The title zwitterion,, C16H11ClN2O, is approximately planar, the dihedral angle between the benzene ring and naphthalene ring system is 1.55 (13)°; an intra-molecular N-H⋯O hydrogen bond stabilizes the planar conformation. In the crystal, π-π stacking between the benzene ring and the naphthalene ring system of adjacent mol-ecules links the mol-ecules into supra-molecular chains running along the b axis, the centroid-centroid distance being 3.765 (2) Å.

In the title compound, C₁₆H₁₂N₂O, the dihedral angle between the planes of the benzene ring and naphthalenone ring system is 1.89 (8)°; an intramolecular N—H...O hydrogen bond occurs between the imino group and the carbonyl group. In the crystal, molecules are linked by weak C—H...π interactions into supramolecular chains propagating along the [01-1] direction.

The title compound, C₁₆H₁₁N₃O₃, belongs to the family of azo dyes. In the light of a single-crystal X-ray study, it is evident that of the tautomeric forms (azo–hydrazone), the hydrazone form is the predominant form in the solid state, namely, (1E)-1-[2-(3-nitrophenyl)hydrazin-1-ylidene]-1,2-dihydronaphthalen-2-one. The naphthol and benzene fragments attached to the –N=N– moiety adopt the s-trans conformation. Furthermore, the molecules are nearly coplanar, subtending a dihedral angle of 2.63 (5)°. An intramolecular N—H...O hydrogen bond occurs. There are only two types of intermolecular interactions in the crystal structure: strong hydrogen-bonding C—H...O interactions and π–π stacking interactions. The importance of C—H...O interactions in the molecular packing is reflected by the relatively high contributions (28.5%) made by O...H/H...O contacts to the Hirshfeld surface.

In the title zwitterion, C17H12N2O2, the dihedral angle between the benzene ring and naphthalene ring system is 11.76 (7)° and an intra-molecular N-H⋯O hydrogen bond exists. In the crystal, molecules are linked via pairs of C-H⋯O hydrogen bonds, forming inversion dimers.