Synthesis, Structure, and Second-Harmonic Generation of Noncentrosymmetric Cocrystals of 2-Amino-5-nitropyridine with Achiral Benzenesulfonic Acids

A series of noncentrosymmetric cocrystals were prepared from 2-amino-5-nitropyridine (2A5NP) and
achiral benzenesulfonic acids (Ar-SO3H), which were designed for nonlinear optical materials. The cocrystals are
colorless, and the melting points are fairly high, at around 200 °C. Both components are commonly crystallized in
1:1 ionic forms of 2A5NP+·Ar-SO3-. The crystal of 2A5NP and 3-methyl-4-nitrobenzenesulfonic acid belongs to chiral
space group P212121, in which the 2A5NP+ cation and the anion are alternately stacked with a dihedral angle due
to the large sulfonic anion to form column structures. Crystals of 2A5NP with both p-toluenesulfonic acid and
p-chlorobenzenesulfonic acid are crystallized into noncentrosymmetric space group Pc and are isomorphic with each
other. The 2A5NP+ cation and anion in each crystal are also alternatively stacked to form independently elongated
zigzag and layer arrays in perpendicular directions. The crystal of 2A5NP and p-phenolsulfonic acid belongs to
acentric space group Pna21, in which herringbone networks of 2A5NP+ cations are arranged in the lattice. The
feature of these organic−organic crystals is that the molecular packings are controlled by the aromatic−aromatic
interactions as well as multidirectional hydrogen bondings between the 2A5NP+ cations and anions. Second-harmonic
generation (SHG) powers measured by the SHEW (second-harmonic generation with evanescent wave) technique
revealed the high SHG efficiencies of cocrystals of 2A5NP with p-toluenesulfonic and p-phenolsulfonic acid, comparable
to those of the well-known m-nitroaniline and 4-(dimethylamino)-3-acetamidonitrobenzene (DAN), respectively.