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Recrystallization of (E)-5-phenyl-1-(pyridin-2-yl)pent-2-en-4-yn-1-one at room temperature from ethylene glycol in daylight afforded [3,4-bis(phenylethynyl)cyclobutane-1,2-diyl)bis(pyridin-2-ylmethanone], C32H22N2O2 (3), while (E)-5-(4-methylphenyl)-1-(pyridin-2-yl)pent-2-en-4-yn-1-one, C17H13NO (2), remained photoinert. This is the first experimental evidence that pentenynones can be photoreactive when fixed in nearly coplanar parallel positions. During the photoreaction, the bond lengths and angles along the pentenyne chain changed significantly, while the disposition of the pyridyl ring towards the keto group was almost unchanged. The cyclobutane ring adopts an rctt conformation.

In the title compound, C21H22ClNO3, the pentadiene unit is nearly planar [maximum deviation = 0.023 (1) Å], but the carbonyl O atom deviates significantly [by 0.304 (1) Å] from its mean plane, which is twisted with respect to the phenyl and chlorobenzene rings by 71.34 (13) and 46.40 (13)°, respectively. In the crystal, inversion-related molecules are linked by two pairs of O—H...O hydrogen bonds, forming chains propagating along [01-1], enclosing R22(16) and R22(22) ring motifs. The chains are linked via C—H...O hydrogen bonds and C—H...π interactions into a three-dimensional supramolecular architecture.

In the title compound, C24H18N4O4, the plane of the phenyl ring is inclined to those of the toluene ring and the dinitro-substituted benzene ring by 66.96 (19) and 47.06 (18)°, respectively, while the planes of the two benzene rings are inclined to one another by 36.26 (19)°. There is an intramolecular N—H...O hydrogen bond between the NH group and the O atom of a nitro group, forming an S(6) ring motif. In the crystal, molecules are linked by C—H...O hydrogen bonds and C—H...π interactions, forming a three-dimensional network. There are also weak π–π interactions present involving the phenyl ring and the dinitro-substituted benzene ring [inter-centroid distance = 3.741 (2) Å].