Isoniazid nitrate: synergistic dual π-conjugation triggering prominent second-harmonic generation.

Nonlinear optical (NLO) crystals are among the key materials in laser science and technology. In the present work, an NLO crystal, isoniazid nitrate (C6H8N3O)NO3, composed of a π-conjugated organic cation (C6H8N3O+) and an inorganic nitrate anion (NO3-), was synthesized via a dual π-conjugation-driven synergistic strategy. This crystal exhibits a remarkable second-harmonic generation (SHG) response (∼2.1 × KH2PO4) and a large birefringence (0.279 at 546.1 nm). In-depth theoretical calculations reveal that the favorable SHG performance originates from the π-conjugated orbitals formed by p-orbitals, while bonding electrons and s-orbital electrons contribute negligibly to the SHG effect. This work provides a case study for the systematic investigation of the dual π-conjugation strategy in both experimental implementation and theoretical exploration.