light causes changes of the absorption properties, this phenomenon is called
photochromism.
Since most of the photochromic molecules only show very limited
photoswitching in the pristine solid, the majority of investigations on their photochromic properties has been performed in solution. As an example, spiropyrans with
substantial light-induced structural changes have almost exclusively been investigated in solution [1–4], when embedded in polymers [5–9] or nanoporous materials
[10–12]. Several efforts were being made to incorporate the photoactive part into
zeolites, silica matrices, or polymers to form smart materials. About 20 years ago,
Caro and co-workers started to implement the photoactive moiety as a guest
molecule into a porous host matrix (porous material, PM) [13]. In a first attempt,
the authors embedded azobenzene into nanoporous zeolites, forming the first
photoswitch@PM composite material. In addition to other azobenzene@zeolite
materials [14, 15], also spiropyran@zeolite composites were synthesized [16–
18]. For both types of switches, photochromic properties of the switchable part
were observed, making these hybrid compounds promising candidates for the
development of functional materials. Another elegant way to obtain these smart
materials is the embedment of photochromic dyes into metal-organic frameworks
(MOFs) to form functional switch@MOF composites. MOFs represent a class of
porous materials with a high crystallinity combined with the ability to be systematically functionalized, opening a wide range of applications.
This chapter will give an overview of switch@MOF systems with the photoactive
part being non-covalently attached to the MOF scaffold. Firstly, the terms photochromism, solvatochromism and metal-organic frameworks will be briefly introduced, and finally, as the main part of this chapter, the hitherto known switch@MOF
hybrid systems will be thoroughly discussed.
1.1 Photochromism
The phenomenon of photochromism was firstly discovered by Fritzsche [19], when
he observed bleaching of an orange-colored tetracene solution in the daylight that
retrieved its color overnight. Almost 100 years later in the 1950s, Hirshberg [20]
was the first to define photochromism as the reversible transformation of a molecular
species between two isomers A and B with different absorption properties upon
irradiation with electromagnetic radiation of a specific wavelength. Here, reorganization of the ground state can be either achieved by light, such molecules that are
called P-type chromophores, or by heat supply, which makes these molecules T-type
chromophores [21]. Two different types of photochromism are distinguished:
positive and negative photochromism. The photochromism is called positive, if the
thermodynamically more stable species A is found to be non- or slightly colored,
while the irradiated one B is deeply colored. Here, λ max (B) of the irradiated moiety is
red-shifted in comparison to λ max (A) of the ground state. If λ max (A) > λ max (B), the
photochromism is called inverse, antidromic, or negative [21].
108
H. A. Schwartz and U. Ruschewitz
photochromism.
Since most of the photochromic molecules only show very limited
photoswitching in the pristine solid, the majority of investigations on their photochromic properties has been performed in solution. As an example, spiropyrans with
substantial light-induced structural changes have almost exclusively been investigated in solution [1–4], when embedded in polymers [5–9] or nanoporous materials
[10–12]. Several efforts were being made to incorporate the photoactive part into
zeolites, silica matrices, or polymers to form smart materials. About 20 years ago,
Caro and co-workers started to implement the photoactive moiety as a guest
molecule into a porous host matrix (porous material, PM) [13]. In a first attempt,
the authors embedded azobenzene into nanoporous zeolites, forming the first
photoswitch@PM composite material. In addition to other azobenzene@zeolite
materials [14, 15], also spiropyran@zeolite composites were synthesized [16–
18]. For both types of switches, photochromic properties of the switchable part
were observed, making these hybrid compounds promising candidates for the
development of functional materials. Another elegant way to obtain these smart
materials is the embedment of photochromic dyes into metal-organic frameworks
(MOFs) to form functional switch@MOF composites. MOFs represent a class of
porous materials with a high crystallinity combined with the ability to be systematically functionalized, opening a wide range of applications.
This chapter will give an overview of switch@MOF systems with the photoactive
part being non-covalently attached to the MOF scaffold. Firstly, the terms photochromism, solvatochromism and metal-organic frameworks will be briefly introduced, and finally, as the main part of this chapter, the hitherto known switch@MOF
hybrid systems will be thoroughly discussed.
1.1 Photochromism
The phenomenon of photochromism was firstly discovered by Fritzsche [19], when
he observed bleaching of an orange-colored tetracene solution in the daylight that
retrieved its color overnight. Almost 100 years later in the 1950s, Hirshberg [20]
was the first to define photochromism as the reversible transformation of a molecular
species between two isomers A and B with different absorption properties upon
irradiation with electromagnetic radiation of a specific wavelength. Here, reorganization of the ground state can be either achieved by light, such molecules that are
called P-type chromophores, or by heat supply, which makes these molecules T-type
chromophores [21]. Two different types of photochromism are distinguished:
positive and negative photochromism. The photochromism is called positive, if the
thermodynamically more stable species A is found to be non- or slightly colored,
while the irradiated one B is deeply colored. Here, λ max (B) of the irradiated moiety is
red-shifted in comparison to λ max (A) of the ground state. If λ max (A) > λ max (B), the
photochromism is called inverse, antidromic, or negative [21].
108
H. A. Schwartz and U. Ruschewitz