New acentric materials constructed from aminopyridines and 4-nitrophenol

Co-crystallization of 4-nitrophenol (I) with five aminopyridines (4-aminopyridine 1, 3,4-diaminopyridine 2, 2,3-diaminopyridine 3, 3-aminopyridine 4, 2-amino-6-methylpyridine 5) and 2,4-diaminopyrimidine 6 resulted in six adducts with the ratio of components 2 : 1 in five and 1 : 1 in one final compounds. Single crystals were grown by slow evaporation technique using ethanol as a solvent. Five adducts with 1-5 crystallize in acentric P2(1) and Pna2(1) space groups, and one, 2(I)center dot 6-in centrosymmetric P2(1)/c space group. Compounds 2(I)center dot 1, 2(I)center dot 2, 2(I)center dot 3 are isomorphous, and demonstrate similar H-bonding patterns despite the differences in aminopyridine molecules. Compound 2(I)center dot 5 is isomorphous to two previously reported compounds. Adducts 2(I)center dot 1, 2(I)center dot 2, 2(I)center dot 3, 2(I)center dot 5, 2(I)center dot 6 represent organic salts composed of pyridinium/pyrimidinium cation, 4-nitrophenolate anion, and 4-nitrophenol neutral molecule. The H-bonded 4-nitrophenol-4-nitrophenolate anionic dimers were found in all compounds with 2 : 1 molar ratio. In adduct I center dot 4 both molecules are in neutral form. The IR spectral data support crystallographic conclusions on salts formation. Plane wave pseudopotential density functional theory calculations were used to predict hyperpolarizability tensor components. Our calculations suggest 2(I)center dot 3 as the best candidate for nonlinear optical materials (14 times more active than urea).