Cooperative Regulation of <i>n</i>s² Lone-Pair Expression Realizes Distinct Excitonic Emissions in Hybrid Germanium, Tin, and Lead Halides

Abstract

Lone-pair expression is significantly influenced by geometric constraints in hybrid metal halides (HMHs). Two-dimensional (2D) HMHs possess reduced structural dimensionality, allowing for a wide range of off-center displacement of the metal center. However, the effect of lone-pair-induced off-center distortion on the geometric configuration of inorganic units, electronic properties, and exciton emissions in 2D HMHs remains unclear. In this study, 2D DMPMBr₄ (DMP = <i>N</i>,<i>N</i>'-dimethylpiperazine) HMHs of group 14 elements (M = Ge, Sn, and Pb) are developed, exhibiting pronounced stereochemical activity of <i>n</i>s² lone-pair electrons. Such 2D HMHs are chosen as a model system to demonstrate the influence of the stereochemical activity of <i>n</i>s² lone-pair electrons on the geometric configuration of inorganic units, electronic properties, and exciton emissions. The off-center distortion <i><b>D</b></i> is introduced to describe the degree of lone-pair expression in these HMHs, and a quantitative relationship between the <i><b>D</b></i> and FE/STE emissions is established. When the <i><b>D</b></i> is reduced to less than 0.24, the off-center distortion of the units is sufficiently suppressed to limit the lone-pair expression, facilitating the excitonic transition from the STE state to the FE state upon compression. Substituting metal cations with those having more inactive <i>n</i>s² lone-pair electrons exerts a similar effect to pressure in promoting the excitonic transitions in the DMPMBr₄ series. This study uncovers the relationship between the stereochemical activity of <i>n</i>s² lone-pair electrons and excitonic emissions, which could accelerate the development of HMHs with the desired optical properties.