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The development of new photochromic compounds, and the
optimization of their photophysical and switching properties are prerequisites
for accessing new functions and opportunities that are not possible with
currently available systems. To this end we recently developed a new
bistable hydrazone switch that undergoes efficient photoswitching and emission
ON/OFF toggling in both solution and solid-state. Here, we present a systematic
structure-property analysis using a family of hydrazones, and show how their properties,
including activation wavelengths, photostationary states (PSSs),
photoisomerization quantum yields, thermal half-lives (t1/2),
and solution/solid-state fluorescence characteristics vary as a function of electron
donating (EDG) and/or withdrawing (EWG) substituents. These studies resulted in
the red-shifting of the absorption profiles of the Z and E isomers of the
switches, while maintaining excellent PSSs in almost all of the compounds. The introduction
of para-NMe2, and/or para-NO2 groups improved the photoisomerization
quantum yields, and the extremely long thermal half-lives (tens to thousands of
years) were maintained in most cases, even in a push-pull system, which can be
activated solely with visible light. Hydrazones bearing EDGs at the stator
phenyl group are an exception and show up to 6 orders of magnitude acceleration
(i.e., days)because of a change in the isomerization mechanism.
Moreover, we discovered that a para-NMe2
group is required to have reasonable fluorescence quantum yields in solution,
and that rigidification enhances the emission in the solid-state. Finally,
X-ray crystallography analysis showed that the switching process is more
efficient in the solid-state when the hydrazone is loosely packed.