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 1, pyridine-N-oxides 2, and 2-halo-3-aminopyridine 3 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 (mu) halogenated N-oxides 2 (mu>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.