Crystal structure of 2-(4-tert-butylphenyl)-3-hydroxy-4H-chromen-4-one

The title compound is relatively planar with the benzene ring being only slightly twisted with respect to the mean plane of the 4H-chromene-4-one moiety (r.m.s. deviation = 0.0191 Å) by 10.53 (8)°. In the crystal, molecules are linked by pairs of O—H⋯O hydrogen bonds, forming inversion dimers with an (10) ring motif.


Chemical context
The flavonol 3-hydroxy-2-phenyl-4H-chromen-4-one (common name: 3-hydroxyflavone) and its derivatives are present in a wide variety of plants as phytochemical compounds (Havsteen, 1983;Aherne & O'Brien, 2002). They have been investigated for many years owing to their chemical, structural, biological and fluorescent properties (Smith et al., 1968;Sengupta & Kasha, 1979;Etter et al., 1986;Klymchenko & Demchenko, 2002;Pivovarenko et al., 2005;Choulier et al., 2010). The phenomenon of dual fluorescence due to excited states intramolecular proton transfer (ESIPT) has attracted much attention (Dick, 1987), as compounds exhibiting such properties can be used as fluorescent probes for sensing and imaging. The fluorescence of flavonols has been shown to be related to the angle between the 4H-chromene-4-one moiety and the attached benzene ring (Klymchenko et al. 2003). The effect of the intramolecular hydrogen bond of flavonols, with an OH group in position 3, for the stabilization of the molecular conformation is also important and this has been confirmed by theoretical calculations reported in a computational study on flavonoids (Aparicio, 2010). As a part of our search for new luminescent materials, we report herein on the synthesis and crystal structure of the title compound, the 4-tert-butylphenyl derivative of 3-hydroxyflavone.

Supramolecular features
In the crystal of the title compound, molecules are linked via pairs of O-HÁ Á ÁO hydrogen bonds, forming inversion dimers with an R 2 2 (10) ring motif (Table 1 and Fig. 2). The dimers are linked by C-HÁ Á Á interactions between neighbouring molecules, forming sheets parallel to (101); see Table 1

Figure 3
View of the crystal packing of the title compound. Dashed lines indicate the C-HÁ Á Á interactions (ring centroids are shown as coloured spheres; see Table 1 for details). H atoms that do not participate in these interactions have been omitted for clarity. 31.2 . Hence, in DUMFAS and LUBBIV there are also short intramolecular C-HÁ Á ÁO interactions, similar to those in the title compound. In the crystals of these two compounds, molecules are also linked via O-HÁ Á ÁO hydrogen bonds, but form chains. along [001] for DUMFAS and along [100] for LUBBIV, rather than inversion dimers as in the crystal of the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydroxyl and C-bound H atoms were included in calculated positions and treated as riding atoms: O-H = 0.84 Å , C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (C,O) for the methyl and hydroxyl H atoms and 1.2U eq (C) for other H atoms.  SHELXL2014 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2014 (Sheldrick, 2015) and PLATON (Spek, 2009).

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. Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane