Efficient Lone-Pair-Driven Luminescence: Structure-Property Relationships in Emissive 5s2 Metal Halides

Abstract

Low-dimensional metal halides have been the focus of intense investigations in recent years following the success of hybrid lead halide perovskites as optoelectronic materials. In particular, the light emission of low-dimensional halides based on the 5s² cations Sn²⁺ and Sb³⁺ has found utility in a variety of applications complementary to those of the three-dimensional halide perovskites because of its unusual properties such as broadband character and highly temperature-dependent lifetime. These properties derive from the exceptional chemistry of the 5s² lone pair, but the terminology and explanations given for such emission vary widely, hampering efforts to build a cohesive understanding of these materials that would lead to the development of efficient optoelectronic devices. In this Perspective, we provide a structural overview of these materials with a focus on the dynamics driven by the stereoactivity of the 5s² lone pair to identify the structural features that enable strong emission. We unite the different theoretical models that have been able to explain the success of these bright 5s² emission centers into a cohesive framework, which is then applied to the array of compounds recently developed by our group and other researchers, demonstrating its utility and generating a holistic picture of the field from the point of view of a materials chemist. We highlight those state-of-the-art materials and applications that demonstrate the unique capabilities of these versatile emissive centers and identify promising future directions in the field of low-dimensional 5s² metal halides.

Figure 1

Structural versatility of octahedral metal halides: dimensional reduction of the aristotypical AMX₃ 3D perovskite to low-dimensional metal halides.

Figure 2

Atomic environments of 0D ns² metal halides, with the lone pair visualized in orange.

Figure 3

(A) Schematic representation of the relationship between the atomic orbital model, the molecular orbital model, and the semiconductor model of an extended solid. (B) Energy band diagram associated with the free ns² ion. (C) Energy band diagram of the metal–halide molecular orbitals. (D) Configurational coordinate diagram of the simplified STE model in 0D 5s² metal halides. (E) Unified model with the configurational coordinate diagram, in which the ground and excited states are described using their atomic character as derived from the active ns² metal ion.

Figure 4

(A) 0D structures with statically expressed lone pairs (SnX₄, SbX₅) and the corresponding energy band diagram. (B) Dynamically distorted octahedral MX₆ and the corresponding band diagram. (C) Excitation-dependent PL of (bmpip)₂SnBr₄ showing singlet and triplet emission. (D) PL and PL excitation (PLE) spectra of (TTA)₂SbCl₅ showing the separate excitation of the triplet and singlet states (reproduced using the previously reported synthesis and characterized using an optical setup described elsewhere). (E) PL and PLE spectra of Cs₄SnBr₆, (C₄N₂H₁₄Br)₄SnBr₆, and [18-crown-6]₂Cs₃SbBr₆.

Figure 5

(A) Radioluminescence spectra of (bmpip)₂SnBr₄ and NaI:Tl under 50 kV Ag tube X-ray irradiation, with images of the NaI:Tl commercial scintillator and a (bmpip)₂SnBr₄ pellet under ambient light and X-ray irradiation. Adapted from ref (34). Copyright 2019 American Chemical Society. (B) Configurational coordinate diagram showing the change in energy barrier with different shifts of the excited state, with red and blue representing high and low ΔE (and thus quenching temperatures), respectively. (C) PL lifetime vs temperature for Rb₇Sb₃Cl₁₆, Cs₄SnBr₆, and (C₄N₂H₁₄I)₄SnI₆.

Figure 6

Atomic environments of polynuclear 0D 5s² metal halides derived from square pyramids and octahedra, with the lone pairs visualized in orange.