measurements, tracing modulations in the reflection intensities, while the peak
positions remained mainly unaltered. For SP-Nitro@MIL-53(Al), surface adsorption
instead of embedment into the MOF pores was found and verified via XPS studies.
Remarkably, in MOF-5 the open MC form of the inserted spiropyran is already
present without UV light treatment. Since MC-Nitro exhibits a drastically higher
dipole moment than its ground state counterpart (SP form), the chemical environment of the MOF-5 pore was suggested to be rather polar and, therefore, preferred by
molecules with a high dipole moment such as merocyanines. Upon UV light
exposure, an increase in the MC absorption for SP-Nitro@MOF-5 indicated proceeding formation of the MC moiety, which is referred to as positive photochromism. For host lattices of the MIL-68 series, no MC absorption band was found
directly after loading. SP-to-MC conversion occurred for all SP-Nitro@MIL-68
composites upon UV light excitation, and, similar to MOF-5 as host material,
positive photochromism was observed for the spiropyran embedded in the respective
porous host. The successful isomerization of the closed SP form to its open MC form
was traced both by UV/vis and IR spectroscopic measurements.
Interestingly, the absorption properties of MC-Nitro strongly depend on the MOF
scaffold (see Fig. 21, left). When embedded in MOF-5, MC-Nitro shows an absorption comparable to being dissolved in methanol or ethanol. For hosts of the MIL-68
family, an absorption resembling that of an acetone solution is observed. For
MC-Nitro, surface-bound on MIL-53(Al), the absorption is similar to that of a
toluene solution. Remarkably, solvatochromic behavior cannot only be observed
in solution but also in the solid state. Both the ability to successfully isomerize from
the closed spiropyran to the open merocyanine form and the dependence of the
absorption properties on the respective host lattice pretty much resemble the behavior of SP-Nitro in solution. Therefore, the authors denoted MOFs as solid solvents
for these dye molecules and were the first to describe a combined photochromic and
solvatochromic response of a dye molecule inside a MOF host.
Taking advantage of SP-Nitro’s solvatochromic response inside the different
MOFs, the authors classified the polarity of MOF-5, MIL-68(In/Ga), and MIL-53
(Al) according to the elutrope series of Snyder [153] by applying SP-Nitro as a
polarity sensor. By doing so, MOF-5 represents the most polar porous material,
followed by MIL-68(In), MIL-68(Ga), and MIL-53(Al). Please note that for MIL-53
(Al) only surface adsorption has to be considered. The correlation between the
polarity of MOFs and solvents is shown in Fig. 22.
In a subsequent study, the authors performed detailed IR und UV/vis measurements to monitor the photostability of the inserted spiropyran upon prolonged and
repetitive UV light treatment. However, only little fatigue resistance was found for
both cases, making the applicability of the nitro-substituted spiropyran in functional
materials difficult.
Photoactive Molecules within MOFs
137
positions remained mainly unaltered. For SP-Nitro@MIL-53(Al), surface adsorption
instead of embedment into the MOF pores was found and verified via XPS studies.
Remarkably, in MOF-5 the open MC form of the inserted spiropyran is already
present without UV light treatment. Since MC-Nitro exhibits a drastically higher
dipole moment than its ground state counterpart (SP form), the chemical environment of the MOF-5 pore was suggested to be rather polar and, therefore, preferred by
molecules with a high dipole moment such as merocyanines. Upon UV light
exposure, an increase in the MC absorption for SP-Nitro@MOF-5 indicated proceeding formation of the MC moiety, which is referred to as positive photochromism. For host lattices of the MIL-68 series, no MC absorption band was found
directly after loading. SP-to-MC conversion occurred for all SP-Nitro@MIL-68
composites upon UV light excitation, and, similar to MOF-5 as host material,
positive photochromism was observed for the spiropyran embedded in the respective
porous host. The successful isomerization of the closed SP form to its open MC form
was traced both by UV/vis and IR spectroscopic measurements.
Interestingly, the absorption properties of MC-Nitro strongly depend on the MOF
scaffold (see Fig. 21, left). When embedded in MOF-5, MC-Nitro shows an absorption comparable to being dissolved in methanol or ethanol. For hosts of the MIL-68
family, an absorption resembling that of an acetone solution is observed. For
MC-Nitro, surface-bound on MIL-53(Al), the absorption is similar to that of a
toluene solution. Remarkably, solvatochromic behavior cannot only be observed
in solution but also in the solid state. Both the ability to successfully isomerize from
the closed spiropyran to the open merocyanine form and the dependence of the
absorption properties on the respective host lattice pretty much resemble the behavior of SP-Nitro in solution. Therefore, the authors denoted MOFs as solid solvents
for these dye molecules and were the first to describe a combined photochromic and
solvatochromic response of a dye molecule inside a MOF host.
Taking advantage of SP-Nitro’s solvatochromic response inside the different
MOFs, the authors classified the polarity of MOF-5, MIL-68(In/Ga), and MIL-53
(Al) according to the elutrope series of Snyder [153] by applying SP-Nitro as a
polarity sensor. By doing so, MOF-5 represents the most polar porous material,
followed by MIL-68(In), MIL-68(Ga), and MIL-53(Al). Please note that for MIL-53
(Al) only surface adsorption has to be considered. The correlation between the
polarity of MOFs and solvents is shown in Fig. 22.
In a subsequent study, the authors performed detailed IR und UV/vis measurements to monitor the photostability of the inserted spiropyran upon prolonged and
repetitive UV light treatment. However, only little fatigue resistance was found for
both cases, making the applicability of the nitro-substituted spiropyran in functional
materials difficult.
Photoactive Molecules within MOFs
137