Angewandte
Chemie
DOI: 10.1002/anie.201310211
Metal–Organic Frameworks
Azobenzene-Functionalized Metal–Organic Polyhedra for the
Optically Responsive Capture and Release of Guest Molecules**
Jinhee Park, Lin-Bing Sun, Ying-Pin Chen, Zachary Perry, and Hong-Cai Zhou*
Abstract: Stimuli-responsive metal–organic polyhedra
(srMOPs) functionalized with azobenzene showed UV-irradi-
ation-induced isomerization from the insoluble trans-srMOP
to the soluble cis-srMOP, whereas irradiation with blue light
reversed this process. Guest molecules were trapped and
released upon cis-to-trans and trans-to-cis isomerization of
the srMOPs, respectively. This study provides a new direction
in the ever-diversifying field of MOPs, while laying the
groundwork for a new class of optically responsive materials.
isomerization.[8] Upon irradiation with UV and visible light,
azobenzene and its derivatives undergo reversible trans–cis
isomerization, which alters the distance between the two end
para carbon atoms from 9 to 5.5 ꢀ and the dipole moment
from 0.52 to 3.08 D.[8c] On the basis of the photochemical and
photophysical properties of azobenzene, optically triggered
solubility changes of polymers, morphological changes of
micelles, and gas-adsorption changes in metal–organic frame-
works (MOFs) have been widely investigated.[9]
Self-assembled molecular cages have attracted consider-
able research attention as a result of their aesthetic structures
and intriguing potential for a wide range of applications, such
as gas storage, separation, drug delivery, and catalysis.[10]
Among these cages, metal–organic polyhedra (MOPs) con-
sisting of metal and organic components can be structurally
tuned through both inorganic and organic syntheses.[11] The
judicious choice of functional groups on geometrically fixed
building blocks can precisely tune the functions and proper-
ties of the MOPs, thus enabling them to respond to external
stimuli.[12] A MOP built with a bulky organic linker was found
by our research group to exhibit a thermally sensitive
molecular-sieving effect.[13] Previously, a MOP for the con-
trolled capture and release of guest molecules upon irradi-
ation with light has rarely been reported.[6a] Herein we report
for the first time optically responsive MOPs derived from
organic linkers containing azobenzene units. Because MOPs
are suitable for capturing guest molecules, such as gas, drug,
and dye molecules, the capture and release of the guest
molecules becomes controllable through the isomerization of
the azobenzene units.
As compared with other types of MOPs, cuboctahedra
contain more organic linkers.[11a] The presence of these 24
organic linkers increases the isomerization effect of the
pendent azobenzene units. A ligand with a 1208 bridging
angle, 2,4-dimethylphenyldiazenylisophthalate (L1), was syn-
thesized on the basis of the Mills reaction.[14] The trans-to-cis
isomerization of L1 upon exposure to UV light (365 nm) was
confirmed by 1H NMR spectroscopy (Figure 1a; see also
Figure S10 in the Supporting Information). A reaction
between Cu(OAc)2 and the acidic form of the ligand afforded
a green precipitate, srMOP-1 (srMOP stands for stimuli-
responsive MOP). The as-synthesized srMOP-1 was soluble in
chloroform. Vapor diffusion of diethyl ether into a solution of
srMOP-1 in chloroform yielded crystalline srMOP-1. How-
ever, owing to light-induced isomerization of L1, it was
difficult to grow crystals large and robust enough for structure
determination, even under light- and temperature-controlled
conditions. Our previous report showed that coordination
assemblies are generally determined by the bridging angle of
the organic linker.[11b] On the basis of this knowledge, we used
T
remendous research effort has been devoted to the study of
stimuli-responsive materials because of their potential appli-
cations in drug delivery, adaptable surface coating, and
molecular machines.[1] As compared with traditional stimuli,
such as a change in temperature[2] or pH value,[3] electric or
magnetic fields,[4] and chemicals,[5] light has distinct advan-
tages. Light with wavelengths near the visible region is
generally nondestructive, has high spatial and periodic
resolution, and generates few side products. Since optically
responsive materials can function more accurately and
predictably, systems for the capture and release of guest
molecules upon irradiation with light have played a significant
role in drug-delivery studies.[6] For example, light-induced
coumarin release was demonstrated in polymeric micelles
consisting of photochromic spiropyran units.[7]
Azobenzene is one of the most studied light-responsive
units owing to its robustness, as well as rapid and reversible
[*] Dr. J. Park
Nano Hybrid Technology Research Center
Creative and Fundamental Research Division
Korea Electrotechnology Research Institute
Changwon, 641-120 (Republic of Korea)
Dr. L.-B. Sun, Y.-P. Chen, Z. Perry, Prof. Dr. H.-C. Zhou
Department of Chemistry, Texas A&M University
College Station, TX 77842 (USA)
E-mail: zhou@tamu.edu
Y.-P. Chen, Prof. Dr. H.-C. Zhou
Department of Materials Science and Engineering
Texas A&M University
College Station, TX 77843 (USA)
[**] This research was partially supported as part of the Center for Gas
Separations Relevant to Clean Energy Technologies, an Energy
Frontier Research Center funded by the US Department of Energy
(DOE), Office of Science, Office of Basic Energy Sciences under
Award Number DE-SC0001015. We also acknowledge the US DOE
(DE-AR000073 and DE-FC36-07GO17033), the National Science
Foundation (NSF CBET-0930079), and the Welch Foundation (A-
1725). We thank Dr. Joseph P. Riebenspies for structure refinement
and advice in crystallography.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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