Flipping the Switch: Fast Photoisomerization in a Confined Environment

Stimuli-responsive materials are
vital for addressing emerging
demands in the advanced technology sector as well as current industrial
challenges. Here, we report for the first time that coordinative integration
of photoresponsive building blocks possessing photochromic spiropyran
and diarylethene moieties within a rigid scaffold of metal–organic
frameworks (MOFs) could control photophysics, in particular, cycloreversion
kinetics, with a level of control that is not accessible in the solid
state or solution. On the series of photoactive materials, we demonstrated
for the first time that photoisomerization rates of photochromic compounds
could be tuned within almost 2 orders of magnitude. Moreover, cycloreversion
rates of photoresponsive derivatives could be modulated as a function
of the framework structure. Furthermore, through MOF engineering we
were able to achieve complete isomerization for coordinatively immobilized
spiropyran derivatives, typically exhibiting limited photoswitching
behavior in the solid state. For instance, spectroscopic analysis
revealed that the novel monosubstituted spiropyran derivative grafted
to the backbone of the MOF pillar exhibits a remarkable photoisomerization
rate of 0.16 s–1, typical for cycloreversion in
solution. We also applied the acquired fundamental principles toward
mapping of changes in material properties, which could provide a pathway
for monitoring material aging or structural deterioration.