1,4-Bis[(2,6-dimethoxyphenyl)ethynyl]benzene

The title compound, C26H22O4, is a derivative of 1,4-bis(phenylethynyl)benzene substituted by four methoxy groups at the terminal benzene rings. The asymmetric unit consists of two half-molecules; one centrosymmetric molecule is planar but the other is non-planar, with dihedral angles of 67.7 (1)° between the central benzene ring and the terminal benzene rings. In the crystal structure, molecules form a zigzag molecular network due to π–π [the interplanar and centroid–centroid distances between the benzene rings are 3.50 (1) and 3.57 (1) Å, respectively] and C—H⋯π interactions (2.75 Å). Introduction of the four methoxy groups results in the supramolecular architecture.

The title compound, C 26 H 22 O 4 , is a derivative of 1,4bis(phenylethynyl)benzene substituted by four methoxy groups at the terminal benzene rings. The asymmetric unit consists of two half-molecules; one centrosymmetric molecule is planar but the other is non-planar, with dihedral angles of 67.7 (1) between the central benzene ring and the terminal benzene rings. In the crystal structure, molecules form a zigzag molecular network due to -[the interplanar and centroidcentroid distances between the benzene rings are 3.50 (1) and 3.57 (1) Å , respectively] and C-HÁ Á Á interactions (2.75 Å ). Introduction of the four methoxy groups results in the supramolecular architecture.

Comment
The synthetic research of ethynylated aromatic compounds has attracted considerable attention because of interests in their molecular structures (Bunz et al., 1999;Kawase et al., 2003), optical properties (Beeby et al., 2002;Bunz, 2000) and molecular electronics (Tour, 2000). Among these ethynylated aromatic compounds, 1,4-bis(phenylethynyl)benzene derivatives have been extensively studied. These compounds have stiff, linear molecular structures and are used as building blocks in the applications such as liquid-crystalline materials (Dai et al., 1999) and electron-conducting molecular wires (Moore et al., 2006). According to the X-ray crystallographic analyses of 1,4-bis(phenylethynyl)benzene, the molecules crystallize in two crystal forms with the P1 and Pbcn space groups (Watt et al., 2004;Li et al., 1998). In both crystals, the molecules are linear and planar. In P1, the molecules are aggregated by the face-to-edge interactions based on C-H···π contacts (2.74-2.89 Å).
In Pbcn, the molecules form π-dimers with an intermolecular distance of 3.49 Å. The π-dimers are aggregated by the faceto-face interactions based on π···π contacts (3.45 Å) and the face-to-edge interactions based on C-H···π contacts (benzene ring) (2.85-2.88 Å) and C-H···π contacts (C≡C bond) (2.79-2.87 Å). Furthermore, the X-ray crystallographic analysis carried out on 1,4-bis(p-tolylethynyl)benzene in P2 1 /c (Filatov & Petrukhina, 2005) again showed the molecule to be linear and planar. The molecules are stacked along the b axis to form a column with intermolecular distances of 3.51 and 3.56 Å. This result indicates that the introduction of two methyl groups to the terminal benzene rings provides the modification of molecular assembly. With regard to this, we investigated the molecular and crystal structure of the title compound, (I), which is a derivative substituted by four methoxy groups at the terminal benzene rings.
Single crystals of (I) were grown by recrystallization from dichloromethane. These produce a structure in P2 1 /c that shows two crystallographically independent molecules, each possessing an inversion centre (Fig. 1). One molecule is planar and strained at the C≡C bonds. The other molecule is a linear, nonplanar structure with a dihedral angle of 67.7 (1)°b etween the central benzene ring and the terminal benzene rings. The C≡C bond lengths are 1.200 (2) Å (C7-C8) and 1.199 (2) Å (C20-C21). These values are analogous to those of 1,4-bis(phenylethynyl)benzene (1.202-1.205 Å).
In summary, we studied the molecular and crystal structure of 1,4-bis[(2,6-dimethoxyphenyl)ethynyl]benzene, which is a 1,4-bis(phenylethynyl)benzene derivative substituted by four methoxy groups at the terminal benzene rings. The methoxy supplementary materials sup-2 groups fixed a π-stacking geometry between the terminal benzene rings resulting in the formation of the zigzag molecular network. The introduction of methoxy groups provided a supramolecular architecture of 1,4-bis(phenylethynyl)benzene.
The reaction mixture was stirred for 16 h at 80 °C. After removal of the solvent, dichloromethane (30 ml) and aqueous disodium ethylenediaminetetraacetate (Na 2 edta) solution (5%, 30 ml) were added. The organic layer was separated and washed with water (30 ml). The organic solution was dried over Na 2 SO 4 and concentrated. The residue was separated by column chromatography on silica gel (CH 2 Cl 2 /hexane = 9: 1) to afford compound (I) (52 mg, 56%) as a yellow powder.
Yellow crystals of (I) suitable for X-ray analysis were grown from a dichloromethane solution.

Refinement
All the H atoms were placed in geometrically calculated positions, with C-H = 0.95 (phenyl) and 0.98 (methyl) Å, and refined using a riding model with U iso (H) = 1.2U eq (C) (phenyl) and 1.5U eq (C) (methyl).