2,3,6,7-Tetramethoxy-9,10-anthraquinone

Molecules of the title compound, C18H16O6, are almost planar [maximum deviation = 0.096 (4) Å] and reside on crystallographic centres of inversion. They adopt a conformation in which the Cmethyl—O bonds are directed along the molecular short axis [C—C—O—C torsion angles of −175.3 (3) and 178.2 (3)°]. In the crystal, molecules adopt a slipped-parallel arrangement with π–π stacking interactions along the a axis with an interplanar distance of 3.392 (4) Å. Weak C—H⋯O interactions link the molecules into sheets parallel to (10-2).

Molecules of the title compound, C 18 H 16 O 6 , are almost planar [maximum deviation = 0.096 (4) Å ] and reside on crystallographic centres of inversion. 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 À175.3 (3) and 178.2 (3) ]. In the crystal, molecules adopt a slipped-parallel arrangement withstacking interactions along the a axis with an interplanar distance of 3.392 (4) Å . Weak C-HÁ Á ÁO interactions link the molecules into sheets parallel to (102).

D-HÁ
Data collection: RAPID-AUTO (Rigaku, 1999); cell refinement: PROCESS-AUTO; data reduction: PROCESS-AUTO; program(s) used to solve structure: SIR2004 (Burla et al., 2005); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999). Acta Cryst. (2012). E68, o2587 [doi:10.1107/S1600536812033119] 2,3,6,7-Tetramethoxy-9,10-anthraquinone Akira Ohta, Kazuki Hattori, Takeshi Kawase, Takashi Kobayashi, Hiroyoshi Naito and Chitoshi Kitamura Comment 9,10-Anthraquinone is an important molecule in the field of industrial dyes. We have recently been interested in the tuning of the solid-state optical properties by the introduction of substituents. As part of our program aimed at the elucidation of the effects of alkoxy substituents on the optical properties in the solid state (Ohta, et al., 2012), we are in need of the information on the crystal structures of a variety of 2,3,6,7-tetraalkoxy-9,10-anthraquinones in order to clarify the correlation between crystal structures and the solid-state photophysics. Although the title compound is already known (Boldt, 1967), the X-ray structure was not reported to date. We report herein the crystal structure of the title compound (I).
To examine the influence of crystal packing on the solid-state fluorescences, the fluorescence spectrum and the absolute quantum yield of (I) 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 = 0.002). The fluorescence quenching would be due to the π-stacked structure.

Experimental
The title compound was prepared according to the literature procedure (Boldt, 1967). Single crystals suitable for X-ray analysis were obtained by recrystallization from DMF.

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 (I), showing the atomic numbering numbering and 30% probability displacement ellipsoids. Symmetry code: (i) -x, -y + 1, -z.

Figure 2
The packing diagram of (I). Hydrogen atoms are omitted for clarity. 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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq C1 0.3162 (