For stilbenes and azobenzenes, irradiation with UV light causes an E-to-Z
conversion of the molecules, for spiropyrans/spirooxazines and diarylethenes/
dithienylethenes (denoted as DAE and DTE in the following), light exposure results
in ring opening or ring closure to yield the excited state, respectively. Spiropyrans
and spirooxazines are switched between their closed, colorless spiropyran form
(denoted as SP form) and the open, deeply colored merocyanine form (denoted as
MC form). The isomerization process is accompanied by a drastic increase in the
dipole moment. In contrast to that, DAEs/DTEs exhibit ring closure of the o-DAE/
DTE to the c-DAE/DTE (o denotes open- and c closed-ring form, respectively). The
isomerization processes are shown in Fig. 1. For a more detailed description of the
switching mechanisms, we refer to the literature, e.g., by Bouas-Laurent and Dürr
[21] and others [22, 23, 25, 29, 31–34].
The light-induced structural changes require a certain degree of spatial freedom
and are strongly related to the type of isomerization. For many DAEs/DTEs,
photoswitching is even possible in the pure solids for both crystalline powders and
single crystals, since the light triggered re-organization between the ground and the
excited state is almost non-spacious [35]. In contrast to that, azobenzenes and their
fluorinated derivatives differ significantly in size of the E and Z isomers [36]. Consequently, the photochromic response in the solid state is limited or even completely
suppressed. A similar case is found for spiropyrans and spirooxazines. Despite of
few exceptions (e.g., when deposited as thin films [37–39], at very low temperatures
[40] or when irradiated with intense UV light [41]), the SP-to-MC isomerization is
sterically hindered in the pristine solid materials. This limited applicability remains a
challenging task, as the dissolved state is obviously disadvantageous for the construction and design of functional materials.
Since the light-induced structural changes cause alterations in absorption properties (color change), dipole moment, and/or refractive index [42], photochromic
compounds are intensively studied as they are expected to pave the way to new
responsive materials. The first example of an application of a spiropyran composite
was realized in 1989 by Rentzepis and Parthenopoulos, who published a spiropyranbased three-dimensional optical data storage device [5]. Additionally, photochromic
compounds were employed as synthetic molecular machines [43–46], whose most
famous representatives were awarded with the Nobel Prize in 2016 [47]. Ever since,
several efforts for the construction of responsive functional materials have been
made, also with an increasing interest in MOFs as host matrices for the
photochromic dye.
1.2 Solvatochromism
The class of spiropyrans and spirooxazines shows solvent-dependent absorption
maxima of the excited species, which is referred to as solvatochromism. As
Reichhardt comprehensively dealt with solvent effects in organic chemistry, his
110
H. A. Schwartz and U. Ruschewitz
conversion of the molecules, for spiropyrans/spirooxazines and diarylethenes/
dithienylethenes (denoted as DAE and DTE in the following), light exposure results
in ring opening or ring closure to yield the excited state, respectively. Spiropyrans
and spirooxazines are switched between their closed, colorless spiropyran form
(denoted as SP form) and the open, deeply colored merocyanine form (denoted as
MC form). The isomerization process is accompanied by a drastic increase in the
dipole moment. In contrast to that, DAEs/DTEs exhibit ring closure of the o-DAE/
DTE to the c-DAE/DTE (o denotes open- and c closed-ring form, respectively). The
isomerization processes are shown in Fig. 1. For a more detailed description of the
switching mechanisms, we refer to the literature, e.g., by Bouas-Laurent and Dürr
[21] and others [22, 23, 25, 29, 31–34].
The light-induced structural changes require a certain degree of spatial freedom
and are strongly related to the type of isomerization. For many DAEs/DTEs,
photoswitching is even possible in the pure solids for both crystalline powders and
single crystals, since the light triggered re-organization between the ground and the
excited state is almost non-spacious [35]. In contrast to that, azobenzenes and their
fluorinated derivatives differ significantly in size of the E and Z isomers [36]. Consequently, the photochromic response in the solid state is limited or even completely
suppressed. A similar case is found for spiropyrans and spirooxazines. Despite of
few exceptions (e.g., when deposited as thin films [37–39], at very low temperatures
[40] or when irradiated with intense UV light [41]), the SP-to-MC isomerization is
sterically hindered in the pristine solid materials. This limited applicability remains a
challenging task, as the dissolved state is obviously disadvantageous for the construction and design of functional materials.
Since the light-induced structural changes cause alterations in absorption properties (color change), dipole moment, and/or refractive index [42], photochromic
compounds are intensively studied as they are expected to pave the way to new
responsive materials. The first example of an application of a spiropyran composite
was realized in 1989 by Rentzepis and Parthenopoulos, who published a spiropyranbased three-dimensional optical data storage device [5]. Additionally, photochromic
compounds were employed as synthetic molecular machines [43–46], whose most
famous representatives were awarded with the Nobel Prize in 2016 [47]. Ever since,
several efforts for the construction of responsive functional materials have been
made, also with an increasing interest in MOFs as host matrices for the
photochromic dye.
1.2 Solvatochromism
The class of spiropyrans and spirooxazines shows solvent-dependent absorption
maxima of the excited species, which is referred to as solvatochromism. As
Reichhardt comprehensively dealt with solvent effects in organic chemistry, his
110
H. A. Schwartz and U. Ruschewitz