Intrazeolite photochemistry. 12. Ship-in-a-bottle synthesis and control of the photophysical properties of 9-(4-methoxyphenyl)xanthenium ion imprisoned into large-pore zeolites

Article abstract of DOI:10.1021/jp960729n

The 9-(4-methoxyphenyl)xanthylium ion (AnX⁺), a bulky cation that according to molecular modeling simulations cannot enter through the 7.4 A pore opening of tridirectional large-pore zeolites Y and β, has been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure involves two steps: (i) generation of xanthylium cation (X⁺) adsorbed within the zeolite by treatment of xanthydrol onto the H⁺ form of these solids; (ii) electrophilic attack of the cation to anisole within the framework of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with some unidentified adventitious material. Textural and acidic properties of these composites were tested using α-methylstyrene dimerization and were found to be very similar to the original HY or Hβ zeolites, thus indicating an internal location for AnX⁺. Entrapment within the zeolite framework restricts AnX⁺ conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds), as well as the observation of room-temperature fluorescence in the nanosecond range.

Full text of DOI:10.1021/jp960729n

J. Phys. Chem. 1996, 100, 18145-18151
18145
Intrazeolite Photochemistry. 12. Ship-in-a-Bottle Synthesis and Control of the
Photophysical Properties of 9-(4-Methoxyphenyl)xanthenium Ion Imprisoned into
Large-Pore Zeolites
Mar´ıa Luz Cano,† Frances L. Cozens,*,‡ Vicente Forne´s,† Hermenegildo Garc´ıa,*,† and
J. C. Scaiano‡
Instituto de Tecnolog´ıa Qu´ımica CSIC-UPV, UniVersidad Polite´cnica de Valencia, 46071 Valencia, Spain, and
Department of Chemistry, UniVersity of Ottawa, Ottawa, Canada, K1N 6N5
ReceiVed: March 8, 1996; In Final Form: August 2, 1996X
The 9-(4-methoxyphenyl)xanthylium ion (AnX+), a bulky cation that according to molecular modeling
simulations cannot enter through the 7.4 Å pore opening of tridirectional large-pore zeolites Y and â, has
been prepared by ship-in-a-bottle synthesis within the supercages of these two zeolites. The synthetic procedure
involves two steps: (i) generation of xanthylium cation (X+) adsorbed within the zeolite by treatment of
xanthydrol onto the H+ form of these solids; (ii) electrophilic attack of the cation to anisole within the framework
of the zeolite. Carrying out the same procedure using toluene or benzene or using an alternative process
involving the reaction of xanthone with anisole, led to the corresponding 9-arylxanthylium ions together with
some unidentified adventitious material. Textural and acidic properties of these composites were tested using
R-methylstyrene dimerization and were found to be very similar to the original HY or Hâ zeolites, thus
indicating an internal location for AnX+. Entrapment within the zeolite framework restricts AnX+
conformational mobility, thwarting the deactivation pathway occurring in solution. This has allowed for the
first time characterization of its triplet excited state as a long-lived transient species (tens of microseconds),
as well as the observation of room-temperature fluorescence in the nanosecond range.
Introduction
These intracrystalline voids of zeolites are accessible to guest
molecules whose molecular sizes are smaller than the apertures
Zeolites have proved to be very convenient hosts to study
the photophysical and photochemical properties of persistent,
adsorbed organic cations.1-5 Stabilization of these reaction
intermediates arises not only from the internal electrostatic fields
occurring inside the voids of these microporous solids6-8 but
also in many cases from the tight fit of the rigid, inert framework
that protects these cations from the attack of nucleophilic
species, and in particular from physisorbed water. Thus, we
have previously observed that samples of xanthylium ions (X+)
included within mordenite or ZSM-5 can be stored for a period
of years without any noticeable alteration of their spectroscopic
properties.2 However, despite these very promising properties,
the number of studies dealing with the use of zeolites as a solid
matrix is still very small and much work remains to be done in
this area. To further expand the type of cations that can be
incorporated, the preparation and characterization of bulky
organic cations (those whose smallest cross section is larger
than 7.4 Å) inside the cages of large-pore zeolites is particularly
challenging. This is due to the special geometry of tridirectional
zeolites such as Y and â, that, despite having cages and cavities
of large dimensions (about 13 Å), are accessible only through
much smaller round pore openings, (typically around 7.4 Å)
that severely limit the size of the species able to gain access to
these cages.
of the pores. The ability of zeolites to incorporate guest species
gives rise to a supramolecular chemistry and several terms such
as molecular sieves,7 microscopic reactors,10 molecular pock-
ets,11 or reaction cavities12 have been coined to describe the
porosity of zeolites at a molecular level. Up to six years ago,
it was a clear limitation in the size of a molecule that could be
hosted in zeolites. However, more recently has been reported
a novel family of extra-large-pore aluminosilicates,13 generally
referred to as mesoporous materials to distinguish from classical
microporous zeolites. Thus, MCM-41 is formed by an array
of parallel hexagonal channels whose diameter can be controlled
from 20 to 60 Å by varying the chain length of the surfactant
used as template during its synthesis.14
Apart from the dimensions of the micropores, the geometry
of the internal voids is another important factor to be considered
since it determines the diffusion coefficient of the guest and,
related to this, the uptake that can be incorporated in the internal
surfaces. In this respect, zeolites can be classified as mono-
directional (system of parallel channels), bidirectional (two
system of channels crossing), or tridirectional (system of cages
interconnected).
In the present series of articles, we have employed zeolites
of medium-, large-, and extra-large-pore size as well as mono-,
bi-, and tridirectional topologies to control and modify the
photochemical properties of embedded organic species. The
structure of some representative zeolites is illustrated in Chart
1. Our systems provides new examples of the potential of
zeolites as rigid matrixes to stabilize and immobilize organic
species.
Ship-in-a-bottle methodologies have to be devised to syn-
thesize large cations that afterward will remain imprisoned inside
the zeolite supercages.1,3,15-17 This methodology has sometimes
failed in the past to produce well-defined species entrapped
Zeolites are crystalline aluminosilicates whose crystalline
structure is formed by channels and cavities of strictly regular
dimensions called micropores. According to the dimensions
of these internal voids, zeolites have been classified as small,
medium, and large pore, when their pore apertures are formed
by rings containing 8, 10, or 12 oxygen atoms, respectively.9
† Universidad Politecnica de Valencia.
‡ University of Ottawa.
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)00729-0 CCC: $12.00 © 1996 American Chemical Society

18146 J. Phys. Chem., Vol. 100, No. 46, 1996
Cano et al.
CHART 1
inside the voids of large-pore zeolites,18-21 and therefore a
careful characterization of the resulting zeolite-bound organic
species is necessary in order to establish their purity and the
absence of adventitious material. We have already successfully
reported the synthesis of the 2,4,6-triphenylpyrylium cation
encapsulated within zeolite Y by emulating homogeneous phase
procedures.1 In the present work, we take advantage of the
extraordinary ability of the zeolite to spontaneously generate
and stabilize positively charged species from neutral precursors
to prepare the 9-(4-methoxyphenyl)xanthylium ion (AnX+)
inside the micropores of zeolites Y and â. The presence of
AnX+ as the sole species, together with the absence of detectable
amounts of X+ was ascertained by IR spectroscopy of the
resulting samples. To the best of our knowledge, the procedure
that was carried out to generate the cation within the zeolite
has not been reported in liquid solution.
We have previously studied the photophysical properties of
the parent unsubstituted X+ incorporated within medium- and
large-pore zeolites.2 Herein, we have found that AnX+ im-
mobilized inside the zeolite supercages shows photochemical
behavior similar to that of X+. Thus, besides intense, long-
lived fluorescence we have observed a strong band in the visible
region for the triplet excited state of the cation, together of an
absorption at 340 nm attributed to the corresponding AnX•
radical. These findings sharply contrast with previous studies
of AnX+ in solution which failed to observe any excited state
of AnX+ at room temperature.22
only the external, more exposed surface of the zeolite particles
was accessible to PhXOH under these adsorption conditions.
In contrast, an alternative strategy depending on the prior
generation of X+ followed by treatment of the resulting
composite at reflux temperature with anisole turned out to be a
successful procedure to prepare AnX+ incorporated inside Y
and â zeolites.
First we prepared the less bulky parent X+ by adsorption of
XOH from a dichloromethane solution onto the H+-form of
zeolites Y and â. The diffuse reflectance and IR spectra of the
resulting complexes were coincident with those previously
reported for these types of samples.2 After the characterization
of the solids the zeolites containing X+ were reacted with neat
anisole. The large excess of anisole compared to X+, as well
as the relatively high reaction temperature ensure that essentially
all X+ is consumed during the synthesis. This was confirmed
by IR spectroscopy (vide infra). Once the reaction was
complete, the samples were submitted to exhaustive solid-liquid
extraction until no more organic material could be recovered
from the orange solids. The reaction sequence for the synthesis
of AnX+ is outlined in Scheme 1; the observation (GC-MS) of
the formation of minor amounts of a product compatible with
AnXH (MW 288) in the supernatant lends support to the
proposed mechanism.
Formation of a C-C bond between X+ and the aromatic ring
of anisole can be easily rationalized as an aromatic electrophilic
Friedel-Crafts reaction between X+ and the electron-rich
aromatic compound to give an intermediate cyclohexadienyl
cation.26 In addition, attempts to reproduce this treatment using
toluene or benzene as the aromatic instead of anisole did not
lead with the same efficiency to the formation of the corre-
sponding 9-aryl-substituted xanthylium ion. This was assessed
by the impossibility to obtain for these samples a defined IR
spectrum corresponding to these cations embedded in the zeolite
matrix. However, the formation of 9-phenyl and 9-p-tolylxan-
thyliums in some extent did occur since the characteristic long
wavelength absorption around 500-nm was observed as a low-
intensity broad band in the diffuse reflectance spectra of these
samples. This is in agreement with the decreasing stability of
the intermediate cyclohexadienyl cation upon changing the aryl
substituent from 4-MeO to 4-Me or 4-H. Once formed, the
cyclohexadienyl cation will deprotonate to generate AnXH
within the zeolite framework.
Results and Discussion
Preparation of the AnX+-HZeolite. Molecular modeling
predicts that although 9-aryl-substituted xanthylium ions can
be accommodated inside the Y supercage, they cannot enter
through the 12-member-ring pore opening. A visualization of
the optimized conformation of AnX+ within the supercage of
zeolite Y is provided in Figure 1. In fact, it has been previously
reported that similarly shaped 9-phenylanthracene cannot enter
through the zeolite Y windows.23-25
In addition, we tried to generate the 9-phenylxanthylium ion
(PhX+) by adsorbing PhXOH from a dichloromethane solution
onto acid Y zeolite. Although the suspension become yellow
in color, it faded completely almost immediately after filtering
the solid. The amount of organic material adsorbed on the solid
was very low (<0.5%) as determined by combustion chemical
analysis and thermogravimetry. Loss of color of the sample
combined with the low amount of organic material indicate that
Much less documented is the nature of the proposed hydride
abstraction from AnXH which leads to the formation of the
organic AnX+ (Scheme 1). However, we have previously
reported that the hydride abstraction yielding X+ occurs
spontaneously by adsorption of the corresponding XH into acid
zeolites.27 Furthermore, this phenomenon, although still poorly
understood, appears to be general since other carbenium ions
can also be generated from their hydrocarbons.28,29
Characterization of the resulting zeolite samples containing
AnX+ was accomplished by combustion chemical analysis,
thermogravimetric-differential scanning calorimetry (TG-DSC),

Intrazeolite Photochemistry. 12
J. Phys. Chem., Vol. 100, No. 46, 1996 18147
Figure 1. Molecular modeling visualization of AnX+ cation inside zeolite Y supercage. Docking was performed to minimize overlapping of
host-guest atoms. The view is taken through one of the four windows facing the xanthylium core, while the MeO group can be seen partially
penetrating through another opening.
SCHEME 1
TABLE 1: Some Relevant Physicochemical Parameters of
the Samples Containing AnX+ Cation Incorporated within
Zeolites
AnX+
AnX+-HY AnX+-Hâ MCM-41
zeolite Si/Al ratioa
2.6
720
39
13.0
666
36
11.3
928
54
b
BET surface area (m2 g-1
)
Figure 2. Thermogravimetric profile (a: left axis) and DSC curve (b:
right axis) of AnX+ incorporated within zeolite HY. The loss of weight
(3.9%) between 300 and 700 °C corresponds to the exothermic
decomposition of organic cation.
amount of organic material
(mg g-1 zeolite)c
UV/vis absorption bands (nm) 371, 515
carbon content (%)d
2.51
370, 495
2.49
365, 497
3.74
a Measured by atomic absorption spectroscopy after HF disgregation.
b For the original zeolite prior to the adsorption procedure; calculated
according to the algorithm proposed by (BET): Brunaer, S.; Emmett,
P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309. c Measured by the
loss of weight between 300 and 700 °C in the TG profiles. d Determined
by combustion chemical analysis.
zeolite matrixes was obtained from the TG-DSC curves. In this
technique the sample is placed in the pan of a microbalance,
while it is being progressively heated to 900 °C under air stream.
The loss of weight due to the desorption/degradation of the
organic material is recorded at each temperature (TG) and
simultaneously the absorbed or evolved heat from the sample
is measured by comparing to an inert standard of kaolin (DSC).
The values obtained by combustion analysis also agree fairly
well with the loss of weight observed in the TG curve from
300 to 700 °C, corresponding to an exothermic process peaking
at 515 °C (Figure 2). We rationalize this exothermic weight
loss as owing to the decomposition of included organic cation.
Diffuse reflectance of these samples exhibited the very
characteristic absorption of AnX+ dye and closely resembled
the UV-vis spectrum of AnX+ in solution. Figure 3 provides
a comparison of the UV-vis diffuse reflectance spectra of some
diffuse reflectance (DR), and IR spectroscopy. Identity of AnX+
chemical structure and purity was confirmed by comparison with
an authentic sample prepared by adsorbing AnXOH onto extra
large pore MCM-4113,14,30,31 (array of parallel hexagonal chan-
nels, 20 Å diameter), where geometrical restrictions do not
apply. Some of the relevant physicochemical parameters of the
samples that have been prepared are given in Table 1.
The amount of organic material (between 3 and 6 wt %)
adsorbed by the solid was determined by combustion elemental
analysis of carbon. Alternatively, the AnX+ content of the

18148 J. Phys. Chem., Vol. 100, No. 46, 1996
Cano et al.
SCHEME 2: Dimer Ratios at Low Conversions: 2a (9),
2b (1), 3 (4)
SCHEME 3: Characteristic Dimers Formed Under
Radical (4) or Radical Cation (5 and 6) Reaction
Mechanisms
Figure 3. Diffuse reflectance (1/R) of the AnX+ incorporated in MCM-
41 by adsorption of AnXOH (plot a) and synthesized in zeolite Y from
X+ (plot b).
comparison, the spectrum of a sample of X+ adsorbed on zeolite
has also been included (Figure 4B).
We attempted an alternative procedure for the preparation of
AnX+ consisting of the treatment of xanthone with anisole in
the presence of HY. Although, DR spectra of the samples
prepared in this way did show the formation of AnX+, IR spectra
were complicated by strong absorption bands in addition to those
associated with AnX+ cation. This indicates that some adventi-
tious, unidentified material has also been formed, making these
samples inappropriate for their photochemical study.
External versus Internal Location of AnX+ on the Zeolite
Particles. After the ship-in-a-bottle synthesis, the zeolites
contain a considerable amount of AnX+ corresponding to one
cation for every two-to-three supercages. We addressed the
possibility that a large portion of AnX+ could be located on
the outer surface of the particles and not within the micropores.
In heterogeneous catalysis a common, indirect procedure to
determine changes in the microporosity of the catalysts owing
to the presence of adsorbed, organic material is to use a test
reaction to compare the activity of the prepared solid with a
fresh sample of the same catalyst.32,33
Figure 4. (A) Aromatic region of the FT-IR spectra of AnX-HY (trace
a) prepared by ship-in-a-bottle synthesis from xanthylium and AnX-
MCM-41 (trace b) obtained by adsorbing AnXOH onto dehydrated
MCM-41. Spectra were recorded after outgassing at 200 °C under
dynamic vacuum (10-2 Torr) for 1 h. (B) IR spectra recorded under
the same conditions of of X+-HMor (trace a) and AnX+-MCM-41 (trace
b).
For this purpose we have chosen the acid-catalyzed dimer-
ization of R-methylstyrene (1), which is known to give rise to
the olefinic dimers 2 together with the indanic dimer 3 (Scheme
2).34-38 Other reaction mechanisms afford different dimers such
as 4 or 5 and 6 (Scheme 3).39-42 Thus, by merely determining
the product distribution, it is possible not only to estimate
changes in acid strength but also to assess if other reaction
mechanisms besides that involving a carbenium ion are inter-
vening.
of the samples prepared in this work. These DR spectra,
especially the long-wavelength, broad bands, are very different
from those corresponding to the precursor X+ in both solution
and zeolite.2
The aromatic region of the IR spectrum of AnX+ showed
many similarities with the parent X+ when adsorbed on zeolites.
However, there are appreciable shifts of the stronger absorption
bands, and the appearance of new bands of medium intensity
around 1500 cm-1. This is the main piece of evidence to rule
out the presence of some residual X+. As already mentioned
above, a definitive piece of evidence supporting the formation
of AnX+ within zeolites Y and â was the remarkable good
match between of the IR spectra of these samples with that
obtained after adsorption of AnXOH onto MCM-41 (Figure 4).
The background intensity increase at 1350 cm-1 and weak broad
absorption peaking at 1750 cm-1 observed in the spectrum of
AnX-HY (trace a in Figure 4A) are characteristic of the HY
matrix and are due to the intense Si-O and Al-O stretching
bands (1050 cm-1) and lattice overtones. For the sake of
Therefore, treatment of a solution of 1 in the presence of
zeolites containing AnX+ gave rise to the same product
distribution and at very similar initial rate and kinetics than the
original samples of zeolites (Scheme 2). This indicates that
the textural properties of the zeolites (surface acidity, mi-
croporosity) have not been substantially altered during the
preparation of AnX+. This would not be the case if AnX+
would have been located on the external surface of the particle,
blocking the pores or neutralizing the acid sites.43
It has to be remarked that according to Scheme 1 the actual
location of AnX+ depends to a large extent on the distribution
of X+ before the addition of anisole, since the relocation of the
much bulkier AnXH or AnX+ seems unlikely. In this sense
we have already presented experimental evidence supporting

Intrazeolite Photochemistry. 12
J. Phys. Chem., Vol. 100, No. 46, 1996 18149
Figure 5. Emission spectra upon 375 nm excitation of AnX+ in Hâ.
the internal location of X+ within the micropores of medium
and large pore zeolites.2
Photophysical Properties. Xanthylium ions, especially
9-aryl substituted, are among the best studied carbenium ions
with regards to their photophysical and photochemical prop-
erties.44-48 In solution they generally exhibit strong fluorescence
(ΦF between 0.55 and 0.88) with lifetimes from 1 to 50 ns22
while the triplet excited state is characterized by a large
extinction coefficient absorption below 300 nm accompanied
by a much less intense broad band in the 400-500 nm region
and bleaching of ground-state absorption.22,49 It has been found
that substitution at the 9-position of the tricyclic aromatic rings
does not substantially modify this general behavior.50 However,
AnX+ constitutes a remarkable exception in this family. Thus,
no fluorescence could be observed at room temperature in
solution,22 while the singlet lifetime estimated by indirect
measurements at 77 K in a mixture of H2SO4/CH3CN was found
to be unusually short (0.14 ns).51 These anomalies were
rationalized assuming a deactivation by intramolecular quench-
ing of the xanthylium moiety by the anisyl substituent occurring
in a twisted conformation.22 This type of deactivation pathway
requiring conformation mobility can be anticipated to be greatly
minimized by entrapment of AnX+ within the zeolite supercages
and by the anisotropy of the zeolitic host. Thus, perusal of
Figure 1 shows AnX+ must adopt a more frozen conformation
to fit within zeolite supercages. In fact for zeolite Y the
methoxy substituent is clearly located in one of the four openings
of the cage.
We have been able to observe room-temperature fluorescence
from AnX+ embedded within zeolites. Figure 5 shows the
fluorescence spectra of AnX+ in Hâ after 375-nm excitation.
The shape of the spectrum is very similar to that of other
xanthylium cations, whereas the confined environment is
manifested in the good spectral resolution. The fluorescence
lifetime was measured and found to be around 4 ns, which is
in the same range as previously reported fluorescence lifetimes
for other xanthylium ions in solution. This indicates that the
intramolecular quenching of the xanthylium moiety by the anisyl
substituent is less effective in the restricted environment
provided by the zeolite cage. A similar but weaker emission
was also observed for the sample AnX+-MCM-41 (prepared
by adsorbing AnXOH), although the lifetime was somewhat
shorter (∼1.3 ns), as expected from the increased freedom of
movement in this zeolite. This shows that the thwarting of the
nonradiative deactivation pathway is a general phenomenon of
the incorporation of AnX+ on solid surfaces and rules out the
possibility that trace amounts of X+ (absent in the AnX+-MCM-
41 preparation) were responsible for the fluorescence.
Figure 6. Transient reflectance spectra (∆J/J0) recorded 2 µs after
266 nm excitation of the AnX+ in Hâ (A) and adsorbed in MCM-41
(B).
The transient DR spectra of the AnX+ zeolite complexes were
found to be very similar to the transient DR spectra of the parent
X+ incorporated within the zeolite framework.2 The charac-
teristic features (Figure 6) include a large band in the visible
region (600 nm), corresponding to the triplet excited state of
the cation. Along with the 600 nm band the spectra show
reflectance at 300 nm which is also attributed to the triplet state
of the cation, while bleaching of the cation is observed at 370
nm. The decay of the 300 and 600 nm bands is accompanied
with an increase in reflectance at 370 nm due to the recovery
of AnX+ ground state. This feature, along with appreciable
oxygen quenching are consistent with the assignment of the 300
and 600 nm bands to the triplet state of the cation. The 600
nm band is twice as intense as the 300 nm band for AnX+-HY
and AnX+-Hâ. The transient spectrum of AnX+-MCM-41 also
exhibits these two absorptions (Figure 6), but in contrast with
the samples of AnX+ confined in more restricted spaces, the
extinction coefficient of the long-wavelength band is weaker
than that of the band centered at 300 nm. In fact, the transient
spectrum of AnX+-MCM-41 is remarkably similar to the T-T
absorption spectra for the triplet of many other xanthenium ions
in solution.49
In addition to the similar triplet spectra, we also observed a
small amount of reflectance at 340 nm upon excitation of the
cation.2 We attribute this reflectance to the AnX• generated by
electron transfer from a donor molecule and/or oxygens of the
framework to the excited AnX+. In solution, AnX• has also
been observed during laser flash photolysis work with AnX+,22
and the proposed mechanism for its formation was single
electron transfer (SET) from adventitious AnXOH to AnX+.
We think that in our case, a SET from electron-rich oxygens of
the framework to excited AnX+ is a more likely explanation.
Clearly, the unusual photophysical behavior of AnX+ has been
overcome by entrapment within zeolite cavities.

18150 J. Phys. Chem., Vol. 100, No. 46, 1996
Cano et al.
Conclusions
the liquid phase was finally analyzed by 1H NMR (CDCl3, 400
MHz Varian spectrometer). Identity of the products was
established after chromatographic isolation by comparison of
their spectroscopic properties with those previously reported.36
The composites containing 9-arylxanthenium ions were
analyzed by combustion chemical analysis using a Fisons EA
1108 CHNS-O elemental analyzer. Thermogravimetric profiles
were measured using a Netzsch-STA 409 EP thermobalance
under air flow. IR spectra were obtained at room temperature
using greaseless quartz cells fitted with CaF2 windows in a
Nicolet spectrophotometer controlled by a work station. Wafers
(10 mg) were pressed into disks and outgassed under 10-2 Torr
at 200 °C for 1 h before to record the spectra. The IR spectrum
of unsubstituted X+ ion in HMor corresponds to the same
sample prepared in ref 2. DR spectra were carried out using a
Shimadzu UV-2101 PC fitted with an integrating sphere setup
and using BaSO4 as standard.
Fluorescence spectra were recorded using a Perkin-Elmer LS-
50 luminescence spectrometer with a front-face attachment for
solids. Time-resolved fluorescence and diffuse reflectance
measurements were carried out using the third harmonic (355
nm; e10 ns, e2 mJ/pulse) from a Surelite Nd:YAG laser as
the excitation source. The signals from the monochromator/
photomultiplier detection system were captured by a Tektronix
Model 2440 transient digitizer and transmitted to a Power-
Macintosh that controlled the experiment and provided suitable
processing and storage capabilities. The software for data
acquisition and instrument control has been developed in the
LabView 3.1.1 environment from National Instruments. Fun-
damentals54,55 and details56 of similar time-resolved diffuse
reflectance laser setup have been reported elsewhere. The
samples were contained in cells constructed of 3 × 7 mm2 quartz
tubing and purged with nitrogen for 30 min before any laser
experiments were conducted.
We have provided spectroscopic evidence showing that AnX+
can be cleanly prepared by ship-in-a-bottle synthesis from the
parent unsubstituted X+, through an unprecedented reaction
mechanism that relies on the ease of hydride transfer on zeolite
surfaces. We have used the dimerization of R-methylstyrene
as a test reaction to establish that the textural and acid properties
of the zeolites particles are not substantially altered by the
preparation procedure or the presence of the organic material,
thus supporting that AnX+ is located within the zeolite super-
cages.
Incorporation of AnX+ within the large-pore zeolite frame-
work modifies the photophysical properties of this cation with
respect to solution by increasing the fluorescence lifetime at
least 30-fold, allowing its measurement at room temperature,
as well as to detect for the first time the triplet excited state.
This can be reasonably attributed to immobilization of AnX+
that hinders the deactivation mechanism occurring in solution.
These results combined with the ship-in-a-bottle synthesis of
AnX+ nicely show the opportunities that zeolites offer as solid
matrixes to control and modify the photochemistry of organic
carbenium ions.
Experimental Section
Zeolite HY was prepared starting from a commercial Na+
form (Union Carbide, SK-40) by three consecutive ion exchange/
calcination cycles using aqueous solutions of ammonium acetate
following the procedure previously reported.52 Zeolite â was
prepared by thermal decomposition of an as-synthesized tetra-
ethylammonium sample.53 Mesoporous MCM-41 was obtained
from amorphous silica (Aerosil 200, Degussa) using a 25%
aqueous mixture of tetraethylammonium hydroxide and hexa-
decyltrimethylammonium bromide following reported proce-
dures.14 Pore diameter was measured by Ar adsorption.
9-Xanthydrol, 9-phenylxanthydrol, and xanthone were com-
mercially available (Aldrich) and used as received, while 9-(4-
methoxyphenyl)-9-xanthydrol was prepared by Grignard reaction
of xanthone with 4-methoxyphenylmagnesium bromide follow-
ing the procedure previously reported.47
The time-resolved fluorescence measurements were carried
out using a Hamamatsu C-4334 streakscope coupled with a
spectrograph and capable of simultaneous spectral and time-
resolved data acquisition. Further details of the technique are
provided in part 15 in this series of papers. Molecular modeling
was performed using Insight II Molecular Modeling package
of programs using a Silicon Graphics work station.
Samples containing AnX+ were prepared by stirring at reflux
temperature a mixture of xanthydrol (50 mg) in CH2Cl2 (30
mL) and the corresponding dehydrated (550 °C, overnight)
zeolite (1.00 g) for 1 h. After this time, the suspension was
filtered and submitted to continuous solid-liquid extraction for
10 h with CH2Cl2 as solvent using a micro-Soxhlet equipment.
The resulting yellow zeolites were characterized by DR and IR
and subsequently reacted with neat anisole (5 mL) at reflux
temperature for 30 min. The solids were recovered, washed
with CH2Cl2, and thoroughly extracted (CH2Cl2) until no more
organic material could be observed in the liquid extracts. The
combined supernatants were analyzed by GC-MS (Hewlett-
Packard 5899A spectrometer). Similar procedures were fol-
lowed using benzene or toluene instead of anisole. Reactions
of xanthone (50 mg) with benzene derivatives (5 mL) in the
presence of activated zeolite (1.00 g) were carried out at reflux
temperature (5 h) and submitted to exhaustive solid-liquid
extraction. Finally all the samples were pump out under 10-1
Torr for 2 h.
Acknowledgment. Financial support by the Natural Sciences
and Engineering Research Council of Canada through an
operating Grant (J.C.S.) and Spanish DGICYT (HG, Project no
PB93-0380) are gratefully acknowledged. M.L.C. also thanks
to the Spanish Ministry of Education for a Grant. Rosa Torrero
is also thanked for her technical assistance in recording the IR
spectra. J.C.S. is the recipient of a Killam Fellowship awarded
by the Canada Council.
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