Hybrid mesoporous-silica materials functionalized by PtII complexes: Correlation between the spatial distribution of the active center, photoluminescence emission, and photocatalytic activity

Article abstract of DOI:10.1002/chem.201200959

[Pt(tpy)Cl]Cl (tpy: terpyridine) was successfully anchored to a series of mesoporous-silica materials that were modified with (3-aminopropyl) triethoxysilane with the aim of developing new inorganic-organic hybrid photocatalysts. Herein, the relationship between the luminescence characteristics and photocatalytic activities of these materials is examined as a function of Pt loading to define the spatial distribution of the Pt complex in the mesoporous channel. At low Pt loading, the Pt complex is located as an isolated species and exhibits strong photoluminescence emission at room temperature owing to metal-to-ligand charge-transfer (³MLCT) transitions (at about 530 nm). Energy- and/or electron-transfer from ³MLCT to O₂ generate potentially active oxygen species, which are capable of promoting the selective photooxidation of styrene derivatives. On the other hand, short Pt...Pt interactions are prominent at high loading and the metal-metal-to-ligand charge-transfer (³MMLCT) transition is at about 620 nm. Such Pt complexes, which are situated close to each other, efficiently catalyze H₂-evolution reactions in aqueous media in the presence of a sacrificial electron donor (EDTA) under visible-light irradiation. This study also investigates the effect of nanoconfinement on anchored guest complexes by considering the differences between the pore dimensions and structures of mesoporous-silica materials. Anchorman: [Pt(tpy)Cl]Cl (tpy: terpyridine) was anchored to a series of mesoporous-silica materials that were modified with (3-aminopropyl)trimethoxysilane to afford new inorganic-organic hybrid photocatalysts (see graphic). Copyright

Full text of DOI:10.1002/chem.201200959

FULL PAPER
DOI: 10.1002/chem.201200959
II
Hybrid Mesoporous-Silica Materials Functionalized by Pt Complexes:
Correlation between the Spatial Distribution of the Active Center,
Photoluminescence Emission, and Photocatalytic Activity
Kohsuke Mori, Kentaro Watanabe, Yoshikazu Terai, Yasufumi Fujiwara, and
[
a]
Hiromi Yamashita*
Abstract: [Pt
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(tpy)Cl]Cl (tpy: terpyri-
luminescence emission at room temper-
ature owing to metal-to-ligand charge-
transfer ( MLCT) transitions (at about
and the metal-metal-to-ligand charge-
transfer ( MMLCT) transition is at
about 620 nm. Such Pt complexes,
which are situated close to each other,
3
dine) was successfully anchored to a
series of mesoporous-silica materials
that were modified with (3-aminopro-
pyl)triethoxysilane with the aim of de-
veloping new inorganic–organic hybrid
photocatalysts. Herein, the relationship
between the luminescence characteris-
tics and photocatalytic activities of
these materials is examined as a func-
tion of Pt loading to define the spatial
distribution of the Pt complex in the
mesoporous channel. At low Pt load-
ing, the Pt complex is located as an iso-
lated species and exhibits strong photo-
3
530 nm). Energy- and/or electron-trans-
3
fer from MLCT to O generate poten-
efficiently catalyze H -evolution reac-
2
2
tially active oxygen species, which are
capable of promoting the selective pho-
tooxidation of styrene derivatives. On
the other hand, short Pt···Pt interac-
tions are prominent at high loading
tions in aqueous media in the presence
of a sacrificial electron donor (EDTA)
under visible-light irradiation. This
study also investigates the effect of
nanoconfinement on anchored guest
complexes by considering the differen-
ces between the pore dimensions and
structures of mesoporous-silica materi-
als.
Keywords: luminescence · mesopo-
rous materials · photocatalysis ·
platinum · silica
[3]
Introduction
metal cations. It was found that the increased intensity of
the phosphorescence emission collaborated well with the in-
creased turnover number (TON) to promote the photoin-
duced oxidation reaction by using molecular oxygen. On the
other hand, the use of mesoporous-silica materials with uni-
form channels (2–50 nm) is of particular interest because
they can be easily tailored to afford preferred characteristics
by anchoring functional groups onto their surfaces or by
The integration of metal complexes with rigid inorganic ma-
trices, such as clays, zeolites, and mesoporous materials, has
been intensively pursued in the drive to construct functional
[1]
inorganic–organic supramolecular devices. In comparison
with their properties in solution, such materials often exhibit
unexpected physicochemical properties that are induced by
both steric- and electrostatic constraints within the restricted
void spaces, as well as by the nature of the functional groups
on the host surface. In the field of optical materials, there
have been numerous reports that utilize zeolites as promis-
ing supports to host small molecules and micron-scale pho-
tonic band-gap materials within their restricted supercages
[4]
doping metal atoms into the silica framework. The high
surface areas, as well as pore volumes, facilitate the opportu-
nity of doping large-sized molecules whilst being capable of
a holding high concentration of these molecules; moreover,
the good optical transparency in the visible-light region also
[5]
provides great potentials for optical applications.
Luminescent polypyridine–metal complexes, in particular
[
2]
(
about 1.3 nm). In our previous work, the physicochemical
2+
2+
8
II
properties of [Fe
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bpy) ] and [Ru
A
H
U
G
R
N
N
(bpy) ] complexes that
square-planar d Pt complexes, have received recent atten-
tion owing to their unique spectroscopic and photophysical
3
3
were included within the cavities of Y zeolites were exam-
ined by continuously varying the extra-framework alkali-
[6]
II
properties. The photoluminescence from such Pt com-
plexes originates from some of the lowest triplet-excited
states, including metal-to-ligand charge-transfer (MLCT)-
and metal-metal-to-ligand charge-transfer (MMLCT) excit-
[
a] Dr. K. Mori, K. Watanabe, Dr. Y. Terai, Prof. Y. Fujiwara,
Prof. H. Yamashita
[7]
Division of Materials and Manufacturing Science
Graduate School of Engineering, Osaka University, 2-1 Yamada-oka
Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7457
E-mail: yamashita@mat.eng.osaka-u.ac.jp
ed states. A low-lying MMLCT excited state appears in
non-covalent dimers and aggregates with short Pt···Pt sepa-
ration (<3.5 ꢀ) and the resulting emission is observed at
3
longer wavelengths than those of unimolecular MLCT emis-
sions. These characteristics provide significant motivation
for developing new materials with new functions or with
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201200959.
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combinations of their inherent properties to expand the
range of possible applications.
With this goal in mind, we have developed a number of
new inorganic–organic hybrid photocatalysts by anchoring a
chloro(2,2’:6’,2’’-terpyridine)platinum(II) complex to meso-
tained. N -adsorption/desorption isotherms of all of the sam-
2
ples were type-IV without a hysteresis loop (see the Sup-
porting Information, Figure S1). The Supporting Informa-
tion, Table S1 shows that the high BET surface areas and
pore volumes were retained, even after deposition of the Pt
complex. The diffraction peaks that were due to the hexago-
nally packed mesoporous structure were almost at the same
locations after deposition of the Pt complex, which indicated
that the long-range order was retained. The estimated diam-
porous-silica hosts. The choice of this [Pt ACHUTNGRNENUG( tpy)Cl]Cl complex
for the evaluation of host–guest interactions is motivated by
the following factors: The Pt complex can be successfully
anchored to the mesoporous silica through (3-aminopropyl)-
triethoxysilane linkers to create new PtꢀN bonds with
eter of the [Pt ACHTUNGRNENUG( tpy)Cl]Cl complex (about 0.9 nm) is less than
amino groups. Because the interactions between each Pt
complex can be easily controlled by varying the Pt loading,
the effect of the spatial distribution of the Pt complex in the
mesoporous channel on the phosphorescence emission and
photocatalytic activity can be fully investigated. Herein, we
report that this isolated Pt complex enables a selective pho-
half that of the MCM-48 channel (2.5 nm). Therefore, the Pt
precursors should diffuse through the channel to a sufficient
extent to form surface-attached complexes that are homoge-
neously distributed across the channel at this loading level.
Figure 1A shows normalized X-ray absorption near-edge
structure (XANES) spectra at the Pt L -edge of the [Pt-
III
tooxidation reaction with O by promoting energy- and/or
ACHTUNGTRENNUN(G tpy)Cl]Cl complex and anchored samples with different Pt
2
3
electron-transfer from MLCT to O , whilst H -production
2
2
in aqueous media is promoted by the proximal Pt com-
plexes, which is capable of the visible-light photosensitiza-
3
tion that is associated with the MMLCT excited states. In
addition, the differences between the pore dimensions and
structures of mesoporous-silica materials were evaluated to
elucidate the effect of nanoconfinement on the anchored
guest molecules. The exploitation of host–guest interactions
within the restricted pore arrangement is expected to confer
unanticipated photochemical properties on the anchored
complexes.
Results and Discussion
Characterization of the anchored Pt complex: Mesoporous-
silica material MCM-48, which consisted of a uniform array
of 3D-connected tubular pores, was prepared according to a
literature procedure by using a surfactant self-assembly ap-
[8]
proach. The obtained MCM-48 was dehydrated under a
dynamic vacuum and the surface OH sites were modified
with (3-aminopropyl)triethoxysilane (APTMS). The [Pt-
Figure 1. A) Pt LIII-edge XANES spectra, and B) FT-EXAFS spectra of
powdered [Pt(tpy)Cl]Cl (a) and Pt-complex-anchored MCM-48 at a Pt
loading of 0.2 wt.% (b), 0.4 wt.% (c), 0.8 wt.% (d), 1.2 wt.% (e), and
.4 wt.% (f).
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(tpy)Cl]Cl complex was anchored onto the silica support by
ACHTUNGTRENNUNG
reacting a solution of the complex in CHCl with APTMS-
3
2
modified mesoporous silica at room temperature to generate
a new PtꢀN bond through the loss of HCl (Scheme 1). In
this way, samples of Pt ACHUTNGRNENGU( tpy)/MCM-48 with different amounts
of Pt loading (Pt: 0.2, 0.4, 0.8, 1.2, and 2.4 wt.%) were ob-
loadings. The white line at around 11565 eV is an absorption
threshold resonance that is due to electronic transitions
from 2p3/2 to unoccupied states above the Fermi level; this
line intensifies with an increase in the number of d-band va-
cancies as a result of oxidation. Therefore, the white-line
absorption peaks of more-oxidized Pt species are more in-
tense than those of the reduced species. All of the anchored
samples afforded more-intense peaks than free [Pt-
[9]
AHCTUNGTRENNUNG( tpy)Cl]Cl, thus suggesting that the surface-anchored Pt spe-
cies were in a slightly electron-deficient state because of the
decrease in the number of s-donor electrons on replacement
of the fourth coordinated ligand from chloride to a nitrogen
atom.
Scheme 1. Anchoring of the [Pt
modified silica surface.
ACHTUNGTRNENUNG( tpy)Cl]Cl complex onto the APTMS-
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II
Hybrid Mesoporous Silica Functionalized by Pt Complexes
FULL PAPER
Fourier transforms of the Pt L -edge X-ray absorption
is assigned to a MMLCT (ds*(Pt)ꢀp*(terpyridine)) excited
III
fine structure (EXAFS) data are shown in Figure 1B. All of
the spectra show a strong peak at around 1.5 ꢀ, owing to a
PtꢀN bond, and a small second shell at about 2.4 ꢀ, which
sate that originates from relatively short Pt···Pt separations
[11]
(<3.5 ꢀ), which are observed in dimers and aggregates.
Such interactions between the 5d orbitals of neighboring
z2
II
is due to adjacent carbon atoms. This result confirms a tri-
Pt centers give rise to a ds* HOMO, whilst the LUMO is
II
dentate binding structure for the Pt center. In the case of
predominantly centered on the aromatic nitrogen-donor
ligand. This phenomenon can be attributed to interactions
between narrowly separated Pt···Pt pairs in the mesoporous
silica channel, which may lead to distortion—to some
extent—by steric constraints, as evidenced by the FT-
EXAFS analysis.
The [Pt AHCTUNGTRENNUN(G tpy)Cl]Cl complex is luminescent in 77 K solution
and exhibits vibronically structured emission within the
range 460–570 nm (Figure 3A). This phenomenon is attrib-
the [Pt ACHTUNGTRENNUNG( tpy)Cl]Cl complex, an additional peak, which was
due to the PdꢀCl bond, was observed at around 1.8 ꢀ,
which completely disappeared after the reaction with
APTMS-modified MCM-48 to generate the new PtꢀN bond,
and was accompanied by elimination of the PtꢀCl bond. No-
tably, samples with high Pt loading exhibited an additional
peak at around 3.5 ꢀ, which may be ascribed to short non-
covalent Pt···Pt interactions (see below). Moreover, the first
peaks of Pt ACHTUNGTRENNUNG( tpy)/MCM-48 were slightly shifted towards
shorter interatomic distances with increasing Pt content.
The Supporting Information, Table S2 summarizes the
curve-fitting analysis by using the PtꢀN shell parameter in
[Pt ACHTUNGTRENNUNG( tpy)Cl]Cl as a standard (CN=3.0, R=2.06 ꢀ). The Pt-
anchored samples exhibit average distances of 2.00–2.06 ꢀ
with coordination numbers of 3.9–4.2. The average PtꢀN
distances shortened as the Pt loading increased, which sug-
gests that the anchored Pt complexes may undergo slight
distortion within the channel, induced by the steric con-
straints at high Pt loading.
As shown in Figure 2A, the UV/Vis absorption spectrum
of the [Pt
ACHTUNGTRENNUNG( tpy)Cl]Cl solution displays intense absorption
bands in the high-energy region (l<350 nm), which are as-
Figure 3. A) Photoluminescence spectra of [Pt ACHUNTGRNENGU( tpy)Cl]Cl complex in
MeCN at 77 K (a) and at room temperature (b), lex =330 nm; B) photo-
luminescence spectra of Pt-complex-anchored MCM-48 at room tempera-
ture with various amounts of Pt loading (lex =450 nm).
3
utable to the formally LC excited state, with a substantial
3
[12]
contribution from close-lying MLCT levels. A relatively
narrow and unstructured luminescence is also observed
within the range 600–800 nm, which is assigned to a
3
[13]
MMLCT band that originates from Pt···Pt interactions.
However, the room-temperature luminescence is almost
non-emissive owing to the presence of low-lying metal-cen-
tered (MC) excited states, which can be easily populated by
thermal activation and provide a fast deactivation pathway
through molecular distortion.
The situation is different for Pt ACHTUTGNRNENUNG( tpy)/MCM-48, which ex-
Figure 2. UV/Vis spectra of: A) [Pt ACHUTNRGNENUG( tpy)Cl]Cl in MeCN, and B) Pt-com-
plex-anchored MCM-48 with Pt loading from 0.2 to 2.4 wt.%.
[14]
hibits strong luminescence at room temperature under de-
gassing conditions (Figure 3B). It is likely that the rigid
matrix within the mesoporous-silica channel efficiently
blocks deactivation processes through molecular distor-
signed to a LC (pꢀp*) transition, and moderately intense
low-energy bands in the range 350–450 nm, which can be as-
cribed to spin-allowed MLCT transitions that involve a
Pt 5dp orbital as the donor orbital and a p* terpyridine orbi-
[13]
tion.
The intensity of the peak near 530 nm (owing to
[10]
3
tal as the acceptor orbital. As shown in Figure 2B, all of
the absorption bands of Pt(tpy)/MCM-48 are in good agree-
MLCT transitions) was most intense at 0.4 wt.% Pt load-
ing; a further increase in the Pt loading resulted in a signifi-
cant decrease in emission intensity. On the other hand, a
AHCTUNGTRENNUNG
ment with those of the free complex and the intensities of
the absorption bands increase steadily over the range of in-
vestigated loading levels. At high Pt loading, there is an ad-
ditional long-wavelength absorption feature at around
3
new emission appeared at around 620 nm ( MMLCT transi-
tion), which was most intense at 2.4 wt.% Pt loading. A sim-
ilar phenomenon was also observed upon excitation at
330 nm (see the Supporting Information, Figure S2). These
510 nm that is not present in the dilute solutions. This band
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H. Yamashita et al.
results clearly show that the Pt complex of the anchored sys-
tems exists in its isolated form without aggregation at low
loading levels, whereas the average separation between ad-
jacent Pt complexes becomes smaller and the contribution
of aggregation becomes dominant with increased Pt load-
Photocatalytic reactions with different Pt loadings: The po-
tential photocatalytic activity of the anchored Pt complexes
was first evaluated in the selective oxidation of styrene de-
rivatives in the presence of molecular oxygen at room tem-
perature. The turnover numbers (TON) based on Pt content
are shown in Figure 5A. The major products were benzalde-
[
15]
ing.
To more-fully characterize the complex that is anchored
on the mesoporous-silica materials, time-resolved lumines-
cence measurements were performed. Figure 4 shows the
decay in the photoluminescence intensity of Pt-complex-
Figure 4. Decay in the photoluminescence intensity of Pt-complex-anch-
ored MCM-48 (Pt: 2.4 wt.%, lex =337 nm).
anchored MCM-48 (Pt: 2.4 wt.%) upon excitation by a
pulsed N laser (337 nm, pulse width: 1 ns) under a vacuum.
2
3
Obviously, the photoluminescence decay owing to MLCT at
around 540 nm is slower than that owing to MMLCT at
3
around 645 nm.
A similar tendency was observed at
0.4 wt.% Pt loading (see the Supporting Information, Figur-
es S3 and S4). The emission-lifetime data is summarized in
Table 1. It has been reported that the lifetimes of the free
[ ACHTUNGTRENNUNG( tpy)Cl]Cl complex at 77 K upon excitation at 337 nm are
Pt
3
3
[8a]
15.0 and 3.1 ms for MLCT and MMLCT, respectively,
which are slightly longer than in the anchored samples. Be-
cause strong luminescence can be observed upon anchoring
at room temperature, an increase in nonradiative processes
is ruled out. Thus, the reason for the shorter lifetime may be
ascribed to the enhancement in transition probability.
Figure 5. Effect of Pt loading on Pt-complex-anchored MCM-48 for:
A) the photocatalytic oxidation of styrene derivatives in the presence of
O
2
(lex >400 nm), and B) photocatalytic production of H
2
from an aque-
ous acetate buffer solution and EDTA (lex >420 nm).
Table 1. Emission lifetimes for Pt-complex-anchored mesoporous silicas
(lex =337 nm).
hyde and acetophenone (>90% selectivity) through C=C
bond cleavage. No significant reaction was observed under
either dark conditions in the presence of the photocatalyst
Sample
Pt [wt.%]
0.4
l
max [nm]
lifetime (t) [ms]
3
MCM-48
540 ( MLCT)
6
540 ( MLCT)
6
540 ( MLCT)
6
540 ( MLCT)
6
8.9
0.3–1.2
8.5
0.3–1.0
8.6
0.3–1.3
7.4
3
35 ( MMLCT)
or without O under UV irradiation; nor did photooxidation
3
2
MCM-48
SBA-15
2.4
2.4
2.4
3
occur to any significant extent in the presence of a homoge-
neous solution of [Pt AHCTUNETGRNNUN(G tpy)Cl]Cl under identical reaction con-
ditions. Herein, 0.4 wt.% Pt loading was the most-efficient
catalyst for the photooxidation reaction and the TON in-
creased in the order 2.4<1.2<0.8<0.2<0.4 wt.%. This
35 ( MMLCT)
3
3
35 ( MMLCT)
3
MCM-41
3
35 ( MMLCT)
0.2–0.9
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II
Hybrid Mesoporous Silica Functionalized by Pt Complexes
FULL PAPER
order corresponded with the increasing intensity of the lumi-
talyst or without EDTA under visible-light irradiation. The
use of triethanolamine and KI instead of EDTA gave poor
results. Notably, the photocatalytic activity varied signifi-
3
nescence emission that was due to the MLCT states. In the
3
absence of O , the excited MLCT states exhibit phosphor-
2
escence emission, whilst potentially active oxygen species,
cantly according to the Pt loading: no H -evolution occurred
2
1
ꢀ
such as O and O * , are generated by energy- and/or elec-
to any significant extent at less than 0.4 wt.% Pt loading,
whilst the photocatalytic activity gradually increased at
more than 0.8 wt.% Pt loading. This behavior was in sharp
2
2
tron-transfer reactions from the excited triplet state to
[
13a,c,16]
O2.
The TON for photooxidation increases with in-
creasing luminescence-emission intensity of the excited trip-
let states. We expected that the enhanced excitation rate
and quantum efficiency of the anchored Pt complex increase
contrast to that in the photocatalytic oxidation with O , but
2
was in good accordance with the increasing intensity of the
3
luminescence emission owing to the MMLCT states. Signifi-
the energy- and/or electron-transfer to O , which ultimately
enhances the photooxidation activity.
The efficient quenching of the photoluminescence by O2
cantly, the maximum activity that was obtained with
2.4 wt.% Pt loading was higher than that of its homogene-
ous counterparts under identical conditions.
2
indicates that the Pt complex that is anchored to mesopo-
The classical molecule-based photocatalytic H -evolution
2
3
rous silica easily interacts with O in its MLCT states com-
system consists of multiple components, including EDTA
2
3
2+
pared to the MMLCT states, as shown in Figure 6. The
(sacrificial electron donor), [Ru ACHUTNGERTNNUNG( bpy) ] (bpy=2,2’-bipyri-
3
2+
Stern–Volmer equation can be used to determine the
dine, photosensitizer), methylviologen (MV , electron
[17]
relay), and colloidal Pt (H -evolving catalyst). The overall
2
reaction is considered to be a visible-light-driven reduction
of protons by EDTA to form molecular hydrogen through
an electron relay. Notably, our photocatalytic system is a
single-component system in which the anchored Pt com-
plexes at high Pt loading possess bifunctionality, that is, they
are capable of visible-light photosensitization, which is asso-
3
ciated with the MMLCT excited states, and hydrogenic acti-
vation to evolve H without an electron relay. The formation
2
of Pt complexes with close proximity to one another is cru-
3
cial because short-lived emissive MMLCT excited states are
3
a more-suitable photosensitizer than long-lived MLCT tran-
sitions. The favorable formation of the postulated hydride–
diplatinum ACHUTNRGNENUG( II, III) intermediate by cooperative action of ad-
jacent Pt complexes also accounts for the high photocatalyt-
ic activity at high Pt loadings.
Figure 6. A) Effect of O
cence spectra of Pt(tpy)/MCM-48 at 2.4 wt.% Pt loading, and B) Stern–
Volmer plot of the I /I values for the yields of the photoluminescence re-
action versus O pressure for MLCT (a) and MMLCT (b), lex =330 nm.
2
addition on the intensity of the photolumines-
ACHTUNGTRENNUNG
0
3
3
Effect of the structure of the mesoporous silica: MCM-48
2
consists of a uniform array of 3D-connected tubular pores
[
18]
(
see above). MCM-41 has a 1D, hexagonally ordered, un-
[19]
quenching of the photoluminescence with O , according to
connected-but-regular cylindrical pore structure.
SBA-15
2
Equation (1), where I and I represent the intensities of pho-
exhibits a regular 1D array of tubular channels that are con-
0
[20]
toluminescence in the absence and presence of O , respec-
nected through lateral connections. Beside its large hexag-
onal channels, which range from 5–30 nm in diameter, SBA-
15 has thicker pore walls (3–6 nm) than MCM-41. In addi-
tion, this molecular sieve often exhibits surface roughness,
including a certain amount of disordered micropores and
2
tively, and Ksv and [Q] are the quenching rate constant and
the concentration of the quencher (i.e., O ), respectively.
2
I =I ¼ 1þK ½Qꢁ
ð1Þ
0
sv
[21]
small mesopores. Such differences in the pore dimensions
and structures of these materials make it possible to investi-
gate the effect of nanoconfinement on anchored guest mole-
cules.
3
The quenching rate constant for MLCT is 0.056, which is
3
quite a bit larger than that for MMLCT (0.015). This result
3
clearly suggests that the excited MMLCT state is hard to
interact with O and, thus, that the photooxidation ability
decreased with higher Pt loading.
Next, to evaluate the potential use of these Pt catalysts,
The Pt-complex-anchored mesoporous-silica samples with
different Pt loadings (Pt: 0.4 and 2.4 wt.%) were obtained
by using the same method with SBA-15 and MCM-41, re-
spectively. The textural properties are given in the Support-
ing Information, Table S1. Maintenance of the mesoporous
structure was confirmed by X-ray diffraction (XRD) and by
the N -adsorption/desorption isotherms. Pt L -edge XAFS
2
H -evolution in aqueous media in the presence of ethylene-
2
diaminetetraacetic acid (EDTA) under visible-light irradia-
tion (l>420 nm) was performed. The progress of H -pro-
2
duction over time, normalized based on the amount of Pt, is
shown in Figure 5B. No significant reaction was observed
either under dark conditions in the presence of the photoca-
2
III
showed no significant changes in the electronic structure
II
and chemical environment of the anchored Pt complexes
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H. Yamashita et al.
on SBA-15 and MCM-41 compared to those on MCM-48
see the Supporting Information, Figure S5). In the UV/Vis
(
spectra, all of the absorption bands of the Pt-complex-anch-
ored SBA-15 and -MCM-41 samples were in good agree-
ment with those anchored on MCM-48 (see the Supporting
Information, Figure S6).
Interestingly, the emission intensities varied according to
the type of mesoporous-silica material: the intensity near
3
5
30 nm, owing to MLCT transitions, increased in the order
MCM-41<SBA-15<MCM-48 at 0.4 wt.% Pt loading (Fig-
ure 7A). At 2.4 wt.% Pt loading, the emission at longer
3
wavelength, which was ascribed to the MMLCT transition,
also varied in the same order at 0.4 wt.% Pt loading (Fig-
ure 7B). The slight shift towards the longer-wavelength
Figure 7. Photoluminescence spectra of various Pt-complex-anchored
mesoporous-silica samples at: A) 0.4wt.% Pt loading (lex =330 nm), and
B) 2.4wt.% Pt loading (lex =450 nm).
region was observed in the case of MCM-48 and MCM41,
which may be attributed to the substantial contribution by
the excimeric species that originated from pꢀp interactions
Figure 8. Effect of the mesoporous structure in the Pt-complex-anchored
samples for: A) the photocatalytic oxidation of styrene derivatives in the
presence of O (lex >400 nm), and B) the photocatalytic production of H
2 2
[22]
between the partially stacked terpy ligands. The emission
spectra are dependent on the ligand field strength, the redox
properties of the metal and the ligands, and the intrinsic
from an aqueous acetate buffer solution and EDTA (lex >420 nm).
[23]
properties of the ligands. Therefore, a slight change in the
environment of the Pt complex, such as pore dimension and
structure, significantly influences the orbital nature of the
3
tensity in the luminescence emission owing to the MLCT
states. As mentioned earlier, a similar phenomenon was ob-
served by varying the Pt loading in MCM-48. Thus, the
anchored Pt complex, with high emission intensity owing to
[24]
excited state. In this case, the smaller hexagonal channels
of MCM-48 (2.5 nm) and MCM-41 (2.6 nm) compared to
SBA-15 (6.3 nm) may lead to much-shorter Pt···Pt interac-
tions, owing to pꢀp stacking between terpyridine ligands.
3
MLCT transitions, increases the energy- and/or electron-
With respect to the emission lifetime, slightly shorter life-
times can be observed in the case of MCM-41 for both the
transfer to O , which eventually enhances the photooxida-
2
tion activity.
3
3
MLCT and MMLCT transition states, which may be attrib-
In addition, the diffusion of O within the mesoporous-
2
uted to the increase in nonradiative process within the 1D-
and small hexagonal channel structure of MCM-41
silica channels may determine the rate of photooxidation. In
the Stern–Volmer equation, by using O as a quencher, Pt-
2
(
Table 1).
ACHTUNGTRENNUNG
As expected, the type of mesoporous-silica material that
2
was used significantly influenced its photocatalytic activities.
The total TON in the aerobic photooxidation of styrene de-
rivatives, by using samples with 0.4 wt.% Pt loading, in-
creased in the order MCM-41<SBA-15<MCM-48 (Fig-
ure 8A). This tendency corresponded with an increasing in-
and 0.05, which were observed for Pt
ACHTUNGTRENNUNG( tpy)/SBA-15 and Pt-
AHCTUNGTR(EUNNNG tpy)/MCM-41, respectively (Figure 9). This increase may be
[25]
due to the 3D-connected channel structure of MCM-48,
which enhances the diffusion of O within the channels and
2
provides ease of access to the anchored Pt complex. For
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II
Hybrid Mesoporous Silica Functionalized by Pt Complexes
FULL PAPER
reactants compared to the 1D hexagonal channel structures
of SBA-15 and MCM-41. This study unambiguously demon-
strates that the use of a mesoporous-silica support allows
the control of the photoluminescence properties and photo-
catalytic activities that are associated with the electronic
configuration of the lowest triplet state of the Pt complex,
according to the loading of Pt complexes and the type of
hexagonal channel structure.
Experimental Section
Synthesis of Anchored Pt complexes on mesoporous silica: To remove
any physisorbed water before surface-modification, the silica support
(MCM-41, SBA-15, and MCM-48) was dried at 393 K for 1 h under
vacuum. A toluene mixture (40 mL) that contained the support (1.0 g)
and APTMS (0.27 g) was stirred at RT for 20 h and then heated at reflux
for 4 h. The product was recovered by vacuum filtration, washed with
Figure 9. Stern–Volmer plot of the I
0
/I values for the yields of the photo-
luminescence reaction versus O
and MCM-41 (c).
2
pressure for MCM-48 (a), SBA-15 (b),
EtOH, and dried under vacuum overnight. A solution of [Pt
(0.012 g) in CHCl (200 mL) was stirred with a sample of APTMS-modi-
fied MCM-41 (1.0 g) at RT for 24 h. The product was recovered by
vacuum filtration, washed with CHCl , and dried under vacuum over-
night to give Pt(tpy)/MCM-41. Pt(tpy)/SBA-15 and Pt(tpy)/MCM-48
were prepared by using the same method with SBA-15 and MCM-48, re-
spectively. Other samples with different Pt loadings were also prepared
by varying the initial concentration. The Pt loadings were determined by
inductively coupled plasma (ICP) analysis.
ACHTUNGTRENNUNG( tpy)Cl]Cl
SBA-15 and MCM-41, with 1D hexagonal channel struc-
tures, the efficiency increases as the pore-size increases.
The influence of mesoporous structure was also examined
3
3
A
H
U
G
R
N
U
G
A
H
U
G
R
N
U
G
ACHTUNGTRENNUNG
in the photocatalytic H -production reaction in aqueous
2
media, in which the amount of evolved H when using the
2
samples with 2.4 wt.% Pt loading increased in the order
MCM-41<SBA-15<MCM-48 (Figure 8B). This tendency
also collaborated well with the increasing intensity of the lu-
Characterization: Powder X-ray diffraction patterns were recorded on a
Rigaku RINT2500 diffractometer with Cu Ka radiation (l=1.5406 ꢀ).
BET surface-area measurements were performed on a BEL-SORP max
3
minescence emission owing to the MMLCT states. As men-
tioned earlier, a similar phenomenon was observed by vary-
ing the Pt loading on MCM-48. These results clearly support
(
Bel Japan, Inc.) at 77 K. The sample was degassed under vacuum at
373 K for 2 h prior to data-collection. UV/Vis diffuse-reflectance spectra
of powdered samples were collected on a Shimadzu UV-2450 spectropho-
tometer. BaSO was used as a reference and the absorption spectra were
4
obtained by using the Kubelka–Munk function. Inductively coupled
plasma optical emission spectrometry (ICP-OES) was performed on a
Nippon Jarrell-Ash ICAP-575 Mark II. Photoluminescence measure-
ments were recorded on a fluorolog-3 spectrofluorometer (Horiba) at
II
our proposal that the anchored Pt complexes, which are
closely related to each other, behave as single-component
bifunctional catalysts that enable both visible-light sensitiza-
3
tion owing to MMLCT excited states and H -evolution by
2
the cooperative action of adjacent Pt complexes without the
need for an additional electron relay.
2
2
93 K. In the time-resolved PL (TR-PL) measurements, a pulsed N laser
of 337.1 nm and a tunable dye laser that operated at 453 nm (1 ns pulse
width, 10 Hz repetition) were used as the excitation sources. TR-PL spec-
tra were recorded by a gated ICCD camera with a 0.15 m monochroma-
tor. Pt LIII-edge XAFS spectra were recorded at RT in fluorescence
mode at the BL-7C facility of the Photon Factory at the National Labora-
Conclusion
tory for High-Energy Physics, Tsukuba (2009G221). A Si ACHUTNRGNENUG( 111) double
Anchoring square-planar complex [Pt ACHUTNGRNENGU( tpy)Cl]Cl onto
crystal was used to monochromatize the X-rays from the 2.5 GeV elec-
tron-storage ring. In a typical experiment, the sample was loaded into an
in situ cell with plastic windows. EXAFS data were examined by using
the EXAFS analysis program, Rigaku EXAFS. The pre-edge peaks in
the XANES regions were normalized for atomic absorption, based on
the average absorption coefficient of the spectroscopic region. Fourier
APTMS-modified mesoporous silica successfully generates a
new type of photocatalyst with unique photoluminescence
properties and photocatalytic activity. Strong photolumines-
cence is observed upon anchoring, whereas a solution of the
II
free Pt complex is non-emissive at room temperature.
3
transformation (FT) of k -weighted normalized EXAFS data was per-
There is a correlation between the photoluminescence inten-
sity and the photocatalytic activity by varying the spatial dis-
tribution of the Pt complexes. Isolated Pt complexes, which
exhibit high photoluminescence intensity owing to the
formed over the range k=3.5–12 ꢀ to obtain the radial structure func-
tion. The CN (coordination number of scatters), R (distance between the
absorbing atom and the scatterer), and Debye–Waller factor were esti-
mated by curve-fitting analysis with inverse FT in the range R=0.8–2.8 ꢀ
assuming single scattering.
3
MLCT states, promote a selective oxidation reaction in the
Photocatalytic liquid-phase oxidation: The powdered Pt catalyst (0.01 g),
styrene (10.0 mmol), and MeCN (15 mL) were introduced into a quartz
reaction vessel (30 mL) that was then sealed with a rubber septum. The
resulting mixture was bubbled with oxygen for 30 min in the dark. Subse-
quently, the sample was irradiated from a sideways direction by using a
Xe lamp (500 W; SAN-EI ERECTRIC CO., Ltd. XEF-501S) for 24 h
with magnetic stirring at ambient pressure and temperature. After the re-
action, the resulting solution was recovered by filtration and analyzed by
an internal-standard technique on a Shimadzu GC-14B with a flame-ioni-
presence of O at room temperature. On the other hand, the
2
formation of proximal Pt complexes intensified the emission
3
that was due to the MMLCT transition, which leads to the
photocatalytic evolution of H2 as
a single-component
system. The 3D-connected tubular structure of MCM-48 is
suitable as a host material both in terms of the enhancement
in phosphorescence intensity and in terms of the diffusion of
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H. Yamashita et al.
zation detector that was equipped with TC-1 columns. The turnover
number (TON) was defined as follows: TON= mol products/mol Pt
atoms on catalyst.
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30 mm) were introduce into a reaction vessel (30 mL), which was then
2
-evolution: The powdered Pt catalyst (Pt: 4 mmol),
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Received: March 21, 2012
Published online: && &&, 0000
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II
Hybrid Mesoporous Silica Functionalized by Pt Complexes
FULL PAPER
Photocatalysis
K. Mori, K. Watanabe, Y. Terai,
Y. Fujiwara, H. Yamashita* &&&& — &&&&
Hybrid Mesoporous-Silica Materials
II
Functionalized by Pt Complexes:
Correlation between the Spatial Distri-
bution of the Active Center, Photolu-
minescence Emission, and Photocata-
lytic Activity
Anchorman: [Pt
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(tpy)Cl]Cl (tpy: ter-
thoxysilane to afford new inorganic–
organic hybrid photocatalysts (see
graphic).
pyridine) was anchored to a series of
mesoporous-silica materials that were
modified with (3-aminopropyl)trime-
Chem. Eur. J. 2012, 00, 0 – 0
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