(C 4 H 6 N 3 O)(HSO 4 ): A Cytosinium Bisulfate with Large Birefringence and Moderate Second Harmonic Generation Effect Produced via Combining a Promising Planar Nonlinear Optical-Active Motif with a Tetrahedral Group.

Investigating novel nonlinear optical (NLO) active units serves as a valuable method for broadening the research landscape of NLO materials. This study showcases the potential of the cytosinium cation (C 4 H 6 N 3 O) + as a novel NLO-active motif through theoretical calculations. The title compound exhibited a wide band gap of 3.85 eV, along with a moderate second harmonic generation (SHG) response of 1.65 times that of KH 2 PO 4 (KDP) and significant birefringence of 0.47. Its exceptional optical properties are primarily attributed to the synergy interaction between cations and anionic groups in the asymmetric unit.