Enhanced photocatalysis of rhenium(I) complex by light-harvesting periodic mesoporous organosilica

Article abstract of DOI:10.1021/ic1000914

This paper describes a new conceptual design for enhancement of photocatalytic CO₂ reduction of a rhenium(I) complex by light harvesting of periodic mesoporous organosilica (PMO). Mesoporous biphenyl-silica (Bp-PMO) anchoring fac-[Re^I(bpy)(CO)₃(PPh ₃)]⁺(OTf)^- (bpy =2,2′-bipyridine; OTf = CF₃SO₃) in the mesochannels was synthesized by co-condensation of two organosilane precursors, 4,4′-bis(triethoxysilyl) biphenyl and 4-[4-{3-(trimethoxysilyl)propylsulfanyl}butyl]-4′-methyl-2, 2′-bipyridine in the presence of a template surfactant, followed by coordination of a rhenium precursor, [Re^I(CO)₅(PPh ₃)]⁺(OTf)^- to the bipyridine ligand in the mesochannels. The 280 nm light was effectively absorbed by the biphenyl groups in Bp-PMO, and the excited energy was funneled into the Re complex by resonance energy transfer, which enhanced photocatalytic CO evolution from CO₂ by a factor of 4.4 compared with direct excitation of the Re complex. Bp-PMO had an additional merit to protect the Re complex against a decomposition by UV irradiation. These results demonstrate the potential of PMOs as a light-harvesting antenna for designing various photoreaction systems, mimicking the natural photosynthesis.

Full text of DOI:10.1021/ic1000914

4554 Inorg. Chem. 2010, 49, 4554–4559
DOI: 10.1021/ic1000914
Enhanced Photocatalysis of Rhenium(I) Complex by Light-Harvesting Periodic
Mesoporous Organosilica
Hiroyuki Takeda,†,§ Masataka Ohashi,†,§ Takao Tani,†,§ Osamu Ishitani,*,‡,§ and Shinji Inagaki*,†,§
†Toyota Central R&D Laboratories, Inc., Nagakute, Aichi 480-1192, Japan, ‡Department of Chemistry,
Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-E1-9 O-okayama,
Meguro-ku, Tokyo 152-8551, Japan, and §Core Research for Evolutional Science and Technology (CREST),
Japan Science and Technology Agency (JST), Kawaguchi, Saitama 332-0012, Japan
Received January 15, 2010
This paper describes a new conceptual design for enhancement of photocatalytic CO2 reduction of a rhenium(I)
complex by light harvesting of periodic mesoporous organosilica (PMO). Mesoporous biphenyl-silica (Bp-PMO)
anchoring fac-[ReI(bpy)(CO)3(PPh3)]þ(OTf)- (bpy =2,20-bipyridine; OTf = CF3SO3) in the mesochannels was
synthesized by co-condensation of two organosilane precursors, 4,40-bis(triethoxysilyl)biphenyl and 4-[4-{3-
(trimethoxysilyl)propylsulfanyl}butyl]-40-methyl-2,20-bipyridine in the presence of a template surfactant, followed by
coordination of a rhenium precursor, [ReI(CO)5(PPh3)]þ(OTf)- to the bipyridine ligand in the mesochannels. The
280 nm light was effectively absorbed by the biphenyl groups in Bp-PMO, and the excited energy was funneled into the
Re complex by resonance energy transfer, which enhanced photocatalytic CO evolution from CO2 by a factor of 4.4
compared with direct excitation of the Re complex. Bp-PMO had an additional merit to protect the Re complex against a
decomposition by UV irradiation. These results demonstrate the potential of PMOs as a light-harvesting antenna for
designing various photoreaction systems, mimicking the natural photosynthesis.
Introduction
a quantum efficiency of almost unity.4 For construction of
artificial photosynthesis systems, the three-dimensional or-
ganization of molecular parts, that is, light absorbers and
multielectron catalysts, at appropriate positions is of parti-
cular importance, because the RET efficiency is strongly
dependent on the distance between the energy donor and
acceptor molecules and their orientation.5
A range of supramolecular antenna systems, such as
dendrimers,5b porphyrin arrays,5b organogels,6 dye-loaded
zeolites,7 and photoactive polymers,5a with controlled three-
dimensional architectures have been reported and exhibit
strong light-absorption and efficient funneling of captured
energy into a small amount of acceptors by RET. However,
it has not been reported to construct supramolecular photo-
catalyst systems for CO2 reduction with an efficient light-
harvesting antenna like in the natural photosynthesis. The
limited application of the conventional supramolecular antenna
materials to a RET type photocatalysis system is mainly
Solar energy conversion systems that involve water split-
ting1 and CO2 reduction2 photocatalysts are attracting much
attention as an alternative to energy production from fossil
fuels.3 The development of CO2 reduction photocatalysts is
particularly important with respect to recycling of exhausted
CO2 into useful fuels and organic compounds.4a Natural
photosynthesis shows the efficient photocatalysis of CO2
reduction to form carbohydrates. One of the key components
of natural photosynthesis is a light-harvesting antenna,
typically observed as wheel-like arrays of chlorophylls in
LH1 and LH2 of purple photosynthetic bacteria, which
absorbs sunlight effectively and funnel the captured energy
to a reaction center by resonance energy transfer (RET) with
*To whom correspondence should be addressed. E-mail: ishitani@
chem.titech.ac.jp (O.I.), inagaki@mosk.tytlabs.co.jp (S.I.).
(1) (a) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851–7861.
(b) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.;
Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76–80. (c) Kudo, A.; Miseki, Y.
Chem. Soc. Rev. 2009, 38, 253–278. (d) Dempsey, J. L.; Esswein, A. J.; Manke,
D. R.; Rosenthal, J.; Soper, J. D.; Nocera, D. G. Inorg. Chem. 2005, 44, 6879–
6892. (e) Du, P.; Schneider, J.; Luo, G.; Brennessel, W. W.; Eisenberg, R. Inorg.
Chem. 2009, 48, 4952–4962.
(2) (a) Arakawa, H.; et al. Chem. Rev. 2001, 101, 953–996. (b) Fujita, E.
Coord. Chem. Rev. 1999, 185-186, 373–384. (c) Takeda, H.; Ishitani, O. Coord.
Chem. Rev. 2010, 254, 346–354.
(3) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15729–15735.
€
(4) (a) Pullerits, T.; Sundstrom, V. Acc. Chem. Res. 1996, 29, 381–389.
(b) Pascal, A. A.; Liu, Z.; Broess, K.; van Oort, B.; van Amerongen, H.; Wang, C.;
Horton, P.; Robert, B.; Chang, W.; Ruban, A. Nature 2005, 436, 134–137.
(5) (a) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg.
Chem. 2005, 44, 6802–6827. (b) Balzani, V.; Credi, A.; Venturi, M. Chem-
SusChem 2008, 1, 26–58.
(6) Ajayaghosh, A.; Praveen, V. K. Acc. Chem. Res. 2007, 40, 644–656.
(7) Calzaferri, G.; Lutkouskaya, K. Photochem. Photobiol. Sci. 2008, 7,
879–910.
r
pubs.acs.org/IC
Published on Web 04/21/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4555
attributed to difficulty in the organization of chromophores and
binding of reaction sites at appropriate positions in the antenna
and/or limited internal void space that hinders efficient mass
transfer for catalytic reactions.
Periodic mesoporous organosilicas (PMOs)8 are a new
class of functional materials with well-defined mesopores and
organic-inorganic hybrid frameworks in a supramolecular
architecture of organic moieties covalently fixed within a
siloxane network. They have been revealed to show unique
fluorescence,9 hole-transportation,10 and electron donation
properties.11 Recently, the organosilica framework was also
found to exhibit unique light-harvesting antenna properties;
funneling of light energy absorbed by approximately 125
chromophores of the framework into a single dye molecule
doped in the mesochannels by RET, with almost 100%
quantum efficiency.12 The well-defined mesopores of PMOs,
whose pore can be controlled in the range of 1.5-30 nm in
diameter, are also advantageous for the construction of
heterogeneous photocatalysis systems, because (i) photoac-
tive species can be placed precisely in the pore space13 into
which light energy is funneled, and (ii) sufficient internal void
spaces for efficient mass transfer can be retained, even after
the placement of photocatalysts, because of larger pore sizes14
compared with those of layered compounds,15 and zeolites.16
Furthermore, the high surface areas of mesoporous struc-
tures can increase reaction sites and thus enhance photo-
catalytic activity. Therefore, PMOs are considered to have a
potential as a solid-state supramolecular light-harvesting
antenna for enhancement of photocatalysis.
Figure 1. Schematic representation of light-harvesting by PMO and
enhancing of photocatalysis of Re complex.
to CO.2b,c,17 A PMO with biphenyl chromophores in the
framework (Bp-PMO)8f,9c,12a was used as a light-harvesting
antenna because of its well-studied optical properties and
good spectral overlap of its emission band with the absorp-
tion band of the Re bipyridine complex for efficient RET. An
elegant route for covalent immobilization of the Re complex
was developed to fix them homogeneously within appropri-
ate distances from Bp (light absorber) in the framework also
for efficient RET. The obtained system successfully funneled
the light energy absorbed in the framework into the Re
complex, placed in the mesochannels and enhanced photo-
catalytic reduction of CO2 (Figure 1).
This paper describes a new conceptual design for enhance-
ment of photocatalytic CO2 reduction of a rhenium(I)
complex placed in the mesochannels of PMO by its light-
harvesting antenna property. A rhenium (Re) bipyridine
complex was chosen as a reaction center because it is
well-known two-electron-reduction photocatalysis of CO2
Experimental Section
Chemicals. All reagents and solvents were of highest com-
mercial quality and used without further purification. The
organosilane precursors, 1,4-bis(triethoxysilyl)biphenyl (BTEBP,
(C2H5O)3Si-C6H4C6H4-Si(OC2H5)3, MW=478.73) and 4-[4-{3-
(trimethoxysilyl)propylsulfanyl}butyl]-40-methyl-2,20-bipyridine18
(SiBPy, (CH3O)3Si-C3H6-S-C4H8-C11N2H9, MW=420.65),
were purchased from Nard Institute, Ltd., Japan. The cationic
surfactant, octadecyltrimethylammonium chloride (C18TMACl,
C18H37N(CH3)3Cl, MW = 348.05) was purchased from TCI.
The Bp-PMO powder was synthesized according to the litera-
ture.8f fac-[Re(dmb)(CO)3(PPh3)](PF6) (dmb=4,40-dimethyl-2,
20-bipyridine) was prepared by a literature method.19
€
(8) (a) Hoffmann, F.; Cornelius, M.; Morell, J.; Froba, M. Angew. Chem.,
Int. Ed. 2006, 45, 3216–3251. (b) Kapoor, M. P.; Inagaki, S. Bull. Chem. Soc.
Jpn. 2006, 79, 1463–1475. (c) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20,
891–908. (d) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O.
J. Am. Chem. Soc. 1999, 121, 9611–9614. (e) Inagaki, S.; Guan, S.; Ohsuna, T.;
Terasaki, O. Nature 2002, 416, 306–307. (f) Kapoor, M. P.; Yang, Q.; Inagaki, S.
J. Am. Chem. Soc. 2002, 124, 15176–15177.
(9) (a) Goto, Y.; Mizoshita, N.; Ohtani, O.; Okada, T.; Shimada, T.; Tani,
T.; Inagaki, S. Chem. Mater. 2008, 20, 4495–4498. (b) Mizoshita, N.; Goto, Y.;
Tani, T.; Inagaki, S. Adv. Funct. Mater. 2008, 18, 3699–3705. (c) Tani, T.;
Mizoshita, N.; Inagaki, S. J. Mater. Chem. 2009, 19, 4451–4456. (d) Mizoshita,
N.; Goto, Y.; Kapoor, M. P.; Shimada, T.; Tani, T.; Inagaki, S. Chem.;Eur. J.
2009, 15, 219–226. (e) Mizoshita, N.; Goto, Y.; Tani, T.; Inagaki, S. Adv. Mater.
2009, 21, 4798–4801.
Synthesis of BPy-Bp-PMO Powder. A mixture of BTEBP
(5.63 g) and SiBPy (3.12 g) was slowly added to a mixture of
C18TMACl (6.7 g), 6 M sodium hydroxide (18 mL), and
deionized water (240 g). The suspension was sonicated for 20 min,
stirred at room temperature (rt) for 18 h, and heated at 95 °C
for further 20 h under a static condition. The resultant white
precipitate was recovered by filtration, washed with water,
and finally vacuumed-dried to yield the as-made BPy-
Bp-PMO powder. The surfactant was extracted from the as-
made material by refluxing the obtained powder (1 g) in ethanol
(200 mL) added with conc. hydrochloric acid (9 g). The extrac-
tion treatment was repeated twice. Finally, the surfactant-free
(10) Mizoshita, N.; Ikai, M.; Tani, T.; Inagaki, S. J. Am. Chem. Soc. 2009,
131, 14225–14227.
(11) Ohashi, M.; Aoki, M.; Yamanaka, K.; Nakajima, K.; Ohsuna, T.;
Tani, T.; Inagaki, S. Chem.;Eur. J. 2009, 15, 13041–13046.
(12) (a) Inagaki, S.; Ohtani, O.; Goto, Y.; Okamoto, K.; Ikai, M.;
Yamanaka, K.; Tani, T.; Okada, T. Angew. Chem., Int. Ed. 2009, 48,
4042–4046. (b) Ito, S.; Fukuya, S.; Kusumi, T.; Ishibashi, Y.; Miyasaka, H.;
Goto, Y.; Ikai, M.; Tani, T.; Inagaki, S. J. Phys. Chem. C 2009, 113, 11884–
11891.
(13) (a) Itoh, T.; Yano, K.; Inada, Y.; Fukushima, Y. J. Am. Chem. Soc.
2002, 124, 13437–13441. (b) Furukawa, H.; Kuroda, K.; Watanabe, T. Chem.
Commun. 2001, 2002–2003.
(14) (a) Lin, W.; Frei, H. J. Am. Chem. Soc. 2005, 127, 1610–1611.
(b) Sung-Suh, H. M.; Kim, D. S.; Lee, C. W.; Park, S.-E. Appl. Organomet.
Chem. 2000, 14, 826–830.
(15) Takagi, S.; Eguchi, M.; Tryk, D. A.; Inoue, H. Langmuir 2006, 22,
1406–1408.
(16) (a) Borja, M.; Dutta, P. K. Nature 1993, 362, 43–45. (b) Yoon, K. B.
Chem. Rev. 1993, 93, 321–329. (c) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E.
J. Am. Chem. Soc. 1988, 110, 8232–8234.
(17) (a) Hawecker, J.; Lehn, J.-M.; Ziessel, R. Helv. Chim. Acta 1986, 69,
1990–2012. (b) Kurz, P.; Probst, B.; Spingler, B.; Alberto, R. Eur. J. Inorg.
Chem. 2006, 2966–2974. (c) Takeda, H.; Koike, K.; Inoue, H.; Ishitani, O. J. Am.
Chem. Soc. 2008, 130, 2023–2031.
(18) Nguyen, J. V.; Jones, C. W. Macromolecules 2004, 37, 1190–1203.
(19) Tsubaki, H.; Sekine, A.; Ohashi, Y.; Koike, K.; Takeda, H.; Ishitani,
O. J. Am. Chem. Soc. 2005, 127, 15544–15555.

4556 Inorganic Chemistry, Vol. 49, No. 10, 2010
Takeda et al.
BPy-Bp-PMO (3.27 g) was treated with a 0.5 M triethylamine
ethanol solution (100 mL) for one night to neutralize the
bipyridine ligands which had been protonated during the sur-
factant extraction process. The elemental analysis of sulfur (S)
indicates 0.47 mmol g-1 of S in BPy-Bp-PMO, which suggests
that the same amount of BPy are attached in Bp-PMO because
the S-C bond cleavage does not usually occurr in the synthesis
condition.
thermal conductivity detector (TCD) and an active carbon
(mesh 60/80, 3 m long) column (Supporting Information, Figure
S1). For comparison, the photocatalysis test was carried out also
for a 50 mL homogeneous solution containing rhenium(I)
complex, fac-[Re(dmb)(CO)3(PPh3)](PF6) (0.06 mM) instead of
the Re/Bp-PMO suspension.
Quantum Yield of CO Formation. An apparent quantum yield
of CO formation was determined using a following equation:
Synthesis of Re/Bp-PMO Powder. [Re(CO)5(PPh3)]þ(OTf)-
(OTf=CF3SO3) was prepared according to the literature.20 The
BPy-Bp-PMO powder (300 mg) was dispersed in a toluene
solution of [Re(CO)5(PPh3)]þ(OTf)- (104 mg, 0.141 mmol in
60 mL of toluene), and refluxed for 5 h under an argon atmo-
sphere. After cooling to room temperature the resultant yellow
powder was recovered by filtration, repeatedly washed with
toluene and acetone, and vacuum dried. The incorporated
amount of Re in Re/Bp-PMO was determined by inductively
coupled plasma (ICP) analysis of both the PMO powder and the
residuals in the filtrate, which showed a good mass balance. The
amount of Re in Re/Bp-PMO was 0.29 mmol g-1, indicating
that 79 mol % of the bipyridine ligands formed [Re(dmb)-
(CO)3(PPh3)]þ complex in Re/Bp-PMO.
app
ΦCO ¼ ðnumber of CO formed ½molꢁÞ=
ðnumber of irradiated photon ½einsteinꢁÞ
Characterization of Rhenium Complex after Photoreaction.
The infrared (IR) spectra of the Re complex after the photo-
reactions were recorded on a Nicolet AVATAR-360 spectro-
meter. The Re/Bp-PMO powder (1.5 mg) before and after the
photocatalytic reaction was diluted with KBr (300 mg) by
mixing and pelleted (200 mg) for IR measurements. The Re
complex after homogeneous photoreactions was characterized
by IR and electrospray ionization mass spectroscopy (ESI-MS)
(using a Micromass Q-TOF mass-spectrometer), after extrac-
tion with CH2Cl2/H2O into a CH2Cl2 phase, evaporation of
CH2Cl2, and redissolution with CH3CN, to avoid baseline
perturbation by TEOA contained in the reaction solution.
13CO2 Tracer Experiment. 13CO2 (ISOTEC, > 99 atom %
13C) instead of CO2 was used for the photocatalysis test. After
photoirradiation, a fraction of the gas phase was collected and
analyzed by the same method as described above using GC-
TCD for determination of a total amount of CO formed.
Furthermore, the gas in the reactor was trapped as solid with
liquid N2 cooling and vaporized in a sample bag. Then, the gas-
phase containing CO formed was analyzed using an Agilent
6890-5973 gas chromatograph-mass spectrometer (GC-MS)
for determination of a fraction of 13CO in CO formed. After
the photocatalytic reaction, the Re/Bp-PMO powder was re-
covered by filtration and analyzed by IR spectroscopy
(Attenuated Total Reflectance; ATR).
General Measurements. The UV-vis diffuse reflectance (DR)
spectra were recorded on a JASCO V-670 spectrophotometer
with an integrating sphere unit (JASCO ISN-723). The longitu-
dinal axes of the spectrum were converted using the Kubelka-
Munk function from reflectance (%R) to K/S unit. The emission
spectra were measured by a JASCO FP6500 spectrofluorom-
eter. The PMO powder (1.0 mg) was dispersed in CH3CN (4 mL)
and degassed by Ar bubbling prior to the measurement. The
nitrogen adsorption/desorption isotherms were measured using
a Quantachrome Nova-3000e sorptometer at -196 °C. Prior to
the measurements, the samples were outgassed at 60 °C for 4 h.
The Brunauer-Emmett-Teller (BET) surface areas were cal-
culated from a linear section of the BET plot (P/P0=0.05-0.2).
The DFT (Density Functional Theory) pore diameter was
calculated using a DFT kernel (N2 at 77 K on silica, cylindrical
pore, NLDFT equilibrium model). The transmission electron
microscopic (TEM) observation was conducted using a JEOL
JEM-2000EX microscope. For observation, the powder sample
was dispersed in ethanol after grinding with a mortar and
deposited to a supporting grid. The powder X-ray diffraction
(XRD) patterns were measured on a Rigaku RINT-TTR dif-
fractometer with Cu KR radiation (50 kV, 300 mA).
Photocatalytic Reaction. The Re/Bp-PMO powder (10 mg)
was dispersed in a 5:1 v/v mixed solution (50 mL) of CH3CN
(Aldrich Biotech grade,>99.93%) and triethanolamine (TEOA,
Fluka 99.5%) as a sacrificial reductant. The suspension was
introduced in a pyrex vessel covered with a quartz cap equipped
with a pit reaching to the liquid surface in the vessel for insertion
of an optical fiber. The vessel was connected to a closed gas
circulation and evacuation system, and then degassing under
vacuum and purging of CO2 (Taiyo Nippon Sanso 99.9%) gas
were repeated three times to remove air from the reactor, and
finally CO2 was introduced until an inner pressure reached
1 atm. The light was irradiated from an end of a fiber with a
300 W xenon lamp (Asahi spectra, MAX-301) equipped with a
mirror module and a band-pass filter (280 or 365 nm, half-width
of 10 nm). The irradiated-light intensity was monitored with a
Molectron PowerMAX500AD power meter and adjusted to
1.7 ꢀ 10-8 einstein/s-1 with a ND filter. The reactor was kept at
20 °C in a thermostat bath (As-one CB-15). The suspension was
stirred with a magnetic bar, and the upper gas phase was pumped
and circulated through the suspension during the photoreac-
tion. A fraction of the gas phase was sampled and analyzed using
a Shimadzu GC-8A gas-chromatograph (GC) equipped with a
Results and Discussion
Re/Bp-PMO powder was prepared by two step processes
as shown in Figure 2. In the first step, Bp-PMO anchoring
bipyridine (BPy) ligands on the walls of the mesochannels
with Si-O-Si covalent bonds (BPy-Bp-PMO) was obtained
by co-condensation of two-organosilane precursors, BTEBP
and SiBPy, in the presence of a C18TMACl surfactant
template under a basic condition. In this route, BPy can be
homogeneously dispersed,8a,21 especially inthe mesochannels
because the bipyridine unit and the attached alkyl chain are
hydrophobic in nature and tend to enter into a core of the
rod like surfactant micelles during the co-condensation
process,8b,22 which can promote homogeneous fixation of
the Re complex in the mesochannels. The rhenium bipyridine
complex, fac-[Re(BPy)(CO)3(PPh3)]þ(OTf)- (BPy=the bi-
pyridine unit anchored on the walls of the mesochannels) was
formed by subsequent coordination of [Re(CO)5(PPh3)]þ-
(OTf)- to the bipyridine unit in BPy-Bp-PMO by refluxing in
a toluene suspension.
The transmission electron microscopy (TEM) (Figure 3a)
confirmed the successful formation of an ordered mesoporous
(21) Ariga, K.; Vinu, A.; Hill, J. P.; Mori, T. Coord. Chem. Rev. 2007, 251,
2562–2591.
(22) (a) Yang, Q.; Kapoor, M. P.; Inagaki, S. J. Am. Chem. Soc. 2002, 124,
9694–9695. (b) Huh, S.; Wiench, J. W.; Yoo, J.-C.; Pruski, M.; Lin, V. S.-Y.
Chem. Mater. 2003, 15, 4247–4256. (c) Kapoor, M. P.; Yang, Q.; Goto, Y.;
Inagaki, S. Chem. Lett. 2003, 32, 914–915.
(20) Nitschke, J.; Schmidt, S. P.; Trogler, W. C. Inorg. Chem. 1985, 24,
1972–1978.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4557
Figure 2. Schematic representation of two-step synthesis of a rhenium-
(I) bipyridine complex ([Re(BPy)(CO)3(PPh3)]þ) fixed in biphenyl-
bridged periodic mesoporous organosilica (Re/Bp-PMO).
Figure 4. UV-vis diffuse reflectance spectrum of Re/Bp-PMO (red
line) and emission spectra of Bp-PMO (black) and Re/Bp-PMO (blue)
excited at 260 nm.
Figure 5. CO generation by photocatalysis for CO2 reduction on Re/
Bp-PMO (Re: 3 μmol) under photoirradiation at 280 (b) and 365 nm (2),
and in a homogeneous [Re(dmb)(CO)3(PPh3)]þ solution (0.06 mM; Re:
3 μmol) under photoirradiation at 365 nm (4). No CO generation was
observed for Bp-PMO without the Re complex under photoirradiation at
280 nm (().
Figure 3. Structural properties of Re/Bp-PMO. (a) TEM image of Re/
Bp-PMO, (b) XRD patterns and (c) nitrogen adsorption/desorption
isotherms at 77 K: the Bp-PMO (black), BPy-Bp-PMO (blue), and Re/
Bp-PMO (red).
The UV-vis diffuse reflectance spectrum of Re/Bp-PMO
(Figure 4: red) shows a metal-to-ligand charge transfer
(MLCT) absorption band (λmax = 345 nm) which corre-
sponds to that of fac-[Re(dmb)(CO)3(PPh3)]þ (dmb =4,40-
dimethyl-2,20-bipyridine) in CH3CN (λmax=340 nm).23 The
ν(CꢂO) peaks at 2040, 1950, and 1930 cm-1 in the Fourier
transform infrared (FT-IR) spectrum of Re/Bp-PMO indi-
cate fac-Re(CO)3 symmetry, in agreement with those of
fac-[Re(dmb)(CO)3(PPh3)]þ (2037, 1948, and 1925 cm-1 in
CH3CN; data not shown).
Figure 4 also displays the emission spectra of Bp-PMO and
Re/Bp-PMO excited at 260 nm. Bp-PMO showed strong
fluorescence (λmax=380 nm, Φ=0.42 ( 0.02) because of the
biphenyl groups in the framework,12a whereas Re/Bp-PMO
showed almost no emission from the biphenyl group but
exhibited a new emission band at 450-700 nm because of the
Re complex. The results suggest that the excitation energy of
the biphenyl groups is completely transferred into the Re
complex in spite of the low Re/Bp molar ratio. Although
the Re complexes also absorb the excitation light (260 nm),
their contribution is estimated to be quite low (∼5%),
with consideration of the molar extinction coefficients (ε) in
CH3CN (ε=27200 and 11700 M-1 cm-1 for the organosilane
structure consisting of one-dimensional uniform channels and
periodic arrangement of Bp in the pore walls8f of Re/Bp-PMO
without any dark spots corresponding to metallic clusters.
The XRD patterns (Figure 3b) of Re/Bp-PMO and BPy-Bp-
PMO also reflected the existence of ordered mesochannels
(2θ = 1.9°) along with lamellar periodicity of the organic
bridges in the pore walls (2θ = 6-40°).8f The nitrogen
adsorption/desorption isotherms revealed that Re/Bp-PMO
had large pore-volume (0.21 cm3 g-1), specific surface area
(520 m2g-1) and pore-diameter (2.9 nm), sufficient for mass
transfer and catalytic reactions (Figure 3c). The pore-volume
of Re/Bp-PMO was smaller than those of Bp-PMO (0.49 cm3
g-1) and BPy-Bp-PMO (0.33 cm3 g-1) without any change in
d-value (4.7 nm), which indicates that the Re complexes were
mostly located within the mesochannels, because the decrease
in pore-volume agreed well with the volume estimated
from the molecular volume ([Re(BPy)(CO)3(PPh3)](OTf),
859.3 A3) and the number of the Re complex introduced in
Bp-PMO. The elemental analysis showed that the content of
the Re complex in Re/Bp-PMO was 0.43 mmol g-1-Bp-PMO,
which corresponds to a Re/Bp molar ratio of 0.11 (Bp in
Bp-PMO was 3.77 mmol g-1).
(23) Hori, H.; Koike, K.; Ishizuka, M.; Takeuchi, K.; Ibusuki, T.;
Ishitani, O. J. Organomet. Chem. 1997, 530, 169–176.

4558 Inorganic Chemistry, Vol. 49, No. 10, 2010
Takeda et al.
Figure 6. Suppression of CO ligand dissociation from [Re(BPy)(CO)3(PPh3)]þ in Bp-PMO. (a) CO generation from a [Re(dmb)(CO)3(PPh3)]þ
homogeneous solution (0.06 mM; Re: 3 μmol) under photoirradiation at 280 nm with (2) and without (4) CO2 in the atmosphere. CO generation from
Re/Bp-PMO (Re: 3 μmol) under photoirradiation at 280 nm without CO2 in the atmosphere (O). (b) FT-IR spectral changes in the CO stretching region of
Re/Bp-PMO during photocatalytic CO2 reduction for 0-24 h. FT-IR spectra of a [Re(dmb)(CO)3(PPh3)]þ homogeneous solution after photoirradiation at
280 nm for 24 h under an Ar atmosphere (black). (c) Schematic representations of the protection effect of Bp-PMO from (d) the CO-ligand dissociation of
[Re(dmb)(CO)3(PPh3)]þ by UV irradiation.
precursor (1,4-bis(triethoxysilyl)biphenyl; BTEBP) and fac-
[Re(dmb)(CO)3(PPh3)]þ, respectively, at 260 nm) and the
molar ratio of Re/Bp (0.11). The measurement of the excited
state lifetime of Re/Bp-PMO also indicated dynamic quench-
ing of the Bp excited state in the presence of the Re complex
(Supporting Information, Figure S2). Thus, the light-energy
captured on the Bp-PMO antenna was efficiently funneled
into the Re complex mainly by the RET mechanism.
The photocatalysis of CO2 reduction was evaluated for Re/
Bp-PMO (10 mg) dispersed in a mixture of an organic solvent
(CH3CN) and a sacrificial agent (triethanolamine; TEOA)
(5:1 v/v, 50 mL). Monochromic light irradiation at 280 nm
(excitation of Bp) under a CO2 atmosphere generated CO
(Figure 5). For initial 5 h, CO was constantly evolved with an
apparent quantum yield (ΦCOapp) of 1.2%, as estimated from
the linear CO evolution rate and the light intensity (1.7 ꢀ
10-8 einstein s-1). The total CO production for 24 h reached
evolution (1.4 μmol for 24 h) (Figure 5). A homogeneous
solution of fac-[Re(dmb)(CO)3(PPh3)]þ also evolved a small
amount of CO (1.4 μmol for 24 h at 365 nm) in CH3CN/
TEOA (5:1 v/v, 50 mL) with the same amount of the Re
complex (3 μmol, 0.06 mM) in the reaction vessel as that in
Re/Bp-PMO (Figure 5).24 Comparison of the amounts of CO
evolved from Re/Bp-PMO under irradiation at 280 and 365
nm for 24 h indicates that the excitation of Bp enhanced the
photocatalysis of the Re complex by a factor of 4.4 (Figure 1).
This result clearly shows an antenna effect of Bp-PMO for
enhancing the photocatalysis of the Re complex. However,
the antenna effect should be much larger than the observed
value because the absorption efficiency of the Bp groups is
approximately 99 times larger than that of the Re complex in
Re/Bp-PMO, as estimated from the Bp/Re ratio (9.1) and the
molar extinction coefficients of BTEBP (ε=15200 M-1 cm-1
at 280 nm) and fac-[Re(dmb)(CO)3(PPh3)]þ (ε=1400 M-1
cm-1 at 365 nm) in CH3CN. The deviation was mainly
attributed to a smaller difference in the actual absorption
efficiency under irradiation at 280 and 365 nm for the Re/Bp-
PMO suspension because of a smaller penetration length of
280 nm light than that of 365 nm light.
The Bp-PMO antenna showed an additional advantage in
stabilization of the Re complex in the mesochannels against
UV light. Irradiation of a homogeneous solution of fac-[Re-
(dmb)(CO)3(PPh3)]þ at 280 nm evolved a considerable
amount of CO gas (132 mol % CO/Re complex after 24-h
irradiation), regardless of the presence of CO2 (Figure 6a),
because of CO ligand dissociation from Re complexes, which
has been known to result from the formation of vibrationally
hot states of the Re complexes by absorption of high-energy
UV light (λ < 313 nm).25 However, Re/Bp-PMO showed
a much smaller amount of CO generation (8.3%) than
that for the homogeneous Re complex solution under
6.2 μmol, which corresponds to a turnover number (TNCO
)
of 2.2 based on the amount of Re atoms (2.9 μmol) in the
slurry, although the catalyst was gradually deactivated after
5 h. The 13CO2 (>99 atom % enriched) tracer experiment
produced 5.6 μmol of CO (over 24 h) consisting of 13CO
(68%) and 12CO (32%) (Supporting Information, Figure
S3). The presence of 12CO can be reasonably explained by a
ligand exchange from 12CO to 13CO in the photocatalytic
cycle,17 which was also confirmed by FT-IR spectroscopy of
the recovered sample (Supporting Information, Figure S3c).
On the other hand, the tricarbonyl structure ([Re(BPy)-
(CO)3L]þ, L=PPh3 or OCHO-) was completely preserved
for Re/Bp-PMO without CO dissociation during the photo-
reaction, as discussedlater. NoCO was detected for Bp-PMO
without the Re complex under photoirradiation at 280 nm,
which indicates that the evolved CO does not originate from
the organic component (Bp) of Bp-PMO (Figure 5). These
results clearly show the photocatalytic formation of CO from
CO2 by Re/Bp-PMO.
(24) Hori, H.; Johnson, F. P. A.; Koike, K.; Takeuchi, K.; Ibusuki, T.;
Ishitani, O. J. Chem. Soc., Dalton Trans. 1997, 1019–1023.
(25) Sato, S.; Sekine, A.; Ohashi, Y.; Ishitani, O.; Blanco-Rodrıguez,
´
Photoirradiation to Re/Bp-PMO at 365 nm (direct excita-
tion of the Re complex) with the same light intensity (1.7ꢀ
10-8 einstein s-1) resulted in a smaller amount of CO
ꢀ
A. M.; Vlcek, A., Jr.; Unno, T.; Koike, K. Inorg. Chem. 2007, 46, 3531–3540.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4559
irradiation at 280 nm without CO2 in the atmosphere
(Figure 6a). The FT-IR spectra of the recovered Re com-
plexes after 24 h UV-light irradiation revealed the forma-
in which the bipyridine ligand was anchored on the walls of
the mesochannels of Bp-PMO. The subsequent coordination
of the Re precursor to the bipyridine unit gave Re/Bp-PMO
in which the Re complexes are homogeneously fixed on the
walls of the mesochannels. The light energy absorbed effec-
tively by the biphenyl groups in Bp-PMO was funneled into
the Re complex by RET and sensitized photocatalytic CO
evolution from CO2 by a factor of 4.4 compared with direct
excitation of the Re complex because of the light-harvesting
of the PMO antenna. The PMO antenna also showed a
photoprotection effect on CO-ligand dissociation from the
Re complex by UV-light irradiation. These results evidently
demonstrate the potential of PMOs as a light-harvesting
antenna for designing various photocatalytic systems using
PMOs9c,26 mimicking the natural photosynthesis.
tion of Re biscarbonyl complexes (1936 and 1866 cm-1
)
such as [Re(dmb)(CO)2(PPh3)2]þ (m/z= 951.2 from ESI-
MS; Supporting Information, Figure S4) and Re monocar-
bonyl complexes (1825 cm-1) for the naked Re complex (in
solution), but not for Re/Bp-PMO (Figure 6b, black). Sup-
pression of the CO ligand dissociation for Re/Bp-PMO can
be explained by the lack of a formation of higher excited
1
states of Re such as, π-π* because of absorption of high-
energy photons by the Bp-PMO antenna, followed by relaxa-
tion to the lowest excited or excimer states of Bp and transfer
to the Re complex making its 1MLCT and 3MLCT by first
intersystem crossing (Figure 6c). The FT-IR spectra also
showed that the Re complex in Bp-PMO gradually changed
to Re-formato species [Re(BPy)(CO)3(OCHO)] (2018, 1912,
and 1894 cm-1) with almost complete formation during the
photoreaction taking 5 h (Figure 6b). A formation of the
Re-formato complex by ligand substitution from PPh3 to
OCHO- is also well-known as a deactivation process in a
homogeneous fac-[Re(dmb)(CO)3(PPh3)]þ system during
photocatalytic CO2 reduction.24 The formation of the for-
mato complex would cause the deactivation of Re/Bp-PMO
for photocatalysis of CO2 reduction (Figure 5). However, the
time-course of the deactivation for Re/Bp-PMO is much
longer than that for homogeneous fac-[Re(dmb)(CO)3-
(PPh3)]þ which usually forms theRe-formato complexwithin
a few tens of minutes.24 Fixation of the Re complex in the
mesochannels would hinder a chain reaction between one-
electron reduced and original Re complexes,24 which would
suppress dissociation of the PPh3 ligand from the Re complex
and the subsequent formation of the deactivated Re-formato
species.
Acknowledgment. The authors thank to Mr. Y. Kawai
(TCRDL) for ICP measurement, Mr. M. Yamamoto
and Ms. A. Ohshima (TCRDL) for GC-MS analysis,
Ms. K. Fukumoto (TCRDL) for NMR measurement,
Mr. S. Kajiya(TCRDL) forESI-MSanalysis, Dr. T. Ohsuna
(TCRDL) for TEM observation, Mr. K. Yamanaka
(TCRDL) and Dr. T. Okada (Toyota Phys. & Chem.
Res. Inst.) for laser spectroscopic measurement, and
Dr. K. Koike (AIST) and Prof. O. Terasaki (Stockholm
Univ.) for helpful discussion.
Supporting Information Available: Complete ref 2a, detailed
description of CO quantification, excitation lifetime of the Re/
Bp-PMO, GC-MS analysis of the evolved gas and FT-IR
spectrum of the rhenium complex after photocatalysis of Re/Bp-
PMO under a 13CO2 atmosphere, and ESI-MS analysis of the
rhenium complex after photzodecomposition of [Re(dmb)(CO)3-
(PPh3)](PF6). This material is available free of charge via the
Internet at http://pubs.acs.org.
(26) (a) Maegawa, Y.; Goto, Y.; Inagaki, S.; Shimada, T. Tetrahedron
Lett. 2006, 47, 6957–6960. (b) Goto, Y.; Nakajima, K.; Mizoshita, N.; Suda, M.;
Tanaka, N.; Hasegawa, T.; Shimada, T.; Tani, T.; Inagaki, S. Microporous
Mesoporous Mater. 2008, 117, 535–540. (c) Takeda, H.; Goto, Y.; Maegawa,
Y.; Ohsuna, T.; Tani, T.; Matsumoto, K.; Shimada, T.; Inagaki, S. Chem.
Commun. 2009, 6032–6034.
Conclusion
Co-condensation of two organosilane precursors contain-
ing bridged biphenyl group and terminal bipyridine ligand in
the presence of a template surfactant afforded BPy-Bp-PMO