Manipulating the cavity of a porous material changes the photoreactivity of included guests

Article abstract of DOI:10.1039/b805895d

Changing an ether to a ketone within the framework of a bis-urea macrocycle has little effect on the supramolecular assembly of this building block into porous crystals but introduces a triplet sensitizer into the framework that dramatically alters the ph

Full text of DOI:10.1039/b805895d

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Manipulating the cavity of a porous material changes the photoreactivity
of included guestsw
Mahender B. Dewal, Yuewen Xu, Jun Yang, Fiaz Mohammed, Mark D. Smith and
Linda S. Shimizu*
Received (in Austin, TX, USA) 9th April 2008, Accepted 7th May 2008
First published as an Advance Article on the web 30th June 2008
DOI: 10.1039/b805895d
4
Changing an ether to a ketone within the framework of a bis-
urea macrocycle has little effect on the supramolecular assembly
of this building block into porous crystals but introduces a triplet
sensitizer into the framework that dramatically alters the photo-
chemical reactions of included guests.
[2+2]-photocycloadditions of enones. We synthesized benzophe-
none macrocycle 2 in which the ether oxygen of host 1 was
replaced with a carbonyl group. Host 2 was synthesized in two
5
steps from the dibromide and masked urea (triazinanone).
Cyclization under basic conditions yielded the urea-protected
macrocycle, which crystallized as a chloroform solvate (S.I.w).
Deprotection with diethanolamine followed by recrystallization
of 2 from hot DMSO afforded crystals suitable for X-ray
analysis.z
Supramolecular assembly deftly and efficiently creates materi-
als with interesting structures and properties from small
molecule building blocks. Some supramolecular assemblies,
as well as more traditional porous materials and crystals, can
be used as templates or vessels to promote chemical reactions
The crystal structure of 2 revealed the expected bis-urea
structure (Fig. 2) that assembles into columnar structures similar
1
3
to host 1, highlighting the predictability of this assembly motif.
with high regio- and stereoselectivity. It is not yet possible to
predict how systematic changes in these nanoreactors influence
the reactants. In this paper, we examine how a change in the
framework of a self-assembling macrocycle influences the UV-
absorption of the macrocycle and subsequently the reaction
environment of the cavity.
The macrocycles display strong urea–urea hydrogen bonds
(Fig. 3) that extend along both sides of the tube with Hꢀ ꢀ ꢀO
˚
distances from 2.17 to 2.30 A. The individual monomers are also
held together by edge to face aryl-stacking interactions. The
sizeable channel of the assembly was filled with DMSO guests
in a 1 : 1 (macrocycle : DMSO) ratio.
We have previously reported bis-urea 1 synthesized from a
phenylether spacer that assembled into columnar structures
Heating removed the DMSO guest from the crystals, forming a
host that maintains permanent porosity. The porosity of the
2
,3
that pack against each other to give porous crystals. We now
report a similar macrocycle 2 that is synthesized from a
benzophenone spacer and also forms porous crystals. The
two macrocycles differ in their linking groups (ether 1 versus
ketone 2). The homogeneous channels of the two systems have
comparable diameters by gas adsorption. They absorb a
similar series of guests. Despite these similarities, the new
benzophenone framework of host 2 facilitates different photo-
chemical reactions within its nanochannels. Host 1 induces the
2
empty host 2 was established by gas absorption using CO at
273 K. The evacuated crystals displayed a type 1 gas adsorption
6
isotherm (Fig. 4A), consistent with a microporous material. The
Brunauer–Emmett–Teller method was applied to the isotherm at
P/P
between 0.012 and 0.028 and the calculated surface area
0
2
ꢁ1
of 320 m
g
, comparable to the original framework 1
[
2+2]-photocycloaddition of a,b-unsaturated ketones (Fig. 1),
whereas host 2 is ineffective in facilitating this photocycload-
dition. Conversely, only host 2 induces a rapid photoisome-
rization of trans-b-methylstyrene to the less stable cis-isomer
(
II), which is a reaction that does not proceed within host 1.
Macrocycle 1 self-assembled into columnar structures via
urea–urea hydrogen bonds and aryl-stacking interactions forming
3
crystals that display permanent porosity. These crystals reversi-
bly bound guests with matched shapes and sizes and facilitated
Department of Chemistry and Biochemistry, University of South
Carolina, Columbia, South Carolina, USA. E-mail:
shimizul@mail.chem.sc.edu; Fax: +1 803-777-9521; Tel: +1 803-
777-2066
w Electronic supplementary information (ESI) available: Experimental
details including the synthesis and characterization of host 2 and
crystallographic analysis. CCDC reference numbers 684399–684400.
For ESI and crystallographic data in CIF or other electronic format
see DOI: 10.1039/b805895d
Fig. 1 The characteristics of these porous nanoreactors were probed
using reactive guests (3–7) in two photochemical reactions: [2+2]-
0
photoadditions (I) where R, R and n depend on the identity of the
starting cyclic enone and cis–trans photoisomerization (II) of styrene 7.
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Fig. 2 Comparison of the cavities of two bis-urea macrocycles from
X-ray crystal structures. The dimensions of the channel between the
van der Waals surfaces of the pore defining atoms (host 1: C16–H8 ꢃ
Fig. 4 (A) Carbon dioxide adsorption isotherms at 273 K for empty
host 2. (B) Following the reversible binding by PXRD: 2ꢀDMSO
(
bottom), empty host 2 (middle), and refilled 2ꢀDMSO (top).
˚
˚
˚
H7–H23 = 4.8 ꢃ 3.8 A, host 2: H8–H8* ꢃ H7–H7* = 3.7 A ꢃ 2.7 A).
2
316 m g ). This corresponds to a total pore volume of 0.059
ꢁ1 3,7
vapour lamp at B30 1C for 24 h. Next, each of the host–guest
complexes was directly dissolved in d -DMSO and monitored
(
3
ꢁ1
6
˚
cm g for pores smaller than 6.9 A in diameter, which was
similar to the values for host 1.
1
by H NMR spectrometry. Only peaks that corresponded to
the host macrocycle 2 and the starting enones were observed.
These results are in stark contrast to the selective [2+2]-
cycloadditions facilitated by host 1.
The reversible binding of guests was followed by several
methods including powder X-ray diffraction (PXRD) and TGA
(S.I.w). Crystals of synthesized 2ꢀDMSO were ground to a powder
How is one to explain this drastic difference in observed
reactivity? Both hosts load the enones with high binding ratios
and examined by PXRD (Fig. 4B). The observed PXRD pattern
closely matched simulated patterns based on the single-crystal
structure (S.I.w), indicating that the bulk powder retains a
structure similar to that of the single crystal. Subsequent heating
to 160 1C facilitated the removal of the included DMSO guest
forming empty host 2. The empty host was highly crystalline by
PXRD (Fig. 4B middle). Exposure of the empty host 2 powder to
guest vapour resulted in reabsorption of the guests to reform host
(
2.5 : 1 for host 1ꢀ4 vs. 2 : 1 for host 2ꢀ4), yet no reaction is
observed within the second host. We attribute these differences to
the UV-absorption characteristics of the two hosts. Host 1 does
not have any significant absorbance in the wavelength ranging
from 300 to 450 nm, where most enones have strong absorption
bands. Therefore, within host 1 these enones undergo the normal
[2+2]-cycloaddition; however, host 2 displays a strong absorption
2
ꢀDMSO, providing further evidence of the reversible nature of
in this range. Literature studies with 2-cyclopentenone in solution
suggest that benzophenone transfers energy to the enone but that
the absorption/desorption process. Both hosts absorbed small
molecule solvents such as EtOAc, THF, and AcOH in identical
host : guest ratios (S.I.w). Larger reactants 3–7 were also absorbed
by both hosts.
8
this triplet excited state is not conducive for the photoreaction.
We next investigated both host 1 and host 2 as confined
environments for the cis–trans photoisomerization of trans-b-
methylstyrene 7 (Fig. 2, reaction II), which is a process known to
Although they have many similarities, the two hosts exhibit
markedly different UV-absorbance spectra in DMSO (Fig. 5).
Host 1 has no significant absorbance in the 320–380 nm range,
which is important for [2+2]-cycloadditions of enones. In
comparison, the benzophenone host 2 has a strong absorbance
in this region with a lmax at 339 nm, similar to what is observed
for 2-cyclohexenone (lmax = 335 nm).
9
require a triplet sensitizer. Can the covalent sensitizer of host 2
transfer energy to included guests? There are examples of
sensitized photoreactions within zeolites loaded with benzophe-
none or in cases where acetophenone or benzophenone is
10
covalently attached to small receptors.
˚
˚
Styrene 7 (B4.3 A ꢃ 7.7 A) was absorbed by both hosts
through vapour loading to give stable inclusion complexes
after 24 h, which display similar host : guest ratios (host 1ꢀ7
(2.7 : 1 host : guest), host 2ꢀ7 (2.5 : 1 host : guest)). Prolonged
exposure to the styrene 7 vapour (72 h) did not alter the
Previous work demonstrated that enones 4–6 underwent
facile [2+2]-cycloaddition reactions to yield the exo head-to-
tail (HT) photodimers as the major product in high conversion
4
(
UV-irradiated using a 450 W Hannovia high pressure mercury
Table 1). The new solid inclusion crystals (host 2ꢀenone) were
Fig. 3 Views from the crystal structure of host 2ꢀDMSO. (A) View
along a single column that contains a channel filled with DMSO
guests. (B) Top view of column.
Fig. 5 UV absorbance of macrocycles 1 and 2 in DMSO.
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3
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Table 1 Comparison of [2+2]-cycloadditions of a,b-unsaturated
ketones (3–6) and photoisomerization of trans-b-methylstyrene (7) in
the presence of host 1 and host 2
host 2 arises from the benzophenone anchored in the framework.
We are now investigating the scope of host 2 to facilitate other
photochemical reactions that may proceed selectively in the
presence of a triplet sensitizer. We will also investigate if host 2,
with its broad UV-absorbance, can induce photoreactions at
longer wavelengths than typically required.
The authors gratefully acknowledge support in part for this
work from the NSF (CHE-071817), from the Petroleum
Research Fund (44682), and from a grant from the University
of South Carolina, Office of Research and Health Sciences.
Notes and references
z Crystallographic data for 2: C34
34 4 5
H N O S, M = 610.71. Colourless
3
binding ratios. The inclusion complex 2ꢀ7 (25 mg) was irra-
diated using a 450 W Hannovia high pressure mercury vapour
lamp at B30 1C for 10 min to 24 h. The reaction progress was
followed by two methods: (A) direct dissolution of a sample in
block crystal, 0.16 ꢃ 0.10 ꢃ 0.08 mm , T = 150 K. Monoclinic, space
˚
1
/c, a = 9.4680(6), b = 23.055(2), c = 13.3023(9) A,
3
group P2
b = 92.765(2)1, U = 2900.3(3) A , Z = 4, D
F(000) = 1288. Bruker SMART APEX CCD-based diffractometer
system, l = 0.71073 A (Mo-Ka), ymax = 22.541, 21 351 reflections
ꢁ3
,
˚
c
= 1.399 Mg m
˚
measured, 3813 unique, 2456 with I 4 2s(I), Rint = 0.0801. Final
R-values: R1 (F, I 4 2s(I)) = 0.0675; wR2 (F , all data) = 0.1722.
d
6
-DMSO and integration of the proton NMR spectra; and
B) removal of guests from the porous framework by washing
the crystals with CH Cl and analysis by GC/MS. In the latter
2
(
2
2
1
D. M. Vriezema, M. C. Aragones, J. A. A. W. Elemans, J. J. L. M.
Cornelissen, A. E. Rowan and R. J. M. Nolte, Chem. Rev., 2005,
105, 1445; J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon
and K. Kim, Nature, 2000, 404, 982; C. D. Wu, A. Hu, L. Shang
and W. Lin, J. Am. Chem. Soc., 2005, 127, 8940; A. Corma and H.
Garcia, Chem. Commun., 2004, 1443; D. Fiedler, D. H. Leung, R.
G. Bergman and K. N. Raymond, Acc. Chem. Res., 2005, 38, 351;
M. Conn and J. Rebek, Jr, Acc. Chem. Res., 1997, 97, 1647; B. W.
Purse and J. Rebek, Jr, Proc. Natl. Acad. Sci. U. S. A., 2005, 102,
case, the host was recovered by filtration and reused.
After 30 min of UV irradiation, a rapid photoisomerization
was observed in the host 2ꢀ7 complex yielding 59% conversion
to the cis-isomer. Further UV-irradiation for 2 or 24 h yielded
identical trans : cis ratios, indicating that this is the photosta-
tionary state within the crystals under broad band irradiation. In
contrast, neat, degassed trans-b-methylstyrene (25 mg) shows
little photoisomerization under similar UV-irradiation, display-
ing o5% cis after 24 h. Literature reports show UV-irradiation
1
0777; A. Natarajan, L. S. Kanumalle, S. Jockusch, C. L. D. Gibb,
B. C. Gibb, N. J. Turro and V. Ramamurthy, J. Am. Chem. Soc.,
2007, 129, 4132; D. B. Varshney, X. Gao, T. Friscic and L. R.
MacGillivray, Angew. Chem., Int. Ed., 2006, 45, 646; D. N.
Dybstev, A. L. Nuzhdin, H. Chun, K. P. Bryliakov, E. P. Talsi,
V. P. Fedin and K. Kim, Angew. Chem., Int. Ed., 2006, 45, 916; T.
Arai, H. Sakuragi and K. Tokumaru, Bull. Chem. Soc. Jpn., 1982,
55, 2204; R. A. Caldwell, G. W. Sovocool and R. J. Peresie, J. Am.
Chem. Soc., 1973, 95, 1496.
(366 nm, 26 1C) of trans-b-methylstyrene with benzophenone
sensitizer in benzene solution gave a comparable percentage of
9
cis-b-methylstyrene (63%) at the photostationary state (3 h).
To further test whether the photosensitizer was responsible
for this photoisomerization, the inclusion complex 1ꢀ7 was
subjected to UV-irradiation for 30 min to 24 h. Host 1 is of
similar size and shape yet contains no triplet sensitizer to
facilitate the cis–trans photoisomerization. Indeed, no cis-
isomer was detectable by proton NMR after 24 h of UV-
irradiation. These results suggest that it is the benzophenone
within the framework that is important for the rapid isomer-
ization of 7 within host 2. Furthermore, these results indicate
that the benzophenone within the covalent framework of
macrocycle 2 is able to act as a triplet sensitizer for this
reaction, transferring energy to the included styrene guest.
We reported a new bis-urea macrocycle that self-assembles
predictably into columnar structures containing DMSO guests.
The guests can be removed from these crystals by heating to form
a host that maintains permanent porosity. This new macrocycle is
similar in dimensions to a previously reported phenylether deri-
vative and absorbs similar guests with nearly the same host : guest
ratios. Although similar in size, the two hosts promote different
photochemical reactivity for absorbed guests. Host 1 enables the
facile [2+2]-cycloaddition to yield HT photodimers in high
selectivity and conversion. In contrast, host 2 facilitated the triplet
sensitized cis–trans photoisomerization of trans-b-methylstyrene.
These results show that small changes within the framework of
porous materials can greatly influence the reactions that occur
within these channels. The most intriguing property of porous
2
L. S. Shimizu, A. D. Hughes, M. D. Smith, M. J. Davis, P. Zhang,
H.-C. zur Loye and K. D. Shimizu, J. Am. Chem. Soc., 2003, 125,
14972.
3 M. B. Dewal, M. W. Lufaso, A. D. Hughes, S. A. Samuel, P.
Pellechia and L. S. Shimizu, Chem. Mater., 2006, 18, 4855.
4
5
J. Yang, M. B. Dewal and L. S. Shimizu, J. Am. Chem. Soc., 2006,
28, 8122; J. Yang, M. B. Dewal, S. Profeta, Y. Li, M. D. Smith
and L. S. Shimizu, J. Am. Chem. Soc., 2008, 130, 612.
A. R. Mitchell, P. F. Pagoria, C. L. Coon, E. S. Jessop, J. F. Pococ,
C. M. Tarver, R. D. Breithaupt and G. L. Moody, Propellants,
Explos., Pyrotech., 1994, 19, 232.
1
6 K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A.
Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem.,
1
985, 57, 603.
7
8
T. J. Barton, L. M. Bull, W. G. Klemperer, D. A. Loy, B.
McEnaney, M. Misono, P. A. Monson, G. Pez, G. W. Scherer,
J. C. Varuli and O. M. Yaghi, Chem. Mater., 1999, 11, 2633; J. C.
Groen, L. Peffer and J. A. Perez-Ramirez, Microporous Mesopor-
ous Mater., 2003, 12, 56.
P. de Mayo, J.-P. Pete and M. Tchir, J. Am. Chem. Soc., 1967, 89,
5
712.
9 T. Arai, H. Sakuragi and K. Tokumaru, Bull. Chem. Soc. Jpn.,
982, 55, 2204; M. G. Rockley and K. Salisbury, J. Chem. Soc.,
1
Perkin Trans. 2, 1973, 1582.
1
0 A. Corma and G. Hermenegildo, Chem. Commun., 2004,
1443–1459; M. V. Baldovi, A. Corma, H. Garcia and V. Marti,
Tetrahedron Lett., 1994, 35, 9447; K. Kokubo, K. Kitasaka and T.
Oshima, Org. Lett., 2006, 8, 1597; M. V. Baldovi, F. L. Cozens, V.
Fornes, H. Garcia and J. C. Scaiano, Chem. Mater., 1996, 8, 152;
H. Nishiguchi, S. Okamoto and M. Nishimura, Res. Chem.
Intermed., 1998, 24, 849.
This journal is ꢂc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 3909–3911 | 3911