SHG active salts of 4-nitrophenolate with H-bonded helical formations: Structure-directing role of ortho-aminopyridines

Among the several approaches developed for the fabrication of molecular crystals with noncentrosymmetric lattices, incorporation of chirality is the only one which guarantees it for fundamental symmetry reasons. One of the few quadratic nonlinear optical chromophores which form a large number of noncentric cocrystals/salts without the assistance of chirality is 4-nitrophenolate; hence it is an ideal candidate to explore new avenues to noncentric lattice formation. The utility of a structure-directing counterion in such an approach is explored in the present study. In contrast to para-aminopyridine, it is found that ortho-aminopyridines (2-aminopyridine and 2,6-diaminopyridine) which support bent pathways of supramolecular assembly through H-bond formation lead to noncentric cocrystalline salts with 4-nitrophenol. The molecular assembly in the crystals can be visualized in the form of H-bonded helical structures. The second harmonic generation observed in the new materials is analyzed in terms of the contributions from the partners in the ternary system consisting of 4-nitrophenolate, pyridinium, and 4-nitrophenol.