2-{(E)-[(2Z)-2-(1,2-Di-hydro-phthalazin-1-yl-idene)hydrazinyl-idene]meth-yl}phenol.

The title compound, C15H12N4O, adopts an E conformation with respect to the azomethine bond and crystallizes in its hydrazinyl-idene tautomeric form. The dihedral angle between the ring systems is 15.98 (7)°. The phenol O-H group forms an intra-molecular O-H⋯N hydrogen bond. In the crystal, pairs of N-H⋯N and C-H⋯O hydrogen bonds link neighbouring mol-ecules into centrosymmetric dimers. These dimers are inter-connected by means of three types of π-π stacking inter-actions. One, with a centroid-centroid distance of 3.577 (1) Å [inter-planar separation = 3.4673 (6) Å], connects adjacent mol-ecules into centrosymmetric dimers. The other two inter-actions, on the outward facing sides of the dimers, are between phenol rings of neighboring mol-ecules [centroid-centroid separation = 3.7907 (13) Å and inter-planar separation = 3.5071 (8) Å], and between phthalazin units [centroid-centroid separation = 3.6001 (12) Å and inter-planar separation = 3.4891 (7) Å]. In combination, the π-π inter-actions lead to the formation of infinite layers with mol-ecules stacked along [0-11]. These layers are, in turn, connected with neighbouring layers through the N-H⋯N and C-H⋯O hydrogen bonds, yielding a three-dimensional supra-molecular architecture.