When embedded inside the MOF thin films, AZB undergoes photochromism,
which was followed by UV/vis spectroscopy (see Fig. 16a): UV light irradiation
causes the E-to-Z conversion (365 nm; red line), while after irradiation with blue
light (455 nm; blue line), a spectrum almost identical to the spectrum of the pristine
E-AZB-loaded SURMOF is obtained. The photostability as well as fatigue resistance was studied through several cycles of UV light and visible light irradiation,
respectively. The authors found the intensity shifts of both the E and Z isomers to be
stable with no significant bleaching, see Fig. 16b). Similar to Ruschewitz and
co-workers [129], IR spectroscopy was applied to determine the switching yield.
Here, the amount of E-AZB decreases from 100% to 35% (65% Z-AZB) after UV
light irradiation, which is similar to results obtained for thin films with AZB side
groups as substituents of a MOF linker [120]. Furthermore, AZB only slowly
desorbs from the host framework, as determined from long-term storage experiments
at room temperature and at 60
C (see Fig. 16c). Thus, AZB@HKUST-1 represents a
rather stable switch@MOF hybrid system.
Based on the results of Ruschewitz and co-workers [137] on fluorinated
azobenzenes, the authors chose tF-AZB as a potential guest molecule. As described
before, the isomerization processes of tF-AZB can be triggered with visible light,
hence no UV light is necessary [136]. Moreover, tF-AZB exhibits both a larger
thermal stability and a higher switching yield. The latter originates from the electronic structures of the isomers: The energies for the n-πà bands of the E and
Z isomer markedly differ as well as the respective excitation wavelengths.
For tF-AZB@MOF thin films, a light response similar to tF-AZB dissolved in
ethanol is observed upon illumination with 400 nm and 530 nm. Photoswitching is
reversible for five switching cycles without significant fatigue. Both the light
response and reversibility of switching are depicted in Fig. 17a, b.
In order to study the functional response of the obtained (tF-)AZB@HKUST-1
thin films, gas uptake and release experiments were performed before and after
irradiation with the polar molecule 1,4-butanediol: the initial argon flow was
enriched with butanediol during the uptake measurements, and the gas uptake was
detected via a quartz crystal microbalance (QCM) [145]. The results of these
measurements before and after UV light irradiation are shown in Fig. 18. After
reaching the equilibrium uptake amount of E-AZB@HKUST-1, the sample was
irradiated with 365 nm for 20 min. The light-induced E-to-Z isomerization of AZB
caused an increase of the butanediol uptake of approx. 8% [144], similar to
photoswitchable SURFMOFs with azobenzene side-groups [120, 146]. The changing uptake of butanediol in these experiments was assumed to derive from the
different dipole moments of E- and Z-AZB, which is 0 D in E-AZB and 3 D in ZAZB [34]. Similar experiments were performed on tF-AZB@HKUST-1 thin films.
Even though an increase of the butanediol uptake was found, the total amount was
significantly smaller, probably as a result of a smaller amount of tF-AZB embedded
inside the MOF host. To sum up, Heinke and co-workers were the first to present
photoswitchable thin films with azobenzene and tF-azobenzene as non-covalently
attached guest molecules, which were used to remote control the gas uptake amount.
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H. A. Schwartz and U. Ruschewitz
which was followed by UV/vis spectroscopy (see Fig. 16a): UV light irradiation
causes the E-to-Z conversion (365 nm; red line), while after irradiation with blue
light (455 nm; blue line), a spectrum almost identical to the spectrum of the pristine
E-AZB-loaded SURMOF is obtained. The photostability as well as fatigue resistance was studied through several cycles of UV light and visible light irradiation,
respectively. The authors found the intensity shifts of both the E and Z isomers to be
stable with no significant bleaching, see Fig. 16b). Similar to Ruschewitz and
co-workers [129], IR spectroscopy was applied to determine the switching yield.
Here, the amount of E-AZB decreases from 100% to 35% (65% Z-AZB) after UV
light irradiation, which is similar to results obtained for thin films with AZB side
groups as substituents of a MOF linker [120]. Furthermore, AZB only slowly
desorbs from the host framework, as determined from long-term storage experiments
at room temperature and at 60
C (see Fig. 16c). Thus, AZB@HKUST-1 represents a
rather stable switch@MOF hybrid system.
Based on the results of Ruschewitz and co-workers [137] on fluorinated
azobenzenes, the authors chose tF-AZB as a potential guest molecule. As described
before, the isomerization processes of tF-AZB can be triggered with visible light,
hence no UV light is necessary [136]. Moreover, tF-AZB exhibits both a larger
thermal stability and a higher switching yield. The latter originates from the electronic structures of the isomers: The energies for the n-πà bands of the E and
Z isomer markedly differ as well as the respective excitation wavelengths.
For tF-AZB@MOF thin films, a light response similar to tF-AZB dissolved in
ethanol is observed upon illumination with 400 nm and 530 nm. Photoswitching is
reversible for five switching cycles without significant fatigue. Both the light
response and reversibility of switching are depicted in Fig. 17a, b.
In order to study the functional response of the obtained (tF-)AZB@HKUST-1
thin films, gas uptake and release experiments were performed before and after
irradiation with the polar molecule 1,4-butanediol: the initial argon flow was
enriched with butanediol during the uptake measurements, and the gas uptake was
detected via a quartz crystal microbalance (QCM) [145]. The results of these
measurements before and after UV light irradiation are shown in Fig. 18. After
reaching the equilibrium uptake amount of E-AZB@HKUST-1, the sample was
irradiated with 365 nm for 20 min. The light-induced E-to-Z isomerization of AZB
caused an increase of the butanediol uptake of approx. 8% [144], similar to
photoswitchable SURFMOFs with azobenzene side-groups [120, 146]. The changing uptake of butanediol in these experiments was assumed to derive from the
different dipole moments of E- and Z-AZB, which is 0 D in E-AZB and 3 D in ZAZB [34]. Similar experiments were performed on tF-AZB@HKUST-1 thin films.
Even though an increase of the butanediol uptake was found, the total amount was
significantly smaller, probably as a result of a smaller amount of tF-AZB embedded
inside the MOF host. To sum up, Heinke and co-workers were the first to present
photoswitchable thin films with azobenzene and tF-azobenzene as non-covalently
attached guest molecules, which were used to remote control the gas uptake amount.
130
H. A. Schwartz and U. Ruschewitz