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.

聚集诱导发光（Aggregation-induced emission, AIE）现象在近数十年来广受关注，因其在固态发光领域具有重要应用价值。然而，受限于固态分子的表征手段不足，阐明其工作机制颇具挑战；若伴随动态结构变化，问题将更为复杂。本研究合成了一系列双芳酰腙（bis-arylacylhydrazones, BAHs），其AIE特性仅能通过水分子的可逆吸附激活。本研究采用微晶电子衍射（microcrystal electron diffraction, MicroED）技术，直接从未刻意生长晶体的粉体样品（分别在无水吸附与有水吸附条件下制备）中解析了两种BAH的分子结构。本研究明确表征了晶体堆积方式与水合物特定共晶行为的依赖关系。结构分析表明，水分子可与三个相邻的BAH-1分子形成强氢键，使该分子几乎完全平面化，并限制其分子内旋转。微晶电子衍射在为水分子吸附诱导的可逆多晶型转变（该转变可调控发光性能）提供决定性线索方面发挥了关键作用。