The investigation of novel D-π-A type dyes (MK-3 and MK-4) for visible light driven photochemical hydrogen evolution

Article abstract of DOI:10.1016/j.dyepig.2019.107710

Two novel donor-π-acceptor (D-π-A) semiconductor organic dyes have been synthesized for the photochemical hydrogen evolution reaction (HER) to sensitize TiO₂ for the first time. The molecular structures of D-π-A semiconductor organic dyes, which are entitled as MK-3 and MK-4, have been characterized by NMR spectroscopy method; and also electrochemical and optical properties have been investigated by cyclic voltammetry and UV–Vis absorption techniques, respectively. Amount of dye loading on TiO₂ surface has been investigated by EDX method. The HER activities have been explored in the presence of triethanolamine (TEOA) as a electron donor reagent under sunlight (solar simulator illumination limited by cut-off filter λ ≥ 420 nm) in the absence and presence of co-catalysts (Pt and Cu₂WS₄). Transient photocurrent densities of MK-3/TiO₂ and MK-4/TiO₂ electrodes have been reached to 110 μA cm^?1 and 275 μA cm^?1, respectively. The photocatalytic HER activities have been relatively enhanced in the presence of the Pt or Cu₂WS₄ co-catalysts (dye/TiO₂/Cu₂WS₄ or dye/TiO₂/Pt) when compared to only dye/TiO₂. The HER rates have been found as 427, 1277, 675, 682, 1027 and 795 μmolg^?1h^?1 for the MK-3/TiO₂, MK-3/TiO₂/Pt, MK-3/TiO₂/Cu₂WS₄, MK-4/TiO₂, MK-4/TiO₂/Pt and MK-4/TiO₂/Cu₂WS₄, respectively. The differences of photochemical activities of MK-3/TiO₂ and MK-4/TiO₂ have been stated taking into account dye molecule structures. Moreover, the HER mechanism have been described by using electrochemical band energy levels of dyes, TiO₂ and Cu₂WS₄ co-catalyst.

Full text of DOI:10.1016/j.dyepig.2019.107710

Accepted Manuscript
The investigation of novel D-π-A type dyes (MK-3 and MK-4) for visible light driven
photochemical hydrogen evolution
Emre Aslan, Merve Karaman, Gizem Yanalak, Mustafa Can, Faruk Ozel, Imren Hatay
Patir
PII:
S0143-7208(19)30767-3
DOI:
https://doi.org/10.1016/j.dyepig.2019.107710
Article Number: 107710
Reference:
DYPI 107710
To appear in: Dyes and Pigments
Received Date: 8 April 2019
Revised Date: 8 May 2019
Accepted Date: 10 July 2019
Please cite this article as: Aslan E, Karaman M, Yanalak G, Can M, Ozel F, Patir IH, The investigation of
novel D-π-A type dyes (MK-3 and MK-4) for visible light driven photochemical hydrogen evolution, Dyes
and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.107710.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
The investigation of novel D-π-A type dyes (MK-3
and MK-4) for visible light driven photochemical
hydrogen evolution
Emre Aslan,[a] Merve Karaman,[b] Gizem Yanalak,[c] Mustafa Can,[b] Faruk Ozel,[d,e] and Imren
Hatay Patir, [f]*
[a] Selcuk University, Department of Chemistry, 42030, Konya, Turkey
[b] Izmir Katip Celebi University, Faculty of Engineering and Architecture, Department of
Engineering Sciences, Cigli, 35620 Izmir, Turkey
[c] Selcuk University, Department of Biochemistry, 42030, Konya, Turkey.
[d] Karamanoglu Mehmetbey University, Department of Metallurgical and Materials
Engineering, 70200, Karaman, Turkey
[e] Karamanoglu Mehmetbey University, Scientific and Technological Research and
Application Center, 70200, Karaman, Turkey
[f] Selcuk University, Department of Biotechnology, 42030, Konya, Turkey e-mail:
imrenhatay@gmail.com
Abstract: Two novel donor-π-acceptor (D-π-A) semiconductor organic dyes have been
synthesized for the photochemical hydrogen evolution reaction (HER) to sensitize TiO2 for
the first time. The molecular structures of D-π-A semiconductor organic dyes, which are
entitled as MK-3 and MK-4, have been characterized by NMR spectroscopy method; and also
electrochemical and optical properties have been investigated by cyclic voltammetry and UV-

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Vis absorption techniques, respectively. Amount of dye loading on TiO2 surface has been
investigated by EDX method. The HER activities have been explored in the presence of
triethanolamine (TEOA) as a electron donor reagent under sunlight (solar simulator
illumination limited by cut-off filter λ ≥ 420 nm) in the absence and presence of co-catalysts
(Pt and Cu2WS4). Transient photocurrent densities of MK-3/TiO2 and MK-4/TiO2 electrodes
have been reached to 110 µA cm-1 and 275 µA cm-1, respectively. The photocatalytic HER
activities have been relatively enhanced in the presence of the Pt or Cu2WS4 co-catalysts
(dye/TiO2/Cu2WS4 or dye/TiO2/Pt) when compared to only dye/TiO2. The HER rates have
been found as 427, 1277, 675, 682, 1027 and 795 µmolg-1h-1 for the MK-3/TiO2, MK-
3/TiO2/Pt, MK-3/TiO2/Cu2WS4, MK-4/TiO2, MK-4/TiO2/Pt and MK-4/TiO2/Cu2WS4,
respectively. The differences of photochemical activities of MK-3/TiO2 and MK-4/TiO2 have
been stated taking into account dye molecule structures. Moreover, the HER mechanism have
been described by using electrochemical band energy levels of dyes, TiO2 and Cu2WS4 co-
catalyst.
Keywords: photocatalysis, solar energy conversion, hydrogen evolution, D-π-A organic dyes
1. INTRODUCTION
The production of cheap, renewable and environmentally friendly fuel hydrogen by
using solar energy has attracted much attention due to the increasing energy crisis. The sun’s
energy converts to hydrogen via photoelectrochemical and photocatalytic reaction systems
from water in the presence of semiconductor photocatalysts. Until now, large-scale
photocatalytic HER systems have been thoroughly examined by using a myriad of catalysts
including metal oxides, chalcogenides, oxynitrides, metal free catalysts, sulfides, phosphides,
borides, carbides [1]. The most commonly used TiO2 photocatalyst possess huge potential

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hydrogen-generating capabilities from water splitting. Beside this advantage, hydrogen
production efficiency of a bare TiO2 still remains limited owing to the wide band gap, the
high recombination of electron-hole pairs and the fast-backward reaction [2-4]. These
obstacles can be decreased by dye sensitization and using co-catalyst for the absorption
visible light and the minimization of recombination rates, respectively [4]. Currently,
semiconductor donor-π-acceptor (D-π-A) structured semiconductor organic dyes have been
aroused interest in photovoltaic and photocatalytic conversion applications due to the
properties of configurable electrochemical band levels, absorption and high intermolecular
charge transfer (ICT) performance [5-8]. These D-π-A structures have been composed of π-
electron-poor (electropositive) acceptor (A) groups and π-electron-rich donor (D) groups
linked by a π bridge, which make possible the separation of electrons and holes to reduce the
recombination events [9-13]. Photocatalytic HER by D-π-A dye, which are consisted of
triphenylamine based donor group, sensitized TiO2 have been investigated by changing
hydrophilic and steric effect, the amount of loading dye, spacer length and the amount of
anchoring groups of dyes [14-19]. D-π-A structured semiconductor organic dyes have also
been employed in the covalently functionalized p-type photocathodes and the modification of
graphene/Pt electrodes [20-24]. Konieczna et al. have been explored that ionic structures of
semiconductor D-π-A sensitizers displayed higher photochemical HER activity than that of
the neutral D-π-A sensitizers [25]. We have shown that the photocatalytic HER activity by
different D-π-A dye (changing electron donating groups) sensitized TiO2 is decreased by the
addition of donor groups because of the steric effect of dyes on TiO2 [26]. In addition, effect
of additional π group has been reported by our group and founded out D-π-A structure is
efficient than D-A structure [27]. D π A conjugated polymeric photocatalyst have also been
used on photocatalytic HER under UV–Vis light irradiation (λ > 300 nm) [28]. The HER
activities of phenothiazine based D-A type organic semiconductor dyes sensitized TiO2

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photocatalyst have been also investigated by Tiwari et al. under solar irradiation [29, 30].
Besides, both the photocatalytic HER and the photovoltaic application of two D A π A
structured semiconductor organic dyes have been investigated under limited light (420
nm<λ<780 nm) illumination. It has been shown that both high photocatalytic HER activity
and photoelectric conversion efficiency have been observed due to the absorption properties
of the dyes with high molar extinction coefficient [31]. Dye sensitized photocatalytic HER
studies are generally performed by using co-catalyst in order to decrease charge
recombination rates and photocorrosion, and to increase stability of reactions and
photogenerated charge transport efficiency, and to host active sites. Co-catalysts for the
photocatalytic HER are usually consisted of platinum group noble metals (PGMs) such as Pt,
Pd, Ru or Ir [7]. Metal sulfide based catalysts like WS2 and MoS2 have been attracted great
attentions as an alternative to PGMs. Transition metal doping or alloyed structures are given
rise to enhance the photocatalytic activities of MS2 (M = Mo and W) [26, 32, 33].
In this study, we have reported first time in the literature the synthesis of two D-π-A
type semiconductor organic dyes, which are entitled MK-3 ((2E) -3- [5 '- {6- [5- (4- {bis [4-
(hexyloxy) phenyl] amino} phenyl) -4- (2-ethylhexyl) -2-thienyl] -1,2 , 4,5-tetrazin-3-yl} -2,2'-
bithien-5yl]
-2-cyanoacrylic
acid)
and
MK-4
((2E)-3-{5-[5-{6-[5-(4-{bis[4-
(hekzilloks)phenyl]amino}phenyl)-4-(2-ethylhexyl)-2-thienyl]-1, 2,4,5-tetrazin-3-yl -3-(2-
ethylhexyl)-2-thienyl]-2-furyl}-2-cyanoacrylic acid), for the visible light sensitization of TiO2.
The molecular structures of MK-3 and MK-4 have been clarified by NMR spectroscopy and
elemental analysis methods. The electrochemical and optical properties of these dyes have
been clarified by the cyclic voltammetry and UV-Vis absorption spectroscopy methods,
respectively. Dye loading rates on TiO2 surface has been also calculated by EDX method. The
photoelectrochemical and the photocatalytic activities of MK-3/MK-4 sensitized TiO2
photocatalysts on the HER have been investigated. The photoelectrochemical HER

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performance of dye sensitized electrodes has been surveyed by chronoamperometry and linear
sweep voltammetry (LSV) methods in the aqueous TEOA/Na2SO4 solution. The
photocatalytic HER activities have been examined in the aqueous TEOA solution as the
sacrificial electron donor medium. Cu2WS4 has also used as the co-catalyst and its catalytic
activity has compared to in situ photodeposited Pt. MK-3/TiO2, MK-3/TiO2/Cu2WS4, MK-
3/TiO2/Pt, MK-4/TiO2, MK-4/TiO2/Cu2WS4 and MK-4/TiO2/Pt have produced 427, 675,
1277, 682, 795 and 1027 µmolg-1h-1, respectively. The differences of HER rates among the
dye sensitized TiO2 photocatalysts explicated by structural differences of D-π-A organic dye.
Moreover, the HER mechanism has been clarified by the energy band levels of the Cu2WS4
co-catalyst, TiO2, and each photosensitizer D-π-A dyes.
2. EXPERIMENTAL SECTION
2.1. Optical and electrochemical experiments
Optical measurements were performed by using
a
UV-Vis absorption
spectrophotometer (Shimadzu UV-1800) and Perkin Elmer LS-50B luminescence
spectrometer. Molar absorption coefficients were calculated according to literature [34]. The
electrochemical characterizations were performed by electrochemical working station (CH
Instruments 760D) in three-electrode cell in acetonitrile solution. Glassy carbon, platinum
wire and Ag/AgCl electrodes were used as working electrode, counter electrode and reference
electrode, respectively. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as
supporting electrolyte.
2.2. Dye sensitization process
TiO2 has been calcined to detract adsorbed and water organic pollutants from the
surface for 45 minutes at 450 ºC. This step is important to generation of binding region
between the surface of dye molecules and TiO2. Then the prepared TiO2 and donor-π-acceptor
dye solutions (10-2 mM) have been mixed and stirred for a while under the darkness (half a

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day to overnight). After the binding process, the solution was filtered and precipitate was
rinsed extensively with tetrahydrofuran (THF) and ethanol, respectively. In a similar method,
for the photoelectrochemical reaction, TiO2 coated FTO electrodes (Dyesol MS001630-1)
have been immersed into the MK-3 or MK-4 solutions (10-5 M) in THF and washed with
ethanol to remove unbinding molecules of dye. Finally, the prepared photocatalyst and
TiO2/electrodes were dried at room temperature and ready for use in the photocatalytic and
photoelectrochemical hydrogen production reaction.
2.3. Photoelectrochemical and photocatalytic hydrogen evolution measurements
Photoelectrochemical treatment was carried out in a three-electrode cell for linear
sweep voltammetry and chronoamperometry, which includes dye-coated FTO (Dyesol MS
001630, 0.4 cm × 0.7 cm, ca. 10 µm in thickness) as a working electrode, Ag/AgCl and
platinum served as the reference and counter electrodes, respectively, in combination with
redox electrolyte solution of TEOA/Na2SO4. Photocatalytic hydrogen evolution was
performed in a single compartment Pyrex glass reaction cell under visible light irradiation (λ
≥ 420 nm, Solar Light XPS-300™). MK-3/TiO2 or MK-4/TiO2 hybrid photocatalysts,
Cu2WS4 or Pt co-catalysts were dispersed in solutions containing TEOA (0.33M) electron
donor into the reaction-cell in the nitrogen gas filled glovebox. Then reaction-cell was
plugged up with septa and ultrasonic treatment was applied for homogeneous distribution.
Subsequently, photocatalytic HER test was initiated in the visible light and the evolved
hydrogen gas was analysed by gas chromatograph using Shimadzu GC-2010 Plus, thermal
conductivity detector (TCD) and argon as the carrier gas.
3. RESULTS AND DISCUSSION
3.1. Synthesis route, optical and electrochemical properties of organic dyes
The synthetic routes of MK-3 and MK-4 dyes are depicted in Scheme 1. 3,6-bis [4-
methylthien-2-yl]-stetrazine (1) was synthesized by using previous literature [35, 36]. 5 '-{6-

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[5-(4-{bis[4-(hexyloxy)phenyl]amino}phenyl)-4-(2-ethylhexyl)-2-thienyl] 1,2,4,5-tetrazin- 3-
yl} -3'- (2-ethylhexyl) 2,2'-bithiophene-5-carbaldehyde (4) and 5-[5-{6-[5-(4-{bis[4-
(hexyloxy)phenyl]amino}phenyl)-4-(2-ethylhexyl)-2-thienyl]-1,2,4,5-tetrazin-3-yl}-3-(2-
ethylhexyl)-2-thienyl]-2-furaldehyde (5) were synthesized by Suzuki reaction between the 4-
[5-{6-[5-Bromo-4-(2-ethylhexyll)-2-thienyl]-1,2,4,5-tetrazin-3-yl}-3-(2-ethylhexyl)-2-thienyl
-N,N-bis[4-(hexyloxy)phenyl]aniline (3) and the boronic acids (5-formyl-2-furanboronic acid
5-formyl-2-thienylboronic acid). Molecules (4) and (5) were converted by Knoevenagel
condensation reaction to MK-3 and MK-4 dyes. The synthetic process, structural and
elemental characterization of MK-3 and MK-4 are given in the SI in detail (Figure S1-10).
H3C
CH3
H3C
CH3
N
N
N
N
N
N
N
N
NBS
H3C
CH3
CHCl3/CH3COOH
H3C
CH3
S
S
S
S
Br
Br
(1)
(2)
CH3
CH3
H13C6
O
CH3
Br
Br
CH3
H13C6
O
OC6H13
S
S
N
N
Pd(dppf)Cl2 K2CO3
DME
N
B
N
S
N
N
N
N
N
N
H3C
H13C6
O
S
H3
C
HO
OH
Br
CH3
CH3
(3)
H
O
CH3
CH3
S
CH3
CH3
Br
S
H13C6
O
S
H13C6
O
N
N
N
H
N
O
N
N
S
N
S
N
N
Pd(dppf)Cl2 K2CO3
DME
N
S
H3C
H3C
B
OH
H13C6
O
HO
H13C6
O
H3C
H3C

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(4)
OH
O
H
H
O
N
N
CH3
CH3
CH3
CH3
S
S
S
S
H13C6
O
H
13C6O
N
N
N
N
N
CNCH2COOH
S
N
S
N
Piperidine
CH3Cl
N
N
H3C
H3C
H
13C6O
H
13C6O
H3C
H3C
MK-3
H
O
CH3
CH3
O
CH3
CH3
Br
S
H13C6
O
S
H13C6
O
N
N
N
H
N
O
N
N
S
N
S
N
N
Pd(dppf)Cl2 K2CO3
DME
N
O
H3C
H3C
B
OH
H13C6
O
HO
H
13C6O
H3C
H3C
(5)
OH
O
H
H
O
N
N
CH3
CH3
CH3
CH3
O
O
S
S
H13C6
O
H13C6
O
N
N
N
N
N
CNCH2COOH
Piperidine
S
N
S
N
CH3Cl
N
N
H3C
H3C
H13C6
O
H
13C6O
H3C
H3C
MK-4

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Scheme 1. Synthetic pathway of the MK-3 and MK-4 dyes.
Binding ratios between TiO2 and MK-3/MK-4 dyes have been found out from the
energy dispersive X-ray spectroscopy (EDX) data’s. EDX spectra and of dye sensitized TiO2
have been displayed in the figure S11 (see SI). Elemental analysis obtained from EDX spectra
of dye sensitized TiO2 have been given in the Table 1. The binding ratio between TiO2 and
dye has been calculated from N/Ti ratio because of the common elements in the each sample.
N/Ti ratios have been turned out to be 3.25x10-3 and 3.16x10-3 for the MK-3/TiO2 and MK-
4/TiO2, respectively. It is obvious that the dye contents are quite similar in both photocatalyst.
The corresponding SEM images of MK-3/MK-4 sensitized TiO2 have also been given in the
supporting information as the figure S12.
Table 1. EDX spectra data’s of dye sensitized TiO2
Element Normal Normal Atom
Error
(%wt) (%wt) (%atom) (%)
Titanium 49.06
Oxygen 32.62
56.34
37.46
2.21
0.05
3.94
100
1.4
37.2
0.4
0.1
0.2
30.73
61.16
4.80
0.10
3.21
100
Carbon
1.92
Nitrogen 0.05
Sulfur
Total
3.43
87.08
Titanium 49.46
Oxygen 38.02
53.94
41.54
1.78
0.04
2.70
100
1.4
24.5
0.3
0.2
0.1
28.45
65.57
3.75
0.09
2.13
100
Carbon
1.64
Nitrogen 0.04
Sulfur
Total
2.48
91.63
When the optical properties of dyes are examined, sharp absorption peaks at 426 nm
and 286 nm for MK-3, and a weak peak at 362 nm while a sharp peak at 282 nm for MK-4

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was observed as shown in Figure 1a-b. The first peaks at 286 and 282 nm for the MK-3 and
MK-4, respectively, are originated from localized π-π* transitions. The second peaks at 426
and 362 nm for the MK-3 and MK-4, respectively, are arise from delocalized π-π* transitions
thanks to intermolecular charge transfer (ICT) properties between the donor and acceptor
groups [37]. Using the UV-Vis absorption spectra, the molar absorption coefficients were
calculated for MK-3 and MK-4 dyes as 2142 M-1 cm-1 (426 nm) and 2570 M-1 cm-1 (365 nm)
[34]. The photoluminescence emission spectra of MK-3 and MK-4 have been given in the
supporting information as the figure S13. Maximum emission peaks of MK-3 and MK-4 have
been 575 nm and 538 nm, respectively. The electrochemical behaviour of dyes was
investigated by the cyclic voltammetry. In cyclic voltammograms as shown in Figure 1c-d,
the oxidation potentials which originated from triphenylamine groups (donor moieties) were
found to be 0.86 V and 0.83 V for MK-3 and MK-4, respectively. The reduction peaks, which
are thought to belong to π groups in the molecules, are found to be -0.79 V and -0.83 V for
MK-3 and MK-4, respectively. Finally, the peaks observed in -1.54 V and -1.25 V are pointed
out the acceptor group (the cyanocarboxylic acid) of MK-3 and MK-4 dyes, respectively
(Table 2).

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Figure 1. Absorption spectrums (a-b) and cyclic voltammograms (c-d) of MK-3 (red
line) and MK-4 (blue line)
Table 2. Optical and electrochemical parameters of MK-3 and MK-4 dyes
Oxidation
Wavelength
Molar Absorption
Coefficients (ε) / M-1 cm-1
Reduction
Dyes
Potentials
/ V
(λ) / nm
Potentials / V
MK-3
MK-4
426
362
2142
2570
0.86
-0.79 / -1.54
-0.83 / -1.25
0.83
3.2. Photochemical Hydrogen Evolution Reactions
Photoelectrochemical (PEC) hydrogen evolution characteristics of MK-3/MK-4
sensitized TiO2 electrodes have been carried out in the aqueous electrolyte solution
(Na2SO4/TEOA) containing of a three-electrode setup with Pt counter electrode, Ag/AgCl
reference electrode and dye sensitized TiO2 working electrodes under solar simulator (Solar

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Light XPS-300TM) illumination limited by cut-off filter (λ ≥ 420 nm). PEC experiments,
which are consisted of chronoamperometry and linear sweep voltammetry (LSV) techniques,
have been performed by open/close of irradiation. LSV studies have been recorded from 0.5
V to -0.5 V, under light/dark cycles. The dye sensitized electrodes display very stable
behavior in this potential frame (Figure 2a). Then, PEC experiments have been studied by the
chronoamperometric method during 350 second with 50 second light off and 50 second light
on at the 0 V potential as shown in figure 2b. MK-3/MK-4 sensitized TiO2 electrodes display
enhancing transient photocurrent density when compared to non-sensitized TiO2 electrode due
to the photogenerated electron-hole separation efficiency of D-π-A organic dyes [38].
Transient photocurrent densities for MK-3/TiO2 and MK-4/TiO2 photoelectrodes have been
attained to 110 and 275 µA cm-1, respectively, while non-sensitized TiO2 electrode reached to
1 µA cm-2 [39]. These comparable photocurrent values of MK-3/TiO2 and MK-4/TiO2 could
be cleared up by differences of intermolecular charge transfer properties [5].
Figure 2. Photoelectrochemical response of TiO2 (black line), TiO2 / MK-3 (red line)
and TiO2 / MK-4 (blue line) by (a) LSV (at a scan rate of 100 mV s-1) and (b)
chronoamperometry techniques in the TEOA (5%) and Na2SO4 (0.1 M) solution at pH 9.

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In addition, photoactivities of MK-3 and MK-4 dyes have been investigated by
photocatalytic HER using dye/TiO2 (10 mg) and aqueous TEOA (5%) solution as a
photocatalyst and sacrificial electron donor, respectively, under solar simulator illumination
(light limited by cut-off filter λ ≥ 420 nm). First of all, optimal pH studies have been carried
out under changing pH from 7 to 10. Among these pH values, the best HER activity has been
shown at pH 9 according to the hourly evolved hydrogen amount (Figure S14 in SI). HER
rates have been decreasing remarkably at more acidic or basic conditions. TEOA is
protonated under pH values of 7, which leads to weakened giving electron of TEOA as a
sacrificial electron donor agent. Hydrogen generation rates thermodynamically become
unfavorable at strong alkaline media. These results have been also in accordance with the
previous published papers using TEOA sacrificial electron donor [17, 26, 27, 40-43]. HER
rates at the pH 9 have been found 427 µmolg-1h-1 and 682 µmolg-1h-1 for the MK-3/TiO2 and
MK-4/TiO2, respectively. The differences of HER rates according to pH can be explained by
electron donor roles of TEOA. The acidic pH of solution is inhibited the ability of electron
donation due to the protonation of TEOA. The increasing pH of alkaline TEOA solution is
lead to decrease in the HER rates because the driving force of HER from water (H+/H2) is
become more negative at high pH [44-46]. The HER activities of dye sensitized TiO2 have
been also explored in the absence and presence of co-catalysts (Pt and Cu2WS4). Pt co-
catalyst has been obtained by the reduction of H2PtCl6. The other co-catalyst ternary metal
sulfide Cu2WS4 as an alternative to Pt has been supplied from our previous published papers
[26, 47] and used as the PGM-free co-catalyst. The HER rates have been found as 427, 675,
1277, 682, 795 and 1027 µmolg-1h-1 for the MK-3/TiO2, MK-3/TiO2/Cu2WS4, MK-3/TiO2/Pt,
MK-4/TiO2, MK-4/TiO2/Cu2WS4 and MK-4/TiO2/Pt, respectively. The evolved amounts of
H2 have been figured out after 8 hour of photocatalytic HER for MK-3/TiO2, MK-
3/TiO2/Cu2WS4, MK-3/TiO2/Pt, MK-4/TiO2, MK-4/TiO2/Cu2WS4 and MK-4/TiO2/Pt as the

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2071, 3170, 15780, 3740, 4200 and 4972 µmolg-1, respectively (Figure 3a-b). When using
only TiO2, Cu2WS4 or TiO2/Cu2WS4, there is no H2 evolved in the same conditions. This
results shows that light is absorbed by only dye molecules. Photoelectrochemical and
photocatalytic HER results in accordance with each other. The differences of HER rates using
D-π-A organic dye sensitized TiO2 have been related to structural variation of MK-3 and MK-
4 dyes. The strong acceptor groups of the MK-3 and MK-4 are displayed similar properties
for cyanocarboxylic acid that attached on the TiO2 surface. On the other hand, the thiophene
(MK-3) and furan (MK-4) spacers linked to this strong acceptor can be distinguished from
their different electronegativity effect. Here, the electronegativity of the oxygen in the furan
group is higher value than the sulphur in the thiophene group, so that intramolecular electron
transfer activity and the photoactivity are more effective.
Figure 3. Photocatalytic hydrogen evolution activities of (b) TiO2 / MK-3, TiO2 /
MK-3 / Cu2WS4, TiO2 / MK-3 / Pt and (c) TiO2 / MK-4, TiO2 / MK-4 / Cu2WS4, TiO2 / MK-4
/ Pt (10 mg TiO2 / dye; 20 ml TEOA (5%), pH=9).

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3.3. The mechanism of Hydrogen Evolution Reaction
The mechanism of HER has been proceeded by electron transfer mechanism as
displayed in the figure 4. This mechanism is taken place in three step: (i) Photons are
absorbed by dyes and electrons excited from HOMO level to LUMO. (ii) Then, the
photogenerated electrons are injected to conduction band (CB) energy level of TiO2. These
photogenerated electrons can be transfused to CB of co-catalyst in the presence of Cu2WS4,
which is thermodynamically favorable since the CB of Cu2WS4 is located between CB of
TiO2 and redox level of H2O/H2 [26]. (iii) Finally, photogenerated electrons on the CB are
reduced the water to produce H2 gas, and holes on the oxidative state of the dye molecules are
refilled with electrons by sacrificial electron donor TEOA to regenerate photocatalytic HER at
the same time. Herewith, HER reaction is readily cycled according to electrochemical band
levels of each component.
Figure 4. The proposed mechanism of HER by using dye sensitized TiO2 as the
photocatalyst in the presence of Cu2WS4 as the co-catalyst.

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4. CONCLUSIONS
In conclusion, the photochemical HER has been investigated by using tetrazine based
novel D-π-A type MK-3 and MK-4 dyes for the sensitization of TiO2 photocatalyst in the
absence and presence of co-catalysts (Pt or Cu2WS4). The photochemical HER differences
can be attributed to the structural variation of MK-3 and MK-4 dyes. The thiophene (MK-3)
and furan (MK-4) spacers linked to cyanocarboxilic acid acceptor give rise to the different
electronegativity effect. MK-4 dye causes the more photoactivity for the HER due to the high
electronegativity of the furan spacer group. In addition, photocatalytic activities of dye
sensitized TiO2 have been increased by addition of Cu2WS4 or Pt co-catalysts. The
photoactivities shows that these D-π-A dyes (MK-3 and MK-4) may be used different solar
energy conversion applications such as dye sensitized solar cells.
ASSOCIATED CONTENT
Supporting Information Available. Detailed synthesis, structural and elemental
characterization of dyes, EDX spectra and SEM images of dye sensitized TiO2,
photoluminescence emission spectra of dyes and pH dependency of photocatalytic HER rates
are given in Supporting Information.
AUTHOR INFORMATION
Corresponding Author
* E-mail: imrenhatay@gmail.com
Notes. The authors declare no competing financial interest.

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ACKNOWLEDGMENT
Imren Hatay Patir would like to thank to The Scientific and Technological Research
Council of Turkey (TUBITAK) (Grant No: 215M309), for Woman in Science program by
UNESCO-Loreal, GEBIP fellowship by Turkish Academy of Sciences and Selcuk University
Scientific Research Foundation (17201020) for financial support this study. This paper is
consisted of part of Ph.D thesis prepared by Emre Aslan.

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Figure and Scheme Captions
Figure 1. Absorption spectrums (a-b) and cyclic voltammograms (c-d) of MK-3 (red
line) and MK-4 (blue line)
Figure 2. Photoelectrochemical response of TiO2 (black line), TiO2 / MK-3 (red line)
and TiO2 / MK-4 (blue line) by (a) LSV (at a scan rate of 100 mV s-1) and (b)
chronoamperometry techniques in the TEOA (5%) and Na2SO4 (0.1 M) solution at pH 9.
Figure 3. Photocatalytic hydrogen evolution activities of (b) TiO2 / MK-3, TiO2 /
MK-3 / Cu2WS4, TiO2 / MK-3 / Pt and (c) TiO2 / MK-4, TiO2 / MK-4 / Cu2WS4, TiO2 / MK-4
/ Pt (10 mg TiO2 / dye; 20 ml TEOA (5%), pH=9).
Figure 4. The proposed mechanism of HER by using dye sensitized TiO2 as the
photocatalyst in the presence of Cu2WS4 as the co-catalyst.
Scheme 1. Synthetic pathway of the MK-3 and MK-4 dyes.
Table 1. EDX spectra data’s of dye sensitized TiO2
Table 2. Optical and electrochemical parameters of MK-3 and MK-4 dyes

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•
•
•
•
Tetrazine based two novel D-π-A organic dyes are synthesized for the first time
First time MK-3 and MK-4 D-π-A dyes are used on photochemical hydrogen evolution
Photoactivity differences are explained by changing spacer group
HER rates are clarified by structural and optical properties of dyes