Increasing the heavy atom effect of xenon by adsorption to zeolites: Photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene

Article abstract of DOI:10.1021/ja961546h

The distribution of products in the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is determined by the electronic spin state of the cyclohexane-1,4-diyl intermediate. Triplet cyclohexane-1,4-diyl yields 1,5-hexadiene (HD) while singlet cyclohexane-1,4-diyl produces bicyclo[2.2.0]hexane (BCH) as the stable product. Intersystem crossing (ISC) between the two diyls is enhanced by an external heavy atom effect. Direct photolysis (366 nm) of DBO in NaY zeolite containing adsorbed xenon yields a product ratio of 75:25 in favor of the triplet product, HD. This is a 24% increase in triplet product from direct photolysis in n-octane. The dramatic enhancement of ISC may be caused by polarization of the Xe atom in the faujasite cage, thereby allowing it to reach its full potential as a heavy atom perturbant. The 3:1 product ratio derives from complete equilibration of the three triplet electron spin states and the singlet spin state either in the short-lived diazenyl diradical or on a high-energy surface of the 1,4-cyclohexanediyl in which the singlet and triplet states are degenerate. The same 75:25 product ratio is achieved (without Xe adsorbed to the zeolite) when the Na⁺ cation of the zeolite is exchanged with Cs⁺ cation. This is not surprising since Xe and Cs⁺ are isoelectronic and therefore should share similar spin-orbit coupling characteristics. Also reported are the product ratios when photolysis of DBO is carried out in zeolites containing other monovalent cations (Li⁺, Na⁺, K⁺, Rb⁺).

Full text of DOI:10.1021/ja961546h

9552
J. Am. Chem. Soc. 1996, 118, 9552-9556
Increasing the Heavy Atom Effect of Xenon by Adsorption to
Zeolites: Photolysis of 2,3-Diazabicyclo[2.2.2]oct-2-ene
Mark A. Anderson and Charles B. Grissom*
Contribution from the Department of Chemistry, UniVersity of Utah, Salt Lake City, Utah 84112
X
ReceiVed May 8, 1996
Abstract: The distribution of products in the photolysis of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) is determined by
the electronic spin state of the cyclohexane-1,4-diyl intermediate. Triplet cyclohexane-1,4-diyl yields 1,5-hexadiene
(HD) while singlet cyclohexane-1,4-diyl produces bicyclo[2.2.0]hexane (BCH) as the stable product. Intersystem
crossing (ISC) between the two diyls is enhanced by an external heavy atom effect. Direct photolysis (366 nm) of
DBO in NaY zeolite containing adsorbed xenon yields a product ratio of 75:25 in favor of the triplet product, HD.
This is a 24% increase in triplet product from direct photolysis in n-octane. The dramatic enhancement of ISC may
be caused by polarization of the Xe atom in the faujasite cage, thereby allowing it to reach its full potential as a
heavy atom perturbant. The 3:1 product ratio derives from complete equilibration of the three triplet electron spin
states and the singlet spin state either in the short-lived diazenyl diradical or on a high-energy surface of the 1,4-
cyclohexanediyl in which the singlet and triplet states are degenerate. The same 75:25 product ratio is achieved
+
+
(
without Xe adsorbed to the zeolite) when the Na cation of the zeolite is exchanged with Cs cation. This is not
+
surprising since Xe and Cs are isoelectronic and therefore should share similar spin-orbit coupling characteristics.
Also reported are the product ratios when photolysis of DBO is carried out in zeolites containing other monovalent
+
+
+
+
cations (Li , Na , K , Rb ).
Introduction
the ability to adsorb other molecules in close proximity and, in
the case of the Xe atom, slightly polarize the filled valence
electron shell.
Heavy atom (HA) solvents are commonly used in photo-
chemistry to enhance intersystem crossing (ISC) in the excited
state and in reactive radical pairs.1 An increase in ISC can alter
fluorescence or phosphorescence quantum yields, or change the
distribution of stable products that arise from singlet and triplet
precursors.2 Intermolecular HA-induced ISC arises via spin-
orbit coupling (SOC) interactions, with the magnitude of SOC
increasing as the nuclear charge increases and the outer valence
shell begins to fill.1 Bromine-containing and iodine-containing
hydrocarbon solvents offer some of the most convenient and
widely-used perturbants, but they have an undesirable reactivity
4
The faujasite framework is comprised of a three-dimensional
-
network of oxygen-sharing AlO4 and SiO4 tetrahedra that form
small and relatively spherical structures known as sodalite cages.
The sodalite cage has an opening that is too small to allow the
entrance of most organic molecules. Multiple sodalite cages
are joined together to form and surround even larger void spaces
in the faujasites to form supercages. Each supercage is ≈13 Å
d
3
across and has a volume of ≈800 Å . The supercage has four
“
windows”, each ≈7.5 Å in diameter, to allow guest molecules
-
access to the interior. Each negatively charged AlO4 is
countered by a cation (M ), and the overall formula for the
faujasites is M (AlO3) (SiO3)n. The cations occupy three
3
toward many free radical species. Xenon offers the possibility
+ 4
of even higher SOC with no reactivity toward radical abstraction,
but with a filled valence shell, its ability to promote ISC is
minimal in a nonpolarizing environment.
Zeolites offer an attractive alternative to solution chemistry
when looking for heavy atom effects in the photochemistry of
organic compounds. The faujasite class of zeolites contains
+
-
separate sites of the faujasites (I, II, III). Two of these sites
(II, III) allow interaction between the cation and adsorbed
organic molecules. The final site for the cation (I) is located
inside a sodalite cage and is thus precluded from interaction
5
with a guest molecule.
+
+
cations that may be exchanged for heavy cations (i.e. Rb , Cs ,
The arrangement of the supercage results in an electrostatic
gradient within the supercage such that the overall polarity of
+
Tl ) that can increase the rate of intersystem crossing between
singlet and triplet electron spin multiplicities. In general, the
faujasite cage itself is non-reactive in organic systems and allows
products to be removed by extraction with organic solvents, or
by dissolution of the matrix by acid. The faujasites also offer
+
+
the supercage decreases with increasing cation size (Li > Na
+
+
+
>
K > Rb > Cs ). Previous studies have shown that the
photochemistry of a guest molecule can be altered by the heavy
atom influence of the cation through the spin-orbit coupling
+
+
parameter (with Rb and Cs+), or the light atom effect (Li
*
Author to whom correspondence should be addressed.
Abstract published in AdVance ACS Abstracts, September 15, 1996.
+
X
and Na ) that affects the guest by high charge density. Lastly,
the photochemistry may be changed simply by the steric factors
associated with the supercage. For this investigation, the NaY
(1) (a) McGlynn, S. P.; Azumi, T.; Kinoshita, M. Molecular Spectroscopy
of the Triplet State; Prentice-Hall: Engelwood Cliffs, 1969. (b) Turro, N.
J. Modern Molecular Photochemistry; Benjamin Cummings: Menlo Park,
5
zeolite was chosen for study.
1
978. (c) Koziar, J. C.; Cowan, D. O. Acc. Chem. Res. 1978, 11, 334. (d)
Khudyakov, I. V.; Serebrennikov, Y. A.; Turro, N. J. Chem. ReV. 1993,
(4) (a) Cheung, T. T. P.; Fu, C. M.; Wharry, S. J. Phys. Chem. 1988,
92, 5170. (b) Ito, T.; Fraissard, J. J. Chem. Phys. 1982, 76, 5225. (c) Ito,
T.; Fraissard, J. J. Chem. Soc., Faraday Trans. 1 1987, 83, 451.
9
3, 537.
(2) (a) Horrocks, A. R.; Wilkinson, F. Proc. R. Soc., Ser. A 1968, 306,
2
57. (b) Robinson, G. W. J. Mol. Spectrosc. 1961, 6, 58. (c) Burton, C. S.;
(5) (a) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res.
1992, 25, 299. (b) Borecka, B.; Gunmundsdottir, A. D.; Olorsson, G.;
Ramamurthy, V.; Scheffer, J. R.; Trotter, J. J. Am. Chem. Soc. 1994, 116,
10322. (c) Ramamurthy, V. In Photochemistry in Organized and Con-
strained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; p 429.
Noyes, W. A., Jr. J. Chem. Phys. 1968, 49, 1705.
3) SOC constants: Br, 2460 cm , I, 5069 cm-1. Murov, S. L.;
-
1
(
Carmichael, I.; Hug, G. L. Handbook of Photochemistry, 2nd ed.; Marcel
Dekker Inc.: New York, 1993; p 339.
S0002-7863(96)01546-6 CCC: $12.00 © 1996 American Chemical Society

Photolysis of 2,3-Diazabicyclo[2.2.2]oct-2-ene
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9553
Scheme 1
DBO Photolysis. In this study we have chosen 2,3-
diazabicyclo[2.2.2]oct-2-ene (DBO) as a reporter molecule for
ISC through SOC. DBO undergoes deazatization following
direct irradiation of the NdN chromophore, or singlet sensitiza-
tion to yield 1,5-hexadiene (HD) and bicyclo[2.2.0]hexane
Results and Discussion
Xenon Heavy Atom Effect in Solution. Photolysis of DBO
was carried out in solution (0.03 M DBO/n-octane) as a function
of increasing Xe pressure. The sample was degassed by a series
of 5 freeze-pump-thaw cycles prior to admission of Xe at a
final pressure of 5-120 psi, with 120 psi corresponding to a
Xe concentration of 1.3 M in n-octane at 25 °C. Photolysis
was carried out at 366 nm as previously described.6 Only a
3% increase in the amount of triplet derived product, from 51:
49 to 54:46 HD:BCH, was observed. By comparison, a 1.1 M
6
(BCH) with a product ratio of 51:49 in n-octane. ISC is
possible at two points. The first is the diazenyl diradical formed
when the first C-N bond is broken. Rapid ISC here would
yield a 1:3 ratio of singlet:triplet cyclohexane-1,4-diyl after
deazatization. Rapid ISC may also occur as the actiVated
cyclohexane-1,4-diyl forms to determine the stable product
outcome at a point when the energy difference between the
singlet and triplet spin states is small.6 Further ISC does not
occur in the short-lived 1,4-cyclohexanediyl boat intermediate
because of the large ∆Est (Scheme 1). The ratio of hexadiene:
bicyclohexane can be increased to a limiting value of ≈75:25
HD:BCH by increasing amounts of bromooctane or iodooctane
cosolvent.6 This feature makes the photolysis of DBO a
convenient and sensitive probe of SOC-induced ISC in reactive
diradicals.
1
1
6
iodooctane solution increases the triplet product by ≈10%.
Xenon Heavy Atom Effect in Zeolites. Xenon exhibits a
much larger HA effect when bound in the zeolite cage with
DBO. The extent of photolysis was typically 1-2% in the
zeolite and GC analysis was used to determine the ratio of
products. This procedure was repeated with the other noble
gases, He and Ar, as controls for which no HA-induced SOC
is expected. [It should be noted that, in all cases, the zeolite
samples were handled in a dry N2 atmosphere or under vacuum.
In high amounts, water can displace the guest molecule from
the supercage. In smaller amounts it may collect around the
openings of the supercage and hinder loading of DBO, or
Xenon Heavy Atom Effects. The noble gases, Xe in
particular, have been used extensively to enhance ISC rates.
Carroll and Quina used Xe as a HA fluorescence quencher in
7
the determination of ISC quantum yields in benzene derivatives.
5
c,12
extraction of the product.
]
At the same time, Macbeath and Unger showed experimentally
that Xe promoted ISC in the photolysis of monofluorobenzene,
as indicated by a decrease in the fluorescence quantum yield
The DBO/Xe/NaY adsorbate exhibited a dramatic increase
in the amount of triplet product to ≈75:25 HD:BCH upon
photolysis throughout the range of 5-120 psi Xe (Figure 1).
The 3:1 ratio of triplet:singlet products is consistent with
complete equilibration of the three triplet states and the singlet
state if the ∆EST ) 0. This equilibrium is reached at relatively
low pressures of Xe because of specific adsorption in the
faujasite cage and localization of DBO in close proximity to
Xe. Other studies using Xe to enhance ISC have shown that
and by an increase in the rate of cis-2-butene photoisomeriza-
tion.8 More recently, Morgan and Pimentel used steady-state
and time-resolved emission spectroscopy to show that Xe
produced an increase in the phosphorescence quantum yield
and a concomitant decrease in the fluorescence quantum yield
of (dimethylamino)benzonitrile in cryogenic Xe matrices. This
was attributed to an increase in the T1 f S0 and S1 h
T1 rates via HA-induced SOC.9 Other examples of Xe-
induced ISC in anthracenes and â,γ-unsaturated ketones are
(
(
9) Morgan, M. A.; Pimentel, G. C. J. Phys. Chem. 1989, 93, 3056.
10) (a) Tanaka, F.; Hirayama, S.; Shobatake, K. Chem. Phys. Lett. 1992,
1
0
known.
1
1
95, 243. (b) Calcaterra, L. T.; Schuster, D. I. J. Am. Chem. Soc. 1981,
03, 2460.
(
(
6) Anderson, M. A.; Grissom, C. B. J. Am. Chem. Soc. 1995, 17, 5041.
7) (a) Carroll, F. A.; Quina, F. H. J. Am. Chem. Soc. 1972, 94, 6246.
(11) Solubility Data Series Vol. 2 Krypton, Xenon and Radon-Gas
Solubilities; Clever, H. L., Ed.; Pergamon Press: New York, 1979; p 159.
(12) Turro, N. J.; Cheng, C.; Abrams, L.; Corbin, D. R. J. Am. Chem.
Soc. 1987, 109, 2449.
(
b) Carroll, F. A.; Quina, F. H. J. Am. Chem. Soc. 1976, 98, 1.
(
8) MacBeath, M. E.; Unger, I. Can. J. Chem. 1971, 49, 594.

9554 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Anderson and Grissom
Table 1. Diene and Bicyclohexane Production in the Direct
Photolysis of DBO as a Function of Faujasite Y Cation
zeolite
% 1,5-hexadiene
% bicyclo[2.2.0]hexane
LiY
NaY
KY
RbY
CsY
62.6 ( 0.8
64.8 ( 0.5
65.6 ( 0.8
72.5 ( 0.5
74.8 ( 0.9
38.4 ( 0.8
35.2 ( 0.5
34.4 ( 0.8
27.5 ( 0.5
25.2 ( 0.9
NaA
65.7 ( 0.7
34.3 ( 0.7
Table 2. Estimated Spin-Orbit Coupling Constants for Metal
Cations and the Isoelectronic Noble Gas Atoms
cation (isoelectronic
noble gas)
electronic
configuration
spin-orbit coupling
-1 a
constants (ú/cm )
+
1s2
He 2s 2p
Ne 3s 3p
Ar 4s 4p
Li (He)
0.7
+
2
6
6
Na (Ne)
520
940
3480
6080
Figure 1. Percent 1,5-hexadiene produced in the direct photolysis (366
nm) at 25 °C of DBO in NaY zeolite as a function of xenon (b), argon
+
2
K (Ar)
+
2
6
Rb (Kr)
(9), and helium (2) gas pressures.
+
2
6
Cs (Xe)
Kr 5s 5p
a
-1
p
Reference 3. The ú/cm values were estimated by finding ú for
the first Xe molecule provides most of the enhanced ISC effect.
Additional Xe atoms have a smaller or an inconsequential added
effect.13
n
an excited sp configuration. A p-electron was promoted to form the
excited configuration.
The effect of Xe is to increase ISC in the diazenyl diradical
or after the second C-N bond cleavage has occurred as the
cyclohexane-1,4-diyl is forming on an activated (high energy)
surface. On this high-energy surface the energies of the singlet
and triplet states are degenerate. There is no effect of Xe on
the 1(nfπ*) and 3(nfπ*) transition in DBO before bond
cleavage, as indicated by the unaltered fluorescence lifetime
and emission spectrum of DBO and the DBO/zeolite adsorbate
in the presence and absence of Xe (see supporting information).
Phosphorescence has never been observed in DBO.1
The value of SOC for ground-state noble gases is quite small,
as only a trivial amount of polarization exists in the filled
which phosphorescence had never been observed previously.15a
Furthermore, when faujasites are exchanged with heavy cations
+
+
+
(
e.g. Rb , Cs , and Tl ), the normal fluorescence emission
spectrum of excited naphthalene can be replaced with a low-
energy emission band ascribed to phosphorescence. This effect
15a,c
has been seen with several other organic guests.
Another
excellent example is the observation of phosphorescence at 298
+
K from trans-stilbenes in Tl -containing faujasites. This is
significant in that only weak phosphorescence from stilbenes
4
has been recorded at 77 K in an organic glass containing ethyl
16
iodide as the heavy atom perturbant.
These effects are
attributed to the enhanced intersystem crossing efficiency of
the heavy cation zeolites along with their ability to encapsulate
the guest and keep it close to the cation. In this study, we have
photolyzed DBO in light and heavy cation exchanged Y zeolites
to compare with the effects induced by Xe.
(
closed-shell) valence octet. Spin-orbit coupling values for the
-
1
excited state are reported to be as follows: He, 0.7 cm ; Ne,
5
-
1
-1
-1
-1 3
20 cm ; Ar, 940, cm ; Kr, 3480 cm ; and Xe, 6080 cm .
These should be regarded as being close to maximum values
that will only be realized in a high-energy environment. If the
+
+
+
+
+
The results for Li , Na , K , Rb , and Cs exchanged
zeolites are shown in Table 1. Two trends are readily apparent.
-1
SOC of Xe were as high as 6080 cm , then a Xe concentration
of 1.3 M should have produced more than a 3% increase in
triplet product, by comparison to the 10% increase in triplet
product realized with 20% iodooctane (1.1 M) solution. We
believe adsorption of the Xe atom in the electrostatic environ-
ment of the zeolite cavity polarizes the Xe atom and increases
its ability to induce SOC. Although this polarization may be
small in NaY,4 this slight polarization may be sufficient to
induce a high level of SOC. This polarization will decrease
the energy gap between the ground state and the lowest excited
state and thereby increase SOC.1 Alternatively, a van der
Waal’s complex of Xe with DBO during photolysis may be
sufficient to induce SOC.
+
+
First, the heavy atom cations, Rb and Cs , show an increase
in the percentage of diene. The amount of diene formed when
Cs is included in the zeolite matrix is the same as that produced
+
when Xe is adsorbed in the zeolite. The percentage of diene
remains at ≈75% when the DBO/CsY adsorbate is flushed with
Xe and photolyzed. This observation suggests that ISC has
produced an equilibrium between singlet and triplet spin states
a
+
and that Cs and adsorbed Xe have roughly the same effect on
the rate of ISC. This is not surprising, since these species are
isoelectronic and should have similar SOC parameters.
+
SOC Constant for Cs . The SOC constant that is often
+
-1
reported for Cs is 840 cm , but this is derived for the first
Effects of Heavy Atom Cations in the Faujasite Cage. A
+ 3,5a
excited electronic state, and not for Cs .
correct, Cs would have only a minimal ability to induce SOC.
If this value were
large volume of work detailing the enhancement of ISC rates
+
by heavy atom cations in zeolites is available.5
,15
In elegant
+
+
+
+
+
The electronic states for Li , Na , K , Rb , and Cs compare
more closely with the noble gases He, Ne, Ar, Kr, and Xe and
should have similar SOC constants. In fact, since the metal
cations contain an additional proton in the nucleus, their
respective SOC constants may be slightly greater than the
corresponding noble gases. The contrasting values are given
in Table 2.
work, Ramamurthy et al. have shown that heavy atom cations
have been used to record phosphorescence from polyenes for
(
13) Kim, T.; Choi, Y. S.; Yoshihara, K. Chem. Phys. Lett. 1995, 247,
41.
14) (a) Engel, P. S.; Steel, C. Acc. Chem. Res. 1973, 6, 275. (b) Clark,
5
(
W. D. K.; Steel, C. J. Am. Chem. Soc. 1971, 93, 6347. (c) Engel, P. S.;
Nalepa, C. J. Pure Appl. Chem. 1980, 52, 2621.
It has been suggested that the electron cloud of the AlO4-
moiety, which forms part of the faujasite wall, is highly extended
(15) (a) Ramamurthy, V.; Caspar, J. V.; Eaton, D. F.; Kuo, E. W.; Corbin,
D. R. J. Am. Chem. Soc. 1992, 114, 3882. (b) Ramamurthy, V.; Corbin, D.
R.; Kumar, C. V.; Turro, N. J. Tetrahedron Lett. 1990, 31, 47. (c)
Ramamurthy, V.; Caspar, J. V.; Corbin, D. R.; Eaton, D. F. J. Photochem.
Photobiol A 1989, 50, 157.
(16) (a) Saliel, J.; Khalil, G. E.; Schange, K. Chem. Phys. Lett. 1980,
70, 233. (b) Garner. H. J. Phys. Chem. 1989, 93, 1826.

Photolysis of 2,3-Diazabicyclo[2.2.2]oct-2-ene
J. Am. Chem. Soc., Vol. 118, No. 40, 1996 9555
spatially and may tend to completely shield the cationic charge.
However, it was pointed out in the same work that the larger
little or no effect on the product ratio. This leaves us with no
definite explanation for the increase in diene/bicyclohexane ratio
for the light cation zeolites relative to the product ratios observed
in homogeneous solution photolysis.
+
-
cations (>Na ) protrude beyond the electronic cloud of AlO4 .
4
a
In this case, the nature of the species will clearly be cationic.
+
To consider these cations to have the low SOC value of the
excited ground state atom seems unreasonable.
It might be argued that an increase in size of the cation (Rb ,
+
Cs ) and the added steric hindrance of a multiple-occupied
The second trend from Table 1 is the increased fraction in
diene production relative to photolysis of DBO in n-octane. The
increase in HD:BCH product ratio from 50:50 to 65:35 in the
light cation exchanged zeolites at first seemed to suggest that
the micropolarity of the cage is increasing the rate of ISC.
Zeolites containing light cations have a greater electrostatic field
supercage might lead to the increase in the HD:BCH ratio when
RbY and CsY are utilized as hosts. We feel this is unlikely,
since our previous work with heavy atom containing solutions
shows the same enhanced ratio. In addition, since it is generally
accepted that the cyclohexane-1,4-diyl must relax into the twist
boat or chair conformation (from the initial boat form) to open
to the 1,5-hexadiene, any steric interference with this process
should lead to more bicyclohexane, and not the diene, being
formed preferentially.
20
+
+
+
+
+
within the cage (Li > Na > K > Rb > Cs ) and are known
to produce changes in the photophysics of organic mol-
ecules.5a It is possible that the electrostatic field within the cage
may be polarizing the DBO molecule (most likely after at least
one C-N bond has broken) thereby leading to an increased rate
of ISC.
Mechanism of Product Formation in the Photolysis of
DBO. These results have forced us to slightly modify our initial
proposal for the mechanism of product formation in the
6
photolysis of DBO. No bicyclohexane can be formed from
To evaluate this possibility, DBO was photolyzed while
adsorbed to the surface of NaA zeolite. NaA has much smaller
openings to the cage system (≈2 Å) thereby excluding most
organic molecules from the supercage. No difference in product
yield was observed in the photolysis in the supercage and on
the surface. DBO was adsorbed onto celite as a further
investigation of surface photolysis. The HD:BCH product ratio
was within 1% of the NaA results. These results suggest that
the electrostatic field inside the supercage is not responsible
the singlet twist boat conformation. Before or simultaneous
with deazatization, ISC can be increased by neighboring heavy
atoms up to the 75:25 equilibrium between the singlet and triplet
electron spin multiplicities. The singlet diazenyl diradical closes
to bicyclohexane with concomitant loss of N2 while the triplet
diazenyl diradical relaxes to the twist-boat conformation,
extrudes N2, and eventually undergoes ISC to the singlet
diradical and undergoes ring opening to give 1,5-hexadiene
(Scheme 1).
Direct irradiation of the diazene chromophore produces the
excited singlet state of DBO. This species can lose energy
through internal conversion, fluorescence emission, or homolysis
of the C-N bond to produce the diazenyl diradical. [Phos-
phorescence has never been observed in intact DBO.]
Rapid ISC may occur at either of two points in the reaction.
Rapid mixing of the spin states may occur in the diazenyl
diradical to produce the 3:1 ratio of singlet to triplet cyclohex-
ane-1,4-diyl directly. Alternatively, rapid ISC may occur in
the 1,4-cyclohexanediyl, but only on a very high energy surface,
where the energy of the singlet and triplet electron spin state is
equal (hence the 3:1 statistical mixture). Since C-N bond
cleavage requires thermal activation (Ea ) 8-9 kcal/mol), bond
homolysis and N2 extrusion must lie on a reaction surface that
is even higher in energy than the reaction surface produced by
photochemical excitation alone. Fast loss of N2 would produce
+
+
+
for the increase in diene production in the Li , Na , and K Y
faujasites.
The surface photolysis results also seem to minimize the
importance of the “lebensraum”, or void space effect, on the
product distribution. Turro and co-workers have shown the
effect of varying the supercage size on product ratios in the
photolysis of dibenzyl ketone (DBK).17 DBK photolysis in
homogeneous solution yields diphenylethane (DPE) as the only
isolated product. When the DBK is introduced inside MY
zeolites, the yield of DPE decreases and the appearance of two
new products, o-DBK and p-DBK, increases as the exchanged
+
+
+
cation increases in size (Cs > Rb > K ). It is suggested
this effect is caused by the steric constraints imposed by the
larger cations and thereby leads to a reduction in void space. If
proper separation of the primary DBK radical pair is limited
by these steric factors, decarbonylation cannot occur and the
radical pair will react to form the isomeric ketones DBK,
o-DBK, and p-DBK depending on the amount of movement
1
,4-cyclohexanediyl in a boat geometry with the unpaired
electronic orbitals oriented orthogonally. Calculations at the
-31G* level reveal than the boat conformation is not an energy
minimum, but relaxes to the twist-boat or chair conformers that
17
allowed by the supercage. The MX faujasites, which have
roughly double the amount of exchangeable cations, and thus
even more cramped supercages, show an even larger lebensraum
effect than their MY counterparts. Other lebensraum effects
6
21
are local minima on the energy surface. In the boat conforma-
tion of 1,4-cyclohexanediyl, the energy gap between the singlet
and triplet state is large (∆Ea ) 22 kcal/mol) and this precludes
reversible ISC in the true boat geometry. This mechanism is
not vastly different than that proposed by Edmunds.2
1
8
on Norrish type I and II reactions are also well-known.
The similar HD:BCH product ratios observed in our study
+
+
+
of DBO contained in the light cation (Li , Na , K ) zeolite
and surface photolysis experiments indicate that interior size
has little to do with product ratio in our system. The relatively
heavy loading of DBO in our samples suggests that many
0b
Conclusions
1
9
supercages may contain up to three DBO molecules. Even
this added steric hindrance inside the void space seems to have
The spin-orbit coupling afforded by Xe is enhanced through
polarization by adsorption in the zeolites or adsorption in a
microenvironment that will influence the distribution of the
(
17) (a) Ramamurthy, V.; Corbin, D. R.; Eaton, D. F. Tetrahedron Lett.
989, 30, 5833. (b) Garcia-Garibay, M. A.; Zhang, Z.; Zhang, Z. J. Am.
Chem. Soc. 1991, 113, 6212. (c) Zhang, Z.; Turro, N. J, Tetrahedron Lett.
1
(19) This estimation is based on the calculated size of DBO (4.62 Å ×
4.28 Å × 3.05 Å) and the space available inside the NaY zeolite. DBO is
not assumed to “stack” well.
(20) (a) Engel, P. S.; Horsey, D. W.; Keys, D. E.; Nalepa, C. J.; Soltero,
L. R. J. Am. Chem. Soc. 1983, 105, 7108. (b) Edmunds, A. J. F.; Samuel,
C. J. J. Chem. Soc., Perkins Trans. 1 1989, 1267.
1
3
987, 28, 5637. (d) Zhang, Z.; Turro, N. J. Tetrahedron Lett. 1989, 30,
761.
(
18) (a) Ramamurthy, V.; Corbin, D. R.; Turro, N. J.; Sato, Y.
Tetrahedron Lett. 1989, 30, 5829. (b) Ramamurthy, V.; Sanderson, D. R.
Tetrahedron Lett. 1992, 33, 2757. (c) Ramamurthy, V.; Corbin, D. R.;
Johnston, L. J. J. Am. Chem. Soc. 1992, 114, 3870.
(21) Roberson, M.; Simons, J., University of Utah, personal communica-
tion.

9556 J. Am. Chem. Soc., Vol. 118, No. 40, 1996
Anderson and Grissom
closed electronic octet through noncovalent interactions. Xenon
has the added advantage of being non-reactive toward atom
abstraction and it is transparent at the wavelengths commonly
used to initiate photolysis.
(366 nm) and the quantum yield for deazatization of DBO at 25 °C is
25
low (Φ
r
6 6
) 0.018, C H ). Lower loading levels do not produce
sufficient of product for analysis. The DBO loaded zeolite (≈70 mg)
was placed in a closed end Pyrex tube (10 cm long, o.d. 9 mm, i.d. 5
mm). The sample tube was evacuated at 0.02 Torr for 30 min and
flame-sealed.
The product ratio reaches a level of 3:1 1,5-hexadiene:
bicyclohexane when DBO is photolyzed inside heavy atom
containing faujasites. The heavy atoms may be in the form of
The DBO/zeolite samples were photolyzed for 6 h using a 450-W
compact Hg arc lamp that was filtered to allow passage of a 100-nm
band centered at 366 nm. After 1 h, the sample was rotated 180°, and
after 2 h, the sample was agitated thoroughly. This process was
continued for the entire photolysis period to provide the maximum
conversion of azoalkane to products. After photolysis, the zeolite was
stirred in 0.5 mL of dry toluene (Omnisolv) for 10 h to extract the
photolysis products and any excess DBO. The zeolite was separated
from the toluene by centrifugation. Several samples were extracted
three times to determine if either product was preferentially adsorbed.
The first extraction typically yielded ≈85% of the products, the second
+
+
exchangeable cations (Rb , Cs ) or adsorbed Xe. The product
ratio of 1,5-hexadiene:bicyclohexane is controlled by the
electron spin multiplicity of the intermediate diradical. The spin
multiplicity reaches an equilibrium of 3:1 (three triplet states
for each singlet state) when ISC crossing, via SOC, is
maximized by the use of heavy atoms in the zeolite cage. The
zeolites also have the advantage of keeping the diradical in close
proximity to the heavy atom.
Rapid ISC induced by heavy atoms may take place in the
diazenyl diradical or in the high-energy cyclohexane-1,4-diyl.
The singlet diazenyl diradical closes to form bicyclohexane,
whereas the triplet diazenyl of the diyl relaxes from the boat
conformation to the twist-boat, and then possibly to the chair,
before undergoing ISC to the singlet and ring opening to the
≈
10%, and the third ≈5%. GC analysis indicated there was no
difference in product ratio from one extraction to the next. The extent
of photolysis was typically 1-2%. It should be noted that the samples
must be sealed under vacuum and photolyzed soon after removal of
all pentane (1-2 days). Older non-vacuum sealed samples (≈2 weeks)
showed a small amount of 1,5-hexadiene and bicyclohexane product
upon extraction even without photolysis. Freshly prepared (<2 days),
vacuum sealed samples showed no products in the absence of
photolysis.
1
,5-hexadiene product.
Experimental Section
DBO/NaY/Xe Sample Preparation. A Pyrex sample tube was
attached to a 4 in. section of 3/8 in. copper tubing with Torr Seal. The
connection was cured at 110 °C for 12 h. The Pyrex/copper tube
assembly was filled with the NaY/DBO and a 200 test stop valve was
fitted to the assembly. The assembly was connected to a 25 L tank of
Xe gas (Spectra Gases, 99.999%) equipped with a 200 psi regulator.
The sample and gas line were evacuated at 0.02 Torr for 30 min. The
system was flushed with Xe gas to pressures of 5-120 psi. The valve
was closed and the sample was allowed to incubate for 30 min prior to
photolysis. The DBO/zeolite/Xe samples were photolyzed and ex-
tracted, and the products were analyzed as described above.
Fluorescence Studies (Available as Supporting Information).
Fluorescence studies on DBO/n-octane, DBO/n-octane/Xe, DBO/NaY,
and DBO/NaY/Xe systems were carried out with an Aminco-Bowman
spectrofluorimeter. The apparatus was used in the reflectance mode
for all measurements. The excitation wavelength was 337 nm (the
n-π* transition of the -NdN- chromophore) and the emission
wavelength was scanned from 350 to 500 nm. No change in emission
intensity or any shift in λmax (405 nm) was observed for any of the
four measurement conditions.
2
2
General. The synthesis of DBO and the identification of products
6
for DBO photolysis have been reported previously. All solvents used
for loading and extracting zeolites were of the highest possible purity.
Zeolite Preparation and DBO Loading. The NaY zeolite powder
was purchased from Aldrich. The zeolite was calcined at 565 °C for
2 h to destroy any organic contaminants. Cation exchange (Li , K ,
Rb , Cs ) of the zeolite was carried out according to literature
+
+
1
+
+
18c,23
methods.
by contacting the powder with the appropriate 10% nitrate solution
LiNO , NaNO , etc.) at 90 °C for 4 h. For each gram of zeolite, 10
The cation of interest was exchanged into the NaY zeolite
(
3
3
mL of the cation nitrate solution was used. This cycle was repeated
three times. The zeolite was filtered, washed thoroughly, and dried at
1
20 °C for 30 min. The zeolite cake was crushed into a fine powder
and heated under vacuum (0.02 Torr) at 300 °C for 3 h. This process
was also carried out with NaNO to ensure that all samples were
manipulated in the same manner. From this point, the zeolite sample
was handled in a N atmosphere.
3
2
To load the dry zeolite, 50 mg of DBO was dissolved in 4 mL of
dry pentane (Omnisolv). To this solution was added 350 mg of the
dry NaY and the slurry was stirred for 10 h. The excess pentane was
removed and the zeolite was washed with three 4-mL aliquots of fresh
pentane. The mother liquid and the three washings were combined
and the solvent was removed by rotary evaporation. Typically, <2
mg unadsorbed DBO was recovered. The DBO/zeolite powder was
dried by passing N gas over the surface for 15 min. The sample was
evacuated at 0.02 Torr for 48 h until the amount of pentane was <0.5%
of that of DBO. The high loading level of DBO (14%) is needed
because the zeolite absorbs at the wavelength for photolysis of DBO
Acknowledgment. The authors would like to thank Mr.
Mark Roberson for his helpful discussions of the mechanism
of DBO photolysis and his help with the 6-31G* calculations.
We would also like to thank Professor William Breckenridge
for his discussions of SOC values. This work is supported by
a grant from the National Institute of Environmental Health
Sciences (ES 05728).
2
4
2
(
22) (a) Cohen, S. G.; Zand, R. J. Am. Chem. Soc. 1962, 84, 586. (b)
Askani, R. Chem. Ber. 1965, 98, 2551.
23) Ramamurthy, V.; Sanderson, D. R.; Eaton, D. F. Photochem.
Photobiol. 1992, 56, 297.
24) Considering the entropic driving forces that derive from the ratio
Supporting Information Available: Fluorescence emission
spectra of (a) DBO/heptane (0.03 M); (b) DBO/heptane (0.03
M)/60 psi Xe; (c) DBO/NaY zeolite (14% loading); and (d)
DBO/NaY zeolite (14% loading)/60 psi Xe (1 page). See any
current masthead page for ordering and Internet access
instructions.
(
(
of internal to external surface areas (1000-10000) and the enthalpic driving
forces from the electronic interactions between guest molecules and the
cations of the zeolites, we assume that the majority of the DBO is taken
into the supercage and not adsorbed onto the outer surface. (a) Turro, N.
J.; Zhang, Z. In Photochemistry on Solid Surfaces; Anpo, M., Matsuura,
T., Eds.; Elsevier: New York, 1989. (b) Turro, N. J. Pure Appl. Chem.
JA961546H
(25) Engel, P. S.; Keys, D. E.; Kitamura, A. J. Am. Chem. Soc. 1985,
107, 4964.
1
986, 58, 1219. (c) Suzuki, I.; Oki, S.; Namba, S. J. Catal. 1986, 100, 219.