advantages – high thermal stability, high fatigue resistance, etc. – which were
described in the respective sections.
It should be stressed here that the embedment of photochromic dyes into porous
hosts is not restricted to MOFs, as also COFs (covalent organic frameworks),
cyclodextrin cages, or zeolites are suitable candidates. Notably, the first work on
the latter is dated back to the late 1990’s [13], a few years before MOFs entered the
scientific world. And for current applications, photochromic dyes embedded
in/being part of polymeric layers are probably still the best choice. But none of
these alternative hosts show the high versatility of MOFs.
Surely, there are also disadvantages of photochromic dyes embedded as
non-covalently attached guests in MOF hosts. The difficulties in obtaining detailed
information about the highly mobile guests have already been mentioned. Additionally, the low thermal stability of these hybrid materials might be considered as a
drawback, as the embedded guests will leave the MOF matrix upon heating.
However, long-term investigations on thin films of type HKUST-1 with embedded
azobenzene proved that at least at room temperature this material is stable for several
months, whereas upon heating to 60
C its integrity suffers significantly. However, it
should be stated that azobenzene is the most volatile guest used in these investigations and with the other less volatile guests the thermal stability increases.
What are the tasks and aims of this research field for the future? As was already
presented in this chapter, the use of high-quality thin films seems to be indispensable
for possible applications. And here is still a high need for further developments, as
the large variety of MOF structures available as bulk materials (single crystals,
powders) cannot be synthesized to the same extent as thin films. Nevertheless,
investigations on bulk switch@MOF systems are still necessary, as in most cases
they paved the way for the development of functional responsive thin film materials.
That is, new MOFs as hosts for such switch@MOF hybrid materials need to be
developed and tested, which influence the photochromic properties of the embedded
dyes by specific spatial or electronic situations. On the other hand, there is still a
large variety of interesting photochromic molecules, which have not been investigated as possible guests for incorporation into MOFs, e.g., fulgides. Indeed, almost
every month a new and interesting photochromic dye molecule is published, which
might be an interesting candidates for a guest@MOF system, e.g., the recently
published iminothioindoxyls [164]. Another approach only poorly being addressed
up to now, is the directed embedment of two or more guests in a MOF matrix, as also
specific guest-guest interactions like charge-transfer or electron hopping will lead to
materials with new and fascinating properties.
Nevertheless, it is our deepest conviction that more detailed information about the
structures of these switch@MOF systems is needed to develop them further, as from
these structures the underlying host-guest and guest-guest interactions can be
deduced and, at best, be used to design new hybrid materials with improved
properties. At the moment, the structural information is low and mainly restricted
to results of high-resolution synchrotron powder diffraction data. However, this
approach is difficult due to the high mobility of the guests and their possible disorder
in high-symmetric hosts. Therefore, more local probes like solid-state NMR or total
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H. A. Schwartz and U. Ruschewitz
described in the respective sections.
It should be stressed here that the embedment of photochromic dyes into porous
hosts is not restricted to MOFs, as also COFs (covalent organic frameworks),
cyclodextrin cages, or zeolites are suitable candidates. Notably, the first work on
the latter is dated back to the late 1990’s [13], a few years before MOFs entered the
scientific world. And for current applications, photochromic dyes embedded
in/being part of polymeric layers are probably still the best choice. But none of
these alternative hosts show the high versatility of MOFs.
Surely, there are also disadvantages of photochromic dyes embedded as
non-covalently attached guests in MOF hosts. The difficulties in obtaining detailed
information about the highly mobile guests have already been mentioned. Additionally, the low thermal stability of these hybrid materials might be considered as a
drawback, as the embedded guests will leave the MOF matrix upon heating.
However, long-term investigations on thin films of type HKUST-1 with embedded
azobenzene proved that at least at room temperature this material is stable for several
months, whereas upon heating to 60
C its integrity suffers significantly. However, it
should be stated that azobenzene is the most volatile guest used in these investigations and with the other less volatile guests the thermal stability increases.
What are the tasks and aims of this research field for the future? As was already
presented in this chapter, the use of high-quality thin films seems to be indispensable
for possible applications. And here is still a high need for further developments, as
the large variety of MOF structures available as bulk materials (single crystals,
powders) cannot be synthesized to the same extent as thin films. Nevertheless,
investigations on bulk switch@MOF systems are still necessary, as in most cases
they paved the way for the development of functional responsive thin film materials.
That is, new MOFs as hosts for such switch@MOF hybrid materials need to be
developed and tested, which influence the photochromic properties of the embedded
dyes by specific spatial or electronic situations. On the other hand, there is still a
large variety of interesting photochromic molecules, which have not been investigated as possible guests for incorporation into MOFs, e.g., fulgides. Indeed, almost
every month a new and interesting photochromic dye molecule is published, which
might be an interesting candidates for a guest@MOF system, e.g., the recently
published iminothioindoxyls [164]. Another approach only poorly being addressed
up to now, is the directed embedment of two or more guests in a MOF matrix, as also
specific guest-guest interactions like charge-transfer or electron hopping will lead to
materials with new and fascinating properties.
Nevertheless, it is our deepest conviction that more detailed information about the
structures of these switch@MOF systems is needed to develop them further, as from
these structures the underlying host-guest and guest-guest interactions can be
deduced and, at best, be used to design new hybrid materials with improved
properties. At the moment, the structural information is low and mainly restricted
to results of high-resolution synchrotron powder diffraction data. However, this
approach is difficult due to the high mobility of the guests and their possible disorder
in high-symmetric hosts. Therefore, more local probes like solid-state NMR or total
148
H. A. Schwartz and U. Ruschewitz