Modification of photochemical reactivity by nafion. Photocyclization and photochemical cis-trans isomerization of azobenzene

Article abstract of DOI:10.1021/jo960946k

The photophysical and photochemical behavior of azobenzene (AB) incorporated into solvent-swollen acid form Nafion (Nafion-H⁺) membranes were examined. In water-swollen Nafion-H⁺ membrane AB exhibits strong fluorescence at room temperature, which has never been observed in isotropic solvents. Photolysis of AB adsorbed in water-swollen Nafion-H⁺ results in its cyclization to give benzo[c]cinnoline (BC) and benzidine (BZ) in quantitative yield. The product distribution is very dependent upon the number of AB molecules in each water cluster of the Nafion membrane (occupancy number). In the case of the occupancy number greater than 2, BC and BZ are formed equimolarly, while when only one AB molecule exists in each water cluster, BC is exclusively produced. The AB molecules incorporated into methanol-swollen Nafion-H⁺ membrane do not emit fluorescence and only undergo cis-trans isomerization when they are photoirradiated. These observations suggest that in water-swollen Nafion-H⁺ AB molecules are solublized in the fluorocarbon/water interface, and the protons of the Nafion-H⁺ participate in the photochemical and photophysical processes of AB. On the other hand, in methanol-swollen Nafion-H⁺ membrane AB molecules are located in the methanol pools, and their photochemical and photophysical behaviors are not intervened by the Nafion protons.

Full text of DOI:10.1021/jo960946k

J . Org. Chem. 1996, 61, 9417-9421
9417
Mod ifica tion of P h otoch em ica l Rea ctivity by Na fion .
P h otocycliza tion a n d P h otoch em ica l Cis-Tr a n s Isom er iza tion of
Azoben zen e
Chen-Ho Tung* and J ing-Qu Guan
Institute of Photographic Chemistry, Chinese Academy of Sciences, Beijing 100101, China
Received May 22, 1996X
The photophysical and photochemical behavior of azobenzene (AB) incorporated into solvent-swollen
acid form Nafion (Nafion-H+) membranes were examined. In water-swollen Nafion-H+ membrane
AB exhibits strong fluorescence at room temperature, which has never been observed in isotropic
solvents. Photolysis of AB adsorbed in water-swollen Nafion-H+ results in its cyclization to give
benzo[c]cinnoline (BC) and benzidine (BZ) in quantitative yield. The product distribution is very
dependent upon the number of AB molecules in each water cluster of the Nafion membrane
(occupancy number). In the case of the occupancy number greater than 2, BC and BZ are formed
equimolarly, while when only one AB molecule exists in each water cluster, BC is exclusively
produced. The AB molecules incorporated into methanol-swollen Nafion-H+ membrane do not emit
fluorescence and only undergo cis-trans isomerization when they are photoirradiated. These
observations suggest that in water-swollen Nafion-H+ AB molecules are solublized in the
fluorocarbon/water interface, and the protons of the Nafion-H+ participate in the photochemical
and photophysical processes of AB. On the other hand, in methanol-swollen Nafion-H+ membrane
AB molecules are located in the methanol pools, and their photochemical and photophysical
behaviors are not intervened by the Nafion protons.
In tr od u ction
Selectivity in organic phototransformations continues
to be one of the main topics of current interest.1 Of the
various approaches use of constrained and ordered media
had shown considerable promise.1,2 Nafion represents a
novel and unique family of polymers which consists of a
perfluorinated backbone and short pendant chains ter-
minated by sulfonic groups.
[(CF2CF2)mCFCF2]n
(OCF2CF)kCF3
m = 5–15.3
n = ca. 1000
k = 1, 2, 3
O
F igu r e 1. Schematic representation of the two-phase cluster-
network model for Nafion membranes.
CF2CF2SO3H
trations of aromatic hydrocarbons and organic dyes, thus
raising the possibility of obtaining high local concentra-
tion of organic molecules and inorganic cations. These
optically transparent membrane systems are readily
amenable to spectroscopic investigations. Because of
these attractive properties, this polymer has been utilized
as a medium for photophysical and photochemical studies
in recent years.9-12 Up to now, the majority of the
photochemical studies within the water-swollen Nafion
have made use of this material just as a heterogeneous
and micellelike medium to gain control of the pathways
undergone by the substrate excited states. However, this
polymer in its protonated form is a superacid.13 The acid
groups can act as active sites and directly intervene in
the chemical process occurring in the water-containing
pockets of the polymer.
When swollen in water, the structure of Nafion is
believed to resemble that of a reverse micelle (Figure
1).3-5 It is suggested that the hydrated SO3- head groups
are clustered together in a water-containing pocket of ca.
50 Å in diameter, which are interconnected by short
channels (ca. 10 Å in diameter) within the perfluorocar-
bon matrix (Figure 1). The polymer backbone of Nafion
provides exceptional chemical, thermal, and mechanical
stability while the sulfonic acid groups provide ion-
exchange and swelling abilities. It has been established6-8
that water-swollen Nafion can incorporate high concen-
X Abstract published in Advance ACS Abstracts, December 1, 1996.
(1) Ramamurthy, V., Ed. Photochemistry in Organized and Con-
strained Media; VCH: New York, 1991.
(2) Fox, M. A., Ed. Organic Phototransformations in Non-homogeous
Media; American Chemical Society: Washington D. C., 1985.
(3) Komoroski, R. A.; Mauritz, K. A. J . Am. Chem. Soc. 1978, 100,
7487.
(4) Lee, P. C.; Meisel, D. J . Am. Chem. Soc. 1980, 102, 5477.
(5) Sondheimer, S. J .; Bunce, N. J .; Fyfe, C. A. J . Macromol. Sci.,
Rev. Macromol. Chem. Phys. 1986, C26, 353.
(6) Lee, P. C.; Meisel, D. Photochem. Photobiol. 1985, 41, 21.
(7) Szentirmay, M. N.; Prieto, N. E.; Martin, C. R. J . Phys. Chem.
1985, 89, 3017.
(9) Niu, Er-Ping; Mau, A. W.-H. Aust. J . Chem. 1991, 44, 695.
(10) Mohan, H.; Iyer, R. M. J . Chem. Soc., Faraday Trans. 1992,
88, 41.
(11) Mika, A. M.; Lorenz, K.; Azczuek, A. J . Memb. Sci. 1989, 41,
163.
(12) Priydarsini, K. I.; Mohan, H.; Mittall, J . P. J . Photochem.
Photobiol. A. Chem. 1993, 69, 345.
(8) Niu, E.-P.; Ghihhino, K. P.,;Smith, T. A.; Mau, A. W.-H. J .
Lumin. 1990, 46, 191.
(13) Sondheimer, S. T.; Bunce, N. J .; Lemke, M. E.; Fyfe, C. A.
Macromolecules 1986, 19, 339.
S0022-3263(96)00946-2 CCC: $12.00 © 1996 American Chemical Society

9418 J . Org. Chem., Vol. 61, No. 26, 1996
Tung and Guan
Ta ble 1. Occu p a n cy Nu m ber of AB in Wa ter -Sw ollen
Na fion Mem br a n e w ith Va r iou s Am ou n t of AB a n d Wa ter
concentration of AB in Nafion (¥103 M)
water contents
(wt %)
3.8
19.0
22.0
30.8
40.0
23
17
12
10
6
0.34
0.30
0.25
0.236
0.204
1.82
1.49
1.26
1.18
1.02
2.00
1.73
1.48
1.37
1.18
2.77
2.40
2.05
1.92
1.65
3.56
3.14
2.66
2.49
2.01
the ns f π1*, π1 f π1* and Φ1 f Φ1* transitions
respectively.14 The ns f π1* transition is slightly red-
shifted while that of the π1 f π1* slighly blue-shifted
compared to those in water (415, 318, and 228 nm in
water). This observation suggests that the environment
around AB molecules in Nafion is slightly less polar than
that in water. It has been established that AB can
undergo protonation in acidic aqueous solution.14 Thus,
it might be expected that AB exists as its protonated form
(ABH+) when it is incorporated into Nafion-H+ mem-
brane. However, this suggestion cannot be substantiated
only by UV absorption, since the absorption spectra for
the protonated and neutral AB are identical.15 Consider-
ing the hydrophobicity and the positive charge of ABH+,
it is likely that the cationic ABH+ molecules are located
close to the sulfonate head groups in the fluorocarbon/
water interface of the membrane.
The solubility of AB in water-swollen Nafion mem-
brane is rather high (4 × 10-2 M, calculated in its swollen
form). We have prepared the Nafion-H+ membrane
samples adsorbing various amount of water and AB. By
using the parameters reported in the literature,5,16 we
could calculate the occupancy numbers (the number of
AB molecules contained in each water cluster of Nafion,
F igu r e 2. Absorption spectra of azobenzene: incorporated
into water-swollen Nafion membrane (s); in aqueous solution
(‚‚‚‚); background absorption of Nafion membrane (----).
In the present paper we illustrate the influence of the
chemical environment of Nafion host on the photochem-
istry of azobenzene (AB) adsorbed in water- and methanol-
swollen acid form Nafion (Nafion-H+) and provide an
example that a cooperative contribution of the acid sites
of the Nafion-H+ is required in the course of photochemi-
cal reaction performed within swollen Nafion. The azo-
benzene system follows two different reaction pathways
depending on the photolysis condition.14 Thus, upon
irradiation in neutral medium cis-trans isomerization
occurs and photostationary mixtures of both stereoiso-
mers are obtained. However, when the photolysis is
performed in the presence of acids, azobenzene undergoes
the photochemical cyclization to benzo[c]cinnoline (BC),
and variable amounts of benzidine (BZ) are produced. In
this case, the conjugated acid of azobenzene (not the
neutral molecule) is the species which undergoes ring
closure. We demonstrate that by including the azoben-
zene molecules in solvent-swollen Nafion one can control
the reaction pathways. Furthermore, we observed the
fluorescence of azobenzene at room temperature for the
first time.
n
AB/cluster) of the samples, which are listed in Table 1.
Results show that each cluster can hold 3.5 AB molecules.
In order to obtain information about the adsorption
process, we analyzed the concentration of AB incorpo-
rated into the Nafion membrane as a funcion of soaking
time. The following eqution was employed for the study
of the adsorption kinetics of AB into the membrane.17
x
x
Ct/C∞ ) 4 DT/T π
(1)
Ct and C∞ represent the concentrations of AB in the
membrane at time t and after equilibration, respectively.
D is the diffusion coefficient in cm2 s-1, and T is the thick-
Resu lts a n d Discu ssion
In cor p or a tion a n d Absor p tion Sp ectr u m of Azo-
ben zen e (AB) in Na fion Mem br a n e. Azobenzene
normally exits in the stable trans-form, and the cis-
isomer easily thermally reverted to the trans-isomer.14
Thus, in the present study we only investigate trans-
azobenzene. We could successfully incorporate AB into
water-swollen Nafion membrane by soaking the mem-
brane in an AB aqueous solution. Figure 2 shows the
absorption spectrum of the Nafion membrane after
equilibrating with a 2 × 10-5 M solution of AB in water.
Undoubtedly, this absorption spectrum is mainly origi-
nated from the AB incorporated into the Nafion. The
background absorption of the membrane itself is also
shown in Figure 2, and is rather low in the near UV
region. Substraction of the background absorption from
the total absorption spectrum of the sample yielded the
absorption spectrum of AB in Nafion, the maxima of
which are at ca. 418, 315, and 228 nm, corresponding to
x
ness in cm. The plot of Ct vs t is linear. From the
plot, the diffusion coefficient was determined to be 7.6 ×
10-10 cm2 s-1. This confirms the applicability of eq 1 and
shows that the sorption is diffusion-controlled, like the
sorption process of Rhodamine 6G in Nafion membrane.10
The adsorption behavior of AB into methanol-swollen
Nafion-H+ membrane is completely different from that
for the water-swollen one. The enhancement of UV
absorption of AB in methanol-swollen Nafion membrane
fails to occur. When the membrane sample is immersed
in the desired methanol solution of AB and reaches
equilibration, the concentration of AB in the membrane
is almost the same as that in the bulk solution. This
observation suggests that in methanol-swollen Nafion AB
(15) Sadtler Research Laboratories, Sadtler Ultra Violet Spctra,
136UV.
(16) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J . Polym. Sci., Polym.
Phys. Ed. 1981, 19, 1687.
(14) Griffiths, J . Chem. Soc. Rev. 1972, 1, 481.
(17) Helfferich, F. Ion Exchange; McGraw Hill: New York, 1962.

Modification of Photochemical Reactivity by Nafion
J . Org. Chem., Vol. 61, No. 26, 1996 9419
Sch em e 1. Mech a n ism of th e F or m a tion of
Ben zo[c]cin n olin e (5) a n d Ben zid in e (7)
F igu r e 3. Fluorescence spectra of azobenzene incorporated
into water-swollen Nafion-H+ membrane: water content of 23
wt% (s); water content of 6 wt % (‚‚‚‚). All of the spectra were
obtained at azobenzene concentration of 3.8 × 10-3 M, excita-
tion at 410 nm.
We could not detect any fluorescence emission from AB
incorporated into methanol-swollen Nafion-H+ membrane.
This observation again suggests that AB is solublized in
the methanol pool, and the protons of the Nafion-H+ do
not intervene in the photochemical process of AB.
P h otocycliza tion of AB In cor p or a ted in to Wa ter -
Sw ollen Na fion Mem br a n e. Photoirradiation of water-
swollen Nafion-H+ membrane soaking AB results in the
cyclization of AB to give benzo[c]cinnoline (BC). Variable
amounts of benzidine (BZ) is also produced if the oc-
cupancy number of AB in the water cluster of the Nafion
is large, as shown in Scheme 1. This reaction could be
followed by the UV absorption of the sample. In general,
after 1 h irradiation with a 150-W Hanovia high-pressure
mercury lamp the absorption of AB completely disap-
pears. Instead, an absorption spectrum with maxima at
248 and 354 nm is detected, which is assignable to the
absorption spectrum of BC in protonated form (BCH+).19
The products can be easily isolated and purified by the
following procedure. Extraction of the irradiated mem-
brane with methanol gives BC. Since BZ is also proto-
nated and may strongly interact with the sulfonate
groups of the Nafion-H+, this compound cannot be
extracted from the Nafion membrane by methanol.
However, after the residual membrane is neutralized
with 1 N NaOH aqueous solution, extraction with diethyl
ether readily gives BZ. The yield of the sum of BC and
BZ was close to 100% based on the consumption of the
starting material. The assignments of the products relies
on their UV absorptions, 1H NMR, and MS spectroscopies
which are given in the Experimental Section.
molecules are located in the methanol pool, as evidenced
by fluorescence and photochemical studies (see below).
F lu or escen ce of AB in Na fion Mem br a n e. It has
been established that AB does not exhibit fluorescence.18
The reason for this is that the first singlet excited state
of this compound is of 1(nπ*) character. Moreover, a
3(ππ*) state lies below but close to the 1(nπ*) state. Thus,
1
radiationless decay of the (nπ*) via efficient intersystem
3
1
crossing to the (ππ*) shortens the lifetime of the (nπ*)
state. On the other hand, Rau18 could detect the fluo-
rescence from the protonated AB (ABH+) at 77 K. This
observation was attributed to the fact that protonation
of the azo-group results in the inversion of the nπ* and
ππ* singlet states, and the radiation transition from the
latter lower energy state to the ground state is orbital
overlap allowed. However, this fluorescence can only be
observed at 77 K, since in fluid solution the twisting of
the molecule about the NdN bond makes the 1(ππ*) state
mix with the 3(nπ*) state, thus enhancing the intersystem
crossing to the triplet state. By incorporating AB mol-
ecules into water-swollen Nafion-H+ membrane, we could
successfully detect the strong fluorescence of AB at room
temperature for the first time. Figure 3 shows the
fluorescence spectra of AB incorporated into water-
swollen Nafion-H+ membrane. The general feature of
this spestrum is identical to that reported in the litera-
ture.18 Obviously, AB is protonated. By reference to the
UV absorption we propose that the molecules of AB are
located in the fluorocarbon/water interface in the mem-
brane, and the constrained environment surrounding the
AB molecule restricts its twisting. Furthermore, for the
sample soaking less amounts of water, the environment
surrounding AB becomes more constrained. Thus, the
fluorescence emission is enhanced. To ascertain the fact
that the acid groups of the Nafion play a role in AB
fluorescence emission, we investigated the photophysical
property of AB incorporated into water-swollen Nafion
membrane in sodium form (Nafion-Na+). No fluorescence
was detected in this case. This suggests that the
cooperative contribution of the acid sites is a requirement
for the fluorescence of AB.
The product distribution was found to be dependent
on the occupancy number of AB in the water cluster of
the Nafion sample, as shown in Table 2. Photolysis of
the sample with an occupancy number greater than 2
leads to the formation of BZ and BC with a ratio of ca.
50:50. However, for the sample with an occupancy
number less than 0.4, we could only isolate the photo-
cyclization product BC in a quantitative yield and no BZ
was detected.
(19) Sadtler Research Laboratories, Sadtler Ultra Violet Spectra,
23847 UV. The UV absorption spectrum of protonated BC in this
handbook also exhibits a maximum at 308 nm in methanol. We found
that this band should be assigned to the absorption of the unprotonated
BC.
(18) Rau, H. Ber. Bunsenges. Phys. Chem. 1967, 71, 48.

9420 J . Org. Chem., Vol. 61, No. 26, 1996
Tung and Guan
Ta ble 2. Mola r Ra tio of BZ to BC in th e P r od u cts of
P h otocycliza tion a s a F u n ction of th e Occu p a n cy
Nu m ber of AB In cor p or a ted in to Wa ter -Sw ollen Na fion
nAB/cluster 3.56 2.40 1.82 1.49 1.18 0.34 0.25 0.016
m
BC/mBZ
0.96 0.96 0.77 0.48 0.12
0
0
0
We have measured the quantum yield for the photo-
cyclization of AB incorporated into the Nafion-H+ mem-
brane and found that the plot of the quantum yield as a
function of the square of the light intensity is linear. This
suggests that the photocyclization is a two-photon pro-
cess. We demonstrated by experiment that photoirra-
diation of AB incorporated into water-swollen Nafion-Na+
membrane could not yield any cyclization product. All
of the observations mentioned above can be rationalized
by the well-established mechanism14,20 for the photocy-
clization which is given by eqs 2 and 3 in Scheme 1.
The conjugated acid of azobenzene, not the neutral
molecule, is the species which undergoes photocyclization.
The first photon converts the protonated trans-isomer of
AB (1) into the cis-isomer (2), and the second photon
results in the cyclization of the cis-isomer to the inter-
mediate 3. Rapid conversion of 3 into the more stable 4
can occur by two prototropic shifts. BC (5) is then formed
by dehydrogenation of 4. The nature of the latter
oxidation step is dependent on the reaction condition.
When the occupancy number of AB in the water cluster
of the Nafion membrane is greater than 2, the probability
of a water cluster to contain two AB molecules would be
high. The dehydrogenation of 4 is caused by ABH+
according to the disproportionation mechanism as shown
by eq 3 in Scheme 1. Thus, for one BC molecule formed,
one molecule of hydrozobenzene (6) is produced. Under
the acidic condition in the water cluster of Nafion-H+, 6
is efficiently rearranged to benzidine (BZ, 7). Conse-
quently, the ratio of BZ to BC in the products is close to
1:1 (Table 2). On the other hand, in the case of the
occupancy mumber less than 0.4 (Table 2), the probability
of one water cluster to host two AB molecules would be
rather low. The dehydrogenation of 4 might be caused
either by oxygen presented in the water cluster of Nafion
or by the -SO3H groups of the Nafion. It has been well
documented21 that cis-stilbenes, like cis-azobenzene, can
undergo photocyclization to give phenanthrene in the
presence of oxygen. The first step of this photocyclization
yields dihydrophenanthrene, which then is oxidized to
phenanthrene by oxygen. By reference to this oxidiza-
tion, 4 might be dehydrogenated by oxygen. On the other
hand, it has been established22 that photoirradiation of
azobenzene in 98% sulfuric acid results in BC in high
yield. In this case, the solvent is the effective dehydro-
genating agents for 4, being reduced during the reaction
to sulfur dioxide. We found that the Nafion-H+ mem-
brane as the medium of the photochemical reaction can
be recycled, but the efficiency for the catalysis of the
photocyclization of azobenzene decreased slightly after
many cycles. Thus, we infer that the SO3H groups of the
Nafion also might be the dehydrogenating agents for 4.
The above two mechanisms for the dehydrogenation of 4
(oxygen vs. SO3H group) cannot be substantiated without
further information. In any case, BC is the unique
product in the photocyclization when the occupancy
number is less than 0.4.
Figu r e 4. Absorption spectra of AB incorporated into methanol-
swollen Nafion-H+ membrane before (s) and after (‚‚‚‚) ir-
radiation.
just a passive medium. The acid sites of the Nafion also
play an essential role in the reaction.
P h otoch em ica l Tr a n s-Cis Isom er iza tion of AB
In cor p or a ted in to Meth a n ol-Sw ollen Na fion Mem -
br a n e. cis-Azobenzene absorbs at shorter wavelength
than the trans-isomer.14 Furthermore, the extinction
coefficient of the ns f π* transition for the cis-isomer is
greater than that for the trans-isomer.14 Thus, the
trans-cis isomerization of AB is easily detected by UV
absorption spectra. By this technique, we successfully
observed the trans-cis isomerization of AB incorporated
into methanol-swollen Nafion-H+ membrane. Figure 4
shows the UV absorption spectra of the samples before
and after photoirradiation. We could obtain a photosta-
tionary mixture of the cis- and trans-isomers but could
not detect any photocyclization product, neither BC nor
BZ. This observation is consistent with the proposal that
in methanol-swollen Nafion-H+ the AB molecules are
solublized in the methanol pool. Thus AB is not proto-
nated and the photocyclization cannot occur.
Con clu sion s
This work illustrates that solvent-swollen Nafion can
play an important role in the selective phototransforma-
tions of azobenzene. In water-swollen Nafion-H+, azoben-
zene molecules are most likely located in the fluorocarbon/
water inteface and exist as their protonated form. In this
constrained medium the protonated azobenzene exhibits
strong fluorescence at room temperature. Photoirradia-
tion of these samples results in cyclization of AB to give
BC and BZ in quantitative yield. Great influence of the
occupancy number of AB in the water cluster of Nafion
on the product distribution was observed. In the case of
one water cluster containing two AB molecules, the
reaction intemediate 4 was dehydrogenated by dispro-
portionation mechanism. Consequently BC and BZ were
formed equimolarly. When one water cluster only hosts
one AB molecule, 4 was probably oxidized by -SO3H
groups of Nafion and/or by oxygen presented in the water
cluster exclusively to yield BC. On the other hand, in
methanol-swollen Nafion AB molecules are solublized in
the methanol pool and cannot be protonated. No fluo-
The results mentioned above illustrates that in the
photocyclization reaction of AB the Nafion is more than
(21) Muszkat, K. A. Top. Curr. Chem. 1980, 88, 89.
(22) Badger, G. M.; J oshua, C. P.; Lewis, G. E. Autr. J . Chem. 1965,
18, 1639.
(20) Badger, G. M.; Drewer, R. J .; Lewis, G. E. Aust. J . Chem. 1966,
19, 643.

Modification of Photochemical Reactivity by Nafion
J . Org. Chem., Vol. 61, No. 26, 1996 9421
by the difference in the absorbance of the solution before and
after the addition of the Nafion membrane. The amount of
the solvent (water or methanol) adsorbed into the membrane
was determined by the difference of the sample weights before
and after soaking.
rescence emission from AB could be detected. Photoir-
radiation of these samples leads to the cis-trans isomer-
ization of AB.
Exp er im en ta l Section
P h otoir r a d ia tion a n d P r od u ct An a lysis. Photoirradia-
tion was carried out in a quartz reactor. A 150-W high-
pressure mercury lamp was used as the excitation source, and
K2Cr2O4 aqueous solution as the filter. The cis-trans isomer-
ization of AB incorporated into methanol-swollen Nafion
membrane was detected by UV absorption spectra. For the
water-swollen samples, after irradiation the membrane was
extracted with methanol, and BC was obtained. The residual
membrane was neutralized with 1 N NaOH aqueous solution
and then extracted with diethyl ether to give BZ. BC and BZ
Ma t er ia ls. Nafion membrane 117 in acid form (Nafion-
H+) with an equivalent weight of 1100 and thickness of 0.0175
cm was a product of Du Pont and was kindly donated by Dr.
A. W.-H. Mau of CSIRO, Division of Chemicals and Polymers.
Prior to use, the membrane was cleaned by boiling in concen-
trated nitric acid for 4 h, and then throughly washed with
distilled water and finally immersed in water for 24 h. The
membrane in sodium form (Nafion-Na+) was obtained by
immersing the pretreated Nafion-H+ membrane in 1 N NaOH
aqueous solution. Excess base was then removed by stirring
the sample in water. Azobenzene (AB) was recrystallized three
times from ethanol before use. Doubly-distilled water was
used throughout this work.
1
were identified by H NMR and MS spestroscopies. BC: m/z:
180 (M+); 1H NMR (CDCl3, ppm): 8.9 (2H, d), 8.6 (2H, d), 8.0
(4H, m). BZ: 1H NMR (CD3COCD3, ppm): 7.3 (4H, d), 6.7
(4H, d), 4.4 (4H, s).
P r ep a r a t ion of Na fion Sa m p les. The weighed Nafion
membrane samples were immersed in a solution of AB in
water or in methanol (2.5 × 10-5 M), and the solution was
kept continuously stirred. At certain intervals the samples
were taken out from the solution, thus samples adsorbing
various amount of AB were prepared. To reach equilibration,
the samples were kept in the solution for at least one day.
The amount of uptake of AB by the membrane was determined
Ack n ow led gm en t. We thank the National Science
Foundation of China for financial support. We also
thank Dr. A. W.-H. Mau of CSIRO, Division of Chemical
and Polymers, Australia for donation of Nafion 117
membrane and for encouraging discussion.
J O960946K