The Renaissance of Functional Hybrid Transition-Metal Halides

ConspectusThere is an extensive history of research on both inorganic and hybrid metal halides, with the latter being first reported in the 1960s. Although work on hybrid systems has progressed steadily over the last 60 years, it has enjoyed a major renaissance during the last 5 years. This has arisen as a consequence of the 2009 discovery of the outstanding optoelectronic properties of hybrid lead halides, such as (MA)PbI3 (MA = methylammonium), and the recognition that there are many opportunities for equally exciting discoveries with compounds of the transition metals. Some of the early work on hybrid transition-metal halides put more emphasis on crystal structures but less on properties. In the modern era, we aim to grasp both the structure and properties, with a new twist. In this Account, we shall explore the recent developments in hybrid transition-metal halides with a focus on work in four main areas: magnetism, photoluminescence, semiconductivity, and spintronics. Our work on magnetism centers on the Ru-based hybrid halides, where the structural types are diversely composed of vacancy-ordered double perovskite, as well as chain-like one-dimensional structures and layered double perovskite (LDP) when paired with a (1+) metal. We explore their magnetic properties and find that their spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. In the luminescence section, we focus on our recent works on hybrid tetrahedral Mn(II) bromides and Cu(I) and Ag(I) iodides. We correlated our newly discovered 0D AmMnBr4 (A = organic cation, m = 1 or 2) compounds with previous reports, and generated a trend where the photoluminescence quantum yield (PLQY) increases with larger Mn-Mn distances. The flexible organic cation becomes the most important tool here to tune the structure-PLQY relations. Cu(I) and Ag(I) iodides coordinated with iodides and organic ligands produce new crystal structures with intense PL. For the semiconducting properties, we explore the Pt-based vacancy-ordered double perovskite and hybrid bismuth and indium-based LDPs to show the structural evolution with different choices for the organic cation, the metal and the halide; these have a strong influence on the optical properties. The LDPs specifically exhibit high structure tunability, with a wide range of (1+) and (3+) metal choices, and are exempt from some of the limitations of 3D double perovskite. In the last section, we introduce the recent progress on hybrid transition-metal-based ferroelectrics and spintronic materials. We successfully demonstrate the utilization of chiral Cu(II) chlorides for circularly polarized light (CPL) detection, showing the high anisotropy of the photoresponsivity. We also highlight the work that the authors have contributed in these areas and suggest several exciting opportunities for future developments.