Optical and photovoltaic properties of salicylaldimine-based azo ligands

Article abstract of DOI:10.1016/j.jphotochem.2009.12.012

A series of azo dyes containing salicylaldimine-based ligands as side chains were prepared and characterized. Absorption and emission data in five solvents of different polarities were studied. Photoirradiation studies under an oxygen atmosphere in water showed that the Schiff base side chains enhanced the photo-oxidative stability of the azo chromophore. The electrochemical properties of the dyes were investigated by a cyclic voltammetry. The synthesized salicylaldimine-based azo dyes gave two irreversible oxidation potentials. Complexation behavior of synthesized compounds with titanium (IV) ions was illustrated by the change in their absorption spectra. These ligands are appropriate sensitizers for anchoring to the TiO₂ surface chemically in dye-sensitized solar cell (DSSC) productions. Electron injection capacities to TiO₂ and photovoltaic performance of the synthesized salicylaldimine-based azo dyes were tested with DSSC.

Full text of DOI:10.1016/j.jphotochem.2009.12.012

Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Optical and photovoltaic properties of salicylaldimine-based azo ligands
Haluk Dinc¸ alpa,∗, Sinem Yavuz , Özgül Haklı , Ceylan Zafer , Cihan Özsoy , Inci Durucasu , Sıddık Ic¸ li
a
b
c
c
a
c
˙
˙
a Department of Chemistry, Faculty of Art and Science, Celal Bayar University, Muradiye, 45030 Manisa, Turkey
b Department of Chemistry, Faculty of Art and Science, Mug˘la University, Kötekli, 48000 Mug˘la, Turkey
c Solar Energy Institute, Ege University, Bornova, 35100 Izmir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 October 2009
Received in revised form 4 December 2009
Accepted 15 December 2009
Available online 23 December 2009
A series of azo dyes containing salicylaldimine-based ligands as side chains were prepared and character-
ized. Absorption and emission data in five solvents of different polarities were studied. Photoirradiation
studies under an oxygen atmosphere in water showed that the Schiff base side chains enhanced the photo-
oxidative stability of the azo chromophore. The electrochemical properties of the dyes were investigated
by a cyclic voltammetry. The synthesized salicylaldimine-based azo dyes gave two irreversible oxidation
potentials. Complexation behavior of synthesized compounds with titanium (IV) ions was illustrated
by the change in their absorption spectra. These ligands are appropriate sensitizers for anchoring to
the TiO2 surface chemically in dye-sensitized solar cell (DSSC) productions. Electron injection capacities
to TiO2 and photovoltaic performance of the synthesized salicylaldimine-based azo dyes were tested
with DSSC.
Keywords:
Azo dye
Solvatochromism
Photodecomposition
Complex formation
DSSC
© 2009 Elsevier B.V. All rights reserved.
1. Introduction
Ni(II) and Zn(II) [15,16], Co(II) [16], Fe(III) [17]. Also, they are used as
optical materials because of their interesting photochromic prop-
Azo dyes are well-known class of organic photoactive mate-
rials due to their excellent optical switching properties, good
chemical stabilities and high solution process abilities [1,2]. These
materials are widely used in heat transfer printing and textile
industries [3,4], optical data storage [5], switching technologies
[6] and photo-refractive polymer industries [7]. Also, azo dyes are
used as sensitizers in dye-sensitized solar cells (DSSCs) based on
photosensitization of nanocrystalline titanium dioxide (nc-TiO2)
[8]. Chelating activity with metal ions, easy and low cost produc-
tion of azo dyes are some of their advantages with reference to
other organic sensitizers used in DSSCs. In order to improve the
photon-to-electric conversion efficiency (PCE), the dye has to be
in close contact with the semiconductor surface. As a result, elec-
tron injection rate from the excited state HOMO level of the dye to
the conduction band (CB) of metal oxides reaches to femto second
level. In particular, the preferred dyes including anchoring groups
such as carboxylates and phosphonates that provide an efficient
interaction with the surface of semiconductor give a simple way to
inject the electrons to the conduction band of the metal oxide [9].
Salicylaldimine-based azo ligands [10–13] are the most impor-
tant class of chelating ligands that are extensively studied in
coordination chemistry of transition metals. The Schiff bases form a
series of complexes with most of the metal ions such as Cu(II) [14],
erties [18,19]. Schiff bases may become promising dye sensitizer in
molecular photovoltaic cells if combined their chelating activities
and other properties. In particular, the use of salicylaldimine-based
azo ligands as sensitizers in DSSCs provides a direct interaction with
the surface of TiO2.
In the present work, we present three new azo dyes containing
salicylaldimine group as sensitizers for DSSCs. The aim of attach-
ing a nitro group at one end of the molecule is to obtain stable
compound against oxidation. Nitro aromatic compounds are resis-
tant to oxidative attack because of their electron-withdrawing
nature [20]. We have synthesized the azo monomers com-
bined with the salicylaldimine-based ligands substituted with
different electron-withdrawing groups (Scheme 1). The character-
istics of the absorption and emission spectra of the synthesized
salicylaldimine-based azo ligands have been identified in five
solvents of different polarities. Also, we have investigated the com-
plexation behavior of the synthesized compounds with Ti(IV) ions.
Photovoltaic performance of DSSCs based on these new type sali-
cylaldimine compounds has been investigated.
2. Experimental details
2.1. Materials and methods
All reagents and solvents used were purchased from Merck
Chemical Company. 1H and 13C NMR spectra were measured on
a Bruker spectrometer operating at 400 MHz. The IR spectra of
∗
Corresponding author. Tel.: +90 236 2412151/2543; fax: +90 236 2412158.
E-mail address: haluk.dincalp@bayar.edu.tr (H. Dinc¸ alp).
1010-6030/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2009.12.012

H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
9
Scheme 1. Synthetic routes of azo dyes 1–4.
the synthesized compounds were measured on a Perkin Elmer-
Spectrum BX spectrophotometer by dispersing samples in KBr
pellets. LC–MS spectroscopy was obtained on an Agilent 1100 MSD
spectrometer.
UV–vis spectra measurements were performed by a JASCO
V-530 UV–VIS diode array spectrophotometer, and fluorescence
spectra measurements were performed with a PTI QM1 fluores-
cence spectrophotometer. Quantum yields of fluorescence were
measured by comparing the fluorescence intensity of the sample
to that of the optically dilute solutions of Riboflavin in ethanol
(˚f = 0.30, ꢀexc = 450 nm) [21]. All the experiments were carried out
at 25 ◦C, and all the compounds were analyzed at an optical density
of below 0.1.
terpineol, the obtained paste was sonicated again by ultrasonic
horn at 200 W power for 10 min. All the ethanol in the paste was
removed by rotary evaporator.
The TiO2 paste was coated on transparent SnO2 coated glass
electrodes (SnO2: F, TEC15, Rsheet: 15 ohm/ꢀ) by screen printing
technique (polyester screen with 77 T mesh). Dried TiO2 electrodes
were sintered at 500 ◦C for 1 h with 10 ◦C/min heating rate. Finally,
mesoporous TiO2 film with 20 nm particle size and 4 ␮m thick-
nesses was obtained.
2.3. Sensitization with dye and DSSC assembly
Electrochemical properties of the materials were studied by
cyclic voltammetry (CV). CV measurements were recorded by
CH 660B model potentiostat from CH instruments using a three-
electrode one-compartment cell equipped with a glassy carbon
working electrode (WE), a platinum counter electrode (CE), and
Ag/AgCl (in 3 M KCl solution) reference electrode (RE). The support-
ing electrolyte was 0.1 M [TBA][PF6] in acetonitrile. The milimolar
solutions of the dyes were prepared in spectroscopic grade ace-
tonitrile. The oxidation potential of ferrocene/ferrocenium redox
couple (Fe/Fe+) used as an internal reference was observed at
+0.41 V.
TiO2 electrodes were immersed in dye solution containing
0.3 mM Z-907 in acetonitrile:tert-butanol (1:1), 0.3 mM azo dyes 3
and 4 in chloroform:methanol (1:1) overnight while electrode tem-
perature is around 100 ◦C. Sensitized TiO2 electrodes were rinsed
with pure acetonitrile and kept in desiccator. Platinized FTO coated
electrode was used as a counter electrode was prepared by thermal
reduction of 1% (v/v) 2-propanol solution of hexachloroplatinic acid
to the metallic platinum. Drop casted electrodes annealed at 380 ◦C
for 30 min. The DSSCs were prepared by placing the electrodes in
sandwich geometry top of each other and in the middle 50-␮m
thick thermoplastic polymer frame Surlyn® 1702 (DuPont). The
electrodes sealed by heating around 100 ◦C and pressing slightly.
Electrolytes consist of iodide/triiodide redox couple filed into cell
via pre-drilled small hole by vacuum. The electrolyte composi-
tion was 0.6 M 1-butyl-3-methyl-imidazolium iodide (BMII), 0.1 M
lithium iodide (LiI), 0.05 M iodine (I2) dissolved in 3-methoxy
propyonitrile (MPN). The active areas of the prepared solar cells
adjusted to 1.0 cm2 by using mask.
2.2. Synthesis of TiO2 nano-particles and electrode preparation
Synthesis of TiO2 nano-particles was achieved by modification
of procedure reported by Graetzel and co-workers [22]. 58.6 g tita-
nium tetra isopropoxide Ti(OiPr)4 was added to 12 g glacial acetic
acid. The solution was added dropwise into the 290 ml cooled
deionized water under vigorous stirring, pH of the solution was
adjusted to 1–2 by adding 5.4 ml nitric acid (65% HNO3). The sol
was peptized at 78 ◦C for 75 min in the oven, and then diluted to
370 ml by adding deionized water. The sol was transferred to the
Teflon beaker equipped autoclave and heated at 235 ◦C for 12 h for
hydrothermal growth of the particles. After cooling down the sus-
pension, 2.4 ml nitric acid (65% HNO3) was added and sonicated
with ultrasonic horn 200 W power for 10 min in order to break
agglomerates. Then, the suspension was concentrated to 16.5%
(w/w) TiO2. Remaining all water was exchanged with ethanol by
centrifuging and 40% (w/w) TiO2 paste was obtained. After the
addition of 4.5% ethyl cellulose in ethanol and 79 g anhydrous ␣-
The performance of the dye-sensitized TiO2 solar cells
was compared to the commercially available ruthenium
dye
(Z-907)
Ru(4,4ꢀ-dicarboxy-2,2ꢀ-bipyridine)(4,4ꢀdinonyl-
2,2ꢀbipyridine)(NCS)2. The photovoltaic characterizations of the
DSSCs were done under the dark and standard conditions by
illumination of AM1.5 global radiation with 100 mW/cm2 light
intensity.
The photovoltaic parameters of DSSC solar cells can be obtained
from the following equations:
V
mIm
FF =
(1)
VocIsc

10
H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
and
atoms); 61.76 (–O–CH2–); 14.50 (–CH2–CH3) ppm. LC/MS–API-
ES: [M]•+ = 419 molecular ion peak; 348 [M]•+–COOC2H5; 314
[M]•+–NO2–OH–OC2H5; 286 [M]•+–NO2–OH–COOC2H5; 270
[M]•+–C8H8NO2; 256 [M]•+–C8H8N2O2; 167; 149; 113.
I
scVocFF
ꢁ =
(2)
Plight
A
where FF is filling factor, Voc is the open circuit voltage, Isc is the
short current. Im and Vm are the current and potential maximum
power point, respectively, Plight is the intensity of incident light,
and A is the cell area.
2.4.4. Synthesis of 1-[3-(((4-carboxypyridyl)imino)methyl)-4-
hydroxyphenylazo]-4-nitrobenzene
(3)
50 mg (0.12 mmol) of azo dye 2 was added to a solution of
16 mg (0.12 mmol) of potassium carbonate in 4 ml of methanol.
The mixture was heated to boiling for 30 min or until no start-
ing compound could be detected on TLC (toluene:methanol/90:10)
plate. The reaction mixture was cooled to room temperature. A
solution of the concentrated hydrochloric acid in methanol was
added dropwise into the reaction mixture with vigorous stirring
and the pH value of the solution was adjusted to 5. A red pre-
cipitate was collected by a suction filtration. The pure product
was dried at 80 ◦C overnight. C19H13O5N5, Yield: 92%, FT-IR (KBr,
2.4. Synthesis
2.4.1. Synthesis of 6-aminoethyl nicotinate
A mixture of 2 g (14.5 mmol) of 6-aminonicotinic acid, 6.2 ml
(106.5 mmol) of absolute ethanol and 2.7 ml of concentrated sul-
furic acid on a steam bath was refluxed for 5 h. The solution was
cooled to room temperature and slowly poured on to 12 g of
crushed ice. A sufficient ammonia solution was added until the
resulting solution was alkaline. The mixture was extracted with
three 25 ml portions of ether. The combined ethereal extracts were
dried over magnesium sulfate. Then, the ether was removed by
flash distillation. The purity of the compound was controlled by
TLC (chloroform:methanol/60:40). C8H10O2N2, Yield: 78%, FT-IR
cm−1): 3434 (ꢂO–H), 1717 (ꢂC
carboxylic acid), 1616 (ꢂCH N),
O
1588, 1558, 1518 and 1474 (ꢂN N; cis and trans), 1342 (ꢂNO2 ),
1286, 1104, 1004 cm−1 1H NMR [400 MHz, DMSO-d6, ı 2.42 ppm
.
(5 peaks) and 3.29 H shift of water]: ı = 10.35 (1H, s); 8.48 (1H,
d, J = 2.3 Hz); 8.41 (2H, d, J = 9.4 Hz); 8.24 (1H, d, J = 2.3 Hz); 8.14
(1H, dd, J1 = 8.6 Hz, J2 = 2.3 Hz); 8.04 (2H, d, J = 8.6 Hz); 7.81 (1H,
dd, J1 = 8.6 Hz, J2 = 2.3 Hz); 7.19 (1H, d, J = 9.4 Hz); 6.78 (1H, s);
6.44 (1H, d, J = 9.4 Hz) ppm. LC/MS–API-ES: [M]•+ = 391 molec-
ular ion peak; 348 [M]•+–COOH; 314 [M]•+–NO2–2OH; 286
[M]•+–NO2–OH–COOH; 267; 258; 167; 149; 113.
(KBr, cm−1): 3411 and 3322 (ꢂN–H), 3137, 1692 (ꢂC ester), 1647,
O
1600, 1513, 1367, 1272, 1135 cm−1
.
1H NMR (400 MHz, CDCl3, ı
7.26 ppm): ı = 8.67 (1H, s); 8.02 (1H, dd, J1 = 8.6 Hz, J2 = 2.3 Hz); 6.51
(1H, d, J = 8.6 Hz); 5.41 (2H, broad s); 4.33 (2H, q, J = 7.0 Hz); 1.35
(3H, t, J = 7.0 Hz) ppm.
2.4.2. Synthesis of 3,5-diisopropyl-4-aminobenzenesulfonic acid
8.7 ml of concentrated sulfuric acid was added dropwise to the
10 ml (50 mmol) of 92% 2,6-diisopropyl aniline in an ice-bath with
vigorous stirring. The mixture was stirred at 180 ◦C for 5 h. Then, the
reaction mixture was cooled to room temperature and poured on to
80 g of crushed ice. The black precipitate was collected by a suction
filtration. The precipitate was dissolved in 100 ml of boiling water
and the solution was treated with 1 g of decolorizing charcoal. The
mixture was stirred for 15 min and clarified by a suction filtration.
The precipitate was collected by a suction filtration, washed well
with cold water and recrystallized from water to give white crys-
tals. C12H19O3NS, Yield: 40%, FT-IR (KBr, cm−1): 3400 (ꢂO–H), 3126
and 3092 (ꢂN–H), 2969 and 2873 (ꢂC–H isopropyl), 1625, 1549, 1233,
2.4.5. Synthesis of 1-[3-(((2,6-diisopropyl-4-
sulfophenyl)imino)methyl)-4-hydroxyphenylazo]-4-nitrobenzene
(4)
A similar procedure to the one given above was followed. A
mixture of 0.3 g (1.1 mmol) of azo dye 1 and 0.28 g (1.1 mmol) of
3,5-diisopropyl-4-aminobenzenesulfonic acid in 10 ml of ethanol
was refluxed under a nitrogen atmosphere with stirring for 6 h.
The mixture was cooled to room temperature and a pale red
precipitate was filtered by a suction filtration. The purity of the
compound was controlled by TLC (chloroform:methanol/90:10).
C25H26O6N4S, Yield: 87%, FT-IR (KBr, cm−1): 3434 (ꢂO–H), 2969
(ꢂC–H isopropyl), 1625 (ꢂCH N), 1527 (ꢂN N), 1342 (ꢂNO2 ), 1183,
1177, 1076, 1040, 735, 640 cm−1 1H NMR [400 MHz, DMSO-d6, ı
.
1043 cm−1 1H NMR [400 MHz, DMSO-d6, ı 2.47 ppm (5 peaks)
.
2.48 ppm (5 peaks)]: ı = 7.41 (2H, s); 3.15 (2H, m, J = 7.0 Hz); 1.17
(12H, d, J = 7.0 Hz) ppm. 13C NMR [100 MHz, DMSO-d6, ı 40.85 ppm
(7 peaks)]: 146.64; 139.87; 129.53; 121.84; 27.74 (–CH(CH3)2);
24.00 (–CH(CH3)2) ppm.
and 3.29 H shift of water]: ı = 8.75 (1H, s); 8.44 (1H, d, J = 1.6 Hz);
8.41 (2H, d, J = 8.6 Hz); 8.10 (1H, dd, J1 = 8.6 Hz, J2 = 1.6 Hz); 8.04
(2H, d, J = 8.6 Hz); 7.45 (2H, s); 7.20 (1H, d, J = 8.6 Hz); 2.90 (2H,
m, J = 7.0 Hz); 1.12 (12H, d, J = 7.0 Hz) ppm. 13C NMR [100 MHz,
DMSO-d6, ı 40.49 ppm (7 peaks)]: 119–166 (15 different C atoms
including aromatic and imine
C atoms); 28.47 (–CH(CH3)2);
2.4.3. Synthesis of 1-[3-(((4-ethylcarboxylate
23.85 (–CH(CH3)2) ppm. LC/MS–API-ES: [M]•+ = 510 molecu-
lar ion peak; 422 [M]•+–2CH(CH3)2; 391 [M]•+–C6H4NO2; 348
[M]•+–2CH(CH3)2–SO3H; 286 [M]•+–NO2–OH–2CH(CH3)2–SO3H;
267 [M]•+–C12H17O3S; 258 [M]•+–C12H17NO3S; 167; 149; 113.
pyridyl)imino)methyl)-4-hydroxyphenylazo]-4-nitrobenzene (2)
The title compound was prepared using a procedure given
in the patent application [23]. A mixture of 0.5 g (1.84 mmol)
of azo dye
1 [26] and 0.31 g (1.84 mmol) of 6-aminoethyl
nicotinate in 20 ml of ethanol was refluxed under a nitrogen
atmosphere with stirring for 8 h. The reaction was monitored by
TLC (toluene:methanol/90:10). The mixture was cooled to room
temperature and an orange precipitate was filtered by a suction
filtration. The solid was recrystallized from ethyl acetate to afford
orange crystals. C21H17O5N5, Yield: 85%, FT-IR (KBr, cm−1): 3316
3. Results and discussion
3.1. Spectral properties of dyes
Normalized steady-state absorption spectra of the reference
compound azo dye 1 and salicylaldimine-based azo ligands in THF
at room temperature are shown in Fig. 1. The reference compound
azo dye 1 shows characteristic absorption peaks at 362 nm and a
shoulder band at 457 nm, which are denoted to its (0,1) and (0,0)
bands, respectively. Upon the introduction of salicylaldimine group
to the azo benzene core, a bathochromic shift is observed.
(ꢂO–H), 1709 (ꢂC O ester), 1622 (ꢂCH N), 1588, 1558 and 1521 (ꢂN
;
N
cis and trans), 1342 (ꢂNO2 ), 1104, 1018 cm−1 1H NMR (400 MHz,
.
CDCl3, ı 7.25 ppm): ı = 9.65 (1H, s); 9.15 (1H, s); 8.42 (1H, d,
J = 1.6 Hz); 8.40 (2H, dd); 8.37 (1H, d); 8.24 (1H, d, J = 1.6 Hz); 8.14
(1H, dd, J1 = 9.4 Hz, J2 = 2.3 Hz); 8.02 (2H, d, J = 8.6 Hz); 7.40 (1H, d,
J = 7.8 Hz); 7.17 (1H, d, J = 9.4 Hz); 4.45 (2H, q, J = 7.0 Hz); 1.44 (3H,
t, J = 7.0 Hz) ppm. 13C NMR [100 MHz, CDCl3 ı 77 ppm (3 peaks)]:
110–166 (17 different C atoms including aromatic and imine C
We have studied the absorption and the fluorescence behaviors
of azo dyes 1–4 in solvents of having different dielectric constants

H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
11
Fig. 1. Normalized UV–vis absorption spectra of azo dyes 1–4 in tetrahydrofuran.
Fig. 2. Comparison of normalized fluorescence spectra of azo dye 3 in solvents of
different polarities (ꢀexc = 450 nm).
summarized in Table 1 and shown in Fig. 2. All of the compounds
exhibit a major emission peak at a wavelength shorter than the
minor peak. The major peak appears around 525 nm. The minor
peak is seen as a red-shifted shoulder on the major emission peak.
The emission spectra of all compounds in water (data not shown)
are structureless while the same spectra give a good shape of bands
in other organic solvents. The structurelessness of the emission
bands in water solution may be attributed to the aggregation of
azo dyes in water.
Another striking feature of the studied compounds is detected
in the excitation spectra. It is well known that many salicylidene
anilines exhibit inter- and intramolecular proton transfer reactions
and give more than one structural form in the ground and excited
states [24–27]. In our previous work, we demonstrated that the
excited enol form of azo chromophores containing thiophene moi-
ety and salicylaldimine-based ligand was converted to the keto
form [28]. Also, it was noted that the hydrogen bonding ability of
the solvents facilitated the formation of intramolecular hydrogen
bonded enolic forms [29]. Fig. 3 gives a comparison of the UV–vis
and the excitation spectra of azo dye 3 in ethyl acetate. Excita-
tion spectrum of azo dye 3 at the collected emission wavelength
of 530 nm is not identical with S0–S1 absorption band of the dye.
Fig. 3. Normalized UV–vis absorption, fluorescence and excitation spectra (with
ball) of azo dye 3 in ethyl acetate (ꢀexc = 450 nm, ꢀem = 530 nm).
Table 1
The visible absorptions, fluorescence emissions, and Stokes shifts (ꢃꢀ) data of azo dyes 1–4 in solvents of different polarities (ꢀ (nm), ε (l mol−1 cm−1)) (ꢀexc = 450 nm).
longest
Solvent
εa
4.8
Compound
ꢀmax
εmax
ꢀ
ꢀem (max)
ꢃꢀ
max
1
2
3
4
359
379
378
267
20,900
20,830
17,270
2660
466
485
485
457
529
534
534
528
79
84
84
78
Chloroform
6.0
1
2
3
4
356
373
370
277
21,800
21,270
11,450
3600
465
482
485
457
527
526
526
517
77
76
76
67
Ethyl acetate
Tetrahydrofuran
1-Butanol
7.6
1
2
3
4
362
271
273
268
20,180
8610
12,150
6500
468
484
484
457
528
528
530
528
78
78
80
78
17.8
80.2
1
2
3
4
380
407
276
380
16,830
7520
2990
9940
461
451
454
467
525
536
536
528
75
86
86
78
Water
1
2
3
4
381
447
448
379
1140
2010
1960
5670
457
447
448
453
533
530
529
529
83
80
79
79
a
Dielectric constant, ε, is taken from Ref. [30].

12
H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
Table 2
Fluorescence emission data, fluorescence quantum yields (˚f), radiative lifetimes (ꢄ0 (ns)), fluorescence lifetimes (ꢄf (ns)), fluorescence rate constants (kfr × 108 (s−1)),
non-radiative rate constants (knr × 1011 (s−1)), and singlet energies (Es (kcal mol−1)) of azo dyes 1–4 in solvents of different polarities (ꢀexc = 450 nm)a.
Solvent
Compound
Фf
ꢄ0
ꢄf
0.002
0.004
0.003
0.006
kfr
5.2
6.1
6.3
2.1
knr
Es
1
2
3
4
0.0011
0.0025
0.0016
0.0012
1.9
1.6
1.6
4.9
4.9
2.5
3.8
1.7
62.2
59.8
59.8
63.4
Chloroform
1
2
3
4
0.0049
0.0012
0.0023
0.0011
1.7
1.6
2.4
5.6
0.009
0.002
0.006
0.006
5.7
6.4
4.1
1.8
1.1
5.3
1.8
1.6
62.4
60.2
59.8
63.4
Ethyl acetate
Tetrahydrofuran
1-Butanol
1
2
3
4
0.0012
0.0013
0.0024
0.0017
1.5
2.2
1.5
2.9
0.002
0.003
0.004
0.005
6.6
4.6
6.7
3.4
5.6
3.7
2.8
1.9
62.0
60.0
59.9
63.4
1
2
3
4
0.0007
0.0013
0.0018
0.0008
2.0
7.0
6.1
3.6
0.001
0.009
0.011
0.003
5.1
1.4
1.7
2.8
7.5
1.1
0.9
3.3
63.0
64.3
63.8
62.1
Water
1
2
3
4
0.0016
0.0008
0.0005
0.0005
1.4
0.4
0.2
3.3
0.002
<0.001
<0.001
0.002
7.3
23.5
55.3
3.1
4.4
63.5
64.8
64.7
64.0
30.4
101.0
6.6
Photophysical parameters are calculated with the formulas: ꢄ0 = 3.5 × 108 (ꢅm2 ax εmax ꢃꢅ1/2), kf = 1/ꢄf = kfr + knr , kfr = 1/ꢄ0, ꢄf = ꢄ0 ˚f [31,32].
a
It shows a sharp peak at 480 nm and a minor peak at 450 nm.
Additionally, the normalized emission spectra of the azo dye 3
have no mirror-image relationship with their respective absorption
spectrum. These results support the intramolecular proton transfer
reaction in the excited state and explain the formation of keto form
of the compound. All of the excitation measurements of azo dyes
1–4 with the employed solvents have given similar results (data
not shown). Although the reference compound azo dye 1 has no
salicylaldimine group as side chains, p-hydroxyl group initiates the
formation of hydrazone tautomer. However, in other structures,
the most possible position of the tautomerization is between the
hydroxyl group and the Schiff base group.
recorded in chloroform, ethyl acetate, and tetrahydrofuran. Hydro-
gen bonding ability of the compounds with 1-butanol or water is
evidently stronger than that of the solvents containing no hydroxyl
moieties. Proton donating ability of the solvents stabilizes the
charge transfer state better than the ground state of the molecule
due to the interaction of the protons with the unshared electron
pairs of the enol oxygen and the diazo nitrogen atoms and enhance
the formation of inter- and intramolecular hydrogen bonded eno-
lic form of the molecule, which creates a charge transfer acceptor
form in the excited state. As a result, energy difference between the
ground state and the excited state of the molecule decreases and a
bathochromic shift occurs.
No clear relationship is determined between the solvent polar-
ity and the absorption or emission wavelengths of the studied
molecules, excluding those in 1-butanol and water solutions. The
compounds give a marked bathochromic shift in absorption and
emission maxima in these solutions compared to the same spectra
Aggregation tendency of the molecules in water is responsi-
ble for the lowest fluorescence quantum yield values among the
studied solutions. Fluorescence quantum yields and calculated
photophysical data of the compounds in solvents of different polar-
ities are summarized in Table 2. High non-radiative decay rate
constant values in water support the aggregation behaviors of the
compounds in water solution. The longest radiative lifetimes are
observed for azo dye 4. Generally, fluorescence quantum yields of
azo dye 4 in the studied solvents are lower than that of the other
compounds. This behavior may be explained by the low tautomer-
ization capability of the azo dye 4 in the respective solvents. Bulky
isopropyl groups may prevent the replacement of hydrogen atom in
the tautomerization process so that the less planar structural form
of azo dye 4 is obtained. Conformational relaxation is increased
with respect to the other studied compounds. As a result, fluores-
cence quantum yield of azo dye 4 decreases.
Table 3
Photodecomposition rate constants (kp × 10−6 (s−1)) of azo dyes 1–4 obtained under
Xe lamp exposure in the fluorescence spectrophotometer in water (ꢀexc = 254 and
450 nm) for 1 h at the collected emission wavelength of 530 nm under an argon or
an air oxygen atmosphere.
Dye
Saturated with argon
Saturated with air oxygen
254 nm
450 nm
254 nm
450 nm
Fig. 4. Stern–Volmer plots of azo dyes 1–4 irradiated under Xe lamp exposure
in the fluorescence spectrophotometer in water at the excitation wavelength of
450 nm for 1 h (ꢀem = 530 nm) under an air oxygen atmosphere. Lineer equations:
(1: −2.2 × 10−5X − 0.0076; R2: 0.97), (2: −7.2 × 10−6X − 3.8 × 10−5; R2: 1.0), (3:
−9.6 × 10−6X + 6.6 × 10−5; R2: 0.99) (4: −8.8 × 10−6X − 3.7 × 10−4; R2: 1.0).
1
2
3
4
21.2
44.5
18.7
22.9
3.5
2.8
1.8
34.8
4.7
52.1
11.5
22.1
7.2
9.6
34.0
8.8

H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
13
Table 4
Cyclic voltammetry data for azo dyes 1–4a.
Dye
Er0ed 3 (V)
Er0ed 2 (V)
Er0ed 1 (V)
Eo0x 1 (V)
Eo0x 2 (V)
LUMO-1 (eV)
LUMO (eV)
HOMO (eV)
HOMO-1 (eV)
Egap1 (eV)
Egap (eV)
1
2
3
4
−1.33
−1.33
−1.40
−1.25
−1.00
−0.98
−0.98
−1.00
−0.72
−0.81
−0.81
−0.79
1.18
1.18
1.18
0.98
−3.67
−3.58
−3.58
−3.60
−5.57
−5.57
−5.57
−5.37
1.90
1.99
1.99
1.77
1.48
1.48
1.38
−3.41
−3.41
−3.39
−5.87
−5.87
−5.77
2.46
2.46
2.38
ELUMO = −(4.8 + Eroendset), EHOMO = ELUMO − Egap [35].
a
3.2. Photostability of dyes
are incorporated into the ligand backbone have been shown in the
literature [36–39].
The dyes were irradiated by Xe lamp exposure in the flu-
orescence spectrophotometer in water for 1 h at the excitation
wavelengths of 254 and 450 nm under argon or air atmosphere.
Photodecomposition behaviors of the compounds have been
detected by monitoring the decrease of the emission intensity at
the collected emission wavelength of 530 nm (Fig. 4). Photode-
composition rate constants of azo dyes 1–4 are calculated by the
formula [33], ln(I0/I) = kp × t, where I0 and I are the emission inten-
sities of the dyes before and after the irradiation, respectively, kp
is the photodegradation rate constant and t is the irradiation time.
The results were summarized in Table 3. Photodecomposition rate
constants at 254 nm are much higher than that of at 450 nm. Higher
energy radiation at 254 nm initiates numerous radicalic reactions
which accelerate the decomposition of molecule. As seen, it is obvi-
ous that molecular oxygen or excited oxygen species play a major
role in the photo-fading of the azo dyes in aerated water solutions.
Self-sensitized photo-oxygenation of azo dyes has attracted a con-
siderable attention in the photodegradation of azo dyes even if
their quantum yields of singlet oxygen are very low (10−3) [34].
While azo dye 1, which does not contain imine bond, shows a
higher photodecomposition rate constant under illumination in the
presence of oxygen, azo dye 2 is the most resistant molecule to
photo-oxidative degradation among the studied compounds. Inter-
action of excited state of azo molecule with the singlet oxygen is a
crucial point in the clarification of photo-fading pathway.
The FT-IR spectrum recorded for the dye 3-adsorbed TiO2 sur-
face gives a clear observation of interaction of dye structure with
TiO2 surface (Fig. 6a). The Schiff base CH N vibration band at
1616 cm−1, the azo group N N isomers vibration bands at 1474 and
1518 cm−1 and carboxylic acid C O vibration band at 1717 cm−1 of
dye 3 on FT-IR spectra disappear, but new bands appear on the
graph. An intense band at 2137 cm−1 is assigned to N–N stretching
frequencies of dye 3–Ti complex. It assumes that dinitrogen species
have been greatly modified by coordination onto titanium. There
is a rare example of a coordination process of Ti with azo dyes in
the literature [40–42]. Also, FT-IR bands at 1706 and 1836 cm−1
are attributed to the stretching vibrations of the Schiff base CH
N
group and the carboxylic acid C O group of the complex structure.
Also, iminium N–H infrared band of complex is observed at 3327
3.3. Electrochemical measurements
CV has been applied to investigate the redox behaviors of
azo dyes 1–4 and to find out HOMO/LUMO energy levels. Fig. 5a
and b shows cyclic voltammograms of the two studied com-
pounds, azo dyes 2 and 4 in three-electrode cell. Each compound
exhibits three reduction waves. The second reduction wave shows
reversible behavior, and the others are irreversible. While two irre-
versible oxidation peaks are observed for azo dyes 2, 3, and 4, only
one irreversible oxidation peak is detected for azo dye 1. Cyclic
voltammetry data including HOMO and LUMO energy levels of
the compounds are also summarized in Table 4. Electron-releasing
capacity estimated from the HOMO/LUMO energy levels decreases
as follows: 4 > 3 ≈ 2 > 1. Azo dye 4, having the highest values of
HOMO energy level, exhibits the lowest oxidation state among the
studied compounds. This is attributed to the presence of isopropyl
group that is donating its electrons to the molecule via sigma bond.
Better stability exhibition of azo dyes 2 and 3 is because of the pres-
ence of the carboxylate groups that gain an extra conjugation to the
molecule.
3.4. Complex formation with titanium (IV) ions
Azo compounds and Schiff bases can bind to the nano-particles
of most of the metal atoms upon complex formation or charge
transfer complex via electron transfer from the dye molecule to
the metal surface. A number of Ti(IV) complexes with bi-, tri- and
tetra-dentate salen-type ligands in which the different amino units
Fig. 5. Cyclic voltammograms at a glassy carbon electrode of (a) azo dye 2 (1 mM),
and (b) azo dye 4 (1 mM) in MeCN containing 100 mM [TBA][PF6] and ferrocene as
an internal electrode (Eox = 0.41 V).

14
H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
Fig. 6. (a) FT-IR/ATR and (b) UV–vis absorption spectrum of dye 3 adsorbed on TiO2 surface. (c) Spectrophotometric titrations of dye 3 (4 × 10−5 M) with the addition of
Ti(OiPr)4 in isopropanol. Changes in the absorbance with the addition of Ti4+ were illustrated by the arrows. [Ti4+] (From down to up) = 0 M, 1.8 × 10−3 M, 3.7 × 10−3 M,
5.5 × 10−3 M, 7.3 × 10−3 M, 9.1 × 10−3 M, 10.9 × 10−3 M, 12.6 × 10−3 M, 14.4 × 10−3 M, 16.1 × 10−3 M, 17.8 × 10−3 M, 19.5 × 10−3 M, 21.2 × 10−3 M and 22.9 × 10−3 M.
and 3393 cm−1 as weak bands. UV–vis absorption spectrum of dye
3-adsorbed TiO2 surface is displayed in Fig. 6b. It shows a maximum
at 394 nm corresponding to the electronic transition of the complex
structure formed between TiO2 particles and the dye structure.
In our study, at each addition 15 ␮l of titanium tetra isopropox-
ide Ti(OiPr)4 solution prepared in isopropanol at the concentration
of 0.375 M was poured into the 3 ml isopropanol solution of
4 × 10−5 M compound 3. The considerable amount of shift was
observed in the absorption spectrum of dye molecule. Hyp-
sochromic shift was observed from 399 to 385 nm for azo dye
3. Also, absorption band at 490 nm disappears upon adding the
Ti(IV) solution (Fig. 6c). This is attributed to the electronic interac-
tion between the Ti(IV) ions and non-bonding electrons of oxygen
or nitrogen atoms of salicylaldimine group. No obvious shift was
observed for dye 4–Ti complex formation. Also, bathochromic shifts
were observed from 379 to 399 nm and 367 to 402 nm for azo dyes
2 and 1, respectively (data not shown).
3.5. Photovoltaic properties
Current–voltage (I–V) characteristics of the solar cells fabricated
using azo dyes 3–4 were determined at standard conditions under
illumination with simulated sunlight (AM1.5, 100 mW/cm2 light
intensity). Fig. 7 reveals the current density–voltage (J–V) charac-
Table 5
Photovoltaic parameters of DSSCs containing nc-TiO2/azo dyes 3–4 photoactive
electrodes, and standard dye Z-907 under illumination with 100 mW/cm2 light
intensity.
Dye
Azo dye 3
Azo dye 4
Z-907
Voc [V]
0.33
0.80
0.24
0.56
0.14
0.51
0.14
0.35
0.65
0.26
0.45
0.12
0.52
0.12
0.60
12.66
0.38
9.33
3.55
0.47
3.55
Isc [mA/cm2]
Vmpp [V]
Impp [mA/cm2]
MPP [mW/cm2]
FF
Fig. 7. Current–voltage graphs of azo dyes 3–4 in dark and under illumination with
ꢁ [%]
100 mW/cm2 light intensity.

H. Dinc¸ alp et al. / Journal of Photochemistry and Photobiology A: Chemistry 210 (2010) 8–16
15
Table 6
One of the advantages of dye 3 is lower HOMO energy level with
respect to dye 4 resulting the lower charge recombination rate
and higher driving force for dye regeneration. An important fac-
tor that produces low yield of efficiency for both of the dyes is the
photodegradation pathway of the dyes in the presence of reactive
oxygene species generated by TiO2 catalyst under illumination. In
addition, self-sensitized photooxidation of the azo dyes is crucial
point in understanding the photodegradation pathways. Dyes show
much higher photodecomposition rate constants in the presence of
oxygen than that of a non-oxygenated atmosphere because of the
formation of oxidation products resulted from self-sensitized pho-
tooxidation of azo dyes 1–4. The tautomerization behavior and the
steric factor play a role in the formation of charge transfer complex
between the singlet oxygen and azo dyes 2–4.
Advantages vs. disadvantages of dye 3 and 4 relative to one another.
Characteristics
Azo dye 3
Azo dye 4
FT-IR (cm−1
)
1616 (m, ꢂCH
490
)
1625 (s, ꢂCH
375
)
N
N
Vis absorptiona
(nm)
Emissionb (nm)
Fluorescence
quantum yieldb
Photodecomposition
526–536
517–528
0.0008–0.0017
0.0016–0.0024
9.6 × 10−6
+14
8.8 × 10−6
rate constantc (s−1
)
Complex formation
+4
with Ti (IV)a
(hypsochromic
shift)
(bathochromic
shift)
HOMO (eV)
Band gap energy
(eV)
−5.57
−5.37
1.99
1.77
a
In isopropanol.
4. Conclusion
b
In the studied solvents (chloroform, ethyl acetate, tetrahydrofuran, 1-butanol),
ꢀexc = 450 nm.
We have synthesized new water-soluble azo ligands contain-
ing a salicylaldimine group as a side chain to fabricate the DSSC.
Also, we have studied the optical and electrochemical properties
of the dyes to evaluate the photovoltaic performances. Steady-
state spectroscopic measurements were carried out in solvents
with increasing polarities. Aggregation tendency of the molecules
in water was responsible for an unusual form of the steady-state
measurements.
Chelating possibilities and steric factors change the favor-
able interaction of dyes with TiO2 surface which affects overall
photon-to-electric conversion efficiency (PCE) of DSSC. These pre-
liminary studies show that salicylaldimine-based azo ligands are
appropriate sensitizers for coordinating with TiO2 surface in DSSC
applications. New molecular modeling of azo dyes, which consist
of donor-linker-acceptor moieties, is necessary to improve DSSC
efficiency.
c
In water, ꢀexc = 450 nm, under O2 atmosphere.
teristic of the photovoltaic devices. Photovoltaic performance data
of the dyes are also given in Table 5. Open circuit voltage (Voc) and
short circuit photocurrent density (Isc) for azo dye 3 are 0.33 V and
0.80 mA/cm2, respectively, and those for azo dye 4 are 0.35 V and
0.65 mA/cm2. These data implies much better performance of azo
dye 3 with an overall conversion efficiency of 0.14% under illumi-
nation. The DSSC with azo dye 4 gives 0.52 fill factor yielding 0.12%
efficiency. Several commercial mono and bis-azo dyes were stud-
ied as sensitizers in DSSCs containing TiO2 photoanode and yielded
good performance with Voc. Especially, studied azo dyes contain-
ing salicylate chelating group were more strongly anchored to TiO2
surface and produced higher short circuit photocurrent greater
than 0.2 mA [43]. According to literature studies related to adsorp-
tion of azo dyes to TiO2 surface, it is reported that some phenyl
azo dyes functionalized with carboxyl and hydroxyl group were
easily adsorbed on TiO2 and ZnO electrodes and gave a strongly col-
ored electrodes [44]. Selected azo dyes containing salicylate groups
were also been used as sensitizers in Graetzel-type solar cells by
El Mekkawi and Abdel-Mottaleb [45]. Salicylate groups facilitate
the direct interaction of the dyes with TiO2 surface, thereby conve-
nient route to electron transfer from the LUMO level of dye to the
conduction band of semiconductor occurred.
Acknowledgements
This project was supported by the Research Council of Celal
Bayar University, the State Planning Organization of Turkey (DPT),
the Research Center of Ege University, and Alexander von Humboldt
Foundation of Germany.
References
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