Synthesis, optical and electrochemical properties of carbazole sensitizers and their interaction with TiO2

Article abstract of DOI:10.1016/j.molstruc.2013.12.017

Two carbazole derivatives (CN, CR) were synthesized and characterized by ¹H, ¹³C NMR, Mass, IR and cyclic voltammetry measurements. The absorption of CR appears at higher wavelength than CN. Effect of solvents on the absorption and e

Full text of DOI:10.1016/j.molstruc.2013.12.017

Journal of Molecular Structure 1060 (2014) 191–196
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, optical and electrochemical properties of carbazole sensitizers
and their interaction with TiO2
⇑
S. Jagadeeswari, G. Paramaguru, S. Thennarasu, R. Renganathan
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
h i g h l i g h t s
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
Carbazole with two different electron withdrawing group were synthesized.
ICT transition is observed by introducing the electron withdrawing anchoring groups.
Carbazole having rhodhanine group shows better electron withdrawing character.
Specific solvent effect in charge transfer transition is observed.
Carboxylate anchoring group have higher binding ability than nitro group with TiO2.
a r t i c l e i n f o
a b s t r a c t
Two carbazole derivatives (CN, CR) were synthesized and characterized by 1H, 13C NMR, Mass, IR and cyc-
lic voltammetry measurements. The absorption of CR appears at higher wavelength than CN. Effect of sol-
vents on the absorption and emission properties of the sensitizers was analyzed. Interaction of these
sensitizers with TiO2 was studied by using absorption and fluorescence measurements. Ground state
complex formation was observed from steady state measurements. Binding constant (K) was calculated
from fluorescence quenching measurements. The possibility of electron transfer from the excited state
sensitizers to the conduction band of TiO2 was analyzed using energy levels and Rehm–Weller equation.
Ó 2013 Elsevier B.V. All rights reserved.
Article history:
Received 7 July 2013
Received in revised form 15 December 2013
Accepted 16 December 2013
Available online 22 December 2013
Keywords:
Carbazole
TiO2
Electron transfer
Fluorescence quenching
Binding constant
1. Introduction
sensitizer into the conduction band of TiO2. Hence by utilizing a
D–A type organic molecule, the light-induced intramolecular
Donor–acceptor (D–A) organic molecules have considerable
attention due to their potential applications in various fields such
as photovoltaic materials for solar cells [1], electroluminescent
(EL) materials for organic light-emitting diodes (OLEDs) [2]. The
physical and chemical properties of D–A materials can be modified
by selecting suitable donor and acceptor moieties. Studies on var-
ious D–A chromophore revealed that these molecules possess an
intramolecular charge transfer characteristic which is one of the
essential parameter for solar cell applications typically in dye sen-
sitized solar cells (DSSCs). In DSSC, the presence of anchoring group
such as carboxyl, hydroxyl, nitro, phosphonic moiety in the accep-
tor part enables efficient binding with TiO2 [3–5]. The sensitizer
which is anchored to the semi conducting TiO2 surface absorbs
the incoming light and electron injection occurs from the excited
electron transfer can easily occur from the electron donor to the
electron acceptor which favors photocurrent generation [2,6]. D–
A organic dyes have several advantages, such as their ease of
synthesis, high molar extinction coefficient, tunable absorption
spectral response from the visible to the near infrared (NIR) region,
and inexpensive production techniques compared to those for
inorganic complexes in the photovoltaic field [7].
So far, many metal free organic dyes containing donor acceptor
moiety have been studied theoretically and experimentally. Tria-
rylamines, carbazoles, fluorenes, thiophenes and oligothiophenes
[8–12] are successfully utilized as electron donating moieties,
whereas oxadiazoles, diarylborons, quinolines, quinoxalines,
thienopyrazines, and benzothiadiazoles [13–17] are commonly
used as electron accepting moieties. Carbazole and its derivatives
are N-Heterocyclic compounds, possess desirable electronic and
charge-transport properties, as well as large
and various functional groups can be easily introduced into the
p-conjugated system,
⇑
Corresponding author. Tel.: +91 431 2407053; fax: +91 431 2407045.
E-mail address: rrengas@gmail.com (R. Renganathan).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.12.017

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S. Jagadeeswari et al. / Journal of Molecular Structure 1060 (2014) 191–196
structurally rigid carbazole ring [18]. Considering all the above
points, we have chosen carbazole as a common donor fragment
and introduced two different types of electron withdrawing accep-
tor parts. This will provide information about the strength of these
different acceptor parts in their electron withdrawing properties
and altering the photophysical properties of carbazole donor frag-
ment. Further, the strength of electronic binding between the sen-
sitizers and TiO2 with their anchoring groups can be understood.
Hence, the selection of electron withdrawing anchoring groups in
designing the sensitizers not only focuses on adsorption ability to
the semiconductor surface but also the extend of its electron
accepting behavior. The consequences of different electron with-
drawing segments in their optical properties were investigated
through absorption and fluorescence measurements. The binding
ability of the sensitizers with TiO2 nanoparticles was probed
through steady state measurements. Energy level calculations
and Rehm–Weller equation were applied to analyse the feasibility
of electron injection from the excited sensitizers to the conduction
band of TiO2. The structures of carbazole sensitizers studied in the
present work are shown below.
mixture was stirred for 30 min and then 1-bromopropane
(1.1 mol ratio) was added. The reaction was stirred at room tem-
perature and monitored by TLC. Water was added, the mixture
was extracted with CHCl3 and dried with anhydrous sodium sul-
fate, and evaporated. The residue was purified by column chroma-
tography with Petroleum ether/ethyl acetate as the eluent to give
CrANA. Yield – 86%, Color: Colorless solid, 1H NMR, (400 MHz,
CDCl3): d (ppm), 0.9–0.99 (t, 3H), 1.87–1.96 (m, 2H), 4.26–4.29(t,
2H), 7.2–7.3 (m, 2H), 7.4–7.5 (m, 4H), 8.1 (t, 2H).
N
2.2.2. Preparation of 9-propyl-9H-carbazole-3-carbaldehyde
(CrACHO)
A round bottom flask was charged with a solution of DMF
(2.01 mol ratio) and 1,2-dichloroethane (3 ml) at 0 °C. POCl3
(1.25 mol ratio) was slowly added to the mixture. Then, CrANA
(1 mol ratio) in 1,2-dichloroethane (3 ml) was added dropwise to
the mixture. The mixture was stirred for 12 h at 90 °C. Next, it
was poured into ice water and the compound was filtered and
dried. Yield – 94%, Color: yellow solid. 1H NMR, (400 MHz, DMSO):
d (ppm), 0.82–0.88 (t, 3H), 1.77–1.86 (m, 2H), 4.41–4.45 (t, 2H), 8.7
(m, 2H), 8.3 (d, 1H), 7.9 (d, 1H), 7.8 (d, 1H) 7.8 (d, 1H), 7.6 (t, 1H),
7.5 (t, 1H), 7.3 (d, 1H), 10 (s, 1H), IR, (KBr, cmꢂ1): CAN – 1339, C@O
N
N
O
O
N
CN
OH
O
S
CR
CN
- 1685 cmꢂ1
.
NO2
2. Experimental section
N
2.1. Materials and methods
Titanium (IV) isopropoxide, 4-Nitrophenylacetonitrile and 4-
Oxo-2-thioxo-3-thiazolidinylacetic acid were purchased from Sig-
ma–Aldrich and used without further purification. Carbazole and
1-bromopropane, piperidine, POCl3 were obtained from LOBA and
Rankem chemicals. Other chemicals and solvents were of analyti-
cal grade and purchased from LOBA chemicals (India). All measure-
ments were performed at room temperature (25 °C).
CHO
2.2.3. Preparation of carbazole sensitizers
To a mixture of CrACHO (1 mol ratio), corresponding active
methylene compound (1 mol ratio) in chloroform, piperidine
(0.01 mol ratio) was added and refluxed until the disappearance
of aldehyde spot in thin layer chromatography. After the comple-
tion of the reaction, the precipitated solid was filtered and dried
in vacuum to yield the desired product.
Absorption spectra were recorded using JASCO 300 UV–Visible
spectrometer. Steady state fluorescence quenching measurements
were carried out with JASCO FP-6500 spectrofluorimeter. The exci-
tation and emission slit width (each 5 nm) and scan time rate
(500 nm/min) were kept constant for all the experiments. Quartz
cells (4 ꢁ 1 ꢁ 1 cm) with high vacuum Teflon stopcocks were used
for spectral measurements. The excitation wavelength of CN and
CR in CHCl3 were 415, 445 nm and emission occurred at 548,
502 nm respectively. Cyclic voltammetry was acquired with
Princeton EG and G-PARC model potentiostat using Glassy carbon
working electrode, Ag/AgCl reference electrode and platinum wire
counter electrode. Tetrabutyl ammonium hexafluoro phosphate
(0.1 M) is used as supporting electrolyte for carbazole sensitizers
in DMF solvent. All samples were deaerated by bubbling with
nitrogen gas for ca. 5 min at room temperature. Preparation and
characterization of colloidal TiO2 nanoparticles have already been
reported [19]. The absorption of the colloidal TiO2 in water is ob-
served at 330 nm. The diameter of the particles determined from
2.2.3.1. 2-(4-Nitro-phenyl)-3-(9H-cabazol-3-yl)-acrylonitrile (CN).
Yield – 32%, Color: orange solid, 1H NMR, (400 MHz, DMSO): d
(ppm), 0.869–0.905 (t, 3H), 1.81–1.86 (m, 2H), 4.42–4.45 (t, 2H),
8.8 (s, 1H), 7.3 (m, 1H), 7.6 (t, 1H), 7.7 (d, 1H), 7.8 (d, 1H), 8 (d,
2H), 8.1 (d, 1H), 8.2 (d, 1H), 8.3 (d, 2H), 8.4 (s, 1H). IR, (KBr,
cmꢂ1): CAN – 1338, C@C – 1580, C„N – 2206, NO2 – 1520 cmꢂ1
.
Mass spectrum (ESI–MS):C24H19N3O2 Calculated = 381.15, [M+H]
Obtained = 380.7.
N
CN
the relationship between band gap shift (DEg) and radius (R) of
quantum-size particles [20] is about 4.1 nm.
2.2. Synthesis of carbazole sensitizers
NO2
2.2.1. Preparation of 9-propyl 9H-carbazole (CrANA)
Carbazole was added to a suspension of sodium hydroxide
(1.2 mol ratio) in DMSO (13 V) under nitrogen atmosphere. The

S. Jagadeeswari et al. / Journal of Molecular Structure 1060 (2014) 191–196
193
2.2.3.2.
[4-Oxo-5-(9-propyl-9H-carbazol-3-ylmethylene)-2-thioxo-
absorption of the latter spreads more in the visible region. This
suggests that rhodanine moiety act as a better electron withdraw-
ing group compared to nitrophenyl acetonitrile segment. Hence,
the observed absorption in the visible region instigates due to
intramolecular charge transfer from the carbazole donor to the
acceptor groups [23]. This illustrates the importance of incorporat-
ing EW group in the sensitizer in tuning their optical properties.
The absorption spectrum of CN and CR in different solvents was
shown in SI 10 and 11. The kmax values and the molar extinction
thioazolidin-3-yl]-acetic acid (CR). Yield – 52%, Color: Red solid, 1H
NMR, (400 MHz, DMSO): d (ppm), 0.84–0.88 (t, 2H), 1.79–1.84
(m, 2H), 4.39–4.43(t, 2H), 4.747 (s, 2H), 7.2 (m,1H), 7.5 (m, 1H),
7.7 (m, 1H), 7.6 (t, 1H), 7.8 (d, 1H), 8 (s, 1H), 8.2 (d, 1H), 8.4 (s,
1H); 13C NMR (100 MHz, DMSO); d 11.25, 21.03, 22.17, 43.95,
45.23, 110.06, 110.58, 117.23, 120.82, 121.80, 122.92, 123.56,
124.35, 126.78, 128.56, 135.62, 140.76, 141.52, 166.47, 167.34,
193.11. IR, (KBr, cmꢂ1): CAN 1329, C@C 1575, C@O 1698, OAH
3480, C@S 1190 cmꢂ1
.
coefficient (
shoulder band appeared for CR in some solvents (Fig. 3) around the
390–420 nm region is due to
* transition localized on the car-
bazole unit. However, charge transfer band (
p* transition
localized towards acceptor fragments) is more prominent and the
p* transition localized on carbazole moiety is appeared as
e) of all the compounds are listed in Table 1. The small
p
? p
p
?
N
O
p
?
O
small shoulder. The absorption maxima of the sensitizers are al-
tered by varying the solvents which signifies the solvent effect in
N
OH
charge transfer transition. The molar extinction coefficient (
CR is found to be higher than the CN.
e) of
O
S
Fig. 2 shows the emission spectrum of carbazole sensitizers in
CHCl3 solvent. The emission spectrum of CR and CN appears at
502 and 548 nm. Figs. 3 and 4 show the emission spectrum of CN
and CR in different solvents and the data are listed in Table 1.
The emission maxima lie between 494–579 nm and 498–523 nm
for CN and CR respectively in various solvents. Lippert Mataga plot
is generally used to understand the solvatochromic effect on the
optical properties of the sensitizers. SI 12 depicts the Lippert–
Mataga plot for carbazole sensitizers. Generally, a linear and
non-linear plot corresponds to general and specific solvatochromic
effect respectively [24]. The observed non-linearity of the
Lippert–Mataga correlation shows the existence of specific solvent
effect. Factors such as hydrogen bonding, stabilization of the intra-
molecular charge transfer states between the sensitizers and the
solvents molecules were liable for the observed specific solvent
effect [24].
3. Results and discussion
3.1. Synthesis and characterization of carbazole derivatives
Scheme 1 shows the synthetic route for the carbazole sensitiz-
ers. Carbazole sensitizers are synthesized according to the
literature method [21,22]. Carbazole was alkylated with 1-bromo-
propane followed by Vilsmeier reaction and subjected to Knoeve-
nagel condensation using corresponding active methylene
compounds yielding the final products and successfully character-
ized by NMR and IR spectroscopy measurements.
The synthesised CN and CR were characterized by using NMR,
IR, absorption, fluorescence and cyclic voltammetry measure-
ments. For CN and CR, all aromatic protons appeared in the range
of 7.2–8.4 ppm and the signal around 8.45–8.82 ppm corresponds
to characteristic olefin proton and aliphatic protons appeared at
0.8–4.4 ppm. The ACH2 peak in CR appears at 4.7 ppm. For 13C of
CR, the carboxylic acid carbon appears at 193.1 ppm and olefin car-
bon signal around 166–167 ppm. In the case of IR, the olefinic C@C
stretching appears at 1575 cmꢂ1 for CR and 1580 cmꢂ1 for CN.
3.3. Binding of CN and CR with TiO2 nanoparticle
From the optical properties of the sensitizers we inferred that
by introducing these EW groups will result in ICT transition and
absorption in the visible range region which gratifies one of the
parameter for its application in DSSC. Binding between the sensi-
tizers with semiconductor TiO2 is another parameter in improving
DSSC efficiency. The ‘COOH’ and ‘NO2’ group in CR and CN respec-
tively will act as anchoring moiety with TiO2. Absorption measure-
ments were performed to analyze the ground state interactions of
3.2. Absorption and fluorescence characteristics of CN and CR
Fig. 1 shows the absorption spectrum of carbazole derivatives in
CHCl3 solvent. The maximum absorption wavelength (kmax) of CR
appears at 445 nm which is 30 nm higher than the kmax of CN.
Although the CN possess extended
p
conjugation than CR, the
H
N
N
N
NaOH in DMSO
POCl3, DMF
+
Br
N2 , RT , 4h
°
ClC2H4Cl, 90 C, 12h
CHO
N
R
Piperidine, CHCl3
R
O
CN
OH
O
N
R
=
,
S
S
NO2
Scheme 1. Synthetic routes of carbazole sensitizers.

194
S. Jagadeeswari et al. / Journal of Molecular Structure 1060 (2014) 191–196
1.05
0.9
1.05
CR
CN
CN
CR
0.9
0.75
0.6
0.75
0.6
0.45
0.3
0.45
0.3
0.15
0.15
0
455
0
360
480
505
530
555
580
605
630
655
680
385
410
435
460
485
510
535
Wavelength (nm)
Wavelength (nm)
Fig. 2. Emission spectrum of CN and CR in CHCl3 solvent.
Fig. 1. Absorption spectrum of CN and CR in CHCl3 solvent.
these sensitizers with TiO2. SI 13 shows the absorption spectrum of
CN in the absence and presence of TiO2 nanoparticles. While
increasing the concentration of TiO2 the absorption of CN de-
creases. Similarly, the absorption spectrum of CR decreases along
with the formation of isobestic point with increasing concentration
of TiO2 (Fig. 5). These observed changes clearly indicate the forma-
tion of ground state complex between the carbazole sensitizers
and TiO2. Generally, after photoexcitation the electron from the
ground state of the sensitizers will transfer to its excited state.
Hence it is important to analyse the excited state interaction of
the sensitizers with TiO2. Fluorescence quenching measurements
have been widely used to analyse the various type of interactions
between the molecules in their excited state [25,26]. Based on this,
the excited state interactions of these sensitizers with TiO2 were
analyzed through fluorescence measurements. SI 14 and Fig. 6
show the emission spectrum of CN and CR in the absence and pres-
ence of TiO2. The emission intensity of CN was significantly re-
duced by TiO2 at different concentrations (1–5 ꢁ 10ꢂ4 M). On
increasing the concentration of TiO2, emission spectrum of CR de-
creases i.e., quenching occurs. From the fluorescence quenching
measurements, the binding constant (K) was calculated from the
following equation.
1.05
0.9
Toluene
THF
CN
CHCl3
EtOAC
Acetone
DMF
0.75
0.6
0.45
0.3
0.15
0
450
500
550
600
650
700
750
Wavelength (nm)
Fig. 3. Emission spectrum of CN in different solvents.
Toluene
THF
1.05
0.9
CR
CHCl3
EtOAC
Acetone
ACN
0.75
0.6
1
1
1
¼
þ
½Qꢃ
DMF
ðF0 ꢂ FÞ ðF0 ꢂ FÞ KðF0 ꢂ FÞ
0.45
0.3
where F0 is the fluorescence intensity of free sensitizer, F0 is the
fluorescence intensity of sensitizer in the presence of TiO2 and F
is the observed fluorescence intensity at its maximum. A plot be-
tween 1/(F0 ꢂ F) vs reciprocal concentration of TiO2 results in
straight line (Fig. 7) and the calculated value of K from the slope
is given in Table 2. The binding constant of CR with TiO2 is higher
than CN. The results suggest that carboxylate anchoring group hav-
ing electron withdrawing rhodanine moiety shows better binding
with TiO2 than nitro anchoring group. The carboxylate type anchor-
ing group can effectively bind with the hydroxyl groups of the TiO2
0.15
0
455
495
535
575
615
655
695
735
Wavelength (nm)
Fig. 4. Emission spectrum of CR in different solvents.
surface as evidenced in the literature [5d,e] and results in stronger
binding affinity than the nitro anchoring group.
Table 1
Absorption and emission properties of carbazole sensitizers in various solvents.
Toluene
THF
CHCl3
EtOAc
Acetone
ACN
DMF
akabs
be
ckemi
akabs
be
ckemi
akabs
be
ckemi
akabs
be
ckemi
akabs
be
ckemi
akabs
be
ckemi
akabs
be
ckemi
CN
CR
409
438
0.1
0.2
494
498
406
436
0.3
0.4
536
500
415
445
0.2
0.4
548
502
406
434
0.2
0.4
530
500
405
430
0.1
0.2
566
517
411
430
0.3
0.3
541
523
418
434
0.3
0.4
576
518
a
b
c
nm.
105 Mꢂ1 cmꢂ1
nm.
.

S. Jagadeeswari et al. / Journal of Molecular Structure 1060 (2014) 191–196
195
0.045
0.5
0.4
0.3
0.2
0.1
0
0.036
0.027
0.018
0.009
0
365
415
465
515
565
615
665
0
0.2
0.4
0.6
0.8
1
1.2
Wavelength (nm)
1/[TiO2]
Fig. 5. Absorption spectrum of CR (1 ꢁ 10ꢂ5 M) in the presence of TiO2in the
concentration range of 15 ꢁ 10ꢂ4 M in CHCl3.
Fig. 7. Comparison of binding constant plot for carbazole sensitizers.
Table 2
Binding constant (K) and thermodynamic data of carbazole sensitizers.
3.4. Possibility of electron transfer process
K ꢁ 104 Mꢂ1
0.267
1.67
Es (eV)
Es*/s+ (V)
DGet (eV)
The excited state carbazole sensitizers may undergo energy or
electron transfer with TiO2 nanoparticles. The decrease in fluores-
cence intensity of the sensitizers in the presence of TiO2 is usually
attributed to electron transfer to the conduction band of semicon-
ductor nanoparticles [27]. The possibility of electron transfer pro-
cess can be probed through energy level diagram and Rehm
Weller equation. The oxidation potential value of the sensitizers
provides information for the possibilities of electron injection from
the excited sensitizers into the TiO2 semiconductor. Electrochemi-
cal measurements were performed to analyse the ground state oxi-
dation potential of the carbazole sensitizer (Fig. 8). The oxidation
potential of CN and CR are 1.78 and 1.68 V. The reversible nature
of oxidation peaks appeared for CN and CR suggest the oxidation
process is stable and implying its advantage for its application in
DSSC [28]. From the ground state oxidation potentials, the excited
state potentials were calculated by the equation
CN
CR
2.68
2.65
0.9
0.97
0.2
0.3
0.00006
CN
CR
0.00005
0.00004
0.00003
0.00002
0.00001
0.00000
-0.00001
-0.00002
ꢄ
Es =sþ ¼ Es=sþ ꢂ Es
where Es/s+ is the oxidation potential of carbazole sensitizers, Es is
the singlet state energy of carbazole derivatives. The calculated val-
ues are summarized in Table 2. The excited state oxidation potential
(Es*/s+) of carbazole sensitizers are found to be favorable for inject-
ing the electrons into the conduction band of TiO2 (ꢂ0.69 V vs Ag/
AgCl) [29]. Rehm Weller equation was applied to calculate the ther-
modynamic feasibility of excited state electron transfer reactions
[30].
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential (V)
Fig. 8. Cyclic voltammogram recorded for carbazole sensitizers in DMF.
e-
D
Get ¼ Eð1o=x2iÞ ꢂ Eð1r=e2dÞ ꢂ Es
O-
NC
N
O-
N
ν
h
CN
700
600
500
400
300
200
100
0
TiO2
OH
O
O
N
N
S
S
CR
Scheme 2. Binding mode of carbazole sensitizers.
430
480
530
580
630
680
ðoxiÞ
1=2
ðredÞ
1=2
Wavelength (nm)
where E
is the oxidation potential of the donor, E
is the
reduction potential of the acceptor, Es is the excited state energy
of the carbazole sensitizers. The calculated Get values are negative
Fig. 6. Fluorescence quenching of CR (1 ꢁ 10ꢂ5 M; kexi = 445 nm; kemi = 502 nm) in
the presence of TiO2 (1–5 ꢁ 10ꢂ4 M) in CHCl3.
D

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Carbazole sensitizers (CN and CR) with electron withdrawing
anchoring group were synthesized and characterized by NMR,
Mass, IR and cyclic voltammetry measurements. Intramolecular
charge transfer transition in the visible region is observed by intro-
ducing the electron withdrawing anchoring groups. Absorption
measurements revealed that the carbazole having rhodhanine
group shows better electron withdrawing character than nitro-
phenyl acetonitrile moiety. The optical and emission properties
of CN and CR were altered by varying the solvents which signifies
the solvent effect in charge transfer transition. The observed non-
linearity of the Lippert–Mataga correlation shows the existence
of specific solvent effect. The interaction between carbazole sensi-
tizers and TiO2 was studied by absorption and fluorescence mea-
surements. Ground state complex formation of CN and CR with
TiO2 was observed in absorption measurements. Binding ability
of the sensitizers with TiO2 was calculated from the fluorescence
data which implies that carboxylate anchoring group (CR) have
higher binding ability than nitro group (CN). From the overall re-
sults, it is observed that the electron withdrawing anchoring
groups have influence in optical and binding properties. These re-
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sitizers with various anchoring groups with predetermined
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