Influence of alkyl chain length on the crystal structures and optical SHG of N-n-alkyl-2,4-dinitroanilines:: Role of dipolar and dispersion energies

The length of alkyl chains attached to NLO chromophores exerts a critical control on their crystal structures and SHG capability. Systems with short as well as long alkyl chains form centrosymmetric lattices whereas those with chains of intermediate length, typically butyl chains, form noncentrosymmetric lattices and show SHG. Investigation of a series of N-n-alkyl-2,4-dinitroanilines is reported along with crystal structure analysis of the propyl, butyl, and pentyl derivatives. Packing energy calculations are carried out for the three structures. There are no intermolecular H-bonds in these crystals. The maximum values of dipole-dipole and dispersion energies in the set are found in the centrosymmetric crystals of the propyl and pentyl derivatives, respectively. Intermediate values of the two energy components are observed in the butyl derivative, which alone has a noncentrosymmetric crystal structure and is SHG active.