Molecular complexation as a design tool in the crystal engineering of noncentrosymmetric structures. Ideal orientation of chromophores linked by O-H···O and C-H···O hydrogen bonds for nonlinear optics

Abstract

Six molecular complexes formed between 4-hydroxy-4'-nitrobiphenyl/stilbene and a 4-substituted pyridine-1-oxide [methyl (1,2), cyano (3,4), and nitro (5,6)] have been studied with the specific aim of assessing a new design strategy for the molecular complexation of new materials that show quadratic nonlinear optical behavior. Five of them (1-4 and 6) exhibit second harmonic generation (SHG) activity when illuminated with 1064-nm Nd³⁺:YAG laser light and, hence, crystallize in noncentrosymmetric space groups. The biphenyl/stilbene component forms a two-dimensional slab structure mediated by phenyl···phenyl (C···H and C···C) interactions, and the polar axes of the biphenyl/stilbene are in an antiparallel alignment. In complexes 1-5, the pyridine-1-oxide component occupies the interslab spaces and is bound to the slabs with strong O-H···O and O-H···N and weak C-H···O hydrogen bonds. In complexes 1-4, the pyridine-1-oxide component is arranged in a herringbone motif, with an optimal orientation thus contributing favorably to the bulk NLO efficiency. This efficiency is equivalent to that of 3-methyl-4-nitropyridine-1-oxide (POM). Complexes 1 and 2 have similar crystal structures in space group P2₁ and comparable lattice constants. Similarly, 3 and 4 have identical crystal packing patterns in space group Pca2₁. In 5 (space group P2₁/a), the 4-nitropyridine-1-oxide occupies the space between the slabs in the form of antiparallel dimers. In complex 6 (space group P2₁), the slab structure is much changed, without any interslab spacing, and the 4-nitropyridine-1-oxide is also involved in slab formation. Crystals of 6 show a detectable SHG activity equivalent to that of urea.