Energy Transfer in a Hybrid IrIII Carbene-PtII Acetylide Assembly for Efficient Hydrogen Production

Article abstract of DOI:10.1002/chem.201500193

A new heterometallic supramolecular complex, consisting of an iridium carbene-based unit appended to a platinum terpyridine acetylide unit, representing a new Ir^III-Pt^II structural motif, was designed and developed to act as an activ

Full text of DOI:10.1002/chem.201500193

DOI: 10.1002/chem.201500193
Communication
&
Photoreduction of Water
III
II
Energy Transfer in a Hybrid Ir Carbene–Pt Acetylide Assembly
for Efficient Hydrogen Production
[a]
[a]
[b]
[a]
[a]
Zhen-Tao Yu,* Yong-Jun Yuan, Xin Chen,* Jian-Guang Cai, and Zhi-Gang Zou
Indeed, the solar-to-energy conversion in nature is realized
through energy and electron transfer in a delicate supramolec-
ular organization. In this respect, of particular interest and in-
spired by nature, is the use of a highly organized molecular as-
sembly of individual functional components, that is, photosen-
sitizer–catalyst conjugates, for the development of a single-
component system capable of performing such process
Abstract: A new heterometallic supramolecular complex,
consisting of an iridium carbene-based unit appended to
a platinum terpyridine acetylide unit, representing a new
III
II
Ir –Pt structural motif, was designed and developed to
act as an active species for photocatalytic hydrogen pro-
duction. The results also suggested that a light-harvesting
process is essential to realize the solar-to-fuel conversion
in an artificial system as illustrated in the natural photo-
synthetic system.
[6–15]
types.
Such architectures enable efficient collaboration in
collecting the light energy and converting it efficiently into
stored chemical energy. Great efforts are being directed to the
design of simple integrated-photocatalyst molecular assem-
blies carrying two or more spatially well-defined metal-based
components, which demonstrates the feasibility of creating ar-
tificial photosynthetic devices. However, these single-compo-
nent systems are still hampered by relatively low efficiency for
hydrogen evolution, and only a few systems have reached
a turnover number (TON) on the order of hundreds versus the
catalysts, a value that is still far less than that achieved with
Constructing artificial systems for capture and conversion of
solar energy to make energy carriers such as hydrogen is im-
perative for creating promising opportunities for the future
[
1]
mitigation of energy requirements. Thus far, partial mimick-
ing of the energy- and electron-transfer processes in natural
photosynthesis, along with viable photoelectrochemical and
photobiological catalytic splitting of water have been devel-
[16]
the multicomponent systems. Therefore, it is highly desirable
to seek new active assemblies that allow their implementation
into original supramolecular light-harvesting devices with the
goal of improving the efficiency of light-to-chemical energy
conversion.
[2]
oped for such a task. Such catalytic reactions have utilized
classical multicomponent systems, incorporating at least
a light sensitizer for light harvesting and a proton reduction
catalyst for receiving the electrons from the excited molecules
and using them to spur hydrogen evolution, in addition to
a sacrificial electron donor such as triethylamine (TEA). There
have been significant advances in such complex reaction mix-
tures in the pursuit of outstanding hydrogen production effi-
In the development of metallosupramolecular assemblies ca-
pable of giving rise to fuel production, we were interested in
merging the properties of the cyclometalated iridium chromo-
phore with platinum acetylide fragments. Such a platinum spe-
cies, in which the carbonÀplatinum bond of the acetylide link-
age may lead to effective electronic coupling with the adjacent
fragments, was selected for its potential ability to drive photo-
[
3,4]
ciency.
However, the optimization of the active components
remains a major obstacle toward improved catalytic efficiency,
mainly because the key parameter is governed by a diffusion-
controlled process and critically depends on the efficiency of
[
17,18]
catalytic hydrogen production.
In this work, we describe
[5]
intermolecular collisions between the species involved.
the formation of a new assembly (1), constructed from a iridi-
um moiety linked to a platinum(II) acetylide terpyridine com-
[
a] Dr. Z.-T. Yu, Y.-J. Yuan, J.-G. Cai, Prof. Dr. Z.-G. Zou
Collaborative Innovation Center of Advanced Microstructures
National Laboratory of Solid-State Microstructures
College of Engineering and Applied Sciences
Nanjing University
plex (Figure 1), and its use in H -evolving photocatalysis. A
2
comparison with corresponding model complexes, 2 and 3, in-
dicates that such a molecular device enables high-efficiency
photoreduction of water, via a sacrificial electron donor, into
hydrogen. The strong N-heterocyclic carbene donor from the
cyclometalating chelate is required to assist in contributing to
the high luminescence quantum yield and high stability transi-
and
Kunshan Innovation Institute of Nanjing University
Nanjing 210093 (P. R. China)
E-mail: yuzt@nju.edu.cn
[19]
tion-metal complexes, which may benefit solar fuel produc-
tion implementation. Such a sensitizer species is expected to
be a complement to the traditional phenylpyridine-based iridi-
um complex in sensitizer design and solar-to-energy conver-
sion applications.
[
b] Dr. X. Chen
National Laboratory for Infrared Physics
Shanghai Institute of Technical Physics
Chinese Academy of Sciences
Shanghai 200083 (P. R. China)
E-mail: xinchen@mail.sitp.ac.cn
Photocatalytic reactions were first conducted in deaerated
acetone/water solutions (100 mL) under visible-light irradiation
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500193.
Chem. Eur. J. 2015, 21, 10003 – 10007
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Communication
between pH 6.0 and pH 9.0,
with all other conditions held
constant. A maximum catalytic
activity was obtained at pH 9.0,
which is near the pKa of TEA,
suggesting that the unprotonat-
ed TEA is the active reductant.
The highest hydrogen-produc-
tion activity coincides with the
pH range at which the system
displays its highest electron-
transfer ability, as confirmed by
the change in intensity of emis-
sion of complex 1 in the pres-
ence of TEA at different pH
levels. Figure S1b indicates that
the quenching of the excited
state of 1 by TEA decreases as
the pH is decreased beyond the
6À
Figure 1. Structures of the complexes in this study (as their PF salts).
pK of TEA. The most efficient
a
quenching takes place between
1 and TEA at pH 9.0, with 95% quenching. Fluorescence stud-
ies revealed that the initial electron-transfer ability can be re-
duced by lowering the pH of the system. Thus, the protonation
state of TEA is important in determining the hydrogen-produc-
tion performance.
of the system (l>420 nm) with complex 1 as a catalyst at
fixed concentration (10 mm) and a sacrificial reducing (SR)
agent of TEA at 0.18m. The ratio of the reaction medium of
acetone/water mixture was 9:1 (v/v) based on optimizing ex-
periments where the ratio of acetone/water was varied. Con-
trol experiments demonstrated that both the components of
Moreover, the concentrations of 1 have a significant and
direct impact on the amount of hydrogen produced. As can be
clearly seen in Figure 2, the total amount of hydrogen produc-
tion increased on increasing the concentration of 1 while keep-
ing the TEA concentration constant at 0.18m, an observation
1
and TEA, as well as light, are necessary for the observed hy-
drogen-evolution photocatalysis.
As shown in Figure S1a (in the Supporting Information), at
pH 9.0 (the natural pH value of the mixed solution containing
0.18m TEA in the present system), hydrogen production grew
continuously and approximately linearly with time for the first
À1
3
0 h of photolysis (average rate, 19 mmolh ), and then the re-
action rate gradually slows down. When the experiment was
stopped after 60 h, the catalytic TON [TONs=n(H)/n(catalyst)]
of 1672 hydrogen units versus the catalyst could be achieved
on the basis of the final amount of hydrogen generated at this
time (836.2 mmol; Table 1, entry 1). In the system promoted by
1
, hydrogen is produced efficiently by photocatalytic water re-
duction in the presence of an electron donor. As seen, the re-
sponse of the photocatalytic system is highly pH-dependent
Figure 2. Hydrogen production using varying concentrations of 1 at the
original pH of 9.0 (l>420 nm).
[a]
Table 1. Photoinduced hydrogen evolution under various conditions.
Entry
Complex
2
H [mmol]
1
2
3
1
2+3
3
3
1
2
836.2
327.3
18.8
8.2
1308.8
0
that is directly related to the absorbance of the reaction solu-
tion (Figure S2 in the Supporting Information). When the con-
centration of 1 was increased from 5 mm to 50 mm, the amount
of hydrogen produced increased from 251.3 (TON=1005) to
1565.4 mmol (TON=626) after 60 h of irradiation, indicating
a direct correlation between hydrogen formation and the cata-
lyst concentration. The maximum TON (1672) of the catalytic
system under visible-light irradiation is significantly higher
than those of other artificial supramolecular catalysts such as
Ru–Pd systems, Ir–Co systems and the trimetallic Ru–Rh–Ru
[
b]
c]
4
5
6
[
[
a] Unless specified, the reactions contained 10 mm bimetallic complex (or
monometallic complex) and 0.18m TEA in an acetone/water (9:1, v/v,
00 mL) solution at an initial pH 9.0; irradiation light: l>420 nm; TONs=
1
n(H)/n(catalyst). [b] In an acetonitrile/water (1:1, v/v) solution. [c] Irradia-
tion with Xenon-arc light.
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Communication
[
6–14]
systems.
Figure S3 (see the Supporting Information) shows
the quantum efficiency of the hydrogen production by 1 as
a function of the incident monochromatic light wavelength in
the ranges of 400–550 nm. The maximum efficiency of 1.32%
was achieved under irradiation at 450 nm after 10 h of reac-
tion. The curve shows a similar result as the absorption spec-
trum, that is, clearly indicating that the hydrogen-production
reaction is driven by the excited-state of 1.
To ensure the assembly results in effective hydrogen evolu-
tion because of the linked light-harvesting and catalytic com-
ponents, comparison experiments were performed by employ-
ing an equimolar mixed solution consisting of complex 2 as
a sensitizer and 3 as a catalyst in place of complex 1 (Table 1,
entry 2). Considerable amounts of hydrogen gas were found
after 24 h of irradiation, when production plateaued, and no
further increases with prolonged catalysis times could be ob-
served. The TON value of this three-component system is
Figure 3. Absorption spectra (left axis) and normalized emission spectra
(
2 2
right axis) of complexes 1–3 in CH Cl (10 mm) at room temperature.
clear complex 1 compared with that of 3. This enhancement
was accompanied by quenching of the Ir-based emission of
the Ir-based unit in complex 1, implying that the occurrence of
the corresponding sensitization of the Pt–acetylide moieties by
the excited Ir moiety is a possible consequence of a downhill
intramolecular energy transfer from the Ir moiety to the Pt
subunit at the end of the molecule, which is in line with the
relative energy levels of their excited states. This is also con-
firmed by the fact that a rise emission lifetime and an increase
in quantum efficiency of 1 relative to that of 3 can be ob-
2.6 times lower than the value observed for the combined
system under conditions of equivalent component concentra-
tions. According to previous reports, in this system, which has
a much larger concentration of TEA (0.18m) than catalyst
(10 mm), reductive quenching is proposed to be dominant over
electron transfer. A low but significant hydrogen evolution
(
2%) was detected when compound 3 was used alone
(
Table 1, entries 3 and 4), which is in line with previous obser-
II
[22]
vations of Pt terpyridine complexes in photochemical hydro-
served. The time-resolved luminescence decays for 1 were
[
18]
gen formation.
The photocatalytic activity of 3 decreased
recorded as a function of the emission wavelengths. The life-
time decays are wavelength dependent when the monitoring
wavelength is changed from 580 nm to 680 nm (Figure S6 in
the Supporting Information), indicating of the occurrence of
energy migration.
rapidly, and full decolorization was observed by visualizing the
color change of the solution after the reaction was complete
within 12 h of irradiation; however, no such color change was
observed when 1 was used as a photocatalyst. The poor hy-
drogen-producing capability of 3 indicates that the platinum
acetylide chromophore cannot to act as an effective catalytic
species for generating hydrogen, as described in the previous
DFT calculations were performed for complex 1 to ascertain
the origins of its electronic properties (Table S2 in the Support-
ing Information). The highest-occupied molecular orbital
(HOMO) involves significant contributions from both the
p(tpmi) orbitals/d(Ir) of the terminal Ir units (tpmi=1-(4-tri-
fluorophenyl)-3-methylimidazolin-2-ylidene), which is similar to
that calculated for control compound 2. HOMOÀ1 is delocal-
ized across the p system of the epip ligand (epip= 2-(4-ethy-
nylphenyl)-1H-imidazo[4,5-f][1,10]phenanthroline), with only
a minor extension onto the Pt center. The lowest unoccupied
molecular orbital (LUMO) is located mainly on the Pt(terpy)
fragment, thereby favoring efficient electron transfer to reduci-
ble substrates, such as water for photocatalytic hydrogen pro-
duction, and the LUMO+1 is localized almost exclusively on
the terpy moiety. The LUMO level of the bimetallic complex is
mainly populated on the Pt moiety rather than that involving
the Ir center, which gives it an important role in governing the
emissive exited state. The time-dependent DFT calculations in-
dicate that the lowest-lying transition for complex 1 is domi-
nated by HOMO!LOMO excitation. This transition can be
[
20]
report.
The photophysical and electrochemical properties of the
complexes 1–3 were investigated (Table S1 in the Supporting
Information) to supply useful information for understanding
the benefits of the introduction of a supramolecular device for
solar hydrogen production application. At room temperature
in dilute deoxygenated CH Cl , complex 1 exhibits a single
2
2
emission band centered at 617 nm upon irradiation (Figure 3),
3
which is assigned as an excited state of predominantly MLCT/
3
LLCT character. This emission maxima of complex 1 was found
to be redshifted about 60 nm from that of the Ir-based model
compound 2 (i.e., ~556 nm), and is rather similar in energy to
the shorter-lived emission obtained from the Pt-based model
complex 3 at about 603 nm, which is associated with
3
[21]
a dp(Pt)!p*(terpy) MLCT state (terpy=terpyridine). The ex-
citation spectrum recorded for this heterometallic complex at
617 nm corresponds reasonably well to its absorption profile
[23]
(Figure S4 in the Supporting Information), indicating the pho-
viewed as a hybrid between the two metal–ligand subunits,
tons absorbed by the Ir group are collected by the Pt unit. In
addition, by measuring the luminescence intensities of isoab-
sorptive solutions of 1 and 3, under identical experimental
conditions (Figure S5 in the Supporting Information), twofold
enhancement of the emission intensity was seen for the binu-
and charge transfer may be considered as occurring partly
from the Ir to the Pt moiety with a contribution from the ter-
minal Ir units to the terpy ligand. Owing to a large admixture
of tpmi p orbitals into the HOMO, this transition may be mixed
with a ligand-to-ligand charge-transfer (LLCT) transition. There-
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fore, it must be considered that the emission may occur from
the excited state partly involving the Ir center, because the
characterized. ESI-MS analysis (Figure S8 in the Supporting In-
formation) of samples of the reaction mixture of 1 that had un-
dergone irradiation for 60 h, showed no peak for complex 1
(m/z=1557.50), whereas the peak of terpy as found at m/z=
402.58, which revealed that complex 1 decomposes to uniden-
tified compounds (m/z=751.67, 696.42, and 651.58) and mon-
[24]
emission is rather complicated and not fully understood.
This spatial separation of HOMO and LUMO for the complex il-
lustrates the thermodynamic possibility of generating
a charge-separated excited state between the terminal Ir and
[
25]
Pt metal center in the complex. In addition, coupling be-
tween the metal centers can be enhanced through the HOMO
of the bridge because of an efficient mixing of the dp metal
orbitals with filled p orbitals of the bridge.
ometallic Ir compounds lacking the Pt unit such as a TEA-coor-
+
dinated adduct [Ir(tpmi) (TEA)] (m/z=744.58) during the pho-
2
tocatalytic reaction. These results indicated the decomposition
of the assembly occurred to release Pt species. To obtain infor-
mation about the oxidation state of the decomposition spe-
cies, X-ray photoelectron spectroscopy (XPS) measurements
were performed on evaporated samples after photolysis as
well as on the isolated powders of 1. The binding energies (BE)
were determined to be 61.49 eV (Ir 4f7/2), 72.92 eV (Pt 4f7/2),
and 399.85 eV (N1s) for the original compound (Figure S9 in
the Supporting Information). After the photoreduction reac-
tion, much broader and relative lower intensities were ob-
served, and the resulting BE values related to decomposition
products were found at 61.26, 72.24, and 398.96 eV for the se-
lected atomic levels, respectively (Figure S10 in the Supporting
Information). The energy shift is much lower than the recorded
The ability to photoinduce electron transfer in supramolec-
ular devices has been exploited for solar-energy conver-
[
7–15]
sion.
However, if it is possible to sensitize catalytic sites by
means of unidirectional energy transfer powering the intermo-
lecular electron-transfer process, which is a key step in natural
[
26]
photosynthetic reactions, improving features of these assem-
[27]
bles for use in photocatalysis may become feasible. Based
on the above results, the importance of the supramolecular
complex is clear in the formation in hydrogen. We attribute
the superior activity of the molecular assembly to the intramo-
lecular energy transfer between the light-harvesting unit and
the catalytic center to form Ir–Pt* species. Energy transfer was
exploited in the photocatalysis by sensitization of specific cata-
lytic sites through which the water reduction is catalyzed, fol-
lowing the fast intermolecular electron transfer that is generat-
ed through the quenching of the excited state of the complex
in the presence of TEA. With TEA, the decay time of complex 1
was observed to be 88 ns. The reductive quenching of the
II
0 [32]
change (DE=2.4 eV) from Pt to Pt . Therefore, these data
provide support for the idea that colloidal species do not par-
[31]
ticipate in the catalysis. Quantitative poisoning experiments
with carbon disulphide (CS ) were performed to identify the
2
true catalytically active species in the given photocatalytic re-
action. These show that ten equivalents of CS are required in-
2
3
MLCT excited state of 1 with TEA was monitored with a linear
hibit about 90% of the activity of the catalysis system
(Figure 4), indicating that the catalytic reaction seems to be
Stern–Volmer plot, which gave a rate constant (k ) of 6.84”
q
10
À1 À1
[33]
1
0 m s . Our investigation demonstrates that energy trans-
homogeneous. In the future, a further unravelling of the un-
fer in the present supramolecular systems is helpful for the
light-harvesting process for hydrogen-production reactions, in
which the photoharvesting unit transfers the light energy di-
rectly to the reaction center and thus increases the efficiency
of the system, as in natural light-harvesting complexes.
derlying mechanism involved in this complex photocatalytic
process should assist in establishing the correlation between
structural features and function of this type of complexes.
In this study, we have described the design and preparation
of a new supramolecular structural motif for the development
of a water reduction photocatalyst. It was constructed with an
photosensitizer part, using an iridium(III) carbene-based sensi-
tizer, coupled through a rigid conjugated phenanthroline de-
rived spacer to a catalyst part based on the platinum(II) acety-
The detailed mechanistic pathway for hydrogen production
is not yet well elucidated for platinum(II) organometallic com-
[
28–30]
pounds.
As in previous related studies, a high yield of the
Pt reduced state is required for the photoreduction of the cat-
alyst in the presence of an electron donor, and the subsequent
reduction of the protons occurs at the Pt site to achieve the
[
10,31]
observed hydrogen.
In the present system, in situ forma-
tion of colloidal Pt metal, which act as the actual active sites
for the hydrogen reduction, cannot be fully excluded though
no distinct metallic particles were observed as precipitates
during photocatalysis with complex 1 (even at 50 mm). When
the photocatalytic reaction occurred with notable deactivation
after 60 h of illumination, UV/Vis spectra were recorded for
samples of the reaction mixture (1+TEA) in acetone/water
(
9:1, v/v). We found a single absorption tail extending into the
visible region, and the characteristic absorbance attributed to
the catalyst decreased dramatically compared with that prior
to photolysis (Figure S7 in the Supporting Information), sug-
gesting the decomposition of 1 occurs as hydrogen evolution.
To shed more light on the mechanism of decomposition, the
decomposition products were extracted from the mixture and
Figure 4. Effect of CS
a system of 1+TEA. Inset: Evaluation of hydrogen production in the pres-
ence of various equivalents of CS catalyst poison in a system of 1+TEA. Re-
action conditions: 0.18m TEA and 10 mm 1 in a 100 mL 9:1 acetone/water
solution (l>420 nm).
2
poisoning on the photogeneration of hydrogen in
2
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lide complex. In this assembly the energy-transfer process
takes place from the excited iridium center to the energy ac-
ceptor, a platinum fragment. The use of supramolecular cata-
lysts not only extends carbene chemistry to photocatalysis but
also indicates that the possibility exists to employ molecular-
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·
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Published online on June 10, 2015
Chem. Eur. J. 2015, 21, 10003 – 10007
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10007
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