A Noble-Metal-Free Heterogeneous Photosensitizer-Relay Catalyst Triad That Catalyzes Water Oxidation under Visible Light

Article abstract of DOI:10.1002/anie.201811570

An entirely earth-abundant chromophore-relay water oxidation catalyst triad system, which is robust and efficient at neutral pH, is presented. The synthesis involves the coordination of a porphyrin derivative to a bridging Fe(CN)₅ group, which is then reacted with Co ions to prepare a covalently linked chromophore–Prussian blue analogue assembly. Light-driven water oxidation studies in the presence of an electron scavenger indicate that the triad is active and it maintains a steady activity for at least three hours. Transient absorption experiments and computational studies reveal that the Fe(CN)₅ group is more than a linker as it takes part in electron-transfer and co-operates with porphyrin in the charge separation process.

Full text of DOI:10.1002/anie.201811570

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: A Noble-Metal-Free Heterogeneous Photosensitizer–Relay–
Catalyst Triad Catalyzes Water Oxidation under Visible Light
Authors: T. Gamze Ulusoy Ghobadi, Elif Akhuseyin Yildiz, Muhammed
Buyuktemiz, Sina Sadigh Akbari, Derya Topkaya, Ümit İsci,
Yavuz Dede, H. Gul Yaglioglu, and Ferdi Karadas
This manuscript has been accepted after peer review and appears as an
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published online in Early View as soon as possible and may be different
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content of this Accepted Article.
To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201811570
Angew. Chem. 10.1002/ange.201811570
Link to VoR: http://dx.doi.org/10.1002/anie.201811570
http://dx.doi.org/10.1002/ange.201811570

10.1002/anie.201811570
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A Noble-Metal-Free Heterogeneous Photosensitizer–Relay–
Catalyst Triad Catalyzes Water Oxidation under Visible Light
T. Gamze Ulusoy Ghobadi, Elif Akhuseyin Yildiz, Muhammed Buyuktemiz, Sina Sadigh Akbari, Derya
Topkaya, Ümit İsci, Yavuz Dede*, H. Gul Yaglioglu*, Ferdi Karadas*
cyanide-based assemblies have been prepared previously via a
building block approach, similar to the preparation of dyads.[21]
Different perspectives of cobalt based Prussian blue analogues
(PBAs) as WOCs have recently been explored by us and other
researchers.[22–31] PBAs consist of earth-abundant elements,
perform at relatively low overpotentials (<500 mV for a current
density of 1 mA cm−2), exhibit exceptional stabilities in a wide pH
range (from 1 to 13), and can operate even with molecular
chromophores for light-driven catalysis.[32] Recently, we employed
a pentacyanoferrate complex rather than a hexacyanometal
precursor to prepare an amorphous PBA with enhanced
electrocatalytic activity.[25] The aforementioned study and many
others reveal that the ammino group of [Fe(CN)5(NH3)]3− complex
can be substituted with pyridyl containing organic moieties via a
straightforward synthesis.[33,34] Moreover, our preliminary results
suggesting that [Fe(CN)5] could facilitate the electron transfer
between the chromophore and the catalyst encouraged us to use
it for the development of a molecular sensitizer-heterogeneous
catalyst system. For this purpose, a symmetrically substituted
Abstract: An entirely earth abundant chromophore-relay-water
oxidation catalyst triad system, which is robust and efficient at neutral
pH, is presented. The synthesis involves the coordination of a
porphyrin derivative to a bridging Fe(CN)5 group, which is then
reacted with Co ions to prepare a covalently linked chromophore-
Prussian blue analogue assembly. Light driven water oxidation
studies in the presence of an electron scavenger indicate that the triad
is active and it maintains a steady activity for at least 3 hours.
Transient absorption experiments and computational studies reveal
that the Fe(CN)5 group is more than a linker as it takes part in electron-
transfer and co-operates with porphyrin in the charge separation
process.
Artificial photosynthesis, which harnesses sunlight to split water,
is a chemical transformation of utmost interest.[1] Several
innovative photoelectrochemical cells for water splitting have
been proposed since the pioneering work by Fujishima and
Honda.[2,3] Of these, dye-sensitized photoelectrochemical cells
(DS-PECs) utilize covalently coordinated chromophore-water
oxidation catalyst (WOC) dyad assemblies[4–6] in an effort to mimic
natural photosynthesis. This approach has further been expanded
by integrating suitable donor and acceptor units to prepare
molecular triads, tetrads, and pentads for efficient charge
porphyrin
derivative,
5,10,15,20-tetra(4-pyridyl)-21H,23H-
porphyrin [TPyP], was selected as the chromophore since it can
be produced in bulk scale, it has a high molar absorption
coefficient in the visible region, and it contains available pyridyl
groups for coordination to pentacyanoiron(II) group.
separation.[7–9]
Molecular
ruthenium
complexes
(as
photosensitizers and/or WOCs) remain the most widely used
despite their high cost and easy decomposition under
photocatalytic conditions.[10–19] Although the use of robust
heterogeneous assemblies such as oxides could be a promising
solution to improve stability, the lack of complete control on the
coordination of metal ions in oxide chemistry does not allow the
preparation of oxide-based dyads.[20] On the other hand, many
[*] T. G. Ulusoy Ghobadi, Prof. F. Karadas
UNAM - National Nanotechnology Research Center,
Institute of Materials Science and Nanotechnology,
Bilkent University, 06800 Ankara, Turkey.
T. G. Ulusoy Ghobadi
Department of Energy Engineering,
Faculty of Engineering,
Ankara University, 06830 Ankara, Turkey.
E. Akhuseyin Yildiz, Prof. H. G. Yaglioglu
Department of Engineering Physics,
Faculty of Engineering,
Ankara University, 06100 Ankara, Turkey.
Email: gul.yaglioglu@eng.ankara.edu.tr
M. Buyuktemiz, Prof. Y. Dede,
Department of Chemistry,
Faculty of Science,
Gazi University, Teknikokullar, 06500 Ankara, Turkey.
Email: dede@gazi.edu.tr
Prof. D. Topkaya
Department of Chemistry, Faculty of Sciences, Dokuz Eylul University,
Tınaztepe Campus, Izmir, Turkey.
Prof. Ü. İşci
Department of Chemistry,
Gebze Technical University, 41400 Kocaeli, Turkey.
S. Sadigh Akbari, Prof. F. Karadas
Department of Chemistry,
Faculty of Science,
Bilkent University, 06800 Ankara, Turkey.
E-mail: karadas@fen.bilkent.edu.tr
Scheme 1. General synthetic scheme and molecular structures of [TPyP], [Fe–
TPyP], and [CoFe–TPyP].
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The amminopentacyanoferrate complex, [Fe(CN)5NH3]3− (Figure
S1), was first reacted with [TPyP] to prepare [Fe‒TPyP], which
was then reacted with Co2+ ions to prepare the triad, [CoFe‒TPyP]
(Scheme 1). An assembly, in which a molecular chromophore is
coordinated to a heterogeneous cyanide-based catalyst, was
prepared with this synthetic route. The synthetic pathway adopts
a molecular bottom-up approach similar to the ones previously
reported for chromophore-catalyst assemblies,[7-19] while it differs
significantly from those by the formation of a heterogeneous
assembly in the final step.
The H-NMR spectrum of [Fe–TPyP] exhibits the characteristic
chemical shifts of [TPyP], which indicates that the porphyrin ring
is still intact and each pyridyl group is coordinated to iron sites
(Figure S2). Infrared studies also confirm the complexation and
the formation of a Prussian blue network (Figure S3). XPS spectra
of N1s and Fe2p bands (Figures S4-S6 and Table S1) are also in
good agreement with the expected structures.[35] The
corresponding SEM-EDX elemental mapping reveals the
homogenous distribution of the elemental species (Figures S7
and S8) in [Fe–TPyP] and [CoFe–TPyP]. The energy dispersive
X-ray spectroscopy (EDS) of [CoFe–TPyP] (Figure S9) also
reveals a stoichiometric ratio of 3:2 (Co:Fe), which is in line with
the expected formula based on charge balance.
TPyP] moiety in the [CoFe–TPyP] system is presumably the
photochemically active species and the role of the Co centers is
similar to what we have previously reported.[26,36] We therefore
studied the electronic structure and excitation characteristics of
[CoFe–TPyP] and [Fe–TPyP] with Density Functional Theory
(DFT). For simplicity we discuss the excitation motif of [Fe–TPyP]
below and emphasize that [CoFe–TPyP] computations yield
exactly the same results (Figures S18 and S19). Due to the
complexity in reliable computation of the electronic structures of
multiple high spin Co centers, electron transfer dynamics of
[CoFe–TPyP] were studied with TA spectra.
For comparison, [ZnFe-TPyP] was also prepared and
characterized with Infrared, XPS, and SEM studies (Figures S10
and S11). The cyclic voltammogram (CV) of [CoFe–TPyP]
modified FTO electrode exhibits an irreversible peak at high
anodic potentials corresponding to catalytic water oxidation
process. This feature is absent in the CV of [ZnFe-TPyP] modified
FTO electrode, which indicates that water is oxidized only at
cobalt sites (Figure S12).
Figure 1. Photocatalytic oxygen evolution by [CoFe–TPyP] (10 mg) and K2S2O8
(20 mM) in pH 7, 50mM phosphate buffer solution. Same batch was used in
each cycle.
Characterization studies and unique coordination mode of
cyanide ligand suggest the presence of random sized PB
structures with limited dimensionalities due to surrounding
molecular porphyrin rings in [CoFe‒TPyP] (Note that the
compound has an amorphous nature, see Figure S13). The
coordination of each Fe site to a porphyrin macrocycle reduces
the dimensionality of PB structures significantly, which in return
yields cobalt sites coordinated to at least one H2O molecule that
is essential for water oxidation.
The ground state of [Fe–TPyP] is found to be a closed shell singlet
(Table S2) with four electronically identical Fe centers having low
spin configurations (Table S3) as expected for Fe in strong ligand
environment. From the frontier orbitals HOMO is comprised of
Fe–d and porphyrin–π orbitals. Two other high lying occupied
orbitals involved in the excitation are of porphyrin π and Fe–d +
porphyrin–π in character (Figures 2 and S19). These three
molecular orbitals (MOs) delocalized on the porphyrin ring and Fe
center are the domain for the excitation.
Photocatalytic O2 evolution profile of [CoFe–TPyP] in the
presence of a sacrificial electron acceptor (K2S2O8) reveals a
steady increase during a 1 h experiment (Figure 1). The curve
yields a turnover frequency of 3.2×10−4 sec−1 (1.2 h−1), which is
similar to the one obtained with PBAs in the presence of a
Ru(bpy)32+/S2O82− system and much higher than the one obtained
when [TPyP] was mixed with a cobalthexacyanoferrate (Figure
S14).[32] Similar profile was obtained for three cycles of the same
batch, which indicates that [CoFe‒TPyP] is stable during
photocatalytic water oxidation process. This is also confirmed with
comprehensive characterization studies performed on the post-
catalytic powder sample (Figures S15-S17). Overall,
photocatalytic tests confirm that porphyrin and PB structures
collaborate for light-assisted water oxidation catalysis, which
could be due to an efficient pathway for electron transfer between
the chromophore and the catalytic cobalt sites. This mechanism
has been investigated by computational and femtosecond
transient absorption (TA) studies.
The two lowest lying unoccupied MOs, both of porphyrin–π*
character, are the degenerate pair of LUMO and LUMO+1. The
calculated excitation motif suggests that an electron hosted in a
hybrid Fe–d + porphyrin–π orbital is transferred to porphyrin
based π* orbital upon irradiation. Thus, Fe(CN)5 fragment is partly
oxidized during the excitation and an internal charge transfer state
with one unpaired electron on Fe and one on the porphyrin is
accessed. Such an excited state of charge transfer nature can
easily hop to the triplet surface (within only 0.1 eV, Tables S4 and
S5) and live long enough to further oxidize the Co-centers.
Excitations and proton coupled electron transfer events studied
for the [CoFe‒TPyP] shed light on the nature of the intermediates
(Figures S20 and S21). Comparison with the [ZnFe‒TPyP]
system (Figure S22) reveal that the Zn center does not possess
the key electronic structure (Table S6) to carry the O–O bond
formation.1 The effect of cobalt sites on the electron transfer is
supported by the transient absorption studies (vide infra).
Previously, we have explored the electronic structure details of
similar Co sites used as WOCs and have shown that it is critical
to oxidize the Co site and increase its electrophilicity in order to
enhance the nucleophilic attack of water.[26,36] The reactivity
parameters in the current study suggest a similar Co centered O–
O bond formation under photocatalytic conditions. Moreover, [Fe–
Quantum chemical calculations thus suggest that i) Fe-centers
are involved in donating electrons and porphyrin is an electron
acceptor. ii) This internal charge transfer repositions the electrons
in two distinct fragments of the system and decreases the
likelihood of charge recombination, thereby allowing the excited
state to live long enough and to undergo other important
1
A
detailed computational study about the electronic structure of the
intermediates and their transformations will be reported as a subsequent work.
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electronic changes. iii) The electron transfer yields an MLCT state. [CoFe–TPyP], excited state absorption with long lifetime around
iv) The internally oxidized Fe-centers act as holes to further
oxidize Co-sites – an essential feature for O–O bond formation at
the Co-site as shown previously.[26] These results are also verified
by steady state absorption (SSA), fluorescence (FL), and
transient absorption (TA) measurements.
430 nm appears at zero time delay suggesting that this is not a
T1–Tn absorption (Figures S32, S33 and Scheme 2).[39] This profile
also implies that the deactivation pathway of [Fe–TPyP] excited
state is altered with the presence of Co-sites due to further
electron transfer, which yields a longer living [CoFe–TPyP] state.
TA measurements, thus, conclude that the Co centers are
internally oxidized with the assistance of Fe(CN)5 group, hence
the photocatalytic water oxidation utilizes the [Fe–TPyP] moiety
as the photosensitizer and Co sites as the catalytic centers.
Figure 3. Transient absorption spectra at zero time delay and time evolution of
the bleach signals at 424 nm wavelength for [TPyP], [Fe–TPyP], and [CoFe–
TPyP] compounds.
In conclusion, an entirely earth abundant cyanide-based
chromophore-relay-catalyst triad system for light driven water
oxidation was reported for the first time by engaging cyanide
chemistry with the dyad concept. The reported assembly is
genuine due, not only to the simplicity of its synthesis but also to
the bifunctionality of each of its components. Porphyrin fragment
is both a chromophore and a capping ligand for reducing the
dimensionality of PB structures, thus, increasing the number of
active catalytic sites. Transient absorption measurements and
computational studies show that [Fe(CN)5] moiety serves both as
a robust linker and a relay group that facilitates the electron
transfer from the catalyst to the chromophore. Co-sites are
essential for catalytic water oxidation. They also play a critical role
in the formation of a stable heterogeneous assembly. Overall, this
proof-of-concept study indicates that cyanide-based systems
could be the key element for the development of earth abundant,
robust, and efficient dye-sensitized photoanodes. The modular
and versatile Fe(CN)5 chemistry is further explored in our labs by
introducing other chromophores to generalize the concept of
photocatalytic water oxidation with cyanide-based assemblies.
Figure 2. TD-DFT excitation behavior for [Fe–TPyP] at UB3LYP/def2-TZV level
of theory. A hybrid Fe–d + porphyrin–π → porphyrin–π* state is generated.
SSA graphs of the investigated compounds show typical
characteristic of porphyrins as shown in Figure S23.[37] FL
measurements are given in Figures S24 and S25.
TA spectra for [TPyP], [Fe–TPyP], and [CoFe–TPyP] at zero time
delay and decay traces of the bleach signals at 424 nm probe
wavelength are given in Figure 3 along with the proposed electron
transfer mechanism (Scheme 2). [TPyP] shows typical features
including a characteristic ground state bleaching and a stimulated
emission around 420 nm, Q bands bleaching around 520 and 650
nm, and T1–Tn absorption around 440 nm developing by
intersystem crossing (ISC) process after 48 ps time delay (Figures
S26 and S27). Although TA measurement of [Fe–TPyP] is similar
to that of [TPyP], it shows enhanced bleaching of Q bands as well
as T1–Tn absorption band (Figures S28 and S29).[38] These data
show the electron transfer from Fe-center to porphyrin as
supported by the TD-DFT calculations (Figures 2 and S30). On
the other hand, decay trace of 430 nm bleach signal for [Fe–
TPyP] exhibits a longer lifetime (from pumped state to ground
state) than that of [TPyP], which is required for efficient
photocatalytic process (Table S7 and Figure S31). In the case of
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Scheme 2. Schematic illustration of the energy diagram for (a) [TPyP], (b) [Fe–TPyP], and (c) [CoFe–TPyP] compounds.
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Acknowledgements
This work is supported by the Scientific and Technological
Research Council of Turkey (TUBITAK), grant number 215Z249.
F.K. and Y.D. thank TÜBA-GEBİP for young investigator awards
and BAGEP for young scientist award.
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Keywords: water oxidation • Prussian blue • porphyrin • water
splitting• triad • dye-sensitized
E. P. Alsaç, E. Ülker, S. V. K. Nune, Y. Dede, F. Karadas, Chem. -
A Eur. J. 2018, 24, 4856–4863.
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Entry for the Table of Contents
COMMUNICATION
Text for Table of Contents
T. Gamze Ulusoy Ghobadi, Elif
Akhuseyin Yildiz, Muhammed
Buyuktemiz, Sina Sadigh Akbari,
Derya Topkaya, Ümit İşçi, Yavuz
Dede*, H. Gul Yaglioglu*, Ferdi
Karadas*
Page No. – Page No.
A Noble-Metal-Free
Heterogeneous Photosensitizer–
Relay–Catalyst Triad Catalyzes
Water Oxidation under Visible
Light
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