Weak Interactions in Benzylidene Anilines With fluorine atoms Three derivatives of E,E-1,4-bis[2-(2,3,4,5,6-pentafluorophenyl)ethenyl]benzene, two of which bear nitrogen atoms in the ethenyl spacers, while in a third the central benzene ring is replaced by a pyrazine moiety, have been synthesized. The supramolecular structures of the resulting set of four compounds have been studied using single-crystal X-ray diffraction to gauge the influence of the position of the nitrogen atoms on the organisation of the molecules in the solid state. The crystal packing patterns were analyzed in terms of intermolecular interactions involving the fluorine and nitrogen atoms, i.e., CH...F, F...F, F...pi and CH...N interactions. The analysis shows that in two of the three solid-state structures the main effect of the nitrogen atoms in an indirect one: they do not participate in intermolecular contacts themselves, but activate nearby hydrogen atoms and phenyl rings in fluorine synthons to form new interactions. Collas, A.; De Borger, R.; Amanova, T.; Blockhuys, F. CrystEngComm 2011, 13, 702-710. With nitro groups Two polymorphs of (E,E)-N,N'-bis(4-nitrobenzylidene)benzene-1,4-diamine have been identified. In each case, the molecules lies across a crystallographic inversion centre. the supramolecular structure of the first polymorph features stacking based on pi...pi interactions assisted by weak hydrogen bonds involving the nitro groups. The second polymorph displays a perpendicular arrangement of molecules linked via the nitro groups, combined with weak CH...O hydrogen bonds. Both crystal structures are compared with that of the carbon analogue (E,E)-1,4-bis[(4-nitrophenyl)ethenyl]benzene. Collas, A.; Zeller, M.; Blockhuys, F. Acta Cryst. C 2011, 67, o171-o174. With methoxy groups The solid-state structures of three push-pull acceptor-pi-donor (A-pi-D) systems differing only in the nature of the pi-spacer have been determined. (E)-1-nitro-4-[2-(3,4,5-trimethoxyphenyl)ethenyl]benzene and its 'bridge-flipped' imine analogues, (E)-3,4,5-trimethoxy-N-(4-nitrobenzylidene)aniline and (E)-4-nitro-N-(3,4,5-trimethoxybenzylidene)aniline, display different kinds of supramolecular networks, viz. corrugated planes, a herringbone pattern and a layered structure, respectively, all with zero overall dipole moments. Only the latter crystallizes in a noncentrosymmetric space group and is, therefore, a potential material for second-harmonic generation (SHG). Collas, A.; Blockhuys, F. Acta Cryst. C 2011, 67, o364-o369.