Crystal structure of (E)-4-{2-[4-(allyloxy)phenyl]diazenyl}benzoic acid

The title compound has an E conformation about the azobenzene linkage and the benzene rings are almost coplanar to one another [dihedral angle = 1.36 (7)°]. In the crystal, a combination of O—H⋯O and C—H⋯O hydrogen bonds and C—H⋯π interactions leads to the formation of slabs parallel to (001).


Chemical context
It is interesting to note that the title compound shows a nematic phase (Cr 190 N 218 I) . Hence, liquid crystallinity may be induced by the formation of hydrogen-bonded dimers. A number of liquid crystal (LC) systems containing hydrogen bonds that function between identical molecules have been reported (Kang & Samulski, 2000;Rahman et al., 2012). Much attention has been paid to hydrogen-bonded supramolecular LCs, including LC dimers based on hydrogen-bonding interactions and several supramolecular LC trimers based on hydrogen-bonding interactions (Lee et al., 2001;Paleos & Tsiourvas, 2001;Takahashi et al., 2003;Bai et al., 2007). A particular aspect of photonics, in which the molecular geometry can be controlled by light, is being proposed as a future technology for optical storage devices (Ikeda & Tsutsumi, 1995;Jayalaxmi et al., 2009). The heart of the phenomenon in such systems is the reversible photo-induced shape transformation of the molecules containing the photochromic azobenzene groups. The title compound contains an azo (-N N-) linkage, it was easy to synthesize and hence costeffective for the possibility of photochromism and photoisomerization usage (Lutfor et al., 2013a,b). We report herein on its synthesis and crystal structure.

Synthesis and crystallization
The title compound was synthesized by a literature procedure (Rahman et al., 2012). The diazonuim salt was prepared with sodium nitrite and subsequent coupling with phenol to afforded the ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate, which was purified by crystallization and recrystallization from methanol. The azobenzene compound was alkylated with allyl bromide to give ethyl 4-{[4-(allyloxy)phenyl]diazenyl}benzoate, which was purified by crystallization from methanol/ chloroform. The terminal double bonds-containing azobenzene compound was hydrolysed under basic conditions to yield the title compound. The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 30% probability level. Only one component of the disordered carboxylic acid group is shown.

Figure 2
A partial view along the a-axis of the crystal packing of the title compound, with hydrogen bonds shown as dashed lines (see Table 1

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Atoms O2 and O3 of the carboxylic acid group are each disordered over two positions and were refined with half occupancy each. The position of the Obound H atom was located in a difference Fourier map and refined as a riding atom: O-H = 0.82 Å with U iso (H) = 1.5 U eq (O). The C-bound H atoms were positioned geometrically and refined using a riding model: C-H = 0.93-0.97 Å with U iso (H) = 1.2U eq (C). Two outlier reflections, 341 and 309, were omitted from the refinement. A partial view of the crystal packing of the title compound. Blue dashed lines represent the intermolecular hydrogen bonds within two-molecule-thick chains and the green dashed lines represent the weak intermolecular C-HÁ Á Á interactions (see Table 1 for details). Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008) and PLATON (Spek, 2009).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.