(a) Loading via diffusion processes:
The MOF is dispersed in a solution of the photochromic dye. Here, the host
material is loaded via diffusion of the guest molecules into the MOF pores. A
possible disadvantage of this method is that the solvent molecules may remain
together with the chromophore inside the pores. Hence, resulting properties
might not exclusively originate from the MOF host, and care must be taken to
remove all residual solvent molecules after the loading process.
(b) Loading via melting processes:
The activated MOF and the dye are mixed and heated to the melting point of the
respective guest molecule. Subsequently, the molten dye will enter the pores of
the host lattice. Here, no solvent molecules have to be excluded in further
considerations.
(c) Loading via gas phase loading processes:
The activated MOF and the dye are thoroughly ground and heated under reduced
pressure to the sublimation point of the photochromic molecule. Here, the guest
molecules are embedded into the host material via the gas phase. Hence, no
solvent molecules have to be considered. However, sublimation temperatures
must be chosen carefully so that embedded dye molecules are not removed again
and no residual guest outside the MOF pores remains. Additionally, care must be
taken to avoid thermal decomposition of the photochromic dye and the MOF.
(d) Loading via crystallization inclusion:
MOF precursors and the photochromic dye form the switch@MOF composite in
a one pot synthesis. During the construction of the MOF network, the switches
used in the starting solution are integrated simultaneously. Obviously, this is a
time-saving method. Anyhow, chromophores with non-coordinating substituents must be chosen to avoid competing effects with the linker molecules.
Furthermore, solvent molecules remaining together with the chromophore inside
the pores might occur in this synthesis procedure. Similar to loading via diffusion processes, care must be taken to remove all solvent molecules afterwards.
All these loading procedures were used to synthesize various switch@MOF
systems with fascinating and even surprising properties. In a next step, the resulting
systems must be thoroughly analyzed. Adequate characterization of these functional
materials is indispensable to understand the occurring optical properties. Here,
different methods were applied, all regarding to Incorporation, Composition, and
resulting Effects, so that Ruschewitz and co-workers named it the ICE-principle
[152]. In Table 1, the different methods used for the characterization of
switch@MOF systems together with the obtainable information are summarized.
The categorization refers to the literature discussed in Chap. 2 “Switch@MOF
Systems.”
Since the first synthesis and investigation of a hybrid switch@MOF systems, not
only the variety of these systems increased from comparably simple to complex dye
molecules, but also the characterization of the obtained materials evolved in a way
that the interaction of host and guest molecule is now understood in more detail. In
the course of the following chapter, various synthesis as well as methodological
Photoactive Molecules within MOFs
115
The MOF is dispersed in a solution of the photochromic dye. Here, the host
material is loaded via diffusion of the guest molecules into the MOF pores. A
possible disadvantage of this method is that the solvent molecules may remain
together with the chromophore inside the pores. Hence, resulting properties
might not exclusively originate from the MOF host, and care must be taken to
remove all residual solvent molecules after the loading process.
(b) Loading via melting processes:
The activated MOF and the dye are mixed and heated to the melting point of the
respective guest molecule. Subsequently, the molten dye will enter the pores of
the host lattice. Here, no solvent molecules have to be excluded in further
considerations.
(c) Loading via gas phase loading processes:
The activated MOF and the dye are thoroughly ground and heated under reduced
pressure to the sublimation point of the photochromic molecule. Here, the guest
molecules are embedded into the host material via the gas phase. Hence, no
solvent molecules have to be considered. However, sublimation temperatures
must be chosen carefully so that embedded dye molecules are not removed again
and no residual guest outside the MOF pores remains. Additionally, care must be
taken to avoid thermal decomposition of the photochromic dye and the MOF.
(d) Loading via crystallization inclusion:
MOF precursors and the photochromic dye form the switch@MOF composite in
a one pot synthesis. During the construction of the MOF network, the switches
used in the starting solution are integrated simultaneously. Obviously, this is a
time-saving method. Anyhow, chromophores with non-coordinating substituents must be chosen to avoid competing effects with the linker molecules.
Furthermore, solvent molecules remaining together with the chromophore inside
the pores might occur in this synthesis procedure. Similar to loading via diffusion processes, care must be taken to remove all solvent molecules afterwards.
All these loading procedures were used to synthesize various switch@MOF
systems with fascinating and even surprising properties. In a next step, the resulting
systems must be thoroughly analyzed. Adequate characterization of these functional
materials is indispensable to understand the occurring optical properties. Here,
different methods were applied, all regarding to Incorporation, Composition, and
resulting Effects, so that Ruschewitz and co-workers named it the ICE-principle
[152]. In Table 1, the different methods used for the characterization of
switch@MOF systems together with the obtainable information are summarized.
The categorization refers to the literature discussed in Chap. 2 “Switch@MOF
Systems.”
Since the first synthesis and investigation of a hybrid switch@MOF systems, not
only the variety of these systems increased from comparably simple to complex dye
molecules, but also the characterization of the obtained materials evolved in a way
that the interaction of host and guest molecule is now understood in more detail. In
the course of the following chapter, various synthesis as well as methodological
Photoactive Molecules within MOFs
115