methylpyridinium} 4-chlorobenzene- sulfonate nitrate Hoong-Kun Fun,a*‡Ching Kheng Quah,a Nawong

disorder in main residue; R factor = 0.074; wR factor = 0.254; data-to-parameter ratio = 9.8. In the title mixed salt, 2C16H15N2 +C6H4ClO3SNO3, one of the cations shows whole molecule disorder over two sets of sites in a 0.711 (7):0.289 (7) ratio. The 4-chorobenzenesulfon-ate anion is also disordered over two orientations in a 0.503 (6):0.497 (6) ratio. The cations are close to planar, the dihedral angles between the pyridinium and indole rings being 1.48 (3) in the ordered cation, and 5.62 (3) and 2.45 (3), respectively, for the major and minor components of the disordered cation. In the crystal, the cations are stacked in an antiparallel manner approximately along the a-axis direction and linked with the anions via N—H O hydrogen bonds and C—H O interactions, generating a three-dimensional network. Weak C—H and – interactions [with centroid–centroid distances of 3.561 (2)–3.969 (7) Å] are also observed. Related literature For related structures, see: Chantrapromma et al. (2008); Chantrapromma & Fun (2009). For background to non-linear optical materials, see: Dittrich et al. (2003); Nogi et al. (2000)