Reversal Enhancement of Birefringence by Stereochemically Active Lone Pairs in Sn(II)-Based Mixed Halides

The indispensability of birefringent materials in advanced optical technologies stems from their pivotal role in the generation and manipulation of polarized light. Sn2+ with stereochemically active lone pairs have widespread applications in synthesizing novel birefringent materials, which are attributed to their significant enhancement of birefringence. Although the activity of lone pair electrons is influenced by different anions, there remains a lack of understanding regarding the relationship between the activity of lone pairs and birefringence in mixed halides. Herein, through an analysis of three Sn(II)-based mixed halides (SnFCl, Sn2F3I, and SnClI), we demonstrate that the activity of lone pairs is governed by the energy lever difference between cation s-states and halogen p-states, with a decreasing trend from F to I. Subsequently, we find a gradual decrease in the lone pair activity of SnFCl, Sn2F3I, and SnClI while their birefringence gradually increases. The reversal enhancement of birefringence with respect to lone pair electron activity can be ascribed to the distinct spatial orientations of stereochemically active lone pairs. These findings provide significant perspectives for comprehending birefringent materials that contain lone pair electrons.