Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase

Article abstract of DOI:10.1016/j.jphotochem.2015.06.026

Effect of chemical structures of some 2,2′-bipyridinium salts (BP²⁺) as the electron carrier molecules on the visible-light induced conversion of CO₂ to formic acid with the system consisting of water-soluble zinc tetraphneylporphyrin tetrasulfonate (ZnTPPS) and formate dehydrogenase (FDH) in the presence of triethanolamine (TEOA) as an electron donor molecule was investigated. Irradiation of a CO₂ saturated solution containing TEOA, ZnTPPS, BP²⁺ and FDH with visible light resulted in production of formic acid. By using 1,1′-ethylene-2,2′-bipyridinium dibromide (DB²⁺) as an electron carrier molecule, the effective formic acid production was observed compared with the other 2,2′-bipyridinium salt derivatives.

Full text of DOI:10.1016/j.jphotochem.2015.06.026

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JPC 9950 No. of Pages 5
Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effect of chemical structure of bipyridinium salts as electron carrier on
the visible-light induced conversion of CO2 to formic acid with the
system consisting of water-soluble zinc porphyrin and formate
dehydrogenase
d
d
Yutaka Amaoa,b,c, , Ryutaro Abe , Sachina Shiotani
*
a
Advanced Research Institute for Natural Science and Technology, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan
Research Centre for Artificial Photosynthesis (ReCAP), Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan
Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency, 4-1-8 Honcho Kawaguchi, Saitama 332-0012,
b
c
Japan
d
Department of Applied Chemistry, Oita University, Dannoharu 700, Oita 870-1192, Japan
A R T I C L E I N F O
A B S T R A C T
Effect of chemical structures of some 2,20-bipyridinium salts (BP2+) as the electron carrier molecules on
the visible-light induced conversion of CO2 to formic acid with the system consisting of water-soluble
zinc tetraphneylporphyrin tetrasulfonate (ZnTPPS) and formate dehydrogenase (FDH) in the presence of
triethanolamine (TEOA) as an electron donor molecule was investigated. Irradiation of a CO2 saturated
solution containing TEOA, ZnTPPS, BP2+ and FDH with visible light resulted in production of formic acid.
By using 1,10-ethylene-2,20-bipyridinium dibromide (DB2+) as an electron carrier molecule, the effective
formic acid production was observed compared with the other 2,20-bipyridinium salt derivatives.
ã 2015 Elsevier B.V. All rights reserved.
Article history:
Received 18 March 2015
Received in revised form 18 June 2015
Accepted 24 June 2015
Available online xxx
Keywords:
Artificial photosynthesis
Bipyridinium salt
CO2 reduction
Formate dehydrogenase
Formic acid synthesis
Zinc porphyrin
1. Introduction
formate (FDH), aldehyde (AldDH) and alcohol dehydrogenase
(ADH) for the synthesis of valuable organic compounds such as
Photoredox system consisting of an electron donating molecule,
a photosensitizer, an electron carrier molecule and catalyst is
widely used for the photoenergy conversion to chemical energy
system [1–5]. This system so called artificial photosynthesis is a
simplification of the photoinduced electron transfer process in
natural photosynthesis reaction. By using reduced electron carrier
molecule as a substrate or coenzyme for the catalyst or enzyme in
this system, hydrogen production and CO2 to the organic material
conversion systems would be developed. For visible light-induced
hydrogen production systems, platinum nano-particle or enzyme
hydrogenase has been used as hydrogen producing catalyst with
the visible light sensitizers such as water soluble zinc porphyrins,
ruthenium polypyridyl coordination complexes and chlorophyll-a
[6–18]. The visible light induced chemical and enzymatic synthesis
of organic compounds has also been developed using this system
in the presence of various dehydrogenases such as lactate (LDH),
lactic acid [19], formic acid [20–28], methanol [29–30] etc. Among
these dehydrogenases, FDH catalyzes the conversion of CO2 to
formic acid in the presence of a suitable electron donating
molecules as a coenzyme such as NADH, reduced form of
bipyridinium salts and so on. Thus, visible light-induced CO2
conversion to organic material system will be developed with the
combination of photoredox system and FDH as an artificial
photosynthesis model.
Photochemical and enzymatic formic acid synthesis from CO2
(hydrogen carbonate ion) with formate dehydrogenase (FDH) and
methylviologen (MV2+) photoreduction with a system containing
ruthenium(II) coordination compound e.g. trisbipyridinium ru-
thenium (III) [Ru(bpy)3]2+as a photosensitizer and mercaptoetha-
nol (RSH) as an electron donor has been reported for the first time
[20–23]. In contrast, we previously reported the photochemical
and enzymatic formic acid synthesis from CO2 or hydrogen
carbonate ion with the system consisting of MV2+, FDH and water-
soluble zinc porphyrin [25–27] or chlorophyll-a [28] in the
presence of triethanolamine (TEOA) as an electron donor. To
* Corresponding author. Fax: +81 6 6605 3726.
E-mail address: amao@ocarina.osaka-cu.ac.jp (Y. Amao).
http://dx.doi.org/10.1016/j.jphotochem.2015.06.026
1010-6030/ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Amao, et al., Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light
induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase, J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.026

G Model
JPC 9950 No. of Pages 5
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improve the yield of formic acid from CO2 in the system consisting
of a photosensitizer, an electron carrier molecule and FDH, an
effective electron carrier molecule for activation of FDH activity of
CO2 to formic acid conversion is desirable. 2,20-Bipyridinium salt
derivatives also act as the effective electron carrier molecules for
the formic acid production from CO2 in the system consisting of a
photosensitizer, an electron carrier molecule and FDH instead of
MV2+. By using 1,10-alkyl-2,20-bipyridinium salt or 1,10-dimethyl-
2,20-bipyridinium salt, the redox potential and dihedral angle
between pyridine rings will be controlled by the methylene chain
length bridged between pyridine rings in bipyridinium salt.
In this work, effect of chemical structures of 2,20-bipyridinium
salt derivatives (chemical structures in Fig. 1) as the electron
carrier molecules on the visible-light induced conversion of CO2 to
formic acid with the system consisting of water-soluble zinc
tetraphneylporphyrin tetrasulfonate (ZnTPPS) and FDH in the
presence of TEOA as an electron donor molecule (Fig. 2) was
investigated. By using 1,10-ethylene-2,20-bipyridinium dibromide
(DB2+) as an electron carrier molecule, the effective formic acid
production observed compared with the other 1,10-alkyl-2,20-
bipyridinium salt derivatives.
Fig. 2. Photochemical and enzymatic synthesis of formic acid from CO2 with the
system consisting of TEOA, ZnTPPS, bipyridinium salt (BP2+) and FDH.
of zinc acetate in 100 ml of methanol at 40 ꢀC for 2 h. The insert of
zinc ion into H2TPPS was monitored by UV–visible absorption
spectra using Shimadzu Multispec 1500 spectrophotometer.
During the reaction, the characteristic absorption band of ZnTPPS
at 550 nm increased and the absorbance at 650 nm of H2TPPS
decreased gradually. After evaporation of the solvent, the product
was dissolved in distilled water. The purification of ZnTPPS was
carried out by dialysis (pore diameter of the tube 2.0 nm) in
distilled water. UV–vis absorption spectrum of ZnTPPS in an
2. Experimental
2.1. Materials
FDH from Candida boidinii was purchased from Roche
Diagnostics K.K. In accordance with the literature, the molecular
weight of FDH from Candida boidinii was estimated to be 74 kDa.
MV2+, tetraphenylporphyrin tetrasulfonate (H2TPPS), TEOA, 2,20-
bipyridine, methyliodide, 1,2-dibromoethane, 1,3-diboromopro-
pane and 1,4-dibromobutane were supplied by Tokyo Kasei Co. Ltd.
The other chemicals were analytical grade or the highest grade
available.
2.2. Preparation of zinc tetraphenylporphyrin tetrasulfonate (ZnTPPS)
Zinc tetraphenylporphyrin tetrasulfonate (ZnTPPS) was syn-
Fig. 3. UV–vis absorption spectrum of ZnTPPS in 3.0 ml of 1.0 mM sodium
pyrophosphate buffer (pH 7.4).
thesized by refluxing H2TPPS with about 10 times molar equivalent
Fig. 1. Chemical structures of bipyridinium salt derivatives.
Please cite this article in press as: Y. Amao, et al., Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light
induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase, J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.026

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3
aqueous solution was shown in Fig. 3. The molar coefficients for
ZnTPPS also were indicated in Fig. 3. The absorption spectrum
shows a typical Soret band with peak at 422 nm as well as Q-band
at 555 and 590 nm.
Although small amount of zinc acetate was not removed from
ZnTPPS solution, it has been confirmed that zinc acetate has no
influence on the following photoreactions [7,8]. Moreover, the
photo-stability of ZnTPPS was checked as a following experiment.
dithionite (0.1 M) in distilled water was deaerated with argon gas
bubbling for 5 min. The production of reduced form of 2,20-
bipyridinium salt derivative was monitored by UV–vis absorption
spectrum using Shimadzu Multispec 1500 spectrophotometer.
Molar coefficients for reduced form of 2,20-bipyridinium salt
derivative was determined by the absorbance of maximum in
absorption spectrum using Beer–Lambert law.
The solution of ZnTPPS (10
mM) in 3.0 ml of 1.0 mM sodium
2.5. Redox potential measurement of 2,20-bipyridinium salt
derivatives
pyrophosphate buffer (pH 7.4) was deaerated by freeze-pump-
thaw cycles repeated 6 times. The sample solution was irradiated
with a 250 W halogen lamp at a distance of 3.0 cm with a Toshiba L-
39 cut-off filter at 30 ꢀC. The degradation of ZnTPPS was
determined by the absorbance at 422 nm using the molar
coefficient. For 7 h continuous irradiation, little change of the
absorbance at 422 nm was observed. Thus, ZnTPPS has a good
stability for continuous irradiation.
The redox potentials for 2,20-bipyridinium salt derivatives were
determined by cyclic voltammetry (Hokuto Denko HZ-3000). All
measurements were carried out under nitrogen-saturated solution
containing 0.2 M potassium chloride and 1.0 mM sodium pyro-
phosphate buffer (pH 7.4) at
a carbon-working electrode. A
platinum wire was used as a counter electrode. All potentials
were relative to the Ag/AgCl electrode used as the reference.
2.3. Preparation of 2,20-bipyridinium salt derivatives
2.6. Photoreduction of 2,20-bipyridinium salt derivatives by
photosensitisation of ZnTPPS
1,10-Alkyl-2,20-bipyridinium salts was synthesized by following
method as shown in Scheme 1. 2,20-Bipyridine (10 mmol) was
dissolved in 300 ml of acetonitrile and then
a
,
v
-dibromoalkane
A solution containing ZnTPPS (10 m
M), 2,20-bipyridinium salt
(n = 2–4) (100 mmol) was added to the reaction mixture with
stirring at 100 ꢀC for 48 h. 1,10-Alkyl-2,20-bipyridinium salt was
produced as a bright yellow precipitate. The precipitate was
collected by suction filtration and washed with acetonitrile. The
desired product was recrystallized from ethanol and water and
dried under vacuum overnight. Proton nuclear magnetic resonance
derivative (0.1 mM) and TEOA (0.3 M) as an electron donor
molecule in 3.0 ml of 1.0 mM sodium pyrophosphate buffer
(pH 7.4) was deaerated by freeze-pump-thaw cycles repeated 6
times. The sample solution was irradiated with a 250 W halogen
lamp at a distance of 3.0 cm with a Toshiba L-39 cut-off filter at
30 ꢀC. Reduced 2,20-bipyridinium salt derivative concentration was
determined by the absorbance at 605 nm using the molar
coefficient.
(1H NMR) in D2O:
d
(ppm) 1,10-ethylene-2,20-bipyridinium
dibromide (DB2+): 4.78 (s, 4H), 7.95 (m, 2H), 8.10 (m, 2H), 8.78
(m, 2H), 9.02 (m, 2H). 1,10-trimethylene-2,20-bipyridinium dibro-
mide (TB2+): 2.59 (m, 2H), 4.37 (m, 4H), 7.94 (m, 2H), 8.08 (m, 2H),
8.78 (m, 2H), 9.20 (m, 2H). 1,10-tetramethylene-2,20-bipyridinium
dibromide (QB2+): 2.07 (m 4H), 4.59 (m, 4H), 7.93 (m, 2H), 8.06 (m,
2H), 8.79 (m, 2H), 9.02 (m, 2H). 1H NMR spectra were recorded on a
Varian GEMINI-200. The chemical shifts were referenced to the
solvent peak calibrated against tetramethylsilane (TMS).
2.7. Visible light-induced formic acid synthesis form CO2 with ZnTPPS,
2,20-bipyridinium salt derivatives and FDH
A solution containing ZnTPPS (10
derivative (0.1 mM), TEOA (0.3 M) and FDH (9.3
m
M), 2,20-bipyridinium salt
M) in 3.0 ml of
m
1.0 mM sodium pyrophosphate buffer (pH 7.4) was deaerated by
freeze-pump-thaw cycles repeated 6 times and then flushed with
CO2 gas for 5 min. The sample solution was irradiated with a 250 W
halogen lamp and wavelengths of less than 390 nm were blocked
with a cut-off filter at 30 ꢀC. The amount of formic acid produced
was detected by an ionic chromatograph system (Dionex IC2000).
1,10-Dimethyl-2,20-bipyridinium salt (DM2+) was synthesized by
following method as shown in Scheme 2. 2,20-Bipyridine
(17.6 mmol) was dissolved in 300 ml of acetonitrile and then
methyliodide (176 mmol) was added to the reaction mixture with
stirring at 100 ꢀC for 48 h. 1,10-Dimethyl-2,20-bipyridinium salt was
produced as a bright yellow precipitate. The precipitate was
collected by suction filtration and washed with acetonitrile. The
desired product was recrystallized from ethanol and water and
3. Results and discussion
dried under vacuum overnight. 1H NMR in D2O:
d
(ppm) DM2+
:
3.1. Molar coefficient for reduced form of 2,20-bipyridinium salt
4.23 (s, 6H), 7.94 (m, 2H), 8.07 (m, 2H), 8.78 (m, 2H), 9.02 (m, 2H).
derivatives
2.4. Determination of molar coefficient for reduced form of 2,20-
bipyridinium salt derivatives
Molar coefficients for dithionite reduced 2,20-bipyridinium salt
derivatives were determined by UV–vis absorption spectra. Molar
coefficients for reduced DB2+, TB2+, QB2+ and DM2+ were estimated
Molar coefficients for reduced form of 2,20-bipyridinium salt
derivatives were determined by following method. A solution
containing 2,20-bipyridinium salt derivative (0.1 mM) and sodium
to be 4313.5 (lmax: 450 nm), 1253.9 (lmax: 450 nm), 4132.8 (lmax
:
436 nm) and 2166.8 Mꢁ1 cmꢁ1
(lmax: 436 nm), respectively.
Scheme 1. Synthesis procedure of 1,10-alkyl-2,20-bipyridinium salt.
Please cite this article in press as: Y. Amao, et al., Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light
induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase, J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.026

G Model
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Scheme 2. Synthesis procedure of 1,10-dimethyl-2,20-bipyridinium salt.
3.2. Redox potential for 2,20-bipyridinium salt derivatives
ZnTPPS depends on the redox potential of BP2+. By using DB2+ with
the lowest redox potential among BP2+s, the concentration of DB.+
Redox potentials for 2,20-bipyridinium salt derivatives were
determined by cyclic voltammetry using Ag/AgCl electrode as the
reference. The first cathodic peak potentials for DB2+, TB2+, QB2+
and DM2+ were estimated to be ꢁ0.65, ꢁ0.82, ꢁ0.92 and ꢁ0.99 V
(vs. Ag/AgCl), respectively. The corresponding anodic peak
potentials for DB2+, TB2+, QB2+ and DM2+ were estimated to be
ꢁ0.45, ꢁ0.62, ꢁ0.72 and ꢁ0.79 V (vs. Ag/AgCl), respectively. Thus,
the first reduction potentials (E1/2) for DB2+, TB2+, QB2+ and DM2+
were calculated to be ꢁ0.55, ꢁ0.72, ꢁ0.82 and ꢁ0.89 V (vs. Ag/
AgCl), respectively.
produced was estimated to be 80 mM after 180 min irradiation.
3.4. Visible light-induced formic acid synthesis from CO2 with ZnTPPS,
2,20-bipyridinium salt derivatives and FDH
Photochemical formic acid synthesis from CO2 was investigated
by adding FDH to the BP2+ photoreduction system using
photosensitization of ZnTPPS in the presence of TEOA. When the
reaction mixture containing ZnTPPS, BP2+, TEOA and FDH in CO2
saturated sodium pyrophosphate buffer was irradiated with visible
light at 30 ꢀC, formic acid production was observed with irradiation
time as shown in Fig. 5. The formic acid production from CO2 with
the visible-light sensitization of ZnTPPS also depends on the redox
potential of BP2+. In contrast, no formic acid production was
observed under dark or in the absence of CO2 condition. Moreover,
no formic acid production also was observed in the absence of FDH.
These results indicate that the photochemical and enzymatic
synthesis of formic acid from CO2 with FDH via the photoreduction
of BP2+ using ZnTPPS photosensitization.
Next let us focus on the interaction between the reduced form
of BP2+ and FDH activity. Kodaka and Kubota reported that effect of
chemical structures of various BP2+ on the photochemical and
enzymatic formic acid synthesis from CO2 with the system
consisting of TEOA, [Ru(bpy)3]2+, BP2+ and FDH [24]. In this
system, the formic acid production was order of TB2+, MV2+, and
QB2+ after 7 h irradiation. TB2+ was the most active electron carrier
and has a redox potential located between those of *[Ru(bpy)3]2
+/[Ru(bpy)3]3+ and CO2/HCOOꢁ. For the system using ZnTPPS, the
redox potentials of 3ZnTPPS*/ZnTPPS+ and CO2/HCOOꢁ were
3.3. Photoreduction of 2,20-bipyridinium salt derivatives with visible-
light sensitization of ZnTPPS
When the reaction mixture containing ZnTPPS, bipyridinium
salt derivative (BP2+) and TEOA was irradiated with visible light, the
.+
absorbance attributed to the absorption band of reduced BP2+ (BP )
increased with irradiation time as shown in Fig. 4. In this system,
the photoinduced electron transfer from the photoexited triplet
state of ZnTPPS (3ZnTPPS*) to BP2+ proceeded. The oxidation
potential for TEOA was reported to be 0.93 V (vs Ag/AgCl) [31]
Energy level of first singlet state for ZnTPPS was estimated to be
2.07 eV using the average value of the frequencies of the longest
wavelength of the absorption maxima and the shortest wavelength
of the fluorescence emission maxima (606 nm by exciting the
ZnTPPS at 422 nm). The redox potentials of the exited triplet state
of ZnTPPS, E (ZnTPPS+/3ZnTPPS*) and E (3ZnTPPS*/ZnTPPSꢁ) were
reported to be ꢁ0.75 and 0.45 V, respectively [32,33]. Thus, the
reduction efficiency of BP2+ with the visible-light sensitization of
Fig. 4. Time dependence of reduced bipyridinium salt derivative production under
Fig. 5. Time dependence of formic acid production under steady state irradiation
steady state irradiation with visible light using a 250 W halogen lamp at a distance
with visible light using a 250 W halogen lamp at a distance of 3.0 cm. The solution
contained ZnTPPS (10 m
M), 2,20-bipyridinium salt derivative (0.1 mM), TEOA (0.3 M)
of 3.0 cm. The solution contained ZnTPPS (10
m
M), 2,20-bipyridinium salt derivative
(0.1 mM) and TEOA (0.3 M) in 3.0 ml of 1.0 mM sodium pyrophosphate buffer
(pH 7.4).
and FDH (9.3
(pH 7.4).
mM) in 3.0 ml of 1.0 mM CO2 saturated sodium pyrophosphate buffer
Please cite this article in press as: Y. Amao, et al., Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light
induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase, J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.026

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JPC 9950 No. of Pages 5
Y. Amao et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
5
estimated to be ꢁ0.75 and ꢁ0.61 V, respectively. DB2+ was the most
active electron carrier and has a redox potential located between
those of 3ZnTPPS*/ZnTPPS+ and CO2/HCOOꢁ.
Technology Agency JST), Grants-in-Aid for Scientific Research (C)
(Japan Society for the Promotion of Science) (23560947), and
Grant-in-Aid for Scientific Research on Innovative Areas “Artificial
Photosynthesis (2406)”.
Moreover, relationship among the dihedral angle of reduced
form of BP2+, the redox potential of BP2+ and the conversion of CO2
to formic acid with the system consisting of [Ru(bpy)3]2+ and FDH
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Thus, the reduced form of BP2+ with small dihedral angle acts as an
effective electron carrier for FDH.
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Acknowledgment
This work was partially supported by Precursory Research for
Embryonic Science and Technology (PRESTO, Japan Science and
Please cite this article in press as: Y. Amao, et al., Effect of chemical structure of bipyridinium salts as electron carrier on the visible-light
induced conversion of CO2 to formic acid with the system consisting of water-soluble zinc porphyrin and formate dehydrogenase, J.
Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.06.026