Chain-Head Engineering Dictates Structural Dimensionality and Bright Emission in Long-Chain A2MnCl4

The rise of emissive low-dimensional hybrid metal halides has led to new materials for applications ranging from solid-state lighting to scintillation detectors. The plethora of potential organic counter-cations offers potential to design these halides, but their variety poses a challenge in predicting structure and tailoring the optical behavior of these hybrid compounds. Here, we explore the role of A-cation chain head and chain length on the structural and optical properties of alkylammonium metal halides, using the well-studied manganese halides A2MnCl4 as a test system. We report the discovery of four new A2MnCl4 compounds based on ammonium (C12-Am and C18-Am) and trimethyl (C14-TMA and C18-TMA) chain heads. (C12-Am)2MnCl4 and (C18-Am)2MnCl4 adopt 2D Ruddlesden-Popper perovskite structures, with weak red emission characteristic of octahedrally-coordinated Mn2+. Meanwhile, the larger trimethyl chain head drives (C14-TMA)2MnCl4 and (C18-TMA)2MnCl4 to form 0D structures with bilayers of isolated [MnCl4]2- tetrahedra spaced out by nonpolar chains, with bright green emission and quantum yields of 30-45 %. While chain length plays a minimal role in the optical properties, the chain head shape directly dictates them through structural control of the Mn2+ emission. This work delivers a clear design principle for hybrid manganese chlorides, aiding the development of next-generation emissive compounds.