An Organic Molecular Photocatalyst Releasing Oxygen from Water

Article abstract of DOI:10.1002/cssc.201901899

Artificial photosynthesis employing solar energy is one of the best means of reaching a sustainable energy cycle, in which water is oxidized to oxygen and supplies electrons for fuel generation. The development of suitable oxygen evolution materials driven by sunlight is hence the most challenging research topic in the field of photocatalysis. Herein, photocatalytic O₂ production from water at a rate of 22.6 μmol h^?1 was performed by using a pyrene-based organic molecule constructed through the Ullmann coupling reaction. Its significantly improved light-harvesting ability and notably accelerated separation and transfer of photogenerated charges enhanced the O₂ evolution efficiency. This study highlights the structural design of organic molecules applied to photocatalytic water splitting.

Full text of DOI:10.1002/cssc.201901899

Accepted Article
Title: An Organic Molecular Photocatalyst Releasing Oxygen from
Water
Authors: Xinchen Wang
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10.1002/cssc.201901899
ChemSusChem
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An Organic Molecular Photocatalyst Releasing Oxygen from
Water
Qinghe Li, Feng Liu, Feng Lin, Wei Lin, and Xinchen Wang*
Abstract: Artificial photosynthesis employing solar-energy is one of
the most prospective means to reach a sustainable energy cycle,
where water is oxidized to oxygen with supplying of electrons for fuel
generation. The development of resultful oxygen evolution materials
driven by sunlight is hence the most challenging research topic in
the field of photocatalysis. Herein, the photocatalytic O2 production
from water with 22.6 μmol h-1 rate was performed using the pyrene-
based organic molecule constructed by Ullmann coupling reaction. It
is the significantly improved light harvesting ability and notably
accelerated separation and transfers of photogenerated charges that
enhanced the O2 evolution efficiency. This study highlights the
structural design of organic molecule applied to photocatalytic water
splitting.
catalysts are noble metal Ru-, and Ir-based inorganic
semiconductors, despite the restricted availability and
tremendous cost.[8] As an alternative material, organic
semiconductors possessing the superiorities of structural
multiformity, regulable synthetic process and precise tuning the
optical and/or optoelectronic and physicochemical properties,
volunteered as the prospective materials in energy catalytic
conversion application.[9] Among which, the metal-free
conjugated polymeric materials featured as the delocalized p-
electron system i.e. binary polymeric carbon nitride (PCN),[10]
ternary boron carbon nitride (BCN),[11] and versatile conjugated
microporous polymer (CMP)[12] coalesced around a number of
articles that facilitated an explosion of research in photocatalytic
releasing O2. Nonetheless, the construction of sufficiently
oxygen-evolving material is still a bottleneck. [13]
Natural being, the small organic molecules e.g. riboflavine,
fluorescein, rose bengal, and rhodamine etc. have been
reported in photocatalytic selective organic synthesis and
pollutant degradation.[14] To date, there is an increasing
tendency to pullulate the organic molecular photocatalyst to
better perform their light-harvesting ability and photo-induced
charge carries efficiency in solar-energy conversion regulated
through molecular engineering.[15] Herein, we present a facile
synthesis method to furnish a photoactive pyrene based organic
molecular photocatalyst constructed by Ullmann coupling
reaction that has a functionality to promote the O2 evolution rate
from water (Scheme 1).
Introduction
Oxygen, appearing in the stable forms of O(3P), O2 and O3, is
regarded as a vital ingredient in the evolution of biology and
geochemistry, and thus understanding its producing approaches
is essential for elucidating the atmosphere chemistry even
arohebiont in general.[1] In the primitive atmosphere during
prebiotic, the exclusive abiotic process identified as the viable
way to generate oxygen before the appearance of
photosynthesis is termed as the three-body recombination
reaction, namely O + O + M → O2 + M, among which the
reactant two O root in the photodissociation of CO2 and M used
as an energy transfer medium.[2] In addition, plant
photosynthesis converting sunlight into chemical energy in the
form of carbohydrates to release O2 simultaneously using CO2
and water as the raw materials has been considered as the most
important chemical reaction on the Earth, and also deemed to
be conducive to explore the appearance of early life.[3] Since
1972, Fujishima and Honda reported the pioneering work for
water splitting into O2 at a semiconductor electrode via the
electrochemical photolysis process,[4] which has inspired the
increasing development of artificial versions of O2
photosynthesis from water splitting. [5]
Scheme 1. Chemical structure of pyrene and pyrene based organic molecular.
Results and Discussion
The burgeoning interest in dowsing the photocatalytic
formation progress of O2 through duplicating plant
photosynthesis may avail to address the urgency of technology
in learning the origin of life.[6] Among, the four electron transfer
process of water oxidation to O2 is supposed as the most
challenging task in solar energy conversion, in which catalysts
lie at the heart.[7] At present, the most effective O2 production
To verify the successful formation of C-Br and C-N bond via
bromination and Ullmann Coupling into pyrene rings, the
products were characterized by fourier transform infrared (FTIR)
spectroscopy and solid-state 13C nuclear magnetic resonance
(NMR) spectroscopy. In the FTIR spectra (Figure 1a), the
intrinsic peak centered at 3131 cm-1 is belonged to the N-H
stretching mode, and peaks at 1484, 1274, 1143, and 979 cm-1
are corresponding to the motions of Tz rings. Furthermore, the
shift of C-Br bond could be observed from 690-515 to 811 cm-1
in Pyr-Br. Compared with Tz and Pyr-Br, the disappearance of
N-H and C-Br signals and shifts of the two strong absorption
peaks at 1509 and 1287 cm-1 are assigned to the C-N stretching
and breathing modes supporting the successful formation of Pyr-
Tz. Moreover, the solid-state 13C NMR spectra also confirm the
Q. Li, F. Liu, F. Lin, Prof. W. Lin, Prof. X. Wang
State Key Laboratory of Photocatalysis on Energy and Environment,
College of Chemistry, Fuzhou University
Fuzhou, 350002, People’s Republic of China.
E-mail: xcwang@fzu.edu.cn
Homepage: http://wanglab.fzu.edu.cn
Supporting information for this article is given via a link at the end of the
document.
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10.1002/cssc.201901899
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belonged to Co2+, respectively (Figure 1d).[17] Besides, the Cu
signal was not detected by the XPS characterization (Figure S4),
verifying the elimination of the copper species from the as-
prepared material. What’s more, the field emission scanning
electron microscopy (SEM) of Pyr-Tz demonstrates its
morphology, which is further remarked by the transmission
electron microscopy (TEM) image presenting with a neural-like
shape with inhomogeneous width 6-20 nm labeled by white
arrow (Figure S5).
In addition, the intrinsic light absorbing region is extended to
approximately at 700 nm for Pyr-Tz among the entire
wavelength range studied, which might be originating from the
π-π* electron transition of sp2-hybridized C and N in the skeletal
structure (Figure 2a). Then, the good matching of conduction
band (CB) and valence band (VB) levels of a photocatalyst with
the proper redox potentials of the photocatalytic processes is of
significance.[18] From the results of DFT calculations (Figure 2b
and Figure S6), it is possible to achieve the process of
photoreduction proton to H2 because the reduction level for H2 is
negative below the LUMO of Pyr-Tz. Furthermore, the HOMO
position is suitable for water oxidation to release O2 attribute to
its strong oxidizing ability.[19]
Whereafter, for a proof-of-concept advance, the optical
properties are adequate to provide charge carries on Pyr-Br and
Pyr-Tz which are confirmed by transient photocurrent and
electrochemical impedance spectroscopy (EIS) tests. The radii
of the semi-circular Nyquist plot for Pyr-Tz electrode is
significantly smaller than Pyr-Br, suggesting the rapid interfacial
charge transfer in Pyr-Tz (Figure 2c, black dashed lines).
Compared with the relative counterparts in the dark, the radii of
the semicircular Nyquist plots for both two electrodes (Figure 2c,
blue dashed lines) become smaller under UV-light irradiation, so
we can make a conclusion that the fast generation and
separation of the photoinduced electron-hole pairs could be
achieved smoothly. As shown in Figure 2d, when the electrode
is illuminated with UV/Vis light, an evidently enhanced transient
photocurrent could be observed which further confirmed the
feasibility of using Pyr-Tz as photocatalyst ascribing to the fast
transfer of charged carriers. The photocurrent was measured
under visible-light irradiation further (Figure S7) and also show
significant signal confirming the excellent separation and
transfer efficiency in Pyr-Tz material. It is witnessed that the
photocurrent intensity decreased when increasing the incident-
light wavelength from 420 nm to 730 nm.[19]
Figure 1. (a) FTIR spectra of Tz, Pyr-Br and Pyr-Tz (b) XRD patterns (c)
Room temperature EPR images of Pyr-Br and Pyr-Tz (d) Co XPS spectrum of
2 wt.% Co/Pyr-Tz.
products of the bromination and Ullmann coupling reaction
(Figure S1 and S2). The XRD patterns confirm that there is no
obvious change about the crystal structure of Pyr-Br sample
after modification of Tz (Figure 1b and Figure S3). Also, the
electron paramagnetic resonance (EPR) spectra were carried
out to analysis the electronic band structure for Pyr-Br and Pyr-
Tz. As shown in Figure 1c, both Pyr-Br and Pyr-Tz showed a
Lorentzian line centering at g = 1.89, which indicated that the
unpaired electrons exist on its framework. When the C-N bond is
formed, the Lorentzian line are enhanced vastly, probably
attribute to the redistribution of p electrons in the interlinkage of
the N-heterocycle and aromatic rings, which might be helpful in
optimizing the band structure for efficient charge migration and
separation. As the fine-scanned Co 2p XPS spectrum of 2 wt.%
Co/Pyr-Tz displays that the peaks at 776.9, 792.2 and 805.2 eV
are assigned to Co3+, the peaks at 781.3 and 797.4 eV are
Under UV light irradiation, using AgNO3 as the electron
scavenger and La2O3 as the pH buffer agent, the photocatalytic
performance of the as-prepared pyrene-based organic molecular
was evaluated in photocatalytic O2 evolution in a standard
system. As shown in Figure 3a, the photo-promoted O2 evolution
process could be realized using Pyr-Tz as the effective light
absorber with 3.26 μmol h-1 O2 evolution rate under no
cocatalyst reaction conditions. The cobalt species reported as
[12a]
an efficient water oxidation cocatalyst, previously
was
selected as the thruster to accelerate the transfer of photo-
induced electron and hole pairs to consummate the
photocatalytic performance of the organic molecular system.
When the metal additives were added into the system, six times
Figure 2. (a) DRS spectra of Pyr-Br and Pyr-Tz (b) DFT calculations of Pyr-Tz
(c) Electrochemical impedance spectra (d) Photocurrent response of Pyr-Br
and Pyr-Tz.
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Experimental Section
Synthesis of Pyr-Br. Bromination is an important class of organic
transformation essential for both laboratory research and modern
industry.[22] However, to keep this process sufficiently controlled with
alternative bromination of Pyr is still a challenging task. Here, the
bromine substituted Pyr (1,3,6,8-tetrabrominepyrene, abbreviate to Pyr-
Br) was constructed with liquid bromine under mild conditions.[23]
Typically, Pyr (1g), Br2 (4 ml), and CCl4 (40 ml) were added into a three-
necked flask under vigorous stirring. Before heating up, it costs nearly 10
min to expel the whole oxygen in the system and the solution was then
heated to 120 °C for 6 h under nitrogen atmosphere. Prudently, the
reaction mixture was filtered and washed with acetone for three times,
then dried at 60 °C under vacuum. The faint yellow powder could be
obtained with 86% yield.
Synthesis of Pyr-Tz. 1,3,6,8-tetra(1,2,4-triazole)pyrene (abbreviate to
Pyr-Tz) was synthesized with high yield in a controllable manner via
copper-catalyzed Ullmann Coupling reaction from Pyr-Br and Tz, a
momentous tactics for the formation of C-C, C-N, C-O and C-S (Se)
bonds.[24] Pyr-Br (0.513 g), Tz (0.69 g), Cu2O (0.025 g) and K2CO3 (0.138
g) were mixed in 50 ml DMF with vigorous stirring. Before heating up, it
costs nearly 10 min to expel the whole oxygen in the system. Then, the
mixture was heated to 110 °C for 48 h under nitrogen atmosphere. Later,
Figure 3. (a) The cocatalyst effect (d) The effect of Co loading on the O2
evolution rate (c) Recycling test and (d) Time course of O2 evolution.
of O2 production rate (22.6 μmol h-1) was observed with the
addition of only Co species, while Ir, Ru and Fe show no obvious
promoting effect in the photocatalytic activity. Later, we
investigated the effect of different amounts of the cobalt species
on photocatalytic water splitting activity (Figure 3b). Incipiently,
the O2 evolution rate of the Pyr-Tz markedly accelerated with
increasing the amount of Co2+ due to the increased number of
catalytic active sites. Nevertheless, when the amount reached
2.5 wt.%, the photocatalytic performance declined owing to the
reported fact that the excessive cobalt species generate photo-
induced electron-holes recombination traps leading to the
decrease of the hot carrier yield,[12a] all these results suggest that
the O2 evolution rate reach the maximum with 2.0 wt.% of Co2+
species. The activity of the recycled sample and the continuous
long time reaction shows the reduced activity due to the loading
of Ag nanoparticles on its surface leading to the light shading
effect and hindering the optical absorption (Figure 3c and 3d).[21]
the reactant was rinsed
a few times with ultrapure water and
concentrated hydrochloric acid, dried at 60 °C for 2 h under vacuum.
Acknowledgements
This work was financially supported by the National Key
Technologies R & D Program of China (2018YFA0209301), the
National Natural Science Foundation of China (21802022,
21425309, 21761132002, and 21861130353), 111 Project
(D16008), and Chang Jiang Scholars Program of China
(T2016147).
Conflict of interest
The authors declare no conflict of interest.
Keywords: Artificial photosynthesis • oxygen evolution • pyrene
• organic molecular photocatalyst • Ullmann Coupling
Conclusions
[1]
[2]
X. D. Wang, X. F. Gao, C. J. Xuan, S. X. Tian, Nat. Chem. 2016, 8,
258-63.
In summary, the Ullmann Coupling reaction was employed to
construct photoactive organic molecular Pyr-Tz for O2 evolution
a) H. D.Holland, Phil. Trans. R. Soc. B. 2006, 361, 903-915; b) J. F.
Kasting, J. B. Pollack, J. Atmos. Chem. 1984, 1, 403-428; c) J. F.
Kasting, J. C. G. Walker, J. Geophys. Res. 1981, 86, 1147-1158; d) J.
F. Kasting, S. C. Liu, T. M. Donahue, J. Geophys. Res. 1979, 84, 3097-
3107.
from water under moderate conditions.
A
detailed
physicochemical characterization indicated that the reinforced
photocatalytic water splitting efficiency is chiefly derived from the
greatly improved light response capacity and the obviously
improved separation and transfer efficiency of photo-induced
electron-hole pairs. This study highlights the molecular
engineering in structural design of organic molecule with the
application on photocatalytic water splitting. It is anticipated that
the proposal should bring promising prospect to artificial
photosynthesis by rational design and engineering of the
functional materials with suitable optical, physical, and chemical
properties.
[3]
a) J. D. Rochaix, Biochim. Biophys. Acta 2011, 1807, 878-886; b) W.
Tezara, V. J. Mitchell, S. D. Driscoll, D. W. Lawlor, Nature 1999, 401,
914-917; c) C. Lunde, P. E. Jensen, A. Haldrum, J. Knoetzel, H. V.
Scheller, Nature 2000, 408, 613-615.
[4]
[5]
A. Fujishima, K. Honda, Nature 1972, 238, 37-38.
a) G. G. Zhang, S. H. Zang, X. C, Wang, ACS Catal. 2015, 5, 941-947;
b) J. Li, H. Li, G. M. Zhan, L. Z. Zhang, Account Chem Res 2017, 50,
112-121; c) K. Maeda, K. Domen, J. Phys. Chem. Lett. 2010, 1, 2655–
2661; d) M. Grzelczak, J. S. Zhang, J. Pfrommer, J. Hartmann, M.
This article is protected by copyright. All rights reserved.

10.1002/cssc.201901899
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FULL PAPER
Driess, M. Antonietti, X. C. Wang, ACS Catal. 2013, 3, 383-388; e) Z. M.
Y. Zhang, X. L. Zhang, G. Q. Zhang, Y. J. Xiong, X. J. Wu, J. L. Yang,
H. X. Xu, Adv. Mater. 2017, 29 (1702428), 1-8; c) L. H. Lin, H. H. Ou, Y.
F. Zhang, X. C. Wang, ACS Catal. 2016, 6 (6), 3921-3931.
[13] a) A. Kudo, Y. Miseki, Chem. Soc. Rev. 2009, 38 (1), 253-278; b) M. W.
Kanan, D. G. Nocera, Science 2008, 321, 1072-1075.
Pan, P. P. Niu, Y. D. Hou, Y. X. Fang, M. H. Liu, X. C. Wang,
ChemSusChem, 2019, 12, 1911-1915.
[6]
[7]
J. Seo, H. Nishiyama, T. Yamada, K. Domen, Angew. Chem. Int. Edit.
2018, 57, 8396-8415.
a) B. Li, F. Li, S. Y. Bai, Z. J. Wang, L. C. Sun, Q. H. Yang, C. Li, Energ.
Environ. Sci. 2012, 5, 8229-8233; b) D. S. Dhawale, P. Bodhankar, N.
Sonawane, P. B. Sarawade, Sustainable Energy Fuels, 2019,
doi.org/10.1039/C9SE00208A; c) S. G. Peera, T. G. Lee, A. K. Sahu,
Sustainable Energy Fuels, 2019, doi.org/10.1039/C9SE00082H.
a) J. Hessels, R. J. Detz, M. T. M. Koper, J. N. H. Reek, Chem. Eur. J.
2017, 23, 16413–16418; b) R. Lalrempuia, N.D. McDaniel, H. M. Bunz,
S. Bernhard, M. Albrecht, Angew. Chem. Int. Edit. 2010, 49, 9765-9768;
c) M. Baumung, F. Schönewald, T. Erichsen, C. A. Volkert, M. Risch,
Sustainable Energy Fuels, 2019, doi.org/10.1039/C8SE00551F.
a) H. Huang, L. Yang, A. Facchetti, T. J. Marks, Chem. Rev. 2017, 117,
10291-10318; b) Y. Z. Chen, W. H. Li, L. Li, L. N. Wang, Rare Met.
2017, 37, 1-12.
[14] a) N. A. Romero, D. A. Nicewicz, Chem. Rev. 2016, 116, 10075−10166;
b) W. Schilling, D. Riemer, Y. Zhang, N. Hatami, S. Das, ACS Catal.
2018, 8, 5425−5430.
[15] Y. S. Zhao, H. B. Fu, A. D. Peng, Y. Ma, Q. Liao, J. N. Yao, Accounts
Chem. Res. 2009, 43, 409-418.
[8]
[9]
[16] J. Liu, Y. Liu, N. Y. Liu, Y. Z. Han, X. Zhang, H. Huang, Y. Lifshitz, S. T.
Lee, J. Zhong, Z. H. Kang, Science 2015, 27, 970-974.
[17] Q. H. Li, F. Lin, F. Liu, X. C. Wang, Chem. Commun., 2019, 55, 3903-
3906.
[18] S. Y. Wang, Y. Y. Gao, S. Miao, T. F. Liu, L. C. Mu, R. G. Li, F. T. Fan,
C. Li, J. Am. Chem. Soc. 2017, 139 (34), 11771-11778.
[19] Q. Wang, T. Hisatomi, Q. X. Jia, H. Tokudome, M. Zhong, C. Z. Wang,
Z. H. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. B. Li, L. D. Sharp,
A. Kudo, T. Yamada, K. Domen, Nat. Mater. 2016, 15, 611–615.
[10] a) X. C. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M.
Carlsson, K. Domen, M. Antonietti, Nat. Mater. 2008, 8, 76-80; b) X. C.
Wang, X. F. Chen, A. Thomas, X. Z. Fu, M. Antonietti, Adv. Mater. 2009,
21, 1609-1612; c) X. C. Wang, K. Maeda, X. F. Chen, K. Takanabe, K.
Domen, Y. D. Hou, X. Z. Fu, M. Antonietti, J. Am. Chem. Soc. 2009,
131, 1680-1681; d) L. H. Lin, Z. Y. Yu, X. C. Wang, Angew. Chem. Int.
Ed., 2019, 58, 6164-6175.
[20]
P. J. Yang, R. R. Wang, M. Zhou, X. C. Wang, Angew. Chem. Int. Edit.
2018, 57, 8674-8677.
[21] a) G. G. Zhang, S. H. Zang, X. C. Wang, ACS Catal. 2015, 5, 941−947;
b) G. G. Zhang, S. H. Zang, L. H. Lin, Z. A. Lan, G. S. Li, X. C. Wang,
ACS Appl. Mater. Interfaces 2016, 8, 2287-2296.
[22] B. Sels, D. D. Vos, M. Buntinx, F. Pierard, A. K. D. Mesmaeker, P.
Jacobs, Nature 1999, 400, 855-857.
[11] a) C. J. Huang, C. Chen, M. W. Zhang, L. H. Lin, X. X. Ye, S. Lin, M.
Antonietti, X. C. Wang, Nat. Commun. 2015, 6, 1-7; b) F. S. Guo, P. J.
Yang, Z. M. Pan, X. N. Cao, Z. L. Xie, X. C. Wang, Angew. Chem. Int.
Edit. 2017, 56, 8231-8235; c) M. Zhou, P. J. Yang, S. B. Wang, Z.S.
Luo, C. J. Huang, X. C. Wang, ChemSusChem, 2018, 11, 3949-3955.
[12] a) Z. A. Lan, Y. X. Fang, Y. F. Zhang, X. C. Wang, Angew. Chem. Int.
Edit. 2018, 57, 470-474; b) L. Wang, Y. Y. Wan, Y. J. Ding, S. K. Wu,
[23] S. Bernhardt, M. Kastler, V. Enkelmann, M. Baumgarten, K. Mullen,
Chem. Eur. J. 2006, 12, 6117–6128.
[24]
a) F. Ullmann, Ber. Dtsch. Chem. Ges. 1903, 36, 2382-2384; b) F.
Monnier, M. Taillefer, Angew. Chem. Int. Edit. 2009, 48 (38), 6954-6971.
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An organic molecular photocatalyst
Pyr-Tz constructed by Ullmann
Q. Li, F. Liu, F. Lin, W. Lin, X. Wang*
Page No. – Page No.
An Organic Molecular Photocatalyst
Releasing Oxygen from Water
coupling reaction reveals improved
light absorption endowing its relative
high photoactivity for O2 evolution.
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