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Series of Photoswitchable Azobenzene-Containing Metal–Organic Frameworks with Variable Adsorption Switching Effect
journal contribution
posted on 2018-07-19, 00:00 authored by Zhengbang Wang, Kai Müller, Michal Valášek, Sylvain Grosjean, Stefan Bräse, Christof Wöll, Marcel Mayor, Lars HeinkeNanoporous
metal–organic frameworks (MOFs) equipped with
light-responsive azobenzene pendant groups present a novel family
of smart materials, enabling advanced applications like switchable
guest adsorption, membranes with tunable molecular separation factors,
and photoswitchable proton conduction. Although it is obvious that
for small pore sizes, steric constraints may prohibit azobenzene switching,
guidelines for optimizing the MOF architecture to achieve large switching
effects have not yet been established. Here, a series of five different
photoswitchable azobenzene-containing pillared-layer MOF structures
is presented. The switching effect is quantified by the light-induced
increase of the uptake amount of butanol as the probe molecule. For
fast and reproducible measurements, thin well-defined MOF films, referred
to as surface-mounted MOFs (SURMOFs), were used in combination with
a quartz crystal microbalance. Although the series comprises similar
MOF structures, the magnitude of the switching effect considerably
differs, here by a factor of 5. The uptake data show that, rather
than the pore size or the number of azobenzene molecules per pore,
the density of azobenzene per pore volume is crucial. The finding
that a large switching effect is reached for a high density of azobenzene
moieties per MOF unit cell provides the basis for further applications
of photoswitchable MOFs and SURMOFs.
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uptake amountMOF architectureserieslight-induced increaselight-responsive azobenzene pendant groupsphotoswitchable azobenzene-containing pillared-layer MOF structuresMOF structurespore sizequartz crystal microbalancesteric constraintsphotoswitchable proton conductionseparation factorsprobe moleculepore sizesazobenzene moietiesuptake data shownovel familyswitchable guest adsorptionsurface-mounted MOFspore volumeMOF unit cellazobenzene moleculesSURMOFMOF filmsphotoswitchable MOFs
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