1-[(E)-2-(5-tert-Butyl-2-hy-droxy-phen-yl)diazen-1-yl]naphthalen-2-ol.

The non-H atoms of the title compound, C20H20N2O2, is located on a mirror plane except two methyl groups of the tert-butyl group. Intra-molecular N-H⋯O hydrogen bonds exist between the hy-droxy and diazenyl groups. In the crystal, mol-ecules are linked by weak C-H⋯O hydrogen bonds into supra-molecular chains running along the a-axis direction.

(E)-1-(2-Phenyl-diazen-2-ium-1-yl)naph-thalen-2-olate.

In the title zwitterionic compound, C16H12N2O, the dihedral angle between the phenyl ring and the naphthalene ring system is 17.85 (8)°; an intra-molecular N-H⋯O hydrogen bond occurs. In the crystal, π-π stacking is observed between naphthalene ring systems of adjacent mol-ecules, the centroid-centroid distance being 3.

(E)-1-[2-(2-Cyano-phen-yl)diazen-2-ium-1-yl]naphthalen-2-olate.

There are two independent zwitterion mol-ecules (A and B) in the asymmetric unit of the title compound, C17H11N3O, which belongs to the family of azo dyes. The dihedral angle between the benzene ring and the naphthalene ring system is 6.99 (6)° in mol-ecule A and 4.

1-(3-Acetyl-phen-yl)-2-(2-oxidonaph-thalen-1-yl)diazen-1-ium.

The title compound, C18H14N2O2, crystallized with two independent zwitterion mol-ecules (A and B) in the asymmetric unit. They are both close to planar, the dihedral angle between the benzene ring and naphthalene ring system being 4.30 (9)° in A and 4.