The long-lived electron transfer state of the 2-phenyl-4-(1-naphthyl) quinolinium ion incorporated into nanosized mesoporous silica-alumina acting as a robust photocatalyst in water

Article abstract of DOI:10.1039/c3cc41575a

A simple electron donor-acceptor linked dyad, the 2-phenyl-4-(1-naphthyl) quinolinium ion (QuPh⁺-NA), was incorporated into nanosized mesoporous silica-alumina to form a composite, which is highly dispersed in water and acts as an efficient and robust photocatalyst for the reduction of O₂ by oxalate to produce hydrogen peroxide with a quantum yield of 10%.

Full text of DOI:10.1039/c3cc41575a

Chemical Communications
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Volume 49 | Number 45 | 7 June 2013 | Pages 5093–5238
ISSN 1359-7345
COMMUNICATION
Shunichi Fukuzumi et al.
The long-lived electron transfer state of the
2-phenyl-4-(1-naphthyl)quinolinium ion incorporated into nanosized
mesoporous silica–alumina acting as a robust photocatalyst in water
1359-7345(2013)49:45;1-#

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The long-lived electron transfer state of the
2-phenyl-4-(1-naphthyl)quinolinium ion incorporated
into nanosized mesoporous silica–alumina acting as a
robust photocatalyst in water†
Cite this: Chem. Commun., 2013,
49, 5132
Received 1st March 2013,
Accepted 14th March 2013
Yusuke Yamada,a Akifumi Nomura,a Kei Ohkubo,a Tomoyoshi Suenobua and
Shunichi Fukuzumi*ab
DOI: 10.1039/c3cc41575a
www.rsc.org/chemcomm
A simple electron donor–acceptor linked dyad, the 2-phenyl-4- Thus, efficient photocatalytic processes using a long-lived ET
(1-naphthyl)quinolinium ion (QuPh+–NA), was incorporated into state in water are highly desired. However, organic donor–
nanosized mesoporous silica–alumina to form a composite, which acceptor linked molecules are yet to be employed as photo-
is highly dispersed in water and acts as an efficient and robust catalysts in water because of their insolubility in water.
photocatalyst for the reduction of O2 by oxalate to produce
hydrogen peroxide with a quantum yield of 10%.
We report herein the incorporation of the 2-phenyl-4-
(1-naphthyl)quinolinium ion (QuPh+–NA), which also forms the
ET state acting as a strong oxidant and reductant (Ered = 1.87 V
Artificial photosynthesis attracts many researchers to realize and Eox = À0.90 V vs. SCE in MeCN) upon photoirradiation,5
a sustainable society depending mainly on solar energy.1 into nanosized mesoporous silica–alumina with a spherical
A variety of electron donor–acceptor linked molecules, which shape (sAlMCM-41) by cation exchange and utilization of the
mimic charge-separation processes in the photosynthetic reac- composite (QuPh+–NA@sAlMCM-41) as a photocatalyst in
tion centre, have been utilized in photocatalytic systems to aqueous media. The composite affords the extremely long-lived
produce high-energy chemicals.2,3 The donor–acceptor linked ET state (QuPhꢀ–NAꢀ+@sAlMCM-41) upon photoirradiation
molecules form an electron-transfer (ET) state upon photoirra- even in the presence of water. Thus, the photocatalytic activity
diation in reaction media, usually in organic solvents.4–6 Even if of the composite was investigated for O2 reduction by the
an electron donor–acceptor linked molecule exhibits a long- oxalate anion to produce H2O2 in water. Oxalate is converted
lived ET state in the isolated state, e.g., in frozen media, the into two equivalents of CO2 after the reaction, therefore, pure
lifetime of its ET state in a solution is much shorter than that H2O2 in water can be easily obtained by just removing the
in the isolated state, because of facile intermolecular back catalytic nanocomposite by filtration or sedimentation.
electron transfer.6
Na+-exchanged sAlMCM-41 has been prepared according to
To avoid such an intermolecular event in solution, electron a reported method with modifications.8 TEM images of the
donor–acceptor linked molecules have been isolated on a metal synthesized Na+-exchanged sAlMCM-41 are displayed in Fig. 1a
oxide support. For example, 9-mesityl-10-methylacridinium ion and Fig. S1 in ESI.† N2 adsorption and desorption isotherms for
(Acr+-Mes), which forms an ET state upon photoirradiation, Na+-exchanged sAlMCM-41 afforded
a Brunauer–Emmett–
has been supported on mesoporous silica–alumina by ion Teller (BET) surface area of 520 m2 gÀ1 with a pore diameter
exchange, exhibiting the ET state lifetime longer than a second of 1.8 nm determined by the Barrett–Joyner–Halenda (BJH)
when suspended in acetonitrile (MeCN) at room temperature.7 method (Fig. S2 in ESI†). The Na+-exchanged sAlMCM-41
Among the reaction media used for chemical reactions, water is was ion-exchanged with QuPh+–NA to prepare the QuPh+–
the most commonly used solvent owing to its abundance, NA@sAlMCM-41 composite. Because the molecular size of
ubiquitous nature and environmentally benign properties. QuPh+–NA is small compared with the pore size of sAlMCM-41,
cation exchange with QuPh+–NA occurs spontaneously upon
mixing Na+-exchanged sAlMCM-41 with QuPh+–NA(ClO4À) in
a Department of Material and Life Science, Graduate School of Engineering,
Osaka University, ALCA, Japan Science and Technology (JST), Suita, Osaka 565-0871, MeCN as observed by UV-vis absorption of the supernatant
(Fig. S3, ESI†). The amount of adsorbed QuPh+–NA was deter-
Japan. E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7370
b Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea
mined by the decrease in absorption of the solution at 333 nm
† Electronic supplementary information (ESI) available: Experimental details,
Fig. S1–S4 (TEM images of Na+-exchanged sAlMCM-41, the N2 isotherm curve
owing to QuPh+–NA. The number of Al atoms on the surface of
sAlMCM-41 estimated from the used amount of Al source was
and the BJH plot, UV-vis absorption change and time courses of EPR signals). See
DOI: 10.1039/c3cc41575a
1.05 Â 10À3 mol gÀ1, thus, 7.9% of all the Na+-sites was
c
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Fig. 1 (a) Transmission electron microscope (TEM) image of Na+-exchanged
sAlMCM-41. (b) Diffuse reflectance UV-vis spectrum of the QuPh+–NA@sAlMCM-41
composite (red solid line) compared with the UV-vis absorption spectrum of the
QuPh+–NA ion in MeCN (black dotted line). Positions of peaks and shoulders are
indicated by broken lines.
occupied by QuPh+–NA. The diffused reflectance UV-vis spectrum
of the QuPh+–NA@sAlMCM-41 composite shown in Fig. 1b
(red solid line) shows that the positions of peaks and shoulders
of the spectrum are quite similar to those of the QuPh+–NA ion
in MeCN (black dotted line in Fig. 1b).
Fig. 2 (a) EPR spectrum of the QuPh+–NA@sAlMCM-41 composite in the
Photoirradiation of the QuPh+–NA@sAlMCM-41 composite presence of adsorbed water under photoirradiation using
a high-pressure
mercury lamp and a UV cut-off filter (l > 340 nm). (b) Time profile of the EPR
signal intensity for QuPhꢀ–NAꢀ+@sAlMCM-41 in the absence (red dotted) or
presence (black solid) of water at 316 K. (c) Time profile of the EPR signal intensity
for QuPhꢀ–NAꢀ+@sAlMCM-41 in the presence of water upon intermittent
using a 1000 W high-pressure mercury lamp through a UV-light
cutting filter (l > 340 nm) results in the formation of the ET
state (QuPhꢀ–NAꢀ+@sAlMCM-41) via photoinduced electron
transfer from the naphthalene (NA) moiety to the singlet excited
photoirradiation for 2 seconds followed by 48 seconds in the dark at 316 K.
state of the quinolinium ion (QuPh+) moiety as evidenced by EPR (d) Plot of ln(k
T
) vs. TÀ1 for intramolecular back electron transfer of QuPh+–
À1
BET
NA@sAlMCM-41 in the presence of water. The water-adsorbed sample was
prepared by exposure of QuPh+–NA@sAlMCM-41 to water vapour at 313 K
overnight under reduced pressure. The time profile at each temperature is
indicated in Fig. S4 in ESI.†
measurements. An EPR signal at g = 2.0031 appearing upon
photoirradiation (Fig. 2a) assures formation of a radical
species. The decrease in the EPR signal intensities observed
by cutting off the light obeyed first-order kinetics for a pro-
longed time after 20 s as shown in Fig. 2b (red dotted line),
revealing the intramolecular back electron transfer of the ET content of B1 wt%.11 Nature utilizes oxalate as an electron
state of QuPh+–NA. Based on the rate constant of the decay donor for producing H2O2 in the enzymatic active centre of
curve, the lifetime of the ET state is longer than 190 s at 316 K. oxalate oxidase, which catalyses the oxidation of oxalate,
The ET state lifetime in the presence of water significantly reduction of O2 to H2O2 and formation of two moles of CO2
decreased as compared with that observed in the absence of [eqn (1)].12 In this process, CO2 is solely produced as a
water (Fig. 2b, black solid line). Water vapour was adsorbed on
(COOÀ)2 + 2H+ + O2 - 2CO2 + H2O2
(1)
the composite at 313 K under reduced pressure for several
hours, because the presence of bulk water strongly interferes by-product. However, no functional model of oxalate oxidase has
the observation of EPR signals. The EPR signal intensity been reported, because the C–C bond of oxalate is kinetically
oscillated by the intermittent photoirradiation for 2 seconds stable. The property of oxalate, which can be converted into
followed by 48 s in the dark as indicated in Fig. 2c. The similar CO2 after oxidation, benefits efficient photocatalytic reactions
signal intensity obtained at each cycle resulted from the photo- because of prevention of back electron transfer. In our photo-
robustness of the composite. An Eyring plot in Fig. 2d for the catalytic system, oxalate can be used as a natural electron donor
rate constant of intramolecular back electron transfer in the ET to reduce O2 to H2O2 using the QuPh+–NA@sAlMCM-41 com-
state of the composite indicates that the activation enthalpy posite as a photocatalyst in water. Scheme 1 depicts the overall
(DH‡) of the composite was determined to be 12(Æ2) kcal molÀ1
.
catalytic cycle for H2O2 production by O2 reduction using
A large DH‡ value [20 (Æ1) kcal molÀ1] has also been reported QuPh+–NA and oxalate as a photocatalyst and an electron
for Acr+-Mes supported on AlMCM-41 in MeCN,7 resulting from donor, respectively.
a large driving force for the back electron transfer in the Marcus
inverted region,9 because of decreased solvation in the pores of photoirradiation (l > 340 nm) of an oxygen-saturated aqueous
sAlMCM-41.
suspension (1.3 mM, 2.0 mL) containing QuPh+–NA@sAlMCM-41
The photocatalytic H2O2 production has been performed by
The QuPh+–NA@sAlMCM-41 composite was examined as a (QuPh+–NA, 0.22 mM) and oxalate (100–400 mM). Fig. 3a shows
photocatalyst for the production of H2O2 in an aqueous the time courses of H2O2 production in the photocatalytic
solution. H2O2 can be simply prepared by reduction of atmo- reaction. The initial H2O2 formation rate (1 h) increased from
spheric O2 by oxalate, which is capable of acting as a strong 1.6 mmol hÀ1 to 2.5 and 3.5 mmol hÀ1 upon increasing the
reductant (Eo = À0.48 V vs. NHE).10 Oxalate can be easily concentration of oxalate from 100 mM to 200 and 400 mM in
found in the soil and leaves of various vegetables at a high the reaction solution. The pH of the reaction solution was
c
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photogenarated QuPhꢀ–NAꢀ+, which are sufficient for both the
oxidation of oxalate and the reduction of O2.5
In summary, a long-lived ET state was attained in pure water
for the first time by supporting QuPh+–NA on nanosized
mesoporous silica–alumina. The formation of the long-lived
ET state of the QuPh+–NA@sAlMCM-41 composite was con-
firmed by EPR measurements. The composite has been shown
to act as an efficient photocatalyst for H2O2 production by
reducing O2 with oxalate in water. The high quantum yield of
10% for H2O2 production in water without any organic solvent
was comparable to that achieved in the homogeneous reaction
system using an organic solvent.15
Scheme 1 Chemical structure of QuPh+–NA and overall catalytic cycle for H2O2
production.
Notes and references
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Fig. 3 (a) Time courses of H2O2 production by photoirradiation (l > 340 nm) of
an aqueous suspension containing (COOÀ)2 (400 mM, red; 200 mM, blue;
100 mM, black, [(COOK)2]/[(COOH)2] = 3) and the QuPh+–NA@sAlMCM-41
composite ([QuPh+–NA]: 0.2 mM). (b) Time courses of H2O2 production under
different pH conditions. The pH value of each aqueous medium was controlled to
1.1 (black), 4.5 (red) or 5.5 (blue) by changing the ratio of (COOH)2 to (COOK)2.
The total concentration of the oxalate anion was fixed at 200 mM.
controlled by changing the ratio of the amount of oxalic acid to
that of disodium oxalate keeping the concentration of oxalate
constant (200 mM) as shown in Fig. 3b. The initial H2O2
formation rate (1 h) reached the maximum around pH 4.5.
A further increase or decrease in pH decelerated the H2O2
formation. At lower pH, the oxalate dianion is protonated to
produce the monoanion and oxalic acid, which cannot act as an
electron donor in the photocatalytic system. At higher pH,
reduction of O2 is thermodynamically unfavourable, because
the one-electron reduced species of O2 (O2 À) cannot be proto-
ꢀ
nated to produce HO2ꢀ that disproportionates to yield H2O2 and
O2. Thus, the H2O2 formation rate reached maximum around
pH 4.5, which is close to the pKa2 of oxalic acid (4.27) and the
pKa of HO2 (4.9).13
ꢀ
The efficiency of the photocatalytic reaction can be evaluated
by the quantum yield (F) of the products, where the F value is
defined as the mole number of H2O2 produced divided by that
of photons absorbed by the photocatalyst. In previous reports,
the F value of H2O2 production has been determined to be
4.2% by photoirradiation (340 nm) of an oxygen-saturated
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13 (a) R. C. Weast, Handbook of Chemistry and Physics 56th Edition, CRC,
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Cleaveland, OH, 1975–1976; (b) B. H. J. Bielski, D. E. Cabelli,
0.2 mM) and oxalate (200 mM) was determined to be 10%
using a ferrioxalate actinometer under photoirradiation with
monochromatised light of 334 nm. Such a high quantum yield
originated from the high oxidizing and reducing abilities of
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5134 Chem. Commun., 2013, 49, 5132--5134
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