Controlled Photocyclization, Photodimerization, and Photoisomerization of Stilbazole Salts within Nafion Membranes

Article abstract of DOI:10.1021/ol025615a

matrix presented Water- and methanol-swollen Nafion membranes were used as microreactors to successfully control the photochemical reaction pathway of stillbazole derivatives. Of particular interest is the production of azaphenanthrene (the product not ob

Full text of DOI:10.1021/ol025615a

ORGANIC
LETTERS
2002
Vol. 4, No. 7
1175-1177
Controlled Photocyclization,
Photodimerization, and
Photoisomerization of Stilbazole Salts
within Nafion Membranes
Xiao-Hong Li, Li-Zhu Wu,* Li-Ping Zhang, and Chen-Ho Tung*
Technical Institute of Physics and Chemistry, The Chinese Academy of Sciences,
Beijing 100101, China
chtung@ipc.ac.cn
Received January 24, 2002
ABSTRACT
Water- and methanol-swollen Nafion membranes were used as microreactors to successfully control the photochemical reaction pathway of
stilbazole derivatives. Of particular interest is the production of azaphenanthrene (the product not obtainable in homogeneous solution) in
high yield under high bulk concentration.
Selectivity in organic phototransformations is one of the main
topics of current interest. Of the various approaches, use of
constrained and organized media as microreactors to control
the reaction pathways has shown considerable promise.1,2
Nafion represents an unique family of polymers which
consist of a perfluorinated backbone and short pendent chains
terminated by sulfonic groups. When swollen in water, its
structure resembles that of a reverse micelle. The hydrated
sulfonic headgroups are clustered together in water-contain-
ing pockets of ca. 50 Å in diameter, which are interconnected
by short channels (ca. 10 Å in diameter) within the
perfluorocarbon matrix.3-5 This water-swollen Nafion can
incorporate high concentrations of organic molecules and
thus may act as microreactor to gain selectivity. In the present
Letter we report an investigation on the photochemistry of
the salts of trans-4-stilbazole (1a) and trans-1-(4-octyl-
oxyphenyl)-2-(4-pyridyl)ethylene (1b) within Nafion mem-
branes. Stilbazole salt and its derivatives can, in principle,
undergo a variety of photochemical reactions (Scheme 1),
Scheme 1
(1) Ramamurthy, V. In Surface Photochemistry; Anpo, M., Ed.; John
Wiley: New York, 1996; p 65.
(2) Tung, C.-H.; Song, K.; Wu, L.-Z.; Li, H.-R.; Zhang, L.-P. In
Molecular and Supramolecular Photochemistry; Ramamurthy, V., Schanze,
K. S., Eds.; Marcel Dekker: New York, 2001; Vol. 18, Chapter 5.
(3) Komoroski, R. A.; Mauritz, K. A. J. Am. Chem. Soc. 1978, 100,
7487.
including cyclization to azaphenanthrenes, dimerization to
cyclobutanes, and cis-trans isomerization.6-10 Although the
photodimerization and cis-trans photoisomerization have
(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) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1975, 97, 1602.
(7) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 877.
10.1021/ol025615a CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/08/2002

been extensively investigated,6-10 the photocyclization prod-
ucts have never been isolated. Furthermore, in the photo-
dimerization the main product is the head-to-tail (h-t) dimer.
Many efforts have been made to prepare the head-to-head
(h-h) dimer.6-10 We found that one can use water- and
methanol-swollen Nafion membranes as microreactors to
direct the photochemical reaction selectively toward either
the cyclization or the dimerization, and in the case of
dimerization to increase the yield of the h-h dimer.
The trans-isomer (1a) of stilbazole was used as the starting
material throughout the present study. For the photolysis in
aqueous solution, this starting material was dissolved in 1
N HCl. The solution was degassed with nitrogen and
irradiated with a 500-W high-pressure mercury lamp. After
irradiation, the solution was neutralized with 1 N NaOH and
extracted with CH2Cl2. The products were analyzed by HPLC
and separated by column chromatography on silica gel with
ethyl ether as the eluting solvent. The main products were
the cis-isomer 5a and the photodimers 3a and 4a (Scheme
1). The structures proposed for the photodimers rests mainly
on their 1H NMR spectra, which are in close agreement with
those reported in the literature.6-11 Furthermore, in the mass
spectra the fragment ions from the h-t dimer (4a) involve
the monomer unit and those from the h-h dimer (3a) involve
stilbene and dipyridylethylene moieties in addition to the
monomer unit.11 Table 1 shows the product distribution for
1:12). No syn h-h dimer (2a) was detected. This regio-
selectivity is obviously due to the minimization of the like-
charge repulsion. Since it has been well documented that
cis-stilbene can undergo photocyclization to give phenan-
threne in the presence of oxygen,12 originally we expected
that the generated cis-isomer 5a might photocyclize to
dihydroazaphenathrene which then is oxidized to azaphenan-
threne (6a, Scheme 1) during the product isolation. However,
upon prolonged irradiation of the diluted solution (1 × 10-5
M) of 1a, we could not detect any trace of 6a. We attribute
the absence of 6a formation to the fast cis-trans isomer-
ization and/or dimerization of the excited state of 5a.
The Nafion membranes used for the reaction media were
the water- or methanol-swollen acid form (Nafion-H+). 1a
was easily adsorbed into Nafion by immersing the polymer
in its well-stirred solution in water or methanol. In these
membranes 1a exists in the stilbazolium salt form where the
counterion is the sulfonate of Nafion. The solubility of 1a
in Nafion-H+ can be rather high (ca. 0.1 mol/g Nafion,
calculated in its swollen form). By changing the ratio of the
substrate to Nafion membranes, we could prepare the samples
with any loading in the region of 0-0.1 mol of 1a in each
gram of Nafion. Using the parameters of Nafion membrane
reported in the literature,5,13 the occupancy number (n, the
number of the substrate molecules contained in each water
cluster of Nafion) of the samples was calculated and is given
in Table 1. Photoirradiation was carried out in a Pyrex reactor
with a 500-W high-pressure mercury lamp as the light source.
Prior to irradiation, the samples were degassed with nitrogen
for 15 min. The photochemical reaction was monitored by
UV absorption spectroscopy. Generally, after 5 h of pho-
tolysis the conversion of the starting material was close to
100%. After irradiation, the excess acids of Nafion-H+
membranes were neutralized with a 1 N NaOH aqueous
solution, and then the products were extracted with methanol.
The products could be quantitatively extracted, and the
material balance was generally near 100%. The product
distribution within Nafion membranes is dramatically altered
compared with that in aqueous solution (Table 1). First, it
was found that the product distribution is remarkably
dependent on the substrate loading. At high loading (n )
70), 1a mainly undergoes photodimerization, although a
small amount of the cis-isomer 5a was detected. The ratio
of the isomerization product 5a to the dimers is changed
from 1.0:1.8 in aqueous solution to 1.0:13 in water-swollen
Nafion and 1.0:19 in methanol-swollen Nafion. Furthermore,
in the photodimerization the yield of the h-h dimer 2a was
Table 1. Product Distribution in the Photochemical Reaction
of 1a and 1b in Aqueous Hydrochloric Acid Solution and within
Nafion Membranes
product (%)a
h-h/
h-t
cyclomer/
dimers
media
1a HCl solution
2
3
4
5
6
0
5 60 35
0
1.0:12
0
1a H2O-swollen Nafion
n ) 70
16 0 77
14 0 73 0 13
7
0
1.0:4.8
0
n ) 2
1.0:5.2 1.0/6.8
1.0:5.6 1.0/1.1
1.0:5.0 1.8/1.0
n ) 1
n ) 0.25
8
6
0 45 0 47
0 30 0 64
1a methanol-swollen Nafion
n ) 70
13 0 82
5
0
1.0:6.3
1.0:5.0 2.0/1.0
0
n ) 0.25
5
0 25 8 62
1b HCl solution
1b water-swollen Nafion
n ) 70
75
2
1 22 0 77:1.0 0
28 18 44 10
0
0
1.0:1.0
1.0:1.6
0
0
methanol-swollen Nafion 23 13 57
n)70
7
a Error was estimated to be 2%.
irradiation of a3 × 10-4 M aqueous solution of 1a. The ratio
of the cis-isomer 5a to the photodimers was found to be
dependent on the concentration of 1a, with high concentration
favoring the formation of the photodimers. The main
photodimer was the syn h-t isomer (4a), although a small
amount of the anti h-h isomer (3a) was produced (3a:4a )
(11) 2a:1H NMR (CDCl3, ppm) 8.30 (d, 4H), 7.18 (m, 14H), 4.57 (d,
2H), 4.59 (d, 2H); MS m/z 364 (M+ + 2), 362 (M+), 182, 181, 180. 4a:
1H NMR (CDCl3, ppm) 8.27 (d, 4H), 7.17 (m, 14H), 4.57 (m, 2H), 4.60
(m, 2H); MS m/z 364 (M+ + 2), 362 (M+), 182, 181, 180. 2b: 1H NMR
(CDCl3, ppm) 8.42 (d, 4H), 7.06 (d, 4H), 6.97 (d, 4H), 6.73 (d, 4H), 4.42
(d, 2H), 4.37 (d, 2H), 3.89 (t, 4H), 1.76 (t, 4H), 1.3 (m, 20H), 0.9 (t, 6H);
MS m/z 619 (M+ + 1), 618 (M+), 436, 309, 180. 3b: 1H NMR (CDCl3,
ppm) 8.4 (d, 4H), 7.22 (m, 8H), 6.91 (d, 4H), 3.97 (t, 4H), 3.71 (t, 2H),
3.54 (t, 2 H), 1.76 (t, 4H), 1.3 (m, 20H), 0.9 (t, 6H); MS m/z 619 (M+
+
(8) Takagi, K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten,
D. G. J. Am. Chem. Soc. 1986, 108, 7865.
(9) Usami, H.; Takagi, K.; Sowaki, Y. J. Chem. Soc., Perkin Trans. 2
1990, 1723.
1), 436, 309, 180. 4b: 1H NMR (CDCl3, ppm) 8.4 (d, 4H), 7.22 (d, 4H),
6.96 (d, 4H), 6.74 (d, 4H), 4.46 (d, 2H), 4.50 (d, 2H), 3.84 (t, 4H), 1.76 (t,
4H), 1.3 (m, 20H), 0.9 (t, 6H); MS m/z 619 (M+ + 1), 309.
(12) Muszkat, K. A. Top. Curr. Chem. 1980, 88, 89.
(10) Usami, H.; Takagi, K.; Sowaki, Y. J. Chem. Soc., Faraday Trans.
1992, 88, 77.
(13) Gierke, T. D.; Munn, G. E.; Wilson, F. C. J. Polym. Sci., Polym.
Phys. Ed. 1981, 19, 1687.
1176
Org. Lett., Vol. 4, No. 7, 2002

increased. The ratio of the h-h to h-t dimers was increased
from 1.0:12 in aqueous solution to 1.0:4.8 in Nafion. These
observations can be interpreted in terms of the location and
preorientation of 1a molecules in Nafion membranes.
Considering the hydrophobicity and the positive charge of
the substrate, it is likely that the molecules of 1a are located
in the fluorocarbon/water (or fluorecarbon/methanol) inter-
face of the membrane. The phenyl group of 1a should reside
in the fluorocarbon phase, while the pyridyl group is
anchored among the sulfonated groups at the interface of
the water pool. Evidently, the viscous environment at the
fluorocarbon/water interface prevents 1a from undergoing
cis-trans photoisomerization, and the photodimerization of
the two neighboring stilbazolium ions favors the formation
of the h-h dimer. Second, for the sample with small
occupancy numbers, a great amount of the photocyclization
product14 6a was produced upon irradiation (Table 1). At
the occupancy numbers of less than 0.5 substrate molecules
in each water cluster, azaphenanthrene was the main product
of the photochemical reaction. The occurrence of the
photocyclization is attributed to the compartmentalization and
viscosity effects of Nafion. At low loading, the probability
of finding two substrate molecules in one water cluster is
small. Thus, the photodimerization is prohibited. The high
viscosity at the fluorocarbon/water interface prevents the
excited state of the cis-isomer 5a from isomerization to the
trans-isomer. Although the high viscosity also reduces the
rate of 5a formation, once formed, 5a can undergo photo-
cyclization. Therefore, cyclization becomes the main pho-
tochemical decay path of the excited state of 5a. The
formation of azaphenanthrene in water- and methanol-
swollen Nafion membranes is of significance. In many cases
a molecule may undergo both intramolecular and intermo-
lecular reactions. The rate of the latter is dependent on the
concentration of the substrate, while that of the former is
not. Hence, high substrate concentrations favor intermolecu-
lar reaction while intramolecular reaction proceeds in good
chemical yield only at low concentrations. Furthermore, there
are some examples for which even at low concentrations the
intramolecular reaction products cannot be produced (for
example, in solution at the concentrations as low as 1 ×
10-5 M, the photocyclization of 1a was not observed). In
general, the intramolecular reaction has to be performed at
substrate concentrations as low as 1 × 10-5 M. In previous
studies15,16 we reported an approach to synthesize intramo-
lecular products in high yields under high substrate concen-
trations. Our approach involves microporous solids or low-
density polyethylene films as microreactors for intramolecular
reaction. In each microreactor only one substrate molecule
is loaded. Thus, intermolecular reactions are hindered and
intramolecular reactions can occur without competition. Since
the concentration of the microreactor is high, the intramo-
lecular products can be synthesized in high yields under high
substrate concentrations. The production of azaphenanthrene
within Nafion membranes reported in the present work is
another example of such an approach. For the sample of 1a
in water-swollen Nafion membranes, the occupancy number
of 0.5 corresponds to 6 × 10-4 mol of the substrate in each
gram of Nafion (the concentration of the water cluster is ca.
1.2 × 10-3 mol/g Nafion),5,13 or 9.6 mol of the substrate in
each liter of Nafion (the density of water-swollen Nafion is
ca. 1.61 kg/L).5,13 Thus, even when the bulk concentration
is as high as 9.7 M, the main product is azaphenanthrene.
As mentioned above, in the photodimerization of 1a within
Nafion membranes the yield of the h-h dimer was evidently
increased compared with that in aqueous solution, because
of the preorientation of the substrate molecules at the
fluorocarbon/water interface. However, the products were
still dominated by the h-t dimer. Thus, we incorporated an
octyloxy group into the stilbazole moiety to synthesize 1b
and expected that the hydrophobic hydrocarbon chain would
increase the extent of preorientation of the substrate mol-
ecules at the fluorocarbon/water interface by insertion into
the fluorocarbon phase of Nafion. In sharp contrast with 1a,
photoirradiation of 1b in aqueous hydrochloric acid solution
at concentrations above 4.2 × 10-5 M mainly gave the syn
h-h dimer 2b in addition to a small amount of the cis-isomer
5b (Table 1). This observation originates from the formation
of micelles. We have demonstrated by surface tension
measurements that the alkylated stilbazole salt 1b as a
surfactant forms micelles with a critical micelle concentration
(CMC) of ca. 4.2 × 10-5 M in a 1 N HCl solution. In
micelles 1b molecules are preorientated in such a way that
the alkyl chains are located in the hydrophobic core while
the cationic chromophores are directed to the water interface.
Consequently, the photodimerization of two neighboring
stilbazolium ions favors the formation of the h-h dimer.
When 1b was incorporated into water- or methanol-swollen
Nafion membranes, the yield of the h-h dimer was indeed
increased compared with that of 1a. The ratio of the h-h to
h-t dimers in water-swollen Nafion was increased from 1.0:
4.8 for 1a to 1.0:1.0 for 1b. These observations suggest that
the extent of preorientation of 1b at the fluorocarbon/water
interface is much better than that of 1a, but not as perfect as
in its micelles.
The above results demonstrate that water- or methanol-
swollen Nafion membranes can act as microreactors to
control the pathways of the photochemical reactions of
stilbazole salts.
Acknowledgment. Financial support from the Ministry
of Science and Technology of China [Grants G2000078104
and G2000077502] and the National Natural Science Foun-
dation of China are gratefully acknowledged.
(14) 6a: 1H NMR (CDCl3, ppm), 8.8 (d, 2H), 8.0 (m, 2H), 7.7-7.8 (m,
5H). MS m/z 179 (M+), 151, 152.
(15) Tung, C.-H.; Wu, L.-Z.; Yuan, Z.-Y.; Su, N. J. Am. Chem. Soc.
1998, 120, 11594.
(16) Tung, C.-H.; Yuan, Z.-Y.; Wu, L.-Z.; Weiss, R. G. J. Org. Chem.
1999, 64, 5156.
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