C₆N₇(NH₂)₃·H₃PO₄: Strong Second Harmonic Generation and Giant Birefringence Benefiting from Amino Substitution Strategy

Abstract With the advancement of laser technology, searching for new nonlinear optics (NLO) active units is necessary for designing NLO materials with excellent performance. In this study, an amino substitution strategy is proposed to explore new planar π‐conjugated groups as functional building units (FBUs) for NLO and birefringent materials. Accordingly, a new NLO‐active molecule melem C₆N₇(NH₂)₃ is identified, which is characterized by significant polarization anisotropies and large hyperpolarizabilities. Subsequently, C₆N₇(NH₂)₃·H₃PO₄ is screened out and successfully synthesized through a simple aqueous method. The compound exhibits a strong second harmonic generation (SHG) response of 8.9 times that of KDP and a large birefringence of 0.609 at 1064 nm. As far as is known, the calculated birefringence of C₆N₇(NH₂)₃·H₃PO₄ is the largest among the organic–inorganic hybrid crystals in which the birefringence originates from π‐conjugated groups. The theoretical computation supports that the amino (─NH₂) substitution strategy is effective for designing and synthesis of materials with strong SHG response and large birefringence.