Molecular complexation as a design tool in the crystal engineering of noncentrosymmetric structures. Ideal orientation of chromophores linked by O-H center dot center dot center dot O and C-H center dot center dot center dotO 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 st rategy for the molecular complexation of new materials that show quadratic nonlinear optical behavior. Five of them (1-4 and 6) exhibit second harmoni c generation (SHG) activity when illuminated with 1064-nm Nd3+: YAG laser l ight and, hence, crystallize in noncentrosymmetric space groups. The biphen yl/stilbene component forms a two-dimensional slab structure mediated by ph enyl 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 wi th strong O-H . . .O and O-H . . .N and weak C-H O hydrogen bonds. In compl exes 1-4, the pyridine-1-oxide component is arranged in a herringbone motif , with an optimal orientation thus contributing favorably to the bulk NLO e fficiency. This efficiency is equivalent to that of 3-methyl-4-nitropyridin e-1-oxide (POM). Complexes 1 and 2 have similar crystal structures in space group P2(1) and comparable lattice constants. Similarly, 3 and 4 have iden tical crystal packing patterns in space group Pca2(1). In 5 (space group P2 (1)/a), the 4-nitropyridine-1-oxide occupies the space between the slabs in the form of antiparallel dimers. In complex g (space group P2(1)), the sla b structure is much changed, without any interslab spacing, and the 4-nitro pyridine-1-oxide is also involved in slab formation. Crystals of 6 show a d etectable SHG activity equivalent to that of urea.