Phenothiazine-based organic dyes with two anchoring groups on TiO 2 for highly efficient visible light-induced water splitting

Article abstract of DOI:10.1039/c2cc36501d

New phenothiazine-based organic dyes with two anchoring groups at 3,7 positions and systematic alkyl chains on nitrogen were prepared. Their TiO ₂-Pt composites showed excellent photo-catalytic activities in visible light-induced water splitting. Interestingly, phenothiazine dyes with longer alkyl chains showed better stability in catalytic systems.

Full text of DOI:10.1039/c2cc36501d

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COMMUNICATION
Phenothiazine-based organic dyes with two anchoring groups on TiO2 for
highly efficient visible light-induced water splittingw
Jeho Lee,a Jiwon Kwak,b Kyoung Chul Ko,b Ji Hoon Park,b Ju Hong Ko,b Nojin Park,a
Eunchul Kim,a Do Hyun Ryu,b Tae Kyu Ahn,*a Jin Yong Lee*b and Seung Uk Son*ab
Received 6th September 2012, Accepted 9th October 2012
DOI: 10.1039/c2cc36501d
New phenothiazine-based organic dyes with two anchoring
groups at 3,7 positions and systematic alkyl chains on nitrogen
were prepared. Their TiO2–Pt composites showed excellent
photo-catalytic activities in visible light-induced water splitting.
Interestingly, phenothiazine dyes with longer alkyl chains
showed better stability in catalytic systems.
As renewable energy resources become an important issue, use
of solar energy for diverse chemical transformations has been
an attractive research subject.1 Especially, the production of
hydrogen from water using solar energy has attracted special
attention from scientists.2 Since the early 1970s, it has been
known that photo energy from ultraviolet (UV) light can be used
to split water into hydrogen and oxygen using wide-bandgap
semiconductors such as TiO2.3 Moreover, utilizing the appropriate
dyes with semiconductors, visible light can be used for water
splitting.4 Although there has been some progress in the develop-
Fig. 1 Structural analysis of the phenothiazine-based organic dyes
ment of dyes, there should definitely be more extensive studies for
more active systems.
used in this study and an illustration of visible light-driven water
splitting.
As visible light sensitizers on TiO2, ruthenium-based dyes have
been extensively used for water splitting.5 Although there have
been recent studies on organic dye systems, efficient systems are
rare due to the relative instability of organic systems.4,6 During
the operation of catalytic systems, the photo-excited dyes transfer
electrons to semiconductor materials to form transient cationic
radical species which retrieve electrons from additional sacrificing
compounds. In this process, the dyes are subject to decomposition,
resulting in low turnover number (TON). Thus, more exploration
for stable and efficient dyes is required.
behavior, diverse redox devices including electrochromic displays
have been developed using phenothiazines.7,8 However, as far as
we are aware, the application of phenothiazine derivatives for
photocatalytic water splitting has not been reported.
For controlling the chemical surroundings, diverse functional
groups can be easily introduced at the nitrogen of phenothia-
zines. It is noteworthy that the substituents in dyes such as
long alkyl chains could enhance electrochemical performance.9
In addition, two anchoring groups can be introduced at the 3
and 7 positions of phenothiazine systems. The multi-anchoring
groups can enhance the stability of catalytic systems during the
photocatalytic reactions.10 Usually, N-substituted simple
phenothiazines are colorless and cannot absorb visible light.
However, further introduction of electron withdrawing groups
into aromatic rings in phenothiazines can induce shift of the
absorption band into the visible light region. It has been
Phenothiazines are cyclic compounds with amine and sulfide
moieties.7 One of the interesting aspects of phenothiazines is the
intriguing stability of their cationic radical formed by one electron
oxidation7 (Fig. 1). Thus, it can be speculated that the phenothia-
zine-based dyes can have enhanced stability in photo-induced
electron transfer from dyes to TiO2. Based on this unique redox
reported that phenothiazine-based dyes with
a directly
a Department of Energy Science, Sungkyunkwan University,
Suwon 440-746, Korea
connected anchoring group showed higher visible light har-
vesting efficiencies in dye-sensitized solar cells than typical
donor–acceptor type dyes with additional p-conjugated
bridges.11 Thus, we designed the straightforward phenothia-
zine-based dyes (P1–P5) with two anchoring groups, as shown
in Fig. 1.
b Department of Chemistry, Sungkyunkwan University,
Suwon 440-746, Korea. E-mail: sson@skku.edu;
Fax: +82-031-290-4572
w Electronic supplementary information (ESI) available: Experimental
details, characterization data of new compounds, UV-vis spectra of
dyes. See DOI: 10.1039/c2cc36501d
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11431–11433 11431

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In this work, we report on the preparation of phenothiazine-
based organic dyes with systematic alkyl chains and two
anchoring groups, their TiO2–Pt composites, and the promising
photo-catalytic activities in visible light-induced water
splitting.
Table 1 Optical and electrochemical properties of P1–P5
b
c
l
max/ea
Dyes (nm/MÀ1 cmÀ1
E0–0
(eV)
Eox1/2
(V)
HOMOd LUMOe
(eV)
)
(eV)
P1
P2
P3
P4
P5
476/13 200
467/13 400
466/13 300
464/13 600
463/13 500
2.19
2.21
2.23
2.23
2.23
0.52
0.54
0.55
0.55
0.55
À5.40
À5.42
À5.43
À5.43
À5.43
À3.21
À3.21
À3.20
À3.20
À3.20
For preparation of the target dyes, alkyl groups were first
introduced into nitrogen by reaction of phenothiazine with
alkyl halides (Scheme 1). Through Vilsmeier reaction,12 two
formyl groups were introduced at the 3 and 7 positions. Then,
Knoevenagel condensation12 with cyanocarboxylic acid in the
presence of base resulted in target dyes with two anchoring
groups. The introduction of cyanocarboxylic acid to N-alkyl-
phenothiazines induced a color change from colorless to
deep red.
a
UV-vis absorption band obtained in g-butyrolactone (GBL).
0–0 Transition energy measured at the onset of absorption spectra.
Oxidation potential (vs. ferrecene, Fc/Fc+) measured using 0.10 M
b
c
Bu4NPF6 in GBL, ITO and Ag/Ag+ as the electrolyte solution,
d
working and reference electrode respectively. HOMO value calcu-
e
lated using potential value of oxidative waves. LUMO value calcu-
lated by HOMO + E0–0
.
Fig. 2, Fig. S1 in the ESIw and Table 1 show optical and
electrochemical properties of the prepared dyes (P1, N-ethyl;
P2, N-butyl; P3, N-octyl; P4, N-dodecyl; P5, N-hexadecyl).
The dyes P1–P5 showed maximum absorption peaks at
476–463 nm in UV/vis absorption spectroscopy, corres-
ponding to deep red color. The optical properties of dyes were
retained on TiO2 (Fig. 2a). Cyclic voltammetry of dyes
revealed reversible one-electron oxidation behavior (Fig. 2b).
The length of alkyl chains on nitrogen showed little effect on
the optical and electrochemical properties. Based on UV/vis
absorption spectra and cyclovoltammograms of P1–P5, the
HOMO and LUMO values were calculated. The 0–0 transition
energies of dyes were calculated in the range of 2.19–2.23 eV,
indicating suitable values for visible light absorption. The
locations of HOMO and LUMO were calculated as À5.4
and À3.2 eV respectively. Considering the location of conduction
and valence bands of TiO2,13 P1–P5 are suitable as dyes for visible
light-driven water splitting through complexation with TiO2.
To develop the visible light-driven catalytic systems for
water splitting, Pt nanoparticles were loaded on commercial
TiO2 nanoparticles by a known method in the literature.14
Then, the prepared dyes were adsorbed on TiO2–Pt composites
by treating composites with 3 Â 10À4 M methanolic solutions
of the dyes. The concentration of the dyes was optimized by
screening concentrations (see Fig. S2 in the ESIw). After
retrieving the dye–TiO2–Pt composites by centrifugation, the
remaining solution became colorless, indicating that the used
dyes were completely adsorbed on TiO2–Pt nanoparticles.
Using the dye-adsorbed TiO2–Pt composites, photocatalytic
performances were investigated in aqueous solution with a
200 W xenon lamp and triethanolamine (TEOA) as sacrificing
reagents (see the ESIw for details). Light with wavelengths
shorter than 420 nm was cut off with an optical filter. For
comparison experiments, dye–TiO2–Pt composite materials
were prepared using commercial N719 and eosin Y via the
same experimental procedure. Fig. 3 shows the photocatalytic
performances of the prepared catalytic systems.
According to Fig. 3 and Table 2, the photocatalytic activities
are summarized by two striking features. First, the phenothia-
zine-based dyes showed significantly higher TONs (380–1026
after 5 hours) than N719 (120 TON after 5 hours) and eosin Y
(160 TON after 5 hours). It is noteworthy that a TON result
(100 after 8 hours) similar to ours for N719 was reported in
recent ruthenium-based water splitting.15 Moreover, P2 and P5
with two anchoring groups showed the higher TONs than dyes
with one anchoring group (see Fig. S3 in the ESIw). The higher
TON values of the P1–P5 could have originated from the
stability of cationic species of phenothiazine-based dyes, as
shown in their electrochemical behavior. Second, the photo-
catalytic activities of P1–P5 were quite dependent on the alkyl
groups on nitrogen. As the length of alkyl chains on nitrogen
became longer, higher turnover numbers (TON) were observed.
Thus, among the prepared dyes, the P5 with the hexadecyl
group on nitrogen of phenothiazine showed the best result with
254 mmol H2 generation after 5 hours, corresponding to 1026
TON. These observations imply that the longer alkyl chains
play a positive role in maintaining the stability of catalytic cycles.
Recently, it was reported that the introduction of long alkyl
chains to ruthenium dyes enhanced the performance of solar cells.9
Scheme 1 Synthetic route for the phenothiazine-based organic dyes
used in this study.
Fig. 2 UV-vis absorption spectra and cyclic voltammograms of dyes.
11432 Chem. Commun., 2012, 48, 11431–11433
c
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Fig. 4 HOMO and LUMO states of P5 and their electron densities.
This work was supported by grants NRF-2011-0029186
(Midcareer Researcher Program) and NRF-2011-0031392
(Priority Research Centers Program). JL thanks for grant
R31-2008-10029 (WCU program). JYL acknowledges the
financial support (No. 2007-0056343) through the NRF of
Korea funded by the MEST.
Fig. 3 Photocatalytic activities of the dyes (P1–P5, N719 and eosin
Y) in visible light-driven water splitting; reaction conditions: 5 mL
10% (v/v) TEOA, 16.5 mg dye–TiO2–Pt composites (0.495 mmol dye),
pH 7.0.
Notes and references
Table 2 Photocatalytic performances of dyes (P1–P5)a
1 Recent reivews: (a) S. C. Roy, O. K. Varghese, M. Paulose and
C. A. Grimes, ACS Nano, 2010, 4, 1259; (b) J. M. R. Narayanam
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b
Dyes
Amount of H2 (mmol)
TONc
TOFd (hÀ1
)
P1
P2
P3
P4
P5
94
109
150
198
254
380
440
606
800
76
88
121
160
205
1026
a
A 200 W Xe lamp was used (the light below 420 nm was cut off by an
b
c
optical filter). Amount of H2 after 5 hours. TON = (2 Â amount
d
of H2)/amount of dye after 5 hours. Values after 5 hours.
5 (a) J. Kiwi and M. Gratzel, J. Am. Chem. Soc., 1979, 101, 7214;
¨
¨
(b) D. Duonghong, E. Borgarello and M. Gratzel, J. Am. Chem.
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It has been reported that the long alkyl chains retard the
decomposition of excited states by blocking the possible
reactions with unfavorable chemical quenchers.9 We speculate
that a similar effect can be valid for this work. Moreover, the
alkyl groups on nitrogen can induce a favorable orientation of
dyes on TiO2, which may result in the efficient electron
injection from excited dyes to TiO2.9 As shown in Fig. 4,
density functional theory (DFT) calculation was performed
for P5. The major electron densities of HOMO and LUMO
states of the P5 are located on the phenothiazine ring and
cyanoacetate groups, respectively. Thus, it is expected that
photo-excitation can induce intramolecular charge transfer
from the phenothiazine ring to cyanoacetate groups.
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In conclusion, phenothiazine-based organic dyes were
prepared and showed highly efficient performances in visible
light-driven water splitting. The alkyl chains on the nitrogen of
the phenothiazine ring further enhanced the TON values up to
1016 after 5 hours. This work shows that phenothiazines are
promising building blocks for visible light harvesting materials
in photocatalytic water splitting.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 11431–11433 11433