Design of High-Performance Infrared Nonlinear Optical PAsS with Perfectly Aligned Polar Molecular Cage via a Bipolar-Axis-Symmetry Coupling Strategy.

Strong polar molecular cages have recently emerged as novel functional building units for high-performance infrared nonlinear optical (IR NLO) crystals. However, these highly polar molecular cages often arrange themselves in a way that cancels out their polarity, leading to a more energetically stable state. As a result, most cage crystal formations tend to crystallize in centrosymmetric space groups, which conflicts with the primary requirement for NLO crystals. Herein, we address the challenge of polar molecular cage arrangement through bipolar-axis-symmetry coupling strategy, utilizing classical NLO parent compounds. By substituting the C symmetric [BO] groups with polar C symmetric [PAsS] cages within the β-BBO polar aixs lattice, we successfully synthesized a new compound, PAsS (PAS), which exhibits a consistent arrangement of polar molecular cages - crucial for maximizing NLO performance. Additionally, due to the non-covalent interactions among [PAsS] polar molecular cages, PAS demonstrates an unexpectedly strong second harmonic generation (SHG) about 8 times that of AgGaS, along with a significant band gap of 2.75 eV. Furthermore, PAS exhibits remarkable stability against air and moisture. These findings validate our design strategy and position PAS as a promising candidate for applications in IR NLO crystals.