Using Halogen···Halogen Interactions to Direct Noncentrosymmetric Crystal Packing in Dipolar Organic Molecules

Halogen atoms Cl, Br, and I steer crystallization of 2-halo-3-hydroxypyridine <b>1</b>, pyridine-<i>N</i>-oxides <b>2</b>, and 2-halo-3-aminopyridine <b>3</b> in noncentrosymmetric space groups efficient for nonlinear optical materials. Whereas the strong hydrogen-bond chains are aligned antiparallel, the L- or V-geometry of weak interhalogen interaction steers polar alignment in the solid state, even for high dipole moment (μ) halogenated <i>N</i>-oxides <b>2</b> (μ > 5.0 D). Six out of seven structures evaluated crystallize in polar/chiral space groups due to interhalogen interactions compared to the one in six probability of noncentrosymmetric packing for achiral molecules.