Density functional theory study of carbazole dyes: Potential application of carbazole dyes in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.molstruc.2018.02.094

Dyes applied in dye synthesized solar cells are an important class of organic compounds. In order to extension novel dyes, three organic dyes (TB-CH, TB-CB and DTB-CB) which contain hexyl and butyl moieties as a branch and thiobarbutiric acid moiety as an electron acceptor, connected by carbazole unit have been studied. We report the synthesis, molecular structures, and study of adsorption of dyes on TiO₂ anatase by density functional theory (DFT). Molecular orbital analysis study indicated all dyes can give suitable electron injection from their LUMO orbitals to the TiO₂ conduction band. Also, dyes were investigated by UV spectroscopy and cyclic voltammetry (CV). The synthesized dyes were characterized extensively by IR, ¹HNMR, ¹³CNMR, mass spectroscopy and CHN analysis. Results of Photo physical, electrochemical properties and density functional theory investigation indicated TB-CH, TB-CB and DTB-CB have potential for application in dye sensitized solar cell. The adsorption energy of each dye/TiO₂ complex was ?1.05, ?1.01 and ?1.00 eV for TB-CH, TB-CB and DTB-CB, respectively.

Full text of DOI:10.1016/j.molstruc.2018.02.094

Accepted Manuscript
Density functional theory study of carbazole dyes: Potential application of carbazole
dyes in dye-sensitized solar cells
Alireza Salimi Beni, Maryam Zarandi, Behzad Hosseinzadeh, A. Najafi Chermahini
PII:
S0022-2860(18)30248-5
10.1016/j.molstruc.2018.02.094
MOLSTR 24916
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 2 October 2017
Revised Date: 25 January 2018
Accepted Date: 24 February 2018
Please cite this article as: A. Salimi Beni, M. Zarandi, B. Hosseinzadeh, A. Najafi Chermahini, Density
functional theory study of carbazole dyes: Potential application of carbazole dyes in dye-sensitized solar
cells, Journal of Molecular Structure (2018), doi: 10.1016/j.molstruc.2018.02.094.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Density functional theory study of carbazole dyes:
potential application of carbazole dyes in dye-
sensitized solar cells
a
b
c
d
Alireza Salimi Beni *, Maryam Zarandi** , Behzad Hosseinzadeh , A. Najafi Chermahini
a
Department of Chemistry, Faculty of Science, Yasouj University, Yasouj, 75918-74831, Iran
b
Young Researchers and Elite Club, Qom Branch, Islamic Azad University, Qom, Iran
c
Department of Chemistry, Mazandaran University, Babolsar, Iran
d
Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran
*
Tel: +98 741 2223048, fax: +98 741 3342172
E-mail: salimibeni@mail.yu.ac.ir
*E-mail: Maryam.zarandi88@gmail.com
*
*
Abstract: Dyes applied in dye synthesized solar cells are an important class of organic
compounds. In order to extension novel dyes, three organic dyes (TB-CH, TB-CB and DTB-CB)
which contain hexyl and butyl moieties as a branch and thiobarbutiric acid moiety as an electron
acceptor, connected by carbazole unit have been studied. We report the synthesis, molecular
structures, and study of adsorption of dyes on TiO anatase by density functional theory (DFT).
2
Molecular orbital analysis study indicated all dyes can give suitable electron injection from their
LUMO orbitals to the TiO conduction band. Also, dyes were investigated by UV spectroscopy
2
and cyclic voltammetry (CV). The synthesized dyes were characterized extensively by IR,
1
13
HNMR, CNMR, mass spectroscopy and CHN analysis. Results of Photo physical,
electrochemical properties and density functional theory investigation indicated TB-CH, TB-CB
and DTB-CB have potential for application in dye sensitized solar cell. The adsorption energy of
each dye/TiO complex was -1.05, -1.01 and -1.00 eV for TB-CH, TB-CB and DTB-CB,
2
respectively.
1

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Keyword: Organic dye, Carbazole, Density functional theory, Dye sensitized solar cell
1
. Introduction
One of the important demands in all over world is to find renewable energy sources. Therefore
developing solar cells is one of the important researchers aim. In 1991 Grätzel and co-workers
have been developed third generation of solar cells, namely nanocrystalline dye-sensitized solar
cell (DSSC) [1-3]. Dye sensitized solar cells (DSSCs) have attracted much attention, due to their
high light-to-electrical conversion efficiency, low cost, and long-term stability[4,5]. The function
of DSSC usually depends to three factors: 1) the mesoporous oxide semiconductor nanoparticle
layers, 2) dye sensitizers, which attached to the surfaces of the oxide semiconductors and 3)
electrolyte that is the medium containing redox system. The fundamental concept of DSSC
function is based on photon to current conversion mechanism. Sunlight passes through the
transparent electrode into the dye layer where it can excite electrons that then flow into the
titanium dioxide. The electrons flow toward the transparent electrode where they are collected
for powering a load. After flowing through the external circuit, they are re-introduced into the
cell on a metal electrode on the back, flowing into the electrolyte. The electrolyte then transports
the electrons back to the dye molecules. [6]. Up to now different scientist focused on design and
developing dye-sensitized solar cells. Therefore different metallic and organic dyes have been
developed [7-16].
The metallic dyes have some shortcoming include: high cost, highly energy consuming method
preparation, toxicity and Low natural abundance [7,8]. In order to overcome these problems new
generation of dyes developed which contain no metals. One way to investigate new dyes
efficiency is theoretical approach. Density functional theory (DFT) is a useful method for the
theoretical treatment of molecular structures and electronic absorption spectra [17,18]. Titanium
dioxide (TiO ) has been extensively applied as a model metal oxide with a wide range of
2
applications in catalysis, photochemistry, and electrochemistry [19]. So many academic
researchers studied versatile compound adsorption on TiO anatase surfaces by density functional
2
theory [20-22].
In this study, in continues our investigation about organic compound [23-24], we report synthesis
and characterization of several dyes contain thiobarbutiric acid moiety as anchoring group. We
2

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investigate theoretical electronic structures, the optical properties and adsorption on TiO
surfaces of dyes. Their potential for application in dye synthesized solar cells discussed.
2
anatase
N
N
N
O
O
O
O
O
O
O
HN
NH
HN
NH
HN
HN
NH
O
S
NH
S
S
S
TB-CH
DTB-CB
TB-CB
Figure 1.Chemical structure of designed and synthesized novel dyes TB-CH, TB-CB and BTD-
CB
2
2
. Experimental
. 1.Materials and characterization
All solvents and other chemicals were reagent grade and used without further purification. All
chemical were purchased from Merck and Sigma Aldrich. Melting points were measured on an
elecrtothermal KSB1N apparatus. TLC was performed on TLC-Grade silica gel-G/UV 254 nm
1
13
plates (n-hexane and ethyl acetate). H and CNMR spectra were recorded on a FT-NMR
1
Bruker Advance ultra-shield spectrometer (frequency line: 400, MHz for HNMR and 100 MHz
1
3
for C NMR; solvents, CDCl and DMSO-d ). FT-IR spectra were recorded in the matrix of
3
6
KBr with JASCO FT-IR-680 plus spectrometer. Mass spectra of the products were obtained with
a HP Agilent technologies 5937with mass selective detector. Elemental analyses (C, H, N) were
performed with a Heraeus CHN-O-Rapid analyzer.
2
.2. Photo physical and electrochemical measurements
Absorption spectra were measured with Perkin Elmer (Lambda 25) UV–VIS spectrophotometer.
−
4
Cyclic voltammetry experiments were performed in dimethylformamide (DMF) (3.0×10 M)
containing 0.1 M n-Bu NPF as the supporting electrolyte by biologic SP 150 electrochemical
4
6
−
1,
analyzer with scan rate of 100 mVs the platinum wire as counter and working electrodes and
Ag/AgCl as reference electrode.
2
.3. Synthesis of dyes
3

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2
.3.1. Synthesis of 9-Hexyl-9H-carbazole
In a dried 100 ml, round bottom flask carbazole (0.20 g, 1.10 mmol) were dissolved in DMSO (3
mL), NaOH (0.40 g) added and then the hexylbromide (0.33g, 1.01 mmol) was added drop wise.
After complete addition, the reaction mixture was heated at 90 ̊C overnight. The organic layer
was separated, washed three times with 50 ml water, dried over MgSO and concentrated. Pure
4
product was obtained after silica gel column chromatography (normal hexane) as a white solid.
White powder, yield 85%, mp = 75-77 ̊C
-1
IR (KBr, cm ): 3051, 2955, 2923, 2869, 2856, 1628, 1594, 1487, 1453, 1376, 1327, 1241, 1217,
1
192, 1153 and 1128.
1
H NMR (400 MHz, CDCl , 20
̊
3
2
1
3
C NMR (100 MHz, CDCl , 20 ̊C ) δ (ppm): 141.3, 126.4, 123.4, 121.1, 119.4, 109.5, 43.6,
3
3
2.4, 29.6, 27.7, 23.1 and 14.8
.3.2. Synthesis of 9-Butyl-9H-carbazole
2
To a stirred solution of carbazole (0.20 g, 1.1 mmol) in DMSO (3 mL), NaOH (0.40 g) was
added and then the corresponding n-butylbromide (0.13g, 1.01 mmol) was added drop wise.
After complete addition, the reaction mixture was heated at 90 for
was separated, washed with water, dried over Na SO and concentrated. Pure product was
obtained after silica gel column chromatography (normal hexane) as a white solid.
̊C overnight. The organic layer
2
4
White powder, Yield: 83%, mp: 54-55 ̊C
-1
FT-IR (KBr, cm ): 3043, 2953, 2928, 1918, 1625, 1591, 1482, 1463, 1348 and 1325.
1
3
HNMR (300 MHz, CDCl ) (ppm): 8.23 (m, 2H), 7.54 (m, 4H), 7.39 (m, 2H), 4.45 (t, J = 6 Hz,
2
H), 1.96 (m, 2H), 1.54 (m, 2H), 1.11 (t, J= 7Hz, 3H)
2
.3.3. Synthesis of butyl-carbazole-3,6-dicarbaldehyde (1)
4

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Phosphorylchloride (2.31 mL, 2.5eq) was added drop wise to DMF (1.76 mL, 2.3 eq) for 1 h at
0
̊
C. The mixture was stirred and then N-(butyl) carbazole (0.22 g, 1 mmol) added over 1 h at
room temperature. After standing for overnight at 90 C, the mixture was poured into ice–water
30 mL), stirred 5h, and neutralized with sodium hydroxide. The solution was extracted three
̊
(
times with ethyl acetate and dried with MgSO . The solvent was removed under reduced
4
pressure. The residue was purified by silica gel column chromatography (eluent: ethyl acetate/ n-
hexane =1:9).
Cream powder, Yield: 60%, mp: 145-150 °C.
-1
FT-IR (KBr, cm ): 2956, 2863, 2811, 2722, 1685, 1629, 1627, 1592, 1488, 1205 and 1125.
1
6
HNMR (400 MHz, DMSO-d ) δ (ppm): 10.94 (s, 2H), 8.88 (s, 2H), 8.79 (d, J= 8 Hz, 2H), 8.13
(d, J = 8 Hz, 2H), 4.53 (t, J = 12 Hz, 2H), 1.77 (m, 2H), 1.32 (m, 2H) and 0.89 (t, J = 1Hz, 3H).
1
3
CNMR (100 MHz, DMSO-d ) δ (ppm): 190.9, 144.1, 128.9, 127.7, 124.6, 122.6, 110.9, 42.6,
6
3
0.8, 19.8 and 13.9.
2
.3.4. 9-Butyl-9H-carbazole-3-carbaldehyde (2)
Phosphorylchloride (1.55 mL, 1.25eq) was added drop wise to N, N-dimethylformamide (DMF,
.88 mL, 1.15 eq) for 1 h at 0 C. The mixture was stirred and then N-(butyl) carbazole (0.25 g, 1
mmol) added over 1 h at room temperature. After standing for 5 hr. at 90 C, the mixture was
poured into ice–water (30 mL), stirred 5h, and neutralized with sodium hydroxide. The solution
was extracted three times with ethyl acetate and dried with Na SO . The solvent was removed
0
̊
̊
2
4
under reduced pressure. The residue was purified by silica gel column chromatography (eluent:
ethyl acetate/ n-hexane =1:9).
Cream powder, Yield: 55%, mp: 62-64 ̊C .
-1
FT-IR (KBr, cm ): 2985, 1691, 1625, 1594, 1473, 1352, 1133 and 808.
1
HNMR (400 MHz, DMSO-d ) δ (ppm): 10.06 (s, 1H), 8.76 (s, 1H), 8.30 (d, J = 8 Hz, 1H), 7.99
6
(m, 1H), 7.79 (d, J = 8 Hz, 1H), 7.70 (d, J = 8 Hz, 1H), 7.54 (m, 1H), 7.31 (t, J = 7.6 Hz, 1H),
4
.46 (t, J = 8 Hz, 2H),1.76 (m, 2H),1.29 (m, 2H) and 0.87 (t, J = 8 Hz, 3H).
5

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1
3
6
CNMR (100 MHz, DMSO-d ) δ (ppm): 192.1, 142.8, 140.9, 128.7, 126.6, 124.4, 122.4, 120.9,
1
20.44, 110.2, 110.4, 42.7, 30.9, 19.8 and 13.3.
2
.3.5. 9-Hexyl-9H-carbazole-3-carbaldehyde (3)
Phosphorylchloride (1.55 mL, 1.25eq) was added drop wise to DMF (0.88 mL, 1.15 eq) for 1 h
at 0 C. The mixture was stirred and then N-(hexyl) carbazole (0.25 g, 1 mmol) added over 1 h at
room temperature. After standing for 5 hr. at 90 C, the mixture was poured into ice–water (30
̊
̊
mL), stirred 5h, and neutralized with sodium hydroxide. The solution was extracted three times
with ethyl acetate and dried with MgSO . The solvent was removed under reduced pressure. The
4
residue was purified by silica gel column chromatography (eluent: ethyl acetate/ n-hexane =1:9).
Cream powder, Yield: 55%, mp: 70-74
̊
-1
1
329, 1240, 1177, 1135, 807, 765, 748 and 730.
1
HNMR (400 MHz, DMSO-d ) δ (ppm): 10.06 (s, 1H), 8.77 (s, 1H), 8.30 (d, J = 8 Hz, 1H), 7.79
6
(
d, J = 8Hz, 1H), 7.70 (m, 1H), 7.55 (m, 1H), 7.31 (t, J = 8Hz, 1H), 4.46 (t, J = 8 Hz, 2H), 1.77
m, 2H), 1.25 (m, 6H) and 0.87 (t, J = 8 Hz, 3H).
(
1
3
CNMR (100 MHz, DMSO-d ) δ (ppm): 13.7, 22, 26, 28.4, 30.9, 42.5, 109.7, 110.2, 120, 120.7,
6
1
22.11, 122.2, 124, 126.6, 126.7, 128.2, 140.4, 142.6 and 192.
2
9
.3.6. Synthesis of 5-[6-(4,6-dioxo-2-thioxohexahydro-5-pyrimidinylidenmethyl)-9-butyl-
H-3-carbazolylmethylene]-2-thioxohexahydro-4,6-pyrimidinedione (DTB-CB)
In a dried 100 ml, round bottom flask N-butylcarbazole-3,6-dicarbaldehyde (0.28 g, 1 mmol) and
thiobarbituric acid (0.29 g, 2 mmol) were dissolved in absolute ethanol (3 mL). After heating the
reaction mixture for 5 h, the bright red dye was obtained and filtered, washed with 30 ml water
three times. The dye BD-CH obtained was purified by crystallization from EtOAc/n-hexane.
Dark red powder, Yield: 82% .
-1
FT-IR (KBr/cm ): 3419, 3202, 3066.26, 2958, 1665, 1524, 1483, 1376, 1287, 1242, 1167 and
027.
1
6

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1
HNMR (400 MHz, DMSO-d
6
) δ (ppm): 12.44 (s, 1H), 11.81 (s, 1H), 9.33 (s, 2H), 8.51 (d, J = 4,
2
H), 7,80 (s, 2H), 7.28 (d, J = 8 Hz, 2H), 4.50 (t, J = 6, 2H), 1.77 (m, 2H), 1.29 (m, 2H), 0.9 (t, J
=
8, 3H).
1
3
6
CNMR (100 MHz, DMSO-d ) δ (ppm): 178.2, 163, 160, 157.3, 144, 134.5, 128.8, 125, 122.5,
1
15,110, 43, 30.5, 19.5 and 13.5. Mass: m/z 532 (M+1), 529, 523, 503, 479,444 and 430.
Anal. Calcd. for C26
N, 13.16.
21 5 4 2
H N O S (531.10): C, 58.74; H, 3.98; N, 13.17, Found: C, 58.72; H, 3.99;
2
4
.3.7. Synthesis of 5-(9-Hexyl-9H-carbazol-3-ylmethylene)-2-thioxo-dihydro-pyrimidine-
,6-dione (TB-CH)
In a dried 100 ml, round bottom flask 9-Hexyl-9H-carbazole-3-carbaldehyde (0.28 g, 1 mmol)
and thiobarbituric acid (0.15 g, 1 mmol) were dissolved in absolute ethanol (3 mL). After heating
the reaction mixture for 5 h, the bright red dye was obtained and filtered, washed with 30 ml
water three times. The dye BD-CH obtained was purified by crystallization from EtOAc/n-
hexane.
Orange powder, Yield: 75% .
-1
FT-IR (KBr/cm ): 3411, 3154, 2159, 1661, 1514, 1463, 1389, 1290, 1187 and 1067.
1
HNMR (400 MHz, DMSO-d ) δ (ppm): 12.41 (s, 1H), 12.32 (s, 1H), 9.35 (s, 1H), 8.37 (d, J = 8,
6
2
H), 8.54 (s,1H), 8.18 (d, J = 8, 2H), 7.71( m, 2H), 7.54 (t, J = 8 1H), 7.33 (t, J = 8, 1H), 4.45 (t,
J = 8, 2H), 3.53 (m, 2H), 2.51(m, 2H), 1.77 (m, 2H), 1.24(m, 2H), 0.79 (t, J = 8, 3H).
1
3
CNMR (100 MHz, DMSO-d ) δ (ppm): 178.1, 162.5, 160, 158, 143.5, 141, 134, 130, 127,
6
1
23.5, 122.5, 121, 114, 110, 109, 43, 31, 30.5, 28.5, 26, 26.5, 22 and 14.
Mass: m/z 406 (M+1), 389, 360, 334, 232 and 204.
Anal. Calcd. for C H N O S (405.15): C, 68.16; H, 5.72; N, 10.76, Found: C, 68.15; H, 5.70;
2
3
23
3
2
N, 10.75.
2
.3.8. Synthesis of 5-(9-Butyl-9H-carbazol-3-ylmethylene)-2-thioxo-dihydro-pyrimidine-4,6-
dione (TB-CB)
In a dried 100 ml, round bottom flask 9-Butyl-9H-carbazole-3-carbaldehyde (0.25 g, 1 mmol)
and thiobarbituric acid (0.15g, 1 mmol) were dissolved in absolute ethanol (3 mL). After heating
7

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the reaction mixture for 5 h, the bright red dye was obtained and filtered, washed with 30 ml
water three times. The dye BD-CH obtained was purified by crystallization from EtOAc/n-
hexane.
Orange powder, Yield: 78%.
-1
FT-IR (KBr/cm ): 3407, 3122, 2924, 2856, 1649, 1598, 1513, 1494, 1386, 1301, 1190 and 1063.
1
HNMR (400 MHz, DMSO-d ) δ (ppm): 12.42 (s, 1H), 12.33 (s, 1H), 9.34 (s, 1H), 8.67 (d, J= 8,
6
2
H), 8.50 (s,1H), 8.18 (d, J = 8, 2H), 7.71( m, 2H), 7.54 (t, J = 8 1H), 7.32 (t, J = 8, 1H), 4.50 (t,
J = 8, 2H), 1.77 (m, 2H), 1.29(m, 2H), 0.80 (t, J = 8, 3H).
1
3
CNMR (100 MHz, DMSO-d
27, 125, 123.5, 122.5, 120, 114, 110.5, 109.5, 42.5, 30.5, 19.5 and 13.5.
Mass: m/z 388 (M+1), 334, 261, 204, 190and 176.
Anal. Calcd. for C21 S (377.12): C, 68.82; H,5.07; N, 11.13, Found: C, 68.78; H, 5.05; N,
6
) δ (ppm): 191.7, 178, 162.5, 160, 158, 143, 140.6, 133.7, 130.2,
1
19 3 2
H N O
1
1.10.
3
. Computational details
To theoretically understand the donor effects and the sensitized mechanism at a molecular level,
the geometric and electronic structures of dyes were studied in detail using DFT calculations of
dyes by Gaussian 03 package. The geometries of BD-CH, BTD-CH and DT-CH were optimized
by using the B3LYP method with the 6-31 G (d) basis set. Salvation effects were introduced by
the SCRF method, via the conductor polarizable continuum model (CPCM) implemented in the
Gaussian program, for both geometry optimizations and TD-DFT calculations. To model the
adsorption modes of the dyes on the TiO
2
anatase(101) surface, periodic density functional
3
theory calculations were carried out using the Dmol package as implemented in Material Studio
(
version 5.5). The TiO anatase (101) surface is modeled using a slab with a thickness of six
2
atomic layers. Periodic boundary conditions are employed with a surface super cell of 21.77 ×
̊2
̊
1
5.10 A and a vacuum layer of at least 20 A to prevent interaction between adjacent slabs. For
these calculations, we used the generalized gradient approximation (GGA) functional with
Perdew and Wang (PW91) formulation. The electronic properties of core electrons were treated
with DFT semi-core pseudoplots in term of the DNP basis set. The convergence energy
8

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-5
̊- 1
tolerance, gradient, and displacement convergence were 1.0 × 10 Ha, 0.004 Ha A , and 0.005
respectively.
4
4
. Results and discussion
.1. Synthesis strategy
Dye molecules have an essential role in DSSC performance; the capability of solar cell in sun
power harnessing depends to dye structure. Molecular designs for organic sensitizers are very
flexible. Therefore; to achieve dyes contain thiobarbutiric acid as anchoring group, and study the
potential of them for application in solar cell, TB-CH, TB-CB and BTD-CB dyes were
synthesized in three-step reactions that is shown in Scheme 2. First by the reaction of carbazole
and hexylbromide or butylbromide in the presence of NaOH in DMSO as solvent, 9-Hexyl-9H-
carbazole and 9-butyl-9H-carbazole were synthesized. The yield of reaction was satisfactory and
was about 85%. In the next step by application of Vilsmeier–Haack reaction, alkyl-carbazole was
formylated. For this purpose, 9-hexyl-9H-carbazole and 9-butyl-9H-carbazole reacted with
3
POCl in the presence of DMF as the reagent and solvent. The yield of this reaction was 60%. In
the final step corresponding carbaldehyde reacted with thiobarbutiric acid via Knoevenagel
condensation. Synthesis of TB-CH, TB-CB and DTB-CB was carried out in the presences of
ethanol without using any basic catalyst. The yields of reactions were 86, 82, and 88%,
respectively. The synthesis of TB-CH, TB-CB and BTD-CB dyes was confirmed by IR and
1
13
HNMR, CNMR, mass spectroscopy and CHN analysis.
9

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H
N
(a)
N
(b)
N
O
(d)
1
(c)
O
N
N
N
(d)
O
2
3
O
N
DTB-CB
O
O
(e)
O
O
1
+
HN
NH
HN
NH
O
O
S
S
HN
NH
S
O
O
N
(e)
TB-CB
2
+
HN
NH
S
O
O
HN
NH
S
N
O
O
TB-CH
(e)
3
+
HN
NH
O
O
S
HN
NH
S
Figure 2. Synthetic routes of TB-CH, TB-CB and DTB-CB (a) 1-Bromohexyl, NaOH, DMSO,
10 C, overnight (b) POCl , DMF, 95 C, overnight (c) 1-Bromobutyl, NaOH, DMSO, 110 C
overnight (d) POCl , DMF, 95 C, 5 h (e) Ethanol, 55 , 5 h.
1
̊
̊
̊ ,
3
̊
̊С
3
1
0

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4
.2. Photo physical characterization by UV–Vis absorption spectra of dyes
UV–Vis analysis is a useful method for evaluation of sun power harnessing capability of dyes
25]. In order to study dye structure in function of light harvesting, we measured the absorption
spectra of the dyes in THF solution and at a monolayer of adsorbed dyes on the TiO film. The
absorption spectra of dyes (TB-CH, TB-CB and DTB-CB) in THF solution at a concentration of
[
2
-4
3˟
10 M have shown in Figure. 3, and data are listed in Table 1. All dyes showed broad
absorption spectra in the visible region. Generally, absorption bands at around 300 nm are related
to π-π electron transition. The λmax of TB-CH, TB-CB and DTB-CB on the film was 456, 457
and 475, respectively. The result revealed that length of alky group in dye structure has not
significant effect on the absorption bands. It is notable DTB-CB contains two acceptor unit and
therefor shows higher λmax in comparison to TB-CH and TB-CB. Also, fluorescence emission
was recorded in THF and spectra are displayed in figure 4. The λex of TB-CH, TB-CB and DTB-
CB was 456, 457 and 475 nm, respectively. In addition, the fluorescent intensity of TB-CH, TB-
CB and DTB-CB was 3.44, 3.77 and 8.01 respectively. The maximum emission of TB-CB is
shifted toward the longer-wavelength region. It is due to the presence of butyl moiety in its
structure. DTB-CB emission is shifted toward the lower-wavelength region. This observation
may be due to the presence of second group of thiobarbutiric acid as the anchoring part and
higher resonance in DTB-CB molecule.
Figure 3. Absorption spectra of dyes TB-CH, TB-CB and DTB-CB in THF.
1
1

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Figure 4. Emission spectra of dyes TB-CH, TB-CB and DTB-CB in THF λex = 456,457 and 475
-5
nm; Conc= 10 M.
In order to monitor the photophysical properties, Absorption spectral of dyes TB-CH, TB-CB
and DTB-CB on TiO film is presented in Figure 5. DTB-CB shows higher absorbance near 370
2
nm, but near 470 nm its absorption decrease significantly. Two other dyes represent smoother
absorption by little changes around 370 and 470 nm.
Figure 5. Absorption spectral of dyes TB-CH, TB-CB and DTB-CB on TiO film.
2
4
.3 Electrochemical properties of dyes
One way to study of electrochemical properties is evaluation of LUMO and HOMO levels of
dyes molecules. LUMO and HOMO levels of dyes must be suitable to match the conduction
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-
-
band (E cb) of TiO
2 3 2
electrode and the HOMO level of I /I , respectively. E cb of TiO electrode
must be more positive than LUMO of dye molecules for the injection of electrons, and for
-
3
-
accepting electron, the level of I /I must be more negative than the HOMO level of dye
molecules, and regenerate from the oxidized state [26].
Table 1. Band gap (calculated by DFT/B3LYP), absorption, and electrochemical
parameters for organic dyes.
parameter
TB-CH
370
TB-CB
370
DTB-CB
370
a
UV (λmax/nm)
b
UV (λmax/nm)
456
457
475
c
Oxidation (V vs. NHE)
Reduction (V vs. NHE)
1.5
1
1.2
d
-0.95
2.45
-1.47
2.47
-1.13
2.33
e
E0-0
f
(
HOMO/LUMO) (ev)
-0.24/ -0.30
0.6
-0.09 / -0.21
0.12
-0.25/ -0.30
0.5
f
Band gap
a
Maximum absorption on TiO film.
2
b
c
Maximum absorption in THF.
Oxidation of dyes measured by cyclic voltammetry in 0.1 M Bu
4
NPF
6
in THF solution
as supporting electrolyte, Ag/AgCl as reference electrode and Pt as counter electrode.
Potentials measured vs Ag/AgCl was converted to normal hydrogen electrode (NHE) by
addition of +0.2V
d
Reduction was calculated by HOMO–E .
0
–0
e
E0–0 transition energy measured at the onset of absorption spectra.
f
DFT/B3LYP calculated values
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Therefore, the electrochemical properties of dyes were studied to explain their HOMO and
LUMO levels. Potentials measured vs Ag/AgCl was converted to normal hydrogen electrode
(NHE) by addition of +0.2 V.
Figure 6. CVs of the TB-CH, TB-CB and DTB-CB dye films in DMF containing 0.1 M Bu NPF
4
6
-1
at a scan rate of 100 mVs , vs Ag/AgCl.
As shown in Figure. 6, the HOMO levels of TB-CH, TB-CB and DTB-CB are 1.5, 1 and 1.2 V
-
-
(
vs. NHE) respectively. In comparison to I /I , redox potential value of dyes showed higher
3
oxidative potential levels, indicating that the oxidized dyes formed after electron injection to the
TiO and therefore could accept electrons from iodide ions thermodynamically. Results revealed
2
that the longer alkyl group caused higher HOMO level and also DTB-CB by having two acceptor
units has higher HOMO level potential in comparison with TB-CB. On the other hand, the
LUMO level of TB-CH, TB-CB, and DTB-CB are -0.95, -1.47, -1.13 (vs. NHE), respectively,
2
which all are more negative than the conduction-band edge of the TiO . Furthermore; band gaps
were 2.45, 2.47 and 2.33 eV for the BD-CH, BTD-CH and DT-CH, respectably. These results
clarify that the dyes are potentially efficient sensitizer for dye synthesized solar cells. The
schematic energy levels of TB-CH, TB-CB and DTB-CB based on absorption and
electrochemical data are shown in Figure. 7.
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4

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Figure 7. Schematic energy levels of TB-CH, TB-CB and DTB-CB based on absorption and
electrochemical data.
4
.4. Molecular electrostatic potential, dipole moments and polarizability of dyes
Total charge distribution of a molecule at a point in space around the molecule produce net
electrostatic effect. This phenomenon is known as molecular electrostatic potential (ESP).
Electrostatic potential is related to the total charge distribution with dipole moments, partial
charges, electronegativity and site of chemical reactivity of a molecule. It provides a way to
visualize the relative polarity of a molecule. The different values of the electrostatic potential at
surface are represented by different colors; potential increases in the order blue > green > yellow
>
orange > red. Electrostatic potential maps of TB-CH, TB-CB, and DTB-CB are presented in
Figure. 8. Dipole moment and polarizability are very important physical quantities of dye
molecules in DSSCs [28]. Molecular electrostatic potential calculations were performed in order
to analyze the charge populations of TB-CH, TB-CB and DTB-CB. In this work, the electron
donor units of the dyes are hexyl and butyl moieties. As presented in Figure. 8 electrostatic
potential of dyes were mostly over alkyl groups and π bridge (carbazole).
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5

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TB-CH
TB-CB
DTB-CB
Figure. 8 Molecular electrostatic potential mapped on the isodensity surface of TB-CH, TB-CB
and DTB-CB
4
.5. Frontier molecular orbitals of dyes and intramolecular electron transfer
To get further insights into the molecular structure and electronic distribution of these organic
dyes, we performed a molecular orbital analysis (MOA). The frontier molecular orbital
contribution is very important in determining the charge-separated states of dye sensitizers [24].
Oxidation potential of dye sensitizer is directly related to the highest occupied MO (HOMO)
level and driving force for the reduction of oxidized dye increase at the larger oxidation
potential. In a solar cell, the dye molecule absorbs light energy, and it injects an electron into the
2
LUMO of the dye molecule, and then ultimately injects it into the conduction band of TiO . The
electron distribution of the HOMO and LUMO of TB-CH, TB-CB and DTB-CB are shown in
Figure. 9. The HOMO and LUMO energies, as well as the HOMO-LUMO gaps (Eg) of TB-CH,
TB-CB and DTB-CB are listed in Table 1. As indicated from the Figure. 9, the HOMO is
delocalized over the entire thiobarbutiric acid ring, especially S atom. In the LUMO orbital, the
electron density has shifted toward the carbazole as π bridge and acceptor end of the molecule.
The band gap for TB-CH, TB-CB and DTB-CB is 0.6, 1.2 and 0.5 eV respectively. Due to the
position of LUMO connected to the acceptor group, therefore, the dyes can give suitable electron
injection from their LUMO orbitals to TiO conduction band [28], so the dyes would show the
2
satisfactory solar cell efficiency as a result of good absorption characteristics.
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Figure 9. The frontier molecular orbitals of the HOMO and LUMO levels calculated with
B3LYP/6-31G (d) of the synthesized dyes.
4
.6. Optic absorption properties of dyes
In order to understand electronic transitions of TB-CH, TB-CB and DTB-CB; calculations of
absorption spectra in THF were done by the time-dependent (TD) method on the optimized
geometries. The comparison UV-Vis absorption of TB-CH, TB-CB and DTB-CB based on
calculation and experiments are presented in Table 3. Calculations showed that the visible
absorption maxima of this kind of dyes correspond to the electron transition from HOMO to
LUMO. The λmax is a function of the electron availability. It is due to electronic transition from
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carbazole ring, and acceptor unit (transition of π–π* type). The observed and computed
absorption maxima are in agreement.
Table 3. EIS parameters obtained from modelling the EIS result.
a
b
Compound
λ
cal (nm)
λobs (nm)
TB-CH
452.37
452.50
477.95
456
457
475
TB-CB
DTB-CB
a
Visible absorption maxima calculated by DFT
b
The observed visible absorption
4
.7 Adsorption of dyes on TiO anatase (101) surface
2
The optimized geometries of dye/TiO systems are shown in Figure. 10 and the corresponding
2
bond distances as well as adsorption energies are provided in Table 4. As shown in Table 4, all
the dyes show a rather strong adsorption onto the TiO surface, with binding distances of 2.69,
2
2
.69, and 2.86–2.68 Aº for TB-CH, TB-CB, and DTB-CB, respectively. The adsorption energy,
Eads is calculated using the following expression
Eads = E(slab+dye) - Eslab - Edye
Where Eslab represents the energy of the clean slab, Edye represents the anchored dye in the gas
phase, and E(slab+dye) is the total energy of the complex of the slab with anchored dye. A negative
value of Eads indicates stable adsorption. As shown in Table 4, the adsorption energy (Eads) of
each dye/TiO complex was calculated to be -1.05, -1.01 and -1.00 eV for TB-CH, TB-CB and
2
DTB-CB, respectively, which are sufficiently large to allow the assumption that the dye
2 2
molecules are chemisorbed on the surface of TiO . The HOMOs and LUMOs of dye–TiO
complexes were also studied. Fig. 10 shows the electron density distribution in the systems, TB-
CH, TB-CB and DTB-CB. The electron density distribution in the HOMOs and LUMOs of these
three dye@TiO systems showed no reasonable deviation. In the case of HOMOs of DTB-CB,
2
despite the presence of two thiobarbituric rings, the electron density was localized mainly over
2
one thiobarbituric ring attached directly to the TiO . On the contrary, TB-CH, TB-CB presents
HOMOs that are delocalized throughout the organic molecule.
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On the other hand, in the case of the LUMOs, the electron density was shifted completely to the
TiO . The shifting of the electron density from the dye to the TiO indicates the direct electron
2
2
injection process from the excited state of the dye to the conduction band of TiO2.
Table 4. Selected bond lengths (Aº) and adsorption energies (eV) of dye/TiO systems.
2
Ti–S (ºA) Ti–S (ºA) Eads (eV)
TB-CH
TB-CB
2.693
2.686
2.864
-
-
-1.05
-1.01
-1.00
DTB-CB
2.684
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Figure 10. Frontier orbitals of the dye–TiO complexes.
2
5
. Conclusion
In the present work, we designed and synthesized TB-CH, TB-CB and DTB-CB organic dyes.
Also we evaluated the electronic and polarizabilities of them by DFT method. The electronic
absorption properties were investigated using TD-DFT. Generally, the present theoretical results
agree with the experimental results. Polarizabilities applied in order to study the anchoring
properties of synthesized dyes and the efficiency of electron injection to TiO . Also, the light
2
harvesting abilities of the dyes have been studied by investigation of electronic structures. The
polarizability suggests that the charges of similar acceptors in different dyes are very similar.
The longer alkenyl in conjugate bridge result in the higher HOMO energy, and then induce
higher driving force for the reduction of oxidized dyes. The introducing two acceptors in DTB-
CB stabilized the HOMO and LUMO, and reduce the Eg. The absorption properties based on
experiments and calculations indicate that the, DTB-CB has higher absorption in UV-Vis region.
In evaluation of anchoring properties, absorption spectral of dyes on TiO film is important.
2
Results indicate all dyes show week absorption especially TB-CH. This malfunction may be duo
to large size ring, presence of sulfur atom simultaneously. Adsorption energy is one of the
factors to determine dye efficiency for solar cells theoretically, dyes have higher adsorption
energy have better function and higher light harvesting in solar cell [23, 27]. By the result of
absorption dyes on TiO film and adsorption energy; dyes contain thiobarbutiric acid moiety as
2
an electron acceptor are not prone to produce high efficiency in solar cells. But application of
carbazole and alkyl groups as bridge and donor moiety is suggested.
Acknowledgments
We are grateful to the Yasouj University and Sharif Univercity of technology, Iran, for financial
supports.
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Highlights
I.
II.
III.
We applied carbazole base as π bridge unit in all synthesis dyes
3 novel dyes synthesized and the structure of dyes studied
Dyes were investigated by UV spectroscopy and cyclic voltammetry (CV)