The catalytic Z to E isomerization of stilbenes in a photosensitizing porous coordination network

Article abstract of DOI:10.1002/anie.201001902

(Figure Presented) One way only: The Z → E photoisomerization of stilbene under visible light occurs within a porous coordination network (see picture). The reaction proceeds through the photoexcitation of a chargetransfer complex between the highly elec-tron-deficient ligand and the stilbene in the pore. The in situ formed E isomer rapidly exchanges with unreacted Z isomer in solution, hence a catalytic amount of the network is sufficient to promote the reaction.

Full text of DOI:10.1002/anie.201001902

Angewandte
Chemie
DOI: 10.1002/anie.201001902
Photoisomerization
The Catalytic Z to E Isomerization of Stilbenes in a Photosensitizing
Porous Coordination Network**
Kazuaki Ohara, Yasuhide Inokuma, and Makoto Fujita*
The tris(4-pyridyl)triazine ligand (1) is an important organic
[1]
building block for self-assembled coordination cages and
[2]
networks. Typically employed because of its rigid planarity,
triangular shape, and commercial availability, ligand 1 is
extremely electron-deficient and, upon coordination of the
pyridyl arms, 1 can become electro- and
photochemically active. Guest interactions
with the low-lying lowest unoccupied
molecular orbital (LUMO) of 1 in tria-
zine-based hosts regularly generate host–
guest charge-transfer complexes, and pho-
toirradiation can induce efficient energy
[
3]
transfer or, in some cases, photoinduced
[
4]
electron transfer.
We hypothesized that network 2, which is generated from
1
and ZnI , could display similar photochemistry in the solid
2
Scheme 1. a) Preparation of porous coordination networks 2 and 2’.
b) Z!E photoisomerization of stilbenes within network 2’.
state; thus we examined the photoinduced isomerization of
stilbene within coordination network 2 (Scheme 1a). Enclath-
rated within the pores of 2, (Z)-stilbene selectively isomerized
to (E)-stilbene under visible light irradiation (Scheme 1b);
the Z/E equilibrium ratio typical for the photostationary state
proximity of the guest with 1 in the network pore play a
crucial role in inducing effective CT interactions.
(
Z/E = 92:8 at l = 313 nm) was not obtained. As guest
Crystals of 2’, which were suspended in a solution of (Z)-
3a in cyclohexane, were photoirradiated with a Xe lamp
ex
molecules can freely diffuse from the pores of 2 into the
solution, crystals of 2 efficiently catalyzed the one-way Z!E
isomerization of stilbene in cyclohexane.
(l = 400–500 nm) for 83 h. This procedure resulted in
ex
greater than 98% conversion into (E)-3a in both the crystal
1
The porous network complex [{(ZnI ) (1) }·x(C H NO )]
and in the supernatant, as determined by H NMR spectros-
2
3
2
6
5
2
n
(
2, x ꢀ 5.5) employed in this work was prepared according to
copy. To analyze the stilbene contained in network 2’, the
crystals were decomposed with hydrochloric acid and
[
5]
the reported procedure. When the as-synthesized network 2
was soaked in a solution of (Z)-stilbene (3a) in cyclohexane,
the crystals immediately turned from pale to bright yellow.
Elemental analysis showed the inclusion of approximately
one molecule of (Z)-3a per unit with a formula of
extracted with CHCl . No other photo-by-products, for
3
example, dihydrophenanthrene or photooxidized products,
were detected. Finally, X-ray diffraction analysis of the
photoirradiated network 2’ provided convincing evidence of
the formation of (E)-3a in the pores of network 2’ (Figure 1
and the Supporting Information). Enclathrated molecules of
(E)-3a exhibited only minor disorder and are distributed over
three non-equivalent positions, one of which interacts with a
nearby triazine moiety 1 by aromatic–aromatic interactions
[
{(ZnI ) (1) }·x((Z)-3a)·y(cyclohexane)] (2’, x ꢀ 1.1, y ꢀ 1.0).
2
3
2
n
The diffuse reflectance UV/Vis spectrum showed a new,
broad charge-transfer (CT) band at approximately 450 nm.
Since this CT band was not observed in a solution of ligand 1
and (Z)-3a in toluene, coordination of 1 to zinc(II) ions and
(interplanar distance ca. 3.4 ꢀ; Figure 1b).
Based on the following observations, we believe the
[
*] K. Ohara, Dr. Y. Inokuma, Prof. Dr. M. Fujita
Department of Applied Chemistry, School of Engineering
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-8656 (Japan)
Fax: (+81)3-5841-7257
selective photoisomerization of (Z)-3a to (E)-3a only occurs
within the pores of 2’: 1) In the absence of network 2,
photoisomerization did not occur and even the individual
network component(s) (ligand 1 and/or ZnI ) were insuffi-
2
E-mail: mfujita@appchem.t.u-tokyo.ac.jp
cient to catalyze the conversion. 2) When crystals of 2’ were
removed during photoirradiation, isomerization stopped.
[
**] This research was supported by the CREST project of the Japan
Science and Technology Agency (JST), for which M.F. is the principal
investigator, and also in part by KAKENHI (20044006), JSPS, and
Global COE Program (Chemistry Innovation through Cooperation
of Science and Engineering), MEXT (Japan).
3
) Photoisomerization was dramatically retarded when
[
5a]
pyrene, which is strongly bound by 2’ and inhibits guest
exchange, was added to the reaction mixture. 4) The E/Z ratio
of stilbene increases faster within the crystals of 1 than in the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201001902.
[
6]
supernatant (Figure 2). These results demonstrate that
Angew. Chem. Int. Ed. 2010, 49, 5507 –5509
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Communications
Scheme 2. Selective conversion of E/Z mixtures of 3 into the E isomer
with a catalytic amount of crystals 2’ (ca. 20 mol% of the [(ZnI ) (1) ]
2
3
2
unit).
observed for the electron-poor 4,4’-dinitrostilbene (3d; 85:15
[
7]
E/Z), which is a poor guest for network 2 and was not
included, thus further supporting that the reaction occurs
[
8–10]
within the porous network 2.
In conclusion, the electron-deficient triazine moieties 1 in
coordination network 2 are photoactive and catalyze the Z!
E photoisomerization of stilbenes under visible light. The
photoisomerization occurs within the pores of 2 and the
resulting (E)-stilbene rapidly exchanges with (Z)-stilbene in
solution so that only a few crystals suffice to completely
convert the entire solution. Although free ligand 1 is photo-
chemically inert, it becomes photoactive when incorporated
into three dimensional coordination cages and porous net-
works. We believe that photoelectron-transfer catalysis by
porous-network solids holds great promise, and we are
currently investigating further applications.
Figure 1. a) X-ray crystal structure of network 2’ with stilbene guest
E)-3a, obtained by irradiation of as-synthesized 2 with visible light for
55 h in a solution of (Z)-3a in cyclohexane. b) Aromatic–aromatic
(
1
interactions between triazine moiety 1 and (E)-3a formed in situ.
Received: March 31, 2010
Revised: May 21, 2010
Published online: June 28, 2010
Keywords: coordination networks · host–guest systems ·
.
metal–organic frameworks · photochemistry · self-assembly
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once formed, the (E)-stilbene in the pores rapidly exchanges
with unreacted (Z)-stilbene in solution. These equilibrium
processes continue until nearly all (> 98%) of stilbene is
converted into the E isomer.
1
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A variety of stilbene derivatives was converted to the
E isomer in the presence of catalytic quantities of 2
(
(
Scheme 2). When a solution of stilbene 3b in cyclohexane
30 mm; 45:55 E/Z mixture) was irradiated in the presence
[
7]
of a catalytic amount of crystalline 2 for 3 days, almost-pure
E)-3b was obtained (> 98% conversion). 4,4’-Dibromostil-
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[
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greater than 98% conversion. However, no conversion was
5
508
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Angewandte
Chemie
[
6] Similar behavior was also observed in our previous study on the
thermal all-E to 13-Z isomerization of retinal within network 2:
K. Ohara, M. Kawano, Y. Inokuma, M. Fujita, J. Am. Chem. Soc.
mechanism operates within the pores of network 2’ where the
proximity of 3 and acceptor 1, which is evidenced by the CT band
in the UV/Vis spectrum and by X-ray analysis, facilitates excited
state electron transfer (refs [9,10]).
2010, 132, 30 – 31.
[
[
7] Obtained from irradiating the commercially available E isomer
at 366 nm for 18 h.
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involves a radical cation mechanism and the generation of cation
radicals through electron transfer to photoexcited electron
acceptors is well known. We thus believe that a similar
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Angew. Chem. Int. Ed. 2010, 49, 5507 –5509
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5509