2 and and middle frequency (left), respectively, correspond to the
reaches 70 % of an N719-based DSSC fabricated and measured small semicircle (left) and large semicircle (right) in the Nyquist
under similar conditions. We have observed that the Voc of RL2 plot (Figure 8, a). The reciprocal of the peak frequency for the
was higher than those of other dyes, indicating the high tend- middle-frequency peak is regarded as the electron lifetime, be-
ency to inject electrons from the LUMO to TiO2 conduction cause it represents the charge transfer process at the TiO2/dye/
band. This improved Voc value is attributed to suppression of electrolyte interface. In other words, a shift to low frequency
dark current.[19] In addition, remarkably, the DSSC based on RL5 corresponds to a longer electron lifetime. The electron lifetimes
Figure 8. Electrochemical impedance spectra: (a) Nyquist plot and (b) Bode plot for DSSCs based on RL1–5 under dark conditions.
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tion. THF was distilled from sodium/benzophenone. DMF was dried
with and distilled from CaH2 under an atmosphere of Ar. DI water
used in the Suzuki coupling reaction was degassed by Ar bubbling
for 10 min. All reactions were performed under Ar atmosphere and
purified by column chromatography on silica gel (70–230 mesh). 1H
and 13C NMR spectra were recorded with Bruker DPX-300 and
Bruker Avance 400 instruments in CDCl3, CD2Cl2 or [D6]DMSO. High-
resolution mass spectra were recorded with a HR-TOF-MS Micro-
mass model VQ-TOF2. The UV/Vis absorption spectra of the dyes
of RL1–5 estimated by fitting the middle frequency of the Bode
phase plot into the equation τe = 1/(2πf), (Figure 8, b) are 12.85,
12.85, 12.85, 16.77, and 1.51 ms, respectively.[30] The longest
electron lifetime was found in RL4 (16.77 ms), indicating less
possibility of the back reaction of the injected electron with the
–
I3 in the electrolyte. Despite the longest electron lifetime, its
efficiency value (4.57 %) is comparable to that of RL2 (4.54 %).
These results reveal that, although insertion of an additional
thiophene spacer at the donor site of the carbazole-based dye
were measured either in CH2Cl2 (1 × 10–5
M) or on the adsorbed
RL4 did not contribute to a significant increase in the efficiency TiO2 film at room temperature by Cary 60 UV/Vis spectrophotome-
ter.
of light harvesting, its highly conjugated system led to a longer
electron lifetime (ca. 4 ms) than its shorter π-electron conjuga-
tion length analogues (RL2, RL3). In contrast, RL5 exhibited the
shortest electron lifetime of 1.51 ms in accordance with the
lowest Voc of 0.65 V, which should be attributed to the low dye
loading (0.55 × 10–6 mol/cm2). Compared with DSSCs based on
other RL dyes, the low dye loading of RL5 probably results in
larger regions of bare conductive FTO surface, which favors the
reduction of the oxidative species, mainly I3– at these bare sites
causing electron recombination.
Cyclic voltammetry (CV) measurements using μAutolab Type III po-
tentiostat-galvanostat (Metrohm Autolab B.VEco-Chemie B.V.,
Utrecht, The Netherlands) with NOVA 1.5 software were performed
in a three-electrode measuring device with a glassy carbon working
electrode, a platinum counter electrode, and a Ag/Ag+ reference
electrode, which was calibrated with ferrocene/ferrocenium (Fc/Fc+)
as the external reference. 0.1
M Tetrabutylammonium hexafluoro-
phosphate (TBAPF6) in CH2Cl2 was used as electrolyte. The measure-
ments were carried out at a scan rate of 50 mV/s. The current den-
sity voltage (J–V) characteristics of the DSSCs were carried out using
a Keithley 2420 source meter under 100 mW/cm2 irradiation of a
150 W Xe lamp with a global AM 1.5 filter for solar spectrum simula-
tion. The electrochemical impedance spectroscopy (EIS) was re-
corded in the dark with the same instrument used for CV. A 10 mV
(rms) AC sinusoidal signal was employed over a frequency range of
0.1–105 Hz at bias potential of –0.71 V.
Conclusions
Five new organic dyes RL1–5 were designed and synthesized
to investigate the effects of the thiophene spacer position on
the light absorption, electrochemical properties, and the per-
formance of carbazole dyes. The introduction of the thiophene
at the acceptor site extends the conjugation of the carbazole
dyes, leading to redshifts of their absorption spectra and en-
hanced molar extinction coefficients. In contrast, insertion of
the thiophene at the donor site produces hyperchromicity but
almost no shift in the wavelength of maximum absorption. RL2
and RL4, with the thiophene at the acceptor site, gave an im-
proved photovoltage and efficiency of DSSCs compared with
those of dyes RL3 and RL5, with the thiophene at the donor
site. RL5, with two secondary donor units, shows lowest photo-
voltage, JSC values and thus lower efficiency owing to the de-
creased amount of dye. The electron lifetime of RL5 is much
lower than that of RL3 (with one secondary donor unit), indicat-
ing that the bulky structure of the terminal thiophene in carb-
azole dyes is likely a drawback. The best performance was
achieved with the solar cell based on RL4 (with thiophene
Computational Methods: All calculations were performed with
Gaussian 09 program package.[31] Ground-state electronic struc-
tures were fully optimized by B3LYP[32] /6-31G(d).[33] Electronic tran-
sitions were studied by time-dependent density functional theory
(TD-DFT) with the hybrid MPW1K/6-31G(d) functional.[34] This func-
tional was used for TD-DFT calculation of dyes by several groups.[35]
The solvent (dichloromethane) effects were included in conductor-
like polarizable continuum model (CPCM)[36] and the 10 lowest sin-
glet-singlet transitions were taken into account.
Fabrication of Dye-Sensitized Solar Cells: To prepare the DSSC
working electrodes, the FTO glass plates (Solar 4 mm thickness,
Nippon Sheet Glass, Japan) were first cleaned three times in an
ultrasonic bath for 15 min with DI water, detergent solution and DI
water in subsequent order, and then rinsed with EtOH and acetone.
The FTO glass plates were immersed into a 40 mM TiCl4 in 10 % HCl
solution at 70 °C for 30 min and washed with DI water, EtOH and
acetone, respectively (Note that the electrode contact area was pro-
tected from the TiCl4 solution by using a common Scotch® Tape). A
layer of Ti-nanoxide T/SP (Solaronix) was coated on a 0.25 cm2 FTO
conducting glass by screen-printing and then dried for 6 min at
125 °C. This procedure was repeated twice and the resulting surface
was finally coated by a scattering layer of Ti-nanoxide R/SP (Solar-
onix). The TiO2 electrodes were gradually heated under air flow at
150 °C for 15 min, 325 °C for 10 min, and 500 °C for 30 min. The
groups on both acceptor and donor sites) with η = 4.57 %, Jsc
=
8.59 mA/cm2, Voc = 0.75 V, and FF = 0.71. Although the perform-
ance is only slightly better than RL2 (with one thiophene on
the acceptor site; η = 4.54 %, Jsc = 8.47 mA/cm2, Voc = 0.76 V,
and FF = 0.70), the electron lifetime of RL4 is greatly enhanced
to 16.77 ms, indicating a much improved charge transport
property. We have concluded that having thiophene units at
both sides of carbazole to construct the D′′–π–D–π–A sensitizer
as in RL4 can be an effective and promising pattern for organic
dye-sensitized solar cells. Our work provides new insight into
the thiophene position related structure–property relationship
of carbazole dyes for DSSCs.
heated electrodes were impregnated with a 40 m
M TiCl4 in 10 %
HCl at 70 °C for 30 min (note that the electrode contact area was
protected from the TiCl4 solution again using a common Scotch®
Tape) and fired again. After cooling to room temperature, the TiO2
electrodes were soaked in a solution with 0.2 mM dyes and kept at
room temperature in the dark for 20 h. To prepare the counter
electrode, a hole was drilled in the FTO glass (Solar 4 mm thickness,
Nippon Sheet Glass, Japan) by using a drill. The perforated glass
Experimental Section
Materials and Instruments: All chemicals and reagents were pur-
was washed with 0.1 M HCl solution in EtOH and cleaned with ultra-
chased from chemical suppliers and used without further purifica-
sound in an acetone bath for 10 min. After heating in air for 20 min
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at 400 °C, H2PtCl6 was deposited on the cleaned FTO glass by
(3 × 15 mL). The organic layer was dried with anhydrous Na2SO4
screen-printing technique. The counter electrodes were subse- and concentrated under reduced pressure. The desired product was
quently heated at 400 °C for 15 min. The working and counter
electrodes were assembled into a sandwiched type cell and sealed
purified by silica gel column chromatography (100 % hexane) to
give 4 (6.23 g, 93 %) as a white solid. 1H NMR (300 MHz, CDCl3):
with Surlyn. The redox electrolyte containing 1
imidazolium iodide (PMII), 0.03 I2, 0.05 LiI, 0.5
idine (4-TBP), 0.1 guadinium thiocyanate (GNCS) in a mixture of
acetonitrile and valeronitrile (volume ratio, 85:15) was dropped H, CH2), 1.00 (t, J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3):
M
1-methyl-3-propyl- δ = 8.16 (d, J = 7.8 Hz, 2 H, ArH), 7.41 (ddd, J = 7.5, 7.5, 1.1 Hz, 2
4-tert-butylpyr- H, ArH), 7.46 (d, J = 7.9 Hz, 2 H, ArH), 7.25–7.30 (m, 2 H, ArH), 4.35
(t, J = 7.2 Hz, 2 H, NCH2), 1.86–1.96 (m, 2 H, CH2), 1.39–1.52 (m, 2
M
M
M
M
through a hole drilled in the back of counter electrode by using
vacuum backfilling. Finally, the hole was sealed with Surlyn and a
cover glass (1.2 mm thickness).
δ = 140.41, 125.52, 122.78, 120.31, 118.66, 108.63, 42.79, 31.10,
20.55, 13.87 ppm. HRMS (ESI): m/z calcd. for C16H17N [M + H+]
224.1439; found 224.1432.
Synthesis of Dyes
3,6-Dibromo-9-butyl-9H-carbazole (5):[38] To a solution of 4
(3.89 g, 17.42 mmol) in THF (90 mL) at room temperature was
added NBS (6.51 g, 36.57 mmol) in small portions in the absent of
light. After stirring for 2 h at room temperature, water (45 mL) was
added. The mixture was then extracted with CH2Cl2 (3 × 45 mL).
The combined organic phase was dried with anhydrous Na2SO4,
and filtered. The solvent was removed to dryness and the residue
was purified by silica gel column chromatography (100 % hexane)
2-(4-Methoxyphenyl)thiophene (1): To a degassed solution of 2-
iodothiophene (169.9 μL, 1.54 mmol) in DMF (15 mL) was added
(4-methoxyphenyl)boronic acid (0.30 g, 2 mmol), Pd(OAc)2 (17.3 mg,
0.08 mmol), and K2CO3 (0.43 g, 3.08 mmol) at room temperature.
After degassing and purging with Ar, the reaction mixture was
stirred at 90 °C for 2.5 h, cooled to room temperature, and diluted
with H2O (60 mL). The mixture was then extracted with CH2Cl2
(3 × 45 mL). The organic portion was combined, dried with anhy-
drous Na2SO4, and filtered. The solvents were removed by rotary
evaporation. The residue was purified by column chromatography
on silica gel (EtOAc/hexanes, 10 %) to give 1 (0.27 g, 93 %) as a
1
to give 5 (6.13 g, 92 %) as a white solid. H NMR (300 MHz, CDCl3):
δ = 8.07 (d, J = 1.8 Hz, 2 H, ArH), 7.52 (dd, J = 8.7, 1.9 Hz, 2 H, ArH),
7.21 (d, J = 8.7 Hz, 2 H, ArH), 4.16 (t, J = 7.1 Hz, 2 H, NCH2), 1.72–
1.82 (m, 2 H, CH2), 1.27–1.40 (m, 2 H, CH2), 0.93 (t, J = 7.3 Hz, 3 H,
CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 139.14, 128.85, 123.27,
123.08, 111.80, 110.26, 42.94, 30.88, 20.40, 13.76 ppm. HRMS (ESI):
m/z calcd. for C16H15Br2N [M + H+] 381.9629; found 381.9630.
1
white solid. H NMR (300 MHz, CDCl3): δ = 7.53 (d, J = 8.9 Hz, 2 H,
ArH), 7.19–7.22 (m, 2 H, ArH), 7.05 (dd, J = 5.0, J = 3.7 Hz, 1 H, ArH),
6.92 (d, J = 8.9 Hz, 2 H, ArH), 3.84 (s, 3 H, OCH3) ppm. 13C NMR
(75 MHz, CDCl3): δ = 159.17, 144.33, 127.91, 127.30, 127.22, 123.82,
122.07, 114.26, 55.35 ppm. HRMS (ESI): m/z calcd. for C11H10OS [M
+ H+] 191.0531; found 191.0535.
3-Bromo-9-butyl-6-(4-methoxyphenyl)-9H-carbazole (6): A mix-
ture of 5 (0.76 g, 2 mmol), K2CO3 (1.14 g, 8.18 mmol), Pd(PPh3)4
(33.0 mg, 0.03 mmol), DI water (4 mL), and THF (8 mL) was heated
to 45 °C for 1 h under an argon atmosphere. 4-Methoxyphenyl-
boronic acid (0.46 g, 3 mmol) dissolved in THF (8 mL) was added.
The reaction mixture was subsequently heated to reflux for 24 h
and cooled to room temperature. The reaction was quenched with
water and extracted with CH2Cl2 (3 × 16 mL). The organic layer was
dried with anhydrous Na2SO4 and concentrated under reduced
pressure. The crude product was purified by column chromatogra-
phy on silica gel (EtOAc/hexanes, 5 %) to give 6 (0.40 g, 48 %) as a
pale-brown solid. 1H NMR (400 MHz, CDCl3): δ = 8.26 (d, J = 1.8 Hz,
1 H, ArH), 8.21 (d, J = 1.5 Hz, 1 H, ArH), 7.69 (dd, J = 8.5, 1.7 Hz, 1
H, ArH), 7.64 (d, J = 8.6 Hz, 2 H, ArH), 7.55 (dd, J = 8.6, 1.8 Hz, 1 H,
ArH), 7.42 (d, J = 8.5 Hz, 1 H, ArH), 7.27 (d, J = 8.6 Hz, 1 H, ArH),
7.05 (d, J = 8.6 Hz, 2 H, ArH), 4.25 (t, J = 7.1 Hz, 2 H, NCH2), 3.89 (s,
3 H, OCH3), 1.81–1.88 (m, 2 H, CH2), 1.35–1.44 (m, 2 H, CH2), 0.96 (t,
J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 158.60,
139.77, 139.39, 134.37, 132.35, 128.24, 128.14, 125.55, 124.59,
123.04, 122.21, 118.39, 114.20, 111.49, 110.17, 109.07, 55.31, 42.95,
31.01, 20.47, 13.82 ppm. HRMS (ESI): m/z calcd. for C23H22BrNO [M
+ H+] 408.0963; found 408.0957.
2-Bromo-5-(4-methoxyphenyl)thiophene (2): Compound
1
(0.27 g, 1.43 mmol) was placed in DMF (20 mL), then NBS (0.13 g,
0.71 mmol) was added at 0 °C. After stirring at 0 °C for 2 h, NBS
(0.13 g, 0.71 mmol) was added again at the same temperature. After
the reaction mixture was warmed to room temperature, water
(20 mL) was added and the reaction mixture was extracted with
CH2Cl2 (3 × 20 mL). The organic layer was then dried with an-
hydrous Na2SO4 and the solvents were removed. The obtained
crude product was purified by column chromatography on silica
gel (CH2Cl2/hexanes, 20 %) to give 2 (0.25 g, 65 %) as a white solid.
1H NMR (300 MHz, CDCl3): δ = 7.43 (d, J = 8.9 Hz, 2 H, ArH), 6.99
(d, J = 3.9 Hz, 1 H, ArH), 6.92 (d, J = 4.0 Hz, 1 H, ArH), 6.90 (d, J =
9.0 Hz, 2 H, ArH), 3.83 (s, 3 H, OCH3) ppm. 13C NMR (75 MHz, CDCl3):
δ = 159.45, 145.80, 130.71, 126.94, 126.50, 122.18, 114.37, 110.14,
55.35 ppm. HRMS (ESI): m/z calcd. for C11H9BrOS [M + H+] 268.9636;
found 268.9630.
[5-(4-Methoxyphenyl)thiophen-2-yl]boronic Acid (3):[37] To a so-
lution of 2 (0.25 g, 0.93 mmol) in anhydrous THF (5 mL) cooled to
–78 °C for 5 min, a solution of nBuLi (1.6
M in THF, 0.70 mL,
9-Butyl-6-(4-methoxyphenyl)-9H-carbazole-3-carbaldehyde (7):
To a solution of 6 (0.19 g, 0.46 mmol) in anhydrous THF (2 mL) was
1.12 mmol) was injected slowly and stirred at –78 °C. After 15 min,
triisopropylborate (1.30 mL, 5.60 mmol) was added at –78 °C and
the mixture was stirred for 2 h. The reaction mixture was warmed
added nBuLi (1.6
M in hexanes, 0.37 mL, 0.60 mmol) dropwise at
–78 °C. After this temperature was maintained for 1 h, DMF
(0.10 mL, 1.30 mmol) was introduced. The resulting solution was
stirred for an additional 30 min at –78 °C and then warmed to room
temperature. The reaction was quenched with 10 % HCl (aq.) and
adjusted to pH 3. After extraction with Et2O (3 × 5 mL), the com-
bined organic phase was dried with anhydrous Na2SO4 and concen-
trated in vacuo. The residue was purified by silica gel column chro-
to room temperature and acidified with 1
N HCl (8 mL). The organic
layer was separated and the aqueous layer was extracted with
CH2Cl2 (3 × 5 mL). The combined organic phases were dried with
anhydrous Na2SO4 and concentrated in vacuo. The crude product
3 was used without further purification.
9-Butyl-9H-carbazole (4): A mixture of carbazole (5.02 g, 30 mmol)
and potassium hydroxide (10.10 g, 180 mmol) in DMF (15 mL) was matography (CH2Cl2/hexanes, 50 %) to give 7 (0.48 g, 61 %) as a
1
stirred at room temperature for 1 h. 1-Iodobutane (6.50 mL, white solid. H NMR (400 MHz, CDCl3): δ = 10.10 (s, 1 H, CHO), 8.63
60 mmol) was then added. After stirring for 20 h, the reaction was
quenched with water (15 mL), and extracted with CH2Cl2
(d, J = 0.9 Hz, 1 H, ArH), 8.30 (d, J = 1.3 Hz, 1 H, ArH), 8.01 (dd, J =
7.9, 1.3 Hz, 1 H, ArH), 7.72 (dd, J = 8.5, 1.6 Hz, 1 H, ArH), 7.64 (d, J =
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8.6 Hz, 2 H, ArH), 7.47 (d, J = 8.5 Hz, 1 H, ArH), 7.46 (d, J = 8.5 Hz,
1 H, ArH), 7.04 (d, J = 8.7 Hz, 2 H, ArH), 4.33 (t, J = 7.1 Hz, 2 H,
NCH2), 3.89 (s, 3 H, OCH3), 1.85–1.92 (m, 2 H, CH2), 1.37–1.46 (m, 2
H, CH2), 0.97 (t, J = 7.4 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz,
CDCl3): δ = 191.91, 159.03, 144.61, 140.43, 134.34, 133.73, 128.68,
128.45, 127.31, 126.10, 124.30, 123.67, 123.33, 118.85, 114.52,
109.78, 109.24, 55.57, 43.46, 31.27, 20.70, 14.03 ppm. HRMS (ESI):
m/z calcd. for C24H23NO2 [M + H+] 358.1807; found 358.1799.
123.24, 122.95, 122.67, 119.12, 118.34, 118.22, 114.34, 110.14,
109.97, 106.89, 55.17, 42.31, 30.77, 19.78, 13.71 ppm. HRMS (ESI):
m/z calcd. for C31H26N2O3S [M + Na+] 529.1562; found 529.1561.
6-Bromo-9-butyl-9H-carbazole-3-carbaldehyde (9): Compound 9
was synthesized according to the same procedure as 7. The reaction
of 5 (0.52 g, 1.36 mmol) in THF (3 mL) with nBuLi (1.6
M in hexane,
0.94 mL, 1.50 mmol), followed by addition of DMF (0.12 mL,
1.5 mmol) provided the crude product. The residue was purified by
silica gel column chromatography (CH2Cl2/hexanes, 50 %) to give 9
(E)-3-[9-Butyl-6-(4-methoxyphenyl)-9H-carbazol-3-yl]-2-cyano-
acrylic Acid (RL1): A mixture of 7 (0.27 g, 0.75 mmol), cyanoacetic
acid (0.13 g, 1.50 mmol), piperidine (0.1 mL), and acetonitrile
(30 mL) was heated to reflux for 16 h. The organic layer was sepa-
rated and the water layer was extracted with CH2Cl2, washed with
distilled water, dried with Na2SO4, filtered, and evaporated to re-
move the solvent. The crude product was purified by column chro-
matography on silica gel (100 % CH2Cl2 then MeOH/CH2Cl2, 20 %)
to give RL1 (0.20 g, 63 %) as a yellow solid. 1H NMR (300 MHz,
[D6]DMSO): δ = 8.70 (s, 1 H, ArH), 8.34 (s, 1 H, ArH), 8.23 (s, 1 H, CH),
8.16–8.18 (m, 1 H, ArH), 7.66–7.76 (m, 5 H, ArH), 7.04 (d, J = 8.8 Hz,
2 H, ArH), 4.42 (t, J = 6.7 Hz, 2 H, NCH2), 3.79 (s, 3 H, OCH3), 1.70–
1.78 (m, 2 H, CH2), 1.20–1.31 (m, 2 H, CH2), 0.85 (t, J = 7.3 Hz, 3 H,
CH3) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 165.26, 158.51, 150.96,
142.22, 139.84, 133.36, 132.24, 127.92, 127.00, 125.41, 124.27,
123.65, 122.73, 122.61, 119.55, 118.00, 114.50, 110.42, 110.14,
106.27, 55.26, 43.67, 30.84, 19.85, 13.80 ppm. HRMS (ESI): m/z calcd.
for C27H24N2O3 [M + Na+] 447.1685; found 447.1683.
1
(0.48 g, 61 %) as a white solid. H NMR (300 MHz, CDCl3): δ = 10.08
(s, 1 H, CHO), 8.53 (d, J = 1.1 Hz, 1 H, ArH), 8.24 (d, J = 1.8 Hz, 1 H,
ArH), 8.02 (dd, J = 8.6, 1.6 Hz, 1 H, ArH), 7.60 (dd, J = 8.7, 1.9 Hz, 1
H, ArH), 7.46 (d, J = 8.6 Hz, 1 H, ArH), 7.32 (d, J = 8.7 Hz, 1 H, ArH),
4.30 (t, J = 7.2 Hz, 2 H, NCH2), 1.80–1.90 (m, 2 H, CH2), 1.35–1.45
(m, 2 H, CH2), 0.95 (t, J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz,
CDCl3): δ = 191.46, 144.19, 139.80, 129.39, 128.87, 127.44, 124.67,
124.33, 123.49, 121.97, 113.14, 110.82, 109.26, 43.33, 30.97, 20.45,
13.76 ppm. HRMS (ESI): m/z calcd. for C17H16BrNO [M + Na+]
352.0313; found 352.0309.
9-Butyl-6-[5-(4-methoxyphenyl)thiophen-2-yl]-9H-carbazole-3-
carbaldehyde (10): Compound 10 was synthesized according to
the same procedure as 6. The reaction of 9 (0.33 g, 1 mmol) with
3 (0.10 g, 0.76 mmol), Pd(PPh3)4 (16.5 mg, 0.01 mmol) and K2CO3
(0.57 g, 4.10 mmol) in THF (12 mL) and DI water (6 mL) provided
the desired product. Purification by column chromatography on sil-
ica gel (CH2Cl2/hexanes, 60 %) gave 10 (0.33 g, 75 %) as a pale-
green oil. 1H NMR (300 MHz, CDCl3): δ = 10.09 (s, 1 H, CHO), 8.60
(d, J = 1.3 Hz, 1 H, ArH), 8.34 (d, J = 1.5 Hz, 1 H, ArH), 8.00 (dd, J =
8.6, 1.5 Hz, 1 H, ArH), 7.76 (dd, J = 8.5, 1.8 Hz, 1 H, ArH), 7.58 (d, J =
8.8 Hz, 2 H, ArH), 7.44 (d, J = 8.6 Hz, 1 H, ArH), 7.40 (d, J = 8.6 Hz,
1 H, ArH), 7.29 (d, J = 3.7 Hz, 1 H, ArH), 7.20 (d, J = 3.7 Hz, 1 H, ArH),
6.94 (d, J = 8.8 Hz, 2 H, ArH), 4.29 (t, J = 7.2 Hz, 2 H, NCH2), 3.84 (s,
3 H, OCH3), 1.81–1.91 (m, 2 H, CH2), 1.34–1.46 (m, 2 H, CH2), 0.96 (t,
J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 191.62,
159.11, 144.37, 143.18, 142.80, 140.45, 128.61, 127.24, 127.17,
127.14, 126.76, 124.73, 124.21, 123.40, 123.20, 122.98, 122.90,
117.53, 114.29, 109.68, 109.12, 55.33, 43.23, 31.02, 20.45, 13.79 ppm.
HRMS (ESI): m/z calcd. for C28H25NO2S [M + Na+] 462.1504; found
462.1496.
5-[9-Butyl-6-(4-methoxyphenyl)-9H-carbazol-3-yl]thiophene-2-
carbaldehyde (8): Compound 8 was synthesized according to the
same procedure as described for 6. The reaction of 6 (0.18 g,
0.44 mmol) with 5-formyl-2-thiopheneboronic acid (82.1 mg,
0.53 mmol), Pd(PPh3)4 (10.1 mg, 0.009 mmol), and K2CO3 (0.24 g,
1.75 mmol) in THF (5 mL) and DI water (2.5 mL) provided the crude
product. Purification by column chromatography on silica gel
(CH2Cl2/hexanes, 50–75 %) gave 8 (79.7 mg, 41 %) as a yellow solid.
1H NMR (400 MHz, CDCl3): δ = 9.89 (s, 1 H, CHO), 8.43 (d, J = 1.3 Hz,
1 H, ArH), 8.29 (d, J = 1.2 Hz, 1 H, ArH), 7.76–7.79 (m, 2 H, ArH), 7.71
(dd, J = 8.5, 1.6 Hz, 1 H, ArH), 7.65 (d, J = 8.7 Hz, 2 H, ArH), 7.42–
7.47 (m, 3 H, ArH), 7.04 (d, J = 8.7 Hz, 2 H, ArH), 4.33 (t, J = 7.1 Hz,
2 H, NCH2), 3.89 (s, 3 H, OCH3), 1.85–1.92 (m, 2 H, CH2), 1.37–1.47
(m, 2 H, CH2), 0.97 (t, J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz,
CDCl3): δ = 182.82, 158.94, 156.53, 141.68, 141.40, 140.38, 138.12,
134.60, 133.04, 128.46, 125.88, 124.70, 124.30, 123.75, 123.33,
123.06, 118.79, 118.76, 114.50, 109.61, 109.53, 55.61, 43.36, 31.36,
20.76, 14.08 ppm. HRMS (ESI): m/z calcd. for C28H25NO2S [M + Na+]
462.1498; found 462.1499.
(E)-3-{9-Butyl-6-[5-(4-methoxyphenyl)thiophen-2-yl]-9H-carb-
azol-3-yl}-2-cyanoacrylic Acid (RL3): RL3 was synthesized by fol-
lowing the procedure described for RL2. The reaction of 10 (0.33 g,
0.75 mmol) with cyanoacetic acid (0.22 g, 2.58 mmol), and ammon-
ium acetate (0.14 g) in acetic acid (14 mL) provided the required
compound. The crude product was purified by column chromatog-
raphy on silica gel (100 % CH2Cl2, then MeOH/CH2Cl2, 25 %) to give
(E)-3-{5-[9-Butyl-6-(4-methoxyphenyl)-9H-carbazol-3-yl]thio-
phen-2-yl}-2-cyanoacrylic Acid (RL2): A mixture of 8 (77.8 mg,
0.18 mmol) and cyanoacetic acid (51.8 mg, 0.61 mmol) in acetic acid
(3 mL) was heated to reflux for 12 h under an argon atmosphere in
the presence of ammonium acetate (32.1 mg). Water (3 mL) was
then added and the reaction mixture was extracted with CH2Cl2
(3 × 5 mL). After removal of solvents under vacuum, the crude com-
pound was purified by column chromatography on silica gel (100 %,
CH2Cl2 then MeOH/CH2Cl2, 25 %) to give RL2 (62.3 mg, 69 %) as a
1
RL3 (0.34 g, 89 %) as a yellow solid. H NMR (300 MHz, [D6]DMSO):
δ = 8.61 (br. s, 1 H, ArH), 8.36 (br. s, 1 H, ArH), 8.26 (s, 1 H, CH), 8.14
(d, J = 8.6 Hz, 1 H, ArH), 7.64 (d, J = 8.4 Hz, 1 H, ArH), 7.50 (d, J =
8.6 Hz, 2 H, ArH), 7.43–7.49 (m, 2 H, ArH), 7.35 (d, J = 3.4 Hz, 1 H,
ArH), 7.25 (d, J = 3.5 Hz, 1 H, ArH), 6.89 (d, J = 8.8 Hz, 2 H, ArH),
4.18 (br. t, 2 H, NCH2), 3.71 (s, 3 H, OCH3), 1.57 (br. s, 2 H, CH2), 1.09–
1.12 (m, 2 H, CH2), 0.71 (t, J = 7.2 Hz, 3 H, CH3) ppm. 13C NMR
(75 MHz, [D6]DMSO): δ = 166.02, 158.88, 151.16, 142.62, 142.13,
141.65, 140.04, 127.01, 126.57, 126.46, 126.01, 124.36, 124.28,
123.91, 123.79, 123.65, 122.60, 122.32, 119.65, 116.88, 114.58,
110.46, 110.08, 106.53, 55.28, 42.45, 30.76, 19.81, 13.73 ppm. HRMS
(ESI): m/z calcd. for C31H26N2O3S [M + Na+] 529.1562; found
529.1563.
1
red solid. H NMR (300 MHz, [D6]DMSO): δ = 8.66 (d, J = 1.5 Hz, 1
H, ArH), 8.54 (d, J = 1.3 Hz, 1 H, ArH), 8.20 (s, 1 H, CH), 7.75–7.78
(m, 2 H, ArH), 7.70–7.73 (m, 3 H, ArH), 7.65 (d, J = 3.9 Hz, 1 H, ArH),
7.58–7.62 (m, 2 H, ArH), 7.03 (d, J = 8.8 Hz, 2 H, ArH), 4.35 (br. t, 2
H, NCH2), 3.79 (s, 3 H, OCH3), 1.69–1.74 (m, 2 H, CH2), 1.21–1.28 (m,
2 H, CH2), 0.83 (t, J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz,
[D6]DMSO): δ = 164.43, 158.33, 151.04, 141.58, 140.74, 139.77,
136.96, 134.73, 133.37, 131.52, 127.75, 124.97, 124.16, 123.92,
3-Bromo-9-butyl-6-[5-(4-methoxyphenyl)thiophen-2-yl]-9H-
carbazole (11): Compound 11 was synthesized according to the
same procedure as 6. The reaction of 5 (0.46 g, 1.38 mmol) with 3
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(0.39 g, 1.66 mmol), Pd(PPh3)4 (31.9 mg, 0.03 mmol) and K2CO3
2.73 mmol) in THF (6 mL) and DI water (1.5 mL) provided the de-
(0.77 g, 5.52 mmol) in THF (13 mL) and DI water (6.5 mL) provided sired product. Purification by column chromatography on silica gel
the required compound. The crude product was purified by column
(CH2Cl2/hexanes, 50 %) to give 13 (0.15 g, 92 %) as a pale-yellow
oil. H NMR (300 MHz, CDCl3): δ = 10.11 (s, 1 H, CHO), 8.64 (d, J =
1.2 Hz, 1 H, ArH), 8.37 (d, J = 1.4 Hz, 1 H, ArH), 8.02 (dd, J = 8.6,
1.6 Hz, 1 H, ArH), 7.79 (dd, J = 8.5, 1.8 Hz, 1 H, ArH), 7.43–7.49 (m,
1
chromatography on silica gel (EtOAc/hexanes, 10 %) to give 11
1
(0.23 g, 34 %) as a pale-green solid. H NMR (400 MHz, CDCl3): δ =
8.26 (d, J = 1.2 Hz, 1 H, ArH), 8.24 (d, J = 1.7 Hz, 1 H, ArH), 7.75 (dd,
J = 8.5, 1.6 Hz, 1 H, ArH), 7.59 (d, J = 8.8 Hz, 2 H, ArH), 7.55 (dd, J = 2 H, ArH), 7.37 (dd, J = 3.6, 1.1 Hz, 1 H, ArH), 7.29 (dd, J = 5.1, 1.1 Hz,
8.6, 1.9 Hz, 1 H, ArH), 7.38 (d, J = 8.5 Hz, 1 H, ArH), 7.28 (d, J =
3.7 Hz, 1 H, ArH), 7.27 (d, J = 8.7 Hz, 1 H, ArH), 7.21 (d, J = 3.7 Hz,
1 H, ArH), 6.95 (d, J = 8.8 Hz, 2 H, ArH), 4.27 (t, J = 7.1 Hz, 2 H,
NCH2), 3.86 (s, 3 H, OCH3), 1.80–1.88 (m, 2 H, CH2), 1.33–1.43 (m, 2
H, CH2), 0.95 (t, J = 7.4 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz,
1 H, ArH), 7.13 (dd, J = 5.1, 3.6 Hz, 1 H, ArH), 4.35 (t, J = 7.2 Hz, 2
H, NCH2), 1.84–1.94 (m, 2 H, CH2), 1.36–1.46 (m, 2 H, CH2), 0.97 (t,
J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3): δ = 191.51,
144.90, 144.21, 140.37, 128.46, 128.01, 127.06, 126.94, 124.96,
123.99, 123.22, 122.75, 122.30, 117.78, 109.54, 108.97, 43.05, 30.90,
CDCl3): δ = 159.07, 143.64, 142.51, 140.13, 139.50, 128.55, 127.39, 20.34, 13.70 ppm. HRMS (ESI): m/z calcd. for C21H19NOS [M + Na]+
126.78, 126.12, 124.57, 124.46, 123.23, 122.97, 122.94, 122.22,
117.50, 114.30, 111.79, 110.32, 109.25, 55.36, 43.07, 31.05, 20.50,
13.83 ppm. HRMS (ESI): m/z calcd. for C27H24BrNOS [M+] 490.0762;
found 490.0757.
356.1085; found 356.1081.
9-Butyl-6-(3,5-dibromothiophen-2-yl)-9H-carbazole-3-carbalde-
hyde (14): Compound 14 was synthesized following the procedure
described for 5. The reaction of 13 (0.31 g, 0.92 mmol) with NBS
(0.35 g, 1.93 mmol) in THF (3 mL) provided the required compound.
The crude product was purified by column chromatography on sil-
ica gel (CH2Cl2/hexanes, 50 %) to give 14 (0.31 g, 67 %) as a yellow
solid. 1H NMR (400 MHz, CDCl3): δ = 10.09 (s, 1 H, CHO), 8.60 (d, J =
1.1 Hz, 1 H, ArH), 8.33 (d, J = 1.4 Hz, 1 H, ArH), 8.02 (dd, J = 8.5,
1.4 Hz, 1 H, ArH), 7.71 (dd, J = 8.5, 1.7 Hz, 1 H, ArH), 7.46–7.49 (m,
2 H, ArH), 7.06 (s, 1 H, ArH), 4.32 (t, J = 7.2 Hz, 2 H, NCH2), 1.84–
5-{9-Butyl-6-[5-(4-methoxyphenyl)thiophen-2-yl]-9H-carbazol-
3-yl}thiophene-2-carbaldehyde (12): Compound 12 was synthe-
sized according to the same procedure as 6. The reaction of 11
(0.23 g, 0.46 mmol) with 5-formyl-2-thiopheneboronic acid
(86.8 mg, 0.56 mmol), Pd(PPh3)4 (10.7 mg, 0.009 mmol) and K2CO3
(0.26 g, 1.85 mmol) in THF (5 mL) and DI water (2.5 mL) provided
the required compound. The crude product was purified by column
chromatography on silica gel (CH2Cl2/hexanes, 50–90 %) to give 12 1.91 (m, 2 H, CH2), 1.37–1.46 (m, 2 H, CH2), 0.97 (t, J = 7.4 Hz, 3 H,
1
(0.11 g, 43 %) as a yellow solid. H NMR (300 MHz, CDCl3): δ = 9.89 CH3) ppm. 13C NMR (100 MHz, CDCl3): δ = 191.53, 144.46, 141.03,
(s, 1 H, CHO), 8.44 (d, J = 1.6 Hz, 1 H, ArH), 8.35 (d, J = 1.5 Hz, 1 H,
ArH), 7.76–7.81 (m, 3 H, ArH), 7.60 (d, J = 8.8 Hz, 2 H, ArH), 7.48 (d,
J = 4.0 Hz, 1 H, ArH), 7.43 (d, J = 8.5 Hz, 1 H, ArH), 7.42 (d, J =
8.6 Hz, 1 H, ArH), 7.32 (d, J = 3.7 Hz, 1 H, ArH), 7.22 (d, J = 3.7 Hz,
1 H, ArH), 6.95 (d, J = 8.8 Hz, 2 H, ArH), 4.33 (t, J = 7.1 Hz, 2 H,
NCH2), 3.86 (s, 3 H, OCH3), 1.86–1.91 (m, 2 H, CH2), 1.38–1.45 (m, 2
H, CH2), 0.97 (t, J = 7.3 Hz, 3 H, CH3) ppm. 13C NMR (75 MHz, CDCl3):
δ = 182.57, 159.16, 156.14, 143.60, 142.69, 141.51, 141.36, 140.48,
137.79, 127.43, 126.84, 126.55, 124.73, 124.65, 124.39, 123.41,
123.13, 123.09, 123.02, 122.97, 118.69, 117.62, 114.36, 109.52,
109.48, 55.39, 43.21, 31.15, 20.53, 13.84 ppm. HRMS (ESI): m/z calcd.
for C32H27NO2S2 [M + H+] 522.1561; found 522.1562.
140.60, 133.58, 128.93, 127.65, 127.52, 124.11, 124.09, 123.13,
122.85, 121.36, 111.07, 109.48, 109.23, 106.39, 43.35, 31.03, 20.48,
13.77 ppm. HRMS (ESI): m/z calcd. for C21H17Br2NOS [M + Na]+
511.9295; found 511.9291.
6-[3,5-Bis(4-methoxyphenyl)thiophen-2-yl]-9-butyl-9H-carb-
azole-3-carbaldehyde (15): Compound 15 was synthesized ac-
cording to the same procedure as 6. The reaction of 14 (0.23 g,
0.47 mmol) with (4-methoxyphenyl)boronic acid (0.21 g,
1.41 mmol), Pd(PPh3)4 (20.8 mg, 0.02 mmol), and K2CO3 (0.52 g,
3.76 mmol) in THF (8 mL) and DI water (2 mL) provided the desired
product. Purification by column chromatography on silica gel
(CH2Cl2/hexanes, 50 %) gave 15 (0.15 g, 59 %) as a pale-yellow solid.
1H NMR (400 MHz, CD2Cl2): δ = 10.06 (s, 1 H, CHO), 8.53 (d, J =
1.0 Hz, 1 H, ArH), 8.20 (d, J = 1.2 Hz, 1 H, ArH), 8.00 (dd, J = 8.6,
1.4 Hz, 1 H, ArH), 7.62 (d, J = 8.8 Hz, 2 H, ArH), 7.53 (d, J = 8.6 Hz,
1 H, ArH), 7.45 (dd, J = 8.5, 1.6 Hz, 1 H, ArH), 7.38 (d, J = 8.5 Hz, 1
H, ArH), 7.32 (s, 1 H, ArH), 7.30 (d, J = 8.8 Hz, 2 H, ArH), 6.97 (d, J =
(E)-3-(5-{9-Butyl-6-[5-(4-methoxyphenyl)thiophen-2-yl]-9H-
carbazol-3-yl}thiophen-2-yl)-2-cyanoacrylic Acid (RL4): Com-
pound RL4 was synthesized by following the procedure described
for RL2. The reaction of 12 (0.11 g, 0.20 mmol) with cyanoacetic
acid (59.0 mg, 0.69 mmol) and ammonium acetate (30.0 mg) in
acetic acid (3 mL) afforded the desired compound. The crude prod- 8.8 Hz, 2 H, ArH), 6.83 (d, J = 8.8 Hz, 2 H, ArH), 4.34 (t, J = 7.3 Hz, 2
uct was purified by column chromatography on silica gel (100 %
CH2Cl2 then MeOH/CH2Cl2, 30 %) to give RL4 (74.0 mg, 62 %) as a
scarlet powder. 1H NMR (300 MHz, [D6]DMSO): δ = 8.67 (s, 1 H, ArH),
8.60 (s, 1 H, ArH), 8.17 (s, 1 H, CH), 7.76–7.81 (m, 3 H, ArH), 7.68 (d,
J = 3.8 Hz, 1 H, ArH), 7.61–7.65 (m, 4 H, ArH), 7.51–7.52 (m, 1 H,
ArH), 7.39–7.40 (m, 1 H, ArH), 7.00 (d, J = 8.5 Hz, 2 H, ArH), 4.38
(br. t, 2 H, NCH2), 3.78 (s, 3 H, OCH3), 1.73–1.76 (m, 2 H, CH2), 1.19–
1.28 (m, 2 H, CH2), 0.84 (t, J = 6.8 Hz, 3 H, CH3) ppm. 13C NMR
(75 MHz, [D6]DMSO): δ = 165.32, 159.30, 151.64, 143.31, 142.42,
141.89, 141.26, 140.53, 137.77, 135.17, 127.04, 126.88, 125.97,
124.91, 124.62, 124.50, 124.06, 123.88, 123.18, 123.06, 119.61,
118.76, 117.81, 115.07, 110.75, 110.67, 106.95, 55.74, 42.83, 31.24,
20.27, 14.18 ppm. HRMS (ESI): m/z calcd. for C35H28N2O3S2 [M + H+]
589.1620; found 589.1619.
H, NCH2), 3.85 (s, 3 H, OCH3), 3.78 (s, 3 H, OCH3), 1.84–1.91 (m, 2 H,
CH2), 1.37–1.46 (m, 2 H, CH2), 0.96 (t, J = 7.4 Hz, 3 H, CH3) ppm. 13C
NMR (100 MHz, CDCl3): δ = 191.95, 159.45, 158.76, 144.56, 142.23,
140.56, 138.25, 136.90, 130.38, 129.48, 128.83, 128.50, 127.33,
127.31, 127.08, 127.04, 125.67, 124.48, 123.35, 123.18, 121.43,
114.56, 114.06, 109.54, 109.31, 55.58, 55.43, 43.52, 31.27, 20.72,
14.03 ppm. HRMS (ESI): m/z calcd. for C35H31NO3S [M + H]+
546.2103; found 546.2105.
(E)-3-{6-[3,5-Bis(4-methoxyphenyl)thiophen-2-yl]-9-butyl-9H-
carbazol-3-yl}-2-cyanoacrylic Acid (RL5): Compound RL5 was
synthesized by following the procedure described for RL2. The reac-
tion of 15 (0.15 g, 0.28 mmol) with cyanoacetic acid (80.9 mg,
0.95 mmol) and ammonium acetate (50.3 mg) in acetic acid (6 mL)
afforded the required compound. The crude product was purified
9-Butyl-6-(thiophen-2-yl)-9H-carbazole-3-carbaldehyde (13): by column chromatography on silica gel (100 % CH2Cl2, then
Compound 13 was synthesized according to the same procedure MeOH/CH2Cl2, 25 %) to give RL5 (0.11 g, 63 %) as an orange solid.
as 6. The reaction of 9 (0.22 g, 0.67 mmol) with 2-thienylboronic 1H NMR (400 MHz, [D6]DMSO): δ = 8.58 (br. s, 1 H, ArH), 8.23 (d, J =
acid (0.13 g, 1 mmol), Pd(PPh3)4 (10.9 mg, 0.01 mmol), K2CO3 (0.38 g,
1.5 Hz, 1 H, ArH), 8.17 (dd, J = 8.8, 0.9 Hz, 1 H, ArH), 8.11 (s, 1 H,
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CH), 7.71 (dd, J = 8.7, 1.8 Hz, 1 H, ArH), 7.65 (d, J = 7.9 Hz, 2 H, ArH),
7.57 (dd, J = 8.5, 2.2 Hz, 1 H, ArH), 7.49 (s, 1 H, ArH), 7.31 (d, J =
8.5 Hz, 1 H, ArH), 7.25 (d, J = 8.0 Hz, 2 H, ArH), 7.00 (d, J = 8.2 Hz,
2 H, ArH), 6.84 (d, J = 7.8 Hz, 2 H, ArH), 4.37 (br. t, 2 H, NCH2), 3.79
(s, 3 H, OCH3), 3.70 (s, 3 H, OCH3), 1.70–1.73 (m, 2 H, CH2), 1.24–1.29
(m, 2 H, CH2), 0.84 (t, J = 6.7 Hz, 3 H, CH3) ppm. 13C NMR (100 MHz,
[D6]DMSO): δ = 164.91, 159.09, 158.37, 150.35, 142.05, 141.21,
139.99, 137.94, 135.96, 129.99, 128.51, 127.78, 127.15, 126.63,
126.30, 125.74, 125.61, 124.01, 123.91, 122.32, 122.22, 120.71,
119.56, 114.65, 114.00, 110.28, 107.11, 55.34, 55.13, 42.55, 30.82,
19.84, 13.77 ppm. HRMS (ESI): m/z calcd. for C38H32N2O4S [M + H]+
635.1980; found 635.1984.
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Acknowledgments
This research project was financially supported by Mahidol Uni-
versity. The authors gratefully acknowledge financial support
from the Office of the Higher Education Commission, Thailand
and Mahidol University under the National Research Universi-
ties Initiative, and the Center of Excellent for Innovation in
Chemistry (PERCH-CIC), Faculty of Science, Mahidol University.
Mr. Kasempong Srisawad, Ms. Jutarat Sudchanham, Assistant
Prof. Dr. Pasit Pakawatpanurat, and Assistant Prof. Dr. Tienthong
Thongpanchang are thanked for their valuable experimental
support.
[17]
Keywords: Sensitizers · Solar cells · Energy conversion ·
Conjugation · Dyes/Pigments
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Received: March 21, 2016
Published Online: ■
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Full Paper
Sensitizers
R. Samae, P. Surawatanawong,
U. Eiamprasert, S. Pramjit,
L. Saengdee, P. Tangboriboonrat,
S. Kiatisevi* ....................................... 1–15
Effect of Thiophene Spacer Position
in Carbazole-Based Dye-Sensitized
Solar Cells on Photophysical, Elec-
trochemical and Photovoltaic Prop-
erties
Five new carbazole-based dyes were nor causes hyperchromicity but almost
synthesized. Whereas redshifted spec- no shift in λmax. The power conversion
tra can be achieved by inserting a thio- efficiency of the DSSCs depends on
phene next to the acceptor, the inclu- the position of the thiophene.
sion of a thiophene as part of the do-
DOI: 10.1002/ejoc.201600353
Eur. J. Org. Chem. 0000, 0–0
www.eurjoc.org
15
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim