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

Abstract

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.

Inter- and Intra-Hydrogen Bonding Strategy to Control the Fluorescence of Acylhydrazone-Based Conjugated Microporous Polymers and Their Application to Nitroaromatics Detection

“…We found in our previous work that the emission of the BAH moiety is enhanced by planarization and J-aggregation induced by water-specific polymorphism rather than by the RIM mechanistic pathway exclusively [19], which led us to introduce the arylacylhydrazone into a CMP to obtain minimal intramolecular motion. We designed two types of acylhydrazone-based CMPs, i.e., with and without intramolecular hydrogen-bond donors (Scheme 1).…”

“…After adding water molecules to the BH-CMP dispersion, the carbonyl oxygen and amide proton in BH-CMP are locked via intermolecular hydrogen bonding. Furthermore, this hydrogen bonding induces the planarization and formation of J-aggregates, resulting in water-specific PIEE [19]. Unlike BH-CMP, ABH-CMP exhibits a planar conformation due to intramolecular hydrogen bonding between the amine group on the central benzene and the carbonyl oxygen (Figure 2b).…”

“…The possible mechanisms of emission of AIEgens mainly include J-aggregate formation [13,14], twisted intramolecular charge transfer [15,16], and restriction of intramolecular motions (RIM) [17,18], among which the latter has been most commonly proposed. In a previous study, we discovered that the solid-state emission of bis-arylacylhydrazone (BAH)-based luminogens can be turned on and off by reversible adsorption of water vapors, which enabled the fabrication of rewritable papers with "water-ink" [19]. Specifically, we found that the formation of interlayer hydrogen-bonding networks in the presence of water results in parallel J-type aggregates with planarization, thereby enhancing the fluorescence.…”

“…They attributed the origin of emission of the acylhydrazone-based COFs to the restriction of intramolecular rotation (RIR) mechanism. However, we previously showed that the emission of BAH moieties occurs not by RIR but by planarization and formation of J-aggregates [19]. Thus, to gain more insight into the origin of the emission in BAH, we were interested in investigating network polymer systems by introducing acylhydrazone into CMPs with limited intramolecular motion.…”

Electron Diffraction of 3D Molecular Crystals

“…213−224 These structures include two noncanonical amino acids bearing all-carbon quaternary stereocenters, 213 a trio of organic semiconductors solved at ultrahigh resolution (one of which is depicted in Figure 9E), 54 a family of electron-deficient expanded helicenes, 216 a pentacyclic indole-derived ester, 217 and a synthetic mimic of the cuboidal subunit in the oxygen-evolving complex of photosystem II. 220 Finally, 3D ED has also tackled a bevy of small-molecule active pharmaceutical ingredients (APIs), including carbamazepine, 200,225 niacin (nicotinic acid), 225 bismuth subgallate, 226 ibuprofen, 200 ethisterone, 200 progesterone, 200 biotin, 200 paracetamol (acetaminophen), 143,199,200 cimetidine, 227 loratadine, 125 sofosbuvir, 143 ramelteon, 228 tolvaptan, 228 olanzapine, 229 epicorazine A, 144 dehydrocurvularin, 144 metaxalone, 230 teniposide, 231 remdesivir, 232 and indomethacin. 126 Because many APIs exist natively as microcrystalline powders, 3D ED could potentially revolutionize solid-state structure determination in the pharmaceutical industry, 231,233 where size-limited single-crystal XRD is currently the gold standard.…”