www.rsc.org/pps | Photochemical & Photobiological Sciences
PAPER
Effect of electron-acceptor strength of zeolite on the luminescence decay rate
of Ru(bpy)32+ incorporated within zeolites†‡
Nobuyuki Taira,a Masashi Saitoh,a Shuichi Hashimoto,*b Hyung Rang Moonc and Kyung Byung Yoon*c
Received 18th April 2006, Accepted 22nd June 2006
First published as an Advance Article on the web 6th July 2006
DOI: 10.1039/b605469b
We carried out time-resolved luminescence and transient absorption studies of
tris(2,2 -bipyridine)ruthenium(II) complex, Ru(bpy)3 2+ assembled in the supercages of zeolites X and Y
exchanged with various alkali metal cations. The average lifetime of the luminescence decay, a measure
of the photoinduced electron transfer (PET) rate, of Ru(bpy)3 2+ * was found to decrease with increasing
the electron-acceptor strength of the host which is represented by the Sanderson’s electronegativity
scale. This result strongly suggests that the zeolite host plays the role of electron acceptor for
Ru(bpy)3 2+ *. However, we could not detect Ru(bpy)3 3+ in the transient absorption spectra, most likely
due to very low absorption coefficient of Ru(bpy)3 3+ and to the low efficiency of net PET. For the above
observation to be made, it is essential to employ the dehydrated zeolite hosts to allow direct interaction
between the guest Ru(II) complex and the host framework. The present study demonstrates the active
role of the zeolite hosts during the PET of incorporated Ru(bpy)3 2+ under the carefully controlled
experimental conditions. This report demonstrates the fact that the zeolite hosts can serve as electron
acceptors although in the past zeolites were shown to play the role of electron donors.
Introduction
Zeolites are a class of nanostructured materials that have welldefined channels and cavities.1,2 They can incorporate atoms
and molecules inside these void spaces and offer environment
significantly different from that in solution in carrying out
chemical reactions.3 The zeolites X and Y used in this work
have the same framework structure. They differ by the siliconto-aluminum (Si/Al) ratio of the framework. When Si/Al ratio
lies between 1 and 1.5, the zeolite is called as zeolite X and when
it is higher than 1.5, the zeolite is called as zeolite Y. Thus, zeolite
X is richer in Al content than zeolite Y.1 Consequently, zeolite
X framework is usually basic whereas zeolite Y framework is
usually acidic. Regardless of the Si/Al ratio, they are constructed
by interconnecting ‘sodalite units’ in such a way that the so-called
‘supercages’ are created (Fig. 1a).
The diameter of a pseudospherical supercage is 1.3 nm and the
size of the window to the supercage is 0.74 nm.1 The neighboring
supercages are interconnected through the windows in a tetrahedral symmetry. Ru(bpy)3 2+ (diameter ∼1.2 nm) has often been
assembled within the supercages of zeolites X and Y (Fig. 1b)
by introducing its components via a ‘ship-in-a-bottle’ synthesis
technique.4–7 The intercage migration of Ru(bpy)3 2+ cannot occur
because the framework is very rigid and the diameter of the
supercage window is much smaller than the size of Ru(bpy)3 2+ .
a
Chemistry Department and Advanced Engineering Courses, Gunma College
of Technology, Maebashi, Gunma, 371-8530, Japan
b
Department of Ecosystem Engineering, The University of Tokushima,
Tokushima, 770-8506, Japan
c
Center for Microcrystal Assembly and Department of Chemistry, Sogang
University, Seoul, 121-742, Korea
† The HTML version of this article has been enhanced with colour images.
‡ Electronic supplementary information (ESI) available: Fig. S1–S3. See
DOI: 10.1039/b605469b
822 | Photochem. Photobiol. Sci., 2006, 5, 822–827
Fig. 1 Framework structure of supercage in zeolites X and Y (a) and
chemical structure of Ru(bpy)3 2+ (b).
Photoinduced electron transfer (PET) between the Ru(bpy)3 2+
assembled in the supercages of zeolite Y and various acceptor
compounds incorporated within the supercages or placed on
the external surfaces of the zeolite has extensively been studied
for many years as model systems to develop the methods of
achieving long-lived charge-separated states through retardation
of back electron transfer processes.8–15 In relation to the above,
the photophysics of the zeolite-confined Ru(bpy)3 2+ has also been
extensively studied.
In fact Dutta and coworkers16–19 have investigated the luminescence properties of Ru(bpy)3 2+ in the triplet excited-state,
*Ru(bpy)3 2+ , entrapped inside Na+ –Y. They found that the
emission decay rate increases remarkably with increasing the
excitation light intensity and with increasing the loading level of
the Ru-complex. In hydrated Na+ –Y, the luminescence decay of
This journal is © The Royal Society of Chemistry and Owner Societies 2006
*Ru(bpy)3 2+ obeyed the simple first-order kinetics at low loadings
and low excitation intensities but deviated otherwise. The fast
decay rates and the nonexponentiality of the decays were ascribed
to the following two mechanisms. The quenching of *Ru(bpy)3 2+
by the Ru(bpy)3 2+ complexes placed in the neighboring supercages
occurs at high loadings (loading dependence) and triplet–triplet
annihilation occurs between the two *Ru(bpy)3 2+ complexes placed
in the neighboring supercages (excitation intensity dependence).18
It has been widely known that *Ru(bpy)3 2+ can serve as both
an electron donor and an acceptor depending on the redox
properties of the partner.20 In fact, any material or compound
can behave as a donor or an acceptor depending on the relative
electron density of the interacting counterpart. Zeolite should
not be an exception. Indeed, a great number of experimental
results have verified that zeolite frameworks actively participate
in PET reactions as electron donors for a variety of intercalated
compounds and the charge-balancing cations act as the electron
acceptors.21,22 However, the nature of PET between the zeoliteintercalated Ru(bpy)3 2+ and the zeolite hosts has not been known,
although PET between zeolite-intercalated Ru(bpy)3 2+ and various
electron acceptors placed in the pores and on the external surfaces
of zeolites has been extensively studied.8–14 We now report that PET
between zeolite-intercalated Ru(bpy)3 2+ and the charge-balancing
cations occurs.
0.243 cm3 g−1 for Na+ –Y. Dehydration of the zeolite samples was
performed at room temperature in the vacuum line at <0.4 Pa for
48 h. By this treatment, the degree of dehydration was 97% (3% of
remaining water content) for Na+ –X and 94% for Na+ –Y. Heating
the samples in the vacuum line to approximately 100 ◦ C will
improve the degree of dehydration of the samples. However, this
caused the color change of the Ru(II)–zeolite complex presumably
due to the release of bpy ligands.16 We were therefore forced to
accept the incomplete dehydration of the samples.
The reflectance spectra of powdered zeolite samples were collected on a Shimadzu UV-3101PC double-monochromator spectrophotometer equipped with an integrating sphere coated with
BaSO4 . The reference was BaSO4 (Kodak white reflectance standard). Absorption spectra were obtained by using the Kubelka–
Munk function. Fluorescence spectra were recorded on a Hitachi
F-3010 spectrofluorometer in a front face geometry. For the
measurements of emission decays, samples were excited at 460 nm
by an OPO laser (Continuum, Surelite-OPO). The emission decay
signals were detected by a photomultiplier (Hamamatsu, R-928),
fed to a digitizing oscilloscope (Tektronix, TDS540A), through a
monochromator (Ritsu, MC-10DG).
Transient absorption spectra excited at 460 nm were measured on a diffuse reflectance laser photolysis setup described
previously.23 All the measurements were carried out at ambient
temperature.
Experimental
Na+ -exchanged zeolites X and Y were purchased from UOP. Various alkali metal ion-exhanged zeolites X and Y were prepared
according to the standard procedure. The Ru(bpy)3 2+ -encapsulated
zeolites X and Y were prepared with the cation-exchanged zeolites
according to the method described in the literature.4–7 The XRD
study showed that the crystal structure of both zeolites X and Y are
not affected by the procedure of Ru(bpy)3 2+ -encapsulation. Table 1
gives the compositions of the zeolites prepared in this work, which
were determined by ICP analyses.
The hydrated zeolite samples were prepared by exposure of the
samples to the atmosphere for a sufficient period of time. The water
content of the hydrated zeolites was 0.187 cm3 g−1 for Na+ –X and
Table 1 Composition of zeolites, Sanderson’s electronegativity and
Sanderson’s partial negative charge density on the framework oxygen
(−d O )a
Zeolite
Unit cell composition
Sanderson’s electronegativity −d O
Li+ –Y
Na+ –Y
K+ –Y
Cs+ –Y
Na+ –X
K+ –X
Cs+ –X
Li34 Na18 Al52 Si140 O384
Na52 Al52 Si140 O384
Na2 K50 Al52 Si140 O384
Na17 Cs34 H1 Al52 Si140 O384
Na82 H1 Al83 Si109 O384
Na5 K70 H8 Al83 Si109 O384
Na42 Cs34 H7 Al83 Si109 O384
2.672
2.606
2.561
2.483
2.404
2.387
2.323
0.2461
0.2628
0.2741
0.2936
0.3135
0.3177
0.3339
The Sanderson’s electronegativity of each M+ -exchanged zeolite Sz was
calculated according to the equation Sz = (SM p SH q SAl r SSi t SO u )1/(p + q + r + t + u) ,
where SM , SH , SAl , SSi , and SO represent the Sanderson’s electronegativities
of the alkali metal, hydrogen, aluminum, silicon, and oxygen, respectively
and p, q, r, t, u represent the number of the corresponding element in a unit
cell, respectively. The Sanderson’s partial charge of the framework oxygen
(d O ) was calculated using the equation d O = (Sz − SO )/(2.08 SO 1/2 ). The
values are taken from: J. E. Huheey, E. A. Keiter and R. L. Keiter, Inorganic
Chemistry, Harper Collins College Publications, New York, 4th edn, 1993.
a
Results and discussion
Steady-state absorption and luminescence spectra in Na+ –X and
Na+ –Y
A series of zeolites Na+ –X and Na+ –Y loaded with the Ru(II)
complex in three different loadings was prepared. Note that the
two zeolites belong to the same crystallographic structure of
faujasite with a difference in Si/Al ratio giving rise to distinct
chemical properties (see Table 1).1 The average occupancy of
Ru(bpy)3 2+ in the supercage (n1̄ = [Ru(II)/[supercage]) employed
presently is 1/7, 1/58, and 1/163 for Na+ –X and 1/7, 1/59,
and 1/172 for Na+ –Y. Here the loading level of the Ru(II)
complex was determined by the X-ray fluorescence analysis. The
diffuse reflectance absorption spectra of a MLCT transition was
measured for hydrated and dehydrated Na+ –X and Na+ –Y (see
supplementary information, Fig. S1‡). Practically, no difference
was observed for the spectra regardless of the host material and the
state of hydration: both the peak position and spectral envelope of
the absorption in Na+ –X were basically similar to those observed
in Na+ –Y.§ Additionally, the absorption intensity was represented
by a linear function of the loadings.
We measured the luminescence spectra of Ru(bpy)3 2+ in the
zeolites Na+ –X and Na+ –Y at three different loading levels (supplementary information, Fig. S2‡). In contrast to the absorption
spectra, we found a remarkable luminescence spectral change
caused by dehydration: we observed that the peak position of
the luminescence spectra undergoes blue shift on dehydration.
For instance, a 30 nm shift from 620 to 590 nm was observed
§ A small shoulder observed for the spectra in Na+ –X is not due to an
impurity, rather due to an artifact associated with the diffuse reflectance
measurement of powdered zeolite samples. See ref. 7 for the detail.
This journal is © The Royal Society of Chemistry and Owner Societies 2006
Photochem. Photobiol. Sci., 2006, 5, 822–827 | 823
in Na+ –Y. This observation is essentially the same as that by
Incavo and Dutta.16 They interpreted the spectral shift as arising
from rigidly held Ru(bpy)3 2+ molecules which prevents the charge
transfer triplet state from relaxing due to lack of solvation. The
luminescence spectra in dehydrated Na+ –X are red-shifted by
10 nm compared to those observed in dehydrated Na+ –Y, and
the intensity increased significantly. This observation is consistent
with remarkably longer lifetimes in Na+ –X as will be given below.
A slight blue shift of the luminescence spectra observed from the
dehydrated, highly loaded Na+ –X and Na+ –Y can be ascribed to
the R(II)–Ru(II) interaction at high loadings.
Luminescence decay kinetics in zeolites X and Y
Fig. 2 compares the time-dependent luminescence decays in
dehydrated system with that in hydrated system both for Na+ –
X (n1̄ = 1/58) and for Na+ –Y (n1̄ = 1/59).
Fig. 2 Effect of dehydration on the luminescence decay profiles of
Ru(bpy)3 2+ assembled in zeolites Na+ –X (occupancy: n1̄ = 1/58) and
Na+ –Y (occupancy: n1̄ = 1/59) excited at 460 nm (2.5 lJ cm−2 ) and
detected at 610 nm: (A) dehydrated Na+ –X; (B) dehydrated Na+ –Y; (C)
hydrated Na+ –Y; (D) hydrated Na+ –X.
Most significantly, a much slower decay rate was observed
for the dehydrated sample of zeolite Na+ –X than that for the
dehydrated sample of Na+ –Y while the difference in the decay rates
is minor for the hydrated samples. The origin of this difference
in the zeolite X and Y is ascribed to the difference in the
acceptor strength of the cations coordinated to the frameworks
with different Si/Al ratios, namely, with different basicity. As
a general rule, the acceptor strength of a cation increases with
decreasing the basicity of the coordinating ligands. Likewise, it is
expected that acceptor strengths of cations in dehydrated zeolites
are expected to increase with decreasing the Si/Al ratio of the
framework since the framework basicity increases accordingly.24
Therefore, the Na+ ions in zeolite Y possess a stronger electronaccepting property for Ru(bpy)3 2+ * than Na+ ions in zeolite X
since the framework of zeolite Y possesses weaker donor strength
than that of zeolite X. Taking this point into consideration, the
experimental result is readily explained by the strong luminescence
quenching ability of zeolite Na+ –Y compared to Na+ –X due to a
charge transfer interaction from Ru(bpy)3 2+ * to the cations.
Consistent with the above interpretation, the role of alkali metal
ions in zeolites as electron acceptors is well documented.25 For
instance, photoexcitation of anthracene or pyrene incorporated
824 | Photochem. Photobiol. Sci., 2006, 5, 822–827
within dehydrated Na+ –Y and Na+ –X leads to photoexcited
singlet states of the arenas which subsequently undergoes electron
transfer to a group of four sodium ions residing in sodalite
cages.26 Cyanoarene sensitizers also readily undergo photoinduced
electron transfer to a group of four sodium ions.27 Zeoliteintercalated naphthalene and biphenyl were also proposed to
undergo photoinduced electron transfer to groups of two and
three sodium ions even in hydrated Na+ –X and and Na+ –Y.28 In
an extreme case, the reduction of a group of four sodium ions
residing in sodalite cages in zeolites X and Y at room temperature
upon contact in the solid state is also well established.29
The decay rates in hydrated systems are basically similar to
those of Ru(bpy)3 2+ * in water.30 This result suggests that, in the
hydrated zeolites, both Ru(II) complexes and alkali metal cations
are shielded by water molecules preventing their direct interaction.
Therefore, dehydration of the zeolite hosts is important for the
observation of the effect of electron-accepting properties of the
cations. We believe the nonexponentiality of the luminescence
decay kinetics of the Ru complexes in dehydrated zeolites is also
related to the photoinduced electron transfer from Ru(bpy)3 2+ to
the cations. Previously, Incavo and Dutta reported the effect of dehydration of Na+ –Y on the luminescence lifetime of Ru(bpy)3 2+ *.16
Consistent with our result, they observed a single-exponential
decay with a lifetime of sF = 440 ns in hydrated system and a
nonexponential decay which is approximated by a sum of two
exponential function (sF1 = 98 ns (75%) and sF2 = 440 ns (25%))
in dehydrated system. Clear interpretation for the mechanism of
their result was, however, not given at that time.
We examined both the loading dependence and the excitation
light intensity dependence of the luminescence decays with a hope
to gain other possible clues to the origin of the nonexponentiality
in dehydrated system. Fig. 3 shows the loading dependence, and
Fig. 4, the excitation light intensity dependence of the decays.
Fig. 3 Loading-dependent luminescence decay curves of Ru(bpy)3 2+ in
dehydrated Na+ –X and Na+ –Y excited at 460 nm and detected at 610 nm
with a laser power of 2.5 lJ cm−2 : (A) Na+ –X (n1̄ = 1/163); (B) Na+ –X
(n1̄ = 1/58); (C) Na+ –X (n1̄ = 1/7); (D) Na+ –Y (n1̄ = 1/172); (E) Na+ –Y
(n1̄ = 1/59); (F) Na+ –Y (n1̄ = 1/7).
The results indicate that the nonexponential decays are still
observable even at a very low loading level with the lowest
excitation intensity under our experimental condition both in
dehydrated Na+ –X and Na+ –Y. This observation is remarkably
different from that in hydrated system by Sykora et al. because
they observed a single-exponential decay at a sufficiently low
This journal is © The Royal Society of Chemistry and Owner Societies 2006
Fig. 4 Excitation laser intensity-dependent luminescence decay curves of Ru(bpy)3 2+ in dehydrated zeolites excited at 460 nm and detected at 610 nm;
(a) Na+ –X (n1̄ = 1/7) and (b) Na+ –Y (n1̄ = 1/7).
loading at a low excitation intensity.18 Here, we should point out
two possibilities of this unusual observation.
One is that even at a very loading level such as n1̄ = 1/163 there
exists a chance that more than one guest species occupy the intimate neighboring supercages due to inhomogeneous distribution
of the Ru complexes. It is widely recognized that the distribution of
guest species in zeolite crystals is inhomogeneous arising from very
slow diffusion of the molecules from outermost part to the inside
of the crystals. In addition, a recent publication has shown that
the distribution of guest molecules is non-uniform at the level of
individual particles.31 Since the two factors are closely interrelated,
an unexpectedly non-uniform distribution of molecules can result
in even if the average loading level is very small.
The other possibility is the heterogeneous nature of the cationic
sites leading to site-dependent forward and back electron transfer
rate from Ru(bpy)3 2+ * to cations and from the reduced cations to
Ru(bpy)3 3+ , respectively. Although we have pointed out two possibilities to explain the observed nonexponential decays, we cannot
decide at this stage which of the two mechanisms contributes most
due to the lack of experimental evidence.
values of the average lifetime s̄ defined by:
s=
A1
A2
A3
s1 +
s2 +
s3
A1 + A2 + A3
A1 + A2 + A3
A1 + A2 + A3
are obtained and listed in Table 2.
The evaluated s̄ value ranges from 360 ns in Na+ –Y to 1.64 ls in
+
K –Y. It is reminiscent that large values of luminescence lifetime
are obtained normally in solid systems: for instance, 1.55 ls
for Ru(bpy)3 2+ adsorbed on zirconium phosphate32 and 3.13 ls
in crystals33 while much smaller values are known in solutions,
e.g. 600 ns in water.30 Therefore, the value of 360 ns in Na+ –
Y is considerably short for adsorbed systems suggesting that the
occurrence of appreciable luminescence quenching by the Na+ ions
in Na+ –Y as described above.
Fig. 5 represents the correlation of s̄ vs. the Sanderson’s
electronegativity scale,24 a generally accepted measure of the
electron-accepting ability of zeolite hosts.
Correlation of luminescence lifetime with electron donor property
of zeolite host
We measured the luminescence decays of Ru(bpy)3 2+ * in various
alkali metal-exchanged zeolites X (n1̄ = 1/58) and Y (n1̄ =
1/59) which were dehydrated by the same method used for the
dehydration of Na+ –X and Na+ –Y. The variation of cations
allows the variation of the acceptor strength of the cations. The
result indicated that the decay rates are clearly dependent on
the cations although nonexponential decay kinetics are generally
observed even at the minimum excitation light intensity under our
experimental condition (supplementary information, Fig. S3‡).
Since we could not single out a unique luminescence lifetime or an
excited state decay rate constant, we tentatively used an average
lifetime to gain an approximate picture of the PET rate. For this
purpose, we employed a sum of three exponential fitting to the
experimental decay curves:
t
t
t
+ A2 exp −
+ A3 exp −
F(t) = F0 A1 exp −
s1
s2
s3
Here F(t) represents the luminescence intensity at time t; F 0 , the
luminescence intensity at time 0; si , ith component of the lifetime;
Ai , ith component of a contribution factor of si . Accordingly, the
Fig. 5 Correlation of average lifetimes with the Sanderson’s electronegativity for alkali metal cation-exchanged zeolites X and Y (n1̄ = 1/58,
1/59); the straight line represents the fitting by the least squares method
with a correlation coefficient of 0.81.
Clearly, the average lifetime of Ru(bpy)3 2+ * decreases (or the
decay rate increases) with increasing the Sanderson’s electronegativity of the zeolite hosts. Since the acceptor strength of a
cation is supposed to increase with increasing the Sanderson’s
electronegativity of the zeolite, the above result certainly shows
that the cations act as electron acceptors for Ru(bpy)3 2+ *, a
finding first elucidated by this work. The rather poor correlation
coefficient of 0.81 of the plot is ascribed to the nonexponentiality
of the decays and the unavoidable insufficient dehydration of the
zeolites.
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Photochem. Photobiol. Sci., 2006, 5, 822–827 | 825
Table 2 Sum of multi-exponentials fit to the observed luminescence decays
Laser power/mJ pulse−1
0.0025
a
Zeolite
Occupancy ([Ru(II)]/[cage])
si
Hydrated Na+ –Y
1/59
0.126 (7)c
0.697 (93)
0.135 (13)
0.749 (87)
0.050 (6)
0.620 (94)
0.147 (8)
0.632 (92)
0.050 (13)
0.362 (25)
1.143 (62)
0.037 (23)
0.217 (47)
0.819 (30)
0.047 (27)
0.261 (44)
1.112 (29)
0.046 (18)
0.307 (38)
1.135 (44)
0.078 (11)
0.590 (36)
1.640 (53)
0.071 (19)
0.495 (43)
1.780 (38)
0.048 (12)
0.461 (29)
1.538 (59)
0.120 (9)
0.879 (29)
1.773 (62)
0.049 (5)
0.615 (21)
1.423 (74)
0.074 (5)
0.830 (22)
2.009 (73)
0.036 (4)
0.602 (19)
1.605 (77)
1/172
+
Hydrated Na –X
1/58
1/163
+
Li –Y
1/59
Na+ –Y
1/7
1/59
1/172
K+ –Y
1/59
Cs+ –Y
1/59
Na+ –X
1/7
1/58
1/163
K+ –X
1/58
Cs+ –X
1/58
0.025
s̄
b
0.655
0.669
0.589
0.593
0.809
0.359
0.448
0.625
1.089
0.906
1.050
si
a
0.633
0.118 (52)
0.474 (37)
1.139 (11)
0.110 (49)
0.461 (32)
1.290 (19)
0.047 (17)
0.299 (39)
1.124 (44)
0.045 (9)
0.484 (36)
1.579 (55)
—
0.358
1.641
1.350
—
1.180
s̄
0.090 (9)
0.689 (91)
0.118 (15)
0.742 (85)
0.074 (7)
0.619 (93)
0.066 (9)
0.616 (91)
—
0.131 (23)
0.610 (34)
1.592 (43)
0.047 (7)
0.556 (22)
1.564 (71)
0.056 (6)
0.524 (21)
1.406 (73)
—
1.369
0.50
b
0.652
0.580
0.568
0.449
0.614
1.041
0.923
1.235
1.145
si a
s̄b
0.087 (17)
0.687 (83)
0.081 (20)
0.747 (80)
0.101 (16)
0.614 (84)
0.079 (14)
0.614 (86)
0.583
0.055 (53)
0.260 (34)
0.867 (13)
0.103 (54)
0.452 (29)
1.261 (17)
0.040 (22)
0.279 (38)
1.121 (40)
0.054 (16)
0.455 (37)
1.541 (47)
0.227
0.052 (40)
0.328 (35)
1.339 (25)
0.040 (10)
0.389 (28)
1.509 (62)
0.046 (9)
0.455 (24)
1.402 (67)
0.614
0.529
0.538
0.403
0.562
0.908
0.474
1.044
1.050
a
si represents the ith components of the excited-state lifetimes in ls. b s̄ represents the average lifetimes in ls. c The number in parentheses represents
relative contribution of given component to the total intensity in percent.
Transient absorption spectroscopic studies
We measured the transient absorption spectra (not shown) of
Ru(bpy)3 2+ * both in dehydrated Na+ –X and Na+ –Y in an attempt
to obtain clear evidence for electron transfer from Ru(bpy)3 2+ * to
the cations: formation of Ru(bpy)3 3+ as a product, in particular, in
Na+ –Y in which a strong luminescence quenching was observed.
According to a previous study, Ru(bpy)3 3+ has a weak absorption
band (e = 440 M−1 cm−1 in acidic aqueous solutions) with a peak
at 675 nm.14,34 Recording the transient spectra prior to 5 ls by the
reflectance method was severely hampered by the intense emission
superimposed on the observing region. The observed spectra are
basically assignable to the decaying Ru(bpy)3 2+ * which is known
to occur in a similar spectral region to that of the luminescence.35
Thus the decay rate is greater for the sample of Na+ –Y than that
of Na+ –X consistent with the observation of the luminescence
826 | Photochem. Photobiol. Sci., 2006, 5, 822–827
decays. We found it extremely difficult to detect the transient
absorption spectrum of Ru(bpy)3 3+ for the present samples by
our measurement system, and could not obtain the solid evidence
for the PET. One reason for this failure is obviously a very low
absorption coefficient of Ru(bpy)3 3+ .
Summary
Ru(bpy)3 2+ -incorporating zeolites have been widely studied as the
artificial photosynthetic systems. However, no investigation has
previously been made on the effect of zeolite hosts on the decay
rate of Ru(bpy)3 2+ *. As a possible means to investigate the above,
we prepared various alkali metal-exchanged zeolites X and Y
incorporated with Ru(bpy)3 2+ with different degrees of loading
aiming at the observation of photoinduced electron transfer (PET)
This journal is © The Royal Society of Chemistry and Owner Societies 2006
between Ru(bpy)3 2+ * and the zeolite hosts. The experimental
results demonstrated a positive correlation between the luminescence decay rate (inverse of luminescence lifetimes) of Ru(bpy)3 2+ *
in dehydrated zeolites and the Sanderson’s electronegativity, which
is a good measure of the electron-accepting nature of the zeolite
hosts. Based on the previous knowledge that alkali metal ions
in zeolites act as electron acceptors for the photoexcited arene
donors, we interpreted the above phenomenon as the electrontransfer quenching of Ru(bpy)3 2+ * by the alkali metal cations.
Acknowledgements
SH is grateful to the Japanese Ministry of Education, Culture,
Sports, Science and Technology: Grant-in-Aid for Scientific
Research on Priority Areas 417 (#17029067) for financial support
of this work. KBY thanks the Ministry of Science and Technology
of Korea for supporting this work through the Creative Research
Initiatives program.
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