2,6-Dimethoxy-9,10-anthraquinone

The title compound, C16H12O4, crystallizes with two half-molecules in the asymmetric unit, each of which is completed by a crystallographic inversion center. The two crystallographically independent molecules have almost the same geometry and are almost planar [maximum deviations = 0.018 (3) and 0.049 (3) Å]. They adopt a conformation in which the Cmethyl—O bonds are directed along the molecular short axis [C—C—O—C torsion angles of 179.6 (2) and 178.0 (2)°]. In the crystal, the molecular packing is characterized by a combination of a columnar stacking and a herringbone-like arrangement. The molecules form slipped π-stacks along the b axis, in which there are two kinds of columns differing from each other in their slippage. The interplanar distances between neighboring molecules are 3.493 (3) for one column and 3.451 (2) Å for the other.

The title compound, C 16 H 12 O 4 , crystallizes with two halfmolecules in the asymmetric unit, each of which is completed by a crystallographic inversion center. The two crystallographically independent molecules have almost the same geometry and are almost planar [maximum deviations = 0.018 (3) and 0.049 (3) Å ]. They adopt a conformation in which the C methyl -O bonds are directed along the molecular short axis [C-C-O-C torsion angles of 179.6 (2) and 178.0 (2) ]. In the crystal, the molecular packing is characterized by a combination of a columnar stacking and a herringbone-like arrangement. The molecules form slipped -stacks along the b axis, in which there are two kinds of columns differing from each other in their slippage. The interplanar distances between neighboring molecules are 3.493 (3) for one column and 3.451 (2) Å for the other.

Related literature
For a study of the effects of alkoxy substituents on the structures and solid-state photophysics of anthraquinones, see: Ohta, Hattori, Kusumoto, et al. (2012). For the synthesis, see: Keller & Rü chardt (1998 Hattori, Kusumoto, et al., 2012). We have revealed the cystal structure of 2,3,6,7-tetramethoxy-9,10-anthraquinone (Ohta, Hattori, Kawase, et al., 2012). In order to elucidate the substitution effects of the methoxy groups on the crystal packing, the X-ray analysis of the title compound was performed.
The molecular structure of the title compound is shown in Fig. 1. The title compound crystallizes with two halves of the molecule in the asymmetric unit of the unit cell. The complete molecules are located on crystallographic inversion centers. The molecules are almost planar with the maximum deviation of 0.018 (3) Å for C8 in one molecule and 0.049 (3) Å for C16 in another molecule. The molecules prefer the conformations in which the C methyl -O bonds are directed along the short molecular axis. Thus, the torsion angles of C3-C2-O2-C8 and C11-C10-O4-C16 are 179.6 (2) and 178.0 (2)°, respectively. Theses conformations are similar to the coressponding moiety in 2,3,6,7-tetramethoxy-9,10-anthraquinone (Ohta, Hattori, Kawase, et al., 2012). However, there is a large difference in crystal packing between the title compound and 2,3,6,7-tetramethoxy-9,10-anthraquinone. As shown in Fig. 2, the crystal structure is characterized by a columnar stacking and a herrinbone-like arrangement, although 2,3,6,7-tetramethoxy-9,10-anthraquinone molecules took a slipped-parallel arrangement. Along the b axis, there are two columns in which molecules form slipped π-stacks. The interplanar distances between neighboring molecules are 3.493 (3) Å for one column and 3.451 (2) Å for another column. Furthermore, the translational shifts of neighboring molecules in the stacks are as follows: For molecule 1 (C1-C8, O1, and O2), the slip distance between neighboring molecules is 3.94 Å, and the anthraquinone rings in the column slipped relative to each other along the long molecular axis by 0.59 Å and along the short molecular axis by 1.72 Å. In contrast, for molecules 2 (C9-C16, O3, and O4), the slip distance between neighboring molecules is 3.94 Å, and the anthraquinone rings in the column slipped relative to each other along the long molecular axis by 1.89 Å and along the short molecular axis by 0.05 Å.
To examine the influence of crystal packing on the solid-state fluorescence, the fluorescence spectrum and the absolute quantum yield of the title compound were measured with a Hamamatsu Photonics PMA11 calibrated optical multichannel analyzer with a solid-state blue laser (λ ex = 377 nm) and a Labsphere IS IS-040-SF integrating sphere, respectively. The crystals showed negligible fluorescence (Φ < 0.001). The fluorescence quenching may result from the π-stacked structure.

Experimental
The title compound was prepared according to the literature procedure (Keller & Rüchardt, 1998). Single crystals suitable for X-ray analysis were obtained by recrystallization from toluene.

Refinement
All the H atoms were positioned geometrically and refined using a riding model with C-H = 0.94 Å and U iso (H) = 1.2U eq (C) for aromatic C-H, and C-H = 0.97 Å and U iso (H) = 1.5U eq (C) for CH 3 . The positions of methyl H atoms were optimized rotationally.

Figure 1
The molecular structure of the title compound, showing the atomic numbering and 50% probability displacement ellipsoids. Symmetry code: (i) -x, -y + 1, -z. (ii) -x + 1, -y + 1, -z + 1.  The packing diagram of the title compound, viewed down the b axis. Hydrogen atoms are omitted for clarity. where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.26 e Å −3 Δρ min = −0.28 e Å −3 Special details Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(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.