A structural study of 1-phenyl-1,2,3,4-tetrahydroquinoline-based dyes for solid-state DSSC applications

Article abstract of DOI:10.1016/j.dyepig.2014.01.010

D-π-A architecture metal-free organic dyes, with a tetrahydroquinoline unit as electron donor, were designed and synthesized for solid-state dye-sensitized solar cells. The sensitizer series was designed to develop a structure-property relationship. These

Full text of DOI:10.1016/j.dyepig.2014.01.010

Dyes and Pigments 104 (2014) 211e219
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
A structural study of 1-phenyl-1,2,3,4-tetrahydroquinoline-based dyes
for solid-state DSSC applications
,
Kasparas Rakstys a, Joana Solovjova a, Tadas Malinauskas a, Ingmar Bruder b
**
,
,
Robert Send b, Algirdas Sackus a, Rüdiger Sens b, Vytautas Getautis a
*
a Department of Organic Chemistry, Kaunas University of Technology, Radvilenu pl. 19, LT-50254 Kaunas, Lithuania
b BASF SE, Carl-Bosch-Str. 38, Ludwigshafen 67056, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
D-p-A architecture metal-free organic dyes, with a tetrahydroquinoline unit as electron donor, were
Received 23 September 2013
Accepted 6 January 2014
Available online 19 January 2014
designed and synthesized for solid-state dye-sensitized solar cells. The sensitizer series was designed to
develop a structureeproperty relationship. These dyes are obtained from relatively cheap starting ma-
terials, without the use of expensive catalysts, rigorously anhydrous or oxygen-free conditions. The
highest solid-state device conversion efficiency (
h
) 3.3% (JSC ¼ ꢀ5.9 mA cmꢀ2, VOC ¼ 780 mV) and fill
Keywords:
Electro-optical material
Solar cell
Titanium dioxide
Tetrahydroquinoline
Hydrazone
factor FF ¼ 0.72 under 100 mW cmꢀ2 (AM 1.5G) solar irradiation was achieved with dye D4 employing a
hydrazone fragment as the spacer between the donor 3-alkoxy-1-phenyl-1,2,3,4-tetrahydroquinoline
and the rhodanine acceptor of the sensitizer.
Ó 2014 Elsevier Ltd. All rights reserved.
Organic sensitizer
1. Introduction
electrode. Due to insufficient conductivity of solid organic HTMs,
the ssDSSCs are currently restricted to using titania films <3 mm.
Dye-sensitized solar cells (DSSCs) are regarded as the coming
photovoltaics generation, and significant progress has been made
since their inception in 1991 [1,2]. DSSCs are fabricated from cheap,
easily processable materials, deriving their competitive perfor-
mance from judicious molecular design and control of
nanoarchitecture.
Ruthenium complexes as molecular sensitizers have shown
impressive solar-to-electric power conversion efficiencies (PCE) in
liquid electrolyte based devices, with the PCE reaching over 11%
under standard AM1.5G full sunlight [3e7]. However, the disad-
vantages of liquid electrolyte based DSSCs, such as solvent evapo-
ration and leakage, as well as corrosive and photochemical
properties of iodine are strong driving forces in the search for al-
ternatives. One of the possible solutions could be solid-state DSSCs
(ssDSSCs) [8,9].
Therefore, high molar extinction coefficient dyes are of great
importance for this type of device in order to allow the use of
thinner films.
In recent years, metal-free organic dyes have attracted
increasing attention as they do not contain any toxic or costly metal
and their properties are easily tuned by facile structural modifica-
tion. In addition, they generally have much higher extinction co-
efficients when compared to Ru(II) polypyridyls, making them
excellent for use in solid state DSSCs in combination with hole
transporting materials such as P3HT [10] or spiro-MeOTAD [11].
A number of organic dye sensitizers have so far been developed,
and the relationship between their chemical structures and
photovoltaic performances of DSSC’s based on the dyes has been
examined. Generally, metal-free organic dyes possess the evident
molecular structure on the donor and acceptor parts bridged by the
conjugated fragment [12]. A large variety of donors has been
investigated, including derivatives of indoline, coumarin, triaryl-
amine, heteroanthracene, carbazole, phenothiazine, N,N-
dialkylaniline, fluorine, and efficiencies of w10% with liquid elec-
trolytes [13e17] and up to 6% with solid HTM [18,19] have been
reached.
In ssDSSCs
a solid hole-transporting material (HTM) is
employed to replace the liquid redox electrolyte which is respon-
sible for the dye regeneration and hole transfer to the counter
* Corresponding author. Tel.: þ370 37 300196; fax: þ370 37 300152.
** Corresponding author. Tel.: þ49 621 60 22828.
E-mail addresses: ingmar.bruder@basf.com (I. Bruder), vytautas.getautis@ktu.lt
(V. Getautis).
Among other factors, the structure of the electron-donating
group has a crucial influence on the conversion efficiency of the
solar energy into electricity in DSSCs. To get a better understanding
0143-7208/$ e see front matter Ó 2014 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2014.01.010

212
K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
of the relationship between dye structure, photophysical and
photoelectrochemical properties, and performance of the DSSCs, a
were coated on Al plates pre-coated with w0.5
ylmethacrylate and methacrylic acid copolymer adhesive layer.
The thickness of the transporting material layer was 0.5e1 m.
mm thick meth-
series of different D-
p-A organic dyes (Fig. 1) has been designed,
m
synthesized, and investigated for the application in ssDSSCs. These
dyes have been constructed based on the electron-donating moi-
ety: 3-hydroxy-1-phenyl-1,2,3,4-tetrahydroquinoline. Tetrahy-
droquinoline donor is synthesized using simple and practical one-
pot reaction route [20,21].
Usually the photoemission experiments are carried out in vac-
uum and high vacuum is one of the main requirements for these
measurements. If vacuum is not high enough, the sample surface
oxidation and gas adsorption are influencing the measurement
results. In our case, however, the organic materials investigated
are stable enough to oxygen and the measurements may be
carried out in air.
2. Experimental details
The samples were illuminated with monochromatic light from
a quartz monochromator with deuterium lamp. The power of the
incident light beam was (2e5)$10ꢀ8 W. A negative voltage
of ꢀ300 V was supplied to the sample substrate. The counter-
electrode with a 4.5 ꢁ 15 mm2 slit for illumination was placed
at 8 mm distance from the sample surface. The counter-electrode
was connected to the input of the BK2-16 type electrometer,
working in the open input regime, for the photocurrent mea-
surement. A 10ꢀ15e10ꢀ12 A strong photocurrent was flowing in
the circuit under illumination. The photocurrent I is strongly
2.1. General
The purification of the reaction mixtures was performed by flash
chromatography using a glass column with silica gel (0.035e
0.070 nm, pore diameter ca. 6 nm). For thin layer chromatographic
(TLC) analysis, Merck precoated TLC plates (silica gel 60 F254) were
used. Melting points were determined on a Kleinfeld melting point
apparatus (Capillary Melting point Apparatus) and are uncorrected.
1H (300 MHz) and 13C NMR (75 MHz) NMR spectra were recorded
at room temperature with a Varian Unity Inova spectrometer, 1H
(400 MHz) and 13C NMR (100 MHz) spectra were recorded at room
temperature with a Bruker Avance III 400 spectrometer. The
chemical shifts, expressed in ppm, were relative to tetramethylsi-
lane (TMS). Infrared spectra were recorded on a Bruker Tensor 27
spectrometer using KBr pellets. UVeVis spectra were recorded on
Perkin Elmer Lambda 35 UV/Vis spectrometer. Fluorescence spectra
were recorded on a Perkin Elmer LS 55 spectrometer. Elemental
analysis (C, H, and N) was done using Exeter Analytical CE-440
elemental analyzer, Model 440 C/H/N/.
dependent on the incident light photon energy h
n
. The I0.5 ¼ f(h
n)
dependence was plotted. Usually the dependence of the photo-
current on incident light quanta energy is well described by
linear relationship between I0.5 and h
n
near the threshold. The
linear part of this dependence was extrapolated to the h axis and
n
the Ip value was determined as the photon energy at the inter-
ception point.
2.4. Computational details
All structures were optimized in the ground state at density
functional theory level using the BP86 functional [24,25]. The res-
olution of the identity approximation and def2-SV(P) double-zeta
basis sets with one set of polarization functions except on hydro-
gens were used in all calculations [26]. Vertical excitation energies
and oscillator strengths were obtained without geometry reop-
timization at the time-dependent density functional theory level
using the B3LYP functional [27,28]. Due to the charge transfer
character of the lowest excitation, excitation energies were also
calculated using the algebraic diagrammatic construction method
to approximate second order (ADC(2)) [29] in combination with the
resolution of the identity-approach [30] and a reduced virtual space
with a cutoff level of 50 eV (RVS-50) [31]. def2-TZVP-basis sets were
used in the RVS-50 ADC(2) calculations. To obtain the electron af-
finities and ionization potentials, the ground state structures were
reoptimized with charges þ1 and ꢀ1. Single point calculations at
BP86/def2-TZVP/COSMO level were done using the anion, cation
2.2. Cyclic-voltammetry measurements
The electrochemical studies were carried out by a three-
electrode assembly cell from Bio-Logic and a Bio-Logic SP-150
potentiostat-galvanostat. The measurements were carried out with
a glassy carbon electrode in dichloromethane solutions containing
0.1 M tetrabutylammonium hexafluorophosphate as electrolyte,
Ag/AgNO3 as the reference electrode and a Pt wire counter
electrode.
2.3. Ionization potential measurements
The ionization potential Ip of the layers of the synthesized
compounds was measured by the electron photoemission in air
method [22,23]. The samples for the ionization potential mea-
surement were prepared by dissolving materials in CHCl3 and
Fig. 1. Structures of 1-phenyl-1,2,3,4-tetrahydroquinoline-based dyes D1eD5.

K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
213
and neutral structures, to obtain electron affinities and ionization
potentials [32,33]. The TURBOMOLE program suite was used for all
calculations [34].
2.5.3. 3-Nonyloxy-1-phenyl-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde (5b)
Compound 5b was prepared similarly to 5a, except that 1-
bromononane (11.3 mL, 59.22 mmol) was used in the reaction.
Yield of 5b was 58.7% (4.4 g).
1H NMR (300 MHz, CDCl3;
d, ppm): 9.68 (s, 1H, CHO), 7.62e7.22
2.5. Synthesis of dyes
(m, 7H, Ar), 6.54 (d, J1 ¼ 8.5 Hz, 1H, 8-H of Ht), 4.02e3.91 (m, 1H,
CH), 3.81 (ddd, J1 ¼12.0 Hz, J2 ¼ 3.4 Hz, J3 ¼ 1.7 Hz, 1H, HA of NCH2),
3.62 (ddd, J1 ¼ 12.0 Hz, J2 ¼ 6.8 Hz, J3 ¼ 1.2 Hz, 1H, HB of NCH2),
3.55e3.42 (m, 2H, OCH2), 3.16 (dd, J1 ¼15.9 Hz, J2 ¼ 4.4 Hz,1H, HA of
CH2CH), 2.96 (dd, J1 ¼ 15.9 Hz, J2 ¼ 7.2 Hz, 1H, HB of CH2CH), 1.60e
1.42 (m, 2H, (OCH2CH2), 1.36e1.10 (m, 12H, ((CH2)6CH3), 0.84 (t,
J1 ¼ 6.9 Hz, 3H, CH3).
Reagents and solvents were purchased from commercial sup-
pliers and used without further purification. 3-Acetyl-1-phenyl-
1,2,3,4-tetrahydroquinoline-6-carbaldehyde (3) was synthesized as
previously reported [21].
Anal. calcd. for C25H33NO2: C, 79.11; H, 8.76; N, 3.69. Found, %: C,
78.82; H, 8.43; N, 3.35.
2.5.1. 3-Hydroxy-1-phenyl-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde (4)
Aldehyde 3 (22.1 g, 75 mmol) was dissolved in 20 ml of meth-
anol by heating, powdered 85% KOH (4.9 g, 75 mmol) was dissolved
in 10 ml of H2O and added to the mixture and it was refluxed for
35 min. At the end of the reaction half of the methanol was
removed and the mixture was extracted with ethyl acetate. The
organic layer was dried over anhydrous Na2SO4, filtered and ethyl
acetate was removed. The residue was dissolved in 40 ml of
methanol and crystals formed upon standing at the ꢀ5 ꢂC tem-
perature. The crystalline product was filtered off, washed with 2-
propanol and recrystallized from methanol. Yield: 13.8 g (72.8%);
m.p.: 198e200 ꢂC (methanol).
2.5.4. 1-Phenyl-3-propoxy-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde N-phenylhydrazone (6a)
5a (5.00 g,19.74 mmol) was dissolved in 2-propanol (25 mL) and
warmed to 85 ꢂC, phenylhydrazine (2.1 mL, 21.31 mmol) was added
and mixture stired for 3 h at 85 ꢂC. After the end of the reaction (TLC
control, n-hexane/diethyl ether, 3/2, v/v) the mixture was cooled
down, obtained crystals were filtered off and washed with a
mixture of n-hexane and 2-propanol (2/1, v/v) to give phenyl-
hydrazone 6a (6.2 g, 95.4%), which was used in the next reaction
without further purification.
1H NMR (400 MHz, CDCl3;
d, ppm): 7.65 (s, 1H, CH]N), 7.42e
1H NMR (300 MHz, CDCl3,
d, ppm): 9.61 (s, 1H, CHO), 7.55e7.52
7.36 (m, 4H, Ar), 7.29e7.25 (m, 4H, Ar), 7.22 (dd, J1 ¼ 8.6 Hz,
J2 ¼ 1.8 Hz, 1H, Ar), 7.15 (t, J1 ¼7.3 Hz, 1H, Ar), 7.02 (d, J1 ¼7.7 Hz, 2H,
Ar), 6.85 (t, J1 ¼ 7.3 Hz, 1H, Ar), 6.75 (d, J ¼ 8.5 Hz, 1H, 8-H of Het),
3.99e3.89 (m, 1H, CH), 3.82 (ddd, J1 ¼ 11.5, J2 ¼ 3.5 Hz, J3 ¼ 1.4 Hz,
HA of NCH2), 3.59e3.44 (m, 3H, HB of NCH2, OCH2), 3.18 (dd,
J1 ¼ 15.8 Hz, J2 ¼ 4.7 Hz, 1H, HA of CH2CH), 2.92 (dd, J1 ¼ 15.8 Hz,
J2 ¼ 8.0 Hz, 1H, HB of CH2CH), 1.68e1.54 (m, 2H, CH2), 0.92 (t,
J1 ¼ 7.4 Hz, 3H, CH3).
(split s, 1H, H-5 of Het), 7.47e7.22 (m, 6H, Ar), 6.52 (d, J ¼ 8.6 Hz,1H,
8-H of Het), 4.43e4.33 (m, 1H, CH of Het), 3.79 (ddd, J1 ¼ 12.1 Hz,
J2 ¼ 2.9 Hz, J3 ¼ 1.4 Hz, 1H, one H of NCH2), 3.62 (ddd, J1 ¼ 12.1 Hz,
J2 ¼ 5.5 Hz, J3 ¼ 1.7 Hz, 1H, other H of NCH2), 3.16 (dd, JAB ¼ 16.2 Hz,
JAx ¼ 4.1 Hz, 1H, HA of CH2Ar), 2.93 (dd, JAB ¼ 16.2 Hz, JBx ¼ 5.3 Hz,
1H, HB of CH2Ar), 2.87e2.78 (m, 1H, OH).
IR (KBr; n
, cmꢀ1): 3600e3200 (broad): 3036, 2896, 2666, 1653,
1604, 1555, 1491, 1214, 943, 822, 791, 768, 702.
Anal. calcd. for C16H15NO2: C, 75.87; H 5.97; N 5.53. Found: C,
75.78; H 5.65; N 5.64.
13C NMR (100 MHz, CDCl3;
d, ppm): 147.5, 145.6, 144.5, 138.3
(N]CH), 129.6, 129,3, 127.9, 125.9, 125.3, 124.8, 124.3, 122.3, 119.6,
115.2, 112.7, 71.4 (CH), 70.5 (OCH2), 54.1 (NeCH2), 34.4 (CH2), 23.3
(CH2), 10.72 (CH3).
IR (KBr; n
, cmꢀ1): 3271, 3056, 3028, 2962, 2863, 1593, 1493,
2.5.2. 1-Phenyl-3-propoxy-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde (5a)
1265, 1098, 1082, 746, 699, 693.
Aldehyde 4 (5.0 g, 19.74 mmol) was dissolved in 50 mL of DMF
and 1-brompropane (5.1 mL, 59.22 mmol) was added. Na2SO4
(1.12 g, 7.90 mmol) and freshly powdered 85% KOH (3.9 g,
59.22 mmol) were added to the reaction mixture in three portions
in 1 h intervals. The reaction mixture was heated at 60e65 ꢂC with
intensive stirring. After starting the aldehyde was consumed (TLC
control, eluent: acetone/n-hexane, 5/20, v/v) the reaction mixture
was cooled to room temperature and extracted with ethyl acetate.
The organic layer was dried over MgSO4, filtered off and concen-
trated under reduced pressure. The residue was purified by silica
gel column chromatography (eluent: acetone/n-hexane, 1/24, v/v).
Yield of 5a was 60.0% (3.5 g).
2.5.5. 3-Nonyloxy1-phenyl-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde N-phenylhydrazone (6b)
Hydrazone 6b was prepared similarly to 6a, except that 5b
(3.8 g, 10 mmol) and an equivalent amount of phenylhydrazine
(1.08 g, 10 mmol) were used in the reaction. After the end of the
reaction (TLC control, n-hexane/diethyl ether, 3/2, v/v) the organic
solvent was removed and the oil residue was used in the next re-
action without further purification.
2.5.6. 1-Phenyl-3-propoxy-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde N-(4-formyl)phenyl-N-phenylhydrazone (7a)
1H NMR (300 MHz, CDCl3;
d
, ppm): 9.70 (s, 1H, CHO), 7.60e7.23
To the solution of compound 6a (6.5 g, 16.86 mmol) in dry DMF
(40 mL) p-fluorobenzaldehyde (3.15 g, 25 mmol) and K2CO3 (7.0 g,
50 mmol) were added and stirred at 90 ꢂC under argon atmosphere
for 3 h while all starting material was consumed (TLC control,
acetone/n-hexane, 1/4, v/v). Then reaction mixture was quenched
with water and extracted with ethylacetate. The organic layers
were combined, washed with brine, dried over Na2SO4 and evap-
orated under reduced pressure. The crude product was purified by
(m, 7H, Ar), 6.55 (d, J1 ¼ 8.5 Hz, 1H, 8-H of Het), 4.01e3.92 (m, 1H,
CH), 3.80 (ddd, J1 ¼12.0 Hz, J2 ¼ 3.4 Hz, J3 ¼ 1.7 Hz, 1H, HA of NCH2),
3.63 (ddd, J1 ¼ 12.0 Hz, J2 ¼ 6.8 Hz, J3 ¼ 1.2 Hz, 1H, HB of NCH2),
3.56e3.43 (m, 2H, OCH2), 3.16 (dd, J1 ¼15.9 Hz, J2 ¼ 4.4 Hz,1H, HA of
CH2CH), 2.96 (dd, J1 ¼ 15.9 Hz, J2 ¼ 7.2 Hz, 1H, HB of CH2CH), 1.66e
1.52 (m, 2H, CH2), 0.90 (t, J1 ¼ 7.4 Hz, 3H, CH3).
IR (KBr; n
, cmꢀ1): 3062, 3038, 2962, 2935, 2875, 1680, 1607,
1589, 1561, 1507, 1493, 1103, 946, 914, 870, 769, 730, 700.
Anal. calcd. for C19H21NO2: C, 77.26; H, 7.17; N, 4.74. Found, %: C,
77.42; H, 7.23; N, 4.65.
silica
gel
column
chromatography
(eluent:
acetone/
tetrahydrofuran/n-hexane, 0.5/0.5/24, v/v/v) to afford 7a. The
yield was 42.4% (3.5 g).

214
K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
1H NMR (300 MHz, CDCl3;
d, ppm): 9.80 (s, 1H, CHO), 7.72 (d,
2.5.9. 2-[4-oxo-5-(1-phenyl-3-propoxy-1,2,3,4-tetrahydroquinolin-
6-methylene)-2-thioxo-thiazolidin-3-yl]acetic acid (D2)
J ¼ 9.0 Hz, 2H, Ar), 7.62 (t, J ¼ 7.4 Hz, 2H, Ar), 7.52 (t, J ¼ 7.4 Hz, 1H,
Ar), 7.41e7.31 (m, 3H, Ar), 7.28e7.09 (m, 9H, Ar), 6.68 (d, J ¼ 8.6 Hz,
1H, 8-H of Het), 3.98e3.86 (m, 1H, CH), 3.83e3.73 (m, 1H, HA of
NCH2), 3.61e3.39 (m, 3H, OCH2, HB of NCH2), 3.15 (dd, J1 ¼ 15.7 Hz,
J2 ¼ 4.6 Hz, 1H, HA of CH2CH), 2.89 (dd, J1 ¼ 15.9 Hz, J2 ¼ 7.9 Hz, 1H,
HB of CH2CH), 1.65e1.50 (m, 2H, CH2), 0.89 (t, J ¼ 7.4 Hz, 3H, CH3).
Aldehyde 5a (1.0 g, 3.38 mmol), rhodanine-3-acetic acid (0.75 g,
3.93 mmol) and ammonium acetate (0.08 g, 1.04 mmol) were
stirred at 100 ꢂC temperature in 2.5 mL of glacial acetic acid for
20 min. After the termination of the reaction (TLC, n-hexane/
toluene/methanol, 2/2/1, v/v/v) the reaction mixture was cooled to
room temperature. Obtained crystals were filtered off, washed with
water, then with the mixture of n-hexane/2-propanol (2/1, v/v). The
product was recrystallized from ethanol for three times. The yield is
57.0% (0.9 g). M.p.: 191e193 ꢂC (methanol).
13C NMR (75 MHz, CDCl3;
d, ppm): 190.9 (CHO), 152.4, 147.6,
145.2, 140.1, 138.8 (N]CH), 131.6, 131.2, 130.1, 129.7, 129.4, 128.8,
128.6, 125.9, 125.6, 125.1, 124.6, 122.2, 115.1, 114.1, 71.3 (CH), 70.6
(OCH2), 54.2 (NCH2), 34.4 (CH2), 23.4 (CH2), 10.8 (CH3).
IR (KBr;
n
, cmꢀ1): 2958, 2926, 2870,1688,1600,1588,1504,1494,
1H NMR (300 MHz, DMSO-d6;
d, ppm): 13.42 (s, 1H, COOH), 7.66
1384, 1302, 1221, 1157, 1100, 1061, 826, 699, 545.
Anal. calcd. for C32H31N3O2: C, 78.50; H, 6.38; N, 8.58; found: C,
78.41; H, 6.12; N, 8.32.
(s, 1H, CH¼), 7.51e7.46 (m, 2H, Ar, 5-H of Het), 7.32e7.27 (m, 4H,
Ar), 7.20 (d, J ¼ 8.8 Hz, 1H, 7-H of Het), 6.49 (d, J ¼ 8.8 Hz, 1H, 8-H of
Het), 4.70 (s, 2H, CH2COOH), 3.97 (br.s., 1H, CH), 3.85e3.75 (m, 1H,
HA of NCH2), 3.64 (dd, J1 ¼12.4 Hz, J2 ¼ 4.8 Hz, 1H, HB of NCH2), 3.43
(t, J ¼ 6.6 Hz, 2H, OCH2), 3.22e3.08 (m, 1H, HA of CH2CH), 2.90 (dd,
J1 ¼ 16.2 Hz, J2 ¼ 4.9 Hz, 1H, HB of CH2CH), 1.56e1.39 (m, 2H,
CH2CH3), 0.81 (t, J ¼ 7.4 Hz, 3H, CH3).
2.5.7. 3-Nonyloxy-1-phenyl-1,2,3,4-tetrahydroquinoline-6-
carbaldehyde N-(4-formyl)phenyl-N-phenylhydrazone (7b)
7b was prepared similarly to that of 7a, except that hydrazone
6b (2.3 g, 4.9 mmol) was used in the reaction. Yield of 7b was 57%
(4.4 g).
13C NMR (75 MHz, DMSO-d6;
d, ppm): 192.6 (CS), 167.4 (COOH),
166.3 (CO), 147.2, 145.5, 134.8, 133.5, 131.0, 130.0, 126.1, 121.6, 121.5,
114.3, 113.4, 69.2 (CH), 69.0 (OCH2), 53.5 (CH2N), 44.8 (CH2COOH),
32.7 (CH2CH), 22.1 (CH2CH3), 11.7 (CH3).
1H NMR (400 MHz, CDCl3;
d, ppm): 9.80 (s, 1H, CHO), 7.73 (d,
J ¼ 9.0 Hz, 2H, Ar), 7.68e7.08 (m, 15H, Ar), 6.69 (d, J ¼ 8.6 Hz, 1H, 8-
H of Het), 3.98e3.86 (m, 1H, CH), 3.83e3.73 (m, 1H, HA of NCH2),
3.61e3.39 (m, 3H, OCH2, HB of NCH2), 3.15 (dd, J1 ¼ 15.7 Hz,
J2 ¼ 4.6 Hz, 1H, HA of CH2CH), 2.89 (dd, J1 ¼ 15.9 Hz, J2 ¼ 7.9 Hz, 1H,
HB of CH2CH), 1.60e1.48 (m, 2H, OCH2CH2), 1.38e1.18 (m, 12H,
(CH2)6CH3), 0.87 (t, J ¼ 6.9 Hz, 3H, CH3).
IR (KBr;
n,
cmꢀ1): 3400e2200, 2961, 2933, 2875, 1730,
1705,1578, 1551, 1512, 1492, 1426, 1396, 1368, 1324,1305, 1277, 1251,
1229, 1197, 1111, 1056, 641, 698.
UV: lmax, nm (
*104): 472 (5.06), 322 (1.31), 206 (3.63).
3
Anal. calcd. for C24H24N2O4S2: C, 61.52; H, 5.16; N, 5.98. Found, %:
C, 61.27; H, 5.46; N, 6.32.
13C NMR (75 MHz, CDCl3;
d, ppm): 190.8 (CHO), 152.3, 147.3,
145.0, 139.8, 138.5 (N]CH), 131.4, 131.0, 130.0, 129.5, 129.2, 128.6,
128.3, 125.7, 125.4, 124.9, 124.4, 122.0, 114.9, 113.8, 71.1 (CH), 68.8
(OCH2), 53.9 (NeCH2), 34.1 (OCH2CH2), 31.9, 29.9, 29.5, 29.4, 29.2,
26.1, 22.6 (CH2), 14.1 (CH3).
2.5.10. 2-{4-oxo-5-[4-(1-phenyl-2-(1-phenyl-3-propoxy-1,2,3,4-
tetrahydroquinolin-6-methy-lene)hydrazinyl)benzylidene]-2-
thioxothiazolidin-3-yl}acetic acid (D3)
Aldehyde 7a (1.0 g, 2.04 mmol), rhodanine-3-acetic acid (0.5 g,
2.62 mmol) and ammonium acetate (0.05 g, 0.65 mmol) were
stirred at 100 ꢂC temperature in 2 mL of glacial acetic acid for 2 h.
After the termination of the reaction (TLC, acetone/n-hexane,1/4, v/
v) the reaction mixture was extracted with ethyl acetate, the
organic layer was dried over Na2SO4 and concentrated in vacuum.
The product was purified by silica gel flash chromatography
(eluent: toluene/methanol, 24/1, v/v). The yield of D3 as dark red
solid was 41.5% (0.5 g).
IR (KBr; n
, cmꢀ1): 2958, 2926, 2870,1688,1600,1588,1504,1494,
1384, 1302, 1221, 1157, 1100, 1061, 826, 699, 545.
Anal. calcd. for C32H31N3O2: C, 78.50; H, 6.38; N, 8.58; found: C,
78.41; H, 6.12; N, 8.32.
2.5.8. 2-[5-(3-hydroxy-1-phenyl-1,2,3,4-tetrahydroquinolin-6-
methylene)-4-oxo-2-thioxothia-zolidin-3-yl]acetic acid (D1)
Aldehyde 4 (1.0 g, 3.95 mmol), rhodanine-3-acetic acid (0.9 g,
4.74 mmol) and ammonium acetate (0.1 g, 1.33 mmol) were stirred
at 100 ꢂC temperature in 2.5 mL of glacial acetic acid for 20 min.
After the termination of the reaction, the obtained crystals were
filtrated off, washed with water, followed by the mixture of hex-
ane:propanol (1/1, v/v). The product was recrystallized from
ethanol for three times. The yield was 59.5% (1.0 g). M.p.: 218e
220 ꢂC (methanol).
1H NMR (300 MHz, CDCl3;
d, ppm): 7.82e7.05 (m, 15H, Ar), 6.94
(dd, J1 ¼8.5 Hz, J2 ¼ 2.1 Hz,1H, 7-H, Het), 6.76e6.68 (m,1H, Ar), 6.41
(d, J ¼ 8.9 Hz, 1H, 8-H, Het), 4.89 (s, 2H, CH2COOH), 4.04e3.89 (m,
1H, CH), 3.89e3.41 (m, 4H, OCH2, NCH2), 3.27e2.84 (m, 2H, CH2CH),
1.71e1.52 (m, 2H, CH2CH3), 1.03e0.83 (m, 3H, CH3).
IR (KBr;
1380, 1295, 1196, 1176, 1103, 1053, 1002, 822, 751, 697, 640, 524.
UV: lmax, nm (
*104): 469 (3.93), 373 (2.22), 324 (1.99), 206
(5.16).
n
, cmꢀ1): 3421, 3032, 2930, 2869, 1706, 1576, 1505, 1420,
1H NMR (300 MHz, DMSO-d6;
d, ppm): 7.65 (s, 1H, CH]), 7.51e
3
7.45 (m, 2H, Ar, 5-H of Het), 7.33e7.27 (m, 4H, Ar), 7.20 (dd,
J1 ¼8.8 Hz, J2 ¼ 2.1 Hz, 1H, 7-H of Het), 6.49 (d, J ¼ 8.8 Hz, 1H, 8-H of
Het), 4.66 (s, 2H, CH2COOH), 4.22e4.16 (m, 1H, CH), 3.75e3.70 (m,
1H, HA of NCH2), 3.49 (dd, J1 ¼12.0 Hz, J2 ¼ 5.8 Hz, 1H, HB of NCH2),
3.10e3.03 (m, 1H, HA of CH2CH), 2.79 (dd, J1 ¼ 15.9 Hz, J2 ¼ 6.6 Hz,
1H, HB of CH2CH).
Anal. calcd. for C37H34N4O4S2: C, 67.05; H, 5.17; N, 8.45. Found: C,
67.17; H, 5.31; N, 8.58.
2.5.11. 2-{4-oxo-5-[4-(1-phenyl-2-(3-nonyloxy-1-phenyl-1,2,3,4-
tetrahydroquinolin-6-methy-lene)hydrazinyl)benzylidene]-2-
thioxothiazolidin-3-yl}acetic acid (D4)
13C NMR (75 MHz, DMSO-d6;
d, ppm): 192.6 (CS), 167.4 (COOH),
166.4 (CO), 147.1, 145.5, 134.6, 133.7, 130.8, 129.9, 126.2, 121.8, 121.5,
114.3, 113.2, 61.3 (CH), 55.9 (CH2NCH), 45.2 (CH2COOH), 35.4
(CH2CH).
D4 was prepared similarly to D3, except that aldehyde 7b (1.5 g,
2.04 mmol) was used in the reaction. The product was isolated as
dark red solid. The yield was 46.1% (0.9 g).
IR (KBr;
1201, 1243, 1214, 1116, 1067, 926, 698.
UV: lmax, nm (
*104): 473 (3.59), 323(0.81), 206 (2.21).
n
, cmꢀ1): 3462, 2918, 2612, 1710, 1576, 1551, 1512, 1319,
1H NMR (400 MHz, CDCl3;
d, ppm): 7.86e6.80 (m, 16H, Ar),
6.68e6.62 (m, 1H, Ar), 6.50 (d, J ¼ 8.9 Hz, 1H, 8-H, Het), 4.65 (s, 2H,
CH2COOH), 3.95e2.65 (m, 7H, CH2CH, OCH2, NCH2), 1.60e1.42 (m,
2H, OCH2CH2), 1.28e1.08 (m, 12H, (CH2)6CH3), 0.92e0.81 (m, 3H,
CH3).
3
Anal. calcd. for C21H18N2O4S2: C, 59.14; H, 4.25; N, 6.57. Found, %:
C, 59.17; H, 4.46; N, 6.52.

K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
215
Scheme 1. Synthesis of the aldehyde 4.
IR (KBr;
n
, cmꢀ1): 3468, 3062, 3034, 2924, 2853, 1709, 1576,
Research Monochromator using additional white background light
1504, 1426, 1384, 1297, 1197, 1176, 1104, 1054, 1003, 823, 752, 698,
664, 526.
illumination.
UV: lmax, nm (
(5.15).
3
*104): 473 (3.83), 372 (2.30), 321 (2.06), 206
3. Results and discussion
Anal. calcd. for C43H46N4O4S2: C, 69.14; H,6.21; N, 7.50. Found: C,
68.87; H, 5.91; N, 7.18.
3.1. Synthesis and characterization
The synthetic strategy (Scheme 1) displays the chemical reac-
tion scheme that was used to synthesize the key compound 3-
hydroxy-1-phenyl-1,2,3,4-tetrahydroquinoline-6-carbal-dehyde
(4). 3-Hydroxy-1-phenyl-1,2,3,4-tetrahydroquinoline (1) was ob-
tained by a one-pot reaction of diphenylamine with epichlorohy-
drin [20,21]. In order to protect the hydroxyl group,1 was converted
to the acylated derivative 2. Vilsmeier-Haack formylation of 2, fol-
lowed by the deprotection of the hydroxyl group in 3 yielded the
desired aldehyde 4.
2.5.12. 2-Cyano-3-{4-[1-phenyl-2-(1-phenyl-3-propoxy-1,2,3,4-
tetrahydroquinolin-6-methy-lene)hydrazinyl]phenyl}acrylic acid
(D5)
Aldehyde 7a (0.4 g, 0.8 mmol), cianoacetic acid (0.085 g,
1.0 mmol) and ammonium acetate (0.02 g, 0.25 mmol) were stirred
at 100 ꢂC temperature in 2 mL of glacial acetic acid for 6 h. After the
termination of the reaction (TLC, acetone/n-hexane, v/v 1/4) the
reaction mixture was extracted with ethyl acetate, the organic layer
was dried over Na2SO4 and concentrated in vacuum. The product
was purified by flash chromatography (eluent: toluene/methanol,
24/1, v/v). The yield was 66.7% (0.3 g).
The synthesis procedure of the dyes D1, D2 possessing a
pe
conjugated methine unit is depicted in Scheme 2. Condensation of
the respective aldehyde 4 with rhodanine-3-acetic acid by the
Knoevenagel reaction in the presence of ammonium acetate gave
the desired dye D1 in moderate yield. In order to improve its sol-
ubility in organic solvents and reduce the tendency to aggregate,
propylation of the hydroxyl group of 4 was carried out. The
resulting alkylated compound 5a with substituted hydroxyl group
was used in condensation reaction with rhodanine-3-acetic acid by
the Knoevenagel reaction in the presence of ammonium acetate to
yield D2.
1H NMR (400 MHz, CDCl3;
d, ppm): 7.88e7.11 (m, 18H, Ar, N]
CH), 6.80e6.67 (m, 1H, Ar), 3.98e3.69 (m, 2H, CH, HA of NCH2),
3.60e3.30 (m, 3H, OCH2, HB of NCH2), 3.21e3.01 (m, 1H, HA of
CH2CH), 2.99e2.37 (m, 1H, HB of CH2CH),1.66e1.45 (m, 2H, CH2),
0.99e0.72 (m, 3H, CH3).
IR (KBr;
1590, 1504, 1382, 1316, 1302, 1222, 1177, 1064, 830, 699, 515.
n
, cmꢀ1): 3426, 3059, 3035, 2959, 2929, 2872, 2214,
UV: lmax, nm (
*104): 425 (3.95), 357 (1.83), 3.27 (1.60), 244
3
(1.36), 206 (4.23).
Anal. calcd. for C35H32N4O3: C, 75.52; H, 5.79; N, 10.06. Found: C,
75.22; H, 5.51; N, 9.78.
Most of the organic sensitizers applied in DSSCs follow the
characteristical pattern of donor and acceptor fragments being
linked by a
p-conjugation bridge, so called D-p-A structure.
Recently it was shown that a bathochromic shift and broadening of
the absorption range as well as enhanced absorptivities can be
obtained by incorporating a hydrazone fragment as p-conjugation
bridge [36]. In the current research we have decided to test a
similar approach utilizing hydrazone moieties to form D-hydra-
zone-A configuration sensitizers. Synthesis of the dyes D3eD5
2.6. Fabrication and characterization of the solid-state dye-
sensitized solar-cells
A TiO2 blocking layer was prepared on a fluorine-doped tin
oxide (FTO)-covered glass substrate using spray pyrolysis [35]. A
TiO2 paste (Dyesol), diluted with terpineol, was applied by screen
possessing
a phenylhydrazone moiety and alkoxy groups of
different chain length attached to the 1-phenyl-1,2,3,4-
tetrahydroquinoline donoric core is shown in Scheme 2. Conden-
sation of the aldehydes 5a and 5b with phenylhydrazine afforded
hydrazones 6a and 6b in quantitative yields. Subsequently, com-
pounds 6a and 6b were used in arylation reaction with 4-
fluorobenzaldehyde to obtain hydrazones 7a, 7b. Finally, conden-
sation of 7a and 7b with rhodanine-3-acetic acid or cyanoacetic
acid by the Knoevenagel reaction in the presence of ammonium
acetate yielded the dyes D3eD5.
printing, resulting in a film thickness of 1.7 mm. All films were then
sintered for 45 min at 450 ꢂC, followed by treatment in a 40 mM
aqueous solution of TiCl4 at 60 ꢂC for 30 min, followed by another
sintering step. The prepared samples with TiO2 layers were pre-
treated with
5 mM solutions of 2-(p-butoxyphenyl)acetohy-
droxamic acid sodium salt or 2-(p-butoxyphenyl)acetohydroxamic
acid tetrabutylammonium salt in ethanol. The electrodes were then
dyed in 0.5 mM dye solution in CH2Cl2. Spiro-MeOTAD was applied
by spin-coating from a solution in DCM (200 mg/mL) also con-
taining 20 mM Li(CF3SO2)2N. Fabrication of the device was
completed by evaporation of 200 nm of silver as the counter elec-
trode. The active area of the ssDSSC was defined by the size of these
contacts (0.13 cm2), and the cells were masked by an aperture of the
same area for measurements. The Currentevoltage characteristics
for all cells were measured with a Keithley 2400 under 1000 W/m2,
AM 1.5G conditions (LOT ORIEL 450 W). The incident photon to
current conversion efficiencies (IPCE) were obtained with an Acton
3.2. Optical, electrochemical and photophysical properties
The absorption, emission, and electrochemical properties of the
dye series are listed in Table 1. The UVeVis spectra of the dyes D1
and D2 show absorption bands appearing at 250e360 nm, which
are assigned as the
pe
p* transitions of the conjugated system
(Fig. 2). When phenylhydrazone is used as
p
-conjugation bridge,

216
K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
Scheme 2. Synthesis route to 1-phenyl-1,2,3,4-tetrahydroquinoline-based dyes D1eD5.
there are two absorption bands in this region, one at 322 nm and
another one at 374 nm for the dyes D3 and D4. In case of sensitizer
D5, the second band is hypsochromically shifted by w20 nm. All of
the synthesized dyes, except for D5, have absorption maxima at
around 470 nm originating from intramolecular charge transfer
(ICT) from the tetrahydroquinoline donor moiety to the rhodanine
acceptor. Comparing structurally similar dyes D3 and D5 bearing
different electron accepting units (rhodanine or cyanoacetic acid),
Table 1
Table 2
Optical characteristics, HOMO, LUMO, Ip and band gap energies for D1eD5a.
Quantum chemical calculation results.
F1e
S2
F2g
S1
F1i
abs
abs
Compound
l
3
l
FLmax EHOMO ELUMO EgCV
(nm) (V vs (V vs
Ip
(eV)c (eV)d
Molecule IP
(eV)a (eV)b (eV)c (nm)d
IP*
EA
S1
max
max
(nm)b (Mꢀ1 cmꢀ1
)
(nm)
(nm)f
(nm)h
(on TiO2)
NHE) NHE)
D1
D2
D3
D4
D5
5.69
5.63
5.18
5.18
5.26
2.94
2.86
2.61
2.61
2.58
3.00
2.71
3.00
3.00
2.64
450
447
483
484
463
0.85 386
0.85 388
1.23 383
1.24 384
1.14 347
0.00 435
0.00 436
0.00 508
0.00 508
0.38 477
0.83
0.17
0.98
0.99
0.88
D1
D2
D3
D4
D5
spiro
473
472
469
473
425
387
35890
50590
39260
38290
39520
69600
472
450
480
485
412
e
543 1.15
540 1.16
627 0.75
630 0.74
531 0.73
ꢀ1.26 2.41 5.54
ꢀ1.28 2.44 5.39
ꢀ1.32 2.07 5.15
ꢀ1.29 2.03 5.18
ꢀ1.82 2.55 5.11
ꢀ2.33e 2.95f 5.00
a
Ionization potential.
Excited state ionization potential (RVS-50 RI-ADC2/def2-TZVP).
Electron affinity.
e
0.62
b
c
a
CV measurements were carried out at a glassy carbon electrode in dichloro-
methane solutions containing 0.1 M tetrabutylammonium hexafluorophosphate as
electrolyte and Ag/AgNO3 as the reference electrode. Each measurement was cali-
brated with ferrocene (Fc). Potentials measured vs Fcþ/Fc were converted to normal
hydrogen electrode (NHE) by addition of þ0.63 V.
d
First and second excited states with oscillator strengths (RVS-50 RI-ADC2/def2-
TZVP).
e
First and second excited states with oscillator strengths (RVS-50 RI-ADC2/def2-
TZVP).
UVeVis spectra measured in 10ꢀ4 M THF solution.
First and second excited states with oscillator strengths (RVS-50 RI-ADC2/def2-
TZVP).
b
f
c
ECgV ¼ EHOMOꢀELUMO
.
d
g
Ionization potential (Ip) was measured by the photoemission in air method from
First and second excited states with oscillator strengths (RVS-50 RI-ADC2/def2-
TZVP).
films.
e
h
ELUMO ¼ EHOMOꢀEgopt
.
First excited state with oscillator strength (TDDFT/B3LYP/def2-SV(P)).
First excited state with oscillator strength (TDDFT/B3LYP/def2-SV(P)).
f
i
Optical band gap Eogpt estimated from the edge of electronic absorption spectra.

K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
217
Fig. 2. UV/Vis and fluorescence spectra of 1-phenyl-1,2,3,4-tetrahydroquinoline-based
dyes D1eD5.
Fig. 4. Photoemission in air spectra of the investigated dyes D1eD5.
the introduction of the latter contributed to the hypsochromic shift
of the absorption spectra by w40 nm. Obviously, rhodanine-3-
acetic acid has stronger electron-withdrawing ability than cyano-
acetic acid. It is also interesting to note that the introduction of the
shifted when the hydrazone unit was introduced as
peconju-
gated bridge into the structures of the sensitizers D3, D4
(FLmax z 630 nm), compared with dyes D1, D2 (FLmax z 540 nm).
Change of the rhodanine-based acceptor with weaker cyanoacetic
acid in D5 also led to the noticeable blue shift of the emission
maximum (FLmax z 530 nm). Dyes D3eD5 exhibited relatively
large Stokes shifts, which could be attributed to the geometrically
relaxed structure of the donor center upon excitation [37].
The absorption spectra of D1, D2 on a TiO2 electrode are broader
and absorption maxima are hypsochromically shifted, compared to
results obtained in the THF solutions (Fig. 3), signifying that energy
levels of the sensitizer molecules have somewhat changed due to
the interaction with TiO2. It could also indicate that these dye
molecules form H-aggregates on the TiO2 surface. Under similar
conditions, the sensitizers D3 and D4 have a slight red shift of
w10 nm after being adsorbed on the TiO2 electrode, indicating
different type of packing of the hydrazone containing dyes
hydrazone moiety as
a p-conjugation bridge between the
tetrahydroquinoline-based donor and the rhodanine-based
acceptor in sensitizers D3, D4, provided no significant shift in the
ICT band compared with D1, D2, implying that this unit doesn’t lead
to better coplanarity than that achieved in the case of dyes D1 and
D2. However, ICT bands are broader and the onset of the absorption
spectra of D3 and D4 reaches 542 nm which is red-shifted by about
16 nm relative to that of dyes D1 and D2.
D1eD5 exhibit high extinction coefficients (Table 1), for
example
times and
3
of D2 (5.1 ꢁ 104 Mꢀ1 cmꢀ1 at 472 nm) is more than four
3
of D3 (3.9 ꢁ 104 Mꢀ1 cmꢀ1 at 469 nm) is more than
three times higher than that of the standard Z907 ruthenium
sensitizer (
especially suitable for solid-state devices, where the film thickness
is crucial.
3
¼ 1.2 ꢁ 104 Mꢀ1 cmꢀ1 at 521 nm) [18]. This makes them
compared with sensitizers without p-conjugated bridge D1 and D2.
The EHOMO and ELUMO energies of D1eD5 were measured
employing the cyclic voltammetry (CV) technique (Table 1). These
values do not represent any absolute solid-state or gas-phase
ionization energies, but can be used to compare different com-
pounds relative to one another. The cyclic voltammograms of all
synthesized compounds show quasi-reversible oxidation and
reduction couples. The energy offset of the ELUMO of the dye D1eD4
molecules is ca. ꢀ1.3 V vs normal hydrogen electrode (NHE), and for
the D5 it’s ꢀ1.8 V vs NHE. These ELUMO energy levels, with respect to
the titania conduction band edge ꢀ0.5 vs NHE [38,39], provides the
thermodynamic driving force for charge injection. The energy
offset of the EHOMO ca. 1.15 V for D1, D2 and ca. 0.75 V for D3eD5
relative to that of the spiro-MeOTAD (0.62 V, measured using CV)
presents enough driving force for the dye regeneration process. The
differences in the CV measurement results for the similar structures
are quite small. Addition of the hydrazone fragment in the struc-
tures D3eD5 lowers the EHOMO by ca. 0.4 V. Substitution of the
rhodanine acceptor with a weaker cyanoacetic acid in D5 lowers
the ELUMO by ca. 0.5 V. These variations in energy levels influence
the size of the bandgap. Eg is larger in the structures without
From the fluorescence spectra of the dyes it could be seen that
the maximum emission wavelengths in THF solution were red-
hydrazone p-bridge (D1, D2) or with weaker acceptor unit (D5).
When considering the use of an organic material for optoelec-
tronic applications it is important to have an understanding of its
solid state ionization potential (Ip). This understanding can help in
identifying suitable organic transport and inorganic electrode
Fig. 3. UVeVis spectra of the dyes D1eD5 absorbed on TiO2 surface.

218
K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
Table 3
containing dye D3 and nonyl one D4 is just 0.03 eV. The smaller
electron accepting unit in D5 allows for a different, perhaps tighter,
packing of the molecules. Results of these changes could be seen in
Ip measurements, D5 has the lowest ionization potential of all
investigated materials. The ionization potential of spiro-MeOTAD
was determined to be 5.0 eV, therefore, there is enough driving
force for the dye regeneration process.
Photovoltaic performance of ssDSSCs based on the D1eD5 dyes.
Dye
JSC [mA cmꢀ2
]
VOC [mV]
FF [%]
h [%]
D1
D2
D3
D4
D5
4.52
4.75
6.08
5.87
4.55
460
720
780
780
660
44
71
68
72
58
0.9
2.4
3.2
3.3
1.8
3.3. Quantum chemistry calculations
materials. The ionization potential was measured by the electron
photoemission in air method (Fig. 4) and results are presented in
Table 1.
The distinct absorption peaks in the solution spectrum of D1e
D4 are described by the RVS RI-ADC(2)-calculations as a mixed pie
pi*/charge-transfer excitation. The second excited states of D1-D4
are dark, locally excited states in the near UV region. In D5, the
absorption splits up into a charge-transfer absorption (first excited
state) and a piepi*-absorption (second excited state). This is re-
flected also in the different spectral appearance of D5. Furthermore,
this explains the systematic shifts from calculation results for ab-
sorption wavelengths of D1eD4, but the overestimation of the
absorption wavelength for D5. Calculated ionization potentials
reproduce the experimental values up to a systematic shift (see
Table 2).
As with the CV experiments, solid state Ip values for the dyes D1,
D2 without hydrazone bridge are noticeably higher compared with
D3eD5 that do possess it. The effect of aliphatic chains is much
more pronounced in solid state than in solution for small molecules
of D1, D2 as they influence the way they pack and therefore have
significant impact on energy levels. D1 without any additional
aliphatic fragments has the highest Ip among the investigated
materials. Additional propyl chain in D2 lowers the Ip by 0.15 eV. In
case of larger and somewhat more complex dyes D3eD5 these
effects are far less pronounced, difference between propyl
3.4. Photovoltaic performance
Owning to their high absorption extinction coefficient,
tetrahydroquinoline-based dyes D1eD5 were evaluated as sensi-
tizers for the solid-state DSSCs using spiro-MeOTAD as HTM
(photovoltaic performance is summarized in Table 3).
Fig. 5 shows the incident photon-to-current conversion effi-
ciency (IPCE) spectra and the JeV characteristics under standard
global AM 1.5 solar conditions of D1-D5 based ssDSSCs. Dyes D3
and D4 possessing hydrazone demonstrate respectable external
quantum efficiencies of around 65%. Sensitizers without hydrazone
p-conjugated bridge D1, D2 demonstrate more modest results, 30%
and 50% respectively. Interestingly, compound D5 with cyanoacetic
acid acceptor exhibited very good external quantum efficiency,
reaching 75% in the short wavelength region. Unfortunately, due to
relatively weak electron accepting capabilities, compared with the
rhodanine-based acceptor, absorbance of the dye D5 in the visible
light region is very limited and overall device efficiency is only 1.8%.
Presence of an unsubstituted hydroxy group in D1 also has pro-
nounced negative effect on solar cell performance. Simple alkyl-
ation of the OH group in the sensitizer D2 increases efficiency more
than twofold, from 0.9% to 2.4%. As expected, addition of the
hydrazone p-conjugated bridge in D3 and D4 improves the ssDSSC
performance, compared with D2 it increases by approximately 35%.
Interestingly, the length of the aliphatic chain, attached to the
donor end of the molecule, wasn’t a major factor. Both sensitizers,
D3 with propyl and D4 with nonyl chain, performed similarly. The
best results were obtained using dye D4, showing overall conver-
sion efficiencies (
FF ¼ 0.72).
h
) of 3.3% (JSC ¼ 5.9 mA cmꢀ2, VOC ¼ 780 mV,
4. Conclusions
In conclusion, we have demonstrated the design and synthesis
pathway of D- -A metal-free organic dye for ssDSSCs, employing a
p
1-phenyl-1,2,3,4-tetrahydro-quinoline fragment electron donating
moiety. These dyes could be obtained using relatively cheap start-
ing materials and simple synthetic procedures, without the use of
expensive catalysts, rigorously anhydrous or oxygen-free condi-
tions. Compared to the unalkylated dye D1 and the structure
Fig. 5. a) IPCE spectra of D1eD5 dye-sensitized devices and b) Currentevoltage
characteristics of ssDSSCs based on D1eD5 under 100 mW cmꢀ2
,
AM 1.5G
without hydrazone
p-bridge (D2), the more complex dyes pos-
illumination.
sessing phenylhydrazone moiety and alkoxy group attached to 1-

K. Rakstys et al. / Dyes and Pigments 104 (2014) 211e219
219
[17] Zeng W, Cao Y, Bai Y, Wang Y, Shi Y, Zhang M, et al. Efficient dye-sensitized
solar cells with an organic photosensitizer featuring orderly conjugated eth-
ylenedioxythiophene and dithienosilole blocks. Chem Mater 2010;22:1915e
25.
phenyl-1,2,3,4-tetrahydroquinoline donoric core (D3, D4) exibit
higher VOC and ISC, which results in the overall conversion efficiency
over 3% under 100 mW cmꢀ2 solar irradiation. The proposed
tetrahydroquinoline-based dyes pave the way for future structural
design of new efficient and cheap metal-free sensitizers for use in
thin film devices.
[18] Cai N, Moon SJ, Cevey-Ha L, Moehl T, Humphry-Baker R, Wang P, et al. An
organic d-p-a dye for record efficiency solid-state sensitized heterojunction
solar cells. Nano Lett 2011;11:1452e6.
[19] Liu X, Zhang W, Uchida S, Cai L, Liu B, Ramakrishna S. An efficient organic-dye-
sensitized
ethylenedioxythiophene) as
solar
cell
with
a
in
situ
polymerized
poly(3,4-
hole-transporting material. Adv Mater
Acknowledgments
2010;22:E150e5.
[20] Vorozhtsov NN, Kutkevichus SI. Study of products of interaction of epichlo-
rohydrin with aromatic amines. Khim Geterotsikl 1965:374e8.
[21] Getautis V, Stanisauskaite A, Malinauskas T, Stumbraite J, Gaidelis V,
Jankauskas V. Hydrazones possessing a phenyl-1,2,3,4-tetrahydroquinoline
moiety as hole transporting materials. Monatsh Chem 2006;137:1401e9.
[22] Miyamoto E, Yamaguchi Y, Yokoyama M. Ionization potential of organic
pigment film by atmospheric photoelectron emission analysis. Electropho-
tography 1989;28:364e70.
[23] Daskeviciene M, Getautis V, Grazulevicius VJ, Stanisauskaite A, Antulis J,
Gaidelis V, et al. Crosslinkable carbazolyl-containing molecular glasses for
electrophotography. J Imaging Sci Technol 2002;46:467e72.
[24] Becke AD. Density-functional exchange-energy approximation with correct
asymptotic behavior. Phys Rev A 1988;33:3098e100.
[25] Perdew JP. Density-functional approximation for the correlation energy of the
inhomogeneous electron gas. Phys Rev B 1986;33:8822e4.
[26] Weigend F, Ahlrichs R. Balanced basis sets of split valence, triple zeta valence
and quadruple zeta valence quality for H to Rn: design and assessment of
accuracy. Phys Chem Chem Phys 2005;7:3297e305.
[27] Marques MAL, Ullrich CA, Noguiera F, Rubio A, Burke K, Gross EKU. Time-
dependent density functional theory: lecture notes in physics. Berlin:
Springer; 2006.
[28] Becke AD. Density-functional thermochemistry. III. The role of exact ex-
change. J Chem Phys 1993;98:5648e52.
[29] Schirmer J. Beyond the random-phase approximation: a new approximation
scheme for the polarization propagator. Phys Rev A 1982;26:2395e416.
[30] Hättig C. Structure optimizations for excited states with correlated second-
order methods: CC2 and ADC(2). Adv Quant Chem 2005;50:37e60.
[31] Send R, Kaila VRI, Sundholm D. Reduction of the virtual space for coupled-
cluster excitation energies of large molecules and embedded systems.
J Chem Phys 2011;134:214114.
[32] Eichkorn K, Weigend F, Treutler O, Ahlrichs R. Auxiliary basis sets for main
row atoms and transition metals and their use to approximate coulomb po-
tentials. Theor Chem Acc 1997;97:119e24.
[33] Schaefer A, Klamt A, Sattel D, Lohrenz JCW, Eckert F. COSMO implementation
in TURBOMOLE: extension of an efficient quantum chemical code towards
liquid systems. Phys Chem Chem Phys 2000;2:2187e93.
[34] TURBOMOLE 6.2. Karlsruhe, Germany: TURBOMOLE GmbH; 2010 [accessed
July 2013], http://www.turbomole.com.
This research was supported by the European Social Fund under
the Global Grant measure (VP1-3.1-SMM-07-K 01-078) and BASF
SE. J. Solovjova acknowledges EU Structural Funds project “ Post-
doctoral Fellowship Implementation in Lithuania” for funding her
postdoctoral fellowship. We gratefully acknowledge habil. dr. V.
Gaidelis for the help with ionization potential measurements.
ꢀ
References
[1] O’Regan B, Grätzel M. A low-cost, high-efficiency solar cell based on dye-
sensitized colloidal TiO2 films. Nature 1991;353:737e40.
[2] Grätzel M. Photoelectrochemical cells. Nature 2001;414:338e44.
[3] Nazeeruddin MK, De Angelis F, Fantacci S, Selloni A, Viscardi G, Liska P, et al.
Combined experimental and DFT-TDDFT computational study of photo-
electrochemical cell ruthenium sensitizers. J Am Chem Soc 2005;127:16835e40.
[4] Chiba Y, Islam A, Watanabe Y, Komiya R, Koide N, Han LY. Dye-sensitized solar
cells with conversion efficiency of 11.1%. Jpn J Appl Phys 2006;45:L638e40.
[5] Gao F, Wang Y, Shi D, Zhang J, Wang MK, Jing XY, et al. Enhance the optical
absorptivity of nanocrystalline TiO2 film with high molar extinction coeffi-
cient ruthenium sensitizers for high performance dye-sensitized solar cells.
J Am Chem Soc 2008;130:10720e8.
[6] Cao YM, Bai Y, Yu QJ, Cheng YM, Liu S, Shi D, et al. Dye-sensitized solar cells
with a high absorptivity ruthenium sensitizer featuring a 2-(Hexylthio)thio-
phene conjugated bipyridine. J Phys Chem C 2009;113:6290e7.
[7] Chen CY, Wang M, Li JY, Pootrakulchote N, Alibabaei L, Ngoc-le CH, et al.
Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-
sensitized solar cells. ACS Nano 2009;3:3103e9.
[8] Snaith HJ, Schmidt-Mende L. Advances in liquid-electrolyte and solid-state
dye-sensitized solar cells. Adv Mater 2007;19:3187e200.
[9] Yum JH, Chen P, Grätzel M, Nazeeruddin MK. Recent developments in solid-
state dye-sensitized solar cells. ChemSusChem 2008;1:699e707.
[10] Mor GK, Kim S, Paulose M, Varghese OK, Shankar K, Basham J, et al. Visible to
near-infrared light harvesting in TiO2 nanotube arrayꢀP3HT based hetero-
junction solar cells. Nano Lett 2009;9:4250e7.
[11] Snaith HJ, Moule AJ, Klein C, Meerholz K, Friend RH, Grätzel M. Efficiency
enhancements in solid-state hybrid solar cells via reduced charge recombi-
nation and increased light capture. Nano Lett 2007;7:3372e6.
[35] Peng B, Jungmann G, Jager C, Haarer D, Schmidt HW, Thelakkat M. Systematic
investigation of the role of compact TiO2 layer in solid state dye-sensitized
TiO2 solar cells. Coord Chem Rev 2004;248:1479e89.
[12] Oayama Y, Harima Y. Molecular designs and syntheses of organic dyes for
dye-sensitized cells. Eur J Org Chem 2009:2903e34.
[13] Hara K, Sato T, Katoh R, Furube A, Ohga Y, Shinpo A, et al. Molecular design of
[36] Urnikaite S, Malinauskas T, Gaidelis V, Bruder I, Send R, Sens R, et al. Simple
and inexpensive organic dyes with hydrazone moiety as p-conjugation bridge
for solid-state dye-sensitized solar cells. Chem Asian J 2013;8:538e41.
[37] Lai RY, Fabrizio EF, Lu L, Jenekhe SA, Bard AJ. Synthesis, cyclic voltammetric
studies, and electrogenerated chemiluminescence of a new donor-acceptor
coumarin dyes for efficient dye-sensitized solar cells.
2003;107:597e606.
J Phys Chem B
[14] Horiuchi T, Miura H, Sumioka K, Uchida S. High efficiency of dye-sensitized
solar cells based on metal-free indoline dyes. J Am Chem Soc 2004;126:
12218e9.
[15] Kim S, Lee JK, Kang SO, Ko J, Yum JH, Fantacci S, et al. Molecular engineering of
organic sensitizers for solar cell applications. J Am Chem Soc 2006;128:
16701e7.
[16] Hagberg DP, Edvinsson T, Marinado T, Boschloo G, Hagfeldt A, Sun LC. A novel
organic chromophore for dye-sensitized nanostructured solar cells. Chem
Commun 2006:2245e7.
Molecule: 3,7-[Bis[4-phenyl-2-quinolyl]]-10-methylphenothiazine.
J Am
Chem Soc 2001;123:9112e8.
[38] Jiang X, Karlsson KM, Gabrielsson E, Johansson EMJ, Quintana M, Karlsson M,
et al. Highly efficient solid-state dye-sensitized solar cells based on triphe-
nylamine dyes. Adv Funct Mater 2011;21:2944e52.
[39] Delcamp JH, Shi Y, Yum JH, Sajoto T, Dell’Orto E, Barlow S, et al. Chem Eur J
2013;19:1819e27.