4-{4-[(E)-(2-Hydroxyphenyl)iminomethyl]phenoxy}benzene-1,2-dicarbonitrile

The asymmetric unit of the title compound, C21H13N3O2, contains two independent molecules with a similar structure. In one molecule, the central benzene ring is oriented with respect to the terminal benzene rings at 27.23 (7) and 67.96 (7)°; in the other molecule, the corresponding dihedral angles are 12.42 (7) and 64.55 (7)°. In both molecules, there is a short O—H⋯N interaction involving the OH group and the adjacent N atom. In the crystal, there are O—H⋯N hydrogen bonds, and C—H⋯O and N—H⋯O interactions linking the molecules to form a three-dimensional network. π–π stacking between the pyridine and benzene rings and between the benzene rings [centroid–centroid distances = 3.989 (2), 3.705 (2) and 3.607 (2) Å] may further stabilize the structure. A weak C—H⋯π interaction is present in the crystal.

The asymmetric unit of the title compound, C 21 H 13 N 3 O 2 , contains two independent molecules with a similar structure. In one molecule, the central benzene ring is oriented with respect to the terminal benzene rings at 27.23 (7) and 67.96 (7) ; in the other molecule, the corresponding dihedral angles are 12.42 (7) and 64.55 (7) . In both molecules, there is a short O-HÁ Á ÁN interaction involving the OH group and the adjacent N atom. In the crystal, there are O-HÁ Á ÁN hydrogen bonds, and C-HÁ Á ÁO and N-HÁ Á ÁO interactions linking the molecules to form a three-dimensional network.stacking between the pyridine and benzene rings and between the benzene rings [centroid-centroid distances = 3.989 (2), 3.705 (2) and 3.607 (2) Å ] may further stabilize the structure. A weak C-HÁ Á Á interaction is present in the crystal.

Related literature
For the use of phthalonitriles for preparing symmetrically and unsymmetrically substituted phthalocyanine complexes, see: Leznoff & Lever (1996). For the widespread applications of phthalocyanines in photodynamic therapy, see: Kartal et al.   Table 1 Hydrogen-bond geometry (Å , ).
Phthalocyanines have currently been the topic of research because of their wide application fields, such as thin film fabrication, organic pigments, chemical sensors, electrochromic display devices, molecular epitaxic deposition and composites, liquid crystals, photovoltaic cells self-assembled materials. In addition to their extensive use as dyes and pigments, phthalocyanines have found widespread application, in photodynamic therapy (Kartal et al., 2006;Tüfekçi et al., 2009). The fundamental optical and electronic properties of these materials are explained and their potential in nonlinear optics, optical data storage, electronic sensors, xerography, solar energy conversion, nuclear chemistry, molecular magnetism, electrochromic displays and heterogeneous catalysis is evaulated by McKeown (1998 (Kartal et al., 2006) have also been determined.

Experimental
The title compound has been prepared in two steps. In the first step; 4-hydroxybenzaldehyde (1.86 g, 15.2 mmol) and 4nitrophthalonitrile (2.64 g, 15.2 mmol) were heated at 353 K in dry DMF (20 ml) with stirring under argon atmosphere.
The mixture was heated for a further 18 h. After cooling, the mixture was added into ice-water (200 g). The product was filtered off and washed with NaOH solution (10% w/w) and water until the filtrate was neutral. In the second step; 4-(4formylphenoxy)benzene-1,2-dicarbonitrile (1.88 g, 7.6 mmol), the product obtained in the first step, and 2-hydroxyaniline (0.83 g, 7.6 mmol) were reacted at 333 K for 4 h in absolute ethanol (50 ml). Recrystallization from absolute ethanol gave a yellow product (yield: 1.55 g, 60%). Single crystals suitable for X-ray diffraction measurement were obtained by

Refinement
Atoms H7 and H7′ (for CH) were located in a difference Fourier map and were refined by applying restraints. The remaining H-atoms were positioned geometrically with O-H = 0.84 Å for OH H-atoms, and C-H = 0.95 Å for aromatic H-atoms, and constrained to ride on their parent atoms, with U iso (H) = k × U eq (C,O), where k = 1.5 for OH Hatoms and k = 1.2 for aromatic H-atoms.     (11) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.