Schiff bases are used as substrates in the preparation of number of industrial and biologically active compounds via ring closure, cycloaddition and replacement reactions. Some Schiff base derivatives are also known to have biological activities such as antimicrobial (El-Masry et al., 2000; Pandeya et al., 1999); antifungal (Singh & Dash 1988; Varma et al., 1986) and antitumor (Hodnett & Dunn 1970; Misra et al., 1981; Agarwal et al., 1983). There are two characteristic properties of Schiff bases, viz. photochromism and thermochromism (Cohen et al., 1964; Moustakali-Mavridis et al., 1978). Schiff bases display two possible tautomeric form, namely the phenol-imine (O—H···N) and keto-amine (N—H···O) forms. In the solid state, the keto-amine tautomer has been found in naphthaldimines (Hökelek et al., 2000; Odabaşoğlu et al., 2003), while the phenol-imine form exists in salicylaldimine Schiff bases (Kaitner & Pavlovic, 1996; Yıldız et al., 1998).

In the title compound, (I), the phenol-imine tautomer is favoured over the keto-amine form, and there is an intramolecular O—H···N hydrogen bond (Fig. 1 and Table 1). It is known that Schiff bases may exhibit thermochromism or photochromism, depending on the planarity or non-planarity of the molecule, respectively. This planarity of the molecule allows the H atom to be transferred through the hydrogen bond in the ground state with a low energy requirement (Hadjoudis et al., 1987). Therefore, one can expect thermochromic properties in (I) caused by planarity of the molecule: the dihedral angle between rings A (C1—C6) and B (C8—C13) is 17.33 (16)° (Fig. 1). In (I), the C8—C7, C4—N1, C7=N1 and O1—C13 bond lengths of 1.441 (4), 1.417 (3), 1.263 (3) and 1.338 (3) Å, respectively are in good agreement with those observed in (E)-2[(3-Fluoropheng)iminomethy]-4-(trifluoromethoxy)phenol [1.447 (4), 1.420 (3), 1.268 (3) and 1.343 (3) Å, Bingöl Alpaslan et al., 2010]. The C5—C4—N1=C7 and N1=C7—C8—C13 torsion angles are -19.0 (5)° and 1.2 (5)°, respectively. In crystal packing, the interactions [C2—H2···Cg1(x, 1 - y, z - 1/2)] and [C17—H17A···Cg1(1/2 - x, 1/2 + y, 3/2 - z)] are effective (Table 1 and Fig. 2.)

The title compound was prepared by refluxing a mixture of a solution containing 5-(diethylamino)-2-hydroxybenzaldehyde (0.5 g, 2.59 mmol) in 20 ml e thanol and a solution containing 4-ethoxyaniline (0.4 g, 2.59 mmol) in 20 ml e thanol. The reaction mixture was stirred for 1 h under reflux. The crystals of (E)-5-(diethylamino)-2-[(4-ethoxyphenylimino)methyl]phenol suitable for x-ray analysis were obtained by slow evaporation from ethyl alcohol (yield % 82;).

All H atoms were refined using a riding model with O—H=0.82 Å and C—H = 0.93 to 0.97 Å, and with U_iso(H) = 1.2–1.5 U_eq (C,O).

Symmetry codes: (i) x, −y+1, z−1/2; (ii) −x+1/2, y+1/2, −z+3/2.