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

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 Nd 3+: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 P 2 1 and comparable lattice constants. Similarly, 3 and 4 have identical crystal packing patterns in space group Pca 2 1 . In 5 (space group P 2 1 / a ), the 4-nitropyridine-1-oxide occupies the space between the slabs in the form of antiparallel dimers. In complex 6 (space group P 2 1), 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.