Dynamics of the transient species generated upon photolysis of diarylmethanes within zeolites - Deprotonation and oxidation reactions

Article abstract of DOI:10.1139/v05-208

The oxidation of diarylmethanes is a multistep process involving initial formation of a radical cation, deprotonation of the radical cation to the radical, and oxidation of the radical to the carbocation. The dynamics and efficiency of the last two steps in this process, namely deprotonation and oxidation, in acidic zeolites and non-acid zeolites are examined in the present work as a function of the acidity of the diarylmethane radical cations and the oxidation potential of the diarylmethyl radicals. Our results indicate that rate constants for deprotonation strongly depend on the acidity of the radical cations, but not on the composition of the zeolites. In addition, oxidation of the radicals to the diarylmethyl cations is strongly dependent on both the oxidation potential of the radicals and the oxidizing ability of the zeolite. This dependence allows oxidation potentials of the zeolites to be estimated.

Full text of DOI:10.1139/v05-208

1637
Dynamics of the transient species generated upon
photolysis of diarylmethanes within zeolites —
Deprotonation and oxidation reactions1
Suzanne Shea, Norman P. Schepp, Amy E. Keirstead, and Frances L. Cozens
Abstract: The oxidation of diarylmethanes is a multistep process involving initial formation of a radical cation,
deprotonation of the radical cation to the radical, and oxidation of the radical to the carbocation. The dynamics and ef-
ficiency of the last two steps in this process, namely deprotonation and oxidation, in acidic zeolites and non-acid
zeolites are examined in the present work as a function of the acidity of the diarylmethane radical cations and the oxi-
dation potential of the diarylmethyl radicals. Our results indicate that rate constants for deprotonation strongly depend
on the acidity of the radical cations, but not on the composition of the zeolites. In addition, oxidation of the radicals to
the diarylmethyl cations is strongly dependent on both the oxidation potential of the radicals and the oxidizing ability
of the zeolite. This dependence allows oxidation potentials of the zeolites to be estimated.
Key words: radical cations, carbocations, zeolites, laser flash photolysis.
Résumé : L’oxydation de diarylméthanes est un processus en plusieurs étapes qui implique la formation initiale d’un
cation radical, une déprotonation du cation radical et une oxydation du radical en carbocation. On a étudié la dyna-
mique et l’efficacité des deux dernières étapes de ce processus, soit la déprotonation et l’oxydation, dans des zéolites
acides et des zéolites non acides, en fonction de l’acidité des cations radicaux diarylméthanes et du potentiel
d’oxydation des radicaux diarylméthyles. Les résultats obtenus indiquent que les constantes de vitesse pour la déproto-
nation dépendent fortement de l’acidité des cations radicaux, mais pas sur la composition des zéolites. De plus,
l’oxydation des radicaux en cations diarylméthyles est fortement dépendante à la fois sur le potentiel d’oxydation des
radicaux et sur l’habilité des zéolites à oxyder. Cette dépendance permet d’évaluer les potentiels d’oxydation des zéo-
lites.
Mots clés : cations radicaux, carbocations, zéolites, photolyse éclair au laser.
[Traduit par la Rédaction] Shea et al. 1648
Introduction
well-suited for the generation of positively charged species
and in fact, carbocations can be thermally generated within
proton-exchanged zeolites (15). The redox properties of the
zeolite are highly dependent on the nature of the charge-
balancing cations (16), and it has been shown that proton-
exchanged zeolites are much more powerful oxidizing agents
than alkali cation-exchanged zeolites (17). These acidic sol-
ids have been the subject of extensive research because of
their catalytic ability and wide use as industrial catalysts,
particularly for the refinement of petroleum products (4, 18,
19).
Zeolites have been shown to be good hosts for the study
of cationic intermediates because of the stabilization pro-
vided by the internal electrostatic fields within the voids of
the microporous solids and also by the confined environ-
ment, which provides protection of the cationic center from
Zeolites are crystalline aluminosilicate materials com-
prised of SiO4 and AlO4 tetrahedra that are arranged in a
–
three-dimensional array to form an open framework struc-
ture that consists of pores and channels whose size and
shape vary widely (1–6). The presence of tetravalent alumi-
num gives rise to a net negative charge in the backbone,
which must be balanced with a charge-balancing cation, typ-
ically alkali metals or protons. While the void cavities and
channels of the zeolite are capable of accommodating mo-
lecular guests, zeolites are not simply inert vessels, but have
been shown to participate as electron donors and acceptors
in both thermal (7, 8) and photoinduced (9–14) electron
transfer reactions. The high density of negative charge on
the zeolite framework is thought to provide an environment
Received 4 May 2005. Published on the NRC Research Press Web site at http://canjchem.nrc.ca on 8 December 2005.
S. Shea, N.P. Schepp,2 A.E. Keirstead, and F.L. Cozens.3 Department of Chemistry, Dalhousie University, Halifax, NS B3H 4J3,
Canada.
1This article is part of a Special Issue dedicated to organic reaction mechanisms.
2Corresponding author (e-mail: nschepp@dal.ca).
3Corresponding author (e-mail: fcozens@dal.ca).
Can. J. Chem. 83: 1637–1648 (2005)
doi: 10.1139/V05-208
© 2005 NRC Canada

1638
Can. J. Chem. Vol. 83, 2005
CR2
-e-
CR2
CR2OH
CR2H
MY
CR2H
[1]
H2O
(-H+)
-H+
-e-
R
R
R
H
R
R
hν
266 nm
MY
H
H
H
H
[2]
-e-
-H+
R1
An2CH2 : R1 = R2 = OCH3
AnPhCH2 :
R2
R1
R2
R1
R2
An2CH•
+•
+•
+•
+•
An2CH2
AnPhCH2
Tol2CH2
Ph2CH2
R1 = H, R2 = OCH3
AnPhCH•
Tol2CH•
Tol2CH2 : R1 = R2 = CH3
Ph2CH2 : R1 = R2 = H
Ph2CH•
nucleophilic species (11, 20, 21). When a positively charged
species is encapsulated within these solids, the whole lattice
of the zeolite can act as a counteranion, insulating and pro-
tecting the carbocation from attack by external nucleophiles.
This stabilization has been found to be dramatic in many
cases in which the tight fit between the organic guest and
the zeolite occurs (22). These characteristics, combined with
the wide range of crystalline structures available and the
ability to tailor the zeolite to the reaction of interest, have
made zeolites the host of choice for many studies of organic
cations (9, 17, 23). Organic radical cations formed within
the zeolite framework have also been the subject of much in-
terest. Photoionization reactions leading to radical cation
formation have been shown to be easily carried out within
zeolite frameworks, making zeolites convenient host materi-
als to study the photoinduced formation of radical cations
(10, 21).
It has long been known that arylmethanes can be oxidized
when incorporated into acidic zeolites (17, 24, 25). The
mechanism for this reaction is thought to involve initial one-
electron oxidation to the arylmethane radical cation that then
deprotonates to yield the corresponding arylmethyl radical.
A second one-electron oxidation can occur to convert the
radical to the carbocation, which is then trapped by water to
give the arylmethyl alcohol (eq. [1]).
ions depends both on the oxidation potential of the radical
and the oxidizing ability of the zeolite.
Results
Dianisylmethane (An2CH2) in NaY, KY, CsY, and HY
Laser flash photolysis (266 nm) of dianisylmethane in
NaY under vacuum conditions resulted in the transient dif-
fuse reflectance spectra shown in Fig. 1a. These spectra
show the presence of absorption at 450 nm formed promptly
within the laser pulse. With a time resolution of ca. 100 ns
for the diffuse reflectance laser system, prompt formation of
the band at 450 nm indicates that it was generated with a
rate constant >107 s–1. The band at 450 nm was not stable
and decayed close to baseline (inset, Fig. 1a) over the 40 µs
timescale of the experiment. Nonlinear least-squares analysis
using a first-order rate expression4 gave a rate constant (k =
1.2 × 105 s–1) for the decay of the 450 nm band. After satu-
ration of the zeolite with molecular oxygen, the band at
450 nm was still observed (Fig. 1b) and its decay remained
unchanged. In solution, the dianisylmethane radical cation
(An2CH2·+) is known to have an absorption maximum at
450 nm and to be unaffected by oxygen (29, 30). Thus, the
·+
transient at 450 nm is assigned to An2CH2 (eq. [2], R1 =
R2 = OCH3).
While several studies have appeared concerning the iden-
tification of radical cations and carbocations upon incorpora-
tion of arylmethane derivatives in acidic zeolites (26–28),
little is known about the absolute kinetics of the individual
reactions involved in the overall oxidation process. In addi-
tion, quantitative information regarding the effect of the acidity
of the arylmethane radical cations and the oxidation poten-
tial of the arylmethyl radicals on the dynamics of these pro-
cesses is not currently available. In the present work, we
examine the intrazeolite behaviour of a series of diarylmeth-
anes incorporated into both proton-exchanged (HY) and al-
kali cation-exchanged (NaY, KY, CsY) zeolites. Our results
show that the dynamics of the deprotonation of the diaryl-
methane radical cations to diarylmethyl radicals strongly de-
pends on radical cation acidity, and that oxidation of the
diarylmethyl radicals to the corresponding diarylmethyl cat-
The radical cation is presumably formed by rapid photo-
ionization of the incorporated dianisylmethane. Consistent
2+
with this mechanism is the observation of short-lived Na3
clusters at wavelengths greater than 510 nm (Fig. 1a) pro-
duced when electrons are trapped by the counterbalancing
sodium cations (10, 13, 31–33).
·+
As An2CH2 decayed, a new transient species with a sharp
absorption band at 340 nm grew in with a rate constant of
1.4 × 105 s–1. Under vacuum conditions the transient respon-
sible for this absorption did not decay over the timescale of
the experiment, while the transient had a very short lifetime
in the presence of oxygen (Fig. 1b). This transient is attrib-
uted to the dianisylmethyl radical (An2CH·), which in solu-
tion is known to have an absorption maximum at 340 nm
(34) and to react rapidly with oxygen. Radical formation
likely occurs by deprotonation (vide infra) of the initially
4 See the Experimental section for explanation of data analysis.
© 2005 NRC Canada

Shea et al.
1639
Fig. 1. Transient diffuse reflectance spectra recorded 400 ns (᭹), 4.00 µs (᭺), 18.8 µs (᭿), and 32.0 µs (ٗ) after 266 nm laser flash
photolysis of An2CH2 in: (a) NaY under vacuum conditions; (b) NaY under oxygen conditions; (c) KY under vacuum conditions;
(d) CsY under vacuum conditions; (e) HY under vacuum conditions; and ( f ) HY under oxygen conditions. Insets show kinetic traces
collected at 340 nm (᭹), 440 nm (᭺), and 500 nm (᭿).
0.30
(b)
NaY, O2
(a)
NaY, vac
0.25
0.20
0.15
0.10
0.05
0.00
0
10 20
50
Time 3(0µs)40
0
10 20 30 40 50
Time (µs)
300
700
400
500
600
700
300
400
500
Wavelength (nm)
600
Wavelength (nm)
0.20
0.15
0.10
0.05
0.00
(d)
CsY, vac
(c)
KY, vac
0
10 20 30 40 50
0
10 20 30 40 50
Time (µs)
Time (µs)
300
400
(e)
500
600
700
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
0.20
0.15
0.10
0.05
0.00
(f)
HY, O2
HY, vac
0
30
50
10 Ti2m0 e (µs)40
0
10 20 30 40 50
Time (µs)
300
700
400
500
Wavelength (nm)
600
700
300
400
500
Wavelength (nm)
600
formed radical cation (eq. [2]), which is typically the domi-
nant reaction of the An2CH2 in solution (29, 30). This
ser pulse, a small fraction were generated more rapidly
within the laser pulse with a rate constant >107 s–1.
·+
mechanism for radical formation is supported by the similar-
A fourth transient species with strong absorption at
500 nm is also present in the spectra in Fig. 1a. This tran-
sient was long-lived and did not decay under both vacuum
and oxygen-saturated conditions over the longest timescale
available on our laser system (10 ms). The location of the
absorption band and the lack of sensitivity toward oxygen
are consistent with the identification of the 500 nm transient
as the dianisylmethyl cation (An2CH+) (34).
·+
ity between the rate constants for the decay of An2CH2 at
450 nm (1.2 × 105 s–1) and the growth of the radical at
340 nm (1.4 × 105 s–1).
Close examination of the spectral data in Fig. 1a also
shows some prompt absorption at 340 nm due to An2CH·
prior to the time-resolved growth. Thus, while most of the
radicals were formed in a time-resolved manner after the la-
© 2005 NRC Canada

1640
Can. J. Chem. Vol. 83, 2005
Fig. 2. Transient diffuse reflectance spectra recorded 400 ns (᭹),
4.00 µs (᭺), 18.8 ␮s (᭿), and 32.0 µs (ٗ) after 266 nm laser
flash photolysis of AnPhCH2 in: (a) NaY under vacuum condi-
tions; and (b) HY under vacuum conditions. Insets show kinetic
traces collected at 340 nm (᭹), 430 nm (᭺), and 460 nm (᭿).
Like An2CH·, whose formation had a small prompt com-
ponent and a larger time-resolved component, the formation
of An2CH+ under vacuum conditions took place over two
different time regimes. In this case, An2CH+ was mostly
formed promptly within the laser pulse with a rate constant
>107 s–1, with the time-resolved component being compara-
tively small. The rate constant for the growth was measured
to be 1.5 × 105 s–1, which is similar to the rate constant for
the decay of An2CH2·+. Under oxygen, both the prompt and
time-resolved components of carbocation formation were still
readily observable, but with substantially reduced intensity.
The results obtained upon laser flash photolysis of
dianisylmethane in KY under vacuum conditions (Fig. 1c)
0.30
(a)
0.25
0.20
0
10 20 30 40 50
0.15
0.10
0.05
0.00
Time (µs)
·+
were similar to those obtained for zeolite NaY. An2CH2
with absorption at 450 nm was formed promptly within the
laser pulse and then decayed with a rate constant of 1.3 ×
105 s–1. Concomitant with this decay was a growth (k =
1.4 × 105 s–1) at 350 nm attributed to the formation of
An2CH·. The long-lived An2CH+ was also observed at
500 nm and exhibited both prompt formation and a small
amount of time-resolved growth (k = 0.9 × 105 s–1), similar
to that seen in zeolite NaY.
300
400
500
600
700
Wavelength (nm)
·+
0.25
0.20
0.15
0.10
0.05
0.00
In CsY, both An2CH2 and An2CH· were again observed
(b)
(Fig. 1d). The decay of the radical cation and the growth of
the radical took place with similar rate constants of 1.4 ×
105 s–1. However, in this case, no distinct absorption at
500 nm was evident, indicating that An2CH+ was not gener-
ated in this zeolite.
In the acidic zeolite HY, laser irradiation of dianisylmeth-
ane under hydrated vacuum conditions led to the formation
of An2CH2·+ (Fig. 1e), which decayed with a rate constant of
1.2 × 105 s–1 that is similar to the rate constants previously
measured in the alkali metal-exchanged Y zeolites. The de-
0
10 20 30 40 50
Time (µs)
·+
cay of An2CH2 did not lead to a significant buildup of the
radical An2CH· at 350 nm. Instead, absorption at 510 nm
owing to An2CH+ grew in with a rate constant of 0.9 ×
105 s–1. Some An2CH· was formed promptly within the laser
pulse. The radical was unusually reactive in HY and decayed
with a rate constant of 1.1 × 105 s–1, similar to the decay of
300
400
500
600
700
Wavelength (nm)
·+
An2CH2 and the growth of An2CH+.
was therefore carried out using a double first-order expres-
sion that led to a rate constant of 1.4 × 105 s–1 for the decay
of AnPhCH2·+. The spectra also feature an absorption band
at 340 nm due to the anisylphenylmethyl radical (AnPhCH·)
(29, 34). The radical grew in with a rate constant of 1.8 ×
105 s–1. This is similar to the rate constant for the decay of
AnPhCH2·+, indicating that the radical was formed by
The addition of oxygen to the dianisylmethane–HY com-
posite did not significantly alter the transient diffuse
·+
reflectance spectra (Fig. 1f), which again showed An2CH2
decaying to give An2CH+ at 510 nm. The rate constants for
decay of the radical cation and growth of the carbocation
were not influenced by the presence of oxygen; however, the
amount of An2CH+ formed was reduced to approximately
half that observed under vacuum conditions.
·+
deprotonation of AnPhCH2 (eq. [2], R1 = H, R2 = OCH3).
Significant absorption was present at 340 nm immediately
after the laser pulse. Thus, some prompt radical formation
did occur prior to the time-resolved component.
Anisylphenylmethane (AnPhCH2) in NaY, KY, CsY, and
HY
After complete decay of AnPhCH2·+, considerable absorp-
tion remained with a maximum at 460 nm. This matches
well with the absorption spectrum of the anisylphenylmethyl
cation (AnPhCH+) (29). Due to the overlap in absorption of
The transient diffuse reflectance spectra obtained upon
266 nm laser irradiation of AnPhCH2 in NaY under vacuum
conditions showed the prompt formation of a transient with
strong absorption at 440 nm (Fig. 2a) assigned to the anisyl-
phenylmethane radical cation (AnPhCH2·+) (29). The radical
cation was not stable, but its decay was difficult to quantify
because of the presence of residual absorption at 460 nm
(vide infra) that also decayed slowly over the timescale of
the experiment, as well as the presence of absorption due to
Na+-electron complexes. Analysis of the decay at 440 nm
·+
AnPhCH2 and AnPhCH+, it is difficult to determine if
AnPhCH+ was generated promptly within the laser pulse, or
if its formation followed the decay of AnPhCH2·+. Signifi-
cant prompt formation likely did occur, as suggested by the
shoulder at 460 nm in the diffuse reflectance spectrum re-
corded immediately after the laser pulse (Fig. 2a). The addi-
tion of oxygen to the sample did not significantly affect the
© 2005 NRC Canada

Shea et al.
1641
decay of the band at 440 nm, but the relative amount of re-
sidual absorption at 460 nm was reduced (see Supplemental
material).5 Absorption due to AnPhCH· at 340 nm was
quenched upon addition of oxygen to the sample, which is
consistent with the known reactivity of carbon-centered radi-
cals.
The ditolylmethane radical cation (Tol2CH2·+) has not
been observed in solution, but its absorption spectrum
should be similar to the 1,4-dimethylbenzene radical cation,
which has a weak absorption band at 450 nm together with a
stronger band at 305 nm in acetonitrile (35). A broad ab-
sorption centered at 460 nm is evident in Fig. 3a. However,
this band is not likely due to the formation of Tol2CH2·+. In
particular, the decay at 460 nm did not lead to the formation
of the radical at 340 nm as would be expected if the band at
460 nm was due to Tol2CH2·+. In addition, the absorption
band at 460 nm changed under oxygen conditions to give a
transient with a narrow absorption band at 470 nm. The lo-
cation and shape of this band is not consistent with the
known spectrum of the Tol2CH2·+, but does agree well with
the presence of the ditolylmethyl cation (Tol2CH+) (34).
Thus, the broad absorption at 460 nm under vacuum condi-
tions is most likely due to prompt formation of Tol2CH+, to-
gether with an oxygen sensitive transient such as sodium
clusters (21) or the triplet excited state of ditolylmethane,
which has broad absorption from 330 to 530 nm (36, 37).
Results similar to those described were obtained upon
laser photolysis of AnPhCH2 in both KY and CsY under
vacuum conditions. The transient diffuse reflectance spectra
·+
again featured absorption of AnPhCH2 that decayed on the
timescale of the experiment and a band at 340 nm owing to
AnPhCH· that exhibited a time-resolved growth. The rate
·+
constants for the decay of AnPhCH2 and the growth of
AnPhCH· in both KY and CsY were very similar (Table 1).
The only major difference in the results obtained in these
two zeolites was that substantial absorption at 460 nm due to
the AnPhCH+ was observed after the decay of AnPhCH2·+ in
·+
KY, while AnPhCH2 decayed to the baseline in CsY. Thus,
AnPhCH+ was generated in KY, but not in CsY.
In the acidic zeolite HY under hydrated vacuum condi-
tions, the spectrum immediately after the laser pulse con-
sisted of a strong band at 450 nm and a weaker band 340 nm
(Fig. 2b). The band at 340 nm was quenched by oxygen and
can be attributed to AnPhCH·. However, unlike the situation
in the non-acidic zeolites where both prompt and time-
resolved components were observed for the formation of
AnPhCH·, only the prompt component was observed in HY.
In addition, AnPhCH· was not stable in HY and decayed
with a rate constant of 1.2 × 105 s–1.
·+
The inability to detect Tol2CH2 suggests that it is very un-
stable in NaY and undergoes deprotonation within the dura-
tion of the laser pulse to rapidly generate Tol2CH·.
·+
The absence of absorption due to Tol2CH2 was also evi-
dent in the spectra generated upon irradiation of ditolyl-
methane in KY. Only prompt formation of a sharp band at
340 nm owing to Tol2CH· was observed, together with broad
absorption due to trapped electrons or triplet ditolylmethane.
In this zeolite, no substantial absorption due to Tol2CH+ at
470 nm was apparent. Thus, Tol2CH+ was not generated in
KY. Results obtained in CsY were very similar to those in
KY: only the prompt formation of Tol2CH· was observed,
with no evidence for the presence of Tol2CH2·+ or the forma-
tion of Tol2CH+.
The absorption band at 450 nm did not decay, but instead
increased in intensity as a function of time with a rate con-
stant of 0.9 × 105 s–1 and shifted to 460 nm. These results in-
·+
dicate that both AnPhCH2 and AnPhCH+ were formed
promptly within the laser pulse to give a composite band
with a maximum at 450 nm. The small decrease in absorp-
tion at 420 nm on the side of the initially formed 450 nm
composite band was presumably due to the decay of
AnPhCH2·+. The time-resolved increase leading to the final
spectrum with a maximum at 460 nm is consistent with an
additional time-resolved component for the formation of
AnPhCH+.
Upon the addition of oxygen to the sample, the absorption
spectrum immediately after the laser pulse showed a strong
band at 450 nm due to AnPhCH+. A distinct shoulder at
410–430 nm was also evident, presumably because of
AnPhCH2·+. Under these conditions, there was no time-
resolved growth of the carbocation at 450 nm; only a small
decay due to the disappearance of AnPhCH2·+ was observed.
Both Tol2CH· and Tol2CH+ were readily observed upon
laser irradiation of Tol2CH2 in HY (Fig. 3b). The absorption
at 470 nm due to Tol2CH+ under vacuum conditions con-
sisted of a large prompt component as well as a small time-
resolved component. The rate constant for this delayed com-
ponent was determined to be 0.9 × 105 s–1. Tol2CH· absorp-
tion at 340 nm was completely formed promptly within the
laser pulse, with no time-resolved growth component. The
radical was long-lived, but did show some decay over the
timescale of the experiment. This decay could not be reli-
ably analyzed because of the large residual absorption re-
maining 40 µs after the laser pulse. The time-resolved
component of Tol2CH+ formation, together with absorption
due to Tol2CH·, was quenched under oxygen-saturated con-
ditions, but prompt carbocation formation was still observed.
Ditolylmethane (Tol2CH2) in NaY, KY, CsY, and HY
The transient diffuse reflectance spectra obtained upon la-
ser irradiation of ditolylmethane in NaY under vacuum con-
ditions (Fig. 3a) clearly show a sharp absorption band at
340 nm formed promptly within the laser pulse. This tran-
sient species, which reacted rapidly with oxygen, can be
identified as the ditolylmethyl radical (Tol2CH·) (34).
Diphenylmethane (Ph2CH2) in NaY, KY, CsY, and HY
The time-resolved diffuse reflectance spectra obtained
upon laser flash photolysis (266 nm) of diphenylmethane in
each of the four zeolites (NaY, KY, CsY, and HY) under vac-
uum conditions showed a strong absorption band centered at
330 nm (Figs. 4a (NaY) and 4b (HY)) because of the forma-
5 Transient diffuse reflectance spectra of AnPhCH2, Tol2CH2, and Ph2CH2 in NaY and HY under oxygen conditions and in KY and CsY un-
der vacuum conditions are provided. Supplementary data for this article are available on the Web site or may be purchased from the Deposi-
tory of Unpublished Data, Document Delivery, CISTI, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. DUD 4051. For
more information on obtaining material refer to http://cisti-icist.nrc-cnrc.gc.ca/irm/unpub_e.shtml.
© 2005 NRC Canada

1642
Can. J. Chem. Vol. 83, 2005
Fig. 3. Transient diffuse reflectance spectra recorded 400 ns (᭹),
4.00 µs (᭺), 18.8 µs (᭿), and 32.0 µs (ٗ) after 266 nm laser
flash photolysis of Tol2CH2 in: (a) NaY under vacuum condi-
tions; and (b) HY under vacuum conditions. Insets show kinetic
traces collected at 340 nm (᭹) and 470 nm (᭿).
0.30
(a)
0.25
0.20
0
10 20
40 50
Time3(0µs)
0.15
0.10
0.05
0.00
300
400
500
Wavelength (nm)
600
700
(b)
0.15
0.10
0.05
0.00
0
10 20 30 40 50
Time (µs)
300 350 400 450 500 550 600 650 700
Wavelength (nm)
tion of the diphenylmethyl radical (Ph2CH·) (34, 38). No ab-
·+
sorption due to Ph2CH2 or Ph2CH+ was evident in any of
the four zeolites examined.
The transient diffuse reflectance spectra obtained in NaY
and KY did include broad absorption stretching from ca. 400
to 700 nm that can be attributed to trapped electrons. In ad-
dition, an unexpected transient with absorption at 530 nm
was observed in both KY and CsY. The transient species re-
sponsible for this absorption was quenched by oxygen. The
absorption at 530 nm is not consistent with the presence of
the diphenylmethane radical cation (λmax for toluene radical
cation ≈ 430 nm) (35) or the diphenylmethyl cation (λmax
=
435 nm) (34), but may be due to the diphenylmethane radi-
cal anion (λmax = 530 nm) (39) generated by trapping of a
photoionized electron by diphenylmethane, or by direct elec-
tron transfer from the framework to excited diphenylmethane.
Discussion
Generation and deprotonation of diarylmethane radical
cations
It is well-established that radical cations of organic mate-
© 2005 NRC Canada

Shea et al.
1643
Fig. 4. Transient diffuse reflectance spectra recorded 400 ns (᭹),
4.00 µs (᭺), 18.8 µs (᭿), and 32.0 µs (ٗ) after 266 nm laser
flash photolysis of Ph2CH2 in: (a) NaY under vacuum condi-
tions; and (b) HY under vacuum conditions. Insets show kinetic
traces collected at 340 nm (᭹).
radical generation occurred by deprotonation of the radical
cations. Some prompt formation of An2CH· and AnPhCH·
was observed upon irradiation of An2CH2 and AnPhCH2 in
the alkali-metal-exchanged zeolites. This suggests that radi-
cal cations generated by photoionization exist in at least two
sites within the zeolites that exhibit considerably different
deprotonation abilities (40, 41). One of these sites induces
rapid deprotonation that leads to prompt radical growth,
while the other induces a slower deprotonation.
0.24
(a)
0.18
It has previously been demonstrated that rate constants for
deprotonation of arylmethyl radical cations in solution in-
versely correlate with thermodynamic stability of the radical
cations (29). Thus, stabilized arylmethyl radical cations, such
as those bearing electron-donating methoxy groups on the
aromatic ring, undergo deprotonation reactions more slowly
than nonstabilized radical cations. The relatively slow depro-
0.12
0.060
0.0
0
10 20 30 40 50
Time (µs)
·+
tonation recorded in the present work for An2CH2 and
·+
AnPhCH2 radical cations compared to the fast deproto-
·+
·+
nation for Tol2CH2 and Ph2CH2 therefore agree well with
expectations based on the relative stability of these radical
cations.
300
400
500
600
700
Wavelength (nm)
The absolute rate constants for the deprotonation reaction
can in some cases be compared with those measured in solu-
tion. In particular, the rate constant for the conversion of
0.20
0.15
0.10
0.05
0.00
(b)
·+
An2CH2 to the dianisylmethyl radical in acetonitrile is ap-
proximately 2.5 × 105 s–1 (29), a value that is slightly greater
than the rate constant of 1.1 × 105 s–1 in NaY. While com-
peting processes may contribute to the decay of the radical
cation, the differences in the rate constants from solution to
NaY suggest that deprotonation in NaY is somewhat slower
than in acetonitrile. It has also been demonstrated that the
0
10 20 30 40 50
Time (µs)
·+
lifetime of An2CH2 in acetonitrile is very similar to the
·+
lifetime of AnPhCH2 (29). The small difference measured
in the present work between the decays of these two radical
cations in zeolites is thus consistent with results obtained in
solution.
300
400
500
600
700
Wavelength (nm)
While rate constants for the deprotonation of Tol2CH2 and
Ph2CH2 radical cations have not been measured, previous re-
sults (42) have shown that the 1,4-dimethylbenzene radical
cation is almost four orders of magnitude more reactive than
the 4-methylanisole radical cation. Thus, replacing a
methoxy group with a methyl group greatly decreases the
lifetime of the radical cation. This is consistent with our in-
ability to observe neither the ditolylmethane nor the
diphenylmethane radical cations in the present work.
rials incorporated within zeolites can be generated by direct
irradiation of the precursor with the zeolite framework act-
ing as the electron acceptor (10, 21). The results obtained in
the present work provide either direct or indirect evidence
that each of the four diarylmethanes do undergo photoioni-
zation within the cation-exchanged Y-zeolites. For An2CH2
and AnPhCH2, direct evidence supporting photoionization
comes from the observation of the absorption bands associ-
ated with their radical cations. For Tol2CH2 and Ph2CH2, the
radical cations were not observed. However, prompt forma-
tion of the corresponding diarylmethyl radicals was observed,
and the presence of these species can most reasonably be ex-
plained by initial formation of short-lived radical cations
generated by photoionization of the neutral precursors, fol-
lowed by rapid deprotonation (>107 s–1) within the time du-
ration of the laser pulse.
Diarylmethyl radicals were also observed upon direct
266 nm irradiation of An2CH2 and AnPhCH2, but with these
two substrates, most of the observed radicals were formed in
a time-resolved manner with rate constants that ranged from
1.3 × 105 to 1.8 × 105 s–1 in NaY, KY, and CsY. These rate
constants quite closely matched the rate constants for the de-
cay of the radical cations, which provides good evidence that
·+
For An2CH2 and AnPhCH2·+, which could be observed
in all of the zeolites, the nature of the counterbalancing cat-
ion had little influence on the rate constant for the conver-
sion of the radical cation to the radical. Thus, the increased
Brønsted basicity associated with zeolites (11) as the
counterion is changed from Na+ to Cs+ is not manifested in
the radical cation decay kinetics, despite the evidence that
deprotonation of the radical cations is a significant process.
In the strongly acidic HY zeolite, the diarylmethyl radi-
cals that would be formed by deprotonation of diaryl-
methane radical cations should rapidly reprotonate, thus
effectively shifting the radical cation – radical acid–base
equilibrium to the radical cation side. Under these circum-
stances, the radical cations would be expected to have con-
siderably longer lifetimes in HY compared with the non-
·+
acidic zeolites. Instead, An2CH2 decayed in HY with a rate
© 2005 NRC Canada

1644
Can. J. Chem. Vol. 83, 2005
H
H
H
H
+
[3]
-e-
-H+
R1
R2
R1
R2
R1
R2
constant that was very similar to its decay in the non-acidic
alkali-metal-exchanged zeolites. Furthermore, no increase in
Second, under oxygenated conditions, the time-resolved
growth of An2CH+ was almost completely quenched in NaY,
and was considerably reduced in HY. Similarly, the delayed
growth of AnPhCH+ and Tol2CH+ observed in evacuated HY
was no longer present under oxygen-saturated conditions. In
addition, while prompt formation of An2CH+, AnPhCH+,
and Tol2CH+ in NaY continued to be observed in the pres-
ence of oxygen, there was a general reduction in intensity
compared with non-oxygenated conditions. Thus, since
carbocations themselves are not directly influenced by the
presence of oxygen, the quenching effect of oxygen strongly
suggests that the precursor to the carbocation is a species
such as a radical or triplet that is reactive toward oxygen.
One mechanism for carbocation formation that agrees
well with many of the trends and observations previously
outlined involves zeolite-induced oxidation of the radical
generated upon deprotonation of the diarylmethane radical
cations (eq. [3]).
This mechanism is nicely consistent with the effect of
changing counterion and substrate structure on the yield of
the carbocations. In particular, the oxidizing ability of Y
zeolites follows the order HY > NaY > KY > CsY (16), and
this order matches the yield of time-resolved carbocation
formation as a function of counterion. Furthermore, An2CH+
was generated under milder conditions than Tol2CH+, while
Ph2CH+ was not detected even in the highly oxidizing HY
zeolite. Since the oxidation potentials of the radicals in-
crease in the order An2CH· < AnPhCH· < Tol2CH· < Ph2CH·
(Table 2), the mechanism in eq. [3] agrees well with the ef-
fect of substrate structure on carbocation formation.
In addition, the efficiency of carbocation formation was
substantially diminished upon addition of oxygen to the zeo-
lite samples. Since neither the radical cations nor the
carbocations are sensitive to oxygen, while the diarylmethyl
radicals are quenched by oxygen, the observed decrease in
the formation of the carbocation under oxygen conditions is
consistent with the radicals being their immediate precursor.
If carbocation formation did take place via oxidation of
the radical, a correlation would normally be expected
between the decay of the radical and the growth of the
carbocation. The current results show that such correlations
are sometimes present: the radicals (An2CH·, AnPhCH·, and
Tol2CH·) in HY did decay as their corresponding
carbocations grew in. However, a more typical correlation
observed in the present work is that between the decays of
the radical cations and the growths of the carbocations, par-
ticularly for the formation of An2CH+ in NaY, KY, and HY.
As well, the radicals (An2CH·, AnPhCH·, and Tol2CH·) ob-
served in NaY under vacuum conditions were long-lived,
and were not converted to their corresponding carbocations
over the timescale of the experiments.
·+
·+
the lifetimes of Tol2CH2 and Ph2CH2 was evident in HY,
as these radical cations were still too short lived to be ob-
served. The continued rapid decay of these radical cations,
particularly in the case of An2CH2·+, is likely due to rapid
oxidation of the diarylmethyl radicals in HY, which does not
allow reprotonation of the radical to take place. This is dis-
cussed in further detail in the following.
Diarylmethyl cation formation
To discuss the mechanism for carbocation formation, it is
useful to first summarize some relevant results and establish
some revealing trends. First, in evacuated zeolites, formation
of the diarylmethyl cations took place over one or two dis-
tinct time regimes, depending on the identity of the
counterion and the structure of the substrate. A substantial
fraction of An2CH+ in NaY and KY was formed promptly
within the laser pulse with a rate constant >107 s–1, and a
smaller fraction was generated more slowly with rate con-
stants similar to those for the decay of the radical cation and
the growth of the radical. In HY, only time-resolved forma-
tion of An2CH+ was observed, while this carbocation was
not detected in CsY. For AnPhCH+, overlapping absorption
of the radical cation and the carbocation made it difficult to
evaluate the fraction of carbocation derived from prompt and
time-resolved processes, but time-resolved formation was
distinctly observed in HY. Only prompt formation of
Tol2CH+ was observed in NaY, but a small amount of time-
resolved formation was present in HY. No carbocation was
observed upon irradiation of Ph2CH2 in any of the zeolites.
Since the diphenylmethyl cation generated by other means is
quite long-lived in HY (41), its absence upon irradiation of
Ph2CH2 in HY in the present experiments suggests that it is
not formed.
The results outlined above reveal several important trends
about the conditions needed for delayed formation of the
carbocations. First, in the non-acidic zeolites, only the most
stable carbocation (An2CH+) showed time-resolved forma-
tion. In addition, this delayed formation of An2CH+ was ob-
served in NaY and KY, but not in CsY. In the acidic HY,
three carbocations (An2CH+, AnPhCH+, and Tol2CH+)
showed delayed formation, but the efficiency decreased upon
going from the more stable An2CH+ to the less stable
Tol2CH+. Thus, these trends show that the presence of de-
layed carbocation formation depends both on the stability of
the carbocation and the nature of the counterion.
Trends concerning prompt carbocation formation can also
be extracted from the results described above. Prompt
carbocation formation was observed in NaY for An2CH+ and
Tol2CH+ (and likely AnPhCH+), but not for the less stable
Ph2CH+. However, in KY only An2CH+ and AnPhCH+
showed evidence for prompt carbocation formation. Thus,
prompt carbocation formation also depends both on the sta-
bility of the carbocation and the nature of the counterion.
These results can be rationalized on the assumption that,
in the inhomogeneous environment of the zeolites (40), the
radicals may initially have been generated in or near sites
that have substantially different oxidizing abilities. Thus, in
© 2005 NRC Canada

Shea et al.
1645
Table 2. Summary of time-resolved diarylmethyl cation (C+) formation in Y zeolites by oxidation of diarylmethyl radicals, and oxida-
tion potentials (vs. SCE in acetonitrile) for the diarylmethyl radicals.
Eox (R·/R+ vs.
SCE)
HY (Ered
≈
NaY (Ered
≈
KY (Ered
≈
CsY (Ered <
Radical
0.23–0.35 V)a
0.04–0.16 V)a
0.04–0.16 V)a
0.04 V) a
-e-
-e-
-e-
-e-
H
H
≈0.04b
0.16c
0.23d
0.35d
Time resolved
Time resolved
Time resolved
No C+
No C+
No C+
No C+
An An
An An
H
H
Time resolved
Time resolved
No C+
Only prompt
Only prompt
No C+
Only prompt
No C+
An Ph
An Ph
H
H
Tol Tol
Tol Tol
H
H
No C+
Ph
Ph
Ph
Ph
Note: Also included are estimates of reduction potentials of zeolites (vs. SCE, see text).
aEstimated as described in the text (vs. SCE).
bOxidation potential not available. However, the oxidation potential for An2C+CH3 has been measured as Eox = –0.06 V vs. SCE in acetonitrile (43); re-
placing the methyl group in An2C+CH3 with hydrogen to give An2CH+ should increase the oxidation potential by ca. 0.10 (44), leaving an oxidation poten-
tial for An2CH+ of ca. 0.04.
cReference 45.
dReference 46.
some cases, the radicals generated by deprotonation were
created in a nonoxidizing environment and were not rapidly
converted to the carbocations. These radicals may slowly
migrate to an oxidizing site and eventually be oxidized to
the carbocation, but this process would not be observed over
the 40 µs timescale of our experiments, and the radicals
would appear as long-lived. In other cases, the radicals were
generated in an environment where they were oxidized as
rapidly as they were formed. Under these conditions, depro-
tonation of the radical cations would be the rate-determining
step, and the growth of the carbocations would match the
decay of the radical cations.
The rapid oxidation of the radicals appeared to reach an
extreme in the highly oxidizing HY zeolite. In zeolite, very
little An2CH· or AnPhCH· radicals were observed. Presum-
ably, almost all these radicals were generated near highly
oxidizing sites so that the majority of deprotonation events
were coupled with an oxidizing event. Such rapid oxidation
of radicals may also explain the higher than expected reac-
tivity of the radical cations in HY. Instead of a radical cation –
radical equilibrium being established in favour of the radical
cation in the highly acidic medium, the radicals formed by
deprotonation are immediately trapped by oxidation to the
carbocation. Essentially, the radicals are diverted away from
a reprotonation pathway by the presence of the oxidation
pathway, and no equilibrium state is established.
An2CH2 and AnPhCH2 in NaY and KY and Tol2CH2 in
NaY, KY, and HY. It is also possible that prompt
carbocation formation is aided by a two-photon process
whereby one photon generates the diarylmethane radical cat-
ion that rapidly deprotonates to give the radical, and a sec-
ond photon within the same laser pulse is absorbed by the
radical to give the excited radical (34). Since excited radi-
cals are known to be more easily oxidized than ground-state
radicals (47), this two-photon sequence would be expected
to promote prompt carbocation formation.
Table 2 presents a qualitative summary to illustrate
whether or not prompt and (or) time-resolved formation of
the diarylmethyl cations was observed in the zeolites stud-
ied, and also includes oxidation potentials for the diaryl-
methyl radicals. Using the assumption that the rapid radical
oxidations are exergonic, we can use the information in Ta-
ble 2 to estimate “upper” and “lower” limits for the oxidiz-
ing ability of the cation- and proton-exchanged Y zeolites
(22). Due to the possibility that prompt carbocation may be
caused by two-photon ionization of the radicals, only the
time-resolved formation of the diarylmethyl cations is used
to signal that a particular zeolite is a sufficiently strong oxi-
dant to induce oxidation of the radicals to carbocations.
Time-resolved formation of the dianisylmethyl cation was
observed in HY, NaY, and KY, but not in CsY. Given that the
oxidation potential for An2CH· ≈ 0.04 V,6 we can estimate
the reduction potentials for sites within HY, NaY, and KY
that oxidize the radical to be >0.04 V, and <0.04 V for CsY.
No obvious time-resolved growth of AnPhCH+ was present
in NaY or KY, thus limiting the oxidizing ability of these
zeolites to <0.16 V. Time-resolved growth of Tol2CH+ was
observed only in HY, which further indicates that the reduc-
tion potential for HY is >0.23 V. Finally, Ph2CH· was not
As previously mentioned, prompt radical formation occurs
to some extent by rapid deprotonation of a small fraction of
the radical cations. If this situation was coupled with rapid
oxidation of the radical, one would expect to observe prompt
carbocation formation. This combination of rapid deproto-
nation and rapid oxidation provides a reasonable explanation
for prompt carbocation formation upon irradiation of
6 All oxidation potentials are vs. SCE in acetonitrile.
© 2005 NRC Canada

1646
Can. J. Chem. Vol. 83, 2005
oxidized to Ph2CH+ in any of the zeolites, even HY. Thus,
the reduction potential for HY should be <0.35 V.
(10–3 torr) for several hours, and sealed prior to laser irradia-
tion.
The amount of organic material incorporated into the zeo-
lite was calculated from the difference between the initial
amount of the diarylmethane and the amount that was recov-
ered in the combined solutions measured by UV absorption.
With hexane as the carrier solvent, it was found that >90%
of the diarylmethanes were incorporated into the zeolite.
In each case, the loading level for the diarylmethanes was
<S> = 0.10 0.01 (i.e., one molecule in every 10 super-
cages).
No color change was observed upon incorporation of the
diarylmethanes into the alkali cation-exchanged zeolites, in-
dicating that the substrates were not thermally converted to
coloured carbocations within these zeolites. The thermal sta-
bility of An2CH2 and AnPhCH2 within the alkali-exchanged
zeolite NaY was studied in more detail by examining the
material obtained after continuous extraction of An2CH2–
NaY and AnPhCH2–NaY composites prepared as previously
described for the laser experiments. The extraction proce-
dure involved placing the organic/zeolite composite in a fil-
ter thimble and subjecting the sample to continuous
extraction using a Soxlet apparatus and refluxing CH2Cl2 for
7 days. GC analysis (Varian Saturn 11, 35 m capillary col-
umn crosslinked 5% phenylmethylsilicone) of the extracted
material indicated that 80% of unreacted diarylmethanes was
recovered (relative to the amount originally incorporated).
No other materials were observed in the extracted material
by GC, leading to the conclusion that the diarylmethanes
were stable within the zeolites.
In HY, no colour change was observed upon incorporation
of Tol2CH2 and Ph2CH2, again suggesting that these materi-
als were thermally stable within the acidic zeolite. On the
other hand, the zeolite samples displayed a pale pink colour
upon incorporation of An2CH2 and AnPhCH2. Steady-state
diffuse reflectance of these samples showed absorption at
510 nm for the An2CH2–HY composite, and at 460 nm for
the AnPhCH2–HY composite, indicating that the coloration
was due to thermal conversion of the substrates to their
corresponding carbocations. The colour was completely re-
moved upon exposure of the samples to atmospheric mois-
ture because of quenching of the carbocations with water.
After continuous extraction of these hydrated samples,
An2CH2 or AnPhCH2 were still the only compounds de-
tected by GC. No evidence of the diarylmethanols or
diarylketones, which would be formed by oxidation of the
diarylmethanols, was found in the GC traces. Thus, while
some decomposition took place in HY, the lack of substan-
tial colour together with exclusive extraction of the
unreacted diarylmethanes suggests that decomposition was
minor. To ensure that no carbocations were present in the la-
ser experiments, samples of An2CH2 and AnPhCH2 in HY
were initially prepared in the same way as previously de-
scribed for the other samples, but then hydrated by exposure
to the atmosphere to remove the coloration and reevacuated
prior to laser irradiation. There was no further evidence for
the presence of carbocations following this supplementary
treatment.
Conclusions
The results clearly show that conversion of photogen-
erated diarylmethane radical cations to diarylmethyl radicals
by deprotonation is an efficient process, with the dynamics
of deprotonation depending on the acidity of the radical cat-
ions, but not the basicity of the zeolites. Subsequent conver-
sion of the radicals to the diarylmethyl cations can also be
rapid, but only when the oxidizing ability of the zeolite ex-
ceeds the oxidation potential of the radical. HY is the stron-
gest oxidizing zeolite of the four Y zeolites used, with a
reduction potential between 0.23 and 0.35 V. CsY is the
poorest oxidant, with a reduction potential at least 0.20 V
lower than HY.
Experimental
Materials
Faujasite zeolite NaY was commercially available from
Aldrich (LZY52 molecular sieves, Si/Al = 2.4) and used as
received. HY was prepared from NH4 Y (Aldrich) by calcin-
+
ing the zeolite at 500 °C to promote loss of NH3. The cation-
exchanged zeolites were prepared by washing the zeolites
with their corresponding chloride salts. The % exchange was
typically 97% for KY and 47% for CsY (16). The solvents
used for the preparation of zeolite samples were spectro-
scopic grade (99.98%, Omnisolv, EMD Chemicals) and used
as received.
Diphenylmethane was commercially available from
Aldrich and was purified by distillation and column chroma-
tography (silica, 70–270 mesh) prior to use. Dianisylmeth-
ane (bis(4-methoxyphenyl)methane), anisylphenylmethane
(4-methoxyphenylphenylmethane),
and
ditolylmethane
(bis(4-methylphenyl)methane) were prepared from their cor-
responding ketones (Aldrich) using the Huang–Minlon pro-
cedure (48). The products were purified on silica by column
chromatography using hexane as the eluant. Melting point
determinations and (or) gas chromatography (GC) were used
to establish purity. All melting points and H and 13C NMR
1
(Bruker 250 MHz using CDCl3 as the solvent) matched liter-
ature values (49, 50).
Zeolite sample preparation
The zeolites were activated at 475 °C for at least 12 h be-
fore use; typically ~300 mg of zeolite was activated over-
night and allowed to cool to room temperature in a dry
environment. A solution of the diarylmethane (about 5 mg)
dissolved in 15 mL of hexanes was rapidly added to 5 mL of
hexanes containing the activated zeolite. The sample was
magnetically stirred for 1 h after which time the sample was
centrifuged and the solvent was decanted. Fresh solvent was
added to wash the zeolite surface. The sample was stirred for
another 30 min, centrifuged, and the solvent was decanted.
The diarylmethane–zeolite complex was transferred to a
dessicator and dried under vacuum of 10–2 torr (1 torr =
133.322 4 Pa) for at least 4 h. The samples were then
transferred to 3 mm × 7 mm quartz sample cells, evacuated
Laser flash photolysis
The nanosecond laser flash photolysis system with diffuse
© 2005 NRC Canada

Shea et al.
1647
12. F. Gessner and J.C. Scaiano. J. Photochem. Photobiol. A, 67,
91 (1992).
13. M. O’Neill, F.L. Cozens, and N.P. Schepp. J. Phys. Chem. B,
reflectance detection at Dalhousie University is of standard
design and has been previously described (51). The samples,
typically ~300 mg of zeolite, were contained in 3 mm ×
7 mm laser cells made out of Suprasil quartz tubing and de-
gassed at reduced pressure (10–3 torr) prior to photolysis.
For the oxygen-quenching experiments, oxygen was passed
through the sample for at least 30 min prior to photolysis.
The excitation source was the fourth harmonic from a
Continuum Nd:YAG NY-61 laser (266 nm, ≤8 ns/pulse,
≤15 mJ/pulse). Transient signals from a monochromator/
105, 12746 (2001).
14. S. Hashimoto. J. Chem. Soc. Faraday Trans. 93, 4402 (1997).
15. F.L. Cozens, H. Garcia, and J.C. Scaiano. Langmuir, 10, 2246
(1994).
16. F.L. Cozens, M.L. Cano, H. Garcia, and N.P. Schepp. J. Am.
Chem. Soc. 120, 5667 (1998).
17. M.L. Cano, A. Corma, V. Fornes, and H. Garcia. J. Phys.
Chem. 99, 4241 (1995).
18. B.W. Wojciechowski and A. Corma. Catalytic cracking, cataly-
sis kinetics and mechanisms. Marcel Dekker, New York. 1984.
19. H. Choukroun, D. Brunel, and A. Germain. J. Chem. Soc.
Chem. Commun. 6 (1986).
photomultiplier system were initially captured by
a
Tektronix 620A digital oscilloscope and transferred to a
Power Macintosh computer. The experiments are computer
controlled using the Power Macintosh and software written
using the LabviewTM programming language.
20. V. Ramamurthy, D.F. Eaton, and J.V. Caspar. Acc. Chem. Res.
25, 299 (1992).
Data analysis
21. K.B. Yoon. In Handbook of zeolite science and technology.
Edited by S.M. Auerbach, K.A. Carrado, and P.K. Dutta. Mar-
cel Dekker, Inc., New York. 2003. p. 591.
The data analysis was based on the fraction of reflected
light absorbed by the transient (reflectance change, R =
∆J/Jo), where Jo is the reflectance intensity before excitation
and ∆J is the change in the reflectance after excitation. Zeol-
ites are known to contain several different microenvironments,
and as a result, time-resolved decays or growths of transients
in zeolites frequently fit better to a multiple first-order rate
equation (Rt = R∞ + ∆R1e–k1t + ∆R2ek2t + …) than to a single
first-order rate equation (Rt = R∞ + ∆R1e–k1t). In the present
work, the growths and decays of transients sometimes fit
very well to a simple first-order expression; other times, the
data fit better to a double first-order expression, but were
still well-described by a single first-order rate equation.
Thus, for consistency across all time-resolved processes, all
rate constant calculations, unless otherwise specified, were
carried out by nonlinear squares fitting of the data to a single
first-order rate equation.
22. V. Ramamurthy, J.V. Caspar, and D.R. Corbin. J. Am. Chem.
Soc. 113, 594 (1991).
23. P.L. Corio and S. Shih. J. Phys. Chem. 75, 3475 (1971).
24. S.N. Khadzhiev, N.G. Yaralov, and Y.V. Shumorskii. React.
Kinet. Catal. Lett. 24, 297 (1984).
25. M.L. Poutsma. In Zeolite chemistry and catalysis. Edited by
J.A. Rabo. American Chemical Society, Washington, D.C. 1976.
p. 437.
26. H. Garcia, V. Marti, I. Casades, V. Fornes, and H.D. Roth.
Phys. Chem. Chem. Phys. 3, 2955 (2001).
27. T.M. Leu and E. Roduner. J. Catal. 228, 397 (2004).
28. H. Garcia and H.D. Roth. Chem. Rev. 102, 3947 (2002).
29. E. Baciocchi, T. Del Giacco, and F. Elisei. J. Am. Chem. Soc.
115, 12290 (1993).
30. E. Baciocchi, T. Del Giacco, F. Elisei, and O. Lanzalunga. J.
Am. Chem. Soc. 120, 11800 (1998).
31. P.H. Kasai and J.R.J. Bishop. J. Phys. Chem. 77, 2308 (1973).
32. J.A. Rabo, C.L. Angell, P.H. Kasai, and V. Schomaker. Dis-
cuss. Faraday. Soc. 41, 328 (1966).
33. X. Liu and J.K. Thomas. Langmuir, 8, 1750 (1992).
34. J. Bartl, S. Steenken, H. Mayr, and R.A. McClelland. J. Am.
Chem. Soc. 112, 6918 (1990).
35. K. Sehested, J. Holcman, and E.J. Hart. J. Phys. Chem. 81,
1363 (1977).
36. M. Fujiwara and A. Yamasaki. J. Chem. Soc. Faraday Trans.
Acknowledgments
We gratefully acknowledge the Natural Sciences and En-
gineering Research Council of Canada (NSERC) for finan-
cial support of this research.
References
94, 2525 (1998).
37. M. Fujiwara, A. Yamasaki, K. Mishima, and K. Toyomi. J.
Chem. Phys. 109, 1359 (1998).
38. A. Bromberg and D. Meisel. J. Phys. Chem. 89, 2507 (1985).
39. T. Shida. Electronic absorption spectra of radical ions. Elsevier,
New York. 1988.
1. M. Barrer. Zeolites and clay minerals as sorbents and molecu-
lar sieves. Academic Press, London. 1978.
2. D.W. Breck. Zeolite molecular sieves: Structure, chemistry and
use. John Wiley and Sons, New York. 1974.
3. A. Dyer. An introduction to zeolite molecular sieves. John
Wiley and Sons, Bath, UK. 1988.
40. J. Sivaguru, J. Shailaja, and V. Ramamurthy. In Handbook of
zeolite science and technology. Edited by S.M. Auerbach, K.A.
Carrado, and P.K. Dutta. Marcel Dekker, Inc., New York. 2003.
p. 515.
4. J.A. Rabo. In ACS Monograph 171. American Chemical Soci-
ety, Washington, D.C. 1976.
5. N.J. Turro. Pure Appl. Chem. 58, 1219 (1986).
6. H. van Bekkum, E.M. Flanigen, and J.C. Jansen. Introduction
to zeolite science and practice. Elsevier, Amsterdam. 1991.
7. H. Garcia, S. Garcia, J. Perez-Prieto, and J.C. Scaiano. J. Phys.
Chem. 100, 18158 (1996).
8. B.D. Flockhart, L. McLoughlin, and R.C. Pink. J. Catal. 25,
305 (1972).
9. F.L. Cozens, H. Garcia, and J.C. Scaiano. J. Am. Chem. Soc.
41. F.L. Cozens, H. Garcia, and J.C. Scaiano. J. Am. Chem. Soc.
115, 11134 (1993).
42. C. Russo-Caia and S. Steenken. Phys. Chem. Chem. Phys. 4,
1478 (2002).
43. H. Ikeda, T. Nakamura, T. Miyashi, J.L. Goodman, K.
Akiyama, S. Tero-Kubota, A. Houmam, and D.D.M. Wayner.
J. Am. Chem. Soc. 120, 5832 (1998).
115, 11134 (1993).
10. K.K. Iu and J.K. Thomas. J. Phys. Chem. 95, 506 (1991).
11. K.B. Yoon. Chem. Rev. 93, 321 (1993).
44. D.D.M. Wayner, D.J. McPhee, and D. Griller. J. Am. Chem.
Soc. 110, 132 (1988).
© 2005 NRC Canada

1648
Can. J. Chem. Vol. 83, 2005
45. L.J. Johnston and T. Kanigan. J. Am. Chem. Soc. 112, 1271
(1990).
46. R. Popielarz and D.R. Arnold. J. Am. Chem. Soc. 112, 3068
(1990).
49. Y. Makioka, S. Uebori, M. Tsuno, Y. Taniguchi, K. Takaki,
and Y. Fujiwara. J. Org. Chem. 61, 372 (1996).
50. H. Mayr, G. Lang, and A.R. Ofial. J. Am. Chem. Soc. 124,
4067 (2002).
47. B.R. Arnold, J.C. Scaiano, and W.G. McGimpsey. J. Am.
51. M. O’Neill, F.L. Cozens, and N.P. Schepp. J. Am. Chem. Soc.
122, 6017 (2000).
Chem. Soc. 114, 9978 (1992).
48. Huang-Minlon. J. Am. Chem. Soc. 68, 2487 (1946).
© 2005 NRC Canada