Microwave assisted synthesis of a series of charge-transfer photosensitizers having quinoxaline-2(1H)-one as anchoring group onto TiO2 surface

Article abstract of DOI:10.1016/j.molstruc.2016.12.021

In this work, we present the synthesis of novel donor-acceptor compounds based on 3-methylquinoxaline-2(1H)one which follow an easy synthetic route, involving Knoevenagel reaction with electron-donor groups such as N,N-dimethylaminobenzene, ferrocene, tri

Full text of DOI:10.1016/j.molstruc.2016.12.021

Accepted Manuscript
Microwave assisted synthesis of a series of charge-transfer photosensitizers having
quinoxaline-2(1H)-one as anchoring group onto TiO surface
2
Mauricio Caicedo, Carlos A. Echeverry, Robson R. Guimarães, Alejandro Ortiz, Koiti
Araki, Braulio Insuasty
PII:
S0022-2860(16)31311-4
10.1016/j.molstruc.2016.12.021
MOLSTR 23219
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 18 October 2016
Revised Date: 5 December 2016
Accepted Date: 6 December 2016
Please cite this article as: M. Caicedo, C.A. Echeverry, R.R. Guimarães, A. Ortiz, K. Araki, B. Insuasty,
Microwave assisted synthesis of a series of charge-transfer photosensitizers having quinoxaline-2(1H)-
one as anchoring group onto TiO surface, Journal of Molecular Structure (2017), doi: 10.1016/
2
j.molstruc.2016.12.021.
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ACCEPTED MANUSCRIPT
Microwave assisted synthesis of a series of charge-transfer photosensitizers having
Quinoxaline-2(1H)-one as anchoring group onto TiO2 surface
Mauricio Caicedo1,2, Carlos A. Echeverry1, Robson R. Guimarães3, Alejandro Ortiz1,2, Koiti
Araki3* and Braulio Insuasty1,2*
1
Heterocyclic Compounds Research Group, Department of Chemistry, Universidad del Valle, A.A.
25360, Cali, Colombia.
2 Centre for Bioinformatics and Photonics-CIBioFI, Calle 13 No. 100-00, Edificio 320, No. 1069, Cali,
Colombia.
3 Laboratory of Supramolecular Chemistry and Nanotechnology, Institute of Chemistry, University
of Sao Paulo, Av. Lineu Prestes 748, Butanta, 05508-000, Sao Paulo, SP, Brazil.
Correspondence: Dr. Prof. Braulio Insuasty, Faculty of Science, Department of Chemistry,
Universidad del Valle, Santiago de Cali, Colombia.
E-mail: braulio.insuasty@correounivalle.edu.co
FAX: +57-2-3339240
Keywords: Quinoxaline, electron-transfer, anchor group, Knoevenagel reaction.
HIGHLIGHTS
•
Efficient preparation of push-pull dyes having quinoxaline-2(1H)-one as anchoring group
by microwave irradiation method.
•
•
Ferrocene group acts as a quencher inhibiting photoinduced electron injection onto TiO2.
The quinoxaline-2(1H)-one was linked to several electro-donor fragments through
Knoevenagel reaction.
•
•
Calculated electronic properties were consistent with the experimental data, confirming the
feasibility of photoinduced electrons in TiO2.
The efficiency of push-pull compounds depend on the molecular structure and the nature
of the donor-groups

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GRAFICAL ABSTRACT
Microwave assisted synthesis of a series of charge-transfer photosensitizers having
Quinoxaline-2(1H)-one as anchoring group onto TiO2 surface
Mauricio Caicedo1,2, Carlos A. Echeverry1, Robson R. Guimarães3, Alejandro Ortiz1,2, Koiti
Araki3* and Braulio Insuasty1,2*
1
Heterocyclic Compounds Research Group, Department of Chemistry, Universidad del Valle, A.A.
25360, Cali, Colombia.
2 Centre for Bioinformatics and Photonics-CIBioFI, Calle 13 No. 100-00, Edificio 320, No. 1069, Cali,
Colombia.
3 Laboratory of Supramolecular Chemistry and Nanotechnology, Institute of Chemistry, University
of Sao Paulo, Av. Lineu Prestes 748, Butanta, 05508-000, Sao Paulo, SP, Brazil.

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ABSTRACT:
In this work, we present the synthesis of novel donor-acceptor compounds based on 3-
methylquinoxaline-2(1H)one which follow an easy synthetic route, involving Knoevenagel
reaction with electron-donor groups such as N,N-dimethylaminobenzene, ferrocene,
triphenylamine (TPA) and ((E)-4,4'-(ethene-1,2-diyl)bis(N,N-diphenylaniline). Additionally,
the optical properties were measured by means of the absorption and emission spectroscopy
suggesting a push-pull behavior which was further confirmed by electrochemical experiments.
Finally, the quinoxaline-2(1H)one fragment not only bestow wide absorption, but also can
chelate to titanium ions on the TiO2 surface, allowing a strong electron coupling between the
excited-state energy level of the dyes and the conduction band of TiO2.
Keywords: Triphenylamine, TiO2, anchor group, qinaxoline, push-pull, microwave irradiation.
1. INTRODUCTION
Organic molecular systems can be assembled (anchored) onto a metal oxide substrate in
different ways: covalent attachment, electrostatic interaction, hydrogen bonding, hydrophobic
interaction, van der Waals forces, or physical entrapment [1]. However, finding new anchoring
groups for the preparation of contact structures, that provide both stability and high contact
transparency, is a critical issue in molecular materials and molecular electronics [2-4].
Actually, the development of a wide variety of materials for areas such as electrical conducting
materials, non-linear optics (NLO), molecular rectification, dyes, solvatochromism, and
photovoltaics (PV) [5], employ interactions between organic molecules and semiconductor
oxides. For example, dye-sensitized solar cells (DSSCs), firstly reported by Grätzel in 1991[6],
absorbed dye molecules on the surface of titanium dioxide (TiO2).
The DSSCs have the potential to improve the present conventional solar cells based on silicon
[7-8]. The immobilization of the light harvesting molecules onto the surface of semiconductor
metal oxide nanoparticles, which collectively act as the working electrode in a DSSCs, is
crucial for initiating a charge separation process via the injection of electrons from the excited
organic molecule (dye) into the conduction band of the metal oxide; thus mimicking the natural

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photosynthesis[9]. The most featured and commonly used anchoring groups for DSSCs are
carboxylic acid and cyanoacrylic acid group[10-12].
On the other hand, new anchoring groups have emerged which not only improve the
understanding of the fundamental processes involved in DSSCs but also increase the number of
new sensitizing materials. For example, recently a novel electron acceptor 2-(1,1-
dicyanomethylene)rhodanine, exhibiting high light harvesting and conversion efficiency [13-
14] was reported.
Microwave irradiation (MWI) is a technique that has been widely employed in synthetic
chemistry owing several advantages compared to traditional methods, especially those related
to selective heating, which minimizes or avoids side reactions. Microwave assisted organic
synthesis (MAOS) has also emerged as a powerful tool for high-throughput procedures.
Accordingly, this technique can improve the yield and purity of the final compounds while
shortening the reaction times by means of a precise control of parameters, such as, irradiation
power, pressure and temperature [15-19].
In this work, we report the microwave assisted synthesis of a novel series of photosensitizers
based on triphenylamine (TPA), N,N-dimethylaminobenzene
,
((E)-4,4'-(ethene-1,2-
diyl)bis(N,N-diphenylaniline) and Ferrocene as electron-donor groups and quinoxaline-2(1H)-
one as anchoring group. A detailed study was conducted to understand the relevance of
electronic structure on the photoinjection properties, as well as on the electrochemical
properties and impedance spectroscopy of devices containing the new dyes.
2. EXPERIMENTAL
2.1 Materials and equipments.
Reagents were used as purchased without further purification unless stated otherwise. Solvents
used in photochemical measurements were spectroscopic grade. All air-sensitive reactions were
conducted under argon atmosphere using dried solvents prepared according to recommended
standard procedures. FTIR spectra were recorded using a Shimadzu FT-IR 8400
spectrophotometer. Absorption spectra were registered in a Jasco V-730 spectrophotometer

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using fused quartz glass cuvettes with 10.0 mm optical path. Fluorescence measurements were
performed in a Jasco spectrofluorometer (FP-8500). NMR spectra were recorded on a
BRUKER 400 MHz UltraShield Advance II NMR (1H: 400 MHz, 13C: 100 MHz) spectrometer
at 298 K using deuterated solvents with TMS as internal standard. Coupling constants (J) are
reported in Hz and chemical shifts (δ) in ppm. Nuclear spin coupling multiplicities are denoted
as follows: s = singlet, d = doublet, t = triplet, m = multiplet, dd = doublet-doublet. Mass
spectra were recorded on a Shimadzu MS-QP 2010 spectrometer operating at 70 eV. Analytical
thin layer chromatography (TLC) was performed using aluminum coated Sorbent Technologies
60 UV254 plates. Microwave experiments were carried out on a focused microwave reactor
(300W CEM Discover).
2.2. Electrochemical measurements.
Electrochemical measurements were performed in a METRΩN Autolab PGSTAT 300N
potentiostat/galvanostat, using a three-electrode configuration system. The measurements were
carried out using a 0.1 M tetrabutylammoniun hexafluorophosphate (TBAPF6) solution in
electrochemical grade CH3CN as electrolyte. A glassy-carbon electrode (3 mm diameter) was
used as the working electrode, and a platinum wire and an Ag electrode were employed as the
counter and the pseudo-reference electrodes respectively. The ferrocene/ferrocenium (Fc/Fc+)
redox couple was used as the internal reference to measure the potentials. Both the counter and
the reference electrodes were directly immersed in the electrolyte solution. The surface of the
working electrode was carefully polished to mirror like finish with 0.3 and 0.5 alumina slurries
prior use. Solutions were deaerated by bubbling argon for few minutes prior each voltammetric
scan at 100 mV/s, unless otherwise specified.
2.3. DSSCs Fabrication
The MQ1-MQ5 dyes described below were applied in DSSCs according to an
adapted literature procedure [20]. Colloidal TiO2 nanoparticles (25 nm large) paste were
made by dispersing the nanoparticles in terpineol, ethylcellulose and ethanol mixture as
described previously [21]. Mesoporous TiO2 nanoparticles electrodes were prepared
spreading the colloidal paste on pieces of conducting FTO (resistance ~15 ꢀ sq-1) that were
subsequently dried in air, heated to 120 ºC for an hour and sintered for 30 min at 450 ºC.

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After cooling down to room temperature, the electrodes were dipped into 1.0 x 10-4 mol L-1
solution of MQ1-MQ5 dyes in acetonitrile. The dye-sensitized electrodes were hot-pressed
(110 ºC) together with a platinum doped FTO counter electrode and a 40 mm thick Surlyn
spacer intercalated in between. Finally, a hole was drilled and the electrolyte solution (0.5
M tert-butylpyridine, 0.6 M tetrabutylamonium iodide, 0.1 M LiI, 0.1 M I2 in
methoxypropionitrile) introduced in after making vacuum inside. Then I x V curves and
electrochemical impedance spectroscopy (EIS) measurements were registered using a
AutoLab PGSTAT30 potentiostat/galvanostat while irradiating the devices with an ABB
class AM 1.5 Oriel solar simulator (ASTM, JIS, IEC) calibrated with a standardized Si light
meter (VISI standards, oriel P/N 91150V). Electrochemical impedance spectra were
registered from 0.01 to 100,000 Hz at cell open circuit potential (VOC) modulating the
frequency of the superimposed sinusoidal potential wave (amplitude = 30 mV). IPCE
spectra were registered using an Oriel Spectral Luminator as light irradiation source.
2.4. Synthesis and Characterization
The starting materials 4-formyltriphenylamine 3 and 4,4'-diformyltriphenylamine 4, were
prepared following literature procedures [22]. The compound 5 was prepared by Michaelis-
Arbuzov reaction [23] and the starting material 6 was obtained by Horner-Emmons reaction
[24]. The preparation of the target molecules (MQ1-MQ5) was performed using a
Knoevenagel condensation with compound 1 under basic conditions and using microwave
irradiation, as shown in Scheme 1.
2.4.1. Synthesis of MQ1
A mixture of formyltriphenylamine 3 (80 mg, 0.29 mmol), 3-methylquinoxalin-2(1H)-one 1
(39 mg, 0.24 mmol) and piperidine (3 drops), were subjected to microwave irradiation, at 120
°C for 2 min and a maximum power of 300 W. Then, the reaction mixture was purified by
column chromatography using a mixture of CH2Cl2/ethyl acetate (15:1) as eluent, to get pure
MQ1 as orange solid, 78% yield.
1H NMR (DMSO-d6, 400 MHz) δ: 12.45 (s, 1H), 8.02 (d, J = 16.0 Hz, 1H), 7.76 (d, J = 8.4
Hz, 1H), 7.63 (d, J = 6.8 Hz, 2H), 7.45-7.52 (m, 2H), 7.37 (t, J = 8.0 Hz, 4H), 7.28-7.31 (m,
2H), 7.10-7.16 (m, 6H), 6.95 (d, J = 12.0 Hz, 2H) ppm; 13C NMR (DMSO-d6, 100 MHz) δ:

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155.3, 153.7, 149.0, 147.0, 137.2, 133.0, 132.0, 130.2, 129.9, 129.8, 129.5, 128.6, 125.5, 124.5,
123.9, 121.9, 120.0, 115.7 ppm; FTIR ν= 3415, 1662 cm-1, MS (IE): m/z 415 [M+].
2.4.2. Synthesis of MQ2
A mixture of formylferrocene 7 (100 mg, 0.47 mmol), 3-methylquinoxalin-2(1H)-one 1 (63
mg, 0.39 mmol) and piperidine (3 drops), were subjected to microwave irradiation, at 120 °C
for 3 min and a maximum power of 300 W. Then, the reaction mixture was purified by column
chromatography using CH2Cl2 as eluent, to get pure MQ2 as dark purple solid, 62% yield.
1H NMR (DMSO-d6, 400 MHz) δ: 12.38 (s, 1H), 7.96 (d, J = 16.0 Hz, 1H), 7.76 (d, J = 16.0
Hz, 1H), 7.18-7.47 (m, 4H), 4.73-4.81 (m, 4H), 4.16-4.31 (m, 5H) ppm; 13C NMR (DMSO-d6,
100 MHz) δ: 164.2, 163.4, 155.3, 149.5, 139.0, 136.4, 133.8, 132.7, 125.2, 123.7, 71.0, 70.4,
69.9, 69.8, 68.7, 68.5 ppm; FTIR ν = 3395, 1661 cm-1; MS (IE): m/z 356 [M+].
2.4.3 Synthesis of MQ3
A mixture of (E)-4-((4-(4-(diphenylamino)styryl)phenyl)(phenyl)amino)benzaldehyde 6 (100
mg, 0.18 mmol), 3-methylquinoxalin-2(1H)-one 1 (25 mg, 0.16 mmol) and piperidine (3
drops), were subjected to microwave irradiation, at 130 °C for 2.2 min and a maximum power
of 300 W. Then, the reaction mixture was purified by column chromatography using a mixture
of CH2Cl2/ethyl acetate (15:1) as eluent, to get pure MQ3 as orange solid, 58% yield.
1H NMR (CDCl3, 400 MHz) δ = 12.03 (s, 1H), 8.18 (d, J = 16.8 Hz, 1H), 7.88 (d, J = 8.0 Hz,
1H), 7.68 (d, J = 16.0 Hz, 1H), 7.62 (d, J = 7.6 Hz, 1H), 7.28-7.51 (m, 12H), 7.21 (d, J = 6.8
Hz, 2H), 7.14- 7.17 (m, 7H), 6.96-7.12 (m, 10H); 13C NMR (CDCl3, 100 MHz) δ: 156.7, 153.3,
148.8, 147.6, 147.2, 147.0, 146.3, 138.0, 133.7, 133.0, 131.8, 130.7, 130.3, 129.5, 129.5, 129.3,
129.2, 128.2, 127.3, 127.3, 127.3, 126.4, 125.4, 125.2, 125.1, 124.9, 124.7, 124.5, 124.2, 124.0,
123.7, 123.03, 122.6, 122.3, 119.4, 115.5 ppm; FTIR ν = 3320, 1660 cm-1; MS (IE): m/z 684
[M+].
2.4.4. Synthesis of MQ4
A mixture of 4,4’-diformyltriphenylamine 4 (100 mg, 0.33 mmol), 3-methylquinoxalin-2(1H)-
one 1 (80 mg, 0.50 mmol) and piperidine (3 drops), were subjected to microwave irradiation, at
150 °C for 5 min and a maximum power of 300 W. Then, the reaction mixture was purified by

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column chromatography using a mixture of CH2Cl2/ethyl acetate (15:1) as eluent, to get pure
MQ4 as orange solid, 52% yield.
1H NMR (DMSO-d6, 400 MHz) δ: 12.48 (s, 1H), 12.31 (s, 1H), 8.05 (d, J = 16.0 Hz, 1H), 7.78
(d, J = 8.0 Hz, 2H), 7.70 (d, J = 8.0 Hz, 3H), 7.57 (d, J = 8.0 Hz, 1H), 7.39-7.52 (m, 4H), 7.26-
7.32 (m, 4H), 7.12-7.23 (m, 4H), 7.07 (d, J = 8.0 Hz, 3H) ppm; 13C NMR (DMSO-d6, 100
MHz) δ: 159.7, 158.1, 155.3, 153.6, 137.01, 136.4, 130.4, 129.8, 129.6, 129.6, 128.7, 128.4,
126.3, 125.3, 124.0, 123.6, 123.5, 120.1, 120.7, 115.7 ppm; FTIR ν = 3455, 1660 cm-1; MS
(IE): m/z 585 [M+].
2.4.5. Synthesis of MQ5
A mixture of 4-(dimethylamino)benzaldehyde 8 (120 mg, 0.81 mmol), 3-methylquinoxalin-
2(1H)-one 1 (107 mg, 0.67 mmol) and piperidine (3 drops), were subjected to microwave
irradiation, at 150 °C for 2 min and a maximum power of 300 W. Then, the reaction mixture
was purified by column chromatography using CH2Cl2 as eluent, to get pure MQ5 as a red
solid, 65% yield.
1H NMR (CDCl3, 400MHz) δ = 12.26 (s, 1H), 8.16 (d, J = 16.0 Hz, 1H), 7.84 (t, J = 8.0 Hz,
2H), 7.64 (d, J = 16.0 Hz, 1H), 7.51 (d, J = 8.0 Hz, 1H, 7.45 (d, J = 8.0 Hz, 1H), 7.38 (d, J =
8,0 Hz, 2H), 6.75 (d, J = 8.0 Hz, 2H), 3.06 (s, 6H); 13C NMR (CDCl3, 100 MHz) δ: 166.1,
165.6, 165.3, 165.1, 165.0, 164.9, 161.2, 160.8, 160.6, 159.26, 153.0, 152.3, 152.1, 152.0,
148.7, 148.5, 57.3 ppm; FTIR ν = 3325, 1668 cm-1; MS (IE): m/z 291 [M+].
3. RESULTS AND DISCUSSION
3.1. Synthesis
We reported the syntheses of new dye molecules having quinoxaline-2(1H)one as surface
anchoring moiety and triphenylamine as electron-donor units, based on a simple synthetic
method (scheme 1). The 3-methylquinoxaline-2(1H)one starting material was obtained by a
procedure described in literature [25]. The formyl derivatives 3 and 4 were obtained by a
Vilsmeier-Haack reaction. Subsequently, the derivative 6 was synthesized starting from
dialdehyde 4 and diethyl (4-(diphenylamino)benzyl)phosphonate 5 at reflux in dry THF
under argon. Finally, in order to diminish the amount of solvent and save time,

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Knoevenagel reactions were carried out between the 3-methylquinoxaline-2(1H)one 1 and
the starting materials 3, 4, 6, 7 and 8 to obtain the target molecules MQ1-MQ5 in good
yields (52-79%).
The structures of the new compounds were confirmed using analytical and spectroscopic
1
techniques such as H and 13C nuclear magnetic resonance (NMR), fourier infrared (FTIR)
and mass spectrometry. FT-IR spectra of the target molecules showed the band
corresponding to ν(C=O) stretching mode around 1660 cm-1.
Scheme 1. Reaction conditions: i) POCl3, DMF, reflux; ii) piperidine, MW, 2-5 min, 120-150 °C,
300 W; iii) 4, THF (dry), t-BuOK, argon, reflux.

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Moreover, the 1H NMR spectra of these compounds not only showed the aromatic protons,
but a singlet around 12.45 ppm corresponding to the N-H proton of the quinoxaline ring,
Furthermore, compounds MQ1-MQ5 showed two doublets corresponding to the vinyl
protons around 7.96-818 and 7.64-7.70 ppm with a coupling constant about 16.0 Hz. The
mass spectrometry experiments showed that the m/z molecular ion peak in agreement with
the molecular weight of the expected dye molecules.
3.2. Absorption and emission spectroscopy
The absorption and emission spectra of compounds MQ1-MQ5 measured in acetonitrile are
shown in figure 1. The absorption spectra of MQ1, MQ3 and MQ4 and MQ5 solution exhibit
the typical features of TPA based compounds: an absorption band around 250 nm assigned to a
π-π* electronic transition and a second band around 320 nm attributed to a n-π* transition.
Additionally, the band at 385-451 nm can be attributed to intramolecular charge transfer (ICT)
from the TPA donor to the quinoxaline-2(1H)one acceptor [14].
a)
0,5
b)
MQ1
1,0
0,4
0,3
0,2
0,1
0,0
Toluene
1,4-dioxane
THF
DCM
Acetone
Acetonitrile
MQ2
MQ3
MQ4
MQ5
0,8
0,6
0,4
0,2
0,0
500
550
600
650
700
200
300
400
500
600
700
λ
(nm)
λ
(nm)
Figure 1. a) Absorption spectra of MQ1-MQ5 measured in acetonirile. b) Absorption spectra o
MQ1 in diferent solvents.
The obtained compounds exhibit the same acceptor and π spacer segments, therefore the
changes in their absorption behaviors will be affected by the strength of the donor parts

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under investigation. For instance, the absorption maxima of MQ1 and MQ2 appear at 434
and 386 nm, respectively. Their structures differ in their electron-donor fragments: the
former sensitizer has a relatively strong donor TPA group, whereas the latter has a
relatively weaker donor ferrocene group. In this manner, replacing the strong electron
donor TPA for a weaker donor resulted in a blue-shifted absorption maxima by 48 nm.
The normalized absorption ICT band of MQ1 (figure 1b) is dependent on the solvent,
exhibiting a bathochromic shift as a function of the solvent polarity (donor number or
basicity), as expected for charge-transfer transitions. In this case, the positive and
negatively charged moieties are stabilized more efficiently by polar solvents thus
decreasing the energy of the excited state species. Similar behavior was observed for MQ5,
which exhibits a typical absorption spectrum pattern of a TPA derivative.
Fluorescence spectra measured upon excitation at λmax in air-equilibrated acetonitrile
solution, at 298 K, are shown in figure 2. The emission spectra of MQ1-MQ5 compounds,
exhibit a band at 565-570 nm, which is slightly red-shifted as compared to the respective
ICT absorption band. This is consistent with a lowest energy charge-transfer excited state
which favors electron-donor capability of the triarylamine and the other fragments, and
eventually favoring the electron injection in TiO2 thus leading to convertion of light into
electricity. On the other hand, the weak solvatochromism for MQ1, and the separated
charges in the ICT excited state seems to be more or less delocalized (figure 2b), indicating
that this is the lowest excited state species. The absorption and emission wavelengths are
summarized in Table 1.
a)
1,0
b)
Toluene
1,4-dioxane
THF
DCM
Acetone
Acetonitrile
1,0
0,8
0,6
0,4
0,2
0,0
MQ1
MQ2
MQ3
MQ4
MQ5
0,8
0,6
0,4
0,2
0,0
550
600
650
700
480
500
520
540
560
580
λ
(nm)
λ
(nm)

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Figure 2. a) Normalized emission spectra of MQ1-MQ5 measured in acetonirile. b) Normalized
emission spectra of MQ1 in diferent solvents.
Table 1. Absorption and emission data of MQ1-MQ5 measured in acetonitrile
λICTmax/nm
(ε/M-1 cm-1)a
434 (26490)
386 (12064)
385 (19164)
451 (16746)
442 (47875)
λEmb/nm
Compound
569
599
570
565
570
MQ1
MQ2
MQ3
MQ4
MQ5
a) The ε was measured for the ICT band for the compounds at concentrations of 1x10-5 M, b) The λEm
was meausuring according to λmax for each compound at concentrations of 1x10-5 M.
3.3. DSSC Performance and electrocemical analysis
Accordingly with the above observations, we decided to prepare DSSCs, which were
assembled using mesoporous TiO2 electrodes and sensitized with the MQ1-MQ5 dyes,
their electron photoinjection and recombination properties were also evaluated.
The curves of short-circuit current density as a function of potential are showed in figure
3. The MQ3 and MQ4 dyes showed higher power efficiency as expected for their higher
values of Jsc and Voc (table 2). Coincidentally, these dyes are those with larger structures.
This suggests that these structures decrease the recombination processes by preventing the
approach of triiodide ions to semiconductor surface, thus decreasing the electron transfer
from TiO2 conduction band to triiodide ions electrolyte. On the other hand, the higher
efficiency of the MQ3 and MQ4 can also be attributed due to the greater contribution of
triphenylamine units and the presence of more anchoring amide groups which improve the
electron injection into semiconductor conduction band and therefore improving the
photovoltaic performance. It is to be noted that the lower efficiency of the MQ2 dye,
indicate that the ferrocene group is acting as an electron recombination site and/or
preventing the vectorial transfer of electrons to TiO2 due to its electron acceptor
characteristics.

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Figure 3. a) The short-circuit current density vs potential curve and b) the power vs potential curve of
DSSCs built with non-sensitized a mesoporous TiO2 electrode and electrodes sensitized with MQ1-
MQ5.
Table 2. Short circuit current density (Jsc), open circuit voltage (Voc), Fill factor (FF), maximum power
(Pmax) and efficiency parameters obtained through curves I vs. V.
Compounds
MQ1 1
MQ1 2
MQ1 3
MQ2 1
MQ2 2
MQ2 3
MQ3 1
MQ3 2
MQ3 3
MQ4 1
MQ4 2
MQ4 3
MQ5 1
MQ5 2
MQ5 3
TiO2 1
TiO2 2
TiO2 3
Voc (V)
0.410
0.418
0.425
0.120
0.086
0.025
0.457
0.459
0.474
0.479
0.486
0.465
0.399
0.403
0.411
0.418
0.404
0.418
Jsc (mA/cm2) Pmax (mW)
FF (%)
59.5
59.8
61.3
26.6
24.5
24.3
65.3
65.5
65.9
66.0
67.5
61.4
60.1
60.8
61.5
56.4
46.7
60.1
η (%)
0.07
0.07
0.306
0.295
0.296
0.013
0.007
0.003
0.924
0.921
0.985
1.094
1.221
1.088
0.378
0.357
0.299
0.206
0.179
0.199
0.012
0.012
0.012
6.73E-05
2.46E-05
2.65E-06
0.044
0.044
0.049
0.055
0.064
0.050
0.015
0.014
0.012
0.008
0.005
0.008
0.08
4.20E-04
1.54E-04
1.65E-05
0.28
0.28
0.31
0.35
0.40
0.31
0.09
0.09
0.08
0.05
0.03
0.05

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A study of the electrical properties of the solar cells interfaces/junctions was carried
out by electrochemical impedance spectroscopy (EIS) (figure 4). Nyquist impedance
spectra provide information about charge transfer resistance of interface/junction based on
the size of the respective arc in spectrum [26]. On the other hand, the time scales of the
charge transfer processes are best seen in the Bode phase plot. By simulation of EIS
spectrum using an equivalent circuit based on the transmission line model, parameters such
as the electron recombination resistance (Rr), electron diffusion resistance (Rt), chemical
capacitance (Cµ), electron diffusion coefficient (Dn), electron lifetime (tn) and electron
diffusion length (Ln) in TiO2 conduction band can be determined. Thus, the EIS technique
can be used to characterize the electrical properties of all interfaces/junctions presented in
running DSSCs [27-28]. The electron transfer resistance in the counter electrode/electrolyte
interface is assigned to the first arc at high frequencies in the Nyquist spectrum (figure 5a).
The electron recombination resistance (Rr) and the electron diffusion resistance (Rt) in the
TiO2 mesoporous film can be determined by analyzing the second arc at intermediate
frequencies. Finally, the resistance for ion diffusion in the electrolyte is associated with the
last arc at low frequency region.
Figure 4. Nyquist and Bode phase spectra of running DSSCs prepared with the MQ1-MQ5 dyes
and non-sensitized TiO2 electrode for comparison.
The diameter of the arc at intermediate frequency of MQ1-MQ5 exhibit variations that can
be attributed to Rr values change (figure 4a and table 3). All photosensitizers showed lower

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recombination resistance than the device prepared with non-sensitized mesoporous TiO2
electrode which exhibited much larger electron recombination rates. In Bode phase
spectrum (figure 4b), the band assigned to electron recombination process (100-102 Hz) in
DSSCs incorporating MQ1-MQ5 showed lower frequency that can be associated with
shorter electron lifetimes as confirmed in table 3. Despite the impedance technique is a
powerful tool to analyze the electron recombination and diffusion process in TiO2
conduction band, it was not possible to find a clear tendency correlating the DSSCs
performance with those impedance parameters. This fact can be attributed to other more
relevant processes determining the overall device efficiency. Accordingly, the primordial
electron photoinjection process by the excited photosensitizers was studied based on their
photoaction spectra.
Table 3. Diffusion resistance (Rt), recombination resistance (Rr) and lifetime (tn) of electrons in
TiO2 conduction band determined by simulation of EIS using an equivalent circuit based on the
transmission line model.
Rt (ꢀ)
43.1
34.7
56.6
57.7
43.1
26.9
Rr (ꢀ)
28.1
6.5
12.7
11.2
5.6
τn (ms)
11.4
5.0
9.4
3.5
1.9
28.7
Compound
MQ1
MQ2
MQ3
MQ4
MQ5
52.1
TiO2
The incident photo-to-current efficiency (%IPCE) of DSSCs sensitized with the new
photosensitizers are shown in figure 5. The MQ1, MQ3, MQ4 and MQ5 exhibited bands
around 480 nm associated with the ICT transition. The MQ3 and MQ4 dyes presented the
highest photoaction efficiencies indicating more efficient electron injection to TiO2
conduction band from photosensitizers excited charge-transfer state species. The
photoaction efficiency of MQ2 was virtually zero, confirming the hypothesis that the
ferrocene group probably is acting as a quencher which inhibits the electron injection from
the photosensitizer to TiO2 conduction band. As expected, the DSSCs overall efficiency
(figure 3) tended to increase as a function of the phoaction efficiencies (figure 5), showing
a clear dependence of devices performance with dyes electron photoinjection properties.

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Figure 5. Photo-action spectra of DSSCs sensitized with MQ1-MQ5 dyes. A device prepared with
non-sensitized mesoporous TiO2 was included for comparison.
The MQ1-MQ5 dyes were electrochemically characterized in solution by cyclic voltammetry
to evaluate the energy levels and explain the photo-injection properties described above.
The cyclic voltammetry experiments of compounds MQ1-MQ5 was measured acetonitrile
containing 0.1 M of TBAPF6 (Figure S3 of Supporting Information). The first oxidation
potential peak (Eox vs. NHE) can be associated to the energy of the highest occupied molecular
orbital (HOMO) and the first reduction potential peak (Ered vs. NHE) can be associated to the
lowest unoccupied molecular orbital (LUMO). The LUMO energy can be calculated from
experimental values of Eox (HOMO) and E0-0 (zero-zero excitation energy) using the equation
Ered = Eox - E0-0. Table 4 summarizes the redox potentials for MQ1-MQ5
.
Table 4. Electrochemical data of compounds MQ1-MQ5 measured by cyclic voltammetry experiments
in acetonitrile solution at 100 mV scan rate with Fc/Fc+ as reference system.

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Compound
MQ1
E0-0a/ eV Eox vs NHE/V Ered vs NHE/V
Egapb/V
0.62
0.61
0.73
0.64
0.45
λint/nm
503
646
510
512
2.47
1.92
2.43
2.42
2.37
1.35
0.81
1.20
1.28
1.42
-1.12
-1.11
-1.23
-1.14
-0.95
MQ2
MQ3
MQ4
MQ5
524
a) E0-0 values were calculated from the intersection of the normalized absorption and the emission spectra
(λint): E0-0 = 1240/λint. b) Egap is the energy gap between the Ered of the compound and the conduction band
level of TiO2 (−0.5 V vs NHE).
The oxidation potential vs. NHE for the novel dyes were found at 0.81-1.35 V range. MQ1-
-
MQ4 photosensitizers showed a favorable redox potential relative to the I-/I3 couple (~ 0.4
V vs. NHE), providing enough driving force for efficiently regenerating the oxidized dye
after photo-induced electron injection, thus minimizing charge recombination. MQ5
exhibited a higher oxidation potential peak at 1.42 V. On the other hand, the reduction
potential vs. NHE and relative LUMO’s of those molecules were found to be significantly
higher in energy than the energy of TiO2 conduction band (-0.5 V vs. NHE).
The negative shift of these potentials with respect to the lower edge of TiO2 conduction
band indicates that the excited dyes can inject electrons in TiO2 conduction band, as
confirmed by the photo-action spectra (figure 6).
In summary, we present the synthesis and characterization of novel dyes MQ1-MQ5
possessing quinoxaline-2(1H)one as surface anchoring group. EIS investigation and the
photovoltaic performance confirm that the anchoring group promotes photo-induced
electron transfer into TiO2 conduction band.

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-1.23 V
-1.14 V
-1.11 V
-1.12 V
-0.95 V
-0.5 V
BC TiO2
1.92
2.43
1.20 V
MQ3
2.42
2.47
2.37
0.81 V
~0.4V
I-/I3
-
1.28 V
1.35 V
1.42 V
MQ1
MQ2
MQ4
MQ5
Figure 6. Energy level diagram of MQ1-MQ5 dyes. The redox potential of the iodine couple and
the lower edge of TiO2 conduction band (CB) are included for comparison purpose.
4. CONCLUSION
A series of soluble compounds consisting on quinoxaline-2(1H)one anchoring group and
TPA derivatives, ferrocene and 4-(dimethylamino)benzaldehyde as electron-donor sites
were synthetized and characterized. All compounds were prepared by means of a
convenient and efficient microwave irradiation method and tested as photosensitizers in
Grätzel type solar cells. Unfortunately, the poor electron-acceptor character of the
quinoxaline-2(1H)one anchoring group seems to decrease the vectorial energy transfer
processes from the TPA donor to semiconductor conduction band thus decreasing the
photoinjection efficiencies. Also, this fundamental process was shown to be limiting the
overall energy conversion efficiencies that can eventually be improved by introducing an
electron-withdrawing substituent is bond in 5-, 6-, 7- and 8-position of the quinoxaline ring.
5. Acknowledgements
Authors wish to thank the Universidad del Valle, COLCIENCIAS and the Science,
Technology and Innovation Fund-General Royalties System (FCTeI-SGR) under contract
BPIN 2013000100007 for financial support. K. Araki and Robson R Guimaraes

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acknowledge the financial support from Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq) and Fundação de Amparo à Pesquisa do Estado de São Paulo
(FAPESP).
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Supporting Information
Microwave assisted synthesis of several electroactive molecules based in Quinoxaline-
2(1H)-one like efficient anchor group on TiO2 surface.
Mauricio Caicedo1,2, Carlos A. Echeverry1, Robson R. Guimarães3, Alejandro Ortiz1,2, Koiti
Araki3* and Braulio Insuasty1,2*
1
Heterocyclic Compounds Research Group, Department of Chemistry, Universidad del Valle, A.A.
25360, Cali, Colombia.
2 Centre for Bioinformatics and Photonics-CIBioFI, Calle 13 No. 100-00, Edificio 320, No. 1069, Cali,
Colombia.
3 Laboratory of Supramolecular Chemistry and Nanotechnology, Institute of Chemistry, University
of Sao Paulo, Av. Lineu Prestes 748, Butanta, 05508-000, Sao Paulo, SP, Brazil.
Figure S1. Compounds (dyes) solutions in acetonitrile. The DSSCs electrodes were prepared by
immersing the TiO2 films to solutions of dyes in acetonitrile (figure S1). The concentration of the
MQ1-MQ5 solutions is 1.0x10-4 M.

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-
Figure S2. a) TiO2 films with MQ1-MQ5 absorbed, b) DSSCs prepared with I3 /I- and FTO.
MQ4
MQ1
MQ2
MQ5
MQ3
0
-2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5
-2,5 -2,0 -1,5 -1,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5
Potencial (V)
Potencial(v)
Figure S3. Cyclic voltammogram vs. (Fc/Fc+) of MQ1-MQ5 dyes in acetonitrile solution at 100 mV
scan rate.