Microcrystal Electron Diffraction Elucidates Water-Specific Polymorphism-Induced Emission Enhancement of Bis-arylacylhydrazone

Aggregation-induced
emission (AIE) phenomena have gained intense
interest over the last decades because of its importance in solid-state
emission. However, the elucidation of a working mechanism is difficult
owing to the limited characterization methods on solid-state molecules,
further complicated if dynamic structural changes occur. Here, a series
of bis-arylacylhydrazones (BAHs) were synthesized, for
which their AIE properties are only turned on by the reversible adsorption
of water molecules. We used microcrystal electron diffraction (MicroED)
to determine the molecular structures of two BAHs directly
from bulk powders (without attempting to grow crystals) prepared in
the absence or presence of water adsorption. This study reveals the
unambiguous characterization of the dependence of crystal packing
on the specific cocrystallization with hydrates. The structural analysis
demonstrates that water molecules form strong hydrogen bonds with
three neighboring BAH-1, resulting in the almost complete
planarization and restriction of the intramolecular rotation of the
molecule. MicroED plays an important role in providing a decisive
clue for the reversible polymorphism changes induced by the adsorption
of water molecules, regulating emissive properties.