Based on IR spectroscopy, the authors proposed formation of π-stacking complexes
of E-azobenzene with the BPDC linkers at the pore entrances. Upon irradiation,
these complexes change from an E-AZB stacking to a Z-AZB stacking. E and
Z isomer profoundly differ in size and dipole moment, and thus, the MOF pore
window opens and closes with great efficacy. With these results, Caro and
co-workers were the first group that could show that an impressive separation factor
for gases can be achieved with a switch@MOF composite with non-covalently
attached guest molecules.
2.3 Diarylethenes in MOFs
2.3.1 Bulk Materials/Single Crystals
Stilbenes and azobenzenes are rather simple photoswitches with comparatively small
changes in their absorption properties, which are almost invisible for the human eye.
Replacement of these dyes with photochromic units that undergo both drastic
geometrical changes and large alterations in their spectral profiles could be of
great potential to remote control MOF pore properties.
In 2013, Benedict and co-workers were the first group to study the photochromic behavior of a dye molecule meeting these requirements inside a MOF.
Already described DMOF-1 [127] was synthesized and single crystals of this
material were loaded with the dithienylethene 1,2-bis(2,5-dimethyl-thien-3-yl)perfluorocyclopentene [147, 148] (denoted as DTE) via a melting process
[149]. The initially formed DTE@DMOF-1 hybrid material shows no optical difference compared to plain DMOF-1 (Fig. 20a, c).
Upon UV light exposure (365 nm), the colorless DTE@DMOF-1 crystals immediately turn dark red, exhibiting strong linear dichroism for incident light, presumably originating from DTE alignment inside the MOF host. In Fig. 20, top, polarized
light images along [001] and [100] and the corresponding absorption spectra,
Fig. 20, bottom, are shown. The authors found the orientation of the electric dipole
transition moment to be responsible for the coloration. For the DTE@DMOF-1
single crystals, the dichroic ratio was calculated to be 0.29. Consequently, the DTE
guest molecule (and its corresponding electric dipole transition moment, EDTM) is
preferentially aligned approx. 28
from the [001] axis in the ac plane.
Unfortunately, the structural characterization of the DTE@DMOF-1 single crystal was not successful due to a combination of inhomogeneous loading and positional disorder of the guest molecule (note: DMOF-1 crystallizes with a high
tetragonal symmetry). By
1 H NMR spectroscopy, the amount of DTE loaded into
DMOF-1 was found to vary significantly, making an accurate structure solution
impossible.
Although switching of DTE inside DMOF-1 can be repeated many times, the
initial open-ring state is not completely retrieved, even after long exposure times
with visible light. Furthermore, the generation of the colored species occurs faster
134
H. A. Schwartz and U. Ruschewitz
of E-azobenzene with the BPDC linkers at the pore entrances. Upon irradiation,
these complexes change from an E-AZB stacking to a Z-AZB stacking. E and
Z isomer profoundly differ in size and dipole moment, and thus, the MOF pore
window opens and closes with great efficacy. With these results, Caro and
co-workers were the first group that could show that an impressive separation factor
for gases can be achieved with a switch@MOF composite with non-covalently
attached guest molecules.
2.3 Diarylethenes in MOFs
2.3.1 Bulk Materials/Single Crystals
Stilbenes and azobenzenes are rather simple photoswitches with comparatively small
changes in their absorption properties, which are almost invisible for the human eye.
Replacement of these dyes with photochromic units that undergo both drastic
geometrical changes and large alterations in their spectral profiles could be of
great potential to remote control MOF pore properties.
In 2013, Benedict and co-workers were the first group to study the photochromic behavior of a dye molecule meeting these requirements inside a MOF.
Already described DMOF-1 [127] was synthesized and single crystals of this
material were loaded with the dithienylethene 1,2-bis(2,5-dimethyl-thien-3-yl)perfluorocyclopentene [147, 148] (denoted as DTE) via a melting process
[149]. The initially formed DTE@DMOF-1 hybrid material shows no optical difference compared to plain DMOF-1 (Fig. 20a, c).
Upon UV light exposure (365 nm), the colorless DTE@DMOF-1 crystals immediately turn dark red, exhibiting strong linear dichroism for incident light, presumably originating from DTE alignment inside the MOF host. In Fig. 20, top, polarized
light images along [001] and [100] and the corresponding absorption spectra,
Fig. 20, bottom, are shown. The authors found the orientation of the electric dipole
transition moment to be responsible for the coloration. For the DTE@DMOF-1
single crystals, the dichroic ratio was calculated to be 0.29. Consequently, the DTE
guest molecule (and its corresponding electric dipole transition moment, EDTM) is
preferentially aligned approx. 28
from the [001] axis in the ac plane.
Unfortunately, the structural characterization of the DTE@DMOF-1 single crystal was not successful due to a combination of inhomogeneous loading and positional disorder of the guest molecule (note: DMOF-1 crystallizes with a high
tetragonal symmetry). By
1 H NMR spectroscopy, the amount of DTE loaded into
DMOF-1 was found to vary significantly, making an accurate structure solution
impossible.
Although switching of DTE inside DMOF-1 can be repeated many times, the
initial open-ring state is not completely retrieved, even after long exposure times
with visible light. Furthermore, the generation of the colored species occurs faster
134
H. A. Schwartz and U. Ruschewitz