Highly efficient photocatalytic water reduction with robust iridium(III) photosensitizers containing arylsilyl substituents

Article abstract of DOI:10.1002/anie.201305684

Waterproof complexes: Cationic Ir^III photosensitizers (PSs) with an ancillary 4,4′-bis(4-(triphenylsilyl)phenyl)-2,2′-bipyridine ligand enabled hydrogen evolution from water with high turnover numbers (TONs; see scheme). The peripheral tripheny

Full text of DOI:10.1002/anie.201305684

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Angewandte
Communications
DOI: 10.1002/anie.201305684
Hydrogen Production
Highly Efficient Photocatalytic Water Reduction with Robust
Iridium(III) Photosensitizers Containing Arylsilyl Substituents**
Dong Ryeol Whang, Ken Sakai, and Soo Young Park*
As initiated by the pioneering studies of the Bernhard
research group,[1] enormous advances in cyclometalated IrIII
photosensitizers have been made in the pursuit of highly
efficient visible-light-driven water-reduction systems.[1–16]
Many different classes of cationic IrIII photosensitizers have
been developed with the general structure [IrIII(C^N)2-
(N^N)]+, in which C^N is a monoanionic cyclometalating
ligand and N^N is a neutral ancillary ligand.[4] Photocatalytic
water-reduction systems are typically benchmarked in terms
of their turnover number (TON), which reflects the stability
of the system under the operating conditions and is thus
deeply related to the monetary expense for solar-to-hydrogen
energy conversion.[7] DiSalle and Bernhard designed a series
of IrIII photosensitizers with pendant pyridine moieties, which
directed the adsorption of the complexes onto a colloidal
platinum catalyst to generate H2 with a TON value of 8800.[5]
However, it has been shown that the overall photo-
catalytic performance of these systems is primarily limited by
the fundamental degradation of photosensitizers during
photolysis.[11,12] Practically, the degradation of IrIII photo-
sensitizers has been suggested to occur by the water-assisted
alkyl groups on the N^N ligand,[4] and III) by increasing the
intrinsic stability of IrIII photosensitizers by changing the
framework from [Ir(C^N)2(N^N)]+ (TON = 86) to [Ir-
(C^N^N)2]+ (TON = 273).[9]
In previous studies of light-emitting electrochemical cells,
attempts were made to inhibit unwanted chemical reactions
so that the fundamental inertness of cationic IrIII complexes
could be considerably improved. In this context, N^N ligands
with bulky aromatic substituents, such as phenyl[19,20] and
Frꢀchet-type dendrons,[21] were designed and tested. We
believed that this approach could be applied to the improve-
ment of photosensitizers for photocatalytic water reduction
according to the above-mentioned strategy II.
We focus herein on the use of triphenylsilyl (TPS) groups
as bulky moieties to prevent ligand substitution and thus
further improve the inertness as well as the sensitivity of IrIII
photosensitizers. In our previous studies, TPS groups together
with related groups were successfully attached to neutral IrIII
complexes to provide highly efficient organic light-emitting
devices.[22–24] These studies showed that arylsilyl groups, such
as TPS groups, provide sufficient steric hindrance to protect
typical reactive centers by imparting a so-called “site-
isolation effect” to the chromophores.[22–27] Herein we show
that the three aryl rings around the silicon atom of TPS groups
are remarkably efficient in protecting the complex from
ligand substitution only if the groups are introduced at the
N^N site. The enhanced stability of a series of TPS-containing
IrIII photosensitizers are examined in detail in conjunction
with their actual photosensitization characteristics in the
photoreduction of water.
À
rupture of metal N(N^N) bonds and subsequent solvolysis of
the complex with the formation of solvento complexes.[17,18]
Some studies have shown that the stability of IrIII photo-
sensitizers can be improved I) by changing the organic
cosolvent from acetonitrile (MeCN) to weakly coordinating
tetrahydrofuran (THF),[2] II) by introducing sterically bulky
[*] D. R. Whang, Prof. S. Y. Park
Center for Supramolecular Optoelectronic Materials
WCU Hybrid Materials Program and Department of Materials
Science and Engineering, Seoul National University
Seoul 151-744 (Korea)
E-mail: parksy@snu.ac.kr
Homepage: http://csom.snu.ac.kr
A series of IrIII photosensitizers with TPS substituents in
various positions were designed and synthesized together
with related control compounds (Scheme 1). Moreover, we
employed two different C^N ligands, 2-(2,4-difluorophenyl)-
pyridine (dfppy) and 2-phenylpyridine (ppy), to tune the
photophysical and electrochemical properties of the IrIII
photosensitizers. We also synthesized and examined an IrIII
photosensitizer with 4,4’-diphenyl-2,2’-bipyridine as the N^N
ligand (Irbpbpy) to explore the relationship between sub-
stituent size and the photosensitizer performance. All IrIII
photosensitizers were synthesized according to standard
procedures (see the Supporting Information).
Prof. K. Sakai
Department of Chemistry, Faculty of Sciences, Kyushu University
Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581 (Japan)
and
International Institute for Carbon-Neutral Energy Research
(WPI-I2CNER), Kyushu University (Japan)
and
International Research Center for Molecular Systems (IRCMS)
Kyushu University (Japan)
Photocatalytic water-reduction efficiency with the IrIII
photosensitizers was evaluated by visible-light irradiation
(l > 400 nm) of the system in cooperation with colloidal
platinum as a water-reduction catalyst (WRC) and triethyl-
amine (TEA) as a sacrificial reducing agent. All experiments
were carried out until gas evolution ceased, and the total
TON values of the IrIII photosensitizers were calculated on
the basis of the number of single-electron-transfer processes
[**] This research was supported by the National Research Foundation
of Korea (NRF) through a grant funded by the Korean government
(MSIP; No. 2009-0081571). The research at Kyushu University was
supported by the International Institute for Carbon-Neutral Energy
Research (WPI-I2CNER), sponsored by the World Premier Interna-
tional Research Center Initiative (WPI), MEXT (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201305684.
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Although the TON of
phenyl-substituted Irbpbpy
(4100) is higher than that of
the unfunctionalized IrIII
photosensitizer
Irbpy
(TON = 530), it is still much
smaller than that of the IrIII
photosensitizer IrTPS with
a
bulkier
substituent
(TON = 17000). The use of
bulkier substituent groups
presumably minimizes the
accessibility of the iridium
center to the solvent and
thus prevents undesired pho-
toinduced solvolysis of the
photosensitizer. Notably, the
TON values of the IrIII pho-
tosensitizers containing TPS
groups in the cyclometalat-
ing ligands (C^N), that is,
TPSppyIr and ppyTPSIr,
were observed to be 570
and 800, respectively, which
are much smaller than those
of IrTPS and FIrTPS. Thus,
the introduction of TPS sub-
stituents on the N^N ligand
Scheme 1. Chemical structures of the IrIII photosensitizers studied.
resulted in
a
dramatic
improvement in the stability
of the photosensitizer, whereas TPS substituents on the C^N
ligands did not provide any stabilizing effect at all. This
observation clearly indicates that the stability of these IrIII
photosensitizers is fundamentally governed by the inertness
(see Table 1). Figure 1 shows the kinetic traces of hydrogen
evolution driven by the IrIII photosensitizers. IrTPS and
FIrTPS showed total TON values of 17000 and 10000,
respectively. To the best of our knowledge, these are the
highest TONs observed for water-reduction catalysis with IrIII
photosensitizers. The control IrIII photosensitizers without
TPS moieties, Irbpy and FIrbpy, showed total TON values of
530 and 480, respectively. The effect of the size of substituent
groups on the photosensitizer activity can be rationalized by
comparing the TON values of IrTPS, Irbpbpy, and Irbpy.
Table 1: Photophysical and photocatalytic properties of IrIII photosensi-
tizers.
[a,b]
[a,c]
[d]
[d]
Entry
lPL
[nm]
tav
[ms]
Eox
[V]
Ered
[V]
TON
(tirr [h])[e]
IrTPS
605
531
601
608
590
589
529
0.47
0.95
0.41
0.22
0.44
0.42
1.3
1.11
1.44
1.16
1.07
1.12
1.10
1.45
À1.26
À1.17
À1.17
À1.28
À1.29
À1.33
À1.23
17000 (144)
10000 (116)
4100 (72)
570 (24)
800 (24)
530 (24)
FIrTPS
Irbpbpy
TPSppyIr
ppyTPSIr
Irbpy
FIrbpy
480 (25)
[a] Value for a 10 mm solution of the photosensitizer in argon-saturated
THF. [b] lex =300 nm. [c] lex =377 nm. [d] The oxidation and reduction
potentials were determined by cyclic voltammetry (versus Ag+/Ag).
[e] TON=2n(H2)/n(photosensitizer), as calculated for reactions carried
out under the experimental conditions described in the Supporting
Information. tirr is the irradiation time.
Figure 1. Kinetic traces of hydrogen evolution. A mixture of the IrIII
photosensitizer (0.50 mmol), K2[PtCl4] (0.50 mmol), TEA (1 mL), H2O
(1 mL), and THF (8 mL) in a 40 mL air-tight vial was irradiated with
a 150 W Xe lamp with an interference filter that eliminated UV light of
wavelengths below 400 nm. Fitted curves are shown with dashed lines.
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of the N^N chelate during the light-induced events. As
mentioned above, one possible interpretation is that ligand
substitution of the N^N chelate is considerably retarded by
the presence of bulky/protective TPS substituents, as sub-
stitution of the N^N ligand is likely to proceed by the so-
called interchange associative mechanism.[4,19–21] When the
transition state for this substitution is reached, the reorgan-
ization energy required to relocate or twist the leaving
bipyridyl chelate must be substantially larger when relatively
large substituents, such as TPS groups, are tethered to the
N^N ligand. This mechanism rationalizes well the higher
stability of compounds with TPS moieties on the N^N ligand.
To confirm whether the high performance of IrTPS and
FIrTPS was due to their intrinsic photochemical stability, we
investigated the degradation of the photosensitizer itself in
the absence of the sacrificial reagent and the WRC. Each
sample containing IrTPS or Irbpy (50 mm) in THF and H2O
(9:1 v/v) was irradiated with visible light (l > 400 nm), and the
absorption spectral changes in the UV/Vis range were
recorded (Figure 2; see the Supporting Information for
other IrIII photosensitizers). Whereas changes in the absorp-
tion of the [IrIII(C^N)2(N^N)]+ framework. To ascertain the
termination mechanism of the IrTPS-containing water-reduc-
tion system, we also checked the long-term stability (up to
120 h) of IrTPS and colloidal platinum (see the Supporting
Information for a discussion).
To further ascertain the role of TPS substituents, we
explored the photophysical and electrochemical properties of
the IrIII photosensitizers (Table 1). All synthesized IrIII photo-
sensitizers were found to be luminescent in THF. The
photoluminescence (PL) energies and lifetimes of the IrIII
photosensitizers are primarily governed by the chemical
structure of their parent C^N ligand (Table 1). The PL spectra
of IrIII photosensitizers incorporating ppy-based C^N ligands
(IrTPS, Irbpbpy, TPSppyIr, ppyTPSIr, and Irbpy) are char-
acterized by their emission maximum at 589–608 nm, whereas
photosensitizers incorporating dfppy-based C^N ligands
(FIrTPS and FIrbpy) showed an emission maximum at 529–
531 nm. A similar trend was recognized in the PL lifetimes of
ppy-based (tav = 0.22–0.47 ms) and dfppy-based IrIII photo-
sensitizers (tav = 0.95–1.3 ms). This trend is consistent with the
electrochemical properties of the IrIII photosensitizers. The
first reduction potentials of all the IrIII photosensitizers are
virtually identical (Ered = À1.17–À1.33 V) and are character-
istic of bpy-centered reduction. On the other hand, the
oxidation potentials are governed by the parent C^N ligand
(Table 1, Eox = 1.07–1.12 V for ppy-based IrIII photosensitizers
and Eox = 1.44–1.45 V for dfppy-based IrIII photosensitizers),
in agreement with the fact that the electronic states of the IrIII
photosensitizers are largely governed by the parent C^N
ligand with a minimal contribution from the TPS or phenyl
substituents.
The above photophysical properties of the IrIII photo-
sensitizers can be rationalized by the results of DFT
calculations. In both systems containing TPS-substituted
bipyridyl ligands, no hybridization is observed between the
phenyl rings of the TPS groups and the parent bipyridyl
system. Thus, the electronic effects of TPS are limited to
effects on the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) of the
IrIII photosensitizers. Figure 3 displays the optimized geom-
etry of IrTPS and the HOMO/LUMO contour diagrams,
which show negligible contributions of TPS to these frontier
orbitals. Mulliken population analysis (MPA) revealed that
the contributions of TPS fragments to the HOMO and
LUMO are 0.01 and 0.63%, respectively. Similar results were
Figure 2. Absorption spectra of a) IrTPS and b) Irbpy at various time
intervals during photoirradiation. In each case, a mixture of the IrIII
photosensitizer (50 mm) in THF and H2O (9:1 v/v) was irradiated with
a 150 W Xe lamp with an interference filter that eliminated UV light of
wavelengths below 400 nm.
tion spectra of bpy- and bpbpy-bearing IrIII photosensitizers
clearly showed degradation upon photoirradiation, the spec-
tra of IrTPS and FIrTPS barely changed. Thus, IrTPS and
FIrTPS demonstrated outstanding photochemical stability.
The increase in absorbance at around 300 nm upon photo-
degradation was assigned to the p–p* transition of the N^N
ligand by comparison with the spectral features of free bpy
and bpbpy (see Figure S2 in the Supporting Information);
thus, this increase is due to the formation of free NN species
upon the photodegradation of the IrIII photosensitizers. These
results clearly indicate that the IrIII photosensitizers with
phenyl groups on the N^N ligand (bpbpy) or TPS groups on
the C^N ligands are rather susceptible to water-assisted
solvolysis, in good agreement with the observed trend in the
TON values for water-reduction catalysis, and further support
our conclusion that the introduction of TPS substituents on
the N^N ligand plays a crucial role in the dramatic stabiliza-
Figure 3. HOMO and LUMO isosurface plots for IrTPS. An isodensity
value of 0.05 eꢀÀ3 was applied for plotting the surfaces.
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found for other TPS-containing IrIII photosensitizers (see the
Supporting Information). We can therefore conclude that the
significant effects of TPS groups on the photochemical
stability and TON of IrIII photosensitizers are indeed caused
by steric factors rather than electronic factors.
In this study, we successfully synthesized a series of
[IrIII(C^N)2(N^N)]+ complexes with bulky/protective TPS
substituent groups with the aim of developing new photo-
catalytic water-reduction systems with highly improved
durability. Systematic exploration of the role of the TPS
substituents revealed that the photosensitizers containing
TPS groups at the N^N ligand exhibited dramatically
enhanced durability. More importantly, the use of TPS
groups led to the observation of exceedingly high TON
values: the highest TON found was 17000 and thus the
highest value reported to date. We believe that the results of
this study indicate important new strategies for the design of
more durable systems in various areas of photochemistry.
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Keywords: hydrogen production · iridium · photocatalysis ·
photosensitizers · water reduction
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