SP-Nitro@UiO-67 thin films are colorless, whereas light exposure causes the isomerization process and the films become colored. Thus, the closed spiropyran form
is exclusively present in the dark. The switching yield was determined by IR
spectroscopy, yielding 70% MC form upon irradiation with UV light.
In order to study the conduction properties of the presented material via DC
measurements, thin films of SP-Nitro@UiO-67 were prepared on interdigitated gold
electrodes. The results of these measurements are depicted in Fig. 26. In the initial
state with spiropyrans being exclusively present in their SP form, almost no current
is observed (see Fig. 26a). Upon UV light exposure (λ ¼ 365 nm), the conductivity
increases by a factor of ten as a result of the SP-to-MC conversion. When turning off
the UV light, reconversion to the ground state is accompanied by a decreasing
current. This conductance photoswitching was repeated for three consecutive cycles
(see Fig. 26b) with a slightly decreasing conductivity, which the authors explained
with the known fatigue of spiropyrans [158]. In Fig. 26c the current-voltage characteristic of SP-Nitro@UiO-67 is depicted. The current increases over the whole
voltage range, when the sample is exposed to UV light. The details of the underlying
charge transfer processes were examined precisely in accompanying DFT calculations. Instead of a ballistic transport, charge hopping is present. For the MC moieties,
the electronic coupling is stronger than for the SP species. Therefore, an increased
charge hopping rate is found, when SP-Nitro is converted to its open merocyanine
form. The stronger electronic coupling and the hole injection from the gold electrodes result in efficient electron hole conduction. In conclusion, this
SP-Nitro@UiO-67 thin film represents the first example for reversible
photoswitching of the electronic properties of a hybrid MOF material.
2.5 Spirooxazines in MOFs
2.5.1 Bulk Materials
For spiropyrans, prolonged UV light irradiation results in photodegradation predominantly via a bimolecular process, which involves the triplet excited state of the
spiropyran moiety [159]. Bimolecular processes are related to the concentration of
spiropyran molecules. By immobilization of single dyes on, e.g., supports, those
processes are easily suppressed [160]. Diamond and co-workers observed an
increased fatigue resistance of spiropyran dyes being immobilized in comparison
to those in the dissolved state [161]. Nonetheless, spiropyrans show comparably low
fatigue resistance in comparison to other photochromic molecules. For instance,
spirooxazines, structurally related molecules, exhibit a significantly higher
photoresistance and reversibility of switching than most of the spiropyrans. Both
dye molecules only differ structurally by an additional nitrogen atom in the
spiropyran chromene ring, which is denoted as benzoxazine part for spirooxazines.
However, in contrast to spiropyrans, the synthesis and also publications on
spirooxazines went almost unnoticed, even though this class of photochromic
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H. A. Schwartz and U. Ruschewitz
is exclusively present in the dark. The switching yield was determined by IR
spectroscopy, yielding 70% MC form upon irradiation with UV light.
In order to study the conduction properties of the presented material via DC
measurements, thin films of SP-Nitro@UiO-67 were prepared on interdigitated gold
electrodes. The results of these measurements are depicted in Fig. 26. In the initial
state with spiropyrans being exclusively present in their SP form, almost no current
is observed (see Fig. 26a). Upon UV light exposure (λ ¼ 365 nm), the conductivity
increases by a factor of ten as a result of the SP-to-MC conversion. When turning off
the UV light, reconversion to the ground state is accompanied by a decreasing
current. This conductance photoswitching was repeated for three consecutive cycles
(see Fig. 26b) with a slightly decreasing conductivity, which the authors explained
with the known fatigue of spiropyrans [158]. In Fig. 26c the current-voltage characteristic of SP-Nitro@UiO-67 is depicted. The current increases over the whole
voltage range, when the sample is exposed to UV light. The details of the underlying
charge transfer processes were examined precisely in accompanying DFT calculations. Instead of a ballistic transport, charge hopping is present. For the MC moieties,
the electronic coupling is stronger than for the SP species. Therefore, an increased
charge hopping rate is found, when SP-Nitro is converted to its open merocyanine
form. The stronger electronic coupling and the hole injection from the gold electrodes result in efficient electron hole conduction. In conclusion, this
SP-Nitro@UiO-67 thin film represents the first example for reversible
photoswitching of the electronic properties of a hybrid MOF material.
2.5 Spirooxazines in MOFs
2.5.1 Bulk Materials
For spiropyrans, prolonged UV light irradiation results in photodegradation predominantly via a bimolecular process, which involves the triplet excited state of the
spiropyran moiety [159]. Bimolecular processes are related to the concentration of
spiropyran molecules. By immobilization of single dyes on, e.g., supports, those
processes are easily suppressed [160]. Diamond and co-workers observed an
increased fatigue resistance of spiropyran dyes being immobilized in comparison
to those in the dissolved state [161]. Nonetheless, spiropyrans show comparably low
fatigue resistance in comparison to other photochromic molecules. For instance,
spirooxazines, structurally related molecules, exhibit a significantly higher
photoresistance and reversibility of switching than most of the spiropyrans. Both
dye molecules only differ structurally by an additional nitrogen atom in the
spiropyran chromene ring, which is denoted as benzoxazine part for spirooxazines.
However, in contrast to spiropyrans, the synthesis and also publications on
spirooxazines went almost unnoticed, even though this class of photochromic
142
H. A. Schwartz and U. Ruschewitz