Elucidating the Impact of Bi 3+ Lone Pair on the Optical and Electronic Properties of Phosphonium‐Based Halide Semiconductors

Hybrid metal halide semiconductors exhibit intriguing photophysical properties that are attributed to strong electron‐phonon coupling mediated by lattice deformations. Although extensive research has established a relatively clear picture of the effects of both intra‐octahedral and inter‐octahedral distortions on the optical and electronic behavior of 3D and 2D metal halide materials, there is still a lack of distortion descriptors to quantify octahedral distortions in edge‐sharing 0D dimers (oligomers). Here, we provide new structure distortion descriptors to accurately quantify the off‐centering of the metal cation from the geometric center of the octahedron and the twist angle between face‐sharing octahedra. As a testing platform, we synthesized two new 0D metal halides based on [Bi 2 I 9 ] 3− dimers using phosphonium organic cations, with general formula (L) 3 Bi 2 I 9 (L = butyl(trimethyl)phosphonium bromide (P5‐P), benzyl(triethyl)phosphonium bromide (P3‐P)). We correlated the inter‐octahedral twisting within the metal halide dimers with their photoluminescence at RT and 80 K, while their inherent water stability enabled the evaluation of their redox and sensing properties toward persistent organic pollutants. The assembly of flexible, wearable electrodes demonstrated the simultaneous determination of per‐ and polyfluoroalkyl substances (PFAS) and polychlorinated biphenyls (PCBs), underscoring their promise for portable field applications.