Time resolved CW-EPR spectroscopy of powdered samples: Electron spin polarization of a nitroxyl radical adsorbed on NaY zeolite, generated by the quenching of excited triplet ketones

Article abstract of DOI:10.1021/jp0110861

Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NAY) by the interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation, or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule. The identification of the polarized nitroxide structure was confirmed by employing both ¹⁴N and ¹⁵N 4-oxo-TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection laser flash photolysis.

Full text of DOI:10.1021/jp0110861

J. Phys. Chem. B 2001, 105, 7477-7481
7477
Time Resolved CW-EPR Spectroscopy of Powdered Samples: Electron Spin Polarization of
a Nitroxyl Radical Adsorbed on NaY Zeolite, Generated by the Quenching of Excited
Triplet Ketones
†
†
‡
,†
Steffen Jockusch, Zhiqiang Liu, M. Francesca Ottaviani, and Nicholas J. Turro*
Department of Chemistry, Columbia UniVersity, New York, New York 10027, USA, and Institute of Chemical
Sciences, UniVersity of Urbino, 61029 Urbino, Italy
Chemically induced dynamic electron polarization (CIDEP) generated in a faujasite zeolite (NaY) by the
interaction between a stable free radical (4-oxo-TEMPO) and the triplet state of benzophenone was investigated
by time-resolved electron spin resonance spectroscopy (TR-CW-EPR). The TR-CW-EPRs were performed
by either pulling a long tube containing powdered zeolite through the EPR cavity during the laser irradiation,
or by flowing a liquid transport medium (polydimethylsiloxane) for the zeolite powder, through a flat cell in
the EPR cavity. CIDEP was observed for intermolecular triplet quenching (benzophenone triplets with 4-oxo-
TEMPO) and intramolecular triplet quenching using a covalently linked TEMPO-benzophenone molecule.
1
4
15
The identification of the polarized nitroxide structure was confirmed by employing both N and N 4-oxo-
TEMPO isotopomers. The kinetics of the triplet quenching inside the zeolite were studied by diffuse reflection
laser flash photolysis.
Introduction
irradiation to produce the radicals.14 We have previously
reported CW-EPR spectra for benzyl radicals on porous silica
The quenching of excited triplet state ketones by nitroxides
in isotropic solutions has been the subject of considerable
generated by continuous irradiation of dibenzylketone adsorbed
1
5
on silica with a HgXe lamp. To reduce photo decomposition
during the irradiation, the sample-containing cell was slowly
moved through the EPR cavity. In this paper, we apply two
techniques, pulling a long tube containing powdered zeolite and
the flowing slurry technique, through the EPR cavity to employ
TR-EPR to investigate photochemical processes of molecules
adsorbed in zeolites. We report the nitroxide CIDEP that is
generated by the quenching of benzophenone triplets with 4-oxo-
TEMPO, both of which were adsorbed on NaY zeolite.
1-4
attention. For sensitizers with triplet energies higher than 225
kJ/mol, quenching rate constants close to the diffusion limit were
4
,5
found. As a result of triplet quenching, chemically induced
dynamic electron spin polarization (CIDEP) of the nitroxides
has been observed by time resolved electron paramagnetic
2,3,5
resonance (TR-EPR).
For quenching of aromatic ketone
triplet states by nitroxides, exclusively emissive electron spin
polarization was observed, which was explained by a radical-
triplet pair mechanism (RTPM).1,6,7
TR-EPR experiments to generate spin polarization of nitrox-
ides by the quenching of ketone triplets have been reported
The faujasite cavities of NaY zeolites (Nax(AlO2)y(SiO2)z;
dehydrated: y + z ) 192, x/y ) 0.7 to 1.1, z/y ) 1.5 to 3)16
consist of supercages possessing a diameter of ca. 13 Å.
Therefore, both an aromatic ketone as triplet sensitizer and a
nitroxide as triplet quencher can occupy the same supercage of
NaY. When both the triplet sensitizer and ketone occupy the
same supercage, no long range diffusion is necessary for
efficient quenching. For studies in NaY zeolites, benzophenone
8
mainly in solutions. Because of our combined interest in CIDEP
and the photochemistry of ketones adsorbed on zeolites, we
9
have initiated TR-EPR investigations of the photochemistry of
molecules adsorbed in zeolites. The motion of guest molecules
adsorbed on zeolites is limited compared to molecules in fluid
solutions. Therefore, processes that lead to electron spin
polarized radicals can be influenced.
(1) was selected as the triplet sensitizer because quenching of
benzophenone triplets by TEMPO derivatives (2) generated one
of the strongest observed electron spin polarization of 2 in
TR-EPR in solutions is usually performed by irradiation of a
sample inside the EPR cavity, usually with a pulsed laser.
1
0-12
2
,5,6
solutions.
Because of photo decomposition due to laser irradiation, the
solutions of the sample are flowed through the cavity in order
to ensure a fresh sample at each laser pulse. Flowing a powdered
sample through the cavity is problematic and is a reason for
the dearth of reports on TR-EPR of powders. Blaettler and Paul
Experimental Section
Materials. Unless otherwise stated, all reagents were pur-
chased from Aldrich Chemical Co. and used as received.
n-Pentane (HPLC grade; Fisher Scientific) and dichloromethane
(Optima; Fisher Scientific) were used as received. Benzophe-
none (1, Chart1) was recrystallized twice from ethanol and once
from hexane.
solved the flow problem by using a liquid transport medium
for particles of zeolite.13 Zeolite-polydimethylsiloxane-slurries,
which are relatively transparent, were flowed through the EPR
cavity during laser irradiation.13 Forbes et al. studied photo-
chemically generated radical pairs at solid/solution interfaces.
Ketones were covalentely anchored to silica surfaces, and the
silica slurries in organic solvents were flowed during laser
1
5
15
14
The N labeled 4-oxo-TEMPO (2- N, Chart1) was prepared
1
5
as follows: 2,2,6,6-tetramethyl-4-piperidone- N was synthe-
sized based on a literature procedure by stirring 6.47g phorone
46.8mmol) in 15 mL of benzene with 2 g of ( NH4)2SO4
1
7
15
(
†
‡
(14.9mmol), 1.56 g of NaOH (39mmol), 3.67g Na2HPO4‚7H2O,
Department of Chemistry, Columbia University.
Institute of Chemical Sciences, University of Urbino.
o
and 5 mL of water at 95 C in a heavy-walled glass pressure
1
0.1021/jp0110861 CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/13/2001

7478 J. Phys. Chem. B, Vol. 105, No. 31, 2001
Jockusch et al.
CHART 1: Structures of molecules investigated in this
report.
vessel for 10 days. The reaction mixture was diluted with 50
mL of water and 50 mL of ether. The aqueous layer was washed
three times with 50 mL of ether, and the combined ether extracts
were dried over anhydrous MgSO4 and concentrated under
reduced pressure. Purification was achieved by flash column
chromatography on silica gel using solvent mixtures of increas-
ing polarity (5:1 to 2:1; hexane:ethyl acetate), and then methanol
Figure 1. Schematic representation of the instrumentation used to
measure electron spin polarization in powder samples by TR-EPR.
15
yielding 4g of 2,2,6,6-tetramethyl-4-piperidone- N (25.8mmol,
1
5
8
2
6% yield). 2- N was then synthesized by oxidation of crude
15
,2,6,6-tetramethyl-4-piperidone- N according to a published
1
8
15
To obtain spectra using a fluidized transport medium, a slurry
of NaY containing 3 and polydimethylsiloxane (viscosity: 500
cP) was prepared as follows: NaY containing 3 was evacuated
procedure using H2O2 and Na2WO4. 2- N was purified by
flash column chromatography using 4:1 hexane:ethyl acetate
and was obtained in 10% yield as an amber-colored solid. EPR,
GC/MS (EI): m/z ) 171 (M ). The purity (>98 %) of 2- N
was determined by GC.
-
4
+
15
to at least 1 × 10 Torr to remove traces of water, solvents
and oxygen and was then filled with argon. Deoxygenated
polydimethylsiloxane was added to the zeolite complex and
homogenized in an ultrasonic bath for about 20 min. The
emulsion was transferred into a gas tight Hamilton syringe and
bubbled with argon. To measure TR-EPR spectra the emulsion
was flowed through a flat quartz cell (∼0.3 mm thick) in the
rectangular cavity of the EPR spectrometer with a flow rate of
The TEMPO labeled benzophenone (3) was synthesized by
1
9
a procedure analogous to that described by Hassner. 3-Ben-
zoylbenzoic acid was esterified with 4-hydroxy-TEMPO in the
presence of 1,3-dicyclohexylcarbodiimide and 4-dimethylamino-
pyridine in THF under argon for 10 h. After removal of the
THF, the residue was dissolved in methylene chloride, washed
with water, 5% acetic acid, and chromatographed over silica
gel (9:1 to 8:2; hexane: ethylacetate) to obtain 3 in 85% yield.
The structure and purity (>98 %) of 3 were determined by NMR
and EPR.
1
mL/min.
CW-EPR. CW-EPR experiments were performed on a
Bruker EMX-EPR spectrometer.
Diffuse Reflectance Laser Flash Photolysis. Diffuse reflec-
tance laser flash photolysis experiments employed pulses (308
nm, 10 ns) from a Lambda Physik Lextra 50 excimer laser and
a computer-controlled system that has been described else-
NaY (CBV100, SiO2/Al2O3 ) 5.1) was purchased from
Zeolyst International. The unit cell formula of this sample was
16
calculated to be Na54(AlO2)54(SiO2)138 when dehydrated. NaY
2
2
o
where. The optical configuration was similar to that used by
was activated at 500 C in an oven for at least 8 h and cooled
2
3
Wilkinson and Kelly. The data analysis was based on the
fraction of reflected light absorbed by the transient (reflectance
change (∆J/Jo) where Jo is the reflectance intensity before laser
excitation and ∆J is the change in the reflectance after excitation.
to room temperature in a desiccator before use. NaY was added
to a solution of the ketone and nitroxide in n-pentane and
dichloromethane (5 mL of n-pentane and 3.2 mL of dichlo-
romethane per gram of NaY). The slurry was stirred for at least
8
h in a closed vessel, and then the solvent was removed by
Results and Discussion
argon flow and subsequently placed in a vacuum chamber for
1
4
2
h.
Figure 2a shows the CW-EPR of 2- N loaded into NaY,
which was coadsorbed with 1 at a loading of 2% and 10%,
respectively (w/w). Assuming a random adsorption in super-
cages, this loading corresponds in average to one molecule of
Time-Resolved CW-EPR. TR-CW-EPR experiments em-
ployed the pulses (308 nm, 10 ns) from a Lambda Physik Lextra
0 excimer laser, a Bruker ER 100D X-band EPR spectrometer,
5
1
4
and a PAR boxcar integrator and signal processor (Models 4420
and 4402, respectively).2 To obtain spectra from dry powders,
quartz tubes (diameter 4 mm, length 1 m) filled with NaY
containing the ketones and TEMPO derivatives were evacuated
2- N and five molecules of 1 in five supercages of NaY. For
direct comparison with the TR-CW-EPR spectra, the CW
spectrum is presented in its integrated form in Figure 2. Spectra
simulation showed that the observed CW-EPR spectrum cor-
responds to 2- N in the slow motion limit (see Supporting
Information 1). The magnetic parameters used in the simulation
are in good agreement with previously published values for
TEMPO in NaX zeolite. For efficient triplet quenching, it
would be desired that each loaded supercage of NaY contains
only one molecule of 1 and only one molecule of 2. Because
the molecules are loaded randomly, the occupation of the
supercages probably deviate from the ideal condition. Further-
more, the maximum amount of nitroxide which can be loaded
is limited by spin-spin exchange interactions, which would
complicate the interpretation of the CIDEP data, i.e., if nitroxide
0,21
-
5
14
to at least 5 × 10 Torr to remove traces of water, solvents
and oxygen and closed with a vacuum stopcock. During the
irradiation with laser light, the sample container was pulled
through the cavity of the EPR spectrometer at a constant rate
of 5 cm/min. A schematic of the experimental setup is shown
in Figure 1. CW-EPR measurements were recorded before and
after the TR-EPR experiments and showed no difference by
visual inspection of the line shape and intensity of the CW-
spectra, demonstrating that the laser irradiation during the TR-
ESR experiments did not cause significant decomposition of
the TEMPO derivatives.
2
4

TR-CW-EPR Spectroscopy of Powdered Samples
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7479
Figure 2. CW-EPR (a, b) and TR-EPR (c, d) spectra of powder
1
4
15
samples of 2- N (a, c) and 2- N (b, d) in NaY (one molecule of 2 in
five supercages), which also contains 1 (one molecule of 1 in one
supercage). TR-EPR spectra (c, d) were recorded 1.0 - 1.7 µs following
laser excitation (308 nm, 10 ns). The CW-EPR spectra (a, b) are
displayed in its integrated form.
Figure 3. TR-EPR spectra recorded 1.0 - 1.7 µs following 308 nm
laser excitation of 3 in NaY (one molecule in five supercages) as powder
sample (a), or as slurry in polydimethylsiloxanes (5% zeolite complex)
as transport medium (b).
molecules are located close to each other, then Heisenberg
exchange interactions alter the EPR spectrum.25-27 This is
expected to be the case when on average one molecule of 2
occupies each supercage. Therefore, for our experiments, the
loading of nitroxides was reduced to one molecule of 2 in five
supercages, where spectral changes due to spin-spin exchange
interactions are minor.
shows that intramolecular triplet quenching inside NaY super-
cages also produces spin polarized nitroxides. The nitroxide
linked benzophenone (3) was loaded into NaY (one molecule
of 3 per five supercages of NaY). The CW-EPR spectrum of 3
Laser irradiation (308 nm) of NaY containing on average five
14
molecules of 1 for every one molecule of 2- N (natural isotope)
affords a broad emissive polarized TR-EPR spectrum shown
in Figure 2c. The spectrum obtained by TR-EPR exhibits the
14
adsorbed on NaY is identical to that obtained for 2- N (Figure
a). Laser irradiation (308 nm) produced a strong emissive spin
2
14
polarized TR-EPR spectrum (Figure 3a), which is identical to
the TR-EPR spectrum generated by intermolecular triplet
quenching (Figure 2c).
same features as the spectrum of 2- N obtained by CW-EPR
spectroscopy (Figure 2a). The observation of emissive spin
polarization is in agreement with TR-EPR experiments in
28,29
solution, where emissive spin polarization was also observed.
The time evolution of the nitroxide spin polarization after
irradiation with a short laser pulse (308 nm, 10 ns) was measured
at 3485 G for inter- and intramolecular triplet quenching (Figure
4a,b). In both cases, the buildup of the polarization occurred
within the first 300 ns, which is the instrument response
function. The decay of the electron spin polarization of the
In normal EPR practice, the identity of the paramagnetic
species under study is uniquely determined by measuring
coupling constants and g factors. However, due to the broaden-
ing of the EPR spectrum of the nitroxide on NaY surface, these
parameters cannot be reliably obtained from the TR-EPR
spectra. The anisotropic components of the magnetic tensor
parameters are detectable, which is common for free radicals
under slow motion conditions.2 To circumvent this problem
and unambiguously support the proposed interpretation, ben-
1
4
nitroxides 2- N and 3 followed first order kinetics. The spin
14
polarization lifetimes of both nitroxides 2- N and 3 are the same
within experimental error (3.9 and 3.8 µs, respectively), and
are in agreement with reported values of TEMPO spin polariza-
5,26
1
5
15
1,29
zophenone and 4-oxo-TEMPO- N (2- N) were employed.
tion lifetimes in solution.
1
5
1
Because N possesses a spin multiplicity of /2, in contrast to
the spin multiplicity of N of 1, both nitroxide EPR spectra
are readily distinguishable from each other.
Figure 2b shows the CW-EPR of 2- N adsorbed in NaY,
which also contains the sensitizer ketone 1 (one molecule of 2
and five molecules of 1 in five supercages of NaY), which is
easily distinguishable from the 14N isotope (Figure 2a). Spectra
simulation is in good agreement with previously published
To study the benzophenone triplet quenching kinetics by
nitroxides inside the zeolites in more detail, diffuse reflectance
laser flash photolysis was performed. Laser flash photolysis (308
nm, 10 ns) of 1 loaded on NaY affords a readily detectable
transient absorption at 525 nm corresponding to the triplet state
14
25,26
1
5
31
of 1 (Figure 5a). The transient decayed with multiexponential
kinetics, as expected for intermediates in a heterogeneous
4
environment. In the presence of the nitroxide 2 the triplets of
1
5
3
values for 2- N in slow motion condition (see Supporting
1 are strongly quenched (Figure 5b). Most of (1)* is quenched
Information 1).2
5,26
Laser irradiation (308 nm) of 1 and 2- N
15
within the first 300 ns. This is consistent with the buildup
kinetics of the electron spin polarization measured by TR-EPR,
which occurred within e300 ns (Figure 4c). Intramolecular
triplet quenching of 3 occurred faster (Figure 5c) than the
response time of our experimental setup for transient diffuse
reflectance. The fast decay is consistent with the short triplet
lifetime of 3, which was determined in benzene solution by laser
in NaY produced an emissive polarized TR-EPR spectrum
shown in Figure 2d. The spectrum obtained by TR-EPR (Figure
2d) exhibits the same features as the spectrum obtained by CW-
EPR spectroscopy (Figure 2b).
The electron spin polarizations shown above were generated
by intermolecular triplet quenching of benzophenone (1) by a
nitroxide (2). If both ketone and nitroxide are chemically linked
through a short tether (i.e., 3, Chart 1), then triplet quenching
can occur intramolecularly, as has been demonstrated success-
fully in homogeneous solution.3,5,30 The following experiment
5
flash photolysis (τT ) 85 ns).
The TR-EPR experiments presented above were performed
on zeolite powder samples, where the sample containing cell
was slowly moved through the EPR cavity during laser

7480 J. Phys. Chem. B, Vol. 105, No. 31, 2001
Jockusch et al.
molecules are too large to enter the NaY cavity, and (iii) the
refractive indexes of polydimethylsiloxane and zeolite crystals
are similar to produce relatively transparent slurries.13
Accordingly, we used polydimethylsiloxane as the transport
medium for zeolite complexes to study electron spin polarization
of a nitroxide generated by quenching of a triplet ketone inside
of NaY. Laser irradiation of an emulsion of polydimethylsi-
loxane and NaY loaded with 3 (one molecule in five supercages)
that is flowed through the EPR cavity affords a broad emissive
polarized TR-EPR spectrum (Figure 3b) which is experimentally
identical to the TR-EPR spectrum obtained from photolysis of
3
adsorbed on zeolite powder (Figure 3a). The time evolution
of the nitroxide spin polarization after laser irradiation using
polydimethylsiloxane as transport medium (Figure 4c) was
compared to powder samples (Figure 4b). The spin polarization
lifetimes of 3 in polydimethylsiloxane slurries and in the zeolite
powder sample NaY are the same within experimental error (3.4
and 3.8 µs, respectively).
Using a liquid transport medium for zeolites loaded with
organic molecules has the risk of the organic molecules
relocating from the zeolite phase to the liquid phase of the
transport medium. To ensure that 3 remained inside NaY, CW-
EPR experiments were performed (experimental spectra shown
in Supporting Information). The nitroxide 3 dissolved in
polydimethylsiloxane (500 cP) shows a three line isotropic EPR
spectrum typical for nitroxides in fast motion condition.2 The
CW-EPR powder spectrum of 3 loaded into NaY cavities is
broadened due to the hindered motion (resolution of the
anisotropic components of the magnetic tensor parameters) and
easily distinguishable from the spectrum in polydimethylsiloxane
solution. The CW-EPR spectrum of a slurry of polydimethyl-
siloxane and NaY loaded with 3 is identical within the
experimental error to the spectrum of 3 adsorbed in powdered
NaY without the transport medium. This demonstrates that 3
remained in the NaY zeolite phase and that no nitroxide is
extracted into the liquid phase of the transport medium.
5,26
Figure 4. Transient EPR decay kinetic observed at 3485 G following
1
4
laser excitation (308 nm, 10 ns) excitation of 1 and 2- N in NaY (five
molecules of 1 and one molecule of 2 in five supercages) (a), and 3 in
NaY (one molecule in five supercages) (b) as powder samples (a, b),
and 3 in NaY (one molecule in five supercages) as slurry in
polydimethylsiloxanes as transport medium (c).
Conclusion
The use of TR-CW-EPR to investigate photochemical reac-
tions has been mainly limited to systems in solutions, which
can be flowed through a flat cell in the EPR cavity to prevent
chromophore depletion due to laser irradiation. TR-CW-EPR
investigations of photochemical reactions of molecules adsorbed
in zeolites can be performed conveniently by pulling a long
tube containing powdered zeolite through the EPR cavity during
the laser irradiation, or by using a liquid transport medium
(polydimethylsiloxane) to produce a slurry of zeolite powder,
which can be flowed through a flat cell in the EPR cavity. Both
methods showed a strong CIDEP, generated by quenching of
benzophenone triplets with TEMPO, which were adsorbed on
NaY zeolite. These findings suggest that it should be possible
to employ TR-CW-EPR to investigate a range of photochemical
reactions of molecules adsorbed on zeolites and other porous
solids. Because the generation of electron polarization from
radicals produced by photolysis depends on a number of factors,
including the extent of diffusional separation and the diffusional
and rotational motions of radicals, the investigation of CIDEP
in porous solids promises to be a method that will be of value
for determining the dynamics of reactive intermediates in
constrained spaces.
Figure 5. Transient diffuse reflectance kinetic traces observed at 525
nm following laser excitation (308 nm, 10 ns) of NaY containing one
molecule of 1 in one supercage (a), five molecules of 1 and one
molecule of 2 in five supercages (b), and one molecule of 3 in five
supercages (c).
irradiation to reduce photo decomposition. Blaetter and Paul
showed that transport of zeolite particles through the EPR cavity
during irradiation can also be achieved by using a liquid
transport medium, which is flowed through a flat cell in the
13
EPR cavity. Polydimethylsiloxane (viscosity 200 cP) was used
for the following reasons: (i) it is viscous enough to prevent
sedimentation of the zeolite particles, (ii) polydimethylsiloxane
Acknowledgment. The authors at Columbia thank the
National Science Foundation for support of this research through
Grant No. CHE98-12676. This work was also supported in part

TR-CW-EPR Spectroscopy of Powdered Samples
J. Phys. Chem. B, Vol. 105, No. 31, 2001 7481
by the National Science Foundation and the Department of
Energy under Grant No. NSF CHE98-10367 to the Environ-
mental Molecular Science Institute (EMSI) at Columbia Uni-
versity.
(14) Forbes, M. D. E.; Ruberu, S. R.; Dukes, K. E. J. Am. Chem. Soc.
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Supporting Information Available: CW-EPR spectra and
14
14
15
simulation of 2- N, 3- N , and 2- N loaded into NaY zeolite.
(
14
CW-EPR spectra of 3- N in polydimethylsiloxane solution,
NaY zeolite powder, and slurry of NaY in polydimethylsiloxane.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(
4
(
(
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