Investigation of crystal structures of eight quasi-planar (E)-1,5-diarylpentenyn-1-ones has been carried out. Five of these compounds crystallize in chiral space groups. Intermolecular interactions responsible for the loss of planarity and steric hindrances that prevent molecules to form centrosymmetric dimers were suggested to be the reasons of the crystallization in chiral groups. Intermolecular bonding has been investigated by means of the Hirshfeld surfaces, and the role of the C–H···O interactions as driving force for crystal structure formation has been demonstrated. Three types of C–H···O bonded synthons have been suggested for this family. Those synthones are found to be in accordance with the charge distribution along the conjugated system estimated with natural population analysis. Nonlinear optical (NLO) properties for five (E)-1,5-diarylpent-2-en-4-yn-1-ones crystallizing in chiral space groups have been investigated both theoretically and experimentally. It was shown that crystalline NLO susceptibility of the Br-derivative is comparable to such an NLO material as N-(4-nitrophenyl)-l-prolinol. The crystal packing effects on calculated NLO properties have been estimated based on the recently proposed Charge Model. Second-harmonic generation measurements of selected (E)-1,5-diarylpent-2-en-4-yn-1-ones confirmed a validity of the Charge Model and demonstrated an efficiency of this family for potential application as materials for nonlinear optics.

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The cyclocondensation of cross-conjugated enynones, dienynones, and trienynones (easily available due to low-cost starting compounds) with arylhydrazines leads to the regioselective synthesis of pyrazole derivatives (dihetaryl-substituted ethens, buta-1,3-diens, and hexa-1,3,5-triens) or results in 4,5-dihydro-1H-pyrazoles in good yield. The reaction path is controlled by the character of the substituent in enynone: the pyrazoles are obtained from the reaction of substrates that contain five-membered heteroaromatic substituents with arylhydrazines, and the 4,5-dihydro-1H-pyrazoles are obtained from the reaction of 1,5-diphenylpent-1-en-4-yn-3-one with arylhydrazines consistently. Despite the presence of a substituent, cyclocondensation of 2-hydrazinylpyridine with all of examined cross-conjugated enynones leads to the formation of pyrazoles. The reaction does not require special conditions (temperature, catalyst, inert atmosphere). The cyclocondensation pathways are determined by the electronic effect of an electron-rich five-membered heteroaromatic ring in the substrate. The synthesis allows use of various substituents and functional groups in enynone and hydrazine. The present method features high yields and simplicity of the product purification. The obtained pyrazoles possess fluorescent properties with a quantum yield up to 31%.

An interaction of 1,5-diaryl-3-X-pent-4-yn-1-ones (where X stands for piperidin-1-yl, morpholin-4-yl, 4-methylpiperazin-1-yl) with arylhydrazines proceeds at room temperature and results in 3-aryl-5-arylethynyl-1-phenyl-4,5-dihydro-1H-pyrazoles with up to 57–73% yields. Under similar conditions, the cyclocondensation of conjugated 2,4,1-enynones with arylhydrazine proceeds only in the presence of cyclic amines. 1,5-Diaryl-3-X-pent-4-yn-1-ones are reported as synthetic equivalents of conjugated 2,4,1-enynones in reactions with arylhydrazines. On the basis of obtained data, there are highly efficient methods developed for the synthesis of 5-arylethynyl-substituted 4,5-dihydro-1H-pyrazoles, as well as for similarly structured 1H-pyrazoles prepared by oxidation in AcOH. Presented products possess quite marked fluorescent abilities. Emission maximum wavelengths are located at 453–465 and 363–400 nm, respectively; certain compounds show extremely large Stokes shifts that may reach 91,000 cm–1.

Condensation of 1,5-disubstituted pent-1-en-4-yn-1-ones with arylhydrazines in acidified alcohol results mainly in the corresponding arylhydrazones with traces of the side products of cyclization at the double bond – 1,5-diaryl-3-(arylethynyl)-4,5-dihydro-1H-pyrazoles (pyrazolines)....