Molecular Crystals Constructed by Polar Molecular Cages: A Promising System for Exploring High-performance Infrared Nonlinear Optical Crystals

Polar molecular crystals, with their densely stacked polar nonlinear optical (NLO) active units, are favored for their large second harmonic generation (SHG) responses and birefringence. However, their potential for practical applications as Infrared (IR) NLO materials has historically been underappreciated due to the weak inter-molecular interaction forces that may compromise their physicochemical properties. In this study, we propose molecular crystals with polar molecular cages as a treasure-house for the development of superior IR NLO materials and a representative system, binary chalcogenide molecular crystals, composed of [P4Sn] (n=3-9) polar molecular cages, is introduced. These crystals may not only achieve wide band gap, large SHG response, and birefringence in a single structure, but also exhibit favorable physicochemical properties. We subsequently obtained a polar molecular crystal, alpha-P4S5, which demonstrated exceptional IR optical properties, including a strong SHG response (1.1xAGS), wide band gap (3.02 eV), large birefringence (0.134@2050 nm), and a broad transmission range (0.41-14.7 mu m). Moreover, it showed excellent water resistance and hardness. These findings highlight the potential of polar molecular crystals as a promising platform for the development of high-performance IR NLO materials. Molecular crystals with polar molecular cages were firstly proposed as a promising system for exploring superior IR NLO materials and a representative system, the binary chalcogenide family P4Sn (n=3-9), is introduced. As a member of the system, alpha-P4S5 possesses a robust SHG response (1.1xAGS), a broad band gap (3.02 eV), ample birefringence (0.134@2050 nm), a wide optical transmission range (0.41-14.7 mu m), and exceptional water-resistance. image