Synthesis and characterization of organic dyes containing 2,7-disubstituted carbazole π-linker

Article abstract of DOI:10.1016/j.tetlet.2013.05.069

New organic dyes containing diphenylamine donor, 2,7-carbazolyl π-linker, and cyanoacrylic acid acceptor have been synthesized and characterized. They possess red-shifted absorption with high molar extinction coefficient when compared to the corresponding

Full text of DOI:10.1016/j.tetlet.2013.05.069

Tetrahedron Letters 54 (2013) 3985–3989
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and characterization of organic dyes containing
2,7-disubstituted carbazole p-linker
b
b
A. Venkateswararao a, K. R. Justin Thomas a, , Chuan-Pei Lee , Kuo-Chuan Ho
⇑
a Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee 247 667, India
b Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
New organic dyes containing diphenylamine donor, 2,7-carbazolyl p-linker, and cyanoacrylic acid accep-
Received 30 March 2013
Revised 14 May 2013
Accepted 17 May 2013
Available online 23 May 2013
tor have been synthesized and characterized. They possess red-shifted absorption with high molar
extinction coefficient when compared to the corresponding dyes with fluorene or phenyl units in the con-
jugation. The dye-sensitized solar cells fabricated using these dyes as sensitizers exhibited power conver-
sion efficiencies as high as 6.8% which increased to 7.2% when chenodexoycholic acid (CDCA) was used to
impede dye aggregation. The DSSCs with ionic liquid electrolytes displayed marked stability over 1000 h.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Organic dyes
Carbazole
Absorption spectra
TDDFT computations
Dye-sensitized solar cells
Organic materials suitable for application in electronic devices
such as organic light-emitting diodes,1 photovoltaics,2 and thin
film transistors3 have attracted wide attention in recent years
due to the demand for alternate energy resources and energy effi-
cient consumer electronic appliances. In most of these devices, the
organic dyes serve as emitters, sensitizers, or charge transporter.
So, the synthesis and characterization of organic dyes possessing
the above mentioned functional properties are actively pur-
sued.1–3 Among the photovoltaic devices, the dye-sensitized solar
cells (DSSC or Grätzel cells) demonstrated by Grätzel are consid-
ered as a potential alternative as their performance can be fine
tuned by architectural alternations and molecular engineering.4
In the Grätzel cells, organometallic and organic dyes have been
widely used as sensitizers for nanocrystalline TiO2 semiconductor.
Carbazole, a fused aromatic heterocyclic compound, has been
widely used in the development of electronic materials due to its
excellent hole transporting capability and facile functionalization
at various nuclear positions.9 Molecular materials bearing func-
tional chromophores in the C-3, C-6, and N-9 positions of the car-
bazole have been widely exploited for application in OLED and
photovoltaic devices. However, the utility of 2,7-disubstituted car-
bazole derivatives is limited due to the non-availability of direct
methods for chemical modifications at the C-2 and C-7 positions.10
In this work, we present the synthesis and characterization of two
new metal-free organic dyes (Fig. 1) possessing 2,7-carbazolyl
bridging unit, diphenylamine donor, and cyanoacrylic acid accep-
tor. Even though organic dyes containing 2,7-functionalized carba-
zole in the conjugation have been recently reported, they utilized
pyridine as the anchoring group and yielded very low efficiency
(1.89%) in the DSSCs.11 The dyes reported in this work showed
excellent light-harvesting properties when used as sensitizer in
Organic dyes possessing donor–p–acceptor configuration have
been found to function efficiently in DSSCs.5 The following molec-
ular engineering methods have been found to improve the effi-
ciency of the DSSCs: (a) amplification of the electron richness of
the donor unit by the introduction of an additional electron donat-
ing unit.6 (b) Incorporation of electron-rich or low band gap chro-
mophores in the conjugation pathway.7 (c) Introduction of
auxiliary acceptors in the vicinity of the acceptor end.8 All these
methodologies were found to be fruitful for the dye molecules
which ensured a facile charge migration from the donor to the
NC
COOH
N
S
n
N
acceptor via a
p
-spacer.
Bu
C1
C2
(n = 1)
(n = 2)
⇑
Corresponding author. Tel.: +91 1332 285376; fax: +91 1332 286202.
E-mail address: krjt8fcy@iitr.ernet.in (K.R. Justin Thomas).
Figure 1. Structures of the carbazole-based organic dyes.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.05.069

3986
A. Venkateswararao et al. / Tetrahedron Letters 54 (2013) 3985–3989
40000
30000
20000
10000
0
DSSCs which illustrate the benefits of the cyanoacrylic acid unit as
an efficient acceptor and anchoring segment.
1.00
0.75
0.50
0.25
0.00
C1 (abs)
C2 (abs)
C1 (em)
C2 (em)
The dyes were synthesized by following a three-step protocol
involving Hartwig–Buchwald C–N coupling reaction,12 Stille reac-
tion13, and Knoevenagel14 condensation (Scheme 1). In the first
step diphenylamine was reacted with a slight excess of 2,7-dibro-
mo-9-butyl-9H-carbazole to obtain 7-bromo-9-butyl-N,N-diphe-
nyl-9H-carbazol-2-amine (2) in good yield (78%). It was then
subjected to the Stille reaction with the tin reagents of the pro-
tected aryl aldehydes and subsequent acid hydrolysis generated
the required conjugated aldehydes 3a and 3b. They were then eas-
ily converted to the desired dyes C1 and C2 by condensing with
cyanoacetic acid in the presence of ammonium acetate in acetic
acid. The dyes were thoroughly characterized by NMR and HRMS
studies (Electronic Supplementary data).
300
400
500
600
700
800
The absorption and emission spectra of the dyes recorded in
dichloromethane solution are displayed in Figure 2 and the perti-
nent data presented in Table 1. Both the dyes displayed four distin-
guishable absorption peaks attributable to the carbazole and
Wavelength, nm
Figure 2. Absorption and emission spectra of the dyes recorded in
dichloromethane.
diphenylamine localized
p–
p⁄ transitions, electronic excitation be-
tween the
p–
p⁄ orbitals delocalized over the entire conjugation
jugation pathway and hike the transition probability of the corre-
sponding transitions.16 It is interesting to note that the
absorption wavelengths of these dyes (C1 and C2) are significantly
red-shifted when compared to those containing biphenyl,17 fluo-
rene,18 or 9,10-dihydrophenanthrene19 unit substituted for carba-
zole or dyes with a pyridine acceptor11 in a similar structural
composition.
Electronic distributions observed in the frontier molecular orbi-
tals of the dyes and their energies are illustrated in Figure 3. HOMO
of the dyes is mainly located on the amine donor unit and spread
up to the carbazole and thiophene units. The LUMO of the dyes
are contributed by the cyanoacrylic acid and the thiophene units.
pathway and amine to cyanoacrylic acid charge transfer transition
in that order. The peaks that appeared at around 260 nm and
310 nm are insensitive to the nature of the p-spacer and are similar
to those observed for carbazole and diphenylamine localized elec-
tronic transitions.15 The longer wavelength absorptions observed
at 357 nm and 473 nm for the dye C1 is red shifted to 375 nm
and 489 nm, respectively in C2. They have also displayed incre-
ment in molar extinction coefficients attributable to the elongation
of conjugation due to the insertion of additional thiophene unit in
C2. Extension of conjugation by oligothiophene unit has been
found to be beneficial to enhance the electron-richness in the con-
Pd(dba)2
dppf
Br
Br
+
HN
N
Br
tert-BuONa
Toluene
78%
N
Bu
N
Bu
1
2
O
O
(i)
Bu3Sn
S
n
Pd(PPh3)2Cl2, DMF, 80 °C
AcOH/H2O
(ii)
NC
COOH
CNCH2COOH
N
N
CHO
S
S
n
n
AcOH
NH4OAc
N
Bu
N
Bu
C1 : n = 1 (83%)
C2
3a : n = 1 (95%)
3b : n = 2 (79%)
: n = 2 (85%)
Scheme 1. Synthesis of the dyes.
Table 1
Optical and electrochemical data for the dyesa
EÃox, Vc
Dye
kabs, nm (emax  103 MÀ1 cmÀ1
)
kem (nm)
Eox (D
EP), Vb
C1
C2
473 (29.3), 357 (18.0), 311 (16.8), 267 (33.3)
489 (34.3), 375 (23.5), 311 (16.9), 264 (34.6)
572
579
0.38 (65), 0.86 (63)
0.33 (62), 0.71 (73)
À1.18
À1.17
a
b
c
Measured in dichloromethane.
Oxidation potentials are reported with reference to the ferrocene internal standard and measured at 100 mV/s.
EÃox is the excited state oxidation potential with reference to NHE.

A. Venkateswararao et al. / Tetrahedron Letters 54 (2013) 3985–3989
3987
Table 2
Performance parameters of the DSSCs fabricated using the dyes C1 and C2
Dye
Jsc
Voc
(mV)
ff
g
(%)
Rrec
Rct2
se
(ms)
(mA cmÀ2
)
(
X)
(
X)
C1
C2
15.30
18.10
660
630
628
620
616
0.60 6.05 22.50
0.60 6.83 12.00
17.55
14.72
13.02
13.77
18.49
12.50
3.12
3.80
3.80
2.09
C2+Aa 19.30
C2+Bb 18.30
C2+Cc 13.60
0.59 7.20
0.61 6.92
0.65 5.45
—
—
—
a
b
c
1 mM CDCA.
10 mM CDCA.
20 mM CDCA.
more negative than the conduction band of the TiO2 (À0.5 V vs
NHE)21 which provides the required thermodynamic driving force
for the electron injection from the photo-excited dye to the con-
duction band of TiO2. Similarly, the oxidized dyes can be effectively
regenerated only if their ground state oxidation potentials are
more anodic than the redox potential of the redox electrolyte.
The oxidation potentials of the dyes (1.15 V for C1 and 1.10 V for
C2 vs NHE) are larger than the redox potential of the IÀ=I3À redox
couple (0.4 V vs NHE).20 This predicts a facile regeneration of the
dyes by the IÀ=IÀ3 electrolyte after electron injection.4b
Figure 3. Energy level diagram showing the electronic distributions observed in the
frontier orbitals of the dyes along with the redox potentials.
This clearly indicates an efficient charge migration from the donor
unit to the acceptor segment on HOMO to LUMO electronic
excitation.
For efficient electron injection into the conduction band of the
TiO2, the excited state oxidation potential of the dyes must be
more negative than the conduction band of the TiO2.4b The ex-
cited-state oxidation potentials (Table 1) of the sensitizers are
DSSCs were fabricated using the dyes as sensitizers, and an elec-
trolyte composed of 0.1 M LiI, 0.6 M 1-propyl-2,3-dimethyl
21
100
C1
C2
(a)
C2 + 1 mM CDCA
75
C2 + 10 mM CDCA
14
C2 + 20 mM CDCA
N719
50
C1
C2
C2 + 1 mM CDCA
C2 + 10 mM CDCA
C2 + 20 mM CDCA
N719
7
25
(b)
0
9
0
400
0
150
300
450
600
750
500
600
700
800
Wavelength, nm
Cell Voltage (mV)
12.5
10.0
7.5
C1
C2
(c)
(d)
C2 + 1 mM CDCA
C2 + 10 mM CDCA
C2 + 20 mM CDCA
6
3
0
5.0
C1
C2
C2+1 mM CDCA
C2+10 mM CDCA
C2+20 mM CDCA
2.5
0.0
100
101
102
103
104
105
20
30
40
50
Z', Ohm
Frequency, Hz
Figure 4. (a) I–V and (b) IPCE (c) Nyquist and (d) Bode phase plots for the devices measured under 100 mW cmÀ2 illumination.

3988
A. Venkateswararao et al. / Tetrahedron Letters 54 (2013) 3985–3989
imidazolium iodide (DMPII), 0.05 M I2, and 0.5 M tert-butylpyri-
dine (TBP) in a 1:1 solvent mixture of acetonitrile/3-methoxypro-
pionitrile. The device performance data under AM 1.5,
100 mW cmÀ2 illumination are compiled in Table 2. The photocur-
rent–voltage (I–V) curves and the incident photon-to-current con-
version efficiencies (IPCE) of the DSSCs are plotted in Figure 4. Both
6.0
5.0
4.0
3.0
C2
C2 +1 mM CDCA
the dyes gave impressive power conversion efficiencies (g = 6.01%
and 6.83%) in the DSSCs. The superior performance of the device-
based on the dye C2 is originating from the higher short circuit cur-
rent attributable to the better light absorption characteristics of C2.
The dye C2 exhibits highest IPCE at shorter wavelength region and
the devices produced superior photon conversion up to 520 nm.
The devices were studied by electrochemical impedance spec-
troscopy (EIS) to understand the kinetics of electron transfer at
the interfaces. The Nyquist and Bode phase plots observed for
the devices under illumination are displayed in Figure 4c and d
and the relevant data listed in Table 2. The bigger semicircle in
the Nyquist plot (Fig. 4c) corresponds to the electron transport
resistance (Rct2) and the calculated value is smaller for the device
with C2. It further lowered on addition of chenodexoycholic acid
(CDCA) up to 10 mM. Smaller Rct2 indicates a favorable electron
collection at the photoanode which results in a high photocurrent.
This is attributable to the thermodynamically more favorable
LUMO in C2 (Fig. 3).5b The electron lifetime extracted from the
0
200
400
600
800
1000
Time, h
Figure 5. Overall efficiencies of the DSSCs with C2 and with C2/CDCA, obtained at
different times for 1000 h; the efficiencies were all measured at 100 mW cmÀ2
.
In summary, we have synthesized new simple organic sensitiz-
ers having diphenylamine donor, 2,7-disubstituted carbazole-
based
p-spacers and cyanoacrylic acid acceptors. They possess an
angular frequency (
phase plot (Fig. 4d) using
x
min) at the mid-frequency peak in the Bode
e = 1/ min significantly small for the de-
intense absorption with the peak wavelength longer than
470 nm. High performance liquid DSSCs exhibiting power conver-
sion efficiency as much as 7.20% can be achieved using these dyes
as sensitizers. This is the first example of the DSSC with highest
power conversion efficiency for a carbazole-based organic sensi-
tizer. Photoanodes sensitized with the newly developed metal-free
organic dyes have been found to be stable up to 1000 h. This dem-
onstrates the potential use of the carbazole moiety in the construc-
tion of efficient and stable DSSCs and warrants more systematic
physicochemical exploration of carbazole-based dyes. The dyes
developed in this work may serve as promising candidates for
co-sensitized solar cells. We are currently working to optimize
the molecular structure as well as device parameters to improve
DSSC efficiency.
s
x
vices fabricated using C2 which suggests faster charge recombina-
tion and larger dark current leading to comparatively low open
circuit voltage. Slight enhancement in the electron lifetime was ob-
served on the addition of CDCA. It probably indicates that the addi-
tion of CDCA inhibits the intermolecular dye association at the
surface of TiO2.22
Generally, dye aggregation at the surface of TiO2 reduces the
electron lifetime and facilitates charge recombination in the DSSCs
fabricated using organometallic or organic dyes.22 To examine the
possibility of dye aggregation, DSSCs were fabricated with various
amounts of CDCA co-adsorbed photoanodes. The photocurrent
density and power conversion efficiency increased on addition of
1 mM CDCA to the dye solution. However, on addition of higher
amounts (10 mM or 20 mM) of CDCA to the dye solution reduced
the Voc and Jsc which led to comparatively low power conversion
efficiency than that observed for the dye alone (Table 2). This out-
come can be rationalized by the following explanations: (1) pure
dye solution leads to aggregation of the dyes on the TiO2 electrode
which causes inefficient electron injection; (2) an optimum
amount of CDCA (1 mM) is sufficient to impede the dye aggrega-
tion; (3) the high CDCA concentration reduces the adsorbed
amount of the dye on TiO2 films and the ability to harvest light.
Acknowledgments
K.R.J.T. is thankful to the Department of Science and Technol-
ogy, New Delhi, India (Ref: DST/TSG/ME/2010/27) and BASF, India
for financial support. DST FIST is acknowledged for mass spectral
facility.
Supplementary data
As a consequence, the highest
when 1 mM CDCA was present in the dye solution.
g value of 7.20% has been obtained
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.05.
069.
In order to investigate the durability of the photoanodes sen-
sitized with C2 or C2/CDCA, DSSCs were also fabricated using
binary-ionic liquid electrolyte composed of 1-methyl-3-propy-
limidazolium iodide (PMII) and 1-ethyl-3-methylimidazolium
tetrafluoroborate (EMIBF4) (65:35) in 0.2 M I2 and 0.5 M TBP
due to their advantages such as negligible vapor pressure, high
thermal stability, wide electrochemical window, and high ionic
conductivity.23 Though these DSSCs based on ionic liquid electro-
lytes and sensitized with C2 or C2/CDCA showed slightly lower
power conversion efficiencies (4.06% and 4.59%, respectively)
than the liquid DSSCs, they were found to be stable for long
term up to the tested 1000 h (Fig. 5). These results indicate that
the photoanodes sensitized with C2 or C2/CDCA can survive for
long duration without significant loss in overall conversion
efficiency.
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