HSAB-Guided Design of Chiral Osmium Halides With Both Efficient Nonlinear Optical Properties and High Stability.

Chiral hybrid organic-inorganic metal halides (HOIMHs) hold great promise for nonlinear optical (NLO) applications. However, their practical applications are severely hindered by the poor environmental stability and photostability. Herein, guided by the hard and soft acids and bases (HSAB) theory, we quantitatively calculated and compared the chemical hardness of all elements in the periodic table based on density functional theory, and found osmium (Os) exhibits the highest chemical hardness among Group VIII transition metals. Thus, we synthesized the chiral osmium-based HOIMHs, (R/S-3BrMBA)2OsCl6 (R/S-Os), and systematically investigated their NLO properties and stability. As expected, the obtained chiral osmium halides remain stable after two months of exposure to air, and the second-harmonic generation (SHG) intensity remains almost unchanged after the same period, confirming their outstanding long-term stability. The efficient SHG responses with effective second-order nonlinear susceptibility (χeff) of 6.4 pm/V and strong SHG circular dichroism (SHG-CD) with anisotropy factor (|gSHG-CD|) of 0.33 are also observed. Our work provides valuable HSAB-guided screening strategy for predicting and designing the highly stable NLO materials.