(shortest O À HÁÁÁC distance: 281 pm compared to 265 pm in AZB 0.50 @MIL-53
(Al) [129]) originating from the high degree of fluorination of the pF-AZB guest and
the inversion of its quadrupole moment [138]. Secondly, the phenyl rings are
arranged almost directly one above each other with only a very small shift, which
is in-between a stacked and an offset-stacked arrangement [139] being also the result
of the inverted quadrupole moment of the perfluorinated ring. This was confirmed by
a small shift in the frequency of e ν (C¼C) from 1,506 cm
À1 in the IR spectrum of
unloaded MIL-53(Al) to 1,513 cm
À1 in the IR spectrum of pF-AZB 0.34 @MIL-53
(Al). Besides the determination of host-guest interactions, IR spectroscopy allows
for a direct quantitative analysis of the switching process by tracking the E and
Z bands of the different azobenzenes, respectively. The results of these investigations are summarized in Table 2.
It is a very remarkable result of this work that for tF-AZB embedded in different
MOF hosts almost quantitative photoswitching (>90%) is found in a solid-state
Fig. 9 (Left) space-filling presentation of the crystal structure of pF-AZB 0.34 @MIL-53(Al) in a
view along [100]; (middle) side view with an “opened” channel; (right) details of the crystal
structure emphasizing some of the underlying host-guest interactions. Reprinted (adapted) with
permission from Wiley and Sons, Ref. [137]; copyright 2019 Wiley-VCH
Table 2 Maximum amounts of the E and Z isomers for the investigated guest@MOF composites,
reached by illumination with λ ¼ 405 nm and λ ¼ 532 nm, respectively
AZB
max. E/max. Z,
[initial state]
tF-AZB
max. E/max. Z,
[initial state]
oF-AZB
max. E/max. Z,
[initial state]
pF-AZB
max. E/max. Z,
[initial state]
MOF-5 100%/25% [100%
E]
93%/82% [52% E] n.d. [62–88% E]
100%/50% [80%
E]
MIL-53
(Al)
100%/0% [100%
E]
90%/100% [37%
E]
100%/56% [99%
E]
100%/little
Z [100% E]
MIL-53
(Ga)
100%/n.d. [100%
E]
90%/94% [37% E] n.d. [little Z]
100%/n.d. [100%
E]
MIL-68
(Ga)
100%/27% [100%
E]
96%/93% [45% E] n.d. [62–88% E]
100%/40% [87%
E]
MIL-68
(In)
100%/30% [100%
E]
95%/95% [51% E] 88%/90% [62% E] 100%/79% [55%
E]
Reprinted (adapted) with permission from Wiley and Sons, Ref. [137]; copyright 2019 Wiley-VCH
n.d. not determined
Photoactive Molecules within MOFs
123
(Al) [129]) originating from the high degree of fluorination of the pF-AZB guest and
the inversion of its quadrupole moment [138]. Secondly, the phenyl rings are
arranged almost directly one above each other with only a very small shift, which
is in-between a stacked and an offset-stacked arrangement [139] being also the result
of the inverted quadrupole moment of the perfluorinated ring. This was confirmed by
a small shift in the frequency of e ν (C¼C) from 1,506 cm
À1 in the IR spectrum of
unloaded MIL-53(Al) to 1,513 cm
À1 in the IR spectrum of pF-AZB 0.34 @MIL-53
(Al). Besides the determination of host-guest interactions, IR spectroscopy allows
for a direct quantitative analysis of the switching process by tracking the E and
Z bands of the different azobenzenes, respectively. The results of these investigations are summarized in Table 2.
It is a very remarkable result of this work that for tF-AZB embedded in different
MOF hosts almost quantitative photoswitching (>90%) is found in a solid-state
Fig. 9 (Left) space-filling presentation of the crystal structure of pF-AZB 0.34 @MIL-53(Al) in a
view along [100]; (middle) side view with an “opened” channel; (right) details of the crystal
structure emphasizing some of the underlying host-guest interactions. Reprinted (adapted) with
permission from Wiley and Sons, Ref. [137]; copyright 2019 Wiley-VCH
Table 2 Maximum amounts of the E and Z isomers for the investigated guest@MOF composites,
reached by illumination with λ ¼ 405 nm and λ ¼ 532 nm, respectively
AZB
max. E/max. Z,
[initial state]
tF-AZB
max. E/max. Z,
[initial state]
oF-AZB
max. E/max. Z,
[initial state]
pF-AZB
max. E/max. Z,
[initial state]
MOF-5 100%/25% [100%
E]
93%/82% [52% E] n.d. [62–88% E]
100%/50% [80%
E]
MIL-53
(Al)
100%/0% [100%
E]
90%/100% [37%
E]
100%/56% [99%
E]
100%/little
Z [100% E]
MIL-53
(Ga)
100%/n.d. [100%
E]
90%/94% [37% E] n.d. [little Z]
100%/n.d. [100%
E]
MIL-68
(Ga)
100%/27% [100%
E]
96%/93% [45% E] n.d. [62–88% E]
100%/40% [87%
E]
MIL-68
(In)
100%/30% [100%
E]
95%/95% [51% E] 88%/90% [62% E] 100%/79% [55%
E]
Reprinted (adapted) with permission from Wiley and Sons, Ref. [137]; copyright 2019 Wiley-VCH
n.d. not determined
Photoactive Molecules within MOFs
123