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New Journal of Chemistry
Page 2 of 7
DOI: 10.1039/C5NJ01718A
ARTICLE
Journal Name
linker, UBMOF-2 (Figure 1). Repeated photo-cycling of reaction which is thermally accessible at room temperature.36
UBMOF-2 results in the loss of photo-responsivity. Analysis of A fit of the time-dependent absorbance of a solution of
the fatigued framework suggests that the degradation is the PhTPDC-c at room temperature was well-modelled using single
result of the formation of photo-inactive atropisomers of exponential decay with a rate constant of 1.43(4) x 10-3 s-1 for
PhTPDC which are stabilized by the framework. The a 3.27x10-3 M solution in MeOH. Thus for PhTPDC in solution,
stabilization of these atropisomers is likely a property unique the cycloreversion reaction is driven by both light and heat. It
to microenvironments which place moderate constraints on remains unknown if thermally activated back reactions are a
molecular reorganization, as is the case in MOFs. general property of phenanthrene based DAEs as previous
Understanding and ultimately controlling atropisomer reports have not specifically examined this property.37,38
formation remains
conformationally-flexible
materials.
a
key design challenge for UBMOF-2 structure and photophysical properties
DAE-based photo-responsive
The solvothermal reaction of PhTPDC, with zinc(II) nitrate in
dimethylformamide yielded large photo-responsive single
crystals of UBMOF-2 suitable for X-ray diffraction structure
determination and spectroscopic characterization (Figure 1). In
these crystals, deprotonated PhTPDC coordinates to Zn4O
secondary building units (SBUs) to form frameworks of
Zn4O(PhTPDC)3, which was named UBMOF-2 (University at
Buffalo MOF-2, Figure 1). Single crystal X-ray diffraction
experiments revealed that UBMOF-2 crystallizes in the cubic
Results and Discussion
Phenanthrene-based
diarylethenes
exhibit
reversible
photocyclization in solution,37,38 while simultaneously
possessing a rigid backbone capable of supporting functional
groups for use in the construction of MOFs.39 The 5-phenyl 2-
methyl thiophene derivative was targeted for synthesis as the
phenyl group has been shown to bathochromically shift the
absorption band in related DAEs.37 We were also interested in
assessing the influence of the bulkier functional group on the
chemical and physical properties of subsequently derived
MOFs. In addition to potentially exhibiting a greater impact on
the cavity or pore size in these materials and thus a greater
impact on the porosity and selectivity, the larger aromatic
group should also exhibit enhanced intermolecular
interactions potentially impacting the distribution of
thiophene orientations within the lattice.
ꢂ
space group ꢀꢁ3ꢁ (a=34.250 Å) and is isostructural with
UBMOF-1, IRMOF-10, and IRMOF-14.39,40
Similar to UBMOF-1, the low symmetry linker PhTPDC in
UBMOF-2 resides on a high site symmetry position (mm2).
While atomic positions of the metal SBU and the
phenanthrene portion of the linker were crystallographically
resolvable, the strong disorder of the photo-reactive
thiophene groups prevented reliable refinement of their
positions on both irradiated and non-irradiated crystals. Acid
digestion experiments performed on UBMOF-2 confirmed that
the PhTPDC linker remained chemically intact throughout the
MOF synthesis and subsequent acid digestion (See supporting
information).
In a solution of methanol, the ring-open form, PhTPDC-o, is
readily converted to the closed form PhTPDC-c upon
irradiation with UV light (365 nm). The initially colourless
solution turns blue following irradiation and the formation of
the ring closed species can be monitored by the appearance of
new absorption bands in the UV/Visible spectrum of the
solution (Figure 2). Upon exposure to visible light (>495 nm),
the blue solution reverts back to colourless and the original
spectrum is obtained. The presence of an isosbestic point at
317 nm indicates the photochemical reaction does not
generate any side products. While ring closed isomers of DAEs
generally exhibit excellent thermal stability, in the absence of
visible light, the PhTPDC coloured solution slowly reverts back
to a colourless solution indicating the presence of a back
Upon irradiation with 365 nm light, single crystals of UBMOF-2
readily transform from colourless to dark blue (Figure 1).
Visible absorption spectra of irradiated individual single
crystals confirmed the presence of an absorption band
identical to that of the PhTPDC-c in solution (Figure 2).
Shown in Figure
3 are traces of the time-dependent
absorbance at 600 nm of a single crystal of UBMOF-2 following
irradiation with UV light (365 nm) at four different
temperatures. Following the cessation of irradiation with UV
light, the absorbance of a photoisomerized single crystal,
UBMOF-2-c, slowly decays back to that of the original crystal
containing only the ring-open species, UBMOF-2-o. In order to
assess the degree to which the ring-opening reaction is light or
thermally driven, the light of the microscope which is used to
monitor the reaction (and could potentially drive the
cycloreversion reaction) was blocked for a short amount of
time during the decay measurement. As shown in Figure 3,
the ‘light blocked’ traces are nearly identical to the traces in
which the light of the microscope illuminates the sample for
the entire duration of the measurement at all temperatures.
These experiments indicate that the cycloreversion reaction in
UBMOF-2 is entirely thermally driven.
The decay of the absorption band at 600 nm follows bi-
exponential kinetics with both measured decay rates
Figure 2. Top: UV/Visible absorption spectra of the open (solid blue line)
and closed form (dashed red line) of PhTPDC (2.5x10-5 M in MeOH). Inset:
The difference absorption spectrum of a UBMOF-2 single crystal following
irradiation with 365 nm light.
2 | J. Name., 2012, 00, 1-3
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Walton, Ian M.
