Temperature-Dependent Reversible Optical Properties of Mn-Based Organic–Inorganic Hybrid (C₈H₂₀N)₂MnCl₄ Metal Halides

Organic–inorganic metal halides (OIMHs) have abundant optical properties and potential applications, such as light-emitting diodes, displays, solar cells, and photodetectors. Herein, we report zero-dimensional Mn-based OIMH (C 8 H 20 N) 2 MnCl 4 single crystals synthesized by a simple slow evaporation method, which exhibit intense green emission at 520 nm originating from 4 T 1 – 6 A 1 transition of Mn 2+ ions. Large organic cations in the crystal structure result in the isolated [MnCl 4 ] 2– tetrahedrons, and the closest Mn–Mn distance reaches 9.07 Å, which effectively inhibits the migration of excitation energy between adjacent Mn 2+ emission centers, thus achieving a high quantum yield (∼87%) and a long photoluminescence (PL) lifetime (3.42 ms). The different optical and structural properties at low and high temperatures are revealed by temperature-dependent PL and X-ray diffraction spectra. The PL spectra and lifetimes under the heating and cooling processes indicate that the optical property transitions are reversible at 220/240 K. Our work provides a promising strategy for building multifunctional optoelectronic materials and insights into the understanding convertible photophysical properties from isomers of metal halides.