Role of Hydrocarbon Chain Length and Head Shape in Design of Low-Dimensional Metal Chlorides with Efficient Emission

The recent surge in low-dimensional metal halides with efficient luminescence has delivered new compounds for applications including LED phosphors, luminescent solar concentrators, and scintillation radiation detectors. The vast array of potential counter-cations offers similarly wide potential for design of these compounds, but this diversity poses a challenge to predictability of the resulting crystal structure. Here, we explore a) the role of hydrocarbon chain length and b) the impact of cation shape through comparison amongst a family of 0D antimony chlorides and a family of 2D and 0D manganese chlorides, respectively. This presentation will provide an overview of long-chain cation-based structure types compatible with efficient light emission, contributing to development of design principles for this burgeoning class of materials.<br/><br/>To illustrate the role of chain length, we developed a series of compounds A₂SbCl₅based on the characteristic SbCl₅ square pyramid. We report three novel hybrid compounds [BDXA]₂SbCl₅ where BDXA are three quaternary ammonium cations that structurally identical save for having a terminal 14-, 16-, and 18-carbon chain, enabling analysis of the impact of this chain length on the optical properties_.The compounds were synthesized via crystallization in DMF with several drops of HCl. Single crystal X-ray diffraction reveals that all three compounds are isostructural, crystallizing in a monoclinic crystal system comprising isolated [SbCl₅]^2- square pyramids spaced out by A-cations. Intriguingly, all compounds exhibit bending of the long chain in 50% of the A-site cations, and this bent portion of the chain contributes to crystallographic disorder at room temperature. All three compounds exhibit a strong bright orange triplet emission, <i>e.g.</i> [BDTA]₂SbCl₅emits at 620nm for a low energy excitation at 360nm, and exhibits singlet emission near 470nm for high energy excitation at 310nm. These compounds have a large Stokes shift of ~1.5 eV and a photoluminescent quantum yield (PLQY) above 90%, making them promising candidates for applications in lighting and scintillation. This presentation will present a novel structure type based on these mixed bent and straight chains, and relate the impact of the chain length by tying subtle changes in the structural details to the optical features of these candidate phosphors.<br/><br/>To probe the role of cation shape in long-chain hybrid metal halides, we report the discovery of 4 members of a series of A₂MnCl₄ based on long-chain A-site cations with two chain heads – one with a terminal ammonium group (denoted C_n-MA) while the other chain ends in a trimethylammonium head (denoted C_n-TMA). This chain-head engineering dictates the dimensionality of the final crystal structure, as the ammonium-terminated (C₁₂-MA)₂MnCl₄ and (C₁₈-MA)₂MnCl₄ form a 2D Ruddlesden-Popper perovskite structure with a 1-layer thick slab of corner-shared octahedra, while trimethyl-terminated (C₁₄-TMA)₂MnCl₄ and (C₁₈-TMA)₂MnCl₄ crystallize in a novel 0D structure with isolated MnCl₄ tetrahedra that pack in layers with the larger TMA chain heads; these layers are separated by the nonpolar carbon chains. The driver behind these structures is the size of the chain head: the methyl-terminated head fits in the terminal voids of the 2D perovskite framework, while the larger trimethyl-terminated cation is too large to coexist near this framework, splitting the would-be octahedra into isolated MnCl₄ tetrahedra. The 2D perovskite structures emit the faint red luminescence characteristic of MnCl₆ octahedra, while the 0D (C₁₄-TMA)₂MnCl₄ and (C₁₈-TMA)₂MnCl₄ emit bright green luminescence characteristic of tetrahedral MnCl4 coordination. These green emitters offer near-unity PLQY due to isolation of the tetrahedra provided by the large trimethyl chain heads. This study illustrates the role of cation shape in directing the structure of these hybrid halides, with implications for development of novel halide emitters.