2-[(E)-2-(3-Hydr­oxy-4-methoxy­phen­yl)ethen­yl]-1-methyl­quinolinium 4-methyl­benzene­sulfonate

The search for new organic materials that exhibit large molecular second-order optical nonlinearities is of considerable interest because of their wide range of potential applications, such as in the frequency doubling of lasers and optical data storage applications (Chemla & Zyss, 1987; Marder et al., 1991; Usman et al., 2000; Umezawa et al., 2002; Feng et al., 2005; Ye et al., 2005). Some organic materials are transparent in the visible region, which is good for nonlinear optical (NLO) materials compared with inorganic crystals [what exactly is being compared here?] (Chemla & Zyss, 1987; Chia et al., 1995; Zhengdong et al., 1997). An organic crystal with delocalized π-electrons usually displays a large NLO response (Prasad & William, 1991). Most of these crystals are composed of aromatic molecules that are substituted with π-electron donors and acceptors, which exhibit intramolecular charge transfer. One generally successful and popular approach to obtaining NLO materials is to find a substance that crystallizes in a non-centrosymmetric space group. Oudar & LePerson (1975) have also reported the effect of conjugation length, using stilbene instead of a benzene π-system. In our continuing research on NLO materials (Rahman et al., 2003; Jindawong et al., 2005; Chantrapromma et al., 2005), we expected that the two rings of quinoline, with the ethylenic C═C lying in the same molecular plane, would change the molecular packing from centrosymmetric, as in our previous report (Rahman et al., 2003), to non-centrosymmetric. For this reason, the title compound, (I), was designed and synthesized. The X-ray study of (I) was carried out in order to obtain detailed information on its crystal structure.

Compound (I) is found to crystallize in the centrosymmetric space group P2₁/n and, therefore, has no second-order nonlinear optical properties. The molecular structure of (I) is illustrated in Fig. 1, and selected bond distances and angles are given in Table 1. The asymmetric unit of (I) consists of a C₁₉H₁₈NO₂⁺ cation and a C₇H₇SO₃⁻ anion. The quinolinium ring system [C16–C24/N1] is essentially planar, with a maximum deviation of 0.029 (2) Å for atom N1. The H atoms attached to C14 and C15 are trans to each other; thus the cation exists in an E configuration [the C13—C14—C15—C16 torsion angle is −178.1 (2)°]. The bond distances and angles (Table 1) in both the cation and the anion have normal values (Allen et al., 1987) and are comparable to those in a related structure (Rahman et al., 2003). The cation is almost planar, the dihedral angle between the benzene and quinolinium rings being 2.59 (9)°. The methoxy substituent deviates slightly from the plane of the benzene ring, with torsion angles of 7.5 (3)° for C25—O4—C10—C9 and 177.0 (2)° for C8—C9—C10—O4. The benzene ring in the 4-methylbenzenesulfonate anion makes dihedral angles of 60.59 (10)° with the benzene ring of the cation and 59.17 (9)° with the quinolinium ring system.

In the crystal structure, atoms O2 and O3 of the anion are involved in weak C—H···O interactions, while atom O1 is involved in O—H···O hydrogen bonds (Table 2). The molecules form alternate layers of cations and anions along the b axis. These layers are interconnected through O—H···O hydrogen bonds and weak C—H···O interactions to form a three-dimensional network (Fig. 2 and Table 2). The crystal structure is further stabilized by C—H···π interactions involving the centroid, Cg1, of the benzene ring (C8–C13) of the cation (Table 2).