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The tautomerism of 1-phenyl-1,2-dihydro-3H-pyrazol-3-one was investigated. An X-ray crystal structure analysis exhibits dimers of 1-phenyl-1H-pyrazol-3-ol units. Comparison of NMR (nuclear magnetic resonance) spectra in liquid state (1H, 13C, 15N) with those of “fixed” derivatives, as well as with the corresponding solid state NMR spectra reveal this compound to exist predominantly as 1H-pyrazol-3-ol molecule pairs in nonpolar solvents like CDCl3 or C6D6, whereas in DMSO-d6 the corresponding monomers are at hand. Moreover, the NMR data of different related 1H-pyrazol-3-ol derivatives are presented.

An efficient synthetic route to construct diverse pyrazole-based chalcones from 1-phenyl-1H-pyrazol-3-ols bearing a formyl or acetyl group on the C4 position of pyrazole ring, employing a base-catalysed Claisen–Schmidt condensation reaction, is described. Isomeric chalcones were further reacted with N-hydroxy-4-toluenesulfonamide and regioselective formation of 3,5-disubstituted 1,2-oxazoles was established. The novel pyrazole-chalcones and 1,2-oxazoles were characterized by an in-depth analysis of NMR spectral data, which were obtained through a combination of standard and advanced NMR spectroscopy techniques.

Organic nanoparticles exhibiting tunable emission properties in response to morphology changes are attractive for application in low-cost fluorescence sensors, e.g., for sensing vapors, temperature, etc., and therefore, convenient ways for altering nanoparticle morphology are highly desired. In this work, phenylenediacetonitrile-based molecules featuring aggregation-induced emission enhancement which are suitable for the realization of morphologytunable nanoparticles by precipitation method have been designed. The morphology tuning was enabled by rational functionalization of the molecular backbone with pyrazole moieties and adjacent alkoxy/cyclic side-groups, which were varied in size and branchiness. The longer alkoxy chains generally caused formation of crystalline nanostructures, while shorter ones resulted in preferably amorphous nanoaggregates. Remarkably, the sharp tuning of the nanoparticle morphology (crystalline → amorphous → crystalline) with the subsequent highcontrast emission switching (emissive → nonemissive → emissive) was achieved for phenylenediacetonitrile bearing dihexylmethoxy-type side-groups by adjusting solvent/nonsolvent ratio (1/1 → 1/4 → 1/9) in the mixture with the dissolved compound. Electron and polarized optical microscopy data confirmed the intermediate nonemissive state (Φ F = 2%) emanates from amorphous spherical aggregates, whereas two highly emissive states (Φ F up to 70%) originate from nanowire-like crystalline particles, which were attributed to different phenylenediacetonitrile polymorphs. Finally, the applicability of the phenylenediacetonitrile-based fluorescent nanoparticles for organic vapor sensing was demonstrated.

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