Photophysical properties of dibenzotropylium cation incorporated within acidic ZSM-5 zeolite

Article abstract of DOI:10.1016/S0009-2614(01)00913-7

Dibenzotropylium ion (DT⁺) has been generated as an indefinitely persistent species within the channels of ZSM-5 in its H⁺-form. Diffuse reflectance laser flash photolysis has allowed detection of a transient (two bands: 300 nm, sharp and ~440 nm broad) decaying in the μs time-scale that has been assigned to the corresponding triplet excited state. The tight fit of DT⁺ within the straight channels (5.2×5.7A?²) of ZSM-5 and the presence of coadsorbed water explain why the DT⁺ triplet excited state is not quenched by oxygen but interacts with triethylamine, which is highly water-soluble. In the latter case, formation of a new transient compatible with DT^· (λ_max=270 and 350 nm) through electron transfer from the amine to DT⁺ triplet is observed.

Full text of DOI:10.1016/S0009-2614(01)00913-7

2
1 September 2001
Chemical Physics Letters 345 /2001) 409±414
www.elsevier.comꢀlocateꢀcplett
Photophysical properties of dibenzotropylium cation
incorporated within acidic ZSM-5 zeolite
Mar ꢀõ a Luz Cano , Michelle N.Chr ꢀe tien , Hermenegildo Garc ꢀõ a a,*,
a
b
b,
*
J.C. Scaiano
a
Departamento de Quimica, Instituto de Tecnolog ꢀõ a Qu ꢀõ mica, Universidad Polit eꢀ cnica de Valencia,
CSIC-UPV, Apartado 22012, 46071 Valencia, Spain
b
Department of Chemistry, Centre for Catalysis, Research and Innovation, University of Ottawa,
Ottawa, Canada, K1N 6N5
Received 11 June 2001; in ®nal form 2 August 2001
Abstract
‡
Dibenzotropylium ion /DT ) has been generated as an inde®nitely persistent species within the channels of ZSM-5 in
‡
its H -form.Di€use re¯ectance laser ¯ash photolysis has allowed detection of a transient /two bands: 300 nm, sharp
and ꢀ440 nm broad) decaying in the ls time-scale that has been assigned to the corresponding triplet excited state.The
‡
ꢁ
2
tight ®t of DT within the straight channels /5.2´5.7 A ) of ZSM-5 and the presence of coadsorbed water explain why
the DT triplet excited state is not quenched by oxygen but interacts with triethylamine, which is highly water-soluble.
‡
Å
In the latter case, formation of a new transient compatible with DT /kmax ˆ 270 and 350 nm) through electron transfer
‡
from the amine to DT triplet is observed. Ó 2001 Published by Elsevier Science B.V.
1
. Introduction
has enabled the study of the photophysical and
photochemical properties of some organic reaction
intermediates that cannot be examined in solution
[4±9].In other cases, encapsulation of organic
carbocations has allowed control to be gained over
the photochemical properties with respect to the
system in solution.For example, 9-/4-methoxy-
phenyl)xanthyl cation, which does not ¯uoresce in
solution, exhibits a strong emission when included
in large pore zeolites [8], and 2,4,6-triphenylpyry-
lium incorporated within Y zeolite acts as a
photocatalyst in aqueous media [10] while in so-
lution it undergoes hydrolytic ring opening.
Zeolites are a large family of microporous alu-
minosilicates di€ering in their crystal structure [1±
].The pore system of the zeolite particles are open
3
to the external surface allowing mass transfer from
the exterior to the interior of the grains.Zeolite
intracrystalline void spaces are available in a va-
riety of sizes and geometries and are able to host
organic species provided that their molecular size
is smaller than the pore dimensions.Zeolites are
also convenient solid matrices for the generation
and stabilization of organic carbocations [4].This
In a study of the electron-transfer ability of
‡
acidic zeolites dibenzotropylium ion /DT ) in-
cluded in medium- and large- pore zeolites was
observed to form by adsorption of dibenzosube-
*
Corresponding author.Fax: +34-96-387-9349.
E-mail address: hgarcia@qim.upv.es /H. Garc ꢀõ a).
0
009-2614ꢀ01ꢀ$ - see front matter Ó 2001 Published by Elsevier Science B.V.
PII: S 0 0 0 9 - 2 6 1 4 / 0 1 ) 0 0 9 1 3 - 7

410
M. Luz Cano et al. / Chemical Physics Letters 345 ꢀ2001) 409±414
Di€use re¯ectance UV±Vis /DR) spectrum of the
samples /Fig.1) matches well with the spectrum of
‡
an authentic sample of DT obtained by dissolv-
ing DTOH in sulfuric acid [18], and is coincident
‡
with the DR spectra of DT adsorbed on HY
previously reported by us [11].
‡
The IR spectrum of cationic DT has not been
Scheme 1.
previously reported.Given that the aluminosilicate
framework of the zeolites has spectral windows in
the IR, it has been possible to record the IR
spectrum of DT incorporated inside HZSM-5
/Fig.2).In Fig.2 we have compared the aromatic
region of the IR spectrum of DTOH, with the
spectra of DT ±HZSM-5 and an authentic DT
cation obtained by dissolving DTOH in H SO .
The remarkable coincidence between the spectra of
‡
rene onto the acidic H -form of the host through a
formal hydride loss mechanism [11].Some of the
‡
‡
samples containing DT incorporated within me-
dium-pore sized or monodirectional, large-pore
sized zeolites do not undergo any detectable vari-
ation in their spectroscopic properties for periods
longer than years.
‡
‡
2
4
‡
‡
The ¯uorescence of DT has been previously
the two cation samples indicates that DT must be
observed in the adiabatic photodehydroxylation of
DTOH /Scheme 1) [12].In this case, upon light
absorption DTOH is promoted to its singlet ex-
cited state and subsequently undergoes dehydr-
the predominant species present in the zeolite.
Importantly, the aromatic region of the IR
spectrum of the precursor /DTOH) is remarkably
di€erent /the central cycloheptatriene ring of
‡
‡
oxylation to form DT in the singlet excited state.
The consequence is the simultaneous observation
DTOH is not aromatic) from that of DT .
Therefore, it can be concluded that any residual
DTOH presents in the zeolite must be below the
detection limit of this technique.This point is
relevant for subsequent photochemical studies
since in a previous report on the photochemical
‡
of ¯uorescence from DTOH and DT .
‡
Although DT has occasionally been used as a
photosensitizer in electron transfer reactions [13]
and photooxidations [14], the triplet excited state
has never been characterized in solution, probably
‡
generation of DT in tri¯uorethanol a large excess
‡
‡
due to the diculty in obtaining DT free from the
interference of quenchers.In the present work, the
of DTOH was present and DT was detected only
‡
persistence of DT incorporated inside HZSM-5
has allowed us to study the photophysical prop-
erties of this aromatic cation in this solid matrix.
‡
Thus, we report the photophysics of DT included
‡
in the H -form of ZSM-5 zeolite, these results are
markedly di€erent from those obtained by excita-
tion of DTOH in 1,1,1-tri¯uoroethanol or in
aqueous acetonitrile solutions [15±17].
2
. Results and discussion
‡
The samples of DT incorporated in ZSM-5
used in this work were prepared by stirring a
suspension DTOH in CH Cl at re¯ux tempera-
ture in the presence of thermally dehydrated
2
2
‡
HZSM-5.Formation of DT results in the ap-
pearance of an orangeꢀred color in the zeolite.
Fig.1.Di€use re¯ectance UV±Vis spectrum /plotted as the
‡
inverse of the re¯ectance, R) of DT in HZSM-5.

M. Luz Cano et al. / Chemical Physics Letters 345 ꢀ2001) 409±414
411
‡
Fig.3 .E mission spectrum of DT
in HZSM-5 upon excitation
at 355 nm using an ns YAG laser.The inset shows the emission
decay at 580 nm.
‡
Fig.2 .A romatic region of the IR spectra of DT
‡
in HZSM-5
SO /b), and
/a), DT obtained by dissolving DTOH in H
DTOH /c).
2
4
‡
The ¯uorescence lifetime of DT in HZSM-5
was measured using an ns laser as the excitation
source.The ¯uorescence decay can be adequately
®tted to mono-exponential ®rst-order kinetics and
the estimated lifetime is 55 ns /inset in Fig.3).This
as a transient [19].Thermogravimetric analyses of
the prepared solids indicated that the cation con-
tent of the solids is about 3 wt% and calorimetry
gives the decomposition peak of this cation at 510
value is similar to the 40 ns previously reported for
‡
DT in 98% H
2
SO
4
solution [12] or 34 ns in tri-
°
C.This is similar to, although lower than, the
¯uoroethanol [15].Although, in general, incorpo-
ration within zeolites causes an increase of several
orders of magnitude in the triplet excited state
lifetime of organic guests, this is not the case for
singlet excited states which typically exhibit a
decomposition temperature of the related xanthyl
cation adsorbed in zeolites [6].This gives an indi-
‡
cation of the high stability of DT adsorbed in
‡
ZSM-5.In fact, a DT ±HZSM-5 sample stored in
a capped vial will persist for longer than ®ve years
without any spectroscopic variation.
lifetime similar to that found in solution [4].The
‡
fact that the lifetime of DT /S
) is not greatly in-
1
‡
The above data show that ZSM-5 in its H
form is a solid host suitable for generating and
¯uenced by incorporation within zeolites is not
unprecedented.
Time resolved di€use±re¯ectance laser ¯ash
‡
stabilizing DT with suciently high purity in an
`
‡
inert' environment free from electron donors.This
photolysis of DT adsorbed within ZSM-5 shows
the generation of a transient species that exhibits a
fact, combined with the transparency of the zeolite
aluminosilicate framework to wavelengths longer
than 220 nm, makes it possible to study the
photophysics of DT included in HZSM-5.
After sample characterization the photophysics
narrow band at ꢀ300 nm and a broader one at
k
max ꢀ440 nm accompanied by a weak positive
‡
absorption extending towards the infrared /Fig.4).
These spectroscopic features are most likely due to
a single species since identical decay pro®les are
recorded over the entire spectrum.This transient
has a lifetime of less than 50 ls.
‡
of DT incorporated in HZSM-5 were studied.
‡
The emission spectrum of DT incorporated
within HZSM-5 /Fig.3) exhibits a ¯uorescence
band centred at 550 nm which is coincident with
‡
Previous assignments of photochemically gen-
erated transient species within zeolites have been
supported by the coincidence of the UV±Vis
spectrum with that of the same species in solution.
that previously reported for DT in H
2
SO
4
solu-
tion [12].The excitation spectrum matches fairly
‡
well with the absorption spectra of DT proving
‡
‡
that DT is the emitting species.
However, in the case of DT previous laser ¯ash

412
M. Luz Cano et al. / Chemical Physics Letters 345 ꢀ2001) 409±414
while the remarkable increase in the lifetime of the
triplet excited state upon zeolite incorporation is a
common pattern also observed previously for ze-
olite-bound xanthylium and pyrylium ions [4].
The decay of the transient species generated
‡
upon excitation of DT ±ZSM-5 is una€ected by
oxygen purging.Triplet excited states are some-
times not readily quenched by oxygen when they
are generated inside the voids of zeolite, particu-
larly if there is a tight ®t of the organic guest inside
the straight channels as is the case with ZSM-
ꢁ
2
5
/5.2´5.7 A ) [21].Lack of reactivity with oxygen
is also caused by the impeded di€usion of oxygen
through the channels of fully hydrated ZSM-5 due
to the poor solubility of oxygen in water.Ac-
cordingly, water-soluble molecules should be able
‡
Fig.4. Transient di€use re¯ectance spectrum of DT
in
HZSM-5 recorded 0.96 ls /d), 2.56 ls /s), and 9.60 ls /h)
after the 355 nm laser pulse.
‡
to quench the triplet excited state of DT .
To test this possibility, quenching of the DT
‡
photolysis studies have failed to observe any
transient [19].This failure is largely due to the fact
that in the case of DT the transient spectrum is
complicated at short time-scales by the intense
bleaching of the highly colored ground state and
the occurrence of long-lived ¯uorescence.Never-
theless, we have recorded the time-resolved UV±
triplet by triethylamine was carried out.It is
known that triplet excited states of aromatic car-
benium ions are exceedingly good electron accep-
tors and, therefore, should be readily quenched by
amines as electron donors.In solution, this study
is not easily done since amines are strong nucleo-
philes and react readily with any carbenium ion in
its ground state.However, we reasoned that ther-
‡
‡
Vis spectra upon laser excitation of DT generated
by dissolving DTOH in tri¯uorethanol containing
a few drops of tri¯uoroacetic acid /TFAA).The
use of the second harmonic of an Nd:YAG laser
‡
mal reaction of aliphatic amines with DT incor-
porated within ZSM-5 should be hindered by
spatial restrictions, for the same reasons that
‡
/
/
532 nm) ensures the selective excitation of DT
k
‡
max ˆ 500 and 540 nm, see Fig.1), since in a
DT ±ZSM-5 is inde®nitely persistent even in the
control experiment no transients were recorded for
DTOH using this laser wavelength.The transient
spectrum recorded 0.16 ls after laser excitation is
dominated by the intense negative absorption due
presence of coadsorbed water.In fact, after ad-
sorption of triethylamine the transient spectrum
upon 532 nm excitation changes and the bands at
kmax ꢀ300 and ꢀ450 nm of the triplet decay with
the same kinetics as the growth of two new bands
‡
to the bleaching of the ground state DT spec-
trum, but there is de®nitely a short-lived transient
absorbing at 330 and 420 nm.
A reasonable assumption is that this transient
at 270 and 350 nm /Fig.5).The later can be as-
Å
signed to DT radical, since the same transient is
observed in the photolysis of DTCl in hexane,
accompanied by the short-lived triplet excited state
of DTCl absorbing at 420 nm.The triplet excited
state of DTCl is not observed in the photolysis
under oxygen and under these conditions the life-
‡
species, generated upon excitation of DT , in tri-
¯
uorethanol is the same as that observed inside
ZSM-5 and corresponds to the triplet excited state
of the cation.This would be consistent with the
photochemical behavior observed for structurally
related aromatic carbenium ions, particularly
xanthylium and pyrylium ions, for which triplet
excited states have been observed [20].The shift in
position of kmax between tri¯uoroethanol and
ZSM-5 probably re¯ects a solvatochromic e€ect,
Å
time of DT is signi®cantly shorter.
In conclusion, medium-pore sized ZSM-5 is a
‡
very convenient zeolite for the generation of DT
as a persistent species in high purity.This persis-
tence has enabled the recording of a transient
‡
spectrum attributable to the DT triplet excited

M. Luz Cano et al. / Chemical Physics Letters 345 ꢀ2001) 409±414
413
and placed in sealed greaseless cells with CaF
windows.The cells were outgassed at 200 °C under
2
À2
10
Pa for 1 h before recording the spectrum at
room temperature.
DTCl, used in this work for the comparative
Å
generation of DT radical in solution, was pre-
pared by the reaction of DTOH with an excess of
thionyl chloride.The product chloride was puri®ed
1
by sublimation and characterized by UV, IR, H-
NMR and GC±MS.
3.2. Laser ¯ash photolysis
‡
Nanosecond laser ¯ash photolysis experiments
Fig.5 .T ransient di€use re¯ectance spectrum of DT
with tri-
ethylamine in HZSM-5 recorded 9.60 ls after the 355 nm laser
pulse.
with zeolitic samples were carried out using a time
resolved di€use re¯ectance setup.Samples were
excited with either the third /355 nm, 6 ns pulse
width, ꢀ15 mJꢀpulse) or second /532 nm, 6 ns
pulse width, ꢀ25 mJꢀpulse) harmonic of a Con-
tinuum Surelite Nd-YAG laser.Signals from the
monochromatorꢀphotomultiplier were captured by
a Tektronix 2440 digitizer and transferred to a
PowerMacintosh computer programmed in the
LabVIEW 4.1 environment from National In-
struments.Detailed descriptions of this system can
be found elsewhere [23±25].
state and demonstrated its electron transfer
quenching with amines.In solution, the photo-
‡
chemistry of DT is complicated by the short
lifetime of the triplet excited state and the reac-
‡
tivity of DT with nucleophiles.
3
. Experimental
Laser ¯ash photolysis experiments involving
DTCl and DTOHꢀacid /to generate the cation in
solution) were carried out in spectroscopic grade
hexane and tri¯uoroethanol respectively.Solutions
were purged with O or N for 30 min prior to each
3
.1. Sample preparation
ZSM-5 /SiꢀAl 34) was prepared using tetrapro-
pylammonium as a structure directing agent ac-
cording to the method described in the literature
2
2
experiment.Samples were excited with 308 nm
pulses /6 ns pulse width, ꢀ85 mJꢀpulse) from a
Lumonics EX-530 excimer laser using a XeꢀHClꢀ
Ne mixture.Signal capture and processing were
accomplished as described above.Time resolved
photoluminescence studies were carried out using
the third /355 nm) harmonic pulse of the Nd-YAG
laser as the excitation source.Samples were irra-
[
22] and the crystallinity was checked by XRD.
‡
DT incorporated in ZSM-5 was prepared by
stirring a suspension of 50 mg of DTOH in
CH Cl /20 ml) at re¯ux temperature in the pres-
ence of thermally dehydrated /500 °C, overnight)
HZSM-5 /1 g).After 1 h the solid was ®ltered and
Soxhlet extracted until no more organic material
could be recovered.Extraction solutions were an-
alyzed by GC±MS and only unreacted DTOH was
observed.
2
2
2
diated in 3´7 mm Suprasil quartz cells and were
purged with nitrogen for at least 30 min prior to
each experiment.
Di€use re¯ectance spectra were recorded on a
Cary 5G spectrophtometer using a praying mantis
attachment with BaSO4 as a standard.FT±IR
spectra were obtained using a Nicolet 710 FT
Acknowledgements
spectrophotometer.Self-supported zeolite wafers
2
Financial support by the Spanish DGICYT
/HG Grant MAT97-1016-CO2) and the NSERC
/
ꢀ10 mg) were compressed at 1 tonꢀcm for 3 min

414
M. Luz Cano et al. / Chemical Physics Letters 345 ꢀ2001) 409±414
[
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