(E)-1-[2-(3,4-Dimethylphenyl)diazen-2-ium-1-yl]naphthalen-2-olate

The title zwitterion, C18H16N2O, features an intra­molecular N—H⋯O hydrogen bond. The di­methyl­benzene ring is rotationally disordered about the N—C bond over two adjacent orientations in a 0.75:0.25 ratio. The dihedral angle between the major orientation of the benzene ring and the naphthalene ring system is 6.06 (2)°. In the crystal, aromatic π–π stacking occurs [shortest centroid–centroid distance = 3.574 (3) A] and C—H⋯O inter­actions are also observed.