Synthesis and characterization of 2D-D-π-A-type organic dyes bearing bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline as donor moiety for dye-sensitized solar cells

Article abstract of DOI:10.1002/ejoc.201201479

A series of novel 2D-D-π-A-type organic dyes, namely CCTTnA (n = 1-3), bearing bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline as an electron-donor moiety (2D-D), oligothiophene segments with a number of thiophene units from one to three units as π-conjugated spacers (π), and cyanoacrylic acid as the electron acceptor (A) were synthesized and characterized as dye sensitizers for dye-sensitized solar cells (DSSCs). These compounds exhibit high thermal and electrochemical stability. Detailed investigations of these dyes reveal that both peripheral carbazole donors (2D) have beneficial influence on the red-shifted absorption spectrum of the dye in solution and dye adsorbed on TiO₂ film, and the broadening of the incident monochromatic photon-to-current conversion efficiency (IPCE) spectra of the DSSCs, leading to enhanced energy conversion efficiency (η). Among these dyes, CCTT3A shows the best photovoltaic performance, and a maximal incident monochromatic photon-to-current conversion efficiency (IPCE) value of 80 %, a short-circuit photocurrent density (J_sc) of 9.98 mA cm^-2, open-circuit voltage (V_oc) of 0.70 V, and fill factor (FF) of 0.67, corresponding to an overall conversion efficiency η of 4.6 % were achieved. This work suggests that organic dyes based on this type of donor moiety or donor molecular architecture are promising candidates for improved performance DSSCs. New 2D-D-π-A-type organic dyes incorporating bis(3,6-di-tert-butylcarbazol-9- ylphenyl)aniline as an electron-donor moiety show great potential as sensitizers for dye-sensitized solar cells. Copyright

Full text of DOI:10.1002/ejoc.201201479

FULL PAPER
DOI: 10.1002/ejoc.201201479
Synthesis and Characterization of 2D-D-π-A-Type Organic Dyes Bearing
Bis(3,6-di-tert-butylcarbazol-9-ylphenyl)aniline as Donor Moiety for Dye-
Sensitized Solar Cells
Tanika Khanasa,[a] Nittaya Jantasing,[a] Somphob Morada,[a] Nararak Leesakul,[b]
Ruangchai Tarsang,[a] Supawadee Namuangruk,[c] Tinnagon Kaewin,[a]
Siriporn Jungsuttiwong,[a] Taweesak Sudyoadsuk,[a] and Vinich Promarak*[d]
Keywords: Donor–acceptor systems / Dyes / Sensitizers / Conjugation
A series of novel 2D-D-π-A-type organic dyes, namely
CCTTnA (n = 1–3), bearing bis(3,6-di-tert-butylcarbazol-9-
ylphenyl)aniline as an electron-donor moiety (2D-D), oligo-
thiophene segments with a number of thiophene units from
one to three units as π-conjugated spacers (π), and cyano-
acrylic acid as the electron acceptor (A) were synthesized
and characterized as dye sensitizers for dye-sensitized solar
cells (DSSCs). These compounds exhibit high thermal and
electrochemical stability. Detailed investigations of these
dyes reveal that both peripheral carbazole donors (2D) have
beneficial influence on the red-shifted absorption spectrum
of the dye in solution and dye adsorbed on TiO2 film, and the
broadening of the incident monochromatic photon-to-current
conversion efficiency (IPCE) spectra of the DSSCs, leading
to enhanced energy conversion efficiency (η). Among these
dyes, CCTT3A shows the best photovoltaic performance, and
a maximal incident monochromatic photon-to-current con-
version efficiency (IPCE) value of 80%, a short-circuit photo-
current density (Jsc) of 9.98 mAcm–2, open-circuit voltage
(Voc) of 0.70 V, and fill factor (FF) of 0.67, corresponding to
an overall conversion efficiency η of 4.6% were achieved.
This work suggests that organic dyes based on this type of
donor moiety or donor molecular architecture are promising
candidates for improved performance DSSCs.
of Ru-based DSSCs,[4] Ru dyes are nevertheless costly, diffi-
cult to attain, and normally have only moderate absorption
intensity.[5] Enormous effort is also being dedicated to the
development of new and efficient dyes that are suitable with
respect to their modest cost, ease of synthesis and modifica-
tion, large molar extinction coefficient, and long-term sta-
bility. Organic dyes meet all these criteria. Thus, there have
been remarkable developments in organic dye-based DSSCs
in recent years,[6] and efficiencies exceeding 10% have been
achieved by using dyes that have broad, red-shifted, and
intense spectral absorption in the visible light region.[7] Al-
though remarkable progress has been made in the develop-
ment of organic dyes as sensitizers for DSSCs, their chemi-
cal structures still require optimization for further improve-
ment in performance. Most of the developed organic dyes
are composed of donor, π-conjugation, and acceptor moie-
ties, thereby forming a D-π-A structure, and broad ranges
of conversion efficiencies have been achieved.[6–8] Most of
the highly efficient DSSCs based on organic dyes have long
π-conjugated spacers between the donor and acceptor, re-
sulting in broad and intense absorption spectra, aromatic
amines as donor moieties, a strong electron-withdrawing
group (cyanoacrylic acid) as acceptor, and anchoring moie-
ties. However, the introduction of long π-conjugated seg-
ments results in rod-like molecules, which can lead to re-
combination of the electrons to the triiodide and magnify
Introduction
Since the report by O’Reagan and Grätzel in 1991 on
dye-sensitized solar cells (DSSCs) with dramatically in-
creased light harvesting efficiency,[1] this type of solar cell
has attracted considerable and sustained attention because
it offers the possibility of low-cost conversion of photo-
energy.[2] To date, DSSCs with a validated efficiency record
of more than 11% have been obtained with Ru complexes
such as the black dye.[3] More recently, a DSSC submodule
of 17 cm2 consisting of eight parallel cells with a conversion
efficiency of more than 9.9% was fabricated by Sony.[3] Al-
though there is still room for improvement of the efficiency
[a] Center for Organic Electronic and Alternative Energy,
Department of Chemistry, Faculty of Science, Ubon
Ratchathani University,
Ubon Ratchathani 34190, Thailand
[b] Department of Chemistry, Faculty of Science, Prince of
Songkhla University,
Hat Yai, Songkhla 90110, Thailand
[c] National Nanotechnology Center, 130 Thailand Science Park,
Paholyothin Rd., Klong Luang Pathumthani 12120, Thailand
[d] School of Chemistry and Center of Excellence for Innovation
in Chemistry, Institute of Science, Suranaree University of
Technology,
Muang District, Nakhon Ratchasima 30000, Thailand
Fax: +66-44-224648
E-mail: pvinich@sut.ac.th
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201479.
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2D-D-π-A-Type Organic Dyes
aggregation between molecules.[9] The close π-π aggregation lems associated with close π-π aggregation of the dye mole-
can not only lead to self-quenching and reduction of elec- cules. The inclusion of tert-butyl substituents can increase
tron injection into TiO2, but can also result in instability of the solubility of the dye, and also form a hydrophobic
the organic dyes due to the formation of excited triplet blocking layer on the TiO2 surface to suppress the approach
–
states and unstable radicals under light irradiation.[10] On of iodide/triiodide (I–/I3 ) electrolyte to the TiO2, conse-
the basis of these criteria, recently, organic dyes with 2D-π- quently leading to an improvement in the open-circuit volt-
A structures have been reported by several groups.[11] Their age. Herein, we report a detailed synthesis and the physical
studies suggest that good performance for organic dyes and photophysical properties of these 2D-D-π-A-type dyes
based on 2D-π-A structure over the simple D-π-A configu- (CCTTnA). Furthermore, their use as dye sensitizers in
ration could be achieved by the molecular design.
DSSCs in comparison with N3 dye and D-π-A-type dye
We anticipated that further improvements could be made (TT2A),[15] having triphenylamine as a donor, are also re-
by introducing an additional donor moiety into the aro- ported.
matic amine to form a 2D-D-π-A structure, and envisaged
that such architectures would extend the absorption region,
Results and Discussion
enhance the molar extinction coefficient, reduce the ten-
dency to aggregate, and lead to better thermal stability
compared with simple D-π-A structures. To this end, we
prepared a set of new, simple 2D-D-π-A-type organic dyes.
Synthesis and Characterization
The dyes were synthesized as outlined in Scheme 1. First,
In our design (Figure 1), triphenylamine and carbazole a key intermediate, N,N-bis[4-(3,6-di-tert-butylcarbazol-9-
moieties were employed as the aromatic and additional do- yl)phenyl]-4-iodoaniline (3), was synthesized from Ullmann
nors, respectively, thereby forming a bis(3,6-di-tert-butyl- coupling of tri(p-iodophenyl)amine (1) with 3,6-di-tert-but-
carbazol-9-ylphenyl)aniline donor moiety (2D-D). The cya- ylcarbazole (2) obtained from directed iodination of tri-
noacrylic acid and oligothiophenes were incorporated as ac- phenylamine with I2, and from an alkylation of carbazole
ceptor (A) and simple π-spacer (π), respectively. Recently, with tert-butyl chloride, respectively. A stoichiometric reac-
efficient DSSCs that incorporate various triphenylamine tion of 1 (2 equiv.) and 2 (1 equiv.) under a catalytic system
and carbazole-based dyes have been reported, indicating the of CuI as catalyst, (Ϯ)-trans-1,2-diaminocyclohexane as co-
importance of their further investigation in DSSCs.[12] In catalyst, and K3PO4 as base in toluene, afforded intermedi-
addition, it has been found that by incorporating an elec- ate 3 in moderate yield (44%). From 3, intermediates 5 and
tron-donor group into the dye molecule, the physical sepa- 7 were synthesized by using a combination of Suzuki cross-
ration of the dye cation from the electrode surface can be coupling and bromination reactions in an iterative manner.
increased, which facilitates a high rate of charge separation Suzuki cross-coupling reaction with 2-thiophene boronic
and collection compared with interfacial charge-recombina- acid catalyzed by [Pd(PPh3)4]/Na2CO3 (aq.) in tetra-
tion processes.[13] Furthermore, the use of triphenylamine hydrofuran (THF) was employed to achieve an increased
and carbazole have aroused great interest because of their number of thiophene units in the molecule, whereas bromi-
excellent hole-transport capability, and their derivatives nation with N-bromosuccinimide (NBS) in THF selectively
have become classic hole-transporting materials.[14] More- introduced a bromo function to the α-position of the ter-
over, the bulky, nonplanar structure of this 2D-D moiety minal thiophene ring, allowing a further Suzuki cross-cou-
may prove to be important for solving the intractable prob- pling reaction to be performed. The Suzuki coupling of 3
and 5 with 2-thiophene boronic acid afforded thiophene
compounds 4 and 6 in good yields, whereas bromination of
4 and 6 with NBS yielded bromo intermediates 5 and 7 in
91 and 87%, respectively. Subsequent coupling of interme-
diates 3, 5 and 7 with 5-formylthiophene-2-boronic acid un-
der the same Suzuki coupling conditions gave 8, 9, and 10
in yields of 75, 70, and 60%, respectively. Final Knoevena-
gel condensation of these aldehydes with cyanoacrylic acid
in the presence of a catalytic amount of piperidine in CHCl3
heated at reflux for 18 h gave (E)-3-[5-(4-{bis[4-(3,6-di-tert-
butylcarbazol-9-yl)phenyl]amino}phenyl)thiophen-2-yl]-
2-cyanoacrylic acid (CCTT1A), (E)-3-[5Ј-(4-{bis[4-(3,6-di-
tert-butylcarbazol-9-yl)phenyl]amino}phenyl)-(2,2Ј-bithio-
phen)-5-yl]-2-cyanoacrylic acid (CCTT2A) and (E)-3-[5ЈЈ-
(4-{bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}-
phenyl)-(2,2Ј:5Ј,2ЈЈ-terthiophen)-5-yl]-2-cyanoacrylic acid
(CCTT3A) in yields of 58, 57, and 48%, respectively. All
dyes can be crystallized to give intensely colored solids. The
colors of these solid products varied from orange to dark-
Figure 1. Molecular structures of the designed dyes.
red as the number of thiophene units in the molecule in-
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Scheme 1. Synthesis of CCTTnA (n = 1–3) and TT2A dyes.
creased from CCTT1A, CCTT2A to CCTT3A. In the syn- ably as a result of their steric bulk and the presence of tert-
thesis of (E)-3-{5Ј-[4-(diphenylamino)phenyl]-2,2Ј-bi- butyl substituents, allowing dye adsorption on TiO2 film
thiophen-5-yl}-2-cyanoacrylic acid (TT2A), intermediate and fabrication of DSSCs.
2,2Ј-bithiophene-5-carbaldehyde (11) was first selectively
iodinated at the α-position of the terminal thiophene ring
with NIS in THF to give 5Ј-iodo-2,2Ј-bithiophene-5-carb-
Optical Properties
aldehyde (12) in 83% yield. Subsequently, Suzuki coupling
UV/Vis absorption spectra of CCTTnA and TT2A in
of 12 with 4-(diphenylamino)phenylboronic acid[16] af- CH2Cl2 are shown in Figure 2 and the characteristic data
forded 5Ј-[4-(diphenylamino)phenyl]-2,2Ј-bithiophene-5- are summarized in Table 1. The spectra of CCTTnA exhib-
carbaldehyde (13) in a yield of 82%. Knoevenagel conden- ited three strong absorption bands at 297–298, 330–340,
sation of this aldehyde with cyanoacrylic acid catalyzed by and 455–464 nm, respectively (Figure 2, a). The first and
piperidine produced TT2A in 79% yield. The structures of second absorption bands were attributed to localized π-π*
all dye molecules were characterized unambiguously on the transitions of the carbazole and triphenylamine donors
1
basis of FTIR, H NMR, and 13C NMR spectroscopy as (2D-D), respectively. The third band was ascribed to intra-
well as high-resolution mass spectrometry. These com- molecular charge transfer (ICT) transition from the 2D-D
pounds show good solubility in organic solvents, presum- moiety to the cyanoacrylic acid acceptor. This was con-
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2D-D-π-A-Type Organic Dyes
Figure 2. UV/Vis absorption spectra of dyes (a) in CH2Cl2 solution and (b) adsorbed on TiO2 film. (c) UV/Vis absorption spectra of
CCTT1A in different solvents. (d) PL spectra of dyes in CH2Cl2 solution.
Table 1. Optical, thermal, electrochemical, and electronic properties of the dyes.
[a]
[a]
[e]
[f]
Dye
Abs.max
[nm]
ε
Abs.[b]
[nm]
Em.max
[nm]
τ
χ2[c]
E1/2 vs. Ag/Ag+[d] T5d
Eg
Eg cal[g]
[eV]
HOMO/LUMO[h]
[eV]
[m–1 cm–1
]
[ns] (%)[c]
[V] [°C]
[eV]
CCTT1A 458
CCTT2A 455
CCTT3A 464
25225
436
442
450
646
658
672
1.14 (74.6)
0.88 (61.0)
0.85 (80.5)
0.60
0.73
1.12
–1.61, 0.83, 1.08, 284
1.40
–1.86, 0.81, 1.08, 285
1.35, 1.50
–1.61, 0.78, 0.99, 353
1.06, 1.31, 1.37
2.25
2.38
2.19
2.08
–5.22/–2.97
–5.20/–2.99
–5.16/–3.03
29322
33926
2.21
2.13
TT2A
N3[19]
405
530
20206
14500
422
–
651
–
1.30 (97.6)
–
0.90
–
–1.55, 0.92, 1.35 290
2.25
1.86
2.38
1.86
–5.28/–3.03
–5.52/–3.84
–
–
[a] Absorption and emission measured in CH2Cl2 solution. [b] Absorption of the dyes adsorbed on TiO2 film. [c] Fluorescence lifetime
measured in CH2Cl2 solution at 25 °C. [d] Measured by CV using a glassy carbon working electrode, Pt counter electrode and Ag/Ag+
reference electrode with 0.1 m nBu4NPF6 as supporting electrolyte in CH2Cl2. [e] Measured by TGA at a heating rate of 10 °C/min under
N2. [f] Estimated from the absorption onset: Eg = 1240/λ. [g] Calculated by at the TDDFT/B3LYP/6-31G (d,p) level. [h] Calculated from
ox
oxidation onset potential: HOMO = –(Eonset + 4.44); LUMO = HOMO – Eg.
firmed by a negative solvatochromic shift, i.e., hypso- bathochromic shifts and increased ε values. The absorption
chromic shift of maximum wavelength (λmax) of the peaks at spectra and the ε values of conjugated chromophores in-
longer wavelength in more polar solvents, whereas the posi- crease as the degree of conjugation is extended.[17]
tions of the first and second absorption bands were nearly
Comparison of the absorption spectra and ε values of
independent of solvent polarity (Figure 2, c). The molar ex- 2D-D-π-A dyes CCTT1A and CCTT2A to those of (E)-3-
tinction coefficients (ε) of the first and second absorption {5-[4-(diphenylamino)phenyl]thiophen-2-yl}-2-cyanoacrylic
bands of all compounds were nearly identical (ε = 42326 acid (λICT = 379 nm, ε = 17000 m–1 cm–1 in CHCl3)[18] and
and 33061 m–1 cm–1, respectively), as they have the same do- TT2A (λICT = 405 nm, ε = 20206 m–1 cm–1), having simple
nor. The ε values of the ICT bands of CCTTnA were mod- D-π-A structures, revealed significant bathochromic shifts
erate to high, ranging from 25225 to 33926 m–1 cm–1. As the (50–79 nm) of the ICT peaks and increases in the ε values
number of thiophene units in the molecules increase from (1.45–1.48 fold). These results suggest that the use of carb-
CCTT1A, CCTT2A to CCTT3A, the ICT bands showed azole as an additional or secondary donor to triphenyl-
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V. Promarak et al.
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amine donor to form a 2D-D-π-A structure dye may be an agreed with the removal of electrons from the tri-
effective way to bathochromically shift and enhance the ε phenylamine donor to give the corresponding radical cat-
value of the dye’s absorption spectra, which is desirable for ion, whereas those of CCTTnA corresponded to removal
harvesting more solar light. Moreover, the ε values of the of electrons from the peripheral carbazole donor moieties
ICT peaks of these dyes are considerably larger than that to give the radical cation. The first oxidation potentials of
of the Ru dye (N3, ε730 nm = 14500 m–1 cm–1),[19] indicating CCTTnA decreased from 0.88, 0.83 to 0.78 V when the
good light harvesting ability. The higher ε values of the dyes number of thiophene units in the molecule or the length
allow a correspondingly thinner nanocrystalline film to be of the π-conjugated spacers were increased from CCTT1A,
used, so avoiding a decrease in the mechanical strength of CCTT2A to CCTT3A, respectively, as observed in other
the film. This is also advantageous for electrolyte diffusion oligothiophenes.[22] These values were lower than that of
in the film and reduces the possibility of recombination of TT2A (0.92 V), indicating the advantage of having periph-
the light-induced charges during transportation.[20] The ab- eral carbazoles as extra donors. The multiple CV scans of
sorption spectra of CCTTnA adsorbed on TiO2 films are CCTTnA revealed identical CV curves, with no additional
shown in Figure 2 (b). The spectra were slightly blueshifted peak at lower potential on the cathodic scan (Epc) being
(13–22 nm) compared with spectra measured in CH2Cl2. observed. This indicates that no oxidative coupling at the 3
Such a phenomenon is commonly observed in the spectral or 6 positions of the peripheral carbazole, leading to elec-
response of other organic dyes, and may be ascribed to the tropolymerization, took place, presumably due to the pres-
H-aggregation of the dye molecules on the TiO2 surface ence of the 3,6-di-tert-butyl groups. This type of electro-
and/or the interaction of the anchoring groups of the dyes chemical coupling reaction can be detected in some carb-
with the surface of TiO2.[21] The fluorescence emission spec- azole derivatives with unsubstituted 3,6-positions[23] and
tra of the dyes in CH2Cl2 were redshifted with increasing might occur upon charge separation, which hampers the
numbers of thiophene units in the molecule, which was dye regeneration. Therefore, importantly, it is clear that the
roughly parallel to the trend of the absorption spectra (Fig- dyes are electrochemically stable molecules.
ure 2, d). The fluorescence decay curves of the dyes were
The HOMO and LUMO energy levels of the dyes calcu-
measured in CH2Cl2 at 25 °C and their decay parameters lated from the onset potential of the CV curves are summa-
giving the best fit are summarized in Table 1. As the rized in Table 1. The HOMO of CCTT1A (–5.22 eV) was
number of thiophene units in the molecule increased from lower than those of both CCTT2A (–5.20 eV) and
CCTT1A, CCTT2A to CCTT3A, the major relaxation time CCTT3A (–5.16 eV), but all were much lower than the re-
–
decreased from 1.14, 0.88 to 0.83 ns, respectively.
dox potential of the I–/I3 couple (–4.8 eV), therefore, dye
regeneration should be thermodynamically favorable and
should compete efficiently with the recapture of the injected
electrons by the dye radical cation. The LUMOs (–2.97 to
Electrochemical and Thermal Properties
The electrochemical properties of the dyes were studied –3.03 eV) of these dyes calculated from the HOMOs and
by cyclic voltammetry (CV) in CH2Cl2 solution with 0.1 m energy gaps (Eg) estimated from the optical absorption edge
nBu4NPF6 as a supporting electrolyte. The results are were less negative than the conduction band of the TiO2
shown in Figure 3 (a) and all data are listed in Table 1. The electrode (–4.00 eV vs. vacuum)[24] and the LUMO of N3
CV curves of all dyes exhibited multi quasireversible oxi- dye (–3.84 eV).[19] To ensure efficient charge injection, the
dation events and one irreversible reduction process. The LUMO level of the dye should be more than 0.3 eV above
reduction wave was attributed to the reduction of the the conduction band of the TiO2. Therefore, all dyes have
cyanoacrylic acid acceptor moiety, which was in the range sufficient driving force for electron injection from the ex-
of –1.55 to –1.86 V. The first oxidation wave of TT2A cited dyes to the conduction band of TiO2. As a result,
Figure 3. (a) CV curves measured in CH2Cl2 solution with nBu4NPF6 as supporting electrolyte at a scan rate of 100 mV/s. (b) TGA
thermograms measured at a heating rate of 10 °C/min under N2.
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2D-D-π-A-Type Organic Dyes
CCTTnA have sufficient energetic driving force for efficient
The molecular orbital distribution is very important in
DSSCs using a nanocrystalline titania photocatalyst and determining the charge-separated states of organic dyes.
–
the I–/I3 redox couple. Moreover, having high LUMO po- The electron distributions of the HOMO and LUMO of
tentials, these dyes become very attractive for other metal CCTTnA and TT2A are shown in Figure 5 and in the Sup-
oxide semiconductors with higher conduction bands than porting Information. To create an efficient charge-transfer
that of TiO2 such as ZnO, Nb2O3, SrTiO3 and their com- transition, the HOMO must be localized on the donor unit
posites[25] to achieve high open-circuit voltage (Voc) DSSCs. and the LUMO on the acceptor unit.[29] The HOMOs of
The thermal properties were investigated by thermogravi- these compounds are calculated to delocalize over the do-
metric analysis (TGA), and the results suggested that dyes nor moiety. In CCTTnA dyes, the major contribution of the
CCTTnA were thermally stable materials, with temperature donor moiety comes from both triphenylamine and each
at 5% weight loss (T5d) well over 285 °C (Figure 3, b). The peripheral carbazole. The LUMOs are delocalized across
better thermal stability of the dye is important for the life- the oligothiophene groups and cyanoacrylic acid acceptor.
time of the solar cells.[26]
The results indicate that the distribution of the HOMO and
LUMO of all dyes is well-separated, suggesting that the
HOMO–LUMO transition and can be considered as an
ICT transition. The lowest transition (S0ǞS1; Ͼ 93%
HOMOǞLUMO) of CCTTnA dyes corresponds to a
charge-transfer excitation from the carbazole-triphenyl-
amine donor to the oligothiophene π-spacer and cyano-
acrylic acid acceptor (see the Supporting Information). The
S2 transition state (S0ǞS2; Ͼ 88%) is related to a HOMO–
1ǞLUMO transition and has a charge shift mainly from
the two peripheral carbazole moieties to the oligothiophene
π-spacer and cyanoacrylic acid acceptor, which is reflected
in the molecular orbital involved in the transition as well as
in the low oscillator strength due to the long-range charge
shift. For the S3 transition state (S0ǞS3; Ͼ 88% HOMO–
2ǞLUMO), a charge shift mainly originates from the do-
nor moiety and oligothiophene π-spacer to the oligothio-
phene π-spacer and cyanoacrylic acid acceptor with mod-
erate to high oscillator strengths. This means that these ad-
ditional carbazole donor units may cause a cascade effect
that aids charge separation. Therefore, it is believed that
Quantum Chemical Calculations
To gain insight into the geometrical, electronic, and op-
tical structures of these new dyes, quantum chemistry calcu-
lations were performed at the TDDFT/B3LYP/6-31G (d,p)
level;[27] the results are summarized in Figure 4 and detailed
in the Supporting Information. A major factor leading to
low conversion efficiencies of many organic dyes in DSSCs
is the formation of dye aggregation on the semiconductor
surface.[10] We therefore included a bulky donor moiety
equipped with two 3,6-di-tert-butylcarbazole units con-
nected to a triphenylamine unit to increase the steric bulk
of the molecule. The optimized structures of CCTTnA re-
vealed that the dihedral angles formed between carbazole
(D1) and phenyl (Ph) planes in all molecules were as large
as 54.34–55.63° due to the steric bulk of the structure,
which could help to prevent close π-π aggregation effec-
tively between the dye molecules. The noncoplanar geome-
try can also reduce contact between molecules and enhance
their thermal stability.[28] The aromatic rings of the π-conju-
gated spacers adopted more planar conformations, with the
dihedral angles between the benzene and thiophene (T1)
planes ranging from 19.60 to 22.80°, and the thiophene (T1)
and thiophene (T2, T3) planes (CCTT2A and CCTT3A)
ranging from 4.23 to 10.54°, whereas the thiophene (T3)
and acrylic acid (A) planes were nearly coplanar (dihedral
angle of less than 0.15°). This suggests that π-electrons from
the donor moiety can delocalize effectively to the acceptor
moiety, which can subsequently transfer to the conduction
band of TiO2.
Figure 4. Schematic views of the ground state structures for
CCT3A.
Figure 5. The HOMO and LUMO orbitals of TT2A (top) and
CCTT2A (bottom).
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both S0ǞS2 and S0ǞS3 transitions can also inject electrons typical for organic adsorbates into nanoporous inorganic
either directly into the conduction band of the TiO2, or matrices.[31] It was found that, at the equilibrium maximum,
indirectly through internal conversion to S0ǞS1, which uptakes of each dye were 44.44ϫ1015, 50.65ϫ1015,
would result in an improved short-circuit current (Jsc) of 53.90ϫ1015 and 83.14ϫ1015 molecules cm–2 for CCTT1A,
DSSCs. As expected, the calculated HOMOs, LUMOs, and CCTT2A, CCTT3A and TT2A, respectively (Table 2). The
energy gaps (Eg cal) of CCTTnA decrease when the number lower dye uptake of CCT1A on the TiO2 film compared to
of thiophene units in the molecule is increased, in agree- others can be rationalized by the steric hindrance of the
ment with experimental absorption spectra and electro- donor moiety around the carboxylic acid anchoring group,
chemical results. These Eg cal values were slightly higher which arises from a considerably shorter π-spacer of
than those estimated from the optical absorption edge (Eg) CCT1A as shown in Figure 7. As a result, CCTT3A and
(Table 1), however, the orbital energy difference between CCT2PA dyes with longer π-conjugated spacers have more
HOMO and LUMO is still an approximate estimation of space to accommodate the donor moiety, allowing in-
the transition energy because the transition energy also con- creased levels of dye uptake. For the same reason, CCTTnA
tains significant contributions from some two-electron inte- had significantly lower levels of dye uptake on TiO2 than
grals. The real situation is that an accurate description of TT2A. The chemisorption of all dyes onto the surface of
the lowest singlet excited state requires a linear combination TiO2 films was confirmed by FTIR spectroscopic analysis,
of a number of excited configurations.
which showed feature absorption peaks of both the dyes
Dye Adsorption on TiO2
The performance of a DSSC is not only based on the
absorption of the harvesting dye but also on the total
amount of dye present. Therefore, dye uptake was deter-
mined by using spectrophotometric measurements of UV/
Vis absorption according to a reported method.[30] Figure 6
(a) shows the dye uptake profiles as a function of time for
CCTTnA and TT2A (see also the Supporting Information).
In all cases, dye uptake markedly increased at the beginning
Figure 7. Space-filling molecular models of the optimized confor-
until it reached a plateau at about 30 h. These profiles are mation of the dyes.
Figure 6. (a) The adsorption data for the dyes onto TiO2 films measured over a period of 50 h at room temperature (line represents the
numerical regression fit). (b) Linear relation between Jsc and εϫ(dye uptake). The linear regression line is Jsc = 3.536 [ε ϫ (dye uptake)]
with R2 = 0.995.
Table 2. Performance parameters of DSSCs constructed using the dyes.
[b]
Dye
Dye uptake[a]
IPCE [%]
Jsc [mAcm–2] Voc [V]
FF
η [%]
Cal. Jsc [mAcm–2]
(ϫ1015 molecule cm–2)
CCTT1A
CCTT2A
CCTT3A
TT2A
44.44Ϯ1.95
50.65Ϯ1.81
53.90Ϯ0.50
83.14Ϯ2.43
55.13Ϯ1.02
70
80
76
69
68
7.53
9.02
9.98
6.89
12.18
0.72
0.71
0.70
0.69
0.65
0.69
0.68
0.67
0.71
0.66
3.70
4.34
4.62
3.38
5.20
6.67
8.76
9.91
6.81
11.43
N3
[a] Obtained from dye adsorption measurement. [b] Obtained from integration of the corresponding IPCE spectra.
2614 Eur. J. Org. Chem. 2013, 2608–2620
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2D-D-π-A-Type Organic Dyes
and TiO2 at 2965 (C–H), 2207 (CϵN), 1611 (C=O), 1508 IPCE (80%) occurred at 475 nm, which is higher than the
(C=C), 1365 (C=O), 1317, 1292, and 1226 cm–1 (see the IPCE values of both CCTT1A- (70%) and CCTT3A
Supporting Information). The characteristic vibration (76%)-sensitized DSSCs. This observation deviates from
modes of the carboxylate group, both symmetric our expectation on the basis of the ε values of their absorp-
(1365 cm–1) and asymmetric (1611 cm–1), of all dyes were tion spectra, but is also observed in other types of organic
identical and were similar to that reported for other dyes dyes.[33] In all devices, we observed a decrease in the IPCE
independent of the molecular volume.[32] This indicates that above 600 nm in the long-wavelength region that can be at-
all dyes bind in the same way to the TiO2, therefore, the tributed to a decrease in the light-harvesting efficiency of
difference in the observed performance can be directly re- these dyes. The slightly lower IPCE value of CCTT3A-sen-
lated to the effect of molecular volume and how much dye sitized DSSC compared with that of CCTT2A-sensitized
is absorbed.
DSSC is probably due to extended π-conjugation elong-
ation of CCTT3A, which may lead to decreased electron-
injection yield relative to that of the CCTT2A dye.[34] This
suggests that the structural modification of the dyes
Photovoltaic Properties of CCTTnA
CCTTnA and TT2A dyes were used as sensitizers for strongly influences electron-injection and the collection effi-
dye-sensitized nanocrystalline anatase TiO2 solar cells ciencies, which, in turn, has a significant effect on the IPCE
(DSSCs). Cells with an effective area of 0.25 cm2 (0.5 cm and overall conversion efficiency (η) of the devices. More-
ϫ0.5 cm) were fabricated with 11 μm (9.5 μm transparent over, the IPCE values of all synthesized dyes were higher
+ 1.5 μm scattering) thick TiO2 working electrode, platinum than that of the N3 dye due to the larger molar extinction
(Pt) counter electrode, and an electrolyte composed of coefficients of these dyes, however, the N3 dye showed a
0.03 m I2/0.6 m LiI/0.1 m guanidinium thiocyanate/0.5 m broader IPCE spectrum, which is consistent with its wider
tert-butylpyridine in a 15:85 (v/v) mixture of benzonitrile/ absorption spectrum.
acetonitrile solution. The reference cell with the same struc-
Under continuous visible-light irradiation (AM 1.5G,
ture based on N3 dye as the sensitizer, was made for com- 100 mWcm–2), the CCTT3A-sensitized DSSC showed the
parison. To assess photovoltaic performance, five cells were highest η among these dyes and gave a short-circuit photo-
prepared and measured under the standard conditions. The current density (Jsc) of 9.98 mAcm–2, open-circuit voltage
corresponding current density-voltage (J-V) characteristics (Voc) of 0.70 V, and fill factor (FF) of 0.67, corresponding
and the incident monochromatic photon-to-current conver- to a η of 4.62%. The Jsc and η values of the DSSCs were
sion efficiency (IPCE) plots are shown in Figure 8, and the in the order: CCTT3A (9.98 mAcm–2, 4.62%) Ͼ CCTT2A
resulting photovoltaic parameters (average values) are sum- (9.02 mAcm–2, 4.34%) Ͼ CCTT1A (7.53 mAcm–2, 3.70%)
marized in Table 2. The IPCE spectra of CCTTnA-sensi- Ͼ TT2A (6.89 mAcm–2, 3.38%). The measured Jsc values
tized DSSCs plotted as a function of excitation wavelength of these solar cells were also cross-checked with the Jsc val-
were broadened and redshifted as the number of thiophene ues calculated from integration of their corresponding
units in the molecule increased, which is coincident with the IPCE spectra (Cal. Jsc), thus supporting the reported effi-
absorption spectra. The IPCE action spectra for DSSC ciency, with the results being in agreement to within 5%
based on CCTT2A were considerably redshifted in com- (Table 2). The larger Jsc and η of the CCTT2A (2D-D-π-
parison with that of a DSSC with TT2A as a result of hav- A)-sensitized solar cell compared with that of a solar cell
ing additional donors in CCTT2A; which is consistent with based on TT2A (D-π-A) demonstrates the beneficial influ-
the UV/Vis absorption spectra of the dye-loaded TiO2 films. ence of the redshifted absorption spectrum of CCTT2A on
The photocurrent response of CCTT2A-sensitized DSSC TiO2 film and the broadening of the IPCE spectrum of
also had a higher value, exceeding 76% between 435 and CCTT2A-sensitized solar cell. This could result from cas-
504 nm, compared with the other dyes. The maximum cading electron transfer from the additional carbazole do-
Figure 8. (a) IPCE plots and (b) J-V curves for the DSSCs.
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2615

V. Promarak et al.
FULL PAPER
nors to the acceptor moiety through the S0ǞS2 and S0ǞS3 cell. The DSSCs based on 2D-D-π-A-type dyes show larger
transitions, as stated earlier. The use of tert-butyl groups as Jsc and η compared with those of a solar cell based on a
substituents on the peripheral carbazole might also play a D-π-A dye. This work suggests that the organic dyes based
–
role in shielding the TiO2 surface from I–/I3 in the electro- on this 2D-D-π-A-type molecular architecture are promis-
lyte, thus reducing charge recombination or dark reaction, ing candidates for the fabrication of DSSCs with improved
leading to the larger η of the device. In the CCTTnA dye performance. Moreover, structural modifications of these
series, the superior solar cell performance (highest η and dyes to increase the molar extinction coefficient and further
Jsc) of the CCTT3A-sensitized DSSC could be rationalized redshift the absorption spectrum is anticipated to give even
by the redshifted absorption spectrum and broader IPCE better performance.
response. Whereas the lower efficiency of the CCTT1A-
based cell could be attributed to both the poorer spectral
response and the lower dye content on TiO2 film. The effi-
Experimental Section
ciency of CCTT3A-sensitized DSSC reaches 89% of the ref-
Materials and Instruments: All reagents were purchased from Ald-
erence N3-based cell (η = 5.20%). Interestingly, the effi-
rich, Acros or Fluka, and used without further purification. All
ciency of the CCTT1A-based cell can also reach more
solvents were supplied by Thai companies and used without further
than 71% that of the N3-based cell, even though the
distillation. THF was heated to reflux with sodium and benzophe-
none, and distilled prior to use. CH2Cl2 for electrochemical mea-
surements was washed with conc. H2SO4 and distilled twice from
CCTT1A dye has a lower IPCE value and narrower IPCE
spectrum than those of both CCTT2A and CCTT3A.
The ε value (in mm–1 cm–1) multiplied by dye uptake (in calcium hydride. Chromatographic separations were carried out on
molcm–2) makes a dimensionless value that corresponds to
silica gel Merck Silica gel 60 (0.0630–0.200 mm). 3,6-Di-tert-butyl-
carbazole (1) was prepared by adopting a literature procedure.[35]
the absorbance of optical spectra. For the present dyes,
1H and 13C NMR spectra were recorded with a Bruker AVANCE
300 MHz spectrometer with TMS as the internal reference using
CDCl3 or CDCl3/[D6]DMSO as solvents. IR spectra were measured
with a Perkin–Elmer FTIR spectroscopy spectrum RXI spectrome-
ter as KBr discs. UV/Vis spectra were recorded as dilute solutions
in spectroscopic grade CH2Cl2 with a Perkin–Elmer UV Lambda
25 spectrometer. Diffuse reflectance spectra of the dye-sensitized
TiO2 were performed at room temperature with a Shimadzu UV-
3101 spectrophotometer; barium sulfate was used as a standard.
The measured reflectance spectra were then converted into absorp-
tion spectra by the Kubelka-Munk method. Steady-state fluores-
cence measurements were performed with a Jobin Yvon Fluoro-
Max 2 fluorescence spectrometer with a slit width of 5 nm (for
both excitation and emission slits). Lifetime measurements were
performed by using subpicosecond laser based time-correlated sin-
gle photon counting (TCSPC) with apparatus described by Land-
graf including LED 450 nm, blue additive (BA) excitation filter and
long pass filter 550 nm.[36] Ludox (colloidal silica) was used as scat-
tering solution. All measurements were carried out in CH2Cl2 at
25 °C and deaerated with N2 gas for 5 min before measuring. Ther-
mal gravimetric analyses (TGA) were performed with a Rigaku
TG-DTA 8120 thermal analyzer with a heating rate of 10 °C/min
under an N2 atmosphere. CV measurements were carried out with
an Autolab potentiostat PGSTAT 12 with a three electrode system
(platinum counter electrode, glassy carbon working electrode, and
Ag/Ag+ reference electrode) at a scan rate of 50 mV/s in CH2Cl2
under an argon atmosphere. The concentration of analytical mate-
rials and tetrabutylammonium hexafluorophosphate (nBu4NPF6)
were 1ϫ10–3 m and 0.1 m, respectively. Melting points were mea-
sured with an Electrothermal IA 9100 series digital melting point
instrument and are uncorrected. HRMS analysis was performed by
the Mass Spectrometry Unit, Mahidol University, Thailand.
these ε ϫ (dye uptake) values showed excellent linear corre-
lation to the observed Jsc, as shown in Figure 6 (b), for
which the intercept was approximately zero (the zero inter-
cept implies that no current density is observed for zero
absorbance). This linear correlation holds true when the Jsc
is determined only by the absorbance of photons. The
length of the thiophene chain does not affect the mecha-
nism of injection, charge separation, or charge recombina-
tion, but controls Jsc through ε and dye uptake efficiency.
This length independence of the injection mechanism
strongly suggests the direct injection mechanism for
CCTTnA because such a mechanism may not explicitly in-
volve the electronic states of the thiophene units.
Conclusions
We have developed a series of novel 2D-D-π-A-type or-
ganic sensitizers, namely CCTTnA, using bis(3,6-di-tert-
butylcarbazol-9-ylphenyl)aniline as the electron-donor moi-
ety (2D-D), oligothiophene segments as π-conjugated spa-
cers (π), and cyanoacrylic acid as the electron acceptor (A).
The study demonstrates the effect of the π-spacer length on
controlling optical and photovoltaic properties. These dyes
exhibit high thermal and electrochemical stability, which
can enhance the stability of the solar cell. DSSCs con-
structed using these dyes as sensitizers exhibit efficiencies
ranging from 3.7 to 4.62% under visible-light irradiation.
The best performance among these dyes was found in
CCTT3A, which shows a Jsc value of 9.98 mAcm–2, Voc
value of 0.70, and FF value of 0.67, which correspond to
a η of 4.62% (89% of the reference N3-based cell). The
theoretical, optical, and electrochemical cell performance
studies suggest that this type of donor architecture (2D-
D) has a beneficial influence, with a redshifted absorption
spectrum of the dye in solution and dye adsorbed on TiO2
Dye Adsorption Measurements: The absorption spectra of the initial
dye solution (2.6–2.7ϫ10–5 m) in CH2Cl2 were measured with a
Shimazu MultiSpec-1501 spectrophotometer. The TiO2 substrates
with an active area of 0.25 cm2 (0.5 cmϫ0.5 cm), prepared in the
same manner as the substrate used for fabrication of DSSCs, were
baked at 450 °C for 30 min and allowed to cool to 70–80 °C for
before immersion into the dye solution in a screw cap quartz cu-
film, and a broadening of the IPCE spectrum of the solar vette (1 cmϫ1 cm). The absorption changes were monitored
2616 Eur. J. Org. Chem. 2013, 2608–2620
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2D-D-π-A-Type Organic Dyes
hourly for 6 h and then very 2 h for 50 h. The amount of dye uptake
was calculated from the calibration plot of known dye concentra-
tion. The chemisorption of all dyes onto TiO2 film was charac-
terized with a Perkin–Elmer FTIR spectroscopy spectrum RXI
spectrometer.
Quantum Chemical Calculation: Ground-state geometries were fully
optimized at the DFT level with the B3LYP hybrid functional. All
optimizations were calculated without any symmetry constraints
using the 6-31G(d,p) basis set with the Gaussian09 software pack-
age.[27]
Fabrication and Characterization of DSSCs. Device Fabrication:
The photoanodes composed of nanocrystalline TiO2 were prepared
by using a previously reported procedure.[37] Fluorine-doped SnO2
(FTO) conducting glasses (8 Ω/sq, TCO30-8, Solaronix) were used
for transparent conducting electrodes. The double nanostructure
thick film (ca. 11 μm thickness) consisting of transparent (Ti-
Nanoxide 20T/SP, Solaronix) and scattering (Ti-Nanoxide R/SP,
Solaronix) TiO2 layers were screen-printed on TiCl4-treated FTO.
The thickness of the TiO2 film was controlled by selection of screen
mesh size and repetition of printing. Prior to dye sensitization,
TiO2 electrodes with cell geometry of 0.5ϫ0.5 cm2 were treated
with an aqueous solution of 4ϫ10–2 m TiCl4 at 70 °C in a water-
saturated atmosphere, heated to 450 °C for 30 min and then cooled
to 80 °C. The TiO2 electrodes were immersed in the dye solution
(3ϫ10–4 m N3 in ethanol, and 5ϫ10–4 m organic dyes in CH2Cl2)
in the dark at room temperature for 30 h to stain the dye onto the
TiO2 surfaces. Excess dye was removed by rinsing with appropriate
solvent. To ensure maximum dye adsorbed on the TiO2 film, higher
dyes concentration (Ͼ 10 fold) than that used for dye adsorption
experiment was used. The Pt counter electrode was prepared on a
predrilled TCO30-8 FTO glass (Solaronix) by thermal decomposi-
tion of 7ϫ10–3 m [H2PtCl6] in 2-propanol solution at 385 °C. The
dye-adsorbed TiO2 photoanode and Pt counter electrode were as-
sembled into a sealed cell by heating a gasket Meltonix 1170–25
film (25 μm thickness, Solaronix) as a spacer between the elec-
trodes. An electrolyte solution of 0.6 m LiI, 0.03 m I2, 0.1 m guanid-
inium thiocyanate, and 0.5 m tert-butylpyridine in 15:85 (v/v) mix-
ture of benzonitrile and acetonitrile was filled through the pre-
drilled hole by vacuum backfilling. The hole was capped by using
hot-melt sealing film (Meltonix 1170–25, 25 μm thickness, So-
laronix) and a thin glass cover. Finally, Schotch 3m conducting tape
and silver paint (SPI supplies) were coated on the electrodes to
enhance the electric contact. For each dye, five devices were fabri-
cated and measured for consistency and the averaged cell data was
reported. The reference cells with the same device configuration
based on N3 dye as the sensitizer were also fabricated for compari-
son.
Tris(4-iodophenyl)amine (2): A mixture of triphenylamine (9.8 g,
40 mmol), KI (14.6 g, 88 mmol), and KIO3 (9.4 g, 44 mmol) in gla-
cial acetic acid (300 mL) was heated at 110 °C for 6 h. The mixture
was cooled and the formed precipitated was collected by suction
filtration. The crude product was thoroughly washed with water
(200 mL), and the solid was dissolved in CH2Cl2 (200 mL) and con-
secutively washed with water (2ϫ 100 mL), aqueous NaHCO3 (2ϫ
100 mL), and brine solution (100 mL), dried with anhydrous
Na2SO4, and filtered. After solvent evaporation, the pure com-
pound (22.4 g, 90%) was obtained by recrystallization from a mix-
ture of CH2Cl2/MeOH as a light-gray solid (m.p. 178–179 °C).
FTIR (KBr): ν = 3056, 1574, 1482, 1311, 1284, 1004, 814 cm–1. 1H
˜
NMR (300 MHz, CDCl3): δ = 7.52 (s, 6 H), 6.79 (s, 6 H) ppm. 13
C
NMR (75 MHz, CDCl3): δ = 146.0, 138.5, 126.0, 86.6 ppm.
HRMS: m/z calcd. for C18H12I3N [MH+] 622.8104; found
623.8187.
N,N-Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]-4-iodoaniline (3):
A mixture of 2 (3.0 g, 4.8 mmol), 1 (2.7 g, 9.6 mmol), CuI (0.9 g,
4.8 mmol), K3PO4 (3.3 g, 24.1 mmol), and (Ϯ)-trans-1,2-diami-
nocyclohexane (0.6 mL, 4.8 mmol) in toluene (60 mL) was de-
gassed with N2 for 5 min and then heated at reflux under an N2
atmosphere for 24 h. After cooling, the solid residue was filtered
out and washed with CH2Cl2 (50 mL). The organic filtrate was
washed with water (2ϫ 100 mL) and brine (100 mL), dried with
anhydrous Na2SO4, and the solvents evaporated to dryness. Purifi-
cation by silica gel column chromatography (CH2Cl2/hexane, 1:9)
gave 3 (1.9 g, 44%) as a light-gray solid (m.p. Ͼ 250 °C). FTIR
1
(KBr): ν = 3042, 2955, 1507, 1483, 1312, 1295, 1262, 810 cm–1. H
˜
NMR (300 MHz, CDCl3): δ = 8.37 (s, 4 H), 7.80 (d, J = 7.2 Hz, 2
H), 7.50–7.68 (m, 16 H), 7.20 (d, J = 7.2 Hz, 2 H), 1.67 (s, 36
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 147.2, 145.9, 145.6,
142.6, 139.4, 133.3, 130.9, 128.7, 127.8, 126.8, 125.3, 124.5, 123.6,
123.3, 122.7, 116.3, 110.8, 109.2, 34.8, 32.0 ppm. HRMS: m/z calcd.
for C58H60IN3 [MH+] 925.3832; found 926.3902.
N,N-Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]-4-(thiophen-2-yl)-
aniline (4): A mixture of 3 (2.0 g, 1.9 mmol), 2-thiopheneboronic
acid (0.27 g, 1.9 mmol), [Pd(PPh3)4] (0.05 g, 0.04 mmol), aqueous
2 m Na2CO3 (16 mL) in THF (40 mL) was degassed with N2 for
5 min and then heated at reflux under N2 atmosphere for 24 h.
After cooling, CH2Cl2 (100 mL) was added and the organic layer
was washed with water (2ϫ 100 mL) and brine solution (100 mL),
dried with anhydrous Na2SO4, filtered, and the solvents evaporated
to dryness. Purification by silica gel column chromatography
(CH2Cl2/hexane, 1:9) gave 4 (1.07 g, 63%) as a light-yellow solid
Device Characterization: The current density–voltage of the DSSCs
was measured with a Keithley 2400 source meter unit in a 4-ter-
minal sense configuration. The data were averaged from forward
and backward scans with a bias step and a delay time of 10 mV
and 40 ms, respectively, according to the method of Koidea and
Han.[38] Simulated sunlight was provided by a Newport sun simula-
tor 96000 equipped with an AM 1.5G filter. To minimize error,
the irradiation intensity of 100 mWcm–2 was approximated with a
calibrated BS-520 Si photodiode (Bunnkoukeiki Co., Ltd., Japan),
which has a spectral response very similar to that of the DSSCs.
The spectral output of the lamp was also matched to the standard
AM 1.5G solar spectrum in the region of 350–750 nm with the aid
of a KG-5 filter, with a spectral mismatch of less than 2%, as re-
ported by Yanagida et al.[39] The IPCE of the device under short-
circuit conditions were measured with an Oriel 150W Xe lamp fit-
ted with a CornerstoneTM 130 1/8 m monochromator as a mono-
chromatic light source, a Newport 818-UV silicon photodiode as
power density calibration and a Keithley 6485 picoammeter. All
measurements were performed by using a black plastic mask with
an aperture area of 0.180 cm2 and no mismatch correction for the
efficiency conversion data.
(m.p. Ͼ 250 °C). FTIR (KBr): ν = 3041, 2958, 1507, 1473, 1316,
˜
1
1294, 1262, 809 cm–1. H NMR (300 MHz, CDCl3): δ = 8.11 (d, J
= 5.4 Hz, 4 H), 7.59 (s, 2 H), 7.24–7.48 (m, 20 H), 7.05 (s, 1 H),
1.40 (s, 36 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 146.6, 145.9,
143.6, 142.7, 139.1, 132.7, 129.5, 128.5, 127.7, 121.1, 125.2, 124.9,
123.8, 123.1, 116.5, 109.5, 34.8, 32.2 ppm. HRMS: m/z calcd. for
C62H63N3S [MH+] 881.4743; found 882.4819.
4-(5-Bromothiophen-2-yl)-N,N-bis[4-(3,6-di-tert-butylcarbazol-9-yl)-
phenyl]aniline (5): N-Bromosuccinimide (0.35 g, 2.0 mmol) was
added in small portions to a solution of 4 (1.7 g, 1.9 mmol) in THF
(30 mL). The mixture was stirred at room temperature under N2
for a further 1 h. Water (30 mL) and CH2Cl2 (100 mL) were added,
and the organic phase was separated, washed with water (2 ϫ
Eur. J. Org. Chem. 2013, 2608–2620
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V. Promarak et al.
FULL PAPER
100 mL), brine (100 mL), dried with anhydrous Na2SO4, filtered,
and the solvents were removed to dryness. Purification by silica gel
column chromatography (CH2Cl2/hexane, 1:9) gave the brominated
product (1.5 g, 91%) as a light-yellow solid (m.p. Ͼ 250 °C). FTIR
123.3, 116.2, 109.2, 34.7, 32.0 ppm. HRMS: m/z calcd. for
C67H65N3OS2 [M+] 991.4569; found 991.4728.
5ЈЈ-(4-{Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}phenyl)-
[2,2Ј:5Ј,2ЈЈ-terthiophene]-5-carbaldehyde (10): Compound 10
(0.72 g, 60%) was prepared from 7 by using a similar method to
that described above for 8, as an orange solid (m.p. Ͼ 250 °C).
(KBr): ν = 3041, 2959, 1507, 1316, 1294, 1263, 809 cm–1. 1H NMR
˜
(300 MHz, CDCl3): δ = 8.15 (d, J = 1.2 Hz, 4 H), 7.26–7.52 (m, 21
H), 7.03 (s, 1 H), 1.48 (s, 36 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 147.3, 145.7, 142.9, 139.3, 138.5, 133.4, 127.7, 126.2, 125.2,
123.6, 123.3, 116.3, 109.2, 86.3, 34.7, 32.0 ppm. HRMS: m/z calcd.
for C62H62BrN3S [MH+] 959.3848; found 960.3908.
FTIR (KBr): ν = 3041, 2957, 1665 (C=O), 1601, 1508, 1460, 1317,
˜
1
1294, 1262, 808 cm–1. H NMR (300 MHz, CDCl3): δ = 9.88 (s, 1
H), 8.17 (d, J = 1.5 Hz, 4 H), 7.70 (d, J = 4.2 Hz, 1 H), 7.61 (d, J
= 8.4 Hz, 2 H), 7.23–7.55 (m, 22 H), 7.17 (d, J = 3.9 Hz, 1 H), 1.50
(s, 36 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 182.3, 147.2,
146.8, 145.8, 144.0, 142.8, 141.6, 139.3, 139.3, 137.2, 135.0, 134.3,
133.3, 128.7, 127.7, 126.9, 125.5, 124.3 124.0, 123.5, 123.3, 116.2,
109.2, 34.7, 32.0 ppm. HRMS: m/z calcd. for C71H67N3OS3 [MH+]
1073.4446; found 1074.4470.
4-([2,2Ј-Bithiophen]-5-yl)-N,N-bis[4-(3,6-di-tert-butylcarbazol-9-
yl)phenyl]aniline (6): Compound 6 (1.8 g, 63%) was prepared from
5 by using a similar method to that described above for 4 and
obtained as a light-yellow-green solid (m.p. Ͼ 250 °C). FTIR
(KBr): ν = 3041, 2953, 1507, 1316, 1293, 1262, 808 cm–1. 1H NMR
˜
(300 MHz, CDCl3): δ = 8.16 (s, 4 H), 7.04–7.62 (m, 25 H), 1.48 (s,
36 H) ppm. 13C NMR (75 MHz, CDCl3): δ = 146.7, 145.9, 142.8,
139.4, 133.2, 129.2, 127.8, 127.7, 126.7,125.1, 124.6, 124.6, 124.6,
124.2,123.9, 123.3, 123.2, 116.2, 109.2, 34.8, 32.0 ppm. HRMS: m/z
calcd. for C66H65N3S2 [MH+] 963.4620; found 964.4637.
2,2Ј-Bithiophene-5-carbaldehyde (11): Compound 11 (2.05 g, 89%)
was prepared from 2-thiopheneboronic acid and 5-formyl-2-
bromothiophene by using a similar method to that described above
for 8, as a light-yellow solid. FTIR (KBr): ν = 3409, 1649 (C=O),
˜
1048, 798 cm–1. 1H NMR (300 MHz, CDCl3): δ = 9.86 (s, 1 H),
7.66 (d, J = 3.9 Hz, 1 H), 7.35 (d, J = 4.4 Hz, 1 H), 7.25 (d, J =
3.9 Hz, 1 H), 7.08 (t, J = 4.4 Hz, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 182.52, 147.17, 141.74, 137.29, 136.05, 128.37, 127.10,
126.16, 124.27 ppm. HRMS: m/z calcd. for C9H6OS2 [M+]
193.9860; found 193.9755.
4-(5Ј-Bromo-[2,2Ј-bithiophen]-5-yl)-N,N-bis[4-(3,6-di-tert-butyl-
carbazol-9-yl)phenyl]aniline (7): Compound 7 (1.3 g, 87%) was pre-
pared from 6 by using a similar method to that described above for
5 and obtained as a yellow-green solid (m.p. Ͼ 250 °C). FTIR
(KBr): ν = 3041, 2959, 1507, 1318, 1294, 1263, 808 cm–1. 1H NMR
˜
5Ј-Iodo-2,2Ј-bithiophene-5-carbaldehyde (12): N-Iodosuccinimide
(2.45 g, 10.89 mmol) was added in small portions to a solution of
11 (1.76 g, 9.07 mmol) in THF/CH3COOH (30/30 mL). The mix-
ture was stirred at room temperature under N2 for a further 1 h.
Water (30 mL) and CH2Cl2 (100 mL) were added, and the organic
phase was separated, washed with water (2 ϫ 100 mL), brine
(100 mL), dried with anhydrous Na2SO4, filtered, and the solvents
were removed to dryness. Purification by silica gel column
chromatography (CH2Cl2/hexane, 1:9) gave the iodinated product
(300 MHz, CDCl3): δ = 8.16 (s, 4 H), 7.26–7.60 (m, 21 H), 7.18 (d,
J = 3.6 Hz, 1 H), 7.09 (d, J = 3.9 Hz, 1 H), 6.98 (d, J = 3.6 Hz, 1
H), 6.94 (d, J = 3.9 Hz, 1 H), 1.49 (s, 36 H) ppm. 13C NMR
(75 MHz, CDCl3): δ = 147.0, 145.8, 143.3, 142.8, 139.3, 139.0,
135.0, 133.2, 130.6, 127.7, 126.8, 125.2, 124.9, 124.4, 123.5, 123.3,
123.1, 116.2, 109.2, 34.7, 32.0 ppm. HRMS: m/z calcd. for
C66H64BrN3S2 [MH+] 1041.3725; found 1042.3773.
5-(4-{Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}phenyl)thio-
phene-2-carbaldehyde (8): A mixture of 3 (1.5 g, 1.8 mmol), 5-for-
myl-2-thiopheneboronic acid (0.15 g, 1.1 mmol), [Pd(PPh3)4]
(0.02 g, 0.01 mmol), and 2 m aq Na2CO3 (12 mL) in THF (25 mL)
was degassed with N2 for 5 min and then heated at reflux under
N2 atmosphere for 24 h. After cooling, CH2Cl2 (100 mL) was
added and the organic layer was washed with water (2ϫ 100 mL)
and brine solution (100 mL), dried with anhydrous Na2SO4, fil-
tered, and the solvents evaporated to dryness. Purification by silica
gel column chromatography (CH2Cl2/hexane, 1:5) gave 8 (0.75 g,
(2.40 g, 83%) as a yellow solid. FTIR (KBr): ν = 3409, 1635 (C=O),
˜
1
1613, 1048, 617 cm–1. H NMR (300 MHz, CDCl3): δ = 9.87 (s, 1
H), 7.66 (d, J = 3.9 Hz, 1 H), 7.22 (d, J = 3.8 Hz, 1 H), 7.19 (d, J
= 3.9 Hz, 1 H), 7.02 (d, J = 3.8 Hz, 1 H) ppm. 13C NMR (75 MHz,
CDCl3): δ = 182.50, 145.59, 142.07, 141.88, 138.19, 137.12, 127.32,
124.57, 75.38 ppm. HRMS: m/z calcd. for C9H5IOS2 [M+]
319.8826; found 319.8811.
5Ј-[4-(Diphenylamino)phenyl]-2,2Ј-bithiophene-5-carbaldehyde (13):
Compound 13 (0.71 g, 82 %) was prepared from 12 and 4-(di-
phenylamino)phenylboronic acid by using a similar method to that
75%) as a yellow solid (m.p. Ͼ 250 °C). FTIR (KBr): ν = 3042,
˜
2958, 1670 (C=O), 1599, 1507, 1446, 1317, 1294, 1263, 808 cm–1.
1H NMR (300 MHz, CDCl3): δ = 9.89 (s, 1 H), 8.17 (s, 4 H), 7.74
(d, J = 3.9 Hz, 1 H), 7.66 (d, J = 8.4 Hz, 2 H), 7.42–7.56 (m, 15
H), 7.37 (d, J = 3.8 Hz, 1 H), 7.30 (d, J = 8.1 Hz, 2 H), 1.49 (s, 36
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 182.5, 154.1, 148.7,
145.4, 142.9, 141.7, 139.3, 137.5, 133.9, 127.8, 127.6, 127.3, 125.8,
123.6, 123.4, 123.4, 123.2, 116.2, 109.2, 34.7, 32.0 ppm. HRMS:
m/z calcd. for C63H63N3OS [M+] 909.4692; found 909.4761.
described above for 8, as an orange solid. FTIR (KBr): ν = 3045,
˜
1671 (C=O), 1599, 1511, 1441, 1319, 1290, 1260, 807 cm–1 1H
.
NMR (300 MHz, CDCl3): δ = 9.85 (s, 1 H), 7.66 (d, J = 3.6 Hz, 1
H), 7.45 (d, J = 7.5 Hz, 2 H), 7.24–7.31 (m, 6 H), 7.06–7.18 (m, 9
H) ppm. 13C NMR (75 MHz, CDCl3): δ = 182.37, 148.07, 147.44,
147.46, 146.29, 141.30, 137.41, 134.08, 129.41, 127.24, 127.10,
126.63, 124.83, 123.74, 123.48, 123.19, 123.14, 29.71 ppm. HRMS:
m/z calcd. for C27H19NOS2 [M+] 437.0908; found 437.0915.
5Ј-(4-{Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}phenyl)- (E)-3-[5-(4-{Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}-
[2,2Ј-bithiophene]-5-carbaldehyde (9): Compound 9 (0.64, 70%) was
prepared from 5 by using a similar method to that described above
phenyl)thiophen-2-yl]-2-cyanoacrylic Acid (CCTT1A): A mixture of
8 (0.3 g, 0.3 mmol), cyanoacetic acid (0.038 g, 0.4 mmol) and piper-
idine (2 drops) in chloroform (20 mL) was degassed with N2 for
5 min and then heated at reflux under N2 atmosphere for 18 h.
After cooling, the reaction was quenched with water (5 mL) and
extracted with CH2Cl2 (2ϫ 50 mL). The combined organic layer
was washed with water (2ϫ 50 mL) and brine (50 mL), dried with
anhydrous Na2SO4, filtered, and the solvents evaporated to dry-
ness. Purification by silica gel column chromatography (MeOH/
for 8, as a yellow solid (m.p. Ͼ 250 °C). FTIR (KBr): ν = 3042,
˜
2959, 1665 (C=O), 1601, 1507, 1456, 1318, 1294, 1263, 808 cm–1.
1H NMR (300 MHz, CDCl3): δ = 9.87 (s, 1 H), 8.18 (d, J = 1.3 Hz,
4 H), 7.67 (d, J = 3.9 Hz, 1 H), 7.61 (d, J = 8.6 Hz, 2 H), 7.25–
7.55 (m, 21 H), 1.50 (s, 36 H) ppm. 13C NMR (75 MHz, CDCl3):
δ = 182.3, 147.7, 147.2, 145.9, 145.7, 142.8, 141.4, 139.3, 137.3,
134.5, 133.4, 128.2, 127.8, 127.2, 127.0, 125.4, 124.1, 123.8, 123.5,
2618
www.eurjoc.org
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 2608–2620

2D-D-π-A-Type Organic Dyes
CH2Cl2, 2:98) afforded the product (0.16 g, 58%) as an orange so- search Network for Research Groups (grant number CHE-RES-
lid (m.p. Ͼ 250 °C). FTIR (KBr): ν = 3422 (O–H), 3042, 2958,
RG50) funded by the Office of the Higher Education Commission,
Thailand, and the Electricity Generating Authority of Thailand
(EGAT). The authors acknowledge scholarship support from the
˜
2214 (CϵN), 1582 (C=O), 1508, 1316, 1294, 1263, 808 cm–1. 1H
NMR (300 MHz, CDCl3/[D6]DMSO): δ = 8.28 (s, 4 H), 8.17 (s, 1
H), 7.75 (d, J = 8.4 Hz, 3 H), 7.61 (d, J = 8.4 Hz, 5 H), 7.23–7.55 Royal Golden Jubilee (RGJ) Ph.D. Program and the Center of Ex-
(m, 12 H), 7.27 (d, J = 8.4 Hz, 2 H), 1.41 (s, 36 H) ppm. 13C NMR cellence for Innovation in Chemistry (PERCH-CIC) to T. K.,
(75 MHz, CDCl3/[D6]DMSO): δ = 164.3, 150.1, 148.0, 145.5, 142.8,
139.0, 138.0, 135.5, 133.2, 127.9, 127.8, 125.9, 124.2 124.0, 123.5,
123.3, 116.8, 109.6, 34.9, 32.2 ppm. HRMS: m/z calcd. for
C66H64N4O2S [M+] 976.4750; found 976.7170.
S. M., and N. J.
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phenyl)-(2,2Ј-bithiophen)-5-yl]-2-cyanoacrylic Acid (CCTT2A):
Compound CCTT2A (0.12 g, 57%) was prepared from 9 by using
a similar method to that described above for CCTT1A, as a red
solid (m.p. Ͼ 250 °C). FTIR (KBr): ν = 3422 (O–H), 3042, 2958,
˜
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2211 (CϵN), 1610 (C=O), 1508, 1363, 1317, 1294, 1263, 809 cm–1.
1H NMR (300 MHz, CDCl3/[D6]DMSO): δ = 8.27 (d, J = 1.2 Hz,
4 H), 8.00 (s, 1 H), 7.73 (d, J = 8.4 Hz, 2 H), 7.35–7.63 (m, 20 H),
7.26 (d, J = 8.4 Hz, 2 H), 1.40 (s, 36 H) ppm. 13C NMR (75 MHz,
CDCl3/[D6]DMSO): δ = 163.8, 147.2, 145.7, 144.1, 142.8, 141.7,
139.0, 136.1, 134.7, 133.0, 128.4, 127.9, 127.2, 125.6, 124.7, 124.5,
124.0, 123.1, 119.6, 116.8, 109.6, 34.8, 32.2 ppm. HRMS: m/z calcd.
for C70H66N4O2S2 [MH+] 1058.4627; found 1059.3502.
(E)-3-[5ЈЈ-(4-{Bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]amino}-
phenyl)-(2,2Ј:5Ј,2ЈЈ-terthiophen)-5-yl]-2-cyanoacrylic Acid
(CCTT3A): Compound CCTT3A (0.19 g, 48%) was prepared from
10 by using a similar method to that described above for CCTT1A,
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as a dark-red solid (m.p. Ͼ 250 °C). FTIR (KBr): ν = 3419 (O–H),
˜
3042, 2960, 2213 (CϵN), 1609 (C=O), 1508, 1364, 1317, 1294,
1263, 808 cm–1. 1H NMR (300 MHz, CDCl3/[D6]DMSO): δ = 8.27
(s, 4 H), 8.05 (s, 1 H), 7.70 (d, J = 8.4 Hz, 2 H), 7.64 (d, J = 4.2 Hz,
1 H), 7.57 (d, J = 8.4 Hz, 4 H), 7.32–7.47 (m, 17 H), 7.25 (d, J =
8.4 Hz, 2 H), 1.39 (s, 36 H) ppm. 13C NMR (75 MHz, CDCl3/
[D6]DMSO): δ = 164.5, 147.0, 145.7, 143.2, 142.8, 142.3, 141.8,
139.0, 137.7, 137.2, 135.8, 134.8, 134.5, 132.9, 128.5, 127.8, 127.1,
127.0, 126.2, 125.5, 125.3, 124.8, 124.4, 123.9, 123.1, 118.9, 116.7,
109.5, 34.9, 32.2 ppm. HRMS: m/z calcd. for C74H68N4O2S3 [M+]
1140.4504; found 1140.5029.
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cyanoacrylic Acid (TT2A): Compound TT2A (0.20 g, 79%) was
prepared from 13 by using a similar method to that described above
for CCTT1A, as a red-orange solid (m.p. Ͼ 250 °C). FTIR (KBr):
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ν = 3418 (O–H), 3044, 2964, 2211 (CϵN), 1611 (C=O), 1512, 1364,
˜
1315, 1284, 1260, 808 cm–1. 1H NMR (300 MHz, [D6]DMSO): δ =
8.00 (s, 1 H), 7.58–7.62 (m, 3 H), 7.40–7.45 (m, 3 H), 7.30–7.35 (m,
4 H), 7.04–7.10 (m, 6 H), 6.93 (d, J = 8.4 Hz, 2 H) ppm. 13C NMR
(75 MHz, [D6]DMSO): δ = 147.72, 147.19, 144.32, 141.53, 140.20,
136.56, 136.18, 134.32, 130.15, 127.29, 127.19, 126.97, 124.96,
124.62, 124.55, 124.10, 123.13, 119.80 ppm. HRMS: m/z calcd. for
C30H20N2O2S2 [M+] 504.0966; found 504.0983.
Supporting Information (see footnote on the first page of this arti-
cle): Quantum chemical calculations, multiple CV scans, FTIR
spectra, and UV/Vis absorption spectra of dyes adsorbed on TiO2,
1
and H and 13C NMR spectra of the dyes and intermediates.
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This work was supported by the Thailand Research Fund (grant
number RMU5080052), the Strategic Scholarships for Frontier Re-
Eur. J. Org. Chem. 2013, 2608–2620
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
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Received: November 6, 2012
Published Online: March 13, 2013
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