Structure Property Analysis of the Solution and Solid-State Properties of Bistable Photochromic Hydrazones

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 (τ1/2), 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 in τ1/2 (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.