Gascon and co-workers presented a comprehensive review on this topic, assigning
the photoactive function to either being a part of the framework itself as intrinsic or
being a guest molecule as extrinsic introduction to the MOF [98]. Further reviews on
this topic have been published [99–106]. Generally, it is possible to introduce the
photoactive functionality in four different ways, which will be shortly described in
the following:
1. Intrinsic introduction
(a) The photoactive part is part of the linker backbone itself.
(b) The photoactive part is a substituent of the linker.
(c) The photoactive part is coordinatively immobilized inside the MOF scaffold.
2. Extrinsic introduction
(d) The photochromic dye is inserted as a non-covalently attached guest molecule inside the MOF lattice.
For the intrinsic introduction, three ways are distinguished: Firstly, the photochromic moiety is part of the linker backbone itself. As aforesaid, the light-induced
isomerization processes often require a certain degree of spatial freedom. Hence,
large structural change, e.g., E/Z isomerization, is sterically hindered [107] or even
leads to a collapse of the framework in such systems [108, 109]. Therefore, only few
studies have been published with promising functionalities [110–112]. Secondly, the
photochromic unit is introduced as a substituent of the linker [113]. Here, both bulk
materials and thin films have been successfully prepared [112, 114–116]. It has been
demonstrated that the uptake amount of gases can be controlled by light
[114, 117]. Particularly for thin films with azobenzene side groups, the diffusion
properties [118, 119], permeation and separation factor of membranes [120], or the
proton-conduction of guest molecules [121] was remote-controlled by light. In a
recently published work, Heinke and co-workers were even able to show that
switching of the enantioselective adsorption capacity by light is possible [122]. However, isomerization processes are hindered in some cases [123]. Moreover, the high
synthetic efforts to obtain such functionalized linker molecules is the major drawback, which limits broad applications of photoactive MOFs constructed that way.
The latter also applies to coordinative immobilization of the photoswitch, which has
been presented by Shustova and co-workers [124] for spiropyran and diarylethene
derivatives.
Via extrinsic introduction, the photoactive moieties are embedded as guest
molecules inside the MOF pores. This strategy is challenging, as the mobility
triggers a significantly more complex situation than the guest being fixed at the
MOF scaffold. For instance, the degree of filling (guest-guest interactions) and the
physicochemical environment given by the MOF voids (host-guest interactions) are
expected to affect the preferable orientation and configuration of the guest molecule
and the photostationary state (PSS).
With respect to extrinsic inclusion, at least four different methods for incorporation were described in the literature:
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H. A. Schwartz and U. Ruschewitz
the photoactive function to either being a part of the framework itself as intrinsic or
being a guest molecule as extrinsic introduction to the MOF [98]. Further reviews on
this topic have been published [99–106]. Generally, it is possible to introduce the
photoactive functionality in four different ways, which will be shortly described in
the following:
1. Intrinsic introduction
(a) The photoactive part is part of the linker backbone itself.
(b) The photoactive part is a substituent of the linker.
(c) The photoactive part is coordinatively immobilized inside the MOF scaffold.
2. Extrinsic introduction
(d) The photochromic dye is inserted as a non-covalently attached guest molecule inside the MOF lattice.
For the intrinsic introduction, three ways are distinguished: Firstly, the photochromic moiety is part of the linker backbone itself. As aforesaid, the light-induced
isomerization processes often require a certain degree of spatial freedom. Hence,
large structural change, e.g., E/Z isomerization, is sterically hindered [107] or even
leads to a collapse of the framework in such systems [108, 109]. Therefore, only few
studies have been published with promising functionalities [110–112]. Secondly, the
photochromic unit is introduced as a substituent of the linker [113]. Here, both bulk
materials and thin films have been successfully prepared [112, 114–116]. It has been
demonstrated that the uptake amount of gases can be controlled by light
[114, 117]. Particularly for thin films with azobenzene side groups, the diffusion
properties [118, 119], permeation and separation factor of membranes [120], or the
proton-conduction of guest molecules [121] was remote-controlled by light. In a
recently published work, Heinke and co-workers were even able to show that
switching of the enantioselective adsorption capacity by light is possible [122]. However, isomerization processes are hindered in some cases [123]. Moreover, the high
synthetic efforts to obtain such functionalized linker molecules is the major drawback, which limits broad applications of photoactive MOFs constructed that way.
The latter also applies to coordinative immobilization of the photoswitch, which has
been presented by Shustova and co-workers [124] for spiropyran and diarylethene
derivatives.
Via extrinsic introduction, the photoactive moieties are embedded as guest
molecules inside the MOF pores. This strategy is challenging, as the mobility
triggers a significantly more complex situation than the guest being fixed at the
MOF scaffold. For instance, the degree of filling (guest-guest interactions) and the
physicochemical environment given by the MOF voids (host-guest interactions) are
expected to affect the preferable orientation and configuration of the guest molecule
and the photostationary state (PSS).
With respect to extrinsic inclusion, at least four different methods for incorporation were described in the literature:
114
H. A. Schwartz and U. Ruschewitz