[66–68], heterogeneous catalysis [69], drug encapsulation and delivery [70, 71],
incorporation of fluorescent dyes [72, 73], and electronic devices [74, 75]. Even
structure elucidation of organic molecules via embedment into a MOF framework
has been shown by Fujita and co-workers, who established this as “crystalline
sponge method” [76–83]. Detailed information on possible industrial applications
of MOFs can be found in several reviews [84, 85]. Also in modern physics
concerning, e.g., semiconductors, MOFs started to gain more and more
attention [86].
These diverse applicabilities are the result of the structural design of MOFs. The
linker molecules as well as the metal nodes can be variably designed. Yaghi and
co-workers synthesized isoreticular MOFs (IR-MOFs), which show similar network
topologies by simultaneous elongation and modification of the linker molecules
[87]. That way, pore diameters up to 98 Å were realized [88]. However, further
elongation of the linker molecule does not always lead to larger pores, but to
interpenetration, which has been observed, e.g., for PIZOFs (porous interpenetrated
zirconium-organic frameworks) [89]. With respect to potential applications such as
chemical sensors, smart membrane separation or data storage devices, MOFs as thin
films are preferable to powder MOFs. The thin film approach allows to fabricate
devices and, additionally, films possess a more flexible design than bulk materials.
For the production of thin MOF films, various methods have been developed [90–
92]. Rather straightforward ways are spin-coating a suspension of MOF crystals onto
a specific surface [93, 94], direct growth of MOF films on surfaces by solvothermal
methods [95, 96] or controlled layer-by-layer synthesis. The latter are referred to as
surface-mounted MOFs – SURMOFs [97].
The systematic design and functionalization of MOF pores, as well as the
opportunity to prepare MOFs as thin films, make this class of materials promising
candidates to control the guest uptake, host-guest interactions, and, additionally,
guest-guest interactions.
While most applications since 1999 focus on gas adsorption and storage, the field
of photoactive MOFs has been given only more attention in recent years and is now a
growing field of research. This is not surprising, since MOFs appear to be ideal host
matrices for photoactive molecules. In the following, these guest@MOF materials
will be introduced in a general way. First, methods on how to incorporate the
photochromic guest molecules and how to characterize the resulting hybrid materials
are described. In a second step, different switch@MOF systems will be introduced,
starting with the rather simple photochromic dye stilbene. Each section will always
start with the work published on crystalline bulk materials and will then, if available,
continue with thin film materials.
1.4 Implementing Photoswitches into MOFs
The construction of switch@MOF hybrid materials can be achieved by either
extrinsic or intrinsic introduction of the photoactive part, respectively. In 2016,
Photoactive Molecules within MOFs
113
incorporation of fluorescent dyes [72, 73], and electronic devices [74, 75]. Even
structure elucidation of organic molecules via embedment into a MOF framework
has been shown by Fujita and co-workers, who established this as “crystalline
sponge method” [76–83]. Detailed information on possible industrial applications
of MOFs can be found in several reviews [84, 85]. Also in modern physics
concerning, e.g., semiconductors, MOFs started to gain more and more
attention [86].
These diverse applicabilities are the result of the structural design of MOFs. The
linker molecules as well as the metal nodes can be variably designed. Yaghi and
co-workers synthesized isoreticular MOFs (IR-MOFs), which show similar network
topologies by simultaneous elongation and modification of the linker molecules
[87]. That way, pore diameters up to 98 Å were realized [88]. However, further
elongation of the linker molecule does not always lead to larger pores, but to
interpenetration, which has been observed, e.g., for PIZOFs (porous interpenetrated
zirconium-organic frameworks) [89]. With respect to potential applications such as
chemical sensors, smart membrane separation or data storage devices, MOFs as thin
films are preferable to powder MOFs. The thin film approach allows to fabricate
devices and, additionally, films possess a more flexible design than bulk materials.
For the production of thin MOF films, various methods have been developed [90–
92]. Rather straightforward ways are spin-coating a suspension of MOF crystals onto
a specific surface [93, 94], direct growth of MOF films on surfaces by solvothermal
methods [95, 96] or controlled layer-by-layer synthesis. The latter are referred to as
surface-mounted MOFs – SURMOFs [97].
The systematic design and functionalization of MOF pores, as well as the
opportunity to prepare MOFs as thin films, make this class of materials promising
candidates to control the guest uptake, host-guest interactions, and, additionally,
guest-guest interactions.
While most applications since 1999 focus on gas adsorption and storage, the field
of photoactive MOFs has been given only more attention in recent years and is now a
growing field of research. This is not surprising, since MOFs appear to be ideal host
matrices for photoactive molecules. In the following, these guest@MOF materials
will be introduced in a general way. First, methods on how to incorporate the
photochromic guest molecules and how to characterize the resulting hybrid materials
are described. In a second step, different switch@MOF systems will be introduced,
starting with the rather simple photochromic dye stilbene. Each section will always
start with the work published on crystalline bulk materials and will then, if available,
continue with thin film materials.
1.4 Implementing Photoswitches into MOFs
The construction of switch@MOF hybrid materials can be achieved by either
extrinsic or intrinsic introduction of the photoactive part, respectively. In 2016,
Photoactive Molecules within MOFs
113