(E)-4-[(4-Nitrophenyl)diazenyl]phenyl anthracene-9-carboxylate

In the title compound, C27H17N3O4, the azo group displays a trans conformation and the dihedral angles between the central benzene ring and the pendant anthracene and nitrobenzene rings are 82.94 (7) and 7.30 (9)°, respectively. In the crystal structure, weak C—H⋯O hydrogen bonds, likely associated with a dipole moment present on the molecule, help to consolidate the packing.

In the title compound, C 27 H 17 N 3 O 4 , the azo group displays a trans conformation and the dihedral angles between the central benzene ring and the pendant anthracene and nitrobenzene rings are 82.94 (7) and 7.30 (9) , respectively. In the crystal structure, weak C-HÁ Á ÁO hydrogen bonds, likely associated with a dipole moment present on the molecule, help to consolidate the packing.

Related literature
This structure is similar to the perviously reported compound (E)-2-{Ethyl[4-(4-nitrophenyldiazenyl)phenyl]amino}ethyl anthracene-9-carboxylate (Rodriguez, et al., 2008). For general background, see: Atassi et al. (1998);Becke (1993 Table 1 Hydrogen-bond geometry (Å , ).  et al. (1998) has documented photoisomerization of the azobenzene in Disperse Red 1 (DR1) to a cis conformation under UV light, with decay back to the equilibrium trans species with removal of the UV light. In this manuscript we present another compound, (I), containing a trans azobenzene conformational state (Fig. 1). The displacement ellipsoids for most of the atoms are well defined. However, the O1 and O2 atoms at the termination of the nitroazobenzene unit do show subtle enlargement. Figure 2 shows a packing arrangement and intermolecular interactions for (I). The nitroazobenzene portion is nearly planar as is the anthracene portion of the molecule. The anthracene is rotated from the nitroazobenzene through the carboxyl group. The title compound displays a head-to-toe configuration via weak C-H···O bonds as shown in Figure 2. Specifically, an O2 atom of one molecule makes a weak bond to H26 of the neighboring molecule with a bond length of 2.55 Å. The calculated dipole moment for a molecule of (I) is 7.6806 Debye using the B3LYP functional (Becke, 1993) with the 6-311 G(d,p) triple-zeta basis. This dipole moment likely drives the head-to-toe alignment of the molecules as illustrated in Figure   2.
The structure of (I) is similar in form to that of the previously reported ester (E)-2-{ethyl[4-(4nitrophenyldiazenyl)phenyl]amino}ethyl anthracene-9-carboxylate (Rodriguez, et al., 2008), with the subtle difference relating to the absence of the ethyl-amino ligand in (I). As with the aformentioned compound, intermolecular interactions for the title compound are exclusively C-H···O in nature (Table 2). An additional interaction which bridges molecules in the a axis direction is also shown in Figure 2. This weak hydrogen bond is between the terminal carboxyl oxygen O4 and the neighboring H17 atom. The hydrogen bond shows a length of 2.57 Å and symmetrically bonds the two H atoms of the anthracene of each molecule.

Experimental
The title compound was synthesized from 9-anthracenecarboxylic acid and 4-(4-nitrophenyl)azophenol via a dicyclohexylcarbodiimide esterification in anhydrous dichloromethane. After filtration of insoluble side products and removal of solvent by rotary evaporation, the crude product was dissolved in dichloromethane and filtered through a silica gel plug. Evaporation of the solvent gave a red powder that was characterized by 1H-NMR, UV/Vis and FTIR. Red crystals of (I) were obtained by recrystallization from hot dichloromethane.
supplementary materials sup-2 Figures Fig. 1. The molecular structure of (I) with 50% probability displacement ellipsoids for non-H atoms. Fig. 2. A packing diagram of (I) illustrating weak C-H···O hydrogen-bond interactions associated with terminal oxygen atoms O2 and O4.