Crystal engineering of some 2,4,6-triaryloxy-1,3,5-triazines: Octupolar nonlinear materials

Abstract

The principles of crystal engineering have been used to design a family of structures with potential as octupolar nonlinear optical (NLO) materials. The major aim in such an exercise, a carry-over of molecular symmetry into the crystal, is possible with a retrosynthetic approach. An appropriate choice of precursor trigonal molecules leads from the concept of the dimeric Piedfort unit. The crystal structures and NLO properties of a series of 2,4,6-triaryloxy-1,3,5-triazines, 1-6, are reported. These compounds consistently form quasitrigonal or trigonal networks that are two-dimensionally noncentrosymmetric. Substitutional variations on the phenyl moieties that were expected to maintain or to perturb this trigonal network have been explored. Molecular nonlinearities have been measured by Harmonic Light Scattering (HLS) experiments. Among the compounds studied, 2,4,6-triphenoxy-1,3,5-triazine, 1 adopts a noncentrosymmetric crystal structure with a measurable SHG powder signal. All these crystal structures are stabilized by weak intermolecular interactions such as herringbone, π...π, C-H...O, and C-H...N hydrogen bonding. These octupolar molecules are more isotropic than the classical p-nitroaniline based dipolar NLO molecules, and this is advantageous from the viewpoint of potential electrooptic applications.