Cooperative\nRegulation of <i>n</i>s<sup>2</sup> Lone-Pair Expression\nRealizes Distinct Excitonic Emissions\nin Hybrid Germanium, Tin, and Lead Halides

Lone-pair expression is significantly\ninfluenced by geometric constraints\nin hybrid metal halides (HMHs). Two-dimensional (2D) HMHs possess\nreduced structural dimensionality, allowing for a wide range of off-center\ndisplacement of the metal center. However, the effect of lone-pair-induced\noff-center distortion on the geometric configuration of inorganic\nunits, electronic properties, and exciton emissions in 2D HMHs remains\nunclear. In this study, 2D DMPMBr<sub>4</sub> (DMP = <i>N</i>,<i>N</i>′-dimethylpiperazine) HMHs of group 14\nelements (M = Ge, Sn, and Pb) are developed, exhibiting pronounced\nstereochemical activity of <i>n</i>s<sup>2</sup> lone-pair\nelectrons. Such 2D HMHs are chosen as a model system to demonstrate\nthe influence of the stereochemical activity of <i>n</i>s<sup>2</sup> lone-pair electrons on the geometric configuration\nof inorganic units, electronic properties, and exciton emissions.\nThe off-center distortion <i><b>D</b></i> is introduced\nto describe the degree of lone-pair expression in these HMHs, and\na 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\nof the units is sufficiently suppressed to limit the lone-pair expression,\nfacilitating the excitonic transition from the STE state to the FE\nstate upon compression. Substituting metal cations with those having\nmore inactive <i>n</i>s<sup>2</sup> lone-pair electrons\nexerts a similar effect to pressure in promoting the excitonic transitions\nin the DMPMBr<sub>4</sub> series. This study uncovers the relationship\nbetween the stereochemical activity of <i>n</i>s<sup>2</sup> lone-pair electrons and excitonic emissions, which could accelerate\nthe development of HMHs with the desired optical properties.